J MATER SCI 41 (2006)2319–2325 Thermal evolution of ZnO-Bi 2 O 3 -Sb 2 O 3 system in the region of interest for varistors M. PEITEADO ∗ , M. A. DE LA RUBIA, J. F. FERN ´ ANDEZ, A. C. CABALLERO Departamento de Electrocer ´ amica, Instituto de Cer ´ amica y Vidrio, CSIC, Madrid, 28049, Spain E-mail: peitead@icv.csic.es Published online: 3 March 2006 In facing the design of new processing strategies for ZnO based ceramic varistors, a precise control of its microstructural development during sintering is demanded. Addition of dopants to zinc oxide results in the formation of secondary phases that to a large extent determine the macroscopic electrical properties of the ceramic. In a varistor system based on ZnO with small additions of Bi 2 O 3 and Sb 2 O 3 these three oxides govern the reactions at high temperature that give place to the secondary phases. These reactions become then the head point from which the functional microstructure is configured. In this way the present work deals with the thermal evolution of the ZnO-Bi 2 O 3 -Sb 2 O 3 system in the region of interest for varistors, revealing the existence of two simultaneous reactions paths during sintering these ceramics. C  2006 Springer Science + Business Media, Inc. 1. Introduction ZnO-Bi 2 O 3 -Sb 2 O 3 (ZBS) based varistors are electroce- ramic devices which exhibit highly nonlinear current- voltage characteristics that make them suitable for protecting against transient voltage surges [1–3].The non- ohmic behaviour is strongly related with the polycrys- talline microstructure, in which ZnO grains separated by electrically active grain boundaries represent the major- ity phase [4–6]. Together with zinc oxide particles of a Zn 7 Sb 2 O 12 spinel type phase, as well as a continuous in- terconnected bismuth rich phase that percolates through the whole ceramic body are also common in ZBS varis- tors. The presence of a very thin Bi-rich amorphous film (1–2 nm) or intergranular segregation of Bi atoms at ZnO/ZnO grain boundaries contributes to the formation of potential barriers to electrical conduction in the vicinity of ZnO interfaces, which are responsible for the device non- linearity [7–9]. Moreover depending on the composition, a small amount of a pyrochlore phase can also form [10]. Ceramic varistors are produced by sintering zinc ox- ide powders with small amounts of metal oxides such as Bi 2 O 3 , Sb 2 O 3 , CoO, MnO, Cr 2 O 3 , etc. As a result of the heating treatment, several thermally activated pro- cesses give place to chemical reactions between the varis- tor components that will lead to the different crystalline phases [3, 6]. The definite composition of the microstruc- ture is therefore dependent on the existence and evolution of such reactions during sintering, and the selection of a ∗ Author to whom all correspondence should be addressed. proper sintering strategy becomes decisive to achieve a microstructure with the desired electrical properties [11]. For example the varistor breakdown voltage, i.e., the on- set voltage magnitude of nonlinear conduction, is directly proportional to the average size of ZnO grains [2, 12]; hence the need for a controlled microstructural develop- ment which could be attained trough carefully controlling the sintering reactions. In a varistor system with ZnO, Bi 2 O 3 and Sb 2 O 3 these three oxides govern the reac- tions at high temperature. It was on 1975 when Wong first suggested a possible sequence of reactions implying the formation of a pyrochlore type compound of formula Bi 2 (Zn 4/3 Sb 2/3 )O 6 [13]. At higher temperatures this phase reacted with ZnO, readily available from bulk matrix, to form a spinel type compound as well as a Bi-rich liq- uid phase; bismuth oxide in the pyrochlore was totally replaced by an equivalent amount of ZnO without dis- turbing the (Zn 4/3 Sb 2/3 ) sublattice: Bi 2 (Zn 4/3 Sb 2/3 )O 6 + ZnO   −→ Zn(Zn 4/3 Sb 2/3 )O 4 + Bi 2 O 3 (liq) (1) The presence of the liquid phase promotes densification as well as ZnO grain growth whereas the spinel phase is reported to act as grain growth inhibitor [4, 5]. Later in 1980 Inada proposed a similar sequence of reactions in 0022-2461 C  2006 Springer Science + Business Media, Inc. DOI: 10.1007/s10853-006-7168-5 2319