Jesper Brandt

Protein forming — a novel shaping technique for ceramics

Abstract A new consolidation method for the forming of ceramics using globular proteins as consolidators/binders was developed. Various protein products: bovine serum albumin (BSA), albumen (egg white powder) and whey protein concentrate (WPC), were shown to work as gelling agents, which was confirmed by rheological measurements and forming experiments. Suspensions of Si3N4, Al2O3 and ZrO2 powders with various solids loadings (44–55 vol.%) were prepared, proteins were added and the resulting slips were poured into simply shaped moulds and consolidation (gelling) took place through heating at 80°C. Through an optimisation of the process, it was possible to sinter to high density values (>99% of theoretical) using gas pressure sintering (GPS) of Si3N4 materials, and using pressureless sintering in air of the oxides. Among the globular proteins studied, WPC showed the most favourable properties with less serious slip thickening, limited foam formation, faster gelling, sufficiently high green body strength to make demoulding in wet state possible and less cracking during drying. However, a larger amount of WPC was needed to achieve a proper gelling compared with the other proteins. This negatively influenced the sintering conditions by lower density of shaped bodies, which required ceramic powders that sinter more easily to full density. The most critical steps in the processing were to efficiently remove air, to break foam formations, and to avoid cracking during the demoulding and drying of the cast bodies. None of the proteins gave rise to critical stresses during the burnout operation and sintering was carried out without deformation or cracking.

Sol-gel coating of alumina fibre bundles

Abstract A procedure for coating alumina fibre bundles with fibrillar boehmite particles is described. Fibrillar boehmite particles, having a specific surface area of 150 m2/g, were adsorbed onto the surface of the alumina fibres. The surface of the alumina fibres was first charge reversed by adsorption of a charge reversing agent. Polyvinylsulphate (PVS) proved to be an excellent charge reversing agent, whereas citric acid and Tiron (3,5-pyrocatecholdisulphonic acid disodium salt) were unable to reverse the surface charge of the alumina fibres. By using polyvinyl-sulphate (PVS), it was possible to adsorb an arbitrary number of layers of fibrillar boehmite particles. The coating of the alumina fibre bundles resulted in an increase in specific surface area from 0.09 m2/g for the uncoated fibres, to 0.7 m2/g for alumina fibres coated with four layers of fibrillar boehmite particles. The adsorbed boehmite particles were fixed onto the alumina fibre surface by calcination at 550 °C for 120 min, during which the boehmite was transformed into γ-Al2O3. The coated alumina fibre bundles were characterized by krypton adsorption and scanning electron microscopy (SEM).