Nanoporosity in Ceramics from Polymeric Precursors
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A.W. REID, B. RAND AND R.J.P. EMSLEY School of Materials, University of Leeds, Leeds LS2 9JT, UK. ABSTRACT It is shown that ceramics derived from polycarbosilane polymers may develop an open nanoporous network after heat treatment to a temperatures between 1300 and 1550'C in argon. The resulting SiC-based ceramics were characterised by N2 gas adsorption analysis and X-ray diffraction. The apparent surface area, and pore volume increase with increasing heat treatment temperature, reaching values of 170 m2g-' and 0.12 cm 3'- respectively. The pore network develops as the SiC crystals grow and as carbon is ejected from the structure. It is thought that the porosity may reside within the carbon phase, but this remains to be confirmed.
INTRODUCTION Polymers such as polycarbosilanes and polysilazanes are used in making fibres [1], matrices for composites and binders for SiC particles [2]. Fibres are usually stabilised by oxygen cross-linking prior to heat treatment to retain morphology. Polymers which undergo thermal cross-linking, and therefore do not need oxidative stabilisation, usually cannot be melt spun since they degrade during spinning but may be suitable for other applications. However, because of the presence of highly reactive Si-H groups, these polymers may also contain a certain amount of oxygen through reaction with the ambient atmosphere.
The ideal 1:1
stoichiometry of Si and C is never obtained in pre-ceramic polymers due to the presence of carbon containing side chains such as phenyl and methyl groups. Nicalon for example contains 5% free carbon [3]. The effect of carbon, in carbon rich polymers, on the thermal degradation of resultant ceramics has been investigated by Delverdier et al [4]. They conclude that excess free carbon need not be avoided since it inhibits SiC crystal growth. The effect of oxygen is more deleterious. As a consequence of oxygen content, high temperature reactions take place in which CO and SiO gases are evolved [1]. Crystallisation and grain growth also occur at high temperature and this is well documented for "Nicalon" fibre [5]. Lipowitz [6] characterised ceramic fibres, including Nicalon, by low angle X-ray scattering and observed an internal closed porosity due to globular nanopores. As the density of these fibres increased with heat treatment, an increase in average pore size and a reduction in pore volume fraction was observed. In this study we have investigated ceramics derived from polymeric precursors by N2 gas adsorption and it is shown that the porosity that develops can be open and often of nano size. Developments in porosity accompany the changing SiC crystal structure.
843 Mat. Res. Soc. Symp. Proc. Vol. 346. 01994 Materials Research Society
EXPERIMENTAL
The polymer investigated, designated PSS 400, was supplied by Nippon Soda Company. The technical data supplied describes it as a polysilastyrene but FTIR analysis suggests it is chemically closer to a polycarbosilastyrene. Its weight average molecular weight was measured as 3200 by GPC analysis. It contains r
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