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Thermodynamic studies of the ionic conductor Ba β-Al2O3
Solid State Ionics 128 (2000) 141–144 www.elsevier.com / locate / ssi
Thermodynamic studies of the ionic conductor Ba b-Al 2 O 3 a, b b S. Dunn *, R.V. Kumar , D.J. Fray a Department of Nanotechnology, Building 70, Cranfield University, Cranfield MK43 0 AL, UK University of Cambridge, Department of Material Science and Metallurgy, Pembroke Street, Cambridge CB2 3 QZ, UK
b
Received 24 September 1999; received in revised form 5 November 1999; accepted 10 November 1999
Abstract Samples of Ba b-Al 2 O 3 with the stoichiometries BaAl 9.2 O 14.8 to BaAl 14 O 22 were prepared by a solid state route from Al 2 O 3 and BaCO 3 . The activity of Al 2 O 3 was determined in each of the samples of Ba b-Al 2 O 3 and for samples of a mixture of Al 2 O 3 and phase I Ba b-Al 2 O 3 with the composition BaAl 14 O 22 and BaAl 2 O 4 mixed with phase II Ba b-Al 2 O 3 having a composition of BaAl 9.2 O 14.8 . In all cases the activity of Al 2 O 3 for the mixtures or compounds was determined against Al 2 O 3 using the solid electrolyte Na b-Al 2 O 3 as the membrane. It is shown that the trend exhibited by the BaO–Al 2 O 3 compositions are that exhibited by a solid solution of the two compositions BaAl 9.2 O 14.8 and BaAl 14 O 22 . 2000 Elsevier Science B.V. All rights reserved. Keywords: b-Al 2 O 3 ; Synthesis; Activity; Thermodynamics
1. Introduction The scientific and technical interest in the properties exhibited by b-Al 2 O 3 has grown over the past few decades. These interests range from fields as diverse as batteries [1] to the chemiluminescence [2] exhibited by some doped b-Al 2 O 3 . The use of bAl 2 O 3 as sensors has also been widely investigated [3,4]. While interest in the laboratory has centred around the ion-exchange, conductivity [5–8], and structural properties [9–11] of these materials. In the Na b-Al 2 O 3 structure, the unit cell is composed of two spinel blocks of O 22 and Al 31 ions, separated by a mirror plane. The Na 1 ions can easily move in two dimensions along the layer *Corresponding author. E-mail address: [email protected] (S. Dunn)
between the spinel blocks. In the case of b0-Al 2 O 3 , the structure is the result of stacking of three spinel blocks and is usually stabilised by adding MgO or Li 2 O. Some of the ion-exchanged b and b0-Al 2 O 3 are unstable at high temperatures, for example Li b-Al 2 O 3 will decompose at approximately 1273 K, and cannot be produced by direct synthesis. The ion-exchanged Ca b-Al 2 O 3 has also been seen to be more stable but in the closely related magnetoplumbite phase. It is however possible to produce a number of b-Al 2 O 3 by solid state sintering [12]. Ba b-Al 2 O 3 has been produced by both solid state [13,14] and ion-exchange [7] routes. The material has been shown to have a b-Al 2 O 3 type structure, with two types of structures being determined: phase I and phase II [10,14]. These two structures have been shown to have the nominal stoichiometry of BaAl 14 O 22 and BaAl 9.2 O 14.8 , having been produced
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00307-0
S. Dunn et al. / Solid State Ionics 128 (2000) 141 – 144
142
by the floating zone method [10,14]. The composition of BaAl 12 O 19 is shown to have a b-Al 2 O 3 structure. The addition of BaCO 3 to high surface area g-Al 2 O 3 has been used to produce a material that retains a high surface area at high temperatures [15,16]. This technique has been used in the production of thermally stable catalyst supports [17]. Recent work completed by Van de Cruys et al. [18] has shown that the activity of Na 2 O in the Na 2 O–Al 2 O 3 phase diagram in the region of Na b-Al 2 O 3 and Na b0-Al 2 O 3 is not steady. The implication of this is that a solid solution of Na b-Al 2 O 3 and b0-Al 2 O 3 exists. In this study we show that a similar phenomena occurs for the BaO–Al 2 O 3 system.
2. Experimental
2.1. Sample preparation Samples of BaO–Al 2 O 3 were made in the range BaO ? XAl 2 O 3 with 3 # X # 8 by a solid state synthetic route. Dry powders of BaCO 3 (Aldrich, 99.9%) and Al 2 O 3 (Aldrich, 99.9%) were mixed together in the correct stoichiometry for the desired compositions and then milled in isopropyl-alcohol (IPA, Aldrich reagent grade)) for 24 h. This was then
dried, sieved and calcined at 12508C for 2 h. Some samples of this powder were analysed by XRD to determine the composition produced. This powder was then re-milled in for 24 h. The resulting slurry was dried and sieved. The powder was then fired in a graphite furnace under Argon at 17808C. After cooling the powder was analysed by XRD to determine the composition produced.
2.2. Thermodynamic analysis of various BaO– Al2 O3 compositions Cells of the type: Pt / unknown Al 2 O 3 / / Na b-Al 2 O 3 / /Al 2 O 3 / Pt % were constructed where the unknown is the sample of BaO–Al 2 O 3 under evaluation. A schematic of the cell is shown in Fig. 1. The experiments were carried out in air at 8508C. The compositions of the BaO– Al 2 O 3 investigated are shown in Table 1.
2.3. X-ray analysis XRD was used to investigate the various phases. The XRD pattern generated was converted for use on a PC. Samples were studied in the powder form or in
Fig. 1. Schematic of apparatus used to perform thermodynamic experiments.
S. Dunn et al. / Solid State Ionics 128 (2000) 141 – 144 Table 1 Composition of BaO–Al 2 O 3 investigated: values of X in BaO? XAl 2 O 3 X
Compositions present
8 7.5 7 6.5 6 5.5 5 4.5 4 3
Al 2 O 3 1Phase I, Ba b-Al 2 O 3 Al 2 O 3 1Phase I, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 BaAl 2 O 4 1Phase II, Ba b-Al 2 O 3 BaAl 2 O 4 1Phase II, Ba b-Al 2 O 3
pellet form. The sample was studied from 2u 55– 858.
3. Results and discussion The XRD completed on the samples after firing to 17808C showed that a range of materials from a mixture of BaAl 2 O 4 –Ba b-Al 2 O 3 (phase II) through the Ba b-Al 2 O 3 compositional range to Al 2 O 3 –Ba b-Al 2 O 3 (phase I) was formed. Table 1 shows the compositions produced and the XRD results obtained. Samples of these powders were then tested in
143
the thermodynamic cell, with the emf generated for each composition being recorded. The results for emf generated for the materials of composition BaO–Al 2 O 3 against Al 2 O 3 are shown in Fig. 2. Here, it is clear that the emf in the region of the Ba b-Al 2 O 3 compositions is not steady. The changing emf and associated activity of the Al 2 O 3 in the Ba b-Al 2 O 3 indicates that it is not possible for two phases to be present. This is perhaps somewhat surprising as it is possible to differentiate the patterns of both BaAl 9.2 O 14.8 and BaAl 14 O 22 in the powder XRD pattern for BaAl 12 O 19 . In previous work [14], it was reported that the intimate mixing of BaAl 9.2 O 14.8 and BaAl 14 O 22 produced the same powder pattern as that of BaAl 12 O 19 . The question is then raised as to how a sample that possess the XRD pattern of two components can only have one phase when viewed thermodynamically. In this case we believe that it is possible for the two compositions to share lattice boundaries. This has been previously demonstrated by the work of Yamamoto and O’Keeffe [9] when they produced HREM micrographs showing the intergrowth of Ba b-Al 2 O 3 phase I and phase II (Fig. 3). The demonstration that the two structures can share lattice boundaries allows the two structures present to have a chemical feel for each others presence, this feel
Fig. 2. EMF generated by a cell of Al 2 O 3 and BaO?XAl 2 O 3 with a Na b-Al 2 O 3 membrane.
144
S. Dunn et al. / Solid State Ionics 128 (2000) 141 – 144
Acknowledgements We would like to acknowledge the help and support of all the students and research fellows in the Materials Chemistry Group at Cambridge, UK and the financial support supplied by Engineering Physics Science Research Council and Cookson Group plc.
References
Fig. 3. HREM micrograph showing intergrowth of Ba(I) and Ba(II) phases (Yamamoto and O’Keeffe [9]).
then produces a system that thermodynamically has only one phase. The number of layers of each of the two structures present can explain the XRD pattern anomaly. In this case we have a material which consists of perhaps 100–1000 layers of each of the two structures. These regions are intimately joined by the intergrowth. We can then view the complete structure of the material as at a macroscopic level as one phase or at the microscopic level as two phases.
4. Conclusions The thermodynamic experiments conducted on the samples of BaO–Al 2 O 3 have shown that the thermodynamic characteristics for materials with the composition BaAl 9.2 O 14.8 through to BaAl 14 O 22 indicate only one phase is present. We report a possible mechanism for this which explains the difference in number of phases as described by XRD and thermodynamics due to stacking of different compositions and the sharing of lattice parameters.
[1] R. Collongues, D. Gourier, A. Kahn, J. Phys. Chem. Solids 45 (10) (1984) 981. [2] A.L. N Stevels, J. Lumin. 17 (1978) 121. [3] B. Bellotto et al., J. Solid State Chem. 117 (1995) 85. [4] J. Zhouhua, D.U. Gang, P. Granati, J. Northeast Univ. Technol. 12 (3) (1991) 241. [5] M.W. Breiter, G.C. Farrington, Mater. Res. Bull. 13 (1978) 1213. [6] N. Iyi, S. Takekawa, S. Kimura, J. Solid State Chem. 59 (1985) 250. [7] R. Seevers, J. DeNuzzio, G.C. Farrington, B. Dunn, J. Solid State Chem. 50 (1983) 146. [8] B. Dunn, R.M. Ostrom, Solid State Ionics 5 (1981) 203. [9] N. Yamamoto, M. O’Keefee, Acta. Crystallogr., Sect. B 40 (1984) 21. [10] N. Iyi, Z. Inoue, S. Takekawa, S. Kimura, J. Solid State Chem. 52 (1984) 66. [11] N. Iyi, Z. Inoue, S. Takekawa, S. Kimura, J. Solid State Chem. 47 (1983) 34. [12] M. Lacerda, J.T.S. Irvine, E.E. Lachowski, F.P. Glasser, A.R. West, J. Br. Ceram. Trans. 87 (1988) 191. [13] M. Machida, K. Eguchi, H. Arai, J. Catal. 103 (1987) 385. [14] S. Dunn, R.V. Kumar, D. J Fray, Solid State Ionics 124 (1999) 133. [15] G. Groppi, F. Assandri, M. Bellotto, C. Cristiani, P. Forzatti, J. Solid State Chem. 114 (1995) 326. [16] M. Johnson, J. Catal. 123 (1990) 245. [17] A. Stiles, Catalysis Today 14 (1992) 269. [18] F. Van de Cruys et al., 2000 (in press).
Thermodynamic studies of the ionic conductor Ba b-Al 2 O 3 a, b b S. Dunn *, R.V. Kumar , D.J. Fray a Department of Nanotechnology, Building 70, Cranfield University, Cranfield MK43 0 AL, UK University of Cambridge, Department of Material Science and Metallurgy, Pembroke Street, Cambridge CB2 3 QZ, UK
b
Received 24 September 1999; received in revised form 5 November 1999; accepted 10 November 1999
Abstract Samples of Ba b-Al 2 O 3 with the stoichiometries BaAl 9.2 O 14.8 to BaAl 14 O 22 were prepared by a solid state route from Al 2 O 3 and BaCO 3 . The activity of Al 2 O 3 was determined in each of the samples of Ba b-Al 2 O 3 and for samples of a mixture of Al 2 O 3 and phase I Ba b-Al 2 O 3 with the composition BaAl 14 O 22 and BaAl 2 O 4 mixed with phase II Ba b-Al 2 O 3 having a composition of BaAl 9.2 O 14.8 . In all cases the activity of Al 2 O 3 for the mixtures or compounds was determined against Al 2 O 3 using the solid electrolyte Na b-Al 2 O 3 as the membrane. It is shown that the trend exhibited by the BaO–Al 2 O 3 compositions are that exhibited by a solid solution of the two compositions BaAl 9.2 O 14.8 and BaAl 14 O 22 . 2000 Elsevier Science B.V. All rights reserved. Keywords: b-Al 2 O 3 ; Synthesis; Activity; Thermodynamics
1. Introduction The scientific and technical interest in the properties exhibited by b-Al 2 O 3 has grown over the past few decades. These interests range from fields as diverse as batteries [1] to the chemiluminescence [2] exhibited by some doped b-Al 2 O 3 . The use of bAl 2 O 3 as sensors has also been widely investigated [3,4]. While interest in the laboratory has centred around the ion-exchange, conductivity [5–8], and structural properties [9–11] of these materials. In the Na b-Al 2 O 3 structure, the unit cell is composed of two spinel blocks of O 22 and Al 31 ions, separated by a mirror plane. The Na 1 ions can easily move in two dimensions along the layer *Corresponding author. E-mail address: [email protected] (S. Dunn)
between the spinel blocks. In the case of b0-Al 2 O 3 , the structure is the result of stacking of three spinel blocks and is usually stabilised by adding MgO or Li 2 O. Some of the ion-exchanged b and b0-Al 2 O 3 are unstable at high temperatures, for example Li b-Al 2 O 3 will decompose at approximately 1273 K, and cannot be produced by direct synthesis. The ion-exchanged Ca b-Al 2 O 3 has also been seen to be more stable but in the closely related magnetoplumbite phase. It is however possible to produce a number of b-Al 2 O 3 by solid state sintering [12]. Ba b-Al 2 O 3 has been produced by both solid state [13,14] and ion-exchange [7] routes. The material has been shown to have a b-Al 2 O 3 type structure, with two types of structures being determined: phase I and phase II [10,14]. These two structures have been shown to have the nominal stoichiometry of BaAl 14 O 22 and BaAl 9.2 O 14.8 , having been produced
0167-2738 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0167-2738( 99 )00307-0
S. Dunn et al. / Solid State Ionics 128 (2000) 141 – 144
142
by the floating zone method [10,14]. The composition of BaAl 12 O 19 is shown to have a b-Al 2 O 3 structure. The addition of BaCO 3 to high surface area g-Al 2 O 3 has been used to produce a material that retains a high surface area at high temperatures [15,16]. This technique has been used in the production of thermally stable catalyst supports [17]. Recent work completed by Van de Cruys et al. [18] has shown that the activity of Na 2 O in the Na 2 O–Al 2 O 3 phase diagram in the region of Na b-Al 2 O 3 and Na b0-Al 2 O 3 is not steady. The implication of this is that a solid solution of Na b-Al 2 O 3 and b0-Al 2 O 3 exists. In this study we show that a similar phenomena occurs for the BaO–Al 2 O 3 system.
2. Experimental
2.1. Sample preparation Samples of BaO–Al 2 O 3 were made in the range BaO ? XAl 2 O 3 with 3 # X # 8 by a solid state synthetic route. Dry powders of BaCO 3 (Aldrich, 99.9%) and Al 2 O 3 (Aldrich, 99.9%) were mixed together in the correct stoichiometry for the desired compositions and then milled in isopropyl-alcohol (IPA, Aldrich reagent grade)) for 24 h. This was then
dried, sieved and calcined at 12508C for 2 h. Some samples of this powder were analysed by XRD to determine the composition produced. This powder was then re-milled in for 24 h. The resulting slurry was dried and sieved. The powder was then fired in a graphite furnace under Argon at 17808C. After cooling the powder was analysed by XRD to determine the composition produced.
2.2. Thermodynamic analysis of various BaO– Al2 O3 compositions Cells of the type: Pt / unknown Al 2 O 3 / / Na b-Al 2 O 3 / /Al 2 O 3 / Pt % were constructed where the unknown is the sample of BaO–Al 2 O 3 under evaluation. A schematic of the cell is shown in Fig. 1. The experiments were carried out in air at 8508C. The compositions of the BaO– Al 2 O 3 investigated are shown in Table 1.
2.3. X-ray analysis XRD was used to investigate the various phases. The XRD pattern generated was converted for use on a PC. Samples were studied in the powder form or in
Fig. 1. Schematic of apparatus used to perform thermodynamic experiments.
S. Dunn et al. / Solid State Ionics 128 (2000) 141 – 144 Table 1 Composition of BaO–Al 2 O 3 investigated: values of X in BaO? XAl 2 O 3 X
Compositions present
8 7.5 7 6.5 6 5.5 5 4.5 4 3
Al 2 O 3 1Phase I, Ba b-Al 2 O 3 Al 2 O 3 1Phase I, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 Phase I1Phase II, Ba b-Al 2 O 3 BaAl 2 O 4 1Phase II, Ba b-Al 2 O 3 BaAl 2 O 4 1Phase II, Ba b-Al 2 O 3
pellet form. The sample was studied from 2u 55– 858.
3. Results and discussion The XRD completed on the samples after firing to 17808C showed that a range of materials from a mixture of BaAl 2 O 4 –Ba b-Al 2 O 3 (phase II) through the Ba b-Al 2 O 3 compositional range to Al 2 O 3 –Ba b-Al 2 O 3 (phase I) was formed. Table 1 shows the compositions produced and the XRD results obtained. Samples of these powders were then tested in
143
the thermodynamic cell, with the emf generated for each composition being recorded. The results for emf generated for the materials of composition BaO–Al 2 O 3 against Al 2 O 3 are shown in Fig. 2. Here, it is clear that the emf in the region of the Ba b-Al 2 O 3 compositions is not steady. The changing emf and associated activity of the Al 2 O 3 in the Ba b-Al 2 O 3 indicates that it is not possible for two phases to be present. This is perhaps somewhat surprising as it is possible to differentiate the patterns of both BaAl 9.2 O 14.8 and BaAl 14 O 22 in the powder XRD pattern for BaAl 12 O 19 . In previous work [14], it was reported that the intimate mixing of BaAl 9.2 O 14.8 and BaAl 14 O 22 produced the same powder pattern as that of BaAl 12 O 19 . The question is then raised as to how a sample that possess the XRD pattern of two components can only have one phase when viewed thermodynamically. In this case we believe that it is possible for the two compositions to share lattice boundaries. This has been previously demonstrated by the work of Yamamoto and O’Keeffe [9] when they produced HREM micrographs showing the intergrowth of Ba b-Al 2 O 3 phase I and phase II (Fig. 3). The demonstration that the two structures can share lattice boundaries allows the two structures present to have a chemical feel for each others presence, this feel
Fig. 2. EMF generated by a cell of Al 2 O 3 and BaO?XAl 2 O 3 with a Na b-Al 2 O 3 membrane.
144
S. Dunn et al. / Solid State Ionics 128 (2000) 141 – 144
Acknowledgements We would like to acknowledge the help and support of all the students and research fellows in the Materials Chemistry Group at Cambridge, UK and the financial support supplied by Engineering Physics Science Research Council and Cookson Group plc.
References
Fig. 3. HREM micrograph showing intergrowth of Ba(I) and Ba(II) phases (Yamamoto and O’Keeffe [9]).
then produces a system that thermodynamically has only one phase. The number of layers of each of the two structures present can explain the XRD pattern anomaly. In this case we have a material which consists of perhaps 100–1000 layers of each of the two structures. These regions are intimately joined by the intergrowth. We can then view the complete structure of the material as at a macroscopic level as one phase or at the microscopic level as two phases.
4. Conclusions The thermodynamic experiments conducted on the samples of BaO–Al 2 O 3 have shown that the thermodynamic characteristics for materials with the composition BaAl 9.2 O 14.8 through to BaAl 14 O 22 indicate only one phase is present. We report a possible mechanism for this which explains the difference in number of phases as described by XRD and thermodynamics due to stacking of different compositions and the sharing of lattice parameters.
[1] R. Collongues, D. Gourier, A. Kahn, J. Phys. Chem. Solids 45 (10) (1984) 981. [2] A.L. N Stevels, J. Lumin. 17 (1978) 121. [3] B. Bellotto et al., J. Solid State Chem. 117 (1995) 85. [4] J. Zhouhua, D.U. Gang, P. Granati, J. Northeast Univ. Technol. 12 (3) (1991) 241. [5] M.W. Breiter, G.C. Farrington, Mater. Res. Bull. 13 (1978) 1213. [6] N. Iyi, S. Takekawa, S. Kimura, J. Solid State Chem. 59 (1985) 250. [7] R. Seevers, J. DeNuzzio, G.C. Farrington, B. Dunn, J. Solid State Chem. 50 (1983) 146. [8] B. Dunn, R.M. Ostrom, Solid State Ionics 5 (1981) 203. [9] N. Yamamoto, M. O’Keefee, Acta. Crystallogr., Sect. B 40 (1984) 21. [10] N. Iyi, Z. Inoue, S. Takekawa, S. Kimura, J. Solid State Chem. 52 (1984) 66. [11] N. Iyi, Z. Inoue, S. Takekawa, S. Kimura, J. Solid State Chem. 47 (1983) 34. [12] M. Lacerda, J.T.S. Irvine, E.E. Lachowski, F.P. Glasser, A.R. West, J. Br. Ceram. Trans. 87 (1988) 191. [13] M. Machida, K. Eguchi, H. Arai, J. Catal. 103 (1987) 385. [14] S. Dunn, R.V. Kumar, D. J Fray, Solid State Ionics 124 (1999) 133. [15] G. Groppi, F. Assandri, M. Bellotto, C. Cristiani, P. Forzatti, J. Solid State Chem. 114 (1995) 326. [16] M. Johnson, J. Catal. 123 (1990) 245. [17] A. Stiles, Catalysis Today 14 (1992) 269. [18] F. Van de Cruys et al., 2000 (in press).