Supercritical drying critical temperature

Carbon dioxide, as can most other substances, can exist in any one of three phases—solid, liquid, or gas—depending on temperature and pressure. At low temperatures, carbon dioxide exists as a solid ("dry ice") at almost any pressure. At temperatures greater than about -76°F (-60°C), however, carbon dioxide may exist as a gas or as a liquid, depending on the pressure. At some combination of temperature and pressure, however, carbon dioxide (and other substances) enters a fourth phase, known as the supercritical phase, whose properties are a combination of gas and liquid properties. For example, supercritical carbon dioxide (often represented as SCC02, SC-C02, SC-CO2, or a similar acronym) diffuses readily and has a low viscosity, properties associated with gases, but is also a good solvent, a property one often associates with liquids. The critical temperature and pressure at which carbon dioxide becomes a supercritical fluid are 31.1°C (88.0°F) and 73.8 atm (1,070 pounds per square inch). 

The PCA process uses supercritical fluid drying to help preserve fine microstructures in the material. Supercritical fluid drying is a technique that has been used for many years to dry biological materials and, more recently, aerogels (qv). The original solvent is replaced by exchange with a supercritical fluid, such as C02, and the system is depressurized above the critical temperature of the SCF. SCFs have no vapor—liquid interface. Thus fine microstructures are... 

Some of the particles were studied after supercritical point drying. The particles were dispersed in acetone by several centrifugation steps (45,000 X g). In a critical point dryer (Balzers Union, CPD 020), the carbon-coated grids and the dispersion in acetone were placed. Under pressure, the acetone was exchanged against liquid CO2 after increasing the temperature and passing the critical point, the particles adsorbed on the film were supercritically dried. 

Cracks can also appear during the pressure release in the autoclave. In the supercritical drying process, the gel is subjected to high temperature and high pressure. When the critical point is reached, the pressure of the autoclave is decreased while the temperature is kept constant. At this instant, the pressure applied to the supercritical fluid is equal to that within the pores. The supercritical fluid has a very low density and viscosity compared with that of the liquid at room temperature however, the low permeability of the gel resists the flow of the supercritical fluid out of the gel. In other words, if the supercritical fluid release is performed too fast a pressure gradient appears. In this case the supercritical fluid within the gel, which is in compression, suddenly expands and the solid part suffers tensile stress. Experiments show that cracking depends on the pressure release rate, on the nature of the gel (basic or neutral), and on its geometrical dimensions. 

Supercritical drying can be performed at high temperature, the lower limit being the critical point of the filling solvent (i.e. methanol Pc=80.9bar, Tc=512.6K, ethanol Pc=61-4bar, Tc=513.9K, acetone Pe=47.0bar, 7 =508.1 K) or at low... 

Use supercritical (hypercritical) drying the liquid is removed above its critical temperature, T, and critical pressure, p. The values of T and for the commonly... 

Supercritical drying [33,34] was initially developed by Kistler to obtain materials having large pore volume and specific surface area. A fluid is qualified as supercritical when its pressure and temperature exceed values, e.g., carbon dioxide has the low critical parameters (T = 31°C, = 7.29 MPa),... 

In contrast to supercritical drying, operating parameters for subcritical drying including temperature and pressnre are below the critical point. Wei et al. [35] nsed E-40 (multi-polysiloxane) as silicon source and isobutyl alcohol as solvent to prepare Si02 gel. The prepared gel was placed in autoclave, adding an appropriate amonnt of isobutyl and surfactant, preserved sometime under pressure of 2.3-2.6 MPa, temperature of 240°C-260°C, and then deflated slowly and cooled naturally hydrophobic SiOj aerogel was finally obtained. 

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