Aristides Dr Naoumidis

Joining of ceramics demonstrated by the example of SiC/Ti

The reaction behaviour of titanium sputtered on silicon carbide was investigated in a series of tests. It was ascertained that, in the temperature range between 1250 and 1500° C, mainly the ternary phase Ti3SiC2 is formed (>90%). In joining experiments of silicon carbide with 1–3 μm thick titanium layers at 1450° C at compaction pressures between 5 and 30 MPa the formation of the ternary phase leads to a high joining strength. The achieved bending strength (σM ≈ 286 MPa; m ≈ 10) is comparable to that of the starting materials (σM ≈ 303 MPa; m ≈ 15).

Synthesis, forming and characterization of ceramic materials for the planar solid oxide fuel cell (SOFC)

Chemically-homogeneous and well-characterized ceramic powders have been used for the preparation and investigation of the SOFC components. The powders were synthesized by spray drying and subsequent calcination. The resulting powders were characterized with regard to the crystalline phases, chemical analysis, specific surface area, particle size and particle size distribution, morphology and particle agglomerates. These parameters are very important for forming thin, flat ceramic sheets and components by tape-casting and screen-printing technology. Forming process, sintering, electrical conductivity and compatibility are discussed.

Microstructure and Interdiffusion Phenomena in YSZ-CGO Composite Electrolyte

In order to investigate the co-firing feasibility of a double-layer ceria-zirconia electrolyte for low-temperature Solid Oxide Fuel Cells (SOFCs), interdiffusion phenomena between yttria-stabilized zirconia (YSZ) and cerium-gadolinium oxide (CGO) were studied in the temperature range 1200°C to 1500°C with the aid of SEM/EDX-WDS. Significant solid state reaction and interdiffusion phenomena across the YSZ-CGO interface were found to occur already at 1200°C. The extent of the interdiffusion zone formed at the interface as well as the elemental distributions in this zone showed no dependence on sintering temperature and holding time. The difference of the diffusivity of Ce 4+ and Gd 3+ into YSZ and Y 3+ and Zr 4+ into CGO resulted in porosity emerging at the YSZ/CGO interface. The use of a microstructure with a graded YSZ/CGO 0.5 YSZ 0.5 /CGO composition was found to reduce the cation diffusion rates across the interface. However, the low conductivity of the CGO 0.5 YSZ 0.5 solid solution phase used requires further optimization of the composition of the interlayer to be used.

Preparation of ThO2/Y2O3 powders by the sol-gel method for solid electrolytes

Abstract Stable Th/Y oxide sols were prepared by peptization of the washed hydroxides and converted by different solidification methods to sinterable ThO 2 /Y 2 O 3 powders which were compared with coprecipitated ThO 2 /Y 2 O 3 powder. The best powder properties were obtained if the Th/Y oxide sol was solidified by internal gelation. Slip casting and subsequent cold isostatic pressing of the sol-gel-derived powder resulted in densities of sintered crucibles or plates of 98·8%, direct cold isostatic pressing in 99·5% of the theoretical density. EMF measurements using sol-gel-derived ThO 2 /Y 2 O 3 plates gave results which agreed with data from measurements using ZrO 2 /Y 2 O 3 solid electrolytes and with literature data.