JOURNALOF MATERIALS SCIENCE: MATERIALS IN ELECTRONICS 13 (2002) 253±256 Varistor behavior of the system SnO 2 ? CoO ? Ta 2 O 5 ? Cr 2 O 3 G. F. MENEGOTTO, S. A. PIANARO, A. J. ZARA Departamento de Engenharia de Materiais, Universidade Estadual de Ponta Grossa, LIMAC ± Laborato  rio Interdisciplinar de Materiais Cera à micos, 84030-900 Ponta Grossa Pr, Brazil S. R. M. ANTUNES, A. C. ANTUNES Departamento de QuõÂmica, Universidade Estadual de Ponta Grossa, LIMAC ± Laborato Ârio Interdisciplinar de Materiais Cera à micos, 84030-900 Ponta Grossa Pr, Brazil E-mail: acantunes@uepg.br In this work the system (98.95  x)% SnO2  1%CoO  0:05%Ta2O5  x %Cr2O3 (mol%) was studied, with x  0.05 and 0.10, prepared by the conventional method of oxides mixture, and sintered at 1300 and 1350  C for 2 h. The non-linear J versus E electrical characteristics a  25 were obtained in the Ta 2 O 5 and Cr 2 O 3 ±CoO doped highly densi®ed SnO 2 ceramics. X-ray diffraction analysis showed that these ceramics are apparently single phase. Electrical properties and microstructure are highly dependent on the Cr 2 O 3 concentration and on the sintering temperature. Excess of Cr 2 O 3 leads to porous ceramics destroying the electrical characteristics of the material. Dopant solid solution formation in the SnO 2 may be responsible for the formation of electrical barriers in the grain boundaries. # 2002 Kluwer Academic Publishers 1. Introduction The current density J and the applied electrical ®eld E of non-ohmic ceramic materials are related by the following equation: J  C ? E a 1 where C is a constant that is related to the microstructure and a is the non-linear coef®cient. The non-linear coef®cient is conventionally obtained for current densities around 1 mA cm  2 [1]. At this current density the breakdown ®eld can be calculated through the equation: E r  v b d 2 with v b the barrier voltage and d the average grain size. For an electrical ®eld higher than E r the electrical conduction starts to be highly non-linear [1, 2]. The leakage current, measured in the linear region of the curve characteristic ln E versus ln J, for a constant electric ®eld, is another important characteristic of the varistor. The SnO 2 is an n-type semiconductor [3] that presents electrical conductivity due to formation of intrinsic defects (vacancies of oxygen or Sn 1 interstitial) or extrinsic ones (due to dopants). The application of the SnO 2 as a dense ceramic has been limited due to its behavior when sintered. The SnO 2 ceramic when sintered by the conventional methods presents minimum densi®cation. The densi®cation can be promoted by appropriately doping the SnO 2 with dopants [4±6]. In a previous study [6±8], we have shown that ceramics can present non- ohmic properties. 2. Experimental procedure Analytical grades of SnO 2 , CoO, Ta 2 O 5 , and Cr 2 O 3 were used in this work. The oxides were ball-mill mixed in alcohol media with the following composition (in mol %): (98.95  x)% SnO2  1% CoO  0:05% Ta2O5 x% Cr2O3, with x ranging from 0.05 to 0.10 mol %. The resulted powders were uniaxially dry pressed 75 MPa in a cylindrical shape. The samples were sintered in a box furnace at 1300 and 1350  C for 2 h, and slowly cooled to room temperature. The ceramic samples were thermally etched at 50  C below the sintering tempera- ture for 15 min and a scanning electron microscope (JEOL model JSM T330A) was used for microstructural analysis. Ceramic crystalline phases were determined by X-ray diffraction (Siemens model D-5000). Current versus voltage electrical characteristics were determined by using a stabilized voltage source along with two digital multimeters. The non-linear coef®cient values (a) were obtained by linear regression of the log-scale plot of current density versus applied electrical ®eld in the region around 1 mA cm  2 and the breakdown electrical ®elds were obtained at this current density. Grain sizes were determined by using SEM micrographs and the equation proposed by Mendelson [9]. 0957±4522 # 2002 Kluwer Academic Publishers 253