Little has been published about dry synthesis of hydroxyapatite. Some insight can perhaps be drawn from the crystallization investigation of calcium phosphosilicate glasses, where it was suggested that needle-like tricalcium phosphate (TCP) nanocrystals, Ca3(PO4)2, may transform to oxyapatite (OAp) ─ the ideal composition being Ca5(PO4)3O ─ at around 750ºC. The very unstable and reactive OAp readily converts to hydroxyapatite in open atmosphere. Notice, additionally, that the composition of amorphous calcium phosphate (ACP) is typically close to that of TCP, being the conversion of ACP to alpha-TCP (or vice versa) possible. For more information see: C. M. Queiroz, M. H. Fernandes, J. R. Frade, “Early steps of orthophosphate crystallisation in a Ca-Mg-K phosphosilicate glass frit”, Materials Science Forum, Vols. 455-456 (2004), 402-405; and references herein.

Article Early Steps of Orthophosphate Crystallisation in a Ca-Mg-K P...

First, sintering or calcination improves crystallinity, which is easily seen on X-ray diffraction patterns. Second, it removes all moisture, as well as other volatile admixtures, remaining from the preparation stage (solvents, ammonia, carbonates, organic compounds, etc.). There is a recent review on this topic by E. Champion, entitled “Sintering of calcium phosphate bioceramics” Acta Biomaterialia 9 (2013) 5855–5875.

Dear Sir,

according to this study

Wang et al.,

Journal of the European Ceramic Society 24 (2004) 693–698doi:10.1016/S0955-2219(03)00248-6

temperatures up to 500°C eliminate extrinsic water in form of adsorbates at the surface or within pores.

In a second temperature regime, ranging from about 700°C to 1100°C, dehydroxylation takes place. In this process, "intrinsic" water is removed

from the crystal itself. Two hydroxygroups transform into one water molecule

and one oxygen ion, i.e.

2 (OH)^- ---> H2O + O^2-

In terms of crystal structure, this means that one (OH)- is replaced by an O^2-

ion and one (OH)^- is replaced by a vacancy #, i.e.,

Ca10(PO4)6(OH)2 ---> Ca5(PO4)3O + Ca5(PO4)3# + H2O

Hope this helps (or just have a look at the Wang et al. paper).

Sincerely,

FM

Hi there,

From a bone point of view it's interesting to note that in it's natural form, bone mineral is not a hydroxyapatite, the hydroxyl group is incorporated during the ashing process used to remove the organic material. For more info on the microstructure on bone you may want to check out the following publications:

http://www.ncbi.nlm.nih.gov/pubmed/1740237

http://www.ncbi.nlm.nih.gov/pubmed/18685796

Doesn't help your question, but thought it may be of interest anyway.

Cheers,

David

you can see my article "Effect of Annealing on Nanoparticles of Hydroxyapatite Synthesized via Microwave Irradiation: Structural and Spectroscopic Studies" for same.