951 research outputs found
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Effect of Processing Parameters on the Density and Microstructure of Direct Laser Sintered Al-12Si Powders
The effect of processing parameters on the sintering behaviour of gas atomised Al-12Si
powders has been investigated. Laser power, scanning rate, scan spacing and layer
thickness are found to control the densification and the resultant microstructural
characteristics of the laser sintered parts. It was found that sintered density increased as
the energy density increased reaching a maximum of 80.2% at an energy input per unit
volume of 67 J mm-3. For parts produced with a slightly lower power density (50 J mm-3), the microstructure consisted of fine dendrites with interconnected porosity while parts
fabricated with a slightly higher power density (100 J mm-3) were noted to have a
preponderance of coarse dendrites with a discontinuous network of irregular shaped pores
surrounded by a fully dense aluminium-silicon matrix.Mechanical Engineerin
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Selective Laser Sintering of Polymer Nanocomposites
This paper describes the fabrication and characterization of polymer nanocomposite (PNC)
materials for use in the selective laser sintering (SLS) process. PNC materials are of great
interest generally because of their excellent physical properties, and offer excellent potential
in rapid manufacturing of structural polymeric parts. Three different nano additive materials
have been used: cerium oxide IV, yttrium stabilized zirconia, and layered Hectorite clay.
These materials have been used to reinforce PA6 polymer using solution blending and spray
drying to create powder with particle sizes in the range of 5-40 µm. The mechanical
properties and microstructure of the PNC materials have been evaluated and the results
compared to those of unfilled polymer.Mechanical Engineerin
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Mass Customization of Foot Orthosis for Rheumatoid Arthritis
Rheumatoid arthritis (RA) is an inflammatory disease, which can cause pain, stiffness, and
swelling in the joints of hands and feet. The foot is a major site for RA involvement and a
major source of disability resulting from this disease. This paper introduces research which
aims to create a mass customisation process for customised orthoses for patients with RA.
3D laser scanning, and gait analysis will be used to generate the orthosis geometry and
rapid manufacturing, namely the selective laser sintering (SLS) process, will be used to
produce the orthoses. The SLS process enables the incorporation of compositional
functional elements, such as locally adjusted stiffness or flexibility, into the orthosis
design.
The process involved two central elements. The first was a literature survey to identify
orthotic design rules for foot impairments in RA. This survey will form a platform for the
design rule development and will be complemented by data obtained from two patient
trials. The second is a virtual three-segment foot model, created in Anybody dynamics
modelling software which can be motivated by data measured from patients using 3D
motion capture and force plate systems. Once the measured data has been applied to the
model, a virtual insole can be used to simulate the effects of various features in the
orthosis.
Considerable variation was noted in the literature for types of material, design and
methods of orthotic construction. Pressure redistribution using cushioning materials was
consistently mapped to painful deformed joints. Orthoses with contoured surfaces, either
custom- or mass produced in thermoplastic materials of varying stiffness and density were
mapped to joint motion control and deformity prevention. The paper will also describe
applying patient gait data to the Anybody model, and then altering the gait pattern by
applying the insole model. Future work will also be discussed.Mechanical Engineerin
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Strength of the DTM RapidSteel 1.0 Material
This paper reports the results of a study into the strength of the DTM RapidSteel 1.0 material.
Elastic modulus and strength of the metal/copper composite material was investigated as a
function of the distance from the point of copper infiltration, the furnace cycle duration, and the
furnace type. The microstructure of the RapidSteel material was also examined in an attempt to
understand the science behind the infiltration process. The results have implications for the
design of tools to be made using the RapidTool process in situations where the tool will be used
as a production tool, rather than a prototype tool.Mechanical Engineerin
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Mechanical Properties and Biological Responses of Bioactive Glass Ceramics Processed using Indirect SLS
This paper will report on research which aims to generate bone replacement components by
processing bioactive glass-ceramic powders using indirect selective laser sintering. The indirect
SLS route has been chosen as it offers the ability to tailor the shape of the implant to the
implantation site, and two bioactive glass ceramic materials have been processed through this
route: apatite-mullite and apatite-wollostanite. The results of bend tests, to investigate
mechanical properties, and in vitro and in vivo experiments to investigate biological responses of
the materials will be reported, and the suitability of completed components for implant will be
assessed.Mechanical Engineerin
Inferring physical conditions in interstellar clouds of H_2
We have developed a code that models the formation, destruction, radiative
transfer, and vibrational/rotational excitation of H_2 in a detailed fashion.
We discuss how such codes, together with FUSE observations of H_2 in diffuse
and translucent lines of sight, may be used to infer various physical
parameters. We illustrate the effects of changes in the major physical
parameters (UV radiation field, gas density, metallicity), and we point out the
extent to which changes in one parameter may be mirrored by changes in another.
We provide an analytic formula for the molecular fraction, f_H2, as a function
of cloud column density, radiation field, and grain formation rate of H_2. Some
diffuse and translucent lines of sight may be concatenations of multiple
distinct clouds viewed together. Such situations can give rise to observables
that agree with the data, complicating the problem of uniquely identifying one
set of physical parameters with a line of sight. Finally, we illustrate the
application of our code to an ensemble of data, such as the FUSE survey of H_2
in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the
elevated UV radiation field intensity and reduced grain formation rate of H_2
in those low- metallicity environments.Comment: 33 pages (aastex, manuscript), 9 figures (3 color). accepted to Ap
Coulomb interactions in single, charged self-assembled quantum dots: radiative lifetime and recombination energy
We present results on the charge dependence of the radiative recombination
lifetime, Tau, and the emission energy of excitons confined to single
self-assembled InGaAs quantum dots. There are significant dot-to-dot
fluctuations in the lifetimes for a particular emission energy. To reach
general conclusions, we present the statistical behavior by analyzing data
recorded on a large number of individual quantum dots. Exciton charge is
controlled with extremely high fidelity through an n-type field effect
structure, providing access to the neutral exciton (X0), the biexciton (2X0)
and the positively (X1+) and negatively (X1-) charged excitons. We find
significant differences in the recombination lifetime of each exciton such
that, on average, Tau(X1-) / Tau(X0) = 1.25, Tau(X1+) / Tau(X0) = 1.58 and
Tau(2X0) / Tau(X0) = 0.65. We attribute the change in lifetime to significant
changes in the single particle hole wave function on charging the dot, an
effect more pronounced on charging X0 with a single hole than with a single
electron. We verify this interpretation by recasting the experimental data on
exciton energies in terms of Coulomb energies. We show directly that the
electron-hole Coulomb energy is charge dependent, reducing in value by 5-10% in
the presence of an additional electron, and that the electron-electron and
hole-hole Coulomb energies are almost equal.Comment: 8 pages, 7 figures, submitted to Phys. Rev.
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