Study On Doping Modification And Ultra-low Temperature Stable Dielectric Properties Of SnO₂ Based Defect-induced Colossal Permittivity Ceramics

Colossal permittivity（CP）materials are the materials with the dielectric permittivity higher than 10³.Due to the large dielectric permittivity,it enables a higher capacitance in a smaller volume,which is promising for the miniaturization of devices,and has become a hotspot of dielectric materials research.In recent years,donor and acceptor co-doped TiO₂systems have achieved high dielectric permittivity and low dielectric loss with good frequency-and temperature-stability.This kind of new CP behavior arises from electron-pinned defect-dipole polarization mechanism caused by co-doping,and is suggested to be universally appeared in many co-doped binary metal oxides（i.e.TiO₂,ZnO,SnO₂,NiO）.The important signature of this new CP materials induced by electron-pinned defect-dipole is an electron thermally activated dielectric relaxation（TADR）phenomenon at low temperature.Its dielectric relaxation temperature is characterized by an defect-dipole freezing temperature T_f（about 100 K-200 K）.Currently,most doping strategies are focused on non-magnetic dopants,such as acceptor elements from main IIIA or IIA groups,in which cases the Coulomb force between defect-dipole dominates the thermal dynamic behavior of the delocalized electrons,and thermally activated dielectric relaxation phenomenon is discovered.In current studies,stable CP properties below 50 K have not been obtained.In order to meet the needs of some ultra-low temperature environments（such as the aerospace field,ultra-low temperature electronic storage,etc.）,it has important basic theoretical significance and practical application value to develop colossal permittivity materials that are stable at ultra-low temperature.This paper has taken SnO₂ as the host material and the magnetic acceptor elements and donor elements are co-doped to study the effect of magnetic elements doping on the dielectric properties at ultra-low temperature.In this paper,magnetic acceptor Co²⁺and donor Nb⁵⁺were first selected,and（Co+Nb）co-doped SnO₂（CNSO）and Co doped SnO₂（CSO）were prepared by solid-state reaction method.It is found that although the samples of CNSO and CSO ceramics still exhibit TADR at low temperature,the temperature T_f of CSO is 20 K and T_f of CNSO is 75 K,are both significantly lower than that of the existing co-doped SnO₂（about 150 K-300 K）.When the temperature is below 50 K,CSO ceramics exhibits antiferromagnetism,while CNSO directly changes to paramagnetism.In thermodynamic theory,Landau free energy of the system is related to the polarization or magnetic state.The magnetic ions doping in metal oxides can easily induce large sp-d exchange interaction between the magnetic ions and the band electrons.Based on this,we studied the influence of the combined effect of the magnetic dipole and electric dipole on the free energy of the system after the magnetic ions doping.Simulation calculations show that under appropriate competitions between electric dipole and magnetic dipole,the free energy of the system could become flat.And it is verified that the introduction of appropriate magnetic dipole can adjust the thermally activated dielectric relaxation behavior at low temperature from simulations,and thus we can obtain ultra-low temperature stable CP properties.Bearing these in mind,we choose donor Ta⁵⁺with much smaller magnetic moment compared with Nb⁵⁺as donor and Co²⁺for co-doping.The ceramic samples exhibit ultra-low temperature stable CP properties,without any signature of TADR phenomenon.The dielectric permittivity is～1500 and dielectric loss is～0.01 in the temperature range of 2 K-300 K（1kHz、10 kHz、100 kHz）,which are basically unchanged.（Co+Ta）co-doped SnO₂ ceramic samples（CTSO）exhibit the coexistence of the antiferromagnetism and paramagnetism when the temperature is below 50 K,and therefore have the appropriate interactions between electric dipole and magnetic dipole.The ultra-low temperature stable CP properties of CTSO are derived from the spin-defect mediated super electric polarization state caused by the exchange interaction between local spin from Co and charge carrier spin from Ta.The spin-defect mediated super electric polarization state is expected to be applied in other colossal permittivity material systems.