Research On Magnetocaloric Properties And Cycle Stability Of LaFeSi-system Alloys Near Ambient Temperature

Due to its energy-efficiency and environmentally friendliness, room-temperature magneticrefrigeration technology has great potential in substituting conventional vapor-compression one.LaFeSi-system alloys as attractive candidate for magnetic refrigerants were observed a changeof lattice parameter and volume when subjected to magnetic phase transition. The volumetricchange can inevitably cause structural deficiency and the reduction of magnetic refrigerationcapability during thermal and magnetic field cycling. This trouble is a significant challenge forthe integration of these materials in the emerging magnetic cooling technology. So, toaccelerate the application of rare earth magnetic refrigerant material in both practical andscientific ways, it has great significance to investigate the stability of this kind of material suchas the magnetization fatigue degradation, the aging when it is refrigerated and the structuralevolution generated by the volume fluctuation.The as-cast La(Fe_1-xCo_x)_13-ySi_yalloys with different Co and Si contents were prepared byelectric arc melting. The alloys were annealed at1373K for120hours. The phasecompositions and the microstructure of these alloys were systematically analyzed using XRD,SEM and TEM. The magnetic phase transition and magnetocaloric performance of the alloyswere investigated by Superconducting Quantum Interference Device （SQUID） magnetometerand man-made magnetocaloric effect directly measuring instrument. In the process ofapplication and removal of the magnetic field, the cycle stability of the rare earth magneticrefrigerant material was researched through the relationship between the maximum adiabatictemperature change and the number of field cycles. The lattice structural evolution ofLa(Fe_0.94Co_0.06)_11.8Si_1.2alloy in the process of cyclic magnetization, including phases andboundaries of grains, were investigated by using high-resolution transmission electronmicroscope （HRTEM）. The major conclusions are as below:The results indicate that the main phase of the alloy annealed for120h is NaZn₁₃-type phase. For single phase alloy, its grain size is in the range of micron magnitude; for theincomplete annealing alloy, the impurity α-Fe phase, which minimum grain size less than50nm,dispersed in the NaZn₁₃phase base. The element substitution not only benefits the formation ofthe NaZn₁₃-type phase and constrains the appearance of the impurity α-Fe phase in the processof heat treatment，but also affects the magnetocaloric properties of the alloys. The fluctuatingof the Curie temperature Tc and adiabatic temperature change ΔTab with the content variationof substitution element was discussed in detail in this work. For La(Fe_I1-xCo_x)_11.8Si_1.2alloy,increasing Co content can raise the value of the Curie temperature Tc, and restrain thermalhysteresis, and reduce the value of magnetic entropy change ΔS and adiabatic temperaturechange ΔTad. For the incomplete annealing alloy La（Fe0.94Co0.06）13-ySiyandLa(Fe_0.93Co_0.07)_13-ySi_y, the more content of Si element, the less impurity phase in the materialhas.The ideal room temperature magnetic refrigeration materials La(Fe_0.94Co_0.06)_11.8Si_1.2wereobtained. Its Curie temperature Tc is287K, and the effective operating temperature range ismore than20K. The value of magnetic entropy change ΔS and adiabatic temperature changeΔTad of this alloy are14.1J/kg·K （H=5T） and2K （H=1.4T） respectively. These resultsdemonstrate that only one week is enough for annealing operation to obtain the roomtemperature magnetic refrigeration materials with larger refrigerating capacity.The cycle stability of La(Fe_0.94Co_0.06)_11.9Si_1.1and La(Fe_0.94Co_0.06)_11.8Si_1.2alloys areoutstanding comparatively. The mechanical integrity did not appear for these alloys when becycled1000times. Furthermore, when be cycled to10000times, the mechanical integrity alsodid not occur. These results imply that the magnetic refrigeration materials which prepared inthis work have potential to practical applications. With the increasing of cycling times, theLattice distortion of grain boundary of La(Fe_0.94Co_0.06)_11.8Si_1.2alloy become more and moreremarkable, even lose the lattice-like stripes characteristic, and the grain boundary width arealso on the increase from atom-thick layers evolution to nanometers and even dozens ofnanometers. Inside the grain of material, there are many subboundary and dislocation structure occurred by cycling. These defects can increase the static magnetic energy of ferromagneticstate and hinder magnetic moment moving. That is probably the leading reason for the drop ofmagnetic entropy change and refrigerating capacity.