Optical properties and oxidation of α-phase Ag-Al thin films

© 2017 IOP Publishing Ltd. We investigate a series of Ag-Al thin films containing up to 12 at% Al with the purpose of discovering whether these alloys would be a better choice for nanophotonic applications than pure Ag. Variable angle spectroscopic ellipsometry, AFM, x-ray diffraction and density functional theory are applied to explore and characterize the materials. Electromagnetic simulations of optical properties are used to place the results into a theoretical framework. We find that the increase in electron-to-atom ratio associated with the Al additions changes the optical properties: additions of the order of 1-2 at% Al are beneficial as they are associated with favorable changes in the dielectric function, but for greater additions of Al there is a flattening of the absorption edge and an increase in optical loss. In addition, contents of more than about 2 at% Al are associated with the onset of time-dependent intergranular oxidation, which causes a pronounced dip in the reflectance spectrum at about 2.3-2.4 eV (∼500-540 nm)

Dielectric function and its predicted effect on localized plasmon resonances of equiatomic Au-Cu

© 2015 IOP Publishing Ltd. Equiatomic (Au,Cu) solid solution orders below 658 K to form a tetragonal AuCu (I) phase with significant changes in physical properties and the crystal structure. The effect of ordering on the dielectric function of the material is controversial however, with inconsistent results reported in the literature. Since the nature of any localized surface plasmon resonance (LSPR) in the nanostructures is very sensitive to the dielectric function, this uncertainty hinders the use of AuCu in plasmonic devices or structures. Therefore, we re-examine the question using a combination of measurements and computations. We find that no significant change in the dielectric function occurs when this material becomes ordered, at least over the range of photon energies relevant to LSPRs. The likely properties of LSPRs in plasmonic devices made of AuCu are analyzed. Use of the alloy offers some advantages over pure Cu, however pure Au would still be the superior option in most situations

Optical properties and electronic structure of the Cu-Zn brasses

© 2015 Elsevier B.V. The color of Cu-Zn brasses range from the red of copper through bright yellow to grey-silver as the Zn content increases. Here we examine the mechanism by which these color changes occur. The optical properties of this set of alloys has been calculated using density functional theory (DFT) and compared to experimental spectroscopy measurements. The optical response of the low Zn content α-brasses is shown to have a distinctly different origin to that in the higher content β′, γ and ε-brasses. The response of β′-brass is unique in that it is strongly influenced by an overdamped plasmon excitation and this alloy will also have a strong surface plasmon response

Flux pinning mechanisms in graphene-doped MGB2 superconductors

The effects of graphene doping on the superconducting properties of MgB2 were studied. We found that small addition of graphene significantly improves the superconducting properties of MgB2, with only a small reduction in Tc. Low resistivity, high critical fields and enhanced flux-flow activation energy were observed for the optimally doped bulk sample. The spatial fluctuation in the transition temperature (dTc pinning) is the flux pinning mechanism in graphene-doped MgB2. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Thermal stability of nanoporous raney gold catalyst

© 2015 by the authors; licensee MDPI, Basel, Switzerland. Nanoporous “Raney gold” sponge was prepared by de-alloying an Au-Al precursor alloy. Catalytic tests using a micro-reactor confirmed that Raney gold can serve as an active heterogeneous catalyst for CO oxidation, reduction of NO to N2, and oxidation of NO to NO2. In general, the specific surface area of a heterogeneous catalyst has an influence on its catalytic efficacy. Unfortunately, gold sponges coarsen readily, leading to sintering of their structure and reduction in surface area. This potentially places constraints on their upper operating temperature in catalytic reactors. Here we analyzed the behavior of Raney gold when the temperature was raised. We examined the kinetics and mechanism of coarsening of the sponge using a combination of in situoptical measurements and Metropolis Monte Carlo modeling with a Lennard-Jones interatomic potential. Modeling showed that the sponges started with an isotropic “foamy” morphology with negative average “mean curvature” but that subsequent thermally activated coarsening will drive the morphology through a bi-continuous fibrous state and on, eventually, to a sponge consisting of sintered blobs of predominantly positive “mean curvature”

Evidence for transformation from δt<inf>c</inf> to δl pinning in MgB<inf>2</inf> by graphene oxide doping with improved low and high field J<inf>c</inf> and pinning potential

Flux pinning mechanism of graphene oxide (GO) doped MgB2 has been systematically studied. In the framework of the collective pinning theory, a B-T phase diagram has been constructed. By adjusting the GO doping level, the pinning mechanism in MgB2 transformed from transition temperature fluctuation induced pinning, δTc pinning, to mean free path fluctuation induced pinning, δl pinning, is observed. Furthermore, in terms of the thermally activated flux flow model, the pinning potential in high field (B > 5 T) is enhanced by GO doping. The unique feature of GO is the significant improvement of both low field Jc and high field J c. © 2013 American Institute of Physics

Effect of sintering temperature on the superconducting properties of graphene doped MgB<inf>2</inf>

A comprehensive study on the effects of sintering temperature on graphene-doped MgB2 superconductor was conducted. Graphene has emerged as an effective dopant that is capable of improving the critical current density (J-{\rm c}) and flux pinning at a very low doping level, with only a slight reduction of the critical temperature (T-{\rm c}). MgB2 undoped and graphene-doped bulk samples were prepared by the in situ method and sintered within a temperature range from 650 to 950 {\circ}\hbox{C}. It is surprising to note that at the doping level of 1 at.% the sample sintered at 850 {\circ}\hbox{C} shows a J\rm c of 5.6 \times 10{3}\ \hbox{A/cm} 2, which is nearly two times higher than that of the undoped sample, with a slight reduction in T\rm c of 0.5 K. The effects of the sintering temperature on the lattice parameters, resistivity, grain to grain connectivity, lattice disorder, and critical fields have also been investigated. The results are compared with those for undoped samples subjected to the same sintering conditions, and the origins of the differences in the critical current density are discussed. © 2002-2011 IEEE

Improving superconducting properties of MgB<inf>2</inf> graphene doping

we report the synthesis and characterization of MgB2 made from nano-boron and doped with graphene in the following mole percentages, X = 0, 3.0 and 12.0. The effect of graphene doping on the normal state resistivity (ρ), superconducting transition temperature Tc, irreversibility and upper critical fields (Hirr and Hc2), and critical current density Jc, as well as the pinning force (Fp) were evaluated. We found that the graphene doping has a positive impact on the above mentioned properties. In the case of the optimally doped (X = 3.0%) sample, the critical current density at 5 K corresponds to 1.4×105 A/cm2 for 2 T field, whereas the undoped sample showed 9.6×104 A/cm2 for the same field, i.e., 1.5 times improvement. Furthermore, the optimally doped sample showed a Jc of nearly 1×104 A/cm2 at 5K, 8 T, which is a significantly high value. The upper critical field has been enhanced to 13 T at 20Kfor the optimal doping level. The flux pinning behavior has been evaluated from the curve of flux pinning force against applied magnetic field, and it reveals that the maximum pinning has been improved by nearly 1.2 times at 20 K, due to the graphene doping. © 2010 IEEE

The effects of graphene doping on the in-field J <inf>c</inf> of MgB <inf>2</inf> wires

The field and temperature dependence of the critical current density J ct were measured for both un-doped and graphene doped MgB 2/Fe wires manufactured by 99.999% Crystalline Boron and 10% excess Magnesium (99%, 325 mesh). At 4.2 K and 10 T, J ct was estimated to be for the wire sintered at 800 °C for 30 minutes, the doped sample is almost improved as one order, compared with the best un-doped sample. At the same time, the temperature dependence of the upper critical field (H c2 and the irreversibility field (Hirr for the samples will also be included from the resistance (R)-temperature (T ). A significant increase in the upper critical field is the main cause of the enhancement of the critical current density, J ct , in the high field region. The calculated active crosssectional area fraction (AF represents the connectivity factor between adjacent grains. This value is decreased with wire samples, which is why the improvement of transport J ct is lower than the improvement of magnetic J cm in diffusion bulk sample. Copyright © 2012 American Scientific Publishers