engleski

Crystallization of porous yttria-stabilized zirconia for energy applications

Stabilized zirconia is diversely applied due to its excellent thermal, hydrothermal, chemical and mechanical stability. It is used as a catalyst or as catalyst support, oxygen sensor, solid electrolyte and as high-temperature thermal isolation. For all these applications, controlled porosity of the ceramic is necessary. We have investigated two methods of preparation, mechanochemical milling and sol-gel process, of yttria-stabilized zirconia. Some samples were modified with addition of alumina, known to improve mechanical and ion-conductive properties of zirconia. The influence of addition of alumina and milling of sol-gel powders on crystallization and morphology was examined. Powders were calcined at 400 – 1200 °C and compressed tablets at 1400 °C. Crystallization was monitored by X-ray diffraction analysis (XRD), Raman spectroscopy (RS) and simultaneous differential scanning calorimetry and thermogravimetric analysis (DSC-TGA). The morphology of the tablets was observed by scanning electron microscope (SEM) and their density was determined by Archimedes method. The presence of alumina was found to delay crystallization of zirconia, the effect being more pronounced at higher alumina content. Otherwise, the crystallization of two oxides is independent and they form separate phases easily distinguished by SEM. In sol-gel prepared powders tetragonal zirconia evolves at 600 °C (800 °C with addition of alumina), the crystallite size growing with increased calcining temperature. Alumina first crystallizes as θ Al2O3 at 1200 – 1300 °C and quickly transforms to corundum as the final form. The crystallization pathways were corroborated by DSC-TGA. Milling lowers the crystallization temperatures of the sol-gel powders (both zirconia and alumina) since nuclei are formed and smaller particle size allows easier removal of organic chelating agent used to stabilize the sol. The milling also results in crystallization of monoclinic zirconia, probably due to impurities that entered the powder during the milling and whose presence was confirmed by RS. In case of mechanochemical preparation monoclinic zirconia is also eventually formed. There is no significant difference in pore size and relative density of tablets pressed from powders obtained by the mechanochemical preparation and sol-gel process. Both mechanochemical preparation and milling of sol-gel powders give tablets with homogeneous morphology, while the advantage of sol-gel process is preparation of pure tetragonal phase.

sol-gel; mechanochemical preparation; yttria stabilized zirconia; alumina; porosity; crystallization

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