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Composite amorphous powder core made by field-annealing with new insulations could improve the high frequency AC magnetic and mechanical properties. All of the amorphous ribbons with composition Fe78B16Si5C and Fe40Ni38Mo4B17Si were annealed at temperature higher than their embrittlement temperature but lower than the crystallization temperature. After the ribbons become brittle, they were mechanically crushed and milled to produce powders with size about 50 ∼ 250μm. To make a core, the amorphous powders combined with insulation binder were put into an evacuted toroidal followed by hot mechanical compaction. The weight percentages of binder were about 3 wt%, 5 wt% and 10 wt%. Field-annealings were carried out for different conditions in an Ar (or N2) atmosphere chamber at elevated temperature with applied field about 10 Oe. The initial magnetization curve and saturation magnetization were measured by vibrating sample magnetometer (O≤H≤10 KOe). DC and AC magnetic properties such as permeability, coercive force, core loss, etc. were tested by B-H loop tracer. It shows the AC properties can be improved by increasing the particle size of the amorphous powder and by increasing the interparticle contacts at high frequency.
Amorphous ribbons (FeNi)78Mo4B17Si, made by melt—spinning technique were prepared as magnetic shielding materials. Cylindrical shields (~0.05mm thick) were fabricated by wrapping ribbons on 20cm long pyrex tubes with 45mm diameter. DC and AC (60Hz, 400Hz, 800Hz, 1 kHz) magnetic shielding data were obtained by following the modified ASTM (A698–74) procedure. For DC magnetic fields, Ni—rich shields showed maximum shielding values at lower applied fields (0.1 0e~l 0e), while Fe—rich shields showed maximum shielding values at higher applied fields (1~5 0e). For AC magnetic fields, the magnetic fields, the magnetic shielding effectiveness was also influenced by skin effect at shield' surfaces. By comparing the data to calculations from shielding theory, there is about 8% magnetic shielding effectiveness induced by skin effect at 1 kHz.
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