Microwave-Assisted Green Synthesis and Temperature-Dependent Dielectric Relaxation of Nominal MgFe₂₋ₓNiₓO₄ Spinel Ferrites Using Tinospora Cordifolia

Abstract

Nominal MgFe_2-xNiₓO₄  spinel ferrites (x = 0.05, 0.10, 0.15 and 0.20) were prepared by a microwave-assisted green combustion route using Tinospora cordifolia stem extract as a natural complexing/combustion medium. The precursor solution was subjected to 2.45
GHz and 600 W for approximately 5 min and used without a separate post-combustion calcination step. X-ray diffraction patterns indicated the presence of a cubic spinel structure. Crystallite-size estimates obtained using the Scherrer, Williamson-Hall and size-strain plot methods gave values of 15.87–17.82, 21.69–26.07 and 18.81–21.76 nm, respectively. The differences among the three methods reflect the contributions of crystallite-size and lattice-strain broadening. The lattice parameter was narrowly dispersed
between 8.37083 and 8.38871 Å. Pellets sintered at 600 °C for 4 h had bulk densities of 3.95–4.08 g.cm⁻³ and apparent porosities of 9.79-12.66%. Dielectric measurements from 1 Hz to 100 kHz and room temperature to 400 °C showed strong frequency dispersion and a marked suppression of 1 kHz low-temperature dielectric constant from 111.42 at x = 0.05 to 14.326 at x = 0.20. Broadened, frequency-dependent maxima of M″ showed a thermal shift, reflecting distributed thermally activated relaxation. Arrhenius analysis of M″ maxima provided values of effective activation energy as 0.779 eV for x = 0.05, 0.484 eV for x = 0.10, 0.480 eV for x = 0.15 and 0.662 eV for x = 0.20. The R² value for x = 0.20 was lower (0.849); therefore, this result should be interpreted with caution. The findings show that nominal Ni-for-Fe substitution combined with microwave-assisted synthesis using plant material can modify the electrical characteristics of the ferrite series.