Ceramic conversion

The polymer-to-ceramic conversion was performed in flowing ammonia to 1000°C (heating rate 25°C/h), then in flowing nitrogen to 1800°C (heating rate 100°C/h).29 30 Some of the properties of the final BN fibers are summarized in Table 4. 

Schwartz, R. W. Lakeman, C. D. E. Payne, D. A. 1990. The effects of hydrolysis conditions, and acid and base additions, on the gel-to-ceramic conversion in sol-gel derived PbTi03. In Better Ceramics Through Chemistry IV, edited by Zelinski,... 

These last two points concern how precursor design and synthesis are influenced by the precursor-to-ceramic conversion process. The next set of criteria identify materials properties that drive precursor design and synthesis including product selectivity. 

B[C2H4Si(R)NH]3 is mainly a function of the silicon-bonded moiety R, whereas the reaction pathway applied does not significantly influence the polymer-to-ceramic conversion. 

In contrast, thermal stability is not necessarily a direct function of the molecular structure. However, simply reducing the nitrogen content by performing the reaction with excess chlorosilanes is not possible, since the released polymers possess undesired large amounts of chlorine which lead to uncontrollable reactions during the polymer-to-ceramic conversion. 

Thermal treatment of the polymeric precursor (3P) and molecular precursors (4) result in an amorphous material (Scheme 2). After the precursor-to-ceramic conversion, a black, dense ceramic was obtained. The density of the material was determined to be 1.6 — 1.7 g/cm. One of our main interests was to reduce the accompanying formation of gaseous by-products during pyrolysis. As shown in Fig. 2, TG measurements up to 1500 °C reveal a weight loss of ca. 37% in the case of 3P... 

Fig. i Compositional diagram (volume fractions) of preceramic paper. Arrows indicate compositional change upon paper-to-ceramic conversion initial fiber fraction may transform to fiber template porosity upon sintering in air. 

Therefore PSE might be a useful model system for the study of the precursor-to-ceramic conversion, despite its relatively high costs. 

The use of polymer pyrolysis for the fabrication of bulk ceramics has been hindered mainly by the significant volume shrinkage (>50%) accompanying the polymer-to-ceramic conversion, often resulting in the formation of cracks which destroys the structural integrity of the ceramic component. 

Fig. 8.10 Photograph of the mixed-metal garnet single crystal by using solid-state ceramic conversion process (polished sample with thickness = 0.8 mm). Reproduced with permission from [65]. Copyright 2014, Elsevier...

It has been confirmed that the presence of bridge-type bonds in such poly (B-alkylaminoborazines) confers flexibility and an inaeased melt-spirmability, thus leading to the conclusions that melt-spinnability inaeases from 1- to 4-daived polymers. In addition, the ceramic yield is reduced with the increased proportion of -NlCHj) in the B-(alkylamino)borazine leading to important shrinkages during the furlha- polymer-to-ceramic conversion of green fibers derived therefrom. Hence, it is relevant to assume... 

P. J. Fazen, J. S. Beck, A. T. Lynch, E. E. Remsen, and L. G. Sneddon (1990), Thermally induced borazine dehydropolymerization reactions. Synthesis and ceramic conversion reactions of a new high-yield polymeric precursor to boron nitride, Chem. Mater. 2, 96. 

Abstract - Studies of the syntheses, properties and ceramic conversion reactions of the new processable borazine-derived polymer systems, poly(vinylborazine), poly(vinylborazine/styrene) and poly(borazylene), along with selected applications of these polymer precursors for the formation of boron nitride ceramics are described. 

The poly(B-vinyllborazine) polymers satisfy a number of the criteria previously identified for preceramic materials and their use as precursors to boron nitride was investigated. It was found that depending upon the polymer and pyrolysis conditions, a variety of ceramic materials may be produced, ranging from black, high carbon content materials to white h-BN. In each case, however, the polymer/ceramic conversion was found to take place with both high ceramic and chemical yields and give materials with B/N ratios of 1.0. 

TGA and DSC studies (Figure) of the polymer/ceramic conversions showed that the polymer has a well defined decomposition path, with the majority of the weight loss... 

Poly(B-vinylborazine) shows excellent conversions to boron nitride, but because of it.s carbon backbone, it requires pyrolysis under an ammonia atmosphere in order to achieve low-carbon boron nitride. In an effort to develop new boron nitride precursors that give improved ceramic yields and/or do not require the use of ammonia during the ceramic conversion step, we initiated investigations of the syntheses of alternative types of borazinc based polymers. 

---