Static capacity method specific surface area and core technology of aperture analyzer - Database & Sql Blog Articles

In China, the specific surface area and aperture analyzer () mainly applies two major test principles. The first is dynamic chromatography, also known as dynamic method; the second is static capacity method, as the name suggests is static. The static capacity method is currently the mainstream testing technology in the world. It is included in the ISO, ASTM and other standards committees, and is also included in the Chinese national standard. The specific surface area and pore size analyzer () of the static capacity method works as follows: under low temperature (liquid nitrogen bath), a certain amount of adsorbate gas is introduced into the sample tube, and the sample tube is controlled. The equilibrium pressure directly measures the adsorption partial pressure, and the adsorption amount of the partial pressure point is obtained by the gas state equation; the adsorption equilibrium temperature is increased by gradually introducing the adsorbate gas to obtain the adsorption isotherm; and the adsorption equilibrium pressure is lowered by gradually extracting the adsorbate gas. The desorption isotherm is obtained. Since the sample to be tested is in a fixed volume of the sample tube, the adsorbate does not flow relative to the dynamic chromatography, so it is called the static volume method. For the specific surface area and pore size analyzer () using the static capacity method, the most important technical points are the pressure sensor, the performance of the vacuum pump, and the adjustment of the liquid nitrogen surface. The system, the sealing performance of the test chamber, the performance of the Dewar used in the test, the temperature control of the experimental system, the overall stability and automation of the instrument. The following will introduce you to these technical points one by one, and explain what kind of technology is a useful and reliable technology, which is the technology that works on the stability, accuracy and repeatability of instrument testing. I hope to help the majority of users, and do not be fooled by false blowing information during the purchase of the specific surface area and aperture analyzer (). 1. Pressure sensor During the test, the cavity gas of the whole system is constantly changing, and the pressure of the cavity changes accordingly. This requires us to have a special accessory to detect the pressure changes caused by these gas changes, then we introduce the pressure sensor as an important accessory. The sensitivity, frequency response characteristics, stability, and measurement accuracy of the pressure sensor directly determine the quality of the pressure sensor. The use of pressure sensors from internationally renowned brands guarantees good performance and ensures accuracy in reading pressure. In addition, the accuracy of the reading pressure can be guaranteed without using a well-known brand, and a pressure sensor suitable for a static specific surface area and aperture analyzer is also selected. Pressure sensors are classified into different types, such as resistance strain gauge pressure sensors, semiconductor strain gauge pressure sensors, piezoresistive pressure sensors, inductive pressure sensors, capacitive pressure sensors, resonant pressure sensors, and the like. Different pressure sensors are available for different industries. Since the specific surface area and the aperture analyzer are evacuated during the test, the entire test chamber is in a low vacuum state. If the piezoresistive pressure sensor is selected, it is obviously inappropriate. The capacitive pressure sensor must be selected. . Capacitive pressure sensors are mainly used to measure low pressure and have high load capacity; while piezoresistive pressure sensors are suitable for measuring pressure changes under high pressure conditions. 2. Vacuum Pump A vacuum pump is a device or device that uses a mechanical, physical, chemical, or physical chemical method to evacuate a pumped container to obtain a vacuum. For the specific surface area and pore size analyzer of the static volumetric method, the good vacuum of the overall test chamber can ensure the accuracy of the experimental data. Therefore, a reliable vacuum pump is very important for gas adsorbers such as specific surface area and pore size analyzers! From the current market point of view, there is still a certain gap between the vacuum pump produced in China and the imported vacuum pump, regardless of the internal rotary vane quality and detail design of the vacuum pump, the noise level of the pump, the vibration amplitude or the oil leakage from the pump. The handling of instant noodles has to be improved. If a domestic vacuum pump is provided for the static volumetric specific surface area and pore size analyzer, high temperature and high pressure gas adsorption instrument, etc., the vacuum degree of the instrument test chamber will be affected, so that the reliability of the final data has to be questioned. Some customers will make assumptions. If the domestic vacuum pump of the specific surface area and pore size analyzer, high temperature and high pressure gas adsorption instrument is also replaced by the imported vacuum pump, the accuracy of the test data should be credible? If you think so, it is a big mistake. First of all, if the vacuum pump is replaced, it needs to have a running-in period with the specific surface area and pore size analyzer, high temperature and high pressure gas adsorption instrument, and the direct replacement will cause the other data of the instrument to deviate from the data when the domestic vacuum pump is equipped. During this period, continuous debugging and technical improvement are required. For the vacuum pump built-in or externally good, many customers have always held different attitudes, which is mainly related to the propaganda of the specific surface area and pore size analyzer, high temperature and high pressure gas adsorption instrument. When the vacuum pump is placed outside the instrument, the automation requirements will not be met. Manually turn on and off the vacuum pump manually during each experiment. The long-term placement of the vacuum pump in the air will also affect the life of the vacuum pump. Over time, the quality will also be affected. The exterior is not beautiful. If it is placed inside the instrument, it can solve the above problems, but the problems caused by the vibration and heat generated by the vacuum pump will affect the overall performance of the specific surface area and the analyzer of the aperture analyzer, high temperature and high pressure gas adsorption instrument. How to deal with these problems is a big technical problem for manufacturers who replace imported vacuum pumps and change vacuum pumps from external to built-in! 3. Liquid Nitrogen Surface Adjustment System In the process of testing the specific surface area and pore diameter using a static volumetric apparatus, the sample tube is placed in a Dewar filled with liquid nitrogen. The portion of the sample tube below the liquid nitrogen surface is called the cold volume, and the portion above the liquid nitrogen surface is called the warm volume. As the experiment progresses, the liquid nitrogen will automatically evaporate continuously, so that the ratio of cold volume to warm volume will gradually change, requiring specific surface area and pore size analyzer to have special techniques to deal with the problem caused by liquid nitrogen volatilization. In order to keep the ratio of cold volume to warm volume unchanged, reliable and accurate data can be obtained. However, the amount of liquid nitrogen volatilization is undetermined by the tester. Therefore, the specific surface area and pore size analyzer are required to monitor the liquid nitrogen surface drop in real time to ensure that the cold volume ratio under the liquid nitrogen surface remains unchanged. 4. Sealing performance of the test cavity For the specific surface area and the aperture analyzer using the static capacity method, if the sealing of the test cavity is not superior enough, the air leakage phenomenon is extremely likely to occur, and the test data is unreliable at the end. Therefore, the specific surface area and pore size analyzer test chamber sealing performance is also a very important parameter! 5. Use of Dewar One of the essential parts of the specific surface area and pore size analyzer using the static volume method is the Dewar. The quality of Dewar directly affects the stability of the test process and the accuracy of the sample data. We first came to understand the history of the Dewar. The Dewar was originally invented by the Scottish physicist and chemist Sir James Dewar. In 1892, Dewar ordered Berg to blow a glass into a special glass bottle. . This is a double-glazed container with two layers of glass walls coated with silver and then the air between the two walls is removed to create a vacuum. The silver on the two layers of the gallbladder prevents radiation from dissipating heat, and the vacuum prevents convection and conduction heat dissipation, so the temperature of the liquid contained in the bottle is not easily changed. Later, Berg made the outer shell of nickel to protect the fragile glass bottle. On January 20, 1893, Dewar announced the invention of a special cryostat that was later called Dewar. The specific surface area and pore size analyzer is always placed in a low temperature liquid nitrogen bath during the experiment. The container for the liquid nitrogen bath (Dova) must meet the following hard conditions, otherwise it will easily affect the specific surface area. Testing of data such as pore size and pore volume. 1. Quality should be safe and reliable. Since the specific surface area and pore size data were tested, the Dewar was bathed with liquid nitrogen (or liquid argon, etc.) at minus 196 °C. If the selection of Dewar is not correct, or the raw materials do not meet the requirements of the specification, and the conditions for the freezing of nitrogen are not met, it is highly probable that the Dewar explosion will occur. Lighter accidents may occur after skin contact. The safety of the operator is endangered by objects such as debris generated during the explosion, and reports of such accidents are frequently reported in the media headlines! Therefore, the quality of Dewar must be reliable and guaranteed! 2. The insulation performance is better. The dewar is composed of inner and outer double bile. The outer surface of the inner liner is wrapped with a multi-layer thermal insulator. Once the reflective material and the thermal insulation material fall off, the adjacent reflective material will directly contact the outer bladder, so that the liquid nitrogen of the inner liner is rapidly Expand and gasify until an explosion occurs. In addition, the liquid nitrogen bath placed in the Dewar with poor thermal insulation has a very fast evaporation rate, which accelerates the consumption of liquid nitrogen and additionally increases the experimental cost. 3. Can support the test time for a long time. When testing the specific surface area, it is completed in one hour or two. However, when testing microporous materials, especially for samples with very rich micropore diameters, the test time can be up to 3 days. This requires that the Dewar flask capacity of the specific surface area and pore size analyzer must be large, the insulation performance must be good, and the test time that can be supported must be long! Otherwise, the liquid nitrogen consumption will be exhausted in the middle of the test, at which point the tester will have to retest. Not only is it time-consuming and labor-intensive, but it has to be manually guarded during the next test to prevent liquid nitrogen from running out. Perhaps you will ask: Will there be any remedy for filling the liquid nitrogen bath in time when the liquid nitrogen is almost exhausted? This method is certainly feasible, but through our many special experiments, the following conclusions are drawn: (1) When the liquid nitrogen bath is filled in time, the adsorption data of adjacent P/P0 points tested at that time will be greatly affected, of course, The accuracy of the final data. (2) You will find that the Dewar lifting motor is at the top of the test station when filling the liquid nitrogen bath. At this time, adding a liquid nitrogen bath is difficult to operate, and liquid nitrogen splashing is prone to occur. In summary, when purchasing the specific surface area and the aperture analyzer, it is absolutely impossible to use the dewar with glass material, which is not only vulnerable to external human factors, but also more likely to cause explosion due to the quality problem of the Dewar itself. occur. Moreover, the insulation performance of the Dewar must be good and the capacity must be large. If you need to test microporous materials, the support test time must be long.