Ordered mesoporous silicoboron carbonitride ceramics from boron-modified polysilazanes: Polymer synthesis, processing and properties

Ordered two-dimensional (2D) mesoporous silicoboron carbonitride (SiBCN) ceramics were prepared by a nanocasting approach of a boron-modified polysilazane of the type [B(C2H4SiCH3NH)(3)](n) (C2H4 = CHCH3, CH2CH2) ([Si3B1.1C10.5N3.0H25.5](n)) using mesoporous CMK-3 carbon as hard template. The polymer was synthesized according to a monomer route by hydroboration of CH2 = CHSiCH3Cl2 followed by reaction of the as-made tris(dichloromethylsilylethyl)borane (B(C2H4SiCH3Cl2)(3) (TDSB, C2H4 = CHCH3, CH2CH2) with lithium amide (LiNH2). It was generated as a highly soluble compound which could easily impregnate mesoporous CMK-3 carbon. The derived [B(C2H4SiCH3NCH3)(3)](n)-carbon composite was directly pyrolyzed in flowing nitrogen at 1000 degrees C to generate a SiBCN-carbon composite. The carbon template was subsequently removed through thermal treatment at 1000 degrees C in a mixture of ammonia and nitrogen to generate ordered mesoporous Si3.0B1.0C4.2N2.4 structures. XRD and TEM analyses revealed that the obtained amorphous mesoporous ceramic exhibits open, continuous, and ordered 2D hexagonal frameworks which are strongly dependent on the number of impregnation cycles and the carbon removal step. Using a double impregnation cycle combined with a pyrolysis process up to 1000 degrees C in flowing nitrogen and a carbon removal step at 1000 degrees C for 3 h in a volumetric flow ratio between ammonia and nitrogen of 1, the ordered mesoporous SiBCN ceramic displays high surface area (630 m(2) g(-1)), high pore volume (0.91 cm(3) g(-1)), and narrow pore-size distribution (around 4.6 nm) with a thermal stability which extends up to 1180 degrees C under nitrogen. (C) 2010 Published by Elsevier Inc.