Ind. Eng. Chem. Res. 2004, 43, 3137-3140
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MATERIALS AND INTERFACES Glass-Carbon Composite Hollow Fibers Shaomin Liu and George R. Gavalas* Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125
Glass-carbon composite hollow fibers were prepared by extruding a suspension of glass particles in an N-methylpyrrolidone solution of poly(ether sulfone). The fibers were gelled in water and pyrolyzed for 0.5-30 min in a furnace preheated to 1100-1200 °C. The resulting composite carbon-glass fibers had about 2.1 mm o.d. and 1.3 mm i.d. Scanning electron microscopy was used to examine fiber morphology, while the nitrogen permeance was about 7 × 10-5 mol/m2‚ Pa‚s, indicating pores of micron size. Compared to pure glass fibers, which are easily deformed at 900 °C, the composite fibers could withstand temperatures up to 1200 °C without suffering deformation. 1. Introduction Hollow fibers can be made by extrusion of a paste of inorganic material and a polymer or some other binder1,2 or by extrusion of a particle-polymer-solvent mixture, followed by gelation (phase inversion) in a nonsolvent.3-6 In both techniques, the hollow fiber precursors containing the ceramic powder and the polymer are heattreated in air to pyrolyze the polymer and burn out the residual carbon. The ceramic particles are then sintered to a dense or porous structure. In contrast to ceramic fibers, glass fibers can also be made by extrusion of the melt, which, however, requires special equipment.7-9 We have recently carried out extrusion of a glass powder-polymer-solvent mixture, followed by subsequent gelation, drying, and sintering to prepare porous glass hollow fibers for possible application as membranes. In the process, we observed that if, instead of slow heating and sintering in air, fast heating in a preheated furnace is employed, composite glass-carbon fibers are obtained. In this brief paper, we describe the preparation and properties of such carbon-glass fibers. 2. Experimental Section 2.1. Materials. Glass powder of composition 74.58.0-9.0-5.0-2.0-1.0-0.5 (wt %) SiO2-B2O3-Na2OK2O-Al2O3-ZnO-BaO supplied by Changzhou Benniu Co. (China) was used as the fiber material. The particle size, softening point, and density of the powder were 2-10 µm, 740 °C, and 2.36 g/cm3, respectively. For comparison purposes, fibers were also made using pure silica and pure zirconia as the inorganic component. Poly(ether sulfone) [PESf; Radel A-300; Solvay Advanced Polymers LLC] and N-methyl-2-pyrrolidone [NMP; EMD Chemicals Inc.] were used for preparing the suspension. Poly(vinylpyrrolidone) [PVP, K90; Alfa Aesar, A Johnson Matthey Co., GAF ISP Technologies, * To whom correspondence should be addressed. Tel.: 626395-4152. Fax: 626-568-8743. E-mail: gavalas@ cheme.caltech.edu.
Mw ) 630 000] was used as an additive. Tap water was used as both the internal and external gelation medium (nonsolvent). 2.2. Fiber Preparation. PESf was slowly added to NMP under stirring to form the polymer solution. Glass powder was then added to form a suspension of glass, NMP, and PESf with a 5:4:1 weight ratio. The suspension was stirred for 24 h to ensure uniform distribution of the particles and subsequently degassed at room temperature. It was then transferred to a stainless steel reservoir pressurized to 40 psig with nitrogen. Extrusion was carried out through a tube-in-orifice spinneret with orifice and inner diameters of 2.5 and 0.72 mm, respectively. The extruded fibers emerging from the spinneret at 10 m/min were passed through an air gap of 4 cm and immersed in a water bath to complete gelation. After thorough washing in water, the gelled hollow fibers referred to in the following as glass-polymer fibers were pyrolyzed in a furnace preheated to 1100 or 1200 °C to decompose the polymer and form the glasscarbon fibers. For that purpose, an alumina tube of 4 mm i.d., 6 mm o.d., and 25 cm length was used as the sample holder into which one glass-polymer fiber of approximately 8 cm length was inserted near the plugged end. The tube was then inserted in a preheated furnace so that the fiber was located in the central, relatively uniform temperature section. The open end of the alumina sample holder protruded 8 cm outside of the furnace and was open to the atmosphere. For comparison purposes, silica-carbon and zirconia-carbon fibers were prepared by the same procedure using silica and zirconia powders. 2.3. Fiber Characterization. The glass-polymer and glass-carbon fibers were examined with a scanning electron microscope (LEO 1550 VP field emission SEM). The glass-polymer fibers after immersion in liquid nitrogen for about 5 min were slowly flexed until a clear cross-sectional fracture occurred. These fibers were subsequently gold-coated using sputter coating under vacuum. In the case of the glass-carbon fibers, the clear cross-sectional fracture was obtained by directly snap-
10.1021/ie0308644 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/15/2004
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Figure 1. SEM of glass powder used as a membrane material.
ping the fibers, and gold coating was not necessary in view of the conducting property of the carbon. SEM micrographs of the surface and cross section of the fibers were taken at various magnifications. The presence of any crystalline phases in the original glass powder and the crushed composite samples was examined using an X-ray diffractometer. To measure the nitrogen permeance, one end of a fiber was sealed by a quick-setting epoxy resin and the fiber was then placed within a concentric steel tube. Nitrogen was passed at ambient temperature and about 2 bar of pressure through the annulus, while the permeate collected at the lumen of the fiber was conducted to a bubble flowmeter. The permeance was calculated from the fiber dimensions, permeate flow rate, and feed pressure by the equation
J ) Q/πLD∆p where J, Q, D, L, and ∆p are the gas permeance (mol/ m2‚Pa‚s), permeate flow rate (mol/s), fiber o.d. (m), length (m), and partial pressure difference across the fiber wall (Pa), respectively. To measure the electrical resistivity, platinum wires were sealed at each end of a fiber using a silver conductive paste, and after drying at 150 °C for 10 h, the electrical resistance was measured with a multimeter. 3. Results and Discussion
Figure 2. SEM of a glass-polymer hollow fiber: (a) cross section; (b) membrane wall; (c) outside membrane surface.
Glass-polymer fibers were made from an initial suspension containing 50% glass, 10% PESf, and 40% NMP (by weight) and gelled in water at room temperature. Subsequently, pyrolysis was conducted in a tubular furnace preheated to 1100 or 1200 °C. During pyrolysis, the temperature of the fibers was measured using a thermocouple inserted inside the alumina sample holder. The fiber temperature lagged significantly behind the fixed furnace temperature, and the final temperature achieved after about 2 or 3 min was lower by 100-150 °C because of heat losses from the alumina sample holder that extended outside of the furnace. SEM of the original glass particles in Figure 1 shows most particles being about 1 µm, but some are clumped into agglomerates of several microns. Compact particles of up to 10 µm are also present. SEM of the cross section and the o.d. surface of a glass-poymer fiber are shown
in Figure 2a-c. The fiber o.d and i.d. obtained from Figure 2a are 2.4 and 1.5 mm, respectively. Figure 2b shows short fingerlike structures near the inner surface and spongelike structures near the center and outer surface, resulting from complicated diffusion and phase separation phenomena occurring upon immersion in the water. The micrograph of the fiber surface in Figure 2c shows glass particles embedded in the polymer. Figure 3 shows micrographs of a glass-carbon fiber produced by pyrolysis for 3 min in a furnace preheated to 1200 °C. From Figure 3a, the o.d. and i,d, of the pyrolyzed fiber are 2.1 and 1.3 mm, reduced from 2.4 and 1.5 mm of the glass-polymer fiber. Figure 3b shows a cross-sectional structure very similar to that of the glass-polymer fiber. However, a comparison of the o.d. surfaces (Figures 2c and 3c) reveals higher porosity in the pyrolyzed fiber, obviously due to removal of some
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Figure 3. SEM of a glass-carbon composite fiber pyrolyzed at a furnace temperature of 1200 °C for 3 min: (a) cross section; (b) membrane wall; (c) outside membrane surface; (d) magnification of part b.
of the products of polymer decomposition. Figure 3d is a higher magnification SEM of the fiber cross section at point A of Figure 3b. The N2 permeance of several fibers examined was in the range of (4-12) × 10-5 mol/ m2‚s‚Pa, indicating pores in the micron range. The raw glass particles shown in Figure 1 generally have an irregular shape, but after 3 min of pyrolysis with a furnace temperature of 1200°C, the smaller particles have assumed nearly spherical shape while the large have retained their irregular shape, albeit with some smoothing of sharp edges. Some of the larger particles shown in Figure 3 may be agglomerates of particle clusters shown in Figure 1. The carbon content calculated from the weight difference before and after oxidation was about 9%, approximately independent of the pyrolysis conditions. The maximum carbon content of the glass-polymer composite (based on the total carbon in the polymer) was 12.4%; hence, 73% of the carbon in the polymer remained in the glass-carbon composite. X-ray analysis did not identify any crystalline phases. The glass has a softening point of 740 °C, at which it deforms under its own weight. The glass-carbon fibers, on the other hand, maintain their shape and sufficient mechanical strength to be routinely handled at temperatures as high as 1200 °C, indicating that the carbon forms a connected network in the composite. The connectivity of the carbon phase was confirmed by the electrical resistance measurements that yielded resistivity of 12 and 4 Ω‚mm after pyrolysis for 1 and 30 min, respectively, in a furnace preheated to 1200 °C. The resistivity of graphite is much lower, 0.0138 Ω‚mm,
but the composite is only 9 wt % carbon, and the carbon is amorphous. After complete carbon removal (ascertained by the disappearance of black or gray color) by oxidation at 600 °C for 40 h in an air atmosphere, the fibers do not completely collapse and, if undisturbed, retain their shape. However, they break easily upon handling. When gently rubbed, the glass-carbon fiber surface does not shed carbon, in contrast to the surfaces of silica-carbon and zirconia-carbon fibers. These observations indicate that the glass phase of the glasscarbon fibers has developed a very tenuous continuity not evident in the two-dimensional micrographs shown in Figure 3. 4. Conclusion Composite glass-carbon hollow fibers were prepared by extrusion-gelation of a glass-polymer-solvent mixture, followed by pyrolysis in a preheated furnace. Compared to pure glass fibers, which are easily deformed at 900 °C, the composite fibers are mechanically stable at temperatures as high as 1200 °C. Electrical resistance measurements and other observations show that the carbon phase is connected and the glass phase is weakly connected. Permeation measurements yielded nitrogen permeance around 7 × 10-5 mol‚m-2‚Pa-1‚s-1, indicating micron-sized pores of little interest as a separation membrane material. Acknowledgment The authors gratefully acknowledge the research funding provided by the U.S. Department of Energy (Grant DE-FG26-00NT40817).
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(6) Liu, S.; Li, K. Preparation of TiO2/Al2O3 Composite Hollow Fiber Membranes. J. Membr. Sci. 2003, 218, 269. (7) Schnabel, R.; Holdel, A.; Gotter, K. Process for Production of Porous Glass Membrane Tubes. U.S. Patent 4,042,359, 1989. (8) Hammel, J. J. Porous Inorganic Siliceous-Containing Gas Enriching Material and Process of Manufacture and Use. U.S. Patent 4,853,001, 1989. (9) Kuraoka, K.; Hirano, T.; Yazawa, T. High-Selectivity, HighFlexibility Glass Hollow-Fiber Membrane for Gas Separation. Chem. Commun. 2002, 6, 664.
Received for review December 9, 2003 Revised manuscript received April 7, 2004 Accepted April 16, 2004 IE0308644
