1. |
- Melin, L. Gunnar, et al.
(författare)
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A study of Mode-I delamination cracks by high-magnification moiré interferometry
- 1998
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Ingår i: Composites Science And Technology. - 0266-3538 .- 1879-1050. ; 58:3-4, s. 515-525
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Tidskriftsartikel (refereegranskat)abstract
- Delamination cracks in carbon-fibre/epoxy composites loaded in Mode-I have been studied by high-magnification moiré interferometry. The moiré interferometer has the capability of switching between laser illumination for acquiring the displacement field and white-light illumination for visualisation of the underlying microstructure. Four specimens with different stacking geometries have been investigated. The delamination cracks have been simulated by using Teflon films. The resulting displacement fields have been compared to numerical solutions calculated by the finite-element method. The comparisons indicate that the Teflon films for some of the specimens did not delaminate fully. Otherwise the agreement is good, confirming that the displacement field around such cracks is satisfactorily described by linear-elastic fracture mechanics. The agreement was improved when a matrix-rich region in front of the crack tip was included in the finite-element model.
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2. |
- Adey, J., et al.
(författare)
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Degradation of boron-doped Czochralski-grown silicon solar cells
- 2004
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 93:5, s. 055504-1
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Tidskriftsartikel (refereegranskat)abstract
- The formation mechanism and properties of the boron-oxygen center responsible for the degradation of Czochralski-grown Si(B) solar cells during operation is investigated using density functional calculations. We find that boron traps an oxygen dimer to form a bistable defect with a donor level in the upper half of the band gap. The activation energy for its dissociation is found to be 1.2 eV. The formation of the defect from mobile oxygen dimers, which are shown to migrate by a Bourgoin mechanism under minority carrier injection, has a calculated activation energy of 0.3 eV. These energies and the dependence of the generation rate of the recombination center on boron concentration are in good agreement with observations.
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3. |
- Adey, J., et al.
(författare)
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Theory of boron-vacancy complexes in silicon
- 2005
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 71:16, s. 165211-
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Tidskriftsartikel (refereegranskat)abstract
- The substitutional boron-vacancy BsV complex in silicon is investigated using the local density functional theory. These theoretical results give an explanation of the experimentally reported, well established metastability of the boron-related defect observed in p-type silicon irradiated at low temperature and of the two hole transitions that are observed to be associated with one of the configurations of the metastable defect. BsV is found to have several stable configurations, depending on charge state. In the positive charge state the second nearest neighbor configuration with C1 symmetry is almost degenerate with the second nearest neighbor configuration that has C1h symmetry since the bond reconstruction is weakened by the removal of electrons from the center. A third nearest neighbor configuration of BsV has the lowest energy in the negative charge state. An assignment of the three energy levels associated with BsV is made. The experimentally observed Ev+0.31 eV and Ev+0.37 eV levels are related to the donor levels of second nearest neighbor BsV with C1 and C1h symmetry respectively. The observed Ev+0.11 eV level is assigned to the vertical donor level of the third nearest neighbor configuration. The boron-divacancy complex BsV2 is also studied and is found to be stable with a binding energy between V2 and Bs of around 0.2 eV. Its energy levels lie close to those of the V2. However, the defect is likely to be an important defect only in heavily doped material.
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4. |
- Huntley, Jonathan M., et al.
(författare)
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Microstructural strain analysis by high magnification moiré interferometry
- 1995
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Ingår i: Interferometry VII. - Bellingham, Wash : SPIE - International Society for Optical Engineering. - 9780819419040
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Konferensbidrag (refereegranskat)abstract
- A high magnification phase-stepping interferometer has been constructed which has a spatial resolution of the order of one micrometers and a sub-mm field of view. Laser illumination is delivered to the microscope head by polarization-preserving single-mode optical fibers. The head itself is a compact unit consisting of collimating optics, objective lens, CCD camera, and a separate white light source. Phase gratings are cast on the polished specimen surface by replication from a master grating, in either silicone rubber or epoxy resin. Subsequent evaporation of a thin layer of gold onto the grating increases the reflectivity and reduces the speckle noise in the images. By switching between the laser illumination and the white light unit, it is possible to view the underlying microstructure in exact registration with the measured displacement fields. The instrument is illustrated with several applications including the visualization of delamination cracks in graphite-epoxy composites and measurement of the strain-to-failure of polymer-bonded-explosives.
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5. |
- Larsen, A. Nylandsted, et al.
(författare)
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E center in silicon has a donor level in the band gap
- 2006
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Ingår i: Physical Review Letters. - 0031-9007 .- 1079-7114. ; 97:10, s. 106402/1-106402/4
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Tidskriftsartikel (refereegranskat)abstract
- It has been an accepted fact for more than 40 years that the E center in Si (the group-V impurity-vacancy pair)-one of the most studied defects in semiconductors-has only one energy level in the band gap: namely, the acceptor level at about 0.45 eV below the conduction band. We now demonstrate that it has a second level, situated in the lower half of the band gap at 0.27 eV above the valence band. The existence of this level, having a donor character, is disclosed by a combination of different transient-capacitance techniques and electronic-structure calculations. The finding seriously questions some diffusion-modeling approaches performed in the past.
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6. |
- Melin, L. Gunnar, et al.
(författare)
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A study of the displacement field around embedded fibre optic sensors
- 1999
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Ingår i: Composites. Part A, Applied science and manufacturing. - 1359-835X .- 1878-5840. ; 30:11, s. 1267-1275
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Tidskriftsartikel (refereegranskat)abstract
- Deformation fields around optical fibres embedded in carbon fibre/epoxy composite specimens have been measured using moiré interferometry. The inclusion of the optical fibre resulted in large strain gradients. Calculated displacements from finite element analysis were compared to the experimental results. The numerical analysis showed that the displacement field on the specimen surface is smoothed out through the moiré grating thickness, an effect which is most pronounced at the material interfaces. With this influence taken into consideration a reasonable good quantitative agreement between the experiments and the finite element analysis was obtained. The finite element analysis also showed that the grating stiffness did not affect the measured displacements as long as the grating had a lower stiffness than the specimen
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7. |
- Pinto, H., et al.
(författare)
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First-principles studies of the effect of (001) surface terminations on the electronic properties of the negatively charged nitrogen-vacancy defect in diamond
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 86:4
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Tidskriftsartikel (refereegranskat)abstract
- Density functional calculations have been carried out on (001)-orientated slabs of diamond with different surface terminations. A negatively charged nitrogen-vacancy defect (NV-) is placed in the middle of the slab approximately 1 nm from each surface and the effect of the surface on the internal optical transition in NV- investigated. The calculations show that the chemical nature of the surface is important. We find that although the clean surface does not lead to charge transfer between the defect and the surface, there is a splitting of the empty excited state, the final state in optical absorption, arising from a strong hybridization of the surface and defect bands. This leads to a broadening of the 1.945-eV transition of the NV- defect. OH- and F-terminated surfaces have no surface states in the band gap and again charge transfer between the defect and surface does not occur. The splitting of the e levels responsible for the optical transitions for OH or F termination is similar to that found in periodic boundary condition simulations for bulk diamond where the defects are separated by 1 nm, and thus the calculations show that hydroxylated or fluorinated surfaces give favorable optical properties.
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8. |
- Pinto, H., et al.
(författare)
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On the diffusion of NV defects in diamond
- 2012
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 209:9, s. 1765-1768
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Tidskriftsartikel (refereegranskat)abstract
- Besides their importance for quantum information processing, NV defects are crucial agents for the diffusion and aggregation of nitrogen in diamond. In the absence of transition metals, it is thought that the first stage of nitrogen aggregation, where close neighbour nitrogen pairs are formed, is mediated by NV defects. Here we use density functional theory to explore the barriers to NV diffusion. We conclude that the barrier is around 5 eV when there is a ready source of vacancies and that this barrier is weakly dependent on pressure.
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9. |
- Pinto, H., et al.
(författare)
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Theory of the surface effects on the luminescence of the NV(-) defect in nanodiamond
- 2011
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Ingår i: Physica Status Solidi (a) applications and materials science. - : Wiley. - 1862-6300 .- 1862-6319. ; 208:9, s. 2045-2050
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Tidskriftsartikel (refereegranskat)abstract
- A vacancy with one of the carbon neighbours replaced by a nitrogen atom in diamond (the NV centre) is a defect of particular interest due to its many potential applications. In the negatively charged state, the defect is paramagnetic with spin 1 and under optical excitation it exhibits an intense luminescence with a zero-phonon line at 1.945eV. This fluorescence is found in nanodiamonds even as small as 5nm and an important question is the effect of the surface of the nanodiamond on the optical emission of NV-.Density functional calculations are used in this work to investigate the effect of the bare (001) and (001)-OH diamond surfaces on the electronic structure of NV-. We show that the (001)-OH diamond surface has the minimum interaction with the defect and is the ideal terminating surface of nanodiamonds, while the bare (001) diamond surface has a strong effect on broadening the emission.
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10. |
- Sugita, S., et al.
(författare)
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The geomorphology, color, and thermal properties of Ryugu: Implications for parent-body processes
- 2019
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Ingår i: Science. - : AAAS. - 0036-8075 .- 1095-9203. ; 364:6437
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Tidskriftsartikel (refereegranskat)abstract
- Asteroids fall to Earth in the form of meteorites, but these provide little information about their origins. The Japanese mission Hayabusa2 is designed to collect samples directly from the surface of an asteroid and return them to Earth for laboratory analysis. Three papers in this issue describe the Hayabusa2 team's study of the near-Earth carbonaceous asteroid 162173 Ryugu, at which the spacecraft arrived in June 2018 (see the Perspective by Wurm). Watanabeet al.measured the asteroid's mass, shape, and density, showing that it is a “rubble pile” of loose rocks, formed into a spinning-top shape during a prior period of rapid spin. They also identified suitable landing sites for sample collection. Kitazatoet al.used near-infrared spectroscopy to find ubiquitous hydrated minerals on the surface and compared Ryugu with known types of carbonaceous meteorite. Sugitaet al.describe Ryugu's geological features and surface colors and combined results from all three papers to constrain the asteroid's formation process. Ryugu probably formed by reaccumulation of rubble ejected by impact from a larger asteroid. These results provide necessary context to understand the samples collected by Hayabusa2, which are expected to arrive on Earth in December 2020.Science, this issue p.268, p.272, p.eaaw0422; see also p.230
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