Magnetocaloric Effect Of The CoMnSb Half-Heusler Alloys At Room Temperature

The room-temperature magnetic refrigerant has drawn an increasing attention in recent years. The traditional vapor-compression refrigeration technology presents some defects difficult to be overcome, such as destroying the environment and low cooling efficiency. The magnetic refrigeration technology is a viable alternative technique since it shows some advantages, such as high cooling efficiency, energy conservation, not releasing any harmful gas to deplete ozonosphere and aggravate greenhouse effect, etc. As a green technology, it takes advantage of magnetocaloric effect of magnetic materials, get cooling by means of magnetization and demagnetization process. Therefore, the magnetic refrigeration technology with solid magnetic material instead of other working substance (e.g. gas) is a huge promising candidate in practical application.CoMnSb Heusler alloy system presents high magnetic entropy change over room temperature. Since the magnetocaloric effect and the Curie temperature of alloys are sensitive to the compositions, it’s possible to obtain a large room temperature magnetic entropy change if the alloy compositions are carefully designed. Therefore in this thesis, we emphasize on the composition design, Curie temperature and the magnetocaloric properties of CoMnSb alloy system. It is revealed that the CoMnSb alloys show high magnetic entropy change and the Curie temperature around room temperature for the alloys with the appropriate compositions. The detailed results were summarized as follows:(1) Cox(MnSb)1+x (x=0.07,0.15,0.24) alloy system.The ingots with the nominal compositions above were melted by high vacuum arc melting furnace, then annealed at873K for30h and subsequently quenched in cooling water. The Cox(MnSb)1-x (x=0.07,0.15,0.24) alloys are composed of the major phase of Mn1.09Sb and minor phases of CoSb3. When x=0.24, an additional phase of CoMnSb alloy appears. It is revealed that the Curie temperature TC and the magnetic entropy change△SM are sensitive to the Co content x. When x=0.15, the MCE of Co1.15(MnSb)0.85alloy is optimal with△SM=1.8J/kg.K at324K under an applied magnetic field of3T. A second-order phase transformation occurs around TC, and the magnetic hysteresis loss thermal lag is negligible. These features demonstrate that Co0.15(MnSb)0.85alloys are promising room-temperature magnetocaloric materials.(2)Co0.4Mn1.5Sb1.1compound.The Coo.4Mn1.5Sb1.1compound was prepared by arc melting furnace under high vacuum, then annealed at873K for30h and subsequently quenched in cooling water. The microstructure of the compound contains the major phases of CoMnSb and Mn1.09Sb and minor mixed phases of CoSb3, Mn3Co7and CoSb2. The magnetocaloric properties indicates that the maximum value of△SM achieves2.4J/kg.K around320K in an applied magnetic field of3T. The Arrott plots showed a second-order transition around Curie temperature for the Coo.4Mn1.5Sb1.1alloy. Therefore, no thermal lag was observed in magnetization v.s. temperature curve. The result suggests the potential application of the Co0.4Mn1.5Sb1.1compound near room temperature.(3) Seeking the appropriate Co-Mn-Sb compositions for optimization of magnetic entropy change.In order to find out the optimal Co-Mn-Sb composition region, we investigated a large amount of alloy compositions according to the Co-Mn-Sb element triangle. Each compound was melted by arc melting furnace under high vacuum, then annealed at873K for30h and subsequently quenched in cooling water. The composition dependence of TC and magnetic entropy change were measured by vibrating sample magnetometer (Versalab free, Quantum Design Co.). The3D maps and contour plot of Curie temperature and magnetic entropy change in this triangle were plotted. It is revealed that the significant magnetocaloric effect with Curie temperature around room temperature exist in a certain Co-Mn-Sb composition region.