Stable α-Al2O3 Research Articles

Structural characterization of aluminized steel heat treated in different environments

Thermal and plasma based heat treatments of Aluminized P91 steel under different environments such as 100%O2, 100%Ar and 50%O2+50% Ar are attempted to investigate microstructural defects. Heat treatment of Al coated P91 steel involving normalizing treatment at 980°C and tempering at 750°C for 0.5h and 1.5h respectively in above mentioned environments under thermal and plasma processing conditions at 5mbar pressure was conducted. Detailed study of microstructural evaluation of the resultant diffusion coating under different environment is reported with SEM-EDS and EBSD characterization in current study. Further the vacancy concentration, CV was evaluated based on the precise combination of the changes in the bulk length and the X-ray lattice parameter from XRD data. The effect of vacancy concentration, grain size and strain on resultant coating has been presented. Presence of stable α-Al2O3 and metastable θ-Al2O3 along with FeAl was confirmed by XRD analysis in both thermal and plasma processed samples under 100%O2 and 50%O2+50%Ar environment while in Ar environment minor presence of θ-Al2O3 was observed during plasma treatment. Plasma heat treatment under 50%O2+50%Ar environment results in coating with stable α-Al2O3 followed by diffusion zone of FeAl with lowest vacancies, residual strain and area fraction of kirkendall porosity with maximum length of diffusion zone of FeAl phase for aluminized P91 steels than plasma treatment under 100% Ar environment and 100% O2.

Oxidation mechanism of micron-sized aluminum particles in Al-CO2 gradually heating system

Micron-sized aluminum powders were heated in carbon dioxide atmosphere through differential scanning calorimetry (DSC) method in this work. Aluminum powders were oxidized into four distinct stages from room temperature to 1500°C. Stage I, amorphous alumina shell turns to γ-Al2O3 phase from room temperature to 620°C. Stage II, accompany with the aluminum core melting, alumina shell becomes thicker and fragile at the temperature around 667°C which is the melting point of aluminum. Stage III, in the temperature range of 690-1150°C, alumina shell was broken partially because of the inside pressure. Liquid aluminum spurts out through the weak point which becomes cracks on the surface just like volcano eruption and then oxidized by CO2 while the temperature increased from 700°C to 900°C. Stage IV, alumina changes to stable α-Al2O3. From what was presented above, a mechanism of micron-sized aluminum particle oxidation in CO2 under gradually increasing temperature condition was proposed as “eruption model”.

Effect of KOH to Na2SiO3 Ratio on Microstructure and Hardness of Plasma Electrolytic Oxidation Coatings on AA 6061 Alloy

In this study, plasma electrolytic oxidation (PEO) process has been employed to fabricate alumina coatings on AA 6061 aluminum alloy from an electrolyte containing water glass (Na2SiO3) and alkali (KOH). The effect of deposition time and the alkali to water glass (KOH: Na2SiO3) composition ratio on the coating morphology and properties are studied. The different phases of the oxide layer and microstructure are investigated by x-ray diffraction, scanning electron microscopy, and atomic force microscopy. The results indicate that initially γ-Al2O3 forms in the coating, and as the processing time is increased from 5 to 60 minutes, α-Al2O3 phase becomes prominent. Further, higher the content of Na2SiO3, higher is the hardness and coating growth rate due to the formation of stable α-Al2O3 and Al-Si-O phase. It has been reported that the optimum properties of the PEO coatings can be obtained at a ratio of KOH: Na2SiO3 ≈ 15:10 followed by 10:10.

Chemical reaction mechanism, microstructural characteristics and mechanical properties of in situ (α-Al2O3+ZrB2)/Al composites

In-situ (α-Al2O3+ZrB2) particle reinforced aluminum matrix composites were fabricated from Al-ZrO2-B2O3 powder mixtures using chemical reaction method. The chemical reaction mechanism, microstructural characteristics and mechanical properties of the composite were investigated. The chemical reaction process was found to comprise three major steps: step 1: 2Al + B2O3→2[B]+Al2O3(amorphous), 2[B]+Al→AlB2,ZrO2+2B→Zr(O,B)2; step 2:Al + ZrO2→Al2O3(amorphous)+[Zr], [Zr]+Al→Al3Zr, [Zr]+Al→AlZr, 3Zr(O,B)2+4Al→2Al2O3+3ZrB2;and step3:Al3Zr + AlZr+2AlB2→6Al+2ZrB2, Al2O3(amorphous)→α-Al2O3. The transitional phases were found to be AlB2 and amorphous Al2O3. Notably, with increasing temperature, amorphous Al2O3 began to crystallize and eventually transformed to more stable α-Al2O3. The activation energies of the three step reactions were calculated and found to be 641, 330 and 192 kJ/mol, respectively. The reaction rate of the third step was observed to be much higher than that of the second step reaction. Reinforcement particles have a uniform distribution in the aluminum matrix. The α-Al2O3 particles exhibit a size ranging from several hundred nanometers to a few micrometers, while the size of ZrB2 particles is less than 3 μm. The tensile strength and elongation rate of the composites are determined to be 226 MPa and 4.6%, respectively.

Mechanical properties of Al2O3 ceramic coatings prepared by plasma spraying on magnesium alloy

An Al2O3 coating has been successfully prepared on an AZ91HP magnesium (Mg) alloy by plasma spraying technology. Al–Si eutectic alloy was used as the transition layer in order to ease the physical and chemical differences between the Al2O3 and the Mg alloy. The mechanical properties of the coating were studied. The results show that the coating consists mainly of stable α-Al2O3 and metastable γ-Al2O3 phases, with a lamellar microstructure. Nano-indentation results exhibit that the peak loads of the plasma-sprayed Al2O3 layer, the Al–Si transition layer, and the Mg alloy are 2.2, 0.6 and 0.25mN, respectively. The elastic modulus and microhardness of the plasma-sprayed Al2O3 layer are 250 and 7.45–8.90GPa, respectively, which are higher than those of the Mg alloy (50 and 0.8GPa). The wear volume of the plasma-sprayed Al2O3 coating is two orders of magnitude smaller than that of the Mg alloy.

NiAl Oxidation Reaction Processes Studied In Situ Using MEMS-Based Closed-Cell Gas Reaction Transmission Electron Microscopy

The nanoscale oxidation mechanisms and kinetics of a model β-NiAl system were investigated using in situ closed-cell gas reaction scanning transmission electron microscopy (STEM). Here, we directly visualize the dynamic structural and chemical changes that occur during high-temperature oxidation at a high spatial resolution of 50.3Ni–49.7Al (at.%) nanoparticles under static air conditions at 730 Torr with heating up to 750 °C at 5 °C/s. A MEMS-based gas cell system, with microfabricated heater devices and a gas delivery system, was used to reveal site-specific oxidation initiation sites. Through time-resolved annular dark-field STEM imaging, we tracked the nanoscale oxidation kinetics of Al2O3. After oxidation at 750 °C, nucleation of voids at the Ni/Al2O3 interface was observed along a NiAl grain boundary, followed by the formation of faceted NiO crystals. Small faceted cubic crystals of NiO were formed at the initial stage of oxidation at high PO2 due to the outward self-diffusion of Ni2+ ions, followed by the formation of a mixture of metastable and stable α-Al2O3 at the oxide/metal interface that is attributed to a PO2 decrease with oxidation time, which agreed with thermodynamic modeling calculations. The results from these in situ oxidation experiments in the β-NiAl system are in agreement with the established oxidation mechanisms; however, with in situ closed-cell gas microscopy it is now feasible to investigate nanoscale oxidation mechanisms and kinetics in real time and at high spatial resolution and can be broadly applied to understand the basic high-temperature oxidation mechanisms for a wide range of alloy compositions.

Catalytic pyrolysis of glycerol into syngas over ceria-promoted Ni/α-Al2O3 catalyst

This paper reports on the catalytic pyrolysis of glycerol into syngas over a 3 wt%Ce-20 wt%Ni/77 wt% α-Al2O3 catalyst and at reaction temperatures of 973 K, 1023 K and 1073 K. NH3- and CO2-TPD analyses have revealed that the as-synthesized catalyst was net acidic with acid-to-basic site ratio of 1.24. This provides ideal conditions for chemisorption of glycerol. In addition, the BET specific surface area was 2.89 m2 g−1. The small surface area can be attributed to the thermally stable α-Al2O3 support. In addition, the average crystallite size was 40.22 nm. The catalytic glycerol pyrolysis produced gaseous products that were comprised of H2, CO, CO2 and CH4 only with H2:CO ratios that were consistently less than 2.0. This ratio is suitable for Fischer-Tropsch synthesis. The yields of CO2 and CH4 were several folds lower than the yields of H2 and CO, indicating that the latter were from primary reaction, viz. glycerol decomposition whilst the former were from secondary competing reactions. In addition, the activation energy obtained via Langmuir-Hinshelwood model was 25.34 kJ mol−1. Used catalyst characterization showed that the carbonaceous deposit was in the forms of whisker-type. This type of carbon deposit would not physically deactivate the catalyst.

Microencapsulated phase change materials with high heat capacity and high cyclic durability for high-temperature thermal energy storage and transportation

Latent heat storage (LHS) technology employing phase change materials (PCMs) has received great attention as an alternative to conventional solid sensible heat storage (SHS) for future high-temperature energy utilisation systems. In this study, we report the synthesis of a core-shell type microencapsulated PCM (MEPCM) consisting of Al-25wt% Si microspheres (mean diameter of 36.3μm and melting temperature of 577°C) as the core (PCM) and Al2O3 as the shell. The MEPCM was prepared in two steps involving (1) the formation of an AlOOH precursor shell on the PCM microspheres by a hydroxide precipitation process in hot water and (2) heat-oxidation treatment in an O2 atmosphere to form a stable Al2O3 shell. In particular, the effects of heat-oxidation temperature on the shell morphology, shell crystal structure, mechanical strength, heat capacity, and cyclic durability of the prepared MEPCMs were examined. The resultant MEPCM is composed of a stable α-Al2O3 shell and Al-25wt% Si core with an effective void inside the core to allow for volume expansion of the PCMs during solid-liquid phase transitions. The heat capacity measured for this material is five times higher than that of conventional solid SHS materials. Additionally, the MEPCM exhibits excellent durability up to 300 heating and cooling cycles under oxygen atmosphere. Consequently, it can potentially be used in the next-generation LHS-based high-temperature thermal energy storage and transportation systems.

Two-Step Process in Homogeneous Nucleation of Alumina in Supersaturated Vapor

Multiple pathways in crystal nucleation are now known to be more common than previously predicted; it is, therefore, crucial to understand the early stages of crystallization. Even in relatively simple vapor-phase homogeneous nucleation, the process has significant potential diversity. Here, we experimentally show crystalline Al2O3 nanoparticles forming via precisely two steps in the nucleation process from supersaturated vapor with a moderate cooling rate. In situ FT-IR measurement of nucleation allowed us to observe the formation of Al2O3 nanoparticles. Liquid-like particles first nucleated from the vapor before crystallizing. The crystalline phase was preserved by quenching without further transformation into the most stable α-Al2O3 polymorph. The precipitated phase changed from δ-Al2O3 for pure Al2O3 to γ-Al2O3 or θ-Al2O3 by adding Sb or Cr, respectively. We demonstrate that a two-step process occurs in homogeneous nucleation of refractory materials from supersaturated vapor, which may facilitate poly...

Modeling of θ → α alumina lateral phase transformation with applications to oxidation kinetics of NiAl-based alloys

In the present work, a physical model has been developed to predict the growth of Al2O3 scale, which involves the thickening of metastable transient θ-Al2O3 phase and finally thermodynamically stable α-Al2O3 phase, accompanied by the concurrent θ→α-Al2O3 phase transformation. Based on nucleation and growth theory, the phase transition kinetics and alumina growth were modeled for both isothermal and continuous heating paths. Using this model, the phase transformation kinetics, evolution of grain size and Al2O3 scale thickness can be predicted for a given thermal path. A new scaling parameter has been proposed that quantitatively reveals the relative significance of nucleation and grain growth to the overall phase transformation kinetics. Predictions based on the model are in good agreement with the experimental observations. Limitations of the model are also presented and discussed.

Cooperative luminescence from low temperature synthesized α-Al2O3: Yb3+ phosphor by using solution combustion

Ytterbium (Yb3+) doped aluminium oxide (Al2O3) powder phosphor was successfully synthesized by solution combustion method. The structure, vibrational bending modes, particle morphology, chemical composition, and photoluminescent properties were analyzed using X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS) and photoluminescent (PL) spectroscopy respectively. The XRD patterns confirmed that Al2O3 crystallized in its stable polymorphic hexagonal α-Al2O3 phase with space group R3c. The average crystallite size estimated from Debye-Scherrer equation was 29nm. The FTIR confirmed the Al-O vibrations associated with α-Al2O3 and the Yb-O vibrations. The SEM data showed that the powders were made up of particles with different shapes but well defined boundaries. Furthermore, the SEM images show that the Yb3+ ions resided on the grain boundaries. When the powders were excited using a 325nm He-Cd laser, the emission was observed in the near infrared (NIR) at 975nm due to the 2F5/2→2F7/2 transition of Yb3+. However, the bluish green emission with a maxima at ~ 480nm was observed as a result of cooperative luminescence of Yb3+ when the powders were excited in the NIR with an excitation wavelength of 980nm. Cooperative energy transfer (CET) mechanism producing NIR emission for the 325nm laser excited Al2O3:Yb3+ powders is presented and discussed.

Metastable-Stable Phase Transformation Behavior of Al2O3 Scale Formed on Fe–Ni–Al Alloys

The oxidation behavior of Ni–Fe–41.5at.%Al alloys with different Fe/Ni ratios was investigated in air at 1000 °C in order to clarify the effect of Fe on the phase transformation of Al2O3 scale, using in-situ high-temperature X-ray diffraction by means of synchrotron radiation. The oxidation mass gain of alloys after 25 h of oxidation generally decreased with increasing Fe content; however, the initial oxidation mass gain was significantly decreased by increasing alloy Fe content. In-situ X-ray diffraction analysis indicated that higher alloy Fe contents promoted rapid formation of the stable α-Al2O3, while lower Fe in the alloy maintained the metastable Al2O3 for longer time oxidation. The effect of Fe on promoting α-Al2O3 formation can be explained by the initial formation of α-Fe2O3, whose structure is isomorphous with α-Al2O3. The additional effect of Fe on the growth rate of α-Al2O3 is also discussed.

Mechanically induced formation of metastable χ- and κ-Al2O3 from boehmite

Nanocrystalline boehmite (γ-AlOOH) powder was treated by high-energy planetary ball milling. With an increase in milling time and centrifugal force, boehmite was transformed into χ- and κ-Al2O3 as a metastable phase of alumina. These metastable alumina phases were confirmed by comparing with X-ray diffraction patterns of the thermally treated gibbsite (γ-Al(OH)3). Eventually, the milling of boehmite resulted in the formation of thermodynamically stable α-Al2O3. Although boehmite is normally decomposed into γ-Al2O3 by heating, the phase transformation route of boehmite induced by mechanical milling differed from that induced by thermal process. We have proposed that the different phase transformation route can be attributed to the crystal deformation of boehmite by milling.

The crystallographic template effect assisting the formation of stable α-Al2O3 during low temperature oxidation of Fe–Al alloys

The role of thermally grown α-Fe2O3 on the nucleation of α-Al2O3 during oxidation of binary Fe–Al alloys with 15 and 26 at.%Al at 700°C was investigated. Surface morphology of the oxide scales indicated direct nucleation of α-Al2O3 preferentially instead of conversion from metastable Al2O3 polymorphs. Oxide scale development over time was also monitored by use of synchrotron X-ray diffraction and Raman spectroscopy. The results showed that the α-Fe2O3 crystal lattice decreases in volume as oxidation progresses, which was found to be consistent with an Al3+ enrichment of α-Fe2O3 as confirmed by the change in relative intensity of α-Fe2O3 Raman peaks.

The Influence of Manganese Amount in Oxyhydroxide of Aluminium – Precursor of MnO<sub>x</sub>-Al<sub>2</sub>O<sub>3</sub> Catalysts on Catalytic Activity in Reaction of Oxidation of C<sub>1</sub>-C<sub>3</sub> of Hydrocarbons

The authors have synthesized nanofibrous aluminum oxyhydroxide (AlOOH), modified with different amounts of manganese ions (II), which is the precursor for methane deep oxidation catalysts. It was shown that the catalytic activity of the manganese-based system obtained depends on manganese amount and thermal activation conditions. It is approved that as the catalyst for deep oxidation of hydrocarbons the system with manganese content of 10.5 wt. % is the most perspective despite the fact that the oxidation rate of methane is lower in it than in the catalyst with Mn content of 5.7 wt. %. As opposed to the catalyst with Mn content of 5.7 wt. %., where the aluminium oxide is in metastable form (s-Al2O3), the catalyst with manganese content of 10.5 wt. % contains in its structure a thermodynamically and thermally stable α-Al2O3 phase.

Effect of environmental sulfur on the structure of alumina scales formed on Ni-base alloys

Short-term oxidation exposures of an alumina-scale forming γ′-Ni3Al-based model alloy in air and O2+0.1%SO2 at 900°C revealed that the presence of sulfur can affect the kinetic competition between the θ and α structural isomorphs of Al2O3. After 2h exposure, metastable θ-Al2O3 growth predominated in air alone; whereas, a much larger percentage of stable α-Al2O3 formed during oxidation in O2+0.1%SO2. This promotion of α-Al2O3 establishment was due to sulfur enrichment on the alloy surface, which occurred even when samples were exposed to O2+0.1%SO2 in a low-temperature, pre-test position (∼150°C), i.e., prior to insertion into the hot zone. It was determined from XPS measurements that the sulfur was mainly in the S6+ valence state and, correspondingly, in the form of NiSO4. Cross-sectional scanning transmission electron microscopy (STEM) using energy-dispersive X-ray spectroscopy (EDS) corroborated the XPS results by detecting that a ∼20nm zone of sulfur enrichment within the surface region of a ∼90nm oxygen-rich layer formed during the pre-test exposure. A systematic explanation for this intriguing observation of sulfur promoting α-Al2O3 establishment is provided from the perspective of kinetics competition between θ and α. This explanation was supported by kinetic calculations and complementary tests in a low pO2 atmosphere.

First-principles study of Cd impurities localized at and near the (0 0 1) α-Al2O3 surface

A combination of two first-principles electronic structure calculation methods in the framework of density-functional theory was applied to investigate the (001) α-Al2O3 surface reconstruction and its structural, electronic, magnetic, and hyperfine properties when doped with Cd impurities at different depths from the surface. The SIESTA approach was used to obtain the equilibrium positions of all atoms and the Full-Potential Augmented Plane Wave plus local orbital (FP-APW+lo) method was employed in order to obtain the electronic structure at these equilibrium positions and all the other physical properties. For the most stable (001) α-Al2O3 surface, we have demonstrated that the inclusion of the Cd atom at substitutional Al sites at and near the surface produces a ground state magnetic behavior. The largest principal component V33 of the electric-field-gradient (EFG) tensor at the Cd atom localized just above the α-Al2O3 terrace showed the same [001] orientation and a dominating p-character as Cd does when it is localized at bulk α-Al2O3, but exhibits an anomalous V33 magnitude four times larger than its value in bulk. Just below the surface, the non symmetric structural relaxation around the Cd impurity is responsible for the strong change in the asymmetry, magnitude, and orientation of the EFG tensor. The changes in the hyperfine properties have been correlated with the modifications observed on the electronic charge density at the different Cd sites and on the p-states of the Cd-projected partial density of states. Accordingly, the significant differences on the hyperfine parameters showed for different depths suggest that 111Cd probe-atoms used in Perturbed γ–γ Angular Correlation experiments could be used for evaluating geometrical and electronic distortions, particularly for positions quite close to the reconstructed surface, as well as contributing to studies of growth, adsorption, and diffusion of atoms in oxide surfaces and interfaces.

Comparison of structure and phase change characteristic of microencapsulated core/shell Al–Si alloy microparticles synthesized by two methods

Pre-oxidation process and modification with silane coupling agent (SCA) of original Al–Si alloy particles were selected to synthesize inorganic microencapsulated core/shell Al–Si alloy microparticles based on the sol–gel technology, respectively. The microstructure and phase change characteristic were measured and investigated by means of Fourier transform infrared spectra, X-ray diffraction, scanning electron microscopy, thermogravimetry and differential scanning calorimeter. These two methods can both realize the microencapsulation and form stable dense α-Al2O3 shell. It is speculated that the microencapsulated reason is mainly attributed to the change in surface electric behaviors of Al–Si alloy particles according to the results of zeta potential measure. The latent heats are 416.92 and 307.21 kJ/kg for specimens treated by pre-oxidation and the modification of SCA, respectively. The latter renders thicker and more uniform shell layer.

Formation of α-alumina scales in the Fe–Al(Cr) diffusion coating on China low activation martensitic steel

To study the formation mechanism of stable α-Al2O3 scales, the oxidation behavior of Fe–Al(Cr) diffusion coating on China low activation martensitic steel has been investigated under the oxygen partial pressure ranging from 1 to 20,000Pa at 1253K. A single, continuous Al2O3 scale with the maximum thickness of about 2000nm was formed on the Fe–Al(Cr) diffusion layer. The phase transformation of alumina scales on the surface of Fe–Al(Cr) layer was studied at different oxidation times ranging from 3 to 180min. With the increase in oxygen partial pressure, the phase transformation time of α-Al2O3 is decreased. The metastable γ-Al2O3 and transition α-(Al0.948Cr0.052)2O3 phases were formed in the earlier oxidation process and finally transformed to the stable α-Al2O3 phase, which were detected by grazing incidence angle X-ray diffraction and confirmed by transmission electron microscopy. This implies that Cr shows the third element effect and serves as a template for the nucleation of the stable α-Al2O3.

Microencapsulation of metal-based phase change material for high-temperature thermal energy storage.

Latent heat storage using alloys as phase change materials (PCMs) is an attractive option for high-temperature thermal energy storage. Encapsulation of these PCMs is essential for their successful use. However, so far, technology for producing microencapsulated PCMs (MEPCMs) that can be used above 500°C has not been established. Therefore, in this study, we developed Al-Si alloy microsphere MEPCMs covered by α-Al2O3 shells. The MEPCM was prepared in two steps: (1) the formation of an AlOOH shell on the PCM particles using a boehmite treatment, and (2) heat-oxidation treatment in an O2 atmosphere to form a stable α-Al2O3 shell. The MEPCM presented a melting point of 573°C and latent heat of 247 J g−1. The cycling performance showed good durability. These results indicated the possibility of using MEPCM at high temperatures. The MEPCM developed in this study has great promise in future energy and chemical processes, such as exergy recuperation and process intensification.