Oxide Barrier Layer Research Articles

Pitting of carbon steel in the synthetic concrete pore solution

AbstractPitting corrosion is a possible mode of failure of the carbon steel overpack of the Belgian supercontainer concept for the isolation of high‐level nuclear waste (HLNW). However, no firm experimental data are currently available to estimate the probability of failure over the extended storage time (100,000 years). Extensive work shows that passivity breakdown results from the condensation of cation vacancies (CVs) at the metal/barrier layer (m/bl) interface, in response to the absorption of Cl− into oxygen vacancies at the surface of the barrier oxide layer. The CVs migrate across the bl to the m/bl interface where they condense, leading to the separation of the bl from the metal. The resulting blister prevents the growth of bl into the metal and dissolution results in blister rupture, marking a passivity breakdown event. Stabilization via differential aeration produces a potentially damaging, stable pit. We review our work on passivity breakdown and the nucleation of pits on P355 QL2 carbon steel in high‐pH aqueous media typical of concrete pore solution, with emphasis on the mechanistic aspects. We conclude that failure of the carbon steel overpack containing the HLNW over a storage horizon of 100,000 years is improbable.

Ultra-short pulse simulation for characterising oxide layer formation on stainless steel during μECM

Electrochemical machining is a controlled metal shaping process based on anodic dissolution performed by the electrode reactions. When pulse signals, in the micro-second or less range, are combined with a small inter-electrode gap, the process is referred to as Micro-Electro-Chemical machining or μECM. The application of using nano-second/micro second pulses for μECM for certain passivation metals can be very limited by the formation of an oxide layer that prohibits the penetration of ultra-short pulse durations due to the capacitive effect of the oxide layer. This paper will present findings from the development of a μECM numerical model that incorporates an advanced hybrid time stepping approach for representing nano-second pulse interactions with the physical phenomena of the electrolytic environment associated with μECM, thereby enabling the characterisation of the capacitive model of the formed oxide barrier layer. Experimental work was conducted using nano and micro-second pulses on the machining of 18CrNi8 and copper using NaNO3 electrolyte. The μECM simulation model contributed to quantifying the lack of faradic machining of 18CrNi8 that was experimentally observed. A particular solution, believed to be effective in breaking through the oxide film, is to directly pre-polarize the layer by super imposing a DC signal prior to the ultra-short pulses, such that its capacity has been already loaded when the short pulse signal is applied. However, too long of a DC signal application time can lead to machining damage. Through the development of an ultra-short pulse simulation model, this work has established, that we can numerically determine an optimal DC signal duration necessary to load the oxide layer just enough for facilitating anodic dissolution during physical μECM process, thus minimising machining damage.

The role of barrier layer temperature in the formation of long and small-diameter TiO2 nanotube arrays

Small-diameter TiO2 nanotubes (TNTs) are fabricated at a fast growth rate by developing an effective anodization method in an organic electrolyte. Two different temperatures are applied to both sides of sample in order to increase the current density during the anodization process. Here, we use a high temperature for the backside of the sample (direct heating of the barrier oxide layer) in order to increase the current density while keeping the electrolyte at a low temperature to decrease the chemical etching at top of the TNT arrays. Increasing the backside temperature up to 55 °C leads to the formation of longest TNTs with an average diameter of about 17 nm at high-speed TNT growth of about 2000 nm/h under 20 V. Based on the high-field theory and accurate estimation of the barrier layer (BL) temperature, the incremental effect of increasing the BL temperature on the anodization current is also investigated.

Electrochemical corrosion behaviour of Sn–Sb solder alloys: the roles of alloy Sb content and type of intermetallic compound

ABSTRACT Sn–Sb solder alloys (2; 5.5 and 10wt-% Sb) were directionally solidified with a view to permitting the effect of a wide range of solidification cooling rates to be related to the resulting microstructure, which is shown to be formed by a cellular Sn-rich matrix with Sn–Sb intermetallic particles (IMCs) randomly distributed in the matrix The corrosion resistance of these alloys is investigated by electrochemical impedance spectroscopy (EIS), equivalent circuit and linear polarisation and the results correlated with microstructural features. The increase in the alloy Sb content is shown to change the nature/morphology of the Sn–Sb IMCs, consequently affecting the corrosion resistance. The EIS data indicated that the Sn–10wt-%Sb alloy has the best corrosion resistance because of the high resistance of its oxide barrier layer. The polarisation curves also indicate lowest corrosion rate and nobler potential related to the Sn–10wt-%Sb alloy, which has also been confirmed by calculations of polarisation resistance.

Barrier performance of ITO film on textured Si substrate

Indium tin oxide (ITO) film is the most widely used as front electrodes in solar cells with a copper metallization scheme. No work has focused on the barrier properties of the ITO layer on the textured silicon for solar cells. In this work, a thin indium tin oxide barrier layer and copper metal layer were deposited on textured (001) silicon by a sputtering. The stacks present Cu/ITO/Si. The structures of Cu/ITO/Si characterized by scanning transmission electron microscope, energy-dispersive X-ray spectrometer, and powder X-ray diffraction. The results show that the stacks of Cu/ITO/Si can be preserved up to 600 °C. The 35-nm thickness ITO layer was found to be a diffusion barrier against Cu up to 600 °C. The copper thin films were agglomerated and formed the particle at a temperature of 700 °C. The failure of Cu/ITO/Si can be attributed to agglomerate copper thin films and breakdown of ITO thin films at a temperature of 700 °C.

Inverse‐direction Growth of TiO2 Microcones by Subsequent Anodization in HClO4 for Increased Performance of Lithium‐Ion Batteries

AbstractThe effect of second anodization on anodically fabricated TiO2 microcones is investigated with the aim of enhancing the lithium‐ion battery performances; these microcones are treated with a trace concentration of an electrolyte of HClO4. Unlike in the case of second anodization in H2SO4 or H3PO4, which leads to breakdown of the structures of the microcones, the HClO4‐treated TiO2 microcones maintain their unique morphological crystalline structures; this enables the inverse growth of an oxide layer. Particularly, protruding nanoparticles provide a large surface area and thus a higher areal capacity, wherein lithium ions can be stored; these particles are formed on the back‐hemisphere layers of the microcones with an increase in the overall thickness of the barrier oxide layer. The HClO4‐treated TiO2 microcones exhibit an areal capacity of approximately 1.6 times than that of pristine TiO2 microcones as well as an excellent cycling stability and capacity retention.

Process deviation based electrical model of voltage controlled magnetic anisotropy magnetic tunnel junction and its application in read/write circuits

As one of the primary elements in magnetoresistive random access memory (MRAM), voltage controlled magnetic anisotropy magnetic tunnel junction (VCMA-MTJ) has received wide attention due to its fast read and write speed, low power dissipation, and compatibility with standard CMOS technology. However, with the downscaling of VCMA-MTJ and the increasing of storage density of MRAM, the effect of process deviation on the characteristics of MTJ becomes more and more obvious, which even leads to Read/Write (R/W) error in VCMA-MTJ circuits. Taking into account the depth deviation of the free layer (<i>γ</i><sub>tf</sub>) and the depth deviation of the oxide barrier layer (<i>γ</i><sub>tox</sub>) in magnetron sputtering technique as well as the etching process stability factor (<i>α</i>) caused by the sidewall re-deposition layer in the ion beam etching process, the electrical model of VCMA-MTJ with process deviation is presented in the paper. It is shown that the VCMA-MTJ cannot achieve the effective reversal of the magnetization direction when <i>γ</i><sub>tf</sub> ≥ 13% and <i>γ</i><sub>tox</sub> ≥ 11%. The precession of magnetization direction in VCMA-MTJ also becomes instable when <i>α</i> ≤ 0.7. Furthermore, the electrical model of VCMA-MTJ with process deviation is also applied to the R/W circuit to study the effect of process deviation on the R/W error in the circuit. Considering the fact that all of <i>γ</i><sub>tf</sub>, <i>γ</i><sub>tox</sub>, and α follow Gauss distribution, The 3<i>σ</i>/<i>μ</i> is adopted to represent the process deviation, with using Monte Carlo simulation, where <i>σ</i> is the standard deviation, and <i>μ</i> is the average value. It is shown that the write error of the circuit goes up to 30 % with 3<i>σ</i>/<i>μ</i> of 0.05 and the voltage (<i>V</i><sub>b</sub>) of 1.15 V. At the same time, the read error of the circuit is 20% with 3<i>σ</i>/<i>μ</i> of 0.05 and driving voltage (<i>V</i><sub>dd</sub>) of 0.6 V. Both the read error rate and the write error rate of the VCMA-MTJ circuit increase as process deviation increases. It is found that the write error rate can be effectively reduced by increasing <i>V</i><sub>b</sub> and reducing the voltage pulse width (<i>t</i><sub>pw</sub>). The increasing of <i>V</i><sub>dd</sub> is helpful in reducing the read error rate effectively. Our research presents a useful guideline for designing and analyzing the VCMA-MTJ and VCMA-MTJ read/write circuits.

Characterization of ceramic layers for thermal barriers coatings

Thermal barrier coatings (TBC) represent the most effective system to protect structural components from damage caused by high temperature and corrosive/erosive environments. Yttria-stabilized zirconia (YSZ) is among the most studied ceramics for such applications. The adherence of the ceramic layer to the metallic object needing thermal protection is one of the critical issues in real operation conditions at high temperature or under thermal shocks. A bond coat is necessary to ensure the adherence of the YSZ top coat to the substrate. In our work, Y, Ta doped NiCrAl compounds bond coat obtained by air plasma spray (APS – Fig. 1) and high velocity air fuel (HVAF) have been subjected to increasing temperature thermal treatments (700 – 1 300 oC/5 h). The system behavior under thermal stress has been investigated by microstructural methods including XRD, SEM and TEM. The TEM/STEM results (Fig. 2) show the oxidation resistance of the HVAF vs APS deposited bond coats. Spurious Y-Ta-O structures have been evidenced which otherwise failed to be observed by conventional XRD/SEM-EDS (Fig. 3). The identified microphases which appear at the high temperature represent the nucleation point for the layer of thermally grown oxide (TGO) with a decisive role in the delaminating process at high temperature. We show that in the APS-deposited bond coat the TGO was distributed in the whole mass of the bond coat, while in the HVAF-deposited bond coat the TGO is pushed toward its outer surface. Formation of TGO is initiated both at the ceramic-bond layer interface and into the pores existing inside the bond layer. The size and morphology of the pores inside the deposited layers depend on the deposition method (ex: APS or HVAF). Fracture occurs at the YSZ/TGO interface. Below fracture, the TGO to bond coat transition is abrupt (~ 5 – 10 µm). Diffuse Al2O3 can be observed in TGO/BC region (2), which segregates at the BC/substrate interface (3); TGO does not built in a compact oxidation barrier layer. Ta, Y, Ti segregations form into the top coat, bond coat and substrate.

Effect of Cr content on the corrosion resistance of Ni\u2013Cr\u2013P coatings for PEMFC metallic bipolar plates

In here, we report on the pulse electrodeposition of nickel–chromium–phosphorous (Ni–Cr–P) coatings on AISI 1020 low carbon steel using an aqueous electrolyte consisting of NiCl2, CrCl3, and NaH2PO2. We evaluated the effectiveness of Ni–Cr–P coatings for polymer electrolyte membrane fuel cell metallic bipolar plates. Coatings deposited at pH 3.0 and room temperature show nearly three orders improvement in corrosion resistance compared to bare AISI 1020. The corrosion current (Icorr) of Ni–Cr–P samples coated at 25 °C is 1.16 × 10−4 A/cm2, while that of bare carbon steel is 1.05 × 10−2 A/cm2. The improvement in corrosion resistance is due to the increase in Cr content in the Ni–Cr–P coatings. Cr forms a stable oxide barrier layer and inhibits pitting corrosion. The interfacial contact resistance increases with an increase in Cr content and immersion time in the corrosion media. The increase in interfacial contact resistance is also due to the formation of a stable oxide barrier.

Ferroelectric Tunnel Junctions Enhanced by a Polar Oxide Barrier Layer

Ferroelectric tunnel junctions (FTJs) have recently aroused significant interest due to the interesting physics controlling their properties and potential application in nonvolatile memory devices. In this work, we propose a new concept to design high-performance FTJs based on ferroelectric/polar-oxide composite barriers. Using density functional theory calculations, we model electronic and transport properties of LaNiO3/PbTiO3/LaAlO3/LaNiO3 tunnel junctions and demonstrate that an ultrathin polar LaAlO3(001) layer strongly enhances their performance. We predict a tunneling electroresistance (TER) effect in these FTJs with an OFF/ON resistance ratio exceeding a factor of 104 and ON state resistance as low as about 1 kΩμm2. Such an enhanced performance is driven by the ionic charge at the PbTiO3/LaAlO3 interface, which significantly increases transmission across the FTJ when the ferroelectric polarization of PbTiO3 is pointing against the intrinsic electric field produced by this ionic charge. This is due to the formation of a two-dimensional (2D) electron or hole gas, depending on the LaAlO3 termination being (LaO)+ or (AlO2)-, respectively, which is formed to screen the polarization charge of the nonuniform polarization state. This 2D electron (hole) gas can be switched ON and OFF by the reversal of ferroelectric polarization, resulting in the giant TER effect. The proposed design suggests a new direction for creating FTJs with a stable and reversible ferroelectric polarization, a sizable TER effect, and a low-resistance-area product, as required for memory applications.

Nickel Electrochemical Deposition onto the Surface of Anodized Aluminum

The use of ordered anodic aluminum oxide for nickel electrochemical deposition is studied. The conditions for aluminum anodization in a bath based on oxalic acid, thinning of the barrier oxide layer, and nickel electrochemical deposition from a sulfamate bath are described. The deposited nickel has a lattice constant of 3.51 A and a texture in which the (100) grain planes are parallel to the surface. The surface morphologies of the anodic oxide layer and the upper and lower parts of the deposited nickel layers are examined using electron microscopy. The mean size of the oxide pores is 50 nm, the mean interpore diameter is 102 nm, and the oxide porosity is 21.5%. The surface topography of the bottom of the deposited nickel layer features a nanoscopic pattern formed by rods, which is identical in pore size and morphology to the oxide surface. The rod length is defined by the thickness of the anodic oxide layer formed on the aluminum-substrate surface. This feature is due to the homogeneous filling of oxide pores during nickel electrodeposition. We find that thinning of the anodic oxide layer to below 10 nm may impair adhesion of the oxide layer to the aluminum substrate and thus lead to nickel deposition on both sides of the oxide layer. The formation of dendrites is noted, and the possible causes of this as well as conditions favoring their growth are discussed.

Impedance of an Aluminum Electrode with a Nanoporous Oxide

The methods of electrochemical impedance spectroscopy and electron microscopy with X-ray electron probe analysis are used to study two types of oxide films on aluminum: a smooth compact one and a porous one (nanooxide) formed by two-stage oxidation. An equivalent circuit is chosen that agrees well with the obtained impedance spectra and allows estimating the thickness and conductivity of barrier oxide layers and also the area occupied by nanopores for various film types.

In-situ Ti-6Al-4V/TiC composites synthesized by reactive spark plasma sintering: processing, microstructure, and dry sliding wear behaviour

Titanium carbide (TiC) reinforced Titanium Matrix Composites (TMCs) have been synthesized via an in-situ reactive spark plasma sintering (SPS) process using commercial Ti-6Al-4V spherical powders pre-coated with 1 wt% carbon nanoparticles by low-energy ball milling. Graphite flakes are used as carbon source, which aids powder flow during mixing as lubricant. Graphite transforms to nano-crystallite carbon during mixing which is favourable for the rapid formation of TiC second phase in the following SPS process. The composites exhibited a novel honeycomb-like cellular microstructure with the formation of 5–6 vol% fine TiC submicron grains interconnected in the titanium α/β matrix. In addition, the reinforcement of the TiC phase with a nano-hardness of 12.4 GPa, improves the wear resistance of the parent alloy matrix (5.1 GPa), with a reduction of 26–28% in wear rate during dry reciprocating sliding tests against Si3N4 balls. During sliding, the wear debris (predominantly anatase TiO2) builds up on the raised TiC hard phase forming a barrier layer of adhered oxide that can protect the alloy matrix underneath from abrasion and oxidation, leading to a reduced wear rate.

Opto-electronic properties of anodized TiO2 nanotube arrays investigated using electron energy loss spectroscopy

A study of the nanoscale crystallinity of anodized TiO2 nanotubes is reported with the aim of demonstrating its influence on the localized optical and electronic properties of the structure. By employing scanning transmission electron microscopy, coupled with electron energy loss spectroscopy, x-ray diffraction and electron energy filtered jump-ratio imaging to probe changes in the electron near edge fine structure of the Ti L3,2 ionization edge, it is found that nanotubes annealed at 450 °C in air for 3 h crystallize in the anatase polymorph along their walls, with the underlying thick oxide barrier layer being predominantly rutile. An increase in annealing temperature to 600 °C results in the co-existence of anatase and rutile along the nanotube length with the nanotube top and bottom showing crystallization in anatase and rutile, respectively. Valence EELS shows that the localized energy band-gap is inconsistent along the nanotube length, deviating between that typically measured for anatase (3.2 eV) and rutile (3.0 eV). The obtained band-gap of 3.21 eV at the nanotube top and 3.44 eV at nanotube bottom is attributed to the co-existence of anatase and rutile phases even at a low annealing temperature of 450 °C.

On the nature of the electric field within the barrier layer of a passive film

In this work, the nature of the electric field (E) in the defective oxide barrier layer on iron has been explored theoretically, within the classical regime, with the goal of ascertaining the validity of the postulates underlying the Point Defect Model (PDM). The analysis shows that if the thickness of the barrier layer under steady-state condition, L, is sufficiently small, the electric field strength and concentrations of point defects are essentially constants in the main part of this layer (with the exceptions in the very thin region near the metal/barrier layer (m/bl) and barrier layer/solution (bl/s) interfaces, depending upon the defect). A quantitative criterion was developed to decide when the barrier layer of the passive film can be considered to be sufficiently thin under these conditions. The potential drops inside the barrier layer and across the m/bl and bl/s interfaces can be represented as linear functions of applied voltage, V. This is the consequence of the change in thickness with applied voltage, with E being approximately (but with high accuracy) independent on the position inside the barrier layer and applied voltage. All conclusions stated above were derived from the solution of the system of mass transport equations for point defects and Poisson equation for the electric potential without applying any additional assumption except the small thickness of the barrier layer and without taking into account quantum (band-to-band) tunneling.

Native Oxide Barrier Layer for Selective Electroplated Metallization of Silicon Heterojunction Solar Cells

The metallization of silicon heterojunction (SHJ) solar cells by electroplating of highly conductive copper onto a multifunctional patterned metal layer stack is demonstrated. The approach features several advantages: low temperature processing, high metal conductivity of plated copper, no organic making, and low material costs (almost Ag‐free). A PVD layer stack of copper and aluminum is deposited onto the cell subsequently to TCO deposition. The aluminum layer is patterned with a printed etchant and its native oxide on the remaining areas inhibits plating. The full area aluminum layer while electroplating supports plating current distribution and allows homogeneous plating height distributions over the cell. The NOBLE (native oxide barrier layer for selective electroplating) approach allows reaching a first encouraging SHJ solar cell efficiency of 20.2% with low contact resistivity.

Fabrication of porous anodic alumina (PAA) templates with straight pores and with hierarchical structures through exponential voltage decrease technique

The present work aims to become a standard step for the fabrication of the next generation porous anodic alumina (PAA) templates based devices and to provide new insights on the mechanism involved in the porous anodic alumina formation. The oxide barrier layer at the bottom of the pores has been successfully thinned by applying an exponential voltage decrease process followed by a wet chemical etching. The impact of the potential drop on the PAA structure has been deeply investigated, as well as the electrolyte temperature, the number of potential steps and the exponential decay rate. The presented results underline that a smart adjustment between the anodization conditions and exponential voltage decay parameters can simultaneously give PAA structures with straight pores and remove the dielectric layer in spite of applying the exponential voltage decay step. Additionally, the PAA structure has been tuned to fabricate hierarchically nanoporous templates with secondary pores ranging from 2 up to 10 branches.

Time-dependent water vapor permeation through multilayer barrier films: Empirical versus theoretical results

Multilayer films that comprise alternating inorganic barrier and polymer layers deposited on a flexible substrate are often used to protect organic electronic devices from degradation caused by oxygen and water vapor. We tested films consisting of up to 11 inorganic silicon oxide barrier and polymer layers to characterize the water vapor permeability, solubility and diffusivity of each layer based on measurements of water sorption, transient water vapor permeation and lag times. The time-dependency of transient water vapor transmission rates (WVTR) of the structures containing a barrier-polymer-barrier sequence was also estimated using a so-called quasi-steady-state (QSS) approximation. The permeability values and lag times in multilayer structures predicted by QSS approximation agreed with the values determined empirically based on time-dependent WVTR measurements. The steady-state WVTR of the multilayer barrier films was dominated by the permeability of the inorganic barrier layers, whereas the solubility coefficient and thickness of the interleaved polymer layers determined the lag times. The barrier performance and lag time of multilayer structures can be predicted using the parameters obtained from sorption measurements, thus significantly reducing the time required for experiments. The results of this study will allow us to design multilayer barrier films according to the lifetime requirements of flexible organic electronic devices.

Enhancing the long-term stability of Ag based seals for solid oxide fuel/electrolysis applications by simple interconnect aluminization

Ag-(0–8 mol%) CuO is used to successfully join aluminized ferritic stainless steel interconnect to the ceria-gadolinia (CGO) barrier layer of a solid oxide fuel/electrolysis cell by reactive air brazing at 1000 °C in air. The wetting of AgCuO on CGO is tailored by varying the CuO content. The effects of the CuO content on the joint microstructure are discussed. The long-term stability of brazed joints is evaluated by aging in oxidizing (air) and reducing (4% H2–50% H2ON2) atmospheres at 800 °C for 250 h. An Ag-2mol% CuO braze results in the best joint stability during aging. Aluminization of the steel to create an alumina surface layer provides excellent protection of the steel both during the joining process and aging in the 2 atm. No degradation related to steel corrosion and outward diffusion of elements from the steel can be observed.

Experimental and computational studies of a graphene oxide barrier layer covalently functionalized with amino acids on Mg AZ13 alloy in salt medium.

Magnesium alloys are promising materials for the biomedical and automobile industries. The Mg alloy's light-weight property leads to numerous industrial applications. However, the magnesium alloy oxide layers are not stable in salt environments. Organic inhibitors and epoxy coatings fail as long term barriers in such media. Recently, carbon based functionalized materials, graphene oxides, were shown to be promising materials for improving corrosion resistance in acid and salt environments. Our research considered graphene oxide covalently functionalized with the amino acid leucine to form anticorrosion coating materials. The functionalized materials were characterized by XRD, Raman, FESEM, HRTEM, FTIR, and AFM methods. The corrosion inhibition efficiency was monitored by electrochemical methods. The novelty of the functionalized graphene oxide materials is that they are water impermeable, and thus could enhance the anticorrosion resistance in salt environments.