B. Li

Photoemission studies of chemical bonding and electronic states at the Fe/Si interface

Chemical bonding and reactions at the Fe/Si interface have been studied as a function of Fe overlayer thickness and annealing temperature. The chemical properties (composition, electronic structure, and reactivity) were observed under ultrahigh vacuum conditions using x‐ray photoemission, ultraviolet photoemission (UPS), and Auger electronic spectroscopy. Both core line and valence‐band states have been measured. At room temperature, intermixing of atoms across the clean Fe/Si interface occurs. In the initial stage the Fe2p3/2 core line shifts 0.4 eV to a higher binding energy relative to pure Fe metal. This is nearly the same as the chemical shift of the Fe2p3/2 core line in FeSi2. With increasing coverage, the Fe2p3/2 core line shifts decrease and subsequently approach that of pure Fe metal. In the meanwhile, the Fe2p3/2 core line shapes changed gradually with greater width and asymmetry upon metal deposition. At the lower coverage, the UPS spectra are dominated by a broadband, but at high coverage, two...

Microstructure effects on shock-induced surface jetting

We investigate with large-scale molecular dynamics simulations shock-induced surface jetting from grooved Cu as regards microstructure effects, including jetting mass/velocity ratios, directionality, jetting phase diagram, secondary jetting, and underlying mechanisms. The grooves are of wedged, cylindrical, and rectangular shapes. Other microstructure features explored are half angles, crystal structure asymmetry as represented by grain boundaries, geometrical asymmetry, and deformation heterogeneity. The common fundamental mechanism is that jetting is driven by stress gradients due to transverse mass collision. For symmetrical wedged grooves, the velocity ratio (maximum jet head velocity/free surface velocity of flat surface) increases linearly with decreasing half angle, with a slope similar for different materials and at nano- to macroscales, as indicated by our simulations and previous experiments. However, the jetting factor or mass ratio reaches the maximum at certain intermediate half angle. An imp...

Formation of silicides in the Ti, Ti(Ox)/Si(111), and Ti/SiO2/Si(111) systems

Reactions at Ti/Si and Ti/SiO2/Si interfaces under the conditions of steady‐state thermal annealing and cw Ar+ laser annealing have been studied using a variety of techniques including Auger electron spectroscopy (AES), x‐ray photoemission spectroscopy (XPS), Rutherford backscattering, and x‐ray diffraction. Ti thin films on silicon have been transformed to Ti5Si3, TiSi, and TiSi2 after steady‐state annealing at 500–600u2009°C, and only TiSi2 can be observed after annealing above 650u2009°C. In the case of the Ti/SiO2/Si system a Ti5Si3 thin film with a TiO overlayer has been formed in the temperature range 750–1000u2009°C. The growth of the Ti–Si2 layer on silicon and the Ti5Si3 layer on SiO2 shows a diffusion‐limited behavior, i.e., x∝t1/2. Continuous‐wave Ar+ laser scanned annealing can induce the solid‐phase reaction of Ti/Si at a power density less than 2.7 kW/cm2, but a liquid‐phase reaction at a power density of about 3.8 kW/cm2. A mixture of TiSi2 and pure Si was observed after the liquid‐phase reaction. Ti(O...

Shock response of a model structured nanofoam of Cu

Using large-scale molecular dynamics simulations, we investigate shock response of a model Cu nanofoam with cylindrical voids and a high initial porosity (50% theoretical density), including elastic and plastic deformation, Hugoniot states, shock-induced melting, partial or complete void collapse, nanojetting, and hotspot formation. The elastic-plastic and overtaking shocks are observed at different shock strengths. The simulated Hugoniot states can be described with a modified, power-law P−α (pressure–porosity) model, and agree with shock experiments on Cu powders, as well as the compacted Hugoniot predicted with the Gruneisen equation of state. Shock-induced melting shows no clear signs of bulk premelting or superheating. Voids collapse via plastic flow nucleated from voids, and the exact processes are shock strength dependent. With increasing shock strengths, void collapse transits from the “geometrical” mode (collapse of a void is dominated by crystallography and void geometry and can be different from that of one another) to “hydrodynamic” mode (collapse of a void is similar to one another); the collapse may be achieved predominantly by flow along the {111} slip planes, by way of alternating compression and tension zones, by means of transverse flows, via forward and transverse flows, or through forward nanojetting. The internal jetting induces pronounced shock front roughening, leading to internal hotspot formation and sizable high speed jets on atomically flat free surfaces.

Thermally driven grain boundary migration and melting in Cu

With molecular dynamics simulations, we systematically investigate melting of a set of Σ3〈110〉70.53° tilt grain boundaries (GB) in Cu bicrystals, including coherent twin boundaries (CTBs), 12 asymmetric tilt grain boundaries (ATGBs), and symmetric incoherent twin boundaries (SITBs), in the order of increasing length weight of SITB or GB energy. ATGBs decompose into CTBs and SITBs, which migrate and coalesce as a result of internal stress relaxation. GBs can be superheated or premelted, and GB melting temperature decreases exponentially with increasing SITB weight, owing to the systematics in GB microstructure. GB melting nucleates at disordered CTB-SITB junctions, and grows along SITBs and then into grain interiors, with the solid-liquid interfaces preferentially aligned with {111}.

Shock-induced melting of honeycomb-shaped Cu nanofoams: Effects of porosity

We investigate shock-induced melting in honeycomb-shaped Cu nanofoams with extensive molecular dynamics simulations. A total of ten porosities ( ϕ) are explored, ranging from 0 to 0.9 at an increment of 0.1. Upon shock compression, void collapse leads to local melting followed by supercooling at low shock strengths. Superheating occurs at ϕ≤0.1. Both supercooling of melts and superheating of solid remnants are transient, and the equilibrated shock states eventually fall on the equilibrium melting curve for partial melting. However, phase equilibrium has not been achieved on the time scale of simulations in supercooled Cu liquid (from completely melted nanofoams). The temperatures for incipient and complete melting are related to porosity via a power law, (1−ϕ)k, and approach the melting temperature at zero pressure as ϕ→1.

Electron spectroscopic studies of the Dy/Si(111) interface

Low‐energy electron diffraction (LEED), x‐ray photoelectron (XPS), ultraviolet photoelectron (UPS), and Auger electron (AES) surface spectroscopic techniques were used to study the chemical interaction between Dy atoms and Si atoms at the Dy–Si interface at room temperature, as well as Dy silicides formed after annealing at 400u2009°C for 30 min. Experiment shows that at low coverages [≤4 monolayers (ML)] atom intermixing takes place, causing chemical shifts of Dy 4f and Dy 3d. However, no significant chemical shift of Si 2p has been observed, suggesting a lack of strong chemical bonds of the Dy–Si type. With coverage increasing, a (7×7) LEED pattern from a clean Si(111) surface turns into a (1×1), and finally becomes ambiguous. After annealing for 30 min at 400u2009°C, the Dy silicides formed with an atomic concentration ratio of Dy to Si of ∼1:2. The binding energies of Dy 4fu2009(Dy Si2), Dy 3d(Dy Si2), and Si 2p(Dy Si2) are 5.5, 1295.2, and 98.65 eV, respectively.

Shock response of open-cell nanoporous Cu foams: Effects of porosity and specific surface area

We investigate the effects of porosity or relative mass density and specific surface area on shock response of open-cell nanoporous Cu foams with molecular dynamics simulations, including compression, shock velocity–particle velocity, and shock temperature curves, as well as shock-induced melting. While porosity still plays the key role in shock response, specific surface area at nanoscales can have remarkable effects on shock temperature and pressure, but its effects on shock velocity and specific volume are negligible. Shock-induced melting of nanofoams still follows the equilibrium melting curve for full-density Cu, and the incipient and complete melting temperatures are established as a function of both relative mass density and specific surface area.

Shock response of He bubbles in single crystal Cu

With large-scale molecular dynamics simulations, we investigate shock response of He nanobubbles in single crystal Cu. For sufficient bubble size or internal pressure, a prismatic dislocation loop may form around a bubble in unshocked Cu. The internal He pressure helps to stabilize the bubble against plastic deformation. However, the prismatic dislocation loops may partially heal but facilitate nucleation of new shear and prismatic dislocation loops. For strong shocks, the internal pressure also impedes internal jetting, while a bubble assists local melting; a high speed jet breaks a He bubble into pieces dispersed among Cu. Near-surface He bubbles may burst and form high velocity ejecta containing atoms and small fragments, while the ejecta velocities do not follow the three-dimensional Maxwell-Boltzmann distributions expected for thermal equilibrium. The biggest fragment size deceases with increasing shock strength. With a decrease in ligament thickness or an increase in He bubble size, the critical shock strength required for bubble bursting decreases, while the velocity range, space extension and average velocity component along the shock direction, increase. Small bubbles are more efficient in mass ejecting. Compared to voids and perfect single crystal Cu, He bubbles have pronounced effects on shock response including bubble/void collapse, Hugoniot elastic limit (HEL), deformation mechanisms, and surface jetting. HEL is the highest for perfect single crystal Cu with the same orientations, followed by He bubbles without pre-existing prismatic dislocation loops, and then voids. Complete void collapse and shear dislocations occur for embedded voids, as opposed to partial collapse, and shear and possibly prismatic dislocations for He bubbles. He bubbles lower the threshhold shock strength for ejecta formation, and increase ejecta velocity and ejected mass.