Jean-Laurent Gardarein

Evidence of second order transition induced by the porosity in the thermal conductivity of sintered metals

In this paper, using both experimental data and theoretical modelling, we investigate the degradation of the thermal conductivity of sintered metals due simultaneously to the grain boundary thermal resistance and the porosity. We show that the porosity dependence of the thermal conductivity of sintered material from spherical particle powder, exhibits a critical behaviour associated with a second order phase transition. An analytical model with a single parameter is proposed to describe the critical behaviour of the thermal conductivity of sintered metals versus porosity.

Experimental study of the thermal conductivity of sintered tungsten: Evidence of a critical behaviour with porosity

Rear face flash experiments were performed in order to determine the thermal conductivity of sintered tungsten at room temperature. Ten different samples were synthesized with the spark plasma sintering technique. The microstructure obtained from the sintering is porous and consists of angular grains with medium sphericity. The average grain size (d) and the porosity (P) of the samples lie within the ranges of 2 mu m <= d <= 7 mu m and 0 <= P <= 0.35. We show that the dependence of the thermal conductivity of the sintered tungsten samples on the porosity shows a critical behaviour. A theoretical explanation of this behaviour and a predictive model for this porosity dependence are proposed

Thermal transport properties of multiphase sintered metals microstructures. The copper-tungsten system: Experiments and modeling

The thermal diffusivity of Cu-W sintered alloys microstructures is measured at room temperature at different compositions, using rear face flash experiments. The samples are synthesized with the Spark Plasma Sintering technique. The resulting microstructures are slightly porous and consist of angular nanoscale grains of tungsten with medium sphericity in a copper matrix. The tungsten particles are at the nanoscale with an average grain size of 250 nm in contrast to the copper matrix for which the average grain size lies in the range 20 mu m-30 mu m; this is large enough to avoid the grains boundary effect upon the thermal transport. The overall porosity of the microstructures lies within the range: 6% <= P <= 12%. Along with the experimental work, a predictive model describing the effective thermal conductivity of multiphasic macrostructures is proposed in order to explain the obtained experimental results. The model was developed based only on physical considerations and contains no empirical parameters; it takes into account the type of microstructure and the microstructure parameters: porosity, grain shape, grain size, and grain size distribution. The agreement between the experiments and the model is found to be excellent

Formation of thin tungsten oxide layers: characterization and exposure to deuterium

Thin tungsten oxide layers with thicknesses up to 250 nm have been formed on W surfaces by thermal oxidation following a parabolic growth rate. The reflectance of the layers in the IR range 2.5-16 mu m has been measured showing a decrease with the layer thickness especially at low wavelengths. Raman microscopy and x-ray diffraction show a nanocrystalline WO3 monoclinic structure. Low energy deuterium plasma exposure (11 eV/D+) has been performed inducing a phase transition, a change in the sample colour and the formation of tungsten bronze (DxWO3). Implantation modifies the whole layer suggesting a deep diffusion of deuterium inside the oxide. After exposure, a deuterium release due to the oxidation of DxWO3 under ambient conditions has been evidenced showing a reversible deuterium retention.

Identification of space and time varying thermal resistance: Numerical feasibility for plasma facing materials

Abstract The present paper deals with a non-linear unsteady calculation combined with the conjugate gradient method (CGM) and the adjoint state, in order to characterize in-situ the spatial and time variation of the thermal resistance of a surface layer. This paper presents the numerical feasibility of this method for the plasma-facing components (PFC), and precisely on the surface carbon layer (SCL), usually poorly attached to the PFC in the fusion machines, a realistic experiment design was used. The accuracy of the method is examined by using simulated inexact infrared measurements obtained on the SCL surface. The advantages of applying the CGM with the adjoint state in the present study, are that no prior information is needed on the time variation and for the initial guesses of the unknown thermal resistance.

Inverse heat conduction problem using thermocouple deconvolution: application to the heat flux estimation in a tokamak

Abstract Internal components of magnetic confinement fusion machines are subjected to significant heat fluxes. A large part of this power is directed towards plasma facing components. Even if these components are designed to receive about , surface temperature and heat flux measurements are important issues to guarantee safe plasma operations. In JET tokamak, few embedded thermocouples (TC) located 1 cm below the tile surface are used to measure the bulk temperatures of the Carbon Fiber tiles (coated with about 20 m of tungsten with the ITER-like wall). We propose here to use an inverse thermal calculation based on Thermal Quadrupole method to locally deduce the deposited heat flux. The calculation requires the location of the peak and the normalized 1D-shape of the heat flux deposited on the target.

Prediction of spatial resolutions of future IR cameras at ITER

Here are presented the predictions of the spatial resolutions of one of the future IR camera that will survey the divertor of the Tokamak ITER. The objective is to associate, in Fourier space, the optical transfer function and the detector transfer function to calculate the total transfer function (TTF) of the virtual IR camera. The modulation transfer function (modulus of TTF) quantifies the ‘imaging’ performances of the virtual camera. Its ‘measuring’ performances are estimated by the simulation of the slit response function experiment. Finally, some results of sharp temperature profile measurements in realistic plasma situations are presented.

Miniaturized heat flux sensor for high enthalpy plasma flow characterization

An improved miniaturized heat flux sensor is presented aiming at measuring extreme heat fluxes of plasma wind tunnel flows. The sensor concept is based on an in-depth thermocouple measurement with a miniaturized design and an advanced calibration approach. Moreover, a better spatial estimation of the heat flux profile along the flow cross section is realized with this improved small sensor design. Based on the linearity assumption, the heat flux is determined using the impulse response of the sensor relating the heat flux to the temperature of the embedded thermocouple. The non-integer system identification (NISI) procedure is applied that allows a calculation of the impulse response from transient calibration measurements with a known heat flux of a laser source. The results show that the new sensor leads to radially highly resolved heat flux measurement for a flow with only a few centimetres in diameter, the so far not understood non-symmetric heat flux profiles do not occur with the new sensor design. It is shown that this former effect is not a physical effect of the flow, but a drawback of the classical sensor design.

Multi parametric sensitivity study applied to temperature measurement of metallic Plasma Facing Components in fusion devices

In nuclear fusion experiments, the protection system of the Plasma Facing Components (PFCs) is commonly ensured by infrared (IR) thermography. Nevertheless, the surface monitoring of new metallic plasma facing component, as in JET and ITER is being challenging. Indeed, the analysis of infrared signals is made more complicated in such a metallic environment since the signals will be perturbed by the reflected photons coming from high temperature regions. To address and anticipate this new measurement environment, predictive photonic models, based on Monte-Carlo ray tracing (SPEOS®CAA V5 Based), have been performed to assess the contribution of the reflective part in the total flux collected by the camera and the resulting temperature error. This paper deals with the effects of metals features, as the emissivity and reflectivity models, on the accuracy of the surface temperature estimation. The reliability of the features models is discussed by comparing the simulation with experimental data obtained with the wide angle IR thermography system of JET-ITER like wall. The impact of the temperature distribution is studied by considering two different typical plasma scenarios, in limiter (ITER start-up scenario) and in X-point configurations (standard divertor scenario). The achievable measurement performances of IR system and risks analysis on its functionalities are discussed.