Sodium ferrites: new materials to be applied in energy storage devices in a wide frequency range

Abstract

Materials for energy storage have been a subject of high interest in recent times. The development of new materials with high dielectric constant and low losses is one of the main goals in scientific research for electronic applications. These properties allow reduction of the size and weight of the electronic devices. In this work, powders of sodium ferrite were prepared by sol-gel, using iron nitrate and sodium acetate as raw materials, according to the Pechini route. In order to optimize the synthesis parameters, the materials were heat-treated, and structural, morphological and dielectric characterization was performed to find the most suitable ones for storing energy. The sample structure was characterized by X-ray diffraction (XRD), Raman spectroscopy and the morphology surface by scanning electron microscopy (SEM). The dielectric properties were studied in a frequency range between 100 Hz and 1 MHz at temperatures from 200 up to 370 K by impedance spectroscopy measurements. The complex permittivity at 2.7 GHz and 5 GHz (T = 300 K) was determined using the small perturbation method through resonant cavities. The sample of sodium ferrite heat-treated at 1100 °C is the most promising for energy storage with a dielectric constant of ≈818 (f = 1 kHz; T = 300 K) and ≈6 (f = 5 GHz; T = 300 K), with low losses. The dielectric constant increases with the presence of Na3Fe5O9 phase and with the grain size and consequently porosity decreases. At low frequencies, this sample presents two relaxation processes described by Cole-Cole model and the relaxation time versus temperature has an Arrhenius behaviour.