No description available.

<p> Working within the framework of the linearized Boltzmann equation for the conduction electrons the existing theoretical treatments of ultrasonic attenuation in metals are extended to include realistic descriptions of the electronic structure and electron-lattice interaction. A variational solution of the Boltzmann equation which allows the inclusion of phonon drag effects is derived. An anisotropic scattering time solution is also presented. Both of these solutions are applied to calculation of the attenuation coefficient in pure metals and dilute alloys. </p> <p> The theory of the effects of electron-electron collisions on the ultrasonic attenuation in metals is also examined. </p> / Thesis / Doctor of Philosophy (PhD)

The impact phenomena of high velocity micron-size particles, although commonly considered and described as detrimental in numerous engineering applications, can be used in a beneficial way if properly understood and controlled. The Cold Gas Dynamic Spray (CGDS) process, known as a surface modification, repair and additive manufacturing process, relies on such high velocity impacts. In the process, solid particles are accelerated by a supersonic gas flow to velocities up to 1200 m/s and are simultaneously heated to temperatures lower than their melting point. When propelled under proper velocity and temperature, the particles can bond onto a target surface. This bonding is caused by the resulting interfacial deformation processes occurring at the contact interface. Hence, the process relies heavily on the gas/particle and particle/substrate interactions. Although numerous experimental and/or numerical studies have been performed to describe the phenomena occurring during particle flight and impact in the CGDS process, numerous phenomena remain poorly understood. First, the effect of substrate surface topographical condition on the particle deformation and ability to successfully adhere, i.e. atomically and/or mechanically, has not been thoroughly investigated such that its influence is not well understood. Another aspect of the process that is generating the largest gap between experimental and numerical studies in the field is the lack of particle in-flight temperature measurements. Obtaining such data has proven to be technically difficult. The challenges stem from the short particle flight time, low particle temperature and small particle size preventing the use of established thermal spray pyrometry equipment. Relatedly, lack of such measurements precludes a proper experimental study of the impact related phenomena at the particle/substrate interface. As a result, the effect of particle size dependent temperature on overall coating properties and atomic bonding relies currently on estimates. Finally, the effect of particle impact characteristics on interfacial phenomena, i.e. grain size and geometry, velocity/temperature, and oxide scale thickness, on adhesion and deformation upon single particle collision has also been scarcely studied for soft particle depositions on hard substrate. Hence, the current research work aims at studying fundamental aspects of particle/gas heat transfer and particle/substrate impact features in goals to improve the understanding of the CGDS process. Different surface preparation methods will be used to create various surface roughness and topographical features, to provide a clear understanding of the target surface state influence on coating formation and adhesion. Additionally, new equipment relying on novel technology, i.e. high-speed IR camera, will be utilized to obtain particle in-flight temperature readings with sequence recordings. Subsequently, the experimental particle in-flight temperature readings will be used to develop a computational fluid dynamics model in goals to validate currently used Nusselt number correlations and heat transfer equations. The particle size-dependent temperature effect on the particle’s elastic and plastic response to its impact with a targeted surface and its ability to successfully bond and form a coating will be studied experimentally. A thorough CFD numerical work, based on experimental findings, will be included to provide full impact characteristics (velocity, temperature, size and trajectory) of successfully deposited particles. Finally, the numerical results will be utilized in the ensuing study to correlate single particle deformation, adhesion and interfacial features to impact characteristics. A finite element model will be included to investigate the effect of particle size dependent temperature on single particle interfacial pressure, temperature and bonding ability.

No description available.

In the present study the results of investigations on polycrystalline MgB2 and on single crystals of YNi2B2C and HoNi2B2C are presented. In particular, measurementes of specific electrical resistance, thermal conductivity, thermoelectric power, and of the linear thermal expansion coefficient were performed. Moreover, the specific heat of polycristalline borocarbide samples was evaluated. From the measured data, the temperature dependencies of the Lorenz number and of the Grueneisen parameter can be determined, also the pressure dependence of the superconducting transition temperature using the Ehrenfest relation. At low temperatures a characteristic deviation of the resistivity from the Bloch-Grueneisen law in the normal state for all investigated substances was observed. A reentrant behaviour in resistivity and thermoelectric power occurs at the antiferromagnetic phase transition of HoNi2B2C. The thermal conductivity of MgB2 below 7 K is dominated by the scattering of phonons at grain boundaries. The absence of both, a maximum of thermal conductivity in the superconducting state, and the change of its slope at the superconducting transition temperature points to the validity of the two-band model that also describes the temperature dependence of specific heat. Measurements of thermoelectric power confirm the different normal-state character of the charge carriers of the investigated superconductors. Diffusion thermopower and phonon drag describe the measured data of all investigated compounds ov a wide range of temperature. The thermal expansion of HoNi2B2C below 10 K is dominated by the magnetic contribution. For all investigated substances the Grueneisen parameter features very large values in selected temperature ranges. In the case of MgB2, its temperature dependence is evidently connected with the properties of the relevant phonon mode. For the borocarbides, the electrical resistance depends very weakly on the crystallographic direction, but in contrast the thermal conductivity does in a quite strong manner. Despite of the antiferromagnetic phase transition in the case of HoNi2B2C, thermoelectric power and thermal expansion show minor anisotropy. / In der vorliegenden Arbeit werden Ergebnisse von Untersuchungen an polykristallinem MgB2 sowie an YNi2B2C- und HoNi2B2C-Einkristallen analysiert. Dafür erfolgten Messungen des spezifischen elektrischen Widerstands, der Wärmeleitfähigkeit, der Thermokraft und des linearen thermischen Ausdehnungskoeffizienten. Zudem wurde die spezifische Wärmekapazität polykristalliner Borkarbide bestimmt und aus den erhaltenen Daten die Temperaturabhängigkeit der Lorenz-Zahl und des Grüneisen-Parameters sowie mittels der Ehrenfest-Relation die Druckabhängigkeit der Sprungtemperatur ermittelt. Bei tiefen Temperaturen findet man im normalleitenden Zustand für alle betrachteten Substanzen ein charakteristisches Abweichen des Widerstands vom Bloch-Grüneisen-Gesetz. Bei HoNi2B2C tritt beim antiferromagnetischen Phasenübergang im Widerstand und in der Thermokraft ein reentrant-Verhalten auf. Die thermische Leitfähigkeit von MgB2 wird unterhalb von 7 K durch die Streuung der Phononen an Korngrenzen bestimmt. Das Fehlen eines Maximums in der Wärmeleitfähigkeit im supraleitenden Zustand und einer Anstiegsänderung bei der Sprungtemperatur liefert einen Hinweis auf die Gültigkeit des Zweibandmodells, mit welchem auch der Temperaturverlauf der Wärmekapazität erklärt werden kann. Messungen der Thermokraft bestätigen den unterschiedlichen Charakter der Ladungsträger im normalleitenden Zustand der untersuchten Supraleiter, wobei Elektronendiffusion und Phonon Drag die Messdaten aller betrachteten Verbindungen in weiten Temperaturbereichen beschreiben. Für HoNi2B2C wird die thermische Ausdehnung unterhalb von 10 K durch den Beitrag der magnetischen Ordnung bestimmt. Der Grüneisen-Parameter weist für alle untersuchten Substanzen in Teilbereichen sehr große Beträge auf. Sein Temperaturverlauf hängt bei MgB2 offenbar mit Eigenschaften der maßgeblichen Phononenmode zusammen. Für die Borkarbide ist die Richtungsabhängigkeit des elektrischen Widerstandes sehr schwach, in der Wärmeleitfähigkeit hingegen recht stark ausgeprägt. Abgesehen vom antiferromagnetischen Phasenübergang bei HoNi2B2C weisen Thermokraft und Ausdehnungskoeffizient eine geringe Anisotropie auf.

In the present study the results of investigations on polycrystalline MgB2 and on single crystals of YNi2B2C and HoNi2B2C are presented. In particular, measurementes of specific electrical resistance, thermal conductivity, thermoelectric power, and of the linear thermal expansion coefficient were performed. Moreover, the specific heat of polycristalline borocarbide samples was evaluated. From the measured data, the temperature dependencies of the Lorenz number and of the Grueneisen parameter can be determined, also the pressure dependence of the superconducting transition temperature using the Ehrenfest relation. At low temperatures a characteristic deviation of the resistivity from the Bloch-Grueneisen law in the normal state for all investigated substances was observed. A reentrant behaviour in resistivity and thermoelectric power occurs at the antiferromagnetic phase transition of HoNi2B2C. The thermal conductivity of MgB2 below 7 K is dominated by the scattering of phonons at grain boundaries. The absence of both, a maximum of thermal conductivity in the superconducting state, and the change of its slope at the superconducting transition temperature points to the validity of the two-band model that also describes the temperature dependence of specific heat. Measurements of thermoelectric power confirm the different normal-state character of the charge carriers of the investigated superconductors. Diffusion thermopower and phonon drag describe the measured data of all investigated compounds ov a wide range of temperature. The thermal expansion of HoNi2B2C below 10 K is dominated by the magnetic contribution. For all investigated substances the Grueneisen parameter features very large values in selected temperature ranges. In the case of MgB2, its temperature dependence is evidently connected with the properties of the relevant phonon mode. For the borocarbides, the electrical resistance depends very weakly on the crystallographic direction, but in contrast the thermal conductivity does in a quite strong manner. Despite of the antiferromagnetic phase transition in the case of HoNi2B2C, thermoelectric power and thermal expansion show minor anisotropy. / In der vorliegenden Arbeit werden Ergebnisse von Untersuchungen an polykristallinem MgB2 sowie an YNi2B2C- und HoNi2B2C-Einkristallen analysiert. Dafür erfolgten Messungen des spezifischen elektrischen Widerstands, der Wärmeleitfähigkeit, der Thermokraft und des linearen thermischen Ausdehnungskoeffizienten. Zudem wurde die spezifische Wärmekapazität polykristalliner Borkarbide bestimmt und aus den erhaltenen Daten die Temperaturabhängigkeit der Lorenz-Zahl und des Grüneisen-Parameters sowie mittels der Ehrenfest-Relation die Druckabhängigkeit der Sprungtemperatur ermittelt. Bei tiefen Temperaturen findet man im normalleitenden Zustand für alle betrachteten Substanzen ein charakteristisches Abweichen des Widerstands vom Bloch-Grüneisen-Gesetz. Bei HoNi2B2C tritt beim antiferromagnetischen Phasenübergang im Widerstand und in der Thermokraft ein reentrant-Verhalten auf. Die thermische Leitfähigkeit von MgB2 wird unterhalb von 7 K durch die Streuung der Phononen an Korngrenzen bestimmt. Das Fehlen eines Maximums in der Wärmeleitfähigkeit im supraleitenden Zustand und einer Anstiegsänderung bei der Sprungtemperatur liefert einen Hinweis auf die Gültigkeit des Zweibandmodells, mit welchem auch der Temperaturverlauf der Wärmekapazität erklärt werden kann. Messungen der Thermokraft bestätigen den unterschiedlichen Charakter der Ladungsträger im normalleitenden Zustand der untersuchten Supraleiter, wobei Elektronendiffusion und Phonon Drag die Messdaten aller betrachteten Verbindungen in weiten Temperaturbereichen beschreiben. Für HoNi2B2C wird die thermische Ausdehnung unterhalb von 10 K durch den Beitrag der magnetischen Ordnung bestimmt. Der Grüneisen-Parameter weist für alle untersuchten Substanzen in Teilbereichen sehr große Beträge auf. Sein Temperaturverlauf hängt bei MgB2 offenbar mit Eigenschaften der maßgeblichen Phononenmode zusammen. Für die Borkarbide ist die Richtungsabhängigkeit des elektrischen Widerstandes sehr schwach, in der Wärmeleitfähigkeit hingegen recht stark ausgeprägt. Abgesehen vom antiferromagnetischen Phasenübergang bei HoNi2B2C weisen Thermokraft und Ausdehnungskoeffizient eine geringe Anisotropie auf.