Theoretical Studies On Wide Temperature Range Of Electrocaloric Effect Tuned By Ferroelectric Phase Transition

The application of refrigeration technology covers industrial and agricultural production,biomedical,scientific research and daily life.Current refrigeration relies mainly on conventional compressor refrigeration,but compressor refrigeration cannot escape the use of refrigerants and emits a large amount of gas to trigger a global greenhouse effect.Electric card refrigeration is to induce the phase change of ferroelectrics by electric field to change the entropy of the material,and then control the absorption/discharge process of the material,which can realize heat transfer and refrigeration.Electrocaloric refrigeration does not require environmentally hazardous refrigerants,is more environmentally friendly,and is simple and lightweight.Therefore,the electrocaloric refrigeration technology has become an important development direction of new refrigeration,and it is also a hot spot in material research.The reported temperature change of the electrocaloric effect is usually present in a narrow temperature range,even around the temperature of FE-PE phase transitions above room temperature,which severely limits their applications.In order to solve the existing problems,we theoretically proposed the ideas of designing the electrocaloric effect with wide-temperature span,using the stress,strain,phase change,respectively.Firstly,a thermodynamic model for the synergistic effect of tensile stress and compressive stress on the electrocaloric effect wide temperature range of the component gradient barium titanites ferroelectric bilayer films was designed.And,the model is used to simulate the temperature range corresponding to the electrocaloric effect caused by composition and stresses.It is shown that the tunable temperature range is derived from the application of compressive and tensile stresses on the component gradient barium titanites ferroelectric bilayer films simultaneously.When the applied stress is 0.4 GPa,the adiabatic temperature change and temperature range are 3.55 K and 267K,respectively.The calculation results demonstrated that the compositionally graded and applied stress can regulate the adiabatic temperature change and broaden the temperature range.Secondly,the research model of wide electrocaloric temperature range broaden by mismatch strain in superlattice system is designed and constructed.Lead zirconate titanate/titanate lead superlattice system was selected as the research object.The influence of misfit strain on the electrocaloric effect in superlattice was studied using the thermodynamic calculation method.By analyzing the structure of the ferroelectric superlattice,it reveals that the interlayer misfit strain and the substrate misfit strain existing in superlattice can affect the electrocaloric effect.The phase diagram between interlayer misfit strain and substrate misfit strain can be obtained by calculation.Two misfit strain considered can widen the temperature range to 281 K.Meanwhile,the results show that the thickness ratio also regulates the transition temperature and temperature range.Not only the ferroelectric-paraelectric phase transition can affect the electrocaloric effect,but also the antiferroelectric-ferroelectric phase transition,ferroelectric-antiferroelectric phase transition and antiferroelectric-paraelectric phase transition in some other ferroelectric materials have an impact on the electrocaloric effect.Finally,the Sm-doped BiFeO3 was selected as the research object to study the effect of ferroelectric-antiferroelectric phase transition on electrocaloric effect.The influence of doping composition on the phase transition is calculated by thermodynamic model.Then the effects of doping composition and hydrostatic stress on the temperature range of electrocaloric effect are analyzed further.The results show that both the Sm doping composition and hydrostatic pressure can induce the antiferroelectric phase,which broaden the temperature range.In summary,in order to widen the operating temperature range of the electrocaloric,this paper designs the material model from different aspects(stress,strain,phase change).Using thermodynamic calculation method,the effects of bending,misfit strain and metal doping of ferroelectric materials on the electrocaloric are studied systematically,which could provide insights into the design and development of high efficiency refrigeration equipment in the future.