US3009780A - Process for the production of large single crystals of boron phosphide - Google Patents

Description

United States Patent Oflice 3,009,780 Patented Nov. 21, 1961 3 009,780 PROCEQS FOR THE PRODUC1ION F LARGE SINGLE CRYSTALS 0F BORON PHOSPHIDE Bobbie D. Stone, Miamisburg, Ohio, or to Monsanto Chemical Company, St. Louis, Mo., a eorpora- (ion of Delaware No Drawing. Filed June 22, 1959, Ser. No. 821,632

9 Claims. (Cl. 23-204) The present invention relates to a new method for the production of a large single crystal form of boron phosphide. It is an object of this invention to provide a new and economical method for the production of boron phosphide characterized as having a cubic crystalline structure and existing as well defined elongated single crystals which are larger than those obtained in conventional methods. It is a further object to provide a method for the production of single crystals of boron phosphide from elemental phosphorus and phosphorus compounds including phosphides which are reacted with metal-boron compounds or elemental boron in solution in a liquid metal. Further objects and advantages of my invention will be apparent from the following description.

The present process for the production of crystalline boron phosphide in a single crystal form is based upon a chemical reaction which occurs under specified conditions within the molten metal matrix composed of a single metal or an alloy. A phosphorus source such as elemental phosphorus, a phosphorus alloy or a metal phosphide reacts with the boron source such as elemental boron, a boron alloy or a metal boride which is dissolved in this medium or matrix. In the present description, the term "alloy" refers to all combinations of metals and also metalloids including those with boron or phosphorus. According to this definition a chemical compound, and also compositions of non-stoichiometric proportions are included in the term alloy. The metal-phosphorus alloys which are contemplated in the present invention are those of copper, aluminum, gallium, indium, silicon, titanium, zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. The boron source for the present reaction is a boron alloy of copper, aluminum, gallium, indium, silicon, titanium, zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium, platinum and carbon used singly or in combination, for example as an iron-nickel-boron alloy.

The matrix in which the boron source and the phosphorus source react is a single metal or mul-ti-component metal alloy selected from the group consisting of copper, aluminum, gallium, indium, silicon, titanium, zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. In carrying out the growth of single crystals, the components are heated to a temperature above the freezing point and below 1800 C., so that the metal or alloy selected must melt below '1 800 C.

The term single crystal as employed in the present invention refers to crystalline material in which the said single crystals have gross physical dimensions such that at least one dimension of the crystalline product is at least 0.1 millimeter.

It has been found that a critical limitation which must be observed if single crystalforms of boron phosphide are to be obtained, is that the molten metal matrix containing the dissolved boron source and the dissolved phosphorussourcemustbecooledataratevaryingwithin the range of from 1' C. to 300 C. per hour from the molten condition to the freezing point, a generally preferred range being 3 C. to 60 C. per hour. A specific preferred cooling rate with regard to iron as the matrix is from 5 C. to 60 C. per hour, a preferred cooling rate in the use of metallic copper as the molten matrix is from 3 C. to 60 C. per hour and a prefered rate of cooling with nickel as the metallic matrix is from 3 C. to 60' C. per hour.

It has also been found that a critical limitation in the production of a single crystal form of boron phosphide is the control of the boron content dissolved in the molten metal matrix. It has been found that a concentration range broadly from 0.1% to 50% by weight of boron, is essential in order successfully to produce the single crystal type. Preferred ranges of boron concentration with respect to the use of different metals as the matrix are set forth in the table below. In this table the percent of boron by weight in the matrix is shown both with respect to a broad range and also a narrow or preferred range for the respective metal matrices.

Whenacombinationofmetalsisusedasthemauix, the weight average boron ranges in accordance with relative proportions are applicable.

It has been found that the use of concentrations of boron other than in the critical range set forth above either gives no boron phosphide or yields products which are not single crystals. It is also preferred to dissolve the boron component completely in the molten matrix before adding the phosphorus component. However the boron or the phosphorus sources may be added simultaneously or in this sequence in the instant process.

In conducting the present process, the molten matrix, containing boron and phosphorus is subjected to cooling. A study of the cooling curve shows that a temperature is reached at which the amount of boron and phosphorus in solution are equal to the saturation value at such temperature. As the solution is cooled further, boron phos phide precipitates. However, for the production of large single crystals, the present controlled rate of cooling is necessary.

In a preferred embodiment of the invention, the process begins with a metal boride, which is dissolved in the desired metal matrix, such metal matrix being either the same as that of the starting boride or a difierent metal of the group defined above. For example, in employing nickel boride as the source of the boron, the metal matrix may be any of the preferred group consisting of copper, aluminum, gallium, indium, silicon, titanium zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum. When the working matrix has thus been obtained with the boron present in the desired proportions as set forth herein, the phosphorus may be introduced directly as phosphorus vapor which is passed into contact with the molten metal solution. However, it is also contemplated that a phosphorus source such as a metal phosphide or alloy may be added directly. It is found that when the phosphorus source is thus dissolved in the matrix, containing the boron, and the entire system cooled to the freezing point at the specified rate, the desired single crystal form of boron phosphide is precipitated. To prevent excessive thermal shock to the boron phosphide crystals in the matrix it is advantageous to cool the solidified ingot slowly to room temperature. However, this cooling rate from the freezing point to room temperature need not nwessarily be within that specified for cooling the melt to the freezing point.

In carrying out this process, certain modifications corning within the broad scope of the invention may be used. In one embodiment the entire charge is heated to the high temperature desired, e.g., above the melting point, but below 1800 C. and then cooled at the rates described above. This procedure may be modified further by placing the charge in an area with a temperature gradient, i.e., having one end hotter than the other, and cooling slowly.

Another embodiment is the application of the present method to zone melting in which the body of the metalboron alloy is reacted in an atmosphere of phosphorus vapor. This is conducted by slowly pulling the charge through a very narrow induction-heating coil so that only a small cross section is molten at one time. Boron and phosphorus then react in the molten none, and as the reaction mixture leaves the hot zone, it cools at the rate described above. This gives rise to large crystals. Since fewer crystal nuclei are formed when only a small zone is molten at one time, this process gives even larger crys tals than in the case where the entire charge is molten at once.

In all of the above methods, after the molten matrix containing the single crystals of boron phosphide has completely solidified, the boron phosphide is obtained by dissolving away the metal with a reagent which does not dissolve crystalline boron phosphide. Examples of such reagents are hydrochloric acid, nitric acid, sulfuric acid and aqua regia.

The following examples illustrate specific embodiments of this invention:

Example 1 Nickel powder 100 mesh) and boron powder (-325 mesh) were thoroughly mixed in proportions to give a mixture containing 93% Ni and 7% B by weight. A charge of 20.80 g. of this mixture was loaded in a graphite boat and the boat placed in a mullite tube closed on one end and sealed to Pyrex glass on the other. The tube was evacuated to a pressure of 10- mm. Hg and then sealed from the vacuum system. After breaking a Pyrex breakseal leading to a phosphorus reservoir, about g. white phosphorus was distilled into the ceramic tube. The Pyrex end of the tube was sealed and the tube placed in two adjoining furnaces in such a manner that the boat containing the nickel-boron mixture was in the center of a large Globar tube furnace, while the Pyrex end of the tube was enclosed in a small Nichrome resistance-wound furnace. The Globar furnace was heated to 1400" C. and then the small furnace was heated rapidly to 280 C. and gradually raised to 410 C. At this temperature, the vapor pressure of phosphorus inside the tube was about one atmosphere. The temperature of the Globar furnace was then lowered at the uniform rate of 5/hour until it reached the solidification point, 1170 C. Both furnaces were turned off and allowed to cool to room temperature. After cooling, the tube was opened by breaking the Pyrex section and the ingot containing crystalline boron phosphide in the nickel phosphide matrix Example 2 The experiment was carried out in the same manner as Example 1 except that a mixture containing 96% Ni and 4% B was used. A charge weighing 16.00 g. was

placed in the ceramic tube as described above and about 15 g. white phosphorous distilled in. After heating to 1400 C. and heating the phosphorus reservoir to 410 C., the charge was cooled at a uniform rate of 5 C./hr. Upon leaching away nickel phosphide with hot dilute nitric acid, the product was obtained as transparent crystals.

Example 3 A charge of 96% Ni4% B mixture was placed in a graphite boat in a mullite tube. The tube was evacuated and phosphorus distilled in as in Example 1. After heating the Globar furnace to 1400" C. and raising the phosphorus reservoir temperature to 410 C., the charge was cooled at a uniform rate of 37 /hour until the freezing point was reached. Upon cooling and leaching the ingot, the product was obtained as transparent red crystals, some of which were 5 mm. in one dimension.

Example 4 A 10.97 g. charge of 92.5% Fe-7.5% B mixture was placed in a graphite boat in a mullite tube. After evacuating, white phosphorus was distilled into the tube which was then sealed. The tube was placed in a Globar furnace with an auxiliary furnace around the phosphorus reservoir. The charge was heated to 1400 C. and the phosphorus reservoir heated to 410 C. to drive the phosphorus into contact with the boron-containing melt. The charge was cooled at a uniform rate of 6 per hour to the freezing point at 1260 C. The furnaces were turned off and allowed to cool to room temperature. The ingot was removed from the tube and iron phosphide was dissolved away with hot concentrated aqua regia. Crystalline boron phosphide was left behind as transparent crystals.

Example 5 This experiment was carried out at atmospheric pressure using a continuous flow gas system. A 32.44 g. charge of a 92.5% Fe-7.5% B mixture was placed in a graphite boat in an open-ended mullite tube connected in such a manner that gas could be passed through the tube. The tube was placed in a Globar furnace with the charge located in the center of the furnace and with a thermocouple reading the temperature at the location of the charge. A graphite boat containing red phosphorus was placed in the tube at one end protruding from the Globar furnace and the tube was surrounded with an auxiliary Nichrome furnace at the point occupied by the phosphorus boat. The Globar furnace was heated to 1400' C. and the phosphorus boat heated to 450 C. and angon was passed through the tube at 50 mL/min. so that the gas stream first passed over the phosphorus boat and then over the charge. The charge was then cooled at a uniform rate of about 5 lhour until a temperature of 1300 C. was reached at which point the phosphorus furnace was turned oif and the charge cooled to room temperature under a stream of argon. After leaching away the iron phosphide matrix with hot concentrated aqua regia, crystalline boron phosphide was left behind in the form of transparent red crystals, some of which were 2-3 mm. in one dimension.

Example 6 The method of Example 5 was repeated with the modification that a 16.88 gm. sample of a 93% Ni-7% B alloy was placed in a refractory boat, and placed in a mullite reaction tube. The Globar furnace surrounding the charge was heated to 1400 C. and the phosphorus furnace heated to 425 C. A stream of argon was passed through the tubular reactor at the rate of 20 ml./min., so that the partial pressure of phosphorus vapor was only about one-tenth as much as in the preceding example.

While the argon-phosphorus vapor stream was passed over the melt, the temperature was lowered at a uniform rate of about 6 C. per hour until a temperature of 1230 C. was reached, slightly above the solidification point. After cooling the reaction mass to room temperature, the nickel phosphide matrix containing some unreacted nickel was dissolved away with hot diluted nitric acid. The product left after the acid treatment consisted predominantly of black crystals of about 2 to 3 mm. length. This product has less phosphorus than cubic crystalline 'BP, and can be controlled to have a formula ranging from B P to 3 F. It was found to crystallize in the monoclinic system.

I claim:

1. Process for the production of single crystals of boron phosphide which comprises dissolving a boron source selected from the group consisting of elemental boron and boron alloys in a molten metal matrix selected from the group consisting of at least one of the metals of the group consisting of copper, aluminum, gallium, indium, silicon, titanium, zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, the proportion of boron calculated as elemental boron in the matrix being in the range of from 0.1% to 50% by weight, and after the said boron component is completely dissolved, introducing into the said mixture a phosphorus source selected from the group consisting of elemental phosphorus, and metal phosphides in suflicient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from the molten condition to the freezing point.

2. Process for the production of single crystals of boron phosphide which comprises dissolving a boron source selected from the group consisting of elemental boron and metal borides in a molten metal matrix selected from the group consisting of at least one of the metals of the group consisting of copper, aluminum, gallium, indium, silicon, titanium, zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, the proportion of boron calculated as elemental boron in the matrix being in the range of from 0.1% to 50% by weight, and after the said boron component is completely dissolved, introducing into the said mixture a phosphorus source selected from the group consisting of elemental phosphorus, and metal phosphides in sufiicient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 3 C. to 60 C. per hour from the molten condition to the freezing point, and thereafter isolating the said crystal form of boron phosphide from the metallic matrix.

3. Process for the production of single crystals of boron phosphide which comprises dissolving a boron source selected from the group consisting of elemental boron and metal borides in a molten metal matrix selected from the group consisting of at least one of the metals of the group consisting of copper, aluminum, gallium, indium, silicon, titanium, zirconium, germanium, chromium, manganese, iron, cobalt, nickel, ruthenium, rhodium, palladium, osmium, iridium and platinum, the proportion of boron in the matrix being in the range of from 0.1% to 50% by weight, after the said boron component is completely dissolved, introducing into the said mixture a phosphorus source selected from the group consisting of elemental phosphorus, and metal phosphide in suflicient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 3 C. to 60 C. per hour from the molten condition to the freezing point, and thereafter isolating the said crystal form of boron phosphide from the metallic matrix, by dissolving the said metal in a mineral acid to leave the said crystalline boron phosphide undissolved.

4. Process for the production of single crystals of boron phosphide which comprises dissolving elemental boron in a molten matrix of nickel metal, the proportion of boron being in the range of from 0.1% to 50% by weight, and after the boron is completely dissolved, introducing into the said mixture elemental phosphorus in sufiicient proportion to combine with the boron, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from the molten condition to the freezing point.

5. Process for the production of single crystals of boron phosphide which comprises dissolving elemental boron in a molten matrix of nickel metal, the proportion of boron being in the range of from 0.1% to 50% by weight, and after the boron is completely dissolved, introducing into the said mixture elemental phosphorus in sufiicient proportion to combine with the boron, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from the molten condition to the freezing point, and thereafter isolating the said crystal form of boron phosphide from the metallic matrix, by dissolving the said metal in a mineral acid to leave the said crystalline boron phosphide undissolved.

6. Process for the production of single crystals of boron phosphide which comprises dissolving elemental boron with metallic iron, the proportion of boron in the matrix being in the range of from 0.1% to 50% by weight, after the said boron is completely dissolved, introducing into the said mixture elemental phosphorus in sufficient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from the molten condition to the freezing point.

7. Process for the production of single crystals of boron phosphide which comprises dissolving elemental boron with metallic iron, the proportion of boron in the matrix being in the range of from 0.1% to 50% by weight, after the said boron is completely dissolved, introducing into the said mixture elemental phosphorus in sufficient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from the molten condition to the freezing point, and thereafter isolating the said crystal form of boron phosphide from the metallic matrix by dissolving the said metal in a mineral acid to leave the said crystalline boron phosphide undissolved.

8. Process for the production of single crystals of boron phosphide which comprises dissolving elemental boron with metallic copper, the proportion of boron in the matrix being in the range of from 0.1% to 50% by weight, after the said boron is completely dissolved, introducing into the said mixture elemental phosphorus in sufiicient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from the molten condition to the freezing point.

9. Process for the production of single crystals of boron phosphide which comprises dissolving elemental boron with metallic copper, the proportion of boron in the matrix being in the range of from 0.1 to 50% by weight, after the said boron is completely dissolved, introducing into the said mixture elemental phosphorus in sufficient proportion to combine with the boron which is present, and thereafter cooling the said molten mixture at a rate within the range of from 1 C. to 300 C. per hour from 7 1 s I the molten cowition to the freezing point, and thereafter OTHER REFERENCES isolating the said crystal form of boron phosphide from the metallic matrix by dissolving the said metal in a min- Popper ct Nature 1075 7 eral acid to leave the said crystalline boron phosphide van Wale Phosphorus and Its C0mPounds, 1958' undissolved. 5 vol. 1, pp. 131, 146.

' Stroughton et al.: Engineering Metallurgy, 2d ed.,

References Cited in the file of this patent 1930, PP-

Hansen: Constitution of Binary Alloys, 2d ed.-, 1958,

UNITED STATES PATENTS pp. 248, 249-252, 256, 257. 2,124,509 McKenna July 19, 1938 m

Claims (1)

1. PROCESS FOR THE PRODUCTION OF SINGLE CRYSTALS OF BORON PHOSPHIDE WHICH COMPRISES DISSOLVING A BORON SOURCE SELECTED FROM THE GROUP CONSISTING OF ELEMENTAL BORON AND BORON ALLOYS IN A MOLTEN METAL MATRIX SELECTED FROM THE GROUP CONSISTING OF AT LEAST ONE OF THE METALS OF THE GROUP CONSISTING OF COPPER, ALUMINUM, GALLIUM, INDIUM, SILICON, TITANIUM, ZIRCONIUM, GERMANIUM, CHROMIUM, MANGANESE, IRON, COBALT, NICKEL, RUTHENIUM, RHODIUM, PALLADIUM, OSMIUM, IRIDIUM AND PLATINUM, THE PROPORTION OF BORON CALCULATED AS ELEMENTAL BORON IN THE MATRIX BEING IN THE RANGE OF FROM 0.1% TO 50% BY WEIGHT, AND AFTER THE SAID BORON COMPONENT IS COMPLETELY DISSOLVED, INTRODUCING INTO THE SAID MIXTURE A PHOSPHORUS SOURCE SELECTED FROM THE GROUP CONSISTING OF ELEMENTAL PHOSPHORUS, AND METAL PHOSPHIDES IN SUFFICIENT PROPORTION TO COMBINE WITH THE BORON WHICH IS PRESENT, AND THEREAFTER COOLING THE SAID MOLTEN MIXTURE AT A RATE WITHIN THE RANGE OF FROM 1*C. TO 300*C. PER HOUR FROM THE MOLTEN CONDITION TO THE FREEZING POINT.