Cellular Ceramics / p2
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132 Part 2 Manufacturing
sisal, jute, and hemp fibers with AlCl3 and TiCl4, respectively. Ota et al. [54] produced biomorphous oxide ceramics by infiltration of Japanese wood species with titanium isopropoxide. After high-temperature treatment in air the wood structures were converted into porous TiO2 ceramics. Shin et al. [55] synthesized biomorphous SiO2 ceramics from wood by a surfactant-templated sol–gel process. Rattan palm preforms were converted to biomorphous Al2O3, ZrO2, and mullite (Al6Si2O13) ceramics by a sol–gel process with metal alkoxides, metal oxide chlorides, and SiO2 nanopowders [56]. Singh and Yee [57] infiltrated jelutong wood with ZrO2 sol for manufacturing monoclinic ZrO2 ceramics with biomorphous structure. Highly porous, biomorphous Al2O3, TiO2, and ZrO2 ceramics were prepared from pinewood and from cellulose fiber preforms by a sol–gel process with metal alkoxides [58–60].
As an example, the manufacture of microcellular oxide ceramics from pinewood will be described [58]. The process starts with infiltration of the dried native pine specimen with liquid organometallic precursors. Low-viscosity, stable oxide sols were prepared for the infiltration process. The properties of the different sols (e.g., concentration, viscosity, stability) had to be adjusted for optimal infiltration behavior into the pinewood preforms. After sol infiltration the samples were dried in air to form in situ gels of the respective oxide. This procedure was repeated several times to increase the precursor content in the raw samples. Then the samples were pyrolyzed at 800 C to carbonize the wood template and decompose the organometallic precursor into the respective oxide. Subsequent infiltrations were performed into the pyrolyzed pinewood templates. For consolidation and sintering into microcellular oxide ceramics, the specimens were annealed in air at temperatures up to 1550 C. The processing scheme is summarized in Fig. 8. The final oxide ceramics are pseudomorphous to the initial pinewood. Additional infiltration processes, followed by repeated annealing, were performed to increase the density of the final materials and to fill the pores left by burning out the CB template.
Figure 8 Processing scheme for manufacturing microcellular oxide ceramic from wood (after [58]).
2.5 Microcellular Ceramics from Wood 133
The technique was used for conversion of pinewood samples to biomorphous Al2O3, TiO2, and ZrO2 ceramics. After processing, the biocarbon of the pine chars was completely burned out to give brightly colored biomorphous oxide ceramics maintaining the macroand microstructural features of the biological template. However, due to Ca impurities in the native pinewood, small amounts of mixed Ca oxides were also observed in the microcellular oxide ceramics [58].
Figure 9 illustrates the cellular microstructure in the axial direction of biomorphous oxide ceramics derived from pinewood. The initial cellular anatomy was reproduced in the ceramic products. In the biomorphous Al2O3 ceramics, burning the carbon out of the pinewood cell walls left small holes in the earlywood regions, where the cell walls of the specimens were not completely consolidated. Due to the smaller cell diameter and thicker cell walls in the earlywood region, the morphology is characterized by a hollow-fiber structure, separated by voids of up to 5 mm in size. In contrast to Al2O3, the cell walls of pinewood were replaced by dense layers of TiO2 in the biomorphous TiO2 ceramics. Most of the vessels are kept open after conversion to ceramic. The microstructure of biomorphous ZrO2 ceramics looks similar to that of biomorphous Al2O3 and TiO2 ceramics. The size of the Al2O3, TiO2, and ZrO2 grains after sintering is about 3–5 mm. Similar results were obtained by using rattan palms as bioorganic template structures [59, 60] or cellulose fiber felts [61].
SEM images: microcellular Al2O3, TiO2, and ZrO2 ceramics derived from pinewood after annealing at 1550, 1200, and 1500 C, respectively, for 1 h in air (see also [58]).
134 Part 2 Manufacturing
Table 2 summarizes the material properties of biomorphous oxide ceramics made from pinewood. The porosity was calculated from the difference between the geometrical and skeletal densities measured by He pycnometry. The skeletal densities of biomorphous Al2O3 and TiO2 ceramics are very close to the theoretical density of Al2O3 (3.97 g cm–3) and TiO2 (4.26 g cm–3), and they indicate only small amount of closed porosity. Due to the content of monoclinic ZrO2 (theoretical density 5.68 g cm–3) and impurities, the skeletal density of the biomorphous ZrO2 is only 5.81 g cm–3 (theoretical density 6.7 g cm–3).
Table 2 Density and porosity values of biomorphous oxide ceramics derived from pinewood by infiltration with different organometallic precursors (after [59]).
Oxide |
Density [g cm–3] |
|
Porosity [%] |
|
|
|
|
|
geometrical |
skeletal |
|
|
|
|
|
a-Al2O3 |
0.22 |
3.93 |
94 |
Rutile TiO2 |
0.86 |
4.26 |
80 |
Cubic ZrO2 |
1.11 |
5.81 |
81 |
2.5.5
Summary
Microcellular carbide and oxide ceramics with cell diameters from a few up to several hundred micrometers can be manufactured from bioorganic structures by utilizing the open-porous morphology of wood materials and applying different bioor- ganic-to-ceramic conversion technologies. Due to the unidirectional pore morphology in the native wood templates, biomorphous ceramics exhibit pronounced anisotropy in physical properties such as penetrability, heat transfer, electrical conductivity, and mechanical stiffness, and are thus suitable for different applications as high-temperature or corrosion-resistant materials.
Biological templates from wood and wood-derived materials are available on a large scale, and they are cheap and regenerable. Wood and products derived therefrom, such as paper and cardboard, can be easily machined and preformed into complex three-dimensional shapes of different porosities and cellular morphologies. The large variety of natural plant tissues, the established forming and shaping technologies for wooden materials, and the utilization of different conversion technologies facilitate the manufacture of microcellular ceramics and ceramic composites with tailored microstructures and compositions.
2.5 Microcellular Ceramics from Wood 135
Acknowledgements
Financial support from the Volkswagen Foundation under contract I/73 043 is gratefully acknowledged. H.S. wants to thank C. Zollfrank, C.R. Rambo, E. Vogli, J. Cao, and P. Greil for many helpful discussions during preparation of biomorphic ceramics.
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