Application of thermal analysis to the study of the kinetics of two-stage reactions

A method is suggested for studying the kinetic parameters of two-stage reactions from thermoanalytical experimental data. The method allows determination of the kinetic characteristics of the process on the basis of integral thermoanalytical data, without separation of the individual stages. Parallel, independent and successive reactions taking place under isothermal and non-isothermal conditions are considered.

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Zusammenfassung Eine Methode zur Untersuchung der kinetischen Parameter von Zweistufenreaktionen unter Verwendung experimenteller thermoanalytischer Daten wird vorgeschlagen. Die Methode ermöglicht, die kinetischen Kennwerte des Prozesses basierend auf integralen thermoanalytischen Werten ohne Trennung der individuellen Schritte zu bestimmen. Unter isothermen und nichtisothermen Bedingungen parallel, unabhängig voneinander und nacheinander verlaufende Reaktionen werden diskutiert.

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Effect of anisotropic diffusion and external electric field on the rate of diffusion-controlled reactions

In this paper we investigate theoretically the effect of an external electric field on the rate constant of steady-state bulk diffusion-controlled reactions. We generalize previously derived results for isotropic diffusion in the absence of interparticle interaction [J. Chem. Phys. 87, 4622 (1987)] to the case where translational diffusion is anisotropic. A frequently occurring situation of transverse isotropy where D(x)=D(y) not equal to D(z) is considered in detail. We derive the first-order expansion for the reaction rate constant in terms of the electric field strength E, k(E)=k(0) (1+1/2epsilongamma), where gamma=k(0)/4piRD( perpendicular ), epsilon=qER/k(B)T, q is the charge, R is the contact distance, and D( perpendicular ) is the transverse diffusion coefficient. Numerical calculations show that this first-order expansion works well in the whole range of applicability of the Nernst-Einstein relation, i.e., for epsilon<1.     

Thermoanalytical studies of the kinetic of reversible reactions

A method is described for determination of the kinetic parameters of reversible reactions under isothermal and non-isothermal conditions in thermal analysis. Reversible reactions of the first and second orders and of mixed types are considered. The kinetic parameters of the forward and reverse reactions are determined on the basis of the initial integral data of the thermoanalytical experiment. The results of processing a computerized experiment demonstrated that the suggested method is satisfactorily applicable.

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Zusammenfassung Es wurde eine Methode zur Bestimmung kinetischer Parameter von reversiblen Reaktionen unter isothermen bzw. nicht-isothermen Umständen in der Thermoanalyse beschrieben. Umkehrbare Reaktionen erster Ordnung, zweiter Ordnung sowie Reaktionen vom Mischtyp wurden betrachtet. Die kinetischen Parameter der Hin- bzw. Rückreaktionen wurden auf der Grundlage der Anfangsintegrale der termoanalytischen Experimente bestimmt. Die Ergebnisse eines computermodellierten Experimentes zeigen, dass die vorgeschlagene Methode befriedigend angewendet werden kann.

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Theory of Spin Echo in Restricted Geometries under a Step-wise Gradient Pulse Sequence

A closed matrix form solution of the Bloch-Torrey equation is presented for the magnetization density of spins diffusing in a bounded region under a steady gradient field and for the Stejskal-Tanner gradient pulse sequence, assuming straightforward generalization to any step-wise gradient profile. The solution is expressed in terms of the eigenmodes of the diffusion propagator in a given geometry with appropriate boundary conditions (perfectly reflecting or relaxing walls). Applications to rectangular, cylindrical, and spherical geometries are discussed. The relationship with the multiple propagator approach is established and an alternative step-wise gradient discretization procedure is suggested to handle arbitrary gradient waveforms.     

Electrical transport and magnetic ordering in R 2Ti3Ge4 (R=Dy, Ho and Er) compounds

New R ₂Ti₃Ge₄ (R=Dy, Ho and Er) intermetallic compounds have been synthesized and characterized by X-ray diffraction and low temperature ac magnetic susceptibility, electrical resistivity and thermoelectric power measurements were carried out. The compounds crystallize in the parent, Sm₅Ge₄-type orthorhombic structure (space group Pnma) and lanthanide contraction is observed as one moves along the rare-earth series. The changeover from paramagnetic to antiferromagnetic phase happens at low temperatures and the ordering temperature scales with the de Gennes factor. The electrical resistivity is metallic with a negative curvature above 100 K. Thermopower displays a weak maximum at temperatures less than 50 K signifying the possible phonon and magnon drag effects.