Biomorphous SiOC/C-ceramic composites from chemically modified wood templates

Anisotropic, biomorphous SiOC/C-ceramic composites with different porosities and designed microstructure were manufactured from native beech and pine wood. In a first step, biotemplate/polysiloxane composites were prepared by infiltration and reaction of the wood preforms with a Si-H functionalised preceramic polymer (polymethylhydrosiloxane-PMHS). Curing of the infiltrated PMHS was achieved by temperature treatment at 120 °C for 12 h. Subsequent pyrolysis of the biopolymer/polysiloxane hybrid materials in inert atmosphere at 800 °C yields a biomorphous SiOC/C-ceramic composite. FTIR, TGA and SEM analysis was applied to monitor structural changes and phase formation processes during thermal treatment. The esterification of the wood with maleic acid anhydride (MA) was used to alter the chemical properties of the wood cell wall and to introduce CC-double bonds for further reaction with PMHS. The PMHS-infiltration reduced the anisotropic shrinkage associated with the thermal decomposition of the biopolymers compared to the native wood. MA-modified, PMHS-infiltrated samples exhibit an improved ceramic yield after pyrolysis, which may be attributable to a facilitated penetration of the PMHS into the wood cell wall. Additionally, the influence of low-molecular weight wood compounds on the PMHS-infiltration as well as on the microstructural evolution was assessed by extraction techniques. Extraction of the low-molecular weight wood compounds yielded an additional porosity and finally void formation inside the SiOC-phase, which stabilizes the specimen during the pyrolysis and prevents the sample from cracking.