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Fabrication and synthesis of α-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH)
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Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 415–418
Fabrication and synthesis of ␣-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) Yong-Taeg O a , Sang-Woo Kim b , Dong-Chan Shin a,∗ a
b
Department of Advanced Materials Engineering, Chosun University, Gwangju 501-759, Republic of Korea Materials Science & Technology Division, Nano-Materials Research Center, KIST, Seoul 136-791, Republic of Korea Received 27 October 2006; accepted 30 April 2007 Available online 9 June 2007
Abstract An ␣-Al2 O3 nanopowder was prepared via the thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). The AACH precursor, which had an average crystallite size of 5–8 nm, was fabricated at a reaction temperature of 8 ◦ C with the pH of the aqueous ammonium hydrogen carbonate (AHC) precursor solution set to 10, which afforded the highest [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ] ionic concentration. The AACH precursor was transformed to NH4 Al(SO4 )2 , rhombohedral (Al2 (SO4 )3 ), amorphous-, -, and ␣-Al2 O3 according to the heat-treatment temperature. A time-temperature-transformation (TTT) diagram for thermal decomposition in air was determined. The critical particle size of -Al2 O3 in the → ␣ phase transformation was ∼30 nm. Homogeneous, spherical ␣-Al2 O3 nanopowders with an average particle size of 70 nm were prepared by firing the precursor crystallites at 1150 ◦ C for 3 h in air. © 2007 Elsevier B.V. All rights reserved. Keywords: Ammonium aluminum carbonate hydroxide (AACH); pH; ␣-Al2 O3 ; Nanopowders; TTT diagram
1. Introduction Alumina for transparent ceramics and laser devices has to have very pure, small and homogeneous particles [1]. The fabrication methods of the alumina included thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), organic aluminum hydrolysis, ethylene chlorohydrin, and Bayer method [2,3]. In all methods, alumina is obtained from sintering of salts and the sinterability of the alumina is determined by the chemical composition of the salts. Among the various methods the thermal decomposition of AACH is known to be the best process [4]. In the method, AACH (NH4 AlO(OH)HCO3 ) is firstly fabricated by the reaction of an ammonium hydrogen carbonate (AHC; NH4 HCO3 ) water solution and an ammonium aluminum sulphate (AAS; NH4 Al(SO4 )2 ) water solution and the subsequent thermal decomposition. During the process, the pH and ion content of the water solution are the most important experimental variables. Using this method we cannot obtain alumina powder sized smaller than 200 nm. However, 60 nm average size
∗
Corresponding author. Tel.: +82 62 230 7191; fax: +82 62 236 3775. E-mail address: [email protected] (D.-C. Shin).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.123
alumina powders has recently been obtained through the heattreatment of crushed powder after thermal decomposition [5–7]. However, that method has the disadvantage of impurity generation during the crushing process. The final grain size is generally determined by the size of ␣-Al2 O3 powder size. In this study, we have tried to fabricate high purity and homogenous ␣-Al2 O3 nanopowder by the thermal decomposition of AACH precursor. The effect of pH and reaction temperature of the water solution on the size of AACH powder has been investigated. In addition, the time-temperaturetransformation (TTT) diagram of NH4 Al(SO4 )2 , rhombohedral (Al2 (SO4 )3 ), amorphous-, -, and ␣-Al2 O3 has been studied to determine the heat-treatment condition. 2. Experiment The starting materials for the synthesis of AACH were AAS (NH4 Al(SO4 )2 ·12H2 O, sigma E.P.) and AHC (NH4 HCO3 , sigma E.P.). AACH was prepared by adding an aqueous solution of AAS (0.1 mol/l) dropwise to an aqueous solution of AHC (1.5 mol/l) at a flow rate of 10 ml/min. The aqueous AHC solution was kept at a pH ranges from 8 to 11 with HC1 (1 mol/l; Wako Pure Chemical Industries, Ltd. G.R.) and
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NH4 OH (4 mol/l; 25% NH3 in water, Wako Pure Chemical Industries, Ltd. G.R.), and the reaction temperature ranged from 8 to 20 ◦ C. The compacts were formed through thermal decomposition by isothermal heat-treatment in air. During isothermal heattreatment, the temperature was varied at 50 ◦ C intervals from 100 to 1200 ◦ C and the heat-treatment time was varied from 102 to 105 s. The heat-treatment compacts were identified by X-ray diffractometer (XRD: Rigaku D-Max III A). The microstructure, average size and distribution of the heat-treatment compacts were identified by cold field emission scanning electron microscopy (FE-SEM: Hitachi, S-4700) and transmission electron microscopy (TEM: Jeol, JEM-2000 FX II). 3. Results and discussion Fig. 1 shows the effect of the pH of the AHC aqueous solution on the AACH crystallite size, ionic species concentrations [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ], and reaction temperature. As shown, the maximum concentration product of the ions necessary for AACH formation and the minimum crystallite size occurred at the same pH level. Therefore, the aqueous AHC solution prepared at pH 10 and reaction temperature 8 ◦ C gave the minimum AACH crystallite size of about 5 nm. It is considered that AACH, deposited during the precipitation reactions, increased in supersolubility in the reactant solution as the concentration product of [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ] that was necessary for the formation of AACH increased [8]. It is also considered that the supersolubility increased as the reaction temperature decreased at the same pH level, similar to the general solubility behavior of a solid. Therefore, as the AACH solubility increased in supersolubility, AACH decreased in crystallite size, due to the accelerated formation of the fine nuclei. Therefore, the optimum condition for the synthesis of AACH precursor powder was with an AHC aqueous solution at pH above 10 and a reaction temperature of 8 ◦ C.
Fig. 1. Change of crystallite size of synthesized AACH and [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ][HCO3 − ] calculated by the master variable technique of ionic equilibrium in AHC and AA aqueous solutions as a function of pH of AHC aqueous solution.
Fig. 2. XRD patterns of phase transformation by the thermal decomposition of AACH.
Fig. 2 shows the XRD patterns obtained from isothermal heat-treatment of the AACH precursor powder at various temperatures for 3 h. All the synthetic powder formed under these experimental conditions was AACH. An absence of XRD peaks was observed in hexagonal NH4 Al (SO4 ) at 200–400 ◦ C, rhombohedral (Al2 (SO4 )3 ) at 400–600 ◦ C, amorphous alumina at 700 ◦ C, -Al2 O3 phase at 950 ◦ C, and ␣- and -Al2 O3 phases at 1050 ◦ C. The high temperature stability of the ␣-Al2 O3 phase at 1150 ◦ C was confirmed.
Fig. 3. TTT diagram by the thermal decomposition of AACH.
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Fig. 4. TEM photographs of products after heating at (a) 700 ◦ C, (b) 950 ◦ C, and (c) 1150 ◦ C for 3 h.
Fig. 3 shows the TTT diagrams, based on the results summarized in Fig. 2, for the thermal decomposition of AACH into ␣-alumina in air. Five phase areas were formed during isothermal heat treatment: AACH at temperatures under 200 ◦ C; between 200 and 400 ◦ C AACH decomposes to form NH4 Al(SO4 )2 ; between 400 and 700 ◦ C NH4 Al(SO4 )2 decomposes to form rhombohedral (Al2 (SO4 )3 ); between 800 and 900 ◦ C the rhombohedral (Al2 (SO4 )3 ) decomposes into amorphous alumina; between 1000 and 1100 ◦ C it forms -Al2 O3 , with the noncrystallization of amorphous alumina as an intermediate step; between 1100 and 1200 ◦ C the → ␣ transformation of alumina takes place. These results indicate that the transformation of AACH into ␣-Al2 O3 passes through the formation of -Al2 O3 for all heat treatments. Fig. 4 shows bright-field TEM images and diffraction patterns of alumina nanoparticles captured during the amorphous to ␣Al2 O3 transformation. The amorphous alumina of the sample annealed at 700 ◦ C for 3 h (Fig. 4(a)) contained agglomerates formed by primary particles with diameters between 10 and 30 nm. The -Al2 O3 of the sample annealed at 950 ◦ C for 3 h (Fig. 4(b)) contained nearly spherical particles with diameter
ranging from 10 to 20 nm. Fig. 4(a) and (b) shows the diffraction patterns corresponding to -Al2 O3 and amorphous alumina. The ␣-Al2 O3 of the sample annealed at 1150 ◦ C for 3 h (Fig. 4(c)) contained nearly spherical particles with diameter ranging from 50 to 100 nm. None of the diffraction patterns were indicative of -Al2 O3 , but some corresponded to the (0 0 1) crystallographic planes of ␣-Al2 O3 . Fig. 5 shows the particle-size distributions for amorphous-, -, and ␣-Al2 O3 in the samples heat-treated at 700, 950, and 1150 ◦ C for 3 h. The particle distribution taken from about 200 particles was obtained from dark-field TEM images. The distribution curves had maxima at 15 nm for amorphous alumina, 22 nm for -Al2 O3 , and 70 nm for ␣-Al2 O3 . All amorphous alumina and -Al2 O3 particles had diameters below ∼30 nm. These results indicate that the critical particle size of -Al2 O3 for the - → ␣-Al2 O3 phase transformation is ∼30 nm. 4. Conclusion The effects of pH and reaction temperature of an AHC water solution on the size of the formed AACH powder were investigated. The optimum process condition was a reaction temperature of 8 ◦ C, an AHC solution pH of 10 and a minimum precursor powder size of 5–8 nm. At pH of 8 or 11, the ion concentration of [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ] was reduced which drastically increased the crystal size of the precursor powder. Through thermal decomposition of the AACH precursor powder, the phase transition temperatures of NH4 Al(SO4 )2 , rhombohedral (Al2 (SO4 )3 ), amorphous-, , and ␣-Al2 O3 were identified. The TTT diagram of those phase transitions was determined by isothermal heat-treatment. We could obtain ␣-Al2 O3 crystallite of 70 nm size through 3-h heat-treatment at 1150 ◦ C. The TTT diagram and ␣Al2 O3 nanopowder can be applied to the fabrication of high strength–high transparent ceramics, as well as several kinds of structural ceramics. Acknowledgment
Fig. 5. Particle size distribution of amorphous-, -, and ␣-alumina during the amorphous- to ␣-alumina transformation. These distributions were determined from TEM photographs.
This study was supported by research funds from Chosun University, 2004.
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References [1] Y.T. O, J.B. Koo, K.J. Hong, J.S. Park, D.C. Shin, Mater. Sci. Eng. A 374 (2004) 191. [2] J.G. Li, X. Sun, Acta Mater. 48 (2000) 3103. [3] C.C. Ma, X.X. Zhou, X. Xu, T. Zhu, Mater. Chem. Phys. 72 (2001) 374. [4] K. Hayashi, S. Toyoda, K. Nakashima, K. Morinaga, J. Ceram. Soc. Jap. Int. Ed. 98 (1990) 29.
[5] S. Fujino, T. Torikai, Y. Miyake, K. Morinaga, in: 18th International Japan–Korea Seminar on Ceramics, 2001, pp. 305. [6] K. Morinaga, T. Torikai, K. Nakagawa, S. Fujino, Acta Mater. 48 (2000) 4735. [7] T. Torikai, K. Nakagawa, K. Morinaga, Eng. Sci. Report Arch. 22 (2000) 1. [8] S. Kato, T. Iga, S. Hatano, Y. Izawa, Yogyo-Kyokai Shi. 84 (1976) 255.
Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 415–418
Fabrication and synthesis of ␣-alumina nanopowders by thermal decomposition of ammonium aluminum carbonate hydroxide (AACH) Yong-Taeg O a , Sang-Woo Kim b , Dong-Chan Shin a,∗ a
b
Department of Advanced Materials Engineering, Chosun University, Gwangju 501-759, Republic of Korea Materials Science & Technology Division, Nano-Materials Research Center, KIST, Seoul 136-791, Republic of Korea Received 27 October 2006; accepted 30 April 2007 Available online 9 June 2007
Abstract An ␣-Al2 O3 nanopowder was prepared via the thermal decomposition of ammonium aluminum carbonate hydroxide (AACH). The AACH precursor, which had an average crystallite size of 5–8 nm, was fabricated at a reaction temperature of 8 ◦ C with the pH of the aqueous ammonium hydrogen carbonate (AHC) precursor solution set to 10, which afforded the highest [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ] ionic concentration. The AACH precursor was transformed to NH4 Al(SO4 )2 , rhombohedral (Al2 (SO4 )3 ), amorphous-, -, and ␣-Al2 O3 according to the heat-treatment temperature. A time-temperature-transformation (TTT) diagram for thermal decomposition in air was determined. The critical particle size of -Al2 O3 in the → ␣ phase transformation was ∼30 nm. Homogeneous, spherical ␣-Al2 O3 nanopowders with an average particle size of 70 nm were prepared by firing the precursor crystallites at 1150 ◦ C for 3 h in air. © 2007 Elsevier B.V. All rights reserved. Keywords: Ammonium aluminum carbonate hydroxide (AACH); pH; ␣-Al2 O3 ; Nanopowders; TTT diagram
1. Introduction Alumina for transparent ceramics and laser devices has to have very pure, small and homogeneous particles [1]. The fabrication methods of the alumina included thermal decomposition of ammonium aluminum carbonate hydroxide (AACH), organic aluminum hydrolysis, ethylene chlorohydrin, and Bayer method [2,3]. In all methods, alumina is obtained from sintering of salts and the sinterability of the alumina is determined by the chemical composition of the salts. Among the various methods the thermal decomposition of AACH is known to be the best process [4]. In the method, AACH (NH4 AlO(OH)HCO3 ) is firstly fabricated by the reaction of an ammonium hydrogen carbonate (AHC; NH4 HCO3 ) water solution and an ammonium aluminum sulphate (AAS; NH4 Al(SO4 )2 ) water solution and the subsequent thermal decomposition. During the process, the pH and ion content of the water solution are the most important experimental variables. Using this method we cannot obtain alumina powder sized smaller than 200 nm. However, 60 nm average size
∗
Corresponding author. Tel.: +82 62 230 7191; fax: +82 62 236 3775. E-mail address: [email protected] (D.-C. Shin).
0927-7757/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.04.123
alumina powders has recently been obtained through the heattreatment of crushed powder after thermal decomposition [5–7]. However, that method has the disadvantage of impurity generation during the crushing process. The final grain size is generally determined by the size of ␣-Al2 O3 powder size. In this study, we have tried to fabricate high purity and homogenous ␣-Al2 O3 nanopowder by the thermal decomposition of AACH precursor. The effect of pH and reaction temperature of the water solution on the size of AACH powder has been investigated. In addition, the time-temperaturetransformation (TTT) diagram of NH4 Al(SO4 )2 , rhombohedral (Al2 (SO4 )3 ), amorphous-, -, and ␣-Al2 O3 has been studied to determine the heat-treatment condition. 2. Experiment The starting materials for the synthesis of AACH were AAS (NH4 Al(SO4 )2 ·12H2 O, sigma E.P.) and AHC (NH4 HCO3 , sigma E.P.). AACH was prepared by adding an aqueous solution of AAS (0.1 mol/l) dropwise to an aqueous solution of AHC (1.5 mol/l) at a flow rate of 10 ml/min. The aqueous AHC solution was kept at a pH ranges from 8 to 11 with HC1 (1 mol/l; Wako Pure Chemical Industries, Ltd. G.R.) and
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NH4 OH (4 mol/l; 25% NH3 in water, Wako Pure Chemical Industries, Ltd. G.R.), and the reaction temperature ranged from 8 to 20 ◦ C. The compacts were formed through thermal decomposition by isothermal heat-treatment in air. During isothermal heattreatment, the temperature was varied at 50 ◦ C intervals from 100 to 1200 ◦ C and the heat-treatment time was varied from 102 to 105 s. The heat-treatment compacts were identified by X-ray diffractometer (XRD: Rigaku D-Max III A). The microstructure, average size and distribution of the heat-treatment compacts were identified by cold field emission scanning electron microscopy (FE-SEM: Hitachi, S-4700) and transmission electron microscopy (TEM: Jeol, JEM-2000 FX II). 3. Results and discussion Fig. 1 shows the effect of the pH of the AHC aqueous solution on the AACH crystallite size, ionic species concentrations [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ], and reaction temperature. As shown, the maximum concentration product of the ions necessary for AACH formation and the minimum crystallite size occurred at the same pH level. Therefore, the aqueous AHC solution prepared at pH 10 and reaction temperature 8 ◦ C gave the minimum AACH crystallite size of about 5 nm. It is considered that AACH, deposited during the precipitation reactions, increased in supersolubility in the reactant solution as the concentration product of [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ] that was necessary for the formation of AACH increased [8]. It is also considered that the supersolubility increased as the reaction temperature decreased at the same pH level, similar to the general solubility behavior of a solid. Therefore, as the AACH solubility increased in supersolubility, AACH decreased in crystallite size, due to the accelerated formation of the fine nuclei. Therefore, the optimum condition for the synthesis of AACH precursor powder was with an AHC aqueous solution at pH above 10 and a reaction temperature of 8 ◦ C.
Fig. 1. Change of crystallite size of synthesized AACH and [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ][HCO3 − ] calculated by the master variable technique of ionic equilibrium in AHC and AA aqueous solutions as a function of pH of AHC aqueous solution.
Fig. 2. XRD patterns of phase transformation by the thermal decomposition of AACH.
Fig. 2 shows the XRD patterns obtained from isothermal heat-treatment of the AACH precursor powder at various temperatures for 3 h. All the synthetic powder formed under these experimental conditions was AACH. An absence of XRD peaks was observed in hexagonal NH4 Al (SO4 ) at 200–400 ◦ C, rhombohedral (Al2 (SO4 )3 ) at 400–600 ◦ C, amorphous alumina at 700 ◦ C, -Al2 O3 phase at 950 ◦ C, and ␣- and -Al2 O3 phases at 1050 ◦ C. The high temperature stability of the ␣-Al2 O3 phase at 1150 ◦ C was confirmed.
Fig. 3. TTT diagram by the thermal decomposition of AACH.
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Fig. 4. TEM photographs of products after heating at (a) 700 ◦ C, (b) 950 ◦ C, and (c) 1150 ◦ C for 3 h.
Fig. 3 shows the TTT diagrams, based on the results summarized in Fig. 2, for the thermal decomposition of AACH into ␣-alumina in air. Five phase areas were formed during isothermal heat treatment: AACH at temperatures under 200 ◦ C; between 200 and 400 ◦ C AACH decomposes to form NH4 Al(SO4 )2 ; between 400 and 700 ◦ C NH4 Al(SO4 )2 decomposes to form rhombohedral (Al2 (SO4 )3 ); between 800 and 900 ◦ C the rhombohedral (Al2 (SO4 )3 ) decomposes into amorphous alumina; between 1000 and 1100 ◦ C it forms -Al2 O3 , with the noncrystallization of amorphous alumina as an intermediate step; between 1100 and 1200 ◦ C the → ␣ transformation of alumina takes place. These results indicate that the transformation of AACH into ␣-Al2 O3 passes through the formation of -Al2 O3 for all heat treatments. Fig. 4 shows bright-field TEM images and diffraction patterns of alumina nanoparticles captured during the amorphous to ␣Al2 O3 transformation. The amorphous alumina of the sample annealed at 700 ◦ C for 3 h (Fig. 4(a)) contained agglomerates formed by primary particles with diameters between 10 and 30 nm. The -Al2 O3 of the sample annealed at 950 ◦ C for 3 h (Fig. 4(b)) contained nearly spherical particles with diameter
ranging from 10 to 20 nm. Fig. 4(a) and (b) shows the diffraction patterns corresponding to -Al2 O3 and amorphous alumina. The ␣-Al2 O3 of the sample annealed at 1150 ◦ C for 3 h (Fig. 4(c)) contained nearly spherical particles with diameter ranging from 50 to 100 nm. None of the diffraction patterns were indicative of -Al2 O3 , but some corresponded to the (0 0 1) crystallographic planes of ␣-Al2 O3 . Fig. 5 shows the particle-size distributions for amorphous-, -, and ␣-Al2 O3 in the samples heat-treated at 700, 950, and 1150 ◦ C for 3 h. The particle distribution taken from about 200 particles was obtained from dark-field TEM images. The distribution curves had maxima at 15 nm for amorphous alumina, 22 nm for -Al2 O3 , and 70 nm for ␣-Al2 O3 . All amorphous alumina and -Al2 O3 particles had diameters below ∼30 nm. These results indicate that the critical particle size of -Al2 O3 for the - → ␣-Al2 O3 phase transformation is ∼30 nm. 4. Conclusion The effects of pH and reaction temperature of an AHC water solution on the size of the formed AACH powder were investigated. The optimum process condition was a reaction temperature of 8 ◦ C, an AHC solution pH of 10 and a minimum precursor powder size of 5–8 nm. At pH of 8 or 11, the ion concentration of [NH4 + ][AlO(OH)n (SO4 )− 3−n/2 ]][HCO2 − ] was reduced which drastically increased the crystal size of the precursor powder. Through thermal decomposition of the AACH precursor powder, the phase transition temperatures of NH4 Al(SO4 )2 , rhombohedral (Al2 (SO4 )3 ), amorphous-, , and ␣-Al2 O3 were identified. The TTT diagram of those phase transitions was determined by isothermal heat-treatment. We could obtain ␣-Al2 O3 crystallite of 70 nm size through 3-h heat-treatment at 1150 ◦ C. The TTT diagram and ␣Al2 O3 nanopowder can be applied to the fabrication of high strength–high transparent ceramics, as well as several kinds of structural ceramics. Acknowledgment
Fig. 5. Particle size distribution of amorphous-, -, and ␣-alumina during the amorphous- to ␣-alumina transformation. These distributions were determined from TEM photographs.
This study was supported by research funds from Chosun University, 2004.
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Y.-T. O et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 313–314 (2008) 415–418
References [1] Y.T. O, J.B. Koo, K.J. Hong, J.S. Park, D.C. Shin, Mater. Sci. Eng. A 374 (2004) 191. [2] J.G. Li, X. Sun, Acta Mater. 48 (2000) 3103. [3] C.C. Ma, X.X. Zhou, X. Xu, T. Zhu, Mater. Chem. Phys. 72 (2001) 374. [4] K. Hayashi, S. Toyoda, K. Nakashima, K. Morinaga, J. Ceram. Soc. Jap. Int. Ed. 98 (1990) 29.
[5] S. Fujino, T. Torikai, Y. Miyake, K. Morinaga, in: 18th International Japan–Korea Seminar on Ceramics, 2001, pp. 305. [6] K. Morinaga, T. Torikai, K. Nakagawa, S. Fujino, Acta Mater. 48 (2000) 4735. [7] T. Torikai, K. Nakagawa, K. Morinaga, Eng. Sci. Report Arch. 22 (2000) 1. [8] S. Kato, T. Iga, S. Hatano, Y. Izawa, Yogyo-Kyokai Shi. 84 (1976) 255.