Fabrication of metal microfibers by melt-spinning

Metal microfibers have a wide range of industrial applications, e.g. as filters, fiber-reinforced composites, electrodes, catalysts, sensors, or magnetic shielding materials. In this project, we modified melt-spinning device and its experimental parameters to fabricate metal microfibers. It was shown for the first time that metal microfibers down to 5 μm could be fabricated using a melt spinning device. The size and circularity of formed fibers could be controlled by experimental parameters.e.g. slit distance to the wheel, chamber pressure, slit size, wheel speed. The mechanism of fiber formation relies on two main steps; i)thin film formation on the rotating wheel ii)spontaneous breaking of the film to smaller widths, dewetting the wheel. It was shown that this process is reproducible and could be used for different classes of materials. e.g. intermetallic alloys, conventional alloys, metal elements and amorphous alloys. The modification of the melt spinning device leads to higher quenching rates up to 108!C/s. The high quenching rate made it possible to make fully amorphous stainless steel fibers for the very first time. Heat-treatment of amorphous stainless steel leads to dual-phase microstructure (nanocrystals embedded together with a glassy phase) which was responsible for its ultra-high hardness value, 14GPa. This value is 7 times higher than the original stainless steel hardness. Thus, the technique opens new possibilities for working with conventional and amorphous alloys e.g. mechanically improved conventional alloy microfibers/ribbons, introducing new alloy microstructures

Ninth European Powder Diffraction Conference – Prague, September 2-5, 2004

Zeitschrift für Kristallographie. Supplement Volume 23 presents the complete Proceedings of all contributions to the IX European Powder Diffraction Conference in Prague 2004: Method Development and Application, Instrumental, Software Development, Materials Supplement Series of Zeitschrift für Kristallographie publishes Proceedings and Abstracts of international conferences on the interdisciplinary field of crystallography

Distinguishing the Role of Structure, Vibrational Dynamics, and Composition in Electron-Phonon Coupling of Nb(100) Surfaces

Niobium is the current material of choice for superconducting radiofrequency (SRF) cavities in particle accelerators. Nb found its ubiquitous use and extensive study in SRF cavities due to both its normal and superconducting state properties. Experimental and theoretical studies have documented and studied mechanisms in which even local hot spots from inhomogeneities, defects, and topographical variations can heat and quench entire SRF cavities. Thus, optimal preparation of Nb SRF cavity surfaces are required to prepare energetically efficient SRF cavities and keep costs of operation from being prohibitively high. In other words, the relationship between atomic-scale surface structure and the resulting superconducting properties at the surface is critical in improving and developing next generation SRF cavity materials. While well-studied, the formation and evolution of surface defects and compositional inhomogeneities remains a challenging part of SRF cavity treatment design and implementation. Such an understanding of the role of surface structure and chemical composition as well as their resulting effects on superconductivity at the surface remains elusive. This thesis seeks to make a start to this foundational understanding with in situ high temperature measurement of atomic scale surface structure, vibrational dynamics, and surface EPC on the metallic Nb(100) surface and its surface oxide reconstruction (3x1)-O/Nb(100). These measurements begin with the discovery of the (3x1)-O/Nb(100)’s high temperature stability and structural/ compositional persistence under SRF cavity preparation conditions. Then, a key driving force for the unusual stability of the (3x1)-O/Nb(100) was elucidated with inelastic HAS TOF measurements and ab initio density-functional theory (DFT) calculations, revealing abnormally strong Nb-O and Nb-Nb bonds that make up the characteristic ladder 10 structure. This means the ladder crests of the (3x1)-O/Nb(100) not only introduces new, relatively strong Nb-O interactions, but it significantly strengthens the Nb-Nb interactions. In this way the (3x1)-O/Nb(100) passivates and stabilizes the surface. These results demonstrate the significant role niobium oxides play in the optimization of growth strategies and coating procedures for next–generation SRF materials. Next the atomic-scale structure’s effect on the superconductivity was investigated using HAS’s sensitivity to EPC. HAS simultaneously measured the surface electron-phonon coupling (EPC, SEPC) constant (, ) and in situ high temperature atomic-scale surface structure of the unreconstructed, metallic Nb(100) surface as well as the (3x1)-O/Nb(100) oxide reconstruction. Ab initio DFT with local averaging agrees well with the HAS data. Furthermore, some variations in subsurface C and O and their effect on the SEPC are discussed. The Nb(100) surface is 0.50 ± 0.08 while that of the (3x1)-O/Nb(100) surface oxide reconstruction is is 0.20 ± 0.06. The measured for the Nb(100) surface is ~1/2 the reported bulk Nb values. The significance of Nb(100)’s diminished EPC was elucidated by estimating relevant superconducting properties from the measured , surface Debye temperature, known material parameters, and well-established equations. These results indicate that the Nb(100) surface has decreased superconducting properties relative to the bulk. This study shows that these effects may be due also to the interface itself even without oxygen. Additionally, the λ S measured for the (3×1)-O surface reconstruction is further diminished from the metallic, unreconstructed Nb(100) value and the reported bulk Nb λ values. Furthermore, varying subsurface O has no significant effect on the λ S of the (3×1)-O reconstruction. While the metallic, unreconstructed Nb(100) surface is significantly affected by accumulated subsurface C and O, the (3×1)-O reconstruction stabilizes its λ S against the effects of subsurface O. These results contain the first measured for the metallic 11 Nb(100), (3x1)-O/Nb(100), and any Nb surface. These measurements begin a fundamental understanding of atomic-scale surface structure’s effect on EPC and superconductivity in Nb

Measurements and numerical models for the evaluation of performance in microwave ovens

Le informazioni elettromagnetiche e termiche sono aspetti molto importanti per analizzare i processi di riscaldamento in forni a microonde ad uso domestico. In questo lavoro vengono valutati l'efficienza del sistema e la distribuzione delle fonti di calore nel carico, utilizzando differenti forni a microonde. Per raggiungere questo obiettivo, sono stati sviluppati  simulazioni numeriche e diversi test sperimentali. In particolare, per quanto riguarda quest'ultima parte si confrontano due diversi metodi di misura: l'uso di telecamera a infrarossi e una serie di misure mediante sonde a termocoppia. Questo progetto anche interessante perché ci permette di validare uno strumento per simulazioni numeriche, COMSOL Multi-phisics, che viene spesso utilizzato per la progettazione di dispositivi elettromagnetici e per l'indagine sui problemi multi-fisici. Il primo capitolo è un background generale che introduce alle microonde. Nel secondo capitolo viene presentata una larga parte teorica circa i forni a microonde. Qui  vengono anche descritti i tre componenti principali che caratterizzano il processo fisico: il magnetron come sorgente di campo, la guida d'onda che porta le onde dall’alimentazione all'ultimo componente, la cavità risonante. Il terzo capitolo introduce alle proprietà dielettriche e termiche che caratterizzano il processo di riscaldamento del carico. Sono inoltre descritti i fenomeni che influenzano le prestazioni del processo di riscaldamento e l’uniformità delle fonti di calore. Il quarto capitolo riguarda la parte di misure sperimentali con diverse attrezzature di prova, confrontando i risultati ottenuti. Il quinto capitolo descrive il software numerico (COMSOL Multi-phisics), vengono presentati i risultati ottenuti e la validazione delle simulazioni. Il sesto e ultimo capitolo presenta il confronto tra le prove sperimentali e i modelli numerici. Qui vengono sviluppate alcune conclusioni e vengono presentate le prospettive per le indagini successive nel campo di studio delle microond

Elementary Scattering Theory

The opportunities for doing scattering experiments at synchrotron and neutron facilities have grown rapidly in recent years and are set to continue to do so into the foreseeable future. This text provides a basic understanding of how these techniques enable the structure and dynamics of materials to be studied at the atomic and molecular level. Although mathematics cannot be avoided in a theoretical discussion, the aim has been to write a book that most scientists will still find approachable. To this end, the first two chapters are devoted to providing a tutorial background in the mathematics and physics that are implicitly assumed in other texts. Thereafter, the philosophy has been one of keeping things as simple as possible

Technical Design Report for the PANDA Solenoid and Dipole Spectrometer Magnets

This document is the Technical Design Report covering the two large spectrometer magnets of the PANDA detector set-up. It shows the conceptual design of the magnets and their anticipated performance. It precedes the tender and procurement of the magnets and, hence, is subject to possible modifications arising during this process