Research On The Energy Storage Property And Electrocaloric Effect Of The Flexible Na_0.5Bi_0.5TiO₃-based Thin Films

Ferroelectrics is an essential member of functional dielectric materials.The polarization and depolarization of the ferroelectrics correspond to the charging and discharging processes of the capacitors,making it irreplaceable in the advanced pulsed power systems.Based on the electrocaloric effect of the ferroelectric materials,the electrocaloric refrigeration is regarded as a novel effective and environmentally friendly cooling technology,having great significance for tackling the cooling problems of electronic components.The ferroelectrics in its film embodiment features with salient advantages of small size,good compatibility with the integrated circuit,and being in line with the miniaturization trend of the future electronic devices,which in turn drives the ferroelectric films to be hot points in the realms of energy storage and solid-state refrigeration.Coinciding with the current remarkable progress in flexible technique,advances in flexible electronic devices are imposing higher demand for the core functional materials,which are spurring unprecedented challenges in the research and development of ferroelectric films.It means that a material should not only fulfill the request of high performance of an electronic device,but also address the demand for deformations at a curved circumstance.Hence,some problems caused by the aforementioned cases,including the fabrication of high-quality flexible films,and the realization of the properties of the films being irrespective of the mechanical flexing,are now urgently awaiting to be solved.Sodium bismuth titanate(Na_0.5Bi_0.5TiO₃),as a ferroelectric at ambient temperature,possesses various properties like ferroelectricity,piezoelectricity,and pyroelectricity,having attracted intensive research interest in the field of the lead-free ferroelectrics.The complex phase transitions of Na_0.5Bi_0.5TiO₃,especially the ferroelectric-antiferroelectric/relaxor phase transition occurred near depolarization temperature,endow it with great potential for applications in the fields of dielectric energy storage and electrocaloric refrigeration.To the best of our knowledge,for the Na_0.5Bi_0.5TiO₃-based films,the preparation of the high-performance bendable films is the key to practical application and even commercialization for flexible energy storage and personal wearable cooling systems.Nonetheless,the Na_0.5Bi_0.5TiO₃-based films are still relatively weak in the energy storage properties and electrocaloric effect,which restricts their further development and applications for flexible electronics and smart wearable devices.To tackle the problems above,in this paper,the temperature-endurable two-dimensional fluorophlogopite mica?F-Mica?was selected as the flexible substrate for the direct growth of the high-performance flexible Na_0.5Bi_0.5TiO₃-BaTiO₃-SrTiO₃ solid solution films via the simple“one-step”approach.With the assistance of the F-Mica flexible platform,the bendability of the whole structure was achieved by thinning the substrate using mechanical exfoliation.And the energy storage and electrocaloric properties are investigated systematically.The main contents of the research work are as follows:1.The 2mol%Mn-modified?1-x?(0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃)-xSrTiO₃?x=0.30,0.45,0.60,0.75?films were fabricated on the flexible Pt/F-Mica substrate platform via a metal organic decomposition method combined with spin-caoting technique.The effects of incorporation of SrTiO₃ on the energy storage properties for the flexible films were investigated systematacially.Results show that the sample with x=0.45 presents the optimal energy storage property with a desirable recoverable energy storage density of 76.1J/cm³ and a high efficiency of 80.0%at 2813k V/cm,which benefits from its weak antiferroelectric-like behavior and predominant relaxor feature.The studies on the energy storage properties of the x=0.45 film at various working conditions reveal that the film has good frequency stability in the range of 500Hz-20kHz,and excellent thermal stability in an ultra-wide working temperature range of-100-200?,as well as robust fatigue and retention endurance over1?10⁸ cycles and 1?10³s.Furthermore,the satisfactory energy storage performance of the x=0.45 sample in the original flat state can be maintained well irrespective of the mechanical bent to the small radius of 2mm and even 10⁴ bending-unbending cycles at radius of curvature of 4mm.2.The 2mol%Mn-doped 0.65(0.94Na_0.5Bi_0.5TiO₃-0.06BaTiO₃)-0.35SrTiO₃ flexible fim for dual use in electrocaloric refrigeration and dielectric energy storage was prepared on the Pt/TiO₂/F-Mica platform via a metal organic decomposition method combined with spin-caoting process.The electrocaloric effect of the film was emphatically studied.Results show that the film exhibits an appreciable electrocaloric effect with maximum adiabatic temperature change?35?T of 12 K and isothermal entropy?35?S of 18 J/?K?kg?at 1642kV/cm near room temperature?40??,which is related with the ferroelectric-relaxor phase transition around the depolarization temperature.And the corresponding electrocaloric strength of?T/?E is 0.007?K?cm?/kV.Owing to the enhanced ferroelectric relaxor behavior through the introduction of SrTiO₃,the film presents a broad operating temperature span over 25?.Moreover,no obvious deterioration occurs for the high electrocaloric performance of the film under bending deformation at radius of 5 mm and after undergoing repeated bending-unbending process for 10⁴ cycles.Also,the flexible film shows good energy storage property with relatively large recoverable energy storage density of 56J/cm³ and high efficiency of 66%.