Showing papers on "Annealing (metallurgy) published in 2011"

L-cysteine-assisted synthesis of layered MoS₂/graphene composites with excellent electrochemical performances for lithium ion batteries.

Abstract: A facile process was developed to synthesize layered MoS2/graphene (MoS2/G) composites by an l-cysteine-assisted solution-phase method, in which sodium molybdate, as-prepared graphene oxide (GO), and l-cysteine were used as starting materials. As-prepared MoS2/G was then fabricated into layered MoS2/G composites after annealing in a H2/N2 atmosphere at 800 °C for 2 h. The samples were systematically investigated by X-ray diffraction, field emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and high-resolution transmission electron microscopy. Electrochemical performances were evaluated in two-electrode cells versus metallic lithium. It is demonstrated that the obtained MoS2/G composites show three-dimensional architecture and excellent electrochemical performances as anode materials for Li-ion batteries. The MoS2/G composite with a Mo:C molar ratio of 1:2 exhibits the highest specific capacity of ∼1100 mAh/g at a current of 100 mA/g, as well as excellent cycling stability and hig...

The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to SiO2

Abstract: The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical of the transferred graphene are studied X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 × increase in average mobility from 1400 to 2700 cm2/Vs The electrical results are compared with graphene doping measurements by Raman spectroscopy

Toward intrinsic graphene surfaces: a systematic study on thermal annealing and wet-chemical treatment of SiO2-supported graphene devices.

Abstract: By combining atomic force microscopy and trans-port measurements, we systematically investigated effects of thermal annealing on surface morphologies and electrical properties of single-layer graphene devices fabricated by electron beam lithography on silicon oxide (SiO(2)) substrates. Thermal treatment above 300 °C in vacuum was required to effectively remove resist residues on graphene surfaces. However, annealing at high temperature was found to concomitantly bring graphene in close contact with SiO(2) substrates and induce increased coupling between them, which leads to heavy hole doping and severe degradation of mobilities in graphene devices. To address this problem, a wet-chemical approach employing chloroform was developed in our study, which was shown to enable both intrinsic surfaces and enhanced electrical properties of graphene devices. Upon the recovery of intrinsic surfaces of graphene, the adsorption and assisted fibrillation of amyloid β-peptide (Aβ1-42) on graphene were electrically measured in real time.

The lifetime of the deviations from bulk behaviour in polymers confined at the nanoscale

Abstract: Monitoring the impact of annealing on nanometre-thick polymer layers provides new insight into the changes in the performance of macromolecular materials. Here, the authors present results showing a correlation between the deviations from bulk behaviour and the growth of an irreversibly adsorbed layer.

Sheet texture modification in magnesium-based alloys by selective rare earth alloying

Abstract: The current study examines the influence of select rare earth elements; Gd, Nd, Ce, La and mischmetal (MM) on the sheet texture modification during warm rolling and annealing of a ZEK100 magnesium alloy, and the resulting formability and anisotropy during subsequent tensile testing at room temperature. It was found that all the investigated RE elements led to weak sheet textures and hence promoted enhanced ductility and reduced anisotropy over conventional Mg sheet. Gd was of a particular interest because it gave rise to a desired Mg sheet texture despite its coarsest grain size resulting in promising mechanical properties. It is suggested that solute related effects on the grain boundary migration and the relative strengths of different deformation mechanisms are responsible for altering the common concepts of recrystallization and grain growth during annealing, and the activation scenarios of slip and twinning during deformation.

Austenite Stability Effects on Tensile Behavior of Manganese-Enriched-Austenite Transformation-Induced Plasticity Steel

Abstract: Manganese enrichment of austenite during prolonged intercritical annealing was used to produce a family of transformation-induced plasticity (TRIP) steels with varying retained austenite contents. Cold-rolled 0.1C-7.1Mn steel was annealed at incremental temperatures between 848 K and 948 K (575 °C and 675 °C) for 1 week to enrich austenite in manganese. The resulting microstructures are comprised of varying fractions of intercritical ferrite, martensite, and retained austenite. Tensile behavior is dependent on annealing temperature and ranged from a low strain-hardening “flat” curve to high strength and ductility conditions that display positive strain hardening over a range of strain levels. The mechanical stability of austenite was measured using in-situ neutron diffraction and was shown to depend significantly on annealing temperature. Variations in austenite stability between annealing conditions help explain the observed strain hardening behaviors.

Germanium Nanotubes Prepared by Using the Kirkendall Effect as Anodes for High‐Rate Lithium Batteries

Abstract: The specific energy and the charge/discharge rate (power) of the existing batteries severely limit their use in many applications, such as electric vehicles and smart grids where batteries are needed to store energy from renewable sources of intermittent nature (for example, solar and wind). While replacing a graphitic carbon anode by metallic Si, Ge, or Sn may increase the anode capacity about three times, the rapid capacity fading during cycling due to large volume change has severely hindered their applicability to practical lithium-ion cells with operating voltages of between 3 and 4.3 V. Germanium has gained much attention as a promising anode material for lithium-ion batteries because of its excellent lithium-ion diffusivity (400 times faster than in Si), high electrical conductivity (10 times higher than Si), and large theoretical specific capacity (ca. 1600 mAhg , corresponding to Li4.4Ge). However, mechanical stresses induced by the volume changes during cycling result in pulverization and exfoliation from current collector, leading to capacity fading and poor cycling life. Among various approaches to minimize the mechanical stresses induced by volume change, the use of nanotubes (NTs) is proven to be most effective in accommodating the volume changes of electrode materials during cycling. Accordingly, anodic aluminum oxide (AAO) templates have been commonly used to produce NTs of many different materials. Unfortunately, this approach results in very low yields, produces NTs with non-uniform wall thicknesses, and is vulnerable to contamination by impurities involved in the reaction process. In 2004, a unique process based on the Kirkendall effect was successfully used to convert metal nanoparticles into hollow metal oxide and sulfide. Kirkendall voids form as a consequence of the difference in diffusion fluxes of atoms across an interface in a diffusion couple at elevated temperatures. When outward diffusion of one atom is much faster than the inward diffusion of another, vacancies are created on the side with higher diffusivity. These vacancies can coalesce into voids under proper conditions. To date, however, similar structures of semiconductors (e.g., Ge and Si) are yet to be produced. Herein we present a high-yielding synthetic method for fabrication of ultralong GeNTs from core–shell Ge–Sb nanowires (NWs) by utilizing the Kirkendall effect at 700 8C. The GeNTs displayed exceptionally high rate capability of up to 40C (40 Ag ) while maintaining a reversible capacity of > 1000 mAhg 1 over 400 cycles with minimal capacity fading when paired with a LiCoO2 cathode in a lithium-ion cell. TEM images showing the evolution of a GeNW (1a) to a GeNT (1e) by the Kirkendall effect are given in Figure 1a–e. First, a mixture of antimony acetate and polyvinyl pyrrolidone (PVP) was coated on the GeNW (1b). The amount of antimony acetate and PVP were varied to adjust the coating concentration of antimony to about 2 wt% of the GeNWs used. Upon annealing at 700 8C, voids start to form inside the core–shell interface (1c) and continue to grow until the conversion is completed (1d). Finally, nanotubular morphology is formed after annealing for 5 h (1e), resulting in GeNTs with diameters varying from 200 to 250 nm (see the Supporting Information, Figure S1–S4 for more detailed morphology evolution from NW to NT). More importantly, the wall thickness of the GeNTs after annealing for 5 h is relatively uniform (ca. 40 nm; Supporting Information, Figure S5c). The selected area diffraction pattern indicates that the NTs are amorphous (Supporting Information, Figure S5c inset). Furthermore, to enhance the long-term stability, the GeNTs were exposed to C2H2 gas for the last 20 min of the 5 h annealing to introduce a thin-film carbon coating on the surface of the GeNTs (Supporting Information, Figure S5). These results show that GeNTs are formed by a multiplestep process (see Figure 1 f with 2D cross-sectional views of the NW/NT at each step of the conversion): a) coating GeNWs with an antimony precursor; b) Ge atoms diffuse outward, leaving voids inside the NWs (note that germanium can react with a variety of metals, such as Au, Ag, Bi, and Sb, with lower atomic diffusivity, thereby creating a void between the inner wire and the outer tube); c) the outer germanium layer continues to diffuse and completely separates from antimony and a portion of GeNW; d) the inner [*] M.-H. Park, Y. Cho, Prof. J. Cho Interdisciplinary School of Green Energy Ulsan National Institute of Science and Technology (UNIST) Ulsan, 689-798 (Korea) E-mail: jpcho@unist.ac.kr Homepage: http://jpcho.com

Enhancement of the capacitance in TiO2 nanotubes through controlled introduction of oxygen vacancies

Abstract: The many applications of high energy storage devices have forged an increasing interest in research areas related to electrochemical capacitors. Here, in this work, we present a facile method for the fabrication of self-organized titania nanotubes grown by anodic oxidation of titanium foil with different subsequent heat-treatment regimes for use as binder-free working electrodes in supercapacitor applications. The capacitance of these highly ordered titania nanotubes, when exposed to a reductive atmosphere during annealing, was determined to be well above 900 µF cm−2, confirming that the capacitance contribution was pseudocapacitive in nature. The behaviour of oxygen depleted titania in the anatase to rutile (A → R) phase transformation and also in electrochemical charge storage has been studied in detail. It was found that upon the reduction of Ti4+ to Ti3+, with oxygen depletion of the structure, the A → R phase transformation was promoted. In addition, the fabricated electrodes showed highly reversible charge–discharge stability.

Drastic Control of Texture in a High Performance n-Type Polymeric Semiconductor and Implications for Charge Transport

Abstract: Control of crystallographic texture from mostly face-on to edge-on is observed for the film morphology of the n-type semicrystalline polymer {[N,N-9-bis(2-octyldodecyl)naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bithiophene)}, P(NDI2OD-T2), when annealing the film to the polymer melting point followed by slow cooling to ambient temperature. A variety of X-ray diffraction analyses, including pole figure construction and Fourier transform peak shape deconvolution, are employed to quantify the texture change, relative degree of crystallinity and lattice order. We find that annealing the polymer film to the melt leads to a shift from 77.5% face-on to 94.6% edge-on lamellar texture as well as to a 2-fold increase in crystallinity and a 40% decrease in intracrystallite cumulative disorder. The texture change results in a significant drop in the electron-only diode current density through the film thickness upon melt annealing, while little change is observed in the in-plane transport of bott...

Enhancing the Curie Temperature of Ferromagnetic Semiconductor (Ga,Mn)As to 200 K via Nanostructure Engineering

Abstract: We demonstrate by magneto-transport measurements that a Curie temperature as high as 200 K can be obtained in nanostructures of (Ga,Mn)As. Heavily Mn-doped (Ga,Mn)As films were patterned into nanowires and then subject to low-temperature annealing. Resistance and Hall effect measurements demonstrated a consistent increase of T(C) with decreasing wire width down to about 300 nm. This observation is attributed primarily to the increase of the free surface in the narrower wires, which allows the Mn interstitials to diffuse out at the sidewalls, thus enhancing the efficiency of annealing. These results may provide useful information on optimal structures for (Ga,Mn)As-based nanospintronic devices operational at relatively high temperatures.

Solvent‐Annealed Crystalline Squaraine: PC70BM (1:6) Solar Cells

Abstract: It is demonstrated that squaraine:PC70BM blends can result in solar cells with high efficiency and fill factor when the nano­structure scale is increased to lead to conduction of photogenerated carriers to the electrodes. Morphological control occurs by a combination of thermal and solvent annealing of these small molecule, solution-processed bulk heterojunction cells. Peak efficiencies of a population of devices is 5.2% at fill factors of 0.5 are achieved under optimized conditions.

Microstructure and residual stress of laser rapid formed Inconel 718 nickel-base superalloy

Abstract: The microstructure and residual stress of laser rapid formed (LRFed) nickel-base superalloy Inconel 718 was investigated. The as-deposited microstructure of an LRFed Inconel 718 alloy is composed of columnar dendrites growing epitaxially along the deposition direction, and the columnar dendrites transformed to unevenly distributed equiaxed grains after annealing treatment at high temperature. Residual stress evaluation in microstructure scale by Vickers micro-indentation method indicates that the residual thermal stress is unevenly distributed in the LRFed sample, and it has a significant effect on the recrystallization during solution annealing treatment. The residual stress is introduced by rapid heating and cooling during laser rapid forming. There is an alternative distribution between high residual stress regions and low residual stress regions, within a single deposited layer, resulting in a similar distribution of recrystallized grain size.

Method of material processing by laser filamentation

Abstract: A method is provided for the internal processing of a transparent substrate in preparation for a cleaving step. The substrate is irradiated with a focused laser beam that is comprised of pulses having an energy and pulse duration selected to produce a filament within the substrate. The substrate is translated relative to the laser beam to irradiate the substrate and produce an additional filament at one or more additional locations. The resulting filaments form an array defining an internally scribed path for cleaving said substrate. Laser beam parameters may be varied to adjust the filament length and position, and to optionally introduce V-channels or grooves, rendering bevels to the laser-cleaved edges. Preferably, the laser pulses are delivered in a burst train for lowering the energy threshold for filament formation, increasing the filament length, thermally annealing of the filament modification zone to minimize collateral damage, improving process reproducibility, and increasing the processing speed compared with the use of low repetition rate lasers.

Real-Time Investigation of Crystallization and Phase-Segregation Dynamics in P3HT:PCBM Solar Cells During Thermal Annealing

Abstract: Crystallization and phase segregation during thermal annealing lead to the increase of power-conversion efficiency in poly(3-hexylthiophene) (P3HT):[6,6]-phenyl C61-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. An understanding of the length and time scale on which crystallization and phase segregation occur is important to improve control of the nanomorphology. Crystallization is monitored by means of grazing incidence X-ray diffraction in real time during thermal annealing. Furthermore, the change in film density is monitored by means of ellipsometry and the evolution of carrier mobilities by means of field effect transistors, both during annealing. From the combination of such measurements with those of device performance as a function of annealing time, it is concluded that the evolution of microstructure involves two important time windows: i) A first one of about 5 minutes duration wherein crystallization of the polymer correlates with a major increase of photocurrent; ii) a second window of about 30 minutes during which the aggregation of PCBM continues, accompanied by an increase in the fill factor.

Synthesis of highly symmetrical BiOI single-crystal nanosheets and their {001} facet-dependent photoactivity

Abstract: Highly symmetrical BiOI single-crystal nanosheets (BiOI SCNs) with dominant exposed {001} facets (up to 95%) have been synthesized by annealing BiI3 and characterized by X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, transmission electron microscopy, selected area electron diffraction, high-resolution transmission electron microscopy, fast-Fourier transform pattern, UV-vis diffuse reflectance and photoluminescence. The thickness and the {001} facets percentage of BiOI SCNs can be tuned by changing the annealing temperature. The thermal decomposition process of BiI3 and the formation mechanism of BiOI SCNs were investigated. BiOI SCNs exhibit higher photoactivity (about 7 times) than irregular BiOI for degradation of Rhodamine B (RhB) dye under visible light irradiation. The {001} facets are the reactive facets of BiOI. The origin of {001} facets-dependent photoactivity is due to an improvement of the separation efficiency of photo-induced electrons and holes.

Dynamics of Crystallization and Disorder during Annealing of P3HT/PCBM Bulk Heterojunctions

Abstract: A real-time crystallographic analysis of P3HT/PCBM films during thermal annealing is reported, detailing the temporal variation of crystallization, disorder, and orientational spread during the annealing. Five P3HT/PCBM chlorobenzene solutions with different P3HT concentrations (0, 33, 50, 67, 100 wt %) were spin coated on SiO2 substrates. The thick films (100 nm) were studied during annealing (50 min at 140 °C), with in situ synchrotron grazing incidence X-ray diffraction (GI-XRD) and a sampling time <8 s. For the first time, the evolution of the crystal structure is analyzed taking into account P3HT paracrystallinity. The following were observed: a predominance of edge-on P3HT lamellae in the as-spun and annealed films; changes in concentration-dependent edge-on lamellar orientation spread along the alkyl-stacking direction and paracrystalline disorder after annealing; a permanent lamellar stretching just along the alky-stacking direction after annealing; an increase in the P3HT domain size along only the alkyl-stacking direction for the edge-on lamellae, with dynamics consistent with PCBM acting as a plasticizer for P3HT; and finally, an increase in the PCBM concentration at the sample−air interface. We show that the paracrystalline correction is important to calculate correctly the domain size as deduced from GI-XRD.

Influence of intermetallic phases and Kirkendall-porosity on the mechanical properties of joints between steel and aluminium alloys

Abstract: The formation of intermetallic reaction layers and their influence on mechanical properties was investigated in friction stir welded joints between a low C steel and both pure Al (99.5 wt.%) and Al–5 wt.% Si. Characterisation of the steel/Al interface, tensile tests and fractography analysis were performed on samples in the as-welded state and after annealing in the range of 200–600 ◦ C for 9–64 min. Annealing was performed to obtain reaction layers of distinct thickness and composition. For both Al alloys, the reaction layers grew with parabolic kinetics with the phase (Al5Fe2) as the dominant component after annealing at 450 ◦ C and above. In joints with pure Al, the tensile strength is governed by the formation of Kirkendall-porosity at the reaction layer/Al interface. The tensile strength of joints with Al–5 wt.% Si is controlled by the thickness of the phase (Al5Fe2) layer. The pre-deformation of the base materials, induced by the friction stir welding procedure, was found to have a pronounced effect on the composition and growth kinetics of the reaction layers.

Thermal stability and oxidation resistance of laser clad TiVCrAlSi high entropy alloy coatings on Ti–6Al–4V alloy

Abstract: TiVCrAlSi high entropy alloy coatings were deposited on Ti–6Al–4V alloy by laser cladding. SEM, XRD and EDS analyses show that, the as-clad coating is composed of (Ti,V) 5 Si 3 and a BCC solid solution. After annealing at 800 °C for 24 h under vacuum, the coating is composed of (Ti,V) 5 Si 3 , Al 8 (V,Cr) 5 , and a BCC solid solution. The temperature-dependent phase equilibrium for the coating material calculated by using the CALPHAD method, indicates that above 880 °C the stable phases existing in the coating material are a BCC solid-solution and (Ti,V) 5 Si 3 . When the temperature is below 880 °C, the stable phases are (Ti,V) 5 Si 3 , Al 8 (V,Cr) 5 , and a BCC solid solution. In order to validate the calculation results, they were compared with TiVCrAlSi alloy samples prepared by arc melting, encapsulated in quartz tubes under vacuum, annealed at 400–1100 °C for 3 days and water-quenched. XRD analysis shows that the experimental phase composition agrees with the thermodynamic calculations. After vacuum annealing, there is a small increase of hardness for the laser clad TiVCrAlSi coating, which is due to the formation of Al 8 (V,Cr) 5 . The oxidation tests show that the TiVCrAlSi coating effectively improves the oxidation resistance of Ti–6Al–4V at 800 °C in air. The formation of a dense and adherent scale consisting of SiO 2 , Cr 2 O 3 , TiO 2 , Al 2 O 3 and a small amount of V 2 O 5 is supposed to be responsible for the observed improvement of the oxidation resistance.

Structural and elemental characterization of high efficiency Cu2ZnSnS4 solar cells

Abstract: We have carried out detailed microstructural studies of phase separation and grain boundary composition in Cu2ZnSnS4 based solar cells. The absorber layer was fabricated by thermal evaporation followed by post high temperature annealing on hot plate. We show that inter-reactions between the bottom molybdenum and the Cu2ZnSnS4, besides triggering the formation of interfacial MoSx, results in the out-diffusion of Cu from the Cu2ZnSnS4 layer. Phase separation of Cu2ZnSnS4 into ZnS and a Cu–Sn–S compound is observed at the molybdenum-Cu2ZnSnS4 interface, perhaps as a result of the compositional out-diffusion. Additionally, grain boundaries within the thermally evaporated absorber layer are found to be either Cu-rich or at the expected bulk composition. Such interfacial compound formation and grain boundary chemistry likely contributes to the lower than expected open circuit voltages observed for the Cu2ZnSnS4 devices.

Particle effects on recrystallization in magnesium?manganese alloys: Particle pinning

Abstract: Dispersoid particles are widely used in wrought aluminium alloys to control grain structure during thermomechanical processing. The aim of this work was to investigate whether this approach could be utilized in wrought magnesium alloys to obtain better control of recrystallization. A binary magnesium–manganese alloy was heat treated to produce a fine dispersion of manganese precipitates. The effect of this dispersion on dynamic and static recrystallization during channel die deformation (at a slow strain rate), hot rolling, and annealing was studied and compared with that of an alloy free of fine particles. It was found that the presence of particles did not suppress dynamic recrystallization during channel die deformation. Fine particles did lead to a much reduced recrystallized fraction after hot rolling, attributed to a retardation of static recrystallization kinetics. Although the presence of pinning particles greatly slowed recrystallization kinetics on annealing, for no conditions studied was it possible to prevent recrystallization of the as-deformed structure using particles.

Microstructure and mechanical properties of Fe–0.2C–5Mn steel processed by ART-annealing

Abstract: Microstructure and mechanical properties of medium manganese steel (Fe–0.2C–5Mn) processed by annealing at 650 °C with annealing time up to 12 h after accelerated cooling were studied. It was found that the martensite structure was gradually transformed into a superfine ferrite and austenite duplex structure by austenite reverted transformation during annealing process. The annealing heat treatment results in a large volume fraction of austenite (34%) and an excellent combination of ultimate tensile strength (∼1000 MPa) and total elongation (>40%) at room temperature. The ultrahigh tensile strength and improved ductility of present steel were mainly attributed to the enhanced phase transformation induced plasticity due to the large fractioned austenite.

High-performance low-temperature solution-processable ZnO thin film transistors by microwave-assisted annealing

Abstract: Oxide semiconductors afford a promising alternative to organic semiconductors and amorphous silicon materials in applications requiring transparent thin film transistors (TFTs). We synthesized an aqueous inorganic precursor by a direct dissolution of zinc hydroxide in ammonium hydroxide solution from which a dense and uniform ZnO semiconducting layer is achieved. Solution-processed ZnO-TFTs prepared at 140 °C by microwave irradiation have shown enhanced device characteristics of ∼1.7 cm2 V−1s−1 mobility and a ∼107 on/off current ratio, with good air stability. Spectroscopic analyses confirmed that such a device improvement originates from accelerated dehydroxylation and better crystallization at low temperature by microwave irradiation. Our results suggest that solution-processable oxide semiconductors have potential for low-temperature and high-performance applications in transparent devices.