24 research outputs found

    Screening and Fabrication of Half-Heusler phases for thermoelectric applications

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    Half-Heusler phases have gained recently much interest as thermoelectric materials. Screening of possible systems was performed by ab-initio simulation using VASP-software. The energy-versus-Volume (E(V)) curves were calculated and calibrated. For TiCoSb, NbNiSn, FeMoSb the stability of Half-Heusler phase against concurrent crystal structures like TiNiSi, ZrCoAl, ZrBeSi, FeSiV, ZrNiP and Full Heusler was confirmed. However, the thermo-dynamical driving force as calculated from the difference in lattice energies is less than 0.1eV/atom. Hence, the fabrication of Half Heusler phases is a challenge and requires three steps, surface activation of the raw material by ball milling, arc-melting of pressed pellets and finally long-term annealing treatment in a vacuum furnace. On doped TiCoSb specimens, Seebeck coefficients up to 0.1 mV/K, on NiNbSn 0.16 mV/K were measured, although the microstructure was not yet optimized.Comment: 6 pages, 5 figures, 2 table

    Interaction of fcc-Palladium nano-crystals with Hydrogen during PECVD growth of Carbon nanotubes

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    Using plasma-enhanced chemical vapor deposition (PECVD) with Acetylene and Ammoniac on Pd-specimens Carbon nanotubes (CNT) could be produced successfully. Two different devices are compared and the conditions for best growth conditions are explained. The detailed analysis of electron diffraction pattern obtained by transmissions electron microscopy (TEM) of as-grown specimen showed an expansion of the Pd lattice, which can be explained by the formation of fcc-PdHx for x=0...1. For x=1..2 the first investigation of hexagonal -PdH2 is reported, which lattice spacing is independent on the hydrogen content. The amount of fcc-PdH and hcp-PdH2 increases when the specimens are treated subsequently with Hydrogen. A growth model is provided.Comment: 6 pages, 6 figure

    Phonons in SrTiO3 analyzed by difference bond-length spectrum

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    Phonons change remarkable the interatomic bond length in solids and this work suggest a novel method how this behavior can be displayed and analyzed. The bond-length spectrum is plotted for each of the different atomic bonding types. When comparing the bond-length to an un-deformed crystal by the so-called difference bond length spectrum, the effect of phonons is clearly visible. The Perovskite lattice of SrTiO3 is used as an example and several lattice vibration modes are applied in a frozen phonon calculation in a 2x2x2 supercell. Ab-initio DFT simulations using the Vasp software were performed to calculate the density of states. The results show the important finding reported here first, that certain phonon interactions with shorter Ti-O bonds decrease the band gap, while changes in the Sr-Ti bond length enlarge the band gap.Comment: 6 pages, 6 figure

    SPS-Sintered NaTaO3-Fe2O3 Composite exhibits Large Seebeck Coefficient and Electric Current

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    NaTaO3-50wt% Fe2O3 composite ceramics showed a large Seebeck voltage of -300 mV at a temperature gradient of 650 K yielding a constant Seebeck coefficient of more than -500 microV/K over a wide temperature range. We report for the first time that SPS sintering at low temperature 870K could maintain the short-circuit current of -80 microA, which makes this thermoelectric material a possible candidate for high-temperature applications up to 1623 K. The reason for the good performance is the interface between Fe2O3 and surrounding NaTaO3 perovskite. When spark-plasma sintering (SPS) is used, constitutional vacancies disappeared and the electric conductivity increases remarkably yielding ZT of 0.016.Comment: 5 pages, 6 figures, 1 tabl

    Ab-initio calculations of the Optical band-gap of TiO2 thin films

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    Titanium dioxide has been extensively studied in recent decades for its important photocatalytic application in environmental purification. The search for a method to narrow the optical band-gap of TiO2 plays a key role for enhancing its photocatalytic application. The optical band gap of epitaxial rutile and anatase TiO2 thin films deposited by helicon magnetron sputtering on sapphire and on SrTiO3 substrates was correlated to the lattice constants estimated from HRTEM images and SAED. The optical band-gap of 3.03 eV for bulk-rutile increased for the thin films to 3.37 on sapphire. The band gap of 3.20 eV for bulk-anatase increases to 3.51 on SrTiO3. In order to interpret the optical band gap expansion for both phases, ab-initio calculations were performed using the Vienna ab-initio software. The calculations for rutile as well anatase show an almost linear increase of the band gap width with decreasing volume or increasing lattice constant a. The calculated band gap fits well with the experimental values. The conclusion from these calculations is, in order to achieve a smaller band-gap for both, rutile or anatase, the lattice constants c has to be compressed, and a has to be expanded.Comment: 4 pages, 4 figures, 1 tabl
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