Mechanical activation of the gamma to alpha transition in Al 2O 3 (original) (raw)
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Transition of γ-Al 2O 3 into α-Al 2O 3 during vibro milling
Powder Technology - POWDER TECHNOL, 2000
The results obtained in studying the phase transition of γ- into α-Al2O3 during intense mechanical activation by vibro milling are presented in this paper. It was found that depending on experimental conditions, the γ→α-Al2O3 transition through δ- and θ-phases, can be initiated. X-ray and SEM analyses of activation products show that the phase transition is preceded by intense reduction of γ-phase particles. The produced α-Al2O3 crystallites have a distorted lattice and are larger than the initial crystallites in the starting powder. The change of specific surface area (Sp) with activation time shows the presence of high internal porosity of γ-Al2O3 which disappears during the complex process of the phase transition. An attempt was made to present the specific surface area change via its γ- and α-phase contributions. In this respect, detailed analyses of γ-Al2O3 particle size reduction along with reduction of their internal porosity and agglomeration of ground powder were performed, which explain the change of the specific surface area with activation time.
Investigation of phase transition of γ-alumina to α-alumina via mechanical milling method
Phase Transitions, 2008
To cite this Article Bodaghi, M. , Mirhabibi, A. R. , Zolfonun, H. , Tahriri, M. and Karimi, M.(2008) 'Investigation of phase transition of γ-alumina to α-alumina via mechanical milling method', Phase Transitions, 81: 6, 571 -580 To link to this Article:
Journal of The American Ceramic Society, 1991
Nanocrystalline Al203 thin films (50 nm in thickness) have been synthesized by rf reactive sputtering deposition and subjected to annealing at temperatures ranging from 800° to 1200°C. TEM analysis indicated that the as-deposited alumina films contained both amorphous phase and metastable γ phase. Structural texture evolved in the films annealed at 800°C for 24 h; the texture had a [00l] preferred orientation and occurred along the {400} and {440} planes of γ Al203, . In the films annealed a t 1200°C for 2h, nucleation and concurrent anomalous grain growth of α-Al203took place in a fine-grained, polycrystalline γ-Al203 matrix. The anomalously grown γ-AL2O3 grains were primarily [0001]-oriented single crystals with grain sizes varying from 3 to 15 pm, while the γ-Al2O3 matrix had an average grain size of 50 nm. The γ-Al2O3 matrix was also strongly textured along the [001] axis and exhibited a heavily faulted, layered micro-structure. Most of these layers were oriented along the {220} crystallographic planes. Periodic superstructure was identified in the layered γ- Al2O3. The formation of layered structure in γ- Al2O3 is attributable to the change of stacking sequence of atomic layers along the {220} orientations. An atomic model is presented to explain the formation of layered structure in γ- Al2O3. The nucleation of α- Al2O3 appears to occur along the {220} crystallographic planes of γ- Al2O3. The explosive grain growth of α- Al2O3 during the γαphase transformation is explained by a mechanism involving interface boundary migration and lattice epitaxy. The orientation relationships between γ- and α- Al2O3 are determined.
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.
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.
Phase Transformation in Nanometer-Sized gamma-Alumina by Mechanical Milling
Journal of the American Ceramic Society, 2005
Nanocrystalline c-alumina powders of 50-nm size were milled by high-energy ball milling. It was found that the pure c-alumina phase showed great stability and did not transform to any other phase even after a long milling time. On the other hand, c-alumina, which contained a small amount of the a-alumina phase, showed a gradual phase transformation from cto a-alumina on milling. The phase transformation mechanism during milling appears to be nucleation and growth type and promoted by the a-alumina seed. I. Introduction
Equivalence of ball milling and thermal treatment for phase transitions in the Al2O3 system
Journal of Alloys and Compounds, 1994
High energy ball milling has recently become a very popular research topic, since virtually any composition can be manufactured using a mixture of elemental (mechanical alloying) or readily available master alloy powders (mechanical grinding). Many solid state reactions, which normally occur at elevated temperatures, can be facilitated by high energy milling [1, 2]. However, a very important parameter of mechanical milling, the powder temperature during milling (called an effective local temperature or a peak temperature at the collision site) is still surrounded by controversy formation of local melts [3], well below the melting point [4], a maximum temperature rise of only 38 °C [5], an excessive heating during milling [6], of up to 407 °C [7], as high as 180 °C [8], about 280 °C [9], exceeding 570 °C [10], over 500 °C [11]). Using the well-known temperatureinduced transition sequence of aluminas, we present experimental evidence that the effective local temperature should be even higher than 700 °C.
Kinetic Analysis Crystallization of α-Al2O3 by Dynamic DTA Technique
2001
The phase transformation of seeded (5 mass% Fe 2 O 3 as a Fe(NO 3) 3 solution) boehmite derived alumina gel to α-Al 2 O 3 was studied with DTA technique and compared with unseeded and α-Al 2 O 3 seeded boehmite gels. Data for kinetic analysis of α-Al 2 O 3 crystallization were obtained from quantitative DTA curves. The kinetic parameters were analysed by traditional Kissinger analysis and Friedman and Ozawa-Flynn-Wall methods using the Netzsch Thermokinetics program. Results of the comparison of values of activation energies for all three gels and methods are the process of α-Al 2 O 3 transformation for originally γ-AlOOH/Fe(NO 3) 3 gels goes like that of unseeded boehmite gels, only under lower temperatures (lower about 200°C).