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Solicitation | difficulties and solutions of pore size analysis of microporous materials

In recent years, the development and application of porous materials have made rapid progress, such as porous polymers, porous ceramics, foamed plastics, porous metal materials and so on. These materials have some common characteristics:low density, high porosity and large specific surface area. They play a unique and important role in the fields of chemical industry, electrochemistry, architecture, military industry and aerospace.

At the same time, porous materials are increasingly used in some emerging fields to solve related problems. For example, a newly launched electric vehicle battery adopts porous spongy nano porous silicon, which can inhibit the expansion of silicon carbon cathode, so as to greatly improve the capacity of lithium batteries and improve the endurance of electric vehicles.

Porous silicon used as cathode of lithium battery

Accurate and concise characterization techniques are needed to study the pore structure of porous materials. According to the detection purpose, it can be generally divided into X-ray small angle diffraction method, gas adsorption method, electron microscope observation method, mercury intrusion method, bubble method, centrifugal force method, transmission method, nuclear magnetic resonance method, etc.

At present, the most common method to characterize the specific surface area and pore diameter of materials is the gas adsorption method, that is, the adsorption of gas molecules (adsorbates) on the surface of the tested material (adsorbent) due to van der Waals force. By measuring the adsorption isotherm of samples, the characteristics of the specific surface area and pore diameter of materials are calculated by equivalent substitution method. At present, there are mainly two kinds of instruments for measuring specific surface area in China, both dynamic chromatography and static volumetric method, but the pore diameter measurement method is the internationally used static volumetric method, which measures the pore diameter from 0.three5nm to more than 100nm. IUPAC classifies the pore diameter, as shown in the figure below,



Macropores:pores with pore width greater than 50nm; FethreeOfourDiatomite and other materials contain such holes;

Mesopores:pores with a width between twonm and 50nm; Most ultra-fine powders are in this range;

Micropore:a hole with a hole width less than twonm; Activated carbon, molecular sieve, zeolite, MOF and other materials mostly contain micropores, which are subdivided and supplemented later;

Extremely microporous:wider micropores with a pore width greater than 0.7nm;

Ultra micropore:narrow micropore with pore width less than 0.7nm.

1. Difficulties in micropore testing

It is difficult to analyze the pore diameter and pore volume of microporous materials. As shown in the figure below, the relative two pore walls in the micropore are very close, the interaction potential generated by the pore wall overlaps, and the force on the adsorbate molecules is larger than that of the mesopore and macropore. The N at the liquid nitrogen temperature of 77Ktwo吸附是微孔和介孔分析最常用的吸附质,此时气体分子的扩散速度和吸附平衡都很慢,填充0.three5nm~1nm的孔要在相对压力10-9<P/P0<10-5In order to achieve the low relative pressure required for micropore filling, turbomolecular pump level vacuum is required, that is, the whole vacuum system needs to reach a very high vacuum.

two、 静态法高性能仪器

Aiming at the testing difficulties of micropores, ultra micropores and extremely micropores, Guoyi precision measurement has launched a static high-performance ultrasorb instrument.

Static method high performance ultrasorb instrument

As shown in the above figure, in order to ensure the high vacuum degree of the whole test process, the whole system of ultrasorb instrument from molecular pump, vacuum pipeline to sample tube is sealed by metal surface and connected by VCR metal gasket. The instrument does not use the quartz sample tube used by conventional instruments, but a new type of sample tube – stainless steel welded quartz tube. The characteristics of this sample tube are:the upper stainless steel part is hard connected with the high-performance ultrasorb instrument through metal gaskets, which further improves the sealing performance of the whole instrument. The lower quartz glass part of the stainless steel welded quartz glass tube gives play to the low thermal conductivity of the quartz glass sample tube, which can reduce the evaporation of coolant (liquid nitrogen) in the experimental test, so as to improve the service time of liquid nitrogen.

In order to obtain the low relative pressure required for testing micropores, high-performance ultrasorb instruments include the following key points in improving the vacuum degree of the vacuum system:

1) A two-stage mechanical pump and a turbo molecular pump are used to work together to achieve a higher vacuum. The vacuum pump draws vacuum to reduce the instrument system to a certain vacuum degree, and then starts the turbomolecular pump. The gas molecules diffused into the molecular pump are discharged through the high-speed rotating blades, so as to reduce the gas molecules in the vacuum system and further achieve a higher vacuum degree.




5) The supporting VCR interface pneumatic valve eliminates the measurement error caused by the local heating of the solenoid valve.


three、 总结

Static method high performance ultrasorb instrument测试微孔标样测试结果见下图所示,相对压力P/P0At least 10-7,位于微孔分析相对压力区间,测试微孔的中值孔径为0.8fournm,符合微孔标样的标准值,证明仪器在温度77K下氮气测试微孔完全可以满足要求。

(contributed by Guoyi precision measurement)