The Energy Storage And Interface Polarization Of Na₂O-K₂O-Nb₂O₅-SiO₂-based Glass-ceramics

With the rapid development of human society and science and technology,the energy problem is increasingly serious,and the development of new energy materials is imminent.Dielectric energy storage materials are one kind of new energy materials.Dielectrics are widely used in MLCC(multilayer ceramic capacitors)and other fields because of their low cost,large specific capacity and high-power density.However,the low energy storage density of dielectric is the main reason limiting its further application.Glass-ceramics have high permittivity and breakdown strength(BDS),which can realize high energy storage density theoretically.However,the interface polarization of the glassceramics can greatly reduce its energy storage performance.Na2O-K2O-Nb2O5SiO2-based glass-ceramics have high permittivity,but the glass-ceramics fail to achieve high energy storage density because of their low BDS.It is found that the space charge will gather at the interface of the glass ceramic to balance the poor polarization due to the large difference in electrical properties between the glass phase and the ceramic phase,thus,the interface polarization is generated,which decrease the electrical uniformity of the glass-ceramics.In this dissertation,the BDS of the Na2O-K2O-Nb2O5-SiO2-based glass-ceramics is improved and high energy storage density is realized by reducing the interface polarization.Na2O-K2O-Nb2O5-SiO2-based glass-ceramics are prepared by traditional melting and controlled-crystallization.The compositions of the glassceramics are designed by adding oxides with different field strength parameter(Fs),and the permittivity and the BDS of the glass-ceramics are coordinated to achieve high energy storage performance.Firstly,according to the Maxwell-Wagner model,the electrical performance of the glass phase dominates the effect on the interface polarization of glassceramics.The interface polarization of the glass-ceramic can be reduced by changing the ratio of Na2O/K2O to regulate the electrical properties of the glass phase.There is a constant phase element(CPE)in the equivalent circuit of the glass-ceramics,and the dispersion index of the constant phase element(CPE-P)in glass phase is related to the interface polarization.When the amount of K2O is 10 mol%,the interspace of the glass network is filled with a great number of K+with a large ionic radius,and the interface polarization relaxation activation energy(Ei)is reduced to 1.59 eV with the increase of CPE-P to the maximum value,which increases the highest value of BDS of 440 kV/cm for the glassceramics.Based on Maxwell-Wagner model,the model of the effect of glass network structure on the interface polarization of the glass-ceramics is established(For short:Glass network structure-Interface polarization model).Secondly,coordinating the permittivity and BDS of the glass-ceramics to achieve high energy storage performance through the composition design,the electrical properties of the glass phase are regulated by changing the structure of the glass phase with the addition of metal oxides with different field strength parameters(Fs),which reduce the interface polarization of Na2O-K2O-Nb2O5SiO2-based glass-ceramics.As intermediates in the glass network structure,high price metal cations possess high electric field,which restricts the movement activity of carriers because of these large attractive force in the glass structure.The addition of Zr4+ with large Fs increases the degree of polymerization(DOP)of the glass network,which significantly increase the value of CPE-P of the glass phase.Thus,the ability of movement of space charge carriers to the interface is reduced because the long-range migration of space charge carriers is frozen.Therefore,the Ei of glass-ceramics is reduced to 0.82 eV,and the electrical uniformity is improved.With the decrease of the interface polarization of the glass-ceramics,the BDS increases significantly,and the maximum BDS reaches to 1770 kV/cm when the amount of Zr4+ is 10 mol%.However,with the addition of Zr4+,the main crystalline phase NaNbO3 gradually disappear,resulting in the decrease of dielectric constant,and a high energy storage density failes to obtain.Thirdly,metal cation Bi3+with the high-valence also has large Fs.The interface polarization is reduced and the energy storage performance is improved while the main crystalline phase NaNbO3 remains unchanged with the addition of Bi3+.It is found that the DOP of the glass network structure firstly increases and then decreases because the CPE-P of the glass phase firstly increases and then decreases,and the interface polarization firstly decreases and then increases with the addition of Bi3+.When the amount of Bi3+is 1 mol%,CPE-P reaches the maximum value and Ei decreases to 1.31 eV.The addition of Bi3+ increases the BDS of the glass-ceramics to a certain extent,and the highest BDS reaches up to 1220 kV/cm.However,ferroelectric characteristics appear in the glass-ceramics due to the addition of Bi3+,which increases the energy loss.Thus,the high energy storage density failes to obtain.Finally,as the glass network modifier of the glass network,metal cations with large ionic radii reduce the interspace of the glass network,which reduces the movement activity of carriers.With the addition of earth-alkaline cations with different ionic radii(M2+=Ca2+,Sr2+,Ba2+),the DOP of the glass network structure increases,which increases the value of CPE-P of the glass phase.When the amount of Ba2+is 3 mol%,CPE-P increases to the maximum value of 0.63,and the interface polarization of the glass-ceramics decreases to 1.05 eV.With the decrease of interface polarization,the addition of Sr2+ and Ba2+ significantly increases the BDS of the glass-ceramics,but the addition of Ca2+ failes to increase the BDS of the glass-ceramics.According to the correlated barrier hopping(CBH)model,the addition of Sr2+ and Ba2+ increase the activation energy and the distance of the electron transition,which improve the BDS of the glass-ceramics,but the addition of Ca2+ reduces the activation energy and the distance of the electron transition,which failes to the BDS of the glass-ceramics.When the amount of Ba2+ is 3 mol%,the BDS of glass-ceramics reaches to 1780 kV/cm.The maximum energy storage density of 23.1 J/cm3 is achieved by coordinating the permittivity and the BDS of the glass-ceramics.In summary,CPE-P of the glass phase is negatively correlated with Ei,which verifies the Glass network structure-Interface polarization model.Zr4+with high valence state and Ba2+ with large radius effectively reduce interface polarization.High dielectric constant of 165 and high BDS of 1780 kV/cm are obtaind for the glass-ceramics.Thus,the maximum energy storage density of 23.1 J/cm3 is obtained with the amount of 3 mol%of Ba2+.