Al Concentration Gradient Research Articles

The effect of Al on the corrosion resistance of binary Mg-Al solid solutions: Combining in-situ electrochemistry with combinatorial thin films

The effect of varying Al concentrations on the electrochemical corrosion resistance of binary Mg-Al solid solutions thin films under alkaline immersion conditions was investigated via a combination of in-situ flow-cell, scanning vibrating electrode technique and microscopy analysis. These spatially resolving characterization techniques are employed along the Al concentration gradient of the combinatorically grown thin films enabling efficient screening of the Al concentration dependent electrochemical corrosion behaviour. The analysis revealed an increasing corrosion resistance with increasing Al concentration, as a consequence of Al induced hydroxide reinforcement. Specifically, the addition of >4 wt.% Al decreases the corrosion current density in the range of 70–90 % compared to pure Mg.

Stoichiometric Cr2AlC MAX phase coatings deposited by HPPMS from composite targets using industrial deposition technology

Compositionally-graded Cr2AlC coatings were deposited from composite targets with Al concentration gradients along the long-axis of the targets, ranging from 20.0 to 30.0 at.%, in an industrial plant. Stoichiometric Cr2AlC coatings deposited by HPPMS with a peak-power density of 217 W/cm2 and at substrate bias potential of −100 V require an Al concentration in the target of 30.0 at.% to compensate for the Al-loss induced by preferential re-sputtering and thermally induced desorption at these deposition conditions. The HPPMS deposition of a stoichiometric Cr2AlC coating at floating potential can be attained from a target segment containing 26.4 at.% of Al. Hence, during synthesis at 560 °C in an industrial deposition system with two-fold rotation significant deviations between target and coating composition were observed. It was demonstrated that these ion energy and power density induced reductions in Al concentration can be compensated for by utilizing Al-rich composite targets.

Friction stir high-throughput synthesis to evaluate the effect of Al on AZ Mg alloys

Friction stir high-throughput synthesis (FSHS), a simple and efficient high-throughput synthesis method for bulk sample with compositional gradient, was developed combining friction stir welding and friction stir processing. The ability and efficiency of this method was demonstrated by using AZ31 and AZ91 as base materials to synthesize AZ Mg alloy samples with Al concentration gradient. Bulk sample with Al concentration range of 3.35 to 8.52 wt% was synthesized within one hour, from which 34 tensile specimens can be obtained. Compositional homogeneity within cross section of stir zone was achieved rapidly via multiple passes of friction stir processing. With Al concentration increasing, grain size decreased and basal texture weakened, resulting in enhancement of mechanical properties and different strain hardening behaviors. This preliminary study shows FSHS is a powerful method for studying the effect of alloying element, highly efficient in screening compositions for new alloy development.

Polarization Effects in Graded AlGaN Nanolayers Revealed by Current-Sensing and Kelvin Probe Microscopy.

We experimentally demonstrate that the conductivity of graded AlxGa1-xN increases as a function of the magnitude of the Al concentration gradient (%Al/nm) due to polarization doping effects, without the use of impurity dopants. Using three up/down-graded AlxGa1-xN nanolayers with Al gradients ranging from ∼0.16 to ∼0.28%Al/nm combined in one structure, the effects of polarization engineering for localized electric fields and current transport were investigated. Cross-sectional Kelvin probe force microscopy and conductive atomic force microscopy were used to directly probe the electrical properties of the films with spatial resolution along the thickness of the growth. The experimental profiles of the built-in electric fields and the spreading current found in the graded layers are shown to be consistent with simulations of the field distribution as well as of the electron and hole densities. Finally, it was directly observed that for gradients less than 0.28%Al/nm the native n-type donors still limit polarization-induced hole doping, making p-type conductivity still a challenge due to background impurities and defects.

High-voltage electrochemical performance of LiNi0.5Co0.2Mn0.3O2 cathode materials via Al concentration gradient modification

Al2O3-modified LiNi0.5Co0.2Mn0.3O2 cathode material is successfully synthesized via a facile carboxymethyl cellulose (CMC)-assisted wet method followed by a high-temperature calcination process. Al concentration gradient doping and accompanying formation of Al-coating are simultaneously accomplished in the modified samples. XRD and EDS analysis demonstrate that Al element is successfully doped into the crystal lattice with concentration gradient distribution inside the particles, reducing the Li/Ni cation mixing and stabilizing the layered structure. The compact distribution of Al on the surface forms a protective layer between the electrodes and the electrolyte, prohibiting the harmful side reactions and phase transition on the interphase. Compared with the pristine, the modified material with 2000 ppm Al2O3 (Al-2000) shows the best high-voltage performance with the capacity retention increased by ~13.3% from 138.3 to 163.0 mAh g−1 at 1 C in 3.0–4.6 V after 100 cycles. Even under the high current rate of 8 C (1240 mAh g−1) after 200 cycles, the Al-2000 still exhibits a capacity retention of 88.6%, greater than 80.3% for the pristine. Furthermore, results from the cyclic voltammetry (CV) and the electrochemical impedance spectroscopy (EIS) measurements confirm the roles of the Al2O3 modification in decreasing polarization and electrochemical resistances, contributing to the kinetic process of electrodes.

Kinetics of Transformation of Al2O3 to MgO·Al2O3 Spinel Inclusions in Mg-Containing Steel

During ladle furnace refining, initial Al2O3 inclusions generally transform into MgO·Al2O3 spinel inclusions; these generated spinel inclusions consequently deteriorate the product quality. In this study, the transformation from Al2O3 to MgO·Al2O3 was investigated by immersing an Al2O3 rod into molten steel, which was in equilibrium with both MgO and MgO·Al2O3 spinel-saturated slag. A spinel layer, with a thickness of 4 μm, was generated on the Al2O3 rod surface just 10 s after its immersion at 1873 K (1600 °C). The thickness of the formed spinel layer increased with the immersion period and temperature. Moreover, the MgO content of the generated spinel layer also increased with the immersion period. In this study, the chemical reaction rate at 1873 K (1600 °C) was assumed to be sufficiently high, and only diffusion was considered as a rate-controlling step for this transformation. By evaluating the activation energy, MgO diffusion in the generated spinel layer was found to be the rate-controlling step. In addition, this estimation was confirmed by observing the Mg and Al concentration gradients in the generated spinel layer. The results of this study suggest that the MgO diffusion in the spinel inclusions plays a substantial role with regard to their formation kinetics.

Photoelectric and passivation properties of atomic layer deposited gradient AZO thin film

Gradient Al-doped ZnO (AZO) thin films were deposited at 150 °C by atomic layer deposition (ALD) with different Al concentration gradient, and their photoelectric and passivation properties were investigated. With increasing Al concentration gradient from 0.09 to 1.21%/nm, Hall-effect showed that the resistivity of gradient AZO thin films deteriorates. The minimal resistivity (2.81 × 10−3 Ω cm), the maximum mobility (9.03 cm2/Vs) and the maximum carrier concentration (2.46 × 1020 cm−3) were obtained at 0.09%/nm Al concentration gradient. The average transmittance of all the gradient AZO films can be more than 85% in the visible region. In addition, gradient AZO thin films demonstrated excellent passivation properties. The maximum minority carrier lifetime (120.6 μs) and the minimal surface recombination velocity (≤208.3 cm/s) were obtained at 0.71%/nm Al concentration gradient.

Optical and electrical characterization of gradient AZO thin film by magnetron sputtering

Gradient Al-doped ZnO (AZO) thin films were deposited by magnetron sputtering. The effects of Al concentration gradient on the structure, optical and electrical properties of gradient AZO films were investigated by X-ray diffraction, X-ray photoelectron spectroscopy, four probe tester and UV–visible spectrophotometer. Results show that all of the gradient AZO films have the hexagonal wurtzite structure with c axis preferential orientation and no other secondary phase exits. With the increase of the Al concentration gradient, the resistivity of AZO thin films deteriorates. The resistivity was least, 1.18 × 10−2 Ω cm, for magnetron sputtering-gradient AZO films at 0.02 % nm−1 Al concentration gradient. The average transmittance of the gradient AZO thin films on the glasses in the band of visible light is about 80 %.

Efficiency improvements in AlGaN-based deep ultraviolet light-emitting diodes using inverted-V-shaped graded Al composition electron blocking layer

This paper principally presents the numerical investigation of electron blocking layers (EBL) structures with different Al concentration gradient changing in AlGaN-based deep ultraviolet light emitting diodes (DUV-LEDs). Compared to conventional EBL structure with constant Al composition, the LED with inverted-V-shaped EBL structure has higher output power and carriers recombination rate, but the efficiency droop will decrease obviously while the electron leakage current can reduce much as well. Therefore, the result indicates that appropriate Al component in LED can enhance electron and hole recombination rate in the active region. The improved performance is mainly attribute the sufficient electron-barrier height and relatively higher hole injection efficiency which results from the mitigated band-bending.

Synthesis and characterization of Al–Mg–Zn thin film alloys co‐deposited from vapour phase

AbstractMg–Zn thin film alloys were produced by thermal co‐evaporation using single source and dual source geometries. The films containing high amount of Zn (66 at.%) were prone to formation of metallic micro spheres on the surface when high evaporation rates (10 nm s−1) were used. The Mg–27.1 at.% Zn bulk alloy was found to be suitable for co‐deposition with Al for producing Al–Mg–Zn thin film alloys. A low Al concentration gradient (5 at.%) could be obtained along the sample due to the high deposition rates of Mg–Zn (7.5 nm s−1) combined with low Al deposition rates (0.2 nm s−1). The Al–Mg–Zn thin films showed an improved resistance against chemical attack when higher amounts of Al are used, as shown by downstream analytics investigations using a flat flow electrochemical cell coupled to an AAS system.

Low temperature conversion of linear C4 olefins using acid ZSM-5 zeolites with aluminum concentration gradient

Two ZSM-5 materials with high and low overall Si/Al ratio, each with an Al concentration gradient across the crystals (G-samples) have been synthesized, physico-chemically characterized with several techniques (XRD, SEM, N2 adsorption, 27Al MAS NMR, NH3-TPD, FT-IR and TOF-SIMS), and catalytically tested in the low-temperature dimerization of n-butenes, paying attention to coke formation. Their acidic properties were found consistent with their framework aluminum content, except for a lower than expected number of Brønsted acid sites as measured by FTIR of adsorbed pyridine. The dimerization – cracking – realkylation mechanism, previously observed for n-butenes dimerization over acid ZSM-5 with homogeneous aluminum distribution, was confirmed over G-type catalysts. The influence of hard coke content, observed in the previous work, on product selectivity and distributions was also found for G-type catalysts. The siliceous G-type catalyst, with low amount of hard coke and micropores partly accessible to reactants, shows shape-selectivity properties consistent with dominating intra-crystalline catalysis. The aluminous G-type catalyst, with high amount of hard coke, and micropores completely filled with aromatic carbonaceous residue, shows thermodynamically equilibrated products selectivity and distributions in agreement with extra-crystalline catalysis.

Aluminizing of TiAl-based alloy using thermal spray coating

Coating of aluminide on a TiAl-based alloy (49.1at.% Al) was carried out by thermal spraying pure aluminum and subsequent diffusion treatment at 1100°C. The growth of a Ti–Al intermetallic layer in the coating layers during diffusion treatment for 120–1800s as well as the oxidation resistance of the aluminized TiAl-based alloy was investigated. The outermost surface layer was comprised of Al-rich intermetallic TiAl3 and contained pores. In contrast, an Al concentration gradient layer consisting of Ti2Al5, TiAl2, and Al-rich TiAl containing 55at.% Al was formed between the outermost layer and the substrate. The thickness of the outermost layer decreased with increasing diffusion time, while the thickness of the intermediate layer grew to approximately 30μm. In addition, the coating/substrate interface changed from a wavy to a linear form with the growth of the intermediate layer. The aluminized coating, at all diffusion times, showed good oxidation resistance in cyclic oxidation tests at 900°C in air.

Order–disorder transformation in the Ni–4.49 at.% Al alloy

The order–disorder transformation in the Ni–4.49 at.% Al alloy was studied using electrical resistivity measurements, microhardness measurements, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The results confirmed the ordering behavior expected for Ni–Al dilute alloys and the suggested relation between resistivity changes and microhardness changes with anti-ferromagnetic spin ordering. The higher value obtained for the activation energy of vacancy migration was associated with a decrease in the Al concentration gradient near solute-depleted regions.

The effect of Si and Al concentration gradients on the mechanical and magnetic properties of electrical steel

Abstract Si and Al addition to electrical steel has a beneficial effect on its soft magnetic properties, which are optimal for 6.5 Si wt%. At this concentration the power losses are low and the magnetostriction is almost zero. However, the increase of these alloying elements has two drawbacks: both magnetic saturation and ductility are reduced. The use of materials with a Si and Al concentration gradient appears as an interesting alternative because it presents the advantages of homogeneous 6.5 wt% Si steel without the cited disadvantages. Samples are produced by hot dipping in a Si–Al hypereutectic bath, followed by annealing treatments for diffusion, leading to different concentration profiles. The effect of the Si and Al gradients on the magnetic saturation, hysteresis losses as well as on the tensile stress–strain behaviour are described.

Magnetic Properties of Electrical Steel With Si and Al Concentration Gradients

Electrical steel with a Si content up to 6.5 wt% is a good soft magnetic material, because the power losses are reduced through the increased electrical resistivity of the bulk material by the presence of alloying elements like Si and Al. Nevertheless, with the increasing of the alloying elements, the concentration of Fe atoms in the bulk is reduced and a lower magnetic saturation is reached. Samples are produced by hot dipping in a Si-Al hypereutectic bath, followed by annealing treatments for diffusion. Series of different concentration profiles have been obtained and the magnetic properties measured. A short immersion is sufficient to form a Si-Al enriched layer with D0/sub 3/ ordered structure (25at% Si-Al) in the steel surface: 10 /spl mu/m of D0/sub 3/ or more can be obtained after 20 s immersion (dependent on substrate and experimental conditions). A short diffusion annealing lowers and broadens this level, e.g., to a classical level of 6.5 wt% of Si, while the center of the steel remains at the original level (e.g., 3 wt% Si). Similar gradients are obtained with the simultaneous presence of Si and Al. Magnetic measurements were performed at 50 and 400 Hz after different thermal cycles, leading to different gradients of Si and Al. The power losses are reduced more than 50% in the final material compared with the original. It appears that a short annealing after the dipping already gives low power losses, even before a homogeneous concentration profile is obtained over the sample thickness. A smooth concentration gradient is preferable, with a lower Si content in the center, in order to maintain a good magnetic saturation. This can also be shown through modeling of the magnetic behavior with the concentration gradient. The reduction of power losses is more pronounced at 400 Hz, proving the importance of the skin effect. A further advantage of this procedure is the shorter annealing time after hot dipping.

Characterization of the breakaway al content in alumina-forming alloys

Residual Al contents were measured in Fe- and Ni-base alloys in order to improve lifetime prediction models based on the consumption of the Al reservoir. Measurements were made using electron probe microanalysis (EPMA) on both foil (50–125 μm thickness) and plate (0.5–2mm) material. For FeCrAl, NiCrAl and Fe3Al plate specimens, significant Al concentration gradients were measured. An Al gradient is not considered in current reservoir type models. Residual Al contents at the onset of breakaway oxidation were somewhat lower than previously reported for several FeCrAl alloys and significantly higher (»10at.%) for Fe3Al and Fe–Al alloys. The implications of these results for performance and lifetime predictions are discussed.

Multicomponent diffusion in the molten system K2O-Na2O-Al2O3-SiO2-H2O

We have measured multicomponent chemical diffusion coefficients in a melt near to the low pressure water-saturated eutectic granite composition in the system K 2 O-Na 2 O-Al 2 O 3 -SiO 2 -H 2 O at 1.0 GPa and temperatures of 1300 and 1600 degrees C. The measured diffusion profiles can be accounted for within the analytical error by diffusion coefficients, which are not dependent on composition within the range of compositions accessed by our experiments. The multicomponent diffusion coefficient matrix [D] has a highly degenerate set of real, positive eigenvalues that show a regular relation to melt viscosity on an Arrhenius diagram. The smallest eigenvalue is that associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated predominantly with Si-Al exchange. The larger two eigenvalues are those associated with Si-Na and Si-K exchange and are effectively degenerate, with the result that exchanges of alkalis for silica or for each other can proceed in pseudo-binary fashion without inducing fluxes of other components. The eigenvalue associated with H-Si exchange is smaller than the alkali-silica eigenvalues, but analytical uncertainties make it also effectively degenerate with the alkalis. Uphill diffusion, notably of water and alkalis, was observed in several experiments, and this would lead to transient partitioning of water and alkalis across diffusion interfaces showing large Al 2 O 3 concentration gradients. Such partitioning in natural systems would persist until Al concentration gradients were erased by continued, much slower Al-Si interdiffusion.

β-Si 3N 4 grain growth, part II: Intergranular glass chemistry

The intergranular chemistry and the substitution level of the β′-grains in a Si 3N 4 ceramic fabricated with 9.5 mol% Al 2O 3 and Y 2O 3 have been characterised by high resolution analytical transmission electron microscopy. The Y 2O 3 Al 2O 3 molar ratio of the starting powder mixture corresponded to that of the Y,Al-garnet (5Al 2O 3·3Y 2O 3). Al and O were incorporated into the Si 3N 4 structure during densification and a dilute β′-Si 3N 4 formed. Al concentration gradients were not observed in the β′-grains, although the Al substitution level varied within and between β′-grains (0.16 ≤ z ≤ 0.24). The consequently reduced Al content of the oxynitride liquid phase sintering medium resulted in a limited formation of N-α-wollastonite (Y 2SiAlO 5N) during cooling from the densification temperature. Y and Al were not homogenously distributed in the residual glass pockets; an enrichment close to the β-Si 3N 4 grains could be observed. Both Y and Al were observed also in the glassy films between adjacent β′-grains. Growth ledges on facets of β′-grains facing curved grain boundaries resulted in a variation of the intergranular film thickness. These features are consistent with grain growth by coalescence.

The effect of excess gallium vacancies in low-temperature GaAs/AlAs/GaAs:Si heterostructures

This article shows that the presence of low-temperature-grown GaAs (LT-GaAs) in LT-GaAs/AlAs/GaAs:Si heterostructures increases the Al/Ga interdiffusion at the heterostructure interfaces. The interdiffusion enhancement is attributed to the presence of Ga vacancies (VGa) in the As-rich LT-GaAs, which diffuses from a supersaturation of VGa frozen-in during sample growth. Chemical mapping, which distinguishes between the AlAs and GaAs lattices at an atomic scale, is used to measure the Al concentration gradient in adjacent GaAs:Si layers. A correlation is observed between the Al/Ga interdiffusion and the gate breakdown voltage in metal-insulator field-effect transistor structures containing LT-GaAs.

Effect of Partial Dissolution During LPE Growth of AlxGa1 − x As on the Efficiency of Diffused Light‐Emitting Diodes

Light‐emitting diodes have been prepared by Zn diffusion to form p‐n junctions in Te‐doped layers grown by liquid phase epitaxy. When the growth process is interrupted by a partial dissolution cycle, the external quantum efficiencies for junctions formed in the material grown after dissolution are about five times greater than those obtained without a dissolution cycle. Efficiencies of about have been measured at about 6750Aå for uncoated diodes. Electron microprobe analysis shows that in the regrown material there is a sharp initial drop in Al concentration, after which the Al concentration gradient becomes considerably less than in layers grown without dissolution. In addition the dislocation density in the regrown layers is about a factor of two lower than in material grown without dissolution.