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Phase development of nanocrystalline cerium oxide via cerium sulfate
ARTICLE IN PRESS
Journal of Crystal Growth 289 (2006) 351–356 www.elsevier.com/locate/jcrysgro
Phase development of nanocrystalline cerium oxide via cerium sulfate Ming-Shyong Tsai, Xu-Zhu Xiao Department of Chemical and Material Engineering, Southern Taiwan University of Technology, Tainan, Taiwan, ROC Received 23 March 2005; received in revised form 25 October 2005; accepted 11 November 2005 Available online 4 January 2006 Communicated by J.M. Redwing
Abstract Nanograde cerium oxide is formed successfully by a normal-pressure aging process via cerium sulfate. The nanograde cerium oxide is formed by the precursor of cerium sulfate decomposition during aging in urea solution. There are two pH stages in the aging process: one is in pH ¼ 5–6 and the other is in pH ¼ 7–8. The first pH stage is the particle decomposed stage in which the particles were decomposed gradually. In the last stage, the final product is uniform aggregated particles which are formed by complete decomposition. The crystalline size of cerium oxide formed by this method is about 6–7 nm. The decomposed rate of precursor is increased by increasing urea concentration or increasing the aging time, respectively. r 2005 Elsevier B.V. All rights reserved. PACS: 6182.Rx; 81.05.Y; 81.10.A Keywords: A1. Crystal morphology; A1. Nano-structure; B1. Rare-earth compound
1. Introduction Cerium oxide (CeO2) is a material with a rapidly increasing number of applications, such as gas sensors, electrode materials of solid oxide fuel cell [1,2], amperometric oxygen monitors, automobile exhaust system’s catalysis, and, especially, abrasives of chemical mechanical polishing (CMP) slurries [3–10]. Kim et al. [10] indicated that the very important process of CMP application is in the shallow trench isolation (STI) process on the silicon substrate. A proper global planarization is required to secure a sufficient depth of focus to make the process margin widely related to the exposure equipment used in fine-pattern process technology. There are both chemical and mechanical interactions between ceria particles and the oxide film during polishing [10]. The selectivity between silicon oxide and silicon nitride of ceria slurry is good. Using ceria slurry, the oxide layer polishing can stop on the nitrate surface and provide global planarization of the STI process. Several methods were investigated to produce the fine cerium oxide powders [6–16]. Wang [6] synthesized Corresponding author. Tel.: +0922739828; fax: +00288662425741.
E-mail address: [email protected] (M.-S. Tsai). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.081
cerium oxide with a particle size range of 30–50 nm via a hydrothermal process. Wang and Lu [16] used cerium nitrite and urea as reactants to form cerium hydroxide carbonate via hydrothermal process. Fluorite structure cerium oxide was synthesized by further calcinations of the intermediate at 500 1C. In the present work, a synthesis method of nanograde cerium oxide is studied. Cerium sulfate is hydrolyzed in DI water to form an amorphous precursor. The precursors were decomposed to form a nanograde crystalline cerium oxide by a normal-pressure aging process. Urea concentration and aging time are studied in this article.
2. Experimental procedure The experimental flow chart of this study is presented in Fig. 1. Precursors were formed by titrating the desired concentration of 200 ml urea solution into 200 ml 0.12 M cerium sulfate solution. The mixed solutions were heated with the reflux apparatus and with the stirred rate of 500 rpm at 100 1C for 5 h. Products of different aging time were sampled for studying the formation process of this route. Products were characterized crystalline structure by
ARTICLE IN PRESS M.-S. Tsai, X.-Z. Xiao / Journal of Crystal Growth 289 (2006) 351–356
352
Urea solution
Ce(SO4)2 solution
Precursor
Aging
Cerium oxide
TEM & XRD Fig. 2. Morphology of precursor.
Fig. 1. Experimental flow chart of formation nanograde cerium oxide via cerium sulfate route.
9
3. Results and discussion Fig. 2 is the morphology of precursor formed by hydrolysis of cerium sulfate in urea solution. The precursor is a smooth particle approximately 100 nm in diameter. During the aging process, pH value of heated solution is divided into two stages: the first stage is in pH ¼ 5–6 and the last stage is in pH ¼ 7–8, as shown in Fig. 3. Aging time for reaching the last stage is short as urea concentration increases. However, only one stage was found at 0.5 M urea solution after 5 h of aging. Urea is decomposed to ammonium hydroxide by heating at above 70 1C. In Fig. 3, increasing the pH value of the heated solution may be due to decomposed urea reacting with precursor at 100 1C. The chemical reaction equations may be written as follows: COðNH2 Þ2ðaqÞ þ 3H2 O ! 2NH4 þ ðaqÞ þ 2OH ðaqÞ þ CO2ðgÞ ;
(1)
CeðSO4 Þ2ðpptÞ þ 2OH ðaqÞ ! CeðOHÞ4ðsÞ þ 2SO4 2 ;
(2)
CeðOHÞ4ðsÞ ! CeO2ðsÞ þ 2H2 O:
(3)
Reactions of Eqs. (2) and (3) may be the reactions that occur in the first pH stage. After completely precipitating,
8 7 6
pH value
X-ray diffraction (XRD) (Rigaku MultiFlex, Tokyo, Japan). The scan range is 20–701 with the scan step of 0.021 and the scan rate of 41/min. The slow scan diffraction peaks of (1 1 1) with scan rate of 0.51/min were used to evaluate the crystalline size of the products by Scherrer formular [17]. The ratio (2 0 0)/(1 1 1) was calculated by Kirk and Wood’s method [18]. Morphologies of products were observed by transmission electron microscopy (TEM; JEM-1200EX, JEOL, Japan).
5 4 (a) (b) (c) (d) (e)
3 2 1 0 0
1
2
3 Aged time (hr)
4
5
6
Fig. 3. Variation of pH value of heated solution aged in (a) 0.5 M, (b) 0.75 M, (c) 1.0 M, (d) 1.25 M, and (e) 1.5 M urea solution.
the excess urea is continuously decomposed, which induces pH increase in the last stage of Fig. 3. The product of 0.5 M urea reaction shows the fluorite structure of cerium oxide after aging longer than 4 h, as shown in Fig. 4. The precursor and the products of aging time less than 4 h are shown in Fig. 4(a)–(d), an unknown structure which may be the complex cerous-ceric sulfate that was similar to the information reported in the literature [19]. Fig. 5 is the XRD patterns of the products formed in 1.5 M urea reaction. The fluorite structure was
ARTICLE IN PRESS M.-S. Tsai, X.-Z. Xiao / Journal of Crystal Growth 289 (2006) 351–356
353
formed after aging longer than 1 h. The intensities of diffraction peaks of Figs. 4 and 5 are increased with increase in aging time. Kirk and Wood [18] indicated that the intensity ratio (2 0 0)/(1 1 1) of XRD peak of cerium oxide is changed with calcination temperature. The accelerated growth of (1 1 1) plane occurs because its free
(f) (e)
(d)
0.5
8
(c)
0.45 7
(a)
20
25
30
35
40
45 2Π
50
55
60
65
70
crystilline size (nm)
0.4
Fig. 4. Phase developments of (a) precursor aged in 0.5 M urea solution for (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 5 h.
0.35
6
0.3
(200)/(111) intensity ratio
(b)
5 0.25
4
0.2 0
0.5
1 Urea concentration (M)
1.5
2
Fig. 7. Crystalline size and intensity ratio of (2 0 0)/(1 1 1) of the powders aged in the different urea concentration for 5 h.
(f) (e)
7
0.32
(d) (c)
6.8 0.31
20
25
30
35
40
45 2θ
50
55
60
65
70
Fig. 5. Phase developments of (a) precursor aged in 1.5 M urea solution for (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 5 h.
Crysatlline size (nm)
6.6 (a)
0.3 6.4 0.29 6.2
0.28
(200)
(200)
(200)/(111) intensity ratio
(b)
6 (111)
(111) 0.27
5.8 0
Fig. 6. Model of crystalline changed in aging process.
1
2
3 time (hr)
4
5
6
Fig. 8. Variation of crystalline size and intensity ratio of (2 0 0)/(1 1 1) of the powders aged in 1.5 M urea solution.
ARTICLE IN PRESS 354
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energy is lower than that of (2 0 0) planes during grain growth. Cerium oxide would change its shape from hexagon (truncated octahedron) to tetragon (a similar structure of octahedron), as shown in Fig. 6 [17]. Figs. 7 and 8 show the results of the variations of crystalline size and the intensity ratio of (2 0 0)/(1 1 1) with different urea concentration and with different aging times. The crystalline size of cerium oxide is increased with increase in urea concentration, as shown in Fig. 7. The intensity ratio of (2 0 0)/(1 1 1) decreases rapidly as urea concentration reaches to 0.75 M and then decreases slightly to a stable value, approximately 0.28. Value of (2 0 0)/ (1 1 1) is very close to the minimum value (0.29) of Kirk and Wood’s results [18], which was calcined at 1150 1C at a very high temperature. The growth rate of (1 1 1) plane is faster than that of (2 0 0) plane in this aging process. Fig. 8 shows that the aging time helps to reduce the intensity ratio of (2 0 0)/(1 1 1) and to increase the crystalline size of cerium oxide. The particle shapes during the first pH stage are shown in Fig. 9. The particle sizes of larger particles in Fig. 9 are similar to particle sizes of precursor, as shown in Fig. 2. However, smaller particles may be the decomposed particles during aging process. After 5 h of aging, pH value
reaches the range of the last pH stage, and particle shape changes to a uniform and aggregate particle. The crystalline size is about 6–7 nm, which was observed by TEM photographs; it is consisted with the result of Fig. 10. However, it still has some particles without decomposing, as shown in Fig. 10(a), which is the case aged in 0.5 M urea solution. From the results of Figs. 4 and 10(a), we can conclude that 0.5 M urea solution is not enough for reaction with cerium sulfate. The nanograde cerium oxide can be formed by cerium sulfate in higher urea concentration and longer aging time. Fig. 11 shows the enlarged scale of particle morphologies. After aging 2 h, the particles are shown as hexagon in shape; however, they are changed to tetragon in shape after aging for 5 h. This result is consistent with the model of Fig. 6. Nanograde cerium oxide is grain growth in the aging process. 4. Conclusions Nanograde cerium oxide is formed successfully by a normal-pressure aging process. Crystalline size of cerium oxide is increased by increasing concentration of urea and increasing aging time. On the other hand, intensity ratio of
Fig. 9. Morphologies of formation particles aged at the first pH stage (a) in 0.5 M urea solution for 2 h, (b) in 0.75 M urea solution for 2 h, (c) in 1 M urea solution for 1.5 h, and (d) in 1.5 M urea solution for 1 h.
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Fig. 10. Morphologies of formation particles aged in (a) 0.5 M, (b) 0.75 M, (c) 1.0 M, and (d) 1.5 M urea solution for 5 h.
Fig. 11. Morphologies of formation particles aged in 1.5 M urea solution for (a) 2 h and (b) 5 h.
(2 0 0)/(1 1 1) decreases with concentration of urea and increases with aging time. Acknowledgement The authors gratefully acknowledge the funding from the National Science Council (NSC93-2214-E-218-002).
References [1] C. Xia, M. Liu, Solid State Ionics 144 (2001) 249. [2] K. Kendall, M. Palin, J. Power Source 71 (1998) 268. [3] M. Pierre, et al., Cerium based glass polishing composition, US Patent 4601755, 1986. [4] M. Pierre, et al., Cerium/rare earth polishing composition, US Patent 4786325, 1988.
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[5] L. Le, Polishing compositions based on cerium and process for the preparation thereof, US Patent 5,026,421, 1991. [6] J.F. Wang, Oxide particles and method for producing them, US Patent 5398352, 1995. [7] K. Hanawa, N. Mochizukl, N. Ueda, Cerium oxide ultrafine particles and method for preparing the same, US Patent 5938837, 1999. [8] M. Yoshida, T. Ashizawa, H. Terazaki, Y. Kurata, J. Matsuzawa, K. Tanno, Y. Ootuki, Cerium oxide abrasive and method of polishing substrates, US Patent 6221118, 2001. [9] H. Kenzo, Cerium oxide ultra-fine particles and method for preparing the same, US Patent 5938837, 1999. [10] J.Y. Kim, S.M. Kim, U. Paik, J. Korean Phys. Soc. 41 (4) (2002) 413. [11] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, Acta Mater. 49 (2001) 419.
[12] Boro Djuricˇic´, Stephen Pickering, J. Eur. Ceram. Soc. 19 (1999) 1925. [13] M. Hirno, E. Kato, J. Am. Ceram. Soc. 79 (3) (1996) 777. [14] V.D. Kosynkin, A.A. Arzgatkina, E.N. Ivanov, M.G. Chtoutsa, A.I. Grabko, A.V. Kardapolov, N.A. Sysina, J. Alloys Compounds 303–304 (2000) 421. [15] F. Bondioli, A. Bonamartini Corradi, C. Leonelli, T. Manfredini, Mater. Res. Bull. 34 (14/15) (1999) 2159. [16] H.C. Wang, C.H. Lu, Mater. Res. Bull. 37 (2002) 783. [17] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wiley Publishing Co. Inc., 1977 (p. 102). [18] N.B. Kirk, J.V. Wood, J. Mater. Sci. 30 (1995) 2171. [19] A. Paulenova, S.E. Creager, J.D. Navratil, Y. Wei, J. Power Source 109 (2002) 431.
Journal of Crystal Growth 289 (2006) 351–356 www.elsevier.com/locate/jcrysgro
Phase development of nanocrystalline cerium oxide via cerium sulfate Ming-Shyong Tsai, Xu-Zhu Xiao Department of Chemical and Material Engineering, Southern Taiwan University of Technology, Tainan, Taiwan, ROC Received 23 March 2005; received in revised form 25 October 2005; accepted 11 November 2005 Available online 4 January 2006 Communicated by J.M. Redwing
Abstract Nanograde cerium oxide is formed successfully by a normal-pressure aging process via cerium sulfate. The nanograde cerium oxide is formed by the precursor of cerium sulfate decomposition during aging in urea solution. There are two pH stages in the aging process: one is in pH ¼ 5–6 and the other is in pH ¼ 7–8. The first pH stage is the particle decomposed stage in which the particles were decomposed gradually. In the last stage, the final product is uniform aggregated particles which are formed by complete decomposition. The crystalline size of cerium oxide formed by this method is about 6–7 nm. The decomposed rate of precursor is increased by increasing urea concentration or increasing the aging time, respectively. r 2005 Elsevier B.V. All rights reserved. PACS: 6182.Rx; 81.05.Y; 81.10.A Keywords: A1. Crystal morphology; A1. Nano-structure; B1. Rare-earth compound
1. Introduction Cerium oxide (CeO2) is a material with a rapidly increasing number of applications, such as gas sensors, electrode materials of solid oxide fuel cell [1,2], amperometric oxygen monitors, automobile exhaust system’s catalysis, and, especially, abrasives of chemical mechanical polishing (CMP) slurries [3–10]. Kim et al. [10] indicated that the very important process of CMP application is in the shallow trench isolation (STI) process on the silicon substrate. A proper global planarization is required to secure a sufficient depth of focus to make the process margin widely related to the exposure equipment used in fine-pattern process technology. There are both chemical and mechanical interactions between ceria particles and the oxide film during polishing [10]. The selectivity between silicon oxide and silicon nitride of ceria slurry is good. Using ceria slurry, the oxide layer polishing can stop on the nitrate surface and provide global planarization of the STI process. Several methods were investigated to produce the fine cerium oxide powders [6–16]. Wang [6] synthesized Corresponding author. Tel.: +0922739828; fax: +00288662425741.
E-mail address: [email protected] (M.-S. Tsai). 0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2005.11.081
cerium oxide with a particle size range of 30–50 nm via a hydrothermal process. Wang and Lu [16] used cerium nitrite and urea as reactants to form cerium hydroxide carbonate via hydrothermal process. Fluorite structure cerium oxide was synthesized by further calcinations of the intermediate at 500 1C. In the present work, a synthesis method of nanograde cerium oxide is studied. Cerium sulfate is hydrolyzed in DI water to form an amorphous precursor. The precursors were decomposed to form a nanograde crystalline cerium oxide by a normal-pressure aging process. Urea concentration and aging time are studied in this article.
2. Experimental procedure The experimental flow chart of this study is presented in Fig. 1. Precursors were formed by titrating the desired concentration of 200 ml urea solution into 200 ml 0.12 M cerium sulfate solution. The mixed solutions were heated with the reflux apparatus and with the stirred rate of 500 rpm at 100 1C for 5 h. Products of different aging time were sampled for studying the formation process of this route. Products were characterized crystalline structure by
ARTICLE IN PRESS M.-S. Tsai, X.-Z. Xiao / Journal of Crystal Growth 289 (2006) 351–356
352
Urea solution
Ce(SO4)2 solution
Precursor
Aging
Cerium oxide
TEM & XRD Fig. 2. Morphology of precursor.
Fig. 1. Experimental flow chart of formation nanograde cerium oxide via cerium sulfate route.
9
3. Results and discussion Fig. 2 is the morphology of precursor formed by hydrolysis of cerium sulfate in urea solution. The precursor is a smooth particle approximately 100 nm in diameter. During the aging process, pH value of heated solution is divided into two stages: the first stage is in pH ¼ 5–6 and the last stage is in pH ¼ 7–8, as shown in Fig. 3. Aging time for reaching the last stage is short as urea concentration increases. However, only one stage was found at 0.5 M urea solution after 5 h of aging. Urea is decomposed to ammonium hydroxide by heating at above 70 1C. In Fig. 3, increasing the pH value of the heated solution may be due to decomposed urea reacting with precursor at 100 1C. The chemical reaction equations may be written as follows: COðNH2 Þ2ðaqÞ þ 3H2 O ! 2NH4 þ ðaqÞ þ 2OH ðaqÞ þ CO2ðgÞ ;
(1)
CeðSO4 Þ2ðpptÞ þ 2OH ðaqÞ ! CeðOHÞ4ðsÞ þ 2SO4 2 ;
(2)
CeðOHÞ4ðsÞ ! CeO2ðsÞ þ 2H2 O:
(3)
Reactions of Eqs. (2) and (3) may be the reactions that occur in the first pH stage. After completely precipitating,
8 7 6
pH value
X-ray diffraction (XRD) (Rigaku MultiFlex, Tokyo, Japan). The scan range is 20–701 with the scan step of 0.021 and the scan rate of 41/min. The slow scan diffraction peaks of (1 1 1) with scan rate of 0.51/min were used to evaluate the crystalline size of the products by Scherrer formular [17]. The ratio (2 0 0)/(1 1 1) was calculated by Kirk and Wood’s method [18]. Morphologies of products were observed by transmission electron microscopy (TEM; JEM-1200EX, JEOL, Japan).
5 4 (a) (b) (c) (d) (e)
3 2 1 0 0
1
2
3 Aged time (hr)
4
5
6
Fig. 3. Variation of pH value of heated solution aged in (a) 0.5 M, (b) 0.75 M, (c) 1.0 M, (d) 1.25 M, and (e) 1.5 M urea solution.
the excess urea is continuously decomposed, which induces pH increase in the last stage of Fig. 3. The product of 0.5 M urea reaction shows the fluorite structure of cerium oxide after aging longer than 4 h, as shown in Fig. 4. The precursor and the products of aging time less than 4 h are shown in Fig. 4(a)–(d), an unknown structure which may be the complex cerous-ceric sulfate that was similar to the information reported in the literature [19]. Fig. 5 is the XRD patterns of the products formed in 1.5 M urea reaction. The fluorite structure was
ARTICLE IN PRESS M.-S. Tsai, X.-Z. Xiao / Journal of Crystal Growth 289 (2006) 351–356
353
formed after aging longer than 1 h. The intensities of diffraction peaks of Figs. 4 and 5 are increased with increase in aging time. Kirk and Wood [18] indicated that the intensity ratio (2 0 0)/(1 1 1) of XRD peak of cerium oxide is changed with calcination temperature. The accelerated growth of (1 1 1) plane occurs because its free
(f) (e)
(d)
0.5
8
(c)
0.45 7
(a)
20
25
30
35
40
45 2Π
50
55
60
65
70
crystilline size (nm)
0.4
Fig. 4. Phase developments of (a) precursor aged in 0.5 M urea solution for (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 5 h.
0.35
6
0.3
(200)/(111) intensity ratio
(b)
5 0.25
4
0.2 0
0.5
1 Urea concentration (M)
1.5
2
Fig. 7. Crystalline size and intensity ratio of (2 0 0)/(1 1 1) of the powders aged in the different urea concentration for 5 h.
(f) (e)
7
0.32
(d) (c)
6.8 0.31
20
25
30
35
40
45 2θ
50
55
60
65
70
Fig. 5. Phase developments of (a) precursor aged in 1.5 M urea solution for (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, and (f) 5 h.
Crysatlline size (nm)
6.6 (a)
0.3 6.4 0.29 6.2
0.28
(200)
(200)
(200)/(111) intensity ratio
(b)
6 (111)
(111) 0.27
5.8 0
Fig. 6. Model of crystalline changed in aging process.
1
2
3 time (hr)
4
5
6
Fig. 8. Variation of crystalline size and intensity ratio of (2 0 0)/(1 1 1) of the powders aged in 1.5 M urea solution.
ARTICLE IN PRESS 354
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energy is lower than that of (2 0 0) planes during grain growth. Cerium oxide would change its shape from hexagon (truncated octahedron) to tetragon (a similar structure of octahedron), as shown in Fig. 6 [17]. Figs. 7 and 8 show the results of the variations of crystalline size and the intensity ratio of (2 0 0)/(1 1 1) with different urea concentration and with different aging times. The crystalline size of cerium oxide is increased with increase in urea concentration, as shown in Fig. 7. The intensity ratio of (2 0 0)/(1 1 1) decreases rapidly as urea concentration reaches to 0.75 M and then decreases slightly to a stable value, approximately 0.28. Value of (2 0 0)/ (1 1 1) is very close to the minimum value (0.29) of Kirk and Wood’s results [18], which was calcined at 1150 1C at a very high temperature. The growth rate of (1 1 1) plane is faster than that of (2 0 0) plane in this aging process. Fig. 8 shows that the aging time helps to reduce the intensity ratio of (2 0 0)/(1 1 1) and to increase the crystalline size of cerium oxide. The particle shapes during the first pH stage are shown in Fig. 9. The particle sizes of larger particles in Fig. 9 are similar to particle sizes of precursor, as shown in Fig. 2. However, smaller particles may be the decomposed particles during aging process. After 5 h of aging, pH value
reaches the range of the last pH stage, and particle shape changes to a uniform and aggregate particle. The crystalline size is about 6–7 nm, which was observed by TEM photographs; it is consisted with the result of Fig. 10. However, it still has some particles without decomposing, as shown in Fig. 10(a), which is the case aged in 0.5 M urea solution. From the results of Figs. 4 and 10(a), we can conclude that 0.5 M urea solution is not enough for reaction with cerium sulfate. The nanograde cerium oxide can be formed by cerium sulfate in higher urea concentration and longer aging time. Fig. 11 shows the enlarged scale of particle morphologies. After aging 2 h, the particles are shown as hexagon in shape; however, they are changed to tetragon in shape after aging for 5 h. This result is consistent with the model of Fig. 6. Nanograde cerium oxide is grain growth in the aging process. 4. Conclusions Nanograde cerium oxide is formed successfully by a normal-pressure aging process. Crystalline size of cerium oxide is increased by increasing concentration of urea and increasing aging time. On the other hand, intensity ratio of
Fig. 9. Morphologies of formation particles aged at the first pH stage (a) in 0.5 M urea solution for 2 h, (b) in 0.75 M urea solution for 2 h, (c) in 1 M urea solution for 1.5 h, and (d) in 1.5 M urea solution for 1 h.
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355
Fig. 10. Morphologies of formation particles aged in (a) 0.5 M, (b) 0.75 M, (c) 1.0 M, and (d) 1.5 M urea solution for 5 h.
Fig. 11. Morphologies of formation particles aged in 1.5 M urea solution for (a) 2 h and (b) 5 h.
(2 0 0)/(1 1 1) decreases with concentration of urea and increases with aging time. Acknowledgement The authors gratefully acknowledge the funding from the National Science Council (NSC93-2214-E-218-002).
References [1] C. Xia, M. Liu, Solid State Ionics 144 (2001) 249. [2] K. Kendall, M. Palin, J. Power Source 71 (1998) 268. [3] M. Pierre, et al., Cerium based glass polishing composition, US Patent 4601755, 1986. [4] M. Pierre, et al., Cerium/rare earth polishing composition, US Patent 4786325, 1988.
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[5] L. Le, Polishing compositions based on cerium and process for the preparation thereof, US Patent 5,026,421, 1991. [6] J.F. Wang, Oxide particles and method for producing them, US Patent 5398352, 1995. [7] K. Hanawa, N. Mochizukl, N. Ueda, Cerium oxide ultrafine particles and method for preparing the same, US Patent 5938837, 1999. [8] M. Yoshida, T. Ashizawa, H. Terazaki, Y. Kurata, J. Matsuzawa, K. Tanno, Y. Ootuki, Cerium oxide abrasive and method of polishing substrates, US Patent 6221118, 2001. [9] H. Kenzo, Cerium oxide ultra-fine particles and method for preparing the same, US Patent 5938837, 1999. [10] J.Y. Kim, S.M. Kim, U. Paik, J. Korean Phys. Soc. 41 (4) (2002) 413. [11] J.G. Li, T. Ikegami, J.H. Lee, T. Mori, Acta Mater. 49 (2001) 419.
[12] Boro Djuricˇic´, Stephen Pickering, J. Eur. Ceram. Soc. 19 (1999) 1925. [13] M. Hirno, E. Kato, J. Am. Ceram. Soc. 79 (3) (1996) 777. [14] V.D. Kosynkin, A.A. Arzgatkina, E.N. Ivanov, M.G. Chtoutsa, A.I. Grabko, A.V. Kardapolov, N.A. Sysina, J. Alloys Compounds 303–304 (2000) 421. [15] F. Bondioli, A. Bonamartini Corradi, C. Leonelli, T. Manfredini, Mater. Res. Bull. 34 (14/15) (1999) 2159. [16] H.C. Wang, C.H. Lu, Mater. Res. Bull. 37 (2002) 783. [17] B.D. Cullity, Elements of X-ray Diffraction, Addison-Wiley Publishing Co. Inc., 1977 (p. 102). [18] N.B. Kirk, J.V. Wood, J. Mater. Sci. 30 (1995) 2171. [19] A. Paulenova, S.E. Creager, J.D. Navratil, Y. Wei, J. Power Source 109 (2002) 431.