Phase formation during liquid phase sintering of ZnO ceramics Mohammad Lutful Arefin 1 *, Friedrich Raether 1 , David Dolejš 2,3 and Andreas Klimera 1 1 Fraunhofer-Institut Silicatforschung (ISC), Neunerplatz 2, 97082 Würzburg, Germany, 1 [email protected], +49(0)931-4100-248, +49(0)931-4100-299 2 Bayerisches Geoinstitut, University of Bayreuth, 95440 Bayreuth, Germany 3 Institute of Petrology and Structural Geology, Charles University, 12843 Praha 2, Czech Republic Abstract ZnO doped with Bi 2 O 3 and Sb 2 O 3 (ZBS), is the basic system for ceramic varistors. Phase formation during sintering of ZBS was measured in situ, using 1 mm thick samples and synchrotron X-rays. Sintering shrinkage was measured in different atmospheres by an optical method. Thermodynamic calculations were performed to explain phase formation, composition, stability of additive oxides and influence of the oxygen fugacity on sintering. Sb 2 O 4 , pyrochlore, trirutile and spinel were formed at temperatures of 500°C to 800°C. The oxidation of antimony was controlled by the oxygen partial pressure and affected both, phase formation and sintering kinetics, in the ZBS system. Keywords: Liquid phase sintering, electroceramics, varistors, high temperature X-ray diffraction, thermodynamic simulation, oxygen fugacity Introduction The highly non-linear electrical properties of the ZnO ceramics have made it an indispensable material for varistor industries. It was studied since the early seventies [1]. The exceptional current-voltage behaviour of ZnO varistors was attributed mainly to the presence of the Bi 2 O 3 additive. In addition, Bi 2 O 3 strongly alters the sintering behaviour of ZnO. It produces a liquid phase with ZnO by eutectic melting at 738°C [2] and enables liquid phase sintering. During cooling, it forms tunnel barriers at the ZnO grain boundaries and this leads to the non-ohmic behaviour [3]. Besides Bi 2 O 3 , Sb 2 O 3 is used as a major additive in ZnO varistor ceramics, leading to the ZBS (ZnO + Bi 2 O 3 + Sb 2 O 3 ) system. Apart from the melt phase, many other phases exist during sintering of the ZBS system, known from early works by Wong and Inada [4, 5, and 6]. In these studies phases were identified by X-ray diffraction (XRD) at ambient temperature. All three ZBS components form a pyrochlore phase which has the nominal stochiometry Zn 2 Bi 3 Sb 3 O 14 [4]. Miles and West showed that two different pyrochlore phases with slightly different stoichiometry coexist in the ZBS system at ambient temperature [7]. ZnO and Sb 2 O 3 form a trirutile phase with the stochiometry ZnSb 2 O 6 [8] and spinel phases. The spinel phases have a cubic (Zn 2.33 Sb 0.67 O 4 ) and orthorhombic (Zn 7 Sb 2 O 12 ) structure [5, 9]. Leite et al. [10] studied the microstructural
Phase formation during liquid phase sintering of ZnO ceramics
Jun 29, 2023
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