92 research outputs found
Nucleation mechanism for the direct graphite-to-diamond phase transition
Graphite and diamond have comparable free energies, yet forming diamond from
graphite is far from easy. In the absence of a catalyst, pressures that are
significantly higher than the equilibrium coexistence pressures are required to
induce the graphite-to-diamond transition. Furthermore, the formation of the
metastable hexagonal polymorph of diamond instead of the more stable cubic
diamond is favored at lower temperatures. The concerted mechanism suggested in
previous theoretical studies cannot explain these phenomena. Using an ab initio
quality neural-network potential we performed a large-scale study of the
graphite-to-diamond transition assuming that it occurs via nucleation. The
nucleation mechanism accounts for the observed phenomenology and reveals its
microscopic origins. We demonstrated that the large lattice distortions that
accompany the formation of the diamond nuclei inhibit the phase transition at
low pressure and direct it towards the hexagonal diamond phase at higher
pressure. The nucleation mechanism proposed in this work is an important step
towards a better understanding of structural transformations in a wide range of
complex systems such as amorphous carbon and carbon nanomaterials
Primary growth hormone insensitivity (Laron syndrome) and acquired hypothyroidism: a case report
<p>Abstract</p> <p>Introduction</p> <p>Primary growth hormone resistance or growth hormone insensitivity syndrome, also known as Laron syndrome, is a hereditary disease caused by deletions or different types of mutations in the growth hormone receptor gene or by post-receptor defects. This disorder is characterized by a clinical appearance of severe growth hormone deficiency with high levels of circulating growth hormone in contrast to low serum insulin-like growth factor 1 values.</p> <p>Case presentation</p> <p>We report the case of a 15-year-old Caucasian girl who was diagnosed with Silver-Russell syndrome at the age of four and a half years. Recombinant growth hormone was administered for 18 months without an appropriate increase in growth velocity. At the age of seven years, her serum growth hormone levels were high, and an insulin-like growth factor 1 generation test did not increase insulin-like growth factor 1 levels (baseline insulin-like growth factor 1 levels, 52 μg/L; reference range, 75 μg/L to 365 μg/L; and peak, 76 μg/L and 50 μg/L after 12 and 84 hours, respectively, from baseline). The genetic analysis showed that the patient was homozygous for the R217X mutation in the growth hormone receptor gene, which is characteristic of Laron syndrome. On the basis of these results, the diagnosis of primary growth hormone insensitivity syndrome was made, and recombinant insulin-like growth factor 1 therapy was initiated. The patient's treatment was well tolerated, but unexplained central hypothyroidism occurred at the age of 12.9 years. At the age of 15 years, when the patient's sexual development was almost completed and her menstrual cycle occurred irregularly, her height was 129.8 cm, which is 4.71 standard deviations below the median for normal girls her age.</p> <p>Conclusion</p> <p>The most important functional tests for the diagnosis of growth hormone insensitivity are the insulin-like growth factor 1 generation test and genetic analysis. Currently, the only effective treatment is daily administration of recombinant insulin-like growth factor 1 starting from early childhood. However, these patients show a dramatically impaired final height. In our case, unexplained central hypothyroidism occurred during treatment.</p
Association of early life factors and acute lymphoblastic leukaemia in childhood: historical cohort study
In a historical cohort study of all singleton live births in Northern Ireland from 1971–86 (n=434 933) associations between early life factors and childhood acute lymphoblastic leukaemia were investigated. Multivariable analyses showed a positive association between high paternal age (⩾35 years) and acute lymphoblastic leukaemia (relative risk=1.49; 95% confidence interval (CI)=0.96–2.31) but no association with maternal age. High birth weight (⩾3500 g) was positively associated with acute lymphoblastic leukaemia (relative risk=1.66; 95% CI=1.18–2.33). Children of mothers with a previous miscarriage or increased gestation (⩾40 weeks) had reduced risks of ALL (respective relative risks=0.49; 95% CI=0.29–0.80, and 0.67; 95% CI=0.48–0.94). Children born into more crowded households (⩾1 person per room) had substantially lower risks than children born into less crowded homes with also some evidence of a lower risk for children born into homes with three adults (relative risks=0.56; 95% CI=0.35–0.91 and 0.58; 95% CI=0.21–1.61 respectively). These findings indicate that several early life factors, including living conditions in childhood and maternal miscarriage history, influence risk of acute lymphoblastic leukaemia in childhood
Waste Generation, Incineration and Landfill Diversion: De-Coupling Trends, Socio-Economic Drivers and Policy Effectiveness in the EU
Unraveling the Impurity Location and Binding in Heavily Doped Semiconductor Nanocrystals: The Case of Cu in InAs Nanocrystals
The doping of colloidal semiconductor
nanocrystals (NCs) presents
an additional knob beyond size and shape for controlling the electronic
properties. An important problem for doping with aliovalent elements
is associated with resolving the location of the dopant and its structural
surrounding within small NCs, an issue directly connected with self-purification.
Here we used a postsynthesis diffusion-based doping method for introducing
Cu impurities into InAs quantum dots. X-ray absorption fine structure
(XAFS) spectroscopy experiments along with first-principle density
functional theory (DFT) calculations were used to probe the impurity
sites. The concentration dependence was investigated for a wide range
of doping levels, helping to derive a self-consistent picture where
the Cu impurity occupies an interstitial site within the InAs lattice.
Moreover, at extremely high doping levels, Cu–Cu interactions
are identified in the XAFS data. This structural model is supported
by X-ray diffraction data, along with the DFT calculation. These findings
establish the reproducibility of the diffusion-based doping strategy
giving rise to new opportunities of correlating the structural details
with emerging electronic properties in heavily doped NCs
Demonstrating the Transferability and the Descriptive Power of Sketch-Map
Increasingly, it is recognized that new automated forms of analysis are required to understand the high-dimensional output obtained from atomistic simulations. Recently, we introduced a new dimensionality reduction algorithm, sketch-map, that was designed specifically to work with data from molecular dynamics trajectories. In what follows, we provide more details on how this algorithm works and on how to set its parameters. We also test it on two well-studied Lennard-Jones clusters and show that the coordinates we extract using this algorithm are extremely robust. In particular, we demonstrate that the coordinates constructed for one particular Lennard-Jones cluster can be used to describe the configurations adopted by a second, different cluster and even to tell apart different phases of bulk Lennard-Jonesium. © 2013 American Chemical Society
Charting Digital Literacy: A Framework for Information Technology and Digital Skills Education in the Community College
Literacy, literacies, and the digital in higher education
This paper is a critical review of some recent literature around the "literacies of the digital" in Schools and Higher Education. It discusses the question: 'what does the conjoining of the terms "digital" and "literacy" add to our understanding of teaching and learning in higher education'? It explores the continuing role of critical literacy in relation to the idea that digital literacies are transformative for pedagogy in this sector
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