Synthesis and Structural Investigations of Infrared Transmitting Materials in Rare Earth Chalcogenide Systems.
gamma rm La _2S_3 (lanthanum sulfide) shows potential as a candidate window material for far IR transmission in the 8-14 μm spectral region. Novel low temperature routes using metalorganic solutions have been developed for the synthesis of gamma and beta rm La_2S_3. Two different amorphous precursors (20 nm size and SSA of 74 m^2 /gm) were synthesized at room temperature which undergo transformation in a sulfur atmosphere to generate sulfide particles (1-3 μm size). A sulfur content of 8.50 wt% in the precursor is critical for the transformation to the cubic (gamma ) form of rm La_2S_3 . The transformation controlled by the extent of hydrolysis of the alkoxide as determined by thermal stability and chemical analysis of the precursor. Detailed microstructural characterization of the precursors and phase evolution mechanisms were studied. Hot pressed disks (9mm thick) of the cubic sulfide ceramic show a far IR cut-off at 13 mum. Structural analysis by convergent beam electron diffraction (CBED) confirmed the BCC cell structure and the space group of I43d for the powders and ceramic. Amorphous IR materials in the rm La_2S_3-GeS_2 system were also studied with an emphasis on bulk glass formation and structural characteristics. Glasses containing 92.5 mol% GeS_2 show evidence of primary (60-800 A) and secondary (30-130 A) phase separation at the molecular level. The effect of composition on the microstructure and thermal stability of these glasses were also investigated. The Molecular Dynamics simulation technique was applied to study the structure of IR transmitting materials in liquid and glassy forms. An empirical model was developed which successfully describes the liquid and glass structures of a representative IR material (ZnCl_2 ) and studies were performed to assess the effect of the mass of the borders (W) of the simulation cell on the glass transition temperature. An extension of the model to simulate crystalline rm La_2S _3 was also attempted.
- Pub Date:
- January 1990
- Engineering: Materials Science; Engineering: Electronics and Electrical; Physics: Electricity and Magnetism