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David Scholz, M. Sc.

Centre for Applied Research

Technology Campus Teisnach Sensor Technology I 4.0

Academic Staff

TCTS O 06

09923/80108-515

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Zeitschriftenartikel

  • David Scholz

Computersimulation von Hämatit. Preisträgerartikel zur Verleihung des Georg-Simon-Ohm-Preises der Deutschen Physikalischen Gesellschaft.

In: Physik Journal vol. 8 pg. 63-65.

  • (2020)
In der Debatte zur Energiewende spielen Lithium-Ionen-Akkus eine wichtige Rolle. Hämatit gilt aufgrund seiner hohen Ladungskapazität als mögliches Anodenmaterial anstelle von Graphit. Seine vorteilhaften Materialeigenschaften, die niedrigen Kosten und die unproblematische Entsorgung machen das Mineral für weitere Anwendungsfelder interessant. Die aktuelle Forschung zielt darauf ab, die Eigenschaften unterschiedlicher Hämatit-Oberflächen besser zu verstehen und damit neue Anwendungen in der Sensorik und in der Katalyse zu ermöglichen. Bereits die Neandertaler verwendeten Hämatit (α-Fe2O3) in einer Mischung mit Ton in Form von rotem Ocker (Rötel) als Pigment. Die tiefrote Färbung gab dem eisenhaltigen Mineral Hämatit – auch Blutstein genannt – seinen Namen. Hämatit kristallisiert in der Korundstruktur, es ist also ebenso wie Korund (α-Al2O3) im trigonalen Kristallsystem aufgebaut, das durch eine hexagonale Elementarzelle beschreibbar ist. Neben Magnetit ist Hämatit das wichtigste Eisenerz. Hämatit ist anti­ferromagnetisch. Oberhalb der sog. Néel-Temperatur von 960 K wird es paramagnetisch. Darunter weist Hämatit einen schwachen Ferromagnetismus auf, da seine magnetischen Momente leicht verkippt sind. Dieser schwache Ferromagnetismus geht unterhalb der Morin-Temperatur von 260 K verloren. Die hohe spezifische Oberfläche von Hämatit-Nanokristallen sowie die Korrosionsbeständigkeit bieten Vorteile für technologische Anwendungen. Zudem ist Hämatit kostengünstig und nicht giftig. Vor allem in der Katalyse, der licht­induzierten Spaltung von Wasser, als chemischer oder magnetischer Sensor und als Anodenmaterial für Lithium-Ionen-Batterien kommt Hämatit infrage [1, 2]. In einer aktuellen Arbeit wurden kristalline Hämatit-Oberflächen mit verschiedenen Millerschen Indizes mittels Hartree-Fock-Methode und einer a-posteriori-Berechnung der Korrelationsenergie simuliert. Die Millerschen Indizes geben die Orientierung einer Oberfläche im Kristallgitter an.
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Vortrag

  • David Scholz
  • Thomas Stirner

Hartree-Fock simulation of hematite surfaces with a posteriori calculation of correlation energy . Preisträgervortrag zur Verleihung des Georg-Simon-Ohm-Preises der Deutschen Physikalischen Gesellschaft an David Scholz

In: DPG-Frühjahrstagung, Dresden (abgesagt wegen Covid-19, gehalten an THD)

Dresden

  • 2020 (2020)
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Zeitschriftenartikel

  • Thomas Stirner
  • David Scholz
  • J. Sun

Convergence of surface energy calculations for various methods: (001), (012), (100) hematite and the applicability of the standard approach

In: Journal of Physics: Condensed Matter vol. 32 pg. 185002 (5pp).

  • (2020)

DOI: 10.1088/1361-648X/ab6f88

Three different methods for the calculation of the surface energy, namely the standard approach, the Boettger relation and the linear-fit method, are applied to the (0 0 1), (0 1 2) and (1 0 0) hematite surfaces. The standard approach was previously shown to suffer from a divergence problem, and the Boettger relation was shown to exhibit quantum size effects. While the linear-fit method, in general, leads to a good convergence behavior of the surface energy, the questions arise whether the relative order of the calculated surface energies depends on the chosen calculation method, and whether there is any merit at all in employing the standard approach. The present work investigates these questions with hematite as a benchmark material system. The simulations show that, for the surface facets and slab thicknesses studied here, the relative order of the surface energies is unaffected by the chosen calculation method. A regime is found where the three methods are in reasonably good agreement with respect to the obtained surface energies. Finally, a procedure is put forward to extract meaningful surface energy values from the standard approach.
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Zeitschriftenartikel

  • David Scholz
  • Thomas Stirner

Convergence of surface energy calculations for various methods: (0 0 1) hematite as benchmark

In: Journal of Physics: Condensed Matter vol. 31 pg. 195901-19508.

  • (2019)

DOI: 10.1088/1361-648X/ab069d

Different methods for calculating the surface energy from ab initio simulations are applied to the relaxed (0 0 1) surface of the metal oxide hematite (α-Fe2O3). The simulations are carried out with a rather moderate k-point grid with shrinking factors of (6 6 6) for all bulk and (6 6) for all slab simulations. Very good convergence is obtained if a linear fit of the slab energies with respect to the number of layers in the slab is performed. In comparison to the other methods employed, this procedure is ultimately the most accurate and reliable method for extracting convergent surface energies from (0 0 1) hematite slabs. Additionally, we propose a way to determine the least possible starting point for calculating the surface energy by the linear-fit method. Furthermore, we find the Boettger method to perform nearly equally well, if the bulk energy is extracted from the energy difference per layer between the slabs with 12 and 18 layers thickness. Both methods give a surface energy of 2.43 J m-2 with a deviation of less than ±0.005 J m-2. The standard approach, which uses a separate bulk simulation, instead shows a significant linear divergence with increasing number of layers in the slab. We also carried out bulk simulations with a surface-oriented bulk unit cell, but found it in our case not to improve the convergence of the standard approach.
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Zeitschriftenartikel

  • Thomas Stirner
  • David Scholz
  • J. Sun

Ab initio simulation of structure and surface energy of low-index surfaces of stoichiometric alpha-Fe2O3

In: Surface Science vol. 671 pg. 11-16.

  • (2018)

DOI: 10.1016/j.susc.2018.01.010

The structure and surface energy of a series of low-index surfaces of stoichiometric α-Fe2O3 (hematite) are investigated using the periodic Hartree–Fock approach with an a posteriori correction of the correlation energy. The simulations show that, amongst the modeled facets, (012) and (0001) are the most stable surfaces of hematite, which is consistent with the fact that the latter are the dominant growth faces exposed on natural α-Fe2O3. The Fe-terminated (0001) surface is shown to exhibit a large relaxation of the surface atoms. It is argued that this arises mainly due to the fact that the surface cations are located opposite empty cation sites in the filled-filled-unfilled cation sequence along the c-axis. In contrast, the (012) plane cuts the crystal through a plane of empty cation sites, thus giving rise to relatively small relaxations and surface energies. The small relaxations and concomitant exposure of five-coordinate cation sites may be important for the catalytic activity of hematite. The simulations also show that the relative stability of the investigated surfaces changes after a full lattice relaxation with the (0001) and (116) facets relaxing disproportionately large. Wherever possible, the simulations are compared with previous simulation data and experimental results. A Wulff–Gibbs construction is also presented.
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Zeitschriftenartikel

  • Thomas Stirner
  • David Scholz
  • J. Sun

Hartree-Fock simulation of the (0 0 0 1) surface of hematite with a posteriori calculation of the correlation energy

In: Computational Materials Science vol. 137 pg. 340-345.

  • (2017)

DOI: 10.1016/j.commatsci.2017.06.011

The results of Hartree-Fock simulations of the (0 0 0 1) surface of hematite including an a posteriori calculation of the correlation energy are presented. Structural as well as electronic and magnetic properties of the surface atoms are considered. Infrared and Raman bands of the hematite slab are also presented. The calculated surface relaxation is shown to be in good agreement with experimental data originating from a recent LEED analysis. The iron-oxygen bond at the single-iron-terminated surface is shown to be less ionic than in the bulk. Also, the magnetic dipole moment of the surface iron atom is smaller than in the bulk. Finally, the surface energy of the (0 0 0 1) facet is presented for hematite, and for comparison also for α-alumina and α-chromia for various hybrid functionals. Here it is shown that the effects of electron correlation play an important role for hematite and chromia due to their localized 3d electrons.
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