Varistor Properties and Aging Behavior of ZnO-V₂O₅-MnO₂-Co₃O₄-La₂O₃Ceramics Modified with Various Additives (Cr, Nb, Dy, Bi)

Abstract

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-

Thermal balance diagram modelling of surge arrester for thermal stability analysis considering ZnO varistor degradation effect

“…ZnO varistors are typical semiconducting electro‐ceramic devices which exhibit a non‐linear V – I behaviour due to the grain boundary effect formed by sintering ZnO powder with minor additives [16]. Therefore, ZnO ceramics cannot exhibit a varistor behaviour without adding heavy elements such as Bi, Pr, Ba and so on.…”

Electro-thermal modeling of surge arrester based on adaptive power loss estimation using finite element method

“…The varistor properties are specified by U–I characteristics . Varistor construction materials , operating history , and temperature are three main factors that have influence on U–I characteristics. In order to investigate the influence of material, temperature, and degrading on MOSA U–I characteristic, experimental setup as shown in Figure was arranged.…”