Refractory Metal Surfaces Research Articles

Research progress in modification of MoSi2 coatings on surface of refractory metals and their alloys: a review

Research progress in modification of MoSi2 coatings on surface of refractory metals and their alloys: a review

The Effect of Aluminizing Nb Refractory Metal Surface on Increasing Oxidation Resistance

In order to increase oxidation resistance of Nb refractory metal, Aluminizing and Aluminizing + Siliconizing with co-deposition were applied to its surface through the method of Halide-activated pack cementation (HAPC). The characterization of diffusion coating was conducted via the analyses of SEM, XRD, and EDS. After that, oxidation tests were conducted, and the effect of coating on oxidation resistance was examined. As a result, according to XRD analysis, it was found that there were Nb5Si3 phase structures in both coatings. In addition to this, Nb5Si3 and Nb3Si phases emerged in co-deposition coating. Protective NbSi2 phase was not observed because an activator with fluoride was used but an activator with chloride was not. Both methods increased the oxidation resistance of Nb. However, it was observed that the target value which was “metal loss of 25μm at 1300°C 100 hours after oxidation” was not obtained only through cementation aluminizing process. We are indebted to TUBITAK for their support. Keywords: Refractory metals, oxidation, pack cementation, co-deposition, aluminizing DOI: 10.7176/JSTR/5-10-12

A method for measuring the high temperature emittance of refractory metal surfaces

For high temperature applications, such as lighting components or heating elements, the thermal emittance of refractory metals such as tungsten can be improved by altering the surface of the material. Laser structuring or porous sintered coatings have been shown to increase the emittance, which leads to improvements in process stability and component lifetime.Measuring the emittance is generally performed at room temperature by reflectometry as high temperature measurement is extremely difficult. This has to be taken into account when comparing surface modifications, as emittance at application temperature might differ. In this paper, a method is described with which samples are heated by a direct current flow and the emittance is measured with an optical pyrometer by comparing temperature values with the actual surface temperature.The method allows a comparative measurement of different surface treatments at the application temperature. Degradation of the surface emittance at the application temperature is also possible to observe. Measured values for tungsten with different surface modifications are shown and discussed.

Microwrinkle structures on refractory metal surfaces irradiated with noble gas plasma species

Microwrinkle structures with a pitch of less than 100 nm and up to 600 nm on refractory metals like tungsten (W) and molybdenum, which are irradiated by noble gas ions such as neon and helium, have been studied systematically. The wrinkle formation mechanism is thought to be a buckling of the hard surface layer supported by the soft elastic substrate, which is induced by a penetration of noble gas species from the irradiated surface. Microwrinkle forms on this structure under lateral compressive strain/stress fields coming from thermal constriction on the way to substrate cooling. Such a process should be anticipated when walls in a fusion reactor are attacked by heat pulses such as edge localized modes and/or vertical displacement events, and therefore might be an initial stage of W surface damage.

Nanostructuring of Refractory Metal Surfaces by Electrochemical Oxidation: Nb and the Binary Systems Ti-Ta and Nb-Ta

Nanostructuring of Refractory Metal Surfaces by Electrochemical Oxidation: Nb and the Binary Systems Ti-Ta and Nb-Ta

Nanostructuring of Refractory Metal Surfaces by Electrochemical Oxidation: Nb and the Binary Systems Ti-Ta and Nb-Ta

Metal oxide nanotubes were produced on Ti, Nb, and TiTa and TiNb alloy surfaces by anodic oxidation in fluoride containing electrolytes. Tube diameters from 7 to 110 nm and tube lengths from 50 nm to 1.6 μm were achieved, depending on the substrate material, the anodization parameters, and the composition of the electrolyte. With the application of a voltage of 20 V for 1 h to Nb substrates tubes with 124.3±8.3 nm length were formed, while the same anodization conditions resulted in a tube length of only 66.7±14.2 nm, when applied in a glycol based electrolyte. While there was a linear connection between tube diameter and voltage for Ti anodization, Nb showed limits in the achievable tube dimensions. Furthermore, significant influences on the ion transport mechanisms were found for variations of electrolyte viscosity and stirring frequency, which highly affected the morphology and the size distribution of the obtained nanotubes. Very different anodization behaviour was found for alloys of Ti with Nb resp. Ta, leading to the conclusion that the Pilling- Bedworth ratio must be kept at an average value below 2.44, if fabrication of isolated nanotubes on an alloy surfaces should be successful. Keywords: Nanotubes, anodization, self-organization, metal oxide, Pilling-Bedworth ratio

Nanostructure Formation on Refractory Metal Surfaces Irradiated by Helium Plasmas

Nanostructure Formation on Refractory Metal Surfaces Irradiated by Helium Plasmas

Development of Functional Scales for Joining of Divertor Components Based on Electrochemical Plating Technology

In fusion technology, functional scales are required for various application fields like first wall tungsten coating, anti-corrosion or tritium permeation barriers, and brazing layers in joining technology. Established processes for layer deposition exhibit several kinds of drawbacks ranging from difficulty controlling layer thickness, inhomogeneity of coatings, application limits because of geometrical reasons, or critical thermal loading. Inherently, electrochemical plating technology does not exhibit these critical features. Growing of galvanic layers depends on the transported charge and thus can easily be controlled by current flow and/or deposition time. The main part of this development work was focused on voltammetric analyses to assist the deposition of transition metals on refractory metal surfaces, e.g., tungsten and Eurofer steel, and to deliver boundary conditions for electrolytes. Typical elements that can be used in joining may range from Ti, V by Ni, Fe up to Pd, and Cu. However, a direct joining of tungsten onto Eurofer steel by copper is metallurgically impossible due to missing miscibility of copper with tungsten. Thus, interlayers with an active functionality are required, which interact with both bulk components and filler to obtain a sound braze joint brazing. For both W-W and W-Eurofer joints, demonstrators were successfully fabricated and analyzed by metallurgical and physical methods.

Quantum confinement effects in thin metal layers on the surface of single crystals and their analysis

This paper presents a brief overview of the results of the studies of quantum confinement effects in thin layers of metals (Au, Ag) on the surface of refractory metals (W(110), Mo(110)) and d metals (Ni(111)) starting with submonolayer coverages up to a thickness of about ten monolayers, which are performed using angle-resolved photoelectron spectroscopy. Series of experimental spectra of quantum-well electronic states and their variation with increasing thickness of the films for both sp and d valence states are presented. A theoretical analysis of the energies of forming quantum-well states is carried out in terms of the classical phase model and the extended phase model with the use of elements of the linear atomic chain approximation. It is shown that theoretical estimates agree well with experimental data.

Laser synthesis of nitrides on the surface of refractory metals immersed in liquid nitrogen

In this paper we describe an approach for the formation of composite layers on the surface of refractory metals. We show that laser radiation on refractory metals (Ti, V, Zr, Mo, Hf, Ta, and W) immersed in liquid nitrogen can provide a chemical synthesis of nitride phases on the surface of metals. The metals were subjected to pulsed laser radiation with a wavelength of 1.06 μm. The power density ranged from 10 4 to 10 9 W cm −2. The synthesis of nitrides began with the formation of Me x N y ( x > y) phases with low contents of nitrogen. When the melting point was reached at the metal surface, the quantity of MeN phases increased sharply. Study of the melting zone showed that it contained a non-uniform distribution of nitride phases. The quantity of nitrides was a maximum on the surface and decreased with the increase of the depth of melting zone. Due to the high-cooling rates, titanium nitride crystallized in the form of columns. Maximum microhardness in the Ti surface layer was up to 20,000 MPa.

Morphological evolution in oxygen-induced faceting ofRe(123¯1)

We have studied oxygen-induced faceting of the atomically rough $\mathrm{Re}(12\overline{3}1)$ surface by means of Auger electron spectroscopy, low energy electron diffraction, and scanning tunneling microscopy (STM). In contrast to previous faceting studies on other refractory metal surfaces, where simple morphologies of the facets were reported, we find a coverage-dependent morphological evolution of the facets ranging from long sawtooth ridges to complex structures exposing four different facets. The faceting occurs only when oxygen coverage $(\ensuremath{\theta})$ exceeds 0.5 monolayer (ML) and the surface is annealed at $\ensuremath{\ge}700\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. At low oxygen coverage $(0.5\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l0.7\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$, the $\mathrm{O}∕\mathrm{Re}(12\overline{3}1)$ surface becomes partially faceted upon annealing; further increasing of oxygen coverage $(0.7\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l0.9\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$ causes the surface to become completely faceted, forming long sawtooth ridges along the $[\overline{2}113]$ direction with typical dimensions of $\ensuremath{\sim}8\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in width and $g50\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ in length upon annealing at $1000\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The size of the ridges grows with annealing temperature and annealing time, and the distance between the ridges is quite uniform. The two sides of each ridge have $(01\overline{1}0)$ and $(11\overline{2}1)$ orientations, and atomic-resolution STM images reveal that the edge of the ridge is atomically sharp. For $0.9\phantom{\rule{0.3em}{0ex}}\mathrm{ML}\ensuremath{\le}\ensuremath{\theta}l1\phantom{\rule{0.3em}{0ex}}\mathrm{ML}$, a third set of facets, identified as $(10\overline{1}0)$, emerges and truncates the original ridges. With the surface fully covered by oxygen $(\ensuremath{\theta}=1\phantom{\rule{0.3em}{0ex}}\mathrm{ML})$, a fourth facet $(01\overline{1}1)$ also becomes prominent upon annealing. This morphological evolution is accompanied by a reduction of the average ridge length along $[\overline{2}113]$, indicating that the $(11\overline{2}1)$ facet is metastable. Our work demonstrates that even in a simple adsorbate/substrate system, the adsorbate-induced modification of the anisotropy of surface free energy can induce a complex sequence of changes in the surface morphology. The faceted Re surfaces may be model systems to study structure sensitivity in catalytic reactions, and may also provide promising templates to grow nanostructures.

Catalytic surface oxidation by rare-earth metals: A photoemission study ofGd- andY-promoted oxidation ofW(110)

Catalytic oxidation of the substrate has been observed for rare-earth metals on refractory metal surfaces, where the mechanism responsible has been attributed to the ability of the rare earth to form oxides with more than one oxidation state. It has been postulated that, as a result, only rare earths with more than one oxidation state will exhibit catalytic activity, at least in terms of surface oxidation. We show, using high-resolution core-level photoemission, that submonolayer coverages of $\mathrm{Gd}$ and $\mathrm{Y}$, two rare-earth metals with only one stable oxide each, promote catalytic oxidation of $\mathrm{W}(110)$. Initial oxygen dosing oxidizes the rare-earth overlayer, followed by oxidation of $\mathrm{W}(110)$ at higher rates, and with higher resulting oxidation states, than are observed for oxygen dosing on the clean surface. We suggest that the lowering of the $\mathrm{W}(110)$ work function, due to the presence of the rare-earth metals and∕or their oxides, is responsible for the promoted oxidation. Catalytic activity of rare-earth metals is therefore not limited to those with more than one oxidation state.

The competition between aluminum and group IV (Si, C) atoms on the rhenium surface

Electropositive atoms of aluminum adsorbed on a (\({{P_{As_4 } } \mathord{\left/ {\vphantom {{P_{As_4 } } {P_{Ga} }}} \right. \kern-\nulldelimiterspace} {P_{Ga} }} = \gamma \))Re surface exhibit competition with non-metals (Si, C) for the adsorption sites at high temperatures (1200–1500 K). In this system, silicon displaces aluminum, while aluminum displaces carbon. The mechanism of this competition is fully analogous to that observed previously for Si, C, and S atoms on the surface of refractory metals, despite the fact that aluminum atoms (in contrast to the nonmetals) in the adsorbed state possess a positive charge.

Formation of intermetallic layers on the surface of refractory metals in a lead melt under conditions of mass transfer

We investigate the kinetics of the interaction between refractory metals (Mo, Nb, and V) and Kh18N10T steel under the conditions of concentration mass transfer by a lead melt at a temperature of 1000°C. In the experiment, the sealed ampul made of austenitic steel is a source of nickel, iron, and chromium. Refractory metals are practically insoluble in lead. Among the components of steel, nickel is transported first (due to the highest solubility) and forms a continuous nickel layer on the surface of refractory metals (Me). In the course of time, the mass transfer of nickel stops owing to a decrease in its concentration in the surface layers of steel, while iron and chromium are transported further and deposit on the nickel substrate. The conditions of dissolution of the components and the formation of intermetallic compounds in the case of interdiffusion of the components on the Me−Ni and Fe(Cr)−Ni interfaces are considered. It is shown that, at the initial stage of the interaction, the nickel interlayer facilitates the interdiffusion of a refractory metal and the transported components (Fe, Cr). The intermetallic layer forms gradually in a rather wide concentration range, which is associated with stress relaxation. It adheres well to the matrix and is free of cracks, flakes, and other damages, caused by abrupt volume changes on the formation of intermetallic compounds.

Charge state of alkali atoms scattered by Mo and W surfaces

From an experimental study it has been established that the charge states of particles scattered by the surface of refractory metals (W, Ta, Mo) during bombardment by neutral atom and ion beams of alkali metals (Li, Na, K), do not depend on charge states of the primary particles. It has been shown that angular and energy distributions of atoms and ions scattered from the bombarded surface of both ion and atom beams are similar to each other. A method for the direct detection of the neutral fraction yield of particles scattered from metal surface has been developed. In this method a second (auxiliary) target surface was used which had a higher work function than the target surface under investigation. In the yield of scattered particles, the ratio of the ionized component to the neutral component is the same for both target surfaces. This fact has made it possible to measure the yields of neutral particles. Thus, the scattering coefficients for ion and neutral fractions of scattered alkali atoms were determined. The measured and calculated dependencies of the ionization degree of η + for alkali particles are compared.

Sticking probability of metal halide molecules incident upon refractory metal surfaces heated in high vacua

The sticking probability (σ) of metal halides (MX, for example, NaBr, CsF and TlCl) impinging with various fluxes (10 10–10 14 molecules cm −2 s −1) upon polycrystalline surfaces of refractory metals (Re and W) heated to a high temperature ( T S, 900–2300 K) in readily attainable high vacua (10 −5–10 −3 Pa) was determined by a thermal positive ionization method. Namely, (1) a molecular beam of MX effusing from a Knudsen cell heated to various temperatures ( T C, 550–900 K) was directed onto the surface, (2) the ion current ( I +) of positive ions (M +) emitted from the surface was measured as a function of either T S or T C, (3) the apparent vapor pressure ( σP) of MX in the cell was evaluated from the data on I + and T C according to our method and (4) our data on σP was compared with literature values of P determined by various methods. In all the sample/surface systems thus studied, σ was essentially unity irrespective of the condition whether the surface was virtually clean or fully covered with residual gases below 10 −3 Pa.

Diamond deposition at low substrate temperatures

Three different experimental approaches are reported for the deposition of diamond at low substrate temperatures. These are a remote thermal CVD technique, and two different means for achieving remote microwave plasma enhanced CVD. Using these approaches it has been shown that diamond can he deposited at low temperatures and on potentially reactive substrates such as fused silica. In addition it is clearly shown that the presence of the substrate within a plasma discharge or immediately adjacent to a hot refractory metal surface is not a prerequisite for the crystallization of diamond from hydrocarbon-hydrogen mixtures. Well crystallized as well as nanocrystalline diamond coatings were obtained at substrate temperatures as low as 430 to 450 °C which offers significant promise for future developments. Growth rates varied with the method used, from 200 to 300 A/h for well crystallized diamond to 0.5 to 1.0 ltm/h for nanocrystalline coatings.

Surface Residence Time Measurements of Ba Atoms by Molecular Beam Relaxation Spectrometry

Molecular beam relaxation spectrometry has been applied to measure the residence time of Ba atoms on such refractory metal surfaces as W and W-26Re, which are commonly used as substrates for dispenser cathodes. It is found that there are at least two chemisorption states on both metal surfaces, which yield different residence times at the same temperature. The chemisorption energies of Ba on W-26Re surface are found to be 2.4 and 0.4 eV respectively. The dependency of the residence time (signal phase shift) on the Ba impinging flux suggests the existence of a third adsorption state. In the dispenser cathode operation temperature range, 900–1200°C, the role of the highest energy chemisorption sites is crucial in determining the surface coverage of the mono-atomic order layer, the electric dipole of which reduces the work function, thus enhancing electron emission of dispenser cathodes.

The influence of platinum implantation on the hydrogen embrittlement of tantalum

The present study is an attempt to investigate the hydrogen embrittlement of tantalum in order to explore the possibility of modifying the surface of refractory metals by ion beam techniques with respect to improved resistance against this form of degradation. Tantalum foils were implanted in the asdelivered state and also after anodizing in order to prepare a thicker defined oxide on the metal with 5 × 10 16 Pt + cm −2 at 80 keV. These specimens were charged cathodically with hydrogen and subjected to a bending test until fracture. The ratio of the number of bends until fracture with charging to the number of bends until fracture without charging was used as a measure of the degree of embrittlement. It turned out that specimens implanted in the as-delivered state were less resistant to hydrogen than were unimplanted samples with undamaged oxide films. However, implanted specimens with oxide layers thicker than 100 nm showed an increased resistance to embrittlement compared with that of unimplanted specimens. This behaviour is discussed in terms of desorption catalysis which is only effective if the implanted platinum has no direct contact with metallic tantalum. A good long-term protective effect is expected for the systems studied.

Current densities and closure rates in diodes containing laser-driven, cesium-coated thermionic cathodes

Cathodes emitting electron current pulses of 102–103 A/cm2 for durations of longer than 1 μs, with very low emittance, are desirable for free electron laser and high-power microwave tube applications. Not all of these criteria can be satisfied by either dispenser thermionic cathodes or plasma forming field emitters. However, pulse-laser heated smooth cesiated refractory metal surfaces have the potential to achieve these conditions. In this paper, test results from low-voltage (2–20 kV) diodes containing cesiated tungsten cathodes are discussed. Space-charge-limited current densities of up to 30 A/cm2 were obtained in fields with maximum strengths of 40 kV/cm. Plasma closure rates at these currents were measured to be 0.17–0.4 cm/μs. Such slow velocity fronts are postualted to be due to the domination of the plasma by massive cesium ions generated from species desorbed from the cathode, rather than to light hydrogen ions (as in the case of field-emission-type cathodes).