MATERIALS—GRAPHITE FOAM

Carbon—graphite foam is a unique material that has yet to find a place among the various types of commercial specialty graphites. Its low thermal conductivity, mechanical stabiHty over a wide range of temperatures from room temperature to 3000°C, and light weight make it a prime candidate for thermal protection of new, emerging carbon—carbon aerospace reentry vehicles. 

Conduct research on advanced matrix manufacturing of materials with tunable thermal, mechanical, neutronic, and environmental resistance properties at high temperatures, such as electron-beam vapor deposition of rhenium and iridium on graphitized foam. 

A small physical model was constructed, as well, using the graphite foam and electric heaters coupled with a computer model to benchmark codes, heat transfer, and overall material performance. 

Studies described in the previous section had been based on the assumption that the voids in the graphite foam could be filled with uranium metal. Materials investigations conducted showed that such a production process was not feasible. Previous works at ORNL studied possible methods of impregnating foam with uranium and are discussed below. 

The use of UC2 or U2C3 as the souree of uraniiun seems the most likely route. There are several approaches one might take to produce a graphite foam eontaining one or the other of these materials. One could simply infiltrate the foam with a fluid suspension of particles in a solvent. Typically, one can achieve upwards of 50 to 60 vol % solids in a suspension and still have it be flowable. Assuming the lower number, 50 vol %, one could incorporate 37.5 vol % U2C3 or UC2 in the foam. One real problem with this approach is that fme-particle-size uraniiun carbides are pyrophoric, and, therefore, safe production processes are difficult to achieve. 

From Fig. 4.38 it is observed that commercial graphite foam exhibits considerably higher density and thermal conductivity values than the graphite foam utilized for the irradiation studies of this project. It is, therefore, anticipated that irradiated samples made from this material would exhibit higher thermal conductivities than the samples used in this study. Hence, the data generated in this study are a good conservative estimate of the thermal properties of commercial foams under irradiation conditions. 

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. 

---