Mechanical and fracture behaviors of defective silicon nanowires: combined effects of vacancy clusters,temperature, wire size,and shape

Abstract The coupled effects of vacancy clusters (VCs), temperature, wire size, and geometry on the mechanical and fracture behaviors of defective silicon nanowires (Si NWs) were investigated using molecular dynamics modeling with Tersoff potential. The formation energies (E _v ) of a monovacancy (3.933 eV) and a tetrahedron vacancy (10.189 eV) obtained in this study agree well with experimental results and ab initio calculation. Simulation results show that the slip deformations of defective Si NWs are triggered at the wire’s surface and edge due to the number of dangling bonds on the wire’s surface being much greater than that inside a vacancy defect. VC defects barely affect to the value of Young’s modulus, but substantially weaken the ultimate strength of wires with a small cross-sectional size. With decreasing wire size and increasing operation temperature, significant reductions in Young’s modulus and fracture strength were observed. The average Young’s modulus for square NWs was about 3.7 % higher than that of wires with a circular shape due to the surface facet effect. A brittle-to-ductile transition (BDT) occurred for [001]-oriented Si NWs with a lateral size≤5.43 nm and an operation temperature T≥300 K.     

Atomistic simulations of solid-state pressure welding of metallic nanowires

Newly discovered micro and nanoscale cold welding has already exhibited great potential in up-to-date nanofabrication processes. In this paper, the atomistic-scale pressure welding processes for metallic nanowires (NWs) are studied using embedded-atom molecular dynamics (MD) simulations. The mechanical behavior and structural evolution of the metallic nanowires, including Au, Ag, and Cu materials that experienced a mechanical stretching break and solid-phase pressure welding process, were thoroughly investigated. The welding temperatures (T _w) ranging from 100 to 900 K were systemically examined for the effects of welding strength. The ratio of welding strength, R _ws, defined as the ratio between the welding strength and the original yield strength of NWs, was employed to identify the welding quality. Simulation results show that the R _ws of Au NWs is better than those of Ag and Cu welded at room temperature; however, for welding at high temperatures (600?C900?K), the R _ws value of Ag NWs is the best. The R _ws values of Au NWs using cold welding show less variance than with high temperature welding, reflecting that the application of cold welding on the Au NWs is highly feasible. The R _ws values for NWs with small diameters are generally higher than those with large diameters. The breaking places of the tensile test for the post-welded NWs did not occur at the welding region, indicating that the broken wires can be robustly reconnected through solid-phase mechanically-assisted welding devices.     

SU-8 flow field plates for a micro PEMFC

A SU-8 photoresist microfabrication process was developed for micro proton exchange membrane fuel cell flow structures for both anode and cathode flow field plates with a cross section of 5 cm² (22.5 mm×22.5 mm) and thickness (for a single cell) of about 750 µm. The new design for flow field plates would have SU-8 used as not only a photoresist but also as a microstructure material. A thickness of 30 nm Pt sputter loading deposited onto a Nafion 117 for membrane electrode assembly was made, with both scanning electron microscopy and atomic force microscopy characterization. Air flows were completed in hydrogen fuel cells with air breathing and forced air flows of low input pressure as well as low velocity. Performance tests of polarization curves and power density distribution as well as impendence measurements were conducted and discussed to examine the effects of orientation of the cathode surface with five hydrogen feeding rates as well as different airflow feeding modes.     

Characteristic simulation with various anode support thicknesses of membrane electrode assembly in SOFC

This study focuses on the research of solid oxide fuel cell (SOFC) and proposes reasonably practical designs, analyses, and numerical analyses with coupling software in physics, COMSOL Multiphysics, as the analysis tool to discuss the effects on the SOFC performance. This research applies the design of electrode support (anode support) to substitute the original electrolyte support, Yttria-stabilized zirconia, so that the electrolyte membrane could form a membrane to reduce ohmic resistance and increase power density. This study further discusses the effects of various flow fields (counterflow and co-flow) on internal mass transfer and SOFC performance. The findings show that the cell performance of SOFC with co-flow is better than counterpart with counterflow under anode support thickness 1,000 μm. Regarding the analyses of porosity effect with the porosity 0.7 and tortuosity 4.5, the power density reaches the maximum that could enhance the cell performance.     

Thermal stretching of defective nanowires: the coupled effects of vacancy cluster defects,operating temperature,and wire cross-sectional area

Extensive atomistic simulations of the thermal stretching of defective nanowires (NWs) were performed using the embedded-atom molecular dynamics modeling approach. The nucleation and propagation of dislocations are described via quantitative dislocation-based analyses. The investigation focuses on the coupled effects of various vacancy cluster (VC) defects, operating temperature, and wire cross-sectional area on the mechanical properties and plastic deformations of defective NWs. With increasing internal stress of a stretched wire, a rapidly moving dislocation loop that transferred atoms to fill up the original vacancy cluster before the wire yielded was found (i.e. it carried the vacancies away from the inside of the wire and formed a notch at the wire edge). The heterogeneous nucleation of dislocations from the notch site propagated along the {111}〈112〉 partial dislocations and formed stacking faults or perfect dislocations on the {111} activated planes. Simulation results show a decreasing yield strength with increasing VC size for a given wire sectional area and temperature. Quasi-linear decreasing Young’s moduli were observed with increasing operation temperature. For a given operation temperature, NW Young’s modulus increased with increasing NW size. Two typical deformation regimes under various operation temperatures were found: (i) a high-temperature-induced pre-melting phenomenon and a thermal softening effect caused low-stress plastic flow and rapid pillar-necking deformation, and (ii) step-wise glides, slip bands, and cross-slips proceeded along the activated glide planes in the low-temperature hard-brittle structure. These two regimes were thoroughly characterized via the evolutions of microscopic dislocations and the changes of true stress. For operation at high temperatures, the ultra-thin 1/5-type pentagonal ring chains exhibit a relatively robust structure, which can potentially be used as building blocks and components for high-temperature nanoelectromechanical systems (NEMS) devices in the future.     

Bifurcation and chaos for porous squeeze film damper mounted rotor–bearing system lubricated with micropolar fluid

This study presents a dynamic analysis of a flexible rotor supported by two porous squeeze micropolar fluid-film journal bearings with nonlinear suspension. The dynamics of the rotor center and bearing center are studied. The analysis of the rotor–bearing system is investigated under the assumptions of non-Newtonian fluid and a short bearing approximation. The spatial displacements in the horizontal and vertical directions are considered for various nondimensional speed ratios. The dynamic equations are solved using the Runge–Kutta method. The methods of analysis employed in this study are inclusive of the dynamic trajectories of the rotor center and bearing center, power spectra, Poincaré maps, and bifurcation diagrams. The maximum Lyapunov exponent analysis is also used to identify the onset of chaotic motion. The numerical results show that the stability of the dynamic system varies with the nondimensional speed ratios, the nondimensional parameter, and permeability. The modeling results obtained by using the method proposed in this paper can be employed to predict the dynamics of the rotor–bearing system, and the undesirable behavior of the rotor and bearing centers can be avoided.