Heat transfer at short time and length scales

Resume : Engineering of thermal properties of materials by nanostructuration has made important progress in the last two decades. Besides phononics, the idea of phonon blocking as a phonon engineering approach is to reduce the lattice thermal conductivity of a material through decreasing the scale or introducing nanostructures. Once the distances between inclusions are comparable to the phonon mean free path within the material the phonons are scattered by the inclusions, resulting in a prohibited heat transport. In this paper, we present a new semi-conductor material made off Ge3Mn5 nano-inclusions embedded into an epitaxial Ge matrix. The size distribution (within 5 and 50 nm) and the concentration of the inclusions can be accorded with growth parameters. The structural features of this material have been fully studied using transmission electron microscopy. To measure the thermal conductivity of the Ge:Mn thin films, we have developed a sensitive 3-omega method adapted for measurements on thin films. Measurements have been carried out at room temperature on Ge:Mn samples having different manganese concentrations and treated at different annealing temperatures. They show that depending on the Mn concentration the thermal conductivity of the Ge:Mn thin films can be reduced by a factor of 4 to 20 comparing to the value for Ge bulk material. Numerical modeling are under way to investigate the phonon scattering mechanisms behind this significant reduction of thermal conductivity.

Resume : Silicon is one of the most abundant element in the world and Si based materials are involved in a broad range of application. With the evolution of the nanofabrication methods, one can tailor the physical properties of nanostructured materials and tune specific properties for specific purposes. For example, there is an increasing interest of nanostructured Si for thermoelectric applications. To enhance the poor thermoelectric conversion efficiency of bulk Si, a successful strategy is to reduce the thermal conductivity (TC) by nano-structuring the materials (nanowires, superlattices, nanopores). For nanoporous materials, the long mean free path phonons are scattered on the pores, driving to an important reduction of the TC. Nevertheless, making nano-pores in Si is often accompanied by an amorphization of the structure around the cavity. With the current work, we study with molecular dynamics simulations the effect of the amorphous edge presence on the TC. The pore size and the a-Si thickness around the pores are the study parameters. Increasing the amorphous fraction leads to the drastic decrease of the TC. Besides, phonon transport in nanoporous Silicon is also studied by the means of Boltzmann transport equation resolution through Monte Carlo method. Using this tool, several porosities are considered in order to make comparisons with other numerical studies and experimental data.

Resume : In recent works, we have explored the prospects for thermoelectric (TE) efficiency enhancement in non-uniform nanostructures above the quantum confinement regime [1-2]. In this regime, electrons and phonons can be treated as bulk-like carriers experiencing effects of the non-uniformity as they move through the nanostructure. Main effects on their transport properties originate from scattering on boundaries/interfaces and energy barriers. In one-dimensional modulated nanostructures (nanowires), a TE power factor enhancement was predicted in the presence of two phases for the electron transport and non-uniform thermal conductivity. We have explored the influence of the nanostructure dimensionality on the transport properties and the heat transfer. The two- and three- dimensional non-uniform nanostructures have been modeled by a network model. Our results indicate two regimes depending on the distribution of the non-uniformity in the nanostructures. The thermal conductivity can be interpreted by either average properties or by the formation of percolation paths for the heat transfer. We will present and discuss our findings on the heat transfer in the presence of non-uniformity due to interfaces and multiple phases in nanocomposite materials. [1] Neophytou N., Zianni X., Kosina H., Frabboni S., Lorenzi B., and Narducci D., Nanotechnology 24 (20) (2013) 205402 [2] Zianni X., Narducci D., Journal of Applied Physics 117 (2015) 035102

Resume : The length dependence of the thermal conductivity is calculated in silicon nanowires and compared to silicon bulk. We use approach-to-equilibrium molecular dynamics (AEMD). AEMD is a transient temperature method for determining thermal conductivity in thermal conductors. Two delimited portions are heated at different temperatures before the temperature difference is monitored during the approach-to-equilibrium. The temperature profile has a sinusoidal form with an amplitude that slowly decreases with time. The temperature difference between the portions decays exponentially with time. We compare the numerical results with the solution of the heat equation in the same conditions. The sinusoidal form of the temperature profile and the exponential decay of the temperature difference are in agreement with the heat equation. Therefore we use the same relation to obtain the thermal conductivity. The method has been previously used to study bulk silicon supercells longer than 1 um. We apply the approach to nanowires. We first study the equilibration of the temperature difference, and show it is longer in nanowires due to surface effects. We validate the use of simultaneous Nosé-Hoover thermostats to obtain the initial temperature diffence. Finally, we obtain the thermal conductivity for several radius and lengths and compare the length dependence to the bulk.

Resume : For many years the ZnO micropowders are successfully and widely used as one of the main components at manufacturing a variety of, mainly low-cost, commercial thermal greases. The value of the coefficient of thermal conductivity of these greases is in the range from 0.6 to 1.5 W/(m∙K). The thermal conductivity of the composite material based on ZnO with different particle sizes (from micrometer to nanometer) and silicone oil was measured using the radial heat flow method. The thermal conductivity of the composite material based on the commercial ZnO micropowder with an average particle size of 50 μm was found to be 0.8 W/(m∙K). The thermal conductivity of the composite material based on the wet chemistry methods synthesized ZnO nanopowder with an average particle size less than 10 nm was found to be 2.5 W/(m∙K). Increased value of the thermal conductivity would be considered as a manifestation of the quantum size effect. Since the particle sizes are commensurate with the exciton Bohr radius (2 nm) and smaller than the mean free path of phonons (30 nm) and de Broglie wavelength in ZnO (14 nm), it is convenient to propose the ballistic mechanism of conductivity. When the particle sizes are commensurate with the exciton Bohr radius, their exciton binding energy on the main level is 4 times higher than those for the bulk sample. Therefore, ZnO nanocomposite material manifests a considerably higher exciton thermal conductivity.

Resume : We herein report the determination of the intrinsic thermal conductivity (κ) and interfacial thermal conductance (g) of single-walled carbon nanotube thin films (50 nm) on top of a SiO2 substrate. The study was performed as a function of temperature (300-450 K) using the opto-thermal technique. The value of κ decreases nonlinearly by approximately 60% from a value of 26 W/Km at 300 K to a value of 9 W/Km at 450 K. This effect stems from the increase of multi-phonon scattering at higher temperatures. The g increases with temperature, reaching a saturation plateau at 410 K. These findings may contribute to a better understanding of the thermal properties of the supported carbon nanotube thin films, which are crucial for any heat dissipation applications.

Resume : Among the variety of stress conditions organisms must survive, temperature increases or decrease damages cellular structures and interferes with cell functions. Often, indirect consequences of a temperature shift are detected. Indirect signals may be accumulation of heat shock proteins (HSPs). Also, structural proteins can denaturate at a critical temperature, as in the thermal transition of red-blood cells at 40 C: spectrin seems to be involved in the transition, as shown by its characteristic unfolding at 40 C. Cold is another temperature condition frequently encountered in nature, causing physiological problems entirely different from heat stress. Indirect effects include reduced enzyme activity, decreased membrane fluidity, stable RNA structures that interfere with translation. Induction of cold shock proteins (CSPs) enables efficient translation and maintains membrane integrity after a temperature down-shift. However, proteins, nucleic acids, lipids, can also act as direct thermosensors. Since the conformation of virtually every biomolecule is susceptible to temperature changes, all such biomolecules could act as primary thermal sensors. For example: temperature changes on DNA curvature or supercoiling can influence transcription initiation; changes to the 3-D structure of mRNA modulate translation efficiency. Here we focus on the structures of DNA and proteins susceptible to temperature changes, to show how several biomolecular thermometers may have evolved in nature.

Resume : Metallic nanoparticles heated by a laser pulse have aroused great interest due to their ability to heat up the surrounding medium as shown in recent theoretical and experimental studies [1,2,3]. This faculty opens the way to numerous applications, notably in biomedicine [4], where for instance, the destruction of a tumor may occur by means of the hyperthermia generated from a gold NP. The objective of this theoretical work is to quantify the heat transfer in a system consisting of a nanoparticle (NP) immersed in water and illuminated by a laser pulse. We investigate different types of NPs including hollow bare metallic NPs, core-shell gold-silica nanoparticles and nanomatryoshkas. For all these types, we account for interfacial resistances at the different interfaces, especially the influence of the electron-phonon interfacial conductance on thermal transport at interfaces. We compare the performance of the different types of NPs to the case of bare gold nanoparticles, with the aim to determine which type of NPs is more efficient to heat up the surrounding medium in the shortest time. We found that the core-shell gold-silica configuration may be the most efficient if the silica shell is thin. This efficiency is to a large extent explained by the electron-phonon coupling taking place at the gold-silica interface. [1]Baffou and Rigneault, PRB (2011); [2]Govorov and Richardson, NT(2007); [3]Lombard, Biben, Merabia, PRL(2014); [4]Baffou and Quidant, LPR(2013).

Resume : Graphene, despite of being a very interesting material, presents several difficulties when our goal is to use it in applications at very high temperatures. We report here that it is possible to achieve a controlled multi-layer graphene burning, layer by layer, through a laser beam focused on suspended samples in cold-wall reactor configurations. Extremely high temperatures (up to ~ 3000K) can be attained without destroying the samples. Further insights on the details of these phenomena were gained through fully atomistic molecular dynamics simulations. These results demonstrate how to solve, with simple setup changes, two challenging graphene problems: high quality controlled thinning of multi-layer graphene and working at high temperatures, even with samples exposed directly to the air. This simple method can be very effective in the production of multi-layer graphene with controlled thickness, paving the way for new graphene applications, such as continuous radiation source and/or scaffolding material at extremely high temperatures.

Resume : The low dimensional materials are becoming one of the most promising for modern electronic due to their outstanding properties. One of them is germanium selenide, a IV - VI narrow band chalcogenide, which could be an alternative for the lead chalcogenides due to its environmental sustainability and high stability and also its less toxicity. We have produced thin films of this materials via mechanical exfoliation and performed temperature-dependent Raman measurements in temperature range of 70 ? 350 K for the germanium selenide thin films. We have observed temperature-dependent non-linear phonon shift in low temperature which could be explained by optical phonon decay onto two or three acoustic phonons with equal energies. The first order temperature coefficient χ for higher temperature for B3g mode is χ = -0.025 cm-1/K, for Ag(1) mode is χ = -0.0197 cm-1/K and for Ag(2) mode is χ = -0.0288 cm-1/K and their values are comparable to those noticed for graphene or phosphorene. Our results are useful for further analysis of phonon and thermal properties of germanium selenide.