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Characterization of sodium β-alumina prepared by sol-gel method
!SolidStateIonics40/41 North-Holland
(1990) 111-114
CHARACTERIZATION Kazuya TEBABE,
OF SODIUM
Shu YAMAGUCHI,
B-ALUMINA
PREPARED
BY SOL-GEL
METHOD
Yoshiaki IGUCHI
Nagoya Insiitute of Technology, Department ofMaterials Science and Engineering, Metals Section, Gokiso-Cho, Showa-ku, Nagoya 466, Japan
and Atsuo IMAI Nagoya Municipal Industrial Research Institute, Rokuban, Atuta-ku, Nagoya 456, Japan
Electrical properties of both the transparent gel and sodium h-alumina with the composition of NaiO*6Al,O, prepared by the sol-gel method were studied. The gel showed a relatively high electrical conductivity. The increase rate in the conductivity was changed at both temperatures, where shrinkage and transformation to sodium g-alumina started to occur. The B-alumina crystallized by the heat treatment at 1150” C was a good sodium ion conductor, though the structure detected by X-ray analysis was highly defective.
1. Introduction Sodium B-alumina is a well-known high sodium ion conductor [ 11. However it has been difficult to prepare B-alumina ceramics suitable for practical applications by conventional processes, because of the necessity of high sintering temperature, which causes an evaporation of alkali and an undesirable grain growth. The sol-gel method using metal alkoxide is one of the most promising approaches to prepare advanced ceramics [2,3]. By using this method, various uniform glasses and ceramics have been prepared at relatively low temperatures without melting or conventional sintering processes. Furthermore, crystallization and uniformity of precursors, which are important problems for formation of ceramics, can be controlled by this method. Therefore, this is a potential method to prepare the p-alumina, especially 0”-alumina ceramics. Although Yoldas [ 41 formed continuous B-alumina films and coatings, details of the crystallization processes were not reported. We reported previously the crystallization reaction of the transparent gels with a wide range of composition in the Na*O-A1203 system [ 5 1. In the 0167-2738/90/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland )
present study, electrical properties of the sample with the composition of Na20-6A1203 prepared by the sol-gel method was studied. Furthermore, the relation between the electrical property and shrinkage behavior of the precursor gel and crystalline phase was examined.
2. Experimental
The condition for the formation of transparent gels was reported in the previous paper [ 5 1. However, the small pieces of the gels prepared under the same condition as the previous one were not suitable for electrical conductivity measurement. To prepare a large gel block sample, amount of water for the hydrolytic reaction of the alkoxides was examined, and was determined to be larger than that for the complete hydrolysis of the alkoxides. The water was diluted to about 2 wt% by organic solvent to hydrolyze the alkoxides slowly. In addition, acetic acid as a peptizing agent was necessary to prevent the precipitation of boehmite. The solution for the hydrolysis
K. Terabe et al. /Sodium &alumina prepared by sol-gel method
112
was prepared by mixing 0.35 mol acetic acid/ (mol Al alkoxide), 200 ml of organic solvent (benzene 140 ml, ethylene glycol monoethyl ether 30 ml, ethanol 30 ml) and 6 mol H,O/(mol Al alkoxide). To prepare the sample for the electrical conductivity measurement, the gels containing the solvent were dried for several weeks in air at room temperature. Then, the samples were heated at 0.1 “C/min to 500°C kept there for five hours to remove the volatile species. After the treatment, only a small amount of remains in the sample was observed by a thermogravimetric analysis. The prepared gels were transparent plates about 10 mm wide, 2 mm thick. ac conductivity measurement with Pt blocking electrodes was made in frequency range of 1- 1O5 Hz using a frequency response analysis in argon. By thermal mechanical analysis (TMA), a shrinking behavior of the gel was measured on heating to 1000°C at 2”C/min in air.
3. Results and discussion Electrical conductivity measured on heating the gel at 2”C/min up to 1150°C and cooling to 260°C is shown in fig. 1. For comparison, the conductivity of B-alumina ceramics doped with MgO (Toshiba
0.6
0.8
10
1.2
1.4
1.6
Ceramics Co. Ltd.) is plotted in the figure. The crystal structure change detected by X-ray diffraction (XRD) analysis were as follows: Amorphous or yalumina-like (state 1) -+ B-alumina (state 2 ). The product in state 1 showed very broad peaks with weak correlation with y-alumina peaks. The broad peaks approached to those of y-alumina after annealing at 800°C for 100 h. In state 2, the broad XRD pattern of B-alumina was observed (see fig. 4). Similar XRD pattern of B-alumina has been reported by Morgan [ 6 1, Poulieff [ 7 ] and Hodge [ 8 1. They suggested that it was caused by highly-disordered structure. The temperature, where the conductivity increased suddenly (indicated (B ) by arrow), coincided with the transition point to sodium B-alumina in the previous paper [ 5 1. The conductivity results in state 2 show that a good conductor of B-alumina was obtained by heat treatment at 1150’ C without conventional sintering processes. It should be noted that the B-alumina with the composition of Na20-6A1203 could not be prepared by any other conventional sintering processes, because the B-alumina containing excess sodium ion as much as the present sample without doping was unstable at ordinary sintering temperature. Though the sample in this study contain more sodium ions than that of Toshiba Ceramics (Na/Alo l/9), the conductivity is slightly low. In this study, the gel showed the relatively large shrinkage observed by TMA, as shown in fig. 2. Since the sintering occurs during the impedance measure-
1.6
1 OOO/T.K Fig. I. Ionic conductivity of the sample as a function of temperature measured on heating and cooling at heating rate of 2”C/ min. State 1 and state 2 correspond to amorphous or y-aluminalike and b-alumina, respectively. Point (A) and point (B) correspond to beginning of shrinkage and transition point to @ammina, respectively.
16’ 200
I
400
600 Temperature
600
I 1000
/*C
Fig. 2. Thermal shrinkage curve of the gel on heating min. Point (A) is the same temperature as in fig. 1.
at 2”C/
K. Terabe et al. /Sodium &alumina prepared by sol-gel method
ment, effect from the pores of the sample should be taken into account. Yoldas reported that b-alumina film prepared by sol-gel method with heat treatment at 1200’ C has pores with radii < 7 nm [ 4 1. The existence of the pores may be one of the causes for the low conductivity. The product in state 1 showed relatively high electrical conductivity. At this conduction, proton can be mobile as well as sodium. The increase rate in the conductivity was changed at the temperature (A) in fig. 1, which is lower than the transition point to b-alumina. This temperature corresponds to the beginning of the shrinkage, as shown in fig. 2. As the gel shrunk above temperature (A), the broad XRD pattern approached to those of y-alumina. The shrinkage may be caused by both sintering of gel and crystallization to y-alumina phase. The former may contribute predominantly to the sintering. The contact area of colloidal particles of the gel increased, so that the conductivity could be increased gradually. In order to examine the changes in the conductivity caused by the transformation, electrical conductivity data were measured during isothermal aging at 950°C and are shown in fig. 3. The XRD patterns of the samples annealed at 950°C for respective times indicated as 1 through 4 are shown in fig. 4. The formation of metastable h-Naz0*X-A1,03 (h-phase) between amorphous or y-alumina-like and b-alumina phase was observed on prolonged aging near the transition point, although the h-phase could not be
4.5 c
4.0 -
Y
3.5 -
% * F:
3.0
ob s 9
t 4 ;
t 3
1
2.5
lee
l 2.0
t
f 2
1 1.5 1.0
”
” 1.5
” 2.0
2.5
“I’ 3.0
3.5
4.0
log(TimeVmin Fig. 3. Change in conductivity aging at 950°C. Each symbol respectively.
of the sample during isothermal (1,2,3,4) shows the aging time,
113
)r
C m c
2
-c
>;I 2
30
35
40
45
50
55
28 Fig. 4. Powder X-ray diffraction patterns of the samples annealed at 950°C for different times. Symbol ( 1,2,3,4)are the same as in fig. 3.
detected on other heat treatment conditions. The hphase has been reportedly formed by a uniform mixing of the reactants on an atomic level using various synthetic methods [8-lo]. The present results show that some parts of the uniform mixing was attained in the sample by the prolonged aging. In fig. 3, the sudden increase in the conductivity occurred in the region between 2 and 3. The XRD pattern of samples heated in the same aging condition showed the formation of the mixed phase of h-phase and p-alumina. In the region between 3 and 4, where L-phase transformed to b-alumina completely, such a sudden increase was not observed. For estimating activation energy (E,) of the conductivity in fig. 1, effect of the shrinkage on the conductivity should be taken into account. The gel was annealed at a desired temperature for five hours, which is long enough for major shrinkage caused by sintering to finish at this temperature. Then, the conductivity was measured on cooling by about 200°C. To plot the Ea against temperatures as shown in fig. 5, the measurements were repeated from 500°C to 1050 oC. The temperatures (A ) and ( B ) in fig. 5 are the same as those indicated in fig. 1. The E, values were constant between 500°C and the temperature
K. Terabe et al. /Sodium p-alumina prepared by sol-gel method
114
5 \ >
composition of NazO-6A1203 prepared by the sol-gel method. The gel showed relatively high electrical conductivity influenced by both the shrinkage behavior and the transformation to B-alumina. Possibility of manufacturing dense beta alumina ceramics at lower temperature has been confirmed by the fact that the prepared B-alumina showed high ionic conductivity.
1.2
1.0.
P t W
0.6
5
0.6
z : z 2
0.4
0.2 I
0 400
I
600
I
I.
600
I
El,
Acknowledgement
1000
Temperature/?2 Fig. 5. Change in activation energy of the sample annealed at each temperature for five hours. Point (A) and (B) are the same temperatures as in fig. 1.
A part of the present work was supported by Grantin Aid for Scientific Research, Ministry of Education of Japan.
References (A), and then decreased. The Ea also changed at both temperatures of the shrinkage beginning (A) and the transition (B). E, in the state 1 was much higher than that of B-alumina. In this state, sodium and proton conduction may occur in adsorbed water layer on the surface of amorphous or y-alumina particles. But detailed mechanism remained to be solved.
4. Conclusion Electrical properties were studied on both the transparent gel and the crystallized beta alumina with
[ 1] Y.F. Yao and J.T. Kummer, J. Inor. Nucl. Chem. 29 ( 1967) 2453. [2] S. Sakka, Am. Ceram. Sot. Bull. 64 (1985) 1463. [ 31 D.R. Uhich, J. Non-Ctyst. Solids, 100 (1988) 174. [4] B.E. Yoldas and D.P. Partlow, Am. Ceram. Sot. Bull. 59 ( 1980) 640. [ 51 S. Yamaguchi, K. Terabe, Y. Iguchi and A. Imai, Solid State Ionics, 25 (1987) 171. [6] P.E.D. Morgan, Mat. Res. Bull. 11 (1976) 233. [ 71 C.N. Poulieff et al., Mat. Res. Bull. 13 (1978) 323. [ 81 J.D. Hodge, Am. Ceram. Sot. Bull. 62 (1983) 244. [9] A.J. Perrotta and J.E. Young, J. Am. Ceram. Sot. 57 (1974) 405. [10]A.G.ElliotandR.A.Huggins,J.Am.Ceram.Soc.58 (1975) 497.
(1990) 111-114
CHARACTERIZATION Kazuya TEBABE,
OF SODIUM
Shu YAMAGUCHI,
B-ALUMINA
PREPARED
BY SOL-GEL
METHOD
Yoshiaki IGUCHI
Nagoya Insiitute of Technology, Department ofMaterials Science and Engineering, Metals Section, Gokiso-Cho, Showa-ku, Nagoya 466, Japan
and Atsuo IMAI Nagoya Municipal Industrial Research Institute, Rokuban, Atuta-ku, Nagoya 456, Japan
Electrical properties of both the transparent gel and sodium h-alumina with the composition of NaiO*6Al,O, prepared by the sol-gel method were studied. The gel showed a relatively high electrical conductivity. The increase rate in the conductivity was changed at both temperatures, where shrinkage and transformation to sodium g-alumina started to occur. The B-alumina crystallized by the heat treatment at 1150” C was a good sodium ion conductor, though the structure detected by X-ray analysis was highly defective.
1. Introduction Sodium B-alumina is a well-known high sodium ion conductor [ 11. However it has been difficult to prepare B-alumina ceramics suitable for practical applications by conventional processes, because of the necessity of high sintering temperature, which causes an evaporation of alkali and an undesirable grain growth. The sol-gel method using metal alkoxide is one of the most promising approaches to prepare advanced ceramics [2,3]. By using this method, various uniform glasses and ceramics have been prepared at relatively low temperatures without melting or conventional sintering processes. Furthermore, crystallization and uniformity of precursors, which are important problems for formation of ceramics, can be controlled by this method. Therefore, this is a potential method to prepare the p-alumina, especially 0”-alumina ceramics. Although Yoldas [ 41 formed continuous B-alumina films and coatings, details of the crystallization processes were not reported. We reported previously the crystallization reaction of the transparent gels with a wide range of composition in the Na*O-A1203 system [ 5 1. In the 0167-2738/90/$ 03.50 0 Elsevier Science Publishers B.V. ( North-Holland )
present study, electrical properties of the sample with the composition of Na20-6A1203 prepared by the sol-gel method was studied. Furthermore, the relation between the electrical property and shrinkage behavior of the precursor gel and crystalline phase was examined.
2. Experimental
The condition for the formation of transparent gels was reported in the previous paper [ 5 1. However, the small pieces of the gels prepared under the same condition as the previous one were not suitable for electrical conductivity measurement. To prepare a large gel block sample, amount of water for the hydrolytic reaction of the alkoxides was examined, and was determined to be larger than that for the complete hydrolysis of the alkoxides. The water was diluted to about 2 wt% by organic solvent to hydrolyze the alkoxides slowly. In addition, acetic acid as a peptizing agent was necessary to prevent the precipitation of boehmite. The solution for the hydrolysis
K. Terabe et al. /Sodium &alumina prepared by sol-gel method
112
was prepared by mixing 0.35 mol acetic acid/ (mol Al alkoxide), 200 ml of organic solvent (benzene 140 ml, ethylene glycol monoethyl ether 30 ml, ethanol 30 ml) and 6 mol H,O/(mol Al alkoxide). To prepare the sample for the electrical conductivity measurement, the gels containing the solvent were dried for several weeks in air at room temperature. Then, the samples were heated at 0.1 “C/min to 500°C kept there for five hours to remove the volatile species. After the treatment, only a small amount of remains in the sample was observed by a thermogravimetric analysis. The prepared gels were transparent plates about 10 mm wide, 2 mm thick. ac conductivity measurement with Pt blocking electrodes was made in frequency range of 1- 1O5 Hz using a frequency response analysis in argon. By thermal mechanical analysis (TMA), a shrinking behavior of the gel was measured on heating to 1000°C at 2”C/min in air.
3. Results and discussion Electrical conductivity measured on heating the gel at 2”C/min up to 1150°C and cooling to 260°C is shown in fig. 1. For comparison, the conductivity of B-alumina ceramics doped with MgO (Toshiba
0.6
0.8
10
1.2
1.4
1.6
Ceramics Co. Ltd.) is plotted in the figure. The crystal structure change detected by X-ray diffraction (XRD) analysis were as follows: Amorphous or yalumina-like (state 1) -+ B-alumina (state 2 ). The product in state 1 showed very broad peaks with weak correlation with y-alumina peaks. The broad peaks approached to those of y-alumina after annealing at 800°C for 100 h. In state 2, the broad XRD pattern of B-alumina was observed (see fig. 4). Similar XRD pattern of B-alumina has been reported by Morgan [ 6 1, Poulieff [ 7 ] and Hodge [ 8 1. They suggested that it was caused by highly-disordered structure. The temperature, where the conductivity increased suddenly (indicated (B ) by arrow), coincided with the transition point to sodium B-alumina in the previous paper [ 5 1. The conductivity results in state 2 show that a good conductor of B-alumina was obtained by heat treatment at 1150’ C without conventional sintering processes. It should be noted that the B-alumina with the composition of Na20-6A1203 could not be prepared by any other conventional sintering processes, because the B-alumina containing excess sodium ion as much as the present sample without doping was unstable at ordinary sintering temperature. Though the sample in this study contain more sodium ions than that of Toshiba Ceramics (Na/Alo l/9), the conductivity is slightly low. In this study, the gel showed the relatively large shrinkage observed by TMA, as shown in fig. 2. Since the sintering occurs during the impedance measure-
1.6
1 OOO/T.K Fig. I. Ionic conductivity of the sample as a function of temperature measured on heating and cooling at heating rate of 2”C/ min. State 1 and state 2 correspond to amorphous or y-aluminalike and b-alumina, respectively. Point (A) and point (B) correspond to beginning of shrinkage and transition point to @ammina, respectively.
16’ 200
I
400
600 Temperature
600
I 1000
/*C
Fig. 2. Thermal shrinkage curve of the gel on heating min. Point (A) is the same temperature as in fig. 1.
at 2”C/
K. Terabe et al. /Sodium &alumina prepared by sol-gel method
ment, effect from the pores of the sample should be taken into account. Yoldas reported that b-alumina film prepared by sol-gel method with heat treatment at 1200’ C has pores with radii < 7 nm [ 4 1. The existence of the pores may be one of the causes for the low conductivity. The product in state 1 showed relatively high electrical conductivity. At this conduction, proton can be mobile as well as sodium. The increase rate in the conductivity was changed at the temperature (A) in fig. 1, which is lower than the transition point to b-alumina. This temperature corresponds to the beginning of the shrinkage, as shown in fig. 2. As the gel shrunk above temperature (A), the broad XRD pattern approached to those of y-alumina. The shrinkage may be caused by both sintering of gel and crystallization to y-alumina phase. The former may contribute predominantly to the sintering. The contact area of colloidal particles of the gel increased, so that the conductivity could be increased gradually. In order to examine the changes in the conductivity caused by the transformation, electrical conductivity data were measured during isothermal aging at 950°C and are shown in fig. 3. The XRD patterns of the samples annealed at 950°C for respective times indicated as 1 through 4 are shown in fig. 4. The formation of metastable h-Naz0*X-A1,03 (h-phase) between amorphous or y-alumina-like and b-alumina phase was observed on prolonged aging near the transition point, although the h-phase could not be
4.5 c
4.0 -
Y
3.5 -
% * F:
3.0
ob s 9
t 4 ;
t 3
1
2.5
lee
l 2.0
t
f 2
1 1.5 1.0
”
” 1.5
” 2.0
2.5
“I’ 3.0
3.5
4.0
log(TimeVmin Fig. 3. Change in conductivity aging at 950°C. Each symbol respectively.
of the sample during isothermal (1,2,3,4) shows the aging time,
113
)r
C m c
2
-c
>;I 2
30
35
40
45
50
55
28 Fig. 4. Powder X-ray diffraction patterns of the samples annealed at 950°C for different times. Symbol ( 1,2,3,4)are the same as in fig. 3.
detected on other heat treatment conditions. The hphase has been reportedly formed by a uniform mixing of the reactants on an atomic level using various synthetic methods [8-lo]. The present results show that some parts of the uniform mixing was attained in the sample by the prolonged aging. In fig. 3, the sudden increase in the conductivity occurred in the region between 2 and 3. The XRD pattern of samples heated in the same aging condition showed the formation of the mixed phase of h-phase and p-alumina. In the region between 3 and 4, where L-phase transformed to b-alumina completely, such a sudden increase was not observed. For estimating activation energy (E,) of the conductivity in fig. 1, effect of the shrinkage on the conductivity should be taken into account. The gel was annealed at a desired temperature for five hours, which is long enough for major shrinkage caused by sintering to finish at this temperature. Then, the conductivity was measured on cooling by about 200°C. To plot the Ea against temperatures as shown in fig. 5, the measurements were repeated from 500°C to 1050 oC. The temperatures (A ) and ( B ) in fig. 5 are the same as those indicated in fig. 1. The E, values were constant between 500°C and the temperature
K. Terabe et al. /Sodium p-alumina prepared by sol-gel method
114
5 \ >
composition of NazO-6A1203 prepared by the sol-gel method. The gel showed relatively high electrical conductivity influenced by both the shrinkage behavior and the transformation to B-alumina. Possibility of manufacturing dense beta alumina ceramics at lower temperature has been confirmed by the fact that the prepared B-alumina showed high ionic conductivity.
1.2
1.0.
P t W
0.6
5
0.6
z : z 2
0.4
0.2 I
0 400
I
600
I
I.
600
I
El,
Acknowledgement
1000
Temperature/?2 Fig. 5. Change in activation energy of the sample annealed at each temperature for five hours. Point (A) and (B) are the same temperatures as in fig. 1.
A part of the present work was supported by Grantin Aid for Scientific Research, Ministry of Education of Japan.
References (A), and then decreased. The Ea also changed at both temperatures of the shrinkage beginning (A) and the transition (B). E, in the state 1 was much higher than that of B-alumina. In this state, sodium and proton conduction may occur in adsorbed water layer on the surface of amorphous or y-alumina particles. But detailed mechanism remained to be solved.
4. Conclusion Electrical properties were studied on both the transparent gel and the crystallized beta alumina with
[ 1] Y.F. Yao and J.T. Kummer, J. Inor. Nucl. Chem. 29 ( 1967) 2453. [2] S. Sakka, Am. Ceram. Sot. Bull. 64 (1985) 1463. [ 31 D.R. Uhich, J. Non-Ctyst. Solids, 100 (1988) 174. [4] B.E. Yoldas and D.P. Partlow, Am. Ceram. Sot. Bull. 59 ( 1980) 640. [ 51 S. Yamaguchi, K. Terabe, Y. Iguchi and A. Imai, Solid State Ionics, 25 (1987) 171. [6] P.E.D. Morgan, Mat. Res. Bull. 11 (1976) 233. [ 71 C.N. Poulieff et al., Mat. Res. Bull. 13 (1978) 323. [ 81 J.D. Hodge, Am. Ceram. Sot. Bull. 62 (1983) 244. [9] A.J. Perrotta and J.E. Young, J. Am. Ceram. Sot. 57 (1974) 405. [10]A.G.ElliotandR.A.Huggins,J.Am.Ceram.Soc.58 (1975) 497.