Several boron-modified polysilazanes of general type {B[C2H4Si(R)NH]3} n (C2H4 = CHCH3 or CH2CH2) were synthesized and their thermal behavior studied. In contrast to the known derivatives with R = alkyl or aryl, we describe ceramic precursors in which the bulky moieties R are substituted with lower weight groups and/or reactive entities. Reactive units enable further cross-linking of the polymeric framework and therefore minimize depolymerization during ceramization. The polymer-to-ceramic conversion of all synthesized polymers was monitored by thermogravimetric analysis. Both low molecular weight substituents and/or cross-linking units increase the ceramic yield from 50% (R = CH3) to 83−88%. High-temperature thermogravimetric analysis in an inert gas atmosphere indicates the ceramics obtained are stable up to ∼2000 °C. XRD studies of the fully amorphous materials point out that, with increasing temperature, formation of α-SiC or α-SiC/β-Si3N4 crystalline phases occurs at 1550−1750 °C, depending on the material's composition. The resistance of these novel materials toward oxidative attack was investigated by TGA in air up to 1700 °C and SEM/EDX, indicating that the materials efficiently self-protect toward oxidation.

The synthesis, detailed spectroscopic characterization, polymer-to-ceramic conversion, and high-temperature behavior of a new class of polymeric precursors for Si-B-C-N composites are discussed. The title compoundsCH 2 -CH 2 ; 5a: R 1 , R 2 ) H; 5b: R 1 ) H, R 2 ) CH 3 ; 5c: R 1 , R 2 ) CH 3 ) were designed especially for the preparation of ceramic films and fiber-reinforced ceramic composite matrixes. They are obtained in quantitative yields by the reaction of oligovinylsilazane [(H 2 CdCH)SiH-NH] n (4) with tris(hydridosilylethyl)boranes of general type B(C 2 H 4 SiHR 1 R 2 ) 3 (C 2 H 4 ) CHCH 3 , CH 2 CH 2 ; 3a: R 1 , R 2 ) H; 3b: R 1 ) H, R 2 ) CH 3 ; 3c: R 1 , R 2 ) CH 3 ) in a thermally induced hydrosilylation reaction without catalyst and/or solvent and without the formation of byproducts. Ceramic yields are 83% for 5a, 82% for 5b, and 63% for 5c as shown by thermogravimetric analysis (TGA). High-temperature TGA of the as-obtained amorphous ceramic materials, carried out in an argon atmosphere, reveals a thermal stability toward degradation of the 5b-derived material 6b up to 2000 °C. In contrast, the 6a material, which was obtained from 5a, decomposes around 1850 °C. The least stable is the 6c ceramic, which decomposes at 1450 °C. The microstructure development of 6a-6c was investigated in the temperature range of 1400-2000 °C by X-ray diffraction (XRD), indicating that preferentially crystalline R-silicon carbide is formed at 1700 °C for 6a, 1500 °C for 6b, and 1600 °C for 6c. In addition, there are less intensive reflections observed in the XRD patterns of 6a, caused by the formation of β-silicon nitride.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

No abstract