A Study Of Structures,Magnetic And Magnetocaloric Effects Of Low-temperature Magnetocaloric Oxides

Magnetic refrigeration,one new type of refrigeration technology based on the magnetocaloric effect,displays a potential application prospect in the field of refrigeration,which is due to its advantages containing low power consumption,low noise,environmental protection,no pollution,and high efficiency,etc.At present,finding high-efficiency magnetic refrigeration materials working in different temperature regions has become one of the urgent problems to be solved in the field of magnetic refrigeration.Among them,low-temperature magnetic refrigeration materials have a wide range of applications in the fields of military defense,industrial production,and measurements of basic physical properties,etc.Therefore,it is very important to study the magnetocaloric effects（MCE）of low-temperature magnetic refrigeration materials.Rare earth-transition metal oxides with large angular momentum of J have received extensive attention,which can display giant MCE in the low temperature region.In this thesis,we focus on study of structures,magnetic and magnetocaloric effects of low-temperature magnetocaloric oxides with the following three works:（1）The DyCoO₃nanocrystalline powders were prepared by sol-gel method,and its structure,morphology and composition were analyzed.The experimental data show that the sample is of single phase with an orthorhombic structure and Pbnm space group.Thermal magnetization curves show that Dy Co O₃exhibits a magnetic phase transition of AFM-PM induced by Dy³⁺-Dy³⁺antiferromagnetic interaction at T=3.9 K.The investigation of magnetocaloric effects show that the maximum magnetic entropy changes and relative refrigeration capacity（RCP）of Dy Co O₃are16.6 J/kg K and 354 J/kg under the magnetic field change 0-60 k Oe,which are comparable with or much larger than the values of other Dy-based oxides.（2）The Gd-based material Gd Fe_1-xCr_xO₃（x=0.0,0.1,0.3,0.5,0.7,0.9 and 1.0）series samples were prepared by solid state reaction method.X-ray diffraction datas show that these samples are of single phase with an orthorhombic structure and Pbnm space group.The magnetization measurement results show that the Gd Fe_1-xCr_xO₃（x=0.0,0.3,0.5,0.7,0.9 and 1.0）samples undergo AFM-PM magnetic phase transitions around the Néel temperature,and the phase transition type is second-order phase transition.The investigation of magnetocaloric effects show that the maximum magnetic entropy changes of Gd Fe_1-xCr_xO₃（x=0.3,0.5 and 0.9）are 37.65,39.26 and38.29 J/kg K and the corresponding relative cooling capacity（RCP）are 364.24,367.65 and 357.77 J/kg under the magnetic field change 0-60 k Oe respectively.（3）The Gd-based material Gd₂Cu O₄polycrystalline powder sample was prepared by solid state reaction method.The XRD data shows that the crystal structure of the sample is a tetragonal T’structure with a space group of I4/mmm.The magnetization results show that Gd₂Cu O₄has a complex magnetic phase transition.The Gd³⁺magnetic sublattice shows a typical PM to AFM around 6.5 K,and the Cu²⁺magnetic sublattice shows spin reorientation transitions around 7.0 K and 7.8 K as well as PM to AFM transition around 284 K,respectively.Arrott diagram analysis found that the compound has a second-order magnetic phase transition.For the magnetic field change 0-70 k Oe,Gd₂Cu O₄shows the maximum magnetic entropy change value and RCP value of 12.78 J/kg K and 203.16 J/kg,respectively,and the inverse magnetocaloric effect appears at low temperature.The above research results show that Dy Co O₃nanocrystals,Gd Fe_1-xCr_xO₃（x=0.3,0.5,0.9）and Gd₂Cu O₄polycrystalline powders can be used as alternative low-temperature magnetic refrigeration working fluids.