396 research outputs found
Exposure of high-temperature alloys in carbonaceous gas atmospheres
Light and scanning electron microscopy were used to evaluate changes resulting from exposure of eight commercial high temperature alloys in carbonaceous atmospheres. Samples were exposed in CO, CH, or CO/CH mixtures up to 900C and 6.2 MPa. All alloys were actively attacked. Surface finish and preoxidation influenced the extent of corrosion. Reduction of NiO, giving a high surface concentration of Ni, promoted catalytic decomposition of CH. Further work is indicated before the performance of these alloys may be ranked with confidence. (auth
Theory of spin wave excitations of metallic A-type antiferromagnetic manganites
The spin dynamic of the metallic A-type antiferromagnetic manganites is
studied. An effective nearest-neighbour Heisenberg spin wave dispersion is
derived from the double exchange model taking into account the superexchange
interaction between the core spins. The result of inelastic neutron scattering
experiment on is qualitatively reproduced.
Comparing theory with experimental data two main parameters of the model:
nearest-neighbour electron transfer amplitude and superexchange coupling
between the core spins are estimated.Comment: to appear in Phys. Rev.
Hopping Conduction in Disordered Carbon Nanotubes
We report electrical transport measurements on individual disordered carbon
nanotubes, grown catalytically in a nanoporous anodic aluminum oxide template.
In both as-grown and annealed types of nanotubes, the low-field conductance
shows as exp[-(T_{0}/T)^{1/2}] dependence on temperature T, suggesting that
hopping conduction is the dominant transport mechanism, albeit with different
disorder-related coefficients T_{0}. The field dependence of low-temperature
conductance behaves an exp[-(xi_{0}/xi)^{1/2}] with high electric field xi at
sufficiently low T. Finally, both annealed and unannealed nanotubes exhibit
weak positive magnetoresistance at low T = 1.7 K. Comparison with theory
indicates that our data are best explained by Coulomb-gap variable range
hopping conduction and permits the extraction of disorder-dependent
localization length and dielectric constant.Comment: 10 pages, 5 figure
Polaronic Signatures in Mid-Infrared Spectra: Prediction for LaMnO3 and CaMnO3
Hole-doped LaMnO3 and electron-doped CaMnO3 form self-trapped electronic
states. The spectra of these states have been calculated using a two orbital
(Mn eg Jahn-Teller) model, from which the non-adiabatic optical conductivity
spectra are obtained. In both cases the optical spectrum contains weight in the
gap region, whose observation will indicate the self-trapped nature of the
carrier states. The predicted spectra are proportional to the concentration of
the doped carriers in the dilute regime, with coefficients calculated with no
further model parameters.Comment: 6 pages with 3 figures imbedde
Morphological and Structural Characterization of Cro2/Cr2o3 Films Grown by Laser-CVD
This work reports on the synthesis of chromium (III, IV) oxides films by KrF
laser-assisted CVD. Films were deposited onto sapphire substrates at room
temperature by photodissociation of Cr(CO)6 in dynamic atmospheres containing
oxygen and argon. A study of the processing parameters has shown that partial
pressure ratio of O2 to Cr(CO)6 and laser fluence are the prominent parameters
that have to be accurately controlled in order to co-deposit both crystalline
oxide phases. Films consistent with such a two-phase system were synthesised
for a laser fluence of 75 mJ cm-2 and a partial pressure ratio about 1.
PACS: 81.15.Fg, 81.15.Kk, 81.05.JeComment: 17 pages, 4 figure
Simulation of small-angle scattering from large assemblies of multi-type scatterer particles
A SANS and APT study of precipitate evolution and strengthening in a maraging steel
In this work a combination of the characterisation techniques small angle neutron scattering (SANS) and atom probe tomography (APT) are used to study the precipitation in a maraging steel. Three similar maraging steel alloys were aged at different temperatures and ageing times, and then characterised using SANS, APT and microhardness. The alloys consist of two types of precipitates, namely Laves phase and β-NiAl, the precipitates have different composition and hence precipitate ageing, which makes it complicated to model. The SANS experimental set-up was relatively simple and allowed the precipitate size and fraction of a large number of samples to be measured in a single experiment. The APT results were used for constraining the SANS modelling, particularly the composition, shape and distribution of phases. The characterisation led to the following description of precipitation: NiAl phase reaches coarsening at early stages of ageing and shifts its strength mechanisms from shearing to Orowan looping, which cause the characteristic peak strength; the Laves phase is in growth throughout and its strength contribution increases with ageing time. These observations were shown to be consistent with precipitate evolution and strengthening models, and the work of others. Although, there are some issues with the combination of SANS and APT approach, which are discussed, the methodology provides a valuable tool to understand complex precipitation behaviours
Cr2O3 thin films grown at room temperature by low pressure laser chemical vapour deposition
Chromia (Cr2O3) has been extensively explored for the purpose of developing
widespread industrial applications, owing to the convergence of a variety of
mechanical, physical and chemical properties in one single oxide material.
Various methods have been used for large area synthesis of Cr2O3 films.
However, for selective area growth and growth on thermally sensitive materials,
laser-assisted chemical vapour deposition (LCVD) can be applied advantageously.
Here we report on the growth of single layers of pure Cr2O3 onto sapphire
substrates at room temperature by low pressure photolytic LCVD, using UV laser
radiation and Cr(CO)6 as chromium precursor. The feasibility of the LCVD
technique to access selective area deposition of chromia thin films is
demonstrated. Best results were obtained for a laser fluence of 120 mJ cm-2 and
a partial pressure ratio of O2 to Cr(CO)6 of 1.0. Samples grown with these
experimental parameters are polycrystalline and their microstructure is
characterised by a high density of particles whose size follows a lognormal
distribution. Deposition rates of 0.1 nm s-1 and mean particle sizes of 1.85
{\mu}m were measured for these films.Comment: 16 pages, 6 figures, accepted for publication in Thin Solid Film
The Value of Information for Populations in Varying Environments
The notion of information pervades informal descriptions of biological
systems, but formal treatments face the problem of defining a quantitative
measure of information rooted in a concept of fitness, which is itself an
elusive notion. Here, we present a model of population dynamics where this
problem is amenable to a mathematical analysis. In the limit where any
information about future environmental variations is common to the members of
the population, our model is equivalent to known models of financial
investment. In this case, the population can be interpreted as a portfolio of
financial assets and previous analyses have shown that a key quantity of
Shannon's communication theory, the mutual information, sets a fundamental
limit on the value of information. We show that this bound can be violated when
accounting for features that are irrelevant in finance but inherent to
biological systems, such as the stochasticity present at the individual level.
This leads us to generalize the measures of uncertainty and information usually
encountered in information theory
Last Glacial Period Cryptotephra Deposits in an Eastern North Atlantic Marine Sequence: Exploring Linkages to the Greenland Ice-Cores
The establishment of a tephra framework for the Greenland ice-cores spanning the last glacial period, particularly between 25 and 45 ka b2k, provides strong potential for precisely correlating other palaeoclimatic records to these key archives. Tephra-based synchronisation allows the relative timing of past climatic changes recorded within different depositional environments and potential causal mechanisms to be assessed. Recent studies of North Atlantic marine records have demonstrated the potential of tracing cryptotephra horizons in these sequences and the development of protocols now allows a careful assessment of the isochronous nature of such horizons. Here we report on tephrochronological investigations of a marine sequence retrieved from the Goban Spur, Eastern North Atlantic, covering ?25–60 ka b2k. Density and magnetic separation techniques and an assessment of potential transport and depositional mechanisms have identified three previously unknown isochronous tephra horizons along with deposits of the widespread North Atlantic Ash Zone II and Faroe Marine Ash Zone III. Correlations between the new horizons and the Greenland ice-core tephra framework are explored and despite no tie-lines being identified the key roles that high-resolution climatostratigraphy and shard-specific trace element analysis can play within the assessment of correlations is demonstrated. The previously unknown horizons are new additions to the overall North Atlantic tephra framework for the last glacial period and could be key horizons for future correlations
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