Heat Capacity Studies of Nanocrystalline Magnetite (Fe₃O₄)

Abstract

The Verwey transition in magnetite (Fe 3 O 4 ) has been studied extensively with a wide assortment of experimental techniques to investigate the effects of impurities, oxygen stoichiometry, and crystal quality; however, no studies have tested the effects of crystal size on the Verwey transition. In this study, the heat capacity of a magnetite powder with an average crystallite size of 13 nm was measured in the temperature range of 0.5-350 K. No obvious anomaly was observed in the heat capacity in the vicinity of 120 K, yet the measurements exhibited unusual thermal behaviors between 50 and 90 K with relaxation times increasing from an average of 25 min to as much as 5 h in this temperature interval. The behavior is typical of a phase transition, so the lack of a distinct anomaly suggests either the phenomenon is spread out over a large temperature interval with little enthalpy or an insulating phase with different thermal conductivities appears. The heat capacity below 90 K was dependent on cooling rate with an inconsistent anomaly found below 4 K. The results suggest that the Verwey transition has a particle-size dependence. Additionally, theoretical fits below 15 K suggest that magnetite nanoparticles display anisotropic ferrimagnetic behavior and a superparamagnetic contribution to the heat capacity which are properties not observed in bulk magnetite.

Specific heat of metal oxide nanofluids at high concentrations for heat transfer

“…This behavior agrees with most reported studies about nanocrystalline materials when compared to bulk minerals [66,67], which attribute this feature to the surface energy, distortion and lattice energy of the nanoparticles [68,69]. Concerning nano-sized ZnO and ZrO 2 particles, they present lower experimental C p than bulk mineral with differences up to 13% [65] and 12% [63] for ZnO and 10% [63] and 18% [64] for ZrO 2 samples.…”

Nanoscale Effects on Thermodynamics and Phase Equilibria in Oxide Systems

“…Cryogenic calorimetry also addresses the heat capacity of the adsorbed water in more detail. [44,47] Reflecting strong interaction of water with the TiO 2 surface, as already seen in the H 2 O vapor adsorption calorimetry, the heat capacity and entropy of surface-confined water is somewhat smaller than that of bulk ice. This might be surprising at first glance if one expects higher heat capacities and entropies of water due to disordering and/or mobility at the interface.…”

“…The magnetic transition in CoO [46] and the Verwey transition in Fe 3 O 4 [47] are affected by particle size, but the effect on the lattice (vibrational) heat capacity is probably minor. Cryogenic calorimetry also addresses the heat capacity of the adsorbed water in more detail.…”

Accurate heat capacity measurements on powdered samples using a Quantum Design physical property measurement system

“…The powdered benzoic acid used in this study was standard reference material 39j from the US National Bureau of Standards. The hematite (a-Fe 2 O 3 ) sample used in this study was a powdered sample sintered at T = 1473 K for 48 h. Powder X-ray diffraction confirmed the sample to be phase pure, and quantitative analysis of the chemical impurities showed this sample to have 0.9992 mass fraction pure [23].…”