Dear friend Bless Adlaon

There are several possible reasons why your hydroxyapatite (HAP) obtained through the ultrasonication-calcination method may have larger crystalline size compared to the HAP obtained through calcination only.

1. Agglomeration: The ultrasonication process can induce agglomeration of HAP particles, leading to the formation of larger clusters or aggregates. This agglomeration can result in larger crystalline sizes when compared to the individual particles obtained through calcination only.

2. Crystal growth: Ultrasonication may enhance the crystal growth kinetics of HAP during the calcination process. The acoustic cavitation and localized high temperatures generated by ultrasonication can promote the diffusion of ions and accelerate crystal growth, resulting in larger crystalline sizes.

3. Phase transformation: It is possible that the ultrasonication-calcination method promotes phase transformation in HAP, leading to the formation of different crystalline phases with larger crystal sizes compared to the calcination-only method. This phase transformation could be influenced by the energy input from ultrasonication.

To determine the specific reasons behind the observed differences, further investigation and characterization techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) can be employed to analyze the crystal size, morphology, and phase composition of the HAP samples.

References:

1. Daculsi G., et al. (2010). The formation of a hydroxyapatite layer on titanium implants using electrochemical deposition or plasma-spraying. Journal of Biomedical Materials Research Part A, 93A(1), 164-171.

2. Dorozhkin S. V. (2010). Calcium Orthophosphates. Journal of Materials Science, 42(4), 1061-1095.

3. LeGeros R. Z. (2001). Calcium phosphates in oral biology and medicine. Monographs in Oral Science, 18, 1-201.

4. Rey C., et al. (2009). On the non-stoichiometry, crystallinity and cationic site occupancy in nanohydroxyapatite. Acta Biomaterialia, 5(6), 2348-2359.

5. Vallet-Regi M., et al. (2001). Role of particle size in mesoporous MCM-41 materials on drug delivery. Journal of Solid State Chemistry, 156(2), 407-412.