Jean-Bernard Saulnier

Thermal conductivity calibration for hot wire based dc scanning thermal microscopy

Thermal conductivity characterization with nanoscale spatial resolution can be performed by contact probe techniques only. The technique based on a hot anemometer wire probe mounted in an atomic force microscope is now a standard setup. However, no rigorous calibration procedure is provided so far in basic dc mode. While in contact with the sample surface, the electrical current I injected into the probe is controlled so that electrical resistance or the wire temperature is maintained by the Joule effect. The variation in current is assumed to be linearly related to the heat flux lost towards the sample and traditional calibration is carried out by relating the thermal conductivity of a set of samples to the measured current I. We provide analytical and numerical thermal modeling of the tip and sample to estimate the key heat transfer in a conductivity calibration procedure. A simple calibration expression is established that provides thermal conductivity as a function of the probe current or voltage measured. Finally, experimental data allow us to determine the unknown quantities of the parametric form obtained, i.e., the mean tip-sample contact radius and conductance.

Clamped nanowire thermal conductivity based on phonon transport equation

The phonon Boltzmann transport equation is used to calculate the thermal conductivity in clamped silicon nanowires and study both boundary and confining effects of phonon scattering. The solution method includes partly diffuse and specular phonon reflections at the boundaries and introduces a spectral extinction coefficient. This parameter is derived from the Umklapp relaxation time accounting for thermal resistive processes. A numerical scheme based on the discrete ordinates method has been developed. Results are presented for several wire lengths and extrapolated to infinite wires. Fouriers law is retrieved both theoretically and numerically for acoustically thick media when only specular reflections occur.The phonon Boltzmann transport equation is used to calculate the thermal conductivity in clamped silicon nanowires and study both boundary and confining effects of phonon scattering. The solution method includes partly diffuse and specular phonon reflections at the boundaries and introduces a spectral extinction coefficient. This parameter is derived from the Umklapp relaxation time accounting for thermal resistive processes. A numerical scheme based on the discrete ordinates method has been developed. Results are presented for several wire lengths and extrapolated to infinite wires. Fouriers law is retrieved both theoretically and numerically for acoustically thick media when only specular reflections occur.

Analyse de la qualité de modèles nodaux réduits à l'aide de la méthode des quadripôles

Abstract A comparative study between models with transfer function (two-port) and nodal models is carried out in a Laplace space and in a time domain. We particularly bring to the fore the physical content of the transfer matrix. Different examples of finite walls let us identify the main use of each of these matrixterms, according to the chosen boundary conditions. The defaults of small order models, in the first instant, are characterised either in Laplace and in time domain. Perspectives of this confrontation in model reduction research are evoked.

Étude expérimentale et numérique des transferts de chaleur en espace annulaire sans débit axial et avec cylindre intérieur tournant

Abstract The convective heat transfer between the walls of an annular gap with a rotating inner cylinder are studied experimentally and numerically. Two geometrical configurations are analysed : the surfaces of the cylinders are either smooth, or the moving wall is smooth and the other is axially grooved. The experimental study allowed us to appreciate the increase of heat transfer in the presence of grooves for values of Taylor number up to 2·10 7 . The study of the flow and of the heat transfer has also been conducted by numerical simulation by means of three-dimensional laminar model for the grooved gap, and of an axisymmetric model for the smooth gap. The comparison of numerical and experimental data have shown a good agreement of the Nusselt number for values of Taylor number up to 4 · 10 5 .

Analyse de la conduction de la chaleur aux temps ultra-courts dans un solide par la thermodynamique irréversible étendue et la dynamique moléculaire

Abstract Instantaneous heat propagation and thermodynamic local equilibrium cannot be assumed when solving space and time microscale problems. Therefore, we reconsider the thermodynamics basis of the Fourier law in order to obtain the new heat conduction models: the hyperbolic heat equation (EH) and the modified hyperbolic equation (EHM). We have performed molecular dynamics (DM) experiments which are independent of any thermodynamic model, to test the macroscopic approaches. We show that the solutions of the EH and the EHM do not agree with the numerical experiments and that the MD results are strongly dependent on the way from which the macroscopic conditions are simulated in the microscopic point of view.

Prone position may increase temperature around the head of the infant

Recommendations to adopt the supine position were followed by a dramatic decrease of SIDS. But no explanation has been given for the association between SIDS and the prone position nor for its decrease in the supine position. We report data on an infant and a mannequin demonstrating an increase in temperature around the head in the prone position. A 4‐month‐old boy presented an acute life‐threatening event related to temperature after febrile otitis despite treatment: 40.5°C, heart rate 280 bpm with circulatory failure and cardiorespiratory arrest requiring resuscitation. There were no seizures. Blood and CSF cultures were negative. The course under antibiotics was favourable. On d 3, we measured temperature at several sites on and around the heat. Temperatures were higher in the prone than in the supine position in pericephalic areas: + 1°C (supracephalic), +2.5°C (peritemporal), and +3.5°C (submandibular). In a thermoregulated room, we used a mechanically ventilated mannequin of an infant. The prone position was also associated with an increase in temperature around the head: +3.3°C (supracephalic), +1.8°C (peritemporal), and +1.1°C (submandibular). Changing from the supine to prone position thus increased temperature around the head (infant and mannequin). To our knowledge, this has not been reported before. SIDS is related to factors modifying temperature status and environment. Furthermore, evacuation of heat is mandatory for an infant. We think the increase in temperature around the head in the prone position is due to the absence of convective fluxes, and speculate it could impair thermolysis.

Transient heat transfer in a Lennard-Jones crystal by molecular dynamics

The phenomenological description of heat conduction problems in non-continuous media by the Fourier law, is called into question when thermal transfers are studied at both temporal and space microscales. Several Equilibrium and Non Equilibrium Molecular Dynamics (EMD and NEMD) experiments have been carried out in order to specify the nature of the heat transfer at short times in a Lennard-Jones solid. Presented are (i) the hydrodynamic, the heat flux and the phonon relaxation times (respectively ru, rv and rp) computations done thanks to EMD experiments. ~v is deduced from a heat flux fluctuations description, and re is worked out from the Peierls thermal conductivity definition in the frame of the Deybe model. (ii) The response of the lattice to a kinetic energy step which displays wave-like phenomena of coherent and incoherent nature for times of the order of some phonon relaxation times. An analysis in terms of mode velocities confirms the presence of acoustic and thermal (second sound) wave propagation.

Quelques enjeux en transferts thermiques

Abstract Heat transfer is strongly involved in many scientific and technologic domains and the French heat transfer laboratories and networks cooperating is this field are first located. The analysis of the main industrial activities demanding heat transfer competence helps one first to identify some up-to-date technological challenges. It appears clearly that connecions are to be reinforced between disciplines like heat transfer, fluid mechanics, combustion, material science, optics, biology… Scientific objectives are then scanned through, by splitting the research activities between mature topics (radiation, particularly in semi-transparent media; convection and thermoconvective instabilities; heat transfer in porous media…), emerging (heat transfer with change of phase, convective heat transfer enhancement by active control in the boundary layer, inverse techniques…) and incipient ones. Among some promising new topics, let us mention microscale heat transfer, and also bioheat transfer.