Description du sujet de thèse

The CNRS, within the Pprime Institute, on the Poitiers-campus site, is recruiting a doctoral student as part of the research project in collaboration with Loma (laboratory based in Bordeaux).
The thesis subject: The subject of the thesis addresses the thermal properties of VO2 and the work will be divided in two parts. In the first part, we will endeavor to make an in-depth study as a function of temperature, of the behaviors of the thermal conductivity k and the specific heat cv of thin crystalline films of VO2 which will be provided to us by our IRCER collaborators in Limoges. The work will be experimental and theoretical in nature. Several parameters can be considered, such as the thickness of the VO2 film, the nature of the substrate, the nature and percentage of doping that VO2 can undergo, etc. The objective is to carry out the most complete study possible, combining experimental measurements and a theoretical analysis in order to provide more elements of understanding of the thermal behaviors of k and cv in the two regimes; heating and cooling, at the level of the VO2 phase transition zone and the impact that the various parameters mentioned above can have on the hysteresis loop in terms of Tc position, width and amplitude. A particular point to be clarified in this first part, will concern the nature of the apparent peak in the behavior of k in the cooling regime which has been revealed in the few previous studies.
In this first part, the experimental work will make use of a very powerful technique at the forefront of the state of the art in the field of thermal metrology, it is the technique of “Time Domain Thermoreflectance (TDTR)”. This technique has excellent temporal resolution thanks to the use of picosecond or femtosecond pulsed lasers, and has become in the last two decades an essential tool for measuring thermal properties, particularly of thin layers. Its use will also allow us to have access to the thermal behavior of the interface thermal resistance between the metallic layer (generally Al) used as a transducer in the TDTR and the VO2 film. This last point, on its own, is very rich on a fundamental level, since the study will allow us to understand a little more the nature of the coupling between electrons and phonons at the interface and the way in which this coupling evolves during the phase transition undergone by the VO2 film as a function of temperature.
In the second part, we will exploit the thermal behaviors of k and cv of VO2 to make a detailed study of the behavior of a VO2 based conductive thermal diode in the two operating regimes: steady-state and transient. It is worth mentioning that until now, only the steady-state regime has been studied in detail and few studies have been carried out for the transient regime of this type of diode. The VO2 based conductive thermal diode will indeed be a very good example of smart thermal insulation in buildings or other sectors. The possibility of reducing the transition temperature of VO2 to temperatures close to room temperature is an interesting point to exploit. This study once successful, will lay the foundations for a potential experimental realization and proof of concept of the VO2 based conductive thermal diode.
The work that we propose to carry out in this thesis will be added to a series of works already started within our team « Thermal Nanoscience and Radiation (TNR) » of the PPRIME institute in Poitiers in collaboration with teams from the Nouvelle Aquitaine region, in the manufacture and structural characterization of VO2 crystalline samples (IRCER in Limoges) as well as the characterization of the thermal properties of thin films by TDTR (LOMA in Bordeaux). This study will take place in close collaboration and complementarity of the experimental and theoretical aspects, between the TNR team of PPRIME and the « Transport of Energy at Small Scales (TESS) » group of the Photonics and Materials team of LOMA in Bordeaux. The experimental part concerning the exploitation of the TDTR will be carried out within the TESS group of LOMA while the theoretical part and the modeling will be conducted within the TNR team of PPRIME.

Contexte de travail

Vanadium dioxide is one of the most interesting thermochromic materials and has been the subject of many research studies in the last few years. This material manifests a reversible metal-insulator phase transition around a critical temperature of Tc~68°C. This transition occurs over an ultra-short interval of time (few picoseconds) and is a direct consequence of the structural transformation that undergoes this material passing from a monoclinic structure at low temperatures to a tetragonal-rutile structure at high temperatures. In the vicinity of Tc, the physical properties; electrical, optical and thermal of VO2, undergo abrupt variations. These variations are very advantageous and allow several promising technological applications to be contemplated; thermal sensors, optical, electrical and thermal switches, smart windows and surfaces, surface treatment to improve thermal comfort in buildings and the design and realization of thermal components having analogous behaviors to the electronic components such as the diode, the transistor and the memristor. This last application would open the way to the realization of a whole information processing architecture that is purely photonic and contactless or purely phononic or a combination of the two processes that one could name « Thermotronics ».

Le poste se situe dans un secteur relevant de la protection du potentiel scientifique et technique (PPST), et nécessite donc, conformément à la réglementation, que votre arrivée soit autorisée par l'autorité compétente du MESR.

Contraintes et risques

The PhD student will be based in Poitiers and Bordeaux.
- the theoretical part and the modeling will be conducted within the TNR team of PPRIME Institute in Poitiers.
- the experimental part of the thesis work concerning the exploitation of the TDTR will be carried out within the TESS group of LOMA

Short trips, in France and abroad, are to be expected.

Informations complémentaires

The candidate must hold a Master's degree or equivalent.
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The doctoral student will be registered at the doctoral school of the University of Poitiers.
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The candidate must have a physicist profile with good skills in condensed matter physics, heat transfer, laser optics and numerical calculation.
He (she) must be motivated to carry out both theoretical and experimental works (instrumentation, measurement, design and analysis).
A good knowledge of written/spoken English is essential, as well as a good ability to work in a team.
Particular attention will be paid to the candidate's maturity and autonomy and to his (her) scientific curiosity.