Influence of the interphasal surface energy on the stability of solid disperse systems

The extension of the size-distribution function of dispersed particles and the dependence of their individual radii on the time permitted going over to the solution of a problem on the stability of different dispersed systems in its more general formulation. It is shown that the conditions of growth, solution, and transformation of the size distribution of microparticles are substantially determined by the kind of dependence of the interphasal surface energy on the radius. Possibilities of the experimental detection of established, unknown earlier, kinetic regularities of the growth and solution of microparticles are discussed.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 7–12, November, 1978.     

Morphogenesis of distributions of microparticles by dimensions in the coarsening of dispersed systems

For a wide class of dispersed systems, a basis is given for the possibility of a complete theoretical analysis of their evolution, owing to Ostwald coagulation of microparticles. Theoretical solutions are obtained to describe distributions of microparticles by dimensions with positive and negative asymmetry, kinetic dependences, and lifetimes of the microparticles of any given dimension, taking into account various mechanisms of their reactive interaction with the substance of the medium. In an example of change of the morphological indicators of experimental histograms of microparticles of Al₃Mg₂ under isothermal heating of the aluminum-magnesium alloy, their correspondence is shown to the nature of the transformation of the theoretical distribution functions.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 53–58, December, 1990.     

On the technique for structural analysis of solid dispersed systems

Analytical relationships are proposed for performing a system analysis of structural coarsening of microparticles of a dispersed phase. For this purpose, a method is developed for revealing the similarities and differences between characteristics of the experimental histogram and the theoretical distribution of microparticles over sizes. This approach to the identification of distributions makes it possible to establish a correlation between the specific features of the transformation of the experimental distributions (histograms) and intrasystem processes.     

Nondiffusive coagulation of microparticles in solid-state dispersions

An expression is derived for the size distribution of dispersed particles and other relations are derived as well, on the basis of a more general equation for the rate of change of the upper and lower limits of the size spectrum. An analysis of these relations reveals the characteristics of transition between two processes, namely from formation of a dispersed phase (crystallization) to coagulation of particles through a disperse system where no coagulation of microparticles occurs. It is shown that in many real disperse systems (dispersion-hardened alloys) microparticles grow by the mechanism of nondiffusive coagulation, the latter being limited by the rate at which atoms build up at the boundaries of particles.     

Analysis of EXAFS data of complex systems

Analysis of EXAFS data of complex systems containing more than one phase and one type of coordination, has been discussed. It is shown that a modified treatment of EXAFS function as well as the amplitude ratio plots provide useful means of obtaining valuable structural information. The systems investigated are: biphasic Ni+NiO mixture, NiAl₂O₄ with two coordinations for Ni, NiO+NiAl₂O₄ mixture, CoS+CoO system and Ni dispersed on Al₂O₃. The results obtained with these systems have been most satisfactory and serve to illustrate the utility and the applicability of the innovations described in this paper.Contribution No. 522 from Solid State and Structural Chemistry Unit     

Ultrasound-assisted fast encapsulation of metal microparticles in SiO2 via an interface-confined sol-gel method

Although the traditional Stoˇber process-based methods are widely used for encapsulation of metal nanoparticles in SiO₂, these time-consuming methods are not effective for coating metal microparticles with a uniform SiO₂ layer of desired thickness. Herein, an ultrasound-assisted, interface-confined sol–gel method is proposed for fast encapsulation of metal microparticles in SiO₂, and the encapsulation of Sn microparticles is chosen as an example to illustrate its feasibility. The proposed method involves covering metal microparticles with liquid films that contain water, alcohol, surfactant (Span-80) and catalyst (NH₄F) and then ultrasonically dispersing these particles into cyclohexane, where tetraethylorthosilicate (TEOS) is added. To ensure the hydrolysis-condensation reactions of TEOS occurring at the particle-cyclohexane interface so that the formed SiO₂ is coated on the particles, the microparticles should be well dispersed into cyclohexane with the liquid films being not broken away from their surfaces. It is found that the assistance of probe sonication and the addition of surfactant are crucial to achievement of a good dispersion of metal microparticles in cyclohexane. And using high-viscosity alcohol (namely glycerol), controlling the volume ratio of water to alcohol and the amount of water, and choosing a suitable ultrasonic power are essential for preventing the formation of free SiO₂ (namely SiO₂ that is not coated on the particles), which is a result that the liquid films escape from the particle surfaces under ultrasonic cavitation. Our results have also revealed that the thickness of SiO₂ layer can be adjusted by changing the reaction time or the total amount of water. In particular, the thickness of SiO₂ layer can be easily raised by simply repeating the encapsulation procedure. Compared with the traditional Stoˇber process-based methods, the proposed method is time-saving (reaction time: about 30 min vs. more than 12 h) and extremely effective for coating microparticles with a continuous, uniform SiO₂ layer of desired thickness.     

分散相颗粒几何因素对电流变液体反应时间的影响

瞬变的反应时间是电流变液体的一个重要参数．本文通过实验和理论分析，建立了考虑分散相颗粒几何因素的电流变反应时间模型根据此模型，长椭球状颗粒的电流变液体反应时间比等效球状颗粒的短，扁椭球状和片状颗粒的电流变液体反应时间比等效球状颗粒的长，而棒状颗粒的电流变液体反应时间则视ε_p/ε_f比值大小或快于或慢于等效球状颗粒的反应时间．且反应时间与分散相尺寸的关系是（１）随体积增大，各种形状颗粒的电流变液体反应时间均变短；（２）同体积下，随长径比增大，长椭球状颗粒的电流变液体反应时间变短，而扁椭球状颗粒和片状颗粒的电流变液体反应时间变长，棒状颗粒的电流变液体反应时间则视。ε_p/ε_f比值大小或增快或减慢．因此，要获得反应快的电流变液体，颗粒形状除选择球状外，还可在使雷诺数较小范围内，选择一定尺寸的长椭球状或棒状颗粒。 关键词：     

Thermal equilibrium in composites

The features of the establishment of thermal equilibrium between phases are investigated for an arbitrary P-component composite, and the characteristic time taken for the temperatures to equalize is calculated. The problem of how the distribution of the impurity phase over the dimensions affects the time behavior of the temperature of the composite is solved. Using a Laplace transformation, a general expression is obtained for σT₀(t) for the particles of a finely dispersed phase having a Poisson distribution, and the specific form of this functional relationship is completely calculated. N. N. Semenov Institute of Chemical Physics, Russian Academy of Sciences. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 15–19, October, 1998.     

An analysis of the change in the Al3Mg2 microparticle size distribution upon heating of an aluminum-magnesium alloy

Ostwald coarsening of dispersed Al₃Mg₂ particles distributed in an aluminum-magnesium alloy (80% Al+20% Mg) is used to illustrate a method of determining the similarities and differences between an experimental histogram (from an image) and a corresponding theoretical distribution, chosen from a collection of different solutions. The determination of a link between the indicators in experimental histograms and the processes generating the transformation is demonstrated. In particular, solubility and growth of Al₃Mg₂ microparticles upon heating of an alloy strongly depend on the variations in structure and properties of material near interphase boundaries. Zaporozhian State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 8–13, February, 1993.     

Optimal oscillation-center transformations

Systems with Hamiltonians of the form H₀(p) + H₁(q,p,t) are considered. A variational principle is proposed for defining that canonical transformation, continuously connected with the identity transformation, which minimizes the residual, coordinate-dependent part of the new Hamiltonian. The principle is based on minimization of the mean square generalized force over phase space and time. The transformation reduces to the action-angle transformation in that part of the phase space of an integrable system where the orbit topology is that of the unperturbed system, or on primary KAM surfaces. General arguments in favour of this definition are given, based on Galilean invariance, decay of the Fourier spectrum, and its ability to include external fields or inhomogeneous systems. The optimal oscillation-center transformation for the physical pendulum (or particle in a sinusoidal potential) is constructed analytically. A modified principle for relativistic systems is presented in an appendix.     

Observation of carbon‐facilitated phase transformation of titanium dioxide forming mixed‐phase titania by confocal Raman microscopy

Confocal Raman microscopy, a relatively new and advanced technique, is found to be suitable for imaging the chemical morphology below the submicrometer scale. It has been employed to probe the phase transformation of carbon‐containing titania (TiO₂) nanopowder and titania thin film subjected to laser annealing. The observation of phase transformation from the anatase phase to the rutile phase at high laser power annealing is attributed to carbon inclusion inside or on the surface of titania. Upon annealing, carbon could react with the oxygen of titania and create oxygen vacancies favoring the transformation from the anatase to the rutile phase. This study provides evidence for the carbon‐assisted phase transformation for creating carbon‐containing mixed‐phase titanium dioxide by laser annealing. We explicitly focus on the presence of carbon in the phase transformation of TiO₂ using confocal Raman microscopy. In all of the investigated samples, mixed anatase/rutile phases with carbon specifically was found at the rutile site. X‐ray diffraction (XRD), scanning electron microscopy (SEM) and energy‐dispersive spectroscopy (EDS) studies have been performed in addition to Raman mapping to verify the mixed‐phase titania formation. Copyright © 2010 John Wiley & Sons, Ltd.     

Enhanced Near‐Infrared Shielding and Light Scattering Using Surface‐Roughened Hybrid Hollow Microparticles Synthesized with Polymer and TiO2@Al(OH)3 for Cosmetic Applications

Some materials and their micro‐/nanostructures are explored to shield near‐infrared (NIR) light. However, the structural role of polymeric matrices in terms of the sensitivity to NIR light and the scattering/absorption characteristics of particles bearing inorganic colloids lack understanding. To understand this issue further, a polymer–inorganic hybrid microparticle is synthesized, where submicrometer‐sized TiO₂ core‐thin aluminium hydroxide shell colloids (TiO₂@Al(OH)₃) are dispersed in a roughened polymer hollow particle matrix. They exhibit higher light extinction at NIR frequencies and higher light scattering efficiencies in the NIR regions compared to hybrid solid microparticles and a simple mixture of inorganic and polymer hollow microparticles. Owing to these characteristics, a cosmetic formulation containing the roughened hybrid hollow microparticles effectively suppresses the increase in the temperatures of artificial skin upon the illumination of a simulated sunlight, without displaying skin whitening which is caused by including much inorganic colloids in the formulation. The present results are helpful to those who manipulate the optical characteristics of inorganic particles whose geometries are hardly tailored. The results are also practically helpful to those who want to block NIR light by reducing the amount of inorganic particles.     

Laser ablation of microparticles for nanostructure generation

The process of laser ablation of microparticles has been shown to generate nanoparticles from microparticles; but the generation of nanoparticle networks from microparticles has never been reported before. We report a unique approach for the generation of nanoparticle networks through ablation of microparticles. Using this approach, two samples containing microparticles of lead oxide (Pb₃O₄) and nickel oxide (NiO), respectively, were ablated under ambient conditions using a femtosecond laser operating in the MHz repetition rate regime. Nanoparticle networks with particle diameter ranging from 60 to 90 nm were obtained by ablation of microparticles without use of any specialized equipment, catalysts or external stimulants. The formation of finer nanoparticle networks has been explained by considering the low pressure region created by the shockwave, causing rapid condensation of microparticles into finer nanoparticles. A comparison between the nanostructures generated by ablating microparticle and those by ablating bulk substrate was carried out; and a considerable reduction in size and narrowed size distribution was observed. Our nanostructure fabrication technique will be a unique process for nanoparticle network generation from a vast array of materials.     

Modelling study on the three-dimensional neutron depolarisation response of the evolving ferrite particle size distribution during the austenite–ferrite phase transformation in steels

The magnetic configuration of a ferromagnetic system with mono-disperse and poly-disperse distribution of magnetic particles with inter-particle interactions has been computed. The analysis is general in nature and applies to all systems containing magnetically interacting particles in a non-magnetic matrix, but has been applied to steel microstructures, consisting of a paramagnetic austenite phase and a ferromagnetic ferrite phase, as formed during the austenite-to-ferrite phase transformation in low-alloyed steels. The characteristics of the computational microstructures are linked to the correlation function and determinant of depolarisation matrix, which can be experimentally obtained in three-dimensional neutron depolarisation (3DND). By tuning the parameters in the model used to generate the microstructure, we studied the effect of the (magnetic) particle size distribution on the 3DND parameters. It is found that the magnetic particle size derived from 3DND data matches the microstructural grain size over a wide range of volume fractions and grain size distributions. A relationship between the correlation function and the relative width of the particle size distribution was proposed to accurately account for the width of the size distribution. This evaluation shows that 3DND experiments can provide unique in situ information on the austenite-to-ferrite phase transformation in steels.     

Picosecond nonlinear optical properties of cuprous oxide with different nano-morphologies

Cuprous oxide nanoclusters, microcubes and microparticles were successfully synthesized by a simple co-precipitation method. Phase purity and crystallinity of the samples were studied by using X-ray powder diffraction. Transmission electron microscopy (TEM) images show different morphologies like nanoclusters, microcubes and microparticles. For linear and nonlinear optical measurements, the as-synthesized Cu₂O with different morphologies were dispersed in isopropanol solution. The absorption spectrum recorded in the visible regions shows peaks that depend on the morphology of the particles and the peak shifts towards red region as one goes from nanoclusters to microparticles. Simple open-aperture Z-scan technique is used to measure nonlinear optical properties of cuprous oxide at 532 nm, 30 ps excitation at 10 Hz repetition rate. Cuprous oxide nanoclusters show reverse saturable absorption (RSA) behaviour, the microcubes and microparticles at a similar concentration exhibit saturable absorption (SA) type of behaviour at lower peak intensities and exhibit RSA within SA at higher peak intensities. The results show that the transition from SA to RSA can be ascribed to the two-photon absorption (TPA) process.     

Analysis of the coarsening of solid dispersed systems

A method is proposed for computer analysis of the kinetics of coarsening of solid dispersed phases due to the Ostwald ripening of microparticles and its associated processes. The proposed method provides useful information on the occurrence of various processes in a dispersed system as it approaches the equilibrium state. In this method, the similarities and differences between the experimental and theoretical size distributions of microparticles are preliminarily determined by comparing the characteristics of the microparticle size distributions with the corresponding moments. The quality and reliability of identifying the density function of the microparticle size distribution are evaluated from an analysis of the relationships between these characteristics.     

Fabrication of L10 FePtAg nanoparticles and a study of the effect of Ag during the annealing process

L1₀ ferromagnetic phase FePt nanoparticles containing Ag atoms (FePtAg) were synthesized by means of a liquid phase process, followed by annealing. The addition of Ag to FePt nanoparticles permits annealing to be conducted at a lower temperature (350 °C). This is further accompanied by a subsequent transformation in the crystal phase from the FCC superparamagnetic phase to the FCT (L1₀) ferromagnetic phase. The effects of annealing temperature and the Ag atoms inside the nanoparticles on the magnetic properties of the FePt nanoparticles have been studied. Using electron spectroscopy for the chemical analysis (ESCA), Ag atoms in the L1₀ phase FePtAg nanoparticles were found to be localized on the surface region of the annealed nanoparticles. The Ag atoms function to inhibit the oxidation of FePt, causing the particles to become more stable and to have ferromagnetic properties.     

Graphitic encapsulation of micron- and nano-sized Ni particles using ethylene as precursor

This work deals with the catalytic decomposition of carbon-containing gases (ethylene) over nickel catalysts to grow graphitic capsules at relatively low temperatures (853 K). The influence of different experimental conditions, such as catalyst size, reactant flow ratio (C₂H₄:O₂), gas flow rate and temperature, is studied. The size of the nickel particles determines the type of structures; while nano-sized nickel particles favor the growth of nanofilaments, graphitic capsules occur over nickel microparticles. The use of reducing and oxidizing environments in the growth of graphitic structures over microparticles are compared, the latter being more appropriate for growing nanofilaments. Below 853 K, no graphitic structures are formed. Moreover, a new model is proposed for the encapsulation of the catalyst metal particles by graphite layers.     

Multivariate analysis of spatially heterogeneous phase synchronisation in complex systems: application to self-organised control of material flows in networks

Networks of interacting components are a class of complex systems that has attracted considerable interest over the last decades. In particular, if the dynamics of the autonomous components is characterised by an oscillatory behaviour, different types of synchronisation can be observed in dependence on the type and strength of interactions. In this contribution, we study the transition from non-synchronised to synchronised phase dynamics in complex networks. The most common approach to quantify the degree of phase synchronisation in such systems is the consideration of measures of phase coherence which are averaged over all pairs of interacting components. However, this approach implicitly assumes a spatially homogeneous synchronisation process, which is typically not present in complex networks. As a potential alternative, two novel methods of multivariate phase synchronisation analysis are considered: synchronisation cluster analysis (SCA) and the linear variance decay (LVD) dimension method. The strengths and weaknesses of the traditional as well as both new approaches are briefly illustrated for a Kuramoto model with long-range coupling. As a practical application, we study how spatial heterogeneity influences the transition to phase synchronisation in traffic networks where intersecting material flows are subjected to a self-organised decentralised control. We find that the network performance and the degree of phase synchronisation are closely related to each other and decrease significantly in the case of structural heterogeneities. The influences of the different parameters of our control approach on the synchronisation process are systematically studied, yielding a sequence of Arnold tongues which correspond to different locking modes.     

The solid state phase transformation of potassium sulfate

Potassium sulfate single crystals that are grown from aqueous solutions lose, upon the first heating, up to 1% of mass that is assumed to be water. This mass loss occurs in the vicinity of the PT from orthorhombic to hexagonal K₂SO₄. Only in the first heating run of K₂SO₄ that has not yet released water, pretransitional thermal effects can be observed in the DTA curve. If K₂SO₄ crystals are grown from solutions containing 4 wt% Cd, Cu, or Fe, only Cu or Fe can be incorporated significantly with concentrations of several 0.1%. The phase transformation temperature measured for such solid solutions depends on the heating rate. For pure K₂SO₄, the phase transformation temperature is independent of heating rate 581.3 ^°C and the enthalpy of transformation is (5.8±0.2) kJ/mol.