303 research outputs found
Thermoelectric prospects of nanomaterials with spin-orbit surface bands
Nanostructured composites and nanowire arrays of traditional thermoelectrics
like Bi, Bi(1-x)Sb(x) and Bi(2)Te(3) have metallic Rashba surface spin-orbit
bands featuring high mobilities rivaling that of the bulk for which topological
insulator behavior has been proposed. Nearly pure surface electronic transport
has been observed at low temperatures in Bi nanowires with diameter around the
critical diameter, 50 nm, for the semimetal-to semiconductor transition. The
surface contributes strongly to the thermopower, actually dominating for
temperatures T < 100 K in these nanowires. The surface thermopower was found to
be -1 T microvolt/(K^2), a value that is consistent with theory. We show that
surface electronic transport together with boundary phonon scattering leads to
enhanced thermoelectric performance at low temperatures of Bi nanowire arrays.
We compare with bulk n-BiSb alloys, optimized CsBi(4)Te(6) and optimized
Bi(2)Te(3). Surface dominated electronic transport can be expected in
nanomaterials of the other traditional thermoelectrics.Comment: 18 pages, 3 figure
Pressure effects on the transport coefficients of Ba(Fe1-xCox)2As2
We report the temperature dependence of the resistivity and thermoelectric
power under hydrostatic pressure of the itinerant antiferromagnet BaFe2As2 and
the electron-doped superconductor Ba(Fe0.9Co0.1)2As2. We observe a hole-like
contribution to the thermopower below the structural-magnetic transition in the
parent compound that is suppressed in magnitude and temperature with pressure.
Pressure increases the contribution of electrons to transport in both the doped
and undoped compound. In the 10% Co-doped sample, we used a two-band model for
thermopower to estimate the carrier concentrations and determine the effect of
pressure on the band structure
Thermoelectric properties of the bismuth telluride nanowires in the constant-relaxation-time approximation
Electronic structure of bismuth telluride nanowires with the growth
directions [110] and [015] is studied in the framework of anisotropic effective
mass method using the parabolic band approximation. The components of the
electron and hole effective mass tensor for six valleys are calculated for both
growth directions. For a square nanowire, in the temperature range from 77 K to
500 K, the dependence of the Seebeck coefficient, the electron thermal and
electrical conductivity as well as the figure of merit ZT on the nanowire
thickness and on the excess hole concentration are investigated in the
constant-relaxation-time approximation. The carrier confinement is shown to
play essential role for square nanowires with thickness less than 30 nm. The
confinement decreases both the carrier concentration and the thermal
conductivity but increases the maximum value of Seebeck coefficient in contrast
to the excess holes (impurities). The confinement effect is stronger for the
direction [015] than for the direction [110] due to the carrier mass difference
for these directions. The carrier confinement increases maximum value of ZT and
shifts it towards high temperatures. For the p-type bismuth telluride nanowires
with growth direction [110], the maximum value of the figure of merit is equal
to 1.3, 1.6, and 2.8, correspondingly, at temperatures 310 K, 390 K, 480 K and
the nanowire thicknesses 30 nm, 15 nm, and 7 nm. At the room temperature, the
figure of merit equals 1.2, 1.3, and 1.7, respectively.Comment: 13 pages, 7 figures, 2 tables, typos added, added references for
sections 2-
Lorenz function of BiTe/SbTe superlattices
Combining first principles density functional theory and semi-classical
Boltzmann transport, the anisotropic Lorenz function was studied for
thermoelectric BiTe/SbTe superlattices and their bulk
constituents. It was found that already for the bulk materials BiTe
and SbTe, the Lorenz function is not a pellucid function on charge
carrier concentration and temperature. For electron-doped
BiTe/SbTe superlattices large oscillatory deviations
for the Lorenz function from the metallic limit were found even at high charge
carrier concentrations. The latter can be referred to quantum well effects,
which occur at distinct superlattice periods
Improved Thermoelectric Cooling Based on the Thomson Effect
Traditional thermoelectric Peltier coolers exhibit a cooling limit which is
primarily determined by the figure of merit, zT. Rather than a fundamental
thermodynamic limit, this bound can be traced to the difficulty of maintaining
thermoelectric compatibility. Self-compatibility locally maximizes the cooler's
coefficient of performance for a given zT and can be achieved by adjusting the
relative ratio of the thermoelectric transport properties that make up zT. In
this study, we investigate the theoretical performance of thermoelectric
coolers that maintain self-compatibility across the device. We find such a
device behaves very differently from a Peltier cooler, and term self-compatible
coolers "Thomson coolers" when the Fourier heat divergence is dominated by the
Thomson, as opposed to the Joule, term. A Thomson cooler requires an
exponentially rising Seebeck coefficient with increasing temperature, while
traditional Peltier coolers, such as those used commercially, have
comparatively minimal change in Seebeck coefficient with temperature. When
reasonable material property bounds are placed on the thermoelectric leg, the
Thomson cooler is predicted to achieve approximately twice the maximum
temperature drop of a traditional Peltier cooler with equivalent figure of
merit (zT). We anticipate the development of Thomson coolers will ultimately
lead to solid state cooling to cryogenic temperatures.Comment: The Manuscript has been revised for publication in PR
A simple model for the vibrational modes in honeycomb lattices
The classical lattice dynamics of honeycomb lattices is studied in the
harmonic approximation. Interactions between nearest neighbors are represented
by springs connecting them. A short and necessary introduction of the lattice
structure is presented. The dynamical matrix of the vibrational modes is then
derived, and its eigenvalue problem is solved analytically. The solution may
provide deeper insight into the nature of the vibrational modes. Numerical
results for the vibrational frequencies are presented. To show that how
effective our method used for the case of honeycomb lattice is, we also apply
it to triangular and square lattice structures. A few suggested problems are
listed in the concluding section.Comment: 9 pages, 12 figures, submitted to American Journal of Physic
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
Electrochemically copper-doped bismuth tellurium selenide thin films
We report the first results of a study on electrochemically doped copper bismuth tellurium selenide thin films electrodeposited from aqueous nitric acid electrolytes containing up to 2 mM of Cu(NO3)2. The effect of Cu(NO3)2 concentration on the composition, structure and thermoelectric properties of the bismuth tellurium selenide films is investigated by scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray diffraction and Seebeck and Hall effect measurements. A Cu(NO3)2 concentration of 1.5 mM is found to offer a Seebeck coefficient of up to −390 μV K−1 at room temperature, which is the highest reported to date for an electrodeposited bismuth tellurium compound
- …