Study On The Preparation And Properties Of WO₃-based Low Voltage Varistor Ceramics

As an inorganic functional material,WO₃ has very important scientific value and huge potential capabilities.It has been actively researched in many aspects,such as electrochromic,photocatalytic degradation,toxic gas sensor,microwave absorption and new battery applications,but the research in low voltage varistor ceramics is very few.As WO₃-based ceramics have low voltage and low working current,WO₃-based ceramics can be considered as low voltage varistor resistors for practical applications in low-voltage and low-current electronic circuits.However,the exploration of preparation process and formulation system of WO₃ ceramics are very few currently,and their preparation process is single and nonlinear coefficient is low.Therefore,it is particularly important to develop new sintering technologies and formulations to improve nonlinearity.Based on the spark plasma-assisted cold sintering process,this article explored the effects of new preparation processes and new doping systems on the microstructure and macroscopic electrical properties of WO₃ ceramics.At the same time,WO₃-based ceramics with high nonlinear coefficients were prepared and the relationship between its microstructure and macroscopic electrical properties was revealed.The spark plasma-assisted cold sintering process was used fabricate WO₃-based ceramics.And the influence of different Mn O₂ doping content on the electrical properties and microstructure of WO₃-based varistor ceramics were studied.Research shows that high density WO₃-based ceramics can be successfully prepared by the spark plasma-assisted cold sintering process.Compared to the un-doped WO₃ sample with breakdown strength of 12.3 V/mm and nonlinear coefficient of 2.5,varistor performance of 5 mol%Mn O₂-doped sample is significantly improved（i.e.,breakdown strength of 12.5 V/mm and nonlinear coefficient of 10.3）,which nonlinearity is about 4times higher than un-doped WO₃ with similar breakdown field.Meanwhile,the microstructure and phase distribution of the samples were analyzed by XRD,SEM and EDS.It was found that the Mn₂₊WO₄ will affect the grain interface and thus affect the varistor properties of WO₃.The grain and grain boundary resistance of the samples were analyzed by impedance spectrum.It was found that the grain boundary resistance of the samples increased from 6.29×10⁴Ωto 6.75×10⁶Ωafter the doping of Mn O₂.Appropriate doping can increase the dielectric constant of WO₃ ceramics and reduce the dielectric loss tanδto 0.08.On the basis of the same process,the effects of different content of NiO doping on the electrical properties and microstructure of WO₃-based varistor ceramics were further studied.The experimental results show that Ni O doping can also get WO₃-based varistor ceramics with high density.When the Ni O doping content is 1 mol%,the varistor performance of the sample is significantly improved with a breakdown strength of 11 V/mm and a nonlinear coefficient of 16.According to XRD,SEM and EDS analysis,the addition of Ni O can inhibit the grain growth of WO₃.However,further increase of Ni O doping content would lead to decrease of breakdown voltage and nonlinear coefficient.With introduce of Ni O,new phases Ni WO₄ are formed in WO₃ceramics,which agglomerate on grains and affect their grain interface,thus affecting their varistor and dielectric properties.It is shown that appropriate amount of Ni O doping can effectively improve the grain boundary resistance of WO₃,and the dielectric loss tanδ value decreases to 0.47 near 1 kHz.