C ceramics by sol–gel and carbothermal reduction processing

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Fabrication of biomorphic ZrC/C ceramics by sol– gel and carbothermal reduction processing Haitang Wu, Tingting Zhang, Yi Li

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S0272-8842(15)01290-0 http://dx.doi.org/10.1016/j.ceramint.2015.07.004 CERI10888

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Cite this article as: Haitang Wu, Tingting Zhang, Yi Li, Fabrication of biomorphic ZrC/C ceramics by sol–gel and carbothermal reduction processing, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2015.07.004 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Fabrication of biomorphic ZrC/C ceramics by sol–gel and carbothermal reduction processing Haitang Wu*, Tingting Zhang, Yi Li College of Forestry, Northwest A&F University, Yangling 712100, China *Corresponding author: Haitang Wu

Tel.: +86 29 87099075; E-mail:

[email protected]

Abstract Biomorphic ZrC/C ceramics were fabricated by infiltration of wood-derived biocarbon template with a novel ZrO2-sol using inorganic precursor zirconium oxynitrate as the source of zirconium and acetylacetone as chemical modifier, followed by subsequent high temperature pyrolysis and reaction treatment under argon at 1500 °C. The microstructure, phase composition, pore size distribution and mechanical properties of ZrC/C ceramic, as well as the conversion mechanism of wood to ZrC/C were investigated. The results showed that the impregnant was firstly decomposed to zirconia, and then reacted with carbon in the cellular wall to form ZrC at high temperature. The cellular morphology of biocarbon template was retained well in the resulting ceramics consisting of cubic ZrC and residual amorphous carbon. The biomorphic ZrC/C displayed a multimodal pore size distribution in the macro-size scale. Moreover, it was found that the mechanical properties of ZrC/C improved significantly on the basis of biocarbon template due to the formation of ZrC layer at inner cell walls.

1

Keywords:

Biomorphic

ceramics;

ZrC;

Microstructure;

Sol–gel

processes;

Carbothermal reduction 1. Introduction In recent years, the manufacturing of ceramic materials from biological templates has attained particular interest due to the possibility of producing novel ceramic materials with a unique microstructure pseudomorphous to that of naturally grown plant [1-4]. Wood is an excellent biotemplate which exhibits the ease of chemical modification, high degree of organization, and inherent self-assembly properties. So far, biomorphic ceramics or composites such as SiC [5], SiOC [6], Al/C [7], TiN/C [8] and C/SiC–ZrC [9] have been prepared from wood or wood wastes through the use of the structural features of naturally grown materials. Such wood-derived ceramics are promising new porous materials with outstanding performances like good friction and wear resistance, corrosion resistance and high specific surface, which may be suitable for a variety of industrial applications, such as hot gas or molten metal filters, heat insulators, catalyst carriers, battery electrodes, immobilization supports for living cells, as well as biomedical materials. Previously, biomorphic SiC ceramic was intensively studied because of its low production costs and excellent properties [10-14]. Among carbide ceramics, ZrC is also one of the most technologically important materials. Besides high melting point, ZrC has a unique combination of high hardness, high modulus of elasticity, resistance to wear and high electrical conductivity, making it a promising candidate for many applications such as wear resistant parts, electronic devices used for thermoionic transducers and components used in ultra-high

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temperature environments [15,16]. As yet, there have been few attempts to produce biomorphic ZrC ceramics from wood. Generally, there are several methods for the preparation of these biomorphic ceramics, including the infiltration of the pyrolyzed biocarbon template with gaseous or liquid precursors, chemical vapor infiltration and sol–gel infiltration. The sol–gel method has proved to be an effective method for preparing ceramic materials, owing to low processing temperatures, simple procedure and the intimate contact of the reactants [17,18]. Rambo et al produced biomorphic ZrC ceramic by sol–gel and carbothermal reduction techniques using zirconium n-propoxide and pine wood as starting materials [19]. Although ZrC ceramic can be synthesized via sol–gel method using Zr-alkoxides as precursors, there are still several drawbacks such as high cost, toxicity and the difficulty to determine C/Zr molar ratio due to the pyrolyzed carbon brought by the alkoxide. To overcome those drawbacks, inorganic precursors like zirconium oxychloride previously have been used to synthesize some advanced ZrC materials such as ultra-fine ZrC powders [20] and ZrC-based biomorphic ceramics [21] by sol–gel method. However, so far to the authors’ best knowledge, no experimental study has been published pertaining to the application of zirconium oxynitrate as zirconium source in the fabrication of biomorphic ZrC ceramics by sol–gel and carbothermal reduction processing. In this study, an attempt was made to manufacture biomorphic ZrC/C ceramics through a combination of sol–gel and carbothermal reduction process using zirconium oxynitrate as zirconium source. The microstructure, composition and formation

3

mechanism of biomorphic ZrC/C ceramic were studied. Meanwhile, the pore size distribution and mechanical properties of the prepared ZrC/C ceramic were investigated. 2. Experimental procedure 2.1. Sample preparation Pine wood (a softwood) was used as plant template for the fabrication of biomorphic ZrC/C ceramics because of its much simpler and more uniform structure compared with hardwood. After drying at 70 °C for 24 h, the wood specimens were pyrolyzed into porous biocarbon template (charcoal) at 850 °C for 2 h in a vacuum furnace. Zirconium oxynitrate (ZrO(NO3)2·2H2O, Sinopharm Chemical Reagent Co., Ltd., Shanghai, China) used as the source of zirconium was of analytical grade. All other chemicals used in this study were obtained commercially and were of analytical grade. ZrO2-sol was prepared by a sol–gel process based on literature with some modifications [22]. Firstly, ZrO(NO3)2·2H2O solution (0.2 mol/l) was prepared by dissolving certain amount of ZrO(NO3)2·2H2O in a mixed solution in which the volume ratio of C2H5OH/H2O was 4:1. Then acetylacetone was added to the solution at a zirconium oxynitrate/acetylacetone molar ratio of 1:1 to improve the stability of the solution. Meanwhile, the pH value of the mixed solution was adjusted to between 6 and 7 using dilute ammonia under stirring. The mixture was heated in water bath at 80 °C for 1 h, and then the ZrO2-sol was obtained. The samples of charcoal were subsequently vacuum infiltrated with the ZrO2-sol in a self-made equipment. After that, the ZrO2-sol contained in charcoal was gelled at 60 °C for 12 h and dried at

4

110 °C for 2 h to remove other solvents. This treatment procedure of impregnation, gelling and drying, namely cycle of impregnation procedure, was repeated several times, to achieve a high content of ZrO2 precursor in the charcoal. After each impregnation cycle, the weight gain of specimen was measured to determine the ZrO2-sol uptake. Finally, the dried ZrO2-gel/charcoal sample was heat-treated in a tube furnace under argon atmosphere. The furnace was heated to the desired temperature with a heating rate of 2 °C/min, and then held for 2 h to carry out the carbothermal reduction of the as-prepared ZrO2-gel/charcoal composite, resulting in the formation of ZrC/C ceramic. 2.2. Characterization The pyrolytic behaviors of pine wood and the ZrO2-gel were characterized by thermal gravimetric analysis and differential scanning calorimetry (TGA-DSC, Netzsch STA 449C) in an argon atmosphere with a heating rate of 10 °C/min. Fourier transform infrared spectra (FTIR, Nicolet Avatar360) detection was performed in the wavenumber range of 4000–400 cm-1 to investigated structural changes of the sample during heating. The phase composition of the sample was identified by X-ray diffraction (XRD, Rigaku Dmax-rb) using Cu Kα radiation. The microstructures of the prepared biomorphic ceramics were characterized by scanning electron microscopy (SEM, Hitachi S4800). The pore size distributions of the samples were determined by means of mercury intrusion method (Micromeritics AutoPore IV 9500). The specific surface areas were measured by BET method. For the evaluation of mechanical properties of materials, the samples (10 mm × 10 mm × 50 mm chamfered bars) were

5

divided to axial samples and radial samples according to the direction of length perpendicular and parallel to the direction of wood growth, respectively. Flexural strength was tested by the three-point bending method. Fracture toughness was tested by the single edge notched beam (SENB) method. All the mechanical properties were measured using an Instron universal testing machine (M1185) with a 20 mm span and a crosshead speed of 0.5 mm/min. Five samples were measured for each group. 3. Results and discussion 3.1. The thermal decomposition of pine wood and ZrO2-gel In order to study the pyrolytic behavior of pine wood, small pieces were heated in the thermal analyzer in an Ar atmosphere. Fig. 1a shows TGA–DSC curves of pine wood up to 900 °C. It can be seen that the pyrolysis of wood proceeded in three stages. The first stage occurred below 130 °C with a little weight loss of about 6%, which should be caused by the desorption of adsorbed water. The second stage occurred in the temperature range of 250–450 °C. In this stage, the three major components of wood, namely hemicellulose, cellulose and lignin, broke down in a stepwise manner [23]. Numberous volatile products were released (CO2, CO and other organics vapors) due to chain scissions, or depolymerization, and the breaking of C–O and C–C bonds, which caused a fast and significant weight loss of about 70%. Additionally, aromatization occurred above 400 °C, resulting in the forming of graphitic layers and a rapid increase in carbon content of the material [24]. The third stage of pyrolysis took place from 450 °C to 900 °C. During this process, a further weight loss of about 14% was detected as the thermal induced decomposition and rearrangement reactions

6

went on. The carbon content in the decomposed product became higher and higher, and finally a carbon template structure was formed. The residual carbon mass was about 19 wt% at 900 °C. The TGA–DSC curves of pine wood exhibited slight differences compared with those of other types of woods such as tilia amurensis wood [24], which should be due to differences in the chemical contents of tilia amurensis and pine woods. The TGA–DSC curves of the ZrO2-gel precursor are shown in Fig. 1b. As can be seen, the decomposition of the gel precursor started below 200 °C and mainly occurred between 250 °C and 350 °C. Above 400 °C, the weight loss decelerated and tended to stabilize at temperatures above 800 °C. The total weight loss of the gel precursor was about 69.3 wt% up to 1000 °C. During the sol–gel process, inorganic metal salts can partially, or completely, hydrolyse and polymerize to form a sol after mixing with solvents (mostly water). It is well known that the metal precursors are easy to react spontaneously with water and form precipitates due to successive hydrolysis and polymerization reactions. In order to control the reactivity between Zr4+ and water and obtain a sol with an appropriate viscosity, Zr4+ can be chemically modified by acetylacetone which acts as chelating and bridging ligands [25,26]. According to previous studies [27], every four Zr4+ could join together with the chelating agent of acetylacetone bonded with OH- to form Zr4(acac)4(OH)12 in the reaction system, which further polymerized and dehydrated for Eqs. (1) and (2). Thus, the endothermic peak observed at 133.6 °C in the DSC curve could be attributed to the evaporation of physical absorption water. The sharp endothermic peak observed at

7

327.1 °C could be caused by the removal of water molecules derived from the dehydroxylation

of

the

reagents

along

with

the

decomposition

of

acetylacetone organic groups connected with Zr4+ in the precursor, which was in agreement with the TGA result. Additionally, the decomposition of NH4NO3 had some effects on this endothermic peak (Eq. (3)). The exothermic peak at 576.3 °C might be ascribed to the crystallization of zirconia from amorphous phase to tetragonal for Eq. (4). The exothermic peak at 765.4 °C could be due to the growth of the grain. As might be expected, the weight loss accelerated again when the temperature exceeded 1100 °C, which was believed to be the carbothermal reduction process. Condensation

[Zr4(acac)4(OH)12] + nH2O Zr-OH + Zr-OH NH4+ + NO3-

Dehydration

→

Neutralization

→

Zr-O-Zr(amorphous)

→

[Zr4(acac)4(OH)12+n]n- + nH+

Zr-O-Zr(amorphous) + H2O Decomposition

NH4NO3 →

Crystallization

→

N2O + 2H2O

t-ZrO2

(1) (2) (3) (4)

3.2. Weight change The weight gain and zirconia content in the ZrO2-gel/charcoal composite may be dependent on the number of impregnation cycles. The weight change of the charcoal sample after repeated sol-impregnation and drying cycles is shown in Fig. 2. As can be seen, the weight gain increased quickly prior to the fifth impregnation due to the large accumulation of ZrO2-gel at the inner cell walls of the large vessels, then increased tardily owing to the clogging of the pores on the surface of the specimens by the gel, and subsequently became constant after eight impregnation/drying cycles.

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After the treatment procedure with ZrO2-sol was repeated nine times, the final weight gain of the sample was about 365 wt%. This value was higher than those of other ZrO2-sol infiltrated specimens reported in the literature [19], which could be due to the high porosity of the pine charcoal sample, an intermediate viscosity of the ZrO2-sol as well as its good wetting ability for charcoal. For the complete conversion of carbon present in charcoal to ZrC during the carbothermal reaction, a sufficient amount of ZrO2 should be introduced into the sample. The total ZrO2 to charcoal mass ratio should be about 3.42 which may be calculated from the stoichiometry of the carbothermal reduction reaction. After heat treatment at 1000 °C, the mass yield of the ZrO2-gel dried at 110 °C (Ygel) was only 48 wt% due to the mass decrease during pyrolysis. In view of these facts, the dried gel mass (mg, g) to charcoal mass (mc, g) ratio (coefficient K) could be calculated by Eq. (5). The theoretical value of K was equal to 7.13 in this case. K=

mg mc

=

3.42 Ygel

(5)

Fig. 2 also shows the variation of calculated coefficient K value with the number of impregnation cycles. It is clearly shown that the coefficient K value increased gradually as the number of impregnation cycles increased. However, the K value for obtained sample after nine impregnation/drying cycles was still lower than theoretical value, therefore it was impossible to introduce the necessary amount of ZrO2 into the charcoal for ZrC synthesis. Advanced densification of the ZrO2-gel/charcoal composite with a higher amount of ZrO2 might be possible by high-pressure infiltration using less impregnation cycles, which was not carried out in this study. 9

Consequently, a complete conversion of carbon to ZrC could not take place in this case and the resulting biomorphic ceramic was actually composed of carbon and ZrC. 3.3. FTIR analysis To study the structural evolution of ZrO2-gel/charcoal composite during the heat treatment, FTIR spectra of the composite were measured from room temperature to 900 °C. The results are shown in Fig. 3. In the IR spectrum (Fig. 3a) of the ZrO2-gel/charcoal composite, the strong absorption band at 3400 cm-1 was due to the stretching vibrations of O-H which was present in gel. The absorption bands at 1590 cm-1 (C=O), 1529 cm-1 (C=C) and 1278 cm-1 (C-CH3) indicated the existence of acetylacetone ligand in ZrO2-gel at room temperature. The absorption bands at 540 cm-1 and 775 cm-1 were attributed to Zi-O and Zr-O-Zr bonds, respectively, which implied that the Zr-O-Zr chain, the backbone of polymer, existed in ZrO2-gel. After heat treatment of the composite at 300 °C, the above-mentioned absorption bands assigned to ZrO2-gel weakened in Fig. 3b. In the IR spectrum of the product obtained at 600 °C (Fig. 3c), the peaks assigned to O-H bond and acetylacetone ligand in ZrO2-gel became negligible. This phenomenon suggested that most of the organic functional groups of gel were decomposed at relatively low temperatures, which was consistent with the TGA–DSC analysis. When the sample was heated to 900 °C, only the peaks assigned to Zr-O and Zr-O-Zr bonds existed, and other bands such as 3400 cm-1 (attributed to -OH) disappeared in the FTIR spectrum (Fig. 3d), indicating that the conversion of ZrO2-gel from organic to inorganic was completed below 900 °C. And the conversion from amorphous to crystalline phase during the pyrolysis process

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was further studied by XRD analysis. 3.4. XRD analysis The XRD patterns of the as-produced biomorphic ceramic samples obtained at different temperatures are shown in Fig. 4. In the XRD pattern of the zirconia/charcoal composite obtained at 1100 °C for 2 h in an Ar atmosphere, peaks due to tetragonal ZrO2 phases were observed indicating the gel had been decomposed to t-ZrO2 under the experimental condition of the present work. Then, some t-ZrO2 transformed to m-ZrO2 when the temperature increased to 1200 °C. Meanwhile, the initial formation of cubic ZrC was detected at 1200 °C. Upon heat treatment at 1300 °C, t-ZrO2 had transformed to m-ZrO2 completely. With the temperature increasing from 1300 °C to 1400 °C, the intensities of the diffraction peaks due to ZrC were further increased, indicating the increase of ZrC amount and the enhancement of crystallization. At 1500 °C, m-ZrO2 phase disappeared and ZrC with a high degree of crystallinity was the main phase, which suggested that nearly the whole amount of zirconia had been consumed during the carbothermal reaction. The crystallite size was calculated from the full-width at half-maximum of the (111) peak of ZrC using the Sherrer equation. It was about 59 nm. Also, it is worth noting that the broad peak positioned around 26° due to carbon was observed for all the samples obtained at different temperatures, suggesting that residual amorphous carbon still existed in the final materials. This was consistent with the previous results of K value analysis. As a result, the resulting ceramic prepared by this technique was a diphase composite consisting of amorphous carbon and cubic ZrC.

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3.5. SEM analysis The microstructures of pine wood charcoal, the ZrO2-gel/charcoal composite and the ZrC/C composite are shown in Fig. 5. Pine is a coniferous wood which exhibits a narrow pore size distribution with a mean pore diameter of about 20 µm [8]. Despite the high weight loss and anisotropic shrinkage of the wood during the pyrolysis process, the channel structure of the initial wood was not damaged and was still maintained well in the carbon preform (Fig. 5a and b). It can be seen that the charcoal exhibited the morphology of hollow channels with a elliptical or rectangular shape, where the black part was lumen and grey part was carbon wall formed by carbonization of cell wall (Fig. 5a). Hollow channels of charcoal had uniform arrangement with carbon wall joining each other. The diameters of a majority of channels were in the range of 10–30 µm, and the thickness of carbon wall was about 2–3 µm. The difference in diameters of hollow channels was due to the uneven distribution of texture of wood. The channels of the carbon preform would be utilized for ZrO2-sol infiltration. The microstructure of the ZrO2-gel/charcoal composite after sol infiltration is shown in Fig. 5c. During the impregnation process, ZrO2-sol infiltrated into the channels and pores of the charcoal and was retained in these pores after gelation. It is noteworthy that the ZrO2-gel/charcoal composite perfectly retained porous carbon of its natural counterparts. The shape, size and distribution of ZrO2-gel were controlled by the channel structure of the charcoal. With increasing the number of impregnation cycles, the amount of ZrO2-gel deposited increased, however, a few large pores were

12

not completely filled. Hence the complete conversion of the biocarbon template into ZrC was not achieved in this study. Fig. 5d shows the SEM image of the as-prepared ZrC/C sample at 1500 °C for 2 h. The main microstructural features of the biocarbon template were well reproduced in the biomorphic ZrC/C ceramic. Based on this behavior, it can be speculated that the in situ reaction between ZrO2 from impregnant and carbon template had occured in the cellular wall to form ZrC. The pore structures composed of ZrC that replaced the carbon as skeleton became the basic microstructure of the biomorphic ZrC/C ceramic, which would improve mechanical properties and did not destroy the structural integrity of the products. In addition, it was observed that the resulting ceramics had coarse surfaces of pores, which could be attributed to the recrystallization of fabricated ZrC and the release of some gaseous products such as carbon monoxide during the high temperature reaction. 3.6. Composition, surface area, porosity and pore size distribution Table 1 shows the compositions, porosities and surface areas of pine wood, charcoal and ZrC/C ceramic obtained in present investigation. In order to calculate the content of ZrC, the ZrC/C was oxidized thoroughly into ZrO2 during the process of burning off the residual carbon at 800 °C for 2 h. As the amount of Zr remained unchanged in this process, the content of ZrC in biomorphic ZrC/C ceramic could be calculated from the content of ZrO2 oxidized from ZrC. Furthermore, the volume content of carbon could be determined by subtracting the sum of percentages of ZrC and porosity from 100. As can be seen, the volume content of residual carbon was

13

higher than that of ZrC in the ZrC/C sample. The low value for the obtained ZrC content might be due to the clogging of the pores and the non-homogeneity of the sol-infiltration after multiple infiltration/drying cycles as well as the large volume loss during the decomposition of the ZrO2-gel. In the pyrolysis of pine into charcoal, a large number of organic gases were released, which caused an anisotropic shrinkage of about 20% in axial but 27% in radial and 32% in tangential directions in the experiment. From Table 1, it also can be seen that the specific surface area and porosity of charcoal were higher than those of the original wood. Compared with charcoal, both the specific surface area and porosity of ZrC/C sample increased slightly, indicating the introduced impregnant had been consumed after carbothermal reduction. The formed ZrC layer by the reaction of the impregnant with the carbon template in the cell wall and the expulsion of some gaseous products in the carbothermal processing could contribute to the higher surface area and porosity. For porous materials, it is of great importance to investigate the pore size distribution. The pore size distributions of pine wood, porous carbon template and ZrC/C ceramic were analyzed by mercury porosimetry. As shown in Fig. 6, all the samples had multimodal pore size distributions. The main pore size of pine wood ranged between 0.1 µm and 0.5 µm, and residual pore size ranged mostly between 10 µm and 40 µm. From the comparison of the pore size distribution between original wood and carbon template, it can be found that the big peak in the range of 0.1–0.5 µm shifted to right with a significant increase of incremental intrusion volume

14

whereas the two other peaks in the size range of 10–40 µm exhibited no obvious shift and their intrusion volumes did not change much. Therefore, carbonisation generated a carbon template with an abundance of macropores which led to a high macroporosity. Comparing the pore size distribution of carbon template in Fig. 6b with that of ZrC/C shown in Fig. 6c, it can be found that there was no apparent change in the pore size distribution after infiltration and pyrolysis of ZrO2-sol precursor. The result also proved that the texture of the carbon template was replicated very well in the biomorphic ceramic. It is notable that the pore size distributions of samples measured by mercury intrusion method were different from SEM observations in Fig. 5a and d, which displayed a uniform pore structure with pore size being mainly in the range of 10–30 µm. In fact, a great quantity of micropores were located in the walls of the vessel as shown in Fig. 5b, which might cause this discrepancy. Additionally, the bottleneck effect in the mercury intrusion method could have an effect on the difference of pore size distribution in the experiment. Similar results were also reported in the literature [28]. 3.7. Mechanical properties The mechanical properties of carbon template and the as-fabricated ZrC/C ceramic were preliminarily investigated. The results are given in Table 2. For all the samples, the flexural strength and fracture toughness of the axial samples were much higher than those of the radial samples, which should be attributed to the loading direction and the anisotropic pore orientation derived from microstructures of pine wood. Since all the samples were porous materials, the pores and cracks within the samples were

15

believed to act as the failure origin. Compared with the axial samples, cracks were easy to expand in the radial samples [29]. For biomorphic ZrC/C, the axial flexural strength, radial flexural strength, axial fracture toughness and radial fracture toughness were 144 MPa, 62 MPa, 2.9 MPa·m1/2 and 1.5 MPa·m1/2, respectively, which were far higher than those of the carbon template from pine wood. The improvement in mechanical properties should be due to the formation of ZrC layer at inner cell walls. In the present work, repeated infiltration increased the amount of ZrO2-gel, which was transformed to ZrO2 during heat treatment in Ar. The in situ generated ZrO2 was then further reacted with carbon to produce ZrC on the surfaces of carbon by carbothermal reduction reaction: ZrO2(s) + 3C(s) → ZrC(s) + 2CO(g)

(6)

In fact this reaction was a gradual transformation which could proceed through several stages with the formation of gaseous intermediate ZrO and solid intermediate oxycarbide compound [30]: ZrO2(s) → ZrO(g) + 0.5O2(g)

(7)

0.5O2(g) + C(s) → CO(g)

(8)

ZrO2(s) + CO(g) → ZrO(g) + CO2(g)

(9)

C(s) + CO2(g) → 2CO(g)

(10)

Obtained zirconium monoxide reacted with carbon ZrO(g) + C(s) → ZrCxOy(s) + CO(g)

(11)

The resulting oxycarbide further reacted with CO, and the synthesis of pure ZrC phase was the result of the decrease of the oxygen content in intermediate oxycarbide

16

ZrCxOy(s) + CO(g) → ZrC(s) + CO2(g)

(12)

During this process, the release of gaseous CO resulted in the increase in porosity of carbon struts and provided the paths for the diffusion of ZrO gas molecules into carbonaceous cell wall, allowing reactions (11) and (12) to continue to form the final ZrC products. Once ZrC was produced on carbon, it became increasingly difficult for either the diffusion of gaseous ZrO or the solid diffusion of carbon through ZrC layer, and the reaction rate decreased. At the end of this process, a tubular layer of ZrC was formed at inner surface since the shape of charcoal channel was elongated cylindric and the synthesized ZrC was strongly dependent on the carbon source. When the ZrC/C composite went through heat treatment at 1500 °C for a long time, sintering might occur leading to the densification of the strut material, which would further improve mechanical property and did not incur damage to the structural integrity of the products. Therefore, the resulting biomorphic ceramic can be considered as porous composite material with amorphous carbon matrix reinforced with ZrC tubes.

4. Conclusions A novel ZrO2-sol was successfully synthesized using zirconium oxynitrate, acetylacetone, ethanol and water as starting materials. Infiltration of ZrO2-sol into pine wood derived carbon templates followed by high temperature pyrolysis of the dried ZrO2-gel into ZrO2 and carbothermal reduction of the ZrO2 with the biocarbon template resulted in the formation of ZrC phase. The resultant material was composed of cubic ZrC and residual amorphous carbon, and the ZrC content was directly correlated with the number of cycles of impregnation procedure. The obtained ZrC

17

phase was mainly distributed on the surface layer of the cell wall. The biomorphic cellular microstructure of wood remained intact in the porous ZrC/C ceramics. The pore size distribution of the ZrC/C was found to be multimodal in the macro-size scale. Furthermore, the mechanical properties of ZrC/C improved noticeably on the basis of carbon template. The production of the biomorphic ZrC/C is environmentally benign and cost effective due to the use of a renewable resource and cheap starting materials. The ceramic produced by this method might be suitable for various applications such as high temperature resistant gas filters, catalyst carriers and corrosion resistant immobilization supports for microbes.

Acknowledgement Financial support from the National Natural Science Foundation of China (No. 51202198) is gratefully appreciated.

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Figure captions Fig.1. TGA–DSC curves of pine wood (a) and the ZrO2-gel precursor (b). Fig. 2. Weight gain and the coefficient K of the ZrO2-gel/charcoal composite as a function of the number of impregnation cycles. Fig. 3. FTIR spectra of (a) ZrO2-gel/charcoal and the composites after heat treatment at (b) 300 °C for 2 h, (c) 600 °C for 2 h and (d) 900 °C for 2 h. Fig. 4. XRD pattens of zirconia/charcoal samples after heat treatment at different temperatures. Fig. 5. SEM images of biomorphic ceramic samples: (a) cross-section perpendicular to axial direction of biomorphic carbon template, (b) cross-section parallel to axial direction of biomorphic carbon template, (c) ZrO2-gel/charcoal composite after infiltration by ZrO2-sol, and (d) ZrC/C ceramic obtained at 1500 °C for 2 h. Fig. 6. Pore size distributions of samples: (a) pine, (b) carbon template from pine and (c) biomorphic ZrC/C ceramic obtained at 1500 °C.

Table Table 1 Compositions, porosities and surface areas of the wood, charcoal and ZrC/C ceramic. Table 2 Comparison of mechanical properties of carbon template and ZrC/C.

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Table1 Compositions, porosities and surface areas of the wood, charcoal and ZrC/C ceramic. Pine wood

Charcoal ZrC/C

Volume content of carbon (%)

—

21.2

10.6

Volume content of ZrC (%)

—

—

7.7

Surface area (m2·g-1)

30.3

39.4

40.5

Porosity (%)

65.1

78.8

81.7

Table 2 Comparison of mechanical properties of carbon template and ZrC/C.

a

Samples

Carbon template

ZrC/C

Axial flexural strength (MPa) a

27 ± 2.2

144 ± 8.1

Radial flexural strength (MPa) b

13 ± 1.5

62 ± 4.3

Axial fracture toughness (MPa·m1/2) a

0.7 ± 0.1

2.9 ± 0.3

Radial fracture toughness (MPa·m1/2) b

0.5 ± 0.1

1.5 ± 0.2

The length of the sample and loading direction are parallel and perpendicular to the

direction of wood growth, respectively.

b

The length of the sample and loading

direction are perpendicular and parallel to the direction of wood growth, respectively.

24

25

26

27

28