Search results for "Thermal Conductivity"
showing 10 items of 160 documents
Two-dimensional phononic thermal conductance in thin membranes in the Casimir limit
2011
We discuss computational analysis of phononic thermal conduction in the suspended membrane geometry, in the experimentally commonly appearing case where heat can flow out radially in two dimensions from a central source. As we are mostly interested in the low-temperature behavior where bulk scattering of phonons becomes irrelevant, we study the limit where all phonon scattering takes place at the membrane surfaces. Moreover, we limit the discussion here to the case where this surface scattering is fully diffusive, the so called Casimir limit. Our analysis shows that in the two-dimensional case, no analytic results are available, in contrast to the well known 1D Casimir limit. Numerical solu…
From kinetic to collective behavior in thermal transport on semiconductors and semiconductor nanostructures
2013
We present a model which deepens into the role that normal scattering has on the thermal conductivity in semiconductor bulk, micro and nanoscale samples. Thermal conductivity as a function of the temperature undergoes a smooth transition from a kinetic to a collective regime that depends on the importance of normal scattering events. We demonstrate that in this transition, the key point to fit experimental data is changing the way to perform the average on the scattering rates. We apply the model to bulk Si with different isotopic compositions obtaining an accurate fit. Then we calculate the thermal conductivity of Si thin films and nanowires by only introducing the effective size as additi…
Heat diffusion in the disordered electron gas
2015
We study the thermal conductivity of the disordered two-dimensional electron gas. To this end we analyze the heat density-heat density correlation function concentrating on the scattering processes induced by the Coulomb interaction in the sub-temperature energy range. These scattering processes are at the origin of logarithmic corrections violating the Wiedemann-Franz law. Special care is devoted to the definition of the heat density in the presence of the long-range Coulomb interaction. To clarify the structure of the correlation function, we present details of a perturbative calculation. While the conservation of energy strongly constrains the general form of the heat density-heat densit…
Thermoelectric Effects: Semiclassical and Quantum Approaches from the Boltzmann Transport Equation
2013
The thermoelectric efficiency of a material depends on its electronic and phononic properties. It is normally given in terms of the dimensionless figure of merit Z T = σ S 2 T ∕ κ. The parameters involved in Z T are the electrical conductivity σ, the Seebeck coefficient S, and the thermal conductivity κ. The thermal conductivity has two contributions, κ = κ e + κ L , the electron thermal conductivity κ e and the lattice thermal conductivity κ L . In this chapter all these parameters will be deduced for metals and semiconductors, starting from the Boltzmann transport equation (BTE). The electrical conductivity, the Seebeck coefficient, and the electronic thermal conductivity will be obtained…
Refrigeration of an Array of Cylindrical Nanosystems by Flowing Superfluid Helium
2016
We consider the refrigeration of an array of heat-dissipating cylindrical nanosystems as a simplified model of computer refrigeration. We explore the use of He II as cooling fluid, taking into account forced convection and heat conduction. The main conceptual and practical difficulties arise in the calculation of the effective thermal conductivity. Since He II does not follow Fourier’s law, the effective geometry-dependent conductivity must be extracted from a more general equation for heat transfer. Furthermore, we impose the restrictions that the maximum temperature along the array should be less than (Formula presented.) transition temperature and that quantum turbulence is avoided, in o…
Effective thermal conductivity of helium II: from Landau to Gorter–Mellink regimes
2014
The size-dependent and flux-dependent effective thermal conductivity of narrow channels filled with He II is analyzed. The classical Landau evaluation of the effective thermal conductivity of quiescent He II is extended to describe the transition to fully turbulent regime, where the heat flux is proportional to the cubic root of the temperature gradient (Gorter–Mellink regime). To do so, we use an expression for the quantum vortex line density L in terms of the heat flux considering the influence of the walls. From it, and taking into account the friction force of normal component against the vortices, we compute the effective thermal conductivity as a function of the heat flux, and we disc…
Effective thermal conductivity of superuid helium: Laminar, turbulent and ballistic regimes
2016
Abstract In this paper we extend previous results on the effective thermal conductivity of liquid helium II in cylindrical channels to rectangular channels with high aspect ratio. The aim is to compare the results in the laminar regime, the turbulent regime and the ballistic regime, all of them obtained within a single mesoscopic formalism of heat transport, with heat flux as an independent variable.
Semiquantum molecular dynamics simulation of thermal properties and heat transport in low-dimensional nanostructures
2012
We present a detailed description of the semi-quantum approach to the molecular dynamics simulation of stochastic dynamics of a system of interacting particles. Within this approach, the dynamics of the system is described with the use of classical Newtonian equations of motion in which the quantum effects are introduced through random Langevin-like forces with a specific power spectral density (the color noise). The color noise describes the interaction of the molecular system with the thermostat. We apply this technique to the simulation of the thermal properties of different low-dimensional nanostructures. Within this approach, we simulate the specific heat and heat transport in carbon n…
The boson peak
2013
The vibrational properties of glasses in the THz range differ very much from what is expected from Debye's elasticity theory: the density of states (DOS) deviates from Debye's ω2 law [the “boson peak” (BP)], the sound velocity shows a negative dispersion in the BP frequency regime and there is a strong increase in the sound attenuation near the BP frequency. These anomalies are related to an anomalous temperature dependence of the specific heat and thermal conductivity in the 10 K regime. An overview of the heterogeneous-elasticity theory is given, by means of which all these anomalies can be explained and shown to arise from the structural disorder, leading to spatial fluctuations of the s…
Non-local heat transport in static solar coronal loops
1991
We investigate the limits of applicability of the Spitzer-Harm thermal conductivity in solar coronal loops and show that the ratio λ0/LTof electron mean-free path to temperature scale height in large-scale structures can approach the limits of the Spitzer-Harm theory. We use a non-local formulation of heat transport to compute a grid of loop models: the effects of non-local transport on the distribution of differential emission measure are particularly important in the coronal part of loops longer than the pressure scale height sp.We derive a scaling law for λ0/LTin the corona, showing that it grows exponentially with L/sp, and discuss effects of non-local heat transport in the transition r…