Sounding-rocket microgravity experiments on alumina dust

Ishizuka, Shinnosuke; Kimura, Yuki; Sakon, Itsuki; Kimura, Hiroshi; Yamazaki, Tomoya; Takeuchi, Shinsuke; Inatomi, Yuko

doi:10.1038/s41467-018-06359-y

Cited by 17 publications

(10 citation statements)

References 32 publications

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“…crystal structures) of alumina have been suggested (Sargent 2019). A recently conducted microgravity experiment in a sounding-rocket has shown that solid Al 2 O 3 exhibits broad emission in the 11−12 µm wavelength range (Ishizuka et al 2018). Other dust features often seen in oxygenrich AGB stars are located at around 10 µm and 18 µm and they are attributed to Si−O streching and Si−O−Si bending modes, respectively (Hackwell et al 1970).…”

Section: Introductionmentioning

confidence: 99%

Bottom-up dust nucleation theory in oxygen-rich evolved stars

Gobrecht

Plane

Bromley

et al. 2022

A&A

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Context. Aluminium oxide (alumina; Al2O3) is a promising candidate as a primary dust condensate in the atmospheres of oxygen-rich evolved stars. Therefore, alumina ‘seed’ particles might trigger the onset of stellar dust formation and of stellar mass loss in the wind. However, the formation of alumina dust grains is not well understood. Aims. We aim to shed light on the initial steps of cosmic dust formation (i.e. nucleation) in oxygen-rich environments via a quantum-chemical bottom-up approach. Methods. Starting with an elemental gas-phase composition, we construct a detailed chemical-kinetic network that describes the formation and destruction of aluminium-bearing molecules and dust-forming (Al2O3)n clusters up to the size of dimers (n = 2) coagulating to tetramers (n = 4). Intermediary species include the prevalent gas-phase molecules AlO and AlOH as well as AlxOy clusters with x = 1–5, y = 1–6. The resulting extensive network is applied to two model stars, which represent a semi-regular variable and a Mira type, and to different circumstellar gas trajectories, including a non-pulsating outflow and a pulsating model. The growth of larger-sized (Al2O3)n clusters with n = 4–10 is described by the temperature-dependent Gibbs free energies of the most favourable structures (i.e. the global minima clusters) as derived from global optimisation techniques and calculated via density functional theory. We provide energies, bond characteristics, electrostatic properties, and vibrational spectra of the clusters as a function of size, n, and compare these to corundum, which corresponds to the crystalline bulk limit (n →∞). Results. The circumstellar aluminium gas-phase chemistry in oxygen-rich giants is primarily controlled by AlOH and AlO, which are tightly coupled by the reactions AlO+H2, AlO+H2O, and their reverse. Models of semi-regular variables show comparatively higher AlO abundances, as well as a later onset and a lower efficiency of alumina cluster formation when compared to Mira-like models. The Mira-like models exhibit an efficient cluster production that accounts for more than 90% of the available aluminium content, which is in agreement with the most recent ALMA observations. Chemical equilibrium calculations fail to predict both the alumina cluster formation and the abundance trends of AlO and AlOH in the asymptotic giant branch dust formation zone. Furthermore, we report the discovery of hitherto unreported global minimum candidates and low-energy isomers for cluster sizes n = 7, 9, and 10. A homogeneous nucleation scenario, where Al2O3 monomers are successively added, is energetically viable. However, the formation of the Al2O3 monomer itself represents an energetic bottleneck. Therefore, we provide a bottom-up interpolation of the cluster characteristics towards the bulk limit by excluding the monomer, approximately following an n−1∕3 dependence.

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Section: Introductionmentioning

confidence: 99%

Bottom-up dust nucleation theory in oxygen-rich evolved stars

Gobrecht

Plane

Bromley

et al. 2022

A&A

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“…We recently succeeded in reproducing the small fwhm of the astronomical 13 μm band in experiments under μG environment . Then, we found that the peak wavelength was shifted toward longer wavelength (smaller wavenumber) compared to the astronomical 13 μm band, which was attributed to growing anisotropy of aluminum oxide nanoparticles . Although the degree of anisotropy may be different, the new pathway will be available in both the previous under μG experiment and astronomical environments.…”

Section: Discussionmentioning

confidence: 90%

Immiscibility of Nucleating Aluminum Oxide Nanoparticles in Vapor

et al. 2018

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Multistep pathways in nucleation are now known to be ubiquitous and thus crucial to understand crystal formation processes. In a supersaturated mother phase, many types of source molecules are coagulated to form an amorphous or liquid-like intermediate, which subsequently transforms to a crystal. Such an intermediate possibly has a chemical formula that is inconsistent with that of the bulk crystal. Even in relatively simple vapor-phase homogeneous nucleation, the roles of the multicomponent mixtures at the nanoscale remain unknown. Here, by combining an experimental approach using in situ infrared (IR) measurements of homogeneous nucleation in the vapor phase and automatic quantum-chemical explorations of reaction routes, we show that a liquid-like nature of nucleating nanoparticles in the Al–O binary system induces immiscible phase separation. In an oxygen-deficient atmosphere, aluminum oxide nanoparticles form from supersaturated vapor with a unique shape composed of an Al metal head and an anisotropic Al2O3 crystalline tail. The anisotropic nanoparticles are larger than spherical Al2O3 nanoparticles formed in an atmosphere with sufficient oxygen. This indirectly shows that fewer nuclei are available in the precursor gas, indicating that homogeneous nucleation is initiated by oxygen-bearing species. We propose that miscible O-rich species act as seeds, and subsequently, the phase separation is induced in the liquid-like nanoparticles. This is supported by quantum-chemical calculations that show aggregation of Al atoms in oxygen-deficient (AlO) n clusters of as few as 16 molecules. In situ IR measurements revealed that the degree of anisotropy continues to increase even after the source gas molecules are exhausted, suggesting that oxygen-bearing species migrate to the tail through the molten Al-rich head. Liquid-like nature of nucleating nanoparticles is the key to control shapes and structures of nanomaterials and to elucidate the origin of cosmic dust.

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“…Describing clusters growth remains a vivid and challenging subject for laboratory cluster sources and nanoparticles production, but also atmospheric aerosol, − cosmic dust, , and combustion. , Because of their tiny size, clusters cannot be described within a condensed matter concept, and classical nucleation theory (CNT) fails to explain their growth . Indeed, CNT remains controversial regarding quantitative results, − and its conceptual limitations have been raised by several authors. ,− As an alternative to CNT which assumes that the seed is homogeneous and isostructural to the bulk crystal, the Ostwald rule − states that the first growing phase is not always the most stable in bulk.…”

Section: Introductionmentioning

confidence: 99%

“…Describing clusters growth remains a vivid and challenging subject for laboratory cluster sources and nanoparticles production, 1 but also atmospheric aerosol, 2−7 cosmic dust, 8,9 and combustion. 10,11 Because of their tiny size, clusters cannot be described within a condensed matter concept, and classical nucleation theory (CNT) fails to explain their growth.…”

Section: ■ Introductionmentioning

confidence: 99%

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters

et al. 2020

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Nucleation kinetics in gas phase remains an open issue with no general model. The derivation of the reaction constants assuming a canonical ensemble fails to describe anisotropic materials such as oxides. We have developed a general and versatile model using activated complex kinetics with a microcanonical approach. This approach handles the kinetics issue in cluster growth when the transient nature of the processes hinders the use of the canonical ensemble. The model efficiently reproduces experimental size distributions of alumina clusters formed by laser ablation with different buffer gas densities, including magic numbers. We show that the thermodynamic equilibrium is not reached during the growth. The bounding energy measured is 10 times lower than the one deduced from DFT calculation, but also the one expected from the bulk cohesive energy.

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Sounding-rocket microgravity experiments on alumina dust

Cited by 17 publications

References 32 publications

Bottom-up dust nucleation theory in oxygen-rich evolved stars

Bottom-up dust nucleation theory in oxygen-rich evolved stars

Immiscibility of Nucleating Aluminum Oxide Nanoparticles in Vapor

Microcanonical Nucleation Theory for Anisotropic Materials Validated on Alumina Clusters

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