| Gd exhibits giant magnetocaloric effect at room temperatures. Perfect toughness and workability makes it to be one of the most important magnetic refrigerant materials at room temperature. It was widely used in magnetic refrigerators. The valence electrons are scattered by magnetism in magnetic materials, so that the resistance to the migration of valence electrons are usually high. As a result, the heat-transfer capability of magnetic materials is poor in common. It can't exchange the heat with the fluid in time when Gd used as a magnetic refrigerant while the magnetic refrigerators was working. So, this drawback affects the availability and power of magnetic refrigerant machines.In order to solve the problem of poor heat conductor preferably, this paper discusses how to join metals with high heat conductivity, namely Cu or Al, with Gd . Firstly, we analyzed that composites not only exhibit higher heat-teansfer capability than Gd, also can save the Gd metal. According to the composites model, we deduced heat-transfer efficiency compared with Gd, and calculated the heat-transfer efficiency with different ratio of thickness of components in composites. The maximum of heat-transfer efficiencies under the best demensional scale ratio of the components were analyzed. This made the theoretical foundation for experiments.The processes that Gd substrates were bonded with Cu and Al separately by means of vacuum diffusion welding technique are introduced in this paper. According to orthogonal experimental design, the results indicate the best poccessing parameters to produce Gd-Cu composites is 600°C under pressure 16MPa 120minutes, and that of Gd-Al is 600°C under pressure 12.1MPa for 30minutes. All properties related to bonded interfaces were tested by SEM, XRD, EDX, microhardness test and home made experimental devices. And then the outcomes show that vacuum diffusion bonding is a feasible method.The research results show that diffusion bonding can easily achieve the linkage between Gd and Cu,Al although they possess large differences in their own characters. Perfect interfaces of Gd-Cu and Gd-Al were achieved and no defects were found such as seam or cavity both in macroscopy and microscopy. X ray diffraction test results show no intermetallics compounds existed in the interfaces of the composites and the bond strengths between two metals were satisfactory. So it can meet the basic requirements of bonding techniques without changing the magnetic property of Gd. Because of the difference of bonded model between two composites, the bonding strength of Al-Gd is higher than that of Cu-Gd. As far as Cu, the trend of bond strength of composites doesn't increase purely with increasing of holding time, there is a maximum value when the time increases at 120 min. After that, the bonding strength decrease with the holding time extent. The diffusion rate of three kinds of atoms are different. The volume of Gd is bigger than those of the other two, so the main diffusions happened Cu to Gd or Al to Gd during bonding processes. Due to Cu as diamagnetic material, Al as paramagnetic substance, the small quantity of Cu or Al in the composites rarely influence the magnetic properties of the composites. the magnetocaloric property of the composites were improved in some extent, that is, the Curie temperatures of the composites are a little higher than that of Gd, thus the composites are more suitable used in magnetic refrigerators at room temperatures.This work possesses characteristics of exploration and innovation at some extent. The results of this experimental provide a valid method to produce magnetic refrigerants with high coefficient of heat transfer used in magnetic refrigerators.
|
PDF Full Text Request