How to judge the isotherm containing micropores - Database & Sql Blog Articles

RF cable can be customized for other specifications

Weimeng ME2188A33PG

Kaixin micro test ESD18VB03-0201

Once the sample is cleaned, it is transferred to an external Dewar (or other constant temperature bath), where it is kept at a stable temperature. Then, a small amount of gas is introduced into the system.

The adsorbate gradually enters the evacuated sample tube. The adsorbent molecules in the sample quickly reach the solid surface of the sample, which acts as the adsorbent.

If the sample contains both micropores and mesopores, the adsorption isotherm should show several distinct stages:

1) Micropore filling (at very low relative pressure, less than 0.01): The initial part of the isotherm shows a steep rise, followed by a plateau. This stage helps determine the micropore volume and structure. Since the pore size is close to the size of gas molecules, selecting the right adsorbate is crucial.

2) Single-layer adsorption: As more gas is introduced, the micropores become filled, and the adsorbate forms a thin layer on the entire surface of the adsorbent. The isotherm shows a knee-like curve at this point.

3) Multi-layer adsorption: After entering the plateau region, multi-layer adsorption begins. This is where the BET theory comes into play, allowing the calculation of the specific surface area based on the adsorption data from this segment.

4) Capillary condensation: When the relative pressure exceeds 0.4, the adsorption process is accompanied by capillary condensation. This phenomenon occurs when gas molecules condense into liquid within the pores due to increased partial pressure. The Kelvin equation is used to describe this process, quantifying the relationship between pore size and condensation pressure. Methods like BJH (Barrett-Joyner-Halenda) can be used to calculate pore size distribution from the equilibrium gas pressure data.

As the adsorption reaches equilibrium near saturation, the pores are completely filled with the adsorbate. Knowing the density of the adsorbate allows us to calculate the volume it occupies, and thus estimate the total pore volume of the sample.

If we reverse the process by reducing the gas amount, a desorption isotherm can be obtained. However, because the mechanisms of adsorption and desorption differ, the two isotherms rarely overlap perfectly. The hysteresis observed in the isotherm is closely related to the shape and structure of the pores in the solid material.