Key parameters for spark plasma sintering of wet-precipitated iodate-substituted hydroxyapatite

A possible strategy for the management of radioactive iodine-129 is to incorporate it under the form of iodate in hydroxyapatite matrices (HA-CaI) intended to be stored in deep geological repositories. To meet disposal requirements, powder materials that were here obtained by a wet-precipitation route required shaping. In this work, we demonstrate that sintering of HA-CaI could be achieved by spark plasma sintering (SPS) at low temperature (T <300 °C) without iodine volatilization. Such densification was only possible if a non apatitic hydrated calcium phosphate layer was present at the surface of grains, the content of which directly depended on synthesis conditions. The higher the proportion of hydrated layer, the better uniaxial cold compaction and densification during sintering. For an optimised HA-CaI powder, a relative density of 88% could be reached using a pressure of 100 MPa, a dwell temperature of 250 °C and a heating rate of 30 °C min−1.