Coupling of Magnetite Particles with Microwaves at Temperatures lower than the Curie Point

Effect of the particle size (d) and apparent density on the coupling of microwaves with Fe₃O₄ was investigated at temperatures lower than the Curie point, T_C ≃ 585 °C. Two samples in the form of tablet with particle sizes of 45-75 μm (MT75) and <45 μm (MT45) and one sample in the form of powder with a particle size of <45 μm (MP45) were heated in multi-mode and maximum E- and H-field modes using a microwave generator at a frequency of 2.45 GHz. According to the results, an earlier temperature increase and also a higher temperature was achieved in the sample heated in the maximum H-filed mode. Regarding the incubation time in all samples, the particle size of Fe3O4 has no significant effect on the time required for the initial temperature increase in the presence of the H-field. In the maximum E-field mode, a shorter time was required for the temperature increase in the MT75 sample than MT45. At T≤T_c, magnetic loss and Joule loss are the dominant heating mechanisms in the presence and absence of the H-field, respectively. Magnetic loss is independent of the particle size whereas Joule loss which is influenced by electrical conductivity, affected by particle size. Therefore, above-mentioned effect of the particle size is attributed to the dominant heating mechanism. Also, some of the small particles seems to be transparent owing to a greater penetration depth (δ), ca. 80 μm at room temperature causing an earlier onset of temperature increase in sample with larger particle size, MT75. Moreover, microwave absorption in a sample with higher apparent density, MT45, was lower because of a higher electrical conductivity of sample in tablet form, MT45, than powder form, MP45.