946 resultados para tensile strain
Resumo:
Molecular dynamics simulations with the Tersoff potential were used to study the response of twinned SiC nanowires under tensile and compressive strain. The critical strain of the twinned nanowires can be enhanced by twin stacking faults, and their critical strains are larger than those of perfect nanowires with the same diameters. Under axial tensile strain, the bonds of the nanowires are stretched just before failure. The failure behavior is found to depend on the twin segment thickness and the diameter of the nanowires. An atomic chain is observed for thin nanowires with small twin segment thickness under tension strain. Under axial compressive strain, the collapse of twinned SiC nanowires exhibits two different failure modes, depending on the length and diameter of the nanowires, i.e., shell buckling for short nanowires and columnar buckling for longer nanowires.
Resumo:
Tensile-strained GaAsP/GaInP single quantum well (QW) laser diode (I-D) structures have been grown by low-pressure metal organic chemical vapor deposition (LP-MOCVD) and related photoluminescence (PL) properties have been investigated in detail. The samples have the same well thickness of 16 nm but different P compositions in a GaAsP QW. Two peaks in room temperature (RT) PL spectra are observed for samples with a composition larger than 0.10. Temperature and excitation-power-dependent PL spectra have been measured for a sample with it P composition of 0.15. It is found that the two peaks have a 35 meV energy separation independent of temperature and only the low-energy peak exists below 85 K. Additionally, both peak intensities exhibit a monotonous increase as excitation power increases. Analyses indicate that the two peaks arise from the intrinsic-exciton recombination mechanisms of electron-heavy hole (e-hh) and electron-light hole (e-hh). A theoretical calculation based oil model-solid theory, taking, into account the spin-orbit splitting energy, shows good agreement with our experimental results. The temperature dependence of PL intensity ratio is well explained using the spontaneous emission theory for e-hh and e-hh transitions. front which the ratio can be characterized mainly by the energy separation between the fill and Ill states.
Resumo:
The hole subband structures and effective masses of tensile strained Si/Si1-yGey quantum wells are calculated by using the 6x6 k.p method. The results show that when the tensile strain is induced in the quantum well, the light-hole state becomes the ground state, and the light hole effective masses in the growth direction are strongly reduced while the in-plane effective masses are considerable. Quantitative calculation of the valence intersubband transition between two light hole states in a 7nm tensile strained Si/Si0.55Ge0.45 quantum well grown on a relaxed Si0.5Ge0.5 (100) substrates shows a large absorption coefficient of 8400 cm(-1).
Resumo:
A novel broadband superluminescent diode (SLD), which has a symmetric graded tensile-strained bulk InGaAs active region, is developed. The symmetric-graded tensile-strained bulk InGaAs is achieved by changing the group III TMGa source flow only during its growth process by low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE), in which the much different tensile strain is introduced simultaneously. At 200mA injection current, the full width at half maximum (FWHM) of the emission spectrum of the SLID can be up to 122nm, covering the range of 1508-1630nm, and the output power is 11.5mW.
Influence of AlN thickness on strain evolution of GaN layer grown on high-temperature AlN interlayer
Resumo:
The strain evolution of a GaN layer grown on a high- temperature AlN interlayer with varying AlN thickness by metalorganic chemical vapour deposition is investigated. In the growth process, the growth strain changes from compression to tension in the top GaN layer, and the thickness at which the compressive- to- tensile strain transition takes place is strongly influenced by the thickness of the AlN interlayer. It is confirmed from the x- ray diffraction results that the AlN interlayer has a remarkable effect on introducing relative compressive strain to the top GaN layer. The strain transition process during the growth of the top GaN layer can be explained by the threading dislocation inclination in the top GaN layer. Adjusting the AlN interlayer thickness could change the density of the threading dislocations in the top GaN layer and then change the stress evolution during the top GaN layer's growth.
Resumo:
High-quality AlInGaN quaternary layers were grown on c-Al2O3 using a thick GaN template. A full width at half maximum of 0.075 degrees from AlInGaN(0004) rocking curve and a minimum yield of 5.6% from Rutherford backscattering/channelling spectrometry (RBS) prove the AlInGaN layer of a comparable crystalline quality with GaN layers. The chemical compositions (both of Al and In contents) of AlInGaN layers are directly obtained from RBS and elastic recoil detection analysis. The lattice parameters both in perpendicular and parallel directions are deduced from X-ray diffraction. The AlInGaN layer is found to process a compressive strain in parallel direction and a tensile strain in perpendicular direction. (c) 2006 Elsevier B.V. All rights reserved.
Resumo:
The strain evolution of the GaN layer grown on a high-temperature AlN interlayer with GaN template by metal organic chemical vapor deposition is investigated. It is found that the layer is initially under compressive strain and then gradually relaxes and transforms to under tensile strain with increasing film thickness. The result of the in situ stress analysis is confirmed by x-ray diffraction measurements. Transmission electron microscopy analysis shows that the inclination of edge and mixed threading dislocations rather than the reduction of dislocation density mainly accounts for such a strain evolution. (c) 2006 American Institute of Physics.
Resumo:
By using reflectance difference spectroscopy we have studied the in-plane optical anisotropy of GaAs surfaces covered by ultrathin InAs layers. The strain evolution of the GaAs surface with the InAs deposition thickness can be obtained. It is found that the optical anisotropy and the surface tensile strain attain maximum values at the onset of the formation of InAs quantum dots (QDs) and then decrease rapidly as more InAs QDs are formed with the increase of InAs deposition. The origin of the optical anisotropy has been discussed.
Resumo:
The reduction of residual strain in cubic GaN growth by inserting a thermoannealing process is investigated. It is found that the epilayer with smaller tensile strain is subject to a wider optimal "growth window." Based on this process, we obtain the high-quality GaN film of pure cubic phase with the thickness of 4 mum by metalorganic chemical vapor deposition. The photoluminescence spectrum at room temperature shows the thick GaN layer has a near-band emission peak with a full width at half maximum of 42 meV which confirms its high crystal quality, further supported by the x-ray (002) diffraction measurement. A simplified model is demonstrated to interpret this strain effect on the growth process. (C) 2003 American Institute of Physics.
Resumo:
We investigated AlGaN layers grown by metalorganic chemical vapor deposition (MOCVD) on high temperature (HT-)GaN and AlGaN buffer layers. On GaN buffer layer, there are a lot of surface cracking because of tensile strain in subsequent AlGaN epilayers. On HT-AlGaN buffer layer, not only cracks but also high densities rounded pits present, which is related to the high density of coalescence boundaries in HT-AlGaN growth process.The insertion of interlayer (IL) between AlGaN and the GaN pseudosubstrate can not only avoid cracking by modifying the strain status of the epilayer structure, but also improved Al incorporation efficiency and lead to phase-separation. And we also found the growth temperature of IL is a critical parameter for crystalline quality of subsequent AlGaN epilayer. Low temperature (LT-) A1N IL lead to a inferior quality in subsequent AlGaN epilayers.
Resumo:
Strain-dependent microstructural modifications were observed in epitaxial BiCrO3 (BCO) thin films fabricated on single crystalline substrates, utilizing pulsed laser deposition. The following conditions were employed to modify the epitaxial-strain: (i) in-plane tensile strain, BCOSTO [BCO grown on buffered SrTiO3 (001)] and in-plane compressive strain, BCONGO [BCO grown on buffered NdGaO3 (110)] and (ii) varying BCO film thickness. A combination of techniques like X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (TEM) was used to analyse the epitaxial growth quality and the microstructure of BCO. Our studies revealed that in the case of BCOSTO, a coherent interface with homogeneous orthorhombic phase is obtained only for BCO film with thicknesses, d < 50 nm. All the BCOSTO films with d = 50 nm were found to be strain-relaxed with an orthorhombic phase showing 1/2 <100> and 1/4 <101> satellite reflections, the latter oriented at 45° from orthorhombic diffraction spots. High angle annular dark field scanning TEM of these films strongly suggested that the satellite reflections, 1/2 <100> and 1/4 <101>, originate from the atomic stacking sequence changes (or “modulated structure”) as reported for polytypes, without altering the chemical composition. The unaltered stoichiometry was confirmed by estimating both valency of Bi and Cr cations by surface and in-depth XPS analysis as well as the stoichiometric ratio (1 Bi:1 Cr) using scanning TEM–energy dispersive X-ray analysis. In contrast, compressively strained BCONGO films exhibited monoclinic symmetry without any structural modulations or interfacial defects, up to d ~ 200 nm. Our results indicate that both the substrate-induced in-plane epitaxial strain and the BCO film thickness are the crucial parameters to stabilise a homogeneous BCO phase in an epitaxially grown film.
Resumo:
A series of quasi-static and dynamic tensile tests at varying temperatures were carried out to determine the mechanical behaviour of Ti-45Al-2Nb-2Mn+0.8vol.% TiB2 XD as-HIPed alloy. The temperature for the tests ranged from room temperature to 850 ∘C. The effect of the temperature on the ultimate tensile strength, as expected, was almost negligible within the selected temperature range. Nevertheless, the plastic flow suffered some softening because of the temperature. This alloy presents a relatively low ductility; thus, a low tensile strain to failure. The dynamic tests were performed in a Split Hopkinson Tension Bar, showing an increase of the ultimate tensile strength due to the strain rate hardening effect. Johnson-Cook constitutive relation was used to model the plastic flow. A post-testing microstructural of the specimens revealed an inhomogeneous structure, consisting of lamellar α2 + γ structure and γ phase equiaxed grains in the centre, and a fully lamellar structure on the rest. The assessment of the duplex-fully lamellar area ratio showed a clear relationship between the microstructure and the fracture behaviour.
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Ripples, present in free standing graphene, have an important influence in the mechanical behavior of this two-dimensional material. In this work we show through nanoindentation simulations, how out-of-plane displacements can be modified by strain resulting in softening of the membrane under compression and stiffening under tension. Irradiation also induces changes in the mechanical properties of graphene. Interestingly, compressed samples, irradiated at low doses are stiffened by the irradiation while samples under tensile strain do not show significant changes in their mechanical properties. These simulations indicate that vacancies, produced by the energetic ions, cannot be the ones directly responsible for this behavior. However, changes in roughness induced by the momentum transferred from the energetic ions to the membrane, can explain these differences. These results provide an alternative explanation to recent experimental observations of stiffening of graphene under low dose irradiation, as well as paths to tailor the mechanical properties of this material via applied strain and irradiation.
Resumo:
Graphene, due to its exceptional properties, is a promising material for nanotechnology applications. In this context, the ability to tune the properties of graphene-based materials and devices with the incorporation of defects and impurities can be of extraordinary importance. Here we investigate the effect of uniaxial tensile strain on the electronic and magnetic properties of graphene doped with substitutional Ni impurities (Ni_sub). We have found that, although Ni_sub defects are non-magnetic in the relaxed layer, uniaxial strain induces a spin moment in the system. The spin moment increases with the applied strain up to values of 0.3-0.4 \mu_B per Ni_sub, until a critical strain of ~6.5% is reached. At this point, a sharp transition to a high-spin state (~1.9 \mu_B) is observed. This magnetoelastic effect could be utilized to design strain-tunable spin devices based on Ni-doped graphene.
Resumo:
To analyse mechanotransduction resulting from tensile loading under defined conditions, various devices for in vitro cell stimulation have been developed. This work aimed to determine the strain distribution on the membrane of a commercially available device and its consistency with rising cycle numbers, as well as the amount of strain transferred to adherent cells. The strains and their behaviour within the stimulation device were determined using digital image correlation (DIC). The strain transferred to cells was measured on eGFP-transfected bone marrow-derived cells imaged with a fluorescence microscope. The analysis was performed by determining the coordinates of prominent positions on the cells, calculating vectors between the coordinates and their length changes with increasing applied tensile strain. The stimulation device was found to apply homogeneous (mean of standard deviations approx. 2% of mean strain) and reproducible strains in the central well area. However, on average, only half of the applied strain was transferred to the bone marrow-derived cells. Furthermore, the strain measured within the device increased significantly with an increasing number of cycles while the membrane's Young's modulus decreased, indicating permanent changes in the material during extended use. Thus, strain magnitudes do not match the system readout and results require careful interpretation, especially at high cycle numbers.