Directional Phonon Suppression Function as a Tool for the Identification of Ultralow Thermal Conductivity Materials

Boundary-engineering in nanostructures has the potential to dramatically impact the development of materials for high-efficiency conversion of thermal energy directly into electricity. In particular, nanostructuring of semiconductors can lead to strong suppression of heat transport with little degradation of electrical conductivity. Although this combination of material properties is promising for thermoelectric materials, it remains largely unexplored. In this work, we introduce a novel concept, the directional phonon suppression function, to unravel boundary-dominated heat transport in unprecedented detail. Using a combination of density functional theory and the Boltzmann transport equation, we compute this quantity for nanoporous silicon materials. We first compute the thermal conductivity for the case with aligned circular pores, confirming a significant thermal transport degradation with respect to the bulk. Then, by analyzing the information on the directionality of phonon suppression in this system, we identify a new structure of rectangular pores with the same porosity that enables a four-fold decrease in thermal transport with respect to the circular pores. Our results illustrate the utility of the directional phonon suppression function, enabling new avenues for systematic thermal conductivity minimization and potentially accelerating the engineering of next-generation thermoelectric devices

Anisotropic hardening in cold forging

The goals of metal forming process design have long exceeded the mere shaping of components. Changes of the component properties which are caused by forming, including residual stresses, damage and work-hardening have received increasing attention in the last years. If done right, the incorporation and control of property changes of cold forged components in terms of numerical process simulations could significantly improve the energy- and resource-efficiency of metal forming processes as well as the components’ service life and performance. To predict and exploit the property changes by means of numerical simulations, the exact incorporation of the workpiece material behavior is of utmost importance. Up to now, anisotropic hardening is rarely considered in the field of cold bulk metal forming making impossible a flawless prediction of a component’s properties and its performance. In the scope of this thesis, typical cold forging materials are characterized with regard to their anisotropic work-hardening behavior exhibited at large strains. Tension, torsion and upsetting of material specimens pre-strained by forward rod extrusion reveal the material’s work-hardening behavior under a variety of different strain paths. It was shown that all investigated materials exhibit an extensive Bauschinger effect, workhardening stagnation and permanent softening which, up to now, are rarely considered in cold forging simulations. All anisotropic hardening phenomena intensify drastically, with the pre-strain. The experimental data is utilized to select, modify, and fit constitutive models of increasing complexity with the goal to capture all relevant work-hardening phenomena exhibited in the course of strain path changes. A modified version of the Yoshida- Uemori multi-surface model is successfully implemented and applied to improve the prediction accuracy of cold forging simulations. Various hardening models were applied to the simulation of basic single-stage cold forging processes, revealing, that the flow stress and residual stresses as well as the ejector forces are strongly affected by strain path changes, which cannot be captured with the common assumption of isotropic workhardening. While the forming forces of single-stage cold forging processes are hardly affected by anisotropic hardening, despite the occurrence of intrinsic strain path changes, the forming forces in multi-stage forming operations are reduced significantly, if large regions of the workpiece experience a strain path reversal. Lastly, it was shown that heat-treatments subsequent to cold forging at temperatures between 300 °C and 600 °C lead to a decrease of the Bauschinger effect, whereas work-hardening stagnation and permanent softening decrease only at larger temperatures.Die Ziele bei der Gestaltung von Umformprozessen gehen längst über die reine Formgebung hinaus. Die Vorhersage von Produkteigenschaften wie Eigenspannungen, Schädigung und Kaltverfestigung, welche durch die Umformung verändert werden, hat in den letzten Jahren zunehmend an Bedeutung gewonnen. Eine aktive Beeinflussung und Ausnutzung der veränderten Bauteileigenschaften würde die Ressourcen- und Energie- Effizienz von Kaltumformprozessen sowie die Leistungsfähigkeit der erzeugten Produkte deutlich steigern. Um diese Änderungen der Eigenschaften mittels Simulationen vorherzusagen und auszunutzen ist die exakte Einbeziehung des Werkstoffverhaltens der Werkstücke während der Umformung von größter Bedeutung. Das anisotrope Verfestigungsverhalten wird im Bereich der Kaltmassivumformung aktuell nur selten berücksichtigt, wodurch eine Vorhersage der Bauteilleistungsfähigkeit nicht möglich ist. Im Rahmen dieser Arbeit werden typische Werkstoffe der Kaltmassivumformung hinsichtlich ihres anisotropen Verfestigungsverhaltens bei großen Umformgraden charakterisiert. Durch Zug-, Torsions- und Stauchversuchen an Werkstoffproben, die durch Voll-Vorwärts-Fließpressen umgeformt wurden, konnte das anisotrope Verfestigungsverhalten unter einer Vielzahl unterschiedlicher Dehnpfade charakterisiert werden. Alle untersuchten Werkstoffe zeigen dabei einen ausgeprägten Bauschingereffekt, Verfestigungsstagnierung und eine bleibende Entfestigung, welche in der Kaltmassivumformung bisher nicht berücksichtigt wurden. Sämtliche Effekte intensivieren sich drastisch mit der Vordehnung. Die experimentellen Daten werden verwendet, um konstitutive Modelle mit zunehmender Komplexität auszuwählen, zu modifizieren und anzupassen, mit dem Ziel, alle relevanten Verfestigungsphänomene zu erfassen. Das Mehrflächenmodell von Yoshida- Uemori wird genutzt, um die Vorhersagegenauigkeit von Kaltumformsimulationen zu steigern. Bei Verwendung des Verfestigungsmodells in Simulationen einstufiger Kaltumformprozesse wurde gezeigt, dass die Fließspannung, Eigenspannungen und Auswerferkräfte stark von einer Dehnpfadumkehr beeinflusst werden, welche durch konventionelle isotrope Verfestigungsmodelle nicht abgebildet werden können. Während die Prozesskräfte bei einstufigen Kaltumformverfahren, trotz intrinsischer Dehnpfadwechsel, kaum von anisotroper Verfestigung beeinflusst werden, führt eine Dehnpfadumkehr bei mehrstufigen Umformvorgängen zu einer deutlichen Verringerung der Umformkräfte. Weiterhin wurde gezeigt, dass eine Wärmebehandlung in Temperaturbereichen zwischen 300 °C und 600 °C zu einer Verringerung des Bauschingereffektes führt, während die Verfestigungsstagnation und bleibende Entfestigung erst bei höheren Temperaturen abnehmen

The Role of Macropinocytosis in Sonic Hedgehog-Induced Axon Growth and Guidance: A Dissertation

Axon pathfinding is an important process required for the establishment of proper neuronal connections during development. An increasing number of secreted and membrane-anchored molecules have been identified as axon guidance cues, which can act as positive or negative factors to increase or decrease the growth of axons and influence the direction of axonal growth. These axon guidance factors present in the extracellular environment interact with receptors present on the growth cone, a structure located at the tip of the axon which functions as the motor unit for the axon. Upon binding to their receptors on the growth cone, the guidance factors then elicit an intracellular signaling cascade within the axon that ultimately influences the direction of axon growth, often through a direct, non-transcriptional mechanism. In this dissertation, we show that Sonic hedgehog (Shh) acts as an axon guidance factor for chick retinal ganglion cell (RGC) axons in a concentration-dependent manner. At a low concentration, Shh functions as a positive factor that induces axon growth and attractive turning while, at a high concentration, Shh functions as a negative factor that induces axon retraction and repulsive axon turning. We further characterized the effects of Shh on macropinocytosis, a fluid-phase type of endocytosis, in the axons. A high concentration of Shh significantly increased macropinocytosis in the axons. Macropinocytosis resulted in the generation of large, dextran-positive, clathrinindependent vesicles in the axonal growth cones, prior to growth cone collapse, axon retraction and repulsive axon turning. These vesicles were found to require dynamic F-actin, nonmuscle myosin II and dynamin for their formation but were formed independently of PI3 kinase signaling. Interestingly, a low concentration of Shh had an opposite effect on macropinocytosis. A low concentration of Shh and soluble laminin decreased macropinocytosis and additionally increased the turnover of these vesicles within the axons, suggesting positive axon guidance factors can additionally regulate downstream processing or maturation of these vesicles. The effect of Shh on regulating the motility of macropinosomes within the axons was investigated. A low concentration of Shh appeared to increase the motility of these vesicles along axonal microtubules in a cAMPdependent manner. However, a high concentration of Shh did not appear to affect the motility of the macropinosomes, suggesting that it likely plays a more predominant role in the formation of these vesicles within the growth cone. When we began this work, a large body of research existed describing the effects of guidance factors on regulating the cytoskeleton during axon motility. However, the role of membrane trafficking events during axon growth and guidance were very poorly characterized. Since we began this project, an increasing number of reports have shown that endo- and exocytosis are important for axon growth and, here, we show that macropinocytosis induced by negative axon guidance factors plays a critical role in growth cone collapse, axon retraction and repulsive axon turning. Positive axon guidance factors also affect macropinocytosis within the axons and additionally regulate their maturation, suggesting that membrane trafficking events mediated by axon guidance factors are important for regulating axon growth and pathfinding

Ferroelectricity in ultra-thin perovskite films

We report studies of ferroelectricity in ultra-thin perovskite films with realistic electrodes. The results reveal stable ferroelectric states in thin films less than 10 \AA thick with polarization normal to the surface. Under short-circuit boundary conditions, the screening effect of realistic electrodes and the influence of real metal/oxide interfaces on thin film polarization are investigated. Our studies indicate that metallic screening from the electrodes is affected by the difference in work functions at oxide surfaces. We demonstrate this effect in ferroelectric PbTiO$_3$ and BaTiO$_3$ films.Comment: 4 pages in REVTEX4, 4 epsf figure

Temperature-dependent thermal conductivity in nanoporous materials studied by the Boltzmann Transport Equation

Nanostructured materials exhibit low thermal conductivity because of the additional scattering due to phonon-boundary interactions. As these interactions are highly sensitive to the mean free path (MFP) of a given phonon mode, MFP distributions in nanostructures can be dramatically distorted relative to bulk. Here we calculate the MFP distribution in periodic nanoporous Si for different temperatures, using the recently developed MFP-dependent Boltzmann Transport Equation. After analyzing the relative contribution of each phonon branch to thermal transport in nanoporous Si, we find that at room temperature optical phonons contribute 18 % to heat transport, compared to 5% in bulk Si. Interestingly, we observe a steady thermal conductivity in the nanoporous materials over a temperature range 200 K < T < 300 K, which we attribute to the ballistic transport of acoustic phonons with long intrinsic MFP. These results, which are also consistent with a recent experimental study, shed light on the origin of the reduction of thermal conductivity in nanostructured materials, and could contribute to multiscale heat transport engineering, in which the bulk material and geometry are optimized concurrently