Improved electrical properties of silicon-incorporated anodic niobium oxide formed on porous Nb-Si substrate

In the present study, porous Nb-Si alloy films with isolated nano-column morphology have been successfully developed by oblique angle magnetron sputtering on to aluminum substrate with concave cell structure. The deposited films are amorphous with the 15 at% silicon supersaturated into niobium. The porous Nb-15 at% Si films, as well as niobium films with similar morphology, are anodized at several voltages up to 50 V in 0.1 mol dm⁻³ ammonium pentaborate electrolyte. Due to the presence of sufficient gaps between neighboring columns, the gaps are not filled with anodic oxide, despite the large Pilling-Bedworth ratio (for instance, 2.6 for Nb/Nb₂O₅) and hence, a linear correlation between the reciprocal of capacitance and formation voltage is obtained for the Nb-15 at% Si. From the comparison with the anodic films formed on porous niobium films, it has been found that silicon addition improves the thermal stability of anodic niobium oxide; the change in capacitance and increase in leakage current become small for the Nb-Si. The findings indicate the potential of oblique angle deposition to tailor porous non-equilibrium niobium alloy films for high performance niobium-base capacitor.     

Adhesion of niobium films to differently oriented α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> surfaces

The atomic and electronic structures of the Nb/α-Al₂O₃ interface are studied by the electron density functional method. The structural and electronic properties of three corundum surfaces, as well as chemical bonds produced by metallic niobium films at variously oriented interfaces, are discussed. Relations between the electronic structure, geometry, and mechanical properties of the interfaces are analyzed. It is shown that the adhesion of niobium films to a great extent depends on the type of oxide surface.     

Improving Fe3Al alloy resistance against high temperature oxidation by pack cementation process

The paper presents the results of oxidation tests of Fe₃Al-based alloys containing additions of Cr, Zr, B, and C, with and without an aluminide coating. The coating was formed by a pack cementation process in which the surface of material got enriched in aluminum. The Al-rich layer was intended to enhance the tendency of Al₂O₃ formation. The slow-growing Al₂O₃ scale provides the best corrosion protection for structural materials at high temperatures. The cyclic oxidation tests were performed in laboratory air at 1373 K. The structure and composition of oxide scales as well as their adherence were evaluated and compared for the materials with and without aluminide coatings. Surface enrichment in aluminum and effect minor addition of Zr on oxidation behavior was discussed.     

Point-defect properties of and sputtering events in the {001} surfaces of Ni3Al I. Surface and point-defect properties

Constant-area and fully relaxed molecular dynamics methods are employed to study the properties of the surface and point defects at and near {001} surfaces of bulk and thin-film Ni, Al and Ni₃Al respectively. The surface tension is larger than the surface energy for all {001} surfaces considered in the sequence: Al (1005?mJ?m^?2)<?Ni₃Al (mixed Ni–Al plane outermost, 1725?mJ?m^?2)<?Ni₃Al (all-Ni-atoms plane outermost, 1969?mJ?m^?2)<?Ni (1993?mJ?m^?2). For a surface of bulk Ni₃Al crystal with a Ni–Al mixed plane outermost, Al atoms stand out by 0.0679?Å compared with the surface Ni atoms and, for the all-Ni-atoms surface, Al atoms in the second layer stand out by 0.0205?Å compared with Ni atoms in the same layer. Vacancy formation energies are about half the bulk values in the first layer and reach a maximum in the second layer where the atomic energy is close to the bulk value but the change in embedding energy of neighbouring atoms before and after vacancy formation is greater than that in the bulk. Both the vacancy formation energy and the surface tension suggest that the fourth layer is in a bulk state for all the surfaces. The formation energy of adatoms, antisite defects and point-defect pairs at and near {001} surfaces of Ni₃Al are also given.     

Temperature dependent preferential sputtering in CoSi2 and NbSi2

Sputtering of CoSi₂ and NbSi₂ has been carried out by Xe ion bombardment at room temperature, as well as at elevated temperatures putting these systems in their radiation-enhanced diffusion regimes. The range of the Xe ions (at 200–260 keV) was appreciably less than the thickness of the silicides. The samples were analyzed by 2 MeV He⁺ backscattering spectrometry, x-ray diffraction and optical microscopy. The ratio of the sputtering yield of Si to that of the metal (i.e., Co or Nb) always exceeds the stoichiometric ratio 21, leading to Si depleted surface layers. The amount of the sputtered species increases almost linearly with dose until intermixing of the silicide with the underlying Si becomes appreciable. This happens at lower doses in the radiation-enhanced diffusion regime than at room temperature. Irradiation of CoSi₂ samples at high temperature leads to a broadening of the implanted Xe profile compared to the room temperature profile. No such phenomenon has been found in NbSi₂. The effect of Xe broadening on the sputtering yields is discussed.     

Molecular depth profiling of multi-layer systems with cluster ion sources

Cluster bombardment of molecular films has created new opportunities for SIMS research. To more quantitatively examine the interaction of cluster beams with organic materials, we have developed a reproducible platform consisting of a well-defined sugar film (trehalose) doped with peptides. Molecular depth profiles have been acquired with these systems using C₆₀⁺ bombardment. In this study, we utilize this platform to determine the feasibility of examining buried interfaces for multi-layer systems. Using C₆₀⁺ at 20 keV, several systems have been tested including Al/trehalose/Si, Al/trehalose/Al/Si, Ag/trehalose/Si and ice/trehalose/Si. The results show that there can be interactions between the layers during the bombardment process that prevent a simple interpretation of the depth profile. We find so far that the best results are obtained when the mass of the overlayer atoms is less than or nearly equal to the mass of the atoms in buried molecules. In general, these observations suggest that C₆₀⁺ bombardment can be successfully applied to interface characterization of multi-layer systems if the systems are carefully chosen.     

Deposition of potassium-oxygen on silicon surfaces by pulsed laser ablation of potassium superoxide: Study of work function changes

Potassium-oxygen species were deposited on pure, Si nanoparticles coated and H-terminated Si nanoparticles coated p-Si(1 0 0) surfaces by pulsed laser ablation of potassium superoxide (KO₂) target. The deposition properties, composition and the work function changes of the deposited species were investigated in situ using an X-ray photoelectron spectroscopy (XPS) and a Kelvin probe measurement. The deposited species were assigned to K₂O₂ and KO₂, and they can be selectively deposited by controlling the laser fluence: i.e., at 200 mJ/cm² and at those more than 300 mJ/cm², respectively. Experimental results showed that the work function decreased drastically with depositing of KO_x (x = 1 or 2), and the minimum work function values observed were 1.0 eV and 0.7 eV for pure p-Si(1 0 0) and Si nanoparticles coated substrates, respectively. The study demonstrates the formation of the surface species with minimum work function can be identified by XPS.     

Structural and electrical properties of AlN films deposited using reactive RF magnetron sputtering for solar concentrator application

AlN is an interesting material with some excellent properties like high hardness (>11 GPa), high temperature stability (>2400 °C), good electrical resistivity (>10¹⁰ Ω cm), and good thermal conductivity (>100 W/m K). These properties make it useful in the field of photo voltaic systems. Cooling of solar cells in solar concentrator application is of major concern because high temperature reduces their efficiency. In the present work we deposited AlN coating, with and without an Al interlayer, on various substrates like Si, quartz, and copper using RF magnetron sputtering. Deposition conditions such as Al interlayer (deposition time = 5-20 min), Ar:N₂ ratio (N₂% = 0-75%) and substrate bias (0 and −50 V) were changed in order to study their effect on coating properties. Coating surface roughness increased from 0.05 to 0.15 μm with increase in Al interlayer thickness. The coating thickness decreased from 4.4 to 3.1 μm with increase in N₂ gas % and films grew in (0 0 2) orientation. Films deposited on copper using Al interlayer showed good electrical resistance of ∼10¹³ Ω. Films deposited on copper without Al interlayer showed presence of voids or micro cracks and poor electrical properties. AlN films deposited at −50 V bias show cracking and delamination.     

Structure and adhesion of MoSi2/Ni interfaces: Evaluation of MoSi2 as an alternative bond coat alloy

We use density functional theory to evaluate the stability of molybdenum disilicide coatings on a nickel substrate, as a possible bond coat alloy for high temperature coating applications. We consider the MoSi₂(0 0 1)/Ni(1 1 1), MoSi₂(1 0 0)/Ni(1 1 1), and MoSi₂(1 1 0)/Ni(1 1 1) interfaces and predict quite strong (3.5-3.8 J/m²) adhesion of this metal-silicide ceramic to nickel. The origin of this strong adhesion is elucidated by examining the geometric and electronic structure of the interfaces. We predict that Mo and Si atoms at the interface primarily occupy Ni 3-fold hollow sites, the typical adsorption site on Ni(1 1 1). Projected local densities of states and electron density difference plots reveal a mixture of localized, covalent Si-Ni bonds and more delocalized metallic Mo-Ni bonding, as the origin of the strong interfacial bonding. As emphasized in our earlier work, creation of strong covalent bonds at interfaces results in very strong adhesion. Such strong adhesion makes MoSi₂ a potential candidate for use in thermal barrier applications, in conjunction with a yttria-stabilized zirconia topcoat.     

The role of adsorption of dodecylethyldimethylammonium bromide and benzyldimethyldodecylammonium bromide surfactants in wetting of polytetrafluoroethylene and poly(methyl methacrylate) surfaces

The role of adsorption of dodecylethyldimethylammonium bromide (C₁₂(EDMAB)) and benzyldimethyldodecylammonium bromide (BDDAB) at water-air and polytetrafluoroethylene (PTFE)-water and poly(methyl methacrylate) (PMMA)-water interface, in wetting of PTFE and PMMA surface, was established from the measured values of the contact angle (θ) of aqueous C₁₂(EDMAB) and BDDAB solutions in PTFE (PMMA)-solution drop-air system, and from the measured values of the surface tension of aqueous C₁₂(EDMAB) and BDDAB solutions. Adsorption of C₁₂(EDMAB) and BDDAB at water-air interface was determined earlier from the Gibbs equation. Adsorption at solid-water interface was deduced from the Lucassen-Reynders equation based on the relationship between adhesion tension (γ_LV cos θ) and surface tension (γ_LV). The slope of the γ_LV cos θ-γ_LV curve was found to be constant and equal to −1, and about −0.3 for PTFE and PMMA surface, respectively (in the case of both surfactant studied: C₁₂(EDMAB) and BDDAB, and in the whole range of surfactants concentration in solution). It means that the amount of the surfactant adsorbed at the PTFE-water interface, Γ_SL, was essentially equal to its amount adsorbed at water-air interface, Γ_LV. However, Γ_SL at the PMMA-water interface was about three times smaller as compared to that at water-air interface. By extrapolating the linear dependence between γ_LV cos θ-γ_LV and dependence between cos θ-γ_LV and cos θ = 1 we determined the value of the critical surface tension of PTFE and PMMA surface wetting, γ_c. The obtained values of γ_c for PTFE surface were equal 23.4 and 23.8 mN/m, 23.1 and 23.2 mN/m for C₁₂(EDMAB) and BDDAB, respectively and they were higher than the surface tension of PTFE (20.24 mN/m). On the other hand, the obtained values of γ_c for PMMA surface were equal 31.4 and 30.9 mN/m, 31.7 and 31.3 mN/m for C₁₂(EDMAB) and BDDAB, respectively and they were smaller than the surface tension of PMMA (39.21 mN/m). Using the values of PTFE and PMMA surface tension and the measured values of the surface tension of aqueous C₁₂(EDMAB) and BDDAB solutions in the Young equation, the PTFE (PMMA)-solution interfacial tension, γ_SL, was also determined. Next, the work of adhesion (W_A) was deduced, and it occurred that the dependence between the W_A and the surface tension (γ_LV) for both studied solids was linear. However, the values of the W_A for PMMA change as a function of log C (C—surfactant concentration) changed from 91.7 to 68.5 mJ/m² and from 91.8 to 65.1 mJ/m² for C₁₂(EDMAB) and BDDAB, respectively. On the other hand, the work of adhesion of both studied surfactants solutions to the PTFE surface was practically constant (an average value was equal 45.8 and 45.4 mJ/m², respectively). These values were close to the value of the work of water adhesion to PTFE surface (45.5 mJ/m²).     

Interfacial microstructures and impact properties for Nb/MOSi2 laminate composites

The purpose of present work is to investigate effects of fabricating temperature and ZrO₂, SiC and NbSi₂ addition on interfacial reaction layer and impact properties for Nb/MoSi₂ laminate composites. Four types of laminate composites alternating four layers of Nb foil with each MoSi₂, mixture layer containing ZrO₂. SiC and NbSi₂ particles were fabricated by hot pressing. The volume fraction of Nb foil involved in these system was nominally 10 vol%. It has been found that the impact value of Nb/MoSi₂ laminate composites decreased at a fabricating temperature higher than 1523 K, since the thickness of reaction layer between Nb and MoSi₂ increased along with fabricating temperature. However, the addition ofZrO₂ particles to Nb/MoSi₂ laminate composites fabricated at 1623 K resulted in a change of the interfacial microstructure as well as a reduction of the reaction layer. Nb/MoSi₂-ZrO₂ laminate composites maintained the same density as that of Nb/MoSi₂ laminate composites fabricated at 1773 K and showed a higher impact value than that of Nb/MoSi₂ laminate composites at 1523 K.     

Niobium based coatings for dental implants

Niobium based thin films were deposited on stainless steel (SS) substrates to evaluate them as possible biocompatible surfaces that might improve the biocompatibility and extend the life time of stainless steel dental implants. Niobium nitride and niobium oxide thin films were deposited by reactive unbalanced magnetron sputtering under standard deposition conditions without substrate bias or heating. The biocompatibility of the surfaces was evaluated by testing the cellular adhesion and viability/proliferation of human cementoblasts during different culture times, up to 7 days. The response of the films was compared to the bare substrate and pieces of Ti6Al4V; the most commonly used implant material for orthopedics and osteo-synthesis applications. The physicochemical properties of the films were evaluated by different means; X-ray diffraction, Rutherford backscattering spectroscopy and contact angle measurements. The results suggested that the niobium oxide films were amorphous and of stoichiometric Nb₂O₅ (a-Nb₂O₅), while the niobium nitride films were crystalline in the FCC phase (c-NbN) and were also stoichiometric with an Nb to N ratio of one. The biological evaluation showed that the biocompatibility of the SS could be improved by any of the two films, but neither was better than the Ti6Al4V alloy. On the other hand, comparing the two films, the c-NbN seemed to be a better surface than the oxide in terms of the adhesion and proliferation of human cemetoblasts.     

X-ray photoelectron spectroscopy of nano-multilayered Zr-O/Al-O coatings deposited by cathodic vacuum arc plasma

Nano-multilayered Zr-O/Al-O coatings with alternating Zr-O and Al-O layers having a bi-layer period of 6-7 nm and total coating thickness of 1.0-1.2 μm were deposited using a cathodic vacuum arc plasma process on rotating Si substrates. Plasmas generated from two cathodes, Zr and Al, were deposited simultaneously in a mixture of Ar and O₂ background gases. The Zr-O/Al-O coatings, as well as bulk ZrO₂ and Al₂O₃ reference samples, were studied using X-ray photoelectron spectroscopy (XPS). The XPS spectra were analyzed on the surface and after sputtering with a 4 kV Ar⁺ ion gun. High resolution angle resolved spectra were obtained at three take-off angles: 15°, 45° and 75° relative to the sample surface.It was shown that preferential sputtering of oxygen took place during XPS of bulk reference ZrO₂ samples, producing ZrO and free Zr along with ZrO₂ in the XPS spectra. In contrast, no preferential sputtering was observed with Al₂O₃ reference samples. The Zr-O/Al-O coatings contained a large amount of free metals along with their oxides. Free Zr and Al were observed in the coating spectra both before and after sputtering, and thus cannot be due solely to preferential sputtering.Transmission electron microscopy revealed that the Zr-O/Al-O coatings had a nano-multilayered structure with well distinguished alternating layers. However, both of the alternating layers of the coating contained of a mixture of aluminum and zirconium oxides and free Al and Zr metals. The concentration of Zr and Al changed periodically with distance normal to the coating surface: the Zr maximum coincided with the Al minimum and vice versa. However the concentration of Zr in both alternating layers was significantly larger than that of Al. Despite the large free metal concentration, the Knoop hardness, 21.5 GPa, was relatively high, which might be attributed to super-lattice formation or formation of a metal-oxide nanocomposite within the layers.     

Adsorption of sodium bis(2-ethylhexyl) sulfosuccinate and wettability in polytetrafluoroethylene-solution-air system

The role of sodium bis(2-ethylhexyl) sulfosuccinate (AOT) adsorption at water-air and polytetrafluoroethylene-water (PTFE) interfaces in wetting of low energy PTFE was established from measurements of the contact angle of aqueous AOT solutions in PTFE-solution drop-air systems and the aqueous AOT solution surface tension measurements. For calculations of the adsorption at these interfaces the relationship between adhesion tension (γ_LV cos θ) and surface tension (γ_LV), and the Gibbs and Young equations were taken into account. On the basis of the measurements and calculations the slope of the γ_LV cos θ-γ_LV curve was found to be constant and equal −1 over the whole range of surfactant concentration in solution. It means that the amount of surfactant adsorbed at the PTFE-water interface, Γ_SL, is essentially equal to its amount adsorbed at water-air interface, Γ_LV. By extrapolating the linear dependence between γ_LV cos θ and γ_LV to cos θ = 1 the determined value of critical surface tension of PTFE surface wetting, γ_C, was obtained (23.6 mN/m), and it was higher than the surface tension of PTFE (20.24 mN/m). Using the value of PTFE surface tension and the measured surface tension of aqueous AOT solution in Young equation, the PTFE-solution interface tension, γ_SL, was also determined. The shape of the γ_SL-log C curve occurred to be similar to the isotherm of AOT adsorption at water-air interface, and a linear dependence existed between the PTFE-solution interfacial tension and polar component of aqueous AOT solution. The dependence was found to be established by the fact that the work of adhesion of AOT solution to the PTFE surface was practically constant amounting 46.31 mJ/m² which was close to the work of water adhesion to PTFE surface.     

Influence of preparation technology on the microstructure and anti-oxidation property of SiC-Al2O3-mullite multi-coatings for carbon/carbon composites

Oxidation protective SiC-Al₂O₃-mullite multi-coatings for carbon/carbon (C/C) composites were prepared with a two-step pack cementation process. The influence of preparation temperature and SiO₂/Al₂O₃ ratio of the pack powder on the phase, microstructure and oxidation resistance of the multi-coatings were investigated. It showed that the multi-coatings that contained mullite could be produced at 1700-1800 °C. A denser coating surface was acquired with the decrease of SiO₂/Al₂O₃ ratio in the pack chemistries while a little damnification to the interface of the coating and C/C substrate. The as-prepared coating could effectively protect C/C composites from oxidation at 1600 °C for 81 h.     

Electronic structures of Al–Si clusters and the magic number structure Al8Si4

The low-energy structures of Al₈Si_m (m = 1–6) have been determined by using the genetic algorithm combined with density functional theory and the Second-order Moller-Plesset perturbation theory (MP2) models. The results show that the close-packed structures are preferable in energy for Al–Si clusters and in most cases there exist a few isomers with close energies. The valence molecular orbitals, the orbital level structures and the electron localisation function (ELF) consistently demonstrate that the electronic structures of Al–Si clusters can be described by the jellium model. Al₈Si₄ corresponds to a magic number structure with pronounced stability and large energy gap; the 40 valence electrons form closed 1S²1P⁶1D¹⁰2S²1F¹⁴2P⁶ shells. The ELF attractors also suggest weak covalent Si–Si, Si–Al and Al–Al bonding, and doping Si in aluminium clusters promotes the covalent interaction between Al atoms.     

High electronic specific heat of some cubic UX3 intermetallic compounds

The low temperature specific heat of cubic UX₃ intermetallic compounds with X = Al, Ga, In, Si, Ge and Sn have been measured. High values for the coefficient of the electronic specific heat have been found, ranging from 14 to 169 mJ/mol K².     

Nuclear charge radii of Al and Si from elastic electron scattering between 25 and 60 MeV

Elastic electron scattering cross sections of²⁷Al and Si (natural isotopic mixture) have been measured relative to carbon. The rms charge radiiR _m , deduced with partial wave calculations, are (3.01±0.05) fm for²⁷Al and (3.06±0.05) fm for Si, in good agreement with results from muonic X-ray energies. The values given are those for a Fermi charge distribution with skin thickness 2.5 fm; harmonic oscillator shell model distributions yield radii smaller by 0.03 fm. The ratioR _m (²⁷Al)/R_m(Si) is 0.984±0.016.     

First-principles study of electronic properties and stability of Nb5SiB2（001） surface

The density functional calculations are performed to study the electronic structure and stability of Nb 5 SiB 2(001) surface with different terminations.The calculated cleavage energies along the(001) planes in Nb 5 SiB 2 are 5.015 J · m 2 and 6.593 J · m 2 with the break of Nb-Si and Nb-NbB bonds,respectively.There exists a close correlation between the surface relaxation including surface ripple and the cleavage energy:the larger the cleavage energy,the larger the surface relaxation.Moreover,the surface stability of the Nb 5 SiB 2(001) with different terminations has been investigated by the chemical potential phase diagram.From a thermodynamics point of view,the four terminations can be stabilized under different conditions.In chemical potential space,NbB(Nb) and Nb(Si) terminations are just stable in a small area,whereas Si(Nb) and Nb(NbB) terminations are stable in a large area(the letters in brackets represent the subsurface atoms).     

Studies on hydrogen diffusivity in iron aluminides using the Devanathan–Stachurski method

An electrolytic method for cathodic hydrogen saturation developed by Devanathan and Stachurski was successfully used to study hydrogen diffusivity in iron aluminides. Both an appropriate electrolyte and a saturating current density are required for this method. A proper form of the saturation curve was only obtained with 35% NaCl electrolyte, which removed the oxide film blocking hydrogen penetration without further corrosive destruction of the iron aluminide. The optimum saturation current density for determining the most reliable diffusivity was 1.91 A/cm², yielding D_eff,H = 4.81 × 10⁻⁶ cm²/s for Fe–40 at.% Al.