Material specific surface area test

　　Using nitrogen adsorption method to determine the specific surface, the key is to obtain the saturation adsorption capacity of a single layer, but the adsorption characteristics of most materials are not single-layer adsorption, but the so-called multi-layer adsorption. BRUMAUER-EMMETT-TELLER proposed the multi-molecular layer adsorption theory in 1938, through the thermodynamic and kinetic analysis of the gas adsorption process, The famous BET equation of nitrogen adsorption with nitrogen partial pressure (P/Po) is derived：

　　P/V（Po-P）=1/VmC+[（C-1)/VmC](P/Po)

　　The analysis shows that (P/Po) in the range of 0.05-0.35, BET equation is a linear equation, and the reciprocal of the sum of the slope and intercept of the line is exactly the saturated adsorption capacity of a single layer. Therefore, in this range, 3-5 pressure points are selected to measure the adsorption capacity of each pressure point, and then the saturated adsorption capacity of a single layer can be obtained by plotting according to the BET equation. Then calculate the ratio surface (figure below). The specific surface obtained by BET equation has become an international common method, also known as "BET specific surface". BET specific surface is the sum of the outer surface area and the inner surface area of all through holes. Change the BET equation to the following form：

　　V/Vm =[ P/(Po-P)]/{(1/C)+[(C-1)/C](P/Po)}

　　V/Vm That's the number of adsorbed layers. It depends on C and P/Po, the same material, at different pressures, the number of adsorbed layers is different, different materials, at the same pressure, the number of adsorbed layers is different。

　　In recent years, it has been found that for microporous materials, because monolayer adsorption can be completed under very low pressure, the choice of pressure point also affects the movement of low pressure direction when testing the BET ratio surface. In other words, the linear range of BET equation moves in the direction of low pressure. For X molecular sieve, the pore size is very small, and the linear range should be 0.005 ~ 0.01. The P/Po linear range of microporous materials should be 0.005 ~ 0.1; The P/Po of mesoporous and microporous composites should be 0.01 ~ 0.2; Only the P/Po linear range of mesoporous materials was 0.05 ~ 0.35; For microporous materials, which are closer to the characteristics of monolayer adsorption, the Langmuier ratio should be of great reference significance. The three-parameter equation can also be applied to the above situations 。