The effects of additives (Cr, Nb, Dy, and Bi) on microstructure, electrical properties, dielectric characteristics, and aging behavior of ZnO-V 2 O 5 -MnO 2 -Co 3 O 4 -La 2 O 3 (ZVMCL) ceramics were systematically investigated. The phase formed in common for all ZVMCL ceramics modified with various additives consisted of ZnO grain as a main phase, and Zn 3 (VO 4 ) 2 and ZnV 2 O 4 as the secondary phases. The sintered density and average grain size were in the range of 5.4-5.54 g/cm 3 and 3.7-5.1 μm, respectively. The ZVMCL ceramics modified with Cr exhibited the highest breakdown field (6,386 V/cm) and the ZVMCL ceramics modified with Nb exhibited the lowest breakdown field (3,517 V/cm). All additives enhanced the nonlinear coefficient (α), by a small or large margin, in particular, additives such as Bi and Nb noticeably increased the nonlinear coefficient, with α=25.5 and α=23, respectively. However, on the whole, all additives did not improve the stability against a DC stress, compared with ZVMCL ceramics.Keywords: Additives, Nonlinear properties, Aging behavior, ZnO-V 2 O 5 -based varistor ceramics Regular Paper INTRODUCTIONZnO is a versatile material that possesses a wide range of applications as active component in gas sensors, optical devices, grain boundary effect devices, etc. ZnO varistors are typical semiconducting electro-ceramic devices, which exhibit a nonlinear V-I relation due to the grain boundary effect formed by sintering ZnO powder with minor additives [1]. The sintering process gives rise to a microstructure, which consists of semiconducting n-type ZnO grains surrounded by very thin insulating intergranular layers. Each ZnO grain acts as if it has a semiconductor junction at the grain boundary [2,3]. Since nonlinear electrical behavior occurs like back-to-back zener diode at each boundary, the impurity doped ZnO varistors can be considered as a multi-junction device composed of many series and parallel connection of grain boundaries [2,3]. The grain size distribution plays a major role in electrical behavior. Electrically, ZnO varistors exhibit highly nonlinear voltage-current (V-I) properties expressed by the relation I = CV α , where I is the current, V is the voltage, C is a constant, α is the nonlinear coefficient, which characterizes the nonlinear properties of the varistors [4,5].ZnO ceramics cannot exhibit a varistor behavior without adding heavy elements with large ionic radii such as Bi, Pr, Ba, etc. Commercial ZnO-Bi 2 O 3 -based ceramics and ZnO-Pr 6 O 11 -based ceramics cannot be co-fired with a silver inner-electrode (m.p. 961℃) in multilayered chip varistors because of the relatively high sintering temperature above 1,000℃ [6,7]. Therefore, new varistor ceramics are required in order to use a silver innerelectrode. Among the various ceramics, one candidate is the ZnO-V 2 O 5 ceramics [8,9]. This system can be sintered at a rela-

The increase in the amount of Tb 4 O 7 increased from E B = 7,473 to 10,035 V/cm and from α = 39.7 to 52.2. In particular, it was found that the ceramics modified with 1.0 mol% in the amount of Tb 4 O 7 are suited for the varistors for high voltage in the light of a high sintered density and a high voltage gradient.