Supercritical methanol synthesis and antioxidizability of ultrafine cobalt powder

Abstract Ultrafine cobalt powders are indispensable for high-end applications (cemented carbides, magnetic alloys, and new-energy batteries) owing to their unique properties. However, conventional methods have the drawbacks of high cost and energy consumption, coarse sizes, agglomeration, and complex processes. This study used a one-step supercritical methanol hydrothermal method to synthesize ultrafine Co powder, investigating key parameters (reducing/chelating agents, concentrations, and pH). The powder was characterized using scanning electron microscopy, x-ray diffraction, Fourier-transform infrared spectroscopy, x-ray photoelectron spectroscopy, and (derivative) thermogravimetry. Under optimal conditions (2 mol l −1 glycerol, 0.2 mol l −1 polyvinylpyrrolidone (PVP), pH = 10.82, and 0.05 mol l −1 precursor), face-centered-cubic/hexagonal-close-packed Co powders with a minimum average size of 522 ± 117 nm were obtained. PVP inhibited agglomeration via C=O–Co coordination, and the powders had excellent oxidation resistance (∼300 °C onset, stable up to 600 °C). This study demonstrates, for the first time, the controlled synthesis of ultrafine cobalt powder using a supercritical methanol system.