Permporometry

This technique, developed by Eyraud [140] modified by Katz et al. [143] and recently by Cuperus et al. [141], is based on the controlled blocking of pores by capillary condensation of a vapour (e.g. CCli, methanol, ethanol, cyclohexane), present as a component of a gas mixture, and the simultaneous measurement of the gas flux through the remaining open pores of the membrane. The capillary condensation process is related to the relative vapour pressure by the Kelvin equation. Thus for a cylindrical pore model and during desorption we have 

Careful control of the relative vapour pressure permits the stepwise blocking of pores. Starting from a relative pressure equal to 1, all the pores of the 

The features of the more important characterisation techniques described here are summarised in Table 4.2. As is evident from this review, there is no technique which is universally applicable for the characterisation of the porous properties of all materials. The choice is made on the basis of many criteria, such as the range of pore size, the nature of the material and its form, together with the application envisaged. Frequently, more than one technique is required in a detailed examination. In the case of membranes, particular problems are encountered because of their form and the small quantity of active material involved. Furthermore, other complexities arise in the case of microporous thin films, although, as we have noted here, currently this is an area of active progress. This involves the development of new techniques and advances in phenomenological theories to describe the properties of such nanostructured supported materials. 

The techniques given in Table 4.2 are well established and have been sub-divided into those which are described as either static or dynamic. We feel this distinction is of particular importance in the characterisation of the porous structure of membranes. Here the performance is determined by the complex link between the structural texture and transport behaviour. An insight into this complexity is frequently provided by dynamic techniques, which are not restricted by the limited quantity of membrane material and are sensitive to the active pathways through the porous structure. Further developments are required in this area both in the improvement of existing techniques and introduction of new techniques. Progress will also come from advances in the theory and modelling of flow behaviour in such porous media, which involve percolation theory and fractal geometry for example. With the refinement of such 

Detailed summary of the main characterisation techniques for the determination of the texture of porous membranes 

The principle of the method is shown schematically in figure IV - 23. At a relative pressure p (p - p/p°) equal to unity, all the pores are filled with liquid and no gas penneation occurs. On reducing the relative pressure, the condensed vapiour is removed from the largest pores in accordance with the Kelvin equation (eq. IV - 7), and the diffusive gas flow through these open pores is measured. On reducing the relative pressure still further, smaller pores become available for gas diffusion. When the relative pressure is reduced to zero, all the pores are open and gas flow proceeds through them all. 

Because a certain pore radius (Kelvin radius ) is related to a specific vapour pressure (eq. IV - 7), a measurement of the gas -flow prbvides information about the number of these specific pores. Reducing the vapour pressure allows the pore size distribution to be obtained. 

This particular membrane has a narrow pore size distribution, which is somewhat unusual for polymeric membranes obtained by phase inversion. Furthermore, the agreement between the methods is quite reasonable, with permporometry giving the highest value and adsorpdon-desorption the lowest. It should be noted that permporometry only measures active pores whereas adsorption-desorption and thermoporometry measure active, deadend and even small pores in the sublayer. 

In summary, permporometry is more complicated than any of the other methods discussed so far because of experimental difficulties. The principal problem is the difficulty of maintaining the same vapour pressure on both sides of the membrane so that some time is necessary before thermodynamic equilibrium is attained and to control the gas flow accurately. Furthermore, the method is difficult to employ with hollow fibers.The advantage of this method is that only active pores are characterised. 

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The pore-size of the resulting mesoporous membranes was determined by permporometry. This method and the home-built equipment that we used for the measurements have been described in detail elsewhere [24], During the measurements the following processes took place ... 

The surface area of the membrane materials was measured before and after SASRA treatment by a single point BET instrument with a TC-detector. Three samples were run in parallel and the amount of adsorbed nitrogen on the sample surface was measured and used for calculating the surface area. The phase composition was characterised by XRD. After SASRA treatment the adherence of the membranes to the support was tested by the Scotch Tape Test [26], In this test, a piece of Scotch Tape was applied firmly with the sticky side onto the membrane surface and torn off rapidly. If the membrane layer was torn off together with the tape, it was concluded that delamination had occurred. For membranes that showed no sign of delamination, the pore-size was measured with permporometry for a second time. 

Table 1 summarises the most important results from the investigation of metal doping. In this table the results of MAP treatment are combined with effects of firing temperature and doping. As can be seen in Table 1, y-alumina membranes with pore radii as low as 2.0 nm (Kelvin radius) may be obtained after firing at 600°C. Note that an instrumental standard error of 0.5 nm (90% reliability) is common in permporometry. This technique should therefore only be used for comparison purposes and to obtain a qualitative impression of the pore-size and pore-size distribution of the material under investigation. 

All coating work is performed under class 100 cleanroom conditions to avoid large defects due to dust particles, resulting in highly reproducible membrane properties, such as layer thickness and pore-size. A typical thickness of the deposited y-alumina layer is 3 im and the mean pore size is 2.5 nm as determined by permporometry. 

Before and after experiments the pore sizes of the membranes were measured by permpo-rometry [16], a technique based on blocking of smaller pores by capillary condensation of cyclohexane and the simultaneous measurement of the permeance of oxygen gas through the larger, open pores. The measurements are performed at 20°C on an area of 8.5 10 4 m2. The pore size distribution (Kelvin radii) is determined in the desorption stage using the Kelvin equation. More details on the permporometry technique can be found in [17] and all experimental details of the permporometry apparatus are presented in [16],... 

To determine the change in pore size of a mesoporous membrane during CVI, the pore-size was measured before and after a CVI-experiment by permporometry. Typical results are shown in Figure 4, in which raw permporometry data are shown before and after CVI with SiAc4. 

Figure 4 Typical permporometry plot before and after CVI with SiAc4.

The pore size distribution or its mean value of a porous inorganic membrane can be assessed by a number of physical methods. These include microscopic techniques, bubble pressure and gas transport methods, mercury porosimetry, liquid-vapor equilibrium methods (such as nitrogen adsorption/desorption), gas-liquid equilibrium methods (such as permporometry), liquid-solid equilibrium methods (thcrmoporometry) and molecular probe methods. These methods will be briefly surveyed as follows. 

Permporometry. Permporometry is based on the well known phenomenon of capillary condensation of liquids in mesopores. It is a collection of techniques that characterize the interconnecting active pores of a mesoporous membrane as-is and nondestiuctively using a gas and a liquid or using two liquids. The former is called gas-liquid permporometry [Eyraud, 1986 Cuperus et al., 1992b] and the latter liquid-liquid permporometry [Capanneli et al., 1983 Munari et al., 1989]. The "active" pores are those pores that actually determine the membrane performance. 

The gas-liquid permporometry combines the controlled stepwise blocking of membrane pores by capillary condensation of a vapor, present as a component of a gas mixture, with the simultaneous measurement of the free diffusive transport of the gas through the open pores of the membrane. The condensable gas can be any vapor provided it has a reasonable vapor pressure and does not react with the membrane. Methanol, ethanol, cyclohexane and carbon tetrachloride have been used as the condensable gas for inorganic membranes. The noncondensable gas can be any gas that is inert relative to the membrane. Helium and oxygen have been used. It has been established that the vapor pressure of a liquid depends on the radius of curvature of its surface. When a liquid is contained in a capillary tube, this dependence is described by the Kelvin equation, Eq. (4-4). This equation which governs the gas-liquid equilibrium of a capillary condensate applies here with the usual assumption of a=0 ... 

Capillary condensation provides the possibility of blocking pores of a certain size with the liquid condensate simply by adjusting the vapor pressure. A permporometry lest usually begins at a relative pressure of 1, thus all pores filled and no unhindered gas transport. As the pressure is reduced, pores with a size corresponding to the vapor pressure applied become emptied and available for gas transport. The gas flow through the open mesopores is dominated by Knudsen diffusion as will be discussed in Section 4.3.2 under Transport Mechanisms of Porous Membranes. The flow rate of the noncondensable gas is measured as a function of the relative pressure of the vapor. Thus it is possible to express the membrane permeability as a function of the pore radius and construct the size distribution of the active pores. Although the adsorption procedure can be used instead of the above desorption procedure, the equilibrium of the adsorption process is not as easy to attain and therefore is not preferred. 

This method has been applied to ceramic membranes (e.g., gamma-alumina membranes) and compared to other methods such as nitrogen adsorption/desorption and thermoporometry (to be discussed next) in Figure 4.13. It can be seen that the mean pore diameter measured by the three methods agrees quite well. The pore size distribution by permporometry, however, appears to be narrower than those by the other two techniques. Similar conclusions have been drawn regarding the comparison between permporometry and nitrogen adsorption/desorption methods applied to porous alumina membranes [Cao et al., 1993]. The broader pore size distribution obtained from nitrogen adsorption/desorption is attributable to the notion that the method includes the contribution of passive pores as well as active pores. Permporometry only accounts for active pores. 

Eyraud, C., 1986, Applications of gas-liquid permporometry to characterization of inorganic ultrafiliers, in Proc. Europc-Japan Congr. Membr. Membr. Processes, Stresa, Italy (1984), p. 629. 

Various improvements have broadened the research in the held of zeoUte membranes and films, such as the development of new synthesis procedures, the use of new supports with speciUc characteristics (monoliths, foams, etc.) or the use of modified supports by means of masking or grafting techniques, the appUcation of new analytical techniques (isotopic-transient experiments, permporometry, etc.), the control of the orientation of the crystals (by means of covalent Unkages, synthesis conditions, etc.) and of the thickness of the membranes, and the preparation of new zeolites as membranes or new zeoUte related-materials. In addition, a variety of zeoUtes can now be prepared as coUoidal systems with particle dimensions ranging from tens to a few hundred nanometers. 

Alternatively, selective blocking of membrane pores by condensing vapors combined with permeation measurements (permporometry) could be used to evaluate defects [18]. However, this method gives only pore sizes and does not consider other specific interactions between the membrane and the permeating molecules. Bemal et al. [19] have developed a fast and simple technique to assess membrane quality taking into account the nature (organophilic vs. hydrophUic) of the zeolite... 

Tsuru T, Tomoya H, Tomohisa Y, and Masashi A. Permporometry characterization of microporous ceramic membranes. J Membr Sci 2001 186 257-265. 

With the advance of porous membranes, the permporometry methods gained fresh impetus. The basic idea is to block pores of some sizes by a wettable liquid and measure either permeation of counter-current diffusion fluxes through the open pores. 

In liquid-expulsion permporometry [8], the porous solid is saturated with a liquid and by application of a pressure difference across the sample the liquid is forced out of the largest pores. The rate of gas permeating through these pores is then measured. Then, the pressure difference is increased which frees another pores, etc. As a result, pore-size distribution is obtained. 

A.2.2 Liquid/gas methods (bubble point, liquid expulsion permporometry)... 

Liquid-liquid displacement porosimetry (or biliquid permporometry) [124-128]... 

P. Sneider and P. Uchytil, Liquid expulsion permporometry for characterization of porous membranes. /. Membr. Sci., 95 (1994) 29. 

M.G. Liu, R. Ben Aim and M. Mietton Peuchot, Characterization of inorganic membranes by permporometry method importance of non equilibrium phenomena, in A.J. Burggraaf, J. Charpin and L. Cot (Eds.), Inorganic Membranes. Key Engineering Materials 61 62, Trans Tech Publications, Zurich, 1991, pp. 603-605. 

C. Eyraud, Application of gas-liquid permporometry to characterization of inorganic ultrafilters, in E. Dridi and M. Nakagaki (Eds.), Proc. Eur.-fPN Cong. Memb. Processes, 1984, pp. 629-634. 

A series of more sophisticated permeation techniques have been recently proposed in order to better characterise the quality of the membrane and to evaluate qualitatively the contribution of non-zeolite pores to the transport. A number of methods derived from permporometry [125] are under study [111,129], The method used in [111] consists in measuring the permeance of an inert gas such as He through a MFI membrane, while increasing the partial pressure (p/pn) of a condensable gas which can enter into the zeolite pores, such as w-hcxanc (Okmaic = 0.43 nm). Zeolite pores arc then blocked first by -hexane adsorption, and when the proportion of n-... 

Fig. 6. Principle of permporometry for the dynamic characterisation of zeolite membranes. When the partial pressure of n-hexane (p/p ) increases, it blocks first zeolite pores and then larger and larger defects sizes, thus blocking progressively the permeation of He.

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