Mean-field theory about the critical size in ferroelectricity

The problem of critical size in ferroelectric crystals and films is considered within the mean-field theory. It is shown that, when the boundary conditions are correctly chosen, the critical size can be either very small or even absent; i.e., ferroelectricity can exist in one unit cell.     

X-ray diffuse scattering from spherically symmetric clusters: The effect of defect size fluctuations

A theory of X-ray diffuse scattering from crystals with randomly distributed spherically symmetric clusters characterized by a statistical spread of sizes with respect to their mean has been developed. This consideration is performed within the model of defects whose strains, in contrast to the well-known Coulomb clusters, are spatially confined by a sphere of fixed radius. The effect of defect size fluctuations on the formation of reciprocal space maps is investigated.     

The dependence of the surface energy of regular clusters and small crystallites on the particle size

Whereas in the case of liquids the theories on the dependence of the surface energy on the particle size are uniform in general, describing a decrease of σ with decreasing particle size, in the case of small solid particles also a dependence with the inverse sign has been assumed in the literature. Equations of the size dependence of σ and γ are derived for icosahedral and cuboctahedral clusters of noble gases considering interactions only with next neighbours. It will be shown that the character of the size dependence is dominated by the choice of the position of the dividing surface. Compared to that, such equations are derived also for ion crystallites. The resulting differences can be explained by the differences in structure and bonding.     

Dielectric relaxation of water clusters encapsulated in carbon nanotubes – Computer simulation study

We have performed a molecular dynamics simulation of water clusters encapsulated in single-walled carbon nanotubes and calculated the dielectric relaxation loss spectra of these systems. We have found out that the nanotube radius considerably affects the relaxation time as well as the shape of spectra.     

The effect of silica substitution on electrical conduction in nickel ferrite

The electrical conductivity and Seebeck coefficient for nickel ferrite and nickel ferrite with 5% silica has been studied as a function of temperature. The lattice constant and the existence of a single phase are established from X-ray studies. Both the ferrites exhibit a transition near the Curie temperature with a change in slope of the conductivity (log σT) versus temperature (10³/T) curves. It is found that the activation energy in the paramagnetic region is higher than that in the ferrimagnetic region for both the ferrites.     

Simulation study of size distributions and magic number sequences of clusters during the solidification process in liquid metal Na

To investigate the size distributions of various clusters formed during solidification processes, a molecular dynamics (MD) simulation study has been performed for a system consisting of 10⁶ liquid metal Na atoms. With the cluster-type index method (CTIM), it is demonstrated that the basic clusters of (13 3 6 4), (13 1 10 2), (14 2 8 4), (14 4 4 6) and (12 0 12 0) and their combinations play a critical role in the microstructure transitions. Also, using a new method to classify all the clusters in the system, the size distributions of various clusters clearly reveal magic number characteristics. The total magic number sequence can be regarded as the superposition of all partial magic numbers corresponding to the related group levels of clusters. The first 10 magic numbers are present in order 14, 22, 28, 34, 41(43), 46(48), 52(54), 57(59), 61(66), 70(74),… (the numbers in the parentheses are the second magic numbers corresponding to the same group level of clusters). This magic number sequence is compared with the experimental and computational results of other authors.     

Preparation and characterization of nickel-modified carbon cryogel beads with uniform particle size

Nickel-modified carbon cryogel beads (Ni/CCBs) with uniform particle size were successfully prepared using the syringe injection method, employing sol–gel polycondensation of a resorcinol–formaldehyde (RF) solution containing Ni acetate as a catalyst. This process was followed by solvent exchange, freeze-drying and carbonization in a nitrogen atmosphere. The effect of viscosity, which increased as the sol–gel polycondensation progressed, of the RF solution on both the particle size distributions of the Ni/CCB and the porous properties was investigated. The influence of Ni content in the initial reactants on the porous properties of the Ni/CCBs was also studied. Monodisperse Ni/CCBs could be obtained by injection during the initial reaction period, before the steep increase in the viscosity of the RF solution. The porous properties of the Ni/CCBs were independent of the viscosity of the RF solution. The BET surface area, micropore volume and mesopore volume of Ni/CCBs decreased with increased Ni content, whereas the macropore volume increased with increased Ni content. The size of the Ni nanoparticles dispersed in the carbon matrix of Ni/CCBs was confirmed to range from 30 to 60 nm.     

Determination of the solid liquid interfacial energy and thereby the critical nucleus size of paracetamol in different solvents

The effect of the type of solvent on the solid liquid interfacial energy was determined by performing induction time measurements of paracetamol in methanol, 1‐propanol, acetone and water at a constant supersaturation temperature of 30 °C and different levels of supersaturation (a/a*) ranging from 1.03 to 1.24. At equal supersaturation level and temperature the induction time increases with decreasing solubility whereas the solid liquid interfacial energy decreases with increasing solubility. The interfacial energy has a minimum value of 1.45 mJ/m² in the solvent where paracetamol has a maximum solubility (methanol) whereas it has its maximum value of 2.91 mJ/m² in the solvent with minimum solubility. The interfacial energy is a function of the solubility has been established. The critical radius for homogeneous nucleation was found to be minimum in the solvent of highest solubility. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Icosahedral symmetry in clusters

First principles calculations and simulations based on interatomic potentials together with experimental studies of abundance spectrum suggest icosahedral structures to be common for some magic clusters of diverse systems such as rare gases, metals, covalently bonded systems and water. Close packing models obtained from pair potentials are shown to be good representations of the structure of rare gas clusters. However, the electronic structure is found to play the important role in the atomic structure and related properties of other clusters. Results of recent studies of fullerenes, their derivatives as well as some large icosahedral metal clusters containing several thousand atoms are also presented. Further results on doped icosahedral clusters are discussed which hold promise for the development of new materials and for understanding the occurrence of icosahedral order in several aluminum alloys.     

Pyroelectric properties of bismuth ferrite in the low-temperature range

The pyroelectric effect in bismuth ferrite single crystals is studied in the temperature range from 4.2 to 200 K (limited from above only by the high conductivity of the sample). A nonmonotonous character of the temperature dependence of the pyroelectric coefficient is revealed in the range 120–160 K: at T = 140 K, the pyroelectric coefficient reaches the maximum value equal to 1.2 × 10^?4 C/(m² K). The crystallophysical analysis of the BiFeO₃ structure based on the structural data confirms that the octupole moment of the structure makes the main contribution to the spontaneous polarization. This circumstance leads to a small (in comparison with isostructural compounds) spontaneous polarization of bismuth ferrite and anomalous behavior of the pyroelectric coefficient caused by the contribution of local vibrations of strongly distorted TiO₆ octahedra.     

The features of X-ray topographic contrast formation in silicon with dislocation clusters

The features of formation of diffraction images of edge dislocation sets forming clusters (of two, three and more dislocations) as well as small-angle dislocation boundaries (walls) have been studied. A variety of diffraction effects of wave fields created in strongly distorted crystals regions along dislocation lines have been observed. Various intensity interference effects of rescattering and internal reflection of the newly formed and already existing wave fields on thickness distributions of intensity for the case of presence in the same glide plane of edge dislocations with parallel and anti-parallel Burgers vectors were discovered.     

The growth of carbon nanostructures on cobalt-doped carbon aerogels

By carbonizing cobalt-doped aerogel precursors directly at various temperatures, or by carbon monoxide decomposition of cobalt-doped carbon aerogels, different carbon nano-features such as carbon nano-filaments and graphitic nano-ribbons were grown on cobalt-doped carbon aerogel samples. Transmission electron spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction characterization results showed that metallic cobalt nano-particles form when heating the cobalt-doped aerogel samples over 500 °C. At low heating temperature, many highly oriented carbon thin films can be found on metallic cobalt nano-particles. When heating the samples at 850 °C, some carbon nano-filaments are obtained. While heating the samples at 1050 °C, many graphitic nano-ribbons are grown and the framework of the interconnected carbon particles of the sample is changed. Graphitic nano-ribbons can also be grown by CO decomposition of the cobalt-doped carbon aerogels. We can therefore control and modify the nanostructures of cobalt-doped carbon aerogels by heating them at different temperatures or by using CO decomposition.     

The liquid state of large clusters with pairwise atomic interactions

Formation of the liquid state for clusters with a pair interaction between atoms is examined within the framework of the void model, in which configurational excitation of atoms results from formation of voids. Void parameters are found from computer simulation by molecular dynamics methods for Lennard-Jones clusters and from real thermodynamic parameters of the liquid states of condensed inert gases. Phase transitions are analyzed in terms of two aggregate states. This information allows us to divide the entropy jump during the solid-liquid phase transitions in two parts, so that one corresponds to configuration excitation at zero temperature, and the other is a contribution from thermal vibrations of atoms. The latter part contributes from approximately 40% for Lennard-Jones clusters consisting of 13 and 55 atoms up to 56% for bulk inert gases. These magnitudes explain the validity of melting criteria based on thermal motion of atoms, even though this phase transition results from configurational excitation of ensembles of bound atoms. It is shown that the void concept allows us to analyze various aspects of the liquid state of clusters including the existence of the freezing temperature below which no metastable liquid state exists, and the properties of glassy states which may exist below the freezing point.     

The effect of inter-cluster interactions on the structure of colloidal clusters

Colloidal systems present exciting opportunities to study clusters. Unlike atomic clusters, which are frequently produced at extremely low density, colloidal clusters may interact with one another. Here we consider the effect of such interactions on the intra-cluster structure in simulations of colloidal cluster fluids. A sufficient increase in density leads to a higher population of clusters in the ground state. In other words, inter-cluster interactions perturb the intra-cluster behaviour, such that each cluster may no longer be considered as an isolated system. Conversely, for dilute, weakly interacting cluster fluids little dependence on colloid concentration is observed, and we thus argue that it is reasonable to treat each cluster as an isolated system.     

Phase transitions of bismuth clusters

X-ray diffraction patterns, extended X-ray absorption fine structure, and optical absorption coefficients were measured in order to investigate the size-dependence of structural and optical phase transitions of bismuth clusters. Contrary to the case of 15-nm thick films, peaks due to crystalline Bi were not observed in X-ray diffraction patterns of as-deposited 0.5-nm thick films. Optical absorption coefficients of the 0.5-nm thick films are small compared with those of 100-nm thick films, and an optical gap appears in the 0.5-nm thick films. These results show directly the phase transition of bismuth clusters from semi-metallic nanocrystalline to semiconducting amorphous-like clusters with decreasing size.     

SANS study of clusters in aqueous magnetic fluids

Small-angle neutron scattering studies of the structure of clusters formed in aqueous magnetic fluids are reviewed. The possibilities of contrast variation using hydrogen-deuterium isotopic substitution in these complex systems are shown. Examples of the use of the method in study of the kinetics of cluster organization of aqueous magnetic fluids upon different external conditions (temperature and magnetic field) are presented. Urgent problems related to aqueous magnetic fluids, in which the method seems to be most promising, are outlined.     

Structure and properties of clusters in FCC materials

Properties of small clusters with icosahedric symmetry, isolated or embedded in an fcc matrix, were investigated for different metals by the molecular-dynamics method. It is shown that the energy specific features of clusters and the height of the barrier for the transition from an fcc to icosahedric structure for isolated clusters are determined to a great extent by the properties of the interatomic-interaction potentials. In this case, the lower the transition barrier, the “softer” the potential is. Vice versa, the clusters embedded in a matrix are most stable in metals with a rigid potential. It is established that the stability of an embedded cluster increases the number of vacancies at the boundary between two structures and depends on the orientation of the cluster with respect to the matrix. An embedded cluster introduces significant perturbations into the surrounding matrix, which extend to distances of about one and a half cluster sizes.