Search

There have been extensive experimental observations of the anisotropic corrosion behavior of metals and alloys, and their mechanisms were assumed to be correlated with the so-called surface energy or the work function. However, to date, a specified mechanism or theory to interpret anisotropic corrosion behavior remains unclear. Here, we determine the anisotropic anodic dissolution of metals and alloys in corrosive environments by developing a formula to specify the relationship between the electrode potential (U) and the current density (I) by considering the basic parameters of our defined surface energy density ( E s u r f / ρ ) and the work function ( Φ ). Therefore, we build an ab initio model to evaluate the anisotropic anodic dissolution behavior of metals and alloys using the inputs obtained within density functional theory. This theory is further validated in the case of variations in the crystallographic planes of Mg. Moreover, some selected alloying additions such as Ga, Cd, Hg, In, As, and Cr are theoretically elucidated to effectively reduce the anodic dissolution rates of the Mg matrix to some extent, in close agreement with available experimental observations. This model is capable of predicting the anisotropic anodic dissolution behavior, providing a promising perspective for designing better corrosion-resistant alloys.

With the rapid development of liquid crystal display technology, product design focuses on high quality, high resolution, thin and lightweight specifications and uniform display brightness. However, the liquid crystal display thickness is determined by the design of the backlight module light source layout. Different light source designs with different optical materials can realise different optical effects. Hence, this study discusses the impact of the surface roughness of backlight module light guide plate on light transmission. With an edge backlight module as an example, this study assumes that the incident surface roughness of the light guide plate depends on control of CNC processing parameters, using the technique for order preference by similarity to ideal solution and back-propagation neural network/genetic algorithm to identify key factors from relevant factors, followed by validation of the optimisation parameter process capability. The roughness Cpk process capability is improved from ...

Structural and luminescence properties of GaSe1−x S x (0 ≤ x ≤ 1) series optical materials have been studied by X-ray diffraction, photoluminescence (PL), and piezoreflectance (PzR) measurements. Powder X-ray diffraction patterns showed the whole series layers present three different kinds of stacking formula with respect to the compositional change of sulfur from x = 0 to x = 1. The comparison of PL and PzR spectra reveals that the GaSe1−x S x layers have three different kinds of stacking phase from x = 0 to x = 1. The PL results show the whole series GaSe1−x S x layers emit the luminescences from red to blue visible region. The PL and PzR spectra of the GaSe1−x S x are analyzed. The structural variation in between the layers is discussed.

Although doping with appropriate heteroatoms is a powerful way of increasing visible light absorption of wide-bandgap metal oxide photocatalysts, the incorporation of heteroatoms into the photocatalysts usually leads to the increase of deleterious recombination centers of photogenerated charge carriers. Here, a conceptual strategy of increasing visible light absorption without causing additional recombination centers by constructing an ultrathin insulating heterolayer of amorphous boron oxynitride on wide-bandgap photocatalysts is shown. The nature of this strategy is that the active composition nitrogen in the heterolayer can noninvasively modify the electronic structure of metal oxides for visible light absorption through the interface contact between the heterolayer and metal oxides. The photocatalysts developed show significant improvements in photocatalytic activity under both UV-vis and visible light irradiation compared to the doped counterparts by conventional doping process. These results may provide opportunities for flexibly tailoring the electronic structure of metal oxides.

Two sharp bands at ∼1350 and ∼1600 cm−1 were observed in the Raman spectra of carbon-implanted GaN after postimplantation annealing treatments. The intensities of these two bands increased while their full widths at half maximum decreased with increasing annealing temperature. The observation of these two bands indicates the formation of microcrystalline graphite in C-implanted GaN. Hall measurements demonstrated that some dispersed C in GaN acted as acceptors and played a role in reducing electron concentration and Hall mobility. The facts that in 1100 °C furnace annealing the intensities of these two Raman peaks decreased rapidly to zero and the resistivity increased by 3 orders of magnitude indicate the dissolution of microcrystalline graphite at this temperature.

This paper presents, discusses, and compares results from experimental and computational studies of the plastic encapsulation process for a 144-lead TQFP package. The experimental results were obtained using an instrumented molding press, while the computational predictions were obtained using a newly-developed software for modeling transfer molding processes. Validation of the software is emphasized, and this was done mainly by comparing the computational results with the corresponding experimental measurements for pressure, temperature, and flow front advancement in the cavities and runners. The experimental and computational results were found to be in good agreement, especially for the flow-front shapes and locations. [S1043-7398(00)00502-8]

5 pages; International audience; The chiral antiferroelectric structure of an achiral bent-core liquid crystal is characterized by resonant x-ray scattering at chlorine K edge. The "forbidden" reflections resulting from the glide or screw symmetry elements are restored by the anisotropy of the tensor structure factor, which we calculate for two possible structural models. A careful analysis of the polarization states of the restored "forbidden" reflections enables an unambiguous identification of a chiral structure (i.e., the so-called anticlinic, antiferroelectric smectic-Cor Sm-CAPA) coexisting with the achiral synclinic antiferroelectric smectic-C or Sm-CSPA. The method proves to be quite powerful as it identifies the chiral structure within coexisting phases despite an imperfect orientation of the sample. The volume fraction of the chiral phase and the distribution of alignment are extracted from the data.

Null transmission ellipsometry and resonant x-ray diffraction are employed to study the effect of enantiomeric excess (EE) on the phase behavior of antiferroelectric liquid crystal 10OTBBB1M7. Phase sequence, layer spacing, and pitch of the helical structures of the smectic-$C$${}_{\ensuremath{\alpha}}^{*}$ and smectic-$C$${}^{*}$ phases are studied as a function of temperature and EE. Upon reducing EE, a liquid-gas-type critical point of the smectic-$C$${}_{\ensuremath{\alpha}}^{*}$ to smectic-$C$${}^{*}$ transition is observed, as well as the disappearance of the smectic-$C$${}_{d4}^{*}$ and the smectic-$C$${}_{d3}^{*}$ phases. Results are analyzed in a mean-field model.

In free-standing smectic films, layers near the surfaces of the film often contain molecules tilted away from the layer normal, while in the bulk of the film the magnitude of the tilt decays exponentially with distance from the surface. We have identified the detailed molecular tilt orientations in the surface layers of films for one antiferroelectric liquid crystal compound. A series of five surface structures exists with different nonplanar tilt arrangements for each structure. The molecular orientations in the surface layers evolve with temperature. The polarization of the film also evolves with temperature, corresponding to the tilt arrangements. Using ellipsometric data, we reconstruct the changes in the magnitude and azimuthal direction of the tilt as functions of temperature. We have also studied films of several different thicknesses. We present a phase diagram for the five surface structures showing the dependence on temperature and film thickness.

Nanometer SiO2/nanometer Si/nanometer SiO2 double-barrier (DB) structures, with Si layers having eleven different thicknesses from 2 to 4 nm, were deposited on n(+)-Si substrates using the magnetron sputtering technique. Strong electroluminescence (EL) from semitransparent Au film/DB/n(+)-Si structure was observed under reverse bias in a range of about 5-7 V. It is found that every EL spectrum of the structure can be decomposed into two Gaussian bands with peaks at around 1.85 and 2.25 eV, and their intensities and current swing synchronously with increasing nanometer Si layer thickness; the periodic length of swing is consistent with half of the de Broglie wavelength of the carriers. A comparison was carried out between EL from the Au/DB/n(+)-Si structure under reverse bias and that from the Au/DB/p-Si structure under forward bias reported previously. (C) 2000 American Institute of Physics. [S0003-6951(00)01736-8].

Surface effects cause tilted molecular arrangements in smectic layers near the surface of a free-standing liquid-crystal film in which the bulk of the film is in the smectic- A phase. One recent work has shown that the tilt directions in adjacent surface layers may be nonplanar. In this paper we study films with thicknesses of two to six smectic layers. Surface effects dominate in these very thin films. We show that the molecular tilts are nonplanar even in these very thin films.

We prepared silane cured PP/metallocene polyethylene to form a thermoplastic vulcanizate. Metallocene polyethylenes (mPE) with two different levels of comonomer contents were grafted with various amounts of vinyltriethoxy silane. Thermal behaviors remained largely unchanged for PP, indicating a dominant effect of cured mPE domains on mechanical properties for the vulcanizates. Tensile strength decreased with increasing the levels of silane concentrations but leveled off at higher silane concentrations at all test temperatures. Tear strength generally decreased with reduced energy dissipation at higher degrees of cure. By relating tensile strength to tear strength, the corrected depth of flaw was ca. 85±8μm which was about twice higher than the natural flaw size of 40±20μm obtained from several different approaches for conventional elastomer, but comparable for other types of thermoplastic vulcanizates. The application of Rivlin and Thomas's theory for this newly developed TPV should be cautious due to its limitation. A further modification of previous theory for thermoplastic elastomers is necessary. The crack tip diameter (effective diameters of fracture zones) deduced from intrinsic energy density and tear strength gave a similar order in comparison with several materials.

Steady-state permeation rates for pure CO2 and CH4 and the binary mixture through homogeneous dense polysulfone and polycarbonate membranes have been measured at 40°C and upstream pressures up to 26 atm. The pressure dependence of the mean permeability coefficients for CO2 and CH4 for all of the systems was simulated equally well in terms of the dual-mode mobility models driven by gradients of chemical potential and concentration. The dual-mode sorption and transport parameters used for evaluation of the mobility models are only those determined from sorption and permeation runs for a single gas. The approximate solution for the mean permeability coefficient of each component in a binary mixture was derived on the basis of the same mobility model driven by gradients of chemical potential. The deviation of approximate solutions from numerical ones remains less than 0.3%.

We conducted time-dependent, in situ x-ray reflectivity measurements on the formation of substrate-supported lipid monolayers and bilayers at solid-liquid interfaces, buried under an aqueous buffer with various concentrations (5, 10, 20, 40, and $50\text{ }\ensuremath{\mu}\text{g}/\text{ml}$) of lipid 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). The DOPC bilayer is formed on the hydrophilic surface of a bare Si substrate, while the DOPC monolayer is formed on a hydrophobic octadecylthricholorsilane (OTS) monolayer-coated Si substrate. The evolution of the reflectivity curves from the lipid bilayers is well described by lateral growth of bilayer islands, consistent with the rupture and fusion model for the adsorption of lipid vesicles to solid-liquid interfaces. By contrast, the formation of the lipid monolayer on OTS-coated Si occurs through a relatively fast coverage of the entire interfacial area, followed by an increase in the monolayer thickness. For both monolayers and bilayers, the rate of lipid layer growth increases with increasing lipid concentration in the buffer solution.

A binary mixture of an antiferroelectric liquid-crystal material containing a selenium atom and a highly chiral dopant is investigated using resonant X-ray scattering. This mixture exhibits a remarkably wide four-layer intermediate smectic phase, the structure of which is investigated over a temperature range of 16K. Analysis of the resonant X-ray scattering data allows accurate measurement of both the helicoidal pitch and the distortion angle as a function of temperature. The former decreases rapidly as the SmC* phase is approached, whilst the latter remains constant over the temperature range studied at 8 degrees +/-3 degrees. We also observe that the senses of the helicoidal pitch and the unit cell of the repeating four-layer structure are opposite in this mixture and that there is no pitch inversion over the temperature range studied.

We have investigated the smectic-Calpha*-smectic-C* (SmCalpha*-SmC*) transition in a series of binary mixtures with resonant x-ray diffraction, differential optical reflectivity, and heat capacity measurements. Results show that the phases are separated by a first-order transition that ends at a critical point. We report the observation of such a critical point. We have proposed the appropriate order parameter and obtained values of two critical exponents associated with this transition. The values of the critical exponents suggest that long-range interactions are present in the SmCalpha*-SmC* critical region.

Null transmission ellipsometry has been employed to study the molecular arrangements in freestanding films of two no-layer-shrinkage liquid crystal homologous compounds above the bulk smectic $A$--smectic ${C}^{*}$ (Sm ${C}^{*}$) transition temperature. An unusual nonplanar-parallel or nonplanar-antiparallel-parallel transition has been observed in both compounds under a proper electric field. With the addition of one ${\mathrm{CH}}_{2}$ group, while the $\mathrm{Sm}{C}^{*}$ phase is more stable thermally, the magnitude of the critical field needed to induce a parallel-antiparallel transition decreases dramatically.

We have studied tilted surface layers in the bulk SmA temperature window of one liquid crystal using null transmission ellipsometry. Five distinct nonplanar surface structures were observed. We present analyses of the tilt and azimuthal profiles of the structures and examine the transitions between the structures. The transitions are identified as a double reentrant synclinic-anticlinic-synclinic-anticlinic transition. Meanwhile, the transitions also display reentrant ferroelectric-antiferroelectric-ferroelectric behavior.

Employing both null transmission ellipsometry and resonant x-ray diffraction, we confirmed the SmC{sub F12}{sup *}-SmC* phase sequence reversal in one liquid crystal compound and specially prepared binary mixtures. This phase sequence reversal was predicted by two recent theoretical advances. Moreover, the temperature range for the SmC{sub F12}{sup *} phase increases significantly in the mixture suggesting that such a phase sequence may exist in other compounds.

We have measured the temperature variation of film tension above the bulk smectic-$A$\char21{}isotropic transition in freestanding films of two liquid-crystal compounds. Above the transition, the tension increases sharply with temperature, and the slope is proportional to the film's thickness regardless of whether or not the compound exhibits regular layer-by-layer thinning. The data can be interpreted in the context of theoretical models for layer thinning.

A realistic heterojunction potential model is adopted to discuss the magnetic field and pressure effects on a bound polaron near a GaAs/AlxGa1−xAs semiconductor heterojunction interface by considering the longitudinal optical phonons and two branches of interface optical phonons. An improved Lee‐Low‐Pines transform method is used to deal with the electron‐phonon and impurity‐phonon interactions. The binding energies of bound polarons as functions of pressure, magnetic field and impurity position are obtained by a variational method.

Several unique optical properties have been obtained from freestanding films of a bent-core compound. Our experimental results indicate the existence of the antiferroelectric biaxial smectic-A (Sm A) phase. The critical exponent associated with the biaxiality through the uniaxial-antiferroelectric biaxial Sm A transition has been measured to be 0.82+/-0.04 , which is in good agreement with our theoretical calculation. Our theoretical advances further demonstrate that the critical behavior of the optical biaxiality with the order parameter being a vector is described by the secondary-order parameter of the three-dimensional XY model. We also observe a remarkable even-odd layering effect exhibited by the surface layers of freestanding films under an applied electric field (approximately 20 V/cm) in the film plane.

This study focuses on the development of next generation integrated CAD-based software tools to simulate reactive flow phenomena during plastic encapsulation of ICs. These tools are applicable to both existing production packages and future configurations, such as moulded MCMs, chip scale packages, ball grid arrays, and ultra-thin QFPs. Successful flow simulation for accurate encapsulation process modelling is strongly dependent on input data for cure kinetics and moulding compound rheology. Studies of rheokinetic relations for epoxy systems have investigated model systems rather than commercial materials, mainly because model systems have slower cure kinetics that are better understood than fast (

Simulations of the step-impedance low-pass filter (LPF) in gallium arsenide (GaAs) substrate show that, in contrast to a 50Ω microstrip, the magnetic fields of the propagating microwaves are heavily concentrated in its inductive elements. The concentrated local magnetic fields facilitate enhanced coupling into an yttrium iron garnet/gadolinium gallium garnet layer over the GaAs substrate and, thus, significantly increase the microwave power absorptions at the ferromagnetic resonance (FMR) frequencies. The large tunable range of FMR frequency and the corresponding S21 and S11 parameters measured have verified the simulation results. Simultaneous enhancement of FMR absorption level and widening of absorption bandwidth has been accomplished in a band-stop filter that utilizes a microstrip meander line with four identical step-impedance LPFs, each of 5.70mm long, inserted in a nonuniform bias magnetic field. A tunable FMR absorption frequency range of 5–21GHz, an absorption level of −35.5dB, and a correspondi...

Dipoles and quadrupoles have been developed for use in a 12.5 GeV storage ring at the ZGS. The winding bore of these magnets is 10.16 cm with a useful aperture of 7.6 cm. The magnets produce a high quality field (±0.1% nonuniformity including the ends) that requires no correction at any field level. An operating current of 190 A at the design field of 30 kG allows the use of a very low heat leak cryostat design. A 3 meter length of magnets is expected to require only 2 liters helium per hour. Construction of 5 prototype modules, consisting of 2 dipoles and 2 quadrupole each, complete with cryostats, is almost completed.

The recovery of a normal zone produced in a superconducting magnet following a mechanical disturbance depends on the heat transfer rate to the liquid helium cooling channels. One of the factors which affects the heat transfer to the channel is the amount of vapor accumulated in the channel during and following a disturbance. The present work is undertaken to determine the void fraction in a liquid helium channel and to study the effect of the amount of vapor in the channel on the heat transfer characteristics. Heat pulses of 30 to 300 ms duration are introduced into the simulated conductor, and observations are made of the resulting temperature response and of the vapor fraction in the channel. The vapor fraction is determined by means of a capacitance bridge which responds to the small difference in dielectric constant between liquid and vapor helium. For completeness steady state measurements of temperature and vapor fraction are also reported.

Argonne National Laboratory is constructing a large superconducting MHD magnet to be used in the Coal-Fired Flow Facility (CFFF) at the University of Tennessee Space Institute. Measurements of the mechanical properties of all the materials used in the superstructure (bore tube, cast flanges and deposited weld metal, cast girder segments, aluminum girders, tie plates and connecting pins) have been performed. These measurements consisted of tensile strength, yield strength, % elongation, and reduction of area at 295, 76 and 4 K. Charpy "V" notch impact tests at 23 K were used for material acceptance with additional impacts done at 76 and 295 K. To verify ductility at the 4 K operating temperature, elastic-plastic J-integral fracture toughness tests at 295, 76 and 4 K along with associated fatigue crack growth rate data were generated. Results of these measurements are discussed.

The ANL-TEPR has a major radius, R = 6.25, and a plasma radius, a = 2.1 m. Sixteen, pure-tension-D superconducting magnets, with minor bore, R/sub bore/ = 7.7 m, and vertical bore, Z = 11.9 m, provide a toroidal field of 34 kG in the plasma, with a maximum field ripple of 2 percent. A stainless-steel-B/sub 4/C blanket/shield, which is 1 m on the inside and 1.3 m on the outside, protects the magnets and converts the nuclear energy to sensible heat, which is removed by the primary coolant, helium or H/sub 2/O. The first wall is 2-cm-thick stainless steel with a 100-..mu..m low-Z coating on the plasma side, and operates at less than or equal to 550/sup 0/C. The toroidal vacuum chamber is pumped down from 10/sup -3/ to 10/sup -5/ torr between burn pulses by thirty-two 25,000 l/s cryosorption pumps, and 50,000 l/s cryosorption panels maintain the vacuum in the neutral beam injectors, which are used to heat the plasma.

When a cryostable composite conductor carrying current experiences a heat input from a mechanical perturbation, a normal region develops which initially propagates and then either collapses or continues to propagate. A computer model has been devised to study this phenomenon. The model incorporates initial or continuing heat input from mechanical perturbations, heat conducted to the neighboring elements of the conductor and, if appropriate, heat conducted through insulation to neighboring turns. Heat is transferred to the helium coolant according to a specified heat transfer coefficient. If the element of conductor is in a normal or current-sharing state, resistive heating also occurs. The (unstable) equilibrium state of heat generation and conduction has been studied; results agree with those of a static calculation. The model has been validated against experimental measurements of response to heat pulses. A study of the trade-off between cross-sectional areas and wetted perimeter of a conductor is included, with recovery current presented as a function of perturbing energy.

Measurements are reported in which photodepopulation and thermal bleaching techniques have been applied to the study of radiation induced trapped positive charge in the Al2O3 substrate of n-channel MOS/SOS devices. Photodepopulation data indicates an optical depth for the hole traps of 2.5 eV. Thermal bleaching studies yield a corresponding thermal depth of 0.75 eV. Some preliminary thermally stimulated current measurements which reveal additional low temperature stable traps are also reported.

The conceptual design of a 10-Tesla pure tension superconducting toroidal field coil (TFC) system has been developed in sufficient detail to define a realistic design for the TFC that could be built based upon the current state of technology with minimum technological extrapolations.

The Argonne National Laboratory has designed, constructed and tested a large aperture superconducting MHD magnet for use in the coal-fired flow facility at Tullahoma, Tennessee. The magnet generates a peak on-axis field of 6 T in a MHD warm bore of 80 cm diameter at inlet, 100 cm diameter at outlet, and 300 cm effective length. The stored energy of the magnet is 210 MJ, the cold mass (4.2 K) is 131 metric tons and the ampere-turns, 13.7 × 10 6 . The final design of the magnet will be briefly introduced. Described in detail are the experiences of coil winding and coil assembly, the assembly of force-containment superstructure, assembly of magnet cryostat and results of magnet performance tests.

As an integral effort in the Argonne Tokomak Experimental Power Reactor Conceptual Design, design studies on the superconducting ohmic heating (OH) coils and the superconducting equilibrium field (EF) coils were completed. These conceptual designs are developed in sufficient detail with clear information on high current ac conductor design, cooling, venting provision, coil structural support, and zero loss poloidal coil cryostat design. Also investigated is the EF field penetration into the blanket and shield.

This work was undertaken to study the vapor formation and heat transfer of multiple layers under both steady state and transient conditions. The change in capacitance between the two sides of the heated channel is measured to determine the volume fraction of the vapor in the cooling channel. It is found that the heat transfer characteristics depend on the vapor accumulation and the velocity of vapor flow. The vapor locking heat flux is decreased when the vapor accumulation is increased.

A thermodynamic model of Fe-C martensite is proposed, which has a distinct physical meaning and is easily treated mathematically (high order derivation). When applying this model to the spinodal decomposition in Fe-C martensite during aging, the theoretical values (critical compositions, sizes of products) are in good agreement with experimental data. The miscibility gap of Fe-C martensite is set up. From this diagram, it is clear that spinodal decomposition may affect the later stages of tempering of medium and high carbon steels. Finally, the discussion shows that A1, A2 stages of aging are both spinodal decomposition, but to different extents.

A conductor for a cryostable magnet is usually made by soldering a superconducting matrix to an OFHC copper strip. The presence of voids between the matrix and the stabilizer due to the imperfection in the soldering bond is of great concern to magnet designers. An experiment was set up to investigage the effect of the void on the characteristics of hot end recovery and cold end recovery for CFFF superconductors. Tests were made on samples having different lengths of soldering void under various magnetic field strengths and injecting energies. The recovery current and the temperature distributions along the matrix and the copper stabilizer were mentioned. The test results are presented.

The Argonne National Laboratory has constructed a large aperture superconducting MHD magnet for use in the Coal-Fired Flow Facility at the University of Tennessee Space Institute at Tullahoma, Tennessee. The magnet will generate a peak on-axis field of 6 T in a MHD warm bore of 80 cm diameter at inlet, 100 cm diameter at exit, and 300 cm effective length. The stored energy of the magnet will be 168 MJ, the cold mass (4.2 K) will be 131 metric tons and the ampere-turns, 13.7 × 106. The final design of the magnet will be briefly introduced. Described in detail are the experiences of coil winding and coil assembly, the assembly of force-containment superstructure and assembly of magnet cryostat.

The advance of optoelectronic devices for long wavelength optical fiber communication in China is reviewed. The main features of the long wavelength semiconductor lasers and photodetectors are presented.

Superconductors used in a large superconducting magnet system are generally designed to be cryostable. There exists a strong possibility that cryostable conductor will burn out in a helium gas environment should a normal zone be generated from any point of the coil above the liquid helium level. An experiment is set up to investigate the heating of a cryostable conductor in a helium gas environment. Results will be presented and the ultimate safety of a cryostable conductor will be discussed.

In a large superconducting magnet, such as the superconducting magneto-hydrodynamic (MHD) dipole system for the National Coal-Fired Flow Facility (CFFF SCMS) at the University of Tennessee Space Institute (UTSI), the conductor joints must be strong, highly reliable, of low resistance, reproducible, and made with reasonable speed. These aspects of conductor joining are being studied.

The influence of processing parameter on the incidence of dielectric breakdowns in thermally grown thin oxides has been investigated. Statistical data shows that the difference between wet and dry oxides is not significant. However, the addition of 5% HCl during oxidation drastically improves both the yield and breakdown strength and reduces the incidence of breakdowns in the thin oxides.