Magnetocaloric Improvements in Doped Heusler Alloys
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Magnetocaloric Improvements in Doped Heusler Alloys Michael V. McLeod1, Bhaskar S. Majumdar1, and Zafer Turgut2 1 Materials Department, New Mexico Institute of Mining and Technology, Socorro, NM 87801 2 Air Force Research Laboratory, Wright-Patterson AFB, OH 45433 USA ABSTRACT Magnetocaloric materials have gained significant interest as an environment friendly and efficient refrigeration technology. We have been working on Heusler alloys, primarily the NiMn-Ga system for improved magnetocaloric effect (MCE). We have observed significant MCE increase in stress assisted thermally cycled samples and demonstrated that one primary mechanism is texture change. More recently, we have observed volumetric decrease as well as anisotropy changes due to stressed cycling. The influence of these parameters on magnetic anisotropy and MCE are discussed. We have also utilized isoelectronic Al substitution at the Ga lattice sites for optimizing atomic distances and exchange interactions in an effort to bring about magnetostructural transformation closer to room temperature (RT) while retaining high MCE. The results show that Al substitutions can bring about large decreases (50 – 70K) in both the Curie (TC) and martensite start (Ms) temperatures, and permit fine tuning the magnetostructural transformation temperature. INTRODUCTION Solid state magnetic refrigeration offers drastic reduction in environmental pollution from common day compressors, higher Carnot efficiencies, and a decrease in power consumption. Given that up to 40% of non-automotive energy is consumed by air conditioners and refrigeration, the payoffs can be significant [1, 2]. The materials that make solid state magnetic refrigeration possible are those that exhibit a giant magnetocaloric effect (MCE) due to coupled structural and magnetic transformations, i.e., a magnetostructural transformation [2-4]. Some of these compounds are, but are not limited to, Gd5(SiGe)2 [5, 6], lanthanum based such as LaFeSi [7], manganese based intermetallics, like MnFePGe [8], and Heusler based intermetallics such as non-stoichiometric Ni2MnGa, Ni2MnIn, Ni2MnSn, etc.. In recent articles we have reported our findings on the influence of thermomechanical processing on the change in magnetic entropy (SM) of Ni2MnGa alloys, particularly of the nonstoichiometric Ni2+xMn1-xGa variety [4, 9]. Based on the concept of stress induced texture in shape memory alloys [10], we demonstrated that the SM of an alloy with x=0.16 increased by about 75% through stress assisted thermal cycling (SATC) between the fully austenite and martensite temperatures [4, 9]. These results suggest that a possible approach in the enhancement of MCE would be the utilization of novel processing techniques. This is especially important because alloying techniques, at least in Heusler alloys, have reached a level of saturation. In [4], we showed that the primary mechanism for the enhancement of the MCE in nonstoichiometric Ni2+xMn1-xGa alloys through SATC treatment was the establishment of preferred orientation of the marten
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