Electrocaloric And Energy Storage Characteristics For Bi Series Lead-free Perovskite Films

The miniaturization and integration of devices are increasingly required in microelectronic devices,such as computer chips that contain billions of transistors of only nanoscale size.However,the generation of waste heat will seriously affect the working efficiency and working life of the device.Most of heat dissipation methods only use mechanical fans or compression coolers as heat dissipation devices,which are far from effective refrigeration of devices.Moreover,compression refrigeration is not only inefficient,but also accompanied by a large amount of energy loss in the process of refrigeration.And the materials used in it have an immeasurable impact on the environment and human health.Electrocaloric effect is a new type of solid-state refrigeration effect that have wide working temperature range,high efficiency and low pollution etc.The electrocaloric films device not only can apply to various requirements of integrated circuit,implementing punctiform cooling way,and can be adapted to the shape of the heating device for its tailored,realizing effective cooling of integrated circuit devices.Electrocaloric effect is mainly in polar materials,in the case of applying or removing the applied electric field caused by material change of polarity,which make the material itself produces the change of entropy??S?and the change of temperature??T?.However,there are more or less inevitable problems in electrocaloric refrigeration,including:how to induce electrocaloric effect at room temperature,improve the refrigerating capacity of electrocaloric films at room temperature,improve the working stability of electrocaloric device,reduce leakage of electrocaloric device and predict the real electrocaloric behavior.At the same time,polar film is accompanied by energy storage and release in the process of polarization and depolarization,which can realize energy storage.And this kind of energy storage is called dielectric energy storage.Dielectric energy storage film can quickly realize the charging and discharging process,so it can quickly realize the switch between different energy sources.High power density,high voltage resistance and long service life characteristics make it be able to power grid interconnection of green energy,electric energy of the vehicle power supply system,as well as providing high energy storage density,high power density and high voltage resistant thin-film capacitors in advanced military technologies such as electromagnetic catapults,electromagnetic guns and laser weapons.However,due to the small size and content of thin-film capacitors,the energy stored by thin-film materials during charging and discharging is relatively small.Only by constantly increasing its energy density can make thin-film dielectric energy storage capacitors have a better application prospect.At present,the methods to improve the energy storage performance of thin-film dielectric capacitors are mainly divided into the following two types:the first is doping modification and the formation of solid solution,which can improve the energy storage density by adjusting the structure or type of domain,phase structure and defect concentration.However,its drawbacks are also very obvious.The increase of energy storage density is usually at the sacrifice of polarization or breakdown electric field,which will greatly limit the increase of energy storage density.The second method is to build a composite system,which can often achieve the improvement of breakdown electric field and the effective maintenance of polarization strength.However,there is a lack of effective research and expansion on such key issues as how to build a composite system and which materials to use.In view of the above problems,this paper attempts to seek breakthroughs in the following aspects:1.In Aurivillius bismuth layered perovskite Na_0.5Bi_4.5Ti₄O₁₅?NBTO?and single perovskite(Na_0.85K_0.15)_0.5Bi_0.5TiO₃?BKNT15?ferroelectric films,by adjusting phase structure,two-phase coexistence points or multi-perovskite units can be constructed to improve ferroelectric polarity,so as to induce substantial entropy change and polarization behavior and obtain significant electrocaloric effect and energy storage performance.2.The crystal structure of BBT film was effectively regulated by the doping of rare-earth element Pr³⁺in Aurivillius bismuth series relaxation ferroelectric perovskite BaBi₄Ti₄O₁₅?BBT?.At low doping concentration?0.0-0.4?,BBT gradually transitions from the A21am group to the Fmm2 group in the orthorhombic phase.When the doping concentration is 0.1,it shows the coexistence of two phases and the dominant role of A phase,which makes BaBi_3.9Pr_0.1Ti₄O₁₅?BBPT10?thin film show greater lattice distortion and enhance ferroelectric polarity.Moreover,due to Pr³⁺doping,BBT film has a stronger relaxation behavior,which leads to decrease hysteresis behavior causing a relatively high energy storage density for BBPT10.With the further doping of Pr³⁺?0.5-0.9?,BBT thin film gradually transitions from the initial orthorhombic phase?polar phase?to the tetragonal phase?non-polar phase?.As a result,the polarization strength of BBT films gradually weakens,but due to the gradually enhanced relaxation characteristics,the size of domains and the interaction between domains gradually decrease,which leads to the gradually weakened hysteresis behavior of Pr³⁺doped BBT and an obvious increase in energy storage efficiency.The crystal structure of BBT-based films can be effectively regulated by rare-earth element Pr³⁺doping,so as to regulate ferroelectricity and hysteresis,and finally realize effective regulation of BBT-based films'energy storage behavior.3.Formed by different dielectric constant and conductivity Na_0.5Bi_3.25La_1.25Ti₄O₁₅/BaBi_3.4La_0.6Ti₄O₁₅?NBLT/BBLT?composite film effectively block the leakage of charge through the interface layer,which greatly improves the breakdown electric field,and promote its polarization.Relaxation behavior leads to a smaller hysteresis behavior and residual polarization strength.High breakdown electric field,large polarization intensity and small hysteresis behavior and residual polarization intensity make NBLT/BBLT composite film obtain excellent energy storage performance,and its energy storage density and efficiency are 92.67 J/cm³ and 75.55%,respectively.Moreover,?Bi₂O₂?²⁺in layered perovskite has a good insulation effect,which can effectively inhibit the flow of leakage charge in the process of repeated polarization and prevent electric breakdown.Combined with the effect of interfacial charge accumulation,it can more effectively inhibit the breakdown in the fatigue process and improve the anti-fatigue performance.The suppression of leakage current and relaxation behavior reduce the energy loss of NBLT/BBLT composite film at high temperature and low frequency,so that NBLT/BBLT film has a good temperature stability?20?-160??and frequency stability?200 Hz-20 kHz?.4.Studying on the Aurivillius bismuth multilayer composite films(Au/Na_0.5Bi_3.25La_1.25Ti₄O₁₅/BaBi_3.4Pr_0.6Ti₄O₁₅/Na_0.5Bi_3.25La_1.25Ti₄O₁₅/Pt?A/NBLT/BB PT60/NBLT/P?)of the energy storage performance.The BBPT60 and NBLT have obvious relaxation ferroelectric properties,which indicates that NBLT/BBPT60/NBLT is a kind of relaxation ferroelectric composite film.Through the design and use of n-type?BBPT60?and p-type?NBLT?dielectric energy storage materials with different defect types,it is found that multilayer films with pnp configuration can greatly improve the breakdown electric field,maximum polarization strength and leakage performance of A/NBLT/BBPT60/NBLT/P films.This is due to the pnp structure on the interface to realize the effective carrier recombination and induction of wider high resistance layer width,reduce the concentration of defects,and realize the coordinated sharing the voltage of double high resistance layer,eventually owning the breakdown electric field and polarization increase at the same time to effectively improve the composite film system of the energy storage performance.Its energy storage density and efficiency are 159.76 J/cm³ and 70%,respectively.