Effects of solution exposure on the combined axial-shear behaviour of unidirectional CFRP rods

In fibre reinforced polymer (FRP) prestressed concrete applications, an FRP tendon must sustain high axial tensile stresses and, if cracks occur, additional dowel forces. The tendon may also be exposed to solutions and so the combined axial-shear stress performance after long-term environmental exposure is important. Experiments were conducted to investigate the combined axial-shear stress failure envelope for unidirectional carbon FRP tendons which had been exposed to either water, salt water or concrete pore solution at 60 °C for approximately 18 months. The underlying load resisting mechanisms were found to depend on the loading configuration, restraint effects and the initial stress state. When saturated, CFRP tendons are likely to exhibit a reduced shear stiffness. However, the ultimate limit state appeared to be fibre-dominated and was therefore less susceptible to reductions due to solution uptake effects. © 2012 Elsevier Ltd. All rights reserved.

Bifurcations and instability in the adhesion of intrinsically curved rods

Motivated by applications such as gecko-inspired adhesives and microdevices featuring slender rod-like bodies, there has been an increase in interest in the deformed shapes of elastic rods adhering to rigid surfaces. A central issue in analyses of the rod-based models for these systems is the stability of the predicted equilibrium configurations. Such analyses can be complicated by the presence of intrinsic curvatures induced by fabrication processes. The results in the present paper are used to show how this curvature can lead to shear-induced bifurcations and instabilities. To characterize potential instabilities, a new set of necessary conditions for stability are employed which cater to the possible combinations of buckling and delaminating instabilities. © 2013 Elsevier Ltd. All rights reserved.

Effects of shape and magnetic field on the optical properties of wurtzite quantum rods

The optical properties of quantum rods in the absence and presence of the magnetic field are studied in the framework of effective-mass envelope function theory. The two-dimensional (2D) and 1D transition dipoles of wurtzite quantum rods are investigated. It is found that the transition dipoles change from 2D to 1D as the aspect ratio of the ellipsoid increases, in agreement with the experimental results. The linear polarization factors of optical transitions of quantum rods with critical aspect ratio are zero at every orientation of the wave propagation. So quantum rods with critical aspect ratio have isotropic transition dipoles. Due to the 2D or 1D transition dipoles, the linear polarization factors of optical transitions of quantum rods change from negative or positive values to zero as the orientation of the wave propagation changes from the x axis of the crystal structure to the z axis, in agreement with the experimental results. Under magnetic field applied along the z axis of the crystal structure, the negative linear polarization factors in the 2D transition dipole case decrease as the magnetic field increases, while under magnetic field applied along the x axis, the negative linear polarization factors increase as the magnetic field increases. The antisymmetric Hamiltonian is very important to these effects of the magnetic field. It is found that quantum rods with a given radius at a given temperature have dark excitons in a range of aspect ratio. The dimensions along the x, y axes of the crystal structure play opposite roles to the dimension along the z axis on the dark exciton phenomenon. Dark excitons become bright under appropriate magnetic field.

Pressure behaviour of the UV and green emission bands in ZnO micro-rods

The pressure behavior of the ultraviolet (UV) and green emission bands in ZnO tetrapod-like micro-rods has been investigated at 300 and 70 K, respectively. The pressure coefficient of the UV band at 300 K is 24.5 meV/GPa, consistent with that of the band gap of bulk ZnO. However, the pressure coefficient of the green band is 25 meV/GPa, far larger than previous literature reports. The green band in this work originates from Cu-related emission, as confirmed by the fine structure observed in the spectra at 10 K. The pressure coefficients of four phonon replicas of the free exciton emission (FX) at 70 K are 21.0, 20.2, 19.8, and 19.3 meV/GPa, respectively. The energy shift rate of the FX emission and the LO phonon energies is then determined to be 21.4 and 0.55 meV/GPa. The pressure coefficient of the neutral donor bound exciton ((DX)-X-0) transition is 20.5 meV/GPa, only 4% smaller than that of FX. This confirms that the (DX)-X-0 emission corresponds to excitons bound to neutral shallow donors. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Optical properties and g factors of GaAs quantum rods

The Hamiltonian of the zinc-blende quantum rods in the framework of eight-band effective-mass approximation in the presence of external homogeneous magnetic field is given. The electronic structure, optical properties and electron g factors of GaAs quantum rods are investigated. We found that the electron g factors are very sensitively dependent on the dimensions of the quantum rods. As some of the three dimensions increase, the electron g factors decrease. The more the dimensions increase, the more the electron g factors decrease. The dimensions perpendicular to the direction of the magnetic field affect the electron g factors more than the other dimension. (c) 2006 Elsevier B.V. All rights reserved.

Effects of electric field on the electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of wurtzite quantum rods with an ellipsoidal boundary under electric field is given after a coordinate transformation. The electronic structure and optical properties are studied in the framework of the effective-mass envelope-function theory. The quantum-confined Stark effect is illustrated by studying the change of the electronic structures under electric field. The transition probabilities between the electron and hole states decrease sharply with the increase of the electric field. The polarization factor increases with the increase of the electric field. Effects of the electric field and the shape of the rods on the exciton effect are also investigated. The exciton binding energy decreases with the increase of both the electric field and the aspect ratio. In the end, considering the exciton binding energy, we calculated the band gap variation of size- and shape-controlled colloidal CdSe quantum rods, which is in good agreement with experimental results.

Electronic structure and optical properties of quantum rods with wurtzite structure

The Hamiltonian of the wurtzite quantum rods with an ellipsoidal boundary is given after a coordinate transformation. The energies, wave functions, and transition possibilities are obtained as functions of the aspect ratio e with the same method we used on spherical dots. With an overall consideration of both the transition matrix element and the Boltzmann distribution we explained why the polarization factor increases with increasing e and approaches a saturation value, which tallies quite well with the experimental result. When e increases more and more S-z states are mixed into the ground, second, and third states of J(z)=1/2, resulting in an increase of the emission of z polarization. It is just the linear terms of the momentum operator in the hole Hamiltonian that cause the mixing of S and P states in the hole ground state. The effects of the crystal field splitting energy, temperature, and transverse radius to the polarization are also considered. We also calculated the band gap variation with the size and shape of the quantum rods.

Linear-Polarization Optical Property of CdSe Quantum Rods

The linear-polarization optical property of CdSe quantum rods is studied in the framework of effective-mass envelope function theory.The effects of shape and magnetic field on the linear polarization factors are investigated.It is found that CdSe quantum spheres have negative polarization factors (xy-polarized emission)and quantum long rods with small radius have positive linear polarization factors (z-polarized emission).The z-direction is the direction of the c axis.Quantum long rods with large radius have negative linear polarization factors,due to the hexagonal crystal symmetry and the crystal field splitting energy.The linear polarization factors decrease and may change from a positive value to a negative value;i.e.,the z-polarized emissions decrease relative to xy-polarized emissions as the magnetic field applied along the z direction increases.