Initial Oxidation Process Research Articles

Electrochemical Studies of Gold in Mops Buffer System

Recent work in this laboratory has investigated interactions of L-cysteine with metal ions.1 For this purpose the MOPS [(3-N-morpholino)propanesulfonic acid] aqueous buffer system has been employed in order to minimize interactions between the metal ions and buffer anions.2 Gold electrodes are expected to be useful in the investigation of L-cysteine and related substances,3 so it is useful to ascertain the background behavior of gold in the aqueous MOPS buffer system. It is the purpose of the present work to determine the voltammetric features in this situation for use in amino acid/metal ion studies.As shown in the accompanying figure, an initial positive-going sweep has a very small oxidation process at +0.45 V vs Ag/AgCl, presumably for the initial oxidation of the gold surface in pH 7.4 MOPS buffer. As this sweep continues, a much larger cathodic process corresponding to bulk gold oxidation is observed. On the return negative-going sweep, there is only a reduction process corresponding to the initial surface oxidation process. It is interesting to note that this reduction process is not observed upon reversing the sweep at only +0.50 V. On the second scan, the initial oxidation process has increased, possibly due to surface roughening in the first scan. Another investigation has involved addition of EDTA to the MOPS solution, resulting in an increase of the second (or bulk) gold oxidation process. This increase is apparently due to the ability of the EDTA to complex the gold cations formed in the oxidation process. This overall behavior is somewhat different compared to that found in other buffer systems such as phosphate,3 and contrasts to such behavior are planned for presentation in this talk. T. Cheek and D. Peña, J. Electrochem. Soc., 167, 155522 (2020).M. H. Ferreira, I. S. S. Pinto, E. V. Soares, and H. M. V. M Soares, RSC Advances, 5(39), 30989 (2015). G.T. Cheek and M.A. Worosz, ECS Trans., 72(27), 1 (2016). doi:10.1149/07227.0001ecst Figure 1

Initial stage oxidation corrosion of commercial ferritic stainless steels with different Cr contents at 650 °C for solid oxide fuel cells

Selecting adequate ferritic stainless steel (FSS) with a high corrosion resistance and a low cost is critical for solid oxide fuel cells (SOFCs) operating at intermediate temperature. In this study, the corrosion behaviors of four commercial FSSs involving TS430, TY441, YG442, and TY445 with a Cr content ranging from 16.18 wt.% to 21.73 wt.% are investigated at 650 °C. The oxidation mass gains, microstructures of surface oxide scale, and electrical conductivities are measured. The effects of grain size as well as doped elements are estimated together with the Cr volatilization. Flaky Cr2O3 particles are formed on TS430 and TY441 dominated by the outward migration of Cr3+. In comparison, a thin and dense layer of chromia is observed on YG442 and TY445. A high Cr content and a uniformly distributed grain size are conducive to the formation of a thin and dense chromia scale on the FSS surface during the initial oxidation process. On the other hand, the addition of Nb, Ti, and Mo weakens the outward diffusion of Cr3+ and reduces the particle size of chromia. After oxidation at 650 °C for 120 h, scattered (Mn, Cr)3O4 spinel particles occur on TS430, YG442, and TY445. TY445 and YG442 exhibit a higher conductivity although all the results of area specific resistance (ASR) are less than 6 mΩ·cm2. Meanwhile, the effect of Cr volatilization is enlarged on the estimation of mass gain at 650 °C compared with even higher temperatures.

Comprehensive evaluation of spontaneous combustion phenomenon from asphaltite perspective: Comparison with coal and clues of a universal process

Asphaltite holds importance as an energy raw material for Turkey. This study explores the potential for spontaneous combustion during underground mining production of Şırnak Üçkardeşler vein asphaltites and describes a generalized process by highlighting similarities and differences with self-heating of coal. Incubation tests, gas composition analysis, and infrared spectroscopy were conducted. Additionally, thermal analysis, surface area measurements, pore diameter analysis, mechanical testing, proximate analysis, and ultimate analysis were performed on the samples. The results reveal key differences in the spontaneous combustion behavior of asphaltites compared to low-rank coals. Melting, occurring between 60 and 80 °C, replaces the moisture-related stagnation observed in coals. The role of different functional groups, such as OH, CH, and CH2, in fueling the initial oxidation processes and other radicals initiating thermal runaway is identified. The reduction in surface area and pore diameters of asphaltites at higher temperatures limit their oxidation capacity. The compressive and tensile strengths of asphaltites significantly decrease with temperature. Similar to coals, carbon dioxide and carbon monoxide are indicative of spontaneous combustion, with carbon monoxide exhibiting a regular increase in temperature. The Arrhenius form confirms the occurrence of homogeneous reactions in asphaltite spontaneous combustion after 75 °C. These findings provide valuable insights into a universal mechanisms and warning signs of spontaneous combustion, aiding in the development of effective prevention strategies.

Mechanistic and kinetic aspects of florfenicol degradation by [rad]OH: Chloride moiety resistance

Antibiotic residues in aquatic environments can be effectively degraded by hydroxyl radical (OH)-based advanced oxidation processes (AOPs). However, the reaction kinetics and mechanisms have not been comprehensive determined due to the limitations of conventional competition kinetic methods and mass spectrometry-based product identification, as these methods fail to exclude interference from secondary radicals and are unable to capture unstable intermediates or transient species. In this study, these limitations were overcome using laser flash photolysis (LFP) and density functional theory (DFT) calculations, allowing accurate determination of the OH-initiated reaction kinetics and degradation mechanisms of florfenicol (FF), a model veterinary antibiotic compound that is frequently detected in the environment. Based on the LFP experiment results, the second order rate constant (k) between OH and FF was determined to be 1.96 × 109 M−1 s−1 by tracking the typical signal of (SCN)2−. Furthermore, DFT calculations revealed two distinct mechanisms for OH addition to the benzene ring and identified hydrogen atom abstraction (HAA) reaction from chiral carbons as being the most favorable initial reaction for OH. In addition, the destruction of chlorine moiety occurred via hydroxylation-chlorine abstraction, rather than direct chlorine abstraction or substitution reactions. The resistance of chlorine moiety to degradation during the initial oxidation process, resulted in the formation of chlorine-containing by-products. This study provided a novel approach to comprehensively investigate the mechanisms of emerging contaminants elimination during OH-based AOPs.

(Best Paper Award) Aluminum Josephson Junction Formation on 200mm Wafers Using Different Oxidation Techniques

For superconducting quantum circuits with a large number of Qubits, reproducible components are crucial for reducing entanglement decoherence. Particularly for reliable industrial manufacturing on full-scale 200 mm' wafers, a very high uniformity level is required to ensure sufficient coherence times. In the present work the special focus was put on manufacturing Al/AlOx/Al Josephson junctions (JJ), which are the most important component of many quantum circuit. Fully Al-based CMOS-compatible JJ’s were produced using a double dry etch process. After patterning the first Al metallization several oxidation processes have been investigated.Static oxidation has been performed by first removing the native AlOx in a multi-chamber system with Ar milling. The final tunneling oxide was controlled by applying a specific pressure in the chamber under a pure O2 atmosphere. Afterwards, without breaking the vacuum, the second Al metallization has been deposited by sputtering. Oxide thicknesses between 1 and 2.5 nm were achieved. A full mapping of the process homogeneity will be given.On the other hand, a dynamic recipe controlled plasma oxidation process was performed, where the native AlOx was first removed by a H2 plasma followed by a defined reoxidation with an oxygen plasma. The resulting oxides had thicknesses up to 10 nm. The second Al metallization was again deposited by sputtering.Both oxidation processes were carefully studied to understand the initial oxidation process of the aluminum surface. Special attention was devoted to the non-destructive removal of the native AlOx with respect to the Al interface. Because the oxide thicknesses varied between 1 and 10 nm, the transition between direct and Fowler-Nordheim tunneling could be investigated. The process-stability on full scale 200 mm wafers and on chip size could be determined via test structures as well as the resistance variation of the Josephson junctions. Furthermore, the electrical properties of the different oxides could be measured and analyzed on wafer level. These studies provide insight into the structure and composition of the aluminum oxides and the applicability for Qubits.

In situ monitoring of initial plasma electrolytic oxidation process on 60 vol. % SiCp/2009 aluminum matrix composite by sound and vibration measurement techniques.

The initial discharge process of plasma electrolytic oxidation (PEO) on the 60 vol. % SiCP/2009 aluminum matrix composite in silicate solution was in situ monitored by sound and vibration measurement techniques. The underwater sound, airborne sound, and sample vibration signals were detected in the initial 120s of the PEO process, and their generation mechanism was discussed. In terms of waveforms and spectrograms of the sound and vibration signals, the initial PEO process can be divided into five stages: conventional anodizing stage (I), glow discharge stage (Ⅱ), tiny spark discharge stage (Ⅲ), large spark discharge stage (Ⅳ), and strong spark discharge stage (Ⅴ). The sound and vibration signals during the PEO process are attributed to the evolution of bubbles, which are from the plasma discharge, electrochemical reactions, and vaporization of electrolyte under Joule heat. In stage I, these signals completely come from the bubbles produced by the evaporative electrolyte and electrochemical reactions. In stages Ⅱ-Ⅴ, the bubbles from the plasma discharge gradually become the main source of these signals with increasing discharge intensity. In addition, the spike peaks on the waveforms of these signals at stage Ⅴ are related to the strong discharge sparks. These results demonstrate that sound and vibration measurement techniques can effectively monitor the PEO discharge process.

Widespread detection of chlorine oxyacids in the Arctic atmosphere

Chlorine radicals are strong atmospheric oxidants known to play an important role in the depletion of surface ozone and the degradation of methane in the Arctic troposphere. Initial oxidation processes of chlorine produce chlorine oxides, and it has been speculated that the final oxidation steps lead to the formation of chloric (HClO3) and perchloric (HClO4) acids, although these two species have not been detected in the atmosphere. Here, we present atmospheric observations of gas-phase HClO3 and HClO4. Significant levels of HClO3 were observed during springtime at Greenland (Villum Research Station), Ny-Ålesund research station and over the central Arctic Ocean, on-board research vessel Polarstern during the Multidisciplinary drifting Observatory for the Study of the Arctic Climate (MOSAiC) campaign, with estimated concentrations up to 7 × 106 molecule cm−3. The increase in HClO3, concomitantly with that in HClO4, was linked to the increase in bromine levels. These observations indicated that bromine chemistry enhances the formation of OClO, which is subsequently oxidized into HClO3 and HClO4 by hydroxyl radicals. HClO3 and HClO4 are not photoactive and therefore their loss through heterogeneous uptake on aerosol and snow surfaces can function as a previously missing atmospheric sink for reactive chlorine, thereby reducing the chlorine-driven oxidation capacity in the Arctic boundary layer. Our study reveals additional chlorine species in the atmosphere, providing further insights into atmospheric chlorine cycling in the polar environment.

Divalent Oxidation State Ni as an Active Intermediate in Prussian Blue Analogues for Electrocatalytic Urea Oxidation.

Urea degradation is one of the most crucial links in the natural nitrogen cycle. Exploring the real active species in the urea electro-oxidation process is of great significance for understanding the urea electro-oxidation mechanism and designing catalysts. A highly active and stable Prussian blue analogue catalyst (PBA@NiFe/NF) loaded on nickel foam was synthesized for electro-oxidation of urea. In situ Raman spectra revealed that Ni in PBA@NiFe/NF was able to maintain a stable divalent nickel (Ni(II)) state for up to 3.5 h during the initial urea oxidation process, which is rarely reported in previous research studies. In addition, with the participation of iron, the Ni-Fe bimetallic center significantly improves the electro-oxidation of urea. Our work provides a new idea for prolonging the Ni(II) activity in electrocatalytic oxidation of urea.

Effects of chemical complexity on the initial oxidation resistance of HfC1−xNx ceramics

The initial oxidation process of refractory alloy ceramics is closely related to their intrinsic properties such as surface adsorption or diffusion of oxygen atoms. We devise a machine learning model that predicts the full spectrum of adsorption energies for an oxygen atom on HfC1−xNx ceramic surfaces with quantum accuracy. With this approach, we show that the chemical complexity of carbonitride makes HfC1−xNx ceramics exhibit multiple types of adsorption sites with competing oxygen adsorption energies, leading to fewer preferable adsorption sites. In particular, we find that heavily doped N can change the stable adsorption site from the 3-fold hollow between metals and C atoms (MMC) to the top of Hf atoms (top-Hf), and the total number of preferable adsorption sites is regulated by their competing energies. In this scenario, we predict HfC0.76N0.24 has superior anti-oxidation performance, consistent with existing experimental measurements. Our findings can stimulate new strategies to enhance the oxidation resistance of refractory alloy ceramics.

First-principles study of different oxidation process on Al(111) and Cu(111): Metal pulled-off effect

The typical FCC Al and Cu with distinct electronic configurations performing the difference in oxidation behaviors are our focus. The optimized reconstruction, thermodynamical energies, electronic properties, and diffusion dynamics of Al(111) and Cu(111) adsorption and penetration systems with on-surface O coverages from 0.25 ∼ 1 ML have been studied using DFT methods. The qualitatively different binding behaviors of O-Al/Cu(111) adsorption systems as the increasing coverage can be illustrated systemically by the relaxations of outermost layers, electron transfer, and orbital hybridization. Based on this benchwork, it can be seen that the initial O-penetrating configurations are energetically less favorable and present more significant surface reconstructions mainly due to the strong interactions between on-surface and sub-surface oxygen in the crystal lattice. A metal pulled-off effect with a larger missing-row stepped on-surface reconstruction only in Cu can be observed over the whole pre-coverage in the existence of sub-surface O, mainly due to the significant strain-induced interactions dominate in the Cu lattice. This reconstruction will suggest the possible formation of a larger channel to further oxidize the inner Cu atoms. On-surface/sub-surface O species play a cooperative role in surface reconstruction and stability of Al/Cu(111) and enlighten us on the distinct atomic-level mechanisms of the initial oxidation process.

Role of Nb in promoting protective scale formation on Al2O3-forming austenitic alloys

The high-temperature oxidation behavior of Fe–25Ni–17Cr–7.7Al–0.03Zr alloys with and without 0.5 or 1 at% Nb addition was investigated at 800 °C in air. Nb addition was found to promote the formation of an external Al2O3 scale. The Nb-free alloy formed a scale consisting of a thick Fe/Ni-rich oxide layer and a Cr2O3 layer, beneath which Al was internally oxidized. However, a continuous protective Al2O3 scale was developed below a thin Cr2O3 layer on the 1 Nb alloy, which significantly decreased the oxidation rate. Examination of the initial oxidation process during heating to 800 °C revealed that all of the alloys formed a Cr2O3 layer and an amorphous Al2O3 layer upon heating to 650 °C, but these layers became discontinuous upon further heating to 800 °C for the Nb-free alloy. The beneficial role of Nb was ascribed to its ability to maintain the initially formed thin Cr2O3 layer by the formation of a Nb-containing Cr-rich oxide layer at the interface between the Cr2O3 and amorphous Al2O3 layers. This Nb-containing Cr-rich layer prevented the breakdown of the transient amorphous Al2O3 layer until its transition to a protective crystalline Al2O3 scale.

Isothermal oxidation resistance and microstructure evolution of VPS-TiAlCrY coating on TiAl single crystals at 1100–1200 °C

TiAlCrY coating was fabricated on TiAl single crystals by vacuum plasma spray (VPS) technique to investigate the oxidation resistance and microstructure evolution via isothermal oxidation tests at 1100 and 1200 ℃, respectively. The results indicated that the oxidation resistance of TiAlCrY coating was excellent with the maximum spallation degree of oxide layer extremely below the critical value of coating failure. The oxide growth kinetic curves of the TiAlCrY coating complied well with parabolic law, and the oxidation rate constants were comparable to those of typical MCrAlY coatings on nickel superalloys. In particular, the strong α-Al2O3 formation ability in the initial oxidation process and the great chemical compatibility at coating-substrate interface contributed to the durable oxidation resistance of TiAlCrY coating. This work highlights the potential application of VPS-TiAlCrY coating for TiAl single crystals at 1100–1200 ℃.

Effect of Pt-doping on the oxidation behaviors of the γ'-Ni3Al and β-NiAl phases in the NiSiAlY alloy

The oxidation behaviors of the NiSiAlY+xPt (x = 0, 1, 5 and 10 wt%) alloys were investigated at 900 ℃, with the focus on the Pt effect, phase and oxidation stage. The doped Pt improved the oxidation resistance but caused different oxidation behaviors in β-NiAl and γ'-Ni3Al phases. During the initial oxidation process, the Pt-doped NiSiAlY alloys showed a faster increase in mass gain, which was mainly contributed by the β-NiAl phase. However, at the stable oxidation stage, the undoped NiSiAlY alloy had the highest oxide growth rate, it was dominated by the oxidation behavior of the γ'-Ni3Al phase.

Thermal oxidation process on Si(113)-(3 × 2) investigated using high-temperature scanning tunneling microscopy.

Thermal oxidation of Si(113) in a monolayer regime was investigated using high-temperature scanning tunneling microscopy (STM). Dynamic processes during thermal oxidation were examined in three oxidation modes – oxidation, etching, and transition modes – in the third of which both oxidation and etching occur. A precise temperature–pressure growth mode diagram was obtained via careful measurements for Si(113), and the results were compared with those for Si(111) in the present work and Si(001) in the literature. Initial oxidation processes were identified based on high-resolution STM images.

Molecular dynamics simulations of the initial oxidation process on ferritic Fe-Cr alloy surfaces.

Oxidation processes of metallic interconnects are crucial to the operation of solid oxide fuel cells (SOFCs), and ferritic Fe–Cr alloy is one of the most important metallic interconnect materials. Based on the ReaxFF reactive potential, the interaction of O2 molecules with three types of surfaces (100, 110, 111) of ferritic Fe–Cr alloy has been studied by classical molecular dynamics at constant O2 concentrations and temperatures. The initial oxidation process is systematically studied according to the analysis of O2 absorption rate, charge variations, charge distributions, mean squared distributions, and oxidation rate. The results reveal that it is easier and faster for the Cr atoms to lose electrons than for the Fe atoms during the oxidation process. The obtained oxidation rate of Cr atoms is larger and the formation of Cr2O3 takes precedence over that of FeO. And the thickness of oxidation layers of different surfaces could be determined quantitatively. We also find that the high O2 concentration accelerates the oxidation process and obviously increases the thickness of oxidation layers, while the temperature has a weaker effect on the oxidation process than the O2 concentration. Moreover, the (110) surface presents the best oxidation resistance compared to the other two surfaces. And the (110) surface is efficient in preventing Fe atoms from being oxidized. Here we explore the initial oxidation process of Fe–Cr alloy and the corresponding results could provide theoretical guides to the related experiments and applications as metallic interconnects.

Surface phase stability of surface segregated AgPd and AgCu nanoalloys in an oxygen atmosphere

The geometric structure of 38-atom AgPd and AgCu nanoalloys is obtained by the genetic algorithm and density functional theory for all compositions and their surface phase stability diagrams are established to provide a clear image for the initial oxidation process. Ag surface segregation is confirmed for both AgPd and AgCu nanoalloys in vacuum. Pure Pd and Cu nanoparticles have lower surface phase stability than bulk metals to be oxidized and the alloying can improve the oxidation resistance. Segregated AgCu nanoalloys have higher phase stability than mixed AgCu in vacuum and have lower surface phase stability than mixed AgCu nanoalloys in an oxygen atmosphere. Unexpectedly, segregated AgPd nanoalloys have both higher phase stability in vacuum and higher surface phase stability in an oxygen atmosphere than that of mixed AgPd nanoalloys. The higher surface phase stability of segregated AgPd nanoalloy could be attributed to the slight elevation of Pd Milliken charge and negative shift of d-band center. Compared with the selective oxidation in conventional alloy oxidation models where the selective oxidation is related to alloy composition, the nanoalloy oxidation is proposed to correlate with surface segregation in AgPd and AgCu nanoalloys in this work, which promotes the development of the conventional alloy oxidation models in that the surface segregation is a precursor to the surface oxidation of nanoalloy and plays a critical role in the selective oxidation of nanoalloy.

ReaxFF reactive molecular dynamics study on oxidation behavior of 3C-SiC in H2O and O2

Simulations of the initial oxidation process of 3C-SiC involving different polar surfaces C (1-00), C(1-1-1-),Si (100), and Si (111) exposed to H2O were studied by ReaxFF. The results show that SiC is gradually converted into silicon oxide and forms low-density carbon chains. The activation energy of the C face is lower than that of the Si face, which reflects lower oxidative stability. In addition, an oxidation reaction was carried out on a C (100) surface with different concentration ratios of H2O and O2. The results indicate the reaction rate of O2 is much higher than that of H2O, while the oxidation capacity of O2 is effectively improved with the addition of water. It is clearly observable that the addition of water leads to the porosity of the oxide layer and the separation of Si atoms.

Initial atomic-scale oxidation pathways on a Ni\u201315Cr(100) alloy surface

To understand the atomistic phenomenon behind initial oxidation processes, we have studied the nanoscale evolution of oxide growth prior to the formation of a complete layer on a Ni–15 wt%Cr(100) alloy surface using scanning tunneling microscopy/spectroscopy (STM/STS). At the onset of oxidation, a NiO superlattice forms oxide wedges across the step edges, eventually growing across the terraces. The completion of the NiO layer is followed by nucleation of the next layer, which always commences at the groove site of the superlattice. The Cr-oxide formation initiates as disk-shaped oxide particles early in the oxidation process, which Monte Carlo simulations reveal are likely caused by Cr clustering across the alloy surface. Upon further oxidation, a Cr(100)-p(2 × 2)O reconstructed surface is observed, indicating phase separation of Cr predicates the formation of the passive Cr-oxide film. The STS results vary across the oxide–alloy interface and between each oxide, providing greater insight into the origins of electronic heterogeneity and their effect on oxide growth. Using these data, we propose an oxidation model that highlights the growth of partial oxide layers on Ni–Cr(100) alloys within the pre-Cabrera–Mott regime.

Crystal Orientation-Dependent Oxidation of Epitaxial TiN Films with Tunable Plasmonics

Titanium nitride (TiN) is a paradigm of refractory transition metal nitrides with great potential in vast applications. Generally, the plasmonic performance of TiN can be tuned by oxidation, which was thought to be only temperature-, oxygen partial pressure-, and time-dependent. Regarding the role of crystallographic orientation in the oxidation and resultant optical properties of TiN films, little is known thus far. Here we reveal that both the oxidation resistance behavior and the plasmonic performance of epitaxial TiN films follow the order of (001) < (110) < (111). The effects of crystallographic orientation on the lattice constants, optical properties, and oxidation levels of epitaxial TiN films have been systematically studied by combined high-resolution X-ray diffraction, spectroscopic ellipsometry, X-ray absorption spectroscopy, and X-ray photoemission spectroscopy. To further understand the role of crystallographic orientation in the initial oxidation process of TiN films, density-functional-theory calculations are carried out, indicating the energy cost of oxidation is (001) < (110) < (111), consistent with the experiments. The superior endurance of the (111) orientation against mild oxidation can largely alleviate the previously stringent technical requirements for the growth of TiN films with high plasmonic performance. The crystallographic orientation can also offer an effective controlling parameter to design TiN-based plasmonic devices with desired peculiarity, e.g., superior chemical stability against mild oxidation or large optical tunability upon oxidation.

The initial wet oxidation process on Fe-Cr alloy surface: Insights from ReaxFF molecular dynamic simulations

The durable use of stainless steels depends on the formation of a protective chromium-rich oxide layer. To better understand the atomistic mechanism of surface oxide film formed on Fe-Cr alloy, the initial wet oxidation process on Fe-Cr alloy surface was investigated using ReaxFF molecular dynamic simulations. The oxidation process initiates from the adsorption and local dissociation of the water molecules near Cr atoms, followed by the segregation of Cr and the formation of cation vacancies on alloy surface. Meanwhile, the dissociated O and H atoms diffuse into the alloy substrate, which consequently facilitates to form a Cr-rich oxide layer. The compositions of oxide film possess distribution from Cr hydrates and hydroxides, Fe hydroxides and oxides to hydrogen ions with negative charge along depth. It was also demonstrated that there is a charge transfer between the metal atoms and hydrogen atoms. The effects of Cr content (0–20 at.%) and crystal surface orientation on the initial wet oxidation process were further discussed. These results can provide a microstructural understanding of the wet oxidation process on the surface of stainless steels from an atomistic point of view.