Defect simulation in MgAl<sub>2</sub>O<sub>4</sub>spinels

Souza, Saulo Soares de; Blak, Ana Regina

doi:10.1080/10420159808220285

Cited by 15 publications

(13 citation statements)

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“…In an equimolar synthetic spinel, due to their smallest energy increase in comparison with other calculated point defect energies, 16–20 the predominant intrinsic defects are those resulting from site exchange of cations (so‐called antisite defect pair). Furthermore, the Schottky defect has a smaller energy than either cation or anion Frenkel defects.…”

Section: Discussionmentioning

confidence: 99%

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

Nuns¹,

béclin²,

Crampon³

2009

Journal of the American Ceramic Society

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The grain-boundary chemistry of fine-grained spinel MgO . nAl 2 O 3 (mean grain size below micron) has been investigated by STEM microanalysis. We have quantified the concentration of each element across the grain boundaries. Stoichiometry variations are observed from the grain-boundary region to the bulk. The Al/Mg ratio increases from 2.1 in the bulk to 2.35 at the grain-boundary regions. X-ray quantification allows us to reveal and to characterize the space-charge layer in the subgrain boundary. The grain-boundary cores are negatively charged due to V 00 Mg vacancies in excess, and in the subgrainboundary region, an opposite, positive space-charge layer is obtained. The point defect composition and the characteristic (sign, space-charge potential F N ) of the space-charge layer are discussed. H. M. Chan-contributing editorThis work was financially supported by the ''Re´gion Nord-Pasde Calais'' and the ''Fonds Europe´en de De´veloppement Re´gional (Feder)'' for supporting this work.

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Section: Discussionmentioning

confidence: 99%

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

Nuns¹,

béclin²,

Crampon³

2009

Journal of the American Ceramic Society

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“…Based on thermodynamic considerations, Nuns et al . also suggested a positive space‐charge with segregated

{Al}_{normalMg}^{∙}

and assigned a negative grain‐boundary core, but based on simulations assumed it to be composed of

V_{Mg}^{″}

. For MgO‐rich compositions, Chiang and Kingery again assumed a positive space‐charge, this time occupied by

{Al}_{i}^{∙ ∙ ∙}

to account for Al 2 O 3 ‐rich stoichiometry at the boundaries.…”

Section: Stoichiometrymentioning

confidence: 99%

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl₂O₄) Spinel

et al. 2013

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The need for materials for demanding optical applications has engendered a resurgent interest in transparent ceramics. Transparent polycrystalline magnesium aluminate spinel is one especially promising and rapidly maturing technology that can fill this niche. Although it has been studied for over 50 yr, it is only recently that highly transparent components with acceptable mechanical properties have been reliably fabricated at reasonable cost. Development has been hindered by the inherent difficulty in sintering spinel to the near-theoretical density required for transparency, a high sensitivity to powder and processing parameters, variable stoichiometry, and a lack of understanding of the synthesis-processing-property relationships. The driver of recent success is an emerging understanding of complex, multiscale, multivariable interactions that occur during green-body formation and sintering. In particular, certain key variables play a decisive role in determining compact properties and their evolution must be controlled from synthesis to the finished product. This article features the interactions between these key variables during processing and gives an expos e of the state of the art in transparent polycrystalline spinel fabrication.

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“…Besides the calculation for the normal structure, computer simulation has also been carried out in the inverse structure, where the stability of the lattice was obtained for the Imma space group. The calculated cohesion energy was -202.38 eV, while the same theoretical quantity in the case of normal spinel was -201.92eV [12] compared to the experimental value -200.1eV [13]. The calculated defect energies for cationic and anionic vacancies have already been presented in a previous work [12].…”

Section: Resultsmentioning

confidence: 97%

“…The great advantage of the computer simulation technique is to predict a defect configuration and to test the possibility of the defect formation through experimental observations. In the A1 sample, the dipole origin of the 160K peak has been attributed to intrinsic defects originated from the interchange of Mg 2+ and Al 3+ sites [12]. The samples that do not present this band have no interchange of sites.…”

Section: Resultsmentioning

confidence: 97%

“…The calculated cohesion energy was -202.38 eV, while the same theoretical quantity in the case of normal spinel was -201.92eV [12] compared to the experimental value -200.1eV [13]. The calculated defect energies for cationic and anionic vacancies have already been presented in a previous work [12]. To study the interchange of sites between Mg 2+ and Al 3+ , the energies of the substitution processes were calculated taking into account the previous formation of cationic vacancies.…”

Section: Resultsmentioning

confidence: 99%

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Characterization of dipole defects in MgAl ₂ O ₄ spinel

Carvalhaes¹,

Rocha²,

Souza³

et al. 2005

phys. stat. sol. (c)

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PACS 61.80.Ed, 61.82.Ms Dipole defects in gamma-irradiated and thermal treated MgAl 2 O 4 samples have been studied through thermally stimulated depolarisation currents(TSDC) technique and computer modelling methods. The presence of TSDC bands varies from sample to sample and some crystals do not present any band. The origin of these bands has been investigated in several different samples. In the spectra of spinels showing TSDC peaks, three bands at 130K, 160K and 320K are observed. The peaks at 130K and 160K have been attributed to dipole defects. After 1200kGy of gamma irradiation the broad band at 320K dislocates to 290K and increases ten times. Pulsed thermal treatments between 350K and 470K produce a progressive reduction of the peak area and a shift in the peak position back to 320K. A detailed analysis of the curve indicates the possibility of a superposition of peaks. Gamma irradiation restores the 320K TSDC peak. Taking into account optical absorption(OA) and electron paramagnetic resonance(EPR) results, the thermal reduction of the 320K TSDC band was attributed to V-type centres as a result of hole trapping at tetrahedral and octahedral cation vacancies. Computer modelling methods, based on lattice energy and defect minimisation, were applied to identify dipole defects that occur in these crystals. The calculations were made in normal and inverse spinel structures, doped with Cr, Co, Mn and Fe in order to justify the presence of dipole bands.

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Defect simulation in MgAl₂O₄spinels

Cited by 15 publications

References 4 publications

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl₂O₄) Spinel

Characterization of dipole defects in MgAl ₂ O ₄ spinel

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Defect simulation in MgAl2O4spinels

Cited by 15 publications

References 4 publications

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

Grain‐Boundary Characterization in a Nonstoichiometric Fine‐Grained Magnesium Aluminate Spinel: Effects of Defect Segregation at the Space‐Charge Layers

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl2O4) Spinel

Characterization of dipole defects in MgAl 2 O 4 spinel

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Product

Resources

About

Defect simulation in MgAl₂O₄spinels

Fifty Years of Research and Development Coming to Fruition; Unraveling the Complex Interactions during Processing of Transparent Magnesium Aluminate (MgAl₂O₄) Spinel

Characterization of dipole defects in MgAl ₂ O ₄ spinel