New experimental insight into the mechanisms of nanoplasticity

The evolution of microstructure and texture of a nanocrystalline Pd–10 at.% Au alloy (initial grain size 16 nm) subjected to severe plastic deformation by high-pressure torsion (HPT) at room temperature is investigated by X-ray line profile analysis and X-ray microdiffraction, respectively. In addition, changes in the microhardness are measured and the texture is modeled. During HPT the microstructure changes: the crystallite size goes over the maximum, the dislocation density goes through a minimum and the density of stacking faults decreases at/up to a shear strain of ～1, corresponding to a grain size of 20 nm. Starting with a random texture, typical brass-type shear components develop at a shear strain above ～1. The microhardness with decreasing crystallite size goes over a maximum at ～20 nm. The correlated changes in microstructure, texture and strength strongly suggest the transition from a dislocation slip to a grain boundary sliding (GBS)-dominated deformation mechanism. The unexpected brass-type texture and its deviation from the ideal position can be simulated with the Taylor model assuming dominant partial dislocation slip and a certain contribution of GBS, respectively. Taken together, the results of many techniques applied to the same material, in particular those of the texture investigations, provide a more comprehensive and consistent picture of nanoplasticity than reported before for face-centered cubic metals.     

Supercapacitor performance of porous carbon nanofiber composites prepared by electrospinning polymethylhydrosiloxane (PMHS)/polyacrylonitrile (PAN) blend solutions

Porous carbon nanofiber composites (NFCs) were prepared by electrospinning blended solutions of polyacrylonitrile (PAN) and polymethylhydrosiloxane (PMHS) in N,N-dimethylformamide (DMF). A PMHS concentration of 5 wt% was regarded as the optimum concentration to obtain fibers of a uniform size with a homogeneous dispersion of silica, the maximum specific surface area and the highest conductivity (4.91 S cm^?1) after heat treatment at 800 °C. The supercapacitor electrode prepared with 5 wt% PMHS had the highest specific capacitance, 126.86 F/g, and the highest energy density, 17.0–10.0 Wh/kg, in the range of 400–20,000 W/kg in a 6 M KOH aqueous solution.     

Electrochemical characterization of in situ polypyrrole coated graphene nanocomposites

Graphene/polypyrrole nanocomposites were prepared by in situ oxidative polymerization method by varying the weight percentage of graphene. FTIR study confirmed the formation of polypyrrole in presence of graphene. Field Emission Scanning Electron Microscopy (FESEM) and High Resolution Transmission Electron Microscopy (HRTEM) were used to characterize the morphology of the nanocomposites which showed a uniform coating of graphene with the polypyrrole. From the cyclic voltammetry (CV) measurement it was found that the capacitances of the nanocomposites were increased up to a certain percentage of graphene and after which it showed a downward trend. The maximum capacitance value and energy density, among the composites studied, were found to be 409 F/g and 227.2 Wh/Kg at 10 mV/s scan rate. However, maximum power density achieved was 4617 W/kg at a scan rate of 200 mV/s.     

An investigation of the Al–Pd–Rh phase diagram between 50 and 100 at% Al

Partial 1100, 1000, 900 and 790 °C isothermal sections of the Al–Pd–Rh phase diagram were studied. The isostructural binary AlPd and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. The Al–Pd and Al–Rh ε-phases form another continuous range. The C–Al₅Rh₂ phase dissolves up to 13 at% Pd, Al₉Rh₂ and Al₇Rh₃ are extended up to 3 at% Pd. Two ternary phases: cubic C₂ (a=1.5483 nm) and hexagonal C₃ (a=1.09159, c=1.3386 nm) were revealed. The former extends along about 65 at% Al from 4 to 27 at% Pd.     

Electrochemical performance of a high cation-deficiency Li2Mn4O9/active carbon supercapacitor in LiNO3 electrolyte

A hybrid supercapacitor based on spinel Li₂Mn₄O₉ and activated carbon (AC) was fabricated. The electrochemical performance of the capacitor was studied by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge in different aqueous electrolytes such as 1 M LiNO₃, Li₂SO₄, NaNO₃ and KNO₃ solution. A maximum specific capacitance of 261 F g^?1 was obtained for the Li₂Mn₄O₉ single electrode between 0 and 1.4 V. The Li₂Mn₄O₉/AC hybrid supercapacitor showed a sloping voltage profile from 0 to 1.4 V and delivered an energy density of 53 Wh kg^?1 based on the total weight of the active electrode materials. The hybrid capacitor exhibited a desirable profile and maintained over 80% of its initial energy density after 1000 cycles, indicating that Li₂Mn₄O₉ has excellent cycling performance and structural stability in aqueous electrolyte. The hybrid supercapacitor also exhibited an excellent rate capability, even at a power density of 1250 W kg^?1, it had a specific energy 29 Wh kg^?1 compared with 48 Wh kg^?1 at the power density of about 417 W kg^?1.     

Behavior of palladium and its impact on intermetallic growth in palladium-coated Cu wire bonding

This paper describes the behavior of palladium in palladium-coated Cu (PdCu) wire bonding and its impact on bond reliability by utilizing transmission electron microscopy (TEM). A Pd layer approximately 80 nm thick, which is coated on the surface of Cu wire, dissolves into the Cu matrix during ball formation (under N₂ gas protection) when the wire tip is melted to form a ball. As a result of dissolving the very thin Pd layer into the ball, Pd is almost undetectable along the entire bond interface between the ball and the Al pad. The behavior of Pd during thermal aging in air, however, is different for central and peripheral interfaces. At the central interface, less than 5 at.% Pd is present after 168 h aging at 175 °C. At the periphery, however, Pd diffuses back and congregates, reaching a level of ～12 at.% after 24 h, and a Pd-rich (Cu,Pd)₉Al₄ layer (>40 at.% Pd) forms after 168 h. Pd acts substitutionally in Cu₉Al₄ but cannot penetrate into the CuAl₂ or CuAl. By comparison of intermetallic thickness and interfacial morphology between PdCu and bare Cu wire bonds, it is concluded that the presence of Pd reduces intermetallic growth rate, and is associated with numerous nanovoids in PdCu bonds.     

Microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites

The microstructure and grain boundary relaxation in ultrafine-grained Al/Al oxide composites were studied by electron microscopy observation and internal friction measurement, respectively. Both the microstructure and the internal friction behavior of the composites were strongly influenced by the thermomechanical treatment parameters. All the Al particles were still covered by the native amorphous oxide shells in those composites sintered at T < 823 K, and no indication of Al grain boundary relaxation was detected. Some Al oxide shells were cracked, resulting in the formation of a few Al–Al grain boundaries between adjacent particles in the sample sintered at 823 K, and one internal friction peak centered at ～440 K was detected. All the oxide shells were broken into small fragments in those samples sintered at T ? 843 K, and two internal friction peaks were detected, one prominent peak at ～440 K and one weak peak at ～540 K. A microstructure with a bimodal grain size distribution of Al was formed via partial recrystallization after thermomechanical treatment of the sample sintered at 893 K, and two internal friction peaks with comparable intensity were detected. The internal friction peaks were associated with the relaxation of Al grain boundary in the composites.     

Fabrication and characterization of flexible and high capacitance supercapacitors based on MnO2/CNT/papers

Flexible paper-based supercapacitors were fabricated using carbon nanotubes (CNTs) and manganese oxides (MnO₂), and their electrochemical properties were characterized in a three-electrode system. CNTs were synthesized via water-assisted chemical vapor deposition (CVD) and dispersed in water using the surfactant sodium dodecylbenzenesulfonate (SDBS). The solution containing dispersed CNTs was simply coated on papers by drop-dry method. MnO₂ was then electrochemically deposited on the CNT-coated papers. The MnO₂/CNT/paper supercapacitors showed high specific capacitance of 540 F/g. Specific energy and specific power were 20 Wh/kg and 1.5 kW/kg, respectively, at current density of 5 A/g in 0.1 M sodium sulfate (Na₂SO₄) aqueous solution. Demonstrated high capacitance of the paper-based electrochemical capacitor makes it a promising candidate for flexible and low-cost energy storage device applications.     

Fabrication and catalytic property of an Ag@poly(3,4-ethylenedioxythiophene) yolk/shell structure

Poly(3,4-ethylenedioxythiophene) (PEDOT) hollow spheres with the size ranged from 210 to 850 nm and a conductivity of 6 × 10^?2 S cm^?1 have been prepared via electrochemical polymerization of 3,4-ethylenedioxythiophene in the presence of poly(vinyl pyrrolidone) as a soft template. Then, a novel Ag@PEDOT yolk/shell structure has been prepared simply and directly by a penetration and reduction approach. The morphology, formation, and the catalytic activity of the as-prepared Ag@PEDOT yolk/shell structure have been investigated. The experimental result shows that the as-prepared Ag@PEDOT yolk/shell structure exhibits excellent catalytic activity on reduction of p-nitrophenol. The employed approach may shed some light on preparing other noble metal@conductive polymer core/shell structure.     

Characterization of titanium aluminide alloy components fabricated by additive manufacturing using electron beam melting

Intermetallic, γ-TiAl, equiaxed, small-grain (～2 μm) structures with lamellar γ/α₂-Ti₃Al colonies with average spacing of 0.6 μm have been fabricated by additive manufacturing using electron beam melting (EBM) of precursor, atomized powder. The residual microindentation (Vickers) hardness (HV) averaged 4.1 GPa, corresponding to a nominal yield strength of ～1.4 GPa (～HV/3), and a specific yield strength of 0.37 GPa cm³ g^?1 (for a density of 3.76 g cm^?3), in contrast to 0.27 GPa cm³ g^?1 for EBM-fabricated Ti–6Al–4V components. These results demonstrate the potential to fabricate near net shape and complex titanium aluminide products directly using EBM technology in important aerospace and automotive applications.     

Effect of SPFGraphene dopant in MEH–PPV organic light-emitting devices

In this paper, graphene acts as the acceptor material in the luminous layer. The doping behavior of graphene in MEH–PPV has improved the device performance due to the efficient electron injection and transport through highly conductive graphene. When the graphene content is 0.02 wt%, the highest EL brightness reaches 1960 cd/m² and the threshold voltage declines from 8 V to 5 V. When the graphene content is 0.02 wt%, the device has the highest EL brightness compared with the devices of other graphene content at the same current density. The doping graphene into MEH–PPV results in the best luminous efficiency and balanced electron and hole mobilities in the active layer.     

Strengthening mechanisms in a high-strength bulk nanostructured Cu–Zn–Al alloy processed via cryomilling and spark plasma sintering

A bulk nanostructured alloy with the nominal composition Cu–30Zn–0.8Al wt.% (commercial designation brass 260) was fabricated by cryomilling of brass powders and subsequent spark plasma sintering (SPS) of the cryomilled powders, yielding a compressive yield strength of 950 MPa, which is significantly higher than the yield strength of commercial brass 260 alloys (～200–400 MPa). Transmission electron microscopy investigations revealed that cryomilling results in an average grain diameter of 26 nm and a high density of deformation twins. Nearly fully dense bulk samples were obtained after SPS of cryomilled powders, with average grain diameter 110 nm. After SPS, 10 vol.% of twins is retained with average twin thickness 30 nm. Three-dimensional atom-probe tomography studies demonstrate that the distribution of Al is highly inhomogeneous in the sintered bulk samples, and Al-containing precipitates including Al(Cu,Zn)–O–N, Al–O–N and Al–N are distributed in the matrix. The precipitates have an average diameter of 1.7 nm and a volume fraction of 0.39%. Quantitative calculations were performed for different strengthening contributions in the sintered bulk samples, including grain boundary, twin boundary, precipitate, dislocation and solid-solution strengthening. Results from the analyses demonstrate that precipitate and grain boundary strengthening are the dominant strengthening mechanisms, and the calculated overall yield strength is in reasonable agreement with the experimentally determined compressive yield strength.     

Sn buffered by shape memory effect of NiTi alloys as high-performance anodes for lithium ion batteries

By applying the shape memory effect of the NiTi alloys to buffer the Sn anodes, we demonstrate a simple approach to overcome a long-standing challenge of Sn anode in the applications of Li-ion batteries – the capacity decay. By supporting the Sn anodes with NiTi shape memory alloys, the large volume change of Sn anodes due to lithiation and delithiation can be effectively accommodated, based on the stress-induced martensitic transformation and superelastic recovery of the NiTi matrix respectively, which leads to a decrease in the internal stress and closing of cracks in Sn anodes. Accordingly, stable cycleability (630 mA h g^?1 after 100 cycles at 0.7C) and excellent high-rate capabilities (478 mA h g^?1 at 6.7C) were attained with the NiTi/Sn/NiTi film electrode. These shape memory alloys can also combine with other high-capacity metallic anodes, such as Si, Sb, Al, and improve their cycle performance.     

Response of Ni/Al laminates to laser-driven compression

Ni/Al laminates with bilayer thicknesses in the micrometer (～5 μm) and nanometer (～50 nm) range were subjected to exothermic reactions induced by laser-driven compression. The initial shockless compression steepened into shock in the microscaled laminates generating a pressure pulse duration of several tens of nanoseconds, which induced strain rates varying from 10⁷ to 10⁸ s^?1. The laser energies applied, 650, 875, and 1305 J, generated peak compression stresses of 30, 75, and 118 GPa, respectively, at the plasma stagnated Al surface. Large differences in flow stresses and bulk compression moduli of Ni and Al introduced shear localization in the Ni/Al interfaces. The nanoscale Ni/Al laminates were fully reacted, producing NiAl with grain sizes less than 500 nm. The NiAl intermetallic phases, B2 (β) phase (fcc) and martensitic phase (bcc), coexist in the NiAl nanograins. It was confirmed that the intermetallic reaction in the Ni/Al microlaminate cannot self-sustain for the short duration, laser-driven compressive loading. The intermetallics NiAl (equiaxed grains) and NiAl₃ (dendrites) were identified on the plasma stagnated surface of Ni/Al microlaminates. The distribution of intermetallic phases varied according to the incident laser energies.     

Titanium to steel joining by spark plasma sintering (SPS) technology

The bonding of Ti–6Al–4V to low alloy steel (AISI4330) using SPS technique in the 850–950 °C temperature range was examined. The formation of a thin (～1 μm) titanium carbide interfacial layer was observed with a thickness only slightly dependent on the joining temperature. This layer separates the joined metals and prevents the formation of Fe–Ti intermetallics in the bonding zone. The maximal tensile strength of the joints (of about 250 MPa) was achieved for bonding at 950 °C for 3.6 ks. The formation of the titanium carbide layer and its evolution are discussed based on the isothermal section of the ternary Fe–Ti–C phase diagram.     

Laves phases in the ternary systems Ti–{Pd,Pt}–Al

Formation and crystal structure of Laves phases in the systems Ti–{Pd,Pt}–Al were investigated employing XPD (X-ray powder diffraction), XSCD (X-ray single crystal diffraction) and EPMA (electron probe microanalysis) techniques. Laves phases with MgZn₂ type (space group: P6₃/mmc) and its variant with the Nb(Ir,Al)₂-type (a√3 × a√3 × c supercell of MgZn₂-type, space group: P6₃/mcm) were found in both systems. Formation of a particular structure type is dependent on temperature and composition. Laves phases with the Nb(Ir,Al)₂-type form around 25 at.% of Pd,Pt at 950 °C. The MgZn₂-type Laves phase Ti(Pt,Al)₂ was not observed at 950 °C, but it forms in as-cast alloys at a slightly lower Pt content, Ti_37.8Pt_19.0Al_43.2. In the Ti–Pd–Al system at 950 °C the MgZn₂-type phase exists at the Pd-poor side of the homogeneity region whilst the Nb(Ir,Al)₂-type phase is slightly richer in Pd. Phase relations associated with the Ti–Pt–Al Laves phase were established at 950 °C and reveal a new compound TiPtAl that derives from hexagonal ZrBeSi-type (ordered Ni₂In-type, a = 0.43925(4) nm, c = 0.54844(5); space group P6₃/mmc; R_F2 = 0.015 from single crystal data). Atom distribution in the compound shows a slight deviation from full atom order Ti(Pt_0.97Al_0.03)(Al_0.98Pt_0.02).     

Improved electron injection in organic LED with lithium quinolate/aluminium cathode

Lithium quinolate (Liq), covered with aluminium, was used as an electron injection layer in a double layer organic light emitting diode consisting of NPD as the hole transport layer and Alq as the emitting layer resulting in lower turn on voltage and increased power efficiency. The driving voltage required to achieve a luminance of 100 cd/m² decreased from 5.8 V for the Ca/Al to 4.2 V when Liq/Al was used, improving device power efficiency from 2.3 to 4.1 lm/W. The performance tolerance to layer thickness of Liq devices is also better than that of the devices with lithium fluoride (LiF). Due to the highly insulating nature of LiF, it can only be used when deposited as an ultra-thin layer, while the Liq can be deposited into layers as thick as 5 nm without significantly affecting the EL properties. An Liq electron injecting layer has also been tried in combination with Ca, Mg and Ag cathodes. Our experiments support the assumption that free lithium is released from lithium quinolate, as in the case of lithium fluoride, when Liq is over coated with active metals such as Al.     

Pseudocapacitance properties of AC/LiNi1/3Co1/3Mn1/3O2 asymmetric supercapacitor in aqueous electrolyte

NH₂NH₂·H₂O which was used as controlling agent was applied to prepare the precursor Ni_1/3Co_1/3Mn_1/3(OH)₂ in the hydroxide co-precipitation method. The precursor was used to synthesize LiNi_1/3Co_1/3Mn_1/3O₂. The samples were characterized by XRD, XPS and SEM. It has been found that sintered sample at 800 °C for 16 h is considered as the optimal synthetic condition. The LiNi_1/3Co_1/3Mn_1/3O₂ was used as positive electrode and the activated carbon as negative electrode of the asymmetric supercapacitor. The electrochemical capacitance performance was tested by cyclic voltammetry, electrochemical impedance spectroscopy and galvanostatic charge/discharge. The results indicate that species of aqueous electrolyte, current density, scan rate and potential limit, etc. have influence on the capacitance property of AC/LiNi_1/3Co_1/3Mn_1/3O₂ supercapacitor. The initial discharge specific capacitance of 298 F g^?1 was obtained in 1 mol L^?1 Li₂SO₄ solution within potential range 0–1.4 V at the current density of 100 mA g^?1 and was cut down less than 0.058 F g^?1 per cycling period in 1000 cycles. The asymmetric supercapacitor exhibited a good cycling performance.     

Novel polymeric light-emitting devices based on bipolar copolymers containing quinoline aluminum moieties and N-vinylcarbazole segments

A series of bipolar copolymers containing quinoline aluminum moieties and N-vinylcarbazole segments exhibited excellent photoluminescence performance and improved hole-transporting ability, were used as light-emitting materials to fabricate polymeric light-emitting devices (PLEDs). Six simple devices with structure of ITO/PEDOT (40 nm)/copolymer (80 nm)/CsF (1.5 nm)/Al (120 nm) were fabricated by solution spin-coating. These devices gave dominant yellow-green emission from quinoline aluminum with excellent stability. Together with electrochemical data, the electroluminescence (EL) mechanism of the devices was studied. Moreover, effects of substituted group and quinoline aluminum's content on the EL properties of the devices were investigated in detail. The results indicate that the device based on the copolymer (P2-1) with substituted group of carbazole and 2 mol% quinoline aluminum possess optimal EL properties with a maximum luminance of 487 cd/m² and a peak current efficiency of 0.545 cd/A.     

Electrodeposited Ni microcones with a thin Au film bonded with Au wire

In wire bonding, the bonding quality between the substrate Au film and metal wire has an effect on productivity and reliability. Au film thickness is important for substrate bondability. It is required to reduce the Au film thickness as thin as possible without deteriorating the level of bondability to cut down extremely high cost of Au consumption. In this study, electrodeposited Ni/Au microcones were fabricated and thermosonic bonded with Au wire. The thickness of Au film was only 0.05 μm. Bonded with Au wire 17.5 μm in diameter, the 0.4 μm-height microcones showed excellent and stable bondability with average pull strength 6.29 gf and small standard deviation. Microscopic observation showed that Ni/Au microcones inserted into Au wire effectively, thus insertion weld between microcones and Au wire was formed. Pull fracture scanning electron microscopy (SEM) images showed an improvement of stitch bonding quality when using Ni/Au microcones. Mechanism of bonding process between the Ni/Au microcones and Au wire was put forward by three stages.