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Queen Mary University of LondonQueen Mary University of London
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School of Engineering and Materials Science
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PhD Thesis: A novel microwave synthesis of calcium hydroxyapatite: optimisation and investigation of a microwave-assisted reaction route

Author: ZAWAHREH, Yousef

Year: 2001

Supervisor(s): Liz Tanner

Hydroxyapatite is a bioactive calcium phosphate used in non-load bearing applications, such as space-filling in maxillofacial reconstruction. As a coating, hydroxyapatite is used on load-bearing orthopaedic metal prostheses to improve fixation and/or biocompatiibility. Conventional synthesis processes for the production of hydroxyapatite are time-consuming and labour intensive. Microwave irradiation was investigated as a means to enhance the synthesis reaction using calcium hydroxide (Ca(OH)2) and orthophosphoric acid (H3PO4) as reactants.

An initial set of reactions indicated the feasibility of the microwave synthesis route. Optimisation reactions were then performed followed by investigation sets of reactions. Parameters such as microwave power, irradiation time, and reactant concentrations were varied. Using 0.5M Ca(OH)2 and 0.3M H3PO4, a phase-pure hydroxyapatite powder with a stoichiometric molar Ca/P ratio of 1.67 was produced in 60 seconds at 450W and 2.45GHz.

The microwave synthesis reaction only reached completion when conducted at moderate temperatures (20-30°C). At approximately 5°C and at temperatures in excess of 45°C, the microwave revealed a 3 to 4-fold increase in the activation energy of the microwave reaction compared to a non-microwave equivalent. The Arrhenius pre-exponential factor was half that of the non-microwave reaction. The kinetic data suggsts two energetically different pathways for the reaction between Ca(OH)2 and H3PO4. A fast high-energy pathway is powered by microwave irradiation, while a slow low-energy pathway prevails under conventional synthesis conditions.

Microwave synthesis has potential for scaling-up to commercial amounts of hydroxyapatite powder production. Control over the product powder morphology may be achieved with further optimisation.