| Magnetic materials with special functions play an important role in the development of modern society.The magnetocaloric materials applied to magnetic refrigeration technology is the mainly research for magnetic materials fields.The research goal of this field is to develop high magnetocaloric performance materials with suitable working temperature region,large magnetocaloric effect,good thermal and magnetic reversibility.Based on the above requirements,we mainly studied the magnetocaloric materials in the low temperature region including liquid nitrogen(77 K),liquid hydrogen(20 K)and liquid helium(4.2 K)temperature region,as well as he lower temperature close to 2 K.The details are as follows:1.Influence on magnetocaloric effect of magnetic refrigeration materials in low temperature region by magnetic atom substitutionThe influence of Ho substitution for Tm atoms on the magnetic properties,magnetic transition and magnetocaloric effect(MCE)of TmGa was systematically investigated according to magnetic measurements and neutron powder diffraction(NPD)experiments.The magnetic transitions of Tm1-xHoxGa compounds show different types by Ho substitution for TmGa and the complete magnetic diagram of Tm1-xHoxGa compounds was obtained.The spin reorientation(SR)transition of Tm0.1Ho0.9Ga compound was directly confirmed by variable-temperature NPD experiments.Furthermore,MCE of Tm1-xHoxGa compounds was significantly affected by Ho substitution.When x=0.15,the peak value of magnetic entropy change reaches the maximum value of 18.0 J/kg K under field change of 0-2 T.The refrigerant temperature span and refrigeration capacity of Tm0.85Ho0.15Ga compound show enhancement of 23.0 and 21.6%,correspondingly,compared with TmGa compound.The balanced MCE performance,indicating that Tm0.85Ho0.15Ga shows potential application value for magnetic refrigeration in the application of hydrogen liquefaction.The complex multiple magnetic phase transitions of Pr1-xHoxGa compounds show three different types by Ho substitution.The low Ho content introduces new antiferromagnetic(AFM)states at low temperatures.The high Ho content introduces the spin reorientation phase transition in the low temperature region.Obvious inverse MCE was obtained at low temperature and low field in low Ho content,which is different from high Ho content only showing direct MCE.The peak values of ΔSM at TSR and Tc both increase with Ho content increasing.(-ΔSM)max at TSR is larger than that at Tc,indicating that the increase of Ho content enhances SR transition.Moreover,approximate table-like ΔSM curves are observed in some components,such as Ho content is 0.6,0.7,0.8,which is beneficial to the practical application of magnetic refrigeration material in wide temperature.2.Study on the magnetic properties and magnetocaloric effect of TmGa by nonmagnetic atom substitutionThe research contents of this part are about Tm1-xYxGa(0≤x≤0.8)and TmGa/In/Ge systems.The influence of Y substitution for Tm atoms and In/Ge substitution for Ga atoms on the magnetic ordering,magnetic transition and MCE of TmGa was systematically studied according to magnetic measurements and variable temperature XRD experiments.For TmGa0.9In0.1,it shows FM-to-AFM and AFM-to-PM transitions with temperature increasing,which is similar with TmGa.For TmGa0.9Ge0.1,only one transition from AFM-to-PM occurs.It can be well explained by lattice distortion based on RKKY indirect exchange interactions.Variable temperature XRD experiments confirm that FM-to-AFM is related to the lattice distortion.The magnetic ordering and different types of transition in TmGa(In/Ge)compounds arise from the change of bond length between the nearest neighboring Tm atoms.Magnetic transition temperatures can be effectively adjusted to the low temperatures with increasing Y content in Tm1-xYxGa compounds.Particularly,Tm0.4Y0.6Ga exhibits the optimized working temperatures around the boiling point of liquid helium and large MCE.It undergoes ferromagnetic(FM)to antiferromagnetic(AFM)transition at TFA=2.8 K and AFM to paramagnetic(PM)transition at TN=5.4 K with increasing temperature,respectively.The(-ΔSM)max is 6.4,10.1 and 15.6 J/kg K,(-ΔTad)max is 2.4,4.2 and 8.4 K under the field changes of 0-1,0-2 and 0-5 T.correspondingly.Thus,Tm0.4Y0.6Ga compound with working temperature region around liquid helium temperature and large MCE could be suitable for the ap plication of helium liquefaction.3.Study on materials with large magnetocaloric effect in ultralow temperature and large low-field magnetocaloric effect in low temperatureGiant low-field MCE in Er1-xTmxAl2 compounds was obtained and studied in detail.For Er0.7Tm0.3Al2 compound,it showed the largest low-field-ΔSM with the peak value of 17.2 and 25.7 J/kg K for 0-1 T and 0-2 T,respectively.The(-ΔSM)max up to 17.2 J/kg K of Er0.7Tm0.3Al2 for 0-1 T is the largest among the intermetallic magnetocaloric materials ever reported at the temperatures below 20 K.The peak value of adiabatic temperature change(ΔTad)max was determined as 4.13 K and 6.87 K for 0-1 T and 0-2 T,respectively.The outstanding low-field MCE performance with low working temperatures indicates that Er1-xTmxAl2(0≤x≤1)compounds are promising candidates of magnetic refrigeration materials at liquid hydrogen and liquid helium temperatures.For ErCu2Si2,it shows AFM to PM transition at TN≤2 K.TN can be adjusted to the lower temperature by 10%Y,20%Al,20%Ge atoms substitution and the MCE performance has been improved as well.(-ASM)max of Er0.9Y0.iCu2Si2,ErCu2Si1.8Al0.2 and ErCu2Si1.8Ge0.2 compounds are 25.8,25.1 and 27.7 J/kg K for 0-5 T,respectively,which shows great improvement of 13.2%,10.1%and 21.5%compared with parent ErCu2Si2(22.8 J/kg K).Moreover,(-ASM)max at low field of 0-2 T has been maintained by Ge substitution.Er0.9Y0.iCu2Si2,ErCu2Si1.8Al0.2 and ErCu2Si18Ge0.2 compounds exhibit large MCE with ultralow transition temperatures which make them be promising for magnetic refrigeration application at low working temperatures. |
PDF Full Text Request