Continuous Hydrothermal Flow Synthesis and Characterisation of Nano-Bioceramics and their Rapid Consolidation Using Spark Plasma Sintering

Abstract

Accidents, surgery and disease often result in the use of biomimetic materials that can replace human hard tissue and calcium phosphate bioceramics are ideally suited for this purpose. Indeed, biological apatite is a poorly crystalline, non-stoichiometric carbonated hydroxyapatite. The composition, crystallinity and particle size of synthetic calcium phosphate bioceramics directly affect their biological, mechanical and thermal performance. Hence control over these properties in synthetic bioceramics is essential in order to mimic human hard tissue in functionality. The existing methods of synthesis of calcium phosphate bioceramics are multi-step, time consuming and require strict control over synthesis conditions. Therefore, there is a requirement of a one-step, rapid synthesis technology which allows control over particle properties. The continuous hydrothermal flow synthesis (CHFS) technique addresses all such issues but it has not been used to synthesise calcium phosphate based nano-bioceramics. The work in this thesis involves the use of CHFS technology to synthesise calcium phosphate bioceramics. It was demonstrated that the rapid crystallising environment in a CHFS system resulted in phase-pure crystalline hydroxyapatite (HA). Traditionally required long ageing times and heat-treatment were avoided. Furthermore, variations in the CHFS system parameters were correlated with properties of the synthesised nanobioceramics. The CHFS system was also used to substitute biologically beneficial ions (C03'-, Si044-, Mg2+ and Zn2) into HA. Some ionic substitutions affected thermal stability and phase composition. For example, increase in magnesium contents in solution resulted in precipitation of a phase pure Mg-Whitlockite phase. Conventional consolidation methods of HA powders require several hours of exposure to elevated temperatures which results in large grains, phase decomposition and poor mechanical properties. Spark Plasma Sintering on the other hand is capable of very high heating and cooling rates. Phase-pure and ion-substituted calcium phosphates and zirconia-hydroxyapatite phase mixtures were spark plasma sintered to high densities with these materials displaying good mechanical properties.