Hydrothermal Synthesis of Fibrous Calcium Vanadate Crystal in the System CaO-V205-H2O

Fibrous calcium vanadate crytals in the system CaO-V205-H2O were hydrothermally synthesized over the temperature range of 200 to 600° C and pressure of 20 to 350 atm. The reaction lasted up to one month. Chemicals used as starting materials, V2O5 and Ca (OH)2were of reagent grade. Techniques involving X-ray diffraction, microscopy and differential thermal analysis were applied for the examination of vanadate crystals. The pH of the solution after the reaction was also recorded. The calcium vanadate fiber tended to increase in length with increasing temperature and pressure. The fiber also grew in a different way according to the reaction time at a given pressure (200 atm); up to 320° C it tended to lengthen with increasing reaction time, but at 400° C it decreased in length with the time involved. The best result was yielded by treating the reaction mixture for 1 hr at 450° C under a pressure of 300 atm. The length of fiber was also dependent on the pH of the solution for the reaction conditions involving temperature of 250, 320 and 400° C-pressure of 200 atm-treatment time of 2 days. Within the temperature range given about the same length of fiber was grown, as far as the pH was held constant. The pH 4 was found to yield the longest fiber after the reaction. According to X-ray diffraction measurements smerwoodite became a stable phase below about 320° C, while above 400° C both hendersonite and simplotite grew together. An unknown phase with a platy crystal habit was produced together with smerwoodite at temperature range above 500° C. DTA and TG curves indicated that a large peak near 645° C for the endothermic reaction should be attributed to the release of bonded water. A fiber (smerwoodite) having a maximum length of 3.4 mm and 10 μ m in diameter was synthesized in this way, which exhibited a light brown color and flexibility.