Phase Transformation Of Al2O3 Research Articles

High-temperature protection performance of Mg-doped Al2O3 protective layers on the thin film thermocouples

Mg-doped Al2O3 protective layers were deposited on Pt–PtRh10 (S-type) thin film thermocouples (TFTCs) by reactive magnetron sputtering. The changes in the composition, microstructure, and element distribution of the Mg-doped Al2O3 films before and after annealing at 1200 °C for 3 h were investigated using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS). The results indicate that incorporating MgO into Al2O3 as an enhancement phase not only inhibits the crystallization and phase transformation of Al2O3 but also promotes the formation of MgAl2O4, sequentially further enhancing the material's high-temperature protective properties. The thermoelectric properties of S-type TFTCs with 1.6 μm thick Al2O3 and Mg-doped Al2O3 protective layers were explored, respectively. The results of static calibration at high temperatures demonstrate that the TFTCs with the Mg-doped Al2O3 protective layer can operate at 1148 °C for over 96 h and at 1480 °C for a brief period. Following 4 calibration cycles, compared to the TFTCs with the Al2O3 protective layer, the average Seebeck coefficients of the TFTCs with the Mg-doped Al2O3 protective layer exhibit a slower decline rate, and its drift rate during the 4th cycle is merely 3.28 °C/h. The TFTCs featuring Mg-doped Al2O3 protective layers demonstrate extended lifetimes, superior high-temperature stability, and enhanced reliability when contrasted with those utilizing undoped Al2O3 protective layers. These attributes offer valuable insights into the progression of TFTC technology.

Effect of β phase size on the microstructure of thermally grown oxide in water vapor atmosphere at 1100 °C

The arc-melted and vacuum-casted CoNiCrAlHf alloys with a 7YSZ top coat were designed to develop a protective alumina scale at 1100 °C in water vapor content. Arc-melted CoNiCrAlHf alloys provided substantial improvements in oxidation resistance. In this work, the oxidation behavior and void distribution of thermally grown oxide (TGO) formed on CoNiCrAlHf alloys were analyzed. The Al2O3 crystal structure and void distribution were studied as a function of the grain size of the CoNiCrAlHf substrate. Numerous voids developed in the oxide scale, and coarse HfO2 particles segregated at the Al2O3 grain boundaries of vacuum-cast CoNiCrAlHf alloys. The uniformly distributed HfO2 particles prevented void formation in the oxide scale. As a result, the phase transformation of Al2O3 was effectively impeded in water vapor.

Study on one-step fast CRN mechanism for preparing fine AlON powder using carbon coated γ-Al2O3/C precursor

By pre-milling in glucose aqueous solution, γ-Al2O3 particles were uniformly mixed with carbon via hydrothermal treatment technique. Then, directly heating the γ-Al2O3/C precursor at 50 °C/min to 1775 °C for 15–30 min in N2, single phase AlON powder was fast obtained through one-step carbothermal reduction and nitridation (CRN). By approaching the evolution of phase assemblages and morphology during heating and dwelling, the one-step fast CRN mechanism for preparing fine AlON powder was revealed. The carbon around γ-Al2O3 particles delayed the phase transformation of Al2O3 from γ phase to α one. The undesired particle aggregation and growth before formation of AlON was also auxiliarily inhibited by swiftly achieving AlON formation temperature via high heating speed. Further elevating synthesis temperature enabled high reaction kinetics to boost the speed of AlON formation. By these ways, CRN reactions were quickly completed to fast obtain pure AlON powder with small primary particles due to limited particle aggregation and growth. The as-prepared fine AlON powder was easy to be ball-milled to prepare highly transparent AlON ceramics by pressureless sintering.

Possibility of Phase Transformation of Al2O3 by a Laser: A Review

Aluminum (Al) components of high quality often require an optimal ratio of lightness and favorable mechanical properties. In order to improve the physical-mechanical properties of Al, an aluminum oxide (Al2O3) film is usually formed on the surface of Al, which itself is characterized by high strength, hardness, corrosion resistance, and other technical properties. Unfortunately, depending on the conditions, the oxide film may be formed from different crystal phases on the Al surface, which are not always of desirable quality, i.e., the α-Al2O3 phase. The present review demonstrates that the properties of the Al2O3 film may be improved by Al processing with a laser beam according to the scheme: Al (Al alloy) → electrochemical anodizing → treatment with laser irradiation → α-Al2O3. Both Al substrate and the anodizing electrolyte affect the phase transformation of anodic Al2O3. Laser irradiation of the Al2O3 surface leads to high heating and cooling rates, which may promote the formation of a highly crystalline α-Al2O3 phase on anodic Al2O3.

Synergistic effect of Pt and Hf on the early-stage oxidation behaviour of NiAl coating at 1000 °C

The initial-stage oxidation behaviours of unmodified, Pt and Pt+Hf modified NiAl coating are systematically investigated by employing the photo-stimulated luminescence spectroscopy (PSLS) and transmission electron microscopy (TEM) technique. Both Pt and Hf additions can retard the θ- to α- Al2O3 phase transformation. The oxidation rates of the modified NiAl coatings are initially higher than that of the unmodified NiAl coating due to the longer presence of θ-Al2O3. However, they decrease drastically when the oxide in the scale turns to be α-Al2O3 dominantly. The delayed phase transformation in the modified coatings results in lower residual stress within the oxide scale.

Phase Transformation of Alumina, Silica and Iron Oxide during Carbothermic Reduction of Fly Ash for Ceramics Production

Fly ash is a by-product from burning of coal. Utilization of fly ash by carbothermic reduction is an effective way to recover aluminum, silicon, and iron to enhance product-added value. This work is focused on the phase transformation of Al2O3, SiO2 and Fe2O3 during carbothermic reduction of fly ash in air. A comparative analysis of carbothermic reduction of fly ash in air and in nitrogen was made. Thermodynamics analysis was performed to illustrate the possible reactions for residue and condensate. X-ray diffraction (XRD), scanning electronic microscope (SEM), and energy dispersive spectrometry (EDS) were employed to characterize the phase composition, surface morphology, and microstructure of the reduced products. Results show that Fe3Si and Fe2Si appear sequentially with increasing of temperature. Al5O6N is an intermediate compound. Residue of Al9FeSi3, Al, and Si, and condensate of SiC, AlN and C are obtained. β-SiAlON was not found in the residue. Nitrogen is involved in the reduction of Al2O3 but not in the reduction of SiO2 and Fe2O3. Carbothermic reduction of fly ash in air did not behave the same as fly ash in nitrogen.

Preparation and cyclic oxidation behaviour of Re doped aluminide coatings on a Ni-based single crystal superalloy

Three Re doped aluminide coatings with different Re content were prepared by electroplating and subsequent arc ion plating (AIP) method. Cyclic oxidation behaviour of all coatings was carried out at 1100 ℃. The results indicated that the oxidation resistance was significantly improved with an appropriate amount of Re addition and electroplating Ni. Besides, Re accelerated the θ-to-α Al2O3 phase transformation. The oxidation mechanisms of aluminide coatings with Re doped and Ni electroplated were discussed as well.

Benefits of Zr addition to oxidation resistance of a single-phase (Ni,Pt)Al coating at 1373 K

A single-phase (Ni,Pt)Al coating with lean addition of Zr was prepared by co-electroplating of Pt-Zr composite plating and subsequent gaseous aluminization treatments. Isothermal and cyclic oxidation behavior of the Zr-doped (Ni,Pt)Al coating samples was assessed at 1373 K in static air in comparison with plain nickel aluminide (NiAl) and normal (Ni,Pt)Al coatings. Results indicated that Zr-doped (Ni,Pt)Al coating demonstrated a lower oxidation rate constant and reduced tendency of oxide scale spallation as well as surface rumpling, in which the enhanced oxidation performance was mainly attributed to the segregation of Zr at oxide scale grain boundaries and the improved Young’s modulus of the coating. Besides, the addition of Zr effectively delayed oxide phase transformation of Al2O3 from θ phase to α phase in the early oxidation stage and coating degradation of β-NiAl to γ'-Ni3Al in the stable oxidation stage.

Effect of preparation conditions on MnxOy/Al2O3 sorbent for H2S removal from high-temperature synthesis gas

A series of MnxOy /Al2O3 sorbents with different tissue and chemical structures were prepared by repeated incipient wet impregnation method with different calcination conditions, including calcination temperature from 500 °C to 900 °C and calcination time from 3 h to 9 h. The H2S absorption performances of the prepared sorbents were investigated by a model synthesis gas containing 2000 mg/m3 H2S at 850 °C. The chemical and physical structures of the samples were characterized by Brunauer–Emmett–Teller, X-ray diffraction and scanning electron microscopy techniques to obtain fundamental information about the deactivation of MnxOy/Al2O3 sorbents during desulfurization–regeneration cycles. Results showed that the structural features of the sorbents at high reaction temperature played an important role in the stability of desulfurization performance. The analysis of the characteristics of the sorbents indicated that the sorbents obtained stabilized chemical and physical structures during desulfurization–regeneration cycles when the calcination temperature was 900 °C and the calcination time was above 6 h. The sorbent suffered serious deactivation during five successive desulfurization–regeneration cycles when the calcination temperature of sorbents was below 850 °C. The sulfur capacity decreasing of MnxOy/Al2O3 sorbents may probably be attributed to a part of MnxOy covered by Al2O3, which can un-sulfide during adsorption process. The coverage of active phase MnxOy may be caused by the shrinking and disappearing of some micropores, and phase transformation of Al2O3, as well as the transformation of MnAl2O4 lead to a part of MnxOy on the sorbents being covered by alumina.

Friction and abrasive wear behaviour of Al 2 O 3 -13TiO 2 and Al 2 O 3 -13TiO 2 +Ni Graphite coatings

Abstract The friction and abrasive wear of plasma sprayed Al2O3-13TiO2 coating, with and without the addition of graphite was investigated. XPS analysis was used to determine the extent of γ-α Al2O3 phase transformation after wear. γ-α Al2O3 phase transformation had different influence on friction and wear of coatings. Al2O3-13TiO2+15% Ni Graphite coating exhibited lower coefficient of friction and lower abrasive wear rates than Al2O3-13TiO2 coating; the magnitude of decrease was dependent on load. The better abrasive wear resistance and lower coefficient of friction of Al2O3-13TiO2+15% Ni Graphite coating was due to the synergistic effect of self-lubricating film of graphite and tribochemical reactions. The severity of contact could identify transitions in abrasive wear rates, operating wear mechanisms and associated failure modes.

Effects of process parameters of vacuum pre-oxidation on the microstructural evolution of CoCrAlY coating deposited by HVOF

The CoCrAlY coatings deposited using the high velocity oxygen fuel process (HVOF) onto the Inconel 718 superalloy were employed by three different vacuum pre-oxidation heat treatments, and subsequently, the specimens were subjected to isothermal oxidation exposure testing in air at 1050 °C for up to 100 h. The spinel oxides were found in the CoCrAlY coatings during the high velocity oxygen fuel process, and subsequently, after isothermal oxidation exposure in air, both the spinel and CoO oxides were formed in the TGO of the NVHT specimen. The evolution of both the phase transformation and the thermally grown oxides' (TGO) growth rate could be significantly influenced by adjusting certain process parameters of vacuum pre-oxidation heat treatments. The TGO of the VHT specimen was predominantly composed of θ-Al2O3 and α-Al2O3; however, Co(Cr, Al)2O4 and alumina were observed in the TGO of the VHT950 and VHT1000 specimens. The θ-Al2O3 to α- Al2O3 phase transformation in the TGO of the VHT specimen could be delayed after isothermal oxidation exposure. The TGO growth rate was significantly suppressed after the vacuum pre-oxidation heat treatments, and the order of the TGO growth rate was as follows: NVHT<VHT950<VHT<VHT1000. The tensile stress at the alumina area of the TGO in the VHT and VHT950 specimens reduced considerably compared to that in the NVHT specimen, and finally, for the VHT1000 specimen, the tensile stress turned to compressive stress. The elemental interdiffusion behaviour between the HVOF-coating and nickel-based superalloy was characterized, and the diffusion activation energy of cobalt, aluminium and chromium were attempted to be estimated.

One‐Pot Synthesis of Ordered Mesoporous NiSiAl Oxides for Catalyzing CO2 Reforming of CH4

Ordered mesoporous NiSiAl composite oxides with an amorphous phase were prepared by an improved evaporation‐induced self‐assembly strategy. The silica was distributed homogeneously in the mesoporous skeleton and improved the ordered mesoporous structure of NiAl. Catalytic tests toward the CO2 reforming of CH4 indicated that the NiSiAl exhibited higher catalytic activity and stability over 100 h at 700 °C than NiAl. The enhanced performance results from the high thermal stability induced by Si doping, which hinders the Al2O3 phase transformation from amorphous to γ‐Al2O3; therefore, the uniform dispersion of the Ni nanoparticles is retained owing to the confinement effect of the ordered mesopores. The stabilized Ni nanoparticles reinforced the resistance of the catalyst to carbon deposition and, thus, boosted its durability. Notably, even under more stringent conditions (750 °C, 200 h), NiSiAl‐0.10 retained its amorphous Al2O3 structure and showed outstanding durability, almost without any deactivation.

The Reaction Behavior of AlN Powder in Wet Air Between 1573 K and 1773 K

The reaction behavior of aluminium nitride (AlN) powder at the temperature range of 1573 K to 1773 K in wet air was investigated using thermo-gravimetric analysis, x-ray diffraction and scanning electron microscopy. The results show that the reaction behavior of AlN powder in wet air depends on temperature and water content. Initially, weight increases quickly due to oxidation. Once the oxide product Al2O3 is formed, the volatile reaction caused by Al2O3 and H2O occurs simultaneously. Because of the appearance of cracks caused by the phase transformation of Al2O3, the reaction behavior is controlled both by oxidation and volatile reaction at a later stage. This phenomenon becomes more obvious with increasing water content. Based on the experimental result, the reaction kinetics is in reasonable agreement with the real physical picture model.

A New Kinetics-Based Approach to Quantifying the Extent of Metastable → Stable Phase Transformation in Thermally-Grown Al2O3 Scales

A novel kinetics analysis method to quantifying the extent of metastable → stable Al2O3 phase-transformation in thermally-grown alumina scales is presented. This analysis involved assessing thermogravimetric data and the time dependence of the associated value of the instantaneous time exponent, n i . It was found that if there is θ → α transformation, the n i -value curve characteristically decreases to a minimum and then increases. The time corresponding to the minimum was defined as a transition time, t tr . It was found that after this t tr , the α phase was significantly established as a continuous layer and the overall scaling kinetics were greatly reduced. Furthermore, using a kinetics scenario for the θ and α co-formation, measured weight-gain kinetics could be well simulated from the initial co-formation stage to the steady-state single-α-phase-growing stage. Finally, it is shown that the transformation kinetics in the lateral direction can also be determined by this kinetics analysis method.

The Preparation and Structure Characterization of Nano-Alumina Fiber Doped Cerium by Electrospinning

Alumina fiber, a kind of inorganic fiber, was widely used in many fields owing to its excellent performance. CeCl3/AlCl3/PVP was used as spinning solution for producing organic-inorganic hybrid fiber by electrospinning, then, alumina fiber doped cerium can be successfully obtained after high temperature calcination. In the paper, the influence of CeCl3 to the crystallization and thermal stability was discussed. The results show that alumina nanofiber with average diameters of 200nm-800nm are obtained by calcination of the as-prepared fibers .The addition of CeCl3 significantly delay the crystallization transformation process of alumina fiber, while the formation of CeO2 reduce diffusion rate of Al3+, eventually, imped the phase transformation of Al2O3 and improve its thermal stability.

Microscale lamellar NiCoCrAlY coating with improved oxidation resistance

A novel NiCoCrAlY coating was fabricated by plasma activated electron beam-physical vapor deposition (PA EB-PVD). Within the duplex coating, the β-NiAl phase precipitated as small lamellae (less than 1μm thick) in a γ matrix, predominantly oriented perpendicular to the coating surface. The oxidation results at 1373K show that the scaling rate constant during the steady-state oxidation of the novel coating is reduced by approximately 70% as compared with the conventional coating of similar composition produced by electron beam-physical vapor deposition (EB-PVD). This reduction in oxidation rate is attributed to a slower θ→α Al2O3 phase transformation during the initial stages of oxidation.

Oxidation of TiAl3 and L12 Coatings on Ti-45Al-5Nb Alloy at 1173K

Oxidation behavior of TiAl3 and L12 coated Ti-45at%Al-5at%Nb alloys at 1173K in air was investigated using mass gain measurement, field emission scanning electron microscopy, transmission electron microscopy, electron-probe microanalysis, glow discharge optical electron spectroscopy, and glancing angle X-ray diffraction analysis. The TiAl3 formed meta-stable alumina θ-Al2O3, while the oxide scale on the L12 consisted of a duplex structure with an outer rutile TiO2 and an inner α-Al2O3. During the very initial stages of oxidation both Ti and Al in the L12 coating could be oxidized, and then the faster diffusing Ti goes out to form an outer, continuous Ti-rich oxide which covers the slow growing Al2O3. In case of the TiAl3 an outer, continuous TiO2 layer was not observed because of the smaller amount of Ti in the TiAl3. It could be concluded that the outer, Ti-rich oxide enhanced a phase transformation of Al2O3 from θ to α. To elucidate the Ti effect, Ti-vapor treated TiAl3 and Ni-50at%Al were oxidized at 1173K in air, and showed formation of an α-Al2O3, in contrast to θ-Al2O3 on their bare alloys.

Early-Stage Oxidation Behavior of Pt-Modified γ′-Ni3Al-Based Alloys with and without Hf Addition

The early-stage oxidation behavior in air of Pt-modified γ′-Ni3Al-based alloys of composition (in at.%) Ni–22Al–30Pt with and without 0.5Hf was investigated in terms of oxidation kinetics, scale evolution and Al2O3 phase transformation. Oxidation exposures included heating to and short-term holds at 1,150 °C. Hafnium addition did not appear to affect microstructural evolution and growth rate of the oxide scales during heating to 1,150 °C; however, it was found that Hf delayed the metastable-to-α-Al2O3 phase transformation, thus allowing continued fast growth of oxide scale. After the transient oxidation stage of up to about 10 min (including heating time), Ni-rich metallic particles precipitated in the lower part of the metastable Al2O3 layer, due to a decrease in the oxygen potential resulting from scale evolution. The present results indicated that the period of oxide phase transformation was followed by the establishment of steady-state oxidation kinetics. However, the steady-state kinetics were different for the two alloy systems. Specifically, after complete phase transformation to α-Al2O3, rapid growth of oxide grains occurred on the Hf-free alloy; whereas, the oxide grain size remained small for the Hf-containing alloy. Such a difference of transformation and subsequent grain-growth behavior greatly affected oxide thickening kinetics.

Solid Solution of Zr into Anhydrous Al2(SO4)3 Crystal Lattice and .ETA..RAR..ALPHA. Phase Transformation of Al2O3 Formed by Thermal Decomposition of the Sulfate Solid Solution.

More than 60mol% Zr4+ ion (ZrO2/(Al2O3+ZrO2)) is soluble into anhydrous Al2(SO4)3 crystal lattice. The apparent crystallite size decreases with increasing Zr4+ content, and the thermal decomposition temperature reduces at contents of zirconia >20mol%. Alumina/zirconia composite powders, in which an amorphous zirconia phase is dispersed in a matrix of fine η-alumina particles, are obtained by thermal decomposition of the sulfate solid solution. Tetragonal phase of zirconia crystallizes at 1100°C and the crystallinity increases with increasing the temperature of the heat-treatment. When mono-dispersed ultra fine particles of zirconia are added to η-alumina formed by thermal decomposition of pure anhydrous aluminum sulfate, the η→α phase transformation of alumina is suppressed, and the effect increases with increasing the amount of the additive. On the contrary, a small quantity of ZrO2<1mol% drastically suppresses the phase transformation of alumina formed by decomposition of sulfate solid solution. But, when ZrO2 content is more than 1mol%, the η→α phase transformation of alumina is accelerated. The acceleration effect becomes larger with increasing the content of ZrO2.

Influence of stabilizers on Na-β-Al2O3 phase formation in Li2O(MgO)-Na2O-Al2O3 ternary systems

The influences of stabilizers on β- and β′′-Al2O3 phase formations in Li2O(MgO)-Na2O-Al2O3 systems were investigated. When stabilized with 4MgCO3ċMg(OH)2ċ5H2O, most of the β′′-Al2O3 phase formed below 1200°C and further β- to β′′-Al2O3 transformation with an increase of temperature was not observed. On the other hand, when stabilized with Li2CO3,β′′-Al2O3 formation occurred by two steps. First, β′′-Al2O3 was partly formed below 1200°C, and, second, noticeable transformation from β-Al2O3 to β′′-Al2O3 occurred at higher temperature ranges. It was shown that transient eutectic liquid in the Li2O-Na2O-Al2O3 system promoted the β- to β′′-Al2O3 transformation at higher temperatures. Uniform distribution of both Mg2+ and Li+ stabilizing ions enhanced β′′-Al2O3 formation at low temperatures. In the Li-stabilized systems, however, homogeneous distribution of Li+ ions hindered both the formation of transient eutectic liquid and the second β- to β′′-Al2O3 phase transformation at high temperatures.