Reduction Of Film Thickness Research Articles

D-A-π-A Motif Quinoxaline-Based Sensitizers with High Molar Extinction Coefficient for Quasi-Solid-State Dye-Sensitized Solar Cells.

To meet the requirement of high molar extinction coefficient, broad absorption spectrum, and photo/thermal stability for sensitizers of quasi-solid-state dye-sensitized solar cells (Qs-DSSCs) with reduced film thickness, a novel D-A-π-A configuration organic sensitizer IQ22 was specifically designed, in which the conjugation bridge of cyclopentadithiophene (CPDT) unit was incorporated to widen the light response and enhance molar coefficients for increasing the short-circuit current density (JSC), and the octane chain on CPDT was targeted for suppressing the charge recombination and improving the open-circuit voltage (VOC). As a result, the Qs-DSSC based on IQ22 exhibits very promising conversion efficiency as high as 8.76%, with a JSC of 18.19 mA cm-2, a VOC of 715 mV, and a fill factor (FF) of 0.67 under AM 1.5 illumination (100 mW cm-2), standing out in the Qs-DSSCs utilizing metal-free organic sensitizers.

Thickness-dependent glass transition temperature and charge mobility in cross-linked polyfluorene thin films.

We report thickness-dependent glass transition temperature (T_{g}) and charge mobility in cross-linked thin films made of conjugated polymer poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB). Monotonic T_{g} depressions with reducing film thickness in thermally and UV cross-linked TFB thin films supported on Si-SiOx substrates are observed through ellipsometry measurements, suggesting that a surface mobile layer with enhanced chain dynamics still exists in cross-linked TFB thin films, even with a high cross-linking percentage. Data fitting using a three-layer model shows that the T_{g} in the interface, bulk and surface layer both increases with increasing cross-linking, while the thickness of the interface and surface layer increases and reduces, respectively. Cross-linking of TFB thin film generates traps that hinder charge transport and consequently reduce charge mobility. The charge mobility converges in thick (>140 nm) and thin (<40 nm) TFB films but shows strong thickness dependence in between, reducing from 4.0×10^{-4}cm^{2}/Vs in a 180-nm film to 0.1×10^{-4}cm^{2}/Vs in a 20-nm thin film.

Contact Analysis and Simulation of Rolled Plastic Film Used for Roof Ventilation in Japanese Greenhouses

<abstract> <b><sc>Abstract.</sc></b> Greenhouses are capable of producing a variety of high-value crops year-round. A novel Japanese greenhouse design is gaining popularity because of its automated roll-up ventilation system that is integrated into the roof. However, due to the frequent movement of the roll-up system, the plastic film deteriorates rapidly and typically lasts for only three or four months. In order to better understand the film deterioration, we studied the mechanics involved at the point of contact between the film and the metal greenhouse frame. We found that film deformation and failure were closely related to stretching and creasing, and these processes were observed at the microscopic level. An experimental device was developed to further study the damage to greenhouse film due to the roll-up movement. We concluded that the reduction in film thickness due to static loading or rolling contact was the best predictor of future film damage.

A numerical study of film condensation on a metallic foam-sintered plate with considering convection and super-cooling effects

A numerical solution was presented for film condensation on a vertical plate sintered with metallic foam. In the metallic foam region, the Brinkman-Darcy model was employed to establish the fluid flow equation, and the thermal equilibrium model was used to describe local heat transfer. Through introduction of dimensionless distance, namely, the ratio of the coordinate normal to the plate and the local film thickness, the computational domain was regularized in a rectangular zone. The effects of convection and explicit heat of super-cooled liquid on the condensation film were considered. The temperature and velocity profile, film thickness, and the local and average Nu number were obtained. The effects of Ja and Da number on the velocity profile and heat transfer performance were investigated. The numerical model was verified through comparison of the predicted results with those in the literature, which neglected convection and explicit heat of super-cooling effects. Convection and explicit heat of super-cooling can play a positive role in reducing the velocity level in the film. The two effects played a positive role in heat transfer improvement when the dimensionless location along the gravity direction (X) was >0.15, which was associated with reduced film thickness. The two effects also had a mild influence on the linear distribution of dimensionless temperature. The heat transfer performance deteriorated with an increase in Ja number or Da number.

Thin-Film Phase Plates for Transmission Electron Microscopy Fabricated from Metallic Glasses.

Thin-film phase plates (PPs) have become an interesting tool to enhance the contrast of weak-phase objects in transmission electron microscopy (TEM). The thin film usually consists of amorphous carbon, which suffers from quick degeneration under the intense electron-beam illumination. Recent investigations have focused on the search for alternative materials with an improved material stability. This work presents thin-film PPs fabricated from metallic glass alloys, which are characterized by a high electrical conductivity and an amorphous structure. Thin films of the zirconium-based alloy Zr65.0Al7.5Cu27.5 (ZAC) were fabricated and their phase-shifting properties were evaluated. The ZAC film was investigated by different TEM techniques, which reveal beneficial properties compared with amorphous carbon PPs. Particularly favorable is the small probability for inelastic plasmon scattering, which results from the combined effect of a moderate inelastic mean free path and a reduced film thickness due to a high mean inner potential. Small probability plasmon scattering improves contrast transfer at high spatial frequencies, which makes the ZAC alloy a promising material for PP fabrication.

Influence of nonlinear thermal radiation and variable viscosity on hydromagnetic heat and mass transfer in a thin liquid film over an unsteady stretching surface

This paper describes the influence of nonlinear thermal radiation on magnetohydrodynamic heat and mass transfer in a thin liquid film over a permeable unsteady stretching surface with temperature-dependent viscosity with convective boundary condition in the presence of heat source/sink and chemical reaction. The momentum, energy and mass-diffusion equations are highly non-linear. Thus the problem is solved numerically using Runge–Kutta–Fehlberg method with shooting technique. The shooting technique is essential to convert the boundary value problem to an initial value type. The results reveal that increase in the unsteadiness parameter leads to increase in the velocity distribution, temperature gradient and concentration gradient due to reduction in the thin film thickness. Further, increase in the value of magnetic parameter results in a decrease in the velocity profile and increase in the temperature and concentration gradient. Also, increase in the thermal radiation parameter decreases the temperature gradient. But there is decrease in the value of the concentration gradient by increasing the value of the Schmidt number.

Combined Effects of Nonlinear Thermal Radiation and Internal Heat Generation/Absorption on Heat and Mass Transfer in a Thin Liquid Film on a Permeable Unsteady Stretching Surface with Convective Boundary Condition

An analysis has been carried out to study the combined effects of nonlinear thermal radiation and internal heat source/sink on heat and mass transfer in a thin liquid film over a permeable unsteady stretching sheet with suction/injection in the presence of chemical reaction. Since the momentum, energy and mass diffusion equations are highly non-linear, the problem is solved numerically. The governing equations are first reduced to a set of ordinary differential equations by using the similarity transformations and then the resulting nonlinear ordinary differential equations are solved using Runge–Kutta–Fehlberg method with shooting technique. The effects of various physical parameters on heat and mass transfer in a thin liquid film are presented graphically and in tabular forms. Numerical results are compared with the published results for some limiting cases. It is found that increase in the unsteadiness parameter leads to increase in the velocity distribution, temperature gradient and concentration gradient due to reduction in the thin film thickness. Also, increase in the thermal radiation parameter decreases the temperature gradient whereas reverse effect is seen on the concentration gradient by increasing the values of the Schmidt number.

Effects of out-of-contact lubricant channeling on friction and film thickness in starved elastohydrodynamic lubrication point contacts

This study presents experimental results on the effect of out-of-contact lubricant channeling on the tribological performance of nonconformal contacts under starved lubrication. Channeling of lubricant was carried out by adding a slider with a limited slot for scraping the displaced lubricant on one of mating surfaces (ball). Thus, the scraped lubricant is forced to flow back into the depleted track through the limited slot resulting in robust replenishment. The measurements have been conducted using optical tribometer (ball-on-disc) equipped with a digital camera and torque sensor. The effect of lubricant channeling was compared to the original contact condition by means of measuring friction and film thickness. The results show that the out-of-contact lubricant channeling leads to a significant enhancement of film thickness and friction reduction under starved conditions. Indeed, the starved elastohydrodynamic lubrication contacts transformed to the fully flooded regime after introducing the flow reconditioning. Moreover, the film thickness decay over time, which is common with starved elastohydrodynamic lubrication contacts, has not been observed in the case of lubricant channeling. However, the beneficial effect of lubricant channeling diminishes as the original contact condition tends to the fully flooded regime. The results of this study can be easily implemented in practical applications such as radial and thrust rolling-element bearings.

Effects of strain on the magnetic and transport properties of the epitaxial La0.5Ca0.5MnO3 thin films

The epitaxial strain can considerably modify the physical properties of thin films compared to the bulk. This paper reports the effects of substrate-induced strain on La0.5Ca0.5MnO3 (LCMO) thin films, grown on (100) SrTiO3 (STO) and LaAlO3 (LAO) substrates by pulsed laser deposition technique. Transport and magnetic properties were found to be strongly dependent on strain type. It is also shown that compressive (tensile) strain leads to the increase (decrease) in the magnetization of the films. Moreover, it was observed that all LCMO films deposited on both LAO and STO substrates behave as an insulator, but LCMO/LAO thin films with compressive strain have lower resistivity than LCMO/STO thin films with tensile strain. Applying magnetic field to LCMO/STO thin films with thickness of 25 and 50nm leads to very small change in the resistivity, while the effects of magnetic field on the sample with thickness of 125nm leads to an insulator–metal transition. For LCMO/LAO thin films, the magnetic field has a strong impact on the resistivity of samples. The results show that the magnetoresistance (MR) is enhanced by increasing film thickness for LCMO/LAO samples, due to the relatively stronger phase separation. For LCMO/STO thin films MR is drastically decreased by reduction of film thickness, which is attributed to the enhancement of the charge–orbital order (CO–O) accompanying the complex spin order (the so-called CE type). The changes of the antiferromagnetic structure from the CE to C type and the enhancement of the CE type could be attributed to the in-plane compressive and tensile strain, respectively.

Frictional behaviour of AA7075-O aluminium alloy in high speed tests

The influence of strain rate on the frictional behaviour of AA7075 aluminium alloy, in the O-annealed temper state, was studied. Attention was focused on the evaluation of the mechanisms affecting friction at high strain rate. To this purpose, axisymmetric and ring compression tests were carried out using the split Hopkinson tension-compression bar in the direct version. The flow curves were investigated in a wide range of strain rates using cylindrical specimens characterised by different slenderness ratios. The results of the axisymmetric compression tests showed an appreciable strain rate sensitivity of the deforming material with a marked increase in flow stress with strain rate in the dynamic loading condition. As far as the ring compression tests are concerned, different strain rates were obtained using hollow cylinders with different initial sizes. The analysis of the results showed that strain rate affects the variation of the inner ring diameter, irrespective of the height reduction and frictional condition. Axysimmetric and ring compression tests were also performed under the quasi-static loading condition and the experimental results did not show any appreciable effect of strain rate on flow curves and frictional behaviour. Finally, the different frictional behaviour between the quasi-static and dynamic loading conditions was attributed both to the reduction in the lubricant film thickness and change in the viscous actions at the specimen-tool interface.

Spectroscopy and Dynamics of a Two-Dimensional Electron Gas on Ultrathin Helium Films on Cu(111).

Electrons in image-potential states on the surface of bulk helium represent a unique model system of a two-dimensional electron gas. Here, we investigate their properties in the extreme case of reduced film thickness: a monolayer of helium physisorbed on a single-crystalline (111)-oriented Cu surface. For this purpose we have utilized a customized setup for time-resolved two-photon photoemission at very low temperatures under ultrahigh vacuum conditions. We demonstrate that the highly polarizable metal substrate increases the binding energy of the first (n=1) image-potential state by more than 2 orders of magnitude as compared to the surface of liquid helium. An electron in this state is still strongly decoupled from the metal surface due to the large negative electron affinity of helium and we find that even 1 monolayer of helium increases its lifetime by 1 order of magnitude compared to the bare Cu(111) surface.

Microphysics of mass-transport in coupled droplet-pairs at low Reynolds number and the role of convective dynamics

Interfacial mass-transport and redistribution in the micro-scale liquid droplets are important in diverse fields of research interest. The role of the “inflow” and the “outflow” type convective eddy-pairs in the entrainment of outer solute and internal relocation are examined for different homogeneous and heterogeneous water droplet pairs appearing in a tandem arrangement. Two micro-droplets of pure (rain) water interact with an oncoming outer air stream (Re ≤ 100) contaminated by uniformly distributed SO2. By virtue of separation/attachment induced non-uniform interfacial shear-stress gradient, the well-defined inflow/outflow type pairs of recirculating eddy-based convective motion quickly develops, and the eddies effectively attract/repel the accumulated outer solute and control the physical process of mass-transport in the droplet-pair. The non-uniformly shear-driven flow interaction and bifurcation of the circulatory internal flow lead to growth of important micro-scale “secondary” eddies which suitably regroup with the adjacent “primary” one to create the sustained inflow/outflow type convective dynamics. The presently derived flow characteristics and in-depth analysis help to significantly improve our understanding of the micro-droplet based transport phenomena in a wider context. By tuning “Re” (defined in terms of the droplet diameter and the average oncoming velocity of the outer air) and gap-ratio “α,” the internal convective forcing and the solute entrainment efficiency could be considerably enhanced. The quantitative estimates for mass entrainment, convective strength, and saturation characteristics for different coupled micro-droplet pairs are extensively examined here for 0.2 ≤ α ≤ 2.0 and 30 ≤ Re ≤ 100. Interestingly, for the compound droplets, with suitably tuned radius-ratio “B” (of upstream droplet with respect to downstream one) the generated “inflow” type coherent convective dynamics helped to significantly augment the centre-line mass flow, which in turn facilitate faster saturation of the upstream droplet. However, for heterogeneous droplet-pairs containing solid nucleus, while increased solid-fraction “S” (the ratio between the radius of the solid nucleus and that of the droplet) through 0.25 ≤ S ≤ 0.45 caused gradual reductions of convective strength and mass absorption rate (RSO2) for the upstream droplet, beyond a critical value S ≥ 0.45 the RSO2 therein continued to rise again owing to the reduced film thickness.

Investigation of optical and microstructural properties of RF magnetron sputtered PTFE films for hydrophobic applications

The deposition time dependence of optical, structural and morphological properties of thin as well as ultrathin Polytetrafluoroethylene (PTFE) sputtered films have been explored in the present communication. The films were prepared by RF magnetron sputtering under high vacuum condition, as a function of deposition time. The ellipsometry as well as X-ray reflectivity data show a drastic reduction in film thickness as the deposition time increases from 5s to 10s, possibly as a consequence of back sputtering. With subsequent deposition, back sputtering component decreases and hence, thickness increases with increase in deposition time. Atomic force microscopy (AFM) images show a slight change in growth morphology although roughness is independent of deposition time. Attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) measurements showed the presence of CC and CFx (x=1–3) bonds in all the PTFE films. Supporting this, corresponding X-ray photoelectron spectroscopy (XPS) curves fitted for C-1s and F-1s peaks revealed a major contribution from CF2 bonds along with significant contribution from CF3 bonds leading to an F/C ratio of ∼1.5 giving hydrophobic nature of all the films.

Mechanical integrity of hybrid indium-free electrodes for flexible devices

Maintaining electrical conductivity, optical transparency, and mechanical integrity against bending and stretching are key requirements for flexible transparent electrodes. Transparent conducting oxides (TCOs) are widely used thin film electrodes in optoelectronic devices. However, these materials are brittle and reducing film thickness to improve their mechanical integrity compromises their electrical performance. Here we combine TCO thin films with metal grids embedded in a polymer substrate to create hybrid electrodes with low sheet resistance and high resilience to bending. Amorphous zinc tin oxide (ZTO) and aluminum-doped zinc oxide (AZO) films sputtered onto polyethylene-terephthalate (PET) substrates with and without embedded metal grids are studied. The hybrid electrodes have an optical absorptance below 5% in the visible range and their electrical sheet resistance is less than 1 Ω/sq. The critical strain for tensile failure is analyzed through a combination of electrical measurements and in-situ observations of crack initiation and propagation during tensile loading. The mean critical strain for failure of the AZO/metal grid is 8.5% and that of the ZTO/metal grid is as high as 10%. The AZO and ZTO films alone present critical strain values around 0.6% and 1% respectively, demonstrating that the addition of the metal grid considerably improves the resistance onset strain of the electrodes far beyond these critical strain limits.

Multiscale imaging and characterization of the effect of mixing temperature on asphalt concrete containing recycled components.

When producing asphalt concrete mixture with high amounts of reclaimed asphalt pavement (RAP), the mixing temperature plays a significant role in the resulting spatial distribution of the components as well as on the quality of the resulting mixture, in terms of workability during mixing and compaction as well as in service mechanical properties. Asphalt concrete containing 50% RAP was investigated at mixing temperatures of 140, 160 and 180°C, using a multiscale approach. At the microscale, using energy dispersive X-ray spectroscopy the RAP binder film thickness was visualized and measured. It was shown that at higher mixing temperatures this film thickness was reduced. The reduction in film thickness can be attributed to the loss of volatiles as well as the mixing of RAP binder with virgin binder at higher temperatures. X-ray computer tomography was used to characterize statistically the distribution of the RAP and virgin aggregates geometric features: volume, width and shape anisotropy. In addition using X-ray computer tomography, the packing and spatial distribution of the RAP and virgin aggregates was characterized using the nearest neighbour metric. It was shown that mixing temperature may have a positive effect on the spatial distribution of the aggregates but did not affect the packing. The study shows a tendency for the RAP aggregates to be more likely distributed in clusters at lower mixing temperatures. At higher temperatures, they were more homogeneously distributed. This indicates a higher degree of blending both at microscale (binder film) and macroscale (spatial distribution) between RAP and virgin aggregates as a result of increasing mixing temperatures and the ability to quantify this using various imaging techniques.

Micellar core-shell-type acrylic-polyurethane hybrid materials with self-polishing property

Marine fouling can be a serious problem in the shipping industry, since it increases the surface roughness of the hull and hence its frictional resistance to its movement through water. Antifouling paint can be defined as preventing the attachment of marine organisms onto surfaces. However, the most commonly used antifouling coating which is the tributyltin-based self-polishing copolymer causes the severe pollution of marine environment. Ammonium salt-based paints include tertiary amines as biocides which have effective biocidal and biodegradable properties without accumulation in the sea environment. However, ammonium salt-based coatings were too sensitive to seawater and became swollen. In this study, polyurethane-acrylic copolymers were synthesized by radical polymerization. These hybrid materials were found to form core–shell structures in aqueous media. Synthesis and properties of copolymers were investigated by Fourier transform-infrared spectrometer, proton-nuclear magnetic resonance, transmission electron microscopy, and dynamic light scattering. The polishing rate of self-polishing copolymer was determined from the reduction in dry film thickness after artificial seawater immersion under a dynamic condition.

Hydrodynamic effects on three phase micro-packed bed reactor performance – Gold–palladium catalysed benzyl alcohol oxidation

The hydrodynamics of a three-phase micro-packed bed reactor and its effect on catalysed benzyl alcohol oxidation with pure oxygen were studied in a silicon–glass microstructured reactor. The microreactor was operated at 120°C and 1barg and contained a channel with a 300μm×600μm cross-section, packed with 1wt% Au–Pd/TiO2 catalyst, 65μm in average diameter. Improvements in the conversion of benzyl alcohol and selectivity to benzaldehyde were observed with increasing gas-to-liquid ratio, which coincided with a change in the flow pattern from a liquid-dominated slug to a gas-continuous flow regime. The observed enhancement is attributed to improved external mass transfer, associated with an increase in the gas–liquid interfacial area and reduction in the liquid film thickness that occur with gradual changes in the flow pattern. A maximum selectivity of 93% to benzaldehyde was obtained under partial wetting – which introduced the added benefit of direct gas–solid mass transfer – outperforming the selectivity in a conventional glass stirred reactor. However, this was at the expense of a reduction in the conversion. A response surface model was developed and then used to predict optimal operating conditions for maximum benzaldehyde yield, which were in the gas-continuous flow regime. This corresponded to relatively high gas flow rate in conjunction with moderate liquid flow rate, ensuring sufficient catalyst wetting with a thin film to reduce transport resistances.

Effects of thickness on optical characteristics and strain distribution of thin-film GaN light-emitting diodes transferred to Si substrates

We investigated the effect of device thickness on the internal quantum efficiency (IQE) of thin-film GaN light-emitting diodes (LEDs), which were grown on Si substrates and transferred to other Si substrates with reduced film thickness. It was confirmed by Raman spectroscopy and photoluminescence measurement that the compressive strain is released and the quantum-confined Stark effect (QCSE) is suppressed after reducing the thickness. The best IQE of 62.9% was reached with a large suppression of the band tilting by QCSE, from 7.9 meV in the original structure to 2.4 meV in the thinnest sample, and this value can compete with that of GaN-based LEDs grown on a sapphire substrate.

Lubricant film thickness and friction force measurements in a laser surface textured reciprocating line contact simulating the piston ring–liner pairing

Applying surface texture to piston liners may provide an effective means of controlling friction and hence improving engine efficiency. However, little is understood about the mechanisms by which pockets affect friction, primarily because of a lack of reliable experimental measurements. To address this, the influence of surface texture on film thickness and friction force was measured simultaneously in a convergent-divergent bearing, under conditions that closely replicate an automotive piston ring-liner conjunction. Film thicknesses were measured using a modified version of the ultra-thin film optical interferometry approach, enabling film thicknesses <50nm to be measured under transient, mixed lubrication conditions. This involved using the out-of-contact curvature of the specimens in place of a spacer layer and analysing multiple interference fringes to avoid fringe ambiguity. Tests were performed on both a textured sample (with features oriented normal to the direction of sliding) and a non-textured reference sample, while angular velocity, applied normal load and lubricant temperature were controlled in order to study the effect of varying lubrication regime (as typically occurs in service). Results showed that the presence of surface pockets consistently enhances fluid film thickness in the mixed lubrication regime by approximately 20nm. Although this is only a modest increase, the effect on friction is pronounced (up to 41% under these conditions), due to the strong dependence of friction on film thickness in the mixed regime. Conversely, in the full film regime, texture caused a reduction in film thickness and hence increased friction force, compared with the non-textured reference. Both textured and non-textured friction values show nearly identical dependence on film thickness, (showing that, under these conditions, texture-induced friction reduction results entirely from the change in film thickness). These results are important in providing film thickness data to validate piston-ring lubrication models and also in helping to understand the effect of surface roughness on texture performance.

Measurement of the Temperature Coefficient of Resistance in Metallic Films with Nano-thickness

The temperature coefficient of resistance (TCR) values of gold and aluminum films deposited on glass substrates were obtained in the range of thickness from 20 nm to 200 nm at 298 K and atmospheric pressure conditions. Applying an electrical current and measuring simultaneously the corresponding changes of voltage (i.e., electrical resistance), and the change of temperature on the thin films, the TCR value was estimated. The measured TCR values show a decrement with the film thickness reduction, and their values are approximately 13.0 % lower than their corresponding bulk values mainly for thinner films. A comparison with previously reported cooper TCR values and the values estimated with the Tellier–Tosser model show good agreement with differences of about 5.0 % between them.