Hydrothermal Synthesis And Properties Of Magnetic Niobate Nanostructures

As a class of high performance materials, niobates have attracted much attentiondue to their interesting optical, dielectric, magnetic and catalytic properties. However,the traditional methods for the preparation of niobate materials always have somedisadvantages, including high cost, complex equipments, tedious processes, poorquality products, which greatly restrict their further application and development.Hydrothermal synthesis is a ideal route for the synthesis of niobate materials, inwhich the reaction conditions can be readily adapted, the operation can be easilymanipulated, and the shape, size and structure of the products can be accuratelycontrolled. The obtained samples usually have the small diameters, the narrowparticle size distribution, and the high purity.In this thesis, therefore, the hydrothermal synthesis of a series of niobatenanostructures have been completed. It is meaningful to improve the syntheticmethods for niobate materials by summarizing the regularity and experience of theresearch. What’s more, the magnetic properties of all the samples have also beeninvestigated.First, the ellipsoidal MnNb2O6crystals have been successfully prepared by afacile hydrothermal method. X-ray powder diffraction patterns show that the productshave the typical orthorhombic columbite structure. The electron microscopy analysesreveal that the obtained MnNb2O6ellipsoids have a very rough surface with flakyhierarchical structures composed of nanoparticles with the diameters of10–20nm.Based on the time-dependent experiments, a possible growth mechanism of theellipsoidal hierarchitectures is also proposed. The magnetization studies demonstratethat the products exhibit an antiferromagnetic behavior with the Néel temperature ofabout4K. The microwave absorption properties of the ellipsoidal MnNb2O6hierarchical structures are also investigated with a vector network analyzer. Theabsorbing peak position moves to lower frequency with increasing the thickness ofthe sample. The value of the minimum reflection loss is11.6dB at7.8GHz with amatching thickness of3.5mm, and the reflection loss lower than10dB can be obtained in the frequency range of7.2–8.3GHz.Second, the FeNb2O6, CoNb2O6and NiNb2O6nanoparticles have beensuccessfully prepared via a facile hydrothermal method. X-ray powder diffractionpatterns show that all the products have the typical orthorhombic columbite structure.The electron microscopy analyses reveal that the obtained nanoparticles have thediameters of50–100nm. The magnetic property results demonstrate that themagnetically ordered state is hard to be observed down to1.8K for FeNb2O6sample,while the magnetic transition temperatures of TN=3K and TN=6K can be obtainedfor CoNb2O6and NiNb2O6samples, respectively. A weak ferromagnetic moment canbe detected below5K for both CoNb2O6and NiNb2O6samples. Furthermore, theNiNb2O6sample even exhibits a metamagnetic transition at about9kOe due to thespin flipping of the ferromagnetic chains.