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Effect of measuring atmosphere on the electrical properties of nanoporous (Ba,Sr)(Ti,Sb)O3 ceramics
Materials Chemistry and Physics 80 (2003) 162–168
Effect of measuring atmosphere on the electrical properties of nanoporous (Ba,Sr)(Ti,Sb)O3 ceramics Jun-Gyu Kim∗ , Weon-Pil Tai Institute of Advanced Materials, Inha University, 253 Yonghyun-dong, Nam-ku, Inchon 402-751, South Korea Received 30 May 2002; received in revised form 29 August 2002; accepted 16 September 2002
Abstract Nanoporous (Ba,Sr)(Ti,Sb)O3 ceramics were fabricated by the addition of partially oxidized Ti powder of 4 wt.%, corn- and potato-starch of 20 wt.% into the (Ba,Sr)(Ti,Sb)O3 powder, respectively. The effect of measuring atmosphere on the electrical properties of the porous (Ba,Sr)(Ti,Sb)O3 ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the (Ba,Sr)(Ti,Sb)O3 ceramics were investigated. The electrical resistivity of the porous (Ba,Sr)(Ti,Sb)O3 ceramics was measured in the air, O2 , N2 and H2 atmospheres during heating and cooling cycles. It was found that the PTCR characteristics and room-temperature resistivity of the porous (Ba,Sr)(Ti,Sb)O3 ceramics decreased in reducing atmosphere mainly because of the decrease in potential barrier height, whereas it increased in oxidizing atmosphere mainly because of the increase in potential barrier height. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous (Ba,Sr)(Ti,Sb)O3 ceramics; Partially oxidized Ti; Starch; PTCR; Microstructure
1. Introduction The dramatical increase of the resistivity in barium titanate (BaTiO3 ) ceramic was observed near the Curie point (TC ) [1–11]. This behavior is known as the positive temperature coefficient of resistivity (PTCR) characteristics [12–20]. The most accepted model to explain the abrupt PTCR effect above TC in BaTiO3 ceramics is the Heywang model [14,15]. Here it is assumed that a two-dimensional layer of acceptor states at grain boundary results in a grain boundary barrier. The nature of surface acceptor states was assumed to be oxygen species chemically adsorbed on the grain surfaces in the Heywang model, and this was confirmed from the studies on the PTCR effect in porous BaTiO3 ceramics [8,14,15]. The PTCR characteristics when annealed under vacuum or in reducing gas atmospheres undergo changes that critically reduce the magnitude of the resistivity jump [21–24]. Porous BaTiO3 has been reported to exhibit large PTCR effects [8,25–29]. Oxygen can be adsorbed at the grain boundaries due to the presence of pores in the porous ceramics, which are more favorable to form surface acceptor states compared with ordinary dense ceramics [29]. In this study, porous (Ba,Sr)(Ti,Sb)O3 ceramics was fabricated by ∗ Tel.: +82-32-8608229; fax: +82-32-865546. E-mail address: [email protected] (J.-G. Kim).
adding TiO2 (Ti) powder (4 wt.%), corn- and potato-starch (20 wt.%), respectively. The aim of this study is to investigate the effect of measuring atmosphere on the electrical properties of the porous (Ba,Sr)(Ti,Sb)O3 ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the (Ba,Sr)(Ti,Sb)O3 ceramics.
2. Experimental procedure The (Ba,Sr)(Ti,Sb)O3 ceramic powder (Kyoritsu Yogyo Co. Ltd., Japan) utilized in this study was commercially obtained high-purity BaTiO3 powder with Sb2 O3 (0.2 mol%) and SrO (20 mol%). The mean particle size and ferroelectric Curie temperature of the powder is 0.9 m and 60 ◦ C, respectively. Ti powder with a mean particle size of 18 m was heated at 600 ◦ C for 1 h in flowing oxygen gas to produce partially oxidized Ti powder. The Ti powder was covered with TiO2 and is labeled as TiO2 (Ti) powder. The TiO2 (Ti) powder of 4 wt.%, corn- (Shinyo Pure Chemicals Co. Ltd., Japan) and potato-starch (Kanto Chemicals Co. Ltd., Japan) of 20 wt.%, respectively, were added to the (Ba,Sr)(Ti,Sb)O3 powder and then mixed in a mortar for 1 h. The mixed powder was compacted by a die-pressing at a pressure of 40 MPa to prepare the green compacts (15 mm × 12 mm × 7 mm). The green compacts were sintered at 1350 ◦ C for 1 h in air and vacuum. The samples
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were heated up to the sintering temperature with an increase in heating rate by 3 ◦ C min−1 and cooled with a cooling rate of 10 ◦ C min−1 from the sintering temperature to 300 ◦ C and then furnace cooled. Both faces of the samples (15 mm × 12 mm) were contacted with ohmic electrodes. The commercial ohmic paste (Ag–7 mass% Ni) of ∼10 m thickness was thinly spread on the two sides of surfaces. After the paste was dried at room temperature, the Ag paste of ∼10 m thickness was again covered on the ohmic paste layers, and then baked at 580 ◦ C for 5 min with a heating rate of 10 ◦ C min−1 in air. Subsequently, the leading copper wires of 0.58 mm diameter were joined to the central portion of the baked surfaces with solder (Pb–2 mass% Sn). Paste of Ga–40 mass% In composite was also spread for complex impedance measurements. The samples prepared in this study are summarized in Table 1. The microstructure of the (Ba,Sr)(Ti,Sb)O3 ceramic was analyzed by scanning electron microscopy (SEM: S-4200, Hitachi). The average grain size of the ceramic was estimated by the line-intersecting method. During heating and cooling cycles in the temperature range of 25–300 ◦ C, the electrical resistance was measured with a digital multi-meter under air, O2 , N2 and H2 atmospheres. Both the heating and cooling rates during resistance measurements were 5 ◦ C min−1 . Capacitance–voltage (C–V) characteristic was measured at 10 kHz using an impedance analyzer at room temperature in order to calculate the electrical potential barrier of grain boundaries and the donor concentration of grains. Complex impedance analysis was also conducted with an impedance
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analyzer at room temperature in the frequency range of 5 Hz–13 MHz in order to determine the electrical properties of grain boundaries and grains separately.
3. Results and discussion It was found that the (Ba,Sr)(Ti,Sb)O3 ceramics by adding TiO2 (Ti) powder, corn- and potato-starch, respectively, had a high porosity and fine grain size. The porosity of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 45.0, 42.3 and 41.2%, respectively. The mean grain
Table 1 Summary of the samples prepared in this study Sample
Measuring atmosphere
Sintering atmosphere
Additive
A1H A1C A2H A2C A3H A3C A4H A4C
Air Air O2 O2 N2 N2 H2 H2
Heating Cooling Heating Cooling Heating Cooling Heating Cooling
Vacuum
4 wt.% TiO2 (Ti)
B1H B1C B2H B2C B3H B3C B4H B4C
Air Air O2 O2 N2 N2 H2 H2
Heating Cooling Heating Cooling Heating Cooling Heating Cooling
Air
20 wt.% corn-starch
C1H C1C C2H C2C C3H C3C C4H C4C
Air Air O2 O2 N2 N2 H2 H2
Heating Cooling Heating Cooling Heating Cooling Heating Cooling
Air
20 wt.% potato-starch
Fig. 1. SEM micrograph of the fractured (Ba,Sr)(Ti,Sb)O3 surfaces for the ceramics containing: (a) 4 wt.% TiO2 (Ti); (b) 20 wt.% corn-starch; and (c) 20 wt.% potato-starch.
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size of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 1.2, 3.1 and 3.2 m, respectively. These porous ceramics are advantageous to oxidize grain boundaries and to produce surface acceptor states [29,30]. The porosity and the grain size were confirmed by the fractured surface observations. Fig. 1 shows the SEM micrographs of the fractured surfaces for the ceramics containing (a) 4 wt.% TiO2 (Ti), (b) 20 wt.% corn-starch and (c) 20 wt.% potato-starch. The morphology of (Ba,Sr)(Ti,Sb)O3 ceramic prepared using corn- and potato-starch is quite different from that of the (Ba,Sr)(Ti,Sb)O3 ceramic prepared using TiO2 (Ti) powder. Figs. 2 and 3 show the electrical resistivities of the porous (Ba,Sr)(Ti,Sb)O3 ceramic containing (a) TiO2 (Ti) powder, (b) corn-starch and (c) potato-starch measured in O2 , N2 atmospheres respectively, during heating and cooling cycles, and the electrical resistivity (ρ 25 ◦ C ) of 25 ◦ C and maximum electrical resistivity (ρ max ) for all the samples measured in air, O2 , N2 and H2 atmospheres are summarized in Table 2. The PTCR characteristics during cooling are nearly the same as those during heating in air atmosphere (Table 2). However, the PTCR characteristics during cooling are significantly different from those during heating in O2 , N2 and H2 atmospheres (Table 2). The resistivity during cooling in O2 atmosphere is slightly higher than that during heating cycle (Fig. 2). This is due to a small increase in potential barrier because of a small decrease in a number of conduction electrons owing to the adsorption of oxygen at the grain boundaries. In reducing atmospheres, the PTCR characteristics on cooling cycles are quite different from those on heating cycles (Fig. 3 and Table 2) and are summarized as follows: (1) the resistivity at low temperatures (<50 ◦ C) during cooling is much lower than that during heating; (2) the magnitude of resistivity jump during cooling is low com-
pared to heating; (3) the magnitude of electrical resistivity at high temperatures (>200 ◦ C) strongly depends on reducing atmospheres. This can be explained by the fact that nitrogen or hydrogen atoms formed from reducing N2 or H2 gases, respectively, diffuse along the grain boundaries and then react with chemisorbed oxygen atoms, and thus, chemisorbed oxygens are consumed and also conduction electrons are released. This leads to a decrease in the potential barrier because of an increase in the number of conduction electrons, degrading the PTCR effects. These results indicate that the porous (Ba,Sr)(Ti,Sb)O3 ceramic is strongly reduced and also the pores are important factors in this process [29]. It has already been reported that the degradation of BaTiO3 occurred upon annealing in reducing gases or in vacuum [21,30]. In addition, at room temperature the electrical resistivity of the (Ba,Sr)(Ti,Sb)O3 ceramic with TiO2 (Ti) powder is higher than that of the (Ba,Sr)(Ti,Sb)O3 ceramic with corn- and potato-starch during heating in air, O2 , N2 and H2 atmospheres. In order to investigate the reason for the discrepancy in the room-temperature resistivities during heating and cooling cycles under various atmospheres, and the reason behind the high electrical resistivity of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder at room temperature, the electrical potential barrier of grain boundaries (Φ) and the donor concentration of grains (Nd ) at room temperature were calculated using the following equation [31]: 1 2 2(Φ + qV) 1 − = (1) C 2C0 q 2 εs N d where C is the capacitance per unit area of a grain boundary, C0 is the capacitance at zero applied voltage, ε s is the dielectric constant of (Ba,Sr)(Ti,Sb)O3 , V is the applied voltage per grain boundary and q is the electronic charge. If the
Fig. 2. Electrical resistivities of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch measured during heating and cooling in O2 atmosphere.
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Fig. 3. Electrical resistivities of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch measured during heating and cooling in N2 atmosphere.
barrier model of grain boundary region is acceptable, plotting the left-hand term of Eq. (1) with the applied voltage yields a straight line. From Eq. (1), Nd and Φ can be calculated from the slope of the line and the intercept of the line on the voltage axis, respectively. In this measurement, the Table 2 Room-temperature and maximum electrical resistivity as well as the PTCR jumps of all the samples Sample
ρ 25 ◦ C ( cm)
ρ max ( cm)
PTCR jump (ρ max /ρ 25 ◦ C )
A1H A1C A2H A2C A3H A3C A4H A4C
3.75 × 104
>108 a
3.91 × 104 6.46 × 104 3.71 × 104 1.00 × 104 3.61 × 104 0.17 × 104
>108 a >108 a >108 a 3.13 × 107 >108 a 5.15 × 105
>2.67 × 103 >2.65 × 103 >2.56 × 103 >1.55 × 103 >2.69 × 103 3.13 × 103 >2.77 × 103 3.03 × 102
B1H B1C B2H B2C B3H B3C B4H B4C
3.73 × 102 3.84 × 102 3.81 × 102 1.02 × 103 4.36 × 102 1.52 × 102 4.06 × 102 6.56 × 10
>108 a >108 a >108 a >108 a >108 a 4.17 × 106 >108 a 1.48 × 104
>2.68 × 105 >2.60 × 105 >2.62 × 105 >9.80 × 104 >2.29 × 105 2.74 × 104 >2.46 × 105 2.26 × 102
C1H C1C C2H C2C C3H C3C C4H C4C
5.49 × 102 5.57 × 102 5.56 × 102 1.20 × 103 7.43 × 102 1.26 × 102 6.01 × 102 8.32 × 10
>108 a >108 a >108 a >108 a >108 a 3.28 × 106 >108 a 3.51 × 104
>1.82 × 105 >1.79 × 105 >1.79 × 105 >8.33 × 104 >1.34 × 105 2.60 × 104 >1.66 × 105 4.22 × 102
a
3.78 × 104
>108 a
Higher than the measuring limit (108 cm) of the multi-meter used.
ceramics were assumed to be composed of uniform cubic grains with an average grain size. Fig. 4 shows the capacitance–applied voltage (C–V) relation of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing (a) TiO2 (Ti) powder, (b) corn-starch and (c) potato-starch measured at room temperature during heating and cooling cycles under N2 atmosphere. The calculated donor concentrations of grains and the electrical potential barriers of grain boundaries for all the samples are summarized in Table 3. The calculated donor concentration of the samples during heating under air atmosphere is equal to that of cooling, whereas the calculated donor concentration during heating cycle under oxidizing and reducing atmospheres is different from that of cooling cycle. The calculated donor concentrations of the samples B1H, B1C, B2H, B2C, B3H, B3C, B4H and B4C are 2.44 × 1018 , 2.44 × 1018 , 2.44 × 1018 , 2.25 × 1018 , 2.44 × 1018 , 2.83 × 1018 , 2.44 × 1018 and 2.98 × 1018 cm−3 , respectively. This indicates that the donor concentrations of the samples during cooling under oxidizing and reducing atmospheres slightly decrease and increase, respectively. The electrical potential barrier height of grain boundaries for the samples during heating in the air, O2 , N2 and H2 atmospheres is basically same, and also the potential barrier height during cooling is extremely sensitive to the measuring atmosphere. On cooling cycle, the potential barrier height in oxidizing atmosphere increased whereas the barrier height in reducing atmosphere decreased. The potential barriers heights of the samples B1H, B1C, B2H, B2C, B3H, B3C, B4H and B4C are 0.005, 0.005, 0.005, 0.014, 0.006, 0.002, 0.006 and 0.001 eV, respectively. It is believed that the room-temperature resistivities during cooling are affected by mainly potential barrier heights. In addition, Table 3 indicated that the donor
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J.-G. Kim, W.-P. Tai / Materials Chemistry and Physics 80 (2003) 162–168 Table 3 Calculated donor concentrations, electrical potential barriers of grain boundaries for all samples
Fig. 4. Capacitance–applied voltage (C–V) relation of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch measured at room temperature during heating and cooling under N2 atmosphere.
concentration of grains of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder slightly decreased compared with that of the (Ba,Sr)(Ti,Sb)O3 ceramics with cornand potato-starch, while the electrical barrier height of grain boundaries of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder largely increased. It is concluded that the high room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder is due to the increase in the electrical barrier height of grain boundaries as well as the decrease in the grain size.
Sample
Donor concentration of grains, Nd (no. cm−3 )
Electrical potential barrier of grain boundaries, Φ (eV)
A1H A1C A2H A2C A3H A3C A4H A4C
1.71 × 1017 1.71 × 1017 1.71 × 1017 1.62 × 1017 1.71 × 1017 2.12 × 1017 1.71 × 1017 2.24 × 1017
0.523 0.527 0.545 0.907 0.517 0.137 0.503 0.022
B1H B1C B2H B2C B3H B3C B4H B4C
2.44 × 1018 2.44 × 1018 2.44 × 1018 2.25 × 1018 2.44 × 1018 2.83 × 1018 2.44 × 1018 2.98 × 1018
0.005 0.005 0.005 0.014 0.006 0.002 0.006 0.001
C1H C1C C2H C2C C3H C3C C4H C4C
2.47 × 1018 2.47 × 1018 2.47 × 1018 2.27 × 1018 2.38 × 1018 2.85 × 1018 2.41 × 1018 2.89 × 1018
0.008 0.008 0.008 0.017 0.010 0.002 0.008 0.001
Fig. 5 shows the impedance spectra of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing (a) TiO2 (Ti) powder, (b) corn-starch and (c) potato-starch obtained at 25 ◦ C during heating and cooling cycles in N2 atmosphere. The electrical resistivity of grain boundaries and sintered compacts at 25 ◦ C for all the samples is summarized in Table 4. The electrical resistivity of grain boundaries for the samples during heating cycle is nearly equal to that during cooling cycle in air atmosphere. However, the electrical resistivity of grain boundaries for the samples during heating cycle is quite different from that during cooling cycles in the O2 , N2 and H2 atmospheres. In particular, the grain boundary resistivity is substantially reduced on cooling cycle in reducing N2 or H2 atmosphere, due to the desorption of O2 at the grain boundaries, thus creating donor-type oxygen vacancies [32]. The grain boundary resistivities of samples B1C, B2C, B3C and B4C at room temperature are 3.75 × 102 , 1.01 × 103 , 1.50 × 102 and 6.41 × 10 cm, respectively. The variation in grain boundary resistivity may originate from a change in acceptor state concentration accompanied by adsorption and desorption of oxygen at the grain boundary regions. The increases in the grain boundary resistivity in O2 atmosphere gives rise to an increase in the resistivity of samples, while the decreases in the grain boundary resistivity in reducing atmospheres gives rise to a decrease in the resistivity of samples. This indicates that the grain boundary resistivity contributes largely to the total resistivity of samples. From the above results, the low room-temperature
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Table 4 Electrical resistivity of grain boundaries at 25 ◦ C for all the samples Sample
ρ 25 ◦ C ( cm)
ρ gb,25 ◦ C ( cm)
A1H A1C A2H A2C A3H A3C A4H A4C
3.75 × 104
3.78 × 104 3.91 × 104 6.46 × 104 3.71 × 104 1.00 × 104 3.61 × 104 0.17 × 104
3.74 × 104 3.77 × 104 3.90 × 104 6.45 × 104 3.70 × 104 0.98 × 104 3.60 × 104 0.16 × 104
B1H B1C B2H B2C B3H B3C B4H B4C
3.73 × 102 3.84 × 102 3.81 × 102 1.02 × 103 4.36 × 102 1.52 × 102 4.06 × 102 6.56 × 10
3.64 × 102 3.75 × 102 3.72 × 102 1.01 × 103 4.35 × 102 1.50 × 102 4.01 × 102 6.41 × 10
C1H C1C C2H C2C C3H C3C C4H C4C
5.49 × 102 5.57 × 102 5.56 × 102 1.20 × 103 7.43 × 102 1.26 × 102 6.01 × 102 8.32 × 10
5.47 × 102 5.55 × 102 5.51 × 102 1.18 × 103 7.41 × 102 1.24 × 102 6.00 × 102 8.15 × 10
4. Conclusion
Fig. 5. Impedance spectra of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch obtained at 25 ◦ C during heating and cooling in N2 atmosphere. The arrow F indicates the direction of frequency increase from 5 Hz to 13 MHz.
resistivity in reducing atmospheres is found to be due to the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. Also, the high room-temperature resistivity in oxidizing atmospheres is due to the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries. The present study strongly supports the validity of the Heywang model [14,15] for the explanation of PTCR effect in BaTiO3 -based ceramics.
The (Ba,Sr)(Ti,Sb)O3 ceramics by adding additive utilized in this study had a high porosity and fine grain size. The porosity of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 45.0, 42.3 and 41.2%, respectively. The mean grain size of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 1.2, 3.1 and 3.2 m, respectively. The PTCR characteristics during cooling are nearly the same as those during heating in air atmosphere. The resistivity during cooling in O2 atmosphere is slightly higher than that during heating cycle. In reducing atmospheres, the PTCR characteristics on cooling cycles are quite different from those on heating cycles. Namely, (1) the resistivity at low temperatures (<50 ◦ C) during cooling is much lower than that during heating, (2) the magnitude of resistivity jump during cooling is low compared to heating, (3) the magnitude of electrical resistivity at high temperatures (>200 ◦ C) strongly depends on reducing atmospheres. PTCR characteristics and room-temperature resistivity of the porous (Ba,Sr)(Ti,Sb)O3 ceramics decreased in reducing atmosphere mainly because of the decrease in potential barrier height, whereas it increased in oxidizing atmosphere mainly because of the increase in potential barrier height. References [1] H. Nagamoto, H. Kagotani, T. Okubo, J. Am. Ceram. Soc. 76 (1993) 2053.
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[17] T. Miki, A. Fujimoto, S. Jido, J. Appl. Phys. 83 (1998) 1592. [18] J.-S. Kim, S.-J.L. Kang, J. Am. Ceram. Soc. 82 (1999) 1196. [19] K. Hayashi, T. Yamamoto, Y. Ikuwara, T. Sakuma, J. Am. Ceram. Soc. 83 (2000) 2684. [20] S.Y. Yoon, K.H. Lee, H. Kim, J. Am. Ceram. Soc. 83 (2000) 2463. [21] M. Kahn, Am. Ceram. Soc. Bull. 50 (1971) 676. [22] G. Er, S. Ishida, N. Takeuchi, J. Ceram. Soc. Jpn. 106 (1998) 470. [23] N. Kurata, M. Kuwabara, J. Ceram. Soc. Jpn. 106 (1998) 1092. [24] S. Tashiro, A. Osonoi, H. Igarashi, J. Ceram. Soc. Jpn. 107 (1999) 15. [25] S.-M. Su, L.-Y. Zhang, H.-T. Sun, X. Yao, J. Am. Ceram. Soc. 77 (1994) 2154. [26] T.R. Shrout, D. Moffatt, W. Huebner, J. Mater. Sci. 26 (1991) 145. [27] J.H. Lee, J.J. Kim, S.H. Cho, Research Report, Kyungpook National University, Korea, 1990. [28] T. Takahashi, Y. Nakano, N. Ichinose, J. Ceram. Soc. Jpn. 98 (1990) 879. [29] M. Kuwabara, J. Am. Ceram. Soc. 64 (1981) C-170. [30] M. Kuwabara, Sol. State Electr. 27 (1984) 929. [31] K. Mukae, K. Tsuda, I. Nagasawa, J. Appl. Phys. 50 (1979) 4475. [32] D.C. Sinclair, A.R. West, J. Mater. Sci. 29 (1994) 6061.
Effect of measuring atmosphere on the electrical properties of nanoporous (Ba,Sr)(Ti,Sb)O3 ceramics Jun-Gyu Kim∗ , Weon-Pil Tai Institute of Advanced Materials, Inha University, 253 Yonghyun-dong, Nam-ku, Inchon 402-751, South Korea Received 30 May 2002; received in revised form 29 August 2002; accepted 16 September 2002
Abstract Nanoporous (Ba,Sr)(Ti,Sb)O3 ceramics were fabricated by the addition of partially oxidized Ti powder of 4 wt.%, corn- and potato-starch of 20 wt.% into the (Ba,Sr)(Ti,Sb)O3 powder, respectively. The effect of measuring atmosphere on the electrical properties of the porous (Ba,Sr)(Ti,Sb)O3 ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the (Ba,Sr)(Ti,Sb)O3 ceramics were investigated. The electrical resistivity of the porous (Ba,Sr)(Ti,Sb)O3 ceramics was measured in the air, O2 , N2 and H2 atmospheres during heating and cooling cycles. It was found that the PTCR characteristics and room-temperature resistivity of the porous (Ba,Sr)(Ti,Sb)O3 ceramics decreased in reducing atmosphere mainly because of the decrease in potential barrier height, whereas it increased in oxidizing atmosphere mainly because of the increase in potential barrier height. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Porous (Ba,Sr)(Ti,Sb)O3 ceramics; Partially oxidized Ti; Starch; PTCR; Microstructure
1. Introduction The dramatical increase of the resistivity in barium titanate (BaTiO3 ) ceramic was observed near the Curie point (TC ) [1–11]. This behavior is known as the positive temperature coefficient of resistivity (PTCR) characteristics [12–20]. The most accepted model to explain the abrupt PTCR effect above TC in BaTiO3 ceramics is the Heywang model [14,15]. Here it is assumed that a two-dimensional layer of acceptor states at grain boundary results in a grain boundary barrier. The nature of surface acceptor states was assumed to be oxygen species chemically adsorbed on the grain surfaces in the Heywang model, and this was confirmed from the studies on the PTCR effect in porous BaTiO3 ceramics [8,14,15]. The PTCR characteristics when annealed under vacuum or in reducing gas atmospheres undergo changes that critically reduce the magnitude of the resistivity jump [21–24]. Porous BaTiO3 has been reported to exhibit large PTCR effects [8,25–29]. Oxygen can be adsorbed at the grain boundaries due to the presence of pores in the porous ceramics, which are more favorable to form surface acceptor states compared with ordinary dense ceramics [29]. In this study, porous (Ba,Sr)(Ti,Sb)O3 ceramics was fabricated by ∗ Tel.: +82-32-8608229; fax: +82-32-865546. E-mail address: [email protected] (J.-G. Kim).
adding TiO2 (Ti) powder (4 wt.%), corn- and potato-starch (20 wt.%), respectively. The aim of this study is to investigate the effect of measuring atmosphere on the electrical properties of the porous (Ba,Sr)(Ti,Sb)O3 ceramics and the role of oxygen on the grain boundaries in the PTCR characteristics of the (Ba,Sr)(Ti,Sb)O3 ceramics.
2. Experimental procedure The (Ba,Sr)(Ti,Sb)O3 ceramic powder (Kyoritsu Yogyo Co. Ltd., Japan) utilized in this study was commercially obtained high-purity BaTiO3 powder with Sb2 O3 (0.2 mol%) and SrO (20 mol%). The mean particle size and ferroelectric Curie temperature of the powder is 0.9 m and 60 ◦ C, respectively. Ti powder with a mean particle size of 18 m was heated at 600 ◦ C for 1 h in flowing oxygen gas to produce partially oxidized Ti powder. The Ti powder was covered with TiO2 and is labeled as TiO2 (Ti) powder. The TiO2 (Ti) powder of 4 wt.%, corn- (Shinyo Pure Chemicals Co. Ltd., Japan) and potato-starch (Kanto Chemicals Co. Ltd., Japan) of 20 wt.%, respectively, were added to the (Ba,Sr)(Ti,Sb)O3 powder and then mixed in a mortar for 1 h. The mixed powder was compacted by a die-pressing at a pressure of 40 MPa to prepare the green compacts (15 mm × 12 mm × 7 mm). The green compacts were sintered at 1350 ◦ C for 1 h in air and vacuum. The samples
0254-0584/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 4 - 0 5 8 4 ( 0 2 ) 0 0 4 8 7 - X
J.-G. Kim, W.-P. Tai / Materials Chemistry and Physics 80 (2003) 162–168
were heated up to the sintering temperature with an increase in heating rate by 3 ◦ C min−1 and cooled with a cooling rate of 10 ◦ C min−1 from the sintering temperature to 300 ◦ C and then furnace cooled. Both faces of the samples (15 mm × 12 mm) were contacted with ohmic electrodes. The commercial ohmic paste (Ag–7 mass% Ni) of ∼10 m thickness was thinly spread on the two sides of surfaces. After the paste was dried at room temperature, the Ag paste of ∼10 m thickness was again covered on the ohmic paste layers, and then baked at 580 ◦ C for 5 min with a heating rate of 10 ◦ C min−1 in air. Subsequently, the leading copper wires of 0.58 mm diameter were joined to the central portion of the baked surfaces with solder (Pb–2 mass% Sn). Paste of Ga–40 mass% In composite was also spread for complex impedance measurements. The samples prepared in this study are summarized in Table 1. The microstructure of the (Ba,Sr)(Ti,Sb)O3 ceramic was analyzed by scanning electron microscopy (SEM: S-4200, Hitachi). The average grain size of the ceramic was estimated by the line-intersecting method. During heating and cooling cycles in the temperature range of 25–300 ◦ C, the electrical resistance was measured with a digital multi-meter under air, O2 , N2 and H2 atmospheres. Both the heating and cooling rates during resistance measurements were 5 ◦ C min−1 . Capacitance–voltage (C–V) characteristic was measured at 10 kHz using an impedance analyzer at room temperature in order to calculate the electrical potential barrier of grain boundaries and the donor concentration of grains. Complex impedance analysis was also conducted with an impedance
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analyzer at room temperature in the frequency range of 5 Hz–13 MHz in order to determine the electrical properties of grain boundaries and grains separately.
3. Results and discussion It was found that the (Ba,Sr)(Ti,Sb)O3 ceramics by adding TiO2 (Ti) powder, corn- and potato-starch, respectively, had a high porosity and fine grain size. The porosity of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 45.0, 42.3 and 41.2%, respectively. The mean grain
Table 1 Summary of the samples prepared in this study Sample
Measuring atmosphere
Sintering atmosphere
Additive
A1H A1C A2H A2C A3H A3C A4H A4C
Air Air O2 O2 N2 N2 H2 H2
Heating Cooling Heating Cooling Heating Cooling Heating Cooling
Vacuum
4 wt.% TiO2 (Ti)
B1H B1C B2H B2C B3H B3C B4H B4C
Air Air O2 O2 N2 N2 H2 H2
Heating Cooling Heating Cooling Heating Cooling Heating Cooling
Air
20 wt.% corn-starch
C1H C1C C2H C2C C3H C3C C4H C4C
Air Air O2 O2 N2 N2 H2 H2
Heating Cooling Heating Cooling Heating Cooling Heating Cooling
Air
20 wt.% potato-starch
Fig. 1. SEM micrograph of the fractured (Ba,Sr)(Ti,Sb)O3 surfaces for the ceramics containing: (a) 4 wt.% TiO2 (Ti); (b) 20 wt.% corn-starch; and (c) 20 wt.% potato-starch.
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size of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 1.2, 3.1 and 3.2 m, respectively. These porous ceramics are advantageous to oxidize grain boundaries and to produce surface acceptor states [29,30]. The porosity and the grain size were confirmed by the fractured surface observations. Fig. 1 shows the SEM micrographs of the fractured surfaces for the ceramics containing (a) 4 wt.% TiO2 (Ti), (b) 20 wt.% corn-starch and (c) 20 wt.% potato-starch. The morphology of (Ba,Sr)(Ti,Sb)O3 ceramic prepared using corn- and potato-starch is quite different from that of the (Ba,Sr)(Ti,Sb)O3 ceramic prepared using TiO2 (Ti) powder. Figs. 2 and 3 show the electrical resistivities of the porous (Ba,Sr)(Ti,Sb)O3 ceramic containing (a) TiO2 (Ti) powder, (b) corn-starch and (c) potato-starch measured in O2 , N2 atmospheres respectively, during heating and cooling cycles, and the electrical resistivity (ρ 25 ◦ C ) of 25 ◦ C and maximum electrical resistivity (ρ max ) for all the samples measured in air, O2 , N2 and H2 atmospheres are summarized in Table 2. The PTCR characteristics during cooling are nearly the same as those during heating in air atmosphere (Table 2). However, the PTCR characteristics during cooling are significantly different from those during heating in O2 , N2 and H2 atmospheres (Table 2). The resistivity during cooling in O2 atmosphere is slightly higher than that during heating cycle (Fig. 2). This is due to a small increase in potential barrier because of a small decrease in a number of conduction electrons owing to the adsorption of oxygen at the grain boundaries. In reducing atmospheres, the PTCR characteristics on cooling cycles are quite different from those on heating cycles (Fig. 3 and Table 2) and are summarized as follows: (1) the resistivity at low temperatures (<50 ◦ C) during cooling is much lower than that during heating; (2) the magnitude of resistivity jump during cooling is low com-
pared to heating; (3) the magnitude of electrical resistivity at high temperatures (>200 ◦ C) strongly depends on reducing atmospheres. This can be explained by the fact that nitrogen or hydrogen atoms formed from reducing N2 or H2 gases, respectively, diffuse along the grain boundaries and then react with chemisorbed oxygen atoms, and thus, chemisorbed oxygens are consumed and also conduction electrons are released. This leads to a decrease in the potential barrier because of an increase in the number of conduction electrons, degrading the PTCR effects. These results indicate that the porous (Ba,Sr)(Ti,Sb)O3 ceramic is strongly reduced and also the pores are important factors in this process [29]. It has already been reported that the degradation of BaTiO3 occurred upon annealing in reducing gases or in vacuum [21,30]. In addition, at room temperature the electrical resistivity of the (Ba,Sr)(Ti,Sb)O3 ceramic with TiO2 (Ti) powder is higher than that of the (Ba,Sr)(Ti,Sb)O3 ceramic with corn- and potato-starch during heating in air, O2 , N2 and H2 atmospheres. In order to investigate the reason for the discrepancy in the room-temperature resistivities during heating and cooling cycles under various atmospheres, and the reason behind the high electrical resistivity of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder at room temperature, the electrical potential barrier of grain boundaries (Φ) and the donor concentration of grains (Nd ) at room temperature were calculated using the following equation [31]: 1 2 2(Φ + qV) 1 − = (1) C 2C0 q 2 εs N d where C is the capacitance per unit area of a grain boundary, C0 is the capacitance at zero applied voltage, ε s is the dielectric constant of (Ba,Sr)(Ti,Sb)O3 , V is the applied voltage per grain boundary and q is the electronic charge. If the
Fig. 2. Electrical resistivities of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch measured during heating and cooling in O2 atmosphere.
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Fig. 3. Electrical resistivities of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch measured during heating and cooling in N2 atmosphere.
barrier model of grain boundary region is acceptable, plotting the left-hand term of Eq. (1) with the applied voltage yields a straight line. From Eq. (1), Nd and Φ can be calculated from the slope of the line and the intercept of the line on the voltage axis, respectively. In this measurement, the Table 2 Room-temperature and maximum electrical resistivity as well as the PTCR jumps of all the samples Sample
ρ 25 ◦ C ( cm)
ρ max ( cm)
PTCR jump (ρ max /ρ 25 ◦ C )
A1H A1C A2H A2C A3H A3C A4H A4C
3.75 × 104
>108 a
3.91 × 104 6.46 × 104 3.71 × 104 1.00 × 104 3.61 × 104 0.17 × 104
>108 a >108 a >108 a 3.13 × 107 >108 a 5.15 × 105
>2.67 × 103 >2.65 × 103 >2.56 × 103 >1.55 × 103 >2.69 × 103 3.13 × 103 >2.77 × 103 3.03 × 102
B1H B1C B2H B2C B3H B3C B4H B4C
3.73 × 102 3.84 × 102 3.81 × 102 1.02 × 103 4.36 × 102 1.52 × 102 4.06 × 102 6.56 × 10
>108 a >108 a >108 a >108 a >108 a 4.17 × 106 >108 a 1.48 × 104
>2.68 × 105 >2.60 × 105 >2.62 × 105 >9.80 × 104 >2.29 × 105 2.74 × 104 >2.46 × 105 2.26 × 102
C1H C1C C2H C2C C3H C3C C4H C4C
5.49 × 102 5.57 × 102 5.56 × 102 1.20 × 103 7.43 × 102 1.26 × 102 6.01 × 102 8.32 × 10
>108 a >108 a >108 a >108 a >108 a 3.28 × 106 >108 a 3.51 × 104
>1.82 × 105 >1.79 × 105 >1.79 × 105 >8.33 × 104 >1.34 × 105 2.60 × 104 >1.66 × 105 4.22 × 102
a
3.78 × 104
>108 a
Higher than the measuring limit (108 cm) of the multi-meter used.
ceramics were assumed to be composed of uniform cubic grains with an average grain size. Fig. 4 shows the capacitance–applied voltage (C–V) relation of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing (a) TiO2 (Ti) powder, (b) corn-starch and (c) potato-starch measured at room temperature during heating and cooling cycles under N2 atmosphere. The calculated donor concentrations of grains and the electrical potential barriers of grain boundaries for all the samples are summarized in Table 3. The calculated donor concentration of the samples during heating under air atmosphere is equal to that of cooling, whereas the calculated donor concentration during heating cycle under oxidizing and reducing atmospheres is different from that of cooling cycle. The calculated donor concentrations of the samples B1H, B1C, B2H, B2C, B3H, B3C, B4H and B4C are 2.44 × 1018 , 2.44 × 1018 , 2.44 × 1018 , 2.25 × 1018 , 2.44 × 1018 , 2.83 × 1018 , 2.44 × 1018 and 2.98 × 1018 cm−3 , respectively. This indicates that the donor concentrations of the samples during cooling under oxidizing and reducing atmospheres slightly decrease and increase, respectively. The electrical potential barrier height of grain boundaries for the samples during heating in the air, O2 , N2 and H2 atmospheres is basically same, and also the potential barrier height during cooling is extremely sensitive to the measuring atmosphere. On cooling cycle, the potential barrier height in oxidizing atmosphere increased whereas the barrier height in reducing atmosphere decreased. The potential barriers heights of the samples B1H, B1C, B2H, B2C, B3H, B3C, B4H and B4C are 0.005, 0.005, 0.005, 0.014, 0.006, 0.002, 0.006 and 0.001 eV, respectively. It is believed that the room-temperature resistivities during cooling are affected by mainly potential barrier heights. In addition, Table 3 indicated that the donor
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J.-G. Kim, W.-P. Tai / Materials Chemistry and Physics 80 (2003) 162–168 Table 3 Calculated donor concentrations, electrical potential barriers of grain boundaries for all samples
Fig. 4. Capacitance–applied voltage (C–V) relation of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch measured at room temperature during heating and cooling under N2 atmosphere.
concentration of grains of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder slightly decreased compared with that of the (Ba,Sr)(Ti,Sb)O3 ceramics with cornand potato-starch, while the electrical barrier height of grain boundaries of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder largely increased. It is concluded that the high room-temperature electrical resistivity of the (Ba,Sr)(Ti,Sb)O3 ceramics with TiO2 (Ti) powder is due to the increase in the electrical barrier height of grain boundaries as well as the decrease in the grain size.
Sample
Donor concentration of grains, Nd (no. cm−3 )
Electrical potential barrier of grain boundaries, Φ (eV)
A1H A1C A2H A2C A3H A3C A4H A4C
1.71 × 1017 1.71 × 1017 1.71 × 1017 1.62 × 1017 1.71 × 1017 2.12 × 1017 1.71 × 1017 2.24 × 1017
0.523 0.527 0.545 0.907 0.517 0.137 0.503 0.022
B1H B1C B2H B2C B3H B3C B4H B4C
2.44 × 1018 2.44 × 1018 2.44 × 1018 2.25 × 1018 2.44 × 1018 2.83 × 1018 2.44 × 1018 2.98 × 1018
0.005 0.005 0.005 0.014 0.006 0.002 0.006 0.001
C1H C1C C2H C2C C3H C3C C4H C4C
2.47 × 1018 2.47 × 1018 2.47 × 1018 2.27 × 1018 2.38 × 1018 2.85 × 1018 2.41 × 1018 2.89 × 1018
0.008 0.008 0.008 0.017 0.010 0.002 0.008 0.001
Fig. 5 shows the impedance spectra of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing (a) TiO2 (Ti) powder, (b) corn-starch and (c) potato-starch obtained at 25 ◦ C during heating and cooling cycles in N2 atmosphere. The electrical resistivity of grain boundaries and sintered compacts at 25 ◦ C for all the samples is summarized in Table 4. The electrical resistivity of grain boundaries for the samples during heating cycle is nearly equal to that during cooling cycle in air atmosphere. However, the electrical resistivity of grain boundaries for the samples during heating cycle is quite different from that during cooling cycles in the O2 , N2 and H2 atmospheres. In particular, the grain boundary resistivity is substantially reduced on cooling cycle in reducing N2 or H2 atmosphere, due to the desorption of O2 at the grain boundaries, thus creating donor-type oxygen vacancies [32]. The grain boundary resistivities of samples B1C, B2C, B3C and B4C at room temperature are 3.75 × 102 , 1.01 × 103 , 1.50 × 102 and 6.41 × 10 cm, respectively. The variation in grain boundary resistivity may originate from a change in acceptor state concentration accompanied by adsorption and desorption of oxygen at the grain boundary regions. The increases in the grain boundary resistivity in O2 atmosphere gives rise to an increase in the resistivity of samples, while the decreases in the grain boundary resistivity in reducing atmospheres gives rise to a decrease in the resistivity of samples. This indicates that the grain boundary resistivity contributes largely to the total resistivity of samples. From the above results, the low room-temperature
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Table 4 Electrical resistivity of grain boundaries at 25 ◦ C for all the samples Sample
ρ 25 ◦ C ( cm)
ρ gb,25 ◦ C ( cm)
A1H A1C A2H A2C A3H A3C A4H A4C
3.75 × 104
3.78 × 104 3.91 × 104 6.46 × 104 3.71 × 104 1.00 × 104 3.61 × 104 0.17 × 104
3.74 × 104 3.77 × 104 3.90 × 104 6.45 × 104 3.70 × 104 0.98 × 104 3.60 × 104 0.16 × 104
B1H B1C B2H B2C B3H B3C B4H B4C
3.73 × 102 3.84 × 102 3.81 × 102 1.02 × 103 4.36 × 102 1.52 × 102 4.06 × 102 6.56 × 10
3.64 × 102 3.75 × 102 3.72 × 102 1.01 × 103 4.35 × 102 1.50 × 102 4.01 × 102 6.41 × 10
C1H C1C C2H C2C C3H C3C C4H C4C
5.49 × 102 5.57 × 102 5.56 × 102 1.20 × 103 7.43 × 102 1.26 × 102 6.01 × 102 8.32 × 10
5.47 × 102 5.55 × 102 5.51 × 102 1.18 × 103 7.41 × 102 1.24 × 102 6.00 × 102 8.15 × 10
4. Conclusion
Fig. 5. Impedance spectra of the porous (Ba,Sr)(Ti,Sb)O3 ceramics containing: (a) TiO2 (Ti) powder; (b) corn-starch; and (c) potato-starch obtained at 25 ◦ C during heating and cooling in N2 atmosphere. The arrow F indicates the direction of frequency increase from 5 Hz to 13 MHz.
resistivity in reducing atmospheres is found to be due to the decrease in potential barrier height, which originates from an increase in the number of electrons owing to the desorption of chemisorbed oxygen atoms at the grain boundaries. Also, the high room-temperature resistivity in oxidizing atmospheres is due to the increase in potential barrier height, which results from the adsorption of chemisorbed oxygen atoms at the grain boundaries. The present study strongly supports the validity of the Heywang model [14,15] for the explanation of PTCR effect in BaTiO3 -based ceramics.
The (Ba,Sr)(Ti,Sb)O3 ceramics by adding additive utilized in this study had a high porosity and fine grain size. The porosity of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 45.0, 42.3 and 41.2%, respectively. The mean grain size of the ceramics containing TiO2 (Ti) powder, corn- and potato-starch were 1.2, 3.1 and 3.2 m, respectively. The PTCR characteristics during cooling are nearly the same as those during heating in air atmosphere. The resistivity during cooling in O2 atmosphere is slightly higher than that during heating cycle. In reducing atmospheres, the PTCR characteristics on cooling cycles are quite different from those on heating cycles. Namely, (1) the resistivity at low temperatures (<50 ◦ C) during cooling is much lower than that during heating, (2) the magnitude of resistivity jump during cooling is low compared to heating, (3) the magnitude of electrical resistivity at high temperatures (>200 ◦ C) strongly depends on reducing atmospheres. PTCR characteristics and room-temperature resistivity of the porous (Ba,Sr)(Ti,Sb)O3 ceramics decreased in reducing atmosphere mainly because of the decrease in potential barrier height, whereas it increased in oxidizing atmosphere mainly because of the increase in potential barrier height. References [1] H. Nagamoto, H. Kagotani, T. Okubo, J. Am. Ceram. Soc. 76 (1993) 2053.
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