Since the particles of the powder material are very fine, the shape of the particles and the surface morphology are complicated, it is impossible to directly measure the surface area thereof. Only indirect methods can be used. Various measurement methods have been proposed for many years, among which the nitrogen physical adsorption method Recognized as the most mature method, it has been included in the standards of countries around the world and is widely used in practice.
The nitrogen adsorption method is based on the adsorption law of the gas on the solid surface. For example, at a liquid nitrogen temperature (-196 ° C), under a certain nitrogen pressure, the surface of any powder material can adsorb nitrogen molecules, and corresponding to the determination. The equilibrium adsorption amount, which is pure physical adsorption, that is, the binding force of the adsorbed gas molecules to the solid surface is weak and reversible, that is, all the adsorbed nitrogen molecules are returned to the room temperature. They are all detached.
It is assumed that the surface area of ​​the powder particles is completely adsorbed with a layer of nitrogen molecules, and the specific surface area (Sg) of the powder can be determined by the number of adsorbed nitrogen molecules and the area occupied by each molecule:
... Sg = Vm?N?Am / 22400?W .................................... (1)
Where: Vm: monomolecular saturation adsorption of nitrogen (ml)
Am: cross-sectional area of ​​nitrogen molecules (Am = 0.162nm2)
W: weight of powder sample (g)
N: Avogadro constant
Substituting the above specific data into equation (1), the basic formula for calculating the specific surface area by the nitrogen adsorption method is obtained:
Sg = 4.36Vm / W ............................................. (2)
It can be seen that accurately determining the single layer saturated adsorption amount Vm on the surface of the sample to be tested is the key to determining the specific surface area.
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