The principle of measuring specific surface area by gas adsorption method is based on the adsorption characteristics of gas on a solid surface. Under a certain pressure, the surface of the sample particle (adsorbent) under test has a reversible physical adsorption effect on gas molecules (adsorbate) at ultra low temperature, and There is a certain equilibrium adsorption amount corresponding to a certain pressure. By measuring the equilibrium adsorption amount, the specific surface area, pore volume, and pore size distribution of the sample to be measured are equivalently determined using a theoretical model.
High-purity nitrogen and liquid nitrogen (coolant) are the most commonly used adsorbates due to their easy availability and good reversible adsorption characteristics, and are widely used in the measurement of specific surface area. For microporous samples with small pores and slow diffusion, such as molecular sieve and activated carbon; and samples with small specific surface area, such as natural ore, organic materials, etc., nitrogen is a limitation for adsorption gas, and argon can be selected , Carbon dioxide gas, krypton gas, etc. as adsorption gas.
Argon gas can be adsorbed stably on the surface of the material at 87K liquid argon temperature or 77K liquid nitrogen temperature as the adsorption gas, which is widely used in the micropore test of molecular sieve samples. There are three main reasons:
1. Nitrogen molecules are polar molecules and there is a quadrupole dipole distance, which strengthens the force between the adsorbate molecules and the pore wall of the uneven molecular sieve, and characteristic adsorption is easy to occur, which makes it difficult to identify molecular sieve with different pore sizes; Argon gas molecules are spherical, non-polar, monoatomic molecules that can obtain a more precise distribution of micropores.
2. For a certain hole width, nitrogen requires a lower P / P0 than argon. Therefore, argon is used as the adsorption gas, and the microporous adsorption can be carried out at a higher P / P0 point, which is beneficial to improve the test accuracy.
3. Argon gas can be adsorbed at the liquid argon temperature of 87K, increasing the temperature of the cold bath, which is helpful to shorten the equilibrium time and improve the test efficiency.
The limitation of argon as the adsorption gas test is that the capillary condensation will disappear after the pore diameter is greater than 12nm, so it can only be used for micropore testing.
For activated carbon samples with more micropores, you can choose to use carbon dioxide as the adsorbate and adsorb at the freezing point, which is mainly used to test the saturated adsorption capacity of activated carbon. The adsorption temperature of carbon dioxide (273K) has been greatly increased relative to the adsorption temperature of argon and nitrogen (77K or 87K), greatly increasing the gas diffusion rate. Therefore, for the activated carbon sample, the choice of carbon dioxide adsorption at the freezing point has the characteristics of high efficiency, easy diffusion, and easy to obtain the saturated adsorption amount, which is more suitable for the test of the activated carbon saturation adsorption capacity. However, the saturated vapor pressure of the freezing point of carbon dioxide (3485.3KPa) is too high, and can only be adsorbed in the range of micropores, and cannot reach the higher P / P0 pressure point, unless a high-pressure adsorption instrument is selected.
For metal powders with small specific surface area, organic materials and some natural ores can use krypton gas as adsorption gas.
Best Instrument Technology (Beijing) Co., Ltd. is one of the earliest professional companies engaged in the R & D, production and sales of nitrogen adsorption specific surface area instruments. It is a high-tech enterprise recognized by Beijing Zhongguancun Science and Technology Park.
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