Initiation of ecton processes by interaction of a plasma with a microprotrusion on a metal surface

Evolution of rapid (～10 ns) Ohmic overheating of a microprotrusion on a surface in contact with a plasma by emission current is studied taking into account the energy carried by plasma ions and electrons, as well as Ohmic heating, emissive source of energy release (Nottingham effect), and heat removal due to heat conduction. Plasma parameters were considered in the range of n = 10¹⁴?10²⁰ cm^?3 and T _e = 0.1 eV?10 keV. The threshold value of energy transferred to the surface from the plasma is found to be 200 MW/cm²; above this value, heating becomes explosive (namely, an increase in the temperature growth rate (δ² T/δt ² > 0) and in passing current (δJ/δt > 0) is observed in the final stage at T ～ 10⁴ K and j ～ 10⁸ A/cm²). In spite of the fact that Ohmic heating does not play any significant role for plasmas with a density lower than 10 18 cm^?3 because the current is limited by the space charge of electrons, rapid overheating of top of microprotrusion is observed much sooner (over a time period of ～1 ns) when the threshold is exceeded. In this case, intense ionization of vapor of the wall material leads to an increase in the plasma density at the surface, and the heating becomes of the Ohmic explosion type. Such conditions for the formation of a micr?xplosion on the surface and of an ecton accompanying it can be created during the interaction of a plasma with the cathode, anode, or an insulated wall and may lead to the formation of cathode and anode spots, as well as unipolar arcs.     

Search for anomalous transmission of ultracold neutrons through metal foils

Results are presented on the search for anomalous transmission of ultracold neutrons (UCNs) through beryllium (thickness ∼0.14 mm), stainless steel (0.05 and 0.015 mm), and copper (0.01 and 0.018 mm) foils. This anomalous transmission is considered to be a possible reason for the disappearance of UCNs from beryllium bottles, an effect which was discovered in experiments at the St. Petersburg Nuclear Physics Institute and which was recently observed in the experiment of V. E. Varlamov et al., JETP Lett. 66, 336 (1997). No transmission was found in our measurements at the 10⁻⁷ level except in the case of copper foils, which we attribute to the presence in the UCN flux of an admixture of neutrons with energies higher than the boundary energy for copper. Pis’ma Zh. éksp. Teor. Fiz. 67, No. 7, 440–444 (10 April 1998) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Evaluation of plasmon-loss spectra of molybdenum using elastic-peak electron spectroscopy

Plasmon-loss spectra of clean polycrystalline molybdenum surfaces have been determined in the primary energy range E_p = 50–3000 eV. Spectra a distributions (nonderivative mode). A simplified model is described for evaluating plasmon-loss spectra using elastic-peak electron spectroscopy, as de of elastically reflected electrons is determined by integrating the N(E) spectrum of secondary and backscattered electrons. The ratio of the ar (23–24 eV) to that of the elastic peak is Pλ, the product of the probability for creating a volume plasmon loss and the inelastic mean free pat second plasmon-loss peak is (Pλ)². Evaluation of our experimental plasmon-loss spectra gives Pλ = 0.4–0.5 for E_p > 500 eV. Th constitutes ～50% of all losses determining the IMFP, interband loss processes being important in the remainder. For the low energy range, E_p found. For E_p < 100 eV, no volume plasmon-loss peak could be detected in our N(E) spectra. The simplified model proves to be valid fo plasmon-loss peak (11–12 eV), i.e., such that N_pls/N_e ? 10^?2. Some results are presented concerning surface plasmon losses as molybdenum surface.     

Superficial migration of tritium physisorbed on monocrystalline nickel (111)

The aim of this study is the measurement of superficial migration coefficient of tritium physisorbed on monocrystalline nickel without chemisorbed sublayer. The chosen crystalline orientation was (111) because it offers the greatest concentration of adsorption sites per square centimeter. A clean surface sample is obtained by mechanical polishing, chemical etching and finally ionic bombardment by high purity argon gas. The pressure in the experimental vessel is maintained below 10^?9 torr, by liquid helium cryopumping after zeolite sorption pumping.A little spot of adsorbed tritium is produced by introduction of a finite amount of tritium gas on the clean surface of the nickel sample through a stainless steel tube. Temperatures of nickel and of the gas introduction tube are respectively regulated at 5 K and 35 K. Tritium is used as a radioactive marker and its 10 keV β-radiation is measured by a channeltron type detector which permits the localization of the deposit without acting on the surface. We observed that tritium sorbed at 5 K is quite immobile (at the time scale of our experiment). After heating up to a fixed temperature T chosen between 10 K and 20 K, the deposite profile variation in function of time is observed to determine the superficial diffusion coefficient D. For the values of T from 13 K to 20 K, D varies from 10×10^?6 to 150×10^?6 cm² sec^?1. A diffusion activation energy of 200 cal mole^?1 is deduced from the exponential increase of the curve. A vibrational frequency can be evaluated to 3×10¹² sec^?1. The rate of desorption permits the evaluation of sorption energy at about 1800 cal mole^?1 in good agreement with usual results concerning physorption of H₂ on metals.     

Possible mechanism of superconductivity in the CuO-Cu interface

Apparently, a two-dimensional CuO lattice is formed on the surface of copper oxide in the CuO-Cu interface. This lattice consists of Cu²⁺ and O^1? ions, which form a narrow, partially filled two-dimensional band. In this case, local electron pairs (LEPs) can form in the oxygen subsystem as a result of the fulfillment of the Shubin-Vonsovskii conditions. A crude estimate of the formation temperature of LEPs gives T* ～ 10 4 K. At the concentration in the interface layer n～1.6×10²⁰ cm^?3 and the effective mass of carriers m* ～ m _e, the onset temperature of Bose-Einstein condensation may take a value of T _c ～ 1000 K. The estimate obtained for the temperature T _c corresponds to the experimental value by an order of magnitude.     

Observation of selective enhancement of the capture of ultracold neutrons by nuclei

The subbarrier reflection of ultracold neutrons (UCNs) from stainless steel (an alloy of iron, nickel, chromium, and titanium) is investigated by means of neutron-radiation analysis. It is found that the increase in the probability of capture of UCNs by nuclei is large compared to the standard theory. The effect is selective, the enhancement factor varying From 3 for iron to 90 for titanium. Pis’ma Zh. éksp. Teor. Fiz. 65, No. 1, 3–8 (10 January 1997)     

Heterogeneous reactions of H and O<Subscript>2</Subscript> with solid alkanes

The paper describes a new source of surface organic radicals whose special feature is a low rate of the initiation of alkyl radicals in reactions of H atoms with the surface of alkanes. A special reactor design was used to accumulate radicals at room temperature and observe alkoxyl radicals RO by the EPR method. For this purpose, alkanes were deposited on aerosil and placed into an EPR cavity. Thanks to the large area of aerosil loads (～10³ cm²), we were able to obtain a stable signal corresponding to a ～10¹⁷ total amount of alkoxyl radicals (the degree of surface coverage ～0.1%). When O₂ was introduced into the reactor in concentrations of 10¹⁵–10¹⁶ cm^?3, a sharp decrease in the signal from surface organic radicals was observed. The process was described as the first-order (RO) _s + O₂ → HO₂ + ketone reaction with a 1.7 × 10^?17 cm³ s^?1 rate constant.     

Morphology,chemical composition,and electrical characteristics of hybrid (Ni-C) nanocomposite structures grown on the van der Waals GaSe(0001) surface

The morphology and chemical composition of metal (Ni), carbon, and composite (Ni-C) nanostructures grown on oxidized and unoxidized (0001) surfaces of a layered GaSe crystal by electron beam vacuum evaporation of the material from a liquid ion source in an electric field have been investigated using atomic force microscopy and X-ray photoelectron spectroscopy. It has been demonstrated that this technology makes it possible to grow nanostructures with different morphologies depending on the growth mode and substrate surface state. Dense homogeneous arrays of nickel nanoparticles (Ni@C) (with geometrical sizes of ～1–15 nm and a lateral density of higher than 10¹⁰ cm^?2) encapsulated into carbon shells, as well as carbon layers (with a thickness of the order of several nanometers), are grown on the unoxidized van der Waals GaSe(0001) surface, whereas Ni-C composite nanostructures are grown on the oxidized surface. The formation of oxide nanostructures on the van der Waals surface and their chemical composition have been examined. Vertical hybrid Au/Ni/(Ni-C)/n-Ga₂O₃(Ni@C)/p-GaSe structures grown on the GaSe(0001) surface contain Ni@C nanoparticles embedded in the wide-band-gap n-Ga₂O₃ oxide. The current-voltage characteristics of these structures at temperatures close to T = 300 K exhibit specific features of the Coulomb blockade effect.     

Chemisorption of nitrogen on platinum {111}: Reflection-absorption infrared spectroscopy

Reflection-absorption infrared spectroscopy has been combined with thermal desorption and surface coverage measurements to study nitrogen adsorption on a {111}-oriented platinum ribbon under ultrahigh vacuum conditions. Desorption spectra show a single peak (at 180 K) after adsorption at 120 K, giving a coverage-independent activation energy for desorption'of ～40 kJmol^?1. The initial sticking probability at this temperature is 0.15, and the maximum uptake was ～1.1 × 10¹⁴ molecule cm^?2. The adsorbed nitrogen was readily displaced by CO, h₂ and O₂. An infrared absorption band was observed with a peak located at 2238 ± 1 cm^?1, and a halfwidth of 9 cm^?1, with a molecular intensity comparable to that reported for CO on Pt{111}. The results are compared with data for chemisorption on other group VIII metals. An earlier assignment of infrared active nitrogen to B₅ sites on these metals is brought into question by the present results.     

Role of Structural Stresses in the Thermodestruction of Supercoiled Cellulose Macromolecules after Nitration

Thermogravimetry is used to study the thermodestruction of nitrocellulose (NC) with various nitrogen contents at various heating rates. At high degrees of nitration and high heating rates of the sample, the reaction occurs in an explosion mode with a threshold of its weight loss depending on the temperature. To explain this behavior, it is assumed that the nitration of cellulose gives rise to structural stresses, which weaken the covalent bonds in it by ～37 kJ/mol (at a nitrogen content of ～13%). This process apparently involves two different mechanisms of weight loss during heating: (a) conventional thermal destruction of NC macromolecules through the rupture of covalent bonds (with k₀ = 10¹³ s^?1, E = 150.2 kJ/mol, and n = 1) at heating rates of up to 10 K/min and nitrogen content in NC of up to 9%; (b) Zhurkov’s thermofluctuational mechanism of the destruction of strained macromolecules, characterized by a sharp (threshold) dependence of the weight loss on the heating rate, which is operative at heating rates above ～4 K/min and high (>13%) nitrogen contents and at 20 K/min and a low (～9%) nitrogen content. Under conditions of rapid heating, ～10–20 K/min, the work done by stressed states to overcome the potential barrier to the rupture of covalent bonds causes an increase in the decomposition rate by a factor of 2000. The observed threshold pattern of weight loss during the thermodestruction of NC explains the long-known critical dependence of the properties of NC used to manufacture propellants on small changes in the nitrogen content.     

Electron stimulated desorption of H3O from 316L stainless steel

Surface ions generated by electron stimulated desorption from mass spectrometer ion source grids are frequently observed, but often misidentified. For example, in the case of mass 19, the source is often assumed to be surface fluorine, but since the metal oxide on grid surfaces has been shown to form water and hydroxides, a more compelling case can be made for the formation of hydronium. Further, fluorine is strongly electronegative, so it is rarely generated as a positive ion. A commonly used metal for ion source grids is 316L stainless steel. Thermal vacuum processing by bakeout or radiation heating from the filament typically alters the surface composition to predominantly Cr₂O₃. X-ray photoelectron spectral shoulders on the O 1s and Cr 2p_3/2 peaks can be attributed to adsorbed water and hydroxides, the intensity of which can be substantially increased by hydrogen dosing. On the other hand, the sub-peak intensities are substantially reduced by heating and/or by electron bombardment. Electron bombardment diode measurements show an initial work function increase corresponding to predominant hydrogen desorption (H₂) and a subsequent work function decrease corresponding to predominant oxygen desorption (CO). The fraction of hydroxide concentration on the surface was determined from X-ray photoelectron spectroscopy and from the deconvolution of temperature desorption spectra. Electron stimulated desorption yields from the surface show unambiguous H₃O⁺ peaks that can be significantly increased by hydrogen dosing. Time of flight secondary ion mass spectrometry sputter yields show small signals of H₃O⁺, as well as its constituents (H⁺, O⁺ and OH⁺) and a small amount of fluorine as F⁻, but no F⁺ or F⁺ complexes (HF⁺, etc.). An electron stimulated desorption cross-section of σ⁺ ∼ 1.4 × 10⁻²⁰ cm² was determined for H₃O⁺ from 316L stainless steel for hydrogen residing in surface chromium hydroxide.     

Measurement of surface resistivity/conductivity of metallic alloys in aqueous solutions by optical interferometry techniques

Optical interferometry techniques were used for the first time to measure the surface resistivity/conductivity of the pure aluminium (in seawater at room temperature), UNS No.304 stainless steel (in seawater at room temperature), and pure copper (in tap water at room temperature) without any physical contact. This was achieved by applying an electrical potential across the alloys and measuring the current density flow across the alloys, during the cyclic polarization test of the alloys in different solutions. In the mean time, optical interferometry techniques such as holographic interferometry were used in situ to measure the orthogonal surface displacement of the alloys, as a result of the applied electrical potential. In addition, a mathematical model was derived in order to correlate the ratio of the electrical potential to the current density flow (electrical potential/electronic current flow = resistance) and to the surface (orthogonal) displacement of the metallic samples. In other words, a proportionality constant (surface resistivity or conductivity = 1/surface resistivity) between the measured electrical resistance and the surface displacement (by the optical interferometry techniques) was obtained. Consequently the surface resistivity (ρ) and conductivity (σ) of the pure aluminium (in seawater at room temperature), UNS No.304 stainless steel (in seawater at room temperature), and pure copper (in tap water at room temperature) were obtained. Also, electrical resistivity values (ρ) from other source were used for comparison sake with the calculated values of this investigation. This study revealed that the measured value of the resistivity for the pure aluminium (7.7 × 10¹⁰ Ω cm in seawater at room temperature) is in good agreement with the one found in literature for the aluminium oxide, 85% Al₂O₃ (5 × 10¹⁰ Ω cm in air at temperature 30 °C). Unfortunately, there is no measured value for the resistivity of cupric oxide (CuO), cuprous oxide (Cu₂O), or even the oxide of the UNS No.304 stainless steel in literature comparing those values with the measured values in this study.     

Tribological characterization of TiCN coatings deposited by two crossed laser ablation plasma beams

The simultaneous laser ablation of two targets (graphite and titanium) in an Ar-N₂ gas mixture was carried out to deposit thin films of the ternary compound TiCN at room temperature. The base conditions used to produce the TiN without carbon were taken from our previous studies. The experimental conditions for the ablation of the carbon target were varied so that the carbon content in the films could be changed depending on the carbon ion energy. The control of the experimental conditions was carried out using a Langmuir planar probe which permitted the determination of the mean kinetic ion energy. The maximum hardness value of 35 GPa, was obtained with a carbon ion energy of about 250 eV, which corresponds to a film with 5 at% carbon content. In order to perform tribological and scratch tests, two types of substrate were used: nitrided AISI 316 stainless steel and AISI 316 stainless steel previously coated with a thin titanium layer (～50 nm). Values of the wear rate in the range of 1.39×10^?6 to 7.45×10^?5 mm³?N^?1?m^?1, friction coefficient from 0.21 to 0.28 and adhesion from scratch test measurements up to 80 N for final critical load, were obtained.     

Desorption of halogens from some Nb,Mo, Ta and W single crystal surfaces

A systematic investigation of the thermal desorption of halogens from well characterized (111), (100) and (110) 4d (Nb, Mo) and 5d (Ta, W) transition metal surfaces has been carried out under low coverage conditions (θ < 10^?2 of a monolayer). Characterization of the surfaces was achieved by LEED, AES and work function determinations while the desorption kinetics were recorded in a large temperature range (1700–2300 K) using a pulsed ionic beam method. The new data concerning some Ta and W surfaces are presented and the results of this systematic study are discussed. It is shown that the halogen desorption parameters, e.g., desorption energies and preexponential factors, are independent of both surface structure and d bond filling of the substrate; E(F) ～4.75 eV, E(Cl) ～4.1 eV, E(Br) ～3.7 eV and τ₀ ～10^?13 ?10^?14 S. The halogen behaviour is compared with that of other adsorbates and with the predictions of a general chemisorption model.     

Experimental investigation of excitation of argon metastables by an electron swarm in argon nitrogen gas mixtures

Excitation of argon metastable levels (4³ P ₂+4³ P ₀) in a nonselfsustained discharge in Ar(≧99%)+N₂(≦1%) gas mixtures has been investigated by measuring N₂ second positive group which is the result of excitation transfer from Ar metastables to N₂ molecules. The method is based on the assumption, experimentally verified, that addition of small quantities of N₂ do not change markedly the probability of Ar excitation. The obtained excitation coefficients for production of Ar ^m in theE/N range (20–70)×10^?21Vm² are (3.5–13)×10^?22m² with an estimated uncertainty of ±50%. Simultaneously, excitation coefficients for direct excitation of nitrogenC-state by swarm electrons in Ar-N₂ mixtures have been obtained. A comparison of nitrogen spectra excited by electrons and by argon metastables showed significant differences in relative population of vibrational and rotational levels of the N₂(C ³ Π _U state in these two cases.     

Studying the effects of temperature and radiation on the Al x O1 − x /TiNC system

The penetration of hydrogen through protective layers of aluminum oxide fixed on the surface of nanocrystalline (NC) titanium is studied. A film 400 nm thick is prepared by the magnetron sputtering. Radiation- and thermally-induced gas release are employed. It is found that the Al _x O_1?x film prevents the release of hydrogen from a sample under both radiation and thermal effects. The temperature of hydrogen extraction from metal hydride accumulators can be reduced by 200–250°C, provided that heating is performed under conditions of surface irradiation by electrons with energies of ～30 keV and current densities of 2 to 3 μA cm^?2.     

Interaction of aluminum with iridium surface: Adsorption, desorption, dissolution, and formation of surface compounds

The interaction of aluminum with an iridium (111) surface was studied in ultrahigh vacuum by Auger electron spectroscopy over the broad temperature range 300–2000 K. At room temperature, layer-by-layer growth of an aluminum film was observed, with a monolayer forming in coherent relation to the substrate. Deposition at 1100–1300 K gives rise to the formation of surface aluminide Ir₄Al with an adatom concentration N _Al = (4.20 ± 0.15) × 10¹⁴ cm^?2. It was shown that aluminum escapes out of the surface aluminide by thermal desorption in the 1300–1700-K temperature interval, with the desorption activation energy changing from ～4.5 to ～5.7 eV as the coverage decreases from the value corresponding to the surface aluminide (taken for unity) down to zero.     

Interaction of aluminum with the rhenium surface: Adsorption, desorption, and growth of surface compounds

The interaction of aluminum with the ( $10\bar 10$ ) rhenium surface was studied experimentally within a broad temperature range, 300–2000 K. Surface aluminide (SA) ReAl with a concentration of adsorbed Al atoms N _Al=1.6×10¹⁵ cm^?2 was found to form. It was shown that aluminum escapes from the surface by thermal desorption at temperatures from 1300 to 1600 K, with the desorption activation energy changing abruptly from ～3.6 to ～4.2 eV when passing through the concentration corresponding to the SA.     

Small-angle neutron scattering study of radiation defects in synthetic quartz

The superatomic structure of synthetic quartz single crystals with dislocation densities ρ = 54 and 570 cm^?2 was studied in the initial state and after irradiation with fast neutrons with energies E _n > 0.1 MeV in a WWRM reactor (St. Petersburg Nuclear Physics Institute) in the fluence range F = 0.2 × 10¹⁷?5.0 × 10¹⁸ neutrons/cm². Weak irradiation with F = 0.2 × 10¹⁷ neutrons/cm² causes only slight structural changes, whereas appreciable generation of defects with radii of gyration r _g ～ 1–2 nm and R _G ～ 40–50 nm occurs at F = 7.7 × 10¹⁷?5.0 × 10¹⁸ neutrons/cm². As the fluence increases further, the number and volume fraction of point defects, as well as extended (channels ～2 nm in radius) and globular (amorphous phase nuclei) defects, increase.     

Surface lifetime degradation in Si and Ge under low energy electron irradiation

The bombardment of n-type Ge and Si by 10–30 keV electron beams is shown to reduce the surface lifetime τ_P of minority carriers but not to affect mobility μ_P or diffusion constant D_P. For experiments, the electron beam in a scanning electron microscope is replaced a chopped light source to perform modified Haynes-Shockley experiment, and the measurements is used to calculate μ, D and τ. The reduction in τ is interpreted in terms of an increase in surface recombination velocity s. At a dose of approximately 10¹⁷ electrons cm^?2 for Ge and 10¹⁵ electrons cm^?2 for Si, a saturation value for s appears to be reached after which further bombardment has no effect. The values of τ obtained from both light and SEM Haynes-Shockley experiments performed in vacuum agree within approximately 5% both prior to and after bombardment. The initial values of lifetime can be restored by etching the samples.