Barium lanthanum sulfide ceramics as new far-infrared transparent materials

Barium lanthanum ternary sulfide (compositions within the BaLa2S4- γ-La2S3 solid solution of cubic Th3P4-type structure, referred to as BLS) was investigated as a novel infrared transparent ceramic for applications as external structural window material. This research marks the initial efforts to consolidate BLS powders into dense ceramics and reports, for the first time, the processing of polycrystalline BLS infrared transparent ceramics. The powders were produced through a combustion synthesis method followed by a sulfurization treatment in pure H2S. Sintering was carried out using hot-pressing and natural sintering. The characteristics of the powders and ceramics (purity, morphology, microstructure and optical transmission) are detailed and compared to evaluate the effectiveness of the two sintering techniques. A peak transmission of 20% at 16.5 μm was achieved for 1.4 mm-thick ceramics that were hot-pressed for 2 h at 1250 °C and subsequently annealed for 12 h at 1250 °C in pure H2S. With an extended transparency window (up to 20 μm, compared with 12 μm for ZnS), BLS polycrystalline ceramics are uniquely suited for use in harsh environments for space applications in both civil and military fields.