Fabrication of Porous Al2O3 Ceramics with Submicron-Sized Pores Using a Water-Based Gelcasting Method

The gelcasting method is usually employed to fabricate relatively dense ceramics. In this work, however, porous Al2O3 ceramics with submicron-sized pores were fabricated using the water-based gelcasting method by keeping the Al2O3 content at low levels. By controlling the water content in the ceramic slurries and the sintering temperature of the green samples, the volume fractions and the size characteristics of the pores in the porous Al2O3 can be readily obtained. For the porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries, their open porosities were from 38.3% to 47.2%, while their median pore sizes varied from 299.8 nm to 371.9 nm. When there was more Al2O3 content in the slurries (40 vol.% Al2O3), the porous Al2O3 ceramics had open porosities from 37.0% to 46.5%, and median pore sizes from 355.4 nm to 363.1 nm. It was found that a higher sintering temperature and Al2O3 content in the slurries increased the mechanical strength of the porous Al2O3 ceramics.

One of the most important methods for the replica technique is the freeze-casting method [16], in which ice crystals grow in a ceramic slurry to occupy spaces inside the ceramic body. During the freeze-drying process, these ice crystals are directly vaporized by vacuum sublimation and leave pores inside the ceramic green body. The freeze-casting technique is widely used to fabricate different types of porous ceramics [17][18][19][20][21][22][23][24][25][26][27]. However, the freeze-casting process, especially the freeze-drying stage, typically takes quite a long time and consumes much more electrical power. In addition, ice crystals usually grow into dendrites, which make the pore surfaces rough, and the porous ceramics often exhibit anisotropic properties.
Herein, a water-based gelcasting route is presented for fabricating porous Al 2 O 3 ceramics with submicron pores, which could be used for filtration and other purposes. One of the advantages of the gelcasting method is that ceramics with complicated shapes can be readily fabricated [28,29]. In this method, high solid content in the ceramic slurries, or low water content, is usually needed to obtain relatively dense ceramics [28,29]. The purpose of this work, however, is to prepare porous ceramics, rather than dense ceramics. Hence, in this work, the Al 2 O 3 solid content is maintained at a relatively lower level, while the water content is kept at a relatively higher level in the ceramic slurries. Instead of freezing the water in the ceramic slurries into ice crystals, as in the freeze casting method [16], it is evaporated during the drying stage, and pores are retained in the green body. This allows porous Al 2 O 3 ceramics with submicron pores to be successfully fabricated. It is noted that fabrication of porous ceramics with submicron pores using the gelcasting method has scarcely been reported in the literature.

Material Preparation
Alpha Al 2 O 3 powders (99.9% purity, grain size about 1 µm on average, Jiyuan Brother Materials Co. Ltd., Henan, China) were used in this study. The chemicals and fabrication method can be referred to in our previous work [30]. For the fabrication of porous ceramics in the present work, the Al 2 O 3 content in the slurries was kept relatively low, at about 30-40 vol.%. In our previous work [30], however, the Al 2 O 3 content in the slurries was about 55 vol.%, which is much higher than in the present work. Figure 1 illustrates the processing steps for fabricating porous Al 2 O 3 ceramics in this work. Ball-milled Al 2 O 3 suspensions were poured into a metal mold (Figure 1a), and monomers were then polymerized to form crosslinked networks (Figure 1b) at 60 • C, for about 15 min. Then, the wet green bodies were dried at 70-110 • C and pores were retained (Figure 1c). The polymers within the dried green bodies were burnt out at 600 • C in air for 2 h. This process is called "degreasing" (Figure 1d). These samples were then sintered in air at 1300 • C, 1350 • C and 1400 • C for 2 h to obtain porous Al 2 O 3 ceramics (Figure 1e).
Materials 2018, 11, x FOR PEER REVIEW 2 of 8 this method, high solid content in the ceramic slurries, or low water content, is usually needed to obtain relatively dense ceramics [28,29]. The purpose of this work, however, is to prepare porous ceramics, rather than dense ceramics. Hence, in this work, the Al2O3 solid content is maintained at a relatively lower level, while the water content is kept at a relatively higher level in the ceramic slurries. Instead of freezing the water in the ceramic slurries into ice crystals, as in the freeze casting method [16], it is evaporated during the drying stage, and pores are retained in the green body. This allows porous Al2O3 ceramics with submicron pores to be successfully fabricated. It is noted that fabrication of porous ceramics with submicron pores using the gelcasting method has scarcely been reported in the literature.

Material Preparation
Alpha Al2O3 powders (99.9% purity, grain size about 1 μm on average, Jiyuan Brother Materials Co. Ltd., Henan, China) were used in this study. The chemicals and fabrication method can be referred to in our previous work [30]. For the fabrication of porous ceramics in the present work, the Al2O3 content in the slurries was kept relatively low, at about 30-40 vol.%. In our previous work [30], however, the Al2O3 content in the slurries was about 55 vol.%, which is much higher than in the present work. Figure 1 illustrates the processing steps for fabricating porous Al2O3 ceramics in this work. Ball-milled Al2O3 suspensions were poured into a metal mold (Figure 1a), and monomers were then polymerized to form crosslinked networks ( Figure 1b) at 60 °C, for about 15 min. Then, the wet green bodies were dried at 70-110 °C and pores were retained (Figure 1c). The polymers within the dried green bodies were burnt out at 600 °C in air for 2 h. This process is called "degreasing" ( Figure  1d). These samples were then sintered in air at 1300 °C, 1350 °C and 1400 °C for 2 h to obtain porous Al2O3 ceramics (Figure 1e).

Material Characterization
The Archimedes method was used to measure the bulk densities of the porous Al2O3 ceramics, and their flexural strength was measured with an electronic universal testing machine (Sans Materials Testing Co. Ltd., Shanghai, China) under a three-point bending setup with a span length of 30 mm and a crosshead speed of 0.5 mm/min. The size of the sample was 3 mm × 4 mm × 36 mm. For the compressive strength test, the sample size was 5 mm in diameter and 10 mm in height, and it was measured with the same instrument and the same crosshead speed. The porosities and pore sizes of the Al2O3 porous ceramics were measured using the mercury porosimetry analysis method (AutoPore IV 9500, Micromeritics, Norcross, GA, USA).

Material Characterization
The Archimedes method was used to measure the bulk densities of the porous Al 2 O 3 ceramics, and their flexural strength was measured with an electronic universal testing machine (Sans Materials Testing Co. Ltd., Shanghai, China) under a three-point bending setup with a span length of 30 mm and a crosshead speed of 0.5 mm/min. The size of the sample was 3 mm × 4 mm × 36 mm. For the compressive strength test, the sample size was 5 mm in diameter and 10 mm in height, and it was measured with the same instrument and the same crosshead speed. The porosities and pore sizes of the Al 2 O 3 porous ceramics were measured using the mercury porosimetry analysis method (AutoPore IV 9500, Micromeritics, Norcross, GA, USA). A field emission scanning electron microscope (FESEM, Hitachi S4800, Tokyo, Japan) was used to investigate the microstructural characteristics of the porous Al 2 O 3 ceramics. The Al 2 O 3 particle size was analyzed using an image analysis software system (ImageJ, National Institutes of Health, Bethesda, MD, USA).  (Figure 3), respectively. The pore structures can be readily seen in Figures 2 and 3, and the Al 2 O 3 particles can also be clearly identified. As shown in Figures 4 and 5, the Al 2 O 3 particle size and the density of the porous Al 2 O 3 ceramics steadily increased with the sintering temperature. This is generally expected for ceramics [31]. For the porous Al 2 O 3 ceramics prepared with an Al 2 O 3 content of 30 vol.% in the slurries, the particle size and density increased from about 1.03 µm and 1.96 g/cm 3 for sintering at 1300 • C, to 1.52 µm and 2.24 g/cm 3 for sintering at 1400 • C, respectively. For the porous Al 2 O 3 ceramics prepared with Al 2 O 3 content at 40 vol.% in the slurries, the particle size and the density increased from about 1.10 µm and 2.02 g/cm 3 for sintering at 1300 • C, to 1.49 µm and 2.38 g/cm 3 for sintering at 1400 • C, respectively. A field emission scanning electron microscope (FESEM, Hitachi S4800, Tokyo, Japan) was used to investigate the microstructural characteristics of the porous Al2O3 ceramics. The Al2O3 particle size was analyzed using an image analysis software system (ImageJ, National Institutes of Health, Bethesda, MD, USA). Figures 2 and 3 show the microstructural morphologies of the porous Al2O3 ceramics, which were prepared with the Al2O3 content in the ceramic slurries with 30 vol.% ( Figure 2) and 40 vol.% (Figure 3), respectively. The pore structures can be readily seen in Figures 2 and 3, and the Al2O3 particles can also be clearly identified. As shown in Figures 4 and 5, the Al2O3 particle size and the density of the porous Al2O3 ceramics steadily increased with the sintering temperature. This is generally expected for ceramics [31]. For the porous Al2O3 ceramics prepared with an Al2O3 content of 30 vol.% in the slurries, the particle size and density increased from about 1.03 μm and 1.96 g/cm 3 for sintering at 1300 °C, to 1.52 μm and 2.24 g/cm 3 for sintering at 1400 °C, respectively. For the porous Al2O3 ceramics prepared with Al2O3 content at 40 vol.% in the slurries, the particle size and the density increased from about 1.10 μm and 2.02 g/cm 3 for sintering at 1300 °C, to 1.49 μm and 2.38 g/cm 3 for sintering at 1400 °C, respectively.   A field emission scanning electron microscope (FESEM, Hitachi S4800, Tokyo, Japan) was used to investigate the microstructural characteristics of the porous Al2O3 ceramics. The Al2O3 particle size was analyzed using an image analysis software system (ImageJ, National Institutes of Health, Bethesda, MD, USA). Figures 2 and 3 show the microstructural morphologies of the porous Al2O3 ceramics, which were prepared with the Al2O3 content in the ceramic slurries with 30 vol.% ( Figure 2) and 40 vol.% (Figure 3), respectively. The pore structures can be readily seen in Figures 2 and 3, and the Al2O3 particles can also be clearly identified. As shown in Figures 4 and 5, the Al2O3 particle size and the density of the porous Al2O3 ceramics steadily increased with the sintering temperature. This is generally expected for ceramics [31]. For the porous Al2O3 ceramics prepared with an Al2O3 content of 30 vol.% in the slurries, the particle size and density increased from about 1.03 μm and 1.96 g/cm 3 for sintering at 1300 °C, to 1.52 μm and 2.24 g/cm 3 for sintering at 1400 °C, respectively. For the porous Al2O3 ceramics prepared with Al2O3 content at 40 vol.% in the slurries, the particle size and the density increased from about 1.10 μm and 2.02 g/cm 3 for sintering at 1300 °C, to 1.49 μm and 2.38 g/cm 3 for sintering at 1400 °C, respectively.      Tables 1 and 2 list the porosities and median pore diameters of the porous Al2O3 ceramics prepared with the Al2O3 contents in the ceramic slurries at 30 vol.% (Table 1) and 40 vol.% (Table 2), respectively. Figure 6 shows the pore size distribution functions of the porous Al2O3 ceramics sintered at 1300 °C ( Figure 6a) and 1400 °C (Figure 6b). It can be seen from Tables 1 and 2 that the porosity decreased with the sintering temperature. The closed porosities of the porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries were generally smaller than the samples prepared with 30 vol.% Al2O3 content in the slurries. In both of the two series of porous Al2O3 ceramics, the closed porosities were much smaller than the open porosities. This suggests that most of the pores in these samples were open pores [27]. This will be beneficial for filtration applications [1]. For the porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries, the median pore diameter decreased quickly from about 371.9 nm for sintering at 1300 °C, to about 299.8 nm for sintering at 1400 °C (Table 1). For the porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries, however, the pore diameter only slightly decreased ( Table 2). The median pore diameter decreased from about 363.1 nm for sintering at 1300 °C, to about 355.4 nm for sintering at 1400 °C (Table 2). In fact, these results are in good agreement with the microstructural morphologies shown in Figures 2c and 3c. It can be noted that the pore size in Figure 3c of the 1400 °C-sintered porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries was larger than that in Figure 2c of the 1400 °C-sintered porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries.   Tables 1 and 2 list the porosities and median pore diameters of the porous Al2O3 ceramics prepared with the Al2O3 contents in the ceramic slurries at 30 vol.% (Table 1) and 40 vol.% (Table 2), respectively. Figure 6 shows the pore size distribution functions of the porous Al2O3 ceramics sintered at 1300 °C ( Figure 6a) and 1400 °C (Figure 6b). It can be seen from Tables 1 and 2 that the porosity decreased with the sintering temperature. The closed porosities of the porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries were generally smaller than the samples prepared with 30 vol.% Al2O3 content in the slurries. In both of the two series of porous Al2O3 ceramics, the closed porosities were much smaller than the open porosities. This suggests that most of the pores in these samples were open pores [27]. This will be beneficial for filtration applications [1]. For the porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries, the median pore diameter decreased quickly from about 371.9 nm for sintering at 1300 °C, to about 299.8 nm for sintering at 1400 °C (Table 1). For the porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries, however, the pore diameter only slightly decreased ( Table 2). The median pore diameter decreased from about 363.1 nm for sintering at 1300 °C, to about 355.4 nm for sintering at 1400 °C (Table 2). In fact, these results are in good agreement with the microstructural morphologies shown in Figures 2c and 3c. It can be noted that the pore size in Figure 3c of the 1400 °C-sintered porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries was larger than that in Figure 2c of the 1400 °C-sintered porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries.  (Table 1) and 40 vol.% (Table 2), respectively. Figure 6 shows the pore size distribution functions of the porous Al 2 O 3 ceramics sintered at 1300 • C ( Figure 6a) and 1400 • C (Figure 6b). It can be seen from Tables 1 and 2 that the porosity decreased with the sintering temperature. The closed porosities of the porous Al 2 O 3 ceramics prepared with 40 vol.% Al 2 O 3 content in the slurries were generally smaller than the samples prepared with 30 vol.% Al 2 O 3 content in the slurries. In both of the two series of porous Al 2 O 3 ceramics, the closed porosities were much smaller than the open porosities. This suggests that most of the pores in these samples were open pores [27]. This will be beneficial for filtration applications [1]. For the porous Al 2 O 3 ceramics prepared with 30 vol.% Al 2 O 3 content in the slurries, the median pore diameter decreased quickly from about 371.9 nm for sintering at 1300 • C, to about 299.8 nm for sintering at 1400 • C (Table 1). For the porous Al 2 O 3 ceramics prepared with 40 vol.% Al 2 O 3 content in the slurries, however, the pore diameter only slightly decreased ( Table 2). The median pore diameter decreased from about 363.1 nm for sintering at 1300 • C, to about 355.4 nm for sintering at 1400 • C ( Table 2). In fact, these results are in good agreement with the microstructural morphologies shown in Figures 2c and 3c. It can be noted that the pore size in Figure 3c of the 1400 • C-sintered porous Al 2 O 3 ceramics prepared with 40 vol.% Al 2 O 3 content in the slurries was larger than that in Figure 2c of the 1400 • C-sintered porous Al 2 O 3 ceramics prepared with 30 vol.% Al 2 O 3 content in the slurries.

Mechanical Properties
The flexural and compressive strength of the porous Al 2 O 3 ceramics are shown in Figure 7. The flexural and compressive strength of the porous Al2O3 ceramics are shown in Figure 7. Figure 7a shows the dependence of the flexural strength of the porous Al2O3 ceramics on the sintering temperature. The flexural strength increased with the increasing sintering temperature. For the porous Al2O3 ceramics prepared with Al2O3 content at 30 vol.% in the slurries, their flexural strength increased from 15.0 MPa when sintered at 1300 °C, to 36.2 MPa and 61.5 MPa when sintered at 1350 °C and 1400 °C, respectively. For the porous Al2O3 ceramics prepared with Al2O3 content at 40 vol.% in the slurries, their flexural strength increased from 19.6 MPa when sintered at 1300 °C, to 42.5 MPa and 73.1 MPa when sintered at 1350 °C and 1400 °C, respectively. Compared with our previous work on gelcasted Al2O3 ceramics [30], in which 55 vol.% Al2O3 content was used in the slurries, the porous Al2O3 ceramics of the present work had smaller flexural strength.
As shown in Figure 7a, in general, the porous Al2O3 ceramics prepared with 40 vol.% Al2O3 content in the slurries exhibited higher flexural strength than those prepared with 30 vol.% Al2O3 content in the slurries for all three sintering temperatures. Furthermore, the difference in their flexural strength became larger at the higher sintering temperature of 1400 °C (Figure 7a). This can be attributed to the larger total porosity of the porous Al2O3 ceramics prepared with 30 vol.% Al2O3 content in the slurries than those with 40 vol.% Al2O3 content in the slurries (Tables 1 and 2).  As shown in Figure 7a, in general, the porous Al 2 O 3 ceramics prepared with 40 vol.% Al 2 O 3 content in the slurries exhibited higher flexural strength than those prepared with 30 vol.% Al 2 O 3 content in the slurries for all three sintering temperatures. Furthermore, the difference in their flexural strength became larger at the higher sintering temperature of 1400 • C (Figure 7a). This can be attributed to the larger total porosity of the porous Al 2 O 3 ceramics prepared with 30 vol.% Al 2 O 3 content in the slurries than those with 40 vol.% Al 2 O 3 content in the slurries (Tables 1 and 2).
The compressive strength of the porous Al 2 O 3 ceramics increased with the increasing sintering temperature (Figure 7b). This variation in behavior is similar to the flexural strength as shown in Figure 7a. For the porous Al 2 O 3 ceramics prepared with Al 2 O 3 content at 30 vol.% in the slurries, their compressive strength increased from 39.1 MPa when sintered at 1300 • C, to 82.6 MPa and 150.6 MPa when sintered at 1350 • C and 1400 • C, respectively. For the porous Al 2 O 3 ceramics prepared with Al 2 O 3 content at 40 vol.% in the slurries, their compressive strength increased from 43.6 MPa when sintered at 1300 • C, to 96.9 MPa and 182.8 MPa when sintered at 1350 • C and 1400 • C, respectively. Therefore, these porous Al 2 O 3 ceramics are mechanically strong enough for practical applications. The dependence of their compressive strength on the sintering temperature (Figure 7b) is in agreement with the results of the Al 2 O 3 /mullite composite porous ceramics reported by others [32]. However, the compressive strength of the porous Al 2 O 3 ceramics of this work was consistently higher than the Al 2 O 3 /mullite composite porous ceramics [32] and the porous Al 2 O 3 ceramics prepared using carbon black as a pore former [33].

Conclusions
Porous Al 2 O 3 ceramics with submicron pores were fabricated using the water-based gelcasting method. The open porosities and median pore sizes of the porous Al 2 O 3 ceramics with 30 vol.% Al 2 O 3 content in the slurries were 47.2% and 371.9 nm when sintered at 1300 • C, 42.5% and 330.6 nm when sintered at 1350 • C, and 38.3% and 299.8 nm when sintered at 1400 • C. The open porosities and median pore sizes of the porous Al 2 O 3 ceramics with 40 vol.% Al 2 O 3 content in the slurries were 46.5% and 363.1 nm when sintered at 1300 • C, 41.7% and 358.5 nm when sintered at 1350 • C, and 37.0% and 355.4 nm when sintered at 1400 • C. The porous Al 2 O 3 ceramics exhibited high mechanical strength, which increased with both increasing sintering temperature and increasing Al 2 O 3 content in the slurries.