Synthesis and thermal stability of ZrO2@SiO2 core-shell submicron particles

ZrO2@SiO2 core-shell submicron particles are promising candidates for the development of advanced optical materials. Here, submicron zirconia particles were synthesized using a modified sol-gel method and pre-calcined at 400 degrees C. Silica shells were grown on these particles (average size: similar to 270 nm) with well-defined thicknesses (26 to 61 nm) using a seeded-growth Stober approach. To study the thermal stability of bare ZrO2 cores and ZrO2@SiO2 core-shell particles they were calcined at 450 to 1200 degrees C. After heat treatments, the particles were characterized by SEM, TEM, STEM, cross-sectional EDX mapping, and XRD. The non-encapsulated, bare ZrO2 particles predominantly transitioned to the tetragonal phase after pre-calcination at 400 degrees C. Increasing the temperature to 600 degrees C transformed them to monoclinic. Finally, grain coarsening destroyed the spheroidal particle shape after heating to 800 degrees C. In striking contrast, SiO2-encapsulation significantly inhibited grain growth and the t -> m transition progressed considerably only after heating to 1000 degrees C, whereupon the particle shape, with a smooth silica shell, remained stable. Particle disintegration was observed after heating to 1200 degrees C. Thus, ZrO2@SiO2 core-shell particles are suited for high-temperature applications up to similar to 1000 degrees C. Different mechanisms are considered to explain the markedly enhanced stability of ZrO2@SiO2 core-shell particles.