Forging limits for an aluminum matrix composite: Part II. Analysis

Plasticity analysis has been carried out to calculate the forging limits of a particulate-reinforced aluminum matrix composite under various forging conditions. A geometric defect(i.e., variation in cross-sectional area) that can concentrate stresses and strain and accelerate local deformation was assumed to represent all possible defects in this material. Thus, in effect, the local stress concentrations around nondeformable particles, inhomogeneous distribution of particles and grain sizes, porosity and cracked particles,etc., were assumed to be simulated adequately by such a defect factor. The analysis followed a method suggested by Marciniak and Kuczynski (MK) to determine the strain path within the defect region of the composite during multiaxial deformation. A relationship of stress, strain, and strain rate obtained from the uniaxial tension test was used for the calculation of the strains. To terminate the plasticity analysis, a rateindependent fracture criterion was used that is based on Cockcroft’s model of a constant work performed by the tensile component of stress. It was found that the calculated results predicted the experimental forging limits for 2014 Al/15 vol pct A1₂O₃ reasonably well. At 400 °C and a slow strain rate (0.015 s~’), the predicted curve was higher than the experimental result. This was probably because the fracture mechanism, and thus fracture criterion, changed with temperature. The effect of assumed defect factor on predicted forging limits was also studied. It was found that the size of the defect factor did not significantly change the forging limits at 300 °C for strain rates from 0.015 s to 0.5 s^-1; however, it did have a large effect on the forging limit at 400 °C for the lower strain rate of 0.015 s^-1. Formerly Graduate Student, the University of Michigan     

Stress-State dependence of strain-hardening behavior in 2014 Al/15 vol Pct Al2O3 composite

The purpose of this investigation was to examine if the effective stress-strain function for discontinuously reinforced aluminum (DRA) matrix composites is independent of stress state, as they are for aluminum alloys. The rationale for such work is provided by the need to develop constitutive equations for applications in metal forming and forging problems. Experimental effectiveas curves at room temperature were determined for a particulate-reinforced composite, 2014 Al/15 vol pct A1₂O₃, and the matrix material, 2014 Al, under a variety of stress states. The tests consisted of uniaxial tension, equibiaxial tension (bulge test), and compression tests. To eliminate the effects of prior precipitation, all samples were given a solution-heat-treatment prior to tests. It was found that for the composite the effective yield stress in uniaxial tension was higher than that in equibiaxial tension but slightly lower than that in compression. However, the effective yield stresses for the matrix material in uniaxial tension and equibiaxial tension were nearly the same. The strain-hardening rate of the composite under equibiaxial tension was higher than that under either uniaxial tension or compression. It is suggested that nondeformable dead zones can develop around the particles during deformation whose shape changes with the applied stress state, and this is partly responsible for the observed differences in behavior. Formerly Graduate Student, the University of MichiganSenior Engineer     

A numerical approach of conjugate hydrogen permeation and polarization in a Pd membrane tube

A numerical method accounting for conjugate hydrogen permeation in a dense palladium (Pd) membrane tube is developed. In the method, hydrogen permeation across the membrane is treated by introducing a source–sink pair and a gas mixture produced from water gas shift reactions serves as the feed gas of the membrane tube. The influences of flow patterns of feed gas and sweep gas as well as their flow rates on hydrogen separation are investigated. A concentration polarization index (CPI) is also conducted to indicate the extent of polarization along the membrane surface. The predicted results suggest that counter-current modes are able to give the better performance of hydrogen separation compared to co-current modes, and hydrogen can be completely recovered if the flow rate of feed gas is low to a certain extent. However, lower flow rates of feed gas and sweep gas will trigger serious concentration polarization. With counter-current modes, the feed gas sent into the membrane tube from the lumen side or the shell side is flexible. The optimum Reynolds number of sweep gas in accordance with the Reynolds number of feed gas is correlated by an arctangent function. This provides a useful reference for the operation of hydrogen separation by controlling sweep gas.     

Using of Lactobacillus and Bifidobacterium to product the isoflavone aglycones in fermented soymilk

The study determined isoflavone aglycone contents in soymilk and sugar-soymilk fermented with a single culture of two strains of Lactobacillus paracasei, two strains of Lactobacillus acidophilus, and one strain of Bifidobacterium longum respectively at 37 degrees C for 24 h. Isoflavone concentration was analyzed by HPLC. The viable count of bacteria in all of the fermented soymilk ranged from 7 to 9 log CFU/ml. The isoflavone aglycone had a significant increase of 62%-96% of isoflavone in all of the fermented soymilk compared to 17% in non-fermented soymilk (P<0.05). The five strains of microorganisms produced lesser amount of isoflavone aglycones in fermented sucrose-soymilk than in other fermented soymilk.     

Space electric field concentrated effect for Zr:SiO2 RRAM devices using porous SiO2 buffer layer

To improve the operation current lowing of the Zr:SiO₂ RRAM devices, a space electric field concentrated effect established by the porous SiO₂ buffer layer was investigated and found in this study. The resistive switching properties of the low-resistance state (LRS) and high-resistance state (HRS) in resistive random access memory (RRAM) devices for the single-layer Zr:SiO₂ and bilayer Zr:SiO₂/porous SiO₂ thin films were analyzed and discussed. In addition, the original space charge limited current (SCLC) conduction mechanism in LRS and HRS of the RRAM devices using bilayer Zr:SiO₂/porous SiO₂ thin films was found. Finally, a space electric field concentrated effect in the bilayer Zr:SiO₂/porous SiO₂ RRAM devices was also explained and verified by the COMSOL Multiphysics simulation model.     

High performance of graphene oxide-doped silicon oxide-based resistance random access memory

In this letter, a double active layer (Zr:SiO _x /C:SiO _x ) resistive switching memory device with outstanding performance is presented. Through current fitting, hopping conduction mechanism is found in both high-resistance state (HRS) and low-resistance state (LRS) of double active layer RRAM devices. By analyzing Raman and FTIR spectra, we observed that graphene oxide exists in C:SiO _x layer. Compared with single Zr:SiO _x layer structure, Zr:SiO _x /C:SiO _x structure has superior performance, including low operating current, improved uniformity in both set and reset processes, and satisfactory endurance characteristics, all of which are attributed to the double-layer structure and the existence of graphene oxide flakes formed by the sputter process.     

Paraffin wax passivation layer improvements in electrical characteristics of bottom gate amorphous indium–gallium–zinc oxide thin-film transistors

In this research, paraffin wax is employed as the passivation layer of the bottom gate amorphous indium–gallium–zinc oxide thin-film transistors (a-IGZO TFTs), and it is formed by sol–gel process in the atmosphere. The high yield and low cost passivation layer of sol–gel process technology has attracted much attention for current flat-panel-display manufacturing. Comparing with passivation-free a-IGZO TFTs, passivated devices exhibit a superior stability against positive gate bias stress in different ambient gas, demonstrating that paraffin wax shows gas-resisting characteristics for a-IGZO TFTs application. Furthermore, light-induced stretch-out phenomenon for paraffin wax passivated device is suppressed. This superior stability of the passivated device was attributed to the reduced total density of states (DOS) including the interfacial and semiconductor bulk trap densities.     

Electrochemical growth and oxygen reduction property of Ag nanosheet arrays on a Ti/TiO2 electrode

A standing Ag nanosheet array was successfully deposited using an electrochemical method. The results demonstrated that the adsorption of polyvinylpyrrolidone (PVP) over the Ag (111) plane during deposition (1) inhibits the crystalline growth in the [111] direction, (2) stacks atomic faults in the fcc structure, and (3) forms a standing Ag nanosheet composed essentially of (111) planes. In an application, this Ag nanosheet array can be successfully used as an oxygen reduction catalyst. Based on Koutecky-Levich equation, the involved electron number at −0.2 V was 3.97.     

High Activity of Hexagonal Ag/Pt Nanoshell Catalyst for Oxygen Electroreduction

Hexagonal Ag/Pt nanoshells were prepared by using a hexagonal Ag nanoplate as the displacement template and by introducing Pt ions. The prepared Ag/Pt nanoshells played the role of an electrocatalyst in an oxygen reduction process. Compared to spherical Pt and Ag/Pt nanoparticles, the hexagonal Ag/Pt nanoshells showed higher activity for oxygen electroreduction.