Fabrication of Porous Ceramics with Unidirectionally Aligned
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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)
Transcript of Fabrication of Porous Ceramics with Unidirectionally Aligned
Fabrication of Porous Ceramics withUnidirectionally 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 fugitivefiber. Cotton thread was coated with ceramic slurry by pullingit through the slurry, and specimens were produced by spool-ing the coated thread. The obtained porous alumina ceramicshad an average pore diameter of 165mm, 35% open porosity,and a bending strength of 160 MPa. It was suggested that thepore size and the porosity could be adjusted using the diameterof the cotton thread and the solids concentration of the slurry,respectively.
I. Introduction
POROUSceramics have received considerable interest in studiesand have been used as hot gas or molten metal filters, catalyst
carriers, and separation membranes, because of their excellentmechanical properties, high-temperature resistance, and chemicalstability.1,2 Porous ceramics have become more important re-cently, because they are used as filters to reduce environmentalpollution in various industrial fields. For example, cordieriteceramic honeycomb substrates coated with catalysts have beenwidely used as diesel particulate filters, to control automotiveexhaust emissions.3–5
One of the most common methods used to fabricate porousceramics with unidirectionally aligned continuous pores is to usefugitive fibrous materials, which are intended to be burned outduring sintering.6 Biomorphic cellular SiC ceramics with aniso-tropic pores can be prepared using natural wood.7 Coblenz8
developed a technology of pulling a thread of mercerized cottonthrough a ceramic paste to produce fibrous monolithic ceramics.The intent of his work was to prepare dense fibrous monolithicceramics with excellent fracture toughness and damage tolerance;therefore, cold isostatic pressing was performed to close the largevoids that remained after burnout of the cotton threads. In thisstudy, we intend to use the slurry coating of fugitive fiber (SCF) tofabricate porous ceramics with unidirectionally aligned continuouspores. Cotton thread has been selected as the fugitive fibers in thepresent work. Because of the water-absorption ability of cottonthread, obtaining a slurry-coated thread with a uniform thicknessof slurry is not difficult. That is, such a coating process issomewhat similar to slip-casting processes with a gypsum mold.The amount of water absorbed by the cotton thread with a certaindiameter is limited; therefore, the coating thickness can be easily
controlled using the solids concentration of the slurry. The presentcommunication will report the preliminary results on the process-ing and microstructure of porous Al2O3 ceramics that have beenprepared by coating a water-based slurry of Al2O3 on mercerizedcotton thread.
II. Experimental Procedures
The starting powder was Al2O3 (TM-DAR grade, particle size0.2mm, 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 BosekiCo., Tokyo, Japan) was used as the fugitive fiber. The slurry wascomposed of 300 g of Al2O3 and 100 mL of a water solution of5 wt% poly(vinyl alcohol) (PVA). The purpose of the addition ofPVA was to provide the obtained green body with sufficientstrength. The slurry was ball-milled using ZrO2(Y2O3) balls for24 h in a plastic bottle. Then, the cotton thread was coated andspooled, as schematically illustrated in Fig. 1. The obtained greenbodies, with dimensions of 40 mm3 12 mm3 6 mm, were slowlydried at room temperature and then sintered at 1350°C for 60 minin an air atmosphere. To prevent the green body from crackingduring the burnout process of the cotton threads, a slow heatingrate was adopted at low temperatures: i.e., a rate of 1°C/min fromroom 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 sinteringtemperature.
The density and porosity of the obtained sample was measuredvia water immersion. A theoretical density of 3.96 g/cm3 for Al2O3
was used in the calculation. The surfaces of samples that were usedto measure the bending strength were ground using a No. 800 (gritparticle size of 20mm) grinding wheel and then polished with2 mm diamond paste and beveled. The three-point bending strengthin the direction perpendicular to the unidirectionally alignedcontinuous pores was measured using bars with dimensions of2.5 mm3 3 mm3 20 mm; the span was 16 mm and the crossheadspeed 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 observethe microstructure.
III. Results and Discussion
Figure 2 shows the microstructure of the green body. The cottonthread that was used had a diameter of;200 mm; it consisted ofmany cotton fibers;20mm in diameter. The thread was coated bythe slurry homogeneously, and a homogeneous green body wasobtained (see Fig. 2(a)). The slurry could not enter the inside of thecotton thread, because of the fast absorption of water from theslurry by the cotton fiber (see Fig. 2(b)). The microstructure of thecontinuous pores of the sintered specimen is shown in Fig. 3. Thisfigure shows that the unidirectionally aligned continuous pores arehomogeneously 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 SynergyCeramics Project. The authors are members of the Joint Research Consortium ofSynergy Ceramics.
*Member, American Ceramic Society.
1395
journalJ. Am. Ceram. Soc.,84 [6] 1395–97 (2001)
;165 mm. One can expect that porous ceramics with continuouspores of smaller diameters can be fabricated via this techniquewhen finer threads are used. In addition, the porosity can becontrolled using slurries with different concentrations of ceramicpowder. This technique is also applicable to other types ofmaterials, including Si3N4 and SiC.
The microstructure of the section parallel to the continuouspores is shown in Fig. 4. Clearly, the shape of the cotton thread isretained after the sintering. The preferable continuous pores wereobtained because the slurry could not be absorbed inside the thread(see Fig. 2(b)). The rough intersurfaces should increase the surfacearea of the pores; this feature should be advantageous if the
material is intended for use as a filter material. The Al2O3 grainsize was;0.5 mm.
Table I lists the properties of the obtained porous material. Theclosed porosity was 0.41%, which was indicative of the almost-fulldensity of the “rib” portion of the material. The open porosity was34.51%, which could be controlled by the solids content of theslurry. The obtained porous material exhibited a relatively highbending strength of 1556 20 MPa. Ohet al.9 reported that porousAl2O3 ceramics with;31% porosity, prepared either by hotpressing or pulse electric current sintering, exhibited a bendingstrength of;80 MPa. This finding indicates that porous ceramicswith unidirectionally aligned continuous pores and a dense ribportion demonstrate very high strength in the direction perpendic-ular to the continuous pores, in comparison with porous ceramicswith randomly oriented pores of the same porosity.
IV. Conclusions
Porous ceramics with unidirectionally aligned continuous porescan be fabricated via the slurry coating of fugitive fiber. PorousAl2O3 ceramics with unidirectionally aligned continuous pores(with a pore diameter of 165mm and open porosity of 34.51%)were produced via the slurry coating of cotton thread in the presentinvestigation. The porous ceramics exhibited a bending strength of1556 20 MPa. It is believed that (i) the pore size can be adjusted,using cotton threads of different diameter, and (ii) the porosity canbe controlled by changing the solids concentration of the slurry. Inaddition, materials other than Al2O3—such as Si3N4 or SiC—canalso be prepared using this method.
Fig. 1. Flow diagram of the fabrication of porous ceramics with unidirectionally aligned continuous pores via the slurry coating of fugitive fiber (SCF).
Fig. 2. SEM micrographs showing the microstructure of the green bodyof the specimen ((a) cross section and (b) near-surface structure of theslurry-coated cotton thread).
Fig. 3. SEM micrograph showing the cross-sectional microstructure ofthe unidirectionally aligned continuous pores.
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References
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Fig. 4. SEM micrograph showing the microstructure of the normalsection of the continuous pores ((a) low magnification and (b) highmagnification); the rough intersurface of the pores is noticeable.
Table I. Properties of Obtained PorousAlumina Ceramics with Unidirectionally
Aligned Continuous Pores
Property Value
Density 2.577 g/cm3
Relative density† 65.08%Porosity
Open 34.51%Closed 0.41%
Size of continuous pores 165mmBending strength 1556 20 MPa
†Percentage of theoretical density.
June 2001 Communications of the American Ceramic Society 1397
