Study On The Preparation Of Hot Pressed La-Fe-Si Based Bulk Materials For Magnetic Refrigeration And The Phase Evolution Of RE-rich La-Fe-Co-Si Alloys

Magnetic refrigeration technology is expected to be applied in room temperature refrigeration due to its high efficiency,energy saving,non-toxic and environmental protection.Among many magnetocaloric materials,Na Zn₁₃-type La?Fe,Si?₁₃ based alloys have the advantages of large magnetic entropy change,adjustable Curie temperature,non-toxicity,low cost,etc..However,the La?Fe,Si?₁₃ alloy prepared by tranditional method?melting+annealing?requires long phase formation time,and the La?Fe,Si?₁₃ alloy with first order magnetic transition has low Curie temperature,poor magnetic field cycling stability,which limits its commerical application.At present,a large number of studies have focused on the technologies combining atom doping and the addition of substitution atoms.La?Fe,Si?₁₃-based composites with excellent comprehensive properties were fabricated by combining series of processes such as arc melting,melt spinning,phase forming heat treatment,pulverizing and powder metallurgy.In this thesis,aiming at the preparation of La?Fe,Si?₁₃-based composites with excellent magnetocaloric and mechanical properties as well as good thermal conductivity,the routes combining powder metallurgy and grain boundary diffusion are used to shorten the preparation time cost and improve comprehensive properties.By analyzing the evolution law of phase composition and microstructure of the La?Fe,Si?₁₃-based composites,the diffusion principle of different elements and the physical mechanism of material performance improvement were studied.In addition,the phase formation and thermal decomposition of rare earth-rich La_x?Fe,Si?₁₃?x?1?alloys were also studied.It also provides theoretical guidance for the controllable preparation of high performance La-Fe-Si based magnetic refrigeration composites with large magnetic entropy change,high mechanical strength and thermal conductivity.The main research contents and conclusions are as follows:Based on the difficulties of phase formation and forming of La-Fe-Si-based bulk alloys,Ce-Co alloy was used as binder,La Fe_11.6Si_1.4/Ce-Co composites with excellent comprehensive properties were prepared by combining hot pressing and grain boundary diffusion technology.The alloy powders of Ce₄₀Co₆₀ binder and La Fe_11.6Si_1.4 with high 1:13 phase content were homogenously mixed,and then La Fe_11.6Si_1.4/Ce₄₀Co₆₀ bulk composites were prepared by hot pressing at 873 K@600 MPa for 30 minutes.By adjusting the diffusion annealing time at 1373K,the diffusion of Ce and Co atoms from binder into La Fe_11.6Si_1.4 particles were realized.The results show that due to local enrichment of Ce-Co binder,the composites have a two-component main phase?1:13 phase?structure after diffusion annealing at 1373 K over 12 h.Under the applied field change of 0?2 T,the composites showed a table-like???SM??T curves with a temperature width of?30 K and maximum magnetic entropy change???SM?of larger than 3.5J/?kg?K?,and the value of refrigeration capacity was larger than 170 J/kg.By using Ce₂Co₇ as binder,La Fe_11.6Si_1.4/Ce₂Co₇ bulk composites were prepared by one-step hot pressing at 1373 K for 1?6 h.In the process of HP,the diffusion of Ce and Co atoms were realized and 1:13 phase formation was promoted simultaneously.The research found that the diffusion mechanism of Ce and Co atoms is different,that is,Co atoms diffuse through vacancy diffusion mechanism,while Ce atoms diffuse through displacement diffusion mechanism.The composites fabricated by one-step hot pressing method show high compressive strength of 530 MPa and good thermal conductivity of 8.21 W/m?K.Meanwhile,as binder content in the composites increased from 0 wt.%to 15 wt.%,the Curie temperature increased from 212 K to 313 K,and the maximum magnetic entropy change decreased from8.8 J/?kg?K?to 6.0 J/?kg?K?for 5 T field change.With the same Ce₂Co₇ binder,the hot pressing molding process of 1173 K for 30 min was adopted.By varying the diffusion annealing temperature?1173?1373 K?and time?2?24 h?,the diffusion mechanism of Ce and Co atoms and phase evolution in the composites were explored.The results showed that Ce and Co atoms preferentially diffused into 1:1:1 type La Fe Si phase at 1373 K,and then La Fe Si?1:1:1?phase reacted with?-Fe phase to form 1:13 phase,simultaneously achieving Ce and Co diffusion into1:13 phase matrix.When the diffusion heat treatment is extended to 12 h,good magnetocaloric properties?5.01 J/kg?K@2 T,T_C?289 K?,large compressive strength?520 MPa?and thermal conductivity?6.17 W/m?K?were obtained in the composites.It has been reported that the addition of rich rare earth?RE?elements is beneficial to promote the phase formation of 1:13 in Na Zn₁₃-type La-Fe-Si alloys,but there is no report on the related processes of phase formation and thermal decomposition of RE-rich La?Fe,Si?₁₃?x?1?based alloy.In this thesis,the related process of phase formation and thermal decomposition of as-cast RE-rich La?Fe,Co,Si?₁₃?1.0?x?1.6?alloys are systematically studied.The results show that more than 90 wt.%content of 1:13 phase could be obtained through 1323 K heat treatment for different times.It was found that with the increase of La content in as-cast ingots,the growth of?-Fe phase in as-cast microstructure was inhibited.The finer?-Fe and 1:1:1 phase structure were formed,and the phase boundary structure of 1:13phase in heat treatment process was increased,and the 1:13 phase formation in heat treatment process was accelerated.The effect of temperature and time on the process of thermal decomposition and its mechanism were studied by using La?Fe,Co,Si?₁₃?1.0?x?1.6?alloy samples with different La contents.The results show that when thermal decomposition occurred at different temperature?873?1273 K?for 12 h,it was found that the thermal decomposition temperature range of these La-rich samples was 973?1073 K,and the fastest thermal decomposition temperature was 1073K.The additon of La couldeffectively inhibit 1:13 phase thermal decomposition.After thermal decomposition at 1073 K for different times?2?24 h?,it was found that La_1.2?Fe,Co,Si?₁₃ sample has more granular 1:1:1 phase and?-Fe phase,which formed phase decomposition nucleation points,resulting the faster thermal decomposition rate.