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SELECTED APPLICATION AREAS. 93 Figure 28 Infrared-transparent mullite from sol-gel. PERMANENT MAGNETS Rapid solidification (i.e., melt spinning) of Re2Fe14B-type materials has permitted the development of permanent magnets whose performance depends in a very important way on submicron-scale microstructures. Two different types of microstructures have been identified in these materials, which lead to significantly different permanent magnet behavior. The mean grain size of the two microstructures is submicron and very similar, approximately 20 nm to 30 nm. However, another submicron structural feature controls the magnetic performance differences. It is a 1-nm-thick uniform intergranular phase. The presence of the submicron intergranular phase shuts off the short-range intergrain ferromagnetic exchange interactions by effectively isolating the grains from magnetic interactions with each other. When the intergranular phase is absent, ferromagnetic exchange couples the magnetizations of the two grains at their surfaces, leading to significant enhancement of the remanent magnetization of all the grains. Thus the submicron microstructure acts as a switch for enhancement of the magnetic properties in these materials. Specifically, it was shown by Croat et al. (1984a, b) that material with compositions in the vicinity of Re2Fe14B stoichiometry could be melt-spun at specific quench rates (i.e., specific wheel speeds) producing material with coercivities in excess of 10 kOe and energy products up to 14 MGOe. [Similar performances were produced by Koon (1980) with heat treatment of amorphous melt spun material of similar composition.] The values by Croat et al. were in good agreement with the predictions for remanence and energy product of
SELECTED APPLICATION AREAS. 94 crystallographically isotropic permanent magnets predicted by Stoner and Wohlfarth (1948), wherein the limits to the magnitude of the remanent magnetization (4 Mr) are less than one-half the saturation magnetization (4 Ms), and the energy product is less than (4 Mr/Ms )2. Bright-and dark-field transmission electron micrographs of optimum as-quenched material (Croat et al., 1984a, b) are shown in Figure 29. It is clear that the sample consists of a two-phase microstructure with small (approximately 30-nm diameter) grains of the principal phase, Re2Fe14B (Herbst et al., 1984) surrounded by a very thin film of an amorphous phase some 1 to 2 nm in thickness. In permanent magnets with this type of microstructure, it is believed that magnetic domain walls are pinned in the amorphous grain boundary regions, thus generating coercity. The submicron microstructure shown in Figure 29 has no amorphous intergranular phase. The mean grain size for this type of material has been determined to be between 14 and 23 nm (Keem et al., 1988). In contrast to the Figure 29 Bright-field image and selected area diffraction of enhanced remanence material. No evidence of intergranular phases is found in either the image or the selected areas diffraction patterns (Keem et al., 1988).
