A review of the magnetoplumbite magnet development is given from the early magnets of Takei to the recent high energy barium ferrite magnets of Tomholt. A method for rapidly testing a large number of ferrite permanent magnet specimens is described which was used on several thousand specimens. The results are summarized for portions of the ternary systems M1O·M2O·Fe2O, where M1 and M2 stand for Ba, Sr, Pb, or Ca. A modified strontium ferrite magnet could be developed which appears to have a higher magnetic energy than any other oxide magnet material previously known. Typical values are: Br=4100 G, IHc=3000 Oe, (BH)max=4.0 mgo. At lower coercive forces and under careful laboratory conditions, modified strontium ferrite magnets can be prepared with a (BH)max approaching 5 mgo. A number of permanent magnet applications are mentioned for which the new material is most suitable.
Currently used ferrite magnets (also called ceramic magnets) are modifications of the magnetoplumbites MO·6 Fe~03 where M stands for Ba or Sr-with additions of other compounds, off-stoichiometric ratios, flexible binders, etc. The first commercial magnetoplumbite magnets were of the barium ferrite type, but the newer ones are modified strontium ferrites which are about 30% higher in coercive force at the same level of remanence and 4% lower in weight than the barium ferrites. They are also easier to manufacture. The results of hysteresis loop measurements on some 20000 anisotropic different ferrite magnets are summarized. The highest values, which are all obtained on modified strontium ferrites, range from (BH) max= 4.8 MGOe at IHc=2.4 kOe to (BH) max=0.8 MGOe at IHc= 11 kOe depending on process and chemical composition. The permanent magnets with the highest presently known energy/cost ratios are obtained by subdividing a green body of sulfate-modified strontium ferrite using a "cookie-cutting" technique and by quick-firing the magnets so that they contain nonequilibrium structures.
Up to 6.0 wt % SrSO" CaSO" and BaSO, were added to strontium ferrite magnets consisting of the magnetoplumbite phase SrO·6 Fe20s. The hysteresis loop, density, saturation moment per gram, and the degree of orientation were measured. The magnets were analyzed by standard chemical methods and by x-ray diffraction techniques. The microstructures of the magnets were examined with a light microscope and with an electron-beam microanalyzer. It was found that approximately 0.1 to 2.0 wt % SrS04, CaSO" or BaSO. are extremely beneficial in activating the ferrite magnet formation and in improving the magnet properties. SrSO, is particularly effective. With only 0.22 wt % SrS04 and at a sintering temperature of 2250°F, the remanence is increased from 3425 to 4150 G, the saturation moment from 65.2 to 69.1 G·cm 3 /g, the density from 4.21 to 4.86 g/cm 3 , and the degree of orientation from 81 % to 87%. X-ray scanning micrographs show that the solubility of SrS04 in SrO·6 Fe20s is greater than 0.22 and less than 1.1 wt % at 2250°F.
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