Preparation And Energy Storage Characteristics Of Barium Titanate Based Relaxor Ferroelectric Ceramics

Relaxor ferroelectrics with perovskite structure are important materials for advanced pulsed power capacitors,which was regard as one of the most important functional material in the development of China.Owing to the advantage of ultra-fast charging and discharging speed,long cycle life and high thermal stability in advanced pulsed power capacitors,which are widely used in electromagnetic pulse weapons,hybrid electric vehicles,mobile-phone and other consumer electronics,and health care,etc.Because of the increasing demand for energy and environment,it is urgent to develop lead-free relaxor ferroelectric materials with high energy storage performance.However,lead-free energy storage ceramics have the following problems that need to be solved:?1?low energy storage density,less than 1.5 J/cm3;?2?low energy storage efficiency,not more than 75%;?3?the dielectric breakdown strength is less than 200 k V/cm.In this project,Ba Ti O₃-based relaxor ferroelectrics were chosen as our research object.On the one hand,a great deal of efforts including the component doped,phase structure design and the preparation optimization have been made to improve the energy storage properties.On the other hand,the physical mechanism affecting energy storage performance is deeply explored.The main research works are as follows:First,the out-of-order system of?1-x?(Ba_0.65Sr_0.35Ti O₃)-x Bi(Mg_2/3Nb_1/3)O₃??1-x?BST-x BMN?are achieved ultra-high energy density and efficiency successfully,which the energy density is 3.90 J/cm3,recoverable energy density is 3.34 J/cm3,and energy efficiency is 85.71%by introducing the theory of crossover ferroelectrics.The excellent performance of energy storage comes from the polar nanoregions.Crossover ferroelectrics can easily obtain the slim P-E loops and high breakdown strength due to the smaller free energy and less defects.Meanwhile,0.90BST-0.10BMN ceramics show excellent thermal stability which the fluctuations of energy density and efficiency are below 3%over a broad temperature range of 25?to 150?at 300 k V/cm.Second,the?1-x?(Bi_0.47La_0.03Na_0.5)_0.94Ba_0.06Ti O₃-x Sr(Sc_0.5Nb_0.5)O₃??1-x?BLNBT-x SSN?energy storage ceramics were synthesized by traditional solid state method.The results reveal that the?1-x?BLNBT-x SSN energy storage ceramics can form perovskite structure.Meanwhile,the lattice parameter and average grain size of?1-x?BLNBT-x SSN have no significant change with the content of SSN increased.However,with the SSN increasing,the relaxation degree of?1-x?BLNBT-x SSN ceramics are stronger,the permittivity and dielectric loss are decreased,and the T_mmoves towards the lower temperature.Due to ions doped breaking the long-range order of ferroelectric domain,which form more highly dynamic polar nanodomains,which more easily to obtained more energy when the applied electric field is removed.The optimum energy storage performance is achieved in 0.85BLNBT-0.15SSN ceramic,which the energy density is 2.22 J/cm3,the recoverable energy density is 1.83 J/cm3,and the efficiency is 82.32%under the applied electric field of 185 k V/cm.In addition,the excellent electrical stability has been achieved over a wide frequency range of 5 Hz to 200 Hz,which the fluctuations of energy density and efficiency are below 8%.Third,the?1-x?(0.65Ba Ti O₃-0.35Bi_0.5Na_0.5Ti O₃)-x Sr(Sc_0.5Nb_0.5)O₃??1-x?BBNT-x SSN?energy storage ceramics were synthesized by traditional solid state method.The results reveal that the?1-x?BBNT-x SSN energy storage ceramics can form a pure ABO₃-type perovskite structure without any secondary phase.With the content of SSN increased,the lattice constant is increased,and the permittivity and dielectric loss are all decreased.The optimum energy storage performance is achieved in 0.90BBNT-0.10SSN ceramic,which the energy density is 2.24 J/cm3,recoverable energy density is 2.02 J/cm3,and energy efficiency is90.18%under the applied electric field of 206 k V/cm.In summary,a series of high energy storage performance BT-based relaxor ferroelectric ceramics were synthesized via traditional solid-state method.Through analyzing the crystal structure,microstructure,dielectric and ferroelectric properties of all prepared samples,it is found that it is easy to obtain higher energy density and energy efficiency in crossover ferroelectrics,which have more polar nanodomains,smaller free energy,and fewer defects.In addition,the lead-free energy storage ceramics explored in this thesis are promising materials for advanced pulsed power capacitors.More importantly,the theory of crossover ferroelectrics can provide more scientific and feasible guidelines on how to design advanced pulsed power capacitors with a large W_rec,a high?and excellent thermal and electrical stability.