Search for: All records

${\mathrm{Nb}}_3\mathrm{Sn}$film coatings have the potential to drastically improve the accelerating performance of Nb superconducting radiofrequency (SRF) cavities in next-generation linear particle accelerators. Unfortunately, persistent ${\mathrm{Nb}}_3\mathrm{Sn}$stoichiometric material defects formed during fabrication limit the cryogenic operating temperature and accelerating gradient by nucleating magnetic vortices that lead to premature cavity quenching. The SRF community currently lacks a predictive model that can explain the impact of chemical and morphological properties of ${\mathrm{Nb}}_3\mathrm{Sn}$defects on vortex nucleation and maximum accelerating gradients. Both experimental and theoretical studies of the material and superconducting properties of the first 100 nm of ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces are complicated by significant variations in the volume distribution and topography of stoichiometric defects. This work contains a coordinated experimental study with supporting simulations to identify how the observed chemical composition and morphology of certain Sn-rich and Sn-deficient surface defects can impact the SRF performance. ${\mathrm{Nb}}_3\mathrm{Sn}$films were prepared with varying degrees of stoichiometric defects, and the film surface morphologies were characterized. Both Sn-rich and Sn-deficient regions were identified in these samples. For Sn-rich defects, we focus on elemental Sn islands that are partially embedded into the ${\mathrm{Nb}}_3\mathrm{Sn}$film. Using finite element simulations of the time-dependent Ginzburg-Landau equations, we estimate vortex nucleation field thresholds at Sn islands of varying size, geometry, and embedment. We find that these islands can lead to significant SRF performance degradation that could not have been predicted from the ensemble stoichiometry alone. For Sn-deficient ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces, we experimentally identify a periodic nanoscale surface corrugation that likely forms because of extensive Sn loss from the surface. Simulation results show that the surface corrugations contribute to the already substantial drop in the vortex nucleation field of Sn-deficient ${\mathrm{Nb}}_3\mathrm{Sn}$surfaces. This work provides a systematic approach for future studies to further detail the relationship between experimental ${\mathrm{Nb}}_3\mathrm{Sn}$growth conditions, stoichiometric defects, geometry, and vortex nucleation. These findings have technical implications that will help guide improvements to ${\mathrm{Nb}}_3\mathrm{Sn}$fabrication procedures. Our outlined experiment-informed theoretical methods can assist future studies in making additional key insights about ${\mathrm{Nb}}_3\mathrm{Sn}$stoichiometric defects that will help build the next generation of SRF cavities and support related superconducting materials development efforts. Published by the American Physical Society2024 

Formic acid is unique among liquid organic hydrogen carriers (LOHCs), because its dehydrogenation is highly entropically driven. This enables the evolution of high-pressure hydrogen at mild temperatures that is difficult to achieve with other LOHCs, conceptually by releasing the “spring” of energy stored entropically in the liquid carrier. Applications calling for hydrogen-on-demand, such as vehicle filling, require pressurized H 2 . Hydrogen compression dominates the cost for such applications, yet there are very few reports of selective, catalytic dehydrogenation of formic acid at elevated pressure. Herein, we show that homogenous catalysts with various ligand frameworks, including Noyori-type tridentate (PNP, SNS, SNP, SNPO), bidentate chelates (pyridyl)NHC, (pyridyl)phosphine, (pyridyl)sulfonamide, and their metallic precursors, are suitable catalysts for the dehydrogenation of neat formic acid under self-pressurizing conditions. Quite surprisingly, we discovered that their structural differences can be related to performance differences in their respective structural families, with some tolerant or intolerant of pressure and others that are significantly advantaged by pressurized conditions. We further find important roles for H 2 and CO in catalyst activation and speciation. In fact, for certain systems, CO behaves as a healing reagent when trapped in a pressurizing reactor system, enabling extended life from systems that would be otherwise deactivated. 

The proliferation of increasingly useful reactions for hydrogen transfer in organic synthesis has included the introduction of many new homogeneous catalysts into the organic synthesis lexicon. Unlike the proliferation of palladium-based cross-coupling reactions in which the mechanism is generally conserved, we are learning that these emerging hydrogen transfer catalysts have a rich diversity of mechanisms for catalyst activation, speciation, C–H bond cleavage and formation, and ultimately deactivation. We find that an underappreciated commonality in the catalytic activation for some of these systems is the generation of a (carbonyl)metal group, which dominates the downstream speciation of the catalyst system. In this mini-review we highlight a few well-documented cases of this phenomenon as food for thought for those who are designing new catalytic systems to introduce into this dynamic and impactful area.