Quantum thermal transistor

We demonstrate that a thermal transistor can be made up with a quantum system of 3 interacting subsystems , coupled to a thermal reservoir each. This thermal transistor is analogous to an electronic bipolar one with the ability to control the thermal currents at the collector and at the emitter with the imposed thermal current at the base. This is achieved determining the heat fluxes by means of the strong-coupling formalism. For the case of 3 interacting spins, in which one of them is coupled to the other 2, that are not directly coupled, it is shown that high amplification can be obtained in a wide range of energy parameters and temperatures. The proposed quantum transistor could, in principle, be used to develop devices such as a thermal modulator and a thermal amplifier in nano systems.Comment: Physical Review Letters, American Physical Society, 2016, 116, pp.20060

Quantum Thermal Rectification to design thermal diodes and transistors

We study in this article how heat can be exchanged between two level systems (TLS) each of them being coupled to a thermal reservoir. Calculation are performed solving a master equation for the density matrix using the Born markov-approximation. We analyse the conditions for which a thermal diode and a thermal transistor can be obtained as well as their optimization

Asymmetry of tensile vs. compressive elasticity and permeability contributes to the regulation of exchanges in collagen gels

The Starling principle describes exchanges in tissues based on the balance of hydrostatic and osmotic flows. This balance neglects the coupling between mechanics and hydrodynamics, a questionable assumption in strained elastic tissues due to intravascular pressure. Here, we measure the elasticity and permeability of collagen gels under tensile and compressive stress via the comparison of the temporal evolution of pressure in an air cavity sealed at the outlet of a collagen slab with an analytical kinetic model. We observe a drop in the permeability and enhanced strain-stiffening of native collagen gels under compression, both effects being essentially lost after chemical cross-linking. Further, we prove that this asymmetric response accounts for the accumulation of compressive stress upon sinusoidal fluid injection, which modulates the material's permeability. Our results thus show that the properties of collagen gels regulate molecular exchanges and could help understand drug transport in tissues

On the stability of the exact solutions of the dual-phase lagging model of heat conduction

The dual-phase lagging (DPL) model has been considered as one of the most promising theoretical approaches to generalize the classical Fourier law for heat conduction involving short time and space scales. Its applicability, potential, equivalences, and possible drawbacks have been discussed in the current literature. In this study, the implications of solving the exact DPL model of heat conduction in a three-dimensional bounded domain solution are explored. Based on the principle of causality, it is shown that the temperature gradient must be always the cause and the heat flux must be the effect in the process of heat transfer under the dual-phase model. This fact establishes explicitly that the single- and DPL models with different physical origins are mathematically equivalent. In addition, taking into account the properties of the Lambert W function and by requiring that the temperature remains stable, in such a way that it does not go to infinity when the time increases, it is shown that the DPL model in its exact form cannot provide a general description of the heat conduction phenomena

Effect of a metallic coating on the thermal conductivity of carbon nanofiber–dielectric matrix composites

International audienceA theoretical model is developed to evaluate the thermal conductivity of composites made of a dielectric matrix material containing randomly oriented and aligned carbon nanofibers coated with a metallic layer. The effect of the metallic coating on the phononic thermal conductivity of the matrix material and the electron–phonon coupling inside the metallic coating are both taken into account in this model. It is shown that: (1) the metallic coating has an extraordinary effect on the enhancement of the composite thermal conductivity. For a volume fraction of 30% of fibers with radius of 50 nm and 10 nm-coating of copper, the increase in the thermal conductivity is as high as 27%, which increases significantly with the fiber volume fraction. (2) Although the thermal conductivity of silver is 453% as that of indium, the composite thermal conductivity is only increased slightly by changing an indium coating to a much more expensive silver coating, due to the relatively high thermal conductivity of these metals in comparison with the one of the matrix. (3) The composite thermal conductivity increases with the volume fraction of the fibers when their radius and the radial thermal conductivity are greater than the Kapitza radius at the matrix-coating interface and the effective thermal conductivity of the matrix, respectively. The obtained theoretical results agree fairly well with experimental data reported in the literature for the thermal conductivity along the axis of aligned carbon fibers with copper coating and embedded in an epoxy matrix. This model is expected to be valid for composites in the absence of percolation with the length-to-radius aspect ratio of fibers in the range of 10–100, and it provides theoretical guides for optimizing cost-efficient high thermal conductivity composites

Measurement of the phonon mean free path spectrum in silicon membranes at different temperatures using arrays of nanoslits

International audienceKnowledge of the phonon mean free path (MFP) holds the key to understanding the thermal properties of materials and nanostructures. Although several experiments measured the phonon MFP in bulk silicon, MFP spectra in thin membranes have not been directly measured experimentally yet. In this work, we experimentally probe the phonon MFP spectra in suspended silicon membranes. First, we measure the thermal conductivity of membranes with arrays of slits at different temperatures. Next, we develop a fully analytical procedure to extract the accumulated thermal conductivity as a function of the MFP. The measured phonon MFP in 145-nm-thick membranes with the surface roughness of 0.2 nm is shorter than that in bulk due to the scattering at the membrane boundaries. At room temperature, the phonon MFP does not exceed 400 nm. However, at 4 K, the MFP becomes longer, and some phonons can travel ballistically for up to one micrometer. These results thus shed light on the long-lasting question of the range of ballistic phonon transport at different temperatures in nanostructures based on silicon membranes

Simultaneous determination of thermal diffusivity and thermal conductivity of a thin layer using double modulated thermal excitations

International audienceA theoretical model is developed to determine simultaneously and in different ways thermal diffusivity and thermal conductivity of thin layers. This is done by using the accurate expression of the temperature distribution derived from the parabolic heat equation when the front surface of the thin layer is excited by a periodic heat flux, while the rear surface is maintained at one of three different types of boundary conditions: modulated periodic heat flux, modulated temperature, or constant temperature. Our approach exploits the modulation frequencies at which the normalized front surface temperature reaches its first maximum and first minimum. It is shown that (i) these characteristic frequencies can be used to obtain the thermal diffusivity of the finite layer under three different types of boundary conditions. (ii) The ratio between the values of the maxima and minima of the temperature can be utilized to determine the thermal conductivity of the finite layer. These two thermal properties are sensitive to the nature of the boundary conditions as well as the modulation frequency of the heat excitation. This paper provides a theoretical basis for the determination of the thermal diffusivity and thermal conductivity of the finite layer using laser-based heating photothermal techniques