Mohammad Bilal Khan

Towards athermal organic-inorganic guided mode resonance filters

We demonstrate guided-mode resonance filters featuring an amorphous TiO(2) layer fabricated by atomic layer deposition on a polymeric substrate. The thermal properties of such filters are studied in detail by taking into account both thermal expansion of the structure and thermo-optic coefficients of the materials. We show both theoretically and experimentally that these two effects partially compensate for each other, leading to nearly athermal devices. The wavelength shift of the resonance reflectance peak (< 1 nm) is a small fraction of the peak width (~11 nm) up to temperatures exceeding the room temperature by tens of degrees centigrade.

An Investigation of the Ablation Behavior of Advanced Ultrahigh-Temperature EPDM/Epoxy Insulation Composites

Abstract The influence of ultrahigh temperature and shear flow on the ablation behavior of bonded composites is studied. The various composites were fabricated by either reactive injection-molding (isotropic composites) or resin-impregnation-stacking (mat reinforcement) techniques. The host polymer matrices (EPDM or epoxy), containing either carbon/refractory fibers or inert/interactive particles, were examined following exposure to an oxidizing flame at 2750 K and a net effective heat flux of ∼25.0 W/cm2, for a specified duration. Both planer and tubular specimens in the head-on impingement (HOI) and parallel flow (PF) configurations were investigated. Scanning electron microscopy (SEM) of the residual surface morphology (RSM) of the various irradiated specimens revealed that the ablation was a function of both the resin and the reinforcement type. The resistance to ablation/erosion improved with both higher-melting substrates and greater matrix-substrate interaction. The global erosion rate was found to...

Impact of atomic layer deposition to nanophotonic structures and devices

We review the significance of optical thin films by Atomic Layer Deposition (ALD) method to fabricate nanophotonic devices and structures. ALD is a versatile technique to deposit functional coatings on reactive surfaces with conformal growth of compound materials, precise thickness control capable of angstrom resolution and coverage of high aspect ratio nanostructures using wide range of materials. ALD has explored great potential in the emerging fields of photonics, plasmonics, nano-biotechnology, and microelectronics. ALD technique uses sequential reactive chemical reactions to saturate a surface with a monolayer by pulsing of a first precursor (metal alkoxides or covalent halides), followed by reaction with second precursor molecules such as water to form the desired compound coatings. The targeted thickness of the desired compound material is controlled by the number of ALD cycles of precursor molecules that ensures the self limiting nature of reactions. The conformal growth and filling of TiO2 and Al2O3 optical material on nanostructures and their resulting optical properties have been described. The low temperature ALD-growth on various replicated sub-wavelength polymeric gratings is discussed.

Ablation, thermal stability/transport and mechanical investigations of modified nanokaolinite impregnated acrylonitrile butadiene rubber composites

Modified nanokaolinite was uniformly dispersed in acrylonitrile butadiene rubber using dispersion kneader and two roller mixing mill. In order to evaluate ablation characteristics of the fabricated composites, high-temperature ablation testing was carried out. The peak incorporation of nanoclay in the rubber matrix has enhanced the ablation resistance up to 97% and reduced the temperature elevation at the back face of the nanocomposite up to 51% during the 200s oxy–acetylene flame exposure on the surface of the ablator. Thermogravimetric and differential thermal analyses elucidate the thermal endurance improvement with increasing filler contents in the polymer matrix. Thermal conductivity and thermal impedance of the composite specimens were evaluated on the domestically manufactured thermal conductivity measuring apparatus. It is scrutinized that the former property reduced up to 49% at and the subsequent characteristic of the rubber composite is elevated up to 43% at 373 K with the 30 wt% incorporation of the nanoclay into the base rubber composition. Mechanical properties of the acrylonitrile butadiene rubber composite specimens were significantly enhanced with increasing filler introduction into the host matrix. Porous char morphology, polymer pyrolysis and composition of the ablated samples were analyzed using scanning electron microscopy and energy dispersive spectroscopy analyses.

Dynamic Mechanical Characterization of the Crosslinked and Chain-extended HTPB Based Polyurethanes

The viscoelastic response of two different polyurethane (PU) systems (crosslinked and chain-extended) based on hydroxyl terminated polybutadiene (HTPB) were analyzed and compared. The PU elastomers were fabricated by reacting HTPB with isophorone diisocyanate. A tri-functional alcohol (1,2,6 hexanetriol) and a bi-functional aliphatic diol (1,4 butanediol) were used as the crosslinking agent and the chain extender respectively. Hydrogen bonding between the hard segments is indentified by the IR spectroscopy. The segregation of these segments was manifested by dynamic mechanical analysis at characteristic frequency over a wide range of temperature. The intensity and temperature location of the relaxation peaks was identified and discussed. The phenomenon of viscous dissipation was evaluated by recording the viscoelastic parameters as a function of temperature across a series of frequencies. The results obtained indicate that the chain-extended polymer is phase segregated and more sensitive to temperature rise via viscous dissipation as compared to crosslinked polymer. Both the polymers showed similar glass transition temperature (Tg) and characteristic relaxation times. Master curves for the polymer systems investigated are produced and discussed.

Field-induced phase fractionation in multiphase polymer flow systems: a review

Abstract The paper reviews the kinetics and morphology of the various phase-segregation processes that may be encountered in multiphase polymer flow systems. A primary aim of this review has been to investigate how certain processing variables influence the polymer structure and the degree of phase segregation in various geometries of interest. A pronounced distinction is made between the terms “diffusion” and “field-induced segregation.” The former is conventionally regarded as the mass transfer resulting from Brownian motion and, in any case, is a randomizing process. The latter, on the other hand, implies “flow” of matter subject to a “driving potential.” As opposed to diffusion in the classical sense, field-driven transport may be affected against the concentration gradient. It is seen that the application of an appropriate lateral field generates transversal migration of suspended material or additive and, in certain cases, the segment domains in the “matrix” polymer. In particular, the potential of ...

Fabrication of vertically aligned CNT composite for membrane applications using chemical vapor deposition through in situ polymerization

We report the fabrication of vertically aligned carbon nanotubes (CNT) composite using thermal chemical vapor deposition (CVD). A forest of vertically aligned CNTs was grown using catalytic CVD. Fluorocarbon polymer, films were deposited in the spaces between vertically aligned MWCNTs using thermal CVD apparatus developed in-house. The excessive polymer top layer was etched by exposing the sample to water plasma. Infrared spectroscopy confirmed the attachment of functional groups to CNTs. Alignment of CNTs, deposition of polymer and postetched specimens were analyzed by field emission scanning electron microscope (FESEM). Uniform distribution of monomodel vertically aligned CNTs embedded in the deposited polymer matrix was observed in the micrograph. Observed uniform distribution otherwise is not possible using conventional techniques such as spin coating.

Effect of concentration of Surfactant on the Exfoliation of Graphite to Graphene in Aqueous Media

Graphite was exfoliated to graphene by tip sonic using sodium cholate as a surfactant in the presence of Millipore water as a medium. The use of water as a solvent for exfoliation purposes is very important due to its environmentally friendly nature and almost no cost, contrary to organic media. Two different concentration ratios of surfactants are used in the present work. As a result, graphene dispersions with two different concentrations of 5 mg/ml and about 7 mg/ml respectively were obtained in aqueous media. It was observed that the optimum concentration of surfactant has an effective role in the exfoliation of graphite to graphene. Concentrations of graphene dispersions were studied through UV spectroscopy, while Raman spectroscopy, Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM) were used to study the quality of the exfoliated graphene flakes.

Effects of selected size of graphene nanosheets on the mechanical properties of polyacrylonitrile polymer

Mechanical properties of polyacrylonitrile (PAN) polymer can be significantly improved by the incorporation of graphene nano-sheets of different sizes. The graphite was exfoliated to graphene using sonic tip in the presence of N-methyl pyrrolidinone (NMP) as a solvent. Exfoliated graphene was separated from unexfoliated graphitic crystallites using selected speed (rpm) of centrifuge for specific time. The exfoliated graphene nano-sheets were classified into two different groups on the basis of flake size, (i.e. 1 μm and 3.5 μm). Then these graphene sheets were incorporated into PAN to study the effects of their flake size on mechanical properties. Different mechanical properties such as Young’s modulus, ultimate tensile strength (UTS) and elongation at break (dL at break) were studied. Young’s modulus and UTS improved more than 45 % and 25 %, for 3.5 μm graphene flake size respectively. While more than 40 % and 21 %, improvement in modulus and UTS for 1 μm graphene sheet were observed respectively.

Adhesion modification by shear-enhanced localized interfacial segregation in model reaction injection moulded (RIM) polyurethanes

—Polyurethane (PU) adhesives are generally multiphase systems and thus naturally consist of materials or phases of intrinsically different chemical and mechanical properties. We describe here a rheological technique (annular RIM system) which has been used to alter the surface chemistry and mechanical properties, and hence the adhesion of the materials, by imposing a shear strain generated during their fabrication. The results of ATR-IR, WAXS, DMTA, HPLC, friction and adhesion measurements are described to assess the degree of phase segregation developed as a function of the shear strain. In the case of simple PU systems, both the degree of phase segregation and the adhesion were found to increase progressively with the introduction of extended shear strains. Additive [multiphase solid and liquid internal mould release (IMR) systems] doped PUs, on the other hand, exhibited a substantial attenuation in the adhesion following nominal shear strain. It is suggested that the observed effect arises from the vis...