Void Morphology Research Articles

Plasmonic Band Gaps and Trapped Plasmons on Nanostructured Metal Surfaces

Nanostructured metal surfaces comprised of periodically arranged spherical voids are grown by electrochemical deposition through a self-assembled template. Detailed measurements of the angle- and orientation-dependent reflectivity reveal the spectral dispersion, from which we identify the presence of both delocalized Bragg and localized Mie plasmons. These couple strongly producing bonding and antibonding mixed plasmons with anomalous dispersion properties. Appropriate plasmon engineering of the void morphology selects the plasmon spatial and spectral positions, allowing these plasmonic crystal films to be optimized for a wide range of sensing applications.

Experimental observations of void growth in the Zr–2.5Nb pressure tube alloy

Voiding during tensile loading of Zr–2.5Nb pressure tube alloy has been experimentally observed at two temperatures. Tensile specimens cut from the wall of a pre-service pressure tube were tested to various degrees of strain and then sectioned to allow internal voids to be examined. A point counting procedure was used to determine the void volume fraction at various levels of strain. Scanning electron microscopy was completed to determine the degree of voiding, the number and size of voids and the void morphology. Observed voids generally remain small while the overall void fraction increased with increased strain. This suggests that the void formation process in this material is dominated by extensive void nucleation, with little growth of the nucleated voids before void coalescence and final fracture.

Three-dimensional features of void morphology in resin transfer molded composites

Detailed analyses of shape, size, and spatial variations of void morphology are presented for a disk-shaped, resin transfer molded (RTM), E-glass/epoxy composite. The disk is molded at constant injection rate and contains 17.5% E-glass random fiber mats. Voids throughout the composite are evaluated by microscopic image analysis of through-the-thickness and planar surfaces obtained from adjacent radial samples. The void content of 2.15% is obtained from the analysis of through-the-thickness images and believed to be representative of the actual void content in the studied part. Relatively large cylindrical voids are observed in cigar shapes in the planar surfaces, whereas these voids only appear as small irregular or elliptical voids on through-the-thickness surfaces. Along the radial direction, combined effects of void formation by mechanical entrapment and void mobility are shown to yield a complex radial void distribution. It is shown that fewer voids are trapped mechanically with increasing distance from the inlet and most of the medium and small voids that are mobile migrate towards the exit during resin injection. These findings are believed to be applicable not only to RTM, but generally to liquid composite molding processes with varying fluid front velocities.

Effect of packing on void morphology in resin transfer molded E-glass/epoxy composites

Effects of applying a packing pressure on void content, void morphology, and void spatial distribution were investigated for resin transfer molding (RTM) E-glass/epoxy composites. Packing pressures of zero and 570 kPa were respectively applied to center-gated composites containing 17.5% randomly oriented, E-glass fiber preform. Radial samples of these disk-shaped composites were utilized to evaluate voidage via microscopic image analysis. Two adjacent surfaces were cut from each molded disk in order to evaluate void presence from both through-the-thickness and planar views. The packed composite was found to contain almost 92% less void content than the unpacked composite. While void fractions of 2.2 and 2.6% were measured, respectively, from the through-the-thickness and planar surfaces of the unpacked composite, only 0.2% void content was observed in the packed composite from both surfaces. Digital images obtained from through-the-thickness surface showed that average void size dropped from 59.3 μm in the unpacked composite to 31.7 μm in the packed composite. A similar reduction in average void size from 66.7 to 41.1 μm was observed from the planar surfaces. Circular voids were found to experience higher removal rates at 99%, followed by cylindrical and elliptical voids at 83 and 81%, respectively; while irregular voids show slightly lower void removal rates at 67%. Void proximity to fiber bundles was also observed to affect void reduction as voids located inside fiber tows experience lower void reduction rates. Along the radial direction of the molded disks, removal of voids with different proximities to fibers seems to depend on their arrangement at the end of the filling stage. These findings are believed to ascertain packing as an effective void removal method for RTM and similar liquid composite molding processes. POLYM. COMPOS., 26:614–627, 2005. © 2005 Society of Plastics Engineers

Steam oxidation of ferritic steels – laboratory test kinetic data

The oxidation resistance of 9–12% chromium steels in steam-containing environments simulating the service conditions of steam power plant has been investigated for exposure times ranging from 1 h up to 10 000 h. In the long-term experiments the steels were evaluated by the mass changes determined in exposure tests, which were interrupted every 250 h, thus providing information concerning the scale growth and spalling characteristics. The morphologies of the oxide scales were examined using optical microscopy, scanning electron microscopy, secondary neutrals mass spectrometry and Raman spectroscopy. Different mechanisms of oxidation were observed for the various steels in different temperature regimes and exposed for different durations. For some steels, the steam oxidation resistance increased with increasing exposure temperature. The scale thickness at which spalling was observed varied according to the exposure temperature and the interval between thermal cycles, so that a critical scale thickness for spalling cannot be derived. The cracking and spallation of scales was correlated with the type, morphology and growth of pores and voids in the scale and could be influenced by the steel microstructure.

Morphology of Voids in Molecular Systems. A Voronoi−Delaunay Analysis of a Simulated DMPC Membrane

A generalized version of the Voronoi-Delaunay method is used to study relatively large intermolecular voids in a model of the hydrated DMPC bilayer, obtained from all-atom Monte Carlo simulation. Application of the original version of the method for molecular systems has been hampered by the fact that these systems geometrically represent ensembles of partially overlapping spheres of different radii. The generalized version of the method is based on using the additively weighed Voronoi diagram, representing the locus of spatial points being equally far from the surface rather than the center of the corresponding pair of atoms. This version of the Voronoi-Delaunay method can be readily used to reveal and analyze voids accessible for probes of different radii even in rather complex molecular systems. When the properties of the voids present in the simulated DMPC membrane are investigated, their shape, size, and orientation have been analyzed in detail in the different regions of the membrane located at different depths along the membrane normal axis. The characteristics of the voids are found to be different in different regions of the bilayer, namely (i) at the middle of the membrane, in the region of the hydrophobic lipid tails, (ii) in the region of the hydrophilic zwitterionic headgroups, and (iii) in the region of the bulklike water adjacent to the bilayer. The largest and oblong voids are found in the middle of the membrane, with a preferred orientation that is parallel to the bilayer normal axis. A clear correlation between the orientation of the voids and the orientation of the lipid chains is observed. In the bulk water region the fraction of the empty space is even higher than at the middle of the membrane; however, here the voids are distributed more uniformly. Finally, in the high-density region of the hydrophilic headgroups the voids are found, on average, smaller than in the other parts of the system.

Formation of Microscopic Voids in Resin Transfer Molded Composites

Performance of composite materials usually suffers from process-induced defects such as dry spots and microscopic voids. While effects of void content in molded composites have been studied extensively, knowledge of void morphology and spatial distribution of voids in composites manufactured by resin transfer molding (RTM) remains limited. In this study, through-the-thickness void distribution for a disk-shaped, E-glass/epoxy composite part manufactured by resin transfer molding is investigated. Microscopic image analysis is conducted through-the-thickness of a radial sample obtained from the molded composite disk. Voids are found to concentrate primarily within or adjacent to the fiber preforms. More than 93% of the voids are observed within the preform or in a so-called transition zone, next to a fibrous region. In addition, void content was found to fluctuate through-the-thickness of the composite. Variation up to 17% of the average void content of 2.15% is observed through-the-thicknesses of the eight layers studied. Microscopic analysis revealed that average size of voids near the mold surfaces is slightly larger than those located at the interior of the composite. In addition, average size of voids that are located within the fiber preform is observed to be smaller than those located in other regions of the composite. Finally, proximity to the surface is found to have no apparent effect on shape of voids within the composite.

Spatial variation of void morphology in resin transfer molded E-glass/epoxy composites

Due to their light weight and high performance, fiberreinforced composites are being increasingly used in various commercial products in aerospace and automotive industries. Emerging new methods such as liquid composite molding (LCM) and resin transfer molding (RTM) offer an economic, versatile, and efficient means for manufacturing geometrically complex, large composites. RTM process consists of injecting a thermosetting polymer into a mold cavity preloaded with a dry reinforcing preform, which is most often made of E-glass fibers. In RTM, voids and dry spots are reported to originate primarily from mechanical entrapment during mold filling [1, 2]. This entrapment is believed to arise from uneven movement of flow front due to differences in permeability between the fiber tows and the surrounding pore space. Furthermore, several process parameters such as injection pressure, vacuum assistance, fibers surface treatment, and resin viscosity have been shown to influence the overall voidage, as well as the void morphology defined by the size and shape of voids within the composite [3]. Adverse effects of voids in composites have been well documented. For example, a void content as low as 1% is known to result in a decrease in strength up to 30% in bending, 3% in tension, 9% in torsional shear, and 8% in impact [4]. In addition, void presence is shown to affect both the rate and the equilibrium level of moisture absorption [5]. Voids with irregular shapes are also known to induce early crack formation [6]. However, detailed morphology analyses together with spatial variation of voids in resin transfer molded composites are not available. In this work, we present the radial and through-the-thickness variation of void morphology in a resin transfer molded, E-glass/epoxy composite disk. A resin transfer molded composite disk having 152.4 mm diameter is fabricated using a mixture of epoxy resin EPON 815C and curing agent EPICURE 3282 (Shell Chemicals). The molding is achieved by injecting the mixture into a center-gated mold cavity at a constant flow rate of 5.32 cm3/s. The mold cavity is preloaded with 4 layers of E-glass preform (FiberGlast part #250) yielding a fiber volume fraction of 18.1% in a 3.84 mm thick disk. The transient inlet pressure recorded during mold filling is presented in Fig. 1. During filling, inlet pressure

Microvoid Defects in Nitrogen- and/or Carbon-doped Czochralski-grown Silicon Crystals

Microvoid defects in nitrogen- and/or carbon-doped Czochralski-grown silicon crystals grown under various growth conditions were investigated by transmission electron microscopy. The morphology and volume of the void defects depended strongly on the nitrogen concentration and the cooling rate. With increasing nitrogen concentration, the shape of the voids changed from {111} octahedron to {111} parallelepiped-plate or -rod via an unfaceted structure. The average volume of the voids decreased exponentially with the increase in nitrogen concentration. On the other hand, multiple voids consisting of octahedral segments, whose average volume is small, were observed in carbon-doped crystals. In nitrogen- and carbon-codoped crystals, multiple voids consisting of plate and rod segments were observed, whose average volume was very small. The thickness of the inner oxide layers of voids was influenced by the cooling rate, and not by nitrogen and carbon. From these results, it was assumed that nitrogen and carbon play different roles in the void formation. The change of void morphology by nitrogen-doping is discussed in terms of anisotropic void growth.

Influence of Hydrogen Plasma Treatment on He Implantation-Induced Nanocavities in Silicon

ABSTRACTHe implantation followed by thermal anneal is a well-established technique for creating layers or bands of cavities in silicon. This process is a consequence of the interaction between He and ion-implant-induced vacancies. Applications of such cavity layers include gettering and localized minority carrier lifetime control, and compliant substrates for lattice-mismatched heteroepitaxy. Studies have shown that the presence of interstitial-type defects can lead to the shrinkage of He-cavities due to the interstitial capture by the cavities. However, few of them deal with the interaction of cavities with vacancies. Here we present results on the formation of He-cavities in Si in the presence of atomic hydrogen and vacancies produced by effusion of hydrogen. Following a helium implant, samples were hydrogenated with an electron cyclotron resonance (ECR) hydrogen plasma. Control samples without any hydrogenation were also used. We studied the influence of hydrogen on void morphology. While hydrogen enhances void size at higher energy implants, the enhancement effect is absent in lower energy implants. The results underscore the role of vacancies in void formation and growth.

Multiple small-angle neutron scattering studies of anisotropic materials

Building on previous work that considered spherical scatterers and randomly oriented spheroidal scatterers, we describe a multiple small-angle neutron scattering (MSANS) analysis for nonrandomly oriented spheroids. We illustrate this with studies of the multi-component void morphologies found in plasma-spray thermal barrier coatings.

Effect of oxygen precipitation on voids in bulk silicon

The effect of oxygen precipitation on flow pattern defects (FPDs) relative to voids in bulk silicon was investigated. FPDs and voids were measured after different annealing time, and with various interstitial oxygen contents and bulk stacking faults (BSFs). It was found that, during annealing, interstitial oxygen in bulk silicon precipitated, however the FPDs density remained constant. The density of BSFs increased with annealing time at 1100 °C. After annealing, the morphology of voids was found to be octahedral with the oxide films inside them, the same as as-grown samples. Therefore, it is concluded that the interstitial silicon atoms released during oxygen precipitation did not enter the voids because of the resistance of the oxide films inside the voids, but condensed into stacking faults. The dissolution of the oxide films is believed to be the principal cause for the annihilation of voids.

Corrosion-Related Interfacial Defects Formed by Dissolution of Aluminum in Aqueous Phosphoric Acid

The mechanism was investigated by which pit initiation on aluminum foils during anodic etching is affected by the use of phosphoric acid as a pretreatment. Positron annihilation measurements, coupled with atomic force microscope images of foils with chemically stripped oxide layers, show evidence that the pretreatment introduces nanometer-scale voids in the metal, at or near the metal-oxide film interface. The location and morphology of voids compares favorably with those of pits, suggesting that voids act as pit initiation sites. The number of void sites was estimated to be the same magnitude as the maximum number of pits formed by anodic etching. Capacitance measurements further indicate that the treatment decreases the surface oxide thickness to about 2 nm. Formation of large numbers of pits during etching is promoted by either reduced oxide thicknesses or more positive etching potentials. It is suggested that the rate of initiation of pits at interfacial voids is determined by the electric field in the overlying surface oxide. © 2002 The Electrochemical Society. All rights reserved.

The effect of contact resistance on current crowding and electromigration in ULSI multi-level interconnects

Electromigration (EM) has been the most persistent interconnect reliability issue over the decades. In general, EM damages tend to occur at atomic flux divergence sites. The EM failure rate can be further accelerated by current crowding, which occurs when current flows between inter-level wires. In this work, we used two different via etch schemes to study the effect of contact resistance on current crowding and EM. We found that the etch stop structures show longer EM lifetimes than the over etch. The contact resistance of the etch stop is higher than that of the over etch. Two-dimensional simulation results show that the higher contact resistance in the etch stop can suppress current crowding and improve EM lifetimes. Differences in void morphology between the over etch and etch stop as a result of current crowding are discussed.

Void formation and morphology in NiAl

Void formation in stoichiometric NiAl was studied through controlled heat treatments and microstructural characterization through transmission electron microscopy. Voids were observed to form at temperatures as low as 400°C, but were noted to dissolve during annealing at 900°C. Two distinct void shapes, cuboidal and rhombic dodecahedral, were observed, often at the same annealing temperature. At higher temperatures (≥800°C) extensive dislocation climb, rather than void formation, was noted. The relative incidence of void formation and dislocation climb can be related to the mobility of vacancies at each annealing temperature. The shape of void type is described in terms of their relative surface energy and mode of nucleation.

The size and shape of local voids

We study the size and shape of low density regions in the local universe which we identify in the smoothed density field of the PSCz flux limited IRAS galaxy catalogue. After quantifying the systematic biases that enter in the detection of voids using our data set and method, we identify, using a smoothing length of 5 $h^{-1}$ Mpc, 14 voids within 80 $h^{-1}$ Mpc and using a smoothing length of 10 $h^{-1}$ Mpc, 8 voids within 130 $h^{-1}$ Mpc. We study the void size distribution and morphologies and find that there is roughly an equal number of prolate and oblate-like spheroidal voids. We compare the measured PSCz void shape and size distributions with those expected in six different CDM models and find that only the size distribution can discriminate between models. The models preferred by the PSCz data are those with intermediate values of $\sigma_{8} (\simeq 0.83)$, independent of cosmology.

Direct observation of void morphology in step-like electromigration resistance behavior and its correlation with critical current density

Electromigration-induced resistance behavior of via-terminated metal lines was studied. We found a strong correlation between void morphology and step-like resistance behavior. For the lines which exhibit pronounced resistance steps, voids are formed at the top or bottom Al/TiN interface. For the lines which exhibit gradual resistance increases, only complete void formation severing the whole line occurs. The step-like resistance behavior is more pronounced when the current density is close to the critical current density. Possible mechanisms for the resistance steps are discussed.

Estimation of Creep Damage for the Components of Mod. 9Cr-1Mo Steel. 3rd Report, Creep Damage Assessment for Seam Weld in Internally Pressurized Pipe.

In order to establish the assessment method of creep damage for high temperature piping components made of modified 9Cr-1Mo steel with seam weld, subjected to internal pressure, the internal pressure creep tests and FEM stress analysis were done for cylinders with seam weld. In internal pressure creep tests, the creep rupture occurred at fine grained heat affected zone (F. G. HAZ), caused by cracking originated from the formation of voids. The correlation between void area fraction and creep damage was obtained. The morphology of voids were quite similar to that observed in welded joint specimens in uni-axial creep tests. Stress distribution of seam weld cylinder under internal pressure creep showed the higher circumferential stresses in F. G. HAZ where creep rupture occurred. The distribution of circumferential stress and creep damage calculated by using circumferential stress along specimen thickness were quite similar to that of void area fraction in internal pressure creep test. The creep damage in seam weld pipe under internal pressure creep could be estimated by the observation of void area fraction in F. G. HAZ.

An Integrated AFM and SANS Approach toward Understanding Void Formation in Conductive Composite Materials

The morphology of voids within carbon black-filled polyethylene conductive composites has been studied by both atomic force microscopy and small-angle neutron scattering. Both void size and polyethylene crystalline lamellae thickness were measured with both techniques, and the results were compared. Atomic force micrographs indicate that voids form at the carbon black−polyethylene interface as previously hypothesized from the interpretation of neutron scattering experiments alone. This result supports the conclusion that voids provide a stress release mechanism during polyethylene crystallization and that variables such as filler morphology, volume fraction, and polymer crystallinity may be used to quantitatively predict the extent of void incorporation as composite composition is varied.

Surface electromigration in copper interconnects

RF-magnetron sputter deposited copper films have been characterised in terms of microstructure, stress and resistivity. The electromigration behaviour of this copper has been compared to that of sputtered aluminium. Atomic Force Microscopy has been used to study the void sites. The morphology of the damaged regions, together with the microstructural information, has been used to gain an insight into the mechanism by which electromigration voiding proceeds. The observed void sites in copper are entirely different from those in aluminium; hence a different failure mechanism must be responsible. The voiding process for copper can be explained by a grain-boundary grooving model, and it was observed that surface impurities controlled the damage mechanism and the resulting void morphology.