1. Solid State Synthesis of Nanocrystalline Hydroxyapatite by Non-Equilibrium P/M Processing
- Author
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Kouzi Kitahara, Masaaki Naka, and Hiroshi Kimura
- Subjects
Materials science ,Metals and Alloys ,Analytical chemistry ,Superplasticity ,Activation energy ,Strain rate ,Condensed Matter Physics ,Nanocrystalline material ,Mechanics of Materials ,Materials Chemistry ,Newtonian fluid ,Crystallite ,Ball mill ,Powder mixture - Abstract
The solid state P/M processing that consists of rotating-arm reaction ball milling and pulse electric discharge consolidation provide a process control methodology to develop the hydroxyapatite (HA) with unique property inherent to the nanocrystalline (nc) structure. The mechanical alloying of the powder mixture is conducive to the solid state synthesis of HA{Ca10(PO4)6(OH)2} having the average crystallite size of 7 nm according to a reaction, 6CaHPO4•2H2O+4Ca(OH)2→nc-Ca10(PO4)6(OH)2+8H2O; this overall process is given by Johnson-Mehl-Avrami equation with the exponent of 1. The nc-Ca10(PO4)6(OH)2 powder compact can be consolidated at a full density; its rapid densification during heating is expressed by an Arrhenius-type equation of Newtonian viscous flow with the activation energy of 402 kJ•mol-1 under both pressure of 80 and 140 MPa. The average crystallite size of bulky Ca10(PO4)6(OH)2 increases from 10 to 50 nm with increasing temperature up to 1173 K. The high-speed superplastic forging is achieved having the strain rate of nearly 10-2 s-1 and the compressibility of 0.65 at a relatively low temperature of 1137 K, and characterized by the strain rate sensitivity exponent ranging from 0.7 to 1 and the activation energy of 75-113 kJ mol-1.
- Published
- 2006