Jan. 9, 1968 B. MATCHEN 3,362,787 PREPARATION OF MOLYBDENUM SILICEDES Filed Feb. 4, 1964 - Bem MarcHENINVENTOR. 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 steel. 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 silicon dioxide 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 melting point of molybdenum disilicide has vari in a silicon carbide 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 glass 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 nitric acid and dilute hydrofluoric acid to obtain lowing proportions within the above range are preferred: the crystals of molybdenum disiiicide.