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Formation, sintering and electrical conductivity of sol–gel-derived LaCr0.75Mg0.25O3
PERGAMON
Solid State Communications 112 (1999) 281–284 www.elsevier.com/locate/ssc
Formation, sintering and electrical conductivity of sol–gel-derived LaCr0.75Mg0.25O3 K. Azegami, M. Yoshinaka, K. Hirota, O. Yamaguchi* Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Kyo-Tanabe Kyoto 610-0321, Japan Received 8 April 1999; accepted 17 June 1999 by F.J. DiSalvo
Abstract A sinterable LaCr0.75Mg0.25O3 powder consisting of submicrometer-size particles (,0.2 mm) has been obtained at low temperatures (,8508C) by decomposition of LaCr0.75Mg0.25O4 which crystallizes at ,5308C from an amorphous material prepared by a sol–gel technique. Dense ceramics (98.5% of theoretical) can be fabricated by sintering for 2 h at 16008C without any control of oxygen pressure. They show an excellent electrical conductivity of 3:17 × 103 S m21 at 10008C. Their thermal expansion coefficient is 10:1 × 1026 =8C: q 1999 Elsevier Science Ltd. All rights reserved. Keywords: B. Chemical synthesis; C. Scanning and transmission electron microscopy; D. Electronic transport
1. Introduction Lanthanum chromite LaCrO3 (orthorhombic, perovskite structure) and doped LaCrO3 have been widely studied as refractory conducting materials because of their electrical conductivity, high melting point (.24008C) and oxidation (corrosion) resistivity. In particular, doped LaCrO3 ceramics with high electrical conductivity have been used as electrodes for magnetohydrodynamic (MHD) power generators, interconnectors of solid oxide fuel cells (SOFCs) and conducting leads for ZrO2-based heating elements [1]. These compounds have been synthesized by solid-state reactions of oxide mixtures at high temperatures (.12008C) in air [2]. This procedure needs repeated heating and grinding to complete the reaction due to the slow diffusion process in solids. In recent years, several methods for the low-temperature synthesis of LaCrO3 have been developed: hydrothermal reaction [3], crystallization of precipitation [4] and thermal decomposition of double complex La[Cr(CH2(COO)2)3]·6H2O [5]. These methods resulted in the formation of LaCrO3 at low temperatures of ,8008C. Unfortunately, LaCrO3 shows poor sinterability and is very difficult to densify under ambient atmospheric conditions. The effect of additives [6] and sintering atmosphere [7] on * Corresponding author. Tel.: 1 81-774-656691; fax: 1 81774-656849.
the densification of LaCrO3 have been extensively investigated. Moreover, there have been attempts to fabricate dense pure and doped LaCrO3 ceramics using hot pressing [8], hot isostatic pressing [9] and hydrothermal reaction sintering [10]. On the other hand, the heterovalent alloying ability of LaCrO3, in which a fraction of M 31 ion (La 31 or Cr 31) is substituted by an ion of different valence, has been exploited to improve the electrical conductivity of the ceramics. Substituents such as Sr 21, Ca 21 and Mg 21 have been employed for this purpose. Although extensive literature is presented for the former two [11,12], the study of Mg 21substituted ceramics is still lacking with a few exceptions [13,14]. Flandermeyer et al. [13] studied the high-temperature stability of hot-pressed LaCr12xMgxO3 (substitution at the Cr-site) ceramics with . 95% of theoretical density and elucidated the relationship between electrical conductivity, defect structure, and temperature at various oxygen activities and dopant levels. Jin et al. [14] reported that LaCr12xMgxO3, in which x was varied from 0 to 0.15, was formed by a solid-state reaction; however, hardly any densification occurred even after heating for 2 h at 16008C in air. A new powder preparation method using hydrazine monohydrate has recently been developed in some systems [15]. During the course of an investigation to obtain reactive Mg 21-doped LaCrO3 powder by applying this preparation LaCrO3 method, 25 mol% Mg 21-substituted (LaCr0.75Mg0.25O3) was found to form at low temperatures
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00339-7
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Fig. 1. (a) DTA curve for as-prepared powder; (b) TG curve for asprepared powder.
from an amorphous material. Dense ceramics (98.5% of theoretical), that could be fabricated without any control of oxygen pressure, showed an excellent electrical conductivity at elevated temperatures. In this communication, we report the formation, sintering, and electrical conductivity of sol–gel-derived LaCr0.75Mg0.25O3.
2. Experimental procedure Lanthanum chloride (LaCl3·7H2O, 99.9% pure), chromium chloride (CrCl3·6H2O, 99.9% pure), magnesium chloride (MgCl2·6H2O, 99.9% pure), and hydrazine monohydrate
Fig. 2. TEM photograph of as-prepared powder.
Fig. 3. (a) XRD pattern for LaCr0.75Mg0.25O4 (monoclinic); (b) XRD pattern for LaCr0.75Mg0.25O3 (orthorhombic).
((NH2)2·H2O) were used as starting materials. Aqueous solutions of the first and second chlorides and the third chloride were adjusted in concentrations of 0.5 and 0.1 mol/l, respectively, by dissolving in distilled water. A five-necked flask was equipped with a reflux condenser, a dropping funnel, a stirring rod, a thermometer and a thermocontroller. A mixed solution (pH 2) in the mole ratio 4La 31:3Cr 31:1Mg 21 was introduced in the flask and stirred for 10 min at room temperature, and then hydrazine monohydrate was added dropwise to the mixed solution at 808C, with continuous stirring, until the resulting suspension reached pH 11.5. Then, the suspension was heated for 2 h at the same temperature. The product was separated from the suspension by centrifugation, washed more than 10 times in hot water to remove adsorbed hydrazine and chloride ions (tested by adding AgNO3 solution), and then dried at 1208C under reduced pressure. Differential thermal analysis (DTA) and thermogravimetry (TG) were conducted in air at a heating rate of 108C/min; a-Al2O3 was used as the reference in the DTA. The asprepared powder and specimens, obtained from DTA runs after cooling, were examined by X-ray diffraction (XRD) using Ni-filtered CuKa radiation and a goniometer with a scanning speed of 0.258/min. Interplanar spacings were measured with the aid of an internal standard of high-purity Si, and unit-cell values were determined by a least-squares refinement. Particle size and morphology were observed by transmission electron microscopy (TEM). Before sintering, calcined powder was pressed into pellets at 196 MPa and then isostatically cold-pressed at 392 MPa. The green
K. Azegami et al. / Solid State Communications 112 (1999) 281–284
Fig. 4. SEM photograph of fracture surface of LaCr0.75Mg0.25O3 ceramics.
compacts were sintered for 2 h at 16008C in air. Bulk densities after polishing with diamond paste were determined by the Archimedes method. Scanning electron microscopy (SEM) was used for microstructural observations. 3. Results and discussion Fig. 1(a) shows a DTA curve of the as-prepared powder. Two successive endothermic peaks at 3608C are due to the release of absorbed water and hydrated water. A sharp exothermic peak was observed at 480–5308C. As will be described, this was found to result from the crystallization of LaCr0.75Mg0.25O4. In addition, the DTA curve revealed an endothermic peak resulting from decomposition
of LaCr0:75 Mg0:25 O4 ! LaCr0:75 Mg0:25 O3 1 1=2·O2 LaCr0.75Mg0.25O4 at 775–8558C. TG data showed a weight decrease of 6.45% in this temperature range (Fig. 1(b)); this corresponds to the decrease (theoretical value 6.452%) of (1/2)·O2 per LaCr0.75Mg0.25O4. The as-prepared powder, consisting of translucent thin particles (,20 nm) (Fig. 2), was amorphous, no significant change in structure being recognized up to 4708C. The XRD lines of LaCr0.75Mg0.25O4 corresponding to LaCrO4 (monoclinic) [16] began to appear at 4808C and the intensity increased rapidly up to 5308C. After the exothermic peak, the specimen showed the XRD pattern of LaCr0.75Mg0.25O4 (Fig. 3(a)). Only well-crystallized LaCr0.75Mg0.25O4 was observed up to 7758C. The specimens above 8608C gave the characteristic XRD pattern of LaCr0.75Mg0.25O3, which is the same as that of LaCrO3 (orthorhombic) [17] (Fig. 3(b)). No free species were detected throughout the heating process up to 12008C. The results indicate that LaCr0.75Mg0.25O4 decomposed into LaCr0.75Mg0.25O3 and (1/2)·O2 at 775–8558C. All diffraction lines for LaCr0.75Mg0.25O3 (9008C/1 h) were indexed as an orthorhombic unit cell with a 0:5484 nm; b 0:5519 nm and c 0:7760 nm; which has values larger than that a 0:5479 nm; b 0:5513 nm and c 0:7756 nm [17] of pure LaCrO3. Thus, the formation of LaCr0.75Mg0.25O3, as well as the final stage of decomposition of double complex La[Cr(CH2(COO)2)3] [5], were achieved at temperatures as low as ,8508C by decomposition of LaCr0.75Mg0.25O4. The as-prepared powder was calcined for 1 h at 10008C. The calcined LaCr0.75Mg0.25O3 powder was sintered as already described. The bulk density was 6.46 g/cm 3, corresponding to 98.5% of theoretical density. 1 As can be seen from an SEM photograph for the fracture surface (Fig. 4), the texture was of a homogeneous structure whose average grain size, determined by a linear intercept method, was 3 mm. It should be noted that the dense ceramics could be fabricated by heating at 16008C in air. This result indicates that the LaCr0.75Mg0.25O3 powder prepared by the present method is highly sinterable. The electrical conductivities, s, were measured from room temperature to 10008C by the van der Pauw method [18] using samples with four platinum electrodes. The temperature dependence of s is shown in Fig. 5; the value of 3:17 × 103 S m21 ; in which the magnitudes of s are comparable to those of Sr- and Ca-substituted LaCrO3 [12,19], was obtained at 10008C. The activation energy was determined to be 0.15 eV at 600–10008C. The electrical conductivity in LaCrO3 is essentially due to the 3d band of the Cr ions [20]. In LaCr0.75Mg0.25O3, Mg 21 ions are distributed randomly on the Cr 31 lattice sites. Thus, the mechanism for electrical conduction can be attributed to the formation of Cr 41 ions as a result of charge compensation caused by the hopping polarons between Cr 31 and Cr 41 ions 1
Fig. 5. Temperature dependence of electrical conductivity of LaCr0.75Mg0.25O3 ceramics.
283
The theoretical density (6.56 g/cm 3) was calculated from the molecular weight, Z 4; Avogadro’s number, and the lattice parameters.
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[21]. SOFCs are operated at ,10008C. In the application of LaCr0.75Mg0.25O3 ceramics to interconnectors, the agreement of thermal expansion coefficients between these ceramics and cubic ZrO2 as electrolytes is required. Their thermal expansions were measured to 10008C at a heating rate of 108C/min. A thermal expansion coefficient of 10:1 × 1026 =8C was obtained; this value was in good agreement with that 10:0 × 1026 =8C [22] of ZrO2(10Y). In conclusion, the presented results suggest that LaCr0.75Mg0.25O3 ceramics are available for high-temperature electrodes and thermoelectric materials. Acknowledgements This work was supported by a grant to the Research Center for Advanced Science and Technology at Doshisha University from the Ministry of Education, Japan. References [1] D.B. Meadowcroft, P.G. Meier, A.C. Warren, Energy Conversion 12 (1972) 145. [2] S.T. Song, H.Y. Pan, Z. Wang, B. Yang, Ceram. Int. 10 (1984) 143. [3] M. Yoshimura, S.T. Song, S. Somiya, J. Ceram. Soc. Jpn. 90 (1982) 91. [4] M. Inagaki, O. Yamamoto, M. Hirohara, J. Ceram. Soc. Jpn. 98 (1990) 675.
[5] S. Nakayama, M. Sakamoto, J. Ceram. Soc. Jpn. 100 (1992) 342. [6] S. Hayashi, K. Fukaya, H. Saito, J. Mater. Sci. Lett. 7 (1988) 457. [7] L. Groupp, H.U. Anderson, J. Am. Ceram. Soc. 59 (1976) 449. [8] S. Kose, Kagaku-Kogyo 12 (1974) 72. [9] S.T. Song, M. Yoshimura, S. Somiya, J. Ceram. Soc. Jpn. 90 (1982) 484. [10] M. Yoshimura, S.T. Song, S. Somiya, Proc. ICF. 3rd 1980, Cent. Acad. Publ., Tokyo, 1982, p. 429. [11] P.S. Devi, M.S. Rao, J. Solid State Chem. 98 (1992) 237. [12] N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, T. Iwata, J. Mater. Sci. 25 (1990) 4531. [13] B.K. Flandermeyer, M.M. Nasrallah, A.K. Agarwal, H.U. Anderson, J. Am. Ceram. Soc. 67 (1984) 195. [14] F. Jin, H. Takizawa, T. Endo, M. Shimada, Nippon Kagakukaishi 5 (1993) 670. [15] K. Ishida, K. Hirota, O. Yamaguchi, H. Kume, S. Inamura, H. Miyamoto, J. Am. Ceram. Soc. 77 (1994) 1391. [16] Powder Diffraction File, Card No. 36–93, Joint Committee on Powder Diffraction Standards, Swarthmore, PA. [17] Powder Diffraction File, Card No. 24–1016, Joint Committee on Powder Diffraction Standards, Swarthmore, PA. [18] L.J. van der Pauw, Philips Res. Rept. 13 (1958) 1. [19] W.J. Weber, C.W. Griffin, J.L. Bates, J. Am. Ceram. Soc. 70 (1987) 265. [20] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41. [21] K. Gaur, S.C. Varma, H.B. Lal, J. Mater. Sci. 23 (1988) 1725. [22] R. Hayami, T. Yabuki, Osaka Kogyo Gijutsu Shikensho Kiho Jpn. 28 (1977) 98.
Solid State Communications 112 (1999) 281–284 www.elsevier.com/locate/ssc
Formation, sintering and electrical conductivity of sol–gel-derived LaCr0.75Mg0.25O3 K. Azegami, M. Yoshinaka, K. Hirota, O. Yamaguchi* Department of Molecular Science and Technology, Faculty of Engineering, Doshisha University, Kyo-Tanabe Kyoto 610-0321, Japan Received 8 April 1999; accepted 17 June 1999 by F.J. DiSalvo
Abstract A sinterable LaCr0.75Mg0.25O3 powder consisting of submicrometer-size particles (,0.2 mm) has been obtained at low temperatures (,8508C) by decomposition of LaCr0.75Mg0.25O4 which crystallizes at ,5308C from an amorphous material prepared by a sol–gel technique. Dense ceramics (98.5% of theoretical) can be fabricated by sintering for 2 h at 16008C without any control of oxygen pressure. They show an excellent electrical conductivity of 3:17 × 103 S m21 at 10008C. Their thermal expansion coefficient is 10:1 × 1026 =8C: q 1999 Elsevier Science Ltd. All rights reserved. Keywords: B. Chemical synthesis; C. Scanning and transmission electron microscopy; D. Electronic transport
1. Introduction Lanthanum chromite LaCrO3 (orthorhombic, perovskite structure) and doped LaCrO3 have been widely studied as refractory conducting materials because of their electrical conductivity, high melting point (.24008C) and oxidation (corrosion) resistivity. In particular, doped LaCrO3 ceramics with high electrical conductivity have been used as electrodes for magnetohydrodynamic (MHD) power generators, interconnectors of solid oxide fuel cells (SOFCs) and conducting leads for ZrO2-based heating elements [1]. These compounds have been synthesized by solid-state reactions of oxide mixtures at high temperatures (.12008C) in air [2]. This procedure needs repeated heating and grinding to complete the reaction due to the slow diffusion process in solids. In recent years, several methods for the low-temperature synthesis of LaCrO3 have been developed: hydrothermal reaction [3], crystallization of precipitation [4] and thermal decomposition of double complex La[Cr(CH2(COO)2)3]·6H2O [5]. These methods resulted in the formation of LaCrO3 at low temperatures of ,8008C. Unfortunately, LaCrO3 shows poor sinterability and is very difficult to densify under ambient atmospheric conditions. The effect of additives [6] and sintering atmosphere [7] on * Corresponding author. Tel.: 1 81-774-656691; fax: 1 81774-656849.
the densification of LaCrO3 have been extensively investigated. Moreover, there have been attempts to fabricate dense pure and doped LaCrO3 ceramics using hot pressing [8], hot isostatic pressing [9] and hydrothermal reaction sintering [10]. On the other hand, the heterovalent alloying ability of LaCrO3, in which a fraction of M 31 ion (La 31 or Cr 31) is substituted by an ion of different valence, has been exploited to improve the electrical conductivity of the ceramics. Substituents such as Sr 21, Ca 21 and Mg 21 have been employed for this purpose. Although extensive literature is presented for the former two [11,12], the study of Mg 21substituted ceramics is still lacking with a few exceptions [13,14]. Flandermeyer et al. [13] studied the high-temperature stability of hot-pressed LaCr12xMgxO3 (substitution at the Cr-site) ceramics with . 95% of theoretical density and elucidated the relationship between electrical conductivity, defect structure, and temperature at various oxygen activities and dopant levels. Jin et al. [14] reported that LaCr12xMgxO3, in which x was varied from 0 to 0.15, was formed by a solid-state reaction; however, hardly any densification occurred even after heating for 2 h at 16008C in air. A new powder preparation method using hydrazine monohydrate has recently been developed in some systems [15]. During the course of an investigation to obtain reactive Mg 21-doped LaCrO3 powder by applying this preparation LaCrO3 method, 25 mol% Mg 21-substituted (LaCr0.75Mg0.25O3) was found to form at low temperatures
0038-1098/99/$ - see front matter q 1999 Elsevier Science Ltd. All rights reserved. PII: S0038-109 8(99)00339-7
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K. Azegami et al. / Solid State Communications 112 (1999) 281–284
Fig. 1. (a) DTA curve for as-prepared powder; (b) TG curve for asprepared powder.
from an amorphous material. Dense ceramics (98.5% of theoretical), that could be fabricated without any control of oxygen pressure, showed an excellent electrical conductivity at elevated temperatures. In this communication, we report the formation, sintering, and electrical conductivity of sol–gel-derived LaCr0.75Mg0.25O3.
2. Experimental procedure Lanthanum chloride (LaCl3·7H2O, 99.9% pure), chromium chloride (CrCl3·6H2O, 99.9% pure), magnesium chloride (MgCl2·6H2O, 99.9% pure), and hydrazine monohydrate
Fig. 2. TEM photograph of as-prepared powder.
Fig. 3. (a) XRD pattern for LaCr0.75Mg0.25O4 (monoclinic); (b) XRD pattern for LaCr0.75Mg0.25O3 (orthorhombic).
((NH2)2·H2O) were used as starting materials. Aqueous solutions of the first and second chlorides and the third chloride were adjusted in concentrations of 0.5 and 0.1 mol/l, respectively, by dissolving in distilled water. A five-necked flask was equipped with a reflux condenser, a dropping funnel, a stirring rod, a thermometer and a thermocontroller. A mixed solution (pH 2) in the mole ratio 4La 31:3Cr 31:1Mg 21 was introduced in the flask and stirred for 10 min at room temperature, and then hydrazine monohydrate was added dropwise to the mixed solution at 808C, with continuous stirring, until the resulting suspension reached pH 11.5. Then, the suspension was heated for 2 h at the same temperature. The product was separated from the suspension by centrifugation, washed more than 10 times in hot water to remove adsorbed hydrazine and chloride ions (tested by adding AgNO3 solution), and then dried at 1208C under reduced pressure. Differential thermal analysis (DTA) and thermogravimetry (TG) were conducted in air at a heating rate of 108C/min; a-Al2O3 was used as the reference in the DTA. The asprepared powder and specimens, obtained from DTA runs after cooling, were examined by X-ray diffraction (XRD) using Ni-filtered CuKa radiation and a goniometer with a scanning speed of 0.258/min. Interplanar spacings were measured with the aid of an internal standard of high-purity Si, and unit-cell values were determined by a least-squares refinement. Particle size and morphology were observed by transmission electron microscopy (TEM). Before sintering, calcined powder was pressed into pellets at 196 MPa and then isostatically cold-pressed at 392 MPa. The green
K. Azegami et al. / Solid State Communications 112 (1999) 281–284
Fig. 4. SEM photograph of fracture surface of LaCr0.75Mg0.25O3 ceramics.
compacts were sintered for 2 h at 16008C in air. Bulk densities after polishing with diamond paste were determined by the Archimedes method. Scanning electron microscopy (SEM) was used for microstructural observations. 3. Results and discussion Fig. 1(a) shows a DTA curve of the as-prepared powder. Two successive endothermic peaks at 3608C are due to the release of absorbed water and hydrated water. A sharp exothermic peak was observed at 480–5308C. As will be described, this was found to result from the crystallization of LaCr0.75Mg0.25O4. In addition, the DTA curve revealed an endothermic peak resulting from decomposition
of LaCr0:75 Mg0:25 O4 ! LaCr0:75 Mg0:25 O3 1 1=2·O2 LaCr0.75Mg0.25O4 at 775–8558C. TG data showed a weight decrease of 6.45% in this temperature range (Fig. 1(b)); this corresponds to the decrease (theoretical value 6.452%) of (1/2)·O2 per LaCr0.75Mg0.25O4. The as-prepared powder, consisting of translucent thin particles (,20 nm) (Fig. 2), was amorphous, no significant change in structure being recognized up to 4708C. The XRD lines of LaCr0.75Mg0.25O4 corresponding to LaCrO4 (monoclinic) [16] began to appear at 4808C and the intensity increased rapidly up to 5308C. After the exothermic peak, the specimen showed the XRD pattern of LaCr0.75Mg0.25O4 (Fig. 3(a)). Only well-crystallized LaCr0.75Mg0.25O4 was observed up to 7758C. The specimens above 8608C gave the characteristic XRD pattern of LaCr0.75Mg0.25O3, which is the same as that of LaCrO3 (orthorhombic) [17] (Fig. 3(b)). No free species were detected throughout the heating process up to 12008C. The results indicate that LaCr0.75Mg0.25O4 decomposed into LaCr0.75Mg0.25O3 and (1/2)·O2 at 775–8558C. All diffraction lines for LaCr0.75Mg0.25O3 (9008C/1 h) were indexed as an orthorhombic unit cell with a 0:5484 nm; b 0:5519 nm and c 0:7760 nm; which has values larger than that a 0:5479 nm; b 0:5513 nm and c 0:7756 nm [17] of pure LaCrO3. Thus, the formation of LaCr0.75Mg0.25O3, as well as the final stage of decomposition of double complex La[Cr(CH2(COO)2)3] [5], were achieved at temperatures as low as ,8508C by decomposition of LaCr0.75Mg0.25O4. The as-prepared powder was calcined for 1 h at 10008C. The calcined LaCr0.75Mg0.25O3 powder was sintered as already described. The bulk density was 6.46 g/cm 3, corresponding to 98.5% of theoretical density. 1 As can be seen from an SEM photograph for the fracture surface (Fig. 4), the texture was of a homogeneous structure whose average grain size, determined by a linear intercept method, was 3 mm. It should be noted that the dense ceramics could be fabricated by heating at 16008C in air. This result indicates that the LaCr0.75Mg0.25O3 powder prepared by the present method is highly sinterable. The electrical conductivities, s, were measured from room temperature to 10008C by the van der Pauw method [18] using samples with four platinum electrodes. The temperature dependence of s is shown in Fig. 5; the value of 3:17 × 103 S m21 ; in which the magnitudes of s are comparable to those of Sr- and Ca-substituted LaCrO3 [12,19], was obtained at 10008C. The activation energy was determined to be 0.15 eV at 600–10008C. The electrical conductivity in LaCrO3 is essentially due to the 3d band of the Cr ions [20]. In LaCr0.75Mg0.25O3, Mg 21 ions are distributed randomly on the Cr 31 lattice sites. Thus, the mechanism for electrical conduction can be attributed to the formation of Cr 41 ions as a result of charge compensation caused by the hopping polarons between Cr 31 and Cr 41 ions 1
Fig. 5. Temperature dependence of electrical conductivity of LaCr0.75Mg0.25O3 ceramics.
283
The theoretical density (6.56 g/cm 3) was calculated from the molecular weight, Z 4; Avogadro’s number, and the lattice parameters.
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K. Azegami et al. / Solid State Communications 112 (1999) 281–284
[21]. SOFCs are operated at ,10008C. In the application of LaCr0.75Mg0.25O3 ceramics to interconnectors, the agreement of thermal expansion coefficients between these ceramics and cubic ZrO2 as electrolytes is required. Their thermal expansions were measured to 10008C at a heating rate of 108C/min. A thermal expansion coefficient of 10:1 × 1026 =8C was obtained; this value was in good agreement with that 10:0 × 1026 =8C [22] of ZrO2(10Y). In conclusion, the presented results suggest that LaCr0.75Mg0.25O3 ceramics are available for high-temperature electrodes and thermoelectric materials. Acknowledgements This work was supported by a grant to the Research Center for Advanced Science and Technology at Doshisha University from the Ministry of Education, Japan. References [1] D.B. Meadowcroft, P.G. Meier, A.C. Warren, Energy Conversion 12 (1972) 145. [2] S.T. Song, H.Y. Pan, Z. Wang, B. Yang, Ceram. Int. 10 (1984) 143. [3] M. Yoshimura, S.T. Song, S. Somiya, J. Ceram. Soc. Jpn. 90 (1982) 91. [4] M. Inagaki, O. Yamamoto, M. Hirohara, J. Ceram. Soc. Jpn. 98 (1990) 675.
[5] S. Nakayama, M. Sakamoto, J. Ceram. Soc. Jpn. 100 (1992) 342. [6] S. Hayashi, K. Fukaya, H. Saito, J. Mater. Sci. Lett. 7 (1988) 457. [7] L. Groupp, H.U. Anderson, J. Am. Ceram. Soc. 59 (1976) 449. [8] S. Kose, Kagaku-Kogyo 12 (1974) 72. [9] S.T. Song, M. Yoshimura, S. Somiya, J. Ceram. Soc. Jpn. 90 (1982) 484. [10] M. Yoshimura, S.T. Song, S. Somiya, Proc. ICF. 3rd 1980, Cent. Acad. Publ., Tokyo, 1982, p. 429. [11] P.S. Devi, M.S. Rao, J. Solid State Chem. 98 (1992) 237. [12] N. Sakai, T. Kawada, H. Yokokawa, M. Dokiya, T. Iwata, J. Mater. Sci. 25 (1990) 4531. [13] B.K. Flandermeyer, M.M. Nasrallah, A.K. Agarwal, H.U. Anderson, J. Am. Ceram. Soc. 67 (1984) 195. [14] F. Jin, H. Takizawa, T. Endo, M. Shimada, Nippon Kagakukaishi 5 (1993) 670. [15] K. Ishida, K. Hirota, O. Yamaguchi, H. Kume, S. Inamura, H. Miyamoto, J. Am. Ceram. Soc. 77 (1994) 1391. [16] Powder Diffraction File, Card No. 36–93, Joint Committee on Powder Diffraction Standards, Swarthmore, PA. [17] Powder Diffraction File, Card No. 24–1016, Joint Committee on Powder Diffraction Standards, Swarthmore, PA. [18] L.J. van der Pauw, Philips Res. Rept. 13 (1958) 1. [19] W.J. Weber, C.W. Griffin, J.L. Bates, J. Am. Ceram. Soc. 70 (1987) 265. [20] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41. [21] K. Gaur, S.C. Varma, H.B. Lal, J. Mater. Sci. 23 (1988) 1725. [22] R. Hayami, T. Yabuki, Osaka Kogyo Gijutsu Shikensho Kiho Jpn. 28 (1977) 98.