Synthesis, Characterization, and Ceramic Conversion Reactions of Borazine/Silazane Copolymers: New Polymeric Precursors to SiNCB Ceramics

Abstract

The first borazine/silazane backbone copolymers derived from the parent borazine, B3N3H6, have been obtained by the thermal condensation of borazine with two silazanes, tris-(trimethylsilylamino)silane (TTS), and 1,1,3,3,5,5-hexamethylcyclotrisilazane (HCT). Both series of copolymers are proposed to contain borazine-boron to silazane-nitrogen linkages. Elemental analyses of the TTS copolymers are consistent with an approximate [[(-B3N3H4)-NH]3SiHL structural unit, while the HCT copolymers have compositions ranging from (B3N3H4)i.oo(N)i.8i(SiMe2)i.67(H)i.6 to (B3N3H4)i.oo(N)i.o7(SiMe2)i.28(H)o.9. Molecular weight studies indicate large polydispersities, and when molecular weight data are combined with intrinsic viscosity results, highly branched structures are suggested. Despite their similar compositions, the two series of copolymers yield different types of ceramic materials upon pyrolysis. The TTS copolymers yield BNSi ceramics that are amorphous to 1400 °C. Pyrolyses to 1800 °C result in further loss of silicon to produce ceramics of variable silicon contents with compositions ranging from Bi.ooNo.9oSi<o.oiC<o.oi to B1.00N0.91Si0.14C0.07• While their DRIFT spectra are consistent with the presence of boron nitride, the XRD spectra of these materials showed no crystalline boron-containing species, but instead, peaks due to small amounts of crystalline /I-SÍ3N4, ß-SiC and elemental Si. The ceramics derived from the HCT copolymers at 1400 °C were also amorphous, but showed greater retention of silicon and carbon with typical compositions of B1.0N1.5Si0.4C0 2. Pyrolyses of the HCT copolymers to 1800 °C yielded materials with compositions ranging from B1.00N2.5sSi0.94C0.30 to B1.00N0.86-Sio.i6Co.19. The DRIFT spectra of the ceramics obtained at both temperatures again indicate the presence of boron nitride. XRD spectra, however, show crystalline phases of /J-SÍ3N4, ß-SiC, and Si, but no diffraction from any boron-containing species.

Silicon-Based Polymer-Derived Ceramics: Synthesis Properties and Applications-A Review

“…In 1993, Sneddon et al 3,69,70,72,73 reported and investigated in detail the syntheses of SiBCN polymers in which borazine units are directly bonded to polyorganosilazanes as presented in Scheme 5.…”

“…[61][62][63][64][65][66][67] Another route for the synthesis of Si-B-C-N ceramics is by means of thermolysis of borazine-based oligosilazanes 2,6,[68][69][70][71][72][73] or boron-modified polysilylcarbodiimides. [74][75][76] Thus, a great variety of preceramic organosilicon polymers have been developed that could be used to produce ceramic materials with a wide range of compositions in the system Si-B-C-N-O and additional metallic constituents, such as Ti, Al and Zr.…”

Progress of a novel non-oxide Si-B-C-N ceramic and its matrix composites

“…Si-B-C-N ceramic is originally prepared by the organic polymer pyrolyzing route [1,2]. Generally, a kind of special organic polymer precursor, such as solid poly (boro)silazanes or polysilylcarbodiimides, is pyrolyzed at 1000 ℃-1400 ℃ in an inert atmosphere for several hours [3][4][5].…”

Amorphous Multinary Ceramics in the Si-B-N-C System

“…However, as side reactions, silazane cleavages and homocondensation of the borazine also take place. Therefore, the resulting polymer contains some repeating borazine units or borazinyl rings attached to the copolymer backbone [73,83,84].…”