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Agía Paraskeví, Greece

Athanassas C.,Institute of Materials Science
Geochronometria | Year: 2011

Difficulties in evaluating accurate equivalent doses for Late Quaternary sediments from the south coasts of Greece arose in the course of SAR measurements. Large scatter among apparent equivalent doses perplexed the evaluation of accurate mean estimates in circumstances where very large doses were involved. Besides, dose recovery experiments revealed that when intersection of the natural signal occurs onto the saturating segment of the growth curve, a distinct relationship between De and growth curve shape occurs. At very low slope angles, SAR tends to overestimate the recov-ered dose. The mechanism which controls the spread in equivalent dose estimation is investigated. © 2011 Silesian University of Technology, Gliwice, Poland. All rights reserved. Source


Kovalchenko A.M.,Georgia Institute of Technology | Fushchich O.I.,Institute of Materials Science | Danyluk S.,Georgia Institute of Technology
Wear | Year: 2012

Under certain conditions, the metals of some heavily loaded tribological units may slide against each other without the benefit of lubrication. In such cases, the heat or electrical conduction properties of the sliding materials are particularly important. Copper-based materials are relevant for such application. In this study, we investigated the wear and friction that occurs between copper-based composites and copper under dry sliding conditions. We varied the composition levels of molybdenum disulfide and molybdenum diselenite (from 1 to 15 percent) that were incorporated into a Cu-based substrate. The specimen composition (Cu+1; 5; 10 and 15wt% MoS 2 and MoSe 2) was prepared by sintering powders with an average particle size of 35-40μm into a hydrogen atmosphere at a temperature of 780°C for 2h. Pin-on-disk experiments were performed without any lubrication at a speed of 0.15m/s while maintaining a contact pressure of 0.127-1MPa. The prepared samples were used as flat-ended pins and slid against a copper plate to ensure contact conformability. The reference copper rapidly began scuffing and eventually experienced a catastrophic failure of the contact surfaces after a relatively short period of smooth sliding. Scuffing and catastrophic damage was prevented by using powder metallurgy technology to incorporate MoS 2 and MoSe 2 into the copper. Materials with MoS 2 and MoSe 2 had a coefficient of friction close to 0.2, undetectable linear wear under a contact pressure of less than 0.255MPa, and an absence of material transfer between the pin and the copper plate. The solid lubricant embedded in the composite materials disintegrated and lubricated the surfaces; however, a protective layer did not form on the counterface. Heavier loads initiated substantial wear. Material removal resulted from adhesion between the sliding composite materials and the transferred material on the Cu counterface, which forms flaky debris. The mechanism of wear of the composite materials Cu-MoS 2 and Cu-MoSe 2 is consistent with the material removal process. As a function of composition, the lubricating effect is more pronounced at concentrations greater than 5wt% of the solid lubricant. © 2012 Elsevier B.V. Source


Pradhan S.K.,Institute of Materials Science
Journal of Materials Science: Materials in Electronics | Year: 2013

The effect of iron deficiency for the improvement of multiferroic properties of Ho doped BiFeO3 ceramics (BiHoXFe 1-XO3 for x = 0, 0.05, 0.1 and 0.15) prepared by conventional solid state reaction route were investigated carefully. As pure BiFeO3 exhibited antifer-romagnetism, lossy ferroelectric (P-E) hysteresis loop and low dielectric constant value so, device integration is significantly hindered due to the presence of impurity phases associated with it. However, suitable variation of metal ion concentration in Ho doped BiFeO3 significantly enhanced both ferroelectric and ferromagnetic properties of the samples. Therefore, increase of Ho concentration certainly favors for suppression of impurity phases in rhombohedral (R3c) structure of BiFeO3 along with no structural transformation. On the other hand, increase of iron deficiency may suppress transformation of Fe from Fe 3+ to Fe2+ as a result, the dielectric constant value of the sample increases along with large reduction of leakage current behavior. © Springer Science+Business Media New York 2012. Source


Pradhan S.K.,Institute of Materials Science
Journal of Materials Science: Materials in Electronics | Year: 2013

Monophasic rhombohedral structure of BiFeO3 electroceramic is successfully synthesized by conventional solid state reaction route followed by slow step sintering schedule. Effect of sintering temperature is found to greatly influence its structural, dielectric, ferroelectric, capacitance and leakage behavior of bulk ceramic. From XRD analysis it is seen that at lower sintering temperature (750 C) bulk BiFeO3 sample showed rhombohedral structure (R3c) along with few impurity phases, which become suppressed at higher sintering temperature and facilitates the compactness of grains and formation of dense microstructure. The leakage current and capacitive characteristic of the sample was improved significantly with increase in sintering temperature of BiFeO3 (850 C). At higher sintering temperature, ferroelectric behavior of the sample is found to change its shape from semi elliptical lossy P-E features to a typical ferroelectric loop with improvement of its remnant as well as saturation polarization value. Raman spectra over the frequency range of 100-700 cm-1 have been systematically investigated. Besides the changes of the peak position and the line width of all modes, the prominent frequency shift, the line broadening and variation of the intensity were observed with increase in sintering temperature. © 2013 Springer Science+Business Media New York. Source


Carbon nanotube above a photonic crystal waveguide with electrodes. The structure converts electric signals into light. Credit: WWU Worldwide growing data volumes make conventional electronic processing reach its limits. Future information technology is therefore expected to use light as a medium for quick data transmission also within computer chips. Researchers under the direction of KIT have now demonstrated that carbon nanotubes are suited for use as on-chip light source for tomorrow's information technology, when nanostructured waveguides are applied to obtain the desired light properties. The scientists now present their results in Nature Photonics. On the large scale, data transmission by light has long become a matter of routine: Glass fiber cables as light waveguides transmit telephone and internet signals, for instance. For using the advantages of light, i.e. speed and energy efficiency, also on the small scale of computer chips, researchers of KIT have made an important step from fundamental research towards application. By the integration of smallest carbon nanotubes into a nanostructured waveguide, they have developed a compact miniaturized switching element that converts electric signals into clearly defined optical signals. "The nanostructures act like a photonic crystal and allow for customizing the properties of light from the carbon nanotube," Felix Pyatkov and Valentin Fütterling, the first authors of the study of KIT's Institute of Nanotechnology, explain. "In this way, we can generate narrow-band light in the desired color on the chip." Processing of the waveguide precisely defines the wavelength at which the light is transmitted. By engravings using electron beam lithography, the waveguides of several micrometers in length are provided with finest cavities of a few nanometers in size. They determine the waveguide's optical properties. The resulting photonic crystals reflect the light in certain colors, a phenomenon observed in nature on apparently colorful butterfly wings. As novel light sources, carbon nanotubes of about 1 micrometer in length and 1 nanometer in diameter are positioned on metal contacts in transverse direction to the waveguide. At KIT, a process was developed, by means of which the nanotubes can be integrated specifically into highly complex structures. The researchers applied the method of dielectrophoresis for deposition of carbon nanotubes from the solution and their arrangement vertically to the waveguide. This way of separating particles using inhomogeneous electric fields was originally used in biology and is highly suited to deposit nanoscaled objects on carrier materials. The carbon nanotubes integrated into the waveguide act as a small light source. When electric voltage is applied, they produce photons. The compact electricity/light signal converter presented now meets the requirements of the next generation of computers that combine electronic components with nanophotonic waveguides. The signal converter bundles the light about as strongly as a laser and responds to variable signals with high speed. Already now, the optoelectronic components developed by the researchers can be used to produce light signals in the gigahertz frequency range from electric signals. Among the leading researchers involved in the project were Ralph Krupke, who conducts research at the KIT Institute of Nanotechnology and at the Institute of Materials Science of TU Darmstadt, Wolfram H.P. Pernice, who moved from the KIT to the University of Münster one year ago, and Manfred M. Kappes, Institute of Physical Chemistry and Institute of Nanotechnology of KIT. The project was funded by the Science and Technology of Nanosystems (STN) programme of the Helmholtz Association. It is aimed at studying nanosystems of unique functionality and the potential of materials of a few nanometers in structural size. The Volkswagen Foundation financed a Ph.D. student position for the research project. In addition, the project was supported by the Karlsruhe Nano Micro Facility (KNMF) platform. Explore further: World-record micrometer-sized converter of electrical into optical signals More information: Felix Pyatkov et al. Cavity-enhanced light emission from electrically driven carbon nanotubes, Nature Photonics (2016). DOI: 10.1038/NPHOTON.2016.70

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