Development Of Oxygen Fugacity Sensor For High Temperature-High Pressure Hydrothermal Systems

High temperature-high pressure hydrothermal systems exist extensively in nature and industrial processes. As long as there are multi-valent elements in the systems, oxygen fugacity in the systems will be one of the most important physicochemical parameters, like temperature and pressure, which affects the properties of the systems and controls the occurrence and evolution of the processes in the systems. Therefore, both in the field of geochemistry and in the fields of other sciences and technology, it is critically important to monitor oxygen fugacity in situ in hydrothermal systems.To solve the current embarrassments of low working temperature, long response time, and controversy of the measurement principle faced in the application of the YSZ oxygen sensor for measurement of the oxygen fugacity in high temperature-high pressure hydrothermal system, this paper first designed and manufactured a new pattern of Y₂O₃-ZrO₂ (YSZ) solid electrolyte oxygen sensor and autoclave hydrothermal reactor. Then the YSZ oxygen sensor was installed successfully into the autoclave at high temperature-high pressure hydrothermal condition.Before the trial of the whole new system above, we measured the temperatures at the inner center and at the center of the outer wall of the autoclave, and the temperature of the metal cone, which was located next to the YSZ sensor. The results showed that there was an difference of more than 15℃ between the inner temperature and the outer temperature of the autoclave and a gradient of 5-6℃ from the metal cone to the inner center of the autoclave. The temperature difference between the metal cone and YSZ oxygen sensor was measured to be less than 2℃All the feasibility experiments of our YSZ oxygen sensor in both dry systems at ambient pressure and hydrothermal systems at high temperatures and high pressures with controlled oxygen fugacity indicated that the emfs measured using our YSZ oxygen sensor were consistent with the emfs calculated using Nemst equation. These results confirmed that our new type of YSZ oxygen sensor observed excellent Nernstbehavior and, therefore, was applicable to monitoring the oxygen fugacity in situ in high temperature -high pressure hydrothermal systems.Using CU-CU2O, Ni-NiO, and CU2O-CUO as the reference buffers of the tested YSZ oxygen sensor, we monitored in situ the oxygen fugacity of supercritical water with a 40% filling at different temperatures. The results showed that our data were reproducible at the same experimental conditions in different runs and in the heating and cooling processes of the same run. Moreover, the results were consistent each other when different reference buffers were used. In the tested temperature range of 500-580°C, it was found that the oxygen fugacity of supercritical water increased linearly with temperature increase, which could be interpreted by the oxygen-consuming corrosion of the inner wall of the titanium alloy autoclave.Using CU-CU2O as reference buffer, we also measured the oxygen fugacities of NaCl and NaOH supercritical fluids with a 40% filling and different concentrations of initial solutions at 500°C and 540°C. The results revealed that at a given filling percentage, the oxygen fugacities of both systems were closely related to temperature and the concentration of initial solution. As the concentration of the initial solution was fixed, the oxygen fugacity increased with temperature increase in both supercritical NaCl and NaOH fluids. At the same temperature, oxygen fugacities in both systems first increased, and then decreased as the initial solution changed from 0.01M through H2O to 0.01M NaCl, with the highest oxygen fugacity occurring at 0.001M NaOH. At the same time, we also found that in both supercritical NaCl and NaOH fluids, the rate of the oxygen fugacity variation became more and more slowly with the concentration of the initial solution increasing at the same temperature. As in the supercritical water above, all these characteristics could be interpreted by the oxygen-consuming corrosion of the inner wall of the titanium alloy autoclave.