Magnetic Properties And Magnetocaloric Effect Of MnCoGe-based Compounds

Magnetic refrigeration is regarded as an alternative technology to theconventional vapor compression refrigeration due to its high-energyefficiency and environmentally friendly advantages. Magneticrefrigeration is based on the magnetocaloric effect, an adiabatictemperature change of a magnetic material responding to applied fieldchange. Recently there are tremendous progresses in developing newmaterials with large MCE. MnCoGe-based alloys become one of themost potential candidate materials for room-temperature magneticrefrigeration.The main object of this thesis is to study the preparation, structuraland magnetic properties of MnCoGe-based alloys, especially the MCE inthis material system and their possible applications in room-temperaturemagnetic refrigeration. The chapters are organized as follow: The currentdevelopment of room-temperature magnetic refrigeration, thefundamental aspects of magnetocaloric effect, the experimental methodsused in this study were introduced in the first three chapters,respectively.A detailed study on the crystal structure, magnetic propertiesand specific-heat capacity of MnCo_1-xAl_xGe （x=0.00,0.05,0.10,0.15,0.20） alloys was given in Chapter4. The samples were prepared by usingarc-melting in an argon gas flow, followed by annealing in evacuated quartz tube at1173K for148h, and furnace cooled to room temperature.A suitable annealing temperature is crucial for obtaining high MCE.In order to improve the MCE in MnCo_1-xAl_xGe （x=0.00,0.05,0.10,0.15,0.20） alloys, B introduction to the lattice and its effects on thecrystal structure and magnetic properties of MnCo_0.92Al_0.08GeB_x（x=0.00,0.02,0.04,0.06,0.08,0.10,0.20） alloys were reported in Chapter5. Asmall amount of interstitial B introduction into the lattice does not changethe crystal structure of the alloys, but it has positive effect on the increaseof magnetic entropy. A study on the different preparation methods andtheir effects on the crystal structure and magnetic properties of differentprocess of MnCoGeB_x（x=0.02,0.04） alloys was presented in Chapter6.Arc-melting and ball milling were used for the preparation of the alloys.In order to avoid the evaporation loss in the arc-melting process, a ballmilling process was used. In ball milling process Co element tend to stickto the milling tank and balls, so add a small amount of stearic acid toprevent the adhesion of the Co element. The study showed that the crystalstructure of alloys obtained by melting and milling method iscompletely different. The study shows that the magnetocaloric propertiesof the MnCoGe-based alloys can be improved by introducing B to thelattice or by different way of preparation.