Página 1 dos resultados de 2421 itens digitais encontrados em 0.012 segundos

## Estudo do decaimento da susceptibilidade magnética em ligas amorfas ferromagnéticas; Study of magnetic susceptibility decay in ferromagnetic amorphous alloys

Sartorelli, Maria Luisa
Fonte: Biblioteca Digitais de Teses e Dissertações da USP Publicador: Biblioteca Digitais de Teses e Dissertações da USP
Tipo: Dissertação de Mestrado Formato: application/pdf
Relevância na Pesquisa
46.29%

## Luminescence effect in amorphous PLT

Freitas, GFG; Nasar, R. S.; Cerqueira, M.; Melo, DMA; Longo, Elson; Pizani, P. S.; Varela, José Arana
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 1175-1181
ENG
Relevância na Pesquisa
46.29%
Amorphous and crystalline powder of PLT phase was synthesized by using the Pechini method. Infrared (FTIR) analysis of the polymeric resin shows intense bands of organic materials from 250 to 1620 cm(-1). X-ray diffraction (XRD) and Raman spectra of calcined powder at different temperatures show amorphous phase at 450 degrees C/3 h, semi-crystalline phase at 550 degrees C/3 h and a crystalline phase at 800 degrees C/3 h. Luminescence effect was observed in amorphous powder calcined from 300 to 350 degrees/3 h with broad absorption peaks in 579 nm at 300 degrees C/3 h and 603 rum at 350 degrees C/3 h, respectively. The photoluminescence effect is attributed to emissions of Ti -> 0 directly from the oxygen 2p orbital (valence band) to the titanate 3d orbital (conduction bands). (c) 2004 Elsevier Ltd. All rights reserved.

## Photoluminescence in amorphous PLZ

Freitas, GFG; Nasar, R. S.; Cerqueira, M.; Melo, DMA; Longo, Elson; Pizani, P. S.; Varela, José Arana
Fonte: Elsevier B.V. Publicador: Elsevier B.V.
Tipo: Artigo de Revista Científica Formato: 793-799
ENG
Relevância na Pesquisa
46.29%
Amorphous and crystalline powder of PLZ was prepared by using the polymeric precursor method. TGA-DSC (Thermal analysis and Differential Scanning Calorimetry) shows the decomposition of polymeric resin, an amorphous phase and the crystallization of powder. Raman scattering of powder shows an amorphous and semicrystalline phase at 450 and 550 degreesC, respectively. XRD (X-ray diffraction pattern) of powder shows high crystallinity at 700 degreesC/3 h. PL (Photoluminescence) analysis of powder at 300 degreesC/3 h shows a broad asymmetric peak at 585 nm and increases of calcining time led to intense peaks of PL at 300 degreesC/6 h. This emission could be attributed to Zr --> O from the oxygen-2p orbitals to the zirconate-3d orbitals. (C) 2003 Elsevier Ltd and Techna S.r.l. All rights reserved.

## A generalized theory of electrical characteristics of schottky barriers for amorphous materials

Gupta, H. M.
Tipo: Artigo de Revista Científica Formato: 733-745
ENG
Relevância na Pesquisa
56.19%
In the present paper, we discuss a generalized theory of electrical characteristics for amorphous semiconductor (or insulator) Schottky barriers, considering: (i) surface states, (ii) doping impurity states at a single energy level and (iii) energetically distributed bulk impurity states. We also consider a thin oxide layer (≈10 Å) between metal and semiconductor. We develop current versus applied potential characteristics considering the variation of the Fermi level very close to contact inside the semiconductor and decrease in barrier height due to the image force effect as well as potential fall on the oxide layer. Finally, we discuss the importance of each parameter, i.e. surface states, distributed impurity states, doping impurity states, thickness of oxide layer etc. on the log I versus applied potential characteristics. The present theory is also applicable for intimate contact, i.e. metal-semiconductor contact, crystalline material structures or for Schottky barriers in insulators or polymers.

## Room-temperature photoluminescence in amorphous SrTiO3-The influence of acceptor-type dopants

Soledade, L. E B; Longo, Elson; Leite, E. R.; Pontes, F. M.; Lanciotti, F.; Campos, C. E M; Pizani, P. S.; Varela, José Arana
Tipo: Artigo de Revista Científica Formato: 629-632
ENG
Relevância na Pesquisa
46.34%
Room-temperature photoluminescence (PL) was observed in undoped and 2 mol% Cr-, Al- and Y-doped amorphous SrTiO3 thin films. Doping increased the PL, and in the case of Cr significantly reduced the associated PL wavelength. The optical bandgaps, calculated by means of UV-vis absorption spectra, increased with crystallinity and decreased with the doping level. It was considered that yttrium and aluminum substituted Sr2+, whereas chromium replaced Ti4+. It is believed that luminescence centers are oxygen-deficient BO6 complexes, or the same centers with some other defects, such as oxygen or strontium vacancies, or BO6 complexes with some other defects placed in their neighborhood. The character of excitation and the competition for negatively charged non-bridging oxygen (NBO) among numerous types of BO6 defect complexes in doped SrTiO3 results in various broadband luminescence peak positions. The results herein reported are an indicative that amorphous titanates are sensitive to doping, which is important for the control of the electro-optic properties of these materials. The probable incorporation of Cr into the Ti site suggests that the existence of a double network former can lead to materials displaying a more intense photoluminescence.

## Caracterização de materiais amorfos, através de medidas de GMI e GMI-FORC; Characterization of amorphous materials by GMI and GMI-FORC measurement

Lenina Alejandra Valenzuela Acuña
Fonte: Biblioteca Digital da Unicamp Publicador: Biblioteca Digital da Unicamp
Tipo: Tese de Doutorado Formato: application/pdf
Relevância na Pesquisa
46.41%
No nosso trabalho utilizamos o fenômeno da magnetoimpedância gigante (GMI) como uma ferramenta de pesquisa na caracterização de ferromagnetos amorfos. Estes materiais são muito moles do ponto de vista magnético, o que pode servir para diversas aplicações, tais como na construção de sensores e de dispositivos de alta frequencia. Na primeira parte da tese veremos como a GMI se torna uma técnica complementar na caracterização da cristalização em materiais amorfos. Na segunda parte da tese estudamos uma fita que apresenta histerese na GMI, de modo inédito utiliza-se o método de First Order Reversal Curves (FORC), em medidas da impedância. Os resultados apresentam um comportamento complexo, para os quais estamos propondo uma interpretação particular. Os materiais que mostram este comportamento podem ser úteis nas aplicações de, por exemplo, armazenamento de informação. Inicialmente, realizamos tratamento térmico convencional e por aquecimento Joule em fitas amorfas de composição Fe86Zr7Cu1B6 fabricadas pelo método melt spinning. Quando tratadas termicamente ocorre a cristalização de partículas a-Fe, e no geral o tamanho das partículas aumenta com o aumento da magnitude do tratamento (temperatura ou corrente). Nas medidas de magnetização vemos geralmente que a coercividade apresenta baixos valores e há um endurecimento nas amostras onde se observou um aumento no tamanho dos grãos no início da cristalização. Por outro lado...

## Consolidation of the Cu46Zr42Al7Y5 amorphous ribbons and powder alloy by hot extrusion

Melle,Ana Karla; Peres,Mauricio Mhirdaui; Bolfarini,Claudemiro; Botta,Walter José; Jorge Junior,Alberto Moreira; Kiminami,Claudio Shyinti
Fonte: ABM, ABC, ABPol Publicador: ABM, ABC, ABPol
Tipo: Artigo de Revista Científica Formato: text/html
Relevância na Pesquisa
46.29%
The amorphous Cu46Zr42Al7Y5 alloy presents large supercooled liquid region (ΔT X = 100 K), with a viscosity of about 10(6) N.s/m² where the material can flow as a liquid, making it possible an easy deformation in this temperature region. The aim of this work was to analyze processing routes to produce bulks of metallic glasses. Two kinds of materials were used: amorphous powders and ribbons, both were consolidated by hot extrusion in temperatures inside the range between Tg and Tx, with a ram speed of 1 mm/min and extrusion ratio of 3 : 1. Analysis of X-Ray Diffratometry (XRD), Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM), revealed that the proposed consolidation routes were effective to produce large bulks of amorphous materials, even with the strong decreasing of ΔT X observed after deformation by milling and during extrusion.

## Propagating mode-I fracture in amorphous materials using the continuous random network (CRN) model

Heizler, Shay I; Kessler, David A.; Levine, Herbert
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.34%
We study propagating mode-I fracture in two dimensional amorphous materials using atomistic simulations. We used the continuous random network (CRN) model of an amorphous material, creating samples using a two dimensional analogue of the WWW (Wooten, Winer & Weaire) Monte-Carlo algorithm. For modeling fracture, molecular-dynamics simulations were run on the resulting samples. The results of our simulations reproduce the main experimental features. In addition to achieving a steady-state crack under a constant driving displacement (which had not yet been achieved by other atomistic models for amorphous materials), the runs show micro-branching, which increases with driving, transitioning to macro-branching for the largest drivings. Beside the qualitative visual similarity of the simulated cracks to experiment, the simulation also succeeds in explaining the experimentally observed oscillations of the crack velocity.

## Size Effect on the Short Range Order in Amorphous Materials

Bloch, Leonid; Kauffmann, Yaron; Pokroy, Boaz
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.38%
Drawing inspiration from nature, where some organisms can control the short range order of amorphous minerals, we successfully manipulated the short range order of amorphous alumina by surface and size effects. By utilizing the Atomic Layer Deposition (ALD) method to grow amorphous nanometrically thin films, combined with state-of-the-art electron energy loss spectroscopy (EELS) and X-ray photoelectron spectroscopy (XPS), we showed experimentally that the short range order in such films is strongly influenced by size. This phenomenon is equivalent to the well-known size effect on lattice parameters and on the relative stability of different polymorphs in crystalline materials. We also show that the short range order changes while still in the amorphous phase, before the amorphous to crystalline transformation takes place.

## Nonequilibrium Thermodynamics of Amorphous Materials I: Internal Degrees of Freedom and Volume Deformation

Bouchbinder, Eran; Langer, J. S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.15%
This is the first of three papers devoted to the nonequilibrium thermodynamics of amorphous materials. Our focus here is on the role of internal degrees of freedom in determining the dynamics of such systems. For illustrative purposes, we study a solid whose internal degrees of freedom are vacancies that govern irreversible volume changes. Using this model, we compare a thermodynamic theory based on the Clausius-Duhem inequality to a statistical analysis based directly on the law of increase of entropy. The statistical theory is used first to derive the the Clausius-Duhem inequality. We then use the theory to go beyond those results and obtain detailed equations of motion, including a rate factor that is enhanced by deformation-induced noisy fluctuations. The statistical analysis points to the need for understanding how both energy and entropy are shared by the vacancies and their environments.; Comment: 7 pages. First of a three-part series

## Non-negligible Contributions to Thermal Conductivity From Localized Modes in Amorphous Silicon Dioxide

Lv, Wei; Henry, Asegun
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.41%
Thermal conductivity is an important property for almost all applications involving heat transfer, ranging from energy and microelectronics to food processing and textiles. The theory and modeling of crystalline materials is in some sense a solved problem, where one can now calculate the thermal conductivity of any crystalline line compound from first principles [1,2] using expressions based on the phonon gas model (PGM)[3,4]. However, modeling of amorphous materials still has many open questions, because the PGM itself becomes questionable when one cannot rigorously define the phonon velocities. New theories and methods are therefore needed to understand phonon transport in amorphous materials. In this letter, we used our recently developed Green-Kubo modal analysis (GKMA) method to study amorphous silica (a-SiO2). The predicted thermal conductivities exhibit excellent agreement with experiments at all temperatures and anharmonic effects are included in the thermal conductivity calculation for all types of modes in a-SiO2 for the first time. Previously, localized modes (locons) have been thought to have a negligible contribution to thermal conductivity, due to their highly localized nature, which conceptually should prevent them from moving energy to another location. However...

## Identification of structural motifs as tunneling two-level systems in amorphous alumina at low temperatures

Paz, Alejandro Pérez; Lebedeva, Irina V.; Tokatly, Ilya V.; Rubio, Angel
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.31%
One of the most accepted models that describe the anomalous thermal behavior of amorphous materials at temperatures below 1 K relies on the quantum mechanical tunneling of atoms between two nearly equivalent potential energy wells forming a two-level system (TLS). Indirect evidence for TLSs is widely available. However, the atomistic structure of these TLSs remains an unsolved topic in the physics of amorphous materials. Here, using classical molecular dynamics, we found several hitherto unknown bistable structural motifs that may be key to understanding the anomalous thermal properties of amorphous alumina at low temperatures. We show through free energy profiles that the complex potential energy surface can be reduced to canonical TLSs. The predicted tunnel splittings from instanton theory, the number density, dipole moment, and coupling to external strain of the discovered motifs are consistent with experiments.; Comment: 10 pages, 9 figures. Final version, accepted for publication in Physical Review B

## Stress effects on the Raman spectrum of an amorphous material: theory and experiment on a-Si:H

Strubbe, David A.; Johlin, Eric C.; Kirkpatrick, Timothy R.; Buonassisi, Tonio; Grossman, Jeffrey C.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.35%
Strain in a material induces shifts in vibrational frequencies, which is a probe of the nature of the vibrations and interatomic potentials, and can be used to map local stress/strain distributions via Raman microscopy. This method is standard for crystalline silicon devices, but due to lack of calibration relations, it has not been applied to amorphous materials such as hydrogenated amorphous silicon (a-Si:H), a widely studied material for thin-film photovoltaic and electronic devices. We calculated the Raman spectrum of a-Si:H \ab initio under different strains $\epsilon$ and found peak shifts $\Delta \omega = \left( -460 \pm 10\ \mathrm{cm}^{-1} \right) {\rm Tr}\ \epsilon$. This proportionality to the trace of the strain is the general form for isotropic amorphous vibrational modes, as we show by symmetry analysis and explicit computation. We also performed Raman measurements under strain and found a consistent coefficient of $-510 \pm 120\ \mathrm{cm}^{-1}$. These results demonstrate that a reliable calibration for the Raman/strain relation can be achieved even for the broad peaks of an amorphous material, with similar accuracy and precision as for crystalline materials.; Comment: 12 pages, 3 figures + supplementary 8 pages, 4 figures

## Two-dimensional defects in amorphous materials

Moshe, Michael; Sharon, Eran; Levin, Ido; Aharoni, Hillel; Kupferman, Raz
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.31%
We present a new definition of defects which is based on a Riemannian formulation of incompatible elasticity. Defects are viewed as local deviations of the material's reference metric field, $\bar{\mathfrak{g}}$, from a Euclidian metric. This definition allows the description of defects in amorphous materials and the formulation of the elastic problem, using a single field, $\bar{\mathfrak{g}}$. We provide a multipole expansion of reference metrics that represent a large family of two-dimensional (2D) localized defects. The case of a dipole, which corresponds to an edge dislocation is studied analytically, experimentally and numerically. The quadrupole term, which is studied analytically, as well as higher multipoles of curvature carry local deformations. These multipoles are good candidates for fundamental strain carrying entities in plasticity theories of amorphous materials and for a continuous modeling of recently developed meta-materials.; Comment: 5 pages, 3 figures

## Recent Developments in Computer Modeling of Amorphous Materials

Drabold, D. A.; Biswas, P.; Tafen, D.; Atta-Fynn, R.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.39%
In this paper, we review some recent work on amorphous materials using current "first principles" electronic structure/molecular dynamics techniques. The main theme of the paper is to emphasize new directions in the use of such ab initio methods. Some of these, being quite new, need development, but we believe have promise for solving new and important kinds of problems in the physics of glassy and amorphous materials. Initially, we discuss first principles calculations in broad outline and comment on the various approximations in common use. Then, we describe methods for forming a computer model of amorphous materials. This is an area of intense activity and methods beyond the obvious "quench from the melt" method are showing promise and utility. In this paper, we discuss a new method: "Decorate and Relax", and a new implementation of the Reverse Monte Carlo method. Finally, we discuss the computation of electronic properties, especially carrier transport and time evolution of electron states.; Comment: To Appear in "Noncrystalline Materials for Optoelectronics", M. Popescu, Ed. (INOE, Bucharest, 2004)

## Variable-amplitude oscillatory shear response of amorphous materials

Perchikov, Nathan; Bouchbinder, Eran
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
46.4%
Variable-amplitude oscillatory shear tests are emerging as powerful tools to investigate and quantify the nonlinear rheology of amorphous solids, complex fluids and biological materials. Quite a few recent experimental and atomistic simulation studies demonstrated that at low shear amplitudes, an amorphous solid settles into an amplitude- and initial conditions-dependent dissipative limit cycle, in which back-and-forth localized particle rearrangements periodically bring the system to the same state. At sufficiently large shear amplitudes, the amorphous system loses memory of the initial conditions, exhibits chaotic particle motions accompanied by diffusive behavior and settles into a stochastic steady-state. The two regimes are separated by a transition amplitude, possibly characterized by some critical-like features. Here we argue that these observations support some of the physical assumptions embodied in the nonequilibrium thermodynamic, internal-variables based, Shear-Transformation-Zone model of amorphous visco-plasticity; most notably that "flow defects" in amorphous solids are characterized by internal states between which they can make transitions, and that structural evolution is driven by dissipation associated with plastic deformation. We present a rather extensive theoretical analysis of the thermodynamic Shear-Transformation-Zone model for a variable-amplitude oscillatory shear protocol...

## Shear-Transformation-Zone Theory of Yielding in Athermal Amorphous Materials

Langer, J. S.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.15%
Yielding transitions in athermal amorphous materials resemble critical phenomena. Historically, they have been described by the Herschel-Bulkley rheological formula, which implies singular behaviors at yield points. In this paper, I examine this class of phenomena using an elementary version of the thermodynamic shear-transformation-zone (STZ) theory, focusing on the role of the effective disorder temperature, and paying special attention to scaling and dimensional arguments. I find a wide variety of Herschel-Bulkley-like rheologies but, for fundamental reasons not specific to the STZ theory, conclude that the yielding transition is not truly critical. In particular, there is a correlation length that grows rapidly, but ultimately saturates near the yield point.; Comment: 7 pages, 5 figures

## Particle displacements in the elastic deformation of amorphous materials: local fluctuations vs. non-affine field

Goldenberg, C.; Tanguy, A.; Barrat, J. -L.
Tipo: Artigo de Revista Científica
Relevância na Pesquisa
56.19%
We study the local disorder in the deformation of amorphous materials by decomposing the particle displacements into a continuous, inhomogeneous field and the corresponding fluctuations. We compare these fields to the commonly used non-affine displacements in an elastically deformed 2D Lennard-Jones glass. Unlike the non-affine field, the fluctuations are very localized, and exhibit a much smaller (and system size independent) correlation length, on the order of a particle diameter, supporting the applicability of the notion of local "defects" to such materials. We propose a scalar "noise" field to characterize the fluctuations, as an additional field for extended continuum models, e.g., to describe the localized irreversible events observed during plastic deformation.; Comment: Minor corrections to match the published version

## EXAFS Comparison of Crystalline/Continuous and Amorphous/Porous GaSb

Kluth, Susan; Johannessen, Bernt; Kluth, Patrick; Glover, Christopher; Foran, Garry J; Ridgway, Mark C