Siliciding

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. 

The discussion of Rutherford backscattering spectrometry starts with an overview of the experimental target chamber, proceeds to the particle kinematics that detennine mass identification and depth resolution, and then provides an example of the analysis of a silicide. 

After 60 minutes of aimealing, all the Pt has reacted to fonn Pt2Si. Almost immediately thereafter the reaction between Pt2Si and Si to fonn PtSi starts and after a fiirther 60 minutes all the Pt2Si has reacted, resulting in a stable PtSi film on Si. The data of silicide thickness versus ramped temperature can be plotted in reduced fonn in an Arrhenius-like plot to give the activation energy [6, 14] ... 

Gettering is a black art. It consists in forcing selected impurities (typically, transition metals) to diffuse toward unimportant regions of tlie device. This is often done by creating precipitation sites and perfoniiing heat treatments. The precipitation sites range from small oxygen complexes to layers such as an A1 silicide. The foniiation of such a... 

If an excess of magnesium is used, magnesium silicide, Mg2Si, is also produced.) The silicon obtained is a light brown hygroscopic powder. Crystalline or metallic silicon is obtained industrially by the reduction of silica with carbon in an electric arc furnace ... 

Reactions of HCl and nitrides, borides, silicides, germanides, carbides, and sulfides take place at significant rates only at elevated (>650° C) temperatures. The products are the metal chlorides and the corresponding hydrides. The reactions most studied are those involving nitrides of aluminum, magnesium, calcium, and titanium, where ammonia (qv) is formed along with the corresponding metal chloride. 

Miscellaneous. Chloroplatinic acid is used in the production of automobile catalysts. Platino-type prints based on reduction of Pt(II) to Pt(0) by a photosensitive reducing agent such as iron(III) oxalate are used in art photography (261,262). Infrared imaging devices based on a platinum siLicide detector have been developed (263). 

Borides and Silicides. These materials do not show good resistance to oxidation. Some siUcides, however, form Si02 coatings upon heating which retards further oxidation. Molybdenum disiUcide [1317-33-5] MoSi2, is used widely, primarily as an electrical heating element. 

If the rules for volatiles and thermodynamics of the haUdes are followed, the reaction can be used for aluminizing, silicidizing, chromizing, and similar processing. 

Silicon (3), which resembles metals in its chemical behavior, generally has a valence of +4. In a few compounds it exhibits a +2 valence, and in silicides it exists as a negative ion and largely violates the normal valency rules. Silicon, carbon, germanium, tin, and lead comprise the Group 14 (IVA) elements. Silicon and carbon form the carbide, SiC (see Carbides). Silicon and germanium are isomorphous and thus mutually soluble in all proportions. Neither tin nor lead reacts with silicon. Molten silicon is immiscible in both molten tin and molten lead. 

Silicon is soluble in aluminum in the solid state to a maximum of 1.62 wt % at 577°C (2). It is soluble in silver, gold, and 2inc at temperatures above their melting points. Phase diagrams of systems containing silicides are available (2,3). 

More than half of the elements in the Periodic Table react with silicon to form one or more silicides. The refractory metal and noble metal silicides ate used in the electronics industry. Silicon and ferrosilicon alloys have a wide range of applications in the iron and steel industries where they are used as inoculants to give significantly improved mechanical properties. Ferrosilicon alloys are also used as deoxidizers and as an economical source of silicon for steel and iron. 

Rare-Earth Silicides. Rare-earth sihcides, in the form of a ferroalloy that contain up to 33% rare earths, are used increasingly by the iron and steel industries. Whereas the term sihcides is no longer used for alloys of this type, it is stih in common usage for these materials. Eor nodular iron, addition... 

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. 

Table 18. Structure and Physical Constants of Titanium Silicides ...

B. Aronsson, T. Lundstrom, and S. Rundqvist, Borides, Silicides and Phosphides, Methuen, London, U.K., 1965. 

Copper—chromium and copper—nickel—silicon—chromium alloys are also precipitation hardenable. The precipitates are nickel sdicides, chromium silicides, and elemental chromium. If conductivity is critical, the chromium—silicon ratio should be held at 10 1 so that appreciable amounts of either element are not left in soHd solution in the copper after aging. Lithium can be used as a deoxidizer in copper alloys when conductivity is important. For a discussion of the principle of age- or precipitation-hardening copper alloys, see Copperalloys,wrought copperalloys. 

When a mismatch is inevitable, as in the combination Gej-Sii j. — Si, it is found that up to a value of jc = 0.4, there is a small mismatch which leads to a strained silicide lattice (known as commensurate epitaxy) and at higher values of jc there are misfit dislocations (incommensurate epitaxy) at the interface (see p. 35). From tlrese and other results, it can be concluded that up to about 10% difference in the lattice parameters can be accommodated by commensurately strained thin films. 

To return to die problem of die vaporization of die tantalum silicides, which could be transported as the tetraiodide of each element, but not as the elementary species. From these data it can be concluded that whatever die starting point in the composition range, the composition of the surface phase will tend towards Tag Sis, which is die most nearly congruently vaporizing composition. 

Figure 3.3 Chemical potential diagram for the transport of titanium silicides by chlorine, showing that only TaSi2 can provide the proper ratio of Ta and Si for stoichiometric transport...

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