Dynamic method to test specific surface area - Database & Sql Blog Articles

(1) Dynamic method, ie continuous flow chromatography, is to carry out nitrogen adsorption in a flowing nitrogen-containing atmosphere at liquid nitrogen temperature, and achieve dynamic equilibrium of adsorption under different nitrogen partial pressures, if the sample tube is left from liquid nitrogen And rising to room temperature, the sample will completely desorb the adsorbed nitrogen, the dynamic nitrogen adsorption instrument needs to make the sample tube enter and exit the liquid nitrogen cup every time a pressure point is measured;
(2) The dynamic method relies on the use of helium as the carrier gas (because helium is not adsorbed at the liquid nitrogen temperature), by adjusting the nitrogen/rhodium ratio, or by controlling the nitrogen/helium flow rate without changing the mixed gas pressure. Under different conditions, different partial pressures of nitrogen are obtained. The pressure of the flowing mixed gas is approximately atmospheric pressure. The partial pressure of nitrogen = the flow rate of nitrogen gas / the total flow of helium plus nitrogen. The partial pressure of nitrogen can vary from 0 to an atmospheric pressure range. Vacuum is not required. condition;
(3) Continuous flow chromatography is named after the use of a thermal conductivity detector in chromatography to measure the amount of nitrogen adsorption. The sample tube is connected in series between the reference arm of the thermal conductivity detector and the measuring arm, as the thermal conductivity detector The reference arm and the nitrogen concentration in the measuring arm are not simultaneously due to the adsorption (or desorption) of the sample, the thermal conductivity cell is out of balance and an electrical signal is output, resulting in a so-called adsorption peak (or desorption peak), the peak The area is proportional to the amount of adsorption (or desorption) of nitrogen, and the amount of nitrogen adsorption or desorption can be obtained by a standard amount. By the way, since the thermal conductivity of helium and nitrogen differ greatly, æ°¦/nitrogen mixture The change in nitrogen concentration in the gas significantly changes the equilibrium state of the thermal conductivity cell, thereby increasing the sensitivity of the thermal conductivity detector;
(4) The direct comparison of the specific surface by direct contrast is a major advantage of the dynamic surface ratio meter, that is, using a standard sample with a known specific surface as the calibration material, under a fixed nitrogen partial pressure (generally nitrogen/氦 = 0.2 gas mixture), the specific surface area of ​​the sample can be calculated by directly comparing the desorption peak area of ​​the sample with the desorption peak area of ​​the standard sample. This method has a fast test speed and is suitable for on-line rapid detection of a production line. The disadvantage is that the difference in adsorption characteristics of the material is not considered. Therefore, when the adsorption characteristics of the sample to be compared with the standard are large, the test result may have a large deviation;
(5) The BET specific surface measurement method can overcome the above limitations of the direct comparison method. The key to the dynamic method to achieve the BET specific surface measurement is to be able to adjust the partial pressure of nitrogen and reach a steady state. The dynamic BET specific surface measuring instrument can also measure the specific surface by direct comparison method by setting the nitrogen partial pressure to 0.2 or 0.3 and mounting the standard specific surface sample.