14,411 research outputs found
A study of crack initiation in corrosion-fatigue of A.I.S.I 316 stainless steel by dynamic measurement of corrosion characteristics and corrosion current transients
This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.A technique has been developed and applied to monitor corrosion current transients, in phase with a cyclic stress. applied in reverse bending at 24 Ez to AISI 316 stainless steel plate immersed in selected aqueous media. The amplitude and waveform of this fluctuating current component, together with the measurement of the corrosion characteristics during fatigue, yield information on the nature of the stress-environment interaction. In conditions where the metal surface was passive, a mechanism is proposed, which involves the repeated transient depassivation of the surface at persistent slip bands, leaving ultimately to premature crack initiation. In conditions where the metal surface was active (transpassive) a mechanism is proposed, which
involves the preferential dissolution at slip steps with resultant enhancement in plastic strain and premature crack initiation.This study is funded by the Science Research Council
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Partitioning of polycyclic aromatic hydrocarbons in heterogeneous clusters
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Ion-Induced Soot Nucleation Using a New Potential for Curved Aromatics
A potential able to capture the properties and interactions of curved polycyclic aromatic hydrocarbons (cPAHs) was developed and used to investigate the nucleation behaviour and structure of nascent soot particles. The flexoelectric charge polarisation of cPAHs caused by pentagon integration was included through the introduction of off-site virtual atoms, and enhanced dispersion interaction parameters were fitted. The electric polarisation and intermolecular interactions of cPAHs were accurately reproduced compared to ab initio calculations. This potential was used within molecular dynamics simulations to examine the
homogeneous and heterogeneous nucleation behaviour of the cPAH corannulene and planar PAH coronene across a range of temperatures relevant to combustion. The enhanced interactions between cPAHs and potassium ions resulted in significant and rapid nucleation of stable clusters compared to all other systems, highlighting their importance in soot nucleation. In addition, the resulting cPAH clusters present morphologies distinct from the stacked planar PAH clusters.This work used the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk). K.B. is grateful to the Cambridge Trust and the Stanley Studentship at King’s College, Cambridge for their financial support. This project is also supported by the National Research Foundation (NRF), Prime Minister’s Office, Singapore under its Campus for Research Excellence and Technological Enterprise (CREATE) programme
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Atomic structure and electronic structure of disordered graphitic carbon nitride
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Exploring the internal structure of soot particles using nanoindentation: A reactive molecular dynamics study
The mechanical properties and internal structure of soot nanoparticles is investigated using reactive molecular dynamics simulations of nanoindentation of model soot particles. The particles that are provided as inputs to the simulations are generated using reactive molecular dynamics to create 3D networks of crosslinked coronene, circumanthracene and core-shell mixtures of coronene and circumanthracene. The results of the simulated nanoindentation experiments are analysed as a function of the degree of crosslinking (defined as the number of crosslinks per monomer in the particles), the size and the core-shell structure of the particles. In the case of homogeneous particles (i.e. those without a core-shell structure), the simulations show a unique relationship between the degree of crosslinking (CL) and the simulated hardness, Young’s modulus and deformation ratio. In the case of particles with a core-shell structure, a unique relationship was only found by considering the core-shell ratio and the degree of crosslinking in both the core and the shell. Our results allow for interpretation of the nanoindentation experiments as suggesting crosslinks are present in mature soot particles and preliminary evidence that crosslinks also are present within the interior of soot particles
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Dynamic polarity of curved aromatic soot precursors
In this paper, we answer the question of whether polar curved aromatics are persistently polar at flame temperatures. We find, using electronic structure calculations and transition state theory, that the inversion barriers of curved aromatics (cPAH) 0.9–1.2 nm in diameter are high and that they are not able to invert over the timescales and at the high temperatures found in sooting flames. We find a transition for smaller curved aromatics between 11–15 ( ≈  0.8 nm) rings where the increasing strain introduced from the pentagonal ring increases the inversion barrier leading to rigidity. We then performed ab initio quantum molecular dynamics to find the molecular dipole fluctuations of a nanometre sized cPAH at 1500 K. We found the bending mode of the bowl shaped molecule gave rise to the largest fluctuations on the dipole moment by  ± 0.5–1 debye about the equilibrium value of 5.00 debye, indicating persistent polarity. We also observed binding of a chemi-ion at 1500 K over 2 ps, suggesting the molecular dipole of cPAH will be an important consideration in soot formation mechanisms
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Mechanical Properties of Soot Particles: The Impact of Crosslinked Polycyclic Aromatic Hydrocarbons
In this paper, we estimate the degree of crosslinking within soot particles making use of reactive molecular dynamics simulations of mechanical properties of crosslinked polycyclic aromatic hydrocarbons (PAH). Representative systems of PAH (pyrene, coronene, ovalene and circumpyrene) with a density similar to soot and with varying degrees of crosslinking were built. Uniaxial tensile test simulations were carried out on the systems and the yield stress of each sample was calculated. The hardness was estimated from the yield stress using an empirical conversion constant and the obtained values were compared with nanoindentation experiments of soot particles. The results show that mature ethylene and diesel soot particles are expected to present a degree of crosslinking between 2.1–3.0 and 3.0–3.5, respectively, to have a value comparable to the hardness found experimentally. Finally, an MD simulation of nanoindentation of a particle of crosslinked coronene molecules provided an alternative means to compute the empirical constant used to convert the yield stress in hardness. These results reveal the importance of crosslinking reactions during soot maturation that give rise to a structure in which the majority of aromatics are aliphatically linked in a 3D network
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Aromatic penta-linked hydrocarbons in soot nanoparticle formation
A new crosslinking reaction between two pentagonal rings around the periphery of planar pericondensed aromatic molecules is proposed and its impact on soot nanoparticle formation explored. The reaction mechanism was computed, using density functional theory, between an aryl-type σ-radical on arim-based pentagonal ring attacking another rim-based pentagonal ring. A hydrogen migration allowed for the formation of a double bond forming a planar aromatic penta-linked hydrocarbon (APLH) complex, recently experimentally observed. The clustering of this planar species is compared with a pericondensed polyaromatic hydrocarbon (PCAH) and an aromatic aryl-linked hydrocarbon (AALH) using molecular dynamics and metadynamics. Similar clustering is found for the investigated species compared with a peri-condensed structure of similar mass indicating enhanced physical interactions after forming the crosslink. Finally, a further crosslink is possible between the unsaturated pentagonal ring sites forming an aromatic rim-linked hydrocarbon (ARLH) complex of considerable stability. This was con firmed by
simulating the stable molecular dynamics of such a complex with on-the-fly quantum forces from a quantum semi-empirical method, revealing possible
reactions under
ame conditions that might play a role in soot nucleation.National Research Foundation - Singapor
Low agreement for assessing the risk of postoperative deep venous thrombosis when deciding prophylaxis strategies: a study using clinical vignettes
BACKGROUND: Several clinical practice guidelines (CPG) on antithrombotic prophylaxis in surgical patients help to decide about the prophylaxis strategy based on the patient risk of deep venous thrombosis (DVT). However, the physician risk estimates of DVT could have little inter-observer reproducibility, which could lead to different individual prophylaxis practices. METHODS: Physicians were asked to evaluate DVT risk in eight clinical vignettes, describing actual patients cared for in our hospital. The vignettes included all possible levels of DVT risk. RESULTS: The degree of prophylaxis strategies accuracy was 63% (95% CI 523–75%). Overall agreement was 0.32 (z = 7.61, p < 0.001) and for each level of risk kappa was 0.38 (z = 6.50, p < 0.001); 0.1 (z = 1.65, p < 0.049) and 0.5 (z = 8.45, p < 0.001) for small, moderate and high risk group respectively CONCLUSIONS: Our results showed that there is poor agreement when physicians have to evaluate the risk for postoperative DVT, and in the cases of low and moderate risks of DVT there is the smallest agreement. In addition, the data also showed that the overall accuracy of DVT prophylaxis strategy was only moderate and the risk evaluation did not correlate to the selection of the strategy. The issue of inter-observers variability should be taken into account when CPG performance are analysed, especially when considering the risk-evaluation to choose the appropriate actions
Mechanical loading of tissue engineered skeletal muscle prevents dexamethasone induced myotube atrophy
Skeletal muscle atrophy as a consequence of acute and chronic illness, immobilisation, muscular dystrophies and aging, leads to severe muscle weakness, inactivity and increased mortality. Mechanical loading is thought to be the primary driver for skeletal muscle hypertrophy, however the extent to which mechanical loading can offset muscle catabolism has not been thoroughly explored. In vitro 3D-models of skeletal muscle provide a controllable, high throughput environment and mitigating many of the ethical and methodological constraints present during in vivo experimentation. This work aimed to determine if mechanical loading would offset dexamethasone (DEX) induced skeletal muscle atrophy, in muscle engineered using the C2C12 murine cell line. Mechanical loading successfully offset myotube atrophy and functional degeneration associated with DEX regardless of whether the loading occurred before or after 24Â h of DEX treatment. Furthermore, mechanical load prevented increases in MuRF-1 and MAFbx mRNA expression, critical regulators of muscle atrophy. Overall, we demonstrate the application of tissue engineered muscle to study skeletal muscle health and disease, offering great potential for future use to better understand treatment modalities for skeletal muscle atrophy
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