Abstract
It is well stablished that heating efficiency of magnetic nanoparticles under radiofrequency fields is due to the hysteresis power losses. In the case of microwires (MWs), it is not clear at all since they undergo non-coherent reversal mechanisms that decrease the coercive field and, consequently, the heating efficiency should be much smaller than the nanoparticles. However, colossal heating efficiency has been observed in MWs with values ranging from 1000 to 2800 W/g, depending on length and number of microwires, at field as low as H = 36 Oe at f = 625 kHz. It is inferred that this colossal heating is due to the Joule effect originated by the eddy currents induced by the induction field B = M + χH parallel to longitudinal axis. This effect is observed in MWs with nearly zero magnetostrictive constant as Fe2.25Co72.75Si10B15 of 30 μm magnetic diameter and 5 mm length, a length for which the inner core domain of the MWs becomes axial. This colossal heating is reached with only 24 W of power supplied making these MWs very promising for inductive heating applications at a very low energy cost.
Highlights
It is well stablished that heating efficiency of magnetic nanoparticles under radiofrequency fields is due to the hysteresis power losses
The determining factor for these types of processes is that the energy cost has to be much smaller than the regarding current processes, either because the application of the radiofrequency field allows more effective processes or because the required temperature or pressure are reduced by the inductive heating[7,9,10]
The mass susceptibility, χg, are 11.5(5), 40.6(5) and 23.7(5) emu/g·Oe for 1, 5 and 20 MWs, respectively, i.e, the susceptibility first increases from 1 to 5 MWs and decreases for a larger number of MWs, as already reported for other authors[48]. These results suggest that the magnetostatic interactions between MWs play the major role for the heating efficiency at radiofrequency fields
Summary
It is well stablished that heating efficiency of magnetic nanoparticles under radiofrequency fields is due to the hysteresis power losses. There is currently a growing interest in applications to which this concept can be spread out, such as heterogeneous catalysis[4,5,6], electrolysis of water[7], new methods of organic synthesis[8,9], molecularly imprinted polymers[10], and others[11,12] These new applications of the use of magnetic nanoparticles as nano-heaters activated by radiofrequency fields are being recently explored and open a new and wide range of possibilities in this field. One of the fundamental requirements that a material should meet to make these new applications really interesting is that the energy needed to activate these nano-heaters is low, i.e., field amplitude and frequency has to be as low as possible In this way, really efficient processes can be obtained. If magnetic materials have the proper size and conductivity, this effect should be considered
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