Research Areas

Sol-gel Chemistry for Functional Materials

Monomeric and oligomeric siloxane precursors are used to modify surfaces of oxidic ceramic or to prepare three-dimensional polysiloxane networks, which exhibit ion exchange properties or act as starting material for thermal conversion to highly porous hybrid materials. Cross-linking of alkoxy siloxanes is performed via hydrolysis/condensation mechanism, whereby the structure of the network (e.g. branched or chain-like) can be influenced by the use of catalysts or by temperature, respectively. If the precursor contains a thermally degradable blowing agent cross-linking and foaming can be adjusted to take place at the same time leading to macroporous foam structures. For the preparation of metal containing siloxanes with a homogenous distribution of metal ions are siloxanes used with functionalised organic groups to complex the ions prior to cross-linking. By a subsequent pyrolysis step under an atmosphere of inert gas these sol-gel derived materials are converted to hybrid materials or SiOC ceramics with high specific surface areas and embedded metallic nanoparticles

Crack-free Surface Micropatterning by Micromoulding

Micromoulding of aqueous ceramic slurries is a fabrication method for generating well-defined micropatterns. Micropatterned structure sizes down to 10 µm can be replicated by using for instance via polydimethylsiloxane (PDMS) stamps from a pre-structured silicon wafer. After plasma treatment the generated negative microstructures on the PDMS stamp are coated with a droplet of ceramic slurry and subsequently pressed gently on a planar ceramic substrate. After gentle drying, the PDMS stamps can be removed. Hereafter, the microstructured samples are sintered in an oven. The complete processed has been optimised in our group to obtain crack-free structures of high quality.

Extrusion of Diaphragms and Capillaries with well-defined Mesopores

 The fabrication of defect-free ceramic diaphragms and capillaries by extrusion with homogeneously distributed pores with sizes smaller than 100 nm in diameter is investigated for the development of novel membranes or modular biological filters. Green bodies after gel-extrusion and drying. The pore size of the sintered ceramic capillaries is below 100 nm.

Advanced Freeze Casting: the Freeze Gelation Process

In this process, an aqueous ceramic suspension is prepared containing ceramic filler particles as well as a colloidal sol (usually SiO2). The prepared suspension is subsequently poured into an aluminium mould and cooled down below zero degrees Celsius. In the process of the water-to-ice-formation a sol gel transition is induced, resulting in an irreversible consolidation of the green body. The demoulding of the green body can therefore be easily carried out at room temperature without damage. As the sol gel transition is not reversible we call this process freeze gelation process (FGP). This highly versatile process allows the fabrication of any kind of complex geometry which can be obtained by a casting process. Therefore, a variety of applications are feasible. Due to the irreversible sol-gel-transition the green body features already a strength level which is suitable for applications at low loading conditions. Hence, sintering is not always necessary. This process for instance is also used for the fabrication of Biocers which consist of a ceramic framework containing living microorganisms. Applications in the field of waste water cleaning or algae breeding are other fields of on-going research. Furthermore, ceramic implant materials based on hydroxyapatite can be produced via the freeze gelation process. With the adjustment of composition and microstructure a specific biodegradation rate of the created complex shaped implants can be realised. Other applications range from the development of sound absorbing materials to ceramic matrix composites.

Direct Foaming

A simple and low cost processing route based on the direct foaming of emulsified inorganic powder suspensions has been developed over the last years in our group. In this novel process, an alkane phase is emulsified in a stabilised aqueous inorganic powder suspension giving rise to low alkane phase (LAPES) or high alkane phase emulsified suspensions (HAPES) depending on the dispersed phase content. If the parameters of the emulsified suspension and the environmental conditions are favourably adjusted, the evaporation of the alkane droplets during foaming leads to a time-dependent expansion of the emerging foam. Hereby, a controlled formation of highly porous microstructures with interconnected cells adjusted from low micro- up to millimeter scale can be achieved. Given the versatility of the process, it can be easily adapted to different compositions resulting in mechanically stable metallic or ceramic parts including graded structures with open porous interfaces.

Biomineralization Inspired Micromoulding Techniques

Biomineralization is the process by which nature governs the production of mineralized tissues. The fascinating shapes and outstanding material properties of mineralized tissues have inspired many researchers to mimic materials like nacre or bone. In our approach, we want to utilize principles that are derived from biomineralization for designing a novel approach for the fabrication of micromoulded ceramics. Our approach is based on the preparation of a polymer induced liquid precursor phase (PILP) from poly(acrylic acid) (PAA) and calcium carbonate. Depending on the concentration of the substrates, PILP droplets or PAA/calcium coacervates phase-separate from the bulk solution giving rise to droplets that can be micromoulded into a specific shape. Using this process, it should be possible to prepare microstructured ceramics without heat treatment. Accordingly, this process is suited to be used to produce microstructured ceramics in sensitive environments, and opens possibilities of in situ functionalization of ceramics.