Liquid oxide surface layers in metal combustion

We present a model that allows us to define the combustion concepts of ignition and extinction for those metals which are separated from an ambient oxidizing atmosphere by a liquid layer of their oxide during a stage of pre-ignition. To obtain the definition, the oxide layer is treated as an open system capable of multiple stationary states. Ignition and extinction are then associated with stationary states of the layer that are critical, i.e. which are located on the stability boundary. The model is used for an investigation of the particular cases of aluminium and boron which differ as to how the oxide layer is supported during the pre-ignition stage: at ignition, boron is known to be a solid whereas aluminium is in the liquid state. Both boron and aluminium have a promising potential for a number of applications in rocket propulsion. The model does not require the initial symmetry to be preserved and allows symmetry-breaking to occur, and thus goes beyond other current models of metal ignition and extinction. We demonstrate how this can be used to unravel some of the physical causes which have so far prevented the full realization of the potential of metals for propulsion.     

Ultracold neutron detector for the spectrometer of a neutron lifetime measuring

The gas-discharge detector is designed for the neutron lifetime spectrometer. The detector is intended for ultracold neutron flux monitoring in measurement cycles at the specrtometer (ILL, Grenoble, France). The detector has been successively tested with a Pu–Be neutron source under laboratory conditions and as a part of the spectrometer.     

Modification of T15K6 hard-alloy surface layers with titanium and zirconium coatings under the action of compression plasma flows

The effect of compression plasma flows (CPFs) with energy densities of 22 and 27 J/cm² on the T15K6 hard-alloy surface with predeposited titanium and zirconium coatings is investigated. It has been ascertained that tungsten nitride (WN) is formed in the surface layers of the systems under study after the action of CPFs in gaseous nitrogen. In the case of the deposition of a zirconium coating, the (Ti, W)C solid solution is alloyed with zirconium. Treatment by CPFs leads to the alloying of the coating and the hard-alloy substrate, generating two types of modified-surface morphology. These types differ in the number of microcraters and pores per unit of surface area. The combined action with energy densities of 22 and 27 J/cm² on a hard alloy enhances its microhardness by a factor of more than 2.6 and 2.2, respectively.     

Options for the neutron lifetime measurements in traps

Different geometries for the neutron lifetime measurements by the method of ultracold neutron storage in material traps and additional possibilities for the neutron storage in the magnetic storage ring are considered.     

Hydrogen desorption from tantalum with segregated oxide surface layers

The combination of thermal desorption spectroscopy (TDS) and Auger electron spectroscopy (AES) was used to examine the correlation between elementary processes on an oxidized tantalum surface and the acceleration of the dehydriding rates at increasing temperatures. We present the TDS of TaD_x foils (foil thickness ~ 25μm) for various initial concentrations (0.038 ⩽ x ⩽ 0.108; α-phase), various heating rates and for high and ultra high vacuum conditions. Activation energies for desorption were derived from the TDS spectra on the assumption that the rate determining process is the recombination of hydrogen atoms to hydrogen molecules at the surface. We find two different activation energies for the low and for the high temperature region. The AES measurements show that the corresponding change in the desorption process is correlated with the dissolution of the segregated oxide surface layer.     

Problem of the neutron lifetime measurements

The current status of the measurements of the neutron lifetime has been analyzed. The problem of experimental data discrepancy has been analyzed and systematic errors of some experiments have been found out. The corrected table of measurements is given and a new world average value of neutron lifetime 879.9 ± 0.9 s is presented.     

The structure of titanium nickelide surface layers formed by pulsed electron-beam melting

X-ray structure analysis technique has been developed for a material heterogeneous over the depth. The structure of the TiNi intermetallide subjected to surface modification by an electron beam has been investigated. The following three layers differentiated in the microcrystalline structure have been singled out: (i) the external sharply textured layer 1–3 μm in thickness with a texture in the [100] B2 direction and a lattice periodicity parameter reduced by 1% in the direction of the normal, (ii) the intermediate gradient-stressed layer 10–15 μm in thickness with the texture of the initial ingot, and (iii) the original material. The crystal lattice of the surface as well as of the intermediate layer is in a state stretched along the surface and compressed along the normal to it. This lattice distortion is maximal in the surface layer and decreases towards the bulk of the material.     

Local modification of thin oxide films on a titanium surface

It is shown that a controllable structural rearrangement of a thin titanium oxide film, accompanied by a change in the bandgap width, can be accomplished under the probe tip of a scanning tunneling microscope (STM). The mechanism of the rearrangement is considered to be the migration of cations and oxygen vacancies in the electric field of the STM.     

Correlation effects in multiple small-angle neutron scattering at the surface layers of inhomogeneities

The influence of spatial correlations on the angular distribution of multiple small-angle neutron scattering (MSANS) at the surface layers of inhomogeneities is studied. Calculations are carried out by taking instrumental distortions into account when observing multiple small-angle neutron scattering by means of methods of a double-crystal diffractometer and a small-angle diffractometer with a position-sensitive detector. Within the framework of the eikonal approximation, the MSANS line is calculated, and the influence of the surface-layer thickness and the inhomogeneity concentration on its width is studied.     

Studies on pore morphology of titanium and its oxide by small angle neutron scattering

Titanium metal bodies have been prepared from the sintered powder compacts of TiO₂ by a novel molten salt electrochemical approach, known as FFC Cambridge process. The phase and compositional characterizations of both Ti and TiO₂ have been carried out by X-ray diffraction. The pore morphologies of sintered TiO₂ pellet and the metallic Ti pellet, obtained after electrochemical reduction have been studied by SANS over a scattering wave vector q range of 0.003–3.5 nm⁻¹ using a double crystal diffractometer and a pin-hole collimated SANS instrument. In the case of reduced metal pellet, average pore size was found to be larger than that of the oxide pellet as the voids left behind after the oxygen atoms left the oxide matrix, could not coalesce.      

中子管非中子产物性质

采用两只经过标定的ST401闪烁探测器,测量了脉冲中子管的中子产额,在其中一只探测器前端增加铅板屏蔽,1 cm的铅屏蔽使探测器输出减少了18.20%,在加速器中子源上进行的类似实验表明,0.5 cm的铅使探测器的输出减少了2.90%。对两个中子源上测量的情况进行了蒙特卡罗模拟,加速器实验与模拟符合较好,脉冲中子管实验差别较大。对实验和模拟的情况进行了分析,结果表明：中子管除产生中子外,还会产生数量较多的轫致辐射X射线,这些X射线对准确测量中子管中子产额将造成不良影响。     

Shadow-casted ultrathin surface coatings of titanium and titanium/silicon oxide sol particles via ultrasound-assisted deposition

Ultrasound-assisted deposition (USAD) of sol nanoparticles enables the formation of uniform and inherently stable thin films. However, the technique still suffers in coating hard substrates and the use of fast-reacting sol–gel precursors still remains challenging. Here, we report on the deposition of ultrathin titanium and titanium/silicon hybrid oxide coatings using hydroxylated silicon wafers as a model hard substrate. We use acetic acid as the catalyst which also suppresses the reactivity of titanium tetraisopropoxide while increasing the reactivity of tetraethyl orthosilicate through chemical modifications. Taking the advantage of this peculiar behavior, we successfully prepared titanium and titanium/silicon hybrid oxide coatings by USAD. Varying the amount of acetic acid in the reaction media, we managed to modulate thickness and surface roughness of the coatings in nanoscale. Field-emission scanning electron microscopy and atomic force microscopy studies showed the formation of conformal coatings having nanoroughness. Quantitative chemical state maps obtained by x-ray photoelectron spectroscopy (XPS) suggested the formation of ultrathin (<10 nm) coatings and thickness measurements by rotating analyzer ellipsometry supported this observation. For the first time, XPS chemical maps revealed the transport effect of ultrasonic waves since coatings were directly cast on rectangular substrates as circular shadows of the horn with clear thickness gradient from the center to the edges. In addition to the progress made in coating hard substrates, employing fast-reacting precursors and achieving hybrid coatings; this report provides the first visual evidence on previously suggested “acceleration and smashing” mechanism as the main driving force of USAD.     

Nitriding of steel and titanium surface layers under the action of compression plasma flows

The elemental and phase compositions of St3 steel and VT1-0 titanium surface layers nitrided by the action of compression plasma flows (CPFs) have been investigated. The plasma flow parameters are shown to be correlated with the modified-layer nitrogen content. The basic mechanism by which the steel and titanium surface layers are saturated with nitrogen has been revealed. The performed experiments indicate that an increase in the absorbed energy density leads to a decrease in the nitrogen concentration because a shock-compressed layer is formed in the near-surface region, impeding nitrogen diffusion into the sample. The higher nitrogen concentration of surface layers treated by CPFs is achieved by increasing the pressure of the residual nitrogen atmosphere. It has been established that γ_N-Fe nitrous austenite, α″-Fe(N) and α′-Ti(N) martensitic phases, and γ′-Fe₄N and δ-TiN _x nitrides can be produced by nitriding the surface layers of St3 steel and VT1-0 titanium.     

Controlled preparation of well-ordered transition metal oxide layers on a metallic surface

A method to create various well-ordered two dimensional transition metal oxide films on a metallic substrate has been exploited. The formation of an intermediate amorphous layer with controllable metal-oxygen stoichiometry serves as an important precursor condition for the final transformation into a mono-phase, crystalline oxide layer via mild annealing. As a key ingredient serves a Cu₃Au(1 0 0) substrate covered by oxygen. The flat Cu-O topmost layer stops completely intermixing of the substrate material with the subsequently evaporated transition metal film. Likewise the wetting of the surface is considerably enhanced and a homogeneous oxidation of the film is strongly promoted. The proposed technique appears to be widely efficient for preparation of various two dimensional oxide films covering the entire Cu₃Au(1 0 0) substrate. Its usefulness is demonstrated successfully for vanadium, niobium and molybdenum to produce a set of single-phase transition metal oxides of different stoichiometry and geometrical structure. All created oxides are found to be thermally stable at least up to a substrate temperature of 800 K.     

Formation of titanium oxide films on the surface of porous silicon carbide

The influence of rapid thermal annealing (RTA) on the properties of a titanium film on the surface of porous silicon carbide is considered. It is shown that an increase in the RTA temperature to 900°C stabilizes the phase composition of the forming titanium oxide over the film, which is identified as rutile. Due to the formation of titanium oxide nanoclusters under the action of RTA, an additional photoluminescence band arises near 2.5 eV. Based on Auger spectrometry data, a multilayer model to calculate the optical parameters of titanium oxide films covering porous silicon carbide is suggested.     

光电倍增管中子直照灵敏度响应

光电倍增管的中子直照响应对脉冲中子探测器的设计和实际应用具有重要影响。分析了中子束直接照射光电倍增管产生直照响应的过程和机理。中子与光电倍增管入射窗硼硅玻璃中的硼和硅发生(n,α)和(n,p)反应,导致窗玻璃产生荧光,从而使光阴极发射光电子。在5SDH-2小串列加速器上,选用9815B和9850B两种光电倍增管进行实验,得到了光电倍增管的中子直照灵敏度能谱响应曲线。结果表明：中子能量由低到高变化时,光电倍增管的中子直照灵敏度也随之增大;两种光电倍增管的中子直照灵敏度比值为1.9×102~2.74×102,该值与其相应的增益比量级一致。     

光电倍增管中子直照灵敏度响应

 光电倍增管的中子直照响应对脉冲中子探测器的设计和实际应用具有重要影响。分析了中子束直接照射光电倍增管产生直照响应的过程和机理。中子与光电倍增管入射窗硼硅玻璃中的硼和硅发生(n,α)和(n,p)反应,导致窗玻璃产生荧光,从而使光阴极发射光电子。在5SDH-2小串列加速器上,选用9815B和9850B两种光电倍增管进行实验,得到了光电倍增管的中子直照灵敏度能谱响应曲线。结果表明：中子能量由低到高变化时,光电倍增管的中子直照灵敏度也随之增大;两种光电倍增管的中子直照灵敏度比值为1.9×10²~2.74×10²,该值与其相应的增益比量级一致。