Abstract
Nanoparticles of ZnO were prepared by the reaction of ethanolic solutions of zinc acetate and oxalic acid followed by drying (80 °C) and calcination (500 °C). Subsequently varistor materials were fabricated from this nanoparticular ZnO via two separate routes:- a) from a “core shell” material using metal salts as additives; b) by using a conventional solid state mixing of metal oxides. Sintering (1050 °C) and subsequent electrical studies were carried out for each of these samples and they were compared with commercial varistor samples prepared under similar conditions. “Core shell” type varistor material showed considerably higher breakdown voltage (Vc = 850 ± 30 V mm−1) as compared to a sample prepared by mixing with metal oxides (Vc = 683 ± 30 V mm−1) or commercial varistor discs (Vc = 507 ± 30 V mm−1). The high breakdown voltage obtained is attributed to the formation of more varistor-active grain boundaries per unit area.
Highlights
In this paper we report a new procedure for the formation of nanoparticular ZnO and the preparation of varistors from it a) using materials in which the dopant oxides were deposited on the nanoparticular core; b) by mixing with solid metal oxides
It is clear that the varistors prepared from the novel post-treated ‘‘core shell’’ samples have achieved a higher densification than the others at this relatively low processing temperature
A novel method for the preparation for nano-ZnO has been successfully performed and the material characterised by X-ray Diffraction (XRD) and Transmission electron microscopy (TEM)
Summary
Varistors are electronic ceramic devices whose function is to limit voltage surges by becoming strongly conducting at a breakdown voltage (Vc).[1,2,3,4] The method of preparation, crystalline size and additive homogeneity are the critical parameters to produce a better varistor material.[5,6,7,8,9,10,11,12] Varistors with inhomogeneous microstructure can cause a large spread in current/voltage characteristics due to high local currents and this leads to the degradation of the varistor during electrical operation.[2,6,8,11] It is well documented that electrical and electronic characteristics can be altered by varying the microstructure at the grain boundaries,[2,11,12,13,14,15] and careful control of the microstructure is required to produce a high performance varistor. Commercial varistors are usually fabricated by solid state mixing of 0.5 to 3 mm ZnO particles with dopant oxides such as Bi2O3, Sb2O3, CoO, MnO, NiO and Cr2O3,2,5–7,11 the mixed powder being pressed and sintered at higher temperatures. In this paper we report a new procedure for the formation of nanoparticular ZnO and the preparation of varistors from it a) using materials in which the dopant oxides were deposited on the nanoparticular core; b) by mixing with solid metal oxides.
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