Silicides preparation

The structures of the metal silicides (prepared by direct combination of the elements at high temperatures) are diverse, and a full discussion of the structures is beyond the scope of this book. Some examples of their solid state structural types are ... 

H.R. Orthner, R Tomasi, and W. J. Botta F., Reaction sintering of titanium carbide and titanium silicide prepared by high-energy milling. Materials Science and Engineering A336.202 (2002). 

The structures of the metal silicides (prepared by direct combination of the elements at high temperatures) are... 

Higher chlorides, Si2Cle to Si6Cl,4 (highly branched - some cyclic) are formed from SiCU plus Si or a silicide or by amine catalysed disproportionations of Si2Cl,5, etc. Partial hydrolysis gives oxide chlorides, e.g. CUSiOSiCla. SiCU is used for preparation of silicones. 

Titanium Silicides. The titanium—silicon system includes Ti Si, Ti Si, TiSi, and TiSi (154). Physical properties are summarized in Table 18. Direct synthesis by heating the elements in vacuo or in a protective atmosphere is possible. In the latter case, it is convenient to use titanium hydride instead of titanium metal. Other preparative methods include high temperature electrolysis of molten salt baths containing titanium dioxide and alkalifluorosiUcate (155) reaction of TiCl, SiCl, and H2 at ca 1150°C, using appropriate reactant quantities for both TiSi and TiSi2 (156) and, for Ti Si, reaction between titanium dioxide and calcium siUcide at ca 1200°C, followed by dissolution of excess lime and calcium siUcate in acetic acid. 

Similar results are obtained for dre deposition of the carbides of these metals using methane as a source of carbon, atrd silicon tetrahalides for the preparation of silicides. These reactions are more complex than dre preparation of the diborides because of the number of carbides atrd silicides that the tratrsition metals form, some of which have wide ranges of non-stoichiometry. The control of the ratio of the partial pressures of dre ingoing gases is therefore important as a process variable. 

Silicides are usually prepared by direct fusion of the elements hut coreduction of Si02 and a metal oxide with C or A1 is sometimes used. Heats of formation are similar to those of borides and carbides but mps arc substantially lower e.g. TiC 3140°, TiBj 2980°, TiSij 1540° and TaC 3800°, TaB2 3100°, TaSi2 1560°C. Few silicides melt as high as 2000-2500°, and above this temperature only SiC is solid (decomp 2700°C). 

The growing interest in volatile silyl-metal complexes for chemical vapor deposition reactions should also be mentioned. This technique is extremely useful for the preparation of silicide films in microelectronic devices. Further examples of applications of silicon-metal compounds are given in the appropriate sections. 

Aylett, B. J., and Tannahill, A. A., Chemical Vapour Deposition of Metal Silicides from Organometallic Compounds with Silicon-Metal Bonds, SIRA Int. Seminar on Thin Film Preparation and Processing Technolgy, Brighton, UK (March 1985)... 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

The flammability and explosive hazard of ferrosilicon powder is increased substantially during grinding in a vibratory mill [1], Explosion hazards from air-hydrogen,—acetylene, or—propane mixtures formed during preparation of ferrosilicon containing alkaline earth additives are attributed to contact of barium or magnesium carbide or silicide additive with atmospheric moisture [2],... 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

Silicon hydrides can be prepared by several methods. A few methods are outlined below. Silane and its higher homologs can be made by treating magnesium silicide, Mg2Si with 20% hydrochloric acid in an atmosphere of hydrogen. An equation for monosilane is given below ... 

Another preparative method involves treating magnesium silicide with ammonium bromide in liquid ammonia in a current of hydrogen. The process forms 70 to 80% yield of mono- and disilanes. The reaction is shown below ... 

Zinc, lithium, and aluminum silicides also may be used instead of magnesium silicide in the above preparations. 

Berzelius heated a mixture of silica, iron, and carbon to a very high temperature, and obtained iron silicide. When he decomposed this with hydrochloric acid, silica was precipitated, and the amount of hydrogen evolved was in excess of the iron, indicating that some other metal must have been present (9). Berzelius finally showed in 1824 that this other seemingly metallic substance was derived from the silica, and succeeded in preparing the amorphous form of it by two methods. In the first of... 

Pt—Q—Salt, [Pt(NH3)2(HP04)] and [Pt(OH)6]2 (259,260). Chloride-based baths have been superseded by P-Salt-based baths, which are more stable and relatively easily prepared. Q-Salt baths offer even greater stability and produce hard, bright films of low porosity. Plating under alkaline conditions employs salts of [Pt(OH6)]2. These baths are easily regenerated but have low stability. Platinum films have uses in the electronics industry for circuit repair, mask repair, platinum silicide production, and interconnection fabrication (94). Vapor deposition of volatile platinum compounds such as [Pt(hfacac)2] and... 

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