Carbon substitution in MgB2 single crystals: structural and superconducting properties

Growth of carbon substituted magnesium diboride Mg(B1-xCx)2 single crystals with 0<x<0.15 is reported and the structural, transport, and magnetization data are presented. The superconducting transition temperature decreases monotonically with increasing carbon content in the full investigated range of substitution. By adjusting the nominal composition, Tc of substituted crystals can be tuned in a wide temperature range between 10 and 39 K. Simultaneous introduction of disorder by carbon substitution and significant increase of the upper critical field Hc2 is observed. Comparing with the non-substituted compound, Hc2 at 15K for x=0.05 is enhanced by more then a factor of 2 for H oriented both perpendicular and parallel to the ab-plane. This enhancement is accompanied by a reduction of the Hc2-anisotropy coefficient gamma from 4.5 (for non-substituted compound) to 3.4 and 2.8 for the crystals with x = 0.05 and 0.095, respectively. At temperatures below 10 K, the single crystal with larger carbon content shows Hc2 (defined at zero resistance) higher than 7 and 24 T for H oriented perpendicular and parallel to the ab-plane, respectively. Observed increase of Hc2 cannot be explained by the change in the coherence length due to disorder-induced decrease of the mean free path only.Comment: 22 pages, 9 figures, 4 table

WDX Studies on Ceramic Diffusion Barrier Layers of Metal Supported SOECs

Solid oxide electrolyser cells (SOECs) have great potential for efficient and economical production of hydrogen fuel. Element diffusion between the Ni-cermet electrode and the metal substrate of metal supported cells (MSC) is a known problem in fuel cell and electrolysis technology. In order to hinder this unintentional mass transport, different ceramic diffusion barrier layers (DBLs) are included in recent cell design concepts. This paper is based on wavelength dispersive X-ray fluorescence investigations of different SOEC and focuses on Fe, Cr and Ni diffusion between the metal grains of the cathode and the metal substrate. Due to the low detection limits and therefore high analytical sensitivity, wavelength dispersive electron probe microanalysis (EPMA) provides a precise method to determine element distribution, absolute element concentration and changes between the reference material and aged cells on a microstructural level by element mappings and concentration profiles. The results of this work show considerable concentration gradients in the metal grains caused by mass exchange during cell operation. Diffusion can be inhibited significantly by integrating different ceramic DBLs of doped LaCrO₃-type or doped LaMnO₃-type perovskite, either by vacuum plasma spraying (VPS) or physical vapour deposition technique (PVD)