Formation Of Large Voids Research Articles

Evolution of density and velocity profiles of matter in large voids

We analyse the evolution of cosmological perturbations which leads to the formation of large voids in the distribution of galaxies. We assume that perturbations are spherical and all components of the Universe - radiation, matter and dark energy - are continuous media with ideal fluid energy-momentum tensors, which interact only gravitationally. Equations of the evolution of perturbations in the comoving to cosmological background reference frame for every component are obtained from equations of conservation and Einstein's ones and are integrated by modified Euler method. Initial conditions are set at the early stage of evolution in the radiation-dominated epoch, when the scale of perturbation is mush larger than the particle horizon. Results show how the profiles of density and velocity of matter in spherical voids with different overdensity shells are formed.

PITS FORMATION FROM VOLATILE OUTGASSING ON 67P/CHURYUMOV–GERASIMENKO

We investigate the thermal evolution of comet 67P/Churyumov-Gerasimenko's subsurface in the Seth_01 region, where active pits have been observed by the ESA/Rosetta mission. Our simulations show that clathrate destabilization and amorphous ice crystallization can occur at depths corresponding to those of the observed pits in a timescale shorter than 67P/Churyumov-Gerasimenko's lifetime in the comet's activity zone in the inner solar system. Sublimation of crystalline ice down to such depths is possible only in the absence of a dust mantle, which requires the presence of dust grains in the matrix small enough to be dragged out by gas from the pores. Our results are consistent with both pits formation via sinkholes or subsequent to outbursts, the dominant process depending on the status of the subsurface porosity. A sealed dust mantle would favor episodic and disruptive outgassing as a result of increasing gas pressure in the pores, while high porosity should allow the formation of large voids in the subsurface due to the continuous escape of volatiles. We finally conclude that the subsurface of 67P/Churyumov-Gerasimenko is not uniform at a spatial scale of similar to 100-200 m.

Microstructure and nonlinear signatures of yielding in a heterogeneous colloidal gel under large amplitude oscillatory shear

We investigate yielding in a colloidal gel that forms a heterogeneous structure, consisting of a two-phase bicontinuous network of colloid-rich domains of fractal clusters and colloid-poor domains. Combining large amplitude oscillatory shear measurements with simultaneous small and ultra-small angle neutron scattering (rheo-SANS/USANS), we characterize both the nonlinear mechanical processes and strain amplitude-dependent microstructure underlying yielding. We observe a broad, three-stage yielding process that evolves over an order of magnitude in strain amplitude between the onset of nonlinearity and flow. Analyzing the intracycle response as a sequence of physical processes reveals a transition from elastic straining to elastoplastic thinning (which dominates in region I) and eventually yielding (which evolves through region II) and flow (which saturates in region III), and allows quantification of instantaneous nonlinear parameters associated with yielding. These measures exhibit significant strain rate amplitude dependence above a characteristic frequency, which we argue is governed by poroelastic effects. Correlating these results with time-averaged rheo-USANS measurements reveals that the material passes through a cascade of structural breakdown from large to progressively smaller length scales. In region I, compression of the fractal domains leads to the formation of large voids. In regions II and III, cluster-cluster correlations become increasingly homogeneous, suggesting breakage and eventually depercolation of intercluster bonds at the yield point. All significant structural changes occur on the micron-scale, suggesting that large-scale rearrangements of hundreds or thousands of particles, rather than the homogeneous rearrangement of particle-particle bonds, dominate the initial yielding of heterogeneous colloidal gels.

Translational and rotational diffusion of flexible PEG and rigid dendrimer probes in sodium caseinate dispersions and acid gels

The dynamics of rigid dendrimer and flexible PEG probes in sodium caseinate dispersions and acid gels, including both translational diffusion and rotational diffusion, were studied by NMR. Above the onset of the close-packing limit (C ∼ 10 g/100 g H2 O), translational diffusion of the probe depended on its flexibility and on the fluctuations of the matrix chains. The PEG probe diffused more rapidly than the spherical dendrimer probe of corresponding hydrodynamic radius. The greater conformational flexibility of PEG facilitated its motion through the crowded casein matrix. Rotational diffusion was, however, substantially less hindered than the translational diffusion and depended on the local protein-probe friction which became high when the casein concentration increased. The coagulation of the matrix led to the formation of large voids, which resulted in an increase in the translational diffusion of the probes, whereas the rotational diffusion of the probes was retarded in the gel, which could be attributed to the immobilized environment surrounding the probe. Quantitative information from PFG-NMR and SEM micrographs have been combined for characterizing microstructural details in SC acid gels.

Nanovoid Formation and Dynamics in He+-Implanted Nanocrystalline Silicon

Helium implantation in single crystal silicon is known to lead, after a proper thermal treatment, to the formation of voids with diameters ranging between 10 nm and 30 nm. Formation of voids is governed by the coalescence of vacancies created by implantation, initially trapping helium atoms. At high temperatures (\({\ge}700^{\circ }\hbox {C}\)), helium leaves the nanobubbles and outdiffuses, while the now empty voids grow in size and eventually change their shape to form tetrakaidecahedra (Wulff construction). In this communication, we report how He+ implantation in heavily boron-doped nanocrystalline silicon shows a completely different dynamics. Annealing at \(500^{\circ }\hbox {C}\) leads to the formation of large voids, located around grain boundaries, along with a large number of nanovoids with an average diameter of 2–4 nm and an estimated density of \(3\times 10^{17}\,\hbox {cm}^{-3}\) distributed throughout the grains. Annealing at higher temperature (up to \(1000^{\circ }\hbox {C}\)) also induces a decrease of the void size with a change in their density, finally accounting to \(2\times 10^{18}\,\hbox {cm}^{-3}\). The high temperature annealing also causes vacancy evaporation down to a depth of 80–100 nm from the outer surface. The possibility of obtaining a stable, uniform distribution of nanometer-sized voids is of major relevance as a novel tool for phonon and electron engineering in thermoelectric materials.

Influence of shadowing effect on morphology and microstructure of silicon thin film in chemical vapor deposition

Influences of gas incident angle () on surface morphology and microstructure of hydrogenated amorphous silicon (a-Si:H) thin films are investigated, which were grown using an oblique angle hot wire chemical vapor deposition (OAD-HWCVD) technique. An exponential relationship between the tan and RMS roughness is observed. The film surface morphology transforms from a self-affine surface into a mounded surface when the incident angle is larger than a critical angle c(60 c 75). Influences of on the microstructural properties of silicon thin films are characterized using Raman scattering and FT-IR measurements. As c, owing to the qusai-local shadowing effect, increasing increases the quantity and size of micro-voids, leading to the decrease of film density and quality. For c, the nonlocal shadowing effect causes the formation of large voids or cracks and the proportion of multi-hydride (SiHn, n 2) increases. Combined with the scaling theory, the relationship between the shadowing effect and the surface morphologies and microstructures of amorphous silicon thin films is discussed.

Advanced Photoassisted Atomic Switches Produced Using ITO Nanowire Electrodes and Molten Photoconductive Organic Semiconductors

Photoassisted atomic switches (PASs) are those produced using photoconductive organic materials. However, the use of solid photoconductive materials and the formation of a large Ag conductive bridge lead to the formation of large voids during the shrinking of Ag conductive bridges; this may degrade the performance of PASs. A low-melting-point organic semiconductor is used as a molten photoconductive material, and self-assembled ITO nanowires are used as transparent electrodes. Stable atomic switching is observed only under light irradiation.

Low-Temperature Pressure-Less Silver Direct Bonding

A light-emitting diode (LED) die and a glass substrate, both of which were coated with a few-micrometers-thick silver layer, was successfully direct-bonded above 200°C in air without any pressure. In this silver direct bonding, the following three factors are essential: 1) keeping surfaces of silver layers clean; 2) bonding with a suitable solvent that can be reduced and has a boiling point near the sintering temperature; and 3) avoiding excessive heating. Oxygen concentration in a sintering atmosphere has a great impact on bonding. The surface of a silver layer shows roughening by abnormal grain growth as increasing oxygen resulting in increase of bonding strength. The abnormal grain growth on the surface of silver effectively contributes to the increase of junction area. Sintering at too high temperature, however, causes the formation of large voids, which migrated towards silver/glass interfaces resulting in the reduction of bonding strength. An LED die bonded at 240°C for 2 h can maintain high bonding strength through a heat run test at 350°C for 2 h. Furthermore, transient thermal resistance of an LED die bonded with the silver direct bonding is lower than that of an LED die bonded by traditional eutectic bonding with Au-Sn alloy. This bonding method can realize high thermal conductivity with low-temperature pressure-less process and can contribute to high-power semiconductor devices.

Magnesium and magnesium-silicide coated silicon nanowire composite anodes for lithium-ion batteries

We synthesized composites consisting of silicon nanowires (SiNWs) coated with magnesium (Mg) and magnesium silicide (Mg2Si) for lithium-ion battery anodes and studied their electrochemical cycling stability and degradation mechanisms. Compared to bare SiNWs, both Mg- and Mg2Si-coated materials show significant improvement in coulombic efficiency during cycling, with pure Mg coating being slightly superior by ∼1% in each cycle. XPS measurements on cycled nanowire forests gave quantitative information on the composition of the SEI layer and showed lower Li2CO3 and higher polyethylene oxide content for coated nanowires, thus revealing a passivating effect towards electrolyte decomposition. Extensive characterization of the microstructure before and after cycling was carried out by scanning- and transmission electron microscopy aided by focused ion beam cross-sectioning. The formation of large voids between the nanowire assembly and the substrate during cycling, causing the nanowires to lose electrical contact with the substrate, is identified as an important degradation mechanism.

Influence of Coke Breeze Positioning on the Sintering Behavior of Pellets and Raw Material Bed with Embedded Pellets

The influence of coke breeze positioning on the sintering behavior of pellets and raw material bed with embedded pellets was investigated. With increasing the coke breeze ratio in the green pellet made from ore and coke mixture fine of –125 μm, the tensile strength of the green and dried pellets decreased. Based on the evaluations under the green and dried pellet equations concerning particle bond, it was presumed these results were caused by the decrease in the contact points between particles due to the reduction of mixture fine’s specific surface area.With increasing the coke breeze ratio in the nucleus pellet, the tensile strength of the burned pellets also decreased. From the change in the porosity up to 900°C, sintering was predominant to the tensile strength. In the temperature from 900°C to 1200°C, the increase of strength became small by adding coke breeze in the nucleus pellet. It was presumed this result was caused by remarkable increase of void after coke combustion.In the sintering of raw material bed with embedded the coke breeze coating pellet, the surrounding temperature of pellet increased and the large void was generated below the pellet. Based on the 2-dimensional numeric evaluation, it was found the heat amounts of the pellet and gas heated by coke combustion were transported to the region under the pellet where the gas flow rate was low. It was considered this heat concentration caused the remarkable melting and large void formation.

Cyclic oxidation behavior of β-NiAlDy alloys containing varying aluminum content at 1200 °C

β-NiAlDy cast alloys containing varying aluminum content were prepared by arc-melting. The microstructures and cyclic oxidation behavior of the alloys at 1200°C were investigated. Grain refinement was achieved by increasing aluminum content in the alloy, which is beneficial to selective oxidation. The Ni–55Al–0.1Dy alloy showed excellent cyclic oxidation resistance due to the formation of a continuous, dense and slow-growing oxide scale. In contrast to this, severe internal oxidation as well as large void formation at the scale/alloy interface occurred in the Ni–45Al–0.1Dy alloy. The aluminum content dependence of the reactive element effects in β-NiAlDy was established that Dy doping strengthened the scale/alloy interface by pegging mechanism in high-aluminum alloys but accelerated internal oxidation in low-aluminum alloys during high-temperature exposure.

Production, structure and properties of twistless carbon nanotube yarns with a high density sheath

Carbon nanotube web drawn from a vertically aligned carbon nanotube forest is converted into a twistless yarn by a rubbing roller system. The yarn consists of a high packing density sheath with carbon nanotubes lying straight and parallel to the yarn axis and a low density core with many microscopic voids. The rubbing motion causes carbon nanotubes in the surface layers of the yarn to move in opposite directions and consequently large shear strains in the intermediate region tear the partially densified carbon nanotube assembly apart, resulting in the formation of large voids in the yarn core. The specific tensile modulus and sonic velocity of the core-sheath structured, twistless carbon nanotube yarns are significantly higher than that of their corresponding twist-densified yarns. These improvements have been attributed to increased nanotube-to-nanotube contact length in the high packing density sheath and very few carbon nanotubes lying at an angle to the yarn axis.

The importance of the energetic species in pulsed laser deposition for nanostructuring

This work reports on the optical and structural properties of nanostructured films formed by Ag nano-objects embedded in amorphous aluminium oxide (a-Al2O3) prepared by alternate pulsed laser deposition (PLD). The aim is to understand the importance of the energetic species involved in the PLD process for nanostructuring, i.e. for organizing nanoparticles (NPs) in layers or for self-assembling them into nanocolumns (NCls), all oriented perpendicular to the substrate. In order to change the kinetic energy of the species arriving at the substrate, we use a background gas during the deposition of the embedding a-Al2O3 host. It was produced either in vacuum or in a gas pressure (helium and argon) while the metal NPs were always produced in vacuum. The formation of NPs or NCls is easily identified through the features of the surface plasmon resonances (SPR) in the extinction spectra and confirmed by electron microscopy. The results show that both the layer organization and self-assembling of the metal are prevented when the host is produced in a gas pressure. This result is discussed in terms of the deceleration of species arriving at the substrate in gas that reduces the metal sputtering by host species (by ≈58%) as well as the density of the host material (by ≥19%). These reductions promote the formation of large voids along which the metal easily diffuses, thus preventing organization and self-organisation, as well as an enhancement of the amount of metal that is deposited.

Effect of large void formation on strength of cemented glass beads

Gas hydrate dissociation can generate large amounts of gas and water in gas hydrate bearing sediments, which may eventually escape from the soil skeleton and form local voids within the sediments. The loss of fine particles between coarse particles or collapse of cementation due to wetting during heavy or continuous rainfall may form large voids within the soil structure. In this study, the effect of local void formation arising from gas hydrate dissociation, loss of fine particles, or dissolution of vanishing materials is examined in terms of the unconfined compressive strength. Glass beads, cement, and water are mixed and compacted into a cylindrical specimen with five equal layers to simulate the base soil structure. Empty capsules used for medication are embedded in the middle of five equal layers to mimic relatively large voids that are disconnected and permanent. A series of unconfined compression tests was performed on capsule-embedded cemented glass beads by varying the number and direction of capsules in each layer. The unconfined compressive strength of cemented glass beads with capsules decreased up to 35% compared to the case without capsules due to increasing void formation, which depends on the quantity and direction of the embedded capsules. The results of this study can be used to predict the reduction in the strength of soils due to locally disconnected large voids.

Yield asymmetry and fracture behavior of Mg–3%Al–1%Zn–(0–1)%Sr alloys extruded at elevated temperatures

Tensile and compression experiments were carried on Mg–3Al–1Zn alloys containing different levels of Sr (0.05, 0.4 and 0.8 wt%) extruded at elevated temperatures of 250, 300, 350 and 400 °C and the results were compared to AZ31 alloy. The addition of 0.05 wt%Sr to the alloy produces the highest strength and the alloy containing 0.4 wt%Sr has the highest elongation during tensile tests. High levels of Sr (0.8 wt%) reduce the yield strength by decreasing the solid solubility of Al in α-Mg and thereby decreasing the solute pining effect of dislocations under both compression and tension loading. The ultimate tensile strength (UTS) is also reduced due to the cracking of Al–Sr precipitates perpendicular to the tension direction. Cracking is also seen in compressed samples; however, the cracks which initiate are parallel to the compression axis, which impedes crack opening and the formation of large voids. As a result, fracture in compression becomes less sensitive to high concentrations of Sr. It is found that increasing the extrusion temperature increases the yield asymmetry and the improvement in yield symmetry is less significant in alloys containing the high levels of Sr.

A novel ultrafine-grained Ni 3Al with increased cyclic oxidation resistance

An ultrafine-grained (UFG) Ni 3Al was fabricated by annealing an electrodeposited Ni–Al composite in vacuum at 600 °C for 2 h. The UFG Ni 3Al, compared to a compositional-similar but coarse-grained (CG) alloy prepared by arc-melting, exhibited a greatly increased cyclic oxidation resistance at 900 °C. Microstructural investigation showed that the CG alloy grew a scale with a high susceptibility to buckling and cracking because of the formation of large voids at the scale/metal interface, but that the UFG alloy grew an adherent scale, because its typical structure prevented the formation of the interface void during oxidation.

Comment on “Formation of Large Voids in the Amorphous Phase-Change MemoryGe2Sb2Te5Alloy”

A Comment on the Letter by Zhimei Sun, Jian Zhou, Andreas Blomqvist, Borje Johansson, and Rajeev Ahuja, Phys. Rev. Lett. 102, 075504 (2009). The authors of the Letter offer a Reply. (See also preceding Comment and Reply in this issue.)

Comment on “Formation of Large Voids in the Amorphous Phase-Change MemoryGe2Sb2Te5Alloy”

A Comment on the Letter by Zhimei Sun, Jian Zhou, Andreas Blomqvist, Borje Johansson, and Rajeev Ahuja, Phys. Rev. Lett. 102, 075504 (2009). The authors of the Letter offer a Reply. (See also following Comment and Reply in this issue.)

Fluxless Bonding of Ni-P/Cu Plated Al Alloy and Cu Alloy with Lead-Free Sn-Cu Foil

To improve the joint strength of the joint of the Ni-P/Cu plated Al alloy and the Cu alloy for a water cooling system, fluxless bonding was conducted with Sn-Cu solder foil in a vacuum. In the joint bonded at 250°C, fracture easily occurs at the joint interface due to insufficient interfacial reaction and shear strength is relatively low. In the joint bonded at 350°C, the excess growth of the Cu 3 Sn layer in the Cu 6 Sn 5 /Cu interface and the large void formation in the joint degrade shear strength. On the contrary, in the joint bonded at 300°C, both the excess growth of the Cu 3 Sn layer and the large void formation are prevented, and thus shear strength becomes relatively high. When bonding was conducted under the optimum conditions, which are bonding temperature of 300°C, bonding time of 20 min and applied stress of 0.2 MPa, shear strength became 33.4 MPa. This value is approximately four times that of the joint bonded with flux.

Formation of Large Voids in the Amorphous Phase-Change MemoryGe2Sb2Te5Alloy

On the basis of ab initio molecular dynamics simulations, large voids mainly surrounded by Te atoms are observed in molten and amorphous Ge2Sb2Te5, which is due to the clustering of two- and threefold coordinated Te atoms. Furthermore, pressure shows a significant effect on the clustering of the under coordinated Te atoms and hence the formation of large voids. The present results demonstrate that both vacancies and Te play an important role in the fast reversible phase transition process.