Mechanical properties of ZnS nanowires and thin films: Microscopic origin of the dependence on size and growth direction

Mechanical properties of ZnS nanowires and thin films are studied as a function of size and growth direction using all-atom molecular dynamics simulations. Using the stress-strain relationship we extract Young's moduli of nanowires and thin films at room temperature. Our results show that Young's modulus of 0001] nanowires has strong size dependence. On the other hand, 01 (1) over bar0] nanowires do not exhibit a strong size dependence of Young's modulus in the size range we have investigated. We provide a microscopic understanding of this behavior on the basis of bond stretching and contraction due to the rearrangement of atoms in the surface layers. The ultimate tensile strengths of the nanowires do not show much size dependence. To investigate the mechanical behavior of ZnS in two dimensions, we calculate Young's modulus of thin films under tensile strain along the 0001] direction. Young's modulus of thin films converges to the bulk value more rapidly than that of the 0001] nanowire.

Influence of stack geometry and resonator length on the performance of thermoacoustic engine

Thermoacoustic engines convert heat energy into high amplitude sound waves, which is used to drive thermoacoustic refrigerator or pulse tube cryocoolers by replacing the mechanical pistons such as compressors. The increasing interest in thermoacoustic technology is of its potentiality of no exotic materials, low cost and high reliability compared to vapor compression refrigeration systems. The experimental setup has been built based on the linear thermoacoustic model and some simple design parameters. The engines produce acoustic energy at the temperature difference of 325-450 K imposed along the stack of the system. This work illustrates the influence of stack parameters such as plate thickness (PT) and plate spacing (PS) with resonator length on the performance of thermoacoustic engine, which are measured in terms of onset temperature difference, resonance frequency and pressure amplitude using air as a working fluid. The results obtained from the experiments are in good agreement with the theoretical results from DeltaEc. (C) 2012 Elsevier Ltd. All rights reserved.

Radiative and non-radiative effects of a substrate on localized plasmon resonance of particles

Experiments have shown strong effects of some substrates on the localized plasmons of metallic nano particles but they are inconclusive on the affecting parameters. Here, we have used discrete dipole approximation in conjunction with Sommerfeld integral relations to explain the effect of the substrates as a function of the parameters of incident radiation. The radiative coupling can both quench and enhance the resonance and its dependence on the angle and polarization of incident radiation with respect to the surface is shown. Non-radiative interaction with the substrate enhances the plasmon resonance of the particles and can shift the resonances from their free-space energies significantly. The non-radiative interaction of the substrate is sensitive to the shape of particles and polarization of incident radiation with respect to substrate. Our results show that the plasmon resonances in coupled and single particles can be significantly altered from their free-space resonances and are quenched or enhanced by the choice of substrate and polarization of incident radiation. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4736544]

Sensitivity of polyvinylidene fluoride films to mechanical vibration modes and impact after optimizing stretching conditions

The β-phase of polyvinylidene fluoride (PVDF) is well known for its piezoelectric properties. PVDF films have been developed using solvent cast method. The films thus produced are in α-phase. The α-phase is transformed to piezoelectric β-phase when the film is hot-stretched with various different stretching factors at various different temperatures. The films are then characterized in terms of their mechanical properties and surface morphological changes during the transformation from α- to β-phases by using X-ray diffraction, differential scanning calorimeter, Raman spectra, Infrared spectra, tensile testing, and scanning electron microscopy. The films showed increased crystallinity with stretching at temperature up to 80°C. The optimum conditions to achieve β-phase have been discussed in detail. The fabricated PVDF sensors have been tested for free vibration and impact on plate structure, and its response is compared with conventional piezoelectric wafer type sensor. The resonant and antiresonant peaks in the frequency response of PVDF sensor match well with that of lead zirconate titanate wafer sensors. Effective piezoelectric properties and the variations in the frequency response spectra due to free vibration and impact loading conditions are reported. POLYM. ENG. SCI., 2012. © 2012 Society of Plastics Engineers.

Simultaneous Detection of Two Triplets: A Time-Resolved Resonance Raman Study

Solvents are known to affect the triplet state structure and reactivity. In this paper, we have employed time-resolved resonance Raman (TR3) spectroscopy to understand solvent-induced subtle structural changes in the lowest excited triplet state of thioxanthone. Density functional theory (DFT) combined with the self-consistent reaction field (SCRF) implicit solvation model has been used to calculate the vibrational frequencies in the solvents. Here, we report a unique observation of the coexistence of two triplets, which has been substantiated by the probe wavelength-dependent Raman experiments. The coexistence of two triplets has been further supported by photoreduction experiments carried out at various temperatures.

Non-destructive evaluation of porosity and its effect on mechanical properties of carbon fiber reinforced polymer composite materials

In this work, an attempt is made to induce porosity of varied levels in carbon fiber reinforced epoxy based polymer composite laminates fabricated using prepregs by varying the fabrication parameters such as applied vacuum, autoclave pressure and curing temperature. Different NDE tools have been utilized to evaluate the porosity content and correlate with measurable parameters of different NDE techniques. Primarily, ultrasonic imaging and real time digital X-ray imaging have been tried to obtain a measurable parameter which can represent or reflect the amount of porosity contained in the composite laminate. Also, effect of varied porosity content on mechanical properties of the CFRP composite materials is investigated through a series of experimental investigations. The outcome of the experimental approach has yielded interesting and encouraging trend as a first step towards developing an NDE tool for quantification of effect of varied porosity in the polymer composite materials.

Non-destructive evaluation of porosity and its effect on mechanical properties of carbon fiber reinforced polymer composite materials

Structural adhesive bonding is widely used to execute assemblies in automobile and aerospace structures. The quality and reliability of these bonded joints must be ensured during service. In this context non destructive evaluation of these bonded structures play an important role. Evaluation of adhesively bonded composite single lap shear joints has been attempted through experimental approach. Series of tests, non-destructive as well as destructive were performed on different sets of carbon fiber reinforced polymer (CFRP) composite lap joint specimens with varied bond quality. Details of the experimental investigations carried out and the outcome are presented in this paper.

Thermo-mechanical stability of a cellular assembly of carbon nanotubes in air

Carbon nanotubes (CNT) in bulk form offer outstanding structural and functional properties, and are shown to remain viscoelastic over a wide temperature range (77-1273 K) under inert conditions. We examine the quasi-static and dynamic compressive mechanical response of these cellular CNT materials in ambient air up to a temperature of 773 K. In uniaxial quasi-static compression, several displacement bursts are noted at large strains. These are results of the slippage and zipping of the CNT, and lead to significant mechanical energy absorption. Results of the dynamic mechanical analysis experiments show no degradation in storage modulus and loss coefficient for up to 20 h at 673 K. Hence, these stable cellular CNT structures can be utilized up to a maximum temperature of 673 K in air, which is much higher than the best polymers. (C) 2012 Elsevier Ltd. All rights reserved.

Evaluation of mechanical losses in a linear motor pressure wave generator

A moving magnet linear motor compressor or pressure wave generator (PWG) of 2 cc swept volume with dual opposed piston configuration has been developed to operate miniature pulse tube coolers. Prelimnary experiments yielded only a no-load cold end temperature of 180 K. Auxiliary tests and the interpretation of detailed modeling of a PWG suggest that much of the PV power has been lost in the form of blow-by at piston seals due to large and non-optimum clearance seal gap between piston and cylinder. The results of experimental parameters simulated using Sage provide the optimum seal gap value for maximizing the delivered PV power.

Effect of length scale on mechanical properties of Al-Cu eutectic alloy

This paper attempts a quantitative understanding of the effect of length scale on two phase eutectic structure. We first develop a model that considers both the elastic and plastic properties of the interface. Using Al-Al2Cu lamellar eutectic as model system, the parameters of the model were experimentally determined using indentation technique. The model is further validated using the results of bulk compression testing of the eutectics having different length scales. (C) 2012 American Institute of Physics. http://dx.doi.org/10.1063/1.4761944]

Ex Situ X-ray Diffraction, X-ray Absorption Near Edge Structure, Electron Spin Resonance, and Transmission Electron Microscopy Study of the Hydrothermal Crystallization of Vanadium Oxide Nanotubes: An Insight into the Mechanism of Formation

The nucleation and growth of vanadium oxide nanotubes (VOx-NT) have been followed by a combination of numerous ex situ techniques. long the hydrothermal process. Intermediate solid phases extracted at different reaction times have been characterized by powder X-ray diffraction, scanning and transmission electron microscopy, electron spin resonance, and V-K edge :X-ray absorption near-edge structure spectroscopy. The supernatant vanadate solutions extracted during the hydrothermal treatment have been studied by liquid V-51 NMR and flame. spectroscopy. For short durations of the hydrothermal synthesis, the initial V2O5-surfactant intercalate. is progressively transformed into VOx-NT whose crystallization starts to be detected after a hydrothermal treatment of 24 h. Upon heating from 24 h to 7 days, VOx-NT are obtained in larger amount and with an improved crystallinity. The detection of soluble amines and cyclic metavanadate V4O12](4-) in the supernatant solution along the hydrothermal process suggests that VOx-NT result from a dissolution precipitation mechanism. Metavanadate species V4O12](4-) could behave as molecular precursors in the polymerization reactions leading to VOx-NT.

Mechanical and electrical contact resistance characteristics of a cellular assembly of carbon nanotubes

We employ nanoindentation coupled with electrical contact resistance measurements for simultaneous characterization of the electrical and mechanical behaviors of a cellular assembly of carbon nanotubes (CNTs). Experimental results reveal two different responses that correspond to relatively dense and porous regions of the cellular structure. Distinct nonlinear electron transport characteristics are observed, which mainly originate from diffusive conductance in the CNT structure. In the denser region, differential conductance shows asymmetric minima at lower bias, implying that conductivity mainly results from bulk tunneling. However, the porous regions show insignificant differential conduction as opposed to the denser region.

Manifestation of intermediate phase in mechanical properties: Nano-indentation studies on Ge-Te-Si bulk chalcogenide glasses

Nano-indentation studies have been undertaken on bulk Ge15Te85-xSix glasses (0 <= x <= 9), to estimate hardness, H and elastic modulus, E. It is found that E and H increase initially with the increase in the atomic percent of Si. Further, a plateau is seen in the composition dependence of E and H in the composition range 2 <= x <= 6. It is also seen that the addition of up to 2 at% Si increases the density rho of the glass considerably; however, further additions of Si lead to a near linear reduction in rho, in the range 2 <= x <= 6. Beyond x=6, rho increases again with Si content. The variation of molar volume V-m brings out a more fascinating picture. A plateau is seen in the intermediate phase suggesting that the molecular structure of the glasses is adapting to keep the count of constraints fixed in this particular phase. The observed variations in mechanical properties are associated with the Boolchand's intermediate phase in the present glassy system, in the composition range 2 <= x <= 6, suggested earlier from calorimetric and electrical switching studies. The present results reveal rather directly the existence of the intermediate phase in elastic and plastic properties of chalcogenide glasses. (C) 2012 Elsevier Ltd. All rights reserved.

Microstructure and mechanical properties of oxidation resistant suction cast Nb-Si-Al alloy

In this paper, we report a significant improvement in mechanical and oxidation properties of near eutectic Nb-Si alloys by the addition of aluminum (Al) and control of microstructural length scale. A comparative study of two alloys Nb-18.79at%Si and Nb-12.3at%Si-9at%Al were carried out. The processing for microstructure refinements were carried out by vacuum suction casting in water cooled thick copper mould. It is shown that addition of Al suppresses Nb3Si phase and promotes beta Nb5Si3 phase under nonequilibrium solidification condition. The microstructural length scale and in particular eutectic spacing reduces significantly to 50-100 nm in suction cast ternary alloy. A detailed TEM study shows the presence of delta-Nb11Si4 phase in Nb matrix. The hardness of Nb solid solution can be increased as a consequence to a level observed in Nb3Si intermetallic due to the well oriented precipitates. Compression test yields the ultimate strength of 1.8 +/- 0.1 GPa and engineering strain of 2.3 +/- 0.03%. In comparison, the binary Nb-18.79 at% Si alloy possesses an ultimate strength of 1.35 +/- 0.1 GPa and strain of 0.2 +/- 0.01% when processed under identical conditions. The latter exhibits coarser microstructural length scale (300-400 nm) and a brittle behavior. The indentation fracture toughness of Al containing suction cast alloy shows a value of 20.2 +/- 0.5 MPa root m which represents a major improvement over bulk Nb-Si eutectic alloy. The detailed thermal studies confirm a multifold improvement in oxidation resistance up to 1000 degrees C. (C) 2012 Elsevier B.V. All rights reserved.

Microstructural Evolution and Some Unusual Effects During Thermo-Mechanical Cycling of Sn-Ag-Cu Alloys

Sn-Ag-Cu (SAC) solders are susceptible to appreciable microstructural coarsening during storage or service. This results in evolution of joint properties over time, and thereby influences the long-term reliability of microelectronic packages. Accurate prediction of this aging behavior is therefore critical for joint reliability predictions. Here, we study the precipitate coarsening behavior in two Sn-Ag-Cu (SAC) alloys, namely Sn-3.0Ag-0.5Cu and Sn-1.0Cu-0.5Cu, under different thermo-mechanical excursions, including isothermal aging at 150 degrees C for various lengths of time and thermo-mechanical cycling between -25 degrees C and 125 degrees C, with an imposed shear strain of similar to 19.6% per cycle, for different number of cycles. During isothermal aging and the thermo-mechanical cycling up to 200 cycles, Ag3Sn precipitates undergo rapid, monotonous coarsening. However, high number of thermo-mechanical cycling, usually between 200 and 600 cycles, causes dissolution and re-precipitation of precipitates, resulting in a fine and even distribution. Also, recrystallization of Sn-grains near precipitate clusters was observed during severe isothermal aging. Such responses are quite unusual for SAC solder alloys. In the regime of usual precipitate coarsening in these SAC alloys, an explicit parameter, which captures the thermo-mechanical history dependence of Ag3Sn particle size, was defined. Brief mechanistic description for the recrystallization of Sn grains during isothermal aging and reprecipitation of the Ag3Sn due to high number of thermo-mechanical cycles are also presented.