Physico-chemical material delevopment for the manufacturing of thermoelectric generators

Der thermoelektrische Effekt kann zur direkten Umwandlung von thermischer in elektrischeEnergie genutzt werden. Aufgrund der großen Menge Abwäarme in vielen Industriebereichen ist die Anwendung dieses Effekts hier vielversprechend, um die Gesamte Effizienz der Energiewandlung zu erhöhen. Diese Arbeit konzentriert sich auf die zwei Hauptforschungsfelder: Die Erforschung und Verbesserung thermoelektrischer Materialien und die Produktionstechnologie von thermoelektrischen Generatoren (TEGs). Bislang wurden thermoelektrische Materialien vor allem hinsichtlich ihrer resultierenden Effizienz bei der Energieumwandlung bewertet. In dieser Arbeit wird hingegen das Konzept, sich stattdessen auf den Leistungsfaktor zu fokussieren, weiter untersucht. Ein Review zu thermoelektrischen Materialien für Hochtemperaturanwendungen beschreibt dies und vergleicht verschiedene im Fokus stehende Materialklassen sowohl hinsichtlich der Effizienz als auch des Leistungsfaktors. Als Ergebnis dieser Diskussion werden verschiedene Konzepte zur Optimierung von Materialien untersucht: Zur Optimierung der Effizienz wurde ein Hybridmaterial entwickelt, dass verschiedene Klassen von Materialien kombiniert, um synergetische Effekte zu erzielen. Das entwickelte Hybridmaterial zeigte aufgrund der geschaffenen Heteromaterial-Grenzflächen eine außergewöhnlich niedrige thermische Leitfähigkeit. Dadurch eignet sich dieses Konzept besonders wenn eine hohe Effizienz erreicht werden soll. Für eine Optimierung der elektrischen Leistung wurden Cu-Ni-Legierungen als vielversprechende Materialien untersucht. Die Cu-Ni Legierungen wurden aus Nanopartikeln hergestellt und mit schweren Elementen legiert, wodurch sowohl der Leistungsfaktor als auch die Effizienz bei geringen Dotierungsmengen gesteigert werden konnte. Auch bei der Herstellung von TEGs besteht ein Optimierungspotenzial, da die Prdouktionskosten einen großen Anteil an den Gesamtkosten ausmachen. Hierfür wurde eine Fertigungsroute entwickelt, die schnelle und skalierbare Produktionstechnologie mit laserbasierten Prozessen kombiniert. Die Schichten wurden durch Sprühbeschichtung hergestellt und das Design anschließend durch den Einsatz eines CO2-Lasers strukturiert. Der entworfene TEG-Prototyp erreicht eine maximale elektrische Leistung von 1,6 µW cm-2. Die entwickelte Herstellungsroute erlaubt eine präzise Kontrolle über die Schichtdicke und die exakte Geometrie. Schließlich wurde das Konzept laserinduzierter Prozesse für thermoelektrische Anwendung, z.B. zum Sintern, anhand einer Laserbehandlung einer gesprühten CCO Schicht untersucht. Durch den Einsatz eines CO2-Lasers konnte hierbei eine beginnende Versinterung der CCO-Schicht beobachtet werden. Die Kombination aus Laser- und thermischer Behandlung zeigte eine Erhöhung der elektrischen Leitfähigkeit, was das Potenzial dieses Konzept in der Herstellung von thermoelektrischen Materialien und Generatoren integriert zeigt

Malignant infarction in cats following prolonged middle cerebral artery occlusion : volumes of severe blood flow reduction predict fatal outcome

Severity and duration of cerebral blood flow (CBF) reduction are main determinants of injury in core and penumbra zones of focal brain ischemia. To study the putative role and predictive significance of the volume of these zones for induction of a malignant course due to edema formation in large hemispheric stroke, we examined reduction of CBF and oxygen metabolism (CMRO2) by sequential positron emission tomography (PET) in a transient ischemia model in cats that is susceptible to secondary deterioration after reperfusion.peer-reviewe

No secondary elevation of extracellular adenosine in malignant edema formation following transient MCA occlusion

Malignant edema is a relevant, serious complication in various clinical situations including large hemispheric stroke. To date, the roles of purine catabolites and amino acids in the course of malignant edema formation remain obscure. We examined the correlation between secondary perfusional disturbance and elevation of extracellular purine catabolites and amino acids in a transient focal ischemia model in cats that is prone to develop malignant edema and thereby secondary ischemia during reperfusionpeer-reviewe

Glutamate and purine catabolites in relation to free radical production during focal ischemia-reperfusion : an in vivo study in cats

The in-vivo interrelation between excitotoxicity and oxidative stress following cerebral ischemia in cats was investigated. To elucidate the role of this mechanism in cerebral ischemia, the study presented herein sought to investigate the spatial and temporal features of the free radical response to elevations of glutamate and purine catabolites in a reproducible model of in vivo focal ischemia/reperfusion. The time course of neurochemical and Reactive Oxygen Species (ROS) were simultaneously conducted in ischemic focus and perifocal region of the brain.peer-reviewe

Extracellular glutamate accumulates only in final, ischemic stage of progressive epidural mass lesion in cats

Epidural mass lesions may cause ischemia due to progressive intracranial hypertension. In order to 1) investigate the impact of intracranial pressure (ICP) on accumulation of neuroactive substances, and 2) test the significance of neurochemical monitoring for early prediction of fatal outcome, we gradually raised ICP in cats by inflation of an epidural balloon: We assessed extracellular substrate alterations in the contralateral cortex in relation to changes of ICP, cerebral perfusion pressure (CPP) and mean arterial blood pressure (MABP). In a complementary experiment, regional cerebral blood flow was assessed by sequential positron emission tomography (PET).peer-reviewe

In situ observations on deformation behavior and stretching-induced failure of fine pitch stretchable interconnect

Electronic devices capable of performing in extreme mechanical conditions such as stretching, bending, or twisting will improve biomedical and wearable systems. The required capabilities cannot be achieved with conventional building geometries, because of structural rigidity and lack of mechanical stretchability. In this article, a zigzag-patterned structure representing a stretchable interconnect is presented as a promising type of building block. In situ experimental observations on the deformed interconnect are correlated with numerical analysis, providing an understanding of the deformation and failure mechanisms. The experimental results demonstrate that the zigzag-patterned interconnect enables stretchability up to 60% without rupture. This stretchability is accommodated by in-plane rotation of arms and out-of-plane deformation of crests. Numerical analysis shows that the dominating failure cause is interfacial in-plane shear stress. The plastic strain concentration at the arms close to the crests, obtained by numerical simulation, agrees well with the failure location observed in the experiment

Towards a deep reinforcement learning integration into model-based systems engineering

The integration of Deep Reinforcement Learning (DRL) in Model-Based Systems Engineering (MBSE) is a promising approach that can lead to significant benefits for system designers and developers. DRL is a branch of machine learning where an agent learns to make decisions by interacting with an environment, receiving feedback in the form of rewards or punishments that indicate the quality of its actions, and adjusting its decision-making policy to maximize the cumulative reward over time. MBSE provides a structured approach to system design, which can help to clarify system requirements, identify potential issues, and improve the overall efficiency of the system development process. This model-based approach can be particularly useful for DRL, which requires a clear understanding of the system environment and objectives to develop the system’s behavior. We propose a method for integrating DRL into MBSE, where the desired system behavior is defined in a model-based representation using a modeling language to describe the relevant design components for DRL. The method's model framework is applied and evaluated to an example use case using SysML as the modeling language. This integration enables system designers to use DRL with the benefits and support of MBSE

High power factor vs. high zT-A review of thermoelectric materials for high-temperature application

Energy harvesting with thermoelectric materials has been investigated with increasing attention over recent decades. However, the vast number of various material classes makes it difficult to maintain an overview of the best candidates. Thus, we revitalize Ioffe plots as a useful tool for making the thermoelectric properties of a material obvious and easily comparable. These plots enable us to consider not only the efficiency of the material by the figure of merit zT but also the power factor and entropy conductivity as separate parameters. This is especially important for high-temperature applications, where a critical look at the impact of the power factor and thermal conductivity is mandatory. Thus, this review focuses on material classes for high-temperature applications and emphasizes the best candidates within the material classes of oxides, oxyselenides, Zintl phases, half-Heusler compounds, and SiGe alloys. An overall comparison between these material classes with respect to either a high efficiency or a high power output is discussed

Approach for Simultaneous Determination of Thickness and Sound Velocity in Layered Structures Based on Sound Field Simulations

For imaging in NDT or in medical diagnostics, the value of sound velocity is assumed a priori. Interfaces of hidden objects are imaged by the measured time of flight (ToF). The supposed locations and extensions of these objects are incorrect if the actual sound velocity differs from the assumed. For material characterization the thickness of a specimen is determined by mechanical measurements and the sound velocity is determined by ToF-measurements. For multi-layered structures the mechanical determination of the thickness of the different layers is impossible non-destructively. It is necessary to determine both quantities simultaneously to get information about the thickness and the material of the different layers. A variation method is introduced in [1] allowing the simultaneous determination of sound velocity and thickness of up to two layers by focusing with an annular array at a fixed position. It works by varying the focus positions and the assumed sound velocity, which is used to calculate the delay times for each control mode by means of FERMAT’s principle. The amplitude of the echo signals is determined as a function of the control mode. Because the sound field depends on the sound velocity of the medium and the control mode evaluating the amplitude of the echo signals yields additional information besides the time of flight. Alternatively, in [2], a fast and efficient method for a simultaneous determination of sound velocity and thickness of a two- layered structure has been presented. It analyses the different signal parts of an echo reflected from the examined interface. These signal parts correspond to different propagation paths. The difference in time of flight between the signal parts contains the information about thickness and sound velocity of the layer. These time differences are used as an input for an inverse geometric model. Although an accuracy of over 95% had been reached, increasing this accuracy fails, because in both cases the analysis of the signals only uses a geometric model neglecting the wave properties. A half-analytical method based on GREEN’s functions and point sources synthesis is used to calculate the sound field in the multi-layered structures. The echoes of several interfaces are calculated for each element of the used array. Using the same parameter of specimen and the array as in the experiments the evaluation of the simulated signal yields correct time differences based on the wave propagation. They allow assuming effective, corrected source points for the geometric model. With such an optimization of the geometric model an accuracy of 99% can be reached for simulated signals. Measurements are executed on two-layered structures consisting of a first layer of water and a second layer of steel, cupper or aluminum with a thickness of d = 6 mm, 8 mm, 10 mm and 12 mm. For the second layer a deviation for the combined determination of sound velocity and thickness between 3% and 5% is reached with the geometric model for both evaluation methods. With the corrected source point the accuracy can be improved

Scaling of Local Slopes, Conservation Laws and Anomalous Roughening in Surface Growth

We argue that symmetries and conservation laws greatly restrict the form of the terms entering the long wavelength description of growth models exhibiting anomalous roughening. This is exploited to show by dynamic renormalization group arguments that intrinsic anomalous roughening cannot occur in local growth models. However some conserved dynamics may display super-roughening if a given type of terms are present.Comment: To appear in Phys. Rev. Lett., 4 pages in RevTeX style, no fig