Sol-gel processing of varistor powders
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A new sol-gel method for the preparation of ZnO varistor powders, using inexpensive source materials such as acetates and nitrates, is described. It yields powders with a more homogeneous distribution of dopants compared to the commercially used mixed oxide technique. Varistor ceramics made from sol-gel powders can be sintered at lower temperatures and show improved electrical behavior. This is a consequence of formation of more varistor-active grain boundaries within the ceramics.
ZnO ceramics containing several metal oxides, e.g., Bi 2 O 3 , Sb 2 O 3 , CO3O4, Cr 2 O 3 , MnO2 etc., exhibit highly nonlinear current-voltage characteristics.1 This enables their use as protection devices against voltage surges and voltage transients. The varistor effect takes place at the grain boundaries within the ceramics. These varistoractive grain boundaries act as microvaristors with breakdown voltages generally estimated to be 3 to 4 V.2"5 A number of theories have been developed to explain this effect.4>6~8 A common opinion is that oxygen defects are the reason for ZnO-conductivity, but the role of the individual chemical additives is still not sufficiently clear. It is presumed that the homogeneous distribution of dopants and the right oxygen concentration are required for a varistor-active behavior of the grain boundaries to form good varistor ceramics. There also exist grain boundaries with high ohmic or low ohmic behavior in a commercial varistor.9 The electrical behavior of the varistor is the result of a network of grain boundaries with different electrical properties. The best device quality would be achieved by a very homogeneous grain size distribution in the microstructure with each grain boundary following the nonohmic conduction mechanism. Inhomogeneous distribution of dopants is a possible reason for ohmic behavior of grain boundaries. Commercial varistor precursor powders are usually produced using conventional ceramic technology with ZnO and metal oxide additives as starting materials. The process entails ball milling, drying, plastifying, and forming by pressing into the desired geometry. This process is recognized as being unsuitable for achieving homogeneous dopant distribution. Several authors have published new methods for preparing varistor powders.10"15 A sol-gel method10 was also investigated in order to achieve high breakdown fields by small grain size. Samples of these powders were hot pressed to avoid excessive grain growth during heat treatment. This paper describes a different way of processing sol-gel powder for varistor application by using inexpensive source materials such as acetates and nitrates. 546 http://journals.cambridge.org
J. Mater. Res., Vol. 7, No. 3, Mar 1992 Downloaded: 19 Nov 2014
These powders provide improved properties of varistors compared to the conventional mixed oxide technique. The new preparation method was carried out to give the following dopant composition in the final oxide powder: ZnO: 96.99 mol%; Sb 2 O 3 : 1.0mol%; Bi 2 O 3 , Co 3 O 4 , Cr 2 O 3 , MnO 2 : each 0.5
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