Fe-Mn-Si Based Composites For Room-temperature Magnetic Refrigeration

MnFeP1xSixcompound is a new room-temperature magnetic refrigerant.This compound exhibits not only excellent room-temperature magneticcooling performance, but also possesses the advantages of simple preparation,low material cost and tunable cooling temperature range.The main subject of this paper is to study the Si substitution effect on theFe₂P-type phase formation, structure and compositions, impurity in rawmaterials, magnetic and magnetocaloric properties of MnFe（P,Si）compounds，as well as the preparation of refrigerant composites.In order to determine the specific heat of materials precisely, thespecific-heat capacity of metal gadolinium as reference was studied by usingthe differential scanning calorimetry （DSC） method. The results obtainedfrom DSC method are in good agreement with the results reported in theliteratures. On the basis of the Debye model, the lattice heat capacity isseparated. The relationship between the relative magnetic entropy andtemperature was obtained from the magnetic part of specific heat near thephase transformation point. The maximal magnetic entropy change of Gd is4.7J·mol^-1·K^-1near room temperature.In order to test if the DSC method can be used for first-order phasetransition materials, we investigated the specific heat of LaFe_9.6-xCo_xSi_1.4（x=0,0.4,0.5,0.6） compounds by using the DSC method. The structural andmagnetic properties of LaFe_9.6-xCo_xSi_1.4compounds were also reported. Theresults show that the samples crystallize in the NaZn13-type Cubic structurewith space group of Fm-3c. All compounds undergo aFerromagnetic-paramagnetic first-order phase transition. The Curietemperature gradually increases with increasing the Co content. Themagnetic-entropy change of LaFe_9.6-xCo_xSi_1.4（x=0,0.4,0.5,0.6） compound is about19.7J·kg^-1·K^-1，5.1J·kg^-1·K^-1，4.7J·kg^-1·K^-1and3.1J·kg^-1·K^-1in amagnetic field change of1.5T.In the last two chapter, we reported the structural, magnetic andmagnetocaloric properties of Mn_1.28Fe_0.67P_1-xSi_x（x=0.52,0.54,0.56,0.58）and Mn_1.28Fe_0.67P_0.48Si_x（x=0.520,0.525,0.530,0.535,0.540） compounds.The results show that the samples crystallize in the Fe₂P-type hexagonalstructure with space group of P-62m. The lattice constants and unit cellvolume is orderly with increasing the content of Si. All compoundsexperience a ferromagnetic-paramagnetic first-order phase transition. Thethermal hysteresis of the compounds fluctuates between （2.5K to6.1K,2.7K to4.1K） and the minimal thermal hysteresis about2.5K and2.7K isfound in Mn1.28Fe0.67P0.44Si0.56and Mn1.28Fe0.67P0.48Si0.54compound. The Curietemperature gradually increases with increasing the Si content. Themagnetic-entropy change of Mn1.28Fe0.67P0.48Si0.52compound is about10J·kg^-1·K^-1and9.79J·kg^-1·K^-1in a magnetic field change of1.5T. Finally, wehave successfully prepared the Fe-Mn-Si based composites forroom-temperature magnetic cooling，and did machine test in the prototypemachine built by Huang at Baotou Research Institute of Rare Earths.