High disilicides

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. 

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on molybdenum disilicide, is finding increased usage in flame breakout shields for aero-engines where tests have shown (unpublished work) that the coated material can withstand a high pressure torching type flame attack at temperatures in excess of 2(X)0°C. 

Similarly, disilyl iron tetracarbonyl gives as a single product the low-temperature (y3) form of iron disilicide, a semiconductor on annealing at 1000°C for 4 days, the /3 form decomposes to give a mixture of the iron-deficient high-temperature (a) form, together with FeSi. 

Many metals form conductive silicides, which, like SiC, are resistant to oxidation through the formation of stable passivating layers of silicates or silica on their surfaces at high temperatures. Molybdenum disilicide (MoSi2) has been developed as a heating element for use in air at temperatures above 1500 °C. Its resistivity behaves as is expected for a metal, increasing from about 2.5 x 10-7 fim at room temperature to about 4 x 10-6 Qm at 1800 °C. 

Titanium disilicide [12039-83-7] is a silvery-gray, crystalline material that oxidizes slowly in air when heated to 700—800°C. It is resistant both to mineral acids (except hydrofluoric) and to aqueous solutions of alkalies, but reacts with fused borax, sodium hydroxide, and potassium hydroxide. It reacts explosively with chlorine at high temperatures. 

Ml) as Pd, Pt, Ni or Co, which form metal-rich silicides at low T, or whether they are such metals (M ) as V, Cr, Zr, Mo, Ta and W, which form disilicides as a first phase at high T (see Table 3) or a combination of both. For the latter, the subsequent reaction with Si depends on whether the low-T silicide-forming metal or the high-T disilicide-forming metal is in contact with the Si. Examples of the four Si reaction categories are shown schematically in Fig. 4. 

SAFETY PROFILE A highly corrosive irritant to the eyes, skin, and mucous membranes. Mildly toxic by inhalation, Explosive reaction with alcohols + hydrogen cyanide, potassium permanganate, sodium (with aqueous HCl), tetraselenium tetranitride. Ignition on contact with aluminum-titanium alloys (with HCl vapor), fluorine, hexa-lithium disilicide, metal acetylides or carbides (e.g., cesium acetylide, rubidium ace-tylide). Violent reaction with 1,1-difluoro-ethylene. Vigorous reaction with aluminum, chlorine + dinitroanilines (evolves gas). Potentially dangerous reaction with sulfuric acid releases HCl gas. Adsorption of the acid onto silicon dioxide is exothermic. See also HYDROGEN CHLORIDE (AEROSOL) and HYDROCHLORIC ACID. 

Use Electrical resistors, protective coatings at high temperatures, engine parts in space vehicles (molybdenum coated with molybdenum disilicide). 

The deposition processes discussed so far typically operate such that all the material required for the growing film comes from the overlying gas or liquid phase. Other deposition reactions involve reaction (and therefore consumption) of the underlying substrate itself. Examples of such deposition processes include thermal oxidation, nitridation, or silicidation of silicon, which can be accomplished by exposing a silicon wafer at high temperature to oxygen, ammonia, or titanium tetrachloride, respectively, to form silicon dioxide, silicon nitride, or titanium disilicide. Solid-phase diffusion and reaction processes are involved in each case. 

The thermoelectric properties of the plasma spray formed iron disilicide might be influenced by a possible change in material composition due to the high temperatures, and the highly disordered structure. 

Williams et al. recently published [62] a paper reporting crystals of erbium disilicide - a compound with high conductivity - that had been grown on a silicon substrate. These nanowires are remarkably straight, and quite uniform... 

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