Comparative analysis of specific surface test methods - Database & Sql Blog Articles

Dynamic chromatography is a method in which a powder sample to be tested is placed in a U-shaped sample tube, and a mixed gas containing a certain proportion of adsorbate is passed through the sample, and the measured sample is determined to be an adsorbate molecule according to a change in gas concentration before and after adsorption ( The adsorption amount of N2); the static method is divided into the weight method and the volume method according to the method of determining the adsorption adsorption amount; the gravimetric method is to determine the adsorption amount of the sample to the adsorbate molecule (N2) according to the change of the sample weight before and after adsorption, Defects such as low resolution, poor accuracy, and high requirements on equipment have been rarely used. The volumetric method is to load a sample of the powder to be tested into a closed tube-shaped sample tube of a certain volume, and inject a certain pressure of adsorbate into the sample tube. The gas, according to the pressure or weight change before and after adsorption, determines the adsorption amount of the sample to the adsorbate molecule (N2);

The purpose of both dynamic chromatography and static methods is to determine the amount of adsorption of the adsorbate gas. After the adsorption amount of the adsorbate gas is determined, the specific surface of the powder to be tested can be calculated from the adsorption amount of the adsorbate molecule.

There are many theories for calculating the specific surface from the adsorption amount, such as Langmuir adsorption theory, BET adsorption theory, and statistical adsorption layer thickness adsorption theory. Among them, the BET theory agrees well with the actual value in most cases than the surface calculation. It is widely used in the specific surface test. The specific surface calculated by BET theory is also called BET specific surface. The statistical adsorption layer thickness method is mainly used to calculate the external specific surface;

A common principle in dynamic chromatography instruments is the solid standard reference method and the BET multipoint method;
The solid standard reference method for solid standard reference method of dynamic chromatography is also called direct contrast method. The instrument of this method abroad is called direct reading surface meter. The principle of the method test is to use a standard sample with a known specific surface as a reference to determine the adsorption amount of the unknown sample to be compared with the standard sample, thereby obtaining the specific surface area of ​​the sample to be tested by a proportional operation. Taking the nitrogen adsorption BET specific surface standard sample as an example, the method is based on two: First, one of the assumptions of BET theory is that the energy change in the adsorption process after adsorbing one layer is equivalent to the heat of liquefaction of the adsorbate molecule. It is independent of the powder itself; Second, under the same nitrogen partial pressure (5%-30%), the same liquid nitrogen temperature, the thickness of the adsorption layer is the same; this is the specific surface value and BET obtained by this simple method. The reason why the value obtained by the multipoint method is consistent is good;
BET multi-point method for dynamic chromatography
The BET multi-point method is a national standard surface test method. The principle is to determine the absolute adsorption amount of nitrogen to be tested under different partial pressures, and calculate the specific surface area by the BET theory to calculate the specific surface area; Relative to the solid standard reference method, but in actual use, because the test process is relatively complicated and time consuming, the test results repeatability, stability, test efficiency is not superior to the solid standard reference method, which is also solid. The reason why the standard reference value of the standard reference method is higher than the BET multipoint method;

Dynamic chromatography and static volume methods are currently the main specific surface test methods. Compared with the two methods, the chromatographic method is more suitable for testing fast specific surface area test and small specific surface area sample with small and medium adsorption capacity (for medium and large adsorption samples, both static and dynamic methods can be quantitatively accurate), static capacity The method is more suitable for aperture and specific surface testing. Although the static method has the functions of surface test and pore size test, the static method is faster than the vacuum processing of the sample, the adsorption equilibrium process is slow, and it is susceptible to the external environment, which makes the test efficiency relative to the dynamic direct reading method of dynamic chromatography. Low, the stability of the test results for small specific surface area samples is also lower than that of dynamic chromatography, so the static method has no advantage over the dynamic resolution in terms of resolution and stability of the surface test; in the BET multi-point method than the surface analysis, static The method does not require liquid nitrogen cup lifting to adsorb and desorb, so the relative dynamic method saves time; the static method can be easily controlled to close to 1 due to the partial pressure of nitrogen due to the dynamic chromatography, so it is more suitable for pore size analysis. The dynamic chromatography method tests the adsorption amount by the concentration change. When the concentration is 1, there is no concentration change before and after the adsorption, which makes the pore diameter test limited.
Static capacity method Under the condition of low temperature (liquid nitrogen bath), a certain amount of adsorbate gas (N2) is introduced into the sample tube, and the adsorption partial pressure is directly measured by controlling the equilibrium pressure in the sample tube, and the fraction is obtained by the gas state equation. The amount of adsorption at the pressure point;

By gradually inputting the adsorbate gas to increase the adsorption equilibrium pressure, an adsorption isotherm is obtained; by gradually extracting the adsorbate gas to lower the adsorption equilibrium pressure, a desorption isotherm is obtained; and the relative dynamic method does not require a carrier gas (He), and no liquid nitrogen cup is required. Lift

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;

Specific surface area test related instrument introduction Dynamic method specific surface area meter test specific surface area accuracy factor

For a dynamic method instrument with wind-heat assist, detector constant temperature, and low-temperature cold trap, the accuracy of the dynamic method surface surface meter without the device is obviously improved; the dynamic method surface surface meter is similar to other analysis instruments. The accuracy and sensitivity are mainly determined by the signal-to-noise ratio; that is, to improve the accuracy and sensitivity, it is necessary to control the signal strength, suppress the background noise, and eliminate 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;

Accuracy Analysis of Small Specific Surface Area Samples Tested by Static Surface Area Meter

Taking a sample having a specific surface area of ​​1 m 2 /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, and the volume under the liquid nitrogen temperature of the adsorption environment during the test is about 0.03 ml; The remaining volume (ie, the background volume) of the sample tube loading portion is about 3-5 ml. To accurately quantify the total adsorption amount of 0.03 ml in the sample tube volume of 3-5 ml and ensure the accuracy is less than 3%, it can be calculated. The accuracy of the pressure sensor is required to be above 0.03%; however, the accuracy of the best imported pressure sensor is only 0.1%, and the accuracy of the pressure sensor for the surface and aperture analyzer is usually 0.15%, which means that the highest precision pressure is currently available. The sensor, even if the temperature field is ideally determined, the liquid nitrogen surface is ideally constant, and the ambient temperature is ideally accurate. The uncertainty of the amount of adsorption can only reach 0.003ml, that is, the uncertainty reaches 10%; The small or small density of the sample is also a sample with a large amount of sample loading, and the accuracy can be imagined. 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 error of the static method instrument test is relatively large compared with the high precision dynamic instrument; the static method can only reduce the error by increasing the sample loading amount. It is common that the static is generally a small specific surface area. The sample is equipped with a large-capacity sample tube, but since the background volume (the volume of the adsorption chamber) also increases, the accuracy is also limited; this is a factor that should be considered when testing the specific surface area using a static method instrument.