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Effect of nanometer Al2O3 powder on microstructure and properties of alumina ceramics by microwave sintering
Materials Science and Engineering A 546 (2012) 328–331
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Effect of nanometer Al2 O3 powder on microstructure and properties of alumina ceramics by microwave sintering Liu Yin ∗ , Min Fan-fei, Zhu Jin-bo, Zhang Ming-xu School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China
a r t i c l e
i n f o
Article history: Received 4 January 2012 Received in revised form 1 March 2012 Accepted 7 March 2012 Available online 29 March 2012 Keywords: Alumina Nanometer powder Microwave sintering Microstructure Mechanical properties
a b s t r a c t The effect of nanometer Al2 O3 powder on microstructure and properties of alumina ceramics by microwave sintering was investigated. The maximum bending strength and fracture toughness of alumina ceramics reach up to 294.41 Mpa and 4.14 Mpa m1/2 , as the alumina ceramics with 20 wt% nanometer A12 O3 powder was microwave sintered at 1500 ◦ C for 30 min. It may be attributed to the addition of nanometer A12 O3 powder and microwave-sintering process resulting in a synergistic effect on improvement of microstructure and mechanical properties. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Alumina ceramics, as one of the most important engineering ceramics, have great potential to replace high-temperature metals into many demanding applications [1]. However, its use in real life systems is limited due to its high sintering temperature and low fracture toughness resulting in catastrophic failure. Some methods have been developed to overcome these drawbacks. For example, the addition of zirconia in the alumina matrix remarkably improves the toughness on account of stress induced transformation toughening [2,3]. Alumina-matrix composite with nanosized or sub-micrometer-sized metal or alloy dispersion may realize a functionally structural composite material [4,5]. Nano-alumina ceramics can also result in enhancement of the mechanical and thermal performance [6–8]. Addition of nanometer Al2 O3 powder in coarse grain alumina is an effective method to improve properties of alumina ceramics [9,10]. The higher specific surface area of the nanometer Al2 O3 powder offers excellent potential to lower sintering-temperature, because the finer grain size in its microstructure can provide additional driving force for densification during sintering. Although the sinterability, microstructure and mechanical behavior of coarse alumina powder and nanometer Al2 O3 powder have been investigated by many researchers [11,12], the addition of nanometer Al2 O3 powder in coarse grain alumina and its effect on the
∗ Corresponding author. Tel.: +86 554 6668643; fax: +86 554 6668643. E-mail address: [email protected] (Y. Liu). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.03.072
sinterability, microstructure and mechanical properties of alumina ceramics by microwave sintering have not been reported. Microwave sintering technology has attracted much scientific interest due to its unique advantages [13–15]. In conventional heating, heat is generated by heating elements and then transferred to sample via radiation, conduction and convection. It may cause some of the constituents to evaporate and allow undesired grain growth in sintering materials. However, the materials in microwave heating absorb microwave energy and then transform it into heat within the sample volume. It can be fully sintered at lower temperature for shorter time. Major advantages of microwave sintering processing are higher energy efficiency, enhanced reaction rate, faster sintering rate, shorter cycle times and lower cost. Therefore, the fine microstructure and enhanced properties can be obtained by microwave sintering technology. In the present work, it is to evaluate the effect of nanometer Al2 O3 powder on the sinterability and microstructure of alumina ceramics by microwave sintering. The mechanical properties of alumina ceramics with different contents of nanometer Al2 O3 powder were also investigated. 2. Experimental Commercial available coarse grain Al2 O3 powder (grain size: 0.6–0.9 m) and nanometer Al2 O3 powder (grain size: 40–80 nm) were used as raw materials. A typical SEM image of nanometer A12 O3 powder is shown in Fig. 1. The raw materials were weighed according to certain proportions (Table 1). 5 wt% of sintering aid (50 pencil stone, 39.5 SiO2 and 10.5 CaCO3 in wt%)
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Fig. 1. A typical SEM image of nanometer A12 O3 powder. Fig. 3. The line shrinkage ratio of alumina ceramics by different sintering technology.
Table 1 Composition of Al2 O3 powder (wt%). No
Coarse grain Al2 O3
Nanometer Al2 O3
1 2 3 4 5 6
100 95 90 80 70 60
0 5 10 20 30 40
constant cross-head speed of 0.5 mm min−1 . Five samples of each batch were tested to obtain an average value. The microstructure of the fracture surface was characterized by scanning electronic microscope.
was added and mixed with bimodal alumina powders. After ballmilling using ethanol as a mixing medium, drying and pelleting, the mixture of the raw materials was uniaxially pressed into bars with 4 mm × 6 mm × 50 mm, then isostatically cold-pressed at 200 MPa. The green bodies were sintered at different temperatures for 30 min in a microwave sintering furnace. For comparison, the green bodies were conventional sintered at the same temperature for 120 min. The Archimedes method was used to measure the density of the sintered samples. The sintered samples were carefully ground and polished into the size of 3 mm × 4 mm × 40 mm for mechanical behavior measurements. The bending strength was measured by the three-point-bending method. Fracture toughness was measured by the single-edge notched beam method (SENB). An initial notch of 2.5 mm in deptch was cut using a diamond blade (0.15 mm in thickness). Both tests were conducted with a span of 30 mm at a
Fig. 2. Relative density of alumina ceramics by different sintering technology.
Fig. 4. SEM images of fracture surfaces of alumina ceramics with the same content of nanometer A12 O3 by different sintering technology (a) conventional sintered at 1550 ◦ C for 120 min, (b) microwave sintered at 1500 ◦ C for 30 min.
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Fig. 5. SEM images of fracture surfaces of microwave-sintered alumina ceramics with different content of nanometer A12 O3 powder at 1500 ◦ C for 30 min (a) 10%, (b) 20%, (c) 30% and (d) 40%.
3. Results and discussion 3.1. Sintering behavior The alumina powder was sintered at different temperatures by different sintering technologies, and the relative densities are shown in Fig. 2. It can be seen that the relative density of the conventional-sintered samples increases with increased sintering temperature, and it could only get to 96.1% of the theoretical value at 1550 ◦ C for 120 min. The variation of density in the microwavesintered samples is similar to that of the conventional-sintered sample. However, its relative density reaches up to 98.5% of the theoretical value at 1500 ◦ C for 30 min, which is much higher than that of the conventional-sintered samples. The line shrinkage ratio of alumina ceramics by different sintering technologies is shown in Fig. 3. The line shrinkage ratio of the microwave-sintered samples is much higher than that of the conventional-sintered samples, which is consistent with the density of samples by different sintering processes. According to the sintering behaviors of alumina ceramics with addition of nanometer Al2 O3 powder, it can be seen that sintering temperature and duration of alumina ceramics is remarkably lowered by microwave sintering process. 3.2. Microstructure Fig. 4 shows SEM images of the fracture surface of alumina ceramics prepared by different sintering processes. It can be seen that there is large difference between fracture surfaces of alumina ceramics. The conventional-sintered sample obtained at 1550 ◦ C for 120 min still exhibits a porous microstructure (Fig. 4a). There are also discontinuous grain growth and platelet grain in conventionalsintered sample. Whereas, it is nearly fully sintered by microwave sintering at 1500 ◦ C for 30 min (Fig. 4b). The grain size is smaller than that of the conventional-sintered sample, and the distribution of the grain size of alumina ceramics is relatively uniform.
Fig. 5 shows SEM images of the fracture surface of microwavesintered samples with different nanometer A12 O3 powder obtained at 1500 ◦ C for 30 min. The sinterability of the alumina ceramics increases with the increased content of nanometer A12 O3 powder, while its grain size decreases. The microstructure of samples is similar to each other with uniform distribution of fine alumina. As the content of nanometer A12 O3 powder is about 20 wt%, its microstructure is more homogeneous and compacter than those of other samples, as shown in Fig. 5b. With the increase of nanometer A12 O3 powder, the abnormal grain growth and big pore in samples are observed in Fig. 5c and d. The higher content of the nanometer A12 O3 powder is, the larger number of the abnormal grain growth and pore are observed in samples. 3.3. Mechanical properties The mechanical properties of the microwave-sintered alumina ceramics with different nanometer A12 O3 powder are shown in Fig. 6. It can be seen that the bending strength and fracture toughness of the samples firstly increase with nanometer A12 O3 powder. The maximum bending strength and fracture toughness of the microwave-sintered alumina ceramics with 20 wt% of nanometer A12 O3 powder are obtained up to 294.41 Mpa and 4.14 Mpa m1/2 , which is in accordance with the microstructure of the sample revealed by SEM image. Then it decreases with the increased content of nanometer A12 O3 powder. The bimodal alumina powder mixtures improved the density of coarse alumina powder, microstructure and mechanical properties of alumina ceramics. As the content of nanometer A12 O3 powder is about 20 wt%, there is the best packing density by theory and experimental results [16–18]. The maximum bending strength and fracture toughness of microwave-sintered alumina ceramics with 20 wt% of nanometer A12 O3 at 1500 ◦ C for 30 min are attributed to its uniform, fine-grained and dense microstructure. It is found that the addition of nanometer A12 O3 powder and microwave
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microwave sintered at 1500 ◦ C for 30 min. The density is up to 98.5% of the theoretical value. The maximum of bending strength and fracture toughness can be obtained up to 294.41 Mpa and 4.14 Mpa m1/2 . It may be attributed to the addition of nanometer A12 O3 powder and microwave sintering process resulting in a synergistic effect on fine grain size, densitification, low sinteringtemperature and excellent mechanical properties. Acknowledgements This work was supported by National Nature Science Foundation of China (51174006), the Natural Science Foundation of High Education School of Anhui Province, China (KJ2010A095) and Anhui Provincial Natural Science Foundation, China (1208085ME84). References Fig. 6. The bending strength and fracture toughness of microwave-sintered alumina ceramics with different content of nanometer A12 O3 powder at 1500 ◦ C for 30 min.
sintering process result in a synergistic effect on densitification, low sintering-temperature and excellent mechanical properties. As the content of nanometer A12 O3 powder is higher than 20 wt%, nanometer A12 O3 grain were consumed and a few abnormal A12 O3 particles had impinged upon each other. There are significant increase of pores and pronounced abnormal grain growth at 1500 ◦ C. Therefore the mechanical properties of alumina ceramics decrease with increasing nanometer A12 O3 powder. 4. Conclusions To summarize, high quality alumina ceramics by using nanometer A12 O3 and coarse grain A12 O3 as raw materials were fabricated by microwave sintering technology. Nanometer A12 O3 powder can improve its sinterability as well as the mechanical properties. This kind of performance improvement can be obtained only when the content of nanometer A12 O3 powder is in a certain amount. The alumina ceramics with 20 wt% nanometer A12 O3 powder were
[1] C.E. Borsa, N.M.R. Jones, R.J. Brook, R.I. Todd, J. Eur. Ceram. Soc. 17 (1997) 865–869. [2] N.A. Travitzky, A. Goldstein, O. Avsian, A. Singurindi, Mater. Sci. Eng. A 286 (2000) 225–229. [3] R.R. Menezes, R.H.G.A. Kiminami, J. Mater. Process. Technol. 203 (1–3) (2008) 513–517. [4] X.Y. Qin, R. Cao, H.Q. Li, Ceram. Int. 32 (2006) 575–581. [5] K. Niihara, B.S. Kim, T. Nakayama, J. Eur. Ceram. Soc. 24 (2004) 3419–3425. [6] C.L. Huang, J.J. Wang, C.Y. Huang, Mater. Lett. 59 (2005) 3746–3749. [7] J. Li, Y.B. Pan, L.P. Huang, J.K. Guo, Ceram. Int. 35 (2009) 1377–1383. [8] L. Wang, J.L. Shi, H.R. Chen, Z.L. Hua, T.S. Yen, J. Mater. Sci. Lett. 20 (2001) 341–342. [9] J. Li, Y.B. Pan, F.G. Qiu, L.P. Huang, J.K. Guo, Mater. Sci. Eng. A 435–436 (2006) 611–619. [10] G. Li, A. Jiang, L. Zhang, J. Mater. Sci. Lett. 15 (1996) 1713–1715. [11] L. Gao, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, J. Eur. Ceram. Soc. 20 (2000) 2149–2152. [12] Z.P. Xie, J.L. Yang, Y. Huang, Mater. Lett. 3 (4–5) (1998) 215–220. [13] Y. Fang, J.P. Cheng, D.K. Agrawal, Mater. Lett. 58 (3–4) (2004) 498–501. [14] M. Mizuno, S. Obata, S. Takayama, et al., J. Eur. Ceram. Soc. 24 (2) (2004) 387–391. [15] R.R. Menezes, R.H.G.A. Kiminami, J. Mater. Process. Technol. 203 (2008) 513–517. [16] R.J. Hellmig, H. Ferkel, J. Am. Ceram. Soc. 84 (2) (2001) 261–266. [17] J. Li, Y.B. Pan, F.G. Qiu, L.P. Huang, J.K. Guo, Mater. Sci. Eng. A 435-436 (2006) 611–619. [18] J. Li, Y.B. Pan, J.W. Ning, L.P. Huang, J.K. Guo, J. Inorg. Mater. 18 (6) (2003) 1192–1198.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Rapid communication
Effect of nanometer Al2 O3 powder on microstructure and properties of alumina ceramics by microwave sintering Liu Yin ∗ , Min Fan-fei, Zhu Jin-bo, Zhang Ming-xu School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, China
a r t i c l e
i n f o
Article history: Received 4 January 2012 Received in revised form 1 March 2012 Accepted 7 March 2012 Available online 29 March 2012 Keywords: Alumina Nanometer powder Microwave sintering Microstructure Mechanical properties
a b s t r a c t The effect of nanometer Al2 O3 powder on microstructure and properties of alumina ceramics by microwave sintering was investigated. The maximum bending strength and fracture toughness of alumina ceramics reach up to 294.41 Mpa and 4.14 Mpa m1/2 , as the alumina ceramics with 20 wt% nanometer A12 O3 powder was microwave sintered at 1500 ◦ C for 30 min. It may be attributed to the addition of nanometer A12 O3 powder and microwave-sintering process resulting in a synergistic effect on improvement of microstructure and mechanical properties. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Alumina ceramics, as one of the most important engineering ceramics, have great potential to replace high-temperature metals into many demanding applications [1]. However, its use in real life systems is limited due to its high sintering temperature and low fracture toughness resulting in catastrophic failure. Some methods have been developed to overcome these drawbacks. For example, the addition of zirconia in the alumina matrix remarkably improves the toughness on account of stress induced transformation toughening [2,3]. Alumina-matrix composite with nanosized or sub-micrometer-sized metal or alloy dispersion may realize a functionally structural composite material [4,5]. Nano-alumina ceramics can also result in enhancement of the mechanical and thermal performance [6–8]. Addition of nanometer Al2 O3 powder in coarse grain alumina is an effective method to improve properties of alumina ceramics [9,10]. The higher specific surface area of the nanometer Al2 O3 powder offers excellent potential to lower sintering-temperature, because the finer grain size in its microstructure can provide additional driving force for densification during sintering. Although the sinterability, microstructure and mechanical behavior of coarse alumina powder and nanometer Al2 O3 powder have been investigated by many researchers [11,12], the addition of nanometer Al2 O3 powder in coarse grain alumina and its effect on the
∗ Corresponding author. Tel.: +86 554 6668643; fax: +86 554 6668643. E-mail address: [email protected] (Y. Liu). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msea.2012.03.072
sinterability, microstructure and mechanical properties of alumina ceramics by microwave sintering have not been reported. Microwave sintering technology has attracted much scientific interest due to its unique advantages [13–15]. In conventional heating, heat is generated by heating elements and then transferred to sample via radiation, conduction and convection. It may cause some of the constituents to evaporate and allow undesired grain growth in sintering materials. However, the materials in microwave heating absorb microwave energy and then transform it into heat within the sample volume. It can be fully sintered at lower temperature for shorter time. Major advantages of microwave sintering processing are higher energy efficiency, enhanced reaction rate, faster sintering rate, shorter cycle times and lower cost. Therefore, the fine microstructure and enhanced properties can be obtained by microwave sintering technology. In the present work, it is to evaluate the effect of nanometer Al2 O3 powder on the sinterability and microstructure of alumina ceramics by microwave sintering. The mechanical properties of alumina ceramics with different contents of nanometer Al2 O3 powder were also investigated. 2. Experimental Commercial available coarse grain Al2 O3 powder (grain size: 0.6–0.9 m) and nanometer Al2 O3 powder (grain size: 40–80 nm) were used as raw materials. A typical SEM image of nanometer A12 O3 powder is shown in Fig. 1. The raw materials were weighed according to certain proportions (Table 1). 5 wt% of sintering aid (50 pencil stone, 39.5 SiO2 and 10.5 CaCO3 in wt%)
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329
Fig. 1. A typical SEM image of nanometer A12 O3 powder. Fig. 3. The line shrinkage ratio of alumina ceramics by different sintering technology.
Table 1 Composition of Al2 O3 powder (wt%). No
Coarse grain Al2 O3
Nanometer Al2 O3
1 2 3 4 5 6
100 95 90 80 70 60
0 5 10 20 30 40
constant cross-head speed of 0.5 mm min−1 . Five samples of each batch were tested to obtain an average value. The microstructure of the fracture surface was characterized by scanning electronic microscope.
was added and mixed with bimodal alumina powders. After ballmilling using ethanol as a mixing medium, drying and pelleting, the mixture of the raw materials was uniaxially pressed into bars with 4 mm × 6 mm × 50 mm, then isostatically cold-pressed at 200 MPa. The green bodies were sintered at different temperatures for 30 min in a microwave sintering furnace. For comparison, the green bodies were conventional sintered at the same temperature for 120 min. The Archimedes method was used to measure the density of the sintered samples. The sintered samples were carefully ground and polished into the size of 3 mm × 4 mm × 40 mm for mechanical behavior measurements. The bending strength was measured by the three-point-bending method. Fracture toughness was measured by the single-edge notched beam method (SENB). An initial notch of 2.5 mm in deptch was cut using a diamond blade (0.15 mm in thickness). Both tests were conducted with a span of 30 mm at a
Fig. 2. Relative density of alumina ceramics by different sintering technology.
Fig. 4. SEM images of fracture surfaces of alumina ceramics with the same content of nanometer A12 O3 by different sintering technology (a) conventional sintered at 1550 ◦ C for 120 min, (b) microwave sintered at 1500 ◦ C for 30 min.
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Fig. 5. SEM images of fracture surfaces of microwave-sintered alumina ceramics with different content of nanometer A12 O3 powder at 1500 ◦ C for 30 min (a) 10%, (b) 20%, (c) 30% and (d) 40%.
3. Results and discussion 3.1. Sintering behavior The alumina powder was sintered at different temperatures by different sintering technologies, and the relative densities are shown in Fig. 2. It can be seen that the relative density of the conventional-sintered samples increases with increased sintering temperature, and it could only get to 96.1% of the theoretical value at 1550 ◦ C for 120 min. The variation of density in the microwavesintered samples is similar to that of the conventional-sintered sample. However, its relative density reaches up to 98.5% of the theoretical value at 1500 ◦ C for 30 min, which is much higher than that of the conventional-sintered samples. The line shrinkage ratio of alumina ceramics by different sintering technologies is shown in Fig. 3. The line shrinkage ratio of the microwave-sintered samples is much higher than that of the conventional-sintered samples, which is consistent with the density of samples by different sintering processes. According to the sintering behaviors of alumina ceramics with addition of nanometer Al2 O3 powder, it can be seen that sintering temperature and duration of alumina ceramics is remarkably lowered by microwave sintering process. 3.2. Microstructure Fig. 4 shows SEM images of the fracture surface of alumina ceramics prepared by different sintering processes. It can be seen that there is large difference between fracture surfaces of alumina ceramics. The conventional-sintered sample obtained at 1550 ◦ C for 120 min still exhibits a porous microstructure (Fig. 4a). There are also discontinuous grain growth and platelet grain in conventionalsintered sample. Whereas, it is nearly fully sintered by microwave sintering at 1500 ◦ C for 30 min (Fig. 4b). The grain size is smaller than that of the conventional-sintered sample, and the distribution of the grain size of alumina ceramics is relatively uniform.
Fig. 5 shows SEM images of the fracture surface of microwavesintered samples with different nanometer A12 O3 powder obtained at 1500 ◦ C for 30 min. The sinterability of the alumina ceramics increases with the increased content of nanometer A12 O3 powder, while its grain size decreases. The microstructure of samples is similar to each other with uniform distribution of fine alumina. As the content of nanometer A12 O3 powder is about 20 wt%, its microstructure is more homogeneous and compacter than those of other samples, as shown in Fig. 5b. With the increase of nanometer A12 O3 powder, the abnormal grain growth and big pore in samples are observed in Fig. 5c and d. The higher content of the nanometer A12 O3 powder is, the larger number of the abnormal grain growth and pore are observed in samples. 3.3. Mechanical properties The mechanical properties of the microwave-sintered alumina ceramics with different nanometer A12 O3 powder are shown in Fig. 6. It can be seen that the bending strength and fracture toughness of the samples firstly increase with nanometer A12 O3 powder. The maximum bending strength and fracture toughness of the microwave-sintered alumina ceramics with 20 wt% of nanometer A12 O3 powder are obtained up to 294.41 Mpa and 4.14 Mpa m1/2 , which is in accordance with the microstructure of the sample revealed by SEM image. Then it decreases with the increased content of nanometer A12 O3 powder. The bimodal alumina powder mixtures improved the density of coarse alumina powder, microstructure and mechanical properties of alumina ceramics. As the content of nanometer A12 O3 powder is about 20 wt%, there is the best packing density by theory and experimental results [16–18]. The maximum bending strength and fracture toughness of microwave-sintered alumina ceramics with 20 wt% of nanometer A12 O3 at 1500 ◦ C for 30 min are attributed to its uniform, fine-grained and dense microstructure. It is found that the addition of nanometer A12 O3 powder and microwave
Y. Liu et al. / Materials Science and Engineering A 546 (2012) 328–331
331
microwave sintered at 1500 ◦ C for 30 min. The density is up to 98.5% of the theoretical value. The maximum of bending strength and fracture toughness can be obtained up to 294.41 Mpa and 4.14 Mpa m1/2 . It may be attributed to the addition of nanometer A12 O3 powder and microwave sintering process resulting in a synergistic effect on fine grain size, densitification, low sinteringtemperature and excellent mechanical properties. Acknowledgements This work was supported by National Nature Science Foundation of China (51174006), the Natural Science Foundation of High Education School of Anhui Province, China (KJ2010A095) and Anhui Provincial Natural Science Foundation, China (1208085ME84). References Fig. 6. The bending strength and fracture toughness of microwave-sintered alumina ceramics with different content of nanometer A12 O3 powder at 1500 ◦ C for 30 min.
sintering process result in a synergistic effect on densitification, low sintering-temperature and excellent mechanical properties. As the content of nanometer A12 O3 powder is higher than 20 wt%, nanometer A12 O3 grain were consumed and a few abnormal A12 O3 particles had impinged upon each other. There are significant increase of pores and pronounced abnormal grain growth at 1500 ◦ C. Therefore the mechanical properties of alumina ceramics decrease with increasing nanometer A12 O3 powder. 4. Conclusions To summarize, high quality alumina ceramics by using nanometer A12 O3 and coarse grain A12 O3 as raw materials were fabricated by microwave sintering technology. Nanometer A12 O3 powder can improve its sinterability as well as the mechanical properties. This kind of performance improvement can be obtained only when the content of nanometer A12 O3 powder is in a certain amount. The alumina ceramics with 20 wt% nanometer A12 O3 powder were
[1] C.E. Borsa, N.M.R. Jones, R.J. Brook, R.I. Todd, J. Eur. Ceram. Soc. 17 (1997) 865–869. [2] N.A. Travitzky, A. Goldstein, O. Avsian, A. Singurindi, Mater. Sci. Eng. A 286 (2000) 225–229. [3] R.R. Menezes, R.H.G.A. Kiminami, J. Mater. Process. Technol. 203 (1–3) (2008) 513–517. [4] X.Y. Qin, R. Cao, H.Q. Li, Ceram. Int. 32 (2006) 575–581. [5] K. Niihara, B.S. Kim, T. Nakayama, J. Eur. Ceram. Soc. 24 (2004) 3419–3425. [6] C.L. Huang, J.J. Wang, C.Y. Huang, Mater. Lett. 59 (2005) 3746–3749. [7] J. Li, Y.B. Pan, L.P. Huang, J.K. Guo, Ceram. Int. 35 (2009) 1377–1383. [8] L. Wang, J.L. Shi, H.R. Chen, Z.L. Hua, T.S. Yen, J. Mater. Sci. Lett. 20 (2001) 341–342. [9] J. Li, Y.B. Pan, F.G. Qiu, L.P. Huang, J.K. Guo, Mater. Sci. Eng. A 435–436 (2006) 611–619. [10] G. Li, A. Jiang, L. Zhang, J. Mater. Sci. Lett. 15 (1996) 1713–1715. [11] L. Gao, J.S. Hong, H. Miyamoto, S.D.D.L. Torre, J. Eur. Ceram. Soc. 20 (2000) 2149–2152. [12] Z.P. Xie, J.L. Yang, Y. Huang, Mater. Lett. 3 (4–5) (1998) 215–220. [13] Y. Fang, J.P. Cheng, D.K. Agrawal, Mater. Lett. 58 (3–4) (2004) 498–501. [14] M. Mizuno, S. Obata, S. Takayama, et al., J. Eur. Ceram. Soc. 24 (2) (2004) 387–391. [15] R.R. Menezes, R.H.G.A. Kiminami, J. Mater. Process. Technol. 203 (2008) 513–517. [16] R.J. Hellmig, H. Ferkel, J. Am. Ceram. Soc. 84 (2) (2001) 261–266. [17] J. Li, Y.B. Pan, F.G. Qiu, L.P. Huang, J.K. Guo, Mater. Sci. Eng. A 435-436 (2006) 611–619. [18] J. Li, Y.B. Pan, J.W. Ning, L.P. Huang, J.K. Guo, J. Inorg. Mater. 18 (6) (2003) 1192–1198.