Magnetic Properties And Magnetocaloric Effects In (La,R) (Fe,Co,Si)₁₃ Compounds

Magnetic refrigeration, as a high-tech green refrigeration technology, has a bright prospect in replacing the conventional vapor compression technology. NaZn₁₃-type La(Fe,Si)₁₃-based compounds and their derivatives are one of the most promising magnetic refrigerants with large magnetocaloric effect, low cost and environmentally friendliness. In this thesis, the structure, magnetic properties and magnetocaloric effect in NaZn₁₃-type (La,R)(Fe,Co,Si)₁₃ were studied systematically by means of X-ray diffraction (XRD), Scanning electron microscope (SEM) and magnetic measurements.The main results and conclusions of this thesis are as follows: (1) The XRD patterns show that La_1.1-xGd_xFe_11.4Si_1.6 compounds with x=0, 0.05, 0.1, 0.15 and 0.2 crystallize in the NaZn₁₃-type phase with space group Fm-3c. The Curie temperature increases slightly from 195 K to 198 K by adding the Gd concentration in the composition range 0≤x≤0.2. The replacement of La by Gd weaken the field-induced magnetic transition near Curie temperature and, in consequence the transition transforms from first order to second order. The thermal hysteresis of the compounds decreases with increasing the content of Gd. The maximal magnetic entropy change ( ?ΔS M) of La_1.1-xGd_xFe_11.4Si_1.6 compounds with x=0, 0.05, 0.1, 0.15 and 0.2 in a magnetic field change of 0¹.5 T decrease from 14.1 Jkg^-1K^-1 to 9.6 Jkg^-1K^-1 . (2) In order to tune the Curie temperature, we have studied the LaGd_0.1Fe_11.4Si_1.6 (x=0.1, 0.3, 0.5, 0.7, 0.9) and La0.9Ce0.2Fe11.0Si2.0 (x=0, 0.2, 0.4 and 0.8) compounds with Co doping. The Scanning Electron Microscope (SEM) image of LaGd0.1Fe11.1Co0.3Si1.6 compounds show that there are three ingredient regions: the main phase LaGd0.17Fe11.16Co0.43Si1.68 (gray zone), a small amount of Fe rich phase LaGd0.68Fe116.9Co3.77Si4.57 (black zone) and a La rich phase LaGd0.12Fe0.96Co0.14Si1.06 (white zone). The substitution of Fe by Co in LaGd0.1Fe11.4-xCoxSi1.6 and La0.9Ce0.2Fe11-xCoxSi2.0 could turn the Curie temperature to room temperature. The magnetic measurements show that there is no thermal hysteresis in these compounds. However, the maximum value (in a field change of 0¹.5 T) of the magnetic entropy changes decrease from13.8 Jkg^-1K^-1 to 1.5 Jkg^-1K^-1 and from 5.6 Jkg^-1K^-1 to 2.7 Jkg^-1K^-1 for LaGd0.1Fe11.4-xCoxSi1.6 in composition range 0.1≤x≤0.9 and La0.9Ce0.2Fe11-xCoxSi2.0 in composition range 0≤x≤0.8, respectively, indicating the isothermal magnetic entropy change reduces with increasing Co concentration. (3) The influence of partial doping rare-earth elements Ce, Gd, Dy, Er in LaR_0.1Fe_11.4Si_1.6 compounds on the Curie temperature and MCE were studied. The Curie temperature of the compounds is determined to be 196, 190, 196, 191 and 194 K for R=La, Ce, Gd, Dy and Er, respectively. The magnetic entropy change of Ce-doping compounds is largest and of Er-doping compounds is smallest in these compounds. Even so, the relative cooling power of the Ce-doping compounds is close to those in the other compounds.