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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,

Nanopore

Nanopore:包括微孔、介孔和大孔;

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.

two)改进高真空涡轮分子泵连接方式。由于波纹管和O型密封圈在低真空下存在自身放气问题,将涡轮分子泵的连接方式进行了改进,传统仪器采用ISO-K连接方式,分子泵和波纹管通过O型圈密封;高性能仪器连接方式改为CF刀口法兰,即通过铜垫片将涡轮分子泵和高真空微焊管路系统进行连接,这种连接方式可以将分子泵极限真空度提高two个数量级。

three)涡轮分子泵进气口采用轴向直连设置方式。较大的口径更便于气体分子的扩散,为了发挥涡轮分子泵的优势,设置分子泵轴向进气CF法兰连接方式,将工作口径优化到最大,且将涡轮分子泵和高真空微焊管路系统腔体采用CF刀口法兰直接连接的方式,可进一步提高整个系统真空度。

four)优化气体管路,充分发挥分子泵优势。所有管路均为高真空微焊管路系统,全系统内管壁电抛光处理,管路之间采用金属面密封的VCR接口配件连接,克服O型圈密封在低真空下自身放气问题,确保高真空下漏气率达到1*10-11Pa.mthree/S要求。

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

除此之外,高性能仪器还应用了高精度数字化压力测量以及数据采集系统,多量程压力传感器分段测量,工业标准RSfour85或RStwothreetwo通讯模式,以及油浴控温腔,同位预处理方式等措施确保微孔测试数据的准确性。

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)