Zaffar M. Khan

Hydrogen silsesquioxane resist stamp for replication of nanophotonic components in polymers

We investigate an affordable, accurate and large-scale production method to fabricate subwavelength grating structures by hot embossing replication in polycarbonate substrates. We use inorganic hydrogen silsesquioxane (HSQ), a high resolution, binary, negative electron beam resist, on silicon substrate to make a stamp for replication. The stamp is fabricated without any etching processes and with simple process steps. The process starts by spin coating an HSQ-resist layer on a silicon substrate. The desired film thickness is achieved by adjusting the spinning speed and time. The resist material is then subjected to e-beam writing and development followed by a heat treatment to enhance the hardness and to obtain hot embossing stamp material properties comparable with solid SiO2. A comparison with and without the silicon etching is also performed. We demonstrate that a high quality stamp for thermal nano-imprint lithography for optical gratings can be fabricated using an inexpensive process without an etching step. The process results in a uniform imprinting density over the entire grating surface and high imprint fidelity. The reflectance spectra of replicated grating structures are also shown to be in agreement with theoretical calculations.

Effect of hygrothermal conditioning on the fracture toughness of carbon/epoxy composites cured in autoclave/Quickstep

The hygrothermal effect on the flexural, interfacial and glass transition behaviors of 977-2A carbon/epoxy composites cured in an autoclave and by using Quickstep were evaluated in the previous paper. This article concerns with the hygrothermal effect on the mode I and mode II delamination resistance of the samples cured in an autoclave and by Quickstep. It was observed that the mean initiation and propagation GC values (i.e. GIC and GIIC) of both autoclave and Quickstep 60 cured specimens were apparently reduced after hygrothermal conditioning; however, the data obtained from both autoclave and Quickstep 60 cured specimens in dry and wet states were not statistically significant. The scanning electron micrograph confirmed the enhancement of matrix ductility; however, no effect of this matrix ductility was observed in the GIC and GIIC data.

Ablation, thermal stability/transport/phase transition study of carbon nanofiber-reinforced elastomeric nanocomposites

Novelty of presented research is focused on ablation and thermomechanical characteristics of various loadings of carbon nanofibers (CNFs) of polyisoprene elastomer (PR). Conventional method (banbury disperser with open mixing mill) is applied to complete fabrication process of CNF-reinforced elastomeric nanocomposites. Scanning electron micrographs confirm the proper distribution of CNFs within the elastomeric host matrix. The successful incorporation of CNFs into PR has reduced the back-face temperature and increased the ablation resistance CNF/PR elastomeric nanocomposite in front of ultrahigh temperature (oxyacetylene flame exposure). Carbon nanofiber network with the elastomeric chain restricts the thermal transport within the CNF/PR nanocomposite specimens. The synergistic effect of CNFs on crystallization, glass transition, melting temperatures (Tm) of PR elastomeric nanocomposites is studied. Mechanical properties of PR are effectively enhanced by the impregnation of CNFs in the elastomeric nanocomposites.

The drilling-induced failure mechanisms in T800/924C toughened carbon-epoxy composite materials

The process of drilling of toughened carbon fiber composite materials presents numerous problems during structural assembly. The extremely abrasive nature of carbon fibers along with the softer resin quickly dull sharp tools, split, tear, pullout, and push-in the fibers on the hole boundaries. The presence and growth of such flaws seriously impairs the structural stability, durability, and reliability particularly under fatigue loading. This research aims to investigate the failure modes, pattern, and sequence of damage mechanisms in toughened carbon-epoxy composite in relation to the drilling dynamics. The surface morphology of the damaged fibers in the sectioned holes was examined by scanning electron microscopy (SEM). The drill forces were determined by drill dynamometer to investigate the drilling thrust and torque. The photoelastic stress analysis was used to determine the strains around hole of the lowest ply. The most critical failure mode was found to be shear crimping of −45° fibers due to microbuckling. This resulted in formation of damage pits that were spaced apart periodically at an angle of 45° on hole boundaries. The through-the-thickness drilling forces caused delamination in the resin rich region of −45°/90° interlayer. The SEM, dynamometer, and photoelastic strain results were correlated to predict the onset of failure modes. The results have been explained in the light of analytical models based on fracture mechanics. Measures have been suggested for minimizing the damage on carbon-epoxy composite hole boundaries.

Cross-Flow-Induced-Vibrations in Heat Exchanger Tube Bundles: A Review

Over the past few decades, the utility industry has suffered enormous financial losses because of vibration related problems in steam generators and heat exchangers. Cross-flow induced vibration due to shell side fluid flow around the tubes bundle of shell and tube heat exchanger results in tube vibration. This is a major concern of designers, process engineers and operators, leading to large amplitude motion or large eccentricities of the tubes in their loose supports, resulting in mechanical damage in the form of tube fretting wear, baffle damage, tube collision damage, tube joint leakage or fatigue and creep etc.

HSQ resist for replication stamp in polymers

We investigated an affordable, accurate and large scale production method to fabricate sub-wavelength grating structures by replication in polycarbonate substrates by hot embossing. We used hydrogen silsesquioxane (HSQ) a high resolution, binary, inorganic, negative electron beam resist, on silicon substrate to make a stamp for replication. We fabricated the stamp on silicon by using HSQ-resist without any etching process with simple process steps. The process starts by depositing an HSQ-resist layer on a silicon substrate and by a measurement of the desired film thickness by adjusting the spinning speed and time. The resist material is then subjected to e-beam writing followed by a heat treatment to enhance the hardness and to reveal properties analogous to solid SiO2 as a hot embossing stamp material. A comparison study is made with and without the etching process with different etching rates. We demonstrate that an effective and inexpensive stamp for thermal nano-imprint lithography (NIL) for optical gratings is provided without an etching process, which gives a uniform imprinting density over the entire grating surface and high imprint fidelity. The reflectance spectra of replicated grating structures are also shown to be in agreement with theoretical calculations.

Thermal behavior of waveguide gratings

We investigate the design of binary grating structures, e.g. resonance waveguide filters (RWFs), with subwavelength feature sizes, taking the temperature dependence of different material parameters into account. Our final goal is to demonstrate devices with athermal operation. We design the binary grating structures to be made in polymer substrates, such as polycarbonate (PC), due to their potential for low cost, mass fabrication. The high thermal expansion coefficient (TEC) of polymers, compared to inorganic optical materials, enhances the thermal sensitivity of the grating structures. The gratings are designed using Fourier Model Method (FMM) by considering both thermal expansion and thermo-optic effects on the resonance wavelength shift. The fabrication of RWF structures is proposed by e-beam lithography, creating a master stamp and copying the structures into a polymer substrate by some replication techniques, followed by an ALD deposition of TiO2. When the resonance waveguide grating RWG is designed for nearly room temperature operation at a peak wavelength of 633 nm with a full width half maximum FWHM of 3 nm (TM mode reflectance), the peak wavelength shifts 0.2 nm /50C when only the TEC is taken into account. However, taking into account also the thermo-optic coefficients TOCs of PC and TiO2, the peak position shifts to 0.4 nm/ 50C on the opposite side of spectral central wavelength. Thus the overall shift reduces to 0.2 nm /5 0C, illustrating partial athermalization. It was also observed that thermo-optic coefficient TOC contributed more significantly than TEC effect. The wavelengths shift was almost linear with respect to temperature for both effects and showed slopes of 0.0673, 0.0422 and 0.02352 for TOC, TEC and combined effects, respectively.

Horizontal Patterns of Single Wall Carbon Nanotubes by Simple Filtration Method

This work demonstrates the horizontal patterning/alignment of single wall nano tubes (SWNTs) using simple filtration method. In the quest of vertical alignment of SWNTs to fabricate the vertically aligned CNT-membrane, this horizontal patterns as observed by simple filtration technique has been achieved. These horizontal patterns are key arrangements sought in the electronic field and fabrication of CNT-nano composites of required characteristics. In this work SWNTs purified/oxidized treatment with nitric acid at 120-122°C. Oxidized SWNTs further reacted with Octadecyl amine at same temperature to make them significantly dispersible/soluble in solvents especially in Tetrahydrofuran (THF). These modified SWNTs dissolved in THF by sonication and uniform black color solution was obtained that did not settle upon prolonged time of standing. This solution is subsequently filtered through 0.2 micro porous PTFE filter using long neck sintered funnel for horizontal alignment. The inter tube gap between CNTs is initially filled by the precursor reactive vapor stream that is consolidated as a solid polymer through insitu polymerization. Transform Infrared (FTIR) analysis was performed to find the attached functional groups. Alignment of functionalized SWNTs collected on PTFE filter was analyzed by Field-emission scanning electron microscopy (FE-SEM). Ordered horizontal patterns were observed. The contemporary horizontal alignment techniques being used are tedious and need very special facilities. The current method is comparatively quite simple and large quantities of CNTs can be aligned to advantage.

Development of Resin Infusion Process for Ultralight Large Composite Structures

Resin Infusion process is an affordable process for developing composite structures but resin impregnate is made difficult by its large size of engineering products. This research demonstrates development of large structures such as body of a high performance automobile in a single step resin infusion process. Three different scaled down models of the car were developed according to user’s technical requirements focusing on minimal weight, air drag and aesthetics. Pro E and ANSYS were used to determine the optimal shape, geometry, size, aesthetics and strength. The digital model of exterior shape of car body was developed through coordinate measuring machine using selected model instead of Pro-E modeling due to time constraints. The digitized data was used for development of Pro-E model. The Pro-E model was scaled up to generate CAD drawings for tool development. Different stations were marked on the model and sliced virtually for development of pattern. After developing pattern, the mold was manufactured from carbon and glass / polyester composites for prototype manufacturing of the car body. The prototype manufacturing involved placement of specific number of carbon layers as perform on female side of the mold. The vacuum sucked the resin through a number of carefully selected entry ports. Multiple resin delivery ports ensured effective resin distribution and impregnation. After curing the cutting, trimming and drilling operations were carried out to finish car body to actual size. Polyacrylic wind shield was thermoformed in convection current oven according to streamlined geometry of car body. The car body was integrated with the compatible floor panels and accessories. The crumble zone shock absorber in the bumper was manufactured using successive layers of Nomex® honeycomb and PVC rigid foam to dampen the accidental shock. The successful test runs were made to qualify the car body according to user’s technical requirements.

Modeling and Analysis of Thermal Damping in Heat Exchanger Tube Bundles

Most structures and equipment used in nuclear power plant and process plant, such as reactor internals, fuel rods, steam generator tubes bundles, and process heat exchanger tube bundles etc., are subjected to Flow Induced Vibrations (FIV). Costly plant shutdowns have been the source of motivation for continuing studies on cross-flow induced vibration in these structures. Damping has been the target of various research attempts related to FIV in tube bundles. A recent research attempt has shown the usefulness of a phenomenon termed as “thermal damping”. The current paper focuses on the modeling and analysis of thermal damping in tube bundles subjected to cross-flow. It is expected that the present attempt will help in establishing improved design guidelines with respect to damping in tube bundles.Copyright