The effect of citric acid on the sintering of calcium phosphate bioceramics

Biphasic calcium phosphate particles were prepared using a rapid increase of the pH value by adding an amount of concentrated ammonia solution, into a well-mixed solution containing Ca(H2PO4)2·H2O and CaCl2 with Ca/P = 1.667 molar ratio; with or without the presence of citric acid (CiA). The precipitation of Calcium Phosphates took place at 97 °C using high-speed dispersing equipment. The samples were characterized using XRD, FT-IR, BET and SEM techniques. The thermal behaviour was studied by TG, DTG and DTA techniques. The modified precipitation method leads to the formation of biphasic needle-like particles consisted of crystalline hydroxyapatite (HA) and β-tricalcium phosphate (β-TCP). The presence of CiA in the initial solution leads to the formation of aggregated nonporous small spheroidal particles consisted of low crystallinity phases of HA and octacalcium phosphate (OCP). The later facilitates the sintering process. The calcined samples at 900 °C were consisted of calcium pyrophosphate (CPP) and TCP. α-TCP is formed in the presence of a large amount of calcium citrate complexes. The main sintering process took place at the temperature range between 750 and 1150 °C, which were strongly depended on the initial amount of the CiA in the precipitation process. These results indicated that the citrate presence in the initial solution have a strong influence on the nucleation and growth processes by complexation with calcium ions and incorporation into the solid structure during particle growth.