Juan J. Morales

Thermal decomposition reactions of solids controlled by diffusion and phase-boundary processes: possible misinterpretation of the mechanism from thermogravimetric data

Abstract A comparative study of phase -boundary and diffusion-controlled reactions has been carried out using thermogravimetric data. The results reported support the hypothesis that a single TG diagram does not allow the determination of whether a solid decomposition reaction is controlled by a diffusion mechanism or governed by the movement of an interface coming from a nucleation process. However, the analysis of both a single TG diagram and an isothermal curve might be a quick and valid way for discerning between these mechanisms. These statements are confirmed by studying the thermal decomposition kinetics of Ba(OH) 2 and CaCO 3 .

Path integral simulation of the free energy of (Lennard-Jones) neon

The quantum mechanical contribution to the free energy of simple fluids can be conveniently determined by path integral simulation and ‘λ-integration’, choosing the classical fluid as reference system. In this paper the method is applied to liquid Lennard-Jones (LJ) neon. The properties of the quantum mechanical and the classical liquids are compared with each other and with experimental data.

Alteration of kinetics and thermodynamics of thermal decomposition of alkaline-earth carbonates induced by grinding

Abstract The effect of grinding on the kinetics and thermodynamics of thermal decomposition reactions of alkaline-earth carbonates has been studied. Grinding of MgCO 3 , SrCO 3 and BaCO 3 leads to a decrease in the activation energy of the reaction which becomes greater as the lattice energy of the salt decreases. These results can be attributed to the mechanically produced lattice disorder that is facilitated with decreasing lattice energy. A decrease in the enthalpy of decomposition has also been noted in the ground samples of SrCO 3 . By contrast, the enthalpy and activation energy of thermal decomposition of CaCO 3 do not undergo significant alterations. Noticeable changes occur only after quite a long grinding time. This behaviour is probably related to an elimination of the microstrains created during grinding as a result of the phase transformations calcite → aragonite and aragonite → calcite, induced in this salt by grinding. A compensation effect between the enthalpy and entropy of decomposition and also between the activation energy and frequency factor is observed. These correlations seem to indicate that the reaction takes place through the same activated complex, its thermodynamic constants depending on the nature of the cation. Thus, the decrease in the frequency factor of the reaction as the grinding of the salt continues, might be related to an increase in the carbonate ion entropy.

Application of programmed temperature decomposition to the study of solid decomposition reactions taking place through the prout and tompkins mechanism

Abstract A method has been developed to allow kinetic analysis of the Prout and Tompkins mechanism of solid decomposition reactions using a single TG diagram obtained in a linear heating program. On the other hand, provided that the nickel formate decomposition follows such a mechanism, this reaction has been used for checking the kinetic equation obtained.

Efficiency test of the traditonal MD and the link-cell methods

Abstract For very large systems, the earlier method of computer simulation by molecular dynamics (MD) has been compared with the newer link-cell (LC) method. The results show clearly that the LC method does not improve the efficiency of traditional MD simulations in spite of the great memory saving achieved.

Comparison of link-cell and neighbourhood tables on a range of computers

Abstract The recent improvement to the link-cell method (Comput. Phys. Commun. 60 (1990) 195), making it faster on a scalar computer than the traditional neighbour table (NT) method, was tested on different vectorial computers. The efficiency of both methods depends greatly on the particular processor architecture. In contrast to scalar computer results, the NT method was found to be better on the vectorial computers.

ON THE EVALUATION OF KINETIC PARAMETERS FROM THERMOGRAVIMETRIC CURVES

We concluded in a previous work [l] that it is not possible to determine the kinetic law followed by a thermal decomposition reaction of a solid from the kinetic analysis of a single TG curve. It is necessary to carry out at least one isothermal run to determine the actual reaction mechanism without ambiguity. It was shown from a theoretical analysis [l] that TG data of reactions following an Avrami-Erofeev law necessarily fit a first-order kinetic equation. However, it was concluded that very similar activation energies would be obtained whatever the value of the exponent n (1, 2 or 3) in the AvramiErofeev equation. Dharwadkar et al. [2] have studied the mechanism of the thermal decomposition of CdC03 and, from the analysis of both a single TG diagram and a single isothermal curve, they find that the reaction mechanism is described by an Avrami-Erofeev equation with n = 3, concluding that their results support, in part, OUT theoretical conclusions. However, they state [Z] that the apparent activation energies calculated frcm a TG diagram are quite different for different values of n, which is not in agreement with our theoretical deduction [ 11. The aim of the present note is to show that the apparent contradiction between the statement of Dharwadkar et al. and our conclusion is due to the different ways of defining the constant rate. Thus, Dharwadkar et al. [2] seem to express the Avrami-Erofeev isothermal in the form

The Van der Waals model of dense fluids. Dependence of the Helmholtz free energy on temperature and density

Abstract A molecular dynamics simulation is used to establish the dependence of the Helmholtz free energy on temperature and density, and the influence of the attractive forces on the structure of dense fluids. Calculations with a 3D Lennard-Jones system near the triple point show that the van der Waals picture is quantitatively exact.

Path integral molecular dynamics methods: Application to neon

Feynmans path integral formulation of quantum statistical mechanics, which has commonly been applied be Monte Carlo methods, is now also implemented by traditional molecular dynamics simulations of the microcanonical ensemble and in the Nosé‐Hoover method simulating the isothermal‐isobaric ensemble. In this article these two methods are applied to solid and liquid neon, in which quantum effects are not negligible. The validity of the procedure is shown by comparison with Monte Carlo and Brownian Dynamics computer simulations and with experiment.

Statistical error method in computer simulations

Abstract Two methods of studying the statistical error in the sequences of data obtained by computer simulation are compared. The first method divides the sequence into blocks whose length is selected graphically by means of the “statistical inefficiency.” The second method uses the autocorrelation function of all the values obtained and analytically calculates its convergence by means of the “correlation length.” The general relationship between the two parameters is found mathematically and is in good agreement with the experimental data obtained by molecular dynamics simulation in the melting zone when a very accurate algorithm is used. As a consequence, the analytical method is more accurate than the graphical method.