The Preparation And Properties Of Novel Antiferroelectric Materials

Antiferroelectric materials can realize the antiferroelectric（AFE）-ferroelectric（FE）phase transition under the applied electric field.In this process,the dielectric constant of antiferroelectric materials increases first and then decreases with the increase of electric field,thus showing a unique double hysteresis loop and almost zero residual polarization.When the electric field is removed,the charge stored in antiferroelectric materials can be released completely,which is more suitable for application in the field of energy storage and capacitor,and has great application potential in the field of energy storage and pulse power capacitor.In the past,anti-ferroelectric research mainly focused on lead zirconate based anti-ferroelectric materials.To carry out research on new anti-ferroelectric material systems,on the one hand,enrich the anti-ferroelectric research system,such as lead hafnium and other lead based systems,on the other hand,it is also to develop toward the trend of lead free,such as silver niobate and sodium niobate anti-ferroelectric systems.Pb Hf O₃antiferroelectric ceramics,due to the high switching field,it is difficult to obtain high quality single Pb Hf O₃ ceramics by traditional solid-state method;Ag Nb O₃antiferroelectric ceramics,as a new type of lead-free antiferroelectric materials,are limited in the application of energy storage and pulse power capacitors due to its complex preparation process.In this dissertation,the energy storage performance,temperature stability and charge and discharge behavior of Ag Nb O₃ and Pb Hf O₃-based antiferroelectric ceramics are systematically studied.On this basis,the relationship between composition and phase structure,dielectric properties are analyzed in detail and the main results were as follows:1.As for the simple Pb Hf O₃ ceramics,its switching field has reached 230 k V/cm,so we reduce its switching field by doping Ti element,and observe the ferroelectric performance under a low electric field.The switching field of the studied ceramics was reduced and the energy storage density of Pb(Hf_0.99Ti_0.01)O₃ ceramics was 5.49 J/cm³at electric field of 200 k V/cm.The saturation polarization is up to 55μC/cm²,and it has good temperature stability（25-120℃）.XRD results show that the distortion of BO₆octahedron and the increase in the displacement of the A-site cation are the main reasons for the polarization enhancement of Pb(Hf_0.99Ti_0.01)O₃ ceramics.2.Then,we study the ferroelectric and energy storage properties of Pb Hf O₃ceramics under high electric field.We selected the(Pb_0.98La_0.02)(Hf_xSn_1-x)_0.995O₃（0.41≤x≤0.65）ceramics,and adjusted the positions of the components in the antiferroelectric region by changing the Hf/Sn ratio.The results show that the studied ceramics present a slant electric hysteresis loop with small hysteresis and high saturation polarization.The internal reason can be ascribed to the uneven distribution of Sn⁴⁺disrupted long-range order structure,making the local structure disorder,thus reducing the AFE-FE phase transition barrier and making ceramics behave small hysteresis.Finally,the(Pb_0.98La_0.02)(Hf.₆₅Sn_0.35)_0.995O₃ ceramics obtained ultra-high energy storage density of 12.2 J/cm³and energy storage efficiency of 92%.The ceramic also has excellent charge-discharge characteristics,in which the discharge density W_Dreaches 5.3 J/cm³ and the discharge time t_0.9 is 103 ns,at the nanosecond level.In addition,good temperature stability(W_rec>8 J/cm³,minimal variation<0.5%)and efficiency（η>92%）can be obtained over a wide temperature range of 20-120℃.The above results show that the refined grain size and disrupted local structure are the reasons for the greatly improved energy storage performance,which can be confirmed by the microstructure and Raman spectroscopy.This work extended the binary phase diagram of Pb Hf O₃-Pb Sn O₃,which provides a detailed experimental basis for the subsequent exploration of antiferroelectric materials suitable for high field intensity and wide temperature range.3.Simple Ag Nb O₃ ceramics were successfully prepared and double hysteresis loops were also observed.Pure silver niobate ceramics have low energy storage density and high hysteresis.On this basis,firstly,the idea of co-doping of A and B sites is adopted to introduce Ca²⁺and Ta⁵⁺,in order to obtain better energy storage characteristics.The addition of Ca and Ta to Ag Nb O₃ ceramics significantly improves the dielectric properties of the ceramics and expands the phase transition region of anti-ferroelectric phase M₁-M₂.The storage density of(Ag_0.90Ca_0.05)(Nb_0.95Ta_0.05)O₃ceramics is 3.36 J/cm³,the peak discharge current is 91.5 A,and it has good temperature stability in the range of 20-100℃.4.The doping of Sm₂O₃ with high melting point reduces the temperature of antiferroelectric phase transformation and enhances the antiferroelectric property of ag niobate ceramics.The addition of Sm₂O₃ will hinder the growth of Ag Nb O₃ ceramic grains during sintering,which helps to refine the grains and improve the dielectric breakdown strength.In addition,the addition of Sm₂O₃ also reduces the tolerance factor,which is conducive to the improvement of antiferroelectric properties of Ag Nb O₃ceramics.Therefore,when the electric field intensity is 370 k V/cm,the energy storage density of(Ag_0.82Sm_0.06)Nb O₃ ceramics is increased to 5.8 J/cm³,which is nearly 4times higher than that of pure silver niobate,and the efficiency is increased to 77%.At the same time,the storage density change rate of(Ag_0.82Sm_0.06)Nb O₃ ceramics is less than 4%in the temperature range of 25-150℃,showing good temperature stability.The discharge density of the ceramic reaches 2.92 J/cm³ when the electric field intensity is 320 k V/cm and the discharge time is in nanosecond level.5.La and Ta elements were introduced into silver niobate ceramics to improve its energy storage performance.The results show that the introduction of these two elements successfully reduces the grain size of ceramics,which is the reason for the high breakdown strength of ceramics and contributes to the enhancement of antiferroelectric properties of ceramics.The effect of La and Ta on the M₂-M₃ phase transition temperature is successfully reduced to room temperature,which makes the ceramics in the relatively stable anti-ferroelectric M₂ phase region.The energy storage density of 6.9 J/cm³ and breakdown strength of 490 k V/cm were obtained in(Ag_0.94La_0.02)(Nb_0.80Ta_0.20)O₃ antiferroelectric ceramics.