375 research outputs found
Optimization of the processing of bio based polymer sustainable products
Polylactic Acid (PLA) is processed by injection moulding technology. The main aim of this study is to provide guidelines for mould and part design, namely to cope with the shrinkage effect and the ejection forces related to the use of bio based polymers. Furthermore optimization of the overall process will be investigated as well as the influence of different parameters to the process and product properties. Draft angle, mould temperature and holding pressure will be related to the ejection forces and the level of shrinkage that occurs
Photonic crystal carpet: Manipulating wave fronts in the near field at 1550 nm
Ground-plane cloaks, which transform a curved mirror into a flat one, and
recently reported at wavelengths ranging from the optical to the visible
spectrum, bring the realm of optical illusion a step closer to reality.
However, all carpet-cloaking experiments have thus far been carried out in the
far-field. Here, we demonstrate numerically and experimentally that a
dielectric photonic crystal (PC) of a complex shape made of a honeycomb array
of air holes can scatter waves in the near field like a PC with a at boundary
at stop band frequencies. This mirage effect relies upon a specific arrangement
of dielectric pillars placed at the nodes of a quasi-conformal grid dressing
the PC. Our carpet is shown to work throughout the range of wavelengths 1500nm
to 1650nm within the stop band extending from 1280 to 1940 nm. The device has
been fabricated using a single- mask advanced nanoelectronics technique on
III-V semiconductors and the near field measurements have been carried out in
order to image the wave fronts's curvatures around the telecommunication
wavelength 1550 nm.Comment: 6 page
Tight-binding g-Factor Calculations of CdSe Nanostructures
The Lande g-factors for CdSe quantum dots and rods are investigated within
the framework of the semiempirical tight-binding method. We describe methods
for treating both the n-doped and neutral nanostructures, and then apply these
to a selection of nanocrystals of variable size and shape, focusing on
approximately spherical dots and rods of differing aspect ratio. For the
negatively charged n-doped systems, we observe that the g-factors for
near-spherical CdSe dots are approximately independent of size, but show strong
shape dependence as one axis of the quantum dot is extended to form rod-like
structures. In particular, there is a discontinuity in the magnitude of
g-factor and a transition from anisotropic to isotropic g-factor tensor at
aspect ratio ~1.3. For the neutral systems, we analyze the electron g-factor of
both the conduction and valence band electrons. We find that the behavior of
the electron g-factor in the neutral nanocrystals is generally similar to that
in the n-doped case, showing the same strong shape dependence and discontinuity
in magnitude and anisotropy. In smaller systems the g-factor value is dependent
on the details of the surface model. Comparison with recent measurements of
g-factors for CdSe nanocrystals suggests that the shape dependent transition
may be responsible for the observations of anomalous numbers of g-factors at
certain nanocrystal sizes.Comment: 15 pages, 6 figures. Fixed typos to match published versio
Evolution of the electronic structure with size in II-VI semiconductor nanocrystals
In order to provide a quantitatively accurate description of the band gap
variation with sizes in various II-VI semiconductor nanocrystals, we make use
of the recently reported tight-binding parametrization of the corresponding
bulk systems. Using the same tight-binding scheme and parameters, we calculate
the electronic structure of II-VI nanocrystals in real space with sizes ranging
between 5 and 80 {\AA} in diameter. A comparison with available experimental
results from the literature shows an excellent agreement over the entire range
of sizes.Comment: 17 pages, 4 figures, accepted in Phys. Rev.
Electron-Hole Correlations and Optical Excitonic Gaps in Quantum-Dot Quantum Wells: Tight-Binding Approach
Electron-hole correlation in quantum-dot quantum wells (QDQW's) is
investigated by incorporating Coulomb and exchange interactions into an
empirical tight-binding model. Sufficient electron and hole single-particle
states close to the band edge are included in the configuration to achieve
convergence of the first spin-singlet and triplet excitonic energies within a
few meV. Coulomb shifts of about 100 meV and exchange splittings of about 1 meV
are found for CdS/HgS/CdS QDQW's (4.7 nm CdS core diameter, 0.3 nm HgS well
width and 0.3 nm to 1.5 nm CdS clad thickness) which have been characterized
experimentally by Weller and co-workers [ D. Schooss, A. Mews, A. Eychmuller,
H. Weller, Phys. Rev. B, 49, 17072 (1994)]. The optical excitonic gaps
calculated for those QDQW's are in good agreement with the experiment.Comment: 3 figures, to appear in Phys.Rev.
Does Pelletizing Catalysts Influence the Efficiency Number of Activity Measurements? Spectrochemical Engineering Considerations for an Accurate Operando Study
International audienc
Electronic structure and optical properties of ZnS/CdS nanoheterostructures
The electronic and optical properties of spherical nanoheterostructures are
studied within the semi-empirical tight-binding model including
the spin-orbit interaction. We use a symmetry-based approach previously applied
to CdSe and CdTe quantum dots. The complete one-particle spectrum is obtained
by using group-theoretical methods. The excitonic eigenstates are then deduced
in the configuration-interaction approach by fully taking into account the
Coulomb direct and exchange interactions. Here we focus on ZnS/CdS, ZnS/CdS/ZnS
and CdS/ZnS nanocrystals with particular emphasis on recently reported
experimental data. The degree of carrier localization in the CdS well layer is
analyzed as a function of its thickness. We compute the excitonic fine
structure, i.e., the relative intensities of low-energy optical transitions.
The calculated values of the absorption gap show a good agreement with the
experimental ones. Enhanced resonant photoluminescence Stokes shifts are
predicted.Comment: 6 pages, 4 Figures, revtex
Exciton states and optical properties of CdSe nanocrystals
The optical spectra of CdSe nanocrystals up to 55 A in diameter are analyzed
in a wide range of energies from the fine structure of the low-energy
excitations to the so-called high-energy transitions. We apply a symmetry-based
method in two steps. First we take the tight-binding (TB) parameters from the
bulk sp^{3}s^{*} TB model, extended to include the spin-orbit interaction. The
full single-particle spectra are obtained from an exact diagonalization by
using a group-theoretical treatment. The electron-hole interaction is next
introduced: Both the Coulomb (direct) and exchange terms are considered. The
high-energy excitonic transitions are studied by computing the electric dipole
transition probabilities between single-particle states, while the transition
energies are obtained by taking into account the Coulomb interaction. The fine
structure of the lowest excitonic states is analyzed by including the
electron-hole exchange interaction and the wurtzite crystal-field terms in the
exciton Hamiltonian. The latter is diagonalized in the single electron-hole
pair excitation subspace of progressively increasing size until convergence.
The peaks in the theoretical transition spectra are then used to deduce the
resonant and nonresonant Stokes shifts, which are compared with their measured
values in photoluminescence experiments. We find that the final results depend
on the crystal-field term, the relative size of the surface and the degree of
saturation of the dangling bonds. The results show a satisfactory agreement
with the available experimental data.Comment: Revtex, 24 pages, 7 Postscript figure
Drying Shrinkage Mechanisms in Portland Cement Paste
Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65426/1/j.1151-2916.1987.tb05002.x.pd
The war on street 'terror': why tackle anti-social behaviour?
This article examines the rationales of Dutch politicians for tackling the perceived pressing problem of 'anti-social behaviour' (ASB) and the question did they copy the British approach? The first part will describe in short the concept of policy transfer and the recent British fight against ASB. The focus will be on the introduction of the Anti-social Behaviour Order. The second part is an empirical study into the Dutch retreat from 'condoning' ASB, consisting of interviews with Dutch politicians focusing on their ideas for tackling ASB. Those are compared with the British's rationales. This kind of comparative elite ethnography is not common in criminology, but this article aims at providing evidence of its benefits. By answering the research question an insight into the origins of policy in the sphere of criminal justice can be obtained
- …