Low-voltage ZnO varistor fabricated by the solution-coating method
Introduction
Zinc oxide varistors, due to their highly nonlinear current–voltage characteristics and energy handling capabilities, have been widely used as surge absorbers in small current electronic circuits as well as large current transmission lines for over-voltage protection [1], [2], [3], [4]. As the development of large scale integrated electronics, low-voltage varistors have attracted more and more attention [5], [6]. Developing materials that exhibit high nonlinear current–voltage characteristics at lower voltage has become an important aspect in the academe and industry researches of the varistor.
Two principal approaches have been used for making low-voltage ZnO varistors. One approach is thinning the devices, such as multilayering, thin foil, and thick film techniques, etc. [7]. However the strength and energy absorption capabilities of the thin ZnO varistor are very poor due to its small volume. The other is the classical approach of optimizing the processing and composition of conventional varistor in order to maximize ZnO grain size [8]. In this approach even growth of ZnO grains is requested in order to obtain a dense uniform ZnO varistor with large size grains.
The homogeneous starting powder is necessary for the manufacture of high performance ZnO varistors [9], [10], [11]. It is difficult for the conventional oxide mixing route to obtain homogeneous composite powder [9], [12], so several chemical methods have been developed in the last decade, such as coprecipitation [13], sol–gel [3], [11], [14], microemulsion [15] and polymerized complex method [12]. However most of these methods are complicated and costly, or not suitable to the production of low-voltage ZnO varistors.
Solution-coating technique has been reported in preparation of high-voltage ZnO varistors recently [9], [16], but not used in low-voltage ZnO varistors. In the present work, the low-voltage ZnO varistors were prepared by a novel solution nano-coating technique. The homogeneous nano-composite ZnO powders with few aggregates were obtained by coating ZnO powder with a mixed solution of dopants and the low-voltage ZnO varistors were successfully synthesized using these powders.
Section snippets
Experimental procedure
The ZnO mixed powders prepared in the present study contained 97.99% ZnO, 0.5% Bi2O3, 0.5% TiO2, 0.5% MnO, 0.5% CoO and 0.01% Al2O3 (all compositions in mol%). For the solution-coating route, the starting materials for the production of ZnO varistor powder included ZnO (commercial), Bi(NO3)3·5H2O (AR), Co(NO3)2·6H2O (AR), Mn(NO3)2 (AR), Al(NO3)3·9H2O (AR) and C16H36O4Ti (CP). A mixed alcohol solution containing the required amounts of metal nitrates and C16H36O4Ti was prepared. After stirring
Results and discussion
Fig. 1 is SEM images showing the morphology of ZnO powder, in which Fig. 1(a) and (b) shows ZnO powder before and after nano-coating, respectively. The ZnO particle size ranges from 100 to 400 nm and the mean grain size is about 300 nm. The surface of the ZnO particles is smooth for Fig. 1(a) and rough for (b), indicating that ZnO particles were coated with the additives after nano-coating. It can be seen that the size of additives is much smaller compared with that of ZnO particles.
Fig. 2 shows
Conclusions
Homogeneous nano-composite ZnO powders with few aggregates are prepared by a novel solution nano-coating technique. A series of low-voltage ZnO varistors with high performance are successfully synthesized using the as-prepared ZnO powders. The varistors prepared by the solution-coating route are associated with lower breakdown fields and leakage currents, higher nonlinearity coefficients and densities in contrast with the varistors prepared by the conventional oxide mixing route, suggesting
Acknowledgment
The authors would like to thank Mr. M.G. Kong for his technical assistance in SEM characterization.
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