Electrical impedance spectroscopy and structural characterization of liquid-phase sintered ZnO–V₂O₅–Nb₂O₅ varistor ceramics doped with MnO

Abstract

The influence of the MnO content on the microstructure and the electrical response of liquid-phase sintered ZnO–V₂O₅ varistor ceramics wereanalysed using A.C. impedance spectroscopy on samples prepared via the conventional solid state route.The impedance spectra were analysed with the help of one model equivalent circuit at high frequencies and another at low frequencies, involving both resistor and non-Debye constant phase elements (CPEs). The results indicate a significant contribution of grain boundary resistance to the total resistance and non-ohmic characteristic of the studied materials. The Arrhenius plots show two slopes with a turnover at 150 °C/200 °C for both the higher- and lower-frequency time constants. These behaviours can be related to the decrease of the minor charge carrier density. These activation energies were associated with the adsorption and reaction of O₂ (as well as V) species at the grain boundary interface. Consequently, better varistor performance is achieved for 96.9 mol% ZnO+0.5 mol% V₂O₅+0.10 mol% Nb₂O₅+2.5 mol% MnCO₃ with nonlinear coefficient α=21.6, breakdown field E_1mA=191.47 V/mm, leakage current density J_L=36.46 µA/cm² and activation energies of 0.639 eV and 0.644 eV. X-ray diffraction shows that in addition to the major ZnO phases, Zn₃(VO₄)₂ and ZnV₂O₄, were detected as minor secondary phases. SEM analysis of the morphology shows that the grain growth increases with increases in the MnO doping level.

Introduction

Zinc oxide varistors are polycrystalline semiconductor devices that exhibit excellent non-linear properties that make them competent to sense and limit transient surges repeatedly and quickly, protecting semi-conductive devices and power system from being destroyed [1], [2]. ZnO varistors consist of the key dopant varistor forming oxide Bi₂O₃ and several types of additives (such as MnO₂, Cr₂O₃, Sb₂O₃ and Co₃O₄) that can generate excellent non-ohmic conduction as well as high-energy absorption capability and low power loss, thereby improving the electrical properties and the stability. Amongst these additives, Bi₂O₃ and some light metal oxides of transition metal oxides (e.g., V), rare earths (e.g., Pr) and alkaline earths (e.g., Ba) play the most important role and segregate at the grain boundary interface [3], [4], [5].

Classical ZnO–Bi₂O₃ and ZnO–Pr₆O₁₁ based varistor systems [1], [2], [3], [4], [5], [6], [7], [8], [9] cannot be co-fired with a pure silver inner-electrode in multilayered chip varistors because of the high sintering temperature (above 1000 °C) in excess of the melting point of silver [Ag mp 961 °C] [10], [11]. ZnO–V₂O₅ based ceramics can be well sintered at approximately 900–950 °C. This outstanding feature makes the ZnO–V₂O₅ based varistor a potential candidate for the fabrication of a multi-layered chip varistor using pure silver as inner electrode instead of the expensive palladium or platinum metals [10], [11], [12], [13], [14].

However, the binary ZnO–V₂O₅ ceramics exhibit a low nonlinear coefficient [10], [11], [12], [13], [14]. To improve the nonlinear properties and stability, MnO₂-and Mn₃O₄-doped ZnO–V₂O₅ varistor ceramics have been actively studied until now [15], [16], [17], [18]. However, MnO- or MnCO₃-doped ZnO–V₂O₅ varistor ceramics have been rarely studied. One of the reasons why MnO- or MnCO₃-doped ZnO–V₂O₅-based varistor ceramics have rarely been studied is because of the low nonlinear properties compared with those of MnO₂-and Mn₃O₄-doped ZnO–V₂O₅ varistor ceramics. It is assumed that this is attributed to only the valence state between MnO₂, Mn₃O₄ and MnO or MnCO₃ as the Mn-species: Mn in MnO₂, Mn₃O₄ and MnO or MnCO₃ has a valence state +4, +1.33 and +2, respectively. Otherwise, MnO or MnCO₃ has an advantage in terms of cost and availability when compared with MnO₂ and Mn₃O₄. Furthermore, the Nb₂O₅-doped ZnO–V₂O₅ varistor ceramics have a nonlinear coefficient in the range of 4 to 7. However, the ZnO–V₂O₅–Nb₂O₅ ceramics doped with MnO or MnCO₃ produce good nonlinear properties [34]. As a result, MnO or MnCO₃ along with a Nb₂O₅ additive acted as the enhancer of the nonlinear coefficient [7], [16].

The electrical performances of ZnO varistors critically depend on the microstructure characteristics, where the electrical characteristics can be controlled by modifying the microstructure at the grain boundary. The nonlinear characteristics are attributed to the formation of Double Schottky Barriers (DSB) at the ZnO grain boundaries. Therefore, there are numerous studies addressing the grain boundary phenomena of ZnO–Bi₂O₃ [19], [20], [21] and ZnO–V₂O₅-based ceramics systems [22], [23]. It is vital to understand the mechanism controlling the grain boundary processes of ZnO varistor-based ceramics. It may also facilitate a researcher or manufacturer to tailor the grain boundary behaviour of ZnO based varistor ceramics in a more efficient manner according to the specific demands of applications. The grain boundary behaviour can be strongly dependent on the dopants. Guo-hua Chen et al. [21] found that the values of the breakdown field E_1mA and nonlinear coefficient are strongly dependent on the resistivity of the grain boundary. Jun Wu et al. [22], [23] studied the influence of MnO₂, PbO and a mixture of MnO₂, PbO and B₂O₃ on the electrical and dielectric properties of ZnO–V₂O₅ ceramics by impedance and dielectric spectroscopy and found that the Schottky barrier present at the grain boundary is significantfor varistor performance. Tsai et al. [10], [11] found no Debye relaxation peak at room temperature for the ZnO-V₂O₅-based ceramics. In our previous study [20], the electrical conductivity of Mn- and Co-doped ZnO–Bi₂O₃ based varistors were investigated using complex plane modulus analysis and found that the ratio of grain boundaries to grain resistance of Mn-doped samples is higher than that of Co-doped samples. Pandey et al. [34] found that the electrical and dielectric properties of liquid-phase sintered ZnO–V₂O₅ ceramics doped with Nb₂O₅ critically depend on the grain boundary resistance using impedance spectroscopy (IS).

Therefore, it is necessary that MnO- or MnCO₃-doped ZnO–V₂O₅–Nb₂O₅ ceramics will be studied in detail in terms of grain boundary behaviour to tailor multi-layered chip varistors.

The present work further addresses the effect of MnO content on ZnO–V₂O₅–Nb₂O₅-based ceramics systems sintered at approximately 950 °C on the microstructure and the influence of the grain boundary structure on the electrical properties of ZnO based ceramics in a systematic manner using AC impedance spectroscopy (IS).