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Topochemical reactions

Particularities of their mechanism and outlook on their research, part II

Journal of Thermal Analysis and Calorimetry
Author: V. V. Boldyrev

A survey of topochemical reactions is given and the theory of their kinetics is discussed.

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Typical localization forms of topochemical reactions taking place on the surface and inside a crystal have been compared. A kinetic model of triclinic crystal decomposition, which in contrast to known models takes into account that crystal faces react non-simultaneously, has been studied. It has been shown that depending on the value of the induction period that precedes entry of the crystal face into the reaction, and on the ratio of the linear velocities of interface advance in different directions, the decomposition may proceed without self-acceleration, or with one-fold or multi-fold self-acceleration.

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Topochemical reactions

Particularities of their mechanism and outlook on their research, part I

Journal of Thermal Analysis and Calorimetry
Author: V. Boldyrev
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Synthesis of special pigments

A study by means of TA methods

Journal of Thermal Analysis and Calorimetry
Authors: Z. Šolc and M. Trojan

Abstract  

Several TA methods (including dynamic TA, Q-TA, emanation and special conductometric TA) were used in investigations of condensed phosphate synthesis, and in the evaluation and control of the reactivity of the raw materials, when high-temperature stable pigments are developed. Conductometric TA occupies a special position because of its high sensitivity to most changes in crystal structure at the beginning of reactions.

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Abstract  

Tetracyanocomplex clathrates and their changes caused by heating were studied. The intermediates formed were characterized by IR and UV-VIS spectroscopy. Elemental analysis and XRD patterns were also used. For the visualisation of changes occurring on the surface and the distribution of metallic elements therein were studied by electron microscopy and EDX. The extent of the non-stoichiometric changes introduced by the topochemical course of the degradation reactions was correlated with the measured electrical values.

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Abstract  

A study was made on the topochemical processes of hydration-dehydration, ammonation-deammonation, disproportionation and anionic condensation (certain of which can proceed concurrently) that occur during the production, storage and exploitation of phosphoric salts and phosphate materials. These processes which can be accompanied by complicated structural transformations and sometimes by melting were shown to be satisfactorily described by the known kinetic regularities. The detected deviations, that are associated with the anomalous influence of temperature, the gaseous phase, the size and habit of the crystals and the ageing of samples on the process rate, were explained in terms of the localization peculiarities of the reactions proceeding on the surface or in the bulk of the reactant.

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Abstract  

The results of atomic dynamics of ultrafines clusters of iron hydroxide and Fe 2 O 3 with average size 1–3nm are shown. The clusters were synthesised after chemical reaction in solution in micropores of sorbents and in topochemical reactions of thermal decomposition salts. Moessbauer spectroscopy studies are applicated. The effect of Surface Active Substance(SAS) on dynamics of clusters is observed. Thermodynamics and phemonologic models of dynamic state of cluster are considered and the probability of the coexistans solid-liquid state of cluster is discussed.

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–8), 308–317. 20 Romhányi, Gy., Jobst, K.: Topochemical reactions in polarisation microscopy of connective tissue and its intracellular substances. Ann. Histochim

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The kinetics of isothermal dehydration of LiKC4H4O6.H2O single-crystals was investigated in the [001] crystallographic direction under a dynamic vacuum of 6.7×10−5 Pa with a quartz crystal microbalance. The removal of H2O molecules may be described by a diffusion equation for a semi-infinite medium. The diffusion coefficients vary from 2.13×10−11 cm2 s−1 at 391.7 K to 9.9×10−9 cm2 s−1 at 453.2 K. The scanning electron microscope data provide some evidence that the dehydration is not a topochemical reaction. From the experimental data it is concluded that the anhydrous product is in the state of “premelting”. This explains the anomalous diffusion energyE D=37±1 kcal mol−1 and preexponential factorD 0=5×109 cm2 s−1.

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