Study Of Electrical And Magnetic Properties Of Lead-free Energy Storage Ceramics And FeCo Alloy Thin Films

With the increase of industrial capacity and consumption of fossil fuels,energy reserves and supply are seriously insufficient.The researching for sustainable materials with high energy density has attracted wide attention.Lead-free energy storage ceramics have attracted the interest of many researchers because of their ultrafast charge–discharge speed and excellent energy storage performance.Studies of lead-free energy storage ceramics mainly focuses on Na_0.5Bi_0.5Ti O₃（BNT）,Na Nb O₃（NN）and K_0.5Na_0.5Nb O₃（KNN）based ceramics.Many scholars have tried to improve the energy storage performance of ceramics by chemical modification,structural design and improvement of preparation technology.In practical applications,the energy storage density of ceramic capacitors is low,and so it is very important to explore lead-free energy storage ceramics with higher energy density.With the rapid development of information,the demand for high-speed information storage and processing is increasing rapidly.The analysis of ultrafast magnetization（spin）dynamics of magnetic materials provides important theories for the development of ultrafast magnetic devices,high-speed information storage,transmission and processing.Ultrafast demagnetization dynamics have attracted new interest in physics since it was first experimentally observed on subpicosecond time scales,and at present some important advances have been achieved.However,the internal mechanism and the influence of various interaction factors of ultrafast demagnetization dynamics in different materials has not been clarified.Therefore,more in-depth research on the internal mechanism of ultrafast demagnetization dynamics in different materials is needed to better understand these processes and provide guidance for more efficient magnetic materials and magnetic devices.Specific findings of this thesis are as follows:（1）We prepared transparent relaxor ferroelectric ceramics（1-x）Bi_0.5Na_0.5Ti O₃-x Na Nb O₃（BNT-x NN）by conventional solid-state reaction method.We find the NN-doping can enhance the polarization and breakdown strength of BNT by suppressing the grain growth and restrained the reduction of Ti⁴⁺to Ti³⁺.As a result,a high recoverable energy storage density of 5.14 J/cm³and its energy efficiency of 79.65%are achieved in BNT-0.5NN ceramic at 286 k V/cm.Furthermore,NN-doping can promote the densification to improve the optical transmittance of BNT,rising from～26%（x=0.2）to～32%（x=0.5）in the visible light region.These characteristics demonstrate the potential application of BNT-x NN as transparent energy storage dielectric ceramics.（2）0.8Na Nb O₃-（0.2-x）Sr Ti O₃-x Bi Fe O₃ ceramics were prepared by conventional solid-state reaction method.The SEM results show that BFO-doping can inhibit the grain growth of NN-ST ceramics,but excessive BFO-doping will cause pores and defects in the ceramics.0.8NN-0.18ST-0.02BFO has larger E_b and P_max and achieves a W_rec of 4.01 J/cm³ andηof 86%at 233 k V/cm.It means that BFO-doping can improve the energy storage performance of NN-ST ceramics.In addition,（1-x-y）Na_0.5Bi_0.5Ti O₃-x Na Nb O₃-y Bi Fe O₃ ceramics were prepared by conventional solid-state reaction method.The dielectric results show that the simultaneous NN-and BFO-doping can enhance the relaxation behavior of BNT-based ceramics.With larger P_maxandΔP,0.7BNT-0.2NN-0.1BFO ceramics obtain a W_rec of 4.22 J/cm³ andηof 69.1%at～256k V/cm.The results show that the energy storage performance of BNT-based ceramics can be improved by introducing NN and BFO.（3）Finally,the time-resolved magneto-optical Kerr spectrum（TR-MOKE）was used to measure the ultrafast demagnetization dynamics of Fe Co alloy thin film.The convolution model was used to fit the experimental results,and the excitation intensity dependent and external magnetic field dependent ultrafast demagnetization dynamics of sample have been discussed and analyzed.The demagnetization time constantτ_m generally shows excitation intensity dependence,indicating that the demagnetization is dominated by EY mechanism.With the increase of excitation intensity,the spin-lattice relaxation timeτ_s-l increases gradually,which can be attributed to the fact that the magnetic anisotropy weakens more obviously.Thermal diffusion constant τ_th increases as the excitation intensity increases due to the balance temperature of the substrate increasing by the pulse laser with high energy.τ_m of the sample does not show obvious dependence on the external magnetic field because the interaction between the external magnetic field and Fe Co spin is weak.τ_s-l andτ_th show obvious dependence on the external magnetic field,which may be due to the enhanced spin-lattice coupling and weak magnetization precession under the external magnetic field,respectively.These results deepen the understanding of the dynamic characteristics and mechanism of ultrafast demagnetization and magnetization recovery in soft magnetic transition metal alloy films...