Cold sintering to form bulk maghemite for characterization beyond magnetic properties

Maghemite nanoparticles have been sought after for electronic, biomedical, and environment applications, for their soft ferrimagnetic properties and large coercivity. While their magnetic properties are well characterized, their nonmagnetic properties are currently not available because maghemite is prepared as nanoparticles and their bulk forms often have contaminants. In this work, thermodynamically unstable maghemite nanoparticles are cold sintered (130-250°C) to form bulk samples with submicron-size grains. Electrical and thermal conductivities of maghemite were evaluated for the first time: 3.5 × 10⁻⁷ S/m and 0.86-1.30 W/(mK). The relative densities of these cold-sintered samples are low (55.9%-64.2%) but comparable with or slightly lower than those previously achieved with higher sintering temperature (~55% at 500°C and ~76% at 1250°C). Such porous maghemite samples with large surface areas can potentially be used as an anode of lithium-ion batteries, while further densification will be pursued in the future by sintering process modification.