134 research outputs found
Portable implementation of a quantum thermal bath for molecular dynamics simulations
Recently, Dammak and coworkers (H. Dammak, Y. Chalopin, M. Laroche, M.
Hayoun, and J.J. Greffet. Quantumthermal bath for molecular dynamics
simulation. Phys. Rev. Lett., 103:190601, 2009.) proposed that the quantum
statistics of vibrations in condensed systems at low temperature could be
simulated by running molecular dynamics simulations in the presence of a
colored noise with an appropriate power spectral density. In the present
contribution, we show how this method can be implemented in a flexible manner
and at a low computational cost by synthesizing the corresponding noise 'on the
fly'. The proposed algorithm is tested for a simple harmonic chain as well as
for a more realistic model of aluminium crystal. The energy and Debye-Waller
factor are shown to be in good agreement with those obtained from harmonic
approximations based on the phonon spectrum of the systems. The limitations of
the method associated with anharmonic effects are also briefly discussed. Some
perspectives for disordered materials and heat transfer are considered.Comment: Accepted for publication in Journal of Statistical Physic
Characterization of tribological behavior of chromium and zinc coatings electrodeposited on a steel substrate
Three coatings of decorative chromium, hard chromium and zinc, electrolytically deposited on the C45 steel substrate, are considered in this study. Experimental characterization of the tribological response of the substrate and different coatings against spherical 100Cr6 counterbody is developed with a linear reciprocating tribometer. The results show that the three coatings have similar values for the stabilized coefficient of friction which remains substantially higher than the value of the friction coefficient for the substrate. The decorative chromium coating has the lowest wear resistance. A better wear resistance is obtained with the zinc coating. The first place in terms of wear resistance is occupied by the hard chromium coating and the uncoated substrate which have similar resistance.Досліджено трибологічну поведінку трьох видів електролітичних покривів: декоративного хрому, твердого хрому та цинку, сформованих на підкладці зі сталі C45. Використано трибометр за умов зворотно-поступального руху контртіла. Виявлено, що для всіх покривів значення стабілізованого коефіцієнта тертя однакові, однак, істотно вищі, ніж для підкладки. Покрив з декоративного хрому має найнижчий опір зношуванню, а вищу зносотривкість – цинковий. Найвища зносотривкість властива покриву з твердого хрому та підкладці без покриву.Исследовано трибологическое поведение трех видов электролитических покрывов: декоративного хрома, твердого хрома и цинка, сформированных на подкладке из стали C45. Использован трибометр при условиях возвратно-поступательного движения контртела. Обнаружено, что для всех покрывов значения стабилизированного коэффициента трения одинаковые, однако, существенно выше, чем для подкладки. Покрыв из декоративного хрома обладает наиболее низким сопротивлением изнашиванию, а цинковый наиболее износостойкий. Наивысшая износостойкость свойственна покрыву из твердого хрома и подкладке без покрыва
Potential energy threshold for nano-hillock formation by impact of slow highly charged ions on a CaF(111) surface
We investigate the formation of nano-sized hillocks on the (111) surface of
CaF single crystals by impact of slow highly charged ions. Atomic force
microscopy reveals a surprisingly sharp and well-defined threshold of potential
energy carried into the collision of about 14 keV for hillock formation.
Estimates of the energy density deposited suggest that the threshold is linked
to a solid-liquid phase transition (``melting'') on the nanoscale. With
increasing potential energy, both the basal diameter and the height of the
hillocks increase. The present results reveal a remarkable similarity between
the present predominantly potential-energy driven process and track formation
by the thermal spike of swift ( GeV) heavy ions.Comment: 10 pages, 2 figure
Étude du frittage non-conventionnel de céramiques de type YAG:Nd en présence d’ajout de silice
L’objectif de cette étude était de cerner l’influence du procédé de frittage non conventionnel employé pour la mise en forme des pièces (frittage SPS ou post-frittage HIP) sur l’activation des processus densifiants. Il s’avère qu’un traitement SPS des poudres initiales permet d’abaisser la température de début de frittage et conduit dans certaines conditions à des pièces translucides ou transparentes. L’influence de la silice, introduit comme ajout de frittage, et du néodyme, introduit comme dopant, est également discutée dans ce travail. Le post-frittage HIP, quant à lui, permet l’élimination de la porosité résiduelle dans les échantillons, ce qui conduit à la transparence des pièces réalisées
Crater formation by fast ions: comparison of experiment with Molecular Dynamics simulations
An incident fast ion in the electronic stopping regime produces a track of
excitations which can lead to particle ejection and cratering. Molecular
Dynamics simulations of the evolution of the deposited energy were used to
study the resulting crater morphology as a function of the excitation density
in a cylindrical track for large angle of incidence with respect to the surface
normal. Surprisingly, the overall behavior is shown to be similar to that seen
in the experimental data for crater formation in polymers. However, the
simulations give greater insight into the cratering process. The threshold for
crater formation occurs when the excitation density approaches the cohesive
energy density, and a crater rim is formed at about six times that energy
density. The crater length scales roughly as the square root of the electronic
stopping power, and the crater width and depth seem to saturate for the largest
energy densities considered here. The number of ejected particles, the
sputtering yield, is shown to be much smaller than simple estimates based on
crater size unless the full crater morphology is considered. Therefore, crater
size can not easily be used to estimate the sputtering yield.Comment: LaTeX, 7 pages, 5 EPS figures. For related figures/movies, see:
http://dirac.ms.virginia.edu/~emb3t/craters/craters.html New version uploaded
5/16/01, with minor text changes + new figure
Systematic pathway generation and sorting in martensitic transformations: Titanium alpha to omega
Structural phase transitions are governed by the underlying atomic
transformation mechanism; martensitic transformations can be separated into
strain and shuffle components. A systematic pathway generation and sorting
algorithm is presented and applied to the problem of the titanium alpha to
omega transformation under pressure. In this algorithm, all pathways are
constructed within a few geometric limits, and efficiently sorted by their
energy barriers. The geometry and symmetry details of the seven lowest energy
barrier pathways are given. The lack of a single simple geometric criterion for
determining the lowest energy pathway shows the necessity of atomistic studies
for pathway determination.Comment: 11 pages, 2 figure
Exploiting Mitochondrial Dysfunction for Effective Elimination of Imatinib-Resistant Leukemic Cells
Challenges today concern chronic myeloid leukemia (CML) patients resistant to imatinib. There is growing evidence that imatinib-resistant leukemic cells present abnormal glucose metabolism but the impact on mitochondria has been neglected. Our work aimed to better understand and exploit the metabolic alterations of imatinib-resistant leukemic cells. Imatinib-resistant cells presented high glycolysis as compared to sensitive cells. Consistently, expression of key glycolytic enzymes, at least partly mediated by HIF-1α, was modified in imatinib-resistant cells suggesting that imatinib-resistant cells uncouple glycolytic flux from pyruvate oxidation. Interestingly, mitochondria of imatinib-resistant cells exhibited accumulation of TCA cycle intermediates, increased NADH and low oxygen consumption. These mitochondrial alterations due to the partial failure of ETC were further confirmed in leukemic cells isolated from some imatinib-resistant CML patients. As a consequence, mitochondria generated more ROS than those of imatinib-sensitive cells. This, in turn, resulted in increased death of imatinib-resistant leukemic cells following in vitro or in vivo treatment with the pro-oxidants, PEITC and Trisenox, in a syngeneic mouse tumor model. Conversely, inhibition of glycolysis caused derepression of respiration leading to lower cellular ROS. In conclusion, these findings indicate that imatinib-resistant leukemic cells have an unexpected mitochondrial dysfunction that could be exploited for selective therapeutic intervention
Prognostic DNA methylation markers for sporadic colorectal cancer: a systematic review
Background Biomarkers that can predict the prognosis of colorectal cancer (CRC) patients and that can stratify high-risk early stage patients from low-risk early stage patients are urgently needed for better management of CRC. During the last decades, a large variety of prognostic DNA methylation markers has been published in the literature. However, to date, none of these markers are used in clinical practice. Methods To obtain an overview of the number of published prognostic methylation markers for CRC, the number of markers that was validated independently, and the current level of evidence (LoE), we conducted a systematic review of PubMed, EMBASE, and MEDLINE. In addition, we scored studies based on the REMARK guidelines that were established in order to attain more transparency and complete reporting of prognostic biomarker studies. Eighty-three studies reporting on 123 methylation markers fulfilled the study entry criteria and were scored according to REMARK. Results Sixty-three studies investigated single methylation markers, whereas 20 studies reported combinations of methylation markers. We observed substantial variation regarding the reporting of sample sizes and patient characteristics, statistical analyses, and methodology. The median (range) REMARK score for the studies was 10.7 points (4.5 to 17.5) out of a maximum of 20 possible points. The median REMARK score was lower in studies, which reported a p value below 0.05 versus those, which did not (p = 0.005). A borderline statistically significant association was observed between the reported p value of the survival analysis and the size of the study population (p = 0.051). Only 23 out of 123 markers (17%) were investigated in two or more study series. For 12 markers, and two multimarker panels, consistent results were reported in two or more study series. For four markers, the current LoE is level II, for all other markers, the LoE is lower. Conclusion This systematic review reflects that adequate reporting according to REMARK and validation of prognostic methylation markers is absent in the majority of CRC methylation marker studies. However, this systematic review provides a comprehensive overview of published prognostic methylation markers for CRC and highlights the most promising markers that have been published in the last two decades
Nonlinear Dynamics Analysis of FGM Shell Structures with a Higher Order Shear Strain Enhanced Solid-Shell Element
Progress on lead-free metal halide perovskites for photovoltaic applications: a review
ABSTRACT: Metal halide perovskites have revolutionized the field of solution-processable photovoltaics. Within just a few years, the power conversion efficiencies of perovskite-based solar cells have been improved significantly to over 20%, which makes them now already comparably efficient to silicon-based photovoltaics. This breakthrough in solution-based photovoltaics, however, has the drawback that these high efficiencies can only be obtained with lead-based perovskites and this will arguably be a substantial hurdle for various applications of perovskite-based photovoltaics and their acceptance in society, even though the amounts of lead in the solar cells are low. This fact opened up a new research field on lead-free metal halide perovskites, which is currently remarkably vivid. We took this as incentive to review this emerging research field and discuss possible alternative elements to replace lead in metal halide perovskites and the properties of the corresponding perovskite materials based on recent theoretical and experimental studies. Up to now, tin-based perovskites turned out to be most promising in terms of power conversion efficiency; however, also the toxicity of these tin-based perovskites is argued. In the focus of the research community are other elements as well including germanium, copper, antimony, or bismuth, and the corresponding perovskite compounds are already showing promising properties. GRAPHICAL ABSTRACT: [Image: see text
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