The specific surface test method is divided into adsorption method, air permeability method and other methods according to different test ideas. The air permeability method is to fill the powder to be tested in the air pipe to a certain bulk density and determine the specific surface area of ​​the powder according to the air permeability rate. The specific surface test range and accuracy are very limited; other specific surface area test methods include particle size estimation method and microscope observation estimation method, which have been rarely used; the adsorption method is more commonly used and the accuracy is higher than other methods;
The idea of ​​the adsorption method is to allow an adsorbate molecule to adsorb on the surface of the powder sample (adsorbent) to be tested, and evaluate the specific surface size of the powder sample to be tested according to the amount of adsorption. According to the different adsorbate, the adsorption method is divided into low temperature nitrogen adsorption method, iodine absorption method, mercury absorption method and adsorption of other molecular methods; earlier used several methods such as iodine absorption method, mercury absorption method, these methods It has been used in different industries for a long time; but due to the large diameter of the iodine molecule used in the iodine absorption method, it cannot enter many small pores, the measured specific surface area is incomplete, and the activity of the iodine molecule is high, which is a lot of powder The body is not applicable, and the limitation is relatively large; the mercury absorption method is also called the mercury intrusion method, and the adsorbent used-mercury is toxic and rarely used, which is not detailed here. Adsorption of other gas molecules is also rarely used. The most widely used method is nitrogen adsorption with nitrogen molecules as adsorbate; nitrogen adsorption method is also called low temperature nitrogen adsorption method because it needs to be adsorbed at the temperature of liquid nitrogen. The adsorption substance used in this method-nitrogen molecule has stable properties , Small molecular diameter, safe and non-toxic, and wide source, is ideal and is currently the main adsorption method to test the adsorption surface.
Nitrogen adsorption method is divided into dynamic chromatography method and static method according to the different adsorption process and the determination method of adsorbate.
Dynamic chromatography is to put the powder sample to be tested in a U-shaped sample tube, so that a mixed gas containing a certain proportion of adsorbate flows through the sample. According to the change in gas concentration before and after adsorption, the sample to the adsorbate molecule (N2) is determined. Adsorption capacity;
The static method is divided into the gravimetric method and the volumetric method according to the different methods for determining the adsorption amount; the gravimetric method is based on the change of the sample weight before and after adsorption to determine the adsorption amount of the sample to the adsorbate molecule (N2) due to the low resolution and accuracy Poor, high requirements for equipment and other defects have been rarely used; the volume method is to put the powder sample to be tested in a closed volume of a test tube sample tube of a certain volume, and inject a certain pressure of adsorbate gas into the sample tube, according to the before and after adsorption Changes in pressure or weight to determine the amount of adsorbed molecules (N2) adsorbed by the tested sample;
The purpose of both dynamic chromatography and static methods is to determine the amount of adsorbed gas. After the adsorption amount of the adsorbate gas is determined, the specific surface of the powder to be measured can be calculated from the adsorption amount of the adsorbate molecule.
There are many theories for calculating the specific surface from the amount of adsorption, such as Langmuir adsorption theory, BET adsorption theory, statistical adsorption layer thickness adsorption theory, etc. Among them, BET theory in terms of specific surface calculation is in good agreement with the actual value in most cases, and is widely used in specific surface testing. The specific surface calculated by BET theory is also called BET specific surface. The statistical adsorption layer thickness method is mainly used to calculate the external specific surface;
There is a commonly used specific surface test method in dynamic chromatography instruments, called solid standard reference method or direct comparison method. The instrument of this method abroad is called direct reading specific surface instrument. The principle of this method is to use a standard sample with a known specific surface as a reference to determine the amount of adsorption of an unknown test sample relative to the standard sample, so as to obtain the specific surface area of ​​the sample to be tested by ratio calculation. Taking the standard sample of BET specific surface using nitrogen adsorption as an example, the basis of this method is two: 1. One of the assumptions of BET theory. The energy change during the adsorption process after the adsorption of one layer is equivalent to the heat of liquefaction of the adsorbate molecules. It has nothing to do with the powder itself; Second, under the same nitrogen partial pressure (5% -30%) and the same liquid nitrogen temperature, the thickness of the adsorption layer is the same; this is the specific surface value obtained by this simple method and BET The reason for the consistency of the values ​​obtained by the multi-point method;
Dynamic chromatographic method and static volume method are the main specific surface test methods commonly used at present. Comparing the two methods, dynamic chromatography is more suitable for testing specific surface area, and static volume method is more suitable for pore size testing. Although the static method has the functions of specific surface testing and pore size testing, the static method takes a longer time to process the sample in vacuum, the adsorption equilibrium process is slower, and it is susceptible to the external environment. The test efficiency is lower than that of dynamic chromatography, and the test results are stable It is also lower than dynamic chromatography, so it has no advantage over dynamic chromatography in terms of efficiency, resolution, and stability of specific surface testing; but the static method can be easily controlled to 1 because of the partial pressure of nitrogen compared to dynamic chromatography, so it is more suitable for doing Pore ​​size analysis. The dynamic chromatography method uses the concentration change to test the amount of adsorption. When the concentration is 1, there will be no concentration change before and after adsorption, which limits the pore size test.
Author: Bei Shide Instrument Technology Technology
Copyright statement: This article is provided by the technology department of Best Instrument Technology (Beijing) Co., Ltd. and retains the copyright. Please indicate the source when reprinting, otherwise you will be held responsible.
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