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Effect of Y2O3 addition on the electrical properties of BaTiO3-based NTC thermistors
Materials Letters 60 (2006) 1011 – 1013 www.elsevier.com/locate/matlet
Effect of Y2O3 addition on the electrical properties of BaTiO3-based NTC thermistors Ying Luo a,b,c,⁎, Xinyu Liu a,b , Guohua Chen a a
Research Center for Materials Science and Engineering, Guilin University of Electronic Technology, Jinji Road No. 1, Guilin, Guangxi Province, 541004, PR China b College of Materials Science and Engineering, Central south University, Changsha 410083, PR China c Guilin Academy of Air Force, Guilin, 541004, PR China Received 7 August 2005; accepted 21 October 2005 Available online 10 November 2005
Abstract The influence of Y2O3 addition on the electrical properties and the microstructure of BaTiO3-based negative temperature coefficient (NTC) thermistors was studied. All the NTC thermistors prepared showed a linear relationship between log resistivity and the temperature, indicative of NTC characteristics. At a given BaTiO3 and BaBiO3 content, as the amount of Y2O3 in BaTiO3-based ceramics thermistors increased, the resistivity decreased to a minimum value and then increased again. Similarly, Y2O3 addition had the same effect on the coefficient of temperature sensitivity for those samples. © 2005 Elsevier B.V. All rights reserved. Keywords: NTC; BaBiO3; Y2O3; Microstructure; Electrical property
1. Introduction NTC thermistors are semiconducting ceramics that generally consist of transition metal oxides with the general formula AB2O4 [1–4].Their resistivity varies exponentially with temperature, as shown in the following Arrhenius equation [5]: ρ = ρoexp(B / T), where ρ0 is the resistivity of the material at infinite temperature, T is the absolute temperature and B is the B constant, sometimes called the coefficient of temperature sensitivity. In fact, B has the dimensions of temperature and is given by B = ΔE / k, where ΔE is the activation energy for electrical conduction and k is the Boltzmann constant. It is generally accepted that their conductivity is described by a thermally activated phonon-assisted hopping of charge carriers between cations of differing oxidation states on the octahedral sites of spinel structure [6–11]. The selection of a given metal oxide material for applications of NTC thermistors is mainly determined by the required ⁎ Corresponding author. Research Center for Materials Science and Engineering, Guilin University of Electronic Technology, Jinji Road No. 1, Guilin, Guangxi Province, 541004, PR China. Tel.: +86 773 5601 434; fax: +86 773 5605 683. E-mail address: [email protected] (Y. Luo). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.067
resistivity and B constant. For the wide range of applications of the thermistors, it is necessary to adjust the composition. The influence of BaBiO3 content on the electrical properties of BaTiO3-based NTC thermistors has been investigated [12]. In the present study, the effect of Y2O3 addition on the electrical properties of BaBiO3 doping BaTiO3-based NTC thermistors is introduced. 2. Experimental procedures Analytical grade BaCO3, TiO2, Y2O3 and BaBiO3 powders were weighed in appropriate proportions, as shown in Table 1. The weighed powders were agate milled in planetary for 24 h in Table 1 Chemical composition of the samples designed in this study Sample
Composition (molecular ratio)
N0 N1 N2 N3 N4 N5
100BaTiO3 + 10BaBiO3 100BaTiO3 + 10BaBiO3 + 0.05Y2O3 100BaTiO3 + 10BaBiO3 + 0.15Y2O3 100BaTiO3 + 10BaBiO3 + 0.25Y2O3 100BaTiO3 + 10BaBiO3 + 0.40Y2O3 100BaTiO3 + 10BaBiO3 + 0.50Y2O3
1012
Y. Luo et al. / Materials Letters 60 (2006) 1011–1013
alcohol. The ball-milled slurries were dried at 120 °C in an oven for 6 h. The dried powders were ground carefully in mortar and passed through a 250-mesh sieve. Subsequently, the mixture of powders was pressed at 175 MPa into 18 mm diameter and about 2.5 mm high cylindrical pellets. Pellets were sintered at 1250 °C for 1 h in air and then furnace cooled.
Fig. 2. At a given BaTiO3 and BaBiO3 content resistivity–temperature curves of samples doped different Y2O3 content sintered at 1250 °C for 1 h.
The Ag pastes with thickness of about 15 μm were spread on opposite-side surface of the sintered samples using a screen printer. After the pastes were dried at room temperature, the samples were heated at 600 °C for 30 min. The microstructure of the samples was investigated by using a scanning electron microscope (Model: JSM5610LV). The average grain size of the samples was estimated by using the line-intersecting method. The samples of each composition were prepared for measuring electrical resistance. The samples were held with a holder in a tube furnace and their temperatures were measured with a digital thermometer. The electrical resistance of the samples in the furnace was measured with a digital multimeter (Fluke 45) from 25 °C to 350 °C in steps of 20 °C. The accuracy of the furnace measurements is ± 0.5 °C. 3. Results and discussion The SEM images of cross-section of ceramics samples N1, N2, N3, N4 and N5 are shown in Fig. 1(a–e), respectively. Fig. 1(a) reveals that the average grain diameters are about 3.5 μm. In sample (b), the
Fig. 1. SEM images of cross-section of ceramics samples (a) N1 (Y2O3 = 0.05), (b) N2 (Y2O3 = 0.15), (c) N3 (Y2O3 = 0.25), (d) N4 (Y2O3 = 0.4) and (e) N5 (Y2O3 = 0.5) at a given BaTiO3 and BaBiO3 content.
Fig. 3. Specific resistivity at 25 °C and B25/125 value for BaTiO3-based ceramics as a function of Y2O3 content, x.
Y. Luo et al. / Materials Letters 60 (2006) 1011–1013 Table 2 Resistivity at 25 °C, B25/125 constant, and activation energy for the samples sintered at 1250 °C for 1 h Sample (BaTiO3/Y2O3)
Resistivity at 25 °C (kΩ cm)
B25/125 constant (K)
Activation energy (eV)
N0 (100:0) N1 (100:0.05) N2 (100:0.15) N3 (100:0.25) N4 (100:0.40) N5 (100:0.50)
20,063 14,434 2158 48 25,797 53,544
4517 4420 4291 3615 4484 4523
0.390 0.381 0.370 0.312 0.387 0.390
average grain diameters are about 4 μm. The grain size is not influenced significantly by the donor concentration. However, in sample (c), they are up to 15 μm. It is probable that Y2O3 content is up to the concentration of forming any liquid eutectic phase. With an increase in Y2O3 content, the grain size decreases rapidly. In samples (d) and (e), their average grain diameters are about 4.5 and 2.5 μm, respectively. Grain size decreases because of a significant dopant drag on the boundary mobility [13]. Fig. 2 reveals resistivity–temperature curves of the samples doped different Y2O3 content at a given BaTiO3 and BaBiO3 content sintered at 1250 °C for 1 h. According to the figure, these samples also exhibit typical NTC characteristic. The resistivity decreased exponentially with temperature. The B25/125 constant can be calculated by the following equation: B25=125 ¼
lnðq25 =q125 Þ 1=T25 −1=T125
where ρ25 and ρ125 are the resistivities measured at 25 and 125 °C, respectively. The calculated activation energy and B25/125 constant are listed in Table 2, together with the resistivity at 25 °C. The values of ρ25, B25/125 constant and the activation energy of the thermistors are 20,063–48–53,544 kΩ·cm, 4517–3615–4523 K and 0.390–0.312– 0.390 eV, respectively. The dependence of ρ25 and B25/125 constant of the samples on Y2O3 content are shown in Fig. 3. As the amount of Y2O3 added in BaTiO3based ceramics increases, the resistivity decreases to a minimum value probably. At high Y2O3 content (≥ 0.25), the resistivity increased again with increasing Y2O3 content. Furthermore, we observed that the resistivity measured at other temperatures showed basically the same behavior as that at room temperature, irrespective of the measuring temperature. This resistivity dependence on doping Y2O3 is similar to that of conventional BaTiO3 ceramics [13,14]. At small concentrations, donor incorporation by electronic compensation explained the high conductivity. As the average dopant concentration increased, the
1013
local donor concentration at the grain boundary increased rapidly because of segregation. This had one important effect that dopant incorporation at the grain boundary shifted from electronic to vacancy compensation, resulting in the formation of highly resistive layers. Similarly, Y2O3 addition had the same effect on the coefficient of temperature sensitivity for those samples.
4. Conclusion At a given BaTiO3 and BaBiO3 content, the influence of Y2O3 content on the microstructure and the electrical properties of BaTiO3-based NTC thermistors was investigated. At a low dopant concentration, the grain size was not influenced significantly by the donor concentration. With an increase in Y2O3 content, the grain size decreased rapidly. A linear relationship between log resistivity and the temperature for all the prepared Y2O3 doping BaTiO3-based NTC thermistors was observed. As the amount of Y2O3 added in BaTiO3-based ceramics increased, the resistivity decreased to a minimum value probably. At high Y2O3 content (≥ 0.25), the resistivity increased again with increasing Y2O3 content. Similarly, Y2O3 addition had the same effect on the coefficient of temperature sensitivity for those samples. References [1] E.G. Larson, R.J. Arnott, D.G. Wickham, J. Phys. Chem. Solids 23 (1962) 1771. [2] F. Golestani-Fard, S. Azimi, K.J.D. Mackenzie, J. Mater. Sci. 22 (1987) 2847. [3] B. Gillot, J.L. Baudour, F. Bouree, R. Metz, R. Legros, A. Rousset, Solid State Ion. 58 (1992) 155. [4] K. Park, D.Y. Bang, J.G. Kim, J.Y. Kim, C.H. Lee, B.H. Choi, J. Korean Phys. Soc. 41 (2) (2002) 692. [5] P. Fau, J.P. Bonino, J.J. Demai, J. Rousset, Appl. Surf. Sci. 319 (1993) 65. [6] D.S. Erickson, T.O. Mason, J. Solid State Chem. 59 (1985) 42. [7] H.J. Van Daal, A.J. Bosman, Phys. Rev. 158 (1967) 736. [8] F.E. Maranzana, Phys. Rev. 160 (1967) 421. [9] S.E. Dorris, T.O. Mason, J. Am. Ceram. Soc. 71 (1988) 379. [10] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41. [11] E.D. Macklen, J. Phys. Chem. Solids 47 (1986) 1073. [12] Y. Luo, X.Y. Liu, Mater. Lett. 59 (2005) 3381. [13] S.B. Desu, D.A. Payne, J. Am. Ceram. Soc. 73 (1990) 3407. [14] P.J. Wang, Z.Q. Zeng, Z.L. Gui, L.T. Li, Mater. Lett. 30 (1997) 275.
Effect of Y2O3 addition on the electrical properties of BaTiO3-based NTC thermistors Ying Luo a,b,c,⁎, Xinyu Liu a,b , Guohua Chen a a
Research Center for Materials Science and Engineering, Guilin University of Electronic Technology, Jinji Road No. 1, Guilin, Guangxi Province, 541004, PR China b College of Materials Science and Engineering, Central south University, Changsha 410083, PR China c Guilin Academy of Air Force, Guilin, 541004, PR China Received 7 August 2005; accepted 21 October 2005 Available online 10 November 2005
Abstract The influence of Y2O3 addition on the electrical properties and the microstructure of BaTiO3-based negative temperature coefficient (NTC) thermistors was studied. All the NTC thermistors prepared showed a linear relationship between log resistivity and the temperature, indicative of NTC characteristics. At a given BaTiO3 and BaBiO3 content, as the amount of Y2O3 in BaTiO3-based ceramics thermistors increased, the resistivity decreased to a minimum value and then increased again. Similarly, Y2O3 addition had the same effect on the coefficient of temperature sensitivity for those samples. © 2005 Elsevier B.V. All rights reserved. Keywords: NTC; BaBiO3; Y2O3; Microstructure; Electrical property
1. Introduction NTC thermistors are semiconducting ceramics that generally consist of transition metal oxides with the general formula AB2O4 [1–4].Their resistivity varies exponentially with temperature, as shown in the following Arrhenius equation [5]: ρ = ρoexp(B / T), where ρ0 is the resistivity of the material at infinite temperature, T is the absolute temperature and B is the B constant, sometimes called the coefficient of temperature sensitivity. In fact, B has the dimensions of temperature and is given by B = ΔE / k, where ΔE is the activation energy for electrical conduction and k is the Boltzmann constant. It is generally accepted that their conductivity is described by a thermally activated phonon-assisted hopping of charge carriers between cations of differing oxidation states on the octahedral sites of spinel structure [6–11]. The selection of a given metal oxide material for applications of NTC thermistors is mainly determined by the required ⁎ Corresponding author. Research Center for Materials Science and Engineering, Guilin University of Electronic Technology, Jinji Road No. 1, Guilin, Guangxi Province, 541004, PR China. Tel.: +86 773 5601 434; fax: +86 773 5605 683. E-mail address: [email protected] (Y. Luo). 0167-577X/$ - see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2005.10.067
resistivity and B constant. For the wide range of applications of the thermistors, it is necessary to adjust the composition. The influence of BaBiO3 content on the electrical properties of BaTiO3-based NTC thermistors has been investigated [12]. In the present study, the effect of Y2O3 addition on the electrical properties of BaBiO3 doping BaTiO3-based NTC thermistors is introduced. 2. Experimental procedures Analytical grade BaCO3, TiO2, Y2O3 and BaBiO3 powders were weighed in appropriate proportions, as shown in Table 1. The weighed powders were agate milled in planetary for 24 h in Table 1 Chemical composition of the samples designed in this study Sample
Composition (molecular ratio)
N0 N1 N2 N3 N4 N5
100BaTiO3 + 10BaBiO3 100BaTiO3 + 10BaBiO3 + 0.05Y2O3 100BaTiO3 + 10BaBiO3 + 0.15Y2O3 100BaTiO3 + 10BaBiO3 + 0.25Y2O3 100BaTiO3 + 10BaBiO3 + 0.40Y2O3 100BaTiO3 + 10BaBiO3 + 0.50Y2O3
1012
Y. Luo et al. / Materials Letters 60 (2006) 1011–1013
alcohol. The ball-milled slurries were dried at 120 °C in an oven for 6 h. The dried powders were ground carefully in mortar and passed through a 250-mesh sieve. Subsequently, the mixture of powders was pressed at 175 MPa into 18 mm diameter and about 2.5 mm high cylindrical pellets. Pellets were sintered at 1250 °C for 1 h in air and then furnace cooled.
Fig. 2. At a given BaTiO3 and BaBiO3 content resistivity–temperature curves of samples doped different Y2O3 content sintered at 1250 °C for 1 h.
The Ag pastes with thickness of about 15 μm were spread on opposite-side surface of the sintered samples using a screen printer. After the pastes were dried at room temperature, the samples were heated at 600 °C for 30 min. The microstructure of the samples was investigated by using a scanning electron microscope (Model: JSM5610LV). The average grain size of the samples was estimated by using the line-intersecting method. The samples of each composition were prepared for measuring electrical resistance. The samples were held with a holder in a tube furnace and their temperatures were measured with a digital thermometer. The electrical resistance of the samples in the furnace was measured with a digital multimeter (Fluke 45) from 25 °C to 350 °C in steps of 20 °C. The accuracy of the furnace measurements is ± 0.5 °C. 3. Results and discussion The SEM images of cross-section of ceramics samples N1, N2, N3, N4 and N5 are shown in Fig. 1(a–e), respectively. Fig. 1(a) reveals that the average grain diameters are about 3.5 μm. In sample (b), the
Fig. 1. SEM images of cross-section of ceramics samples (a) N1 (Y2O3 = 0.05), (b) N2 (Y2O3 = 0.15), (c) N3 (Y2O3 = 0.25), (d) N4 (Y2O3 = 0.4) and (e) N5 (Y2O3 = 0.5) at a given BaTiO3 and BaBiO3 content.
Fig. 3. Specific resistivity at 25 °C and B25/125 value for BaTiO3-based ceramics as a function of Y2O3 content, x.
Y. Luo et al. / Materials Letters 60 (2006) 1011–1013 Table 2 Resistivity at 25 °C, B25/125 constant, and activation energy for the samples sintered at 1250 °C for 1 h Sample (BaTiO3/Y2O3)
Resistivity at 25 °C (kΩ cm)
B25/125 constant (K)
Activation energy (eV)
N0 (100:0) N1 (100:0.05) N2 (100:0.15) N3 (100:0.25) N4 (100:0.40) N5 (100:0.50)
20,063 14,434 2158 48 25,797 53,544
4517 4420 4291 3615 4484 4523
0.390 0.381 0.370 0.312 0.387 0.390
average grain diameters are about 4 μm. The grain size is not influenced significantly by the donor concentration. However, in sample (c), they are up to 15 μm. It is probable that Y2O3 content is up to the concentration of forming any liquid eutectic phase. With an increase in Y2O3 content, the grain size decreases rapidly. In samples (d) and (e), their average grain diameters are about 4.5 and 2.5 μm, respectively. Grain size decreases because of a significant dopant drag on the boundary mobility [13]. Fig. 2 reveals resistivity–temperature curves of the samples doped different Y2O3 content at a given BaTiO3 and BaBiO3 content sintered at 1250 °C for 1 h. According to the figure, these samples also exhibit typical NTC characteristic. The resistivity decreased exponentially with temperature. The B25/125 constant can be calculated by the following equation: B25=125 ¼
lnðq25 =q125 Þ 1=T25 −1=T125
where ρ25 and ρ125 are the resistivities measured at 25 and 125 °C, respectively. The calculated activation energy and B25/125 constant are listed in Table 2, together with the resistivity at 25 °C. The values of ρ25, B25/125 constant and the activation energy of the thermistors are 20,063–48–53,544 kΩ·cm, 4517–3615–4523 K and 0.390–0.312– 0.390 eV, respectively. The dependence of ρ25 and B25/125 constant of the samples on Y2O3 content are shown in Fig. 3. As the amount of Y2O3 added in BaTiO3based ceramics increases, the resistivity decreases to a minimum value probably. At high Y2O3 content (≥ 0.25), the resistivity increased again with increasing Y2O3 content. Furthermore, we observed that the resistivity measured at other temperatures showed basically the same behavior as that at room temperature, irrespective of the measuring temperature. This resistivity dependence on doping Y2O3 is similar to that of conventional BaTiO3 ceramics [13,14]. At small concentrations, donor incorporation by electronic compensation explained the high conductivity. As the average dopant concentration increased, the
1013
local donor concentration at the grain boundary increased rapidly because of segregation. This had one important effect that dopant incorporation at the grain boundary shifted from electronic to vacancy compensation, resulting in the formation of highly resistive layers. Similarly, Y2O3 addition had the same effect on the coefficient of temperature sensitivity for those samples.
4. Conclusion At a given BaTiO3 and BaBiO3 content, the influence of Y2O3 content on the microstructure and the electrical properties of BaTiO3-based NTC thermistors was investigated. At a low dopant concentration, the grain size was not influenced significantly by the donor concentration. With an increase in Y2O3 content, the grain size decreased rapidly. A linear relationship between log resistivity and the temperature for all the prepared Y2O3 doping BaTiO3-based NTC thermistors was observed. As the amount of Y2O3 added in BaTiO3-based ceramics increased, the resistivity decreased to a minimum value probably. At high Y2O3 content (≥ 0.25), the resistivity increased again with increasing Y2O3 content. Similarly, Y2O3 addition had the same effect on the coefficient of temperature sensitivity for those samples. References [1] E.G. Larson, R.J. Arnott, D.G. Wickham, J. Phys. Chem. Solids 23 (1962) 1771. [2] F. Golestani-Fard, S. Azimi, K.J.D. Mackenzie, J. Mater. Sci. 22 (1987) 2847. [3] B. Gillot, J.L. Baudour, F. Bouree, R. Metz, R. Legros, A. Rousset, Solid State Ion. 58 (1992) 155. [4] K. Park, D.Y. Bang, J.G. Kim, J.Y. Kim, C.H. Lee, B.H. Choi, J. Korean Phys. Soc. 41 (2) (2002) 692. [5] P. Fau, J.P. Bonino, J.J. Demai, J. Rousset, Appl. Surf. Sci. 319 (1993) 65. [6] D.S. Erickson, T.O. Mason, J. Solid State Chem. 59 (1985) 42. [7] H.J. Van Daal, A.J. Bosman, Phys. Rev. 158 (1967) 736. [8] F.E. Maranzana, Phys. Rev. 160 (1967) 421. [9] S.E. Dorris, T.O. Mason, J. Am. Ceram. Soc. 71 (1988) 379. [10] I.G. Austin, N.F. Mott, Adv. Phys. 18 (1969) 41. [11] E.D. Macklen, J. Phys. Chem. Solids 47 (1986) 1073. [12] Y. Luo, X.Y. Liu, Mater. Lett. 59 (2005) 3381. [13] S.B. Desu, D.A. Payne, J. Am. Ceram. Soc. 73 (1990) 3407. [14] P.J. Wang, Z.Q. Zeng, Z.L. Gui, L.T. Li, Mater. Lett. 30 (1997) 275.