Solar Radiation Provides Inexpensive Substitute for Laser Surgical Technology
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Rustum Roy and co-workers at The Pennsylvania State University have demonstrated that crystalline phases can be rendered noncrystalline in a bulk solid and hard magnets can be converted to soft magnets in the solid state, both in a matter of seconds using microwave processing at temperatures far below the melting temperatures of the materials. Their latest findings will appear as a rapid communication in the December issue of the Journal of Materials Research, and is currently available on-line (www.mrs.org). Microwave processing has been explored for the thermal processing, synthesis, and sintering of materials, primarily ceramics, over the last two decades. Most researchers held that the reaction and heating is due to the electric-field vector (E) component of the microwaves. The magnetic-field component (H) was considered to be negligible in the energy loss of the microwave radiation. Earlier, the researchers at Penn State were able to separate the E and H peak intensities of 2.45 GHz (~12-cm wavelength) microwaves in a cavity with a separation distance of 4 cm. This enabled them to subject identical samples at the E and H component peaks and study the effects of each, with the caveat that the materials’ reaction to the fields could distort the pure E and H fields. Using this apparatus, the researchers have previously demonstrated that the magnetic-field losses are probably a significant portion, and in some cases the dominant one, of the overall loss mechanism in several materials. In their present study, the researchers placed pellets of a stoichiometric mixture of the constituent oxides of BaFe12O19 (BaCO3 and Fe3O4) in their apparatus in the E-field and H-field peaks of the 2.45-GHz microwaves. When placed in the H field, the material completely decrystallized in as little as 5 s, as shown by x-ray diffraction. In the E field, the material formed euhedral crystals of BaFe12O19. The temperatures of the specimens were closely monitored and did not go above 1200°C in the H field and 1400°C in the E field, well below the melting temperature. Similarly, pellets of the ferrite Fe3O4 placed in the H field rapidly decrystallized in less than 60 s. When subjected to the E field, Fe2O3 crystal peaks were observed in x-ray diffraction measurements of the samples. The microstructures of the decrystallized specimens consisted of smooth wavelike topologies separated by ~2 µm and made up of small contiguous points. The researchers were unable to explain this microstructural evolution. In addition, the magnetic properties (B–H curves and saturation magnetiza-
tion) of the samples from the E field and H field were measured and compared to a sample subjected to the usual multimode microwaves. The magnetic saturation moments and B–H curves were very different for all three. In particular, the decrystallized BaFe12O19 ferrite sample (subjected to the H field) was found to have become a soft magnet. This was attributed to the fact that different sublattices are excited for the different processing modes, since a crystalline mat
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