Microstructural characterization and electrical conductivity of CuxMn3−xO4 (0.9≤x≤1.3) spinels produced by optimized glycine–nitrate combustion and mechanical milling processes

Abstract

In this research, the electrical conductivity of CuxMn3−xO4 (0.9≤x≤1.3) spinel powders prepared by an optimized glycine–nitrate process (GNP) followed by high-energy mechanical milling was studied. The samples were characterized by X-ray diffraction (XRD), thermal gravimetric and differential thermal analysis (TG/DTA), field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), 4-probe DC and Archimedes methods. Different glycine/nitrate (G/N) ratios of 0.56, 1.11 and 1.67 were employed, from which the 1.11 (stoichiometric ratio) resulted in the formation of desired spinel phase. The as-prepared powder was calcined at 500°C for 2h under Ar atmosphere (referred as sample A). To evaluate the effect of grain size on electrical conductivity, sample A was milled for 1h (sample B) and 3h (sample C) and pelletized under a pressure of 50MPa. The results showed that sample A had a very low relative density after cold pressing (~40%) leading to a significantly lower conductivity (14.4Scm−1 at 750°C) with an activation energy of 0.76eV as compared to the samples B (29Scm−1 at 750°C) and C (23Scm−1 at 750°C), with the activation energies of 0.67 and 0.72, respectively. Sample C (average grain size of ~10nm), with larger fraction of grain boundaries, exhibited lower conductivity with respect to sample B (average grain size of ~22nm) due to the higher level of charge carrier scattering.The optimized GNP-mechanical milling process was then applied on CuxMn3−xO4 (0.9≤x≤1.3) spinels to probe the effect of copper content on electrical conductivity. In this case, all stoichiometries were sintered at 1250°C for 2h. The results showed that the conductivity increases from 52 to 140Scm−1 by increasing the copper content in the range of 0.9≤x≤1.3 at 750°C. This arises from a gradual increase in Mn3+–Mn4+ octahedrally located ion pairs.