Organosilicon carbodiimides have been successfully applied as single-source precursor compounds for the synthesis of novel ternary Si-, C-, and N-containing solid phases. Their thermally induced decomposition gives either amorphous silicon carbonitrides or polycrystalline silicon nitride and silicon carbide mixtures, materials that are presently of technological interest for their exceptional hardness, strength, toughness, and high temperature resistance even in corrosive environments. This review is concerned with the synthesis, characterization, and thermal stability of element carbodiimides. The main part of this paper is focused on polymeric silicon-based carbodiimides obtained by the reaction of chloro(organo)silanes with bis(trimethylsilyl)carbodiimide. In the case of RSiCl 3 , novel poly(silylcarbodiimide) gels are formed. Starting from silicon tetrachloride, new crystalline SiCN phases (namely, SiC 2 N 4 and Si 2 CN 4 ), have been isolated. Their crystal structures as well as their thermal behavior in the range between room temperature and 1600 °C are discussed. Moreover, preliminary results on the synthesis of germanium-and boron-containing carbodiimides are reported. It is also shown that carbon-based carbodiimides can be obtained by the reaction of cyanuric halides with bis(trimethylsilyl)carbodiimide. These materials are investigated as precursors for the synthesis of new carbon nitrides with high hardness.

Coatings comprising various volume fractions of particulate polyaniline dispersed in a polyvinylbutyral binder are applied to iron substrates. A scanning Kelvin probe is used to measure substrate potentials in humid air and follow corrosion-driven coating delamination ͑cathodic disbondment͒ when 5% w/v ͑0.86 M͒ aqueous NaCl contacts a coating defect. Emeraldine base has no effect on substrate potential or delamination kinetics. Emeraldine salts ͑ES͒ doped using p-toluenesulfonic ͑HpTS͒, camphorsulfonic ͑HCS͒, phosphoric ͑H 3 PO 4 ͒, and phenylphosphonic ͑H 2 PP͒ acids increase substrate potentials by up to 0.36 V and inhibit delamination with efficiency order of HpTS Ͻ HCS Ͻ H 3 PO 4 Ӷ H 2 PP. Dopant salts added to the corrosive electrolyte do not inhibit delamination. It is proposed that inhibition arises primarily from cathodic O 2 reduction becoming relocated from the ennobled substrate onto the ES coating. However, Fe 3 ͑PO 4 ͒ 2 and FePP salt films formed at the ES-substrate interface also contribute by hindering interfacial electron transfer.

The sol-gel process for the synthesis of oxidic glasses and ceramics has been the subject of intensive investigations in synthetic inorganic chemistry and is used in a wide range of technical applications.['. However, it was limited hitherto to systems containing oxygen.''] We now report on the first anhydrous sol-gel process for the synthesis of oxygen-free silicon ceramics.The reaction of trichloro(methy1)silane (1) with the carbodiimide synthetic building block bis(trimethylsily1)carbodiimide (2) and a pyridine catalyst produces a transparent, dimensionally stable gel [Eq. (a)]. The reaction is performed without sol- increases from 6 h (reflux temperature) to 24 days (room temperature).Reaction (a) proceeds completely analogously to the known aqueous sol-gel process; the carbodiimide 2 adopts the role played by water in the substitution and condensation steps [Eq. (b)]. During the aging process, the number of bonds in the gel network is increased by progressive polycondensation. The FT-IR spectra of the nonaged gels reveal the presence of trimethylsilyl end groups; these are no longer detectable by spectroscopy after the aging process [Eq. (c)]. Therefore, the condensation reactions are not complete at the gel point, but continue during the aging process. The rate of aging is not only a function of the amount of pyridine, but also a function of the temperature: a higher pyridine content and a higher aging temperature (20-45 "C) accelerate further cross-linking. During the aging process there is no phase change to give a crystalline solid. The xerogel 3 obtained after drying (3 x lo-' mbar, 130°C) is X-ray amorphous. Furthermore, the in-situ X-ray powder diffractograms do not show any reflections in the temperature range between room temperature and 850 "C which could indicate crystalline phases. The controlled aging of the gel is characterized by crack-free, three-dimensional shrinkage. Thus, for a pyridine content of 0.3 equiv with respect to 1, shrinkage is complete after about four weeks. It is irreversible, and the rate of shrinkage is likewise dependent on the temperature and the concentration of catalyst. Immediately after gelation, the gel is transparent. On aging the gel becomes cloudy. On slow evaporation of the liquid phase, it converts into the transparent xerogel3. The gel shrinks during the aging process to 25 % of the original volume. After the liquid phase was decanted off, and the remaining liquid removed by slow evaporation, the gel shrinks linearly by a further 10%. Measurements of the porosity revealed that the gel is very dense; no micropores could be detected.f31In contrast, on treating 1 with hexamethyldisilazane (

A novel polymeric organosilicon gel with the composition [MeSi(NCN)1.5]n synthesized by the reaction of MeSiCl3 with Me3Si–N=C=N–SiMe3 is reported. The reaction is performed without any solvent and with catalytic amounts of pyridine and provides highly cross‐linked poly(methylsilsesquicarbodi‐imide) in the form of a stable non‐oxidic gel with unusual low open porosity (<1 m2 g−1). The Si–C–N gel transforms to an amorphous silicon carbonitride ceramic, SiC1.1N1.6, by the thermally induced ceramization at 1200 °C in inert atmosphere (argon). The gel‐derived silicon carbonitride is thermally stable up to 1450 °C. The synthesis, characterization and pyrolysis behavior of the new polyorganosilicon gel is discussed. © 1997 John Wiley & Sons, Ltd.

No abstract