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Towards High Surface Area α-Al₂O₃-Mn-Assisted Low Temperature Transformation.

ABSTRACT: When impregnated with manganiferous precursors, γ-Al₂O₃ may be converted into α-Al₂O₃ under relatively mild and energy-saving conditions. In this work, a manganese assisted conversion to corundum at temperatures as low as 800 °C is investigated. To observe the alumina phase transition, XRD and solid-state ²⁷Al-MAS-NMR are applied. By post-synthetical treatment in concentrated HCl, residual manganese is removed up to 3 wt.-%. Thereby, α-Al₂O₃ with a high specific surface area of 56 m² g^-1 is obtained after complete conversion. Just as for transition alumina, thermal stability is an important issue for corundum. Long-term stability tests were performed at 750 °C for 7 days. Although highly porous corundum was synthesized, the porosity decreased with time at common process temperatures.

SUBMITTER: Jahnichen T

PROVIDER: S-EPMC10142860 | biostudies-literature | 2023 Apr

REPOSITORIES: biostudies-literature

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Towards High Surface Area α-Al2O3-Mn-Assisted Low Temperature Transformation.

Jähnichen Tim T Carstens Simon S Franz Maximilian M Laufer Otto O Wenzel Marianne M Matysik Jörg J Enke Dirk D

Materials (Basel, Switzerland) 20230412 8

When impregnated with manganiferous precursors, γ-Al2O3 may be converted into α-Al2O3 under relatively mild and energy-saving conditions. In this work, a manganese assisted conversion to corundum at temperatures as low as 800 °C is investigated. To observe the alumina phase transition, XRD and solid-state 27Al-MAS-NMR are applied. By post-synthetical treatment in concentrated HCl, residual manganese is removed up to 3 wt.-%. Thereby, α-Al<su ...[more]

PMID: 37109882

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Project description:The spontaneous crystal surface reconstruction of M-plane α-Al2O3 is employed for nanopatterning and nanofabrication in various fields of research including, among others, magnetism, superconductivity, and optoelectronics. In this reconstruction process the crystalline surface transforms from a planar morphology to one with a nanoscale ripple patterning. However, the high sample temperature required to induce surface reconstruction made in situ studies of the process seem unfeasible. The kinetics of ripple pattern formation therefore remained uncertain, and thus production of templates for nanofabrication could not advance beyond a trial-and-error stage. We present an approach combining in situ real-time grazing incidence small-angle X-ray scattering experiments (GISAXS) with model-based analysis and with ex situ atomic force microscopy (AFM) to observe this morphological transition in great detail. Our approach provides time-resolved information about all relevant morphological parameters required to trace the surface topography on the nanometer scale during reconstruction, i.e., the time dependence of the pattern wavelength, the ripple length, width, and height, and thus their facet angles. It offers a comprehensive picture of this process exemplified by a M-plane α-Al2O3 surface annealed at 1325 °C for 930 min. Fitting the model parameters to the experimental GISAXS data revealed a Johnson-Mehl-Avrami-Kolmogorov type of behavior for the pattern wavelength and a predominantly linear time dependence of the other parameters. In this case the reconstruction resulted in a crystalline surface fully patterned with asymmetric ripple-shaped nanostructures of 75 nm periodicity, 15 nm in height, and 630 nm in length. By elucidating the time dependence of these morphological parameters, this study shows a powerful way to significantly advance the predictability of annealing outcome and thus to efficiently customize nanopatterned α-Al2O3 templates for improved nanofabrication routines.

Project description:The Ni3Al(111) surface was characterized during oxidation within the temperature range of 690-800 K by in situ scanning tunneling microscopy (STM), low energy electron diffraction (LEED) and auger electron spectroscopy (AES). Within this temperature range oxygen dosing always leads to the formation of a surface aluminum oxide layer while Ni atoms remain in their metallic state. The temperature however, affects the kinetics and the structure of the grown oxides. Above 790 K the known (√67 × √67)R12.2° double layer oxide grows, which consists of two Al-O layers. Oxygen dosing at the lower temperature of 740 ± 10 K leads to a single layer oxide with only one Al-O plane. The lattice mismatch of the aligned oxygen and substrate lattices induce a (7 × 7) moiré pattern of this surface phase. Surprisingly, when lowering the sample temperature below 720 K during oxygen exposure, again a bilayer oxide grows on the Ni3Al(111) surface. The formation of this bilayer oxide starts with the growth of the single layer oxide that is subsequently covered by a second Al-O layer. At temperatures close to 720 K, the 2nd layer is ordered and a diffraction pattern is observed indicating a (4√3 × 4√3)R30° unit cell with regard to the oxygen lattice of the surface oxide. A structure model is presented that relates this so far unknown double layer oxide to the building principle of α-Al2O3. The respective growth kinetics and the availability of Al atoms dictate whether the single or the low temperature double layer oxide is formed. The related mass transport on the surface can be observed using in situ STM which allows the qualitative discussion of the growth kinetics. When lowering the oxide growth temperature below 700 K, the 2nd oxide layer is still formed ontop of the single layer oxide but in a disordered state so that the LEED pattern of the single layer oxide with a (7 × 7) moiré unit cell is again observed. This accounts for the confusing fact that the (7 × 7) moiré LEED pattern may indicate either the formation of a single or a low temperature double layer oxide.

Dataset Information

Towards High Surface Area α-Al₂O₃-Mn-Assisted Low Temperature Transformation.

Publications

Towards High Surface Area α-Al<sub>2</sub>O<sub>3</sub>-Mn-Assisted Low Temperature Transformation.

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