Akhilesh Goyal

This work describes the effective use of rice husk ash (RHA) as an alternative to commercial silica in the epoxy–glass (EP/GF) composite to develop radar absorbing structures (RAS). The EP/GF/RHA hybrids with RHA loading of 0 to 15 wt% were prepared and their thermal, mechanical, electromagnetic (EM), and microwave absorption properties were investigated. Scanning electron microscopy showed almost uniform dispersion of RHA in the matrix. The addition of RHA improved the dimensional stability of the hybrids remarkably in the out‐of‐plane direction. The coefficient of thermal expansion (CTE) of the hybrid was decreased by 34.3% as compared to the composite without RHA. The uniform dispersion and better adhesion of RHA with the matrix led to a remarkable improvement in tensile strength and elastic modulus. With increasing RHA loading, the real ( ε ′) and imaginary ( ε ″) parts of the complex permittivity of the EP/GF/RHA hybrids increased slightly. The hybrid with 15 wt% RHA and 4 mm thickness revealed excellent microwave absorption properties with maximum reflection loss (RL _max ) of −26.38 dB at 9.04 GHz and effective absorption bandwidth (< −10 dB) of 1.33 GHz. These results highlight the potential of EP/GF/RHA hybrids as eco‐friendly and sustainable RAS with better thermal and mechanical properties for defense applications.

Three dimensionally printed poly(ether‐ketone‐ketone) (PEKK)/short carbon fiber (SCF) composites are increasingly used for aerospace applications. Due to slow crystallization rate during the 3D‐printing process, 60/40 PEKK grade polymer requires post heat treatment (i.e., annealing) to enhance its crystallinity and mechanical properties. Therefore, the crystallinity of the as‐printed and annealed (2 h at 200°C) specimens was determined using density measurements, differential scanning calorimetry (DSC), and x‐ray diffraction (XRD) methods. The density method indicated an increase in crystallinity of neat PEKK from 0.73% (±0.81) (for as‐printed) to 18.24% (for annealed). DSC results demonstrated a marked increase in crystallinity from 0.96% (for as‐printed) to 27.01% (for annealed) for neat PEKK. The crystallinity of the composites further increased with SCF content. XRD demonstrated slightly higher crystallinity that is, 32.75%, 38.79%, and 44.03%, for the annealed neat PEKK, 10 and 20 wt% composites, respectively. This resulted in significant increase in tensile strength and elastic modulus of the composites compared to neat PEKK, highlighting the importance of post‐heat treatment on mechanical properties of the 3D‐printed PEKK/SCF composites.