A New Preparation Method Of The NaZn₁₃-type Compounds La(Fe, Co, Si)₁₃ And Magnetocaloric Effect Study

NaZn13-type La (Fe, Si)₁₃ compounds with low Si content exhibit large magnetocaloric effect in the vicinity of their Curie temperature. However,the La (Fe, Si)₁₃ compounds exist many problems,such as low Curie temperature,long time annealing and impurity phases etc. In order to improve the preparation method and to increase the Curie temperature, we have studied a new preparation method, making the La (Fe, Si)₁₃ compound with nominal composition La(Fe,T)11 (T : stabilizing element), and the relationship between the preparation method and phase,crystal structure,magnetocaloric effects of the La(Fe,T)11 (T : stabilizing element) compounds.Main results and conclusions of this thesis are following: (1) LaFe11-xSix (x = 1.2, 1.4, 1.8, 2.0) compounds with nominal composition 1:11 were annealed for two hour at 1373 K. Powder X-ray diffraction (XRD) patterns show that the compounds crystallize in NaZn13-type structure (space group Fm3c) except a small amounts ofα-Fe as secondary phase. With increasing the Si content, the Curie temperature increases from 181 K to 261 K. The magnetic entropy change of the LaFe11-xSix (x = 1.2, 1.4, 1.8, 2.0 ) compounds are 14 J/kgK,11J/kgK,2.4 J/kgK,1.9 J/kgK in a magnetic field change 0-1.5 T. We have studied the Scanning Electron Microscope (SEM) and Energy Dispersion Spectrum (EDS) in the LaFe_11-xSi_x (x = 1.2, 1.4) compounds. The main phase of LaFe9.8Si1.2 and LaFe_9.6Si_1.4 compounds are LaFe10.8Si1.09 and LaFe_10.58Si_1.78,which are very close to the 1:13。(2) The prolonged annealing time, for 50h and 100h, can reduce or remove theα-Fe phase in the compounds. (3) The addition of Co element into the LaFe_9.8Si_1.2 and LaFe_9.6Si_1.4 compounds leads to increasing the Curie temperature of LaFe_9.8Si_1.2 and LaFe9.6Si1.4 to room temperature. We use the Landau theory to analyze the influence of Co on phase transition and calculate the magnetic entropy change of the compounds. We also did the SEM and EDS for LaFe_9.8-xCo_xSi_1.2(x = 0.8 and 0.9) and the results show that the main phase compositions are LaFe_9.96Co_0.11Si_1.51 and LaFe10.08Co0.15Si1.21. The direct measurements of the MCE show that the adiabatic temperature change of the LaFe_9.8-xCo_xSi_1.2(x = 0.7, 0.8, 0.9 ) compounds in a field change 1.5 T are 1.7 K,1.5 K,1.1 K。(4) We have studied the structure and magnetocaloric effect of LaFe11-xSix(x = 2.0, 2.2, 2.4, 2.6, 2.8, 3.0, 4.0, 5.0) compounds without annealed. When the Si concentration range is in 2≤x≤2.6, all the compounds crystallize in NaZn13-type structure (space group Fm3c) and exhibit large magnetic entropy change. When the Si range is in2.8≤x≤5, all the compounds crystallize in Ce₂Ni₁₇Si₉-type structure. According to the present study, We have determined the phases and their compositions by SEM and EDS。According to the present study, we conclude that the NaZn₁₃-type LaFe_13-xSi_x compounds can be easily prepared from the nominal composition compounds LaFe_11-xSi_x.