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Gradient Distribution (gradient + distribution)

Distribution by Scientific Domains
Polymers and Materials Science60%

Selected Abstracts

A New Technique for Preparing a Filled Type of Polymeric Gradient Material

MACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2006
Yong-Bin Zhu
Abstract Summary: So-called functionally gradient materials have received increased attention as a new type of composite whose microelements, including composition and structure, change spatially to optimize the gradient properties for a specific application. In this study, a new technique for continuously preparing a filled type of PGM was investigated through co-extrusion/gradient distribution/2-dimensional mixing with conventional polymeric material processing facilities. The processing line from co-extrusion, gradient distribution to 2-dimensional mixing was fulfilled by two extruders, a gradient distribution unit and 2-dimensional mixing units. The gradient distribution unit and 2-dimensional mixing units were designed separately in our group. As an example, a PE/GB PGM was prepared by using this new technique. The gradient variation of composition along the sample thickness direction was studied by TG and SEM. The TG results indicated that a gradient variation of the content of GB was formed along the thickness of the sample. The direct gradient distribution of GB came from SEM observation, which showed an increased stacking density of GB along the sample thickness. Experimental results indicated that the processing method with co-extrusion/gradient distribution/2-dimensional mixing can serve as a new way to produce a filled type of PGM and is worthy of further investigation. The prepared polyethylene/glass bead PGM; the graph illustrates the glass bead concentration gradient across the sample thickness. [source]

Modelling and analysing evolution of dispersal in populations at expanding range boundaries

ECOLOGICAL ENTOMOLOGY, Issue 5 2007
CLARE L. HUGHES
Abstract 1.,Species would be expected to shift northwards in response to current climate warming, but many are failing to do so because of fragmentation of breeding habitats. Dispersal is important for colonisation and an individual-based spatially explicit model was developed to investigate impacts of habitat availability on the evolution of dispersal in expanding populations. Model output was compared with field data from the speckled wood butterfly Pararge aegeria, which currently is expanding its range in Britain. 2.,During range expansion, models simulated positive linear relationships between dispersal and distance from the seed location. This pattern was observed regardless of quantity (100% to 10% habitat availability) or distribution (random vs. gradient distribution) of habitat, although higher dispersal evolved at expanding range margins in landscapes with greater quantity of habitat and in gradient landscapes. Increased dispersal was no longer evident in any landscape once populations had reached equilibrium; dispersal values returned to those of seed populations. However, in landscapes with the least quantity of habitat, reduced dispersal (below that of seed populations) was observed at equilibrium. 3.,Evolutionary changes in adult flight morphology were examined in six populations of P. aegeria along a transect from the distribution core to an expanding range margin in England (spanning a latitudinal distance of >200 km). Empirical data were in agreement with model output and showed increased dispersal ability (larger and broader thoraxes, smaller abdomens, higher wing aspect ratios) with increasing distance from the distribution core. Increased dispersal ability was evident in populations from areas colonised >30 years previously, although dispersal changes were generally evident only in females. 4.,Evolutionary increases in dispersal ability in expanding populations may help species track future climate changes and counteract impacts of habitat fragmentation by promoting colonisation. However, at the highest levels of habitat loss, increased dispersal was less evident during expansion and reduced dispersal was observed at equilibrium indicating that, for many species, continued habitat fragmentation is likely to outweigh any benefits from dispersal. [source]

Porous Polymer Films with Gradient-Refractive-Index Structure for Broadband and Omnidirectional Antireflection Coatings

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2010
Xiao Li
Abstract Porous polymer films that can be employed for broadband and omnidirectional antireflection coatings are successfully shown. These films form a gradient-refractive-index structure and are achieved by spin-coating the solution of a polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA)/PMMA blend onto an octadecyltrichlorosilane (OTS)-modified glass substrate. Thus, a gradient distribution of PMMA domains in the vertical direction of the entire microphase-separated film is obtained. After those PMMA domains are removed, a PS porous structure with an excellent gradient porosity ratio in the vertical direction of the film is formed. Glass substrates coated with such porous polymer film exhibit both broadband and omnidirectional antireflection properties because the refractive index increases gradually from the top to the bottom of the film. An excellent transmittance of >97% for both visible and near-infrared (NIR) light is achieved in these gradient-refractive-index structures. When the incident angle is increased, the total transmittance for three different incident angles is improved dramatically. Meanwhile, the film possesses a color reproduction character in the visible light range. [source]

A New Technique for Preparing a Filled Type of Polymeric Gradient Material

MACROMOLECULAR MATERIALS & ENGINEERING, Issue 11 2006
Yong-Bin Zhu
Abstract Summary: So-called functionally gradient materials have received increased attention as a new type of composite whose microelements, including composition and structure, change spatially to optimize the gradient properties for a specific application. In this study, a new technique for continuously preparing a filled type of PGM was investigated through co-extrusion/gradient distribution/2-dimensional mixing with conventional polymeric material processing facilities. The processing line from co-extrusion, gradient distribution to 2-dimensional mixing was fulfilled by two extruders, a gradient distribution unit and 2-dimensional mixing units. The gradient distribution unit and 2-dimensional mixing units were designed separately in our group. As an example, a PE/GB PGM was prepared by using this new technique. The gradient variation of composition along the sample thickness direction was studied by TG and SEM. The TG results indicated that a gradient variation of the content of GB was formed along the thickness of the sample. The direct gradient distribution of GB came from SEM observation, which showed an increased stacking density of GB along the sample thickness. Experimental results indicated that the processing method with co-extrusion/gradient distribution/2-dimensional mixing can serve as a new way to produce a filled type of PGM and is worthy of further investigation. The prepared polyethylene/glass bead PGM; the graph illustrates the glass bead concentration gradient across the sample thickness. [source]

Solution to shape identification problem of unsteady heat-conduction fields

HEAT TRANSFER - ASIAN RESEARCH (FORMERLY HEAT TRANSFER-JAPANESE RESEARCH), Issue 3 2003
Eiji Katamine
Abstract This paper presents a numerical analysis method for shape determination problems of unsteady heat-conduction fields in which time histories of temperature distributions on prescribed subboundaries or time histories of gradient distributions of temperature in prescribed subdomains have prescribed distributions. The square error integrals between the actual distributions and the prescribed distributions on the prescribed subboundaries or in the prescribed subdomains during the specified period of time are used as objective functionals. Reshaping is accomplished by the traction method that was proposed as a solution to shape optimization problems of domains in which boundary value problems are defined. The shape gradient functions of these shape determination problems are derived theoretically using the Lagrange multiplier method and the formulation of material derivative. The time histories of temperature distributions are evaluated using the finite-element method for a space integral and the Crank,Nicolson method for a time integral. Numerical analyses of nozzle and coolant flow passage in a wing are demonstrated to confirm the validity of this method. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(3): 212,226, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10086 [source]