A Comparative Study of Thermal Conductivity in ZnO- and SnO 2 -Based Varistor Systems P. R. Bueno w and J. A. Varela Instituto de Quı´mica, Universidade Estadual Paulista, 14801-907 Araraquara, SP, Brazil C. M. Barrado, E. Longo, and E. R. Leite Interdisciplinary Laboratory of Electrochemistry and Ceramics, Departmento de Quı´ mica, Universidade Federal de Sa˜ o Carlos, 13565-905 Sa˜ o Carlos, SP, Brazil We performed a comparative study of electrical and thermal properties of ZnO- and SnO 2 -based varistor. The electrical properties of commercial ZnO-based varistor are equivalent to that found in SnO 2 -based varistor system. In spite of this, the SnO 2 showed a thermal conductivity higher than commercial samples of ZnO-based varistor, which allied with its simpler microstructure and lower dopant concentration is a remarkable result that point out to the use of this system to compete com- mercially with ZnO-based varistor devices. Z NO- and SnO 2 -based varistor are multi-junction ceramic semiconductor devices made by sintering ZnO or SnO 2 with other metal oxides of small amount. The commonly metal ox- ides added to ZnO commercial varistors are Bi 2 O 3 , Co 2 O 3 , Pr 6 O 11 , and Mn 2 O 3 , 1,2 while in SnO 2 the main ones remain as CoO, Nb 2 O 5 , and Cr 2 O 3 . 3–7 The main feature of a varistor is that they act as an insulator below the varistor voltage, called breakdown voltage, and conductor thereafter. The nonlinear I–V characteristics of varistors are generated by many double Schottky barriers at the grain-boundary layers, which are es- sentially formed through the segregation of varistor-forming oxide. Moreover, they possess excellent surge-withstanding ca- pability. Therefore, they have been used as a core element of surge absorbers in electronic circuits and as surge arresters in electric power systems. 2 Furthermore, as discussed in earlier reports, 3–9 dense SnO 2 - based systems present values of nonlinear coefficient (a), break- down voltage (E b ) and barrier voltage per grain (n b ) equivalent to those of the traditional and commercial ZnO varistor, which make the SnO 2 -based varistor as one of the most promise can- didates to compete commercially with ZnO-based varistor. It is largely accepted that the nonohmic properties of such devices are controlled by the grain-boundary features. 2,8–10 However, there is more than one conduction paths among grain boundary, particularly for ZnO 10 that posses an evident bulk intergranular material. Despite of this possibility of several con- duction paths throughout the grain boundary, the main one that control the nonohmic properties is believed to be because of double Schottky barrier contacts formed among region closest to the grain–grain contact, which has been proved to be similar in both type of varistor mentioned. 8,9 The conduction path formed by double Schottky contact is believed to be very sensitive to the oxygen treatment whatever type of varistor involved 9 and is also thermally activated, tem- perature-sensitive with leakage conduction related to the barrier height. 10 The leakage current further can progressively increase the temperature until device enters in a thermal runway regime. This kind of failure leads to the formation of a hole through the varistor with distinct signs of melting and vaporization and is commonly associated with thermal runway occurring as a con- sequence of current localization in the dielectric and semicon- ductor junction among the grains. The conceptual picture is a positive feedback mechanism in which current localization oc- curs along some path through the microstructure with the higher current density leading to enhanced local Joule heating. Because of resistance of semiconductor decrease with the increase of temperature, the lowered resistance caused by local heating along the localization path favors further current localization. This feedback continues until melting and electrical shorting occurs, causing the failure of the device. For this reason, it is expected that a good device have more capability of thermal dissipation (a good thermal conductivity) to avoid this thermal runway effect. As obvious consequence, the functionality of the device as a reversible solid-state switches with large-energy-handing capabilities also depend on this fea- ture. Despite of its nonohmic properties equivalent to the ZnO- based varistors, there is no data in literature leading to failure characteristics of SnO 2 -based varistor system. Therefore, we have driven a comparative study of thermal conductivity in commercial ZnO- and SnO 2 -based varistors us- ing the laser pulse method. The ZnO-based varistor samples used in this work were obtained from two different commercial manufacturers that will therein after name as ZnO-C1 and ZnO- C2. The SnO 2 -based varistor system was prepared as described elsewhere. 3–9 For thermal property analysis of the varistor sys- tems it was used the LFA-427 equipment, which directly deter- mines the value of thermal diffusivity. It consists of a laser source, whose pulse impacts the face of the cylindrical sample maintained at a stationary state of temperature, whereas at the opposite face of the sample the increase of temperature as a function of the time is measured by an infrared sensor of worked trigged at the same time of the laser shot. The samples’ microstructure and mean grain size were deter- mined by analyzing the SEM micrographs (ZEISS DSM 940A) using image analysis software (PGT–IMIX). For the electrical measurements, silver contacts were deposited on the samples’ surfaces, after which the pellets were heat-treated at 4001C for 30 min. Then, the current–voltage measurements were taken using a high voltage-measuring unit (KEITHLEY Model 237). Figure 1 shows the SEM micrographs of polycrystalline com- mercial ZnO- and SnO 2 -based ceramics studied here. By this figure, it was confirmed what was already commented in previous paper 8 that the traditional and commercial ZnO Á Bi 2 O 3 -based varistor systems have a complex microstructure containing sev- eral phases such as a bismuth-rich phase, spinel (nominally Zn 7 Sb 2 O 12 ) and pyrochlore (nominally Zn 2 Bi 3 Sb 3 O 14 ), which 2629 J ournal J. Am. Ceram. Soc., 88 [9] 2629–2631 (2005) DOI: 10.1111/j.1551-2916.2005.00469.x r 2005 The American Ceramic Society D. W. Johnson—contributing editor Supported by the Brazilian research funding agencies CNPq and FAPESP. w Author to whom correspondence should be addressed. e-mail: prbueno@iq.unesp.br Manuscript No. 186599. Received October 25, 2002; approved July 25, 2003.