1,721,053 research outputs found
SiGe nanocomposites: novel materials with very low lattice thermal conductivity for efficient thermoelectric conversion
No full textWithin a materials-by-design paradigm we make use of large-scale
atomistic simulations to tailor the structural and chemical characteristics
of SiGe nanocomposites so as to get poor thermal conductors useful for
efficient thermoelectric energy production. Simulations provide evidence
that thermal transport only marginally depends on stoichiometry, while
it is deeply affected by granulometry. The main role in affecting thermal
conduction by grain boundaries is enlightened, which largely affect
vibrational modes with long mean free path
Thermal conductivity in SiGe nanocomposites
No full textWe calculate the lattice thermal conductivity in model SiGe nanocomposites by molecular dynamics in a transient thermal conduction regime. Our simulations provide evidence that thermal transport depend only marginally by stoichiometry in the range , while it is deeply affected by the granulometry. In particular, we show that SiGe nanocomposites have lattice thermal conductivity below the corresponding bulk alloy with same stoichiometry. The main role in affecting thermal conduction is provided by grain boundaries, which largely affect vibrational modes with long mean free path
Lattice Thermal Conductivity of SiGe Nanocomposites
No full textWe calculate the lattice thermal conductivity in model Si_(1−x)Ge_x nanocomposites by molecular dynamics
in a transient thermal conduction regime. Our simulations provide evidence that thermal transport depends
only marginally on stoichiometry in the range 0.2 ≤ x ≤ 0.8, while it is deeply affected by the granulometry. In particular, we show that Si1−xGex nanocomposites have lattice thermal conductivity below the corresponding bulk alloy with the same stoichiometry. The main role in affecting thermal
conduction is provided by grain boundaries, which largely affect vibrational modes with a long mean-free
path
Thermal conduction and rectification phenomena in nanoporous silicon membranes
Full text linkNon-equilibrium molecular dynamics simulations have been applied to study thermal transport properties, such as thermal conductivity and rectification, in nanoporous Si membranes. Cylindrical pores have been generated in crystalline Si membranes with different configurations, including step-like, ordered and random pore distributions. The effect of interface and overall porosity on thermal transport properties has been investigated as well as the impact of the porosity profile on the direction of the heat current. The lowest thermal conductivity and highest thermal rectification for equal porosity have been found for a step-like pore distribution. Increasing interface porosity resulted in an increase of thermal rectification, which has been found to be systematically higher for random pore distribution with respect to an ordered one. Furthermore, a maximum in rectification of 5.5% has been found for a specific overall porosity (phi(tot) = 0.02) in samples with constant interface porosity and ordered pore distribution. This has been attributed to an increased effect of asymmetric interface boundary resistance resulting from increased fluctuations of the latter with altering temperature. The average value of the interface boundary resistance has been found to decrease with increasing porosity for samples with ordered pore distribution leading to a decrease in thermal rectification
Atomistic investigation of Poly(3-hexylthiophene) adehesion on nanostructured titania
No full textWe studied the adhesion of poly(3-hexylthiophene) on a nanostructured titanic surface in vacuo by means of model potential molecular dynamics. We generated large-scale atomistic models of nanostructured titania surfaces [consisting of spherical nanocaps on top of a (110) rutile surface] and we studied the adhesion of an oligothiophene as a function of local curvature and roughness. In the limit of a perfect planar Surface, the maximum adhesion energy is calculated to be as large as 0.6 eV/monomer, and it corresponds to the oligothiophene oriented along the [(1) over bar 10] direction of the surface. Deformations of the polymer are observed due to incommensurability between the titanic and the polymer lattice parameters. When the surface is nanostructured, adhesion of the polymer is affected by the local morphology and it nonmonotonic dependence on the surface curvature is observed. The atomistic results are explained by a simple continuum model that includes the strain energy of the polymer and its electrostatic interaction with the local surface charge
Intrinsic thermal conductivity in monolayer graphene is ultimately upper limited: A direct estimation by atomistic simulations
No full textIn the sample size domain so far explored, the thermal conductivity of graphene shows an intriguing dependence on the sample length Lz along the heat flux direction. An extrapolated infinite value for such a thermal conductivity is sometimes suggested for infinite samples, while other investigations predict anyway an upper limit for it. We address this issue by avoiding any guess or approximation on the underlying microscopic heat transport mechanisms; we rather perform direct atomistic simulations aimed at estimating thermal conductivity in samples with increasing size up to the unprecedented value of Lz=0.1 mm. Our results provide evidence that thermal conductivity in graphene is definitely upper limited in samples long enough to allow a diffusive transport regime for both single and collective phonon excitations
Adhesion and diffusion of Zinc-Phthalocyanines on the ZnO (10-10) surface
No full textWe study the adhesion and the diffusion of single zinc-phthalocyanine (ZnPc) molecules on a ZnO (10 (1) over bar0) surface by means of model potential molecular dynamics. We find that the molecule is easily adsorbed on the surface, and we identify the ZnPc adsorption sites. The diffusion at room temperature is studied by metadynamics. We find that the molecule is able to diffuse by hopping between the surface dimer rows along the [010] crystallographic direction with a free energy barrier as small as similar to 0.4 eV. At temperatures T > 700 K, a different mechanism is found with large diffusion paths along the surface trench groove
Role of electron-phonon scattering on thermoelectric coefficients in pristine Cs2NaYbCl6 perovskite: A full DFT approach
Full text linkDouble-halide perovskites are recently attracting significant interest in the field of thermoelectric research due to the possibility of achieving very low thermal conductivity and retaining relatively high Seebeck coefficient and electrical conductivity. Accurate estimates of the transport related properties are, thus, highly desirable and are strictly linked to an extremely detailed characterization of the microscopic mechanism underlying the transport itself. To address this issue, in this study we conduct a comprehensive theoretical investigation of one of the main process-limiting carrier transports, i.e., electron-phonon scattering, in a typical double-halide perovskite, Cs2NaYbCl6. In order to quantify the specific magnitude of this process, we adopt in this study a comprehensive DFT analysis, with a focus on evaluating the resulting lifetimes (in both hole- and electron-mediated regimes), by solving iteratively the linearized Boltzmann equation. Using the evaluated lifetimes, we found distinct differences in electrical mobilities and Seebeck coefficient as a function of the temperature and carrier concentrations, revealing that the thermoelectric figure-of-merit ZT, while quite low, is significantly higher in the case of electron-mediated regime
Effect of hydrogenation on graphene thermal transport
No full textWe studied thermal conductivity of the three most stable hydrogenated graphene (graphane) conformers by means of non equilibrium molecular dynamics. We estimated thermal conductivity for pristine graphene with sample length 2.1 (2.4) mu m as large as kappa = 745.4+/- 0.3 and 819.1 +/- 0.3 W m(-1)K(-1) in the armchair and zigzag directions, respectively, in very good agreement with previous theoretical results based on the Boltzmann transport equation. In the case of the three graphane isomers we observed a dramatic kappa reduction by at least one order of magnitude with respect to pristine graphene. We elucidated this reduction in terms of different phonon density of states and mean-free path distribution between graphene and graphane. The deterioration of thermal transport upon hydrogenation in graphene, could be proposed as a way to tune thermal transport in graphene for phononic applications such as thermal diodes
Self-assembling of poly3-hexylthiophene
No full textWe study the assembling of P3HT chains in vacuo by means of a combination of first-principles density functional theory and model potential molecular dynamics. We find that, in the absence of any external constraints, the pi-pi interchain interaction between thiophenes is the major driving force for the assembling. Single chains stack in a staggered geometry giving rise to the formation of two-dimensional hydrophobic foils. These, in turn, assemble into a zigzag bulk polymer structure in agreement with experimental findings. Finally, in the presence of some external constraint (like a substrate), when the alignment of single chains is favored instead of the stacking, a different bulk structure is possible where thiophene rings are aligned
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