Laser polishing of niobium for superconducting radio-frequency accelerator applications

Interior surfaces of niobium cavities used in superconducting radio frequency accelerators are now obtained by buffered chemical polish and/or electropolish. Laser polishing is a potential alternative, having advantages of speed, freedom from noxious chemistry and availability of in-process inspection. We studied the influence of the laser power density and laser beam raster rate on the surface topography. These two factors need to be combined carefully to smooth the surface without damage. Computational modeling was used to estimate the surface temperature and gain insight into the mechanism of laser polishing. Power spectral density analysis of surface topography measurements shows that laser polishing can produce smooth topography similar to that obtained by electropolish. This is a necessary first step toward introducing laser polishing as an alternative to the currently practiced chemical polishing

Simulation of nonlinear superconducting rf losses derived from characteristic topography of etched and electropolished niobium surfaces

A simplified numerical model has been developed to simulate nonlinear superconducting radiofrequency (SRF) losses on Nb surfaces. This study focuses exclusively on excessive surface resistance (R-s) losses due to the microscopic topographical magnetic field enhancements. When the enhanced local surface magnetic field exceeds the superconducting critical transition magnetic field H-c, small volumes of surface material may become normal conducting and increase the effective surface resistance without inducing a quench. We seek to build an improved quantitative characterization of this qualitative model. Using topographic data from typical buffered chemical polish (BCP)- and electropolish (EP)-treated fine grain niobium, we have estimated the resulting field-dependent losses and extrapolated this model to the implications for cavity performance. The model predictions correspond well to the characteristic BCP versus EP high field Q(0) performance differences for fine grain niobium. We describe the algorithm of the model, its limitations, and the effects of this nonlinear loss contribution on SRF cavity performance

The production of advanced fine-grained alumina by carbon nanotube addition

Alumina and alumina+1vol.% carbon nanotube (CNT) composites were fully densified by spark plasma sintering. Post-sintering heat treatments (1300–1500 ◦C) were performed to completely oxidize CNTs and then densify the remaining 1 vol.% to produce fine-grained ceramics. The grain size and Vickers hardness of the heat-treated composites were compared with the monolithic alumina sintered without CNT addition. Compared to the initial powder particle size of alumina (D50: 356±74 nm), minimal grain growth (∼450 nm) was observed for the fully dense heat-treated composites. A 25% improvement in Vickers hardness and >10 times finer average grain size were observed for alumina produced by the heat treatment (1300 ◦C) of alumina+1vol.% CNT composite, compared to alumina sintered without CNTs

Strength and hardness enhancement and slip behaviour of high-entropy carbide grains during micro-compression and nanoindentation

Bulk polycrystalline high-entropy carbides are a newly developed group of materials that increase the limited compositional space of ultra-high temperature ceramics, which can withstand extreme environments exceeding 2000°C in oxidizing atmospheres. Since the deformability of grains plays an important role in macromechanical performance, we studied the strength and slip behaviour of grains of a spark-plasma sintered (Hf-Ta-Zr-Nb)C high-entropy carbide in a specific orientation during micropillar compression. Additionally, the hardness of grains of different orientations was investigated by nanoindentation. For comparison, identical measurements were carried out on the monocarbides HfC and TaC. Four micropillars were fabricated by focused ion beam (FIB) in visibly pore free regions of large (Hf-Ta-Zr-Nb)C, HfC and TaC grains of a specific orientation (Φ~14° and φ2~45°) selected by electron backscatter diffraction (EBSD). This resulted in equal Schmid factors for both the and slip systems, which were reported to operate in HfC and TaC. It was revealed that (Hf-Ta-Zr-Nb)C had a significantly enhanced yield and failure strength compared to the corresponding base monocarbides, while maintaining a similar ductility to the least brittle monocarbide (TaC) during the operation of slip systems (Fig. 1). Nanoindentation investigations revealed a significant enhancement in hardness (~30%) of the high entropy (Hf-Ta-Zr-Nb)C material compared to that calculated according to the rule of mixtures from the base monocarbides (HfC, TaC, ZrC, NbC) and in comparison to the hardest monocarbide (HfC). Additionally, it was concluded that the much larger strength enhancement of micropillars compared to the average nanohardness of randomly oriented grains is attributed to the different slip systems. For (Hf-Ta-Zr-Nb)C, the operation of was identified in micropillar experiments, but the dominant slip system in nanoindentation is assumed to be the , possibly via the activation of partial dislocations, which is attributed to the different Schmid factors due to the different stress fields between nanoindentation and micropillar compression. Please click Additional Files below to see the full abstract

The effect of Nd substitution on the electrical properties of Bi3NbTiO9 Aurivillius phase ceramics

The effect of Nd substitution on the electrical properties of Bi3NbTiO9 Aurivillius phase ceramic

Topographic power spectral density study of the effect of surface treatment processes on niobium for superconducting radio frequency accelerator cavities

Microroughness is viewed as a critical issue for attaining optimum performance of superconducting radio frequency accelerator cavities. The principal surface smoothing methods are buffered chemical polish (BCP) and electropolish (EP). The resulting topography is characterized by atomic force microscopy (AFM). The power spectral density (PSD) of AFM data provides a more thorough description of the topography than a single-value roughness measurement. In this work, one dimensional average PSD functions derived from topography of BCP and EP with different controlled starting conditions and durations have been fitted with a combination of power law, K correlation, and shifted Gaussian models to extract characteristic parameters at different spatial harmonic scales. While the simplest characterizations of these data are not new, the systematic tracking of scale-specific roughness as a function of processing is new and offers feedback for tighter process prescriptions more knowledgably targeted at beneficial niobium topography for superconducting radio frequency applications