Improvement of cubic silicon carbide crystals grown from solution

Cubic-silicon carbide crystals have been grown from carbon-rich silicon solutions using the travelling-zone method. To improve the growth process, we investigated the effect of controlling more tightly some of the growth parameters. Using such improved growth conditions, our best sample is a 12 mm diameter and 3 mm long 3C–SiC crystal. It is grown on a (0001) 2 off, 6H–SiC seed and has 111-orientation. The low amount of silicon inclusions results in a reduced internal stress, which is demonstrated by the consideration of μ-Raman spectra collected at room temperature on a large number of samples.     

Antiferromagnetic Mn50Fe50 wire with large magnetostriction

This work presents a study on the relation between the fiber texture and the magnetostrictive performance in an antiferromagnetic Mn₅₀Fe₅₀ alloy wire, which was prepared through the combining process of hot rolling and cold drawing. The face-centered cubic (fcc) crystal structure can be retained during the plastic deformation process. Mixed fiber textures consisting of both 1 1 0 and 1 0 0 components were formed along the drawing direction (DD) in the wire. A large magnetostriction of 750 ppm was obtained along DD under 1.2 T, which can be ascribed to the single γ phase and the formation of preferred crystal orientation.     

A rapid measurement of : The DECPMG sequence

The Driven-Equilibrium Carr–Purcell Meiboom–Gill (DECPMG) pulse sequence is a rapid method for obtaining the average ratio of longitudinal to transverse relaxation times T₁/T₂ as a function of T₂. Since this is a one-dimensional experiment, the T₁/T_2T2 ratio can be acquired, potentially, in just two scans; the second scan being a reference CPMG measurement. Conventionally, T₁/T₂ is determined from a two-dimensional T₁-T₂ relaxation correlation experiment. The method described here offers a significant reduction in experimental time without a reduction in signal-to-noise. The T₁/T₂ ratio is useful for comparing the behaviour of liquids in porous media. Here we demonstrate the application of the DECPMG sequence to the study of oil-bearing rocks by differentiating oil or water saturated rock cores, and by observing the relative strengths of surface interaction for water in two types of rock by measuring T₁/T₂ as a function of magnetic field strength.     

Simulation of kinetically limited growth of electrodeposited polycrystalline Ni films

A kinetic Monte Carlo (KMC) technique was adopted to simulate the growth dynamics of electrodeposited polycrystalline Ni thin films under kinetically limited conditions with a two-dimensional triangle lattice mapping the cross-section of the film. Effects of surface diffusion and surface-energy anisotropy on the microstructures of the plated film such as shape and size of grains, density, surface roughness, and textures were examined. This work focuses on the microstructure evolutions for different deposition parameters (for example, deposition rate and electrolyte temperature). We obtained the growth exponent β=0.46, which characterizes the scaling behavior of this growth. Preferential orientation of 1 1 1 and 1 0 0 have been observed at elevated temperature and at elevated deposition rate, respectively. To simulate the evolution of texture, the influence of hydrogen absorption on the surface energy of six main surfaces of nickel was investigated.     

Far-field properties and beam quality of vectorial Hermite–Laguerre–Gaussian beams beyond the paraxial approximation

The far-field properties and beam quality of vectorial nonparaxial Hermite–Laguerre–Gaussian (HLG) beams are studied in detail, where, instead of the second-order-moments-based M² factor, the extended power in the bucket (PIB) and βparameter are used to characterize the beam quality in the far field and the intensity in the formulae is replaced by the z component of the time-averaged Poynting vector S_z. It is found that the S_z PIB and βparameter of vectorial nonparaxial HLG beams depend on the mode indices n, m, αparameter and waist-width-to-wavelength ratio w₀/λ and the PIB and βparameter are additionally dependent on the bucket's size taken.     

Entanglement evolution of a two-qubit system interacting with a quantum spin environment

We investigate the entanglement dynamics and decoherence of a two-qubit system under a quantum spin environment at finite temperature in the thermodynamics limit. For the case under study, we find different initial states will result in different entanglement evolution, what deserves mentioning here is that the state |Ψ=cosα|01+sinα|10 is most robust than other states when π/2<α<π, since the entanglement remains unchanged or increased under the spin environment. In addition, we also find the anisotropy parameter Δ can suppress the destruction of decoherence induced by the environment, and the undesirable entanglement sudden death arising from the process of entanglement evolution can be efficiently controlled by the inhomogeneous magnetic field ζ.     

Near UV photoluminescence of Hg-doped GaN nanowires

Mass Hg-doped GaN nanowires with an average diameter of about 25 nm and lengths up to hundreds of micrometers are fabricated by chemical vapor reaction. The as-synthesized products have a single crystal phase and grow along the 0 0 1 direction. The growth of Hg-doped GaN nanowires is suggested for quasi vapor–solid mechanism (QVSM). In particular, for as large-scale GaN nanowires like films, a novel strong near ultraviolet PL spectrum appears with a doping Hg where the Hg-doped GaN nanowires are found to be responsible for the different characteristics; the PL mechanism is explained in detail.     

Four-zone solidification microstructure formed by laser melting of copper thin films

Polycrystalline copper film microstructures produced by laser melting and rapid lateral solidification are analyzed using transmission electron microscopy. The microstructure is predominantly composed of directionally solidified grains up to 22 μm long and 1 μm wide lying in the plane of the film. We identify four morphologically solidification zones, corresponding to occlusion, steady lateral growth, defective growth, and nucleation. Electron diffraction analysis indicates clustering of 1 0 0 orientations around the direction of solidification for the grains in the steady lateral growth zone. Simple estimates of solidification times based on heat flow modeling and a critical nucleation temperature suggest that interface velocities of several hundred m/s may be attained during solidification.     

Synthesis and efficient field emission of ZnO nanoinjectors

ZnO nanoinjectors were synthesized on Au-coated Si substrate by direct thermal evaporation of zinc powder at a low temperature of 600 °C and atmospheric pressure. Field-emission scanning electron microscopy and X-ray diffraction were applied to study the structural characteristics of the sample. The result indicated that the nanoinjector sample consisted of single-crystalline wurtzite structures which were preferentially oriented in the 0 0 1 direction. The field emission of the sample started at a turn-on field of 1.5 V/μm at a current density of 1 μA/cm², while the emission current density reached about 1 mA/cm² at an applied field of 5.0 V/μm.     

Both-end opened nanostructure holes by embedded carbon nanotubes realized on thinned membranes on (1 0 0) silicon substrates

Vertically aligned carbon nanotubes (CNT) have been grown in a DC-PECVD apparatus on quartz membranes. 1 0 0-oriented Si wafer has been anisotropically etched in a KOH solution. A mixture of acetylene and hydrogen gases is used to grow CNT while Ni acts as the catalyst layer. As-grown structures have been coated by titanium dioxide using chemical vapor deposition at atmospheric pressure. By means of a polishing and ashing process steps followed by total removing of the quartz membrane both ends of CNTs are opened and nano holes are obtained. SEM analysis is used to study the evolution of such nanostructures.     

Effects of Cr dopant on the microstructure and electromigration performance of Cu interconnects

Cu and Cu(Cr) alloy films were deposited on SiO₂ substrates by magnetron sputtering. The microstructure and electromigration performance of films were investigated. A small amount of Cr refines the Cu grains and improves the surface morphology of Cu films. After annealing at 450 °C, in contrast to the Cu film with large lateral grown grains, the Cu(Cr) alloy film exhibits fine columnar grains with a 1 1 1 preferred orientation. Most of Cr in the annealed Cu(Cr) film has segregated at the film surface and the film/substrate interface. The grain boundary grooving at the film/substrate interface is completely prohibited for Cu(Cr) films. As a result, the electromigration lifetimes of annealed Cu(Cr) lines are 10–100 times longer than those of annealed Cu lines. The final resistivity of the annealed Cu(Cr) film is 2.55 μΩ cm which is close to that of the annealed Cu film. With the improved surface morphology and high electromigration resistance, the dilute Cu(Cr) alloy film can be a viable interconnect material or a seed layer in the Cu-damascene technology.     

Concavity for nuclear bindings, thermodynamical functions and density functionals

Sequences of experimental ground-state energies for both odd and even A are mapped onto concave patterns cured from convexities due to pairing and/or shell effects. The same patterns, completed by a list of excitation energies, give numerical estimates of the grand potential Ω(β,μ) for a mixture of nuclei at low or moderate temperatures T=β⁻¹ and at many chemical potentials μ. The average nucleon number A(β,μ) then becomes a continuous variable, allowing extrapolations towards nuclear masses closer to the drip lines. We study the possible concavity of several thermodynamical functions, such as the free energy and the average energy, as functions of A. Concavity, which always occurs for the free energy and is usually present for the average energy, allows easy interpolations and extrapolations providing upper and lower bounds, respectively, to binding energies. Such bounds define an error bar for the prediction of binding energies. Finally we show how concavity and universality are related in the theory of the nuclear density functional.     

Simulation of a turbulent non-premixed flame using conditional source-term estimation with trajectory generated low-dimensional manifold

Conditional source-term estimation (CSE) is a method to close the mean chemical reaction source-term in an averaged transport equation. It is used with a trajectory generated low-dimensional manifold (TGLDM) to simulate a turbulent non-premixed flame. Integral equations are inverted for two progress variables, Y_CO2|ξ and Y_H2O|ξ, by assuming spatial homogeneity in the conditional averages. Using these two progress variables, the conditional source terms of temperature and other scalars are interpolated from the TGLDM table and mapped back into the physical space to be substituted into the transport equations. Solving a transport equation using a source-term interpolated from the TGLDM is found to improve the prediction of NO over simply interpolating the mass fraction of NO directly from the TGLDM. This method has been applied in a large eddy simulation (LES) of a turbulent non-premixed flame. Both GRI-Mech 3.0 and GRI-Mech 2.11 are found to be able to predict the temperature and major species well. However, only GRI-Mech 2.11 gives an acceptable prediction of NO. It is found that major species can be interpolated from the TGLDM table which can significantly reduce the computational cost.     

Nucleation, growth and morphology of Co thin films on Ag(1 1 0)

Scanning tunneling microscopy (STM) experiments reveal that Co growth on Ag(1 1 0), at coverages of Co < 1 ML and low substrate temperatures (150 K), involves a concomitant insertion of Co into the top Ag layer and exchange of Ag out onto the surface. At 300 K, coverages of Co > 1 ML gives rise to a 3D nanocluster growth on the surface, with the clusters covered by Ag. Depending slightly on coverage, the clusters have a typical diameter of 3 nm and a height of 0.4 nm. Upon annealing to 500 K, major changes are observed in the morphology of the surface. STM and AES show that there is a reduction of the number of Co islands on the surface, partly due to subsurface Co cluster migration and partly due to sintering into larger clusters.     

Studies of the spin-Hamiltonian parameters, d–d transitions and defect structures for two tetragonal Cu centers in Ba2ZnF6:Cu crystal

Two theoretical methods, the perturbation theory method (PTM) and the complete diagonalization (of energy matrix) method (CDM), are applied to calculate the spin-Hamiltonian parameters (g-factors g, g and hyperfine structure constants A, A, obtained from electron paramagnetic resonance (EPR) spectra) and d–d transitions (obtained from optical spectra) for two tetragonal Cu²⁺ centers in Ba₂ZnF₆:Cu²⁺ crystals. The Cu²⁺(I) ion replaces the Zn²⁺ ion at tetragonally compressed octahedral coordination and has the ground state ²A₁(|d_z²), whereas the Cu²⁺(II) ion is at an interstitial site with a square-planar F⁻coordination and has the ground state ²B₂(|d_x²-y²). The calculated spin-Hamiltonian parameters and d–d transitions from the PTM and CDM coincide and are in reasonable agreement with the experimental values. This suggests that both methods are effective for the theoretical studies of EPR and optical spectral data for 3d⁹ ions in tetragonal symmetry with different ground states. The defect structures of the two Cu²⁺ centers in Ba₂ZnF₆:Cu²⁺ are also estimated.     

One-dimensional self-consistent solution of modulation doped nanocrystalline/crystalline Si heterojunctions

A kind of modulation doped structure of n-type nanocrystalline hydrogenated silicon (nc-Si:H) film with intrinsic nc-Si:H layer with p-type bulk Si 100 substrate was proposed. The numerical self-consistent solutions of one-dimensional Schrödinger and Poisson equations along the direction normal to the heterojunction were performed to calculate the distribution of electron density and profiles of conduction-band as a function of ionized donor concentration in doped film, thickness of intrinsic layer, and other device parameters in the junctions. The calculated results are shown to be in agreement with experimental data. The relation of mobility vs sheet density of two-dimensional electron gases under different scattering mechanisms was analyzed. The obtained consequences may be used to evaluate optimum design for the modulation doped nc-Si:H-based devices.     

GaN micromachined FBAR structures for microwave applications

This paper presents the manufacturing of GaN membrane supported F-BAR structures. The 2.2 μm thick GaN layer was grown using MOCVD techniques on a high-resistivity 111-oriented silicon substrate. Conventional contact lithography, electron-gun Ti/Au evaporation and lift-off techniques were used to define top-side metallization of the the FBAR structures. Bulk micromachining techniques were used for the release of the GaN membrane. The bottom-side metallization of the micromachined structure was obtained by means of sputtered gold. S-parameter measurements have shown a pronounced resonance around 1.2 GHz. The extracted value of acoustic velocity is in good agreement with those reported by other authors on materials fabricated by other methods. The demonstrated FBAR function in epitaxially grown GaN layers opens new avenues for a low-cost monolithic integration with GaN-based electronics and sensing devices.     

Optical anisotropy and polarization fields for wurtzite films deposited on ()- and ()-orientated GaN substrates

We analyse the optical properties of GaN homoepitaxies grown on semipolar ()- and ()- orientated GaN substrates. We find the optical anisotropy of the GaN films to be strictly ruled by the angle between the growth plane and the 0001 direction of the crystal. This anisotropy offers a wide range of applications in the domain of surface emitting devices like VCELs and cavity polariton lasers based on GaN but also for similar structures relying on ZnO-related materials.     

Experimental and numerical investigation of the hetero-/homogeneous combustion of lean propane/air mixtures over platinum

The pure heterogeneous and the coupled hetero-/homogeneous combustion of fuel-lean propane/air mixtures over platinum have been investigated at pressures 1 bar  p  7 bar, fuel-to-air equivalence ratios 0.23  φ  0.43, and catalytic wall temperatures 723 K  T_w  1286 K. Experiments were performed in an optically accessible catalytic channel-flow reactor and involved 1-D Raman measurements of major gas-phase species concentrations across the reactor boundary layer for the assessment of catalytic fuel conversion and planar laser induced fluorescence (LIF) of the OH radical for the determination of homogeneous ignition. Numerical predictions were carried out with a 2-D elliptic CFD code that included a one-step catalytic reaction for the total oxidation of propane on Pt, an elementary C₃ gas-phase chemical reaction mechanism, and detailed transport. A global catalytic reaction step valid over the entire pressure–temperature-equivalence ratio parameter range has been established, which revealed a p^0.75 dependence of the catalytic reactivity on pressure. The aforementioned global catalytic step was further coupled to a detailed gas-phase reaction mechanism in order to simulate homogeneous ignition characteristics in the channel-flow reactor. The predictions reproduced within 10% the measured homogeneous ignition distances at pressures p  5 bar, while at p = 7 bar the simulations overpredicted the measurements by 19%. The overall model performance suggests that the employed hetero-/homogeneous chemical reaction schemes are suitable for the design of propane-fueled catalytic microreactors.     

Combustion of nano-aluminum and liquid water

An experimental investigation on the combustion behavior of nano-aluminum (nAl) and liquid water has been conducted. In particular, linear and mass-burning rates of quasi-homogeneous mixtures of nAl and liquid water as a function of pressure, mixture composition, particle size, and oxide layer thickness were measured. This study is the first reported self-deflagration on nAl and liquid water without the use of any additional gelling agent. Steady-state burning rates were obtained at room temperature (25 °C) using a windowed vessel for a pressure range of 0.1–4.2 MPa in an argon atmosphere, particle diameters of 38–130 nm, and overall mixture equivalence ratios () from 0.5 to 1.25. At the highest pressure studied, the linear burning rate was found to be 8.6 ± 0.4 cm/s, corresponding to a mass-burning rate per unit area of 6.1 g/cm² s. The pressure exponent at room temperature was 0.47, which was independent of the overall mixture equivalence ratio for all of the cases considered. The mass-burning rate per unit area increased from 1.0 to 5.8 g/cm² s for an equivalence ratio range of 0.5–1.25. It varied inversely to particle diameter, increasing by 157% when the particle diameter was decreased from 130 to 50 nm at  = 1.0.