Dissipation in Mesoscopic Superconductors with Ac Magnetic Fields

The response of mesoscopic superconductors to an ac magnetic field is investigated both experimentally and with numerical simulations. We study small square samples with dimensions of the order of the penetration depth. We obtain the ac susceptibitity $\chi=\chi'+i\chi''$ at microwave frequencies as a function of the dc magnetic field $H_{dc}$. We find that the dissipation, given by $\chi''$, has a non monotonous behavior in mesoscopic samples. In the numerical simulations we obtain that the dissipation increases before the penetration of vortices and then it decreases abruptly after vortices have entered the sample. This is verified experimentally, where we find that $\chi''$ has strong oscillations as a function of $H_{dc}$ in small squares of Pb.Comment: 4 pages, 2 figure

Sputter deposition of Mo-alumina cermet solar selective coatings: Interrelation between residual oxygen incorporation, structure and optical properties

Mo/alumina cermet-based selective coatings are of great interest for concentrated solar-thermal power systems, in particular, parabolic trough collectors. We report on the sputter deposition of high-performance multilayer Mo/alumina cermet coatings on stainless steel with a solar absorptance of 94% and a thermal emittance of 8% (at 400 °C), and excellent thermal stability. The selective coatings were deposited in a 0.95 m ^3 sputtering chamber in order to correlate the deposition parameters, such as presence of residual gases, deposition power, and sputtering method (DC or RF), with the coating composition and the resulting optical properties. X-ray photoelectron spectroscopy, x-ray diffraction, and Raman spectroscopy have been applied to quantitatively describe the effect of residual oxygen on the distribution of oxidation states of Mo in the metallic infrared reflector layer, the high and low metallic volume fraction cermet layers, as well as the composition of the alumina top layer. The results provide strategies to obtain optimal selective coatings under conditions where residual oxygen cannot be avoided, which are essential for a successful transition from a laboratory to pre-industrial scale of vacuum deposition systems

A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Mo back contact

CdS/CdTe thin film solar cells with an area of 1 cm2 were obtained and studied in detail. A ZnO buffer layer was deposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also by using RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of the solar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched by using Br-Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Mo targets. The procedure developed in this work led us to make systematically solar cells with good efficiency. However, the series resistance has a high value for an area of 1 cm2 (22 Ω cm2). In order to make more detailed study, the solar cell with an area of 1 cm2 was divided in a 3 × 3 matrix. A good homogeneity in cell properties is observed and the efficiency increases to more than 11%, fundamentally through decreasing series resistance

Thermal Characterization of Carbon Fiber-Reinforced Carbon Composites

Carbon fiber-reinforced carbon (C/C) composites consist in a carbon matrix holding carbon or graphite fibers together, whose physical properties are determined not only by those of their individual components, but also by the layer buildup and the material preparation and processing. The complex structure of C/C composites along with the fiber orientation provide an effective means for tailoring their mechanical, electrical, and thermal properties. In this work, we use the Laser Flash Technique to measure the thermal diffusivity and thermal conductivity of C/C composites made up of laminates of weaved bundles of carbon fibers, forming a regular and repeated orthogonal pattern, embedded in a graphite matrix. Our experimental data show that: i) the cross-plane thermal conductivity remains practically constant around (5.3 ± 0.4) W·m −1  K −1 , within the temperature range from 370 K to 1700 K. ii) The thermal diffusivity and thermal conductivity along the cross-plane direction to the fibers axis is about five times smaller than the corresponding ones in the laminates plane. iii) The measured cross-plane thermal conductivity is well described by a theoretical model that considers both the conductive and radiative thermal contributions of the effective thermal conductivity