A comparative study of failure features in aerospace grade unidirectional and bidirectional woven CFRP composite laminates under four-point bend fatigue loads

Damage mechanisms in unidirectional (UD) and bi-directional (BD) woven carbon fiber reinforced polymer (CFRP) laminates subjected to four point flexure, both in static and fatigue loadings, were studied. The damage progression in composites was monitored by observing the slopes of the load vs. deflection data that represent the stiffness of the given specimen geometry over a number of cycles. It was observed that the unidirectional composites exhibit gradual loss in stiffness whereas the bidirectional woven composites show a relatively quicker loss during stage II of fatigue damage progression. Both, the static and the fatigue failures in unidirectional carbon fiber reinforced polymer composites originates due to generation of cracks on compression face while in bidirectional woven composites the damage ensues from both the compression and the tensile faces. These observations are supported by a detailed fractographic analysis.

Enhanced sunlight photocatalytic activity of Ag3PO4 decorated novel combustion synthesis derived TiO2 nanobelts for dye and bacterial degradation

This study demonstrates the synthesis of TiO2 nanobelts using solution combustion derived TiO2 with enhanced photocatalytic activity for dye degradation and bacterial inactivation. Hydrothermal treatment of combustion synthesized TiO2 resulted in unique partially etched TiO2 nanobelts and Ag3PO4 was decorated using the co-precipitation method. The catalyst particles were characterized using X-ray diffraction analysis, BET surface area analysis, diffuse reflectance and electron microscopy. The photocatalytic properties of the composites of Ag3PO4 with pristine combustion synthesized TiO2 and commercial TiO2 under sunlight were compared. Therefore the studies conducted proved that the novel Ag3PO4/unique combustion synthesis derived TiO2 nanobelt composites exhibited extended light absorption, better charge transfer mechanism and higher generation of hydroxyl and hole radicals. These properties resulted in enhanced photodegradation of dyes and bacteria when compared to the commercial TiO2 nanocomposite. These findings have important implications in designing new photocatalysts for water purification.

Single-step synthesis of carbon nanotubes/iron/iron oxide composite films through inert-ambient CVD using ferric acetylacetonate as a precursor

We have developed a unique single-step chemical vapor deposition (CVD) route for the synthesis of composite thin films containing carbon nanotubes (CNTs). CVD was carried out in an inert ambient using only iron(III) acetylacetonate as the precursor. Depositions were conducted at 700 degrees C on stainless steel substrates in argon ambient in the absence of any reactive gases (such as oxygen, hydrogen). By changing the deposition parameters, especially the pressure in the CVD reactor, the form of carbon deposited could be changed from amorphous to carbon nanotubes, the latter resulting in Fe-Fe3O4-CNT films. X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, and electron microscopy together confirm the formation of the three-component composite and illustrate the nanoscale mixing of the components. Elemental iron formed in this process was protected from oxidation by the co-deposited carbon surrounding it. Irrespective of the substrate used, a composite coating with CNTs was formed under optimum conditions, as verified by analyses of the film formed on polycrystalline alumina and silicon substrates.

Defect induced optical bandgap narrowing in undoped SnO2 nanocrystals

Unusual optical bandgap narrowing is observed in undoped SnO2 nanoparticles synthesized by the solution combustion method. The estimated crystallite size is nearly 7 nm. Though the quantum confinement effect predicts a larger optical bandgap for materials with small crystallite size than the bulk, the optical bandgap in the as synthesized materials is found to be 2.9 eV compared to the reported value of 3.6 eV for bulk SnO2 particles. The yellow-green photoluminescence emissions and the observed narrowing of the bandgap can be attributed to the deep donor levels of oxygen vacancies, owing to the high exothermicity of the combustion reaction and the faster cooling rates involved in the process. (C) 2013 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

Growth of TiO2 Films by RF Magnetron Sputtering for MOS Gate Dielectrics: Influence of Substrate Temperature

Titanium dioxide thin films were deposited by RF reactive magnetron sputtering technique on p-type silicon(100) substrates held at temperatures in the range 303-673 K. The influence of substrate temperature on the core level binding energies, chemical bonding configuration, crystallographic structure and dielectric properties was investigated. X-ray photoelectron spectroscopy studies and Fourier transform infrared transmittance data confirmed the formation of stoichiometric films with anatase phase at a substrate temperature of 673 K. The films formed at 303 K were nanocrystalline with amorphous matrix while those deposited at 673 K were transformed in to crystalline phase and growth of grains in pyramidal like structure as confirmed by X-ray diffraction and atomic force microscopy respectively. Metal-oxide-semiconductor capacitors were fabricated with the configuration of Al/TiO2/Si structures. The current voltage, capacitance voltage and conductance voltage characteristics were studied to understand the electrical conduction and dielectric properties of the MOS devices. The leakage current density (at gate voltage of 2 V) decreased from 2.2 x 10(-6) to 1.7 x 10(-7) A/cm(2), the interface trap density decreased from 1.2 x 10(13) to 2.1 x 10(12) cm(-2) eV(-1) and the dielectric constant increased from 14 to 36 with increase of substrate temperature from 303 to 673 K.

Study of Degradation in Composite Lap Shear joints using a Guided Wave Technique

Experimental and numerical investigations were carried out using lamb waves to study the degradation in adhesive joints made of carbon fiber reinforced plastic (CFRP) adherends and epoxy adhesive. Degradation was inducted into the epoxy adhesive by adding different amounts of polyvinyl alcohol. Fundamental lamb wave modes were excited in the CFRP adherends using piezoelectric transducer disks and made to propagate through the adhesive layer. The received waveforms across adhesive joints with varied degradation were studied. A 2D finite element model was utilized to verify the experimental results. Good correlation was observed between numerical and experimental results. Details of the investigation and results obtained are presented in the paper.

High photoconductive combustion synthesized TiO2 derived nanobelts for photocatalytic water purification under solar irradiation

Drinking water scarcity is a major issue that needs to be addressed seriously. Water needs to be purified from organic pollutants and bacterial contamination. In this study, sunlight driven photocatalysis for the degradation of dyes and bacterial inactivation has been conducted over TiO2 nanoparticles (CST) and TiO2 nanobelts (CSTNB). TiO2 nanoparticles were synthesized by a solution combustion process using ascorbic acid as a fuel. Acid etched TiO2 nanobelts (CSTNB) were synthesized using combustion synthesized TiO2 as a novel precursor. The mechanism of formation of TiO2 nanobelts was hypothesized. The antibacterial activity of combustion synthesized TiO2 and acid etched TiO2 nanobelts were evaluated against Escherichia coli and compared against commercial TiO2. Various characterization studies like X-ray diffraction analysis, BET surface area analysis, diffused reflectance measurements were performed. Microscopic structures and high resolution images were analyzed using scanning electron microscopy, transmission electron microscopy. The extent of photo-stability and reusability of the catalyst was evaluated by conducting repeated cycles of photo degradation experiments and was compared to the commercial grade TiO2. The reactive radical species responsible for high photocatalytic and antibacterial activity has been determined by performing multiple scavenger reactions. The excellent charge transfer mechanism, high generation of hydroxyl and hole radicals resulted in enhanced photocatalytic activity of the acid etched TiO2 nanobelts compared to commercial TiO2 and nanobelts made from commercial TiO2.

Novel architecture for anomalous strengthening of a particulate filled polymer matrix composite

We propose an architecture for dramatically enhancing the stress bearing and energy absorption capacities of a polymer based composite. Different weight fractions of iron oxide nano-particles (NPs) are mixed in a poly(dimethylesiloxane) (PDMS) matrix either uniformly or into several vertically aligned cylindrical pillars. These composites are compressed up to a strain of 60% at a strain rate of 0.01 s(-1) following which they are fully unloaded at the same rate. Load bearing and energy absorption capacities of the composite with uniform distribution of NPs increase by similar to 50% upon addition of 5 wt% of NPs; however, these properties monotonically decrease with further addition of NPs so much so that the load bearing capacity of the composite becomes 1/6th of PDMS upon addition of 20 wt% of NPs. On the contrary, stress at a strain of 60% and energy absorption capacity of the composites with pillar configuration monotonically increase with the weight fraction of NPs in the pillars wherein the load bearing capacity becomes 1.5 times of PDMS when the pillars consisted of 20 wt% of NPs. In situ mechanical testing of composites with pillars reveals outward bending of the pillars wherein the pillars and the PDMS in between two pillars, located along a radius, are significantly compressed. Reasoning based on effects of compressive hydrostatic stress and shape of fillers is developed to explain the observed anomalous strengthening of the composite with pillar architecture.

High-Performance Stacked TiO2-ZrO2 and Si-doped ZrO2 Metal-Insulator-Metal Capacitors

Metal-insulator-metal (MIM) capacitors for DRAM applications have been realised using stacked TiO2-ZrO2 (TiO2/ZrO2 and ZrO2/TiO2) and Si-doped ZrO2 (TiO2/Si-doped ZrO2) dielectrics. High capacitance densities (> 42 fF/mu m(2)), low leakage current densities (< 5 x 10(-7) A/cm(2) at -1 V), and sub-nm EOT (< 0.8 nm) have been achieved. The effects of constant voltage stress on the device characteristics is studied. The structural analysis of the samples is performed by X-ray diffraction measurements, and this is correlated to the electrical characteristics of the devices. The surface chemical states of the films are analyzed through X-ray photoelectron spectroscopy measurements. The doped-dielectric stack (TiO2/Si-doped ZrO2) helps to reduce leakage current density and improve reliability, with a marginal reduction in capacitance density; compared to their undoped counterparts (TiO2/ZrO2 and ZrO2/TiO2). We compare the device performance of the fabricated capacitors with other stacked high-k MIM capacitors reported in recent literature.

Probing Ultrafast Photoinduced Electron Transfer to TiO2 from CdS Nanocrystals of Varying Crystallographic Phase Content

The photoinduced electron transfer processes in a nanoheterostructured semiconductor assembly are complex and depend on various parameters Of the constituents of the assembly. We present here the ultrafast electron transfer characteristics of an assembly comprised of a Wide band semiconductor, titanium dioxide (TiO2), attached to light-harvesting cadmium sulfide (CdS) nanotrystals of varying crystallographic phase content. Quantitative analysis of Synchrotron high-resolution X-ray. diffraction data of CdS nanocrystals precisely reveals the presence of both wurtzite and zinc blende phases in varying amounts. The,estimated content of crystal phases is observed to be strongly dependent on an important synthesis parameter, viz., the ratio of the two solvents. The biphasit nature of CdS influences directly the shape of the nanocrystal at long reaction times as well as the transfer of the photoexcited electrons from the CdS to TiO2 as obtained from transient absorption spectroscopy. A higher amount of zinc blende Phase is observed to be beneficial for fast electron transfer across the CdS-TiO2 interface. The electron transfer rate constant differs by one order of magnitude between the CdS nanocryStals and varies linearly with the fraction of the phases.

Superior Performance of a MoS2-RGO Composite and a Borocarbonitride in the Electrochemical Detection of Dopamine, Uric Acid and Adenine

A MoS2-RGO composite and borocarbonitride (BC5N) have been used as electrodes to selectively detect dopamine and uric acid in the presence of ascorbic acid. Both the electrodes show excellent eletrocatalytic activity towards the detection of dopamine, the detection limits being 0.55 mu M and 2.1 mu M in the case of MoS2-RGO and BCN respectively. MoS2-RGO shows a linear range of current over the 1-110 mu M concentrations of dopamine, while BCN shows over the 2.3-20 mu M range. BCN also exhibits satisfactory performance in the oxidation of uric acid with a detection limit of 3.8 mu M and the linear range from 4 to 40 mu M. The MoS2-RGO has also been used to detect adenine as well.

Ultrathin Au nanowires supported on rGO/TiO2 as an efficient photoelectrocatalyst

We report the synthesis of stable rGO/TiO2/Au nanowire hybrids showing excellent electrocatalytic activity for ethanol oxidation. Phase-pure anatase TiO2 nanoparticles (similar to 3 nm) were grown on GO sheets followed by the growth of ultrathin Au nanowires leading to the formation of a multidimensional ternary structure (0-D TiO2 and 1-D Au on 2-D graphene oxide). The oleylamine used for the synthesis of the Au nanowires not only leads to stable Au nanowires anchored on the GO sheets but also leads to the functionalization and room temperature reduction of GO. Using control experiments, we delineate the role of the three components in the hybrid and show that there is a significant synergy. We show that the catalytic activity for ethanol oxidation primarily stems from the Au nanowires. While TiO2 triggers the formation of oxygenated species on the Au nanowire surface at a lower potential and also imparts photoactivity, rGO provides a conducting support to minimize the charge transfer resistance in addition to stabilizing the Au nanowires. Compared with nanoparticle hybrids, the nanowire hybrids display a much better electrocatalytic performance. In addition to high efficiency, the nanowire hybrids also show a remarkable tolerance towards H2O2. While our study has a direct bearing on fuel cell technology, the insights gained are sufficiently general such that they provide guiding principles for the development of multifunctional ternary hybrids.

A Few Case Studies on the Correlation of Particle Network and Its Stability on the Ionic Conductivity of Solid-Liquid Composite Electrolytes

We discuss here the crucial role of the particle network and its stability on the long-range ion transport in solid liquid composite electrolytes. The solid liquid composite electrolytes chosen for the study here comprise nanometer sized silica (SiO2) particles having various surface chemical functionalities dispersed in nonaqueous lithium salt solutions, viz, lithium perchlorate (LiClO4) in two different polyethylene glycol based solvents. These systems constitute representative examples of an independent class of soft matter electrolytes known as ``soggy sand'' electrolytes, which have tremendous potential in diverse electrochemical devices. The oxide additive acts as a heterogeneous dopant creating free charge carriers and enhancing the local ion transport. For long-range transport, however, a stable spanning particle network is needed. Systematic experimental investigations here reveal that the spatial and time dependent characteristics of the particle network in the liquid solution are nontrivial. The network characteristics are predominantly determined by the chemical makeup of the electrolyte components and the chemical interactions between them. It is noteworthy that in this study the steady state macroscopic ionic conductivity and viscosity of the solid liquid composite electrolyte are observed to be greatly determined by the additive oxide surface chemical functionality, solvent chemical composition, and solvent dielectric constant.

Graphene scavenges free radicals to synergistically enhance structural properties in a gamma-irradiated polyethylene composite through enhanced interfacial interactions

A unique strategy for scavenging free radicals in situ on exposure to gamma irradiation in polyethylene (PE) nanocomposites is presented. Blends of ultra-high molecular weight PE and linear low-density PE (PEB) and their nanocomposites with graphene (GPEB) were prepared by melt mixing to develop materials for biomedical implants. The effect of gamma irradiation on the microstructure and mechanical properties was systematically investigated. The neat blend and the nanocomposite were subjected to gamma-ray irradiation in order to improve the interfacial adhesion between PE and graphene sheets. Structural and thermal characterization revealed that irradiation induced crosslinking and increased the crystallinity of the polymer blend. The presence of graphene further enhanced the crystallinity via crosslinks between the polymer matrix and the filler on irradiation. Graphene was found to scavenge free radicals as confirmed by electron paramagnetic resonance spectroscopy. Irradiation of graphene-containing polymer composites resulted in the largest increase in modulus and hardness compared to either irradiation or addition of graphene to PEB alone. This study provides new insight into the role of graphene in polymer matrices during irradiation and suggests that irradiated graphene-polymer composites could emerge as promising materials for use as articulating surfaces in biomedical implants.

Electrochemical behavior of Sn-graphene composite coating

The electrochemical properties of pure Sn and Sn-graphene composite coating have been determined and compared. Coatings were electrodeposited on mild steel substrates. Graphene was synthesized by the electrochemical exfoliation process using SO42- ion as the intercalating agent. Morphological and structural characterization results revealed a clear effect of graphene on altering the texture, grain size and morphology of the coating. Corrosion behavior was analyzed through potentiodynamic polarization and electrochemical impedance spectroscopic methods. A significant improvement in the corrosion resistance in terms of reduction in corrosion current and corrosion rate and increase in polarization resistance was noted in case of Sn coating containing graphene.