Showing papers by "Marc A. Meyers published in 2009"

Interfacial shear strength in abalone nacre.

Abstract: The shear strength of the interface between tiles of aragonite in the nacre of red abalone Haliotis rufescens was investigated through mechanical tensile and shear tests. Dog-bone shaped samples were used to determine the tensile strength of nacre when loaded parallel to the plane of growth; the mean strength was 65 MPa. Shear tests were conducted on a special fixture with a shear gap of 200 microm, approximately 100 microm narrower than the spacing between mesolayers. The shear strength is found to be 36.9+/-15.8 MPa with an average maximum shear strain of 0.3. Assuming the majority of failure occurs through tile pull-out and not through tile fracture, the tensile strength can be converted into a shear strength of 50.9 MPa. Three mechanisms of failure at the tile interfaces are discussed: fracture of mineral bridges, toughening due to friction created through nanoasperities, and toughening due to organic glue. An additional mechanism is fracture through individual tiles.

The role of dislocations in the growth of nanosized voids in ductile failure of metals

Abstract: Dislocations are the most important element in our understanding of the mechanical response of metals. Their postulation in 1934 led to revolutionary advances in our ability to predict the mechanical behavior of materials. The authors recently advanced a dislocation mechanism for void growth in ductile metals. This paper reviews the analytical and atomistic calculations carried out in support of this model. The emission of shear dislocation loops, nucleated at the surface of nanosized voids, is responsible for the outward flux of matter, promoting void growth. This is a new paradigm in the initiation of void growth, which was attributed to convergent vacancy diffusion or to prismatic loops by others. The analytical treatment is based on the emission of a dislocation from a void in the plane along which the shear stresses are maximum. Molecular dynamics calculations performed for different orientations of the tensile axis show how the loops generate and expand outward. These loops involve the emission of partial dislocations and are the counterpart for voids of the Ashby geometrically necessary shear loops postulated for rigid particles. This process is demonstrated for bicrystalline and nanocrystalline copper.

Chapter 89 Dislocations in Shock Compression and Release

Abstract: Shock-wave compression is an extreme regime of deformation characterized by strain rates on the order of 106–1010 s−1, pressures ranging from a few to hundreds of GPa, and shock and residual temperatures that can exceed the melting point. These shock waves can be generated by several means of rapid energy deposition at the material surface. The most common are detonation of explosives in contact with surface, impact of a flyer plate with surface (accelerated by explosives, by compressed gases in gun, or by lasers), or direct laser irradiation. The principal dislocation structures observed after shock-wave compression are illustrated and the principal mechanisms of dislocation generation in shock compression are discussed, with their relative merits and limitations. The effect of polycrystallinity on the shock-wave configuration and on the defects generated is presented. Polycrystallinity creates shock-front irregularities which can contribute to greater dislocation generation. Recent work, in which both shock or isentropic compression was applied to FCC metals, is reviewed. The microstructure changes from cells to stacking faults and eventually to deformation twins as the pressure is increased. An analytical model for the transition from slip to twinning under shock and isentropic compression is presented; a model for the transition from cells to stacking faults is also presented. The results of atomistic simulations are presented and compared with experimental measurements. Interestingly, the dislocation spacing predicted by molecular dynamics (MD) is orders of magnitude lower than transmission electron microscopy measurements on recovered specimens. MD computations reveal that a significant portion of the dislocations generated in shock compression is annihilated when the pressure decays back to the ambient value. Hence, one can postulate that most dislocations generated at the shock front are annihilated upon unloading. The reflections of shock waves at free surfaces generate tensile pulses that can initiate spalling and fragment the back of the specimens if of sufficient amplitude. A mechanism for the early growth of these voids through the emission of shear loops from the void surface and their expansion, is presented.

Analysis and characterization by electron backscatter diffraction of microstructural evolution in the adiabatic shear bands in Fe-Cr-Ni alloys

Abstract: The microstructural evolution inside adiabatic shear bands in Fe-Cr-Ni alloys dynamically deformed (strain rates > 104 s−1) by the collapse of an explosively driven, thick-walled cylinder under prescribed strain conditions was examined by electron backscatter diffraction. The observed structure within the bands consisted of both equiaxed and elongated grains with a size of ∼200 nm. These fine microstructures can be attributed to recrystallization; it is proposed that the elongated grains may be developed simultaneously with localized deformation (dynamic recrystallization), and the equiaxed grains may be formed subsequently to deformation (static recrystallization). These recrystallized structures can be explained by a rotational recrystallization mechanism.

Uniaxial Freezing, Freeze‐Drying, and Anodization for Aligned Pore Structure in Dye‐Sensitized Solar Cells

Abstract: A variety of methods are available for creating the titanium dioxide (TiO2) semiconductor surface layer of dye-sensitized solar cells (DSSCs); however, many of them are used independently to create surface morphologies that are influenced by only one process. A series of experimental techniques are utilized, some not originally used for thin film preparation, to create a semiconductor surface that exhibits variations in morphology on the macro-, micro-, and nanoscales. The techniques used to create the micro- and nanostructures are uniaxial freezing, freeze-drying, and anodization or etching, combined with the macrostructural techniques of the doctor blade method, screen printing, and/or electrophoretic deposition. When several of these techniques are used together to create, and modify, a thin film for DSSC, these techniques can produce a TiO2 semiconductor layer for DSSC that has very high current and voltage characteristics, and a surface morphology more complex than can be created by using any one of the techniques alone. YE-SENSITIZED solar cells (DSSCs) are comprised of three major and two minor components: the semiconductor base layer, the dye, the electrolyte, and the two electrodes, respectively. Titanium dioxide (TiO2) plays a crucial role as the semiconductor base layer material, and its main structural function is to provide a substrate for organic dye attachment, and also to provide a barrier between the charge transfer electrolyte and the conducting glass anode (to prevent charge recombination— where electrons return to the electrolyte instead of traveling to the electrode). Electrochemically, the TiO2 semiconductor accepts electrons injected by the organic dye, and transfers them to the conducting glass anode, with a negligible loss of energy. 1 Other semiconductors of similar nature (ZnO, etc.) can be used, but TiO2 is the focus of most current research. This is due to the close match of the band gap of TiO2 to energy of the electrons emitted by many of the metallo-organic dyes that are readily available on the market, and the ability of these dyes to attach with chelation. 2 Many methods are used to produce the TiO2 surface for DSSCs, including electrophoretic deposition (EPD), screen printing, and doctor blade deposition. The goal of each of these methods is to produce a semiconductor layer whose surface maximizes the amount of dye that can attach, while minimizing the recombination losses. The doctor blade technique was the first technique 3 used to create the semiconductor band gap layer of DSSCs, and is the least sophisticated. The technique consists of using a flat, sharp object (like a razor blade) or a round, thin object (like a glass stirrer) to lay a thin layer of slurry, with a thickness determined by a spacer. The simplest spacer is adhesive tape, which is between 8 and 10 mm thick on average, 4 and it is placed on opposite sides of the area where the film is to be laid, and the doctor blade is dragged across. The surface structure produced by this method is flat, with variations based on rolling technique, flatness of the spacing material, and position of the blade during the process. One of the drawbacks to this method is that there is limited control over the properties of the final surface and its microstructure, and there is no direct way to manipulate the morphology at the nanoscale. Another drawback is that the thickness of the film is directly related to the thickness of the spacers, and it is difficult to find appropriate spacing material with the desired thickness of 1–7 mm. 5 Overall, the doctor blade method is rather limited to produce the complex surface needed for DSSC to maximize dye attachment.

Explosive compations of intermetallic‐forming powder mixtures for fabricating structural energetic materials

Abstract: 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 micro structure and degree of densification. Mechanical properties of the compacts are characterized over the strain‐rate range of 10−3 to 104 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 w...

Laser compression of nanocrystalline metals

Abstract: Shock compression in nanocrystalline nickel is simulated over a range of pressures (10–80 GPa) and compared with experimental results. Laser compression carried out at Omega and Janus yields new information on the deformation mechanisms of nanocrystalline Ni. Although conventional deformation does not produce hardening, the extreme regime imparted by laser compression generates an increase in hardness, attributed to the residual dislocations observed in the structure by TEM. An analytical model is applied to predict the critical pressure for the onset of twinning in nanocrystalline nickel. The slip‐twinning transition pressure is shifted from 20 GPa, for polycrystalline Ni, to 80 GPa, for Ni with g. s. of 10 nm. Contributions to the net strain from the different mechanisms of plastic deformation (partials, perfect dislocations, twinning, and grain boundary shear) were quantified in the nanocrystalline samples through MD calculations. The effect of release, a phenomenon often neglected in MD simulations, o...

Laser Shock Compression and Spalling of Reactive Ni-Al Laminate Composites

Abstract: Reactive laminates produced by successive rolling and consisting of alternate layers of Ni and Al (with bi‐layer thicknesses of 5 and 30 μm) were investigated by subjecting them to laser shock‐wave loading. The laser intensity was varied between ∼2.68×1011 W/cm2 (providing an initial estimated pressure P∼25 GPa) and ∼1.28×1013 W/cm2 (P∼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 μm) laminate exhibited the most intensive localized interfacial reaction at the higher laser intensity (1.28×1013 W/cm2); 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 wit...

Bioinspired Inorganic/polymer Thin Films

Abstract: Studies of hard biological materials such as marine shells, animal teeth, horns and bones have produced fascinating ideas for mimicking their micro/nanostructure in the lab. The nacre in the abalone shell has a well-defined organic/inorganic structure that has a fracture resistance that is much higher than the individual constituents. By using biocompatible materials we have fabricated zirconium nitride/ polymethylmethacrylate alternating layers that are based on the structure of nacre. A combination of DC-magnetron sputtering and pulsed laser deposition on (100) silicon substrates was used to fabricate multilayers in a single chamber without breaking the vacuum. The ZrN films showed nanocrystalline columnar growth on the silicon substrates or on the PMMA nanolayer. High resolution SEM analysis at the inorganic/organic interface revealed well formed, uniform thickness inorganic films which are separated by the polymeric layer (30-90 nm). The ratio of the ceramic/polymer is the same as in nacre. Nanoindentation hardness values of ˜ 20GPa were measured on both the ZrN single film, similar to published values, and the ZrN/PMMA composite layers and the elastic modulus remained constant, independent of the number of layers.