Effect of powder treatment on optical quality of transparent ceramics

Yttrium aluminum garnet (Y3Al5O12) has an isotropic crystal structure that makes it transparent to visible and IR electromagnetic radiation even in polycrystalline ceramic form. Only fully dense and defect-free materials can be transparent. Typical defects in ceramics are pores and secondary phases, both acting as scattering centers lowering the transparency. A common method to produce YAG ceramics is the solid-state reaction sintering under high vacuum of Y2O3 and Al2O3 powders mixed according to the YAG stoichiometry. We compared different powder treatments to identify the process that leads to the least quantity of defects in reaction-sintered YAG ceramics. We tested the effect of a series of modifications in the powder treatment process, starting from a preset combination of Y2O3 and Al2O3 powders. For example, ultrasonication was used to evaluate its ability to break the powders aggregates. Spray drying was used to dry the powders after the ball milling step, and compared to other drying processes. Finally, the use of a dispersant to favor an intimate powder mixing was also investigated. The obtained powders were pressed, calcinated and sintered in a high vacuum furnace. The samples were mirror polished on both the faces, characterized by SEM and by measuring the optical transmittance. A novel approach was adopted to measure the volumetric residual porosity with a digital optical microscope. The amount and size of residual pores are measured in an automatic way by assembling together a subsequent set of pictures taken at different depths.