A. A. Khurram

Optimization of the Carbon Coating of Honeycomb Cores for Broadband Microwave Absorption

A simple and fast coating method of honeycomb cores for microwave absorption has been described. The honeycomb cores with two different thicknesses (5 and 20 mm) coated with thermoplastic resin filled with carbon powder as lossy filler in 5, 10, 15, and 20 wt% have been tested for microwave absorption in 2 - 18-GHz frequency range. The 5-mm-thick honeycomb has shown absorption bandwidth of 14 GHz for maximum absorption of -6 dB (75%) with 15 wt%. filler content. However, the percentage of the filler was decreased to 10 wt% in 20-mm-thick honeycomb absorber for maximum absorption over a wide frequency range. The honeycomb sample with 10 wt% filler has bandwidth of 18 GHz for -7 dB (80%) reflection loss. The reflection loss measurements of coated honeycomb cores have also shown that use of E-glass fiber/epoxy composite can enhance the performance of the honeycomb absorber. The combination of a microwave absorbing nanocomposite and the coated honeycombs has been resulted in inferior absorption properties in 2 - 18-GHz frequency range.

Correlation of electrical conductivity, dielectric properties, microwave absorption, and matrix properties of composites filled with graphene nanoplatelets and carbon nanotubes

The DC electrical conductivity, percolation threshold, and dielectric properties of Graphene Nanoplatelets (GNPs) filled epoxy composites are studied and correlated with microwave absorption. The properties of GNPs filled composites are also compared with multiwalled carbon nanotubes (MWCNTs) composites, and GNPs are observed to have superior conductivity than MWCNTs. In all batches, the nanofillers have 0.5, 1.0, 1.5, 2.0, 2.5, 3.0, and 3.5 wt. %. All composites irrespective of the type of nanofiller and viscosity of the matrix have shown electrical percolation threshold at 3.0 wt. %. The dielectric properties, i.e., complex permittivity, tan loss, and AC conductivity, are studied in 100 Hz–5.5 MHz. The DC and AC electrical conductivities (at and below the percolation) measured in 100 Hz–5.5 MHz are correlated to the GNPs and MWCNTs epoxy composites in the microwave frequency range (11–17 GHz). The maximum return loss of −12 dB and −6 dB was determined for MWCNTs and GNPs, respectively. The effects of na...

Activation energy and excess conductivity analysis of (Ag)x/CuTl-1223 nano-superconductor composites

Silver (Ag) nanoparticles were added into (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (CuTl-1223) high Tc superconducting matrix to get (Ag)x/CuTl-1223, x = 0, 0.5, 1.0, 2.0, and 4 wt. %, nano-superconductor composites. The activation energy {U (eV)} and zero resistivity critical temperature {Tc (0)} were increased with increasing contents of Ag nanoparticles in (CuTl-1223) phase up to x = 2.0 wt. %. The increase of activation energy is most probably due to interaction of carriers with the metallic Ag nanoparticles present at grain boundaries of the host CuTl-1223 superconducting matrix. The systematic increase in Tc (0) and gradual decrease in normal state resistivity {ρ300 K (Ω cm)} may be due to improved inter-grains coupling by filling up the voids and pores with the inclusion of metallic Ag nanoparticles at the grain-boundaries. There are two possible mechanisms associated with the inclusion of Ag nanoparticles, one is the formation of non-superconducting regions causing the increase of activation energy and other (d...

Fluctuation induced conductivity in (Cu0.5Tl0.5−xKx)Ba2Ca3Cu4O12−δ superconductor

The fluctuation induced conductivity (FIC) of (Cu0.5Tl0.5−xKx)Ba2Ca3Cu4O12−y (x=0,0.25) superconductor has been studied after potassium substitution at thallium site in the charge reservoir layer (Cu0.5Tl0.5Ba2O4−y). The samples were also post-annealed in oxygen to change the carrier density in CuO2 planes. The zero resistivity critical temperature Tc(0) was found to increase after K+1 substitution at Tl+3 sites in the Cu0.5Tl0.5Ba2O4−y charge reservoir layer. However, Tc(0) has been suppressed after post-annealing the samples in oxygen. The FIC analysis was performed in the frame work of Aslamasov–Larkin theory. The results of FIC analysis have shown three-dimensional (3D), two dimensional (2D) and short wavelength fluctuations (SWFs) in the order parameter. It has been also observed that there is an increase in the 3D-2D and 2D-SWF cross-over temperatures after potassium substitution along with an increase in Tc(0). However, after post-annealing in oxygen, the dimensional cross-over has been shifted to ...

Suppression of activation energy and superconductivity by the addition of Al2O3 nanoparticles in CuTl-1223 matrix

Low anisotropic (Cu0.5Tl0.5)Ba2Ca2Cu3O10−δ (CuTl-1223) high Tc superconducting matrix was synthesized by solid-state reaction and Al2O3 nanoparticles were prepared separately by co-precipitation method. Al2O3 nanoparticles were added with different concentrations during the final sintering cycle of CuTl-1223 superconducting matrix to get the required (Al2O3)y/CuTl-1223, y = 0.0, 0.5, 0.7, 1.0, and 1.5 wt. %, composites. The samples were characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive X-ray, and dc-resistivity (ρ) measurements. The activation energy and superconductivity were suppressed with increasing concentration of Al2O3 nanoparticles in (CuTl-1223) matrix. The XRD analysis showed that the addition of Al2O3 nanoparticles did not affect the crystal structure of the parent CuTl-1223 superconducting phase. The suppression of activation energy and superconducting properties is most probably due to weak flux pinning in the samples. The possible reason of weak flux p...

A study of the nanocomposite sandwich structures for broadband microwave absorption and flexural strength

Composite sandwich structures are devised to work in a wide frequency of the microwave band. The microwave absorbing properties of composite sandwich structures are studied in 2–18 GHz frequency band. The sandwich structures were manufactured from E-glass fiber/epoxy composites filled with carbon nano-materials and para-aramid honeycomb cores. The complex permittivity of E-glass/epoxy nanocomposites and adhesive films are determined in 8–12 GHz frequency range using free-space measurement setup. The complex permittivity data were used to design the sandwich structures by varying composition and thickness of nanocomposite sheets using a simulation tool Computer Simulation Technology Microwave Studio. In the designing process, the thickness of honeycomb sheets was also varied to get best spacer thickness for the cancellation of reflected and transmitted microwaves. The simulated and measured results have shown that the designed structure can be used for −10 dB Reflection coefficient over a wide frequency ranges in the microwave region. The results of flexural strength of the sandwich structure and tensile strength of facing sheets are also presented.

Mechanical and thermal properties of hybrid carbon fibre–phenolic matrix composites containing graphene nanoplatelets and graphite powder

ABSTRACT Carbon fibre–phenolic matrix (CF–P) composites containing graphene nanoplatelets (GNPs) were manufactured for improved mechanical and thermal properties. For comparison, micrometer-size pyrolytic graphite powder (GP) was also incorporated in CF–P composites. The loading of carbon fibres was kept constant at 60 wt-% while the quantity of GNPs was varied from 0.1 wt-% to 0.3 wt-% and GP from 1.0 wt-% to 3.0 wt-%. Only GNPs were functionalised by ultraviolet-ozone treatment to improve their dispersion in the matrix while all the composites were manufactured by hand layup method and characterised by scanning electron microscopy, impact, flexural, thermogravimetry and ablation tests. The composite containing 0.3 wt-% GNPs showed considerable improvement in ablation, flexural and impact testing as compared to CF-P composites containing GP. Finally, the ablation mechanisms of post-ablated composites were discussed in the light of available data in the literature.

Microwave Properties of Nanocomposites: Effect of Manufacturing Methods and Nanofiller Structure

Nanocomposite materials filled with multiwall carbon nanotubes (MWCNTs) having three types of structures, i.e., longer (200lm), shorter (20‐50lm), and aminated (20‐50lm), are manufactured for microwave absorption (MA) in 11‐17GHz frequency range. Microstructure, dielectric permittivity, direct current (DC) electrical conductivity, and MA properties of the MWCNTs‐epoxy nanocomposite were investigated. A correlation has been developed between the structure (aspect ratio and surface functionality) of MWCNTs, electrical conductivity of the composite, and MA (return loss (RL)). E-glass/epoxy composite filled with longer carbon nanotubes (CNTs) has shown higher RL as compared to that of other two nanocomposites. The measurements have shown that the magnitude of RL of microwaves depends strongly on the structure of MWCNTs used in the composite. Furthermore, the effect of synthesis route followed for the manufacturing of nanocomposite on its electrical conductivity and microwave absorbing properties is also investigated; three different approaches were followed to manufacture CNT/epoxy nanocomposites from longer CNTs (200lm). [DOI: 10.1115/1.4029916]

Optimization of the manufacturing parameters of honeycomb composite sandwich structures for aerospace application

Sandwich structures comprising para-aramid paper honeycomb core and carbon fiber epoxy matrix composite facesheets were fabricated for aerospace applications. For the adhesion of honeycomb core and composite facesheets, two different types of adhesive films were used. The curing parameters for adhesive films, including temperature and time, were optimized for maximum bonding strength.

Improving the performance of conventional glass fiber epoxy matrix composites by incorporating nanodiamonds

Abstract Multiscale glass fiber epoxy matrix composites containing nanodiamonds were fabricated using vacuum bagging technique. Three different loadings of nanodiamonds were incorporated in epoxy resin after their functionalization through ozone-treatment, i.e., 0.1, 0.3 and 0.5 wt%. The functionalization of nanodiamonds was confirmed by infrared spectroscopy, which improved the dispersion of nanodiamond in epoxy resin thus improving the mechanical properties. Tensile, compression, flexural and interlaminar shear properties of the composites were improved. The tensile, compression and flexural strengths improved up to 36, 56 and 30% by the addition of 0.5 wt% nanodiamonds while the corresponding moduli increased to 30, 125 and 46%, respectively. An improvement of 38% in interlaminar shear strength was observed. The microscopy of the composites was performed using optical and electron microscopy and proper impregnation of glass fibers and the absence of the agglomerates of nanodiamonds were ensured. The homogeneous dispersion of nanodiamonds and their adhering role at fiber/matrix interface improved the mechanical properties of the composites. The developed composites are ideal candidate materials for engineering applications demanding high specific mechanical properties.