Porous Polymer-derived Ceramics by Solution-based Freeze Casting for Capillary Transport and Catalysis

Abstract

The precise control of the pore structure is fundamental for the application of porous media. Freeze casting has proved to offer a great variety of possibilities to widely change the resulting pore structure and related properties such as the permeability and the mechanical strength. This work aims to develop porous polymer-derived SiOC monoliths for capillary transport and heterogeneous catalysis by adapting the processing parameters of the solution-based freeze casting and by altering the chemical composition of the starting solution. Hereby, tert-butyl alcohol and cyclohexane act as solvents and polysiloxanes are used as preceramic polymers. In the first part and as base for the subsequent conditioning, the fundamental relationships between the addition of preceramic (SiOC) and ceramic (silica, alumina) filler particles and the resulting pore structure and properties such as hydrophilicity, strength and specific surface area are elucidated. In the second part, the relation between the pore structure and the capillary transport behavior which is a key for an efficient design of capillary active components is investigated. To generate different pore morphologies and pore orientation, different solvents as well as two methods of freezing are used within this work: non unidirectional and unidirectional freezing. Isothermal wicking experiments at room temperature show that a high permeability results in fast wicking. Lastly and regarding the application in heterogeneous catalysis, the underlying principles and processes of generating a macroporous metal containing PDC monolith are investigated. Inherent catalytic active monoliths are successfully prepared by the novel combination of solution-based freeze casting with the generally known principle of in situ formation of nickel particles in SiOC matrices.