Preparation, Characterization And Growth Mechanism Of Boron-Rich Nanostructures

Boron-rich nanostructures, including B₄C and B₆O naonstructures, wereproduced in high yields by conventional solid state sintering. In this dissertationdetailed characterizations have been conducted on the lamellar-twinnedwhiskers and fivefold twinned nanowires to reveal their internal defects, surfacestructure and chemical composition. Based on the insight into the microstructureof boron-rich materials at atomic level, growth models involving the nucleation,growth and the morphology evolution has been proposed.A high yield of boron-rich nanostructure with controllable chemicalcomposition and microstructure has been achieved. The sintering temperatureand the start materials play an import role in the yield, microstructures andcomposition of the boron-rich structures and their effects have been analyzedsystematically. On a basis of the TEM characterization of the boron-richnanostructures and their variants, the structural evolution of boron-richnanostructures is summarized to give guidance on the controllable synthesis ofboron-rich materials.By the combination of series tilt diffraction, high resolution electronmicroscopy and electron energy loss spectroscopy, high density of defects,mainly in the form of microtwins has been disclosed in the boron-rich whiskers.It is demonstrated the growth of the whiskers with 'slab' like morphology iscontrolled by the well know twining plane reentrant edge (TPRE) mechanismwhich is previously assumed as valid in FCC system. Our experimentobservation lends direct support that TPRE is a general crystal growth theoryand can be directly applied to relevant systems with low twinning formationenergy.Through the crystallography analysis of a boron carbide nanorod withpseudo-fivefold twinned cross section, we present a structural model that, theangular misfit of excess5°for the cyclic twinned B4C structures, has beenmostly accommodated in one of the five twinned segments, with other segmentsand the twining boundaries well relaxed. Besides, close insepection of thefivefold twinned boron-rich nanowires implies the five segments (T1^T5) can develop independently, which is not the case in the well analyzed cyclic twinedmetallic nanowires. The intriguing phenomenon is directly related to the highdensity of microtwins in boron-rich star-shaped nanowires.The surface chemistry of both nanowires and whiskers has been closedinspected by Z-contrast technique with atomic resolution. Barium atoms whichcan give much stronger contrast in Z-contrast image compared to lightboron-rich materials tend to segregate along the (001) surface when thesintering temperature is set below1300℃. Strikingly,1300℃is the transitiontemperature above which the barium segregation inclines to be more disorder.The order and disorder transition of surface segregation along the specificcrystallography planes has been firstly revealed in boron-rich materials.Moreover, the surface chemistry dramatically alters the surface structures whichgive rise to the unique transition from the lamella twinning to cyclic twinning at1300℃. The surface structure transformation model successfully explains themorphology of a large number of boron-rich nanostructures observed both inprevious studies and our system.