Preparation, Structure And Magnetocaloric Effect Of Gd-Co Based Amorphous Ribbons And La_1-xCe_x（Fe,Co,Si）₁₃ Compounds

Magnetic refrigeration technology plays an important role in the science and technologyprogress and social development. Due to the needs of the energy crisis, energy conservationand emissions reduction, the development of energy conservation and environmentallyfriendly refrigeration technology has become the technological focus in the world. Magneticrefrigeration technology, based on the magnetocaloric effect of materials, has advantages ofno environmental pollution, high efficiency and energy saving. So far, it has gotten very widerange of applications at low temperature. With the development of technology, magneticrefrigeration is developing gradually to the high temperature applications. Magnetocaloricmaterial is the key to the room temperature magnetic refrigeration technology.Magnetic properties and magnetocaloric effect of Gd₅₉Co₄₁and Gd₅₆Co₄₄melt-spunribbons were studied at first in this thesis work. The influences of crystallization treatment onthe structure, magnetic properties and magnetocaloric effects of Gd₅₉Co₄₁and Gd₅₆Co₄₄ribbons were studied. A small amount of B, Nb and Ge elements were added to adjust themagnetic properties and magnetocaloric effect of Gd₄Co₃amorphous alloys. Aftercrystallization treatment, the influences of doping elements on the structure and the change ofmagnetocaloric effect of these ternary alloys were investigated. The critical behavior of（Gd₄Co₃）100-xGex（x=10,15） amorphous alloys around the Curie temperatures was studied bythermodynamics theory. Finally, La_1-xCex(Fe_0.92Co_0.08)_11.4S_1.6（x=0,0.1,0.3,0.5） ribbons wereprepared by melt-spinning followed by a short time heat treatment, the influences of Ce dopingon the structure, magnetic property and magnetocaloric effect of La(Fe_0.92Co_0.08)_11.4S_1.6compound were studied. For the perspective applications of magnetic refrigeration technology,the preparation method for the bulk materials of this series of alloys were also investigated.The results show that large values of （ΔSM） can be obtained in the amorphous Gd₅₉Co₄₁and Gd₅₆Co₄₄alloys under a low applied field. The Gd₄Co₃phase was homogeniouslyprecipitated after complete crystallization process. Compared with the Gd₅₉Co₄₁and Gd₅₆Co₄₄amorphous alloys, both the saturation magnetization and the maximum isothermal magnetic entropy change of Gd₅₉Co₄₁and Gd₅₆Co₄₄crystalline alloys decreased.（Gd₄Co₃）100-xMx（M=B, Nb, Ge; x=0,5,10,15） amorphous alloys order ferromagneticallyand they undergo the second-order transition at their Curie temperatures. The glass-formingability of （Gd₄Co₃）100-xBxseries of alloys was promoted by B doping, and their Curietemperatures decreased with the increase of B content. Under an applied field change from0to5T, the maximum value of （-ΔSM） for （Gd₄Co₃）85B15amorphous alloy was7.8J kg-1K-1, andlarge value of the refrigerant capacity （RC） was also obtained. For the （Gd₄Co₃）100-xNbxamorphous alloys, the Curie temperatures had no obvious change and the maximum isothermalmagnetic entropy change decreased slightly with the increase of Nb content. The criticalexponent values obtained for （Gd₄Co₃）100-xGex（x=10,15） amorphous alloys were approachedto the values predicted by the mean-field model, which indicated that there are somenanocrystals formed in the melt-spun samples with long-range magnetic coupling.After crystallization, the H_o4Co₃-type hexagonal phase was obtained for crystallized（Gd₄Co₃）100-xMx（M=B, Nb, Ge） alloys. In contrast with the （Gd₄Co₃）100-xMxamorphous alloys,the Curie temperatures increased and the maximum values of （ΔSM） decreased for thecrystalline alloys.With the increase of Ce content from x=0to x=0.5, the lattice constant ofLa_1-xCex(Fe_0.92Co_0.08)_11.4S_1.6alloys decreased and the Curie temperature reduced from323K to286K. All the alloys order ferromagnetically and undergo the second-order transition aroundtheir Curie temperatures. The results show that after substituting Ce for part of La inLa_1-xCex(Fe_0.92Co_0.08)_11.4S_1.6alloys, the cubic NaZn13-type structure phase formed more easilyand the content of α-Fe phase reduced significantly. However, the substitution of Ce has aoptimum value. The magnetic entropy change increases greatly with Ce substitution increasesat the beginning and the other impurity phases appear when Ce substitution is up to0.5, and, asa result, the magnetic entropy change decreases.The heat treatment time has an important impact on the structure and magnetic entropychange of La0.7Ce0.3(Fe_0.92Co_0.08)_11.4S_1.6melt-spun ribbons. The melt-spun ribbons wereannealed at1000℃for1,3and5h, respectively. It was found that the cubic NaZn13-type structure phase formed and α-Fe phase reduced significantly with prolonging the annealingtime. The optimized properties were obtained when the sample was annealed at1000for3h.While the La-rich and Fe-rich phases appeared in the melt spun ribbons with extending theannealing time （>3h）, the Curie temperature and the magnetic entropy change decreased.From the perspective application of magnetic refrigeration technology,La0.7Ce0.3(Fe_0.92Co_0.08)_11.4S_1.6bulk material was prepared from melt-spun powders by warmcompaction followed by annealed at1000℃for3h. The structure and magnetic entropy changeof the bulk were studied. The results show that the warm compaction alloy consists ofNaZn13-type structure phase as the main phase and a small amount of α-Fe and FeSi₂phase asthe impurity phases. Compared to melt-spun ribbons, the compaction has more impurity phasesand shows lower magnetocaloric effect. In a word, compaction process needs to be improved,the melt-spun ribbons with short-time heat treatment shows good magnetocaloric performance.