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abstract: Niobium is the primary material for fabricating superconducting radio-frequency (SRF) cavities. However, presence of impurities and defects degrade the superconducting behavior of niobium twofold, first by nucleating non-superconducting phases and second by increasing the residual surface resistance of cavities. In particular, niobium absorbs hydrogen during cavity fabrication and promotes precipitation of non-superconducting niobium hydride phases. Additionally, magnetic flux trapping at defects leads to a normal conducting (non-superconducting) core which increases surface resistance and negatively affects niobium performance for superconducting applications. However, undelaying mechanisms related to hydride formation and dissolution along with defect interaction with magnetic fields is still unclear. Therefore, this dissertation aims to investigate the role of defects and impurities on functional properties of niobium for SRF cavities using first-principles methods. Here, density functional theory calculations revealed that nitrogen addition suppressed hydrogen absorption interstitially and at grain boundaries, and it also decreased the energetic stability of niobium hydride precipitates present in niobium. Further, hydrogen segregation at the screw dislocation was observed to transform the dislocation core structure and increase the barrier for screw dislocation motion. Valence charge transfer calculations displayed a strong tendency of nitrogen to accumulate charge around itself, thereby decreasing the strength of covalent bonds between niobium and hydrogen leading to a very unstable state for interstitial hydrogen and hydrides. Thus, presence of nitrogen during processing plays a critical role in controlling hydride precipitation and subsequent SRF properties. First-principles methods were further implemented to gain a theoretical perspective about the experimental observations that lattice defects are effective at trapping magnetic flux in high-purity superconducting niobium. Full-potential linear augmented plane-wave methods were used to analyze the effects of magnetic field on the superconducting state surrounding these defects. A considerable amount of trapped flux was obtained at the dislocation core and grain boundaries which can be attributed to significantly different electronic structure of defects as compared to bulk niobium. Electron redistribution at defects enhances non-paramagnetic effects that perturb superconductivity, resulting in local conditions suitable for flux trapping. Therefore, controlling accumulation or depletion of charge at the defects could mitigate these tendencies and aid in improving superconductive behavior of niobium. / Dissertation/Thesis / Doctoral Dissertation Materials Science and Engineering 2019

La réduction du cout de construction et d’exploitation des futurs accélérateurs d particules, a grande et petite échelles, dépend du développement de nouveaux matériaux pour les surfaces actives des structures supraconductrices en radiofréquence (SRF). Les propriétés SRF sont essentiellement un phénomène de surface vu que la profondeur de pénétration (profondeur de pénétration de London, λ) des micro-ondes (RF) est typiquement de l’ordre de 20 à 400 nm en fonction du matériau. Lorsque les procédés de préparation de surface sont optimises, la limite fondamentale du champ RF que les surfaces SRF peuvent supporter est le champ RF maximum, Hc₁, au-delà duquel le flux magnétique commence à pénétrer la surface du supraconducteur. Le matériau le plus utilise pour des applications SRF est le niobium (Nb) massif, avec un champ Hc₁ de l’ordre de 170 mT, qui permet d’atteindre un champ accélérateur de moins de 50 MV/m. Les meilleures perspectives d’amélioration des performances des cavités SRF sont liées à des matériaux et méthodes de production produisant la surface SRF critique de façon contrôlée. Dans cette optique, deux avenues sont explorées pour utiliser des couches minces pour augmenter les performances des structures SRF au-delà du Nb massif, en monocouche ou en structures multicouches Supraconducteur-Isolant-Supraconducteur (SIS) : La première approche est d’utiliser une couche de Nb déposée sur du cuivre (Nb/Cu) à la place du Nb massif. La technologie Nb/Cu a démontré, au cours des années, être une alternative viable pour les cavités SRF. Toutefois, les techniques de dépôt communément utilisées, principalement la pulvérisation magnétron, n’ont jusqu’à présent pas permis de produire des surfaces SRF adaptées aux performances requises. Le récent développement de techniques de dépôt par condensation énergétiques, produisant des flux d’ions énergétiques de façon contrôlée (telles que des sources d’ions ECR sous ultravide) ouvrent la voie au développement de films SRF de grand qualité. La corrélation entre les conditions de croissance, l’énergie des ions incidents, la structure et les performances RF des films produits est étudiée. Des films Nb avec des propriétés proches du Nb massif sont ainsi produits. La deuxième approche est basée sur un concept qui propose qu’une structure multicouche SIS déposée sur une surface de Nb peut atteindre des performances supérieures à celles du Nb massif. Bien que les matériaux supraconducteurs à haute Tc aient un champ Hc₁ inférieur à celui du Nb, des couches minces de tels matériaux d’une épaisseur (d) inférieure à la profondeur de pénétration voient une augmentation de leur champ parallèle Hc₁ résultant au retardement de la pénétration du flux magnétique. Cette surcouche peut ainsi permettre l’écrantage magnétique de la surface de Nb qui est donc maintenue dans l’état de Meissner à des champs RF bien plus importants que pour le Nb massif. La croissance et performance de structures multicouches SIS basées sur des films de NbTiN, pour le supraconducteur, et de l’AlN, pour le diélectrique, sont étudiées. Les résultats de cette étude montrent la faisabilité de cette approche et le potentiel qui en découle pour l’amélioration des performances SRF au-delà du Nb massif. / The minimization of cost and energy consumption of future particle accelerators, both large and small, depends upon the development of new materials for the active surfaces of superconducting RF (SRF) accelerating structures. SRF properties are inherently a surface phenomenon as the RF only penetrates the London penetration depth λ, typically between 20 and 400 nm depending on the material. When other technological processes are optimized, the fundamental limit to the maximum supportable RF field amplitude is understood to be the field at which the magnetic flux first penetrates into the surface, Hc₁. Niobium, the material most exploited for SRF accelerator applications, has Hc₁~170 mT, which yields a maximum accelerating gradient of less than 50 MV/m. The greatest potential for dramatic new performance capabilities lies with methods and materials which deliberately produce the sub-micron-thick critical surface layer in a controlled way. In this context, two avenues are pursued for the use of SRF thin films as single layer superconductor or multilayer Superconductor-Insulator-Superconductor structures: Niobium on copper (Nb/Cu) technology for superconducting cavities has proven over the years to be a viable alternative to bulk niobium. However the deposition techniques used for cavities, mainly magnetron sputtering, have not yielded, so far, SRF surfaces suitable for high field performance. High quality films can be grown using methods of energetic condensation, such as Electron Cyclotron Resonance (ECR) Nb ion source in UHV which produce higher flux of ions with controllable incident angle and kinetic energy. The relationship between growth conditions, film microstructure and RF performance is studied. Nb films with unprecedented “bulk-like” properties are produced. The second approach is based on the proposition that a Superconductor/Insulator/Superconductor (S-I-S) multilayer film structure deposited on an Nb surface can achieve performance in excess of that of bulk Nb. Although, many higher-Tc superconducting compounds have Hc₁ lower than niobium, thin films of such compounds with a thickness (d) less than the penetration depth can exhibit an increase of the parallel Hc₁ thus delaying vortex entry. This overlayer provides magnetic screening of the underlying Nb which can then remain in the Meissner state at fields much higher than in bulk Nb. A proof of concept is developed based on NbTiN and AlN thin films. The growth of NbTiN and AlN films is studied and NbTiN-based multilayer structures deposited on Nb surfaces are characterized. The results from this work provide insight for the pursuit of major reductions in both capital and operating costs associated with future particle accelerators across the spectrum from low footprint compact machines to energy frontier facilities.