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Pressureless sintering of translucent MgO ceramics
Available online at www.sciencedirect.com
Scripta Materialia 59 (2008) 757–759 www.elsevier.com/locate/scriptamat
Pressureless sintering of translucent MgO ceramics Dianying Chen,a Eric H. Jordanb,* and Maurice Gella a
Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA b Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA Received 23 May 2008; accepted 8 June 2008 Available online 20 June 2008
MgO nanocrystalline powders were synthesized via a wet precipitation process. X-ray diffraction analysis of the heat-treated precursor powders shows that a crystalline MgO phase forms at 500 °C. Translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders at 1400 °C for 2 h under ambient atmosphere. The as-sintered MgO ceramics have a relative density of 98.1% with an average hardness of 6.8 GPa. Scanning electron microscope characterization revealed that the translucent MgO ceramics have an average grain size of 6 lm. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: MgO; Precipitation; Optical materials; Sintering
Transparent polycrystalline ceramics are widely used in applications such as solid-state lasers [1–3], high-pressure sodium lamps [4] and scintillators [5]. MgO ceramic has excellent thermal and mechanical properties with a high melting point (2800 °C) and an isotropic, cubic crystal structure. It is a potential candidate for optical applications. Transparent magnesia ceramics have been prepared by the following techniques: (i) pressureless sintering in vacuum with sintering additives, (ii) hot pressing, (iii) hot isostatic pressing (HIP), and (iv) spark plasma sintering. Misawa et al. [6] fabricated transparent MgO ceramic using fine MgO powders and silica and boron oxide as additives by sintering at 1600 °C for 2 h in vacuum. Fang et al. [7] fabricated translucent MgO ceramics by hot pressing MgO nanopowders using LiF as additive. Chaim et al. [8] fabricated fully dense, optically transparent MgO ceramics from nanocrystalline powders at 800 °C and 150 MPa for 5 min using spark plasma sintering. Itatani et al. [9] fabricated transparent MgO ceramics at 1600 °C by HIP of pressureless-sintered MgO compact. However, reports on the pressureless sintering of translucent MgO ceramics under ambient atmosphere are rare. Carbonates have been used as excellent precursors for the preparation of translucent or transparent YAG [10], MgAl2O4 [11] and Y2O3 [12] ceramics. Magnesium car* Corresponding author. Tel.: +1 860 486 2371; fax: +1 860 486 5088; e-mail: [email protected]
bonate is expected to be the ideal precursor to make highly sinterable magnesia powders for transparent magnesia ceramics. In the present research, nanocrystalline MgO powders were prepared by decomposition of the as-precipitated Mg5(OH)2(CO3)44H2O precursor. Translucent MgO ceramics were prepared by pressureless sintering of the nanocrystalline MgO powders at 1400 °C for 2 h under ambient atmosphere without additives. Magnesium nitrate hexahydrate (Mg(NO3)26H2O, 98%, Alfa Aesar), ammonium hydrogen carbonate (NH4HCO3, Alfa Aesar) and ammonium hydroxide (NH4OH, 28.0%, Alfa Aesar) were used as starting materials. The magnesium and ammonium solutions were prepared separately. For the former, a solution of Mg(NO3)2 (0.5 M) in 1000 ml distilled water was made. For the ammonium solution, a NH4HCO3 solution with 1.5 M concentration was made by dissolving ammonium hydrogen carbonate in distilled water and adjusting the pH value to 10.5 using ammonia hydroxide. One liter of 0.5 M Mg(NO3)26H2O solution was added dropwise into 500 ml of 1.5 M NH4HCO3 solution under vigorous stirring to produce a milky, gelatinous precipitate. The precipitate was then aged for 24 h. The resultant suspension was centrifuged and washed five times with distilled water, rinsed with ethanol and dried at 80 °C for 12 h. The dried powders were then heat treated at various temperatures for 2 h. The as-precipitated powder was calcined at 700 °C for 2 h and then compressed into discs in a stainless steel
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.06.007
758
D. Chen et al. / Scripta Materialia 59 (2008) 757–759
mold under a uniaxial pressure of 200 MPa. Sintering of the MgO discs was completed using a muffle furnace under ambient atmosphere. The sintering temperature was 1400 °C for 2 h. Both differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) experiments were performed simultaneously on the as-dried precipitate using a SDTQ6000 thermal analyzer (TA Inc., New Castle, DE). For each thermal analysis run, 30 mg of powder was placed in an Al2O3 crucible. The specific surface area of the calcined powders was determined using a gas sorption analyzer based on the Brunauer–Emmett–Teller (BET) method. The crystalline phase composition of all samples was determined using X-ray diffraction (XRD, Cu-Ka radiation; D5005, Bruker AXS, Karlsruhe, Germany). The density of sintered samples was measured by the Archimedes principle using water as the immersion liquid. Scanning electron microscopy (SEM) analysis of powders and sintered bodies was conducted using a JEOL 6335F field-emission instrument (JEOL, Tokyo, Japan). Prior to examination, the samples were goldcoated to prevent charging in the electron microscope. The Vickers hardness of the as-sintered ceramics was measured on the polished top surface with a 1.96-N normal load and a dwell time of 15 s. The hardness value for each sample is the average of 10 measurements. Figure 1 shows the XRD pattern of the as-precipitated precursor. The precipitate is composed of crystalline Mg5(OH)2(CO3)44H2O phase (JCPDF No. 250513). The XRD patterns of the as-precipitated powders heated in the laboratory furnace at various temperatures are shown in Figure 2. At 400 °C, the powders are amor-
600 M: Mg5(OH)2 (CO ) .4H2O 34 M
400 M
300 M
40
M M
M M M M
100
M
M M M
M
M M
M
0 10
20
30 40 2 θ (º)
50
60
Figure 1. XRD of the as-precipitated powders.
20
100
M
0 133º C
80
-20
483º C 256º C
-40
425º C
-60
60
-80 -100
40
-120
X: Cubic MgO
X
Intensity (cps)
0
o
200
400
600
800
1000 1200
Temperature ºC
X (d) 700 C
Heat flow (mW)
M
200
Weight percent (%)
Intensity (CPS)
500
phous. When the temperature is increased to 500 °C, the MgO crystalline phase begins to form. With an increase of temperature, the crystalline peaks become sharp and intense, which indicates increased MgO crystallinity and grain growth. The weight loss of the precursor as a function of temperature in air was measured using a thermal gravimetric-differential thermal analysis (TG-DTA) instrument after the precursor was dried at 80 °C. Typical TGDTA curves for the crystallization of MgO precursor obtained at a heating rate of 10 °C min 1 are shown in Figure 3. The sample weight decreases with increasing temperature continuously from room temperature to 500 °C. The overall weight loss (57.8 wt.%) of the precursor is very close to the theoretical value (57.2 wt.%) calculated based on Mg5(OH)2(CO3)44H2O. The shallow endothermic peak at 133 °C can be ascribed to the evaporation of residual water as well as physically adsorbed water. The other two endothermic peaks at 256 and 425 °C can be attributed to the decomposition of hydroxides and carbonates, respectively, because of significant weight losses. The exothermic DTA peak at 482 °C can be ascribed to the crystallization of MgO from the amorphous phase, as confirmed by XRD analysis (Fig. 2). The BET specific surface area and grain size of MgO nanocrystalline powders heat treated at various temperatures are summarized in Table 1. The BET specific surface area of the powders decreases from 90.7 to 70.8 m2 g 1 with increasing calcination temperature from 500 to 700 °C. The average grain size determined by the Scherrer equation [13] based on the XRD line broadening after heat treatment at 700 °C is about 16.3 nm. Thus the as-prepared MgO powders have a high specific surface area and nanosized grains. Figure 4 shows the morphology of the powders heat treated at 700 °C for 2 h. Spherical particles with average size of 20 nm are observed, which is consistent with the grain size determined by the Scherrer equation.
X
X X
o
Figure 3. Typical TG-DTA curves of as-precipitated powders at a heating rate of 10 °C min 1.
(c) 600 C o
(b) 500 C
Table 1. Summary of BET surface area and grain size of MgO nanocrystalline powders
o
(a) 400 C
20
30
40
50
60
70
80
2 theta
Figure 2. XRD of MgO powders calcined at various temperatures: (a) 400 °C, (b) 500 °C, (c) 600 °C, and (d) 700 °C.
Heat treatment temperature (°C) BET surface area (m2 g 1) Average crystalline size from XRD line broadening (nm)
500 90.7 10.3
600 86.0 14.7
700 70.8 16.3
D. Chen et al. / Scripta Materialia 59 (2008) 757–759
Figure 4. Microstructure of MgO powders calcined at 700 °C for 2 h.
Figure 5. Photograph of the pressureless-sintered translucent MgO ceramics.
Figure 6. Microstructure of the as-sintered MgO: 1400 °C for 2 h.
Figure 5 shows a photograph of the MgO pellet pressureless sintered at 1400 °C for 2 h. The pellet is 10 mm in diameter and 0.8-mm-thick. The pellet is translucent and the character under the pellet is visible. The as-sintered ceramic has a relative density 98.1% of the theoretical density with an average hardness of 6.8 GPa. Thus, dense and translucent MgO ceramics can be successfully prepared using a pressureless sintering technique. The microstructure of the as-sintered MgO pellet at 1400 °C for 2 h is shown in Figure 6. The average grain size of the translucent MgO ceramics is 6 lm. The preparation of dense MgO ceramics using a pressureless sintering technique without additives in the present experiment can be attributed to the high specific surface area of the nanocrystalline powders, which provides a high driving force for sintering and densification.
759
It is well known that the mechanical strength and thermal shock resistance of ceramics increases with decreasing grain size [14–17]. Therefore, preparation of nanocrystalline transparent MgO ceramics is highly desirable to improve the mechanical properties. Although the average grain size in the pressureless-sintered translucent MgO ceramics in the present research is as large as 6 lm, it offers the possibility of making transparent nanocrystalline MgO ceramics using the as-synthesized, highly sinterable nanocrystalline MgO powders by optimizing the sintering conditions, e.g. by lowering the sintering temperature using hot pressing or HIP techniques. In summary, MgO nanocrystalline powders were synthesized via a wet precipitation process. Optically translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders at 1400 °C for 2 h without sintering additives. The as-sintered MgO ceramics had an average grain size of 6 lm, a relative density of 98.1% and an average hardness of 6.8 GPa. This work was supported by a sub-contract from Raytheon Company, from a prime contract funded by DARPA/ONR. [1] V. Lupei, A. Lupei, A. Ikesue, Appl. Phys. Lett. 86 (2005) 111118. [2] H. Yagi, T. Yanagitani, K. Takaichi, K. Ueda, A.A. Kaminskii, Opt. Mater. 29 (2007) 1258. [3] Y.S. Wu, J. Li, Y.B. Pan, J.K. Guo, B.X. Jiang, Y. Xu, J. Xu, J. Am. Ceram. Soc. 90 (2007) 3334. [4] R. Apetz, M.P.B. van Bruggen, J. Am. Ceram. Soc. 86 (2003) 480. [5] T. Yanagida, T. Roh, H. Takahashi, S. Hirakuri, M. Kokubun, K. Makishima, M. Sato, T. Enoto, T. Yanagitani, H. Yagi, T. Shigetad, T. Ito, Nucl. Instrum. Methods A 579 (2007) 23. [6] T. Misawa, Y. Moriyoshi, Y. Yajima, S. Takenouchi, T. Ikegami, J. Ceram. Soc. Jpn. 107 (1999) 343. [7] Y. Fang, D. Agrawal, G. Skandan, M. Jain, Mater. Lett. 58 (2004) 551. [8] R. Chaim, Z.J. Shen, M. Nygren, J. Mater. Res. 19 (2004) 2527. [9] K. Itatani, T. Tsujimoto, A. Kishimoto, J. Eur. Ceram. Soc. 26 (2006) 639. [10] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, J. Am. Ceram. Soc. 83 (2000) 961. [11] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, J. Am. Ceram. Soc. 83 (2000) 2866. [12] N. Saito, S. Matsuda, T. Ikegami, J. Am. Ceram. Soc. 81 (1998) 2023. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [14] N. Bamba, Y.H. Choa, K. Niihara, Nanostruct. Mater. 9 (1997) 497. [15] B. Liang, C.X. Ding, Surf. Coat. Technol. 197 (2005) 185. [16] B. Jiang, G.J. Weng, Int. J. Plasticity 20 (2004) 2007. [17] S.C. Tjong, H. Chen, Mater. Sci. Eng. R 45 (2004) 1.
Scripta Materialia 59 (2008) 757–759 www.elsevier.com/locate/scriptamat
Pressureless sintering of translucent MgO ceramics Dianying Chen,a Eric H. Jordanb,* and Maurice Gella a
Department of Chemical, Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, Storrs, CT 06269, USA b Department of Mechanical Engineering, University of Connecticut, Storrs, CT 06269, USA Received 23 May 2008; accepted 8 June 2008 Available online 20 June 2008
MgO nanocrystalline powders were synthesized via a wet precipitation process. X-ray diffraction analysis of the heat-treated precursor powders shows that a crystalline MgO phase forms at 500 °C. Translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders at 1400 °C for 2 h under ambient atmosphere. The as-sintered MgO ceramics have a relative density of 98.1% with an average hardness of 6.8 GPa. Scanning electron microscope characterization revealed that the translucent MgO ceramics have an average grain size of 6 lm. Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: MgO; Precipitation; Optical materials; Sintering
Transparent polycrystalline ceramics are widely used in applications such as solid-state lasers [1–3], high-pressure sodium lamps [4] and scintillators [5]. MgO ceramic has excellent thermal and mechanical properties with a high melting point (2800 °C) and an isotropic, cubic crystal structure. It is a potential candidate for optical applications. Transparent magnesia ceramics have been prepared by the following techniques: (i) pressureless sintering in vacuum with sintering additives, (ii) hot pressing, (iii) hot isostatic pressing (HIP), and (iv) spark plasma sintering. Misawa et al. [6] fabricated transparent MgO ceramic using fine MgO powders and silica and boron oxide as additives by sintering at 1600 °C for 2 h in vacuum. Fang et al. [7] fabricated translucent MgO ceramics by hot pressing MgO nanopowders using LiF as additive. Chaim et al. [8] fabricated fully dense, optically transparent MgO ceramics from nanocrystalline powders at 800 °C and 150 MPa for 5 min using spark plasma sintering. Itatani et al. [9] fabricated transparent MgO ceramics at 1600 °C by HIP of pressureless-sintered MgO compact. However, reports on the pressureless sintering of translucent MgO ceramics under ambient atmosphere are rare. Carbonates have been used as excellent precursors for the preparation of translucent or transparent YAG [10], MgAl2O4 [11] and Y2O3 [12] ceramics. Magnesium car* Corresponding author. Tel.: +1 860 486 2371; fax: +1 860 486 5088; e-mail: [email protected]
bonate is expected to be the ideal precursor to make highly sinterable magnesia powders for transparent magnesia ceramics. In the present research, nanocrystalline MgO powders were prepared by decomposition of the as-precipitated Mg5(OH)2(CO3)44H2O precursor. Translucent MgO ceramics were prepared by pressureless sintering of the nanocrystalline MgO powders at 1400 °C for 2 h under ambient atmosphere without additives. Magnesium nitrate hexahydrate (Mg(NO3)26H2O, 98%, Alfa Aesar), ammonium hydrogen carbonate (NH4HCO3, Alfa Aesar) and ammonium hydroxide (NH4OH, 28.0%, Alfa Aesar) were used as starting materials. The magnesium and ammonium solutions were prepared separately. For the former, a solution of Mg(NO3)2 (0.5 M) in 1000 ml distilled water was made. For the ammonium solution, a NH4HCO3 solution with 1.5 M concentration was made by dissolving ammonium hydrogen carbonate in distilled water and adjusting the pH value to 10.5 using ammonia hydroxide. One liter of 0.5 M Mg(NO3)26H2O solution was added dropwise into 500 ml of 1.5 M NH4HCO3 solution under vigorous stirring to produce a milky, gelatinous precipitate. The precipitate was then aged for 24 h. The resultant suspension was centrifuged and washed five times with distilled water, rinsed with ethanol and dried at 80 °C for 12 h. The dried powders were then heat treated at various temperatures for 2 h. The as-precipitated powder was calcined at 700 °C for 2 h and then compressed into discs in a stainless steel
1359-6462/$ - see front matter Ó 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2008.06.007
758
D. Chen et al. / Scripta Materialia 59 (2008) 757–759
mold under a uniaxial pressure of 200 MPa. Sintering of the MgO discs was completed using a muffle furnace under ambient atmosphere. The sintering temperature was 1400 °C for 2 h. Both differential thermal analysis (DTA) and thermal gravimetric analysis (TGA) experiments were performed simultaneously on the as-dried precipitate using a SDTQ6000 thermal analyzer (TA Inc., New Castle, DE). For each thermal analysis run, 30 mg of powder was placed in an Al2O3 crucible. The specific surface area of the calcined powders was determined using a gas sorption analyzer based on the Brunauer–Emmett–Teller (BET) method. The crystalline phase composition of all samples was determined using X-ray diffraction (XRD, Cu-Ka radiation; D5005, Bruker AXS, Karlsruhe, Germany). The density of sintered samples was measured by the Archimedes principle using water as the immersion liquid. Scanning electron microscopy (SEM) analysis of powders and sintered bodies was conducted using a JEOL 6335F field-emission instrument (JEOL, Tokyo, Japan). Prior to examination, the samples were goldcoated to prevent charging in the electron microscope. The Vickers hardness of the as-sintered ceramics was measured on the polished top surface with a 1.96-N normal load and a dwell time of 15 s. The hardness value for each sample is the average of 10 measurements. Figure 1 shows the XRD pattern of the as-precipitated precursor. The precipitate is composed of crystalline Mg5(OH)2(CO3)44H2O phase (JCPDF No. 250513). The XRD patterns of the as-precipitated powders heated in the laboratory furnace at various temperatures are shown in Figure 2. At 400 °C, the powders are amor-
600 M: Mg5(OH)2 (CO ) .4H2O 34 M
400 M
300 M
40
M M
M M M M
100
M
M M M
M
M M
M
0 10
20
30 40 2 θ (º)
50
60
Figure 1. XRD of the as-precipitated powders.
20
100
M
0 133º C
80
-20
483º C 256º C
-40
425º C
-60
60
-80 -100
40
-120
X: Cubic MgO
X
Intensity (cps)
0
o
200
400
600
800
1000 1200
Temperature ºC
X (d) 700 C
Heat flow (mW)
M
200
Weight percent (%)
Intensity (CPS)
500
phous. When the temperature is increased to 500 °C, the MgO crystalline phase begins to form. With an increase of temperature, the crystalline peaks become sharp and intense, which indicates increased MgO crystallinity and grain growth. The weight loss of the precursor as a function of temperature in air was measured using a thermal gravimetric-differential thermal analysis (TG-DTA) instrument after the precursor was dried at 80 °C. Typical TGDTA curves for the crystallization of MgO precursor obtained at a heating rate of 10 °C min 1 are shown in Figure 3. The sample weight decreases with increasing temperature continuously from room temperature to 500 °C. The overall weight loss (57.8 wt.%) of the precursor is very close to the theoretical value (57.2 wt.%) calculated based on Mg5(OH)2(CO3)44H2O. The shallow endothermic peak at 133 °C can be ascribed to the evaporation of residual water as well as physically adsorbed water. The other two endothermic peaks at 256 and 425 °C can be attributed to the decomposition of hydroxides and carbonates, respectively, because of significant weight losses. The exothermic DTA peak at 482 °C can be ascribed to the crystallization of MgO from the amorphous phase, as confirmed by XRD analysis (Fig. 2). The BET specific surface area and grain size of MgO nanocrystalline powders heat treated at various temperatures are summarized in Table 1. The BET specific surface area of the powders decreases from 90.7 to 70.8 m2 g 1 with increasing calcination temperature from 500 to 700 °C. The average grain size determined by the Scherrer equation [13] based on the XRD line broadening after heat treatment at 700 °C is about 16.3 nm. Thus the as-prepared MgO powders have a high specific surface area and nanosized grains. Figure 4 shows the morphology of the powders heat treated at 700 °C for 2 h. Spherical particles with average size of 20 nm are observed, which is consistent with the grain size determined by the Scherrer equation.
X
X X
o
Figure 3. Typical TG-DTA curves of as-precipitated powders at a heating rate of 10 °C min 1.
(c) 600 C o
(b) 500 C
Table 1. Summary of BET surface area and grain size of MgO nanocrystalline powders
o
(a) 400 C
20
30
40
50
60
70
80
2 theta
Figure 2. XRD of MgO powders calcined at various temperatures: (a) 400 °C, (b) 500 °C, (c) 600 °C, and (d) 700 °C.
Heat treatment temperature (°C) BET surface area (m2 g 1) Average crystalline size from XRD line broadening (nm)
500 90.7 10.3
600 86.0 14.7
700 70.8 16.3
D. Chen et al. / Scripta Materialia 59 (2008) 757–759
Figure 4. Microstructure of MgO powders calcined at 700 °C for 2 h.
Figure 5. Photograph of the pressureless-sintered translucent MgO ceramics.
Figure 6. Microstructure of the as-sintered MgO: 1400 °C for 2 h.
Figure 5 shows a photograph of the MgO pellet pressureless sintered at 1400 °C for 2 h. The pellet is 10 mm in diameter and 0.8-mm-thick. The pellet is translucent and the character under the pellet is visible. The as-sintered ceramic has a relative density 98.1% of the theoretical density with an average hardness of 6.8 GPa. Thus, dense and translucent MgO ceramics can be successfully prepared using a pressureless sintering technique. The microstructure of the as-sintered MgO pellet at 1400 °C for 2 h is shown in Figure 6. The average grain size of the translucent MgO ceramics is 6 lm. The preparation of dense MgO ceramics using a pressureless sintering technique without additives in the present experiment can be attributed to the high specific surface area of the nanocrystalline powders, which provides a high driving force for sintering and densification.
759
It is well known that the mechanical strength and thermal shock resistance of ceramics increases with decreasing grain size [14–17]. Therefore, preparation of nanocrystalline transparent MgO ceramics is highly desirable to improve the mechanical properties. Although the average grain size in the pressureless-sintered translucent MgO ceramics in the present research is as large as 6 lm, it offers the possibility of making transparent nanocrystalline MgO ceramics using the as-synthesized, highly sinterable nanocrystalline MgO powders by optimizing the sintering conditions, e.g. by lowering the sintering temperature using hot pressing or HIP techniques. In summary, MgO nanocrystalline powders were synthesized via a wet precipitation process. Optically translucent MgO ceramics were prepared by pressureless sintering the nanocrystalline MgO powders at 1400 °C for 2 h without sintering additives. The as-sintered MgO ceramics had an average grain size of 6 lm, a relative density of 98.1% and an average hardness of 6.8 GPa. This work was supported by a sub-contract from Raytheon Company, from a prime contract funded by DARPA/ONR. [1] V. Lupei, A. Lupei, A. Ikesue, Appl. Phys. Lett. 86 (2005) 111118. [2] H. Yagi, T. Yanagitani, K. Takaichi, K. Ueda, A.A. Kaminskii, Opt. Mater. 29 (2007) 1258. [3] Y.S. Wu, J. Li, Y.B. Pan, J.K. Guo, B.X. Jiang, Y. Xu, J. Xu, J. Am. Ceram. Soc. 90 (2007) 3334. [4] R. Apetz, M.P.B. van Bruggen, J. Am. Ceram. Soc. 86 (2003) 480. [5] T. Yanagida, T. Roh, H. Takahashi, S. Hirakuri, M. Kokubun, K. Makishima, M. Sato, T. Enoto, T. Yanagitani, H. Yagi, T. Shigetad, T. Ito, Nucl. Instrum. Methods A 579 (2007) 23. [6] T. Misawa, Y. Moriyoshi, Y. Yajima, S. Takenouchi, T. Ikegami, J. Ceram. Soc. Jpn. 107 (1999) 343. [7] Y. Fang, D. Agrawal, G. Skandan, M. Jain, Mater. Lett. 58 (2004) 551. [8] R. Chaim, Z.J. Shen, M. Nygren, J. Mater. Res. 19 (2004) 2527. [9] K. Itatani, T. Tsujimoto, A. Kishimoto, J. Eur. Ceram. Soc. 26 (2006) 639. [10] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, J. Am. Ceram. Soc. 83 (2000) 961. [11] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, J. Am. Ceram. Soc. 83 (2000) 2866. [12] N. Saito, S. Matsuda, T. Ikegami, J. Am. Ceram. Soc. 81 (1998) 2023. [13] H.P. Klug, L.E. Alexander, X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials, Wiley, New York, 1954. [14] N. Bamba, Y.H. Choa, K. Niihara, Nanostruct. Mater. 9 (1997) 497. [15] B. Liang, C.X. Ding, Surf. Coat. Technol. 197 (2005) 185. [16] B. Jiang, G.J. Weng, Int. J. Plasticity 20 (2004) 2007. [17] S.C. Tjong, H. Chen, Mater. Sci. Eng. R 45 (2004) 1.