Showing papers on "Annealing (metallurgy) published in 2010"

A Critical Size of Silicon Nano‐Anodes for Lithium Rechargeable Batteries

Abstract: Due to the high theoretical capacity (ca. 4200 mAhg ) of Si when Li4.4Si is formed, it has been extensively investigated for use as a high-capacity anode material that can replace graphite, which is currently used (372 mAhg ). However, Si exhibits significant volume changes (> 360%) during Li alloying and dealloying. These changes cause cracking and crumbling of the electrode material and a consequent loss of electrical contact between individual particles and hence severe capacity drop. However, such mechanical strain induced by volume change can be reduced by employing smaller particles. To this end, synthetic methods such as spark ablation, aerogel techniques, and sputtering have been employed. Formation of crystalline Si nanoparticles requires higher temperatures due to the more covalent nature of these particles compared to Ge particles, and at low temperature amorphous phases become more common. The first commonly recognized successful production of Si nanoclusters was reported byHeath et al. They showed that Si nanocrystals capped with alkyl groups can be produced by reduction of SiCl4 and RSiCl3 (R=H, C8H17) according to the reaction SiCl4+RSiCl3+Na!Si+NaCl. This process was carried out at high temperature (385 8C) and high pressure (>100 atm) in a steel bomb fitted into a heating mantle. A process that utilizes SiCl4 reduction at room temperature under an inert atmosphere was initially reported by Kauzlarich et al. However, the drawback of their method was that the product obtained at room temperature was not fully crystallized and was severely capped with alkyl terminators. Moreover, an annealing process above 900 8C is required to obtain the crystalline phase. Similar solution syntheses have been reported at low or high temperature after reducing Si salts with LiAlH4 [13,14] or alkyl silanes. However, all of these methods produce a broad particle size distribution or involve aggregation of the nanoparticles. Furthermore, they all yield amounts of material too small for use in anode production for lithium secondary batteries. We now report a synthetic method using reverse micelles at high pressure and temperature in a bomb that produces Si nanoparticles (n-Si) with various particle sizes without aggregation and thus enables the optimal nanoparticle size for use in anode materials to be chosen. Figure 1 shows the XRD pattern and TEM images of n-Si prepared with trimethyloctadecylammonium bromide (OTAB) surfactant. The XRD pattern clearly shows forma-

Effects of Thermal Annealing Upon the Morphology of Polymer-Fullerene Blends

Abstract: Grazing incidence X-ray scattering (GIXS) is used to characterize the morphology of poly(3-hexylthiophene) (P3HT)–phenyl-C61-butyric acid methyl ester (PCBM) thin film bulk heterojunction (BHJ) blends as a function of thermal annealing temperature, from room temperature to 220 °C. A custom-built heating chamber for in situ GIXS studies allows for the morphological characterization of thin films at elevated temperatures. Films annealed with a thermal gradient allow for the rapid investigation of the morphology over a range of temperatures that corroborate the results of the in situ experiments. Using these techniques the following are observed: the melting points of each component; an increase in the P3HT coherence length with annealing below the P3HT melting temperature; the formation of well-oriented P3HT crystallites with the (100) plane parallel to the substrate, when cooled from the melt; and the cold crystallization of PCBM associated with the PCBM glass transition temperature. The incorporation of these materials into BHJ blends affects the nature of these transitions as a function of blend ratio. These results provide a deeper understanding of the physics of how thermal annealing affects the morphology of polymer–fullerene BHJ blends and provides tools to manipulate the blend morphology in order to develop high-performance organic solar cell devices.

Microstructure and pseudocapacitive properties of electrodes constructed of oriented NiO-TiO2 nanotube arrays.

Abstract: We report on the synthesis and electrochemical properties of oriented NiO-TiO2 nanotube (NT) arrays as electrodes for supercapacitors. The morphology of the films prepared by electrochemically anodizing Ni−Ti alloy foils was characterized by scanning and transmission electron microscopies, X-ray diffraction, and photoelectron spectroscopies. The morphology, crystal structure, and composition of the NT films were found to depend on the preparation conditions (anodization voltage and postgrowth annealing temperature). Annealing the as-grown NT arrays to a temperature of 600 °C transformed them from an amorphous phase to a mixture of crystalline rock salt NiO and rutile TiO2. Changes in the morphology and crystal structure strongly influenced the electrochemical properties of the NT electrodes. Electrodes composed of NT films annealed at 600 °C displayed pseudocapacitor (redox-capacitor) behavior, including rapid charge/discharge kinetics and stable long-term cycling performance. At similar film thicknesses ...

Effect of grain refinement to 1 μm on strength and toughness of dual-phase steels

Abstract: Large strain warm deformation at different temperatures and subsequent intercritical annealing has been applied to obtain fine grained (2.4m) and ultrafine grained (1.2m) ferrite/martensite dual-phase (DP) steels. Their mechanical properties were tested under tensile and impact conditions and compared to a hot deformed coarse grained (12.4m) reference material. Both yield strength and tensile strength follow a Hall–Petch type linear relationship, whereas uniform elongation and total elongation are hardly affected by grain refinement. The initial strain hardening rate as well as the post-uniform elongation increase with decreasing grain size. Ductile fracture mechanisms are considerably promoted due to grain refinement. Grain refinement further lowers the ductile-to-brittle transition temperature and leads to higher absorbed impact energies. Besides the common correlations with the ferrite grain size, these phenomena are explained in terms of the martensite particle size, shape and distribution and the more homogeneous dislocation distribution in ultrafine ferrite grains.

Defect‐Related Emissions and Magnetization Properties of ZnO Nanorods

Abstract: A clear correlation between defect-related emissions and the magnetization of ZnO nanorods synthesized by a one-step aqueous chemical method is demonstrated. The relative contribution of the emission bands arising from various types of defects is determined and found to be linked with the size of the nanorods and annealing conditions. When the size of the nanorods and the annealing temperature are increased, the magnetization of pure ZnO nanorods decreases with the reduction of a defect-rotated band originating from singly charged oxygen vacancies (V + o ). With a sufficient increase of annealing temperature (at 900 °C), the nanorods show diamagnetic behavior. Combining with the electron paramagnetic resonance results, a direct link between the magnetization and the relative occupancy of the singly charged oxygen vacancies present on the surface of ZnO nanorods is established.

Transparent p-type SnOx thin film transistors produced by reactive rf magnetron sputtering followed by low temperature annealing

Abstract: P-type thin-film transistors (TFTs) using room temperature sputtered SnOx (x<2) as a transparent oxide semiconductor have been produced. The SnOx films show p-type conduction presenting a polycrystalline structure composed with a mixture of tetragonal β-Sn and α-SnOx phases, after annealing at 200 °C. These films exhibit a hole carrier concentration in the range of ≈1016–1018 cm−3; electrical resistivity between 101–102 Ω cm; Hall mobility around 4.8 cm2/V s; optical band gap of 2.8 eV; and average transmittance ≈85% (400 to 2000 nm). The bottom gate p-type SnOx TFTs present a field-effect mobility above 1 cm2/V s and an ON/OFF modulation ratio of 103.

The effect of vacuum annealing on graphene

Abstract: The effect of vacuum annealing on the properties of graphene is investigated by using Raman spectroscopy and electrical measurement. Heavy hole doping on graphene with concentration as high as 1.5 × 1013 cm−2 is observed after vacuum annealing and exposed to an air ambient. This doping is due to the H2O and O2 adsorption on graphene, and graphene is believed to be more active to molecular adsorption after annealing. Such observation calls for special attention in the process of fabricating graphene-based electronic devices and gas sensors. On the other hand, because the quality of graphene remains high after the doping process, this would be an efficient and controllable method to introduce heavy doping in graphene, which would greatly help on its application in future electronic devices. Copyright © 2009 John Wiley & Sons, Ltd.

Influence of Al on the Microstructural Evolution and Mechanical Behavior of Low-Carbon, Manganese Transformation-Induced-Plasticity Steel

Abstract: Microstructural design with an Al addition is suggested for low-carbon, manganese transformation-induced-plasticity (Mn TRIP) steel for application in the continuous-annealing process. With an Al content of 1 mass pct, the competition between the recrystallization of the cold-rolled microstructure and the austenite formation cannot be avoided during intercritical annealing, and the recrystallization of the deformed matrix does not proceed effectively. The addition of 3 mass pct Al, however, allows nearly complete recrystallization of the deformed microstructure by providing a dual-phase cold-rolled structure consisting of ferrite and martensite and by suppressing excessive austenite formation at a higher annealing temperature. An optimized annealing condition results in the room-temperature stability of the intercritical austenite in Mn TRIP steel containing 3 mass pct Al, permitting persistent transformation to martensite during tensile deformation. The alloy presents an excellent strength-ductility balance combining a tensile strength of approximately 1 GPa with a total elongation over 25 pct, which is comparable to that of Mn TRIP steel subjected to batch-type annealing.

Microstructural, Optical, and Electrical Properties of SnO Thin Films Prepared on Quartz via a Two-Step Method

Abstract: A simple, cost-effective, two-step method was proposed for preparing single-phase SnO polycrystalline thin films on quartz. X-ray diffraction (XRD) analysis demonstrated that the annealed films were consisted of polycrystalline α-SnO phase without preferred orientation, and chemical composition analysis of the single phase in nature was analyzed using X-ray photoelectron spectroscopy (XPS). Transmittance spectra in UV−vis−IR range indicated that the average transmittance of both the as-deposited and the annealed SnO thin films was up to 70%. The optical band gap decreased from 3.20 to 2.77 eV after the annealing process, which was attributed to the crystalline size related quantum size effect. Photoluminescence (PL) spectrum of the annealed film showed only a weak peak at 585 nm, and no intrinsic optical transition emission was observed. Moreover, the p-type conductivity of SnO film was confirmed through Hall effect measurement, with Hall mobility of 1.4 cm2 V−1 s−1 and hole concentration of 2.8 × 1016 cm−3.

Effect of heat treatment on the properties and structure of TiO2 nanotubes: phase composition and chemical composition

Abstract: Titanium oxide (TiO2) nanotubes prepared by electrolytic anodisation of a titanium electrode have been systematically heat treated to control the conversion of the as-prepared amorphous structure to nanocrystalline anatase and rutile. Raman spectroscopy revealed that the temperature of calcination is critical in determining the structure and crystallinity of the titania. X-ray Photoelectron Spectroscopy analysis shows the as-prepared film to consist mainly of oxide, although a small amount of fluoride contamination remains from the electrolyte. Organic components from post-anodising cleaning treatments were also present. Fluorine ions are gradually ejected from the anodic layer during annealing and the fluorine concentration is negligible in samples that are heat treated above 400 °C. Choosing the appropriate annealing temperature allows the structure to be made up of defined proportions of anatase and rutile with a reduced contamination of species from the electrolyte or organic solvents. Copyright © 2010 John Wiley & Sons, Ltd.

Improved electrical transport in Al-doped zinc oxide by thermal treatment

Abstract: A postdeposition thermal treatment has been applied to sputtered Al-doped zinc oxide films and shown to strongly decrease the resistivity of the films. While high temperature annealing usually leads to deterioration of electrical transport properties, a silicon capping layer successfully prevented the degradation of carrier concentration during the annealing step. The effect of annealing time and temperature has been studied in detail. A mobility increase from values of around 40 cm2/Vs up to 67 cm2/Vs, resulting in a resistivity of 1.4×10−4 Ω cm has been obtained for annealing at temperatures of 650 °C. The high mobility increase is most likely obtained by reduced grain boundary scattering. Changes in carrier concentration in the films caused by the thermal treatment are the result of two competing processes. For short annealing procedures we observed an increase in carrier concentration that we attribute to hydrogen diffusing into the zinc oxide film from a silicon nitride barrier layer between the zinc...

High-strength Fe–20Mn–Al–C-based Alloys with Low Density

Abstract: Mechanical properties of Fe-20Mn-(10-14)Al-(0-1.8)C (mass%) quaternary and Fe-20Mn-(10-14)Al-(0.75-1.8)C-5Cr (mass%) quinary alloys were investigated by hardness, cold-workability and tensile tests at room temperature. The γ(fcc) alloys in both quaternary and quinary systems with a low density of less than 7.0 g/cm 3 showed an excellent ductility and their hardness and tensile strength increased with increasing Al and C contents. The γ+α(bcc) duplex alloys also exhibited a high tensile strength by controlling the α volume fraction. TEM observation confirmed that high hardness and tensile strength of the alloys with high Al and C contents are caused by the precipitation of nano-size κ-carbide with perovskite structure during cooling from the annealing temperature. Fe-20Mn-11Al-1.8C-5Cr alloy with a density of 6.51 g/cm 3 showed a high specific strength of more than 180 MPa · cm 3 /g with a good tensile elongation of 40 %. The present Fe-20Mn-Al-C(-5Cr) alloys showed a higher specific strength than conventional steels.

Effects of recrystallization annealing temperature on carbide precipitation, microstructure, and mechanical properties in Fe-18Mn-0.6C-1.5Al TWIP steel

Abstract: The microstructure, dimple structure, and mechanical properties of a cold-rolled Fe–18Mn–0.6C–1.5Al TWIP steel were investigated as a function of annealing temperature. The recrystallization started at 600 °C and finished at 700 °C for the holding time of 10 min. The coarsening rate of recrystallized grains was increased over about 840 °C and Rockwell hardness was greatly decreased between 800 and 900 °C, which shows a good agreement with the equilibrium dissolution temperature of M 3 C carbides. The reversion of the tensile strength occurred between 700 and 800 °C because of the carbide precipitation hardening. The precipitation-time-temperature diagram was generated by dilatometric tests, showing a nose temperature of 800 °C. The dimple size was decreased to 700 °C and then increased again with higher annealing temperature, having a strong proportional relationship with austenite grain size.

Reactive ballistic deposition of α-Fe2O3 thin films for photoelectrochemical water oxidation

Abstract: We report the preparation of alpha-Fe2O3 electrodes using a technique known as reactive ballistic deposition in which iron metal is evaporatively deposited in an oxygen ambient for photoelectrochemical (PEC) water oxidation. By manipulating synthesis parameters such as deposition angle, film thickness, and annealing temperature, we find that it is possible to optimize the structural and morphological properties of such films in order to improve their PEC efficiency. Incident photon to current conversion efficiencies (IPCE) are used to calculate an AM1.5 photocurrent of 0.55 mA/cm(2) for optimized films with an IPCE reaching 10% at 420 nm in 1 M KOH at +0.5 V versus Ag/AgCl. We also note that the commonly observed low photoactivity of extremely thin hematite films on fluorine-doped tin oxide substrates may be improved by modification of annealing conditions in some cases.

Thermal stability of nanocrystalline Fe–Zr alloys

Abstract: Fe–Zr nanocrystalline alloys with an as-milled grain size less than 10 nm were synthesized by ball milling. The microstructure changes due to annealing were characterized using X-ray line broadening, microhardness, focused ion beam channeling contrast imaging, and transmission electron microscopy (TEM). Additions of 1/3 to 4 at.% Zr stabilized nanocrystalline grain sizes at elevated annealing temperatures compared to pure Fe. With 4 at.% Zr, a fully nanocrystalline microstructure with a TEM grain size of 52 nm was retained at temperatures in excess of 900 °C. Alloys with lower Zr contents showed less stability, but still significant compared to pure Fe. Bimodal nano–micro grain size microstructures were also observed.

Effects of Annealing Parameters on Optical Properties of Thermochromic VO2 Films Prepared in Aqueous Solution

Abstract: This work confirmed experimentally that the microstructure (grain boundaries, grain sizes, and size distributions) of VO2 films has significant effects on the features of the semiconductor−metal (S−M) transition. This feature enables us to wisely regulate the parameters of the phase transition, which is of great importance in achieving practical applications. Thermochromic VO2 films with various optical properties and phase transition parameters (for example: hysteresis widths ranging from 12 to 50 °C) have been synthesized on fused silica substrates via a simple solution process with inorganic precursors and polyvinylpyrrolidone (PVP). The widths and slopes of the hysteresis loops (i.e., the temperature sensitivity of the transition) can be regulated by controlling grain sizes and grain boundary conditions, which are believed to dominate the generation of the elementary hysteresis loop of each grain and the propagation of the S−M transition, respectively. A film consisting of qusai-isolated small particl...

Annealing effects on the properties of copper oxide thin films prepared by chemical deposition

Abstract: Copper oxide thin films with thickness of 0.45 mu m were chemically deposited on glass substrates by dipping the microscope glass slide for 20 s each in 1 M NaOH and copper complex solutions. Temperature of NaOH solution was increased to 70 degrees C, while the copper solution was maintained at room temperature. Copper oxide thin films were annealed in air at different temperature of 200 - 400 degrees C and as-prepared sample was used as reference. The films structures were studied by XRD. The patterns showed that the films as prepared and annealed at 200 degrees C were cuprite structure with Cu(2)O composition. Films annealed at 300 degrees C consist of mixed tenorite (CuO) and cuprite (Cu(2)O) phases. Annealing the films in air at 400 degrees C completely converts these films to tenorite structure with composition of CuO. The proportion of the two forms of copper oxide varies with oxidation temperature. The surface properties were characterized using scanning electron microscopy. UV-Vis transmittance spectra confirmed the results from the XRD by a shift in the optical band gap from 2.40 to 1.73 eV. The conversion was also confirmed by the FTIR spectroscopy measurement. Photoluminescence intensity is greatly improved with the increase in annealing temperatures.

Vacancy-Mediated Magnetism in Pure Copper Oxide Nanoparticles

Abstract: Room temperature ferromagnetism (RTF) is observed in pure copper oxide (CuO) nanoparticles which were prepared by precipitation method with the post-annealing in air without any ferromagnetic dopant. X-ray photoelectron spectroscopy (XPS) result indicates that the mixture valence states of Cu1+ and Cu2+ ions exist at the surface of the particles. Vacuum annealing enhances the ferromagnetism (FM) of CuO nanoparticles, while oxygen atmosphere annealing reduces it. The origin of FM is suggested to the oxygen vacancies at the surface/or interface of the particles. Such a ferromagnet without the presence of any transition metal could be a very good option for a class of spintronics.

Effect of Zn substitution on magnetic properties of nanocrystalline cobalt ferrite

Abstract: The Zn substituted cobalt ferrite nanoparticles having the generic formula Co1−xZnxFe2O4 (x=0.0–0.7) were prepared by wet chemical coprecipitation technique using analytical reagent (AR) grade sulphates. The prepared samples were heated at 150 °C to remove water molecules and then annealed at 725 °C for 16 h. Investigation of the structural properties were carried out using x-ray diffraction, transmission electron microscopy (TEM), and scanning electron microscopy techniques. The nanocrystalline nature of the samples is confirmed by TEM data. Substitution of the nonmagnetic Zn2+ ions considerably changes the magnetic properties. Neel’s model fails to explain the observed magnetic behavior above x=0.2. For x≥0.2 the Yafet–Kittel model can be fitted. AC susceptibility measurements confirm the decrease in Curie temperature.

Thermal stability of austenite retained in bainitic steels

Abstract: Steels with a microstructure consisting of a mixture of bainitic ferrite and carbon-enriched retained austenite are of interest in a variety of commercial applications because they have been shown to exhibit good combinations of strength, toughness and ductility. However, their use at temperatures moderately above ambient requires a knowledge of the thermal stability of the austenite. The changes that occur during the tempering of a mixture of bainitic ferrite, carbon-enriched retained austenite and martensite have been characterised. An analysis of the volume change due to transformation shows that it is possible to distinguish the decomposition of austenite from the tempering of martensite. The nature of the carbides that form during the heat treatment is discussed as are the implications on the development of mathematical models accounting for calculating the strain during austenite decomposition and martensite tempering. It is found that the early stages of tempering reactions where the austenite content is not greatly reduced, can dramatically influence the stability of the austenite as it is cooled to ambient temperature.

Investigation of the parameters of the cold roll bonding (CRB) process

Abstract: In this study, commercial purity aluminum (AA1100) strips were cold roll bonded at ambient temperature. As the main factors, the effects of different amounts of reduction in thickness, initial thickness, rolling speed, and rolling direction on bond strength were evaluated by the peeling test. Also, the effects of pre- and post-rolling annealing treatments were investigated. It was found that higher reductions, lower initial thickness, and rolling speed were the important factors involved in improving bond strength. Also, annealing treatment before and/or after the CRB process increased bond strength, while the effect of pre-rolling annealing was more pronounced. Furthermore, bond strength decreased by increasing the angle of CRB process with respect to the rolling direction of as-received strips. Finally, optical and scanning electron microscopes were used to evaluate the surface conditions of the peeled surfaces.