<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>Composite materials used in the aviation industry are known to be more complex than their metallic predecessors. This impacts not only the design and manufacturing of composite structures, but also the failure studies when these structures fail and break (as may be the case in an aircraft accident). Additionally, when under combustion, composite materials introduce potential health hazards. At elevated temperatures, the fibers can be released, presenting an inhalation hazard. Similarly, the matrix decomposition results in a series of potentially toxic byproducts. When encountering composite fires at aircraft accident sites, a series of protocols have been delineated by the corresponding agencies. These include wearing personal protective equipment as well as the application of so-called fixant solutions over the burning composites, with the latter being the focus of this study. The purpose of the fixant solutions is to provide a film of protection that – in essence – holds down small fibers and prevents them from becoming airborne. While the use of fixant solutions is necessary to protect the health of individuals in the vicinity of burnt composites, the potential detrimental impact the application thereof has on fractographic evidence should also be considered. Experts in the field have voiced concerns regarding the use of fixants, outlining that these chemicals may wash evidence away, cover up evidence, or interfere with imaging methods needed during the failure analysis. The purpose of the conducted research, thus, was to compare the relative impact of four commonly used fixant solutions – namely water, wetted water, polyacrylic acid (PAA), as well as a mixture of water and floor wax – on fractographic features of failed carbon fiber/epoxy composite specimens. Specifically, fractographic evidence of two forms of damage – impact and tension – were evaluated. With this goal, the methodology included steps to manufacture the specimens of interest, introduce the two forms of damage, burn the specimens, apply fixants, and perform the microscopic analysis via a scanning electron microscope (SEM). The fractographic evidence prior and after the application of fixant was evaluated qualitatively and quantitatively. The results showed that the evaluated fixants did influence the fracture surfaces imaged, and in certain cased obscured evidence of interest. Additionally, differences between the fixants were ascertained for both forms of damage evaluated. The water treatment was found to perform the best, minimizing the disruption of evidence. Nonetheless, while the study did answer the research questions and the different treatments were compared, additional areas of research and factors that should be considered were identified. </p>

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