Thermal Transformations of Aluminas and Alumina Hydrates - Reaction with 44% Technical Acid.

Stumpf, H; Russell, Allen S.; Newsome, J. W.; Tucker, C. M.

doi:10.1021/ie50487a039

Cited by 296 publications

(101 citation statements)

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“…Stumpf, Russell, Newsome & Tucker (1950) determined the cubic lattice constant of ~7-alumina to be 7.9/k. Lippens & DeBoer (1964) found, by selected-area electron diffraction, that rt-alumina is somewhat tetragonally deformed with the c/a ratio varying between 0.985 and 0.993, and y-alumina is more tetragonally deformed with the c/a ratio varying between 0.983 and 0.987, but that the oxygen sublattice of y-alumina is fairly well ordered, much more so than that of ~7-alumina.…”

Section: Crystal Structures Of ~7- Y-and O-aluminamentioning

confidence: 99%

Structures and transformation mechanisms of the η, γ and θ transition aluminas

Zhou¹,

Snyder²

1991

Acta Crystallogr B Struct Sci

753

546

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617B(4,4) = 2rr2At 2 (Yamamoto et al., 1984) AbstractThe defect crystal structures of r/-, y-and 0-alumina obtained from dehydroxylation of well crystallized bayerite and boehmite have been derived from the analysis of their X-ray powder diffraction patterns and from the Rietveld refinement of their neutron powder diffraction patterns. Profile analysis of the various reflection zones in these defect spinel structures shows different coherent domain sizes which can be associated with the tetrahedral and octahedral aluminium and the oxygen sublattices. These observations have been used to define the nature of the crystal structures, and to give insight into the transformation mechanisms. The very large surface energies of these phases are evident in the observation of * Present address: Materials Data, Inc., PO Box 791, Livermore, CA 94550, USA.t To whom all correspondence should be directed.0108-7681/91/050617-14503.00 a nearly three-coordinated surface AI atom in the 7/ phase, and are the reason for the stability of the defect spinel structures of the transition aluminas. The reduction of surface area and the ordering of the tetrahedral AI sublattice which occurs on heating causes the spinel framework to collapse so that the structure, which exhibits tetragonal character at the early stage of the transition, settles into monoclinic 0-alumina displacively at the later stage, and eventually transforms to hexagonal corundum reconstructively. Thus 0-alumina should be considered the ultimate rather than the intermediate structural form into which the transition aluminas could evolve on the way to corundum. The overall crystal structure of the transition aluminas should therefore be viewed intrinsically as spinel deformed rather than as tetragonally deformed. Crystal data at room temperature: r/-alumina, cubic, Fd3m, a = 7.914 (2) A, RB = 6.24, Rp = 6-50, R, = 8.43%; y-alumina, cubic, Fd3m, a = © 1991 International Union of Crystallography 618 r/, ,/ AND 0 TRANSITION ALUMINAS 7.911 (2),~, Rs = 10.53, Rp = 7-61, R,. = 10.25%; 0-alumina, monoclinic, C2/m, a = 11.854(5), b = 2-904 (1), c = 5-622 ,~, /3 = 103.83 (7) °, Re = 15-02, Rp = 9"37, R,. = 11"93%.

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Section: Crystal Structures Of ~7- Y-and O-aluminamentioning

confidence: 99%

Structures and transformation mechanisms of the η, γ and θ transition aluminas

Zhou¹,

Snyder²

1991

Acta Crystallogr B Struct Sci

753

546

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“…Stumpf et al (1950) showed that between the dehydroxilation of the hydroxide and the alpha-alumina crystallization a certain number of intermediate crystalline structures of alumina are formed, which are reproducible; each one has a different crystalline structure and is stable in a given temperature range (Russell & Cochran 1950, Ervin Jr 1952, Brown et al 1953, Day & Hill 1953, de Boer et al 1954, Stirland et al 1958, Sato 1959, Francombe & Rooksby 1959, Wefers 1963, Lima-de-Faria 1963, Aldcroft & Bye 1967, Tanev & Vlaev 1993. The structure of each alumina and its temperature range of existence are determined by the structure of the starting or precursor hydroxide (Wefers & Misra 1987); they are different for gibbsite, bayerite, nordstrandite, boehmite or diaspore.…”

Section: Introductionmentioning

confidence: 99%

“…Extensive literature exists on the dehydroxilation of the crystalline hydroxides, in special on gibbsite, because this latter is the phase formed in the industrial Bayer Process. These aluminas are called "Transition Aluminas" and are identified by Greek letters alpha; gamma; delta and others (Stumpf et al 1950). Brindley (1963) reviewed the results of the simultaneous application of X-ray and electron diffractions and differential thermal analysis to characterize the products of the thermal recrystallization reactions of aluminum hydroxides and other metal hydroxide and hydroxisilicates.…”

Section: Introductionmentioning

confidence: 99%

Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

Kiyohara

Santos

Coelho

et al. 2000

An. Acad. Bras. Ciênc.

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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 .

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“…The presence of transition phases may be related to the small size of aluminium oxide crystalline seeds formed during the carbonization process, due to the inhibitory effect of the carbonaceous scaffold, as commented in section 3.2. Moreover, the relatively low temperature of formation of alpha phase with respect to the typical phase transition scheme proposed by Stumpf et al [57] and Tertian and Papée [58], could be due to various causes, such as the formation of a gel composed of hydrolyzed polynuclear aluminium species during the infiltration step. It has been described that aluminas generated from polyaluminium species, in particular from basic aluminium chlorides, have lower transition temperatures than usual [59,60].…”

Section: Alumina Ceramic Fibresmentioning

confidence: 96%

Characterization of thermally stable gamma alumina fibres biomimicking sisal

Benítez‐Guerrero

Pérez‐Maqueda

Jiménez

et al. 2014

Microporous and Mesoporous Materials

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Mesoporous gamma alumina fibres of high surface area, stable up to 1000 ºC, were synthesized by bioreplica technique using sisal fibres as templates. Alumina formation during pyrolysis and calcination of fibres infiltrated with aluminium chloride solution has been studied, paying special attention to the interaction between the precursor and sisal fibres, using several experimental techniques such as ATR-FTIR, coupled TG-FTIR and thermo-XRD analysis. The morphology and microstructure of the resulting alumina fibres were characterized using SEM and TEM. The crystallographic analysis of the alumina sample performed by electron and Xray diffraction suggests that fibres are constituted by  and -Al 2 O 3 crystallites, whose chemical structure was confirmed by ATR-FTIR and Al 27 -MAS-NMR. The specific surface area and porosity of ceramic fibres were determined by N 2 and CO 2 adsorption-desorption measurements. Resulting alumina fibres retain high specific surface areas of 200 m 2 /g and 150 m 2 /g even after calcination at 1000 °C for 15 h in dry air and for 4 h in wet air, respectively.

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Thermal Transformations of Aluminas and Alumina Hydrates - Reaction with 44% Technical Acid.

Cited by 296 publications

References 0 publications

Structures and transformation mechanisms of the η, γ and θ transition aluminas

Structures and transformation mechanisms of the η, γ and θ transition aluminas

Structure, surface area and morphology of aluminas from thermal decomposition of Al(OH)(CH3COO)2 crystals

Characterization of thermally stable gamma alumina fibres biomimicking sisal

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