Microweighing in Supercritical Carbon Dioxide
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Carbon dioxide (CO2), when heated and pressurized above its critical temperature (31°C) and pressure (7.377 MPa), is identified as being in a fourth, supercritical state with properties intermediate between liquid and gas. At the critical point, the interface of CO2 liquid and vapor starts to vanish. Supercritical CO2 (scCO2) has liquid-like densities providing good solvent capability, gas-like viscosities, and diffusivities to benefit mass transport, and a nonhazardous nature for the environment. Because of these properties and other advantages of CO2 over organic solvents such as low surface tension and low cost, scCO2 has received increasing industrial and research attention in a variety of processes such as extraction, cleaning, polymer synthesis, and more recently microelectronics processing. CO2 has been proposed to serve as an energy-efficient and environmentally benign solvent platform. The real-time monitoring of the time-dependent mass change is crucial to understanding, characterizing, designing, and controlling the aforementioned processes. However, this monitoring presents a number of challenges because of the difficulty in applying various detection methods under high-pressure conditions.