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For the first time, we measured Raman spectra from Li(Al1-xCox)O2 (x = 0.5 to 0.9), a new cathode material for lithium batteries. Whereas LiCoO2 sintered at 400 °C develops a spinel structure, Li(Al1-xCox)O2 sintered at 380 °C is amorphous, as shown by its single broad Raman band. Li(Al1-xCox)O2 sintered at 700 or 900 °C shows Raman peaks independent of x that coincide with those from LiCoO2, indicating that Li(Al1-xCox)O2 has the α–NaFeO2 structure (space group R3m). Traces of the impurity phase Co3O4 appear in samples treated at 900 °C but not at 700 °C. The Raman peak widths exceed those in LiCoO2, suggesting that replacement of Co by Al increases disorder among the Li ions.
Small-angle x-ray scattering (SAXS) technique has been used to investigate nano-sized powders of Y2O3, Y1.8Er0.2O3, and Y1.8Nd0.2O3, obtained by means of combustion synthesis. The results show that the powders have a mass-fractal behavior, with fractal dimension, Df, in the 1.7–1.8 range, and average particle sizes in the 20–30-nm range. These results are supported by transmission electron microscope observations. Moreover, from SAXS data, it has been possible to establish a narrow particle size polydispersity, as well as the presence of particle-diffuse or “fuzzy” interfaces. These results show, for the first time, the fractal behavior of materials prepared by combustion synthesis and are in agreement with a recently developed model. The nanostructural features are discussed within the framework of the peculiar optical properties of the doped luminescent materials.
A surface layer of metal carbides provides an excellent interface to achieve a highly adherent diamondlike carbon (DLC) coating. A plasma immersion ion implantation (PIII)-based procedure is described, which delivers a high retained dose of implanted carbon at the surface of aluminum alloys. A shallow implantation profile, followed by argon sputter cleaning and continued until a saturated carbon matrix is brought to the surface, provides an excellent interface for subsequent growth of DLC. At a carbon retained dose above 1018 atoms/cm2 the DLC adhesion exceeds the coating's cohesion strength. Regardless of the silicon content in the aluminum, the coating produced by this method required tensile strengths typically exceeding 140 MPa to separate an epoxy-coated stud from the coating in a standard pull test. Improved DLC adhesion was also observed on chromium and titanium. The reported tensile strength is believed to substantially exceed performance of DLC coatings produced by any other method.
Diamond deposition on copper is problematic mainly due to the poor affinity between copper and carbon. Therefore, it becomes necessary to pretreat the substrate surfaces prior to diamond deposition. Several surface pretreatments have been investigated, such as polishing using various abrasives and substrate biasing. In this study, we report new results relating diamond nucleation on copper substrates to a combination of surface polishing and biasing pretreatments. The results show that the combined pretreatments give a higher nucleation density than the two individual treatments. It was found that an increase of 70% in the nucleation density was observed when the surfaces were polished with diamond paste and then negatively biased. Copper surfaces polished with diamond powder and then biased displayed the highest nucleation density obtained. Raman spectroscopy revealed that after negatively biasing the substrate for 30 min, broad D- and G-bands of microcrystalline graphite were present, which completely disappeared with subsequent diamond growth, leaving behind a good-quality diamond film.
Effects of the vicinal angle, film thickness, and temperature on the growth modes, microstructures, and electrical properties of YBa2Cu3O7–δ on SrTiO3 were studied. Island growth transition between the initial nucleation and the later coalescence stages was observed with film thickness on a planar SrTiO3, while no islands were observed at the later stage due to the step-flow mode. As the growth temperature increased, a-axis precipitates were transformed to c-axis precipitates (islands), while no islands formed on vicinal SrTiO3. The supercurrent critical temperature was strongly related to the substrate vicinal angle due to the step-flow mode.
Bi-2212 cylindrical rods were obtained using a laser-induced directional solidification system. Although as-grown Bi-2212 samples are well textured, they do not exhibit superconducting behavior and, as a result, need further heat treatments. The modifications taking place during annealing were analyzed in the present work, in particular with respect to the evolution of the microstructure with the annealing time and the phase content. Diffusion processes in which the Bi-2212 phase grows along the thickness of the platelets take place during annealing. The presented results show that the physical properties of these samples improve during the initial approximately 60 h of annealing and that they remain constant thereafter.
Chemical solution epitaxy was used to deposit an epitaxial film of Gd2O3 on roll-textured nickel. A 2-methoxyethanol solution of gadolinium methoxyethoxide was used for spin-coating and dip-coating. Films were crystallized using a heat treatment at 1160 °C for 1 h in 4% H2/96% Ar. Single-layer films were approximately 600 Å in thickness, and thicker films could be produced using multiple coatings. θ/2θ x-ray diffractograms revealed only (0041) reflections, indicating a high degree of out-of-plane texture. A pole-figure about the Gd2O3 (222) reflection indicated a single in-plane epitaxy. Scanning electron microscopy showed that the films were smooth, continuous, and free of pin holes. Atomic force microscopy revealed an average surface roughness of 53 Å. Electron diffraction indicated that the misalignment of the majority of the grains in the plane was less than 10°. High-current (0.4 MA/cm2) Yba2Cu3O7–δ films were grown on roll-textured nickel substrates using Gd2O3 as the base layer in a three-layer buffer structure.
Zinc powder reacts with equivalent elemental selenium in solvent ethylenediamine at 120 °C for 6 h to form a complex, which is converted to ZnSe nanoparticles by pyrolysis or protonization. X-ray diffraction results suggest that the as-formed products have wurtzite structure. Transmission electron microscopy observation show that particles with spherical and laminar morphology were produced by pyrolysis and protonization, respectively. The formation of ZnSe nanoparticles is also investigated by infrared and thermal analysis.
We investigated the atomic structure, electrical, and infrared range optical properties of diamondlike carbon (DLC) films containing alloy atoms (Cu, Ti, or Si) prepared by pulsed laser deposition. Radial distribution function (RDF) analysis of these films showed that they are largely sp3 bonded. Both pure DLC and DLC + Cu films form a Schottky barrier with the measuring probe, whereas DLC + Ti films behave like a linear resistor. Pure DLC films and those containing Cu exhibit p-type conduction, and those containing Ti and Si have n-type conduction. Photon-induced conduction is observed for pure DLC, and the mechanism is discussed in terms of low-density gap states of highly tetrahedral DLC. Our results are consistent with relative absence of gap states in pure DLC, in accordance with theoretical prediction by Drabold et al.37 Temperature dependence of conductivity of DLC + Cu shows a behavior σ ∞ exp(−B/T1/2), instead of the T−1/4 law (Mott–Davis law). Contributions from band-to-band transitions, free carriers, and phonons to the emissivity spectrum are clearly identified in pure DLC films. The amorphous state introduces a large contribution from localized states. Incorporation of a small amount of Si in the DLC does not change the general feature of emissivity spectrum but enhances the contribution from the localized states. Cu and Ti both enhance the free carrier and the localized state contributions and make the films a black body.
Photoluminescence (PL) and photo-induced-current transient spectroscopy measurements on As-doped CdTe/Cd0.96Zn0.04Te heterostructures grown by molecular beam epitaxy were carried out to investigate the optical properties and the annealing effects on the deep levels. The temperature dependence of the PL spectra showed that the luminescence intensity of the exciton peak related to the neutral acceptors (A°, X) decreased with increasing measurement temperature and that the activation energy of the (A°, X) peak for the As-doped CdTe epilayer was 7 meV. Five hole-trap peaks appeared for the annealed As-doped CdTe/Cd0.96Zn0.04Te heterostructure. Two of these peaks, denoted by H1 and H2, might be related to extrinsic impurities, and the other three peaks, represented by H3, H4, and H5, might be attributed to intrinsic impurities. These results can help improve understanding for the application of As-doped CdTe/Cd0.96Zn0.04Te heterostructures in optoelectronic devices.
Dense p-type and n-type SiGe thermoelectric conversion units were fabricated with a double-layer electrode of W/TiB2 or W/MoSi2 by using glass encapsulation hot-isostatic-pressing process. The TiB2 and MoSi2 layers were used to prevent the chemical reaction between the tungsten and SiGe materials. Si3N4 ceramic particles were added into the electrode materials to reduce the mismatch of the thermal expansion between the electrode and the SiGe. Finite element analysis showed that the addition of 40 vol% Si3N4 into the TiB2 layer and 55 vol% Si3N4 into the MoSi2 layer reduced the thermal residual stress to a much lower value than the strength of individual layer. Sintered units had electrical resistivities of (1.5–2.0) × 10−3 Ω cm in the SiGe zone and 10−4 Ω cm in the electrodes. The comparison of the thermoelectric properties of the SiGe sintered with and without electrodes confirmed that the electrodes did not deteriorate the Seebeck coefficient of the SiGe alloys.
The properties of nonequilibrium face-centered-cubic (fcc) and body-centered-cubic (bcc) Fe–Cu alloys were studied using the first-principles full-potential linearized augmented plane wave method within the generalized gradient approximation. The ab initio calculation results are compared quantitatively with the magnetic moment and atomic volume observed for mechanically alloyed FexCu100–x (x = 0 to 100) supersaturated bcc and fcc solid solutions. The calculations show that Cu alloying leads to a small enhancement of the magnetic moment of bcc Fe. The fcc Fe moment, on the other hand, experiences a more pronounced increase into a high-spin state upon alloying with Cu. It reaches approximately the same value as that in the bcc alloys for all Cu concentrations where fcc solutions are obtained in experiments, corroborating previous ab initio calculations using different methods. The magnetic moment increases are accompanied by an atomic volume expansion. Both the calculated moment and volume behavior are in good agreement with those measured for fcc and bcc Fe–Cu solutions. The magnetovolume expansion upon magnetic interaction between the alloyed Fe and Cu, rather than the positive heat of mixing, constitutes the primary reason for the atomic volume increase observed.
The aim of the work described in the present paper was to investigate the microstructural stability during annealing treatments of a Fe–Al alloy obtained by melt spinning. To this purpose internal friction (IF) and dynamic modulus (Md) measurements were employed, and the results correlated with x-ray diffraction, optical microscopy, and scanning and transmission electron microscopy observations. In particular, the B2-ordered Fe–38A1–2Cr–0.015C–0.003B (in at.%) alloy was studied during repeated heating runs from room temperature to 823 K by IF and Md. The modulus exhibited a broad maximum (in the range of 600–800 K) only in the first run. On the basis of transmission electron microscopy and x-ray diffraction analysis, the irreversible transformation was explained by considering a two-stage process that occurs when vacancies in supersaturation move toward dislocations. The first stage is connected to dislocation locking; the second one is due to annihilation of some vacancies by dislocation climb.
The isothermal oxidation of Pd-modified Ni aluminide coatings was studied as a function of Po2 and temperature (900–1200 °C). A kinetic transition was observed between 900 and 1000 °C. Grazing incident x-ray diffraction, thermogravimetric analysis, x-ray photoelectron spectroscopy, scanning electron microscopy/energy dispersive spectroscopy, and secondary ion mass spectrometry analyses are consistent with the growth of δ-alumina or α-alumina below or above this transition temperature. Moreover, because Po2 was established before specimen heating, an effect of heating rate was observed and analyzed. More importantly, no kinetic transition was observed for sand-blasted specimens oxidized at low Po2. Thus conditions for the direct growth of an α-alumina scale could be determined from the reported results.
An analysis is made of contact damage in brittle coatings on metal substrates, using a case study of a dental porcelain coating of thickness between 0.1 and 1 mm fused onto a Pd alloy base, with spherical indenter of radii 2.38 and 3.98 mm. At large coating thicknesses (>300 μm), the first damage takes the form of surface-initiated transverse cone cracks outside the contact. At small coating thicknesses (<300 μm), the first damage occurs as yield in the substrate, with attendant formation of subsurface transverse median cracks in the coating. At high loads and thin coatings, both forms of transverse cracking occur, along with subsequent delamination of the ceramic/metal interface, signalling impending failure. Conditions for avoiding such transverse cracking are considered in terms of minimum coating thicknesses and maximum sustainable contact loads. General implications concerning the design of brittle coating systems for optimum damage resistance are considered, with special reference to dental crowns.
Composites consisting of nanometer-sized nickel–zinc ferrite and α-iron were prepared by subjecting micrometer-sized ferrite particles to a reduction treatment in the presence of α–Fe2O3. The materials were characterized by x-ray diffraction, electron microscopy, Mossbauer spectroscopy, and magnetization measurements. A wide range of saturation magnetization and coercivity can be obtained by changing the reduction schedule. The reduction process appears to break down the particle size of the precursor powder of nickel–zinc ferrite.
Recently we showed that, under nonhydrostatic loading, the FR1 →AO polymorphic transformation of unpoled lead zirconate titanate 95/5-2Nb (PNZT) ceramic began when the maximum compressive stress equaled the hydrostatic pressure at which the transformation otherwise occurred. More recently we showed that this criterion seemed not to apply to poled ceramic. However, unpoled ceramic is isotropic whereas poled ceramic is not. If we further assume that the transformation depends on both the stress magnitude and its orientation relative to PNZT's structure, these disparate results can be resolved. This modified hypothesis makes two predictions for transformation of unpoled ceramic under uniaxial compression: (i) it will begin when the compressive stress equals the hydrostatic pressure for transformation, and (ii) steadily increasing stress will be required to drive it to completion. Here we present experimental results that confirm these predictions. We then revisit our earlier results for poled and unpoled PNZT. The new hypothesis quantifies the observed effect of shear stress on the mean stress for onset of the transformation of unpoled ceramic and explains previously reported kinetic effects.
Plasma-enhanced chemical vapor deposition of SiOx coatings on thermoplastics provides a viable route for production of transparent composite materials with high fracture toughness and high gas barrier properties, which are important considerations in the food packaging and biomedical device industries. By examining several series of systematically varied SiOx/polycarbonate composites, we have identified design correlations between coating characteristics (thickness, density, surface roughness, and O2 transmission) and deposition conditions (time, power, pressure, and flow rates). Of particular interest is the observation that the thermal activation energy for O2 permeation through these composites increases (by up to 17 kJ/mol) as their barrier efficacy increases.
Mullite powder with a nearly stoichiometric composition was doped with 1.5–5 wt% SiO2 or 0.5–1.0 wt% Y2O3 and hot pressed at 1525–1550 °C to produce almost fully dense materials. The effect of the additives on the grain growth of the dense systems was investigated during subsequent annealing at temperatures above that of the eutectic (∼1590 °C) for the SiO2–Al2O3 system. The average length and width of the grains were measured by image analysis of polished and etched sections. At 1750 °C, anisotropic grain growth was relatively rapid, leading to the formation of rodlike grains. Compared to the undoped mullite, the addition of SiO2 and Y2O3 produced a small reduction in the grain growth kinetics. Transmission electron microscopy revealed that the glassy second phase was concentrated at the three-grain junctions or distributed inhomogeneously at the grain boundaries. For the materials annealed at 1750 °C, the indentation fracture toughness at room temperature increased from 2.0 to 2.5 MPa m1/2 for the undoped mullite to values as high as 4.0–4.5 MPa m1/2 for the doped mullite. The implications of the data for enhancing the fracture toughness of mullite by the in situ development of a microstructure of elongated grains are considered.
The average thermal residual stress in a continuous boundary phase in polycrystalline ceramic composites was calculated with a simple thin boundary layer model, and a criterion for the self-cracking of the boundary phase was derived under a certain assumption. From the proposed model, the toughness of the materials can be increased by both tensile and compressive stress at boundaries when the crack propagates transgranularly. The toughness will be increased when the stress at boundary is compressive for intergranular fracture mode. The maximum increase is predicted to be achieved at boundary phase contents below 33%. The experimental results for yttria-stabilized tetragonal zirconia polycrystalline ceramics doped with different kinds of grain-boundary phase is in a qualitative agreement with the prediction by the model, but the toughness increase is largely dependent on the distribution feature of glass phases.