302 research outputs found
Electrical Switching in Thin Film Structures Based on Transition Metal Oxides
Electrical switching, manifesting itself in the nonlinear current-voltage characteristics with S- and N-type NDR (negative differential resistance), is inherent in a variety of materials, in particular, transition metal oxides. Although this phenomenon has been known for a long time, recent suggestions to use oxide-based switching elements as neuristor synapses and relaxation-oscillation circuit components have resumed the interest in this area. In the present review, we describe the experimental facts and theoretical models, mainly on the basis of the Mott transition in vanadium dioxide as a model object, of the switching effect with special emphasis on the emerging applied potentialities for oxide electronics
Semiempirical Hartree-Fock calculations for KNbO3
In applying the semiempirical intermediate neglect of differential overlap
(INDO) method based on the Hartree-Fock formalism to a cubic perovskite-based
ferroelectric material KNbO3, it was demonstrated that the accuracy of the
method is sufficient for adequately describing the small energy differences
related to the ferroelectric instability. The choice of INDO parameters has
been done for a system containing Nb. Based on the parametrization proposed,
the electronic structure, equilibrium ground state structure of the
orthorhombic and rhombohedral phases, and Gamma-TO phonon frequencies in cubic
and rhombohedral phases of KNbO3 were calculated and found to be in good
agreement with the experimental data and with the first-principles calculations
available.Comment: 7 pages, 2 Postscript figures, uses psfig.tex. To be published in
Phys.Rev.B 54, No.4 (1996
Колебательная, релаксационная и трансляционная подвижность ионов Li+ в монокристалле LiNbGeO5.
Dielectric properties of single crystals of LiNbGeO5, crystallizing in the andalusite (β-Al2SiO5) structure, have been studied. Dielectric and conducting properties along the major crystallographic axes were investigated at frequencies from 1 to 1000 kHz and in the range of temperatures between 100 and 600 K. Determined potential barriers between the equilibrium positions of Li+ ions are classified as low energy (0.23-0.79 eV, along the a axis) and high energy (0.9-1.90 eV, along the a, b, and c axes). Additional interstitial sites with the distances between them 0.3-0.4 Å have been found along the a axis, while no such sites observed along the b and c axes. Activation energy (Ea) in the direction a is 0.47 eV and ?a(570 K) = 2.10-5 S/cm, which correspond to the super-ionic conductivity.Исследованы диэлектрические свойства монокристаллов LiNbGeO5 со структурой минерала андалузита (β-Al2SiO5). Диэлектрические и проводящие свойства изучены вдоль главных кристаллографических осей a, b, c при частоте измерительного поля 1-1000 кГц в интервале температур 100-600 К. Найденные значения потенциальных барьеров между положениями равновесия ионов Li+ подразделяются на низкоэнергетические (0.23-0.79 эВ, a-ось) и высокоэнергетические (0.9-1.80 эВ, a, b и c-оси). В направлении оси a обнаружены дополнительные междуузельные позиции с расстояниями между ними 0.3-0.4 A, в то время как для осей b и c такие позиции отсутствуют. В направлении a энергия активации Еa = 0.47 эВ и значение ?a(570 К) = 2.10-5 См/см, что соответствует суперионной проводимости
Theory of bound polarons in oxide compounds
We present a multilateral theoretical study of bound polarons in oxide
compounds MgO and \alpha-Al_2O_3 (corundum). A continuum theory at arbitrary
electron-phonon coupling is used for calculation of the energies of thermal
dissociation, photoionization (optically induced release of an electron (hole)
from the ground self-consistent state), as well as optical absorption to the
non-relaxed excited states. Unlike the case of free strong-coupling polarons,
where the ratio \kappa of the photoionization energy to the thermal
dissociation energy was shown to be always equal to 3, here this ratio depends
on the Froehlich coupling constant \alpha and the screened Coulomb interaction
strength \beta. Reasonable variation of these two parameters has demonstrated
that the magnitude of \kappa remains usually in the narrow interval from 1 to
2.5. This is in agreement with atomistic calculations and experimental data for
hole O^- polarons bound to the cation vacancy in MgO. The thermal dissociation
energy for the ground self-consistent state and the energy of the optically
induced charge transfer process (hops of a hole between O^{2-} ions) have been
calculated using the quantum-chemical method INDO. Results obtained within the
two approaches for hole O polarons bound by the cation vacancies (V^-) in
MgO and by the Mg^{2+} impurity (V_{Mg}) in corundum are compared to
experimental data and to each other. We discuss a surprising closeness of the
results obtained on the basis of independent models and their agreement with
experiment.Comment: 13 pages, 2 figures, 2 tables, E-mail addresses:
[email protected], [email protected]
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Science and Knowledge Informing Policy and People: The Human Dimensions of Wave Energy Generation in Oregon Final Report to the Oregon Wave Energy Trust
In 2007, the Oregon Wave Energy Trust (OWET) put out a request for proposals to begin to discover answers to many of the environmental and human dimensions questions. A multidisciplinary group of social scientists – Flaxen Conway, Brent Steel, Michael Harte, and Bryan Tilt, Oregon State University – responded to this call. Working together, they created a new research program at OSU. The Human Dimension of Wave Energy (HDWE) research program was created to provide the opportunity for a cadre of social scientists – professors and graduate students – to study this new use of the ocean space and place. This report include six research papers. THree areas of concern emerge. First, we learned that in a time where energy demands are increasing and existing supplies are either decreasing or creating other challenges, the ocean energy industry must move in a socially, economically, and environmentally responsible manner. These are keys for sustainability and acceptance. There are legal and regulatory best practices that can be incorporated. Developers will continue to manage their way through an evolving process. Governments will be continuing to develop or build their expertise as they manage the ocean space and place for the public who owns it. Secondly, policies will continue to be made and hopefully monitored, evaluated, and improved. The public and policy actors need to be aware and engaged. Perceptions and opinions will be changed not by pressure or force, but with trusted knowledge and communication. Of special interest to OWET and others could be the trusted sources of information for each audience group. At least four of the six projects report back on this. Third, investments in research and testing are not only sound because they bring about answers to important questions but also because the public and others support and expect this investment. The “jury is still out” on what forms of energy generation should/could be in Oregon’s energy portfolio. Research and testing are important to developers (for technology advancement and sound business planning), policy actors and government (for innovative and effective policies), stakeholders (for site selection, technology improvement and functionality), and the public (for understanding and support).Submitted to OWET September 30, 2009
Relative energetics and structural properties of zirconia using a self-consistent tight-binding model
We describe an empirical, self-consistent, orthogonal tight-binding model for
zirconia, which allows for the polarizability of the anions at dipole and
quadrupole levels and for crystal field splitting of the cation d orbitals.
This is achieved by mixing the orbitals of different symmetry on a site with
coupling coefficients driven by the Coulomb potentials up to octapole level.
The additional forces on atoms due to the self-consistency and polarizabilities
are exactly obtained by straightforward electrostatics, by analogy with the
Hellmann-Feynman theorem as applied in first-principles calculations. The model
correctly orders the zero temperature energies of all zirconia polymorphs. The
Zr-O matrix elements of the Hamiltonian, which measure covalency, make a
greater contribution than the polarizability to the energy differences between
phases. Results for elastic constants of the cubic and tetragonal phases and
phonon frequencies of the cubic phase are also presented and compared with some
experimental data and first-principles calculations. We suggest that the model
will be useful for studying finite temperature effects by means of molecular
dynamics.Comment: to be published in Physical Review B (1 march 2000
Phase formation and relaxor properties of lead-free perovskite ceramics on the base of sodium-bismuth titanate
The work was supported by the Russian Foundation for Basic Research (Projects 16-53-48009, 17-03-00542)
Spectroscopic characterization of atmospheric pressure um-jet plasma source
A radio frequency um-jet plasma source is studied using He/O2 mixture. This
um-jet can be used for different applications as a source of chemical active
species e.g. oxygen atoms, molecular metastables and ozone. Using
absolutely-calibrated optical emission spectroscopy and numerical simulation,
the gas temperature in active plasma region and plasma parameters (electron
density and electron distribution function) are determined. Concentrations of
oxygen atoms and ozone in the plasma channel and in the effluent of the plasma
source are measured using emission and absorption spectroscopy. To interpret
the measured spatial distributions, the steady-state species' concentrations
are calculated using determined plasma parameters and gas temperature. At that
the influence of the surface processes and gas flow regime on the loss of the
active species in the plasma source are discussed. The measured spatial
distributions of oxygen atom and ozone densities are compared with the
simulated ones.Comment: 29 pages, 10 figure
Properties of pattern formation and selection processes in nonequilibrium systems with external fluctuations
We extend the phase field crystal method for nonequilibrium patterning to
stochastic systems with external source where transient dynamics is essential.
It was shown that at short time scales the system manifests pattern selection
processes. These processes are studied by means of the structure function
dynamics analysis. Nonequilibrium pattern-forming transitions are analyzed by
means of numerical simulations.Comment: 15 poages, 8 figure
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