Structure model of γ-Al<sub>2</sub>O<sub>3</sub>based on planar defects

Rudolph, Martin; Motylenko, Mykhaylo; Rafaja, David

doi:10.1107/s2052252518015786

Cited by 43 publications

(39 citation statements)

References 31 publications

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“…From Figs. 5(b) and 5(d), the following OR is obtained: This OR is consistent with the pseudomorphosis relationship between boehmite and cubic structures reported in the literature (Lippens & de Boer, 1964;Wilson, 1979;Rudolph et al, 2019). The width of the crystallite is measured as 66.73 nm before transformation [Fig.…”

Section: Crystallographic Orientation and Dimensional Changessupporting

confidence: 86%

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

Luo¹

2021

Acta Crystallogr Sect B

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γ-Alumina is a used material, while its precise crystal structure and transformation mechanism derived from boehmite have remained unclear in the literature for decades. In this work, quantitative electron microscopy has been applied to study the crystalline structure of γ-alumina and its transformation mechanism from boehmite. Based on Rietveld refinement of electron diffraction patterns, a new tetragonal structure model, with a space group of I41/amd (No. 141), was proposed for the γ-alumina phase, with Al cations on 4a, 8c, 8d and 16g sites and O anions on the 16h site, which could provide better fits than current models. During the boehmite to γ-alumina transformation induced by e-beam irradiation, when the boehmite layers were oriented along the edge-on direction, a shrinkage caused by dehydration was directly observed. Two kinds of boehmite to γ-alumina transformation mechanisms, namely collapse and reaction mechanisms, were elucidated crystallographically in detail with new insights through an intermediate structure, and the reaction mechanism was demonstrated to produce much reduced changes in dimensions and volume, compared with the collapse mechanism. The experimental observations supported the reaction mechanism, which occurred through partial occupation of the dehydrated space by diffusion in the initial stage of the transformation, without the formation of voids that only appeared after the initial stage. Filling tetrahedral interstices of the intermediate structure with Al cations in different ways yields tetragonal or cubic γ-alumina structures, and the tetragonal structure is energetically favorable because of smaller lattice distortions required, compared with the cubic structure. The crystallographic orientation relationships of γ-alumina with the parent boehmite phase deduced from the proposed mechanisms are consistent with the experimental observations.

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Section: Crystallographic Orientation and Dimensional Changessupporting

confidence: 86%

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

Luo¹

2021

Acta Crystallogr Sect B

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“…Highly crystalline -Al 2 O 3 gives sharp peaks, with a maximum count of 33 108 (2 = 43.35 ) while low-crystallinity -Al 2 O 3 gives diffuse profiles with a maximum count of 4779 (2 = 66.90 ), just one-seventh of that of -Al 2 O 3 . Moreover, the relation yð2Þ S ( yð2Þ S holds in all BG regions of the -Al 2 O 3 pattern, which indicates the presence of diffuse scattering from -Al 2 O 3 with the structure of a cubic defect spinel type (Samain et al, 2014;Rudolph et al, 2019). It would be reasonable, however, to assume that these two materials, having the same chemical composition and being measured under the same experimental conditions, have the same scattered intensity (a À1 k ) and almost the same BG height.…”

Section: Wppf-mentioning

confidence: 93%

A practical approach to the direct-derivation method for QPA: use of observed powder patterns of individual components without background subtraction in whole-powder-pattern fitting

Toraya

2019

J Appl Cryst

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The direct‐derivation (DD) method for quantitative phase analysis (QPA) can be used to derive weight fractions of individual phases in a mixture from the sums of observed intensities along with the chemical composition data [Toraya (2016). J. Appl. Cryst. 49, 1508–1516]. The whole‐powder‐pattern fitting (WPPF) technique can be used as one of the tools for deriving the observed intensities of individual phases. In WPPF, the observed powder pattern of a single‐phase sample after background (BG) subtraction can be used as the fitting function in combination with the fitting functions widely used in Pawley and Rietveld refinements. The direct fitting of the observed pattern is a very useful technique when the target component is a low‐crystallinity or amorphous material [Toraya (2018). J. Appl. Cryst. 51, 446–455]. Technical problems in utilizing the BG‐subtracted pattern are the uncertainty associated with the determination of BG height and the parameter interaction between the BG function (BGF) and the BG‐subtracted pattern in the least‐squares fit. In this study, a practical approach in which single‐phase observed patterns are used for the direct fitting without subtracting their BG intensities is proposed. In QPA, the contribution of BG intensities can be neutralized by converting the sum of BG‐included intensities into the sum of BG‐subtracted intensities by multiplying by a conversion factor. When the magnitudes of the conversion factors are almost identical for all components, they can be canceled out under the normalization condition in deriving weight fractions, and they are not required in QPA. The magnitude of the conversion factor for each component can be determined by one of two experimental techniques: using a single‐phase powder of the target component or a mixture containing the target component in a known weight ratio. The theoretical basis of the present procedure is given, and the procedure is experimentally verified. In this procedure, the interaction between the BGF and the BG‐included observed pattern is negligibly small. Least‐squares fitting with a few adjustable parameters is very fast and stable. Accurate QPA could be conducted, as indicated by the average deviation of 0.05% from weighed values in QPA of α‐Al2O3 + γ‐Al2O3 mixtures with five different weight ratios and 0.4% in QPA of an α‐SiO2 + SiO2 glass mixture

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“…During the formation of intermediate phases of aluminum oxide, these vacancies show different degrees of ordering, which usually increases from -Al 2 O 3 to θ-Al 2 O 3 [12]. Different distribution of vacancies over cation sites: one of the reasons why different structural descriptions of -Al 2 O 3 can be found in the literature for samples obtained by different technologies, with different crystallite sizes, etc.…”

Section: Literature Reviewmentioning

confidence: 99%

“…This transformation is initially thermodynamically favorable due to the presence of vacancy defects in the Al cation sublattice of the γ-А1 2 О 3 phase. In the case of alloying with magnesium (AMg3 alloy), which also has a relatively large bond strength with oxygen (according to [12], the free energy of formation of aluminum oxide is about 1100 kJ/mol, and for magnesium oxide, about 1160 kJ/mol), its substitution in the Al 3+ cation sublattice does not lead to a significant change in the kinetics of the γ → α transformation.…”

Section: Fig 11 -Scheme Of Rhombohedral Cell Transformation From a Cmentioning

confidence: 99%

Regularities of the influence of microarc oxidation of aluminum alloys on the phase-structural state of the formed oxide coatings and the peculiarities of γ-AL2O3 → α-AL2O3 polymorphic transformation during their annealing

Sоbоl¹,

Subbotinа²

2020

Bulletin of the National Technical University «KhPI». Series: New Solutions in Modern Technology

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The influence of the technological parameters of microarc oxidation on the regularities of the phase-structural state of coatings formed on D16 aluminum alloys (the main alloying element is Cu) and AMg3 (the main alloying element is Mg) and the effect of annealing in the temperature range 600-1280 °C on the γ-Al2O3 → α-Al2O3 phase transformation are investigated. It was found that in the coatings formed during microarc oxidation in a complex (alkaline-silicate) electrolyte, three main phase-structural states are revealed: γ-Al2O3, α-Al2O3 and mullite (3Al2O3·2SiO2). The conditions of electrolysis allowing the formation of a twophase state (γ-Al2O3 and α-Al2O3) on alloys of both types have been determined. It has been established that alloying elements of the AMg3 alloy provide in MAO coatings a higher stability of the γ-A12O3 structure in comparison with the analogous state in MAO coatings on D16 alloy. High-temperature annealing of MAO coatings made it possible to reveal a more complete phase transformation on D16 alloy, the structural basis of which is the appearance of tetragonality in the defective cubic lattice of the γ-Al2O3 phase. Annealing of MAO coatings stimulates the γ → α transformation with the greatest dynamics of change in the coatings obtained on D16 alloy. At the highest annealing temperature of 1280 °C (for 60 min) as a result of γ→α transformation, the relative content of the α-Al2O3 phase in the coating is 89 % (coating obtained on D16 alloy) and 30 % (coating obtained on AMg3 alloy). A model of the polymorphic γ-Al2O3 → α-Al2O3 transformation in aluminum oxide is proposed, based on the ordering of the metal cationic subsystem in octahedral and tetrahedral internodes and the enhancement of this process upon the weakening of the "metaloxygen" bond as a result of the replacement of Al ions by Cu ions that have a low binding energy with oxygen. A correlation between the relative content of the α-Al2O3 phase and the hardness of the MAO coating was found. With the highest content of the α-Al2O3 phase, the hardness reaches 16000 MPa.

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Structure model of γ-Al₂O₃based on planar defects

Cited by 43 publications

References 31 publications

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

A practical approach to the direct-derivation method for QPA: use of observed powder patterns of individual components without background subtraction in whole-powder-pattern fitting

Regularities of the influence of microarc oxidation of aluminum alloys on the phase-structural state of the formed oxide coatings and the peculiarities of γ-AL2O3 → α-AL2O3 polymorphic transformation during their annealing

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Structure model of γ-Al2O3based on planar defects

Cited by 43 publications

References 31 publications

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

Structure of boehmite-derived γ-alumina and its transformation mechanism revealed by electron crystallography

A practical approach to the direct-derivation method for QPA: use of observed powder patterns of individual components without background subtraction in whole-powder-pattern fitting

Regularities of the influence of microarc oxidation of aluminum alloys on the phase-structural state of the formed oxide coatings and the peculiarities of γ-AL2O3 → α-AL2O3 polymorphic transformation during their annealing

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Structure model of γ-Al₂O₃based on planar defects