Chemical synthesis of a new tin dioxide based (SnO 2 : Co, Al, Nb) varistor
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Chemical synthesis of a new tin dioxide based (SnO2 : Co, Al, Nb) varistor P. N. Santhosh, H. S. Potdar, and S. K. Datea) Physical Chemistry Division, National Chemical Laboratory, Pune 411 008, India (Received 29 May 1996; accepted 16 September 1996)
A new varistor composition based on SnO2 doped with Co, Al, and Nb has been successfully synthesized by a chemical route. It exhibits excellent nonlinear current-voltage (I-V) characteristics with a ø 72 and E1mA ø 1820 Vymm.
Varistors are electroceramic devices widely used for transient voltage surge protection of electronic/electrical appliances.1 The microstructure of a varistor consists of highly conductive n-type semiconducting grains surrounded by electrically insulating grain boundary regions. Nonlinear coefficient sad and breakdown voltage sE1mAd are the two most important performance parameters of a varistor and are controlled by the functional microstructure (grain, grain boundary distribution), dopant distribution, etc.2,3 ZnO-based varistors have exhibited excellent nonlinear behavior both in the bulk and in the thin film forms.4 Other compositions5,6 based on TiO2 and SrTiO3 have also shown relatively good nonlinear behavior. An enhancement in the performance parameters of the varistors has been noticed when prepared by chemical routes7–9 such as sol-gel, microemulsion, liquid mix, etc. As the electronic properties of these materials are highly microstructure dependent, a uniform green microstructure consisting of uniformly packed particles with a narrow size distribution is necessary. In the search for new varistor materials with well-defined functional microstructure, we attempted to synthesize SnO2 -based composition by the chemical route. Small quantities of Al2 O3 , CoO, and Nb2 O5 were used as dopants/additives. This study resulted in an excellent varistor (SnO2 : Al, Co, Nb) with a ø 72 and E1mA ø 1820 Vymm. We have employed a liquid mix technique for the homogeneous synthesis of SnO2 -based composition with Sn, Co, Nb, and Al taken in the molar ratio 98.9 : 1 : 0.05 : 0.05, respectively. The processing step is given in flow chart (Fig. 1). The base materials used were SnCl2 , Co(NO3 )2 ? 6H2 O, Nb2 O5 , and Al(NO3 )3 ? 9H2 O. A solution containing Co(NO3 )2 ? 6H2 O, Nb2 O5 , Al(NO3 )3 ? 9H2 O was prepared in 1000 ml distilled water. SnCl2 was hydrolyzed in 600 ml distilled water and was added to the above solution. The whole solution was continuously stirred. Nb2 O5 was added to this stirring solution along with 18 ml liquid ammonia.
Stirring was continued for 3 h, and the solution was then allowed to settle down by itself. This solution was then dried in a water bath to avoid the possible loss of any dopants during filtration. The dried powder was calcined at 800 ±C for 3 h. The calcined powder was grounded well to avoid agglomeration. This calcined powder was pelletized and then sintered at various temperatures in the range 1100 ±C to 1350 ±C. Here we present only the sample sintered at 1250 ±C, wh
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