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Preceramic polymers; Ordered mesoporous SiBCN; Nanocasting
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(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 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 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 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 m2 g-1), high pore volume
(0.91 cm3 g-1), and narrow pore-size distribution (around 4.6 nm) with a thermal stability which extends up to 1180 C under nitrogen.
