Influence of cobalt oxide addition on electrical properties of ZnO-Pr6O11-based varistor ceramics

Abstract

ZnO varistors are smart ceramic semiconductor components made by ZnO sintered together with minor additives. They exhibit highly nonlinear voltage-current (V –I ) characteristics expressed by I = kV α , where k is a constant and α is nonlinear exponent as an index or figure of merit indicating the effectiveness of a varistor. Furthermore, they possess excellent high energyhandling capabilities. As a result, they have been widely used to protect various semiconductor devices, electronic circuits, and electric power systems from dangerous abnormal transient voltage [1, 2]. ZnO varistors are greatly divided into Bi2O3and Pr6O11-based with varistor-forming oxide (VFO). Most commercial ZnO varistors containing Bi2O3 exhibit excellent varistor properties, but they have a few flaws due to Bi2O3 having high volatility and reactivity [3]. And they need many additives to obtain the high performance. To overcome these problems, Pr6O11-based ZnO varistors are being studied [4–10]. In Pr6O11-based ZnO varistors, the most important two additives are Pr6O11, which in substance gives rise to nonlinear properties, and CoO, which improves them. However, the nonlinear properties in these varistors can be never improved by incorporating of any additives without CoO. Therefore, the CoO is indispensable additives in ZnO varistors. In Bi2O3-based ZnO varistors, CoO or Co3O4 content is limited to 0.5 mol% in general. No study of the influences of CoO additionon electrical properties in Pr6O11-based ZnO varistors has been reported. The goal of this paper is to investigate the influence of CoO addition on electrical properties of ZnO-Pr6O11-based ceramics. Reagent-grade raw materials were prepared for ZnO varistors with composition expression, such as (98.5 − x) mol% ZnO, 0.5 mol% Pr6O11, x mol% CoO (x = 0.5, 1.0, 2.0, 3.0, 5.0), 0.5 mol% Cr2O3, 0.5 mol% Dy2O3. The mixture was calcined in air at 750◦C for 2 hr. The calcined powders were pressed into discs 10 mm in diameter and 2 mm in thickness at a pressure of 80 MPa. The discs were sintered at 1350 ◦C in air for 1 hr. The size of the final samples was about 8 mm in diameter and 1.0 mm in thickness. Silver paste was coated on both faces of the samples and ohmic contacts were formed by heating at 600 ◦C for 10 min. The size of electrodes was 5 mm in diameter. The surface microstructure was examined by scanning electron microscopy (SEM, Model S2400, Hitachi, Japan). The average grain size (d) of varistor ceramics was determined by the lineal intercept method [11]. The density (ρ) of varistor ceramics was measured by the Archimedes method. The V –I characteristics of the varistors were measured using a Keithley 237 unit. The varistor voltage (V1mA) was measured at a current density of 1.0 mA/cm2 and the leakage current (IL) was measured at 0.80 V1mA. In addition, the nonlinear exponent (α) was determined from α = 1/(log E2 − log E1), where E1 and E2 are the electric fields corresponding to 1.0 mA/cm2 and 10 mA/cm2, respectively. The capacitance-voltage (C–V ) characteristics of varistors were measured at 1 kHz using a RLC meter (QuadTech 7600) and an electrometer (Keithley 617). The donor concentration (Nd) and the barrier height (φb) were determined by the equation (1/Cgb − 1/Cb0) = 2(φb + Vgb)/qeNd [12], where Cgb is the capacitance per unit area of a grain boundary, Cb0 is the value of Cgb when Vgb = 0, Vgb is the applied voltage per grain boundary, q is the electronic charge, and e is the permittivity of ZnO (e = 8.5 e0). The density of interface states (Nt) at the grain boundary was determined by the equation Nt = (2eNdφb/q) [12] and the depletion layer width (t) of the either side at the grain boundaries was determined by the equation Nd t = Nt [13]. Fig. 1 shows the SEM micrographs of varistor ceramics sintered with various CoO contents. It is well known that the microstructure of Pr6O11-based ZnO varistor ceramics is consisted of only two phases [8]: ZnO grain (bulk phase, black) and intergranular layer (second phase, whitish) comprising of Prand Dy2O3rich phase located at the boundaries. As the CoO contents increased, the sintered density was increased from 5.25 to 5.55 g/cm3 corresponding 91 to 96% of theoretical density of pure ZnO (5.78 g/cm3) up to 2.0 mol%, whereas the additions further did not affect density, saturating 5.55 g/cm3. Therefore, the ceramics was more densified with increasing CoO contents. The average grain size increases from 9.9 to 27.2 μm with increasing CoO content. As a result, it can be seen that the CoO promotes grain growth. The grain size directly affects varistor voltage in voltage-current characteristics. The detailed V –I microstructural parameters are summarized in Table I. Fig. 2 shows the E–J characteristics of varistors with various CoO contents. The shape of curves is somewhat complex without having any remarkable tendency in the light of CoO content only. The characteristic curves of varistors are greatly divided into two regions,