| Magnetic refrigeration has a bright prospect for replacing the conventional vapor compression technology as a high-tech green refrigeration technology. Since the end of the 1990 s, the magnetic refrigeration technology has been developed rapidly, and plays an important role in the aerospace field. While in the civil field, the magnetic refrigeration technology also has been a hotspot for many countries and companies. At present, the main enterprise and companies about refrigeration technology are American GE company, Germany BASF company, France Cooltech company, Korean Samsung company and Chinese Haier company. In recent years, Mn-based compounds become a promising material for magnetic refrigeration, which exhibit giant magnetocaloric effect(MCE), such as Mn Fe PAs, Mn As and so on. In 2009, Paduani et al. found a large magnetization change of Mn1-xFe1-yAlx+y alloys near room temperature region. Most recently, Wang et al studied on the MCE in Mn42Fe50-xAl8+x alloys. The Mn Fe Al alloys have the advantages of simple preparation, raw materials are easy to be obtained. In this thesis, we report the structural properties, magnetic properties and magnetocaloric effects in Mn50-xFe10+xAl40 and Mn38Fe22Al40(B,C)y alloys. The main results of this thesis are as follows:The preparation and measurement of samples: the starting materials of Mn(purity 99.9%), Fe(99.9%), Al(99.9%), Fe B(B content with 19.52 %) andC(99.9%) mixed with required ratio and melted in arc melting furnace. The obtained samples are annealed and quenched. After that, the structure and magnetic measurements are performed.(1) A detailed analysis of XRD results shows Mn50-xFe10+xAl40 alloys with x = 8, 12, 16 and 20 crystallize in the Cs Cl(B2)-type cubic structure(space group Pm-3m). Magnetization measurement results show the decrease of Curie temperature when the Mn atoms were substituted by Fe atoms. We also found that the Curie temperature of Mn38Fe22Al40 alloy is near room temperature and the magnetic-entropy change is obvious(2) The XRD results show that Mn38Fe22Al40-xBx alloys with x = 0, 1, 2, 3 and 4 alloys crystallize in the Cs Cl(B2)-type cubic structure(space group Pm-3m). Boron atoms occupy interstitial positions in Mn38Fe22Al39 B, Mn38Fe22Al38B2 alloys and play a important role. However, the boron addition slightly changes magnetic-entropy change, but the maximal magnetic-entropy change is about 0.67 J/kg K for x = 1 for a field change of 1.5 T at 292 K. This work confirmed that the atoms in interstitial positions have positive effect on the improvement of magnetic-entropy change of Mn Fe Al alloy.(3) The boron introduction in Mn38Fe22Al40 alloy has strong effects on the structure and magnetocaloric effect in Mn38Fe22Al40 Bx alloys with x = 0, 0.2, 0.4, 0.6, 0.8 and 1.0. The Curie temperature increases when boron atoms enter the interstitial positions(low boron content, x < 0.4), decreases when boron atoms occupy the substitution positions(high boron content, x ≥ 0.4). Borondoping in Mn38Fe22Al40 alloy enhances the saturation magnetization and therefore, an increase of the magnetic entropy change △Sm as well.(4) The carbon doping effect on the structural and magnetocaloric effect in Mn38Fe22Al40 Cx alloys with x = 0, 0.1, 0.3, 0.5, 0.7, 1.5 and 2.5 were investigated. XRD study shows the alloys with x = 0, 0.1, 0.3, 0.5 and 0.7 crystallize in a Cs Cl(B2)-type cubic structure(space group Pm-3m) and the alloys with 1.5 and 2.5 crystallize in(Fe/Mn)3Al(DO3)-type structure with space group Fm-3m, and there is an additional phase α-Mn. When the value of x is from 0 to 0.7, carbon atoms occupy the interstitial positions and carbon addition makes the effective exchange interaction weaker so that it leads to the decrease of Curie temperature. However, the carbon addition with 1.5 and 2.5 makes the Curie temperature and magnetic moment decrease obviously, it is also found that the maximal magnetic-entropy change of carbon addition alloy with x = 0.1 is about 0.71 J/kg K at 297 K(magnetic field change from 0 to 1.5 T). When x = 0.1(annealed 35h), the maximal magnetic-entropy change is about 0.78 J/kg K at 305 K(magnetic field change from 0 to 1.5 T). |
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