Cold sintering to form bulk maghemite for characterization beyond magnetic properties

Abstract

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.

Tuning Interparticle Contacts and Transport Properties of Maghemite–Thermoset Nanocomposites by Applying Oscillating Magnetic Fields

“…The pure epoxy reference samples were measured to have an electrical resistivity of 1.19 ± 0.40 × 10 14 Ω m, which is comparable with the reported value of ∼10 14 Ω m . The cold-sintered maghemite-only reference sample (relative density of 56.4%) was measured to have a resistivity of 9.65 × 10 6 Ω m …”

“…To understand the impact of the particles, the dashed lines in Figure c,d estimate the nanocomposite thermal conductivity using the rule of the mixtures based on the volume fraction and the thermal conductivities of epoxy and maghemite ( k p ). The k p value was varied from the conservatively measured 1.3–5 W/(m K). This approach neglects details of the morphology of the particles and boundary resistance that can significantly alter the effective thermal conductivity of the nanocomposites.…”

“…The cold-sintered maghemite-only reference sample had a thermal conductivity of 0.50 W/(m K) . Using Landauer’s relationship without considering thermal boundary resistances in the sintered samples, the intrinsic thermal conductivity of dense maghemite was conservatively estimated as 1.30 W/(m K) …”

Construction of Inorganic Bulks through Coalescence of Particle Precursors

“…Lots of inorganic materials use cold sintering to build bulk materials, including solid-state electrolytes [ 46 ], ferroelectrics [ 49 ], piezoelectric materials [ 50 ], Li-ion cathodes [ 51 ], structural materials [ 52 ], microwave dielectrics [ 53 ], semiconductors [ 54 ], and magnetic ceramics materials [ 55 ]. In 1986, Yamasaki first published the work about densifying ceramic at temperatures below 200 °C through hydrothermal processing and isostatic pressing [ 56 ].…”

Development of a Magnetic Fluid Heating FEM Simulation Model with Coupled Steady State Magnetic and Transient Thermal Calculation

“…For the thermal conductivity of the carrier fluid, the value of λ f = 0.15 W/mK was estimated for mineral oil, taking literature [39] as a guide. For maghemite, the thermal conductivity falls in the range of 0.86-1.30 W/mK, according to [40]. For the value λ p = 0.92 W/mK, using Equation ( 17), the magnetic fluid thermal conductivity was λ = 0.187 W/mK.…”