MAGNETOCALORIC PROPERTIES OF THE STACKED Ni_(50)Mn_(18.75-x)Cu_(6.25+x)Ga_(25) (x = 0÷1) POLYCRYSTALLINE ALLOYS

摘要

Nowadays, much effort is devoted worldwide to the development of refrigerators utilizing the magnetocaloric effect (MCE). The MCE is observed in the course of heating and cooling of the material during the successive cycles of magnetization and demagnetization of materials exhibiting magnetic transitions, i.e. ferromagnetic - paramagnetic transition at the Curie temperature. To this group belong Heusler phase based Ni2MnGa alloys. In our paper the effect of stacking of NiMnCuGa alloys, having various compositions, was studied towards achieving a broader temperature range of the relatively high magnetocaloric properties. Three stacked alloys, with 6.25, 6.75 and 7.25 at. % of Cu content exhibit a broad spectrum, over 25°C, of the magnetic entropy change (ΔS), calculated on a basis of thermomagnetic curves recorded with 200 kA/m step over the 0-1200 kA/m range. A single alloy, having 6.25 at. % Cu, exhibits high value of the magnetic entropy change, approx. 12 J/kgK, however, the change occurs within a narrow temperature range of a few centigrades. Calculations of the relative cooling power (RCP) revealed the value of 62.5 J/kg for the stacked alloys, over a broad temperature range of 25°C, which exceeds the respective RCP for the single alloy – 46.5 J/kg. Although the maximum value of the ΔS is three times lower than for the single alloy the direct measurement of the adiabatic temperature change (ΔTad) revealed the constant value of the ΔTad equal to 0.4 °C in the temperature range from 35 to 65 °C. NiMnCuGa alloys package seems to be an interesting alternative to expensive alloys based on gadolinium or lanthanum. The temperature change can be relatively easily controlled with the chemical composition and/or the heat treatment of the alloys thereby increasing the operating temperature range of the devices made of them. The alloys described are also simple to prepare by casting and heat treatment.