The Crystallography, Magnetic, Magnetocaloric Properties And Strengthing Of Gd-based Solid Solution Alloys

In this paper, the recent development in the exploration of new magnetic refrigerating materials near room temperature was summarized. An evaluation about magnetocaloric effect of those materials was made based on the thermo-dynamic principles as well as the requirement of magnetic refrigerating system. Accordingly, as a conclusion in comparison with intermetallic compounds, pure metal Gd as magnetic refrigerant had many advantages. However, pure Gd could not meet the demand for refrigerating system because of its poor strength, so it was necessary to modify pure metal Gd. Solid-solution treatment by adding other alloying elements was one way to strengthing Gd. Two aims were proposed in this study. One was optimizing the chemical composition of Gd-based alloy with not only large magnetocaloric effect but also enough strength. Another was to explore some problems such as whether the alloying elements influenced the crystallography, magnetic and magnetocaloric properties of Gd, and what the relationship between the change of magnetic properties of materials and the change of electronic structuremagnetic properties and magnetocaloric effect of Gd-based alloys with Superconducting Quantum Interference Devices (SQUID) and a direct MCE measurement apparatus. Another was the detection of the crystal structure and electronic structure of Gd-based alloys by XRD and XPS. The third work was the measurement of the microhardness of the materials performed on HX-200 Vickers hardness meter. Finally, the following important results were obtained:1 Seven series of Gd-based alloys were designed. The effects of addition of La, Ce, Pr, Nd, C, Y or Dy on magnetocaloric effect and strength of Gd were systematically investigated and the chemical composition for Gd-based alloy with high strength and large magnetocaloric effect comparable to that for Gd was determined, which had great engineering significance.2 The influence of the addition of La, Ce, Pr, Nd, C, Y or Dy on the crystallography and magnetic property of Gd was systematically studied.In crystallography, the results showed that in Gd-based binary solid solution, La,Y or Dy atoms was randomly distributed in crystal lattice, but Ce, Pr, Nd or C was segregated on the (101) crystal plane of Gd. When the amount of La, Ce, Pr, Nd or C reached 5at%, the c/a values of the corresponding binary alloys increased obviously and the c/a value of Gd1-xNdx alloy was the largest. For alloys adding C, Y or Dy, the c/a values decreased with the addition of the alloying element.In magnetic properties, the results showed that except Dy, the addition of all other alloying elements could decrease the specific saturation magnetization of Gd-based binary alloy. The Curie temperatures of alloys adding La, Ce, Pr or Nd decreased fast with the increase of the alloying elements, but the Curie temperature of alloys adding C, Y or Dy decreased slowly with the increase of the alloying elements.The explanation about the effect of the addition of alloying element on the Curie temperature of Gd-based alloys was made based on the Molecular- Field theory and RKKY interaction theory. The reason why the addition of alloying element decreased the Curie temperature of the alloy is thought to origin from the decrease of de Gennes factor whose magnitude reflects the intensity of magneticinteraction between magnetic atoms3 The effects of different alloying elements on isothermal magnetic entropy change and adiabatic temperature change of Gd-based alloys were systematically studied:When the adding amount was within 2.2at% for La, Ce or Nd, 1.4at% for Pr, 5at% for C or Y , 10at% for Dy, the difference of isothermal magnetic entropy change between these Gd-based alloys and pure Gd was not evident at a magnetic field change of 0-2T or 0-5T.Under a magnetic field change of 0-1T for each system of Gd-based alloys, the adiabatic temperature change of the alloy decreased with the addition of alloying elements. Among all the studied alloys, the decreasing rate of adiabatic temperature change alloy was the slowest for Gd-Y alloy and the fastest for Gd-La alloy.The reason why adiabatic temperature change of alloy decreased with the addition of alloying element was explained. It was believed that the addition of alloying elements decreased the magnitude of de Gennes factor and increased the value of heat capacity of alloy, so resulting the decrease of adiabatic temperature change of alloy.4. Based on the Molecular- Field theory, the curve of theoretical calculation for isothermal magnetic entropy change versus temperature was almost identical with the experimental curve, which showed that the model was useful for a rapid screening of potentional magnetocaloric effect materials.5 The mechanical strength of Gd-based alloy was expressed by the microhardness of the alloy. The results showed that among seven series of alloys, the increase of the hardness values of Gd1-xCx alloy and Gd1-xYx alloy were the largest. When the content of C was more than 3.5at%, and the content of Y was more than 5at%, the hardness of alloys increased greatly. The reason why the hardness value of Gd1-xCx alloy was larger than that of other system of alloy was explained based on the classical solid-solution reinforcement theory. It was thought that the large lattice mismatch between C and Gd as well as the interstitial site of C atoms in the Gd crystal lattice led to large reinforcement of Gd1-xCx alloy.6 The electronic structure of Gd-based alloy was investigated by XPS. There existed a connection between the results of the XPS study and the results of magnetic property study. Since Gd 4f-electron is magnetic electron and Gd 5d-electron plays an important role in the interaction between magnetic atoms, the Gd 4f-spectra and 5d-spectra were paid more attention to. The results showed that adding 5at% La, Ce, Pr or Nd into Gd almost did not change the bonding energy of Gd 4f-electrons,but the distances of energy levels between Gd 4f-electrons and 5d-electrons and between Gd 5p-electrons and 5d-electrons changed, which was in accordance with the result that the Curie temperatures of these binary alloys obviously changed in comparison with that of Gd. Adding C into Gd did not change the bonding energy of Gd 4f-electrons , but the distances of energy levels between Gd 4f-electrons and 5d-electrons and between Gd 5p-electrons and 5d-electrons were in accordance with the result that the Curie temperatures of Gd1-xCx changed little with the addition of C. The addition of 11at% Y or 20at% Dy made the bonding energy of Gd 4f-electrons, the distances of energy levels between Gd 4f-electrons and 5d-electrons and between Gd 5p-electrons and 5d-electrons change obviously, which was also in accordance with the result that there were large difference between the Curie temperatures of Gd-based alloys and that of pure Gd.