| Summary: | 碩士 === 大同工學院 === 材料工程研究所 === 87 === A modified hydrothermal method was used to synthesize calcium phosphate powders and coatings. This modified hydrothermal method, that designed by our laboratory, have larger thermal gradient (△T ) in the reaction chamber than the traditional autoclave, and the reactive mechanisms are quite different between these two methods.
The suitable condition for the powder synthesis is at 300℃ for 24hr, and the phases of the calcium phosphate powders synthesized by the hydrothermal method mainly include CaHPO4,
Ca(OH )2, Ca2P2O7, and Ca10(PO4)6(OH)2. With higher temperature and longer heating time, the reaction becomes more completed.
When the Ca/P ratios of the raw materials are low, Ca2P2O7 can be obtained. As the Ca/P ratio increases, CaHPO4 gradually becomes the major phase of the powders. Hydroxyapatite can be produced with more quantity of calcium oxide. However, as the quantity of the calcium oxide powder continues increasing, the peaks of the hydroxyapatite phase does not become stronger; only the peaks of the Ca(OH )2 greatly increase. Adding more phosphoric acid into the system, that is, decreasing the pH value in this system, hydroxyapatite phase could be easier observed.
When synthesize calcium phosphate coatings onto 304 stainless steel substrate, the surface processes of the ceramic coating growth are regarded as function of the concentration of the reactants and the temperature difference immediately at the surface. The process is mainly assumed to occur in some stages: First, the surface of the substrate would be modified by the phosphoric acid; then the powders coming along with the vapor condense into the substrate. Those adsorbed powders are gradually built into the lattice. After the nucleation and growth processes, finally the coating and some crystal could be attained.
The major composition of the coating is iron phosphate hydroxide (Fex(PO4)y(OH)z), and the color of it is green. The crystals obtained in this experiment include calcium pyrophosphate (JCPDS #44-762 Ca2P2O7.H2O), tricalcium phosphate (JCPDS #9-348 Ca3(PO4)2), and monetite (JCPDS #9-80 CaHPO4).
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