| Dielectric energy storage ceramics is widely used in pulse devices and other portable devices because of their high power density,high operating voltage,small size,and high safety features.The application and research of lead–free energy storage ceramics follows the current trend of environmental protection.However,the energy storage density of lead-free dielectric energy storage materials is relatively low,so improving the energy storage density is the top priority of current research work.Potassium sodium niobate（K0.5Na0.5Nb O,KNN）based lead-free piezoelectric ceramics have high dielectric constants enabling them to be an alternative system for dielectric energy storage ceramic materials.However,the large residual polarization strength and low breakdown field strength of KNN-based ceramics limit their development and application as energy storage materials.To solve these problems,this thesis adopts a synergistic approach to doping modification and process improvement to change the electric domain structure and reduce the residual polarization strength by adding multiple valence metal cation doping components to the KNN matrix on the one hand,and to improve the breakdown strength of ceramic materials by treating ceramic powders on the other hand,which together improve the energy storage density of KNN-based energy storage ceramic materials.In this study,KNN-based ceramic materials with different doping components were prepared to use the conventional solid-phase method,and their ceramic powder particles were treated with different ways to investigate their structural characteristics,dielectric properties and energy storage characteristics.The main studies are as follows:（1）The ball milling time of 0.925(K0.5Na0.5)Nb O3–0.075Bi(Zn2/3(Ta0.5Nb0.5)1/3)O3（KNN–0.075BZTN）relaxor ferroelectric energy storage ceramic powder particles and the two–stage sintering regime of the green body were adjusted to obtain ceramic samples with high grain boundary content.The results show that the ceramic grain size decreases as the ball milling time increases,and the reasonable two-stage sintering can produce a small amount of liquid phase at the ceramic grain boundaries,which together enhances the denseness of the ceramics.According to ferroelectric tests,the breakdown field strength of the ceramics was increased from 222 k V/cm to 317 k V/cm,optimizing the ball milling time and two-stage sintering regimes.The final ceramic samples of breakdown field strength of 307 k V/cm,maximum energy storage density of 4.05 J/cm3,and energy storage efficiency of 87.4%were prepared.（2）0.9(K0.5Na0.5)Nb O3–0.1Bi(Zn2/3Nb1/3)O3–x Zn（0.9KNN–0.1BZN–x Zn）relaxor ferroelectric ceramics with different Zn local（grain boundary to intra–grain）concentration gradients were prepared by sintering after homogeneous wrapping of Zn O on the surface of porcelain particles.The results show that the gradient of the grain fraction will further narrow the polar nano region of the ceramics,enhance the relaxation of the ceramics,improve the dielectric response at high electric fields,and thus enhance the energy storage efficiency.Finally,ceramics with breakdown field strength of 326 k V/cm,effective energy storage density of 4.01J/cm3,and energy storage efficiency of 97.1%were prepared.（3）The(K0.5Na0.5)Nb O3–0.05Bi(Zn0.5Ti0.5)O3（KNN–0.05BZT）energy storage ceramic materials with different mass fractions of Si O2 cladding were prepared by water–based cladding method.The results showed that the Si O2 cladding layer improved the denseness of the ceramics,which in turn enhanced their mechanical properties.The hardness value of the 3.5wt%Si O2coated ceramic sample was 7.04 Gpa,the flexural strength was 195 MPa,the compressive strength value was 447 MPa,and the elastic modulus was 0.86 GPa.The good mechanical properties improved the breakdown strength of the ceramic,and the final ceramic with breakdown field strength of 419 k V/cm,energy storage density of 4.64 J/cm3,and energy storage efficiency of 77%was prepared. |
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