<p>    Ceramics with novel optical properties have enabled substantial advances in technologies ranging from medical imaging to fish finding. Further development of optically transparent ceramics will allow the creation of novel devices with new capabilities, capable of functioning in previously inconceivable operating conditions. Hypersonic aerospace applications often utilize IR imaging for guiding and target identification. Sensors utilized in the detection and measurement of IR radiation cannot withstand the extreme environments intrinsic to hypersonic travel and thus must be protected from the surrounding environment while minimizing distortion of incident IR radiation. Towards this end, IR transparent ceramics have been developed that can withstand the extreme environments of hypersonic travel, while maintaining their optical and mechanical properties. </p> <p>The binary II-VI semiconductor Zinc Sulfide (ZnS) has been primarily utilized for this application due to its strong transmission of 8-10 μ𝑚 IR radiation in combination with the stability of its mechanical properties at elevated temperatures encountered at high airspeeds. While it has proven to be a capable material for the application, previous testing has found it to degrade and fail catastrophically when exposed to sand or water at subsonic speeds. This initiated a search for materials with similar IR transmittance properties to ZnS but with higher strength and resistance to degradation. </p> <p>The diamond allotrope of carbon has been found to have the most optimal mechanical properties for this application, but due to obvious limitations from cost and processing in bulk, it is not considered a realistic option for the application. The ternary sulfide Calcium Lanthanum Sulfide (CLS, CaLa2S4) was discovered in the early 1980s, with an extended IR transmission window of 8-12 μ𝑚 in contrast to the 8-10 μ𝑚 transmission window of ZnS. In combination with more favorable mechanical properties than ZnS, CaLa2S4 has become a promising candidate towards the manufacture of stronger IR windows for aerospace applications. To expand the existing body of knowledge on this ternary sulfide and towards the advancement of IR window materials, this work seeks to utilize density functional theory to characterize defects in CLS to guide future investigations of this material system.</p>

<p dir="ltr">Hypersonic flight is the key to unlocking a nation’s strategic advantage in this century’s military theater. Military powerhouses such as the United States, Russia, India, China, Australia, and the EU publicly possess hypersonic weapons capabilities. Such technology enables intercontinental travel orders of magnitude faster than conventional flights. A trip halfway across the world would take not twenty hours, but two. However, the level of thermal and chemical load the aircraft and these electronic equipment experience while at such high speeds cause them to fail. Thus, ceramic window materials are used to act as a barrier between the hypersonic flight environment and this sensitive electronic equipment. Such materials need to be both mechanically robust, but transparent within the relevant infrared ranges used for target detection. Single-crystal sapphire (alumina) is an infrared window material readily available, plentiful, and easy to microstructurally control and manufacture, but not optimal. Its transparency range is limited to the optical and near-infrared, while it exhibits poor mechanical and dielectric strength. Polycrystalline alumina (PCA) has recently been shown to possess more favorable infrared window characteristics as opposed to its single-crystal counterpart. This is achieved by processing using a platelet powder morphology in a single processing step – hot-pressing. Full densification (> 99.5%) of PCA samples was achieved, demonstrating maximum of 84% optical transparency, but accompanied by grain growth (60+µm), resulting in lower mechanical strength. This research thus works on a two-fold approach to minimizing the grain growth of PCA. Optical tests demonstrated favorable results for lowering isothermal temperatures to reduce grain growth. Weibull values of m = 28.8 and m = 9.7 from 4 point-flexure tests were obtained (ASTM 1161a). Thermal loading via ablation testing compared PCA samples to industry alternatives (single-crystal sapphire) and (equiaxed alumina). Ablation tests revealed the benefit of polycrystalline alumina over sapphire. The benefit of lower isothermal sintering temperatures for reduced grain growth resulted in higher peak load before failure, resulting in greater characteristic strength and minimal transmission lost during a minute of oxyacetylene heat flux exposure. Finally, additional work was done on nanoceramic MgO-Y₂O₃, in a ceramic-processing method like that of PCA. These findings will also be discussed.</p>