Doping α-Al2O3 to reduce its hydrogen permeability: Thermodynamic assessment of hydrogen defects and solubility from first principles

Somjit, Vrindaa; Yildiz, Bilge

doi:10.1016/j.actamat.2019.02.031

Cited by 29 publications

(15 citation statements)

References 49 publications

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“…The results for the formation energies are in reasonable agreement with the HSE functional calculations of Na-Phattalung et al [10], which further signifies the conclusions of Somjit and Yildiz [41] regarding applicability of GGA-PBE functional for the purpose of defect thermodynamics in alumina. Not surprisingly, under Al-rich conditions, the formation energy of oxygen vacancy is lowest amongst these defects; therefore, VO is expected to form in considerable concentrations upon sintering at the lowest limits of oxygen partial pressure.…”

Section: Resultssupporting

confidence: 87%

“…1. The fermi energy is set to 0 eV, representing the highest occupied level, and the calculated band gap of the pure crystal is found to be 6.17 eV, with the enthalpy of formation (T = 0 K) being -14.97 eV, which are in agreement with prior theoretical studies using GGA-PBE functional [41,44]. The discrepancy between the calculated and experimental band gap is due to the used functional, which is a well-established shortcoming of DFT [45], however, given that the levels have a systematic error, makes it possible to interpret the data efficiently, by considering appropriate band edges and referencing fermi level to a common potential rather than valence band maximum [46].…”

Section: Resultssupporting

confidence: 80%

“…The total density of states for spin up and spin down electrons shows significant difference in all of the transition metal containing defects, thus causing a net magnetic moment. Although it is possible to correct for this systematic error in DOS through higher levels of theory [45], the energy levels reported using DFT are not considerably erroneous for the purpose of this study, as shown by Somjit and Yildiz [41]. Nevertheless, the optical transitions cannot be deduced using reported DOS, given that a more accurate evaluation of the energy levels becomes necessary for describing absorption and emission wavelengths.…”

Section: Resultsmentioning

confidence: 86%

“…Calculated formation energies are valid for zero kelvin temperature, and should serve the purpose of a qualitative comparison only. It is possible to evaluate the impact of higher temperatures and intermediate partial pressures of gases on the formation energies, using statistical mechanics as shown in the literature [38,41]. However, for the purpose of this study, the defect formation energies are only used as estimates for a comparison between the defects at low temperatures, prior to sintering.…”

Section: Methodsmentioning

confidence: 99%

“…1 and Eq. 2, the defect formation energies are highly dependent on furnace atmosphere, temperature, pressure and presence of other complex or charged defects in the crystal, which can change the equilibrium fermi level and thus significantly impact the defect equilibria [41][42][43]. Given that calculation of complex and charged defects are essential for; plotting Kroger-Vink diagrams, estimating defect concentrations and finding solubility limits of dopants, it is not possible to accurately deduce such information based on calculations done in this study, and as the main purpose of this paper is to find the induced magnetic moments by the defects mentioned above, we have refrained from doing a detailed analysis of defect equilibria.…”

Section: Methodsmentioning

confidence: 99%

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Ab-initio study of paramagnetic defects in Mn and Cr doped transparent polycrystalline Al2O3 ceramics

Mansoor

et al. 2021

Synth. Sinter.

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Birefringence is a major source of difficulty in sintering of transparent polycrystalline alumina ceramics, especially as the grain size exceeds a few hundred nanometers, which ultimately leads to complete opacity, mainly due to scattering of light. Recent studies have made it clear that by application of a strong magnetic field, alumina grains can be aligned along a particular crystallographic orientation, which minimizes scattering due to birefringence, and enhances transparency. Defects that cause spin delocalization are known to induce a paramagnetic behavior in alumina ceramics. Therefore, such defects have become a focal point of research for magnetic field assisted sintering of transparent polycrystalline alumina, in order to reduce the necessary magnetic field strength during production process. In light of recent studies on paramagnetic potentials of transition metal doped alumina, we have applied Spin Polarized Density Functional Theory (SP-DFT) calculations on manganese and chromium doped and co-doped alumina to calculate the magnetic moments, density of states and defect formation energies, which should be expected from this system of dopants, along with their interactions with oxygen vacancies. The results clearly indicate that formation of a point defect comprised of chromium and manganese positioned substitutionally at adjacent aluminum sites, in vicinity of an oxygen vacancy can induce a magnetic moment equivalent to 5 Bohr magnetons (μβ), outperforming previously reported defects. Based on this study we find it likely that chromium and manganese co-doping in alumina can further reduce the required magnetic field strength for production of transparent polycrystalline alumina.

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Section: Resultssupporting

confidence: 87%