jHtfSr /feg

Figure 59. (a) Molecular structure of Sr{Ta(OPr')6}2-Pr' OH. Reprinted with permission from [278], A. C. Jones et al., J. Mater. Chem. 11, 544 (2001). © 2001, Royal Society of Chemistry. (b) Molecular structure of [Sr{Ta(OEt)5(bis-dmap)}2]. Reprinted with permission from [277], M. J. Crosbie et al., Chem. Vap. Deposition 5, 9 (1999). © 1999, Wiley-VCH Publishers.

The continuous quest to obtain materials with improved properties and the possibility of improving the compositional homogeneity and tailoring the composition through new organometallic molecular or polymeric precursors has led to investigations of new preceramic precursors based on additional elements (e.g., B-C-N and Si-B-N-C systems). The approach of synthesizing nonoxide ceramic materials from multicomponent precursor molecules already containing the basic structure (coordination) and bonds desired in the final ceramic product has proved to be of great value. Among ternary nonoxide ceramics, boron-carbon-nitrogen is gaining increasing attention because of the outstanding properties of boron carbonitride, BCxNy. These compounds are superhard like diamond and cubic boron nitride and show remarkable stability against oxidation at elevated temperatures [586].

When compared with diamond, which decomposes to carbon dioxide at 800 °C in air, cubic boron nitride (c-BN) is resistant to oxidation up to 1600 °C. However, the application of c-BN is complicated because of high internal stresses in thin films. Boron carbonitride possesses high hardness and oxidation resistance by low internal stresses; these properties make it an interesting alternative material. The starting materials pyridine borane (C5H5N-BH3) and triazaborabi-cyclodecane (BN3H2(CH2)6) containing boron, carbon, and nitrogen have been used as SSPs to deposit ultrahard BCxN on low-melting-point materials like aluminum and polymers [240]. Riedel et al. have made a comparative evaluation of the suitability of C5H5N-BH3 and BN3H2(CH2)6 in a plasmaassisted CVD process. They found that the atomic hydrogen produced by the decomposition of precursor in the plasma reduces the carbon content in the films. Since BN3H2(CH2)6 contains a higher amount of hydrogen, BCN films produced with this precursor revealed a lower carbon content. This example illustrates the role auxiliary elements (ligands) can play in the final composition of the material [271].

In view of the higher thermal stability of amorphous Si-C-N ceramics compared with binary Si-N or Si-C amorphous compounds, a large number of organometallic precursors such as polyorganosilazane [587], polymethyldis-ilazanes [588], and polyorganosilylcarbodiimides [589] have been used to obtain Si-C-N systems. It was observed that a dual-source precursor system consisting of polysilazane (Si-N) and polycarbosilane (Si-C) did not provide sufficiently homogeneous Si-C-N precursors to prevent the crystallization of SiC. On the other hand, single-source Si-C-N precursors with a similar C:Si ratio led to products that were amorphous at rather high temperatures [590].

Jansen et al. [591] have prepared amorphous ceramics with the chemical compositions Si3B3N7 and SiBN3C by the pyrol-ysis of the single-source trichlorosilylamino-dichloroborane, Cl3Si-NH-BCl2, under ammonia and inert gas atmospheres, respectively. These ceramics possess optimized mechanical properties compared with material prepared from modified polymers or by copolymerization [592]. The borosili-con nitride Si3B3N7 and the borosilicon carbonitride SiBN3C remain amorphous on heating in vacuum or in a N2 atmosphere up to 1800 and 1900 °C, respectively, and no separation into thermodynamically stable crystalline phases was observed. In addition, an extremely high resistance against oxidation up to 1550 °C was observed for SiBN3C, which is simply too high for a metastable nonoxide ceramic [592]. The exceptional properties of these amorphous compounds seem to have their origin in the precursor chemistry and preorga-nized Si-N-B bonds. The atomic arrangement in the precursor (Fig. 60) is an important condition for the homogeneous elemental distribution, and it avoids regional inhomogene-ity due to boron- or silicon-enriched clusters [593]. Si-B-N-C ceramics prepared from other precursors show segregation into crystalline Si3N4 and turbostratic BN at temperatures over 1400 °C [594, 595].

Whereas SiBN3C fibers can easily withstand temperatures of up to 1800 °C in nonoxidizing atmospheres and up to 1500 °C under oxidizing conditions for long periods (<50 h), the commercial HiNicalon fibers (Nippon Carbon), when

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