Rajendra Kumar 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.

The growing demand for stealth technologies has accelerated the development of efficient and easily manufacturable microwave‐absorbing materials. In this study, short carbon fiber (SCF) reinforced poly(ether‐ketone‐ketone) (PEKK) composites were fabricated using fused filament fabrication (FFF). Samples with varying thicknesses (1–4 mm) were printed in both horizontal (H) and vertical (V) orientations to evaluate their microwave absorption behavior. Thermal analysis revealed that incorporating SCF slightly increased the melting temperature and crystallinity of PEKK, with crystallinity rising from 27.1% to 32.51% for the 20 wt.% SCF composite. Improved thermal stability was also observed from thermogravimetric analysis. Dielectric properties increased gradually with increasing SCF loading. Importantly, both printing orientation and sample thickness influenced absorption performance. H‐printed composites demonstrated superior microwave absorption compared to V‐printed ones, with the PEKK/10 wt.% SCF sample achieving a maximum reflection loss (RL _max ) of −34.74 dB at 9.29 GHz for a thickness of 2.5 mm. Unlike conventional studies that focus solely on material composition, this work highlights the combined roles of processing parameters and thickness in tuning absorption behavior. The results demonstrate that 3D‐printed PEKK/SCF composites offer competitive performance, with the added advantages of lightweight, efficient, and customizable microwave absorbing materials.

In this work, the thermal conductivity, linear coefficient of thermal expansion (CTE) and dynamic mechanical properties of the 3D printed high‐performance poly (ether‐ketone‐ketone) (PEKK) composites reinforced with 10 and 20 wt% short carbon fiber (SCF) were investigated. The PEKK/SCF composites were printed in two configurations via edge orientation and base orientation. Edge orientation printed composites exhibited better thermal conductivity and storage modulus than those of base orientation. Scanning electron microscope revealed uniform distribution of SCF in the PEKK matrix and confirmed the fibers' orientation in the printing direction. The CTE of the PEKK/20 wt% SCF composite printed on edge orientation with raster angle 0° reduced by 392% compared to neat PEKK. The thermal conductivity was improved by 136% while the same composite printed on base orientation exhibited ~66% improvement compared to neat PEKK. The storage modulus of the PEKK/20 wt% SCF composite printed on edge orientation exhibited an increase of 133% compared to neat PEKK.

Rice husk is a major agricultural waste generated every year globally. Its open‐air burning produces rice husk ash (RHA), which has a severe impact on soil, air, and human health. However, the use of its fine particles in the polymer matrices can make value addition. In the present research, the RHA was coated with 0.2, 0.5, and 1.0 wt.% graphene nanoplatelets (GNP). These GNP‐coated RHA (G@R) were reinforced into the epoxy matrix. The electrical, electromagnetic (EM), and microwave absorption properties of G@R‐filled epoxy (EP/G@R) nanohybrids were then investigated. The direct current (DC) electrical conductivity and dielectric constant of neat epoxy increased with increasing G@R loading, and the highest values obtained are 9.5 × 10 ⁻⁴ and 5.14, respectively, at 30 wt.% G@R. The real ( ε ′) and imaginary parts ( ε ″) of the complex permittivity of G@R‐filled EP nanocomposites increased marginally with increasing G@R content. The nanocomposite with 30 wt.% 1G@R and lower thickness (2.5 mm) exhibited excellent microwave absorption in the X‐band with wide absorption bandwidth. The maximum reflection loss (RL _max ) is −19.53 dB at 11.87 GHz, and the absorption bandwidth (< −10 dB) is 3.08 GHz (9.32–12.4 GHz).

In the present work, the coefficient of thermal expansion (CTE) of the rice husk ash (RHA) particles embedded epoxy‐glass fiber (EP/GF) hybrids was investigated. The EP/GF/RHA hybrids containing 0–10 wt.% RHA were fabricated and characterized. The density of the hybrids was found in the range of 1.61–1.68 g/cm³. The linear (i.e., in‐plane) CTEs of the EP/GF composites were significantly reduced to about 16.5–19.4 × 10⁻⁶/°C and 17.4–20.6 × 10⁻⁶/°C along the parallel and transverse directions of the fibers, respectively. Additionally, through thickness (i.e., out‐of‐plane) CTEs of the EP/GF/RHA hybrids were also reduced significantly after the addition of RHA particles. The CTE of the EP/GF composite decreased from 66.4 × 10⁻⁶/°C (without RHA) to 42.2 × 10⁻⁶/°C when it was reinforced with 10 wt.% RHA. The hybrids exhibited in‐plane CTEs ~16.5–19.4 × 10⁻⁶/°C, which are close to that of metals due to the good adhesion of the epoxy with the GF and RHA particles. These values are very close to the CTE values of solder and copper. This suggests that the developed hybrids could reduce the problem of warping and thermal stresses. In brief, the use of these hybrids will definitely increase the life of solder joints, electronic packaging substrates, and printed circuit boards.Highlights: Rice husk ash was used as a filler in a glass fiber–epoxy matrix to reduce CTE. SEM revealed good adhesion between epoxy and reinforcements. The in‐plane CTE of 16.5–19.4 × 10⁻⁶/°C indicates excellent dimensional stability. Rice husk ash decreased the out‐of‐plane CTE of the hybrids significantly. Anisotropic CTEs make these hybrids useful for futuristic printed circuit boards.

Light weight carbon‐coated nickel (CCNi)/poly(ether‐ketone) (PEK) nanocomposites reinforced with 0–3.87 vol% (or 5 wt%) multiwalled carbon nanotubes (MWCNTs) have been fabricated using high energy ball mill followed by hot pressing and evaluated for electromagnetic interference (EMI) shielding effectiveness (SE) in the X‐band (8.2–12.4 GHz). The MWCNTs/CCNi/PEK nanohybrids show electrical conductivity and EMI‐SE one order of magnitude and 85% higher than those of nanocomposites without MWCNTs, respectively. Experimentally, it is found that absorption is the major shielding mechanism while reflection is secondary. Such an excellent EMI‐SE (i.e., 29.5 dB) value obtained at a very low density (~ 1.45 g/cm³) makes the nanohybrids potential candidates for defense, space, and aerospace applications.Highlights: Lightweight multifunctional advanced MWCNT/Ni@C/PEK nanohybrids were proposed. Nanohybrids containing MWCNT exhibited EMI‐SE 85% higher than those without MWCNT. Combined addition of Ni@C and MWCNT into PEK yielded synergetic improvement in electrical conductivity. Nanohybrids exhibited an EMI‐SE of about 29.5 dB, which is better than those reported in the literature.

Dynamic mechanical properties of the poly (ether-ketone) (PEK) based nanocomposites reinforced with 0–30 wt% (0–20.2 vol%) SiO ₂ nanoparticles (20–40 nm) were investigated at 1 Hz over the temperature range of 30°C–250°C for the application in printed circuit boards/electronic substrates. It was found that uniform dispersion of SiO ₂ nanoparticles in the PEK matrix resulted in significant increase in storage modulus and microhardness. The highest increase in storage modulus measured at 50°C (glassy region) and 250°C (rubbery region) was more than 100% and 250% than that of neat PEK, respectively. The nanocomposites exhibited lower peak values of the tan δ than that of neat PEK. Scanning electron microscopy revealed good interfacial adhesion between the PEK matrix and the SiO ₂ nanoparticles which was validated by Kubat parameter. The glass transition temperatures determined from the loss modulus and tan δ are slightly increased for the nanocomposites. However, differential scanning calorimetry could not detect glass transition temperature.