Templating Approaches Using Natural Cellular Plant Tissue
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Approaches Using Natural Cellular Plant Tissue Peter Greil
Abstract Biological preforms such as plant tissue offer a novel approach for manufacturing biomorphous ceramics with an anisotropic cellular micro- and macrostructure pseudomorphous to the natural template structure. Mimicking the hierarchical microstructure 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 microstructure in biomorphous ceramics in order to improve mechanical properties at low density. 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 are 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.
(μm to nm) offers the possibility to tailor the local strut microstructure in biomorphous ceramics in order to improve mechanical properties at low density.12
Chemical Reaction Processing Prior to using natural plant tissue for templating, the extractives—low molecular fragments of bio-organic constituents of the cell wall—need to be removed. Otherwise, they may react with the infiltration solution, alter the pore structure, and decrease the thermal stability of the template.13 Extraction procedures generally include treating the wood pieces with organic solvents of varying polarity. Conversion of the bio-organic template into an inorganic material structure can be divided into direct replication of cellular anatomy, indirect replication of the vascular pore system, and multiphase composite formation, Figure 1. Direct replication involves a multistep process resulting in the mineralization of the 3D template. Mineralization may be achieved by intercalation of the cell walls with liquid precursor systems that may be an inorganic (metal salts), metal organic (metal alkoxides), or organometallic (direct metal to carbon bond) sol. After modification of the native cell wall structure (for example with maleic anhydride) to facilitate penetration of the precursor sol into the cell wall,14 acid- or base-catalyzed hydrolysis reactions trigger gel formation via an olation-, oxolation-, alcolation-, or alcoxolation-type of condensation reaction.15 350°C, inert Cellulose + Mn+(OR)n + atmosphere –n H2O → C + Mn+ On/2 ² + n ROH↑ + Gas↑ (II) >600°C, air
C + Mn+On/2 + O2 → MOn/2 + CO2
(III)
Chelating agents forming polybasic organometallic complexes from ino
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