Irradiation-induced heterogeneous nucleation in uranium dioxide

Using classical molecular dynamics simulations, we have studied the first stages of defect cluster formation resulting from 10 keV displacement cascades in uranium dioxide. Nanometre size cavities and dislocation loops are shown to appear as a result of the irradiation process. A specifically designed TEM experiment involving He implanted thin foils have also been carried out to support this modelling work. These results, in conjunction with several other observations taken from the literature of ion implanted or neutron irradiated uranium dioxide, suggest a radiation damage controlled heterogeneous mechanism for insoluble fission product segregation in UO₂.     

Acoustic crystallization and heterogeneous nucleation.

By focusing a high-intensity acoustic wave in liquid helium, we have observed the nucleation of solid helium inside the wave above a certain threshold in amplitude. The nucleation is a stochastic phenomenon. Its probability increases continuously from 0 to 1 in a narrow pressure interval around P(m) + 4.7 bars ( P(m) = 25.3 bars is the melting pressure where liquid and solid helium are in equilibrium). This overpressure is larger by 2 to 3 orders of magnitude than what had been previously observed. Our result strongly supports the recent suggestion by Balibar, Mizusaki, and Sasaki that, in all previous experiments, solid helium nucleated on impurities.     

Phase field theory of heterogeneous crystal nucleation

The phase field approach is used to model heterogeneous crystal nucleation in an undercooled pure liquid in contact with a foreign wall. We discuss various choices for the boundary condition at the wall and determine the properties of critical nuclei, including their free energy of formation and the contact angle as a function of undercooling. For particular choices of boundary conditions, we may realize either an analog of the classical spherical cap model or decidedly nonclassical behavior, where the contact angle decreases from its value taken at the melting point towards complete wetting at a critical undercooling, an analogue of the surface spinodal of liquid-wall interfaces.     

Slow crack propagation in heterogeneous materials

Statistics and thermally activated dynamics of crack nucleation and propagation in a two-dimensional heterogeneous material containing quenched randomly distributed defects are studied theoretically. Using the generalized Griffith criterion we derive the equation of motion for the crack tip position accounting for dissipation, thermal noise, and the random forces arising from the defects. We find that aggregations of defects generating long-range interaction forces (e.g., clouds of dislocations) lead to anomalously slow creep of the crack tip or even to its complete arrest. We demonstrate that heterogeneous materials with frozen defects contain a large number of arrested microcracks and that their fracture toughness is enhanced to the experimentally accessible time scales.     

Mechanoelectrical transformations in heterogeneous materials with piezoelectric inclusions

Mechanoelectrical transformations are studied on models with different geometries and piezoelectric inclusion concentrations. The electrical signal intensity is found to decrease with increasing depth of a piezoelectric source relative to the position of an electrical signal detector. The electrical signal is the difference between signals from the unlikely charged surfaces of the piezoelectric source being deformed by an acoustic excitation wave. The spectral amplitudes of electrical signals coming from different regions of one sample and from identical (in composition) samples containing a large amount of piezoelectric inclusions differ considerably. This difference is due to the random orientation of the quartz piezoelectric axes relative to the electrical detector. Therefore, nondestructive mechanoelectrical techniques for inspection of heterogeneous materials with piezoelectric inclusions must use amplitude-independent criteria.     

Elastic and plastic effects on heterogeneous nucleation and nanowire formation

We investigate theoretically the effects of elastic and plastic deformations on heterogeneous nucleation and nanowire formation. In the first case, the influence of the confinement of the critical nucleus between two parallel misfitting substrates is investigated using scaling arguments. We present phase diagrams giving the nature of the nucleation regime as a function of the driving force and the degree of confinement. We complement this analytical study by amplitude equations simulations. In the second case, the influence of a screw dislocation inside a nanowire on the development of the morphological surface instability of the wire, related to the Rayleigh-Plateau instability, is examined. Here the screw dislocation provokes a torsion of the wire known as Eshelby twist. Numerical calculations using the finite element method and the amplitude equations are performed to support analytical investigations. It is shown that the screw dislocation promotes the Rayleigh-Plateau instability.     

Effective conductivity of heterogeneous multiphase media with circular inclusions

Determining the effective conductivity of heterogeneous media is a central problem in different fields of physics. The medium considered here contains cylinders (inclusions) of random conductivities that are distributed at random in an embedding matrix. For random systems, widely encountered in applications, we derive an approximative analytical solution that applies to significantly denser configurations than Maxwell first-order approximations. The analytic solution is tested against accurate numerical simulations. The widely used effective medium approach is shown to be exact for symmetric conductivity distributions and quite accurate for asymmetrical cases.     

Effects of CNT inclusions on structure and dielectric properties of PVDF/CNT nanocomposites

ABSTRACT

The effects of multiwall carbon nanotube (CNT) inclusions on the crystalline structure of poly(vinylidene fluoride) (PVDF), and on the dielectric properties of PVDF/CNT nanocomposites (NCs), prepared by melt mixing, were investigated by employing X-ray diffraction, differential scanning calorimetry, and dielectric spectroscopy techniques. Our results imply that, in the NCs, the formation of β-phase crystals depends on specific compression treatment in the melt and fast cooling. Dielectric measurements on NCs, with CNT concentrations below the electrical percolation threshold, reveal that the dielectric strength of the two relaxation processes in the amorphous phase and dielectric permittivity, ?′, measured within the broad temperature range from ?150 °C to 60 °C, increase strongly with increasing CNT concentration. This enhancement of amorphous PVDF polarizability has been attributed to the increase of the local electric field, due to local polarization generated at the surface of conductive inclusions/CNT clusters.     

Effect of one-dimensional diffusion on void nucleation

Nucleation of voids via the stochastic accumulation of vacancies is considered when one-dimensionally migrating self-interstitials are present. A system instability signaling a non-equilibrium phase transition is found to occur when the mean free path of the one-dimensionally moving self-interstitials becomes comparable with the average distance between the voids at a sufficiently high void-number density. At this point, due to the exponential dependence of void nucleation probability on the net vacancy flux, the nucleation of voids is much more favored at the void lattice positions. At the same time, voids initially nucleated at positions where neighboring voids are non-aligned will shrink away. The shrinkage of non-aligned voids is caused entirely by the stochastic fluctuations in point-defect fluxes received by the voids. These two processes leave the aligned voids to form a regular lattice. The formation of the void lattice in this way can be accomplished at a void swelling of below 1%, in agreement with experimental observation. PACS 61.80.Az; 61.72.Cc; 05.65.+b     

Simulation of crack growth during fracture of heterogeneous materials

A cellular automaton model for describing the fracture of mechanically loaded heterogeneous materials has been constructed. Two extreme scenarios of the fracture process have been revealed, i.e., the dispersion (percolation) scenario, according to which defects accumulate uniformly throughout the volume of the material, and the correlated scenario (growth of predominantly a single source), which have been observed during the fracture of real materials. It has been shown that, in the case of the correlated fracture, a crack grows through the mechanism of ejection of double kinks of its front. In the intermediate case, the process occurs according to both scenarios: first, the slow accumulating (percolation) fracture and, then, the rapid correlated fracture; by the time the latter process begins, a self-organized critical state with a power-law size distribution of cracks typical of it has been formed.     

Dimple‐assisted dewetting: heterogeneous nucleation in undercooled wetting films

Undercooled wetting films near a first‐order wetting transition exhibit an unusually long lifetime: the thermal nucleation barrier for formation of a critical hole in a film of thickness F diverges according to Γ ∼ exp (ℰ_c/k_BT) where the excess free energy ℰ_c ∼ F^ζ with ζ ≥ 2. Localized perturbations of the liquid‐vapor interface (‘dimples’) are shown to be a useful tool in reducing Γ in a controlled way: they act as heterogeneous nucleation centers for thermal critical nuclei. For ⁴He wetting films on weak‐binding alkali substrates (Cs, Rb) dimples can be generated either by vortices in a superfluid film or by surface electrons. The theory of the heterogeneous nucleation process initiated by the presence of surface dimples (‘dimple‐assisted dewetting’) is developed, accompanied by quantitative predictions for experiment.     

Effect of ultrasound on heterogeneous systems

Examples of sonochemical reactions occuring under heterogeneous conditions are discussed with respect to their mechanism. The activation of a metal surface involves complex phenomena, in which parameters of importance seem to be the hardness and the adhesion of the passivating layer. Some considerations are given on the probable link between sonochemistry and tribochemistry.     

From homogeneous to heterogeneous nucleation of chain molecules under nanoscopic cylindrical confinement

The crystallization of monodisperse linear polyethylene confined in nanoporous alumina is investigated with the calorimetric measurements. We observe a drastic change in crystallization behavior, specifically nucleation, with a decrease in the pore diameter. Crystallization in relatively larger pores with the diameters of 62 and 110 nm occurs at lower temperatures within a very narrow range, whereas crystallization in smaller pores with diameters of 15-48 nm occurs at a higher and broad range of temperatures. Nucleation and crystallization kinetics in nanopores is discussed based on classical nucleation theory as well as the Avrami theory.     

Effect of an external electric field on two-dimensional nucleation

Summary Two-dimensional nuclei of different possible shapes are considered in the presence of an external electric field which is applied either perpendicular or parallel to the nucleation surface. The additional driving force needed for the formation of a nucleus due to the electric field is found and used in the nucleation theory. The results show that the electric field can either stimulate or inhibit the nucleation process depending on the ratio of the dielectric constants and the orientation of the applied electric field with respect to the nucleation surface. The dependence of the two-dimensional nucleation rate on the intensity of electric field is also determined.

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Riassunto Si considerano nuclei bidimensionali di differenti forme possibili in presenza di un campo elettrico esterno che è applicato sia perpendicolare che parallelo alla superficie di nucleazione. Si trova e si usa nella teoria di nucleazione l’ulteriore forza guida necesaria per la formazione di un nucleo a causa del campo elettrico. I risultati mostrano che il campo elettrico può sia stimolare che inibire il processo di nucleazione che dipende dal rapporto delle costanti dielettriche e dell’orientamento del campo elettrico applicato rispetto alla superficie di nucleazione. Si determina anche la dipendenza del valore di nucleazione bindimensionale dall’intensità del campo elettrico.

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Резюме Рассматриваются двумерные ядра различных возможиых форм в присутствии внешнего электрического поля, которое либо перпендикулярно, либо параллельно поверхности вародышеобразования. Определяется и используется в теории зародяшеобразования дополнительная движущая сила, необходимая для образования ядра, всл⪟дствие действия электрического поля. Полученные результатя показяают, что электрическое поле может либо стимулировать, либо тормозить процесс зародышеобразования в зависимости от отношения диэлектричеких постоянных и от ориентации приложенного электричеслог поля отнонительно поверхности зародышеодразования. Такзе определяется зависимость скорости образвания двумерных ядер от интенсивности электрического поля.

     

Athermal heterogeneous nucleation of freezing: numerical modelling for polygonal and polyhedral substrates

In a number of cases of practical interest, nucleation of freezing occurs at small supercooling, on potent nucleant substrates. Nucleation is then deterministic, consisting of the onset of free growth at a supercooling related only to substrate size and shape. Existing analytical treatments have evaluated the critical supercooling only for substrates in the form of a plane circle or a sphere. We develop a numerical treatment for a substrate of arbitrary shape. Coefficients are derived to permit straightforward evaluation of the critical supercooling for shapes typical of actual nucleant particles.