| Magnetic refrigeration technology based on the magnetocaloric effect(MCE)has provided a practical way of realizing refrigeration from ultralow temperature to room temperature.It is considered to be one of the most promising cooling technologies with its advantages of high efficiency,environmental friendliness and energy conservation.The NaZn13-type structure La-Fe based compounds have an adjustable magnetic phase transition temperature,low cost and large magnetocaloric effect,which are most likely to achieve commercial application.In this paper,the phase formation mechanism and molding process of(La,Ce)(Fe,Co,Si)13 melt spun ribbons and strip-cast flakes prepared by melt spinning and strip casting techniques,are systematically investigated,respectively.La(FexSi1-x)13(0.85≤x≤0.9)alloys have a giant magnetocaloric effect(GMCE)near their Curie temperatures(TCs),but the working temperature of these alloys is much lower than room temperature.In this paper,the TC of the alloy is further improved by Co and Ce substituting for part of Fe and La.First of all,the La0.7Ce0.3(Fe0.92Co0.08)11.4Si1.6 melt spun powders were pressed by warm compaction.After 1000℃/3h high temperature and short time sintering,there were only a small amount of α-Fe and LaFeSi phases.Under a maximum applied field of 2T and 5T,the(-ΔSM)max were 4.1 J/(kg·K)and 8.3 J/(kg·K)respectively.The magnetic and magnetocaloric properties of La0.8Ce0.2(Fe0.95Co0.05)13-xSix(x=1.1~1.6)melt spun ribbons were also investigated.The results show that Si can improve the TC,but greatly weakens the magnetocaloric properties of the alloys.The phase formation mechanism of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 strip-cast flakes were systematically studied by annealing heat treatment and the relative content of each phase during the phase formation was quantitatively analyzed using Rietica and Photoshop software.The results show that La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 flakes with more than 93.5wt.% 1:13 phase when annealing at 1323 K for 12 hours.Overhigh annealing temperature or overlong annealing time would result in the decomposition of the 1:13 phase and the aggregation of α-Fe phase.Under a maximum applied field of 2T,the(-ΔSM)max of the samples annealed at 1323 K for 2,6,12,18 and 24 hours are 2.79,10.42,12.50,11.74 and 11.68 J/(kg·K),respectively.Finally,the effects of particle size and its distribution of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2 on the hot-pressing preparation and performance of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites were systematically investigated.The strip-cast flakes annealed at 1323 K for 12 hours were mechanically ground and sieved into four types,<45,45-100,100-180,180-250μm,and mixed homogeously with quantative tin powders(La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2:Sn=9:1,mass fraction)and then hot pressed at 900MPa/150℃ for 2 mins.The results show that with the increase of particle size,the adiabatic temperature change((35)Tad)and(-ΔSM)max of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites increase from 1.74 K,6.38 J/(kg·K)to 2.05 K,7.96 J/(kg·K)under a maximum applied field of 1.4T and 2T respectively in the vicinity of TC,while the compressive strengths(σbcs)of the composites decrease from 180-200 MPa to 136 MPa.When the composites were synthesized by mixing 0,5,10,20,30 wt.% of fine particles(<45μm)into the coarse particles respectively,and then hot pressed with tin powders.The results show that the optimum mechanical and magnetic properties are obtained when the weight ratio between coarse particles and fine particles is 4:1(denoted as 4#).The(35)Tad and(-ΔSM)max of 4# increase from 2.05 K,7.96 J/(kg·K)to 2.14 K,8.71 J/(kg·K)under a maximum applied field of 1.4T and 2T respectively in the vicinity of TC,and the σbc value of 4# reaches 226 MPa.The changes indicate that composites prepared by using a bimodal distribution of particle size can not only solve the problem of poor mechanical properties,but also enhance the magnetocaloric properties of La0.8Ce0.2(Fe0.95Co0.05)11.8Si1.2/Sn composites. |
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