NdFeB Magnets with Improved Temperature Characteristics
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NdFeB MAGNETS WITH IMPROVED TEMPERATURE CHARACTERISTICS B. M. Ma Crucible Research Center, P. 0. Box 88, Pittsburgh, PA 15230 ABSTRACT The temperature dependence of magnetic properties for NdFeCoB alloys were investigated. It is well known that dysprosium substitution for neodymium increases the intrinsic coercivity. Aluminum also improves the intrinsic coercivity, Hci, at room temperature but impairs the Curie temperature and the intrinsic coercivity at elevated temperatures. The rate of increase of the Hci with increasing dysprosium in the NdFeCoB system is less than when dysprosium is added to the NdFeB system. A combination of aluminum and dysprosium is more effective than aluminum alone in raising Hci. Molybdenum addition into the NdDyFeCoB alloy was found to reduce the temperature coefficients of Br and Hci. Temperature coefficients of Br and Hci obtained on a NdloDy 5 Fe67ColoMolAllB6 alloy are -0.07 %/°C and -0.4 %/OC respectively. INTRODUCTION NdFeB permanent magnets exhibit excellent hard magnetic characteristics at room temperature [1]. Unfortunately, because of the low Curie temperature and high temperature coefficients of Br and Hci, operation is restricted to a maximum temperature of about 100 0 C. Improvements are required in order for this material to be used at elevated temperatures. Curie temperature, temperature coefficients of Br and Hci, and the energy product must all be improved in order to increase the maximum operating temperature of these magnets. There are two approaches one can take to improve the temperature dependence of induction. One can either raise the Curie temperature, for example, by substituting Co for Fe, or one can compensate for the rapid decrease of the Nd moment with increasing temperature by alloying with heavy rare earths [2, 3]. Unfortunately, both of these approaches negatively affect other characteristics of the magnet. Cobalt additions tend to reduce the intrinsic coercivity, while heavy rare earths couple antiferromagnetically with the iron and reduce the saturation magnetization. Other alloy additions also result in competing positive and negative effects on the magnetic properties. Alloying elements such as aluminum and molybdenum are found to increase the room temperature intrinsic coercivity of NdFeB magnets. Unfortunately, the Curie temperature drops when either aluminum or molybdenum is added to the alloy. Additionally, the intrinsic coercivity at elevated temperature is significantly reduced by aluminum additions. The combined effects of Al, Mo, Dy and Co additions were investigated. The results are reported in this paper. EXPERIMENTAL PROCEDURES Alloys were prepared by vacuum induction melting or arc melting under an argon atmosphere and processed into magnets by the traditional powder metallurgy (P/M) method [4]. Samples used to measure the Curie temperature, the saturation magnetization, and the temperature dependence of magnetization were prepared by crushing the alloy to -400 mesh, placing the powder in argon filled Vycor capsules and sealing. The Curie
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