Preparation,characterization And Physical Properties Of Nanosized Pseudobinary Rare-earth Hexaborides R_1-xCa_xB₆?R=La,Ce?

In this thesis,the single phase and ternary Ce_1-xCa_xB₆ nanosize powder was synthesized by using a solid state reaction in a continuous vacuum condition with CeO₂—Ca O—Na BH₄ system as precursors.The effects on phase,morphology and optical absorption properties for the various Ca doping content were examined by X-ray diffractometer?XRD?,field emission scanning electron microscope?FESEM?and Perkin Elmer Lambda-35 UV-vis spectrophotometer.The relation between the reaction temperature and optical absorption performance was discussed in terms of Ce_0.8Ca_0.2B₆ nanocrystalline.Moreover,a theoretical illustration was presented for optical absorption properties of Ce_1-xCa_xB₆ with various x values in the framework of density functional theory.And the band structure,density of states and optical properties were calculated by the method of LDA+U approximation.?1?As a result,the X-ray diffraction results show the Ca doping unchanged crystal structure and phase of CeB₆.To determine the synthesis sequence of phase formation,the Ca doping component x = 0.2 as an example was heated at different temperatures.The single-phase Ce_0.8Ca_0.2 B₆ nanometer powder can be obtained under reaction temperature of 900 ? for 2h.The images were captured by FESEM show that when the reaction temperatures are at 900 ? and 1000 ?,the distribution and the degree of crystallinity of Ce_0.8Ca_0.2B₆ samples are poor and the shape is irregular;At the elevated reaction temperature to 1100 ?,the Ce_0.8Ca_0.2B₆ nanometer powder with better cubic shape disperses and crystalizes well,average granularity is 130 nm;As the reaction temperature was further elevated to 1200 ?,the as-prepared product of Ce_0.8Ca_0.2B₆ with perfect cubic shape exhibits superior dispersity and crystallinity,average particle size increase to 150 nm in which it shows the grain growth behavior.Meanwhile,the different values of x of Ce_1-xCa_xB₆ system at 1200 ? was observed by field emission scanning electron microscope.It is found that with the doping concentration decreasing,the uniformity in size to some extent increases and the degree of perfectness of cubic shape observed in the FESEM patterns reduces.The selected area corresponding to the EDS image initially shows that the Ca dopes into Ce B₆ lattice.This area to Element Mapping shows that the distribution of Ca in Ce B₆ nanoparticles is homogeneous;?2?Perkin Elmer Lambda 35 UV-vis spectrophotometer was used to measure the optical absorption property at room temperature.It is found that the absorption valley moves from 604 nm to 685 nm with the Ca contents increasing in the absorption spectra,which indicates an obvious redshift phenomenon.Furthermore,when the doping content is 20%,the location of transmission peak will shift from 662 nm to 621 nm,indicating a blue-shift phenomenon;?3?the first principles calculation results reveal that Ca 3d state near the Fermi energy affects the optical properties of Ce0.875Ca0.125B₆ system and the Ce_1-xCa_xB₆ system Fermi energy decreases with the content x increasing.The result is good agreement with the experimental data.And the absorption spectrum law of theory model is in accordance with the experiment data.On the other hand,this thesis aims to discover a technique for the fabrication of La B₆ ultrafine nanometer powders so as to improve the disadvantages of the current preparation methods.The powders were systematically examined by transmission electron microscope?TEM?,high resolution transmission electron microscope?HRTEM?and other methods.The La B₆ morphology evolution mechanism during cooling crystallization will be discussed on the basis of Ostwald law.The consequences are presented as follows:?1?The XRD measurements show that ultrafine La B₆ nanopowders are single phase and superior crystallinity,particle size?32.7 nm?is deduced from Scherrer formula;?2?The images analysis of FESEM and TEM indicate that the particle with excellent dispersion is 40 nm in mean size.Moreover,it can be clearly seen that spheres,hexagons and cubes coexist,which initially is indicative of morphology evolution during cooling crystallization;?3?the as-obtained product was characterized by HRTEM,which further proves that morphology evolution exists in the crystallization process.Unstable morphologies of spheres and hexagons are firstly formed.With temperature decreasing or time passing,stable morphology of cubes takes shape.The images of the selected single particle area were measured by HRTEM.And the results show that there are no defects such as fault,dislocation or twin crystal in parallel arrangement of crystal.This indicates that the crystallinity of the particles is fine.Interplanar spacing d = 0.42 nm is good agreement with?100?crystal plane spacing corresponding to the cubic structure.Fast Fourier transform further confirms that the as-prepared product has a good crystalline nature and simple cubic structure.