Optical transparency in polycrystalline ceramic oxides can be achieved if the material is fully densified. Spark plasma sintering (SPS) of oxide nanoparticles leads to immediate densification with final-stage sintering. Further densification by annihilation of the isolated pores is associated with diffusional processes, regardless of the densification mechanism during the intermediate stage. Densification equations in conjunction with the concept of grain boundary free volume were used to derive the pore size–grain size–temperature map for designing the nanopowder and SPS process parameters to obtain transparent oxides.
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