Formation Of Α-alumina Research Articles

Rapid thermal process for fabricating α-alumina tight ultrafiltration membrane with narrow pore size distribution

The development of tight ceramic ultrafiltration membranes for water purification facilitates is challenging because of high fabrication costs and low permeate flux. In this study, a low-energy rapid thermal process (RTP) was comprehensively investigated for fabricating of high-precision, permeable α-alumina tight ultrafiltration membranes. Compared to the conventional thermal process (CTP), RTP offers an ultra-fast heating rate and shorter holding time, effectively mitigating grain growth and pore aggregation. Moreover, the decomposition of organic matter is delayed during RTP, providing a lower interfacial energy that enables direct formation of α-alumina at a low temperature of 750 °C. During a secondary subsequent RTP, the residual phase is fully converted to α-alumina by the in-situ seeding effect. By utilizing RTP, we successfully prepared α-alumina ultrafiltration membranes within 30 min by calcination, with an average pore size of approximately 6 nm, a narrow pore distribution coefficient of 1.12, and high permeance of 219 L−1·m−2·h−1·bar−1.

Kinetic oxidation model including the transient regime for a single crystal nickel-based superalloy over the temperature range 750–1300 °C

The oxidation behavior of the Ni-base single-crystal superalloy AM1 was investigated over the temperature range 750–1300 °C. The use of stepwise multi-temperature thermogravimetric analysis (SMT-TGA) showed the presence of a transient regime. This transient regime can be modeled by the initial formation of a fast-growing mixed oxide and then by the formation of α-alumina gradually covering the metal/oxide interface. This model helps to determine the time required for AM1 alloy to be out of the transient regime. Then, the model makes it possible to predict any oxidation kinetics of the AM1 superalloy for the entire temperature range studied.

Synthesis of self-healing NiAl-Al2O3 composite coatings by electrochemical way

Nickel aluminide diffusion coatings act like an aluminium reservoir to form protective alumina scales at high temperatures. However, interdiffusion phenomena play a critical role in the metallurgical integrity of these coatings. This work explores a new type of self-healing coating for high temperature oxidation applications. Micro-reservoirs consisting of an aluminium-rich intermetallic core (Al3Ni2) and an aluminium oxide shell are embedded in a nickel aluminide matrix (NiAl). This new type of coating has been synthesized by an electrochemical chemical route to trap the micro-reservoirs in an electrodeposited nickel matrix followed by aluminization via a slurry route. Under high temperature oxidation conditions, the micro-reservoirs allow to form a diffusion barrier and to release Al into the coating matrix. These two simultaneous phenomena make it possible to maintain a sufficient amount of aluminium and ensure the unique formation of α-alumina at the surface of the coatings, hence to possibly increase the lifespan of the aluminium diffusion coatings.

The activation mechanism of oxalic acid on γ-alumina and the formation of α-alumina

Converting the γ phase into the α phase completely is necessary in the presintering stage of industrial alumina (Al2O3), which requires high temperature and energy consumption. To reduce the presintering temperature, γ-Al2O3 was activated by oxalic acid. XRD, 27Al-MAS-NMR and TG-DSC were used to characterize the γ - alumina before and after activation, and the phase transformation was studied. The formation temperature of α-Al2O3 decreased to 1029 °C for oxalic acid activated γ-Al2O3, and the α-fraction was 100% for activated γ-Al2O3 at 1300 °C. After oxalic acid activation, the diffraction peak intensity of γ-Al2O3 decreased significantly; the results of 27Al-MAS-NMR suggested that octahedral [AlO6] in γ-Al2O3 was easier than tetrahedral [AlO4] to be attacked by oxalic acid, and the formation of pentavalent [AlO5] with higher reaction activity, which was in favour of the lowering formation temperature of α-Al2O3. The dissolution concentration of Al increased after oxalic acid activation, and the dissolution process was controlled by surface reactions. Oxalic acid mainly attacked the octahedral aluminium in γ-Al2O3 and extracted Al as three complexes of [Al(C2O4)]+, [Al(C2O4)2]- and [Al(C2O4)3]3-. Oxalic acid activated γ - Al2O3 with a lower phase transformation temperature has broad application prospects in the alumina industry.

Synthesis and characterization high purity alumina nanorods by a novel and simple method using nanocellulose aerogel template

Highly porous nanofibrillated cellulose aerogel fibers (NFCA) prepared from bagasse pulp was used as a template for in situ preparation of alumina nanorods. NFCA was soaked in aluminum nitrate aqueous solution followed by soaking in ammonium hydroxide solution to generate aluminum hydroxide within the porous structure and at surface of NFCA. Sintering of NFCA/Al (OH)3 was carried out at 1100 °C to produce nano-sized alumina with rod-like structure. The synthesized Al2O3 nanorods were investigated using scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), and high resolution transmitted electron microscope (HR-TEM). The Al2O3 rods had width from 123 to 86 nm while their length was in the micrometer range, as shown from SEM and HR-TEM images. The selected area X-ray diffraction (SEAD) showed rhombohedral crystal structure. XRD pattern confirmed formation of α-alumina. Energy dispersive X-ray spectroscopy (EDX) showed purity of the prepared Al2O3.

Long-term oxidation of MCrAlY coatings at 1000 °C and an Al-activity based coating life criterion

MCrAlY type (M=Ni and/or Co) coatings are widely used for the protection of components in the hot sections of gas turbines at high service temperatures by forming a continuous α-alumina. A reliable criterion to estimate the capability of coating to form α-alumina is of great importance to accurately evaluate coating lifetime. However, some coatings retain the ability to form a continuous α-alumina scale when the concentration of Al in coatings decreases to a critical level, therefore, the empirical Al-concentration based criterion is inadequate to properly predict the formation of a continuous α-alumina. Thus, a new life criterion, namely the critical Al-activity criterion, is proposed.In this work, the critical Al-activity to form a continuous α-alumina was validated by Al-activity calculation using Thermo-Calc software based on survey of research results of critical Al-concentration to form α-alumina on binary Ni-Al and ternary Ni-Cr-Al systems. Long-term oxidation tests were performed to support the criterion: three different MCrAlY coatings coated on IN-792 superalloy substrates were oxidized at 1000°C for various periods of time up to 10,000h. The microstructural evolution of MCrAlY coatings was investigated using Scanning Electron Microscope. The near-surface Al concentration and interdiffusion behaviour between substrate and coating were measured using Energy Dispersive X-ray Spectroscopy. The new critical Al-activity criterion has been successfully adopted in α-alumina formation prediction, showing a good agreement with experiment results. Therefore, it can be concluded that the extrapolation of new criterion from binary and ternary systems to multi-alloyed MCrAlY system is reasonable. Furthermore, the partial pressure of oxygen (PO2) in atmosphere has been taken into consideration by combination with Al-activity to calculate the critical chemical reaction constant (K) of formation of α-alumina. The potential applicability of the methodology to predict MCrAlY life is also discussed.

High temperature oxidation of AlCrFe complex metallic alloys

Isothermal oxidation of Al65Cr27Fe8 and Al80Cr15Fe5 was studied in the 600–1080°C range. Formation of transient alumina layers is obtained up to 900°C. On Al65Cr27Fe8 transient to α-phase transformations occur when performing oxidation at 1000°C, together with the possible appearance of (Al0.9Cr0.1)2O3. At 1080°C, direct formation of α-alumina is obtained. On Al80Cr15Fe5, spallation of the oxide layer during the cooling stage is observed following oxidation at 800 and 900°C, revealing thermal etching of the underneath alloy surface. At 1050°C the α-Al2O3 scale is directly formed but plastic deformation and recrystallization of the underneath alloy into several intermetallic phases is observed.

Sol–Gel Aluminum Hydroxides and Their Thermal Transformation Studies for the Production of α-Alumina

This work reports the formation of α-alumina from pseudoboehmite synthesized from three different hydrated inorganic aluminum salts such as aluminum nitrate, aluminum chloride and aluminum sulfate using the sol–gel method in aqueous medium at 80 °C. All the aluminum hydroxides were thermally treated and all showed similar crystalline conversion behavior; and, their solid-phase transformations to aluminum oxides were investigated. Following the synthesis, all products were characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry–thermogravimetric analysis and X-ray diffraction. The hydrolysis products were calcined at 200, 300, 600, 800, 1100 and 1200 °C for 1-h. The eta (η) and theta (θ) transitional aluminas were detected until complete transformation to highly crystalline α-alumina occurred at ~1200 °C. Moreover, starting at 1100 °C the sample derived from the sulfate salt showed intense crystalline peaks corresponding to α-alumina. The morphological study for the samples of α-alumina calcined at 1200 °C was carried out by field emission scanning electron microscopy where a vermicular structure was observed. These α-alumina samples were also analyzed by high resolution-transmission electron microscopy. In addition, fast Fourier transform and selected area electron diffraction patterns of α-alumina samples calcined at the highest temperature were obtained to calculate the experimental interplanar distances whose values were compared to reported theoretical values.

Solution combustion synthesis of α-Al2O3 using urea

The processes involved in the solution combustion synthesis of α-Al2O3 using urea as an organic fuel were investigated. The data describing the influence of the relative urea content on the characteristic features of the combustion process, the crystalline structure and the morphology of the aluminium oxide are presented herein. Our data demonstrate that the combustion of stable aluminium nitrate and urea complexes leads to the formation of α-alumina at temperatures of approximately 600–800°C. Our results, obtained using differential thermal analysis and IR spectroscopy methods, reveal that the low-temperature formation of α-alumina is associated with the thermal decomposition of an α-AlO(OH) intermediate, which was crystallised in the crystal structure of the diaspore.

Synthesis and characterisation of alumina fibres using organic/inorganic composite sol

The polyvinyl butyral–Al(NO3)3 composite sols and alumina fibres were synthesised by the sol–gel process in an aqueous solution using the polyvinyl butyral (PVB) and Al(NO3)3.9H2O (AN). The viscosity of PVB–AN composite sol increased with increasing AN content and aging time when it was laid at room temperature. The addition of AN leads to the formation of new weak peak and the deviation of diffraction angle to higher degrees according to the X-ray diffraction patterns (XRD). The exothermic peak of PVB disappeared and a weak endothermic peak was observed in differential scanning calorimetry curves of composite powders. The XRD pattern of fibres sintered at 1200°C showed the formation of α-alumina and the fibres showed smooth surface and uniform diameter.

Low-temperature formation of α-alumina from various polyhydroxoaluminum–hydroxy acid composite gels

Low-temperature α-alumina formation was attempted using various polyhydroxoaluminum (PHA)–hydroxy acid composite gels, which were prepared from PHA solutions containing different amounts of hydroxy acids, such as lactic acid, glycolic acid, malic acid, citric acid or mandelic acid. The composite gels began to transform into α-alumina when heated at lower temperatures of around 500 °C and the α-alumina fraction of the heat-treated products increased with increasing temperature. The α-alumina fraction was also dependent on both the type and amount of hydroxy acid additive. Among the composite gels studied, significant low-temperature α-alumina formation was observed for the PHA–mandelic acid, PHA–citric acid and PHA–lactic acid series. Low-temperature α-alumina formation was further promoted by employing a two-step heat-treatment method. The interaction between the functional groups of PHA and hydroxy acid and the seeding effect appear to play important roles in the course of the gelation and calcination processes for low-temperature α-alumina formation.

Synthesis and characterization of polyvinyl butyral–Al(NO3)3 composite sol used for alumina based fibers

The polyvinyl butyral–Al(NO3)3 composite sol used for alumina based fibers was synthesized by the sol–gel process in an aqueous solution using the polyvinyl butyral (PVB) and Al(NO3)3 · 9H2O (AN). The alumina fibers with smooth surface and uniform diameter were prepared. PVB, AN, PVB–AN composite sol and alumina fibers have been studied by X-ray diffraction (XRD), derivative thermo-gravimetric/differential scanning calorimetry (DTG/DSC), Fourier transform infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM). The interaction between PVB and AN was reported. The presence of a new weak peak at low angle and deviation of diffraction angle in XRD patterns implied that the reaction between PVB and AN took place. DTG/DSC curves showed the decomposition temperatures of AN increased and that of PVB decreased in the PVB–AN composite sol, which was considered to be caused by the interaction between PVB and AN. FTIR spectroscopy of PVB–AN gel showed a new absorption peak due to the COOH group, which implied the presence of new reaction product. The schematic reaction formula was shown in this paper. The XRD pattern of fibers sintered at 1,200 °C showed the formation of α-alumina and the fibers showed smooth surface and uniform diameter.

Low-Temperature Formation of .ALPHA.-Alumina from Polyhydroxoaluminum-Lactic Acid Composite Gels

Polyhydroxoaluminum (PHA)-lactic acid (LA) composite gels were prepared from PHA-LA mixed solutions. Phase transition behavior of the composite gels under heat-treatment was studied. The gels transformed into transition aluminas and/or α-alumina, depending on the heat-treatment conditions. Composite gels containing 3-10 mass% LA were found to yield α-alumina when heat-treated at low temperatures of around 500°C. The low-temperature formation of α-alumina was promoted at low heating rates. The fraction of α-alumina formed in the heat-treated products increased with temperature, reaching 40 mass% at ~800°C. The low-temperature formation of α-alumina is induced by the transformation of a partially formed diaspore-like structure, which is enhanced with reducing heating rate. The formation of 6-coordinated Al from 4-coordinated Al via the intermediate 5-coordinated Al during heat-treatment seems to be key to the formation of the diaspore-like structure, leading to the low-temperature formation of α-alumina.

Hydrothermal Synthesis of Submicrometer α‐Alumina from Seeded Tetraethylammonium Hydroxide‐Peptized Aluminum Hydroxide

Submicrometer α‐alumina powder was successfully synthesized from seeded aluminum hydroxide peptized with tetraethylammonium hydroxide (TENOH) and hydrothermally treated at 200°C, using α‐alumina particles as seeds. The powders were characterized by XRD, SEM, DTA‐TG, and BET analyses. Results showed that seeding could greatly enhance the transition to α‐alumina at 200°C without formation of other transient alumina phases. α‐Alumina with some amount of boehmite formed in the seeded samples, whereas boehmite was the exclusive phase formed in the nonseeded sample. The morphology of α‐alumina embedded in the boehmite matrix for the seeded samples suggests a direct transition from aluminum hydroxide to α‐alumina without the formation of transient alumina phases. The formation of α‐alumina in the seeded samples at temperatures as low as 200°C could be attributed to a favored nucleation in the TENOH‐peptized aluminum hydroxide and to the subsequent hydrothermal treatment that supplies the necessary activation energy for crystal growth. Transition of boehmite to α‐alumina in the hydrothermally treated samples with low‐seed contents was significantly promoted by heat‐treating the samples at 500°C.

Low-temperature synthesis of α-alumina from aluminum salt and urea

Studies on low-temperature synthesis of α-alumina from concentrated solutions of aluminum salt and urea at low temperature are reported. Characterization studies by both physical and chemical means reveal formation of α-alumina with density<0.3 g/cm 3, suitable for polishing and grinding use.

Formation of α-Alumina Around 500°C in Alkoxy-Derived Alumina Gels Under Ambient Pressure—Effects of Starting Solution Composition and Seeding

The hydrolysis of aluminum-sec-butoxide (ASB) in the ASB-H2O-C2H5OH-HCl solutions at 0°C for 18 h and subsequent aging of the sol for 30 days at 5°C provided an alumina gel which was partially (20–30 wt%) transformed to α-Al2O3 when heat-treated at 500°–600°C under ambient pressure. In addition to the hydrolysis and aging conditions, the amounts of water and HCl in the starting solution were found to be very important factors for the low temperature formation of α-Al2O3 in the resultant gel. Some difference in the chemical environment of involved OH groups was found between the alumina gel forming α-Al2O3 at 500°–600°C and the conventional gel. Corundum seeding was found to be effective; thus, 1 wt% seeding brought about an increase in the fraction of α-Al2O3 formed at 600°C to 40–50 wt%.

FTIR Study of High Temperature Phase Formation In Sol-Gel Aluminium Titanate

Fourier Transform Infrared Spectroscopic (FTIR) investigation has been carried out to study the structural changes and coordination of Al and Ti in the sol-gel derived aluminium titanate system as a function of temperature (65°–1400°C). An analysis of the FTIR spectral patterns shows that both octahedral as well as tetrahedral coordinations are present up to 800°C. At 1100°C, octahedral coordination is more predominant due to the formation of α-alumina. At 1300°C and above, the coordination is purely octahedral due to the formation of aluminium titanate.

Oxidation Behavior of Fine-Grain MA 956 Superalloy

This investigation was undertaken in an attempt to gain a fundamentalunder-standing of the oxidation behavior of a fine-grain MA 956 in thetemperature range 800 to 1200°C, with emphasis placed on the scalemorphology and oxidation kinetics. Oxidation reaction led to the formationof a thin and dense alumina scale, irrespective of the oxidationtemperature. At intermediate temperatures plateletlike oxides of alumina,probably θ-Al2O3, covered the surface, whereas at othertemperatures small nodules of Fe-, Cr-, Ti or Y-rich oxides were observed inthe outer part of the alumina scale. The temperature dependence of theparabolic rate constant revealed a change in the oxidation mechanism with atransition at about 1000°C. The high-temperature mechanism is controlled bythe formation of α-alumina, whereas the low-temperature oxidationmechanism is controlled by formation of metastable alumina. Comparison withthe reported oxidation kinetics for coarse-grain MA 956 is inconclusive,since microstructure (grain size, texture) of the substrate changed duringoxidation in the high-temperature range.

A process for the production of sub-micron to millimetre sized thermally stable α-alumina spheres

The sol-emulsion-gel method has been employed to produce alumina gel spheres from unseeded as well as seeded alumina sol. Controlled heating of the unseeded gel spheres leads to the formation of α-alumina phase at 1200°C. Various seeds have been tried to determine the effect of seeding on the formation of α-alumina and it is found that seeding with 0.5 wt.% Fe 2O 3 leads to the formation of α-alumina at a relatively low temperature of 950°C. The sizes of the microspheres can be varied in the range between 0.5 μm and 1 mm by suitable choice of the process parameters. The resultant gel spheres on controlled heating transform into thermally stable α-alumina spheres.

Densification of alumina produced by urea formaldehyde sol–gel polymeric route

α-Alumina powder was prepared in uniform spherical particles via urea formaldehyde sol–gel polymeric route using aluminium chloride as precursor. Alumina was both tetrahedrally and octahedrally coordinated within the structure of the resin. The α-phase is formed in the powder calcined at 1000°C when the number of Al 3+ substituted in the resin structure reached 2 moles equivalent oxide. The octahedrally coordinated aluminium form nucleation sites that act as seeds for the formation of α-alumina in powders calcined at 700°C. Bodies processed by cold isostatic press (CIP) under 300 and 600 Mpa from this powder show densification approaching to about 94% total density TD when fired at 1550°C . Grain growth was observed in microstructure of bodies fired at 1700°C and densification was lowered to 90.9% TD.