Synthesis of hydroxyapatite biomedical ceramic by spark plasma sintering and it's characterization

Abstract

This work describes the unique features and controlled sintering ability of modern spark plasma sintering machine for the preparation of transparent hydroxyapatite biomedical ceramic. It also investigated the effect of contact electrical poling to measure and justifies its piezoelectric behaviors. In this research work, highly specialized electric current assisted Spark Plasma Sintering (SPS) technique was used for hydroxyapatite (HA) powder sintering. All the sintering parameters were controlled with high precision by its computerized mechanism that suppressed several limitations of conventional sintering techniques. HA biomedical ceramic pellets were prepared at prolonged temperature ranges around 850oC to 1000oC with calcined and un-calcined HA powder. Transmittances of prepared samples were measured by using UV Vis spectrometer. Phase changes in sintered pellets were analyzed by X- ray diffraction pattern and optical micrograph. XRD analysis confirmed phase purity and crystal orientation of produced samples. Relevant orientation indexes of HA materials were measured from X ray diffraction patterns of planes perpendicular and parallel to the applied pressure direction to identify c-axis elongation so as to evaluate the possibility of introducing piezoelectric property in the HA sample. HA samples prepared at sintering temperature 900oC using calcined powder shows maximum transmittance nearly 75%. On the other hand, HA samples that were prepared from un-calcined powder showed maximum transmittance around 70%. Comparatively, lower sintering pressure 3.9kN was found more favorable for producing transparent HA pellets at temperature 900oC but after crossing 975oC, opposite phenomenon was also observed that is higher pressure was better for producing more transparent HA pellets. Textured transparent HA samples were then applied in high voltage contact electrical poling devices to create a net polarization and therefore to induce piezoelectricity. Here, sufficiently strong electric fields were applied across these ceramics to cause reorientation of spontaneously elongated unit cell into more isotropic alignment along the field lines. Finally, applied field was removed and samples were cooled rapidly to induce a net overall piezoelectric property. Here, the effect of applied voltages and poling temperatures were investigated and found to induce longitudinal piezoelectric coefficient (D33) around 0.8pC and 0.5pC for those sample sintered at temperature 875oC and 975oC when they were polled parallel and perpendicular direction of SPS pressure surface respectively. Keywords: Hydroxyapatite, Spark Plasma Sintering, Transparent HA ceramic, high voltage contact electrical poling, Piezoelectricity.