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Analysis on the aging characteristics of PTCR of donor-doped barium titanate
Materials Science and Engineering B99 (2003) 394 /398 www.elsevier.com/locate/mseb
Analysis on the aging characteristics of PTCR of donor-doped barium titanate Buyin Li *, Dongxiang Zhou, Daoli Zhang, Shenglin Jiang Department of Electronic Science & Technology, Huazhong University of Science & Technology, Wuhan 430074, People’s Republic of China Received 14 June 2002; accepted 10 September 2002
Abstract The resistance versus time curves of room temperature unload ageing, high temperature unload ageing and high temperature continuous load test ageing of PTC thermistors are measured. At the initial aging stage under both high temperature unload ageing and high temperature continuous load test ageing, the resistance decreases first, and then increases, which are different from that of the room temperature aging. Such abnormal phenomena exist in the initial ageing stage and are attributed to buffering of the unstable mechanical stress and desorption of the absorbed oxygen molecules under the thermal action. R /T characteristic curves of aged sample are different from that of oxidized and reduced samples. A new mechanism is proposed to interpret the aging characteristics of the donor-doped BaTiO3 PTC thermistor. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Barium titanate; PTCR; Ageing characteristics
1. Introduction It is well known that donor-doped barium titanate ceramic thermistor shows a positive temperature coefficient of resistance (PTCR) effect after sintering in air. These dramatic characteristics have found wide applications as color TV degausser, motor starter, self-regulating heaters, over current limiters and so on [1]. The characteristics of these elements depend on ambient working environments. The electrical properties of the PTCR elements would deteriorate when they are imposed on high temperature, high humidity environments for a long period, which is called ageing characteristics. Ageing characteristics such as an increase of resistance, a decrease of withstanding voltage will lead to failure of the elements. It is found that the temperature versus time curves of high temperature continuous load test are different from those of the room temperature aging, and the R /T characteristics of aged samples are different from those of reduced and oxidized samples, conventional theory cannot explain * Corresponding author. Tel./fax: /86-27-8754-2994. E-mail address: [email protected] (B. Li).
these phenomena [2 /4], our aim is to propose a new mechanism to explain phenomena of PTCR ageing characteristics under different circumstances.
2. Experiments The samples were prepared by conventional ceramic technique. Both sides of all samples were sintered with two layer electrodes. The inner layer electrode was commercialized Ag-Zn ohmic electrode, the outer layer was Ag surface electrode.
2.1. Room temperature and high temperature unload test ageing The PTCR elements are placed on the air at normal room temperature and ambient temperature 1009/5 8C, respectively, the room temperature resistance is measured at a certain time interval and the high temperature test sample must be cooled at room temperature for 24 h. The resistance versus time curves are shown, respectively, in Fig. 1.
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 5 0 9 - 3
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
Fig. 1. The curves of resistance vs. time of room temperature and high temperature unload test ageing.
2.2. High temperature continuous load test High temperature continuous load test circuit is shown in Fig. 2. Test condition: V /AC 350 V, RL / 30 V, ambient temperature 1009/5 8C. Resistance is measured after the elements are cooled at room temperature for 24 h at same time interval as that of the unload test ageing. The curve of resistance versus time of high temperature continuous load test is shown in Fig. 3. 2.3. Reducing and oxidizing experiments A number of samples are reduced in hydrogen / nitrogen mixed atmosphere and others are oxidized in oxygen gas at 800 8C for 1 h, respectively. 2.4. The R /T characteristics curves Before and after the all above experiments, the R /T characteristic curves are measured by the ZWX-B type PTCR R /T Automatic Test System which was manufactured by Huazhong University of Science and Technology.
Fig. 3. The curves of resistance vs. time of high temperature continuous load test.
aging (Figs. 1 and 3). Usually, the room temperature resistance increase is mainly attributed to the increase of the ceramic body of the sample, increase of the electrode resistance, and increase of contact resistance between the electrode and the ceramic body. It is obvious that the decrease of resistance occurs at the initial aging stage (seen in Fig. 1), such phenomena cannot be explained by the above-mentioned three causes. Sometimes, we can take for granted to think that such a decrease of resistance is due to the temperature of the ceramic body whose temperature is higher than the room temperature, and there is a NTC region below the Curie temperature. Fig. 4 is the R /T curves of the same sample measured at different time, the curve a is the curve of the first measurement, the curve b is that of the second measurement after the sample cooled at room temperature for 24 h, the main parameters are shown in Table 1. In Table 1, R25 8C is the sample’s resistance at 25 8C, Tc is the Curie temperature, Rmin is minimum resistance, Tmin is the corresponding temperature of Rmin, Rmax is the maximum resistance, Tmax is the corresponding temperature of Rmax, Alfa is the temperature coefficient, and Belta is the ratio of Rmax and
3. Experimental results and discussions Fig. 1 shows that the resistance of the room temperature aging samples increase little monotonously with time, while to curves of both the high temperature unload test and high temperature continuous load test, at the initial aging stage, the resistances decreases first, and then increases as those of the room temperature
Fig. 2. Measuring circuits of high temperature load continuous test.
395
Fig. 4. The R /T curves of room temperature aging.
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
396
Table 1 Parameters of the same sample measured at different time Number
R25 8C (V)
Tc (8C)
Rmin (V)
Tmin (8C)
Rmax (V)
Tmax (8C)
Alfa (%)
Trange (8C)
Belta
a b
25.02 23.67
129.5 125.3
20.05 18.36
89.6 85.1
5.6E/07 5.4E/07
211.2 214.5
26.90 25.55
121.6 129.4
2.8E/06 2.9E/06
Rmin, Trange is the difference between the Tmax and Tmin. Fig. 5 is the R /T curves of the same sample measuring at one continuous measurement, the curve a is the curve of resistance with the increase of temperature during the temperature rising period, while the curve b is that of resistance with the temperature during the temperature dropping period, the main parameters are shown in Table 2. From both the above figures and tables, it is clearly seen that there is a tendency of decrease of room temperature resistance, minimum resistance and maximum resistance at both the second measuring and the cooling measuring situations. Therefore, the resistance decrease relates to the thermal action of the elements, at the initial aging stage, the buffering of the mechanical stress and desorption of the absorbed oxygen can probably lead to transition from one meta-stable stage to stable stage of the samples’ interfaces. For example, the desorption of the absorbed oxygen at the grain boundary leads to release of some captured electrons, and therefore the concentration of electrons in grains increases, and the resistivity decreases. R /T curves of high temperature continuous load test is shown in Fig. 6. The curve a is the normal R /T characteristic one, curve b is that of the sample of 1000 h high temperature continuous load test. It can be seen that the curve a and curve b is crossed at one
temperature point, and Rmin of curve a is smaller than that of curve b, but Rmax of curve a is bigger than that of curve b, therefore, the characteristics of the aged sample becomes deteriorative. R /T curves of the reducing and oxidizing samples are shown in Fig. 7. The curve a is the normal R /T characteristic one, curve b is that of the sample treated in hydrogen/nitrogen mixed gas for 1 h, and curve c is that of the sample treated in oxygen gas atmosphere for 1 h. We find that both the room resistance and the maximum resistance increase simultaneously after the sample is oxidized and decrease simultaneously after the sample is reduced (seen from Fig. 7). These situation are completely different from that of aged samples. Usually, causes of aging characteristics of PTC thermistor of donor-doped barium titanate are mostly attributed to the oxygen absorption in the grain boundaries and the oxidation of the electrodes. By comparison with Figs. 6 and 7, the aging causes cannot simply attribute to the ambient oxygen absorption or desorption because the room resistance and the maximum resistance increase or decrease simultaneously under both situation. Similarly, if the aging is only caused by the oxidation of electrodes, the maximum resistance shouldnot decrease. So a new mechanism is proposed to interpret the aging characteristics of the donor-doped BaTiO3 PTC thermistor. So we consider the internal reducing reaction predominates in the grains [5]: Y2 O3 2TiO2 1 2Y+Ba 2TiTi 6OO O2 (g)2e? 2
(1)
At the grain boundaries, the cation vacancy compensation reaction predominates: Y2 O3 3TiO2 2Y+Ba VƒBa 3TiTi 9OO
(2)
During the aging process, the oxidizing reaction occurs at the grain boundaries first: 1 TiO2 O2 (g) VƒBa TiTi 3OO 2h+ 2
Fig. 5. The R /T curves of high temperature continuous load test.
(3)
This reaction is then gradually propagated from the grain boundaries to the internal grains. Eq. (3)/Eq. (1):
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
397
Table 2 Parameters of the same sample measured at temperature increase and decrease Number
R258C (V)
Tc (8C)
Rmin (V)
Tmin (8C)
Rmax (V)
Tmax (8C)
Alfa (%)
Trange (8C)
Belta
a b
25.13 23.70
130.1 128.0
20.10 18.99
89.7 89.3
5.6E/07 4.9E/07
210.3 210.6
26.90 26.74
120.6 121.3
2.8E/06 2.6E/06
However, if the oxygen in Eq. (3) is absorbed from the ambient atmosphere, the resistance of the grain boundaries should increase too. It is obviously contrast to the experimental observation. We consider that it is due to the reduction of grain boundaries themselves. Eq. (2) subtracted by Eq. (3) leads to Eq. (6), Eq. (6) is equivalent to Eq. (1). 1 Y2 O3 2TiO2 O2 (g) 2 2Y+Ba 2TiTi 6OO 2h+
Fig. 6. The R /T curves of high temperature continuous load test.
(6)
So the concentration of the cation vacancies decreases and the electron concentration increases in the grain boundaries, which leads to the decrease of the maximum resistance of aged samples. Therefore, the internal oxidation and reduction reaction is the key of the aging characteristics in donor-doped BaTiO3 PTCR thermistors.
4. Conclusion
Fig. 7. The R /T curves at different gas treatment.
1 Y2 O3 3TiO2 O2 (g) 2 1 2Y+Ba VƒBa 3TiTi 9OO O2 (g)2e? 2 2h+
(4)
The electron compensation occurs: 2e?2h+ null
(5)
which leads to Eq. (2) in the grains, so the concentration of the cation vacancies increases and the electron concentration in the grains decreases, which lead to the increase of the minimum resistance of aged samples.
The curves of resistance /time of are different from that of the room temperature aging. The resistance of the former two curves first decreases at the initial aging stage and then increases with time, but resistance of the room temperature aging element increases little monotonously with time. The decrease of resistance occurs under both the high temperature unload test and continuous load test at the initial aging stage. Thermal action can probably lead to transition from one meta-stable stage to stable stage of the samples’ interfaces, and desorption of the absorbed oxygen at the grain boundary lead to the decrease of both maximum and minimum resistance of PTCR elements. The oxidizing reaction occurs at the grain boundaries first, then gradually propagates from the grain boundaries to the internal grains, which lead to the increase of the minimum resistance of aged samples. Meanwhile, internal reducing reaction occurs at grain boundaries, which leads to the decrease of the maximum resistance of aged samples. The internal oxidation and reduction reaction is the key of the aging characteristics in donordoped BaTiO3 PTCR thermistors.
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B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
References [1] D.X. Zhou, S.P. Gong, PTC Materials and Application, The Press of Huazhong University of Science & technology, Wuhan, China, 1989. [2] M. Drofenik, J. Am. Ceram. Soc. 70 (5) (1987) 311 /314.
[3] W. Heywang, J. Am. Ceram. Soc. 47 (10) (1964) 484 /490. [4] E. Brzozowski, M.S. Castro, Ceram. Int. 26 (2000) 265 /269. [5] B.Y. Li, Study on the Defect Chemistry and Electrical Property of Oxide Semiconducting Ceramics, Doctoral Thesis of Huazhong University of Science & Technology, China, 2000.
Analysis on the aging characteristics of PTCR of donor-doped barium titanate Buyin Li *, Dongxiang Zhou, Daoli Zhang, Shenglin Jiang Department of Electronic Science & Technology, Huazhong University of Science & Technology, Wuhan 430074, People’s Republic of China Received 14 June 2002; accepted 10 September 2002
Abstract The resistance versus time curves of room temperature unload ageing, high temperature unload ageing and high temperature continuous load test ageing of PTC thermistors are measured. At the initial aging stage under both high temperature unload ageing and high temperature continuous load test ageing, the resistance decreases first, and then increases, which are different from that of the room temperature aging. Such abnormal phenomena exist in the initial ageing stage and are attributed to buffering of the unstable mechanical stress and desorption of the absorbed oxygen molecules under the thermal action. R /T characteristic curves of aged sample are different from that of oxidized and reduced samples. A new mechanism is proposed to interpret the aging characteristics of the donor-doped BaTiO3 PTC thermistor. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Barium titanate; PTCR; Ageing characteristics
1. Introduction It is well known that donor-doped barium titanate ceramic thermistor shows a positive temperature coefficient of resistance (PTCR) effect after sintering in air. These dramatic characteristics have found wide applications as color TV degausser, motor starter, self-regulating heaters, over current limiters and so on [1]. The characteristics of these elements depend on ambient working environments. The electrical properties of the PTCR elements would deteriorate when they are imposed on high temperature, high humidity environments for a long period, which is called ageing characteristics. Ageing characteristics such as an increase of resistance, a decrease of withstanding voltage will lead to failure of the elements. It is found that the temperature versus time curves of high temperature continuous load test are different from those of the room temperature aging, and the R /T characteristics of aged samples are different from those of reduced and oxidized samples, conventional theory cannot explain * Corresponding author. Tel./fax: /86-27-8754-2994. E-mail address: [email protected] (B. Li).
these phenomena [2 /4], our aim is to propose a new mechanism to explain phenomena of PTCR ageing characteristics under different circumstances.
2. Experiments The samples were prepared by conventional ceramic technique. Both sides of all samples were sintered with two layer electrodes. The inner layer electrode was commercialized Ag-Zn ohmic electrode, the outer layer was Ag surface electrode.
2.1. Room temperature and high temperature unload test ageing The PTCR elements are placed on the air at normal room temperature and ambient temperature 1009/5 8C, respectively, the room temperature resistance is measured at a certain time interval and the high temperature test sample must be cooled at room temperature for 24 h. The resistance versus time curves are shown, respectively, in Fig. 1.
0921-5107/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 2 ) 0 0 5 0 9 - 3
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
Fig. 1. The curves of resistance vs. time of room temperature and high temperature unload test ageing.
2.2. High temperature continuous load test High temperature continuous load test circuit is shown in Fig. 2. Test condition: V /AC 350 V, RL / 30 V, ambient temperature 1009/5 8C. Resistance is measured after the elements are cooled at room temperature for 24 h at same time interval as that of the unload test ageing. The curve of resistance versus time of high temperature continuous load test is shown in Fig. 3. 2.3. Reducing and oxidizing experiments A number of samples are reduced in hydrogen / nitrogen mixed atmosphere and others are oxidized in oxygen gas at 800 8C for 1 h, respectively. 2.4. The R /T characteristics curves Before and after the all above experiments, the R /T characteristic curves are measured by the ZWX-B type PTCR R /T Automatic Test System which was manufactured by Huazhong University of Science and Technology.
Fig. 3. The curves of resistance vs. time of high temperature continuous load test.
aging (Figs. 1 and 3). Usually, the room temperature resistance increase is mainly attributed to the increase of the ceramic body of the sample, increase of the electrode resistance, and increase of contact resistance between the electrode and the ceramic body. It is obvious that the decrease of resistance occurs at the initial aging stage (seen in Fig. 1), such phenomena cannot be explained by the above-mentioned three causes. Sometimes, we can take for granted to think that such a decrease of resistance is due to the temperature of the ceramic body whose temperature is higher than the room temperature, and there is a NTC region below the Curie temperature. Fig. 4 is the R /T curves of the same sample measured at different time, the curve a is the curve of the first measurement, the curve b is that of the second measurement after the sample cooled at room temperature for 24 h, the main parameters are shown in Table 1. In Table 1, R25 8C is the sample’s resistance at 25 8C, Tc is the Curie temperature, Rmin is minimum resistance, Tmin is the corresponding temperature of Rmin, Rmax is the maximum resistance, Tmax is the corresponding temperature of Rmax, Alfa is the temperature coefficient, and Belta is the ratio of Rmax and
3. Experimental results and discussions Fig. 1 shows that the resistance of the room temperature aging samples increase little monotonously with time, while to curves of both the high temperature unload test and high temperature continuous load test, at the initial aging stage, the resistances decreases first, and then increases as those of the room temperature
Fig. 2. Measuring circuits of high temperature load continuous test.
395
Fig. 4. The R /T curves of room temperature aging.
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
396
Table 1 Parameters of the same sample measured at different time Number
R25 8C (V)
Tc (8C)
Rmin (V)
Tmin (8C)
Rmax (V)
Tmax (8C)
Alfa (%)
Trange (8C)
Belta
a b
25.02 23.67
129.5 125.3
20.05 18.36
89.6 85.1
5.6E/07 5.4E/07
211.2 214.5
26.90 25.55
121.6 129.4
2.8E/06 2.9E/06
Rmin, Trange is the difference between the Tmax and Tmin. Fig. 5 is the R /T curves of the same sample measuring at one continuous measurement, the curve a is the curve of resistance with the increase of temperature during the temperature rising period, while the curve b is that of resistance with the temperature during the temperature dropping period, the main parameters are shown in Table 2. From both the above figures and tables, it is clearly seen that there is a tendency of decrease of room temperature resistance, minimum resistance and maximum resistance at both the second measuring and the cooling measuring situations. Therefore, the resistance decrease relates to the thermal action of the elements, at the initial aging stage, the buffering of the mechanical stress and desorption of the absorbed oxygen can probably lead to transition from one meta-stable stage to stable stage of the samples’ interfaces. For example, the desorption of the absorbed oxygen at the grain boundary leads to release of some captured electrons, and therefore the concentration of electrons in grains increases, and the resistivity decreases. R /T curves of high temperature continuous load test is shown in Fig. 6. The curve a is the normal R /T characteristic one, curve b is that of the sample of 1000 h high temperature continuous load test. It can be seen that the curve a and curve b is crossed at one
temperature point, and Rmin of curve a is smaller than that of curve b, but Rmax of curve a is bigger than that of curve b, therefore, the characteristics of the aged sample becomes deteriorative. R /T curves of the reducing and oxidizing samples are shown in Fig. 7. The curve a is the normal R /T characteristic one, curve b is that of the sample treated in hydrogen/nitrogen mixed gas for 1 h, and curve c is that of the sample treated in oxygen gas atmosphere for 1 h. We find that both the room resistance and the maximum resistance increase simultaneously after the sample is oxidized and decrease simultaneously after the sample is reduced (seen from Fig. 7). These situation are completely different from that of aged samples. Usually, causes of aging characteristics of PTC thermistor of donor-doped barium titanate are mostly attributed to the oxygen absorption in the grain boundaries and the oxidation of the electrodes. By comparison with Figs. 6 and 7, the aging causes cannot simply attribute to the ambient oxygen absorption or desorption because the room resistance and the maximum resistance increase or decrease simultaneously under both situation. Similarly, if the aging is only caused by the oxidation of electrodes, the maximum resistance shouldnot decrease. So a new mechanism is proposed to interpret the aging characteristics of the donor-doped BaTiO3 PTC thermistor. So we consider the internal reducing reaction predominates in the grains [5]: Y2 O3 2TiO2 1 2Y+Ba 2TiTi 6OO O2 (g)2e? 2
(1)
At the grain boundaries, the cation vacancy compensation reaction predominates: Y2 O3 3TiO2 2Y+Ba VƒBa 3TiTi 9OO
(2)
During the aging process, the oxidizing reaction occurs at the grain boundaries first: 1 TiO2 O2 (g) VƒBa TiTi 3OO 2h+ 2
Fig. 5. The R /T curves of high temperature continuous load test.
(3)
This reaction is then gradually propagated from the grain boundaries to the internal grains. Eq. (3)/Eq. (1):
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
397
Table 2 Parameters of the same sample measured at temperature increase and decrease Number
R258C (V)
Tc (8C)
Rmin (V)
Tmin (8C)
Rmax (V)
Tmax (8C)
Alfa (%)
Trange (8C)
Belta
a b
25.13 23.70
130.1 128.0
20.10 18.99
89.7 89.3
5.6E/07 4.9E/07
210.3 210.6
26.90 26.74
120.6 121.3
2.8E/06 2.6E/06
However, if the oxygen in Eq. (3) is absorbed from the ambient atmosphere, the resistance of the grain boundaries should increase too. It is obviously contrast to the experimental observation. We consider that it is due to the reduction of grain boundaries themselves. Eq. (2) subtracted by Eq. (3) leads to Eq. (6), Eq. (6) is equivalent to Eq. (1). 1 Y2 O3 2TiO2 O2 (g) 2 2Y+Ba 2TiTi 6OO 2h+
Fig. 6. The R /T curves of high temperature continuous load test.
(6)
So the concentration of the cation vacancies decreases and the electron concentration increases in the grain boundaries, which leads to the decrease of the maximum resistance of aged samples. Therefore, the internal oxidation and reduction reaction is the key of the aging characteristics in donor-doped BaTiO3 PTCR thermistors.
4. Conclusion
Fig. 7. The R /T curves at different gas treatment.
1 Y2 O3 3TiO2 O2 (g) 2 1 2Y+Ba VƒBa 3TiTi 9OO O2 (g)2e? 2 2h+
(4)
The electron compensation occurs: 2e?2h+ null
(5)
which leads to Eq. (2) in the grains, so the concentration of the cation vacancies increases and the electron concentration in the grains decreases, which lead to the increase of the minimum resistance of aged samples.
The curves of resistance /time of are different from that of the room temperature aging. The resistance of the former two curves first decreases at the initial aging stage and then increases with time, but resistance of the room temperature aging element increases little monotonously with time. The decrease of resistance occurs under both the high temperature unload test and continuous load test at the initial aging stage. Thermal action can probably lead to transition from one meta-stable stage to stable stage of the samples’ interfaces, and desorption of the absorbed oxygen at the grain boundary lead to the decrease of both maximum and minimum resistance of PTCR elements. The oxidizing reaction occurs at the grain boundaries first, then gradually propagates from the grain boundaries to the internal grains, which lead to the increase of the minimum resistance of aged samples. Meanwhile, internal reducing reaction occurs at grain boundaries, which leads to the decrease of the maximum resistance of aged samples. The internal oxidation and reduction reaction is the key of the aging characteristics in donordoped BaTiO3 PTCR thermistors.
398
B. Li et al. / Materials Science and Engineering B99 (2003) 394 /398
References [1] D.X. Zhou, S.P. Gong, PTC Materials and Application, The Press of Huazhong University of Science & technology, Wuhan, China, 1989. [2] M. Drofenik, J. Am. Ceram. Soc. 70 (5) (1987) 311 /314.
[3] W. Heywang, J. Am. Ceram. Soc. 47 (10) (1964) 484 /490. [4] E. Brzozowski, M.S. Castro, Ceram. Int. 26 (2000) 265 /269. [5] B.Y. Li, Study on the Defect Chemistry and Electrical Property of Oxide Semiconducting Ceramics, Doctoral Thesis of Huazhong University of Science & Technology, China, 2000.