For samples with small specific surface area, 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 method test contain a wind-heat removal device and a detector thermostat. The precision of the high-precision dynamic method instrument is large. The reason why the static method is difficult to guarantee accuracy in the testing of small specific surface samples is as follows:
Taking a sample with a specific surface area of ​​1 m2 / g as an example, the adsorption amount of 0.5 g of this sample to nitrogen is about 0.1 ml in the BET partial pressure range under standard conditions, and the volume at the temperature of the adsorption environment during the test is about 0.03 ml; The remaining volume (that is, the background volume) of the sample tube loading part is about 3-5ml. To accurately quantify the total adsorption amount of 0.03ml in the 3-5ml sample tube volume and ensure that the accuracy reaches within 3%, you can calculate The accuracy of the pressure sensor is required to be more than 0.03%; but the accuracy of the best imported pressure sensor is only 0.1%, and it is usually 0.15% higher than that of the pressure sensor used for surface and pore size analyzers, that is, the highest precision pressure The sensor, even if the temperature field is ideally measured, the liquid nitrogen surface is ideally constant, and the ambient temperature is ideal and accurate, the uncertainty of the determined amount of adsorption can only reach 0.003ml, that is, the uncertainty reaches 10%; if for the specific surface The accuracy of small or small packing density, which is difficult to load a large sample, can be imagined. But for medium and large specific surface samples, the adsorption capacity is generally not so small, and the accuracy of the static method is easily guaranteed to be within 2% or even 1%; it is not a problem;
Therefore, in the test of small specific surface samples, the static method can only reduce the error by increasing the loading volume. It is common that statics are generally equipped with large-capacity sample tubes for small specific surface area samples, but due to the background volume (adsorption chamber volume) With the increase, the accuracy improvement is also limited; and some manufacturers claim that the lower limit of the static small surface test can reach 0.0001m2 / g, which is irresponsible;
For the high-precision dynamic method instrument with wind-heat assisted removal, detector constant temperature, and low-temperature cold trap, the accuracy of the standard dynamic method surface spectrometer, which does not have this device, is significantly improved; Similar to the instrument, its accuracy and sensitivity mainly depend on the signal-to-noise ratio; that is, to improve accuracy and sensitivity, it needs to be controlled from three aspects: improving signal strength, suppressing background noise, and eliminating external interference. The methods of increasing signal strength generally include increasing the sample size and increasing the detector current, but increasing the detector current generally increases the noise at the same time, so the detector current will have an optimal range; so in terms of suppressing noise and eliminating external interference More work can be done; the main sources of error originating from the instrument itself are: detector temperature drift, signal sharpness; detector temperature control device to suppress temperature drift, wind-heat removal device can improve signal sharpness, For a sample with a specific surface area of ​​1 m2 / g, 0.5 g of nitrogen is adsorbed at a partial pressure of about 0.2, and the desorption peak area and background can be guaranteed to be within 2% of the error;
Therefore, for small specific surface samples, the advantages of sensitivity and resolution of dynamic method instruments with wind-heat assisted removal, detector constant temperature, and low temperature cold trap are manifested; but for medium and large specific surface samples, due to the strong signal, Both the common dynamic surface area analyzer and the static surface area analyzer can guarantee accuracy; this is like analyzing the difference between a 1 in 10,000 balance and a 1 in 1,000 balance;
However, the vast majority of materials containing micropores, mesopores and other voids will not have a small specific surface; if the material has a small specific surface, the research value of the porosity will be limited;
In summary:
1. For samples with small specific surface (below 10m2 / g), preferentially adopt the specific surface instrument with dynamic chromatographic method with wind-heat assisted removal and constant temperature device of the detector, taking advantage of its high resolution and sensitivity;
2. For medium and large specific 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 rapid determination of the specific surface due to the solid standard sample reference method. Point method has the advantage of less time-consuming liquid nitrogen consumption;
3. For samples that need to measure the specific surface and pore size distribution, it is recommended to use the static volume method for the specific surface and pore size analyzer
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