Nearly fully dense (>96% theoretical maximum density) powder mixture compacts with combinations of Nb, Ni, Mo, W, and Ta, with Al, were produced by explosive consolidation. The quasi-static and dynamic behavior and failure mechanisms were investigated experimentally and computationally. For two mixtures (Ni+Al, W+Al) the Al phase was continuous, while for the other three mixtures (Nb+Al, Ta+Al, Mo+Al), the Al phase was discontinuous. It was found that the continuous phase significantly influenced the mechanical response (in compression) and determined the fracture morphology of the compacts. Accordingly, the mixtures with continuous Al phases had the lowest compressive strength. Two distinct failure mechanisms, axial splitting and shear failure, were observed. Axial splitting occurred when the Al phase was continuous (Ni+Al, W+Al); shear failure was primarily associated with extensive deformation of the Nb, Ta and Mo continuous phases. Finite element simulations provide valuable help in interpreting the experimental results and predicting mechanical strength and failure mechanisms akin to those observed. The interfacial bonding strength is shown to be an important parameter in determining the mechanical response of the compacts.

Ni/Al laminates with bilayer thicknesses in the micrometer (∼5 μm) and nanometer (∼50 nm) range were subjected to exothermic reactions induced by laser-driven compression. The initial shockless compression steepened into shock in the microscaled laminates generating a pressure pulse duration of several tens of nanoseconds, which induced strain rates varying from 107 to 108 s−1. The laser energies applied, 650, 875, and 1305 J, generated peak compression stresses of 30, 75, and 118 GPa, respectively, at the plasma stagnated Al surface. Large differences in flow stresses and bulk compression moduli of Ni and Al introduced shear localization in the Ni/Al interfaces. The nanoscale Ni/Al laminates were fully reacted, producing NiAl with grain sizes less than 500 nm. The NiAl intermetallic phases, B2 (β) phase (fcc) and martensitic phase (bcc), coexist in the NiAl nanograins. It was confirmed that the intermetallic reaction in the Ni/Al microlaminate cannot self-sustain for the short duration, laser-driven compressive loading. The intermetallics NiAl (equiaxed grains) and NiAl3 (dendrites) were identified on the plasma stagnated surface of Ni/Al microlaminates. The distribution of intermetallic phases varied according to the incident laser energies.

Two dynamic compression methods were applied to a monolithic metal and reactive mixtures to investigate their responses: (a) Dynamic experiments using a split Hopkinson pressure bar were applied to reactive mixtures densified by explosive consolidation in order to establish their mechanical response and failure mechanisms. (b) Laser compression and release, which can impart high stresses, up to hundreds GPa, in times of nanoseconds and fractions thereof, was applied to establish the spalling strength of vanadium and the reaction threshold for Ni/Al laminates. The spallation and fragmentation exhibited by recovered mono- and poly-crystalline vanadium prove that the laser intensities and crystal structure play important roles in determining spall strength, fragmentation, and microstructural processes. Densified reactive mixtures with different microstructures (Ni, Mo, W, Nb and Ta with Al) were subjected to the quasi-static and dynamic strain rates. Two distinct failure mechanisms, axial splitting and shear failure, were observed in the recovered specimens. Axial splitting occurred when the bonding between the powders was poor; shear failure was primarily associated with extensive deformation of continuous Ta and Nb phases. Finite element simulations provided valuable information in interpreting the experimental results and predicting failure mechanisms akin to those observed. Ni/ Al laminates were subjected to laser compression. The strain rates varied from 10⁵ to 10⁸ s⁻¹, and the initial stress varied from 30 to 300 GPa. It is found the thickness of the lamellar and the interlaminar bonding strength are the two critical factors in determining mechanical failure. The intermetallic reaction leading to Ni₃Al and NiAl were produced by the laser energies and laser pulse durations in direct laser shock experiments. Laser-driven compression was also applied to study the high temperature synthesis in nano-scale Ni/Al laminates with bilayer thickness 54 nm. Intermetallic phases, NiAl and NiAl₃, were found on the plasma stagnated laminates. However, the self-propagating high temperature synthesis (SHS) did not self-sustain in the micro-scale laminate because of the short duration of the pulse

Reactive Ni/Al laminates (with bilayer thicknesses of 5 and 30μm) were subjected to direct high-intensity laser shock-wave loading. The laser intensity was varied between ∼2.68×1011 and ∼1.28×1013Wcm−2, with two distinct initial pulse durations: 3 and 8ns. Analytical and computational estimations (HYADES) were conducted to simulate the propagation of the shock wave and obtain the initial shock pressure. The thinner bilayer laminate exhibited intense localized interfacial reaction at the higher laser intensity (1.28×1013Wcm−2), but the intermetallic reaction did not propagate through the laminates. The estimated temperature changes inside the sample, cooling rate, and cooling time were calculated by analyzing the intermetallic dendrites. Scanning electron microscopy, electron-dispersive spectroscopy and X-ray diffraction were carried out for identifying the compositions of intermetallic products. Increase in the duration of laser shock wave enhanced the reaction in laminates. It is demonstrated that the methodology of laser shock is suited to investigate the threshold conditions for dynamic mechanical reaction initiation.

The Arapaima gigas scales play an important role in protecting this large Amazon basin fish against predators such as the piranha. They have a laminate composite structure composed of an external mineralized layer and internal lamellae with thickness of 50-60 μm each and composed of collagen fibers with ~1 μm diameter. The alignment of collagen fibers is consistent in each individual layer but varies from layer to layer, forming a non-orthogonal plywood structure, known as Bouligand stacking. X-ray diffraction revealed that the external surface of the scale contains calcium-deficient hydroxyapatite. EDS results confirm that the percentage of calcium is higher in the external layer. The micro-indentation hardness of the external layer (550 MPa) is considerably higher than that of the internal layer (200 MPa), consistent with its higher degree of mineralization. Tensile testing of the scales carried out in the dry and wet conditions shows that the strength and stiffness are hydration dependent. As is the case of most biological materials, the elastic modulus of the scale is strain-rate dependent. The strain-rate dependence of the elastic modulus, as expressed by the Ramberg-Osgood equation, is equal to 0.26, approximately ten times higher than that of bone. This is attributed to the higher fraction of collagen in the scales and to the high degree of hydration (30% H(2)O). Deproteinization of the scale reveals the structure of the mineral component consisting of an interconnected network of platelets with a thickness of ~50 nm and diameter of ~500 nm.

Conventional uniaxial tension tests and laser-shock experiments were carried out to investigate the mechanical properties of aluminum–nickel laminates under quasi-static (∼10−3s−1) and dynamic (∼105s−1) loading conditions. A finite-element code was used to model the experiments and provide insights into the laminates’ mechanical response. It was demonstrated that the geometry of the laminate constituents (i.e. the aluminum and nickel laminae) and the interlaminar bonding are the two critical parameters in determining the accuracy of the numerical calculations. These results are also useful to those interested in improving the mechanical properties of this class of materials since we demonstrate that a material with “perfect” laminae and “perfect bonding” is substantially stronger than the experimental material.

Polycrystalline and monocrystalline (〈100〉 and 〈110〉) vanadium was subjected to shock compression followed by tensile wave release to study spall and fragmentation behavior. The shock pulse was generated by a direct laser drive at energy levels ranging from 11 to 440Jmm–2 (laser beam irradiated area 1.12mm2) and initial pulse durations of 3 and 8ns (approximate initial pressures between 10 and 250GPa). Glass and polycarbonate shields placed at a specific distance behind the vanadium targets were used to collect and analyze the ejected fragments in order to evaluate and quantify the extent of damage. The effects of target thickness, laser energy, polycrystallinity and pulse duration were studied. Calculations show melting at a pressure threshold of ∼150GPa, which corresponds to a laser energy level of ∼180Jmm–2. Consistent with the analytical predictions, the recovered specimens and fragments show evidence of melting at the higher energy levels. Spalling in the polycrystals occurred by a ductile tearing mechanism that favored grain boundaries. In the monocrystals it occurred by a mixture of cleavage fracture along the {010} planes and ductile dimple fracture. This lower spall strength in polycrystals contradicts predictions from the Hall–Petch equation. Experimentally obtained fragment sizes were compared with predictions from the Grady–Kipp model. The spall strength of vanadium under laser loading conditions was calculated from both VISAR pull-back signals and using the spall thickness. It was found to be considerably higher than predictions from gas gun experiments, the monocrystals showing a higher value than polycrystals. This higher spall strength is suggestive of a strong time dependence of the phenomenon, consistent with the nucleation and growth kinetics of voids and the strain rate sensitivity embedded in the Grady theory.

Reactive laminates produced by successive rolling and consisting of alternate layers of Ni and Al (with bi-layer thicknesses of 5 and 30 mu m) were investigated by subjecting them to laser shock-wave loading. The laser intensity was varied between similar to 2.68x10(11)W/cm(2) (providing an initial estimated pressure P similar to 25 GPa) and similar to 1.28x10(13W)/cm(2) (P similar to 333 GPa) with two distinct initial pulse durations: 3 ns and 8 ns. Hydrodynamic calculations (using commercial code HYADES) were conducted to simulate the behavior of shock-wave propagation in the laminate structures. SEM, and XRD were carried out on the samples to study the reaction initiation, and the intermetallic compounds. It was found that the thinner bilayer thickness (5 mu m) laminate exhibited the most intensive localized interfacial reaction at the higher laser intensity (1.28x10(13)W/cm(2)); the reaction products were identified as NiAl and other Al-rich intermetallic compounds. The reaction front and the formation of intermetallic compounds extend into the sample with a thinner bilayer thickness (5 mu m) to a depth of about 50 mu m. Increase in the duration of laser shock wave induces increased reaction, which occurs also in the thicker bilayer laminate samples (30 mu m bi-layer thickness). It is demonstrated that the methodology of laser shock is well suited to investigate the threshold conditions for dynamic mechanical reaction initiation caused by high intensity laser irradiation.

Using the Janus LLNL and Omega facilities, we are using laser energy to generate shock and quasi-isentropic compression of monocrystalline, polycrystalline, and nanocrystalline FCC and BCC metallic specimens(Cu, Ni, V). We have investigated the internal defects generated by experimental and computational (MD) means. By comparing experimentally observed and computationally predicted structures we can obtain new insights into the fundamental deformation mechanisms. We have also investigated the mechanisms of spall initiation, propagation, and fragmentation.

A double-tube implosion geometry is used to explosively shock consolidate intermetallic-forming Ni-Al, Ta-Al, Nb-Al, Mo-Al and W-Al powder mixtures for fabricating bulk structural energetic materials, with mechanical strength and ability to undergo impact-initiated exothermic reactions. The compacts are characterized based on uniformity of microstructure and degree of densification. Mechanical properties of the compacts are characterized over the strain-rate range of 10(-3) to 10(4) s(-1). The impact reactivity is determined using rod-on-anvil experiments, in which disk-shaped compacts mounted on a copper projectile, are impacted against a steel anvil in using a 7.62 mm gas gun. The impact reactivity of the various explosively-consolidated reactive powder mixture compacts is correlated with overall kinetic energy and impact stress to determine their influence on threshold for reaction initiation. The characteristics of the various compacts, their mechanical properties and impact-initiated chemical reactivity will be described in this paper.

A surface-rich platinum/titanium bilayer was deposited on poly(ethylene naphthalate) film by vacuum sputtering as counterelectrode for plastic dye-sensitized solar cells (DSSCs). Compared to the electrodes made of pure Pt layer, this electrode maintained similar electrochemical catalytic effect at relative low Pt usage. Current-voltage characteristics of the plastic DSSC at this stage stand at 0.69V on VOC, 9.97mA∕cm2 on ISC, 0.69 on fill factor, and 4.31% cell efficiency under AM1.5, 100mW∕cm2 illumination.

A PVdF-HFP/PEG/PEGDMA cross-linking film has been prepared as the electrolyte for dye-sensitized solar cell (DSSC). The film can be made porous by controlling the evaporation behavior of solvents. Room temperature ionic conductivity of the micro-porous film exceeds 1mS/cm. In addition, we also evaluated the significance of cell gap in DSSC by analyzing the impedance spectroscopy of the cell with polymer electrolyte. Finally, by decreasing the film thickness, the DSSC equipped with 11μm, micro-porous and cross-linked film showed a conversion efficiency over 4% and 5% under 1 and 0.16Sun, respectively.

Growth kinetics Of CU6Sn5 and Cu3Sn compounds in Cu/Sn thin film couples was studied by in situ resistivity measurements and X-ray diffraction analysis. The Cu6Sn5 and Cu3Sn intermetallic compounds were found to form in sequence when the Cu/Sn bimetallic samples were isochronally annealed from room temperature to 220 degrees C. A kinetic model was presented to explain sequential appearance of the Cu6Sn5 phase and the Cu3Sn phase during the isochronal annealing process. The activation energies of Cu6Sn5 and Cu3Sn compound growth in Cu/Sn thin film couples were determined from the in situ resistivity measurements to be 0.87 eV and 1.05 eV, respectively. (c) 2006 Elsevier B.V. All rights reserved.

Poly(N-vinyl-2-pyrrolidone)-capped platinum nanoclusters were deposited on indium tin oxide glass as a counterelectrode for dye-sensitized solar cells using a “two-step dip coating” process at room temperature. Compared to sputtered-Pt electrodes, an electrode produced by this technique exhibited ultralow Pt-loading at 4.89μg∕cm2 and an acceptable charge-transfer resistance of 5.66Ωcm2. Current-voltage characteristics of the DSSC at this stage stand at 0.66V on VOC, 10.5mA∕cm2 on ISC, 0.41 on fill factor, and 2.84% cell efficiency under AM1.5, 100mW∕cm2 illumination.

Cu 6 Sn 5 and Cu 3 Sn intermetallic compounds are commonly found in the Sn-Cu bimetallic system. Due to the distinct resistivity of these two compounds, the electrical properties of Cu/Sn interfaces, e.g., solder joints on Cu metallization, may be impacted by the formation of Cu-Sn compounds. In this study, the kinetics of Sn-Cu compound formation was investigated by in-situ resistivity measurement, x-ray diffraction, and scanning electron microscopy (SEM). The interfacial reaction of the Cu-Sn bimetallic thin film specimen was monitored by the resistivity change of the specimen during thermal treatment. The activation energy of formation of Cu-Sn compounds was determined to be 0.97±0.07 eV. It is proposed that the Cu 6 Sn 5 compound first forms at Sn/Cu interfaces and then reacts with Cu, forming the Cu 3 Sn compound at elevated temperatures during the thermal ramping process. The effect of thin film thickness on the sequential formation of Sn-Cu compounds is also discussed.