Investigation of the microwave sintering of carbonated hydroxyapatite
Résumé
With improved solubility and biodegradation rate, carbonated hydroxyapatites (CHA) are promising bioceramics for filling bone defect compared to unsubstituted hydroxyapatite (HA, Ca10(PO4)6(OH)2). Carbonate ions in the HA lattice can substitute either for OH- to form A-type CHA (CAHA) through thermal treatment under CO2-rich atmosphere or for PO43- to form B-type CHA (CBHA) through synthesis by wet precipitation. Controlling the amount and location of CO32- as well as the microstructure, which affect the biodegradation, depends on the fabrication process and remains challenging. The sintering of CBHA is usually achieved through resistive sintering under a CO2-rich atmosphere to prevent thermal decomposition. This atmosphere causes an enrichement of A-type CO32- known to hinder densification. Sufficient consolidation thus requires high temperature or longer dwell time which may be accompanied by thermal decomposition or grain growth. To overcome these limitations, we investigate the microwave (MW) sintering which allow faster processing time. In this work, the sintering of CBHA containing 0.8 mol.% of CO32- was studied in a MW multimode cavity under air. Pellets starting from CBHA alone and from CBHA mixed with carbon powder to generate in-situ a CO2-rich atmosphere were considered. Sintering were performed at 900 °C and 1000 °C with different ramps and dwell times. Final densities were ranged between 70 and 85 % of theoretical density. From XRD and FTIR results, it was not possible to sinter a pure CBHA. With carbon, substitution of A-type CO32- appeared. The presence of carbon was thus effective for generating a CO2-rich atmosphere but insufficient, in our conditions, to avoid the decomposition. Further studies are under progress to optimize the sintering conditions.