2,905 research outputs found
Theoretical search for Chevrel phase based thermoelectric materials
We investigate the thermoelectric properties of some semiconducting Chevrel
phases. Band structure calculations are used to compute thermopowers and to
estimate of the effects of alloying and disorder on carrier mobility. Alloying
on the Mo site with transition metals like Re, Ru or Tc to reach a
semiconducting composition causes large changes in the electronic structure at
the Fermi level. Such alloys are expected to have low carrier mobilities.
Filling with transition metals was also found to be incompatible with high
thermoelectric performance based on the calculated electronic structures.
Filling with Zn, Cu, and especially with Li was found to be favorable. The
calculated electronic structures of these filled Chevrel phases are consistent
with low scattering of carriers by defects associated with the filling. We
expect good mobility and high thermopower in materials with the composition
close to (Li,Cu)MoSe, particularly when Li-rich, and recommend this
system for experimental investigation.Comment: 4 two-column pages, 4 embedded ps figure
Atomic force microscopy of supported planar membrane bilayers
Membrane bilayers of dipalmitoyl phosphatidylcholine (DPPC) and dipalmitoyl phosphatidylethanolamine (DPPE) adsorbed to a freshly cleaved mica substrate have been imaged by Atomic Force Microscopy (AFM). The membranes were mounted for imaging by two methods: (a) by dialysis of a detergent solution of the lipid in the presence of the substrate material, and (b) by adsorption of lipid vesicles onto the substrate surface from a vesicle suspension. The images were taken in air, and show lipid bilayers adhering to the surface either in isolated patches or in continuous sheets, depending on the deposition conditions. Epifluorescence light-microscopy shows that the lipid is distributed on the substrate surfaces as seen in the AFM images. In some instances, when DPPE was used, whole, unfused vesicles, which were bound to the substrate, could be imaged by the AFM. Such membranes should be capable of acting as natural anchors for imaging membrane proteins by AFM
Magnetism, Critical Fluctuations and Susceptibility Renormalization in Pd
Some of the most popular ways to treat quantum critical materials, that is,
materials close to a magnetic instability, are based on the Landau functional.
The central quantity of such approaches is the average magnitude of spin
fluctuations, which is very difficult to measure experimentally or compute
directly from the first principles. We calculate the parameters of the Landau
functional for Pd and use these to connect the critical fluctuations beyond the
local-density approximation and the band structure.Comment: Replaced with the revised version accepted for publication.
References updated, errors corrected, other change
Lattice dynamics and reduced thermal conductivity of filled skutterudites
The great reduction in thermal conductivity of skutterudites upon filling the
``void'' sites with Rare Earth (RE) ions is key to their favorable
thermoelectric properties but remains to be understood. Using lattice dynamic
models based on first principles calculations, we address the most popular
microscopic mechanism, reduction via rattling ions. The model withstands
inelastic neutron scattering and specific heat measurements, and refutes
hypotheses of an anharmonic RE potential and of two distinct localized RE
vibrations of disparate frequencies. It does indicate a strong hybridization
between bare La vibrations and certain Sb-like phonon branches, suggesting
anharmonic scattering by harmonic RE motions as an important mechanism for
suppression of heat conductivity.Comment: modified version resubmitted to PRB. Results unchanged, text changed
substantiall
Effect of heat treatments on precipitate microstructure and mechanical properties of CuCrZr alloy
Final report on neutron irradiation at low temperature to investigate plastic instability and at high temperature to study caviation
Quantum Critical Behavior and Possible Triplet Superconductivity in Electron Doped CoO2 Sheets
Density functional calculations are used to investigate the doping dependence
of the electronic structure and magnetic properties in hexagonal NaCoO.
The electronic structure is found to be highly two dimensional, even without
accounting for the structural changes associated with hydration. At the local
spin density approximation level, a weak itinerant ferromagnetic state is
predicted for all doping levels in the range to , with competing
but weaker itinerant antiferromagnetic solutions. The Fermi surface, as
expected, consists of simple rounded hexagonal cylinders, with additional small
pockets depending on the lattice parameter. Comparison with experiment
implies substantial magnetic quantum fluctuations. Based on the Fermi surface
size and the ferromagnetic tendency of this material,it is speculated that a
triplet superconducting state analogous to that in SrRuO may exist
here.Comment: 4 pages, 1 figur
Spin Fluctuations and the Magnetic Phase Diagram of ZrZn2
The magnetic properties of the weak itinerant ferromagnet ZrZn_2 are analyzed
using Landau theory based on a comparison of density functional calculations
and experimental data as a function of field and pressure. We find that the
magnetic properties are strongly affected by the nearby quantum critical point,
even at zero pressure; LDA calculations neglecting quantum critical spin
fluctuations overestimate the magnetization by a factor of approximately three.
Using renormalized Landau theory, we extract pressure dependence of the
fluctuation amplitude. It appears that a simple scaling based on the
fluctuation-dissipation theorem provides a good description of this pressure
dependence.Comment: 4 revtex page
Chemical Removal of Cordon Bunches in Dried Grape Production
The Shaw swing-arm trellis has increased mechanisation in dried grape production as it enables trellisdrying of the grapes in the vineyard. However, cordon bunches that do not dry reduce the quality of driedfruit at harvest. Traditionally, foliar application of ammonium nitrate was used to remove cordon bunches,but results were inconsistent. In recent years, health and safety concerns have heavily restricted the useof ammonium nitrate in the agricultural sector. Use of alternate chemical treatments was investigated tofind an alternative to ammonium nitrate for removal of cordon bunches in Carina Currant, Sultana andSunmuscat dried grape production. Different doses of calcium nitrate and Ethrel® (21.7%, w/v, ethephon)were applied to grapevines at various stages of vine growth. Calcium nitrate applied at a rate of 2.6%(w/v) to the vine pre-bloom at the 6 to 8 leaf stage (LS) and 250 ppm Ethrel® applied at full bloom (10 to 12LS) achieved the most effective results. Both treatments provide a cost effective alternative to ammoniumnitrate without any significant negative impact on growth, fruit bearing of vines and yield in the followingyear. Separate application scheduling of calcium nitrate (at pre-bloom) and Ethrel® (at full-bloom) offersflexibility in spray (application) schedules a management practices
Ab Initio Phonon Dispersions for PbTe
We report first principles calculations of the phonon dispersions of PbTe
both for its observed structure and under compression. At the experimental
lattice parameter we find a near instability of the optic branch at the zone
center, in accord with experimental observations.This hardens quickly towards
the zone boundary. There is also a very strong volume dependence of this mode,
which is rapidly driven away from an instability by compression. These results
are discussed inrelation to the thermal conductivity of the material.Comment: 3 figures; typos corrected. Figure 1 replaced to correct labe
- …