Solution Combustion Synthesis and Characterization of (CoCrFeMnNi)₃O₄ High-Entropy Oxide Using Different Fuels

High-entropy oxides (HEOs) have recently attracted significant interest due to their tunable crystal structures, compositional versatility, and promising functional properties in energy storage, catalysis, and magnetic applications. Among various synthesis routes, solution combustion synthesis (SCS) offers a rapid and energy-efficient pathway for producing phase-pure HEO powders with controlled morphology. In this study, (CoCrFeMnNi)₃O₄ high-entropy oxide was synthesized via SCS using three different fuels, followed by postcombustion heat treatments at 800, 900, and 1000 °C for 1 h. The calcination temperatures were selected based on thermogravimetric analysis (TGA) and literature data. Phase formation, microstructural evolution, and elemental distribution were investigated by X-ray diffraction (XRD) and scanning electron microscopy coupled with energy-dispersive spectroscopy (SEM-EDS), while specific surface area and porosity characteristics of selected samples were evaluated by Brunauer-Emmett-Teller (BET) analysis and its corresponding Barrett-Joyner-Halenda (BJH) method. The results revealed that the choice of fuel significantly influenced the combustion characteristics, phase purity, and particle morphology, while the calcination temperature played a key role in grain growth and densification. Glycine at a stoichiometric ratio (Φ_e = 1) and calcination at 900 °C yielded the most favorable results, producing sharp spinel peaks consistent with the $Fd\stackrel{-}{3}m$ space group and elemental distributions closest to equimolar. TGA confirmed high thermal stability with <3% weight loss up to 1000 °C, while the citric acid route exhibited ∼25% mass loss due to residual organics. Postcalcination SEM analyses showed homogeneous microstructures with well-defined grains, particularly in glycine-derived samples, whereas excess fuel or unsuitable stoichiometry led to porous or amorphous products. BET/BJH analyses of glycine-derived samples prepared at the 1.0× fuel stoichiometry further confirmed the temperature-dependent textural evolution, showing a progressive reduction in specific surface area and porosity with increasing calcination temperature, in agreement with SEM-observed densification. This work provides a systematic comparison of fuel-dependent SCS synthesis for (CoCrFeMnNi)₃O₄ and establishes a synthesis parameter space for obtaining single-phase spinel oxides with controlled microstructures at relatively low processing temperatures.