CN109738473B - Method for measuring porous material pore tortuosity factor - Google Patents
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Abstract
The invention belongs to the field of performance characterization of porous materials, relates to measurement of a porous material pore tortuosity factor, and particularly relates to a method for measuring the porous material pore tortuosity factor, which comprises the steps of firstly scanning to obtain the surface appearance of the porous material or the section appearance vertical to the airflow direction, and analyzing to obtain the average pore diameter value R' of the surface pores of the surface appearance or the section appearance; secondly, measuring the average pore diameter value R of the porous material pores by using an instrument capable of measuring the average pore diameter value of the porous material pores; and finally, calculating by combining the two average pore diameter values to obtain the pore tortuosity factor of the porous material, and solving the problems of long time consumption, high cost, complex measurement process and the like of the conventional measurement method.
Description
Technical Field
The invention belongs to the field of performance characterization of porous materials, relates to measurement of a porous material pore tortuosity factor, and particularly relates to a method for measuring the porous material pore tortuosity factor.
Background
Porous materials are a common form of material in which a fluid, including a gas or a liquid, can flow within the channels. Porous materials are currently widely used in the fields of filtration, catalysis, electrodes, and the like. When gas flows in the porous material body, the pore structure has an important influence on the gas flowing process, and one of the parameters for evaluating the influence degree is the tortuosity factor.
The concept of the tortuosity factor is shown in FIG. 1. FIG. 1(a) is a porous material having a thickness H, with gas being transported from its lower surface to its upper surface. When the air holes are vertical to the surface of the material, the length of a transmission path of the air in the material is the thickness H of the material; in an actual porous material, the pores are generally curved, and the gas transmission path is as shown in fig. 1(b), and the total length of the transmission path is L; when the air hole of length L is straightened, the equivalent transmission is as shown in fig. 1 (c). In fig. 1(c), the gas penetrates the material with a thickness H, travels a path length L, and has an included angle β with the direction perpendicular to the surface of the material.
According to the definition of the tortuosity factor τ, there are:
the tortuosity factor is an important parameter of porous materials, and relates to the transmission rate of gas or liquid in the porous materials. In the prior art, a measurement method for the tortuosity factor is generally complex. Such as the common FIB-SEM method, i.e., focused ion beam-scanning electron microscope dual beam system. As shown in fig. 2, the FIB-SEM method is to bombard the porous material with ion beams, scan the new surface of the porous material with a scanning electron microscope, and then precisely restore the internal three-dimensional structure of the porous material with a computer, thereby precisely calculating the tortuosity factor of the porous material. This requires a layer-by-layer bombardment of the cross-section, which results in irreparable damage to the material, and is time-consuming and costly.
If the porous material is a porous electrode for a fuel cell, analysis of its tortuosity factor may also be by electrochemical means. The method comprises the steps of firstly preparing a complete battery, testing a current-voltage curve of the fuel battery to obtain the limiting current density, and estimating to obtain the tortuosity factor of the electrode. When the method is used, when the working current is increased to a certain degree, the output power can be suddenly reduced to 0 due to concentration polarization. The current density at this time was the limiting current density, and the fuel concentration at the reaction interface was 0. After the critical parameter of the limit current density is obtained, the diffusion coefficient is calculated according to the model, and then the tortuosity factor is estimated. The method has the advantages that the porous electrode material is prepared into the complete battery and can be tested under the working condition of the fuel battery, the testing method is complex and the time consumption is long.
Disclosure of Invention
The invention overcomes the defects of the prior art, provides a method for measuring the pore tortuosity factor of the porous material, and solves the problems of long time consumption, high cost, complex measuring process and the like of the conventional measuring method.
In order to achieve the purpose, the invention adopts the following technical scheme.
A method for measuring the air hole tortuosity factor of a porous material comprises the steps of firstly, scanning to obtain the surface appearance of the porous material or the section appearance vertical to the air flow direction, and analyzing to obtain the average aperture value R' of the surface air hole of the surface appearance or the section appearance; secondly, measuring the average pore diameter value R of the porous material pores by using an instrument capable of measuring the average pore diameter value of the porous material pores; and finally, calculating by combining the two average pore diameter values to obtain the pore tortuosity factor of the porous material.
Further, the method for measuring the pore tortuosity factor of the porous material comprises the following steps:
(1) scanning by a scanning electron microscope to obtain a surface appearance picture of the porous material, and analyzing by combining computer software to obtain an average pore diameter value R' of pores on the surface of the porous material; (2) measuring the average pore diameter value R of the pores of the porous material by an instrument which can measure the average pore diameter value of the pores of the porous material; (3) calculating the pore tortuosity factor of the porous material by the following formula:
further, scanning by a scanning electron microscope to obtain a picture of the surface topography or the cross section topography perpendicular to the airflow direction of the porous material.
Further, the method for obtaining the average aperture value R' through scanning analysis comprises the following steps:
the method comprises the following steps: obtaining a scanning electron microscope picture of the porous material through a scanning electron microscope, and analyzing the aperture of the porous material through software;
step two: measuring the pore size and counting;
step three: analyzing the statistical result and making a frequency distribution graph;
step four: and after the frequency distribution graph is obtained, fitting the pore size distribution of the porous material by utilizing Gaussian distribution, and calculating the average pore size.
Further, the pore size of the porous material was analyzed by Image J software, and the results were counted by Origin software and the average pore size was obtained.
Further, the instrument capable of measuring the average pore diameter value of the pores of the porous material is a bubble point instrument or a gas adsorption analyzer.
Further, when a bubble point instrument is used for measuring the average pore diameter value R of the pores of the porous material, firstly, liquid is used for fully soaking all the pores of the porous material, then certain pressure is applied to one side of the porous material, the pores which are fully soaked with the liquid in the porous material are flushed by airflow along with the increase of the pressure, and then the airflow passes through the pores; and finally, measuring the change of the airflow along with the applied pressure to obtain the average pore diameter of the pores of the porous material.
Further, the range of the applied pressure on one side of the porous material when measured using a bubble point meter is 5MPa to 0.2 kPa.
Further, when a gas adsorption analyzer is used for measuring the average pore diameter value R of the pores of the porous material, nitrogen or carbon dioxide is used as adsorption gas, and a pore diameter distribution test mode is adopted to obtain the average pore diameter of the pores of the porous material through measurement.
Aiming at the porous material with uniformly distributed pores, the invention only needs a microstructure scanning electron microscope picture and combines the pore diameter analysis result of a bubble point instrument or a gas adsorption analyzer to obtain the pore tortuosity factor of the material, and the test process is simple.
Compared with the prior art, the invention has the following beneficial effects.
(1) The method provided by the invention aims at the porous material with uniformly distributed pores, and can obtain the pore tortuosity factor of the porous material by only scanning the surface appearance of the porous material or a scanning electron microscope picture of a microstructure vertical to the gas flow direction and combining the pore diameter analysis result of a bubble point instrument or a gas adsorption analyzer.
(2) The porous material pore tortuosity factor measured by the method is in a reasonable range, and can be popularized and used. For example, research shows that the tortuosity factor of the anode porosity of the fuel cell is in the range of 3-10, and in order to verify the measurement result of the invention, the following two groups of tests are designed.
Experiment one: tortuosity factor measurement for fuel cell anode porous ceramic sample a:
the porosity of the fuel cell anode porous ceramic sample a is 60%, as shown in fig. 3, the scanning electron micrograph of the porous ceramic sample a is shown, the fitting graph of the pore size distribution analyzed by the scanning electron micrograph is shown in fig. 4, the pore size distribution of the porous ceramic sample a measured by a bubble point instrument is shown in fig. 5, and the tortuosity factor measurement result of the fuel cell anode porous ceramic sample a is shown in table 1.
Table 1: tortuosity factor measurement of Fuel cell Anode porous ceramic sample A
Experiment two: tortuosity factor measurement for fuel cell anode porous ceramic sample B:
the porosity of the fuel cell anode porous ceramic sample B was 40%, as shown in fig. 6, which is a scanning electron micrograph of the porous ceramic sample B, the pore size distribution fitting graph analyzed by the scanning electron micrograph is shown in fig. 7, the pore size distribution of the porous ceramic sample B measured by a bubble point instrument is shown in fig. 8, and the tortuosity factor measurement result of the fuel cell anode porous ceramic sample B is shown in table 2.
Table 2: tortuosity factor measurement of Fuel cell Anode porous ceramic sample B
The two groups of experimental results show that the obtained tortuosity factor is in a reasonable range of 3-10, and when the porosity of the material is improved, the tortuosity factor is reduced, which is reasonable and self-correct.
Fig. 9 is an equivalent schematic diagram of the average pore diameter and the tortuosity factor of the porous material. The actual pore structure and form of the porous material are irregular, so that the characterization of pores in the prior art is subjected to approximation and mathematical fitting treatment to different degrees, the pores are equivalent to circular straight pores with the radius of R, and pores exposed on the surface are equivalent to circular holes with the radius of R', and the method is provided on the basis of the circular straight pores. The approximate processing method of the invention is very simple, but effectively reflects the change of the tortuosity factors of different materials, and is a beneficial supplement to the prior art.
Drawings
FIG. 1 is a schematic diagram of the path and tortuosity factor of a porous material through which a gas passes in the background of the invention.
Fig. 2 is a schematic diagram of a focused ion beam-scanning electron microscope dual beam system in the background of the invention.
FIG. 3 is a scanning electron micrograph of anode porous ceramic sample A of a fuel cell according to the present invention.
FIG. 4 is a plot of a fit to the pore size distribution as analyzed by the SEM of FIG. 3.
FIG. 5 is a pore size distribution plot of the porous ceramic sample A of FIG. 3 measured using a bubble point instrument.
Fig. 6 is a scanning electron micrograph of fuel cell anode porous ceramic sample B of the present invention.
FIG. 7 is a plot of a fit to the pore size distribution as analyzed by the SEM of FIG. 6.
Fig. 8 is a pore size distribution diagram of the porous ceramic sample B in fig. 6 measured using a bubble point meter.
FIG. 9 is an equivalent schematic diagram between the average pore size and the tortuosity factor of a porous material.
Detailed Description
The invention is further described below with reference to the following figures and examples.
The invention provides a method for measuring a porous material pore tortuosity factor, which combines the average pore diameter obtained by analyzing a scanning electron micrograph with the average pore diameter obtained by analyzing a bubble point instrument to finally obtain the porous material pore tortuosity factor.
In the characterization of porous ceramics, the specific pore size or pore channel is actually a certain size and length distribution range, and when the average value is used for characterization, the average pore size and the average tortuosity factor can be used for representation. When the structural characteristics of the ceramic are described by these two average values, the structure can be equivalent to that shown in fig. 9. In the figure, a hole is a circular hole, the actual diameter of which is 2R, and the intersection of the hole with the surface, the cross section of which is an ellipse, is approximated to be circular, the diameter is approximated to be 2R', and the intersection angle of the hole with the surface normal is β, the following relationship is given:
in the formula, the R value is obtained by measuring a bubble point instrument, and the R' value is obtained by analyzing a scanning electron microscope. The average tortuosity factor of the material is thus obtained.
The operating principle of the bubble point instrument is that all pores of the porous material are filled with liquid, and then a certain pressure is applied to one side of the porous material. Along with the increase of the pressure intensity, the air holes which are filled with liquid in the porous material are gradually flushed away by the airflow from large to small, and then the airflow passes through the air holes; finally, the pore size distribution and the average pore size of the pores of the porous material can be obtained by measuring the change of the airflow along with the applied pressure.
When the bubble point instrument is used for measurement, the range of the applied pressure determines the range of the aperture which can be measured, and the higher the value of the applied pressure is, the smaller the analyzable aperture is. The operating pressure range of the bubble point apparatus according to the present invention is 5MPa to 0.2kPa, corresponding to a measuring pore diameter range of 0.02 μm to 500. mu.m.
When a gas adsorption analyzer is used for measurement, nitrogen or carbon dioxide is used as adsorption gas, the gas is adsorbed and condensed in the air holes in a low-temperature environment, the pore size distribution and the average pore size are obtained according to adsorption isotherm analysis, and the corresponding measured pore size range is 0.5nm to 500 nm.
The bubble point instrument measures the pore diameter according to the order of pore diameter size, gives the number of pores with certain size in turn, thus obtains the pore diameter distribution, and then calculates a specific average pore diameter according to a certain weight rule. For example, 100 pores with the diameter of 10 μm, 200 pores with the diameter of 5 μm and 300 pores with the diameter of 1 μm are sequentially measured by a bubble point instrument, and finally, the average pore diameter is given according to the formula rule carried by the instrument. For gas adsorbers, the average pore size is also given in this way, except that the gas adsorbers measure the pores first and then the macropores.
SEM pictures were analyzed using Image J and Origin software to count pore size and distribution of the porous ceramic surface to obtain R'. The method comprises the following specific steps.
(1) And obtaining a proper scanning electron microscope picture through a scanning electron microscope, and analyzing the aperture of the sample through Image J software.
Setting a scale: finding a line drawing tool in a toolbar, drawing a straight line by using the line drawing tool, coinciding with the length of the ruler, finding Analyze- - > Set scale in a menu bar of ImageJ, filling the Known length of the ruler in a knock Distance column in a popup window, and changing the Unit of the ruler in Unit of Length. Then, click OK completes setting of the scale.
(2) And (3) measuring the pore size: and (3) marking the diameter of a certain hole by using a line marking tool, clicking Analyze- > measure in a menu, and in a popped result window, length is the measured hole diameter data. In general, the hole statistics should be counted for a minimum of 100 holes, and the statistics are not repeated to ensure the accuracy of the statistics. And clicking analysis- - > Label, and after each nano particle is measured, carrying out Label once so as to avoid repeated statistics. After each line is drawn, the statistics are accumulated by clicking on Ctrl + M.
(3) And (3) statistical result derivation: after all the 100 holes are counted, clicking a result window, selecting save as from a menu for storage, and obtaining an XLS file.
Origin was used for statistical analysis. The xls file is opened and the length column is copied into origin. Selecting the column, and then clicking 'statistics- > description statistics- > frequency distribution' in the Origin menu bar; statistical parameters including the center of the interval, the end of the interval, frequency, etc. may be selected in the pop-up window. Click "confirm" after setting.
(4) And after the frequency distribution graph is obtained, fitting the pore size distribution by utilizing Gaussian distribution, and calculating the average pore size.
Example 1
FIG. 3 is a scanning electron micrograph of a fuel cell anode porous ceramic sample A with a porosity of 60%. 5 different regions are selected, obvious holes observed in the regions are counted, about 400 holes are calculated, the holes are analyzed by the method provided by the invention, the hole diameter distribution of the holes is fitted by Gaussian distribution, and as shown in figure 4, the surface average hole diameter of the sample is 0.84 μm. FIG. 5 shows the pore size distribution measured by a bubble point apparatus, wherein the pore size is 0.45. mu.m.
From this, the tortuosity factor tau of the sample A was calculated60＝(0.84/0.45)2＝3.48。
Example 2
Fig. 6 is a scanning electron micrograph of fuel cell anode porous ceramic sample B having a porosity of 40%. 5 different areas are selected, obvious holes observed in the areas are counted, about 400 holes are calculated, the holes are analyzed by the method provided by the invention, the hole diameter distribution of the holes is fitted by Gaussian distribution, and as shown in figure 7, the surface average hole diameter of the sample is 0.71 mu m. FIG. 8 shows the pore size distribution measured by a bubble point apparatus, and the pore size is 0.27. mu.m.
From this, the tortuosity factor tau of the sample B was calculated40＝(0.71/0.27)2＝6.91。
Example 3
And (3) adopting a fuel cell anode porous activation layer sample C with the porosity of 32%, and obtaining the surface average pore diameter of the sample C according to a scanning electron microscope analysis chart, wherein the surface average pore diameter of the sample C is 0.28 mu m. The average pore diameter was 0.087 μm by gas adsorption analysis.
From this, the tortuosity factor tau of the sample C is calculated32＝(0.28/0.087)2＝10.36。
The above examples are only intended to illustrate the technical solution of the present invention, and not to limit it; although the present invention has been described in detail with reference to the foregoing embodiments, it will be apparent to those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof; and such modifications or substitutions do not depart from the spirit and scope of the corresponding technical solutions.
Claims (5)
1. A method for measuring the pore tortuosity factor of a porous material is characterized in that firstly, a picture of the surface appearance or the cross section appearance vertical to the airflow direction of the porous material is obtained through scanning, and the average pore diameter value R' of the surface pores of the surface appearance or the cross section appearance is obtained through analysis by combining computer software;
secondly, measuring the average pore diameter value R of the porous material pores by using an instrument capable of measuring the average pore diameter value of the porous material pores;
finally, calculating the pore tortuosity factor of the porous material by the following formula:
calculating by combining the two average pore diameter values to obtain the pore tortuosity factor of the porous material;
The instrument capable of measuring the average pore diameter value of the pores of the porous material is a bubble point instrument or a gas adsorption analyzer,
when a gas adsorption analyzer is used for measuring the average pore diameter value R of the pores of the porous material, nitrogen or carbon dioxide is used as adsorption gas, a pore diameter distribution test mode is adopted to obtain the average pore diameter of the pores of the porous material through measurement,
when a bubble point instrument is used for measuring the average pore diameter value R of the pores of the porous material, firstly, liquid is used for fully soaking all the pores of the porous material, then certain pressure is applied to one side of the porous material, the pores fully soaked with the liquid in the porous material are flushed by airflow along with the increase of the pressure, and then the airflow passes through the pores; and finally, measuring the change of the airflow along with the applied pressure to obtain the average pore diameter of the pores of the porous material.
2. The method for measuring the pore tortuosity factor of a porous material according to claim 1, wherein the surface topography or the cross-sectional topography perpendicular to the direction of gas flow of the porous material is obtained by scanning with a scanning electron microscope.
3. The method for measuring the pore tortuosity factor of a porous material according to claim 1 or 2, wherein said method for obtaining the average pore size value R' by scanning analysis comprises the steps of:
The method comprises the following steps: obtaining a scanning electron microscope picture of the porous material through a scanning electron microscope, and analyzing the aperture of the porous material through software;
step two: measuring the pore size and counting;
step three: analyzing the statistical result and making a frequency distribution graph;
step four: and after the frequency distribution graph is obtained, fitting the pore size distribution of the porous material by utilizing Gaussian distribution, and calculating the average pore size.
4. The method for measuring the pore tortuosity factor of a porous material according to claim 1, wherein the pore size of the porous material is analyzed by Image J software, and the result is counted by Origin software to obtain the average pore size.
5. The method for measuring the pore tortuosity factor of a porous material according to claim 1, wherein the pressure is applied to one side of said porous material in the range of 5MPa to 0.2kPa as measured using a bubble point apparatus.
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