Larger Island Size Research Articles

Influence of hexagonal scanning strategy island size on microstructure, residual stress, and corrosion behavior of 17-4 PH SS made by L-PBF

17-4 Precipitation-hardened steel has attracted worldwide attention due to its applications. In the manufacture of 17-4 PH SS using SLM, several parameters play an important role. In the present study, the hexagonal island sizes were varied as 1.25-2.5-3.75-5-6.75-7.5-8.75-10 mm to understand the microstructure, microhardness, residual stresses, and corrosion behavior of 17-4 precipitation hardened stainless steel fabricated by laser-powder bed fusion with a scan rotation angle of 90°. The results indicate that the average time per layer decreases as the island size increases. The quality of the surface increased with a decrease in hexagonal island size as scan length reduced. The hardness varied with island size and higher hardness was exhibited due to fine grain size. The medium island size microstructure consists of regular molten pools and lower heat accumulation. Large and small molten pools were observed on the front and side surface respectively for samples with different hexagonal island sizes. Austenite phase was detected in all samples with the highest intensity at hexagonal inside out sample with an island size of 2.5 mm at 43.500°. Both hexagonal samples with 2.5 mm island size and 10 mm samples almost exhibit ɣ-austenite phase in the finer grain size area, as per EBSD orientation maps. The residual stresses increased as the hexagonal island size increased because when a large island size is used, it produces a long scanning track with a weak pre-heating effect, resulting in a high-temperature gradient and high residual stress. Higher corrosion resistance was obtained for 2.5 mm hexagonal scan strategy island size because of finer grain size and lower defects.

Influence of scanning strategy and parameter on microstructural feature, residual stress and performance of Sc and Zr modified Al–Mg alloy produced by selective laser melting

Selective laser melting (SLM), due to its track-by-track and layer by layer localized additive manufacturing process nature, has significant requirements for the optimization of laser scanning strategy and parameter. In this work, rare earth element Sc- and Zr- modified Al–Mg alloy was processed by SLM using an “island” scanning strategy with different island sizes. The influence of island size on surface quality, microstructural feature, residual stress, tensile property and corrosion resistance of SLM-processed Al–Mg-Sc-Zr alloy was systematically investigated. Experimental results indicated that the surface smoothness was enhanced as the island size decreased, but the influence of island size on microstructural development and tensile properties was not distinct. Considerably high tensile properties above 500 MPa combined with elongations of 10%–12% were generally obtained for Al–Mg-Sc-Zr specimens processed under different island sizes. Minimum residual stress was generated at a moderate island size of 2.4 mm and the residual stress increased both at a small and a large island size. A similar tendency was also observed for the corrosion resistance of SLM processed Al–Mg-Sc-Zr specimens, where a superior corrosion resistance (icorr = 6.82 × 10−6 A cm−2) was generated at the middle island size. The different temperature gradient and thermal behaviors under different island sizes contribute to the variation of surface conditions and residual stresses. The change in corrosion behaviors under different island sizes was related to the different molten pool configurations. These findings provided the knowledge of the relationship between laser scanning strategy and properties of SLM processed Al–Mg-Sc-Zr alloy, which further facilitated process optimization and engineering applications of laser additive manufacturing of high-performance Al–Mg-Sc-Zr alloy.

Shapes of Fe nanocrystals encapsulated at the graphite surface

We describe and analyze in detail the shapes of Fe islands encapsulated under the top graphene layers in graphite. Shapes are interrogated using scanning tunneling microscopy. The main outputs of the shape analysis are the slope of the graphene membrane around the perimeter of the island, and the aspect ratio of the central metal cluster. Modeling primarily uses a continuum elasticity (CE) model. As input to the CE model, we use density functional theory to calculate the surface energy of Fe, and the adhesion energies between Fe and graphene or graphite. We use the shaft-loaded blister test (SLBT) model to provide independent stretching and bending strain energies in the graphene membrane. We also introduce a model for the elastic strain in which stretching and bending are treated simultaneously. Measured side slopes agree very well with the CE model, both qualitatively and quantitatively. The fit is optimal for a graphene membrane consisting of 2–3 graphene monolayers, in agreement with experiment. Analysis of contributions to total energy shows that the side slope depends only on the properties of graphene/graphite. This reflects delamination of the graphene membrane from the underlying graphite, caused by upward pressure from the growing metal cluster. This insight leads us to evaluate the delamination geometry in the context of two related, classic models that give analytic results for the slope of a delaminated membrane. One of these, the point-loaded circular blister test model, reasonably predicts the delamination geometry at the edge of an Fe island. The aspect ratio also agrees well with the CE model in the limit of large island size, but not for small islands. Previously, we had speculated that this discrepancy was due to lack of coupling between bending and stretching in the SLBT model, but the new modeling shows that this explanation is not viable.

Unusual island formations of Ir on Ge (111) studied by STM

Island formation on the Ir/Ge(111) surface is studied using ultrahigh vacuum scanning tunneling microscopy. Ir was deposited at room temperature onto a Ge (111) substrate with coverages between 0.5 and 2.0 monolayers (ML). The samples were annealed to temperatures between 550 and 800 K, and then cooled prior to imaging. With 1.0 ML Ir coverage, at annealing temperatures 650–750 K, round islands form at locations where domain boundaries of the substrate reconstruction intersect. Both the substrate and the islands display a (3x3)R30∘ reconstruction. Additionally, a novel surface formation is observed where the Ir gathers along the antiphase domain boundaries between competing surface domains of the Ge surface reconstruction. This gives the appearance of the Ir in the domain boundaries forming pathways between different islands. The islands formed at higher annealing temperatures resulted in larger island sizes, which is evidence of Ostwald ripening. We present a model for the islands and the pathways which is consistent with our observations.

Analysis of the growth dependences of silicon-on-sapphire heteroepitaxy

The formation of nanoislands of two forms (dome-shaped and a truncated dome) is experimentally found during the initial stage of silicon-on-sapphire heteroepitaxy. Atomic-resolution images of the silicon islands on sapphire are obtained by transmission electron microscopy. A model is proposed to explain the instability of the shape of islands and its transition from isotropic to anisotropic and to describe the evolution of average island sizes during growth. It is shown that the height dependence of the island diameter is approximately linear; however, for large island sizes there is a significant discrepancy with the experimental data due to island coalescence, which is disregarded in the model.

Kinetic-Dynamic Properties of Different Monomers and Two-Dimensional Homoepitaxy Growth on the Zn-Polar (0001) ZnO Surface

Homoepitaxy ZnO monolayer growth was investigated from dynamics to kinetics taking ZnO molecules and Zn–O cluster monomers into account in the atomistic growth by first-principles calculations and Monte Carlo simulations and compared with experimental growth by molecular beam epitaxy. Theoretically, the ZnO molecules were found to scatter equivalently on both the wurtzite sites (WSs) and zincblende sites (ZSs) and stick even at high temperatures. The Zn3O1 monomers resulted in a larger island size and a higher compact degree and the growth approached to the two-dimensional mode at high temperature; the film structure finalized in the single wurtzite phase structure with more vacancies, which agreed with the in situ scanning tunnel microscopy observation for the growth in Zn-rich conditions to form Zn3O1 monomers. For the Zn1O3 monomers, the transformation from ZSs to WSs was more difficult even with temperature increase and they could locate at both WSs and ZSs, consistent with the in situ reflection high energy electron diffraction for the growth in O-rich conditions to form Zn1O3 monomers. Combining the advantages of both cluster monomers, a two step-growth technique was developed by alternatively supplying Zn and O. The resultant ZnO films exhibited flat texture and uniform phase structure as indicated by the atomic steps in the STM images and the streaky RHEED patterns.

Anomalous Scaling in Heteroepitaxial Island Dynamics on Ag(100)

Diffusion and decay of alloyed Cu/Ag islands are investigated in the size range from 1 to 40 nm2 on Ag(100) at room temperature with fast-scanning tunneling microscopy and density functional theory. While islands at sizes above 7 nm2 show the diffusion and decay behavior expected for dynamics based on single atom hopping, islands smaller than 4 nm2 diffuse faster and decay slower than predicted by standard theory. This anomalous behavior at unexpected large island sizes is related to a size dependent dealloying of the Cu/Ag islands.

New Radiocarbon Dates for the Grenadine Islands (West Indies)

Intensified archaeological research in the Caribbean over the past 2 decades has provided a wealth of new information on how and when these islands were settled prehistorically. However, there has been a paucity of research on islands in the southern Lesser Antilles, which would allow for more rigorous testing of migration models and various settlement pattern hypotheses. To address some of these chronological and geographical gaps, we present a corpus of 41 radiocarbon dates from several sites in the Grenadine Island chain. Results to date support a relatively late Ceramic Age settlement of these smaller islands (about AD 400) compared to other nearby, larger islands in the southern Lesser Antilles (about AD 200) as well as the Caribbean as a whole (about 400/500 BC). Intriguing questions also remain as to an apparent, but as yet inadequately tested, pattern where earlier colonization dates are correlated with larger island size.

Phase‐field modeling of the effect of interfacial energy on pyrolytic carbon morphology in chemical vapor deposition

AbstractA conserved phase‐field model is proposed to investigate the effect of interfacial energy on the morphological evolution of the pyrolytic carbon deposit in chemical vapor deposition. The equilibrium geometry of carbon deposit islands is analytically predicted, of which the contact angle was controlled through the boundary conditions of the phase‐field parameter at the substrate surface according to the Young‐Laplace equation. Simulations of deposit growth are carried out for single and multi island nucleation. It is clarified that the island morphology depends on the magnitude of the interface energy. It is also observed that high interface energy results in large island size fluctuation. (© 2010 Wiley‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Multifunctional Nanocomposite Thin Films by Aerosol‐Assisted CVD

AbstractThe aerosol‐assisted (AA)CVD reaction of titanium isopropoxide and a solution of preformed tin dioxide nanoparticles leads to the production of titanium dioxide/tin dioxide nanocomposite thin films on glass substrates. Scanning electron microscopy (SEM) studies show that film composition and morphology are linked, with larger island sizes being observed as more tin dioxide nanoparticles are incorporated into the films. Reflectance measurements performed on the nanocomposite thin films show an increase in reflection in the infrared (IR) portion of the spectrum with increased tin dioxide nanoparticle incorporation. Photocatalysis experiments indicate that the nanocomposite films are photo‐active with irradiation by 365 nm light. The nanocomposite thin films display both low‐emissivity and self‐cleaning functionalities; and as such they are multifunctional.

Universality in size distributions of irreversibly grown epitaxial islands: Kinetic Monte Carlo simulations and the rate equations study

The scaling approach to the irreversible epitaxial growth gained wide recognition due to its ability to describe with the use of a universal function the island size distributions (ISDs) corresponding to a broad range of experimental conditions. The approach, however, is operative only in the case of large average island sizes ${s}_{av}$ and large diffusion to deposition rates ratios $R$. Physically this corresponds to long deposition times and/or high temperatures. We argue that the ISDs exhibit yet another universality property which holds for much broader range of growth conditions, in particular, for low temperatures (small $R$) and small ${s}_{av}$ (short deposition times). We show that the normalized ISDs corresponding to the same ${s}_{av}$ are accurately described by the same universal distribution.

Dependence of pyrocarbon microstructure on the substrate and annealing during the initial stage of chemical vapor deposition

Electron microscopic and electron spectroscopic techniques were applied to study interface properties, microstructure and texture of pyrolytic carbon obtained after short-time chemical vapor deposition (CVD) on planar Si substrates. The pyrolytic carbon was obtained in a hot-wall reactor from methane at a total pressure of 20 kPa and temperature of 1100 °C. Only short depositions between 2.5 and 240 min were performed. The carbon deposition starts with the nucleation of isolated islands. The increase of residence and deposition time leads to the formation of a continuous layer by larger island sizes and higher island densities, a transition from rough to smooth surfaces and formation of pores on smooth surfaces. An increased deposition rate during the first 15 min is observed which is correlated with a granular morphology of the carbon layer. Using BN-covered Si wafers with a surface roughness on a 100 nm scale reduces the texture degree in the vicinity of the interface and strengthens adhesion of the pyrolytic carbon compared to the smooth Si substrate. The texture of high-textured pyrolytic carbon is improved significantly by annealing at 1100 °C.

Phase-field study of interface energy effect on quantum dot morphology

A multi phase-field model is developed to investigate the effect of interface energy between a film and substrate on the morphological evolution of the film during self-assembled quantum dot (QD) growth in the deposition process. The single- and multi-island growth simulations for the SiGe/Si system are carried out in two-dimensional space with various interface energies. From the numerical results, it is clarified that the island shape and the island morphological transition points dramatic change depending on the magnitude of the interface energy. Here, the transition points, such as that from a pyramid to a dome, occurred in an earlier growth stage with higher interface energy. It is also observed that high interface energy results in large island size fluctuation.

Local-strain effects in Si∕SiGe∕Si islands on oxide

The tensile-strained Si, based on the misfit between Si and SiGe gives high speed and high drive current to the metal-oxide-silicon field effect transistors. In order to achieve the total system minimum energy, the island edge, within some characteristic length, bends upwards giving rise to a distorted lattice, as simulated by the finite element method. The finding indicates that the conventional strain partition rule of Si∕SiGe∕Si layers used for a large island size (>10μm) is not adequate for a small island size (<200nm) due to the significant local-strain effect of the edge lattice distortion. For a small island size, the bending from the edge can significantly affect the strain on the surface of the top Si layer, and a compressive strain or reduced tensile strain occurs at the center of the top Si layer, while the conventional strain partition rule predicts a uniform tensile strain on the top Si layer for any size of Si∕SiGe∕Si islands.

Divertors for Helical Devices: Concepts, Plans, Results, and Problems

With Large Helical Device (LHD) and Wendelstein 7-X (W7-X), the development of helical devices is now taking a large step forward on the path to a steady-state fusion reactor. Important issues that need to be settled in these machines are particle flux and heat control and the impact of divertors on plasma performance in future continuously burning fusion plasmas. The divertor concepts that will initially be explored in these large machines were prepared in smaller-scale devices like Heliotron E, Compact Helical System (CHS), and Wendelstein 7-AS (W7-AS). While advanced divertor scenarios relevant for W7-X were already studied in W7-AS, other smaller-scale experiments like Heliotron-J, CHS, and National Compact Stellarator Experiment will be used for the further development of divertor concepts. The two divertor configurations that are being investigated are the helical and the island divertor, as well as the local island divertor, which was successfully demonstrated on CHS and just went into operation on LHD. At present, on its route to a fully closed helical divertor, LHD operates in an open helical divertor configuration. W7-X will be equipped right from the start with an actively cooled discrete island divertor that will allow quasi-continuous operation. The divertor design is very similar to the one explored on W7-AS. For sufficiently large island sizes and not too long field line connection lengths, this divertor gives access to a partially detached quasi-steady-state operating scenario in a newly found high-density H-mode operating regime, which benefits from high energy and low impurity confinement times, with edge radiation levels of up to 90% and sufficient neutral compression in the subdivertor region (>10) for active pumping. The basic physics of the different divertor concepts and associated implementation problems, like asymmetries due to drifts, accessibility of essential operating scenarios, toroidal asymmetries due to symmetry breaking error fields, etc., are discussed.

Structural and Compositional Evolution of Self-Assembled Germanium Islands on Silicon (001) During High Growth Rate LPCVD

AbstractUnderstanding the process of self-organization of Ge nanostructures on Si with controlled size distribution is a key requirement for their application to devices. In this study, we investigate the temporal evolution of self-assembled islands during the low pressure chemical vapour deposition (LPCVD) of Ge on Si at 650°C using high growth rates (6–9 nm/min). The islands were characterized by atomic force microscopy, transmission electron microscopy, Rutherford backscattering spectrometry and micro-Raman spectroscopy. We found that the first nanostructures to assemble were small islands, with a narrow size distribution, typical of the ‘lens-shaped’ structures reported in previous studies. Next to form were a population of larger ‘lens-shaped’ islands with a similar surface density to that of the small islands, but with broad height and width distributions. These islands differ from the pyramid-shaped islands previously reported for a similar size range. On further Ge deposition, the population evolves into one of large square-based truncated pyramids with a very narrow size distribution. Such pyramidal structures were previously reported at smaller sizes. Furthermore, we see no evidence of the multifaceted domes previously reported in this size range. The small ‘lens-shaped’ islands appear to be strained, whilst some of the intermediate-sized islands and all the large truncated pyramids contain misfit strain relaxation induced defects. Additionally, in the both the intermediate size ‘lens-shaped’ islands and in the large size truncated pyramidal islands, there is evidence of Si-Ge strain-induced alloying, more significant in the first than in the latter. Our observation of ‘lens shaped’ islands and truncated pyramids at larger sizes than are normally observed, suggests a kinetically driven process that delays the evolution of energetically favourable island structures until larger island sizes are reached.

Effects of ion beams on the early stages of MgO growth

Molecular dynamics simulations are performed to examine the effects of ion beams in selecting MgO island orientations during ion beam assisted deposition. Sputter yields are determined as a function of ion beam orientation for MgO islands of different sizes. While channeling does occur in small islands, the difference in sputter yields between channeling and nonchanneling orientations is much smaller in small islands than in larger islands or a continuous film. The ion beam is capable of disrupting the entire island structure at small island sizes, while channeling dependent sputtering occurs at larger island size. Since less sputtering occurs for channeling orientations, the ion beam produces an orientation-dependent island growth rate. This leads to island orientation-dependent nucleation rates. A new analytical model parameterized using the molecular dynamics sputter yield data is developed which predicts how growth conditions affect these island-orientation nucleation rates. Based upon these simulations and model results, we suggest a strategy to efficiently grow thin, highly textured MgO films.

Smoluchowski ripening and random percolation in epitaxialSi1−xGex/Si(001)islands

Island size distributions of ripened ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(001)$ $(x=0.3\ensuremath{-}0.7)$ islands exhibit a slow decay at small island sizes and an asymmetric bell-shaped distribution peaked at a large size. We explain the ripening process in ${\mathrm{Si}}_{1\ensuremath{-}x}{\mathrm{Ge}}_{x}/\mathrm{Si}(001)$ islands by proposing a model based on random percolation and Smoluchowski ripening. Drawing an analogy with percolation theory, we use a shifted average height to represent the time variable, which is also applicable in the Smoluchowski ripening model; this shifted average height is used to analyze size distributions and correlation functions. Critical exponents derived from the site percolation model at $d=3$ agree with our measurements. Island diffusion $D\ensuremath{\propto}{s}^{\ensuremath{-}\ensuremath{\alpha}}$ leading to coalescence events, shows a weak dependence on size s: \ensuremath{\alpha}=0.28\ifmmode\pm\else\textpm\fi{}0.20, attributable to the inverse process of sequential incorporation of atoms at step edges due to the high stress concentration at island terrace perimeters. While random percolation dominates for small sizes regime, Smoluchowski ripening controls the distribution peak at large island sizes. Good agreement is obtained between the model and the measured island size distributions.

Nucleation, adatom capture, and island size distributions: Unified scaling analysis for submonolayer deposition

We consider the irreversible nucleation and growth of two-dimensional islands during submonolayer deposition in the regime of large island sizes. A quasihydrodynamic analysis of rate equations for island densities yields an ordinary differential equation (ODE) for the scaling function describing the island size distribution. This ODE involves the scaling function for the dependence on island size of {open_quotes}capture numbers{close_quotes} describing the aggregation of diffusing adatoms. The latter is determined via a quasihydrodynamic analysis of rate equations for the areas of {open_quotes}capture zones{close_quotes} surrounding islands. Alternatively, a more complicated analysis yields a partial differential equation (PDE) for the scaling function describing the joint probability distribution for island sizes and capture zone areas. Then, applying a moment analysis to this PDE, we obtain refined versions of the above ODE{close_quote}s, together with a third equation for the variance of the cell area distribution (for islands of a given size). The key nontrivial input to the above equations is a detailed characterization of nucleation. We analyze these equations for a general formulation of nucleation, as well as for an idealized picture considered previously, wherein nucleated islands have capture zones lying completely within those of existing islands.

Influence of the deposition rate on the structure of thin metal layers

The influence of the deposition rate on the morphology of epitaxial Sb layers on GaAs(110) is investigated. Layers with a nominal thickness of 30 monolayers (ML) were deposited with varying deposition rates (10 to 0.03 ML/min) on substrates kept at room temperature. Characterization techniques employed are Raman scattering, low energy electron diffraction (LEED), X-ray reflectivity and scanning probe microscopy. A wide variation of layer morphology is obtained dependent on the deposition rate: lowering the deposition rate generally results in larger island size and rougher surfaces. In most cases, the Sb layer is of rhombohedral structure. However, in a very narrow range of deposition rates, we found a growth mode of Sb, which is similar to a pseudo-cubic phase. This phase was found so far only for a thickness below 15 ML after annealing at 475 K.