Fabrication of Porous Ceramics with Unidirectionally Aligned Continuous Pores Guo-Jun Zhang,* Jian-Feng Yang,* and Tatsuki Ohji* Synergy Materials Research Center, National Institute of Advanced Industrial Science and Technology (AIST), Nagoya 463–8687, Japan Porous alumina ceramics with unidirectionally aligned contin- uous pores were fabricated via the slurry coating of fugitive fiber. Cotton thread was coated with ceramic slurry by pulling it through the slurry, and specimens were produced by spool- ing the coated thread. The obtained porous alumina ceramics had an average pore diameter of 165 mm, 35% open porosity, and a bending strength of 160 MPa. It was suggested that the pore size and the porosity could be adjusted using the diameter of the cotton thread and the solids concentration of the slurry, respectively. I. Introduction P OROUS ceramics have received considerable interest in studies and have been used as hot gas or molten metal filters, catalyst carriers, and separation membranes, because of their excellent mechanical properties, high-temperature resistance, and chemical stability. 1,2 Porous ceramics have become more important re- cently, because they are used as filters to reduce environmental pollution in various industrial fields. For example, cordierite ceramic honeycomb substrates coated with catalysts have been widely used as diesel particulate filters, to control automotive exhaust emissions. 3–5 One of the most common methods used to fabricate porous ceramics with unidirectionally aligned continuous pores is to use fugitive fibrous materials, which are intended to be burned out during sintering. 6 Biomorphic cellular SiC ceramics with aniso- tropic pores can be prepared using natural wood. 7 Coblenz 8 developed a technology of pulling a thread of mercerized cotton through a ceramic paste to produce fibrous monolithic ceramics. The intent of his work was to prepare dense fibrous monolithic ceramics with excellent fracture toughness and damage tolerance; therefore, cold isostatic pressing was performed to close the large voids that remained after burnout of the cotton threads. In this study, we intend to use the slurry coating of fugitive fiber (SCF) to fabricate porous ceramics with unidirectionally aligned continuous pores. Cotton thread has been selected as the fugitive fibers in the present work. Because of the water-absorption ability of cotton thread, obtaining a slurry-coated thread with a uniform thickness of slurry is not difficult. That is, such a coating process is somewhat similar to slip-casting processes with a gypsum mold. The amount of water absorbed by the cotton thread with a certain diameter is limited; therefore, the coating thickness can be easily controlled using the solids concentration of the slurry. The present communication will report the preliminary results on the process- ing and microstructure of porous Al 2 O 3 ceramics that have been prepared by coating a water-based slurry of Al 2 O 3 on mercerized cotton thread. II. Experimental Procedures The starting powder was Al 2 O 3 (TM-DAR grade, particle size 0.2 mm, purity of 99.99% (main impurities included 7 ppm silicon, 4 ppm iron, and 2 ppm sodium); Taimei Chemical Co., Nagano, Japan). Mercerized cotton thread (thread size No. 80, Fuji Boseki Co., Tokyo, Japan) was used as the fugitive fiber. The slurry was composed of 300 g of Al 2 O 3 and 100 mL of a water solution of 5 wt% poly(vinyl alcohol) (PVA). The purpose of the addition of PVA was to provide the obtained green body with sufficient strength. The slurry was ball-milled using ZrO 2 (Y 2 O 3 ) balls for 24 h in a plastic bottle. Then, the cotton thread was coated and spooled, as schematically illustrated in Fig. 1. The obtained green bodies, with dimensions of 40 mm 3 12 mm 3 6 mm, were slowly dried at room temperature and then sintered at 1350°C for 60 min in an air atmosphere. To prevent the green body from cracking during the burnout process of the cotton threads, a slow heating rate was adopted at low temperatures: i.e., a rate of 1°C/min from room temperature to 600°C, 2°C/min in the 600°–900°C range, and then a rate of 5°C/min was used to get to the sintering temperature. The density and porosity of the obtained sample was measured via water immersion. A theoretical density of 3.96 g/cm 3 for Al 2 O 3 was used in the calculation. The surfaces of samples that were used to measure the bending strength were ground using a No. 800 (grit particle size of 20 mm) grinding wheel and then polished with 2 mm diamond paste and beveled. The three-point bending strength in the direction perpendicular to the unidirectionally aligned continuous pores was measured using bars with dimensions of 2.5 mm 3 3 mm 3 20 mm; the span was 16 mm and the crosshead speed was 0.5 mm/min in the three-point bending test apparatus. Each of the strength data was an average of five measurements. Scanning electron microscopy (SEM) was performed to observe the microstructure. III. Results and Discussion Figure 2 shows the microstructure of the green body. The cotton thread that was used had a diameter of ;200 mm; it consisted of many cotton fibers ;20 mm in diameter. The thread was coated by the slurry homogeneously, and a homogeneous green body was obtained (see Fig. 2(a)). The slurry could not enter the inside of the cotton thread, because of the fast absorption of water from the slurry by the cotton fiber (see Fig. 2(b)). The microstructure of the continuous pores of the sintered specimen is shown in Fig. 3. This figure shows that the unidirectionally aligned continuous pores are homogeneously distributed. The diameter of the pores was R. K. Bordia—contributing editor Manuscript No. 188274. Received September 11, 2000; approved February 13, 2001. This work has been supported by AIST, MITI, Japan, as part of the Synergy Ceramics Project. The authors are members of the Joint Research Consortium of Synergy Ceramics. *Member, American Ceramic Society. 1395 journal J. Am. Ceram. Soc., 84 [6] 1395–97 (2001)