Results Obtained by MSP and Comparison with Other Methods.
A comparison of the pore size distribution obtained by Porotech MSP and by other methods for ceramic Al2O3 based sample.
The data of Standard Porosimetry (MSP) are in a good agreement with the modern methods of research such as Atomic Force Microscopy (AFM), scanning electron microscopy (SEM). Distinctions with Mercury Porosimetry (MMP) should be related to a mistake in the setting of a contact angle of mercury. Usually it is considered that the contact angle of mercury with all materials is equal 140 degrees. However this is far from reality. For example, the angle can have value from 112 up to 150 for the same materials. Moreover it depends on the radius of a capillary.
At the same time, if a sample is strong enough, have no amalgamated inclusions and the contact angle for mercury is exactly known the results of the measurements with the MSP and MMP are practically identical.
The Fig. 7 shows the porograms measured by Method of Standard Porosimetry (MSP) and the Method of Mercury Porosimetry (MMP) for sufficiently firm and not amalgamated Ni and Ti electrodes. It can be seen there is a good agreement between the results, obtained by these two methods.
Sometimes MMP is used in examining samples with a low mechanical strength. To reveal the influence of high mercury pressures the results of measurements by MMP and MSP for such samples were compared at the Fig.8. As an example differential porograms are shown for a fibrous battery separator. It can be seen, that changes of the MMP curves take place according to deformation by high pressure of mercury.
Most metals can form amalgams with mercury. In the case when porous material contains these metals.
Mercury porosimetry always gives increased results. You can see on the Fig.9 differential pore size
distribution for a lead electrode of a lead oxide battery obtained by two methods. Here you can also see the negative influence of high pressure of mercury on the results. The lead oxide battery’s electrode is not strong enough and was destroyed by the pressure.
The knowledge of porous structure of components of Proton Exchange Membrane (PEM) Fuel Cells is important for understanding of mass and heat transfer process inside cells to increase specific energy or power of electrochemical devices in whole. The membrane
electrode assembly (MEA) is the core component of a fuel cell. It consists of the membrane, anode and cathode electrodes. Gas diffusion layers (GDL) are placed from both sides of MEA.
So the fuel cell represents a “sandwich” from five or more porous layers with different functions, porous structure and hydrophilic-hydrophobic properties. Complicated mass-transfer processes of hydrogen, oxygen and water with electrochemical reactions take place inside this “sandwich”. It is obvious, that the fuel cell overall performance depends on porous structure of each layer. MSP allows studying multi-layer systems in real environment: water, temperature, compression (Fig.10, 11, 12).
Proton Exchange Membrane, which has the widest application in electrochemical devices doesn’t have any pores in a dry state. Porosity appears only with application of water (Fig.10, 11 red curves). In this case the Method of standard Porosimetry is the only method, which can give information about porous structure.
Water distribution vs. capillary pressure (Fig.12) is important for optimization of Water Management in the Fuel cell.
MSP can also be used for evaluation of the contact angle and the hydrophilic-hydrophobic properties of multi-component materials. For this purpose porograms with the test liquid, whose contact angle has to be determined, and with a standard liquid whose contact
angle is ~ 0° are measured. The Fig. 13 shows porograms for gas - diffusion layer (GDL) in PEM fuel cell obtained with octane and water (red and blue). Here there is a shift of the curves. Contact angle is responsible for this shift. Contact angle can be easily calculated from this data (green curve). The contact angles depend on the pore radius. There is new information, which we obtained first.
Other porous materials especially having an organic origin (leather, wood, paper, natural textile and soil) are prone to swelling in water.
MSP enables examination of structural changes of swelling. Fig. 14 shows differential porograms for Whatman Filter Paper. A dry structure of the Paper was studied by using octane. The structure of the Paper, as it works during filtration of water solutions, was examined by using water. It can be seen that, as a result of swelling in water, the porosity of the Paper was sharply extended.
MSP gives the possibility to study the porous structure of compressible materials at various level of compression. It is possible to separate primary (inside particles) and secondary (between particles) porous structure for powders as well. An intersection of the curves in Fig. 15 characterizes border between primary and secondary structures.
Porotech will provide you with a complete understanding of the pore structure of your materials.
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