Study On Fabrication And Properties Of High-Breakdown-Field ZnO Varistor Ceramics Based On The "Cold Sintering"

ZnO varistor ceramics are widely used for overvoltage protection in electronic circuits and power systems due to the excellent nonlinear characteristics.However,the traditional ceramic preparation process has high sintering temperature,long sintering time,high energy consumption,and has a series of disadvantages such as volatile additives and difficult to control grain size,which limit the successful preparation of varistor with high breakdown field.It is also difficult to meet the miniaturization of electronic devices,and the current requirements of high-breakdown-field arresters for UHV power grids.Therefore,it is very important to develop new low-temperature sintering technology.In this paper,the influence of different sintering conditions and doping components on the properties of ZnO varistor ceramics were studied based on two"cold sintering"processes.ZnO ceramics with high breakdown field were prepared,and the correlation between their microstructure and macroscopic electrical properties was obtained.Firstly,the effects of different doping components on the electrical properties and microstructure of ZnO varistor ceramics prepared by the"cold sintering"process（CSP）were studied.The research shows that the current-voltage properties of the ZnO ceramics prepared by CSP is significantly affected by the doped components,among which the ternary doping system（Bi-Co-Mn）can significantly improve the varistor performance of the samples.Its nonlinear coefficient reaches 33.5,the breakdown field reaches 3550 V/mm,and the corresponding Schottky barrier height reaches 0.67eV,which are much higher than those of the traditional solid-phase sample with the same formula.At the same time,the grain and grain boundary resistance of the sample at90℃were analyzed by impedance spectroscopy.It was found that the ternary doping increased the grain boundary resistance from 1.75×10³Ωto 8.15×10⁸Ω,while the corresponding activation energy of grain boundary also increased from 0.23 eV to 1.29eV.Secondly,based on the CSP,a plasma-assisted"cold sintering"technology（CSPS）was proposed.The effects of different sintering temperatures（120-300°C）on the density,microstructure,and electrical properties of ZnO ceramics were studied,compared with SPS samples.The research shows that CSPS greatly reduces the densification temperature of ZnO ceramics,and ZnO ceramics with high density（>99%）,high crystallinity,low porosity and small grain size can be prepared at 300°C.At the same time,the grain and grain boundary resistance of the samples at room temperature were analyzed by impedance spectroscopy.It was found that the grain boundary resistance of CSPS samples was higher,which created favorable conditions for the subsequent preparation of high-breakdown-field ZnO varistor ceramics.Finally,under the above sintering conditions,the effects of different doping components（Bi,Co,Mn）on the microstructure and electrical properties of ZnO varistor ceramics prepared by CSPS were further studied.Firstly,through XRD analysis,it was found that due to the inherent low oxygen partial pressure of CSPS,Bi₂O₃was reduced to Bi,which caused the non-linear characteristics of the samples to not be effectively improved.After a post-annealing process at 650°C,the non-linear characteristics of the samples were significantly improved,with a nonlinear coefficient of 35.3 and a breakdown field of 6529 V/mm,indicating that CSPS further improves the electrical performance of ZnO varistor ceramics.