Mattia Biesuz

Theoretical and phenomenological analogies between flash sintering and dielectric breakdown in α-alumina

α-alumina pre-sintered samples were subjected to flash sintering at 1200 °C under differing electric field strengths. The analysis of the relation between the incubation time and pre-sintering temperature clearly shows that the presence of pores and surfaces within the sample plays a central role in field-assisted sintering behavior of the material. The observed behavior is accounted for by the strong non-linear electrical conductivity shown by porous alumina at high field strength. The observed non-ohmic conductivity can also be related to the “pre-breakdown” behavior previously described by Frenkel. Literature results suggest that the field involved in flash sintering of alumina is comparable with the dielectric strength at high temperature. We can also state that dielectric breakdown and flash sintering in alumina are associated with similar physical phenomena.

Effect of the Precipitating Agent on the Synthesis and Sintering Behavior of 20 mol% Sm-Doped Ceria

Nanocrystalline 20 mol% samaria-doped ceria powders (Ce0.8Sm0.2O1.9) were synthesized by coprecipitation techniques using various precipitating agents in aqueous solution: ammonia, ammonium carbonate, tetramethylammonium hydroxide, and urea. The synthesized powders after calcination at 600°C possess a fluorite structure with nanometric size although they are characterized by a very different morphology and degree of agglomeration. Remarkable differences appear in the sintering behavior, especially because of the presence of hard agglomerates. The precipitating agent has therefore a crucial role in the coprecipitation process, which influences the morphology of the powders and in turn the sintering behavior. The obtained results clearly reveal that ammonium carbonate and urea are the best precipitating agents to obtain final dense products after sintering.

Investigation of Electrochemical, Optical and Thermal Effects during Flash Sintering of 8YSZ

This paper reports the electrochemical, optical and thermal effects occurring during flash sintering of 8 mol % yttria-stabilized zirconia (8YSZ). In-situ observations of polycrystalline and single crystal specimens revealed electrochemical blackening/darkening during an incubation period prior to flash sintering. The phenomenon is induced by cathodic partial reduction under DC fields. When using a low frequency AC field (0.1–10 Hz) the blackening is reversible, following the imposed polarity switching. Thermal imaging combined with sample colour changes and electrical conductivity mapping give a complete picture of the multi-physical phenomena occurring during each stage of the flash sintering event. The partial reduction at the cathode causes a modification of the electrical properties in the sample and the blackened regions, which are close to the cathode, are more conductive than the remainder of the sample. The asymmetrical nature of the electrochemical reactions follows the field polarity and causes an asymmetry in the temperature between the anode and cathode, with the positive electrode tending to overheat. It is also observed that the phenomena are influenced by the quality of the electrical contacts and by the atmosphere used.