Temperature and Supersaturation Dependent Nucleation Rates of Heterogeneous Water by Molecular Cluster Model Calculation

A statistical mechanical method to evaluate the energy of formation of water clusters attached to a foreign particle surface is described, with the binding energy being evaluated on a molecular level, using semiempirical modified neglect of diatomic overlap (MNDO) theory. The model is applied to water nucleation on a silicon oxide surface. The binding energy contribution, which represents the energy of formation at T=0 K, is found to slightly (but not negligibly in the thermal sense) increase with the number of hydrogen bonds between the water cluster and the condensation nucleus whose surface is made of silicon oxide. An analytic expression is developed to fit the binding energy contribution as a function of cluster size. At lower temperatures, a linear relationship is found between the log of the nucleation rate and reciprocal temperature for fixed saturation ratio. However, at higher temperatures, this relationship deviates from linearity. The deviation is sufficient to suggest the existence of a critical temperature for which the nucleation rate reaches a maximum. Furthermore, another kind of critical temperature is found, which corresponds to a minimum cluster critical size (at fixed saturation ratio). These are found to almost coincide for the cases of heterogeneous and homogeneous nucleation

Temperature and Supersaturation Dependent Nucleation Rates of Water by Molecular Cluster Model Calculations

Using a microphysical approach to nucleation, we present an extensive study of water nucleation rates for wide ranges of both temperature and supersaturation ratio. Based on the fundamental molecular properties of clusters instead of bulk properties, the microphysical approach is demonstrated to predict good agreement with measured nucleation rates over this broad range of conditions. Predicted critical sizes for nucleation are found to be relatively small, and are in the molecular cluster size regime rather than in a size regime that should be characterized by bulk values. Estimated sticking coefficient values cover the range of ~0.9 to ~0.2 for the temperature range considered, whereas sticking coefficient values corresponding to Becker-Doring theory suffer an unreasonably large three-orders of magnitude decrease for temperature increase from 220K to 285K

Simulation of Bulk Silicon Crystals and Si(111) Surfaces with Application to a Study of Fluorine Coverage of the Surfaces

Computational efficiency for the simulation of bulk crystals and surfaces is highly desirable. In an effort to study semiconductor crystals, we present a self-consistent treatment for the simulation of silicon crystals and surfaces based on the combination of a siligen model and a semiempirical Hamiltonian method. An artificial atom called siligen is introduced for the application of the semiempirical method to finite-size silicon clusters. The calculated average bond energies for the saturated silicon clusters are between 2.045 and 2.568 eV, compared to the measured value of 2.31 eV. A simulated bulk silicon surface using siligens is introduced in order to examine variation of the bond strength between fluorine atoms and the simulated silicon (111) surface. It is found that bond strength computed from the simulated surface, with siligens, rapidly converges to a saturated limit as the number of surface layers increases, while a pure silicon (111) surface without siligens yields no satisfactory convergence

Lanczos exact diagonalization study of field-induced phase transition for Ising and Heisenberg antiferromagnets

Using an exact diagonalization treatment of Ising and Heisenberg model Hamiltonians, we study field-induced phase transition for two-dimensional antiferromagnets. For the system of Ising antiferromagnet the predicted field-induced phase transition is of first order, while for the system of Heisenberg antiferromagnet it is the second-order transition. We find from the exact diagonalization calculations that the second-order phase transition (metamagnetism) occurs through a spin-flop process as an intermediate step.Comment: 4 pages, 4 figure

Pressure and Temperature Effects on the Energy of Formation for Silicon Clusters

At present most theoretical studies of atomic clusters are limited to their physical properties referred to 0 K. To the best of our knowledge, there exists no theoretical study of the simultaneous dependence of cluster formation and cluster-size distributions on both pressure and temperature. In the present work both pressure and temperature effects on the formation of silicon clusters are explored. A universal semiempirical formula is obtained to show a general trend in the variation of binding energy as a function of cluster size for both atomic and molecular clusters

The structure of fluid trifluoromethane and methylfluoride

We present hard X-ray and neutron diffraction measurements on the polar fluorocarbons HCF3 and H3CF under supercritical conditions and for a range of molecular densities spanning about a factor of ten. The Levesque-Weiss-Reatto inversion scheme has been used to deduce the site-site potentials underlying the measured partial pair distribution functions. The orientational correlations between adjacent fluorocarbon molecules -- which are characterized by quite large dipole moments but no tendency to form hydrogen bonds -- are small compared to a highly polar system like fluid hydrogen chloride. In fact, the orientational correlations in HCF3 and H3CF are found to be nearly as small as those of fluid CF4, a fluorocarbon with no dipole moment.Comment: 11 pages, 9 figure

Anharmonicity, vibrational instability and Boson peak in glasses

We show that a {\em vibrational instability} of the spectrum of weakly interacting quasi-local harmonic modes creates the maximum in the inelastic scattering intensity in glasses, the Boson peak. The instability, limited by anharmonicity, causes a complete reconstruction of the vibrational density of states (DOS) below some frequency $\omega_c$, proportional to the strength of interaction. The DOS of the new {\em harmonic modes} is independent of the actual value of the anharmonicity. It is a universal function of frequency depending on a single parameter -- the Boson peak frequency, $\omega_b$ which is a function of interaction strength. The excess of the DOS over the Debye value is $\propto\omega^4$ at low frequencies and linear in $\omega$ in the interval $\omega_b \ll \omega \ll \omega_c$. Our results are in an excellent agreement with recent experimental studies.Comment: LaTeX, 8 pages, 6 figure

Three-dimensional heterostructure of metallic nanoparticles and carbon nanotubes as potential nanofiller

The effect of the dimensionality of metallic nanoparticle-and carbon nanotube-based fillers on the mechanical properties of an acrylonitrile butadiene styrene (ABS) polymer matrix was examined. ABS composite films, reinforced with low dimensional metallic nanoparticles (MNPs, 0-D) and carbon nanotubes (CNTs, 1-D) as nanofillers, were fabricated by a combination of wet phase inversion and hot pressing. The tensile strength and elongation of the ABS composite were increased by 39% and 6%, respectively, by adding a mixture of MNPs and CNTs with a total concentration of 2 wt%. However, the tensile strength and elongation of the ABS composite were found to be significantly increased by 62% and 55%, respectively, upon addition of 3-D heterostructures with a total concentration of 2 wt%. The 3-D heterostructures were composed of multiple CNTs grown radially on the surface of MNP cores, resembling a sea urchin. The mechanical properties of the ABS/3-D heterostructured nanofiller composite films were much improved compared to those of an ABS/mixture of 0-D and 1-D nanofillers composite films at various filler concentrations. This suggests that the 3-D heterostructure of the MNPs and CNTs plays a key role as a strong reinforcing agent in supporting the polymer matrix and simultaneously serves as a discrete force-transfer medium to transfer the loaded tension throughout the polymer matrix