Influence of Spark Plasma Sintering Conditions on the Optical and Mechanical Properties of Spinel.

The influence of spark plasma sintering (SPS) conditions on the optical and mechanical properties of MgAl2O4 spinel was investigated for application to infrared windows as parts of military systems. The thermal conditions of SPS, including the temperature and heating rate, have a significant impact on optical and mechanical properties. This study shows that the formation and growth of abnormal grains cause mechanical degradation with an increasing SPS temperature. In-line transmittance (Tin) was affected by the heating rate due to changes in oxygen vacancy and carbon contamination in SPSed samples. The fabricated spinel exhibited excellent flexural strength of 401 MPa and an average mid-infrared transmittance of 84.8% in the range of 3-5 µm.

Radio-frequency-transmitting hexagonal boron nitride-based anti- and de-icing heating system.

Anti- and de-icing heating systems are used to both prevent the accumulation of ice and to remove it and thus avoid damage. Typically, anti- and de-icing heating systems employ carbon-based materials, metal frames, and bulky ceramic structures. These structures generally lead to the loss of radio-frequency (RF) signals and are also relatively heavy. Therefore, RF equipment such as radar domes (radomes) and antennas require anti- and de-icing systems with high RF transmittance for normal operation. In this work, we fabricated a fluorine-doped tin oxide (FTO) wave pattern covered with hexagonal boron nitride (h-BN) layers (i.e., an h-BN/FTO wave pattern) on a glass substrate for use as an RF-transmitting heating system for anti- and de-icing. The FTO wave pattern and h-BN layer act as the heating element and heat spreader, respectively. The h-BN layer showed a transmittance of approximately 90% for RF waves on glass (X band: 8.2-12.4 GHz) (the 10% loss was attributable to the glass substrate). The differences in the temperatures of the FTO-patterned and non-patterned areas for the h-BN(3.6 nm)/FTO and FTO wave pattern were 19.3 and 25.5 °C, respectively. This means that the h-BN layer improved the heat-spreading performance by 6.2 °C. Furthermore, a de-icing test was performed using the h-BN(3.6 nm)/FTO wave pattern by applying a voltage of 40 V at -20 °C. The ice on the non-patterned area melted within 1 min while that on the FTO-patterned area melted within 30 s. These results suggest that the fabricated h-BN(3.6 nm)/FTO wave pattern for RF-transmitting heating systems is suitable for use with the radomes of drones, unmanned aerial vehicles, aircraft, and spaceships in extremely cold environments.