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Nonlinear electrical properties of TiO2–Y2O3–Nb2O5 capacitor-varistor ceramics
Materials Science and Engineering B85 (2001) 6 – 10 www.elsevier.com/locate/mseb
Nonlinear electrical properties of TiO2 –Y2O3 –Nb2O5 capacitor-varistor ceramics Changpeng Li *, Jinfeng Wang, Xiaosu Wang, Wenbin Su, Hongcun Chen, Dexin Zhuang Department of Physics, Shandong Uni6ersity, Jinan 250100, People’s Republic of China Received 5 September 2000; received in revised form 8 February 2001; accepted 13 February 2001
Abstract The nonlinear electrical properties of TiO2 –Y2O3 –Nb2O5 ceramics were investigated as a new varistor material. It was found that an optimal doping composition of 99.75%TiO2 – 0.60%Y2O5 – 0.10% Nb2O5 was obtained with low breakdown voltage of 8.8 V mm − 1, high nonlinear constant of 7.0 and ultrahigh relative dielectric constant of 7.6 ×104, which is consistent with the highest and narrowest grain boundary barriers in the composition. Samples doped with 0.10 mol.% Nb2O5 exhibit the highest permittivitty and resistivity at low frequencies and comparatively lower values at high frequencies in comparison with other samples studied. In view of these electrical characteristics, the ceramics of 99.75%TiO2 – 0.60%Y2O3 – 0.10%Nb2O5 is a viable candidate for capacitor-varistor functional devices. The performance of the ceramics as a function of Nb-doping depends primarily on the extent of substitution of Ti4 + with Nb5 + . In order to illustrate the role of grain boundary barriers for high Nb-doping co-concentrations in TiO2 –Y2O3 –Nb2O5 varistors, a grain-boundary defect barrier model was introduced. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Varistors; Titanium oxide; Yttrium oxide; Niobium oxide; Electrical properties
1. Introduction Varistor materials with high nonlinearity in their current-voltage characteristics are used as protecting devices against voltage transients in electronic and industrial equipment and as surge arrestors. The most important property of a varistor is its nonlinear current-voltage characteristic, which can be expressed by the equation I = KV h, where h is the nonlinear coefficient, a vital parameter used to scale the nonlinearity. Zinc oxide varistors exhibit highly nonlinear currentversus-voltage (I – V) characteristics and are widely used as voltage suppressors in a large variety of powder systems and electric circuits [1,2]. The recent trend in electrical appliance design, however, requires varistors that contain multifunction and have relatively low breakdown voltage [3,4]. Slightly doped TiO2 ceramics, regarded as ultrahigh * Corresponding author. E-mail address: [email protected] (C. Li).
dielectric capacitors [5], are viable candidates for varistors that could satisfy the new functional demand. Yan and Rhodes [5] firstly reported that (Nb, Ba)-doped TiO2 ceramics possess varistor properties, with a current –voltage nonlinearity index of h= 3–4, and that an oxidizing atmosphere during cooling was necessary in the new varistors [6,7]. Recently, Yang and Wu studied the effects of BaO and Bi2O3 additives and of sintering temperature on the varistor characteristics and other electrical properties of Nb-doped TiO2 ceramics [8]. In our previous work, we had found the (Y, Nb)-doped TiO2 ceramics had good nonlinear electrical properties and ultrahigh dielectric constant. This paper would present the effect of niobium additives on the varistor characteristics and other electrical properties of (Y, Nb)-doped TiO2 ceramics. Sintering was performed in air, without a reducing or oxidizing atmosphere for control. Characteristics such as the frequency dependence of the permittivitty and the resistivity, I–V were measured. To investigate the microstructure, a scanning electron microscope (SEM) was used. The results were analyzed and a defect model is introduced in this paper.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 6 3 - 3
C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
Fig. 1. I– V characteristics of samples with different Nb2O5 dopants.
7
Fig. 2. Characteristic plots of ln J × E 1/2 for samples with different Nb2O5 dopants.
3. Results 2. Experimental procedure The raw chemicals in the present study were analytical grades of TiO2 (99.8%), Y2O3 (99.99%) and Nb2O5 (99.95%). The powder compositions (99.4− X)%TiO2 +0.60% Y2O3 +X%Nb2O5, where X= 0.05, 0.1, 0.15, 0.20, were obtained by conventional mixing using a ZrO2 ball mill. The milled powder was pressed into disks 15 mm in diameter and 1.5 mm thick at a pressure of 160 MPa, which were sintered at the temperature of 1400°C for 1h, cooled at a constant rate of 5°C min − 1 from 1400 to 1100°C, then cooled to room temperature automatically. The green compacts were put into an Al2O3 crucible and fully surrounded with powder of matching composition to reduce the evaporation of low-melting-point components. For microstructural characterization the samples were polished, thermally etched and analyzed in a scanning electron microscope (SEM). The mean grain sizes were determined by the intercept method. For electrical property measurement, silver electrodes were made on both surfaces. The frequency dependence of the permittivitty and the resistivity were investigated by using a Hewlett-Packard LF impedance analyzer (model HP4192A) over a range of frequencies from 10 to 107 Hz. For electrical characterization of current density versus applied electrical field, a semiconductor I–V graph (QT2) was used.
The nonlinear characteristics in electrical properties of the TiO2 –Y2O3 –Nb2O5 ceramics are shown in Fig. 1. The nonlinear coefficient h was obtained by [9]: h=
log(I2/I1) log(V2/V1)
(1)
where V1 and V2 are the voltage at current I1 and I2 , respectively. It was observed from Fig. 1 and Table 1 that the breakdown voltage reached a minimum of 8.8 V mm − 1 and the nonlinear coefficient h reached a maximum, h:7.0, for 99.30%TiO2 –0.60%Y2O3 – 0.10%Nb2O5 composition. The fact that the breakdown voltage is proportional to the thickness of the pellet reveal that the non-ohmic behaviour is an interface property of bulk TiO2 ceramics, and not a property of the ceramic–electrode interface. Considering the Schottky type conduction model, plots of ln J against E 1/2 can be built up to determine values of B, the interface voltage barrier height, and i, a constant related to the potential barrier width, for the TiO2 based varistor with different Nb2O5 dopants, as shown in Fig. 2. B Can be obtained from the intersection of the extrapolated lines of the plot with the ln J axis, and the relative magnitude of constant i, which is inverse to , the potential barrier width, can be derived from the slopes of the plots [10]. Values of B and are shown in Table 1. From Fig. 2 we find that the highest and narrowest grain boundary voltage barrier
Table 1 Some characteristics of the samples doped with different amounts of Nb2O5. Nb2O5 (mol.%)
h
EB (V mm−1)
B (eV)
i×103 (V−1/2.cm1/2)
mr (×104) (at 1 kHz)
0.05 0.10 0.15 0.20
4.2 6.9 5.5 5.0
34.8 8.8 12.8 14.9
0.56 0.65 0.61 0.55
8.1 29.6 18.5 11.2
2.16 7.63 7.53 6.59
8
C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
occurs for the composition 99.30%TiO2 – 0.60%Y2O3 – 0.10%Nb2O5, which is consistent with the observed nonlinear electrical properties for this composition. Table 1 shows that the sample doped with 0.10% Nb2O5 exhibits the highest nolinearity coefficient, the lowest breakdown voltage and highest permittivitty. To further investigate the reason of the variation of electrical properties with different Nb2O5 dopants, the frequency dependence of the permittivitty and the resistivity were measured. Fig. 3 showed the permittivitty vs. frequency of all samples. The results can be roughly divided into two groups: group A which consists of samples doped with 0.05, 0.15, 0.20 mol.% Nb2O5, has comparatively lower permittivitty in the low frequency region, while group B which is the only sample doped with 0.10 mol.% Nb2O5 has much higher permittivitty [11]. Fig. 4 shows the variations in resistivity with frequency. The resitivity decreases with increasing frequency, similar to the behavior of the dielectric constant. It is seen from Figs. 3 and 4 that the samples doped with 0.25 mol.% Nb2O5, in comparison with
Fig. 3. Permittivitty against frequency for samples doped with different amounts of Nb2O5.
Fig. 4. Resistivity against frequency for samples doped with different amounts of Nb2O5.
other samples, exhibit the highest resistivity and permittivitty at low frequencies, and the comparatively low resistivity and permittivitty at high frequencies.
4. Discussion TiO2 ceramics have a wide band gap of about 3.0 eV and are found to be an insulator at room temperature. The dopant of Nb, which has a 5+ valence and an ionic radius approximate to that of Ti4 + , acts as a donor when dissolved in the TiO2 lattice, as indicated by defect Eqs. (2)– (4): − Nb2O5 ¥ 2Nb+ + 4OO + 1/2O2(g) Ti + 2e
(2)
2TiO2 ¥ 2Ti2Ti− + V2O+ + 3OO + 1/2O2(g)
(3)
2− 2V2O+ + V4Ti− 2V+ O + VTi
(4)
where the electron, e−, usually combines with the regular titanium lattice ion to form Ti2Ti− . The dissolution of niobium into TiO2 ceramics increases the concentration of the conduction electrons, which will give rise to the ceramic semiconduction behavior. The substitution of Ti4 + with Nb5 + will promote the formation of acceptor defects in the samples, which create depletion layers at grain boundaries leading to the formation of voltage barriers for the electronic transport. The decrease of the grain resistance also facilitates the formation of the grain boundary barriers. The SEM micrographs show that the grain sizes increase with the introduction of niobium, which imply that the substitution of Ti4 + with Nb5 + facilitates the growth of the grains. According to the boundary barrier model, the reference voltage barrier, Vr, for a varistor is determined by the mean number of barriers n¯ in series multiplied by 6b, that is: Vr = n¯ ·6b,
(5)
where 6b is the voltage barrier at a grain boundary. With the Nb2O5 dopants increasing from 0.05 to 0.10 mol.%, the grain boundary voltage barrier becomes higher but narrower, the n¯ becomes less due to the grain sizes becoming larger, which make the reference voltage barrier reaches a minimum for 99.30%TiO2 – 0.60%Y2O3 –0.10%Nb2O5 composition. Fig. 3 shows that the samples all have ultrahigh permittivitty. Neither TiO2 nor the intergranular material can account for this value. The high permittivitty of the ceramic comes, as Matsuoka pointed out [12,13], from the fact that the resistivity of TiO2 grains is much lower than that of the grain boundary layers, so the entire voltage is sustained across narrow intergranular regions and the polarization is large. The permittivitty of the ceramic is then: m = mBd/tB
(6)
C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
9
2− − charged acceptors (V4Ti− , V2Ti− , Y− Ti , TiTi , e ) at the grain boundary interfaces. The oxygen could also be responsible for Schottky barrier formation if we consider that oxygen can be adsorbed at the interfaces and react with negative defects according to [17]:
Fig. 5. Grain-boundary defect barrier model for Nb2O5 doped TiO2 varistors.
where mB is the internal permittivitty of the barrier material, d is the size of the cube grains and tB is the mean thickness of the insulation barriers. It is shown from Eq. (6) that the dielectric capacitance is proportional to 1/tB, which can be used to explain the permittivitty of the samples increasing with the concentration of Nb2O5 dopants. Figs. 3 and 4 show that the permittivitty and resistivity of group A exhibit much lighter variation with frequency compared with group B. As is known, the properties in the low frequency region result from high resistivity and large width of the interfacial layers, and those measured in the high frequency region reflect the low resistivity of the grains. The introduction of niobium increases the height of the grain boundary barriers, which leads to the increase of the interface resistivity, and decreases the resistivity of the grains. The permittivitty is expected to change rapidly near the relaxation frequency, which increases with the conductivity of the ceramics. The dissolution of niobium into TiO2 ceramics decreases the concentration of oxygen vacancies and increases the height of the grain boundary interface barriers, which leads to the decrease of the resultant relaxation [14]. Thus the higher the amount of niobium dopants, the more variation of relative permittivitty and resistivity in the measured frequency range. However, the solubility of Nb5 + exists a maximal limit. When the dopants of Nb2O5 exceeds this solubility limit, the superfluous Nb5 + , which can’t substitute Ti4 + further, will segregate to grain boundary interface [15]. Thus the segregation of Nb5 + blocks the formation and transport of e− and other defects, which will play an effect on the properties of the TiO2 based samples contrary to the substitution of Ti4 + with Nb5 + , as shown in the Figures and Table 1. Gupla and Carlson [13] developed a grain boundary defect model for ZnO varistors analogous to the band model comprising the Schottky barrier [16]. In order to illustrate the grain boundary barrier formation in TiO.2Y2O.3Nb2O5 varistors, an analogy to this model can be considered. In Fig. 5 the positively charged + donors (V2O+ , V+ O , NbTi ) extending from both sides of a grain boundary are compensated by the negatively
1/2O2 Oxad
(7)
2Oxad + V4Ti− 2O − + V2Ti−
(8)
2Oxad + Ti2Ti− 2O − + TixTi
(9)
V4Ti− + 4O − 4O2 − + VxTi
(10)
2O − + Ti2Ti− 2O2 − + TixTi
(11)
The adsorbed oxygen at the grain boundary captures electrons from acceptor defects negatively charged at the grain boundary and stays at the interface. This effect has been confirmed by impedance analysis [11].
5. Conclusion The main conclusions are as follows: (1) An optimal doping composition of 99.30%TiO2 –0.60%Y2O3 – 0.10%Nb2O5 was obtained with low breakdown voltage of 8.8 V mm − 1, high nonlinear constant of 7.0 and ultrahigh permittivitty of 7.6× 104 (at 1 kHz), which can be used as capacitor-varistor ceramics. Deviation from this doping composition, towards either higher or lower Nb2O5 content, causes deterioration of the I–V characteristics. (2) The frequency dependence of the permittivitty and the resistivity also shows that the samples doped with 0.10 mol.% Nb2O5, in comparison with other samples, exhibit the highest resistivity and permittivitty at low frequencies, and the comparatively low resistivity and permittivitty at high frequencies. The phenomenon can be explained by the substitution of Ti4 + with Nb5 + from 0.05 to 0.10 mol.%. The substitution of Ti4 + with Nb5 + from 0.05 to 0.10 mol.% causes a decrease in the resistivity of the grain and facilitates the formation of a large boundary barrier, which leads to a higher nonlinear characteristic. When doping Nb2O5 more than 0.10 mol.%, the superfluous Nb5 + segregates to the interfaces and play a reverse effect on the electrical properties of the samples. (3) In order to illustrate the grain boundary barrier formation in TiO2 –Y2O3 –Nb2O5 varistors, a barrier model was also introduced.
References [1] [2] [3] [4]
T.K. Gupta, J. Am. Ceram. Soc. 73 (7) (1990) 1817 –1840. R. Einzinger, Annu. Rev. Mater. Sci. 17 (1987) 299 –321. M.F. Yan, W.W. Rhodes, Appl. Phys. Lett. 40 (1982) 536 –537. M.F. Yan, W.W. Rhodes, Grain Boundaries in Semiconductors, Elseviser, New York, 1982.
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C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
[5] M.F. Yan, W.W. Rhodes, Additives and Interfaces in Electronic Ceramics, American Ceramic Society, Columbus, OH, 1983. [6] N. Yamaoka, M. Masuyama, M. Fukui, Am. Ceram. Soc. Bull. 62 (6) (1983) 698 –700. [7] N. Yamaoka, M. Masuyama, M. Fukui, Am. Ceram. Soc. Bull. 62 (6) (1983) 703. [8] S.L. Yang, J.M. Wu, J. Am. Ceram. Soc. 76 (1) (1993) 145 – 152. [9] J.F. Wang, Chin. Phys. Lett 17 (7) (2000) 530 – 531. [10] S.A. Pianaro, J. Mater. Sci. Lett. 16 (1997) 634 –638.
.
[11] [12] [13] [14] [15]
Y.J. Wang, J. Phys. D Appl. Phys. 33 (2000) 96 – 99. M. Matsuoka, Jpn. J. Appl. Phys. 10 (1971) 736. T.K. Gupta, W.G. Carlson, J. Mater. Sci. 20 (1985) 3487. J.M. Wu, C.J. Chen, J. Mater. Sci. 23 (1988) 4157 –4164. S.H. Kim, H.W. Seon, Mater. Sci. Eng. B 60 (1999) 12 – 20. [16] R.E. Leite, J.A. Varela, E. Longo, Mater. Sci. Eng. B 27 (1992) 5325. [17] P.R. Emtage, J. Appl. Phys. 48 (10) (1977) 4372 – 4384.
Nonlinear electrical properties of TiO2 –Y2O3 –Nb2O5 capacitor-varistor ceramics Changpeng Li *, Jinfeng Wang, Xiaosu Wang, Wenbin Su, Hongcun Chen, Dexin Zhuang Department of Physics, Shandong Uni6ersity, Jinan 250100, People’s Republic of China Received 5 September 2000; received in revised form 8 February 2001; accepted 13 February 2001
Abstract The nonlinear electrical properties of TiO2 –Y2O3 –Nb2O5 ceramics were investigated as a new varistor material. It was found that an optimal doping composition of 99.75%TiO2 – 0.60%Y2O5 – 0.10% Nb2O5 was obtained with low breakdown voltage of 8.8 V mm − 1, high nonlinear constant of 7.0 and ultrahigh relative dielectric constant of 7.6 ×104, which is consistent with the highest and narrowest grain boundary barriers in the composition. Samples doped with 0.10 mol.% Nb2O5 exhibit the highest permittivitty and resistivity at low frequencies and comparatively lower values at high frequencies in comparison with other samples studied. In view of these electrical characteristics, the ceramics of 99.75%TiO2 – 0.60%Y2O3 – 0.10%Nb2O5 is a viable candidate for capacitor-varistor functional devices. The performance of the ceramics as a function of Nb-doping depends primarily on the extent of substitution of Ti4 + with Nb5 + . In order to illustrate the role of grain boundary barriers for high Nb-doping co-concentrations in TiO2 –Y2O3 –Nb2O5 varistors, a grain-boundary defect barrier model was introduced. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Varistors; Titanium oxide; Yttrium oxide; Niobium oxide; Electrical properties
1. Introduction Varistor materials with high nonlinearity in their current-voltage characteristics are used as protecting devices against voltage transients in electronic and industrial equipment and as surge arrestors. The most important property of a varistor is its nonlinear current-voltage characteristic, which can be expressed by the equation I = KV h, where h is the nonlinear coefficient, a vital parameter used to scale the nonlinearity. Zinc oxide varistors exhibit highly nonlinear currentversus-voltage (I – V) characteristics and are widely used as voltage suppressors in a large variety of powder systems and electric circuits [1,2]. The recent trend in electrical appliance design, however, requires varistors that contain multifunction and have relatively low breakdown voltage [3,4]. Slightly doped TiO2 ceramics, regarded as ultrahigh * Corresponding author. E-mail address: [email protected] (C. Li).
dielectric capacitors [5], are viable candidates for varistors that could satisfy the new functional demand. Yan and Rhodes [5] firstly reported that (Nb, Ba)-doped TiO2 ceramics possess varistor properties, with a current –voltage nonlinearity index of h= 3–4, and that an oxidizing atmosphere during cooling was necessary in the new varistors [6,7]. Recently, Yang and Wu studied the effects of BaO and Bi2O3 additives and of sintering temperature on the varistor characteristics and other electrical properties of Nb-doped TiO2 ceramics [8]. In our previous work, we had found the (Y, Nb)-doped TiO2 ceramics had good nonlinear electrical properties and ultrahigh dielectric constant. This paper would present the effect of niobium additives on the varistor characteristics and other electrical properties of (Y, Nb)-doped TiO2 ceramics. Sintering was performed in air, without a reducing or oxidizing atmosphere for control. Characteristics such as the frequency dependence of the permittivitty and the resistivity, I–V were measured. To investigate the microstructure, a scanning electron microscope (SEM) was used. The results were analyzed and a defect model is introduced in this paper.
0921-5107/01/$ - see front matter © 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 0 1 ) 0 0 5 6 3 - 3
C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
Fig. 1. I– V characteristics of samples with different Nb2O5 dopants.
7
Fig. 2. Characteristic plots of ln J × E 1/2 for samples with different Nb2O5 dopants.
3. Results 2. Experimental procedure The raw chemicals in the present study were analytical grades of TiO2 (99.8%), Y2O3 (99.99%) and Nb2O5 (99.95%). The powder compositions (99.4− X)%TiO2 +0.60% Y2O3 +X%Nb2O5, where X= 0.05, 0.1, 0.15, 0.20, were obtained by conventional mixing using a ZrO2 ball mill. The milled powder was pressed into disks 15 mm in diameter and 1.5 mm thick at a pressure of 160 MPa, which were sintered at the temperature of 1400°C for 1h, cooled at a constant rate of 5°C min − 1 from 1400 to 1100°C, then cooled to room temperature automatically. The green compacts were put into an Al2O3 crucible and fully surrounded with powder of matching composition to reduce the evaporation of low-melting-point components. For microstructural characterization the samples were polished, thermally etched and analyzed in a scanning electron microscope (SEM). The mean grain sizes were determined by the intercept method. For electrical property measurement, silver electrodes were made on both surfaces. The frequency dependence of the permittivitty and the resistivity were investigated by using a Hewlett-Packard LF impedance analyzer (model HP4192A) over a range of frequencies from 10 to 107 Hz. For electrical characterization of current density versus applied electrical field, a semiconductor I–V graph (QT2) was used.
The nonlinear characteristics in electrical properties of the TiO2 –Y2O3 –Nb2O5 ceramics are shown in Fig. 1. The nonlinear coefficient h was obtained by [9]: h=
log(I2/I1) log(V2/V1)
(1)
where V1 and V2 are the voltage at current I1 and I2 , respectively. It was observed from Fig. 1 and Table 1 that the breakdown voltage reached a minimum of 8.8 V mm − 1 and the nonlinear coefficient h reached a maximum, h:7.0, for 99.30%TiO2 –0.60%Y2O3 – 0.10%Nb2O5 composition. The fact that the breakdown voltage is proportional to the thickness of the pellet reveal that the non-ohmic behaviour is an interface property of bulk TiO2 ceramics, and not a property of the ceramic–electrode interface. Considering the Schottky type conduction model, plots of ln J against E 1/2 can be built up to determine values of B, the interface voltage barrier height, and i, a constant related to the potential barrier width, for the TiO2 based varistor with different Nb2O5 dopants, as shown in Fig. 2. B Can be obtained from the intersection of the extrapolated lines of the plot with the ln J axis, and the relative magnitude of constant i, which is inverse to , the potential barrier width, can be derived from the slopes of the plots [10]. Values of B and are shown in Table 1. From Fig. 2 we find that the highest and narrowest grain boundary voltage barrier
Table 1 Some characteristics of the samples doped with different amounts of Nb2O5. Nb2O5 (mol.%)
h
EB (V mm−1)
B (eV)
i×103 (V−1/2.cm1/2)
mr (×104) (at 1 kHz)
0.05 0.10 0.15 0.20
4.2 6.9 5.5 5.0
34.8 8.8 12.8 14.9
0.56 0.65 0.61 0.55
8.1 29.6 18.5 11.2
2.16 7.63 7.53 6.59
8
C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
occurs for the composition 99.30%TiO2 – 0.60%Y2O3 – 0.10%Nb2O5, which is consistent with the observed nonlinear electrical properties for this composition. Table 1 shows that the sample doped with 0.10% Nb2O5 exhibits the highest nolinearity coefficient, the lowest breakdown voltage and highest permittivitty. To further investigate the reason of the variation of electrical properties with different Nb2O5 dopants, the frequency dependence of the permittivitty and the resistivity were measured. Fig. 3 showed the permittivitty vs. frequency of all samples. The results can be roughly divided into two groups: group A which consists of samples doped with 0.05, 0.15, 0.20 mol.% Nb2O5, has comparatively lower permittivitty in the low frequency region, while group B which is the only sample doped with 0.10 mol.% Nb2O5 has much higher permittivitty [11]. Fig. 4 shows the variations in resistivity with frequency. The resitivity decreases with increasing frequency, similar to the behavior of the dielectric constant. It is seen from Figs. 3 and 4 that the samples doped with 0.25 mol.% Nb2O5, in comparison with
Fig. 3. Permittivitty against frequency for samples doped with different amounts of Nb2O5.
Fig. 4. Resistivity against frequency for samples doped with different amounts of Nb2O5.
other samples, exhibit the highest resistivity and permittivitty at low frequencies, and the comparatively low resistivity and permittivitty at high frequencies.
4. Discussion TiO2 ceramics have a wide band gap of about 3.0 eV and are found to be an insulator at room temperature. The dopant of Nb, which has a 5+ valence and an ionic radius approximate to that of Ti4 + , acts as a donor when dissolved in the TiO2 lattice, as indicated by defect Eqs. (2)– (4): − Nb2O5 ¥ 2Nb+ + 4OO + 1/2O2(g) Ti + 2e
(2)
2TiO2 ¥ 2Ti2Ti− + V2O+ + 3OO + 1/2O2(g)
(3)
2− 2V2O+ + V4Ti− 2V+ O + VTi
(4)
where the electron, e−, usually combines with the regular titanium lattice ion to form Ti2Ti− . The dissolution of niobium into TiO2 ceramics increases the concentration of the conduction electrons, which will give rise to the ceramic semiconduction behavior. The substitution of Ti4 + with Nb5 + will promote the formation of acceptor defects in the samples, which create depletion layers at grain boundaries leading to the formation of voltage barriers for the electronic transport. The decrease of the grain resistance also facilitates the formation of the grain boundary barriers. The SEM micrographs show that the grain sizes increase with the introduction of niobium, which imply that the substitution of Ti4 + with Nb5 + facilitates the growth of the grains. According to the boundary barrier model, the reference voltage barrier, Vr, for a varistor is determined by the mean number of barriers n¯ in series multiplied by 6b, that is: Vr = n¯ ·6b,
(5)
where 6b is the voltage barrier at a grain boundary. With the Nb2O5 dopants increasing from 0.05 to 0.10 mol.%, the grain boundary voltage barrier becomes higher but narrower, the n¯ becomes less due to the grain sizes becoming larger, which make the reference voltage barrier reaches a minimum for 99.30%TiO2 – 0.60%Y2O3 –0.10%Nb2O5 composition. Fig. 3 shows that the samples all have ultrahigh permittivitty. Neither TiO2 nor the intergranular material can account for this value. The high permittivitty of the ceramic comes, as Matsuoka pointed out [12,13], from the fact that the resistivity of TiO2 grains is much lower than that of the grain boundary layers, so the entire voltage is sustained across narrow intergranular regions and the polarization is large. The permittivitty of the ceramic is then: m = mBd/tB
(6)
C. Li et al. / Materials Science and Engineering B85 (2001) 6–10
9
2− − charged acceptors (V4Ti− , V2Ti− , Y− Ti , TiTi , e ) at the grain boundary interfaces. The oxygen could also be responsible for Schottky barrier formation if we consider that oxygen can be adsorbed at the interfaces and react with negative defects according to [17]:
Fig. 5. Grain-boundary defect barrier model for Nb2O5 doped TiO2 varistors.
where mB is the internal permittivitty of the barrier material, d is the size of the cube grains and tB is the mean thickness of the insulation barriers. It is shown from Eq. (6) that the dielectric capacitance is proportional to 1/tB, which can be used to explain the permittivitty of the samples increasing with the concentration of Nb2O5 dopants. Figs. 3 and 4 show that the permittivitty and resistivity of group A exhibit much lighter variation with frequency compared with group B. As is known, the properties in the low frequency region result from high resistivity and large width of the interfacial layers, and those measured in the high frequency region reflect the low resistivity of the grains. The introduction of niobium increases the height of the grain boundary barriers, which leads to the increase of the interface resistivity, and decreases the resistivity of the grains. The permittivitty is expected to change rapidly near the relaxation frequency, which increases with the conductivity of the ceramics. The dissolution of niobium into TiO2 ceramics decreases the concentration of oxygen vacancies and increases the height of the grain boundary interface barriers, which leads to the decrease of the resultant relaxation [14]. Thus the higher the amount of niobium dopants, the more variation of relative permittivitty and resistivity in the measured frequency range. However, the solubility of Nb5 + exists a maximal limit. When the dopants of Nb2O5 exceeds this solubility limit, the superfluous Nb5 + , which can’t substitute Ti4 + further, will segregate to grain boundary interface [15]. Thus the segregation of Nb5 + blocks the formation and transport of e− and other defects, which will play an effect on the properties of the TiO2 based samples contrary to the substitution of Ti4 + with Nb5 + , as shown in the Figures and Table 1. Gupla and Carlson [13] developed a grain boundary defect model for ZnO varistors analogous to the band model comprising the Schottky barrier [16]. In order to illustrate the grain boundary barrier formation in TiO.2Y2O.3Nb2O5 varistors, an analogy to this model can be considered. In Fig. 5 the positively charged + donors (V2O+ , V+ O , NbTi ) extending from both sides of a grain boundary are compensated by the negatively
1/2O2 Oxad
(7)
2Oxad + V4Ti− 2O − + V2Ti−
(8)
2Oxad + Ti2Ti− 2O − + TixTi
(9)
V4Ti− + 4O − 4O2 − + VxTi
(10)
2O − + Ti2Ti− 2O2 − + TixTi
(11)
The adsorbed oxygen at the grain boundary captures electrons from acceptor defects negatively charged at the grain boundary and stays at the interface. This effect has been confirmed by impedance analysis [11].
5. Conclusion The main conclusions are as follows: (1) An optimal doping composition of 99.30%TiO2 –0.60%Y2O3 – 0.10%Nb2O5 was obtained with low breakdown voltage of 8.8 V mm − 1, high nonlinear constant of 7.0 and ultrahigh permittivitty of 7.6× 104 (at 1 kHz), which can be used as capacitor-varistor ceramics. Deviation from this doping composition, towards either higher or lower Nb2O5 content, causes deterioration of the I–V characteristics. (2) The frequency dependence of the permittivitty and the resistivity also shows that the samples doped with 0.10 mol.% Nb2O5, in comparison with other samples, exhibit the highest resistivity and permittivitty at low frequencies, and the comparatively low resistivity and permittivitty at high frequencies. The phenomenon can be explained by the substitution of Ti4 + with Nb5 + from 0.05 to 0.10 mol.%. The substitution of Ti4 + with Nb5 + from 0.05 to 0.10 mol.% causes a decrease in the resistivity of the grain and facilitates the formation of a large boundary barrier, which leads to a higher nonlinear characteristic. When doping Nb2O5 more than 0.10 mol.%, the superfluous Nb5 + segregates to the interfaces and play a reverse effect on the electrical properties of the samples. (3) In order to illustrate the grain boundary barrier formation in TiO2 –Y2O3 –Nb2O5 varistors, a barrier model was also introduced.
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