Ultrasonically accelerated synthesis of hydroxyapatite
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Ultrasonic energy was used to accelerate the formation of hydroxyapatite (HAp). The experiments were carried out in aqueous systems on two different sets of reactants: (1) a mixture of Ca4(PO4)2O(TetCP) and CaHPO4 • 2H 2 O (brushite) and (2) a-Ca 3 (PO 4 ) 2 (a-TCP). The reaction systems were exposed to ultrasound of 20 kHz for various times ranging from 5 to 80 min. The products were characterized by XRD and SEM. Parallel experiments without ultrasound were carried out for calibration. The results show that the ultrasound substantially accelerates both reactions. With ultrasound, the time required for the TetCP-brushite system to complete the reaction forming HAp was reduced from 9 h to 25 min at 25 °C, and from 3 h to 15 min at 38 °C. At 87 °C, a-TCP does not hydrolyze within 1 h in de-ionized water unless the pH is adjusted. Hydrolysis of a-TCP was induced by sonication in less than 20 min, and longer treatment results in the formation of a homogeneous sol of HAp.
I. INTRODUCTION The composition of stoichiometric hydroxyapatite is Ca 5 (PO 4 ) 3 OH. The "HAp" structure is the main inorganic constituent of human hard tissue. Because of its similarity in both composition and structure to bones and teeth, synthetic HAp is the most biocompatible material yet identified, and it has been studied for decades. Conventionally, HAp can be synthesized by various methods, which can be classified into two categories: aqueous phase synthesis or high temperature solid state synthesis. Synthesis involving an aqueous phase includes hydrolysis, acid-base reaction, hydrothermal reaction, etc. For example, the hydrolysis of brushite (CaHPO4 • 2H2O),1 at 6 0 - 7 0 °C and pH 8 or so, or hydrolysis of a-TCP, 2 under similar conditions, will both give HAp powder. In the acid-base reaction method in which two or more phosphate compounds are used, one is relatively more basic and the other more acidic; for example, Ca 4 (PO 4 ) 2 O and CaHPO 4 • 2H 2 O or CaHPO 4 also produce HAp at low temperature.3'4 HAp can be prepared by hydrothermal treatment of the precipitate5 obtained by reaction between Ca(NO 3 ) 2 and H 3 PO 4 . An example of a synthesis of HAp by solid state reaction is reaction between CaCO 3 and Ca 3 (PO 4 ) 2 at 1200-1300 °C in water vapor for 2-3 h.6 These reactions to produce HAp take a few hours to several days to complete. In the case of the acid-base reaction mentioned, HAp can be formed in situ under physiological conditions (at about 38 °C), but the reaction may take about a week to complete.4 The morphologies of the HAp formed by these methods are highly dependent on the nature of starting materials and the processing conditions used. Recently, acoustic waves have been used by Roy et al. to stimulate inorganic chemical reaction at or near 2294
J. Mater. Res., Vol. 7, No. 8, Aug 1992
room temperature.7 The results show that the power ultrasound enhances the reactivities of inorganic systems near room temperature. For example, a 20-min exposure of A12O3 + P 2 O 5 + H 2 O to 20 kHz ultrasonic waves produced a highly
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