Study Of Synthesizing Hydroxyapatite Using Neutralization Method And Mechanism

Hydroxyapatite (Ca10(PO4)6(OH)2, HA) is used widely as replacement and repairing materials of hard tissue, due to excellent bioactivity, biocompatibility and osteoconductivity. In this thesis, the HA has been synthesized via neutralization method. The thermodynamics of synthesizing hydroxyapatite powders were analyzed, and the effect of process factors on HA purity was studied. Then, the phase transformation and kinetic factors were investigated in HA synthesis process in Ca(OH)2-H3PO4 system by neutralization method, and the synthesis process of HA was optimized. Furthermore, the reaction model of actual synthetic process was established. Meanwhile, the influence of reaction temperature, added anhydrous ethanol, reaction time and dispersant on phase composition and morphology of hydroxyapatite was discussed. Finally, doped hydroxyapatite was synthesized using the same method, and as-synthesized powders were characterized.The study on thermodynamics and kinetics of synthesizing hydroxyapatite powders with Ca(OH)2-H3PO4 reaction system showed that three kinds of precipitation products including CaHPO4·2H2O (DCPD), Ca3(PO4)2 (TCP) and HA were appeared. The synthesizing of hydroxyapatite went through DCPD�'TCP�'HA. The optimum condition of HA synthesis was obtained as follows:the Ca(OH)2 particles was in range of 75-100 μm, concentration of Ca(OH)2 was 0.15 mol/L and titration time of H3PO4 was 60 min. The impurities of TCP and CaO appeared in HA powder while the Ca(OH)2 particles were than 100 μm, Ca(OH)2 concentrations were 0.3 mol/L and 0.45 mol/L, and titration rates of H3PO4 were 10 min and 120 min.Synthesizing hydroxyapatite powders with Ca(OH)2-H3PO4 reaction system, the ionization rate of OH- from Ca(OH)2 and the ionization rate of H+ from phosphoric acid reached dynamics balance during the reaction process, and precipitations of the reaction product of H3PO4 and Ca(OH)2 did not from on the surface of Ca(OH)2 particle, ion diffusion and mass transfer did not limit in the liquid-solid interface, the synthetic product was single HA and the synthetic reaction was completely. On the other hand, ion diffusion and mass transfer were affected by slow speed of ionization of Ca(OH)2 and the precipitations of the reaction product on the surface of Ca(OH)2, the purity of the final reaction product was affected. The reaction product of Ca(OH)2-H3PO4 system was treated by means of ball milling with the guide of dynamic model, and the result indicated that the ball-milling of reaction products increased the degree of reaction and improved the purity of HA powders. The synthetic process was optimized and reaction model of actual synthetic process was established based on the experiment.The single phase of HA was fabricated when reaction temperature was within the range of 0 ℃ to 80 ℃, the particle size of synthetic HA powders was increased with the increase of temperature. The water-ethanol was used as mixed solvent for the research of synthesizing in the temperatures below 0 ℃, and the process was affected by the content of ethanol. While the TCP was obtained when the temperature at-10 ℃ and -20 ℃, respectively. When the volume of ethanol was 10% and 27% of the total volume at 0 ℃, the synthesizing powders was HA, but reaction product was TCP when the anhydrous ethanol volume was 38% of the total volume of the solution. The added dispersants in the process of synthesizing HA did not affect the final product, and hexadecyl trimethyl ammonium bromide (CTAB) as dispersant was better than polyethylene glycol (PEG).The cation-doped hydroxyapatite was synthesized using neutralization method. Ag-HA, Zn-HA and Mg-HA powders could be synthesized using Ag+ and Ag2+ as silver source, ZnO and Zn(NO3)2·6H2O as zinc source, MgO and Mg(NO3)2·6H2O as magnesium source, respectively. However, with the increase of doped amount, the degree of crystallinity and thermal stability of as-synthesized powders decreased, and the impurity of TCP appeared in HA powders. Furthermore, the impurities of TCP, Ag3PO4, Zn3(PO4)2 and Mg3(PO4)2 appeared in HA powder when the mole fraction of Ag+, Zn2+、Mg2+ increased to 5%. Under the dynamic model, the doped-HA reaction model was also established. The results suggested that the metal oxides could be used to synthesize doped-hydroxyapatite in Ca(OH)2-H3PO4 reaction system instead of the soluble salts.