Decrystallization effects of BaCO/sub 3/+4Fe/sub 3/O/sub 4/ mixture processed in a 2.45 GHz, H-field and 83 GHz gyrotron cavities

Summary form only given. In spite of overwhelming evidence showing that a variety of solid state reactions were radically different when performed in a multimode microwave chamber compared to conventional heating some still questioned whether there is direct link to the radiation field. The present study has been conducted, for the first time ever, in a single mode TE/sub 103/ cavity at 2.45 GHz. Pellets made out of mixtures of BaCO/sub 3/+4Fe/sub 3/O/sub 4/ have been reacted within the same cavity, at the positions of the E field and H field maxima. The mixture heats up rapidly in H-field and within 5 seconds of irradiation big differences are shown to occur in its crystal structure and morphology. Phases such as Fe/sub 3/O/sub 4/ or bindary compounds such as BaFe/sub 12/O/sub 19/ are rendered non-crystalline to XRD in a few seconds in the H field. The microstructures are unique, showing smooth glass-like regions with regular waves parallel to each other. In the E field node the identical pellet reacts completely and forms large euhedral crystals of the single phase. The magnetic properties of these decrystallized ferro-magnetic phases also show remarkable changes from the original very hard, to very soft, magnets. The same mixture [BaCO/sub 3/+4Fe/sub 3/O/sub 4/] was microwave irradiated in a 83 GHz gyrotron beam system. Twenty seconds of irradiation with 2 to 6 kW has led to the formation of the BaFe/sub 12/O/sub 19/ form in major quantities, with unreacted Fe/sub 2/O/sub 3/ in minor quantities. A combination of 3 kW power and 90 seconds irradiation time resulted in selective absence of some XRD peaks and considerable reduction of other XRD peak intensities, suggesting that the BaFe/sub 12/O/sub 19/ surface has partially decrystallized. 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.