Phase Formation,microstructure And Magnetocaloric Properties Of La-Ce-Fe-Si Alloys

Along with the rapid progress of society,intensive attention has been paid towards the research and exploration with the advantages of green,clean and high efficiency materials.Compared with the traditional gas-pressing refrigeration,magnetic refrigeration is one kind of new emerging technology which has many obvious advantages,e.g.,environmental friendly,no harmful gasses involved,high efficiency,and more compactly,much noisy as well.It is well known that the magnetic refrigeration materials play a key role in the magnetic refrigeration technology.NaZn₁₃-type La?Fe,Si?₁₃-based compounds are regarded as one of the most favorable refrigerants for near room-temperature magnetic refrigeration applications due to its tunable Curie temperature,large magnetic entropy change,nontoxic character,and low costly elements.In this paper,we analyzes the phase mechanism of 1:13 phase under different cooling rate.Furthermore,the influence of preparation and element doping on the microstructure and magnetocaloric properties were investigated.Main experimental results and conclusions are summarized as follows:?1?The formation of the La?Fe,Si?₁₃phase requires homogenization heat treatment for a long time,leading to low efficient preparation and energy consuming.The element of Si intends to stabilize the cubic NaZn₁₃-type structure and promote the formation of the 1:13phase.However,it should be addressed that the MCE of the LaFe_13-x3-x Si_x will decreased with higher Si content.Meanwhile,the entropy change increases after the substitution of Ce for La due to the reduction of lattice parameter and thereby the enhancement of itinerant-electron metamagnetic.Based on this,high Si content and Ce-doped La_0.6Ce_0.4Fe₁₁Si₂ alloy prepared in rapid solidification technology with different cooling rates.The 1:1:1 phase can be suppressed due to the fast cooling rate and the amount of 1:13 phase decreases with increasing speed up to 25m/s.then increased on further higher speed.For the spinning rate of 25 m/s,a maximum amount of 1:13 phase of⁷4.4%is found.The volume fraction of 1:13 phase is related to the competing nucleation rate between?-Fe and 1:13 phases during rapid solidification.For the lower prepared speed,?-Fe phase selected to form in a priority mood.Faster prepared speed gives rise to a larger undercooling and the nucleation rate of 1:13 phase is increased.However,more defects could be introduced by faster speed,which might be unfavorable to the atomic diffusion and smear the formation of the 1:13 phase.But the MCE is low due to the higher Si content in 1:13.?2?In our previous work,we found that the Si content is reduced result from the existed5:3 phase in off-stoichiometric rich La La₂Fe₁₁Si₂ bulk alloy alloys.Where the secondary La₅Si₃ phase accelerates the formation of the 1:13 phase and the annealing time can be considerably shortened to 24 hours.In this work,we have carried out a systematic investigation on the influence of partial Ce substitution for La for a series of La_2-xCe_xFe₁₁Si₂alloys.The formation of high percentage of the 1:13 phase?⁸0 vol.%?after 12h annealing at1423K in La_2-xCe_x Fe₁₁Si₂ alloys.With lower Ce content?x less than 0.6?,the microstructure is similar.The MCE gradually increase due to the increased Ce content in 1:13 phase.For x=0.8,there is no drastic change in terms of the morphology with exception of the precipitation of the minor 2:17 phase.The high of 21.8 J/kgK in 2 T in the La_1.2Ce_0.8Fe₁₁Si₂ alloy was observed.With further increase of Ce content from x=1.0 to 1.2 the quantity of 2:17 phase significantly increases,and the entropy change becomes small due to the precipitation of 2:17ferromagnetic phase.?3?For the La_2-xCe_xFe₁₁Si₂ alloys,in order to adjust to Curie temperature to room temperature,hydrogen absorption was achieved using the method of heat treatment in the hydrogen atmosphere.However,the alloy bluk is pulverization and 1:13 phase grain separating each other after hydrogenation.The smeared magnetostatic interactions and first order phase transition result to the increased entropy change and hysteresis.The lattice expansion caused by interstitial H atoms depresses the overlap between Fe-3d electrons,thus leading to an smeared IEM and increased T_c.The bigger volume change give rise to smaller entropy change.In addition,the lattice entropy decreased due to the increased?V/V.So the entropy change is reduced after hydrogenation for La_2-xCe_xFe₁₁Si₂ alloys,and the higher Ce containing,the smaller entropy change.