Coated particles carbonate

Fig. 2. Fuel for high temperature gas-cooled reactor. Fissile material is coated with carbon and siHcon carbide, fertile material with carbon. Particles mixed...

The protection of components against nuclear radiation is a critical factor in the design of nuclear-fission components.P CVD is used extensively in this area, particularly in the coating of nuclear fuel particles such as fissile U-235, U-233, and fertile Th-232 with pyrolytic carbon. The carbon is deposited in a fluidized-bed reactor (see Ch. 4). The coated particles are then processed into fuel rods which are assembled to form the fuel elements. 

Four low pressure merciuy lamps (15 W, 254 nm) were installed inside the reactor. 4.5kg of Ti02-lfee activated carbon (GAC) or Ti02-coated activated carbon (GAC-Ti) was loaded inside the reactor and aqueous solution of TCE (0.05mg/L concentration) was fed into the bottom of the reactor. The exit stream of the reactor was analysed for TCE concentration. Successfiil fluidization without carryover of the activated carbon particles could be obtained at the flow rates of upward-flowing water between 150 200cm /sec. 

The surface of a commercial catalyst (40=100 m2/g, p0= 3.2 g/cm3) after exploitation in reaction covered with a layer of carbon (pc= 1.7 g/cm3) of thickness / = 1 nm. Assuming uniform coating with carbon of spherical particles of the catalyst calculate the resulting surface area of a carbonized catalyst. 

In order to overcome these problems, hybridization of both materials (C and Si) in one electrode material by HTC seemed to be a promising option [75]. For this purpose, pre-formed silicon nanoparticles were dispersed into a dilute solution of glucose followed by hydrothermal treatment at 180 °C. The carbon-coated particles were then further treated at 750 °C in order to improve the conductivity and structural order of the carbon layer. It was shown that the hydrothermal treatment, following by high temperature carbonization, resulted in formation of a few nanometer thin layer of SiOx layer on the Si nanoparticles, effectively leading to a Si/SiOx/C nanocomposite. Some TEM micrographs of these materials are shown in Fig. 7.8. 

Again, decomposition of urea in an acidic metal salt solution in the presence of preformed particles can yield homogeneous layers of variable thicknesses of metal basic carbonates on the core materials of different chemical composition. In order to obtain uniformly coated particles (rather than a mixture of the latter and of the independently precipitated coating material), a balance between the amount of the initial dispersed matter and the concentration of the reacting solutes must be optimized. 

Even newer generations of nanomaterials are based on carbon nanotubes using the bottom-up approach. The materials are still very expensive, but the technology is evolving rapidly. Another type of nanotube has been prepared based on self-assembly of specific molecules such as chitosan-based nanoparticles of polypeptides, DNA or synthetic polymers. Phospholipids or dendrimer-coated particles are suitable for the entrapment of actives in very small vesicles. The current materials are still lacking in selectivity and yield (costs). 

The present authors proposed ferromagnetic supports, which can withstand liquefaction environment and conditions by their inherent nature or by protection with a carbon coating (141). Carbon has been recognized as an excellent support for FeS and Ni-MoS as described in the previous section (50). Very fine particles of ferrite are available. Coating with carbon can be performed through precipitation of polymers or pitches by the aid of suspension agents followed by carbonization (Ml). Catalyst deactivation by mineral and carbon deposition should be avoided for this approach to be feasible. 

Figure 3. Electron micrograph of the carbon shells of tridymite particles. After diffuse coating with carbon in a vacuum, the tridymite was dissolved away by hydrofluoric acid specific surface area of this sample of relatively coarse crystals is 7.7 sq. meters/gram (X24,000)...

KAERI is joining the PYCASSO experiment to investigate coating behaviour under irradiation (Groot, 2008). The objective of the experiment for KAERI is to analyse the porosity and property changes of a pyrolytic carbon buffer layer as a function of neutron fluence and temperature. Approximately 10 000 coated particles of four different coating layer densities were provided for PYCASSO. 

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