Materials Science and Engineering A 371 (2004) 229–235

Preparation of biomorphic SiC ceramic by carbothermal reduction of oak wood charcoal Junmin Qian∗ , Jiping Wang, Zhihao Jin State Key Laboratory for Mechanical Behavior of Materials, School of Materials Science and Engineering, Xi an Jiaotong University, Xi an 710049, PR China Received 9 August 2003; received in revised form 24 November 2003

Abstract Highly porous silicon carbide (SiC) ceramic with woodlike microstructure has been prepared at 1400–1600 ◦ C by carbothermal reduction reaction of charcoal/silica composites in static argon atmosphere. These composites were fabricated by infiltrating silica sol into a porous biocarbon template from oak wood using a vacuum/pressure infiltration process. The morphology of resulting porous SiC ceramic, as well as the conversion mechanism of wood to porous SiC ceramic, have been investigated by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) techniques. Experimental results show that the biomorphic cellular morphology of oak wood charcoal is remained in the porous SiC ceramic with high precision that consists of ␤-SiC with traces of ␣-SiC. Silica in the charcoal/silica composites exists in the cellular pores in form of fibers and rods. The SiC strut material is formed by gas–solid reaction between SiO (g) and C (s) during the charcoal-to-ceramic conversion. The densification of SiC strut material may occur at moderate temperatures and holding time. © 2003 Elsevier B.V. All rights reserved. Keywords: Biomorphic SiC; Porous ceramics; Wood; Microstructure; Sol–gel process; Carbothermal reduction

1. Introduction Design of novel ceramic materials with specific structures and functional properties by mimicking the hierarchical cellular structure of wood has recently attained particular interest [1–3]. Wood is a naturally grown composite material of complex hierarchical cellular structure, and comprised of elongated tubular cells aligned with the axis of the tree trunk and growth ring structures [4]. The tubular cells of wood with a preferential orientation in axial direction offer the possibility to use various infiltration techniques to transform the bioorganic wood structure into an inorganic ceramic material with tailored physical and mechanical properties [2,5]. Wood-derived cellular ceramics might be of interest for high-temperature exhaust gas filters, catalyst carriers, advanced microreactor systems, immobilization supports for living cells, microbes or enzymes, and waste water treatment, as well as acoustic and heat insulation structures, etc. ∗ Corresponding author. Tel.: +86-29-82667942; fax: +86-29-82665443. E-mail address: [email protected] (J. Qian).

0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.11.051

Wood has been used to prepare many advanced ceramic materials such as woodceramics, woodceramics/metal composites, porous carbide, or oxide ceramics, and biomorphic Si/SiC composites [3,6–11]. Previous work converting wood into various SiC-based ceramic materials focused on the infiltration of the pyrolysed biocarbon template with gaseous or liquid silicon bearing precursors such as silicon melt, silicon, and silicon monoxide vapors and organosilicon compounds at high temperatures [2,12–15], and silica sol from tetraethylorthosilicate (TEOS) at low temperatures followed by pryolysis in inert atmosphere [16]. In terms of economy and efficiency, sol–gel method is the best choice. Sol–gel technology has been utilized to prepare ceramic materials using reactive replica techniques [17], owing to low cost, simple procedure, and lower processing temperatures. Recently, some advanced SiC materials such as nanowires, nanorods, and nanometer-sized powders or whiskers, have been prepared by sol–gel methods [18–21]. For example, fine SiC tubes were prepared by sintering and gasifying carbon fibres covered with a silica layer produced by a sol–gel method [22,23]. Mesoporous SiC powders were produced from TEOS and phenolic resin with nickel nitrate as a pore-adjusting reagent by a modified sol–gel

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method [24]. It appears that in a more general way, SiC materials of desired microstructure may be prepared by an appropriate selection of the silica–carbon artefact. The objective of the present work is to prepare porous SiC ceramic with woodlike microstructure from TEOS and oak wood as starting materials by sol–gel and carbothermal reduction process. The morphology changes and carbothermal conversion of the charcoal/silica composite into SiC ceramic were investigated by SEM, XRD, TGA, and FTIR techniques. The resulting biomorphic SiC ceramic retains the original structures of oak charcoal and possesses higher strength than bulk charcoal.

Wood Drying 110ºC, 2~4d Carbonization Vaccum, 1200ºC, 4h Charcoal Impregnated with SiO2 sol by vaccum/ pressure infiltration process

2. Experimental procedure Gelation

2.1. Material preparation Oak wood was shaped, dried at 110 ◦ C for 2–4 days, and subsequently carbonized under vacuum at 1200 ◦ C for 4 h in a graphite heater furnace with a slow heating rate of 2 ◦ C min−1 up to 600 ◦ C and a higher rate of 5 ◦ C min−1 up to the peak temperature, resulting in a porous biocarbon template (charcoal). Silica sol was prepared from ethanol solutions of tetraethoxysilane [Si(OC2 H5 )4 , TEOS], distilled water and hydrochloric acid at a suitable molar ratio of TEOS:H2 O:HCl by a sol–gel process as that described elsewhere [22,25]. In the present work, the concentration of silica sol was equal to 20% by weight. Charcoal was placed in an infiltration vessel in a self-made equipment. The vessel was evacuated and held for 3 h, and then backfilled with silica sol. After that, the pressure in the vessel was raised to a high atmosphere pressure of 1.5 MPa with 6 h hold. The silica sol contained in charcoal was gelled at 60 ◦ C for 20 h and dried at 120 ◦ C for 8 h to remove other solvents, resulting in silica/charcoal composites. The treatment procedure of impregnation, gelling and drying, namely cycle of infiltration procedure, was repeated several times, to increase the silica content in charcoal/silica composites. Carbothermal reduction of the as-prepared silica/charcoal composites was carried out in static argon atmosphere in a graphite furnace at elevated temperatures with a rate of 600 ◦ C h−1 to form porous SiC ceramic. The reactant silica/charcoal composites loaded in graphite crucibles were placed into the hot zone of the furnace. The temperature of the furnace was raised up to the desired temperature, and then held for 4–8 h to allow complete reaction of silica with the carbon structure to form ␤-SiC. Fig. 1 summarizes the processing scheme.

Drying 120ºC, 8h C/SiO2 composites Carbothermal reduction Ar, 1400-1600ºC, 4-8h Woodlike SiC ceramics Fig. 1. Processing scheme of manufacturing woodlike SiC ceramic from oak wood.

(Model Netzsch Thermische Analyser STA 409C) thermal analyzer with alumina powder as the reference sample. The morphological changes of the starting material during the transformation of the carbon/silica material into SiC ceramic were observed and analyzed by scanning electron microscopy (SEM, Hitachi, S-2700) operated at 20 kV and 20 mA. X-ray diffraction (XRD) was measured on a D/MAX-RA X-ray diffractometer to determine the crystalline phases formed during the carbothermal reduction reaction, using nickel filtered Cu K␣ radiation produced at 35 kV and 20 mA. The samples were mounted on a stub and then metallized with gold. Fourier transform infrared spectroscopy (FTIR) studies were performed with a Fourier transform infrared spectrometer (AVATAR 360 FTIR, Nicolet) in the wavenumber range of 4000–500 cm−1 . The samples’ spectra were recorded by transmission in dry air atmosphere through a pastille made of a few milligrams of sample materials mixed with KBr.

2.2. Characterization 3. Results and discussion The pyrolysis behavior of oak wood was characterized using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) in a N2 flow-rate of 100 ml min−1 with a heating rate of 10 ◦ C min−1 from room temperature to 800 ◦ C. TGA and DSC were performed on DSC–TGA

3.1. Pyrolytic conversion of wood to carbon template In order to study the decomposition behavior of oak wood, small pieces were heated in the thermal analyzer in

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2 TGA DSC

80

0

60 40

-2

20 0

200

400 600 Temperature (˚C )

800

DSC (mW/mg)

Residual weight (%)

100

0

231

-4

Fig. 2. TGA–DSC curves of oak wood.

N2 atmosphere. Fig. 2 shows TGA and DSC curves of oak wood up to 800 ◦ C. It can be seen that weight loss during heating of wood starts below 100 ◦ C, reaches a maximum rate at 330 ◦ C and is almost terminated at 700 ◦ C. Although the TGA curve for oak wood is quite similar in its nature to that of maple wood, the DSC curve shows slight differences due to differences in the chemical contents of oak and maple woods [26]. 3.2. XRD analysis The XRD patterns of charcoal, silica/charcoal composite, and resulting porous SiC ceramics obtained under different conditions are shown in Fig. 3. It can be seen that two broad

Fig. 4. Infrared spectra of (a) silica/charcoal composite and porous SiC ceramics obtained by heating C/SiO2 composites with a C-to-SiO2 molar ratio of ≈3 at (b) 1450 ◦ C for 4 h and (c) 1600 ◦ C for 4 h.

peaks centered around 22 and 44◦ in Fig. 3a and b suggest that both charcoal and silica/charcoal composite are amorphous. Peaks due to ␤-SiC (cubic type) phases are observed together with the broad peaks due to charcoal when the molar ratio of C-to-SiO2 is 7 (Fig. 3c), indicating that the formation of ␤-SiC has taken place and that residual free carbon still exists. The intensity of peaks due to ␤-SiC phases in the XRD patterns increases significantly with increasing silica content, whereas the intensity of peaks (0 0 0 2) and (0 0 0 4) due to residual free carbon disappears. When the charcoal/silica composite with a molar ratio of ≈3 are sintered at 1600 ◦ C for 4 h, one additional line at 2θ = 33.82◦ (d = 0.265 nm) in the XRD patterns of porous SiC ceramic (Fig. 3e) is detected near the (1 1 1 ) line of the cubic structure of the silicon carbide at 2θ = 35.64◦ (d1 1 1 = 0.251 nm) which is characteristic of hexagonal polytypes (␣-SiC phase) [27]. Diffraction peak for crystalline SiO2 (cristobalite) is observed when silica content is further increased. The optimum number of cycles of infiltration procedure is found to be 3, i.e., C/SiO2 ≈ 3, when very well crystallized ␤-SiC and a negligible residue of unreacted reactants are produced. 3.3. FTIR analysis

Fig. 3. Powder XRD patterns of (a) charcoal, (b) charcoal/silica composite, and resulting woodlike SiC ceramics obtained under different conditions: (c) 1600 ◦ C, 4 h, molar ratio of C-to-SiO2 ≈ 7; (d) 1450 ◦ C, 4 h, molar ratio of C-to-SiO2 ≈ 2.8; and (e) 1600 ◦ C, 4 h, molar ratio of C-to-SiO2 ≈ 3.

Fig. 4 shows the FTIR spectra of charcoal/silica composite and porous SiC ceramic prepared from charcoal/silica composites. In the IR spectrum (Fig. 4a) of the charcoal/silica composite, the absorption bands at 1090, 800 and 466 cm-1 are attributed to antisymmetric and symmetric stretching vibrations of Si–O–Si bond, respectively. After sintering at 1450 ◦ C for 4 h, a spectral evidence for the carbothermal reduction of silica is obvious. A new intense broad band centered at 825 cm−1 that appears beside the Si–O vibrations is observed (Fig. 4b), which is ascribed to the Si–C fundamental stretching vibration [25,28], in spite of some discrepancy between the wavenumber values (789–794 cm−1 ) indicated in the literature [29]. The discrepancy between the values is attributed to the different morphology of the analysed SiC. In the IR spectrum (Fig. 4c) of the products obtained at 1600 ◦ C for 4 h, the above-mentioned absorption bands

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assigned to silica gel disappear, and nearly only the Si–C vibration centered at about 825 cm−1 is observed, showing that the carbothermal reduction is almost completed. 3.4. Mechanism of conversion of charcoal/silica composite into SiC ceramic It has been shown [22] that under the experimental conditions of the present work, the overall reaction between charcoal and silica for producing silicon carbide is: SiO2 (s, l) + 3C(s) → SiC(s) + 2CO(g)

(1)

In fact, Eq. (1) proceeds through two stages in which a gaseous intermediate, silicon monoxide (SiO) gas is formed. The first step consists of a solid–solid or solid–liquid type of reaction between carbon and silica leading to the formation of gaseous silicon monoxide (SiO) and carbon monoxide (CO) according to Eq. (2): SiO2 (s, l) + C(s) → SiO(g) + CO(g)

(2)

where s, l, and g refer to the solid state, the liquid state and the gas state, respectively. Eq. (2) is either a purely solid–solid or a liquid–solid reaction (above 1450 ◦ C, quartz melts) [18]. Once carbon monoxide (CO) is formed, SiO can also be produced according to reaction: SiO2 (s) + CO(g) → SiO(g) + CO2 (g)

(3)

Since an significant amount of carbon remains in the material, any CO2 produced will be consumed immediately by the Boudouard reaction, Eq. (4), to form CO gas CO2 (g) + C(s) → 2CO(g).

(4)

In these reactions, carbon is either a CO2 getter or a CO generator, which keeps the CO2 /CO ratio low enough to make the reduction of SiO2 possible by gas phase CO. In a second step, the gaseous silicon monoxide (SiO) subsequently reacts further with carbon according to the following gas–solid reaction: SiO(g) + 2C(s) → SiC(s) + CO(g)

(5)

The SiO vapor from Eqs. (2) and (3) reacts with carbon to yield SiC (s) nuclei heterogeneously on the surfaces of carbon through Eq. (5), which is commonly accepted mechanism of bulk SiC formation [30]. The synthesized SiC is strongly dependent of the carbon source [31], which makes it possible that the resulting SiC ceramic is of a woodlike microstructure. In general, as soon as SiC forms on carbon, the growth process via Eq. (5) can be hindered by either the solid diffusion of carbon or the diffusion of SiO gas molecules through SiC layer. In the present work, the struts are porous after initial carbothermal reduction reaction (Fig. 8b), which provides the paths for diffusion of SiO gas molecules into carbonaceous cell wall, allowing Eq. (5) to continue.

Fig. 5. SEM micrographs of carbonized oak wood obtained at 1200 ◦ C for 4 h: (a) and (b) cross-sections perpendicular to axial direction, and (c) cross-section parallel to axial direction.

Weight change (%)

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233

200

3.5. SEM analysis

160

Fig. 5 shows the cellular microstructures of biocarbon template from oak wood. As seen in Fig. 5, the microstructure of the as-prepared oak wood charcoal shows 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 three groups, depending on their cross-sectional area: large channels (noted by “A”), medium-sized channels (noted by “B”), and small channels (noted by “C”), which form honeycomb structures. The average diameter of each group of cells is 100 ␮m for the large cells, ∼25 ␮m for the medium-sized cells and 6 ␮m for the small cells. Most of the cellular pores show a round or elliptical shape. Silica content in charcoal/silica composite may be controlled by the number of infiltration cycles. Silica amount in the charcoal/silica composite increases as the number of cycles of infiltration procedure increases, as shown in Fig. 6. Silica content quickly increases prior to the fifth infiltration, tardily increases owing to “bottle-neck” effect of the cellular pores, and subsequently becomes constant after the treatment procedure with silica sol is repeated six times. The SEM micrographs of the charcoal/silica composites are shown in Fig. 7. It is seen that fine SiO2 gel fibers or rods appear in the cellular pores after one cycle of infiltration procedure (Fig. 7a). The cellular pores are completely and uniformly filled up with dried SiO2

120 80 40 0

0

2

4

6

8

10

Number of cycles of SiO2 sol infiltration procedure (times)

Fig. 6. Weight changes of SiO2 contained in charcoal, relative to the increasing number of cycles of infiltration procedure.

In our experiments, the carbothermal reduction was carried out in static argon atmosphere, thus the SiO gas pressure could be maintained much higher than when argon gas was flowing into the reaction chamber and carrying the SiO gas out of the furnace. Hence, under very high SiO partial pressure the disproportionation reaction of gaseous SiO into Si and SiO2 can take place according to the reaction 2SiO(g) → Si(s) + SiO2 (s)

(6)

The resulting Si would react with carbon to give rise to spherical SiC particles [32] according to the reaction Si(s) + C(s) → SiC(s)

(7)

Fig. 7. SEM micrographs of charcoal/SiO2 composites when the infiltration procedure is repeated (a) 1 and (b) and (c) nine times, respectively. (a) and (b) Cross-sections perpendicular to axial direction, and (c) cross-section parallel to axial direction.

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Fig. 8. SEM micrographs of woodlike ␤-SiC ceramics obtained from C/SiO2 composites with a C-to-SiO2 molar ratio ≈3 under different conditions: (a) 1600 ◦ C, 4 h; (b) 1500 ◦ C, 4 h; and (c) 1600 ◦ C, 8 h. (a, b, and c) Cross-sections perpendicular to axial direction.

gel when the infiltration procedure is repeated nine times (Fig. 7b). The SEM micrographs of the resulting porous SiC ceramic are shown in Fig. 8. It can be seen that the resulting SiC material is of a microstructure pseudomorphous to biocarbon template derived from oak wood. The density of the struts of the products may be greatly affected by holding time at elevated temperatures. From SEM observations it is concluded that the SiC forming the cell wall material between the cells is highly porous in the initial state of reaction (Fig. 8b), making rapid gas transport possible through SiC layer. In the literature [33,34], half of the initial carbon is supposed to be released from the template via CO (g) (produced mainly by Eqs. (2) and (5)) evaporation leaving a substationally higher porosity in the cell wall. Thus, tailoring of the strut microstructure by suitable processing techniques seems to play a key role for improving the mechanical properties of low-dense biomorphous silicon carbide ceramics. When the charcoal/silica composite is heated at 1600 ◦ C for a longer time, sintering may occur owing to small initial SiC particles (1–5 ␮m) (Fig. 8b), resulting in a densification of the strut material (Fig. 8c), which would improve mechanical strength and does not destroy structural integrity of the products. At the same time, it becomes increasingly difficult to transport SiO to the carbon reaction sites and reaction rate decreases.

4. Conclusions Highly porous SiC ceramic with a woodlike microstructure was prepared at 1600 ◦ C for 4–8 h in a static argon atmosphere by sol–gel and carbothermal reduction techniques using TEOS and oak wood as the starting materials. The resulting SiC ceramic is composed of ␤-SiC with traces of ␣-SiC. The molar ratio of C-to-SiO2 ≈ 3, namely three cycles of impregnation procedure, is established as an optimum value for the preparation of woodlike silicon carbide ceramic. The free carbon content in porous SiC ceramic prepared using this procedure is controllable by number of cycles of infiltration procedure. Silicon carbide struts in the resulting porous SiC ceramic are formed by gas–solid reaction of SiO (g) and C (s) during carbothermal reduction reaction. Because of small initial particles sizes, sintering is likely to occur at moderate temperatures that may be used for increasing the density of the strut material and the mechanical properties. Conversion of wood into ceramic materials with a microstructure pseudomorphous to the bioorganic template anatomy offers a great potential for designing novel ceramics with anisotropic cellular morphologies. Acknowledgements This study is supported by the National Natural Science Foundation of China (No. 50272051 and No. 59872025).

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