- Email: [email protected]
Solubility of lanthanum, nickel and chromium in barium stannate
July 2001
Materials Letters 49 Ž2001. 251–255 www.elsevier.comrlocatermatlet
Solubility of lanthanum, nickel and chromium in barium stannate Shail Upadhyay a , Om Parkash b,) , Devendra Kumar b a
School of Materials Science and Technology, Institute of Technology, Banaras Hindu UniÕersity, Varanasi-221 005, India b Department of Ceramic Engineering, Institute of Technology, Banaras Hindu UniÕersity, Varanasi-221 005, India Received 14 January 2000; received in revised form 26 October 2000; accepted 31 October 2000
Abstract Undoped, donor doped and acceptor doped barium stannate systems viz Ba 1yx La x SnO 3 , and BaSn 1yx Cr xO 3 were prepared by a soil state ceramic method. The effect of these dopants on densification, crystal structure and microstructure structure have been studied. The limit of solid solution formation in lanthanum, nickel and chromium doped barium stannate is up to x F 0.10. Crystal structure of all the samples in various doped systems remains cubic similar too undoped BaSnO 3. Nickel doping seems to promote densification whereas lanthanum and chromium seems to inhibit it. For low values of x, average grain size of the samples in the doped systems is similar to that in barium stannate while for higher x it becomes smaller than 1 mm. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Perovskite; Barium stannate; Solid solution
1. Introduction Properties of stannates having the general formula MSnO 3 Žwhere M s Ca, Sr and Ba. have not been studied in detail through their structure are quite similar to titanates. Wainer paid attention to these materials in 1946 when he used ceramic dielectric bodies comprised of alkaline earth titanates in a series of patents w1,2x. It has been reported that CaSnO 3 , SrSnO 3 and BaSnO 3 are sensitive to humidity and oxygen gas w3,4x. La-doped SrSnO 3 is also used as a humidity sensor and its sensing properties enhance with increasing concentration of lanthanum w5x. Thin films of BaSnO 3 deposited on Al 2 O 3 substrate are used for sensing oxygen, carbon monoxide and oxides of nitrogen w6,7x.
)
Corresponding author. E-mail address: [email protected] ŽO. Parkash..
Despite the recent interest in the use of stannates as ceramic dielectric bodies, humidity sensing, gas sensing, cell devices, and structural similarity with well understood alkaline earth titanates BaTiO 3 and SrTiO 3 , relatively little has been published regarding the effect of donor and acceptor substitutions on crystal structure, microstructure, electrical and dielectric behavior of these stannates. The properties of perovskite oxide ŽABO 3 . ceramics are tailored by isovalent or heterovalent substitution of foreign metal ions on A or B site independently or simultaneously. The heterovalent donor or acceptor ions are compensated by formation of electronic or ionic defects, which in turn control the properties of the resulting material. In a series of investigations, we have synthesized and characterized undoped, independent donor La3q and acceptors Ni 3q or Cr 3q doped BaSnO 3 systems w8x. In this paper, the limit of solid solution formation, crystal structure and microstructure of lanthanum, nickel and chromium doped bar-
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 7 8 - 5
252
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
ium stannate systems have been reported. To the best of our knowledge, these materials have not been studied so far. Results on undoped barium stannate have also been reported for comparison. 2. Experimental Samples in the undoped and doped barium stannate, BaSnO 3 , Ba 1yx La x SnO 3 , BaSn 1yx Ni xO 3 and BaSn 1y x Cr xO 3 were prepared by solid state ceramic method. The compounds used for preparation of these materials were BaCO 3 Ž99.5%., SnO 2 Ž99.9%., lanthanum oxalate Ž99.9%., basic nickel carbonate Ž99.95%. and Cr2 O 3 Ž99.6%.. The number within parentheses indicates %purity of materials. Laoxalate is used in place of lanthanum oxide on account of hygroscopic nature of the later compound; %lanthanum oxide in lanthanum oxalate is estimated by heating it at 9008C for 4 h and then weighing it as La 2 O 3 . Stoichiometric amounts of respective compounds were mixed in a ball mill for 6 h using acetone as the
mixing media. The mixed powders were dried in an oven overnight. The dry mixed powders were calcined in a platinum crucible at 1525 K for 8 h in air. The calcined powders were ground, mixed and pelletized Ža die of 12-mm dia was used for 2–3-mmthick pellets.. These pellets were kept in a platinum crucible and sintered at 1625 K for 12 h. Initially, pellets were heated slowly at a rate of 28Crmin to 725 K. They were maintained at this temperature for 1 h to burn off the binder. After this, pellets were heated at a rate of 48Crmin to the sintering temperature, 1625 K. The pellets were sintered at this temperature for 12 h. After sintering, pellets were cooled to 875 K using a cooling rate of 58Crmin with the help of a programmable temperature controller ŽCentury System Model Cr-7533.. In order to increase density and homogeneity, the process of sintering was repeated. For determination of solid solution solubility limit, X-ray diffraction ŽXRD. patterns of the powders of finally sintered pellets were recorded with a diffractometer ŽRigaku Rotoflex TRC-300. employing
Fig. 1. Powder X-ray diffraction ŽXRD. patterns of the samples Ža. BaSnO 3 , Žb.Ba 0.95 La 0.05 SnO 3 , Žc. BaSn 0.90 Ni 0.10 O 3 and Žd. BaSn 0.90 Cr0.10 O 3 .
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
CuK a 1 radiation. True density or theoretical density is determined from the molecular weight of the sample divided by volume of the unit cell. Bulk density of the material is determined from the mass of the pellet divided by geometrical volume of the pellet; %total porosity of the samples was calculated from the theoretical and measured bulk density. The %porosity of these samples is high, therefore, polishing of the surfaces of the pellets becomes difficult. For scanning electron microscopic studies, freshly fractured surfaces of these pellets were coated with Au–Pd alloy by sputtering method in a sputtering unit ŽCIKO-800, Japan.. Scanning electron micrographs were recorded at different magnifications with the help of a scanning electron microscope ŽJeol JSM 840 A..
253
3. Results and discussion Powder X-ray diffraction patterns ŽXRD. of the representative samples BaSnO 3 , Ba 0.95 La 0.05 SnO 3 , BaSn 0.90 Ni 0.10 O 3 and BaSn 0.90 Cr0.10 O 3 of different systems are shown in Fig. 1. Single-phase solid solution forms for x F 0.10 in the systems BaSn 1yx Ni xO 3 and BaSb 1yx Cr xO 3 whereas in Ba 1y x La x SnO 3 system it forms for x F 0.05. Powder X-ray diffraction of undoped barium stannate is indexed on the basis of the cubic unit cell reported in the literature w9x. Lattice parameter is found to be ˚ which is in good agreement with 4.123 " 0.001 A, the value reported in the literature w10x. Powder X-ray diffraction patterns of higher compositions Ž x s 0.15 and 0.20. in the system Ba 1y x La x SnO 3
Fig. 2. Scanning electron micrographs ŽSEM. of fractured surfaces of samples Ža. BaSnO 3 , Žb.Ba 0.999 La 0.001 SnO 3 , Žc. BaSn 0.90 Ni 0.10 O 3 and Žd. BaSn 0.90 Cr0.10 O 3 .
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
254
show diffraction lines corresponding to the planes of ˚ phase La 2 Sn 2 O 7 . The intensity of the pyrochloro A diffraction lines of the La 2 Sn 2 O 7 phase increases with increasing lanthanum concentration and sintering temperature. For Ni-doped system, higher compositions Ž x s 0.20 and 0.30. show diffraction lines of NiO. In these systems, all the single-phase compositions have a cubic unit cell similar to undoped BaSnO 3 . Lattice parameters of these samples are given in Table 1. Lattice parameter decreases with increasing x in the systems Ba 1y x La x SnO 3 and BasN1y x Cr xO 3 whereas it remains constant in the system BaSn 1y x Ni xO 3 . Solid solubility limit in these doped barium stannate system is low. Limited solubility is due to heterovalent substitution, which gives various defects and hence limited solubility. Decrease of lattice parameter in these systems can be due to smaller ionic radii of dopant ions La3q Ž1.32 ˚ ., Ni 3q Žlow spin—0.56 A, ˚ high spin—0.60 A˚ . A ˚ . in comparison to the host ions and Cr 3q Ž0.615 A ˚ . and Sn4q Ž0.69 A˚ . w11x. Almost Ba2q Ž1.44 A constant lattice parameter in the system BaSn 1y xNi xO 3 may be due to the presence of nickel in q2 ˚ . and Sn4q state because ionic radii of Ni 2q Ž0.70 A ˚ Ž0.69 A. are approximately the same. Theoretical and experimental bulk density of all the samples have been calculated by the procedure described in Section 2. Percentage total porosity is calculated from the theoretical and experimental bulk density. Theoretical density, experimental bulk density and %porosity of all the samples are given in
Table 1. It is observed that %porosity in undoped BaSnO 3 is 23. In order to improve the density, sintered pellets were fired again at 1625, 1650 and 1675 K successively for 12 h. This treatment resulted only in nominal improvement of the bulk density. Finally, %porosity is found to be 21. From Table 1, it is observed that La and Cr doping increases the %porosity. Moreover, in these systems %porosity is almost independent of concentration of dopants. It is observed that in the nickel doped barium stannate system BaSn 1y x Ni xO 3 , %porosity decreases with increasing x. Electron Spin Resonance ŽESR. studies indicate that nickel exists predominantly in divalent state and chromium exists in trivalent state. The stable oxidation state of nickel is q2 and that of chromium is q4 at high temperature of sintering w12x. Presence of Ni 2q on Sn4q site leads to generation of oxygen vacancies to maintain the change neutrality as follows: BaO q NiO ™ Ba Ba q NiYSn q VOv v , where all the species are written in accordance with Kroger–Vink notation w13x. On the other hand, the presence of Cr 4q on Sn4q site at high temperature does not lead to generation of oxygen vacancies. The increasing densification in BaS ny x Ni xO 3 system with increasing x shows that diffusion of oxygen plays an important role in the sintering of these materials. The poor sintering of the samples in the BaSn 1y x Cr xO 3 system seems to be due to insignificant concentration of oxygen vacancies. Diffusion of oxygen has been
Table 1 Lattice parameter, theoretical density, experimental bulk density, %porosity and grain size of the samples in different doped barium stannate systems System
Ba 1y x La x SnO 3
BaSn 1y x Ni x O 3
BaSn 1y x Cr x O 3
Composition Ž x.
Lattice parameter ˚. ŽA
0.00 0.001 0.01 0.05 0.01 0.05 0.10 0.01 0.05 0.10
4.123 " 0.001 4.120 " 0.002 4.116 " 0.002 4.109 " 0.001 4.122 " 0.001 4.121 " 0.001 4.120 " 0.001 4.119 " 0.001 4.116 " 0.002 4.113 " 0.003
Theoretical
Experimental
%
density Žgrcm3 .
density Žgrcm3 .
Porosity
Grain size Žmm.
7.20 7.26 7.17 7.08 7.27 7.22 7.13 7.12 7.08 7.06
5.60 4.88 4.79 4.63 5.61 6.10 6.43 4.93 4.98 5.03
21 33 31 30 19 16 10 31 30 30
;4 ;3 -1 -1 ;1 ;1 ;1 ;3 ;2 ;2
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
reported to control the sintering behaviour of an analogous compound Ba 2 SnO4 w14x. The other reason for the difference in the sintering behaviour of Ni, Cr and La doped systems may be due to higher sintering temperature required for the chromium and lanthanum doped materials. Zinc stannate has been reported to sinter by evaporation and condensation mechanism w15x. This mechanism leads to sintering without densification. Higher porosity in the present materials may be because of this mode of sintering. Scanning electron micrographs of the freshly fractured surfaces of the samples BaSnO 3 , Ba 0.999La 0.001 SnO 3 are shown in Fig. 2. Approximate grain size for all the compositions in various systems is given in Table 1. Scanning electron micrograph of BaSnO 3 shows that grains are in the range 2–4 mm. In La-doped system microstructure of the sample with composition x s 0.001, shows grains in the range 3–6 mm. Grain size of the Ni doped samples is smaller as compared to undoped BaSnO 3 . In Crdoped system the average grain size of the composition with x s 0.01 is almost of the same size as in undoped BaSnO 3 but of higher compositions is smaller than that of BaSnO 3 . In all the systems grain size decreases with increasing x. Heterovalent dopants Ždonors as well as acceptors. have been reported to segregate to the grain boundaries in perovskite titanates w16x. The segregation of dopants at the grain boundaries inhibit the grain growth. Decrease in the average grain size with increasing x in all the systems investigated in this study can be accounted for by the segregation of dopants at the grain boundaries. Slight differences in average grain size observed in different systems may be due to different segregation tendencies of different ions.
Acknowledgements The authors are grateful to Prof. D. Pandey, School of Materials Science and Technology, BHU,
255
Varanasi, India, and Prof. T.K. Gundarao, IIT, Powai, Mumbai, India, for providing XRD facilities and recording ESR spectra of the samples, respectively. One of the authors, Ms. Shail Upadhyay, is grateful to University Grant Commission ŽUGC., New Delhi, India, for financial support in terms of fellowship. Financial support from Department of Science and Technology, Government of India, New Delhi, is gratefully acknowledged.
References w1x E. Wainer, High dielectric materials and method of making, Ža. US Patent 240 2515, June 18, 1946; Ceram. Abstr. 143 Ž1946.; Žb. US Patent 240 2518, June 19 Ž1946.; Ceram. Abstr. 143 Ž1946.. w2x E. Wainer, High dielectric materials and method of making, US Patent 2432532 Oct. 26, Ceram. Abstr. 1109. w3x Y. Shimizu, M. Shimabukuro, H. Arai, T. Seiyama, Chem. Lett. Ž1985. 917. w4x Y. Shimizu, Y. Fukuyama, T. Narikiyo, H. Arai, T. Seiyama, Chem. Lett. Ž1985. 377. w5x Y. Shimizu, M. Shimabukuro, H. Arai, T. Seiyama, J. Electrochem. Soc. 136 Ž1989. 1206. w6x U. Lampe, J. Gebringer, H. Meixner, Sens. Actuators, B 24–25 Ž1995. 657. w7x U. Lampe, J. Geblinger, H. Meixner, Sens. Actuators, B 26–27 Ž1995. 97. w8x S. Upadhyay, PhD Dissertation, Institute of Technology, Banaras Hindu University, India, December, 1998. w9x H.D. Megaw, Proc. Phys. Soc., London 58 Ž1946. 133. w10x A.J. Smith, J.E. Welch, Acta Crystallogr. 13 Ž1960. 653. w11x R.D. Shannan, C.T. Prewitt, Acta Crystallogr., B 25 Ž1969. 925. w12x Y.M. Chaing, T. Takashi, J. Am. Ceram. Soc. 73 Ž1990. 3278. w13x F.A. Kroger, H.J. Vink, in: F. Seitz, D. Turnbull ŽEds.., Solid State Physics, vol. 3, Academic Press, New York, 1976, 307. w14x T. Hashemi, D.L. Hales, A.W. Brinkman, Br. Ceram. Trans. J. 91 Ž1992. 150. w15x T. Heshmi, H.M. Al-Allak, J. Illingsworth, A.W. Brinkman, J. Woods, J. Mater. Sci. Lett. 9 Ž1990. 776. w16x S.B. Desu, D.A. Payne, J. Am. Ceram. Soc. 73 Ž1990. 3398.
Materials Letters 49 Ž2001. 251–255 www.elsevier.comrlocatermatlet
Solubility of lanthanum, nickel and chromium in barium stannate Shail Upadhyay a , Om Parkash b,) , Devendra Kumar b a
School of Materials Science and Technology, Institute of Technology, Banaras Hindu UniÕersity, Varanasi-221 005, India b Department of Ceramic Engineering, Institute of Technology, Banaras Hindu UniÕersity, Varanasi-221 005, India Received 14 January 2000; received in revised form 26 October 2000; accepted 31 October 2000
Abstract Undoped, donor doped and acceptor doped barium stannate systems viz Ba 1yx La x SnO 3 , and BaSn 1yx Cr xO 3 were prepared by a soil state ceramic method. The effect of these dopants on densification, crystal structure and microstructure structure have been studied. The limit of solid solution formation in lanthanum, nickel and chromium doped barium stannate is up to x F 0.10. Crystal structure of all the samples in various doped systems remains cubic similar too undoped BaSnO 3. Nickel doping seems to promote densification whereas lanthanum and chromium seems to inhibit it. For low values of x, average grain size of the samples in the doped systems is similar to that in barium stannate while for higher x it becomes smaller than 1 mm. q 2001 Elsevier Science B.V. All rights reserved. Keywords: Perovskite; Barium stannate; Solid solution
1. Introduction Properties of stannates having the general formula MSnO 3 Žwhere M s Ca, Sr and Ba. have not been studied in detail through their structure are quite similar to titanates. Wainer paid attention to these materials in 1946 when he used ceramic dielectric bodies comprised of alkaline earth titanates in a series of patents w1,2x. It has been reported that CaSnO 3 , SrSnO 3 and BaSnO 3 are sensitive to humidity and oxygen gas w3,4x. La-doped SrSnO 3 is also used as a humidity sensor and its sensing properties enhance with increasing concentration of lanthanum w5x. Thin films of BaSnO 3 deposited on Al 2 O 3 substrate are used for sensing oxygen, carbon monoxide and oxides of nitrogen w6,7x.
)
Corresponding author. E-mail address: [email protected] ŽO. Parkash..
Despite the recent interest in the use of stannates as ceramic dielectric bodies, humidity sensing, gas sensing, cell devices, and structural similarity with well understood alkaline earth titanates BaTiO 3 and SrTiO 3 , relatively little has been published regarding the effect of donor and acceptor substitutions on crystal structure, microstructure, electrical and dielectric behavior of these stannates. The properties of perovskite oxide ŽABO 3 . ceramics are tailored by isovalent or heterovalent substitution of foreign metal ions on A or B site independently or simultaneously. The heterovalent donor or acceptor ions are compensated by formation of electronic or ionic defects, which in turn control the properties of the resulting material. In a series of investigations, we have synthesized and characterized undoped, independent donor La3q and acceptors Ni 3q or Cr 3q doped BaSnO 3 systems w8x. In this paper, the limit of solid solution formation, crystal structure and microstructure of lanthanum, nickel and chromium doped bar-
00167-577Xr01r$ - see front matter q 2001 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X Ž 0 0 . 0 0 3 7 8 - 5
252
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
ium stannate systems have been reported. To the best of our knowledge, these materials have not been studied so far. Results on undoped barium stannate have also been reported for comparison. 2. Experimental Samples in the undoped and doped barium stannate, BaSnO 3 , Ba 1yx La x SnO 3 , BaSn 1yx Ni xO 3 and BaSn 1y x Cr xO 3 were prepared by solid state ceramic method. The compounds used for preparation of these materials were BaCO 3 Ž99.5%., SnO 2 Ž99.9%., lanthanum oxalate Ž99.9%., basic nickel carbonate Ž99.95%. and Cr2 O 3 Ž99.6%.. The number within parentheses indicates %purity of materials. Laoxalate is used in place of lanthanum oxide on account of hygroscopic nature of the later compound; %lanthanum oxide in lanthanum oxalate is estimated by heating it at 9008C for 4 h and then weighing it as La 2 O 3 . Stoichiometric amounts of respective compounds were mixed in a ball mill for 6 h using acetone as the
mixing media. The mixed powders were dried in an oven overnight. The dry mixed powders were calcined in a platinum crucible at 1525 K for 8 h in air. The calcined powders were ground, mixed and pelletized Ža die of 12-mm dia was used for 2–3-mmthick pellets.. These pellets were kept in a platinum crucible and sintered at 1625 K for 12 h. Initially, pellets were heated slowly at a rate of 28Crmin to 725 K. They were maintained at this temperature for 1 h to burn off the binder. After this, pellets were heated at a rate of 48Crmin to the sintering temperature, 1625 K. The pellets were sintered at this temperature for 12 h. After sintering, pellets were cooled to 875 K using a cooling rate of 58Crmin with the help of a programmable temperature controller ŽCentury System Model Cr-7533.. In order to increase density and homogeneity, the process of sintering was repeated. For determination of solid solution solubility limit, X-ray diffraction ŽXRD. patterns of the powders of finally sintered pellets were recorded with a diffractometer ŽRigaku Rotoflex TRC-300. employing
Fig. 1. Powder X-ray diffraction ŽXRD. patterns of the samples Ža. BaSnO 3 , Žb.Ba 0.95 La 0.05 SnO 3 , Žc. BaSn 0.90 Ni 0.10 O 3 and Žd. BaSn 0.90 Cr0.10 O 3 .
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
CuK a 1 radiation. True density or theoretical density is determined from the molecular weight of the sample divided by volume of the unit cell. Bulk density of the material is determined from the mass of the pellet divided by geometrical volume of the pellet; %total porosity of the samples was calculated from the theoretical and measured bulk density. The %porosity of these samples is high, therefore, polishing of the surfaces of the pellets becomes difficult. For scanning electron microscopic studies, freshly fractured surfaces of these pellets were coated with Au–Pd alloy by sputtering method in a sputtering unit ŽCIKO-800, Japan.. Scanning electron micrographs were recorded at different magnifications with the help of a scanning electron microscope ŽJeol JSM 840 A..
253
3. Results and discussion Powder X-ray diffraction patterns ŽXRD. of the representative samples BaSnO 3 , Ba 0.95 La 0.05 SnO 3 , BaSn 0.90 Ni 0.10 O 3 and BaSn 0.90 Cr0.10 O 3 of different systems are shown in Fig. 1. Single-phase solid solution forms for x F 0.10 in the systems BaSn 1yx Ni xO 3 and BaSb 1yx Cr xO 3 whereas in Ba 1y x La x SnO 3 system it forms for x F 0.05. Powder X-ray diffraction of undoped barium stannate is indexed on the basis of the cubic unit cell reported in the literature w9x. Lattice parameter is found to be ˚ which is in good agreement with 4.123 " 0.001 A, the value reported in the literature w10x. Powder X-ray diffraction patterns of higher compositions Ž x s 0.15 and 0.20. in the system Ba 1y x La x SnO 3
Fig. 2. Scanning electron micrographs ŽSEM. of fractured surfaces of samples Ža. BaSnO 3 , Žb.Ba 0.999 La 0.001 SnO 3 , Žc. BaSn 0.90 Ni 0.10 O 3 and Žd. BaSn 0.90 Cr0.10 O 3 .
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
254
show diffraction lines corresponding to the planes of ˚ phase La 2 Sn 2 O 7 . The intensity of the pyrochloro A diffraction lines of the La 2 Sn 2 O 7 phase increases with increasing lanthanum concentration and sintering temperature. For Ni-doped system, higher compositions Ž x s 0.20 and 0.30. show diffraction lines of NiO. In these systems, all the single-phase compositions have a cubic unit cell similar to undoped BaSnO 3 . Lattice parameters of these samples are given in Table 1. Lattice parameter decreases with increasing x in the systems Ba 1y x La x SnO 3 and BasN1y x Cr xO 3 whereas it remains constant in the system BaSn 1y x Ni xO 3 . Solid solubility limit in these doped barium stannate system is low. Limited solubility is due to heterovalent substitution, which gives various defects and hence limited solubility. Decrease of lattice parameter in these systems can be due to smaller ionic radii of dopant ions La3q Ž1.32 ˚ ., Ni 3q Žlow spin—0.56 A, ˚ high spin—0.60 A˚ . A ˚ . in comparison to the host ions and Cr 3q Ž0.615 A ˚ . and Sn4q Ž0.69 A˚ . w11x. Almost Ba2q Ž1.44 A constant lattice parameter in the system BaSn 1y xNi xO 3 may be due to the presence of nickel in q2 ˚ . and Sn4q state because ionic radii of Ni 2q Ž0.70 A ˚ Ž0.69 A. are approximately the same. Theoretical and experimental bulk density of all the samples have been calculated by the procedure described in Section 2. Percentage total porosity is calculated from the theoretical and experimental bulk density. Theoretical density, experimental bulk density and %porosity of all the samples are given in
Table 1. It is observed that %porosity in undoped BaSnO 3 is 23. In order to improve the density, sintered pellets were fired again at 1625, 1650 and 1675 K successively for 12 h. This treatment resulted only in nominal improvement of the bulk density. Finally, %porosity is found to be 21. From Table 1, it is observed that La and Cr doping increases the %porosity. Moreover, in these systems %porosity is almost independent of concentration of dopants. It is observed that in the nickel doped barium stannate system BaSn 1y x Ni xO 3 , %porosity decreases with increasing x. Electron Spin Resonance ŽESR. studies indicate that nickel exists predominantly in divalent state and chromium exists in trivalent state. The stable oxidation state of nickel is q2 and that of chromium is q4 at high temperature of sintering w12x. Presence of Ni 2q on Sn4q site leads to generation of oxygen vacancies to maintain the change neutrality as follows: BaO q NiO ™ Ba Ba q NiYSn q VOv v , where all the species are written in accordance with Kroger–Vink notation w13x. On the other hand, the presence of Cr 4q on Sn4q site at high temperature does not lead to generation of oxygen vacancies. The increasing densification in BaS ny x Ni xO 3 system with increasing x shows that diffusion of oxygen plays an important role in the sintering of these materials. The poor sintering of the samples in the BaSn 1y x Cr xO 3 system seems to be due to insignificant concentration of oxygen vacancies. Diffusion of oxygen has been
Table 1 Lattice parameter, theoretical density, experimental bulk density, %porosity and grain size of the samples in different doped barium stannate systems System
Ba 1y x La x SnO 3
BaSn 1y x Ni x O 3
BaSn 1y x Cr x O 3
Composition Ž x.
Lattice parameter ˚. ŽA
0.00 0.001 0.01 0.05 0.01 0.05 0.10 0.01 0.05 0.10
4.123 " 0.001 4.120 " 0.002 4.116 " 0.002 4.109 " 0.001 4.122 " 0.001 4.121 " 0.001 4.120 " 0.001 4.119 " 0.001 4.116 " 0.002 4.113 " 0.003
Theoretical
Experimental
%
density Žgrcm3 .
density Žgrcm3 .
Porosity
Grain size Žmm.
7.20 7.26 7.17 7.08 7.27 7.22 7.13 7.12 7.08 7.06
5.60 4.88 4.79 4.63 5.61 6.10 6.43 4.93 4.98 5.03
21 33 31 30 19 16 10 31 30 30
;4 ;3 -1 -1 ;1 ;1 ;1 ;3 ;2 ;2
S. Upadhyay et al.r Materials Letters 49 (2001) 251–255
reported to control the sintering behaviour of an analogous compound Ba 2 SnO4 w14x. The other reason for the difference in the sintering behaviour of Ni, Cr and La doped systems may be due to higher sintering temperature required for the chromium and lanthanum doped materials. Zinc stannate has been reported to sinter by evaporation and condensation mechanism w15x. This mechanism leads to sintering without densification. Higher porosity in the present materials may be because of this mode of sintering. Scanning electron micrographs of the freshly fractured surfaces of the samples BaSnO 3 , Ba 0.999La 0.001 SnO 3 are shown in Fig. 2. Approximate grain size for all the compositions in various systems is given in Table 1. Scanning electron micrograph of BaSnO 3 shows that grains are in the range 2–4 mm. In La-doped system microstructure of the sample with composition x s 0.001, shows grains in the range 3–6 mm. Grain size of the Ni doped samples is smaller as compared to undoped BaSnO 3 . In Crdoped system the average grain size of the composition with x s 0.01 is almost of the same size as in undoped BaSnO 3 but of higher compositions is smaller than that of BaSnO 3 . In all the systems grain size decreases with increasing x. Heterovalent dopants Ždonors as well as acceptors. have been reported to segregate to the grain boundaries in perovskite titanates w16x. The segregation of dopants at the grain boundaries inhibit the grain growth. Decrease in the average grain size with increasing x in all the systems investigated in this study can be accounted for by the segregation of dopants at the grain boundaries. Slight differences in average grain size observed in different systems may be due to different segregation tendencies of different ions.
Acknowledgements The authors are grateful to Prof. D. Pandey, School of Materials Science and Technology, BHU,
255
Varanasi, India, and Prof. T.K. Gundarao, IIT, Powai, Mumbai, India, for providing XRD facilities and recording ESR spectra of the samples, respectively. One of the authors, Ms. Shail Upadhyay, is grateful to University Grant Commission ŽUGC., New Delhi, India, for financial support in terms of fellowship. Financial support from Department of Science and Technology, Government of India, New Delhi, is gratefully acknowledged.
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