Study On The Magnetism And Magnetocaloric Effect Of Mo-Based ABO₄ Type Oxides

It has become a green and pollution-free refrigeration technology to realize refrigeration based on magnetocaloric effect.Low-temperature magnetic refrigeration can be used in aerospace and helium,hydrogen,nitrogen liquefaction and other fields.However,the corresponding magnetic refrigeration materials are mostly Gd alloys and Gd oxides.The low storage in the earth’s crust and high price of Gd limit its further application.Looking for rare earth-free magnetic refrigeration materials has become a hot research topic.At present,most of the researches are the alloys of intermetallic compounds.These materials have a large magnetic entropy change.Due to their poor chemical stability,difficult preparation and large thermal hysteresis,there are some difficult bottlenecks in the application.In comparison,transition metal oxides have high chemical stability,simple synthesis,and relatively small thermal hysteresis,which is expected to broaden the application of magnetocaloric refrigeration technology.In this thesis,rare earth-free metal oxidesα-Ni Mo O₄ andα-Co Mo O₄ were synthesized,and their magnetic properties and magnetocaloric effects were explored.The main contents are as follows:1.The synthesis ofα-Ni Mo O₄ and its magnetocaloric effect.Using solid-phase method,Ni Mo O₄ was synthesized with Ni O and Mo O₃ as raw materials.Through powder X-ray diffraction,Raman spectroscopy and Fourier transform infrared spectroscopy,it is proved that the prepared sample isα-Ni Mo O₄ with monoclinic structure,the space group is C2/m（12）,and the lattice parameter is a=9.5828（3）（?）,b=8.7504（3）（?）,c=7.6604（2）（?）,β=114.247°.The isofield magnetization curves ofα-Ni Mo O₄ under different magnetic fields are measured,and it is found thatα-Ni Mo O₄ is antiferromagnetic in a small field,and the Neel temperature T_N=19.7 K.But when a large magnetic field is applied,α-Ni Mo O₄ will undergo a magnetic field-induced metamagnetic phase transition.Analysis based on the calculated Arrott curves,this phenomenoncanbeproved to beafirst-order phase transition.Furthermore,the magnetocaloric effect ofα-Ni Mo O₄ is calculated by using isothermal magnetization curves at different temperatures.The maximum magnetic entropy change(-△⁸（6）)is 3.90 J kg^-1 K^-1 and relative cooling power（RCP）is 58.5 J/kg when the field change（ΔH）is 50 k Oe.WhenΔH=0～50 k Oe,α-Ni Mo O₄ has inverse magnetocaloric effect at low temperature.2.Study on the magnetism and magnetocaloric effect ofα-Co Mo O₄.Pureα-Co Mo O₄ was obtained by solid-phase reaction method.Refinement of powder X-ray diffraction data shows that the lattice parameters ofα-Co Mo O₄ at room temperature are a=9.7169（3）（?）,b=8.8640（3）（?）,c=7.6699（2）（?）,β=114.038°,and it is the same structure asα-Ni Mo O₄.Similarly,in a small magnetic field,theα-Co Mo O₄ sampleexhibits antiferromagnetic behavior,and the isofield magnetization curves and isothermal magnetization curves under a large magnetic field prove thatα-Co Mo O₄is a metamagnetic material.The Arrott curves and the irreversibility of the thermal hysteresis loop prove thatα-Co Mo O₄ has undergone a first-order phase transition,and the measurement of specific heat capacity further proves the existence of this phase transition.Data analysis shows thatthe-△⁽（6）is-22.58 J kg^-1K^-1（△H=50 k Oe）,indicating that there is a large inverse magnetocaloric effect inα-Co Mo O₄.The powder X-ray diffraction data and refinement at different temperatures found thatα-Co Mo O₄ has negative thermal expansion at low temperatures,which may be related to the change in the magnetic properties of the sample.This work first discovered this new type of inverse magnetocaloric materials without rare earth elements.Due to its cheap raw materials,simple preparation,high stability,and large magnetic entropy,it has broad application prospects in the field of low-temperature magnetic refrigeration.