Atomic hydrogen-deuterium mixtures at 1 kelvin: Recombination rates,spin-exchange cross sections,and solvation energies

A new technique has been applied to the study of atomic hydrogen and deuterium mixtures confined by liquid helium coated walls. The method uses standard low-field hyperfine magnetic resonance of the H atoms at 1420 MHz, but takes advantage of the dramatically different spin-exchange broadening for H-H and H-D collisions to simultaneously monitor the H and D densities. This provides a powerful means for studying the interaction of D with itself and with liquid helium, something otherwise difficult to achieve, and it also makes possible the study of spin-exchange and recombination interactions between H and D. A wide range of experimental results are presented, including the rate constants for H-D and D-D recombination, the spin-exchange broadening cross sections for H-D and D-D collisions, the H-D spin-exchange frequency shift cross section and an improved value for the H-⁴He buffer gas shift. Finally, a detailed study of the solvation of D into liquid⁴He has yielded an improved value for the salvation energy, a useful lower bound for the effective mass for D in liquid⁴He, and evidence for the reaction D + HD D₂ + H on the surface under the liquid⁴He film.     

SEM investigation of the magnetic domain structure of sintered SmCo5 permanent magnets

The paper presents scanning electron microscopy (SEM) study of the magnetic microstructure of anisotropic sintered SmCo₅ permanent magnets. Observations were made in the thermally demagnetized state of the magnets at the surfaces both perpendicular and parallel to the alignment axis. Magnetic domains were revealed using the technique of type-I magnetic contrast (for the first time) and the colloid-SEM method. The domain structure consists of main domains (which extend through the whole grain thickness) and surface domains of reverse magnetization (reverse spikes). The main domains form a maze pattern near the surface perpendicular to the alignment axis. The reason for the presence of the maze domain structure and reverse spikes at the surface perpendicular to the alignment axis is the reduction in the magnetostatic energy at the cost of a larger total Bloch wall area. Investigations carried out on the surface parallel to the alignment axis allowed to obtain much better insight into the orientation of grains.     

Heat Transfer in Subsonic Flows of Dissociated Nitrogen: HF Plasmatron Experiment and Numerical Simulation

Experiments on heat transfer in subsonic jets of dissociated nitrogen have been carried out on a IPG-4 induction plasmatron. The heat fluxes to copper, stainless steel, nickel, graphite, and quartz surfaces at the stagnation point of a water-cooled cylindrical flat-faced model 20 mm in diameter and dynamic pressures have been measured at a pressure of 50 hPa in the test chamber and a power of 35–65 kW of the HF generator. The experiments showed the influence of surface catalytic properties on the heat flux in relation to the nitrogen atom recombination. In the conditions of the experiments, a numerical simulation of nitrogen plasma flows in the discharge channel of plasmatron and the subsonic dissociated nitrogen jet flow around the cylindrical model has been carried out. The experimental and calculated data on heat fluxes to cooled copper, stainless steel, nickel, graphite, and quartz surfaces are compared. The quantitative catalyticity scale of the studied materials in relation to the heterogeneous recombination of nitrogen atoms is established.     

Optical Observation of Atomic Hydrogen on the Surface of Liquid Helium

We have optically detected hydrogen atoms adsorbed on the surface of liquid helium, a system relevant for the study of Base degeneracy in two dimensions. The atoms are excited by 121.6 nm light and detected both in fluorescence and in absorption. The optical spectrum of the adsorbed hydrogen atoms was not known a priori. It shows a resonance that is much broader than that of a hydrogen atom in vacuo, and it is shifted to lower frequencies. From the fluorescence intensity we determine that we have reached a surface density corresponding to one atom per square De Broglie wavelength. This means that our experiments take place at the edge of quantum degeneracy. In the regime where the adsorption isotherm is known we can use the measured hydrogen densities to infer the temperature of the helium surface. We use this information to determine the thermal conductance between the surface and the bulk of liquid helium. We find quantitative agreement between the measured temperature drops and the prediction of ripplon-phonon coupling theory.     

A graphical peak analysis method for characterizing impurities in SiC, GaN and diamond from temperature-dependent majority-carrier concentration

A method for uniquely determining the densities and energy levels of impurities from the temperature dependence of the majority-carrier concentration in wide band gap semiconductors (e.g., SiC, GaN, and diamond) is discussed. It is demonstrated that the proposed graphical peak analysis method can evaluate the number of impurity species and can determine those densities and energy levels uniquely and accurately, while fitting a simulation to the experimental temperature-dependent majority-carrier concentration leads to less reliable densities and energy levels of impurities. In the case that the Fermi levels in p-type SiC, GaN and diamond are located between the acceptor level and the valence band maximum, the excited states of acceptors strongly affect the hole concentration. This indicates the distribution function including the influence of the excited states should be applied to determine the densities and energy levels of acceptors from the temperature-dependent hole concentration.     

Desorption of Solid O2 Induced by Vibrational Excitations

Molecular dynamics calculations of the vibrationally induced desorption of a simple low cohesive energy molecular solid, O ₂ have been carried out. The calculations were extended up to 8 ns after the excitation. The desorption process has been found to have an evaporative character. Upon high excitation densities, guest molecules dissolved in the lattice were carried off by the matrix material. The evolution of the bulk of the irradiated material was examined. The transfer of vibrational energy into lattice heating evolves from a steady linear to rapid nonlinear regime. The efficiency of energy transfer to the lattice was found to depend nonlinearly on the density of excited molecules and on the anharmonicity of the intramolecular potential.     

Nano-phases of NaBH4 and KBH4

Phase stability and chemical bonding of beta-NaBH4 and beta-KBH4 derived nano-structures and possible low energy surfaces of them from thin film geometry have been investigated using ab initio projected augmented plane wave method. Structural optimizations based on total energy calculations predicted that, for beta-NaBH4 and beta-KBH4 phases, the (011) and (101) surfaces are more stable among the possible low energy surfaces. The predicted critical size of the nano-cluster for beta-NaBH4 and beta-KBH4 is 1.35 and 1.8 nm, respectively. The corresponding critical diameter for the nano-whisker is 2.6 and 2.8 nm respectively for beta-NaBH4 and beta-KBH4. Structural optimization based on total energy calculations show that the bond distances in the surfaces of nano-whisker are found to be higher than that in the bulk material and the calculated H site energies and bond overlap population analysis suggesting that it is considerably easier to remove hydrogen from the surface of the clusters and nano-whiskers than that from the bulk crystals.     

Elementary processes involved in the interaction of a solid with a bunch of active gas particles

We propose a new method for investigating processes at the solid-gas interface, which is based on the interaction of the solid surface with a bunch of active species carried by a gas. Using this method, the radical recombination luminescence (RRL) kinetics in crystalline phosphors (CaO-Mn, ZnS-Tm, and ZnS-Cu) excited by a bunch of hydrogen atoms was studied during a time interval of 0.1 s at a temporal resolution of 10 ms. From these RRL measurements, data on the stages of the heterogeneous reaction H+H→H₂ were obtained. It was found that the RRL intensity of a ZnS-Cu phosphor decreases with increasing surface electron excitation level. The phenomenon of the total light yield accumulation in ZnS-Cu was observed, which is explained by ionization of the surface electron states.     

Performance and application study of silane coupling agent (KH550) modified wood fiber-fly ash cenosphere/epoxy resin composites

In this paper, the epoxy resin composite filled with wood fiber and fly ash cenosphere was prepared. In order to improve the bonding properties between wooden fiber/fly ash cenosphere and epoxy resin, the grafting treatment of wooden fiber and fly ash cenosphere surfaces was carried out here using KH550 type silane coupling agent. The effects of different process parameters on the surface modification effect of wooden fiber and fly ash cenosphere were investigated, the mechanical properties and energy absorption characteristics of the materials before and after the filler modification were tested, and the microscopic interfacial structures of the matrix with wooden fiber and fly ash cenosphere were investigated by scanning electron microscopy. Meanwhile, based on LS-DYNA simulation software, the energy-absorbing performance of energy-absorbing boxes prepared from AA6061 aluminum alloy and modified wooden fiber-fly ash cenosphere/epoxy resin composites were compared in low-velocity collisions.     

Surface-induced dissociation of ions produced by matrix-assisted laser desorption/ionization in a fourier transform ion cyclotron resonance mass spectrometer

Intermediate pressure matrix-assisted laser desorption/ionization (MALDI) source was constructed and interfaced with a 6-T Fourier transform ion cyclotron resonance mass spectrometer (FT-ICR MS) specially configured for surface-induced dissociation (SID) studies. First MALDI-SID results in FT-ICR are presented, demonstrating unique advantages of SID over conventional FT-ICR MS ion activation techniques for structural characterization of singly protonated peptide ions. Specifically, we demonstrate that SID on a diamond surface results in a significantly better sequence coverage for singly protonated peptides than SORI-CID. A combination of two effects contributes to the improved sequence coverage: shattering of peptide ions on surfaces opens up a variety of dissociation channels at collision energies above 40 eV, and second, wide internal energy distribution deposited by collision with a stiff diamond surface provides an efficient mixing between the primary reaction channels that are dominant at low internal energies and extensive fragmentation at high internal excitation that results from shattering. Activation of MALDI-generated ions by collisions with surfaces in FT-ICR MS is a new powerful method for characterization and identification of biomolecules     

光子在闪烁晶体中传输的蒙特卡罗模拟

为了找到构筑闪烁晶体探测器的优化方法，使用蒙特卡罗方法对闪烁晶体BGO（Bi4Ge3O12，锗酸铋）的光收集效率进行了模拟研究。模拟结果表明：入射面为粗糙面，其余为抛光面，同时外层包装上高反射率的材料，可得到最大的光输出（约59．1％的光子被收集）；耦合剂的折射率的得到高的光输出也起着非常重要的作用。     

Enhanced optical performance by energetic hydrogen passivation at Si/oxide interface

Plasma immersion ion implantation (PIII) of hydrogen can provide appropriate kinetic energy to passivate the Si/SiO₂ interface. To avoid excessive damage of hydrogen, the implantation is performed with low kinetic energy (100 eV). Passivation decreases the dark current and enhances responsivity of metal-oxide-semiconductor tunneling photodetectors. The dependence of photoluminescence (PL) intensity on surface recombination velocity is theoretically studied. The intensity enhancement of PL also indicates that surface recombination velocity at Si/SiO₂ is significantly reduced after the PIII passivation. Since PIII is capable of isotropic implantation, tunable penetration depth, and large area process, it is an ideal tool for Si passivation with high throughput.     

Current trends in biomaterial surface functionalization—nitrogen-containing plasma assisted processes with enhanced selectivity

Low-pressure gas-discharge plasmas are widely used for polymer surface functionalization on industrial scale. For biomaterial applications, the density and selectivity of the functionalization are of particular importance, because functional groups control the immobilization of biomolecules. Therefore, surface modification of biomaterials is a challenging task for low-pressure plasma technique. Plasma processes have been successfully applied to various polymer types in order to generate multifunctional surfaces. This paper discusses the present state and the prospects of non-coating plasma processes to generate mono functional surfaces of controlled amino group density. Such surfaces appear most desirable for many applications. The results of various microwave- and radio-frequency- excited plasma processes reported in the literature are reviewed and compared to a sequence of experiments that was conducted in a UHV reaction environment. Non-thermal plasmas are especially well suited for thermally damageable polymers. The effect of hydrogen admixture to discharges in nitrogen and ammonia is discussed in detail. The optimization of process parameters lead to highly selective amino functionalization of high density. The selectivity reached 100% -NH₂/N at a surface density of amino groups of 3% -NH₂/C.     

Galvanostatic pulse deposition of hydroxyapatite for adhesion to titanium for biomedical purposes

Calcium phosphate coatings, in particular synthetic hydroxyapatite, are applied to the surfaces of titanium and its alloys so as to improve the biocompatibility and biological performance. Currently, plasma spraying is the clinically accepted technique for the deposition of calcium phosphate onto titanium. Electrochemical cathodic deposition is emerging as an alternative technique due to it being a nonline-of-sight technique. In this present study, it is demonstrated that increased thickness, crystallinity and adhesion of calcium phosphate coating on titanium is achieved by periodic pulsed low current densities compared to a constant current deposition method. It is believed that the “off” part of the AC deposition cycle gives the calcium and phosphate ions in the bulk solution sufficient time to diffuse to the titanium's surface maintaining more favourable conditions for HA growth. Unfortunately, although pulsed deposition at high current densities is able to produce thick coatings it cannot avoid problems associated with hydrogen bubbles and thus both AC and DC films deposited at high current densities have low crystallinity and poor adhesion.     

Spectroscopic Probing of Mixed-Mode Adsorption of Ru(bpy)(3)(2+) to Silica

Evanescent wave excitation of fluorescence was used to study the adsorption of Ru(bpy)(3)(2+) from aqueous solution to three types of surfaces: bare silica, a dimethylethylsiloxane (C(2)) monolayer on silica, and a dimethyloctadecylsiloxane (C(18)) monolayer on silica. The solution pH was varied to investigate the nonpolar and electrostatic contributions to the free energy of adsorption for each surface. The pH dependence of the adsorption showed that the pK(a) is the same for each of the three surfaces, consistent with earlier conclusions that the acidity of derivatized silica surfaces is due to areas of exposed silica. The free energies of adsorption for the bare silica surface, -26.2(±0.2) kJ/mol at pH 8, was attributed to electrostatic interactions alone. The free energy of adsorption for the C(2) and C(18) surfaces, -28.5(±0.1) and -31.5(±0.1) kJ/mol, respectively, were found to have both electrostatic and nonpolar contributions, with the latter being larger by 50% for the C(2) surface and 100% for the C(18) surface. Using Gouy-Chapman theory, the surface charge densities on each of the three surfaces, calculated from the electrostatic interaction energy of Ru(bpy)(3)(2+), were found to be within the range of literature values: 8.8(±0.1) × 10(-)(7) mol/m(2) for bare silica and 1.7(±0.1) × 10(-)(7) mol/m(2) for both the C(18) and C(2) surfaces. The results demonstrate that a cationic dye can be used to probe the silanol activity of chemically modified silica surfaces. The results support the picture that these chemically modified silica surfaces are acidic due to molecular-scale areas of contact between the bare silica substrate and the aqueous phase.     

Brownian dynamics simulations of rigid rod-like macromolecular particles flowing in bounded channels

Brownian dynamics simulations have been carried out of the joint probability distribution functions (PDF), P(ξ,θ), for macromolecular rod-like particles in the limit of infinite dilution in a solution under hydrodynamic linear flow. These PDF are calculated as a function of the orientations of the rod-like particles, θ and of the positions, ξ, of their centres of mass measured from a solid surface boundary. These simulations are developed in the neighbourhood of a solid surface boundary and in a confined space bounded by two such boundaries. They are constructed for a wide range of key quantities depicting the ratio of the hydrodynamic shear rate to the rotational Brownian diffusion coefficient. The notion of restitution is introduced to develop an algorithm for the consequences of the Brownian and hydrodynamic collisions of these macromolecules with impenetrable solid surface boundaries, which approach applies to a wide range of surfaces and macromolecules. The simulation results for the PDF distributions are given for typically low and high hydrodynamic flow conditions, and their properties are discussed. We show, for example, for low shear rates that a phenomenon which we call Brownian restitution enables the macromolecular rods to pass through a channel that is narrower than the rod length.     

Hybrid passivated colloidal quantum dot solids

Colloidal quantum dot (CQD) films allow large-area solution processing and bandgap tuning through the quantum size effect. However, the high ratio of surface area to volume makes CQD films prone to high trap state densities if surfaces are imperfectly passivated, promoting recombination of charge carriers that is detrimental to device performance. Recent advances have replaced the long insulating ligands that enable colloidal stability following synthesis with shorter organic linkers or halide anions, leading to improved passivation and higher packing densities. Although this substitution has been performed using solid-state ligand exchange, a solution-based approach is preferable because it enables increased control over the balance of charges on the surface of the quantum dot, which is essential for eliminating midgap trap states. Furthermore, the solution-based approach leverages recent progress in metal:chalcogen chemistry in the liquid phase. Here, we quantify the density of midgap trap states in CQD solids and show that the performance of CQD-based photovoltaics is now limited by electron-hole recombination due to these states. Next, using density functional theory and optoelectronic device modelling, we show that to improve this performance it is essential to bind a suitable ligand to each potential trap site on the surface of the quantum dot. We then develop a robust hybrid passivation scheme that involves introducing halide anions during the end stages of the synthesis process, which can passivate trap sites that are inaccessible to much larger organic ligands. An organic crosslinking strategy is then used to form the film. Finally, we use our hybrid passivated CQD solid to fabricate a solar cell with a certified efficiency of 7.0%, which is a record for a CQD photovoltaic device.     

Theoretical investigations on low energy surfaces and nanowires of MgH(2)

The phase stability, chemical bonding, and electronic structure of MgH(2) nanowires and possible low energy surfaces of α-MgH(2) thin films have been investigated using the ab initio projected augmented plane-wave method. Structural optimizations based on total energy calculations predicted that, for the α-MgH(2) phase, the (101) surface is more stable among the possible low energy surfaces. The electronic structure study reveals that the nanowires also have nonmetallic character similar to that of the bulk and thin film phases. Bonding analysis shows that the character of chemical bonding in nanowires has been considerably changed compared with that in bulk phases. Similarly, the bond distances in the surfaces of nanowires are found to be higher than in the bulk material, suggesting that it is possible to remove hydrogen from the nanowires considerably more easily than from bulk crystals.