Silicon, reduction potentials

These features imply that the cost reduction potential for thin-film technology is very high and it is thus capable, in the longer term, of extending the P V learning curve beyond the point that can be reached by crystalline silicon technology. 

Silicon (continued) reduction potentials, 33 1 IS salts, lattice energy and thermochemistry, 22 70-73... 

Tab. 13 Half-wave reduction potentials at room temperature (in V vs. Fc/Fc+) of representative examples of cycloaddition derivatives of Cgo with oxygen-, carbon- and silicon-containing groups...

Arylsilanes serve as a typical example of this system. The reduction potentials of arylsilanes are slightly less negative than those of the parent aromatic hydrocarbons [199-203]. This seems to be attributed to the dj -pj interaction between the aromatic ring and the silicon atom. The electrochemical behavior of silyl-substituted cyclooctatetraene is interesting [204]. The second reduction potential becomes less negative by the silyl substitution. The stabilization of the dianion (aromatic 10 7r-system) by dj -p interaction seems to be responsible for this phenomenon. 

The low ability of a Si—X bond to react by a one-electron transfer process is also clearly indicated by the electrode potential data derived from the electrochemical reduction of silicon halides (Table 11) (110). 

In some cases, e.g., at the electrochemical reduction of silicon species, the potential of final stage (formation of a metal) turns out to be more negative than the potential of the alkali metal formation (see Chap. 3). Similar situation seems to be true for a number of other polyvalent metals. For example, the reaction of zirconium metal with fluoride melts is known [13] where the potassium metal is a product. 

The metal-nonmetal line passes through the heart of the group, with carbon being a bona fide nonmetal and lead a bona fide metal. In between are two metalloids (silicon and germanium) and a borderline metal (tin). The progression in the acid-base character of the oxides of the elements further emphasizes the trend from nonmetal to metal. The formulas of the oxides and halides show the increasing importance of the +2 oxidation state down the group, and this is reinforced by a consideration of standard reduction potentials. 

This reference electrode system is suitable for use between 700 and 950 °C, below which the reference system has too high impedance (typically above 10 2) and above which the silver melts T = 962 °C). Internal standards are used to measure the stability of the electrode. Here, reduction of silicon dioxide and calcium metal deposition (from Ca " ) can be used as indicator potentials and show that the electrode can give stable potentials for times between hours and days. In practice, while carrying out long electrolysis reactions in chloride melts, the group at CSIRO (Australia) have found that the electrodes typically last about 8 h. This is, however, sufficient to carry out many types of experiments. 

Knowing that the reduction potentials of dichlorosilanes are much less cathodic than that of Me3SiCl, we can assume that the dichlorosilanes are first reduced to monochlorosilyl anions which are then trapped by the Me3SiQ present in large excess rather than by the dichlorosilane. Moreover, the kinetic curves (obtained from GC analysis) of the electrochemical reduction of PhMeSiCl2 with an A1 anode, as an example, show that the two silicon-chlorine bonds are reduced in two well-separated steps that is the monochlorinated disilane was formed initially until 2.1 F/mol of PhMeSiCl2 had been consumed (Equations 11, Figure 4). 

Figure 3.5 shows the positive SSIMS spectrum from a silicon wafer, illustrating both the allocation of peaks and potential isobaric problems. SSIMS reveals many impurities on the surface, particularly hydrocarbons, for which it is especially sensitive. The spectrum also demonstrates reduction of isobaric interference by high-mass resolution. For reasons discussed in Sect. 3.1.3, the peak heights cannot be taken to be directly proportional to the concentrations on the surface, and standards must be used to quantify trace elements. 

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