509 research outputs found
Drops and Bubble in Materials Science
The formation of extended p-n junctions in semiconductors by drop migration, mechanisms and morphologies of migrating drops and bubbles in solids and nucleation and corrections to the Volmer-Weber equations are discussed. Bubble shrinkage in the processing of glass, the formation of glass microshells as laser-fusion targets, and radiation-induced voids in nuclear reactors were examined
Fluoride glass: Crystallization, surface tension
Fluoride glass was levitated acoustically in the ACES apparatus on STS-11, and the recovered sample had a different microstructure from samples cooled in a container. Further experiments on levitated samples of fluoride glass are proposed. These include nucleation, crystallization, melting observations, measurement of surface tension of molten glass, and observation of bubbles in the glass. Ground experiments are required on sample preparation, outgassing, and surface reactions. The results should help in the development and evaluation of containerless processing, especially of glass, in the development of a contaminent-free method of measuring surface tensions of melts, in extending knowledge of gas and bubble behavior in fluoride glasses, and in increasing insight into the processing and properties of fluoride glasses
Precipitation and Crystal Growth from Solution
The precipitation of crystals from solution involves nucleation,
which is usually rapid and heterogeneous, and the subsequent
growth of the crystals. In order to measure the rate of the interface
growth process, the size of the crystals, as well as the rate
of incorporation of material into the crystals, must be measured,
and very small crystals must be studied to insure interface control.
For slightly soluble salts the order of the interface growth
process depends upon the stoichiometry of the salt and its supersaturation.
The interface growth coefficients for different salts,
orders of crystallization, and supersaturation are compared. These
coefficients · depend upon the type of salt crystallizing, but usually
not upon the order or supersaturation
Potentials in Ion-specific Electrodes
Ion-specific electrodes are ideal tools for studies involving
ionic solutions, since no separation of the components present
is necessary. Glass electrodes, typical representatives of these
systems, are amenable to theoretical treatment analogous to that
applied usually to ion-exchangers. Elements of the theory are
given and results of experimental tests obtained in aqueous
solutions and fused salts are discussed
Topologically disordered systems at the glass transition
The thermodynamic approach to the viscosity and fragility of amorphous oxides was used to determine the topological characteristics of the disordered network-forming systems. Instead of the disordered system of atoms we considered the congruent disordered system of interconnecting bonds. The Gibbs free energy of network-breaking defects (configurons) was found based on available viscosity data. Amorphous silica and germania were used as reference disordered systems for which we found an excellent agreement of calculated and measured glass transition temperatures. We reveal that the Hausdorff dimension of the system of bonds changes from Euclidian three-dimensional below to fractal 2.55 ± 0.05-dimensional geometry above the glass transition temperature
Short-time Dynamics of Percolation Observables
We consider the critical short-time evolution of magnetic and
droplet-percolation order parameters for the Ising model in two and three
dimensions, through Monte-Carlo simulations with the (local) heat-bath method.
We find qualitatively different dynamic behaviors for the two types of order
parameters. More precisely, we find that the percolation order parameter does
not have a power-law behavior as encountered for the magnetization, but
develops a scale (related to the relaxation time to equilibrium) in the
Monte-Carlo time. We argue that this difference is due to the difficulty in
forming large clusters at the early stages of the evolution. Our results show
that, although the descriptions in terms of magnetic and percolation order
parameters may be equivalent in the equilibrium regime, greater care must be
taken to interprete percolation observables at short times. In particular, this
concerns the attempts to describe the dynamics of the deconfinement phase
transition in QCD using cluster observables.Comment: 5 pages, 4 figure
Irradiation-induced Ag nanocluster nucleation in silicate glasses: analogy with photography
The synthesis of Ag nanoclusters in sodalime silicate glasses and silica was
studied by optical absorption (OA) and electron spin resonance (ESR)
experiments under both low (gamma-ray) and high (MeV ion) deposited energy
density irradiation conditions. Both types of irradiation create electrons and
holes whose density and thermal evolution - notably via their interaction with
defects - are shown to determine the clustering and growth rates of Ag
nanocrystals. We thus establish the influence of redox interactions of defects
and silver (poly)ions. The mechanisms are similar to the latent image formation
in photography: irradiation-induced photoelectrons are trapped within the glass
matrix, notably on dissolved noble metal ions and defects, which are thus
neutralized (reverse oxidation reactions are also shown to exist). Annealing
promotes metal atom diffusion, which in turn leads to cluster nuclei formation.
The cluster density depends not only on the irradiation fluence, but also - and
primarily - on the density of deposited energy and the redox properties of the
glass. Ion irradiation (i.e., large deposited energy density) is far more
effective in cluster formation, despite its lower neutralization efficiency
(from Ag+ to Ag0) as compared to gamma photon irradiation.Comment: 48 pages, 18 figures, revised version publ. in Phys. Rev. B, pdf fil
Thermodynamic parameters of bonds in glassy materials from viscosity-temperature relationships
Doremus's model of viscosity assumes that viscous flow in amorphous materials is mediated by broken bonds (configurons). The resulting equation contains four coefficients, which are directly related to the entropies and enthalpies of formation and motion of the configurons. Thus by fitting this viscosity equation to experimental viscosity data these enthalpy and entropy terms can be obtained. The non-linear nature of the equation obtained means that the fitting process is non-trivial. A genetic algorithm based approach has been developed to fit the equation to experimental viscosity data for a number of glassy materials, including SiO2, GeO2, B2O3, anorthite, diopside, xNa2O–(1-x)SiO2, xPbO–(1-x)SiO2, soda-lime-silica glasses, salol, and α-phenyl-o-cresol. Excellent fits of the equation to the viscosity data were obtained over the entire temperature range. The fitting parameters were used to quantitatively determine the enthalpies and entropies of formation and motion of configurons in the analysed systems and the activation energies for flow at high and low temperatures as well as fragility ratios using the Doremus criterion for fragility. A direct anti-correlation between fragility ratio and configuron percolation threshold, which determines the glass transition temperature in the analysed materials, was found
Heat capacity of liquids: an approach from the solid phase
We calculate the energy and heat capacity of a liquid on the basis of its
elastic properties and vibrational states. The experimental decrease of liquid
heat capacity with temperature is attributed to the increasing loss of two
transverse modes with frequency , where is liquid
relaxation time. In a simple model, liquid heat capacity is related to
viscosity and is compared with the experimental data of mercury. We also
calculate the vibrational energy of a quantum liquid, and show that transverse
phonons can not be excited in the low-temperature limit. Finally, we discuss
the implications of the proposed approach to liquids for the problem of glass
transition
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