J. Cent. South Univ. Technol. (2009) 16: 0897-0901

DOI: 10.1007/s11771-009-0149-5

Influence of soaking time on nonlinear electrical behavior and

dielectric properties of TiO₂-based varistor ceramics

MENG Fan-ming(孟凡明)^{1, 2, 3}, LU Fei(鲁 飞)^1,3, XIAO Lei(肖 磊)^{1, 3}, SUN Zhao-qi(孙兆奇)^{1, 3}

(1. School of Physics and Materials Science, Anhui University, Hefei 230039, China;

2. State Key Laboratory of Materials Modification by Laser, Ion and Electron Beams,

Dalian University of Technology, Dalian 116024, China;

3. Anhui Key Laboratory of Information Materials and Devices, Anhui University, Hefei 230039, China)

Abstract: The influence of soaking time on the nonlinear electrical behavior and dielectric properties of TiO₂-based varistor ceramics was investigated. Based on single sintering process, six disk samples of (Sr, Bi, Si, Ta)-doped TiO₂-based varistor ceramics were fabricated by sintering at 1 250 ℃ for 0.5-5.0 h. The samples were characterized by X-ray diffraction, voltage-current characteristics, energy spectra, metallographs, breakdown voltages, and apparent dielectric constant. It is found that the breakdown electrical field intensity at a current density of 10 mA/cm² decreases from 5.5 to 4.1 V/mm first and then increases to 7.0 V/mm, the nonlinear coefficient increases from 2.39 to 2.62 first and then decreases to 2.42, and the apparent dielectric constant increases from 98 200 to 115 049 first and then decreases to 73 865 with the soaking time increasing from 0.5 to 5.0 h. These indicate that the optimal soaking time is 2.0-3.0 h considering both nonlinear electrical behavior and dielectric properties.

Key words: TiO₂ varistor ceramics; breakdown voltage; nonlinear coefficient; dielectric constant; soaking time

1 Introduction

Varistors are of high nonlinear voltage-current characteristics, which are used as protecting devices against voltage transients in electronics industrial equipment and surge arresters. As one of commercial varistors, ZnO-based varistor has been widely used as voltage suppressors in a great number of power systems and electronic circuits[1-2]. However, the low permittivity of ZnO varistor limits its ability to absorb the sparks, which makes it usually applied in the low voltage circuit. Recent electrical appliance designs require varistors to possess more functions and have a relatively low breakdown electrical field intensity. To meet these demands, many new varistor materials such as SrTiO₃[3], TiO₂[4], SnO₂[5], V₂O₅[6], and WO₃[7] were developed. SrTiO₃-based varistor ceramic is capable of high energy-absorbing. But it has a serious drawback that its conductance is induced through chemical reduction in the H₂-N₂ atmosphere at a temperature higher than 1 100 ℃ because it is generally prepared by a two-step sintering process. Thus, these devices are comparatively unstable because of aging when they are exposed to air in a high applied electrical field intensity. Meanwhile, the study on SnO₂, V₂O₅ and WO₃ varistor ceramics is immature.

Researchers have been interested in TiO₂-based varistors recently. YAN and RHODES[8] reported that (Nb, Ba)-doped TiO₂-based varistor ceramics are of useful varistor properties, with a nonlinear coefficient of α=3-4. An oxidizing atmosphere is necessary during cooling. Since then, plenty of studies have been carried out to investigate the influence of composition and process condition on microstructures and electrical properties of TiO₂-based varistor ceramics. LI et al[9] reported low-voltage TiO₂ varistor ceramics doped with Nb₂O₅, SiO₂, CeO₂ and CaCO₃. They investigated the effects of different dopants on varistor voltage at 1 mA by orthogonal test method. It is found that there exists the 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 energy dispersive X-ray spectrometry and the content varies with sintering temperature. It is suggested that the second phase segregates on the grain surface during sintering process and forms an insulating layer that results in a higher varistor voltage. NAVALE et al[10] reported that the addition of small quantities of Ta₂O₅ into TiO₂ leads to highly nonlinear electrical characteristics. LUO et al[11] investigated the effect of Ta on the microstructures, nonlinear electrical behavior and dielectric properties of (Ca, Si, Ta)-doped TiO₂ ceramics. It is found that an optimal doping composition of 0.8% (mole fraction) Ta₂O₅ leads to a low breakdown voltage of 14.7 V/mm, a high nonlinear constant of 4.8 and an ultrahigh apparent dielectric constant of 5.0×10⁴ and tanδ=0.66 (dissipation factor, measured at 1 kHz), which are consistent with those of the highest and narrowest grain boundary barriers of the ceramics.

It is reported that dopants with a valence of +5, such as Nb and Ta, have an ionic radius similar to that of Ti⁴⁺[8]. These dopants readily dissolve into TiO₂ lattice, reducing its resistivity by donating conductive electrons. The introduction of SrCO₃, Bi₂O₃ and SiO₂ densifies TiO₂ probably by increasing lattice defects through the formation of a solid solution or a liquid phase[12]. The purpose of this work is to investigate the influence of soaking time on nonlinear electrical behavior and dielectric properties of (Sr, Bi, Si, Ta)-doped TiO₂-based varistor ceramics.

2 Experimental

2.1 Preparation of samples

The raw chemicals (in mass fraction) are analytical grades of TiO₂ (99.9%), Ta₂O₅ (99.99%), SrCO₃ (99%), Bi₂O₃ (99%), and SiO₂ (95.78%). Nominal compositions (mole fraction, %) of the studied samples were: 100TiO₂+0.3SrCO₃+0.3Bi₂O₃+0.3SiO₂+0.075Ta₂O₅. The chemicals were weighed and wet-milled in a polyethylene mortar with ZrO₂ balls. The milled and mixed powders were dried and then pressed into disks of 11 mm in diameter and 2 mm in thickness at 2.5 MPa. The green compacts were put into an Al₂O₃ crucible, fully surrounded with powder of matching composition to reduce the evaporation of low-melting-point components, sintered at 1 250 ℃ for 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 h, respectively, and then cooled to room temperature naturally.

2.2 Characterization of samples

X-ray diffraction (XRD) test was performed using Cu K_α radiation and a Ni filter at a scanning speed of 2(?)/min. Surface morphological features were observed by a metallogenetic microscope (XJG-05). Energy spectra were analyzed by an energy dispersive X-ray spectrometer (S-520/ISIS-300). For measuring electrical properties, In-Ga electrodes were made on both surfaces. An HP 4140B PA analyzer was used to measure the voltage-current characteristics, a MY-4C meter to measure the breakdown voltages, and a low-frequency impedance analyzer (model HP4192A) to measure the frequency dependences of the capacitance.

3 Results and discussion

3.1 X-ray diffraction (XRD) analysis

The XRD patterns of the sintered samples and the green body are given in Fig.1. The XRD pattern of the green sample shows that the raw powder is anatase structure (with major intensity peaks at 2θ=25.30?, 37.33? and 48.09?, which are corresponding to anatase  (101), (004) and (200)). The results of the sintered samples show that all the major peaks match those of the rutile structure (with major intensity peaks at 2θ=27.42?, 36.13?, 41.22?, 54.35? and 69.11?, which are corresponding to rutile (110), (101), (111), (211), and (301)).

Fig.1 XRD patterns of green sample (a) and samples sintered at 1 250 ℃ for 0.5 h (b), 1.0 h (c), 2.0 h (d), 3.0 h (e), 4.0 h (f), and 5.0 h (g), respectively

3.2 Nonlinear electrical properties

The nonlinear coefficient (α) is obtained by the following equation:

                              (1)

where  J₁ and J₂ are the current densities; E₁ and E₂ are the breakdown electrical field intensities corresponding to current densities, J₁ and J₂, respectively. When J₁ and J₂ are 1 and 10 mA/cm², respectively, Eq.(1) can be expressed as

                         (2)

where   and are breakdown electric field intensities corresponding to current densities of 1 and 10 mA/cm², respectively. Figs.2 and 3 show nonlinear electrical properties of all the samples. From Figs.2 and 3, it can be seen that breakdown electric field intensity, , decreases from 5.5 to 4.1 V/mm first and then increases to 7.0 V/mm, while the nonlinear coefficient increases from 2.39 to 2.62 first and then decreases to 2.42 with

Fig.2 Relationship between electric field intensity (E) and current density (J) curves of samples sintered at 1 250 ℃ for 0.5, 1.0, 2.0, 3.0, 4.0, and 5.0 h, respectively

Fig.3 Influence of soaking time on breakdown electric field intensity and nonlinear coefficient

soaking time increasing from 0.5 to 5.0 h.

The nonlinear electrical properties of the (Sr, Bi, Si, Ta)-doped TiO₂-based varistor ceramics were analyzed according to the theory of defects in the crystal. In TiO₂ lattice, titanium ions and oxygen ions can deviate from their normal lattice sites at a high sintering temperature, leaving titanium vacancy ( and oxygen vacancy  behind. The dopant Ta⁵⁺ with an ionic radius similar to that of Ti⁴⁺ acts as a donor when it dissolves into TiO₂ lattice. The radius of Sr²⁺ (116 pm) is larger than that of Ti⁴⁺ (61 pm)[13], so Sr²⁺ may substitute Ti⁴⁺ rather than inhabit TiO₂ lattice. The dissolution of Sr into grains of rutile structure creates one oxygen vacancy and negative charged . Bi plays a similar role as Sr except that Bi easily loses mass through evaporation. Assuming that additives Ta, Sr and Bi are dissolved in TiO₂ grains, the defect equations can be expressed as follows[14]:

         (3)

       (4)

       (5)

         (6)

From Eq.(4), the dissolution of Ta into TiO₂ lattice increases the concentration of oxygen, which counterworks the process presented in Eq.(3), promoting the growth of the grains. Therefore, the grain size increases (Fig.4) and the mean number of barriers (n) decreases with increasing soaking time. According to the boundary barrier model, the breakdown electrical field intensity () is determined by n in series multiplied by V_gb, which is

                           (7)

where  V_gb is the voltage barrier at the grain boundary, which is almost a constant for different samples. Therefore, depends primarily on n. With the increase of the soaking time, n decreases and thus leads to the decrease of.

Furthermore, Sr and Bi play an important role in TiO₂-based ceramics. Because of their great ionic radii (116 pm for Sr²⁺ and 102 pm for Bi³⁺) compared with that of Ti⁴⁺ (61 pm), Sr and Bi are prone in segregate to grain boundary areas (Table 1), especially during cooling, to relieve elastic strain energy. This results in the building up of a negatively charged layer near grain boundaries. Apparently, the acceptor concentration (density of surface states, N_s) increases gradually with increasing soaking time. From Eq.(4), it can be seen that the dissolution of Ta into rutile grains contributes to the generation of electrons for conduction which gives rise to the semi-conducting behavior of the ceramics. Since the radius of Ta⁵⁺ (64 pm) is similar to that of Ti⁴⁺      (61 pm)[13], Ta⁵⁺ is dissolved easily into the TiO₂ lattice. It has been reported that Nb₂O₅ has solubility of a few contents in rutile phase. Since the radius of Ta⁵⁺ is similar to that of Nb⁵⁺, it can be reasonably assumed that  Ta (0.075% mole fraction) is dissolved into the rutile grains at all sintering temperatures. All samples have the same donor density (N_d) of  as added.

According to the discussion above, the acceptor concentration N_s increases gradually and donor density N_d does not change with increasing soaking time. Apparently, the interface voltage barrier height  increases with increasing soaking time through the following equation[15]:

                                 (8)

where  e is electric charge and ε the dielectric constant of material. As is known, the electric conduction above the ohmic region is associated with the thermion

Fig.4 Metallographs of samples sintered at 1 250 ℃ for different time: (a) 0.5 h; (b) 1.0 h; (c) 2.0 h; (d) 3.0 h; (e) 4.0 h; (f) 5.0 h

Table 1 Data from energy spectra of grain and grain boundary in samples sintered at 1 250 ℃ for 2.0 h (mass fraction, %)

emission of Schottky type. For this type of mechanism the current density is related to the electric field intensity and the temperature by the following equation[16]:

               (9)

where  A^* is the effective Richardson constant; E_b is the interface breakdown electric field intensity; and β is a constant. From Eq.(9), it can be seen that the interface breakdown electric field intensity increases in direct proportion to the interface voltage barrier height. Since the barrier height is directly proportional to the soaking time, the interface breakdown electric field intensity increases with increasing soaking time.

The nonlinear coefficient α can be expressed as[12]

                               (10)

From Eqs.(9) and (10), Eq.(11) can obtain.

                              (11)

Apparently, the variation of nonlinear coefficient (α) is in accordance with the variation of interface breakdown electric field intensity. Since the interface breakdown electric field intensity increases with increasing soaking time, the nonlinear coefficient will increase with increasing soaking time.

However, when soaking time is more than 3.0 h, hypo-grain boundary appears, thus the amount of the grain boundary increases and the surface states redistribute. As a result, both the interface voltage barrier height and the nonlinear coefficient decrease. Furthermore, because of appearance of hypo-grain boundary, grain size decreases. Therefore, breakdown voltage increases.

3.3 Dielectric properties

Fig.5 shows the apparent dielectric constant vs soaking time of all samples at 1 kHz. It can be seen that the apparent dielectric constant increases from 98 200 to 115 049 first and then decreases to 73 865 with soaking time increasing from 0.5 to 5.0 h.

Fig.5 Influence of soaking time on apparent dielectric constant

The apparent dielectric constant ε_ra can be expressed as[17-18]:

                                (12)

where  ε_r is the relative dielectric constant of TiO₂,    ε_r=114; d₀ is the grain size; and t_b is the average thickness of the insulation barrier. Generally, d₀=10-20 ?m, t_b=0.01-0.10 ?m. According to Eq.(12), the assessed value of ε_ra is 10⁴-10⁵. Assuming that the average number of grain is constant, when the grain size increases, the average thickness of the insulation barrier will decrease. Thus the apparent dielectric constant increases with increasing soaking time.

However, when soaking time is more than 3.0 h, hypo-grain boundary appears, thus grain size decreases (Fig.4), and the apparent dielectric constant decreases.

4 Conclusions

(1) Six samples of TiO₂-based varistor ceramics are fabricated by sintering at 1 250 ℃ for 0.5-5.0 h. The influence of soaking time on nonlinear electrical behavior and dielectric properties is investigated.

(2) Breakdown voltage decreases first and then increases, while the nonlinear coefficient and the dielectric constant increase first and then decrease with increasing soaking time.

(3) It may be possible to fabricate TiO₂-based varistor devices of high dielectric constant and high nonlinear coefficient with low breakdown voltage by sintering at 1 250 ℃ for 2.0-3.0 h.

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(Edited by CHEN Wei-ping)

Foundation item: Project(50872001) supported by the National Natural Science Foundation of China; Projects(KJ2007B132, KJ2009A006Z) supported by the Scientific Research Foundation of Education Department of Anhui Province, China; Project(XJ200907) supported by the Foundation of Construction of Quality Project of Anhui University, China

Received date: 2009-04-22; Accepted date: 2009-06-27

Corresponding author: MENG Fan-ming, PhD, Associate professor; Tel: +86-551-5107284; E-mail: mrmeng@ahu.edu.cn