| 581. |
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| 582. |
- Lane, Nina J, et al.
(författare)
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First-order Raman scattering of the MAX phases Ta4AlC3, Nb4AlC3, Ti4AlN3, and Ta2AlC
- 2012
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Ingår i: Journal of Raman Spectroscopy. - : John Wiley and Sons. - 0377-0486 .- 1097-4555. ; 43:7, s. 954-958
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Tidskriftsartikel (refereegranskat)abstract
- Herein, we report on the Raman spectra of the following ternary hexagonal carbides and nitrides (MAX phases): Ta4AlC3, Ta2AlC and Ti4AlN3. We also present the Raman-active modes of a- and beta-Ta4AlC3, Nb4AlC3 and Ti4AlN3, also referred to as the 413 MAX phases as predicted from first principles calculations using density functional theory. We compare the obtained experimental and calculated results with previous studies on Ta2AlC and Ti4AlN3. The vibrational behavior associated with the Raman-active modes for the 413 phases has been identified for the first time. In general, the agreement is good between theory and experiment. The experimental and calculated results indicate that the modes at low wavenumbers - dominated by the Al atoms - are a weak function of chemistry and the differences in energy can be traced to variations in the reduced mass. The modes at higher wavenumbers are dominated by the C and N atoms and show a strong dependence on the unit cell chemistry, with the TaC bond being stiffer than the NbC bond, which is in turn stiffer than TiN.
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| 583. |
- Lane, Nina J, et al.
(författare)
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First-principles study of dislocations in hcp metals through the investigation of the (11(2)over-bar1) twin boundary
- 2011
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:18, s. 184101-
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Tidskriftsartikel (refereegranskat)abstract
- Herein, we use first principles calculations to study the energy of the (11 (2) over bar1) twin boundary in Zr, Zn, Mg, Ti, and Be. This boundary is important for understanding the microyielding and damping of hexagonal close-packed metals. The (11 (2) over bar1) twin boundary is unique in that it is composed of-and can form by the glide of-basal dislocations nucleating at every c lattice parameter. The effect of the number of atoms between boundaries on the boundary energy, and the resulting lattice strains of the relaxed structures are quantified. It is shown that the energies obtained converge within 32-64 atoms/supercell. The structures with a higher second-order elastic constant term, c(44), also have higher boundary energies. It is further shown that the critical resolved shear stresses of the basal dislocations at 0 K, which make up the (11 (2) over bar1) twin, are so low as to be below the threshold of the first principles calculations.
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| 584. |
- Lane, Nina J., et al.
(författare)
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High-temperature stability of alpha-Ta(4)AlC(3)
- 2011
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Ingår i: Materials research bulletin. - : Elsevier Science B.V., Amsterdam.. - 0025-5408 .- 1873-4227. ; 46:7, s. 1088-1091
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Tidskriftsartikel (refereegranskat)abstract
- Cold-pressed alpha-Ta(4)AlC(3) powders were annealed up to 1750 degrees C to test first-principles predictions of alpha-beta phase-stability reversal at 1600 degrees C. Up to 1600 degrees C, the alpha-Ta(4)AlC(3) samples were stable with no indications of any alpha-beta transformation, as shown by the strong characteristic X-ray diffraction peaks of alpha-Ta(4)AlC(3) and the zigzag stacking observed by transmission electron microscopy. These results show that, in this experimental situation, high temperature alone is not sufficient to cause the alpha-beta transformation.
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| 585. |
- Lane, Nina J., et al.
(författare)
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Neutron diffraction measurements and first-principles study of thermal motion of atoms in select M(n+1)AX(n) and binary MX transition-metal carbide phases
- 2012
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:21
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Tidskriftsartikel (refereegranskat)abstract
- Herein, we compare the thermal vibrations of atoms in select ternary carbides with the formula M(n+1)AX(n) ("MAX phases," M = Ti, Cr; A = Al, Si, Ge; X = C, N) as determined from first-principles phonon calculations to those obtained from high-temperature neutron powder diffraction studies. The transition metal carbides TiC, TaC, and WC are also studied to test our methodology on simpler carbides. Good qualitative and quantitative agreement is found between predicted and experimental values for the binary carbides. For all the MAX phases studied-Ti3SiC2, Ti3GeC2, Ti2AlN, Cr2GeC and Ti4AlN3-density functional theory calculations predict that the A element vibrates with the highest amplitude and does so anisotropically with a higher amplitude within the basal plane, which is in line with earlier results from high-temperature neutron diffraction studies. In some cases, there are quantitative differences in the absolute values between the theoretical and experimental atomic displacement parameters (ADPs), such as reversal of anisotropy or a systematic offset of temperature-dependent ADPs. The mode-dependent Gruneisen parameters are also computed to explore the anharmonicity in the system.
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| 586. |
- Lane, Nina J., et al.
(författare)
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Structure of a new bulk Ti5Al2C3 MAX phase produced by the topotactic transformation of Ti2AlC
- 2012
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Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 32:12, s. 3485-3491
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Tidskriftsartikel (refereegranskat)abstract
- Upon annealing cold-pressed Ti2AlC,-325 mesh powders, at 1500 degrees C for 8 h in argon, the resulting partially sintered sample contained 43(+/- 2) wt.% of the layered ternary carbide Ti5Al2C3. Herein, the X-ray powder diffraction pattern of Ti5Al2C3 is reported for the first time and its structure and stoichiometry are confirmed through high-resolution transmission electron microscopy. This phase has a trigonal structure (space group P3m1) with a unit cell consisting of 3 formula units and cell parameters of a=3.064 angstrom, c = 48.23 angstrom. The lattice parameters determined through first principles calculations agree reasonably well with the experimentally determined values. At 147.1 GPa, the calculated bulk modulus falls between the bulk moduli of Ti2AlC and Ti3AlC2. The transformation from Ti2AlC to Ti5Al2C3 is topotactic.
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| 587. |
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| 588. |
- Lapauw, T., et al.
(författare)
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Synthesis of the new MAX phase Zr2AlC
- 2016
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Ingår i: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 36:8, s. 1847-1853
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Tidskriftsartikel (refereegranskat)abstract
- This study reports on the first experimental evidence of the existence of the Zr2AlC MAX phase, synthesised by means of reactive hot pressing of a ZrH2, Al and C powder mixture. The crystal structure of this compound was investigated by X-ray and neutron diffraction. The lattice parameters were determined and confirmed by high-resolution transmission electron microscopy. The effect of varying the synthesis temperature was investigated, indicating a relatively narrow temperature window for the synthesis of Zr2AlC. ZrC was always present as a secondary phase by hot pressing in the 1475-1575 degrees C range.
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| 589. |
- Lapauw, T., et al.
(författare)
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Synthesis of the novel Zr3AlC2 MAX phase
- 2016
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Ingår i: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 36:3, s. 943-947
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Tidskriftsartikel (refereegranskat)abstract
- Herein we report, for the first time, on the synthesis and structural characterization of the Zr-based MAX phase, Zr3AlC2, fabricated by reactive hot pressing of ZrH2, Al, and C powders. The crystal structure of Zr3AlC2 was determined by X-ray diffraction and high resolution transmission electron microscopy to be the hexagonal space group P63/mmc. The a and c lattice parameters are 3.33308(6)angstrom and 19.9507(3)angstrom, respectively. The samples include the secondary phases ZrC and Zr-Al intermetallics as confirmed by quantitative electron probe microanalysis. The Vickers hardness, using a force of 30 N, was measured to be 4.4 +/- 0.4 GPa. (C) 2015 Elsevier Ltd. All rights reserved.
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| 590. |
- Lauridsen, Jonas, et al.
(författare)
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AgI as a solid lubricant in electrical contacts
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- AgI coatings have been deposited by electroplating on Ag plated Cu coupons. Electron microscopy shows that the coatings consist of weakly agglomerated AgI grains. X-ray diffraction, differential scanning calorimetry, thermogravimetry and mass spectrometry show that the AgI exhibits a reversible transformation from hexagonal to cubic phase at 150 °C. AgI starts to decompose at 150 °C with an accelerating rate up to the AgI melting temperature (555 °C), where a complex-bonded hydroxide evaporates. Ag-pin-on-disk testing shows that the iodine addition to Ag decreases the friction coefficient from 1.2 to ~0.4. The contact resistance between AgI and Ag becomes less than 100 μΩ after ~500 operations as the AgI deagglomerates and Ag is exposed on the surface, and remains low during at least 10000 reciprocating operations. This makes AgI suitable as a solid lubricant in electrical contacts.
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