Jung-Hye Eom

Processing and properties of macroporous silicon carbide ceramics: A review

Abstract Macroporous silicon carbide is widely used in various industrial applications including filtration for gas and water, absorption, catalyst supports, concentrated solar power, thermoelectric conversion, etc. During the past several years, many researchers have found diverse routes to fabricate macroporous SiC with porosity ranging from 9% to 95%. This review presents a detailed discussion on processing techniques such as partial sintering, replica, sacrificial template, direct foaming, and bonding techniques, as well as the mechanical and thermal properties of macroporous SiC ceramics fabricated using these methods. The full potential of these materials can only be achieved when properties are tailored for a specific application, whereas the control over these properties is highly dependent on the processing route. From the collected data, we have found that the porosity ranges from 9% to 91% with flexural strength of 1–205 MPa, compressive strength of 1–600 MPa, fracture toughness of 0.3–4.3 MPa m1/2, and thermal conductivity of 2–82 W/(m·K). This review will enlighten future investigations on processing of porous SiC and its usage in various applications.

Effect of additive composition on microstructure and strength of porous silicon carbide ceramics

Porous silicon carbide (SiC) ceramics have many potential applications due to their unique properties, which include high temperature stability, chemical stability, excellent abrasion resistance, high thermal shock resistance, high specific strength, and controlled permeability [1–9]. For example, SiC ceramics can be used as catalytic supports, molten metal filters, membrane supports, gas-burner media, and light-weight structural materials for high-temperature applications. It has frequently been observed that the composition of the sintering additives affects the microstructural development of sintered SiC ceramics. Al2O3–Y2O3 [10], Al2O3– Y2O3–CaO [11], and AlN–Y2O3 [12] additives generally lead to the growth of SiC platelet grains when the SiC is sintered or annealed at a temperature above 1950 C. In contrast, Y–Mg–Si–Al oxynitrides [13], B4C–C [14], and AlN [15] additives lead to equiaxed microstructures, regardless of the sintering temperature. Thus, the mechanical properties of porous SiC ceramics may be affected by the sintering additives. Chi et al. [16] investigated the effect of incorporating various amounts of Al2O3 on the strength of porous SiC ceramics. They observed a maximum strength of *17 MPa at a porosity of 61% when 5 wt% Al2O3 was added. Lee and Kim [17] fabricated porous SiC ceramics by powder processing using polymer microbeads as a template. The ceramics showed a strength of *30 MPa at a porosity of 50% when 8 wt% Al2O3–Y2O3 was added in a 7:3 weight ratio. Ma et al. [18] fabricated porous SiC ceramics by adding silicone resin as a binder. The ceramics typically showed a strength of *21 MPa at a porosity of 45%. However, there has been no systematic research on the effect of sintering additives in the processing of porous SiC ceramics. In this study, porous SiC ceramics were fabricated by carbothermal reduction of a polysiloxane-derived SiOC with hollow microspheres, followed by sintering. The effects of the additive composition on the porosity, microstructure, and strength of the resulting porous SiC ceramics were investigated. The potential advantages of using polysiloxane for fabricating porous SiC ceramics are the utilization of low-cost polymer processing such as extrusion and the easiness of porosity control [19]. The following raw materials were used: a polysiloxane (YR3370, GE Toshiba Silicones Co., Ltd, Tokyo, Japan), carbon black (Corax MAF, Korea Carbon Black Co., Ltd., Inchon, Korea), b-SiC (Ultrafine grade, Betarundum, Ibiden Co. Ltd., Ogaki, Japan), hollow microspheres (461DE20, Expancel, Sundsvall, Sweden), Al2O3 (AKP30, Sumitomo Chemical Co., Tokyo, Japan), Y2O3 (H C. Starck GmbH & KG, Goslar, Germany), Y3Al5O12 (YAG, High Purity Chemicals, Osaka, Japan), MgO (High Purity Chemicals, Osaka, Japan), SiO2 (High Purity Chemicals, Osaka, Japan), CaO (High Purity Chemicals, Osaka, Japan), and AlN (grade F, Tokuyama Soda Co., Tokyo, Japan). The SiC was used as an inert filler, while the oxides and AlN were used as sintering additives. The inert filler was added to minimize shrinkage during sintering and to increase the strength of the resulting porous SiC ceramics [20]. The microspheres were hollow poly(methyl methacrylate) spheres with diameters ranging from 15 to 25 lm. Eight batches of powder were prepared (Table 1). The microsphere content was fixed at 5 wt%. An example of the sample notation is as follows: 3A2Y denotes a specimen containing 3 wt% Al2O3 and 2 wt% Y2O3 as sintering J.-H. Eom Y.-W. Kim (&) Department of Materials Science and Engineering, The University of Seoul, 90 Jeonnong-dong, Dongdaemun-gu, Seoul 130-743, Republic of Korea e-mail: ywkim@uos.ac.kr

Fabrication of silicon oxycarbide foams from extruded blends of polysiloxane, low-density polyethylene (LDPE), and polymer microbead

Silicon oxycarbide (SiOC) foams with porosities ranging from 77% to 90% and a cell density higher than 108 cells/cm3 were made from polysiloxane, low-density polyethylene (LDPE), and polymer microbead blends. The polysiloxane, LDPE, and polymer microbeads were compounded directly using a counter-rotated twinscrew extruder. The obtained blends were foamed with gaseous carbon dioxide, cross-linked, and subsequently pyrolyzed. The process resulted in the production of highly porous, open-cell SiOC foams with a bimodal distribution of pore morphology, small spherical pores derived from polymer microbeads and relatively large elongated or equiaxed pores derived from foaming using carbon dioxide and decomposition of LDPE.

Porosity Control of Porous Zirconia Ceramics

A simple pressing process using zirconia and microbead for fabricating porous zirconia ceramics is demonstrated. Effects of microbead content and sintering temperature on microstructure, porosity, compressive and flexural strengths were investigated in the processing of porous zirconia ceramics using microbead as a pore former. By controlling the microbead content and the sintering temperature, it was possible to produce porous zirconia ceramics with porosities ranging from 43% to 70%. Typical compressive and flexural strength values at ~50% porosity were ~150㎫ and ~35㎫, respectively.

Effect of additive composition on mechanical properties of pressureless sintered silicon carbide ceramics sintered with alumina, aluminum nitride and yttria

Silicon carbide (SiC) ceramics were pressureless sintered with 3 vol% Al2O3-Y2O3-AlN additives with the AlN/(Al2O3+AlN) molar ratios of 0-0.75 at 1850-2000 °C for 1 hr and the effects of additive composition (i.e., changes in the AlN/(Al2O3+AlN) molar ratio while maintaining constant Y2O3 content) on the mechanical properties of the pressureless-sintered SiC ceramics were investigated. Self-reinforced microstructures consisting of relatively large platelet SiC grains and relatively small equiaxed grains have been obtained in all specimens when sintered at 1900 °C for 1 h in an argon atmosphere. The achievement of self-reinforced microstructures under such mild conditions (holding for 1 hr at 1900 °C) is caused by the beneficial effects of additive composition and the acceleration of the β→α phase transformation of SiC by seeding, i.e., the addition of 1 vol% α-SiC into β-SiC. The typical flexural strength and fracture toughness of the pressureless-sintered SiC ceramics with an AlN/(Al2O3+AlN) mole ratio of 0.5 were 433 MPa and 6.6 MPa·m1/2 at room temperature, respectively.

CERAMIC MEMBRANES PREPARED FROM A SILICATE AND CLAY-MINERAL MIXTURE FOR TREATMENT OF OILY WASTEWATER

The application of ceramic membranes is limited by the high cost of raw materials and the sintering process at high temperatures. To overcome these drawbacks, the present study investigated both the preparation of ceramic membranes using cost-effective raw materials and the possibility of recycling the membranes for the treatment of oily wastewater. Ceramic membranes with a pore size of 0.29–0.67 μm were prepared successfully at temperatures as low as 1000–1100°C by a simple pressing route using lowcost base materials including diatomite, kaolin, bentonite, talc, sodium borate, and barium carbonate. The typical steady-state flux, fouling resistance, and oil-rejection rate of the low-cost virgin membranes sintered at 1000°C were 2.5 × 10−5 m3m−2s−1 at 303 kPa, 63.5%, and 84.1%, respectively, with a feed oil concentration of 600 mg/L. A simple burn-out process of the used membranes at 600°C in air resulted in >95% recovery of the specific surface area (SSA) of the virgin membranes, a significantly increased steady-state flux, decreased fouling resistance, and increased oil-rejection rate. The typical steady-state flux, fouling resistance, and oil-rejection rate of the low-cost ceramic membrane sintered at 1000°C and subsequently heat treated at 600°C for 1 h in air after the first filtration were 5.4 × 10−5 m3m−2s−1 at 303 kPa, 27.1%, and 92.9%, respectively, with a feed oil concentration of 600 mg/L. The present results suggest that the low-cost ceramic membranes used for oily wastewater filtration can be recycled by simple heat-treatment at 600°C in air. As the fouling resistance of the low-cost ceramic membranes decreased with a decrease in pore size, the preferred pore size of the membranes for oily wastewater filtration is <0.4 μm.

Effect of initial α-phase content on microstructure and flexural strength of macroporous silicon carbide ceramics

The effects of the initial α-phase content on the microstructure and flexural strength of macroporous silicon carbide (SiC) ceramics were investigated. When β powder or a mixture of α/β powders containing small (≤3%) amounts of α powder were used, the grains showed a platelet-shape. In contrast, the grains had an equiaxed-shape when α powder or a mixture of α/β powders containing large (≥50%) amounts of α powder was used. The flexural strength increased with increasing α-SiC content in the starting composition, whereas the porosity decreased with increasing α-SiC content. The strength of the macroporous SiC ceramics was affected mostly by the porosity when the grain size was smaller than 10 μm, whereas the strength was controlled by pore size and grain size when the microstructure consisted of large (>10 μm) platelet grains.