Critical considerations on the methods for evaluating kinetic parameters from nonisothermal experiments

By surveying the most used methods for evaluating the kinetic parameters from nonisothermal experiments, a new classification scheme of the methods is proposed. For each method the number of principles and theoretical approximations required to derive the equation which grounds it, is considered as a comparison criterion. The methods are, finally, classified into classes of equivalence. As a result of the analysis it is also suggested that the activation energy, as calculated from nonisothermal data, should be given as a range of values instead of a unique value. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet 30: 313–327, 1998.     

Discrete rectilinear 2-center problems

Given a set P of n points in the plane, we seek two squares such that their center points belong to P, their union contains P, and the area of the larger square is minimal. We present efficient algorithms for three variants of this problem: in the first the squares are axis parallel, in the second they are free to rotate but must remain parallel to each other, and in the third they are free to rotate independently.     

On the Apparent Compensation Effect Found for Two Consecutive Reactions

A critical analysis of the use of an overall single rate reaction equation instead of the true rate equation corresponding to a complex process consisting in two consecutive reactions is presented. In accordance with this approximation, often used in the kinetic analysis of the system in which several reactions take place, the overall process is described by the apparent activation parameters (the apparent activation energy, E _ap, and the apparent pre-exponential factor, A _ap) and the apparent conversion function. The theoretical isotherms (α=α(t), where a is the conversion degree and t is the time) have been simulated for a system in which two consecutive reactions occur. In this case, the apparent activation parameters depends on: (a) the considered range of the temperature; (b) the temperature, for a given conversion degree. It is shown that the apparent activation parameters are corrrelated by the compensation effect relationship: lnA _ap=α*+β*E _ap where α* and β* are the parameters of the linear regression. The possibility of using the apparent kinetic parameters to predict the isotherms α=α(t) for temperatures lower than those for which these parameters were evaluated, is discussed. This revised version was published online in July 2006 with corrections to the Cover Date.     

Critical Considerations on the Isoconversional Methods. III. On the evaluation of the activation energy from non-isothermal data

The differential and integral isoconversional methods for evaluation the activation energy, described in the first note of this series, were applied for: a) simulated data for two successive reactions; b) dehydration of calcium oxalate monohydrate. It was shown that for these systems the activation energy depends on the conversion degree as well as on the method of evaluation. This revised version was published online in August 2006 with corrections to the Cover Date.     

Application of the Pseudo-Inverse Matrix Method in Non-Isothermal Kinetic Analysis

A method is proposed for evaluation of the activation parameters for reactions which occur under non-isothermal conditions. This method can discriminate between possible differential conversion functions. The proposed method, which was coded into a software package available to the scientific community, is designed to solve an overdetermined systems of equations: da_i/dt=k(T_i)f(a_i) where i equations are to be considered (i can be the number of experimental points). Solution of this overdetermined system with a pseudo-inverse matrix method furnishes the activation parameters and the parameters of the conversion function f(a). Some examples of application of this method in non-isothermal kinetic analysis are presented. This revised version was published online in August 2006 with corrections to the Cover Date.     

On the structure and electrical conductivity of polyaniline/polystyrene blends prepared by an aqueous‐dispersion blending method

This article describes electrically conductive polymer blends containing polyaniline‐dodecyl benzene sulfonic acid (PANI‐DBSA) dispersed in a polystyrene (PS) matrix or in crosslinked polystyrene (XPS). Melt blending of previously mixed, coagulated, and dried aqueous dispersions of PANI‐DBSA and PS latices lead to high conductivities at extremely low PANI‐DBSA concentrations (∼0.5 wt % PANI‐DBSA). In these blends, the very small size of the PANI‐DBSA particles and the surface properties (with surfactants used) of both the PANI and polymer particles play a major role in the PANI‐DBSA particle structuring process. The PANI‐DBSA behavior is characteristic of a unique colloidal polymeric filler with an extremely high surface area and a strong interaction with the matrix, evidenced by a significantly higher glass‐transition temperature of the matrix. The effect of the shear level on the conductivity and morphology of the PS/PANI‐DBSA blends was studied by the production of capillary rheometer filaments at various shear rates. An outstanding result was found for XPS/PANI‐DBSA blends prepared by the blending of aqueous XPS and PANI‐DBSA dispersions. Some of these blends were insulating at low shear levels; however, above a certain shear level, smooth surface filaments were generated, with dramatically increased and stable conductivities. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 611–621, 2001     

On the implementation of symmetric and antisymmetric periodic boundary conditions for incompressible flow

In this paper we consider symmetric and antisymmetric periodic boundary conditions for flows governed by the incompressible Navier-Stokes equations. Classical periodic boundary conditions are studied as well as symmetric and antisymmetric periodic boundary conditions in which there is a pressure difference between inlet and outlet. The implementation of this type of boundary conditions in a finite element code using the penalty function formulation is treated and also the implementation in a finite volume code based on pressure correction. The methods are demonstrated by computation of a flow through a staggered tube bundle.     

Atom Michelson interferometer on a chip using a Bose-Einstein condensate

An atom Michelson interferometer is implemented on an "atom chip." The chip uses lithographically patterned conductors and external magnetic fields to produce and guide a Bose-Einstein condensate. Splitting, reflecting, and recombining of condensate atoms are achieved by a standing-wave light field having a wave vector aligned along the atom waveguide. A differential phase shift between the two arms of the interferometer is introduced by either a magnetic-field gradient or with an initial condensate velocity. Interference contrast is still observable at 20% with an atom propagation time of 10 ms.     

Spin-boson thermal rectifier

Rectification of heat transfer in nanodevices can be realized by combining the system inherent anharmonicity with structural asymmetry. We analyze this phenomenon within the simplest anharmonic system-a spin-boson nanojunction model. We consider two variants of the model that yield, for the first time, analytical solutions: a linear separable model in which the heat reservoirs contribute additively, and a nonseparable model suitable for a stronger system-bath interaction. Both models show asymmetric (rectifying) heat conduction when the couplings to the heat reservoirs are different.