Oxide Growth Rate Research Articles

Mechanical\u2013chemical coupling phase-field modeling for inhomogeneous oxidation of zirconium induced by stress\u2013oxidation interaction

A phase-field model is proposed to study the inhomogeneous growth of zirconia induced by the stress–oxidation interaction, which captures the complex interplay among diffusion, oxidation kinetics, interfacial morphology evolution, and stress variation in an oxidation process. Through this numerical model, many experimentally observed but insufficiently understood phenomena can be well explained. Specifically, the numerical simulations reveal quantitatively the causes of interface roughening or smoothening during the inward oxide growth, the roughness-dependent oxide growth rate, and the nucleation sites of premature cracking. These numerical findings can be used as the theoretical references for the improving the durability of oxide scale and prolonging the service life of zirconium-based alloy cladding used in the nuclear power plant.

Super-fast fabrication of self-ordered nanoporous anodic alumina membranes by ultra-hard anodization

Although the fast fabrication of long-range ordered porous anodic alumina (PAA) membranes has already been realized by hard anodization, achieving super-fast oxide growth rates has been still very challenging. Herein, we report a new generation of the anodization process, called ultra-hard anodization (UHA), in which the mean growth rate of self-ordered PAA reached 2900 µm/h in the first 80 s of the process, resulting in a membrane thickness of 58 µm. By comparison, the same thickness can be obtained using the conventional hard anodization and mild anodization processes in 2400 s and 10 h, respectively. While the current density (J) reaches a high value of 2400 mA/cm2, burning and dielectric breakdown phenomena are avoided by controlling the barrier layer temperature and the diffusion length. In addition, the mechanism of oxide growth is explained on the basis of the high field theory and the diffusion-limited path of ionic species through the nanopores, taking into account the effect of temperature and sprinkling rate of electrolyte on J. By blending 0.3 M oxalic acid with phosphoric or sulfuric acid separately, the UHA process is also performed at target voltages ranging from 85 to 150 V, thereby allowing for a wide range of interpore distances. From a practical point of view, we synthesize high aspect ratio polymer nanowires (up to 320 µm) in the resulting membranes. Our findings open a new approach for the super-fast fabrication of the self-ordered PAA, facilitating the development of nanodevices and their commercial applications.

Mechanism of the oxide scale formation in thermally-sprayed NiCoCrAlY coatings modified by CeO2 nanoparticles

The increased operating temperatures of next-generation gas turbines require high-performance coatings with improved oxidation resistance. NiCoCrAlY coatings have significant potential for achieving this goal however improved insights into their oxidation performance are required. In this study, a high-velocity oxy-fuel (HVOF) process was used to compare NiCoCrAlY/nano-CeO2 with NiCoCrAlY conventional coatings. High-temperature oxidation behaviour of the modified coatings was investigated and compared with the conventional coating. Free-standing coatings were then subjected to short- and long-term isothermal oxidation test at 1000 °C. The microstructural features of the modified powders and coatings before and after oxidation were characterized using a Field Emission Scanning Electron Microscope (FESEM), Energy Dispersive Spectroscopy (EDS), Transmission Electron Microscope (TEM), Raman spectroscopy and X-ray Diffraction (XRD). The oxide growth rate of the coatings was also examined and modeled using various diffusion-based calculations. Results showed that, the modified NiCoCrAlY-1.0 wt% nano-CeO2 coating had the highest oxidation resistance (26 % higher than the conventional NiCoCrAlY coatings). The higher oxidation resistance of the modified NiCoCrAlY-1.0 wt% nano-CeO2 coating was attributed to its higher density as well as its lower structural porosity and oxide growth rate after the short- and long-term oxidation tests. Moreover, acceptable conformity of parabolic rate behaviour was obtained for all types of the conventional and modified coatings. The dominant mechanism of oxidation improvement for NiCoCrAlY-1.0 wt.% nano-CeO2 coating was found to the controlling the oxide scale growth rate. As a consequence, the findings indicated that the modified NiCoCrAlY-1.0 wt.% nano-CeO2 coating can be a candidate for the protection of hot sections of gas turbines in the future.

Environmental barrier coatings using low pressure plasma spray process

AbstractYb2Si2O7/Si bilayer environmental barrier coatings (EBCs) on SiC ceramic substrate were produced by low pressure plasma spray (LPPS) process. Phase composition, microstructure, and thermal durability of LPPS Yb2Si2O7/Si coating were investigated. XRD analysis indicated that the coating is mainly composed of Yb2Si2O7 with ~15.5v% Yb2SiO5 phases. The LPPS EBCs have a dense microstructure with porosity less than 4%. Adhesion strength measurement indicated the LPPS EBCs have an average adhesion strength of 29.1 ± 0.8 MPa. Furnace cycle test (FCT) on the coatings in air at 1316°C was performed and the test ran for 900 cycles and there was no coating spallation/failure for LPPS Yb2Si2O7/Si EBCs. The FCT results demonstrated the excellent thermal cycle durability of LPPS EBCs. Oxidation kinetics investigation of LPPS EBCs in flowing 90% H2O (g)+10% air at 1316°C showed that the thermally grown oxide (TGO) growth rate is close to the oxidation rate of pure Si in dry air and is significantly lower than that in water vapor environment. The LPPS process is promising in making highly durable Yb2Si2O7‐based dense EBCs by impeding diffusion and ingression of water vapor/O2.

Preparation, characterization and oxidation behavior of CeO2-gradient NiCrAlY coatings applied by HVOF thermal spraying process

Conventional and CeO2-gradient NiCrAlY coatings (0.5–2.0 wt% of nano-CeO2 from the inner layer to layers adjacent to the outer surface) were produced by the high-velocity oxy-fuel spraying (HVOF) process. Microstructural characteristics, phases analysis and high-temperature oxidation behavior of the functionally graded (FG) NiCrAlY/nano-CeO2 coatings were investigated and compared with those monolayer (ML) NiCrAlY coatings with without CeO2 nanoparticles. The microstructure of the ML- and FG-coatings, as well as processed powders, were characterized using a transmission electron microscope (TEM), field emission scanning electron microscope (FESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction analysis (XRD), microhardness testing and Raman spectroscopy. On account of the calculation of oxidation behavior of the coating, the ML- and FG-coatings were exposed to laboratory's air at 1000 °C up to 100 h (short-term exposure) and 5000 h (long-term exposure) and moreover, the weight gain versus oxidation time and oxide growth rate were subsequently examined. Obtained results indicated that FG-NiCrAlY/nano-CeO2 coating had a better oxidation resistance as well as a lower oxide growth rate compared to the rest of the ML-coatings. In addition, increasing oxidation resistance of FG-NiCrAlY/nano-CeO2 coating can be attributed by controlling Al and O diffusion due to the formation of relatively thin, dense, and sticky Al2O3 oxide scale. Besides, all cases of the oxidized ML- and FG-coatings had a parabolic rate behavior owing to their diffusion-controlled oxidation under short- and long-term oxidation tests.

Damage Assessment and Fracture Resistance of Functionally Graded Advanced Thermal Barrier Coating Systems: Experimental and Analytical Modeling Approach

Enhancement of stability, durability, and performance of thermal barrier coating (TBC) systems providing thermal insulation to aero-propulsion hot-section components is a pressing industrial need. An experimental program was undertaken with thermally cycled eight wt.% yttria stabilized zirconia (YSZ) TBC to examine the progressive and sequential physical damage and coating failure. A linear relation for parameterized thermally grown oxide (TGO) growth rate and crack length was evident when plotted against parameterized thermal cycling up to 430 cycles. An exponential function thereafter with the thermal cycling observed irrespective of coating processing. A phenomenological model for the TBC delamination is proposed based on TGO initiation, growth, and profile changes. An isostrain-based simplistic fracture mechanical model is presented and simulations carried out for functionally graded (FG) TBC systems to analyze the cracking instability and fracture resistance. A few realistic FG TBCs architectures were considered, exploiting the compositional, dimensional, and other parameters for simulations using the model. Normalized stress intensity factor, K1/K0 as an effective design parameter in evaluating the fracture resistance of the interfaces is proposed. The elastic modulus difference between adjacent FG layers showed stronger influence on K1/K0 than the layer thickness. Two advanced and promising TBC materials were also taken into consideration, namely gadolinium zirconate and lanthanum zirconate. Fracture resistance of both double layer and trilayer hybrid architectures were also simulated and analyzed.

Effect of in situ SiC and MoSi2 phases on the oxidation behavior of HfB2-based composites

Monolithic HfB2 and HfB2-15vol%SiC-15vol%MoSi2 composite samples were oxidized by a conventional electric furnace at 1700 °C for 5 h. Microstructural and phase analysis of the oxidized samples were performed by X-ray diffraction (XRD) analysis and field emission scanning electron microscope (FESEM) equipped with energy-dispersive spectroscopy (EDS). Besides, for analyzing the oxidation mechanism of the samples, thermodynamic calculations were also accomplished by HSC software. The changes in weight and thickness of the oxide scale were measured and the oxide growth rate of the oxidized samples was subsequently calculated. The results showed that HfB2-15vol%SiC-15vol%MoSi2 composite was much more resisted than that monolithic HfB2 due to the formation of a thin Si-rich glass layer on the surface of the composite sample. By acting to fill the porosities between HfO2 grains, Si-based glass phase enhanced the oxidation resistance of HfB2-15vol%SiC-15vol%MoSi2 composite. Conversely, the oxidized monolithic HfB2 had only a thick porous oxide layer (HfO2) which led to considerably lower oxidation resistance. On the other side, three layers containing HfO2 and Si-based glass phases were formed on the oxidized HfB2-15vol%SiC-15vol%MoSi2 composite. Moreover, no porosities and no porous layers were also detected on the oxidized composite sample. Consequently, HfB2-15vol%SiC-15vol%MoSi2 composite had a parabolic behavior owing to its diffusion-controlled oxidation under the isothermal oxidation process.

Influences of coexisting O2 in H2O-annealing ambient on thermal oxidation kinetics and MOS interface properties on 4H–SiC (1–100)

On 4H–SiC (1–100) m-face substrate, the process design of SiO2 growth by a conventional thermal oxidation accompanied with post-oxidation annealing (POA) in H2O ambient was investigated, especially focusing on the impact of O2 or H2O composition in the POA ambient. From the oxidation kinetics study, the surface on m-face was found to be more reactive to O2 than to H2O in thermal oxidation at 1100–1300 °C, while co-existence of both oxidants resulted in the highest oxide growth rate. As for the electrical characteristics of MOS capacitors, it was clarified that the POA process in H2O with low O2 concentration ambient was beneficial in interface trap density (Dit) reduction on m-face, even though the observed competitive relationship between Dit and flat-band voltage (VFB) instability made it challenging to simultaneously suppress both of those two parameters on m-face. High O2 concentration POA ambient worsened both of those two parameters, suggesting additional defects being easily introduced on m-face by the coexisting O2 in the POA ambient.

High Temperature Isothermal Oxidation of Heat-treated HR-120 Ni-based Alloys

The effect of heat treatment of HR-120 Ni-based alloys on the isothermal oxidation at high temperature was studies. HR-120 Ni-based alloys was undergo a heat treatment process at two different temperatures, namely 1000 °C and 1200 °C for 3 h soaking time, followed by water quench. The heat-treated samples were then undergoing an isothermal oxidation test at 500 °C for 500 h exposure time in air. Oxidized samples have been characterized in terms of the kinetics of oxidation, phase analysis using x-ray diffraction (XRD) and oxide surface morphology using scanning electron microscope (SEM) and field emission scanning electron microscope (FESEM) equipped with energy dispersive x-ray (EDX) spectrometer. The heat treatment process exhibits an increasing of average grain size alloy as the temperature increases. Whereas, the oxidation kinetics of oxidized samples exhibits a parabolic rate law, representing a diffusion-controlled oxide growth rate. The oxidized of heat-treated sample at 1000 °C recorded low oxidation rate with low parabolic rate constant values. The oxide surface morphology of oxidized samples indicates the formation of continuous oxide scales with overgrown Nb-rich oxide particles after exposure for 300 h. At 500 h exposure time, the formation of Nb-rich oxide particles was growing with evidence of crack occurred around the overgrown oxide particles.

Investigation on the oxide-oxide galvanic displacement reactions employed in the preparation of electrocatalytic layers

Galvanic displacement reactions between sacrificial oxide layers and metal cations, leading to the formation of secondary oxide layers, have been studied with the aim of collecting new experimental evidence on their mechanism. The study has been carried out by combining electrochemical methods, SEM-EDS and XPS depth profiling. Both porous and compact PbO2 layers have been used as sacrificial oxides and reacted, in different experiments, with: (i) a single low-valent cation, (ii) two cations in sequential exchange reactions or (iii) two cations simultaneously. The experimental results converged to show (i) a lack of correlation between the reaction driving force and the secondary oxide growth rate, (ii) the incorporation of cations from solution at the secondary oxide/electrolyte interface, (iii) the transport of Pb species through the growing secondary oxide layer and (iv) a major effect of mass transport on the growth rate.

In situ oxidation studies of Cu thin films: Growth kinetics and oxide phase evolution

A comprehensive understanding of the oxidation of Cu thin films in the low-temperature regime is of fundamental interest and particularly relevant for applications in the fields of micro- and nanoelectronics, sensors, catalysis, and solar cells. The current study reports on the oxidation kinetics of PVD grown Cu thin films (20–150 nm thick) and the oxide phase evolution from Cu2O to CuO upon thermal oxidation in the temperature range of 100–450 °C. XRD investigations in the laboratory and at the synchrotron show that the oxide phase formation critically depends on the oxidation conditions such as temperature and oxygen partial pressure. The real-time synchrotron XRD measurements reveal that the formation of the CuO phase only starts after complete oxidation of the Cu films to Cu2O films. In situ resistance measurements were performed to follow the oxide growth rate of Cu2O on Cu films in the temperature range of 100–300 °C in air and in 10 mbar pO2. It is found that the oxidation kinetics of Cu films to Cu2O films follows the linear rate law, which is attributed to surface reaction controlled oxidation. The oxygen dissociation rate at the gas–solid interface is the rate-limiting process. A dramatic decrease in the linear oxidation rate is observed at low oxygen partial pressures. The fundamental differences between the oxidation rate-limiting processes of Cu as compared to other transition metal films are discussed.

Depth Profile Analysis of Thin Oxide Layers on Polycrystalline Fe-Cr.

Surfaces of polycrystalline ferritic Fe-Cr steel with grain sizes of about 13 µm in diameter were investigated with surface sensitive techniques. Thin oxide layers, with a maximum thickness of about 100 nm, were grown by oxidation in air at temperatures up to 450°C and were subsequently characterized using time-of-flight secondary ion mass spectrometry (TOF-SIMS) and atomic force microscopy. Correlative microscopy was applied, which allows for element-specific depth profiles on selected grains with a particular crystal orientation. A strong correlation between the grain orientation and the thickness of the oxide layer was found. The sequence in the oxidation growth rate of ferritic Fe-Cr steel crystal planes is found to be {011} > {111} > {001}, which is unexpectedly opposed to known Fe-based systems. Moreover, for the first time, the Cr/Fe ratio throughout the oxide layer has been determined per grain orientation. A clear order from high to low of {001} > {111} > {011} was detected.

Effect of vacuum heat treatment on the oxidation kinetics of freestanding nanostructured NiCoCrAlY coatings deposited by high-velocity oxy-fuel spraying

In this study, the nanostructured NiCoCrAlY powder feedstock was coated on the substrate via the high-velocity oxy-fuel spraying process. Freestanding nanostructured NiCoCrAlY coatings were then vacuum heat-treated at 1100 °C for 1, 2, and 5 h under a pressure of 10−3 Pa. The microstructure examinations of the nanostructured NiCoCrAlY coatings before and after vacuum heat treatment were investigated using x-ray diffraction and the field-emission scanning electron microscope equipped with electron dispersed spectroscopy analysis. In order to measure the high-temperature oxidation resistance, the freestanding coatings were evaluated using the isothermal oxidation test at 1000 and 1100 °C, respectively, under the air atmosphere. The obtained results showed that homogeneous and adherent α-Al2O3 was successfully formed on the surface of the heat-treated coating during isothermal oxidation. Results also indicated that the heat-treated NiCoCrAlY coating had a higher isothermal and cyclic oxidation resistance. As a consequence, a lower oxide growth rate was achieved by controlling diffusion due to the finer and denser structure of Al2O3 on the heat-treated nanostructured coating for 2 h after the oxidation test.

Oxide scale growth on Fe-40Ni-24Cr alloy at high temperature oxidation

Fe-40Ni-24Cr alloy is a Ni-based alloy used at high temperature application. In this study, Fe-40Ni-24Cr alloy had undergo a solution treatment at two different temperatures, namely 1050° and 1150°, for 3 hours soaking times followed by water quench. A solution-treated Fe-40Ni- 24Cr alloy was experienced a high temperature isothermal oxidation test at 500° for 500 hours exposures duration. The growth of oxide scale was characterized in terms of oxidation kinetics and surface morphology using field emission scanning electron microscope equipped with energy dispersive x-ray spectroscopy. The oxidation kinetics of all samples shows an increasing weight gain pattern as the exposure durations increases. The oxidation kinetics was obeyed a parabolic rate law, indicating the oxide growth rate was followed diffusion-controlled mechanism. The surface morphology of oxidized solution-treated Fe-40Ni-24Cr alloy displayed a formation of continuous oxide scale with evidence of overgrown Nb-rich oxide particles distributed along the oxidized samples surface. Oxidized Fe- 40Ni-24Cr alloy which undergo solution treatment at 1150° exhibited the formation of oxide spallation. The spallation has a tendency to occur around the area of overgrown Nb-rich oxide particles.

Dissolved Organic Matter Affects Arsenic Mobility and Iron(III) (hydr)oxide Formation: Implications for Managed Aquifer Recharge.

During managed aquifer recharge (MAR), injected water significantly alters water chemistry in an aquifer, affecting arsenic mobility. To elucidate the effects of dissolved organic matter (DOM) on arsenic mobilization during MAR, this bench-scale study examined arsenic mobilization from arsenopyrite (FeAsS, an arsenic-containing sulfide) in the presence of Suwannee River natural organic matter, humic acid, and fulvic acid (SRNOM, SRHA, and SRFA), alginate (Alg), polyaspartate (PA), and glutamate (Glu). Suwannee River DOM (SRDOM) decreased arsenic mobility in the short term (<6 h) via inhibiting arsenopyrite oxidative dissolution, but increased arsenic mobility over a longer experimental time (∼7 days) via inhibiting secondary iron(III) (hydr)oxide precipitation and decreasing arsenic adsorption onto iron(III) (hydr)oxide. In situ grazing incidence small-angle X-ray scattering measurements indicated that SRDOM decreased iron(III) (hydr)oxide nucleus sizes and growth rates. A combined analysis of SRDOM and other proteinaceous or labile DOM (Alg, PA, and Glu) revealed that DOM with higher molecular weights would cause more increased arsenic mobility. These new observations advance our understanding of the impacts of DOM in injected water on arsenic mobility and secondary precipitate formation during MAR, and in other systems where interactions between DOM, arsenic, and iron(III) (hydr)oxides take place.

Oxidation of metal thin films by atomic oxygen: A low energy ion scattering study

In this study, we combine low-energy ion scattering (LEIS) static and sputter depth profiles for characterization of the oxidation kinetics on Zr, Mo, Ru, and Ta films of various thicknesses, followed by exposure to atomic oxygen at room temperature (∼20 °C). A method for nondestructive determination of the oxide growth rate via LEIS static depth profiling (static DP) is presented in detail. This method shows high sensitivity to the oxide thickness formed, and the results are in agreement with those obtained by X-ray reflectometry and sputter depth profiling (sputter DP). Sequential exposures of oxygen isotopes in combination with LEIS sputter DP are applied to elucidate the growth mechanism of the oxide films. The results indicate that the oxidation kinetics at the applied experimental conditions is directly influenced by the metal work function, characterizing a Cabrera-Mott growth type. The maximum thickness of the formed oxide and oxide growth rate are in the order Zr ≈ Ta &amp;gt; Mo &amp;gt; Ru. The combining of analysis by LEIS static DP and isotope tracing sputter DP is decisive in the characterization of oxidation kinetics in the room temperature regime.

Influence of NiAl2O4 spinel formation on the oxidation behavior of the Ni50Al alloy at 1273 K in air

The influence of NiAl2O4 spinel formation on the oxidation behavior of the Ni50Al alloy at 1273 K in air was investigated. Pulsed electrodeposition was used to introduce a nano layer of Ni prior to the oxidation test of the Ni50Al alloy. Excluding the initial stage of oxidation, the oxidation mass gains of the coated sample significantly decreased, and the oxide scale growth rate was slower than that on the bare Ni50Al alloy sample. Peaks from the metastable Al2O3 phases were detected for the Ni-coated Ni50Al sample in X-ray powder diffraction (XRD) analysis even after 100 h oxidation. Multi-layered oxide scale of facet-NiO, fine-NiAl2O4 and dense Al2O3 has developed on the oxidized coated sample, in the order from outside to inside the oxide scale. NiO was consumed in the formation of denser NiAl2O4 spinel that hindered the oxidation kinetics of the Ni50Al alloy. The results confirmed that the slower growth rate of oxide on the coated sample might be attributed to the blocking effect of the NiAl2O4 spinel even though metastable Al2O3 phases still grow.

Effect of CO2 gas pressure on composition, growth rate, and structure of duplex oxide formed on 9Cr steel at 550 °C

The oxidation behaviour of F91 in 0.1, 5, and 10 MPa CO2 was studied at 550 °C for 500 h. Previous results on the effect of CO2 pressure were summarised and compared with the results in this study. The oxidation kinetics decreased from 0.1 to 5 MPa and thereafter showed a lower dependence on the pressure (5–10 MPa). The carbon concentration at the oxide–alloy interface increased with time, but with different rates depending on the CO2 pressure. The amorphous carbon layers, and M23C6 carbides were identified in the inner oxide layer in the case of 5 MPa CO2.

Anodic oxidations: Excellent process durability and surface passivation for high efficiency silicon solar cells

We investigate the versatility of anodically grown silicon dioxide (SiO2) films in the context of process durability and exceptional surface passivation for high efficiency (>23%) silicon solar cell architectures. We show that a room temperature anodic oxidation can achieve a thickness of ~70 nm within ~30 min, comparable to the growth rate of a thermal oxide at 1000 °C. We demonstrate that anodic SiO2 films can mask against wet chemical silicon etching and high temperature phosphorus diffusions, thereby permitting a low thermal budget method to form patterned structures. We investigate the saturation current density J0 of anodic SiO2/silicon nitride stacks on phosphorus diffused and undiffused silicon and show that a J0 of <10 fA cm−2 can be achieved in both cases. Finally, to showcase the anodic SiO2 films on a device level, we employed the anodic SiO2/silicon nitride stack to passivate the rear surface of an interdigitated back contact solar cell, achieving an efficiency of 23.8%.

GaN/Ga2O3 Core/Shell Nanowires Growth: Towards High Response Gas Sensors

The development of sensors working in a large range of temperature is of crucial importance in areas such as monitoring of industrial processes or personal tracking using smart objects. Devices integrating GaN/Ga2O3 core/shell nanowires (NWs) are a promising solution for monitoring carbon monoxide (CO). Because the performances of sensors primarily depend on the material properties composing the active layer of the device, it is essential to control them and achieve material synthesis in the first time. In this work, we investigate the synthesis of GaN/Ga2O3 core-shell NWs with a special focus on the formation of the shell. The GaN NWs grown by plasma-assisted molecular beam epitaxy, are post-treated following thermal oxidation to form a Ga2O3-shell surrounding the GaN-core. We establish that the shell thickness can be modulated from 1 to 14 nm by changing the oxidation conditions and follows classical oxidation process: A first rapid oxide-shell growth, followed by a reduced but continuous oxide growth. We also discuss the impact of the atmosphere on the oxidation growth rate. By combining XRD-STEM and EDX analyses, we demonstrate that the oxide-shell is crystalline, presents the β-Ga2O3 phase, and is synthesized in an epitaxial relationship with the GaN-core.