24 research outputs found
Screening and Fabrication of Half-Heusler phases for thermoelectric applications
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
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
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
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
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