Subject silicon stabilization

This subject was studied in relation to the silicone breast implant controversy of the 1990s. Silicone-fiUed artificial breasts were coated with thin layers of a TDI-based polyurethane foam to encourage stabilization by cell ingrowth. Residual toluene diamine (TDA) was found in prepolymer-based polyurethanes at the part-per-biUion level and, more importantly, hydrolysis that would lead to the release of more TDA in vivo was suspected. 

It follows that, even though suitable procedures were to be found for the alkylation of longer silicon chains, the products would be subject to oxidation, to thermal dissociation, and to hydrolysis in the presence of alkalies. It seems unlikely that any combination of substituents could stabilize these chains sufficiently to allow their practical application to polymeric materials.1... 

The presence of group 4 metals P to a carbenium ion has a dramatic effect on the stability of the carbenium ion, a remarkable phenomenon known as the P-effect.12 29, 30 The silicon P-effect was discovered in 1937 by Ushakov and Itenberg,31 who noted the exceptional reactivity of P-silyl-substituted halides R3SiCH2CH2X towards elimination (Scheme 1). The effect has since been the subject of many mechanistic32-36 and theoretical studies37-39 and several reviews.10-12,29... 

Reliable stability data of the p-i-n solar cell itself are not easily obtained, especially for non-encapsulated cells or modules. One of these tests e.g. for EN/IEC 61646 certification of modules is the so-called damp-heat test (85°C, 85% humidity, up to lOOOh). Recent studies were performed by Stiebig et al. [50, 51] exposing different types of cells to harsh conditions. One of the most important results was the excellent stability of silicon thin film solar cells. Remarkably, this is also valid for small area modules even without encapsulation [52]. This is of high interest because costs and efforts for module encapsulation strongly depend on the inherent stability of the solar cells. As a more detailed treatment of this subject is beyond the scope of this chapter, the reader is referred to the original papers [50,51]. 

The failure of difluoramine to appear among the final products is not particularly surprising. In the presence of nitric acid and/or nitrogen oxides, it might easily be oxidized and may well constitute the source of the silicon tetrafluoride. The formation of a carbonium ion from trityl-difluoramine would be favored by resonance stabilization. In the tert-butyl case, on the other hand, this driving force is not present and formation of the ion would be expected to occur less readily. In addition, both the tert-butyl carbonium ion and the difluorammonium ion from which it is derived would be more subject to a variety of side reactions than the corresponding trityl species. 

This chapter will concentrate on the chemistry of metal-14-centered anions (Ge, Sn, Pb). These compounds and their silyl analogues are ionic or polarized alkaline and alkaline earth metal-14 compounds, as well as delocalized molecules such as metalloles. Ammonium metallates Mi4 R4N+ or metal-14-centered anion radicals are also considered. The subject was explored during the 1960s and 1970s and thoroughly reviewed in 1982 and 1995 in Comprehensive Organometallic Chemistry, Vols. I and and for silicon species in a previous volume of this series . By that time the main routes to metal-14 anions were known. Since then, the subject has been developed in the topics of particular syntheses, stabilization using steric hindrance, electronic effects and complexation, spectroscopic and structural analyses "... 

The form that silicon carbide takes depends on many factors including thermal history, impurity type and level, and environment. The p form is generally felt to be the stable phase at low temperatures, whereas the a form is the high-temperature form. There are many exceptions to the rule, as the conversion to a from /3 and the converse have been reported. The stability and transformations of the various polytypes vary among themselves and constitute a subject that is too broad for this effort. The basic a and p descriptors will be used for the remainder of this section. 

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