Abstract: Based on years of devoted research in the field of nitrogen adsorption instrument and its testing technology, the author of this paper made a comprehensive analysis of the characterization of the surface characteristics of nano-powder materials, especially the specific mesopores and micropores on the surface of complex porous powders. Representation methods, how to deepen understanding, correct application, and the problems that still exist in theory and application provide unique insights.

One. The surface characteristics of nano-powders are very different from the internal atoms, so the surface effect becomes an important characteristic of nano-powder materials. The surface of nano-powders, especially porous nano-powders, is very complicated and contains a general meaning. The surface also includes the inner surface of the through hole communicating with the surface. In order to facilitate the characterization and analysis, it is necessary to simplify the complex situation, the shape of the hole is generally simplified into two types of slit shape and cylindrical shape; the hole is divided into micropores (Micropores) according to the size <2nm; mesopores or mesopores ( Mesopores) Pore diameter 2-50nm; Macropores (Macropores) pore size> 50nm. The characterization of powder surface characteristics is often divided into two categories: specific surface and porosity. The specific indicators for the characterization of porous nano-powders are very complicated, and the concept must be very clear.
P / Po is less than 0.1, can be used to measure and analyze the distribution of ultra-micropores. P / Po is in the range of 0.05 to 0.35, and the multi-point BET specific surface test and calculation. When P / Po is more than 0.4, capillary condensation occurs. The measurement and analysis of mesopores and macropores are closely related to the characterization of surface characteristics and their measurement methods. Due to the complexity of the surface characteristics of nanopowders, various characteristic indexes need to be measured by gas adsorption method, usually measured by nitrogen adsorption instrument . Nitrogen adsorption method is a very scientific and ingenious method. In layman's terms, it uses gas molecules as a "ruler" to measure the surface of the powder by strictly measuring the adsorption of the substance surface to achieve the description of the surface characteristics of the powder. At a constant temperature, the curve of adsorption capacity with pressure is called the isothermal adsorption curve, which is the most important performance of the adsorption characteristics of solid materials. The measurement and analysis of specific surface and pore size distribution are dependent on the isotherm adsorption curve. The following figure is a typical activated carbon isotherm adsorption curve actually measured by a nitrogen adsorption instrument. According to this isotherm adsorption curve, it is different in different pressure ranges. Surface characteristic analysis: specific surface analysis for nitrogen partial pressure of 0.05-0.35; mesopore and macropore measurement and analysis for nitrogen partial pressure from 0.15 to 0.99 or more; micropore analysis for nitrogen partial pressure of less than 0.15. With the continuous advancement of technology, the accuracy and scope of analysis continue to expand, and the physical models (or methods) used continue to be enriched.

two. Characterization of specific surface area The key to measuring specific surface area by gas adsorption method is to measure the single-layer saturated adsorption amount, but the actual adsorption is not all single-layer adsorption, but the so-called multi-layer adsorption. In 1938, Brumuer, Emmett and Teller Through the thermodynamic and kinetic analysis of the gas adsorption process, the relationship between the actual adsorption amount V and the monolayer saturated adsorption amount Vm is found, which is the well-known BET equation. Therefore, people have measured the multi-point adsorption amount and passed the BET The specific surface obtained by the equation is called the BET specific surface and has always been the standard characterization method for the surface area of ​​powder materials. In addition, Laugmuir also introduced a formula for measuring specific surface based on the assumption of single layer adsorption, called Laugmuir specific surface, which is used to characterize the surface area of ​​materials with single layer adsorption characteristics.
The BET specific surface actually contains the internal surface area of ​​all pores on the surface of the powder. For some materials, such as rubber reinforcement, the internal surface area of ​​the micropores has no effect, so the rubber reinforcement (carbon black) ) Puts forward the concept of so-called external surface area that does not include the internal surface area of ​​the micropores, and proposes a corresponding method for measuring the external surface area.
In recent years, considering the existence of micropores, the pressure application range of the BET equation should be adjusted. For the X molecular sieve, the linear range of BET is 0.005 to 0.01; for microporous materials, 0.005 to 0.1; The porous composite material is 0.01 ~ 0.2; only the mesoporous material P / Po is 0.05 ~ 0.3 is suitable; in fact, the adsorption of microporous materials is closer to the characteristics of single-layer adsorption, Langmuier introduced by the single-layer adsorption theory The specific values ​​should be more in line with them.
The characterization of the surface area of ​​the microporous powder also includes the internal surface area of ​​the micropores and the internal surface area of ​​the mesopores, which will be discussed below.
three. About the characterization of porosity Porosity is different from the general porosity. He is a proper term for the characterization of the surface characteristics of ultrafine powders, including the total pore volume, pore size distribution, average pore size and other connotations. From the analysis theory, it should be divided into mesopores and macropores, and micropores to understand.
Mesoporous and large holes:
The measurement and analysis of mesopores and macropores have a history of more than 60 years, and the BJH (Barrett-Joyner-Halenda) method has been used to this day. BJH pore size analysis is based on macro-thermodynamics. The main theoretical basis is the capillary condensation theory. The main calculation method is to use the Kelvin equation to determine the pore size corresponding to the pressure, and it is assumed that the nitrogen adsorbed in the pore is the density of liquid nitrogen. Exist; as long as the gas isotherm adsorption curve is measured, the pore volume-pore size distribution, total pore volume and average pore size can be calculated one by one.
(1) Characterization range of mesopores and macropores: The control range of the nitrogen partial pressure of all nitrogen adsorption instruments is almost wide at present: the minimum value is close to 0, the maximum value is close to 1, and the lower limit of mesoporous, that is, 2nm corresponds to The partial pressure of nitrogen is 0.14; when the pressure is 0.996, the diameter of the pores can reach 500nm. Therefore, the range of pore size analysis by the BJH method includes mesopores and some large pores. The upper limit of pores measured by the nitrogen adsorption method is generally considered to be 500nm. The characterization of pore characteristics includes total pore volume, pore size distribution, and average pore size.
(2) Total pore volume:
â… ) The total pore volume of adsorption: the adsorption volume under the highest relative pressure of nitrogen is regarded as being completely adsorbed and filled in the pores. The total pore volume calculated from this does not specify the minimum size of the pores, but it must have a pore size The upper limit is defined, for example, the volume of all pores below 300 nm in diameter (P / Po = 0.993). It is not suitable to compare the total pore volume of different pore size ranges. Since the pore volume is proportional to the cube of its diameter, for materials with large pores, a small change in the upper limit of the pore size will cause a large difference in the total pore volume .
Ii) BHJ adsorption (desorption) cumulative total pore volume: using the BJH method, from the isothermal adsorption or isothermal desorption process, the total pore volume obtained by accumulating the pore volumes of different pore sizes calculated step by step, he has a clear pore size The upper and lower limits, the lower limit is generally 2nm, the upper limit is 200 ~ 400nm, should pay attention to the difference in pore size range when comparing various data. The measurement range of the pore size should be matched with the material to be tested. It is not necessarily a good thing to blindly pursue a high pore size measurement upper limit. For example, for nano powders, the particle size is less than 100nm, it is difficult to imagine that the particles themselves have a diameter of more than tens of nm In fact, the measured "large holes" are the gaps between the particles, and it is wrong to characterize the surface characteristics of the powder. Theoretically, the capillary condensation phenomenon occurs when P / P0≥0.4, but the lower limit of the BJH pore size analysis that has been used is far below 0.4, and no corresponding explanation has been made for this.

(3) Aperture distribution:
â… ) Differential distribution: Pore size distribution refers to the quantitative distribution of pore volume of different pore sizes. It is generally used to express it with a bar chart, but for pore distribution, the pore size range is too large, from a few tenths to hundreds of nanometers, and more attention is paid to small The distribution of size pores cannot be achieved with a column chart. Therefore, the differential distribution is used to characterize the method, that is, dV / dr-D or dV / logd-D curve. The points on this figure represent the pore volume with the pore size. The rate of change, his height can not directly correspond to the size of its volume, the rate of change is not equal to the volume, because the pore volume is proportional to the cubic power of the pore size, the volume of a 50nm pore is equivalent to the volume of nearly a million 1nm pores ;
Ⅱ) Integral distribution: also known as cumulative distribution, that is, the volume of pores of different sizes is accumulated step by step from small to large or from large to small. From this figure, the volume of the pore area of ​​any pore size and its total volume can be obtained The percentage distribution of pore volume can be made in any scale. The figure on the right is a typical pore size distribution diagram that combines the differential distribution and the integral distribution on one graph.
(4) Average pore size:
There are three different ways to express the average pore size. They all have specific meanings and should not be confused.
â… ) The average pore diameter of adsorption: the average pore diameter calculated from the total pore volume of adsorption and the BET specific surface area, including all pores, only the upper limit of the pore diameter is defined;
Ii) BJH adsorption average pore diameter: the average pore diameter calculated from the BJH adsorption cumulative total pore volume and the BJH adsorption cumulative total pore surface area, with upper and lower limits of the pore diameter;
â…¢) BJH desorption average pore diameter: the average pore diameter calculated from the BJH desorption cumulative total pore volume and the BJH desorption cumulative total pore internal surface area, which has upper and lower limits of the pore diameter;
(5) The most permissible pore diameter: the pore diameter corresponding to the highest point on the differential distribution curve of pore diameter represents the important feature of the pore size distribution of the tested material;
four. About the characterization of micropores In the case of micropores, the interaction potential energy between the pore walls overlaps each other, and the adsorption in the micropores is larger than that of the mesopores, so the filling in the micropores will occur when the relative pressure is less than 0.01. The pores of 0.5 ~ 1nm can produce the filling of adsorbate even when the relative pressure is 10-5 ~ 10-7, so the measurement and analysis of micropores is much more complicated than mesopores. Obviously, it is wrong to extend the BJH pore size analysis method to the microporous area. There are two reasons. First, the Kelvin equation is not applicable when the pore diameter is less than 2nm; second, the adsorbate in the pores described by the capillary condensation phenomenon is Liquid, and due to the interaction of dense pore walls in the micropores, the adsorbate filled in the micropores is in a non-liquid state, so the law of pore size distribution must have new theories and calculation methods. The macroscopic thermodynamics method has been far not enough.
The t-graph method proposed by Lippens and deBoer is one of the most used in micropore analysis. The amount of adsorption is defined as a function of the statistical layer thickness t, which is calculated from the standard isotherm. From the t-graph, you can calculate the specific surface area, external surface area, and total volume of the micropores containing the micropores. The T-map is used for the analysis method of pore size distribution, that is, the MP method, and the pore size distribution of micropores can be analyzed. The disadvantage of the T graph method and the MP method is that he still regards the adsorbate filled in the micropores as a liquid, and the Kelvin equation is still used for the calculation of the pore size, so the total pore volume of the micropores he obtained has only relative significance , And the range of pore size distribution can not characterize the real small pore area.
The HK and SF methods have introduced a semi-empirical analysis method for calculating the effective pore size distribution from the isothermal adsorption line of microporous samples, which are used in nitrogen / carbon (slit) and argon / zeolite (cylindrical pore) systems, respectively. For a loose fluid, a series of related parameters of adsorbent and adsorbate need to be introduced in these calculations. The selection of these parameters has a great influence on the calculation result. This method proposes a new relationship between the pore diameter and the filling pressure, but has not Solve the density model of nitrogen molecules in micropores, so he can represent the law of adsorption of micropores and the law of pore size distribution of micropores, but it also has no significance for quantitative analysis.
In the past ten years, the non-localized density function theory (NLDFT) and computer simulation methods (such as Monte Carlo fitting) have developed into effective methods for describing the adsorption and phase behavior of restricted non-uniform fluids in porous materials. The NLDFT method is suitable for many An adsorbent / adsorbed substance system, compared with the classical thermodynamics and microscopic model methods, the NLDF method describes the behavior of the fluid restricted in the pores at the molecular level. Its application can be used to compare the molecular properties of the adsorbate gas with them. The adsorption properties in pores of different sizes are related, so the NLDFT method for characterizing the pore size distribution is applicable to the entire range of micropores and mesopores. At present, the NLDFT method is highly respected in foreign instruments, and it is used as a standard to compare the effects of other methods. However, the calculation method of the NLDFT method for different adsorbent / adsorbent systems is different and cannot be used in different systems. General purpose, very inconvenient to apply. The new theory is only to solve two problems, one is to introduce a more reasonable correspondence between the pore filling pressure and pore size, and the other is to propose the state and density of the adsorbate filled in the pores, there is reason to expect a comparison Simpler and more general density function theory makes the analysis of ultra-micropore pore size distribution more extensive and comparable

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