Jan. 9, 1968 B. MATCHEN 3,362,787 PREPARATION OF SILICEDES Filed Feb. 4, 1964

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AT7ORMAY 3,362,787 United States Patent Office Fatented Jan. 9, 968

Sid 3,362,787 tion resistance up to about 1300° C. and it has been re PREPARATION OF MOLY3DENUM, SCDES ported that addition of 10% molybdenum disilicide to Ben Matchen, Niagara Falls, Ontario, Canada, assignor the boride dramatically increases the temperature of effec to Norton Company, Worcester, Mass, a corporation tive oxidation resistance to 1950° C. This property of of Miassachusetts titanium diboride, having good resistance up to about Filed Feb. 4, 1964, Ser. No. 342,357 1400° C., may be similarly improved. 5 Claims. (C. 23-204) Cermets have been produced with molybdenum disili cide as the base material, together with various refractory oxides, to improve thermal shock and oxidation resistance ABSTRACT OF THE DISCLOSURE O and to impart more refractoriness to bodies produced MosSi and MoSi are prepared by heating molybdic therefrom. Suggested applications include kiln furniture, oxide with silicon in the presence of carbon and silicon saggers, sand-blast nozzles, exhaust tube linings, etc. Alu carbide; silica is preferably also present; MoSi may be mina is the most common oxide used because its thermal prepared from MosSi by heating with silicon in the pres expansion coefficient (8.8x10-6/ C.) closely matches ence of carbon; furnace temperatures of 1700 to 1950 that of the disilicide (8.4x10-6/ C.). Other oxides or C. are employed with times of 3 to 8 hours, preferably in mixtures of oxides may be used. The silicide may also be a continuous kiln. Hot pressing of product yields pieces used as a binder for carbide and, as mentioned above, having 94.4% theoretical density, good oxidation resist boride bodies, and has been found to bind with metallic ance. Mixtures of molybdenum silicide with alumina or materials such as nickel, cobalt, iron and stainless . 20 Addition of these metals increases thermal shock resist magnesia were hot pressed to give strong products. alCg. Molybdenum disilicide is ideal for use in chemical proc This invention relates to the preparation of molyb essing equipment because of its resistance to most inor denum silicides from molybdenum oxide employing sili ganic acids including aqua regia, aqueous alkali solutions, con carbide as a source for at least some of the silica. 25 and liquid metals such as sodium, zinc, bismuth, and In particular, the objects of this invention are econom gallium. ically to produce compounds of molybdenum and silicon It has recently been suggested to use the disilicide as having Si to Mo ratios of from 1.6 to 2 or higher and one portion of a thermocouple with boron carbide or from 0.55 to 5.65, corresponding to the theoretical com chromium silicide as the other portion. The potential pounds MoSia and MoSi, respectively. 30 application of the thermocouples is associated with pro The drawing shows a flow sheet illustrating the proc duction of molten ferrous and non-ferrous metals. esses described and claimed for producing molybdenum Metallic molybdenum has relatively poor oxidation re silicides. sistance and the disilicide can be used to form a protec Very few of the less common silicides have reached tive covering thereon. Such coatings having a thickness of commercial prominence. However, one of these which 35 less than 0.125 mm. and provide protection for over 1000 appears headed in this direction is molybdenum disilicide hours in air at 1700° C. Thermal shock resistance of the which has been found to be very stable in the ordinary coating is excellent and a film of on the atmosphere at high temperatures in the range of 2000 silicide heals the cracks which may appear. It has been C. and to exhibit high heat conductivity, resistance to proposed that the molybdenum disilicide coating on me 40 tallic molybdenum consists of a thin intermediate molyb oxidation, and heat shock resistance. For example, the denum-rich layer and a dense silicide layer. Molybdenum heat conductivity of molybdenum disilicide is about 0.075 disilicide, flame-sprayed on molybdenum provides oxida cal./sec./cm.2/ C. (20° to 200° C.), and it has a re tion protection for 60 seconds in a Mach 2 airstream at sistivity of approximately 20 microhms-cms. It has been 3800° F. The disilicide may also be used as a protective found that molybdenum disilicide is useful in electrical coating for graphite linings in uncooled rocket motors. resistance elements and has been used as an impregnant The of molybdenum disilicide has vari in a element to render such resistance bars ously been reported as 1850° C. and as 2030-50° C. serviceable at much higher temperatures than has been It is crystallized in a tetragonal system having lattice possible heretofore. Such a bar is disclosed in the co constants of a=3,200 A., c=7.86 A., c/a=2.457. The pending application of John Fredriksson, Ser. No. 43,877 50 density from X-ray data is 6.24 grams per cubic centi for Electrical Bars, filed July 19, 1960, now Patent No. meter and its tensile strength, like that of graphite, in 3,171,871. creases with increasing temperature. The resistance to oxidation at temperatures up to The silicide of molybdenum having the formula MosSi3 1650-1750° C. makes molybdenum disilicide promising has been considered one of the lesser important of the for electrical heating elements as noted above and as silicides. However, it has been found that this silicide of structural shapes in specific locations of combustion molybdenum may be combined as a major constituent chambers, gas turbines, kilns, high temperature dies, in with various oxides or mixtures of oxides from the sec duction-brazing fixtures, etc. The mechanism of the oxi ond and third groups of the periodic table including alu dation resistance is not precisely known, except that it is mina, calcium aluminate, magnesium aluminate and relatively poor below 1000-1200° C., then becomes 60 magnesia. These compositions exhibit desirable electrical speculated as due to formation of a silica skin stable up resistivity and strength properties for use in high tempera to its melting point of about 1700 C. Another theory ture (1500 to 1700° C.) heating elements. MoSi is also claims that protection is provided by a very adherent useful as a major constituent of elements in electrical glassy layer 0.003 to 0.1 mm. thick consisting of a com appliances. Because of the high quantity of contained plex molybdenum-silicon oxide. It is recommended (with 65 molybdenum (85.1% compared with 63.1% for molyb in this theory) that molybdenum disilicide speciments be denum disilicide) this silicide finds application as an preheated above 1400° C. to insure formation of the additive in metallurgical processes. It is also used as an protective layer and thus protect the material against oxidation-resistant protective coating for molybdenum oxidation at lower temperatures. and may be foamed to provide lightweight, easily-shaped, Molybdenum disilicide greatly improves the oxidation 70 oxidation resistant refractory linings for certain types of resistance of borides. Zirconium diboride has good oxida furnaces and kilns. Additionally, it has been found that this silicide of molybdenum is suitable and may be effec 3,362,787 3. 4. tively utilized as a raw material in one process for the pass through (T) a certain screen size and be retained production of molybdenum disilicide. (on) on a screen of finer mesh (U.S. Sieve Series). MosSi is crystallized in a hexagonal system having TABLE 1. lattice constants of a=7.271 A., c=4.992 A. and c/a=0.687. It is not a true binary compound, but a ter Mo03 Mix SiC Coke nary compound which is stabilized by around 1% by (MoO3-SiO2-C) T54 T24 weight of or by carbon, nitrogen, oxygen or boron. The Percent Percent Percent amounts of these materials can vary considerably without T24 on 44------i6.5 ------48,3 T44 on 72------2.0 37. 26.2 affecting the basic structure of the silicide, although such T72 on iOX--- 9.4 23.5 7.6 variations will cause corresponding variations in the size O TOX on 18X- 7. 2.8 15.9 of the lattice parameters. T18X on 25S 9.3 5.2 0.3 The silicides of molybdenum, and particularly the di T25S------35.7 13.4 1.7 silicide, have heretofore been prepared by melting ele 00, () 100.0 00, 0 mental molybdenum with elemental silica. However, at the temperature required for the fusion of these elements, 5 As noted above, molybdenum oxide mix contains small a reaction may occur between the material and the con amounts of silicon oxide and carbon. It is commercially tainer such that the products are contaminated and the available and has an average product analysis of approxi desirable properties thereof are impaired. Moreover, the mately 77.4% molybdenum oxide (MoC), 10% silicon elemental products of substantial purity which are re oxide (SiO2) and 12% coke. In the process of this in quired, are unduly high in price. 20 vention the reaction mixture contains from about 65 to 75 In attempting to overcome these difficulties, the prior parts of MoC), from about 15 to 25 parts silicon carbide art has suggested reacting molybdenum oxide with silicon and from about 7 to 20 parts carbon (in the form of in a furnace heated by an oxy-hydrogen blow pipe. It coke), each by weight of the total reaction mixture. When has also been proposed to use fluxes such as lime and employing the above specified MoC)3-SiO2-coke mix magnesia or cryolite, the resulting product being leached (available in briquetted form), I have found that the foll with and dilute to obtain lowing proportions within the above range are preferred: the crystals of molybdenum disiiicide. However, none of these attempts has resulted in an economical process. Parts A still more recent proposal has been to heat an inti MoO-SiO-C mix ------86 to 92 mate mixture of comminuted elemental molybdenum and 30 SiC ------5 to 10 comminuted elemental silicon in a vacuum or in an inert Coke ------1 to 5 atmosphere at a temperature below fusion to form the Relatively less amounts of silicon carbide and carbon disilicide. However, the expense of the elemental reactants are necessary since the silicon oxide and carbon con is still present and the ultimate expense of production is tained in the MoC)-SiC-C provide some of the reactants increased even more by the necessity of providing a vac necessary for combination with molybdenum to form the uum or inert atmosphere. silicide with some carbon remaining available for addi It has been found that molybdenum silicide of good tional reaction. It is not certain whether the silicon oxide yield and high purity may be produced by heating a mix and carbon go through a silicon carbide stage before ture of molybdenum oxide, silicon carbide and carbon being converted to the silicide; that is, the silicon oxide at elevated temperatures and for periods of time such 40 (SiO2) may form the silicide with the molybdenum be that the constituents will combine to form the silicide. fore going through a silicon carbide or silicon oxide It has been found that molybdenum oxide may be used (SiO) stage. in the form of a commercially available mix which con In the following example and throughout the specifica tains also silicon oxide and carbon. The molybdenum tion and claims, all parts are parts by weight unless oxide is intimately mixed with silicon carbide and addi otherwise specified. tional carbon and heated at elevated temperatures. The EXAMPLE 1. constituents combine in the solid phase to produce Mossia. Molybdenum oxide ore, silicon carbide and coke having Suitable conditions for the heating portion of the treat a particle size in the range of that shown in Table 1 were ment include temperatures between about 1700 and thoroughly mixed together in the following proportions: 1950° C. and the heating step is continued for a length 50 of time such that there is complete reaction between the Parts particles in the mixture, generally from about three to Molybdenum oxide ore ------267 about eight hours. It has been noted that a second firing Silicon carbide ------22 of the reaction product at a somewhat higher temperature Petroleum coke ------8 for from one to several minutes may increase the purity 55 The mixture was placed in a graphite mold, covered of the yield. with a graphite slab and heated in an induction furnace The reaction times and temperatures within those at 1800 C. for four hours and then heated to a tem given above are regulated according to the "texture" of perature of 1925 C. and held for about one minute. The product desired. For example, a dense product requires resulting product was largely MoSi3 and the chemical a higher temperature and/or longer reaction time, 60 analysis was as follows: whereas a sintered product requires a lower temperature Percent and/or shorter reaction time. If the silicide is to be used Mo ------81.72 as a raw material for the production of molybdenum Si ------13.34 disilicide, a sintered product is desired, as opposed to the 65 C------2.83 fused, metallic appearing dense material, since it is easier to break down for subsequent reaction. 97.89 The particles of molybdenum oxide mix and the coke X-ray analysis showed that substantially all of the ma should generally be not substantially larger than will pass terial was MosSi, the identifying lines being 2.145 A., a 24-mesh screen; the silicon carbide should be of a size 70 2.064 A. and 1.373 A. There was a trace of either MoSi to pass a 54-mesh screen. Smaller particles may be used or MoC. Two lines of spacing, d=1496 A. and d=1.115 to allow even closer contact between the reactants. A., both weak, could not be correlated with any crystal The average particle size of the raw materials used in line compound. this process is set forth in the following table, the nota While the above example illustrates the use of molyb tions regarding the size being read as particles which will 75 denum oxide ore in the process of this invention, molyb

3,362,787 7 The product was somewhat deficient in silicon so the MIXTURE (PERCENT BY WEIGFIT) ingredients in the following examples were proportioned to improve the molybdenum to silicon ratio. Ex. MoC) SiC Si Coke Silica Silica Temp. EXAMPLE 3 Mix Sand (° C.) (Hrs.) 5 m Another portion of molybdenum silicide having the 4------76.25 3.75 2.5 1,800 4 same formula and chemical analysis as that of Example 2 5------75 7.5 1.25 1,750 5-3 6------72.5 17.5 3.75 1,750 53% was intimately mixed with other raw materials having the 7------72.5 7.5 3.75 1,700 4 Re?ired 1,725 proportions listed below: Proportion by Material: weight, percent O Mossia ------70 Silicon ------20 The yield, based on the weight of the respective starting Silica sand ------10 mix, and chemical analysis of the products obtained in 100 Examples 4-7 was as follows: CHEMICAL ANALYSIS (PERCENT) Yield Mo Si C Fe Total Mo-Si Atonic Example Ratio

42.8 67.16 30.26 0,58 0.81 - 98.8 97.42 MoSiss - 46.3 64.9 31.00 2.05 0.74 98.0 95.91 MoSi. 30.9 64,91 31.83 .96 0,7 99.47 96.74 MoSi68 - 58.1 64.83 29.91 0.50 0.69 95.93 94,74 MoSiss While no examination was conducted on the product The mixture was placed in a graphite boat, covered of Example 4, the results of an X-ray examination of the with a graphite slab, and passed through a continuous kiln remaining products is as follows: having the following reaction conditions: Preheat zone power input-15 kW., 74' long 30 Hot zone power input-105 kW., 48' long Temperature reading, hot zone-1850 C. Rate of travel-7'/hr. Example Strongest X-ray Lines (A.) The product (89.5% of theoretical yield based on Mo) 3.9 2.95 2.259 2.022 was dense, semi-fused, silver-gray and quite metallic in 3.91 2.95 2.259 2.020 appearance. Its chemical analysis was as follows: 3.90 2.96 2.264 2. (20 Percent 40 Mo ------62.79 Si ------33.46 While Examples 4-7 illustrate the production of molyb C ------0.41 denum disilicide from the molybdic ore in a batch fur Fe ------0.76 45 nace, a continuous kiln may also be used. A recommended N ------0.04 mixture for production in the kiln is as follows: Material: Proportion by weight, percent Total analyzed ------97.46 Molybdenum oxide mix (Mo.O-SiC-C) ---- 70 Mo-Si ------percent-- 96.25 50 Silicon carbide ------6 Atomic ratio, MoSii.93. Silicon ------20 An X-ray analysis of the above product failed to indi Silica sand ------4 cate the presence of any crystalline material other than Recommended reaction conditions include a hot zone MoSia. The X-ray pattern showed the three strongest 55 temperature of about 1750 C. and a rate of travel of characteristic lines of pure MoSia at 3.96, 2.99 and 2.034 about 6% inches per hour in the furnace mentioned above, A., and was in every way identical to the pattern of Portions of the molybdenum disilicide produced ac MoSi2. No lines characteristic of any other crystalline cording to this invention were tested for hot pressing material could be observed indicating the total absence 60 characteristics, modulus of rupture at room and elevated of any other crystalline material. temperatures, and oxidation resistance at temperatures In Example 3 above, the preheat zone is indicated as from 1000 to 1450° C. The product of Example 3 was having a length of 74 inches. This includes the entire formed into pieces approximately /2-inch in diameter and length before the product reaches the hot zone tempera 2/2 inches in length at a temperature of 1675 C. and 65 a pressure of 3000 p.s. i. The average weight loss of three ture of between 1700° C. and 1850° C. This preheat test pieces was 1.1%, while the average density was Zone may also be considered as including both a true pre 94.4% of theoretical and the average resistivity 32.9 heat Zone of 54 inches and an intermediate zone of 20 microhm centimeters. The modulus of rupture of two of inches. these specimens was 18,100 p.s. i. and 17,390 p.s.l. respec In the following examples the disilicide was prepared 70 tively at room temperature, and both had a value of (directly from molybdic oxide. In each example the reac 14,350 p.s.i.at 1200° C. tion of the mixture indicated was carried out in a mold Additional specimens were placed in a moving-air fur ing furnace. The Moos mix consists of 77.4% Moo, nace for eight hours at 1000, 1200° and 1450° C. with 10%. SiO2 and 12% carbon in the form of coke. 75 the following results: 3,362,787 10 TABLE 3 3. A process according to claim 2 wherein the reac tion mix includes SiO2. Temperature Weight Loss, Remarks 4. A process for producing MoSi2 comprising heating Percent a source of MoSis in the presence of elemental silicon 1,000 C------0.83 No change; surface slightly in the amount of at least 7 moles of silicon for each 5 duller. moles of MoSi produced, said heating taking place under 1,200 C------0.70 Glaze starting to form. 1,200 C.------0.73 Do. non-oxidizing conditions at a temperature of at least i,450 C------0.65 Brown glaze over whole piece. 1700° C. 1,450 C.------0.57 Do. 5. A process according to claim 4 in which the source 10 of MoSis is a mixture of molybdenum trioxide, silicon 1. Sige specimens had previously undergone cross-bending tests at carbide, and carbon. What is claimed is: References (Cited 1. A method of making MosSi3 comprising heating a UNITED STATES PATENTS mixture consisting of molybdenum oxide, 65 to 75 parts, 5 silicon carbide 15 to 25 parts, and carbon 7 to 20 parts, 2,619,406 11/1952 Briney ------23-204 at a temperature of from 1700° C. to 1950° C. for at least three hours. FOREIGN PATENTS 2. A method of making MoSi2 comprising heating a 837,979 6/1960 Great Britain. mixture consisting of molybdenum oxide, 65 to 75 parts, 20 silicon carbide 15 to 25 parts, carbon 7 to 20 parts, and MILTON WEISSMAN, Primary Examiner. silicon 15 to 20 parts at a temperature of from 1700° C. OSCAR R. VERTIZ, Examiner. to 1950° C. for at least three hours. H. S. MILLER, Assistant Examiner.