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Detonation in gaseous mixtures is a phenomenon of great importance for explosion safety assessment in hydrogen economy and for the development of advanced detonation-based propulsion systems. In practical applications, a detonation is generally caused by a deflagration to detonation transition (DDT) since a smaller amount of energy is required compared to a direct initiation. The key issue of DDT is finding the appropriate mechanisms to rapidly generate the detonation waves with a relatively weak ignition source. The objective of this work is to study numerically the possibility of DDT resulting from shock-multiple cylindrical flames interaction. The numerical setup aims to mimic an array of laminar flames ignited at different spark times, artificially inducing chemical activity to stimulate the coupling between the gasdynamics and the chemical energy release for the transition of deflagration-to-detonation. Using numerical simulations, a number of physical parameters are varied to determine their effect on the run-up distance as well as the time until the onset of detonation occurs, and to explore any scaling relationship among different them. The two-dimensional Navier-Stokes equations with one-step Arrhenius chemistry including the effects of viscosity, thermal conduction and molecular diffusion are used for the simulations. For comparison, simulations with Euler equations are also performed. The finite-volume operator splitting scheme used is based on the 2nd order Godunov-type, Weighted Average Flux (WAF) method with an approximate HLLC Riemann Solver. An Adaptive Mesh Refinement (AMR) technique is used to increase the resolution in areas of interest. The simulation results show that the interaction of the weak shock with the first cylindrical flame demonstrates very good agreement with the results in the literature and that a single weak shock–flame interaction was not enough to cause prompt DDT. However, a high degree of Richtmyer-Meshkov instabilities induced by repeated shock-flame interactions along with shock-boundary interactions generate turbulence that accelerates the flame brush, until eventually a hot spot ignition in the unreacted material develops into a multi-headed detonation wave. The simulation results also show that DDT is sensitive to the simulation method and that certain simulation parameters significantly affect the DDT phenomenon.

We developed a sample mounting and transfer mechanism for studying heterogeneous catalytic reactions in a high-pressure ultrahigh vacuum system. The system is optimized for the use of sum frequency generation vibrational spectroscopy to probe surface species in situ during the reactions. The system allows wide-ranging working conditions: a pressure range between 4×10−10 and 760 Torr and a temperature range between 200 and 1200 K. Rigid and reproducible sample positioning as well as precise temperature control can be achieved by compact designs of a sample holder and a sample mount incorporating ultrahigh vacuum compatible plugs. The performance of the system is demonstrated here by monitoring surface intermediates over time during cyclohexene catalytic reactions using a Pt(111) single crystal and sum frequency generation vibrational spectroscopy.

Boron nitride nanotubes with hollow conical-helix geometry have exhibited striking flexibility and elasticity comparable to metals. During an electron-beam induced deformation at room temperature, the nanotubes can be bent by a maximum angle as high as 180° and then retrieve the starting morphology without any evidence of structural failure. The outstanding low-temperature elasticity in this nano-material is interpreted by a theoretical model, displaying deformation processes dominated by slide of filaments along with changes in apex angles stepwise. The specific tubular geometry is believed to take advantages of both high stiffness and extraordinary flexibility of BN filaments, and easiness of interlayer slide in graphitic structure, hence leading to high resistance to fracture.

A high resolution reflectivity diffractometer (HRRD) has been recently commissioned on Station 2.3 at the SRS, Daresbury Laboratory. Using the instrument which is x-ray wavelength tunable, the characterization of the interface structure in an ion-assisted deposited Co/Cu multilayer has been carried out. In order to improve the electron density contrast of the sample, we have used an x-ray energy near the Cu K edge of 8.989 keV. By scanning the normal (specular), near normal (off specular), and parallel axes (transverse) of the surface, we have obtained useful reflectivity intensity data. By applying simulations to the data, we have determined a total roughness (rms) of 6.0±0.5 A with a significant contribution from correlated interfacial roughness (4.2±0.2 A). Other detailed structural information obtained has successfully demonstrated that the commissioned instrument is a viable tool for reflectivity studies.

The effect of growth temperature on morphology, structure, and photoluminescence (PL) of Eu-doped GaN (GaN:Eu) films grown by magnetron sputtering, and the relationships of growth-temperature-structure-PL were investigated by x-ray diffraction, scanning electron microscopy, transmission electron microscopy, and PL. The films grown at room temperature (RT), 573 K, and 773 K are composed of random crystalline grains, c-oriented GaN nanorods with lots of stacking faults, and well-crystalline c-oriented GaN, respectively. The characteristic emission lines of the Eu3+ were observed in the PL spectra at room temperature for the 573-K-grown and 773-K-grown films, while no emission line for the RT-grown film. The PL intensity from the 773-K-grown film is much stronger than that from the 573-K-grown film.

A system for the automatic acquisition and recording of images from video input scenes to a volume holographic optical memory has been designed and constructed. This memory is intended for use as a template database for reference images for an optical correlator. The system has been tested and commissioned; initial results indicate that system parameters such as hologram reconstruction quality are of a sufficiently high standard for the application.

The temperature of single quantum well semiconductor laser facets increases during operation, eventually reaching a critical temperature, thermal runaway, and catastrophic optical damage. A study of changes in composition of the near‐surface region of facets which accompany heating has been carried out for continuously operated, uncoated AlGaAs‐GaAs‐AlGaAs graded index separately confined heterostructure single quantum well lasers. High resolution depth profiles by scanning Auger microscopy show that the laser facets can be quite variable in initial composition, and undergo pronounced stoichiometry changes even during the first few minutes of operation. At longer times a continuing out‐migration of the group III elements is observed. Unlike the double heterojunction lasers, facet oxidation is not pronounced and is not responsible for diffusion of Ga and Al. There are indications, however, that a slow leakage of oxygen into the crystal may occur. Spatially resolved analyses provide evidence that carrier‐me...

We report on the microstructure and photoresponse of ion‐beam‐deposited Y‐Ba‐Cu‐O granular thin films. High‐temperature superconductor thin films of the average composition YBa1.8Cu3.6Ox were ion‐beam deposited onto yttria‐stabilized zirconia and strontium titanate substrates from a composite target. Post‐annealing processes were used to control the degree of granularity in these films, and led to films which varied in superconducting behavior. The microstructures of these films were studied by secondary electron microscopy and Auger analysis. Photoresponse and resistivity were measured as a function of temperature. We found that the degree of granularity in our films correlates with their resistivity and photoresponse which are similar to those of epitaxial or granular films.

We report on several important features of GaAs polarized electron source operation. Specifically we point out the beneficial effect on crystal lifetime produced by the constant low‐level application of cesium from a dispenser embedded in an extraction anode. Using our experience in low‐energy polarized electron‐atom scattering as a reference, we also discuss the importance of frequent energy calibrations of GaAs electron beams for high‐resolution investigations.

We present results of Raman and Rayleigh scattering, near‐band‐edge photoluminescence, and electron microscope examination of GaAs thin films (

In this work, a barrier height of ∼0.7 eV is constantly observed from the In0.53Ga0.47As Schottky diodes, regardless of the utilization of different metals as Schottky contacts. By the addition of Pr2O3 and In2O3 in the liquid phase epitaxy, a very low background impurity concentration is also obtained. The low background concentration is credited to the gettering effect from the addition of Pr and oxygen in the growth melt. The high Schottky barrier is attributed to the formation of a stable oxide layer on the surface of the epilayer, which in turn forms a metal-insulator-semiconductor structure in our Schottky diodes. The high Schottky barrier is very stable even at a high measuring temperature and was repeatedly obtained after four months of exposure to the environment.

The magnetic entropy change, ΔS (hence the heating or cooling) of ferromagnetically coupled magnetic clusters induced by an application of a magnetic field is substantial and occurs over a broad temperature range. This behavior contrasts with a ferromagnetic material, where larger entropy changes can be obtained, but only over small temperature ranges close to the Curie point. Monte Carlo simulations have been carried out to obtain ΔS when the spins are subjected to uniaxial anisotropy fields. While previous calculations for interacting spins showed increases in the Curie temperature and enhancements of ΔS in the vicinity of the Curie point with increasing anisotropy, the present results for interacting clusters show similar increases in the temperature of ΔS peaks accompanied by decreases in the peak values of ΔS. These results show that the flattening of the ΔS peaks, which make superferromagnetic materials valuable for magnetic refrigeration is assisted by the presence of uniaxial anisotropy. Some effe...

The facet heating behavior of AlGaAs double‐heterojunction (DH) lasers was measured as a function of injection current using Raman microprobe spectroscopy. The result from the DH lasers is compared to AlGaAs ridge‐waveguided single quantum‐well (SQW) lasers. A clear discontinuity in the facet temperature rise was found below and above the lasing threshold for the DH lasers, indicating a substantial contribution to facet heating by the photon flux. This is in contrast to the facet heating behavior of SQW lasers in which photon flux has been shown to play only a minor role. The data suggest a large difference in the facet absorption of the lasing photons between the two types of lasers. This is further supported by the effect of argon laser probe beam induced facet heating on the diode laser’s output power.

The strain and structural variations across InGaAs ridges (∼1 μm) and valleys (∼3 μm) grown on patterned GaAs(100) substrates are examined via Raman microprobe spectroscopy. High spatial resolution scans across these structures show significant variations in the Raman features (phonon frequency, linewidth, and intensity). Variation in the GaAs‐like longitudinal optical mode frequency suggests a large difference in strain found in the InGaAs on top of the 1 μm ridges and in the valleys. Concomitant measurement of phonon linewidth and Ar+ laser‐induced heating are also indicative of the same. These results, in conjunction with transmission electron microscope studies, demonstrate that Raman microprobe spectroscopy is a powerful tool for probing patterned semiconductor structures on a microscopic scale.

Cavity‐enhanced detection is used to monitor minute vapor plumes produced by focusing a pulsed laser beam onto a surface placed inside a resonant optical cavity. The photovaporization signals from a variety of different materials are examined, with emphasis being placed on their amplitude and temporal structure.

The facet temperature and output power of uncoated AlGaAs single quantum well lasers operated at constant current were measured as a function of time until catastrophical breakdown. The temperature rise is observed to consist of two regimes−an initial linear temperature rise accompanied by a gradual power degradation followed by a rapid nonlinear temperature rise at what appears to be a critical temperature leading to catastrophical optical damage (COD). The time to COD and the pre‐COD facet temperature rise rate are found to have a strong dependence on the diode output power. This facet temperature behavior plus the data obtained from an argon laser probe beam induced heating experiment provide valuable information regarding the mechanisms leading to COD.

Based on semiempirical generalized embedded atom method (GEAM), we carried out molecular dynamics and Monte Carlo simulations to study the structural properties of FeCo and FeNi nanoparticles. It is found that these two kinds of nanoparticles possess a new stable quasicore-shell structure, no matter whether they are in molten or condensed state and whether they are prepared by annealing or quenching. In FeCo (FeNi) nanoparticles of various sizes and atom compositions, the quasicore-shell structure is always preferred, with the shell formed only by Fe atoms and the core formed by randomly distributed Co(Ni) and Fe atoms. We have also investigated the formation mechanism of the quasicore-shell structure by energy difference analysis of the pure and doped icosahedra structure of FeCo and FeNi nanoparticles.

This letter addresses the issue of the order-disorder and Curie transitions in Ni70Fe30 nanoalloy. The ordered phase is observed at room temperature while the disordered phase appears when the nanoalloy is heated up to 773 K. By means of mechanical spectroscopy, x-ray diffraction, and vibrating sample magnetometer measurements, the order-disorder and Curie transition temperatures of 20 nm Ni70Fe30 nanoalloy are determined to be 636 and 728 K, both lower than the corresponding values in the coarse-grained form. Moreover, the reduction in these two critical temperatures is consistent with the predictions of a thermodynamic analytical model.

Vertically aligned TiO2 nanorods were prepared by its sol-gel which is spun coat onto aluminum anodic oxide template pregrown on an indium tin oxide glass substrate. The poly(3-hexylthiophene) (P3HT), a conjugate polymer infiltrated into the nanorod arrays, and the combined system were used as the active layer. The nanorods/P3HT solar cell with cell size of 0.06cm2 demonstrated a power conversion efficiency of 0.512% while the bilayer TiO2 film/P3HT cell was 0.12%. The current work provides fabrication method for a stable well aligned nanorods/polymer hybrid solar cell production.

The oxygen pressure dependence of the formation of excess O vacancies and interstitials in monoclinic HfO2 was investigated by performing first principles and thermodynamic calculations. Upper and lower critical oxygen pressures are identified that heavily favor the formation of oxygen interstitials and vacancies, respectively. The ratio of these critical pressures can be specified unambiguously as the sum of the formation energies of O vacancies and interstitials at 0K obtained from ab initio calculations.

A two-step treatment by argon (Ar) and hydrogen plasma beam scanning was developed to modify the surface bonds of a hydrogenated amorphous carbon (a-C:H) film on indium tin oxide glass. The modified a-C:H film is an excellent layer for liquid crystal alignment, on which the pretilt angle of a nematic liquid crystal (ZLI-2293) can be adjusted by the two-step scanning treatment. The chemical bonding characteristics of the as-scanned a-C:H films were measured using micro-Raman and x-ray photoemission spectroscopies. The antiparallel cells were filled with the nematic liquid crystal on the as-scanned a-C:H film to measure the pretilt angle. Experimental results indicate that the carbon dangling bonds are generated through a bond-breaking process during the Ar plasma beam scanning step (first step). The hydrogen plasma beam scanning step (second step) can passivate some of the carbon dangling bonds that are generated in the first step. The pretilt angle decreases as the hydrogen plasma beam scanning time incre...

A first principles electronic structure based method is presented to determine the equivalent circuit representations of nanostructured physical systems at optical frequencies, via a mapping of the effective permittivity calculated for a lattice of physical nano-elements using density functional theory to that calculated for a lattice of impedances using circuit theory. Specifically, it is shown that silicon nanowires and carbon nanotubes can be represented as series combinations of inductance, capacitance and resistance. It is anticipated that the generality of this approach will allow for an alternate description of physical systems at optical frequencies, and in the realization of novel opto- and nanoelectronic devices, including negative refractive index materials.

Aluminum borate (Al18B4O33) nanorods doped with Eu3+ and Eu2+ were synthesized via a simple calcination method. Both nanorods are of straight morphology and smooth surface, with the average diameter of ∼80nm. The structural and compositional characteristics have been investigated by x-ray diffraction, infrared spectra, and various microscopy techniques. A possible growth mechanism was proposed for the synthesis of the doped Al18B4O33 nanorods. Photoluminescence measurements indicate that Al18B4O33:Eu3+ nanorods exhibit emission peaks at 590, 595, 612, and 617nm, and Al18B4O33:Eu2+ nanorods display a broad green emission band centered at ∼540nm.

This letter introduces a resonant mass sensor that is based on a lateral extensional mode piezoelectric resonator and has a minimum detectable mass of 10−15g at room temperature and atmospheric pressure. The resonator with size of about 200×50×1μm3 has a quality factor (Q) of >1400 at 60MHz, and as small as 0.1ppm shift of its resonant frequency can be detected. The 0.1ppm detection capability corresponds to a mass uncertainty of only about 4.6fg. We have experimentally demonstrated a minimum detectable frequency shift of ∼1.6ppm due to absorption of isopropanol vapor (73fg) on the sidewalls of the parylene-coated lateral extensional mode piezoelectric resonator.

This letter presents the design, fabrication, and testing results of a piezoelectrically transduced UHF micromechanical resonator. The resonator is designed to vibrate in the (2, i) in-plane mode, and thin-film zinc oxide is used for piezoelectric transduction. A four-mask process is developed to fabricate the resonator on silicon on insulator wafer, and the maximum processing temperature is kept below 250°C. Prototypes with various diameters are fabricated, with a measured resonant frequencies span from 44.97 MHz to 1.14 GHz. Quality factors around 200 are obtained on different devices. Motional impedance in the range of several hundred ohms is demonstrated.

The theory of double Gel’fand polynomials is applied to irreducible representations of the symmetric and SU4 groups with the aim to treat spin–isospin wave functions of nuclear cluster systems. Multiplicity‐free recoupling coefficients of the symmetric group are connected with special types of Clebsch–Gordan coefficients of the unitary group. The standard phase conventions of the Yamanouchi basis and of the multiplicity‐free recoupling coefficients are proved to be derivable from natural phase conventions of double Gel’fand polynomials and these special Clebsch–Gordan coefficients. By extending the concept of double Gel’fand polynomials, useful expansion formulas are derived with respect to the determinant associated with a matrix tensor product. A simple example of their application is given for normalization kernels of two‐body systems composed of s‐shell clusters and for SU4 Clebsch–Gordan coefficients in the spin–isospin representation needed therein.

A combined Raman and photoluminescence study of one‐phonon processes in direct and indirect band‐gap AlxGa1−xAs alloys grown by molecular‐beam epitaxy is presented. The appearance of the ‘‘forbidden’’ TO mode is attributed to twinning effects related to growth rather than the possible experimental deviation from the strict backscattering geometry or the presence of alloy disorder. LO phonons around X as well as Γ and L points are found to contribute to the phonon‐assisted exciton recombination process though the dominant contribution is from phonons around the X point. A discrepancy between the results of existing methods of determining alloy concentration from Raman and photoluminescence data is found, indicating the need for reevaluation and further refinement of such methods.

We report on the development of several operating procedures that promise to make GaAs photoemission electron sources easier to construct, more reliable to operate, and more amenable to use in dynamic vacuum systems. We describe in particular a method for ‘‘ohmically’’ heating a 〈100〉 crystal of GaAs under vacuum to approximately 600 °C. We also discuss our observations of the role of oxygen in the activation of the crystal surface, the use of continuous cesiation, and of the performance of the crystal under varying vacuum conditions.

Rayleigh and Raman scattering studies reveal a direct correlation between the degree of disorder and growth kinetics attendant to the chosen growth conditions in molecular beam epitaxial growth. This inference is derived from studies on AlxGa1−xAs alloys grown under growth conditions indicated by reflection high‐energy electron diffraction intensity dynamics to be near optimal and away from optimal.

A UHV compatible Mott polarization analyzer is described that employs electron accelerating voltages of ∼20 kV. The efficiency of the analyzer, ∼3×10−5, is competitive with those provided by other polarimeters. The present analyzer is considerably simpler and more compact than earlier designs and can be used to undertake energy‐ and angle‐resolved polarization measurements with input beam currents below ∼10−14 A.

High‐purity films of tungsten are deposited from tungsten hexafluoride and hydrogen using plasma‐enhanced deposition (PED). At 400 °C deposition temperature, resistivities of ∼40 μΩ cm are attained. After annealing at 1100 °C, the resistivity falls to ∼7 μΩ cm. Below 400 °C, the as‐deposited film stress is

Plasma‐enhanced chemical vapor deposition of a metastable phase of tungsten ( β‐W) is performed using tungsten hexafluoride and hydrogen as source gases. At 350 °C, the as‐deposited resistivity of these films is ∼50 μΩ cm. After heat treatments between 650 and 750 °C in forming gas, the resistivity drops below 11 μΩ cm. Concomitant with this resistivity change is a phase change to α‐W, the equilibrium, body‐centered‐cubic form.

A simple low‐voltage (20‐kV) UHV‐compatible Mott polarimeter is described that incorporates an in‐line retarding potential energy analyzer that, at incident electron energies ≤50 eV, provides an energy resolution of ≤0.5 eV. The polarimeter is very compact and is suitable for use in a wide variety of applications requiring energy‐ and angle‐resolved polarization measurements.

Data are presented showing that thick scattering foils (or solid targets) can be used in retarding‐potential Mott polarimeters without degrading their performance. Use of thick foils can simplify the design and operation of such polarimeters. A straightforward calibration procedure is described.

Plasma‐density and electron‐number‐density profiles of ionizing‐argon flows behind moving shock waves were measured using a dual‐wavelength Mach–Zehnder interferometer with the hypervelocity shock tube at shock Mach numbers M∼13 and 16 at an initial pressure of 5 Torr and temperature of 300 °K.

Using pseudonatural orbitals as a basis for the superposition of configurations (SOC), we have calculated the adiabatic potential energy surfaces for linear HeHH+ and HHeH+. Except for the different basis set of Gaussian functions used, these calculations were carried out identically to one reported earlier for linear H3. It is found that HeHH+ has a potential energy minimum ∼ 5 kcal deep for RHeH ≅ 2.0 a.u., and RHH ≅ 2.0 a.u. This “hydrogen bonding” effect is given by the Hartree–Fock results and is little changed by SOC. HHeH+ is found to have a high potential energy barrier of ∼ 25 kcal for the symmetric complex, RHHe = RHeH ≅ 2.1 a.u.