Twenty Golden Years of Battery R&D at CSIR,1974–1994

This paper is a personal reflection of 20 years of sodium- and lithium-based battery R&D in South Africa between 1974 and 1994. The impact of the innovative materials science and engineering research conducted during this period is highlighted by the later successful implementation of batteries in powering practical systems, as well as in technology licensing.Keywords: CSIR, battery, Zebra, sodium-metal chloride, sodium-sulphur, LiAl/FeSx, lithium-ion, spine

Rainfall seasonality captured in micromammalian fauna in Late Quaternary contexts, South Africa

There exists ongoing debate regarding shifts in the latitudinal extent of the southern African winter-rainfall zone throughout the late Quaternary. Fossil proxies which can be related directly to rainfall seasonality have the potential to assist in quantifying these shifts. Relationships between mean monthly temperature and mean monthly rainfall in modern environments are quantified to generate a seasonality index associated with summer- or winter-rainfall. These seasonality indices can in turn be related to percentage occurrences reflecting relative abundances of rodent taxa represented in areas within southern Africa. Such data are used together to obtain an equation from which an index of seasonality in rainfall can be calculated, based on relative abundances of rodents in the modern landscape. The equation is applied to rodents represented in a Late Quaternary faunal sequence at Boomplaas Cave in the south-eastern part of the Western Cape Province. Results confirm that this region experienced a predominantly winter-rainfall regime during the Last Glacial Maximum (LGM), though the amount of rain may have been relatively low for the coldest episodes circa 20,000 cal. yr BP in the Boomplaas palaeo-environments.National Research Foundation, Andrew Mellon Foundatio

Spectroscopy and Time Variability of Absorption Lines in the Direction of the Vela Supernova Remnant

We present high resolution (R~75,000), high signal-to-noise (S/N~100) Ca II $\lambda$3933.663 and Na I $\lambda\lambda$5889.951, 5895.924 spectra of 68 stars in the direction of the Vela supernova remnant. The spectra comprise the most complete high resolution, high S/N, optical survey of early type stars in this region of the sky. A subset of the sight lines has been observed at multiple epochs, 1993/1994 and 1996. Of the thirteen stars observed twice, seven have spectra revealing changes in the equivalent width and/or velocity structure of lines, most of which arise from remnant gas. Such time variability has been reported previously for the sight lines towards HD 72089 and HD 72997 by Danks & Sembach (1995) and for HD 72127 by Hobbs et al. (1991). We have confirmed the ongoing time variability of these spectra and present new evidence of variability in the spectra of HD 73658, HD 74455, HD 75309 and HD 75821. We have tabulated Na I and Ca II absorption line information for the sight lines in our sample to serve as a benchmark for further investigations of the dynamics and evolution of the Vela SNR.Comment: 8 pages of text, 4 tables, 16 pages of figures Accepted and to be published in ApJ

Rainfall seasonality captured in micromammalian fauna in Late Quaternary contexts, South Africa

There exists ongoing debate regarding shifts in the latitudinal extent of the southern African winter-rainfall zone throughout the late Quaternary. Fossil proxies which can be related directly to rainfall seasonality have the potential to assist in quantifying these shifts. Relationships between mean monthly temperature and mean monthly rainfall in modern environments are quantified to generate a seasonality index associated with summer- or winter-rainfall. These seasonality indices can in turn be related to percentage occurrences reflecting relative abundances of rodent taxa represented in areas within southern Africa. Such data are used together to obtain an equation from which an index of seasonality in rainfall can be calculated, based on relative abundances of rodents in the modern landscape. The equation is applied to rodents represented in a Late Quaternary faunal sequence at Boomplaas Cave in the south-eastern part of the Western Cape Province. Results confirm that this region experienced a predominantly winter-rainfall regime during the Last Glacial Maximum (LGM), though the amount of rain may have been relatively low for the coldest episodes circa 20,000 cal. yr BP in the Boomplaas palaeo-environments.National Research Foundation, Andrew Mellon Foundatio

Pourbaix-like phase diagram for lithium manganese spinels in acid

Calculations are performed on the free energies for proton-promoted reactions of the lithium-ion-battery electrode material LiMn_(2)O_4 spinel in acid, as a function of lithium excess and lithium deficiency relative to stoichiometry. In particular, we consider the dissolution reaction proposed by Hunter (J. Solid State Chem., 1981, 39, 142), in which protons react with lithium manganate spinel to form λ-MnO2, Li^+, and Mn^(2+) products. The calculations employ a hybrid method developed in previous work in which first principles total energy calculations are applied for the solid phases and free atom energies, and tabulated ionization and hydration energies for the aqueous species. A correction to the atomic energies, derived from analysis of binary oxide dissolution reactions, improves the accuracy of the results. A Pourbaix-like dissolution/stability phase diagram is constructed from the resultant reaction free energies

A spectral and spatial analysis of eta Carinae's diffuse X-ray emission using CHANDRA

The luminous unstable star (star system) eta Carinae is surrounded by an optically bright bipolar nebula, the Homunculus and a fainter but much larger nebula, the so-called outer ejecta. As images from the EINSTEIN and ROSAT satellites have shown, the outer ejecta is also visible in soft X-rays, while the central source is present in the harder X-ray bands. With our CHANDRA observations we show that the morphology and properties of the X-ray nebula are the result of shocks from fast clumps in the outer ejecta moving into a pre-existing denser circumstellar medium. An additional contribution to the soft X-ray flux results from mutual interactions of clumps within the ejecta. Spectra extracted from the CHANDRA data yield gas temperatures kT of 0.6-0.76 keV. The implied pre-shock velocities of 670-760 km/s are within the scatter of the velocities we measure for the majority of the clumps in the corresponding regions. Significant nitrogen enhancements over solar abundances are needed for acceptable fits in all parts of the outer ejecta, consistent with CNO processed material and non-uniform enhancement. The presence of a diffuse spot of hard X-ray emission at the S condensation shows some contribution of the highest velocity clumps and further underlines the multicomponent, non-equilibrium nature of the X-ray nebula. The detection of an X-ray ``bridge'' between the northern and southern part of the X-ray nebula and an X-ray shadow at the position of the NN bow can be attributed to a large expanding disk, which would appear as an extension of the equatorial disk. No soft emission is seen from the Homunculus, or from the NN bow or the ``strings''.Comment: 15 pages, 10 figures, accepted by A&A; paper including images with full resolution available at http://www.astro.ruhr-uni-bochum.de/kweis/publications.htm

Magnetic excitations in the spinel compound Lix_x[Mn1.96_{1.96}Li0.04_{0.04}]O4_4 (x= 0.2, 0.6, 0.8, 1.0): how a classical system can mimic quantum critical scaling

We present neutron scattering results on the magnetic excitations in the spinel compounds Li$_x$[Mn$_{1.96}$Li$_{0.04}$]O$_4$ (x= 0.2, 0.6, 0.8, 1.0). We show that the dominant excitations below T ~ 70 K are determined by clusters of Mn^4+ ions, and that these excitations mimic the E/T-scaling found in quantum critical systems that also harbor magnetic clusters, such as CeRu$_{0.5}$Fe$_{1.5}$Ge$_2$. We argue that our results for this classical spinel compound show that the unusual response at low temperatures as observed in quantum critical systems is (at least) partially the result of the fragmentation of the magnetic lattice into smaller units. This fragmentation in quantum critical systems is the direct and unavoidable result of intrinsic disorder.Comment: 8 pages, 8 figures; to be submitted to PR

Origin of electrochemical activity in nano-Li2MnO3; Stabilization via a 'point defect scaffold'

Molecular dynamics (MD) simulations of the charging of Li2MnO3 reveal that the reason nanocrystalline-Li2MnO3 is electrochemically active, in contrast to the parent bulk-Li2MnO3, is because in the nanomaterial the tunnels, in which the Li ions reside, are held apart by Mn ions, which act as a pseudo 'point defect scaffold'. The Li ions are then able to diffuse, via a vacancy driven mechanism, throughout the nanomaterial in all spatial dimensions while the 'Mn defect scaffold' maintains the structural integrity of the layered structure during charging. Our findings reveal that oxides, which comprise cation disorder, can be potential candidates for electrodes in rechargeable Li-ion batteries. Moreover, we propose that the concept of a 'point defect scaffold' might manifest as a more general phenomenon, which can be exploited to engineer, for example, two or three-dimensional strain within a host material and can be fine-tuned to optimize properties, such as ionic conductivity

Accretion onto the Companion of Eta Carinae During the Spectroscopic Event: III. the He II 4686 Line

We continue to explore the accretion model of the massive binary system eta Carinae by studying the anomalously high He II 4686 line. The line appears just before periastron and disappears immediately thereafter. Based on the He II 4686 line emission from O-stars and their modeling in the literature, we postulate that the He II 4686 line comes from the acceleration zone of the secondary stellar wind. We attribute the large increase in the line intensity to a slight increase in the density of the secondary stellar wind in its acceleration zone. The increase in density could be due to the ionization and subsequent deceleration of the wind by the enhanced X-ray emission arising from the shocked secondary wind further downstream or to accretion of the primary stellar wind. Accretion around the secondary equatorial plane gives rise to collimation of the secondary wind, which increases its density, hence enhancing the He II 4686 emission line. In contrast with previous explanations, the presently proposed model does not require a prohibitively high X-ray flux to directly photoionize the He.Comment: ApJ, in pres