Study On Preparation And Structure-activity Relationship(SAP)of Niobate Glass Ceramics With High Power And Discharge Energy Density

To meet the development trend of miniaturization,lightweight and integration of power electronics and pulsed power devices,dielectric capacitors urgently need to improve their energy storage density and power density.The development of new high-performance dielectric materials,as the core material of dielectric capacitors,is particularly important.Niobate glass ceramics with tungsten bronze structure has the advantages of excellent temperature and chemical stability,tunable composition and electrical properties as well as low preparation cost,which has a broad application prospect in the field of high voltage and power dielectric capacitors.In this paper,K₂O-BaO-Sr O-Nb₂O₅-B₂O₃system glass is taken as the main object of study,and the effects of heat treatment process and A/B site substitution on the phase composition and microstructure of glass ceramics were systematically investigated,as well as the intrinsic relationships between phase composition and crystal structure changes and the dielectric,ferroelectric,energy storage and charging/discharging properties of glass ceramics were explored by using DSC,XRD,SEM,TEM,impedance,dielectric temperature spectroscopy,ferroelectricity,and charge-discharge analysis,and explored new strategies for the performance regulation of glass ceramics.The main research of this paper is as follows:1.K₂O-BaO-Sr O-Nb₂O₅-B₂O₃（KBSN）glass ceramics was prepared by high-temperature melting and controlled precipitation method,and the effects of crystallization temperature on phase composition,structure,and dielectric,ferroelectric and energy storage properties were systematically investigated.The results show that the main crystalline phase precipitated from the microcrystalline glass is KBa Sr Nb₅O₁₅with tungsten bronze structure,and the second phase K₂Sr Nb₅O₁₅ starts to precipitate when the crystallization temperature is greater than 740℃,which affects the dielectric constant of the glass ceramics.In addition,as the crystallization temperature increases,the crystallinity increases,the grain size increases,and the breakdown field strength of the glass ceramics decreases.When the crystallization temperature was 740°C,the best comprehensive properties of the glass ceramics were obtained,including:ε_r=207,tanδ<0.005,andη≈94.9%.In addition,a discharge energy density of about 2.2 J/cm³and a discharge power density of 380.9 MW/cm³ were achieved at an applied electric field of 600 k V/cm with a discharge time of only 11.2 ns.The above results indicate that controlling the crystallization temperature is essential for optimizing the overall performance of the glass ceramics.2.K₂O-BaO-Sr O-Nb₂O₅-B₂O₃-Ag₂O（KBSN-x Ag₂O）glass ceramics were prepared,and the relationship between the crystal structure changes induced by A-site substitution during the crystallization process and the dielectric,ferroelectric and energy storage properties was systematically investigated.The results show that Ag⁺with small ionic radius enters the A-site of KBa Sr Nb₅O₁₅ crystals and replaces K⁺during the crystallization process of glass ceramics,leading to the shrinkage of cell volume and the decrease of lattice constant.Meanwhile,Raman spectrum shows that the substitution of Ag⁺increases the filling of the tungsten bronze structure and also results in the tilting of Nb O₆octahedra,leading to the change of the dielectric constant.In addition,excess Ag₂O increases the DC leakage conductance and deteriorates the BDS of the glass ceramics.The best comprehensive performance was obtained when Ag₂O was added at 1.0 mol%,including:ε_r=258,W_rec=5.8 J/cm³,andη≈91%.In addition,a high discharge energy density of 3.62 J/cm³ and an ultra-high discharge power density of 1054.8 MW/cm³ were achieved under an applied electric field of 800k V/cm with a discharge time of only 6.8 ns.The above results indicate that the dielectric constant can be effectively increased while maintaining a high applied electric field strength by A-site substitution in tungsten bronze structured glass ceramics.Meanwhile,it has been demonstrated that the prepared glass ceramics has a promising application in high-power pulsed dielectric capacitors.3.K₂O-BaO-Sr O-Nb₂O₅-B₂O₃-Ta₂O₅（KBSN-x Ta₂O₅）glass ceramics were prepared and the relationship between the crystal structure changes caused by B-site substitution during the crystallization process and the dielectric,ferroelectric and energy storage properties was systematically investigated.It was demonstrated that no new phase was generated by the addition of Ta₂O₅,and Ta⁵⁺replaced Nb⁵⁺at the B-site in KBa Sr Nb₅O₁₅ crystals during the crystallization of the glass ceramics.The different covalency of Ta⁵⁺and Nb⁵⁺leads to different bond lengths of Nb-O and Ta-O bonds.Therefore,the substitution of Ta⁵⁺for Nb⁵⁺leads to a change in the lattice constant,a smaller value of c/a,and a weaker tetragonality of the tungsten bronze structure.When the addition of Ta₂O₅ is 4 mol%,the spatial point group of the crystal structure changes from a non-centrosymmetric P4/bm to a centrosymmetric P4/mbm,leading to a change in the ferroelectric behavior of the glass ceramics from a relaxed ferroelectric to a paraelectric.In further,the addition of Ta₂O₅ leads to a decrease in the interfacial activation energy,a widening of the band gap,and a significant decrease in the grain size of the glass ceramics,which leads to a substantial increase in the breakdown field strength.When the incorporation of Ta₂O₅ is 4.0 mol%,the glass ceramics achieves the best comprehensive performance,including a high recoverable energy density of 6.20J/cm³ and a high energy storage efficiency of 82%under an applied electric field of1300 k V/cm.Moreover,an ultra-high discharge energy density of 5.62 J/cm³ and an ultra-high discharge power density of 1296.9 MW/cm³ were also achieved under the same applied electric field.The above results indicate that KBSN-x Ta₂O₅ glass ceramics has a prospective application in high-voltage high-power pulsed dielectric capacitors.