Magnetocaloric Effects Of LaCe Mischmetal Doped NaZn₁₃-type Alloys And Their Hydries

With the further development of society,more attention has been paid to green environmental protection technology.Compared with traditional gas compression refrigeration,magnetic refrigeration technology shows many advantages,such as environmental protection,energy saving,high efficiency,small size,low noise.Therefore,it has been attracted worldwide attention.The key factor of magnetic refrigeration technology is the selection of appropriate magnetic refrigerants.LaFe_13-xSi_x alloys with NaZn₁₃3 structure have been widely concerned and studied in the world,due to their advantages such as giant magnetic heating effect,low thermal hysteresis,adjustable Curie temperature and low cost.In the LaFe_13-xSi_x system,partial substitution of Ce for La leads to increase of magnetocaloric effect and decrease of Curie temperature.In order to further reduce economic cost of the alloys,LaCe mischmetal was added to partially replace La element,and the similar effect as the use of Ce element was obtained.In this thesis,the effects of substitution of Co,Al and Mn for Fe on the Curie temperature and magnetocaloric effect in the alloys containing LaCe mischmetal and in their corresponding hydrides.Using industrial metals as starting materials,La_0.7?LaCe?_0.3Fe_11.5-xT_xSi_1.5?T=Co,Al,Mn;x=0.1,0.2,0.3,0.4,0.5?alloys were prepared by melting and annealing.The structural and magnetic properties were characterized by means of XRD,SEM,PCT and VSM.All the hydrides were obtained by absorbing saturated hydrogen with an initial hydrogen pressure about 1 MPa at a reaction temperature of 270?.The XRD results suggest that the alloys and their hydrides mainly crystallize in NaZn₁₃3 cubic structure.In La_0.7?La,Ce?_0.3Fe_11.5-xCo_xSi_1.5?x=0.1,0.2,0.3,0.4,0.5?series of alloys,the Curie temperature gradually increases and the magnetocaloric effect monotonously decreases with the increase of Co.However,the alloys still exhibit large magnetocaloric effect.As the Co content increases from 0.1 to 0.5,the Curie temperature increases from 194 K to239K.For a field change of 3 T,the maximum isothermal magnetic entropy decreases from 26.4J/?kg·K?for x=0.1 to 14.5J/?kg·K?for x=0.5.The Curie temperature can be improved close to room temperature by hydrogen absorption reaction.The hydrides with x>0.2 show large magnetic entropy changes around the Curie temperatures of 273K.The Curie temperature can be also improved by the substitution of Al for Fe in La_0.7?La,Ce?_0.3Fe_11.5-xAl_xSi_1.5.5 alloys.However,with the increase of Al,the magnetocaloric effect decreases significantly in the alloys.The Curie temperature of the alloy with x=0.1 is found to be 211 K.The maximum isothermal magnetic entropy is low as 6.6J/?kg·K?for a field change of 3T.The Curie temperature increases from 211K to 260K after hydrogen absorption reaction.The hydride with x=0.1 shows the highest Curie temperature of 273 K and the largest isothermal magnetic entropy change of 17.3J/?kg·K?for a field change of 3 T.The Curie temperature pronouncedly reduces with the increase of Mn in La_0.7?La,Ce?_0.3Fe_11.5-xMn_xSi_1.5?x=0.1,0.2,0.3,0.4,0.5?series of alloys,but the alloys remain strong itinerant electron metamagnetic first-order phase transition and giant magnetocaloric effect.The Curie temperature is adjustable between 168 K and 88 K when the Mn content x changes from 0.1 to 0.4.After hydrogen absorption reaction,the Curie temperature is adjustable between 258 K and 154 K.For a field change of 3 T,the alloys and their hydrides present similar values of isothermal magnetic entropy change,being 19.8J/?kg·K?and 19.1J/?kg·K?,respectively.In summary,the magnetic and magnetocaloric properties have been systematically investigated in LaCe mischmetal doped La_0.7?La,Ce?_0.3Fe_11.5-xT_xSi_1.5?T=Co?Al?Mn;x=0.1?0.2?0.3?0.4?0.5?alloys.The results indicated that the Curie temperature can be adjusted close to room temperature and the giant magnetocaloric effect can be maintained by means of elemental substitution and hydrogen absorption in the studied alloys.The study offers scientific guidance for developing applicable refrigerants for magnetic refrigeration.