Nanocrystalline and metastable phase formation in vacuum thermal decomposition of calcium carbonate

Well characterised. polycrystalline powders of commercially procured CaCO3 were thermally decomposed in the vacuum as well as in the flowing gas atmos phere for the purpose of studying solid state transformations. The characte risation of tbe end product CaO, obtained from the thermal decomposition, r evealed contrasting features in the powder X-ray diffractograms. While the flowing gas method, conducted inside a thermogravimetric analyser (TGA), in dicated formation of stable microcrystalline calcia, the decomposition unde r dynamic vacuum revealed formation of metastable-nanocrystalline calcia. T he latter study was carried out in an evolved gas analysis-mass spectrometr y (EGA-MS) facility. Experiments were also conducted inside the high temper ature XRD (HTXRD) machine. The paper attempts to bring out possible mechani sms responsible for formation of these end products with such glaring struc tural contrast. Non-equilibrium conditions prevalent under dynamic vacuum c ondition as well as misfit strain energy available from CaCO3/CaO interface are presumed to be the reason behind such metastable transformations. Kine tic analysis of the transformation revealed prevalence of nucleation and gr owth phenomena. Corresponding Arrhenius factors were also calculated. (C) 2 000 Elsevier Science B.V. All rights reserved.