Nanocrystalline Soft Magnetic Alloys for Space Applications
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Nanocrystalline Soft Magnetic Alloys for Space Applications Matthew A. Willard, Thomas Francavilla1, Ramasis Goswami1, and Vincent G. Harris2 U. S. Naval Research Laboratory, Code 6340, Washington, DC 20375, U.S.A. 1 GeoCenters Inc., Arlington, VA 22201, U.S.A. 2 Electrical and Computer Engineering Department, Northeastern University, Boston, MA 02115, U.S.A. ABSTRACT This study focuses on a Co-based nanocrystalline alloy (Co84.55Fe4.45Zr7B4) with potential for long-term high temperature use. As an indication of their performance, core losses were measured on toroidal samples using a Walker AC permeameter over a frequency range of 0.1 to 500 kHz, at induction amplitudes of 100, 300, and 500 mT, and temperatures from 22 to 300°C. For a given frequency and maximum induction amplitude, the losses were invariant as a function of measurement temperature. Vibrating sample magnetometry provided the magnetization and hysteretic losses as a function of temperature. As the temperature of the alloy was raised to 300°C from room temperature, the saturation magnetization (120 emu/g)was reduced by less than 15%. A toroid was aged at 300°C for up to 300 hours and core loss measured as a function of aging time at the previously mentioned frequencies and induction amplitudes. The losses were invariant over the aging time. INTRODUCTION Flywheel and magnetic bearing systems require soft magnetic materials possessing high strength and low eddy current and hysteretic losses. Although amorphous alloy magnets exhibit good loss characteristics, their long-term use at elevated temperatures, near 300 to 400°C, degrades their properties. In recent years, nanocrystalline materials with exceptional soft magnetic properties have been developed with potential use for such applications. Nanocrystalline soft magnetic materials possess higher magnetization and operation temperatures than amorphous alloys with similar compositions. Recently, nanocrystalline alloys with the nominal composition (Fe,Co,Ni)89Zr7B4 have been produced by a single wheel melt spinning technique, followed by an isothermal anneal above the primary crystallization temperature.[1,2] The resulting materials are multi-phase with residual amorphous phase between nanocrystallites. The high frequency response of these materials is aided by the multiphase microstructure, primarily by the reduction of eddy current losses by the high resistivity of the residual amorphous phase. The use of soft magnetic materials in space applications in many cases requires operation at temperatures of 300°C for extended periods of time.[3] Lengthy operation at elevated temperatures has caused degradation of amorphous alloys. As an alternative to the use of amorphous materials in the frequency range from 1 to 100 kHz, the present study examines the losses of a nanocrystalline soft magnetic alloy, aged at 300°C for up to 300 hours. The following sections will explore the losses and microstructure of these alloys under these conditions.
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EXPERIMENT An arc melter was used to form an ingot
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