Trilok Chand Shami

Investigation and performance evaluation of carbon black- and carbon fibers-based wideband dielectric absorbers for X-band stealth applications

This paper presents the design, preparation, and electromagnetic testing of wideband dielectric absorbers based on carbon fibers and carbon black powders for stealth applications. The specially prepared absorbers are characterized using the partially filled waveguide technique for the complex permittivity, and the return loss of the sample is measured using the free-space system in the X-band frequency region. A maximum return loss of 29u2009dB i.e. absorption of 99% at 10.3u2009GHz is achieved with minimum 97% absorption throughout the X-band frequency region. The fabricated absorber having density of 0.28u2009gm/cc and thickness of 2u2009mm makes it a potential candidate for stealth applications especially for the defense targets. A multilevel fast multipole method-based electromagnetic simulation is carried out on the artillery shell model, and on the leading edge model of the aircraft using the measured electromagnetic properties of designed absorber for the radar cross-section (RCS) reduction. The maximum RCS reduction of 27 and 20u2009dB has been observed for the artillery shell model, and for the leading edge of stealthy aircraft model, respectively, as compared to that of the PEC structure at 10.3u2009GHz frequency.

Design, synthesis, characterization and performance evaluation of multi-band perfect metamaterial absorber

This paper presents the design, synthesis, characterization, and performance evaluation of a metamaterial absorber having four distinct absorption peaks. The proposed absorber is based on the periodic array of the hexagonal closed rings and octa-star structure printed over the dielectric substrate. The dimensions of unit cell are optimized such that the absorption takes place at the distinct frequency near the Federal Communications Commission defined radar spectrum e.g. at 4.10, 6.15, 10.05, and 15.52 GHz with the absorptivity of 0.98, 0.99, 0.99, and 0.99, respectively. The proposed structure is fabricated and the experimental result shows high absorptivity under transverse electric and magnetic polarization for the wide angle of incidence angles, which is in concurrence with the simulated results. The equivalent circuit model of the absorber has been developed sequentially for each of the structure. Furthermore, the complex refractive index of the metamaterial structure has been retrieved in order to have a detailed analysis, which supports the absorption phenomena at all the corresponding frequencies. The proposed metamaterial structure appears to be a potential candidate for absorber applications in the radar cross-section reduction, thermal detectors, and thermal imaging.

Evaluation of thermal, morphological and flame-retardant properties of thermoplastic polyurethane/polyphosphazene blends

A polydichlorophosphazene was prepared by thermal ring-opening polymerization of hexachlorophosphonitrile (trimer). The functionalization of polydichlorophosphazene was done via substituting the chlorine groups with sodium salt of trifluroethanol and furfuryl alcohol and then structural properties of functionalized polyphosphazene (PPZ) was confirmed by GPC, FT-IR and NMR spectroscopic analysis. The PPZ was further blended with thermoplastic polyurethane (TPU) using solution casting method in anhydrous tetrahydrofuran solvent. The miscibility behavior of TPU/PPZ blends was observed via FTIR spectroscopy which unfolded the reason of compatibility of the blends due to intermolecular Hydrogen bonding interaction between the TPU and PPZ. Further, the thermal properties of the casted films were analyzed using thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) which revealed the increase in onset of degradation temperature of TPU with the increase of PPZ content. Also the char yields of TPU blends at 600xa0°C were increased upon increasing the amount of polyphosphazene from 5 to 20xa0wt%. The appearance of crystallinity in the TPU/PPZ blend due to heteronucleation effect by PPZ was analyzed by X-ray diffractometer. Besides this, the morphological changes of the TPU/PPZ blended films were investigated by scanning electron microscopy which exposed the homogeneity in the blend (d). The flame-retardant properties were evaluated using limiting oxygen index (LOI) analyzer and UL-94 (vertical burning test) showing the rating of V0 with achieved LOI value of 31.4% for the blend (d). Moreover, the films were characterized in terms of mechanical properties (as per relevant ASTM standards) using UTM which demonstrated that the addition of 20 wt% PPZ in the blend enhanced the tensile strength by about 110% and decreased the elongation at break by nearly 13%.

Polyvinylidenefluoride/Polymethylmethacrylate/Polyphosphazene/Lithium Tantalate Composites: Synthesis and Characterization

This study presents the preparation of poly vinylidene fluoride (PVDF) based polymethyl methacrylate (PMMA), Poly(bis(4-Aminophenoxy)phosphazene)(PPZ) and lithium tantalate (LT) composites films using solution blending process. The fabricated PVDF/PMMA/PPZ/LT composite films were evaluated using Fourier transformed infrared spectroscopy, Scanning electron microscopy, Differential scanning calorimetry and Thermo gravimetric analyses. The changes in microstructure, dielectric, morphological and thermal properties of these films with change in composition of PVDF/PMMA/PPZ/LT have been investigated. The high PMMA content and incorporation of LT favored the PVDF phase transition from ‘α’ to ‘β’ phase as shown by FT-IR analysis. The LT particles were properly dispersed in PVDF/PMMA/PPZ matrix as confirmed by scanning electron micrograph images. The dielectric properties of the PVDF/PMMA/PPZ improved with increasing the concentration of LT. The dielectric constant of the films increased with increase in LT content in the blends. The values of dielectric properties observed were higher at lower frequency at room temperature. The composites having 10, 20, 30 & 40% LT in blend samples showed regular increase in Tons, Tmax and corresponding char yield with increase in the filler content. All the composites demonstrated two steps decomposition due to the interaction of filler with polymers at high temperature leading to oxidative decomposition of polymeric chains.