Scripta Materialia 53 (2005) 1363–1368 www.actamat-journals.com

Preparation and characterization of biomorphic SiC hollow fibers from wood by chemical vapor infiltration Junmin Qian *, Jiping Wang, Guangya Hou, Guanjun Qiao, Zhihao Jin State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi’an Jiaotong University, Xi’an 710049, China Received 3 June 2005; received in revised form 8 August 2005; accepted 24 August 2005 Available online 12 September 2005

Abstract Biomorphic SiC hollow fibers were prepared by the reactive infiltration of SiO vapor into basswood-derived charcoal. Gaseous SiO was produced from a SiO2/Si powder mixture in Ar at elevated temperatures. Scanning electron microscopy, energy dispersive X-ray spectroscopy, X-ray diffraction and Fourier transform-infrared spectroscopy were employed to characterize the structural morphology and phase compositions of the final products. The results show that the tubular cells in bulk charcoal are converted into lots of SiC hollow fibers with pore diameters of 10–50 lm and lengths ranging from hundreds of lm to several mm. Resulting SiC hollow fibers consist of b-SiC with a minute amount of a-SiC. The formation mechanism of SiC hollow fibers is based on the gas–solid reaction between SiO and carbon. Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Biotemplating; Chemical vapor infiltration; Silicon carbide; Hollow fibers; Texture

1. Introduction Biotemplating is a new concept for fabricating ceramic materials with novel hierarchical and complex microstructures using natural biological materials as templates, such as wood [1], rattan [2], organic fiber [3], rice husk [4], paper [5]. Amongst these, wood has been paid considerable attention with respect to the conversion of its tissue to ceramic materials because it has highly anisotropic cellular structures [6,7]. Wood has been used to fabricate various novel ceramics with micro-, meso- and macro-structures, classified into the following two groups according to the density or porosity of the products. Initially, ceramic materials derived from wood were mainly dense or low-porosity SiC, SiC/Si or oxide ceramic composites prepared by the infiltration of silicon melt or oxide precursors into charcoals [8–10]. These materials usually possess high mechanical

*

Corresponding author. Tel.: +86 29 82668614; fax: +86 29 82665443. E-mail address: [email protected] (J. Qian).

performances, but do not effectively utilize the pore microstructure of wood. Now, more attention is focused on fabricating porous ceramics from wood. For example, porous, biomorphic oxide ceramics such as Al2O3 [11], ZrO2 [12], TiO2 [13] and SiO2 [14] are prepared via the combination of the infiltration of metal alkoxide or oxide sols into charcoal and subsequent oxidation in air at high temperatures [15]. Porous carbide ceramics such as SiC, SiC/C, TiC/C keeping the morphology of pyrolyzed wood or with a hybrid pore structure [16] are produced by the reactive infiltration of gaseous metal-containing substances or the carbothermal reduction of mineralized wood with SiO2 or tetrabutyl titanate [17–19]. Ceramic thin-walled microtubes or hollow fibers are attracting a great deal of interest for their potential applications in air separation and selective oxidation of light hydrocarbons to synthesis gas and other products [20–22], and are a promising alternative for membrane catalysis at elevated temperatures and high-pressure separation processing, where organic polymer membranes cannot be used, because of their good mechanical, thermal and

1359-6462/$ - see front matter Ó 2005 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2005.08.029

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chemical stability, and larger surface area-to-volume ratio. They may also be used as supports on which membrane materials are usually deposited as thin layers [23], to reduce membrane resistance and to retain membrane mechanical strength. Until now, glass hollow fibers and carbon hollow fibers have been prepared, and ceramic hollow fibers have been reported in the literature more recently [24,25]. However, a major difficulty of preparing ceramic hollow fiber membranes is the lack of appropriate technologies. The inner diameter of ceramic thin microtubes or hollow fibers prepared by conventional methods is not less than 1 mm [26–28]. Many efforts have been made to produce ceramic hollow fibers [29,30]. In particular, SiC microtubes or hollow fibers are of great potential due to their excellent mechanical strength, good resistance to oxidation and corrosion, thermal stability and chemical inertness, as well as thermal conductivity and electronic conductivity [31,32]. For example, SiC hollow fibers were fabricated by heating carbon fibers coated with silica in air at high temperatures [33]. In the present work, we report a novel method for preparing SiC hollow fibers from wood by using a reactive infiltration technique. The structural morphology and crystalline phase of the final products are investigated by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD) and Fourier transform-infrared (FT-IR) spectroscopy techniques. The formation mechanism of SiC hollow fibers is also discussed. 2. Experimental Wood was purchased from XiÕan Wood Co. (XiÕan, China). Dried wood was carbonized under vacuum at 1200 °C for 4 h in a graphite heater furnace with low heating ramp of 65 °C/min, resulting in a porous biocarbon template (charcoal). Silica sol was prepared by a sol–gel process through the hydrolysis and polycondensation of tetraethoxysilane [Si(OC2H5)4, TEOS] purchased from XiÕan Chemical Reagent Co. (XiÕan, China), as described previously [16], gelled at 80 °C for 3 days, and dried at 120 °C for 12 h, resulting in silica powder. Silica was mixed with silicon powder at a molar ratio of 1:1, with a silicon excess of 5%, ball-milled for 2 days, and sieved with 400 mesh. Charcoal was placed at the constant temperature position of a graphite heater furnace, and reacted with silicon monoxide vapor produced by heating the as-prepared silica/silicon powder mixture in a graphite crucible below the samples at elevated temperatures under an argon atmosphere. The samples were held for 2, 4 and 6 h at the peak temperature, and cooled down to room temperature in argon. The processing scheme for manufacturing of SiC hollow fibers using basswood as a natural biotemplate is described in Fig. 1. The morphology and quantitative elemental analyses of the resulting products and the initial charcoal were ob-

Biotemplate (Basswood) Drying and Shaping Carbonization Vacuum, 1200 ºC, 4 h Biocarbon template (Charcoal) SiO vapor infiltration Ar, 1600-1750 ºC SiC hollow fibers Fig. 1. Flow chart for the manufacturing of biomorphic SiC hollow fibers from basswood.

served and characterized by SEM (Hitachi, S-2700) equipped with an EDX spectrometer. XRD was carried out on a D/MAX-RA X-ray diffractometer to determine the crystalline phases formed during the reactive infiltration, using nickel filtered CuKa radiation produced at 35 kV and 20 mA. The final products were crushed into a powder and mixed with KBr. FT-IR studies were performed with a Fourier transform infrared spectrometer (AVATAR 360 FT-IR, Nicolet) in the wavenumber range from 1400 to 600 cm
1. The specific surface area of samples was measured by the Brunauer, Emmett and Teller method using N2 as the adsorbate. The yield of the C ! SiC conversion (Y), i.e., the mass of SiC in the final product relative to the theoretical mass of SiC which should be obtained following the reaction Eq. (2) based on the starting mass of charcoal, was calculated from: mSiC Y ¼ ; ð1Þ 5 mSiC þ 3 ðmP
mSiC Þ where mSiC and mP are the mass of pure SiC in the final product and the final product, respectively. 3. Results and discussion 3.1. Microstructure characterization Fig. 2 presents the SEM images of basswood-derived charcoal and resulting SiC materials. As seen in Figs. 2a and b, charcoal has hollow channels of various diameters that originate from tracheid cells that are parallel to the axis of the tree. The channels can be classified into two groups, depending on their cross-sectional area: large channels and small channels that are in the vicinity of the larger channels. The average diameter of each group of cells is 50 lm for the large cells and 10 lm for the small cells. Most

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Fig. 2. SEM images of charcoal ((a) cross section perpendicular to axial direction; (b) cross section parallel to axial direction) and SEM images of SiC materials obtained at: (c) 1600 °C for 2 h, (d) 1750 °C for 4 h and (f) 1750 °C for 6 h. (e) Is a high-magnification SEM image of (d).

of the cellular pores show a round or elliptical shape. The topologically uniform arrangement of cells in wood is interrupted by growth ring patterns. After reactive infiltration at 1600 °C for 2 h in Ar atmosphere, the tubular structure of the cells in charcoal was retained due to the ordered semicrystalline cellulose networks of tubular cell walls in wood, but many gaps appear among the tubular cells, as shown in Fig. 2c. As the degree of infiltration reaction increases, the phenomenon becomes more noticeable, as shown in Fig. 2d. The tubular cells tend to become single. After an infiltration at 1750 °C for 6 h, bulk charcoal is nearly completely changed into SiC hollow fibers, namely, SiC microtubes. The SiC hollow fibers possess pore diameters of 10–50 lm, their pore wall thickness is less than 2 lm, their length is in the range from hundreds of lm to several mm, and their specific surface area is 20.8 m2 g
1 (see Table 1). At the same time, we noted that there were lots of particles at the resultant SiC fiber surface, which are the result of the reaction between gaseous SiO and carbon originating from thin film tissue, resin and pectin, etc

Table 1 Specific surface area (SSA) of original wood, charcoal and the product Sample

Basswood

Charcoal

SiC fiber

SSA/m2 g
1

24.7

31.6

20.8

among the tubular cells. The corresponding EDX spectrum of the particle shown in Fig. 3 indicates that the particles consist of only Si and C, and quantitative analysis shows that their average atomic ratio approximates to 1:1. 3.2. XRD analysis The XRD patterns of charcoal and the as-produced SiC hollow fibers are shown in Fig. 4. The two broad diffraction bands of charcoal shown in Fig. 4a are observed, indicating that the charcoal is amorphous. After the reactive infiltration is performed for 2 h, only one diffraction band attributed to amorphous charcoal is visible and its intensity becomes very weak, as shown in Fig. 4b. At the same time,

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coal is nearly completely converted into SiC, and no other impurities exist. However, after a calcination of the final product was performed in air at 560 °C for 2 h, very little mass loss was found due to the oxidation of unreacted charcoal, and the corresponding yield of the C ! SiC conversion was 97.6%. If the reactive infiltration time is further prolonged to 6 h, compared with Fig. 4c, some new diffraction peaks except those of b-SiC and a-SiC are detected, and attributed to crystalline silica (cristobalite) and silicon which are formed via the following steps: starting SiO2 and Si powders gasify, diffuse, and deposit on the surface of SiC microtubes, and have no chance to react with charcoal. Fig. 3. EDX spectrum of the particle.

3.3. FT-IR analysis The FT-IR spectrum of the resulting SiC hollow fiber is presented in Fig. 5. The wide band centered at 830, 870 cm
1 with a shoulder at 950 cm
1 is assigned to the Si–C fundamental vibrations [33]. Pure SiC exhibits a broad peak between 900 and 700 cm
1, with a maximum of absorbance at 870 cm
1 [35]. For a SiC material prepared from a carbon/silica composite, the broad band is located between 825 and 898 cm
1 (centered at 850 cm
1). This discrepancy is related to the different morphology and microstructure of the as-formed SiC crystal. The low intense broad band located around 1085 cm
1 is assigned to the antisymmetric stretching vibrations of Si–O–Si bridging sequences [36,37], indicating the resulting SiC hollow fibers contain a minor fraction of silica or silicon oxycarbide phases, SiOXCY [38]. No diffraction peaks indicative of the impurities are observed in its XRD pattern, because their amounts are negligible with respect to XRD sensitivity. 3.4. Formation mechanism of SiC hollow fibers The reaction to form hollow SiC fibers proceeds via two steps. In the first step of the reaction, SiO vapor is generated

the XRD profile displays four main strong peaks at 2h = 35.71°, 41.46°, 60.06° and 71.84°, which indicate the crystal growth of SiC and are attributed, respectively, to the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes of the cubic type (b-SiC) phase according to the Ramsdell notation. One additional diffraction peak near the (1 1 1) planes of b-SiC (2h = 35.71°) is detected at 2h = 33.74°, which is characteristic of hexagonal polytypes (a-SiC) [34]. The form of SiC that is stable in the domain of the reaction temperature, is indeed the cubic silicon carbide which agrees with the present results. When the reactive infiltration time is lengthened to 4 h, the corresponding XRD pattern indicates the product is essentially constituted of b-SiC with a minute amount of a-SiC (Fig. 4c), suggesting that char-

Transmittance/a.u.

Fig. 4. XRD patterns of: (a) charcoal and SiC ceramics fabricated at 1750 °C for (b) 2 h, (c) 4 h and (d) 6 h.

1400 1300

1200 1100 1000

900

800

700

600

Wavenumber/cm-1 Fig. 5. FT-IR spectrum of the SiC hollow fiber obtained at 1750 °C for 4 h.

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by heating a mixture of silica and silicon at high temperatures according to the oxidation–reduction reaction (2). SiO2 þ Si ! 2SiO

ð2Þ

In the second step of the reaction, the as-formed SiO vapor diffuses and infiltrates into the charcoal via its tubular pores, and synchronously reacts with the carbonaceous pore wall to form SiC according to the following gas–solid reaction (3). SiO þ 2C ! SiC þ CO

ð3Þ

Once carbon monoxide (CO) is formed, SiC can also be produced according to reaction (4). SiO þ 3CO ! SiC þ 2CO2

ð4Þ

In general, reaction (3) produces solid SiC and can retain the morphology of the starting carbonaceous material, whereas reaction (4) tends to form SiC whiskers. Under the reaction conditions applied in this study, SiC whiskers cannot form due to the high reaction temperature and low CO partial pressure, because the reaction of SiO and CO becomes thermodynamically favorable only when the partial pressure of CO is greater than 0.027 MPa [39]; moreover, the higher reaction temperature does not allow the SiC whiskers to exist in a stable manner. During the conversion of charcoal into SiC material, the large volume change occurs because of the different coefficients of thermal expansion between charcoal and SiC, resulting in a large stress at the interfaces among the microtubes. At the same time, reaction (3) partially consumes carbon on the outer surface of every microtube in charcoal, leading to a weaker connection among the microtubes. For these two reasons, the resulting SiC hollow fibers peel off. When the infiltration reaction reaches a certain extent, the whole charcoal block is converted into lots of single SiC hollow fibers. 4. Conclusions We prepared SiC hollow fibers from basswood by the reactive infiltration of gaseous SiO generated from a solid mixture of Si and SiO2 into charcoal at 1750 °C in an argon atmosphere. SEM, EDX, XRD and FT-IR studies show that the as-prepared SiC hollow fibers maintain the original morphology of single tubular pore of charcoal very well, display pore diameters of 10–50 lm and lengths ranging from hundreds of lm to several mm, and are made of bSiC with a minute amount of a-SiC. In general, the cellular pores in wood possess pore diameters ranging from tens of lm to hundreds of lm and their lengths range from 0.1 to 10 mm, depending on the type of wood. The present method may make it easy to produce large-scale amounts of SiC hollow fibers with desired pore diameters and lengths at low cost, because tens of thousands of kinds of wood in the world are available; this will be interesting with regard to the potential future applications of SiC hollow fibers in many fields.

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