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Al doped p-type 6H silicon carbide was irradiated by low energy electrons to create primary defects. Two deep levels at EV+0.36 eV and EV+0.81 eV were created by this irradiation. Isochronal annealing study was carried out on the electron irradiated sample to investigate the annealing out of the two primary defects and the creation of thermal annealing-induced secondary defects. Four more deep hole traps (0.45, 0.56, 0.74, and 0.71 eV above the valence band) were formed during the whole annealing process up to a temperature of 1600 °C. All the electron irradiated deep level defects were annealed out after the 1600 °C annealing. [ABSTRACT FROM AUTHOR]

A model is presented in this article to investigate the dependence of effective magnetostriction of the magnetostrictive composites on the parameters of components including the elastic modulus, permeability, and volume fraction, etc. Concentrating on the two-component magnetostrictive composites and choosing the two components to be general magnetostrictive materials, this model yields, analytically, the effective magnetostriction of composites by means of the method of complex potential. In terms of the analysis of this model, the magnetostrictive composites can be roughly divided into two kinds. One kind is that the matrix material is nonmagnetic or a material with very low magnetostriction, in which the effective magnetostriction is independent of the permeability of components. Another kind is the case in which the two components have close magnetostriction, and the effective magnetostriction of the composites generally depends on the elastic and the magnetic parameters of both components and the volume fraction. Unlike the first kind of composite, in a certain range, effective magnetostriction of this kind of composite can be improved by increasing the permeability of matrix. In addition, dependence of the effective magnetostriction on the other parameters of the components has also been discussed systematically. To evaluate the accuracy of this model, comparisons are made between the theoretical values and the experimental results published in the literature, which indicate that predictions of this model agree qualitatively with the experimental data. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Low-temperature photoluminescence spectroscopy has revealed a series of features labeled S1, S2, S3 in n-type 6H-SiC after neutron and electron irradiation. Thermal annealing studies showed that the defects S1, S2, S3 disappeared at 500 °C. However, the well-known D1 center was only detected for annealing temperatures over 700 °C. This experimental observation not only indicated that the defects S1, S2, S3 were a set of primary defects and the D1 center was a kind of secondary defect, but also showed that the D1 center and the E1, E2 observed using deep level transient spectroscopy might not be the same type of defects arising from the same physical origin. [ABSTRACT FROM AUTHOR]

We investigate with large-scale molecular dynamics simulations texture evolution in nanocrystalline Cu under planar shock wave loading. Five representative initial textures are explored under varying impact strengths. On the basis of Euler angles, we perform orientation mapping and texture analysis, including orientation distribution functions, pole figures, and inverse pole figures. Shock compression induces a weak but apparent ⟨ 110 ⟩ textures in nanocrystalline Cu initially with no texture, and a ⟨ 100 ⟩ fiber texture, and an incomplete weak ⟨ 110 ⟩ texture in nanocrystalline Cu initially with a { 100 } ⟨ 100 ⟩ recrystallization texture; such texture changes can be attributed to deformation twinning and dislocation slip and traced back to grains initially with ⟨ 100 ⟩. A ⟨ 100 ⟩ texture and a { 100 } ⟨ 100 ⟩ cube texture component are induced via twinning in nanocrystalline Cu initially with the ⟨ 111 ⟩ and β fiber textures, respectively, and can be traced back to grains initially with ⟨ 111 ⟩. [ABSTRACT FROM AUTHOR]

We present the width dependent study of edge distribution of graphene nanoribbons unzipped from multi-wall nanotubes. The partial unzipping of the carbon nanotubes yields a mixture of carbon nanotubes and nanoribbons. Comparing atomic resolution images of scanning tunneling microscopy with the lattice of graphene, the edge structures of nanoribbons are identified. Below 10 nm, the edges are closer to armchair type. Above 20 nm, the ribbons prefer to have edges close to zigzag type. In between, a more random distribution of the edges is found. The findings are of potential usages for the edge control in graphene nanoribbon based applications. [ABSTRACT FROM AUTHOR]

Hypereutectic 0.8MgO-0.2Y2O3 nanocomposites were studied using high pressure and high temperature in situ synchrotron energy dispersive x-ray diffractometry at 5.5 GPa and 1273 K. Polymorphic transformations in the Y2O3 phase, which resulted in a quadruple phase equilibria among cubic, hexagonal, and monoclinic phases with cubic MgO, were observed at 298 K under 5.5 GPa-a pressure much smaller than the pressures at which polymorphic transitions in macroscopic Y2O3. Time-resolved diffractometry at (1273 K, 5.5 GPa, 120 min) revealed a 0.87% expansive volumetric lattice strain in MgO, and is attributed to solid solution formation with Y2O3 which is otherwise absent at ambient conditions. The residual MgO unit cell volume expansion is 0.69% at 298 K, which is indicative that yttrium remained in solid solution. The macroscopic shrinkage due to densification at (1273 K, 5.5 GPa, 120 min) is 3.45% by volume. The partial molar volume of Y3+ in the solid solution is smaller than its molar volume in the pure state per thermodynamic considerations. The importance of repulsion among O2- ions in the immediate vicinity of a Mg2+ vacancy as well as misfit strain due to differences in ionic radii upon Y3+ substitution on Mg2+ sites is presented. A self-consistent model and explanation for the observed concomitant occurrence of cubic → monoclinic and cubic → hexagonal polymorphic transitions in Y2O3 and their stabilization at room temperature is proposed. [ABSTRACT FROM AUTHOR]

In this paper, the pressure (P) and shear vertical (SV) wave propagation in the periodically layered fluid and piezoelectric structure are theoretically studied. A single piezoelectric layer is first analyzed for three scenarios: piezoelectricity-ignored, electrically open, or electrically closed. It is observed that both the Lamb wave dispersion curves and the transmission coefficients are affected by piezoelectricity and electrical boundary conditions under any incident angle. From the analyses of the periodically layered structure, it is further found that the piezoelectricity and the electrical boundary conditions have influence on band gaps for a given incident angle. [ABSTRACT FROM AUTHOR]

In this paper, the propagation of the shear horizontal wave (SH-wave) in the single piezoelectric layer and periodically layered piezoelectric structure is studied. Both the dispersion equation and transmission coefficients are derived to reveal the wave behavior of the corresponding structures when the piezoelectricity is ignored or the electrical circuit is open and closed. The zero-order mode of the piezoelectricity-ignored single plate is not dispersive and every higher order mode is dispersive with a cut-off frequency. Same features are found for the electrically open and closed cases except that the zero-order mode of the latter case is no more non-dispersive. The pass bands of the piezoelectricity-ignored periodically layered structure appear when the normalized frequency is an even integer under the normal incidence, and new stop bands will appear from the pass bands as the incident angle increases. The same features are observed for the band gaps of the electrically open and closed cases except that the zero-order mode of the latter case is dispersive. The stop bands of the periodic structure will change in a cycle as the geometric ratio increases from zero to one. [ABSTRACT FROM AUTHOR]

Two powerful synchrotron x-ray scattering techniques for residual strain depth-profiling and tomography-like scatter-intensity profiling of materials are presented. The techniques utilize energy dispersive x-ray scattering, from a fixed microvolume, with microscanning of the specimen being used to profile its interior. The tomography-like profiles exploit scattering-cross-section variations, and can be contrast enhanced by separately monitoring scattering from different crystal structures. The strain profiling technique is shown to finely chronicle the internal strain variation over several mm of steel. Detailed strain profiling for a cantilever spring demonstrates the interplay of residual and external stresses in elastic/plastic deformation. Since surface compression, by shot peening, is a classic method to fortify against fatigue failure, the strain profile for a shot-peened, surface-toughened material is determined and discussed in terms of a simple elastic-plastic stress/ strain model. Finally the lattice strains in a WC/Co coated steel composite material are discussed. [ABSTRACT FROM AUTHOR]

We present systematic investigation on texture evolution in nanocrystalline Ta under planar shock wave loading at different impact velocities. Seven representative initial textures and two loading directions are studied via large-scale molecular dynamics simulations. Orientation mapping and texture analysis, including orientation distribution functions, pole figures, and inverse pole figures, are performed. Shock compression induces a ⟨ 221 ⟩ texture in nanocrystalline Ta initially with no texture, ⟨ 100 ⟩ fiber texture, { 100 } ⟨ 100 ⟩ texture, and θ + γ rolling texture via twinning, which can be traced back to grains initially with ⟨ 100 ⟩. A ⟨ 100 ⟩ texture is induced via twinning for nanocrystalline Ta initially with no texture, ⟨ 110 ⟩ fiber texture, and α + γ rolling texture and can be traced back to ⟨ 110 ⟩. Dislocation slip and grain boundary sliding lead to the movement of ⟨ 110 ⟩ toward ⟨ 111 ⟩ , and the strengthening of ⟨ 100 ⟩ and ⟨ 111 ⟩ orientation densities. The generation of new textures is observed for most cases. However, no new texture is found in the ⟨ 111 ⟩ fiber texture case for shock loading parallel to the fiber, and a much slower elastic–plastic transition occurs due to the lack of deformation twinning. [ABSTRACT FROM AUTHOR]

Nanoparticles with different core@shell structures (NaGdF4:Yb/Tm@NaLnF4 with Ln=lanthanide) were successfully synthesized for luminescence upconversion from 980 nm to 800 nm. The upconversion luminescence of the core@shell nanoparticles (NaGdF4:Yb/Tm@NaGdF4) has a much stronger infrared emission centered at 800 nm than that of the core-only nanoparticles (NaGdF4:Yb/Tm). This is not only due to the removal of surface defects of the core nanoparticles by the shell, which results in the decrease of the luminescence quenching, but also due to the breaking of the crystal field symmetry around the Tm3+ ion. By replacing the NaGdF4 shell with an NaYF4, the emission could further be enhanced. We suggest that the breaking of the crystal field symmetry plays also in this case a key role for further enhancement of upconversion emission. The importance of this asymmetry was further revealed by investigating the energy transfer between Er3+ ion in the shell and the Tm3+ ion of the core nanoparticle. [ABSTRACT FROM AUTHOR]

Nonisothermal densification in 8% yttria doped zirconia (8YSZ) particulate matter of 250 nm median particle size was studied under 215 V/cm dc electric field and 9 °C/min heating rate, using time-resolved in-situ high temperature energy dispersive x-ray diffractometry with a polychromatic 200 keV synchrotron probe. Densification occurred in the 876-905 °C range, which resulted in 97% of the theoretical density. No local melting at particle-particle contacts was observed in scanning electron micrographs, implying densification was due to solid state mass transport processes. The maximum current draw at 905 °C was 3 A, corresponding to instantaneous absorbed power density of 570 W/cm3. Densification of 8YSZ was accompanied by anomalous elastic volume expansions of the unit cell by 0.45% and 2.80% at 847 °C and 905 °C, respectively. The anomalous expansion at 905 °C at which maximum densification was observed is characterized by three stages: (I) linear stage, (II) anomalous stage, and (III) anelastic recovery stage. The densification in stage I (184 s) and II (15 s) was completed in 199 s, while anelastic relaxation in stage III lasted 130 s. The residual strains ([variant_greek_epsilon]) at room temperature, as computed from tetragonal (112) and (211) reflections, are [variant_greek_epsilon](112) = 0.05% and [variant_greek_epsilon](211) = 0.13%, respectively. Time dependence of (211) and (112) peak widths (β) show a decrease with both exhibiting a singularity at 905 °C. An anisotropy in (112) and (211) peak widths of {β(112)/β(211)} = (3:1) magnitude was observed. No phase transformation occurred at 905 °C as verified from diffraction spectra on both sides of the singularity, i.e., the unit cell symmetry remains tetragonal. We attribute the reduction in densification temperature and time to ultrafast ambipolar diffusion of species arising from the superposition of mass fluxes due to Fickian diffusion, thermodiffusion (Soret effect), and electromigration, which in turn are a consequence of a superposition of chemical, temperature, and electrical potential gradients. On the other hand, we propose defect pile-up at particle-particle contacts and subsequent tunneling as a mechanism creating the 'burst-mode' discontinuous densification at the singularities observed at 847 and 905 °C. [ABSTRACT FROM AUTHOR]

We study the temperature and pressure dependence of phase evolution in the 0.5MgO-0.5Y2O3 nanocomposite system using a diamond anvil apparatus in conjunction with in situ synchrotron energy dispersive x-ray diffraction at 7 GPa hydrostatic pressure. At (298 K, 7.0 GPa), structural transformations in the Y2O3 phase are observed, giving rise to the co-existence of its cubic, hexagonal, and monoclinic polymorphs together with cubic MgO. An increase in temperature to 1273 K causes the crystallinity of the Y2O3 hexagonal and monoclinic phases to increase. Isothermal and isobaric hold at (1273 K, 7.0 GPa) for 60 min results in yttrium dissolution in cubic MgO, causing ∼1.0% expansive volumetric lattice strain despite the large differences in the ionic radii of the cations. Cooling the nanocomposite to (298 K, 0 GPa) after a 60 min soak yields four phase co-existence among cubic MgO and cubic, hexagonal, and monoclinic Y2O3. The residual MgO unit cell volume expansion is 0.69% at 298 K, indicating solid solution formation at room temperature despite large differences in the ionic radii of Mg2+ and Y3+. The macroscopic shrinkage due to densification is 3% by volume. Thermodynamic considerations suggest that the relative molar partial volume of Y3+ in MgO is a negative quantity, indicating that the partial molar volume of Y3+ in the solid solution is smaller than its molar volume in the pure state. Aging of the nanocomposites for 240 h does not change the observed 4 phase co-existence. We propose a crystallographic model in which the observed volumetric expansion of the MgO unit cell is primarily attributed to two hydrostatic expansive strain components accompanying solid solution formation: (i) Coulomb repulsion among O2- ions in the immediate vicinity of Mg2+ vacancies, and (ii) misfit strain due to differences in ionic radii upon Y3+ substitution on Mg2+ sites. [ABSTRACT FROM AUTHOR]

Elastic and plastic strain evolution under four-point bending has been studied by synchrotron energy dispersive x-ray diffraction. Measured strain profiles across the specimen thickness showed an increasing linear elastic strain gradient under increasing four-point bending load up to ∼2 kN. The bulk elastic modulus of Ti–6Al–4V was determined as 118 GPa. The onset of plastic deformation was found to set in at a total in-plane strain of ∼0.008, both under tension and compression. Plastic deformation under bending is initiated in the vicinity of the surface and at a stress of 1100 MPa, and propagates inward, while a finite core region remains elastically deformed up to 3.67 kN loading. The onset of the plastic regime and the plastic regime itself has been verified by monitoring the line broadening of the (100) peak of α-Ti. The effective compression/tension stress-strain curve has been obtained from the scaling collapse of strain profile data taken at seven external load levels. A similar multiple load scaling collapse of the plastic strain variation has also been obtained. The level of precision in strain measurement reported herein was evaluated and found to be 1.5×10-5 or better. [ABSTRACT FROM AUTHOR]

In-plane alternating pulses of current applied with an external magnetic field that changes the strength and direction of the exchange bias are observed in NiFe/Cu/NiFe/FeMn spin valves. To provide clear evidence of this spin-polarized current for antiferromagnets incorporated in spin valves, a series of measurements was carried out to rule out the influence of Joule heating caused by the pulse. The measurements revealed that Joule heating plays a minor role in the impact of exchange bias, thus providing more convincing evidence of the spin torque exerted on the interfacial antiferromagnetic moments. [ABSTRACT FROM AUTHOR]

The work reports the effect of a tensile stress on the magnetic domain of FeCoSiB amorphous films. The domain structures of the unstressed and stressed FeCoSiB amorphous films with different tensile strains have been studied by magnetic force microscopy (MFM). With the increase of the tensile stress in the samples, it has been observed that the domain structures transform from irregular domains into parallel strip domains, and magnetic contrasts decrease. The MFM image disappears when the tensile stress is strong enough. A model has been presented to explain the evolution of the domain structures under the tensile stress. The effects of the stress on the domain of the films have been discussed. [ABSTRACT FROM AUTHOR]

Presents information on a study which discussed a method to improve the morphology and microstructure of sputtered bismuth-substituted garnet films for optical storage. Examination of the composition and surface morphology of films by scanning electron microscopy and electron probe microanalysis; Crystallization process of as-deposited amorphous films; Measurement of the magneto-optical Kerr hysteresis.