Microstructural and optical properties of the ZnS ceramics sintered by vacuum hot-pressing using hydrothermally synthesized ZnS powders

ZnS nanopowders annealed at low temperatures (≤550 °C) have a pure cubic structure, while a small amount of hexagonal phase formed in specimens annealed at temperatures ≥700 °C. The particle sizes of the ZnS nanopowders increased with the annealing temperature. ZnS ceramics that were sintered using ZnS nanopowders annealed at low temperatures (≤550 °C) exhibited low transmittance, because of their porous microstructure. ZnS ceramics that were synthesized using ZnS powders annealed at high temperatures (≥800 °C) containing large agglomerated particles, also exhibited low transmittance, due to the presence of a liquid phase. A carbonate absorption band was found from the ZnS ceramics with small grains, because carbon ions diffused from the graphite mold into the ZnS ceramics during sintering, probably through the grain boundaries, and formed carbonates. A ZnS ceramic that was sintered at 1020 °C using the nanopowders annealed at 750 °C exhibited dense microstructure, with a large transmittance, 68%, in the wavelength range 6.0–12 μm.