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Low temperature sintering behavior of B2O3 vapor in BaTiO3-based PTCR thermistors
Sensors and Actuators A 116 (2004) 215–218
Low temperature sintering behavior of B2 O3 vapor in BaTiO3-based PTCR thermistors Jianquan Qi a,b,∗ , Wanping Chen b , Hangyao Wang a , Yu Wang b , Longtu Li a , Helen Lai Wah Chan b b
a Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Department of Applied Physics and Materials, Research Center, The Hong Kong Polytechnic University, Hong Kong, China
Received 18 October 2003; received in revised form 16 March 2004; accepted 17 March 2004 Available online 20 May 2004
Abstract The effects of B2 O3 vapor on the sintering and the PTCR effect of BaTiO3 -based ceramics are investigated in this study. It is revealed that B2 O3 vapor can be doped into both pure and Y-doped BaTiO3 ceramics during sintering. B2 O3 vapor not only obviously influences the densification and the grain growth but also increases the grain lattice parameters. The PTCR effect is improved through the doping of B2 O3 vapor as the room temperature resistivity is considerably decreased and the resistance jump is greatly increased. With the doping of 1 mol% B2 O3 vapor, semiconducting Y-BaTiO3 with the room temperature resistivity of 30 cm is obtained when the ceramics are sintered at as low as 1150 ◦ C; while when the ceramics are sintered at 1350 ◦ C with the doping of 0.25 mol% B2 O3 vapor, the PTCR effect is enhanced by nearly two orders of magnitude and the room temperature resistivity is decreased from 25 to 9 cm. © 2004 Elsevier B.V. All rights reserved. Keywords: BaTiO3 ceramics; B2 O3 ; Vapor; PTCR effect
1. Introduction It is well known that the resistivity of donor-doped BaTiO3 semiconducting ceramics show a dramatic increase near the Curie temperature, Tc , which is called the positive-temperature-coefficient-resistivity (PTCR) effect [1]. Because the sintering temperature of BaTiO3 -based ceramics increases with the doping level of the donors, some sintering aids, such as SiO2 and its inclusions, are often adopted to decrease the sintering temperature. The sintering aids lead to the formation of liquid phases and promote densification during sintering. B2 O3 and boron inclusions are also studied as liquid-phase-sintering aids by previous authors [2,3]. Rhim et al. [2] found that the dielectric and ferroelectric properties of Ba0.7 Sr0.3 TiO3 samples with 0.5 wt.% B2 O3 sintered at <1150 ◦ C were as good as those of undoped Ba0.7 Sr0.3 TiO3 sintered at 1350 ◦ C. Lee et al. [3] reported that various boron source compounds, such as BaB2 O4 , greatly lowered the starting temperature of densi-
fication by the formation of liquid phases around 900 ◦ C, lowered room-temperature-resistivity (ρr ) and enhanced PTCR effect of Y-doped BaTiO3 ceramics. These sintering aids are added before sintering. In our previous studies, we found that the vapor of some oxides can be doped to BaTiO3 -based ceramics during sintering and influences the PTCR effect distinctly [4]. The PTCR effect can be enhanced by two orders of magnitude through a trace amount of Bi2 O3 vapor [5] or Sb2 O3 vapor [6], while the effect is depressed by one order of magnitude through a trace amount of PbO vapor [7]. Recently, we found that B2 O3 can also be adopted as a vapor dopant because B2 O3 volatilizes easily at high temperatures. B2 O3 vapor was found to enhance the PTCR effect distinctly [8], but its sintering effect was not studied. In this paper, we focus our attention on the sintering behavior of B2 O3 vapor in Y-doped BaTiO3 ceramics. The related mechanism is also discussed.
2. Experiments ∗ Corresponding author. Tel.: +86-852-2766-7797; fax: +86-852-2333-7629. E-mail addresses: [email protected], [email protected] (J. Qi).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.064
High purity commercial BaTiO3 (99.99%, Advanced Nano Products Co., Ltd., Chungcheongbuk-do, Korea) and Y2 O3 (99.99%, Beijing Chemical Plant, Beijing, China)
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powders were used as the starting materials. The basic powders were prepared according to the formula of BaTiO3 + 0.25 mol% Y2 O3 . After milling and drying, the powders were calcined in air at 1150 ◦ C for 2 h, and then pressed into small disks 2 mm thick and 10 mm in diameter. Pure BaTiO3 powders were also pressed into disks and sintered for comparison. The disks were placed in three bottom-up alumina crucibles marked as A, B and C, respectively. About 0.25 mol% B2 O3 powders were placed beside the samples in crucible B. An amount of 1 mol% B2 O3 powders were placed beside the samples in crucible C. The samples were sintered at 1150–1350 ◦ C for 1 h in air, and furnace-cooled to room temperature. More details about vapor doping method can be seen in [4,5]. The grain lattice parameters of the ceramics were measured using X-ray diffraction (XRD). Scanning electron microscopy (SEM) was employed in the observation of the ceramic morphology and the estimation of grain sizes. The shrinkage of the ceramics was determined by measuring and comparing the diameter of pellets before and after sintering, i.e., shrinkage = (diameter of pressed pellet − diameter of sintered pellet)/diameter of pressed pellet. On some of the sintered ceramic pellets, In–Ga alloy was pasted on the two major surfaces to form electrodes. Such samples were used in the measurement of the temperature dependence of resistance of the ceramics. The measurement was conducted on a PTCR test system containing a temperature chamber and a precision resistance tester. The temperature range in the measurements usually covered from room temperature to 400 ◦ C.
Fig. 1. The shrinkage and grain size vs. sintering temperature: (a) sintered in crucible A (no B2 O3 ); (b) sintered in crucible B (with 0.25 mol% B2 O3 ).
tice cell, and decrease the activation energy of bulk diffusion. We found that doping B2 O3 vapor in BaTiO3 results in the expanding of the lattice cell [9], and thus boron interstitial may exist in BaTiO3 grain lattice. Therefore, interstitial boron ion may be introduced into BaTiO3 bulk by the doping of B2 O3 vapor during sintering × B2 O3 (g) → 2B··· i + 3OO + 3VBa
(1)
where B··· i stands for the interstitial boron with three positive charges, O× O for electrically neutral oxygen atoms on oxygen for barium vacancy with two negative charges. sites, and VBa Due to the volatility of B2 O3 at high temperatures, interstitial boron can also escape from BaTiO3 lattice as
3. Results and discussion
× ·· 2B··· i + 3OO → 3VO + B2 O3 (g) ↑
The ρr of the Y-doped BaTiO3 samples in crucible A, in which there is no B2 O3 , is greater than 109 cm (we regard it as insulator) when the sintering temperature is lower than 1250 ◦ C. The ρr of the Y-doped BaTiO3 samples in crucible B, in which there is a small amount of B2 O3 as vapor source, is 15 cm and the sample became a dense ceramic at a sintering temperature of 1200 ◦ C; while no samples in this crucible are well densified and they are all insulators when they are sintered at 1150 ◦ C. In crucible C, in which there is a higher amount of B2 O3 , both undoped and Y-doped BaTiO3 can be well densified when they are sintered at 1150 ◦ C; at the same time, Y-BaTiO3 samples became good semiconductors with a ρr of 30 cm. Only doped with B2 O3 vapor, BaTiO3 samples are all insulators. The shrinkage and grain size of Y-BaTiO3 samples sintered in crucibles A and B as function of sintering temperature are shown in Fig. 1. It is clear that the samples started to be densified (estimated by sintering shrinkage) at a lower temperature when B2 O3 vapor was present. Bulk diffusion is one of the most important matter transfer modes for densification during ceramic sintering. Defects and impurities in the grain lattices often increase the distortion of the lat-
where VO·· stands for the oxygen vacancy with two positive charges. Some vacancies are formed in both Eqs. (1) and (2). Vacancy is the necessary passage of ions diffusion. The activation energy of bulk diffusion can be decreased by the formation of vacancies, and the densification temperature can thus be decreased. One can see that 6% shrinkage has happened in the samples sintered in crucible B at 1150 ◦ C, while almost no shrinkage is observed in the samples sintered in crucible A at this temperature. This demonstrates that the sintering densification can be promoted by the doping of B2 O3 vapor. B2 O3 vapor appears to have inconsistent effects on the sintering densification process and the grain growth. Although B2 O3 vapor results in an obvious densification in the sintering at 1150 ◦ C, it has no noticeable effect on the grain growth of sintering at this temperature. On the other hand, B2 O3 vapor greatly promotes the grain growth in the sintering at 1200 ◦ C, while the difference in densification between the samples sintered with and without B2 O3 vapor at this temperature becomes smaller. This inconsistency results from the fact that grain growth during sintering is relatively independent of densification. There are several matter
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transfer modes during sintering. B2 O3 vapor may promote the formation of active liquid phases or make the boundary slide more easily at a temperature above 1200 ◦ C, and thus improve the surface and boundary diffusion during sintering as reported by previous authors [2,3]. These matter transfers mainly promote grain growth. They cannot effectively affect the densification rate during sintering [10]. Thus, the grain growth is greatly increased at 1200 ◦ C. The liquid phases resulted from B2 O3 vapor must be negligible at low temperatures so that the grain growth cannot be promoted at 1150 ◦ C. For the samples sintered in crucibles B and C at temperatures above 1250 ◦ C, the grain size decreases with the sintering temperature. At temperatures above 1250 ◦ C, B2 O3 must form liquid phases along grain boundaries and
the grains are wrapped so the driving force of matter transfer is decreased. Therefore, the grain growth is slowed down and the samples with smaller grain size are observed, as shown in Fig. 2. The semiconducting process can be promoted by the doping of B2 O3 vapor. For example, we obtained semiconducting Y-BaTiO3 ceramics with a ρr of 30 cm sintered at as low as 1150 ◦ C in crucible C with the highest amount of B2 O3 vapor, but we can only obtain insulating ceramics sintered in crucible A without B2 O3 vapor when the sintering temperature is below 1250 ◦ C. The PTCR effect of Y-BaTiO3 can be greatly enhanced and the ρr can be considerably decreased in the presence of B2 O3 vapor when the sample sintered at high temperatures such as above 1250 ◦ C. Fig. 3 shows the temperature dependence of resistivity of three representative samples. These curves were measured through heating from room temperature to 400 ◦ C and the resistivity of the samples showed an excellent reversibility between heating up and cooling down. Sintered at 1350 ◦ C, the sample from crucible B has ρr ∼ 10 cm, resistance jump (Rmax /Rmin ) ∼ 103 , but the sample from crucible A has ρr ∼ 20 cm, log(Rmax /Rmin ) < 1.5. Even the sample sintered at 1150 ◦ C in crucible C has ρr ∼ 30 cm, log(Rmax /Rmin ) ∼ 3.5. The PTCR effect in the samples should be further improved when the cooling rate is decreased to oxidize the defects at grain boundary [11] after the soak stage in sintering. Lee et al. [3] have attributed the decrease of ρr by the doping of BaB2 O4 into Y-BaTiO3 ceramics to the donor behavior of interstitial boron ion, such as 3 B2 O3 → 2B··· i + 6e + 2 O2 (g) ↑
Fig. 2. The grain size of the samples sintered in crucible B (with 0.25 mol% B2 O3 ) at different temperatures: (a) 1150 ◦ C, (b) 1200 ◦ C, (c) 1350 ◦ C.
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Fig. 3. The temperature dependence of resistivity of three samples sintered in crucibles A–C, respectively.
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We found that pure BaTiO3 samples could not become semiconducting through the doping of B2 O3 vapor sintered at any temperature in our experiments. Therefore, we suggest that the interstitial boron ion is compensated by cation vacancy as Eq. (1) rather than by free electrons as Eq. (3). Because the bulk diffusion is promoted by the doping of B2 O3 vapor through the formation of interstitial boron ion, donor dopants can be easily synthesized into grain lattice, and the ρr of the ceramics can be decreased. The PTCR effect of the BaTiO3 -based ceramics is related to the activation of the trap centers during ferroelectric phase transition. The interstitial boron ion and/or related complex would be responsible for the enhancement of the PTCR effect as we reported previously [12].
4. Conclusion Semiconducting Y-BaTiO3 with the room temperature resistivity of 30 cm can be obtained when the ceramics sintered at as low as 1150 ◦ C through the doping of 1 mol% B2 O3 vapor. B2 O3 vapor promotes the surface or boundary diffusion due to the formation of active liquid phases in grain surface or boundary, and accelerates the grain growth. Sintered at temperatures higher than 1250 ◦ C, however, the grain growth is depressed because too much B2 O3 -contained liquid phases are formed which wrap the grains and decrease the driving force of sintering. Interstitial boron ions are formed in BaTiO3 lattice through the doping of B2 O3 vapor. Some vacancies are formed when interstitial boron ions are introduced in and volatilized from BaTiO3 lattice, which promote bulk diffusion and mass transfer. Bulk diffusion helps the incorporation of donor dopants into the grain lattice so the room temperature resistivity of the ceramics can be decreased through the doping of B2 O3 vapor. Acting as trap centers, interstitial boron ion and/or its complex enhance the PTCR effect of the ceramics.
Acknowledgements This work has been supported by the Postdoctoral Research Fellowship scheme and the Center for Smart Materials of The Hong Kong Polytechnic University.
References [1] P.W. Haayman, R.W. Dam, H.A. Klasens, German Patent 929 (1955) 350. [2] S.M. Rhim, S. Hong, H. Bak, O.K. Kim, Effects of B2 O3 addition on the dielectric and ferroelectric properties of Ba0.7 Sr0.3 TiO3 ceramics, J. Am. Ceram. Soc. 83 (5) (2000) 1145–1148. [3] J.H. Lee, Y.W. Heo, J.A. Lee, Y.D. Ryoo, J.J. Kim, S.H. Cho, Grain boundary and its related properties of boron added Y-doped BaTiO3 ceramics, Solid State Ionics 101/103 (1977) 787–791.
[4] J.Q. Qi, Z.L. Gui, L.T. Li, Vapor doping method, a new effective method used in BaTiO3 based PTCR ceramics, J. Inorgan. Mater. 14 (3) (1999) 408–412 (in Chinese). [5] J.Q. Qi, W.P. Chen, Y.J. Wu, L.T. Li, Improvement of the PTCR effect in Ba1−x Srx TiO3 semiconducting ceramics by doping of Bi2 O3 vapor during sintering, J. Am. Ceram. Soc. 81 (1998) 437–438. [6] J.Q. Qi, Z.L. Gui, Y.J. Wu, L.T. Li, Enhancement of PTCR effect of semiconducting Ba1−x Srx TiO3 by Sb2 O3 vapor, Sens. Actuators A 93 (2001) 84–85. [7] J.Q. Qi, Z.L. Gui, L.T. Li, Y.J. Wu, Positive temperature coefficient resistance effect in Ba1−x Srx TiO3 ceramics modified with Bi2 O3 and PbO by vapor doping method, J. Mater. Res. 14 (8) (1999) 3328– 3329. [8] J.Q. Qi, L.T. Li, Q. Zhu, Y.L. Wang, Z.L. Gui, Abnormal behavior of B2 O3 vapor dopants in BaTiO3 based PTCR ceramics, J. Inorgan. Mater. 16 (7) (2001) 739–741 (in Chinese). [9] J.Q. Qi, Y. Wang, W.P. Chen, H.L.W. Chan, Effect of B2 O3 vapor doping on the lattice parameter and electrical properties in BaTiO3 ceramics, Jpn. J. Appl. Phys. 42 (12B) (2003) L1516–L1518. [10] W.D. Kingery, H.K. Bowen, D.R. Uhlmann, Introduction to Ceramics, 2nd ed., Wiley, 1976, (Chapter 10) pp. 448–513 . [11] J.Q. Qi, N.X. Zhang, Z.L. Gui, L.T. Li, The oxidizing of grain boundary defects in BaTiO3 based PTCR ceramics, Ferroelectrics 263 (2001) 107–112. [12] J.Q. Qi, Q. Zhu, Y.L. Wang, Y.J. Wu, L.T. Li, Enhancement of positive temperature coefficient resistance effect of BaTiO3 -based semiconducting ceramics caused by B2 O3 vapor dopants, Solid State Commun. 120 (2001) 505–508.
Biographies Jianquan Qi is an associate professor of Department of Materials Science and Engineering, Tsinghua University. He received BS, MS, PhD degrees from Tsinghua University in 1989, 1992, 2003, respectively. He now works for the Department of Applied Physics, Hong Kong Polytechnic University as a research fellow. His main research interest includes semiconducting ceramics, dielectrics and nanostructure synthesis. Wanping Chen is a research fellow at The Hong Kong Polytechnic University. He obtained his MS degree in 1992 from Huazhong University of Science and Technology and PhD degree in 1998 from Tsinghua University. His current research focuses on the electrochemcial reactions in electronic ceramics and devices. Hangyao Wang received BS degree from Tsinghua University in 2002. He is now a postgraduate student in National University of Singapore. His main research interest includes semiconducting ceramics, computing nanostructures. Yu Wang obtained his BS, MS and PhD degrees, all in materials science and engineering, from Tsinghua University in 1989, 1994 and 1998, respectively. He is now working in The Hong Kong Polytechnic University as a research scientist. Dr. Wang’s research interest focuses on electronic ceramics, thin films and heterostructures. Longtu Li is a professor in the Department of Materials Science and Engineering, Tsinghua University, a member of the Chinese Academy of Engineering and a senior member of IEEE. He graduated from Tsinghua University in 1958. His research fields include ferroelectric, piezoelectric, dielectric and semiconducting ceramic materials and their applications. Helen Lai Wah Chan is a chair professor at the Department of Applied Physics, The Hong Kong Polytechnic University. She received the BSc and MPhil degrees in physics from the Chinese University of Hong Kong in 1970 and 1974, respectively, and the PhD degree from Macquarie University, Australia, in 1987. Her research areas include ferroelectric materials (ceramics, polymers, thin films and composites) and devices. She also has research interest on nano-materials.
Low temperature sintering behavior of B2 O3 vapor in BaTiO3-based PTCR thermistors Jianquan Qi a,b,∗ , Wanping Chen b , Hangyao Wang a , Yu Wang b , Longtu Li a , Helen Lai Wah Chan b b
a Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China Department of Applied Physics and Materials, Research Center, The Hong Kong Polytechnic University, Hong Kong, China
Received 18 October 2003; received in revised form 16 March 2004; accepted 17 March 2004 Available online 20 May 2004
Abstract The effects of B2 O3 vapor on the sintering and the PTCR effect of BaTiO3 -based ceramics are investigated in this study. It is revealed that B2 O3 vapor can be doped into both pure and Y-doped BaTiO3 ceramics during sintering. B2 O3 vapor not only obviously influences the densification and the grain growth but also increases the grain lattice parameters. The PTCR effect is improved through the doping of B2 O3 vapor as the room temperature resistivity is considerably decreased and the resistance jump is greatly increased. With the doping of 1 mol% B2 O3 vapor, semiconducting Y-BaTiO3 with the room temperature resistivity of 30 cm is obtained when the ceramics are sintered at as low as 1150 ◦ C; while when the ceramics are sintered at 1350 ◦ C with the doping of 0.25 mol% B2 O3 vapor, the PTCR effect is enhanced by nearly two orders of magnitude and the room temperature resistivity is decreased from 25 to 9 cm. © 2004 Elsevier B.V. All rights reserved. Keywords: BaTiO3 ceramics; B2 O3 ; Vapor; PTCR effect
1. Introduction It is well known that the resistivity of donor-doped BaTiO3 semiconducting ceramics show a dramatic increase near the Curie temperature, Tc , which is called the positive-temperature-coefficient-resistivity (PTCR) effect [1]. Because the sintering temperature of BaTiO3 -based ceramics increases with the doping level of the donors, some sintering aids, such as SiO2 and its inclusions, are often adopted to decrease the sintering temperature. The sintering aids lead to the formation of liquid phases and promote densification during sintering. B2 O3 and boron inclusions are also studied as liquid-phase-sintering aids by previous authors [2,3]. Rhim et al. [2] found that the dielectric and ferroelectric properties of Ba0.7 Sr0.3 TiO3 samples with 0.5 wt.% B2 O3 sintered at <1150 ◦ C were as good as those of undoped Ba0.7 Sr0.3 TiO3 sintered at 1350 ◦ C. Lee et al. [3] reported that various boron source compounds, such as BaB2 O4 , greatly lowered the starting temperature of densi-
fication by the formation of liquid phases around 900 ◦ C, lowered room-temperature-resistivity (ρr ) and enhanced PTCR effect of Y-doped BaTiO3 ceramics. These sintering aids are added before sintering. In our previous studies, we found that the vapor of some oxides can be doped to BaTiO3 -based ceramics during sintering and influences the PTCR effect distinctly [4]. The PTCR effect can be enhanced by two orders of magnitude through a trace amount of Bi2 O3 vapor [5] or Sb2 O3 vapor [6], while the effect is depressed by one order of magnitude through a trace amount of PbO vapor [7]. Recently, we found that B2 O3 can also be adopted as a vapor dopant because B2 O3 volatilizes easily at high temperatures. B2 O3 vapor was found to enhance the PTCR effect distinctly [8], but its sintering effect was not studied. In this paper, we focus our attention on the sintering behavior of B2 O3 vapor in Y-doped BaTiO3 ceramics. The related mechanism is also discussed.
2. Experiments ∗ Corresponding author. Tel.: +86-852-2766-7797; fax: +86-852-2333-7629. E-mail addresses: [email protected], [email protected] (J. Qi).
0924-4247/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2004.03.064
High purity commercial BaTiO3 (99.99%, Advanced Nano Products Co., Ltd., Chungcheongbuk-do, Korea) and Y2 O3 (99.99%, Beijing Chemical Plant, Beijing, China)
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powders were used as the starting materials. The basic powders were prepared according to the formula of BaTiO3 + 0.25 mol% Y2 O3 . After milling and drying, the powders were calcined in air at 1150 ◦ C for 2 h, and then pressed into small disks 2 mm thick and 10 mm in diameter. Pure BaTiO3 powders were also pressed into disks and sintered for comparison. The disks were placed in three bottom-up alumina crucibles marked as A, B and C, respectively. About 0.25 mol% B2 O3 powders were placed beside the samples in crucible B. An amount of 1 mol% B2 O3 powders were placed beside the samples in crucible C. The samples were sintered at 1150–1350 ◦ C for 1 h in air, and furnace-cooled to room temperature. More details about vapor doping method can be seen in [4,5]. The grain lattice parameters of the ceramics were measured using X-ray diffraction (XRD). Scanning electron microscopy (SEM) was employed in the observation of the ceramic morphology and the estimation of grain sizes. The shrinkage of the ceramics was determined by measuring and comparing the diameter of pellets before and after sintering, i.e., shrinkage = (diameter of pressed pellet − diameter of sintered pellet)/diameter of pressed pellet. On some of the sintered ceramic pellets, In–Ga alloy was pasted on the two major surfaces to form electrodes. Such samples were used in the measurement of the temperature dependence of resistance of the ceramics. The measurement was conducted on a PTCR test system containing a temperature chamber and a precision resistance tester. The temperature range in the measurements usually covered from room temperature to 400 ◦ C.
Fig. 1. The shrinkage and grain size vs. sintering temperature: (a) sintered in crucible A (no B2 O3 ); (b) sintered in crucible B (with 0.25 mol% B2 O3 ).
tice cell, and decrease the activation energy of bulk diffusion. We found that doping B2 O3 vapor in BaTiO3 results in the expanding of the lattice cell [9], and thus boron interstitial may exist in BaTiO3 grain lattice. Therefore, interstitial boron ion may be introduced into BaTiO3 bulk by the doping of B2 O3 vapor during sintering × B2 O3 (g) → 2B··· i + 3OO + 3VBa
(1)
where B··· i stands for the interstitial boron with three positive charges, O× O for electrically neutral oxygen atoms on oxygen for barium vacancy with two negative charges. sites, and VBa Due to the volatility of B2 O3 at high temperatures, interstitial boron can also escape from BaTiO3 lattice as
3. Results and discussion
× ·· 2B··· i + 3OO → 3VO + B2 O3 (g) ↑
The ρr of the Y-doped BaTiO3 samples in crucible A, in which there is no B2 O3 , is greater than 109 cm (we regard it as insulator) when the sintering temperature is lower than 1250 ◦ C. The ρr of the Y-doped BaTiO3 samples in crucible B, in which there is a small amount of B2 O3 as vapor source, is 15 cm and the sample became a dense ceramic at a sintering temperature of 1200 ◦ C; while no samples in this crucible are well densified and they are all insulators when they are sintered at 1150 ◦ C. In crucible C, in which there is a higher amount of B2 O3 , both undoped and Y-doped BaTiO3 can be well densified when they are sintered at 1150 ◦ C; at the same time, Y-BaTiO3 samples became good semiconductors with a ρr of 30 cm. Only doped with B2 O3 vapor, BaTiO3 samples are all insulators. The shrinkage and grain size of Y-BaTiO3 samples sintered in crucibles A and B as function of sintering temperature are shown in Fig. 1. It is clear that the samples started to be densified (estimated by sintering shrinkage) at a lower temperature when B2 O3 vapor was present. Bulk diffusion is one of the most important matter transfer modes for densification during ceramic sintering. Defects and impurities in the grain lattices often increase the distortion of the lat-
where VO·· stands for the oxygen vacancy with two positive charges. Some vacancies are formed in both Eqs. (1) and (2). Vacancy is the necessary passage of ions diffusion. The activation energy of bulk diffusion can be decreased by the formation of vacancies, and the densification temperature can thus be decreased. One can see that 6% shrinkage has happened in the samples sintered in crucible B at 1150 ◦ C, while almost no shrinkage is observed in the samples sintered in crucible A at this temperature. This demonstrates that the sintering densification can be promoted by the doping of B2 O3 vapor. B2 O3 vapor appears to have inconsistent effects on the sintering densification process and the grain growth. Although B2 O3 vapor results in an obvious densification in the sintering at 1150 ◦ C, it has no noticeable effect on the grain growth of sintering at this temperature. On the other hand, B2 O3 vapor greatly promotes the grain growth in the sintering at 1200 ◦ C, while the difference in densification between the samples sintered with and without B2 O3 vapor at this temperature becomes smaller. This inconsistency results from the fact that grain growth during sintering is relatively independent of densification. There are several matter
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transfer modes during sintering. B2 O3 vapor may promote the formation of active liquid phases or make the boundary slide more easily at a temperature above 1200 ◦ C, and thus improve the surface and boundary diffusion during sintering as reported by previous authors [2,3]. These matter transfers mainly promote grain growth. They cannot effectively affect the densification rate during sintering [10]. Thus, the grain growth is greatly increased at 1200 ◦ C. The liquid phases resulted from B2 O3 vapor must be negligible at low temperatures so that the grain growth cannot be promoted at 1150 ◦ C. For the samples sintered in crucibles B and C at temperatures above 1250 ◦ C, the grain size decreases with the sintering temperature. At temperatures above 1250 ◦ C, B2 O3 must form liquid phases along grain boundaries and
the grains are wrapped so the driving force of matter transfer is decreased. Therefore, the grain growth is slowed down and the samples with smaller grain size are observed, as shown in Fig. 2. The semiconducting process can be promoted by the doping of B2 O3 vapor. For example, we obtained semiconducting Y-BaTiO3 ceramics with a ρr of 30 cm sintered at as low as 1150 ◦ C in crucible C with the highest amount of B2 O3 vapor, but we can only obtain insulating ceramics sintered in crucible A without B2 O3 vapor when the sintering temperature is below 1250 ◦ C. The PTCR effect of Y-BaTiO3 can be greatly enhanced and the ρr can be considerably decreased in the presence of B2 O3 vapor when the sample sintered at high temperatures such as above 1250 ◦ C. Fig. 3 shows the temperature dependence of resistivity of three representative samples. These curves were measured through heating from room temperature to 400 ◦ C and the resistivity of the samples showed an excellent reversibility between heating up and cooling down. Sintered at 1350 ◦ C, the sample from crucible B has ρr ∼ 10 cm, resistance jump (Rmax /Rmin ) ∼ 103 , but the sample from crucible A has ρr ∼ 20 cm, log(Rmax /Rmin ) < 1.5. Even the sample sintered at 1150 ◦ C in crucible C has ρr ∼ 30 cm, log(Rmax /Rmin ) ∼ 3.5. The PTCR effect in the samples should be further improved when the cooling rate is decreased to oxidize the defects at grain boundary [11] after the soak stage in sintering. Lee et al. [3] have attributed the decrease of ρr by the doping of BaB2 O4 into Y-BaTiO3 ceramics to the donor behavior of interstitial boron ion, such as 3 B2 O3 → 2B··· i + 6e + 2 O2 (g) ↑
Fig. 2. The grain size of the samples sintered in crucible B (with 0.25 mol% B2 O3 ) at different temperatures: (a) 1150 ◦ C, (b) 1200 ◦ C, (c) 1350 ◦ C.
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Fig. 3. The temperature dependence of resistivity of three samples sintered in crucibles A–C, respectively.
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We found that pure BaTiO3 samples could not become semiconducting through the doping of B2 O3 vapor sintered at any temperature in our experiments. Therefore, we suggest that the interstitial boron ion is compensated by cation vacancy as Eq. (1) rather than by free electrons as Eq. (3). Because the bulk diffusion is promoted by the doping of B2 O3 vapor through the formation of interstitial boron ion, donor dopants can be easily synthesized into grain lattice, and the ρr of the ceramics can be decreased. The PTCR effect of the BaTiO3 -based ceramics is related to the activation of the trap centers during ferroelectric phase transition. The interstitial boron ion and/or related complex would be responsible for the enhancement of the PTCR effect as we reported previously [12].
4. Conclusion Semiconducting Y-BaTiO3 with the room temperature resistivity of 30 cm can be obtained when the ceramics sintered at as low as 1150 ◦ C through the doping of 1 mol% B2 O3 vapor. B2 O3 vapor promotes the surface or boundary diffusion due to the formation of active liquid phases in grain surface or boundary, and accelerates the grain growth. Sintered at temperatures higher than 1250 ◦ C, however, the grain growth is depressed because too much B2 O3 -contained liquid phases are formed which wrap the grains and decrease the driving force of sintering. Interstitial boron ions are formed in BaTiO3 lattice through the doping of B2 O3 vapor. Some vacancies are formed when interstitial boron ions are introduced in and volatilized from BaTiO3 lattice, which promote bulk diffusion and mass transfer. Bulk diffusion helps the incorporation of donor dopants into the grain lattice so the room temperature resistivity of the ceramics can be decreased through the doping of B2 O3 vapor. Acting as trap centers, interstitial boron ion and/or its complex enhance the PTCR effect of the ceramics.
Acknowledgements This work has been supported by the Postdoctoral Research Fellowship scheme and the Center for Smart Materials of The Hong Kong Polytechnic University.
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Biographies Jianquan Qi is an associate professor of Department of Materials Science and Engineering, Tsinghua University. He received BS, MS, PhD degrees from Tsinghua University in 1989, 1992, 2003, respectively. He now works for the Department of Applied Physics, Hong Kong Polytechnic University as a research fellow. His main research interest includes semiconducting ceramics, dielectrics and nanostructure synthesis. Wanping Chen is a research fellow at The Hong Kong Polytechnic University. He obtained his MS degree in 1992 from Huazhong University of Science and Technology and PhD degree in 1998 from Tsinghua University. His current research focuses on the electrochemcial reactions in electronic ceramics and devices. Hangyao Wang received BS degree from Tsinghua University in 2002. He is now a postgraduate student in National University of Singapore. His main research interest includes semiconducting ceramics, computing nanostructures. Yu Wang obtained his BS, MS and PhD degrees, all in materials science and engineering, from Tsinghua University in 1989, 1994 and 1998, respectively. He is now working in The Hong Kong Polytechnic University as a research scientist. Dr. Wang’s research interest focuses on electronic ceramics, thin films and heterostructures. Longtu Li is a professor in the Department of Materials Science and Engineering, Tsinghua University, a member of the Chinese Academy of Engineering and a senior member of IEEE. He graduated from Tsinghua University in 1958. His research fields include ferroelectric, piezoelectric, dielectric and semiconducting ceramic materials and their applications. Helen Lai Wah Chan is a chair professor at the Department of Applied Physics, The Hong Kong Polytechnic University. She received the BSc and MPhil degrees in physics from the Chinese University of Hong Kong in 1970 and 1974, respectively, and the PhD degree from Macquarie University, Australia, in 1987. Her research areas include ferroelectric materials (ceramics, polymers, thin films and composites) and devices. She also has research interest on nano-materials.