Spatial distribution of nuclei in progressive nucleation: modeling and application

Phase transformations ruled by non-simultaneous nucleation and growth do not lead to random distribution of nuclei. Since nucleation is only allowed in the untransformed portion of space, positions of nuclei are correlated. In this article an analytical approach is presented for computing pair-correlation function of nuclei in progressive nucleation. This quantity is further employed for characterizing the spatial distribution of nuclei through the nearest neighbor distribution function. The modeling is developed for nucleation in 2D space with power growth law and it is applied to describe electrochemical nucleation where correlation effects are significant. Comparison with both computer simulations and experimental data lends support to the model which gives insights into the transition from Poissonian to correlated nearest neighbor probability density.Comment: 30 pages; 9 figure

Impact of seed density on continuous ultrathin nanodiamond film formation: an analytical approach

An analytical mean field approach for describing the time evolution of film growth by seeding has been developed. The modeling deals with the generic case of anisotropic growth with different growth rates, respectively on -- and normal to -- the substrate plane. The finite size of the seeds is considered by including spatial correlation effects among seeds through hard-core interactions. The approach, based on probability theory, provides solution in closed form for mean film thickness as a function of substrate coverage, seed density and initial size of the seeds. For negligible values of the initial coverage of the substrate by seeds, manageable analytical expressions are attained. The model has been validated by comparison with experimental data available in the literature. This study is significant in connection to the possibility of determining optimal growth conditions for ultrathin nanocrystalline diamond (NCD) film. In fact, the knowledge of the seeding/nucleation density that allows a given minimum average thickness of continuous film is of utmost importance for the development of technologically advanced applications.Comment: 20 pages, 5 figures, 2 table

Vibrational temperature of the adlayer in "hot atom" reaction mechanism

Hot-atoms reactions mechanisms bring about reaction rates which are several orders of magnitude higher than those expected in the case of ad-atoms which have thermalized with the surface. This paper addresses the issue of a possible thermodynamic characterization of the adlayer under reactive conditions and at the steady state. In turn, this implies to tackle the question of determining the temperature of the ad-atoms. This is done by means of a nonequilibrium statistical thermodynamic approach, by exploiting a suitable definition of the entropy. The interplay between reaction rate, vibrational temperature of the ad-atoms and adsorbed quantities is highlighted. It is shown that the vibrational temperature depends on reaction rate logarithmically and exhibits a non-linear scaling on physical quantities linked to the energetics of the reaction, namely the adsorption energy and the binding energy of the molecule. The present modeling is also discussed in connection with response equations of nonequilibrium thermodynamics.Comment: 29 pages, 8 figure

On the work of adhesion of film-substrate solid junctions

A study of the work W of adhesion of a thin film of thickness h deposited on a substrate is reported. The free energy change occurring during the deposition of the coating from the gas phase is assumed to be negative. In these conditions the thermodynamic stability of the deposited film requires W < 0. To obtain a negative value of the work of adhesion the surface tension 7rs of the film-substrate interface has to satisfy inequalities involving the mechanical properties of the film, the surface tension ~,f~ of the film and the parameters that enter into the film-substrate interaction potential. It is also shown that the 7fs--~sgq-)'fg < 0 condition is not always a sufficient condition to have W < 0

Kolmogorov-Johnson-Mehl-Avrami kinetics for non-isothermal phase transformations ruled by diffusional growth

Abstract We report on the functional form of the rate of the transformed volume fraction in non-isothermal phase transitions occurring by nucleation and diffusional growth. The microscopic growth rate is computed by solving the diffusion problem for time-dependent diffusion coefficient. The growth law is further employed in the Kolmogorov– Johnson–Mehl–Avrami (KJMA) theory for describing the time dependence of the transformed volume at constant heating rate. It is demonstrated that the transformation rate separates in the product of volume fraction and actual temperature functions. In the framework of the KJMA approach this factorization is exact. It is also shown that for real systems (due to the high values of the reduced activation energies for nucleation and growth), the kinetics is in excellent agreement with the stretched exponential function appropriate for isothermal transformations

I fenomeni fisici e chimici irreversibili: la teoria termodinamica dell'osmosi e la sua verifica sperimentale

In questo articolo vengono forniti ai docenti delle scuole di I e II grado alcuni spunti didattici per l’elaborazione di una lezione sull’ irreversibilit`a dei fenomeni naturali. Il processo dell’ osmosi, di straordinaria importanza nella chimica, nella biologia e nella fisiologia, `e analizzato quale caso paradigmatico di un fenomeno irreversibile. La prima parte dell’articolo ne esamina alcuni aspetti termodinamici sulla base del I e del II principio della termodinamica. La seconda parte `e interamente dedicata alla discussione di semplici esperimenti e alla possibilit`a di ottenere informazioni quantitative sul fenomeno in esame

I fenomeni fisici e chimici irreversibili: la teoria termodinamica dell'osmosi e la sua verifica sperimentale

In questo articolo vengono forniti ai docenti delle scuole di I e II grado alcuni spunti didattici per l’elaborazione di una lezione sull’ irreversibilit`a dei fenomeni naturali. Il processo dell’ osmosi, di straordinaria importanza nella chimica, nella biologia e nella fisiologia, `e analizzato quale caso paradigmatico di un fenomeno irreversibile. La prima parte dell’articolo ne esamina alcuni aspetti termodinamici sulla base del I e del II principio della termodinamica. La seconda parte `e interamente dedicata alla discussione di semplici esperimenti e alla possibilit`a di ottenere informazioni quantitative sul fenomeno in esame

Energy disposal via electron-hole pair excitation in atom recombination on metal surfaces: Electron distribution function and detailed balancing

The energy disposal via electron-hole (e-h) pair formation in the highly exoergic recombination of gas atoms on metal surfaces, has been studied. The impact of this excitation process on the energy spectrum of the metal electrons has been investigated at steady state. The energy distribution function of the electrons is shown to be non-thermal to an extent that depends on the reaction rate, probability distribution function for e-h pair creation and physical parameters of the material. The model has been applied to compute the electron spectra of metal surfaces, using kinetic data on H atom recombination that are available from the literature. The number of electrons made available for detection as chemicurrent has been derived and compared to experimental data on the H/Cu system. The interplay between the electron distribution function and the reaction kinetics is analysed by means of the principle of detailed balance. The detailed balancing analysis shows that, depending on the e-h excitation energy, the rate constant of energy transfer from the bath to the adlayer can be orders of magnitude larger than that proper for equilibrium conditions. (c) 2007 Elsevier B.V. All rights reserved

Interplay between Kolmogorov-Johnson-Mehl-Avrami kinetics and Poisson-Voronoi tessellation

In this paper we investigate the connection between Voronoi tessellation and the KJMA approach of space filling. In particular, we study how nuclei, in their growth, cover a given Voronoi cell. This approach leads to an integral equation for the cell-size distribution function. Starting from the 1D case, that is solved exactly, we extend the results to the dDcase. The analysis allows to find a rationale to the phenomenological parameter entering the Gamma distribution function and to improve the description of the transformation through the knowledge of the kinetics of grain formation. Moreover, the nucleus size distribution function has been calculated as a function of the transformed fraction

Idrogeno ed elio : atomi di storia

Viene ripercorsa, molto brevemente, la strana storia della scoperta di un atomo di idrogeno "esotico" nel periodo pioneristico della spettroscopia ottica. Un periodo pieno di fervore e scoperte , che, al di là di qualche incidente di percorso, fornì dati che contribuirono non poco alla definitiva determinazione della struttura dell'atomo di idrogeno, più in generale della tabella periodica degli elementi, e che spianò la strada alla seconda rivoluzione scientifica: la nascita della meccanica quantistica