Surface And Solution Modification Regulation Of Electrocaloric Effect Of Nanofibers At Different Temperature

With the electronic components miniaturization requirements and environmental friendly demand,traditional refrigeration equipment has been unable to meet the needs of modern society.New solid-state refrigeration technology is developing vigorously,among which electrocaloric effect refrigeration has attracted extensive attention because of its low energy consumption,no noise and no pollution.The electrocaloric effect mainly uses the isothermal entropy change inside the polar material to refrigerate,and ferroelectric material has become the first choice for electrocaloric effect refrigeration due to its polarization reversal property.When the adiabatic temperature change((35)T)is greater than 3 K,it can meet the requirements of solid state refrigeration practical application,and the greater the adiabatic temperature change,the lower the cost and energy consumption will be.In order to achieve a large electrocaloric effect in the low temperature range,the experimental literature focuses on the composite film of ferroelectric materials and PVDF ferroelectric polymer materials,and it is concluded that the composite film with ferroelectric nanofibers as the filler shows the electrocaloric effect to meet the demand of solid state refrigeration.Therefore,we will select the ferroelectric nanofibers as the research object,and establish the theoretical model of parallel distribution of nanofibers which is different from the traditional fiber theoretical model,and study the electrocaloric effect of the nanofibers through the method of phase field simulation.The properties of one-dimensional nanomaterials are affected by material size and surface effect due to their special surface morphology.Therefore,we first studied the influence of radius on the electrocaloric effect of Pb TiO₃(PTO)nanofibers,and concluded that the maximum radius of isothermal entropy change((35)S)and adiabatic temperature change((35)T)is 50 nm.Then the surface energy of the nanofibers was introduced to adjust the electrocaloric effect of the 50 nm nanofibers by extrapolation length.The adiabatic temperature variation of nearly 19 K was obtained in the PTO nanofibers with an extrapolated length of 6 and a radius of 50 nm,but the obtained temperature was 660?.Due to the high phase transition temperature,we further induced domain change in the low temperature region of 50 nm PTO nanofibers by surface tension.We found that the effect of surface compression stress can significantly enhance the electrocaloric effect of the nanofibers in the low temperature region.An adiabatic temperature change of about 5 K can be obtained under an electric field of260 k V/cm at an operating temperature of 200?.Although in line with the requirements of practical application,but the operating temperature is still high as expected,and the value of(35)T is much smaller than the phase transition point,so we further proposed the solid solution modification of PTO,in order to achieve in the low temperature zone(Pb_xSr_(1-x))TiO₃ nanofibers to obtain a large electrocaloric effect.Based on the above work,the electrocaloric effect of 50 nm(Pb_xSr_(1-x))TiO₃nanofibers with different Sr and Pb ratios under the effect of electric field intensity of600 k V/cm was further presented.It is found that the adiabatic temperature change of8 K can be obtained in the Pb_0.55Sr_0.45TiO₃ nanofibers at 100?,indicating that the addition of Sr can reduce the phase transition temperature of the fibers and achieve the purpose of obtaining a large electrocaloric effect in the low temperature region.Based on the conclusions in Chapter 3,we further regulate the surface tension of each component of the nanofibers,and give the maximum value of the electrocaloric effect and the corresponding temperature of different components of the nanofibers under different surface tension,which is convenient to find the corresponding parameter combination of the nanofibers with the maximum electrocaloric effect under different service environments.