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Physica E 40 (2008) 2540–2544 High specific surface area porous SiC ceramics coated with reticulated amorphous SiC nanowires Limin Shi à , Hongsheng Zhao, Yinghui Yan, Ziqiang Li, Chunhe Tang Institute of Nuclear and New Energy Technology, Tsinghua University, Beijing 100084, China Available online 18 October 2007 Abstract High specific surface area porous SiC ceramics, coated completely with reticulated amorphous SiC nanowires, have been fabricated with commercially available phenolic resin and silicon powders using a novel method. The results indicate that the specific surface area and porosity of the as-synthesized materials can be up to 112 m 2 g À1 and 81%, respectively. The coated amorphous SiC nanowires have uniform structure and smooth surfaces. The electron emission turn-on field and threshold field are about 2.9 and 6.7 V mm À1 , respectively. This kind of materials may simultaneously possess the unique properties of porous materials, SiC, and nanowires. And they may have promising applications in a wide variety of areas. r 2007 Elsevier B.V. All rights reserved. PACS: 61.46.Àw; 81.05.Je Keywords: Porous; SiC; Nanowires; Ceramics 1. Introduction Due to their excellent performances, such as low density, high permeability, and high specific surface areas, porous materials have found numerous applications, including filters, catalyst supports, sensors, separations, and tissue engineering [1,2]. SiC is a typical transition metal carbide with excellent physical, chemical, mechanical, and electro- nic properties, which make it extensively used in many fields such as metallurgy, refractory, and environment protection [3,4]. During the past two decades, one- dimensional nanomaterials (nanotubes, nanowires, nanor- ods, and nanofibers) have attracted considerable attention because of their unique physico-chemical properties and wide variety of potential applications [5,6]. It has been evidenced that the porous SiC ceramics combine the properties of porous materials with those of SiC. This is the same with SiC nanowires. In order to meet the stringent requirements of some applications, various methods have been developed for fabricating porous SiC ceramics [7,8] and SiC nanowires [9,10]. Similarly, the preparation of high specific surface area porous SiC ceramics coated entirely by reticulated SiC nanowires may make it possible to obtain a kind of functional and structural material, which perhaps can simultaneously possess the excellent properties of porous materials, SiC, and 1D nanomaterials. Unfortunately, few reports on the fabrication of this unique material are available until now. Here, we propose a novel method to synthesize high specific surface area porous SiC ceramics, which are coated entirely with reticulated SiC nanowires. Encouraged by the success in preparing SiC nanowires [11], Al was used as catalyst in the present experiments. The specific surface area of the porous SiC ceramics produced in our process can be up to 112 m 2 g À1 . The porosity is greater than 80%. The electron emission turn-on field is 2.9 V mm À1 and the electron emission threshold field is a s low as 6.7 V mm À1 . The reticulated SiC nanowires, which are coated on the external surface of the as-synthesized porous SiC ceramics, are amorphous. At the same time, the SiC nanowires have uniform structure and smoot h surface. ARTICLE IN PRESS www.elsevier.com/locate/physe 1386-9477/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2007.10.034 à Corresponding author. Tel.: +86 10 89796095; fax: +86 10 69771464. E-mail address: slm02@mails.tsinghua.edu.cn (L. Shi). 2. Experimental Commercially available phenolic resin (barium phenolic resin; Beijing Fiberglass-Reinforced Plastics Research and Design Institute, Beijing, China) and silicon powder (average particle size of 9.4 mm; Beijing Da Di Zelin-Silicon Limited Company, Beijing, China) were used as carbon source and silicon source, respectively. Analytical ethanol was used as solvent and de-ionized water as precipitator. Aluminum powders were used as catalyst. The fabrication of porous SiC ceramics constructed by SiC nanowires consisted of the following steps: (1) Preparation of precursor powders based on coat-mix proces s [12–14]: After being properly weighed, 10 g phenolic resin was first dissolved into 20 g ethanol to form the binder solution, which was stirred vigorously at 45 1C for 1 h. Ten grams of silicon powders were subsequently introduced into the binder solution. After being homogeneously mixed at 45 1C for another 1 h, the obtained slurry was injected into de- ionized water. Then, the aqueous solution was stirred at 45 1C for about 2 h. This step was followed by decanting, filtering and drying. At last, the precursor powders, core–shell silicon-phenolic resin powders, could be ob- tained. (2) Molding: The obtained precursor powder s were pressed into the cylindrical green compacts with a diameter of 10 mm using a stainless steel mould. The temperature, pressure and soaking time were maintained at 80 1C, 0.1 MPa and 1 h, respectively. (3) Carbonizing: This step was performed by heating the green bodies in a program- mable quartz tube furnace in a flowing Ar atmosphere. For the sake of not destroying the samples, they were heated from room ambient temperature to 800 1C with a heating rate of 0.5 1C min À1 . The holding time is 2 h at 800 1C. (4) Sintering: The resul ting carbonized compacts and Al powders, which were held in a graphite crucible with a graphite lid, were sintered in a graphite furnace in Ar atmosphere. Here, the graphite crucible covered by a graphite lid was used to provide a relatively closed environment for the synthesis of porous SiC ceramics coated entirely by SiC nanowires. The heat treatment started at ambient temperature and stopped at 1500 1C with a he ating rate of 5 1C min À1 , then cooled in the furnace to room ambient temperature. The pressure of Ar and soaking time were 0.15 MPa and 2 h, respectively. X-ray diffraction (XRD) pattern of the prepared porous SiC ceramics was recorded at room temperatur e on a Japan D/max-IIIA X-ray diffractometer using standard Cu Ka radiation. Renishaw RM1000 Raman microscope was used to further investigate the structure of the as-synthesized products. A Hitachi S-3000N scanning electron microscope (SEM), operated at an accelerating voltage of 10 kV, was employed to characterize the morphology of the porous ceramics. Further structure cha racterization and fast four- ier transform (FFT) of the SiC nanowires, which coated with the external surface off fabricated porous SiC ceramics, were investigated on a Tecnai TF20 high- resolution transmission electron microscope (HRTEM) operated at 200 kV. Energy dispersive X-ray spectroscopy (EDX; attached to the Technai TF20) was employed for identifying the elemental composition of the SiC nano- wires. The specific surface area, median pore diameter (volume), median pore diameter (area), average pore diameter (4 V A À1 ), bulk density, apparent density, and porosity of the fabricated porous SiC ceramics were determined by mercury intrusion porosimetry (AutoPore IV 9510, Micromeritics, USA). The field emission proper- ties were measured in a vacuum chamber at room temperature. 3. Results and discussion The color of the carbonized compacts is black. However, it can be seen directly that after heating at 1500 1C for 2 h with Ar pressure of 0.15 MPa, the color of the samples has turned into light green. This implies that a complete conversion of silicon and phenolic resin-derived carbon into SiC has been achieved at such an experimental condition. The light green color of the resulted products also indicates that the as-synthesized porous SiC ceramics possess a relatively higher purity. Fig. 1(a) and (b) shows the XRD patterns of the external and internal surfaces of the porous ceramics fabricated at 1500 1C for 2 h with Ar pressure of 0.15 MPa using our process, respectively. It can be found from Fig. 1(a) that no obvious diffraction peaks of SiC crystals existed in the XRD pattern of the external surface of the as-synthes ized materials. This indicates that the external surface of the resulting ceramics is amorphous state. As shown in Fig. 1(b), the five sharp characteristic peaks correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) planes of b-SiC ARTICLE IN PRESS Fig. 1. XRD patterns ((a) external and (b) internal) of high specific surface area porous SiC ceramics coated with reticulated SiC nanowires fabricated at 1500 1C for 2 h with Ar pressure of 0.15 MPa using our proposed process. The amorphous nature of the external of the as- synthesized SiC materials is revealed by the XRD patterns. L. Shi et al. / Physica E 40 (2008) 2540–2544 2541 phase according to the standard JCPDS cards (29-1129). Combined with the above XRD analyses, it can be concluded that the as-synthesized materials are SiC ceramics coated with amorphous layers. The SEM images shown in Fig. 2(a) and (b) are the typical morphologies of the external surface and fracture surface of the resulted porous SiC ceramics coated with reticulated SiC nanowires, respectively . Fig. 2(a) clearly reveals that the external surface of the fabricated ceramics are constructed completely by reticu- lated SiC nanowires with uniform diameter distribution in the range of 30–80 nm, and lengths over several tens of micrometers. It can also be found from Fig. 2(b) that the fabricated ceramics have large amounts of irregular pores. The further morphology characterization of SiC nano- wires, which coated on the surface of the as-synthesized porous SiC ceramics, can be seen clearly by HRTEM imaging shown in Fig. 3(a). It indicates that the SiC nanowires have uniform structure and smooth surface. The chemical composition of the corresponding SiC nanowire presented in Fig. 3(a) is characterized by EDX spectro- scopy, as shown is Fig. 3(b). The EDX peaks at 0.26, 0.52, 1.51, 1.73, 0.94, 8.02, and 8.92 keV correspond to the Ka lines of carbon, oxygen, silicon, aluminum, and the La,Ka, and K b lines of copper, respectively. The peaks of copper are originated from the copper grid, which is used to prepare the TEM sample. The oxygen is probably formed during the TEM sample preparation. This reveals that the nanowires are SiC nanowires with a small amount of ARTICLE IN PRESS Fig. 2. The representative SEM images ((a) external surface and (b) fracture surface) of the as-synthesized porous SiC ceramics. It can be found that the fabricated SiC ceramics with large amounts of pores are coated completely with reticulated nanowires with uniform structure. Fig. 3. (a) Representative TEM images, (b) EDX spectra of SiC nanowires, and (c) HRTEM lattice image of the reticulated SiC nanowires which are coated on the external surfaces of the porous SiC ceramics prepared using our process. It can be evidenced that the SiC nanowire with a uniform structure is amorphous, which is further confirmed by the FFT shown in (d). L. Shi et al. / Physica E 40 (2008) 2540–25442542 aluminum. The corresponding HRTEM lattice image and FFT shown in Fig. 3(c) and (d) indicate that the SiC nanowires are amorphous. Thi s is in accordance with the XRD analysis mentioned above. Table 1 lists the data for the obtained high specific surface area porous SiC ceramics co ated completely by reticulated SiC nanowires measur ed by mercury intrusion porosimetry. It can be found that the median pore diameter (volume) and average pore size are 1.6 mm and 25 nm, respectively. The bulk density is 0.61 g cm À3 and the apparent (skeletal) density is 3.19 g cm À3 . The open porosity can be up to 81%. Moreover, it is surprising to find that the specific surface area of the porous materials prepared by our process is greater than 100 m 2 g À1 . These unique properties may make this novel material an attractive candidate for many potential applications such as catalysis supports. Fig. 4 shows the typical plot of emission current density versus electric field (J–E) for the obtained high specific surface area porous SiC ceramics coated entirely by reticulated SiC nanowires. The electron emission is observed at an electric field of about 2.8 V mm À1 . The electron emission turn-on field (E to ) and threshold field (E thr ), defined as the macroscopic fields required to produce a current density of 10 mAcm À2 and 10 mA cm À2 , are about 2.9 and 6.7 V mm À1 , respectively. The low E thr value indicates that this material produced by our method may have promising applications in flat panel displays. By plotting ln(J Á E À2 ) versus E À1 , a Fowler–Nordheim (F–N) curve can easily be obtained (inset in Fig. 4). The linearity of the curve shows that the obtained materials possess a conventional field emission mechanism. 4. Conclusions In summary, we have demonstrated a process for the fabrication of high specific surface area porous SiC ceramics, which are coated completely with reticulated amorphous SiC nanowires. Commerci ally available phe- nolic resin an d silicon powders are employed as carbon source and silicon source, respectively. The reticulated SiC nanowires coated on the external surface of the as- synthesized materials are amorphous phase. Meanwhile, these SiC nanowires have uniform structure and smooth surface. The as-synthesized porous SiC ceramics, coated completely with reticulated amorphous SiC nanowires, possess high specific surface areas and porosity. The specific surface area and porosity can be up to 112 m 2 g À1 and 81%, respectively. The field emission properties of the obtained high specific surface area porous SiC ceramics coated by reticulated SiC nanowires are measured. The E to and E thr are about 2.9 and 6.7 V mm À1 , respectively. Thus, we can envisage that the prepared materials may find interesting applications in filters, catalyst supports, and electronic devices. Given the simplicity of the procedures and the unique properties of the as-synthesized materials, the method described here should attract a great deal of attention. Other high specific surface area porous carbide ceramics coated with carbide nanowires may also be fabricated by this method. Acknowledgment This work is supported by Key Faculty Support Program of Tsinghua University. References [1] R.A. Caruso, J.H. Schattka, Adv. Mater. 12 (2005) 1921. [2] Y.J. Wang, F. Caruso, Adv. Funct. Mater. 14 (2004) 1012. [3] T. Ishikawa, Y. Kohtoku, K. Kumagawa, T. Yamamura, T. Nagasawa, Nature 391 (1998) 773. [4] D. Nakamura, I. Gunjishima, S. Yamaguchi, T. Ito, A. Okamoto, H. Kondo, S. Onda, K. Takatori, Nature 430 (2004) 1009. ARTICLE IN PRESS Table 1 The major properties of the porous SiC ceramics coated entirely by reticulated SiC nanowires fabricated at 1500 1C for 2 h with Ar pressure of 0.15 MPa using our proposed process Property Value Specific surface area (m 2 g À1 ) 112 Median pore diameter (volume) (mm) 1.6 Median pore diameter (area) (nm) 6 Average pore diameter (4 V A À1 ) (nm) 25 Bulk density (g cm À3 ) 0.61 Apparent (skeletal) density (g cm À3 ) 3.19 Porosity (%) 81 It is surprising to find that the specific surface area of the resulting porous SiC ceramics is greater than 100 m 2 g À1 , which may help this novel material find many applications. Fig. 4. The typical emission J–E plot from the obtained high specific surface area porous SiC ceramics coated with reticulated SiC nanowires. The E to and E thr are about 2.9 and 6.7 V mm À1 , respectively. Inset: F–N plot. 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Luhleich, J. Dias, H. Nickel, Carbon 35 (1997) 95. [14] L. Shi, H. Zhao, Y. Yan, Z. Li, C. Tang, Powder Technol. 169 (2006) 71. ARTICLE IN PRESS L. Shi et al. / Physica E 40 (2008) 2540–25442544 . Physica E 40 (2008) 2540–2544 High specific surface area porous SiC ceramics coated with reticulated amorphous SiC nanowires Limin Shi à ,. 2007 Abstract High specific surface area porous SiC ceramics, coated completely with reticulated amorphous SiC nanowires, have been fabricated with commercially