High-temperature Dielectric Response And Relaxation Properties In Ni₂/Co₂ Z-type Hexaferrites

Hexaferrites,as multifunctional materials,have been researched widely in recent years due to the promising applications in information technology.There are many reports on magnetic properties of hexaferrites,however,dielectric properties were not reported widely.In this paper,the high temperature dielectric properties of several Z-type hexaferrites with nominal composition of?Ba,Sr?₃Me₂Fe₂₄O₄₁?Me is a divalent metal cation?are studied.The main research process and results are as follows:?1?The polycrystalline sample of Ba₃Ni₂Fe₂₄O₄₁1 with nominal composition was successfully synthesized by sol-gel method with clear grains and grain boundaries was identified by the scanning electron microscopy?SEM?.The frequency dependence of complex impedance in different temperature suggests that the sample shows a significant relaxation effect,which is attributed to the thermally activated model after analysis.Through a comprehensive analysis of the conductivity of the sample,we found that the conductivity as well as the dielectric loss of the sample increases with the increase in temperature,and the temperature dependence of DC conductivity follows Arrhenius law,the calculated value of the activation energy is0.58eV.?2?The polycrystalline samples of Sr₃Co₂Fe₂₄O₄₁1 and Ba₃Co₂Fe₂₄O₄₁1 with nominal composition were successfully synthesized by sol-gel method.The XRD data indicate that the samples are Co₂Z-type hexaferrites with the centrosymmetric space group of P63/mmc.The SEM micrographs suggest that mixture plate-like grains are formed and Sr₃Co₂Fe₂₄O₄₁1 shows more dense grain structure.The dielectric measurements indicate that the samples exhibit excellent dielectric properties,and the dielectric relaxation behaviors are obvious at high temperature.The frequency-temperature relationship corresponding to the dielectric relaxation peak of the sample conforms to Arrhenius'law and can be attributed to the thermal activation model.The X-ray photoelectron spectroscopy indicates that Fe²⁺and Fe³⁺coexist in the system and the dielectric relaxations originate from the carriers hopping between Fe²⁺and Fe³⁺.?3?The polycrystalline samples of Ba_1.5Sr_1.5Co₂Fe₂₄O₄₁1 with nominal composition were successfully synthesized by sol-gel method with clear grains and grain boundaries was identified by the scanning electron microscopy.The study through a series of dielectric measurements suggests that the sample exhibits colossal dielectric response at high temperatures,and the colossal dielectric permittivity at low frequency is ascribed to the Maxwell Wagner Polarization Effect.The obvious dielectric relaxation can be seen both in dielectric-frequency relationship and dielectric-temperature relationship which is attributed to the thermally activated model.The complex impedance data is simulated with an equivalent electric circuit and the result suggests that the delectric response originates from both the grains and grain boundaries.The activation energies of the grains and grain boundaries are calculated which are 0.62 and 0.67 eV,respectively.