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Microwave assisted synthesis and sintering of La0.8Sr0.2Ga0.83Mg0.17O2.815
Materials Letters 57 (2003) 1792 – 1797 www.elsevier.com/locate/matlet
Microwave assisted synthesis and sintering of La0.8Sr0.2Ga0.83Mg0.17O2.815 R. Subasri a,*, Tom Mathews b, O.M. Sreedharan b a
Pulvermetallurgisches Laboratorium, Max-Planck-Institut fu¨r Metallforschung, Heisenbergstrasse 3, Stuttgart D-70569, Germany b Thermodynamics and Kinetics Division, Materials Characterization Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu-603 102, India Received 5 July 2002; accepted 22 July 2002
Abstract A novel method to produce dense bodies of SrO- and MgO-doped LaGaO3 (LSGM) ceramics by microwave assisted processing in a very short time of 10 min is reported here. The heating was carried out using Na-hW-alumina as the microwave susceptor. The phase purity and electrical properties of the microwave processed samples were determined and compared with a conventionally processed material. It was found that LSGM with a composition La0.8Sr0.2Ga0.83Mg0.17O2.815 always yielded a two-phase mixture though the presence of second phase did not affect the conductivity significantly at lower temperatures. The microstructure was found to be more uniform in case of the microwave sintered material. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave processing; Microwave susceptor; LSGM; Solid oxide; Fuel cells; Electrical conductivity; Sintering; Electroceramics; Perovskites
1. Introduction Since the discovery of superior oxide-ion conductivity in perovskite-structured doped LaGaO3 materials, there has been unabating research on the use of different methods for the synthesis of this class of compounds and their characterization [1 –6]. Efforts are ongoing to replace the conventional yttria-stabilized zirconia (YSZ) by SrO- and MgO-doped LaGaO3 (usually abbreviated as LSGM) for use as
*
Corresponding author. Tel.: +49-711-6893102; fax: +49-7116893131. E-mail address: [email protected] (R. Subasri).
solid electrolyte in the intermediate temperature solid oxide fuel cells because of its conducive electrical and chemical properties. An extensive investigation on the various compositions of the La1 xSrxGa1 yMgy O3 (x + y)/2 by varying x and y that would yield a single-phase material was carried out by Huang et al. [7]. It was found that the compositions with x = y = 0.20 or more always yielded a multiphasic substance, and when x + y < 0.35, a single-phase material was formed. The electrical properties of all such compositions were also determined and it was found that La0.8Sr0.2Ga0.83Mg0.17O2.815 was the material that exhibited the highest conductivity (r = 0.166 at 1073 K and 0.079 S/cm at 973 K) and also formed a single phase. Amongst the various methods available for
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 7 0 - 4
R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
ceramic processing, the use of microwaves as energy source for synthesis and sintering of ceramics has aroused substantial amount of interest amongst materials scientists due to its unique advantages, namely, shorter processing times and the superior quality of the samples obtained thereafter [8 – 12]. Recently, it was reported that the sodium beta alumina family of compounds are very good absorbers of microwaves and that sodium beta alumina could be used as a microwave susceptor to heat ceramics that do not directly couple with microwaves [13 – 15]. In view of the increasing importance to produce dense LSGM ceramics for use as oxide solid electrolytes, an investigation on the microwave assisted synthesis and sintering of La0.8Sr0.2Ga0.83Mg0.17O2.815 (the composition which was reported to exhibit the highest conductivity) using Na-hW-alumina as the susceptor was carried out and its electrical conductivity measured for comparison with the properties of a conventionally sintered material. This paper reports for the first time an attempt to produce dense LSGM ceramic of the composition La0.8Sr0.2Ga0.83Mg0.17O2.815 by microwave assisted heating and its characterization.
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glycol added was half the number of moles of citric acid. The resultant solution was then slowly evaporated over a hot plate at f 453 K in order to form the gel. The gel so formed was dried in an oven and decomposed at f 1023 K, which resulted in a fine precursor powder. The precursor was then compacted into solid disks at 300 MPa for further heat treatment by exposure to microwaves for 10 min using Na-hW-alumina as the microwave susceptor in a domestic microwave oven (BPL Sanyo, 2.45 GHz; 800 W). A flow sheet for the entire process adopted is given in Fig. 1. Other details
2. Experimental 2.1. Synthesis All reagents used were of purity greater than 99.99%. La(NO3)36H2O, Sr(NO3)2, Mg(CH3COO)4 4H2O and Ga metal were used as the starting materials. Strontia- and magnesia-doped lanthanum gallate corresponding to the formula La1.8Sr0.2Ga1.83Mg0.17 O2.815 was prepared through a citrate-gel combustion technique. In order to obtain a solution of gallium nitrate, a known amount of Ga metal was dissolved in a minimum quantity of concentrated HNO3 and a standard solution was made from which the required amount of Ga(NO3)3 was pipetted out for the synthesis of the precursor. Appropriate amounts of the salts lanthanum nitrate, strontium nitrate and magnesium acetate were dissolved in distilled water separately and homogenised. To this solution, citric acid was added as a complexant and refluxed for 1 h. The amount of citric acid added was three times the total number of moles of cations. To the refluxed solution, ethylene glycol was added to enhance gelation. The amount of ethylene
Fig. 1. Flow sheet for the microwave assisted synthesis cum sintering of LSGM.
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R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
regarding the microwave set-up are as given elsewhere [13]. In addition, another pellet of same precursor was heat treated in a furnace at 1793 K for 12 h in air in order to compare the phase purity and electrical properties of the microwave processed material with that of the conventionally processed one. 2.2. Characterization The X-ray diffraction (XRD) pattern of the products (microwave as well as conventionally heated) in the form of powder as well as solid disks were obtained using a Rigaku model desk top diffractometer using Cu Ka as the incident radiation. In order to measure the conductivity of the samples, the parallel surfaces of the microwave sintered sample were coated with platinum paste (Demetron 308 A, Germany) and fired at 1273 K in air for 5 min in order to form a thin, adherent layer of platinum which acted as the electrode reversible to O2 . An AC amplitude of 10 mV was applied and the response of the sample to the applied voltage over a frequency range of 1 MHz to 1 Hz was measured as a function of temperature in an atmosphere of flowing air. The resistance of the leads (platinum in this case) was measured without the sample and deducted from that of the sample. The measurements were performed for different heating and cooling cycles. The same procedure was repeated for the conventionally sintered sample. Scanning electron micrographs were obtained using a JEOL 6300F microscope using 3 and 5 kV as the accelerating voltage of the electron beam.
3. Results and discussion After exposure to microwaves, the temperature of the sample raised immediately to nearly 1773 K (adjudged from the colour of the sample) just after 10 min when tuned at full power. Though no attempt was made to measure the exact temperature of the sample, it was known from previous investigations that the susceptor (Na-hW-alumina) was capable of reaching very high temperatures ( f 2273 K) within a short period of 30 min when operated at full power. The time of exposure in case of LSGM was optimised to 10 min after performing trial runs by exposing the samples for a longer time when the samples actually
melted. The colour of the precursor powder was light pink and it changed to dark brown after microwave assisted heating, which indicates that the desired product (LSGM) was formed and simultaneously, it was obtained in a dense form. The XRD patterns of the products in both the cases, as depicted in Fig. 2, show the presence of two phases, namely, LaGaO3 and LaSrGa3O7. Recently, the effect of doping and processing conditions on the properties of La1 xSrx Ga1 yMgyO3 d was reported by Gorelov et al [16]. These authors report that the solubility limit of Sr and Mg is f 16 mol% and if excess of either Sr or Mg is added, LaSrGa3O7 and/or La4Ga2O9 phases would be formed in addition to the major LaGaO3 phase. An optimum sintering temperature of 1793 K was found to yield bodies with maximum density. In accordance with their observation, the sample whether heated conventionally at 1793 K or using microwaves showed presence of a second phase (LaSrGa3O7). Hence, it could be concluded that the solubility limits reported by Gorelov et al. should be considered reliable and the composition La0.8Sr0.2Ga0.83Mg0.17 O2.815 may not yield a single-phase material. The pycnometric densities of the samples were measured to be 94% of theoretical values. The impedance plots for the samples ( ZWvs. ZV) showed two semicircles. The intersection of the first semicircle with the x-axis gave the bulk resistance of the sample. After correction for the resistance of the leads, the conductivity of the sample was determined and a plot of log r against reciprocal temperature is depicted in Fig. 3. A comparison of the conductivities of the samples prepared by both routes along with the data reported by Gorelov et al. for different compositions are made. In addition, it can be seen that at lower temperatures (T < 833 K), the conductivity of the microwave sintered sample is comparable to that of the conventionally heated sample, whereas at temperatures greater than 833 K, the conductivity of the microwave sintered sample is lower than that of the conventionally sintered one (0.0196 when compared to 0.0246 S/cm at 900 K). The coefficients of the linear fit of the conductivity values as a function of inverse temperature in both of the cases are presented in Table 1. A lower activation energy of 87.5 kJ/mol could be determined for the microwave sintered material at lower temperatures when compared to a value of 96.2 kJ/mol in the case of conventionally sintered material valid for the same
R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
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Fig. 2. Comparison of the XRD patterns of the microwave sintered material (pellet) with that of a conventionally sintered one at 1793 K.
Fig. 3. Comparison of conductivity data of microwave and conventionally sintered samples with that from Ref. [16].
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Table 1 Coefficients of the linear fit of the conductivity data as a function of inverse temperature [log rT (V 1 cm 1 K) = A + 1000B/T (K)] for the microwave sintered and the conventionally sintered samples Sample
A
B
Microwave sintered Conventionally sintered
6.324 6.928
4.569 5.026
temperature range (715 – 950 K) under consideration. A comparison of the conductivity values from the present studies with those reported for different compositions by Gorelov et al. shows that the composition with 15 mol% of Sr and Mg (a single-phase material) exhibits the highest conductivity value that is comparable to those obtained for the conventionally sintered sample. Hence, it could be concluded that though the material under consideration in the present investigation could not be obtained as a single phase, the presence of second phases does not affect its conductivity at temperatures below f 850 K and still gives a high conductivity comparable to a single-phase material exhibiting the maximum conductivity reported in the literature. The microwave sintered material gave lower values of conductivity at higher temperatures, which means that the mechanism of conduction that is operating at such temperatures must be different from the predicted one at such temperatures. A possible explanation could be offered by comparison of the scanning electron micrographs of the conventionally sintered sample with that of the microwave sintered
one (cf. Figs. 4 and 5). The average grain size of the microwave sintered sample could be determined to be 4 Am and the same for the conventionally sintered sample to be 10 Am. The grains in the former case are found to be more uniform and this could offer a better mechanical strength to the samples. However, on careful examination of Fig. 6, one could observe that there are small islands of the major phase in pockets of secondary phases which could be deleterious to the oxide ion conductivity at high temperatures. However, further studies are required to probe into the exact mechanism of conduction of a microwave sintered material at higher temperatures though at lower temperatures, the conductivity values with the conventionally sintered one are comparable.
Fig. 4. Scanning electron micrograph of conventionally sintered LSGM at 1793 K in air.
Fig. 6. Scanning electron micrograph of microwave sintered LSGM showing secondary phases (E) including the major perovskite phase (.) at some spots.
Fig. 5. Scanning electron micrograph of microwave sintered LSGM for 10 min.
R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
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4. Conclusion
References
It could be concluded from the present studies that a microwave assisted synthesis and sintering of LSGM could be successfully accomplished in a very short period of 10 min using sodium hW alumina as the microwave susceptor yielding 94% dense ceramic bodies. This method of obtaining dense bodies of LSGM is cost-effective and a timesaving process, which should be considered very valuable from a technology point of view. An analysis of the phases in this material showed that this composition always yielded a two-phase mixture and a comparison of the electrical conductivities of the microwave heated material with the conventionally heated material proved that both of them behaved similarly at lower temperatures, but the microwave heated sample gave lower values of conductivity at T>833 K. The microwave sintered samples exhibited an average grain size of 4 Am.
[1] T. Ishihara, H. Matsuda, Y. Takita, J. Am. Chem. Soc. 116 (1994) 3801. [2] M. Feng, J.B. Goodenough, Eur. J. Solid State Inorg. Chem. T31 (1994) 663. [3] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 79 (4) (1996) 1100. [4] K. Huang, M. Feng, J.B. Goodenough, J. Electrochem. Soc. 143 (1996) 3630. [5] P. Huang, A. Petric, J. Electrochem. Soc. 143 (5) (1996) 1644. [6] K. Huang, J.B. Goodenough, J. Solid State Chem. 136 (1998) 274. [7] K. Huang, R.S. Tichy, J.B. Goodenough, J. Am. Ceram. Soc. 81 (10) (1998) 2565. [8] D.E. Clark, F.D. Gac, W.H. Sutton (Eds.), Microwave Theory and Applications in Materials Processing, Ceram. Trans., vol. 21, ACS, Westerville, OH, USA, 1991. [9] P.D. Ramesh, D. Brandon, L. Scha¨chter, Mater. Sci. Eng. A 266 (1999) 211. [10] R. Roy, D. Agarwal, J. Cheng, S. Genevanishvilli, Nature 399 (1999) 668. [11] J. Cheng, D. Agarwal, S. Komarneni, M. Mathis, R. Roy, Mater. Res. Innov. 1 (1997) 44. [12] W.H. Sutton, Ceram. Bull. 68 (2) (1989) 376. [13] T. Mathews, R. Subasri, O.M. Sreedharan, Solid State Ionics 148 (2002) 135. [14] R. Subasri, T. Mathews, O.M. Sreedharan, V.S. Raghunathan, Solid State Ionics Lett. (communicated). [15] R. Subasri, T. Mathews, K. Swaminathan, O.M. Sreedharan, J. Alloys Compd. (communicated). [16] V.P. Gorelov, D.I. Bronin, Ju.V. Sokolova, H. Na¨fe, F. Aldinger, J. Eur. Ceram. Soc. 21 (2001) 2311.
Acknowledgements The authors are grateful to Dr. V.S. Raghunathan (Associate Director of Materials and Metallurgy Group, IGCAR, Kalpakkam) for the keen interest and constant encouragement provided throughout the course of this investigation. The authors are also grateful to Mrs. S. Ku¨hnemann (Max-Planck-Institut fu¨r Metallforschung, Stuttgart, Germany) for performing the scanning electron microscopy.
Microwave assisted synthesis and sintering of La0.8Sr0.2Ga0.83Mg0.17O2.815 R. Subasri a,*, Tom Mathews b, O.M. Sreedharan b a
Pulvermetallurgisches Laboratorium, Max-Planck-Institut fu¨r Metallforschung, Heisenbergstrasse 3, Stuttgart D-70569, Germany b Thermodynamics and Kinetics Division, Materials Characterization Group, Indira Gandhi Centre for Atomic Research, Kalpakkam, Tamil Nadu-603 102, India Received 5 July 2002; accepted 22 July 2002
Abstract A novel method to produce dense bodies of SrO- and MgO-doped LaGaO3 (LSGM) ceramics by microwave assisted processing in a very short time of 10 min is reported here. The heating was carried out using Na-hW-alumina as the microwave susceptor. The phase purity and electrical properties of the microwave processed samples were determined and compared with a conventionally processed material. It was found that LSGM with a composition La0.8Sr0.2Ga0.83Mg0.17O2.815 always yielded a two-phase mixture though the presence of second phase did not affect the conductivity significantly at lower temperatures. The microstructure was found to be more uniform in case of the microwave sintered material. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave processing; Microwave susceptor; LSGM; Solid oxide; Fuel cells; Electrical conductivity; Sintering; Electroceramics; Perovskites
1. Introduction Since the discovery of superior oxide-ion conductivity in perovskite-structured doped LaGaO3 materials, there has been unabating research on the use of different methods for the synthesis of this class of compounds and their characterization [1 –6]. Efforts are ongoing to replace the conventional yttria-stabilized zirconia (YSZ) by SrO- and MgO-doped LaGaO3 (usually abbreviated as LSGM) for use as
*
Corresponding author. Tel.: +49-711-6893102; fax: +49-7116893131. E-mail address: [email protected] (R. Subasri).
solid electrolyte in the intermediate temperature solid oxide fuel cells because of its conducive electrical and chemical properties. An extensive investigation on the various compositions of the La1 xSrxGa1 yMgy O3 (x + y)/2 by varying x and y that would yield a single-phase material was carried out by Huang et al. [7]. It was found that the compositions with x = y = 0.20 or more always yielded a multiphasic substance, and when x + y < 0.35, a single-phase material was formed. The electrical properties of all such compositions were also determined and it was found that La0.8Sr0.2Ga0.83Mg0.17O2.815 was the material that exhibited the highest conductivity (r = 0.166 at 1073 K and 0.079 S/cm at 973 K) and also formed a single phase. Amongst the various methods available for
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 1 0 7 0 - 4
R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
ceramic processing, the use of microwaves as energy source for synthesis and sintering of ceramics has aroused substantial amount of interest amongst materials scientists due to its unique advantages, namely, shorter processing times and the superior quality of the samples obtained thereafter [8 – 12]. Recently, it was reported that the sodium beta alumina family of compounds are very good absorbers of microwaves and that sodium beta alumina could be used as a microwave susceptor to heat ceramics that do not directly couple with microwaves [13 – 15]. In view of the increasing importance to produce dense LSGM ceramics for use as oxide solid electrolytes, an investigation on the microwave assisted synthesis and sintering of La0.8Sr0.2Ga0.83Mg0.17O2.815 (the composition which was reported to exhibit the highest conductivity) using Na-hW-alumina as the susceptor was carried out and its electrical conductivity measured for comparison with the properties of a conventionally sintered material. This paper reports for the first time an attempt to produce dense LSGM ceramic of the composition La0.8Sr0.2Ga0.83Mg0.17O2.815 by microwave assisted heating and its characterization.
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glycol added was half the number of moles of citric acid. The resultant solution was then slowly evaporated over a hot plate at f 453 K in order to form the gel. The gel so formed was dried in an oven and decomposed at f 1023 K, which resulted in a fine precursor powder. The precursor was then compacted into solid disks at 300 MPa for further heat treatment by exposure to microwaves for 10 min using Na-hW-alumina as the microwave susceptor in a domestic microwave oven (BPL Sanyo, 2.45 GHz; 800 W). A flow sheet for the entire process adopted is given in Fig. 1. Other details
2. Experimental 2.1. Synthesis All reagents used were of purity greater than 99.99%. La(NO3)36H2O, Sr(NO3)2, Mg(CH3COO)4 4H2O and Ga metal were used as the starting materials. Strontia- and magnesia-doped lanthanum gallate corresponding to the formula La1.8Sr0.2Ga1.83Mg0.17 O2.815 was prepared through a citrate-gel combustion technique. In order to obtain a solution of gallium nitrate, a known amount of Ga metal was dissolved in a minimum quantity of concentrated HNO3 and a standard solution was made from which the required amount of Ga(NO3)3 was pipetted out for the synthesis of the precursor. Appropriate amounts of the salts lanthanum nitrate, strontium nitrate and magnesium acetate were dissolved in distilled water separately and homogenised. To this solution, citric acid was added as a complexant and refluxed for 1 h. The amount of citric acid added was three times the total number of moles of cations. To the refluxed solution, ethylene glycol was added to enhance gelation. The amount of ethylene
Fig. 1. Flow sheet for the microwave assisted synthesis cum sintering of LSGM.
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R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
regarding the microwave set-up are as given elsewhere [13]. In addition, another pellet of same precursor was heat treated in a furnace at 1793 K for 12 h in air in order to compare the phase purity and electrical properties of the microwave processed material with that of the conventionally processed one. 2.2. Characterization The X-ray diffraction (XRD) pattern of the products (microwave as well as conventionally heated) in the form of powder as well as solid disks were obtained using a Rigaku model desk top diffractometer using Cu Ka as the incident radiation. In order to measure the conductivity of the samples, the parallel surfaces of the microwave sintered sample were coated with platinum paste (Demetron 308 A, Germany) and fired at 1273 K in air for 5 min in order to form a thin, adherent layer of platinum which acted as the electrode reversible to O2 . An AC amplitude of 10 mV was applied and the response of the sample to the applied voltage over a frequency range of 1 MHz to 1 Hz was measured as a function of temperature in an atmosphere of flowing air. The resistance of the leads (platinum in this case) was measured without the sample and deducted from that of the sample. The measurements were performed for different heating and cooling cycles. The same procedure was repeated for the conventionally sintered sample. Scanning electron micrographs were obtained using a JEOL 6300F microscope using 3 and 5 kV as the accelerating voltage of the electron beam.
3. Results and discussion After exposure to microwaves, the temperature of the sample raised immediately to nearly 1773 K (adjudged from the colour of the sample) just after 10 min when tuned at full power. Though no attempt was made to measure the exact temperature of the sample, it was known from previous investigations that the susceptor (Na-hW-alumina) was capable of reaching very high temperatures ( f 2273 K) within a short period of 30 min when operated at full power. The time of exposure in case of LSGM was optimised to 10 min after performing trial runs by exposing the samples for a longer time when the samples actually
melted. The colour of the precursor powder was light pink and it changed to dark brown after microwave assisted heating, which indicates that the desired product (LSGM) was formed and simultaneously, it was obtained in a dense form. The XRD patterns of the products in both the cases, as depicted in Fig. 2, show the presence of two phases, namely, LaGaO3 and LaSrGa3O7. Recently, the effect of doping and processing conditions on the properties of La1 xSrx Ga1 yMgyO3 d was reported by Gorelov et al [16]. These authors report that the solubility limit of Sr and Mg is f 16 mol% and if excess of either Sr or Mg is added, LaSrGa3O7 and/or La4Ga2O9 phases would be formed in addition to the major LaGaO3 phase. An optimum sintering temperature of 1793 K was found to yield bodies with maximum density. In accordance with their observation, the sample whether heated conventionally at 1793 K or using microwaves showed presence of a second phase (LaSrGa3O7). Hence, it could be concluded that the solubility limits reported by Gorelov et al. should be considered reliable and the composition La0.8Sr0.2Ga0.83Mg0.17 O2.815 may not yield a single-phase material. The pycnometric densities of the samples were measured to be 94% of theoretical values. The impedance plots for the samples ( ZWvs. ZV) showed two semicircles. The intersection of the first semicircle with the x-axis gave the bulk resistance of the sample. After correction for the resistance of the leads, the conductivity of the sample was determined and a plot of log r against reciprocal temperature is depicted in Fig. 3. A comparison of the conductivities of the samples prepared by both routes along with the data reported by Gorelov et al. for different compositions are made. In addition, it can be seen that at lower temperatures (T < 833 K), the conductivity of the microwave sintered sample is comparable to that of the conventionally heated sample, whereas at temperatures greater than 833 K, the conductivity of the microwave sintered sample is lower than that of the conventionally sintered one (0.0196 when compared to 0.0246 S/cm at 900 K). The coefficients of the linear fit of the conductivity values as a function of inverse temperature in both of the cases are presented in Table 1. A lower activation energy of 87.5 kJ/mol could be determined for the microwave sintered material at lower temperatures when compared to a value of 96.2 kJ/mol in the case of conventionally sintered material valid for the same
R. Subasri et al. / Materials Letters 57 (2003) 1792–1797
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Fig. 2. Comparison of the XRD patterns of the microwave sintered material (pellet) with that of a conventionally sintered one at 1793 K.
Fig. 3. Comparison of conductivity data of microwave and conventionally sintered samples with that from Ref. [16].
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Table 1 Coefficients of the linear fit of the conductivity data as a function of inverse temperature [log rT (V 1 cm 1 K) = A + 1000B/T (K)] for the microwave sintered and the conventionally sintered samples Sample
A
B
Microwave sintered Conventionally sintered
6.324 6.928
4.569 5.026
temperature range (715 – 950 K) under consideration. A comparison of the conductivity values from the present studies with those reported for different compositions by Gorelov et al. shows that the composition with 15 mol% of Sr and Mg (a single-phase material) exhibits the highest conductivity value that is comparable to those obtained for the conventionally sintered sample. Hence, it could be concluded that though the material under consideration in the present investigation could not be obtained as a single phase, the presence of second phases does not affect its conductivity at temperatures below f 850 K and still gives a high conductivity comparable to a single-phase material exhibiting the maximum conductivity reported in the literature. The microwave sintered material gave lower values of conductivity at higher temperatures, which means that the mechanism of conduction that is operating at such temperatures must be different from the predicted one at such temperatures. A possible explanation could be offered by comparison of the scanning electron micrographs of the conventionally sintered sample with that of the microwave sintered
one (cf. Figs. 4 and 5). The average grain size of the microwave sintered sample could be determined to be 4 Am and the same for the conventionally sintered sample to be 10 Am. The grains in the former case are found to be more uniform and this could offer a better mechanical strength to the samples. However, on careful examination of Fig. 6, one could observe that there are small islands of the major phase in pockets of secondary phases which could be deleterious to the oxide ion conductivity at high temperatures. However, further studies are required to probe into the exact mechanism of conduction of a microwave sintered material at higher temperatures though at lower temperatures, the conductivity values with the conventionally sintered one are comparable.
Fig. 4. Scanning electron micrograph of conventionally sintered LSGM at 1793 K in air.
Fig. 6. Scanning electron micrograph of microwave sintered LSGM showing secondary phases (E) including the major perovskite phase (.) at some spots.
Fig. 5. Scanning electron micrograph of microwave sintered LSGM for 10 min.
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4. Conclusion
References
It could be concluded from the present studies that a microwave assisted synthesis and sintering of LSGM could be successfully accomplished in a very short period of 10 min using sodium hW alumina as the microwave susceptor yielding 94% dense ceramic bodies. This method of obtaining dense bodies of LSGM is cost-effective and a timesaving process, which should be considered very valuable from a technology point of view. An analysis of the phases in this material showed that this composition always yielded a two-phase mixture and a comparison of the electrical conductivities of the microwave heated material with the conventionally heated material proved that both of them behaved similarly at lower temperatures, but the microwave heated sample gave lower values of conductivity at T>833 K. The microwave sintered samples exhibited an average grain size of 4 Am.
[1] T. Ishihara, H. Matsuda, Y. Takita, J. Am. Chem. Soc. 116 (1994) 3801. [2] M. Feng, J.B. Goodenough, Eur. J. Solid State Inorg. Chem. T31 (1994) 663. [3] K. Huang, M. Feng, J.B. Goodenough, J. Am. Ceram. Soc. 79 (4) (1996) 1100. [4] K. Huang, M. Feng, J.B. Goodenough, J. Electrochem. Soc. 143 (1996) 3630. [5] P. Huang, A. Petric, J. Electrochem. Soc. 143 (5) (1996) 1644. [6] K. Huang, J.B. Goodenough, J. Solid State Chem. 136 (1998) 274. [7] K. Huang, R.S. Tichy, J.B. Goodenough, J. Am. Ceram. Soc. 81 (10) (1998) 2565. [8] D.E. Clark, F.D. Gac, W.H. Sutton (Eds.), Microwave Theory and Applications in Materials Processing, Ceram. Trans., vol. 21, ACS, Westerville, OH, USA, 1991. [9] P.D. Ramesh, D. Brandon, L. Scha¨chter, Mater. Sci. Eng. A 266 (1999) 211. [10] R. Roy, D. Agarwal, J. Cheng, S. Genevanishvilli, Nature 399 (1999) 668. [11] J. Cheng, D. Agarwal, S. Komarneni, M. Mathis, R. Roy, Mater. Res. Innov. 1 (1997) 44. [12] W.H. Sutton, Ceram. Bull. 68 (2) (1989) 376. [13] T. Mathews, R. Subasri, O.M. Sreedharan, Solid State Ionics 148 (2002) 135. [14] R. Subasri, T. Mathews, O.M. Sreedharan, V.S. Raghunathan, Solid State Ionics Lett. (communicated). [15] R. Subasri, T. Mathews, K. Swaminathan, O.M. Sreedharan, J. Alloys Compd. (communicated). [16] V.P. Gorelov, D.I. Bronin, Ju.V. Sokolova, H. Na¨fe, F. Aldinger, J. Eur. Ceram. Soc. 21 (2001) 2311.
Acknowledgements The authors are grateful to Dr. V.S. Raghunathan (Associate Director of Materials and Metallurgy Group, IGCAR, Kalpakkam) for the keen interest and constant encouragement provided throughout the course of this investigation. The authors are also grateful to Mrs. S. Ku¨hnemann (Max-Planck-Institut fu¨r Metallforschung, Stuttgart, Germany) for performing the scanning electron microscopy.