Samarpita Roy (Awarded on August 10, 2022)

Processing-Microstructure-Property Relationships of Er2O3 Doped ZnO Based Varistors

This investigation aims to achieve a comprehensive understanding on the processing-microstructure-property correlations of Er2O3 doped ZnO-V2O5 based ceramics with a view to develop fine-grained varistor suitable for high-voltage applications. The selected multicomponent, e.g., (97.4 - x) ZnO - 0.5 V2O5–2.0  MnO2? 0.1 Nb2O5 – x (= 0 -2.0 mol.%) Er2O3, varistor powder mixtures have been subjected to high-energy ball milling up to 35 h followed by compaction and sintering. Conventional single stage sintering has been performed over a wide range of temperature and time combinations. In addition, some selected varistor compositions have also been sintered following two stage technique. Microstructures of the as-received and ball-milled powders as well as sintered pellets have been characterized by employing various techniques like XRD, FESEM, TEM and EDS. Bulk density of sintered specimens has been measured following the Archimedes principle. The mean size of ZnO grains has been determined using ImageJ software considering several FESEM images for each sintered pellet. The electrical properties have been measured with the help of high voltage power supply unit.

The major findings of the present study are: (i) High-energy ball milling of powder mixtures helps to achieve homogeneous distribution of various oxides and reduces the particle size to nanometric level. (ii) Sintering of ball-milled powders develops ZnO grain as a primary phase surrounded by a thin intergranular layer consisting of Zn3(VO4)2, Zn4V2O9, Zn2MnO4, V2O5 and Mn-rich phases with additional ErVO4 and Er-rich phases only for Er2O3 doped samples. (iii) Er2O3 doping diminishes the densification process a little but markedly reduces the coarsening of ZnO grains due to the formation of secondary spinel phases at the grain boundaries and triple points. Appropriate amount of Er2O3 doping is found to reduce the grain size over an order of magnitude. (iv) The detailed analysis of grain growth kinetics reveals that the values of both apparent activation energy and grain growth exponent increase monotonically with Er2O3 concentration establishing its effectiveness as a potential grain growth inhibitor.(v) Er2O3 doping enhances the nonlinear electrical properties of the selected varistors; the magnitudes of the breakdown field (EB) and nonlinear exponent (α) increase with Er2O3 doping up to 0.5 mol.%. Although, higher amount of Er2O3 (> 0.5 mol.%) doping refines the grain size further; however, it does not improve nonlinear electrical property owing to the formation of large volume of spinel phases that not only disturb the homogeneity of the microstructure but also results in poor densification. (vi) A rise in sintering temperature and/or time gradually increases the grain size of the sintered pellets that, in turn, diminishes the nonlinear characteristics. The donor concentration increases while the depletion layer width decreases with increase in sintering temperature and/or time. The barrier height is found to reduce with sintering temperature, although it becomes independent with sintering time. (vii) Two stage sintering of the chosen varistor with an appropriate amount (0.5 mol.%) of Er2O3 doping generates  submicron-sized grain with noticeably improved nonlinear properties (EB = 16.9 kV cm -1 and α = 202). Interrelationships amongst processing parameters, microstructural features and nonlinear electrical properties are established and the underlying mechanisms are discussed.