Method of Comparing Solid‐State Kinetic Data and Its Application to the Decomposition of Kaolinite, Brucite, and BaCO₃

Abstract

A method of comparing the kinetics of isothermal solid-state reactions based on the classical equation for analysis of nucleation-and-growth processes is described. In this method, plots of In In (1-a) vs In (time), where 01 is the fraction reacted, are used to distinguish reaction mechanisms. Even nonintegral slopes obtained for values of the fraction reacted from 0.15 to 0.50 may indicate whether the reaction rate is diffusion-or phase-boundary-controlled. The problems of ascertaining zero time and self-cooling (or heating) of the reacting sample can be observed in the analysis but do not cause severe difficulties in interpretation, as they can for analyses based on reduced-time plots. The analysis is applied to the dehydroxylation of kaolinite and of brucite and the decomposition of BaCO,,.

Kinetics of the chrysotile and brucite dehydroxylation reaction: a combined non-isothermal/isothermal thermogravimetric analysis and high-temperature X-ray powder diffraction study

“…However, their mechanistic interpretation might be influenced by the isothermal approach. van Aken and Langenhorst (2001)verified former findings of Hancock and Sharp (1972) and Kingery (1966, 1967) concerning a R3 mechanism by transmission electron microscopy. They demonstrated a complex multistep reaction mechanism including at least two stages of an interface-controlled process, one at the reaction onset and one towards the end.…”

“…The R-type model suggested by the master plots for the first part of the reaction is supported by kinetic, microscopic and diffraction studies by Kingery (1966, 1967) who proposed a nucleation-and growthgoverned mechanism accompanied by extensive cracking of the primary phase (R3 mechanism). A R3 model was also proposed by Hancock and Sharp (1972) in the range 15 % ≤ α ≤ 50 %. However, their mechanistic interpretation might be influenced by the isothermal approach.…”

“…without independent control if the latter is really describing the rate-limiting step. The coefficient n is than extracted from the best fit, and the corresponding mechanism is presented as the ratelimiting step (Hancock and Sharp 1972).…”

“…This absence may explain the low number of geoscientific papers dealing with modern solidstate kinetic concepts such as isoconversional kinetics, variable activation energies, multi-step reaction mechanisms. In fact, most of the papers written since the millennium are still using the Avrami-Erofe'ev reaction model and isothermal model-fitting functions (Hancock and Sharp 1972;Bamford and Tipper 1980) to describe the experimental data. The Avrami-Erofe'ev model describes reactions whose rates are determined by nucleation and growth steps as well as diffusion steps (e.g.…”

The role of synchrotron radiation in examining the self-assembly of crystalline nanoporous framework materials: from zeolites and aluminophosphates to metal organic hybrids

“…Most commonly used the Avrami and Erofe'ev model via a Hancock-Sharp analysis 58 of the crystallization curve:…”