Surface Analytical Investigations Research Articles

Electrochemical Reactivity of Zirconium-Based Bulk Metallic Glasses

ABSTRACTAmorphous Zr-(Ti)-(Nb)-Al-Cu-Ni alloy samples were prepared by melt-spinning and copper mould casting in an argon atmosphere and characterized regarding their microstructure and thermal behavior. Their anodization behavior in aqueous environments with pH= 0.5 – 13 was studied by electrochemical polarization techniques in combination with surface analytical investigations, i.e. SEM/EDX, AES. In chloride-containing solutions the macroscopic corrosion resistance of bulk amorphous alloys is affected by the presence of heterogeneities, such as concentrated cluster zones of selected components or crystalline defects. Pitting phenomena are studied in neutral and acidic chloride solutions and as a result a local corrosion mechanism is proposed. The cathodic reactivity of alloy samples at different microstructural states and after pre-etching in fluoride solutions was investigated. After pre-etching melt-spun amorphous samples exhibit a significant increase in surface reactivity as expressed by a drastic increase in electrochemical capacities and in cathodic current densities as well as by a significant reduction of overpotentials for the hydrogen reduction reaction. The hydrogen sorption behavior was studied on samples galvanostatically charged at various cathodic current densities by means of XRD, DSC, TEM and thermal desorption analysis TDA. At room temperature Zr-based alloys absorb hydrogen up to H/M=1.65 mainly by interstitial solution of hydrogen atoms in the amorphous structure. The effect of absorbed hydrogen on the thermal stability and the crystallization behavior is described.

Surface Analysis of Impact Compressed Cu-Ni Multilayer

Surface analytical investigation with AES and XPS has been performed on Cu-Ni multilayer and its impact compressed sample. A well formed Cu-Ni multilayer using RF magnetron sputtering system with MMPC has been observed. The possibility of the formation of a Cu-Ni alloy after the high speed impact compression has been found, and this suggests that the impact compression method is useful for the preparation of metal alloys.

Physical and electrochemical properties of vanadium alloy oxide layers prepared by anodic passivation

Thin oxide layers of high purity metals are suitable as redox electrodes in aqueous solutions. Concentrations of hydrogen peroxide could be analyzed potentiometrically. In dependence on the pH-value Nerstian behaviour was observed in the range of 0.1–30 g/l (H2O2) approximately. It is assumed that this electrochemical behaviour of the oxide layer is mainly influenced by electronic and ionic defects. Alloys of vanadium and titanium are included in the investigation. In order to clarify the mechanism in detail, electrochemical measurements (voltammetry, EIS) and surface analytical investigations (XRD, AES, XPS) have been carried out.

Determination of the Real Indenter Shape for Nanoindentation/Nanotribology Tests by Surface Metrological and Analytical Investigations

ABSTRACTIn this paper a critical comparison of different kinds of surface metrological methods like atomic force microscopy (AFM), scanning electron microscopy (SEM) and optical interferometric profilometry is made, as well as analytical investigations based on both Hertz and Sneddon theories of pure elastic contact, to qualify the real indenter shape function. The results obtained show the advantages and disadvantages of the respective methods by describing a rounded indenter tip.

Corrosion Behavior of High-Nickel and Chromium Alloys in Natural Baltic Seawater

Abstract Effect of Cl− ion concentration (0 M sodium chloride [NaCl] to 2 M NaCl) and temperature (25°C to 75°C) on stability of the passive state ofhigh-Ni and Cr alloys: NI-1 (∼ 16% Mo), CR-2 (∼ 6.2% Mo), and NI-3 (3.5% Mo) were investigated in acidic and neutral electrolytes in strictly controlled electrochemical conditions. The anodic behavior of the alloys appeared to depend mostly upon Mo content in the alloy. Thus, the NI-1 was the most stable alloy under the applied experimental conditions. The other alloys were also quite resistant, undergoing pitting only at elevated temperatures, at high anodic potentials, and at a chloride concentration not lower than 1 M. In natural Baltic seawater, these alloys did not exhibit any tendency to pitting, in qualitative agreement with the accelerated electrochemical tests. Complementary microscopic and surface analytical (AES) investigations were carried out to correlate the anodic and corrosion behavior of these materials with their composition and structure, a...

Surface analytical investigation of the electrochemical and corrosion behaviour of nanocrystalline FeAl8

From the higher fraction of grain boundaries in nanocrystalline substances a different corrosion behaviour in comparison to the conventional polycrystalline material can be expected, which may be utilised for the development of new corrosion resistant alloys. Therefore, the oxidation behaviour of these two different crystallisation states of FeAl8 was compared by means of electrochemical and surface analytical experiments. The oxide films formed after electrochemical passivation were investigated by Auger Electron Spectroscopy. The application of inelastic peak shape analysis by the method of Tougaard showed, that for both materials the oxide layer may be described by a model of a (below the contamination) buried layer with a thickness of only a few nanometers depending on the preparation conditions. Factor Analysis was applied for the evaluation of the differentiated low energy Auger electron spectra (20–100 eV) as a function of depth profiling sputtering time. For both, the nanocrystalline and the polycrystalline material, the inner part of the oxide layer was enriched in Al, whereas the very outer part (surface region) was enriched in Fe. No differences concerning the sputtering time for removal of the oxide layers were found for the two alloys.

Surface analytical investigations of the electrochemical double layer on silver electrodes in alkaline media

The electrochemical double layer on Ag in aqueous solutions was examined ex situ with X-ray photoelectron spectroscopy (XPS) and UV-photoelectron spectroscopy (UPS). The specimens were removed from the electrolyte with hydrophobic surfaces and under potential control. The potential dependent surface concentrations of the adsorbed anions, cations and the surface excess charge, as well as the amount of adsorbed water were determined. The results obtained for the double layer on Ag in 1 M NaClO 4+0.1 M NaOH were compared with an acidic perchlorate solution and pure 1 M NaOH. We found evidence for a specific adsorption of hydroxide ions in the basic electrolyte; i.e. the OH −-surface concentration has to be considered for a proper determination of the cationic excess charge and the potential of zero charge. The observed systematic XPS binding energy shifts of the adsorbate species with respect to the metal core levels and the measured work function changes suggest that the electrochemical adsorbates are not in electronic equilibrium with the metal.

Photoluminescence quenching of Alq3 by metal deposition: A surface analytical investigation

The importance of the interfacial properties in organic light emitting devices (OLED) is well recognized. We will describe our recent efforts to understand interfaces in OLEDs using surface/interface analytical techniques in a well controlled ultrahigh vacuum environment. We observed severe photoluminescence quenching of organic thin films comprising of a model OLED material, namely tris-(8-hydroxyquinoline) aluminum (Alq3), upon deposition of a number of metals. Such quenching may severely affect the electroluminescence device efficiency. We have investigated the interfaces using x-ray photoemission spectroscopy and ultraviolet photoemission spectroscopy, and studied the intriguing process of the interface formation at an atomic/molecular level. We will show that microscopic surface and interface properties are intimately related to the device characteristics and performance.

Passivity of Al–Mo amorphous alloys andtheir electrochemical breakdown

Abstract The passivity of Al–Mo amorphous alloys and stability of the passive state in borate buffer solution containing various concentrations of Cl− (0–1 M NaCl) have been investigated by electrochemical, microscopic and surface analytical methods (AES, XPS). The anodic passivating film had an insulating character supporting voltages over several volts. Surface analytical investigations have revealed that the film consists of oxidized Al and O, whereas Mo signals did not suggest any oxidized molybdenum containing species within the film. Microscopic examinations have shown that at and above the pitting potential, E′np in Cl−-containing solution, an isotropic localized dissolution (pitting) took place. The localized current density within the growing pits was ∼105 times higher than that flowing from the remaining passive surface. A tentative mechanism of passivity of the Al–Mo alloys and of its breakdown is proposed.

A surface analytical investigation of the electrochemical double layer on silver electrodes in chloride solutions

Abstract The electrochemical double layer on Ag in NaCl solutions was examined with X-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS). The potential dependent surface concentration of the adsorbed species (Na + , Cl − , H 2 O) as well as the cationic excess charge was determined. The specific adsorption of Cl − is accompanied by coadsorption of cations and water. Angle-resolved XPS measurements and ISS experiments gave more detailed information about the structure of the double layer perpendicular to the silver electrode surface. A layered structure of the specifically adsorbed chloride anions and the coadsorbed cations was determined, in agreement with recently published studies.

Adsorption and growth of dip-coating prepolymer films on silicon wafers. An atomic force microscope study

Ultrathin films of a prepolymer of the dicyanate of bisphenole A were made by dip-coating on commercial silicon wafers. The initial stages of film formation could be characterized in detail by AFM (atomic force microscopy) in the tapping mode and in a newly developed deformation mapping mode. The interpretation is assisted by simple molecular modelling as well as surface analytical investigations. The thinnest films are discontinuous and have a mean thickness <1 nm (ellipsometry). They consist primarily of an amorphous deformable phase which probably represents monomer. Even for the tiny roughness of the wafers (0.13 nm RMS), the initial adsorption is preferred in substrate depressions. High resolution imaging could be reproducibly achieved even for this amorphous film surface. An extended contact time of the wafer with the prepolymer solution induces the adsorption of adsorbates, whose heights correspond to the next larger species of the prepolymer, the trimer. Further increase in contact time results in a close packing of these adsorbates and such with double the height. After a contact time of about one minute, a steady state of the film morphology is reached. Then, the mean film thickness can only be raised by increasing the concentration of the solution. In addition, all films comprise large prepolymer agglomerates which represent conglomerates already present in the solution above a small, critical concentration.

Formation and Properties of Porous GaAs

AbstractPorous structures on n-type GaAs (100) can be grown electrochemically in chloridecontaining solutions. Crystallographic etching of the sample is a precursor stage of the attack. Polarization curves reveal the existence of a critical onset potential for pore formation (PFP). The PFP is strongly dependent on the doping level of the sample and the presence of surface defects. Good agreement between the PFP and the breakdown voltage of the space charge layer is found. Surface analytical investigations by EDX, AES and XPS show that the porous structure consists mainly of GaAs and that anion uptake in the structure can only be observed after attack has been initiated. Photoluminescence measurements reveal – under certain conditions – visible light emission from the porous structure.

Electrochemical, Microscopic and Surface Analytical Investigations of Chemical Breakdown of Passivity

Electrochemical, Microscopic and Surface Analytical Investigations of Chemical Breakdown of Passivity

Surface Analytical Investigations of Electrochemically Formed Passive Layers on Binary Fe/Al Alloys

Passive layers have been formed on Fe/Al alloys under well‐controlled electrochemical conditions. These layers were studied by x‐ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) without any exposure to the laboratory atmosphere during the specimen transfer from the electrolyte to the ultrahigh vacuum. The presence of aluminum reduces the active dissolution of the alloys in weakly acidic electrolytes such as phthalate buffer. This leads to lower anodic current densities compared to pure iron. The passive layer thickness due to the preparation conditions varies from 3 to 6 nm. Both components (Al and Fe) enter the passive layer, but Al is significantly enriched. XPS and especially ISS depth profiles show an Al concentration maximum at a depth of 2 nm which is roughly in the center of the passive layer. Apparently the passive film is a mixed oxide with a pronounced concentration gradient of both cationic species. Galvanostatic reduction and dissolution of the passive layers in combination with XPS permits the determination of an electrochemical depth profile which confirms the results of the sputter depth profiles. A clearly separated sublayer structure of different oxidation states [Fe(II) and Fe(III)] was only found in the passive layers on pure iron. Its existence in the passive layers on Fe/Al alloys could not be confirmed. The preparation parameters, especially the passivation time, cause changes in the concentration gradients. Closer examinations of these effects by time‐resolved measurements provide kinetic information on the growth and chemical changes of the passive layer. A model describing the observed effects during the layer formation is presented.

Surface analytical studies on metal surface and interface phenomena

Surface and interface segregation phenomena on metals can influence many material properties. From equilibrium surface segregation studies on binary model systems (single crystals), thermodynamic data for the segregation systems can be derived and may be used to explain phenomena on more complex systems. Equilibrium surface segregation is normally limited to occur within one atomic surface layer, but phosphorus segregation is an exception. By interaction with crystal defects, P atoms may also be enriched in subsurface layers. This was confirmed by Auger electron-spectroscopy, x-ray excited photoelectron spectroscopy, depth profiling and atom probe studies on surface and grain boundary segregation for several iron base alloys. The influence of segregation phenomena on some material properties is demonstrated by the results of surface analytical investigations.

Surface analytical investigations of Al and (Ti, Al) nitrides formed by ion implantation and their corrosion properties

AbstractNitrides of Ti, Al and TiAlx alloys were prepared by low energy ion implantation (3 keV N2+). The layer thickness was ∼ 10 nm. The nitride layers were characterized using XPS, UPS and a Kelvin probe. The insulating properties known for bulk AlN were found also for the prepared AlNimpl layers. The work functions for the alloys and the nitrides and their variation with the ion dose were determined. Peak parameters for the XP spectra deconvolution were established. Despite their thinness, the layers showed anodic stability in 0.5 M H2SO4 up to 1.3 V (SHE) for (TiAl)Nimpl and even up to 1.8 V (SHE) for AlNimpl. Above these potentials, the nitrides were oxidized to metal oxides.

Electrochemical, ellipsometrical and surface analytical investigations of passive layers on binary Fe/Al alloys

AbstractX‐ray photoelectron spectroscopy, ISS, ellipsometry, electrochemical impedance spectroscopy and electrochemical methods have been used to investigate the passive layers (PAL) on iron/aluminium alloys with low contents of Al (2 up to 12 wt.%). Ellipsometric measurements were performed during the galvanostatic reduction of PAL formed on Fe/8Al in 0.5 M Na2SO4/H2SO4 (pH 3.85) at various potentials. Combining the results of ellipsometric singlelayer model evaluations and of the electrochemical reduction curves it was found that the outer part of the PAL mainly consists of the relatively stable γ‐Fe2O3. Its fraction to the overall thickness of the PAL gives a maximum value when the alloy is passivated at +1 VSHE. At higher potentials, porous films with both decreasing capacities and polarization resistances, measured by impedance spectroscopy, are formed. The XPS and ISS sputter depth profiles of PAL on Fe4/12Al formed in phthalate buffer (pH 5) at +1 VSHE showed an Al enrichment in the centre of the layers. The reductive dissolution of the passive layer in close combination with XPS analysis at distinct states of this process yields similar depth profiles. These results suggest the formation of mixed Fe/Al oxides or oxide compounds.

19.O.03 Surface-analytical investigations on the water up-take of toluene-treated combustion aerosol particles before and after exposure to NO2

19.O.03 Surface-analytical investigations on the water up-take of toluene-treated combustion aerosol particles before and after exposure to NO2

Surface analytical investigations on the oxidation behaviour of TiAl-base intermetallics

The oxidation behaviour of TiAl-base intermetallics has been investigated at 800°C in air. The main emphasis was placed on the mechanism by which niobium additions decrease the oxidation rate of titanium aluminides. For this purpose specimens of Ti50Al and Ti45Al10Nb were oxidized in a two-stage oxidation technique using an 18O-tracer. The scale formed during this oxidation process was analyzed by SNMS. It was found that niobium is mainly incorporated in the titanium dioxide and the nitride scale which forms beneath the initially formed alumina layer. The effect of this behaviour of the niobium on the oxide scale growth rates is discussed.

Surface analytical investigation of the tritium getter ZrCO after exposure to various gases

ZrCo is a hydride forming material frequently investigated and proposed for storage and handling of hydrogen isotopes. It is of special interest for deuterium and tritium storage needed for the operation of a nuclear fusion reactor. An interaction of ZrCo powder with contaminant gases particularly during the thermal release of the hydrogen isotopes from the hydride at temperatures above 300 °C was found to cause a reduction of the reactivity of the powder. Consequently, a serious decrease of the hydrogen storage capacity occurs. From surface analytical investigations of ZrCo after exposure to the contaminant gases CO, CO2, O2, N2, CH4, and C2H4 it was concluded that CH4, C2H4, O2, or N2 react predominantly with Zr, the principal hydride forming alloy component, to carbide, nitride, and/or oxide. The resulting decrease of the storage capacity is possibly due to either a reduction in the amount of unreacted Zr available for the formation of Zr hydrides or to a formation of a thick protective overlayer. CO or CO2, which react mainly with the Co component but with Zr in surface near layers only, cause a less pronounced decrease in storage capacity.