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Effect of Sc2O3 on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors
ARTICLE IN PRESS
Physica B 367 (2005) 29–34 www.elsevier.com/locate/physb
Effect of Sc2O3 on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors$ Guo-Zhong Zang, Jin-Feng Wang, Hong-Cun Chen, Wen-Bin Su, Chun-Ming Wang, Peng Qi School of Physics and Microelectronics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Received 3 April 2004; received in revised form 20 May 2005; accepted 24 May 2005
Abstract The effects of Sc2O3 doping on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors were investigated and the maximal nonlinear coefficient (a ¼ 34) of the sample doped with 0.03 mol% Sc2O3 was obtained. It was found that the breakdown electrical field of samples increased significantly with increasing Sc2O3 concentration and the sample doped with 0.08 mol% Sc2O3 has the highest breakdown electrical field of 3336 V/mm. The measurement of grain size and sample impedances reveal that the increase of grain boundary resistance is the substantial reason for the increase of breakdown electrical field. The increase of grain boundary resistance may be caused by the increase of depletion layer thickness and it can be illustrated by the decrease of relative dielectric constant with increasing Sc2O3 concentration. r 2005 Elsevier B.V. All rights reserved. PACS: 84.32.Ff; 72.20.Ht; 81.20.Ev Keywords: Varistor; Nonlinear coefficient; Sc2O3; SnO2
1. Introduction Varistors can sense and limit high transient voltage surges and can repeatedly endure such surges without being destroyed, they are usually $ Supported by the Natural Science Foundation of Shandong province, China (Grant no. Z2003F04). Corresponding author. Tel.: 86053183770358322; fax: +865318377031. E-mail address: [email protected] (G.-Z. Zang).
used to protect electronic circuits from voltage pulse shock. The most important property of a varistor is its nonlinear voltage–current characteristic, and it can be expressed by equation I ¼ KV a , where the a coefficient gives the degree of nonlinearity and the constant K depends on the microstructure and is related to the electrical resistivity of the material. The earliest used varistors were SiC-based ceramic systems [1,2]. Then in 1969, Matsuoka announced the development of varistors based on
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.05.045
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
3000
(211)
Fig. 1 shows the XRD pattern of the sample doped with 0.08 mol% Sc2O3. Comparing with XRD data of pure SnO2, no other phase besides SnO2 rutile structure in the sample was observed. Co2O3 should then form a solid solution in SnO2 and lead to the high densification of this system [8,9]. The relative densities of all the samples are about 97% as shown in Table 1. The high densification may also be illustrated by the dense microstructure of this system as shown in Fig. 2 and the grain size were list in Table 1. The electrical nonlinear characteristics of the samples are shown in Fig. 3. It is found that all the
(101)
(a)
(310) (112) (301)
1000
(220) (002)
2000 (111) (210)
The raw chemicals used in this study were analytic grades of SnO2 (99.5%), Ta2O5 (99.95%), Sc2O3 (99.27%), Cr2O3 (99.0%) and Co2O3 (98.5%). The compositions were (99.12x) mol%, SnO2+0.75 mol%, Co2O3+0.10 mol%, Ta2O5+0.03 mol% and Cr2O3+x mol% Sc2O3, where x ¼ 0, 0.03, 0.06, 0.08. The SnO2-based varistors were prepared by conventional ceramic processing. The mixed raw chemicals were milled in nylon kettle for 12 h with ZrO2 balls and some distilled water, dried, mixed with 0.5% weight of PVA binder and pressed into disks of 15 mm in diameter and 1.5 mm in thickness at 180 MPa. After burning out the PVA binder at 650 1C, the disks were sintered at 1300 1C for an hour and cooled freely to room temperature. To measure the electrical properties, silver electrodes were made on both surfaces of the sintered disks. The microstructure of the sample surfaces were analyzed by scanning electron microscopy (SEM) using a JEOL (Model JXA-840) microscope and
3. Results and discussion
(200)
2. Experimental procedure
the phases were analyzed by X-ray diffraction (XRD). For electrical characterization of current density versus applied electrical field, a semiconductor I2V grapher (QT2) was used. The frequency dependence of relative dielectric constant and impedance spectra were obtained using an impedance analyzer (Agilent 4294A).
(110)
ZnO compositions, which was a successive improvement, particularly in processing [3]. Up to now, varistors based on ZnO have been most extensively studied [4–7]. However, other ceramic varistor systems are also investigated to meet the demands of various applying fields, such as the varistors with high breakdown voltage, high density and single phased structure. In 1995, S. A. Pianaro, for the first time, found a new varistor material [8], (Co, Nb)-doped SnO2, which has only single phase, high density and rutile structure. In 1998, (Co, Ta)-doped SnO2 varistors were investigated by Antunes et al. [9]. Recently, Cr2O3, In2O3, Er2O3 and Sc2O3 on the properties of (Co, Nb)-doped SnO2 varistors had been investigated by Wang et al. [10–13] and the increase of breakdown electrical fields were attributed to the decrease of grain size. In this work, the effect of Sc2O3 doping on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors was investigated and some new results were obtained.
Intensity (CPS)
30
0 2000
(b)
1000
0 30
45 2θ (deg)
60
Fig. 1. X-ray diffraction picture for the samples (a) without the dopant of Sc2O3; (b) doped with 0.08 mol% Sc2O3.
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
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Table 1 Characteristics of the samples doped with different contents of Sc2O3 Sc2O3 (mol%)
a
Relative density (%)a
E B (V/mm)
r b
Grain size (mm)
0 0.03 0.06 0.08
16.6 34.8 24.6 21.3
98.1 97.3 96.8 97.2
513 630 972 3336
754 563 81 26
3.97 4.81 5.53 3.94
a
Theoretical density of SnO2 is 6.95 g/cm3. r is measured at 1 kHz.
b
Fig. 2. SEM microstructure characteristics of the samples doped with different Sc2O3 contents. (a) x ¼ 0; (b) x ¼ 0:03; (c) x ¼ 0:06, (d) x ¼ 0:08.
samples exhibit varistor behavior and the nonlinear coefficient a was calculated using [5] a¼
lgðI 2 =I 1 Þ , lgðV 2 =V 1 Þ
(1)
where V 2 and V 1 are the voltages at currents I 2 (10 mA) and I 1 (1 mA), respectively. The sample doped with 0.03 mol% Sc2O3 exhibits the highest a value of 34 as shown in Table 1. From Fig. 3 we can also see that the breakdown electrical
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
32
103
x=0 x=0.03 x=0.06 x=0.08
ε/ε0
E (V/mm)
3000
1500
x=0 x=0.03 x=0.06 x=0.08
102
0 0
2
4
6
8
102
J (mA/cm2) Fig. 3. J-E characteristics of the samples doped with different Sc2O3 contents.
field E B (field at current 1 mA) of the samples increased significantly with increasing Sc2O3 concentration. Breakdown electrical field of varistors can be expressed by the following equation [14]: E B ¼ n¯ V g ,
(2)
where n¯ is the average grain number per unit length which is inversely proportional to mean grain size and V g is the breakdown voltage of a grain boundary. Eq. (2) indicates that E B is determined by grain size d and V g . In the studies of Wang et al. [10,11], E B is determined mainly by grain size d and V g is about 3 V of each sample and the decrease of grain size led to the increase of breakdown electrical field. However, from Table 1 we can see that the grain size increased with increase in Sc2O3 content and further doping of Sc2O3 led to a decrease of grain size in the system studied. This phenomenon means that V g is not necessarily equal to about 3 V and it can be affected significantly by Sc2O3. The increase of E B means that the breakdown voltage of one grain boundary increased significantly with increase in Sc2O3 content. Fig. 4 shows the relative dielectric constant (r ¼ =0 ) versus frequency from 40 Hz to 15 MHz for the samples doped with different contents of Sc2O3 and it is found that r decreased obviously with increasing Sc2O3 concentration. r is related to average grain size d and the depletion layer
103
104 105 Frequency (Hz)
106
107
Fig. 4. Relative dielectric constants spectra of samples doped with different Sc2O3 contents.
width tB according to the following equation [15]: r ¼ B d=tB ,
(3)
where B (B ¼ 14 [16]) is the internal permittivity of the barrier material. The significant decrease of r and slight variation of d indicate that tB should increase with increasing Sc2O3 concentration and the widest depletion layer may result in the largest grain boundary resistance RGB . Usually, we take the diameter of a semicircular impedance diagram of Z0 (resistance) versus Z 00 (reactance) as the grain boundary resistance [17]. The whole semicircles of impedance diagrams were gotten as shown in Fig. 5 by measuring the Z 0 and Z 00 at 280 1C since the impedance diagrams presented a part of semicircle at room temperature. As estimated, the sample doped with 0.08 mol% Sc2O3 has the largest RGB . In addition, the centers of all the semicircles are below the resistance (Z 0 ) axes and this problem needs to be further studied. All the results and analyses indicate that tB plays a very important role in the variation of electrical properties and the Sc2O3 dopant is the substantial reason for the increase of tB . Proper addition of Sc2O3 may lead to the following action [18]: SnO2
2þ 1 Sc2 O3 ! 2Sc Sn þ VO þ OO þ 2O2 .
(4)
The substitution of Sn by Sc will increase the depletion layer width tB and the oxygen vacancy originated in the above reaction will accelerate the
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
33
x=0 x=0.03 x=0.06 x=0.08
1
0
Z″ (MΩ⋅cm)
center -1 0
2
4
Fig. 6. Grain boundary defect model for (Co, Ta, Cr, Sc)doped SnO2 varistor system.
3
0
-3 0
4
8 Z′ (MΩ⋅cm)
12
16
Fig. 5. Impedance spectra of the samples doped with different Sc2O3 contents.
diffusion of ions and facilitate SnO2 grains to grow larger. However, excessive Sc2O3 concentration leads to a decrease of grain size as shown in Table 1. By analogy to the rule of Cr2O3 which makes the grain size of (Co, Nb)-doped SnO2 varistors decrease due to the probable presence of CoCr2O4 [19], some compounds of Scandium may be present at gain boundaries to hinder SnO2 grains from combining each other. The grain boundary resistance RGB can be affected by the average grain boundary number per unit length, the depletion layer width tB and the amount of insulating compounds on the grain boundary et al. RGB of the system studied in this paper may be determined mainly by tB and the amount of insulating compounds on the grain boundary considering the slight variation of grain size. The exhibition of minimum RGB of the sample doped with 0.03 mol% Sc2O3 may be caused by the active negatively charged defects Sc0sn at 280 1C. In conclusion, the increase of grain boundary resistance is the essential reason for the increase of breakdown electrical field. The ultrahigh
breakdown electrical field, 3336 V/mm, may be very useful for high voltage protection. The varistor behavior of SnO2 material can be explained by Schottky type potential barrier model proposed by Bueno et al. [20]. The potential barrier is formed by intrinsic defects of SnO2, extrinsic defects created by solid substitution of dopants, and negative charges, O0 and O00 at the interface. These defects create depletion layers at grain boundaries leading to the formation of a voltage barrier for the electronic transport. This transport occurs by tunnelling and is responsible for the nonlinear behavior of current density versus applied electric field. The potential barrier model for (Co, Ta, Cr, Sc)-doped SnO2 varistor system is shown in Fig. 6.
4. Conclusions A higher nonlinear coefficient of 34 for the SnO2 varistor doped with 0.03 mol% Sc2O3 and an composition of SnO2+0.75 mol%Co2O3+0.10 mol% Ta2O5+0.03 mol%Cr2O3+0.08 mol%Sc2O3 with ultrahigh breakdown electrical field of 3336 V/ mm, high nonlinear coefficient of 21 were obtained. The variations of electrical properties are attributed mainly to the variation of grain size, average width of the depletion layer and grain boundary resistance. Sc2O3 in some form of the compounds of Scandium residing at grain boundaries hinders the growth of SnO2 grains.
ARTICLE IN PRESS 34
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References [1] C.J. Frosch, Bell Lab. Rec. 32 (1954) 336. [2] H.F. Dienel, Bell Lab. Rec. 34 (1956) 407. [3] M. Matsuoka, T. Masuyama, Y. Iida, Jpn. J. Appl. Phys. 8 (1969) 1275. [4] H.R. Philipp, L.M. Leivinson, J. Appl. Phys. 50 (1979) 383. [5] T.K. Gupta, J. Am. Ceram. Soc. 73 (1990) 1817. [6] S.A. Pianaro, E.C. Pereira, E. Longo, J. Mater. Sci. 30 (1995) 133. [7] T.R. Narayanan Kutty, S. Ezhilvalavan, Jpn. J. Appl. Phys. 34 (1995) 6125. [8] S.A. Pianaro, P.R. Bueno, E. Longo, J. Mater. Sci. Lett. 14 (1995) 692. [9] A.C. Antunes, S.R.M. Antunes, S.A. Pianaro, J. Mater. Sci. Lett. 17 (1998) 577. [10] W.X. Wang, J.F. Wang, H.C. Chen, Mater. Sci. Eng. B99 (2003) 470.
[11] W.X. Wang, J.F. Wang, H.C. Chen, J. Phys. D: Appl. Phys. 36 (2003) 1040. [12] C.M. Wang, J.F. Wang, H.C. Chen, Chin. Phys. Lett. 21 (2004) 716. [13] G.Z. Zang, J.F. Wang, H.C. Chen, Appl. Phys. A: Mater. Sci. Proc. 80 (2005) 1093. [14] E.R. Leite, A.M. Nascimento, P.R. Bueno, J. Mater. Sci.: Mater. Electr. 10 (1999) 321. [15] P.R. Emtage, J. Appl. Phys. 48 (1977) 4372. [16] P.R. Bueno, M.M. Oliveira, W.K. Bacelar-Junior, J. Appl. Phys. 91 (2002) 6007. [17] J.F. Wang, Y.J. Wang, W.B. Su, Mater. Sci. Eng. B 96 (2002) 8. [18] F.A. Kroger, H. Vink, Solid State Physics, Academic Press, New York, 1956, p. 307. [19] S.A. Pianaro, P.R. Bueno, E. Longo, Ceram. Inter. 25 (1999) 1. [20] P.R. Bueno, S.A. Pianaro, E.C. Pereira, J. Appl. Phys. 84 (1998) 3700.
Physica B 367 (2005) 29–34 www.elsevier.com/locate/physb
Effect of Sc2O3 on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors$ Guo-Zhong Zang, Jin-Feng Wang, Hong-Cun Chen, Wen-Bin Su, Chun-Ming Wang, Peng Qi School of Physics and Microelectronics, State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, PR China Received 3 April 2004; received in revised form 20 May 2005; accepted 24 May 2005
Abstract The effects of Sc2O3 doping on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors were investigated and the maximal nonlinear coefficient (a ¼ 34) of the sample doped with 0.03 mol% Sc2O3 was obtained. It was found that the breakdown electrical field of samples increased significantly with increasing Sc2O3 concentration and the sample doped with 0.08 mol% Sc2O3 has the highest breakdown electrical field of 3336 V/mm. The measurement of grain size and sample impedances reveal that the increase of grain boundary resistance is the substantial reason for the increase of breakdown electrical field. The increase of grain boundary resistance may be caused by the increase of depletion layer thickness and it can be illustrated by the decrease of relative dielectric constant with increasing Sc2O3 concentration. r 2005 Elsevier B.V. All rights reserved. PACS: 84.32.Ff; 72.20.Ht; 81.20.Ev Keywords: Varistor; Nonlinear coefficient; Sc2O3; SnO2
1. Introduction Varistors can sense and limit high transient voltage surges and can repeatedly endure such surges without being destroyed, they are usually $ Supported by the Natural Science Foundation of Shandong province, China (Grant no. Z2003F04). Corresponding author. Tel.: 86053183770358322; fax: +865318377031. E-mail address: [email protected] (G.-Z. Zang).
used to protect electronic circuits from voltage pulse shock. The most important property of a varistor is its nonlinear voltage–current characteristic, and it can be expressed by equation I ¼ KV a , where the a coefficient gives the degree of nonlinearity and the constant K depends on the microstructure and is related to the electrical resistivity of the material. The earliest used varistors were SiC-based ceramic systems [1,2]. Then in 1969, Matsuoka announced the development of varistors based on
0921-4526/$ - see front matter r 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2005.05.045
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
3000
(211)
Fig. 1 shows the XRD pattern of the sample doped with 0.08 mol% Sc2O3. Comparing with XRD data of pure SnO2, no other phase besides SnO2 rutile structure in the sample was observed. Co2O3 should then form a solid solution in SnO2 and lead to the high densification of this system [8,9]. The relative densities of all the samples are about 97% as shown in Table 1. The high densification may also be illustrated by the dense microstructure of this system as shown in Fig. 2 and the grain size were list in Table 1. The electrical nonlinear characteristics of the samples are shown in Fig. 3. It is found that all the
(101)
(a)
(310) (112) (301)
1000
(220) (002)
2000 (111) (210)
The raw chemicals used in this study were analytic grades of SnO2 (99.5%), Ta2O5 (99.95%), Sc2O3 (99.27%), Cr2O3 (99.0%) and Co2O3 (98.5%). The compositions were (99.12x) mol%, SnO2+0.75 mol%, Co2O3+0.10 mol%, Ta2O5+0.03 mol% and Cr2O3+x mol% Sc2O3, where x ¼ 0, 0.03, 0.06, 0.08. The SnO2-based varistors were prepared by conventional ceramic processing. The mixed raw chemicals were milled in nylon kettle for 12 h with ZrO2 balls and some distilled water, dried, mixed with 0.5% weight of PVA binder and pressed into disks of 15 mm in diameter and 1.5 mm in thickness at 180 MPa. After burning out the PVA binder at 650 1C, the disks were sintered at 1300 1C for an hour and cooled freely to room temperature. To measure the electrical properties, silver electrodes were made on both surfaces of the sintered disks. The microstructure of the sample surfaces were analyzed by scanning electron microscopy (SEM) using a JEOL (Model JXA-840) microscope and
3. Results and discussion
(200)
2. Experimental procedure
the phases were analyzed by X-ray diffraction (XRD). For electrical characterization of current density versus applied electrical field, a semiconductor I2V grapher (QT2) was used. The frequency dependence of relative dielectric constant and impedance spectra were obtained using an impedance analyzer (Agilent 4294A).
(110)
ZnO compositions, which was a successive improvement, particularly in processing [3]. Up to now, varistors based on ZnO have been most extensively studied [4–7]. However, other ceramic varistor systems are also investigated to meet the demands of various applying fields, such as the varistors with high breakdown voltage, high density and single phased structure. In 1995, S. A. Pianaro, for the first time, found a new varistor material [8], (Co, Nb)-doped SnO2, which has only single phase, high density and rutile structure. In 1998, (Co, Ta)-doped SnO2 varistors were investigated by Antunes et al. [9]. Recently, Cr2O3, In2O3, Er2O3 and Sc2O3 on the properties of (Co, Nb)-doped SnO2 varistors had been investigated by Wang et al. [10–13] and the increase of breakdown electrical fields were attributed to the decrease of grain size. In this work, the effect of Sc2O3 doping on the electrical properties of (Co, Ta, Cr)-doped SnO2 varistors was investigated and some new results were obtained.
Intensity (CPS)
30
0 2000
(b)
1000
0 30
45 2θ (deg)
60
Fig. 1. X-ray diffraction picture for the samples (a) without the dopant of Sc2O3; (b) doped with 0.08 mol% Sc2O3.
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
31
Table 1 Characteristics of the samples doped with different contents of Sc2O3 Sc2O3 (mol%)
a
Relative density (%)a
E B (V/mm)
r b
Grain size (mm)
0 0.03 0.06 0.08
16.6 34.8 24.6 21.3
98.1 97.3 96.8 97.2
513 630 972 3336
754 563 81 26
3.97 4.81 5.53 3.94
a
Theoretical density of SnO2 is 6.95 g/cm3. r is measured at 1 kHz.
b
Fig. 2. SEM microstructure characteristics of the samples doped with different Sc2O3 contents. (a) x ¼ 0; (b) x ¼ 0:03; (c) x ¼ 0:06, (d) x ¼ 0:08.
samples exhibit varistor behavior and the nonlinear coefficient a was calculated using [5] a¼
lgðI 2 =I 1 Þ , lgðV 2 =V 1 Þ
(1)
where V 2 and V 1 are the voltages at currents I 2 (10 mA) and I 1 (1 mA), respectively. The sample doped with 0.03 mol% Sc2O3 exhibits the highest a value of 34 as shown in Table 1. From Fig. 3 we can also see that the breakdown electrical
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
32
103
x=0 x=0.03 x=0.06 x=0.08
ε/ε0
E (V/mm)
3000
1500
x=0 x=0.03 x=0.06 x=0.08
102
0 0
2
4
6
8
102
J (mA/cm2) Fig. 3. J-E characteristics of the samples doped with different Sc2O3 contents.
field E B (field at current 1 mA) of the samples increased significantly with increasing Sc2O3 concentration. Breakdown electrical field of varistors can be expressed by the following equation [14]: E B ¼ n¯ V g ,
(2)
where n¯ is the average grain number per unit length which is inversely proportional to mean grain size and V g is the breakdown voltage of a grain boundary. Eq. (2) indicates that E B is determined by grain size d and V g . In the studies of Wang et al. [10,11], E B is determined mainly by grain size d and V g is about 3 V of each sample and the decrease of grain size led to the increase of breakdown electrical field. However, from Table 1 we can see that the grain size increased with increase in Sc2O3 content and further doping of Sc2O3 led to a decrease of grain size in the system studied. This phenomenon means that V g is not necessarily equal to about 3 V and it can be affected significantly by Sc2O3. The increase of E B means that the breakdown voltage of one grain boundary increased significantly with increase in Sc2O3 content. Fig. 4 shows the relative dielectric constant (r ¼ =0 ) versus frequency from 40 Hz to 15 MHz for the samples doped with different contents of Sc2O3 and it is found that r decreased obviously with increasing Sc2O3 concentration. r is related to average grain size d and the depletion layer
103
104 105 Frequency (Hz)
106
107
Fig. 4. Relative dielectric constants spectra of samples doped with different Sc2O3 contents.
width tB according to the following equation [15]: r ¼ B d=tB ,
(3)
where B (B ¼ 14 [16]) is the internal permittivity of the barrier material. The significant decrease of r and slight variation of d indicate that tB should increase with increasing Sc2O3 concentration and the widest depletion layer may result in the largest grain boundary resistance RGB . Usually, we take the diameter of a semicircular impedance diagram of Z0 (resistance) versus Z 00 (reactance) as the grain boundary resistance [17]. The whole semicircles of impedance diagrams were gotten as shown in Fig. 5 by measuring the Z 0 and Z 00 at 280 1C since the impedance diagrams presented a part of semicircle at room temperature. As estimated, the sample doped with 0.08 mol% Sc2O3 has the largest RGB . In addition, the centers of all the semicircles are below the resistance (Z 0 ) axes and this problem needs to be further studied. All the results and analyses indicate that tB plays a very important role in the variation of electrical properties and the Sc2O3 dopant is the substantial reason for the increase of tB . Proper addition of Sc2O3 may lead to the following action [18]: SnO2
2þ 1 Sc2 O3 ! 2Sc Sn þ VO þ OO þ 2O2 .
(4)
The substitution of Sn by Sc will increase the depletion layer width tB and the oxygen vacancy originated in the above reaction will accelerate the
ARTICLE IN PRESS G.-Z. Zang et al. / Physica B 367 (2005) 29–34
33
x=0 x=0.03 x=0.06 x=0.08
1
0
Z″ (MΩ⋅cm)
center -1 0
2
4
Fig. 6. Grain boundary defect model for (Co, Ta, Cr, Sc)doped SnO2 varistor system.
3
0
-3 0
4
8 Z′ (MΩ⋅cm)
12
16
Fig. 5. Impedance spectra of the samples doped with different Sc2O3 contents.
diffusion of ions and facilitate SnO2 grains to grow larger. However, excessive Sc2O3 concentration leads to a decrease of grain size as shown in Table 1. By analogy to the rule of Cr2O3 which makes the grain size of (Co, Nb)-doped SnO2 varistors decrease due to the probable presence of CoCr2O4 [19], some compounds of Scandium may be present at gain boundaries to hinder SnO2 grains from combining each other. The grain boundary resistance RGB can be affected by the average grain boundary number per unit length, the depletion layer width tB and the amount of insulating compounds on the grain boundary et al. RGB of the system studied in this paper may be determined mainly by tB and the amount of insulating compounds on the grain boundary considering the slight variation of grain size. The exhibition of minimum RGB of the sample doped with 0.03 mol% Sc2O3 may be caused by the active negatively charged defects Sc0sn at 280 1C. In conclusion, the increase of grain boundary resistance is the essential reason for the increase of breakdown electrical field. The ultrahigh
breakdown electrical field, 3336 V/mm, may be very useful for high voltage protection. The varistor behavior of SnO2 material can be explained by Schottky type potential barrier model proposed by Bueno et al. [20]. The potential barrier is formed by intrinsic defects of SnO2, extrinsic defects created by solid substitution of dopants, and negative charges, O0 and O00 at the interface. These defects create depletion layers at grain boundaries leading to the formation of a voltage barrier for the electronic transport. This transport occurs by tunnelling and is responsible for the nonlinear behavior of current density versus applied electric field. The potential barrier model for (Co, Ta, Cr, Sc)-doped SnO2 varistor system is shown in Fig. 6.
4. Conclusions A higher nonlinear coefficient of 34 for the SnO2 varistor doped with 0.03 mol% Sc2O3 and an composition of SnO2+0.75 mol%Co2O3+0.10 mol% Ta2O5+0.03 mol%Cr2O3+0.08 mol%Sc2O3 with ultrahigh breakdown electrical field of 3336 V/ mm, high nonlinear coefficient of 21 were obtained. The variations of electrical properties are attributed mainly to the variation of grain size, average width of the depletion layer and grain boundary resistance. Sc2O3 in some form of the compounds of Scandium residing at grain boundaries hinders the growth of SnO2 grains.
ARTICLE IN PRESS 34
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References [1] C.J. Frosch, Bell Lab. Rec. 32 (1954) 336. [2] H.F. Dienel, Bell Lab. Rec. 34 (1956) 407. [3] M. Matsuoka, T. Masuyama, Y. Iida, Jpn. J. Appl. Phys. 8 (1969) 1275. [4] H.R. Philipp, L.M. Leivinson, J. Appl. Phys. 50 (1979) 383. [5] T.K. Gupta, J. Am. Ceram. Soc. 73 (1990) 1817. [6] S.A. Pianaro, E.C. Pereira, E. Longo, J. Mater. Sci. 30 (1995) 133. [7] T.R. Narayanan Kutty, S. Ezhilvalavan, Jpn. J. Appl. Phys. 34 (1995) 6125. [8] S.A. Pianaro, P.R. Bueno, E. Longo, J. Mater. Sci. Lett. 14 (1995) 692. [9] A.C. Antunes, S.R.M. Antunes, S.A. Pianaro, J. Mater. Sci. Lett. 17 (1998) 577. [10] W.X. Wang, J.F. Wang, H.C. Chen, Mater. Sci. Eng. B99 (2003) 470.
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