Stress-induced platelet formation in silicon:a molecular dynamics study

The effect of stress on vacancy cluster configurations in silicon is examined using molecular dynamics. At zero pressure, the shape and stability of the vacancy clusters agrees with previous atomistic results. When stress is applied the orientation of small planar clusters changes to reduce the strain energy. The preferred orientation for the vacancy clusters under stress agrees with the experimentally observed orientations of hydrogen platelets in the high stress regions of hydrogen implanted silicon. These results suggest a theory for hydrogen platelet formation

Molecular dynamics simulation of brittle fracture in silicon

The fracture process involves converting potential energy from a strained body into surface energy, thermal energy, and the energy needed to create lattice defects. In dynamic fracture, energy is also initially converted into kinetic energy. This paper uses molecular dynamics (MD) to simulate brittle frcture in silicon and determine how energy is converted from potential energy (strain energy) into other forms

Faster radial strain relaxation in InAs-GaAs core-shell heterowires

The structure of wurtzite and zinc blende InAs-GaAs (001) core-shell nanowires grown by molecular beam epitaxy on GaAs (001) substrates has been investigated by transmission electron microscopy. Heterowires with InAs core radii exceeding 11 nm, strain relax through the generation of misfit dislocations, given a GaAs shell thickness greater than 2.5 nm. Strain relaxation is larger in radial directions than axial, particularly for shell thicknesses greater than 5.0 nm, consistent with molecular statics calculations that predict a large shear stress concentration at each interface corner

Computational fluid dynamics study of dusty air flow over NACA 63415 airfoil for wind turbine applications

Gulf and South African countries have enormous potential for wind energy. However, the emergence of sand storms in this region postulates performance and reliability challenges on wind turbines. This study investigates the effects of debris flow on wind turbine blade performance. In this paper, two-dimensional incompressible Navier-Stokes equations and the transition SST turbulence model are used to analyze the aerodynamic performance of NACA 63415 airfoil under clean and sandy conditions. The numerical simulation of the airfoil under clean surface condition is performed at Reynolds number 460×103, and the numerical results have a good consistency with the experimental data. The Discrete Phase Model has been used to investigate the role sand particles play in the aerodynamic performance degradation. The pressure and lift coefficients of the airfoil have been computed under different sand particles flow rates. The performance of the airfoil under different angle of attacks has been studied. Results showed that the blade lift coefficient can deteriorate by 28% in conditions relevant to the Gulf and South African countries sand storms. As a result, the numerical simulation method has been verified to be economically available for accurate estimation of the sand particles effect on the wind turbine blades

Cracking in hydrogen ion-implanted Si/Si0.8Ge0.2/Si heterostructures

We demonstrate that a controllable cracking can be realized in Si with a buried strain layer when hydrogen is introduced using traditional H-ion implantation techniques. However, H stimulated cracking is dependent on H projected ranges; cracking occurs along a Si0.8Ge0.2 strain layer only if the H projected range is shallower than the depth of the strained layer. The absence of cracking for H ranges deeper than the strain layer is attributed to ion-irradiation induced strain relaxation, which is confirmed by Rutherford-backscattering-spectrometry channeling angular scans. The study reveals the importance of strain in initializing continuous cracking with extremely low H concentrations

Study on fatigue and energy-dissipation properties of nanolayered Cu/Nb thin films

Energy dissipation and fatigue properties of nano-layered thin films are less well studied than bulk properties. Existing experimental methods for studying energy dissipation properties, typically using magnetic interaction as a driving force at different frequencies and a laser-based deformation measurement system, are difficult to apply to two-dimensional materials. We propose a novel experimental method to perform dynamic testing on thin-film materials by driving a cantilever specimen at its fixed end with a bimorph piezoelectric actuator and monitoring the displacements of the specimen and the actuator with a fibre-optic system. Upon vibration, the specimen is greatly affected by its inertia, and behaves as a cantilever beam under base excitation in translation. At resonance, this method resembles the vibrating reed method conventionally used in the viscoelasticity community. The loss tangent is obtained from both the width of a resonance peak and a free-decay process. As for fatigue measurement, we implement a control algorithm into LabView to maintain maximum displacement of the specimen during the course of the experiment. The fatigue S-N curves are obtained

H-induced platelet and crack formation in hydrogenated epitaxial Si/Si <inf>0.98</inf>B <inf>0.02</inf>/Si structures

An approach to transfer a high-quality Si layer for the fabrication of silicon-on-insulator wafers has been proposed based on the investigation of platelet and crack formation in hydrogenated epitaxial Si Si0.98 B0.02 Si structures grown by molecular-beam epitaxy. H-related defect formation during hydrogenation was found to be very sensitive to the thickness of the buried Si0.98 B0.02 layer. For hydrogenated Si containing a 130 nm thick Si0.98 B0.02 layer, no platelets or cracking were observed in the B-doped region. Upon reducing the thickness of the buried Si0.98 B0.02 layer to 3 nm, localized continuous cracking was observed along the interface between the Si and the B-doped layers. In the latter case, the strains at the interface are believed to facilitate the (100)-oriented platelet formation and (100)-oriented crack propagation. © 2006 American Institute of Physics

Increasing the fracture toughness of silicon by ion implantation

This study was motivated by some earlier indications that the fracture toughness of silicon could be increased by ion implantation. The location of fracture in hydrogen implanted silicon was found to change depending on the dose of implanted ions. For relatively low doses, fracture occurred at the center of the damage region created by implantation. However, for larger doses, the fracture location switched to the deeper edge of the implanted zone. This implied that the center of the implanted region experienced toughening due to ion implantation at the higher dose level. In addition, an initial increase in fracture toughness with radiation dose has been observed experimentally in some ceramics. After the initial increase, the fracture toughness reaches a peak and then decreases with further irradiation. The toughness increases found thus far are modest (25-100%). In attempts to explain the experimental results, several toughening mechanisms (such as deflection of the crack by the irradiated damage) have been proposed. However, the proposed mechanisms predict only a 40-80% increase in fracture toughness, which does not account for the highest levels of toughness observed. Our recent molecular dynamics (MD) calculations have found a previously unknown toughening mechanism acting in silicon, which can also explain the earlier experimental observations of toughening induced by irradiation. In our MD simulations, ion implantation produced clusters of disordered atoms. The presence of these clusters allowed silicon to deform plastically as a crack approached, blunting the crack tip and arresting crack growth. The MD calculations show a factor of 3 increase in fracture toughness. We have conducted experiments with silicon implanted with a small dose (ions/cm2) of alpha particles uniformly distributed to a depth of 25 pm. A 20% increase in fracture toughness is observed. Additional experiments with higher implantation doses are planned for the immediate future

Performance of a Wind Turbine Blade in Sandstorms Using a CFD-BEM Based Neural Network

In arid regions, such as the North African desert, sandstorms impose considerable restrictions on horizontal axis wind turbines (HAWTs), which have not been thoroughly investigated. This paper examines the effects of debris flow on the power generation of the HAWT. Computational Fluid Dynamics (CFD) models were established and validated to provide novel insight into the effects of debris on the aerodynamic characteristics of NACA 63415. To account for the change in the chord length and Reynolds number along the span of the blade and the 3D flow patterns, the power curves for a wind turbine were obtained using the Blade Element Momentum (BEM) method. We present a novel coupled application of the neural network, CFD, and BEM to investigate the erosion rates of the blade due to different sandstorm conditions. The proposed model can be scaled and developed to assist in monitoring and prediction of HAWT blade conditions. This work shows that HAWT performance can be significantly diminished due to the aerodynamic losses under sandstorm conditions. The power generated under debris flow conditions can decrease from 10 to 30% compared to clean conditions