J Electroceram (2006) 17:1109–1111 DOI 10.1007/s10832-006-9119-6

Electrically conductive carbon-coated ceramic fiber media Jae Chun Lee · Hyuk Chun Kwon · Yong-Pil Kwon · Sung Park · Hae-Won Lee · Jong-Ho Lee · Joosun Kim

Received: 28 June 2005 / Revised: 1 May 2006 / Accepted: 12 May 2006 C Springer Science + Business Media, LLC 2006 

Abstract Porous ceramic fiber composites were coated with pyrolytic carbon by the decomposition of either propane or infiltrated phenolic resin in a nitrogen atmosphere at 800–900◦ C. The amount of carbon coating was varied to tailor the electrical conductivity of the carbon-coated composites. The electrical and thermal conductivity of the composites were measured at room temperature using a two-point method and a hot-wire one, respectively. Up to 30 wt% pyrolytic carbon, the electrical conductivity σ showed linearly increasing tendency and was fitted by the effective conductivity according to the parallel rule of a mixture σ =Vi · σ i with an effective conductivity of pyrolytic carbon σ c and volume fraction of coated carbon Vc . The electrical conductivity of coated carbons prepared from propane at 900◦ C and phenolic resin at 800◦ C was of the order of 100 and 10−1 S cm−1 , respectively. Keywords Ceramic fiber . Carbon . Coating . Conductivity . Porous

conducted on non-conductive substrates to produce advanced functional materials; electroconductive structural ceramics for self-strain detection [1], and electrically heatable filters for gas removal [2]. This paper concerns the potential applications of electrically conductive carbon-coated porous ceramic fiber media in thermal insulation and electric heating areas. Thermal shielding performance of insulation media made of glass or ceramic fibers is comparable to those of polymers. Moreover, the formers are non-flammable and environmentally friendly products. Carbon is a good electrical conductor and easy to form as thin films. Carbon coatings on ceramics by pyrolysis of hydrocarbon and coated polymeric resin have been often carried out to produce functional media for the adsorption of volatile organic compounds, catalyst and membranes [3–6]. This paper describes the effect of carbon precursors on electrical properties of carbon-coated porous ceramic fiber media. The goal is to provide electrical conductivity suitable for electrical heating while maintaining the thermally insulative properties of the ceramic fiber media.

1 Introduction

2 Experimental procedure

Many of thermal-resistant ceramic materials are electrically and thermally insulative. Their applications are often passive and limited due to such an electrically non-conductive property. Coating of electrically conductive films has been

2.1 Sample preparation

J. C. Lee ( ) · H. C. Kwon · Y.-P. Kwon · S. Park Department of Materials Science and Engineering, Myongji University, Kyunggido 449-728, Korea e-mail: [email protected] H.-W. Lee · J.-H. Lee · J. Kim Nano-Materials Research Center, Korea Institute of Science and Technology, Seoul 136-791, Korea

Disk shaped porous ceramic fiber media with bulk densities of about 0.18 and 0.30 g cm−3 were prepared using ceramic fiber, colloidal silica and organic binder by a vacuum forming process described elsewhere [7]. The ceramic fibers used in this study were fabricated by melt-blown process and provided by Thermal Ceramics Korea. The composition of these fibers was 46–47 wt% SiO2 and 53–54 wt% Al2 O3 . The amorphous character of as-received fibers was confirmed by X-ray diffraction. Fiber media with high bulk density were coated with pyrolytic carbon by the decomposition of Springer

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J Electroceram (2006) 17:1109–1111

Fig. 1 Anisotropic effective electrical conductivity of porous carbon-coated ceramic fiber media as a function of carbon content obtained from different carbon precursors: Normal scale (left) and log scale (right), respectively

propane at 900 ◦ C [8]. For the fiber media with low bulk density, phenolic resin solutions of various concentrations were infiltrated into the open pores of the media. The resin coated composites were subsequently pyrolysed at 800 ◦ C for 1 h in nitrogen under atmospheric pressure to convert the resin to pyrolytic carbon. 2.2 Characterization The electrical resistance R of the carbon-coated sample was measured at room temperature using a two-probe method with Hewlett-Packard 34401A multimeter. All samples of typically ∼10 mm height, ∼31 mm width and ∼33 mm length L had a rectangular cross sectional area S cut from the 48 mm diameter disk shaped sample. The ends of the sample were coated with silver paint to promote consistently good electrical contact with the probes. The electrical conductivity σ = (1/R)(L/S) was calculated by taking the value of resistance and dimensional measurements. Carbon content of the coated sample was estimated by weight change after burning-off the carbon in air from the carbon-coated samples at 800◦ C for 1 h.

3 Results and discussion In Fig. 1,the bulk electrical conductivity of the coated samples σ in the two directions, x and z, is shown as a function of carbon content obtained from different carbon precursors. White symbols denote for the electrical conductivity in xdirection and black symbols for the one in z-direction, i.e., thickness direction. Schematic of structure of ceramic fiber media in this work is shown in Fig. 2.It has been known that fiber media produced by vacuum molding process show preferred orientation of fibers, which results in the microstructure and property anisotropy [9]. In Fig. 2, z-direction is parSpringer

Fig. 2 Schematic structure of porous fiber media [9]

allel to molding direction of fiber slurry. It can be also noticed that x- or y-direction is perpendicular to molding direction and parallel to the fiber array. This means that fiber composite has a layered structure in which the fibers both parallel to x or y are more cross-linked than those to parallel to z. For this preferred fiber orientation property, the electrical connectivity parallel to the fiber array (x- or y-direction) is relatively better than the one perpendicular to the fiber array (z-direction) as shown in Fig. 2. Therefore, the electrical conductivity perpendicular to the fiber array (z-direction) is lower than the one parallel to the fiber array (x- or y-direction) as shown in Fig. 1. Although there is some scattering in the electrical conductivity data in both x- and y-direction, linearly increasing tendency of electrical conductivity can be found with increasing carbon content in the range 0.5–30 wt%. The use of different carbon precursors gave different electrical conductivities for the carbon-coated fiber media, propane giving higher electrical conductivity than the phenolic resin. The electrical conductivity of carbon film coated on the ceramic fiber was estimated from the parallel rule of electrical conductivity in a mixture. For this, assumption was made that the porous carbon-coated ceramic fiber media consisted

J Electroceram (2006) 17:1109–1111

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of electrically non-conductive ceramic fibers coated with conductive carbon parallel to current flow. Then, the effective electrical conductivity σ of the rectangular sample was estimated from the measured resistance R, length L and rectangular cross section S as mentioned earlier. Using V f = S f /S, Vc = Sc /S and V p = S p /S (V is the volume fraction), and the effective conductivity σ (S/cm) according to the parallel rule of a mixture is expressed as

ters in size [13]. Furthermore, the conductivity of carbon films increases with deposition temperatures from the order of 10 S cm−1 at 800◦ C to 102 S cm−1 at 900◦ C. Therefore, it is concluded that the relatively high value of electrical conductivity from propane in Fig. 1 is related to the less pores and higher pyrolysis temperature for the coated carbon.

σ = Vf · σf + Vc · σc + Vp · σp

4 Conclusions

(1)

where subscripts of “f”, “c” and “p” denote the ceramic fiber, coated carbon and pore, respectively (i.e., σf = σp = 0). For carbon-coated fiber media obtained by the decomposition of either propane or phenolic resin, average bulk densities measured by Archimedes’ principle are ∼0.2 g cm−3 and ∼0.3 g cm−3 , respectively. Density of coated carbon is assumed to be 2.0 g cm−3 . Then, the following simple equations for the effective electrical conductivity of carbon-coated samples can be derived from Eq. (1) and in terms of weight fraction of carbon wc , i.e., gram of carbon per carbon-coated sample weight. σ = k · σc · wc

(2)

where the estimated value of k is 0.15 for the coated carbon from propane and 0.10 from phenolic resin, respectively. Equation (2) shows that the electrical conductivity of carbon-coated composite is a linear function of carbon content wc , i.e., the slope is proportional to carbon content wc . It should be noted that the effective electrical conductivity of coated carbon σ c in Eq. (2) is directional as shown in Fig. 1. From the least-squares fit of data points of electrical conductivity, effective electrical conductivity of coated carbon film σ c from propane in x and z-directions is estimated as ∼2.0 S cm−1 and ∼0.33 S cm−1 , respectively. Similarly, σ c from phenolic resin in x and z-directions is estimated as ∼0.42 S cm−1 and ∼0.12 S cm−1 , respectively. These values are about two to three orders of magnitude lower than those of carbon fiber derived from isotropic pitch, which has a higher electrical conductivity of 200 S cm−1 [10]. The low electrical conductivity of coated carbon from phenolic resin appears to result from the porous nature of the coated carbon and low carbonization temperature. It is well known that carbonization of phenolic resin has resulted in the formation of cracks and pores due to the shrinkage and decomposition of the resin during pyrolysis [11–12]. On the contrary, it has been reported that decomposition of hydrocarbons by chemical vapor deposition at temperatures below 1000◦ C produces carbon films of smooth surfaces, containing randomly distributed pores of a few hundred nanome-

Porous ceramic fiber media were coated with pyrolytic carbon by the decomposition of two different carbon precursors, propane and phenolic resin in a nitrogen atmosphere at 800–900◦ C. Anisotropy of porous ceramic fiber media can be explained by different electrical conductivity in the two directions, x and z. The electrical conductivities of porous carbon-coated samples increase linearly with increasing carbon content, which can be estimated using the parallel rule of electrical conductivity in a mixture. The electrical conductivity of carbon film is about two to three orders of magnitude lower than that of pure carbon fiber, which has a higher electrical conductivity of 200 S cm−1 . The low electrical conductivity of coated carbon from phenolic resin appears to result from the porous nature of the coated carbon and low carbonization temperature. Acknowledgment This work was supported by Korea Research Foundation Grant (R05-2004-000-12431-0).

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