Structure and order in thermal dehydrations of crystalline solids

A review of the mutual dependence of Arrhenius parameters evaluated by the thermoanalytical study of solid-state reactions: the kinetic compensation effect

This is an introductory manual on uses of the microscope. The opening chapter deals with the optical system. There is a chapter on metallographic microscopes and illumination of opaque objects, a chapter on light sources and illumination of transparent objects, methods on how to prepare the material to be studied and then how to interpret the physical properties found. Other topics include ultramicroscopy, photomicrography and microprojection, electron microscopy, polarizing microscopes and the study of doubly refractive materials, determination of refractive indices of liquids and solids, chemical crystallography, microscopal measurements, determination of partical size, and quantitative analysis of heterogeneous mixtures. This volume includes diagrams, book references at the end of each chapter, and many drawings of crystals. -- AATA

The handbook of chemical microscopy

Is the science of thermal analysis kinetics based on solid foundations?: A literature appraisal

A physico-geometric approach to the kinetics of solid-state reactions as exemplified by the thermal dehydration and decomposition of inorganic solids

Recent advances in microscope techniques have been used in a reinvestigation of the kinetics of the formation and the growth of nuclei developed during dehydration of cleaved (111) faces of relatively perfect crystals of the alums KAl(SO 4 ) 2 . 12H 2 O and KCr(SO 4 ) 2 . 12H 2 O , and solid solutions of five intermediate compositions, KAl x Cr 1-x (SO 4 ) 2 . 12H 2 O. There was observational evidence that nuclei are usually produced at sites of identifiable surface imperfection and that such sites are not randomly distributed across the reactant surfaces. Nucleation is a deceleratory process and, during the later stages, the continued generation of new reaction sites was associated with surface cracking. The rates of interface advance are constant at the boundaries of established nuclei, though this stability sometimes followed an initial period of either more or less rapid growth. The implications of these observations are discussed for the formulation of yield-time kinetic expressions for solid state decomposition. The activation energies determined for the constant rate of interface advance during dehydrations of chrome alum and the solid solutions were between 70 and 93 kJ mol -1, with no systematic dependence on composition discerned, and the value for aluminium alum was greater (108 kJ mol -1 ). Textural examinations of partially reacted surfaces give evidence that exposure to vacuum results in modification, attributable to water loss, of all the reactant surface and that dehydration is not restricted to the interface comprising the reactant-product contact zone. It is believed that each growth nucleus arises as a consequence of a local recrystallization and this is promoted at the interface by water temporarily retained within cracks and pores of the reactant and between the finely divided product particles. We also conclude that local surface stresses exert a more significant control upon these reactions than has hitherto been generally realized. This model contrasts with many mechanistic features of the accepted theory of the subject. Rates of nucleus growth in the five solid solutions (mixed alums) were less than those of the pure end members, a minimum reactivity was found at the composition 2Al:1Cr. There were no significant changes in activation energy with composition and we discuss the significance of the Arrhenius parameters calculated for these reactions.

Nucleation and Growth Processes Occurring During the Dehydration of Certain Alums: The Generation, the Development and the Function of the Reaction Interface

Some particular methods of collecting and treating DSC data for the determination of kinetic parameters in thermal decompositions of solids are suggested. These are mainly concerned with an extrapolation method to avoid the effect of the sample mass; with an approximate method to obtain the activation energy and the exponent of the decay-law for runs at constant temperature in which the total area under the thermal curve is not known with accuracy; and with a method to ascertain whether a particular decomposition takes place by a single mechanism or by a sequence of different mechanisms.

Some experimental aspects of DSC determination of kinetic parameters in thermal decompositions of solids

The kinetics of the thermal dehydration of various kinds of BaCl2 · 2H2O and of BaCl2 · H2O are investigated using a differential scanning calorimeter. The loss of H2O proceeds in two steps: BaCl2 · 2H2O→BaCl2 · H2O→BaCl2 and is therefore revealed by two endothermic peaks.

DSC study of the kinetics of the thermal dehydration of BaCl2 · 2H2O and BaCl2 · H2O

The presence of graphite is demonstrated to reduce the influence of thermal emissivity on the evaluation of heat changes for reactions of the type: Asolid → Bsolid + Cgas by differential scanning calorimetry.

Use of graphite to improve heat determinations by differential scanning calorimetry

The kinetics of thermal dehydration of microcrystalline powders and single-crystals of barium chlorate monohydrate were studied by differential scanning calorimetry. The computer elaboration of thermal data showed that a single kinetic law could not account for the whole decomposition; moreover, different laws had to be used for powders than for single-crystals, whose reaction appeared diffusion-limited towards the end of the dehydration. Optical microscopy was extensively used as an independent source of information to clarify some of the thermal results and to gain a deeper insigth into the possible reaction mechanism.

R. Spinicci

Papers

Nucleation and Growth Processes Occurring During the Dehydration of Certain Alums: The Generation, the Development and the Function of the Reaction Interface

Some experimental aspects of DSC determination of kinetic parameters in thermal decompositions of solids

DSC study of the kinetics of the thermal dehydration of BaCl2 · 2H2O and BaCl2 · H2O

Use of graphite to improve heat determinations by differential scanning calorimetry

Kinetics of thermal dehydration of Ba(ClO3)2 · H2O