A Research On Modification Design And Energy Storage Properties Of Bismuth Magnesium Niobate Material

In recent years,the potential applications of dielectric tunable materials in microwave tunable devices/systems have attracted the attention of researchers.The non-ferroelectric Bi_1.5Mg Nb_1.5O₇（BMN）cubic pyrochlore material has extremely low dielectric loss,as well as high dielectric constant and dielectric tunability.In addition to beginning to show its peculiar dielectric properties in microwave tunable devices,BMN cubic pyrochlore has application prospect in the field of energy storage devices.The channels constructed by BO₆ octahedral network in crystal structure could accommodate different A cations.Therefore,BMN cubic pyrochlore material shows high flexibility for chemical composition.The dielectric properties of BMN material are closely related to the high flexibility of A-site cations.The change of the occupancy of A-site ions induces A-site and O’to deviate from ideal coordinates.The dielectric tunability of BMN is restrained by the displacement of A-site and O’ions.The thesis is designed to introduce cations,with moderate ionic radius and high polarizability,into the A site to increase disorder degree and polarizability of the whole BMN structure.The ions for doping modification,including Ni²⁺,Ca²⁺,Co²⁺and Sr²⁺,are selected to replace A-site Mg²⁺in BMN.The effect of A-site ion substitution crystal structure and dielectric responding mechanism of Bi_1.5Mg_1-xM_xNb_1.5O₇（x M-BMN,x=0.0～0.5,M=Ni,Ca,Co,Sr）cubic pyrochlores had been systematically investigated.Meanwhile,the energy storage characteristics of BMN cubic pyrochlores were pathbreakingly studied.The main research achievements are as follows:（1）The research on the complete Ni²⁺substitution for Mg²⁺in BMN material was carried out.And Ni²⁺occupied both A and B sites of Bi_1.5Ni Nb_1.5O₇（BNN）.The Ni²⁺doping hardly optimized the dielectric constant and dielectric loss of BMN,which is attributed to the low polarization of Ni²⁺ions as well as the considerable oxygen vacancies and surface adsorbed oxygen in BNN structure.However,Ni²⁺doping enhanced the temperature stability of the BNN material,owing to the weak interaction between BO₆ octahedra.Additionally,the BNN material with a flexible crystal structure showed a higher dielectric tunability（6%）than that of BMN film（3.7%）in 0.7 MV/cm.（2）The research of x Ni-BMN（x=0.0～0.5）,in which Mg²⁺ions were partially replaced by Ni²⁺ions,was carried out.Partial doping of Ni²⁺maintained the cubic pyrochlore phase with（222）preferred orientation of BMN material.And there was no second phase observed in x Ni-BMN.Ni²⁺doping contributed to the formation of new A-O bonds and the bent O’-A-O’chains in x Ni-BMN.The dielectric constant and dielectric loss of x Ni-BMN cubic pyrochlore decreased commonly with the increase of doping amount.The dielectric loss of 0.3Ni-BMN material was as low as 0.00031.x Ni-BMN owned higher dielectric tunability than pure BMN.The maximum tunability of 0.1Ni-BMN film was 14.3%at 1.6 MV/cm.The energy storage density and energy storage efficiency of x Ni-BMN film were lower than that of BMN film,which may be related to the lower intensity of polarization and breakdown strength of x Ni-BMN system.（3）The effect of Ca²⁺doping on microstructure and dielectric response mechanism of BMN cubic pyrochlore was studied.The crystal cell of x Ca-BMN（x=0.0～0.5）gradually shrank with the increasing doping content due to the squeezed Mg-O bond when Ca²⁺with larger ionic radius entered to the lattice.As the content of doping increased,the coupling between BO₆octahedra was enhanced.And the second phase Ca Bi₂Nb₂O₉appeared in 0.5Ca-BMN sample.The 0.1Ca-BMN obtained a higher dielectric constant（181）at optimum sintering temperature and dielectric tunability 10.3%at 0.8 MV/cm.However,its energy storage density and efficiency were lower than those of BMN film,which are 7.2 J/cm~3（1.76 MV/cm）and 70%,respectively.（4）The lattice constant,chemical bond energy and dielectric constant of x Co-BMN first increased and then decreased with the increasing doping content.The dielectric constant of BMN increased significantly from 152 to 192,after A-site Mg²⁺ions were partially replaced by Co²⁺ions（x=0.1）with high polarizability.With the increase of Co²⁺doping content（x=0.1～0.5）,the dielectric constant of x Co-BMN decreased from 192 to156,which is related to the decreasing disorder degree of the system with the weak polarization intensity.The dielectric loss of x Co-BMN was optimized and reduced from0.00057 to 0.00028 after Co²⁺doping.In addition,the slight introduction of Co²⁺led to the deviation of A and O’ions from ideal positions in 0.1Co-BMN material to obtain a higher tunability（11%,1.2 MV/cm）.However,the energy storage density of 0.1Co-BMN film was low,and the maximum was merely 2.4 J/cm~3（1.6 MV/cm）.（5）The substitution of Sr²⁺with large radius only occurring in A-site Mg²⁺,led to an expansion of x Sr-BMN cell.There existed the second phase Sr Bi₂Nb₂O₉ with excessive Sr²⁺doping.Sr Bi₂Nb₂O₉ significantly influenced the crystal structure of the material and weakened the interaction between BO₆ octahedra.The dielectric constant of 0.1Sr-BMN was larger than that of pure BMN.The Sr²⁺doping modification for dielectric loss of BMN was to no avail.The addition of Sr²⁺promoted the sintering of BMN material and reduced the sintering temperature（from 1120°C to 1020°C）.Sr²⁺doping failed to optimize the tunability and energy storage performance of the material.After doping,the maximum tunability was 9%（1.6 MV/cm）,and the maximum energy storage density was7.5 J/cm~3（2.3 MV/cm）.（6）There are differences in occupying situation of BMN with different doped ions.The complete Ni²⁺substitution of BNN tended to occupy B-site,while partial doped ions replaced A-site Mg²⁺firstly,then B-site in the x M-BMN system.Ni²⁺and Ca²⁺ions hardly improve the dielectric constant of BMN,but a small amount of Co²⁺and Sr²⁺could improve dielectric constant.Ni²⁺and Co²⁺reduced the dielectric loss of BMN,while Ca²⁺and Sr²⁺with larger ionic radius led to increasing dielectric loss.In 0.1M-BMN system,Sr²⁺and Ca²⁺reduced the sintering temperature of BMN,while Ni²⁺and Co²⁺reduced the dielectric loss of BMN.The high dielectric tunability of 0.1M-BMN films showed a positive correlation with large dielectric constant.Addition of dopant with x=0.1 would reduce the energy storage property of BMN film.