Hollow-sphere foam

Key words cellular metals, metallic foam, open- and closed-cell foam, hollow-sphere foam, optimized truss structures, honeycomb. 

Cellular solids are a class of materials with low densities and novel physical, mechanical, thermal, electrical, and acoustic properties. Low-density cellular metals can feature a wide variety of topologies to include open-cell foam, closed-cell foam, hollow-sphere foam, periodic/optimized truss structures, and honeycomb. Metallic foams consist of air dispersed in a solid matrix, similar to polymer foams such as polystyrene or food foams such as whipped cream. Closed-cell foams feature solid faces such that each cell is independently sealed from its neighboring cells, whereas open-cell foams (also known as porous metals, metal sponges and truss-type materials) do not contain cell walls they only have cell edges. Hollow-sphere foams consist of an assembly of individual hollow spheres. 

A detailed analysis of the behavior of hollow-sphere foams is available in [18,19], The theoretical performance of hollow-sphere foams is on par with that of closed-cell foams. Since hollow-sphere foams can be produced with fewer defects, they have the potential to perform up to three times better than existing closed-cell foams at a relative density of 10% and ten times better below a relative density of 5% [17], The behavior of simple cubic packed (SC) and face-centered cubic packed (FCC) hollow-sphere foams is shown in Figure 1. The FCC hollow-sphere foam generally represents the best performance of optimally bonded hollow spheres that was measured in this work and SC hollow-sphere foam generally represents the performance of non-optimally bonded, random packed hollow spheres [17]... 

The relative strength of hollow-sphere foams lies between the theoretical performance of open- and closed-cell foams. The performance of optimized truss structures is similar to that of closed-cell foams and, for the Kagome truss, approaches the behavior of a Hashin-Shtrikman porous material. Honeycombs are the most efficient structures when loaded purely out-of-plane. However, plastic buckling can decrease its performance at low relative densities. Further, since honeycomb is highly anisotropic, any inplane loading results in severely reduced performance. Although the theoretical performance of closed-cell foams far exceeds that of open-cell foams, processing defects result in commercially available material that behaves similar to an open-cell material at low relative densities. Commercially available samples of other types of low-density metallic structures behave nearly as predicted. [17]... 

The maximum operating temperature of the insulation would have been approximately 1150 K (1610 °F). At this temperature, the most promising insulation materials for this application were ceramic or metallic foams. Figure 9-44 depicts Inconel 617 hollow sphere foam. 

Syntactic foam contains an orderly arrangement of hollow sphere fillers. They are usually glass microspheres approximately 100 microns (4 mils) in diameter, provide strong, impervious supports for otherwise weak, irregular voids. As a result, syntactic foam has attracted considerable attention both as a convenient and relatively lightweight buoyancy material and as a porous solid with excellent shock attenuating characteristics. The latter characteristic is achieved... 

On the other hand, syntactic materials may also be thought of as reinforced or filled plastics, with the gas-containing particles being the reinforcing component. This classification is also justifiable in view of the manufacturing technology. The matrix is not foamed chemically, but is filled mechanically with the hollow spheres. Syntactic foamed plastics are thus called physical foams 6,7). 

In these Equations, G is the modulus of the syntactic foam, G0 is the modulus of the polymer matrix, v0 is Poisson s ratio of the polymer matrix, and 9 is the maximum packing fraction of the filler phase. For uniform spheres, 9 0.64 (see Sect. 3.6). The volume fraction of spheres in the syntactic foam is 9sph. The slope of the G/G0 vs. 9sph curve depends strongly upon whether or not G/G0 is greater or less than 1.0. The slope is negative if the apparent modulus of the hollow spheres is less than the modulus of the polymer matrix. 

Recently, Kinra and Ker 137) published data of the shear modulus of syntactic foams consisting of hollow glass spheres in a poly(methyl methacrylate) matrix. The glass spheres had a mean radius of 45 pm and a wall thickness of 1.2 pm. Reliable values are known for the shear modulus of the polymer G0, the shear modulus of glass Gs, and Poisson s ratio of the polymer G0 = 1120 MPa, Gs = 2800 MPa, and v0 = 0.35. Using these values, the upper curve 1 of Fig. 24 was calculated by Nielsen for the modulus of the foam as a function of the volume fraction of hollow spheres. These calculated values are, however, too high compared with the experimental values reported by Kinra and Ker. 

Syntactic Foam. Hollow glass, ceramic, or plastic spheres are dispersed in the reactive liquid system before it is cast. When the liquid is polymerized and cured, the hollow spheres make it a unicellular foam. The air bubbles in the cells make it low-density, low dielectric constant and loss, and very resistant to compressive forces such as hydrostatic head in deep-sea equipment. 

When the inclusions are thin-walled, hollow spheres, such as the glass microspheres used in a syntactic foam, then the moduli of the inclusions are given by (9)... 

The materials employed for making hollow microspheres include inorganic materials such as glass and silica, and polymeric materials such as epoxy resin, unsaturated polyester resin, silicone resin, phenolics, polyvinyl alcohol, polyvinyl chloride, polyjM-opylene and polystyrene, among others, commercial jx oducts available are glass, silica, phenolics, epoxy resin, silicones, etc. Table 36 shows low-density hollow spheres. Table 37 shows physical properties of glass microspheres, and Table 38 shows comparison of some fillers on the physical properties of resulting foams (10). 

Syntactic foams are made using a resin matrix to which has been added hollow spheres of various materials. The resultant product is a foamlike material made without the use of a blowing agent. The most common matrix resins are epoxies and polyesters, although urethanes, PVC plastisols, and phenolic resins have also been used. Indeed, any polymer that can be made liquid, either before final polymerization or by heat, can be used as the binding resin. In syntactic foams the resin matrix is the continuous phase and the hollow spheres the discontinuous phase. 

Major polymer applications aircraft interiors, automotive (pump housings, transmission reactors, timing pulleys), marine, construction, coatings, adliesives. carbonless copy paper, abrasives, friction materials, laminates, foimdry resins, battery separators, wood bonding, composites, foam, hollow spheres... 

W.S. Sanders in Mechanical Behavior of Closed-Cell and Hollow-Sphere Metallic Foams, Massachusetts Institute of Technology ScD Thesis,USA, 2002. 

Abstract Nanoparticles (NPs, diameter range of 1-100 nm) can have size-dependent physical and electronic properties that are useful in a variety of applications. Arranging them into hollow shells introduces the additional functionalities of encapsulation, storage, and controlled release that the constituent NPs do not have.This chapter examines recent developments in the synthesis routes and properties of hollow spheres formed out of NPs. Synthesis approaches reviewed here are recent developments in the electrostatics-based tandem assembly and interfacial stabilization routes to the formation of NP-shelled structures. Distinct from the well-established layer-by-layer (LBL) synthesis approach, the former route leads to NP/polymer composite hollow spheres that are potentially useful in medical therapy, catalysis, and encapsulation applications. The latter route is based on interfacial activity and stabilization by NPs with amphiphilic properties, to generate materials like colloidosomes, Pickering emulsions, and foams. The varied types of NP shells can have unique materials properties that are not found in the NP building blocks, or in polymer-based, surfactant-based, or LBL-assembled capsules. 

Binks and Murakami report unique behavior in NP-induced phase transformation in particle-stabilized air-water systems that is not demonstrated by surfactants. It was seen that by altering silica-NP (20-30 nm) hydrophobicity at constant air water ratio or by changing the air water ratio at fixed NP wettability, phase inversion could be induced from air-in-water to water-in-air foams (Fig. 12) [36]. This investigation thus demonstrates that control over interfacial assembly of NPs leads to the formation of stable NP-shelled hollow spheres, thus resulting in the formation of stable foams, dispersions, and powders with far reaching consequences in opening new avenues for advanced encapsulation (Fig. 13). 

Fig. 28. Foam bodies and hollow spheres produced from negatively charged drops of the complex...

B Two foam bodies are still in stage f of Fig. 27, for the rest the coacervate drops have been completely transformed into hollow spheres (196 X lin.). 

There thus remains the question as to what mechanism is concerned in the formation of the foam vacuoles and hollow spheres, which at the beginning stands quite in the foreground. The free energy which is concerned here must be obtained from the original disturbed equilibrium state (bringii the coacervate drops into contact with much water). There are now some indications that in the swelling of the primarily formed vacuoles to foam vacuoles or hollow spheres an electroendosmotic water transport through the coacervate lamellae is involved. 

The conditions already discussed are expressed in Fig. 29 from which one can read ojEf at what mixing ratios and pH s of the original sols foam bodies and hollow spheres are formed after shaking the isolated coacervate layer with a certain amount... 

To test the hypothesis that an electroendosmosis takes place in the formation of foam bodies and hollow spheres the E.M.F. of cells of the following type was also determined ... 

We thus come to the conclusion that for the formation of foam bodies and thin walled hollow spheres two conditions must be fulfilled simultaneously ... 

As already stated the hollow spheres after they have formed from foam bodies continue to exist for some time (for example V2 hour) in their typical form. 

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