Xerogels mercury porosimetry

The slabs of non aggregated monodisperse silica particles of the same size range than the xerogels and aerogels (S1-S3) are only intruded by mercury during mercury porosimetry So a necessary condition for the presence of a crushing mechanism is the aggregation of the particles to form a three-dimensional network. 

The results with the slabs of monodisperse non aggregated silica spheres (of the same size range than the xerogels and aerogels) which undergo only intrusion during mercury porosimetry implies that the particles need to be aggregated so that the compaction mechanism takes place... 

Some materials, among the most porous, show a large volume variation due to mechanical compaction when submitted to mercury porosimetry. High dispersive precipitated silica shows, as low density xerogels and carbon black previously experimented, two successive volume variation mechanisms, compaction and intrusion. The position of the transition point between the two mechanisms allows to compute the buckling constant used to determine the pore size distribution in the compaction part of the experiment. The mercury porosimetry data of a high dispersive precipitated silica sample wrapped in a tight membrane are compared with the data obtained with the same sample without memlM ane. Both experiments interpreted by equations appropriate to the mechanisms lead to the same pore size distribution. 

In a previous study [5], we showed that some materials, in particular the low density silica xerogels, exhibit a remarkable behavior when submitted to mercury porosimetry. At low pressure, the volume variation observed is entirely due to a crushing mechanism, generally irreversible with sometimes a weak elastic component. At high pressure, these xerogels are invaded by mercury which intrudes the pore network. The transition fi om the crushing mechanism to intrusion is sudden at a pressure Pi, characteristic of the material. This particular point can be easily located on the curve of cumulative volume versus logarithm of pressure by... 

Figure 1 shows mercury porosimetry curves on high dispjersive precipitated silica and on a low density xerogel previously examined [5]. The volume variation as a function of logarithm of pressure shows the same behavior. On both curves, one can see a sharp increase of the curve slope for a characteristic transition pressure P,. The value of this transition pressure is 45 MPa for precipitated silica and 27 MPa for the low density xerogel sample. The value of transition pressure Pt is dependent of the compressive strength of the sample. 

Below 45 MPa, the high dispersive precipitated silica sample with or without membrane collapses without mercury intrusion. The buckling mechanism of pores edges can be assumed as in the case of low density xerogels. Consequently, equation (2) can be used to interpret the mercury porosimetry curve in this low pressure domain. The constant A, to be used in equation (2) can be calculated from the P, value using equation (4). With a mercury surface tension 0.485 N/m, a contact angle 0= 130° and P, = 45 MPa, one obtains K = 86.3 nm MPa" . 

R. Pirard, B. Heinrichs, J.P. Pirard, "Mercury porosimetry applied to low density xerogels ... 

R. Pirard, B. Heinrichs, O. Van Cantfort, and J.-P. Pirard, Mercury Porosimetry Applied to Low Density Xerogels Relation Between Structure and Mechanical Properties, J. Sol-Gel Sci. Technol., 13, pp. 335-39, 1998. 

Pirard R, Rigacci A, Marechal JC, Achard P, Quenard D, Pirard JP (2003) Characterization of porous texture of hyperporous polyurethane based xerogels and aerogels by mercury porosimetry using densification equation. Polymer 44 4881 887. 

Pirard R, Heinrichs B, Van Cantfort O et al (1998) Mercury porosimetry applied to low density xerogels relation between structure and mechanical properties. J. Sol-Gel Sci. Tech. 13 335-339... 

Job N, Pirard R, Pirard J P et al (2006) Non intrusive mercury porosimetry Pyrolysis of resorcinol-formaldehyde xerogels. Particle Particle Systems Characterization 23 72-81... 

Figure 5.11a shows mercury porosimetry data for a silica xerogel with mixed behavior the material first shrinks (full circles) up to a critical pressure Per beyond which its small, uncollapsed pores are intruded (open circles). The critical pressure is identified by a sudden change of slope of the volume variation curve. If the sample is depressurized before reaching Per (crosses), then indeed no mercury uptake is detected in the sample. The pore volume distribution in Fig. 5.11b can be computed using either Eqs. 5.4 or 5.5 depending on the considered pressure domain. At P=Pc both equations are valid so that the mechanical constant C - necessary for the hierarchical collapse equation - can be obtained conveniently (Pirard et ah, 1998) ... 

Fig. 5.11 (a) Mercury porosimetry data for silica xerogel, shrinkage followed by intrusion, (b) pore... 

In order to characterize the structure of RF and carbon xerogels, a combination of nitrogen adsorption (for micro- and meso-pores) and mercury porosimetry (for pore diameters from 7.5 to 150 nm) was used to obtain the BET surface area and pore volume (microporous and total) helium and mercury pycnometry were applied to determine the skeletal and bulk density. 

Alie, C., Pirard, R., Pirard, J.-P., 2001. Mercury porosimetry applicability of the budding-intrusion mechanism to low-density xerogels./. Non-Cryst. Solids 292 138-149. 

The mechanical bdbiaviour of two series of silica and of resorcinol xerogels is analyzel by mercury porosimetry. The data are expressed as pressure-density curves, which enables textural infinmation to be obtained. In particular, it is shown that some of the analyzed samples exhibit a maik lowering of their mechanical stiffiiras upon compression, lliis observation is analyzed in terms of tte collapse of the sample s porosity and of the heterogeneity of die microstructure. 

The texture of two series of silica and organic xerogels was analyzed by mercury porosimetry. The samples are mainly compressed but not intruded by mercury in the porosimeter, and flie information obtained is therefore purely mechanical. A textural information can however be extracted from the data by analyzing the way in which a given microstructme should resist a compressive stress. 

Figure 11-5. Mercury porosimetry curve, volume versus pressure, ofa silica xerogel, showing two successive mechanisms. The point ofmechanismchange, Pt, is located at 26 MPa. Reprinted from Pirard (1997a) with permission from the Royal Society of Chemistry.

Figure 11-6. Monolithic silica xerogel before mercury porosimetry (A) the same sample after densification till a pressure just below Pt (B) and the same sample after pressurization till 200 MPa (C). Reprinted from Pirard (2002) with permission from Elsevier.

The wrapping method was used by Pirard et al. on precipitated sihca (Pirard et al., 2000a) and by Ah6 et al. on low-density silica xerogels (Ahe et al., 2001), not only to correctly identify the two successive mechanisms of volume variation, but also to demonstrate the validity of equation (11-7) in the densification part. The mercury porosimetry curves of a sample wrapped in a tight membrane and the same material without membrane are identical between 1 MPa and Pt Fig. 11-9). This confirms that the mechanism of volume variation in this pressure domain is truly crushing, without intrusion. At pressures above P, the two curves are very different, as is expected because the two mechanism are different for the sample wrapped in a membrane, the only possible mechanism is the densification, whereas the sample without membrane is invaded by mercury. Below Pt, the sample with or without membrane collapses without mercury intrusion. Consequently, equation (11-7)... 

Figure 11-17 shows the entire pore volume distribution of a low-density xerogel obtained from a succession of characterization methods. Below 2 nm, the distribution is obtained from analysis of the nitrogen adsorption isotherm by Brunauer s method, and between 2 and 7.5 nm, it is given by the same isotherm analyzed by Broekhoffde Boer s method (Lecloux, 1981). From 7.5 to 53 nm the distribution is obtained from the part of the mercury porosimetry curve that exhibits mercury intmsion, analyzed by equation (11-1) and, between 53 and 350 nm, it is derived from the part of the same mercury porosimetry curve that shows the buckling phenomenon, analyzed by equation (11-7). 

Pirard R., Bonhomme D., Kolibos S., Pirard J.-P. Textural properties and thermal stability ofsihca-zirconia aerogels. J. Sol-Gel Sci. Technol. 1997c 8 831 Pirard R, Heinrichs B., Van Cantfort 0., Pirard J.-P. Mercury porosimetry applied to low density xerogels. Relation between structure and mechanical properties. J. Sol-Gel Sci. Technol. 1998 13 335... 

The preceding, rather qualitative discussion is largely based on gel texture observed by TEM. Quantitative determinations of surface area, pore size distribution, and pore volume may be derived using gas adsorption-condensation and mercury porosimetry. In the following subsections quantitative data obtained using these methods are used to elucidate xerogel porosity. 

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