Comparing Near-Field Thermal Radiation Between Different Materials Including III-V Semiconductors

When the distance between objects decreases below the characteristic wavelength of thermal radiation (few micrometers in the 300-1000 K range), the radiative heat exchanged between these objects is increased beyond the blackbody limit imposed by Planck’s law in the far field. This increase of thermal radiation, which takes place in the near field, can reach several orders of magnitude. Explained by the additional contribution of evanescent waves to the radiative transfer, this phenomenon, which is now well documented theoretically and experimentally, can be of interest for thermal-energy harvesting. For instance, thermophotovoltaics (TPV) can take advantage of this enhancement of the radiative heat flux in order to increase the electrical output power density when the emitter is brought closer to the cell [1]. A further improvement of such a device is to place a light–emitting diode (LED) on the heated emitter side, which allows controlling the emission spectrum; such device is termed thermophotonic (TPX).<br/>Some of us have demonstrated recently that Stefan-Boltzmann’s law is modified in the near field [2] and that the temperature dependence of near-field radiative heat transfer varies with the material considered. Here, we analyse the near-field radiative heat transfer between a heated emitter and different types of samples of interest for energy harvesting. For TPV and TPX devices, III-V semiconductors such as InSb, GaAs, InGaP or AlGaAs are key materials. We report on the experiments, which consist in executing several approaches of a heated micrometric spherical emitter close to the flat studied sample in order to obtain the near-field radiative conductance as a function of distance. Since the increase in the radiative flux depends dramatically on distance, we also report on our recent efforts to determine the flux more accurately in the last 50 nm before contact by means of combined analysis of laser deflection, fiber interferometry and resistive thermometry.<br/><br/>[1] Near-field thermophotovoltaic conversion with high electrical power density and efficiency above 14%, C. Lucchesi, D. Cakiroglu, J.-P. Perez, T. Taliercio, E. Tournié, P.-O. Chapuis and R. Vaillon, Nano Letters 21, 4524 (2021)<br/>[2] Temperature dependence of near-field radiative heat transfer above room temperature, C. Lucchesi, R. Vaillon and P.-O. Chapuis, Materials Today Physics 21, 100562 (2021)<br/><br/>This work has received funding from project EU H2020 FETProactive-2019-2020/GA951976 (TPX-Power).