Studies On The Preparation And Magnetocaloric Effect In （La, R）（Fe, T,Si）₁₃H_x Alloys

NaZn13-type La（Fe, Si）₁₃compounds are a promising candidate formagnetic refrigerant due to their large magnetocaloric effect, low cost andenvironmental-friendly property. Curie temperature of the La（Fe, Si）₁₃basedalloys span a range covering the room temperature after hydrogen absorptionprocess. This paper studies the （La, R）（Fe, T, Si）₁₃H_xalloys, showing thepreparation and magnetocaloric effect. Raw materials with low purity areadopted in this research. the alloy with compostions of La_0.5Pr_0.5Fe_11.4Si_1.6,La_0.6Pr_0.4Fe_11.4Si_1.6B_0.2, LaFe_11.9-xCo_xSi_1.1B_0.2（x=0.4,0.5,0.6） and LaFe11.39Mn0.35Si_1.26B_x（x=0,0.1.0.2）, are produced by vacuum induction melting,annealed at certain temperatures in cacuum furnace heated by electricresistance wire, hydrogenated under low temperature around280℃and lowpressure of hydrogen of0.1-0.2Mpa. X-ray diffractmeter （XRD）, scanningelectron microscopy （SEM）, differential scanning calorimetry （DSC） andvibrating sample magnetometer （VSM） are used to examine themicrostructure, magnetic properties and magnetocaloric effect La_0.5Pr_0.5Fe_11.4Si_1.6alloy are prepared by mid-frequency induction furnace annealed at1150℃under argon annealing for100hours, then hydrogenated at atemperature of280℃for3h-5h under hydrogen pressure of0.1-0.15MPa.Curie temperature of the hydride obtained was increased by140K under0-1.5T magnetic field, and the maximum magnetic entropy change keeps thesame value of20.80J/kgK as the annealed sample. Hydrogen absorption isvery uniform.The hydrogen content of the hydride is about1.7detected byDSC. The hydride was set aside at room temperature for0d,180d and210dleading to a regularly decrease of the Curie temperature from325K to322Kand317K, maximum magnetic entropy change under0-1.5T magnetic fieldfrom20.80J/kgK to19.4J/kgK and18.6J/kgK, respectively. Alloy ingot of 7kg La_0.6Pr_0.4Fe_11.4Si_1.6B_0.2produced by mid-frequency induction furnace inargon atmosphere, annealed at1200℃for16hours, then hydrogenated at atemperature of280℃for5h under the pressure to0.15MPa. The stability ofthe resulted La_0.6Pr_0.4Fe_11.4Si_1.6B_0.2H_xhydride is very good. The latticeconstant increases from1.2295nm to1.2491nm, the Curie temperatureincreases from198K to325K. The alloy La_0.6Pr_0.4Fe_11.4Si_1.6B_0.2and hydrideLa_0.6Pr_0.4Fe_11.4Si_1.6B_0.2H_xhave the maximum magnetic entropy change of9.1J/kgK under0-1.5T magnetic field. And the hydride kept at room temperaturein atmosphere for190d still maintain its original performance. TheLaFe_11.9-xCo_xSi_1.1B_0.2（x=0.4,0.5,0.6） and LaFe_11.39Mn_0.35Si_1.26B_x（x=0,0.1.0.2）alloys were made by a high frequency induction furnace with the addition of afew interstitial atoms B and the substitution of Co, Mn for Fe. The Curietemperature of the LaFe_11.9-xCo_xSi_1.1B_0.2（x=0.4,0.5,0.6） alloy were231K,248K and260K before hydrogen absorption process, increased to378K,389K and391K after hydrogen absorption, respectively. The maximummagnetic entropy changes of the alloys were changed from9.8J/kgK,8.5J/kgK and6.0J/kgK to7.9J/kgK,6.6J/kgK and1.3J/kgK forLaFe_11.9-xCo_xSi_1.1B_0.2（x=0.4,0.5,0.6）, respectively. The Curie temperature ofLaFe_11.39Mn_0.35Si_1.26B_x（x=0,0.1.0.2） were140K,145K and148K beforehydrogenating process, and changed to278K,279K and281K. after hydrogenabsorption. The maximum magnetic entropy changes of the LaFe11.39Mn0.35Si_1.26B_x（x=0,0.1,0.2） alloy were7.2J/kgK,7.1J/kgK and4.4J/kgK,respectively. The ones was changed to6.7J/kgK,6.9J/kgK and2.3J/kgK forits hydrides. Adiabatic temperature changes were2.0K,2.2K and0.9K forLaFe_11.39Mn_0.35Si_1.26B_x（x=0,0.1,0.2）, respectively, after hydrogen absorption.