Fabrication Of Barium Titanate-based Ferroelectric Ceramics/multilayer Structure And Their Electrocaloric Properties

Refrigeration technology can be seen everywhere,such as air-conditioning,the preservation of fresh food in daily life,the low-temperature treatment of steel in the industrial production process,and the preservation of vaccines and organs in the pharmaceutical industry.However,with the continuous improvement of science and technology and the rapid development of the national economy,traditional refrigeration technology on the environmental damage is also increasing,leading to the generation of the greenhouse effect,global warming,and also greatly affecting the ozone layer.In addition,China advocates environmental protection,energy saving and emission reduction,and has certain requirements and specifications for the application of refrigeration technology.Therefore,the research on utilization of new high-efficiency and energy-saving refrigeration technologies have become the main topic now.The electrocaloric effect discovered in dielectric materials decades ago,making it possible for electrocaloric refrigeration technology to replace vapor compression refrigeration.The search for new refrigeration technologies,such as electrostaticaloric refrigeration,electrocaloric refrigeration and magnetocaloric refrigeration,has attracted more and more attention.Some of ferroelectric materials have attracted much attention due to the electrocaloric effect.It is causing a new refrigeration technology revolution,which represents an environmentally friendly,non-greenhouse effect refrigeration technology,and is expected to play a more important role in the future technological revolution.In this thesis,lead-free perovskite barium titanate is selected as the matrix material,and chemical elements are doped to modify it.Firstly,（1-x）Ba(Zr_0.2Ti_0.8)O₃–x(Ba_0.7Ca_0.3)Ti O₃[（1-x）BZT-x BCT]ceramics were prepared by the solid-state reaction.The microstructure and electrical properties of seven different component ceramics were explored,and the relaxation properties and electrocaloric effect of different samples were studied.The study found that the change of composition has little effect on the electrical properties of ceramics.The maximum polarization of ceramic are between 13.6-16.4μC/cm²,and the remanent polarization are between 4-7μC/cm².The Curie temperature are between 40-85℃,the dispersion factorλ,which characterizes the relaxation performance of the material,are between 1.76-1.94,entropy change（ΔS）of the electrocaloric are between 0.4-0.57 J·kg^-1·K^-1,and temperature change（ΔT）of the electrocaloric are between 0.2-0.4 K.We found that the electrocaloric effect of BZT-BCT ceramics are insensitive to the change of composition.Secondly,due to the sintering temperature of（1-x）BZT-x BCT ceramics is too high,the combination of the ceramic layer and the electrode layer is poor,we needs to reduce the ceramic of sintering temperature.In this paper,the optimal composition（（1-x）BZT-x BCT:x=0.4）is selected as the original composition,and Cu O is added as a sintering aid to ensure that the ceramics can still maintain excellent performance after reducing the sintering temperature.The experimental results show that Cu O can reduce the sintering temperature without sacrificing ceramic properties.Finally,we adopt（1-x）BZT-x BCT:x=0.4,0.2wt%Cu O component to prepare multilayer structure ceramics by the tape casting process,and carry out phase structure,cross-section microstructure,electrical properties and electrocaloric performance test.The prepared multilayer structure ceramic includes two active layers,and the other layers are used as auxiliary layers.The entropy change and temperature change of the electrocaloric measured by the indirect method are 1.45 J·kg-¹·K^-1 and 1.16 K,respectively.The electrocaloric effect is three times higher than that of bulk ceramics.Between 25-100°C,temperature change of the electrocaloric is basically above 0.75K,and the operating temperature range of the d multilayer ceramics is wider.In summary,this paper systematically studied the microstructure and electrical properties of（1-x）BZT-x BCT ceramics,explored the influence of the composition on the relaxation performance and electrocaloric effect,and analyzed its physical mechanism.At the same time,multilayer structure ceramics were prepared,and the improvement of electrocaloric performance was attributed to the multi-phase coexistence of the material system,excellent relaxation performance and improvement of breakdown field strength.More importantly,this work opens up a new way for the application of bulk ceramics in solid-state refrigeration,and promotes the development of electrocaloric refrigeration technology from the material end to the device end.