417 research outputs found
Modeling a thermionic energy converter using finite-difference time-domain particle-in-cell simulations
An efficient algorithm to calculate intrinsic thermoelectric parameters based on Landauer approach
The Landauer approach provides a conceptually simple way to calculate the
intrinsic thermoelectric (TE) parameters of materials from the ballistic to the
diffusive transport regime. This method relies on the calculation of the number
of propagating modes and the scattering rate for each mode. The modes are
calculated from the energy dispersion (E(k)) of the materials which require
heavy computation and often supply energy relation on sparse momentum (k)
grids. Here an efficient method to calculate the distribution of modes (DOM)
from a given E(k) relationship is presented. The main features of this
algorithm are, (i) its ability to work on sparse dispersion data, and (ii)
creation of an energy grid for the DOM that is almost independent of the
dispersion data therefore allowing for efficient and fast calculation of TE
parameters. The inclusion of scattering effects is also straight forward. The
effect of k-grid sparsity on the compute time for DOM and on the sensitivity of
the calculated TE results are provided. The algorithm calculates the TE
parameters within 5% accuracy when the K-grid sparsity is increased up to 60%
for all the dimensions (3D, 2D and 1D). The time taken for the DOM calculation
is strongly influenced by the transverse K density (K perpendicular to
transport direction) but is almost independent of the transport K density
(along the transport direction). The DOM and TE results from the algorithm are
bench-marked with, (i) analytical calculations for parabolic bands, and (ii)
realistic electronic and phonon results for .Comment: 16 Figures, 3 Tables, submitted to Journal of Computational
electronic
Wildfire selectivity for land cover type: does size matter ?
Previous research has shown that fires burn certain land cover types disproportionally to their abundance. We used quantile
regression to study land cover proneness to fire as a function of fire size, under the hypothesis that they are inversely
related, for all land cover types. Using five years of fire perimeters, we estimated conditional quantile functions for lower
(avoidance) and upper (preference) quantiles of fire selectivity for five land cover types - annual crops, evergreen oak
woodlands, eucalypt forests, pine forests and shrublands. The slope of significant regression quantiles describes the rate of
change in fire selectivity (avoidance or preference) as a function of fire size. We used Monte-Carlo methods to randomly
permutate fires in order to obtain a distribution of fire selectivity due to chance. This distribution was used to test the null
hypotheses that 1) mean fire selectivity does not differ from that obtained by randomly relocating observed fire perimeters;
2) that land cover proneness to fire does not vary with fire size. Our results show that land cover proneness to fire is higher
for shrublands and pine forests than for annual crops and evergreen oak woodlands. As fire size increases, selectivity
decreases for all land cover types tested. Moreover, the rate of change in selectivity with fire size is higher for preference
than for avoidance. Comparison between observed and randomized data led us to reject both null hypotheses tested
(a = 0.05) and to conclude it is very unlikely the observed values of fire selectivity and change in selectivity with fire size are
due to chance.Funding: This paper was supported by the Fundação para a Ciência e Tecnologia Ph.D. Grant SFRH/BD/40398/2007. JMCP participated in this research under the
framework of research projects ‘‘Forest fire under climate, social and economic changes in Europe, the Mediterranean and other fire-affected areas of the world
(FUME)’’, EC FP7 Grant Agreement No. 243888. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the
manuscrip
Semi-analytical solution to the frequency-dependent Boltzmann transport equation for cross-plane heat conduction in thin films
Thermoelectric properties of lead chalcogenide core-shell nanostructures
We present the full thermoelectric characterization of nanostructured bulk
PbTe and PbTe-PbSe samples fabricated from colloidal core-shell nanoparticles
followed by spark plasma sintering. An unusually large thermopower is found in
both materials, and the possibility of energy filtering as opposed to grain
boundary scattering as an explanation is discussed. A decreased Debye
temperature and an increased molar specific heat are in accordance with recent
predictions for nanostructured materials. On the basis of these results we
propose suitable core-shell material combinations for future thermoelectric
materials of large electric conductivities in combination with an increased
thermopower by energy filtering.Comment: 12 pages, 8 figure
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