Investigation On The Energy-storage Performance And The Electricaloric Effects Of Niobate-based Lead-free Ceramics

Due to their spontaneous polarization and phase transition behavior,ferroelectric materials can achieve efficient and rapid energy storage and conversion under external fields.In recent years,they have shown promising research prospects in the fields of electrical energy storage and solid-state electric refrigeration,making them a new generation of green energy materials in the field of power electronic microdevices and systems.However,the practical application of related ferroelectric materials is limited by their low energy storage density,small refrigeration efficiency,and poor stability.Therefore,it is an urgent scientific task in the fields of materials and physics to explore new multifunctional ferroelectric materials with high energy storage and refrigeration performance and establish a multi-degree-of-freedom control strategy for ferroelectric materials.Ferroelectric materials based on niobate,such as KNbO₃,K_0.5Na_0.5NbO₃,and Ag NbO₃,have larger saturated polarization,rich phase transitions,and higher Curie temperature advantages than other ferroelectric materials,and have been widely studied in the fields of energy storage and electric refrigeration,with a series of research results being reported.However,the key performance parameters of these materials,such as energy storage density,energy storage efficiency,adiabatic temperature change,and refrigeration strength,are still unsatisfactory due to the lack of established key preparation process parameters and ideal composition and structure control methods.Therefore,exploring the optimal preparation process,establishing a multielement control method based on the key structure,and revealing the intrinsic correlation mechanism between structure and performance are effective ways to solve these scientific problems facing niobate-based ferroelectric materials.In this thesis,K_0.5Na_0.5NbO₃-based ceramics with typical relaxor ferroelectric characteristics and Ag NbO₃-based ceramics with antiferroelectric characteristics were selected as the research objects.The preparation process of the two ceramic materials were detailedly explored,and the key process parameters were optimized.Meanwhile,the structural evolution laws of the two ceramic materials with composition and external field changes were deeply studied,and the regulation mechanism of relevant structures on their energy storage and electric refrigeration performance was revealed,achieving breakthroughs in energy storage and refrigeration performance.Firstly,the preparation process of K_0.5Na_0.5NbO₃ based relaxor ferroelectric ceramics and Ag NbO₃ antiferroelectric ceramics was explored.Through the study of key process parameters such as presintering,remove binder process,and sintering,the optimal preparation processes for two types of ceramics were obtained.Among them,K_0.5Na_0.5NbO₃ based relaxor ferroelectric ceramics are suitable for gel discharge using a three-stage remove binder process,and the optimal sintering temperature is around 1100℃;The optimal presintering and sintering temperatures for Ag NbO₃ antiferroelectric ceramics are 850℃and 1080℃,respectively.In addition,selecting Zr O₂ as the buried powder is beneficial for reducing impurity phases and improving the forming quality of the ceramics.Secondly,（1-x）K_0.5Na_0.5NbO₃-x Bi(Mg_0.5Ti_0.5)O₃[（1-x）KNN-x BMT]relaxor ferroelectric ceramics were prepared by traditional solid-state method under optimal process conditions.By doping BMT,the grain growth of ceramics was suppressed,and the grain size decreased from850 nm（x=0.05）to 195 nm（x=0.20）.At the same time,high ceramic density was achieved and porosity was reduced.When x=0.15,the ceramic had the best energy storage performance.When the breakdown electric field was 275 k V/cm,the energy storage density(W_rec)can reach 2.25J/cm³,and the energy storage efficiency（η）can be achieved about 84%and had good frequency and temperature stability in the range of 1-50 Hz and 25℃-125℃.The direct charge-discharge test results showed that the ceramics had an extremely fast full charge rate,and its discharge time(t₉₀)is only 88 ns.Based on structural analysis,the reduction of grain size is the key structural factor in improving ceramic breakdown.More importantly,the addition of BMT can reduce the size of the electric domain,effectively enhance the relaxation behavior of the ceramics,and play a key role in enhancing its energy storage density and efficiency.Once again,high-quality Ag NbO₃ antiferroelectric ceramics were prepared and their energy storage performance was studied.The prepared Ag NbO₃ ceramics had a single perovskite structure and good ceramic density.When the breakdown electric field of the Ag NbO₃ ceramic was 260 k V/cm,the W_rec can reach 3.08 J/cm³,which was higher than the reported energy storage density of pure Ag NbO₃ ceramics.The Ag NbO₃ ceramics had good frequency stability,with a change rate of less than 10%in W_rec within the range of 1-50 Hz.But its energy storage performance had a significant temperature dependence,mainly caused by its rich phase transition in the temperature field.Meanwhile,the regulation of Yb³⁺doping on the energy storage performance of Ag NbO₃ ceramics was studied.The introduction of Yb³⁺stabilized the antiferroelectricity of Ag_1-3xYb_xNbO₃ ceramics.When Yb³⁺content was 0.02 mol,the ceramics obtained 3.51 J/cm³ energy storage density and 63.1%efficiency at 310 k V/cm,and the ceramics also had high power density（P_D=101 MW/cm³）.Finally,based on the Maxwell relationship,we investigated the electric card effect of Ag NbO₃ lead-free antiferroelectric ceramics.Ag NbO₃ antiferroelectric ceramics exhibit a coexistence of positive and negative electric charge effects over a wide temperature range of 40℃to 210℃.At 65℃and 180 k V/cm,a negative charge card effect with a value of-4.38℃was obtained,while at 210℃and 180 k V/cm,a positive charge card effect with a value of 2.3℃was obtained,which is comparable to the charge card effect of lead based ceramics.We propose a mechanism model based on the synergistic effect of temperature and electric field to illustrate the positive and negative electric card effects generated.This new work not only broadens the research field of Ag NbO₃ in refrigeration,but also reveals the application advantages of this system in the field of electrocaloric effect.