Thermodynamic model of hardness: Particular case of boron-rich solids

A number of successful theoretical models of hardness have been developed recently. A thermodynamic model of hardness, which supposes the intrinsic character of correlation between hardness and thermodynamic properties of solids, allows one to predict hardness of known or even hypothetical solids from the data on Gibbs energy of atomization of the elements, which implicitly determine the energy density per chemical bonding. The only structural data needed is the coordination number of the atoms in a lattice. Using this approach, the hardness of known and hypothetical polymorphs of pure boron and a number of boron-rich solids has been calculated. The thermodynamic interpretation of the bonding energy allows one to predict the hardness as a function of thermodynamic parameters. In particular, the excellent agreement between experimental and calculated values has been observed not only for the room- temperature values of the Vickers hardness of stoichiometric compounds, but also for its temperature and concentration dependencies

Surface melting in nanoparticles and nanosystems. 2. Scientific and nanotechnological aspects of the role of surface melting in nanoparticles and nanosystems

Taking into account results of our molecular dynamics experiments, we have concluded that of the three commonly considered alternative models of nanoparticle melting (homogeneous melting, liquid shell, nucleation of liquid and growth), the latter is the most adequate. At the same time, a more adequate model corresponds to a combination of continuous melting at the initial stage of the process with its subsequent abrupt completion. In other words, nucleation and growth of a liquid-like surface layer occur until a certain critical radius of the crystalline core of the particle is reached, and then melting is completed very quickly, almost abruptly (in fractions of a nanosecond) at a temperature interpreted as the nanoparticle melting temperature Tm. Then, the role of surface melting in nanoparticle sintering is discussed. According to our results, the sintering of metal nanoparticles at high temperatures cannot be reduced to a single mechanism: a certain role play surface melting, surface and bulk diffusion, deformation in the contact zone, and collective effects associated with the displacements of groups (clusters) of atoms rather than of individual atoms. We also have put forward and substantiated the hypothesis that the previously introduced redetermined Tamman temperature TT=0,5Tm corresponds to the switching of the scenario of sintering of metal nanoparticles from formation of a dumbbell-shaped nanocrystal at low temperatures to the scenario corresponding to coalescence of solid nanoparticles resulting in the formation of a defective nanocrystal of a shape close to spherical

Comparative molecular dynamics simulation of synthesis of silver nanoparticles from the gas phase

A comparative molecular dynamics simulation of the gas-phase synthesis of Ag nanoparticles is carried out employing two different types of many-particle potentials of the interatomic interaction: a potential corresponding to the embedded atom method and the tight-binding potential. The initial temperature was varied from 1000 to 3000 K, and then it gradually decreased to 77 K, which corresponded to the temperature of liquid nitrogen. The results obtained using alternative force fields are consistent with each other, but, at the same time, they significantly differ both in the dynamics of evolution of the system and in the obtained final configurations of nanoparticles. Increasing the cutoff radius of the tight binding potential significantly changes the rate of the nanoparticle formation. However, an increase in the cutoff radius when using the embedded atom method does not affect the evolution of the system. The configurations obtained as a result of simulation using the embedded atom method are characterized by a smaller size and a shape close to spherical, while when using the tight binding potential, larger nanocrystals with an elongated shape are formed

Molecular dynamics of C70S48: Dielectric function and NMR study

Molecular dynamics of C70S48 have been studied by dielectric relaxation spectroscopy and NMR. In accordance with the NMR data, the rotation of C70 molecules is not hindered by sulfur, and even at room temperature, it is uniaxial like that in pure C70. Numerous phase transitions observed for pure C70 have no analogues in E70S48. The only anomaly of the dielectric function was found at 245 K. In accordance with the NMR data, C70 molecules rotate uniaxially at room temperature, but below 170 K, this rotation begins to freeze, and at 150 K, the rotation is frozen in the NMR scale. Dielectric function temperature dependence does not show any anomalies around 170 K, where the NMR registered a freezing of the rotational freedom of C70 molecules, and the NMR does not show any features around 245 K, where the dielectric function shows a sharp anomaly. Some possible explanations for the nature of the anomaly around 245 K are discussed. © 2001 American Chemical Society.SCOPUS: ar.jinfo:eu-repo/semantics/publishe