Monodisperse silica spheres in the 50-520 nm size range were prepared by using the Stober process. Diffusive growth has been determined from Nielsen chronomal analysis for the 520 and 310 nm monodisperse silica spheres. The densification behaviour and evolution of the microstructure of the sphere compacts indicated an inverse dependence of shrinkage rate on the sphere size due to viscous sintering. The increase in sphere size from 50 to 500 nm shifted the densification temperature from $ 1120 1C to 1240 1C. The amorphous nature of the spheres was conserved up to 1200 1C where cristobalite crystal nucleation started and complete transformation to cristobalite phase has been observed upon heat treatment at 1300 1C. The activation energies for viscous sintering according to the Frenkel and Mackenzie/Shuttleworth models were calculated as 125 and 335 kJ/mol, respectively. These substantially low activation energies can be attributed to the presence of a significant level of silanol groups.

Abstract. Sintering and mechanical behavior of pure and hydroxyapatite (HA) whisker reinforced HA composites were investigated in this work. Pure and composite samples were prepared by using a commercial powder and whiskers prepared by molten salt synthesis. The dry-pressed samples were sintered in the 800 and 1300°C range. The effect of whisker-addition on the mechanical properties of HA was investigated through compression and hardness testing. Compressive strength and fracture strain were observed to increase by the addition of whiskers.

The preparation of calcium phosphate based powders through chemical precipitation in order to determine the optimum conditions for the preparation of stoichiometric, thermally stable hydroxyapatite (HA) powders was investigated in this work. The effects of parameters like pH, aging time, and heat treatment temperature on the phase structure and morphology of the powders were investigated by SEM and XRD.HA was prepared at a Ca/P ratio of 1.67. The precipitation pH had a profound influence on the morphology and crystallite size of the powders in the 4-11 pH range. Relatively large 5µm prismatic particles were obtained at low pH whereas the powders were formed from agglomerates of nano sized particles at high pH. Powders had a HA dominant phase structure above a pH of 8. Phase pure HA powders were obtained with close to theoretical yields at pH = 10 and 11. The crystallite sizes of the powders varied from about 30 nm at 90°C to 50-90 nm at 1250°C with the smallest crystallite sizes obtained for pH 10 and 11 powders at 1250°C. The crystallite size increased significantly with aging temperature.

Abstract.The goal of the present work was to investigate the effects of several cement preparation parameters on setting and hardening reaction mechanisms and hydroxyapatite (HA) cement properties. A central composite experimental design (CCD) was conducted by choosing particle size, solid to liquid ratio, pH, seed concentration and buffer concentration as design parameters along with compressive strength and setting time being the responses. Tetracalcium phosphate (TTCP) powders were prepared by heat treatment of calcium and phosphate source mixtures in the 1200-1400 o C temperature range followed by quenching to room temperature in a dessicator. The second phase used in the formulations (brushite) was prepared by aqueous chemical methods. A series of HA pastes/cements were prepared by changing the above mentioned design parameters. Cements were characterized by a standardized setting time test, mechanical testing machine, SEM and XRD. HA cements with the desired properties can be formulated by using CCD in which the responses were expressed by a second order polynomial equation of the parameters. Compressive strengths for the majority of HA cements were determined to be in the 100-160 MPa range which is significantly higher than those reported in the literature. IntroductionCalcium phosphate cements (CPCs) have been increasingly used for bone repair and regeneration of defects since their discovery in 1983[1]. Moldable CPC paste self setting in vivo offer significant advantages in several clinical applications. CPCs may also have great potential in drug delivery. CPCs are obtained by mixing one or several calcium phosphate phases with an aqueous solution forming a paste which sets and then hardens in minutes. The nature of these setting and hardening reaction mechanisms are still under investigation. The dissolution rates of the components and the nucleation/growth rate of the cement phase have a determining role in establishing the cement properties. The commonly known most important parameters used for controlling the rate of these reactions are solid to liquid ratio, characteristics of the powder phases, pH and composition of the liquid phase, HA seed amount, particle sizes/ratios of the solid phases, use of inhibitors, etc. [2,3].The ability to control these reaction rates is essential in providing sufficient time to the surgeon, necessary viscosity and hardened strength for the success of the application. The main goal of a significantly large number of CPC formulations developed since their discovery is the control of the development of cement paste/structure/properties as a function of time. Various current and proposed CPC applications all have specific setting and hardened cement properties.The goal of the present work was to investigate the dependence of the cement setting time and hardened cement compressive strength on five important cement formulating parameters by using a five-factor central composite experimental design approach. The use of these generated equations relating the two important pr...