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Register a new userA novel route to phase formation of cobalt oxyhydrates using KMnO4 as an oxidizing agent
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We have first succeefully synthesized the sodium cobalt oxyhydrate superconductors using KMnO4 as a de-intercalating and oxidizing agent. It is a novel route to form the superconductive phase of NaxCoO2.yH2O without resorting to the commonly used Br2/CH3CN solution. The role of the KMnO4 is to de-intercalate the Na+ from the parent compound Na0.7CoO2 and oxidize the Co ion as a result. The higher molar ratio of KMnO4 relative to the sodium content tends to remove more Na+ from the parent compound and results in a slight expansion of the c-axis in the unit cell. The superconducting transition temperature is 4.6-3.8 K for samples treated by the aqueous KMnO4 solution with the molar ratio of KMnO4 relative to the sodium content in the range of 0.3 and 2.29.
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This paper has been withdrawn by the author due to a data error in Fig 5.
We have first used the aquesous KMnO4 solutions to de-intercalate and oxideze gamma-phase of Na0.7CoO2 and successfully form the superconductive phase of cobalt oxyhydrate hydrates (Na,K)xCoO2.yH2O with Tc=3.2-4.6K based on the magnetization measurements. The higher molar ratio of KMnO4/Na used to treat Na0.7CoO2 results in more removal of Na+ and leads to a partial or even complete replacement of K+ for Na+. The low molar ratio of KMnO4/Na forms a superconductive phase with the c-axis ca. 19.6 angstrom, whereas the high molar ratio of KMnO4/Na forms a non-superconductive phase with teh c-axis ca. 13.9 angstrom. The superconductive 19.6 angstrom phase is unstable with respect to the ambient air in terms of losing water molecule from the structure; nevertheless, the dehydration/hydration process is reversible when storing the sample in a chamber with sufficient humidity.
Very recently, the tetragonal BiOCuS was synthesized and declared as a new superconducting system with Fe-oxypnictide - related structure. Here, based on first-principle FLAPW-GGA calculations, the structural parameters, electronic bands picture, density of states and electron density distribution for BiOCuS are investigated for the first time. Our results show that, as distinct from related metallic-like FeAs systems, BiOCuS phase behaves as an ionic semiconductor with the calculated indirect band gap at about 0.48 eV. The superconductivity for BiOCuS may be achieved exclusively by doping of this phase. Our preliminary results demonstrate that as a result of hole doping, the [CuS] blocks become conducting owing to mixed Cu 3d + S 3p bands located near the Fermi level. For the hole doped BiOCuS the Fermi surface adopts a quasi-two-dimensional character, similarly to FeAs SCs.
The highly unusual divalent silver in silver difluoride (AgF$_2$) features a nearly square lattice of Ag$^{+2}$ bridged by fluorides. As a structural and electronic analogue of cuprates, its superconducting properties are yet to be examined. Our first principles electronic structure calculations reveal a striking resemblance between AgF$_2$ and the cuprates. Computed spin susceptibility shows a magnetic instability consistent with the experimentally observed antiferromagnetic transition. A linearized Eliashberg theory in fluctuation-exchange approximation shows an unconventional singlet $d$-wave superconducting pairing for bulk AgF$_2$ at an optimal electron doping. The pairing is found to strengthen with a decreasing interlayer coupling, highlighting the importance of quasi-2D nature of the crystal structure. These findings place AgF$_2$ in the category of unconventional high-$T_text{C}$ superconductors, and its chemical uniqueness may help shed new lights on the high-$T_text{C}$ phenomena.
In this paper we report a new synthesis route to produce boron powders characterized as being amorphous and having very fine particle size. This route has been developed to improve the performances of superconducting MgB2 powders, which can be directly synthesized from this nano-structured boron precursor by following the ex-situ or the in-situ P.I.T. method during the manufacturing of tapes, wires and cables. All the procedure steps are explained and the chemical-physical characterization of the boron powder, using x-ray diffraction (Xrd), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques, is reported. Furthermore, a comparison with commercial boron is given. Preliminary results of the magnetic and electrical characterization, such as the critical temperature (TC) and the transport critical current density (JC t), for the MgB2 tape are reported and compared with the tape prepared with commercial boron.
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