Study of the coalescence phenomena in biogenic nano-hydroxyapatite produced by controlled calcination processes at low temperature

Abstract

The coalescence of nanocrystals generated by thermal treatments is a phenomenon observed in many materials. In the case of hydroxyapatite (HAp) derived from a biological source such as bovine bone, this transition was observed at temperatures around 700 °C. However, whether this is also the case at lower temperatures has not yet been investigated. With this aim, this work examined HAp samples from cattle calcined at 400–720 °C in increments of 20 °C, to determine the temperature effect on HAp nanocrystals due to coalescence. Differential scanning calorimetry showed that the raw (as prepared) bovine hydroxyapatite exhibited four thermal transition events at which the coalescence of HAp nanocrystals occurred. Infrared analysis showed that the full width at half maximum (FWHM) of the phosphate band decreases with increasing calcination temperature. The FWHM of the X-ray diffraction peaks showed minima values just at the coalescence events, while the unit cell volumes exhibited an opposite trend. The crystallite size of the HAp increases with increasing calcination temperature during the coalescence events, and the Rietveld method shows that the unit cell volume follows the same behavior and shows an opposite behavior in terms of the micro-strain. Scanning electron microscopy (SEM) images of samples calcined at temperatures above 520 °C show the first signs of coalescence, as necks are formed by the union of adjacent HAp nanocrystals. N2 adsorption-desorption measurements have also shown that the average pore diameter is maintained during thermal processes. Finally, transmission electron microscopy (TEM) was used to observe the coalescence of the nanocrystals of the sample burned at 420 and 520 °C. These thermal events could be because the surface vibrational states of at least two adjacent HAp nanocrystals collapse and form new bulk vibrational states.