Spongelike structures

Skeletal (Raney ) catalysts are made by a very simple technique. An alloy of two metals in roughly equal proportions, where one metal is the desired catalytic material, and the other is dissolvable in hydroxide, is first made. This alloy is crashed and leached in concentrated hydroxide solution. The soluble metal selectively dissolves, leaving behind a highly porous spongelike structure of the desired catalytic metal. Catalysts formed by this technique show high activity and selectivity, and have found wide use in industry, particularly for hydrogenation and dehydrogenation reactions. 

The foam monoliths were a-Al203 samples with an open cellular, spongelike structure. We used samples with nominally 30 to 80 pores per inch (ppi) which were cut into 17 mm diameter cylinders 2 to 20 mm long. A 12 to 20 wt.% coating of Pt or Rh was then applied directly to the alumina by an organometallic deposition. 

Figure 25-5 Scanning electron micrographs of silica chromatography particles, (a) Aggregate of spherical particles with 50% porosity and a surface area of 150 m2/g. (b) Spongelike structure with 70% porosity and a surface area of 300 m2/g. Pores are the entryways into the interior of the particles. In both cases, the nominal pore size is 10 nm, but the distribution of pore sizes is greater in the spongelike structure. The spongelike structure also dissolves more readily in base. [From Hewlett-Packard Co. and R. Majors, LCGC May 1997, p. S8.J...

There are various kinds of polytetrafluoroethylene. One is granular polymer consisting of spongy, white particles having a median size of about 600/l The specific surface of this polymer is on the order of 2 m2/g (determined by nitrogen adsorption and calculations by the method of Brunauer, Emmett, and Teller). Since this specific surface area is about 1700 times the observed outer surface of the particles, these measurements confirm the porous, spongelike structure that can be seen in the photomicrograph of a cross section of several particles in Fig. la. 

Tritium profiles are commonly interpreted with the basic premise that the soil is a uniform medium, with a spongelike structure, through which water infiltrates. However, short-circuit flow through conduits such as cracks, bioturbations, or decayed root channels must also be considered. Cracks, or rodent holes, are efficient intake points of runoff, and occasionally they become filled with coarse particles, providing routes of relatively high conductivity. When a soil profile builds up, these coarsely repacked channels... 

Properties Thermally expanded (exfohated) mag-nesium-iron-aluminum silicate having a minimum of 98% purity and a bulk d of 5-7 lb/cu ft. Expansion occurs by heating at793C. Spongelike structure that absorbs liquids and permits reexpansion after compression to 70-80% of the original heat-expanded volume. 

Porous ceramics are made by the sol-gel process. These ceramics have spongelike structures, with many porelike lacunae, or openings, that can make up from 25 to 70 percent of the volume. The pore size can be large, or as small as 50 nanometers (2 X 10 inches) in diameter. Because of the large number of pores, porous ceramics have enormous surface areas (up to 500 square meters, or 5,382 square feet, per gram of ceramic), and so can make excellent catalysts. For example, zirconium oxide is a ceramic oxygen sensor that monitors the air-to-fuel ratio in the exhaust systems of automobiles. 

Figure 1.5 Representation of a spongelike structure of a bicontinuous microemulsion that consists of a surfactant monolayer separating oil and water domains.

Missaoui et al. 2000 Buzynin et al. 2008). The PS provides a large area composed of an array of voids and interconnected silicon clusters or rods. This spongelike structure could limit strain and crack development after the post-growth cooling (Matoussi et al. 2001). 

It is clear that any responsive structure that is larger could have a finely divided spongelike structure that allows for fluids to flow in and out through channels rather than simply diffusing. If a hypothetical microstructural scale of 100 microns thickness is adopted, a diffusion coefficient of about 3 x 10 cm s can be accepted in a working device. 

Expanded Teflon. Although not typically thought of as a reinforcement. Teflon that has been expanded into a spongelike structure on a microscopic scale is currently being used in applications that require prepregs with very low dielectric constants or loss properties. 

Zilberman Y, Tisch U et al (2009) Spongelike Structures of hexa-peri-hexabenzocoronene derivatives enhance the sensitivity of chemiresistive carbon nanotubes to nonpolar volatile organic compounds of cancer. Langmuir 25(9) 5411-5416... 

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