Thermoelectric effect | EPFL Graph Search

The thermoelectric effect is the direct conversion of temperature differences to electric voltage and vice versa via a thermocouple. A thermoelectric device creates a voltage when there is a different temperature on each side. Conversely, when a voltage is applied to it, heat is transferred from one side to the other, creating a temperature difference. At the atomic scale, an applied temperature gradient causes charge carriers in the material to diffuse from the hot side to the cold side. This effect can be used to generate electricity, measure temperature or change the temperature of objects. Because the direction of heating and cooling is affected by the applied voltage, thermoelectric devices can be used as temperature controllers. The term "thermoelectric effect" encompasses three separately identified effects: the Seebeck effect, Peltier effect, and Thomson effect. The Seebeck and Peltier effects are different manifestations of the same physical process; textbooks may refer to

Réaffectation et développement du secteur de la caserne de la Poya, Fribourg

Christophe Waber

Un nouveau pôle multi-fonctionnel se dessine sur le plateau de Saint-Léonard, sportif, commercial mais aussi résidentiel et administratif avec la caserne de la Poya qui se libèrera à l'horizon 2025. Le développement d'un quartier mixte sur cette parcelle, d'un seul tenant et d'une grande qualité, doit répondre à de nombreux critères. Il doit respecter l'utilisation mesurée et efficace de la ressource mais aussi apporter un bénéfice qualitatif à plus large échelle. Ainsi, la volonté de ne pas fractionner l'image urbaine du projet et de la considérer comme un élément générateur d'urbanité à son échelle, apporte clarté et sentiment d'unité. Le parti pris de bâtir le pourtour du site et de générer un large espace au centre, permet d'offrir aux futurs usagers et habitants de cet ensemble une vaste cour commune qui rappelle les dimensions des espaces militaires précédents, soit la place d'appel. Ce projet représente un état transitoire du développement de ce quartier, avec sur la partie Sud de la parcelle, des jardins familiaux qui pourront ensuite être entourés de petits immeubles d'habitations contigus construits peut-être par des maîtres d'ouvrage non institutionnels. La réussite d'une telle opération dépend d'outils aptes à canaliser son image. Un masterplan, définissant alignement, gabarit, épaisseur, mobilité et accès ainsi que l'image architecturale des façades, permettra de reconnaître le projet comme un seul îlot même si celui-ci devait se voir fractionné en faveur de différents maîtres d'ouvrage. La mixité du programme (bureau, service cantonal, formation et logement) représente un défi supplémentaire pour la cohésion de ce nouveau quartier.

2015

Effect of Heat Current on Magnetization Dynamics in Magnetic Insulators and Nanostructures

Francesco Antonio Vetrò

The term "spin caloritronics" defines a novel branch of spintronics that focuses on the interplay between electron spins with heat currents. In the frame of this research area, this thesis is aimed at investigating the effect of a heat current on magnetization dynamics in two different typologies of systems and materials: magnetic insulators and metallic nanostructures. In the first case we conduct studies on yttrium iron garnet (YIG) samples subjected to a temperature gradient. The irreversible thermodynamics of a continuous medium with magnetic dipoles predicts that a thermal gradient across a YIG slab, in the presence of magnetization waves, produces a magnetic field that is the magnetic analog of the well known Seebeck effect. This thermally induced field can influence the time evolution of the magnetization, in such a way that it is possible to modulate the relaxation of the precession when applying a heat current. We found evidence for such a magnetic Seebeck effect (MSE) by conducting transmission measurements in a thin slab of YIG subjected to an in-plane temperature gradient. We showed how the MSE can modulate the magnetic damping depending on the direction of the propagating magnetostatic modes with respect to the orientation of the temperature gradient. In the second part of the thesis we focus our investigation on metallic nanostructures subjected to a heat current. In a metal, the three-current model (current of entropy, of spin up and spin down electrons) predicts that a heat current induces a spin current which will then influence the magnetization dynamics like a charge-driven spin current would. Hence, we explore what has been called Thermal Spin Torque in electrodeposited Co / Cu / Co asymmetric spin valves placed in the middle of copper nanowires. These samples are fabricated by conventional electrodeposition technique in porous polycarbonate membranes using an original method that allows high frequency electrical measurements. We used a modulated laser to investigate the effect of a temperature gradient. We observed a heat-driven spin torque by measuring electrically the quasi-static magnetic response of a spin valve when subjected to the heat current, generated by two laser diodes heating the electrical contact at one end or the other of the nanowire. Analysing the variation in the resistance induced by a heat-driven spin torque, represented by peaks occurring in correspondence with the GMR transition, we found that a temperature difference of the order of 5 K is sufficient to produce sizeable torque in spin valves.

EPFL2015

Spin dependent thermoelectric effects in magnetic nanostructures

Santiago Serrano Guisan

The aim of this thesis work was to obtain a deeper understanding of spin-dependent transport by the study of mixed effects of heat and charge currents in magnetic nanostructures. Indeed, even thermodynamics establish a relationship between both currents, no previous works were performed in this sense up to now. In order to do this, an original experimental setup was developed which allows to carry out a novel transport measurement called the thermogalvanic voltage (TGV). It measures the AC voltage response of the sample driven by an oscillatory variation of the temperature induced by a chopped laser beam focused on the nanostructure under a direct current flow. The main advantage of this new experimental setup is that it can be used to perform three different transport measurements: resistance, thermoelectrical power and TGV measurements. Furthermore, the well-defined junctions of our magnetic nanowires, in addition to the lock-in detection of signals, allows to obtain a better quality of thermoelectrical measurements than those observed by other groups. Different kind of magnetic nanowires like ferromagnetic/non-magnetic multilayers and ferromagnetic homogeneous nanowires were studied by these three different transport measurements. All these nanowires were 6μm in length and 50nm in diameter. All TGV measurements reveal a linear dependence with currents. This behaviour can be interpreted in terms of adiabatic conductivity for homogeneous nanowires and in terms of Peltier effect for multilayer nanowires produced in ferromagnetic/non-magnetic interfaces. Furthermore, the magnetic field dependence of TGV (MTGV) cannot be accounted for by GMR and MTEP effects. Therefore, a novel spin-dependent mechanism is invoked in such measurements. A simple phenomenological model, ascribed in the framework of thermodynamics of irreversible processes, was developed to explain this effect. It allows to identify this phenomena to a novel spin-dependent variable: The asymmetry of spin-mixing mechanisms (ΔL). This variable will be related to the difference of transition rates between both spin channels (τ-1+- and τ-1-+) developed in ferromagnetic material due to electron-magnon interactions. A large magnetic field dependence of TGV signals were observed in magnetic granular systems. Moreover, MTGV shows a different dependence from GMR on magnetic field, grain size and temperature. This effect is identified as a spin-mixing mechanism produced by the electron spin precession about the magnetic grain moments.