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Influence of doping BiYO3 and Nb2O5 on PTCR characteristics of BaTiO3 thermistor ceramics
Sensors and Actuators A 173 (2012) 158–162
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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Influence of doping BiYO3 and Nb2 O5 on PTCR characteristics of BaTiO3 thermistor ceramics Yongping Pu, Haidong Wu ∗ , Jifeng Wei School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China
a r t i c l e
i n f o
Article history: Received 31 March 2011 Received in revised form 14 October 2011 Accepted 14 November 2011 Available online 22 November 2011 Keywords: PTCR characteristic Thermistor ceramics Ba0.96 Ca0.04 TiO3 –BiYO3 Curie temperature
a b s t r a c t Nb2 O5 -doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 (where x = 0.01, 0.02, 0.03 and 0.04) lead-free PTC thermistor ceramics were prepared by a conventional solid state reaction method. X-ray diffraction, scanning electron microscope, Agilent E4980A and resistivity-temperature measurement instrument, were used to characteristic the lattice distortion, microstructure, temperature dependence of permittivity and resitivity-temperature dependence. It was revealed that the tetragonality c/a of the perovskite lattice, the microstructure and the Curie temperature changed with the BiYO3 content. In order to decrease the room temperature resistivity, the effect of Nb2 O5 on the room temperature resistivity was also studied, and its optimal doping content was finally chosen as 0.2 mol%. The 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 thermistor ceramic exhibited a low RT of 3.98 × 103 cm, a typical PTCR effect of max /min > 103 and a Tc of 153 ◦ C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Barium titanate (BaTiO3 )-based thermistor ceramics are widely used as materials with a positive temperature coefficient of resistivity (PTCR) [1]. Numerous applications, such as temperature sensors [2], self-regulating heaters and over-current protection [3], make full use of the PTCR characteristics. It is well known that the PTCR characteristics are associated with the ferroelectric Curie point, at which the crystal structure transforms from cubic phase to tetragonal phase [4]. Actually, the Tc can be easily controlled by substituting Pb2+ for Ba2+ . Recently, lead-free materials have been demanded from the viewpoint of environmental protection [5,6]. Therefore, many researchers have been trying to search for some bismuth compounds with high Curie point, such as Na0.5 Bi0.5 TiO3 , BiFeO3 , Bi4 Ti3 O12 and so on, so as to substitute Pb and shift the Curie point [7–9]. Although numerous literatures have reported that the Curie temperature could be greatly improved by introducing Na0.5 Bi0.5 TiO3 and so on, it is still far from current needs [10]. It is well known that donor doping can obviously reduce the room temperature resistivity [11]. However, the donor content should be strictly controlled. Otherwise it will lead to a rapid increase of the room-temperature resistivity of ceramics and even insulation. Generally, the voltage withstanding of ceramics and the amount of Bi3+ substitution will markedly increase when CaCO3 is
∗ Corresponding author. Tel.: +86 29 86168803; fax: +86 29 86168688. E-mail address: [email protected] (H. Wu). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.11.020
added into BaTiO3 ceramics, which had been studied by Völtzke [12]. In addition, the grain size of sintered ceramics decreased continuously with the increase of Ca2+ , but excess CaCO3 may lead to a rapid increase of the room temperature resistivity. Therefore, we introduced 4 mol% CaCO3 into BaTiO3 –BiYO3 ceramics in this study. In this study, we selected BiYO3 as another end member of BaTiO3 -based solid solutions to shift the Curie point and prepared Ba0.96 Ca0.04 TiO3 –BiYO3 thermistor ceramics by a reduction–reoxidation method. In order to reduce the room temperature resistivity, Nb2 O5 was introduced into Ba0.96 Ca0.04 TiO3 –BiYO3 thermistor ceramics and the optimal doping content of Nb2 O5 was also studied. In addition, we also investigated the influence of reoxidation temperature and time on PTCR characteristics of Ba0.96 Ca0.04 TiO3 –BiYO3 ceramics and ascertained the optimal reoxidation conditions. Furthermore, the lattice parameters, the microstructure and the PTCR characteristics of Nb2 O5 -doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics were investigated on the basis of the BiYO3 content. 2. Experimental procedures BaCO3 and TiO2 powders with purity of 99.99% were thoroughly mixed according to the formula of BaTiO3 and calcined at 1220 ◦ C for 2 h so as to obtain a pure BaTiO3 phase. In addition, Bi2 O3 and Y2 O3 powders with purity of 99.99% at a molar ratio of Bi:Y = 1:1 were weighted and mixed well, then the mixture was calcined at 900 ◦ C for 2 h to obtain a pure BiYO3 phase. Nb2 O5 and CaCO3 powders with purity of 99.99% and the synthesized BaTiO3 were weighted, mixed and calcined at
Y. Pu et al. / Sensors and Actuators A 173 (2012) 158–162
Fig. 1. XRD patterns: (a) BiYO3 powders; (b) Ba0.96 Ca0.04 TiO3 ceramics.
1220 ◦ C for 2 h. Subsequently, the calcined powder, the synthesized BiYO3 powder according to the nominal composition (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 (x = 0.01, 0.02, 0.03 and 0.04) were mixed in distilled water using a zirconia ball mill in polyethylene pot for 6 h. Meanwhile, in order to improve the densification and reduce the sintering temperature of ceramic samples, 0.06 mol% Al2 O3 and 0.12 mol% SiO2 were added. After drying and granulating with polyvinyl alcohol (PVA, 3 wt%), the well-mixed powders were pressed into a disk (˚ 10 mm × 1.5 mm) at 120 MPa and sintered at a temperature range of 1300–1380 ◦ C in a pure N2 flow atmosphere, then reoxidized at a temperature range of 700–1000 ◦ C in air for several hours. X-ray diffraction (XRD) patterns were obtained by means of an automated diffractometer (D/max-2200PC, RIGAKU, Japan) with Cu K␣1 radiation. A scanning electron microscope (JSM-6460) was used to investigate the microstructure of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics. In addition, the In–Ga paste was applied to both surfaces of the sintered samples as the electrode, then the resitivity-temperature dependence could be obtained by using computer-controlled resistivity-temperature measurement instrument (ZWX-B) from room temperature to 250 ◦ C at the rate of 2 ◦ C/min, and the temperature dependence of permittivity at 1 kHz could be obtained by using Agilent E4980A from room temperature to 200 ◦ C at the rate of 2 ◦ C/min.
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Fig. 2. XRD patterns of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics.
the observation of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 ceramic samples at 2 angles around 31.5◦ , it can be found that the (1 0 1) peaks were shifted to the right side with the increase of BiYO3 . In addition, the peak intensities also changed remarkably. On the basis of the XRD results, the a- and c-axes of the tetragonal structure, respectively decreased and increased with the substitution of Bi3+ and Y3+ to Ba-site in the BaTiO3 , and the unit cell volume decreased in the BaTiO3 –BiYO3 . This was because the ionic radius of Bi3+ (96 pm) and Y3+ (101 pm) was smaller than that of Ba2+ (134 pm). In addition, the axial ration (c/a) increased with the increase of BiYO3 and it closely related to the spontaneous polarization (Ps ). 3.2. Surface morphology
3. Results and discussions
Fig. 3 shows the SEM images of as-sintered surface of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered in N2 atmosphere at 1350 ◦ C for 2 h. It can be seen that the grains were well formed and the grain boundaries were clearcut. In addition, the average grain size decreased with the increase of BiYO3 . In this study, Y2 O3 and Bi2 O3 were produced from the decomposition of BiYO3 , then Y3+ and Bi3+ replaced Ba2+ and Ti4+ till the solubility limit was reached, thus Y3+ and Bi3+ impurities were accumulated at the grain boundaries. Generally, dopant segregation pin grain boundaries mobility during sintering, which inhibits the grain growth. Therefore, the average grain size became smaller with the increase of BiYO3 .
3.1. Crystalline structure
3.3. Electrical characteristics
Fig. 1 shows the XRD patterns of pure BiYO3 powders and sintered Ba0.96 Ca0.04 TiO3 powders. It can be seen that no secondary phase emerged from XRD result. It is indicated that pure BiYO3 powders were prepared successfully. In addition, it can be found that the homogeneous solid solution with perovskite structure was formed because of the dissolution of CaCO3 in BaTiO3 . Fig. 2 shows the XRD patterns of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a pure N2 flow atmosphere. It can be found that there was no secondary phase emerged from XRD results. All the peaks of sintered samples were similar to that of pure BaTiO3 , indicating that the samples consisted of a single perovskite phase. From the inset in Fig. 2, it can be seen that the lattice parameters of sintered samples varied with the BiYO3 content, which demonstrated that the raw materials had formed a solid solution during sintering. Through
3.3.1. Influence of sintering and oxidation temperature Fig. 4 shows the density and RT as a function of sintering temperature. It can be seen that the density and RT changed very little when the sintering temperature was higher than 1350 ◦ C. Therefore, the optimal sintering temperature was chosen as 1350 ◦ C in this study. Fig. 5(a) shows the dependence of reoxidation temperature on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics. From Fig. 5(a) it can be seen that the samples sintered in N2 revealed a low resistivity and weaken PTCR characteristics, which was due to the existence of many oxygen vacancies after sintering in N2 . It is well known that the potential barriers height on grain boundaries and the density of the grain surface acceptor state of BaTiO3 -based ceramics are the two critical factors in the PTCR characteristics [13]. As
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Fig. 3. Microstructure of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered in N2 at 1350 ◦ C for 2 h: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03 and (d) x = 0.04.
can be seen, marked PTCR characteristics were obtained at the reoxidation temperature above 900 ◦ C, because the density of the surface acceptor state of ceramics increased with the increase of reoxidation temperature. However, the further reoxidized degree may lead to a rapid increase of the room-temperature resistivity of ceramics and even insulation [14]. This was because the oxygen diffused faster at grain boundaries than into grains during reoxidation [15,16], and the oxygen vacancy at grain boundaries was more easily compensated. Therefore, it is necessary to choose a suitable reoxidation temperature so as to increase the density of the surface acceptor state at grain boundaries and keep the oxygen vacancies in grains from being compensated. Fig. 5(b) shows the dependence of reoxidation time on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO 3 –0.03BiYO3 –0.002Nb2 O5 ceramics. It can be seen that the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics sintered in N2 were improved by prolonging the reoxidation time. The samples revealed marked PTCR characteristics
when the reoxidation time was longer than 2 h. However, the RT increased to 5.8 × 104 cm when the reoxidation time was extended to 4 h. Therefore, suitable reoxidation time is needed to improve the PTCR characteristics. 3.3.2. Influence of Nb2 O5 concentration Fig. 6 shows the temperature dependence of resistivity of Nb2 O5 doped 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air. As can be seen, the RT of ceramics firstly decreased and then increased with the increase of Nb2 O5 , which could be clarified by the defects compensating mechanisms from electronic compensation to cation vacancies compensation [17]. Generally, different donor concentrations lead to different charge compensation mechanisms, which had been explained by Brzozowski and Castro [18]. When the concentration of Nb5+ was lower, electronic compensation would prevail and the semiconducting behavior could be explained by Eq. (1) [19]. When excess Nb5+ was incorporated into the lattice, charge disequilibria were compensated by ionic defects (Eq. (2)), thus the room temperature resistivity increased with the increase of Nb2 O5 . Therefore, the Nb2 O5 content was chosen as 0.2 mol% in the following study. In addition, it can be found that the Curie temperature was shifted to a lower temperature with the increase of Nb2 O5 . Nb2 O5 → 2NbTi • + 2e + 4Oo x + (1/2)O2 •
2Nb2 O5 → 4NbTi + VTi
Fig. 4. Density and RT as a function of sintering temperature.
+ 10Oo
x
(1) (2)
3.3.3. Influence of BiYO3 concentration Fig. 7 illustrates the dielectric properties of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered in N2 at 1350 ◦ C for 2 h and subsequently reoxidized at 900 ◦ C for 2 h in air as a function of temperature at 1 kHz. It is well known that the peak position of ε-temperature curve corresponds to the Tc , and does not depend on the measurement frequency [20]. From Fig. 7 it can
Y. Pu et al. / Sensors and Actuators A 173 (2012) 158–162
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a
b
Fig. 5. (a) Dependence of reoxidation temperature on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics. (b) Dependence of reoxidation time on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics.
be seen that the Tc was shifted to a higher temperature with the increase of x value. This was because Bi3+ entered Ba-site and the Tc increased with the incorporation of Bi3+ [21]. Fig. 8 shows the temperature dependence of resistivity of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at
Fig. 6. Temperature dependence of resistivity of Nb2 O5 doped 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air.
Fig. 7. Dielectric constant versus temperature (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air.
of at
Fig. 8. Temperature dependence of resistivity of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air.
900 ◦ C for 2 h in air. It can be found that the Curie temperature was shifted to a higher temperature with the increase of x value, which was coincided with the curve of permittivity-temperature dependence. Because the element bismuth has an electronic configuration 6s2 6p3 , it takes a tripositive state in the ordinary compounds [22]. Generally, the incorporation of Bi3+ has effect on the transition temperature (Tc ), at which the crystal structure transforms from cubic phase to tetragonal phase. As the amount of Bi3+ substitution increased, the transition temperature was shifted to a higher temperature. However, the amount of Bi3+ substitution was limited in BaTiO3 -based ceramics, i.e., Bi3+ replaced Ba2+ till the solubility limit was reached, then excess Bi3+ could substitute Ti4+ sites as well as Ba2+ sites, and obstructed the appearance of e , which led to a rapid increase of the room-temperature resistivity and even insulation. From Fig. 8 it can be seen that the room temperature resistivity sharply increased with the increase of x value, which could be explained by Eq. (3) and (4). Generally, two kinds of defect compensation mechanisms may occur with the introduction of M3+ (M = Bi and Y) [23]. When the concentration of M3+ was lower (less than 0.3 mol%)[24], the semiconducting behavior could be explained by Eq. (3). With increasing of M3+ concentration, it was more likely to produce VBa (Eq. (4)), which led to a rapid increase of the room temperature resistivity. Because the value of x was greater than or equal
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Table 1 Variations of RT and Tc for Nb2 O5 doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air. Composition (mol%)
RT ( cm)
Tc (◦ C)
0.99Ba0.96 Ca0.04 TiO3 –0.01BiYO3 –0.002Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.002Nb2 O5 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 0.96Ba0.96 Ca0.04 TiO3 –0.04BiYO3 –0.002Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.000Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.001Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.003Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.004Nb2 O5
1315 2090 3980 ≥106 6.5 × 105 6.6 × 104 2.8 × 104 5.1 × 104
126 140 153 163 154 151 146 142
to 0.01 in this study, more VBa was produced with the introduction of BiYO3 , which may lead to a thick layer rich in barium vacancies at the grain boundaries and the inhibition in the movement of free charges [25]. Therefore, the RT increased with the increase of x value. Ba2+ Ti4+ O3 2− + nM3+ → Ba1−n 2+ Mn 3+ Ti4+ O3 2− + nBa2+ + ne (3) Ba2+ Ti4+ O3 2− + nM3+ → Ba1−(3/2)n 2+ Mn 3+ Ti4+ O3 2− + (3/2)nBa2+ + (1/2)nVBa
(4)
Table 1 shows the RT and Tc of Nb2 O5 doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air. It can be seen that the RT and Tc varied with the BiYO3 content distinctly. With the increase of BiYO3 , the RT of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sharply increased from 1315 cm (x = 0.01) to 106 cm(x = 0.04), which had been explained as above. 4. Conclusions The PTCR characteristics of Nb2 O5 -doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics sintered in N2 and reoxidized in air were investigated. The result showed that the minimum value of RT could be obtained when the Nb2 O5 content was 0.2 mol%. The Tc of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics was shifted to a higher temperature with the increase of x value, but the RT was higher than 106 cm when x = 0.04. The 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air exhibited a low RT of 3.98 × 103 cm, a typical PTCR effect of max /min > 103 and a Tc of 153 ◦ C. Acknowledgements
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Biographies
This research was supported by the National Natural Science Foundation of China (51072106), Research projects of Science and Technology Division, Shaanxi (2010K10-14), Foundation of Shaanxi Educational Committee (112H011), and the Graduate Innovation Fund of Shaanxi University of Science and Technology.
Yongping Pu was born in 1971 in Shanxi Province, PhD (Xi’an Jiaotong University), PhD director, professor. Now he works at School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an. His major research focuses on lead-free PTC thermistor ceramics, temperature sensors, high density capacitors.
References
Haidong Wu received his bachelor degree (2009) from Shaanxi University of Science and Technology, China. His current research interest is temperature sensors.
[1] H. Takeda, H. Harinaka, T. Shiosaki, M.A. Zubair, C. Leach, R. Freer, T. Hoshina, T. Tsurumi, Fabrication and positive temperature coefficient of resistivity properties of semiconducting ceramics based on the BaTiO3 –(Bi1/2 K1/2 )TiO3 system, J. Eur. Ceram. Soc. 30 (2010) 555–559.
Jifeng Wei received his bachelor degree (2008) and master degree (2011) from Shaanxi University of Science and Technology, China. His current research interest is temperature sensors.
Contents lists available at SciVerse ScienceDirect
Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna
Influence of doping BiYO3 and Nb2 O5 on PTCR characteristics of BaTiO3 thermistor ceramics Yongping Pu, Haidong Wu ∗ , Jifeng Wei School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China
a r t i c l e
i n f o
Article history: Received 31 March 2011 Received in revised form 14 October 2011 Accepted 14 November 2011 Available online 22 November 2011 Keywords: PTCR characteristic Thermistor ceramics Ba0.96 Ca0.04 TiO3 –BiYO3 Curie temperature
a b s t r a c t Nb2 O5 -doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 (where x = 0.01, 0.02, 0.03 and 0.04) lead-free PTC thermistor ceramics were prepared by a conventional solid state reaction method. X-ray diffraction, scanning electron microscope, Agilent E4980A and resistivity-temperature measurement instrument, were used to characteristic the lattice distortion, microstructure, temperature dependence of permittivity and resitivity-temperature dependence. It was revealed that the tetragonality c/a of the perovskite lattice, the microstructure and the Curie temperature changed with the BiYO3 content. In order to decrease the room temperature resistivity, the effect of Nb2 O5 on the room temperature resistivity was also studied, and its optimal doping content was finally chosen as 0.2 mol%. The 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 thermistor ceramic exhibited a low RT of 3.98 × 103 cm, a typical PTCR effect of max /min > 103 and a Tc of 153 ◦ C. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Barium titanate (BaTiO3 )-based thermistor ceramics are widely used as materials with a positive temperature coefficient of resistivity (PTCR) [1]. Numerous applications, such as temperature sensors [2], self-regulating heaters and over-current protection [3], make full use of the PTCR characteristics. It is well known that the PTCR characteristics are associated with the ferroelectric Curie point, at which the crystal structure transforms from cubic phase to tetragonal phase [4]. Actually, the Tc can be easily controlled by substituting Pb2+ for Ba2+ . Recently, lead-free materials have been demanded from the viewpoint of environmental protection [5,6]. Therefore, many researchers have been trying to search for some bismuth compounds with high Curie point, such as Na0.5 Bi0.5 TiO3 , BiFeO3 , Bi4 Ti3 O12 and so on, so as to substitute Pb and shift the Curie point [7–9]. Although numerous literatures have reported that the Curie temperature could be greatly improved by introducing Na0.5 Bi0.5 TiO3 and so on, it is still far from current needs [10]. It is well known that donor doping can obviously reduce the room temperature resistivity [11]. However, the donor content should be strictly controlled. Otherwise it will lead to a rapid increase of the room-temperature resistivity of ceramics and even insulation. Generally, the voltage withstanding of ceramics and the amount of Bi3+ substitution will markedly increase when CaCO3 is
∗ Corresponding author. Tel.: +86 29 86168803; fax: +86 29 86168688. E-mail address: [email protected] (H. Wu). 0924-4247/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.sna.2011.11.020
added into BaTiO3 ceramics, which had been studied by Völtzke [12]. In addition, the grain size of sintered ceramics decreased continuously with the increase of Ca2+ , but excess CaCO3 may lead to a rapid increase of the room temperature resistivity. Therefore, we introduced 4 mol% CaCO3 into BaTiO3 –BiYO3 ceramics in this study. In this study, we selected BiYO3 as another end member of BaTiO3 -based solid solutions to shift the Curie point and prepared Ba0.96 Ca0.04 TiO3 –BiYO3 thermistor ceramics by a reduction–reoxidation method. In order to reduce the room temperature resistivity, Nb2 O5 was introduced into Ba0.96 Ca0.04 TiO3 –BiYO3 thermistor ceramics and the optimal doping content of Nb2 O5 was also studied. In addition, we also investigated the influence of reoxidation temperature and time on PTCR characteristics of Ba0.96 Ca0.04 TiO3 –BiYO3 ceramics and ascertained the optimal reoxidation conditions. Furthermore, the lattice parameters, the microstructure and the PTCR characteristics of Nb2 O5 -doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics were investigated on the basis of the BiYO3 content. 2. Experimental procedures BaCO3 and TiO2 powders with purity of 99.99% were thoroughly mixed according to the formula of BaTiO3 and calcined at 1220 ◦ C for 2 h so as to obtain a pure BaTiO3 phase. In addition, Bi2 O3 and Y2 O3 powders with purity of 99.99% at a molar ratio of Bi:Y = 1:1 were weighted and mixed well, then the mixture was calcined at 900 ◦ C for 2 h to obtain a pure BiYO3 phase. Nb2 O5 and CaCO3 powders with purity of 99.99% and the synthesized BaTiO3 were weighted, mixed and calcined at
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Fig. 1. XRD patterns: (a) BiYO3 powders; (b) Ba0.96 Ca0.04 TiO3 ceramics.
1220 ◦ C for 2 h. Subsequently, the calcined powder, the synthesized BiYO3 powder according to the nominal composition (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 (x = 0.01, 0.02, 0.03 and 0.04) were mixed in distilled water using a zirconia ball mill in polyethylene pot for 6 h. Meanwhile, in order to improve the densification and reduce the sintering temperature of ceramic samples, 0.06 mol% Al2 O3 and 0.12 mol% SiO2 were added. After drying and granulating with polyvinyl alcohol (PVA, 3 wt%), the well-mixed powders were pressed into a disk (˚ 10 mm × 1.5 mm) at 120 MPa and sintered at a temperature range of 1300–1380 ◦ C in a pure N2 flow atmosphere, then reoxidized at a temperature range of 700–1000 ◦ C in air for several hours. X-ray diffraction (XRD) patterns were obtained by means of an automated diffractometer (D/max-2200PC, RIGAKU, Japan) with Cu K␣1 radiation. A scanning electron microscope (JSM-6460) was used to investigate the microstructure of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics. In addition, the In–Ga paste was applied to both surfaces of the sintered samples as the electrode, then the resitivity-temperature dependence could be obtained by using computer-controlled resistivity-temperature measurement instrument (ZWX-B) from room temperature to 250 ◦ C at the rate of 2 ◦ C/min, and the temperature dependence of permittivity at 1 kHz could be obtained by using Agilent E4980A from room temperature to 200 ◦ C at the rate of 2 ◦ C/min.
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Fig. 2. XRD patterns of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics.
the observation of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 ceramic samples at 2 angles around 31.5◦ , it can be found that the (1 0 1) peaks were shifted to the right side with the increase of BiYO3 . In addition, the peak intensities also changed remarkably. On the basis of the XRD results, the a- and c-axes of the tetragonal structure, respectively decreased and increased with the substitution of Bi3+ and Y3+ to Ba-site in the BaTiO3 , and the unit cell volume decreased in the BaTiO3 –BiYO3 . This was because the ionic radius of Bi3+ (96 pm) and Y3+ (101 pm) was smaller than that of Ba2+ (134 pm). In addition, the axial ration (c/a) increased with the increase of BiYO3 and it closely related to the spontaneous polarization (Ps ). 3.2. Surface morphology
3. Results and discussions
Fig. 3 shows the SEM images of as-sintered surface of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered in N2 atmosphere at 1350 ◦ C for 2 h. It can be seen that the grains were well formed and the grain boundaries were clearcut. In addition, the average grain size decreased with the increase of BiYO3 . In this study, Y2 O3 and Bi2 O3 were produced from the decomposition of BiYO3 , then Y3+ and Bi3+ replaced Ba2+ and Ti4+ till the solubility limit was reached, thus Y3+ and Bi3+ impurities were accumulated at the grain boundaries. Generally, dopant segregation pin grain boundaries mobility during sintering, which inhibits the grain growth. Therefore, the average grain size became smaller with the increase of BiYO3 .
3.1. Crystalline structure
3.3. Electrical characteristics
Fig. 1 shows the XRD patterns of pure BiYO3 powders and sintered Ba0.96 Ca0.04 TiO3 powders. It can be seen that no secondary phase emerged from XRD result. It is indicated that pure BiYO3 powders were prepared successfully. In addition, it can be found that the homogeneous solid solution with perovskite structure was formed because of the dissolution of CaCO3 in BaTiO3 . Fig. 2 shows the XRD patterns of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a pure N2 flow atmosphere. It can be found that there was no secondary phase emerged from XRD results. All the peaks of sintered samples were similar to that of pure BaTiO3 , indicating that the samples consisted of a single perovskite phase. From the inset in Fig. 2, it can be seen that the lattice parameters of sintered samples varied with the BiYO3 content, which demonstrated that the raw materials had formed a solid solution during sintering. Through
3.3.1. Influence of sintering and oxidation temperature Fig. 4 shows the density and RT as a function of sintering temperature. It can be seen that the density and RT changed very little when the sintering temperature was higher than 1350 ◦ C. Therefore, the optimal sintering temperature was chosen as 1350 ◦ C in this study. Fig. 5(a) shows the dependence of reoxidation temperature on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics. From Fig. 5(a) it can be seen that the samples sintered in N2 revealed a low resistivity and weaken PTCR characteristics, which was due to the existence of many oxygen vacancies after sintering in N2 . It is well known that the potential barriers height on grain boundaries and the density of the grain surface acceptor state of BaTiO3 -based ceramics are the two critical factors in the PTCR characteristics [13]. As
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Fig. 3. Microstructure of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered in N2 at 1350 ◦ C for 2 h: (a) x = 0.01; (b) x = 0.02; (c) x = 0.03 and (d) x = 0.04.
can be seen, marked PTCR characteristics were obtained at the reoxidation temperature above 900 ◦ C, because the density of the surface acceptor state of ceramics increased with the increase of reoxidation temperature. However, the further reoxidized degree may lead to a rapid increase of the room-temperature resistivity of ceramics and even insulation [14]. This was because the oxygen diffused faster at grain boundaries than into grains during reoxidation [15,16], and the oxygen vacancy at grain boundaries was more easily compensated. Therefore, it is necessary to choose a suitable reoxidation temperature so as to increase the density of the surface acceptor state at grain boundaries and keep the oxygen vacancies in grains from being compensated. Fig. 5(b) shows the dependence of reoxidation time on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO 3 –0.03BiYO3 –0.002Nb2 O5 ceramics. It can be seen that the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics sintered in N2 were improved by prolonging the reoxidation time. The samples revealed marked PTCR characteristics
when the reoxidation time was longer than 2 h. However, the RT increased to 5.8 × 104 cm when the reoxidation time was extended to 4 h. Therefore, suitable reoxidation time is needed to improve the PTCR characteristics. 3.3.2. Influence of Nb2 O5 concentration Fig. 6 shows the temperature dependence of resistivity of Nb2 O5 doped 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air. As can be seen, the RT of ceramics firstly decreased and then increased with the increase of Nb2 O5 , which could be clarified by the defects compensating mechanisms from electronic compensation to cation vacancies compensation [17]. Generally, different donor concentrations lead to different charge compensation mechanisms, which had been explained by Brzozowski and Castro [18]. When the concentration of Nb5+ was lower, electronic compensation would prevail and the semiconducting behavior could be explained by Eq. (1) [19]. When excess Nb5+ was incorporated into the lattice, charge disequilibria were compensated by ionic defects (Eq. (2)), thus the room temperature resistivity increased with the increase of Nb2 O5 . Therefore, the Nb2 O5 content was chosen as 0.2 mol% in the following study. In addition, it can be found that the Curie temperature was shifted to a lower temperature with the increase of Nb2 O5 . Nb2 O5 → 2NbTi • + 2e + 4Oo x + (1/2)O2 •
2Nb2 O5 → 4NbTi + VTi
Fig. 4. Density and RT as a function of sintering temperature.
+ 10Oo
x
(1) (2)
3.3.3. Influence of BiYO3 concentration Fig. 7 illustrates the dielectric properties of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered in N2 at 1350 ◦ C for 2 h and subsequently reoxidized at 900 ◦ C for 2 h in air as a function of temperature at 1 kHz. It is well known that the peak position of ε-temperature curve corresponds to the Tc , and does not depend on the measurement frequency [20]. From Fig. 7 it can
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a
b
Fig. 5. (a) Dependence of reoxidation temperature on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics. (b) Dependence of reoxidation time on the PTCR characteristics of 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 ceramics.
be seen that the Tc was shifted to a higher temperature with the increase of x value. This was because Bi3+ entered Ba-site and the Tc increased with the incorporation of Bi3+ [21]. Fig. 8 shows the temperature dependence of resistivity of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at
Fig. 6. Temperature dependence of resistivity of Nb2 O5 doped 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air.
Fig. 7. Dielectric constant versus temperature (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air.
of at
Fig. 8. Temperature dependence of resistivity of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air.
900 ◦ C for 2 h in air. It can be found that the Curie temperature was shifted to a higher temperature with the increase of x value, which was coincided with the curve of permittivity-temperature dependence. Because the element bismuth has an electronic configuration 6s2 6p3 , it takes a tripositive state in the ordinary compounds [22]. Generally, the incorporation of Bi3+ has effect on the transition temperature (Tc ), at which the crystal structure transforms from cubic phase to tetragonal phase. As the amount of Bi3+ substitution increased, the transition temperature was shifted to a higher temperature. However, the amount of Bi3+ substitution was limited in BaTiO3 -based ceramics, i.e., Bi3+ replaced Ba2+ till the solubility limit was reached, then excess Bi3+ could substitute Ti4+ sites as well as Ba2+ sites, and obstructed the appearance of e , which led to a rapid increase of the room-temperature resistivity and even insulation. From Fig. 8 it can be seen that the room temperature resistivity sharply increased with the increase of x value, which could be explained by Eq. (3) and (4). Generally, two kinds of defect compensation mechanisms may occur with the introduction of M3+ (M = Bi and Y) [23]. When the concentration of M3+ was lower (less than 0.3 mol%)[24], the semiconducting behavior could be explained by Eq. (3). With increasing of M3+ concentration, it was more likely to produce VBa (Eq. (4)), which led to a rapid increase of the room temperature resistivity. Because the value of x was greater than or equal
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Table 1 Variations of RT and Tc for Nb2 O5 doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air. Composition (mol%)
RT ( cm)
Tc (◦ C)
0.99Ba0.96 Ca0.04 TiO3 –0.01BiYO3 –0.002Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.002Nb2 O5 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 0.96Ba0.96 Ca0.04 TiO3 –0.04BiYO3 –0.002Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.000Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.001Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.003Nb2 O5 0.98Ba0.96 Ca0.04 TiO3 –0.02BiYO3 –0.004Nb2 O5
1315 2090 3980 ≥106 6.5 × 105 6.6 × 104 2.8 × 104 5.1 × 104
126 140 153 163 154 151 146 142
to 0.01 in this study, more VBa was produced with the introduction of BiYO3 , which may lead to a thick layer rich in barium vacancies at the grain boundaries and the inhibition in the movement of free charges [25]. Therefore, the RT increased with the increase of x value. Ba2+ Ti4+ O3 2− + nM3+ → Ba1−n 2+ Mn 3+ Ti4+ O3 2− + nBa2+ + ne (3) Ba2+ Ti4+ O3 2− + nM3+ → Ba1−(3/2)n 2+ Mn 3+ Ti4+ O3 2− + (3/2)nBa2+ + (1/2)nVBa
(4)
Table 1 shows the RT and Tc of Nb2 O5 doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air. It can be seen that the RT and Tc varied with the BiYO3 content distinctly. With the increase of BiYO3 , the RT of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics sharply increased from 1315 cm (x = 0.01) to 106 cm(x = 0.04), which had been explained as above. 4. Conclusions The PTCR characteristics of Nb2 O5 -doped (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 thermistor ceramics sintered in N2 and reoxidized in air were investigated. The result showed that the minimum value of RT could be obtained when the Nb2 O5 content was 0.2 mol%. The Tc of (1 − x)Ba0.96 Ca0.04 TiO3 –xBiYO3 –0.002Nb2 O5 thermistor ceramics was shifted to a higher temperature with the increase of x value, but the RT was higher than 106 cm when x = 0.04. The 0.97Ba0.96 Ca0.04 TiO3 –0.03BiYO3 –0.002Nb2 O5 thermistor ceramics sintered at 1350 ◦ C in a N2 flow atmosphere and reoxidized at 900 ◦ C for 2 h in air exhibited a low RT of 3.98 × 103 cm, a typical PTCR effect of max /min > 103 and a Tc of 153 ◦ C. Acknowledgements
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Biographies
This research was supported by the National Natural Science Foundation of China (51072106), Research projects of Science and Technology Division, Shaanxi (2010K10-14), Foundation of Shaanxi Educational Committee (112H011), and the Graduate Innovation Fund of Shaanxi University of Science and Technology.
Yongping Pu was born in 1971 in Shanxi Province, PhD (Xi’an Jiaotong University), PhD director, professor. Now he works at School of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an. His major research focuses on lead-free PTC thermistor ceramics, temperature sensors, high density capacitors.
References
Haidong Wu received his bachelor degree (2009) from Shaanxi University of Science and Technology, China. His current research interest is temperature sensors.
[1] H. Takeda, H. Harinaka, T. Shiosaki, M.A. Zubair, C. Leach, R. Freer, T. Hoshina, T. Tsurumi, Fabrication and positive temperature coefficient of resistivity properties of semiconducting ceramics based on the BaTiO3 –(Bi1/2 K1/2 )TiO3 system, J. Eur. Ceram. Soc. 30 (2010) 555–559.
Jifeng Wei received his bachelor degree (2008) and master degree (2011) from Shaanxi University of Science and Technology, China. His current research interest is temperature sensors.