Initial Oxide Layer Research Articles

Bonding adhesive strength for plasma spray with cathode spots of low-pressure arc after grit blasting and removal of oxide layer on metal surface

Cathode spots of a low-pressure arc can remove oxide layers and evaporate impurities on metal surfaces. Removal of the oxide layer using cathode spots is expected to solve recent obstacles due to chemical and mechanical cleaning methods. Various phenomena of cathode spots have been investigated for pre-treatment of atmospheric pressure plasma spray (APPS). This study treated the surface shapes of oxide and non-oxide samples using a composite pre-treating method: cathode spots after grit blasting. In addition, the samples are compared with conventionally treated cathode spots and mechanical blasted surfaces. Results show that roughness on the sample surfaces becomes higher in concomitance with the initial oxide layer thickness. This fact reveals the factors those dominant bonding strength on the pre-treated surface using cathode spots of a low-pressure arc. Bonding strength becomes higher in relation to the arithmetical mean height of the surface roughness caused by the initial oxide layer thickness. Bonding strength is higher when the mean spacing of profile irregularities is narrower. The bonding adhesive strength after treatment using cathode spots after grit blasting is greater than 90 MPa. However, cathode spot treatment must be limited not to destroy the projection formed by melting after grit blasting in an arc time up to 5 s. And basically the removal characteristics of oxide layer using the cathode spots were discussed on the trace, roughness, number of cathode spots in time and pressure, moving speed on the metal surface, etc.

Notch Root Oxide Formation During Fatigue of Polycrystalline Silicon Structural Films

This paper investigates the fatigue behavior of n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> -type 2-mum- thick polycrystalline silicon films that exhibit an initially thin (~2-3 nm) native oxide layer. The testing of kilohertz-frequency resonators provided accurate stress-life fatigue data at 30 and 50% relative humidity (RH) in the low (< 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> )and high (up to 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">11</sup> ) cycle regimes. Long fatigue life specimens were associated with larger decreases in the natural frequency of the resonator and very smooth failure origins (at the notch) that encompassed several grains. Additional testing at various humidity levels highlighted the critical influence of humidity on the fatigue damage accumulation rate, which was measured via changes in the natural frequency. Finally, Auger electron spectroscopy (AES) characterized the formation of a nanometer-scale oxygen-rich reaction layer during cyclic loading. Although AES revealed a thin 2-3-nm initial oxide layer on a control specimen, measurements on a long-life fatigued specimen revealed an increased oxygen concentration over the first 10 nm of the material at the notch root. These findings demonstrate that the reaction-layer fatigue mechanism for silicon structural films operates even when reaction layers are initially very thin.

X-ray photoelectron spectroscopic study of the formation of catalytic gold nanoparticles on ultraviolet-ozone oxidized GaAs(100) substrates

A comprehensive study of the oxide formed on GaAs by exposure to ultraviolet light and ozone (UV-ozone), and the evolution of Au nanoparticles on UV-ozone treated GaAs(100) substrates, has been carried out using x-ray photoelectron spectroscopy. The UV-ozone oxide has been found to consist of oxides of gallium and arsenic in addition to having a significant contribution from elemental arsenic concentrated mostly at the substrate-oxide interface. An intermediate suboxide species −AsxGa2O is found to make up a substantial part of the oxide layer, mostly near the surface region, whereas the interface to bulk GaAs is dominated by Ga2O3. As2O3 is found to be the dominant oxide in the initial UV-ozone oxide layer, while the oxide layer becomes rich in Ga2O3 as the sample is annealed at 500 °C for 300 s in a dry nitrogen environment. It is also found that gold does not alloy with Ga during the formation of gold nanoparticles by dewetting from the UV-ozone oxidized GaAs substrate.

Local study of thickness-dependent electronic properties of ultrathin silicon oxide near SiO2/Si interface

We have used scanning tunnelling microscopy and spectroscopy (STM/STS) to investigate the electronic properties of ultrathin silicon oxide films on silicon, e.g. surface band gap and band offset versus silicon oxide thickness near the interface. The length of the interfacial electronically transitional region is measured to be about 0.9 nm, in which the silicon oxide surface band gap increases gradually with thickness and its evolution is meticulously observed. Both the conduction and valence band offsets between silicon oxide and silicon increase by about 1.0 eV with only a ∼0.3 nm steric difference in this region. Furthermore, in an oxide thinner than ∼0.6 nm, oxide gap states can be discerned from STS which is attributed to the electronic states extending from the silicon substrate over the Si oxide. On the basis of local layer-resolved electronic properties, we corroborate that a usable oxide thickness should be no less than ∼0.9 nm on the Si(1 1 1) substrate, which corresponds to only three initial oxide layers.

DNA microarrays on silicon nanostructures: Optimization of the multilayer stack for fluorescence detection

To improve the sensitivity of fluorescence detection in DNA microarrays, the use of silicon nanostructures based on chemical vapor deposition (CVD) processes adopted for the growth of rough polycrystalline silicon was investigated. These substrates present advantages of two main properties which could lead to an enhancement of the fluorescence detection, i.e. (i) the increase of the available surface area in order to achieve a high loading capacity of biomolecules and (ii) the optimization of the stack of silicon nanostructures support. Indeed, the structures were elaborated on an initial thermal oxide layer and then covered with a silicon oxide layer, obtained by oxidation and allowing the functionalization for the subsequent grafting of DNA probes. Moreover, these oxide layers play a part in the fluorescence detection. The influence of the silicon oxide layer thickness above and below the silicon grains in close relation with the density of nanostructures on the emitted fluorescence was emphasized. This paper presents an experimental characterization of the fluorescence intensity and the optimization of the different layers that composed the substrate used for DNA microarrays. The performances of the microarrays were investigated by means of hybridization experiments using complementary fluorescent labeled-oligonucleotides targets. Our results indicate that an optimized substrate can be designed and that the use of oxidized silicon nanostructures for support of biochip could be a strategy for improving the sensitivity of fluorescence detection.

Atmospheric corrosion of steel reinforcing bars produced by various manufacturing processes

The aim of the present investigation is to evaluate the effect of manufacturing process upon the performance of reinforcing steel bars against atmospheric corrosion. Reinforcing steel bars produced by different manufacturing methods were exposed to the atmosphere in Athens for different periods of time. The progress of corrosion was evaluated by measuring the thickness of the corrosion layer formed on the surface of the bars. The morphology of the corroded layer was studied by optical and scanning electron microscopy (SEM), while X-ray diffraction analysis was used to identify the mineralogical composition of the corrosion products. Independently of the manufacturing process used, the main corrosion product formed was lepidocrocite (γ-FeOOH), followed closely by akaganeite (β-FeOOH) and goethite (α-FeOOH). Furthermore, it was found that the surface condition and the initial oxide layer on the steel bars, which is closely related to the manufacturing process, played a major role in the development of corrosion in atmosphere. Work-hardened reinforcing steel bars were found very sensitive to exhibition in the usual atmospheric conditions, followed by the Tempcore steel bars. The hot-rolled bars, either unalloyed or microalloyed with vanadium, presented the best corrosion behavior.

Characteristics of the kissing-bond in friction stir welded Al alloy 1050

Initial oxide layer on the butt surface fragments during friction stir welding (FSW) often remaining as a faint zigzag-line pattern on the cross section. When remnants of the oxide layer often adversely affects the mechanical properties in the weld, it is called as “kissing-bond”. The present study systematically examines the effect of oxide array on the bend property in the root region of friction stir (FS) welded Al alloy 1050 by transmission electron microscopy (TEM) to clarify the identity of “kissing-bond”.

Pre-treatment on metal surface for plasma spray with cathode spots of low pressure arc

The cathode spots of the low pressure arc can remove the oxide layer and evaporate impurities on the metal surface. The removal of the oxide layer by using the cathode spots is expected to solve the recent problems of the chemical and mechanical cleaning methods. The phenomena of the cathode spots for pre-treatment of Atmospheric Pressure Plasma Spray (APPS) have been investigated. In this paper, the surface shapes of oxide and non-oxide work pieces were treated by using a composite pre-treating method: the cathode spots after grid blasting. In addition, they are compared with conventional cathode spots treated and mechanical blasted surfaces. As the results, the roughness on the surface of work pieces is arisen with the initial oxide layer thickness. It reveals the factors dominating bonding strength on the surface pre-treated by using the cathode spots of the low pressure arc. The bonding strength increases with the arithmetical mean height of the surface roughness due to the initial oxide layer thickness. The bonding strength is raised when the mean spacing of profile irregularities becomes narrow. The bonding adhesive strength treated by the cathode spots after the grid blasting is over 90 MPa.

FIB-assisted TEM study of an oxide array in the root of a friction stir welded aluminium alloy

A local region in the preferentially etched line at the root part of a friction stir welded Al alloy was directly observed by TEM. TEM clarified that the preferentially etched line consisted of zigzag region having a high density of amorphous Al 2O 3 particles which originated from the initial butt surface oxide layer.

Two stage oxidation in epitaxial Ni (111)/GaN (0001) thin films

We present the oxidation process of epitaxial Ni (111)/GaN (0001) thin films studied by in situ synchrotron x-ray scattering, scanning electron microscopy, and transmission electron microscopy. By monitoring the evolution of the Ni (111) Bragg reflection, we reveal that two distinct oxidation processes occur. Initially, a continuous NiO layer of about 50 Å thickness is formed on the surface of Ni. The planar oxide layer saturates immediately and passivates the film from further surface oxidation. From this stage, the oxidation proceeds by means of the growth of surface oxide islands. The Ni atoms diffuse out through the defect sites running vertically through the initial oxide layer to form the oxide islands. Voids are generated underneath the oxide layer in this process. The oxide in the second stage grows logarithmically in time with the activation energy of about 0.15 eV.

In Situ Study of the Surface Oxidation of FeCr Alloys Using Grazing Incidence X‐ray Absorption Spectroscopy (GIXAS)

AbstractThe surface oxidation of FeCr alloys with 18, 28, and 43 mass‐% Cr was investigated in situ using grazing‐incidence X‐ray absorption spectroscopy (GIXAS) at the chromium and iron K‐edges. Oxidation in air was monitored in situ in the temperature range from 290 K to 680 K. The standard GIXAS data analysis is extended for the treatment of a single layer model in order to estimate the chromium concentrations of the oxide layer and of the near‐interface substrate as well as the oxide layer thickness. XANES analysis shows transitions from b.c.c. Fe to corundum type Fe2O3 and from b.c.c. Cr to corundum type Cr2O3. The initial oxide layers are 1.1‐1.4 nm thick and contain 60‐90 mass‐% chromium, while the near‐interface substrate is depleted in Cr. During heating, iron oxide growth dominates up to 560‐600 K. Then the chromium oxide layer loses its passivation effect and Cr oxidation sets in.

Influence of the bath composition and the pH on the induced cobalt–molybdenum electrodeposition

Experimental electrodeposition parameters (pH, cobalt(II), citrate and molybdate concentrations) were varied in order to analyse their influence on induced cobalt–molybdenum codeposition. Voltammetry was the main technique used. In Co–Mo electrodeposition an initial molybdenum oxide layer is formed, over which the alloy is deposited. The process depends on the nature of the species in solution. At quasi-neutral pH, CoCit − is the main Co(II) species when there is sufficient complexing agent. Thus, cobalt is deposited as a result of the reduction of CoCit −, and molybdenum is reduced from MoO 4 2− to MoO 2. This intermediate oxide evolves to molybdenum via the formation of [MoO 2–CoCit −] ads. In acidic media, molybdate complexes with citrate, so molybdenum oxides are formed from H r MoO 4Cit [5− r] . On the other hand, cobalt is deposited mainly by CoHCit reduction. CoCit −/CoHCit and molybdenum oxides are reduced simultaneously, so cobalt nuclei are always needed to induce the codeposition.

Tunnel magnetoresistance with plasma oxidation time in doubly oxidized barrier process

Magnetic tunnel junctions (MTJ) with the tunnel barrier oxidized in two steps with plasma were fabricated to obtain a structurally uniform AlOx insulator. Plasma oxidation of 10 A-thick Al layer for 5–20 sec formed the initial oxide barrier on top of which a second oxide layer was deposited by oxidizing 13 A-thick Al for 120 sec. The doubly oxidized junctions exhibited a magnetoresistance (MR) ratio of 27–31% depending on the oxidation period of the initial oxide layer while only 24% was obtained for the junction deposited in one-step oxidation with a similar stack configuration. The junction resistance of the MTJ increased monotonically with oxidation time while minimal deterioration of the MR ratio was observed when oxidation time increased from 5 sec to 20 sec. Transmission electron microscopy of the junctions also confirmed that the AlOx thickness was thinner for the doubly oxidized junctions compared to the singly oxidized MTJ. X-ray photoelectron spectroscopy of the double junction also strongly suggested that the initial oxide layer prevents over-oxidation of the bottom electrode. Our results suggest that the AlOx oxidized in two steps produces improved junction performance as well as a wider processing window.

Electrochemical investigation of the oxides formed at the surface of brass (Cu–10Zn) by a humid-air plasma treatment

Linear potential sweep voltammetry was used to characterize the metal oxides formed at the surface of brass (Cu–10Zn) foils exposed to a gliding-arc-humid-air plasma. This study confirmed the formation of copper and zinc oxides (i.e. Cu 2O, CuO, and ZnO). The electrochemical reduction curves in aqueous solutions of the different brass samples showed that copper and zinc oxides were characterized by their cathodic peaks. The reduction potentials of the oxides varied with the exposure time to the plasma at fixed working conditions. The analysis of the voltammetry curves showed that the initial oxide layer was formed in majority by the precursor oxide Cu x O ( x>4) and Cu 2O. The oxidation of Cu(I) oxide (Cu x O and Cu 2O) lead to the formation of CuO. When the CuO layer was extensively grown, the ZnO oxide began to grow. A mechanism was proposed for the reduction of thin films containing Cu 2O, CuO and ZnO oxides formed at the surface of the sample.

Magnetic tunnel junctions with doubly-plasma oxidized AlOx insulation layer

Magnetic tunnel junctions (MTJ), with the tunnel barrier plasma oxidized in two steps, were fabricated in order to obtain structurally uniform AlOx insulator. The doubly oxidized junctions exhibited the magnetoresistance (MR) ratio of 27–31% without showing any noticeable drop in the MR ratio even after oxidation time was extended well beyond the optimal oxidation time for the normal junctions. Transmission electron microscopy of the junctions confirmed that the AlOx thickness was thinner for the doubly oxidized junctions compared to the singly oxidized MTJ. X-ray photoelectron spectroscopy of the doubly oxidized junction also strongly suggested that the initial oxide layer prevents the over-oxidation of the bottom electrode. The AlOx tunnel barrier oxidized in two steps improved the junction performance and widened the processing window.

Effects of additive and impurity species on the oxide morphology of silicon nitride

Si3N4 sintered with Lu2O3 was exposed to 1 atm of oxygen and steam environments at 1200–1400 °C. The effects of additive and impurity species on the morphological development of the oxide layer were examined. Oxide layers grown on as-received samples in oxygen generally contained bubbles and cracks and underwent spallation due to the presence of an initial impurity-laden oxide layer. Oxide layers grown on as-received samples in steam exhibited layered morphology: a glassy outer layer and a cristobalite inner layer with a high population density of Lu2Si2O7 particles between. The Lu2Si2O7 particles accumulated at the interface led to extensive spallation of the upper oxide layer. Removal of the initial oxide by polishing resulted in improved oxidation resistance and oxide morphology in oxygen and in steam.

STS investigations to describe corrosion procedures on thin CrN layers.

To represent the corrosion characteristics of thin hard material layers, CrN layers, which were magnetron sputtered, have been exposed to solutions with chloride or sulfate ions. The alterations of the layer surfaces were examined. Static immersion tests were carried out ex-situ in 1 mol L(-1) NaCl or 1 mol L(-1) Na(2)SO(4) solution over a period of 14 months. For in-situ measurements, the layers were inserted into the electrochemical cell of an EC STM as the working electrode. The sample potential was altered cyclically. At the same time, voltammograms were taken and the topography alteration was observed. Alterations in the surface structure were documented as hybrid information resulting from changes in topography and the chemical consistency or the thickness of the oxide layers. These modifications were demonstrated with scanning tunnelling spectroscopy before and after the treatment with solutions. Several chemical phases (initial state and oxide layers) were distinguished with raster tunnelling spectroscopy (measurements I(V) and I(z)). The evaluation of the I(z) spectroscopy showed that the tip was in mechanical contact with the sample if an oxide layer of greater thickness has existed. Exemplary XPS investigations demonstrated corrosive alterations of the CrN layers due to oxidation. Asymmetry and peak locations suggest that in addition to Cr(2)O(3) and CrN a complex mixture of different chromium oxides such as Cr(2)O(3) and Cr(OH)( x) exists.

Study of interface formation of (Ba,Sr)TiO3 thin films grown by rf sputter deposition on bare Si and thermal SiO2/Si substrates.

ABSTRACT(Ba,Sr)TiO3 (BST) thin films were deposited by ion beam sputtering on both bare and oxidized Si. Spectroscopic ellipsometry (SE) model results have shown an increase in the SiO2 layer thickness for bare substrates and those with a 1 nm initial oxide layer, and a decrease for thicker (3.5 nm) initial SiO2 films. This result was confirmed by high resolution electron microscopy (HREM) analysis of the films, and it is believed to be due to simultaneous subcutaneous oxidation of Si and reaction of the BST layer with SiO2. From high-frequency capacitance-voltage (C-V) analysis, a decrease in the interface trap density Dit of an order of magnitude was observed for oxidized Si substrates.

Study of interface formation of (Ba,Sr)TiO3 thin films grown by rf sputter deposition on bare Si and thermal SiO2/Si substrates.

ABSTRACT(Ba,Sr)TiO3 (BST) thin films were deposited by ion beam sputtering on both bare and oxidized Si. Spectroscopic ellipsometry (SE) model results have shown an increase in the SiO2 layer thickness for bare substrates and those with a 1 nm initial oxide layer, and a decrease for thicker (3.5 nm) initial SiO2 films. This result was confirmed by high resolution electron microscopy (HREM) analysis of the films, and it is believed to be due to simultaneous subcutaneous oxidation of Si and reaction of the BST layer with SiO2. From high-frequency capacitance-voltage (C-V) analysis, a decrease in the interface trap density Dit of an order of magnitude was observed for oxidized Si substrates.

A Microscopic and Analytic Study of Aluminum Particles Agglomeration

Aluminum powders added to solid propellants burn either as single particles or agglomerates. Combustion efficiency is very dependent on the final aluminum particle size injected into the chamber flowfield. This work has investigated experimentally the microscopic behavior of powdered aluminum particles during gradual heating in inert and oxidative atmospheres. Effects of surrounding gas composition, particle size, initial oxide layer thickness, and dilution by refractory compound on the particles behavior during heating have been studied. A novel model of a crucial stage in aluminum particles agglomeration based on estimation of mechanical stresses appearing in the oxide layer during particle heating has been suggested. Results of estimation confirm that disruption of oxide layer takes place during aluminum melting. This disruption results in the flow of a molten metal through cracks formed in the oxide layer and in the formation of large liquid drops. Results of theoretical calculations are in a good agreement with experimental data.