Porosity mercury porosimetry

Pore specific volume Porosity Mercury porosimetry Gas adsorption Acid-base sites Selective chemisorption Temp, programmed desorption... 

Porosity and pore-size distribution usually are measured by mercury porosimetry, which also can provide a good estimate of the surface area (17). In this technique, the sample is placed under vacuum and mercury is forced into the pore stmcture by the appHcation of external pressure. By recording the extent of mercury intmsion as a function of the pressure appHed, it is possible to calculate the total pore volume and obtain the population of the various pore sizes in the range 2 nm to 10 nm. 

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). 

Anode electrode Sintering temp. ("C) Initial porosity (%) FHH equation (Nitrogen adsorption) Mercury porosimetry Average Ds- Contact Angle with electrolyte 0C)... 

Mercury porosimetry is based on the fact that mercury behaves as a nonwetting liquid toward most substances and will not penetrate the solid unless pressure is applied. To measure the porosity, the sample is sealed in a sample holder that is tapered to a calibrated stem. The sample holder and stem are then filled with mercury and subjected to increasing pressures to force the mercury into the pores of the material. The amount of mercury in the calibrated stem decreases during this step, and the change in volume is recorded. A curve of volume versus pressure represents the volume penetrated into the sample at a given pressure. The intrusion pressure is then related to the pore size using the Washburn equation... 

From the mercury porosimetry data, porosity can be calculated. A higher porosity means a more open pore structure, thus generally providing a higher permeability of the membrane. Porous inorganic membranes typically show a porosity of 20 to 60% in the separative layer. The porous support layers may have higher porosities. 

Since the porosity of carbons is responsible for their adsorption properties, the analysis of the different types of pores (size and shape), as well as the PSD, is very important to foresee the behavior of these porous solids in final applications. We can state that the complete characterization of the porous carbons is complex and needs a combination of techniques, due to the heterogeneity in the chemistry and structure of these materials. There exist several techniques for the analysis of the porous texture, from which we can underline the physical adsorption of gases, mercury porosimetry, small angle scattering (SAS) (either neutrons—SANS or x-rays—SAXS), transmission and scanning electron microscopy (TEM and SEM), scanning tunnel microscopy, immersion calorimetry, etc. 

The porosity of AKP-30 (AKP-15) tubes, made with optimum [APMA] was 42.5% (43.2%) after firing at 500°C, measured with the Archimedes method by immersion in mercury. The sintered compacts had a porosity of 34.8% (34.5%). Their pore-size distributions, measured by mercury porosimetry are given in Figure 3. The mean pore radius was found to be 60 (92) nm. 

Scaffold porosity and information on the pore size distribution can be obtained from intrusion techniques. The most commonly used methods are mercury porosimetry and capillary flow porometry. In mercury porosimetry the pressure required to fill a tissue scaffold with non-wetting mercury is monitored over a set period of time. Higher pressures are required to fill small pores than large pores a fact that can be exploited using the Washburn equation13 to extract structural information where D is the diameter of the pore at a particular differential... 

Three methods have been used in this investigation to estimate the porosity of PCL tissue scaffolds, namely weight determination, mercury porosimetry and analysis of scanning electron micrographs. The results are shown in Figure 4. 

Table 4 lists porosity values determined from mercury porosimetry. These preliminary values agree with those expected based on the proportion of porogen used to make the scaffolds for samples B and E. However there are significant differences between the porosity estimates for samples A and C and those expected based on sample composition. Intrusion methods were not suitable to characterize sample D due to its low permeability. 

Rieckmann and Keil (1997) introduced a model of a 3D network of interconnected cylindrical pores with predefined distribution of pore radii and connectivity and with a volume fraction of pores equal to the porosity. The pore size distribution can be estimated from experimental characteristics obtained, e.g., from nitrogen sorption or mercury porosimetry measurements. Local heterogeneities, e.g., spatial variation in the mean pore size, or the non-uniform distribution of catalytic active centers may be taken into account in pore-network models. In each individual pore of a cylindrical or general shape, the spatially ID reaction-transport model is formulated, and the continuity equations are formulated at the nodes (i.e., connections of cylindrical capillaries) of the pore space. The transport in each individual pore is governed by the Max-well-Stefan multicomponent diffusion and convection model. Any common type of reaction kinetics taking place at the pore wall can be implemented. 

The effective diffusivity and permeability of the porous alumina sample G1 were measured in the Graham diffusion cell and in the permeation cell (Salejova et al., 2004), cf. Table 1. The porosity e is separated into macro-porosity macro corresponding to large pores and nano-porosity nano corresponding to small pores, and s — smacro + 8nano- The boundary in the classification between macro-and nano-pores is somewhat arbitrary selected as the inflection point on the integral mercury porosimetry curve in Fig. 14. 

Agglomerate porosity can be measured by gas adsorption or mercury porosimetry. However, any breakage or compression of the granules under high pressure during porosimetry can invalidate the results. 

Gas adsorption Is most widely used to assess porosity, especially by analyzing the hysteresis loops appearing in the isotherms due to capillary condensation in pores (fig. 1.13 types IV and V). However, there are a number of alternatives. Including mercury porosimetry, neutron and X-ray scattering. 

A comparison of different methods to characterise the porosity of Egyptian mortars presented. The results obtained using adsorption manometry, mercury porosimetry and thermoporometry are complementary and overlap in certain regions of pore size. 

The case of mEl is somewhat more surprising, whilst mercury porosimetry and thermoporometry detect a peak just above 10 nm, the gas adsorption results show no indication of any porosity in this range. This may be due to a non-rigid structure that can not be detected using the former techniques. 

Fig. 6 Comparison between X-ray refraction and high pressure mercury intrusion results of mean pore sizes and specific surface corresponding to Al203-samples of varying mean porosities. The differences of the calculated mean pore sizes may be explainable by considering the different pore models. The specific surface S measured by X-ray refraction solely is caused by geometric characteristics of the sample and agree well with the results of mercury porosimetry using a cylindrical capillary model.

The openness (e.g., volume fraction) and the nature of the pores affect the permeability and permselectivity of porous inorganic membranes. Porosity data can be derived from mercury porosimetry information. Membranes with higher porosities possess more open porous structure, thus generally leading to higher permeation rates for the same pore size. Porous inorganic membranes, particularly ceramic membranes, have a porosity... 

A closed porosity, for example (Fig. 7.20), will show up very easily in electron microscopy, whereas it will not be directly detected by mercury porosimetry if it is only accessible via micro or mesoporosity. 

A) Pressure-controlled mercury porosimetry procedure. It consists of recording the injected mercury volume in the sample each time the pressure increases in order to obtain a quasi steady-state of the mercury level as P,+i-Pi >dP>0 where Pj+i, Pi are two successive experimental capillary pressure in the curve of pressure P versus volume V and dP is the pressure threshold being strictly positive. According to this protocol it is possible to calculate several petrophysical parameters of porous medium such as total porosity, distribution of pore-throat size, specific surface area and its distribution. Several authors estimate the permeability from mercury injection capillary pressure data. Thompson applied percolation theory to calculate permeability from mercury-injection data. 

The porous supports, in disc or tubular shaped form, were produced commercially (Velterop Company, Netherlands). The discs (25 mm in diameter and 2 mm in thickness) were available with different macropore sizes (0.08, 0.15, 2 and 9 pm). These macropores, which were formed between the sintered alumina grains are shown typically for a disc in Figure 1. The tubes with an outer diameter of 14 mm and a wall thickness of 3 mm were manufactured with pores of 2.5 pm and 9 pm. The macropore structure of these different types of supports was analysed by mercury porosimetry. Changes in the porosity which occurred after the hydrothermal treatment were also monitored. Figure 2 shows the highly uniform pore structure of a series of supports with different nominal pore sizes. 

---