Detailed electronic structure studies on superconducting MgB2_2 and related compounds

In order to understand the unexpected superconducting behavior of MgB$_2$ compound we have made electronic structure calculations for MgB$_2$ and closely related systems. Our calculated Debye temperature from the elastic properties indicate that the average phonon frequency is very large in MgB$_2$ compared with other superconducting intermetallics and the exceptionally high $T_c$ in this material can be explained through BCS mechanism only if phonon softening occurs or the phonon modes are highly anisotropic. We identified a doubly-degenerate quasi-two dimensional key-energy band in the vicinity of $E_{F}$ along $\Gamma$-A direction of BZ which play an important role in deciding the superconducting behavior of this material. Based on this result, we have searched for similar kinds of electronic feature in a series of isoelectronic compounds such as BeB$_2$, CaB$_2$, SrB$_2$, LiBC and MgB$_2$C$_2$ and found that MgB$_2$C$_2$ is one potential material from the superconductivity point of view. There are contradictory experimental results regarding the anisotropy in the elastic properties of MgB$_2$ ranging from isotropic, moderately anisotropic to highly anisotropic. In order to settle this issue we have calculated the single crystal elastic constants for MgB$_2$ by the accurate full-potential method and derived the directional dependent linear compressibility, Young's modulus, shear modulus and relevant elastic properties. We have observed large anisotropy in the elastic properties. Our calculated polarized optical dielectric tensor shows highly anisotropic behavior even though it possesses isotropic transport property. MgB$_2$ possesses a mixed bonding character and this has been verified from density of states, charge density and crystal orbital Hamiltonian population analyses

Anisotropies of Compton profiles in nickel

The Compton profiles of nickel are calculated using an ab initio, spin-polarized self-consistent linear combination of Gaussian orbitals band-structure method within the local-density-functional theory. The resulting anisotropies are compared with recent experimental results of Rollason et al. The present theoretical results, which include the correlation potential, show improved agreement with experiment as compared to an earlier exchange-only calculation, especially in the low- and high-momentum regions. However, the present accurate local-density results still overestimate the anisotropies significantly, indicating the inadequacy of the local-density-functional theory for calculating the ground-state electron momentum density

Correlation between electronegativity and superconductivity

SIGLEITItal

Influence of microstructural changes on corrosion behaviour of thermally aged Ti-6Al-7Nb alloy

Solution treatment and ageing (STA) is an effective strengthening method for α + β titanium alloys. This paper reports the effect of solution treatment and aging on the corrosion behaviour of Ti-6Al-7Nb alloy in a simulated body fluid (Ringer's solution). Ti-6Al-7Nb alloy is hot rolled in the α + β field and subjected to solution treatment above and below its beta transus temperature (1283 K). The solution treated specimens are water quenched (WQ), air-cooled (AC), and furnace cooled (FC) at three different rates, and subsequently aged at 823 K for 4 h. Microstructural changes were examined using optical microscopy and phases developed were analyzed using XRD. The influence of microstructure on the corrosion performance of the alloys are discussed in detail based on the Open Circuit Potential (OCP), passive current density and area of repassivation loop values obtained from the cyclic polarization study in Ringer's solution. The passive current density was low (0.5 μA/cm2) for the specimen with duplex microstructure obtained for specimen solution treated at 1223 K, air-cooled, and aged, in comparison with that for as-rolled specimen (1.5 μA/cm2). The corrosion aspects resulting from various heat treatments are discussed in detail

Sliding wear behavior of plasma sprayed nanoceramic coatings for biomedical applications

This paper reports on the sliding wear performance of the nanostructured Al2O3–13TiO2, ZrO2 and the bilayered (ZrO2/Al2O3–13TiO2) coated biomedical Ti–13Nb–13Zr alloy in simulated body fluid condition. The nanopowders were sprayed using an atmospheric plasma spray technique and the reciprocatory sliding wear behavior of all the above coatings was evaluated using wear testing machine. The bilayered (ZrO2/Al2O3–13TiO2) coating which has not been reported hitherto exhibited two hundred and five hundred fold increase in the wear resistances when compared with that of the nanostructured Al2O3–13TiO2 (AT) and ZrO2 (YSZ) coatings. This substantial improvement in the wear resistance of the bilayered coating is attributed to its lower porosity and higher adhesion strength when compared to the AT and YSZ coatings. This study suggests that this new type of bilayered coating may be a preferred approach to be tried for obtaining much highe

Studies on plasma sprayed bu-layered ceramic coating on bio-medical Ti-13Nb-13Zr alloy

Biomedical Ti alloys are prone to undergo degradation due to the combined effect of wear and corrosion. To overcome these problems, surface modification techniques are being used. In this paper, the biomedical Ti alloy Ti–13Nb–13Zr was plasma sprayed with nanostructured Al2O3–13 wt%TiO2, yttria stabilized zirconia powders and bilayer containing alternate layers of the two coatings to improve the corrosion resistance and microhardness of the substrate. The plasma sprayed coatings were characterized by X-ray diffraction, scanning electron microscopy and Raman spectroscopy. The microstructure, microhardness and surface roughness of the coatings were investigated. The corrosion resistance of the coatings was studied in simulated body conditions. The results show improved corrosion resistance for the bilayered coating compared to the individual plasma sprayed coatings on biomedical Ti–13Nb–13Zr alloy substrate