Unoxidized Surface Research Articles

AES and LEED studies of the initial stages of oxidation of ?-NiAl with and without platinum doping

The initial stages of oxidation of clean (110), (100), and (111) oriented β-NiAl single crystals, with and without Pt doping (50 nm), were followed at 973 K using LEED and AES. Upon oxidation, Al2O3 is selectively formed. On NiAl (110) a complex surface structure is observed which according to Jager et al. [13] is a duplex oxide structure similar to α- or γ-Al2O3. On the (100) face, submicroscopic facetting occurs during oxidation. For the (111) oriented sample the hexagonal structure remains and only an increased background intensity for the LEED pattern is observed for the oxidized surface. After Pt doping, the phenomena observed for the Pt-free sample are not significantly changed. However the following differences exist: 1. the formation of the Al2O3 oxide is more difficult for the (110) and (111) samples but Pt doping facilitates facetting and oxide formation on the (100) oriented sample; 2. sputtering of the oxide layers on Pt-doped samples is retarded in all cases, a very stable and dense oxide seems to be formed which adheres well; 3. Pt seems to hinder the Al diffusion to the unoxidized surface of the samples.

Surface chemistry of new As precursors for MOVPE and MOMBE: phenylarsine and tertiarybutylarsine on GaAs(100)

Surface reactions of PhAs and tBAs on unoxidized and thermally oxidized GaAs(100) were studied in an UHV chamber using a differentially pumped quadrupole mass spectrometer and a molecular beam nozzle. A special sample holder allowed periodic switching between an unoxidized and an oxidized surface to observe small differences in reactivity. The following assumptions are compatible with the experimental results: Adsorption of PhAs or tBAs on GaAs(100) followed by rupture of the As-H bonds is the first step. Most of the organometallic radicals desorb from the surface, a fraction decomposes further by rupture of the As-C bond. The organic radicals react with the surface hydrogen and desorb as benzene and butane or butene, respectively. Low values of the activation energies ( ≤ 0.4 eV) for the different reaction steps suggest a diffusion of PhAs or tBAs on the surface as the rate limiting step for the overall reaction. A comparison of oxidized and unoxidized surfaces exposed to a constant gas flux of PhAs or tBAs revealed a reduced reactivity on the oxidized surface. Thermodesorption experiments with oxidized GaAs(100) surfaces showed that the desorption temperature of GA 2O decreased from 862 K without tBAs to 835 K in a tBAs flux. During this annealing process a reaction between the surface oxide and organic radicals from the tBAs decomposition seems to form a highly stable contamination layer which was observed by X-ray photoelectron spectroscopy. This contamination of the surface can be avoided by annealing in UHV without any As-species present and monitoring the Ga 2O flux from the surface with a mass spectrometer because non-stoichiometric evaporation of GaAs occurs only after desorption of the oxide.

Raman and Ellipsometric Studies of Delta-Doped GaAs

ABSTRACTGaAs (100) samples with multiple δ-doped layers (N2D=∼2×1013/cm2) were studied by Raman scattering (RS) and spectroscopic ellipsometry (SE). A quasithree- dimensional (3D) plasmon-phonon coupled mode (L+), probed at λ= 514.5 nm, from a 9-layer δ-doped GaAs with layer-spacing of 100 Å, was observed at ∼895 cm−1. At similar frequency, a plasmon mode was also detected from another GaAs sample with the same δ-doping periods but doubled layer-spacing (200 Å). This provides evidence of spatial quantization of the electron distributions in δ-doped GaAs. The equivalent 3D electron concentration, estimated from the Raman plasmon mode, is ∼1.1×1019/cm3. The presence of the 3D plasmon mode from a quasi-two-dimensional (2D) electron gas is possibly contributed by the electrons in the high energy subbands in the V-shaped potential well of the δ-doped GaAs. The pseudodielectric function <ε>= <ε1>+i<ε2> of this δ-doped GaAs sample was measured by spectroscopic ellipsometry (SE), from an unoxidized surface in an ultrahigh vacuum (UHV) chamber, in the range of 1.5 to 5.0 eV. Compared with uniformly doped GaAs, our SE data indicates a reduced broadening of the optical transitions between the E1 and E1+Δ1, energies due to the δ-doping.

Study of the electrooxidation of carbon monoxide on rhodium in acid and alkaline media by UV - visible potential-modulated reflectance spectroscopy and cyclic voltammetry

The chemisorption of CO on a Rh electrode in 0.5 M HClO 4 and 1 M NaOH was studied by cyclic voltammetry and UV-Visible Potential-Modulated Reflectance Spectroscopy (PMRS). Electrooxidation of dissolved CO on Rh is negligible in acid. On the contrary, the stationary current in base reaches ∼- 1 5 of the diffusion-limited value. We report a voltammetric peak due to the oxidation of dissolved CO in base, which can be observed only for CO admission potentials < 0.1 V vs. RHE, and which precedes the sharp peak for oxidation of (chemisorbed and dissolved) CO. The potential of this sharp peak in base is independent of the admission potential up to E adm = 0.47 V, but for higher values the peak potential increases linearly with E adm, probably due to the formation of a rhodium oxide prior to CO admission. The amount of CO chemisorbed on Rh in base is independent of the presence on the electrode surface of a thick layer of electrochromic oxide previously grown by potential cycling, which we attribute to CO chemisorption continuing to take place on the unoxidized Rh surface. The PMR spectrum of Rh on which CO is chemisorbed shows a well-defined maximum, which appears at 265 and 295 nm in acid and base, respectively. This PMRS maximum is due to an electric field effect, since its height is independent of the modulation frequency, and grows linearly with the modulation amplitude. Reflectograms recorded at the wavelength of the PMRS maximum during a slow potential sweep prove conclusively that it originates in the chemisorbed CO. The PMRS maximum at 265 nm is attributed to linearly chemisorbed CO, and that at 295 nm to bridged CO.

Electrochemistry of mixed-metal interfaces: Chemisorption and electrochemical reactivity of thiophenols at Au 90Pt 10 electrodes

The surface electrochemical properties of thiophenolic compounds chemisorbed at a smooth polycrystalline Au 90Pt 10 alloy electrode have been studied in comparison with those at the pure monometal electrodes. 2,5-Dihydroxythiophenol (DHT) and pentafluorothiophenol (PFT) were used as model compounds since these are strongly chemisorbed on Au and Pt, and their electrochemical reactivities [reversible redox activity, hydrodesulfurization (HDS), and anodic oxidation] in the chemisorbed state depend strongly upon the composition of the electrode surface. Experimental measurements were based on thin-layer electrochemical methods. The major findings are as follows: (i) The packing density and the mode of binding (through the -S group) of DHT and PFT at Au 90Pt 10 are the same as at the pure Pt and Au electrodes, (ii) Substrate-mediated DHT-DHT interactions which occur on Pt but not on Au are promoted, albeit to a small extent, at the alloy interface, (iii) The extent of HDS is greater at Au 90Pt 10 than at Au but much lower than at Pt. (iv) HDS of DHT at Au 90Pt 10 unlike that at Pt, is not accompanied by hydrogenation and/or hydrogenolysis of the pendant aromatic ring; that is, HDS at the alloy surface is selective towards simple cleavage of the C-S bond. (v) Irreversible oxidation of DHT chemisorbed at Au 90Pt 10 yields two distinct anodic peaks; the first appears to be associated with oxidation involving the Pt surface sites, whereas the second is suggestive of oxidation involving Au surface atoms, (vi) Chemisorbed PFT, which on pure Pt is inert towards anodic oxidation, undergoes irreversible oxidation at Au 90Pt 10 similarly but not identically to that at pure Au. (vii) Based upon the separate Pt-oxide and Au-oxide reduction peaks, the surface composition of the Au 90Pt 10 alloy is the same as that in the bulk. (viii) On the unoxidized alloy surface, the distribution of the Au and Pt sites is predominantly homogeneous, (ix) Phase separation occurs after the alloy surface is oxided; reduction of the surface oxide appears to restore the homogeneity of the bimetallic interface, (x) Under the conditions of the reactions studied, no surface roughening or segregation occurred at the alloy electrode. The present results reflect the complex interplay between the relative surface activities and catalytic reactivities of the Au and Pt sites at the Au 90Pt 10 interface.

Simulation of cutting tool wear by a modified pin-on-disc test

Metal cutting is characterized by extreme contact conditions at the tool-workpiece interface which cannot be simulated in a conventional wear test. Consequently, the performance of the tool material is normally evaluated by full-scale, comparative cutting tests. In this type of test, the observed tool lives are determined by both gradual wear and edge chipping. This creates a problem in the development of new tool materials and coatings since the tool material wear resistance cannot be determined separately. In order to overcome this experimental difficulty, a modified pin-on-disc test has been developed. In this test, the contact conditions in machining are reproduced by letting the pin slide against a newly formed, unoxidized countermaterial surface. In the present work, the performance of this new test has been evaluated for high speed steel tool wear with a quenched and tempered steel, AISI 4340, as countermaterial. The influence of normal force, sliding speed and sliding distance on wear and friction has been investigated. The observed wear rates and wear mechanisms are compared to results from conventional laboratory wear testing (using the same test set-up) and machining. The experimental results demonstrate that the new wear test produces wear characteristics that closely correspond to tool wear. Thus, the modified pin-on-disc test exhibits great potential as a first screening wear test of new tool materials and coatings.

Oxidation of a cobalt chromium alloy: An x-ray photoemission spectroscopy study

The oxidation of a Co86Cr14 alloy surface was studied by x-ray photoemission spectroscopy. Following ion sputtering at 25 °C, the unoxidized alloy surface composition was nearly the same as the bulk composition. Vacuum annealing of the surface caused Cr enrichment at the surface. This is discussed in the context of the expected behavior at thermodynamic equilibrium. Oxidations from 25 to 400 °C were done under controlled exposure conditions using dry oxygen at 10−7 Torr and under atmospheric conditions. A layered mixture of oxides was formed at the surface and was rich in Co oxides near the surface and either rich in Cr oxide or at the bulk Cr/Co ratio in the inner oxide layers, depending on the oxidation conditions. The observed oxide structure is proposed to be a consequence of the competition between the preference for Cr to oxidize because of its higher oxidation potential relative to Co and the relative supply of Co available for oxidation compared to the supply of Cr due to the different diffusivities of Co and Cr in the oxide structure. The results indicate that Co has a much higher mobility than Cr in this temperature range, as is the case above 800 °C. The oxide growth appears to be dominated by the high oxidation potential of Cr prior to the formation of a complete oxide layer and, subsequently, is dominated by the high Co diffusivity, under all conditions examined.

Photoinitiated polymerization on metal surfaces

Irradiation of acrolein vapors in the presence of films of nickel and other metals results in the formation of thin films of polyacrolein on the metal surface. The polyarcrolein film protects the metal surface from abrasion and corrosion. Polymer growth is more rapid on oxidized vs. unoxidized metal surfaces indicating a role for surface oxide sites in the polymerization process. Several other monomers investigated fail to form adhesive polymer films on metal films, but can be copolymerized with acrolein.

Producing selective absorption at a coated metal surface

Unoxidized metal surfaces have inadequate absorption coefficients in the incident wavelength range of 0.2-2.5 micrometers, this range being converted to heat. One way of selectively increasing the absorption coefficient of a photothermal energy converter in this wavelength range is to deposit a layer of material that absorbs much of the incident flux but as far as possible does not increase the emissivity in the range 4-40 micrometers, which corresponds to a working temperature range of about 200-300 degrees C. This method is examined in this article.

The Effect of Cyanide on the Oxidation of Formaldehyde on Au, Cu, and Pd Electrodes

The effect of the CN− ion on the formaldehyde electro‐oxidation reaction has been examined in basic EDTA solutions at 75°C on Au, Pd, and Cu electrodes. The results are compared with the cyclic voltammetry of these three metals in EDTA solutions. The cyanide ion poisons electro‐oxidation on Au and Pd; on Cu, however, substantially higher currents can be sustained in the presence of cyanide. The poisoning on Au and Pd is related to the suppression of dissolution of the metal by the formation of a sparingly soluble metal cyanide on the surface. Metal dissolution is apparently also suppressed on Cu electrodes, but the greatest promoting effect of CN− occurs in a potential range in which CN− adsorbs on the unoxidized Cu surface.

Plasma-sprayed nickel cathode coatings for hydrogen evolution in alkaline electrolytes

Hydrogen evolution cathodes with plasma-sprayed nickel coatings were much more efficient in alkaline electrolytes than either uncoated cathodes or cathodes with porous sintered nickel coatings. This efficiency improvement could not be attributed to either superior coating morphology or increased surface area. The degree of coating oxidation appeared to be the main factor influencing the hydrogen evolution overpotentials at either the plasma-sprayed or sintered nickel cathode coatings studied. Highly-oxidized nickel surfaces produced much lower Tafel slopes than similar unoxidized surfaces. Plasma-sprayed nickel cathode coatings made with fine, high surface area nickel powder produced hydrogen evolution overpotentials in the range 0.10–0.14 V.

The electrical and photovoltaic properties of tunnel metal-oxide-semiconductor devices built on n-InP substrates

Pd-(n-type) InP Schottky barrier devices have been fabricated on unoxidized and oxidized surfaces and their electrical as well as photovoltaic characteristics have been evaluated. Devices built on the oxidized surfaces show considerably improved characteristics compared with those built on unoxidized surfaces. A saturation current density of 9×10−9 A cm−2 and barrier height of 0.85 eV have been observed for tunnel Metal-Oxide-Semiconductor (MOS) devices compared with 1.1×10−6 A cm−2 and 0.62 eV for Metal-Semiconductor (MS) devices. Effective Richardson constants, obtained from activation energy plot of saturation current density, have been found to be 0.66 and 1.9 A cm−2 K−2 for MOS and MS devices, respectively. Such low values of the Richardson constant, when compared with the theoretical value, provide clear evidence of the modification of current transport mechanism in the presence of an interfacial layer. Photovoltaic cells with an efficiency of 7% have been built on oxidized surfaces without the use of a grid structure or antireflection coatings. Under AMl illumination, the oxidized devices show the following parameters: open circuit voltage = 575 mV, short circuit current = 16.8 mA/cm2, and fill factor = 0.73. Using photoresponse techniques, the diffusion length has been calculated to be 1.78 μm for MOS devices, a value similar to that obtained for MS devices.

High resolution studies of glass‐crystal interfaces

SUMMARYHigh resolution electron microscopy has been used to image intergranular glassy phases in sintered and hot‐pressed nitrogen ceramics, and the Si‐SiO2 interfaces important in MOS devices.The thin intergranular glassy phases were found at most grain boundaries in all liquid‐phase sintered or hot‐pressed ceramics examined. Silicon carbide sintered by a solid state diffusion process, however, contained no such phases.Si‐SiO2 interfaces have been examined in cross‐section at about 0ṁ3 nm resolution. The images show that the transition from the Si crystal to the amorphous oxide was abrupt, but that there were 0ṁ3‐0ṁ5 nm high hills on the Si surface, separated by 2–4 nm, and long‐range undulations of 0ṁ6‐0ṁ8 nm height and 20–50 nm periodicity. Ultra‐thin (3 nm) oxides were also examined and were found to be uniform in thinkness, indicating that the roughness at the Si‐SiO2 interfaces is initially caused by irregularities on the original, unoxidized Si surface.

The effect of interfacial bonding on the adhesion of brittle film-polymer multilayered structures

A previously derived model and method to determine the adhesion of thin brittle films coated on flexible substrates are used to study the effect of changing the interfacial bonding on adhesion. Nickel films with oxidized and unoxidized surfaces are prepared by vacuum deposition onto Mylar. Various thickness of selenium are then deposited onto the nickel. By measuring the tensile force required to cause cohesive and adhesive failure, the approximate crack spacing and energy required to separate the interface are calculated. The calculated and measured crack spacings are compared and the chemical structure and fracture surface defined by electron spectroscopy for chemical analysis (ESCA).

Electrocatalytic effect on the oxide film at Pt anodes on Cl* recombination kinetics in chlorine evolution

In aqueous medium, Cl2 evolution on noble metal anodes occurs on a surface oxide film, the properties of which, rather than those of the metal, determine the kinetics of the reaction. The kinetics of Cl2 evolution on an oxidized Pt electrode in aqueous KCl are compared with those for Cl2 evolution on a demonstrably unoxidized Pt surface in anhydrous trifluoroacetic acid (TFA) as solvent. Hence the effects of development of an oxide film on Pt in aqueous medium can be determined in relation to the kinetic behaviour of Cl2 evolution on the unoxidized Pt metal surface. In both TFA and water, and their mixtures, it is shown that Cl2 evolution kinetics are recombination-controlled and non-diffusion-controlled limiting currents, ilim, are observed. The ilim values in aqueous medium, i.e. for the oxide-covered surface, are some 45 times larger than on the free Pt surface in TFA; since the mechanism is shown to be the same in the two solvents, and their mixtures, this observation indicates a substantial electrocatalytic enhancement of the recombination rate for Cl• on the oxide in comparison with the Pt metal surface. This is presumably due to weaker binding of Cl• on oxidized Pt than on the “bare” Pt surface. Effects due to specific adsorption of Cl− ion are also important and give rise to dependence of the recombination catalysis on Cl− ion concentration. A method is proposed for analyzing the recombination kinetics by means of linear plots which give both the recombination rate constant and the quasi-equilibrium constant for the prior Cl− ion discharge step as a function of water content.

Electrocatalytic effect of the oxide film at Pt anodes on Cl • recombination kinetics in chlorine evolution

In aqueous medium, Cl 2 evolution on noble metal anodes occurs on a surface oxide film, the properties of which, rather than those of the metal, determine the kinetics of the reaction. The kinetics of Cl 2 evolution on an oxidized Pt electrode in aqueous KCl are compared with those for Cl 2 evolution on a demonstrably unoxidized Pt surface in anhydrous trifluoroacetic acid (TFA) as solvent. Hence the effects of development of an oxide film on Pt in aqueous medium can be determined in relation to the kinetic behaviour of Cl 2 evolution on the unoxidized Pt metal surface. In both TFA and water, and their mixtures, it is shown that Cl 2 evolution kinetics are recombination-controlled and non-diffusion-controlled limiting currents, i lim, are observed. The i lim values in aqueous medium, i.e. for the oxide-covered surface, are some 45 times larger than on the free Pt surface in TFA; since the mechanism is shown to be the same in the two solvents, and their mixtures, this observation indicates a substantial electrocatalytic enhancement of the recombination rate for Cl • on the oxide in comparison with the Pt metal surface. This is presumably due to weaker binding of Cl • on oxidized Pt than on the “bare” Pt surface. Effects due to specific adsorption of Cl − ion are also important and give rise to dependence of the recombination catalysis on Cl − ion concentration. A method is proposed for analyzing the recombination kinetics by means of linear plots which give both the recombination rate constant and the quasi-equilibrium constant for the prior Cl − ion discharge step as a function of water content.

Surface and emission characterization of the impregnated dispenser cathode

Several osmium/ruthenium-coated and non-coated commercial impregnated cathodes were investigated using scanning Auger microscopy (SAM), photo electron spectroscopy (PES), mass spectroscopy and field-emission retarding potential (FERP) techniques for work function measurements. The ratio of Ba/BaO evaporating from the surface varied from 1–8 at 1418 K with the largest value corresponding to the lowest work function cathode. Work functions measured by the FERP technique varied from 2.7–1.8 eV and correlated well with the emission current density variation among the various cathodes. Richardson work functions in the range 1.5–1.7 eV were obtained, but showed no correlation with cathode current density. PES and SAM results showed that the surface of an operating cathode consisted of a Ba + O chemical combination on an unoxidized tungsten surface. The ratio of Ba/O was larger on the flats than within the pores intersecting the surface. Large beam size Auger spectroscopy gave near identical surface composition for high and low current density cathodes.

Effect of the graphite surface charge on the orientation of nitroalkane molecules in an adsorption layer

1. Measurements have been made of electroosmotic transport of aqueous phosphate nitroalkane solutions through oxidized and unoxidized high-purity graphite diaphragms. 2. The variation of the electroosmotic transport with the molecular weight of the adsorbed compound is such as to indicate that the nitroalkane molecule is preferentially normally oriented with respect to the oxidized surface, and preferentially tangentially oriented with respect to the unoxidized surface.

Design and technique variables affecting fracture resistance of metal-ceramic restorations

An attempt was made in this study to make a clinically meaningful measure of fracture resistance in metal-ceramic restorations. Forty-four solid metal-ceramic crown forms were fabricated and subjected to compressive load testings. Variables included the presence or absence of a metal coating agent, the type of metal preparation (using stones of different abrasives), and three designs of the underlying metal. The following conclusions were arrived at: (1) The design of the underlying metal structure had a significant relation to the ultimate fracture strength. (2) A design with a definite acuteness of the underlying metal structure failed at significantly lower ultimate fracture strengths. (3) A metal conditioning agent did not decrease fracture resistance if applied properly. (4) Fracture strength was severely decreased when (a) improper thickness of the coating agent was used and (b) porcelain was fused to an unoxidized metal surface. (5) Bond strength, although a contributing factor, may not be as important as metal design and proper manipulation of materials during fabrication of the restoration.

Use of liquid chromatography to study the adsorption of aqueous solutions of nitroalkanes on graphite

1. The Chromatographie method was used to determine the adsorption isotherms of aqueous solutions of four nitroalkanes on graphite at 293, 308, and 323°K, and the thermo-dynamic parameters of the adsorbability were calculated. 2. On an unoxidized graphite surface the standard change in the enthalpy of adsorption increases with increase in the chain length of the molecule, and decreases on an oxidized surface.