VITREOUS CARBON FOAM

Vitreous carbon can be produced in the fonn of an open-pore foam, known as reticulated vitreous carbon (RVC). The precursor materials are essentially the same polymers used for solid vitreous carbon, except that they are foamed prior to carbonization. The carbonization process is similar. The open-pore structure of the material is shown in Fig.6.8.i l 

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Kent BL, Mutharasani R (1992), Cultivation of animal cells in a reticulated vitreous carbon foam, J. Biotechnol. 22 311-328. 

Figure 6.8. Open-pore structure of vitreous carbon foam.h ...

Vitreous-carbon foam is very susceptible to oxidation due its large surface area. Any application involving an oxidizing atmosphere above 500°C should not be considered. 

Electrodes. Its chemical inertness, its wide range of usable potential (1.2 to -1.0 V vs. SCE) and the hydrodynamic and structural advantages of its open-pore foam structure make vitreous carbon foam an attractive material for electrodes for lithium-ion and other types of batteries, with many potential applications in electrochemistry.[ l[ ll ]... 

Hlgh>Temperature Thermal Insulation. A potential application of vitreous-carbon foam is high-temperature thermal insulation in vacuum or non-oxidizing atmosphere. Severai factors combine to make this structure an exceiient thermal insulator (a)Vne low volume fraction of the solid phase, which limits conduction (b) the small cell size, which virtually eliminates convection and reduces radiation through repeated absorption/reflection at the cell walls and (c) the poor conductivity of the enclosed gas (or vacuum). An additional advantage is its excellent thermal-shock resistance due to its... 

Adsorption of Hydrocarbons and Other Gases. In the activated form, vitreous-carbon foam could replace activated-carbon granules without the requirement of a container (see Ch. 10, Sec. 4.0). Potential uses are in emission control and recovery. 

Other Applications. Vitreous-carbon foam is being considered as a filter for diesel particulates and for the filtration of non-carbide-forming molten metals. 

Carbon—Graphite electrodes are often used, but they are less stable toward corrosion/phys-ical degradation than platinum. Graphite is available in many forms, including woven cloths, reticulated foam, and glassy (vitreous) carbon rods and plates. Organic products at different types of graphite anodes may differ considerably [60]. 

Two types of FP traps have been installed in JOYO. One is a cesium trap installed in the primary coolant sodium purification system to capture cesium released from failed fuels. An open pore, foam-like glassy carbon that consists of thin struts of Reticulated Vitreous Carbon (RVC) is used as a material for collecting cesium. The capacity of this trap is designed to be 7.4E+12 Bq. The other trap is a Cover Gas Clean-up System (CGCS) to collect and store the noble fission gas released from failed fuels. Although it is planned that only one failed fuel pin will be in the core at any time, the CGCS is designed to handle the releases of up to twelve failed fuel pins. 

Vitreous carbon can be produced in three basic types which have essentially the same microstructure, but different macrostructures solid (or monolithic), foam (or reticulated), and spheres (or particles). Each type is reviewed in the next three sections. 

Vitreous carbon loam is produced in several pore sizes, usually described as number of pores per inch (ppi). Commercially available foams are respectively 60,100, and 200 ppi (24,39 and 78 pores per cm). These foams have low density, with relatively even pore distribution. Their properties are listed in Table 6.4.i i... 

A variant of the enhanced reaction zone concept is to utilize as catalyst support various porous three-dimensional electrodes with thickness between 200 to 2,000 pm. Thus, the electric contact resistance between the individual layers is eliminated. The three-dimensional matrix (such as various graphite felts, reticulated vitreous carbon, metal mesh, felt, and foam) supporting uniformly dispersed electrocatalysts (nanoparticles or thin mesoporous coating) could assure an extended reaction zone for fuel (methanol, ethanol, and formie aeid) electrooxidation, providing an ionic conductor network is established to link the catalytically active sites and the proton exchange membrane. The patent by Wilkinson et al. also suggests such electrode configurations (e.g., carbon foam, expended metal and reticulated metal) but experimental results were not provided [303]. 

One of the newest types of porous, three-dimensional materials to receive attention is reticulated (foam) metal (e,g. Ni, stainless steel, Cu) or carbon, e.g. reticulated vitreous carbon (RVC). The open-cell (alveoli like) porous structure of these materials confers a range of interesting properties ... 

Many concepts were developed to overcome one of the main drawbacks of the lead-acid system the heavy supporting lead structures (grids, connectors, etc.). Lead foam [89], lead-plated carbon rods [90], electroplated vitreous carbon [91], flexible-graphite grids [92], or graphite foams [93] were tested, also lead-plated materials like titanium [94], Ebonex [95], copper mesh [96], polymeric structures [97], polymer foam [98], or glass fiber mesh [99]. Warlimont and Hofmann [100] describe the development of multilayer composite grids. 

Material costs will be a large factor in the total reactor costs. Mainly anodic materials commonly used in MFC reactors, such as graphite foams, reticulated vitreous carbon, graphite, and others, are quite expensive. Simplified electrodes. 

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