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The origin of enhanced conductivity and structure change in defective LiTiO: a study combining theoretical and experimental perspectives
No full textThe spinel LiTiO (LTO) has emerged as a promising anode material for the next generation of all-solid-state Li-ion batteries (ASSB), primarily due to its characteristic “zero strain” charge/discharge behavior and exceptional cycling stability, which significantly prolongs battery lifespan. Pristine LTO, however, is hindered by poor ionic and electronic conductivity. By employing tailored sintering protocols that create oxygen vacancies, a high-performing, blue LTO material is achieved. It has been proposed that the increased electronic conductivity could stem from vacancy-induced polarons. Yet, detailed insights into polaron stability, distribution, and dynamics within both the LTO bulk and surface have remained elusive due to limited information on structural changes. Utilizing Positron Annihilation Lifetime Spectroscopy (PALS) and Coincidence Doppler Broadening Spectroscopy (CDBS), in conjunction with Two Component Density Functional Theory (TCDFT) with the on-site Hubbard U correction, enables us to probe the depth profile of defect species introduced by sintering in a reductive environment. Our research provides direct evidence of oxygen vacancy formation within the subsurface region, an inference drawn from the observation of Ti3+. Our investigation into Li16d vacancy formation within the bulk region uncovers the interactions between mobile species, namely Li-ions and polarons. Furthermore, we delve into the polaron stability on the LTO surface, offering an explanation for the superior performance of the (100) facet exposed LTO nanoparticle, as compared to its (111) exposed counterpart
Prospects for forward emitted positronium from nanoporous membranes at AEgIS
No full textAntihydrogen formation at AEgIS at CERN leverages charge exchange between Rydberg positronium (Ps*) and antiprotons, with cross-sections scaling with the Ps principal quantum number n4 and inversely with relative velocity v−2. However, the motional Stark effect and velocity mismatch between Ps and antiprotons impose stringent constraints, limiting efficiency. Advances in transmission positronium converters mitigate self-ionization losses and improve velocity alignment, promising a significant boost in antihydrogen yield. This work evaluates formation cross-sections, Ps velocity profiles, and the integration of advanced transmission Ps converters for precision gravitational studies
Quantum criticality and dimensional reduction in the frustrated sawtooth chain compound atacamite
No full textWe present evidence for the presence of a field-induced quantum critical point (QCP) at 21.9(1) T (H || c axis) in the frustrated quantum magnet atacamite CuCl(OH), a mineral which we have recently identified as a material realization of sawtooth chains [J ~ 336 K (basal-basal), J' ~ 102 K (basal-apical)]. Residual interchain couplings lead to long-range antiferromagnetic order in atacamite below = 8.9(1) K. Through extensive high-field studies of the heat capacity, we were able to map the entropy landscape of atacamite up to 35 T. It features highly distorted isentropes indicative of quantum criticality in the material. By further combining macroscopic and microscopic high-field measurement techniques, we provide evidence for the QCP at 21.9(1) T to separate field regions with and without long-range magnetic order, but far away from full polarization of the material. Underpinned by numerical results we propose this behavior to be relatedto an effective dimensional reduction of the material, where the chains decouple into independent subunits. We interpret this behavior as a special type of dimensional reduction caused by the interchain network
From structure to electrochemistry: the influence of transition metal ordering on Na /vacancy orderings in P2-type NaMO cathode materials for sodium-ion batteries
No full textP2-type layered oxides are attractive cathode active materials for sodium-ion batteries, however, these materials typically suffer from detrimental Na+/vacancy orderings. In this work, we investigate the origin as well as the influence of the transition metal ratio on Na+/vacancy orderings in P2-type cathode materials. A combination of X-ray diffraction (XRD), neutron diffraction, advanced electrochemical methods, operando XRD and DFT calculations is applied to study Na+/vacancy orderings in P2-NaxNi1/3Mn2/3O2 and P2-NaxMn3/4Ni1/4O2. In P2-NaxNi1/3Mn2/3O2, a honeycomb Ni/Mn superstructure leads to charge ordering within the transition metal slab and pronounced Na+/vacancy orderings, causing distinct voltage jumps at specific sodium contents (x = 2/3, 1/2 and 1/3). For P2-Na0.60Mn3/4Ni1/4O2, the Ni/Mn superstructure is disrupted, resulting in more complex charge orderings within the transition metal slab, partially suppressed Na+/vacancy orderings and an overall smoother potential profile. Based on our findings, guidelines to suppress Na+/vacancy orderings in P2-type cathode materials for sodium-ion batteries are postulated and discussed with respect to electrochemical measurements of various transition metal compositions. These guidelines can serve to predict the tendency towards Na+/vacancy orderings for a given cathode composition or to design new cathode compositions for enhanced cycle life based on the absence of Na+/vacancy orderings
Impact of high heat flux loads on the residual stress in a tungsten-monoblock plasma-facing component
No full textThe stress state in a plasma-facing component (PFC) under high heat-flux (HHF) loads is the most important factor to determine the lifetime of the component. Stresses in a typical tungsten monoblock type PFC are produced by fabrication process and reactor operation where the component is subjected to HHF loads. In this study, both stress contributions were determined non-destructively by means of neutron diffraction technique. To this end, operational HHF loads were simulated using a high-power neutral hydrogen beam facility (GLADIS) to impose cyclic surface heating at 20 MW/m². A dedicated small-scale mock-up was fabricated applying hot radial pressing technique to join four tungsten blocks to a CuCrZr alloy cooling pipe via a soft 0.1 mm thick soft copper interlayer. This thin copper interlayer was used to simplify the residual stress profile for this preliminary test. The neutron diffraction measurements were carried out, at room temperature, at two different high-flux reactors: FRM II and the HANARO. Separate stress-relieved tungsten and CuCrZr samples were examined as reference state. The 3D stress tensor was determined in the same external block of the mock-up for both measurements, scanning it from the front face of the tungsten block towards the inner wall of the CuCrZr pipe. The results obtained at these two neutron sources are in good quantitative agreement. Comparing them with the stress profiles before thermal loading, it appears that after the HHF test at GLADIS compressive stresses up to -800 MPa developed in the tungsten block near the interlayer, while the CuCrZr pipe was scarcely affected, probably since the tungsten block accommodated most of the thermal impact. While stress measurements very close to the interlayer might have been affected by spatial resolution issues and error in the reference lattice parameter, the results of these experiments clearly indicate the significant impact of HHF loads on the stress profiles in the tungsten blocks
