Thermoélectricité | EPFL Graph Search

La thermoélectricité est l'électricité générée par l'effet thermoélectrique, un phénomène physique présent dans certains matériaux, qui lie les flux de chaleur qui les traversent aux courants électriques qui les parcourent. Cet effet est à la base d'applications, dont très majoritairement la thermométrie, puis la réfrigération ( module Peltier) et enfin, très marginalement, la génération d'électricité (par « thermopile » ou « calopile »). Elle a été découverte puis comprise au cours du grâce aux travaux de Seebeck, Peltier ou encore Lord Kelvin. Un matériau thermoélectrique transforme directement la chaleur en électricité, ou déplace de l'énergie thermique par l'application d'un courant électrique. Un grand nombre des matériaux possédant des propriétés thermoélectriques intéressantes ont été découverts au cours des décennies 1950 et 1960. C'est notamment le cas du tellurure de bismuth (Bi2Te3) utilisé dans les modules Peltier commerciaux, ou des alliages silicium-germanium (SiGe) ut

Charge transport properties of spin-orbit coupled materials in different correlation regimes

Soudabeh Sheikholharam Mashhadi

From recent advances in solid state physics, a novel material classification scheme has evolved which is based on the concept of topology and provides an understanding of different phenomena ranging from quantum transport to unusual flavors of superconductivity. Spin-orbit coupling is a major term in defining the topology of materials, and its interplay with electron electron correlations yields novel, intriguing phenomena. In the weak to intermediate correlation regime, spin-orbital entanglement leads to the emergence of topological insulators, which constitute Dirac materials with 2D spin-polarized helical edge or surface states and an insulating bulk. In the specific case of topological insulators, theory predicts that their unique band structure leads to a high thermoelectric performance, which is of practical relevance for energy conversion applications.In the strong correlation regime, the spin-orbit coupling removes the orbital degeneracy, thereby enhancing quantum fluctuations of the spin-orbit entangled states. This in turn can lead to the emergence of novel phases such as quantum spin liquids and unconventional superconductivity. The Kitaev quantum spin liquid (QSL) is a topological state of matter that exhibits fractionalized excitations in the form of Majorana fermions. The main aim of this thesis was to study these novel states of matter in the form of two different materials that fall in the weak and strong correlation regime, respectively. On this basis, it should furthermore be explored whether benefit can be taken of their properties by combining them with other quantum materials into heterostructures.In the first part of this thesis, nanoplatelets of bismuth chalcogenide-based 3D topological insulators were grown by chemical vapor deposition, and characterized by magnetotransport experiments. Having established charge transport characteristics of the single components, in the aim to exploit their high thermoelectric efficiency, the lateral heteroctructures of them are fabricated. Their investigation by scanning photocurrent microscopy revealed strong photocurrent generation at the p-n junction. The obtained results demonstrate that such type of hybrid heterostructure is a promising route to enhance the thermoelectric performance in nanostructured devices. In the second part of this thesis, the Kitaev spin liquid candidate, RuCl3 was studied. Raman spectroscopy on exfoliated RuCl3 revealed a broad magnetic continuumat low energies which according to its particular deviation from bosonic character is assigned to the fractionalized excitations. Furthermore, in complementary charge transport experiments, it was found that themagnetic fluctuations and structural changes in thismaterial are highly entangled, and influence the emergence of the fractionalized excitations. Finally, it was investigated whether the magnetic insulator character of ®-RuCl3 can be exploited to alter the graphene band structure through a proximity effect in vertical graphene/ RuCl3 heterostructures. Measurements of their low-temperature, nonlocal resistance confirmed the possibility to induce pure spin currents in this manner. At the same time, the charge transport through the graphene served as a probe of the magnetic ordering in RuCl3, although the complex magnetic behavior in this material necessitates more studies.

EPFL2018

Tailoring thermal properties of functional materials

Xavier François Mettan

Thermal conductivity (k) plays an essential role in functional devices. In some cases, high value is required to transfer heat efficiently. In others, low k assists in maintaining a temperature gradient necessary for device operation. It is profitable to design materials where one can tune k in a broad range according to its function. Beyond functionality, k is a fundamental property of matter, and a fantastic probe to assess vibrational dynamics and electronic structure in condensed matter. In conjunction with other transport properties such as electrical resistivity and thermoelectric power, k promotes a deeper understanding of the electronic and vibrational structure of materials. In the present Thesis, careful experimental determination of the transport coefficients of a variety of systems over a broad temperature range (4-300 K) let me unveil subtle interactions between phonons and/or electrons. The insights gained from an accurate interpretation of these properties empowers us to engineer and fine-tune the functionality of materials for applications. I first investigate the thermoelectric properties of nickel-based high-entropy alloys (HEAs), believed to host unconventional effects arising from the high level of site-disorder in their crystalline structure. Anomalous resistivity with a low temperature coefficient validates the disordered nature of HEAs. The Seebeck coefficient, reported for the first time for HEAs, is surprisingly low. In the quest to understand transport in HEA, I have discovered that iron-nickel alloys exhibit a promising thermoelectric figure of merit (ZT) of 0.1 at room temperature. One way to further increase ZT is to decrease k (in FeNi alloys or other materials) by texturing the material. Aluminium foams are systems of choice to observe the effect of a modified structure on the thermoelectric properties. Tailoring of the bubbles' size is demonstrated by applying an external acoustic force in the foaming process. Moreover, the influence of the macro- and micro- structure of the foams on thermoelectric properties is discussed. The best example for tuning k by texturing it is anatase titanium dioxide (TiO2). k was varied over three orders of magnitude by adjusting its oxygen contents and by texturing it into a foam, aerogel-like structure. We identified a new, strong diffusion mechanism of heat by polarons, created as a consequence of oxygen vacancies. Furthermore, anatase nanowires organized into foams result in an unprecedented low k = 0.014 W/Km at room-temperature. Doping this anatase foam heralds promising applications, in particular in thermoelectricity. An oxide which has an intrinsically nanoporous structure reminding that of a foam, is Mayenite (Ca12Al14O33). The thermal conductivity of such system is verified to be intrinsically low. The peculiar cage structure could host atomic of molecular species to tune its electrical conductivity, with bright prospects for thermoelectric applications. Finally, the thermoelectric properties of hybrid halide perovskites, cheap and easy-to-synthesise compounds, were studied. Three materials of this family have ultra-low k, due to the vibrational degrees of freedom of their organic cations. Notably, a tin-based organic-inorganic compound delivers ZT = 0.13 at room temperature. The thermoelectric conversion efficiency of this material was measured directly and is believed to yield promising thermoelectric applications.

EPFL2019

Tuning of the Thermoelectric Figure of Merit of CH3NH3MI3 (M=Pb,Sn) Photovoltaic Perovskites

László Forró, Endre Horvath, Péter Krisztián Matus, Xavier François Mettan, Bálint Náfrádi, Davor Pavuna, Andrea Pisoni, Riccardo Pisoni, Massimo Spina

The hybrid halide perovskites, the very performant compounds in photovoltaic applications, possess large Seebeck coefficient and low thermal conductivity, making them potentially interesting high figure of merit (ZT) materials. For this purpose, one needs to tune the electrical conductivity of these semiconductors to higher values. We have studied the CH3NH3MI3 (M-Pb,Sn) samples in pristine form showing very low ZT values for both materials; however, photoinduced doping (in M-Pb) and chemical doping (in M-Sn) indicate that, by further doping optimization, ZT can be enhanced toward unity and reach the performance level of the presently most efficient thermoelectric materials.

American Chemical Society2015