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The orientational growth of grains in doped BaTiO3 PTCR materials by microwave sintering
Journal of Materials Processing Technology 137 (2003) 100–101
The orientational growth of grains in doped BaTiO3 PTCR materials by microwave sintering Aimin Changa,*, Jiawen Jianb a
Xinjiang Institute of Physics, Urumqi 830011, PR China Xinjiang Petroleum Institute, Urumqi 830011, PR China
b
Abstract Doped BaTiO3 (BTO) PTCR materials were sintered by microwave (MW) of 2.45 GHz at 1250 8C for 15 min. For comparison, identical samples ð30 mm 45 mmÞ were also sintered by conventional (CO) SiMo furnace at 1350 8C for 2 h. The experimental results show that the MW-sintered samples have a much greater axial linear shrinkage rate of 48% vs. the rate for the CO-sintered sample of 26%. This result has not yet been reported in other researcher works. Marked differences were also observed in the microstructure of the two samples by SEM, the grains in the CO-sintered sample showing on ecliptic spherical shape, whereas in MW-sintered samples the grains show an orientational striplike microstructure. The XRD analysis indicates that the perovskite structure of MW-sintered BTO is tetragonal, whereas the CO-sintered sample is cubic. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave sintering; Orientational growth; BaTiO3
1. Introduction As a unique technology, microwave (MW) sintering has been applied to the processing of electroceramics in many aspects [1]. The MW-sintering of PTCR materials has been reported for many cases, such as BaTiO3 (BTO)-based ceramics, BTO thick films (Sr0.2Ba0.8)TiO3, (Sr0.4Pb0.6)TiO3, and (Pb0.6Sr0.3Ba0.1)TiO3 [2–8], but in these cases, there are no abnormal densification phenomenon as found in the present experiments, i.e., abnormal orientational growth of grains in the ceramic samples, so that the sample has very large axial linear shrinkage rate, the reason for which phenomenon is still not clear. However, for a practical PTC material, in order to obtain the assumed characteristics, many dopants would be added to the BTO materials, just as in present work, so the MW-sintering of doped BTO PTCR materials needs to be investigated systematically as a new processing technology for materials.
2. Experimental procedure Reagents of BaCO3, TiO2, Y2O3, SiO2, Al2O3, Mn(NO3)2 were mixed in the composition of: BTO þ 1% TiO2 þ 0:2% * Corresponding author. E-mail address: [email protected] (A. Chang).
Al2 O3 þ 0:1% MnðNO3 Þ2 þ 0:25% Y2 O3 þ 2% SiO2 (mole ratio), and calcined at 1150 8C for 2 h, then milled in a nylon jar for 24 h. The powders were pressed into cylinders with the size 30 mm 45 mm. The duplicated samples were sintered by MW (2450 MHz) at 1250 8C for 15 min, and by conventional SiMo furnace at 1350 8C for 2 h. The temperature profile for MW-sintering was measured using an infrared pyrometer. After sintering, the samples were examined by the XRD and SEM methods. The insulating system adopted in the MW-sintering of samples is shown in Fig. 1.
3. Results and conclusions The linear shrinkage rate (Z) was calculated using the following equation: Z¼
L0 L L0
where L0 is the length of the green sample, and L the length of the sample after sintering. Both the axial and the radial linear shrinkage rate were calculated, the results shows that the MW-sintered sample’s axial linear shrinkage rate is 48%, which is much larger than CO-sintered sample’s axial linear shrinkage rate of 26%. This result is shown in Fig. 2. Very different microstructures were observed in the SEM photographs presented in Fig. 2, the CO-sintered sample
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 9 3 - 2
A. Chang, J. Jian / Journal of Materials Processing Technology 137 (2003) 100–101
101
Fig. 1. Illustration of the insulating system: (1) Al2O3 fibre insulator; (2) Al2O3 ceramic insulator; (3) inner insulator; (4) sample; (5) hole for temperature measurement; (6) Al2O3 fibre.
Fig. 4. XRD patterns of doped BTO samples: (A) conventional sintering; (B) MW-sintering.
Fig. 2. Comparison of the sample’s linear shrinkage rate (Z) between MW-sintering and conventional sintering.
showing ecliptic spherical shape, whereas the MW-sintered sample shows an orientational strip-like microstructure. The growth of grains in MW-sintered BTO is obvious orientational, which is very different from the familiar CO-sintered BTO.
The XRD analysis shows that the phase is different in the two samples (see Figs. 3 and 4), the phase of MW-sintered sample being tetragonal (see Fig. 4(B)) with the (0 0 2) and (2 0 0) diffraction peaks being split significantly, indicating that the c/a ratio of the materials is not small. This is significantly different from that of the CO-sintered sample’s cubic phase (see Fig. 4(A)). The reason for this abnormal phenomenon can perhaps be attributed to the polarisation of BTO as a typical piezoelectric material in the MW field, and also to the enhancement of grain diffusion by the MWs. However, the real reason for this abnormal phenomenon still needs to be investigated in detail.
References
Fig. 3. The microstructure of the samples: (A) conventional sintering; (B) MW-sintering.
[1] W.H. Sutton, Microwave processing of ceramic materials, Bull. Am. Ceram. Soc. 68 (2) (1989) 376–386. [2] St. Derling, H.P. Abicht, Microwave sintering of BaTiO3-based ceramics, J. Microwave Power Electromagn. Energy 31 (1996) 221– 227. [3] B. Kang, H.-G. Kim, Microwave sintering of BaTiO3 thick films, J. Mater. Sci. Lett. 13 (1994) 806–809. [4] H.-Y. Chang, K.-S. Lin, I.-N. Lin, Modification of PTCR behavior of (Sr0.2Ba0.8)TiO3 materials by post-heat treatment after microwave sintering, J. Eur. Ceram. Soc. 16 (1996) 63–70. [5] H.-Y. Chang, K.-S. Liu, I.-N. Lin, et al., Electrical characteristics of (Sr0.2Ba0.8)TiO3 positive temperature coefficient of resistivity materials prepared by microwave sintering, J. Appl. Phys. 78 (1995) 423– 427. [6] H.-Y. Chang, K.-S. Liu, H.W. Chen, et al., V-shaped positive temperature coefficient of resistivity (PTCR) characteristics of microwave-sintered (Sr0.4Pb0.6)TiO3, Mater. Chem. Phys. 42 (1995) 258–263. [7] C.-C. Chou, H.-Y. Chang, I.-N. Lin, et al., Microscopic examination of the microwave sintered (Sr0.4Pb0.6)TiO3 positive temperature coefficient resistor material, Jpn. J. Appl. Phys., Part 1 37 (1998) 5269– 5272. [8] C.-T. Hu, H.-W. Chen, H.-Y. Chang, et al., Effect of SiO2 sintering aids on high critical temperature positive temperature coefficient of resistivity properties of (Pb0.6Sr0.3Ba0.1)TiO3 materials prepared by microwave sintering technique, Jpn. J. Appl. Phys., Part 1 37 (1998) 186–191.
The orientational growth of grains in doped BaTiO3 PTCR materials by microwave sintering Aimin Changa,*, Jiawen Jianb a
Xinjiang Institute of Physics, Urumqi 830011, PR China Xinjiang Petroleum Institute, Urumqi 830011, PR China
b
Abstract Doped BaTiO3 (BTO) PTCR materials were sintered by microwave (MW) of 2.45 GHz at 1250 8C for 15 min. For comparison, identical samples ð30 mm 45 mmÞ were also sintered by conventional (CO) SiMo furnace at 1350 8C for 2 h. The experimental results show that the MW-sintered samples have a much greater axial linear shrinkage rate of 48% vs. the rate for the CO-sintered sample of 26%. This result has not yet been reported in other researcher works. Marked differences were also observed in the microstructure of the two samples by SEM, the grains in the CO-sintered sample showing on ecliptic spherical shape, whereas in MW-sintered samples the grains show an orientational striplike microstructure. The XRD analysis indicates that the perovskite structure of MW-sintered BTO is tetragonal, whereas the CO-sintered sample is cubic. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave sintering; Orientational growth; BaTiO3
1. Introduction As a unique technology, microwave (MW) sintering has been applied to the processing of electroceramics in many aspects [1]. The MW-sintering of PTCR materials has been reported for many cases, such as BaTiO3 (BTO)-based ceramics, BTO thick films (Sr0.2Ba0.8)TiO3, (Sr0.4Pb0.6)TiO3, and (Pb0.6Sr0.3Ba0.1)TiO3 [2–8], but in these cases, there are no abnormal densification phenomenon as found in the present experiments, i.e., abnormal orientational growth of grains in the ceramic samples, so that the sample has very large axial linear shrinkage rate, the reason for which phenomenon is still not clear. However, for a practical PTC material, in order to obtain the assumed characteristics, many dopants would be added to the BTO materials, just as in present work, so the MW-sintering of doped BTO PTCR materials needs to be investigated systematically as a new processing technology for materials.
2. Experimental procedure Reagents of BaCO3, TiO2, Y2O3, SiO2, Al2O3, Mn(NO3)2 were mixed in the composition of: BTO þ 1% TiO2 þ 0:2% * Corresponding author. E-mail address: [email protected] (A. Chang).
Al2 O3 þ 0:1% MnðNO3 Þ2 þ 0:25% Y2 O3 þ 2% SiO2 (mole ratio), and calcined at 1150 8C for 2 h, then milled in a nylon jar for 24 h. The powders were pressed into cylinders with the size 30 mm 45 mm. The duplicated samples were sintered by MW (2450 MHz) at 1250 8C for 15 min, and by conventional SiMo furnace at 1350 8C for 2 h. The temperature profile for MW-sintering was measured using an infrared pyrometer. After sintering, the samples were examined by the XRD and SEM methods. The insulating system adopted in the MW-sintering of samples is shown in Fig. 1.
3. Results and conclusions The linear shrinkage rate (Z) was calculated using the following equation: Z¼
L0 L L0
where L0 is the length of the green sample, and L the length of the sample after sintering. Both the axial and the radial linear shrinkage rate were calculated, the results shows that the MW-sintered sample’s axial linear shrinkage rate is 48%, which is much larger than CO-sintered sample’s axial linear shrinkage rate of 26%. This result is shown in Fig. 2. Very different microstructures were observed in the SEM photographs presented in Fig. 2, the CO-sintered sample
0924-0136/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 2 ) 0 1 0 9 3 - 2
A. Chang, J. Jian / Journal of Materials Processing Technology 137 (2003) 100–101
101
Fig. 1. Illustration of the insulating system: (1) Al2O3 fibre insulator; (2) Al2O3 ceramic insulator; (3) inner insulator; (4) sample; (5) hole for temperature measurement; (6) Al2O3 fibre.
Fig. 4. XRD patterns of doped BTO samples: (A) conventional sintering; (B) MW-sintering.
Fig. 2. Comparison of the sample’s linear shrinkage rate (Z) between MW-sintering and conventional sintering.
showing ecliptic spherical shape, whereas the MW-sintered sample shows an orientational strip-like microstructure. The growth of grains in MW-sintered BTO is obvious orientational, which is very different from the familiar CO-sintered BTO.
The XRD analysis shows that the phase is different in the two samples (see Figs. 3 and 4), the phase of MW-sintered sample being tetragonal (see Fig. 4(B)) with the (0 0 2) and (2 0 0) diffraction peaks being split significantly, indicating that the c/a ratio of the materials is not small. This is significantly different from that of the CO-sintered sample’s cubic phase (see Fig. 4(A)). The reason for this abnormal phenomenon can perhaps be attributed to the polarisation of BTO as a typical piezoelectric material in the MW field, and also to the enhancement of grain diffusion by the MWs. However, the real reason for this abnormal phenomenon still needs to be investigated in detail.
References
Fig. 3. The microstructure of the samples: (A) conventional sintering; (B) MW-sintering.
[1] W.H. Sutton, Microwave processing of ceramic materials, Bull. Am. Ceram. Soc. 68 (2) (1989) 376–386. [2] St. Derling, H.P. Abicht, Microwave sintering of BaTiO3-based ceramics, J. Microwave Power Electromagn. Energy 31 (1996) 221– 227. [3] B. Kang, H.-G. Kim, Microwave sintering of BaTiO3 thick films, J. Mater. Sci. Lett. 13 (1994) 806–809. [4] H.-Y. Chang, K.-S. Lin, I.-N. Lin, Modification of PTCR behavior of (Sr0.2Ba0.8)TiO3 materials by post-heat treatment after microwave sintering, J. Eur. Ceram. Soc. 16 (1996) 63–70. [5] H.-Y. Chang, K.-S. Liu, I.-N. Lin, et al., Electrical characteristics of (Sr0.2Ba0.8)TiO3 positive temperature coefficient of resistivity materials prepared by microwave sintering, J. Appl. Phys. 78 (1995) 423– 427. [6] H.-Y. Chang, K.-S. Liu, H.W. Chen, et al., V-shaped positive temperature coefficient of resistivity (PTCR) characteristics of microwave-sintered (Sr0.4Pb0.6)TiO3, Mater. Chem. Phys. 42 (1995) 258–263. [7] C.-C. Chou, H.-Y. Chang, I.-N. Lin, et al., Microscopic examination of the microwave sintered (Sr0.4Pb0.6)TiO3 positive temperature coefficient resistor material, Jpn. J. Appl. Phys., Part 1 37 (1998) 5269– 5272. [8] C.-T. Hu, H.-W. Chen, H.-Y. Chang, et al., Effect of SiO2 sintering aids on high critical temperature positive temperature coefficient of resistivity properties of (Pb0.6Sr0.3Ba0.1)TiO3 materials prepared by microwave sintering technique, Jpn. J. Appl. Phys., Part 1 37 (1998) 186–191.