78,269 research outputs found
Unusual thermoelectric behavior of packed crystalline granular metals
Loosely packed granular materials are intensively studied nowadays.
Electrical and thermal transport properties should reflect the granular
structure as well as intrinsic properties. We have compacted crystalline
based metallic grains and studied the electrical resistivity and the
thermoelectric power as a function of temperature () from 15 to 300K. Both
properties show three regimes as a function of temperature. It should be
pointed out : (i) The electrical resistivity continuously decreases between 15
and 235 K (ii) with various dependences, e.g. at low ,
while (iii) the thermoelectric power (TEP) is positive, (iv) shows a bump near
60K, and (v) presents a rather unusual square root of temperature dependence at
low temperature. It is argued that these three regimes indicate a competition
between geometric and thermal processes, - for which a theory seems to be
missing in the case of TEP. The microchemical analysis results are also
reported indicating a complex microstructure inherent to the phase diagram
peritectic intricacies of this binary alloy.Comment: to be published in J. Appl. Phys.22 pages, 8 figure
Analysis of broadband microwave conductivity and permittivity measurements of semiconducting materials
We perform broadband phase sensitive measurements of the reflection
coefficient from 45 MHz up to 20 GHz employing a vector network analyzer with a
2.4 mm coaxial sensor which is terminated by the sample under test. While the
material parameters (conductivity and permittivity) can be easily extracted
from the obtained impedance data if the sample is metallic, no direct solution
is possible if the material under investigation is an insulator. Focusing on
doped semiconductors with largely varying conductivity, here we present a
closed calibration and evaluation procedure for frequencies up to 5 GHz, based
on the rigorous solution for the electromagnetic field distribution inside the
sample combined with the variational principle; basically no limiting
assumptions are necessary. A simple static model based on the electric current
distribution proves to yield the same frequency dependence of the complex
conductivity up to 1 GHz. After a critical discussion we apply the developed
method to the hopping transport in Si:P at temperature down to 1 K.Comment: 9 pages, 10 figures, accepted for publication in the Journal of
Applied Physic
Optimized White Reflectance in Photonic Network Structures
Three-dimensional disordered networks are receiving increasing attention as
versatile architectures for highly scattering materials. However, due to their
complex morphology, little is still known about the interplay between their
structural and optical properties. Here, we describe a simple algorithm that
allows to generate photonic network structures inspired by that of the
Cyphochilus beetle, famous for the bright white reflectance of its thin
cuticular scales. The model allows to vary the degree of structural anisotropy
and filling fraction of the network independently, revealing the key
contribution of these two parameters to the overall scattering efficiency.
Rigorous numerical simulations show that the obtained structures can exceed the
broadband reflectance of the beetle while using less material, providing new
insights for the design of advanced scattering materials.Comment: 10 pages, 3 figures. peer reviewed version, published in final form
at https://doi.org/10.1002/adom.20190004
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