Synthesis And Mechanism Of Pyrochlore-type H₂W₂O₇ Microstructures Prepared By Hydrothermal Method

Pyrochlore-type H2W2O7 not only could be used as the former body of WO3, but itself has wide application prospect and popularization value with the special reticular formation and metastability. The researches on H2W2O7 are still at starting stage at present and commercial production is difficult to realize. The hydrothermal method is hopeful to solve the problem, but it's limited by the complex construction of the hydrothermal system with sodium tungstate solution. Therefore, intensification of the precipitation process aiming to prepare high quality powders of H2W2O7 and increase precipitation yield holds outstanding significance and applying prospect.From the point of view of increasing precipitation yield, the influence of additive species, acidity of the solution and temperature on the precipitation are investigated in detail, and some kinds of organic additives are studied principally. Meanwhile, the product feature and some related influencing factors are analyzed, and preliminary explanation about the mechanism of hydrothermal synthesis of H2W2O7 is presented.The results indicate that the suitable pH range of solution for preparing H2W2O7 could be extended to 3.5-8.9, and the highest precipitation rate could reach 99%. The pH value and stability of sodium tungstate solution significantly affect the reaction rate and morphology of powders, and various additives can create quite different pH environment of the hydrothermal system, which is the key influence of all the factors. The addition of reactants and temperature mainly affect the reaction rate, and the molar ratio of H+ to WO42- has more possibility to be 2. The higher of the pH, the purer the powders of H2W2O7, and there are scarcely any impurities when pH of the solution is higher than 7.0. The powders (generally less than 0.5μm) agglomerate readily, but the agglomerates could be eliminated to some degree by ultrasonic vibration. The precursor of H2W2O7 has been approximately confirmed as W2O72-, and the multicomponent ionic balance in sodium tungstate solution under neutral or weak alkaline environment have been discussed, both of which help to determine the mechanism of hydrothermal synthesis of H2W2O7.