Carbon sintering mechanism

The Si-B-C system was mainly investigated with view to the understanding of the sintering mechanisms of SiC with boron in combination with carbon and the sintering of boron carbide with silicon [4]. The silicon solubility of about 2.5 at.% in B4C at 2323 K and the comparatively low temperatures of liquid phase formation in the ternary system enhance the sintering of boron carbide. 

Three sintering mechanisms have been proposed to explain ECSA loss of fuel cell catalysts catalyst dissolution/reprecipitation, migration of Pt particles, and carbon corrosion [85]. 

As mentioned earlier, CB is prone to oxidation, the so-called carbon corrosion, which results in the loss of surface area, changes in the pore structure and finally also leads to sintering of the supported nanoparticles and eventually their loss from the support surface. This affects both the kinetics of the reaction and the electrode s mass transport behavior resulting in a significant loss of performance with operation time. Consequently, carbon support durability is considered to be a major barrier for the successful commercialization of fuel cell technology in the automotive sector. So much so, during the last decade, more than 60 publications dealt with carbon corrosion mechanisms in fuel cell apphcation [82]. 

Keywords ceramic matrix composites, carbon nanotubes, mechanical properties, sintering, glasses... 

Three sintering mechanisms have been proposed to explain ECSA loss of fuel cell electrocatalysts (1) dissolution/reprecipitation (Tseung and Dhara 1975 Watanabe et al. 1994 Antolini 2003 Ferreira et al. 2005), (2) migration of platinum particles Bett et al. 1976 Blurton et al. 1978 Wilson et al. 1993 Ferreira et al. 2005 Borup et al. 2006), and (3) carbon corrosion (Roen et al. 2004 Cai et al. 2006 Guilminot et al. 2007). Details of these mechanisms can be found in the chapter High-Tanperature Polymer Electrolyte Fuel Cells Durability Insights. ... 

Inorganic membranes (29,36) are generaUy more stable than their polymeric counterparts. Mechanical property data have not been definitive for good comparisons. IndustriaUy, tube bundle and honeycomb constmctions predominate with surface areas 20 to 200 m. Cross-flow is generaUy the preferred mode of operation. Packing densities are greater than 1000 /m. Porous ceramics, sintered metal, and metal oxides on porous carbon support... 

Barium carbonate of finely controlled particle size reacts in the soHd state when heated with iron oxide to form barium ferrites. Magnetically aligned barium ferrite [11138-11-7] powder can be pressed and sintered into a hard-core permanent magnet which is used in many types of small motors. Alternatively, ground up magnetic powder can be compounded into plastic strips which are used in a variety of appHances as part of the closure mechanism. 

The metal surface area at the inlet end of the catalyst bed in experiment HGR-12 was smaller than that at the outlet end this indicates that a decrease in nickel metal sites is part of the deactivation process. Sintering of the nickel is one possible mechanism, but carbon and carbide formation are suspected major causes. Loss of active Raney nickel sites could also conceivably result from diffusion of residual free aluminum from unleached catalyst and subsequent alloying with the free nickel to form an inactive material. 

Bartholomew and coworkers32 described deactivation of cobalt catalysts supported on fumed silica and on silica gel. Rapid deactivation was linked with high conversions, and the activity was not recovered by oxidation and re-reduction of the catalysts, indicating that carbon deposition was not responsible for the loss of activity. Based on characterization of catalysts used in the FTS and steam-treated catalysts and supports the authors propose that the deactivation is due to support sintering in steam (loss of surface area and increased pore diameter) as well as loss of cobalt metal surface area. The mechanism of the latter is suggested to be due to the formation of cobalt silicates or encapsulation of the cobalt metal by the collapsing support. 

The major problems with Ni-based anodes and NiO cathodes are structural stability and NiO dissolution, respectively (9). Sintering and mechanical deformation of the porous Ni-based anode under compressive load lead to severe performance decay by redistribution of electrolyte in a MCFC stack. The dissolution of NiO in molten carbonate electrolyte became evident when thin electrolyte structures were used. Despite the low solubility of NiO in carbonate electrolytes ( 10 ppm), Ni ions diffuse in the electrolyte towards the anode, and metallic Ni can precipitate in regions where a H2 reducing environment is encountered. The precipitation of Ni provides a sink for Ni ions, and thus promotes the diffusion of dissolved Ni from the cathode. This phenomenon... 

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