Abstract
Thin films are now widely used in technical systems because of their particularly interesting properties compared to conventional "solid" materials. The thermal behaviour of these systems is often mentioned as a technological lock (for power transfer, microelectronics, …). It is therefore essential to control the heating of these materials and to perfectly know the thermal properties of the materials they are made of.Unfortunately, the thermal characterisation of thin films leads to many difficulties as their thermal properties are strongly correlated to their microstructure and depend on the temperature. Moreover, there may be potential presence of non-negligible thermal interface resistances as well as non-ideal experimental effects (semi-transparency, low signal to noise ratio, ...). Furthermore, the extremely low thermal response time of thin films make it necessary to develop specific methods for their characterisation. Two main classes of methods globally exist for this purpose: photothermal methods and electrothermal ones. Contrary to the context of the characterisation of "solid" opaque materials, the characterisation of thin layers does not necessarily imply that optical methods are less intrusive than electrical ones (by contact), in particular due to the possible semi-transparent character of thin layers generally implying the need to cover them with an opaque film to absorb and emit energy on the surface. In this context, an opto-electrothermal characterisation method is developed during this thesis for the determination of the thermal conductivity (and/or of the diffusivity and/or of the effusivity) of orthotropic thin films. Applicable over a wide range of thermal conduction times covering those of solid and thin materials, this method is based on the heating by the mean of an optical excitation of a metallic strip deposited on the front face (excited face) of the film to be characterized, and on the measurement of the variation of the electrical resistance of this strip (serving as an electrothermal transducer).The first part presents the design and characteristics of a new experimental bench for the opto-electrothermal method, dimensioned to be applicable to materials with thermal conduction times ranging from a few seconds to a few tens of nanoseconds. For this purpose, electrothermal models acting as numerical twins of the bench are developed; they allow to consider the degradation of the electronic performances of the measurement chain at high frequencies depending on the frequency dynamic of the thermal diffusion within multilayer samples. The implementation of an inverse method and the analysis of its performance are then detailed in the context of the characterisation of thin and massive media. Methods are proposed to solve theoretical (parameter correlations, model biases, ...) and experimental (low response times, noise, interface resistance, ...) difficulties that may occur. In particular, the electrothermal models developed are used as simulation and/or estimation models. They account for the possibilities of neglecting or not the degradation of the performance of the measurement chain for the estimation of the thermophysical properties and to take it into account if necessary. The inverse method is also implemented on first experimental measurements carried out with the developed bench. Finally, the method is applied in front- or back-facing measurement configurations to thin materials with good thermal conductivity and semi-transparent properties. For this purpose, quadrupole models describing the conducto-radiative transfer within the sample are developed and used.