Thermal and kinetic study of hydroxyapatite formation by solid‐state reaction

Abstract

Hydroxyapatite (HAP) is one of the most attractive calcium phosphates which is used for different applications. While HAP has been mostly synthesized using wet methods, recently solid-state synthesis methods have attracted more attention. Many efforts have been made to better understand the solid-state synthesis mechanism. In this study, the kinetics of the solid-state reaction between hydroxide calcium (Ca(OH) 2) and phosphorus pentoxide (P 2 O 5) was studied under nonisothermal conditions using isoconversional methods. The phase analysis revealed that HAP is the main product of this process with no impurity. Thermogravimetric (TG) curves were recorded at four heating rates (β = 5, 10, 12 and 15 • C⋅min −1) and analyzed by several well-known isoconversional methods. Activation energy values were determined for each extent of reaction, which varied from ∼139 to 100 kJ/mol at the reaction extent of 0.1-0.9. Comparison between activation energy values obtained by isoconversional methods revealed that Flynn-Ozawa-Wall and Vyazovkin's method are more accurate than other. The preexponential factor and the reaction model were determined through model-free methods. The obtained results indicated that the reaction mechanism was chemically controlled or ordered-based model. Furthermore, the results showed that formation of HAP does not depend on the heating rate; however, it proceeds with increasing temperature.

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