Research Areas

Precursor derived Ceramics and Ceramers


Precursor derived ceramics - Ceramers

The conversion of polymeric precursors under an atmosphere of inert gas at moderate temperatures is a versatile processing route for non-oxidic ceramic materials such as SiC, Si3N4 or SiOC as compared to the much more energy consuming conventional sintering of SiC or Si3N4 powders. Generally the starting materials are inexpensive and can easily be processed using well-established shaping methods used for the processing of polymers. By varying the precursor composition or the temperature regime of pyrolysis or by addition of filler particles hybrid materials and advanced ceramics with interesting properties can be obtained. Porous or dense microstructures, high temperature, oxidation and chemical resistance or magnetic and electroconducting behaviour can be provided and engineered for different applications. The purification of exhaust gas using adsorbents is an important demand to reduce the global air pollution. By converting polysiloxane precursors partly into hybrid materials (Ceramers) with high specific surface areas, tuneable pore size distributions and adjustable surface characteristics can be obtained, which are the main requirements to provide new adsorbents with a high cleaning efficiency. One of the key issues is the specific modification of the surfaces by changing the chemical nature of the precursor and thereby the remaining surface groups that will influence the selective interaction with the adsorptive. The investigation and efficient use of surface-substrate interactions are of even higher demand for the development of porous heterogeneous catalysts due to the more complex reaction pathway on the surface of a metallic active site. With the manufacturing of hierarchical ordered foamed or particle hybrid materials containing in-situ generated metallic nanoparticles some drawbacks of conventional supported catalyst are expected to be overcome. For instance, the long-term stability should be increased due to a stronger particle-matrix interaction that avoids extensive sintering of the metal at higher operation temperatures. At the same time, the adjusted porosity should provide a good accessibility of the catalyst particles and a fast heat and mass transfer leading to a controlled catalysed reaction with an increased selectivity towards the desired product. The supply of new materials for a sustainable power generation is one of the most urgent aims of the new century addressing the ebbing of fossil fuels. High-temperature polymer electrolyte fuel cells are discussed to be a promising, resource-saving technology. However, this technology requires proton conducting and temperature resistant (200°C) polymers, which are rare among organic materials. Functionalised and just cross-linked polysiloxanes exhibit these properties and could therefore substitute conventional proton conductors. High conductivities are generally reached for materials using a water based conduction mechanism, which collapses, however, at higher operation temperatures due to the evaporation of water. Functionalised polysiloxanes with organic groups transferring the protons without the help of water are promising candidates to solve this problem. An extension of this development is the production of membrane-electrode-assemblies using electroconducting hybrid materials also based on polysiloxanes as electrocatalyst which are expected to show an improved interaction at the three phase boundary of these novel fuel cell materials.


Hybrid material adsorbents

Surface characteristics of hybrid materials like hydrophobic or hydrophilic properties can be influenced, for instance, by the chemical nature of the siloxane precursors or by the applied pyrolysis temperature. Due to a pronounced hydrophobic nature of surfaces and an adjusted pore size distribution bulk or foamed hybrid materials can be used for volatile organic compounds (VOC) control. Under specific operation conditions, e.g. conditions where only moderate regeneration temperatures are available, hybrid materials are even superior to conventional adsorbents such as activated carbons, which often show the disadvantage of an incomplete regeneration.


Preparation and development of catalytic active hybrid ceramics

For catalysis based on expensive noble metals it is required that the catalyst is homogeneously dispersed as nanoparticles and easily accessible for reactants resulting in an efficient degree of usage. Furthermore, the catalyst should stay dispersed which requires a stable embedding in the supporting matrix. An additional aspect concerning the support geometry is the usage of foams which combines the advantages of traditionally used honeycomb and pellet supports. In order to meet these requirements foamed polysiloxane based hybrid ceramics are prepared as supporting material containing gold, platinum or other metals as catalysts. The conversion of polysiloxanes to hybrid ceramics is realized by inert gas pyrolysis at temperatures of 400-600°C and in order to adjust material properties beneath the pyrolysis temperature the siloxane precursors are varied. As precursor for the catalytic active metal particles the corresponding metal salts or preformed nanoparticles are used.


Polysiloxane based Materials for High-Temperature-Polymer-Electrolyte-Membrane-Fuel Cells

This research project focuses on the development, manufacturing and characterization of functional materials for the use in high temperature polymer electrolyte membrane fuel cells with temperatures up to 160°C. The advantages of higher temperatures include better heat management, higher efficiency of electrocatalysis and better tolerance towards the contamination by carbon monoxide. For this application functionalized siloxanes are synthesized and proton conducting membranes are formed by a sol-gel process. These membranes will be combined with electrocatalysts which are also based on siloxanes and which are joined together in our institute to membrane-electrode-assemblies.