Low-voltage Zno Varistor Growth Of Grain Prepared By The Process And Properties

We can prepare low-voltage ZnO varistor by three main methods such as diminishing the thickness of the ZnO voltage-sensitive ceramic disc, reducing the breakthrough voltage of the single grain boundary layer in ZnO voltage-sensitive ceramic and augmenting the average size of the ZnO grains. What's more, those methods mentioned above can be combined each other properly to be applied to prepare low-voltage ZnO varistor. In this thesis, the preparation process of augmenting the ZnO grains' average size and properties of low-voltage ZnO varistor is investigated originally and systematically. By virtue of advanced test&analysis apparatus such as good-sized omnipotent optical microscope, XRD instrument, EDS instrument and SEM, not only quite a few relatively ideal test results have been obtained, but also some theoretical fruits of a considerable value have been acquired after extensive tests. The above achievement can contribute helpful reference to large-scale industrialization production. They are as follows:The appropriate preparation process of seed crystal has been found. In detail, the formula for seed crystal consists of 99.5% ZnO and 0.5%BaCO₃. First, lasting for ten hours at the temperature of 1420℃according to the process like the typical preparation process of electronic ceramic, secondly, to be poached by boiling distilled water for enough time, thirdly, ZnO seed crystal with the sizes of 38μm～48μm and 48μm～61μm are attained through classifying grains by size depending on sieves of 240, 300 and 400mesh.The above ZnO seed crystal possesses hexagonal wurtzite structure.The formulas for low-voltage ZnO varistor are designed according to the RoHS instructions instituted by Europe Union. These tests in this thesis are arranged and carried out on the basis of L16(4⁵) orthogonal experiment table. And then, the relatively ideal process to voltage-sensitive ceramic discs' demand for lower-voltage has been found and the ZnO voltage-sensitive ceramic of relatively fine electrical properties has been also prepared stably after the test data&results were dealt with and analyzed by polar margin analysis method rationally, which has breakdown voltage of 11V/mm, leakage current of 0.018mA and nonlinear coefficient of 15.6.By dint of EDS instrument, the constituent micro-distribution of ceramic disc is realized and it can be found that not only there are chiefly bismuth accumulation grain boundary layer, bismuth sparsity grain boundary layer and direct contact grain boundary layer, but also there is possibly a equivalent grain boundary layer named close pore containing residual bismuth. In fact, the four kinds of grain boundary layers can have different contribution to the voltage-sensitive effect of ceramic disc. In other words, the four kinds of grain boundary layers have different efficacy, the voltage-sensitive effect of ceramic disc should be statistical average of the different contribution of the four kinds of grain boundary layers in ceramic disc.By dint of SEM, the transect BSEI of ceramic disc can be acquired. Through rational comparison and analysis of the transect BSEI, that high sintering temperature, long soaking time, titanium dioxide and seed crystal are beneficial to the size increase of ZnO grain is confirmed basically. In addition, the conclusion can be drawn from the comparison and analysis that the effect of single titanium dioxide is less than single seed crystal for the size increase of ZnO grain and the effect of single seed crystal is less than titanium dioxide plus seed crystal.What's more, the law can be found that as the average size of the ZnO grains gets smaller the breakdown voltage gets lower when the breakthrough voltage of the single grain boundary layer in ZnO voltage-sensitive ceramic could be approximatively regarded as a constant and the grain size distribution is relatively uniform; As grain average size becomesbigger breakdown voltage gets lower in principle, whereas breakdown voltage gets higher little by little because many equivalent grain boundary layers exist in the grains at the same time.