Microstructure, membranes

The authors discuss Schroeder s paradox, referred to elsewhere in this review, and the fact that liquid water uptake increases but saturated water uptake decreases with temperature. And, at low temperature, the water uptake by membranes in contact with saturated vapor is greater than that by membranes in contact with liquid water, which suggests a fundamental difference in membrane microstructure for the two situations. An energy level diagram of thermodynamic states versus temperature was proposed, based on this Flory—Huggins-based model. 

Using a simple electrostatic interaction-based model factored into reaction rate theory, the energy barrier for ion hopping was related to the cation hydration radius. The conductance versus water content behavior was suggested to involve (1) a change in the rate constant for the elementary ion transfer event and (2) a change in the membrane microstructure that affects conduction pathways. 

More discussions on the membrane microstructure affecting the membrane reactor performance will be made in (Chapter 10 and 11. 

The evaluation of the commercial potential of ceramic porous membranes requires improved characterization of the membrane microstructure and a better understanding of the relationship between the microstructural characteristics of the membranes and the mechanisms of separation. To this end, a combination of characterization techniques should be used to obtain the best possible assessment of the pore structure and provide an input for the development of reliable models predicting the optimum conditions for maximum permeability and selectivity. The most established methods of obtaining structural information are based on the interaction of the porous material with fluids, in the static mode (vapor sorption, mercury penetration) or the dynamic mode (fluid flow measurements through the porous membrane). 

However, in the cell the membrane hydration is affected by generic fuel cell processes, including the supply of humidified reactant gases to the electrodes, electroosmotic drag of water from anode to cathode, backtransport of water in the membrane, and production of water at the cathode. It is, therefore, generally important to consider the internal membrane water balance self-consistently and relate it to the membrane microstructure. 

There are different approaches that incorporate the water balance in the membrane into models of fuel cell performance. They rest on different concepts of membrane microstructure. As a common feature they use local values of transport parameters which are functions of the local water content, w (volume fraction of water relative to the total membrane volume). 

A nonconventional view of membrane microstructure, which neither conforms with the solution nor with the porous rock picture, was recently suggested in Ref. 84. Classical MDs simulations on microstructure and molecular mobility in swollen Nation membranes revealed a picture of a rather dynamic structure of water clusters with temporary formation and break-up of water bridges between them. The frequency of intercluster bridge formation was found to be consistent with the experimental transport coefficients through the membrane. 

Furthermore, which mechanism prevails is also determined by the membrane microstructure and water/polymer interactions. A pronounced hydropho-bic/hydrophilic phase separation will result in a well-developed porous structure and, thereby promote hydraulic permeation as the relevant mechanism. In random polymer membranes, which exhibit a smaller extent of ion clustering, water fractions will be more dispersed in the... 

With the development of more complex and sophisticated inorganic membranes there is a need for a better understanding of membrane structures and their influence on the mechanisms of separation processes. This requirement for a better insight into the relationships between (a) the membrane synthesis route, (b) the membrane microstructure or morphological properties and (c) the permeation properties, has been widely emphasised in the literature. Information on membrane characteristics is essential for membrane users, manufacturers and scientists to choose an appropriate membrane for a specific application, controlling membrane quality and preparation process parameters or understanding transport mechanisms. 

Determination of the coefficients based on understanding of the membrane microstructure and modelling of the interaction between the membrane and the two transported species, i.e. hydronium and water, would be better. Most desirable would be a proper mathematical transition from an exact microscopic description of the interaction of membrane, hydronium and water, towards a macroscopic model. Such information and description being currently unavailable, we have to rely on guidance from knowledge on the membrane morphology to devise assumptions on the functional dependence of the coefficients on temperature and water content. 

To perform the NMR spectroscopy, a clean, umnodified Vycor tube and a modified Vycor tube were crushed to a fine powder. Firstly, Si NMR was performed on unmodified and modified Vycor samples to ensure that the desired surface modification was achieved. C NMR was then carried out on the modified tube before and after exposure to CO2 to show the formation of the carbamate species, as this was the hypothesized mechaifism for CO2 transport through these membranes as shown in the reaction mechanism below (Eq. 7.1). The procedure for the NMR experiments reported by Singh et al. [11] was used in the characterization. The chemical shift in the NMR signal for Si or C atoms depends on their envi-ronment, and thus, the peaks can distinguish various Si and C moieties present in the membrane material and hence provide useful information about the membrane microstructure. 

Some reference values of membrane permeance for thin Pd-Ag membranes (surface rds) are plotted in Fig. 18.35. As expected, the thinner the membrane and higher the operating temperature, the higher the hydrogen permeance is, although some inversions are observed due to the membrane microstructure. 

Another important factor which determines membrane characteristics is the membrane structure, particularly the membrane microstructure, which is also called membrane morphology. The desired membrane characteristic can be obtained by controlling the membrane morphology. The specific membrane morphology is usually achieved by controlling the conversion process from liquid to solid during the production of the membrane. This may be done by controlling the evaporation rate of the solvent in a dry... 

Another type of microstructure in glass-ceramics can best be described by comparing it to the structure of an organic cell. A cell is composed of a very thin membrane that separates and protects the contents of the cell from the neighboring cells. Similarly, in a cellular membrane microstructure, a very thin membrane surrounds the contents of a cell-like entity. 

This type of cellular membrane microstructure can be developed in glass-ceramics that exhibit a very low coefficient of thermal expansion because of the precipitation of p-quartz solid-solution or P-spodumene crystals (Beall 1992). The glass matrix, which is very thin and surrounds the crystals, plays the part of the cellular membrane. As a result, the crystals are separated from each other. The membrane accounts for approximately 10 vol% of the micro-structure and can, for example, act as a diffusion barrier between the crystals. 

Figure 3-2 shows the cellular membrane microstructure of a P-quartz solid solution glass-ceramic. The individual phases are presented in connection with their effect on the different solid-state reactions to provide a better understanding of this microstructure. As described in Section 2.2.2, the nucleation of P-quartz solid-solution crystals is heterogeneously initiated by ZrTiO nucleating agents. These ZrHO crystals represent the nucleus of the... 

Figure 3-2 Cellular membrane microstructure of a B-quartz solid-solution glass-ceramic (bar 200 nm). TEM (Maier and Muller, 1989). Residual glass with ZrTi04 precipitates is observed with cellular structure.

Self-assembled membranes constructed from phospholipids and other surfactants have been extensively investigated to understand their formation, encapsulation and release, and templating properties (7-25). Lipids and surfactants are amphiphilic molecules with hydrophilic, polar headgroups and nonpolar tails. As a result of the hydrogen bonding and electrostatic interactions of the hydrophilic headgroups and the van der Waals interactions between the hydrophobic tails, amphiphiles form organized membrane microstructures when dispersed in water or oil. When... 

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