Foam Wall Structure

While the 3-D foam stmctures are very attractive for rapid mass transport and fast electrochemical reactions, many issues still remain for their applications in practical batteries and fuel cells, in- 

APPLICATIONS OF fflERARCHICAL POROUS STRUCTURES TO FUNCTIONAL ELECTROCHEMICAL DEVICES 

Copper-Ceiia Composite Anode for Solid Oxide Fuel Cell 

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If we imagine a foamed plastic structure consisting entirely of spherical cells of diameter d arranged in a cubic lattice, then for a wall thickness of 5 the weight of the polymer (g ) per cell will be ... 

Fig. 26.6 Representative images as detected by an optical microscope during tack tests through a transparent substrate. Highly inhomogeneous structures develop on a macroscopic scale. Polymeric partition walls keep neighboring cavities separate. A polygonal, foam-like structure develops. The diameter of the punch is 2 mm.

Out-of-plane joints have been reviewed by Junhou and Shenoi, mainly in the context of marine structures [47]. The most common marine topologies are the monocoque or top-hat stiffened structures, using single skin glass/ polyester or vinyl ester laminates, and the sandwich or double wall structures with a foam core. The authors illustrate six different kinds of out-of-plane joints used in this context with FRP. 

In order to increase the specific surface area and enhance the effectiveness/ activity of the porous electrodes, it is necessary to reduce the size of the pores, as well as the branches in the foam or agglomerates of copper grains in the honeycomb-Uke structures [26]. One of the ways to improve micro- and nanostructural characteristics of open porous electrodes is the addition of additives to the electroplating solution [26]. The decrease of diameter of holes, as well as the increase of their number in 3D foam copper structures, can be realized by the addition of acetic acid to the copper sulfate solution [26]. Also, the addition of chloride ions dramatically reduces the size of the copper branches in the walls of holes. The reduction in pore size is a result of lowering hydrophobic force of the generated hydrogen gas by adding bubble stabilizer (e.g., acetic acid) that suppresses the coalescence of bubbles, while the decrease in branch size in the foam wall is a consequence of the catalytic effect of chloride ions on the copper deposition reaction. 

Structural foam is more tolerant of nonuniform wall thicknesses than sohd injection molding and variations of 75 to 100% are acceptable for standard foam and co-injected walls. Thin foamed walls (0.156 in) should not vary more than 50 to 75%. The change in wall thickness should be very gradual and abrupt wall thickness changes must be avoided to reduce stress concentrations. Wall thickness tolerances of 0.005 in can be held. Draft should be specified according to the recommendations outlined in Sec. 8.2.2. 

Overall tolerances in structural foam molding vary with the size of the part. For dimensions of less than 4 in, tolerances of 0.005 in can be held. From 4 to 10 in, the minimum tolerance should be 0.010 in. From 10 to 20 in, tolerances of 0.015 in should be regarded as a minimum and beyond 20 in the tolerance should be 0.050 in. Co-injection structural foam and thin-wall structural foam tolerances would be similar to those for injection molding. 

Aggregation in colloidal systems can be introduced by various mechanism. Attractive interactions between the colloids is the most prominent example. Another possibility is to confine the colloids in one phase of a phase separating mixture, e.g., in the isotropic phase of a liquid crystalline fluid that is undergoing the isotropic-to-nematic transition [52, 53]. This unusual soft solid consists of a foam hke structure, where the bubbles are filled with liquid crystal in the nematic phase and the colloids are confined in the walls separating the bubbles [54]. 

One is the porous structure of foam wall. The wall of copper electro-deposits includes a number of small pores between branches, as seen in Fig. 4(b), whereas that of tin electro-deposits shows little pores inside, as seen in Fig. 6(b). As discussed earlier, hydrogen gas plays a critical role, as a dynamic template, in forming porous structure. In the course of copper deposition, hydrogen gas... 

Unlike copper electro-deposits, morphological change of tin electro-deposits with the tin sulfate content is quite visible. Figure 10 tells us that the 3-D foam structure was barely developed at the veiy low content of tin sulfate (0.1 M). The foam structure starts to form in a 0.5 M tin sulfate solution and the structure formed in a 1.0 M solution is characterized by relatively dense foam wall as compared to that prepared in a 0.5 M solutioa This means the concentration of tin sulfate in the solution critically influences the foam structure and the details of the wall. Under the experimental conditions studied in this work, well-defined 3-D tin foam structure was formed in solutions with tin sulfate concentrations between 0.1 and 1.0... 

Shown in Fig. 12 are the morphological changes in copper foam stractures with the content of acetic acid. It is noted that the local abnormal growth of foam wall is clearly seen at the acetic acid content more than 0.1 M, as shown in the dotted circles of Fig. 12(a)-(c). In order to further investigate the overgrowth, copper was electro-deposited for 90 s at 0.2 M acetic acid (Fig. 13). Overgrown foam wall is much thicker than the normal ones and its apparent porosity becomes significantly reduced. Also, it is noticeable that 3-D foam structure is hardly developed when the content... 

Figure 14. Change in wall structure of the foam with the content of chloride ions.

The transverse modulus is lower partly because the cell wall is less stiff in this direction, but partly because the foam structure is intrinsically anisotropic because of the cell shape. When wood is loaded across the grain, the cell walls bend (Fig. 26.5b,c). It behaves like a foam (Chapter 25) for which... 

Perhaps, however, the greatest virtue of structural foams is the ability to increase the ratio of part rigidity/weight. A foam of half the density of a solid material only requires a 25% increase in wall thickness to maintain the rigidity. 

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