Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences

Levin, Igor; Brandon, David

doi:10.1111/j.1151-2916.1998.tb02581.x

Cited by 1,266 publications

(773 citation statements)

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“…On a free electron model, the HfO 2 plasmon energy can be estimated by taking the density of 9.68 g / cm 3 as for the crystalline phase. 39 The most reasonable assumption ͑12 total valence electrons per molecule by counting eight electrons from O and four electrons from Hf͒ gives a plasmon energy of 21.3 eV, in which case plasmon resonance would contribute to the broad peak observed around that energy. Figure 4 shows that the measured value of the bulk plasmon energy for the whole lanthanide oxide series extended up to the hafnium oxide follows a slightly increasing law, which is similar to that deduced from the free electron model, but with absolute value typically 2 -3 eV lower.…”

Section: Resultsmentioning

confidence: 99%

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Effect of Al addition on the microstructure and electronic structure of HfO2 film

Wang

Egerton

et al. 2007

Journal of Applied Physics

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We have investigated the microstructures and electronic structures of a series of hafnium aluminate ͑HfAlO͒ films with Al concentration ranging from 0% to 100%. When the films evolve from pure HfO 2 to pure Al 2 O 3 by increasing the aluminum content, we find changes in their radial distribution functions, which disclose the short-range order of the materials, despite the amorphous nature of all films. The HfAlO films ͑with Al/ Hf ratio ranging from 0.25 to 5.8͒ appear to be a single glassy phase of Hf, Al, and O, instead of simple mixtures of HfO 2 and Al 2 O 3 . The Hf ͑Al͒-O, Hf ͑Al͒-Al, and Hf-Hf bonds are observed to be insensitive to the amount of Al in the film, except when the Al concentration is large ͑Al/ Hfϳ 5.8͒, in which case the bonding is similar to that in pure Al 2 O 3 . Although the local symmetry of Hf in amorphous HfO 2 is suggested by the electron energy-loss spectrum taken at an oxygen K edge, it is largely disrupted when Al is introduced. The valence electron energy-loss spectroscopy reveals three distinct evolving features as the Al content increases, which we discuss in terms of the electronic structure of HfO 2 .

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Section: Resultsmentioning

confidence: 99%

“…38 Al 2 O 3 has several crystalline phases, with density ranging from ϳ3.5 to 4.0 g / cm 3 . 39 Taking the density value of 3.5 g / cm 3 as more applicable to an amorphous phase, we can estimate the plasmon energy based on a free electron model,…”

Section: Resultsmentioning

confidence: 99%

Effect of Al addition on the microstructure and electronic structure of HfO2 film

Wang

Egerton

et al. 2007

Journal of Applied Physics

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“…Depending on the replica used for the short-wavelength range and under the assumption of thermodynamic equilibrium, the estimated temperature of the amorphous alumina grains at that spatial region would be around 1200-1600 K (Khouri et al 2015). However, amorphous alumina can only exist below 1000 K (Levin et al 1998;Levin & Brandon 2005). At temperatures around 1500 K, crystalline α-alumina can form, while below 1300 K crystalline γ-alumina can exist.…”

Section: Aluminium Depletion Into Grainsmentioning

confidence: 99%

Study of the aluminium content in AGB winds using ALMA

Decin

Richards

Waters

et al. 2017

A&A

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Context. The condensation of inorganic dust grains in the winds of evolved stars is poorly understood. As of today, it is not yet known which molecular clusters form the first dust grains in oxygen-rich (C/O < 1) asymptotic giant branch (AGB) winds. Aluminium oxides and iron-free silicates are often put forward as promising candidates for the first dust seeds. Aims. We aim to constrain the dust formation histories in the winds of oxygen-rich AGB stars. Methods. We obtained Atacama Large Millimeter/sub-millimeter array (ALMA) observations with a spatial resolution of 120 × 150 mas tracing the dust formation region of the low mass-loss rate AGB star, R Dor, and the high mass-loss rate AGB star, IK Tau. We detected emission line profiles of AlO, AlOH, and AlCl in the ALMA data and used these line profiles to derive a lower limit of atomic aluminium incorporated in molecules. This constrains the aluminium budget that can condense into grains. Results. Radiative transfer models constrain the fractional abundances of AlO, AlOH, and AlCl in IK Tau and R Dor. We show that the gas-phase aluminium chemistry is completely different in both stars with a remarkable difference in the AlO and AlOH abundance stratification. The amount of aluminium locked up in these three molecules is small, ≤1.1 × 10 −7 w.r.t. H 2 , for both stars, i.e. only ≤2% of the total aluminium budget. An important result is that AlO and AlOH, which are the direct precursors of alumina (Al 2 O 3 ) grains, are detected well beyond the onset of the dust condensation, which proves that the aluminium oxide condensation cycle is not fully efficient. The ALMA observations allow us to quantitatively assess the current generation of theoretical dynamical-chemical models for AGB winds. We discuss how the current proposed scenario of aluminium dust condensation for low mass-loss rate AGB stars within a few stellar radii from the star, in particular for R Dor and W Hya, poses a challenge if one wishes to explain both the dust spectral features in the spectral energy distribution (SED) in interferometric data and in the polarized light signal. In particular, the estimated grain temperature of Al 2 O 3 is too high for the grains to retain their amorphous structure. We advocate that large gas-phase (Al 2 O 3 ) n clusters (n > 34) can be the potential agents of the broad 11 µm feature in the SED and in the interferometric data and we propose potential formation mechanisms for these large clusters.

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“…2(a)), all of the PHA-hydroxy acid composite gels, except for the PHA-MnA system, partially crystallized into -alumina, while the PHA gel formed only -alumina. This unusual crystallization behavior of the composite gels, i.e., the formation of -alumina at considerably low temperatures, resembles the thermal decomposition route of diaspore (-AlOOH), which directly leads to -alumina [3]. A similar phenomenon has been reported by several researchers in the case of sol-gel-derived precursors from AlCl3•6H2O or aluminum alkoxide as a starting material [5][6][7][8][9].…”

Section: Resultsmentioning

confidence: 55%

“…In general, the preparation of -alumina powders requires calcination of alumina precursors, such as aluminum hydroxides or hydrated aluminas, at temperatures above 1100 °C to allow transformation into -alumina, along with the intermediate formation of metastable transition aluminas at lower temperatures [1,2]. Calcination at such high temperatures inevitably induces unwanted crystal growth and particle coarsening, which makes it difficult to synthesize -alumina powders with fine primary particles [3,4]. Therefore, lowering the temperature of -alumina formation has been a subject of interest to many researchers, because the synthesis of nanosized -alumina powders with controlled morphology could be attained by reduction of the -transformation temperature.…”

Section: Introductionmentioning

confidence: 99%

Low-temperature formation of α-alumina from various polyhydroxoaluminum–hydroxy acid composite gels

Yamaguchi

Mukouyama

Fujita³

et al. 2011

Ceramics International

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Low-temperature -alumina formation was attempted using various polyhydroxoaluminum (PHA)-hydroxy acid composite gels, which were prepared from PHA solutions containing different amounts of hydroxy acids, such as lactic acid, glycolic acid, malic acid, citric acid or mandelic acid. The composite gels began to transform into -alumina when heated at lower temperatures of around 500 °C and the -alumina fraction of the heat-treated products increased with increasing temperature. The -alumina fraction was also dependent on both the type and amount of hydroxy acid additive. Among the composite gels studied, significant low-temperature -alumina formation was observed for the PHA-mandelic acid, PHA-citric acid and PHA-lactic acid series. Low-temperature -alumina formation was further promoted by employing a two-step heat-treatment method. The interaction between the functional groups of PHA and hydroxy acid and the seeding effect appear to play important roles in the course of the gelation and calcination processes for low-temperature -alumina formation.

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Metastable Alumina Polymorphs: Crystal Structures and Transition Sequences

Cited by 1,266 publications

References 69 publications

Effect of Al addition on the microstructure and electronic structure of HfO2 film

Effect of Al addition on the microstructure and electronic structure of HfO2 film

Study of the aluminium content in AGB winds using ALMA

Low-temperature formation of α-alumina from various polyhydroxoaluminum–hydroxy acid composite gels

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