Novel Synthesis,Characterization And Multi-functionality Of La-Fe-Si Magnetocaloric Materials

With the improvement of living standard the call on environmental friendly technology,there has been an increasing demand on new refrigeration solutions to replace conventional air compression cooling systems.The environmental friendly and highly efficient solid-state refrigeration such as magnetocaloric cooling based on magnetocaloric effect(MCE)is regarded as the most promising candidate technology.The MCE materials include a large amount of alloy systems,and their performances are closely related to the microstructure.The solidification microstructure of rare-earth based MCE materials,such as Na Zn₁₃–type La-Fe-Si alloys,is rather cooling-rate sensitive.The traditional one-sample-at-a-time synthesis methods become ineffective when it demands for fast-screening of the prominent microstructure to obtain the desired magnetocaloric effect.The adiabatic temperature change,as an important criterion of a potential magnetocaloric refrigerant,affects the efficiency of thermal transport between heat exchange liquid and MCE materials and determines the temperature span of active magnetic regenerator(AMR).So far the measurement of adiabatic temperature change relies on the home-built measurement system which tests one sample at a time.As the performance of rare earth-based MCE materials could be tuned by doping of a variety of elements,it is necessary to employ high-throughput characterization methods to accelerate the development of new MCE materials.The functional stability of MCE materials also plays an important role for its application.Currently it still needs a characterization method for the determination of adiabatic temperature change under cyclic conditions.Therefore,it is important to develop a new characterization system which is capable of both multi-sample temperature change measurement and cyclic stability evaluation.The last topic is to solve the long-standing challenge for developing a comprehensive MCE material with good shaping ability,large magnetic entropy change and high thermal conductivity.We use gradient cooling as a high-throughput synthesis method for the screening of optimum cooling rate for La-Fe-Si alloys.Finite element modeling reveals that a wide cooling rate distribution of 270-3000 K/s could be reached in one wedge mold casting.It is found that with the optimum cooling rate of 1000-3000 K/s,the formation of magnetocaloric phase is accelerated.Correspondingly the solidified alloy exhibits a large magnetic entropy change of 14 J/kg K under 2 T field after short time annealing.Moreover,we found a needle-like phase which could act as diffusion path of Si element during annealing,which enriched our understanding of its phase formation mechanism.We developed a multi-sample adiabatic temperature change measurement system based on infrared calorimetry,which facilitates the simultaneous non-contact measurement of several samples with different compositions.This approach provides a more reliable evaluation on the functional stability of MCE materials because it avoids the change of test conditions due to expansion/shrinkage of the materials under magnetic field changes.It is found that the adiabatic temperature change decreased from 2.12 K to 1.85 K after 0.1 million cycling.The visualized in-situ thermal image from infrared measurement gives an insight into the structure and performance evolution during cycling,and it turns out that the volume effect and cracking of sample caused the degradation of the magnetocaloric effect.By exploiting the excellent deformability of?-Fe phase in as-cast Fe-rich La-Fe-Si alloy,we propose a near-net shaping method of open die forging to prepare La-Fe-Si thin plates.This novel approach is demonstrated for the first time to exhibit several advantages in producing full-dense materials,facilitating the phase formation,and maintaining large magnetocaloric effect.After annealing,the microstructure comprised of aligned?-Fe and nonequiaxial 1:13 grains elongated normal to the uniaxial deformation.Although pronounced<001>_Fe//FD and<111>_Fe//FD fiber textures formed after deformation,the newly formed functional 1:13 phases exhibited homogeneous crystallographic orientations and refined grain size,indicating the accelerated nucleation of 1:13 phase due to prior deformation.HRTEM and STEM observations at the 1:13/?-Fe interfaces revealed that the strain energy stored in deformation defects enhanced the elemental diffusion and crystal growth.Large magnetic entropy change of 14 J/kg K and adiabatic temperature change of 5.7 K at 2T is obtained in the sample annealed for just 3 days.Furthermore,a unique dual-phase structure consisting of aligned?-Fe phase and non-uniaxial 1:13 grains brings about a significant anisotropy in cross-plane and in-plane thermal conductivity.This new insight would greatly benefit the design of high efficient magnetic refrigerator with one-way enhanced thermal conduction.The multi-functionality of La-Fe-Si has been explored based on its microstructure and metamagnetic transition.First,the nagetive thermal expansion(NTE)behavior of plastically deformed La Fe_13.92Si_1.4H is examined by DIC measurement.By manipulating the morphology and distribution of?-Fe phase which has positive thermal expansion coefficient,the NTE along the direction parallel to the?-Fe alignment could be significantly restricted,leading to an anisotropic thermal expansion behavior.Second,temperature-driven thermal switch and robot have been produced by combination of La-Fe-Si powder with epoxy or molybdenum flakes.The pronounced first-order metamagnetic transition of La-Fe-Si facilitates the fast switch on/off or the actuation of the robot.The choice of elastic matrix with higher elasticity modulus and thermal conductivity could enhance the elastic-kinetic energy conversion during actuation.