Synthesis of nano-crystalline NiFe2O4 powders in subcritical and supercritical ethanol

• Nano NiFe2O4 powders were prepared under sub- and supercritical conditions of EtOH.
• Supercritical synthesis shows higher NiFe2O4 conversion and lower α-Fe2O3 content.
• These NiFe2O4 powder have mesoporous structure with uniformly sized particles.
• Both powders have much better magnetic properties: remanence and coercivity.
• Powder obtained under supercritical conditions have higher squareness ratio.

Nano-crystalline nickel ferrite has a broad range of applications due to its favourable magnetic properties. Those characteristics can be significantly influenced by the synthesis pathway including methods conducted in the presence of supercritical alcohols. Nano-crystalline NiFe2O4 powders were obtained in the reaction under subcritical and supercritical conditions of ethanol. Both high pressure synthesis routes resulted in powders with smaller primary particles and higher mesoporosity than co-precipitation method. Upon the annealing treatment the average crystallite size increased while material structure remained uniform, resulting in significantly enhanced magnetic properties, such as coercivity and remanence. It was found that synthesis under supercritical conditions provides higher conversion but also material with larger average crystallite size upon annealing. The time and temperature of annealing stage significantly influenced the morphology and magnetic properties of the obtained powders.

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