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

Abstract

Ordered two-dimensional (2D) mesoporous silicoboron carbonitride (SiBCN) ceramics were prepared by a nanocasting approach of a boron-modified polysilazane of the type [B(C 2H 4SiCH 3NH) 3] n (C 2H 4 = CHCH 3, CH 2CH 2) ([Si 3B 1.1C 10.5N 3.0H 25.5] n ) using mesoporous CMK-3 carbon as hard template. The polymer was synthesized according to a monomer route by hydroboration of CH 2 = CHSiCH 3Cl 2 followed by reaction of the as-made tris(dichloromethylsilylethyl)borane (B(C 2H 4SiCH 3Cl 2) 3 (TDSB, C 2H 4 = CHCH 3, CH 2CH 2) with lithium amide (LiNH 2). It was generated as a highly soluble compound which could easily impregnate mesoporous CMK-3 carbon. The derived [B(C 2H 4SiCH 3NCH 3) 3] n -carbon composite was directly pyrolyzed in flowing nitrogen at 1000 °C to generate a SiBCN-carbon composite. The carbon template was subsequently removed through thermal treatment at 1000 °C in a mixture of ammonia and nitrogen to generate ordered mesoporous Si 3.0B 1.0C 4.2N 2.4 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 °C in flowing nitrogen and a carbon removal step at 1000 °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 °C under nitrogen.