Factors affecting the measurement of specific surface area and pore size distribution (porosity)
In the test of specific surface area and pore size distribution (porosity) of gas adsorption method, several factors will have a very important influence on the test process and results. These factors need to be considered for effective analysis of test results. These factors include: sample processing conditions, characteristics of adsorbate gas, different test methods, etc., which are described in detail below.
Sample processing conditions
Since the measurement of specific surface area and porosity is closely related to the outer surface of the particles, and the key to the determination by the adsorption method is that the adsorbate gas molecules are "effectively" adsorbed on the surface of the measured particles or filled in the pores, so whether the surface of the sample particles "Clean" is essential. The purpose of sample processing is mainly to let the surface occupied by non-adsorbed molecules be released as much as possible, so as to facilitate the adsorption of the adsorbed molecules on the surface during the test. Generally, the sample needs to be pre-treated before measurement. The measured sample characteristics are selected. Under normal circumstances, most samples need to remove the water molecules adsorbed on the surface, so drying above 100 ℃ (generally 105 ℃ -120 ℃) ​​under normal pressure can achieve this purpose, which is conducive to simplifying the operation process . For samples with micropores or strong adsorption characteristics, it is easy to adsorb impurity molecules at normal temperature and pressure, or cause many other molecules to be adsorbed on the surface during the manufacturing process. It is usually necessary to remove them under vacuum. For gas treatment, sometimes it is necessary to introduce an inert protective atmosphere during the pretreatment process to facilitate the desorption of impurities on the sample surface. In short, the purpose of sample pretreatment is to make the surface of the sample clean to ensure that the measurement results of specific surface area and pore size (porosity) are accurate and effective. Beijing Jinaipu Technology Co., Ltd. has specially developed the F-Sorb sample processor for customers to choose according to their needs. This product is used in conjunction with the F-Sorb X400 series specific surface and pore size analyzer, and can also be used for other purposes that require sample pre-treatment. The product has complete functions and stable performance. It is an ideal sample heating, vacuum processing and air isolation protection device.
Adsorption gas characteristics
In the analysis and test of the specific surface area and pore size distribution of the gas adsorption method, the most basic requirement for the adsorbate gas is that its chemical properties are stable, and it will not have any effect on the performance of the sample itself and the surface adsorption characteristics during the adsorption process, and must be reversible physical Adsorption. Nitrogen is the most commonly used adsorbate. Practice has shown that most of the determination of substances selects nitrogen as the adsorbate, and the accuracy and repeatability of the test results are ideal. For samples containing micropores, if the micropore size is very small and is close to the diameter of the nitrogen molecule, on the one hand, the nitrogen molecule is difficult or impossible to enter the micropore, resulting in incomplete adsorption; on the other hand, the gas molecule is The adsorption characteristics of pores with a diameter equivalent to it are very complicated and are affected by many additional factors, so the amount of adsorption cannot fully reflect the size of the sample surface area. For such samples, argon or krypton gas with a smaller molecular diameter is generally used as the adsorbate to facilitate the adsorption of the sample and ensure the validity of the test results.
Test method factors
Different test methods will have a great influence on the test results, and different test methods have their own advantages and disadvantages. In the continuous flow method, because of the principle of "comparison", compared with the volume method, it can effectively reduce the impact of sample processing on the test results. By comparison, to some extent, the error caused by imperfect processing of the standard sample and the measured sample can be offset, provided that the surface structure and adsorption characteristics of the two samples are similar and the processing conditions are the same. This is very valuable for testing for product quality on-site control purposes, reducing sample processing time and greatly improving testing efficiency. If the same substance is used as the standard sample and the measured sample, due to the similar surface structure and adsorption characteristics, the specific surface area test results will be insensitive to the sample processing conditions, in other words, the error is offset. Therefore, the continuous flow method is very suitable for on-site inspection of product quality. On the contrary, the volumetric method can be said to be very sensitive to sample processing, because it adopts the principle of absolute adsorption measurement. Any unclean surface or other factors that affect the adsorption process of adsorbate will directly affect the product of the measurement result.
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