Synthesis of the Hg Ba2 Ca2 Cu3 O9+x superconductor ceramic: thermodynamic analysis

Eh-pH diagrams of the systems Hg-H2O, Ba, H2O, Ca-H2 O at 25 deg C were constructed for selected activities of the species of interest and then from these diagrams have been extracted values of the couples (activity, pH) for the predominance limits of the species for the Eh-pH level corresponding to the equilibrium of the aqueous solution with an oxygen fugacity equal to 0.21 atm. With these extracted data, the corresponding pa-pH diagrams were constructed; therein being equal to Hg, Ba, Ca or Cu. The four pa-pH diagrams were then overlapped each other and analyzed in terms of the insight into the co-precipitation from an aqueous solution containing all of them. The conclusion taken is that at a pH > 11 and moderately high concentrations of the components it is possible to co-precipitate Hg O, Cu O and Ca (OH)2, but not the barium, once the barium hydroxide is extremely soluble. So, its co-precipitation is only possible by means of the use of ammonium (our sodium) carbonate or, alternatively, through complete evaporation of the solvent. Finally, it is discussed the need for the use of high fugacity of oxygen in order to keep as much as possible the stability of the Hg O, as well as the adversity born from the presence of barium and calcium carbon in the system. (author)