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Physico chemical studies of the reaction of strontium choride with fluorine Rantamaa, Anssi Kalervo

Abstract

The kinetics of the reaction of solid strontium chloride with fluorine gas have been studied by gravimetric, thermometric, and microphotographic methods. ESR and X-ray crystallography were used to study the products. The reaction commenced after an induction period of 1 to 10 minutes. On single crystal specimens studied microscopically, formation and growth of nuclei of SrF₂ thereafter occupied several minutes before the nuclei coalesced to form a continuous SrF₂ layer. By thermometric studies on a polycrystalline boule of reactant on a thermocouple, the extent of reaction during the nucleation period was found to be proportional to t⁴ , suggesting nuclei formed proportional to t² and subsequent two-dimensional growth at constant linear rate. On single crystal specimens, microphotography showed a t² law for number of nuclei only for one specimen with a rough surface. For smooth surfaces, number of nuclei was generally constant, but linear growth was confirmed in many cases. Two growth rates were measured, an initial rate of 6.4 x 10ˉ⁴ mm sec ˉ¹ and a less reproducible rate to which a transition sometimes occurred in later stages of 1.7 x 10ˉ³mm secˉ¹. The nucleation was found to be non-activated and the change in rate was ascribed, together with an increase in the number of nuclei late in the nucleation period, to effects of mechanical strain. The development of the main reaction after establishment of a continuous reaction interface was followed gravimetrically, and found to obey the Ginstling-Brounshtein equation for diffusion through a spherical shell of solid reaction product, having a sharp interface with the reactant. A lower limit of 2 x 10ˉ⁵ cm² secˉ¹ was found for the diffusion coefficient, suggesting that the process is gaseous diffusion in cracks in the product layer. The crystallinity of the product depended on the rate of reaction. For rapid, high-temperature reaction, the product gave a powder diffraction pattern, but for a sample reacted more slowly with a controlled supply of F₂, the product was found to be essentially a single crystal (diffuse diffraction spots indicating ranges of disorientation of no more than about 5°) with the same crystallographic orientation as the reactant. Attempts to locate the ESR signal found in earlier work were only partially successful, but suggest that the signal is largely in the product phase, and that it represents a byproduct rather than a reaction intermediate.

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