The thermal dehydroxylation of kaolinite using thermogravimetric analysis and Controlled rate thermal analysis

The aim of this work was to identify parameters that could influence the conversion of kaolinite into metakaolinite. To this end, the dehydroxylation of four kaolinites, using controlled rate thermal analysis (CRTA) and thermogravimetric analysis (TGA), was investigated. X-ray diffraction (XRD) and Fourier transform infra-red (FTIR) were used to confirm the kaolinitic nature of the samples. Their crystallinity was evaluated using the Hinckley index from the XRD patterns and transmission electron microscopy (TEM) was used for morphological observation of the clay platelets. The thermal analyses of the samples indicate, for all samples, low defects as revealed by the low amount of adsorbed water on the clay surface. The dehydroxylation temperature from both techniques was influenced by the sample crystallinity and particle size. It was observed from both techniques that increase crystallinity resulted in highdehydroxylation temperature. Also, the influence of the clay platelets size on the dehydroxylation temperature, is proposed. All the results indicate that crystallinity and particle size are parameters that could be used to control the temperature for the conversion of kaolinite to metakaolinite. Keywords: Kaolinite; Crystallinity, Dehydroxylation; Thermal analysis

Controlled Rate Thermal Analysis (CRTA) as a Fast and Effective Method for the Development of Ceramic Powders of Synthetic Hydroxyapatite at Low Temperatures

One of the main limitations for applying synthetic hydroxyapatite as a filler in cement and other formulations in orthopedic surgery is its morphology. The present work shows the obtaining of synthetic hydroxyapatite powders at low temperatures such as 300 and 850ºC using the Controlled Rate Thermal Analysis (CRTA) technique. The powders obtained were characterized by IR spectroscopy and X-ray diffraction, showing that the phase formed corresponds to crystalline hydroxyapatite. The specific surface area values determined are between 17 and 66 m2/g with a pore size between 50-300 Å. The transformation of phases in the synthetic hydroxyapatite is studied by Dynamic Thermogravimetric Analysis and CRTA techniques, allowing the kinetic calculations of the transformation using two methods of data processing, determining the activation energy (Ea), pre-exponential factor (A), and model kinetic most likely in each stage. The results show the effectiveness and usefulness of the CRTA technique for preparing synthetic hydroxyapatite powders with different specific surface areas, which makes this technique attractive for medical purposes.