Calcium Phosphate Powders for Biomedical Applications: Synthesis, Thermal Behavior and Non-Conventional Sintering.
The present work was focused on the synthesis of three different calcium phosphate powders with possible application as bioceramics, their chemical, structural and thermal characterization, and finally their consolidation into dense compounds by conventional and flash sintering techniques. In the first part, Mg-doped (0 - 2 mol% Mg2+) tricalcium phosphate powders with micrometric size were produced by solid state reaction, and the influence of dopant on their sintering behavior and, specifically, on the β→α phase transition was studied. It was shown that magnesium stabilizes β-phase and ensures, after conventional sintering, much better densification and final mechanical properties. Moreover, annealing treatments on sintered compounds are suitable to convert the retained α- into β-TCP only in presence of Mg. Un-doped β-TCP was additionally subjected to flash sintering, thus obtaining dense microstructure at temperatures lower than 1000°C in just 10 min and avoiding any phase transition. A specific physical model based on of thermal-balance equations was considered to investigate the flash sintering process in detail; it was possible to point out that thermal runaway is the main mechanisms that triggers the process, which could be described also in terms of electric behavior of the material, real sample temperature and flash onset. Moreover, the observed blackening effect and the development of an additional resistance contribution at the electrodes were taken into account and discussed. In the second part of the work, Mg-doped (0 - 5 mol% Mg2+) tricalcium phosphate nanometric (~ 20 nm) powders were synthetized by chemical precipitation, thus obtaining highly-defected CDHA easily convertible into β-TCP at 750°C. Magnesium doping was found to inhibit the first crystallization and to promote β-TCP formation directly. The nanopowders were conventionally sintered to produce dense (~90%) β-TCP with sub-micrometric gran size. Flash sintering was also carried out on the nanopowders, demonstrating that the flash event can occur only after CDHA→β-TCP reaction, since the precursor is too resistive for allowing the electrical current flow. A non-linear electrical behavior was found for the β-phase, associated with the grain growth. Flash sintering was also applied in isothermal mode, producing dense sub-micrometric β-TCP at 900°C in just few seconds. It was also possible to build two maps relating the processing parameters for flash sintering on the basis of thermal model and the material behavior. Finally, hydroxyapatite nanopowders were synthesized by chemical precipitation with different amount of Sr2+ replacing Ca2+ into the apatite structure (0 - 100 mol%). The nanopowders were deeply characterized from a morphological, chemical and structural point of view (SEM, TEM, ICP, XRD, FT-IR, 31P-NMR, 1H-NMR, N2 sorption) finding a relation between the experimental evidences and the amount of Sr.