Hydrothermal vapor synthesis of inorganic oxides
Creators: name: Kopp, Daniel
Publisher: name: No Publisher Supplied
Year: 2016

description: A new powder synthesis technique called Hydrothermal Vapor Synthesis (HVS) has been proposed and developed. This technique utilizes pressurized, unsaturated water vapor as the reaction medium to facilitate the crystallization of multi-cation, anhydrous inorganic oxides. HVS is based on the principles of both hydrothermal and solid-state chemistry to promote crystallization of thermodynamically favorable and stable phases. In conventional hydrothermal processes, liquid water is heated in a sealed reactor and is used as the solvent that accelerates a thermodynamically favorable reaction. The pressure within the reactor is dictated by the liquid-vapor equilibria. In contrast, the liquid phase is completely vaporized in HVS, and the unsaturated vapor pressure can be engineered independent of temperature by reducing the reactor fill percent. As a result, the reaction medium can be tuned to access a desired thermodynamic property space. This dissertation develops (1) thermodynamic models, and (2) experimental procedures for the synthesis of magnesium aluminate spinel (MgAl2O4) and calcium silicate (β-CaSiO3) powders in unsaturated vapor. Additionally, this dissertation uses enthalpic calculations to qualify HVS as a low energy technique. HVS was used to crystallize MgAl2O4 from a mixture of Mg(OH)2 and α-Al2O3 precursors. Varying the reaction temperature revealed that MgAl2O4 begins to crystallize at temperatures as low as 370 °C. Reaction rate studies at 390, 430, and 470°C and a water partial pressure of 58 atm revealed the apparent activation energy of MgAl2O4 formation to be 60.67 kJ/mol. Additionally, enthalpic analysis indicates that since HVS requires low quantities of water, the synthesis of MgAl2O4 requires up to 42 and 76% less energy than solid-state and supercritical water (SCW) synthesis, respectively. HVS was also used to crystallize β-CaSiO3 from a mixture of CaCO3 (Calcite) and SiO2 (Quartz) precursors at temperatures as low as 310°C. Increasing the reaction temperature from 310 to 430°C enhanced the reaction rate ~4-fold. The 390°C reaction appeared to have the highest reaction rate, yielding 77.3% β-CaSiO3 in 17 h. Enthalpic analysis of the reaction system revealed that the synthesis of β-CaSiO3 requires 13 and 62% less energy than solid-state and SCW synthesis, respectively. This dissertation opens up a path to multiple synthesis reactions, which were not economically feasible before, by significantly reducing reaction temperature and by utilizing a wide-range of precursors (e.g., oxides, hydroxides, carbonates, etc.) without preliminary thermal or chemical treatments. The utilization of unsaturated water vapor circumvented (1) high reaction pressures, (2) the need for high embodied energy precursors, (3) energy intensive heating of saturated liquid/gaseous H2O associated with hydrothermal and supercritical water techniques, and (4) the high temperatures associated with solid-state reactions. These advantages make this technology a promising method for commercial ceramic powder manufacturing.