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.