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Abstract

Structural transformation and sintering processes of tricalcium phosphate (TCP) ceramics prepared from defective hydroxyapatite (Ca9HPO4(PO4)5OH) were studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). Starting powders with Ca/P ratio approximately 1.5 were obtained by adding 0.5 l of 0.3 M H3PO4 solution to an equal volume of 0.45 M Ca(OH)2. In the prepared ceramics, the onset temperature for transformation of defective hydroxyapatite into TCP (witlokite) agrees with the onset temperature for sintering (800 degrees C). Sintering occurs through the formation of a fibrous structure, which resembles biological hard tissue. In the 1000-1200 degrees C range, these fibres coalesce into grains of up to 0.6 microm in size with a fibrous-laminar morphology. At the end of this sintering stage witlokite transforms into alphaTCP. At about 1450 degrees C, partial decomposition of alphaTCP into Ca2P2O7+Ca4P2O9 is observed. AFM observations suggest that Ca2P2O7 is segregated in the liquid state and increases the velocity of grain growth (up to 12 microm).

Impact of Gamma Irradiation on the Properties of Magnesium-Doped Hydroxyapatite in Chitosan Matrix

“…This transformation could occur over a wider range for the (Ca + Mg)/P ratio than that presented in previous studies [ 42 , 43 ]. According to previous studies [ 44 , 45 , 46 ] the transformation of HAp into β-TCP can be influenced by several factors, such as temperature, pH value, substituted ions (it is known that hydroxyapatite is very hospitable and allows for the replacement of the Ca 2+ , PO 4 3− , or OH − groups with other ions), and thermal stability. In agreement with Kannan et al [ 47 ], the HPO 2 4− ions plays an important role in the β-TCP formation (Equation (1)): Ca 10−x (HPO 4 ) x (PO 4 ) 6−x (OH) 2−x → 1−x Ca 10 (PO 4 ) 6 (OH) 2 + 3·Ca 3 (PO 4 ) 2 + xH 2 O …”

Rapid Mix Preparation of Bioinspired Nanoscale Hydroxyapatite for Biomedical Applications

“…This was supported by the lower than stoichiometric Ca:P ratio (1.63) obtained with the XRF data ( Table 1 ). It is understood that a reduced Ca:P ratio is associated with a lower thermal stability20212223. In this method, the rapid addition of the phosphoric acid solution rapidly lowered the pH of the reaction suspension to generate HPO 4 ions.…”