Sol-Gel Synthesis of Ca3(PO4)2 and Ca3-xNa2x(PO4)2 Powders for the Fabrication of Bioceramics by 3D Printing

We report techniques for the synthesis of microcrystalline powders of tricalcium phosphate (TCP) (beta-Ca-3(PO4)(2)) and phase A (Ca2.5Na(PO4)(2), an ordered solid solution based on Ca3-xNa2x(PO4)(2) alpha-rhenanite), by the Pechini sol-gel process. The phosphorus-containing reagents used are triethyl phosphate (TEP), PO(OC2H5)(3), and ethylenediamine(tetramethylenephosphonic acid) (EDTMP), (H2O3PCH2)(2)N(CH2)(2)N(CH2PO3H2)(2). We analyze the phase composition, micromorphology, and sintering behavior of the synthesized powders. The use of TEP as a phosphorus-containing reagents leads to a change in Ca/P ratio (stoichiometry) in the final product as a result of TEP vaporization during gel polycondensation and the formation of a mixture of hydroxyapatite (Ca-10(OH)(2)(PO4)(6)) and calcium oxide. If EDTMP is used, the stoichiometry remains unchanged owing to Ca2+ chelating by the complexone: in TCP synthesis, gel thermolysis at temperatures from 750 to 1000 degrees C leads to beta-TCP crystallization (with an average particle size of 1 mu m); in the synthesis of phase A, gel thermolysis at temperatures from 500 to 800 degrees C leads to the formation of a mixture of beta-TCP, beta-CaNaPO4, and phase A (average particle size of 250 nm in the range 500-700 degrees C). According to dilatometry data, the synthesized powders can be densified more rapidly in comparison with powders prepared by ceramic processing route. The present results suggest that sol-gel processing with the use of EDTMP can be recommended for the synthesis of powders of resorbable phosphates, such as TCP and Ca(3 -x)Na(2)x(PO4)(2), suitable for stereolithographic fabrication of osteoconductive bioceramics.