Analytical Study of Bifurcation and Nonlinear Behavior of Sandwich Columns

Buckling behavior of axially compressed sandwich columns exhibits a variety of interesting phenomena. The column can buckle in a long wave “overall mode” or a short-wave “wrinkling mode,” the latter involving severe bending of the facings and transverse deformation of the core. The full-range nonlinear behavior of these structures is investigated in the elastic range taking typical examples. Columns can be, though not always, imperfection sensitive when wrinkling is the principal mode of buckling. The columns buckling in the overall mode collapse by the formation of localized wrinkles in the postbuckling range. An interesting new finding of this study is that, for certain combinations of core compliance and facing thickness, there can be a bifurcation from the prebuckling path at a load smaller than that predicted by the linear stability analysis. In this scenario, if wrinkling is the dominant mode of buckling, the column turns out to be imperfection sensitive.     

Dielectrophoretic deposition of graphite oxide soot particles

Alternating current dielectrophoresis in water was used to position graphite oxide soot (GO-soot) particles generated by rapid thermal expansion of graphite oxide under inert gas. The dielectrophoretic deposition was carried out at a frequency of 10 MHz and a peak-to-peak voltage of 10 V, and the deposited particles were analyzed using scanning electron microscopy. The vertical cross section, obtained by focused ion beam cutting, shows the wrinkled layers of the GO-soot particles and cavities between the layers. The electrical transport measurements show typical characteristics of metal-like pathways. The improved electrical contact between electrodes and GO-soot, probably due to the thin platelet structure of GO-soot, makes the material favorable for electrical device applications. The results demonstrate that AC dielectrophoresis can be used to selectively deposit graphite oxide soot particles at desired locations.     

Dose evaluation of selective collimation effect in cephalography by measurement and Monte Carlo simulation

Recently, simulations based on the Monte Carlo code have been increasingly applied for physics phenomena, patient dose and quality assurance of radiation systems. The objective of this study was to use Monte Carlo simulation and measurement to verify dose and dose reduction in cephalography. The collimator was constructed with 3-mm thick lead plate, and attached to the tube head to remove regions of disinterest in the radiation field. A digital phantom patient was constructed to evaluate patient dose. In addition, detectors of pixel size 1×1 cm2 and 0.1×0.1 cm2 were constructed to check collimator location. The effective dose according to International Commission on Radiological Protection 103 was calculated with and without collimation. The effective doses for simulation with and without collimation were 5.09 and 11.32 μSv, respectively. The results of the calculated effective dose show 61.7 % reduction of field area and 55 % of effective dose. The Monte Carlo simulation is a good evaluation tool for patient dose.     

Growth of conformal copper films on TaN by electrochemical deposition for ULSI interconnects

Seedless copper electrochemical deposition (ECD) becomes a potential interconnect technology while device dimension keeps shrinking in ULSI design. In seedless copper ECD on TaN, which is a widely used diffusion barrier, uniform growth of copper film on TaN is hindered because a robust native Ta₂O₅ exists on TaN surface. Complete removal of the native Ta₂O₅ can be attained using a saturated KOH solution that is assisted by an anodic voltage. This then permits that copper film grows on the pretreated TaN surface in a copper-citrate (Cu-Cit) complex electrolyte. Its growing morphology and deposition rate are dependent on the etching depth of as-deposited TaN in the KOH solution. Even for a very short etching time of 0.8 s, thin Ta₂O₅ is totally etched off and the activated TaN surface appears. Thin and conformal copper films grown in a layer-by-layer mode on the TaN surface are proper to function as an ECD seed or metal lines for ULSI interconnects.     

Stress behavior of electrodeposited copper films as mechanical supporters for light emitting diodes

Thick copper films electrodeposited on vertically structured GaN-based LEDs play a critical role in supporting thin GaN multi-layers mechanically after laser lift-off (LLO) of the sapphire substrate. The intrinsic stress of electrodeposited copper was measured from the change in the substrate's curvature using a modified Stoney equation. 150-μm thick copper supporters showed very low intrinsic stress of 4-28 MPa (tensile stress), which developed during copper electrodeposition on Au seed. The stress generation was attributed to nuclei coalescence and formation of grain boundaries of which volume increased with the applied current density. The preferential texture plane of thick copper supporters was (2 2 0) at high current densities, while copper films thinner than 1 μm was strongly oriented along (1 1 1) plane. Various seed metals such as Cu, Ni, Au, Ag, and Al were employed to observe the influence of seed material on the stress of copper supporters. The effect of wetting agent on the stress of copper supporters was also monitored in the concentration range of 0.1-1.5 g/L of sodium lauryl sulfate (SLS).     

Effects of internal electrode composition on the reliability of low-firing PMN–PZT multilayer ceramic actuators

We investigated the effects of internal electrode composition on the reliability of low-firing multilayer ceramic actuators using Ag internal electrodes. Ag–ceramic composite pastes were prepared by adding Pb(Mg_1/3Nb_2/3)O₃–Pb(Zr_0.475,Ti_0.525)O₃ (PMNZT) ceramic powders to a commercial Ag paste at concentrations in the range of 0–60 vol%. PMNZT multilayered laminates were fabricated using tape casting, and then cofired at 925 °C for 10 h. The fatigue behaviors of multilayer actuators with Ag internal electrodes having different PMNZT concentrations were compared by applying a 2 kV/mm ac electric field at 50 °C under a relative humidity of 30%. The failure data were analyzed using Weibull statistics. The addition of PMNZT ceramics enhanced the mean time to failure by reducing the densification mismatch between the piezoelectric ceramic and internal electrode layers during the cofiring process.