International Journal of Inorganic Materials 1 (1999) 235–241 q Combustion synthesized ZnO powders for varistor ceramics a b, a a * ˜ V.C. Sousa , A.M. Segadaes , M.R. Morelli , R.H.G.A. Kiminami a ˜ ˜ Federal University of Sao Carlos, Department of Materials Engineering, 13565-905 Sao Carlos SP , Brazil b University of Aveiro, Department of Ceramics and Glass Engineering, UIMC, 3810-193 Aveiro, Portugal Abstract Commercial ZnO varistor ceramics are multicomponent, with minor amounts of added oxides that play important roles, both in the strict electrical sense and for the control of the microstructure. The present work describes the straightforward combustion synthesis of pure and doped ZnO powders from stoichiometric mixtures of the relevant water soluble metal nitrates as cation precursors and urea as fuel. The mixtures were ignited at 5008C resulting in a dry, very fine powder. The as-prepared combustion products, characterized by XRD, SEM, TEM and BET, show high specific surface area, have very small particle sizes and are crystalline, with atomic level homogeneity. Implications on sintering and electrical behaviour are discussed.  1999 Elsevier Science Ltd. All rights reserved. Keywords: A. electronic materials; A. ceramics; B. chemical synthesis; C. electron microscopy 1. Introduction development of a Bi O -rich liquid phase, while oxides 2 3 such as Co O , Cr O , MnO and Sb O are mostly grain 3 4 2 3 2 2 3 Zinc oxide varistors are electronic ceramic devices growth inhibitors and are added because the varistor whose primary function is to sense and limit transient breakdown voltage is inversely proportional to the ZnO voltage surges and to do so repeatedly without being average grain size. These ceramics usually consist of ZnO, destroyed [1]. In 1958, Kosman and Gesse [2] reported, for Zn Sb O spinel and Bi O -rich phases, and sometimes 7 2 12 2 3 the first time, on the non-linear properties of zinc oxide Zn Bi Sb O pyroclore, depending on the additives used 2 3 3 14 based materials but raised little notice in the industrial [4], which suggests that the microstructure development is world. In the early 1970s, Matsuoka et al. [3], of the mostly governed by the phase equilibria in the ternary Matsushita Electronic Components company, patented the system ZnO–Bi O –Sb O [1,4–8]. Part of the grain 2 3 2 3 varistor effect as a result of research carried out on rectifier growth inhibition is due to the formation of the spinel contacts between a semiconducting ceramic (zinc oxide) second phase whose particles pin down the grain and a metal (silver). Since it was discovered that zinc oxide boundaries. If the grain boundary pinning spinel particles ceramics containing Bi O and other metal oxides as are formed early during the sintering process, when the 2 3 additives exhibit highly non-ohmic voltage–current charac- ZnO particles are very small and similar in size, the teristics, these ceramics have been widely used for voltage particle drag mechanism, which reduces the grain growth stabilization and transient surge absorption in electronic rate, also prevents the occurrence of discontinuous or circuits [4]. abnormal grain growth. Thus, it is very important that the Typical ZnO varistor ceramics contain more than 90 element distribution in the microstructure is highly mol% ZnO and the composition is balanced by the homogeneous, which must be achieved prior to sintering, incorporation of such additives as Bi O , Sb O , CoO, during the powder preparation process. These ceramics are 2 3 2 3 MnO, Cr O and sometimes also SiO and SnO . The commonly produced by the conventional solid state route 2 3 2 2 addition of Bi O affects the non-linearity of the current– (mixture of oxides), but several other elaborate wet-chemi- 2 3 voltage characteristics and aids sintering through the cal routes such as sol–gel, coprecipitation and Pechini have also been used and are reported in the literature [9–12]. It is well known that ceramic powder synthesis entails q Paper presented at the First International Conference on Inorganic some difficulties, especially in the case of complex com- Materials, Versailles, France, 16–19 September, 1998. positions. Chemical homogeneity is nearly impossible to *Corresponding author. ˜ E-mail address: segadaes@cv.ua.pt (A.M. Segadaes) guarantee by mechanical blending and grinding processes. 1466-6049 / 99 / $ – see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S1466-6049(99)00036-7