To save this undefined to your undefined account, please select one or more formats and confirm that you agree to abide by our usage policies. If this is the first time you used this feature, you will be asked to authorise Cambridge Core to connect with your undefined account.
Find out more about saving content to .
To save this article to your Kindle, first ensure email@example.com is added to your Approved Personal Document E-mail List under your Personal Document Settings on the Manage Your Content and Devices page of your Amazon account. Then enter the ‘name’ part of your Kindle email address below.
Find out more about saving to your Kindle.
Note you can select to save to either the @free.kindle.com or @kindle.com variations. ‘@free.kindle.com’ emails are free but can only be saved to your device when it is connected to wi-fi. ‘@kindle.com’ emails can be delivered even when you are not connected to wi-fi, but note that service fees apply.
A single layer of La2Zr2O7 (LZO), deposited on textured Ni and Ni–1.7% Fe–3% W (Ni–W) tapes by a low-cost sol-gel process, is used as buffer layer for the growth of YBa2Cu3O7−δ (YBCO) coated conductors. It is shown for the first time that such single buffer layers can be used for the deposition of YBCO yielding critical current densities (Jc) that are comparable to those typically obtained using CeO2/YSZ/Y2O2 trilayers on identical substrates, i.e., in excess of 1 MA/cm2 at 77 K and self-field. The properties of the YBCO films and the dependence of Jc on thickness of the LZO layer are investigated.
In this work, the synthesis of CeO2–10 mol% Y2O3 powders by a nitrate–glycine gel-combustion route was investigated. Special attention was given to the influence of the glycine/metal ratio and calcination temperature on powder morphology. In contrast to the usual reported behavior, the best powder properties (crystallite size, 4.5–7 nm; specific surface area; 25–40 m2/g) were obtained for slow combustion processes with glycine/metal ratios of 1.5–2, whereas energetic reactions resulted in large crystallite and particle sizes. Furthermore, it was found that the crystallite size increases considerably even at moderate calcination temperatures (350–550 °C), showing the high reactivity of these nanopowders.
We report on the synthesis of carbon nanotubular structures produced for the first time by means of pulsed KrF laser ablation of a graphite pellet at high temperature (1150 °C), under high argon gas pressure (500 torr), and at relatively high ultraviolet (UV) laser intensities (8 × 108 W/cm2). The carbon nanotubular structures were directly observed by transmission electron microscopy and characterized by micro-Raman spectroscopy. Nanohorns (∼2.5 nm diameter and ∼10 nm long), a few single-wall nanotubes (1.2 to 1.5 nm diameter), and other nanotubular structures (such as graphitic nanocages and low-aspect-ratio nanotubules) were clearly observed in the carbon deposit. Raman spectra in the low-frequency range confirmed a population of tubular structures with diameters ranging from 0.7 to 2.0 nm. It is shown that the relatively high UV laser intensity used here favors the growth of various nanotubular structures to the detriment of single-wall nanotubes.
Single, epitaxial buffer layers of insulating LaMnO3 (LMO) or conductive La0.7Sr0.3MnO3 (LSMO) have been grown by sputter deposition on biaxially textured Ni and Ni–alloy substrates. We report baseline investigations of their compatibility with the Yba2Cu3O7−δ (YBCO) coatings and demonstrate biaxially textured YBCO films grown by pulsed-laser deposition on these single-buffered tapes. Superconducting property characterizations revealed better properties for YBCO films on LMO-buffered tapes relative to those grown on LSMO layers. Self-field critical current densities (Jc) exceeding 1 × 106 A/cm2 at 77 K have been obtained for the YBCO (200 nm) films on LMO-buffer layers. These results offer prospects for the use of single, LMO-buffered metal tapes in the development of practical YBCO-coated conductors.
Well-dispersed anatase and rutile nano-particles were prepared via hydrothermal treatment of tetrabutylammonium hydroxide-peptized and HNO3-peptized sols at 240 °C. A broad particle size distribution of anatase crystals was observed in the nonpeptized TiO2 species hydrothermally treated at 240 °C. X-ray diffraction and transmission electron microscopy, as well as zeta potential measurement, were used to characterize the particles. The formation of the well-dispersed anatase and rutile particles from the peptized samples could be attributed to (i) homogeneous distribution of the component in the peptized sols, and (ii) the high long-range electrostatic forces between particles in the presence of both peptizers, which were not present in the nonpeptized samples. This work provided a new way to prepare nano-crystals of titania.
Barium indium oxides (BaIn2O4, Ba4In6O13, Ba2In2O5, Ba3In2O6, and Ba5In2O8) were synthesized by the citric process and characterized by powder x-ray diffraction. The optical absorption properties of these compounds were investigated by UV–visible diffuse reflectance spectroscopy. It was found that with the increase of the mole ratio of In2O3 in the formula the optical absorption edges of these oxides shift to the longer wavelength side monotonically. The photocatalytic H2 and O2 evolutions under visible light irradiation (λ > 420 nm) from aqueous CH3OH/H2O and AgNO3/H2O solutions were performed. Among these oxides, BaIn2O4 was the most stable compound, and other compounds were not stable chemically in the case of water and visible light irradiation.
Iodine-doped whiskers of C60 (I–C60 whiskers) with diameters ranging from submicrometers to micrometers and lengths longer than 100 μm were successfully obtained by the use of the liquid–liquid interfacial precipitation method. Transmission electron microscopy observations showed that the I–C60 whiskers were single crystalline and had a growth axis parallel to the close-packed direction of C60 molecules and expanded (002) lattice planes indicative of the intercalation of iodine and oxygen atoms between the (002) planes of a body-centered-tetragonal crystal system. The I–C60 whiskers showed nonlinear I-V curves. The electrical resistivity of the I–C60 whiskers was more than three orders of magnitude lower than that of pristine face-centered-cubic C60 crystals.
We report up to 6 wt% storage of H2 at 2 atm and T = 77 K in processed bundles of single-walled carbon nanotubes. The hydrogen storage isotherms are completely reversible; D2 isotherms confirmed this anomalous low-pressure adsorption and also revealed the effects of quantum mechanical zero point motion. We propose that our postsynthesis treatment of the sample improves access for hydrogen to the central pores within individual nanotubes and may also create a roughened tube surface with an increased binding energy for hydrogen. Such an enhancement may be needed to understand the strong adsorption at low pressure. We obtained an experimental isosteric heat qst = 125 ± 5 meV. Calculations are also presented that indicate disorder in the tube wall enhances the binding energy of H2.
This work presents the preliminary experimental results of Pb(ZrxTi1−x)O3 (PZT) thin films preparation to develop a new combinatorial process by chemical solution deposition (CSD) method. PZT thin films were fabricated from the PbZrO3 (PZ) and PbTiO3 (PT) multilayers on a substrate of Si/SiO2/Ti/Pt. The precursor PT and PZ solutions were coated in different sequences, (i) in the PT start sequence and (ii) in the PZ start sequence, and with different PT and PZ concentrations. It was found that the deposition sequence of PT and PZ led to the differences in composition, microstructure, texture, and ferroelectric property of the resultant thin films. The PT start deposition was suitable for preparation of the PZT thin films, while the PZ start deposition caused composition deviation and bad ferroelectric property. The postannealing had little effect for the formation of the PZT thin films from the multi-PT and PZ layers. A single perovskite phase of PZT can be obtained from the deposited multi-PT and PZ layers even without any postannealing when concentrations of the precursor PT and PZ solution are smaller than 0.025 M and the deposition sequence is in the PT start sequence. This method would be a constructive way to develop and study other thin film materials systems.
The fundamental relations used in the analysis of nanoindentation load–displacement data to determine elastic modulus and hardness are based on Sneddon's solution for indentation of an elastic half-space by rigid axisymmetric indenters. It has been recently emphasized that several features that have important implications for nanoindentation measurements are generally ignored. The first one concerns the measurement of the contact depth, which is actually determined by using a constant value ε = 0.75 for the geometry of a Berkovich indenter and for any kind of material, whereas the reality is that ε is a function of the power law exponent deduced from the analysis of the unloading curve. The second feature concerns the relation between contact stiffness, elastic modulus, and contact area, in which a correction factor γ larger than unity is usually ignored leading to a systematic overestimation of the area function and thus to errors in the measured hardness and modulus. Experimental measurements on fused quartz are presented that show the variation of ε with the geometry of the tip–sample contact; that is to say with the contact depth, as well as the existence of the correction factor γ, as predicted in some recent articles. Effects of both ε and γ on harness and modulus measurements are also shown.
It is known that thin films of polycrystalline silicon, deposited under the right conditions, can be permeable to HF-based etching solutions. While these films offer unique capabilities for microfabrication, both the poor reproducibility of the permeable film properties and the lack of a detailed physical understanding of the material have limited their application. This work provides a methodical study of the relationship between process, microstructure, and properties of permeable polycrystalline silicon thin films. It is shown that the permeability is a result of small pores, on the order of 10 nm, between the 100–200-nm hemispherical grains characteristic of the permeable film morphology. This morphology occurs only in nearly stress-free films grown in a narrow temperature range corresponding to the transition between tensile and compressive film growth regimes. This result strongly suggests that the monitoring of residual film stress can provide the process control needed to reliably produce permeable films. A simple kinetic model is proposed to explain the evolution of the morphology of the permeable films.
The effects of Mn and Fe contents on the mechanical properties of aluminum-based A206 alloys were investigated quantitatively. Results showed that the addition of Fe caused a loss in both ductility and yield strength. Further addition of Mn could recover the ductility, but it caused a further loss in yield strength. In low-Mn alloys (0.29 wt% Mn) the primary constituent was the needle shape of Cu2FeAl7. Upon further addition of Mn, the Chinese script configuration of Mn-bearing particles formed instead. The Cu2Mn3Al20 particles formed in high-Mn alloys during solution treatment and resulted in grain-growth inhibition. The needle, Mn-bearing, and Cu2Mn3Al20 particles caused the solid solution level of copper in the matrix to decrease; meanwhile, increasing the Mn solution level retarded the precipitation of the strengthening phase. Differential scanning calorimetry analyses showed the kinetics and amount of decrease in θ′ phase precipitation when the contents of Fe and/or Mn were increased. The smaller grain size induced by the Cu2Mn3Al20 particles and the θ′ phase were the factors that determined the hardness of A206 alloys under as-quenched and T7-treated conditions, respectively.
A two-dimensional cellular automaton model was developed for the simulation of nucleation and growth of ferrite grains at various cooling rates in low-carbon steels. The model calculates the diffusion of the solute and temperature fields in an explicit finite method and incorporates local temperature and concentration changes into a nucleation or growth function, which is utilized by the automaton in a probabilistic fashion. The modeling provides an efficient way to understand how those physical processes dynamically progress and affect nucleation and growth of ferrite grains.
Transmission losses were monitored in the ultraviolet-visible spectra of irradiated hydroxyethyl methacrylate (HEMA) copolymer at elevated temperatures. The transmission in irradiated HEMA in the ultraviolet and visible wave length range was almost the same for doses 400 kGy ≤ Φ ≤ 1000 kGy, but was smaller than that of the nonirradiated HEMA copolymer. The reduction in transmission in the irradiated specimens was attributed to the presence of color centers. The concentration of color centers was enhanced by thermal annealing. The transmission data (or absorption data) at 467 nm was found in good agreement with the theoretical model in which the color center production followed a first-order kinetic process. The rate constant satisfies the Arrhenius equation, and the corresponding activation energy is 17.37 kJ/mol and is independent of the dosage. The results were compared with those reported in the literature.
A fine NiO powder and a Sm-doped ceria powder with a composition of Ce0.8Sm0.2O1.9 were synthesized by heating the oxalate precursors at 300–1200 °C in air to produce a cermet (anode material) for solid oxide fuel cell. A 0.2 M Ni(NO3)2 solution and a 0.2 M Ce(NO3)3–Sm(NO3)3 solution were mixed with 0.4 M oxalate solution, respectively, to produce the oxalate precursors. Only the cubic phase of Ce0.8Sm0.2O1.9 was formed in the calcined powders from the Sm-doped cerium oxalate. However, the mixed phases of NiO and Ni were produced in the NiO precursor after the calcination at 300–600 °C. At higher temperatures, only NiO was detected. The primary particle sizes, which were determined from the Brunauer-Emmett-Teller analysis surface areas, were 60 nm for NiO and 10 nm for Ru/Sm-doped ceria (SDC) after the heat treatment at 400 °C. The oxalate precursors of SDC and NiO provided 433 and 259 kJ/mol of the activation energy, respectively, for sintering/grain growth in the temperature range from 600 to 1200 °C. As-produced SDC precursor formed platelike secondary particles of 0.5–2-μm length by the heating at 800 °C. Heating of Ni oxalate at 800 °C produced isotropic fine NiO secondary particles of 0.5–2-μm sizes.
Silicon nitride–Si2N2O in situ composites were prepared by hot pressing powder mixtures of α–Si3N4, 6 wt% Y2O3, 1 wt% Al2O3, and 0–12 wt% SiO2. X-ray diffraction (XRD) analysis indicated that the volume percents of Si2N2O were 0, 13, 31, and 54 for the composites prepared with 0, 4, 8, and 12 wt% SiO2, respectively. XRD results also indicated that both silicon nitride grains and Si2N2O grains were laid down perpendicular to hot pressing direction. As the volume percent of Si2N2O increased, the width and the amount of elongated silicon nitride grains decreased, but the fracture toughness increased. Young's modulus of the in situ composites decreased as the Si2N2O content was increased. The erosion rate decreased as the Si2N2O content was increased, in part, due to both the increased fracture toughness and the reduced grain size. Erosion of the composites occurred primarily due to the grain dislodgment. The sample without Si2N2O experienced micro-chipping due to transgranular fracture.
The kinetics of (Bi,Pb)2Sr2Ca2Cu3O10+x phase formation in KCl flux was studied, and kinetic analysis using the Avrami relation for isothermal phase transformation gave the Avrami exponent n = 2.5 at 855 °C for the whole process of (Bi,Pb)-2223 phase formation. The estimated value of the activation energy Ea = 150 kJ/mol for the formation of (Bi,Pb)-2223 phase at 845–855 °C is the lowest among the previously reported values. The low value of activation energy explains the fast formation of single-phase (Bi,Pb)-2223 powder in the KCl flux.
Bi2Sr2Ca2Cu3O10+x and (Bi,Pb)2Sr2Ca2Cu3O10+x single crystals with a sharp superconducting transition at Tc = 109 K were grown using a modified KCl flux technique. The crystals show platelike morphology with typical dimensions of 0.5 × 0.5 × 0.002 mm3 and 0.25 × 0.25 × 0.001 mm3 for Pb-free and Pb-doped compositions, respectively. The formation of Bi-2212 intergrowth in the crystals is suppressed by utilization of a stable MgO crucible, suppression of the KCl evaporation, and isothermal heat treatment at a temperature close to the melting temperature of the oxide precursor in the flux. Morphology, phase purity, and chemical composition of grown crystals were determined by various analysis methods while the superconducting properties were studied by magnetization and resistivity measurements.
Single shear lap specimens were subjected to creep, isothermal aging, and thermomechanical fatigue (TMF). Scanning electron microscopy micrographs of previously polished specimens revealed changes in surface morphology. Orientation imaging microscopy was carried out on the same specimens to study the microstructural evolution and crystal orientation changes. As-fabricated joints consistently show a preferred crystal orientation with a few minority orientations with highly preferred misorientations. Alloy additions caused an increase in the number of statistically significant crystal orientations and misorientations. The solidification microstructure was unchanged due to room-temperature creep. Aging caused development and motion of well-defined subgrain boundaries and removal of most minority orientations. TMF causes heterogeneous refinement of the microstructure that accounts for the localized grain boundary sliding in regions of high strain concentration. This study implies that the lead-free solder joints are not polycrystals, but multicrystals, so that deformation is very heterogeneous and sensitive to strain and temperature history.
A titanium composite reinforced by in situ dual-scale particle, high-aspect-ratio TiB whiskers and fine TiC particulates was fabricated by a reactive hot pressing technique from a B4C–Ti system. The composite was subjected to creep investigations in compression at 873–923 K. This composite exhibited a stress exponent of 4.5–4.6 and a creep activation energy of 298 kJ/mol. By comparison, unreinforced Ti exhibited a stress exponent of 5.2–5.3 and a creep activation energy of 259 kJ/mol. No change in the stress exponent with varying creep rates was observed in both composite and unreinforced Ti under the investigated creep rates. The creep resistance of the composite was more than one order of magnitude higher than that of the unreinforced Ti. The load transfer mechanism accounted for this result. The creep of both composite and unreinforced Ti was controlled by lattice diffusion in the titanium matrix.