Synthesis and Morphology Control of Yttrium Oxide Sub-Micrometric Spheres Composed of Multiple Aggregated Nanoparticles

In the world of advanced materials, yttrium oxide is regarded as a promising material for multiple applications such as lasers, sensors, catalysts, magnets and so on due to its high thermal, mechanical and chemical stability. As with any other ceramic material, its application relies heavily on the morphology and size distribution of its particles. Specifically, yttrium oxide sub-micrometric spheres allow for high packing densities, lowering significantly the temperature required for achieving high density ceramics through conventional sintering techniques. The application of yttrium oxide for scintillators requires transparent, highly dense ceramics, thus the synthesis of yttrium oxide sub-micrometric spheres is of high interest for such applications. A traditional method for the synthesis of these spheres is the urea homogenous precipitation method, which forms yttrium hydroxide carbonate precipitates. By heating the samples to temperatures of several hundred degrees Celsius, the precipitates are transformed into oxides. As such, the present study makes an in-depth investigation of the synthesis of yttrium oxide sub-micrometric spheres by the urea homogenous precipitation method. Through scanning electron microscopy (SEM), observations regarding the precipitation mechanism were made by visualizing the particles’ growth over time during precipitation under different conditions. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) revealed that the precipitated spheres are porous and composed of much smaller, aggregated amorphous nanoparticles. Analyzing the spheres’ size distributions, the possibility of different precipitation and/or aggregation mechanisms taking part in the process was raised. Additionally, investigations of the heat treatment’s temperature demonstrated that this step is crucial for the particles' final shape, altering both their morphology and size as the nanoparticles inside each sphere transform, grow and interact with each other, even in temperatures much below the traditional sintering temperatures. Under higher temperatures, the spheres start to interact with each other, losing their spherical conformation as they irreversibly transform into larger aggregates. However, under lower temperatures, sphericity is preserved, forming spherical particles with various degrees of porosity, which can be sintered into transparent ceramics or used in other applications such as a support for catalysts.