Vacuum encapsulated synthesis of 11.5 K NbC superconductor

Journal of Superconductivity and Novel Magnetism (Impact Factor: 0.93). 05/2012; 25:1421–1425. DOI: 10.1007/s10948-012-1654-6
Source: arXiv

ABSTRACT Bulk polycrystalline NbC samples are synthesized through solid state reaction
route in an evacuated sealed quartz tube. Studied NbC samples are crystallized
in NaCl-type cubic structure with space group Fm-3m. To control cell parameters
and minute un-reacted phases, different samples are synthesized with various
heat treatments. Finally phase pure NbC is achieved. The grain size of the as
systemized material being seen from SEM (scanning electron microscopy) is
non-uniform of around 3-10\mu m size. Crystal structure and lattice parameters
of samples have been calculated by Rietveld analysis of room temperature X-ray
powder diffraction data. The lattice parameter increases with synthesis
temperature and scales with superconducting transition temperature (Tc). Both
AC and DC magnetization exhibited highest Tc at around 11.5 K for an NbC sample
with lattice parameter a = 4.471 {\AA}. The lower critical field (Hc1) and
irreversibility field (Hirr) measured at 3 K are around 250 Oe and 4.5 kOe
respectively. The upper critical field (Hc2) being determined from in-field AC
susceptibility measurements is 7.8 kOe and 11.7 kOe with 50% and 90%
diamagnetism criteria, respectively.

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    ABSTRACT: A niobium carbide diffusion layer with a cubic NbC phase surface layer (˜6 μm) and a Nb and C diffusion subsurface layer (˜1 μm) is fabricated on the surface of AISI 304 stainless steel (304 SS) bipolar plate in a proton exchange membrane fuel cell (PEMFC) using plasma surface diffusion alloying. The electrochemical behaviour of the niobium carbide diffusion-modified 304 SS (Nb-C 304 SS) is investigated in simulated PEMFC environments (0.5 M H2SO4 and 2 ppm HF solution at 80 °C). Potentiodynamic, potentiostatic polarisation and electrochemical impedance spectroscopy measurements reveal that the niobium carbide diffusion layer considerably improves the corrosion resistance of 304 SS compared with untreated samples. The corrosion current density of Nb-C 304 SS is maintained at 0.058 μA cm-2 and 0.051 μA cm-2 under simulated anodic and cathodic conditions, respectively. The interfacial contact resistance of Nb-C 304 SS is 8.47 mΩ cm2 at a compaction force of 140 N cm-2, which is significantly lower than that of the untreated sample (100.98 mΩ cm2). Moreover, only a minor increase in the ICR of Nb-C 304 SS occurs after 10 h potentiostatic tests in both cathodic and anodic environments.
    Journal of Power Sources 01/2014; · 5.21 Impact Factor

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Rajveer Jha