Increase In Domain Size Research Articles

Anisotropy of Unsaturated Layered Soils: Impact of Layer Composition and Domain Size

Soils commonly exhibit hydraulic anisotropy under field conditions due to layering. Previous studies on saturation-dependent anisotropy were mostly focused on the impact of saturation degree, bulk density, and soil texture. In this study, the impact of layer composition and domain size on the anisotropy of unsaturated soils was investigated. We applied the same capillary pressure at top and bottom of the layered domain to simulate a mean unit hydraulic gradient condition and determined the anisotropy factor by the ratio of fluxes in the directions parallel and perpendicular to the layering. The anisotropy factors were then examined in relation to the layer composition, domain size, and mean capillary pressure for both two- and three-layer formations. In addition, the performance of the conveniently used conceptual anisotropy based on the harmonic and arithmetic mean hydraulic conductivities was also investigated for comparison. The maximum anisotropy is reached when the more permeable layer thickness is <50% in the two-layer formation. Both two-layer and three-layer formations exhibit a nonmonotonic anisotropic behavior in relation to the mean capillary pressure head. In the three-layer formations, anisotropy always exists because the minimum anisotropy factor is larger than one. In the dry range, the anisotropy of layered formations decreases significantly with increasing domain size. The anisotropy behavior of layered formations that is influenced by layer composition and domain size should be appropriately quantified, as the commonly used approach based on the harmonic and arithmetic mean of hydraulic conductivities may introduce large errors.

Size and shape dependence study of magnetization reversal in magnetic antidot lattice arrays

Magnetic Antidot Lattice (MAL) arrays of Co have been prepared in micron range using ultraviolet (UV) lithography technique with different shapes and sizes. Magnetization reversal in such MAL systems has been studied by magneto-optic Kerr effect (MOKE) based microscopy by varying the angle between the easy axis and the external magnetic field. The domain images evidence that the magnetization reversal along easy axis is highly dominated by nucleation of domains which is subsequently accompanied by domain wall motion. We have observed that with increase in active magnetic area domain size increases but on the contrary coercivity decreases. The presence of periodic holes turns the MALs magnetically hard when compared to similar thickness of continuous thin film. The magnetization relaxation along easy axis for the Co MAL at constant dc field fits very well with the exponential law of Fatuzzo–Labrune indicating domain nucleated dominant process.

Phase structure and crystallization behavior of polyethylene in its blends with cis-1,4-butadiene rubber

Phase structure and crystallization behavior of polyethylene (PE) in its blends with cis-1,4-butadiene rubber (BR) at different blend ratios and sample preparation conditions were studied. The PE is finely dispersed in the BR matrix. For samples hot pressed at 145 °C, circular PE microdomains with randomly oriented PE lamellar aggregates were produced. The domain size and number increase with increasing PE content. When the PE content is over 10 wt%, most of the PE domains impinged each other. The separated PE domains are connected by PE stripes with parallel arranged lamellar aggregates. For samples hot pressed at 140 °C, elongated PE microdomains with oriented PE lamellar aggregates were obtained due to the shear flow. The crystallization of PE in the blends depends on the phase structure. Confined crystallization of PE occurs in small microdomains at relatively low temperature. With the increase of domain size, the crystallization ability of PE increases while the confined crystallization decreases.

Phase Diagram of Conjugated Polymer Blend P3HT/PF12TBT and the Morphology-Dependent Photovoltaic Performance

In this study, we systematically investigated the morphological evolution of poly(3-hexylthiophene) (P3HT) and poly{2,7-(9,9-didodecyl-fluorene)-alt-5,5-[4′,7′-bis(2-thienyl)-2′,1′,3′-benzothiadiazole]} (PF12TBT) blend thin films cast from o-dichlorobenzene (oDCB) solution and its effect on photovoltaic performance. Blend ratio and film drying time both played a crucial role in determining polymer blend film morphologies, including phase-separated structure and domain size. An apparent morphological transformation from uniform morphology to droplet and bicontinuous structure could be observed with the P3HT/PF12TBT blend ratio ranging from 10/90 to 90/10. Uniform morphology resulted from one component becoming dominated, and thus no obvious phase separation could be observed. The formation of bicontinuous morphology via a spinodal decomposition mechanism which emerged at nearly equal blend ratio could be attributed to the similar and favorable solubility for both polymers in processing solvent, while the asymmetric composition led to the formation of droplet morphology via a spinodal decomposition mechanism. In addition, the increased domain size which resulted from coarsening of adjacent domains could be observed with the extending film drying time. Further expanding drying time, the thin films exhibited unsharp morphology with large domains which could be attributed to the mixing of polymer blend with the formation of vast P3HT crystallites and the aggregation of both donor and acceptor via molecular diffusion. On the basis of the varying morphologies, an approximate phase diagram of P3HT/PF12TBT blend was depicted. In order to demonstrate the effect of various morphologies on photovoltaic performance, devices based on films with three kinds of blend ratios (40/60, 50/50, and 60/40) while undergoing different drying time (20, 30, and 45 s) were prepared. As the drying time extended, a gradual decreased power conversion efficiency (PCE) could be observed for all blend ratio devices, which resulted from the dramatic morphological transformation from small domains to intermediate domains and then upsharp morphology with large domains ultimately.

Low Density Growth of Graphene by Air Introduction in Atmospheric Pressure Chemical Vapor Deposition

Chemical vapor deposition (CVD) is a promising method to produce large-size single-crystal graphene, and further increase in domain size is desirable for electro/optic applications. Here we studied the effect of low amount of air introduction by intentional leak on graphene growth in atmospheric pressure CVD. The air introduction at the heating process resulted in roughening of Cu surface induced by oxygen, while air introduction at the annealing under H2 ambient drastically decreased graphene density due to reduction of active sites for graphene nucleation both by surface oxidation and enlargement of Cu domain. Although air introduction only at the growth stage was ineffective for graphene nucleation, air introduction for both annealing and growth provided great enhancement of domain growth without increasing the density of graphene, which is an optimized condition to obtain a large single-crystal. This controlled introduction of air in atmospheric pressure CVD provided ∼ 2.5 mm hexagonal single layer graphene with high quality. [DOI: 10.1380/ejssnt.2015.404]

Artificial neural network modeling of scale-dependent dynamic capillary pressure effects in two-phase flow in porous media

A computationally efficient and simple alternative platform for the prediction of the domain scale dependence of the dynamic capillary pressure effects, defined in terms of a coefficient named as dynamic coefficient (τ), is developed using an artificial neural network (ANN). The input parameters consist of the phase saturation, media permeability, capillary entry pressure, viscosity ratio, density ratio, temperature, pore size distribution index, porosity and domain volume with corresponding output τ obtained at different domain scales. Different ANN configurations as well as linear and nonlinear multivariate regression models were tested using a number of performance criteria. Findings in this work showed that the ANN structures with double hidden layers perform better than those with a single hidden layer. In particular, the ANN configuration with 13 and 15 neurons in the first and second hidden layers, respectively, performed the best. Using this best-performing ANN, effects of increased domain size were predicted for three separate experimental results obtained from literature and our laboratory with different domain scales. Results showed increased magnitude of τ as the domain size increases for all the independent experimental data considered. This work shows the applicability and techniques of using ANNs in the prediction of scale dependence of two-phase flow parameters.

Domain engineering of physical vapor deposited two-dimensional materials

Physical vapor deposited two-dimensional (2D) materials span larger areas compared to exfoliated flakes, but suffer from very small grain or domain sizes. In this letter, we fabricate freestanding molybdenum disulfide (MoS2) and amorphous boron nitride (BN) specimens to expose both surfaces. We performed in situ heating in a transmission electron microscope to observe the domain restructuring in real time. The freestanding MoS2 specimens showed up to 100× increase in domain size, while the amorphous BN transformed in to polycrystalline hexagonal BN (h-BN) at temperatures around 600 °C much lower than the 850–1000 °C range cited in the literature.

The Role of Solvent Additive Processing in High Performance Small Molecule Solar Cells

The use of small volumes of a high boiling point liquid as a “solvent additive” is a deposition processing method that has been implemented in most high/record performing polymer:fullerene-based bulk heterojunction (BHJ) solar cell devices. Recently, solvent additive processing has been employed in a solution processable small molecule (SPSM) BHJ system, viz., 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene:[6,6]-phenyl C71 butyric acid methyl ester (p-DTS(PTTh2)2:PC71BM), when a small amount, 0.25 v/v %, diiodooctane (DIO) was added to the casting solution, several key device metrics increased, leading to a high power conversion efficiency (PCE) of 6.7%. X-ray diffraction experiments show that the amount of additive added to the casting solution to make p-DTS(PTTh2)2:PC71BM thin films has several effects on the structure at multiple length scales: for example, the number and orientation of p-DTS(PTTh2)2 crystallites, different π–π stacking distances, and the nanoscale domain size. Additionally, we utilize energy filtered transmission electron microscopy (EFTEM), a technique that significantly enhances the contrast between p-DTS(PTTh2)2 and PC71BM in real space, to further verify the effect of increasing domain size as the additive concentration is increased. Tomographic reconstruction of the TEM micrographs provides a 3D representation of the BHJ structure. These studies show how domain size and tortuosity in all dimensions change due to solvent additive processing, and the overall finding is that the nanostructures of p-DTS(PTTh2)2 have enhanced connectivity when 0.25 v/v % DIO was used. Finally, we show evidence of solvent additive retention in p-DTS(PTTh2)2:PC71BM films when 1 v/v % DIO is used (but absent for 0.25%). This finding, in conjunction with the appearance of two populations of π–π stacking distances when 1 v/v % DIO is used, leads to the identification of one of the specific points of interaction between DIO and p-DTS(PTTh2)2.

High Anion Conductivity and Low Water Uptake of Phosphonium Containing Diblock Copolymer Membranes

Poly[(styrene)-block-((2-acryloxy)ethyltributylphosphonium bromide)] diblock copolymers (STBP) were synthesized in two steps. First, reversible addition–fragmentation chain transfer polymerization was used to synthesize the diblock copolymer precursors poly[(styrene)-block-(bromoethyl acrylate)] (SBEA), followed by functionalization with tributylphosphine. Copolymers with overall molecular weights ranging from 31 to 87 kg/mol were synthesized. The volume fraction of the ion-containing monomers in the copolymers was fixed at about 0.57. Self-assembly of these copolymers into ordered morphologies with tunable domain sizes was demonstrated by small-angle X-ray scattering. The effect of morphology on water uptake and bromide ion conductivity was explored in samples equilibrated in liquid water. The use of the pendant tributylphosphonium cations, which have some hydrophobic character, results in low water uptake and high anionic conductivity. The conductivity increases with increasing domain size while water uptake is unaffected by domain size.

Research on Computational Fluid Dynamics with Effect of Grid Quality on the Accuracy of Simulated Results of Two Dimensional Low-Speed Parallel Flow

In the field of computational fluid dynamics, grid generation costs most of work. The accuracy and reliability of the simulated results depend directly on grid quality. The two dimensional parallel flow with low speed was used to study the effect of grid quality on simulated results. Several conclusions are obtained. The computational error decreases with the increase of domain size in flow direction. Too many grids in flow direction will lead to bigger round-off error, while too few grids will make it harder to catch the correct flow. The increase of grid quantity in the direction perpendicular to flow direction will decrease the computational error, but the effect is not obvious. Increasing the grid quantity in the area near the wall will decrease the computational error.

Morphological effects on the small-molecule-based solution-processed organic solar cells.

We report a proof-of-concept study on solution-processed organic solar cells (OSCs) based on [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM) and structurally compact donor molecules which have dithiophene-phenazine-dithiophene (TH-P) and dithiophene-quinoxaline-dithiophene (TH-Q) configurations with decyloxy and methyl side groups, respectively. These molecules formed one-dimensional fibers through self-assembly via weak nonbonding interactions such as π-π and van der Waals interactions even during a fast solvent removal process such as spin-casting. Photophysical and thermal properties of the new donor molecules were characterized with UV-vis absorption and fluorescence spectroscopy, differential scanning calorimetry, and thermogravimetric analysis. The electrochemical data determined experimentally were correlated well with theoretical evaluations. The fibers from the two donor molecules showed distinct morphological differences, allowing for in-depth investigations into their influence on the OSC performance. A continuous three-dimensional network of endless one-dimensional nanofibers, with a width of 300-400 nm, were formed from TH-P regardless of the presence of PC61BM, affording spontaneous nanoscale phase separation that facilitates a large donor/acceptor interfacial area. Bulk (BHJ) and planar heterojunctions (PHJ) from TH-P/PC61BM showed a power conversion efficiency (PCE) of 0.38% and 0.30%, respectively, under optimum device conditions. Post thermal annealing led to the increased domain size and a major decrease in Jsc. Meanwhile, shorter, more rigid needles with a large thickness variation were formed from TH-Q. A continuous network of TH-Q was obtained by spin-coating only in the presence of PC61BM, and the PCE of TH-Q/PC61BM BHJ was found to be 0.36%. However, the PHJ showed poor device performance due to TH-Q's inability to form a continuous film by spin-coating. The present study suggests a basic molecular architecture to drive one-dimensional assembly and demonstrates the significance of fibrillation for small-molecule-based OSCs.

Threefold atmospheric-pressure annealing for suppressing graphene nucleation on copper in chemical vapor deposition

Chemical vapor deposition (CVD) is a promising method of producing a large single-crystal graphene on a catalyst, especially on copper (Cu), and a further increase in domain size is desirable for electro/optic applications. Here, we report on threefold atmospheric-pressure (ATM) annealing for suppressing graphene nucleation in atmospheric CVD. Threefold ATM annealing formed a step and terrace surface of the underlying Cu, in contrast to ATM annealing. Atomic force microscopy and Auger electron mapping revealed that Si-containing particles existed on threefold-ATM- and ATM-annealed surfaces; particles on Cu had a lower density after threefold ATM annealing than after ATM annealing. The formation of a step and terrace surface and the lower density of particles following the threefold ATM annealing would play a role in reducing graphene nucleation. By combining threefold ATM annealing and electropolishing of Cu, the nucleation of graphene was effectively suppressed, and a submillimeter-sized hexagonal single-crystal graphene was successfully obtained.

Organic Photovoltaics: Elucidating the Ultra‐Fast Exciton Dissociation Mechanism in Disordered Materials

AbstractOrganic photovoltaics (OPVs) offer the opportunity for cheap, lightweight and mass‐producible devices. However, an incomplete understanding of the charge generation process, in particular the timescale of dynamics and role of exciton diffusion, has slowed further progress in the field. We report a new Kinetic Monte Carlo model for the exciton dissociation mechanism in OPVs that addresses the origin of ultra‐fast (&lt;1 ps) dissociation by incorporating exciton delocalization. The model reproduces experimental results, such as the diminished rapid dissociation with increasing domain size, and also lends insight into the interplay between mixed domains, domain geometry, and exciton delocalization. Additionally, the model addresses the recent dispute on the origin of ultra‐fast exciton dissociation by comparing the effects of exciton delocalization and impure domains on the photo‐dynamics.This model provides insight into exciton dynamics that can advance our understanding of OPV structure–function relationships.

Organic Photovoltaics: Elucidating the Ultra‐Fast Exciton Dissociation Mechanism in Disordered Materials

Organic photovoltaics (OPVs) offer the opportunity for cheap, lightweight and mass-producible devices. However, an incomplete understanding of the charge generation process, in particular the timescale of dynamics and role of exciton diffusion, has slowed further progress in the field. We report a new Kinetic Monte Carlo model for the exciton dissociation mechanism in OPVs that addresses the origin of ultra-fast (<1 ps) dissociation by incorporating exciton delocalization. The model reproduces experimental results, such as the diminished rapid dissociation with increasing domain size, and also lends insight into the interplay between mixed domains, domain geometry, and exciton delocalization. Additionally, the model addresses the recent dispute on the origin of ultra-fast exciton dissociation by comparing the effects of exciton delocalization and impure domains on the photo-dynamics.This model provides insight into exciton dynamics that can advance our understanding of OPV structure-function relationships.

Parameter Study of Tropical Cyclones in Rotating Radiative–Convective Equilibrium with Column Physics and Resolution of a 25-km GCM

Abstract Rotating radiative–convective equilibrium is studied by extracting the column physics of a mesoscale-resolution global atmospheric model that simulates realistic hurricane frequency statistics and then coupling it to rotating hydrostatic dynamics in doubly periodic domains. The parameter study helps in understanding the tropical cyclones simulated in the global model and also provides a reference point for analogous studies with cloud-resolving models. The authors first examine the sensitivity of the equilibrium achieved in a large square domain (2 × 104 km on a side) to sea surface temperature, ambient rotation rate, and surface drag coefficient. In such a large domain, multiple tropical cyclones exist simultaneously. The size and intensity of these tropical cyclones are investigated. The variation of rotating radiative–convective equilibrium with domain size is also studied. As domain size increases, the equilibrium evolves through four regimes: a single tropical depression, an intermittent tropical cyclone with widely varying intensity, a single sustained storm, and finally multiple storms. As SST increases or ambient rotation rate f decreases, the sustained storm regime shifts toward larger domain size. The storm’s natural extent in large domains can be understood from this regime behavior. The radius of maximum surface wind, although only marginally resolved, increases with SST and increases with f for small f when the domain is large enough. These parameter dependencies can be modified or even reversed if the domain is smaller than the storm’s natural extent.

Stoichiometry effect on Cu2ZnSnS4 thin films morphological and optical properties

Thin films of Cu2ZnSnS4 (CZTS) were prepared by sulfurization of multilayered precursors of ZnS, Cu, and Sn, changing the relative amounts to obtain CZTS layers with different compositions. X-Ray Diffraction (XRD), Energy Dispersive X-Ray spectroscopy, and SEM were used for structural, compositional, and morphological analyses, respectively. XRD quantitative phase analysis provides the amount of spurious phases and information on Sn-site occupancy. The optical properties were investigated by spectrophotometric measurements and Photothermal Deflection Spectroscopy. These films show a clear dependence of the optical and microstructural properties on the tin content. As the tin content increases we found: (i) an increase in both crystalline domain and grain size, (ii) an abrupt increase of the energy gap of about 150 meV, from 1.48 to 1.63 eV, and (iii) a decrease of sub-gap absorption up to two orders of magnitude. The results are interpreted assuming the formation of additional defects as the tin content is reduced.

EFFECTS OF ENHANCED STRATIFICATION ON EQUATORWARD DYNAMO WAVE PROPAGATION

We present results from simulations of rotating magnetized turbulent convection in spherical wedge geometry representing parts of the latitudinal and longitudinal extents of a star. Here we consider a set of runs for which the density stratification is varied, keeping the Reynolds and Coriolis numbers at similar values. In the case of weak stratification, we find quasi-steady dynamo solutions for moderate rotation and oscillatory ones with poleward migration of activity belts for more rapid rotation. For stronger stratification, the growth rate tends to become smaller. Furthermore, a transition from quasi-steady to oscillatory dynamos is found as the Coriolis number is increased, but now there is an equatorward migrating branch near the equator. The breakpoint where this happens corresponds to a rotation rate that is about 3-7 times the solar value. The phase relation of the magnetic field is such that the toroidal field lags behind the radial field by about $\pi/2$, which can be explained by an oscillatory $\alpha^2$ dynamo caused by the sign change of the $\alpha$-effect about the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an $m=1$ mode is excited for a full $2\pi$ azimuthal extent, reminiscent of the field configurations deduced from observations of rapidly rotating late-type stars.

Influence of the antiphase domain structure on the long-range atomic order parameter in the Ni3Mn alloy with superstructure L12

The antiphase domain structure in the Ni3Mn alloy with superstructure L12 with various manganese contents has been studied. It has been found that a deviation of the alloy composition from the stoichiometry leads to the formation of manganese oxides at the antiphase domain boundaries and grain boundaries. With an increase in the antiphase domain sizes, the domain size distribution function changes from the normal to lognormal, and the degree of long-range atomic order decreases.

Atomistic insight into micro-phase separation and gas diffusion in PEO–PBT multiblock copolymers

Atomistic simulation is reported for micro-phase separation and H2 diffusion in poly(ethylene oxide) (PEO)–poly(butylene terephthalate) (PBT) multiblock copolymers with varying PEO segment length. The simulated densities of copolymers agree well with available experimental data with only 1–2% deviations. A strong interaction exists between PEO segments as evidenced by the sharp peaks in radial distribution functions. With increasing PEO segment length, the fractional free volume marginally increases, PEO and PBT segments tend to be more immiscible and segregated domain size increases. This is consistent with experimental and theoretical studies in the literature. The mobility of PBT segments is almost one order of magnitude smaller than that of PEO segments. With increasing PEO segment length, the mobility of PEO–PBT copolymers is enhanced. For H2 diffusion in PEO–PBT copolymers, the diffusivity increases with increasing PEO segment length, as experimentally observed. This simulation study provides atomistic insight into the micro-structure and gas transport properties in PEO–PBT copolymers.

Scaling function in conductivity of planar random checkerboards

Under investigation is the finite-size scaling of the Fourier thermal conductivity in two-phase planar random checkerboard microstructures at 50% nominal volume fraction. Examples considered include Aluminum–Copper, Constantan–Lead, Stainless Steel–Gold, Inconel X-750–Aluminum, Titanium Dioxide–Gold, Carbon Steel–Diamond, Lead–Diamond, Boron–Diamond, Molybdenum–Test, Constantan–Diamond. Mesoscale bounds are obtained using an approach consistent with the Hill–Mandel homogenization condition. Extensive numerical simulations are conducted on 10 types of microstructures with the contrast (k) ranging from 1.54 to 100. The effects of mesoscale (δ) and phases’ contrast are evaluated and generic scaling laws are established quantitatively. This is accomplished using a non-dimensional scaling function derived by contracting the mesoscale conductivity and resistivity tensors. The scaling function very closely fits all the material combinations and is given by g(δ,k)=12(k-1/k)2exp[-0.53(δ-1)0.69]. As a verification of our procedure, it is observed that, with increasing domain size, the mesoscale conductivity tends to the exact theoretical result for macroscopic conductivity of random checkerboards: being the geometric mean of two phases. By choosing an appropriate functional form of the scaling function, a material scaling diagram is constructed with which one can rapidly estimate the size of representative volume element for a given contrast within acceptable accuracy.