Thermal reactions of inorganic hydroxy-compounds under applied electric fields, I

Abstract

A method which has been developed for thermogravimetric study of solids in the presence of applied electric fields has been used in a study of kaolinite dehydroxylation in a controlled inert atmosphere. Electric fields of ~10 ~ V/m lower the initiation temperature for dehydroxylation by as much as 60 ~ in some cases; the activation energy for dehydroxylation is reduced by 3--12 kcal/mole. The rate constants" for electrolysed samples (based arbitrarily on a first-order Saw) are increased by electrolysis but this effect falls off at higher temperatures as the normal thermal processes begin to predominate. The effect of the field on the various processes of ionic migration in the lattice is discussed.In previous papers [1,2] it has been shown qualitatively that the application of an a.c. or d.c. electric field to hydroxyl-containing solids such as kaolinite [A12Si2Os(OH)a ] above the dehydroxylation temperature results in an enhancement of the high-temperature reactions and a significant improvement in the physical and crystallochemical properties of the product at a given temperature. In the case of the applied d.c. field these results have been attributed to the removal of residual protons present in kaolinite even after dehydroxylation is substantially complete [1]. The operation of such a proton-elimination mechanism during the high temperature reactions suggests that electrolysis of inorganic hydroxylcontaining solids at lower temperatures might also influence the course of their dehydroxylation reactions.The purpose of this work was to investigate the manner in which d.c. electric fields interact with solids during their thermal dehydroxylation. The course of the dehydroxylation reaction was followed directly by weight-loss measurements made during the electrolysis. Although this was not primarily a kinetic study, a kinetic analysis was made of the resulting weight-loss curves since this was the most convenient means of comparing the behaviour of the electrolysed solid hydroxides with the unelectrolysed control samples.Previous TG kinetic investigations of kaolinite dehydroxylation have often resulted in differing conclusions; earlier workers generally interpreted their weightloss data in terms of first-order kinetics [3,4] which were taken to indicate a random nucleation process. More recent work in vacuo [5,6] and controlled

Thermal reactions of inorganic hydroxy-compounds under applied electric fields, II

“…The reaction temperature for a powder is much lower than for an unelectrolysed pellet, from which the escape of water is more difficult. The weight-loss data were analysed in terms of the Coats and Redfern equation using a computer programme as previously described [1 ]. The best linear fit of the data was given by a first-order rate law from which activation energies and pre-exponential factors were calculated ( Table 1).…”

“…Weight-loss measurements were made using the electrolysis cell previously described [1] in conjunction with a Stanton TR-1 thermobalance. The samples were heated at a linear temperature rise of 2~ in a flowing atmosphere of dried argon (300 ml/min.).…”

“…Another suggestion [4] is that the elements of water are removed with equal facility from both types of hydroxide, but in the aluminium compounds the comparative lack of mobility of dislocations and defects hinders the changes in oxygen packing necessary for the formation of new compounds. Since the application of an electric field influences both proton migration and to a lesser extent defect movement [1 ] the present electrolysis studies should also provide information on the relative importance of both processes in these compounds.…”

“…In a previous paper [1 ] it was shown that the application of an electric field to the hydroxy-aluminosilicate kaolinite [A12Si20~(OH)~ ] during dehydroxylation resulted in a decreased activation energy for the process, the effect of the field being most pronounced at lower temperatures. Since at higher temperatures the reaction in kaolinite is apparently dominated by defect formation, on which the field exerts less effect, it is interesting to study the influence of electric fields on the dehydroxylation of related simple hydroxides such as Mg(OH)2 and AI(OH)~, which represent opposite extremes of proton retention behaviour.…”

“…As in the previous work on kaolinite [1], TG kinetic measurements were made solely as a means of comparison between electrolysed and unelectrolysed samples. No mechanistic comparison was attempted between the present work on Mg(OH)2 and the results of previous workers [5 -8] although the inhomogeneous mechanism of Ball and Taylor [9] has provided a basis for understanding the electrolysis results, despite the fact that some of the X-ray evidence for this mechanism [9] has now been discounted [10].…”