Poster P1 Synthetic hydroxyapatite (HAP) is of importance as a biomaterial as it is chemically similar to the mineral component of mammalian bone. As such the human body (and for that matter, the bodies of other mammals as well) is quite happy to integrate it into it. HAP is sometimes used in powder form, but in most cases it is used as a bulk material. To produce bulk samples generally requires sintering at elevated temperatures, usually in excess of 1000°C. The products are soluble or resorbable calcium phosphates, which dissolve when exposed to physiological environments Introduction Experiments Acknowledgments This work is financially supported by the Bulgarian Ministry of Education, Youth and Science under Projects DTK 02-70/2009 and CVP MECHANOCHEMICAL TRANSFORMATION OF SINTERED BI-PHASE CALCIUM PHOSPHATE CERAMICS R.Ilieva 1, E.Dyulgerova 2, Chr. Balarew 1, R.Titorenkova 3, O.Petrov 3 1 Institute of General and Inorganic Chemistry, Bulgarian Academy of Sciences,Acad. G. Bontchev Str., Bl.11, 1113 Sofia, Bulgaria, 2 Dental Medicine Faculty, University of Medicine, 1 G. Sofiiski Str., 1431 Sofia, Bulgaria 3 Institute of Laboratory of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bontchev Str., Bl.107, 1113 Sofia, Bulgaria 3 Aims This work aims to study the phase and structural changes of calcium phosphate ceramic, as a result of dray mechanochemical treatment. It is assumed that the resulting nano-crystalline material will exhibit enhanced chemical activity, bioactivity and biodegradability, thus facilitating interaction between single molecules and bulk material. Results Steps of synthesis i. Precipitation of hydroxyapatite Ca 10 (PO 4 ) 6 (OH) 2 from solutions: Ca(NO 3 ) 2 (1 M) and (NH 4 ) 2 HPO 4 (0,6 M); ii. Drying at 100  C for 15h; iii.Sintering at 1100  C for 1h - formation of  -tricalcium phosphate (  -Ca 3 (PO 4 ) 2 ) (degree of conversion of 0.4 to 0.5) iv. Milling in an agate mill (600 rpm, for 5, 10 and 20 hours). Chemical analysis Sample Molar ratio Ca/P Dried 1.67 (HAP) Sintered at 1100  C 1.59 (HAP and β-TCP) Ca 10 (PO 4 ) 6 (OH) 2  3  -Ca 3 (PO4) 2 +CaO+H 2 O Mechanochemically treated 1.59 (HAP and β-TCP) X-ray diffraction X-ray diffraction patterns of calcium phosphate samples: (a)sintered at 1100 ºC; (b)sintered at 1100 ºC, milling 5h; (c)sintered at 1100 ºC, milling 10h; (d)sintered at 1100 ºC, milling 20h. (■) hydroxyapatite ;(●) β–tricalcium phosphate (β-TCP) With increasing time of mechanochemical treatment:  the β-TCP phase was transformed from crystalline to amorphous after 20h milling;  the particle size of HAP phase reduces reaching from 77 nm to 30 nm for 20 h. X-ray Data Scanning electron microscopy SEM images of (a) initial sample sintered at 1100 ºC, (b) sintered at 1100 ºC bi-phase calcium phosphate after mechanochemical treatment for 20 h. The intensity ratio between the most intensive XRD peaks of HAP and β-TCP increases and the crystallite size gradually decreases upon milling. This observation indicates that the amount of crystalline β-TCP phase decreases upon mechanochemical treatment. The increase of the FWHM of the peaks is negligible after 10 h of treatment and significant after 20 h of milling, which is an indication that amorphization of β-TCP occurs. Conclusions Crystal size decrease with the milling time