Hydroxyapatite (Ca10(PO4)6(OH)2, HAp) is one of the most attractive materials for vertebrate and dental implants due to its compositional and biological similarity to native tissues. HAp exists over a compositional range that can be characterized in terms of its Ca/P ratio. Stoichiometric HAp has a Ca/P ratio of 1.67, while calcium-deficient HAp has a Ca/P ratio as low as 1.5. HAp can be prepared in bulk by normal ceramic processing methods using fine powder and a precursor at high temperatures. Such a powder results in easy handling, casting, and sintering, leading to an excellent sintered body. Several investigations on the precipitation of HAp or its precursor phases from Ca2+ and PO^-bearing solutions and an acid-base reaction at constant composition have been carried out; however, these methods need highly qualified and controlled parameters, such as the nature and composition of the starting materials, and the pH and temperature of the solutions prepared to obtain an

HAp monophase. However, high-intensity ultrasound waves can stimulate the reactivity of chemical species involved, resulting in the acceleration of the heterogeneous reactions between liquid and solid reactants. Ultrasound was irradiated to an aqueous suspension containing phosphoric acid (H3PO4) and calcium hydroxide (Ca(OH)2) to investigate the sonochemical effect on the preparation of hydroxyap-atite (Ca10(PO4)6(OH)2, HAp). The HAp monophase could be synthesized from the suspension sonicated for 60 min [110 and references therein].

1. The HAp monophase can be synthesized from an aqueous suspension containing H3PO4 and Ca(OH)2 with the aid of ultrasonic irradiation under atmospheric pressure, and this synthesis reaction progresses more effectively than that by the heating method.

2. The synthesis reaction for HAp in the ultrasonic field is almost completed by 60 min in the present condition. The completion of the reaction is confirmed by monitoring the equilibrium pH of the suspension to be a neutral value.

3. The ultrasonic irradiation leads to the formation of very fine HAp powder with a relatively narrow size distribution.

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