High voltage SnO 2 varistors prepared from nanocrystalline powders Mohammad Maleki Shahraki Æ Seyyed Ali Shojaee Æ Ali Nemati Æ Mohammad Ali Faghihi Sani Received: 18 February 2009 / Accepted: 1 April 2009 / Published online: 18 April 2009 Ó Springer Science+Business Media, LLC 2009 Abstract In this study, SnO 2 -based varistors were pre- pared from mechanically activated nanocrystalline pow- ders. Nanocrystalline powders were derived by subjecting the initial powders to intensive high-energy activation with different times and ball to powder ratio. The effect of activation parameters on the powder properties and sinter- ing temperature, as well as microstructural, micro-electrical and macro-electrical properties of the final specimens was evaluated. Varistors derived from high-energy mechanical activation exhibit a higher density (98.3% relative density) and more refined microstructure upon sintering at 1,300 °C in comparison varistors prepared from conventional pow- ders. Breakdown voltage and nonlinear coefficient were increased up to 24 kV/cm and 45 respectively. 1 Introduction Varistors are electronic ceramic devices whose primary function is to sense and limit (i.e., clamp) transient voltage surges and to do so repeatedly without being destroyed. ZnO- based varistors exhibit a typical non-linear coefficient of 30– 250 and are the most common system of varistors [1–3]. Other semiconductor oxides showing non-ohmic behavior are TiO 2 [4] and SrTiO 3 [5]. In 1995, Pianaro reported a new varistor material [6], (Co, Nb)-doped SnO 2 , which only has a single phase, rutile structure. The significant densification of SnO 2 varistor systems in comparison with pure SnO 2 was attributed to induction of oxygen vacancies in SnO 2 lattice by cobalt. These defects are preferentially created at the surface of SnO 2 grains, increasing oxygen diffusion at grain boundaries and thus promoting densification [7]. Nb 2 O 5 reduces the resistance of SnO 2 grains because it has donor effect in tin oxide varistor [8]. Other dopants such as Cr 2 O 3 and Pr 2 O 3 improve nonlin- earity and inhibit grain growth [9, 10]. SnO 2 based varistors with simple structure, high breakdown field and larger thermal conductivity than ZnO varistors could be a candidate for substituting zinc oxide varistors. On the other hand, their sintering temperature is higher than for ZnO varistors and their nonlinear coeffi- cient is considerably lower than ZnO varistors [11]. Currently, SnO 2 -varistors are produced by standard method, mixing followed by ball milling. This method cannot fulfill the requirement for advanced application. Chemical routes are not widely developed due to the unavailability of water-soluble salts of niobium at a rea- sonable cost [12, 13]. Recently, a method of high-energy mechanical activation changing original powder from micro into nanometer crystallite was utilized to produce high voltage-gradient ZnO varistors [14–16] and effect of mechanical activation on macro-electrical properties such as nonlinear coefficient, breakdown voltage and micro-elec- trical properties such as Schottky barrier height, depletion width and grain boundary resistance was evaluated [17]. In this work, we examined the effect of high-energy mechan- ical activation conditions on sintering temperature, macro and micro electrical properties of SnO 2 varistors. 2 Experimental procedure The molar composition of SnO 2 varistor in this work was 98.5%SnO 2 (Merck), 0.9%Co 2 O 3 (Aldrich), 0.1%Nb 2 O 5 M. M. Shahraki  S. A. Shojaee  A. Nemati (&)  M. A. F. Sani Department of Materials Science and Engineering, Sharif University of Technology, Azadi Ave, 11365-9466 Tehran, Iran e-mail: nemati@sharif.edu 123 J Mater Sci: Mater Electron (2010) 21:199–205 DOI 10.1007/s10854-009-9893-4