The BET test method is an abbreviation of the BET specific surface area test method, which is named because it is based on the famous BET theory. BET is an acronym for three scientists (Brunauer, Emmett, and Teller). Based on the multi-layer adsorption formula derived from the classic statistical theory, the famous BET equation has become the theoretical basis of particle surface adsorption science. And it is widely used in the research of particle surface adsorption performance and data processing of related detection instruments.
The BET test theory is based on the multimolecular layer adsorption model proposed by Hironor, Emmet, and Taylor, and derives the relationship between the single-layer adsorption capacity Vm and the multi-layer adsorption capacity V, which is the well-known BET equation. The BET equation is based on the theory of multi-layer adsorption, which is closer to the actual adsorption process of the substance, so the test results are more accurate. Through the actual measurement of 3-5 groups of tested samples under different nitrogen partial pressures, the multi-layer adsorption capacity, with P / P0 as the X axis and P / V (P0-P) as the Y axis, is linearly fitted by the BET equation. Obtain the slope and intercept of the straight line, and then obtain the Vm value to calculate the specific surface area of ​​the measured sample. Theory and practice show that when the P / P0 point is in the range of 0.05 ~ 0.35, the BET equation is consistent with the actual adsorption process, and the graph linearity is also very good, so the point of choice in the actual test process is within this range.
The BET equation is as follows:
P / V (Pо-P) = [1 / Vm × C] ﹢ [﹙C-1 / Vm × C﹚ × ﹙P / Pо﹚]
In the formula: P: partial pressure of nitrogen
P0: the saturated vapor pressure of nitrogen at the temperature of liquid nitrogen
V: actual amount of nitrogen adsorbed on the sample surface
Vm: Saturated nitrogen adsorption capacity
C: constant related to the adsorption capacity of the sample
The operation procedure of the BET experiment is similar to the direct comparison method. The difference is that the BET method needs to calibrate the volume of the actual nitrogen adsorption of the sample, and the theoretical calculation method is also different. The BET method for measuring specific surface area has a wide range of applications and is currently widely used internationally. The test results have high accuracy and credibility, and are particularly suitable for scientific research institutions. When the tested sample has a strong ability to adsorb nitrogen, the single-point BET method can be used. The test speed is the same as the direct comparison method, and the test result is compared with the multi-point BET method.
The BET nitrogen adsorption method generally takes a long time. It is recommended to use a fully automatic specific surface testing instrument to reduce the test intensity and at the same time the accuracy is also guaranteed. At present, similar instruments abroad are fully automatic.
The BET test theory is based on the multimolecular layer adsorption model proposed by Hironor, Emmet, and Taylor, and derives the relationship between the single-layer adsorption capacity Vm and the multi-layer adsorption capacity V, which is the well-known BET equation. The BET equation is based on the theory of multi-layer adsorption, which is closer to the actual adsorption process of the substance, so the test results are more accurate. Through the actual measurement of 3-5 groups of tested samples under different nitrogen partial pressures, the multi-layer adsorption capacity, with P / P0 as the X axis and P / V (P0-P) as the Y axis, is linearly fitted by the BET equation. Obtain the slope and intercept of the straight line, and then obtain the Vm value to calculate the specific surface area of ​​the measured sample. Theory and practice show that when the P / P0 point is in the range of 0.35-0.05, the BET equation is consistent with the actual adsorption process, and the graph linearity is also very good, so the point of choice in the actual test process is within this range.
The operation procedure of the BET experiment is similar to the direct comparison method, except that the BET method requires the calibration sample to actually adsorb nitrogen.
BJH pore size distribution test
1) Introduction to pore size analysis
Practice has shown that the microscopic characteristics of ultrafine powder particles not only appear as irregular surface shapes, but also have pore structures. The size, shape and number of pores have a great influence on the surface area measurement results. At the same time, the pore size and pore size distribution of the material have a great influence on the adsorption, catalysis and stability of the material itself. therefore
The determination of pore volume and pore size distribution has become another major field of powder material performance testing, and is usually closely related to the measurement of specific surface area.
The so-called pore size distribution refers to the rate of change of pore volume of different pore sizes with pore size. Generally, the pores are divided into three categories according to the average radius of the pores: pores ≤ 2 nm are micropores, pores in the range of 2-50 nm are mesopores, and pore diameters ≥ 50 nm are macropores. Macropores are generally measured by mercury intrusion method, and mesopores and micropores are measured by gas adsorption method.
2) Principle and method of aperture measurement
Gas adsorption method pore size distribution measurement uses the principle of capillary condensation and volume equivalent substitution, that is, the amount of liquid nitrogen filled in the measured pore is equivalent to the pore volume. The adsorption theory assumes that the shape of the hole is a cylindrical tube, thus establishing a capillary condensation model. According to the capillary condensation theory, the pore size range in which capillary condensation can occur under different P / P0
It is different. As the value of P / P0 increases, the radius of pores where aggregation can occur also increases. Corresponding to a certain value of P / P0, there is a critical pore radius Rk. All pores with a radius less than Rk are capillary condensed, liquid nitrogen is filled in them, and pores larger than Rk will not be capillary condensed, and liquid nitrogen will not filling. The critical radius can be given by the Kelvin equation
Out:
Rk =? Log (/414.0) P / P0 ……………
Rk is called the Kelvin radius, and it depends entirely on the relative pressure P / P0. The Kelvin formula can also be understood as that for pores that have undergone condensation, when the pressure is below a certain P / P0, the condensate in pores with a radius greater than Rk will vaporize and desorb.
Theory and practice show that capillary condensation occurs only when P / P0 is greater than 0.4. By measuring the amount of nitrogen condensed at different P / P0, the isothermal adsorption and desorption curve can be drawn. Through different theoretical methods The curve of pore volume and pore size distribution is obtained. The most commonly used calculation method is the use of BJH theory, commonly referred to as BJH pore volume and pore size distribution.
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