P-Olivier CHAPUIS - CNRS research scientist

I am broadly interested by all topics related to energy and nanotechnology/nanoscience. I have been especially dealing with nanoscale thermal radiation and nanoscale heat conduction, and their connection to energy conversion.

Nanoscale (sub-diffusive) heat conduction

Fourier's law, which states that the local conducted heat flux is proportional to the gradient of temperature (see the Mémoire sur la Théorie Analytique de la Chaleur), diverges at small scale. As an example, in 1D, the flux would be inversely proportional to the distance between the hot and the cold sides according to the famous law, which is not possible if this distance is reduced to an arbitrarily-small value. The transferred heat flux can be limited by various effects, the most prominent one being the shift from a diffusive regime of heat conduction to a ballistic regime. The Knudsen number allows to quantify the transition between the regimes, by comparing the size involved in the geometry to the mean free path of the energy carriers. Another effect that should be accounted for is the limitation to the flux that can transmitted accross an interface; this was first evidenced between a solid and a fluid by Pyotr Kapitza, but is general to all pairs of materials.
In electrically-insulating solids or in semiconductors, heat is mainly transferred by phonons. I have been interested by many aspects related to these energy carriers, in particular by the impact of the confinement of their sources (Paris, Barcelona). The analysis can be performed in light of the Boltzmann transport equation for phonons. A particularly-interesting way to analyse heat conduction from a localized heat source is scanning thermal microscopy (see review), a technique for localized thermal probing. We are actively moving toward complex thermal confinement such as two-dimensional one, taking advantage of the progresses that have been made in the last decade in the simple configurations (nanowires, velocities and relaxation times can be measured for a (low-frequency...) portion of the spectrum and compared to acoustic calculations.
We are pursuing this phononic approach (see our paper on phononic resistances). It is especially tempting to analyze these aspects in light of the non-linearities that could take place: they may lead to thermal rectification, which is at the heart of the dreams of a future revolution linked to the mastering of energy in solids.

Link to a basic presentation on heat conduction at nanoscale (in French, no wave regime).

Microscale and nanoscale (sub-wavelength) thermal radiation

When Max Planck wrote his famous book "Theory of Heat Radiation", he mentioned that the theory would not be valid at scales on the order of or smaller than the radiation wavelength. Some efforts have been pursued since that time to develop a theory of thermal radiation valid at all scales. In 1969, an experiment by Hargreaves, who was working at Philips Research Labs led by famous physicist Hendrik Casimir, showed that it may be possible to measure the thermal radiation exchanged by objects spaced by less than 10 microns (Wien's wavelength, where the maximum of thermal radiation occurs, at room temperature). Dirk Polder and M. Van Hove presented the full theory, taking into account Sergueï Rytov's theory of fluctuating fields. It demonstrated theoretically that heat transfer through thermal radiation could be enhanced by orders of magnitudes!
Since a decade, rigorous experiments are possible: the most striking ones have been those in Germany, at MIT (1, 2), in France, in Florida, and in the Czech Republic. In addition to the early preparation of an experiment, I have been involved in the analysis of the experiments dealing with metals, and especially at the limits of fluctuating electrodynamics (spatial non-locality). More recently, information related to the thermal spectra in the near field has been observed (Paris 1&2, Boulder). It is extremely interesting that the spectrum of thermal radiation can change and exhibit peaks where a lot of energy is confined (as predicted by Greffet et al.).
We have recently analyzed the prediction of possible decreases of the heat exchanged by radiating surfaces below the far-field limit for bulk surfaces (see our article and the work by colleagues at Columbia University) and thin films. This is due to the existence of a coherent regime which can start at much larger distances than what was thought previously (link). Thin films and finite objects are also interesting due to their far-field thermal radiation properties which differ from the macroscopic ones and can be optimized.

Link to a basic presentation on thermal radiation at micro and nanoscale (in French).

Nanoscale energy conversion

Carbon-related energy sources may appear as scarce in the future; they may already appear as unappropriate due to the induced climate issues. Nanoscale can provide an avenue for the developement of novel energy sources taking benefit from the heat freely available in the environement.
Among these sources one can find thermoelectricity, the goal of which is to generate electricity from a difference of temperature between two close locations. Peltier and Seebeck are credited with the main discoveries linked to thermoelectricity. Since it was shown that thermal insulation is required for such phenomenon to to take place, nanoscale design is exploited to improve the associated devices.
Another possible way to harvest energy is thermophotovoltaics, which was proposed in particular by Kolm and Aigrain in the 1950s-1960s. This is parallel to photovoltaics, but the sources are radiating bodies at intermediate temperatures (~1000°C) instead of the sun directly, which leads to constraints for the radiating bodies (see e.g. some of them). We are interested in taking advantage of the modification of the flux intensities and spectra in the near field to design near-field thermophotovoltaic devices, which are subject to surface recombinations and high carrier-density photogeneration.

We are performing both experimental investigations (link to a basic presentation on thermal nanometrology, in French), with electro-thermal means as in scanning thermal microscopy and metal line-based deposited devices, and numerical studies, with the Boltzmann transport equation when particle transport is considered, or with wave equations such as the Maxwell ones or those associated to continuum elasticity. They have been or are performed in very close collaborations with friendly colleagues thanks to the support of various institutions.