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Alumina-based monofoams utilizing superplastic deformation facilitated by the addition of magnesia or magnesium aluminate spinel
Journal of Alloys and Compounds 471 (2009) L32–L35
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Alumina-based monofoams utilizing superplastic deformation facilitated by the addition of magnesia or magnesium aluminate spinel Mako Obata, Hidetaka Hayashi, Akira Kishimoto ∗ Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
a r t i c l e
i n f o
Article history: Received 7 February 2008 Received in revised form 24 March 2008 Accepted 30 March 2008 Available online 12 May 2008 Keywords: Sintering Ceramics Oxide materials Spinel Grain boundary
a b s t r a c t Alumina-based ceramic foam utilizing the superplastic deformation after sintering has been proposed and demonstrated by the present authors. Effects of two kinds of additives, magnesia or magnesium aluminate spinel on the figure of alumina-based ceramic foams were examined. In both cases, total porosity increased with increasing the addition amount up to 30 mol%. With the same addition amount, the porosity of spineladded foam was slightly larger than that of magnesia-added one. On pore expansion, relative density of pore shell decreased while the density degradation degree of magnesia-added foam was more serious than that of spinel-added one. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Previously, we fabricated ceramic foams by expanding a oncesintered dense shell utilizing the superplastic deformation of 3 mol% yttria-stabilized zirconia [1,2]. Conventional solid-state processing of porous ceramics includes the partial-sintering method [3] and pore-forming inclusion method [4]. In liquid-phase processing, an air bubble is readily introduced into the precursor slurry or gel. Before solidification, the organic or liquid components evaporate, and porous ceramics that have both high porosity and a high closed pore ratio are rarely attained, similar to solid-state ceramics processing [5]. Sintering after foam introduction reduces the porosity because the sintering process inevitably involves the exclusion of pores. Consequently, the conventional porous ceramics processing or sintering of a foamed precursor results in either insufficient sintering while maintaining porosity or improved inter-grain bonding at the expense of porosity [6]. In our ceramics, the foaming processing is carried out after sintering; consequently, there is no degradation in inter-gain bonding and high porosity is compatible with high structural reliability. Furthermore, the powder compaction process is simple and foaming occurs during heating at normal sintering temperatures under ambient pressures.
∗ Corresponding author. Tel.: +81 86 251 8069; fax: +81 86 251 8069. E-mail address: [email protected] (A. Kishimoto). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.136
In addition to the first successful fabrication of superplastic foam based on 3 mol% yttria-stabilized zirconia (3YSZ) [1,2], we previously fabricated superplastic foams based on alumina, a more widely used ceramic, by dispersing 3YSZ or MgO [7]. However, the total porosity of the MgO-dispersed ceramic foam was less than that of 3YSZ-dispersed ceramic foam. In the former system, the added MgO is converted into magnesium aluminate spinel after foam formation. Therefore, it is interesting to investigate the porosity when spinel is added to the starting alumina powder. In this study, we fabricated alumina-based foams in which the superplastic property was facilitated by adding magnesia or spinel. The effects of the additives on the foam figures and microstructures were examined. 2. Experimental procedure To demonstrate ceramic foaming following the sintering process, macroscopic single foams were fabricated using a method similar to that reported previously [1,2,7]. Alumina powders (AKP-30, average grain size: 0.33 m, specific surface area: 8 m2 /g; Sumitomo Chemical, Tokyo, Japan) with addition of various amounts of magnesia (MgO) or magnesium aluminate spinel to enhance their superplasticity were used as matrixes. Silicon carbide was chosen as it is a high-temperature foam agent that decomposes and evaporates at high temperatures. First, 0.1 g of silicon carbide powder (Grade-UF, average grain size: 0.31 m, specific surface area: 20 m2 /g; Ibiden, Aichi, Japan) was pressed into a pellet in a Ø 10-mm die under 30 MPa. Samples of about 4 g of alumina-based powder were weighed. Half of the alumina-based powder, the compressed silicon carbide powder, and the remaining half of the alumina-based powder were put into a Ø 20-mm steel die and pressed uniaxially at 30 MPa for 1 min, and then hydrostatically at 200 MPa for 1 min. The resultant powder compacts were heated to 1600 ◦ C at a rate of 800 ◦ C/h, kept at that temperature for 4 or 8 h, and then cooled.
M. Obata et al. / Journal of Alloys and Compounds 471 (2009) L32–L35
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Table 1 Comparison of resultant figure of alumina-based superplasticity foams doped with magnesia and magnesia alumina spinel Magnesia addition
Fig. 1. Side view of macroscopic ceramic foams fabricated by foaming after sintering of alumina-based matrix doped with (a) magnesia and (b) magnesium aluminate spinel.
To evaluate the effects of the additive, the other fabrication conditions were fixed, including the type and amount of foaming agent, and the powder compaction and heating programs. As additives to the alumina matrix powder, magnesia (MO-V01P, average grain size: 0.05 m, specific surface area: 30 m2 /g; Ube Materials Industries, Yamaguchi, Japan) or magnesium aluminate spinel was tested. The latter additive was obtained through a solid-state reaction between alumina and magnesia powders as described previously at 1200 ◦ C for 4 h and identified by X-ray diffraction (XRD) as a singlephase spinel. Before addition, they were passed through a stainless mesh. The external dimensions of the alumina-based ceramic foams, such as the foam height and foam diameter, were measured using calipers. The apparent density of the outer shell and of the entire ceramic foam was measured using Archimedes’ method with water as the medium. The porosity of the ceramic foam was estimated from its apparent density and the relative density of the outer shell. The fracture surface of the outer shell was polished with 9-m diamond paste, etched thermally at 1500 ◦ C for 30 min, and subjected to scanning electron microscopy (SEM). The sizes of approximately 100 ceramic grains were calculated using the code method and their distribution was evaluated. Crystalline phases that formed on the inner and outer sides of the shell in addition to the synthesized spinel powder were identified using an XRD method with Cu K␣ radiation.
Spinel addition
10 mol%
20 mol%
30 mol%
10 mol%
20 mol%
30 mol%
Foam P (%) s (%) d (mm)
11.7 97 16.9
14.2 92 16.5
16.5 89 17.2
11.0 97 16.5
17.0 95 17.0
17.9 94 16.8
Bulk b (%)
98
96
94
98
97
96
In both the MgO- and spinel-added systems, the porosity increased with the amount added. As the forces driving pore expansion and gas evolution from the same foam agent at 1600 ◦ C are the same, the greater porosity was attributed to the easier deformation with the addition. In both systems, the relative density of the outer shell decreased with the amount added. Compared with the bulk with the same amount added, the relative density of the foam shells was smaller. Ceramic walls become gas tight when the relative density exceeds 90% of the theoretical value. The gas begins to evolve when the outer shell becomes gas tight while holding at 1600 ◦ C. Subsequently, sintering and expansion of the outer shell proceed simultaneously. The tensile stress should act on the outer shell during macroscopic pore expansion, suppressing densification. The porosity of the spinel-added foams was greater than that of the foams with the same amount of added MgO. The degree
3. Results and discussion Fig. 1 shows the outer figures of alumina-based macrofoams with added MgO (30 mol%) (a) or magnesium aluminate spinel (30 mol%) (b), where the other fabrication parameters were identical, such as the foam agent, compaction pressure, and temperature program. Previously, we fabricated ceramic foams after sintering utilizing 3YSZ, which is widely known to show superplastic deformation [1]. We have also demonstrated how to make aluminabased superplastic foams by adding 3YSZ or magnesia. In this study, solid-state foaming of alumina was also realized by adding spinel. We reported previously that the porosity of MgO-added monofoams is less than that with addition of 3YSZ or silica. We attributed the lower porosity to the lower ductility, as reported by Hiraga et al. [8] The porosity of spinel-dispersed foams is less than 20%, suggesting low ductility similar to that of MgO-added foams. The total porosity, P, relative density of the outer shell, s , shell thickness, t, and foam diameter, d, are listed in Table 1 for both 10, 20, and 30 mol% magnesia- and spinel-added foams. As a reference, bulk materials were fabricated with addition of the same amount without the foaming agent under the same heating conditions and their relative densities are listed in Table 1. In this table, the porosity represents the volume fraction of macropores to the total volume (P = Vp × 100/(Vp + Vb ) (%)). A photo and schematic illustration of the cross-section of a monofoam is shown in Fig. 2(a) and (b) together with the relations of t, d, Vp , and Vb .
Fig. 2. (a) A cross-sectional photo of ceramic monofoam doped with 30 mol% of magnesium aluminate spinel. (b) Schematic illustration of the cross-section of monofoam (t, thickness of pore wall; d, macroscopic pore diameter; Vp , pore volume; Vb , matrix volume).
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M. Obata et al. / Journal of Alloys and Compounds 471 (2009) L32–L35
of degradation of the shell density differs markedly between the two additives. When the amount added was increased from 10 to 30 mol%, the relative density of the shell decreased from 97 to 89% with MgO addition, but only from 97 to 94% with spinel addition. Although the relative density of the bulk decreased slightly with addition, the density decreased markedly in the MgO-added alumina monofoam, suggesting sensitivity to tensile stress. From a technological perspective, the addition of spinel instead of magnesia improves both the porosity and pore wall density, while the added magnesia changes into magnesia aluminate spinel resulting in the same composition as that with added spinel. XRD analyses indicated that all of the added MgO turned into spinel on heating for 8 h without any residual MgO. In the MgOadded system, the pore expansion force was thought to act before spinel transformation, leading to the lower porosity and smaller shell density. The superplastic deformation of MgO-dispersed alumina has been reported to be smaller than of spinel-dispersed alumina [9], which agreed with the present result. There have been few reports on the effects of tensile stress on ceramics; however, the suppressive effect of MgO-dispersed alumina is thought to be large considering the small grain boundary sliding, reflecting a small amount of superplastic deformation. Fig. 3 shows the SEM photos of pore walls for (a) MgO-dispersed and (b) spinel-dispersed monofoams. Grain sizes are both smaller than 10 m and the population of voids seem large in the former. Fig. 4 shows the grain size distribution of the MgO- and spineladded systems. In both systems, the grain size increased with the amount added. A smaller grain size suppressed by the additive sometimes improves the grain boundary sliding, resulting in
Fig. 4. Grain size distribution of alumina-based monofoams dispersed with 10, 20, and 30 mol% of (a) magnesia and (b) magnesium aluminate spinel.
enhanced superplasticity [10,11]. However, this explanation is not applicable to the present results. The average grain size of the MgOadded alumina is smaller than that of the spinel-dispersed alumina (both 30 mol%), while enormous grains were seen in the former. These enormous grains would form before the spinel transformation, which is another factor that suppresses densification. 4. Conclusions
Fig. 3. SEM photos of pore wall for alumina-based monofoams dispersed with 30 mol% of (a) magnesia and (b) magnesium aluminate spinel.
We successfully fabricated alumina-based ceramic foams following sintering by improving the superplasticity by dispersing of magnesia or magnesia aluminum spinel. In both cases, total porosity increased with increasing the addition amount up to 30 mol%. With the same addition amount, the porosity of spinel-added foam
M. Obata et al. / Journal of Alloys and Compounds 471 (2009) L32–L35
was slightly larger than that of magnesia-added one. On pore expansion, relative density of pore shell decreased while the density degradation degree of magnesia-added foam was more serious than that of spinel-added one. Acknowledgements The authors thank Sumitomo Chemical Co. Ltd., Tokyo, Japan, Ube Material Industries, Ltd., Yamaguchi, Japan, for supplying ␣alumina powders, and magnesia powders, respectively. References [1] A. Kishimoto, T. Higashiwada, H. Asaoka, H. Hayashi, Adv. Eng. Mater. 8 (2006) 708–711.
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[2] T. Higashiwada, H. Asaoka, H. Hayashi, A. Kishimoto, J. Eur. Ceram. Soc. 27 (2007) 2217–2222. [3] A. Diaz, S. Hampshire, J. Yang, T. Ohji, S. Kanzaki, J. Am. Ceram. Soc. 88 (2005) 698–706. [4] R. Barea, M.I. Osendi, P. Miranzo, J.M.F. Ferreira, J. Am. Ceram. Soc. 88 (2005) 777–779. [5] Y. Gu, X. Liu, G. Meng, D. Peng, Ceram. Int. 25 (1999) 705–709. [6] J. Saggio-Woyansky, C.E. Scott, W.P. Minnear, Am. Ceram. Soc. Bull. 71 (11) (1992) 1674–1682. [7] A. Kishimoto, T. Higashiwada, M. Takahara, H. Hayashi, Mater. Sci. Forum 544–545 (2007) 641–644. [8] K. Hiraga, K. Nakano, T.S. Suzuki, Y. Sakka, Scripta Mater. 39 (1998) 1273–1279. [9] G. Chen, K. Zhang, G. Wang, W. Han, Ceram. Int. 30 (2004) 2157–2162. [10] Y. Yoshizawa, T. Sakuma, Acta Metall. Mater. 40 (1992) 2943–2950. [11] K. Nakano, T.S. Suzuki, K. Hiraga, Y. Sakka, Scripta Mater. 38 (1997) 33–38.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Letter
Alumina-based monofoams utilizing superplastic deformation facilitated by the addition of magnesia or magnesium aluminate spinel Mako Obata, Hidetaka Hayashi, Akira Kishimoto ∗ Division of Chemistry and Biochemistry, Graduate School of Natural Science and Technology, Okayama University, 3-1-1 Tsushima-naka, Okayama 700-8530, Japan
a r t i c l e
i n f o
Article history: Received 7 February 2008 Received in revised form 24 March 2008 Accepted 30 March 2008 Available online 12 May 2008 Keywords: Sintering Ceramics Oxide materials Spinel Grain boundary
a b s t r a c t Alumina-based ceramic foam utilizing the superplastic deformation after sintering has been proposed and demonstrated by the present authors. Effects of two kinds of additives, magnesia or magnesium aluminate spinel on the figure of alumina-based ceramic foams were examined. In both cases, total porosity increased with increasing the addition amount up to 30 mol%. With the same addition amount, the porosity of spineladded foam was slightly larger than that of magnesia-added one. On pore expansion, relative density of pore shell decreased while the density degradation degree of magnesia-added foam was more serious than that of spinel-added one. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Previously, we fabricated ceramic foams by expanding a oncesintered dense shell utilizing the superplastic deformation of 3 mol% yttria-stabilized zirconia [1,2]. Conventional solid-state processing of porous ceramics includes the partial-sintering method [3] and pore-forming inclusion method [4]. In liquid-phase processing, an air bubble is readily introduced into the precursor slurry or gel. Before solidification, the organic or liquid components evaporate, and porous ceramics that have both high porosity and a high closed pore ratio are rarely attained, similar to solid-state ceramics processing [5]. Sintering after foam introduction reduces the porosity because the sintering process inevitably involves the exclusion of pores. Consequently, the conventional porous ceramics processing or sintering of a foamed precursor results in either insufficient sintering while maintaining porosity or improved inter-grain bonding at the expense of porosity [6]. In our ceramics, the foaming processing is carried out after sintering; consequently, there is no degradation in inter-gain bonding and high porosity is compatible with high structural reliability. Furthermore, the powder compaction process is simple and foaming occurs during heating at normal sintering temperatures under ambient pressures.
∗ Corresponding author. Tel.: +81 86 251 8069; fax: +81 86 251 8069. E-mail address: [email protected] (A. Kishimoto). 0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2008.03.136
In addition to the first successful fabrication of superplastic foam based on 3 mol% yttria-stabilized zirconia (3YSZ) [1,2], we previously fabricated superplastic foams based on alumina, a more widely used ceramic, by dispersing 3YSZ or MgO [7]. However, the total porosity of the MgO-dispersed ceramic foam was less than that of 3YSZ-dispersed ceramic foam. In the former system, the added MgO is converted into magnesium aluminate spinel after foam formation. Therefore, it is interesting to investigate the porosity when spinel is added to the starting alumina powder. In this study, we fabricated alumina-based foams in which the superplastic property was facilitated by adding magnesia or spinel. The effects of the additives on the foam figures and microstructures were examined. 2. Experimental procedure To demonstrate ceramic foaming following the sintering process, macroscopic single foams were fabricated using a method similar to that reported previously [1,2,7]. Alumina powders (AKP-30, average grain size: 0.33 m, specific surface area: 8 m2 /g; Sumitomo Chemical, Tokyo, Japan) with addition of various amounts of magnesia (MgO) or magnesium aluminate spinel to enhance their superplasticity were used as matrixes. Silicon carbide was chosen as it is a high-temperature foam agent that decomposes and evaporates at high temperatures. First, 0.1 g of silicon carbide powder (Grade-UF, average grain size: 0.31 m, specific surface area: 20 m2 /g; Ibiden, Aichi, Japan) was pressed into a pellet in a Ø 10-mm die under 30 MPa. Samples of about 4 g of alumina-based powder were weighed. Half of the alumina-based powder, the compressed silicon carbide powder, and the remaining half of the alumina-based powder were put into a Ø 20-mm steel die and pressed uniaxially at 30 MPa for 1 min, and then hydrostatically at 200 MPa for 1 min. The resultant powder compacts were heated to 1600 ◦ C at a rate of 800 ◦ C/h, kept at that temperature for 4 or 8 h, and then cooled.
M. Obata et al. / Journal of Alloys and Compounds 471 (2009) L32–L35
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Table 1 Comparison of resultant figure of alumina-based superplasticity foams doped with magnesia and magnesia alumina spinel Magnesia addition
Fig. 1. Side view of macroscopic ceramic foams fabricated by foaming after sintering of alumina-based matrix doped with (a) magnesia and (b) magnesium aluminate spinel.
To evaluate the effects of the additive, the other fabrication conditions were fixed, including the type and amount of foaming agent, and the powder compaction and heating programs. As additives to the alumina matrix powder, magnesia (MO-V01P, average grain size: 0.05 m, specific surface area: 30 m2 /g; Ube Materials Industries, Yamaguchi, Japan) or magnesium aluminate spinel was tested. The latter additive was obtained through a solid-state reaction between alumina and magnesia powders as described previously at 1200 ◦ C for 4 h and identified by X-ray diffraction (XRD) as a singlephase spinel. Before addition, they were passed through a stainless mesh. The external dimensions of the alumina-based ceramic foams, such as the foam height and foam diameter, were measured using calipers. The apparent density of the outer shell and of the entire ceramic foam was measured using Archimedes’ method with water as the medium. The porosity of the ceramic foam was estimated from its apparent density and the relative density of the outer shell. The fracture surface of the outer shell was polished with 9-m diamond paste, etched thermally at 1500 ◦ C for 30 min, and subjected to scanning electron microscopy (SEM). The sizes of approximately 100 ceramic grains were calculated using the code method and their distribution was evaluated. Crystalline phases that formed on the inner and outer sides of the shell in addition to the synthesized spinel powder were identified using an XRD method with Cu K␣ radiation.
Spinel addition
10 mol%
20 mol%
30 mol%
10 mol%
20 mol%
30 mol%
Foam P (%) s (%) d (mm)
11.7 97 16.9
14.2 92 16.5
16.5 89 17.2
11.0 97 16.5
17.0 95 17.0
17.9 94 16.8
Bulk b (%)
98
96
94
98
97
96
In both the MgO- and spinel-added systems, the porosity increased with the amount added. As the forces driving pore expansion and gas evolution from the same foam agent at 1600 ◦ C are the same, the greater porosity was attributed to the easier deformation with the addition. In both systems, the relative density of the outer shell decreased with the amount added. Compared with the bulk with the same amount added, the relative density of the foam shells was smaller. Ceramic walls become gas tight when the relative density exceeds 90% of the theoretical value. The gas begins to evolve when the outer shell becomes gas tight while holding at 1600 ◦ C. Subsequently, sintering and expansion of the outer shell proceed simultaneously. The tensile stress should act on the outer shell during macroscopic pore expansion, suppressing densification. The porosity of the spinel-added foams was greater than that of the foams with the same amount of added MgO. The degree
3. Results and discussion Fig. 1 shows the outer figures of alumina-based macrofoams with added MgO (30 mol%) (a) or magnesium aluminate spinel (30 mol%) (b), where the other fabrication parameters were identical, such as the foam agent, compaction pressure, and temperature program. Previously, we fabricated ceramic foams after sintering utilizing 3YSZ, which is widely known to show superplastic deformation [1]. We have also demonstrated how to make aluminabased superplastic foams by adding 3YSZ or magnesia. In this study, solid-state foaming of alumina was also realized by adding spinel. We reported previously that the porosity of MgO-added monofoams is less than that with addition of 3YSZ or silica. We attributed the lower porosity to the lower ductility, as reported by Hiraga et al. [8] The porosity of spinel-dispersed foams is less than 20%, suggesting low ductility similar to that of MgO-added foams. The total porosity, P, relative density of the outer shell, s , shell thickness, t, and foam diameter, d, are listed in Table 1 for both 10, 20, and 30 mol% magnesia- and spinel-added foams. As a reference, bulk materials were fabricated with addition of the same amount without the foaming agent under the same heating conditions and their relative densities are listed in Table 1. In this table, the porosity represents the volume fraction of macropores to the total volume (P = Vp × 100/(Vp + Vb ) (%)). A photo and schematic illustration of the cross-section of a monofoam is shown in Fig. 2(a) and (b) together with the relations of t, d, Vp , and Vb .
Fig. 2. (a) A cross-sectional photo of ceramic monofoam doped with 30 mol% of magnesium aluminate spinel. (b) Schematic illustration of the cross-section of monofoam (t, thickness of pore wall; d, macroscopic pore diameter; Vp , pore volume; Vb , matrix volume).
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M. Obata et al. / Journal of Alloys and Compounds 471 (2009) L32–L35
of degradation of the shell density differs markedly between the two additives. When the amount added was increased from 10 to 30 mol%, the relative density of the shell decreased from 97 to 89% with MgO addition, but only from 97 to 94% with spinel addition. Although the relative density of the bulk decreased slightly with addition, the density decreased markedly in the MgO-added alumina monofoam, suggesting sensitivity to tensile stress. From a technological perspective, the addition of spinel instead of magnesia improves both the porosity and pore wall density, while the added magnesia changes into magnesia aluminate spinel resulting in the same composition as that with added spinel. XRD analyses indicated that all of the added MgO turned into spinel on heating for 8 h without any residual MgO. In the MgOadded system, the pore expansion force was thought to act before spinel transformation, leading to the lower porosity and smaller shell density. The superplastic deformation of MgO-dispersed alumina has been reported to be smaller than of spinel-dispersed alumina [9], which agreed with the present result. There have been few reports on the effects of tensile stress on ceramics; however, the suppressive effect of MgO-dispersed alumina is thought to be large considering the small grain boundary sliding, reflecting a small amount of superplastic deformation. Fig. 3 shows the SEM photos of pore walls for (a) MgO-dispersed and (b) spinel-dispersed monofoams. Grain sizes are both smaller than 10 m and the population of voids seem large in the former. Fig. 4 shows the grain size distribution of the MgO- and spineladded systems. In both systems, the grain size increased with the amount added. A smaller grain size suppressed by the additive sometimes improves the grain boundary sliding, resulting in
Fig. 4. Grain size distribution of alumina-based monofoams dispersed with 10, 20, and 30 mol% of (a) magnesia and (b) magnesium aluminate spinel.
enhanced superplasticity [10,11]. However, this explanation is not applicable to the present results. The average grain size of the MgOadded alumina is smaller than that of the spinel-dispersed alumina (both 30 mol%), while enormous grains were seen in the former. These enormous grains would form before the spinel transformation, which is another factor that suppresses densification. 4. Conclusions
Fig. 3. SEM photos of pore wall for alumina-based monofoams dispersed with 30 mol% of (a) magnesia and (b) magnesium aluminate spinel.
We successfully fabricated alumina-based ceramic foams following sintering by improving the superplasticity by dispersing of magnesia or magnesia aluminum spinel. In both cases, total porosity increased with increasing the addition amount up to 30 mol%. With the same addition amount, the porosity of spinel-added foam
M. Obata et al. / Journal of Alloys and Compounds 471 (2009) L32–L35
was slightly larger than that of magnesia-added one. On pore expansion, relative density of pore shell decreased while the density degradation degree of magnesia-added foam was more serious than that of spinel-added one. Acknowledgements The authors thank Sumitomo Chemical Co. Ltd., Tokyo, Japan, Ube Material Industries, Ltd., Yamaguchi, Japan, for supplying ␣alumina powders, and magnesia powders, respectively. References [1] A. Kishimoto, T. Higashiwada, H. Asaoka, H. Hayashi, Adv. Eng. Mater. 8 (2006) 708–711.
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[2] T. Higashiwada, H. Asaoka, H. Hayashi, A. Kishimoto, J. Eur. Ceram. Soc. 27 (2007) 2217–2222. [3] A. Diaz, S. Hampshire, J. Yang, T. Ohji, S. Kanzaki, J. Am. Ceram. Soc. 88 (2005) 698–706. [4] R. Barea, M.I. Osendi, P. Miranzo, J.M.F. Ferreira, J. Am. Ceram. Soc. 88 (2005) 777–779. [5] Y. Gu, X. Liu, G. Meng, D. Peng, Ceram. Int. 25 (1999) 705–709. [6] J. Saggio-Woyansky, C.E. Scott, W.P. Minnear, Am. Ceram. Soc. Bull. 71 (11) (1992) 1674–1682. [7] A. Kishimoto, T. Higashiwada, M. Takahara, H. Hayashi, Mater. Sci. Forum 544–545 (2007) 641–644. [8] K. Hiraga, K. Nakano, T.S. Suzuki, Y. Sakka, Scripta Mater. 39 (1998) 1273–1279. [9] G. Chen, K. Zhang, G. Wang, W. Han, Ceram. Int. 30 (2004) 2157–2162. [10] Y. Yoshizawa, T. Sakuma, Acta Metall. Mater. 40 (1992) 2943–2950. [11] K. Nakano, T.S. Suzuki, K. Hiraga, Y. Sakka, Scripta Mater. 38 (1997) 33–38.