Mixed Oxide Layer Research Articles

Radiation-induced modifications in copper oxide growth

Cu samples were irradiated with 10 MeV Au3+ ions at 200 °C to damage levels of 5, 10, and 15 displacements per atom (dpa) as an analogue to study long term self-irradiation effects of alpha-decay in Pu. Samples were then subject to accelerated aging at 350 °C for 1 h in air resulting in mixed oxide layer growth (Cu2O and CuO). Raman spectroscopy revealed that the CuO phase fraction was gradually decreased as the damage level increased. These findings indicate that accumulated damage from self-irradiation causes quantifiable modifications in metal oxidation that could serve as a novel forensic signature.

Isothermal oxidation behavior of micro-regions in multiphase Ni3Al-based superalloys

The isothermal oxidation behavior of micro-regions in multiphase Ni3Al-based superalloys (high Fe and Cr contents) was investigated at 600 °C in air. The oxide scale is mainly composed of the outer mixed oxide layer (NiO, NiFe2O4 and Al2O3) and the inner single Al2O3 layer. In particular, Cr2O3 is formed in the outer oxide scale of γ' + γ dendrite with enrichment in Cr element. The relative dense continuous Al2O3 layer is formed in the interdendritic region (β phase with high Al content). In addition, γ' envelope (connecting the γ' + γ dendrite and interdendritic β phase) shows preferential oxidation featuring significant cellular bulge (NiO and NiFe2O4 mixed oxides).

Long-Term Failure Mechanisms of Thermal Barrier Coatings in Heavy-Duty Gas Turbines

Thermal barrier coatings serve as thermal insulation and antioxidants on the surfaces of hot components. Different from the frequent thermal cycles of aero-engines, a heavy-duty gas turbine experiences few thermal cycles and continuously operates with high-temperature gas over 8000 h. Correspondingly, their failure mechanisms are different. The long-term failure mechanisms of the thermal barrier coatings in heavy-duty gas turbines are much more important. In this work, two long-term failure mechanisms are reviewed, i.e., oxidation and diffusion. It is illustrated that the growth of a uniform mixed oxide layer and element diffusion in thermal barrier coatings are responsible for the changes in mechanical performance and failures. Moreover, the oxidation of bond coat and the interdiffusion of alloy elements can affect the distribution of elements in thermal barrier coatings and then change the phase component. In addition, according to the results, it is suggested that suppressing the growth rate of uniform mixed oxide and oxygen diffusion can further prolong the service life of thermal barrier coatings.

High-temperature oxidation behavior of Ti2AlNb alloy with PEO/hBN composite coating at 1000 °C

The oxidation resistance of Ti2AlNb alloy is not ideal when the service temperature is higher than 800 °C. In this work, plasma electrolytic oxidation (PEO) coating and PEO/hBN composite coating were prepared and used to improve the high temperature performance of Ti2AlNb alloy. The microstructure and chemical composition of these coatings were systematically analyzed by FESEM, XPS and XRD. Meanwhile, the high temperature oxidation behaviors of bare and coated Ti2AlNb samples at 1000 °C were studied by isothermal oxidation tests. The results showed that the PEO coating was porous and contained rutile-TiO2, anatas-TiO2, Al2O3, a little Nb2O5 and WO3. PEO/hBN composite coating was denser and also had hBN phase in addition to the original components. The bare sample had the biggest oxidation reaction rate constant (K) and poor oxidation resistance. After being oxidized at 1000 °C for 100 h, the mass gain reached to 12.08 mg/cm2, and a multi-layered oxide scale consisting of loose Al2O3 outer layer, R-TiO2 and AlNbO4 mixed oxide layer and oxygen diffuse zone was formed. PEO coating improved the initial oxidation resistance by hindering the diffusion of oxygen, but the coating began to fail after 30 h due to the presence of micro-pores. In contrast, PEO/hBN composite coating had better protection effect against Ti2AlNb matrix. The oxidation kinetic curve of this sample followed the quasi-parabolic law and did not degrade at 1000 °C up to 100 h. The dense coating structure and the filling effect of nitride (AlN, TiN, etc.) formed during high-temperature process on coating defects were considered to be the main reasons for the excellent oxidation resistance of PEO/hBN coatings.

Characterization of the nitrided γ-Ti-46Al–2Nb and γ-Ti-46Al–2Nb-0.7Cr-0.3Si intermetallic alloys

The thermal, surface properties and microstructures of the nitrided γ-Ti-46Al–2Nb and Ti–46Al–2Nb-0.7Cr-0.3Si intermetallic alloys were investigated. These vacuum-melted alloys were annealed at 1100 °C to stabilize the (α2+γ) lamellae structure. Moreover, the annealed alloys were further annealed at 900 °C under nitrogen gas (nitridation). The alloys microstructures and nitride surface coatings were characterized. The Ti–46Al–2Nb alloy was comprised of γ-TiAl, α2-Ti3Al and β-phases after the annealing, while the Ti–46Al–2Nb-0.7Cr-0.7Si alloy revealed the Ti5Si3, γ-TiAl and α2-Ti3Al phases. Thin nitride layers formed on both alloys. After cyclic oxidation, the nitrided alloys exhibited surface spallation at different rates. The X-ray photoelectron spectroscopy (XPS) revealed the presence of mixed oxide layer of Al2O3, Cr2O3 and SiO2 near the Ti–46Al–2Nb-0.7Cr-0.7Si surface.

Effect of thermal oxidation on surface chemistry and elemental segregation of Al-Cu-Li alloy

The thermal stability of Al-Cu-Li alloys is critical for the extreme working conditions in the aircraft industry. The initial stages of thermal oxidation of an industrial AA 2050-T8 Al-Cu-Li alloy were investigated by in situ XPS and ToF-SIMS.Annealing the sample at 200 °C leads to a surface enrichment in metallic Li and Ag. When exposed to oxygen at low pressure (PO2 < 10−6 mbar) a mixed Li-Al oxide layer is formed. Photoemission intensities (recorded at 45° and 90° relative to the surface) are consistent with the formation of a continuous oxide layer for oxygen exposure from 0 to 5000 langmuirs (L). Assuming a homogeneous composition of the oxide layer, its thickness was 9 ± 1 Å at 5000 L. The relative content of Li and Al in the oxide layer evolves from an initial value of 50% to stabilize at 70% of Li above 100 L of oxygen exposure. Simultaneously, the content of Li at the alloy substrate surface is reduced from 10% to 0%. Moreover, specific segregation sites are evidenced by ToF-SIMS three-dimensional chemical mapping. For Li, a homogenous surface distribution while for Mg the accumulation at the grain boundaries were observed. The segregation of Ag at the oxide/alloy interface was confirmed by ToF-SIMS ion depth profiles.

Evolution of Optical, Electrical, and Structural Properties of Indium Tungsten Oxide upon High Temperature Annealing

Optical, structural and electrical properties of thermally co‐evaporated indium tungsten oxide (IWOx) thin films with varied stoichiometry, from pure tungsten oxide to pure indium oxide (InOx) are investigated upon stepwise annealing, up to 700 °C. The thin films are candidate materials for carrier selective contacts in different types of solar cells, such as silicon hetero junction and perovskite solar cells. Three different phases for the thin films with different stoichiometry and crystallization temperatures of Tc &gt; 500 °C for tungsten‐rich layers and Tc ≈ 200 °C for indium‐rich layers are found. The pronounced optical absorption of the as‐deposited InOx‐rich layers is strongly decreased after crystallization. Tungsten oxide rich layers show low optical absorption in the as‐deposited state as well as for all applied annealing temperatures. The lateral conductivity of the pure indium oxide can be increased from 1.24 × 10−2 up to 0.83 S cm−1 after 700 °C annealing. The conductivity of the pure tungsten oxide increases slightly after crystallization from 2.55 × 10−5 to 8.25 × 10−5 S cm−1 after annealing at 700 °C. However, for mixed oxide layers with ≈25% InOx‐fraction in the mixture, the highest conductivity of 4.0 × 10−6 S cm−1 cannot be increased by the applied annealing process.

Investigation of oxidation behaviors of coated Zircaloy as accident-tolerant fuel with CrAlN and CrAlSiN coatings in high-temperature steam

CrAlN and CrAlSiN coatings were deposited on Zircaloy samples by multi-arc ion plating (MAIP) and the microstructure, hardness, and oxidation resistance of the two types of coatings in high-temperature steams were investigated. The coatings can effectively protect the substrate from oxidation during exposure in 1000 °C steam for 2 h and can delay the oxidation of substrate for more than 15 min in 1200 °C steam. Mixed oxide layers formed and suppressed the elemental diffusion during oxidation. Nitrogen bubbles were found inside the mixed oxide. The CrAlSiN coatings showed better oxidation resistance but they cracked more easily.

Comprehensive study on the microstructure evolution and oxidation resistance performance of NiCoCrAlYTa coating during isothermal oxidation at High temperature

The microstructural evolution and high temperature oxidation resistance performance of NiCoCrAlYTa coating were investigated via isothermal oxidation at 1100℃ and 1150℃ for up to 200 h. The results reveal that the microstructure of oxides scale formed at higher temperature gradually evolve from a dense alumina layer to a mixed oxide layer, consisting of α-Al2O3 and considerable amount of NiAl2O4 spinel, NiO as well as abundant amount of (Y, Ta)-enriched oxides, which not only caused the formation of defects, such as voids and cracks, but also developed a rumpling of coating surface, accelerating the aggregation and growth of cracks. And serious internal oxidation was also detected as the oxidation time increased, which was discussed and explained from the aspect of interdiffusion behavior. At high temperature of 1100℃ and 1150℃, the oxidation kinetics curve deviated from parabolic law, indicating the oxidation resistance property of NiCoCrAlYTa coating compromised under these severe conditions. Based on the result of calculated diffusion activation energy, the failure mechanism of NiCoCrAlYTa coating is largely dependent on the exposure temperature. During the growth of oxides scale, the external diffusion of cations is the mainly process, accompanied with the internal diffusion of oxygen to a certain degree. The results could provide comprehensive understanding of the microstructural evolution during oxidation, which was essential for controlling oxides scale growth and improving the durability of NiCoCrAlYTa coating.

Continuous tuning of the plasmon resonance frequency of porous gold nanoparticles by mixed oxide layers

Porous gold nanoparticles (PGNs) are very popular due to their high surface/volume ratio, moreover they have stronger plasmonic properties than their solid counterparts. These properties make the porous gold nanoparticles very useful for lots of applications, for instance chemical sensors, cancer therapy applications. For applications, however, it is indispensable that the resonance frequency (RF) of a plasmonic structure to be tuneable. In this work we show that the RF can be set in a wide range as desired by coating the PGNs by mixed oxide layers. By changing the composition of the coating layer, that is the mixture ratio, the RF can be shifted practically continuously in a wide range determined by the refractive index of the used oxides. As a demonstration, PGNs were coated with mixed alumina-titania oxide layers (5–7 nm) using plasma-enhanced atomic layer deposition method. The oxide layer, beside as a tuning tool, also stabilises the structure of the PGNs when are exposed to elevated temperature. This is shown by the influence of the temperature (from 350, ^{circ }hbox {C} up to 900, ^{circ }hbox {C}) on the morphology, and as a consequence the optical extinction spectra, of the oxide coated PGNs.

Oxidation behaviors of (Hf0.25Zr0.25Ta0.25Nb0.25)C and (Hf0.25Zr0.25Ta0.25Nb0.25)C-SiC at 1300–1500 °C

In this work, high-entropy ceramics (Hf0.25 Zr0.25Ta0.25Nb0.25)C (HZTNC) and HZTNC doped with 20 vol% SiC (HZTNC-SiC) were fabricated by spark plasma sintering. Their oxidation behavior was investigated over the temperature range of 1300–1500 °C for up to 60 min. Both HZTNC and HZTNC-SiC exhibited good oxidation resistance, and their weight change as a function of oxidation time obeyed a parabolic law. Through XRD, microstructure observation, and elemental mapping analysis of the oxide layers, it was found that the formation of Nb2Zr6O17, Hf6Ta2O17, and (Ta, Nb)2O5 mixed-oxide layers effectively protected the matrix from further oxidation. Microcracks began to appear on the oxide layer of HZTNC at high temperatures after 60 min of oxidation. However, the addition of SiC in HZTNC suppressed these microcracks at high temperatures due to the active oxidation of SiC. Compared with the oxides formed on HZTNC, the additional formation of Hf(Zr)SiO4 on HZTNC-SiC could further improve its oxidation resistance over HZTNC ceramics.

Precursor-surface interactions revealed during plasma-enhanced atomic layer deposition of metal oxide thin films by in-situ spectroscopic ellipsometry

We find that a five-phase (substrate, mixed native oxide and roughness interface layer, metal oxide thin film layer, surface ligand layer, ambient) model with two-dynamic (metal oxide thin film layer thickness and surface ligand layer void fraction) parameters (dynamic dual box model) is sufficient to explain in-situ spectroscopic ellipsometry data measured within and across multiple cycles during plasma-enhanced atomic layer deposition of metal oxide thin films. We demonstrate our dynamic dual box model for analysis of in-situ spectroscopic ellipsometry data in the photon energy range of 0.7–3.4 eV measured with time resolution of few seconds over large numbers of cycles during the growth of titanium oxide (TiO2) and tungsten oxide (WO3) thin films, as examples. We observe cyclic surface roughening with fast kinetics and subsequent roughness reduction with slow kinetics, upon cyclic exposure to precursor materials, leading to oscillations of the metal thin film thickness with small but positive growth per cycle. We explain the cyclic surface roughening by precursor-surface interactions leading to defect creation, and subsequent surface restructuring. Atomic force microscopic images before and after growth, x-ray photoelectron spectroscopy, and x-ray diffraction investigations confirm structural and chemical properties of our thin films. Our proposed dynamic dual box model may be generally applicable to monitor and control metal oxide growth in atomic layer deposition, and we include data for SiO2 and Al2O3 as further examples.

Boosting multiple photo-assisted and temperature controlled reactions with a single redox-switchable catalyst: Solvents as internal substrates and reducing agent

An alternative and economically viable process for the synthesis of β-aryl enals, enones and the aryl amines has been developed by partial oxidation of ethanol, isopropanol and N, N-dimethyl formamide (DMF). The formation of β-aryl enals, enones and the aryl amines was catalyzed by a mixed metal oxides layer of cobalt and chromium supported on halloysite nanotubes, designated as CoCr2O4-HNT. The CC and CN bond formation reactions were found to be influenced by temperature and the nature of base. The condensation of aldehyde with in situ generated acetaldehyde by ethanol oxidation forming β-aryl enals occurred selectively at 120 °C. The partial oxidation of isopropanol to acetone and its condensation with aldehydes forming β-aryl enones occurred at room temperature. Increase in temperature caused the liberation of hydrogen gas from isopropanol and allowed the reversible reduction of aldehydes to alcohols. Increase in temperature in isopropanol and increase in base concentration in ethanol causes the selective reduction of aldehydes to alcohols. Besides being active for the Claisen-Schmidt type of reactions and the aryl halides amination process, the synthesized catalyst was also found to be highly active for the photocatalytic oxidation of benzyl alcohols in absence of any external oxidizing agent. The positive holes (h+) generated at the Co(II) site as evident from EPR analysis was considered to be responsible for high photocatalytic activity of the material reducing the recombination rate of holes and electrons (e−). Density Functional Theory calculations were performed to understand the mechanism of ethanol oxidation to acetaldehyde.

Enhanced chlorine evolution from dimensionally stable anode by heterojunction with Ti and Bi based mixed metal oxide layers prepared from nanoparticle slurry

This study reports enhanced current (CERCS) and energy efficiency (EERCS) of reactive chlorine species (RCS) generation on Ir7Ta3Oy anode by Ti/Bi mixed metal oxide heterojunction layers despite reductions in pseudo-capacitance and film conductivity. In potentiostatic electrolysis of 50 mM NaCl solutions, dramatic improvement (0.61 mmol cm−2 hr−1 at 2.5 V NHE) was noted by simple coating of thin (~2 μm) TiO2 layer from ball-milled TiO2 nanoparticle (80–100 nm) suspension, even with moderate elevation in voltammetric wave. Decoration of Bi2O3 particles (1 – 2 μm) showed limited or adverse effects for RCS generation and stability. However, Bi-doped TiO2 layers prepared from polyol-mediated or co-precipitation methods marked the highest CERCS (~100%) and EERCS (8.16 mmol Wh−1 at 2.5 V NHE) by increased mixing level and effective shift in surface charge. Surface ·OH exclusively mediated the RCS generation whose further transformation to higher oxide could be restrained by the heterojunction layer.

Effects of Al Sputtering Film on the Oxidation Behavior of NiCrAlY Bondcoat

In this study, the oxidation behavior of Al coated NiCrAlY bondcoat is investigated. It is known that many methods are applied to improve the lifetime of bondcoat in thermal barrier coatings. Herein, the Al sputtering method is selected to increase the Al content, which does not change the structure of bondcoat. Thin Al film of ~2 µm was sputtered on the surface of bondcoat, which improved the oxidation resistance of NiCrAlY bondcoat. Experimental results showed that, after oxidation for 200 h at 1200 °C, the formation of a dense and continuous α-Al2O3/Cr2O3 multilayer was observed on the Al coated bondcoat surface. In contrast, a mixed oxides (NiO, Cr2O3 and spinel oxides) layer formed on the surface of the as-sprayed bondcoat samples. Results of the cyclic oxidation at 1050 °C within 204 h indicated that the Al sputtering method can improve the oxidation resistance of bondcoat. This study offers a potential way to prolong the lifetime of thermal barrier coatings and provides analysis of the oxidation mechanism.

Conceptual design of Super FR for MA transmutation with axially heterogeneous core

Supercritical water cooled Fast Reactor (Super FR) is featured with the large coolant density reduction by almost 1/10 from the core inlet to the outlet. Since Minor Actinides (MAs) not only have large neutron capture cross sections for thermal neutrons, but also can fission with fast neutrons, MA transmutation performance of Super FR may greatly depend on MA loading positions in the core axial direction of Super FR.However, such investigations have never been conducted. Hence, this study aims to design a Super FR transmutation core concept with the axial configuration of multiple layers of Mixed Oxide (MOX) and blanket fuels, with a focus on the influence of the large axial coolant density change on MA transmutation and core characteristics.With the design criteria of the negative Void Reactivity Coefficient (VRC), the Maximum Cladding Surface Temperature (MCST) < 650 ℃ and the Maximum Linear Heat Generation Rate (MLHGR) < 39 kW/m, three-dimensional neutronics and thermal-hydraulics coupled core burnup calculations have been carried out. Assembly-wise coolant flow rate distribution is determined to attain high core average outlet temperature and the core characteristics of different designs have been evaluated for the equilibrium core after the cores have reached equilibrium states with given fuel shuffling schemes. It has been shown that the MA transmutation amount is limited by deterioration of VRC due to increase of Pu enrichment for compensating the reactivity penalty by MA loading. Moreover, such influence has been found to be more significant in the lower region of the core, where the coolant density is relatively high. Hence, the core design with MA loading to the upper MOX layer is favorable for improving the MA transmutation performance. However, the trade-off relationship between the MA transmutation amount and thermal-hydraulics performance (increase of MLHGR and decrease of average outlet temperature) has been revealed. To overcome the issue, the core radial zoning has been applied and it has been found effective to suppress the trade-off relationship.

Strength and Mechanism of Adhesion to the Substrate Layer while Applying Plasma Coatings in Oxidizing Environments

It has been examined that when an oxide layer is treated under atmospheric conditions on a treated substrate surface, in the contact zone, an oxide layer is always present. This mixed oxide layer influences the contact temperature and the adhesion strength of the particles that are being coated to the substrate surface.Henceforth the process of heating of the contact layer of surface oxides was investigated. The experimentation was demonstrated by coupling of the particles of the coating layer with the surface layer when wetting is provided by melting of the oxide layer. The conditions of the wettable contact under nonstationary heat exchange were determined depending on the thermophysical properties of the contact pair, the temperatures of the coating particles and the substrate. The wetting time was evaluated depending on the thickness of the oxide layer. The identity of the adhesion strength of the coating and the cohesive strength of the oxide layer was substantiated. It was evaluated that for the inner surface of the coating elastic modulus and breaking stress is about 20% higher than for the external. The detected difference is related to the presence of the temperature gradient across the thickness for coating.

High temperature oxidation behavior of Co-20Re-25Cr-2Si alloy at different oxygen pressures

The oxidation of the quaternary Co-20Re-25Cr-2Si alloy has been studied at 1000−1300 °C for 24 h. On the whole, at high oxygen pressure of 0.1 MPa the alloy shows mass loss while at low oxygen pressure of 3.0×10−5 Pa, however, the alloy shows mass gain. Compared with the irregular kinetics under the oxygen pressure of 0.1 MPa, the kinetics approximately conform to parabolic law under the oxygen pressure of 3.0×10−5 Pa, with the value of oxidation activation energy equal to 262.7 kJ/mol. The scales formed under the oxygen pressure of 0.1 MPa at 1000−1300 °C are very similar. The outermost layer is CoO, followed by a mixed oxide layer probably composed of CoO and CoCr2O4. The innermost layer is probably composed mainly of a thin discontinuous layer of Cr2O3, accounting for the continuous mass loss due to the evaporation of Re element in the form of ReO3. However, the sales formed under the oxygen pressure of 3.0×10−5 Pa are probably composed of an outer thin layer of CoCr2O4 spinel and an inner dense and continuous layer of Cr2O3, which provides the good oxidation resistance for the alloy. The combination of the addition of Si and low oxygen pressure has shown a synergistic effect on the formation of a protective chromia layer.

Electrochemical Deposition of Mixed Ce-Al Oxide Layers on Stainless Steel and Assessment of their Corrosion-Protective Ability

The corrosion protective ability of the electrodeposited mixed nanocrystalline (CeO2)x(Al2O3)1-x layers has been studied in 0.1M HNO3 solution as a function of the changing ratio between CeO2, CeAlO3 and Al2O3 on the modified SS surface. It was ascertained that the mixed layer changes substantially the character of the cathodic corrosion reaction, which leads to changes in the control of the corrosion process and stabilization of the passive state of the SS, the system (CeO2)x(Al2O3)1-x/SS respectively, in case of prolonged corrosion testing.

Anticorrosion Performance of the Electrochemically Grown Mixed Porous Oxide Films on Titanium Alloy in Biological Solution.

A thin porous mixed layer of TiO2-ZrO2 was grown on titanium-zirconium alloy surface by electrochemical oxidation. Further comparison of the corrosion behavior in Fusayama-Meyer biological solution was performed. Scanning electron microscopy surface morphology investigations confirm the presence of porous oxide film, while energy-dispersive X-ray spectroscopy analyses provide a high amount of oxygen element in the porous film and a higher amount of Zr element, concluding the mixed nature of oxide film formed, TiO2-ZrO2. For corrosion investigations, electrochemical techniques such as open-circuit potential, electrochemical impedance spectroscopy, and potentiodynamic polarization were applied. The corrosion investigations reveal better behavior of the thin porous mixed oxide layer on titanium alloy compared to the untreated alloy having on their surface the native oxide formed in contact with the air, confirming the possibility to improve the properties of such implant application in saliva solution.