Kinetic Analysis Crystallization of α-Al2O3 by Dynamic DTA Technique (original) (raw)
Related papers
2013
In this work, boehmite sol was prepared by a previously applied and validated method; hydrolysis of aluminum chloride hexa-hydrate. In order to obtain precise results, the effect of pH after adding precipitating agent, aging time, peptizing temperature and ultrasonic vibration time on the crystallite size of final precipitate were investigated in a narrow range. The preparation conditions applied in the production step of nanocrystalline boehmite affected on the desired alumina phase. Experiments were set based on the statistical design of experiments (Taguchi method). Furthermore the influence of calcination on crystallization and phase transformation of the precipitate was investigated using X-ray diffractometry (XRD) and simultaneous thermal analysis (STA) techniques. To evaluate the results, the obtained data were statistically analyzed. Considering the statisti cal analysis of experiments, the pH after adding precipitating agent is the major parameter affecting crystallite size...
Phase Transformations of α-Alumina Made from Waste Aluminum via a Precipitation Technique
We report on a recycling project in which α-Al 2 O 3 was produced from aluminum cans because no such work has been reported in literature. Heated aluminum cans were mixed with 8.0 M of H 2 SO 4 solution to form an Al 2 (SO 4 ) 3 solution. The Al 2 (SO 4 ) 3 salt was contained in a white semi-liquid solution with excess H 2 SO 4 ; some unreacted aluminum pieces were also present. The solution was filtered and mixed with ethanol in a ratio of 2:3, to form a white solid of Al 2 (SO 4 ) 3 ·18H 2 O. The Al 2 (SO 4 ) 3 ·18H 2 O was calcined in an electrical furnace for 3 h at temperatures of 400-1400 °C. The heating and cooling rates were 10 °C /min. XRD was used to investigate the phase changes at different temperatures and XRF was used to determine the elemental composition in the alumina produced. A series of different alumina compositions, made by repeated dehydration and desulfonation of the Al 2 (SO 4 ) 3 ·18H 2 O, is reported. All transitional alumina phases produced at low temperatures were converted to α-Al 2 O 3 at high temperatures. The X-ray diffraction results indicated that the α-Al 2 O 3 phase was realized when the calcination temperature was at 1200 °C or higher.
Journal of Crystal Growth, 2002
θ- to α-phase transformation of three different nano-sized θ-Al2O3 powders pre-treated with high uniaxial pressure (250–750 MPa) was examined. During phase transformation, the presence of critical crystallite sizes of θ- (dcθ=∼25 nm) and α-Al2O3 (dcα=∼17 nm) and the primary size of α-Al2O3 (dp=∼45 nm) is required. The high pressure induces a removal of the agglomerate state among the θ-Al2O3 crystallites and an increase in bulk density of the green compact. The former eventually creates homogeneity in the interspacing of the θ-particles (=crystallites) that results in a narrowed exothermic peak on the differential thermal analysis (DTA) profile while the latter brings the particles closer in distance, thus resulting in a reduction of the transformation temperature. And the de-agglomeration effect presumably abates the finger growth of α-Al2O3 crystallites.The estimated lowest temperature for the completion of θ- to α-Al2O3 phase transformation using θ-powders derived from boehmite is 1050°C at a heating rate of 10°C/min. And the process duration is about 3.5 min or 35°C on the DTA profile.
Some Issues on Ultrafine Α-Alumina Powders Fabricated by Low Temperature Synthesis
Materials Methods Technologies, 2014
Thermal treatment (>1200 o C) of unstable intermediate phases of aluminum hydroxides and aluminum oxides inspires formation of thermodynamically stable α-Al 2 O 3 which is widely applied in the production of ceramics with different functional purposes. Low-temperature phase transformation and obtaining of α-Al 2 O 3 nanopowders is possible , if in the reaction mixture is added a small amount of up to 5% α-Al 2 O 3 (during decomposition, hydrolysis, drying-heating and annealing of precursors). Modified polymer-precursor and solution combustion techniques were used for obtaining α-alumina. Aluminum nitrates, aluminum alkoxydes, oxidants and bifunctional compounds for producing organic-inorganic gels were used as initial precursors. Organic-inorganic gels were obtained by homogenization of aluminum and organic compounds within the range of 80-200°C and by carbonization of the obtained mass within the temperature range of 300-800°C. The addition of 3-5% α-Al 2 O 3 seeds in the initial reactive mixture decreases γ→α phase transformation temperature (1200°C) to 1020-1080°C. It was established, that during the interaction between aluminum nitrates and urea, adding of aluminum caused obtaining of amorphous aluminum oxide, annealing of which for 2h at 800°C on the open air gave α-Al 2 O 3. There were studied possibilities of obtaining nano-phase powders by low-temperature (20-90 o C). corrosion of aluminum activated with mercury. Subsequent annealing of the obtained Al2O3•nH 2 O amorphous powder at 1000-1100 o C resulted in the formation of α-Al 2 O 3 fibers. XRD analysis showed that Al 2 O 3 fibers contained particles with primary crystallites of 10-30 nm sizes.
Journal of Physics and Chemistry of Solids, 2021
The aim of this work was to derive γ-alumina from gels prepared by hydrolysis of aluminum (Al) sec-butoxide (Asb) chelated with ethyl acetoacetate (Eaa) in various ratios and to determine the effect of Eaa on the properties of obtained alumina. Gels and γ-alumina were investigated by nuclear magnetic resonance, X-ray diffraction, scanning electron microscopy and N 2 adsorption-desorption isotherms. The results show that the Eaa/Asb ratio greatly affects the gelation process. At higher Eaa/Asb ratios, hydrolysis resistant tris(ethyl acetoacetate)aluminum (III) is formed, causing a less complete gelation process. Aluminum coordination in samples varies from six-fold coordination in tris(ethyl acetoacetate)aluminum (III) to less defined five-fold coordinated sites of Al in the gel. Thermal treatment at 800 • C for 2 h induces γ-Al 2 O 3 crystallization. A partial transformation to α-Al 2 O 3 is observed after thermal treatment at 900 • C for 2 h for some samples. Transformation to α-Al 2 O 3 after thermal treatment at 1000 • C for 2 h is complete for all samples. In addition to thermal stability of samples, the Eaa/Asb ratio also influences morphology of γ-Al 2 O 3 crystallized at 800 • C. In samples with lower Eaa/Asb ratio, particles appear compact, but are cavernous for samples with higher Eaa/Asb ratios.
Synthesis of α-alumina from a less common raw material
Journal of Sol-Gel Science and Technology, 2012
A nanostructured α-Al 2 O 3 with particle size lower than 100 nm was obtained from a hazardous waste generated in slag milling process by the aluminium industry. The route developed to synthesize alumina consisted of two steps: in the first one, a precursor of alumina, boehmite, γ-AlOOH was obtained by a sol-gel method. In the second step, the alumina was obtained by calcination of the precursor boehmite (xerogel). Calcination in air was performed at two different temperatures, i.e 1300 and 1400 ºC, to determine the influence of this parameter on the quality of resulting alumina. X-ray diffraction patterns and transmission electron microscopy images of calcined powers revealed beside corundum the presence of transition aluminas and some rest of amorphous phase in the sample prepared at 1300 ºC. The increase of the calcinations temperature to 1400 ºC favors the formation of an almost single-phase corundum powder. The transition of θ-to α-Al 2 O 3 was followed by means of Infrared spectroscopy, since it is accompanied by the disappearance of the he IR band frequencies associated with tetrahedral sites (AlO 4 sites), giving rise to a spectrum dominated by Al 3+ ions in octahedral sites (AlO 6) characteristic of corundum.
Phase transformation of α-alumina from aluminium waste
2011
α-Al 2 O 3 were produced from aluminium wastes (aluminium cans). Roasted aluminium cans were mixed with concentrated H 2 SO 4 to form Al 2 (SO 4 ) 3 solution. The solution was filtered out and mixed with ethanol to form white solid of Al 2 (SO 4 ) 3 .18H 2 O. The Al 2 (SO 4 ) 3 .18H 2 O was calcined for 3 hours at temperatures of 400 to 1400ºC. The phase change was investigated using XRD and FESEM. All transitional alumina produced at low temperatures converts to α-Al 2 O 3 at high temperature, since a series of alumina formation by dehydration and desulphonation of the Al 2 (SO 4 ) 3 .18H 2 O. X-ray diffraction show phase of α-Al 2 O 3 after calcined at temperature 1200 ºC.
Ti4+ addition effect on α-Al2O3 flakes synthesis using a mixture of boehmite and potassium sulfate
Ceramics International, 2010
Single-crystal a-Al 2 O 3 hexagonal flakes with a diameter of about 200 nm and 20 nm in thickness were obtained by mixing different molar ratios of potassium sulfate to boehmite and heating at 1000 8C. Co-doping 1 mol% TiO 2 can increase the shape anisotropy of a-Al 2 O 3 hexagonal flakes, increasing the diameter to 400 nm. The effects of potassium sulfate, Fe 2 O 3 and TiO 2 on the phase transformation and morphology development of alumina were investigated using X-ray diffraction analysis (XRD), differential thermal analysis (DTA) and transmission electron microscopy (TEM). The results indicate that co-doping potassium sulfate, Fe 3+ and Ti 4+ can promote g ! a-Al 2 O 3 phase transformation and change the morphology from a vermicular structure into hexagonal platelets. The shape anisotropy of a-Al 2 O 3 hexagonal flakes can be increased by adding TiO 2 due to the segregation of Ti 4+ ions onto the surfaces of basal planes of a-Al 2 O 3 single crystal particle.
Anais da Academia Brasileira de Ciências, 2000
Crystalline aluminium hydroxiacetate was prepared by reaction between aluminium powder (AL-COA 123) and aqueous solution of acetic acid at 96 • C±1 • C. The white powder of Al(OH)(CH 3 COO) 2 is constituted by agglomerates of crystalline plates, having size about 10µm. The crystals were fired from 200 • C to 1550 • C, in oxidizing atmosphere and the products characterized by X-ray diffraction, scanning electron microscopy and surface area measurements by BET-nitrogen method. Transition aluminas are formed from heating at the following temperatures: gamma (300 • C); delta (750 • C); alpha (1050 • C). The aluminas maintain the original morphology of the Al(OH)Ac 2 crystal agglomerates, up to 1050 • C, when sintering and coalescence of the alpha-alumina crystals start and proceed up to 1550 • C. High surface area aluminas are formed in the temperature range of 700 • C to 1100 • C; the maximum value of 198m 2 /g is obtained at 900 • C, with delta-alumina structure. The formation sequence of transition aluminas is similar to the sequence from well ordered boehmite, but with differences in the transition temperatures and in the development of high surface areas. It is suggested that the causes for these diversities between the two sequences from Al(OH) Ac 2 and boehmite are due to the different particle sizes, shapes and textures of the gamma-Al 2 O 3 which acts as precursor for the sequence gamma-to alpha-Al 2 O 3 .