Biomimetic synthesis of ceramics and ceramic composites

Materials Science and Engineering A 412 (2005) 43–47

Biomimetic synthesis of ceramics and ceramic composites H. Sieber ∗ University of Erlangen-Nuremberg, Department of Materials Science, Glass and Ceramics, D-91058 Erlangen, Germany Received in revised form 11 April 2005

Abstract The processing of biomorphous ceramics and ceramic composites represents an advanced concept for manufacturing of porous or dense ceramic materials with microcellular morphologies. The cellular anatomy of naturally grown plants as well as there derived products such as cellulose fibre felts, cloth, papers and cardboard provide an attractive template for the design of materials with hierarchically ordered structures on different length scales that cannot be processed by conventional processing technologies. The inherent cellular and open porous morphology of the bioorganic materials is easily accessible for liquid or gaseous infiltrants of different compositions. Using high-temperature reaction processes, the bioorganic structures can be converted into oxide or carbide-based biomorphous ceramics within reasonable time, maintaining the morphological features of the native template. © 2005 Published by Elsevier B.V. Keywords: Ceramic; Cellular anatomy; Morphology

1. Introduction Plant materials, such as wood, palms and grasses, are natural, bioorganic composites and exhibit porous, anisotropic morphologies with excellent strength at low density, high stiffness, elasticity and damage tolerance on the micro as well as on the macro-scale [1,2]. In contrast to most technically designed materials, the morphology of bioorganic materials is characterized by a hierarchically built anatomy with microstructural features ranging from the millimeter (e.g. growth ring pattern of wood, papers) to the micrometer-scale (cellulose fibre structures). By using such materials as well as there from derived technical products as structural templates, a large variety of microstructural designed ceramic and composite materials can be processed (Fig. 1). Different biotemplating technologies for conversion of bioorganic, preformed materials into structural ceramics and ceramic composites have been developed in the recent years [2–4]. Generally, they can be divided in reactive and molding techniques. The reactive techniques involve conversion of the bioorganic preforms in biocarbon templates (CB -templates) by pyrolysis in inert atmosphere and subsequent the reaction of the char with

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Si or metals into carbide phases (e.g. SiC or TiC). The molding techniques, on the other hand, reproduces the microstructural morphologies of the bioorganic template by coating of the internal surfaces with low viscous oxide sols followed by annealing in air for burn out of the CB -template and consolidation of the oxide layers. The latter technique was applied for manufacturing of highly porous, biomorphous TiO2 -, Al2 O3 -, mullite(Al6 Si2 O13 ) or ZrO2 -ceramics (Fig. 2). The ceramic conversion of wood derived materials such as cellulose fibre felts, preprocessed papers or corrugated cardboard structures follows a similar approach. By a combination of the transformation and the substitution technique, light-weight carbide/oxide ceramic composites were processed from preformed corrugated cardboard materials. The low cost of the raw materials as well as the availability of well-established paper processing offer an economic way to manufacture light-weight ceramic composites with a uni- or bidirected pore structure [5,6]. The conversion includes the dip-coating of macrocellular templates of corrugated cardboard with cell diameter of a few millimetres with Al/Si-containing slurries followed by the reaction of the Al–Si above the melting temperature with the carbonised perform and resulted in SiC–Al2 O3 –mullite-ceramics after oxidation in air. The cardboard processing showed only minor shrinkage, which facilitates near-net shape manufacturing of large-scale ceramic devices such as panels or corrugated structures. The aim of the present paper is to discuss the technological

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Fig. 1. Conversion of bioorganic materials into structural ceramics and composites: materials.

Fig. 2. Conversion of bioorganic materials into structural ceramics and composites: processing.

approaches for the different biomimetic conversion methods as well as characteristic properties and applications of the resulting biomorphous ceramics and ceramic composite materials. 2. Biomorphous carbide ceramics The reactive conversion of bioorganic materials into carbidebased ceramics follows, in general, a two-step processing: (i) the decomposition of the bioorganic material into a carbon char by pyrolysis in inert atmosphere at temperatures above 800 ◦ C and (ii) the infiltration of the porous biocarbon char at temperatures above 1450 ◦ C with different Si-containing or above 1200 ◦ C with Ti-containing precursors and reaction into SiC- or TiCceramics, respectively. During pyrolysis a significant weight loss in the range of 70–80 wt.% and a shrinkage between 20 and 40% is observed, depending on the kind of bioorganic material [7]. However, despite the weight loss and shrinkage during pyrolysis, the micromorphology of the bioorganic materials is maintained after carbonisation. The use of preprocessed natural materials such bioorganic powders from saw dust or cellulose fibre, plywood or fibre board as well as the infiltration with carbon-containing liquids can reduce the anisotropy as well as the amount of the shrinkage [8–10]. The subsequent conversion into carbide-based ceramics by infiltration with liquid or gaseous Si or metalcontaining precursors does not involve significant geometrical changes and can be viewed as a near-net shape manufacturing [11]. Nearly dense SiSiC-ceramic composites with microcellular morphology were prepared by liquid Si infiltration of the

carbonised biotemplates similar to the conventional liquid silicon infiltration (LSI) processing [12]. Byrne and Nagle [13], Greil et al. [7] as well as Martinez-Fernandez et al. [14,15] and Singh [10] converted different kinds of wood structures by infiltration of the pyrolysed wood preforms with a Si-melt into cellular SiSiC-composites. After the reactive Si-infiltration at temperatures between 1450 and 1600 ◦ C the small pores of the CB -template up to a pore diameter of approximately 50 ␮m are filled up with residual Si. The Si-content of the final cellular SiSiC-ceramic composite as well as the total porosity and phase distribution depend mainly on the morphology of the used biocarbon preform. Fig. 3a shows a pine wood derived SiSiC-ceramics composite, that exhibits only a small amount of residual porosity (about few vol%), but a large amount of unreacted Si (about 50 vol%). Due to the high carbon density in the late wood areas, the conversion of pine char into ␤-SiC is not completed and unreacted carbon of few vol% still remained [16], centre-left side in Fig. 3a. Single-phase, highly porous, biomorphous SiC-ceramics were manufactured using Si-containing vapours as reactants for the pyrolyzed bioorganic chars. The Si-containing vapours were forced to penetrate the pores of the CB -templates and react with the biocarbon to form ␤-SiC. Different reactive, Si-containing vapour sources such as Si [17], SiO [18] or CH3 SiCl3 (MTS, methyltrichlorosilane) [19–21] were applied, resulting in biomorphous SiC-ceramics with controlled porosity and adjustable mechanical strength, depending on the type of bioorganic material (e.g. wood type, cellulose fibre paper) and the applied processing parameters (e.g. infiltration temperature, vapour pressure, gas species). In contrast to the Si-melt

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Fig. 3. SEM micrographs of biomorphous SiC- and TiC-based ceramics derived from bioorganic preforms by reactive melt and gas phase infiltration in inert atmosphere: (a) Si-melt infiltration of pine wood at 1550 ◦ C [15], (b) SiO-gas infiltration of oak wood at 1600 ◦ C [18], (c) processing of Si-filled cellulose fibre paper at 1400 ◦ C [22] and (d) TiCl4 -infiltration of pine wood at 1250 ◦ C [30].

infiltration, no residual Si was observed in the biomorphous SiCceramics after reactive infiltration with Si-containing vapours (Fig. 3b). While the Si-melt infiltration yield SiC-grains of up to 20 ␮m in size, the vapour–solid reactions yield SiC-grains in the submicrometer range [5]. The reactive conversion of Si-powder filled cellulose fibre papers [22] (Fig. 3c), Si-organic polymer [23] or SiO2 -gel/SiO2 nanopowder [24,25] infiltrated biocarbon templates represents a processing route where only one high-temperature reaction step has to be applied. However, the carbothermal reaction of the biocarbon template with SiO2 often yields a low mechanical stability of the obtained SiC-material, due to the formation of volatile CO and CO2 [24,25]. While most investigations in the recent years deal with the synthesis of biomorphous SiC-based ceramics, only few investigations were performed for conversion of bioorganic materials into TiC-based ceramics. Ashitani et al. investigated the synthesis of TiC-based ceramics from Ti-powder filled woody materials by self-propagation high-temperature synthesis [26]. In contrast to the Si-vapour processing, the reactive infiltration of carbonised wood structures with pure Ti-vapour in vacuum at 1600 ◦ C only yields a small surface layer of TiC of few 100 ␮m in depth, due to the clogging of the pores with TiC [27]. Highly porous, biomorphous TiC-ceramics were prepared by infiltration of wood char with tetrabutyl titanate and high-temperature carbothermal reaction in inert atmosphere at temperatures above 1400 ◦ C [28]. The CVI-R processing (chemical vapour infiltration reaction) of carbonised cellulose fibre papers [29] or wood char [30] with titanium tetrachloride (TiCl4 ) at temperatures of about 1200 ◦ C represents a more promising approach for the synthesis of mechanically stable, highly porous and biomorphous TiC-ceramics (Fig. 3d).

3. Biomorphous oxide ceramics Only few investigations have been performed on the synthesis of biomorphous oxide-based ceramics. Yermolenko et al. synthesized ZrO2 -fibres by oxidizing of hydrated cellulose fibres impregnated with Zr-salt [31]. Al2 O3 - and TiO2 -fibers were manufactured by infiltration of natural sisal, jute and hemp fibres with AlCl3 and TiCl4 , respectively [32,33]. Ota et al. [34] produced biomorphous TiO2 -ceramics by infiltration of wood with titanium isopropoxide (TTIP). Highly porous, biomorphous Al2 O3 -, TiO2 -, ZrO2 -ceramics were prepared from pine wood as well as from cellulose fibre preforms via a sol–gel process with metal-alkoxides [35–37]. The infiltration of jelutong wood with ZrO2 -sol and heat-treatment at 1800 ◦ C yielded the manufacturing of monoclinic ZrO2 -ceramics with biomorphous structure [38]. Fig. 4 shows highly porous, biomorphous TiO2 -ceramics derived from cellulose fibre felt and pine wood. The biomorphous TiO2 -ceramics were prepared by vacuum-infiltration of the bioorganic preforms with the liquid TTIP precursor [37]. After TTIP infiltration, the samples were dried in air to form in situ gel and pyrolyzed at 800 ◦ C to carbonise the bioorganic preforms as well as to decompose the TTIP-gel into TiO2 . For consolidation and sintering into porous, microcellular TiO2 ceramics, the TiO2 coated specimens were annealed in air at 1200 ◦ C. The porous morphologies of the bioorganic preforms were well reproduced in the TiO2 -ceramics, with the cell walls of initial cellulose fibres and wood cell walls replaced by dense TiO2 -structures. Most of the pores are kept open after ceramic conversion, only few lumps of TiO2 were observed in the vessels of the TiO2 -pine, Fig. 4b. The size of the TiO2 -grains after sintering at 1200 ◦ C is about 1–3 ␮m.

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Fig. 4. SEM-micrographs of porous TiO2 -ceramics derived from bioorganic preforms by TTIP infiltration and annealing at 1200 ◦ C/4 h in air: (a) cellulose fibre felts and (b) pine wood. Table 1 Properties/applications of biomorphous ceramics and ceramic composite materials Material

Properties

Application

Soft wood Pine, fir

Mono modal pore distribution (5–50 ␮m)

Hot gas filter, sensor substrates

Hard wood Oak, beech, maple

Bi/multi modal pore distribution (1–300 ␮m)

Catalyst and biocatalyst support structures

Tropical plants Balsa Bamboo Rattan

Light-weight wood (0.05–0.13 g/cm3 ), fast growth, stem > 10 cm/year Tube morphology, strong fibres—weak matrix Homogeneous stem, no branches long and large pores (∅ ∼ 300 ␮m)

Kiln furniture, HT isolation materials Ceramic tubes Ceramic nozzles

Wood products Plywood/chipboard Cardboard/paper Organic fibres

Homogeneous and isotropic materials Excellent forming and infiltration abilities Low-cost carbon fibre, highly-porous tissue

Wear, friction materials Heat exchanger, filter LOM techniques Ceramic fibres, felts

Similar results were obtained by using different bioorganic template structure, e.g. rattan palms or corrugated cardboard preforms for conversion into TiO2 -, ZrO2 - or Al2 O3 -ceramics [35–37]. The formation Ca–P–O-ceramics on highly-porous plant structure by a biomimetic deposition technique was investigated for applications as porous bone replacement materials or for mineralisation of tissue engineering scaffolds [39]. 4. Properties/applications Bioorganic plant materials (e.g. wood, palms, organic fibres) are low cost materials and available on a commercial scale. They can easily be machined into complex three-dimensional shapes of different porosity with a large variety in cell diameters. For manufacturing of ceramic and composite materials derived from preformed bioorganic materials different processing techniques can be applied, depending on the properties of the final ceramic materials like mechanical strength, specific surface area, porosity and pore size distribution. Table 1 lists some morphological and processing properties and possible application for biomorphous oxide/carbide and composite ceramics derived from bioorganic materials as well as there from derived technical products suitable for applications especially not only as high-temperature and corrosion resistant materials but also for medical and biocatalyst support structures.

5. Summary Bioorganic materials such as wood and organic fibre plants as well as there from derived products are available on a commercial scale, they are less expensive and natural regenerating. They exhibit a wide range of porous and cellular structure with a large variety in cell diameters ranging from micrometer (wood cell structures) up to the millimetre level (preprocessed papers). The developed reactive conversion techniques of preformed biological structures into oxide/carbidebased ceramics and ceramic composites can overtake these advantages and represents an alternative ceramic manufacturing route compared to the conventional powder processing technologies. The large variety of natural plant morphologies as well as the established shaping technologies available for bioorganic materials facilitates the manufacturing of porous or dense ceramic and ceramic composites with a tailored microstructure and composition, suitable for applications especially as high-strength, high-temperature or/and corrosion resistant materials. Acknowledgements Thanks are due to E. Vogli, C. Cao, O. Rusina and D. Almeida Streitwieser for preparation of the biomorphous specimen and to P. Greil for helpful discussions. The financial support from the

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