Enhanced electron-phonon coupling in NbB2 by nanoscaling the grain size.

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]