| Functional materials with high energy storage density and large electrocaloric effect play a very important role in power electronic systems,so they attract many researchers to carry out research on them,and gradually become a research hotspot.Among all kinds of dielectric materials,antiferroelectric materials have the unique electric dipole arrangement,which makes it show the electric field-induced antiferroelectric-ferroelectric phase transition,which leads to its superior energy storage characteristics compared with ferroelectric materials.In addition,people found in the study of the electrocaloric effect of polar dielectric materials,in addition to the positive electric electrocaloric,there is another electrocaloric effect,that is,negative electrocaloric effect.Studies have shown that these two effects can exist in different materials or in the same material simultaneously.If the positive electrocaloric and negative electrocaloric can be used together,the refrigeration capacity of the electrocaloric device can be greatly improved.It has also been found that negative electrocaloric exists in many antiferroelectric materials.At present,the research of antiferroelectric materials mainly focuses on PbZrO3 series,AgNbO3 series and NaNbO3 series.Although some researchers have long pointed out that PbHfO3 perovskite is also an antiferroelectric material,people have not paid much attention to it.The research on its antiferroelectric properties is mainly focused on theory,while the direct test on the antiferroelectric properties in experiments is rarely reported.In addition,antiferroelectric materials in the form of thin films can meet the requirements of miniaturization and miniaturization of electronic products,and have broad application prospects.In view of this,this study will take PbHfO3 film as the research object,and systematically study its antiferroelectric behavior,energy storage characteristics,electrocaloric effect and antiferroelectric performance regulation,in order to provide reference for the study of PbHfO3 film.The specific content of this study is as follows:1)Since the PbHfO3 antiferroelectric film has not been reported,we first studied its preparation process,mainly through the sol-gel method to prepare the film,discussed the sol-gel preparation process,the type of substrate,annealing temperature,film thickness,buffer layer and annealing process on its antiferroelectric properties.The results show that the stable precursor solution can be prepared by using lead acetate and hafnium acetylacetone as solute,glacial acetic acid and ethylene glycol methyl ether as solvent,and acetylacetone as stabilizer under the condition of proper heating.The concentration of the precursor solution was controlled within 0.2～0.4 mol/L.The substrates can be Pt/TiO2/SiO2/Si and ITO/Glass,and the ITO/Glass substrate is the best.The annealing temperature can be 650～700℃,and the best annealing temperature is 7000℃ for 10 min.The thickness of the film should be controlled within 300～450 nm.The buffer layer does not improve the antiferroelectrie properties of the film.The multilayer annealing process is limited in improving the antiferroelectric properties of films.2)The effect of annealing process on energy-storage performance in PbHfO3 antiferroelectric thin films was studied,mainly including the effect of annealing temperature on the antiferroeleetric properties,energy storage density,energy storage efficiency,thermal stability and fatigue characteristics of PbHfO3 antiferroelectric thin films.X-ray diffraction analysis indicated that annealing temperature played a key role in their crystallinity and phase composition.Due to the coexistent of fine grains and amorphous phase,PHO AFE films annealed at 650℃ displayed not only optimal energy-storage density and energy efficiency,but also excellent thermal stability and fatigue endurance.Compared with films annealed at 7500℃,the recoverable energy-storage density and energy efficiency in samples annealed at 650℃ increased by 50%and 100%,respectively.These results demonstrate that PHO film is a significant and promising candidate for applications in power electronics systems.3)We systematically report the electrocaloric effect of PbHf03 antiferroelectric thin films.PbHfO3 thin films not only shows a large negative electrocaloric effect(up to-7.7 K)at room temperature,but also a large positive electrocaloric effect(8.4 K)at high temperature.In addition,its electrocaloric strength at room temperature(|△T/△E|～0.023 K cm kV-1)is very acceptable.According to the improved Ginzburg-Landau-Devonshire free energy theory,the positive electrocaloric is caused by the coordination of temperature and electric field to promote the antiferroelectric and ferroelectric phase transition,and the positive electrocaloric is caused by the decrease of polarization value caused by the system gradually approach to the paraelectric state.Therefore,the negative electrocaloric can be obtained at room temperature by applying an appropriate applied electric field.If the combination of positive and negative electrocaloric effect is utilized,the refrigeration capacity of electric card will be greatly improved,which can open a new height for electrocaloric refrigeration.4)In order to improve the electrocaloric effect at room temperature,we prepared different(Pb1-xBax)HfO3 films.The results show that the phase composition,ferroelectric and antiferroelectric properties and electrocaloric effect of PbHfO3-based thin films are greatly affected by Ba2+doping.When the amount of Ba2+increases gradually,the(Pb1-xBax)HfO3 film changes from antiferroelectric to ferroelectric,and the Curie temperature of PbHfO3 film gradually approaches room temperature.Among all the samples,(Pb0.2Ba0.8)HfO3 films had the largest △T(～41.1K)at 70℃,while(Pb0.3Ba0.7)HfO3 films had the largest AT(～30 K)at room temperature.These large electrocaloric effects may be related to the transition from ferroelectric phase to paraelectric phase.Doped Ba2+has such a high electrocaloric effect on PbHfO3 thin films that it can be a candidate material for electrocaloric cooling devices in the future.5)Through clever design,we selected Ca2+,Sr2+and Ba2+with different radii to partially replace Pb2+ ions in PbHfO3,and these dopants can introduce different chemical stresses in the system.Among them,small radius ion doping leads to compressive stress,while large radius ion doping leads to tensile stress,and the more doping amount,the more stress is introduced.It has been shown that these tensile and compressive stresses have a significant effect on the antiferroelectric and ferroelectric phase transition in PbHfO3-based films.The tensile stress decreases the critical electric field Ea and Ef,while the tensile stress increases Ea and Ef.This indicates that chemical stress is an effective means to adjust the properties of antiferroelectric materials,and will have a profound impact on the application of antiferroelectric materials. |
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