| PZT95/5 is one kind of ferroelectric(FE) material stablized near the boundary of FE and antiferroelectric(AFE) phases,With rich microstructures and strong mechanical-electric coupling. Pressure, temperature, electric field, or other external field may induce its structural phase transition, accompanied by a significant change of mechanical and electrical properties. Especially, shock wave compression of poled PZT95/5 results in FE-AFE phase transition and rapid(microseconds)depoling(release of bound charge). To generate a high electrical current or voltage,which is the basic mechanism of explosive-driven ferroelectric generator and has very important applications in high-tech devices and special equipment of military industry and nuclear technology.FE-AFE phase transition and rapid release of bounded charge of PZT95/5 ferroelectrics is a multi-disciplinary phenomenon involving materials science, mechanics, electrical engineering fileds,etc. The characteristics of mechanical-electric coupling phase transition,mechanism of discharge and mechanical-electric failure have been the basis of scientific issues and hot topics in the field of engineering and applied research of such ferroelectric functional materials. Aiming at understanding of kinetics of the FE-AFE phase transformation and electric breakdown of mechanical-electric coupling in shocked ferroelectric materials, this thesis carrys out a quantitative, systematic theoretical study on these topics.Firstly, considering the coexistence of FE and AFE phase, based on the phenomenological thermodynamics theory and Kittle’s antiferroelectric expression,a model describing the kinetics of FE-AFE phase transition induced by multi-field(such as an electric field, force field, etc.) is proposed. The influence of the hydrostatic pressure on free energy of phase transition, polarization and mechanics characteristic is investigated using this model, and the temperature dependence of ferroelectric-paraelectric, antiferroelectric-paraelectric phase transiton is revealed.Secondly, in view of the mechanical-electric failure of PZT95/5 ferroelectric ceramic, starting from one kind of typical electrical breakdown failure loaded by a quasi-static electric field, we established a channel electro-mechanical breakdown model of porous PZT95/5 ferroelectric ceramic. It was derived from the electro-mechanical breakdown mechanism of the tubular channel and partial discharges in the channel. By this model, it gives the prediction of a critical dielectric breakdown strength for ferroelectric ceramics, which is related to the permittivity, Young’s modulus and different porosity. The results are in good agreement with experimental results. In addition, the dielectric breakdown strength as a function of porosity and path feature size was also discussed. The dielectric breakdown of ferroelectric ceramics under shock wave loading and its critical field strongly depend on mechanical, electrical and electromechanical coupling factors. The breakdown mechanism in shock-compressed ferroelectrics is still not completely understood. Further, for the other kind of typical electrical breakdown under shock loading, a theoretical prediction of the critical breakdown electric field strength was performed. Based on the Kirchoff’s current law and an external circuit analysis for ferroelectric ceramics under shock wave loading, an analytical current-electric-field relationship for ferroelectric ceramics under the mechanical–electrical conversion process was obtained. Combining the stable value of the maximum of output electric field by shock loading with the breakdown of quasi-static electric field, a model of predicting the range of the breakdown field for ferroelectric ceramics with different porosities under shock wave compression was then proposed theoretically. The experimental breakdown field falls within the predicted region, to show the power of the model. Although the model only predicts a range of the breakdown field, it is the first model that is capable of doing that in the existing literatures. To further examine the validity of this model, the effects of the porosity,the load resistance and the velocity of shock wave on the output electric field during discharge process of poled PZT 95/5 ferroelectric ceramics were addressed, showing qualitatively consistency with the existing experimental results.Finally, to understand the mechanical properties of recovering sample after shock loading, the poled and unpoled PZT95/5, the Meso-micro phase transition characteristics and mechanical properties have been studied by means of nanoindentation method. The Young modulus and stiffiness of samples were obtained based on the continuous stiffness measurement method. The mechanical properties of poled and unpoled PZT95/5 were analyzed comparatively, and the load-displacement curves under different depth of three samples were also obtained. The surface topography of different indentation depth was obtained by using atomic force microscope. The further information on the mechanical deformation and phase transformation in pop-in point of different indentation depth were obtained. |
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