Fabrication and Characterization of Preceramic Polymer Derived Macroporous Silicon Oxycarbide

Abstract

Polymer derived porous silicon oxycarbide (SiOC) ceramics show higher heat resistance than that of SiO2, and are stable in their microstructure and composition in air even at temperatures up to 1200 oC. The structure of SiOC is intermediate between SiC and SiO2 with the chemical formula SiCxOy that exhibits improved stability against decomposition,resistance to crystallization, and outstanding behaviour in oxidative and corrosive environment. Because of such excellent properties SiOC finds application as thermal and structural composites, catalysts and catalyst support, gas adsorbents, filters, and electrode materials for Li-ion battery. There exists various methods of fabricating highly porous SiOC ceramics, including sacrificial templating, direct foaming, and replica technique. Foaming methods suffer from gradient in porosity along the foaming direction, while hollow struts are the characteristic feature of replica methods that degrade the mechanical strength. There are very few reports on the fabrication of highly porous ceramics (porosity ~90%) with dense struts by sponge replication methods. Likewise, there are various methods of fabricating hierarchically porous ceramics that include emulsion technique, soft templating, etching techniques, and freeze casting. However, most of the methods involve multi-step synthesis approach. Thus, it is a challenge to produce hierarchically porous SiOC ceramics in a one-step synthesis approach. Our approach has been to develop highly porous ceramics using positive sponge replication methods and subsequently creating 1D nanostructures by catalyst assisted pyrolysis, thus generating hierarchically porous SiOC ceramics. The first section of results and discussions part analyzes the formation of dense and hollow struts in macroporous SiOC ceramics. We have used a simple method for fabricating macroporous SiOC ceramics by positive sponge replication method to generate porosity in the range 90-95%. The obtained macroporous SiOC ceramics, with dense struts, are thermally stable upto 1200 oC, and have higher compressive strength than similar ceramics with hollow struts. In the next section, the compressive strength of macroporous SiOC ceramics has been quadrupled by increasing the initial viscosity of preceramic polymer solution. One of the most important aspects of the current work is the use of synchrotron
X-ray micro tomography to qualitatively and quantitatively evaluate a plethora of porostructural features, including porosity, pore volume, pore size, strut size, strut density, pore interconnectivity, degree of anisotropy, sphericity, and pore orientation.
The subsequent part of the thesis deals with the fabrication and characterization of hierarchically porous SiOC ceramics. We have made an attempt to generate pore hierarchy by growing one dimensional nanostructures on the strut/cell wall of macroporous SiOC ceramics. The growth of 1D nanostructures were carried out by catalyst assisted pyrolysis, using different metal catalyst (Co, Fe, Ni). Different type of nanostructures were observed by using different catalyst and the possible growth mechanisms were explained. However, we studied in detail the growth of 1D nanostructure using Ni catalyst on reticulated porous SiOC ceramics. It was shown that by varying pyrolysis temperature, and nickel concentration, the length and volume of nanostructures can be varied. We could observed 1D nanostructures of SiOC with an amorphous coating. A unique growth mechanism was proposed for the growth of SiOC nanostructure encapsulated by multiwalled carbon nanotubes. Thus, overlapping of such nanostructures led to the formation of hierarchically porous SiOC ceramics.
The major outcomes from the thesis include, the formation of macroporus SiOC ceramics with dense struts, the ability to manipulate the process conditions to fabricate macroporous SiOC bodies with varying porosity, strut dimension, and mechanical properties, and the fabrication of hierarchical porous ceramics. The former finds excellent applications as porous burners, and materials for sound absorption, while later can serve as an excellent toxin adsorption from contaminated water and also as high temperature stable catalyst support.