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The PTCR effect of Ag+-modified strontium–lead titanate semiconducting ceramics doped with excess PbO
Materials Science and Engineering B99 (2003) 313 /315 www.elsevier.com/locate/mseb
The PTCR effect of Ag-modified strontium lead titanate semiconducting ceramics doped with excess PbO /
Jingchang Zhao *, Longtu Li, Zhilun Gui State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 14 June 2002; received in revised form 9 October 2002
Abstract Y-doped Sr0.5Pb0.5TiO3 semiconducting ceramics were fabricated by adding excess PbO and controlling the sintering conditions. Their room temperature resistivity (rRT) is 101 V cm and there is a jump of about five orders of magnitude in resistivity, showing the positive temperature coefficient of resistance (PTCR) effect. Adding Ag increases both the rRT and the PTCR effect. Ag metal particles were observed to segregate in the 1.0 mol% Ag -doped composition. The influences of excess PbO and Ag on the electrical properties of (Sr, Pb)TiO3-based semiconducting ceramics were discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Strontium /lead titanate; Positive temperature coefficient of resistance
1. Introduction (Sr, Pb)TiO3-based electroceramic materials have received much attention since the composite thermal sensitivities were first observed in 1988 [1]. This class of semiconducting ceramics exhibit negative temperature coefficient of resistance (NTCR) effect below the Curie temperature (Tc) and the PTCR effect above Tc [2,3]. Furthermore, the sintering temperature of (Sr, Pb)TiO3 material is generally lower than that of the conventional BaTiO3 PTCR [4]. Therefore, (Sr, Pb)TiO3 can be developed for fabrication of certain electrical components, such as precise temperature controllers, overflow protect devices etc. [5]. The research for improving the thermal sensitivity and understanding the conduction mechanism in (Sr, Pb)TiO3 are intriguing [6,7]. Wang et al. [8] successfully prepared (Sr, Pb)TiO3based PTCR of low resistivity and weak NTC effect (T B/Tc), via chemical synthesis and rapid thermal sintered (RTS) processing. Analysis of the complex impedance confirmed the NTCR/PTCR phenomenon
* Corresponding author. Tel.: /86-10-627-84579; fax: /86-10-62771160. E-mail address: [email protected] (J. Zhao).
is a grain boundary effect [9]. However, it is difficult to lower its rRT to 103 V cm when it is fabricated by solidstate reaction, while the influence of acceptor on its resistivity-temperature characteristics has been rarely reported. The aim of this work is to synthesize Sr0.5Pb0.5TiO3based PTCR with low rRT by adding excess PbO and controlling the sintering conditions, and to investigate the influence of Ag incorporation on the electrical properties and microstructure of Y-doped Sr0.5Pb0.5TiO3 ceramics.
2. Experimental High purity PbO, TiO2, Y(NO3)3 and SrTiO3 were wet-milled in a plastic pot and preheated at 700 8C for 2 h to synthesize 0.5 mol% Y3-doped Sr0.5Pb0.5TiO3 powders. Subsequently, 4.0 mol% PbO and 0.1 /1.0 mol% AgNO3 (0.1 mol l1 water solution) were added and mixed into the calcined ceramic powders. The powders were then pressed into discs of 10 mm in diameter and about 1 mm in thickness, followed by sintering at 1100 8C for 1 h. During the cooling process, the pellets were soaked at 950 8C for 4 h in order to
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 6 1 - 5
J. Zhao et al. / Materials Science and Engineering B99 (2003) 313 /315
314
Fig. 1. Resistivity-temperature Sr0.5Pb0.5TiO3 ceramics.
plots
of
Ag -modified
Y-
volatilize out the residual PbO from the grain boundaries. The surfaces of sintered ceramics were coated with In /Ga alloy and the resistance-temperature characteristics were measured from room temperature to 400 8C using a dc resistance-temperature test system. Their microstructures were investigated by using JSM-6301F scanning electron microscope (SEM) equipped with energy dispersive analysis of X-ray (EDAX).
3. Results and discussion Fig. 1 shows the dependence of resistivity on the temperature of sintered Y-Sr0.5Pb0.5TiO3 ceramics. Without adding Ag , the ceramic exhibits the lowest rRT (101 V cm) and its resistivity jumps about five orders of magnitude in the PTCR effect region, but the increase of its resistivity with temperature is very slow. With modification of Ag , it was observed that rRT and PTCR effect increased with increasing Ag concentration. The detailed parameters are listed in Table 1, where rmin and rmax are the minimal resistivity and the
maximal resistivity, a 30 is the differential variability of resistivity at 30 8C higher than the switch temperature. The microstructure of 1.0 mol% Ag-modified Ydoped Sr0.5Pb0.5TiO3 ceramic was studied using SEM. Fig. 2 shows that the grains are cubical shape with a degree of abnormal grain growth and second phase particles can be found in some regions. Analysis of EDAX confirmed that the spherical particles are Ag metal, suggesting that the solubility of Ag in (Sr, Pb)TiO3 lattices is less than 1.0 mol%, and the residual Ag is segregated at the grain boundaries. In a previous study [10], we observed that rRT and thermal sensitivity of (Sr, Pb)TiO3 are evidently influenced by PbO loss during calcinations. According to Chang and co-workers [6], conventionally sintered (Sr, Pb)TiO3 contains Pb-deficient grain boundary layers. Therefore, the excess PbO can form a concentration gradient between grain boundaries and grain interiors, preventing Pb2 vacancies and stabilizing Pb/Ti ratio upon calcination. Apparently, the PbO segregated at the grain boundaries also increases the resistivities. The heating treatment in cooling stage was thus adopted in order to remove the residual PbO in this work. This was effective in lowering the resistivity of (Sr, Pb)TiO3 semiconducting ceramics. On the other hand, the resistivity-temperature curves in Fig. 1 shows that all samples with excess PbO additives exhibit typical PTCR characteristics. The variation of resistivity below the switch point is very small. On the basis of this observation, it may be assumed that the NTCR effect of Y-doped (Sr, Pb)TiO3 is closely related to the Pb2 vacancies and it can be adjusted by controlling PbO loss during calcination. Y-doped (Sr, Pb)TiO3 thermistor is an n -type semiconductor, when Y3 ions substitute A positions (A / Sr, Pb) to form the donor defects (YA+ ). The nonequilibrium charges can be compensated by two processes: (i) formation of Ti4+ e states, and (ii) formation of cationic vacancies or acceptor defects. The former improves the electrical conduction, because the trapped electrons on Ti4 ions are removable between Ti4 ions in an electrical field. On the contrary, both cationic vacancies and acceptor defects decrease the electrical conduction by forming defect complexes with donor defects (YA+ ). In Y-doped (Sr, Pb)TiO3, Ag could act
Table 1 Resistivity-temperature parameters of Ag -modified Y-Sr0.5Pb0.5TiO3 ceramics Samples
Undoped
0.1 mol% Ag
0.3 mol% Ag
0.5 mol% Ag
1.0 mol% Ag
rRT (V cm) rmax (V cm) a 30 (%/8C) log (rmax/rmin)
1.0/101 / 6.49 /4.89
1.9/101 / 7.30 /4.99
9.9/101 4.58/106 8.66 4.87
8.0/102 5.04/107 10.60 4.86
7.6 /103 8.80 /108 10.73 4.95
J. Zhao et al. / Materials Science and Engineering B99 (2003) 313 /315
315
Fig. 2. SEM of 1.0 mol% Ag -modified Y-Sr0.5Pb0.5TiO3 ceramics and EDAX analysis of the observed second phase.
as an acceptor. Owing to the size effect, Ag ions preferentially substitute A positions (A /Sr, Pb) in the (Sr, Pb)TiO3 lattices to form acceptor defects (AgA? ). Therefore, the donor defects can be compensated as: YA + Ag?A l(YA + ×Ag?A ) (A Sr; Pb) The above equation can satisfactorily explain the observation that rRT of Y-doped (Sr, Pb)TiO3 increases with increasing Ag concentration as seen in Fig. 1. Meanwhile, AgA? at the grain boundaries increases the density of acceptor state. These acceptor states then act as trapping centers to increase the height of barriers, which resulted in the enhancement of PTCR effect.
4. Conclusions Y-doped Sr0.5Pb0.5TiO3 ceramics were obtained by adding excess PbO and controlling the sintering conditions, resulting in a low resistivity (rRT101 V cm) and typical PTCR effect. Excess PbO at the grain boundaries depresses the formation of Pb2 vacancies, which decreases rRT and NTCR effect below Tc. With the
addition of Ag , both rRT and PTCR effect of Y-doped Sr0.5Pb0.5TiO3 ceramics were enhanced.
References [1] Y. Hamata, H. Takuchi, K. Zomura, Jpn. Patent No. 63-280401 (1988). [2] H.Y. Chang, K.S. Liu, C.T. Hu, et al., Jpn. J. Appl. Phys. 35 (1996) 656 /662. [3] Y.Y. Lu, T.Y. Tseng, Mater. Chem. Phys. 53 (1998) 132 /137. [4] D.J. Wang, J. Qiu, Z.L. Gui, L.T. Li, J. Mater. Res. 14 (7) (1999) 2993 /2996. [5] C. Lee, I.N. Lin, C.T. Hu, J. Am. Ceramic Soc. 77 (5) (1994) 1340 /1344. [6] H.Y. Chang, K.S. Liu, H.W. Chen, et al., Mater. Chem. Phys. 42 (1995) 258 /263. [7] H.F. Cheng, C.T. Hu, Y.Y. Lin, et al., Jpn. J. Appl. Phys. 37 (1998) 1932 /1938. [8] D.J. Wang, Z.L. Gui, L.T. Li, J. Mater. Sci. Mater. electronics 8 (1997) 271 /276. [9] D.J. Wang, J. Qiu, Y.C. Guo, et al., J. Mater. Res. 14 (1) (1999) 120 /123. [10] J. Zhao, L. Li, Z. Gui, J. Eur. Ceramic Soc. 22 (7) (2002) 1171 / 1175.
The PTCR effect of Ag-modified strontium lead titanate semiconducting ceramics doped with excess PbO /
Jingchang Zhao *, Longtu Li, Zhilun Gui State Key Laboratory of New Ceramics and Fine Processing, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 14 June 2002; received in revised form 9 October 2002
Abstract Y-doped Sr0.5Pb0.5TiO3 semiconducting ceramics were fabricated by adding excess PbO and controlling the sintering conditions. Their room temperature resistivity (rRT) is 101 V cm and there is a jump of about five orders of magnitude in resistivity, showing the positive temperature coefficient of resistance (PTCR) effect. Adding Ag increases both the rRT and the PTCR effect. Ag metal particles were observed to segregate in the 1.0 mol% Ag -doped composition. The influences of excess PbO and Ag on the electrical properties of (Sr, Pb)TiO3-based semiconducting ceramics were discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Strontium /lead titanate; Positive temperature coefficient of resistance
1. Introduction (Sr, Pb)TiO3-based electroceramic materials have received much attention since the composite thermal sensitivities were first observed in 1988 [1]. This class of semiconducting ceramics exhibit negative temperature coefficient of resistance (NTCR) effect below the Curie temperature (Tc) and the PTCR effect above Tc [2,3]. Furthermore, the sintering temperature of (Sr, Pb)TiO3 material is generally lower than that of the conventional BaTiO3 PTCR [4]. Therefore, (Sr, Pb)TiO3 can be developed for fabrication of certain electrical components, such as precise temperature controllers, overflow protect devices etc. [5]. The research for improving the thermal sensitivity and understanding the conduction mechanism in (Sr, Pb)TiO3 are intriguing [6,7]. Wang et al. [8] successfully prepared (Sr, Pb)TiO3based PTCR of low resistivity and weak NTC effect (T B/Tc), via chemical synthesis and rapid thermal sintered (RTS) processing. Analysis of the complex impedance confirmed the NTCR/PTCR phenomenon
* Corresponding author. Tel.: /86-10-627-84579; fax: /86-10-62771160. E-mail address: [email protected] (J. Zhao).
is a grain boundary effect [9]. However, it is difficult to lower its rRT to 103 V cm when it is fabricated by solidstate reaction, while the influence of acceptor on its resistivity-temperature characteristics has been rarely reported. The aim of this work is to synthesize Sr0.5Pb0.5TiO3based PTCR with low rRT by adding excess PbO and controlling the sintering conditions, and to investigate the influence of Ag incorporation on the electrical properties and microstructure of Y-doped Sr0.5Pb0.5TiO3 ceramics.
2. Experimental High purity PbO, TiO2, Y(NO3)3 and SrTiO3 were wet-milled in a plastic pot and preheated at 700 8C for 2 h to synthesize 0.5 mol% Y3-doped Sr0.5Pb0.5TiO3 powders. Subsequently, 4.0 mol% PbO and 0.1 /1.0 mol% AgNO3 (0.1 mol l1 water solution) were added and mixed into the calcined ceramic powders. The powders were then pressed into discs of 10 mm in diameter and about 1 mm in thickness, followed by sintering at 1100 8C for 1 h. During the cooling process, the pellets were soaked at 950 8C for 4 h in order to
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 6 1 - 5
J. Zhao et al. / Materials Science and Engineering B99 (2003) 313 /315
314
Fig. 1. Resistivity-temperature Sr0.5Pb0.5TiO3 ceramics.
plots
of
Ag -modified
Y-
volatilize out the residual PbO from the grain boundaries. The surfaces of sintered ceramics were coated with In /Ga alloy and the resistance-temperature characteristics were measured from room temperature to 400 8C using a dc resistance-temperature test system. Their microstructures were investigated by using JSM-6301F scanning electron microscope (SEM) equipped with energy dispersive analysis of X-ray (EDAX).
3. Results and discussion Fig. 1 shows the dependence of resistivity on the temperature of sintered Y-Sr0.5Pb0.5TiO3 ceramics. Without adding Ag , the ceramic exhibits the lowest rRT (101 V cm) and its resistivity jumps about five orders of magnitude in the PTCR effect region, but the increase of its resistivity with temperature is very slow. With modification of Ag , it was observed that rRT and PTCR effect increased with increasing Ag concentration. The detailed parameters are listed in Table 1, where rmin and rmax are the minimal resistivity and the
maximal resistivity, a 30 is the differential variability of resistivity at 30 8C higher than the switch temperature. The microstructure of 1.0 mol% Ag-modified Ydoped Sr0.5Pb0.5TiO3 ceramic was studied using SEM. Fig. 2 shows that the grains are cubical shape with a degree of abnormal grain growth and second phase particles can be found in some regions. Analysis of EDAX confirmed that the spherical particles are Ag metal, suggesting that the solubility of Ag in (Sr, Pb)TiO3 lattices is less than 1.0 mol%, and the residual Ag is segregated at the grain boundaries. In a previous study [10], we observed that rRT and thermal sensitivity of (Sr, Pb)TiO3 are evidently influenced by PbO loss during calcinations. According to Chang and co-workers [6], conventionally sintered (Sr, Pb)TiO3 contains Pb-deficient grain boundary layers. Therefore, the excess PbO can form a concentration gradient between grain boundaries and grain interiors, preventing Pb2 vacancies and stabilizing Pb/Ti ratio upon calcination. Apparently, the PbO segregated at the grain boundaries also increases the resistivities. The heating treatment in cooling stage was thus adopted in order to remove the residual PbO in this work. This was effective in lowering the resistivity of (Sr, Pb)TiO3 semiconducting ceramics. On the other hand, the resistivity-temperature curves in Fig. 1 shows that all samples with excess PbO additives exhibit typical PTCR characteristics. The variation of resistivity below the switch point is very small. On the basis of this observation, it may be assumed that the NTCR effect of Y-doped (Sr, Pb)TiO3 is closely related to the Pb2 vacancies and it can be adjusted by controlling PbO loss during calcination. Y-doped (Sr, Pb)TiO3 thermistor is an n -type semiconductor, when Y3 ions substitute A positions (A / Sr, Pb) to form the donor defects (YA+ ). The nonequilibrium charges can be compensated by two processes: (i) formation of Ti4+ e states, and (ii) formation of cationic vacancies or acceptor defects. The former improves the electrical conduction, because the trapped electrons on Ti4 ions are removable between Ti4 ions in an electrical field. On the contrary, both cationic vacancies and acceptor defects decrease the electrical conduction by forming defect complexes with donor defects (YA+ ). In Y-doped (Sr, Pb)TiO3, Ag could act
Table 1 Resistivity-temperature parameters of Ag -modified Y-Sr0.5Pb0.5TiO3 ceramics Samples
Undoped
0.1 mol% Ag
0.3 mol% Ag
0.5 mol% Ag
1.0 mol% Ag
rRT (V cm) rmax (V cm) a 30 (%/8C) log (rmax/rmin)
1.0/101 / 6.49 /4.89
1.9/101 / 7.30 /4.99
9.9/101 4.58/106 8.66 4.87
8.0/102 5.04/107 10.60 4.86
7.6 /103 8.80 /108 10.73 4.95
J. Zhao et al. / Materials Science and Engineering B99 (2003) 313 /315
315
Fig. 2. SEM of 1.0 mol% Ag -modified Y-Sr0.5Pb0.5TiO3 ceramics and EDAX analysis of the observed second phase.
as an acceptor. Owing to the size effect, Ag ions preferentially substitute A positions (A /Sr, Pb) in the (Sr, Pb)TiO3 lattices to form acceptor defects (AgA? ). Therefore, the donor defects can be compensated as: YA + Ag?A l(YA + ×Ag?A ) (A Sr; Pb) The above equation can satisfactorily explain the observation that rRT of Y-doped (Sr, Pb)TiO3 increases with increasing Ag concentration as seen in Fig. 1. Meanwhile, AgA? at the grain boundaries increases the density of acceptor state. These acceptor states then act as trapping centers to increase the height of barriers, which resulted in the enhancement of PTCR effect.
4. Conclusions Y-doped Sr0.5Pb0.5TiO3 ceramics were obtained by adding excess PbO and controlling the sintering conditions, resulting in a low resistivity (rRT101 V cm) and typical PTCR effect. Excess PbO at the grain boundaries depresses the formation of Pb2 vacancies, which decreases rRT and NTCR effect below Tc. With the
addition of Ag , both rRT and PTCR effect of Y-doped Sr0.5Pb0.5TiO3 ceramics were enhanced.
References [1] Y. Hamata, H. Takuchi, K. Zomura, Jpn. Patent No. 63-280401 (1988). [2] H.Y. Chang, K.S. Liu, C.T. Hu, et al., Jpn. J. Appl. Phys. 35 (1996) 656 /662. [3] Y.Y. Lu, T.Y. Tseng, Mater. Chem. Phys. 53 (1998) 132 /137. [4] D.J. Wang, J. Qiu, Z.L. Gui, L.T. Li, J. Mater. Res. 14 (7) (1999) 2993 /2996. [5] C. Lee, I.N. Lin, C.T. Hu, J. Am. Ceramic Soc. 77 (5) (1994) 1340 /1344. [6] H.Y. Chang, K.S. Liu, H.W. Chen, et al., Mater. Chem. Phys. 42 (1995) 258 /263. [7] H.F. Cheng, C.T. Hu, Y.Y. Lin, et al., Jpn. J. Appl. Phys. 37 (1998) 1932 /1938. [8] D.J. Wang, Z.L. Gui, L.T. Li, J. Mater. Sci. Mater. electronics 8 (1997) 271 /276. [9] D.J. Wang, J. Qiu, Y.C. Guo, et al., J. Mater. Res. 14 (1) (1999) 120 /123. [10] J. Zhao, L. Li, Z. Gui, J. Eur. Ceramic Soc. 22 (7) (2002) 1171 / 1175.