The Lowtemperature Sintering And Electrical Property Study Of ZnVSb Based Varistor Ceramic

The developments of integrated circuit and surface mounting technology have resulted in an increasing demand on the miniaturized low voltage ZnO based varistor and the formation of a fast expanding global market for it. So far, the multilayer approach has been proved to be the most feasible and successful way of producing miniaturized low voltage ZnO based varistor. In order to reduce the fabricating expense of multilayer ZnO varistor without sacrificing its performance, low temperature sintered ZnO based ceramics of high non-ohmic property, homogeneous microstructure are preferred, so that high Ag/Pd ratio inner-electrode or even pure Ag inner electrode may be used to replace the presently used low Ag/Pd ratio inner electrodes. Therefore the research work of this present dissertation was carried out to systematically investigate the low temperature fabrication of ZnVSb based varistor ceramic and its electrical properties. The main contents are as following:This dissertation first present an analysis of dopant elements roles on the microstructure and properties of ZnO based varistor ceramics. In succession, several constituent choosing principles of low-voltage ZnO based varistor ceramic were introduced. Based on these principles, a new ZnO-V₂O₅ based ceramic doped with antimony, as the main microstructure and property modificator, and transient metal elements, such as manganese, cobalt, as the nonlinearity enhancers, was chosen as the basic composition of the dissertation study. The as fabricated ceramics based on this composition here and after were named as ZnVSb based varistor ceramics.At the beginning stage of the later research, P-â… V/Sb precursor, P-â…¡V/Sb precursor andα-Zn₇Sb₂O₁₂ spinel precursor were prepared in advance. Then the effects of P-â… precursor, P-â…¡precursor, Sb₂O₃ and the spinel precursor on the microstructure and properties of the ZnVSb based ceramics were studied respectively at doping level of 0.5mol% (in equivalent amount of Sb₂O₃). In former three ZnVSb ceramics, doping form change of antimony gradually reduced the formation temperature of ZnSb₂O₆, a interim product of ZnSb spinel formation reactions. It, in turn, resulted in an gradual enhancement in spinel formation within the ceramic, so that the average grain size of the ceramic was progressively reduced. The as formed spinel was found having an effect of promoting grain boundary segregation of dopant elements, through which mechanism, the nonlinearity of the P-â…¡precursor and Sb₂O₃ doped ZnVSb based ceramics was progressively enhanced if compared with that of the P-â… precursor doped one. However, the breakdown voltage of latter two ceramics was elevated as well. Similar results were also obtained in ZnVSb based ceramics doped with P-â… precursor and a-Zn₇Sb₂O₁₂ spinel ranging from 0 to 1.5mol% (in equivalent amount of Sb₂O₃). The nonlinearity exponents a, breakdown voltage and the sintering temperature of the ceramic were elevated from 20, 162 V/mm and 900℃to more than 80, 1250V/mm and more than 950℃respectively, as the antimony dopant concentration. increased from 0 mol% to 1.5mol%. As a result, the Sb concentration of the ZnVSb based ceramics for low voltage application should be strictly restricted below 0.5mol% (also in equivalent amount of Sb₂O₃).Quantitative activation analyses on the grain growth of 0.5mol% (in equivalent amount of Sb₂O₃) P-â… precursor, P-â…¡precursor, Sb₂O₃ and that of spinel precursor separately doped ZnVSb ceramics have been attempted by using the phenomenologieal grain growth kinetic equation: Gⁿ-G₀ⁿ=K₀texp(-Q/RT). The obtained grain growth kinetic exponents and apparent activation energies for above four ZnVSb based varistor ceramics are 2.44, 2.49, 4.03, 2.56 and 218KJ/mol, 292KJ/mol, 356KJ/mol, 236KJ/mol respectively. Grain growth of ZnVSb based varistor ceramics was mainly dominated by liquid phase assisting mechanism and that of the spinel particles pinning. The liquid phase and spinel formation within ZnVSb based varistor ceramics can be affected by the variations of antimony doping form, so as to affect the microstructure and properties of the ceramic.Considering from the interest of low voltage application, the optimum doping form of antimony within ZnVSb based varistor ceramics is P-â… V/Sb precursor, as it may enable the ceramic to have the lowest activation grain growth energy, in addition to the good non-ohmic property, homogeneous microstructure and relatively low breakdown voltage. The optimum properties of the ZnVSb based ceramic containing 0.5mol% P-â… precursor (in equivalent amount of Sb₂O₃), sintered at 900℃for 4 hours, are listed as following: its relative density is more than 97%, a 56, breakdown voltage 556 V/mm, leak current density 26μA·cm^-2.The tape-casting technique was employed to fabricate plate varistor of the optimum composition. Through optimizing the fabrication procedure, the camber of the green product of plate varistor was successfully eliminated, so that camber-free plate varistors of less than 500μm in thickness were fabricated. Its nonlinearity exponent is more than 30. Moreover the electric nonlinearity of the plate varistor exhibits a strong dependence on the ceramic thickness, when the thickness is less than 1mm. The main reason has been attributed to the fact that electrical property non-homogeneity of the grain boundaries gradually takes its negative effect on the electric properties of the as fabricated plate varistors as its thickness decreases.Finally, the study on the co-firing behavior of Ag inner electrode and ZnVSb based ceramic showed that diffusion of Ag through the inter-connected pores during the early period of sintering process and inter-granule V-rich phase were the most possible ways of Ag diffusion within the ZnVSb based ceramic layer. Co-firing ZnVSb ceramic with Ag inner-electrodes also resulted in the nonhomogeneous distribution of V-rich inter-granule phase within the ZnVSb based ceramic layer, which phenomenon, consequently, resulted in the observed deterioration in the low temperature sintering capability of the ceramic layer. The above findings may all technically contribute to the future success of the fabrication of multilayered chip varistor of ZnVSb based ceramics.