The principle of determining specific surface area using the gas adsorption method relies on the adsorption behavior of gases on solid surfaces. At a given pressure, the sample's surface (adsorbent) can reversibly adsorb gas molecules (adsorbate) at ultra-low temperatures, and there is a defined equilibrium adsorption amount corresponding to each pressure level. By measuring this equilibrium, the specific surface area, pore volume, and pore size distribution of the tested material can be calculated using theoretical models.
High-purity nitrogen and liquid nitrogen are commonly used as adsorbents due to their availability and good reversible adsorption properties, making them widely applied in surface area measurements. However, for microporous materials like molecular sieves and activated carbon, or samples with low surface area such as natural ores and organic materials, nitrogen may not provide accurate results. In such cases, other gases like argon, carbon dioxide, or helium are often preferred.
Argon gas is particularly suitable for micropore analysis due to its stable adsorption at 87K (liquid argon) or 77K (liquid nitrogen). It offers three main advantages: first, argon is a non-polar, spherical molecule, reducing unwanted interactions that can complicate pore size identification. Second, it allows adsorption at higher P/P0 values compared to nitrogen, improving measurement accuracy. Third, using argon at 87K can shorten equilibration time, enhancing testing efficiency.
However, argon is only effective for pores smaller than 12 nm, as capillary condensation ceases beyond this size, limiting its use to micropore testing. For activated carbon samples rich in micropores, carbon dioxide is often used instead. Adsorbed at 273K (freezing point), COâ‚‚ provides a higher diffusion rate, making it ideal for measuring saturated adsorption capacity. Although the vapor pressure at this temperature is high (3485.3 kPa), it remains effective within the microporous range. Higher P/P0 values require specialized equipment.
For materials with small surface areas, such as metal powders, organic materials, and certain natural ores, helium is a preferred choice due to its low molecular weight and strong diffusivity.
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