A. Degiovanni

Measurement of thermal diffusivities through processing of infrared images

The measurement of the thermal diffusivity of a thin layer in the direction of its plane is usually a difficult operation. The standard ‘‘flash technique’’ is very appropriate for diffusivity measurement in the direction of the thickness of the sample but adaptations of this method to in‐plane measurements remain very sensitive to the position and form of heat excitation and temperature sensors. The new procedure proposed here consists of applying any geometrically nonuniform heat impulse on the front face of the sample and recording the entire transient temperature image on the rear face thanks to an infrared camera. The influence of axial diffusion can be avoided for periods much longer than the axial diffusion characteristic time. Integral transforms on the radial space variables (Fourier transform) are very suitable for treating the temperature field and to estimate radial diffusivity. The main advantage of this method is to avoid any experimental precaution (no knowledge of the geometrical form of th...

A theoretical study of the transient coupled conduction and radiation heat transfer in glass: phonic diffusivity measurements by the flash technique

Abstract Evaluating the phonic conductivity of glasses is difficult because of the combined mode of heat transfer. In this work, a model is developed for the flash experiment, and numerical results are reported. Simulations in the non-gray case have shown that heat transfer in the sample is completely free from any radiative contribution for conditions of small optical thicknesses and reflecting walls. Thus diffusivity identified: from the rear-face thermogram is a quantity of phonic origin. Experimental values for phonic conductivity are presented. The results for float glass are in agreement with literature data. For silica, a new result has been obtained: the temperature dependence of the phonic conductivity is found to be similar to that of its specific heat, confirming microscopic models of thermal conductivity in disordered systems.

Ablative melting of a solid cylinder perpendicularly pressed against a heated wall

Abstract This paper reports an analytical and experimental study on ablative melting of a solid cylinder perpendicularly pressed against a stationary heated surface. An explicit analytic solution is found for the rate of ablation in terms of temperature difference and pressure applied and of geometrical as well as physical properties of the solid and liquid. Data obtained in a limited number of rather crude experiments with rods of melting solids (ice, paraffin) and with rods of wood under flash pyrolysis conditions show a fair agreement with the predictions of the theoretical study thus confirming the “fusion model” of flash pyrolysis of wood in ablation regime.

Mesure de la diffusivité longitudinale de matériaux anisotropes

Abstract Measurement of in-plane diffusivity of anisotropic solid samples. Survey of the techniques developed at LEMTA. Three techniques for the measurement of in-plane diffusivity of anisotropic solid samples have been developed at the Laboratory and are presented here : • the two directional heat pulse (flash) method with local contact measurement of two temperatures; • the fin method with local contact or contactless measurement of two temperatures; • the two directional heat pulse method with measurement of the temperature field by an infrared camera and data processing through integral transformations.

Microscopic observation of weak electric fields

Weak electric fields in vacuum around conductors like small metallic wires or tips are observed using a low-energy electron point projection microscope operating in interferential mode. Observations are made at room temperature with a sensitivity of a few hundred millivolts per micrometer and submicrometer spatial resolution.

Reconstitution of a non uniform interface thermal resistance by inverse conduction

Diffusive heat transfer problem within a non isotropic stratified structure, containing a plane defect of non uniform resistance, is solved analytically by the method of integral transforms. The idea consists in applying Fourier cosine transforms on the space variables and a Laplace transform on the time variable. The thermal quadrupole formalism allows to reduce the mathematical model to a product of matrices in the transformed space. The direct modeling, within the framework of a 2D geometry, has been followed by the construction of an inverse procedure that reconstitutes the variation of the thermal resistance from the measure of the surface temperature. But in the presence of noise, this solution becomes unstable because the problem is ill-posed. To avoid an unstable solution, a method of regularization is necessary. It consists in filtering the data to make our inverse problem well-posed The inversion can then be undertaken in an explicit way either in the transformed space, or in the real space. The...

Experimental determination of the thermal conductivity of liquid UO2 near the melting point

The article gives an account of measurements of the thermal conductivity of liquid UO2. The sample was heated up to above the melting point by a laser pulse of a controlled shape, and the produced thermogram of temperature history was measured by a fast and accurate pyrometer with a time resolution of 10 μs. The experiment shows that the rate of temperature increase during the ascending part of the pulse changes moderately across the melting point. Due to the high power input, this effect cannot be explained in terms of the sole intake of latent heat of fusion. By solving the related heat transfer equation with a 2D-axisymmetric numerical model, it is demonstrated that this feature depends principally on heat conduction in the sample, and proves that the thermal conductivities of solid and liquid are not very different. A theoretical sensitivity study assessing the influence of the liquid thermal conductivity on the pulse temperature evolution showed that the conductivity of the liquid can be deduced from...

Modulated photothermal radiometry applied to semitransparent samples: Models and experiments

Mathematical modeling is presented of the combined conductive and radiative heat transfer occurring in a semitransparent material (STM) subjected to a periodic heat flux. The models rely on the quadrupole method, which is a very powerful tool to obtain analytical solutions in the Fourier or Laplace domain. Photoacoustic or photothermal radiometry techniques are reviewed. Two groups of methods are discussed depending on whether the sample has natural or opaque interfaces to simulate radiative exchanges with the surroundings. The metrological problem of measuring the phonic thermal diffusivity of semitransparent materials is investigated. Theoretical simulations are given. They explain some typical features of the phase-lag signal of temperature responses. Experimental measurements on pure silica validate the results and prove that these methods are efficient for the thermal characterization of STM.

Imaging charged objects using low-energy-electron coherent beams

The microscopic shapes, unobservable by conventional electron optics, of objects surrounded by electrostatic fields are observed. The method directly uses electron-coherent-beam imaging concepts and is successfully applied to recently developed low-energy-electron projection holography. Experimental demonstrations on objects charged in a controlled way like a biased metallic tip as well as on a self-charging insulator object are shown.

Interferometry with low‐energy electrons

The first experimental results combining an electrostatic electron biprism and a low‐energy electron projection microscope are presented. In these experiments, a low‐energy electron beam is split into two coherent beams which interfere afterwards. The interference pattern consists in many parallel fringes. The electron energy ranges from a few tens eV up to a few hundred eV.