Metal from silanes

The thermal decomposition of silanes in the presence of hydrogen into siUcon for production of ultrapure, semiconductor-grade siUcon has become an important art, known as the Siemens process (13). A variety of process parameters, which usually include the introduction of hydrogen, have been studied. Silane can be used to deposit siUcon at temperatures below 1000°C (14). Dichlorosilane deposits siUcon at 1000—1150°C (15,16). Ttichlorosilane has been reported as a source for siUcon deposition at >1150° C (17). Tribromosilane is ordinarily a source for siUcon deposition at 600—800°C (18). Thin-film deposition of siUcon metal from silane and disilane takes place at temperatures as low as 640°C, but results in amorphous hydrogenated siUcon (19). 

Thin-film deposition of siUcon metal from silane and disilane takes place at temperatures as low as 640°C, but results in amorphous hydrogenated siUcon... 

The vinylsilane C-Si bond can also be formed from a silane by reductive cyclization/hydrosilylation of a 1,6- or 1,7-diyne. Reductive cyclization of diynes is an important ring-forming method catalyzed by transition metals, and silanes are common reductants in this process. However, in many cases the silane serves only as a hydride source, and the silyl group is not retained in the isolated product.95 Here, the focus is on the more rare methods which allow simultaneous C-C bond formation and vinylsilane installation. 

At set of toluene soluble oligomers of the type shown in equation 70 were made with various metal to silane diol ratios in order to achieve the appropriate Zn2Si04 stoichiometry. The oligomer molecular weights varied from Mn = ca 1.8 kDa to 11 kDa, as determined by NMR end group analysis. 

Even though much of the purported evidence for silylenoid species is weak or nonexistent upon close examination, such species may nevertheless be produced, especially if the possibility of dinuclear complexes is considered. In fact, there are several lines of evidence which do suggest that some sort of silylenoid species is generated from silanes and low-valent metal complexes. First, as Eq. (74) implies, the same dimethylsili-con metal complex is apparently generated from two diverse silicon compounds. Second, even early in the reaction, the most abundant product in the disproportionation of EE is ED E, which is the result of a SiO/Me exchange. If the redistribution were occurring solely by addition/elimina-tion [e.g. Eqs. (142-145)], then the observed SiO/Me exchange is the result of insertion into Si—O and Si—C bonds [Eqs. (148-149)]. Now,... 

The Rochow Process. Rochow found that alkyl and aryl halides react directly with silicon when their vapors contacted silicon at elevated temperatures to produce complex mixtures of organosilicon halides. The reaction is promoted by a wide variety of metals from both the main group and the transition series, but the most efficient catalyst is copper. The most studied reaction of this type is the reaction between methyl chloride and silicon to give dimethyldichlorosilane and methyltrichlorosilane. Dimethyldichloro-silane is major feedstock silane for methylsilicon polymers. 

Reactions in which transition-metal complexes extrude silylene ligands from silanes have already been mentioned (equations 32-34). Although the mechanisms of these reactions remain ill-defined, it seems possible that terminal silylene complexes may be involved as intermediates. Such a proposal has been made regarding the formation of the silylene-bridged dicobalt species 16 from Co2(CO)8 and tetramethyldisilane (Scheme l)105. 

From Silanes. A summary of the purification of SiH4, including the removal of PH3, is given in Silicon Suppl. Vol. B 1, 1982, pp. 76/7. The removal of small amounts of PH3 in SiH4 succeeds with zeolites [22, 23] and in some cases with zeolites which are cation-exchanged with the bivalent cations of Mg [24, 25], Pb, Mn, Co, and Cd [25]. Contact with Cu on an alumina carrier [26] or with a metalated macroreticular polymer [27] was also used. A solution of NaAlH4 can also remove PH3 selectively from SiH4 [28]. 

Table 1. Average metal particle diameters (from XRD) for some metal precursor/silane combinations [AEAPTS = 3-(2-aminoethylamino)propyltriethoxysilane, APS = aminopropyltriethoxysilane, TAS = (3-trimethoxy-silylpropyl)-diethylentriamine)].

On a laboratory scale, it has been demonstrated in recent years that thin films of silanes applied to metals can protect the metal from many forms of corrosion, including uniform corrosion, pitting corrosion, stress corrosion cracking, crevice corrosion and other forms, in addition to providing excellent and durable paint adhesion (see Durability - fundamentals). Such thin films are typically of not more than 300-mn thickness. They can be applied by immersion of the metal into a dilute silane solution, for example, 5% in water or water/alcohol mixtures, as not all silanes dissolve in water. Brushing, wiping or spraying application methods can also be used. 

The future system (scheme C) involves only two layers, the primer and the topcoat. The VOC problem is solved by dispersing the resin, the crosslinker, the silane and the Cr-free inhibitors into a water-based solution [7-11,27-31]. The silane improves the adhesion of the primer to the metal, the Cr-free inhibitors incorporated in the primer protect the metal from corrosion and the functionality of the silane-containing polymer matrix ensures good adhesion to the subsequently applied topcoat [33]. 

The superprimer network is fairly hydrophilic. X-ray reflection results of silane-resin films have shown that these siloxane-containing primers actually contain small voids that attract water into the coating. As the water penetrates the voids, it does not make the coating swell [64]. It only allows the less soluble Cr-free pigments to move in the voids and leach out on-demand to protect the metal from corrosion. This on-demand protection behavior has been studied by exposing scribed panels to corrosive environments and subsequently the surfaces and the cross-sections of the samples have been examined by SEM/EDX for residues of pigments and corrosion products [7-11, 27-30, 61-63]. 

Inelastic electron tunnelling spectroscopy (lETS) has been used to study some silanes on aluminium oxide. The technique records vibrational spectra of an absorbed monolayer. Silanes can be applied to the oxidised metal from solution or vapour, and devices are completed by evaporation of a top electrode which is usually of lead, because of its superconductivity. The device is cooled to the temperature of liquid helium (4.2 K) to minimise thermal broadening. Most electrons (>99%) pass through the device elastically, but a small number excite vibrational modes. It is these that are detected and displayed as a spectrum. Both IR and Raman modes can be observed the selection rule for lET spectroscopy is one of orientation, in that bonds which are aligned perpendicular to the surface give the most intense peaks. 

A number of complexes have been prepared from the electron-rich ligand 3,5-di(tert-butyl)-l,2,4-triphospholyl these include manganese carbonyl complexes and the manganocene, which has been structurally characterised. A paper explaining the facile silane dissociation in transition metal T -silane complexes Cp(CO)2M[p -H(SiH3 Cl )] (M = Mn, Tc and Re n = 1-3) (see, for example, 28) makes interesting reading. ... 

Ring-Opening Polymerization. As with most other inorganic polymers, ring-opening polymerization of cyclotetrasilanes has been used to make polysilanes (109,110). This method, however, has so far only been used for polymethylphenylsilane (eq. 12). Molecular weights (up to 100,000) are higher than from transition-metal catalyzed polymerization of primary silanes. 

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