Inverse Magnetocaloric Effect In NaZn₁₃-type LaFeSi And NiMnSn Heusler Alloys

In recent years, much attention has been paid to the energy demand and environment resource depletion. With the extensive use of refrigeration and air conditioning, the refrigeration technology has become an indispensable aspect. However, traditional vapor compression/expansion refrigeration technology has a lot of disadvantages, such as low conversion efficiency of vapor compressor, and generation of gas depletion ozonosphere and greenhouse effect gas, etc. While, magnetic refrigeration is a new cooling technology with huge potential application prospect. Therefore, magnetic refrigeration materials, the key materials of the magnetic refrigeration technology, become a very important topic.In this thesis, we simply introduce the research background of the magnetic refrigeration technology, such as the principle of the magnetic refrigeration technology, the development course, the research status of the magnetic refrigeration materials research at first. Then, we present the preparation, microstructure analysis, magnetic and magnetocaloric properties of some useful magnetic refrigeration materials. The detailed investigation results were summarized as follows:Firstly, the Ni43Mn46Sn11Six (x=0.0-0.3) Heusler alloys were prepared by arc melting method at high vacuum. The alloys were annealed at1173K for3days and subsequently quenched in cooling water. The cubic single phase structure was obtained in the alloys. It is revealed that the Si doping gives rise to a slight decrease of magnetic entropy change, while the phase transition temperature shifts in a wide range.Secondly, La0.8-xMxCe0.2Fe11.4Si1.6(M=Al, Mg) compounds were also prepared by arc melting method at high vacuum. The alloys were annealed at1373K for5days and subsequently quenched in cooling water. The results demonstrate that the incorporation of M atoms is unfavorable for the formation of the NaZn13-type phase, however, the thermal hysteresis and the hysteresis loss are dramatically reduced. Substitution of Al for La in the alloys results in an increase of the Curie temperature while the magnetic entropy change reduces. On the other hand, replacing La with Mg leads to a significant improvement of the magnetic entropy change while keeping the Curie temperature almost unchanged.Thirdly, we compared the effect of preparation on the magnetocaloric properties of La0.8-xMgxCe0.2Fe11.4Si1.6alloys by use of different sample preparation methods. One is arc-melting, the other is induction melting. The precursor ingots were annealed at1373K for5days and subsequently quenched in cooling water. The magnetocaloric properties between the arc-melting samples and the induction samples were compared. It is demonstrrated that the spike-like spurious peak in the S-T curve of the alloys can be alleviated by the induction melting method.Finally, we investigate the effect of measurement method on real value of the magnetic entropy change by different methods for La0.8-xMgxCe0.2Fe11.4Si1.6alloys. Three magnetic measurements, normal isothermal measurement (ITN), another isothermal loop process (ITL), and an iosfield measurement process (IFP) were adopted. It is revealed that the "colossal" peaks of the entropy change arise not from the wrong application of the Maxwell relation, but from the incorrect probing of the phase transition.