OutgassingRate

Outgassing rate is the rate at which a material releases volatile gases when placed in vacuum or low-pressure environments. It is used to characterize materials for vacuum systems and spacecraft, where released gases can contaminate surfaces, degrade performance, or affect pressure and optical properties. The rate is typically expressed as a quantity per unit area, such as Torr·liters per second per square centimeter (Torr L s^-1 cm^-2), or in alternative units like molecules cm^-2 s^-1 or Pa m^3 s^-1 m^-2. In practice, the value often refers to the outgassing rate after a defined bakeout or conditioning procedure and under specified temperature and pressure.

Measurement and reporting: Outgassing rate is determined in a controlled vacuum test. A sample is cleaned and subjected to a defined bakeout, then mounted in a vacuum chamber and isolated from pumps to observe the pressure rise over time. The rate can be computed from the slope of the pressure vs time curve, corrected for pumping and volume, and normalized to the surface area. Materials researchers publish representative values for common materials under specified test conditions.

Factors: The rate depends on material composition, surface finish, prior history (cleanliness), temperature, and exposure to vacuum. Water and hydrocarbons are common contributors. Bakeouts, surface treatments, and coatings can substantially reduce outgassing. The rate generally decreases with time as volatile species are depleted.

Applications and relevance: Outgassing rate is a critical parameter in designing high- and ultra-high-vacuum systems, semiconductor fabrication equipment, and spacecraft payloads. Materials are selected for low outgassing, and post-cleaning conditioning is standard practice to minimize contamination risk.