Design and synthesis of polyboro(metal)silazanes for polymer derived SiB(M)CN ceramics

Abstract

With the advent of high-speed aircraft, advancements in defense technologies, and the burgeoning field of aerospace technology, there has been a marked increase in interest towards polymer-derived advanced ceramics as potential alternative materials. Among these, SiB(M)CN ceramics have garnered attention due to their exceptional thermal stability, superior mechanical properties, and promising functional applications, positioning them as critical function-structure materials. Nevertheless, the exploration and application of SiB(M)CN ceramics encounter significant obstacles, including the ambiguous relationship between molecular structure and ceramic properties, a scarcity in the discovery of new functionalities, and an inefficiency in methods for metal incorporation. This thesis addresses these challenges by focusing on the design of the molecular structure of polyborosilazanes. Through systematic characterization and analysis, it delves into the intricate relationships between molecular structure and ceramic properties, explores the functionality of polyborosilazanes and their resultant ceramics, and provides insights into the considerations necessary for the molecular structural design of polyborosilazanes.
Polyborosilazanes PBSZ-P and PBSZ-T were synthesized by incorporating pinacolborane and trimethyl borate into the OLPS
matrix. Both polyborosialzanes are soluble in solvents such as xylene, tetrahydrofuran, and chloroform, with initial melting points of 74.9 °C and 77.1 °C, respectively. PBSZ-P is characterized by a Si-N-Si backbone, interconnected by Si-O-B bridges, and further incorporates a five-membered pinacolborane ring structure. Conversely, PBSZ-T is composed of repeating units of Si-N-Si, Si-N-B, and Si-O-B. Upon heating in nitrogen to 1000 °C, PBSZ-P and PBSZ-T undergo ceramization, yielding ceramic residues of 72.0 wt% and 68.4 wt%, respectively. The ceramic products of both PBSZ-P and PBSZ-T begin to exhibit crystallization of α-Si3N4 and h-BN at temperatures starting from 1600 °C under a nitrogen atmosphere.
UV-curable polyborosilazanes (UV-PBSZs) were synthesized utilizing dichloromethylsilane (DCMS), dichlorovinylmethylsilane (DCMVS), hexamethyldisilazane (HMDS), and a borane sulfide complex. The molecular architecture of UV-PBSZs features a Si-N-Si main chain, with methyl and vinyl groups as side chains and boron atoms serving as interchain linkages. The presence of vinyl groups enables the UV-PBSZs to be cured through UV irradiation. The mechanisms and processes of UV curing were thoroughly investigated, revealing that curing mechanisms can be primarily divided into two categories: the initiation of vinyl cyclization and polymerization through carbon-
based free radicals, and the facilitation of addition and polymerization reactions by silicon-based free radicals. Remarkably, the ceramic yield of UV-cured UV-PBSZs exhibited a significant increase, with the highest observed yield reaching 69.3 wt% from an initial 9.0 wt%. The ceramics derived from UV-PBSZs demonstrated exceptional thermal stability and a pronounced resistance to crystallization, maintaining an amorphous structure even after exposure to temperatures up to 1600 °C, with only trace amounts of SiC, β-Si3N4, and BN detectable.
Polyborotitanosilazanes containing titanium were synthesized through a Lewis neutralization reaction between the titanium chloride (Ti-Cl) moiety from titanium tetrachloride and the amine hydrogen (N-H) from an oligomer of PBSZ. This method effectively incorporated titanium atoms into the precursor molecules, resulting in a molecular structure characterized by a networked, branched polymer. At the core of this structure are titanium atoms, with a Si-N-Si main chain and side groups that include hydrogen, methyl, and vinyl. The maximum ceramic yield of PBTiSZ was found to be 73.8 wt%. The crystallinity of SiBTiCN ceramics showed a direct correlation with the titanium content; at a pyrolysis temperature of 1000 °C, samples with low titanium content remained amorphous, while those with higher titanium content displayed weak crystallization of TiN. Upon sintering at temperatures exceeding 1600 °C, the primary crystalline phases in low titanium content samples were identified as SiCN and TiN, whereas samples with higher titanium content also exhibited SiCN, TiN, and TiB2 phases. The presence of TiB2, alongside TiN, contributes to the formation of an ultra-high temperature ceramic, enhancing the thermal stability of the entire ceramic material. Moreover, the inclusion of TiN and TiB2 has been shown to improve the electromagnetic wave absorption capabilities of the ceramics. As the titanium content increases, so does the efficacy of electromagnetic wave absorption, with the highest observed absorption rate being 43.5 dB in the SiBTiCN10-1600 sample.

Date of Award	17 Mar 2025
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Xiaosu Yi (Supervisor), Ping Cui (Supervisor), Liu He (Supervisor) & Yujie Song (Supervisor)