Comparison of dynamic and static methods for small specific surface area samples - Master's thesis - Dissertation

For small specific surface area samples, such as battery materials, organic materials, biological materials, metal powders, abrasives and other materials with small porosity, due to the small amount of adsorption, the results of the static test are more than the wind-heat assist device and the detector thermostat. The high-precision dynamic method has large instrument error. Why the static method is difficult to guarantee the accuracy of the small surface sample test for the following reasons: Take the sample with a specific surface area of ​​1 m2/g as an example. The adsorption amount of 0.5 g of nitrogen to the sample is about 0.1 ml under the BET partial pressure range. The volume of the adsorption environment at the liquid nitrogen temperature during the test is about 0.03 ml; the remaining volume of the sample tube loading portion (that is, the background volume) is about 3-5 ml, which is accurate in the sample tube volume of 3-5 ml. Quantitative total adsorption amount of 0.03ml and guaranteed accuracy of less than 3%, can calculate the accuracy of the required pressure sensor to reach 0.03% or more; but the current best imported pressure sensor accuracy is only 0.1%, and usually than the surface and pore size analysis The accuracy of the pressure sensor used in the instrument is 0.15%, which means that the current highest precision pressure sensor, even if the temperature field is ideally measured, the liquid nitrogen surface is ideally constant, and the ambient temperature is ideally accurate, and the uncertainty of the amount of adsorption is only Can reach 0.003ml, that is, the uncertainty reaches 10%; if it is smaller than the surface or the bulk density is small, it is difficult to sample the sample, the accuracy can be imagined Cicadas. However, for medium-large ratio surface samples, the general adsorption amount is not so small, and the accuracy of the static method is easily guaranteed to be within 2% or even 1%;
Therefore, in the test of small specific surface samples, the static method can only reduce the error by increasing the amount of sample loading. It is common that static is generally equipped with a large-capacity sample tube for small specific surface area samples, but due to the background volume (the volume of the adsorption chamber) Increasingly, the accuracy is also limited; and some manufacturers claim that the static method is less than the surface test limit of 0.0001m2/g, which is irresponsible;
For the high-precision dynamic method instrument with wind-heat assist, detector constant temperature and low-temperature cold trap, the accuracy of the standard dynamic method surface surface meter without the device is obviously improved; the dynamic method surface surface meter and other analysis Similar to the instrument, its accuracy and sensitivity mainly depend on the signal-to-noise ratio; that is, to improve the accuracy and sensitivity, it needs to be controlled from three aspects: improving signal strength, suppressing background noise, and eliminating external interference. The method of increasing the signal strength generally increases the sample weight and increases the detector current. However, increasing the detector current generally increases the noise at the same time, so the detector current has an optimal range; therefore, in suppressing noise and eliminating external interference. The work that can be done is much more; the source of error originating from the instrument itself is: detector temperature drift, signal sharpness; the detector thermostat is used to suppress temperature drift, and the wind-heat assist device can improve signal sharpness. For the sample with a surface area of ​​1 m 2 /g of 0.5 g of nitrogen adsorption amount at a partial pressure of about 0.2, the peak area and background can be guaranteed to be within 2%;
Therefore, for small surface samples, the advantages of sensitivity and resolution for dynamic instrumentation with wind-heat assist, detector constant temperature, and low-temperature cold trap are reflected; however, for medium-to-large ratio surface samples, due to strong signal, Both the normal dynamic surface area meter and the static surface area meter ensure accuracy; this is like the difference between one-tenth of an analytical balance and a thousandth of a balance;
However, most materials containing voids such as micropores and mesopores are not much smaller than the surface; if the material is smaller than the surface, the research value of the void fraction is limited;
In summary:
1. For small surface samples (below 10m2/g), the dynamic chromatography specific surface instrument with wind-heat assisted and detector thermostat is preferred, and its resolution and sensitivity are utilized.
Second, for medium-to-large ratio surface samples, if only the specific surface area is tested, the dynamic method and the static method have no obvious advantages and disadvantages. The dynamic method has the advantage of quickly determining the specific surface due to the solid standard reference method, and the static method has more BET. The point method is less advantageous than the time-saving liquid nitrogen consumption;
Third, the need to measure the surface and pore size distribution of the sample, it is recommended to use the static volume method of the specific surface and aperture analyzer;