811 research outputs found
Structural transformations in porous glasses under mechanical loading. I. Tension
The evolution of porous structure and mechanical properties of binary glasses
under tensile loading were examined using molecular dynamics simulations. We
consider vitreous systems obtained in the process of phase separation after a
rapid isochoric quench of a glass-forming liquid to a temperature below the
glass transition. The porous structure in undeformed samples varies from a
connected porous network to a random distribution of isolated pores upon
increasing average glass density. We find that at small strain, the elastic
modulus follows a power-law dependence on the average glass density and the
pore size distribution remains nearly the same as in quiescent samples. Upon
further loading, the pores become significantly deformed and coalesce into
larger voids that leads to formation of system-spanning empty regions
associated with breaking of the material.Comment: 27 pages, 13 figure
Distributions of pore sizes and atomic densities in binary glasses revealed by molecular dynamics simulations
We report on the results of a molecular dynamics simulation study of binodal
glassy systems, formed in the process of isochoric rapid quenching from a
high-temperature fluid phase. The transition to vitreous state occurs due to
concurrent spinodal decomposition and solidification of the matter. The study
is focused on topographies of the porous solid structures and their dependence
on temperature and average density. To quantify the pore-size distributions, we
put forth a scaling relation that provides a robust data collapse in systems
with high porosity. We also find that the local density of glassy phases is
broadly distributed, and, with increasing average glass density, a distinct
peak in the local density distribution is displaced toward higher values.Comment: 22 pages, 6 figure
Structural transformations in porous glasses under mechanical loading. II. Compression
The role of porous structure and glass density in response to compressive
deformation of amorphous materials is investigated via molecular dynamics
simulations. The disordered, porous structures were prepared by quenching a
high-temperature binary mixture below the glass transition into the phase
coexistence region. With decreasing average glass density, the pore morphology
in quiescent samples varies from a random distribution of compact voids to a
porous network embedded in a continuous glass phase. We find that during
compressive loading at constant volume, the porous structure is linearly
transformed in the elastic regime and the elastic modulus follows a power-law
increase as a function of the average glass density. Upon further compression,
pores deform significantly and coalesce into large voids leading to formation
of domains with nearly homogeneous glass phase, which provides an enhanced
resistance to deformation at high strain.Comment: 25 pages, 12 figure
Evolution of the pore size distribution in sheared binary glasses
Molecular dynamics simulations are carried out to investigate mechanical
properties and porous structure of binary glasses subjected to steady shear.
The model vitreous systems were prepared via thermal quench at constant volume
to a temperature well below the glass transition. The quiescent samples are
characterized by a relatively narrow pore size distribution whose mean size is
larger at lower glass densities. We find that in the linear regime of
deformation, the shear modulus is a strong function of porosity, and the
individual pores become slightly stretched while their structural topology
remains unaffected. By contrast, with further increasing strain, the shear
stress saturates to a density-dependent plateau value, which is accompanied by
pore coalescence and a gradual development of a broader pore size distribution
with a discrete set of peaks at large length scales.Comment: 30 pages, 16 figure
Definition of thermal resistance of pipe walls of small thickness by thermal flows density change
The provisions basing structural dependence of thermal resistance of materials of pipe walls made of boiler steels in dimensional thickness where effects of bounder jumps of temperatures creating distortions in measurements of heat conductivity factor are shown are presented. The established dependences are explained from positions structural crack formation in undersurface areas
A note on Makeev's conjectures
A counterexample is given for the Knaster-like conjecture of Makeev for
functions on . Some particular cases of another conjecture of Makeev, on
inscribing a quadrangle into a smooth simple closed curve, are solved
positively
Definition of thermal resistance of pipe walls of small thickness by thermal flows density change
The provisions basing structural dependence of thermal resistance of materials of pipe walls made of boiler steels in dimensional thickness where effects of bounder jumps of temperatures creating distortions in measurements of heat conductivity factor are shown are presented. The established dependences are explained from positions structural crack formation in undersurface areas
Roughening of ion-eroded surfaces
Recent experimental studies focusing on the morphological properties of
surfaces eroded by ion-bombardment report the observation of self-affine
fractal surfaces, while others provide evidence about the development of a
periodic ripple structure. To explain these discrepancies we derive a
stochastic growth equation that describes the evolution of surfaces eroded by
ion bombardment. The coefficients appearing in the equation can be calculated
explicitly in terms of the physical parameters characterizing the sputtering
process. Exploring the connection between the ion-sputtering problem and the
Kardar-Parisi-Zhang and Kuramoto-Sivashinsky equations, we find that
morphological transitions may take place when experimental parameters, such as
the angle of incidence of the incoming ions or their average penetration depth,
are varied. Furthermore, the discussed methods allow us to calculate
analytically the ion-induced surface diffusion coefficient, that can be
compared with experiments. Finally, we use numerical simulations of a one
dimensional sputtering model to investigate certain aspects of the ripple
formation and roughening.Comment: 20 pages, LaTeX, 5 ps figures, contribution to the 4th CTP Workshop
on Statistical Physics "Dynamics of Fluctuating Interfaces and Related
Phenomena", Seoul National University, Seoul, Korea, January 27-31, 199
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