Phase Transformation Of Alumina Research Articles

Controlling the Phase Transformation of Alumina for Enhanced Stability and Catalytic Properties

AbstractCurrent transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low‐surface‐area α‐Al2O3 and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ‐Al2O3 to α‐Al2O3 and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. The results presented herein not only provide molecular‐level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation.

Controlling the Phase Transformation of Alumina for Enhanced Stability and Catalytic Properties.

Current transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low-surface-area α-Al2O3 and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ-Al2O3 to α-Al2O3 and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. The results presented herein not only provide molecular-level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation.

Influence of sodium hydroxide on phase transformation of gamma alumina to alpha alumina

AbstractCorundum and transition alumina powders are commonly produced through the Bayer process—that is, by digestion of bauxite in a sodium hydroxide solution, followed by gibbsite precipitation, which contains significant amounts of Na+ ions. During the de‐hydroxylation of gibbsite, Na+ is associated with retardation of the structural phase transformations between transition alumina and corundum (α‐Al2O3) phases, although the exact role of Na is unclear. The influence of Na+ ions on the phase transformation of γ‐Al2O3 to α‐alumina was investigated by varying the concentration of NaOH in aqueous γ‐Al2O3 suspensions. The phase transformation behavior of γ‐Al2O3 was monitored through thermal analysis, X‐ray diffraction, scanning electron microscope, X‐ray photoelectron spectrometer (XPS), and transmission electron microscope. As the Na concentration in γ‐alumina suspensions increased, the amount of α‐Al2O3 decreased in samples that were heat treated at 1200°C for 2 h. XPS analysis indicated that washing transition alumina powders with NH4OH solutions had decreased the surface concentration of Na+ and facilitated the phase transformation to α‐Al2O3.

Irradiation effects on microstructure, mechanical properties, and lead-bismuth eutectic corrosion resistance of alumina coating

A comprehensive evaluation including microstructure, mechanical properties, and lead-bismuth eutectic corrosion resistance of amorphous alumina coatings after different doses of Au2+ irradiation was done using X-ray diffraction, scanning electron microscopy, transmission electron microscopy, nanoindentation, SRIM software, and eRDF Analyzer. The results showed that irradiation decreased the surface roughness and induced crystallization in the middle of the coating and at the interface. The middle crystallization might be dominated by electron energy loss, while the interface had to consider the influence of defects and doping. The regions that remain amorphous after irradiation also showed differences in topological bonding and nanovoid. The changes in topological bonding hardened the coating, while the nanovoids generated by atomic rearrangement weakened the mechanical properties. The irradiation deteriorated the lead-bismuth corrosion resistance of the coating due to the void formation and alumina phase transformation. In addition, enhanced temperature sensitivity at the interface was observed after irradiation.

Evaluation of metal dusting resistance of thermal sprayed NiCrAlSiTaY and NiCrAlSiTaY/laser re-melted Al2O3 coatings on Inconel 625

The present study assessed the resistance against MD corrosion in a single-layer, oxide-forming NiCrAlSiTaY coating, and a double-layer NiCrAlSiTaY/laser-remelted (LRed) Al2O3 coating on an Inconel 625 (IN625) alloy. The NiCrAlSiTaY and Al2O3 coatings were applied onto the IN625 substrate using high velocity oxy-fuel spraying (HVOF) and air plasma spraying (APS) techniques, respectively. Coated and non-coated coupons were exposed, for 70 cycles, to a strong carburizing atmosphere of CO-50 % H2 in the temperature range of 550–750 °C to evaluate MD behavior. The results showed that the non-coated IN625 alloy contained a carbide free zone (CFZ), and secondary carbide precipitations of M23C6 and M6C and their subsequent in-situ oxidation were observed in the sub-surface layer. Such a development was indicative of the type III MD characteristic. The results also showed that the single-layer coating successfully protected the alloy, and only a part of the substrate underwent local oxidation. In addition, performing laser remelting (LR) on the Al2O3 coating significantly increased the resistance against the MD of the alloy through the removal of defects such as interconnected porosities, and via alumina phase transformation (γ → α). The lowest weight gain after 70 cycles of MD test was recorded for the double-layer coating coupons.

The effect of milling time on the alumina phase transformation in the AMCs powder metallurgy reinforced by silica-sand-tailings

This study aims to determine the effect of milling time and sintering temperature parameters on the alumina transformation phase in the manufacture of Aluminium Matrix Composites (AMCs) reinforced by 20 % silica sand tailings using powder metallurgy technology. The matrix and fillers use waste to make the composites more efficient, clean the environment, and increase waste utilization. The milling time applied to the Mechanical Alloying (MA) process was 0.5, 6, 24, 48, and 96 hours, with a ball parameter ratio of 15:1 and a rotation of 93 rpm. Furthermore, hot compaction was carried out using a 100 MPa two-way hydraulic compression machine at a temperature of 300 °C for 20 minutes. The temperature variables of the sintering parameter process were 550, 600 to 650 °C, with a holding time of 10 minutes. Characterization of materials carried out included testing particle size, porosity, X-Ray Diffraction (XRD), SEM-Image, and SEM-EDX. The particle measurement of mechanical alloying processed, using Particle Size Analyzer (PSA) instrument and based on XRD data using the Scherrer equation, showed a relatively similar trend, decreasing particle size occurs when milling time was increased 0.5 to 24 hours. However, when the milling time increases to 48 and 96 hours, the particle size tends to increase slightly, due to cold-weld and agglomeration when the Mechanical Alloying is processed. The impact is the occurrence of the matrix and filler particle pairs in the cold-weld state. So, the results of XRD and SEM-EDX characterization showed a second phase transformation to form alumina compounds at a relatively low sintering temperature of 600 °C after the mechanical alloying process was carried out with a milling time on least 24 hours

Microstructure evolution and mechanical behavior of a mullite fiber heat‐treated from 1100 to 1300°C

AbstractIn this paper, the effect of phase transformation on microstructure evolution and mechanical behaviors of mullite fibers was well investigated from 1100 to 1300°C. In such a narrow temperature range, the microstructure and mechanical properties showed great changes, which were significant to be studied. The temperature of the alumina phase transformation started at below 1100°C. The main phases in fibers were γ‐Al2O3 and δ‐Al2O3 with amorphous SiO2 at 1150°C. The stable α‐Al2O3 formed at 1200°C. Then the mullite phase reaction occurred. As the alumina phase reaction took place, the tensile strength increased with the increasing temperature. In particular, the filaments achieved the highest strength at 1150°C with 1.98 ± 0.17 GPa, and the Young's modulus was 163.08 ± 4.69 GPa, showing excellent mechanical performance. After 1200°C, the mullite phase reaction went on with the crystallization of orthorhombic mullite. The density of surface defects increased rapidly due to thermal grooving, which led to mechanical properties degrade sharply. The strength at 1200°C was 1.01 ± 0.15 GPa with a strength retention of 63.13%, and the Young's modulus was 184.14 ± 10.36 GPa. While at 1300°C, the tensile strength was 0.64 ± 0.14 GPa with a strength retention of only 40.00%.

Mineral phase transformation in coal gangue by high temperature calcination and high-efficiency separation of alumina and silica minerals

Coal gangue is the solid waste from the coal mining process. About 1 billion tons of coal gangue is discharged every year in the world. Coal gangue is usually piled up in the large areas and difficult to be used. Phase transformation and separation of alumina and silica minerals in coal gangue are studied in this paper in order to prompt the resource utilization of coal gangue. The results show that the phase transformation of kaolinite in coal gangue during high-temperature calcination is in the following way: kaolinite → metakaolin → spinel → mullite. Active silica mineral is released during the phase transition process, which can be separated from other minerals by caustic leaching. The optimal calcination temperature of coal gangue is 1050 ± 50 °C while the caustic leaching temperature is 85 °C and the leaching time should be kept between 60 and 80 min. The modulus of the sodium silicate solution can reach 1.0–1.1 and the alumina to silica ratio (A/S) of the desilicated residue can be raised to 1.7.

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.

Influence of hydrothermal conditions on the phase transformations of amorphous alumina

Amorphous Al2O3 transforms into the pseudoboehmite and boehmite phases of AlO(OH) under hydrothermal conditions. Specific surface area, pore volume and the amount of acidic sites decrease upon complete conversion of amorphous Al2O3 into boehmite.

Influence of hydrothermal conditions on the phase transformations of amorphous alumina

Amorphous Al 2 O 3 transforms into the pseudoboehmite and boehmite phases of AlO(OH) under hydrothermal conditions. Specific surface area, pore volume and the amount of acidic sites decrease upon complete conversion of amorphous Al 2 O 3 into boehmite.

Coprecipitation synthesis of Si-modified mesoporous alumina with high thermal stability from coal fly ash

Si-modified mesoporous alumina with high thermal stability was synthesized from coal fly ash through a facile coprecipitation method using NH4HCO3 and NH4Al(SO4)2. The effects of Si dopant contents on the characterization of alumina were investigated using X-ray diffraction, Fourier transform infrared spectroscopy, N2 adsorption–desorption method and transmission electron microscopy. The results show that the introduction of Si effectively inhibits the phase transformation of mesoporous alumina from γ-Al2O3 to α-Al2O3. Compared with alumina obtained without Si dopant, the specific surface area of mesoporous alumina with adding 12 wt% Si improved from 5.2 to 227.5% by increasing the calcination temperature from 500 to 1200 °C, respectively. The specific surface area, pore volume and pore size of mesoporous alumina with 12 wt% Si calcined at 1100 °C are 149 m2/g, 0.52 cm3/g and 8.9 nm, respectively.

Effect of microstructure of a NiCoCrAlY coating fabricated by high-velocity air fuel on the isothermal oxidation

The effect of grain size and porosity of a NiCoCrAlY coating fabricated by high-velocity air fuel on the isothermal oxidation behavior is studied. To understand this effect, the microstructure of coating, the chemistry and microstructure of thermally grown oxides (TGO), the phase transformation of alumina, the oxidation rate, and the TGO stress are investigated. It is demonstrated in this study that the oxidation and spallation resistance of TGO are predominantly determined by the porosity rather than grain size for the long term oxidation, although the nanoscale γ-grains can facilitate the earlier establishment of α-Al2O3 scale.

Formation mechanism of an undesirable by-product in the mild hydro-chemical process for the extraction of alumina from fly ash and its mitigation

The mild hydro-chemical process has been commercially demonstrated as an effective method to extract alumina from high alumina fly ash. This study investigated the formation mechanism of an undesirable aluminum-containing by-product, hydrated sodium calcium aluminosilicate (HSCA, 1.2Na2O·0.8CaO·Al2O3·2SiO2·H2O), produced during the mild hydro-chemical process and its mitigation strategy. The effects of reaction temperature, residence time and mole ratio of calcium to silicon on the alumina extraction and phase transformation of the leaching residues have been investigated, confirming the formation of HSCA phase being responsible for the decrease of alumina extraction efficiency. At the low Ca(OH)2 addition, the HSCA phase could be easily transformed from Na8Al6Si6O24(OH)2(H2O)2 combining with Ca(OH)2. On the other hand, when the addition of Ca(OH)2 was abundant, the existence of the NaCaHSiO4 product layer would insulate most of Ca(OH)2 from the Na8Al6Si6O24(OH)2(H2O)2 causing formation of HSCA. Based on the reaction mechanism and coating effect analysis, the mitigation strategy was also proposed. By increasing stirring speed to break the NaCaHSiO4 coating layer, HSCA phase could be completely avoided and a qualified alumina extraction efficiency of 90.84% could be achieved.

Synthesis and anisotropic properties of alumina-silica aerogels constructed by silica sols infiltrated into unidirectional frozen alumina templates

Abstract Alumina-silica aerogels (ASAs) were firstly prepared by using alumina aerogels as template via ice-template method and then silica sols infiltrated via vacuum impregnation. Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to investigate the microstructures characteristics and phase composites. Compared with the alumina templates (ATs), ASAs presented the porous characteristic of nano-level aerogels. The infiltrated silica can retard phase transformation of alumina. The anisotropic mechanical and thermal conduction properties of ATs in parallel and perpendicular to ice-crystal growth directions were demonstrated, which was related to the anisotropic porous structures of ATs due to the difference of main and secondary skeleton structures forming in unidirectional freeze. After infiltrated silica sols, the mechanical property was increased but the thermal conductivity was decreased. The anisotropic characteristics of ASAs still were maintained. Using ice template as a support, shape-stabilized aerogel composites can be constructed.

Ammonia Synthesis Activity of Alumina-Supported Ruthenium Catalyst Enhanced by Alumina Phase Transformation

The increase of alumina calcination temperature from 800 °C to 1300 °C results in the transformation of γ-Al2O3 to α-Al2O3 phase accompanying a decrease of specific surface area and the amount of tetrahedral Al3+ sites. Over Ru–Ba/alumina catalysts, an increase in alumina calcination temperature would broaden the size distribution of Ru particles, enlarge the metal-to-oxide ratio of Ru, decrease the amount of surface hydroxyl groups, as well as lower the temperature for N2 desorption. As a result, the increase of alumina calcination temperature lessens the effect of hydrogen poisoning and decreases the activation energy for ammonia synthesis. The Ru–Ba/Al2O3 catalyst with alumina calcined at 980 °C having both θ-Al2O3 and α-Al2O3 shows ammonia synthesis rate three times higher than that with alumina calcined at 800 °C having a γ-Al2O3 phase.

ТЕРМОИНДУЦИРОВАННЫЕ ФАЗОВО-МОРФОЛОГИЧЕСКИЕ ПРЕВРАЩЕНИЯ В ПЛЕНКАХ ПОРИСТОГО АНОДНОГО ОКСИДА АЛЮМИНИЯ ИЗ РАСТВОРОВ ГИДРОКСИДА НАТРИЯ

Представлены результаты исследования изменений фазового состава и морфологии пористого анодного оксида алюминия при отжиге 500 °С в течение 40 минут. Методом рентгенофазового анализа исследованы образцы исходных и отожженных пленок анодного оксида алюминия. Установлено, что температурное воздействие привело к образованию низкотемпературной модификации оксида алюминия γ-Al2О3 путем терморазложения «гидроксида-предшественника». Фазовые превращения подтверждены с помощью методов колориметрии и фрактального анализа.

Phase transformation of alumina induced by high pressure spark plasma sintering (HP-SPS)

Spark plasma sintering of nanocrystalline γ-Al2O3 at very high pressure was investigated up to 5 GPa. A SPS pressure/temperature diagram of α-Al2O3 and γ-Al2O3 phase was plotted. An in-house high pressure spark plasma sintering equipment (HP-SPS) was used. The capabilities and parameters control of this equipment greatly reduced the phase transition and sintering temperatures to obtain ceramics in contrast to conventional high pressure and high temperature sintering (HP-HT). The transition temperature could be lowered to ~500 °C from 1 GPa. The grain growth in ceramic was retained by the HP-SPS sintering. The density was also demonstrated to be optimized with HP-SPS.

Effect of tensile load on high temperature oxidation of CoNiCrAlY coating

Oxide growth plays a critical role in the failure mechanisms of thermal barrier coating (TBC) systems. This study is aimed to investigate the chemo-mechanical coupling oxidation behavior of CoNiCrAlY coating which is widely used as bond coat in TBC systems. Oxidation experiments at 1000 °C were conducted on both load-free and tension-loaded specimens. Microscopic examination was also performed using scanning electron microscopy and Raman spectroscopy. The experimental results show that the oxidation of the CoNiCrAlY coating was accelerated by the applied tensile load. In the oxide scale of the load-free specimen, both α-alumina and θ-alumina were present when the oxidation duration was short (≤15 h), but the θ-alumina transformed to α-alumina after 20-h oxidation. By contrast, in the tension-loaded specimen, mixed α-alumina and θ-alumina were found even after oxidation for 20–60 h. This phenomenon suggests that the tensile load can cause delay in the phase transformation from fast-growing θ-alumina to slow-growing α-alumina. A new mechanism based on the theoretically derived effect of mechanical work on the free energy for alumina phase transformation has been proposed to explain the experimental observations.

Effects of chlorides on the phase transformation of alumina

The discovery that γ-alumina can be prepared at lower calcination temperature from an ammonium aluminum carbonate hydroxide (AACH) precursor templated with rosinyl trimethyl ammonium chloride has led to the assumption that Cl- is capable of lowering the temperature of the phase transformation of AACH to γ-alumina. To verify this hypothesis, several chlorides were introduced into the calcination process of the AACH precursor. The samples were characterized using X-ray diffraction, thermogravimetry and differential scanning calorimetry as well as X-ray photoelectron spectroscopy. The results showed that Cl- has great effects on the crystallinity of γ-alumina obtained by the calcination of AACH. The effects varied with the type, dosage and introduction method of the particular chloride. It was also found that Cl- began to take effect only after the AACH had been decomposed to form amorphous alumina. Therefore, the true function of Cl- is to decrease the temperature of the phase transformation from the amorphous phase to γ-alumina: the phase transformation temperature decreases by as much as 115 K.