CA1075877A - Sub-micron refractory metal boride powder and method for preparing same - Google Patents

Abstract

Abstract of the Disclosure Sub-micron titanium diaboride powder and other hard, refractory metal boride powders, such as zirconium diboride and hafnium diboride powders, are prepared by vapor phase reaction of the corresponding metal halide, e.g., titanium halide, and coron source reactante in the presence of hydrogen in a reaction zone and in the substantial absence of oxygen, either combined or elemental. In a preferred embodiment, the metal halide, e.g., titanium tetrachioride, and boron source, e.g., boron trichloride, reactants are mixed with a hot stream of hydrogen produced by heating hydrogen in a plasms heater. The reaction zone is mainitained at metal boride forming temperature and submicron soild metal boride powder is removed promptly from the reactor and permitted to cool. The preponderant number of metal boride particles comprising the powder product have a particle size in the range of between 00.5 and 0.07 microns. By adding a source of carbon in the reaction zone, a metal boride powder product is prepared containing a minor concentration of carbon, e.g., from above 0.1 about 5 percent by weight total carbon, probably as submicron refactory metal carbide. Alternatively, submicron metal carbide powders, e.g., titanium, zirconium, halnium or bores carbide powders, or finely-divided carbon can be blended physically with the sub-micron metal boride powder to provided metal borides containing a minor concentration of carbon in the amounts previouly indicated. Titanium diboride powder compositions containing minor amounts of carbon can be hot pressed, or cold pressed and sintered to densities of at least 95 percent of theoretical.

Description

~'7S87~

Description of the Invention The literature describes a variety of methods for preparing hard refractory metal borides such as titanium diboride. For example, elemental titanium and boron can be fused together at about 3630F.
This method (synthesis by fusion) produces products that are relatively j impure and requires isolation of the boride product by chemical treat-; ment. Other sintering processes involve the reaction of elemental _ titanium with boron carbide (U.S. Patent 2,613,154), the reaction of titanium hydride with elemental boron (U.S. Patent 2,735,155), and the reaction of ferrotitanium and ferroboron alloys in a molten metal matrix, e.g., iron (U.S. Patent 3,096,149). A fused salt bath containing an alkali metal or alkaline earth metal reducing agent and titanium-and boron-containing reactants has been used to produce titanium diboride (U.S. Patent 3,520,656). U.S. Fatent 3,775,271 describes the electrolytic preparation of titanium and zirconium diborides by using a molten sodium salt electrolyte and rutile or zircon concentrates as the source of titanium and zirconium, respectively. -The preparation of the borides of titanium, zirconium, and hafnium ~` by the vapor phase reaction of the corresponding metal l~alide, e.g., titanium tetrachloride, and a boron halide, e.g., boron trichloride or boron tribromide, in the presence of hydrogen at temperatures of from 1000-1330C., 1700-2500C., and 1900-2700C., respectively, has been .~ .j , ,:

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S!37~7 reported in Re~ractory llard Metals, by Schwarzkopf and ~ieffer, the Mac~iillan Company, ~.Y., 1953, pages 277, 281 and 2S5. Typically, these vapor phase reactions have been conducted by heating the reactants in the presence oE an incandescent t~mgsten filament. Such procedures, however, produce a coating of the metal boride on a heated substrate rather than a powdery product. The aforementioned vapor phase reaction for preparing titanium diboride has been conducted at temperatures less than 1200C. using sodium vapor in lieu of hydrogen (U.S. Patent

3,244,482).
A widely reported commercial process used for preparing refractory metal borides, e.g., titanium diboride, is the carbothermic process. In this process9 refractory metal oxide, e.g., titanium dioxide, an oxide of boron, e.g., B203, and carbon are heated in an electric arc or high frequency carbon furnace. As an alternative to the electric arc furnace, it has been proposed to prepare titanium diboride by injecting powdered activated charcoal impregnated with boron oxide and titania (anatase) into an argon plasma (~ritish Patent Specification 1,273,523). This process produces about one gram of product in ten minutes and is not, therefore, considered commercially attractive. The product obtained from the aforementioned carbothermic process is ground in, for example, jaw-crushers and mills, and screened.
To obtain a finely-divided product, extensive milling is required. For example, U. S. Patent 3,052,538 describes the necessity for milling intermetallic compounds such as titanium diboride and titanium carbide to obtain a fine particle si~e useful for dispersion strengthening of titanium A milling time of 300 hours (12-1/2 days) in a porcelain mill using hardened steel balls as the grinding medium is recited as ~751~77 being required.
The reported average si~e oE the product produced from such lengthy milling ranges from about 2 to about 10 mlcrons. Moreover, the product is contaminated with metallic impurities abraded from the materials of construction of the mill and grinding surface. Thus, it is common to find imyurities in the product such as tungsten, iron, chromium, cobalt, and nickel. Moreover, extensive milling produces a significant amount of ultrafine, i.e., less than 0.05 micron, fragments.
These fragments are produced during milling and comprise irregular pieces of the principal particles that have been chipped or ground away from the edge or face of the particle. Thus, extensive milling produces particles having fractured irregular surfaces and a relatively large amount of fines.
It has now been discovered that submicron refractory metal boride powder, such as titanium diboride, zirconium diboride and hafnium diboride powders, can be produced by reacting in the vapor phase, the corresponding metal halide, e.g., titanium halide, and boron source, e.g., boron hydride or boron halide, reactants in the presence of hydrogen, e.g., a hot hydrogen gas stream produced by a hydrogen plasma heater, and in the substantial absence of oxygen, either combined or elemental. Preferably, hydrogen is heated in a plasma heater to form a highly heated hydrogen gas stream, which is introduced into the reactor and into the reaction zone. The metal halide and boron source reactants are introduced into the reactor and preferably into the hot hydrogen stream and the resulting reactant gas mixture permitted to react in a zone maintained at metal boride forming temperatures. The solid titanium diboride formed is removed from the reactor, quenched, . . .
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~()75~377 usually hy indirect heat exchange mealls, and recovered in conventional fine particle collection equipment, e.g., cyclones, electrostatic precipitators, dust collectors, etc. Solid, submicron titanium diboride powder is produced, the particles of which are characterized by well developed individual crystals that have well developed faces. Substantially all, i.e., at least 90 percent, of the particles have a nominal sectional diameter oE less than one micron. '~-~
The preponderant number, i.e., greater than 50 percent, of the particles less than one micron are in the particle size range of between 0.05 and 0.7 microns. The powder product can be produced containing less than 0.25 weight 10 percent oxygen and less than 0.20 weight percent halogen, e.g., chlorine.
Thus, more specifically, the present invention profides submicron titanium diboride powder comprising at least 99 weight percent titanium diboride, said powder having a surface area of between 3 and 35 square meters per gram and containing less than 0.4 weight percent metal impurities and less than 0.1 weight percent carbon, wherein the nominal sectional diameter of at least 90 percent of the titanium diboride particles r-of said powder are less than one micron, said particles being tabular to equidimensional hexagonal crystals having well developed faces, and a number median particle size of between about 0.08 and 0.6 microns, said powders 20 being characterized by the property of being able to be cold formed and sintered to a density of at least 90 percent of the theoretical density for titanium diboride.
An intimate mixture of refractory metal boride powder contain- ,~
ing carbon, either as free carbon or chemically combined carbon and probably as submicron refractory metal carbide, can be produced by introducing a vaporous source of carbon into the reactor as a further reactant. In this manner, coproduced powders of, for example, titanium diboride and titanium `~ carbide in intimate admixture and in most any proportion can be prepared.
Typically, the metal boride powder contains a minor amount of carbon.
30 For use in aluminum reduction or refining electrolytic cells, consolidated F

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:: ' ~()7S877 articles prepared from such re~rnctory metnl borLde powder preferably contain between about 0.1 and about 5 weLght percent of total carbon, which is the sum of the carbon present in the powder as free carbon and chemically r combined carbon. For other uses, a boride powder product containing higher amounts of total carbon can be producecl.
In another aspect, the invention provides a process for ~a preparing submicron refractory metal boride powder selected Erom the diborides of the metals titanium, zirconium and hafnium, capable of being r cold formed and sintered to a density of at least 90 percent of the:Lr theoretical density, by gas phase reaction of the halide of the correspond-ing metal and boron source reactants in the presence of hydrogen in a resctor, which comprises projecting a hot hydrogen gas stream into a ~Y
reaction zone in the reactor, introducing substantially pure gaseous metal halide and boron source reactants into said reaction zone, the heat content of the hydrogen gas stream and reactants being sufficient to establish refractory metal boride forming temperatures in sald reaction zone, reacting said metal halide and boron source reactants in the reaction zone in ththe substantial absence of oxygen and a source of carbon, and removing solid, submicron refractory metal boride powder from the reactor, at least 90 percent of the particles of which have a nominal sectional diameter of less than one micron~ said particles having a number median particle size of between about 0.08 and 0.6 microns and being tabular to equidimensional hexagonal crystals with well developed faces.
In yet another aspect of this invention, there is provided a method of preparing a sintered titanium diboride article comprising the steps of: ' a) bl~nding a small amount of binder with the titanium diboride '. ir powder as previously described, b~ forming the resulting blend of binder and powder into a green article of the desired shape, and ~ a .

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1(37S877 c? sintering the green article at temperatures of from 1950 C.
to 2250 C. for a time su~fic-Lent to obtain a titanium diboride article having a density greater than 90 percent of theoretical.
PreEerably the temperature is from 2000 C to 2100 C and the time at temperature is about 1 hour. It has also been found preferable to include from about 1 to about 5 weight percent of wax binder.
Brief Description of the Drawings The process described hcrein for preparing submicron refractory metal boride powder, the submicron refractory metal borlde particles ` r -5b-' . ~

1~75877 produced ~hereby and articles prepared from such powder can be better understood by reEerence to the accompanying drawings and photomicro-graphs wl-erein:
FIGUR~ 1 is a diagram o~ an assemblage, partially broken away in section, comprising arc plasma gas heating means, two slot reactant mixer means for introducing reactants to the hot gas stream emanating from the plasma heater, reactor means, and auxiliary product recovery equipment means tcyclones and bag filter) for recovering the metal boride powder product suspended in the reactor gaseous effluent;
FIGURE 2 is a diagrammatic sectional view of the lower portion of the arc plasma gas heating means and upper portion of the reactor of FIGURE 1 combined with three slot reactant mixer means in place of the two slot reactant mixer means illustrated in FIGURE l;
FIGURE 3 is a scanning electron micrograph having a magnifica-tion factor of 25,000 of a sample of titanium diboride powder having a B.E.T. surface area of 4.7 square meters per gram that was prepared in a mamler similar to that described in Example XI;
FIGURE 4 is a scanning electron micrograph having a magnifica-tion factor of 25,000 of a sample of titanium diboride powder having a B.E.T. surface area of 11.5 square meters per gram that was prepared in a manner similar to that described in Example XIX.
FIGURE 5 is a transmission electron micrograph having a magnifica-tion factor of 25,000 of a sample of the titanium diboride described in connection with FIGURE 4.
FIGURE 6 is a scanning electron micrograph having a magnification factor of 3,000, of a sample of purchased titanium diboride powder;
FIGURE 7 is a photomicrograph, having a magnification factor of . ~
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~075877 2100, of a polished etched section of the hot pressed plate prepared in Example ~III;
FIGU~E 8 is a photomicrograph, having a magnification factor of 2100, of a polished etched section of the hot pressed plate of Example XII;
FIGURE 9 is a photomicrograph, having a magnification factor of 2100, of a polished etched section of the isostatically pressed and sintered rod prepared in Example XV from 7.0 square meters per gram titanium diboride; and - FIGURE 10 is a photomicrograph, having a magnification factor of 2100, of a polished etched section of the isostatically pressed and sintered rod of Example XVI.

- Detailed Description Refractory metal borides of Group 4b of the Periodic Table of the Elements (Handbook of Chemistry and Physics, 45th edition, published by The Chemical Rubber Co., 19~4) prepared by the process described hereinafter, namely, titanium diboride, zirconium diboride and hafnium diboride, are grey to black powders composed predominantly of well developed crystals having well defined faces. FIGURES 3, 4, and 5, - which are electron micrographs (25,000 magnification) of titanium diboride prepared in accordance with the present invention, show examples of the typical crystalline particles produced. The product ; contains varying proportions of equidimensional and tabular single crystals, which are freely dispersible by virtue of extremely limited crystal intergrowth. The equidimensional crystals are bounded either by planar crystal faces or smooth rounded surfaces. The tabular crystal ~75877 forms consist dominantly of hexagonal prisms terminated by the basal pinacord. The tabular crystals are flattened perpendicular to the c - crystallographic axis as a result of greater development of the pinacoidal faces relative to the prism faces. Consequently, the crystal habit of the product can be described as tabular to equidi-mensional hexagonal. Based on visual observations of the powdery product through an electron microscope, the tabular hexagonal crystals exibit a nominal sectional diameter to thickness ratio within the range of 1.5:1 to 10:1 FIGURE 6 is a scanning electron micrograph (3,000 magnification) of a sample of purchased titanium diboride powder. A comparison of the powder illustrated in FIGURE 6 with that of FIGURES 3, 4, and 5 shows clearly the difference between the purchased titanium diboride powder and the titanium diboride powder prepared according to the present invention. The purchased powder is significantly larger, as ` shown by the linear scale on each FIGURE and their respective magnifica-tion factor. Further, the purchased powder does not contain weli developed titanium diboride crystals with well developed faces. The crystals that are present in FIGURE o are not as well grown as those in FIGURES 3, 4 and 5 and exhibit many irregular or broken edges.
Finally, the presence of a significant amount of ultrafine fragments is evident in the purchased titanium diboride product shown in FIGURE 6, while there is a substantial absence of such ultrafine fragments in the product of FI~URES 3, 4, and 5.
Submicron metal boride powders, e.g., titanium diboride, that can be prepared utilizing the process described in more detail hereinafter ..
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~0758~7 are substantially free of undesirable metal contarninants, i.e., the powders are essentially pure, as established by emission spectrographic analysis.
~ letal impurities (as elemental metal) normally represent less than 4,000 parts per million parts of the boride powder (ppm), i.e., less than 0.4 weight percent, and often represent less than 3,000 ppm (0.3 weight percent). Among the metals that can comprise the afore-mentioned impurities are the -following: aluminum, barium, calcium, chromium, copper, iron, potassium, litllium, magnesium, magnanese, sodium, nickel, silicon, vanadium and tungsten. The source of such ; metal impurities, if present, in the boride powder product is normally the reactants or equipment used to prepare the product.
Oxygen and halogen, e.g., chlorine, normally make up the largest individual non-metallic impurities that are introduced into the product from the reactants. By virtue of the described process, it is readily feasible to obtain boride powders with less than 0.20 weight percent halogen, e.g., chlorine, and less than 0.25 weight percent oxygen. By careful recovery, e.g., degasification, and handling techniques to avoid exposure of the boride powder product to the atmosphere (oxygen~
or moisture, boride powders with less than 0.15, often less than 0.10, weight percent halogen, and less than 0.20, e.g., less than 0.15 weight percent oxygen can be obtained. The aforementioned values for halogen and oxygen are based upon analysis for such impurities obtained by use of X-ray spectrographic analysis and by the use of a Leco oxygen analyæer (model~
534-300) respectively. The aforementioned X-ray spectrographic technique analy7es principally for unreacted metal halides and subhalides present :~, _ g _ :

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~1~37S1977 in the boride powcler. Adsorbecl hydrogen halide, e.g., hydrogen chloride, on the boride powder may not be detected by that technique.
~ en not adclecl intentionally, carbon can also be present in the boride powder product. Wormally, the boride powders are substantially free of carbon, i.e., the carbon level is typically less than 0.1 weight percent. Because of its beneficial effect (describecl hereinafter), carbon is not considered usually as an impurity. Thus, despite the use of substantially pure reactants and careful handling and recovery techniques, a small amount of metal impurities, halogen, oxygen and carbon can be present in the product.
The total amount of contaminants in tlle boride powder product is usually less than 1.0 weight percent, and typically is less than 0.75 weight percent. Stated another way, the refractory metal boride powders of the present process are usually at least 99 percent pure and typically are at least 99.25 percent pure.
As heretofore indicated, refractory metal boride powders of the present invention can be prepared with minor amounts of added carbon. It has been found that carbon notably in the elemental form or as metal carbide, aids the densification of the boride powder (promotes sintering) that is processed by cold pressing and sintering, or hot pressing. As used herein with respect to metal boride powders or compositions, the term "carbon" or "total carbon" unless otherwise defined are, intended to mean the carbon present therein both as elemental carbon and chemically combined carbon, e.g., as a metal carbide. Consolidated articles prepared from refractory metal boride powders containing from above 0.1 to about 5 weight percent total carbon, preferably from above 0.1 to about 2 weight percent, more preferably, ., .

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1075~77 about 1 weigllt percent, total carbon are especially useful in aluminum reduction or aluminum refining cells. For other uses, refractory metal boricle powders containing higher amounts of total carbon, e.g., up to 10 weight percent or more, are contemplatecl.
Thus, powder compositions tand articles prepared therefrom) contain- ~
ing from 0.1 to 10 weigllt percent total carbon are contemplated.
l~hile the carbon can be added as elemental finely-divided carbon, it is preferred that it be present as submicron refractory metal carbide powder, e.g., hafnium carbide, titanium carbide, tantalum carbide, zirconium carbide, boron carbide, silicon carbide, etc. ~_ Preferably, the refractory metal of the carbide will be the same as the boride; but, identity of refractory metal is not necessary. Thus, compositions such as titanium diboride powders containing carbon as hafnium carbide, tantalum carbide, zirconium carbide, boron carbide or silicon carbide are contemplated. Other combinations of refractory metal boride powders and refractory metal carbide powders are also contemplated. Moreover, while the carbon content of the boride powder composition can be introduced by physically mixing the carbon source (carbon or metal carbide) in the amounts desired, it is also possible to introduce the carbon into the powder composition in the metal boride powder forming process, i.e., in the reactor and during the vapor phase reaction.
The metal boride powders produced by the present process, e.g., titanium boride, are9 as indicated, predominantly submicron in size.
The surface area of the boride powder product can vary between about 3 and about 35 square meters per gram, (m /gram), more typically between ~s~7 7 abou~ 4 and about 15 m /gram, e.g., between 5 al~cl 10 m /gram, as measured by the method of Brunauer, Emmett, and Tcller, J. /~m. Chem.
Soc., 60, 309 (1938). This method, which is often referred to as the B.E.T. method, measures the absolute surface area of a material by measuring the amount of gas adsorbed under special conditions of low temperature and pressure. The B.E.T. surface areas reported herein were obtained using nitrogen as the gas adsorbed and liquid nitrogen temperatures (-196C.) and a pressure of 150 mm of mercury (0.2 relative pressure).
The surface area of the boride powder is, of course, a function of the particle size of the boride particles produced, i.e., the smaller the particle size, the higher the surface area. The average spherical particle size diameter, in microns, of the refractory metal boride, e.g., titanium diboride, powder particles produced can be estimated roughly by the expression: -~ Average Spherical Particle Size Diameter = 1.33/Surface Area (m /gram) :' which assumes that each particle is a sphere (regular shaped polygon).
Substantially all, i.e., at least 90 percent (by number) of the metal boride particles comprising the boride powder composition are submicron, i.e., have a nominal sectional diameter of less than one micron.
The nominal sectional diameter is the nominal diameter of a particle viewed under high magnification, e.g., 25,000 magnification, such as seen by an electron microscope and depicted in electron micrographs. The nominal diameter is based on the two dimensional surface viewed under high magnification. The preponderant number, i.e., greater than 50 percent, of the particles less than one micron are in the particle size range of ~(~7S877 bet~een 0.05 and 0.7 microns. Particles as small as 0.03 microns and as large as 2 microns can be present in the powdery product; but, particles greater than 2 microns rarely represent more than one percent by number of the product. The aforesaid crystalline particles less than 0.05 microns are distinguishable Erom the ultrafine fragments less than 0.05 microns found in metal diboride powder that has been milled extensively. The metal diboride powders described herein are substantially free of fragments less than 0.1 micron, e.g., the ultrafine fragments less than 0.05 microns. It is estimated from a study of the refractory metal boride powders of the present invention with a Zeiss TGZ-3 Particle Size Analyzer that at least 60 percent on a number basis, more usually at least 70 percent, e.g., 98 percent, of the boride particles comprising the powder are 0.7 microns or less.
It is not uncommon to find that the aforesaid percentages represent also tlle particles within the particle size range of between 0.05 and 0.7 microns. It is estimated ~urther that less than 10 percent on a number basis of the boride particles are greater than 1 micron.
The aforementioned values respecting the percentage of boride particles 0.7 microns or less depends on the particle size distribution of the powder. Generally, the particle size distribution is relatively narrow.
; The number median particle size of the boride particles comprising the boride powder composition is usually between about 0.08 and about 0.6 micron, more usually between 0.1 and 0.5 microns, and varies directly with the surface area of the powder. Because of its high surface area, the metal boride powder tends to adsorb readily oxygen or moisture.
The refractory metal boride powders of the present process are useful as metallurgical additives, as cermet components, for dispersion ' ~, ' , ' .

1C~7~1!377 strengthenin~ of metals, as components of the so-callecl super alloys ancl nuclear steels, as coatings or materials exposed to molten metals and in refractory applications. When consolidated, those boride powders can be used as high temperature electrical conductors, as electrodes in metal manufacture and refining such as aluminum manufacture. The relatively low electrical resistivities of consolidated shapes prepared from these boride powders make them especially desirable as electrical conductors and electrodes. Moreover, it has been found that the electrical resistivity of hot pressed or cold pressed and sintered forms prepared from the boride powder products, e.g., titanium diboride, produced in accordance with the process described herein are lower than values reported in the literature. For example, electrical resistivity values for titanium diboride have been reported as being greater than 10 microohm centimeters, e.g., from 10 to 30 microohm centimeters and typically from 15 to 25 microohm centimeters. In contrast, hot pressed or cold pressed and sintered titanium diboride forms prepared from titanium diboride powder produced in accordance with the present invention are typically less than 10 microohm centimeters, e.g., usually from 5 to 9 microohm centimeters. The electrical resistivity of zirconium diboride and hafnium diboride is also typically less than 10 microohm centimeters at room temperature, e.g., 25C.
Electrical resistivity can be measured in the conventional manner. Briefly, such measurement is obtained by applying direct current from two electrodes across the specimen to be measured, e.g., a square or rectangular plate, and the potential (voltage) difference between two points on the specimen equidistance from the electrodes recorded by an electrometer. For example, a 2 inch x 2 inch x 1/2 inch refractory metal `';' ~7587~

boride plate is clamped at the 1/2 incll side between two copper electrodes and a direct current appliecl across the plate. A dlstance of 4 centiTneters along the line of current flow (2 centimeters on either side of the ccnter line) ls measured and the end points marked.
The probes from the elsctrometer are placed on the end points of the measured 4 centimeter length and the potential difference measured.
Generally, electrical resistivity :is taken at 25C. and the values reported in the examples herein were measured at that temperature. The electrical resistivity value is calculated from the following expression:

(Potential Difference, volts)(Cross Sectional Area, cm .) Y ( ) (Applied Amperage, AmpsXDistance between voltage probes, cm.) Refractory metal boride powders prepared in accordance with the process described herein can be consolidated into shapes or forms of high density by conventional hot pressing, or cold pressing and sintering techniques. The refractory metal boride powders, e.g., titanium diboride of the present invention can be consolidated by hot pressing by subjecting a mold containing the powders to a continuously applied pressure of from about 0.5 to 50 tons per square inch, e.g., 1 to 3 tons per square inch, while raising slowly its temperature to between 1600C. and 2700C., e.g., 1800C.-2500C. The compacting, heating and subsequent cooling operations are typically carried out in an inert atmosphere, e.g., argon or in a vacuum. The operation is often carried out in a graphite die having a cavity of the appropriate desired cross-sectional shape. The pressure is preferably applied to the powder by plungers acting on opposite ends of the powder, e.g., a column of powder. The nature of the hot pressing process is such as to render it difficult to form, shapes other than flat plates and other relatively simple shapes. Moreover, hot pressing !~

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~75~77 is a relatively expensive process and is hard to adapt to large scale pro(luction by continuous processing.
The reractory metal boride powders of the present invention can be consolidated by cold pressing and sintering by pressing the powder into the desired shape followed by sintering the resulting form at temperatures between 1800C. and 2500C. either in a vacuum or in a neutral (inert) atmosphere. For simple shapes such as cylinders, plates, or the like, the powders can be dry pressed in matched metal dies. For complicated shapes, slip casting, tape casting, pressure casting, compression casting, extrusion or injection molding can be used to cold form the article. Further, a wax binder can be incorpor-ated into the powder by spray drying techniques and the resulting powder blend molded into the desired shape in rubber molds. Typically, the powder composition is mixed with a small portion of binder, e.g., 1 weight percent of paraEfin wax dissolved in l,l,l-trichloroethane solvent.
The solvent is evaporated prior to consolidating the powder. The resulting powder composition-binder mixture can be consolidated by applying pressure to the mixture, e.g., isostatically or between matched metal dies, either at ambient temperature or at slightly elevated temperatures, but, significantly less than sintering temperatures. The pressure applied is in the range of 0.5 to 50 tons per square inch, e.g., 2-10 tons per square inch. Alternatively, the powder composition-binder mixture can be extruded into the desired shape. Sintering is accomplished by heating the consolidated shape in vacuum or inert atmosphere at temper-atures of from 1800C. to 2500C. Prior to sintering, it may be necessary to first heat the green compacts at temperatures sufficient to remove any organic binder material (if used). Heating at about 200 to 400C. for ~75~77 about one hour in a vacuum or inert atmosphere is usually sufficient to remove such binder materials. The term "cold formed" as used herein means that the metal boride powder composition is compacted and shaped, as by pressing or molding, prior to the sintering opera-tion, as distinguished Erom hot formed or hot pressed bodies which are shaped and pressed by the application of pressure during sintering.
According to published reports, cold pressing and sintering of substantially pure refractory metal boride powders, e.g., titanium diboride, has not been employed successfully to prepare sintered articles of greater than 90 percent of theoretical density. For example, R. A. Alliegro describes sintered titanium diboride and zirconium diboride with densities of not greater than about 68 percent and about 81 percent of theoretical on page 518 of his article, "Boride and Boride-Steel Cathode Leads", Extractive Metallurgy of Aluminum~ Volume 1, G. Gerard et al editor, Interscience Publishers, New York, 1962. U. S. Patent 3,028,324 recites that "Current-conducting elements (carbides and borides of titanium, zirconium, tantalum and niobium) made by the use of cold pressing techniques...possess the disadvantage of having a relatively high porosity, e.g., up to 20 percent, and of being permeable so that the elements can be penetrated by undesirable substances..." (column 9, lines 5-10). When used in aluminum reduction or refining electrolytic cells, sintered elements with such high porosity levels are susceptible to penetration to the molten material in the cell, e.g., flux, metal and electrolyte. ~len such penetration occurs, cracking and failure of the sintered element results. Consequently, hot pressing has been used to produce prod~lcts . ~ :
;: .

-~07~877 having densities near or approaching theoretical densitles. llot pressing, however, lilDits to a great extent the shape and si~e of sintered elements.
It has been found that the substantially pure refractory metal boride powders, e.g., titanium diboride powder, of the present invention can be cold pressed and sintered to high densities. In addition, carbon-containing refractory metal boride powder compositions of the present invention also can be cold pressed and sintered to high densities, i.e., at least 90 percent of the theoretical density of the refractory metal boride. Depending upon the particular powder or powder composition, densities in excess of 93 percent of theoretical, e.g., in excess of 95 percent and often in excess of 98 percent of theoretical, can be achieved. Stated another way, cold pressed and sintered elements fabricated from titanium diboride powder and powder compositions having a porosity level of not more than lO percent now can be obtained. The aforesaid refractory metal boride powders and powder compositions can be hot pressed to densities at least equal to that obtained by cold pressing and sintering and more usually to densities approaching the theoretical density.
Hot pressed or cold pressed and sintered articles having densities of greater than 90 percent of theoretical of the refractory metal boride density, e.g., at least 92 or 93 percent of theoretical, are generally considered impermeable. Thus, when such articles are used in, for example, aluminum reduction or refining electrolytic cells, they are substantially impermeable to molten material to which they are exposed in such cells. The refractory metal boride powders and powder compositions of the present invention can be fabricated into ' ',".'~: ' . :

3LCi758~77 articles having sucll densities ancL, accordingly, such articles are useful as current conductillg elements in the aforementioned type electrolytic cells.
The presence of carbon in the boride powder compositions (as free carbon or chemically combined carbon, e.g., as refractory metal carbide) promotes sintering of the boride powder to high densities. ~lile the carbon can be introduced into the boride powder in any convenient manner, it is preferred that the carbon be introduced into the powder in the reactor when the metal boride powder is being formed. Various advantages accrue when the carbon is -~
introduced into the boride powder at that time. First, a more homogeneous distribution of carbon in the boride powder product results than can be achieved by physically blending. A homogeneous distribution of carbon throughout the boride powder hinders grain growth during sintering and helps provide a fine grain structure.
fine grain structure generally has greater strength than a coarse grained structure. Second, elimination of possible oxygen and metal contarnination as a consequence of such blending is achieved. Third, the presence of ultrafine carbon particles in the reaction zone -~
provides also a source of nuclei which often results in a boride powder product of higher surface area than a powder prepared in a reaction system that does not have such nuclei. Finally, it has been observed that less reactor added carbon is required to obtain the same degree of densification than is required with physically blended carbon. ~esults obtained with reactor added carbon compare favorably with those obtained using twice as much carbon that has been physically blended with preformed refractory metal boride. It is postulated that ~7~8~7 the essentialLy homogeneous dispersion of reactor aclded carbon througllout the refractory metal boride powder is a major reason for this result. Iurther, titalllum diboride containing reactor added carbon provides a sintered article having an essentially equiaxed grain structure while titanium diboride containing physically blended carbon provides a sintered article having less pronounced equiaxed grains and more elongated grains.
The apparent grain size, i.e., average diameter, of the refractory metal boride grain as measured on an etched metallographi-cally polished surface of a sintered refractory metal boride specimen is predominantly fine. ~s measured on photomicrographs of the polished surface, the grain size of the boride grains is generally less than 20 microns, and predominantly in the range of about 1 to 10 microns.
The grains are of relatively uniform size and occur in a microstructure characterized by contiguous grain boundaries and low porosity resulting in high density and strength of the sintered bodies.
Refractory metal boride compositions comprising mixtures of more than one metal boride powder are also contemplated herein. Thus, blends of titanium diboride powder with zirconium diboride powder and/or hafnium diboride powder in most any proportion can be cold pressed and sintered, or hot pressed in the same manner as heretofore described.
Such mixtures of boride powders can be prepared by blending the pre-formed boride powders in the relative amounts desired; or, the boride powders can be co-produced by introducing into the reactor, usually simultaneously, the refractory metal halides of the metal borides desired and in the proportion desired in the end product. Further, mixtures of the carbides of the aforementioned refractory metals with .. . . . .
.
.~.. ~ , .

~6)7S877 SUCII boride powder mlxtures can be blended physically with the powder or simultaneously preparecl with the aforementioned refractory metal borides in the amo~mts desc:ribed previously by introducing a carbon source into the reaction zone.
Generally, any volatile inorganic titanium, zirconium or hafnium halide, e.g., a compound of only the aforementioned metal and halogen (chlorine, bromine, fluorine and iodine), can be used as the source of the aforementioned metal in the refractory metal boride powder product prepared by the process described herein. As used herein the terms "metal halide" and "metal boride" or "metal diboride" are intended to mean and include the halides and borideF
respectively of titanium, zirconium and hafnium, i.e., the elements of Group 4b of the aforesaid Periodic Table oE the Elements. However, for the sake of con~enience and brevity, reference will be made ; sometimes to only one of the aforementioned metal halides or borides.
E~cemplary of the refractory metal halides that can be employed in the present process include: titanium tetrachloride, titanium tetrabromide, titanium tetraiodide, titanium tetrafluoride, zirconium tetrabromide, zirconium tetrachloride, zirconium tetrafluoride, zirconium tetraiodide, hafnium tetrabromide, hafnium tetrachloride, hafnium tetrafluoride~ hafnium tetraiodide, as well as subhalides of titanium and zirconium such as titanium dichloride, titanium tri~
chloride, titanium trifluoride, zirconium dibromide, zirconium tri-bromide, zirconium dichloride and zirconium trichloride. Of course, subhalides other than the subchlorides and subfluorides can be used in the same manner. Mixtures of metal halides of the same metal such as the chlorides and the bromides, e.g., titanium tetrachloride and :

- ' ' ~ ,.

~75877 titaniwll tetrabromide can be employed as the metal halide reactant.
Further, mixtures of halides of different metals can be used when it is desired to co-produce more than one metal boride powder, e.g., titanium diboride and zirconium diboride. Preferably, the halogen portion of the metal halide reactant(s) is the same to avoid separation and recovery of different hydrogen halides from the product stream.
The metal halide reactant(s) can be introduced into the reactant inlet assembly (mixer means) used to introduce the reactants into the reactor as a liquid or vapor; but, should be introduced in such a manner that the reactant(s) is a vapor in the reactant mixing zone and subsequent reaction zone. Economically preferred as the metal halide reactant are the tetrachlorides, e.g., titanium tetrachloride. The metal halide reactant(s) snould be substantially pure, i.e., substantially free of metal contaminants and free or chemically combined oxygen so as to produce a metal boride powder having the purity described earlier.
The boron source reactant like the metal halide reactant should be also oxygen-free and substantially pure to avoid the intro-duction of oxygen and metal contaminants into the metal diboride product. By oxygen-free is meant that the boron source is substantially free of chemically combined oxygen, e.g., the oxides of boron, as well as uncombined oxygen. Despite the precautions of reactant purity, a small amount of oxygen contamination occurs in the boride powder, as earlier described. As a suitable source of boron for the metal borides, there can be mentioned inorganic boron compounds such as boron tribromide, boron triiodide, boron trichloride, boron trifluoride and the hydro-borides (boranes), e.g., B2116, B5Hg, B101114, and B6H2, Boron trichloride ' ' .. .
:

1~75B77 is preferred. ~s in the case of the metal halide reactant, the boron source reactant is introduced into the reactor in such a manner that it is present in the reactant mixing zone and reaction zone as a vapor. The metal halide source and boron source should be chosen from those compounds which, in combination, provide a thermodynamically favorable reaction at the desired reaction temper-ature. For example, the reaction of titanium tetrachloride with boron trifluoride is thermodynamically less favorable at 2000K. than at 2500K. Thus, such thermodynamically less favorable reactions will require higher reaction temperatures.
The amount of boron source reactant introduced into the reaction zone in the reactor will be preferably in at least stoichio-metric quantities, i.e., in amounts sufficient to provide at least two atoms of boron for each atom of metal, e.g., titanium, introduced into the reaction zone in the reactor as metal halide, e.g., titanium halide, reactant. The ratio of the boron source reactant to the metal halide reactant can, of course, ~7ary from stoichiometric quantities.
Thus, the boron source reactant can be introduced in amounts sufficient to provide in the reaction zone between about 1.8 and about 3 atoms of boron per atom of metal, e.g., titanium. Preferably, greater than the stoichiometric ratio is used. For example, the mole ratio of reactants boron trihalide to titanium tetrahalide (BX3/TiX4), wherein X is halogen, can vary from about 1.8:1 to 3:1 and preferably is about 2. When a stoichiometric excess of the boron source is used, less residual unreacted metal halide reactant is found in the product. When a stoichiometric excess of metal halide is used, sub-halides of the '~ ' - .. . . , . , , . :
. ~ -, .

~1~)758~7 metal are found ln the product. While it is preferred that the boron source reactant be used in stoichiometric excess either of the metal halide or boron source reactants can be used in stoichiometric excess in amounts of from 5 to 30 percent by weight.
In the embodiment wherein carbon-containing metal diboride powder is produced in the reactor, carbon source reactant i5 also introduced into the reaction zone in the reactor. Such carbon source reactant is of the type that is volatile in the reaction zone and is capable of reacting in a thermodynamically favorable manner at the temperatures at which tlle reaction is conducted. In the aforesaid embodiment, volatile hydrocarbons, halogenated hydrocarbons or mixtures thereof that are substantially pure and oxygen-free, as defined above, can be used as the carbon source. As used herein, the term "halogenated hydrocarbon", e.g., "chlorinated hydrocarbon", is intended to mean and include both compounds of carbon, halogen and hydrogen, and compounds only of carbon and halogen, e.g., carbon tetrachloride.

; ~ypical hydrocarbons that can be used as the carbon source include the normally gaseous or liquid but relatively volatile hydro-carbons including saturated and unsaturated Cl - C12 hydrocarbons, such as methane, ethane, propane, the butanes, the pentanes, decanes, dodecanes, ethylene, propylene, the butylenes and amylenes, synmletrical dimethylethylene and like alkenes, cycloaliphatic and aromatic hydro-carbons, such as cyclopentane, cyclohexane, toluene, benzene, etc., ;~ and acetylenic compounds of which may be noted acetylene, methyl acetylene, ethyl acetylene, and dimethyl acetylene. Methane or propane are economically preferred for this purpose. Rarely are ~' .

1~75~377 hydrocarbolls of more than twelve carbons used.
Examples of halogenated hy(lrocarbons that can be used as the source oE carbon in the process described herein include saturated and unsaturated compounds containing Erom one to twelve, more usually one to eight, carbon atomsS such as methyl chloride, ethyl cllloride, chloroform, methylene chloride, carbon tetrachloride, dichlorodifluoromethane, amyl chloride, chloroethane, vinyl cllloride, l,l-dichloroethylene, 1,2-dichloroethylene, l,l-dichloroethane, 1,2-dichloroethane, ethylene dibromide, trichloroethylene, perchloroethylene, propylene dichloride, 1,1,2-trichloroethane, l,l,l-trichloroethane, 1,1,1,2- and 1,1,2,2-tetrachloroethane, hexachloroethane, and like aliphatic chlorides, fluorides, bromides or iodides containing up to about twelve carbon atoms, most preferably up to about six carbon atoms. Aromatic halocarbon compounds, e.g., chlorocarbon compounds, also can be used. Such compounds include C6 - Cg halogenated aromatic compounds, such as monochlorobenzene, orthodichlorobenzene, paradi-chlorobenzene and the like. Cycloaliphatic halides, such as the C5 -C6 aliphatic halides, e.g., chlorinated cyclopentane and cyclohexane, etc., can also be used.
Typically, the above-described hydrocarbons and halogenated hydrocarbons should be readily vaporizable (volatile) without tar formation since otherwise unnecessary difficulties which are unrelated to the process itself can arise, such as the plugging of transfer lines by decomposition or polymerization products produced in the course of vaporizing the carbon source reactant.
The amount of carbon source reactant, e.g., hydrocarbon or halogenated hydrocarbon, used will of course depend on the amount of :.
..~

.

.~ .
.: - . .
:'` ' '. ' '` .. :: ' ' ' ' 5~

carbon desired in the Einal boride powder product. The amount of total carbon in the metal diboride powder, e.g., titanium diboride powder, or diboride powder composition can range from above 0.1 to about 5 weight percent, preferably between about 0.15 and about 2 weight percent, e.g., 1 weight percent. ~len a carbon source reactant is introduced into the reactor, it is expected that carbide(s) of metal(s) present in the reactor, e.g., titanium carbide, are co-produced with the metal diboride. At low levels of carbon, i.e., less than 1 weight percent total carbon, the X-ray pattern characteristic of metal carbides, such as titanium carbide, in the diboride powder is not fairly evident. By "total carbon" is meant the total amount of both free carbon and chemically combined carbon, e.g., metal carbide, in the metal diboride powder product. If, for example, all of the co-formed carbon in titanium boride powder is present as titanium carbide a total carbon content of between above 0.1 and about 5 weight percent corresponds to a titanium carbide content of between above 0.5 and about 25 weight percent. On the same basis, the preferred level of total carbon corresponds to a titanium carbide content of between about 0.75 and 10, e.g., 5 weight percent. From the evidence at hand, it is believed that when a carbon source is added to the reaction zone, the carbon in the metal boride powder product is present principally as metal carbide.
Metal boride, e.g., titanium diboride, powder containing from above 0.1 to 5 weight percent total carbon can be prepared also by blending physically submicron metal carbide powder, e.g., titanium ; carbide powder, and/or finely-divided carbon with submicron metal boride, e.g., titanium diboride powder in amounts sufficient to provide .
:"~

., ~075877 a total carbon level within the aforesaid range. Submicron titanium carbicle and other metal carbides can be prepared by the process exemplified by U.S. Patents 3,485,586, 3,661,523, 3,761,576, and 3,340,020. Generally, the submicron titanium carbide used will have a number median particle size of between about 0.1 and 0.9 microns.
Submicron carbon is commercially available and such materials can be used directly; a commercial carbon product having a particle size larger than desired can be used, preferably by first being reduced in size by grinding the carbon in conventional milling equipment, e.g., fluid energy mills. For example, commercially available N110 carbon black having a surface area of 11-19 m /gram can be used.
The vapor phase reaction of metal halide and boron source reactants with or without a volatile carbon source is conducted in the presence of hydrogen. The amount of hydrogen utilized in the above-described process is at least that amount which is required stoichio- -metrically to satisfy the theoretical demand of the reaction. Preferably, the amount of hydrogen used is in excess of the theoretical amount. When, for example, the metal halide reactant used is titanium tetrachloride and the boron source reactant used is boron trichloride, the theoretical amount or demand of hydrogen required can be expressed by the equation: _ I. TiC14 + 2~C13 ~ 5 2 > Ti~2 + 10 HCl Often the amount of hydrogen utilized will be in excess of ten times and as high as 100 times the amount of hydrogen shown to be required by the above equation or required to equal the chemical equivalents of halogen of the metal halid~ and/or boron halide reactants. When the boride source ' .

.

' ~75877 is a hydroboride, the hydrogen available from the hydroborlde can be used to satisfy all or a part of the hydrogen demand. Typically, the mole ratio of hydrogen to metal halide reactant ranges between about 20 and 40, e.g., 25 moles of hydrogen per mole of metal halide.
The temperature at which the vapor phase reaction of metal halicle and boron source reactants is conducted will depend on the reactants selected and will be those temperatures at which submicron metal boride powder is produced with the selected reactants under thermodynamically favorable conditions, i.e., metal boride powder forming temperatures. The average reaction zone temperature for the aforementioned vapor phase production of metal boride powder such as titanium diboride powder typically is above 1000C. and usually ranges upwardly of 1000C. to about 3500C. The process can be conducted at subatmospheric, atmospheric, and superatmospheric pressures. Typically, the process is conducted at between about 1 and about 3 atmospheres, normally between 1 and 1.5 atmospheres pressure.
The process and handling equipment utilized in the afore-mentioned process for producing metal diboride powder (as more specifically described hereinafter) are constructed from materials resistant to the temperatures and corrosive environment to which they are exposed during the various steps of the procedure, as outlined hereinafter. The present invention will be more fully understood by reference to the accompanying drawings. Referring now to FIGURE 1, there is shown apparatus comprising plasma generator heating means 1 :' ~ - 28 -~ ;
-~C~7587 7 moun~ed atop reactant inlet assembly (mixer) means 30 which~ inturn, is mountecl atop reactor 34. ~lthough the aforesaid apparatus is sho~l in vertical alignment, other alignments away from the vertical including a horizontal alignment are contemplated. Whlle the plasma generator heating means shown is an arc heater, other plasma heater types, e.g., an induction (high frequency) heater, can also be used. Further, other heating means such as electrical resistance heaters, can be used to heat hydrogen to the temperatures required by the process described herein. The hydrogen is heated typically to temperatures which is suEficient to establish and -~
maintain metal boride forming temperatures in the reaction zone bearing in mind that it is mixed with the metal halide and boron source reactants which are introduced into the reaction zone at below the reaction temperature 3 usually significantly be]ow reaction temperatures. Thus, the principal source of heat for the reaction is generally the highly heated hydrogen gas stream. Plasma heater 1 consists essentially of an annular anode 11 which is aligned coaxially with cathode rod 3.
Both anode and cathode are mounted in a cylindrical sleeve 9 which is electrically non-conductive. In the embodiment illustrated, the cathode rod tapers conically at its end essentially to a point. The anode and cathode are constructed out of conventional electrode type materials, such as copper, tungsten, etc. The cathode often has a thoriated tungsten tip or inserts which assist in cooling of the cathode.
As is conventional with plasma heaters, the anode is surrounded by an annular cooling chamber 13 through which coolant, e.g., water, or other cooling medium is circulated by means (not shown) in order to hold the anode at a suitably low temperature and prevent undue erosion thereof.

.

: :.:

~[1758~7 In a simiLIr manner, the interior of the cathode is provided wltil cooling challlber 7 and with means (not shown) to circulate water or other suitable cooling fluid therein in order to ho:Ld the cathode at a suitable operating temperature. Tube 2 serves to help support and align cathode rod 3 and provide a conduit or coolant flow.
Cathode 3 can be provided with means for moving it in a vertical direction so that the distance between cathode 3 and anode 11 can be varied.
The anode and cathode are axially aligned but spaced longi-tudinally to provide annular space 21 which tapers conically to a _ i~
coaxial outlet conduit 23. The assemblage is also provided with plasma or work gas inlet means 15 having conduit 17 which communicates through annular conical conduit l9 with the annular space 21. The cathode and anode are connected by electrical connecting means (not shown) to a power supply (not shown). Typically, the power source is a direct current power source.
Reactant mixer means 30 is adjacent to the anode end ofcylindrical sleeve 9, and as shown, comprises two coaxial, longitudinally spaced annular conduits 42 and 47 that are provided with inlet nozzle means 40 and 45, respectively. As shown, exit port 48 of annular conduit ~
47 is retracted from exit port 43 of annular conduit 42 to form a conical reactant introduction zone 24. Reactants from reactant supply means (not shown) are introduced into conduits 42 and 47 through nozzle means 40 and 45 respectively. The flow path of the reactants discharged through exit ports 43 and 48 can be perpendicular to the exiting gas from conduit 23, as shown. If desired, exit ports 43 and 48 also can be positioned away from the perpendicular, i.e., downwardly or upwardly, 107587~7 at an angle of from 1 to 45 from the horizontal position shown 90 that the reactant gas Elow is clirected at such angle into or in contact with the stream oE hot gas emanating from the plasma heater.
The reactant gas can be projected radially, tangentially or at any suitable angle therebetween into the downwardly flowing stream of heated plasma gas emanating from outlet conduit 23. The top of reactant mixer means 30 contains opening 31 which is coaxially aligned with outlet conduit 23 of anode 11 to provide an overall direct straight-line path for the heated plasma gas from plasma generator 1 through reactant mixer means 30 into reactor 34. Prefer-ably, the heated plasma gas is introduced into the center of reactor 34 and spaced from the walls thereof to thereby assist in positioning the reaction zone away from the walls of the reactor.
Typically, hydrogen is used as the gas which is heated by the aforementioned heating means, e.g., plasma heater 1; however, other gases, e.g., the noble gases can be used. ~rgon and helium are suitable plasma gases. The use of hydrogen as the plasma gas is advantageous since it insures the establishment of a reducing atmosphere and provides a halogen, e.g., chlorine, acceptor, thereby removing halogen released from the metal halide, boron halide and/or halocarbon compound reactants as hydrogen halide. Mixtures of hydrogen with other gases, such as the noble gases, e.g., argon or helium, can also be employed as the plasma gas. When a noble gas is used as the plasma gas, the hydrogen required for the vapor phase reaction is introduced into the reactor by mixing it with the reactants, as a part of the boron source reactant, e.g., the boranes, and/or as a separate stream through mixer means 30.

:.

~ ~ .

~a75~77 As the heatecl pLasma gas stream moves past the zone of reactant introduction 24, it mixes with the reactants introcluced through reactant mixer means 30. The reactants are introduced usually at below reaction temperatures. Because of the high heat content of the hot hydrogen stream no special efforts to heat the reactants to temperatures above which they are gaseous are required. The resulting gaseous mixture is forwarded into the interior of reactor 34 and reacted therein. Reactor 34 is typically externally water cooled (not shown). Typically, the reactants and reaction mixture are in turbulent flow although laminar flow can be used. The reaction mixture flowing into reactor 34 which is a recirculating-type reactor as opposed to a plug flow-type reactor, typically has an apparent residence time therein of between about 0.05 and about 0.5 seconds, more usually between about 0.1 and 0.2 seconds.
The apparent residence time can be calculated by dividing the reactor volume by the gas flow through the reactor.
As shown in FIGURE 1, finely-divided metal diboride powder product, which is suspended in reaction product gases as well as excess reactant gas, hereinafter collectively referred to as product gases or other equivalent terms, is removed from reactor 34 through conduit 36 and introduced into cyclones 38 and 39, in order to separate the solid metal diboride powder from the product gases. The submicron particles of metal diboride are~formed completely in the reactor and since the reactor effluent is cooled to below metal boride forming temperatures substantially immediately, substantially no metal boride formation or individual particle growth (other than by physical aggregation) occurs outside the reactor. Cyclones 38 and 39 are normally cooled, e.g., .

: L~7S~77 e~ternalLy water cooled to cool the powder procluct. For example the cyclones can be tracecl with tubing through which coolant, e.g., water, is passed. As shown, the discharge ~rom conduit 36 is introduced tangentially into cyclone 38 ancl from there into cyclone 39 by means of conduit 51. Titanium diboride powder drops out into receivers 25 and 26, respectively, while gaseous effluent leaves cyclone 39 through conduit 52 and :into solids separation chamber 28 in which there is disposed a bag filter 29, electrostatic precipitator or other convenient means for separating suspended solids from a gas.
Cyclones 38 and 39, and receivers 25 and 26 are closed to the atmosphere -to prevent contamination of the product with oxygen. Thus, the metal diboride powder that is formed in the reactor at metal diboride forming temperatures is removed immediately from the reactor and forwarded to product collectors that are substantially below temperatures found in the reactor. The powder product is typically cooled or allowed to cool to room temperature. However, if the cooling capacity of the cyclones and receivers is not sufficient to provide a powder product at room temperature, the product in the receivers may be above room temperature, i.e., from about 20C. to 100C., because of the residual heat content of the powder. Higher temperatures in the receiver may be used intentionally, as described hereinafter, to promote degassification of the powder product. Separation chamber 28 as shown also has an exit or exhaust 50 on the opposite side of the bag filter. As shown, the bag filter has engaged therewith a suitable shaking means 59 to clear the filter of metal diboride powder. While only two cyclones and receivers are shown, more than two can be used. Alternatively a single receiver and cyclone can be used.

. . .

~0~5~7 Solids separation chamber 28 can also be a caustic water scrubber, often containing packing of some sort, e.g., balls, saddles, etc. for greater contact. The scrubber separates the fine solids from the gas stream and neutralizes acidic species therein before the gas is discharged to the atmosphere or to a flue. To recover unreacted reactants, hydrogen, hydrogen chloride, etc. from the product gases substantially devoid of its solids burden, conventional separa-tion and recovery means for such materials can be installed between exit conduit 52 and the flue. Further, if the heat removal from the product recovery apparatus, i.e., the cyclones and receivers, is insufficient, the product transfer line 36 can be externally cooled.
Moreover, a cold or cooler compatible gas can be mixed with the exiting product effluent to thereby cool it.
Referring now to FIGURE 2, there is shown a partial assembly, in cross-section, similar to that of FIGURE 1, except that three-slot reactant mixer means 32 instead of two-slot reactant mixer means 30 is shown. In addition to annular conduits 42 and 47, there is shown a coaxial, annular conduit 44 which is spaced longitudinally from annular conduits 42 and 47. The exit port 49 of conduit 44 is retracted from that of conduit 47 to further extend conical reactant introduction zone 24. Annular conduit 4h is connected to nozzle means 41 for introducing reactant gas into said conduit. Nozzle means 41 is~ in turn, connected with reactant gas supply means (not shown). Reactant mixer means 30 and 32 can be constructed of any suitable material, such as graphite, molybdenum, refractory or any other material which will withstand the heat and corrosive environment present in the reactant .~, :.
i~

`;

,~ . ~ . . ,,, . ~ . , ~175~7 introductioll zone 24. The mixer means can be internally cooled thereby permitting the use of conventiona:L metal fabrication.
In the production of refractory metal boride powders, e.g., titanium diboride powder, in the manner described, there is a strong tendency for the metal diboride powder product to deposit and accumu- -late on the surfaces of the reactant mixer means exposed to the reactants. When this occurs, the titanium diboride powder can restrict the reactant exit ports associated with the reactant mixer means, e.g., exit ports 43 and 48 of FIGURE 1. Partial blockage of these exit ports upsets the flow patterns of the reactant and gas streams introduced into reactant introduction zone 24. Such upset in flow patterns can intensify the growth of powder deposits on the exposed surfaces of the reactant mixer means, such as lip 46 of mi~er means 30. The growth of such deposits can continue until the reactant exit ports are completely blocked. Significant blockage of such ports affects product conversion and yield and can cause premature shut down of the process for removal of the deposits.
Addition of anhydrous hydrogen halide, e.g., hydrogen chloride, to reactant introduction zone 24, helps to reduce metal diboride powder deposits on the exposed portions of the reactant mixer means. Typically, the halogen portion of the hydrogen halide corresponds to the halogen ` portion of the metal halide reactant. Thus, when titanium tetrachloride is used as the metal halide reactant, the anhydrous hydrogen halide used is hydrogen chloride. The amount of anhydrous hydrogen halide used can vary; but, typically will range between about 50 and about 350 mole percent hydrogen halide based on the metal halide reactant. Any convenient means can be used to introduce anhydrous hydrogen halide into :'.` ' ' ' . . ' . .. ' :

1~7587~7 ~one 24. Commonly, the anhyclrous hydrogen halide is introduced in admixture with the metal halide reactant; however, it can be introduced with the boron source reactant, the carbon source reactant, if used, or as a separate stream. With reference to FIGURES 1 and 2, it is preferred that the anhydrous hydrogen halide be introduced near the top oE zone 24.
The metal halide and boron source reactants are mixed commonly with a carrier gas to facilitate their introduction into reactant introduction zone 24. The carrier gas can be hydrogen, recycle hydrogen, recycle solids-Eree product gas, or a chemically inert,(i.e., inert with respect to the reactant with which it is admixed) gas such as the noble gases, e.g., argon and helium. Hydrogen is not used commonly with the boron source reactant, e.g., boron tri-chloride, for the reason that hydrogen has been observed to react with the boron halide reactant within the reactant inlet conduits thereby causing blockage thereof. The amount of carrier gas used to facilitate the introduction of the reactants can vary; but, generally will range between 250 and 1200 mole percent based on the reactant with -, ;
which the carrier gas is admixed. The carrier gas assists in cooling the mixer means, in keeping reactant conduits free of condensibles ~ and has some effect in controlling the mixing of the reactants in zone - 24 with a consequent effect on the surface area of the metal boride powder product.
The mean particle size (and thus surface area) of the refractory ~ metal boride particles comprising the powdery product prepared by the -~ process described herein is a function of many variables within the process system some of which can be interrelated. From the evidence . . .
:;',' ' ' ; .:
.: :
`: `

~L~75~77 at hancl some general observations can be made. Particle size tends to increase with an increase in the rate o~ production. Partic].e size does not appear to change significantly with changes in the hydrogen plasma gas flow. Particle size tends to decrease with an increase in the intensity of mixing resulting from the use of larger amounts of carrier gas (or inert gas) introduced into the reactor other than by means of the plasma gas. Finally, increasing the amount of nuclei from additives, such as hydrocarbons, tends to decrease the particle size.
In carrying out the preparation of refractory metal diboride powder by the process and with the apparatus described herein, and particularly with reference to FIGURE 1 adapted with reactant mixer means 32 of FIGURE 2, a hydrogen-containing gas or noble gas, e.g., argon, is introduced into plasma generator means l, through conduit 17 from whence it is directed by means of annular conduit 19, into space 21, between cathode 3 and anode 11. The plasma gas can be introduced in a manner such that the gas flows in a spiral or helical fashion through outlet conduit 23. Alternatively, the plasma gas can be introduced radially into the space 21 between the cathode and anode i so that there is no halical flow pattern established by the plasma gas and tne heated plasma gas exits the plasma heater in a substantially linear flow path.
An electric arc is established between the anode and cathode and as the arc passes through the plasma gas, the gas is heated to high temperatures, usually temperatures above reaction zone temperatures.
A hydrogen-containing plasma gas can have an enthalpy of between 20,000 ~)75~

and 60,000 BTU per pound of gas, more co~lonly between 30,000 and 40,000 ~TU/pound. The heate-l plasma gas is projected directly into reactor 34, passed reactant introduction zone 24 formed by the lower lip of anode 11 and the exit ports of reactant inlet conduits 42, 47 and 44.
Reactant gases, metal halide and boron source reactant, are introduced, in one embodiment, into nozzles 40 and 41, respectively, and thence into reactant introduction zone 24 and into the environment of the downwardly flowing stream of hot plasma gas. The reactant gases can be introduced at a mass velocity such that they are aspirated by the movement oE the projected plasma stream or, they can be introduced into the plasma stream at a mass velocity such that the plasma stream is momentarily constricted. ~Iydrogen can be introduced into nozzle 45 of reactant mixer 32 and thence into the reactant introduction zone 24 thereby acting as a gas sheath between the metal halide and boron source reactants.
The formation of refractory metal diboride powder by the gas phase reaction of the corresponding metal halide and boron source reactants in the presence of hydrogen and in the substantial absence of oxygen (combined or elemental) commences essentially immediately with the mixing of the reactants in the reaction zone at metal boride forming temperatures. Optimally, the gas phase reaction is confined to a zone within reactor 34 away from the hot surfaces of the reactant mixer means and the reactor. This minimizes deposition of the metal boride powder product on the wall surfaces, which, if not otherwise removed, will continue to build-up until causing interruption of the process. The powder that builds-up on the walls of the reactor tends ' :- . . . . ;

to be coarser than the powdery procluct removecl from the reactor soon aEter it is formed. Co-mingling the build-up powder on the wall with the principal diboricle powder product contributes to the production of a non-uniform product. When the principal powder product becomes non-uniEorm because of coarse powder from the reactor wall the powder product should be classified to remove oversi~ed particles before being used.
Finely-divided refractory metal dlboride powder suspended in reactor effluent product gas is removed immediately from reactor 34 and introduced into cyclone 38. A portion of the powder product is removed in cyclone 38 and recovered in receiver 25. Powder product retained in the gas effluent from cyclone 38 is forwarded via conduit 51 to cyclone 39 wherein further amounts of metal diboride powder product are removed and recovered in receiver 26. Additional cyclones and receivers can be used if needed. In most cases, the products from receivers 25 and 26 are blended into a single product.
The reactor effluent product gas, now substantially free of its solid metal diboride powder content, is forwarded to gas separation chamber 28 where it is treated to free it from any remaining suspended metal diboride powder. As shown, the product gas passes through a bag filter 29 and is removed from chamber 28 by means of conduit 50.
The product gas now removed of its metal diboride and/or other solids burden can be treated further to recover valuable by-products and remove noxious components therefrom before being burned or discharged , to the atmosphere. If desired, the product gas can be treated to recover hydrogen and/or hydrogen halide, e.g., hydrogen chloride, for ~3175~77 use in the present process or in sone other process or the cooled procluct effluent stream can be recycled to the reactor as a source of cooling or diluent gas.
The metal diboride powder product prepared in accordance with the aforementioned process is a finely-divided powder that can adsorb gases such ~mreacted reactants that may be present in the receiver in which the product is collected. To avoid contamination by adsorption, receivers 25 and 26 are heated generally to temperatures above about 200F. (93C.), e.g., from about 200F.-600F. (93C.-316C.) to assist in degassing of the product during collection of the product.
Simultaneously, it is advantageous to maintain a stream of hydrogen or an inert noble gas, e.g., argon, percolating through the product to further assist in the degasification step while the product cools. If the product is not substantially free of unreacted reactants such as the metal halide, e.g., titanium halide, and boron source, e.g., boron halide, such compounds can react with moisture or oxygen in the atmos-phere to form oxides or hydroxides of the metal, e.g., titanium or boron, thereby introducing oxygen contamination into the product.
Advan~ageously, the product is handled without exposure to the atmosphere;
however, in some cases, some exposure to the atmosphere cannot be pre-vented. In the event the metal diboride powder product contains adsorbed chlorine-containing species, e.g., the subhalides of the metal halide reactant such as titanium trichloride and titanium dichloride, such species can be removed by heating the product to between about 400 and 1000C., e.g., 500-700C. and preferably about 600C. for between about 1 and 4 hours. In performing such heating step, the metal diboride powder is charged to a calciner or similar furnace, preferably a rotating calciner, and heated to the indicated temperatures for ' , ~La7587~7 the indicatecl tlme. ~ st~eam of hydrogen or inert gas, such as argon, is maintained over the heated product to help remove undesirable adsorbecl gases from the procluct and prevent exposure to oxygen. After degassing, the boride product can be coated with a paraffin wax or otller similar binder material to minimize the rate of oxygen pick-up during storage and handling.
; ~len a source of carbon is introduced into the reactor to prepare a carbon-containing metal diboride powder (presumably as simultaneously produced metal carbide), the carbon source reactant can be introduced by any convenient means. Thus, the carbon source reactant can be introduced into the reactor mixed with one or both of the metal halide and boron source reactants. Alternatively, the carbon source can be introduced as a separate reactant stream. Thus, apparatus such as described and shown in FIGURE 2, provides individual conduits for each of the reactants when the aforesaid embodiment is used. The reactants can be introduced into the reactor in any sequence; however, the metal halide, e.g., titanium halide, reactant is introduced preferably upstream of the boron source reactant. Preferably the carbon source reactant is introduced prior to the metal halide and boron source - reactants. Further, one or more of the reactant gases can be introduced through the same conduit in the reactant mixer means (provided the reactants are at a temperature at which inter-reaction does not occur) thereby leaving a conduit for the use of a sheath gas. Still further, mixer means with four, five or more slots are contemplated so that each reactant and'gas stream introduced through said mixer means can be introduced separately.
When it is desired to produce metal boride, e.g., titanium . :
:
:
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10758~7 diboride, powder in the absence of co-formed metal carbide, metal halide, e.g., titanium tetrachloride, reactant can be introduced through the top slot of the three slot mixer means depicted in FIGURE 2, hydrogen is introduced through the middle slot thereby acting as a sheath gas between the metal halide reactant and the _ boron source reactant, e.g., boron trichloride, which is introduced through the bottom slot of the mixer. ~lternatively, metal halide can be introduced through the top slot, boron source reactant through the middle slot and sheath gas, e.g., hydrogen, through the bottom slot. The sheath gas serves to prevent contact of the reactant gases with exposed surfaces of the mixer means 32, such is lip 75, and the reactor, e.g., the upper lip 76 of reactor 34. When metal boride, e.g., titanium diboride, is to be produced with co-formed metal carbide, e.g., titanium carbide, the carbon source reactant can be introduced through the top slot, the metal halide reactant introduced through the middle slot and the boron source reactant introduced , through the bottom slot. Other reactant introduction sequence can, of course, be used if desired.
The boride powders described herein, particularly titanium and zirconium diboride, when hot pressed or cold pressed and sintered into solid shapes are especially useful as current conducting elements in electrolytic cells for the production of metals, e.g., aluminum. The term "electrolytic cell" as used herein with respect to aluminum production is intended to include both reduction cells and three-layer cells for the refining or purification of aluminum. When used as a current conducting element, titanium and zirconium diboride can comprise at least part of the cathode of the electrolytic cell or of the elements used for conducting '`
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: ~ : :, . ' electrolyzin~ current to and/or from the electrolytic cell, and can be exposed to the molten metal eitheT in the reduction cell or in the purification cell.
Both the aluminum reduction cell and the three-layer purifi-cation cell are of the type in whicn electrolyzing current passss through a body of electrolyte or flux. In the case of the reduction cell, tlle current passes between an anode ancl a cathode having their operative faces in contact with the body of electrolyte whlch has dissolved therein a compound of the metal. The cathode can be the pool of molten metal which collects on the floor of the cell or it can be an emersed electrode presenting a solid surface to the electrolyte.
Such an electrode can extend into the pool of molten metal in which case the latter is also cathodic. In the case of three-layer aluminum puriEication cells, the current passes between the pool of aluminum alloy forming the bottom layer in the cell and the layer of purified molten aluminum forming the top layer in such a cell through the body of electrolyte or flux forming the intermediate layer which is in contact with both the top and bottom layers. The operative face or faces of the current-conducting element, i.e., the face or faces exposed to the deleterious conditions subsisting during the operation of the electrolytic cell, e.g., the face or faces exposed to the molten metal, can be fabricated from the metal diboride,e.g., titanium and zirconium diborides described herein.
Currently, carbon is used extensively for the construction of current-conducting elements in aluminum reduction electrolytic cells.
However, the use of carbon entails a number of very considerably disad-vantages, not the least of which is the fact that the floor of the cell '' ; - 43 -- .....

~7SB77 lining wllicll supports the molten metal must, in practice, be arranged in a substalltially horizontal plane. With such arrangement, the floor space occupied by a single cell is quite extensive and the cost of constructing such large cells is considerable. The necessity for the horizontal arrangement arises from the fact that molten aluminum does not wet carbon. Further, the gradual penetration of molten flux or flux constituents into the cell floor causes the carbon Eloor to swell or disintegrate arld shortens its useEul life. Deposits are formed on the surface of the carbon which increase the voltage drop across the cell and reduce the efficiency of the latter. Still further, the horizontal construction has the urther disa~vantage that the inherent turbulence of the molten metal cathode requires a high inter-polar distance to insure against contact of the molten metal cathode with the anode and with the consequent production of excess heat which has ~ to be dissipated.
;~ Current conducting elements prepared with metal diboride powders of the present invention can be disposed in a vertical or inclined posi-tion in the electrolytic cell for the reason that molten aluminum wets the surface of such elements. Thus, a cathode prepared from the titanium ; diboride powder of the present invention can be arranged in the electrolytic _ cell so that the operative face or faces of the cathode are disposed at a relatively large angle, i.e., 60 or 90 degrees, to the horizontal, thereby ; allowing the deposited aluminum to continuously drain from the face or faces of the cathode and preferably to collect in a pool in contact with a lower part of the cathode from which pool it may be withdrawn from time to time in the usual manner. D~e to the inclined or substantially vertical .` :

.

~75i377 arrangement of the catllode, the floor space occupied by the electrolytic cell is very consiclerably reclucecl in relation to that which is con-ventionally required. Perhaps the largest advantage to the use of inclined or substantially vertically arranged electrodes of the instant metal diborides is that surging oE the molten aluminum is less likely to occur so that the spacing of the anode and cathode can be substantially reduced compared with that adopted in aluminum reduction cells heretofore known and the dissipation of electrical energy in the electrolyte corres-pondingly reduced. Moreover, current conducting elements prepared from titanium diboride compositions have relatively high electrical conduc-, , tivity, i.e., a low electrical resistivity, and therefore the voltagedrop due to the passage of the operating current is less than that experienced in cells of orthodox construction. The effect of sludge formation at the bottom of the cell which causes an undesirable addi-tional voltage drop at the cathode in existing horizontal cells can also be avoided. Thus, the use of current-conducting elements prepared from metal diboride powder of the present invention in aluminum reduction cells improves the passage of electrolyzing current through the cell because of the low electrical resistivity of the compositions, and further, when such elements are used in a substantially vertical or inclined position, the voltage drop across the electrolytic cell is significantly reduced thereby providing significant savings in power. Such power savings have become increasingly more important due to the continuing rising cost of power.
The use of titanium diboride current-conducting elements in electrolytic cells for the production or refining of aluminum is described in the following U. S. patents, 2,915,442, 3,028,324, 3,215,615, 3,314,876, ::, : `

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~C~75877 3,330,756, 3,156,639, 3,274,093 and 3,400,061. Despite the rather extensive efEort these patents incl:icate was mounted and the potential advantages for USillg titaniu.n diboride and titanium diboride compo-sitions as current-conducting elements in electrolytic cells for the production of aluminum as described in the aforementioned patents, such compositions do not appear to have been commercially adopted on any significant scale by the aluminum industry. The reasons for such lack of acceptance are believed to be related to the lack of stability of the current-conducting elements prepared from the titanium diboride powders oE the prior art during service in electrolytic reduction cells.
It has been reported that SUCil current-conducting elements prepared with compositions of the prior art fail after relatively short periods in service. Such failure has been associated in the past with penetra-tion of the current-conducting element structure by the electrolyte, e.g., cryolite, thereby causing critical weakening of the self-bonded structure with consequent cracking and failure. Other reasons proposed have been the solubility of the compositions in molten aluminum, molten flux or electrolyte, or the lack of mechanical strength and resistance to thermal shock.
Ideally, a current-conducting element should have the foIlowing characteristics:
1. Good electrical conductivity.
2. It must not react with nor be soluble in either molten aluminum or, under cathodic conditions, in molten flux or electrolyte, at least to any appreciable extent at the operating temperature of the cell. The ~ ~758~7 solubility of the materi~ molten aluminum is an important consideration as it determines both the usefuL llfe of the current-conducting element and the degree of contamination of the aluminum produced through the agency of such current-conducting element.
3. Wetability by molten aluminum.

4. Capable of being produced and fabricated into required shapes economically.

5. High stability under the conditions existing at the cathode of the cell, i.e., it should possess good resistance to penetration by the molten electrolyte (cryolite) and to cracking.

6. Low thermal conductivity.

7. Good mechanical strength and resistance to thermal shock.
In order to have high stability under service conditions and resistance to penetration by the electrolyte, the current-conducting element prepared of titanium diboride powder compositions must have a relatively high density. In the past, high densities have been achieved with metal boride powder compositions of the prior art by hot pressing only. The metal boride powders of the present invention can be cold-formed and sintered to high densities. These metal boride powders provide the opportunity for preparing current-conducting elements of simple and complex shapes at a reasonable cost. Such current-conducting elements are resistant to the environment existing in electrolytic cells for the reduction or purification .
of aluminum and have improved stability compared to prior art boride composi-tions in such electrolytic cells. The present process is more particularly described in the following examples which are intended as illustrative only since numerous modifications and variations -therein will be apparent to those - ~7 -.

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.:~: . . , ~07S8'77 skilled in the art. tll the following examples, some volumes of gas are expressed in cubic feet per hour at standard conditions rl4.7 pounds per s~uare inch (101.3 kPa) pressure and 70 F (21 C~ or SCFII. Reactant and other gas stream rates were measured at nominal labora-tory conditions, i.e., 1 atmos-phere and 70 F (21 C), and are reported as measured if other than SCFH. Unless otherwise specified all percentages are by weigh-t.
The following examples illustrate the prepara-tion of re~ractory metal carbides by vapor phase reaction of the corresponding metal halide and a boron source in the presence of a hot hydrogen stream and in the substantial absence of oxygen, combined or elemen-tal.
EXAMPLE I
A heated gas stream was produced by supplying an argon-hydrogen gas mixture, which comprised 95.6 SCFH of argon and 29.9 SCFH hydrogen, to an induction plasma heater. The plasma heater was a quar-tz tube surrounded by a cooling jacket, which was provided with an inlet and outlet through which water coolant was passed. The quartz tube had an axial hollow core thxough which the work gas was passed. Surrounding the quartz tube was a 5--turn externally cooled copper coil energized by a 25 kilowatt Toccotron radio frequency generator, operating at about 4.5 megahertz. The radio frequency (R.F.) power level supplied to the induction coil was 18.5 kilowatts. The argon-hydrogen hot gas produced by the induction plasma heater was calculated to have an enthalpy of about 336 BTU/feet .
The plasma heater was mounted atop a copper cylinder which served as the reactor. The reactor was also externally cooled. Between the bottom of the plasma heater and the top of the copper reactor was positioned reactant inlet mixer means containing four in~ection ports which were disposed in a horizontal plane and positioned 90 degrees from each other. Gaseous titanium tetrachloride at a rate OI 2.8 grams/minute (measured at 59 C), together with .

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lO~S877 ll.~ SC~II o~ ar~on as a carrier gas was supplied to two of the injection ports located 180 deyrees apart. Into the other spatially opposed injection ports was introduced 8~l1 cc/minute oE gaseous boron trichloride toyether with one liter per minute of hydrogen as a carrier gas. This represented about a 20 percent stoichiometric excess of boron trichloride based on the production of titanium diboride (TiB2). The reaction was conducted for about 200 minutes.
Reactor product was passed through an externally water cooled copper tube to cool the powder product to a point at which it could be collected in a ; Teflon bag filter. The product obtained was finely-divided and had a surface area of about 13.0 square meters per gram. X-ray diffraction analysis of the product showed it to be titanium diboride (TiB2). The product fumed when exposed to air indicating that the material was pyrophoric.
EXAMPLE II
Run A
The procedure of Example I was repeated except that the R.F. power to the plasma heater was about 21 kilowatts and the plasma gas was 120.2 SCFH
of argon. Seven hundred eighty (7~0) cc/minute of boron trichloride together with one liter per minute of hydrogen as the carrier gas and 11.8 SCFH of argon carrier gas with titanium tetrachloride reactant were introduced into the heated gas emanating from the plasma heater. The aforementioned amount of boron trichloride represented an 8.0 percent stoichiometric excess. The run was continued for 3 hours and the powder product recovered. Analysis of the product showed it to be titanium diboride. The product removed from the reactor was pyrophoric.
Run B
The procedure of Run A was repeated except that 720 cc/minute of boron trichloride were introduced as the reactant and the plasma gas comprised 86 SCFH of argon and 32.5 SCFH of hydrogen. The run was continued for 3 hours ';
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and the recovered powder procluct found to have a B.E.T. surface area of 12.0 square meters per graM. The product was identified as titanium diboride and as removed from the reactor found to be pyrophoric.
Run C
The procedure of Run B was repeated except that the R.F. power to the plasma heater was about 20 kilowatts, the plasma gas rate was 86 SCFH of argon and 32.5 SCFH of hydrogen, and the boron trichloride reactant feed rate was 650 cc~minute. The run was continued for 146 minutes and the recovered powdered product identified as titanium diboride. The product had a B.E.T.
surface area of 10.3 square meters per gram. The product removed from the reactor was observed -to be pyrophoric.
EXAMPLE III
The apparatus of Example I was modified by substitu-ting for the reactant injection ports, reactant inlet assembly means similar to assembly means 30 of FIGVRE 1 of the attached drawings. The R.F. power to the plasma heater was about 24 kilowatts; the hydrogen-argon plasma gas was 78.4 SCFH
, argon and 42.7 SCFH hydrogen. Eight hundred forty-three (843) cc/minute of gaseous boron trichloride, 8.59 grams/minute of gaseous titaniurn tetrachloride, and 1.9 SCFH of argon carrier gas were introduced through the bottom slot of the reactant inlet assembly means and 38 SCFH of argon shroud gas were intro-` duced through the top slot of the reactant inlet assembly means. The run was continued for 200 minutes. The powder product was recovered in cyclone receivers and found to have a B.E.T. surface area of 9.3 square meters per gram. The product was observed to be pyrophoric.
EXAMPLE IV
~- Apparatus similar to FIGURE 1 of the attached drawing was utiliized to prepare titanium diboride. The arc heater utilized was a medium voltage, medium amperage heater hàving a power input of 28 kilowatts. This heater has .~ .
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~075877 been operatccl with an efficiency of between 50 and 60 percent. The reactant inlet assembly means used was one in which the reactants were introduced lnto the hot gas emanating from the plasma heater from ports disposed horizontally 180 degrees apart. ~Iydrogen at a rate of 19 liters per minute was introduced into the plasma heater ancl heated therein. The amount of power supplied to the plasma heater was 7.2 kilowatts. Titanium te-trachloride at a rate of 0.99 grams per minute, together with 3.6 litcrs per minute of argon as the carrier gas and 0.25 liters per minute of boron trichloride together with 3.0 liters per minute of argon as the carrier gas was introduced into the hot hydrogen stream emanating from the plasma heater. The run was continued for 60 minutes producing titanium diboride product at a 98.6 percent yield. The recovered product was identified as titanium diboride by X-ray diffraction analysis. The product fumed when exposed to air, indicating that the product was pyrophoric.
EXAMPLE V
:
Run A
Apparatus similar to FIGURE 1 was used to prepare a titanium di-boride. The arc heater was a medium voltage, medium amperage heater having a power input of 28 kilowatts. The arc heater was operated a-t between 24-28 kilowatts. Hydrogen in the amount of 300 SCFH was introduced into the arc heater as the plasma gas. Gaseous titanium tetrachloride in the amount of 18.7 grams per minute, together with hydrogen as the carrier gas in the amount of 20 SCFH, was introduced through the top slot of the reactant inlet assembly means. Gaseous boron trichloride, in the amount of 26.9 grams per minute with an argon carrier gas in the amount of 22 SCFH was introduced through the bottom slot of the assembly means. The run continued for 95-1/2 minutes and titanium diboride having a B.E.T. surface area of about 14 scluare meters per gram was obtained. Titanium diboride deposits on the bottom lip ~ ' . . .
: ' ' ' ~ , 58~7 of the reactarlt inl~t assembly were observed at the end oE the run.
Run B
Tlle procedure of ~un A was repeated except that boron trichloridewas introduced througll the top slot and titanium tetrachloricle through the bottom slot oE the reactant inlet assembly means. 25.6 grams per minute of gaseous boron trichloride with 22 SCE`H argon and 1~.7 grams per minute of titanium tetrachloride together with 12 SCFH of hydrogen chloride were utilized as the reactants. The run was continued for 120 minutes -to produce titanium diboride, haviny a B.E.T. surface area of about 9.1 square meters per gram. A thin skin of -titanium diboride powder deposi-ts on the inlet assembly were observed at the end of the run. Most of the deposit was found to be attached to the bottom exposed portion of the inlet assembly, i.e., lip 46 of mixer means 30 in FIGURE 1, and the exposed top lip of reactor 34.
- Run C
The procedure of Run A was repeated, except that 12 SCFH of hydrogen chloride was utili~ed as the carrier gas for the titanium tetrachloride instead of the 20 SCFH of hydrogen and 27.8 grams per minute of boron tri-chloride was fed to the reactor. This run continued for 150 minutes and the titanium diboride product was found to have a B.E.T. surface area of about 5.8 square meters per gram. No growth of titanium diboride deposits on the inlet assembly means was observed.
Run D
The procedure of Run C was repeated, except that the titanium tetrachloride feed rate averaged about 21 ~rams per minute and the ~oron trichloride feed rate averaged about 29.8 grams per minute. This run - continued for 975 minutes and the titanium diboride product had a B.E.T.
surface area of about 6.3 square meters per gram. No growth of titanium diboride deposits on the reactant inlet assembly means was observed at the : ' .
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end o~ the run.
In all of the above e~amples, the powder product obtained was calcined in the presenc~ of hydrogen at 1000 C to clegasify the product. Some of the calcined products remained pyrophoric.

Run _ Run D was repeated except that 23 SCFH of hydrogen was added to the titanium tetrachloride reactant introduced through the top slot of the reactant inlet assembly means. The titanium tetrachloride and hydrogen chloride ~; reactant addition rates averaged lg.2 grams per minute and 2.5 SCFH, respectively. Boron trichloride in the amount of 27.0 grams per minute together with 22 SCFH argon was introduced through the bottom slot of the reactant inlet assembly means. This run continued Eor 1,072 minutes and produced titanium diboride having a B.E.T. surface area oE abou-t 14.1 s~uare meters per gram.
EXAMPLE VI
. Apparatus analogous to FIGURE 1 modified with the reactant inlet . : .
assembly means of FIGURE 2 was used to prepare finely-divided titanium di-boride. The power to the plasma heater was 22.5 kilowatts. Hydrogen in the amount of 300 SCFH was used as the plasma gas. 0.71 grams per minute of ~` 20 1,1,2-trichloroethane together with 45 SCFH hydrogen as a carrier gas was introduced through the top slot of the three-slot reactant inlet assembly means, which was fabricated from graphite. Titanium tetrachloride in the amount of 18.8 grams per minu-te together with 20 SCFH hydrogen and 5 SCFH
hydrogen chloride was introduced through the middle slot of the reactant inlet assembly means. Boron trichloride in the amount of 21.7 grams per minute, - together with 22 SCFH argon was introduced through the bottom slot of the reactant inlet assembly means. This run was continued Eor 989 minutes and produced titanium diboride having a B.E.T. surface area o~ 24.0 square meters :, .
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~s~ 77 per gram. The product WLIS ana1yzed ~or carbon alld found to have 0.55 percent total carbon.
E~amples I-VI show that submicron titanium diboride having a surface area of between about 3 and about 35 square meters per yram, more typically be-tween about 4 and about 15 square meters per gram, can be produced by the vapor phase reaction of titanium halide and a boron source compound. The submicron titanium diboride powder are well formed, individual crystals of titanium diboride. Typical scanning and transmission electron micrographs of such titanium diboride is shown in FIGURES 3, ~ and 5 which are described in more detail hereinbefore. A comparison of the photomicrograph of FIGURES 3 and 4 with that of FIGURE 6, which is a scanning electron micrograph of a sample of purchased titanium diboride powder, clearly shows the difference between the two products. The photomicrographs of FIGURE 6 illustrates a product with ill-defined crystals, irregular faces, agglomerated product and a significant amount of fines, which are apparently produced by milling a product that was originally larger in size.
EXAMPLE VII
Apparatus simllar to FIGURE l, which is described in Example V, Run A, was used to prepare zirconium diboride. Hydrogen in the amount of 300 SCFH was introduced into and heated by the medium voltage, medium amperage arc heater. Gaseous zirconium tetrachloride, at a rate of 20.5 grams/minute, - and lO0 SCFH argon were introduced through the bottom slot of the reactant inlet assembly into the hot hydrogen stream emanating from the arc heater.
Gaseous boron trichloride, at a rate of 4.93 liters/minute (a 25 percent stoichiometric excess based on ~irconium tetrachloride), and 22 SCFH of argon - were introduced through the top slot of the reactant inlet assem'oly. The process was continued for 42 minutes. The ~irconium diboride product recovered had a B.E.T. surface area of 7.7 square meters per gram.

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~07~8~7 EXN~PLE VIII
The proced~lre and apparatus of Example VII is used to prepare hafnium diboride and a finely-divided, submicron produc-t similar in size and surface area to the zirconlum diboride of Example VII is recovered.
EX}~IPLE IX
Apparatus analogous to that used in Example VI was used to prepare titanium diboride. 300 SCF~ of hydrocJen was used as the plasma gas. Propane (89 standard cc/minute), and 45 SCF~I hydrogen as a carrier gas were introduced into the reactor through the top slot of the three-slot reactant inlet assembly means. Titanium tetrachloride (52 grams/minute) together with 9 SCFH of hydrogen and 24 SCFH of hydrogen chloride were introduced through the middle slot, and boron trichloride (13,000 standard cc/minute) and 22 SCFH of argon were introduced through the bottom slot of the inlet assembly. Titanium diboride powder product was recovered and degassed under a hydrogen flow of :
11 SCFH at 600 C Eor 4-3/4 hours. The titanium diboride powder product had an elemental analysis of 31.9 percent boron, 0.09 percent oxygen, 0.78 percent carbon and 0.088 percent chlorine, and had a B.E.T. surface area of about 6.4 m2/gram.
EXAMPLE X
Apparatus analogous to that used in Example IX was used to prepare titanium diboride. 300 SCFH of hydrogen was used as the plasma gas. Titanium tetrachloride in the amount of about 41.5 grams/minute, 9 SCFH of hydrogen and 24 SCFH of hydrogen chloride were introduced into the reactor through the top slot of the three-slot reactant inlet assembly means. About 22 SCFH of hydrogen was introduced through the middle slot; and, boron trichloride in -:
the amount of about 10,700 standard cc/minute (about a 10 percent stoichio-metric excess) and about 22 SCFH of argon were introduced through the bottom slot of the inlet assembly. Titanium diboride powder was recovered and - 55 _ .:

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,:. . : , ~75X377 d~cJassed under nydro~en at 600 C Eor 3 hours. The titanium diboride powder product had an elemelltal analysis oE 3Z.3 percerlt boron, 0.~4 percent oxygen and 0.03 pcrcent chlorine, and had a B.E.T. surEace area oE 3.3 m ~gram.
E tPLE X
The procedure of Example X was repeated and titanium diboride powder having an elemental analysis oE 32.3 percent boron, 0.60 percent oxygen and O.lO percent chlorine was recovered. The product had a B.E.T. surface area of 4.5 m2/gram.
The following examples illustrate the utility of the refractory metal borides.
EX~LE XII
A portion of the titanium diboride powder of Example IX was hot pressed at about 2100 C and 3500 pounds per square inch into a plate 2 inches x 2 inches x l/2 inch. The plate had a density oE 97 percent of the theoretical density of TiB2 and a resistivity of about 7 microohm centimeters.
~; The plate was analyzed for oxygen, which was found to be about 0.05 percent.
; The plate was operated as a cathode in an aluminum reduction cell for lO0 hours at 960 C at an anode current density of 6.5 amperes/inch . At the end of the test peribd, the plate was removed, fractured, and inspected. No deteriora-tion of the plate and no penetration of electrolyte into the plate was observed. Fracture of the plate was observed to be primarily transgranular.
A piece of the test plate was cut out after the test was completed and polished and etched. FIGURE 8 is a photomicrograph, having a magnification factor of 2100, of a polished and etched section of the plate. The micro-structure of FIGURE 8 shows a mosaic of equidimensional TiB2 grains with contiguous grain boundaries and a limited grain size range. The TiB2 grains range from about one to fifteen microns in diameter; but, are predominantly in the four to twelve micron range in size. Titanium carbide occurs as .: , .

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~C~75877 occl~lsions less th~n one micron in size within the titallium diboride grains.
EX~MPLE XIII
~ bl~nd of the titanium diboride powders oE Examples X and XI in a weight ratio of about 58.5/~l.S was mixed with about 5 weight percent of titanium carbide powder having a 8 . E . T. surface area of about 4~5 m /gram.
The titanium carbide powder was prepared in accordance with the procedures described in U.S. Patent 3,~85,586. The titanium diboride and titanium carbide powders were mixed with 1 percent paraf-fin wax in l,l,1-trichloroethane with a high speed Cowles mixer. The blended mixture was vacuum dried and hot pressed at about 2000 C and 3500 pounds per square inch in-to a 2 inch x 2 inch x 1/2 inch plate. The plate was allowed to cool overnight in the mold under vacuum.
The plate had a density oE about 93 percent of the theoretical density of TiB2 and was found to have an oxygen content of about 0.33 percent. The electrical resistivity of the plate was 6 microohm centimeters. The plate was operated as a cathode in an aluminum reduction cell under the same conditions as recited in Example XII. At the end of the test period, the plate was removed, fractured and inspected. Some minor spalling and erosion o~ the plate had taken place; but, no penetration of the electrolyte into the plate was observed.
Fracture of the plate was observed to be primarily transgranular.
A piece of the test plate was cut out after the test was completed and polished and etched. FIGURE 7 is a photomicrograph, having a magnification factor of 2100 of a polished and etched section of the plate. The micro-structure of FIGURE 7 is fine and shows interlocking grains of white, lath-i shaped TiB2 with grey TiC grains dispersed in the structure. The TiB2 grains range in size from less than one micron to five microns. TiC grains are up to three microns in diameter.
EXAMPLE XIV
A blend of 95 parts of titanium diboride powder prepared in a manner . .

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similar to Example V, Run ~ and 5 parts of titanium carbidc powder was mixed with about 1 percent paraffin wax in l,l,l-trichloroethane and ball milled for about one hour. The titanium diboride powder had a B.E.T. surface area of 4.9 m /gram and the titanium carbide powder had a B.E.T. surface area of about 5.0 m /gram. The blended mixture was vacuum dried and isostatically presscd at abou-t 20,000 pounds per square inch into a cylindrical rod 1-1/2 inch in diameter x 16-3/8 inches long. A well 3~8 inch in diameter and about 15 inches deep was drilled out of the rod and the resulting rod was vacuum sintered at about 1900C for about 1 hour. The rod had a density of 95 percent of the theoretical density of TiB2. The sintered rod was tested as a thermo-couple well in an aluminum reduction cell. The rod showed excellent thermal shock resistance and resistance to the bath.
. ~
EXAMPLE XV
Rods similar to that of Example XIV were prepared using titanium diboride powder having B.E.T. surface areas of 6.6 m /gram and 7.0 m /gram.
The sintered rods had densities of 96 percent and greater than 99 perFent of the theoretical density of TiB2 respectively. A piece of the rod prepared with the 7.0 m /gram titanium diborlde was polished and etched. FIG~RE 9 is a photomicrograph, having a magnification factor of 2100, of a polished and ` 20 etched section of the rod. The microstructure of FIG~RE 9 shows a mosaic of relatively equidimensional TiB2 grains with the light-grey TiC predominantly localized in interstices between TiB2 grains or occurring as occlusions within the TiB2 grains. Electron microprobe analysis has indicated that a gold color induced in the TiC signifies scavenging of oxygen and nitrogen to produce a solid solution phase represented by Ti (C,O,N).
EXAMPLE XVI
Titanium diboride powder prepared in a manner similar to Example IX
; and having a B.E.T. surface area of 24 m /gram and 0.46 percent carbon was ; ~ - 58 -'.~
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~07~i~77 isostatically pressed at 20,000 pounds per square inch lnto a cylindrical rod.
The rod was vacu~lm sintered at about 2000 C Eor about 30 mlnutes. The sintered rod, whic}l had dimensior.s of about 1 inch x 5 inches, had a density of about 9~ percent of the theoretical density oE TiB2 and a resistivity of about 9 microohm centimeters.
A piece of the rod was polished and etched. FIGURE 10 is a pho-to-micrograph, having a magnification factor of 2100, of a polished and etched section of the rod. The microstructure of FIGURE 10 shows a mosaic of equi-dimensional TiB2 grains with contiguous grain boundaries and a limited grain size range. The TiB2 grains are predominantly three to ten microns in diameter. The Ti (C,O,N) phase occurs as occlusions less than one micron in size within the TiB2 grains.
EXAMPLE XVII
A portion of the titanium diboride powder of Example V, Run B was used to prepare two cold pressed and sintered cylindrical rods. The powder was loaded into pressing molds and dies in dry nitrogen filled glove bags and pressed under a nitrogen atmosphere. Pressing was performed isostatically in a rubber mold at 20,000 and 30,000 pounds per square inch ~psi) respectively.
The rods were buried in graphite powder in a vacuum furnace and vacuum 20 sintered. The furnace was heated to 2000 C in 2-1/2 hours and held at that temperature for 25 minutes. The furnace was turned off and the temperature decreased to 1625 C in 15 minutes. After cooling, the rods were submitted for density measurements. The rod which was pressed at 20,000 psi had a density of 4.44 g/cc. The rod which was pressed at 30,000 psi had a density of 4.40 g/cc. The aforesaid densities are 98 and 97 percent of theoretical based on a theoretical TiB2 density of 4.51 grams/cc.
EXAMPLE XVIII
A portion of the titanium d boride powder of Example V, Run B was ,.
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hot pressed at l600 C and at 4500 pounds per square inch (psi). The tirne interval at 1600 C was 30 minutes. The hot pressed specimen had a density of 4.14 CJ/CC (about 92 percent of the theoretical density of TiB2) and a transverse rupture strength of 34,000 psi.
A Eurther portion oE the titanium diboride powder of Example V, Run B was hot pressed at 1750 C and at 5000 psi for 10 minutes. The hot pressed specimen had a density of ~.46 g/cc (about 98 percent theoretical) and a transverse rupture strength of 4/,000 psi.
The following example illustrates a further preparation of titanium diboride powder in accordance with the present invention.
EXAMPLE XIX
The apparatus and general procedure of Example VI was used except that titanium tetrachloride in the amount of 72.2 yrams per minute and 15 SCE'H
of hydrogen were introduced into the reactor through the top slot of the reactant mixer assembly means. 1.26 grams per minute of 1,1,2-trichloroethane, 45 SCFH of hydrogen and 20 SCFH of hydrogen chloride were introduced through the middle slot and boron trichloride in an amount calculated to represent a 10 percent stoichiometric excess (basis the titanium te-trachloride) and 8 SCFH
of argon were introduced through the bottom slot of the reactan-t mixer assembly. The titanium diboride powder product recovered had a B.E.T. surface area of 11.5 square meters per gram and was found to contain about 31.6 percent boron, 0.08 percent chlorine, 0.19 percent oxygen and 1 percent total carbon.
Although the present process has been described with reference to specific details of certain embodiments thereof, it is not intended that such details should be regarded as limitations upon the scope of the invention except as and to the extent that they are included in the accompanying claims.

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Claims (35)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. Submicron titanium diboride powder comprising at least 99 weight percent titanium diboride, said powder having a surface area of between about 3 and 35 square meters per gram and containing less than 0.4 weight percent metal impurities and less than 0.1 weight percent carbon, wherein the nominal sectional diameter of at least 60 percent of the titanium diboride particles of said powder are less than one micron, said particles being tabular to equidimensional hexagonal crystals having well developed faces, and a number median particle size of between about 0.08 and 0.6 microns, said powders being characterized by the property of being able to be cold formed and sintered to a density of at least 90 percent of the theoretical density for titanium diboride.

2. The titanium diboride powder of Claim 1 wherein the surface area is between about 3 and 15 square meters per gram.

3. The titanium diboride powder of Claim 1 wherein the surface area is between about 5 and 10 square meters per gram.

4. The titanium diboride powder of Claims 1, 2, or 3 wherein at least 98 percent of the titanium diboride particles have a nominal sectional diameter of 0.7 microns or less.

5. The titanium diboride powder of Claim 1,2 or 3 wherein at least 90 percent of the titanium diboride particles have a nominal sectional diameter of 0.7 microns or less.

6. The titanium diboride powder of Claim 1 wherein the ratio of the nominal sectional diameter to thickness of the titanium diboride tabular crystals is from 1.5:1 to 10:1.

7. The titanium diboride powder of Claim? 1 wherein the powder contains less than 0.25 weight percent oxygen.

8. The titanium diboride powder of Claim 7 wherein the titanium diboride contains less than 0.20 weight percent halogen as de-termined by X-ray spectrographic analysis.

9. The titanium diboride powder of Claim 1 wherein the powder is substantially free of titanium diboride fragments less than 0.1 micron.

10. The titanium diboride powder of Claim 1 wherein the number median particle size of the particles is between about 0.1 and about 0.5 microns.

11. The titanium diboride powder of Claims 1 wherein at least 70 percent of the titanium diboride particles have a nominal sectional diameter of 0.7 microns or less and wherein the ratio of the nominal sectional diameter to thickness of the titanium diboride tabular crystals is from 1.5:1 to 10:1.

12. The titanium diboride powder of Claim 11 wherein the titanium diboride contains less than 0.20 weight percent oxygen, and less than 0.15 weight percent halogen as determined by X-ray spectrographic analysis. .

13. The titanium diboride powder of Claim 12 wherein the titanium diboride contains less than 0.3 weight percent metal impurities.

14. The titanium diboride powder of Claim 13 wherein the number median particle size of the particles is between 0.1 and 0.5 microns.

15. A titanium diboride article of manufacture having a density of at least 90 percent of the theoretical density of titanium diboride which is prepared by hot pressing or cold forming and sintering the submicron titanium diboride powder of claims 1, 2, or 3, the article having a carbon content of less than 0.1 weight percent.

16. A titanium diboride article of manufacture having a density of at least 90 percent of the theoretical density of titanium diboride which is prepared by hot pressing or cold pressing and sintering the submicron titanium diboride powder of Claim 7, the article having a carbon content of less than 0.1 weight percent.

17. The article of Claim 16 wherein the titanium diboride powder is the powder of Claim 8.

18. A method of preparing a sintered titanium diboride article comprising the steps of:
a) blending a small amount of binder with the titanium diboride powder of Claim 1, b) forming the resulting blend of binder and powder into a green article of the desired shape, and c) sintering the green article at temperatures of from 1950°C.
to 2250°C. for a time sufficient to obtain a titanium diboride article having a density greater than 90 percent of theoretical.

19. A method of Claim 18 wherein the temperature is from about 2000°C. to 2100°C. and the time at temperature is about 1 hour.

20. The method of Claim 18 wherein from about 1 to about 5 weight percent of wax binder is used.

21. A method of preparing a sintered titanium diboride article comprising the steps of:
a) blending a small amount of binder with the titanium diboride powder of Claim 7, b) forming the resulting blend of binder and powder into a green article of the desired shape, and c) sintering the green article at temperatures of from 1950°C.
to 2250°C. for a time sufficient to obtain a titanium diboride article having a density greater than 90 percent of theoretical.

22. The method of Claim 21 wherein the titanium diboride powder is the powder of Claim 8.

23. The method of Claim 21 wherein the temperature is from about 2000°C. to 2100°C.,the time at temperature is about 1 hour, and from about 1 to about 5 weight percent wax binder is used.

24. A process for preparing submicron refractory metal boride powder selected from the diborides of the metals titanium, zirconium and hafnium, capable of being cold formed and sintered to a density of at least 90 percent of their theoretical density, by gas phase reaction of the halide of the corresponding metal and boron source reactants in the presence of hydrogen in a reactor, which comprises projecting a hot hydrogen gas stream into a reaction zone in the reactor, introducing substantially pure gaseous metal halide and boron source reactants into said reaction zone, the heat content of the hydrogen gas stream and reactants being sufficient to establish refractory metal boride forming temperatures in said reaction zone, reacting said metal halide and boron source reactants in the reaction zone in the substantial absence of oxygen and a source of carbon, and removing solid, submicron refractory metal boride powder from the reactor, at least 60 percent of the particles of which have a nominal sectional diameter of less than one micron, said particles having a number median particle size of between about 0.08 and 0.6 microns and being tabular to equidimensional hexagonal crystals with well developed faces.

25. The process of Claim 24 wherein the metal halide reactant is the tetrachloride of titanium, zirconium or hafnium.

26. The process of Claim 24 wherein the boron source reactant is boron trichloride.

27. The process of Claim 24 wherein the hot hydrogen stream is formed by heating hydrogen in plasma heating means.

28. The process of Claim 24 wherein the reactor is a recircula-ting-type reactor.

29. The process of Claim 24 wherein the metal halide reactant is the tetrachloride of titanium, zirconium, and hafnium, the boron source reactant is boron trichloride and the hot hydrogen stream is formed by heating hydrogen in plasma heating means.

30. The process of Claim 29 wherein the plasma heating means is a plasma arc heater.

31. A process for preparing submicron titanium diboride powder capable of being cold formed and sintered to a density of at least 90 percent of the theoretical density for titanium diboride by gas phase reaction of titanium tetrachloride and boron trichloride reactants in the presence of hydrogen in a reactor, which comprises heating hydrogen in plasma arc heater means and projecting the heated hydrogen as a gas stream into a reaction zone in the reactor, introducing substantially pure gaseous titanium tetrachloride and boron trichloride reactants into said reaction zone, the heat content of said heated hydrogen and reactants being sufficient to establish titanium diboride forming temperatures in the reaction zone, reacting said reactants in the substantial absence of oxygen and a source of carbon, and removing solid, submicron titanium diboride powder from the reactor, at least 60 percent of the particles of which have a nominal sectional diameter of less than one micron, said particles having a number median particle size of between 0.08 and 0.6 micron and being tabular to equidimensional hexagonal crystals with well developed faces, said powder having a surface area of between about 3 and 35 square meters per gram and containing less than 0.4 weight percent metal impurities and less than 0.1 weight percent carbon.

32. The process of Claim 31 wherein the mole ratio of boron trichloride to titanium tetrachloride is from 1.8:1 to 3:1.

33. The process of Claim 31 wherein the principal source of heat for the reaction is the heated hydrogen gas stream.

34. The process of Claim 31 wherein the titanium diboride powder removed from the reactor has a surface area of between about 4 and 15 square meters per gram, and the number median particle size of said particles are between about 0.1 and about 0.5 micron.

35. The process of Claims31 or 34 wherein the solid titanium diboride powder removed from the reactor contains less than 0.25 weight percent oxygen.