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Abstract Nanocrystalline NbB 2 powder has been synthesized via a single step solid-state reaction of precursor materials Nb 2 O 5 and NaBH 4. Phase composition, grain morphology, microstructure, phonon vibrations and superconducting properties were investigated by XRD, FESEM, HRTEM, Raman spectroscopy and PPMS. The results show that the grain size of nanocrystalline NbB 2 is very sensitive to the reaction temperature and easy to aggregate to form micron scale of non-morphology powders. A systematic increase of the electron-phonon coupling constant with the nanoscaling grain size is observed, which we explain in terms of crystal defects and phonon vibrations. The zero-temperature critical fields, H c 1 (0) and H c 2 (0), have been estimated at 85 Oe and 7200 Oe, respectively, and the Ginzburg–Landau parameter κ is 5.1, confirming that NbB 2 is a type-II superconductor. Highlights • Single-phased NbB 2 nanocrystalline has been synthesized by a solid-state reaction in a continuous evacuating process. • The reaction temperature has an important effect on the grain size. • A systematic increase of the electron-phonon coupling constant with the nanoscaling grain size is observed. [ABSTRACT FROM AUTHOR]

Abstract: The high quality La x Ce1− x B6 (x =0, 0.2, 0.4, 0.6, 1) single crystals with large emission current density have been successfully grown by floating zone method. The crystal structure, crystal quality and emission properties of La x Ce1−xB6 single crystals were characterized by 360° Phi scanning single crystal diffractometer, X-ray Laue diffraction and the thermal electron emission measurements. It is found that the as-grown crystals have a good crystal quality without subgrains and twin crystal phenomenon. The thermionic emission measurement results show that the maximum emission current density on (100) surface of La0.2Ce0.8B6 single crystal is 71.2A/cm2 at 1873K under an applied voltage of 800V, which is larger than the reported values in the literatures up to now. Furthermore, the crystals with different components also show a higher emission current density such as 53.8A/cm2 for La0.4Ce0.6B6 and 53.3A/cm2 for CeB6 single crystals. The field emission single tip made by LaB6 single crystal shows a superior performance than the polycrystalline LaB6 field emission tip and its field enhancement factor (β) is 36,231cm−1. Thus, the La x Ce1− x B6 single crystals will have promising application prospects as a hot or cold cathode. [Copyright &y& Elsevier]