The major fields of Peter Greil´s research include fundamental and applied aspects of non-powderous processing of ceramics and composites from polymers and biological preforms. Current research is focused on microstructure evolution and properties of cellular ceramics and on crack healing of ceramic composites.
Polymer-Filler Derived Ceramics
Tailoring of composition and molecular structure combined with excellent thermo-plastic shaping technologies offers a great potential for development of advanced materials from organo-silicon based polymer precursor systems. Polymer derived ceramics with compositions in the system Si-C-N-O(-M) with M = B, Al, Ti, Zr, etc. offer excellent thermal stability as well as interesting electrical, piezo-electrical, magnetical, optical and mechanical properties. Enhancement of mechanical properties in bulk components of larger volume, however, mainly faces two problems: i. control of the nano- and microstructure evolution under the constraints of limited transport of gaseous decomposition products and ii. retardation of strain relaxation upon polymer-to-ceramic conversion (e.g. viscous to brittle elastic behaviour transition).
Basic principle of volume displacive High precision manufacturing of a vacuum pump
filler controlled polymer to ceramic rotor from polysiloxane/filler system showing dense
transformation surface and porous core microstructures
In his pioneering work Peter Greil has successfully demonstrated that the reduction of porosity and dimensional changes associated with the polymer to ceramic conversion may be achieved by loading reactive fillers (metals, intermetallics). Upon thermal degradation the fillers may react with the solid polymer residue and the gaseous decomposition products to form carbides, nitrides, oxides, or mixtures thereof. Volume dilatation of the polymer phase may be compensated by an appropriate expansion of a filler reaction phase. Grain boundaries between the filler reaction phases are formed which generate a rigid skeleton stabilizing the shape and size of a component during thermally induced transformation from polymer to ceramic. Furthermore, release of volatile decompositions products which cause pore formation is reduced and yield of ceramic residue increases significantly. Compared to sintering of ceramic powders which suffers from a linear shrinkage of approximately 15 – 25 % reactive filler controlled polymer derived ceramics attained values of less than 0.05 %.
Current work addresses microstructure evolution and properties of low density cellular ceramics formed by in situ blowing reactions. Healing of surface defects was demonstrated by reaction of a reactive filler phase with an environmental vapour phase (air or nitrogen). Due to the formation of a microstructure gradient with a dense surface reaction layer of approximately 10 – 100 µm covering the porous core material pores and cracks in the reaction layer were healed and superior strength improvement was attained even without surface machining.
Greil´s research on biomorphous ceramics centres on transforming cellular biological preforms such as plant tissue into ceramics with an anisotropic cellular micro- and macro-structure pseudomorphous to the natural template structure. Mimicking the hierarchical micro-structure of the native template at different length scales from large vessels (mm) down to a cell wall microstructure (μm to nm) offers the possibility to tailor the local strut micro-structure in biomorphous ceramics in order to improve mechanical properties at low density. Greil studied physico-chemical routes of manufacturing biomorphous ceramics from plant tissue of various anatomy. Mineralization may be achieved by intercalation of the cell walls with an inorganic, metal organic, or organometallic sol. Heating above the pyrolysis temperature of the hydrocarbons forming the cell wall material in an inert atmosphere finally results in a positive replica of the cellular structure with a metal oxide/carbon composite forming the cell walls. Amorphous, nano- or microcrystalline C/Si-O-C(-N) composite materials were obtained by infiltration with a low viscosity preceramic polymeric precursor, such as polycarbosilane, -silazane, -siloxane, or a copolymer or mixture thereof. Pyrolysis into a biocarbon template and subsequent metal alloy melt or vapor infiltration and reaction at high temperatures above 1000°C is an alternate way to produce single and multiphase carbides and composites.
Multilayer biopolymer composite cell Biomorphous SiC ceramic derived from pinus silv.
wall microstructure of a plant cell by a vapour phase infiltration-reaction process
Based on his fundamental research results potential applications stimulated by a huge diversity of natural template structures with respect to pore volume (30–90%), pore diameter (<0.1–1000 μm), and pore shape (cylindrical, polygonal) were evaluated. While gymnosperms such as Pinus Silv exhibit monomodal tracheidal pore size distribution with a maximum lumae diameter of approximately 20 μm, angiosperms such as Quercus Robur typically contain a multimodal pore size distribution, with pore channel diameter maxima at 0.08 μm, 1–10 μm, and 200 μm (depending on the growth conditions). Due to the accessibility of large surface area coating with functional layers, catalysts such as zeolites and bioactive layers such as bioglass were explored. Furthermore, pulp fibers from wood processing were investigated for shaping of corrugated and light-weight ceramic structures (so called Preceramic Paper Derived Ceramics).