| 1. |
- Aghda, Soheil Karimi, et al.
(author)
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Ion kinetic energy- and ion flux-dependent mechanical properties and thermal stability of (Ti,Al)N thin films
- 2023
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In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 250
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Journal article (peer-reviewed)abstract
- Ion-irradiation-induced changes in structure, elastic properties, and thermal stability of metastable c-(Ti,Al)N thin films synthesized by high-power pulsed magnetron sputtering (HPPMS) and cathodic arc deposition (CAD) are systematically investigated by experiments and density functional theory (DFT) simulations. While films deposited by HPPMS show a random orientation at ion kinetic energies (Ek)>105 eV, an evolution towards (111) orientation is observed in CAD films for Ek>144 eV. The measured ion energy flux at the growing film surface is 3.3 times larger for CAD compared to HPPMS. Hence, it is inferred that formation of the strong (111) texture in CAD films is caused by the ion flux-and ion energy-induced strain energy minimization in defective c-(Ti,Al)N. The ion energy-dependent elastic modulus can be rationalized by considering the ion energy-and orientation -dependent formation of point defects from DFT predictions: The balancing effects of bombardment-induced Frenkel defects formation and the concurrent evolution of compressive intrinsic stress result in the apparent independence of the elastic modulus from Ek for HPPMS films without preferential orientation. However, an ion energy-dependent elastic modulus reduction of similar to 18% for the CAD films can be understood by considering the 34% higher Frenkel pair concentration formed at Ek=182 eV upon irradiation of the experimentally observed (111)-oriented (Ti,Al)N in comparison to the (200)-configuration at similar Ek. Moreover, the effect of Frenkel pair concentration on the thermal stability of metastable c-(Ti,Al)N is investigated by differential scanning calorimetry: Ion-irradiation-induced increase in Frenkel pairs concentration retards the wurtzite formation temperature by up to 206 degrees C.
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| 2. |
- Aghda, Soheil Karimi, et al.
(author)
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Valence electron concentration- and N vacancy-induced elasticity in cubic early transition metal nitrides
- 2023
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In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 255
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Journal article (peer-reviewed)abstract
- Motivated by frequently reported deviations from stoichiometry in cubic transition metal nitride (TMNx) thin films, the effect of N-vacancy concentration on the elastic properties of cubic TiNx, ZrNx, VNx, NbNx, and MoNx (0.72 & LE; x & LE; 1.00) is systematically studied by density functional theory (DFT) calculations. The predictions are validated experimentally for VNx (0.77 & LE; x & LE; 0.97). The DFT results indicate that the elastic behavior of the TMNx depends on both the N-vacancy concentration and the valence electron concentration (VEC) of the transition metal: While TiNx and ZrNx exhibit vacancy-induced reductions in elastic modulus, VNx and NbNx show an increase. These trends can be rationalized by considering vacancy-induced changes in elastic anisotropy and bonding. While introduction of N-vacancies in TiNx results in a significant reduction of elastic modulus along all directions and a lower average bond strength of Ti-N, the vacancy-induced reduction in [001] direction of VNx is overcompensated by the higher stiffness along [011] and [111] directions, resulting in a higher average bond strength of V-N. To validate the predicted vacancy-induced changes in elasticity experimentally, close-to-singlecrystal VNx (0.77 & LE; x & LE; 0.97) are grown on MgO(001) substrates. As the N-content is reduced, the relaxed lattice parameter a0, as probed by X-ray diffraction, decreases from 4.128 & ANGS; to 4.096 & ANGS;. This reduction in lattice parameter is accompanied by an anomalous 11% increase in elastic modulus, as determined by nanoindentation. As the experimental data agree with the predictions, the elasticity enhancement in VNx upon N-vacancy formation can be understood based on the concomitant changes in elastic anisotropy and bonding.
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| 3. |
- Corkett, Alex J., et al.
(author)
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Band Gap Tuning in Bismuth Oxide Carbodiimide Bi2O2NCN
- 2019
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In: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 58:9, s. 6467-6473
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Journal article (peer-reviewed)abstract
- Layered bismuth oxides exhibit a broad range of tunable physical properties as a result of their excellent structural versatility which facilitates compositional substitutions at both cationic and anionic positions. Here we expand this family in a new direction through the preparation of the first example of a bismuth-containing oxide carbodiimide, Bi2O2NCN, which assumes an extended variant of the anti-ThCr2Si2 structure-type adopted by Bi(2)O(2)Ch (Ch = Se or Te) oxide chalcogenides. Electronic structure calculations reveal the title compound to be an indirect band gap semiconductor with a band gap of approximately 1.4 eV, in good agreement with the measured value of 1.8 eV, and intermediate between that of structurally related Bi2O2S (1.12 eV) and beta-Bi2O3 (2.48 eV). Mott-Schottky experiments demonstrate Bi2O2NCN to be an n-type semiconductor with a conduction band edge position of -0.37 V vs reversible hydrogen electrode. This study highlights the pseudochalcogenide nature of the N-=C=N- carbodiimide anion, which may be substituted in place of oxide or chalcogenide anions in this and potentially other structural classes as an effective means of electronic tuning.
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| 4. |
- Fekete, Matej, et al.
(author)
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On the determination of the thermal shock parameter of MAX phases : A combined experimental-computational study
- 2023
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In: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 43:13, s. 5484-5492
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Journal article (peer-reviewed)abstract
- Thermal shock resistance is one of the performance-defining properties for applications where extreme temperature gradients are required. The thermal shock resistance of a material can be described by means of the thermal shock parameter RT. Here, the thermo-mechanical properties required for the calculation of RT are quantum-mechanically predicted, experimentally determined, and compared for Ti3AlC2 and Cr2AlC MAX phases. The coatings are synthesized utilizing direct current magnetron sputtering without additional heating, followed by vacuum annealing. It is shown that the RT of both Ti3AlC2 and Cr2AlC obtained via simulations are in good agreement with the experimentally obtained ones. Comparing the MAX phase coatings, both experiments and simulations indicate superior thermal shock behavior of Ti3AlC2 compared to Cr2AlC, attributed primarily to the larger linear coefficient of thermal expansion of Cr2AlC. The results presented herein underline the potential of ab initio calculations for predicting the thermal shock behavior of ionically-covalently bonded materials.
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| 5. |
- Holzapfel, Damian M., et al.
(author)
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Enhanced thermal stability of (Ti,Al)N coatings by oxygen incorporation
- 2021
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In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 218
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Journal article (peer-reviewed)abstract
- Thermal stability of protective coatings is one of the performance-defining properties for advanced cutting and forming applications as well as for energy conversion. To investigate the effect of oxygen incorporation on the high-temperature behavior of (Ti,Al)N, metastable cubic (Ti,Al)N and (Ti,Al)(OxN1-x) coatings are synthesized using reactive arc evaporation. X-ray diffraction of (Ti,Al)N and (Ti,Al)(OxN1-x) coatings reveals that spinodal decomposition is initiated at approximately 800 degrees C, while the subsequent formation of wurtzite solid solution is clearly delayed from 1000 degrees C to 1300 degrees C for (Ti,Al)(OxN1-x) compared to (Ti,Al)N. This thermal stability enhancement can be rationalized based on calculated vacancy formation energies in combination with spatially-resolved composition analysis and calorimetric data: Energy dispersive X-ray spectroscopy and atom probe tomography data indicate a lower O solubility in wurtzite solid solution compared to cubic (Ti,Al)(O,N). Hence, it is evident that for the growth of the wurtzite, AlN-rich phase in (Ti,Al)N, only mobility of Ti and Al is required, while for (Ti,Al)(O,N), in addition to mobile metal atoms, also non-metal mobility is required. Prerequisite for mobility on the non-metal sublattice is the formation of non-metal vacancies which require larger temperatures than for the metal sublattice due to significantly larger magnitudes of formation energies for the non-metal vacancies compared to the metal vacancies. This notion is consistent with calorimetry data which indicate that the combined energy necessary to form and grow the wurtzite phase is larger by a factor of approximately two in (Ti,Al)(O,N) than in (Ti,Al)N, causing the here reported thermal stability increase. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 6. |
- Holzapfel, Damian M., et al.
(author)
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Influence of ion irradiation-induced defects on phase formation and thermal stability of Ti0.27Al0.21N0.52 coatings
- 2022
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In: Acta Materialia. - : Elsevier. - 1359-6454 .- 1873-2453. ; 237
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Journal article (peer-reviewed)abstract
- The influence of changes induced by ion irradiation on structure and thermal stability of metastable cubic (Ti,Al)N coatings deposited by cathodic arc evaporation is systematically investigated by correlating experiments and theory. Decreasing the nitrogen deposition pressure from 5.0 to 0.5 Pa results in an ion flux-enhancement by a factor of three and an increase of the average ion energy from 15 to 30 eV, causing the stress-free lattice parameter to expand from 4.170 to 4.206 Å, while the chemical composition of Ti0.27Al0.21N0.52 remains unchanged. The 0.9% lattice parameter increase is a consequence of formation of Frenkel pairs induced by ion bombardment, as revealed by density functional theory (DFT) simulations. The influence of the presence of Frenkel pairs on the thermal stability of metastable Ti0.27Al0.21N0.52 is investigated by scanning transmission electron microscopy, differential scanning calorimetry, atom probe tomography and in-situ synchrotron X-ray powder diffraction. It is demonstrated that the ion flux and ion energy induced formation of Frenkel pairs increases the thermal stability as the Al diffusion enabled crystallization of the wurtzite solid solution is retarded. This can be rationalized by DFT predictions since the presence of Frenkel pairs increases the activation energy for Al diffusion by up to 142%. Hence, the thermal stability enhancement is caused by a hitherto unreported mechanism - the Frenkel pair impeded Al mobility and thereby retarded formation of wurtzite solid solution.
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| 7. |
- Liu, Sida, et al.
(author)
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Stress-dependent prediction of metastable phase formation for magnetron-sputtered V1-xAlxN and Ti1-xAlxN thin films
- 2020
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In: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 196, s. 313-324
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Journal article (peer-reviewed)abstract
- Metastable transition metal aluminum nitride (TMAlN, TM = Ti, V) thin films are today deposited utilizing ionized vapor phase condensation techniques where variations in ion flux and ion energy cause compressive film stress, in turn affecting Al solubility. While the metastable phase formation of TiAlN has been modeled, the influence of film stresses on phase formation has so far been overlooked. Using combinatorial deposition via magnetron sputtering, thermodynamic modeling and density functional theory calculations, we investigated the phase formation of V1-xAlxN and Ti1-xAlxN thin films at various substrate temperatures and deposition rates. Ab initio calculations indicate that the maximum solid solubility of Al in face-centered cubic (fcc) V1-xAlxN or fcc-Ti1-xAlxN shows a linear trend as a function of the magnitude of compressive stress. Here, we consider the influence of film stresses on the metastable phase formation of fcc-V1-xAlxN and fcc-Ti1-xAlxN for the first time. Specifically, experimental data from a single combinatorial deposition is utilized to predict the stress-dependent formation of metastable phases based on thermodynamic and ab initio data. Explicit consideration of stress extends the Al solubility limit to higher values for both Ti1-xAlxN and V1-xAlxN thin films, previously unobtainable by energetics, but accessible experimentally. These predictions are experimentally verified and thus provide guidance for experimental efforts with the goal of increasing the Al concentration in fcc-TMAlN thin films. (C) 2020 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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| 8. |
- Sahu, Rajib, et al.
(author)
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Compositional defects in a MoAlB MAB phase thin film grown by high-power pulsed magnetron sputtering
- 2023
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In: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 15:43, s. 17356-17363
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Journal article (peer-reviewed)abstract
- Various compositional defects such as Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo, together with MoB MBene, are observed to be coexisting in a MoAlB MAB phase thin film grown at 800 °C by high-power pulsed magnetron sputtering. An overall film composition of Mo0.29Al0.33B0.38 is measured by time-of-flight elastic recoil detection analysis. The concurrent formation of these compositional defects in the MoAlB matrix occurs during the synthesis without using any chemical reagent, and their coexistence with the MAB phase is thermodynamically possible, as elucidated by density functional theory simulations. These defect phases are imaged at the atomic scale by aberration-corrected scanning transmission electron microscopy. A rough estimation of defect populations of 0.073, 0.037, 0.042 and 0.039 nm−1 for Mo3Al2B4, Mo4Al3B4, Mo6Al5B6 and Al3Mo compositional defects, respectively, is performed within the MoAlB matrix. The calculated energies of formation reveal that the Mo4Al3B4 and Mo6Al5B6 defect phases form spontaneously in the MoAlB host matrix, while the energy barrier towards the formation of the metastable Mo3Al2B4 phase is approx. 20 meV per atom. The small magnitude of this barrier is easily overcome during vapor phase condensation, and the surface diffusion of adatoms during deposition leads to local compositional variations and the coexistence of the defect phases in the host matrix. Additionally, at grain boundaries, the presence of MoB MBene is observed, with an interlayer spacing between two Mo2B2 units increasing up to ∼50% compared to the pristine MoAlB phase.
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| 9. |
- Sahu, Rajib, et al.
(author)
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Direct MoB MBene domain formation in magnetron sputtered MoAlB thin films
- 2021
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In: Nanoscale. - : Royal Society of Chemistry. - 2040-3364 .- 2040-3372. ; 13:43, s. 18077-18083
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Journal article (peer-reviewed)abstract
- Two-dimensional (2D) inorganic transition metal boride nanosheets are emerging as promising post-graphene materials in energy research due to their unique properties. State-of-the-art processing strategies are based on chemical etching of bulk material synthesized via solid-state reaction at temperatures above 1000 degrees C. Here, we report the direct formation of MoB MBene domains in a MoAlB thin film by Al deintercalation from MoAlB in the vicinity of AlOx regions. Hence, based on these results a straightforward processing pathway for the direct formation of MoB MBene-AlOx heterostructures without employing chemical etching is proposed here.
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