A high-temperature Auger electron spectrometer setup and its application to reactive wetting experiments at 1700 K

A high-temperature Auger electron spectroscopy setup and its in situ application to sessile drop experiments of molten silicon on oxide substrates are presented. The experimental setup allows for measurements of previously inaccessible surface reactions at temperatures up to 1700 K. Auger electron spectra of SiO₂, MgO, and liquid Si are presented. Furthermore, the areas of the substrates that have been transiently wetted by the silicon melt are investigated. The results are discussed with respect to questions concerning reactive wetting of oxides by metal melts, which are important for the material science of joining processes.     

Wetting and infiltration of volatile fluorides by In-Ti melt

In this study, the effect of the volatile nature of thermodynamically stable fluorides (MgF₂, CaF₂, and BaF₂) on wetting and infiltration phenomenon is discussed. Specially designed sets of experiments under different conditions of fluoride vapor evacuation were conducted. Experiments with In-Ti (~1 at.%) drops covered by a small cap and with artificially closed and open capillaries suggested that under restricted conditions of vapor evacuation, a thin solid layer is formed on the surface of the liquid metal, mechanically preventing both the spreading of In-Ti melt at the surface of the dense substrates and liquid penetration into the porous preforms.     

Preparation of Thin Lead Stannate Layers on Single-Crystal Silicon

Thin films containing lead stannates were prepared on single-crystal silicon substrates by annealing Sn/Pb/Si and Pb/Sn/Si structures produced by magnetron sputtering with the use of solid Sn targets and solid or liquid (self-sputtering) Pb targets. The structures were annealed in flowing oxygen for 10 min at temperatures in the range 520–1120 K. The phase composition of the films is found to strongly depend on the Pb deposition procedure, the sequence of layers in the as-grown heterostructure, and thermal oxidation conditions.     

Physical mechanism of the wetting of solid surfaces

This article examines physical principles in the theory of wetting. Wetting angles are calculated by using isotherms of the disjoining pressure of wetting liquid films on solid substrates.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 53, No. 5, pp. 795–802, November, 1987.     

Flux-assisted infiltration of liquid Al-6063 into TiC beds in air

This study details trials to produce aluminium metal matrix composites reinforced with TiC particles by means of a flux-assisted infiltration technique. Whilst no infiltration of TiC beds occurred, by using a K-Al-F flux infiltration was successful at temperatures as low as 680°C. Some reaction of TiC with the Al matrix, forming TiAl_2.3Si_0.1 and Al₄C₃, was observed in the microstructure along with flux trapped within the Al-6063 matrix. DSC showed exothermic oxidation of TiC to occur, until the flux melts at 545°C arresting and preventing further oxidation by spreading over, coating and cleaning the particle surfaces. As soon as the flux melts, it also starts dissolving the oxide layer on the Al alloy and prevents any re-oxidation by isolating the surface from the surrounding atmosphere. Sessile drop experiments suggest that when the alloy melts and the oxide layer has been dissolved by the flux, intimate contact occurs between the liquid and the particles. The low tensions for the solid/flux and liquid metal/flux interfaces facilitates spreading and wetting of liquid Al on the TiC particles, followed by infiltration of the bed and the displacement of the flux to the outer surfaces of the sample.     

Oxygen influence on ceramics wettability by liquid metals: Ag/α-Al2O3—Experiments and modelling

A renewed interest in the effects of oxygen on the wetting of solid oxides has recently risen in connection to the development of the technique of “air brazing” which makes use of the strong effect of oxygen to increase the wettability of oxides by means of processes running in air or in atmospheres with high oxygen content. Adsorption of oxygen not only at the liquid–vapour surface but also at the solid–liquid interface has been postulated by many previous researches, mainly on the basis of thermodynamic considerations. Along the same line, new results of the wetting behaviour in the system Ag/α-Al₂O₃ as a function of oxygen partial pressure are presented, with the simultaneous measurement of the liquid surface tension. These results are compared with the existing ones, and discussed on the basis of thermodynamic principles. The resulting work of adhesion is compared with the “work of separation” computed by the density functional theory (DFT) approach. DFT calculations are also employed, at variance with previous models, to investigate the structures that are formed at the interface upon addition of oxygen in different sites energetics, atomic and electronic properties of this oxygen-rich interface are discussed together with the connection with experiments.     

Reactive wetting in metal/metal systems: Dissolutive versus compound-forming systems

Reactive wetting in metal–metal systems involves complex interactions between the molten phase and the solid substrate. The simplest interaction involves only the dissolution of the substrate into the molten phase. The more complex interaction involves both dissolution of the substrate and compound formation at the solid–liquid boundary. The fundamental differences between these two types of reactive wetting are identified by studying the purely dissolutive system Bi–Sn and the compound-forming system Au–Sn. Experiments employ the traditional sessile drop technique as well as a novel two-dimensional drop transfer technique that enables real-time visualization of the solid–liquid interface evolution. Recent results from wetting of pure Sn on Au-coated Cu substrates are also presented and reveal a much richer wetting behavior than either Sn on Au or Sn on Cu binary systems.     

Kinetic and electrical characterization of thin silicon oxide films obtained by electron cyclotron resonance plasma

We report the use of ECR plasma to grow and deposit silicon oxides ranged between 2.5 and 10 nm with rates between 0.04 and 2.0 nm s^–1. The thickness of the films, well grown or deposited, were characterized by means of a spectroscopic ellipsometer, operating in the 1.5–4.5 eV range. Uniformities of thin oxide films were better than 4% on 3 silicon wafers, and ranged from 4% to 6% between different runs. The concentrations of the different chemical species during the first stages of the plasma formation were followed by optical emission spectroscopy to give some insight into the origin of the properties of the different thin SiO _x films. FT-IR spectra show bands with larger FWHM in ECR oxides than in the thermal ones, but the main peak positions were close to one another in the spectra of the three kinds of oxides. The observed mode strength for the LO₄-TO₄ pair of each spectrum is the main aspect that distinguishes the deposited from the grown film, indicating unequal degrees of disorder in both oxides. C–V measurements were performed in MOS capacitors. In the as-deposited SiO _x films a positive net oxide charge density similar to the one in the thermal SiO₂ films is observed. It is worth noting that a post-metallization anneal causes a reduction of the initial net charge density and even a sign change, depending on the treatment conditions.     

Thermal oxidation of transition metal silicides

A survey has been made of most publications on the oxidation of transition metal silicide films on either silicon or an SiO₂ substrate.On silicon substrates the general trend is that SiO₂ forms on the sample surface in preference to metal oxides, with the silicide layer being morphologically preserved. Thermodynamics, in terms of heats of formation and ternary phase diagrams, has been used successfully to explain the general absence of metal oxides and also to explain the exceptions to that rule. Kinetics also plays a part in the determination of the reaction products. The growth rate of SiO₂ on silicon substrates obeys the linear-parabolic law. The parabolic rate constant of silicide oxidation is essentially the same as that of silicon oxidation, indicating that the oxide and its diffusivity for the oxidant are the same for silicon and silicides. However, the linear rate constant of silicides exceeds that of silicon, and its value varies with the silicide. The differences among silicides might be attributed to differences in the atomic transport processes within the silicide; the enhancement with respect to silicon has been ascribed to the metallic nature of the silicides.On SiO₂ substrates, the oxidation ultimately leads to the formation of metal oxides as well. Instabilities of structure and loss of material can occur.The properties of the grown SiO₂ are reviewed and directions for further studies are outlined.     

Fabrication and characterisation of Al–7Si–0.35Mg/fly ash metal matrix composites processed by different stir casting routes

In the present investigation, the effect of three different stir casting routes on the structure and properties of fine fly ash particles (13 μm average particle size) reinforced Al–7Si–0.35Mg alloy composite is evaluated. Among liquid metal stir casting, compocasting (semi solid processing), modified compocasting and modified compocasting followed by squeeze casting routes evaluated, the latter has resulted in a well-dispersed and relatively agglomerate and porosity free fly ash particle dispersed composites. Interfacial reactions between the fly ash particle and the matrix leading to the formation of MgAl₂O₄ spinel and iron intermetallics are more in liquid metal stir cast composites than in compocast composites.     

Electrical properties of r.f. magnetron sputtered BaxSr1−xTiO3 films on multi-layered bottom electrodes for high-density memory application

Ba_xSr_1–xTiO₃ (BST) thin films have been deposited by r.f. magnetron sputtering on silicon and platinum-coated silicon substrates with different buffer and barrier layers. Electrical properties of BST films have been evaluated using both metal–insulator–semiconductor (MIS) and metal–insulator–metal (MIM) structures. MIS capacitor C–V and G–V characteristics have been utilized to determine the fixed charge density, interface trap density and the trap distribution in the silicon bandgap. BST films deposited on Si/SiO₂/SiN/Pt and Si/SiO₂/Ti/TiN/Pt multilayer bottom electrodes have been used for the fabrication of MIM capacitors. The role of bottom electrode, processing temperature and Ba to Sr ratio on the electrical properties of Ba_xSr_1–xTiO₃ films have been investigated. Current–voltage behavior has indicated an ohmic nature at lower voltages and Poole–Frenkel conduction at higher voltages. Deposited films have shown an excellent time-dependent dielectric breakdown under constant-current stressing.     

Wettability and reactivity between B4C and molten Zr55Cu30Al10Ni5 metallic glass alloy

The isothermal wetting and spreading behaviors of molten Zr₅₅Cu₃₀Al₁₀Ni₅ metallic glass alloy on B₄C substrates were studied using a modified sessile drop method at 1133–1253 K in a high vacuum. A distinct reaction layer consisting of ZrB₂ and ZrC_x was produced at the interface and displayed good wettability with the molten alloy. The entire spreading kinetics could be characterized by four representative stages: (i) an initial rapid spreading presumably driven by adsorption of the active Zr atoms at the solid–liquid interface, (ii) a quasi-linear and (iii) a linear spreading stage controlled by the chemical reaction between Zr and B₄C in both cases, and (iv) an approach-to-equilibrium stage with precipitation of crystals in the liquid. An increase in temperature promotes the wetting and reaction. In view of the reasonable wettability and reactivity, there is a potential for preparing Zr-base bulk metallic glass matrix composites reinforced by in situ ZrC–ZrB₂ hybrid ceramic particulates using B₄C as a reaction agent by way of an infiltration synthesis technique.     

Wetting and interfacial interactions in the CaO–Al2O3–SiO2/silicon carbide system

In this article, the wetting of 23 wt% CaO–15 wt% Al₂O₃–62 wt% SiO₂ molten glass on polycrystalline silicon carbide is studied under air at temperatures between 1,100 and 1,590 °C. Wetting experiments are performed by the sessile drop technique. Good wetting (final contact angle lower than 50°) is observed regardless of the experimental temperature when it is higher than 1,300 °C. Moreover, some specific experiments of wetting of glass on platinum, silica and monocrystalline SiC substrates are also performed. The character of molten glass spreading on silicon carbide (reactive or non-reactive) as well as the role of the atmosphere on interfacial interactions with SiC are identified and discussed.     

Sol-gel preparation and characterisation of mixed metal tin oxide thin films

Mixed metal oxide stannates were prepared by sol-gel methods and coated onto solid titanium substrates as thin films using spin and dip coating methods. Metal oxides such as Sb₂O₅, ZrO₂, CuO, MnO_x and PdO were introduced into a SnO₂ host matrix using sol-gel technology. The mixed metal tin oxide materials prepared via the sol-gel route were extensively characterised in terms of surface characterisation and chemical composition. Thermogravimetric analysis was performed to confirm that at 600 °C (the calcination temperature) no further structural changes due to mass loss occur. UV spectroscopy of the liquid gels allowed the determination of the band gap energy. The surface morphology of the thin film electrodes were characterised by atomic force microscopy and scanning electron microscopy and the effect of the coating method employed i.e. spin or dip coating could be clearly seen in the estimated values of surface roughness. These techniques were also able to confirm the thickness of the films in the nano range. Combined nuclear beam techniques such as Rutherford backscattering spectroscopy and particle induced X-ray emission provided some insight into the chemical composition of the mixed metal tin oxides and confirmed the presence of the dopant element in the SnO₂ host material.     

Experimental evidence of liquid drop break-up in complete wetting experiments

We report experiments concerning the deposition of a droplet on a solid surface and the related spontaneous spreading, during which a secondary droplet is ejected. Experimental investigations and theoretical considerations have been performed in order to understand the reasons of the formation of this droplet and of its ejection and to estimate the conditions that induce these phenomena. High-speed imaging and specific deposition conditions have been necessary to visualize such phenomena. It has been shown that the ejection is possible in the complete wetting regime when low impact inertia and high position of the center of mass of the drop before spreading are achieved simultaneously. A model taking into account non-stationarity, inertia, wetting capillarity and viscous effects has been developed. It resulted in two dimensionless numbers Π₁ and Π₂ that characterize the occurrence of the ejection phenomenon.     

Synthesis of carbon nanotube films by thermal CVD in the presence of supported catalyst particles. Part I: The silicon substrate/nanotube film interface

The interface between the silicon substrate and a carbon nanotube film grown by thermal CVD with acetylene (C₂H₂) and hydrogen at 750 or 900 °C has been characterized by high resolution and analytical transmission electron microscopy, including electron spectroscopic imaging. Silicon (0 0 2) substrates coated with a thin (2.8 nm) iron film were heat treated in the CVD furnace at the deposition temperature in a mixture of flowing argon and hydrogen whereby nanosized particles of (Fe,Si)₃O₄ formed. These particles were reduced to catalytic iron silicides with the –(Fe, Si), ₂–Fe₂Si and ₁–Fe₂Si structures during CVD at 900 °C, and multi-wall carbon nanotubes grew from supported particles via a base-growth mechanism. A limited number of intermediate iron carbides, hexagonal and orthorhombic Fe₇C₃, were also present on the substrate surface after CVD at 900 °C. The reduction of the preformed (Fe, Si)₃O₄ particles during thermal CVD at 750 °C was accompanied by disintegration leading to the formation of a number of smaller (<5 and up to 10 nm) iron and silicon containing particles. It is believed that the formation of these small particles is a prerequisite for the growth of aligned multi-wall carbon nanotube films.     

Coating of hydroxyapatite on metal plates using thermal dissociation of calcium-EDTA chelate in phosphate solutions under hydrothermal conditions

Coating of hydroxyapatite on various metal plates was carried out by a homogeneous precipitation technique using hydrothermal reactions in Ca(edta)^2-–NaH₂PO₄ at 140–200°C and pH 3.4–10.0. Hydroxyapatite films were formed on the surface of the iron plates in solutions at an initial pH above 4.7, whereas aggregates consisting of needle-like hydroxyapatite crystal radiating from a point in the form of flower deposited as islands on the surface of aluminum, copper and titanium plates. The upper part of the film formed on the surface of the iron plates consisted of needle-like hydroxyapatite particles whereas the bottom of the film consisted of spherical hydroxyapatite particles. The length of the needle-like hydroxyapatite particles increased with decreasing concentration of Ca(edta)^2-.     

Generation of metal and metal oxide nanoparticles by liquid flame spray process

A liquid flame spray (LFS) process has been investigated for the generation of single component nanoparticles. In the LFS process, a solution consisting of metal nitrate dissolved in water is sprayed into a turbulent, high temperature H₂-O₂-flame. The primary spray droplets evaporate and subsequent reactions in the flame produce metal or metal oxide vapours which nucleate to final particulate form. In the study, the process characteristics were examined to produce 10–60 nm particles from silver, palladium and iron containing precursors. A systematic study using variable process parameters proved that the size of the generated nanoparticles is set by the mass flow rate of the metal precursor, only. The geometric standard deviation of the size distributions was seen to vary in a limited range of 1.35–1.4. The particle size was verified by aerosol instrumentation, the composition and morphology by X-ray diffraction (XRD) and transmission electron microscopy (TEM), correspondingly. The Ag and Pd particles were seen to consist of pure metals. For iron, the presence of all three of the following compounds were detected: Fe, Fe₂O₃ and Fe₃O₄.     

Phosphonate-anchored thin films on titanium and niobium oxide surfaces: Fabrication and characterization

Phosphonate-anchored thin films form on various metal oxide substrates. This paper compares structural details of these covalently anchored films on the oxidized surfaces of titanium, niobium and a Ti45Nb alloy. This is made possible by a sample configuration wherein the alkylphosphonates are coated onto a thin film of metal which is sputtered onto a double-side-polished silicon wafer and then oxidized. Samples are flat and reflective and are suitable for ellipsometry, wetting measurements, X-ray Photoelectron Spectroscopy, Atomic Force Microscopy, and Fourier Transform Infrared-Attenuated Total Reflectance Spectroscopy. Deposition from heated tetrahydrofuran produces ordered films with measurable differences among deposition protocols and among metal oxide substrates. These substrates enable identification of the mildest deposition procedures that still provide uniform, robust surface coatings.