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"Stone, Kevin"
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The best interests assessor practice handbook
The first textbook to consider the Best Interests Assessor (BIA) role in depth, offering practical guidance and exploring its particular challenges in the context of the Mental Capacity Act and Deprivation of Liberty Safeguards legal framework.
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Dynamics of pore formation during laser powder bed fusion additive manufacturing
Laser powder bed fusion additive manufacturing is an emerging 3D printing technique for the fabrication of advanced metal components. Widespread adoption of it and similar additive technologies is hampered by poor understanding of laser-metal interactions under such extreme thermal regimes. Here, we elucidate the mechanism of pore formation and liquid-solid interface dynamics during typical laser powder bed fusion conditions using in situ X-ray imaging and multi-physics simulations. Pores are revealed to form during changes in laser scan velocity due to the rapid formation then collapse of deep keyhole depressions in the surface which traps inert shielding gas in the solidifying metal. We develop a universal mitigation strategy which eliminates this pore formation process and improves the geometric quality of melt tracks. Our results provide insight into the physics of laser-metal interaction and demonstrate the potential for science-based approaches to improve confidence in components produced by laser powder bed fusion.
Laser-matter interactions during laser powder bed fusion additive manufacturing remain poorly understood. Here, the authors combine in situ X-ray imaging and finite element simulations to show how detrimental pores form under printing conditions and develop a strategy to suppress them.
Journal Article
Coupling between oxygen redox and cation migration explains unusual electrochemistry in lithium-rich layered oxides
Lithium-rich layered transition metal oxide positive electrodes offer access to anion redox at high potentials, thereby promising high energy densities for lithium-ion batteries. However, anion redox is also associated with several unfavorable electrochemical properties, such as open-circuit voltage hysteresis. Here we reveal that in Li
1.17–
x
Ni
0.21
Co
0.08
Mn
0.54
O
2
, these properties arise from a strong coupling between anion redox and cation migration. We combine various X-ray spectroscopic, microscopic, and structural probes to show that partially reversible transition metal migration decreases the potential of the bulk oxygen redox couple by > 1 V, leading to a reordering in the anionic and cationic redox potentials during cycling. First principles calculations show that this is due to the drastic change in the local oxygen coordination environments associated with the transition metal migration. We propose that this mechanism is involved in stabilizing the oxygen redox couple, which we observe spectroscopically to persist for 500 charge/discharge cycles.
Lithium ion battery electrodes employing anion redox exhibit high energy densities but suffer from poor cyclability. Here the authors reveal that the voltage of anion redox is strongly affected by structural changes that occur during battery cycling, explaining its unique electrochemical properties.
Journal Article
Metal–oxygen decoordination stabilizes anion redox in Li-rich oxides
Reversible high-voltage redox chemistry is an essential component of many electrochemical technologies, from (electro)catalysts to lithium-ion batteries. Oxygen-anion redox has garnered intense interest for such applications, particularly lithium-ion batteries, as it offers substantial redox capacity at more than 4 V versus Li/Li+ in a variety of oxide materials. However, oxidation of oxygen is almost universally correlated with irreversible local structural transformations, voltage hysteresis and voltage fade, which currently preclude its widespread use. By comprehensively studying the Li2−xIr1−ySnyO3 model system, which exhibits tunable oxidation state and structural evolution with y upon cycling, we reveal that this structure–redox coupling arises from the local stabilization of short approximately 1.8 Å metal–oxygen π bonds and approximately 1.4 Å O–O dimers during oxygen redox, which occurs in Li2−xIr1−ySnyO3 through ligand-to-metal charge transfer. Crucially, formation of these oxidized oxygen species necessitates the decoordination of oxygen to a single covalent bonding partner through formation of vacancies at neighbouring cation sites, driving cation disorder. These insights establish a point-defect explanation for why anion redox often occurs alongside local structural disordering and voltage hysteresis during cycling. Our findings offer an explanation for the unique electrochemical properties of lithium-rich layered oxides, with implications generally for the design of materials employing oxygen redox chemistry.Reversible high-voltage redox is a key component for electrochemical technologies from electrocatalysts to lithium-ion batteries. A point defect explanation for why anion redox occurs with local structural disordering and voltage hysteresis is proposed.
Journal Article
Understanding crystallization pathways leading to manganese oxide polymorph formation
Hydrothermal synthesis is challenging in metal oxide systems with diverse polymorphism, as reaction products are often sensitive to subtle variations in synthesis parameters. This sensitivity is rooted in the non-equilibrium nature of low-temperature crystallization, where competition between different metastable phases can lead to complex multistage crystallization pathways. Here, we propose an ab initio framework to predict how particle size and solution composition influence polymorph stability during nucleation and growth. We validate this framework using in situ X-ray scattering, by monitoring how the hydrothermal synthesis of MnO
2
proceeds through different crystallization pathways under varying solution potassium ion concentrations ([K
+
] = 0, 0.2, and 0.33 M). We find that our computed size-dependent phase diagrams qualitatively capture which metastable polymorphs appear, the order of their appearance, and their relative lifetimes. Our combined computational and experimental approach offers a rational and systematic paradigm for the aqueous synthesis of target metal oxides.
Minor variations in synthesis conditions can redirect crystallization pathways through different nonequilibrium intermediates. Here, the authors present a theoretical framework to predict which polymorphs appear during MnO
2
precipitation, which is validated by in situ X-ray scattering of reaction progression.
Journal Article
Transformation from crystalline precursor to perovskite in PbCl2-derived MAPbI3
Understanding the formation chemistry of metal halide perovskites is key to optimizing processing conditions and realizing enhanced optoelectronic properties. Here, we reveal the structure of the crystalline precursor in the formation of methylammonium lead iodide (MAPbI
3
) from the single-step deposition of lead chloride and three equivalents of methylammonium iodide (PbCl
2
+ 3MAI) (MA = CH
3
NH
3
). The as-spun film consists of crystalline MA
2
PbI
3
Cl, which is composed of one-dimensional chains of lead halide octahedra, coexisting with disordered MACl. We show that the transformation of precursor into perovskite is not favored in the presence of MACl, and thus the gradual evaporation of MACl acts as a self-regulating mechanism to slow the conversion. We propose the stable precursor phase enables dense film coverage and the slow transformation may lead to improved crystal quality. This enhanced chemical understanding is paramount for the rational control of film deposition and the fabrication of superior optoelectronic devices.
The existence of a crystalline precursor is key to perovskite film formation, but the precise chemistry of the precursor and its transformation into perovskite are poorly understood. Here, the authors identify the crystal structure and conversion chemistry of the precursor for PbCl
2
-derived methylammonium lead iodide perovskites.
Journal Article
Persistent and partially mobile oxygen vacancies in Li-rich layered oxides
Increasing the energy density of layered oxide battery electrodes is challenging as accessing high states of delithiation often triggers voltage degradation and oxygen release. Here we utilize transmission-based X-ray absorption spectromicroscopy and ptychography on mechanically cross-sectioned Li
1.18–
x
Ni
0.21
Mn
0.53
Co
0.08
O
2–
δ
electrodes to quantitatively profile the oxygen deficiency over cycling at the nanoscale. The oxygen deficiency penetrates into the bulk of individual primary particles (~200 nm) and is well-described by oxygen vacancy diffusion. Using an array of characterization techniques, we demonstrate that, surprisingly, bulk oxygen vacancies that persist within the native layered phase are indeed responsible for the observed spectroscopic changes. We additionally show that the arrangement of primary particles within secondary particles (~5 μm) causes considerable heterogeneity in the extent of oxygen release between primary particles. Our work merges an ensemble of length-spanning characterization methods and informs promising approaches to mitigate the deleterious effects of oxygen release in lithium-ion battery electrodes.
Oxygen release in Li-rich layered oxides is of both fundamental and practical interest in batteries, but a varied mechanistic understanding exists. Here the authors evaluate the extent of oxygen release over extended cycles and present a comprehensive picture of the phenomenon that unifies the current explanations.
Journal Article
Highly active oxygen evolution integrated with efficient CO₂ to CO electroreduction
Electrochemical reduction of CO₂ to useful chemicals has been actively pursued for closing the carbon cycle and preventing further deterioration of the environment/climate. Since CO₂ reduction reaction (CO₂RR) at a cathode is always paired with the oxygen evolution reaction (OER) at an anode, the overall efficiency of electrical energy to chemical fuel conversion must consider the large energy barrier and sluggish kinetics of OER, especially in widely used electrolytes, such as the pH-neutral CO₂-saturated 0.5 M KHCO₃. OER in such electrolytes mostly relies on noble metal (Ir- and Ru-based) electrocatalysts in the anode. Here, we discover that by anodizing a metallic Ni–Fe composite foam under a harsh condition (in a low-concentration 0.1 M KHCO₃ solution at 85 °C under a high-current ∼250 mA/cm²), OER on the NiFe foam is accompanied by anodic etching, and the surface layer evolves into a nickel–iron hydroxide carbonate (NiFe-HC) material composed of porous, poorly crystalline flakes of flower-like NiFe layer-double hydroxide (LDH) intercalated with carbonate anions. The resulting NiFe-HC electrode in CO₂-saturated 0.5 M KHCO₃ exhibited OER activity superior to IrO₂, with an overpotential of 450 and 590 mV to reach 10 and 250 mA/cm², respectively, and high stability for >120 h without decay. We paired NiFe-HC with a CO₂RR catalyst of cobalt phthalocyanine/carbon nanotube (CoPc/CNT) in a CO₂ electrolyzer, achieving selective cathodic conversion of CO₂ to CO with >97% Faradaic efficiency and simultaneous anodic water oxidation to O₂. The device showed a low cell voltage of 2.13 V and high electricity-to-chemical fuel efficiency of 59% at a current density of 10 mA/cm².
Journal Article
Measurement and Modeling of Short and Medium Range Order in Amorphous Ta2O5 Thin Films
Amorphous films and coatings are rapidly growing in importance. Yet, there is a dearth of high-quality structural data on sub-micron films. Not understanding how these materials assemble at atomic scale limits fundamental insights needed to improve their performance. Here, we use grazing-incidence x-ray total scattering measurements to examine the atomic structure of the top 50–100 nm of Ta
2
O
5
films; mirror coatings that show high promise to significantly improve the sensitivity of the next generation of gravitational-wave detectors. Our measurements show noticeable changes well into medium range, not only between crystalline and amorphous, but also between as-deposited, annealed and doped amorphous films. It is a further challenge to quickly translate the structural information into insights into mechanisms of packing and disorder. Here, we illustrate a modeling approach that allows translation of observed structural features to a physically intuitive packing of a primary structural unit based on a kinked Ta-O-Ta backbone. Our modeling illustrates how Ta-O-Ta units link to form longer 1D chains and even 2D ribbons and how doping and annealing influences formation of 2D order. We also find that all the amorphousTa
2
O
5
films studied in here are not just poorly crystalline but appear to lack true 3D order.
Journal Article