Cellular Ceramics / p2

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2.5

Microcellular Ceramics from Wood

Heino Sieber and Mrityunjay Singh

2.5.1

Introduction

Wood is a natural composite material developed through long-term genetic evolution. It exhibits an anisotropic, porous morphology with excellent strength at low density, high stiffness, elasticity, and tolerance to damage on the microand macroscales [1, 2]. In contrast to most technically advanced man-made materials, the morphology of wood is characterized by a complex, hierarchical anatomy. The microstructural features of wood range from the milli- (growth-ring patterns) via micro- (tracheidal-cell patterns) down to the nanometer scale (molecular cellulose fiber and membrane structures of cell walls). The most distinct feature of the wood structure, however, is the open porous system of the tracheidal cells, which provide the transportation path for water and nutrients in the living wood and form a uniaxial pore structure. The mechanical properties of plant materials like wood and palms are determined by this open cellular structure, which results in pronounced anisotropy [3].

The heterogeneous tissues of wood are made by different types of cells. The wood cell walls are made of biopolymers such as cellulose, hemicellulose, pectin, and proteins. Several cellulose macromolecules are bundled to fibrils, and lignin serves as a binder between the cellulose microfibrils and increases the mechanical strength. The average elemental composition of wood is about 50 wt% C, 43.4 wt% O, 6.1 wt% H, 0.2 wt% N, and 0.3 wt% ash consisting mainly of oxides of K, Ca, Mg, Na, Si, P, and Al [4].

The morphology and the arrangement of the cells vary between coniferous wood, deciduous wood, and wood-forming plants like palms (Fig. 1). Coniferous woods have a relatively uniform structure interrupted by seasonal rings. It consists of 90–95 % tracheids, which are long and slender cells (diameter up to 50 mm, length up to a few millimeters) that are tapered at the ends and form a nearly monomodal pore morphology. Deciduous woods are less homogeneous. They contain additionally tracheary cells (diameter up to 500 mm, length less than 1 mm) which form long tubes of a few centimeters. Tracheids and tracheary cells are oriented in the direction of the trunk axis. Cells arranged in radial direction (rays) and pores in the cell walls create a three-dimensional open-porous network for transportation. The axial microstructure of deciduous woods and palms is characterized by a multimodal porous morphology with pore diameters between a few and a few hundred micrometers.

Cellular Ceramics: Structure, Manufacturing, Properties and Applications.

Michael Scheffler, Paolo Colombo (Eds.)

Copyright 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

ISBN: 3-527-31320-6

2.5 Microcellular Ceramics from Wood 123

Figure 1 SEM images: microstructure of different wood plants.

The cellular anatomy of naturally grown wood provides an attractive template for the design of microcellular ceramic materials with hierarchically ordered pore structures on different length scales. In recent years, different technologies for transforming native wood materials into cellular ceramics and ceramic composites has attained particular interest due to unidirectional pore structures on the microscopic level that can not be produced by conventional ceramic processing technologies [5, 6]. The inherent cellular and open-porous morphology of the bioorganic materials is easily accessible to liquid or gaseous infiltrants of different compositions. By using high-temperature pyrolysis (carbonization) and subsequent infiltration-reaction processes, the bioorganic structures can be converted to biomorphous ceramics within reasonable time and maintain the morphological features of the native template.

124 Part 2 Manufacturing

Wood preforms allow the manufacture of cellular ceramics with cell diameters in the micrometer range. Porous ceramics with cellular structures on a larger scale can be manufactured by conversion of wood-derived materials such as cellulose fiber felts, preprocessed papers, or corrugated cardboard structures. Ohzawa et al. [7] and Almeida, Streitwieser et al. [8] infiltrated cellulose-fiber paper preforms with SiC by pressure-pulsed chemical vapor infiltration for high-temperature filter applications. Sieber et al. [9, 10] used macrocellular templates of corrugated cardboard with cell diameters of a few millimeters for reactive conversion to SiC/mullite-based ceramic composites. The processing of cellulose-fiber papers filled with Si powder yields highly porous SiC ceramics with fibrous morphology [11, 12].

Typically, the fabrication of cellular ceramics and ceramic composites from wood templates involves two processing steps: preparation of porous biocarbon (CB) templates and subsequent conversion to carbide (reactive techniques) or oxide (molding techniques) ceramic structures. While the reactive techniques involve transformation of the CB template into carbide phases by solid/liquid or gas-phase reactions, the molding technique reproduces the microstructural morphologies of the wood template by coating of internal surfaces (Fig. 2). The latter techniques have mainly been applied for manufacturing microcellular, oxide-based ceramics. Both processing routes are discussed in detail in following sections.

CB-Template

CB-Template

Figure 2 Processing scheme for manufacturing of biomorphous carbide and oxide ceramic from wood (MTS: methyltrichlorosilane, TEOS: tetraethylorthosilicate, PMS: polymethylsiloxane).

2.5.2

Fabrication of Porous Biocarbon Templates

The preparation of biocarbon (CB) template structures by pyrolysis of native wood materials is carried out above 700 C. During pyrolysis, a slow heating rate of about 1 C min–1 must be applied up to about 500 C to completely decompose the polyaromatic wood polymers cellulose, hemicellulose, and lignin to carbon. Afterwards, a higher heating rate of about 5 C min–1 up to peak temperature can be used [13, 14].

2.5 Microcellular Ceramics from Wood 125

The formation of carbon template structures from wood is a thermal degradation process [15]. It can be divided in three steps:

The weight loss during pyrolysis of the wood preforms is almost complete at about 600 C and results in a final weight loss in the range of 70–80 wt % [13, 16]. The weight losses of various kinds of wood can differ and are related to the molecular composition of hemicellulose, cellulose, and lignin in the wood. Due to its aromatic structure, lignin yields a much smaller weight loss (ca. 55 wt %) than cellulose (ca. 80 wt %) [16].

The weight loss leads to anisotropic shrinkage of the wood materials during pyrolysis by about 20 % in the axial, 30 % in the radial, and 40 % in the tangential direction on average [13, 16, 17]. It can be explained by decomposition and rearrangement of the oxygen-bonded glucose units of the cellulose fiber during thermal treatment and thus depends on the relative orientation of the cellulose fiber to the wood axis [18]. After pyrolysis, the total porosity of the CB template is 20–25 % higher than that of the native (dried) wood [13, 16]. The weight loss and shrinkage during pyrolysis of pinewood are exemplarily shown in Fig. 3.

Despite the large weight loss and anisotropic shrinkage during pyrolysis, the fine microstructural features of the cellular wood anatomy on the submicrometer scale are retained with high precision in the CB template. By using slow heating rates during pyrolysis, even large wood species can be carbonized into CB template structures maintaining the morphology of the natural wood anatomy and without cracking of the material [13].

Figure 3 Weight loss and linear shrinkage during pyrolysis of pinewood (from [17]).

126Part 2 Manufacturing

2.5.3

Preparation of Carbide-Based Biomorphous Ceramics

Carbide-based biomorphous ceramics can be fabricated by reactive conversion of the porous CB template. To manufacture SiC-based ceramics, different reactive processing routes were applied [19], such as infiltration with Si melts, Si/SiO/CH3SiCl3 vapors, or sols containing SiO2 precursors (Fig. 4). Infiltration with SiO2 precursor sols at ambient temperature is an easier processing route with only one high-tem- perature reaction step, but it often results in low mechanical stability of the obtained SiC material. Examples are the infiltration of organosilcon monoor polymers into charcoal and their conversion to highly porous SiC and SiOC ceramics [20, 21]. Herzog et al. developed a processing route with infiltration of nanosized SiO2 sols into wood chars [22]. Repeated infiltration and carbothermal reduction of the CB template yields beechand pinewood derived SiC ceramics with porosities of 65–75 %.

Figure 4 Processing scheme for manufacturing microcellular

SiC ceramics from wood (after [19]).

Besides the synthesis of biomorphous SiC-based ceramics, a few reports also deal with the conversion of bioorganic materials to TiC-based ceramics. The reactive infiltration of carbonized wood structures with pure Ti vapor in vacuum at 1600 C yields a thin surface layer of TiC of few 100 mm in depth [23]. Sun et al. prepared highly porous, biomorphous TiC ceramics by infiltration of wood char with tetrabutyl titanate and high-temperature reaction in inert atmosphere [24]. A more promising

2.5 Microcellular Ceramics from Wood 127

approach for synthesis of mechanically stable, porous, biomorphous TiC ceramics is chemical vapor infiltration-reaction (CVI-R) of carbonized wood specimens with TiCl4 at about 1200 C [25].

2.5.3.1

Processing by Silicon-Melt Infiltration

SiSiC ceramic composites with microcellular morphology and excellent mechanical stability can be prepared by infiltration of liquid Si into the carbonized wood templates, similar to conventional liquid-silicon infiltration (LSI) processing [26]. Byrne and Nagle [27], Greil et al. [16], Martinez-Fernandez et al. [28, 29], and Singh et al. [5, 30, 31] converted different kinds of wood structures by spontaneous infiltration with an Si melt to microcellular SiSiC composites. Similarly, Shin and Park [32] used charcoal for fabrication of biomorphous SiSiC composites. After reactive infiltration with liquid Si at 1450–1600 C, the carbon of the wood char is transformed into b-SiC. The small pores in the CB template, up to a pore diameter of approximately 50 mm, are filled with residual Si, and an SiSiC ceramic–ceramic composite is formed. The Si content of the final cellular SiSiC composite, as well as the total porosity and pore size distribution, depends on infiltration parameters and the anatomy of the wooden preforms. To fabricate more porous biomorphous SiSiC ceramics, the specimens can be subjected to stoichiometric or near-stoichiometric Si infiltration in which only the Si required for conversion of the biocarbon cell walls is supplied [33], or the unconsumed Si can be removed afterwards by heat treatment [34] or chemical etching [16].

Figure 5a shows redwood-derived microcellular SiSiC ceramics after spontaneous Si-melt infiltration. The pores walls were converted to b-SiC, while the pore volumes were filled with residual Si after processing. To remove the excess Si remaining in the pores, the samples were heat-treated at 1550 C for 4 h (Fig. 5b). After initial infiltration at 1450 C for 30 min, the geometrical density was around 2.4 g cm–3, and it decreased to 2.0 g cm–3 after heat treatment at 1550 C [34].

Biomorphous SiSiC ceramic composites contain about 1–5 vol % of unconverted carbon after processing, due mainly to local density variations in the CB template. Especially in denser areas of the CB template (e.g., latewood areas in pine char), the carbon density can exceed the critical limit of about 0.97 g cm–3, which results in clogging of the pores during Si-melt infiltration and inhibits further SiC formation [35]. The microstructure of the biomorphous SiSiC ceramic composites is characterized mainly by b-SiC grains with a grain size in the micrometer range (up to ca. 10 mm) formed by intimate contact of the SiC phase with the Si melt. Additionally, layers of a nanograined b-SiC phase with average grain diameters of less than 100 nm, formed only between the coarse-grained SiC phase and the unconverted carbon regions, are observed [36, 37]. While the coarse-grained SiC phase is due to dissolution and recrystallization processes of the carbon and SiC in contact with the Si melt, the nanograined SiC phase originates from gas-phase and solid-state reactions of C and Si. The evolution of the different SiC phases during infiltration of liquid Si into carbonized wood templates is a combined infiltration/reaction process influ-

128 Part 2 Manufacturing

SEM images: microcellular SiSiC ceramic derived from redwood a) after Si-melt infiltration, and b) heat-treated at 1550 C to remove excess

silicon (reprinted from [34], with permission by CPRC, Hanyang University, Korea).

enced by the local carbon morphology, as well as by time-dependent dissolution, recrystallization, and SiC grain-growth processes [38].

The microcellular, biomorphous SiSiC ceramics exhibit excellent but anisotropic mechanical properties, due to the unidirectional cellular pore morphology of the wood template [16, 29, 33, 39–41]. The microstructure–strength correlation in porous materials is described in the literature by two main approaches: cellular materials with porosities greater than 70 %, in which the strength is mostly related to the bending strength of the cell walls [42], and systems with lower porosities, in which the strength is assumed to be only dependent on the minimum solid area perpendicular to the applied stress [43]. While the cellular model of Gibson and Ashby exhibits good agreement for the compressive strength of low-density biomorphous SiSiC [33], the model cannot differentiate between different topologies of the materials and thus, for a wide variety of biomorphous SiSiC microstructures, the anisotropic behavior and porosity dependence fit better with the assumption of the mini- mum-solid-area approach [29].

The mechanical properties of porous, biomorphous SiSiC ceramic composites can be retained up to high temperatures until melting of Si starts [33]. Potential applications of microcellular, biomorphous SiSiC ceramics are filters, microreactor devices, and absorbers for high temperatures [37], as well as medical implants [44].

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2.5.3.2

Gas-Phase Processing

In contrast to Si-melt infiltration, highly-porous, single-phase biomorphous SiC ceramics with a cellular morphology on the micrometer scale can be manufactured by using Si-containing vapors as reactants. During processing, the Si-containing vapor penetrates the pores of the CB template and reacts with the carbon to form b-SiC. Different reactive Si-containing vapor species such as Si [17, 45, 46], SiO [47] and CH3SiCl3 (methyltrichlorosilane, MTS) [48] can be used, and the porosity of the final SiC ceramics depends on the reaction products formed:

CB(s) + (1+x) CH3SiCl3(g) + 2 H2(g) fi SiCB(s) + x SiC(s) + 3(1+x) HCl(g) + CH4(g) (3)

where x denotes the amount of additionally deposited SiC from CVI processing. The specific volume change associated with the reaction of the wood-derived car-

bon with the Si-containing vapors to give b-SiC is given by:

where n, M, and denote the number of moles, the molecular weight, and the densities of the indicated phases. The density of SiC is SiC = 3.21 g cm–3. Depending on the density of the carbon after pyrolysis (graphite: C = 2.26 g cm–3, vitreous carbon: C = 1.44 g cm–3) and the kind of reaction (Eqs. 1–3), specific volume changes occur (Fig. 6).

The measured skeletal density of the wood char after pyrolysis at 1600 C is 1.4–1.6 g cm–3, and the specific surface area of a few tens of m2/g indicates a pronounced porous strut morphology [14, 15, 47]. The reaction of the CB template struts with Si vapor (Eq. 1) results in 1:1 conversion of porous carbon to b-SiC and thus in an increase in volume during SiC formation together with densification of the porous carbon strut. CVI processing with gaseous organosilicon precursors (Eq. 3) yields a much denser SiC ceramic due to the deposition of additional SiC. In contrast, processing with SiO vapor results in more porous cellular materials due to the release of carbon by CO and CO2 evaporation during the reaction (Eqs. 2a and 2b).

Figure 7 shows biomorphous SiC ceramics from pinewood after Si-vapor infiltration. In contrast to Si-melt infiltration, no Si was deposited during processing and a highly porous, single-phase SiC ceramic was formed. The final microcellular SiC

2.5 Microcellular Ceramics from Wood 131

ceramics completely reproduce the porous wood morphology down to the micrometer level. The b-SiC grains formed in the reaction are in the submicrometer range and form a nearly dense strut morphology. The microstructure, porosity, and mechanical properties of the biomorphous SiC ceramics after gas-phase infiltration depend on the kind of wood as well as on the infiltrating and reacting vapor species. For instance, SiC ceramics derived from pinewood and different Si-containing vapors exhibit bending strengths in the axial direction between 4 and 21 MPa [49] (compression strength 70–151 MPa [50]) for a porosity in the range between 60 and 80 % (Tab. 1).

Table 1 Properties of biomorphous SiC-based ceramics from pinewood prepared by different technologies [16, 33, 49, 50]. Compression strength was measured in the axial direction at room temperature; radial and tangential strengths are lower.

* Compression strength was measured at 1150 C [33].

** Bending strength was measured by biaxial ball-on-ring testing [49].

2.5.4

Preparation of Oxide-Based Biomorphous Ceramics

Only a few investigations have focused on the synthesis of microcellular oxide ceramics with biomorphous microstructures. Metal alkoxides and metal chlorides can be used as precursors which can form a macromolecular oxide network through hydrolysis and condensation on the inner cell walls of the porous structure of native plants and CB templates. Subsequently, the biocarbon is burned out by heating in air, and a highly porous oxide ceramic or ceramic composite is formed. Further annealing leads to consolidation and sintering of the highly porous structure and results in microcellular, biomorphous oxide ceramics of different compositions.

The technique was used by Yermolenko et al. for the formation of ZrO2 fibers by oxidizing hydrated cellulose fibers impregnated with a zirconium salt [51]. Padel and Padhi [52, 53] manufactured Al2O3 and TiO2 fibers by infiltration of natural