The promising room-temperature magnetic refrigeration materials — Misch-metal (MM)₂Fe_17-xSi_x (x=0–1.5) compounds

The phase structure, magnetic properties, magnetocaloric effect (MCE) and relative cooling power (RCP) of MM₂Fe_17-xSi_x (x = 0–1.5) compounds were studied. The phase transition temperature was increased from 238 K to 366 K with the increase of Si content. The X-ray diffraction (XRD) patterns indicate that the MM₂Fe_17-xSi_x (x = 0–1.5) compounds crystallize primarily in a rhombohedral phase with Th₂Zn₁₇ type crystal structure coexisting with a small amount of the precipitation α-Fe phase. These results are reinforced by backscattered scanning electron microscope (SEM) images and electron energy dispersive X-ray spectroscopy (EDX) analysis. The maximum values of magnetic entropy change (ΔS_M) for MM₂Fe_17-xSi_x (x = 0, 0.3, 0.6, 1.0) compounds are −2.1 J kg⁻¹K⁻¹, −1.3 J kg⁻¹K⁻¹, −1.5 J kg⁻¹K⁻¹, −1.8 J kg⁻¹K⁻¹ for field change of 0–2 T and −4.0 J kg⁻¹K⁻¹, −2.7 J kg⁻¹K⁻¹, −3.1 J kg⁻¹K⁻¹, −3.5 J kg⁻¹K⁻¹ for field change of 0–5 T. The estimated RCP values are larger than 100 J kg^-1 and 350 J kg^-1 for the field change of 0–2 T and 0–5 T, respectively. The working temperature range δT_FWHM is larger than 50 K and 100 K with the field changes of 0–2 T and 0–5 T, respectively. The temperature range and RCP values are comparable to the second-order phase transition (SOPT) materials R₂Fe₁₇ (R = Pr, Nd) and Gd, but the cost of MM₂Fe_17-xSi_x is only about 25% and 10% compared with R₂Fe₁₇ (R = Pr, Nd) and Gd, respectively.