体彩20选5预测:High Permeability Alloys
The soft magnetic nickel-iron alloys, ranging from 36% to 81% nickel, balance iron, with additions of other elements to develop or accentuate particular magnetic characteristics, have been used extensively in transformers, synchros, torque motors, amplifiers, recording heads, and other devices where high flux densities are needed in response to low magnetizing forces; and for shielding sensitive electronic equipment from stray magnetic fields.
Unoriented 48% nickel-iron alloy has a saturation flux density of about 15000 gauss; it has seen wide use as a core material for solenoids and relays. A free-machining grade is available with essentially the same magnetic properties. A nondirectional strip modification is used for 0.004 to 0.02 in. thick laminations; its permeability is uniform in all directions. A variation of the 48% nickel-iron alloy is also produced with very high permeability parallel and perpendicular to the rolling direction; its major application is for transformer laminations.
The 79% nickel-4% molybdenum-iron alloy is available as strip for cores, toroids and laminations. Where compactness and weight are important, and for shielding applications, it can be specially processed to exhibit square hysteresis loop properties and is used in making tapewound toroids to achieve high initial permeability and low hysteresis loss. The alloy when vacuum melted, reaches a minimum permeability of 65 000 at 40 gauss.
The magnetic properties of the nickel-iron high permeability alloys are a function of strip thickness, melting procedure, chemical analysis, and freedom from contaminants such as carbon, sulphur, and oxygen that can be picked up during melting, machining or during annealing. These alloys must be annealed in an inert, dissociated ammonia or dry hydrogen atmosphere to reduce carbon, to prevent surface oxidation dueing the annealing cycle, and to promote optimum magnetic properties.
Metal structures and enclosures, whatever the metal is, tend to shield components or systems from electromagnetic and radio frequency fields, EMI/RFI. However, nonmetallic structural materials, used with increasing frequency because of their light weight, are inherently non-shielding. Engineers at Westinghouse electric corporation recently measured and evaluated several current technologies for metallizing plastic molded housings, chasses and other structural elements to provide RFI/EMI shielding.
They compared surface metallizing by flame or arc spraying of nickel, aluminum or zinc; adhesive bonded aluminum foil; nickel or aluminum-bearing paints, ion palting or vacuum metallizing of nickel, copper or aluminum; and electrodeposition or plating of nickel and copper. Copper-nickel plating was found most effective and practical and had the additional advantages of the abrasion and environmental resistance of the outer nickel layer, the continuity of the coating and the easily-established electrical ground.