اثر آلايش نقره (Ag) و كروم (Cr) بر ويژگي هاي ساختاري و مغناطوگرمايي آلياژهاي حافظه پذير فرومغناطيس Ni-Mn-Sn

Magnetic refrigeration based on the magnetocaloric effect has emerged as a clean an‎d energy-efficient alternative to conventional vapor-compression cooling, an‎d ferromagnetic shape-memory Ni-Mn-Sn Heusler alloys are attractive can‎didates in this field owing to the strong coupling between their magnetic an‎d structural degrees of freedom an‎d the low cost of their constituent elements. A survey of the literature shows that substitution of silver an‎d chromi-um at the Sn site of the Ni₄₇Mn₄₀Sn₁₃ compound has not been previously investigated; ac-cordingly, two alloy series, Ni₄₇Mn₄₀Sn₁₃₋ₓAgₓ an‎d Ni₄₇Mn₄₀Sn₁₃₋ₓCrₓ, were prepared by me-chanical alloying an‎d characterized after annealing an‎d rapid quenching. The crystal structure was examined by X-ray diffraction an‎d Rietveld refinement, the magnetic an‎d magnetocalor-ic behavior was studied using a vibrating-sample magnetometer, an‎d the magnetic entropy change was calculated from magnetization–temperature curves using the Maxwell relations. The results showed that samples with low doping levels are single-phase L2₁ ordered cubic austenite, an‎d that with increasing concentration an orthorhombic Pmma martensite phase appears at room temperature; the orthorhombic distortion is more pronounced in the chromi-um series, consistent with the smaller atomic radius of chromium. Substitution at the Sn site shifts the structural transition temperature strongly toward room temperature, rising from 190 K in the undoped sample to 268 K in Ag0.5 an‎d 274 K in Cr0.75. Comparison of the two series indicated that the valence-electron concentration alone is not a complete descriptor of the transition behavior an‎d that the difference in electronic configuration of the doping ele-ments may play an independent role. The divergence of the ZFC an‎d FC branches at low temperature indicates magnetic irreversibility in the martensite phase. In the magnetocaloric study, the Ag0.5 sample exhibited a maximum entropy change of 16.19 J/kg·K at 5 T with a peak at 281 K, more than twice that of the undoped sample, while the Cr0.75 sample showed the highest refrigerant capacity of 104.6 J/kg. The data revealed no simple correlation be-tween the field-induced shift rate of the transition temperature an‎d the magnitude of the mag-netocaloric response; rather, the rate of change of magnetization with temperature, which also depends on the width of the transition interval, plays a decisive role. These results indicate that controlled substitution of silver an‎d chromium at the Sn site can be an effective route for simultaneously tuning the martensitic transition temperature an‎d the magnetocaloric response of Ni-Mn-Sn alloys.

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حسين احمدوند

اسماعيل عبدالحسيني سارسري , حميده شاكري پور