Study On Fabrication And Physical Properties Of TiO₂ Based Linear Dielectric Ceramics With High Dielectric Breakdown Strength

Pulsed power system has been widely applied in the domains of controlled thermonuclear fusion,particle beam accelerators,proton therapy and ultra-wideband technology?UWB?,and so on.Pulse forming line with a function of pulse forming and transmission is an essential part of pulsed power system.In order to meet the growing demand for developing compact and miniaturized pulsed power system,it is urgent to reduce the size and weight of the pulse forming line.Using dielectric ceramics to replace the liquid dielectrics in the current system is a very promising solution.However,the relatively low dielectric breakdown strength of dielectric ceramics becomes a key factor that impedes its applications.Therefore,how to further improve the dielectric breakdown strength of ceramics has become the research focus.The main purpose of this thesis is to improve the dielectric breakdown strength of Ti O2 based dielectric ceramics.The system of Ca O-Mg O-Al₂O₃-Si O₂-Ti O₂ ceramics was taken as the research subject due to its great potential in achieving high dielectric breakdown strength.In order to greatly improve the dielectric breakdown strength of this material system and explore the mechanism of dielectric breakdown in Ti O₂-based ceramics,we tried to carry out our research from two aspects.On the one hand,considering that the effective liquid phase sintering was difficult to occur during a conventional sintering process in the system of 0.45Ti O₂-0.05Ca O-0.05Mg O-0.09Al₂O₃-0.36Si O₂?mol?dielectric ceramics,the hot-press sintering method was taken to improve sintering quality in this work.On the other hand,considering that introducing a core-shell structure into ceramics could effectively inhibit the extension of electrical branches and reduce the concentration of local electric field,a Ti O₂@x wt%Ca O-Mg O-2Al₂O₃-8Si O₂core-shell structure was designed and the core-shell structure powders were synthesized by chemical solution method,so the microstructure could be controlled precisely.The dielectric breakdown strength of Ti O₂-based dielectric ceramics prepared by hot-press sintering method reaches 77.5 k V/mm,which is 1.8 times as large as samples prepared by conventional sintering method.Besides,it also has good temperature stability.At 10 k Hz,the variation of dielectric constant is only 2.5%between-55??150?and the dielectric loss is less than 10^-2in the whole temperature range.The changes in microstructure,bulk density,dielectric properties,chemical defects and dielectric breakdown strength between conventional sintered and hot-press sintered samples were investigated in this paper.The association rules between ceramic structure and dielectric breakdown strength were summarized by the analysis of breakdown channel,complex impedance spectrum and X-ray photoelectron spectroscopy.The improvement of dielectric breakdown strength could be ascribed to improved bulk density,smaller grain size and less reduction of Ti⁴⁺to Ti³⁺.The improved bulk density means less weak points and better thermal conductivity,smaller grain size brings a higher grain boundary density and less reduction of Ti⁴⁺to Ti³⁺leads to an enhancement of insulation.All these factors can explain the improvement of dielectric breakdown strength well.Well-coated powders can be obtained on the condition of p H=7 and 4 wt%coating amount.With the further increase of coating amount,the dielectric breakdown strength of coated ceramics gradually decreases and the TEM results imply that more coating amount leads to shell clustering itself rather than forming a complete coating layer.After obtaining the well-coated Ti O₂@4 wt%Ca O-Mg O-2Al₂O₃-8Si O₂powders,we prepared bulk ceramics by conventional sintering and hot-press sintering method,respectively.Through the regulation of microstructure,the 4 wt%Ca O-Mg O-2Al₂O₃-8Si O₂coated samples prepared by conventional sintering and hot-press sintering method obtained a higher dielectric breakdown strength of 68.0 k V/mm and 79.0k V/mm,respectively.At the same time,the samples maintained a relatively high dielectric constant.?Conventional sintered samples:79,hot-press sintered samples:70,measured at 1 k Hz,room temperature?...