Ion dynamics in fluoride-containing polyatomic anion cathodes by muon spectroscopy

Polyatomic anion insertion electrodes present compositional and morphological variety, as well as the ability to tune operational voltages by influencing the nature of metal-oxygen bonding. Realizing the application of these compounds as electrodes in Li- and Na-ion batteries requires a detailed understanding of ion dynamics in these systems. Here is presented the microscopic Li-ion and Na-ion diffusion properties in LiFeSO4F and Na2FePO4F, respectively, using muon spin relaxation (μ+SR) spectroscopy for the first time. Li-ion diffusion processes in the tavorite LiFeSO4F phase are found to proceed with an activation energy (Ea) of 48(4) meV and a diffusion coefficient of 1.71 × 10−9 cm2 s−1, while Na-ion mobility in Na2FePO4F has a calculated diffusion coefficient of 3.47 × 10−10cm2 s−1 and a higher energy barrier to ion diffusion at 96(8) meV. This is the first such examination of fluoride-containing polyatomic cathodes using μ+SR, where the presence of the highly electronegative fluoride species was thought to preclude activation energy and diffusion coefficient determination due to strong μ+-F− interactions. These insights open up the possibility of studying a myriad of fluoride-containing electrode materials using the μ+SR technique

Magnetic phase separation in ordered alloys

We present a lattice model to study the equilibrium phase diagram of ordered alloys with one magnetic component that exhibits a low temperature phase separation between paramagnetic and ferromagnetic phases. The model is constructed from the experimental facts observed in Cu$_{3-x}$AlMn$_{x}$ and it includes coupling between configurational and magnetic degrees of freedom which are appropriated for reproducing the low temperature miscibility gap. The essential ingredient for the occurrence of such a coexistence region is the development of ferromagnetic order induced by the long-range atomic order of the magnetic component. A comparative study of both mean-field and Monte Carlo solutions is presented. Moreover, the model may enable the study of the structure of the ferromagnetic domains embedded in the non-magnetic matrix. This is relevant in relation to phenomena such as magnetoresistance and paramagnetism.Comment: 12 pages, 11 figures, accepted in Phys. Rev.

Elucidating local diffusion dynamics in nickel-rich layered oxide cathodes

Elucidating Li-ion transport properties is essential for designing suitable methodologies to optimise electrochemical performance in Ni-rich cathodes for high energy density Li-ion batteries. Here, we report the local-scale Li-diffusion characteristics of a series of nickel-rich layered oxide cathodes, prepared via microwave methods, using muon spin relaxation methods. Our results detail the effects of cation dopants, selected for structure stability, on transport properties in candidate nickel-rich chemistries. We find that the local diffusion properties improve with increasing nickel content. Our results demonstrate that these observations are dependant on substitutional effects

The Detection of Incipient Caries with Tracer Dyes

The purpose of this study was to determine the increase in color contrast produced by the use of a tracer dye in detection of incipient caries lesions with transillumination. Twenty four caries-free first premolars were immersed in an acid gelatin for production of artificial incipient caries lesions. After the lesions had developed, these teeth were photographed by transillumination. Two photographs were taken of each tooth. The first photograph showed the lesion without dye. A blue tracer dye was then added and absorbed by the lesion, and a second photograph was taken. The data on the color difference were obtained by use of a reflectance colorimeter and showed a four-fold increase between the lesion and surrounding area with the dye. A two-way analysis of variance was used for the statistical interpretation. The color difference between the lesion without the dye and then with the dye was significant. The use of the blue tracer dye, therefore, significantly increased the contrast in the images of the artificial incipient lesions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/68289/2/10.1177_00220345890680021101.pd

Tight-binding g-Factor Calculations of CdSe Nanostructures

The Lande g-factors for CdSe quantum dots and rods are investigated within the framework of the semiempirical tight-binding method. We describe methods for treating both the n-doped and neutral nanostructures, and then apply these to a selection of nanocrystals of variable size and shape, focusing on approximately spherical dots and rods of differing aspect ratio. For the negatively charged n-doped systems, we observe that the g-factors for near-spherical CdSe dots are approximately independent of size, but show strong shape dependence as one axis of the quantum dot is extended to form rod-like structures. In particular, there is a discontinuity in the magnitude of g-factor and a transition from anisotropic to isotropic g-factor tensor at aspect ratio ~1.3. For the neutral systems, we analyze the electron g-factor of both the conduction and valence band electrons. We find that the behavior of the electron g-factor in the neutral nanocrystals is generally similar to that in the n-doped case, showing the same strong shape dependence and discontinuity in magnitude and anisotropy. In smaller systems the g-factor value is dependent on the details of the surface model. Comparison with recent measurements of g-factors for CdSe nanocrystals suggests that the shape dependent transition may be responsible for the observations of anomalous numbers of g-factors at certain nanocrystal sizes.Comment: 15 pages, 6 figures. Fixed typos to match published versio

Evidence for Shape Co-existence at medium spin in 76Rb

Four previously known rotational bands in 76Rb have been extended to moderate spins using the Gammasphere and Microball gamma ray and charged particle detector arrays and the 40Ca(40Ca,3pn) reaction at a beam energy of 165 MeV. The properties of two of the negative-parity bands can only readily be interpreted in terms of the highly successful Cranked Nilsson-Strutinsky model calculations if they have the same configuration in terms of the number of g9/2 particles, but they result from different nuclear shapes (one near-oblate and the other near-prolate). These data appear to constitute a unique example of shape co-existing structures at medium spins.Comment: Accepted for publication in Physics Letters

Time–temperature equivalence in the tack and dynamic stiffness of polymer prepreg and its application to automated composites manufacturing

A recently developed peel test designed to simulate the automated tape lay-up (ATL) process was used to measure tack and dynamic stiffness of newly developed ATL prepregs. Resin was extracted from the prepreg process before impregnation of the fibres. Isothermal small amplitude frequency sweeps were carried out in shear rheology to determine time–temperature superposition parameters in the form of Williams–Landel–Ferry equation. Gel permeation chromatography and differential scanning calorimetry demonstrated that the resin was not significantly changed during the prepregging process. The WLF parameters were used to transpose isothermal tack and dynamic stiffness results with excellent agreement found. This relationship offers manufacturers using composite prepreg a method to maximise and maintain tack levels at different feed rates by appropriate changes in temperature. This is of significant importance in improving the reliability of automated composite lay-up processes such as AFP and ATL, whose feed rate must vary to accommodate lay-up operations

Quantum fluctuations of charge order induce phonon softening in a superconducting cuprate

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order

Quantum fluctuations of charge order induce phonon softening in a superconducting cuprate

Quantum phase transitions play an important role in shaping the phase diagram of high-temperature cuprate superconductors. These cuprates possess intertwined orders which interact strongly with superconductivity. However, the evidence for the quantum critical point associated with the charge order in the superconducting phase remains elusive. Here we show the short-range charge orders and the spectral signature of the quantum fluctuations in La$_{2-x}$Sr$_x$CuO$_4$ (LSCO) near the optimal doping using high-resolution resonant inelastic X-ray scattering. On performing calculations through a diagrammatic framework, we discovered that the charge correlations significantly soften several branches of phonons. These results elucidate the role of charge order in the LSCO compound, providing evidence for quantum critical scaling and discommensurations associated with charge order

Spin Susceptibility and Superexchange Interaction in the Antiferromagnet CuO

Evidence for the quasi one-dimensional (1D) antiferromagnetism of CuO is presented in a framework of Heisenberg model. We have obtained an experimental absolute value of the paramagnetic spin susceptibility of CuO by subtracting the orbital susceptibility separately from the total susceptibility through the $^{63}$Cu NMR shift measurement, and compared directly with the theoretical predictions. The result is best described by a 1D $S=1/2$ antiferromagnetic Heisenberg (AFH) model, supporting the speculation invoked by earlier authors. We also present a semi-quantitative reason why CuO, seemingly of 3D structure, is unexpectedly a quasi 1D antiferromagnet.Comment: 7 pages including 4 tables and 9 figure