Powder processing parameters and their influence on the electrical performance of ZnO varistor
Begum, Shahida (1996) Powder processing parameters and their influence on the electrical performance of ZnO varistor. PhD thesis, Dublin City University.
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The preparation of pressing grade electronic ceramic demands powder with a high degree of reliability in terms of compositional and microstructural homogeneity, having proper particle size and shape. The study was mainly concerned with the powder processing parameters such as the spray drying, the type and concentration of organic binder, and the distribution of particles.
Experiments with the spray drying variables were conducted in the laboratory and in the pilot scale dryer Factorial analysis was carried out to estimate the main and interacting effects of the independent variables on the response. A mathematical model for the mean particle size of the spray dried granules was also developed to optimize the operating condition of the dryer. The adequacy of the model was verified by statistical analysis Improved powder characteristics were achieved by optimizing the spray drying variables Varistor performance and powder characteristics had shown significant correlation Optimum spray drying conditions were identified depending on the electrical characteristics of the varistor.
The influence of different latex binders and their blends for a fixed concentration of binder and solid in the slurry was initially evaluated. Among the various binders including the conventional PVA, the latex binder, Ltx C was found to be superior The slurry with a low level of the new binder and a high solid content produced powder with further enhancement in powder properties.
The fracture strength of the green and sintered body made from the powder with different binder systems were analyzed by Weibull approach. The estimation of the Weibull parameters indicated that a more reliable green and sintered body can be generated by using a lower level of Ltx C and a higher level of solid in the slurry However, the best binder, its level and solid concentration were selected on the basis of the performance characteristics of the varistor.
Regarding the particle size fraction, the physical and electrical properties were not very different for the coarse, medium and wide distribution of powder However, the green and the fired density of the discs produced from the fine fraction of powder were higher. Though the varistor with fine fractions exhibited higher nominal voltage but
was poor in terms of energy absorption capability.
In addition to the powder processing, an investigation was conducted to assess the effect of passivation thickness on the performance of the varistor. The energy absorption capability of the varistor was found to be highly sensitive to this thickness Neither too thin nor too a thick coating of glassing was found to be favourable. Failure mode was also changed with the passivation thickness Thicker passivation generated pinholes as a result of higher insulation, whereas, thinner passivation led to flashover due to dielectric breakdown. The analysis of variance also showed the significance of the glass thickness on varistor performance.
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