Influence of Mg-doping, calcium pyrophosphate impurities and cooling rate on the allotropic α⇆β-tricalcium phosphate phase transformations

Alpha and beta-tricalcium phosphates are allotropic phases which play a very important role as bone graft substitutes, namely in calcium phosphate cements. Despite extensive research efforts, contradictory reports exist on the importance of quenching for maintaining alpha-TCP purity. The role of calcium pyrophosphate impurities derived from a certain calcium-deficiency, hydroxyapatite impurities derived from calcium excess, and various ionic substitutions on thermal stability of these phases was not yet fully disclosed. The present work reports on the kinetics of alpha <->beta-TCP phase transformations of calcium-deficient TCP powders with different Mg-doping extents (0-5 mol%) prepared by precipitation. Mg clearly enhanced the thermal stability of beta-TCP. The effect of cooling rate was more complex and interdependent on the Mg content and the heat treatment schedule. High alpha-TCP contents were retained upon cooling at 5 degrees C min(-1) for Mg <= 1 mol% or upon quenching from 1550 degrees C for Mg contents <= 2 mol%. (C) 2015 Elsevier Ltd. All rights reserved.