WO1983000325A1

WO1983000325A1 - Sintered refractory hard metals

Info

Publication number: WO1983000325A1
Application number: PCT/US1982/001004
Authority: WO
Inventors: Lakes Carbon Corporation Great; Louis Arpad Joo'; Kenneth Wayne Tucker; Scott David Webb
Original assignee: Great Lakes Carbon Corp
Priority date: 1981-07-27
Filing date: 1982-07-22
Publication date: 1983-02-03
Also published as: EP0085093A1; BR8207805A; EP0085093A4; JPS58501172A

Abstract

TiB2-carbon composites are produced by mixing the raw materials comprised of carbon, TiB2, pitch, and other reactants, forming a shaped article, processing in a nitrogen atmosphere up to 2100<o>C, and in a noble gas above 2100<o>C, by pressureless sintering of TiB2 or other refractory hard metal powder, or by molding or extrusion of plastic mixes of binder and particulate carbon and refractory hard metal.

Description

SINTERED REFRACTORY HARD METALS

BACKGROUND OF THE INVENTION

Refractory hard metals (RHM) as a class are hard, dense materials with high melting points, and are generally of low solubility and resistant to corrosive attack by most acids and alkalis.

RHMs have high electrical conductivity due to their metallic structure; consequently, this combination of properties has made them candidates for use as electrodes in molten salt electrolysis processes where their corrosion resistance and conductivity are vital properties needed for economical performance.

The RHMs have other properties which have limited their usage up to the present time. Tney are usually brittle, haye little resistance to thermal shock, and are quite expensive to produce and fabricate into useful articles.

RHM articles have been produced by a number of processes including hot pressing of the granular or powdered materials, chemical vapor deposition, and in situ reduction of metals by carbon or other reducing agents. Hot pressing is the most commonly used process for production of shapes. A die and cavity mold set is filled with powder, heated to about 300^ο-800^οC and placed under pressure of about 2 x 10⁸ Pa, then removed from the mold and heated at about 1500^ο-2000^οC, or higher, or sintered in the mold.

Hot pressing has the limitations of applicability to simple shapes only, erosion of the mold, and slow production. The pieces produced by hot pressing are subject to a high percentage of breakage in handling, making this process expensive in terms of yield of useful products.

The RHMs of most interest include the carbides, borides, and nitrides of the metals of IyΑ, IVB, VB, and VIB of the periodic table, particularly Ti, V, Si, and W. Of these, the borides are of most interest as electrodes in high temperature electrolysis applications due to their electrical conductivity, and of the borides, TiB₂ has been extensively investigated for use as a cathode or cathodic element in the Hall-Heroult cell. Typically the Hall cell is a shallow vessel, with the floor forming the cathode, the side walls a rammed coke-pitch mixture, and the anode a block suspended in the molten cryolite bath at an anodecathode separation of a few centimeters. The anode is typically formed from a pitch-calcined petroleum coke blend, prebaked to form a monolithic block of amorphous carbon. The cathode is typically formed from a pre-baked pitch-calcined anthracite or coke blend, with cast-in-place iron over steel bar electrical conductors in grooves in the bottom side of the cathode.

During operation of the Hall cell, only about 25% of the electricity consumed is used for the actual reduction of alumina to aluminum, with approximately 40% of the current consumed by the voltage drop caused by the resistance of the bath. The anode-cathode spacing is usually about 4-5 cm., and attempts to lower this distance result in an electrical discharge from the cathode to the anode through aluminum droplets.

The molten aluminum is present as a pad in the. cell, but is not a quiescent pool due to the factors of preferential wetting of the carbon cathode surface by the cryolite melt in relation to the molten aluminum, causing the aluminum to form droplets, and the erratic moyements of the molten aluminum from the strong electromagnetic forces generated by the high current density.

The wetting of a solid surface in contact with two immisci ble liquids is a function of the surface free energy of the three surfaces, in which the carbon cathode is a low energy surface and consequently is not readily wet by the liquid aluminum. The angle of a droplet of aluminum at the cryolite-aluminum-carbon junction is governed by the relationship

where α₁₂, α₁₃, and α₂₃ are the surface free energies at the aluminum carbon, cryolite-carbon, and cryolite-aluminum boundaries, respectively. If the cathode were a high energy surface, such as would occur if it were a ceramic instead of carbon, it would have a higher contact angle and better wettability with the liquid aluminum. This in turn would tend to smooth out the surface of the liquid aluminum pool and lessen the possibility of interelectrode discharge allowing the anode-cathode distance to be lowered and the thermodynamic efficiency of the cell improved, by decreasing the voltage drop through the bath. Typically, amorphous carbon is a low energy surface, but also is quite durable, lasting for several years duration as a cathode, and relatively inexpensive. However, a cathode or a cathode component such as a TiB₂ stud which has better wettability and would permit closer anode-cathode spacing could improve the thermodynamic efficiency and be very cost-effective.

Several workers in the field have developed refractory high free energy material cathodes. U.S. 2,915,442, Lewis, December 1, 1959, claims a process for production of aluminum using a cathode consisting of the borides, carbides, and nitrides of Ti, Zr, V, Ta, Nb, and Hf. U.S. 3,028,324, Ransley, April 3, 1962, claims a jnethod of producing aluminum using a mixture of TiC and TiB₂ as the cathode. U.S. 3,151,053, Lewis, September 29, 1964, claims a Hall cell cathode conducting element consisting of one of the carbides and borides of Ti, Zr, Ta and Nb. U.S. 3,156,639, Kibby, November 10, 1964, claims a cathode for a Hall cell with a cap of refractory hard metal and discloses TiB₂ as the material of construction. U.S. 3,314,876, Ransley, April 18, 1967, discloses the use of TiB₂ for use in Hall cell electrodes. The raw materials must be of high, purity particularly in regard to oxygen content, Col. 1, line 73-Col. 2, line 29; Col. 4, lines 39-50, Col. 8, lines 1-24. U.S. 3,400,061, Lewis, September 3, 1968 discloses a cathode comprising a refractory hard metal and carbon, which may be formed in a one-step reaction during calcination. U.S. 4,071,420, Foster, January 31, 1978, discloses a cell for the electrolysis of a metal component in a molten electrolyte using a cathode with refractory hard metal TiB₂ tubular elements protruding into the electrolyte. S.N. 043,242, Kaplan et al. (Def. Pub.), filed May 29, 1979, discloses Hall cell bottoms of TiB₂. None of the above disclose the advantageous use of the various inventive improvements on the pressureless sintering of TiB₂ powder in the mold. None disclose the subsequent treatment of the piece formed in this manner by the processes of impregnation. None disclose the formation of a composite by sintering a layer of TiB₂ over a core of graphite.

The level of skill in the art is quite high, with all of the inventors holding advanced scientific degrees.

SUMMARY OF THE INVENTION

Titanium Diboride, TiB₂ has been proposed for use as a cathodic element in Hall cells, giving an improved performance oyer the amorphous carbon and semi-graphite cathodes presently used.

It had previously been known that Titanium Diboride (TiB₂) was useful as a cathode component in the electrolytic production of aluminum, when retrofitted in the Hall cell as a replacement for the carbon or semi-graphite form. The electrical efficiency of the cell was improved due to better conductivity, due mainly to a closer anodecathode spacing; and the corrosion resistance was improved, probably due to increased hardness, and lowex solubility and chemical inertness as compared to the carbon and graphite, forms. If the anode-cathode (A-C) distance could be lowered, the % savings in electricity would be as follows:

A-C distance % sayings

3 .8 cm. std.

1.9 cm. 20% 1.3 cm. 27%

1.0 cm. 30%

The principal deterrent to the use of TiB₂ as a Hall cell cathode has been the sensitivity to thermal shock and the great cost, approximately $25/lb. as compared to the traditional carbonaceous compositions, which cost about $0.60/lb. We have invented an improved process for producing a pure TiB₂ or TiB₂-carbon composite which shows excellent performance as a cathode or cathode component in Hall aluminum cells. The method is markedly more economical, and also produces an unexpectedly improved cathode when its performance is compared to the traditional carbonaceous material.

Our method of producing TiB₂ articles involves a sintering process in which powdered TiB₂, other RHMs or mixture of a KBM and carbon powder are simply poured or packed vertically into a mold with slightly larger dimensions than the desired article, and then fired in a con trolled atmosphere to the sintering temperature for the particular

RHM in process. For TiB₂ the preferred sintering temperature is 1700^ο- 2400^οC. We have made both solid and hollow cylinders, however, there does not appear to be any limitation en the shape or size of the article to be produced with appropriate operating conditions and mold design. During the sintering process, the article shrinks away from the interior mold surfaces and can usually be removed by hand.

Our process has the advantages of savings of time, capital investment, and operating costs due to the fewer operations, improved yield, less wear on the equipment by abrasive RHMs, lower density and ability to use low cost fillers, adaptability to automated production, and less critical control needed for heating and cooling rates.

The articles produced by our process have improved thermal and mechanical shock resistance and a more active surface area, when compared to conventionally pressed and sintered pieces. We have also found that we can use this process to apply a coating to a pre-formed core by this method, particularly a carbon core matching as closely as possible the coefficient of thermal expansion (CTE) of the KHM, making articles produced in this manner much more economical than solid RHM articles. A core with lower strength and more elasticity than the sintered TiB₂may be used to form a composite mechanically bonded article.

After sintering, the article formed may be further treated by impregnation with a carbonizable. binder, baked, and graphitized to form an impervious carbon-TiB₂ structure. The carbon particulate matter found most useful includes fine particle size graphite, calcined petroleum coke, metallurgical coke, and wood charcoal.

Impregnating carbonizable binders that are useful include petroleum and coal tar pitches, phenolic type condensation resins, vegetable pitches, and lignosulfonates from wood. When baking a TiB₂ shape after impregnation with a carbonizable impregnant, we find that we obtain a higher coke yield (75-80%) from the impregnant than when baking a similar shape of baked carbon particu lates after impregnation with the same impregnant (70-75%), apparently due to a catalytic effect by the TiB₂ during the coking reaction. In all of the above processes for manufacture of carbon-RHM composites, we have found superior performance in the Hall cell if the composite is heated as above to about 2100°C or higher to graphitize the carbon.

During processing of carbon-RHM composites the carbon will oxidize to CO or CO₂ if the article is processed in an air atmosphere. Consequently a nitrogen atmosphere has commonly been used to avoid this difficulty. However, nitrogen, while relatively unreactive at ambient temperatures, at elevated temperatures of 2000°C and over, can be highly reactive with many elements and compounds, and is unsatisfactory for use with many RHMs.

We have found that when processing carbon-TiB₂ mixtures in a nitrogen atmosphere, that the observed melting point and percentage of TiB₂ are inversely related, i. e., that higher concentrations of TiB₂ lower the melting point, of the mixture, and that when processing TiB₂ in nitrogen some 3N and TiB are detected by x-ray diffraction in the final material if the temperature is higher than about 2200°C. In pure argon the melting point of the TiB₂-composite is much higher, about 2450°C. (All physical quantities giyen herein use the metric or SI designations unless otherwise stated.) We have found that we obtain better yields of products and can operate more economically by the use of a system of interchangeable or multiple processing atmospheres. At temperatures below about 2100°C we operate in a nitrogen atmosphere, which is inert with the components of the mix below that temperature, and at temperatures above 2100°C, we use a noble gas atmosphere, preferably argon. By using this combination, we maintain an inert atmosphere around the articles being produced at the lowest possible cost, with a high weight percentage yield, based on the raw material weights.

We can vary the porosity, density, and strength by controlling the particle size and shape of the powder mixture used. A mixture of dif ferent sized particles will normally sinter to a higher density piece than one of uniformly sized particles.

As shown below, higher sintering temperatures promote higher densities in the shaped pieces.

Thus, the strength, density, electrical conductivity, chemical resistance, and other parameters can be controlled by varying the particle sizes and mixtures, heat treating temperature, impregnation, and graphitization processes.

DESCRIPTION OF THE PHOTOGRAPHS

All of the articles shown are in graphite molds, exhibited photo graphically to display the grain structure. All proportions are by wt. and all measurements are in metric units unless otherwise stated.

Figure 1 is a yertical view of a cylinder of 100% TiB₂ processed in argon for 2 hours at 2615 + 15°C with a maximum temperature of 2630°C. The TiB has partially melted and reacted with the mold as shown in Figure 2 in a cutaway view.

Figure 3 is a cylinder of 100% TiB₂ processed for 2 hours @ 2490° + 40°C with a maximum temperature of 2530°C in argon, showing fusion of the granules.

Figure 4 is a sintered molding of 100% TiB₂ processed @ 2450°C + 50°C in argon for 1.5 hours, and Figure 5 is a section of the same piece in the mold. There is some slight eyidence of attack on the graphite mold, leading to the conclusion that the^ upper limit of temperature for this particular raw material and process is slightly less than 2450° or probably about 2400°C. Figure 6 is a piece of 100% TiB₂ sintered in argon for 2 hours @ 2260°C + 50°C,and Figure 7 shows the article taken out of the mold. Figure 8 is a piece of 100% TiB₂ processed for 3 hours @ 2200°

+ 15°C in argon, and Figure 9 is the article out of the mold. The filled cavity was 2.86 cm in diameter and the finished piece was 2.41 cm in diameter, for a shrinkage of approximately 15%, although shrinkage When cooling the processed article, we find it necessary to maintain a non-αxidizing or non-reactive atmosphere down to a temperature of 400°C or below, and preferably 200°C and below, to prevent reaction with air from occurring. The method is generally adaptable to any of the various processes for the production of RHM containing articles at temperatures over approximately 2000°C. These processes include sintering of either loose filled or pressed articles; the heat treatment of mixtures of a RHM and carbonaceous materials, e.g., TiB₂ plasticized with coal tar pitch; heat treatment of reactants to form the RHM in situ, e.g., B₄C + C + TiO₂; RHM deposited by chemical vapor deposition, e.g., TiCl₄ + 2 BCl₃ + 5 H₂

TiB₂ + 10 HCl. Powdered TiB₂ is poured into a mold and heated to a sintering temperature of 1700°-2400°C in an inert atmosphere, preferably argon, for about 1 to 4 hours. A graphite mold may be used and is the preferred material of construction. We have found that TiB₂ of the proximate analysis below is useful: Ti - 69.06% B - 31.24% C - 2270 P.P.M. (parts per millon) O₂ - 3490 P.P.M. N₂ - 150 P.P.M.

Al - 0.1%

The data aboye total over 100%, are therefore slightly inaccurate, and should be regarded as approximate concentrations of the elements. The particle size mean is approximately 7 μ, with 90% falling in the 5-15 μ range and none over 44 μ.

All of the following examples used the above TiB₂ powder. The articles produced by this method have unexpectedly good resistance to thermal and mechanical shocks, probably due to the point contact bonding of the particles and the porosity. This porosity may also be filled by impregnants such as pitch or phenolic resins if special enhanced properties of strength, thermal shock resistance or non-porosity are required. A petroleum pitch having a softening point from 110°-120°C is the preferred impregnant, applied under alternate cycles of vacuum and pressure at 175°-250° C and 2-15 x 10⁵ Pa, baked on a cycle rising to 700°-1100° C over a period of 1 to 10 days, then further heated to about 2000°-2400°C to graphitize the carbonized residue.

The process in general comprises gravity filling a mold with the RHM powder, with a small amount of vibration sometimes needed to eliminate voids and air pockets. The mold is then heated to the sintering temperature in a controlled atmosphere and held there for a period of about one to four hours. The atmosphere is generally an inert gas, and preferably a noble gas such as argon. If the RHM being formed is a nitride, nitrogen may be used, but may react undesirably with borides and carbides to form the nitrides.

EXAMPLE 1

A piece (Figure 19) was made from a mixture of 15% graphite flour, having a particle size similar to the TiB₂ powder, and 85% TiB by wt., sintered in a graphite mold in argon at 1900°C. The piece was unloaded from the furnace after 16 hours cooling time, and was strong and homogenous in appearance.

EXAMPLE 2

A mold was filled with a mixture of the graphite flour described in Example 1 and TiB₂ at a 50/50 wt. ratio. It did not sinter at 1900°C in argon. About 20-30% graphite by wt. appears to be a maximum concentration for this system.

EXAMPLE 3

A sample of TiB₂ powder was sintered to form a strong article when processed in argon @ 1700°-2400°C. A series of cylinders treated at temperatures in this range had a positive correlation of apparent density (A.D.) with sintering temperature:

Sintering Temp. A.D. % Porosity (calc.)

1700°C 2.92 g/cc 35%

1900°C 3.04 g/cc 33%

2200°c 3.98 g/cc 12%

2300°C 3.93 g/cc 12%

2400°C 3.92 g/cc 13% Since the known density of pure TiB₂ is 4.52 g/cc, it is seen that these pieces have porosities ranging from 13-35%.

The pieces were unloaded from the furnace after about 40 hours cooling time.

EXAMPLE 4

A mixture of 20% by wt. graphite powder (100% passing through a 79 mesh/cm screen and 95% through a 128 mesh/cm screen) and 80% by wt. of TiB₂ above formed a firm homogenous piece when sintered for one hour at 1900°C (Figure 22). It displayed a very slight tendency to flake, leading to the conclusion that in this system 20% graphite is a practical maximum. Most fully calcined coke powders and graphites may be used as components of the mixture, also other carbonaceous powders such as coal coke and charcoal.

EXAMPLE 5 A TiB₂ piece with a graphite core was made by placing a preformed core insert of Great Lakes Carbon H-303 graphite with the following characteristics into a graphite mold:

A. D. (Apparent Density) - 0.95 CTE - 24-34 x 10^-7/°C Tensile strength - 1000-1200 K Pa

MOR (Modulus of Rupture) - 1400-1700 K Pa Compressive strength - 3200-3400 K Pa Porosity - 55%

Powdered TiB₂ was poured into the mold, the mold placed in an induction furnace, and heat treated at 1900°C in argon. The sample piece was sintered successfully (Figures 12, 13), with the following parameters:

Diameter 7.4 cm Length 6.9 cm A. D. 2.326 g/cc

Using this low strength porous graphite as a core enabled production of a piece with firm mechanical bonding without excessive resistance to the movement caused by differential thermal contraction on cooling. EXAMPLE 6

The same type of core used in Example 5 was used, with the mixture of the powdered graphite (15% by wt.) and TiB₂ (85% by wt.) used in Example 1 poured into the mold surrounding the core, and processed in the same manner at 1900°C in argon.

The piece that was produced (Figures 15, 16) had the following parameters:

Diameter 7.6 cm Length 6.4 cm A. D. 1.411 g/cc

This sample failed, apparently because the graphite-TiB₂ coating was not as strong as the pure TiB₂ coating on the piece made in Example 5.

EXAMPLE 7

A TiB₂ pipe was made by filling a cylindrical mold having a centered wooden dowel rod with the above powdered TiB₂ and sintering as in Example 1. The dowel rod burned out leaving a TiB₂ cylinder with uniform walls (Figures 23, 24, 25).

EXAMPLE 8

After cooling, a sintered 100% TiB₂ piece was impregnated by heat ing it to 240°C, placing it in an autoclaves, and drawing a vacuum. After 1 hour, the piece was impregnated with molten pitch at 240°C under 7 x 10³ Pa pressure (100 PSI).

The A. D. of the piece before impregnation was 2.9, giving a 36% pore volume. Pitch pickup on impregnation was 16%. by wt. After im- pre-gnaticn, the piece was baked for six days on a cycle rising to 720°. The article of sintered TiB₂ iaay be re-impregnated and rebaked as above, to fully impregnate the available porosity. It may further be heated to a graphitizing temperature of 1800 °C to 2400°C.

Pressures used for impregnation may vary widely from about 2 to 15 x 10⁵ Pa, with the fluidity and the temperature of the impregnant used. Temperatures used will usually be in the range of 175° to 250°C.

The impregnant preferred is a petroleum pitch having a softening point of 110°-120°C. The baking cycle is frcm 1 to 10 days, typically 6 days, with the temσerature in the range of 700°-1100°C. Example 9

A graphite mold was filled with TiB₂ powder having a maximum particle size of 44 μ, and a mean particle size of 7 μ. It was heated to 2615 + 15°Cin an argon atmosphere and held there for two hours. The TiB was partially melted, and had attacked the wall of the graphite mold.

Example 10

The same TiB₂ powder used in Example 1 was dispersed in molten coal tar pitch at about 175°C, using 85% TiB₂ - 15% pitch by wt. The plastic mixture was molded into a cylinder, baked on a six day cycle rising to

720°C, removed from the baking oven, and heat treated at 2100°C in nitrogen. The article thus produced was tested as a cathodic component in a Hall cell with excellent results, being wetted by the molten aluminum and showing little visible corrosion or other defect.

Example 11

The same materials and procedures were used as in Example 4 above, except that the atmosphere in the furnace was argon and the final temperature was 2400°C. The piece produced had good performance when tested in the Hall cell.

Example 12

A piece was produced from the following raw material composition: TiB₂ - 72.0 % by wt.

Sponge Coke Flour - 12.7 % by wt. 110 Coal Tar Pitch - 15.3 % by wt. 100.0 %

The mixture was heated to 175°C and the solids dispersed in the molten pitch. The mixture was cooled, then molded to a cylindrical shape. It was baked on a six day cycle rising to 720°C, then cooled, placed in a furnace with a nitrogen atmosphere, and heated tc 2100°C. The piece produced showed good durability with little corrosion and no cracking when tested as a cathodic element in a Hall aluminum cell. Example 13

A mixture of the following composition was used to produce a cathodic element for a Hall cell.

TiB Powder - 72.0 % by wt. Sponge Coke - 12.7 % by wt.

110° Coal Tar Pitch - 15.3 % by wt.

100.0 % The mixture was processed as in Example 4 except in argon. The cathodic element produced was tested in a Hall cell with satisfactory performance.

Example 14

The same raw materials and procedures used in Example 10 are used to make a cathodic element, with a nitrogen atmosphere used up to

2100°C. The article is then further heated to 2400°C in an argon atmo sphere by purging the nitrogen with argon and maintaining the argon atmosphere up to the final temperature and during coolinc of the element .

Example 15

The same materials and procedures used in Example 12 are used to make a cathodic element for a Hall cell. When the element reaches 2100°C, the atmosphere is purged with argon and further heated to 2400°C with argon. On reaching 2400°C, the power is cut off and the furnace allowed to cool. When the temperature has cooled to about 2000°C, the small argon flow required to maintain the atmosphere is replaced by nitrogen and the nitrogen atmosphere maintained to ambient or slightly above.

Ex-3mple 16

A cathodic element for a Hall cell was produced from a mixture of 50% TiB₂ (85% assay), 27% prilled pitch (coal tar pitch, 110° softening point), and 23% calcined sponge coke (particle size 3 mm mean diem.). The mixture was heated and the particulate matter dispersed in a sigma mixer at 170°C, then molded at 1.4 x 10⁷ Pa (2000 PSI). The element was baked to about 720° over a six day period to carbonize the pitch, impregnated with petroleum pitch, re-baxed, and beared in argcn to 2400°C to graphitize the carbon. The element formed had an A.D. (Apparent Density) of 2.26. After a test run in a Hall cell, the element was fully wetted by the aluminum and edges were sharp, indicating good resistance of the element to corrosion by the electrolyte.

Example 17

Small solidified puddles of a material presumed to be a TiB₂ alloy were noted on samples heat treated to 2300°C and higher in a nitrogen atmosphere.

Four samples of different compositions were processed at 2700 and 2400°C with results as follows:

Sample No. Temp.,° C Atm. Carbon-TiB₂ Results

1 2700° 70-30 N₂ Sample stuck to graphite container

2 2700°

N₂ 50-50 Metallic liquid drops on sample side

3 2400° 50-50 Large metallic puddle

N₂ beside sample

4 2400 0-100 Pressed powder preform-

N₂ melted

The puddles found in the above samples were analyzed by x-ray diffraction and found to contain TiB₂, TiO, BN , and C.

Based on the above, a series of tests was run in argon and nitrogen atmospheres. A sample containing 50% TiB₂-50% sponge coke flour was mixed with pitch, baked, impregnated with pitch and re-baked, both bakes rising to about 800° over a six day period. The sample above was then divided and the pieces processed in nitrogen and argon atmospheres: Sample No. Terno.,° C Atm. Results

5 2350° Metallic liquid solidified on surface

N₂

6 2200° Good-black powder on surface

N₂

7 2000° Good-black powder on surface

N₂

8 2400° Ar Good-light gray powder on surface

9 2200° Ar Good-light gray powder on surface

10 2000° Ar Good-light gray powder on surface

11 2340° Ar Good-light gray powder on surface

The results indicated that a nitrogen atmosphere was unsuitable for processing TiB₂ at temperatures of about 2300°C or higher, and that an argon atmosphere is preferable at this temperature range.

Claims

ClaimsWe claim:

1. A process for the production of a refractory article comprising filling a mold by gravity with a refractory hard metal powder, placing the mold in a chamber, controlling the atmosphere in the chamber, to prevent decomposition of the powder, and heating the chamber to a sintering temperature and maintaining the chamber at the sintering temperature until the article is formed.

2. The process of claims 1, 8, or 9 wherein the refractory hard metal powder has a mean particle diameter of 7 μ, with 90% of particles having a diameter from 5 to 15 μ, and 100% of said particles being less than 44 μ in diameter.

3. The process of Claims 1, 8, or 9 wherein the refractory hard metal powder is a mixture of not more than 30% carbon powder by wt. and not less than 70% TiB₂ powder by wt.

4. The process of Claim 3 wherein the carbon powder is selected from the group consisting of graphite, calcined petroleum coke, metallurgical coke, and charcoal.

5. The process of Claim 1 wherein the atmosphere is a noble gas.

6. The process of Claim 1 wherein the chamber is heated to a sintering temperature in the range of 1700° to 2400°C for up to 4 hours.

7. The process of Claims 1, 8, or 9 wherein the article after sintering is cooled, then impregnated under a pressure of 2 to

15 x 10 Pa at 175 to 250°C with a carbonizable impregnant, baked to carbonize the impregnant on a cycle rising to 700° to 1100° over a period of 1 to 10 days, then further heated to 1700° to 2400°C to form a TiB₂-graphite composite structure.

8. In a process for the production of an article comprising a refractory hard metal, the improvement comprising processing said article in an atmosphere of nitrogen at temperatures rising to 2100°C, and processing said article in an atmosphere of noble gas at temperatures rising to above 2100°C.

9. In a process for the production of an article comprising a refractory hard metal, the improvement comprising the use of a nitrogen atmosphere at temperatures up to about 2100°C, purging said atmosphere with a noble gas, and maintaining a noble gas atmosphere up to a final temperature above 2100°C.

10. The process of Claim 8 wherein the noble gas atmosphere is maintained while cooling the article to a temperature below 400°C.

11. The process of Claim 8 wherein the noble gas atmosphere is main tained while cooling the article to approximately 2100°C, and a nitrogen atmosphere maintained to a temperature below 400°C.

12. The process of Claims 8, 9, or 11 wherein the noble gas is argon.

13. The process of Claims 1, 8, or 9 wherein the refractory hard metal is TiB₂.

14. In a process for the production of a cathodic component for a Hall cell comprising TiB₂ wherein said component is heated to a temperature of 2000°C or higher, the improvement comprising the use of a nitrogen atmosphere when heating said component up to abput 2100°C, and the use of an argon atmosphere when heating said component at temperatures rising to above 2100°C.

15. Tne process of Claims 1, 8, or 9 wherein the refractory hard metal comprises from 50 to 100% TiB₂with from 0 to 50% carbon.

16. In a process for producing a TiB₂ article αf known density and porosity comprising filling a graphite mold with a TiB₂ powder having a mean particle size diameter of 7 μ, a maximum particle size diameter of 44 μ, with 90% of particles greater than 5 μ and less than 15 μ in diameter, the improvement comprising sintering said powder in a noble gas at a temperature of approximately 1700° to form an article with an apparent density of about 2.9 g/cc and a porosity of about 35%, or at a temperature of approximately 1900°C to form an article with an apparent density of about 3.0 g/cc and a porosity of about 33%, or at a temperature of approximately 2200°C to form an article with an apparent density of about 4.0 g/cc and a porosity of about 12%, or at a temperature of approximately 2300°C to form an article with an apparent density of about 4.0 g/cc and a porosity of about 12%, or at a temperature of approximately 2400°C to form an article with an apparent density of about 3.9 g/cc and a porosity of about 12%.

17. A process for the production of a hollow refractory hard metal article comprising filling a graphite mold around a wooden core with powdered refractory hard metal and heating said mold in a noble gas atmosphere to a sintering temperature from 1700° to 2400°C, and maintaining said mold in said atmosphere at said temperature for a period of up to four hours.

18. A cathodic component for a Hall-Heroult aluminum cell made by the process of Claims 1 or 19.

19. A process for the production of a cathodic element for a Hall-Heroult aluminum reduction cell comprising selecting a composition of not less than 70% by wt. TiB₂ powder and not more than 30% by wt. carbon powder selected from the group consisting of ground calcined petroleum coke and ground graphite, both of said powders having particle sizes of 44 μ maximum diameter, thoroughly mixing said powders, pouring said powder mixture thus formed into a graphite mold with linear dimensions from 1 to 30% larger than the desired size of said element effective to contain said powder such that the final size of said element produced is approximately of the desired dimensions for said element, placing said mold in a furnace with an argon atmosphere, heating said mold to a temperature in the range of 1700° to 2400°C, holding said mold at said temperature for a period of from 1 to 4 hours, cooling said mold to 150°C or less, removing said mold from said furnace, removing the piece thus produced from said mold, reheating said piece to 175° to 250°C, placing said piece in an autoclave, impregnating said piece at least once under alternate cycles of vacuum and pressure with a petroleum pitch having a softening point from 110° to 120°C, baking said piece at least once under a temperature cycle rising to 700° to 1100° over 1 to 6 days, and graphitizing said piece by heating it to 1800 C to 2400°C to form said cathodic element.