T. Minowa

Magnetic properties of extremely small Nd-Fe-B sintered magnets

Magnetic properties of small Nd-Fe-B sintered magnets were investigated. Thickness of surface damaged layer, which shows low coercivity and causes the deterioration in magnetic properties, was found to be comparable to mean grain size of sintered bodies. Magnets with small grains of around 5 /spl mu/m showed rather higher magnetic performance after machining into small pieces with a surface area/volume ratio of 20-35 mm/sup -1/. Chemical etching with a nitric acid solution was effective in partial recovery of magnetic properties through bringing slightly higher coercivity of surface grains. Heat treatment at higher temperatures than the melting point of Nd-rich phase and subsequent aging enhanced the coercivity of surface grains. Coating with powders of Dy/sub 2/O/sub 3/, DyF/sub 3/, or TbF/sub 3/ before the heat treatment resulted in a great enhancement in coercivity without significant reduction in remanence.

Improvement of the corrosion resistance on Nd-Fe-B magnets with nickel plating

Three kinds of corrosion tests, a humidity test (80 degrees C, 90% RH), an autoclave test (120 degrees C, 2 atm saturated with water vapor), and a salt spray test (35 degrees C, 5% NaCl), were performed on a sintered magnet treated with nickel plating. For comparison, samples with epoxy resin coating, Al ion plating, and without coating were also exposed to the corrosion test. After specified periods of corrosion testing, the permanent flux loss of the remagnetized sample was measured. Changes in appearance were also observed. It was found that the epoxy resin coating could not prevent the penetration of moisture in the humidity and autoclave tests because of its hygroscopic properties. The nickel plating and Al ion plating withstood the humidity and autoclave tests reasonably well, but they suffered slight corrosion in the salt spray test. It is concluded that the use of nickel plating can provide high corrosion resistance and low cost of production. >

Single crystal magnets

Single crystals of the Fe–Cr–Co alloys without and with 3% Mo were prepared by a recrystallization process. The microstructures and the magnetic properties of 〈100〉, 〈110〉, and 〈111〉 single crystals were investigated after the heat treatment in the ridge region of the miscibility gap. The magnetic properties of the Fe–22u2009Cr–17u2009Co ridge single crystals were not affected very much by the crystal orientation. However, the Fe–23u2009Cr–20u2009Co–3u2009Mo alloys show the crystal orientation dependence of the magnetic properties. The single crystal alloys are aged in a magnetic field, the direction of which is varied around 〈100〉 axes. The magnetic properties are measured parallel to the applied field directions and are summarized in view of deviated angle from a 〈100〉 axis. The greater the deviation of the applied field direction from a 〈100〉 axis, the poorer are the magnetic properties of the Mo single crystal alloy. The best magnetic properties are achieved with Fe–22u2009Cr–18.5u2009Co–3u2009Mo 〈100〉 ridge single crystal as Br=1.5...

Fe-Cr-Co permanent magnet alloys heat-treated in the ridge region of the miscibility gap

By heat-treating in the ridge region of the miscibility gap, the magnetic properties of the Fe-Cr-Co permanent magnet alloys have been remarkably enhanced. The optimum procedure of the heat treatments, such as thermomagnetic treatment and step-aging sequence for the ridge alloys, is examined. Such heat treatments are discussed in conjunction with their microstructures. It is found that thermomagnetic treatment in the ridge region is effective in aligning and elongating the FeCo-rich particles parallel to the applied magnetic field direction. An Fe-22Cr-15Co alloy achieves the magnetic properties as Br = 1.56 T (15.6 kG), bHc = 51.6 kA/m (645 Oe) and (BH)max = 66.4 kJ/m3(8.3 MGOe), which are almost comparable to those of the columnar Alnico family.

Microstructure of Nd‐rich phase in Nd‐Fe‐B magnet containing oxygen and carbon impurities (abstract)

The microstructures of Nd‐Fe‐B magnet were examined by scanning electron microscopy (SEM) with wavelength dispersive x‐ray spectrometer (WDS) specially equipped lead stearate crystal for lighter elements detection. Investigations were performed on the samples of (Nd0.9Dy0.1)15FebalB8 magnets with various oxygen and carbon content. Oxygen and carbon were added to the alloy, because magnetic properties were extremely influenced by these elements and commercial magnets could not be produced without these impurities. SEM observations revealed a complex microstructure of Nd‐rich phase1,2 in grain boundary junctions. Nd‐rich phase principally consists of two different phases. One is Nd‐rich phase absorbing oxygen and the other is Nd‐rich phase condensing carbon. Carbon content were quantitatively confirmed by WDS analysis for the first time. The enrichment of carbon into Nd‐rich phases were accelerated by a heat treatment in the temperature range between 500 and 800u2009°C. Coercive forces have significantly decrea...