Phase formation and decrystallization effects on BaCO3 + 4 Fe3O4 mixtures; a comparison of 83 GHz, multimode millimeter-wave and 2.45 GHz single mode microwave H-field processing

Samples of BaCO3 + 4 Fe3O4 mixtures were microwave irradiated in an 83 GHz gyrotron beam system and a 2.45 GHz single mode system. Twenty seconds of irradiation with 2–6 kW and 83 GHz energy lead to the formation of the BaFe12O19 form in major quantities, with unreacted Fe2O3 in minor quantities. A combination of 3 kW power and 90 s irradiation time resulted in selective absence of some X-ray diffraction (XRD) peaks and considerable reduction of other XRD peak intensities, suggesting that the BaFe12O19 surface is partially decrystallized. However, in a 2.45 GHz H-field system, irradiation of the BaCO3 + Fe3O4 mixture resulted in a total disappearance of peaks in the XRD, establishing complete decrystallization in 5 s. The 83 GHz effects appear to be mainly on the surface of the materials processed, whereas for the mixture reacted in the 2.45 GHz H-field the decrystallization effect occurs in bulk. These findings correlate well with the greatly different absorption lengths at the two frequencies.