Effet de la nanostructuration par faisceaux laser ultra-courts sur l’évolution des propriétés thermoélectriques des matériaux

Today, renewable energies such as wind, solar, hydropower and thermoelectricity play an essential role to cover our energy needs. Among these different sources of energy, thermoelectricity, which offers the ability to convert a heat into electricity or vice versa, has attracted a great attention due to its wide field of potential applications. The current advances in thermoelectric research are focusing on the improvement of the conversion efficiency of thermoelectric devices through optimizing and improving the thermoelectric properties of the thermoelectric materials (Seebeck coefficient, electrical conductivity and thermal conductivity). For this, different approaches (doping, new materials, nanostucturing...) have been investigated in the literature. Among these approaches, nanostructuring of materials is the most studied in the literature in order to improve the thermoelectric properties of materials. In this thesis work, we aimed to study the effect of surface nanostructuring of materials (mesoporous silicon and titanium oxide deposited in thin film) by ultra-short laser beams (picosecond and femtosecond) on the evolution of their thermoelectric properties. First, we focused on the study of various physical phenomena involved during the laser-matter interaction that yield to the formation of very different nanostructures in form of ripples, spikes, dots and others as function of the applied laser dose (fluence and number of pulses). The formation of these nanostructures has been studied in two regimes (stationary and dynamic). After optimizing the laser parameters leading to the formation of such nanostructures, a characterization of Seebeck coefficient and the electrical conductivity before and after the nanostructuring of these materials was carried out by using a new experimental setup (ZT-meter) designed and validated in GREMI laboratory. The results of measurements showed an important improvement of Seebeck coefficient and electrical conductivity after nanostructuring. This important improvement observed with the both materials leaded to a strong increase in the thermoelectric power factor (reaching roughly 50000%).Aujourd’hui, les énergies renouvelables comme l’énergie éolienne, l’énergie solaire, l’énergie hydroélectrique et la thermoélectricité jouent un rôle essentiel dans la couverture de nos besoins en énergie. Parmi ces différentes sources d’énergie, la thermoélectricité, qui permet de convertir la chaleur en électricité ou inversement, attire une grande attention grâce à son large champ d’application. Les actuelles avancées dans la recherche thermoélectrique visent l’amélioration du rendement de conversion des modules thermoélectriques, à travers l’optimisation des propriétés thermoélectriques intrinsèques des matériaux utilisés (coefficient de Seebeck, conductivité électrique et conductivité thermique). Pour cela, différentes approches ont été étudiées (dopage, nouveau alliages, nanostucturation …). Parmi ces approches, la nanostructration des matériaux a été largement étudiée pour mener à bien cet objectif. Dans ce travail de thèse, nous nous sommes intéressés à étudier l’effet de la nanostructuration de surface des matériaux (silicium mesoporeux et oxyde de titane déposé en couches minces) par faisceaux laser ultra-court (picoseconde et femtoseconde) sur l’évolution de leurs propriétés thermoélectriques. Dans un premier temps, nous nous sommes focalisés sur l’étude des différents phénomènes physiques impliqués durant l’interaction laser-matière ainsi que sur la formation des différentes nanostructures résultantes (en forme de ripples, spikes, dots et autres) en fonction de la dose laser appliquée (la fluence et le nombre de pulses). La formation de ces nanostructures a été étudiée suivant deux régimes (stationnaire et dynamique). Après l’optimisation des paramètres conduisant à la formation de ces nanostructures, la caractérisation du coefficient de Seebeck et la conductivité électrique avant et après la nanostructuration de ces matériaux a été réalisée grâce à un nouveau dispositif de mesure (ZT-meter) développé au laboratoire GREMI. Les résultats de mesures montrent une importante amélioration du coefficient de Seebeck et la conductivité électrique après la nanostrucutration. Un facteur d’augmentation de la puissance thermoélectrique a été observé pour les deux matériaux étudiés ; notamment dans le cas de couches minces d’oxyde de titane (jusqu’à 500 fois)

Micro-spiked mesoporous silicon formed by UV picosecond laser irradiation

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Nanostructuring of titanium oxide thin film by UV femtosecond laser beam: from one spot to large surfaces

International audienceSurface structuring of titanium oxide thin films by a UV femtosecond laser beam (266 nm, 100 fs) is presented in this paper. Without using laser scanning, the results show the formation of regular dots and laser induced periodic surface structures (LIPSS) with a period close to that of the beam wavelength. These nanostructures seem to be due to free-surface energy minimization. Furthermore, laser irradiation of large homogeneous surfaces by laser scan (5 × 5 mm²) showed more exotic surface morphologies, ranging from regular dots and low spatial frequency LIPSS to microstructures that are completely controlled by the laser fluence and the number of shots. Typically 2D circular dots of 100 nm diameter with two distinct periods (260 and 130 nm) are achieved under a very low fluence of 15 mJ/cm² after 13000 shots

Histological and in situ microscopic observation of femtosecond laser induced incisions in the crystalline lens

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Laser-induced periodic surface structures formation on mesoporous silicon from nanoparticles produced by picosecond and femtosecond laser shots

International audienceThis paper deals with the formation of laser-induced periodic surface structures (LIPSS) on mesoporous silicon thin films induced by two laser regimes in the UV range: picosecond and femtosecond. Different LIPSS formation mechanisms from nanoparticles, mainly coalescence and agglomeration, have been evidenced by scanning electron microscopy analysis. The apparition of a liquid phase during both laser interaction at low fluence (20 mJ/cm2) and after a large number of laser pulses (up to 12,000) has been also shown with 100 nm size through incubation effect. Transmission electron microscopy analyses have been conducted to investigate the molten phase structures below and inside LIPSS. Finally, it has shown that LIPSS are composed of amorphous silicon when mesoporous silicon is irradiated by laser beam in both regimes. Nevertheless, mesoporous silicon located between LIPSS stays crystallized

Effect of micro and nano-structuration by ultrashort laser beam on the thermoelectrical properties of silicon substrates

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Effect of post-deposition thermal treatment on thermoelectric properties of pulsed-laser deposited Ca3Co4O9 thin films

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Micro-spiked mesoporous silicon formed by UV picosecond laser irradiation

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Micro-spikes formed on mesoporous silicon by UV picosecond laser irradiation

International audienceConical spikes with sizes ranging between 0.2 and 5 µm were formed on a mesoporous silicon surface under ultraviolet picosecond laser irradiation in ambient air. Scanning electron microscopy was employed to visualize the successive steps in the formation of the spikes. On increasing the number of shots (up to 1000) at a fluence of 400 mJ/cm2, an increase in the size and an enhancement in the distribution of spikes were observed. The experimental observations revealed that the formation and the growth of these structures were mainly due to material ablation and removal mechanisms. FTIR analysis on several large (5X 5 mm2) irradiated zones showed an enhancement in the absorption coefficient mainly in the IR range from 1 to 8 µm versus the size of the microstructures

Interference between LIPSS and nano-pores in case of mesoporous silicon induced by pico and femto-second laser beams

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