A Study on the Consolidation of Sulfide-Based Infrared Optical Ceramics

Abstract

Due to the favorable infrared transparency, zinc sulfide (ZnS) and calcium lanthanum sulfide (CaLa2S4) are both attractive sulfide-based candidates for infrared optical ceramics. In the current project, detailed studies were performed on the phase development, microstructural evolution, sintering behavior and grain growth kinetics of these sulfide-based ceramics under different pressure-assisted consolidation conditions, in order to process ZnS, CaLa2S4 and ZnS-CaLa2S4 infrared optical ceramics with improved optical and mechanical properties.

The phase transition behavior between sphalerite and wurtzite of ZnS infrared optical ceramics was investigated, by hot pressing and pressureless sintering of two different ZnS powders. This work revealed that, during sintering of ZnS, the phase transition behavior varied based on the starting powder particle size and magnitude of the applied pressure. It was demonstrated that smaller particle sizes led to the more phase transition from sphalerite to wurtzite at 1000C. Additionally, an applied uniaxial pressure during hot press sintering could promote a reverse phase transformation from wurtzite to sphalerite along with enhanced twining and densification, resulting in improved optical and mechanical performances of the ZnS ceramics.

Another study developed hot-pressed Cr2+ doped ZnS ceramics for potential mid-infrared laser applications. Successful consolidation of Cr2+ doped ZnS infrared transparent ceramics (maximum infrared transmittance of 67% at 11.6μm) were achieved by vacuum hot pressing. Infrared absorption and photoluminescence measurements revealed that the Cr2+ ions were tetrahedrally coordinated within the ZnS host lattice.

Different synthetic routes with and without high-temperature sulfurization were investigated and compared for optimization of the synthesis of CaLa2S4. It was found that CaLa2S4 with the cubic phase could be successfully synthesized, by using a wet chemistry method (single-source precursor method) followed by thermal decomposition, which demonstrated the possibility to synthesize CaLa2S4 without any hazardous sulfurization in CS2 or H2S.

CaLa2S4 infrared optical ceramics were sintered via different pressure-assisted sintering techniques (hot pressing and field-assisted sintering) at different temperatures. Through densification curves and microstructural characterizations, densification behavior and grain growth kinetics of the CaLa2S4 ceramics were studied. Based on the established models, densiffication was determined to be controlled by various mechanisms depending upon temperature range. The infrared transmittance of the CaLa2S4 ceramics was observed to reach a maximum of 48.1% at 9.2μm. Furthermore, it was revealed that the hardness was closely correlated with densification and grain growth.

In addition, ZnS and CaLa2S4 composite ceramics were consolidated via pressure-assisted sintering of 0.5ZnS-0.5CaLa2S4 (volume ratio) composite powders. The consolidated composite ceramics were identified to be composed of sphalerite ZnS, wurtzite ZnS and thorium phosphate-structured CaLa2S4. The sphalerite-wurtzite phase transition of ZnS was further demonstrated to be accompanied by formation of stacking faults and twins in the ceramics. Grain boundary diffusion was determined to be the main mechanism controlling grain growth for both phases within the composite ceramics. It was also found that the addition of the CaLa2S4 phase improved the indentation hardness of the ceramics relative to pure ZnS by homogeneous dispersion of ZnS and CaLa2S4 small grains. Further studies on decreasing grain size and impurity removal are critical to improving the infrared transmittance of the ZnS-CaLa2S4 ceramics.