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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: Processing of ceramic foams

Author: NANGREJO, Rafiqu

Year: 2001

Supervisor(s): Mohan Edirisinghe

Three new methods were investigated to produce ceramic foams from polymeric precursors. Firstly, polysilanes were pyrolyzed at 900°C. After pyrolysis silicon carbide (SiC) foams and micro-porous solids were obtained. The effect of the pyrolytic yield on the type of ceramic produced, its pore structure and shape retention were investigated. Polysilanes were blended in various ratios to control the pyrolysis process more precisely. This allowed the type, shape and pore-structure of the SiC foams produced to be controlled more efficiently. There existed a relationship between the composition and structure of the polysilanes and their pyrolytic yield at 900°C and this determines the type, shape retainability and pore-structure of the ceramics produced. Polysilanes or their blends which gave a pyrolytic yield of 50-60 wt.% produced the best SiC foams.

Secondly, a simpler method was investigated. In this approach polyurethane foams were immersed in polysilane precursor solutions to prepare pre-foams. Subsequently, these were heated in nitrogen at different temperatures >900°C. SiC foams produced in this manner retained the shape of the pre-foams and consisted of well-defined open-cell structures with struts free from surface cracks and a central hole. These defects are usually present in foams produced using ceramic slurries. The shrinkage which accompanies pyrolysis of the pre-foams was reduced by increasing the concentration of polysilane in the solutions. Porosity and average compressive strength of foams were in the range of 79-92% and 1.1 MPa, respectively.

Thirdly, the solution method was extended to the production of composite foams using suspensions. Several types of ceramic powders were added to the precursor solutions before immersing the polyurethane foams in the suspensions. Scanning electron micrographs show that these foams have open-cell structures and defect free status. However, some cracks were observed at higher levels of Si3N4, TiC and Al2O3 contents. The shrinkage was reduced considerably due to the addition of Si3N4, TiC and Al2O3 particles. SiC-Si3N4, SiC-TiC and SiC-Al2O3 composite foams prepared have overall porosity in the range of 80-95% and the average compressive strength in the range of 1.2 MPa-1.6 MPa, respectively.