| Dielectric ceramic capacitors have the characteristics of fast charging speed,high power density,safety and reliability,and strong resistance to cycle aging.However,low energy storage density and low energy storage efficiency limit their applications.Based on these,barium strontium titanate?BST?/zirconia?ZrO2?composite powders and ceramics with different ratios were prepared by a sol-gel method and an oxalate precipitation method.The ratio of n?BST?to n?ZrO2?is 4/1,8/1,12/1,16/1,20/1 and 24/1,respectively.The effects of ball milling time of BST powder on the phase,morphology and dielectric properties of BST/ZrO2 ceramics were investigated.To obtain BST/ZrO2 composite ceramics,two-step sintering method,traditional sintering method and buried powder sintering method were employed.The effects of sintering method on BST/ZrO2 ceramic phase,morphology,dielectric properties and energy storage properties were discussed.The main work of this thesis is as follows:Firstly,BST powders with perovskite structure were prepared by the sol-gel method and the BST powders were refined by a ball milling technique.The content of polyethylene glycol 400was studied for dispensability of BST particles in a solution,and 1.2wt%is the appropriate content.Zirconia powders with tetragonal structure were prepared by the oxalate precipitation method,and BST/ZrO2 composite powders with different n?BST?/n?ZrO2?were prepared by a modified oxalate precipitation method.The results of phase analysis and EDS Mapping shows that BST@ZrO2 core-shell structure exits in the composite powders.Secondly,the effects of different BST ball milling time on the structure and dielectric properties of BST/ZrO2?4:1?ceramics were studied.The results show that the particle size and uniformity of BST powders are relatively good when the ball milling time is 2h.At this time,the dielectric permittivity of as-sintered BST/ZrO2 ceramics is 1150 at room temperature.In the range of-60°C-140°C,dielectric permittivity has a good stability and the change rate of dielectric permittivity is-14%to 10%.The dielectric loss is 0.04 at room temperature.Thirdly,the phase structure,microstructure and electrical properties of BST/ZrO2 ceramics with different ratios of n?BST?to n?ZrO2?were studied systematically.Two-step sintering method,traditional sintering method and buried powder sintering method were conducted to fabricate composite ceramics.Both the sintering technique parameters and ceramic properties were optimized.The results show that,through optimizing the two-step sintering process,dense BST/ZrO2?24/1?composite ceramics were obtained with dielectric permittivity of 812,loss tangent of 0.01,discharged energy density of 150×10-3J/cm3 and energy storage efficiency of85%.Using the conventional sintering method,when the sintering temperature is 1325°C,the dielectric permittivity of the BST/ZrO2 ceramics is relatively good and the dielectric loss is small.The dielectric constant and the dielectric loss of the BST/ZrO2?12/1?composite ceramics are 634 and 0.02,respectively.The discharged energy density of this sample is 113×10-3J/cm3,and the energy storage efficiency is 70%.Using the buried powder sintering method,the dielectric constant and the dielectric loss of the BST/ZrO2?12/1?composite ceramics are 730and 0.004,respectively.The discharged energy density is 145×10-3J/cm3,and the energy storage efficiency is 80%.The results of these three sintering techniques show that the discharged energy storage density and the energy storage efficiency of BST ceramics can effectively be improved by adding a small amount of ZrO2. |
PDF Full Text Request