197 research outputs found
Annealing of defects in Fe after MeV Heavy ion irradiation
We report study of recovery dynamics, followed by in-situ resistivity
measurement after 100 MeV oxygen ion irradiation, in cold rolled Fe at 300K.
Scaling behavior with microstructural density and temperature of sample have
been used to establish stress induced defects formed during irradiation as a
new type of sink. The dynamics after irradiation has been shown to be due to
migration of defects to two types of sinks i.e. stress induced defect as
variable sinks and internal surfaces as fixed sinks. Experimental data obtained
under various experimental conditions have been fitted to theoretical curves.
Parameters thus obtained from fitting are employed to establish effect of
electronic energy loss and temperature on recovery dynamics and stress
associated with variable sinks.Comment: 12 pages, 7 figures. Europhysics Letter (in press
Phase Transition and Electrical Properties in Cs2SeO4.Te(OH)6
Dielectric investigations in the temperature and frequency 300- 600 K and 0,1KHz–13MHz, respectively, show that cesium selenate tellurate Cs2SeO4.Te(OH)6 (CsSeTe) exhibits two phase transitions at 490 and 525 K. The a.c. complex impedance measurements performed on CsSeTe material show an important level of conductivity at high temperature, attributed to the motion of H+ proton. This behavior is in agreement with the presence of the super- protonic phase transition in CsSeTe compound at 525K. This assignment was confirmed by the analysis of the M"/M"max spectra. The temperature dependences of ε΄r and tanδ indicate that the anomaly at 490K is attributed to a ferroelectric-paraelectric phase transition. Thermal analysis at high temperature, DSC, DTA, TG, Ms/z= 18 and Ms/z= 32 confirm the presence of the two transitions already cited, the temperature and the nature of the decomposition. Â
Size-dependent infrared properties of MgO nanoparticles with evidence of screening effect
We have investigated the infrared (IR) absorption properties of MgO nanoparticles (NPs) with the means of molecular dynamics simulations. Several size effects have been observed. We show in particular that the absorption of IR radiation does not occur predominantly through the polariton mode but preferentially through surface modes. This enhanced surface absorption is found to result from the absence of dielectric screening of the first atomic layer of the NPs. We demonstrate concomitantly that a macroscopic description of electrodynamics is inadequate to capture these unusual IR properties
Thermal analysis, X-ray diffraction, spectroscopy studies and magnetic properties of the new compound Tl2HAsO4.Te(OH)6
The Tl2HAsO4.Te(OH)6 (TlAsTe) compound crystallizes in the triclinic system P1 with unit cell parameters: a= 7.100(10) Å, b= 7.281(13) Å, c= 8.383(11) Å, α= 76.91(1)°, β= 87.16(1)°, γ= 66.96(2)°, Z= 2 and V= 388.19(1) Å3. This new structure can be described as a lamellar one with the atomic arrangement being built by planes of Te(OH)6 octahedra alterning with planes of arsenate tetrahedra. Raman and infrared spectra recorded at room temperature confirm the presence of As  and Te  groups and characterize the hydrogen bonds present in the crystal lattice. Differential scanning calorimerty shows the presence of three-phase transitions at 396 K, 408 K and 430 K present in the title compound. Typical thermal analyses, such as differential thermal analysis and thermogravimetry show that the decomposition of this material starts at about T= 445 K. Magnetization curve of Tl2HAsO4·Te(OH)6 substance have revealed a diamagnetic response overall temperature range studied
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
The one-dimensional organic–inorganic hybrid: catena-poly[bisÂ[1-(3-ammonioÂpropÂyl)-1H-imidazolium] [[iodidoplumbate(II)]-tri-μ-iodido-plumbate(II)-tri-μ-iodido-[iodidoplumbate(II)]-di-μ-iodido]]
The organic–inorganic hybrid, {(C6H13N3)2[Pb3I10]}n, was obtained by the reaction of 1-(3-ammonioÂpropÂyl)imidazolium triiodide and PbI2 at room temperature. The structure contains one-dimensional {[Pb3I10]4−}n polymeric anions spreading parallel to [001], resulting from face–face–edge association of PbI6 distorted octaÂhedra. One of the PbII cations is imposed at an inversion centre, whereas the second occupies a general position. N—H⋯I hydrogen bonds connect the organic cations and inorganic anions
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
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
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