A kinetics formulation for low-temperature plasticity

The mechanism of low-temperature plastic deformation is controlled by thermally activated dislocation movements. An evolutionary constitutive law based on the principles of deformation kinetics is described in this article. The constitutive law is expressed with a sinh function designed for computational efficiency. It is derived from rigorously defined kinetics principles. The approximation involved in the sinh function is defined so that in applications an exact evaluation can be made of the validity limits. The system of the constitutive law and the external constraints lead to the operational equations. Applications are developed for constant strain-rate loading, constant stress-rate loading, stress relaxation, creep, and ratchetting processes. The analysis provides a unified treatment for low-temperature plastic deformation.     

Deformation twinning in bulk nanocrystalline metals: Experimental observations

Deformation twins have been observed in nanocrystalline (nc) fcc metals with medium-to-high stacking fault energies such as aluminum, copper, and nickel. These metals in their coarse-grained states rarely deform by twining at room temperature and low strain rates. Several twinning mechanisms have been reported that are unique to nc metals. This paper reviews experimental evidences on deformation twinning and partial dislocation emissions from grain boundaries, twinning mechanisms, and twins with zero-macro-strain. Factors that affect the twinning propensity and recent analytical models on the critical grain sizes for twinning are also discussed. The current issues on deformation twinning in nanocrystalline metals are listed.     

Electrochemical hydrogen storage of NdMg12–Ni composites modified with carbon nanotubes and BN particles

In this work, the electrochemical performance of NdMg₁₂–Ni composite electrode in alkaline solution and the effect of the surface modification with carbon nanotubes (CNTs) and boron nitride (BN) particles on the NdMg₁₂–Ni composite were investigated. The NdMg₁₂ alloy was synthesized by a salt-cover-melting and a subsequent quenching process. The NdMg₁₂–Ni–BN and NdMg₁₂–Ni–CNTs composites were prepared by ball-milling NdMg₁₂ alloy, Ni powders and CNTs or BN particles. It is found that CNTs or BN particles are mainly attached onto the surface of the NdMg₁₂–Ni composite after the ball-milling process. The electrochemical experiment results indicate that the NdMg₁₂–Ni composites modified with CNTs or BN particles have the improved electrochemical performance. In particular, the NdMg₁₂–Ni–5 wt.% CNTs and NdMg₁₂–Ni–3 wt.% BN composites have the higher initial discharge capacity of 416.6 mAh/g and 442.9 mAh/g, respectively, larger than the original NdMg₁₂–Ni composite. The large amount of grain boundaries and crystalline defects, induced during the ball-milling process, can accelerate the bulk hydrogen diffusion and provide more surface active sites for the electrochemical reaction of the composites. However, the cycle stability of the composites modified by CNTs or BN particles is still not satisfactory for the practical application.     

Electrochemical investigation on nanoflower-like CuO/Ni composite film as anode for lithium ion batteries

Nanoflower-like CuO/Ni film was prepared by electrodeposition method in an alkaline nickel electroplating solution, and the nanoflower-like CuO film was obtained by direct oxidation on copper substrate. The nanoflower-like CuO was crystalline with space group of C2/c, and the amorphous Ni particle layer on the surface of film contacted well with the nanoflower-like CuO. The electrochemical properties of CuO/Ni film were investigated by cyclic voltammetry (CV) and galvanostatic charge-discharge tests. Since the metallic Ni can act as conductor and catalyst, the CuO/Ni film exhibits higher initial coulombic efficiency (72.1%) than the pure CuO film (57.0%), and better capacity retention (96.3% of the 2nd cycle) than the pure CuO film (67.8% of the 2nd cycle) at the current density of 0.1 mA cm⁻².     

Copolymerization of polytriphenylamine with coumarin to improve the oxidation potential and LiFePO4 battery overcharge tolerance

By an oxidative-coupling copolymerization with coumarin, the oxidation potential of polytriphenylamine was improved to 3.75 V from 3.60 V. The copolymer exhibited excellent electrochemical activity in redox cycling. The separator used in the experiment was prepared by roll pressing the copolymer powder with 30% (wt.) polytetrafluoroethylene into a ∼80 μm thick sheet, which was incorporated into a LiFePO₄-Li cell. Tests showed that when the cell was overcharged to 3.789 V, the copolymer separator was oxidized and became electronically conducting, which caused short circuit formation and prevented the organic electrolyte from being oxidized. Although the cell was overcharged at 0.3 C for 5 h, the voltage was stable at 3.789 V. When discharged, the cell released all its capacity at normal charge. The cell was overcharged seven times with identical results obtained for each cycle. Overcharging at different rates demonstrated that the separator could exhibit reversible, self-activating protection for LiFePO₄-based batteries, even at 1.3 mA/cm² (3.2 C to the cell).     

Size effects on charge ordering and magnetic properties in La0.25Ca0.75MnO3

The size effects on the charge ordering (CO) and magnetic properties in La_0.25Ca_0.75MnO₃ with mean particle size ranging from 40 to 2000 nm were studied. With decreasing particle size the CO transition temperature shifts to lower temperature and the transition width becomes increasingly wide, indicating the weakening of the CO state. Meanwhile the ferromagnetic (FM) cluster glass state appears and the magnetization at low temperature increases significantly. The behaviour is due to the increasing uncompensated surface spins which weaken the antiferromagnetic interaction and disfavour the formation of the CO state. The suppression of the CO state and appearance of the FM cluster glass state are also found in La_0.25Ca_0.75MnO₃ nanowires fabricated by a sol–gel template method. These results indicate that the CO state can be modulated effectively by varying particle size, which has an important implication for nano-device applications of manganites.     

High-pressure sintered yttria stabilized zirconia ceramics

Fast densification of 8YSZ ceramics under a high pressure of 4.5 GPa was carried out at different temperatures (800, 1000, 1450 °C), by which a high relative density above 92% could be obtained. FT-Raman spectra indicate that the 8YSZ underwent a phase transition from partially tetragonal to partially cubic phase as temperatures increase from 1000 to 1450 °C when sintering under high pressure. The electrical properties of the samples under different high-pressure sintering conditions were measured by complex impedance method. The total conductivity of 0.92 × 10⁻² S cm⁻¹ at 800 °C has been obtained for 8YSZ under high pressure at 1450 °C, which is about 200 °C lower than that of the samples prepared by conventional pressureless sintering.     

Investigation of rapid conversion of switchgrass in subcritical water

The reaction characteristics of switchgrass conversion in subcritical water were investigated using a batch reactor under conditions of rapid rising to 250–350 °C and pressure of 20 MPa, with reaction times varying from 1–300 s. The effects of temperature and reaction time on product distribution and yields of chemical products were investigated. High conversion of switchgrass (90 wt.% on dry biomass basis) can be obtained in less than 60 s under a relative lower reaction temperature of 350 °C, compared with that in a switchgrass flash pyrolysis process where switchgrass conversion achieves only 58.9–78.8 wt.% in temperature range of 450–550 °C. The yield of water solubles (WS) can reach 37 wt.% after reaction for 1 s at 250 °C. The increases in temperature and reaction time lead to increases of the biomass conversion and the yield of gas, while WS yield decreases by secondary decomposition reactions. Many lignin-derived compounds were identified by GC-MS analysis and could well be recovered in methanol solubles (MS). Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) analysis of methanol insolubles (MI) indicated that the lignocellulosic matrix could be significantly decomposed, and no char formation was observed, while many lignin structures were left in the MI products. These results provide important information for recovering value-added chemicals from energy crops and biomass waste.     

Application of direct fluid flow oscillations to improve mixing in microbioreactors

This article describes an active mixing method for a microbioreactor that was designed, simulated, tested, and successfully implemented. By applying a varying pressure to a microchannel looping tangentially into a cylindrical microreactor an oscillating fluid flow was shown to occur. Such an oscillating fluid flow improved mixing, both by diffusion and convection. The oscillating fluid flow has a large impact on the ratio between the diffusion domain and the convection domain. A good match was obtained between experimental mixing results, computational fluid dynamics simulation results and the results of a simplified mixing model thus demonstrating the potential of simulation on improving the design of microreactors. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

The effect of polymer surface modification via interfacial polymerization on polymer–protein interaction

Membrane separation is an important processing technology used for separating food ingredients and fractionating value‐added components from food processing byproducts. Long‐term performance of polymeric membranes in food protein processing is impeded by the formation of fouled layers on the membrane surface as a result of protein adsorption onto the membrane surface. Surface modification of synthetic membranes, i.e., changing surface characteristics to reduce protein adsorption permanently, is one of the innovative ways of reducing the fouling of membrane surfaces. In this study, surface modification of flat‐sheet ultrafiltration membrane, polyethersulfone (PES), was investigated in improving the hydrophilicity of PES surfaces, thereby reducing adsorption of the protein caused by hydrophobic–hydrophobic interaction between the protein and the membrane. Hydrophilic polymer grafting through thin‐film composite using interfacial polymerization was employed to improve the hydrophilicity of the commercial PES membranes. Poly(vinyl alcohol), poly(ethylene glycol), and chitosan were chosen as hydrophilic polymers to graft on PES membrane because of their excellent hydrophilic property. Modified PES membranes were characterized by contact angle, FTIR, XPS, and AFM. Contact angles of modified PES membranes were reduced by 25 to 40% of that of the virgin PES membrane. XPS spectrum supported that the PES membranes were successfully modified by interfacial polymerization. Tapping‐mode AFM was used to examine the changes in surface topography of modified PES membranes. The PES membranes modified by interfacial polymerization showed lower roughness (from 1.2 to 2.0 nm) than that of virgin PES membrane (2.1 nm). The results of these instrumental analyses indicated that the PES membranes were successfully enhanced hydrophilically through interfacial polymerization. The protein adsorption on the modified membranes was reduced by 30 to 35% as a result of surface modification of the PES membranes using interfacial polymerization technique. Published 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009