358 research outputs found
Structural properties of a calcium aluminosilicate glass from molecular-dynamics simulations: A finite size effects study
We study a calcium aluminosilicate glass of composition
(SiO)-(AlO)-(CaO) by means of
molecular-dynamics (MD) simulations, using a potential made of two-body and
three-body interactions. In order to prepare small samples that can
subsequently be studied by first-principles, the finite size effects on the
liquid dynamics and on the glass structural properties are investigated. We
find that finite size effects affect the Si-O-Si and Si-O-Al angular
distributions, the first peaks of the Si-O, Al-O and Ca-O pair correlation
functions, the Ca coordination and the oxygen atoms environment in the smallest
system (100 atoms). We give evidence that these finite size effects can be
directly attributed to the use of three-body interactions.Comment: 36 pages, 14 figures. Journal of Chem. Phys., in pres
Detection of the tagged or untagged photons in acousto-optic imaging of thick highly scattering media by photorefractive adaptive holography
We propose an original adaptive wavefront holographic setup based on the
photorefractive effect (PR), to make real-time measurements of acousto-optic
signals in thick scattering media, with a high flux collection at high rates
for breast tumor detection. We describe here our present state of art and
understanding on the problem of breast imaging with PR detection of the
acousto-optic signal
Theoretical study of Acousto-optical coherence tomography using random phase jumps on US and light
Acousto-Optical Coherence Tomography (AOCT) is variant of Acousto Optic
Imaging (called also ultrasonic modulation imaging) that makes possible to get
z resolution with acoustic and optic Continuous Wave (CW) beams. We describe
here theoretically the AOCT e ect, and we show that the Acousto Optic tagged
photons remains coherent if they are generated within a speci c z region of the
sample. We quantify the z selectivity for both the tagged photon eld, and for
the M. Lesa re et al. photorefractive signal
Hyperthermophily and the origin and earliest evolution of life
The possibility of a high-temperature origin of life has gained support based on indirect evidence of a hot, early Earth and on the basal position of hyperthermophilic organisms in rRNA-based phylogenies. However, although the availability of more than 80 completely sequenced cellular genomes has led to the identification of hyperthermophilic-specific traits, such as a trend towards smaller genomes, reduced proteinencoding gene sizes, and glutamic-acid-rich simple sequences, none of these characteristics are in themselves an indication of primitiveness. There is no geological evidence for the physical setting in which life arose, but current models suggest that the Earth’s surface cooled down rapidly. Moreover, at 100°C the half-lives of several organic compounds, including ribose, nucleobases, and amino acids, which are generally thought to have been essential for the emergence of the first living systems, are too short to allow for their accumulation in the prebiotic environment. Accordingly, if hyperthermophily is not truly primordial, then heat-loving lifestyles may be relics of a secondary adaptation that evolved after the origin of life, and before or soon after separation of the major lineages
Optical Detection of Ultrasound by Two-Wave Mixing in Photorefractive Semiconductor Crystals Under Applied Field
The optical detection of transient surface motion has many practical applications which include, in particular, the vibration monitoring of engineering structures (aircraft, power plants,...) and the detection of ultrasound produced by piezoelectric transducer or by pulse laser excitation. This last application where ultrasound is generated and detected by lasers, presents many advantages over conventional piezoelectric based methods. First, laser-ultrasonics is a remote sensing technique. Consequently it can be used, for example, for inspecting hot materials and products moving on a production line. Second, surfaces of complex shapes can also very easily be probed. For many applications, these advantages compensate the usually lower sensitivity of the laser-based technique compared to piezoelectric transduction
Back-translation for discovering distant protein homologies
Frameshift mutations in protein-coding DNA sequences produce a drastic change
in the resulting protein sequence, which prevents classic protein alignment
methods from revealing the proteins' common origin. Moreover, when a large
number of substitutions are additionally involved in the divergence, the
homology detection becomes difficult even at the DNA level. To cope with this
situation, we propose a novel method to infer distant homology relations of two
proteins, that accounts for frameshift and point mutations that may have
affected the coding sequences. We design a dynamic programming alignment
algorithm over memory-efficient graph representations of the complete set of
putative DNA sequences of each protein, with the goal of determining the two
putative DNA sequences which have the best scoring alignment under a powerful
scoring system designed to reflect the most probable evolutionary process. This
allows us to uncover evolutionary information that is not captured by
traditional alignment methods, which is confirmed by biologically significant
examples.Comment: The 9th International Workshop in Algorithms in Bioinformatics
(WABI), Philadelphia : \'Etats-Unis d'Am\'erique (2009
Biochar a partir de biomassa de Miscanthus e Quebracho, produzido em forno tubular em sistema aberto e sob fluxo de nitrogĂŞnio.
bitstream/item/141831/1/Comunicado-Tecnico-375-2015.pd
Molecular dynamics simulation of radiation damage in glasses
Molecular dynamics simulations of the ballistic effects arising from displacement cascades in glasses have been investigated in silica and in a SiO 2 -B 2 O 3 -Na 2 O glass. In both glasses the T-O-T′ angle (where T and T′ are network formers) diminishes, despite radiation causes opposite effects: while the ternary glass swells and silica becomes denser. We show that radiation-induced modifications of macroscopic glass properties result from structural change at medium/range, reflecting an increasing disorder and internal energy of the system. A local thermal quenching model is proposed to account for the effects of ballistic collisions. The core of a displacement cascade is heated by the passage of the projectile, then rapidly quenched, leading to a process that mimics a local thermal quenching. The observed changes in both the mechanical and structural properties of glasses eventually reach saturation at 2 10 18 α/g as the accumulated energy increases. The passage of a single projectile is sufficient to reach the maximum degree of damage, confirming the hypothesis postulated in the swelling model proposed by J.A.C. Marples
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