Thermal, structural, and impedance analysis of nanocrystalline magnesium chromite spinel synthesized via hydrothermal process

Nanocrystalline magnesium chromite spinel was synthesized through hydrothermal reaction of metal nitrate solutions in stoichiometric amount at different pH, temperature and time intervals. The synthesized products were characterized for crystallinity, phase identification, and surface morphology by X-ray diffraction (XRD) and scanning electron microscope (SEM). XRD patterns showed that as-synthesized product remained amorphous up to 250 °C. However, well-crystallized magnesium chromite spinel structure is formed after calcination at 850 °C. Rietveld refinement study confirms the formation of single-phase cubic structure MgCr 2 O 4 with lattice parameter a = 8.3347 Å, and Fd 3 m space group. The as-processed MgCr 2 O 4 products showed extensive XRD line broadening, and the mean crystallite size of such crystals was found to be mainly in size range of 85–124 nm. Surface SEM images of calcined specimens revealed that the matrix is uniform, and no separation of secondary phase was detected. Thermal stability was examined by thermogravimetry (TG), differential thermal analysis (DTA), and differential scanning calorimetry. TG/DTA reveals that MgCr 2 O 4 is thermally stable above 700 °C. Fourier transform infrared (FTIR) spectra studies shows two strong bands, one around 600 cm −1 which is attributed to the intrinsic vibrations of tetrahedral and other at 400 cm −1 is due to octahedral one. FTIR confirms the formation of metal oxides. The bandgap energy was estimated by absorption spectroscopy in ultraviolet–visible range and was found to be 0.693 eV for MgCr 2 O 4 specimen sintered at 1,000 °C. Isothermal shrinkage characteristic and coefficient of thermal expansion were determined by dilatometry. The powder specimens showed excellent densification at 1,250 °C temperature and uniformly fine grain sintered ceramics (>90 % relative density) with submicron grain size (2–5 μm) were obtained after sintering at 1,000–1,250 °C. Impedance studies were carried out at room temperature and equivalent circuit model ( R 1 Q 1 ) ( R 2 Q 2 ) ( R 3 Q 3 ) is used to explain different relaxation processes. We report largest impedance values i.e., 6.74 × 10 8 Ω, reduced dielectric constant (≈1.0), and low tangent loss (0.8) for MgCr 2 O 4 sintered at 1,250 °C.