Study On Synthesis,structure And Performance Of Dinuclear Molecular-based Magnetic Refrigeration Materials Containing Rare Earth With MOF Structure

Compared with traditional refrigeration technology,magnetic refrigeration technology has attracted the attention of countries all over the world because of its environmental protection,high efficiency,energy saving,stability,reliability,low noise,and small size.The performance of the magnetic refrigeration material is the most critical part of the magnetic refrigeration technology,which will directly affect the power and efficiency of the magnetic refrigeration equipment.The molecular-based magnetic refrigeration material has a very significant effect on ultra-low temperature refrigeration,so it has received the attention and research of the majority of scientific researchers.Among them,due to the simple molecular model,binuclear complexes are very suitable for studying the magnetic structure relationship in molecular-based magnetic refrigeration materials and perfecting the related structure-performance control theory.The metal-organic framework structure has the advantages of large specific surface area,adjustable pore size,diverse structure,diverse functions and its promotion of high magnetocaloric effects,which are also the focus of molecular-based magnetic refrigeration materials.Based on this,this thesis has prepared several series of metal-organic framework structures of binuclear complexes with different ligands under hydrothermal or solvothermal conditions,and characterized their structures and magnetic properties.The specific contents are as follows:（1）Using 5-（4-carboxybenzyloxy）isophthalic acid（H₃L）as the ligand,rare earth nitrate hexahydrate as the source of rare earth metal ions,and a mixed solution of DMF and ethanol as the solvent,the reaction was performed at 80℃at a constant temperature3 day.After slow cooling at 3℃/h,filtration separation,three DMF washes,and three ethanol washes,two kinds of dual-nuclear monometallic organic framework materials Ln₂（L）₂（DMF）₄·6H₂O（Ln=Dy（1）,Tb（2））were successfully constructed.The X-ray single crystal diffraction results of complex 1 and complex 2 show that they belong to the monoclinic crystal system,the C2/c space group,and the{Ln₂}dimer unit is used as the node to form a three-dimensional network structure.Thermogravimetric analysis shows that they can be transformed into a stable core-shell structure of porous carbon-coated metal nanoparticles at high temperatures,with high thermal stability.Magnetic studies show that there are antiferromagnetic interactions in their molecules.（2）Using 1,1’-ferrocene dicarboxylic acid as the ligand,and rare earth nitrate hexahydrate as the source of rare earth metal ions,three kinds of dual-nuclear polymetallic organic framework materials[Ln（OH）(FeC₁₂H₈O₄)]·H₂O（Ln=Gd（11）,Dy（17）,Ho（18））were successfully constructed by hydrothermal method at 160℃and alkaline conditions.Through the hydrothermal reaction of gadolinium nitrate and ligand,the effects of temperature,sodium hydroxide concentration,and solvent type on the formation of single crystals were studied in detail,the optimal conditions for the formation of large-size single crystals were determined.The infrared absorption spectra and powder X-ray diffraction of complexes 11,17,and 18 confirmed that they have a pinwheel-like channel MOF structure.Simultaneous thermal analysis shows that they have good thermal stability.Magnetic studies show that there are ferromagnetic interactions in the molecules of complexes 11 and 17.Among them,the complex 11 has a great magnetocaloric effect,and the magnetic entropy value is as high as 49.19 J kg^-1K^-1 when T=2.5 K andΔH=7 T.It is a molecular-based magnetic refrigeration material with considerable application prospects.