Abstract
To enhance the mechanical reliability of porous calcium hydroxyapatite (HAP) ceramics utilized as biocompatible implant materials, hexagonal rod-shaped crystals of HAP were formed using sintering aids of metal salts. The firing conditions and the suitable sintering aid salts were determined from the results of morphological observation of the hexagonal crystals in the porous ceramics. The observation results also deduced the mechanism of crystal growth. Porous Ca-HAP ceramics were prepared by freeze-dry processing as follows. Synthetic hydroxyapatite powders calcined at 800°C were suspended in a metal salt solution as starting feed material for freeze-drying. Spherical drops of the slurry were instantly frozen in a liquid nitrogen bath. The frozen particles were dried in a vacuum chamber. The dried particles were then fired in an oven controlled at 1200°C with 1-hour soaking time. The open porosity induced by sublimation of ice crystals in the freeze-dried particles was retained after firing. XRD, EDS and porosity measurements of the fired samples were performed as well as SEM observation. Monovalent metal sulfate and chloride salts such as K2SO4, Na2SO4 and Li2SO4 behaved as useful sintering aids to form hexagonal rod-shaped crystals with aspect ratios ranging from 2 to 12. All the above sintering aids could form liquid phase during firing process. In the fired body obtained from tightly packed green body, no rod-shaped crystals were observed. For hexagonal crystals to grow into rod-shaped ones, adequate spatial room might be necessary.
