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.
Conductive
nanofillers, if integrated in an organized manner, can
improve the transport properties of polymer matrices without compromising
on their light weight. However, the relationship between the particle
assemblies and transport properties of such nanocomposites, especially
the competing effects of connected nanofiller pathways compared to
resistances at interparticle contacts, has not been quantitatively
studied. In this work, with the model nanocomposite of maghemite nanoparticles
in epoxy, a novel fabrication method has been demonstrated to align
nanofillers and control the interparticle contact amount within such
a nanofiller assembly, using nanoparticle surface functionalization
and oscillating magnetic field application. The nanofiller assembly
cross-sectional areas were measured by processing micro-CT images
and compared with the measured electrical and thermal properties of
the nanocomposites. In terms of thermal transport, when the nanofiller
assembly cross-sectional area was small, the dominance of conductivity
pathways was observed up to ∼4.7 vol %, while interfacial thermal
resistance began to dominate when the nanofiller assembly cross-sectional
area became larger than 2700 μm2.
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