Mercury porosimetry principles

Mercury porosimetry is generally regarded as the best method available for the routine determination of pore size in the macropore and upper mesopore range. The apparatus is relatively simple in principle (though not inexpensive) and the experimental procedure is less demanding than gas adsorption measurements, in either time or skill. Perhaps on account of the simplicity of the method there is some temptation to overlook the assumptions, often tacit, that are involved, and also the potential sources of error. 

Surface Area and Permeability or Porosity. Gas or solute adsorption is typicaUy used to evaluate surface area (74,75), and mercury porosimetry is used, ia coajuactioa with at least oae other particle-size analysis, eg, electron microscopy, to assess permeabUity (76). Experimental techniques and theoretical models have been developed to elucidate the nature and quantity of pores (74,77). These iaclude the kinetic approach to gas adsorptioa of Bmaauer, Emmett, and TeUer (78), known as the BET method and which is based on Langmuir s adsorption model (79), the potential theory of Polanyi (25,80) for gas adsorption, the experimental aspects of solute adsorption (25,81), and the principles of mercury porosimetry, based on the Young-Duprn expression (24,25). 

With these facts in mind, it seems reasonable to calculate the pore volume from the calibration curve that is accessible for a certain molar mass interval of the calibration polymer. A diagram of these differences in elution volume for constant M or AM intervals looks like a pore size distribution, but it is not [see the excellent review of Hagel et al. (5)]. Absolute measurements of pore volume (e.g., by mercury porosimetry) show that there is a difference on principle. Contrary to the absolute pore size distribution, the distribution calcu-... 

In order to visualize the process of non-wetting fluid injection Wood s metal Porosimetry was used [11-16], Wood s metal Impregnation technique is based on the same principles as Mercury Porosimetry, i.e., an immiscible,... 

The principles of mercury porosimetry are further illustrated in figure 2.9. 

Two methods are used to measure the pore size distribution in a powder mercury porosimetry and adsorption-desorption hysteresis. Both methods utilize the same principle capillary rise. A nonwetting liquid requires an excess pressure to rise in a narrow capillary. The pressure difference across the interface is given by the Young and Laplace equation [15]. 

Dullien et. al. [55-57] examined salt particles embedded in a matrix of Wood s metal using the principles of quantitative stereology. They then leached out the salt particles and examined the matrix using mercury porisimetry. Poor agreement was obtained and this they attribute to the mercury porosimetry being controlled by neck diameter. Nicholson... 

We now consider application of percolation theory to describing mercury intrusion into porous solids. First we briefly recall the main physical principles of mercury porosimetry (in particular, the Washburn equation). These principles are treated in detail in many textbooks [e.g., Lowell and Shields 49)]. The following discussions (Sections IV,B and IV,C) introduce general equations describing mercury penetration and demonstrate the effect of various factors characterizing the pore structure on this process. Mercury extrusion from porous solids is briefly discussed in Section IV,D. 

We will provide a succinct introduction to the main textural characterisation techniques for catalysts. As a heterogeneous catalyst comprises a support and an active phase, we will distinguish between techniques intended for studying the support, which will be presented in a first section (Physisorption isotherms and mercury porosimetry) and techniques used to characterise the active phase, in the strict sense of the term, shown in a second section (Chemical adsorption). For each technique, we will show the theoretical principle, the way in which the measurement is carried out and the equipment used. Finally, examples will be used to illustrate the type of response that can be given using these characterisation techniques. 

Because ceramic powders usually have macropores, mercury porosimetry is more suitable than gas adsorption. The principle of the technique is the phenomenon of capillary rise, as shown schematically in Fig. 4.4 [19]. When a liquid wets the walls of a narrow capillary, with contact angle, 9 < 90°, it will climb up the walls of the capillary. If the liquid does not wet the walls of a capillary, with contact angle, 9 > 90°, it will be depressed. When a nonwetting liquid is used, it is necessary to force the liquid to flow up the capillary to the level of the reservoir by applying a pressure. For a capillary with principal radii of curvature rj and r2 in two orthogonal directions, the pressure can be obtained by using the Young and Laplace equation ... 

A low melting point alloy (LMPA) intrusion technique, exactly similar in principle to the well-established mercury porosimetry technique, has been developed as the basis for a new method of visualised characterisation of catalyst pore structure. 

Previous work had demonstrated the use of a Woods Metal, or LMPA, in the production of photographed SEM images of serial sections of impregnated porous media [13]. This sq)proach was adapted and used in a new process, exactly similar in principle to mercury porosimetry. Mercury was replaced by LMPA, which had similar surface tension and contact angle when in liquid form. Serial sections from LMPA intruded samples could then be compared with theoretical visualised sections constructed from data from stochastic pore networks. 

On initial inspection the results obtained from serial sectioning of LMPA intruded samples appear at odds with the principle theory behind intrusion and retraction as predicted by the Washburn equation. But further inspection shows it is not the Washburn equation, but mercury porosimetry that is at fault. Pore network models have often been used to characterise the behaviour of pore structure in relation to mercury porosimetry. But the model is only as good as the assumptions and the data that it is based iqron. Without artificially shielding the network, the model caimot propa ly detomine the correct psd and cannot derive a more spatially accurate structure that could be used for diffusion and reaction modelling. In order to characterise the pore structure more accurately, we need to introduce some of the elements usually revealed by LMPA intrusion tests. 

A new experimental procedure based on the isothermal desorption of vapour is proposed to extend the domain of characterisation of porous solids and powders by capillary condensation until the macropore range. The set-up is based on the use of a Tian-Calvet type microcalorimeter that insures a lull control of temperature gradients around the sample and allows the desorption isotherm to be determined very close to the saturation pressure. The principle of the experiment is described and the first results obtained for water desorption are compared to measurements based on gravimetry as well as to pore size distributions obtained by mercury porosimetry. 

S. Bukowiecki, B. Straube, and K.K. Unger Pore structure analysis of close-packed silica spheres by means of nitrogen sorption and mercury porosimetry, in Principles and Applications of Pore Structural Characterization (Haynes J.M. Rossi-Doria P., eds) Arrowsmith,... 

Mercury porosimetry is a method based on the same principles as the bubble pressure method but now mercury (a nonwetting fluid) is used to fill a dry membrane [19]. 

The high pressures used in mercury porosimetry gives erroneous results with deformable material such as fabric. A more general version of liquid porosimetry, based on the same principles, uses a variety of liquids and can be carried out in the extrusion mode [79,80]. In this technique, a pre-saturated specimen is placed on a microporous membrane supported on a rigid porous plate in an enclosed chamber. The gas pressure within the chamber is increased in steps causing the liquid to flow out of the pores. The amount of liquid removed is monitored by a top-loading recording balance. One also has the... 

The physical principle is in fact close to porosimetry experiments based on mercury injection. However, because of evident environmental disadvantages of mercury manipulation, it appeared convenient to find other (solid-liquid) systems. This difficulty was overcome by using hydrophobic solids and water [4]. 

The principle behind porosimetry is that, by intruding mercury under increasing pressure, the relationship between mercury pressure and pore diameto can be exploited, as governed by the Washburn equation... 

Bulk porosity, hydrophobic/hydrophilic porosity, and pore size distribution are the most important parameters of the GDL microstructure. Bulk porosity and hydrophobic/hydrophilic porosity are always determined as follows [54, 58]. The sample is immersed in water and then in decane. The water fills only the hydrophilic pores while the decane can fill both hydrophilic and hydrophobic pores. By weighing the sample before and after measurement, total pore volume and hydrophilic pore volume can be calculated. The solid volume of the GDL is calculated using the principle of buoyancy. The bulk or hydrophilic porosity of the GDL is defined as its total pore volume or hydrophilic pore volume divided by the sum of its total pore volume and its solid volume. Hydrophobic porosity is determined by subtracting the hydrophilic porosity from the bulk porosity. Pore distribution is determined by mercury-intrusion porosimetry [54]. Measurement is made of the amount of mercury that penetrates the pores of the sample as a function of the applied pressure. The pressure required for mercury to penetrate a certain size of pore is a function of the pore diameter. The pore size distribution of the GDL is then collected and analysis yields the cumulative pore size distribution. 

Few techniques are able to characterise the complex pore structure of hydrated cementitious materials. One of the most used techniques is mercury intrusion porosimetry (MIP). This technique is based on the intrusion of a nonwetting fluid (mercury) into porous structures under increasing pressure. This simple principle often makes users forget about the underlying assumptions and the limitations of the MIP technique. 

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