Preparation of Nanofiber-Reinforced Hydroxyapatite/Boehmite Composite Hollow Spherical Particles Using Spray Drying

Hollow spherical hydroxyapatite (HAp) particles have potential as functional fillers and microcapsules; however, their low mechanical strength caused by their structural fragility limits their use. In this study, high-aspect-ratio boehmite nanofibers were incorporated into hollow spherical HAp particles by spray drying to improve their mechanical strength. A boehmite sol was added to an HAp aqueous solution prepared by the CO2 blowing method, and the mixed solution was spray-dried at 100 °C. Hollow spherical particles with a particle size of approximately 1 μm were obtained at boehmite sol additions up to 0.05 mass%. At 0.10 mass% addition, nozzle clogging and irregularly shaped particles were observed. Transmission electron microscopy observations showed fibrous structures within the hollow walls of the boehmite-containing particles. The average compressive strength increased from approximately 0.6 MPa without boehmite addition to approximately 8.5 MPa at 0.05 mass% addition. This value exceeded the practical benchmark pressure of 8 MPa used in the VSI standard, suggesting that the addition of boehmite nanofibers contributed to improving the mechanical strength of hollow spherical HAp particles.