Disilicides

The transition metal MSij phases with M = Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W crystallize with the tetragonal Cl lb structure, the hexagonal C40 structure or the orthorhombic C49 and C54 structures and show extended mutual solid-solubilities (Kudielka and Nowotny, 1956). These crystal structures are closely related since they only differ in the different stacking of the same structural elements (Nowotny, 1963). 

MoSi2 with a Cl lb structure has found widespread use as heating elements in high-temperature furnaces for temperatures up to 1700°C because of its advanta- 

Presently MoSi2 is being considered for structural, high-temperature applications and various materials developments are in progress (Lugscheider et al., 1991 Kumar and Liu, 1993 Petrovic, 1993 Hardwick et al., 1993). The elastic constants are known (Nakamura et al., 1990 Nakamura, 1991 Srinivasan and Schwarz, 1992), and the plastic deformation behavior 

the application of soft, dislocation emitting surface films (Czarnik et al., 

or the insertion of a ductile second phase which is usually a Nb-rich phase (Lu etal., 1991 Xiao and Abbaschian, 1992 Venkkateswara Rao et al., 1992 Alman and Stoloff, 1992). Toughness can also be increased by embedding hard second phases, in particular ceramics, in an MoSi2 matrix in order to hinder crack growth (see e.g. Bhattacharya and Petrovic, 1991 H. Chang et al., 1992). 

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Tungsten disilicide [12039-88-2] WSi2, forms bluish-gray tetragonal crystals (a = 0.3212 nm, c = 0.7880 nm). It is insoluble in water and melts at 2160°C. The compound is attacked by fluorine, chlorine, fused alkaUes, and HNO —HF. It may be used for high temperature thermocouples in combination with MOSi in an oxidising atmosphere. 

Clearly the best protection from oxidation by a silicide as a coating on a reactive substrate would be the disilicide, which has the highest silicon content, and could be expected to provide a relatively protective silica coating. 

The cohesive, elastic, and dynamic properties of cobalt-disilicide... 

Several coating techniques are now available to overcome the oxidation problems with molybdenum above 300°C. One of these, based on molybdenum disilicide, is finding increased usage in flame breakout shields for aero-engines where tests have shown (unpublished work) that the coated material can withstand a high pressure torching type flame attack at temperatures in excess of 2(X)0°C. 

Silicon diffusion (Ihrigising) is not commonly applied to steels, but is increasingly used to protect the refractory metals - (disilicide coatings). 

The silicides of major industrial importance are the disilicides of the refractory metals molybdenum, tantalum, titanium, tungsten, vanadium, and zirconium.pl] These compounds are of great interest par-... 

Tantalum disilicide, TaSi2, is very refractory and chemically resistant. It is deposited from the reaction of the chloride, TaCl5, with silane (SiH4), or dichlorosilane (SiH2Cl2), the latter precursor being preferred. The reaction takes place in a plasma as follows ... 

Borides, Silicides, III—VandII—VI Compounds 331 2.7 Titanium Disilicide... 

Titanium disilicide (TiSi2) has very low electrical resistivity and is a promising metallization material.P ] It is deposited by the following reaction ... 

Tungsten disilicide (WSi2) is refractory and stable with low resistivity. As with other silicides, a common deposition system uses silane as the silicon source with tungsten hexafluoride as follows ... 

Cobalt disilicide is deposited from the carbonyl Co2(CO)g at 200°C in vacuum, followed by annealing at 700°C... 

The silicide and salicide technologies are particularly important in Japan and may indicate a general switch to these materials whenever appropriate.In one case, epitaxial cobalt disilicide provided a satisfactory diffusion source and is mentioned as a promising alternative for deep submicron devices.1 1... 

A carbon tetrachloride/decaborane (C10H14) mixture detonates on impact. Calcium disilicide detonates when it comes into contact with carbon tetrachloride. 

See Calcium disilicide Carbon tetrachloride Calcium hypochlorite... 

Zirconium Fire and Explosion Incidents, TID-5365, Washington, USAEC, 1956 Calcium disilicide exploded when milled in the solvent. 

See Iron(III) oxide Calcium disilicide Metal fluorides... 

Calcium disilicide ignites in close contact with alkali metal fluorides, forming silicon tetrafluoride. 

Hexalithium disilicide Mellor, 1940, Vol. 6, 169 It incandesces on warming in fluorine. 

Calcium disilicide readily ignites, and lithium hexasilicide becomes incandescent, when warmed in fluorine. 

Calcium cyanamid process, for ammonia synthesis, 11 114, 115 Calcium cyanide, 8 194-197 Calcium cyanide diammoniate, 8 195 Calcium dichromate, molecular formula, properties, and uses, 6 561t 5-Calcium disilicate monosulfate, phase in Portland cement clinker, 5 472t 5-Calcium disilicate monocarbonate, phase in Portland cement clinker, 5 472t Calcium disilicide, 4 530 8-Calcium disodium trialuminate, phase in Portland cement clinker, 5 472t Calcium doping, 23 842-844 Calcium P-alumina, 2 406t Calcium ethylenediaminetetraacetate, 7 596t... 

Disilanes, direct-process residue, 22 552 Disilicide pest, 25 386 Disinfectant/Disinfection Byproducts Rule (D/DBPR), 17 807 Disinfectants, 8 605, 606... 

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