The densification of zeolite/apatite composites using a pulse electric current sintering method: A long-term assurance material for the disposal of radioactive waste

Pulse electric current sintering (PECS) method was applied to the fabrication of zeolite, hydroxyapatite (HAp) and fluorapatite (FAp) sintered bodies that should be long-term assurance materials for the disposal of radioactive waste. The weight ratio of zeolite and apatite was ca. 3/7. Zeolite powder evenly covered with HAp thin layers prepared by a hydrothermal method and spherical FAp powder by spray dryer were employed for the PECS; the sample was sintered at 900 °C for 10 min at a rate of 50 °C/min under a uniaxial pressure of 50 MPa and then cooled to 600 °C at 5 °C/min in vacuum. The powder X-ray diffractions indicated that the structure of zeolite changed to the amorphous. The zeolite powder was well dispersed in FAp matrix as the results of element mapping analyses by energy dispersive X-ray spectrometer. The observations by a scanning transmission electron microscope indicated that amorphous zeolites were covered with needle-like HAp crystals of which layer completely coupled with sintered FAp grains. The HAp thin layers thus play an important role for improving the affinity between FAp matrix and the zeolite. The microhardness and three-point bending strength of the sintered bodies were also elucidated by a dynamic-ultra microhardness tester and a universal tester, respectively.