10,304 research outputs found

    The compressibility and high pressure structure of diopside from first principles simulation

    Get PDF
    The structure of diopside (CaMgSi2O6) has been calculated at pressures between 0 and 25 GPa using the planewaves and pseudopotentials approach to density functional theory. After applying a pressure correction of 4.66 GPa to allow for the under-binding usually associated with the generalized gradient approximation, cell parameters are in good agreement with experiment. Fitting to the third-order Birch-Murnaghan equation of state yields values of 122 GPa and 4.7 for the bulk modulus and its pressure derivative. In addition to cell parameters, our calculations provide all atomic positional parameters to pressures considerably beyond those currently available from experiment. We have analyzed these data in terms of polyhedral rigidity and regularity and find that the most compressible Ca polyhedron becomes markedly less anisotropic above 10 GPa

    The effect of cation order on the elasticity of omphacite from atomistic calculations

    Get PDF
    Omphacite, a clinopyroxene mineral with two distinct crystallographic sites, M1 and M2, and composition intermediate between diopside and jadeite, is abundant throughout the Earth's upper mantle, and is the dominant mineral in subducted oceanic crust. Unlike the end-members, omphacite exists in two distinct phases, a P2/n ordered phase at low temperature and a high-temperature C2/c disordered phase. The crystal structure and full elastic constants tensor of ordered P2/n omphacite have been calculated to 15 GPa using plane-wave density functional theory. Our results show that several of the elastic constants, notably C11, C12, and C13 deviate from linear- mixing between diopside and jadeite. The anisotropy of omphacite decreases with increasing pressure and, at 10 GPa, is lower than that of either diopside or jadeite. The effect of cation disorder is investigated through force-field calculations of the elastic constants of Special Quasirandom Structures supercells with simulated disorder over the M2 sites only, and over both cation sites. These show that cation order influences the elasticity, with some components displaying particular sensitivity to order on a specific cation site. C11, C12, and C66 are sensitive to disorder on M1, while C22 is softened substantially by disorder on M2, but insensitive to disorder on M1. This shows that the elasticity of omphacite is sensitive to the degree of disorder, and hence the temperature. We expect these results to be relevant to other minerals with order-disorder phase transitions, implying that care must be taken when considering the effects of composition on seismic anisotropy

    Implement A Novel Symmetric Block Cipher Algorithm

    Get PDF

    Interactions between bare and protonated Mg vacancies and dislocation cores in MgO

    Get PDF
    Water can be incorporated into the lattice of mantle minerals in the form of protons charge-balanced by the creation of cation vacancies. These protonated vacancies, when they interact with dislocations, influence strain rates by affecting dislocation climb, pinning the dislocation, and, potentially, by altering the Peierls barrier to glide. We use atomic scale simulations to investigate segregation of Mg vacancies to atomic sites within the core regions of dislocations in MgO. Energies are computed for bare and V′′Mg protonated Mg vacancies occupying atomic sites close to ½ 〈110〉 screw dislocations, and ½ 〈110〉 {100} and ½ 〈110〉 {110} edge dislocations. These are compared with energies for equivalent defects in the bulk lattice to determine segregation energies for each defect. Mg vacancies preferentially bind to ½ 〈110〉 {100} edge dislocations, with calculated minimum segregation energies of − 3.54 eV for and − 4.56 eV for 2HxMg . The magnitudes of the minimum segregation energies calculated for defects binding to ½ 〈110〉 {110} edge or ½ 〈110〉 screw dislocations are considerably lower. Interactions with the dislocation strain field lift the threefold energy degeneracy of the 2HxMg defect in MgO. These calculations show that Mg vacancies interact strongly with dislocations in MgO, and may be present in sufficiently high concentrations to affect dislocation mobility in both the glide- and climb-controlled creep regimes

    Lubrication of dislocation glide in forsterite by Mg vacancies: insights from Peierls-Nabarro modeling

    Get PDF
    Dislocation glide is an important contributor to the rheology of olivine under conditions of high stress and low to moderate temperature, such as occur in mantle wedges. Interactions between point defects and dislocation core may alter the Peierls stress, σp, and has been suggested that vacancy-related defects may selectively enhance glide on certain slip systems, changing the olivine deformation fabric. In this study, the Peierls-Nabarro model, parameterized by generalized stacking fault (GSF) energies calculated atomistically using empirical interatomic potentials, is used to determine the effect of bare Mg vacancies on the Peierls stresses of [100](010) and [001](010) dislocations in forsterite. Mg vacancies considerably reduce GSF energies and, consequently, σp for dislocations gliding on (010) in olivine. The magnitude of this decrease depends strongly on dislocation and the type of the lattice site, with vacant M2 sites producing the largest reduction of σp. The [001](010) slip system is found to be more sensitive than the [100](010) slip system to the presence of vacancies. Although, at ambient pressure, σp is lower for [100](010) than [001](010) edge dislocations, dσp/dP is greater for [100](010) dislocations, resulting in a change in the preferred slip system at 1.5 GPa. By preferentially lubricating [001](010) glide, Mg vacancies reduce the pressure at which this cross-over occurs. An M2 vacancy concentration at the glide plane of 0.125 defects/site is sufficient to reduce cross-over to 0.7 GPa. This may account for the existence of the B-type olivine deformation fabric in the corners of mantle wedges

    Autistic adults’ experiences of diagnostic disclosure in the workplace: Decision-making and factors associated with outcomes

    Get PDF
    Autistic individuals often struggle to find and maintain employment. This may be because many workplaces are not suited to autistic individuals’ needs. Among other difficulties, many autistic employees experience distracting or disruptive sensory environments, lack of flexibility in work hours, and unclear communication from colleagues. One possible way of mitigating these difficulties is for employees to disclose their diagnosis at work. While disclosure may increase understanding and acceptance from colleagues, it can also lead to discrimination and stigma in the workplace. Research has shown that disclosure outcomes are often mixed, but it is unclear what factors are associated with either positive or negative outcomes of disclosure for autistic people. This study aimed to identify these factors and explore the reasons why autistic employees choose to disclose or to keep their diagnosis private. Semi-structured interviews were conducted with 24 clinically-diagnosed autistic adults (12 male and 12 female) who were currently, or had been, employed in the UK (mean age = 45.7 years). Through thematic analysis, we identified three main themes under experiences of disclosure: 1) A preference for keeping my diagnosis private; 2) The importance of disclosure in the workplace; and 3) Disclosure has mixed outcomes. We also identified three factors associated with disclosure outcomes: understanding of autism, adaptations, and organisational culture. These results have implications for improving inclusive practices on both the individual and organisational level to ensure more positive disclosure experiences for autistic employees

    Full STEAM ahead: a manifesto for integrating arts pedagogics in to STEM education

    Get PDF
    This paper sets out to challenge the common pedagogies found in STEM education with a particular focus on engineering. The dominant engineering pedagogy remains “chalk and talk”; despite research evidence that demonstrates its ineffectiveness. The paper argues that there is a potential confusion in engineering education around the role of active learning approaches, and that the adoption of these approaches may be limited as a result of this confusion, combined with a degree of disciplinary egocentrism. The paper presents examples of engineering and “engineering like” projects that demonstrate the effectiveness of adopting pedagogies and delivery methods more usually attributed to the liberal arts such as studio based learning. The paper concludes with some suggestions about how best to create a fertile environment from which inquiry based learning can emerge as well as a reflection on whether the only real limitation on cultivating such approaches is the disciplinary egocentrism of traditional engineering educators

    Practice Makes Imperfect: Restorative Effects of Sleep on Motor Learning

    Get PDF
    Emerging evidence suggests that sleep plays a key role in procedural learning, particularly in the continued development of motor skill learning following initial acquisition. We argue that a detailed examination of the time course of performance across sleep on the finger-tapping task, established as the paradigm for studying the effect of sleep on motor learning, will help distinguish a restorative role of sleep in motor skill learning from a proactive one. Healthy subjects rehearsed for 12 trials and, following a night of sleep, were tested. Early training rapidly improved speed as well as accuracy on pre-sleep training. Additional rehearsal caused a marked slow-down in further improvement or partial reversal in performance to observed levels below theoretical upper limits derived on the basis of early pre-sleep rehearsal. This decrement in learning efficacy does not occur always, but if and only if it does, overnight sleep has an effect in fully or partly restoring the efficacy and actual performance to the optimal theoretically achieveable level. Our findings re-interpret the sleep-dependent memory enhancement in motor learning reported in the literature as a restoration of fatigued circuitry specialized for the skill. In providing restitution to the fatigued brain, sleep eliminates the rehearsal-induced synaptic fatigue of the circuitry specialized for the task and restores the benefit of early pre-sleep rehearsal. The present findings lend support to the notion that latent sleep-dependent enhancement of performance is a behavioral expression of the brain's restitution in sleep

    Thermoelastic properties of MgSiO3-majorite at high temperatures and pressures: A first principles study

    Get PDF
    As the major component of garnet, the second most abundant phase in Earth's transition zone, MgSiO3-majorite plays a fundamental role in controlling the state and dynamics of Earth's mantle. However, due to challenges of experiments and simulations, there are still very limited data on the elastic properties of MgSiO3-majorite at simultaneously high temperatures and pressures. In this study, we have carried out extensive first principles calculations to determine the thermoelastic properties of MgSiO3-majorite up to 2000 K and 40 GPa. We find that the elastic constants of MgSiO3-majorite change significantly over the temperature and pressure range studied, with noticeable non-linearities in their pressure dependences. The seismic anisotropy of MgSiO3-majorite is high and generally increases with pressure. It is much higher than that of the other end-members of garnet and ringwoodite, which makes it the most anisotropic mineral in assemblages expected in the lower transition zone. Based on our calculated elastic moduli and with careful elimination of systematic errors, we establish a third-order Birch-Murnaghan-Mie-Grüneisen model for MgSiO3-majorite with the parameters: V0 = 114.1 cm3/mol, K0 = 163.6 GPa, G0 = 86.4 GPa, K0′ = 4.44, G0′ = 1.16, γ0 = 1.08, q0 = 0.48, ηS0 = 0.76, and θ0 = 822.5 K. Integrating our results into a thermodynamic model able to predict the properties of mantle assemblages, we find that a pyrolite composition produces velocities that agree with the seismic model AK135 in the upper transition zone. In the lower transition zone, a pyrolite composition fits well with some specific local observations, but a mechanical mixture with 18% basalt and 82% harzburgite is in better agreement with the global seismic model PREM. The much larger abundance of MgSiO3-majorite in the garnet phase of harzburgite suggests that the anisotropy in the lower transition zone may not be negligible and would be observable at least in the heterogeneous zones near subducting slabs
    corecore