Surface Recombination Coefficient Research Articles

Surface recombination in breakdown time delay experiments: Effect of different cathode materials

The late afterglow in nitrogen with iron electrode is studied by the breakdown time delay method, i.e., by measuring the breakdown time delay td as a function of the afterglow time τ. It is proposed that the cause of the secondary electrons initiating the breakdown is the energy of the surface recombination of nitrogen atoms on the iron electrode. The gas-phase and macrokinetic diffusive models are used to describe the experimental breakdown time delay data. By fitting the theoretical curve to the experimental data: (1) it has been confirmed that the recombination on the molybdenum glass is of the second order and the value of the surface recombination coefficient is determined at 4 mbar; (2) it has been shown that the surface recombination on the iron electrode is of the second order, and the effective recombination coefficients are determined; (3) the analytical form of the recombination coefficient as a function of the adsorption characteristics of surfaces and the pressure of the parent gas has been derived. In addition, the orders of surface recombination on the molybdenum-, aluminum-, and gold-plated electrode were determined by the same method.

Influence of Impurities on Surface Recombination of Nitrogen Atoms in Late Afterglow

The influence of impurity contents (water vapour and oxygen) on surface recombination of nitrogen atoms on the glass walls and the copper electrode surface is studied. The decay of nitrogen atom number density in late afterglow has been detected by the breakdown time delay method and the memory effect was found for nitrogen with large abundance of impurities (technical purity gas). The dominant reaction on the glass walls covered with water vapour was found to be of the second order. The surface recombination coefficient has been increased by about two orders of magnitude compared to the pure gas. Also, the increase of the secondary electron yield by about one order of magnitude occurs caused by chemisorbed oxygen on the electrode surface.

Effect of surface impurities segregation and ion sputtering on ion driven deuterium permeation through copper

The deuterium permeation through a polycrystalline copper membrane driven by an ion beam (3 keV D 2 +) was measured as a function of temperature (573–873 K) and surface impurity concentration. For copper a single-impurity layer can be prepared by combining thermal annealing and argon ion bombardment. The surface recombination coefficient was evaluated from the measured permeation flux. Under assumption that the segregation of sulphur on the surface results in modification of the activation energy for chemisorption, E C, the value of E C was calculated as a function of sulphur concentration.

A model for trapping and re-emission at hydrogen ion implantation

A simple two-parameter model for re-emission at hydrogen ion implantation is proposed. The model takes into account diffusion, molecular desorption and hydrogen-defect interaction. Comparison of the model with experiments on hydrogen and deuterium re-emission from stainless steel, molybdenum, tungsten, boron nitride, carbon and pyrocarbon demonstrates a good agreement. Two fitting parameters are used in the model: (1) the recycling factor R = DK where D is the diffusion coefficient and K the surface recombination coefficient and (2) the defect factor Φd equal to the maximum areal concentration of hydrogen being able to be trapped in defects. At high temperature R(T) and Φd(T) follow exponential dependences in accordance to thermal activation of the processes involved. At low temperatures R(T) and Φd(T) functions deviate from the exponents, presumably due to domination of some radiation enhanced processes. Comparison of our R and Φd values with those available from other experiments shows that the fitting parameters in our model have reasonable values.

Surface recombination of atoms in a nitrogen afterglow

The surface recombination of nitrogen atoms in afterglow is studied by the time delay method, accompanied by the macrokinetic diffusive model. The method consists of the measurement of the dependence of the mean value of the breakdown time delay on afterglow period td=f(τ) and fitting of the data by the model that was developed. Excited N2(A 3∑+u) nitrogen molecules formed in the surface-catalyzed recombination on cathode produce secondary electrons. The electrons entering the interelectrode space determine the time delay in electrical breakdown. The time delay method is very efficient in nitrogen atom detection down to a natural radioactivity level. By fitting the calculated curve to the experimental data, we have: (1) shown that the nitrogen atom recombination on the glass container walls is second-order in N while the recombination on the copper electrode is the first order; (2) determined the value of the surface recombination coefficient for molybdenum glass; (3) determined the combined probability of N2(A 3∑+u) metastable formation by recombination at electrode surface and of secondary electron emission. Furthermore, we derive the adsorption isotherm of nitrogen atoms on molybdenum glass, the type of recombination mechanism and the dependence of the activation energy for desorption (or the heat of adsorption) on the fractional coverage.

Ion-driven permeation and surface recombination coefficients of deuterium for silver

The steady-state flux of deuterium permeation through an iron membrane (99.99 + % purity, 0.14 mm thickness) driven by a deuterium ion beam (5 keV D3+, 1.0 × 1015 D-atoms cm−2s−1) was measured in the range of 30–1050°C. The permeation flux increased with increasing temperature above 200°C whereas it was roughly constant below 150°C. Such temperature dependence was observed for nickel and copper as well, and has been ascribed to the transition in the rate-limiting processes of the deuterium transport in the membrane. The recombination coefficient of deuterium on iron surface was evaluated from the permeation flux density. In the case of α-iron, the evaluated value agreed with a semitheoretical one estimated using literature data of adsorption probability and solubility. This agreement indicates that the release kinetics reflects the activation barrier for hydrogen (deuterium) adsorption.

Recombination coefficients of deuterium on metal surfaces evaluated from ion-driven permeation

The surface recombination coefficients of deuterium for polycrystalline Ni, Fe, Cu, and Ag were evaluated from the flux of ion-driven permeation. These data agreed rather quantitatively with semi-theoretical estimations based on a simple consideration of equilibrium. This demonstrates that the release kinetics is dominated by the activation barrier for adsorption as well as the solubility.

Ion‐driven permeation and surface recombination coefficient of deuterium for copper

The flux density of deuterium permeation through polycrystalline copper membrane driven by an ion beam (5 keV D+3 ) was measured as a function of temperature (80–500 °C) and implantation flux [(0.2–1.0)×1015 D atoms cm−2 s−1 ]. The observed dependence of permeation flux on both parameters changed around 200–300 °C. This was found to be due to the transition in rate‐controlling processes. The surface recombination coefficient of deuterium was evaluated from the permeation flux. In order to discuss it, the model for recombination coefficient based on one‐dimensional potential‐energy surface was extended, taking account of subsurface solution sites. The comparison between the experimentally and theoretically determined values supported the existence of the activation barrier for hydrogen chemisorption on copper.

Determination of the Kinetic Coefficients of Silicon Self‐Interstitials from Back‐Side Oxidation/Front‐Surface Stacking‐Fault Growth Experiments

A series of back‐side oxidation/front‐side stacking‐fault growth experiments have been carried out to determine the kinetic coefficients of self‐interstitials in silicon. In these experiments, wet and dry oxidations of the back side of thinned silicon samples were used to inject self‐interstitials from the back surfaces. The sample front surfaces were capped with oxide or nitride layers, and the concentration of self‐interstitials at the capped surfaces were monitored by the growth or shrinkage of surface stacking faults. Experimental results have been analyzed using steady‐state and transient models, based on the assumption that self‐interstitials dominate the kinetic processes of intrinsic point defects. From these analyses, the relative recombination rates of self‐interstitials at oxide and nitride boundary layers have been obtained, with an oxide layer found to absorb self‐interstitials at about three times the rate of a nitride layer. The results also suggest that the surface recombination coefficients are time dependent rather than constant, as has been previously assumed.

Hydrogen transport behavior of metal coatings for plasma-facing components

Plasma-facing components for experimental and commercial fusion reactor studies may include cladding or coatings of refractory metals like tungsten on metallic structural substrates such as copper, vanadium alloys and austenitic stainless steel. Issues of safety and fuel economy include the potential for inventory buildup and permeation of tritium implanted into the plasma-facing surface. This paper reports on laboratory-scale studies with 3 keV D + 3 ion beams to investigate the hydrogen transport behavior in tungsten coatings on substrates of copper. These experiments entailed measurements of the deuterium re-emission and permeation rates for tungsten, copper, and tungsten-coated copper specimens at temperatures ranging from 638 to 825 K and implanting particle fluxes of approximately 5 × 10 19 D/m 2 s. Diffusion constants and surface recombination coefficients with enhancement factors due to sputtering were obtained from these measurements. These data may be used in calculations to estimate permeation rates and inventory buildups for proposed diverter designs.

A model of polysilicon emitter transistors devoid of a single-crystal emitter region

A one-dimensional model is described for a polysilicon emitter bipolar transistor, whose emitter drive-in has been low enough so that there is no single-crystal part to the emitter. Both direct and trap-assisted tunnelling as well as recombination mechanisms both at the interface and in the space charge region are considered in detail. The model is versatile enough to handle all configurations of the potential barrier presented by the interfacial layer and the semiconductors. A good fit to experimental results is obtained, and a full study of the effect of varying the physical parameters of the model is made. These include the surface recombination coefficients, the band-gap narrowing, the doping concentration (ND) in the emitter and (NA) in the base. An earlier companion paper included effects of varying additional parameters. It is found that the poorer current gain displayed by this kind of device (as compared with an equivalent transistor with a single-crystal region as part of the emitter) is due to increased recombination at the interface. However, a very large gain (in excess of 10000) is predicted, for this type of device, if the interface state density is lowered sufficiently (to a value of 109 cm-2, or below). The cause of a pronounced kink in the base current characteristic, which is observed experimentally, is fully explained and furthermore it is shown that this can lead to a negative differential resistant regime for certain parameter values.

Implantation-Driven Tritium Permeation in Vanadium Alloy Fusion Reactors

The safety concerns resulting from tritium implantation into vanadium and vanadium alloys have been studied. Tests were conducted to measure the physical constants that determine permeation and inventory in these materials. Surface recombination coefficients, enhancement factors due to plasma sputtering, and the diffusion constants were measured. To determine the implications of these results, application was made to the TITAN-I fusion reactor design. For this design, the tritium permeation rate and inventory buildup in the first wall were calculated for a range of surface conditions on the coolant side. 7 refs., 8 figs., 1 tab.

Modeling of pulsed-laser-induced gas detrapping, chemical reactions, diffusion, and desorption

Short, intense laser pulses can thermally desorb hydrogen and other gases from the near surface (∼1 μm) of solids. This can be used, with some advantages, to study the trapping mechanisms, whether physical or chemical, the diffusion, and the surface recombination of these gases. By assuming a laser pulse with a Gaussian time profile, these successive steps are modeled with a finite difference code by using realistic temperature- and concentration-dependent material parameters. We show explicit results for the magnitude of the desorption puff as a function of the laser energy and for the depth profiles of the remaining gas under various assumptions for the detrapping mechanism, the diffusion coefficient, and the surface recombination coefficient. The results demonstrate that in spite of the poorer control on temperature, compared to isochronal or ramp anneals, the rate-limiting process can be identified and the activation energies determined.

Material technologies for fusion reactor. In-vessel materials.

Chemical sputtering and radiation-enhanced sublimation of graphite have been the subject of many intense researches. Chemical form of desorbing species and synergism between thermal atoms and energetic atoms are among the controversial problems of surface erosion of graphite. Self-sustaining surface segregated low-Z layers have been proposed as a new approach to impurity control. Hydrogen isotope dynamics between plasma and first wall is very important in controlling the plasma, and hence the fusion reactor itself. In this sense, surface recombination coefficient of hydrogen must be evaluated for various materials with various surface conditions. Its theory and surface impurity effect are described.

Measurements of density, surface recombination coefficient, and diffusion coefficient of hydrogen atoms by Lα laser fluorescence spectroscopy

The technique of laser fluorescence spectroscopy at the Lyman-alpha (Lα) wavelength in hydrogen was used to measure the temporally and spatially resolved atomic density, the surface recombination coefficient γ, and the diffusion coefficient DH,H2 of the atomic hydrogen in molecular hydrogen. At the same time, it revealed a temporal change of γ, probably due to the change of surface number density of trapped hydrogen atoms. The present technique provides a new and powerful means for determinations of values associated with hydrogen atoms by absolute and local measurements of the atoms at the detection limit of extremely low concentrations.

Ion beam studies of hydrogen in metals

Methods based on ion implantation and nuclear reaction analysis have been developed and used to investigate fundamental aspects of the behavior of hydrogen isotopes in metals. The binding enthalpy of deuterium at irradiation defects, helium bubbles, deuterium bubbles and metal-oxide interfaces was measured for aluminum, iron, nickel, copper, palladium, austenitic stainless steel, Inconel and amorphous Fe 40Ni 40P 14B 6. The binding enthalpies determined for the pure metals are in excellent agreement with mechanistic calculations based on effective medium theory and other information. Surface-limited release of deuterium from iron, stainless steel and Inconel was measured as a function of temperature and the state of surface oxidation. The release rate was accurately proportional to the square of the deuterium concentration in solution, permitting the results to be expressed in terms of a surface recombination coefficient. This quantity was up to four orders of magnitude greater for an ion-sputtered surface than for a surface with electropolish oxide. The diffusion coefficient and solid solubility of tritium in stainless steel were measured for the first time at the ice point, thereby extending downward by three orders of magnitude the diffusivities available from conventional permeation experiments. Deuterium concentration profiles resulting from electrochemical charging of Incology 903 were measured as a function of charging current, thereby providing a direct systematic calibration of such charging in an austenitic material where conventional permeation measurements are precluded by the small hydrogen diffusion rate.

ERD measurement of surface recombination coefficients of implanted hydrogen ions

The elastic recoil detection (ERD) method by 2 MeV He+ ions was applied to observe the behaviour of implanted hydrogen atoms during simultaneous implantation of 25 keV or 50 keV He+ ions upon high purity Al and Al alloy (Al-3 wt% Mg-2.3 wt% Li-0.2 wt% Zr) at room temperature and above.Trapping of hydrogen atoms by lattice defects and presumably by surface oxide film is observed in Al, and by both alloying elements and lattice defects, but not by the surface films, in the alloy. At temperatures higher than 70 ° C, only the surface trapping is observed in Al.Surface recombination coefficients on the Al surface are measured in the temperature region between 70 ° C and 220 ° C and found to range from 10−28 to 10−26 cm4/s, increasing with the increase of temperature.Phenomological diffusion constants of about 10−10 cm2/s at 70 ° C and 170 ° C are obtained by solving the diffusion equation so as to meet these surface recombination coefficients.

Atomic, ionic and molecular hydrogen permeation facility with in situ Auger surface analysis

Hydrogen recycle at the first wall, as well as tritium retention and inventory, are influenced by the surface recombination coefficient Kr of first-wall structures. This parameter is a function of surface composition which itself changes as a result of exposure to the hydrogen plasma. A permeation membrane facility is described which incorporates in situ monitoring of the membrane by Auger analysis. The facility is capable of incorporating various bombarding species, either separately or combined, of H2, sub-eV H0, ~ 60 eV-10 keV H+ and e−. Initial results for Kr on a palladium membrane exposed to H2 and sub-eV H0, in the region of surface recombination limited permeation, are presented. Preliminary observations have been made of the effect of carbon surface impurities on the permeation rates.

Permeation probes for the characterization of the atomic hydrogen flux to a tokamak wall

The first wall of a future magnetic fusion device is essentially defined as the plasma-facing surface of the breeding blankets, which is supposed to be subjected to bi-directional hydrogen isotopes permeation: in one direction by edge plasma-driven permeation (PDP) of deuterium as well as tritium into blankets, and in the other direction by breed tritium gas-driven permeation (GDP) into the edge plasma. Deuterium and tritium PDP will complicate the recovery of tritium from the blanket, while tritium GDP will lead to an unwanted increase of particle recycling in the first wall region, which could even affect core confinement performance. Reduced activation ferritic/martensitic (RAFM) steels are widely proposed as candidate structural materials for the blanket of a DEMO reactor, the surface coatings made of tungsten are necessary to protect the plasma-facing wall from sputtering under high-energy particle bombardment. Therefore, the characterization of hydrogen isotopes transport through a multi-layer W + RAFM wall is of crucial importance to evaluate major reactor design issues including tritium retention, particle recycling and breeding feasibility, etc. This paper is intended to provide a review over the transport parameters of hydrogen isotopes in RAFM steels, including permeability, diffusivity, solubility and surface recombination coefficient. In addition, the present research status of hydrogen isotopes permeation and retention behavior of tungsten coated RAFM steels is briefly introduced.

Defect trapping of ion-implanted deuterium in nickel

Trapping of ion-implanted deuterium by lattice defects in nickel has been studied by ion-beam-analysis techniques in the temperature range between 30 and 380 K. The deuterium-depth profiles were determined by measuring either the α particles or the protons from the 3He-excited nuclear reaction D(3He,α)p, and the deuterium lattice location was obtained by means of ion channeling. Linear-ramp annealing (1 K/min) following a 10-keV D+ implantation in nickel produced two annealing stages at 275 and 320 K, respectively. The release-vs-temperature data were analyzed by solving the diffusion equation with appropriate trapping terms, yielding 0.24 and 0.43 eV for the trap-binding enthalpies associated with the two stages, referred to as an untrapped solution site. The 0.24-eV trap corresponds to deuterium close to the octahedral interstitial site where it is believed to be trapped at a vacancy, whereas it is suggested that the defect correlated with the 0.43-eV trap is a multiple-vacancy defect. The previously air-exposed and electropolished nickel surface was essentially permeable; the surface-recombination coefficient was determined to be K≳10−19 cm4/s at 350 K.