Specific surface area refers to the total area of materials per unit mass.Unit: mtwo/g. It usually refers to the specific surface area of solid materials, such as powder, fiber, particle, sheet, block and other materials.
There is another definition of specific surface area: area/volume.
annotations:Specific surface area refers to the total area of materials per unit mass.branchExternal surface area、Internal surface areaTwo types.National standard unit mtwo/g。Ideal non porous materials only have surface area, such asportland cement , some clay mineral particles, etc;Porous and porous materials have external and internal surface areas, such asAsbestos fibre, rock (mineral) wool, diatomite, etc.Determination methods includeVolumetric adsorption, gravimetric adsorption, flow adsorption, gas permeability, gas attachmentEtc.Specific surface area is an evaluationCatalyst and adsorbentAnd other porous materials such as asbestos, mineral wool, diatomite and clay minerals.The specific surface area of asbestos, its thermal properties, adsorption capacitychemical stability, opening degree, etc.
Measurement:A solid has a certain geometric shape, and its surface area can be obtained by ordinary instruments and calculations.However, it is difficult to measure the surface area of powder or porous materials. They not only have irregular outer surfaces, but also have complex inner surfaces.Generally speaking, the total surface area occupied by 1g solid is the specific surface area S (mtwo/g）。The measurement of specific surface area of porous materials is of great significance in both scientific research and industrial production.Generally, porous materials with large specific surface area and high activity have strong adsorption capacity.Methods for determining specific surface area includeGas adsorptionMethod and solution adsorption method.
The finer the dust particles, the larger the specific surface area.Fine particles often exhibit significant physical and chemical activities, such as oxidation, dissolution, evaporation, adsorption, catalysis, and physiological effects, which can be accelerated due to the large ratio of fine particles to the surface.This is why the explosion risk and toxicity of some dusts increase with the decrease of particle size.The wettability and adhesion of dust are also related to its specific surface area.[1]
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Method summary:Main points of specific surface area test methodContinuous flow method(i.e. dynamic method) andStatic capacity method。
The dynamic method and the static volumetric method are the main specific surface measurement methods commonly used.
The dynamic method is to put the powder sample to be measured in a U-shaped sample tube, so that the mixed gas containing a certain proportion of adsorbate flows through the sample, and determine the adsorption amount of the measured sample to the adsorbate molecule (N2) according to the change of gas concentration before and after adsorption.
The static method can be divided into gravimetric method and volumetric method according to the different methods for determining the adsorption capacity.Gravimetric method is to determine the adsorption amount of the measured sample to the adsorbate molecule (N2) according to the change of sample weight before and after adsorption. Due to the low resolution, poor accuracy and high requirements for equipment, it is rarely used;Volumetric method is to put the powder sample to be measured into a closed tube like sample tube of a certain volume, inject a certain pressure of adsorbate gas into the sample tube, and determine the adsorption amount of the measured sample to the adsorbate molecule (N2) according to the pressure or weight changes before and after adsorption;
Compared with the two methods, the dynamic method is more suitable for testing fast specific surface area test and small specific surface area samples with small adsorption amount (for mediumSample with large adsorption capacity，Static methodThe static volumetric method is more suitable for the measurement of aperture and specific surface area.Although the static method has the function of surface test and aperture test, the static method takes a long time due to the vacuum treatment of samples,adsorption equilibriumThe process is slow and easy to be affected by the external environment, which makes the test efficiency faster than the dynamic methodDirect readingLow, the stability of test results for small specific surface samples is also lower than that of dynamic method, so the static method is more stable than the dynamic methodTest efficiency, resolution and stabilityCompared with the dynamic method, it has no advantage;At multipoint BET methodsurface analysis In terms of static method, there is no need to lift the liquid nitrogen cup to absorbDesorptionTherefore, relative dynamic method saves time;The static method is relative to the dynamic method because of nitrogenpartial pressureIt can be easily controlled to close to 1, so it is suitable for aperture analysis.However, the dynamic method tests the adsorption capacity through the concentration change. When the concentration is 1, there will be no concentration change before and after the adsorption, which limits the pore size test.
The purpose of dynamic method and static method is to determine the adsorption capacity of adsorbate gas.After the adsorption amount of adsorbate gas is determined, the specific surface of the powder to be measured can be calculated from the adsorption amount of the adsorbate molecule.
There are many theories to calculate specific surface area from adsorption amount, such as Langmuir adsorption theory, BET adsorption theory, statistical adsorption layer thickness method adsorption theory, etc.The BET theory in specific surface calculation is in good agreement with the actual value in most cases, and is widely used in specific surface testing. The specific surface calculated by BET theory is also called BET specific surface.StatisticsAdsorption layerThe thickness method is mainly used to calculate the external specific surface;
Continuous flow method
The principles commonly used in dynamic instruments are direct comparison method and multi-point BET method;
Similar to other analytical instruments, the accuracy of dynamic specific surface analyzer is mainly affected by the detection method, pipeline design and whether it has complete automation of operation.
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Test method: nitrogen adsorption dynamic is a relatively mature specific surface area test method in France. In the specific surface area study and related data reports, only the results detected by the BET method are true and reliable. There are many instruments in China that can only detect by direct comparison method, and they are also eliminated in China.The multi-point BET method is used for specific surface area measurement at home and abroad. The specific surface area measurement standards developed at home and abroad are based on the BET test method. Please refer to the national standard (GB/T 19587-2004) - the method for determining the specific surface area of solid substances based on the gas adsorption BET principle;
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Fully automated operation: the specific surface area detection is actually a time-consuming work. Due to the different adsorption capacity of samples, the test of some samples may take a whole day. If the test process is not fully automated, the testers can not leave at any time, and should be highly concentrated to observe the instrument panel and control the knob,A little carelessness will lead to the failure of the test process.This will waste a lot of valuable time of testers.Only truly fully automated intelligent specific surface area tester products can meet the international standards of the testing instrument industry. Similar international products are fully automated, and manually operated instruments have long been eliminated abroad.The truly fully automated intelligent specific surface area analyzer liberates testers from repetitive mechanical operation, greatly reduces their work intensity, makes training simple, and improves work efficiency.The truly automatic intelligent specific surface area tester greatly reduces the error caused by human operation and improves the test accuracy.
Direct comparison method
The direct comparison method is called direct reading surface comparator in foreign countries.The principle of this method is to use the standard sample with known specific surface as a reference to determine the adsorption amount of unknown sample to be tested relative to the standard sample, so as to obtain the specific surface area of the sample to be tested through proportional calculation.Taking the nitrogen adsorption of BET specific surface standard samples as an example, the basis of this method is two: first, one of the assumptions of BET theory is that the energy change in the adsorption process after the adsorption of a layer is equivalent to the heat of liquefaction of adsorbate molecules, that is, it is independent of the powder itself;2、 Under the same nitrogen partial pressure (5% - 30%) and the same liquid nitrogen temperature, the thickness of the adsorption layer is the same;This is why the specific surface value obtained by the direct comparison method is consistent with the value obtained by the BET multipoint method;
Multipoint BET method[2]
The multi-point BET method is a national standard specific surface test method. Its principle is to calculate the absolute adsorption amount of nitrogen on the sample to be tested under different partial pressures, and calculate the adsorption amount of a single layer through the BET theory, so as to calculate the specific surface area;Its theoretical recognition is higher than that of the direct comparison method, but in actual use, because the test process is relatively complex and time-consuming, the repeatability, stability and efficiency of the test results are not superior to that of the direct comparison method, which is also the reason why the repeatability nominal value of the direct comparison method is higher than that of the multi-point BET method;
Static capacity method
At low temperature（liquid nitrogenUnder the condition of bath), a certain amount of adsorbate gas (N2) is introduced into the sample tube, the adsorption partial pressure is directly measured by controlling the equilibrium pressure in the sample tube, and the adsorption amount at this partial pressure point is obtained through the gas state equation;
Increase the adsorption equilibrium pressure by gradually adding adsorbate gas to obtainadsorption isotherm；The desorption isotherm was obtained by gradually extracting adsorbate gas to reduce the adsorption equilibrium pressure;The relative dynamic method does not require carrier gas (He) and repeated lifting of liquid nitrogen cup;
Since the sample to be tested is in the sample tube with a fixed volume, the adsorbate does not flow relative to the dynamic method, so it is called the static volumetric method;
With specific surface area of 1mtwo/For example, 0.5 g of the sample absorbs about 0.1 ml of nitrogen under standard conditions within the range of BET partial pressure, and the volume is about 0.03 ml under the temperature of liquid nitrogen in the adsorption environment during the test;The residual volume (i.e. background volume) of the sample loading part of the sample tube is about 3-5 ml, so the total adsorption capacity of 0.03 ml should be accurately quantified in the 3-5 ml sample tube volume and the accuracy should be within 2%, so the requirements can be calculatedPressure sensorThe accuracy of shall be more than 0.02%;However, the precision of the best imported pressure sensor is only 0.1%, and it is usually higher than that of the surface andAperture analyzerThe accuracy of the pressure sensor used is 0.15%, which means that even if the temperature field is ideal, the liquid nitrogen surface is ideal and constant, and the ambient temperature is ideal and accurate, the uncertainty of the amount of adsorption can only reach 0.003ml, that is, the uncertainty can reach 10%;If the sample size is smaller than the surface or the bulk density is smaller, that is, it is difficult to load a large amount of samples, its accuracy can be imagined.However, for medium and large specific surface samples, the adsorption capacity is generally not so small, and the accuracy of the static method can easily be guaranteed within 2% or even 1%;
Therefore, in the test of small specific surface samples, the error of static method is larger than that of high-precision dynamic method;The static method can only reduce the error by increasing the sample loading. It is common that the static method generally equips small specific surface area samples with large capacity sample tubes, but because the background volume (adsorption chamber volume) also increases, the improvement of accuracy is limited;This is the factor that should be taken into consideration when measuring specific surface area with static instrument.
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There are about a dozen effective national standards for specific surface area testing in China. Here are some commonly used national standard methods:
