Initial nucleation stages and properties of CuCl nanoparticles in glasses

The time variation and temperature dependence of a CuCl phase nucleation in a glass was studied by exciton spectroscopy. The phase formation kinetics at three temperatures was measured. A time delay in attaining a stationary rate of the new phase growth was observed at all temperatures, in agreement with the Zeldovich theory. The kinetic parameters of the CuCl phase formation were determined in the initial stage, when the critical nuclei possessing a zero surface energy (and an effective radius below 1.3 nm) appear in the glass matrix. The first-order phase transition in the new phase is 200 K below the melting temperature of CuCl single crystals. The temperature dependence of the CuCl phase nucleation rate reveals the second and third stages of the new phase formation. The activation energies for diffusion of the CuCl phase components in the glass matrix are determined.     

Exciton absorption at the initial stages of the formation of the CuCl phase in a glass

The fundamental absorption spectra of CuCl nanocrystals in glass samples are investigated in the energy range 3–4 eV at a temperature of 300 K with the aim of elucidating the kinetics of the initial stage of the formation of the CuCl phase in the glass. The CuCl phase is grown in the glass under stepwise annealing at temperatures of 500, 615, and 707°C. The kinetics of variation in the intensity and the shift of the maximum in the exciton absorption spectra of CuCl nanocrystals are studied in the course of annealing of the glass samples. It is established that, for all the temperatures under investigation, the formation of the CuCl phase begins with the transient stage that involves the fluctuation formation of supercritical nuclei of the CuCl nanomelt. At a temperature of 500°C, the transient stage gives way to the stage of a rapid increase in the number of supercritical nuclei of the CuCl phase. At temperatures of 615 and 707°C, the transient stage gives way to the stage of an intensive growth of nuclei without a considerable increase in their number. The number of nuclei formed during the transient stage at 707°C is smaller than that observed after the transient stage at 500°C by a factor of 24. However, the sizes of the nuclei formed at 707°C are larger than those observed after the transient stage at 500°C. This difference is explained by the fact that the diffusion length of Cu⁺ ions controlling the formation of the CuCl phase increases with increasing temperature. The experimental data on the kinetics of the formation of the new phase in the glass are in good agreement with the Zel’dovich-Frenkel classical theory of the formation of a new phase, which accounts for the stage of the formation of critical nuclei.     

Production and decay of subcritical nuclei in a solid solution

The decay of “subcritical” nuclei in a solid solution has been revealed in the investigation of the CuCl phase nucleation kinetics in glass. As soon as “supercritical” nuclei with an average radius R = 1.1 nm are created at 500°C, a sharp temperature increase up to 650°C transforms most nuclei created in the first annealing stage into “subcritical” ones, and this results in the decay of 80% of the nuclei in 5 min, while the remaining 20% of the nuclei grow in size to 2.4 nm. Their growth provides a sixfold increase in the CuCl phase growth rate against that in conventional annealing at 650°C. The kinetic dependences of the nucleation parameters—the amount of the phase and the average radius and concentration of the particles—were determined by the intrinsic absorption spectra of the CuCl nanocrystals. The critical radius of the CuCl nanomelt at 650°C has been estimated as 1.3 nm and the evaporation heat of the CuCl phase molecules in glass, as 13 kJ/mol. It is shown that multistage annealing makes it possible not only to control the parameters of the particles of the new phase, but also to determine the critical parameters of the initial nucleation stage.     

Formation of two distributions of nanoparticles of a new phase in a solid solution

Processes of the formation of the double distribution of CuCl nanoparticles in glass in two-stage annealing have been analyzed. The distribution of CuCl nanoparticles with the average radius R ₁ = 14 and 18 nm appears in 20 and 40 min at temperature T ₁ = 700°C in samples 1 and 2, respectively. By this time, the formation of new clusters ends, and only the growth of previously formed clusters occurs (the second stage of nucleation). Then, the formation of the CuCl phase continues at T ₂ = 500°C. A decrease in the temperature is accompanied by a decrease in the critical radius of particles of a new phase. For this reason, the formation and growth of new CuCl nuclei begin again and the second distribution with a mean radius of 1 nm or larger is formed. As a result, double distributions of CuCl nanoparticles with significantly different mean radii are formed in samples 1 and 2. The concentration and mean radius of CuCl particles in distributions have been determined from the optical absorption spectra of CuCl nanocrystals at 80 K in the wavelength range of 300–500 nm.     

Effect of preliminary low-temperature annealing on the kinetics of nucleation

The changes observed in the kinetics of CuCl nucleation in glass due to preliminary low-temperature annealing have been investigated using optical spectroscopy. The influence of the number of nuclei formed at a temperature of 500°C on the growth rate of the CuCl phase at 650°C has been examined. The amount of the CuCl phase in the sample is determined from the optical absorption coefficient in the range of band-to-band transitions in the CuCl nanocrystals. The average radius of CuCl particles is calculated from the position of the maximum of the exciton band. It has been demonstrated that the preliminary formation of CuCl nuclei at 500°C for 3 h makes it possible to increase the growth rate of the CuCl phase by a factor of 6. In the sample with preliminarily formed nuclei, there occur two opposite processes: thermal decomposition of part of the small nuclei, which have become subcritical at 650°C; and the growth of larger nuclei, which have become supercritical at 650°C due to the diffusive inflow of the components of the new phase. When the equilibrium concentration is reached, the new phase contains particles with a smaller radius but at a higher concentration as compared to those in the case of the conventional nucleation at 650°C.     

Nonisothermal nucleation in the CuCl solid solution in glass: Dissolution of subcritical CuCl nuclei with a positive jump of the nucleation temperature

The processes of nucleation in a CuCl solid solution in glass with a positive temperature jump from T ₁ = 500°C to T ₂ = 550, 600, and 650°C have been investigated using optical spectroscopy and exciton-thermal analysis. The dissolution of a part of the particles of the CuCl nanomelt formed previously at T ₁ has been observed at T ₂. Variations in the shape of the radius distribution curve of the CuCl particles due to the dissolution of initial nuclei have been determined from the melting kinetics of CuCl particles during linear heating of the sample. The nonisothermal nucleation of CuCl in glass under conditions of the temperature jump has been simulated numerically. The results of calculations of the variations in the radius distribution of CuCl particles are in agreement with the experiment. The calculated data on the variation in time of the critical radius r _c and the concentration of CuCl monomers in glass after the temperature jump have been obtained.     

Decline of nucleation in the heating process with a high heating rate

The effect of the heating rate on the nucleation of metallic glass in a rapid heating process starting from the glass transition temperature is investigated. The critical nucleus radius increases with the increase of the temperature of the undercooling liquid. If the increment rate of the critical nucleus radius, owing to the heating process, is higher than the growth rate of the nuclei, the nuclei generated at the low temperature will become the embryos at the high temperature. This means that the high heating rate can make no nucleation happen in the heating process. In consideration of the interfacial energy, the growth rate of the nuclei increases with the increase of their size and the growth rate of the critical nucleus is zero. Thus, the lower heating rate can also make the nuclei decline partially. Finally, this theory is used to analyze the nucleation process during laser remelting metallic glass.     

Influence of selective heating on the kinetics of the late stage of first-order phase transitions

The effect of radiation on the kinetics of the late stage of first-order phase transitions (Ostvald ripening) was studied. In particular, features of the growth kinetics of nuclei were revealed in relation to their size and time of exposure to radiation. Possible approaches to controlling the nucleus growth kinetics were shown.     

Size effects in the exciton energy and first-order phase transitions in CuCl nanocrystals in glass

The absorption spectra and the melting and crystallization kinetics of CuCl nanocrystals in glass are investigated in the range of particle radii 1–30 nm. Three discontinuities are found on the curves representing the size dependence of the melting point T _m(R) and the crystallization point T _c(R). As the particle radius gradually decreases from 30 nm in the range R⩽12.4 nm there is a sudden 60° drop in the temperature T _c in connection with the radius of the critical CuCl nucleus in the melt. A 30° drop in T _m is observed at R=2.1 nm, and a second drop of 16° in the temperature T _c is observed for CuCl particles of radius 1.8 nm. The last two drops are associated with changes in the equilibrium shape of the nanoparticles. In the range of smaller particles, R⩽1.34 nm the T _c(R) curve is observed to merge with the T _m(R) curve, owing to the disappearance of the work of formation of the crystal surface during crystallization of the melt as a result of the zero surface tension of CuCl particles of radii commensurate with the thickness of the effective surface layer. An increase in the size shift of the exciton energy is observed in this same range of CuCl particle radii (1–1.8 nm). The size dependence of the melting and crystallization temperatures of the nanoparticles is attributed to variation of the free energy in the surface layer of a particle. Fiz. Tverd. Tela (St. Petersburg) 41, 310–318 (February 1999)     

Kinetics of the formation and growth of critical nuclei in nanostructured films of borides, nitrides, and silicides

The kinetics of formation and growth of critical nuclei in nanostructured films of borides, nitrides, and silicides grown by ion deposition methods has been studied using X-ray diffraction, electron microscopy, and secondary-ion mass spectrometry. The relationships have been obtained allowing not only the calculation of the critical nucleus size, the rate of their formation and growth in the films, but also the prediction of these parameters based on the data on adhesion, supersaturation, and elastic characteristics of grown phases.     

Use of (<Superscript>3</Superscript>He, <Emphasis Type="Italic">t</Emphasis>) charge-exchange reactions in determining radii of excited states of nuclei

A method for determining the radii of excited states of nuclei by means of (³He, t) charge-exchange reactions was proposed. Two versions of a comparison of differential cross sections for (³He, t) reactions were considered. The first relies on a comparison with cross sections for inelastic-scattering processes leading to the formation of isobaric analog states, while the second involves (³He, t) reactions leading to the production of the ground state. The two versions in question yield similar results and make it possible to determine the radius of the first excited state of the ¹³N nucleus. This state has the excitation energy of E* = 2.37 MeV, lying above the proton-emission threshold. The resulting radius proved to be enhanced in relation to the ground state and is close to the radius of the 3.09-MeV isobaric analog state of the ¹³С nucleus, which has a neutron halo. This permitted drawing the conclusion that the ¹³N nucleus in the 2.37-MeV state has a proton halo. The possibility of revealing a proton halo in other states of light nuclei is considered.     

Nucleation kinetics in a CuCl solid solution in glass: Calculation and comparison with experiment

The kinetics of formation of CuCl nanoparticles in a glass has been studied. The experimental results obtained have been compared with the results of calculations. A method has been developed for calculating the nucleation kinetics, which decreases the time of calculations by a factor of several tens. This has been achieved using the joint kinetic equation for distributions of clusters over the number of particles and over the radius. The distributions over the number of particles and over the radius have been used for small and large clusters, respectively. The concentration of molecules near the surface of clusters has been determined from the asymptotic solution of the diffusion equation. For subcritical clusters, the concentration of molecules near the cluster surface has been taken to be equal to the average concentration in the solid solution. This method has been used to calculate the nucleation kinetics of CuCl nanoparticles in a glass. The results obtained from the calculation of the time dependences of the increase in the concentration and average radius of clusters agree well with experiment.     

Growth, structure, and morphological stability of nuclei growing from eutectic melts

Kinetics of self-consistent growth of new-phase nuclei from eutectic melts have been studied. The growth kinetics of nuclei of eutectic composition are shown to depend on the sum of supersaturations over all components. It has been established that nuclei in a eutectic melt reach a common critical radius determined by supersaturation for both components. The concept of a diffusion dipole is introduced: a two-phase object in a eutectic melt or solid solution, in which two nuclei of different composition are related through a common diffusion field. The morphological stability of such dipoles is studied. It is found that a nucleus of eutectic composition is more stable with respect to small fluctuations in its shape than the corresponding one-component nucleus. It is shown that perturbations resulting in dipole-shape distortions develop perpendicular to the axis connecting the centers of the nuclei of different compositions (i.e. the dipole axis). This is consistent with the well-known experimental observation of the layered structure of eutectic compositions. Fiz. Tverd. Tela (St. Petersburg) 39, 1464–1469 (August 1997)     

Nonisothermal Nucleation in the Solid Solution CuCl in Glass: Temperature Range of the Formation of Supercritical Nuclei of CuCl Phase

Physics of the Solid State - It is established both experimentally and by calculations that the nuclei of CuCl phase do not form in the solid solution of CuCl in the glass at the temperature Tu...     

Kinetics of pore formation upon plastic deformation of crystals with a cesium chloride structure

The kinetics of pore formation upon plastic deformation of crystals with a cesium chloride structure was studied as the first stage of a first-order phase transition in a deformable media. The shape and the critical dimensions of microvoids depending on the critical shear stress were found. The number of critical microvoids per unit volume arising per unit time on the surface of a slip plane was determined based on the kinetics of formation of pairs of charged vacancies. The volume fraction of pores at the initial stage of plastic deformation in CsBr and CsI crystals was estimated.     

Effect of electron irradiation on the oxidation of indium

The oxidation kinetics of indium is studied for two cases, namely, during continuous electron irradiation (E _p = 1800 eV) and without electron irradiation, as a function of the time of exposure to an oxygen medium at a partial oxygen pressure of 10⁻⁴ Pa and room temperature. The initial oxygen exposure was 50 L. The kinetic curves recorded upon continuous electron irradiation have two inflection points, and they can be attributed to the following three states of oxidation: physical adsorption, chemisorption with the formation of a nonstoichiometric oxide layer, and the growth of a homogeneous oxide layer. Only the first inflection point is observed during oxidation without electron irradiation, and further exposure does not lead to the second inflection point within the experimental time.     

On the theory of transient nucleation at the intermediate stage of phase transitions

The evolution of a system of growing aggregates in a macroscopically homogeneous medium with account of both the reduction in metastability and the continuing initiation of new nuclei is studied. The corresponding integro-differential model describing the intermediate stage of phase transitions is solved analytically for arbitrary nucleation kinetics and growth rates of nuclei. An exact solution of the Fokker–Planck equation is found with allowance for the diffusivity along the axis of nucleus radii. In limiting cases of purely kinetic and mixed kinetic-diffusion rates of crystal growth for a special form of diffusivity, the obtained solutions transform to earlier known expressions.     

Kinetic study of phase transformation of n-octane using hydrothermal diamond anvil cell

A kinetic study of phase transformation of n-octane has been performed using a hydrothermal diamond anvil cell. The results show that pressure has a negative effect on the solid–liquid reaction rate. The increase of pressure can accelerate the liquid–solid transformation rate. Upon the liquid–solid transformation, the light transmittance showed a decreased trend with time in the early stage, which was caused by the formation of a large quantity of crystal nuclei. In the later stage, the light transmittance almost remained the same, thus indicating a growth stage of crystal nuclei. The activation volume yields a value of 2.16×10^?5 and –1.35×10^?5 m³/mol for the solid–liquid and liquid–solid transformations. Based on the obtained activation energy, the solid–liquid transformation is dominated by the interfacial reaction and diffusion, and the liquid–solid transformation is controlled by diffusion. This technique is an effective and powerful tool for the transformation kinetics study of n-octane.     

Line tension at fluid membrane domain boundaries measured by micropipette aspiration

Line tension is a determinant of fluid phase domain formation kinetics and morphology in lipid bilayer membranes, which are models for biological membrane heterogeneity. We describe the first direct measurement of this line tension by micropipette aspiration. Our data are analyzed with a model that does not rely on independently measured (and composition dependent) secondary parameters, such as bending stiffness or membrane viscosities. Line tension is strongly composition dependent and decreases towards a critical consolute point in a quasiternary room temperature phase diagram.     

Towards the determination of the charge radius of 11Li by laser spectroscopy

Isotope shift measurements by means of laser resonance ionization spectroscopy are a unique tool to determine the charge radii of halo nuclei. The most prominent halo nucleus ¹¹Li is at the same time the best accessible candidate for such studies. The experimental method to determine the charge radius of this exotic nucleus and first test results on ⁷Li will be presented in this paper. This revised version was published online in August 2006 with corrections to the Cover Date.