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Role of second phase in (Nb,Ce,Si,Ca)-doped TiO2 varistor ceramics
Materials Letters 57 (2003) 3748 – 3754 www.elsevier.com/locate/matlet
Role of second phase in (Nb,Ce,Si,Ca)-doped TiO2 varistor ceramics Jianying Li a,b,*, Shaohua Luo c, Weihua Yao c, Zhongtai Zhang c b
a Multi-Disciplinary Materials Research Center, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China State Key Laboratory of Electrical Insulation for Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China c Department of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
Received 19 December 2002; accepted 3 January 2003
Abstract A low-voltage TiO2 varistor ceramics doped with Nb2O5, SiO2, CeO2 and CaCO3 was systematically researched. The effects of different dopants on varistor voltage V1 mA were investigated by orthogonal test method. SEM, XRD and EDAX were carried out to study the change of microstructure. The results show that there exist second phase on the surface of TiO2 grains, which can facilitate an increasing varistor voltage. The second phase is proved to be Perrierite phase (Ce2Ti2Si2O11) by EDAX and the content varies with sintering temperature. It is suggested that the second phase segregates at the grain surface during sintering process and makes an insulating layer which result in a higher V1 mA. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Varistors; Titanium oxide; Microstructure; Grains; Second phase; Nonlinear phenomena
1. Introduction Varistors are devices that show nonlinear electrical property. Their current –voltage characteristics can be expressed by I = CUa, where a is nonlinear coefficient. Nowadays the most common and best varistors are made from zinc oxide (ZnO) ceramics. It is widely used in various situations to protect electrical or electronic equipments against transient overvoltage. The ZnO varistor is one of the electronic ceramics whose properties are controlled by grain boundary * Corresponding author. State Key Laboratory of Electrical Insulation for Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. Tel: +86-29-2663781; fax: +86-29-2667056. E-mail address: [email protected]. (J. Li).
phenomena. However, it was generally accepted that the breakdown voltage of single grain boundary in ZnO ceramics is finite and may be assumed to be near a constant of 3 V [1]. The total breakdown voltage along the single thread direction of the current flow is the product of breakdown voltage of each grain boundary and the number of grain boundaries. Thus, it is rather restricted for ZnO varistors to be used in low-voltage systems. In an earlier work [2] we reported the relationship between breakdown electric field and thickness of various ZnO varistors. To continue our work on varistors, we studied the nonlinear properties of ceramics based on TiO2 to understand the behavior of a low-voltage varistor. A number of low-voltage varistor materials were reported such as TiO2 and SrTiO3 ceramics. They both exhibit larger dielectrics
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00173-3
J. Li et al. / Materials Letters 57 (2003) 3748–3754
constant as well as lower varistor voltage than ZnO ceramics. Thus they are attracting a great deal of attention for better characteristics of noise absorption and multifunction properties. Yan and Rhodes [3] firstly reported that (Nb,Ba)-doped TiO2 ceramics has useful varistor properties with an a of about 3– 4. Yang and Wu [4,5] did detailed research on (Nb,Ba,Bi)-doped TiO2 ceramics. It was revealed that Ba dominates the varistor action in the material system. Pennewiss and Hoffman [6] studied the effect of oxidizing condition on the electrical properties of Al-doped TiO2. The voltage-dependent resistivity was attributed to the surface oxidation layer rather than the grain boundary effect, which is quite similar to SrTiO3 varistor ceramics [7]. In this paper we are reporting a novel (Nb,Ce,Si,Ca)-doped TiO2 varistor ceramics, which has demonstrated to be potentially useful as new low-voltage varistor. Special attention is paid on the relationship between electrical property and microstructure.
2. Experimental Traditional ceramic process was used to prepare the TiO2 varistor ceramics. Raw materials used in this study were reagent-grade TiO2, Nb2O5, SiO2, CeO2 and CaCO3. The compositions were designed according to orthogonal table. The powder mixtures were wet ball-milled in a polyethylene bottle with ZrO2 balls for 3 h in deionized water. After dried and granulated the powder was pressed into disks of 12 mm in diameter and 1.2 mm in thickness. The green samples were sintered at temperatures ranging from 1300 to 1400 jC for 2– 4 h. For electrical measurements, an Ag electrode was formed on both polished surfaces of the disks. The Ag electrode was prepared at 570 jC for 20 min. The varistor voltage V0.1 mA and V1 mA were measured by using MY-4C meter. a was calculated by the following formula [8]. a¼
logðI2 =I1 Þ ¼ logðV2 =V1 Þ logðV1
1 mA =V0:1
were observed by using SEM (XL-30FEG) technology.
3. Results and discussion 3.1. Effects of dopants on V1
TiO2 1 0 S Nb2 O5 ! 2NbTi þ 2e þ 4O O þ O2 z 2
ð2Þ
In this process, free electrons can be released with the substitution of Ti4 + with Nb5 + when the Nb addition varies from 0.1 to 0.5 mol%. It was also found that the nonlinear coefficient a increases from 4.4 to 4.8 when the Nb2O5 addition changes from 0.2
Table 1 Orthogonal test of the (Nb,Ce,Si,Ca)-doped TiO2 ceramics
ð1Þ
The capacitance C and dielectric loss tand were measured by use of an LCR meter (2693D). The frequency is fixed at 1 kHz. The microstructures
mA
In order to investigate the roles of different dopants, the L9 (34) orthogonal table was used to carry out the experiments. The factor levels were shown in Table 1, which shows the addition of 0.1– 0.5 mol% for CeO2, 0.2 –0.4 mol% for Nb2O5, 0 –0.2 mol% for CaCO3, and 0.1– 0.5 mol% for SiO2. Furthermore, three sintering conditions were considered: 1350 jC 2 h, 1380 jC 2 h, and 1380 jC 4 h. The nonlinear coefficient a and varistor voltage V1 mA are the two most important performance parameters of varistors. In order to achieve low-voltage varistors, the effects of dopants on varistor voltage V1 mA were paid special attention. The corresponding results of Table 1 were shown in Fig. 1. The roles of Ce, Nb, Ca and Si on V1 mA were shown by four groups of curves, respectively. It can be seen that V1 mA increases sharply with increase of CeO2 addition. However, it decreases sharply with increase of Nb2O5, which is quite similar to Yang and Wu’s results. The role of Nb can be explained by the following defect reaction:
Level mA Þ
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Factors A
1 2 3
B
C
D
CeO2
Nb2O5
CaCO3
SiO2
0.1 0.3 0.5
0.2 0.3 0.4
0 0.15 0.2
0.1 0.3 0.5
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Fig. 1. Effects of dopant additions on varistor voltage V1
to 0.4 mol%. Thus, the substitution of Ti4 + with Nb5 + cannot only decrease the grain resistivity by more free electrons, but also leads to a higher grain boundary barrier and, consequently, higher a value. The effects of Ca and Si are less dominant compared with Ce and Nb. Lower varistor voltage can be obtained when the Ca content is 0.15 mol% and sintered at 1380 jC; lower SiO2 content of 0.1 mol% leads to lower V1 mA. It is generally accepted that SiO2 can facilitate the grain growth in ZnO varistor ceramics. However, the role of SiO2 may need to be reconsidered when applied in TiO 2 ceramics based on the results of Fig. 1. The sintering process shows little influence on the roles of above dopants. The curves show similar trends under different sintering conditions. However, generally, V1 mA decreases with the increase of sintering temperature and sintering period. This is due to larger grains caused by elevated sintering temperatures or prolonged sintering period. Such a behavior is similar to ZnO ceramics.
mA.
A: CeO2; B: Nb2O5; C: CaCO3; D: SiO2.
to 1350 jC, the varistor voltage V1 mA is clearly observed with lowest value of about 12 V. Furthur increase of sintering temperature to 1380 jC increases the varistor voltage to 16 V. On the other hand, the nonlinear coefficient a remains increasing monotonously with increasing sintering temperature in the sintering temperature range of 1300 – 1380 jC. It is thus clear that a low varistor voltage of 12 V and an a of 3.7 can be achieved under controlled processing conditions. From the experimental results, it can be seen that an increase of sintering temperature from 1350 to 1380 jC leads to higher varistor voltage,
3.2. The observed second phase and microstructure According to the orthogonal test results, an optimum composition was obtained. Based on this composition, the change of V1 mA and a against sintering temperature in the range from 1300 to 1380 jC was shown in Fig. 2. As the sintering temperature is raised
Fig. 2. Sintering temperature dependence of varistor voltage V1 and nonlinear coefficient a.
mA
J. Li et al. / Materials Letters 57 (2003) 3748–3754
which is different from reported results. It is generally expected that a raised sintering temperature leads to larger grains and thus lower varistor voltage. To understand these results, macro- and microstructural studies were carried out with XRD and SEM techniques. In order to examine whether any additional phases are formed during sintering process, we carried out the structural investigation using the X-ray powder diffraction technique. Powder XRD patterns of different samples sintered at five different temperatures (1100, 1300, 1330, 1350 and 1380 jC) and that of the unsintered raw powder recorded at 25 jC are given in Fig. 3. The XRD pattern of the unsintered powder shows that the raw powder is anatase (with major intensity peaks at 2h = 25.28j, 36.94j, 48.05j, etc.). As the sintering temperature is raised to 1100 jC the evolution of the rutile phase can be seen. The sample sintered at 1100 jC shows all the major peaks that correspond to a rutile structure (with major intensity peaks at 2h = 27.48j, 36.08j, 41.22j, 54.32j, etc.). Furthermore, a very small trace of second phase (marked with ‘‘S’’ in the spectra) present at 2h values of 30.2j can be found. At 1100 jC the second phases are less prominent. As the sintering temperature is raised to 1300 and 1330 jC, two strong intensity
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peaks present at 2h values of 29.08j and 30.2j can be clearly found. However, when the sintering temperature is raised to 1350 jC, the intensity peaks almost disappear. Further increase of temperature to 1380 jC makes two strong intensity peaks again. The presentation of second phase and its relation with sintering temperature can well explain the sintering temperature dependence of varistor voltage V1 mA. It is suggested that the second phase is electrical insulating and stronger peaks in the XRD pattern account for higher varistor voltage in Fig. 2. Since the maximum intensity peaks corresponding to the rutile phase are very sharp, they must represent the TiO2 grains. Thus the second phase is expected to segregate at the grain surface during sintering process and makes an electrical insulating layer, which can facilitate higher varistor voltage. SEM micrographs of these samples sintered at five temperatures (1100, 1300, 1330, 1350 and 1380 jC) are shown in Fig. 4. The grain size of the samples increases with increasing sintering temperature. The sample sintered at 1100 jC shows an average size of less than 1 Am and lots of voids can be seen. Thus the sintering temperature is in the initial stage at this temperature of 1100 jC. The sample sintered at 1300 jC shows an average grain size of 3.3 Am. As
Fig. 3. XRD pattern of samples sintered at different temperatures (S represent second phase).
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J. Li et al. / Materials Letters 57 (2003) 3748–3754
Fig. 4. SEM micrographs of samples sintered at different temperatures. (a) 1100 jC, (b) 1300 jC, (c) 1330 jC, (d) 1350 jC and (e) 1380 jC.
J. Li et al. / Materials Letters 57 (2003) 3748–3754
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Fig. 5. EDAX spectra of the second phase.
the sintering temperature is increased to 1330 jC the grain size also increase to about 4 Am. At 1350 and 1380 jC the grain size increased to 6.5 and 7 Am, respectively. From these SEM micrographs it can be seen that there appear second phase on the surface of TiO2 grains. The second phase appears in two forms: column-like and network-like phase. When the sintering temperature is 1300, 1330 and 1380 jC, the second phase appear in both two forms. When the sintering temperature is 1350 jC, there only exists column-like second phase. Thus there is less second phase at 1350 jC compared with other sintering temperatures. The result matches the XRD results and it can well explain the V1 mA – sintering temperature relation. To investigate the composition of the second phase, EDAX was carried out and the result was shown in Fig. 5. It is clear that the second phase is mainly composed of Ti, Ce and Si, which is most likely to be Perrierite phase (Ce2Ti2Si2O11) [9]. Due to the complicated multi-layer structure formed in the Perrierite phase [10], the migration of electronic carriers is rather limited with the increased content of second phase. Such a result matches the relation between V1 mA and Ce, Si addition as shown in Fig. 1. The increase of Ce and Si addition lead to more insulating second phase on grain surface and, consequently, higher varistor voltage can be obtained.
4. Conclusion (Nb,Ce,Si,Ca)-doped TiO2 ceramics reveals some interesting phenomena in microstructure characteristics when sintering temperature was varied. Conclusions can be drawn from this investigation as follows: 1. The varistor voltage V1 mA shows close dependence on CeO2 and Nb2O5 addition. An increasing addition of CeO2 from 0.1 to 0.5 mol% leads to increasing V1 mA while an increasing addition of Nb2O5 from 0.2 to 0.4 mol% leads to decreasing V1 mA. 2. A second phase mainly composed of Ce, Ti and Si appears on the grain surface region. XRD and SEM results reveal that the content of second phase reaches a minimum at 1350 jC, which accounts for lowest varistor voltage V1 mA of 12 V in the sintering temperature range from 1300 to 1380 jC.
References [1] M. Bartkowiak, G.D. Mahan, F.A. Mdine, M.A. Alim, J. Appl. Phys. 80 (1996) 6516. [2] S.T. Li, J.Y. Li, F.Y. Liu, M.A. Alim, G. Chen, J. Phys. D: Appl. Phys. 35 (2002) 1884. [3] M.F. Yan, W.W. Rhodes, Appl. Phys. Lett. 40 (1982) 536. [4] S.-L. Yang, J.-M. Wu, J. Mater. Res. 10 (1995) 345.
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J. Li et al. / Materials Letters 57 (2003) 3748–3754
[5] S.-L. Yang, J.-M. Wu, J. Am. Ceram. Soc. 76 (1993) 145. [6] J. Pennewiss, B. Hoffmann, Mater. Lett. 9 (1990) 219. [7] J.Y. Li, S.T. Li, M.A. Alim, G. Chen, J. Mater. Sci., Mater. Electron. (in press). [8] P.N. Santhosh, D.K. Kharat, S.K. Date, Mater. Lett. 28 (1996) 37.
[9] H. Miyajima, S. Matsubara, R. Miyawaki, K. Yokoyama, K. Hirokawa, Mineral. Mag. 65 (2001) 111. [10] C. Gueho, D. Giaquinta, J.L. Mansot, T. Ebel, P. Palvadeau, Chem. Mater. 7 (1995) 486.
Role of second phase in (Nb,Ce,Si,Ca)-doped TiO2 varistor ceramics Jianying Li a,b,*, Shaohua Luo c, Weihua Yao c, Zhongtai Zhang c b
a Multi-Disciplinary Materials Research Center, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China State Key Laboratory of Electrical Insulation for Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China c Department of Material Science and Engineering, Tsinghua University, Beijing 100084, People’s Republic of China
Received 19 December 2002; accepted 3 January 2003
Abstract A low-voltage TiO2 varistor ceramics doped with Nb2O5, SiO2, CeO2 and CaCO3 was systematically researched. The effects of different dopants on varistor voltage V1 mA were investigated by orthogonal test method. SEM, XRD and EDAX were carried out to study the change of microstructure. The results show that there exist second phase on the surface of TiO2 grains, which can facilitate an increasing varistor voltage. The second phase is proved to be Perrierite phase (Ce2Ti2Si2O11) by EDAX and the content varies with sintering temperature. It is suggested that the second phase segregates at the grain surface during sintering process and makes an insulating layer which result in a higher V1 mA. D 2003 Elsevier Science B.V. All rights reserved. Keywords: Varistors; Titanium oxide; Microstructure; Grains; Second phase; Nonlinear phenomena
1. Introduction Varistors are devices that show nonlinear electrical property. Their current –voltage characteristics can be expressed by I = CUa, where a is nonlinear coefficient. Nowadays the most common and best varistors are made from zinc oxide (ZnO) ceramics. It is widely used in various situations to protect electrical or electronic equipments against transient overvoltage. The ZnO varistor is one of the electronic ceramics whose properties are controlled by grain boundary * Corresponding author. State Key Laboratory of Electrical Insulation for Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China. Tel: +86-29-2663781; fax: +86-29-2667056. E-mail address: [email protected]. (J. Li).
phenomena. However, it was generally accepted that the breakdown voltage of single grain boundary in ZnO ceramics is finite and may be assumed to be near a constant of 3 V [1]. The total breakdown voltage along the single thread direction of the current flow is the product of breakdown voltage of each grain boundary and the number of grain boundaries. Thus, it is rather restricted for ZnO varistors to be used in low-voltage systems. In an earlier work [2] we reported the relationship between breakdown electric field and thickness of various ZnO varistors. To continue our work on varistors, we studied the nonlinear properties of ceramics based on TiO2 to understand the behavior of a low-voltage varistor. A number of low-voltage varistor materials were reported such as TiO2 and SrTiO3 ceramics. They both exhibit larger dielectrics
0167-577X/03/$ - see front matter D 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0167-577X(03)00173-3
J. Li et al. / Materials Letters 57 (2003) 3748–3754
constant as well as lower varistor voltage than ZnO ceramics. Thus they are attracting a great deal of attention for better characteristics of noise absorption and multifunction properties. Yan and Rhodes [3] firstly reported that (Nb,Ba)-doped TiO2 ceramics has useful varistor properties with an a of about 3– 4. Yang and Wu [4,5] did detailed research on (Nb,Ba,Bi)-doped TiO2 ceramics. It was revealed that Ba dominates the varistor action in the material system. Pennewiss and Hoffman [6] studied the effect of oxidizing condition on the electrical properties of Al-doped TiO2. The voltage-dependent resistivity was attributed to the surface oxidation layer rather than the grain boundary effect, which is quite similar to SrTiO3 varistor ceramics [7]. In this paper we are reporting a novel (Nb,Ce,Si,Ca)-doped TiO2 varistor ceramics, which has demonstrated to be potentially useful as new low-voltage varistor. Special attention is paid on the relationship between electrical property and microstructure.
2. Experimental Traditional ceramic process was used to prepare the TiO2 varistor ceramics. Raw materials used in this study were reagent-grade TiO2, Nb2O5, SiO2, CeO2 and CaCO3. The compositions were designed according to orthogonal table. The powder mixtures were wet ball-milled in a polyethylene bottle with ZrO2 balls for 3 h in deionized water. After dried and granulated the powder was pressed into disks of 12 mm in diameter and 1.2 mm in thickness. The green samples were sintered at temperatures ranging from 1300 to 1400 jC for 2– 4 h. For electrical measurements, an Ag electrode was formed on both polished surfaces of the disks. The Ag electrode was prepared at 570 jC for 20 min. The varistor voltage V0.1 mA and V1 mA were measured by using MY-4C meter. a was calculated by the following formula [8]. a¼
logðI2 =I1 Þ ¼ logðV2 =V1 Þ logðV1
1 mA =V0:1
were observed by using SEM (XL-30FEG) technology.
3. Results and discussion 3.1. Effects of dopants on V1
TiO2 1 0 S Nb2 O5 ! 2NbTi þ 2e þ 4O O þ O2 z 2
ð2Þ
In this process, free electrons can be released with the substitution of Ti4 + with Nb5 + when the Nb addition varies from 0.1 to 0.5 mol%. It was also found that the nonlinear coefficient a increases from 4.4 to 4.8 when the Nb2O5 addition changes from 0.2
Table 1 Orthogonal test of the (Nb,Ce,Si,Ca)-doped TiO2 ceramics
ð1Þ
The capacitance C and dielectric loss tand were measured by use of an LCR meter (2693D). The frequency is fixed at 1 kHz. The microstructures
mA
In order to investigate the roles of different dopants, the L9 (34) orthogonal table was used to carry out the experiments. The factor levels were shown in Table 1, which shows the addition of 0.1– 0.5 mol% for CeO2, 0.2 –0.4 mol% for Nb2O5, 0 –0.2 mol% for CaCO3, and 0.1– 0.5 mol% for SiO2. Furthermore, three sintering conditions were considered: 1350 jC 2 h, 1380 jC 2 h, and 1380 jC 4 h. The nonlinear coefficient a and varistor voltage V1 mA are the two most important performance parameters of varistors. In order to achieve low-voltage varistors, the effects of dopants on varistor voltage V1 mA were paid special attention. The corresponding results of Table 1 were shown in Fig. 1. The roles of Ce, Nb, Ca and Si on V1 mA were shown by four groups of curves, respectively. It can be seen that V1 mA increases sharply with increase of CeO2 addition. However, it decreases sharply with increase of Nb2O5, which is quite similar to Yang and Wu’s results. The role of Nb can be explained by the following defect reaction:
Level mA Þ
3749
Factors A
1 2 3
B
C
D
CeO2
Nb2O5
CaCO3
SiO2
0.1 0.3 0.5
0.2 0.3 0.4
0 0.15 0.2
0.1 0.3 0.5
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J. Li et al. / Materials Letters 57 (2003) 3748–3754
Fig. 1. Effects of dopant additions on varistor voltage V1
to 0.4 mol%. Thus, the substitution of Ti4 + with Nb5 + cannot only decrease the grain resistivity by more free electrons, but also leads to a higher grain boundary barrier and, consequently, higher a value. The effects of Ca and Si are less dominant compared with Ce and Nb. Lower varistor voltage can be obtained when the Ca content is 0.15 mol% and sintered at 1380 jC; lower SiO2 content of 0.1 mol% leads to lower V1 mA. It is generally accepted that SiO2 can facilitate the grain growth in ZnO varistor ceramics. However, the role of SiO2 may need to be reconsidered when applied in TiO 2 ceramics based on the results of Fig. 1. The sintering process shows little influence on the roles of above dopants. The curves show similar trends under different sintering conditions. However, generally, V1 mA decreases with the increase of sintering temperature and sintering period. This is due to larger grains caused by elevated sintering temperatures or prolonged sintering period. Such a behavior is similar to ZnO ceramics.
mA.
A: CeO2; B: Nb2O5; C: CaCO3; D: SiO2.
to 1350 jC, the varistor voltage V1 mA is clearly observed with lowest value of about 12 V. Furthur increase of sintering temperature to 1380 jC increases the varistor voltage to 16 V. On the other hand, the nonlinear coefficient a remains increasing monotonously with increasing sintering temperature in the sintering temperature range of 1300 – 1380 jC. It is thus clear that a low varistor voltage of 12 V and an a of 3.7 can be achieved under controlled processing conditions. From the experimental results, it can be seen that an increase of sintering temperature from 1350 to 1380 jC leads to higher varistor voltage,
3.2. The observed second phase and microstructure According to the orthogonal test results, an optimum composition was obtained. Based on this composition, the change of V1 mA and a against sintering temperature in the range from 1300 to 1380 jC was shown in Fig. 2. As the sintering temperature is raised
Fig. 2. Sintering temperature dependence of varistor voltage V1 and nonlinear coefficient a.
mA
J. Li et al. / Materials Letters 57 (2003) 3748–3754
which is different from reported results. It is generally expected that a raised sintering temperature leads to larger grains and thus lower varistor voltage. To understand these results, macro- and microstructural studies were carried out with XRD and SEM techniques. In order to examine whether any additional phases are formed during sintering process, we carried out the structural investigation using the X-ray powder diffraction technique. Powder XRD patterns of different samples sintered at five different temperatures (1100, 1300, 1330, 1350 and 1380 jC) and that of the unsintered raw powder recorded at 25 jC are given in Fig. 3. The XRD pattern of the unsintered powder shows that the raw powder is anatase (with major intensity peaks at 2h = 25.28j, 36.94j, 48.05j, etc.). As the sintering temperature is raised to 1100 jC the evolution of the rutile phase can be seen. The sample sintered at 1100 jC shows all the major peaks that correspond to a rutile structure (with major intensity peaks at 2h = 27.48j, 36.08j, 41.22j, 54.32j, etc.). Furthermore, a very small trace of second phase (marked with ‘‘S’’ in the spectra) present at 2h values of 30.2j can be found. At 1100 jC the second phases are less prominent. As the sintering temperature is raised to 1300 and 1330 jC, two strong intensity
3751
peaks present at 2h values of 29.08j and 30.2j can be clearly found. However, when the sintering temperature is raised to 1350 jC, the intensity peaks almost disappear. Further increase of temperature to 1380 jC makes two strong intensity peaks again. The presentation of second phase and its relation with sintering temperature can well explain the sintering temperature dependence of varistor voltage V1 mA. It is suggested that the second phase is electrical insulating and stronger peaks in the XRD pattern account for higher varistor voltage in Fig. 2. Since the maximum intensity peaks corresponding to the rutile phase are very sharp, they must represent the TiO2 grains. Thus the second phase is expected to segregate at the grain surface during sintering process and makes an electrical insulating layer, which can facilitate higher varistor voltage. SEM micrographs of these samples sintered at five temperatures (1100, 1300, 1330, 1350 and 1380 jC) are shown in Fig. 4. The grain size of the samples increases with increasing sintering temperature. The sample sintered at 1100 jC shows an average size of less than 1 Am and lots of voids can be seen. Thus the sintering temperature is in the initial stage at this temperature of 1100 jC. The sample sintered at 1300 jC shows an average grain size of 3.3 Am. As
Fig. 3. XRD pattern of samples sintered at different temperatures (S represent second phase).
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Fig. 4. SEM micrographs of samples sintered at different temperatures. (a) 1100 jC, (b) 1300 jC, (c) 1330 jC, (d) 1350 jC and (e) 1380 jC.
J. Li et al. / Materials Letters 57 (2003) 3748–3754
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Fig. 5. EDAX spectra of the second phase.
the sintering temperature is increased to 1330 jC the grain size also increase to about 4 Am. At 1350 and 1380 jC the grain size increased to 6.5 and 7 Am, respectively. From these SEM micrographs it can be seen that there appear second phase on the surface of TiO2 grains. The second phase appears in two forms: column-like and network-like phase. When the sintering temperature is 1300, 1330 and 1380 jC, the second phase appear in both two forms. When the sintering temperature is 1350 jC, there only exists column-like second phase. Thus there is less second phase at 1350 jC compared with other sintering temperatures. The result matches the XRD results and it can well explain the V1 mA – sintering temperature relation. To investigate the composition of the second phase, EDAX was carried out and the result was shown in Fig. 5. It is clear that the second phase is mainly composed of Ti, Ce and Si, which is most likely to be Perrierite phase (Ce2Ti2Si2O11) [9]. Due to the complicated multi-layer structure formed in the Perrierite phase [10], the migration of electronic carriers is rather limited with the increased content of second phase. Such a result matches the relation between V1 mA and Ce, Si addition as shown in Fig. 1. The increase of Ce and Si addition lead to more insulating second phase on grain surface and, consequently, higher varistor voltage can be obtained.
4. Conclusion (Nb,Ce,Si,Ca)-doped TiO2 ceramics reveals some interesting phenomena in microstructure characteristics when sintering temperature was varied. Conclusions can be drawn from this investigation as follows: 1. The varistor voltage V1 mA shows close dependence on CeO2 and Nb2O5 addition. An increasing addition of CeO2 from 0.1 to 0.5 mol% leads to increasing V1 mA while an increasing addition of Nb2O5 from 0.2 to 0.4 mol% leads to decreasing V1 mA. 2. A second phase mainly composed of Ce, Ti and Si appears on the grain surface region. XRD and SEM results reveal that the content of second phase reaches a minimum at 1350 jC, which accounts for lowest varistor voltage V1 mA of 12 V in the sintering temperature range from 1300 to 1380 jC.
References [1] M. Bartkowiak, G.D. Mahan, F.A. Mdine, M.A. Alim, J. Appl. Phys. 80 (1996) 6516. [2] S.T. Li, J.Y. Li, F.Y. Liu, M.A. Alim, G. Chen, J. Phys. D: Appl. Phys. 35 (2002) 1884. [3] M.F. Yan, W.W. Rhodes, Appl. Phys. Lett. 40 (1982) 536. [4] S.-L. Yang, J.-M. Wu, J. Mater. Res. 10 (1995) 345.
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J. Li et al. / Materials Letters 57 (2003) 3748–3754
[5] S.-L. Yang, J.-M. Wu, J. Am. Ceram. Soc. 76 (1993) 145. [6] J. Pennewiss, B. Hoffmann, Mater. Lett. 9 (1990) 219. [7] J.Y. Li, S.T. Li, M.A. Alim, G. Chen, J. Mater. Sci., Mater. Electron. (in press). [8] P.N. Santhosh, D.K. Kharat, S.K. Date, Mater. Lett. 28 (1996) 37.
[9] H. Miyajima, S. Matsubara, R. Miyawaki, K. Yokoyama, K. Hirokawa, Mineral. Mag. 65 (2001) 111. [10] C. Gueho, D. Giaquinta, J.L. Mansot, T. Ebel, P. Palvadeau, Chem. Mater. 7 (1995) 486.