Formation Of Al2O3 Layer Research Articles

Enhancing high-temperature chloride molten salts corrosion resistance of 310S steel with Si-added Ni–Al coating

Ni–Al based coating is one of the common high temperature and corrosion resistant protective coatings. In the present paper, the effect of Si content on the formation of Al2O3 layer and the corrosion resistance of Ni–Al coating to chloride molten salt was studied. The research results show that the Ni–Al coatings with varying Si content mainly form NiAl2O4 and Al2O3 oxide layers after pre-oxidation, and form a three-layer structure after molten salt corrosion. The appropriate amount of Si added to the Ni–Al coating can promote the formation of the α-Al2O3 layer during pre-oxidation and thermal corrosion, improving the corrosion resistance of the coating to high temperature chloride molten salt, and hindering the diffusion of Cr elements. Particularly, the Ni–Al coating with Si content of 1.2 at.% has the thickest and densest Al2O3 protective layer, showing the best corrosion resistance. The simulation results shows that compared with the Ni–Al coating without Si, the Cl atoms in the Si-added Ni–Al coating need to overcome greater energy barriers to diffusion during the molten salt corrosion process, which also indicates that the addition of Si can improve the corrosion resistance of the coating.

Continuously Activated Function of Aluminum in Galvanic Micro Arrays in Contact with Water

Galvanic coupled arrays of Aluminum (Al) and Gold (Au) aligned in microgap has displayed high accuracy and sensitivity towards detection of invisibly small water droplets. With Al in moisture sensing chip, 1) formation of Al2O3 layer around Al and 2) decay of Al electrode with time, may interfere its overall activity. In this study, a continuous activation of Al electrode on the sensor chip was performed and investigated through electrochemical evaluation, output current measurement, and SEM observations. Further pH in its vicinity was simulated for ∼2 years. The results showed that a sensing chip exhibits almost steady output current throughout the period. At relatively high current density of 1.25 × 10−6 A.cm−2, obtained in the short circuit, anodic reaction on the Al electrode generates protons. This production of protons reduces the pH to ∼4, where Al2O3 and Al3+ formed on its surface could get dissolved. In addition, Al consumption in 100 ms, i.e. the time for one closed circuit corresponds to 2.26 × 10−17 g.A−1 s−1, which is negligible. The SEM images confirmed unchanged morphology of Al electrodes. The results support the long-term stability of accurate and sensitive sensor chip with galvanic array of Al and Au electrodes.

Heterostructure formation of group-III sesquioxides via cation-exchange reactions with metal chloride gases

Thermodynamic analyses and experimental demonstrations were performed to investigate whether cation-exchange reactions (CERs) with metal chloride gases are a feasible technique for forming heterostructures of group-III sesquioxides. The thermodynamic analyses revealed the possibility of formation of aluminum oxide (Al2O3) layers on gallium oxide (Ga2O3) or indium oxide (In2O3) substrates via CERs with aluminum chloride gases, as well as formation of Ga2O3 layers on In2O3 substrates via CERs with gallium chloride gases, in a practical temperature range. However, CERs with gallium chloride gases or indium chloride gases did not occur on Al2O3 substrates, suggesting that heterostructures were not formed. On the basis of the results of the thermodynamic analyses, a CER was performed by the reaction of n-type β-Ga2O3(2̄01) substrates with aluminum trichloride gas, resulting in the formation of Al2O3 layers. Under optimal conditions, a dense (001)-oriented κ-Al2O3 layer with a thickness of 30 nm was formed and served as an excellent gate oxide in metal–oxide–semiconductor devices. The relative dielectric constant of κ-Al2O3 was obtained via capacitance–voltage measurements, and a high value of ∼22 was obtained in the several tens of kHz band. CERs using metal chloride gases open a new method for forming heterostructures of group-III sesquioxides.

Atomic level decomposition mechanism of Ti2AlC into TiCx

It has proved that outwards diffusion of Al from A layers of Ti2AlC and Ti3AlC2 leads to the formation of Al2O3 layer and TiC during oxidation above 800 °C. While, no atomic level observation was conducted to reveal this transformation process. We demonstrated that the replacement of Al by Ti atoms in A planes leads to the formation of TiCx rather than the connection of two adjacent Ti2C or Ti3C2 layers after the outwards diffusion of Al atoms from A layers. This replacement arouses an adjustment of atomic distance between two adjacent Ti layers and produces one local strain. This further induces one rotation of lattice and TiCx formation instead of the twin formation Ti2C or Ti3C2 along A layer. Due to the lack of one Ti or Al layer, Edge-like structure similar to out-of-plane dislocations formed at the junctions of Ti3C2 and Ti3AlC2 or Ti2AlC and Ti3AlC2. Our observations prove that it is impossible to produce MXene at 800 °C or even high temperature.

Preparation of a thick NiAl/Al2O3 coating by embedding method and its corrosion resistance under supercritical water environment

One of the critical issues in the application of supercritical water oxidation technology is to improve the corrosion resistance of reactor materials. Use of Al2O3 coating is one of the most promising methods to address this issue. In this study, thick NiAl/Al2O3 coatings on Inconel 625 substrates were prepared by a consecutive pack embedding and in-situ thermal oxidation process. The effect of aluminizing and oxidation temperature on phase structure and coating thickness is studied. Results show the diffusion of Al from the exterior to the interior of the alloy matrix to form intermetallic compounds between Al and metal elements in the matrix (Ni, Cr, Mo, etc.). Moreover, the coating thickness can reach above 300 μm at the aluminizing temperature of 950 °C. Increasing the aluminizing temperature above 950 °C will not increase the coating thickness further. After high temperature oxidation subsequently, only phases of NiAl and Al2O3 were detected. The formation of Al2O3 layer can be ascribed to the surface oxidization of Al. And the NiAl between the alloy substrate and Al2O3 coating provides an interfacial layer that can alleviate the crack or exfoliation of ceramic coating due to the mismatching of thermal expand coefficient. The thick NiAl/Al2O3 coatings prepared by aluminizing 950 °C and oxidizing at 1100 °C exhibit satisfied corrosion resistance after supercritical water test. This work would provide a significant method to develop advanced ceramics coating for the corrosion resistance of alloys.

Thermal stability and electrochemical behavior of nanostructured Ti-Al(∼1%(at.))-Mo-N coatings deposited using the Arc-PVD method

The results of studying the electrochemical behavior and thermal stability of multilayered nanostructured Ti-Al-Mo-N coatings deposited using the Arc-PVD method are reported. The paper shows that passivity of the coatings is determined by the possibility of the formation of Al2O3 layer at the interface between the aggressive environment and the coating. This effect is more marked for the coatings containing 8 at% Mo than in those containing 30 at% of Mo. It can be explained by higher mobility of Al in the coating with 8 % (at.) of Mo than in the coating with 30 % of Mo which have varied thickness of the Mo2N layer that inhibits the diffusion of Al. Investigation of thermal stability of the Ti-Al-Mo-N coatings with Mo content of 30 and 8 at% showed that their multilayered architecture are retained at temperatures up to 600 °C.

Phenomenon of Whiskers Formation in Al2O3−C Refractories

Pure alumina (Al2O3) and carbon‐bonded alumina (Al2O3−C) are used to investigate their interaction with iron (Armco) at 1625 °C, applying the sessile drop method. The investigations are conducted inside a heating microscope for 1 h under argon atmosphere with low oxygen partial pressure. No whiskers formation is observed from the experiments with pure Al2O3 and molten iron. In the case of Al2O3−C/Fe system, several phenomena including the decarburization of Al2O3−C, Al2O3−C dissolution in molten iron, Al2O3 layer formation on the iron droplet, and Al2O3 whiskers formation are observed. To clarify the possible influence of iron on Al2O3 whiskers formation, blank Al2O3−C samples without the presence of iron are heated and held in a half‐sealed platinum crucible at 1550 °C for 2 h. Thereby, the Al2O3 whiskers formation is detected. Finally, the experimental investigations are supported with thermodynamic simulations.

Influence of small amounts of Si and Cr on the high temperature oxidation behavior of novel cobalt base superalloys

The effect of small amounts of Si and Cr on the high temperature oxidation of alloys based on Co-9Al-9W was investigated at 900 °C. The results show that the effect of Si additions on oxidation resistance was more pronounced than for Cr additions. An addition of 4 at.% Cr was insufficient to produce a protective layer, while 2 at.% Si led to the formation of a dense, continuous Al2O3 scale. The thermodynamics and kinetics for Al2O3 layer formation were analyzed by combining CALPHAD and Wagnerʼs theory.

Improving hot corrosion resistance of aluminized TiAl alloy by anodization and pre-oxidation

Abstract To shield TiAl alloy from hot corrosion attack, a compact protective coating was fabricated by the combination of aluminizing, anodization and pre-oxidation. The hot corrosion behavior of the coated-TiAl specimen was investigated in the mixture salt consisting of 75 wt.% Na2SO4 and 25 wt.% NaCl at 700 °C. Results indicated that the anodization and pre-oxidation were beneficial to the generation of Al2O3 layer, which could act as a diffusion barrier to prevent the molten salts and oxygen from diffusing into the alloy during exposure to a hot corrosion environment while the aluminizing coating could provide sufficient aluminum source to support the continuous formation of Al2O3 layer. Moreover, the internal stress of the coating was reduced due to the formation of a gradient coating consisting of TiAl3 and TiAl2.

Experimental investigations on the chlorine-induced corrosion of HVOF thermal sprayed Stellite-6 and NiAl coatings with fluidised bed biomass/anthracite combustion systems

Stellite-6 (Co-based) and NiAl coatings (Ni-based) were deposited via HVOF spraying onto 304 stainless steels and tested in a 20 kWth biomass fired bubbling fluidised bed (BFB) combustor for 20 h and an industrial scale anthracite fired CFB boiler for 1630 h. Stellite-6 showed excellent corrosion resistance in both fluidised bed combustion systems because of the formation of the outermost Cr2O3 layer and the spinel CoCr2O4 beneath, whereas NiAl coatings’ anti-corrosion performance was significantly depleted due to the chlorine attack, and the resultant formation of Al2O3 layer at the coating/substrate interface finally led to coating spallation in both systems.

Facile synthesis of aluminum branched oligo(phenylphosphonate) submicro-particles with enhanced flame retardance and smoke toxicity suppression for epoxy resin composites

A novel and submicro-scale aluminum branched oligo(phenylphosphonate) (AHPP) has been successfully synthesized and embedded into a polymeric substrate to improve the fire safety of epoxy resin (EP). The chemical structures of intermediates and target products were characterized using the nuclear magnetic resonance spectroscopy, X-ray diffraction and Fourier transform infrared analysis. Morphology analysis confirmed that all of the as-synthesized AHPP submicro-particles are mutually well-separated. Combustion results demonstrated that the limiting oxygen index value is increased to 30.5% from 23.5% while the PHRR and THR are decreased by ca. 68.1% and 41.2%, respectively for the EP/AHPP-7.5 composite compared to the corresponding values for pure EP. In addition, the binary blends display the satisfying smoke toxicity suppression performance during combustion. The total smoke production and the total CO yield for EP/AHPP-7.5 are dramatically reduced by 62.0% and 32.3%, respectively, which may mainly be ascribed to the catalytic carbonization performance of the polymers and formation of Al2O3 layers on the surface of the char residues. As a result, the findings in this study enabled the submicro-scale phosphorus-containing flame retardant to be a potential candidate as an efficient additive for reducing smoke toxicity of polymer composites.

TiN/Al2O3/ZnO gate stack engineering for top-gate thin film transistors by combination of post oxidation and annealing

Control of fabrication processes for a gate stack structure with a ZnO thin channel layer and an Al2O3 gate insulator has been examined for enhancing the performance of a top-gate ZnO thin film transistor (TFT). The Al2O3/ZnO interface and the ZnO layer are defective just after the Al2O3 layer formation by atomic layer deposition. Post treatments such as plasma oxidation, annealing after the Al2O3 deposition, and gate metal formation (PMA) are promising to improve the interfacial and channel layer qualities drastically. Post-plasma oxidation effectively reduces the interfacial defect density and eliminates Fermi level pinning at the Al2O3/ZnO interface, which is essential for improving the cut-off of the drain current of TFTs. A thermal effect of post-Al2O3 deposition annealing at 350 °C can improve the crystalline quality of the ZnO layer, enhancing the mobility. On the other hand, impacts of post-Al2O3 deposition annealing and PMA need to be optimized because the annealing can also accompany the increase in the shallow-level defect density and the resulting electron concentration, in addition to the reduction in the deep-level defect density. The development of the interfacial control technique has realized the excellent TFT performance with a large ON/OFF ratio, steep subthreshold characteristics, and high field-effect mobility.

Surface modification of polymeric substrates to enhance the barrier properties of an Al2O3 layer formed by PEALD process

Aluminum oxide (Al2O3) layers were deposited on various polymeric substrates by a low frequency plasma-enhanced atomic layer deposition (PEALD) process. Polyethylene naphthalate (PEN), polyethylene terephthalate (PET), and polyethersulfone (PES) were tested as substrates for barrier films. Each substrate has its own characteristics to have influences on the Al2O3 layer formation and penetration into the substrate, which greatly affected the barrier properties. Prior to the deposition process, polymeric substrates were pretreated in argon and oxygen plasmas, and surface energy was leveled up due to the formation of polar group. Characterizations of the Al2O3 layer by Time of Flight - Secondary Ion Mass Spectrometry (ToF-SIMS) revealed that plasma treatment lowered the level of OH− in Al2O3 layer. X-ray photoelectron microscopy (XPS) confirmed that A12p peak of Al2O3 layer was shifted to a higher core level by plasma treatment. Density of the layer on the plasma treated surface was greater than that of untreated surface. It was found that plasma treatment of the surface had significant effects on the formation of the Al2O3 layer, which much improved the barrier performance. Optical transmittance was little affected by plasma treatment and PEALD process. After oxygen plasma pretreatment, the WVTR of the Al2O3 layer deposited on the plasma-treated PEN substrate was around 7.2 × 10−4 g/m2day, which is significantly lower than that of the untreated substrate.

Examining surfaces of oxidized aluminum exposed to СО2 laser pulses

Comprehensive microstructure studies of the surfaces of duralumin sheets exposed to an intense microsecond СО2 laser pulse (Е ~ 500 J) are performed for the first time. The irradiated area is ~100 cm2. A pulse with a duration of ~5 μs has an ~200 ns leading peak. Passivated duralumin sheets subjected to pressure and thermal treatment resulting in the formation of Al2O3 layers ~7 μm thick are irradiated. Electron and atomic force microscopy, X-ray spectral analysis, and optical profilometry are used in the study. Detected traces of nonequilibrium evaporation contained a polycrystalline aggregate of several stoichiometric and nonstoichiometric phases of aluminum oxide and its compounds with manganese and magnesium.

A novel way to enhance hydrogenation resistance of nano-layered titanium silicon carbide by the doping of aluminium

The outstanding irradiation and corrosion resistance of nano-layered Ti3SiC2 make it suitable for cladding materials in advanced nuclear systems. However, Ti3SiC2 shows the relatively poor thermal stability in hydrogen circumstance at high temperature, which is a big obstacle for its nuclear related applications. In this paper, we proposed an effective approach to improve the hydrogenation resistance of Ti3SiC2 by the doping of Al. The experimental results demonstrated that compared to pure Ti3SiC2, Ti3Si0.9Al0.1C2 (TSAC) displayed much better hydrogenation resistance. Through first-principles calculation, it was concluded that the introduction of H interstitial atom made the formation energy of Al vacancy much lower, so Al atoms became much easier to remove from TSAC than Si atoms and Ti atoms. The preferable loss of aluminium from TSAC substrate gave rise to the formation of Al2O3 layer, which improved the hydrogenation resistance of TSAC by inhibiting further reaction between TSAC and hydrogen.

High temperature oxidation behaviour of Al2O3/Al composite coating on γ-TiAl

An Al2O3/Al composite coating was introduced on the substrate γ-TiAl alloy by magnetron sputtering. The isothermal oxidation behaviour of the coated γ-TiAl alloy was investigated at 1000°C. The results suggested that the Al2O3/Al composite coating improved the oxidation resistance of γ-TiAl alloy at 1000°C air exposure. No spallation or crack was observed in the oxide scale of coated specimen, which was composed of α-Al2O3 and TiAl3 after the oxidation test. The outward Al diffusion from interlayer provided sufficient Al source for the formation of Al2O3 layer in the coating surface during the high temperature oxidation test. As an active intermetallic element with Ti, Al suppressed the outward of Ti effectively due to the formation of Ti–Al intermetallics. Owing to inward and outward diffusion, the Al interlayer was consumed after 100 h oxidation test and a Ti–Al interdiffusion zone was formed, which was beneficial for the adhesion between coating and substrate.

Al<sub>2</sub>O<sub>3</sub>/SiO<sub>2</sub> Materials with Bimodal Pore Size Distribution as Support for Heterogeneous Catalysts

The Al2O3/SiO2 materials prepared by impregnation of mesoporous silica with water solution of Al(NO3)3·9H2O were suggested to be used as supports for catalyst of high-temperature catalytic reactions. It was shown that the distribution of Al2O3 component inside wide pores of silica (10-50 nm) occurs with the formation of Al2O3 layer in case of low loading and porous layer or particles, if Al2O3 loading increases. Thus, materials with bimodal pore size distribution were obtained. The Cr-containing catalysts were prepared on the basis of these supports and tested in the process of isobutane dehydrogenation at 540 °C. It was observed that the activity of the obtained Cr/Al2O3/SiO2 materials was comparable with the one for Cr/Al2O3 catalyst. Thus, the suggested Al2O3/SiO2 materials with unique porous structure are promising to be used as supports for catalysts for high-temperature reactions.

Secondary ion mass spectrometry and photoluminescence study on microstructural characteristics of chemically synthesized ZnO nanowalls

We report on the thermal effect on the microstructural modification of self-assembled ZnO nanowalls grown on Al substrates. The morphology of the as-synthesized nanowalls has been investigated using scanning electron microscopy, while the X-ray diffraction study ensures the formation of hexagonal wurtzite crystal structure of ZnO and also indicates the formation of Al2O3 layer at the interface of Al and ZnO nanowalls. The unidirectional vertical growth of ZnO wire-like network formed at an intermediate stage has been anticipated as the evolution mechanism of the nanowalls. Upon annealing in air and vacuum environment, significant crystallographic modification of the ZnO nanowalls has been observed through X-ray diffraction. The appearance of various native defects (e.g. oxygen vacancies, zinc interstitials, etc.) in the ZnO crystals of the annealed nanowalls has been evidenced through photoluminescence spectroscopy. The luminescence response of the nanowalls is found to be dependent on the surface adsorbant species like OH−, O2−, etc., whose existence on the nanowall surface have been detected through secondary ion mass spectrometry (SIMS). Again the oxygen deficiency in the vacuum-annealed nanowalls has also been confirmed through SIMS measurements. Apart from this microstructural alteration of the nanowalls, annealing also has aggravated the dominance of insulating nature of the Al2O3 interface.

Chemical-state-resolved depth profiles of Al/Pr0.7Ca0.3MnO3 stacked structures for application in resistive switching devices

We have investigated the interfacial chemical reaction between Al metal electrodes and Pr0.7Ca0.3MnO3 perovskite oxides to shed light on the underlying mechanism of resistive switching phenomenon that can be utilized in random access memory devices. Depth profiles of the Al/Pr0.7Ca0.3MnO3 interface show the formation of Al2O3 layers resulting from redox reactions between Al metal and perovskite oxide. Angle-resolved Mn 2p core-level photoemission spectra, which are independent of the Al metal electrode thickness, show that a Mn-Al alloy is formed at the interface between Al metal and Al2O3 layers during the initial growth of the Al metal electrode. This suggests that the Mn impurity level in the band gap of Al2O3 layers plays an important role in resistive switching characteristics.

Corrosion Resistance of 9 - 15% Cr ODS Steels and its Comparison with Austenitic Stainless Steel

An oxide dispersion strengthened steels are one of the most promising high temperatures, and high performance advanced structural material being developed for future fast reactors and high-temperature Generation IV reactors. In the present work, the corrosion resistance and its correlation with the passive film compositions of 11% Cr F/M and 9-15% Cr (with Zr or Hf) ODS steels is examined and compared with AISI type 304L stainless steel in boiling 60 - 62% (~13 M) HNO3. The corrosion rate measured in 62% HNO3 for 240 h of 11% Cr F/M, 9% Cr and 15% Cr (Zr) ODS steels show high corrosion rate. On the other hand, low corrosion rate was observed in 304L stainless steel (0. 21 to 23 mm y-1). However, severe intergranular corrosion attack was revealed in type 304L SS after 240 h exposure, but none in ODS steels. Such an intergranular corrosion attack seen in type 304L stainless steel is undesirable. On the contrary, low corrosion rate (0.04 0.15 mm y-1) of 15% Cr (Hf) ODS steel in 3 M, 6 M and 9 M HNO3, comparable to that of type 304L stainless steel was observed. The improved corrosion resistance of 15% Cr (Hf) ODS steel was attributed to enrich (20 at. %) and protective Al2O3 layer formation in addition to Cr2O3 in the passive film.