Magnetocaloric Effect Of Fe₂P-type (Mn, Fe)₂(P, Ge) Compounds

Magnetic refrigeration is a cooling method based on the magnetocaloric effect (MCE), which is a versatile, energy-efficient and environmentally friendly refrigeration technology. The MnFeP1-xAsx compounds, in which the operating temperature can be tuned from about 150 K to about 335 K by adjusting the P/As ratio without losing the large MCE, is considered as one of the most promising candidates for magnetic refrigeration materials. This is not only because of its large MCE but also because of its low material cost. However, the toxic element As in the MnFeP1-xAsx compounds raises the question whether this material can be an environmentally friendly commercial material or not. Recently a surprisingly large MCE has been observed in the As partly-replaced compounds Mn1.1Fe0.9P0.7As0.3-xGex and the As all-replaced compounds Mn1.1Fe0.9P0.7-xGex at room temperature. This thesis focuses on the study of the effects of the compositions and the preparation techniques on the thermal hysteresis. The purpose is to improve the magnetocaloric effect of these materials and to determine the optimal compositions and the preparation methods.The magnetic and magnetocaloric properties of the Mn1.2Fe0.8P1-xGex compounds with x = 0.2, 0.22, 0.3, 0.4 and 0.5 was investigated in this thesis. The Curie temperatures in these compounds can be tuned by adjusting the P/Ge ratio. The magnetic moments of the Mn1.2Fe0.8P1-xGex compounds measured at 5 K and 5 T increase with increasing Ge content. The magnetic-entropy change has a maximum around 233 K for the compound with x = 0.22, which is about 19 and 31 J/kgK for a field change of 2 and 5 T, respectively.In chapter 5 of this thesis, the effects of the sample preparation on the thermal hysteresis have been studied. First the melt spinning technique was used for the preparation of the Mn0.8Fe1.2P1-xGex compounds with x = 0.1, 0.2, 0.3 and 0.4. The Mn0.8Fe1.2P0.8Ge0.2 ribbon shows a very small thermal hysteresis and a relatively large relative cooling power. Then the relationship between the thermal hysteresis and the particle size was studied, and the results show the thermal hysteresis doesn't change too much when changing the particle size of the Mn1.1Fe0.9P0.84Ge0.16 compound. But in the meantime, the virgin effect was decreased when decreasing the particle size. This is probably due to micro strains. Finally, the MnFeP0.8Ge0.2 compound quenched from different sintering temperatures shows giant MCE in a large temperature range. The maximum magnetic-entropy change is about 27 and 75 J/kgK for a field change of 2 and 5 T, respectively. The Curie temperature can be tuned by sintering at different temperatures. An Arrott-plot method was used for the confirmation that the transition is a first-order magnetic transition. Since the quenched samples show large thermal hysteresis in the range between 10 and 15 K. A measurement method, which simulates a real heat cycle, different from the previous one was used for the determination of the entropy change. The results show that the magnetic entropy change is smaller, but it is still large, than that we obtained by using the previous method.Taking into account the fact that TC in these compounds can be tuned by the variation of composition and sintering at different temperatures for certain compositions, thus we expect to have the possibility of designing composites with tunable working temperature which can operate in a wide temperature region.