A series of higher-order Ruddlesden-Popper phase materials - La
3
PrNi
3
O
10−
δ
, La
2
Pr
2
Ni
3
O
10−
δ
and LaPr
3
Ni
3
O
10−
δ
- were synthesised and investigated by neutron powder diffraction to ...understand the oxygen defect structure and propose possible pathways for oxygen transport in these materials. Further complimentary DFT calculations of the materials were performed to support the experimental analysis. All of the phases were hypostoichiometric and it was observed that the majority of the oxygen vacancies were confined to the perovskite layers, with a preference for equatorial oxygen sites. A particular preference for vacancies in O(1) and O(5) sites at high temperatures was observed from neutron diffraction measurements which were further complimented by DFT calculations wherein the vacancy formation energy was found to be lowest at the O(1) site. Also, a preference for a curved oxygen transport pathway around the NiO
6
octahedra was observed which agrees with the published literature for Ruddlesden-Popper phase materials. Lattice parameters for all three compositions showed a linear increase with increasing temperature, but the increase was greatest in the
c
parameter while the
b
parameter showed only a slight increase when compared to the
a
parameter. The thermal expansion coefficient was calculated for all compositions and was found to be in the range 13.0-13.4 × 10
−6
°C
−1
, which is compatible with the commonly used electrolyte materials for solid oxide fuel cells.
The plot of formation energy for oxygen vacancies at the O1O5 sites in the
Bmab
primitive cell structures of La
4
Ni
3
O
10
, La
2
Pr
2
Ni
3
O
10
and Pr
4
Ni
3
O
10
, showing oxygen vacancies prefer O1, O2 and O3 crystallographic sites in perovskite layers.
Activation losses at solid oxide fuel cell (SOFC) electrodes have been widely attributed to charge transfer at the electrode surface. The electrostatic nature of electrode–gas interactions allows us ...to study these phenomena by simulating an electric field across the electrode–gas interface, where we are able to describe the activation overpotential using density functional theory (DFT). The electrostatic responses to the electric field are used to approximate the behavior of an electrode under electrical bias and have found a correlation with experimental data for three different reduction reactions at mixed ionic–electronic conducting (MIEC) electrode surfaces (H2O and CO2 on CeO2; O2 on LaFeO3). In this work, we demonstrate the importance of decoupled ion–electron transfer and charged adsorbates on the performance of electrodes under nonequilibrium conditions. Finally, our findings on MIEC–gas interactions have potential implications in the fields of energy storage and catalysis.
The mass analysis of gas mixtures using quadrupole mass spectrometers is a well developed analytical technique which is routinely used with great success. The spectrometer's ionisation source is ...commonly one in which electrons from a heated filament are accelerated to around 70 eV before impacting with the gas samples. There are significant advantages, however, in being able to use other electron energies. Lower energies are particularly useful and are used in a so-called Threshold Ionisation mass spectrometry (TIMS) mode. The work described here applies the TIMS technique to a variety of problems which historically have proved to be difficult, and in some cases intractable, using the conventional mass resolved mode of mass spectrometry. The problems tackled include 4He/D2 and 3He/HD separation in nuclear fusion research, measuring trace levels of NH3 in air, and the separation of contributions from nitrogen and carbon monoxide to the QMS mass peak at m/z = 28. Finally, for measurements at higher gas pressures in the mass spectrometer, data for the production of long-lived metastable species are described.
•Threshold ionisation MS (TIMS) quantitatively complements conventional MS analysis.•TIMS can quantitatively resolve challenging MS problems (e.g. NH3/OH and D2/He).•Novel high source pressure QMS to identify metastables in plasma and gas analysis.
Experimental techniques that probe the local environment around O in paramagnetic Li-ion cathode materials are essential in order to understand the complex phase transformations and O redox processes ...that can occur during electrochemical delithiation. While Li NMR is a well-established technique for studying the local environment of Li ions in paramagnetic battery materials, the use of 17O NMR in the same materials has not yet been reported. In this work, we present a combined 17O NMR and hybrid density functional theory study of the local O environments in Li2MnO3, a model compound for layered Li-ion batteries. After a simple 17O enrichment procedure, we observed five resonances with large 17O shifts ascribed to the Fermi contact interaction with directly bonded Mn4+ ions. The five peaks were separated into two groups with shifts at 1600 to 1950 ppm and 2100 to 2450 ppm, which, with the aid of first-principles calculations, were assigned to the 17O shifts of environments similar to the 4i and 8j sites in pristine Li2MnO3, respectively. The multiple O environments in each region were ascribed to the presence of stacking faults within the Li2MnO3 structure. From the ratio of the intensities of the different 17O environments, the percentage of stacking faults was found to be ca. 10%. The methodology for studying 17O shifts in paramagnetic solids described in this work will be useful for studying the local environments of O in a range of technologically interesting transition metal oxides.
Background. Nutrition interventions targeted to individuals are unlikely to significantly shift US dietary patterns as a whole. Environmental and policy interventions are more promising for shifting ...these patterns. We review interventions that influenced the environment through food availability, access, pricing, or information at the point-of-purchase in worksites, universities, grocery stores, and restaurants.
Methods. Thirty-eight nutrition environmental intervention studies in adult populations, published between 1970 and June 2003, were reviewed and evaluated on quality of intervention design, methods, and description (e.g., sample size, randomization). No policy interventions that met inclusion criteria were found.
Results. Many interventions were not thoroughly evaluated or lacked important evaluation information. Direct comparison of studies across settings was not possible, but available data suggest that worksite and university interventions have the most potential for success. Interventions in grocery stores appear to be the least effective. The dual concerns of health and taste of foods promoted were rarely considered. Sustainability of environmental change was never addressed.
Conclusions. Interventions in “limited access” sites (i.e., where few other choices were available) had the greatest effect on food choices. Research is needed using consistent methods, better assessment tools, and longer durations; targeting diverse populations; and examining sustainability. Future interventions should influence access and availability, policies, and macroenvironments.
Ionic and Electronic Conduction in TiNb2O7 Griffith, Kent J; Seymour, Ieuan D; Hope, Michael A ...
Journal of the American Chemical Society,
10/2019, Volume:
141, Issue:
42
Journal Article
Peer reviewed
Open access
TiNb2O7 is a Wadsley–Roth phase with a crystallographic shear structure and is a promising candidate for high-rate lithium ion energy storage. The fundamental aspects of the lithium insertion ...mechanism and conduction in TiNb2O7, however, are not well-characterized. Herein, experimental and computational insights are combined to understand the inherent properties of bulk TiNb2O7. The results show an increase in electronic conductivity of seven orders of magnitude upon lithiation and indicate that electrons exhibit both localized and delocalized character, with a maximum Curie constant and Li NMR paramagnetic shift near a composition of Li0.60TiNb2O7. Square-planar or distorted-five-coordinate lithium sites are calculated to invert between thermodynamic minima or transition states. Lithium diffusion in the single-redox region (i.e., x ≤ 3 in Li x TiNb2O7) is rapid with low activation barriers from NMR and D Li = 10–11 m2 s–1 at the temperature of the observed T 1 minima of 525–650 K for x ≥ 0.75. DFT calculations predict that ionic diffusion, like electronic conduction, is anisotropic with activation barriers for lithium hopping of 100–200 meV down the tunnels but ca. 700–1000 meV across the blocks. Lithium mobility is hindered in the multiredox region (i.e., x > 3 in Li x TiNb2O7), related to a transition from interstitial-mediated to vacancy-mediated diffusion. Overall, lithium insertion leads to effective n-type self-doping of TiNb2O7 and high-rate conduction, while ionic motion is eventually hindered at high lithiation. Transition-state searching with beyond Li chemistries (Na+, K+, Mg2+) in TiNb2O7 reveals high diffusion barriers of 1–3 eV, indicating that this structure is specifically suited to Li+ mobility.
Achieving profound pulpal anaesthesia in a mandibular molar diagnosed with irreversible pulpitis can be argued to be the most testing of dental anaesthetic challenges. Following discussion on the ...possible reasons for this occurrence in part 1, part 2 outlines the various local anaesthetic techniques that practitioners can use to overcome the acutely inflamed mandibular molar. They should then be able to apply these same principles to help anaesthetise any other tooth presenting with an acutely inflamed pulp. Techniques are discussed in detail along with key variables that have been associated with having an impact on the anaesthetic efficacy. This is to bring to light factors that can aid anaesthetic success as well as dispel common misnomers.
Substituted Li-layered transition-metal oxide (LTMO) electrodes such as Li x Ni y Mn z Co1–y–z O2 (NMC) and Li x Ni y Co1–y–z Al z O2 (NCA) show reduced first cycle Coulombic efficiency (90–87% under ...standard cycling conditions) in comparison with the archetypal Li x CoO2 (LCO; ∼98% efficiency). Focusing on Li x Ni0.8Co0.15Al0.05O2 as a model compound, we use operando synchrotron X-ray diffraction (XRD) and nuclear magnetic resonance (NMR) spectroscopy to demonstrate that the apparent first-cycle capacity loss is a kinetic effect linked to limited Li mobility at x > 0.88, with near full capacity recovered during a potentiostatic hold following the galvanostatic charge–discharge cycle. This kinetic capacity loss, unlike many capacity losses in LTMOs, is independent of the cutoff voltage during delithiation and it is a reversible process. The kinetic limitation manifests not only as the kinetic capacity loss during discharge but as a subtle bimodal compositional distribution early in charge and, also, a dramatic increase of the charge–discharge voltage hysteresis at x > 0.88. 7Li NMR measurements indicate that the kinetic limitation reflects limited Li transport at x > 0.86. Electrochemical measurements on a wider range of LTMOs including Li x (Ni,Fe) y Co1–y O2 suggest that 5% substitution is sufficient to induce the kinetic limitation and that the effect is not limited to Ni substitution. We outline how, in addition to a reduction in the number of Li vacancies and shrinkage of the Li-layer size, the intrinsic charge storage mechanism (two-phase vs solid-solution) and localization of charge give rise to additional kinetic barriers in NCA and nonmetallic LTMOs in general.
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