Computer-aided instruction to improve pass rates of first-year chemistry students

Published ArticlePast imbalances in the South African education system continue to perpetuate in poorly resourced schools and inadequately skilled teachers, resulting in under-prepared university students. At Tshwane University of Technology (TUT) a computer-based intervention was developed to address two of the conceptual difficulties identified in prospective first-year Chemistry students. After implementation of the intervention, average improvements of 13.6% and 6.4% were obtained for the concepts of conservation of matter and physical and chemical change respectively. The students' attitudes towards computer-aided study, assessed using a semi-structured questionnaire, were found to be extremely positive despite limited computer access

First principles electronic structure of spinel LiCr2O4: A possible half-metal?

We have employed first-principles electronic structure calculations to examine the hypothetical (but plausible) oxide spinel, LiCr2O4 with the d^{2.5} electronic configuration. The cell (cubic) and internal (oxygen position) structural parameters have been obtained for this compound through structural relaxation in the first-principles framework. Within the one-electron band picture, we find that LiCr2O4 is magnetic, and a candidate half-metal. The electronic structure is substantially different from the closely related and well known rutile half-metal CrO2. In particular, we find a smaller conduction band width in the spinel compound, perhaps as a result of the distinct topology of the spinel crystal structure, and the reduced oxidation state. The magnetism and half-metallicity of LiCr2O4 has been mapped in the parameter space of its cubic crystal structure. Comparisons with superconducting LiTi2O4 (d^{0.5}), heavy-fermion LiV2O4 (d^{1.5}) and charge-ordering LiMn2O4 (d^{3.5}) suggest the effectiveness of a nearly-rigid band picture involving simple shifts of the position of E_F in these very different materials. Comparisons are also made with the electronic structure of ZnV2O4 (d^{2}), a correlated insulator that undergoes a structural and antiferromagnetic phase transition.Comment: 9 pages, 7 Figures, version as published in PR

A stochastic exposure assessment model to estimate vanadium intake by beef cattle used as sentinels for the South African vanadium mining industry

This paper presents an environmental exposure assessment model for estimating chronic intake of vanadium (a transition metal) by cattle farmed extensively in areas contaminated by vanadium pollutants. The exposure model differs from most other models in several ways: (1) it does not rely heavily on extrapolating information from the point source (e.g. stack height, exit velocity, exit diameter) to the point of exposure. (2) It incorporates the physiological constraints of the species exposed. (3) It takes into account oral as well as inhalation exposure. (4) It addresses terrain, by using measurements at the point of exposure. (5) It accounts for existing background concentrations of pollutants and pollutants from multiple sources. (6) It uses a stochastic process with distribution functions to account for variability in the data over time. Environmental inputs into the model included aerial fall-out sample vanadium (n = 566), unwashed grass sample vanadium (n = 342) and soluble soil sample vanadium (n = 342). Physiological cattle inputs were derived from two cohorts of Brahman-cross sentinel cattle (n = 30). The model provided an estimate of the chronic external exposure dose of vanadium for two separate groups of cattle grazing over a 5-year period (1999–2004) immediately adjacent (median dose = 2.14 mg vanadium/kg body weight/day) and 2 km away (median dose = 1.07 mg/kg/day) from a South African vanadium-processing plant, respectively. The final output of the model is a distribution curve of the probable vanadium intake based on the variability within the inputs over the 5-year period of the study. The model is adaptable enough for application to other transition metals and species (including man), and could be used as an alternative to plume-dispersion modelling

Recent progress in the development of Li2MnSiO4 cathode materials

Lithium ion batteries are under intense development to meet the performance specifications for consumer applications in portable electronic devices, electric vehicle batteries and stationary storage as back-up for intermittent renewable energy generation technologies. The most expensive and capacity-limiting component in these battery systems is the cathode material. Research has been directed to the development of novel cathode materials with high capacity and energy density and the lithium transition metal orthosilicates have been identified as possible high performance cathodes. In this review we focus on recent developments in the study of Li2MnSiO4 and its derivatives as a lithium-ion battery cathode material. Preparation techniques, structural issues, conductivity enhancement and complex morphologies are discussed, to show the recent improvements and limitations in synthesis and electrochemical performance

A stochastic model for estimating intake of vanadium by beef cattel used as sentinels

This paper presents an environmental exposure assessment model for estimating chronic intake of vanadium (a transition metal) by cattle farmed extensively in areas contaminated by vanadium pollutants. The exposure model differs from most other models in several ways: (1) it does not rely heavily on extrapolating information from the point source (e.g. stack height, exit velocity, exit diameter) to the point of exposure. (2) It incorporates the physiological constraints of the species exposed. (3) It takes into account oral as well as inhalation exposure. (4) It addresses terrain, by using measurements at the point of exposure. (5) It accounts for existing background concentrations of pollutants and pollutants from multiple sources. (6) It uses a stochastic process with distribution functions to account for variability in the data over time. Environmental inputs into the model included aerial fall-out sample vanadium (n = 566), unwashed grass sample vanadium (n = 342) and soluble soil sample vanadium (n = 342). Physiological cattle inputs were derived from two cohorts of Brahman-cross sentinel cattle (n = 30). The model provided an estimate of the chronic external exposure dose of vanadium for two separate groups of cattle grazing over a 5-year period (1999–2004) immediately adjacent (median dose = 2.14 mg vanadium/kg body weight/day) and 2 km away (median dose = 1.07 mg/kg/day) from a South African vanadium-processing plant, respectively. The final output of the model is a distribution curve of the probable vanadium intake based on the variability within the inputs over the 5-year period of the study. The model is adaptable enough for application to other transition metals and species (including man), and could be used as an alternative to plume-dispersion modelling

An investigation of spinel-related and orthorhombic LiMn0₂ cathodes for rechargeable lithium batteries

Cathode materials that have been synthesized by reduction of lithium-manganese-oxide and manganese-oxide precursors with hydrogen at 300 to 350°C and with carbon at 600°C have been evaluated in rechargeable lithium cells. The cathodes which initially have a composition close to LiMnO₂ contain structures related to the lithiated-spinel phase Li₂[Mn₂]0₄ and/or orthorhombic LiMnO₂. The orthorhombic LiMnO₂ component transforms gradually to a spinel structure on cycling. These cathodes are significantly more tolerant to repeated lithium insertion and extraction, when cycled over both the 4 and 3 V regions, than a standard Liₓ[Mn₂]0₄ spinel electrode (0 < x < 2)

The effect of multivalent cation dopants on lithium manganese spinel cathodes

The cycling stability of 4 V LiₓMn₂O₄ electrodes in lithium, flooded electrolyte glass cells has been improved by the addition of multivalent cation dopants (Mg²⁺, Zn²⁺ and Al³⁺). Optimal dopant levels to achieve maximum capacity and the greatest stability with repeated cycling have been determined. The effect of doping the oxygen-rich spinel Li₂Mn₄O₉, was also determined and shown to make no significant improvement in the life cycle stability in the 3 V region

Crystal chemistry of the Pmnb polymorph of Li2MnSiO4

The crystal structure of the Pmnb polymorph of Li2MnSiO4 (prepared by solid-state synthesis in argon at 900 °C) is characterized by Rietveld refinement of structural models using high resolution synchrotron X-ray and neutron powder diffraction data. The crystal structure is confirmed to be isostructural with Li2CdSiO4 with lattice parameters a=6.30694(3), b=10.75355(4), and c=5.00863(2) Å, which are in good agreement with previously published data. No evidence was found for mixed lithium/manganese sites. Testing of the material as a cathode in a lithium cell shows that 1.3 lithium ions per formula unit can be extracted on the first charge cycle but very little lithium can be re-inserted. These results are compared with those of other phase-pure Li2MnSiO4 polymorphs

An electron energy loss spectroscopy and electron diffraction study of the Pmnb polymorph of Li2MnSiO4

The Mn valency and the crystallinity of Li₂MnSiO₄ cathodes (Pmnb form) were examined with electron energy-loss spectroscopy (EELS) and selected area electron diffraction (SAED) both before and after electrochemical lithium extraction. A decrease in the crystallinity of the delithiated charged cathode particles compared to the as-prepared material was observed. The decrease in crystallinity varied from particle to particle. EELS analysis showed that the non-uniform decrease in crystallinity was due to a non-uniform extraction of lithium from the particles. The observed decrease in discharge capacity of the Pmnb polymorph of Li₂MnSiO₄ with cycling was attributed to the progressive loss of crystallinity and the structural collapse of Li diffusion pathways