Solid Growth Model Research Articles

Waiting-Time Scaling of a Restricted Solid on Solid Growth Model in $d = 1+1$

Waiting-Time Scaling of a Restricted Solid on Solid Growth Model in $d = 1+1$

A thick-walled fluid-solid-growth model of abdominal aortic aneurysm evolution: application to a patient-specific geometry.

We propose a novel thick-walled fluid-solid-growth (FSG) computational framework for modeling vascular disease evolution. The arterial wall is modeled as a thick-walled nonlinearly elastic cylindrical tube consisting of two layers corresponding to the media-intima and adventitia, where each layer is treated as a fiber-reinforced material with the fibers corresponding to the collagenous component. Blood is modeled as a Newtonian fluid with constant density and viscosity; no slip and no-flux conditions are applied at the arterial wall. Disease progression is simulated by growth and remodeling (G&R) of the load bearing constituents of the wall. Adaptions of the natural reference configurations and mass densities of constituents are driven by deviations of mechanical stimuli from homeostatic levels. We apply the novel framework to model abdominal aortic aneurysm (AAA) evolution. Elastin degradation is initially prescribed to create a perturbation to the geometry which results in a local decrease in wall shear stress (WSS). Subsequent degradation of elastin is driven by low WSS and an aneurysm evolves as the elastin degrades and the collagen adapts. The influence of transmural G&R of constituents on the aneurysm development is analyzed. We observe that elastin and collagen strains evolve to be transmurally heterogeneous and this may facilitate the development of tortuosity. This multiphysics framework provides the basis for exploring the influence of transmural metabolic activity on the progression of vascular disease.

Template synthesis and characterization of carbon nanomaterials from ferrocene crystals

Filamentous ribbon-like structures of highly disordered carbon of thickness 10–100 nm built from merged individual carbon nanofibers were synthesised by chemical vapour deposition from saturated ferrocene–benzene solution at 950 K. The materials obtained were characterized by electron microscopy, x-ray and electron diffraction, Raman spectroscopy and a possible growth mechanism for their formation was proposed and discussed. The synthesis demonstrates the possibility of a template growth of carbon nanomaterials and supports the vapour–solid–solid growth model of carbon materials because the catalysing metal particles are solid under the experimental conditions. Due to the large number of structural defects, filamentous structure, submicrometer thickness and low intraparticle diffusion of the nanomaterials, they can find application in catalysis as catalyst supports and sorbents.

Controllable growth of GeO 2 nanowires with the cubic and hexagonal phases and their photoluminescence

Single crystalline GeO 2 nanowires with the cubic and hexagonal structures were synthesized via heating Germanium wafer coated with an Au film at 500–600 °C under the flow of H 2O vapor/N 2. The Germanium wafer was used as both reagent and substrate for the growth of GeO 2 nanowires. The control over the GeO 2 nanowires with the cubic and hexagonal phases, the diameters of 30–75 nm and the lengths of 2.5–200 μm can be achieved by varying the heating temperature and time. The GeO 2 nanowires follow a top-Vapor–Liquid–Solid growth model. Ultraviolet, violet and blue emissions peaked at 351.9, 396.8 and 480.2 nm, respectively, are observed from the GeO 2 nanowires, which indicate that the GeO 2 nanowires may have potential applications in nanoscale photonic and electronic devices. This low temperature Vapor–Liquid–Solid growth process may be employed for the controllable synthesis of other oxide nanowires with different crystalline phases, and provides opportunities for both fundamental research and technological applications.

Growth of copper sulfide dendrites and nanowires from elemental sulfur on TEM Cu gridsunder ambient conditions

Copper sulfide dendrites and subsequent uniform nanowires up to tens of micrometers longcan be grown on carbon-coated transmission electron microscopy (TEM) Cu grids fromelemental sulfur at room temperature under ambient conditions without any solvent andsurfactants. TEM and high-resolution TEM studies demonstrated the morphology evolution ofCu2S from dendrites into ultra-long nanowires with increasing ageingtime. The sulfur species influenced significantly the growth rate ofCu2S dendrites and nanowires, but the final morphology remained the same. The nativeoxide on the surface of Cu grids played a critical role in the formation ofCu2S dendrites and nanowires. The crystal structures and phase purity ofCu2S samples were confirmed by x-ray diffraction (XRD) and energy dispersive x-ray spectroscopy(EDX). A solid–liquid–solid growth model may be considered a potential mechanism inCu2S morphology evolution on the basis of the experimental results. Most importantly, the presentstudy provides a simple and environmentally friendly route for the growth of one-dimensional (1D)Cu2S on Cu substrate.

Synthesis of nano onion-like fullerenes by chemical vapor deposition using an iron catalyst supported on sodium chloride

Nano-carbon materials were synthesized by the catalytic decomposition of acetylene at 420 °C using iron supported on sodium chloride as catalyst. The catalysts contain about 0.3, 1.6, 3.3, and 5.2 wt% iron. The samples were examined by scanning and transmission electron microscopy, energy dispersive spectroscopy, and X-ray diffraction. The results show that nano onion-like fullerenes (NOLFs) surrounding an Fe3C core were obtained using the catalyst containing 0.3 wt% iron. These had a structure of stacked graphitic fragments, with diameters in the range 15–50 nm. When the product was further heat treated under vacuum at 1,100 °C, NOLFs with a clear concentric graphitic layer structure were obtained. The growth mechanism of NOLFs encapsulating metallic cores is suggested to be in accordance with a vapor–solid growth model.

A semi-empirical molecular orbital study of freestanding and fullerene-encapsulated Mo nanoclusters

Nanoscopic transition metal clusters are widely used in the catalytic growth of carbon nanotubes (CNTs) synthesised using a chemical vapour deposition process. It is known that addition of oxygen and heavier chalcogens such as sulphur can both increase CNT growth rate and maximum length, and also promote selective growth of single and double wall CNTs. We describe results of semi-empirical molecular orbital calculations, using the AM1* Hamiltonian as implemented in the VAMP module of Materials Studio®, of both freestanding and fullerene-encapsulated molybdenum (Mo) clusters, which show that metal clusters above a critical size cause the formation of defects by opening of hexagon–hexagon (6–6) carbon bonds that lead eventually to a splitting open of the fullerene cage. Furthermore, we demonstrate that the addition of oxygen, nitrogen and sulphur to the molybdenum cluster cause changes to its electronic and geometric structure. Our simulations clearly demonstrate the strong interaction of the metal catalyst particle with the fullerene cap of a growing CNT, which may affect one or more stages of carbon incorporation into CNTs according to the commonly accepted vapour–liquid–solid growth model, although the precise mechanism requires further investigation.

Self-catalyzed growth of magnesium oxide nanorods

MgO nanorods with spherical particles at the tips were fabricated through direct reaction of Mg and oxygen. The nanorods have diameters ranging from 20 to 50 nm and their lengths are up to 10 μm. The diameters of the spherical particles are about 3–4 times larger than that of the nanorods. X-ray diffraction and energy-dispersive X-ray fluorescence results prove that the nanorods are composed of face-centered cubic (fcc) MgO. The transmission electron microscopy observations show that the nanorods are single crystalline. The growth direction of the nanorod is along [111]. A self-catalyzed vapor–liquid–solid growth model is adopted to account for the formation of the MgO nanorods.

Catalytic growth of α-FeSi 2 and silicon nanowires

Via vaporization of Fe and Si powders mixed with a molar ratio of 1:1, α-FeSi 2 nanowires were synthesized on one Si wafer, where the temperature is higher than 950 °C and Fe supply is enough, and meantime Si nanowires were obtained on the other Si wafer, where the temperature is lower than 500 °C and Fe atoms are much less than Si atoms in vapor. The two kinds of nanowires have diameters of 40–80 nm and lengths of 3–10 μm. Their shapes, crystallinity, and compositions are studied in terms of field emission scanning electron microscopy, X-ray diffraction spectra, and energy dispersive X-ray spectroscopy. From the obtained experimental results, we explain the growth mechanism of the two kinds of nanowires using the vapor–liquid–solid growth model. Two important conditions, high temperatures and enough Fe supply, are emphasized for syntheses of the α-FeSi 2 nanowires.

Growth mechanism of planar-type GaAs nanowhiskers

The mechanism for the lateral growth of ultrathin GaAs whiskers is discussed in connection with the vapor–liquid–solid growth model. The observed growth rate shows that the migration of the source material plays an essential role in such whisker growth.