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1.
  • Cabioch, Thierry, et al. (författare)
  • Tailoring of the thermal expansion of Cr2(Alx,Ge1−x)C phases
  • 2013
  • Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 33:4, s. 897-904
  • Tidskriftsartikel (refereegranskat)abstract
    • We report thermal expansion coefficients of the end members and solid-solution compounds in the Cr2(Alx,Ge1−x)C system. All samples studied were essentially phase-pure Cr2AlxGe1−xC except the Cr2GeC sample, which contained a substantial fraction of Cr5Ge3Cx. X-ray diffraction performed in the 25–800 °C temperature range shows that the in-plane thermal expansion remains essentially constant at about 14 ± 1 × 10−6 K−1 irrespective of Al content. The thermal expansion of the c axis decreases monotonically from 17 ± 1 × 10−6 K−1 for Cr2GeC to ∼12 ± 1 × 10−6 K−1 with increasing Al content. At around the Cr2(Al0.75,Ge0.25)C composition, the thermal expansion coefficients along the two directions are equal; a useful property to minimize thermal residual stresses. This study thus demonstrates that a solid-solution approach is a route for tuning a physical property like the thermal expansion. For completeness, we also include a structure description of the Cr5Ge3Cx phase, which has been reported before but is not well documented. Its space group is P63/mcm and its a and c lattice parameters are 7.14 Å and 4.88 Å, respectively. We also measured the thermal expansion coefficients of the Cr5Ge3Cx phase. They are found to be 16.3 × 10−6 K−1 and 28.4 × 10−6 K−1 along the a and c axes, respectively. Thus, the thermal expansion coefficients of Cr5Ge3Cx are highly anisotropic and considerably larger than those of the Cr2(Alx,Ge1−x)C phases.
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2.
  • Anasori, Babak, et al. (författare)
  • A Tungsten-Based Nanolaminated Ternary Carbide: (W,Ti)(4)C4-x
  • 2019
  • Ingår i: Inorganic Chemistry. - : AMER CHEMICAL SOC. - 0020-1669 .- 1520-510X. ; 58:2, s. 1100-1106
  • Tidskriftsartikel (refereegranskat)abstract
    • Nanolamellar transition metal carbides are gaining increasing interests because of the recent developments of their twodimensional (2D) derivatives and promising performance for a variety of applications from energy storage, catalysis to transparent conductive coatings, and medicine. To develop more novel 2D materials, new nanolaminated structures are needed. Here we report on a tungsten based nanolaminated ternary phase, (W,Ti)(4)C4-x, synthesized by an Al catalyzed reaction of W, Ti, and C powders at 1600 degrees C for 4 h, under flowing argon. X-ray and neutron diffraction, along with Z-contrast scanning transmission electron microscopy, were used to determine the atomic structure, ordering, and occupancies. This phase has a layered hexagonal structure (P6(3)/mmc) with lattice parameters, a = 3.00880(7) angstrom, and c = 19.5633(6) angstrom and a nominal chemistry of (W,Ti)(4)C4-x (actual chemistry, W2.1(1)Ti1.6(1)C2.6(1)). The structure is comprised of layers of pure W that are also twin planes with two adjacent atomic layers of mixed W and Ti, on either side. The use of Al as a catalyst for synthesizing otherwise difficult to make phases, could in turn lead to the discovery of a large family of nonstoichiometric ternary transition metal carbides, synthesized at relatively low temperatures and shorter times.
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3.
  • Anasori, Babak, et al. (författare)
  • Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3
  • 2015
  • Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 118:9, s. 094304-
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein, we report on the phase stabilities and crystal structures of two newly discovered ordered, quaternary MAX phases-Mo2TiAlC2 and Mo2Ti2AlC3-synthesized by mixing and heating different elemental powder mixtures of mMo:(3-m) Ti:1.1Al:2C with 1.5 less than= m less than= 2.2 and 2Mo: 2Ti:1.1Al:2.7C to 1600 degrees C for 4 h under Ar flow. In general, for m greater than= 2 an ordered 312 phase, (Mo2Ti) AlC2, was the majority phase; for mless than 2, an ordered 413 phase (Mo2Ti2)AlC3, was the major product. The actual chemistries determined from X-ray photoelectron spectroscopy (XPS) are Mo2TiAlC1.7 and Mo2Ti1.9Al0.9C2.5, respectively. High resolution scanning transmission microscopy, XPS and Rietveld analysis of powder X-ray diffraction confirmed the general ordered stacking sequence to be Mo-Ti-Mo-Al-Mo-Ti-Mo for Mo2TiAlC2 and Mo-Ti-Ti-Mo-Al-Mo-Ti-Ti-Mo for Mo2Ti2AlC3, with the carbon atoms occupying the octahedral sites between the transition metal layers. Consistent with the experimental results, the theoretical calculations clearly show that M layer ordering is mostly driven by the high penalty paid in energy by having the Mo atoms surrounded by C in a face-centered configuration, i.e., in the center of the Mn+1Xn blocks. At 331 GPa and 367 GPa, respectively, the Youngs moduli of the ordered Mo2TiAlC2 and Mo2Ti2AlC3 are predicted to be higher than those calculated for their ternary end members. Like most other MAX phases, because of the high density of states at the Fermi level, the resistivity measurement over 300 to 10K for both phases showed metallic behavior. (C) 2015 AIP Publishing LLC.
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4.
  • Anasori, Babak, et al. (författare)
  • Mo2TiAlC2: A new ordered layered ternary carbide
  • 2015
  • Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 101, s. 5-7
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein we report on the synthesis of a new layered ternary carbide, Mo2TiAlC2, that was synthesized by heating an elemental mixture at 1600 degrees C for 4 h under an Ar flow. Its hexagonal, a and c lattice parameters were calculated via Rietveld analysis of powder X-ray diffraction patterns to be, respectively, 2.997 angstrom and 18.661 angstrom. High-resolution scanning transmission electron microscopy showed that this phase is ordered, with Ti layers sandwiched between two Mo layers in a M(3)AX(2) type ternary carbide structure. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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5.
  • Anasori, Babak, et al. (författare)
  • Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)
  • 2015
  • Ingår i: ACS Nano. - : AMER CHEMICAL SOC. - 1936-0851 .- 1936-086X. ; 9:10, s. 9507-9516
  • Tidskriftsartikel (refereegranskat)abstract
    • The higher the chemical diversity and structural complexity of two-dimensional (2D) materials, the higher the likelihood they possess unique and useful properties. Herein, density functional theory (DFT) is used to predict the existence of two new families of 2D ordered, carbides (MXenes), MM-2 C-2 and MM-2 C-2(3), where M and M are two different early transition metals. In these solids, M layers sandwich M" carbide layers. By synthesizing Mo2TiC2Tx, Mo2Ti2C3Tx, and Cr2TiC2Tx (where T is a surface termination), we validated the DFT predictions. Since the Mo and Cr atoms are on the outside, they control the 2D flakes chemical and electrochemical properties. The latter was proven by showing quite different electrochemical behavior of Mo2TiC2Tx and Ti3C2Tx. This work further expands the family of 2D materials, offering additional choices of structures, chemistries, and ultimately useful properties.
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6.
  • Badr, Hussein O., et al. (författare)
  • Bottom-up, scalable synthesis of anatase nanofilament-based two-dimensional titanium carbo-oxide flakes
  • 2022
  • Ingår i: Materials Today. - : ELSEVIER SCI LTD. - 1369-7021 .- 1873-4103. ; 54
  • Tidskriftsartikel (refereegranskat)abstract
    • Two-dimensional (2D) materials offer advantages that their 3D counterparts do not. The conventional method for the bulk synthesis of 2D materials has predominantly been through etching layered solids. Herein, we convert - through a bottom-up approach - 10 binary and ternary titanium carbides, nitrides, borides, phosphides, and silicides into 2D flakes by immersing them in a tetramethylammonium hydroxide solution at temperatures in the 25-85 degrees C range. Based on X-ray diffraction, density functional theory, X-ray photoelectron, electron energy loss, Raman, X-ray absorption near edge structure spectroscopies, transmission and scanning electron microscope images and selected area diffraction, we conclude that the resulting flakes are carbon containing anatase-based layers that are, in turn, comprised of approximate to 6 x 10 angstrom(2) nanofilaments in cross-section some of which are few microns long. Electrodes made from some of these films performed well in lithium-ion and lithium-sulphur systems. These materials also reduce the viability of cancer cells thus showing potential in biomedical applications. Synthesizing 2D materials, at near ambient conditions, with non-layered, inexpensive, green precursors (e.g., TiC) is paradigm shifting and will undoubtedly open new and exciting avenues of research and applications.
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7.
  • Bentzel, Grady W., et al. (författare)
  • High-Temperature Neutron Diffraction, Raman Spectroscopy, and First-Principles Calculations of Ti3SnC2 and Ti2SnC
  • 2016
  • Ingår i: Journal of The American Ceramic Society. - : WILEY-BLACKWELL. - 0002-7820 .- 1551-2916. ; 99:7, s. 2233-2242
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein, we report-for the first time-on the additive-free bulk synthesis of Ti3SnC2. A detailed experimental study of the structure of the latter together with a secondary phase, Ti2SnC, is presented through the use of X-ray diffraction (XRD), and high-resolution transmission microscopy (HRTEM). A previous sample of Ti3SnC2, made using Fe as an additive and Ti2SnC as a secondary phase, was studied by high-temperature neutron diffraction (HTND) and XRD. The room-temperature crystallographic parameters of the two MAX phases in the two samples are quite similar. Based on Rietveld analysis of the HTND data, the average linear thermal expansion coefficients of Ti3SnC2 in the a and c directions were found to be 8.5 (2).10(-6) K-1 and 8.9 (1) . 10(-6) K-1, respectively. The respective values for the Ti2SnC phase are 10.1 (3) . 10(-6) K-1 and 10.8 (6) . 10(-6) K-1. Unlike other MAX phases, the atomic displacement parameters of the Sn atoms in Ti3SnC2 are comparable to those of the Ti and C atoms. When the predictions of the atomic displacement parameters obtained from density functional theory are compared to the experimental results, good quantitative agreement is found for the Sn atoms. In the case of the Ti and C atoms, the agreement is more qualitative. We also used first principles to calculate the elastic properties of both Ti2SnC and Ti3SnC2 and their Raman active modes. The latter are compared to experiment and the agreement was found to be good.
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8.
  • Byeon, Ayeong, et al. (författare)
  • Two-Dimensional Titanium Carbide MXene As a Cathode Material for Hybrid Magnesium/Lithium-Ion Batteries
  • 2017
  • Ingår i: ACS Applied Materials and Interfaces. - : AMER CHEMICAL SOC. - 1944-8244 .- 1944-8252. ; 9:5, s. 4296-4300
  • Tidskriftsartikel (refereegranskat)abstract
    • As an alternative to pure lithium-ion, Lit, systems, a hybrid magnesium, Mg2+, and Li+ battery can potentially combine the high capacity, high voltage, and fast Li+ intercalation of Li-ion battery cathodes and the high capacity, low cost, and dendrite-free Mg metal anodes. Herein, we report on the use of two-dimensional titanium carbide, Ti3C2Tx (MXene), as a cathode in hybrid Mg2+/Li+ batteries, coupled with a Mg metal anode. Free-standing and flexible Ti3C2Tx/carbon nanotube composite "paper" delivered-,100 mAh at 0.1 C and similar to 50 mAh g(-1) at 10 C. At 1 C the capacity was maintained for amp;gt;500 cycles at 80 mAh g(-1). The Mo2CTx MXene also demonstrated good performance as a cathode material in this hybrid battery. Considering the variety of available MXenes, this work opens the door for exploring a new large family of 2D materials with high electrical conductivity and large intercalation capacity as cathodes for hybrid Mg2+/Li+ batteries.
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9.
  • Dahlqvist, Martin, et al. (författare)
  • MAX phases – Past, present, and future
  • 2023
  • Ingår i: Materials Today. - : Elsevier B.V.. - 1369-7021 .- 1873-4103.
  • Tidskriftsartikel (refereegranskat)abstract
    • The MAX phases are a class of nanolaminated materials composed of an early transition-metal (M), an A-group element (A) and C, N, B and/or P (X). Progress in MAX phase research in recent years has increased their number from the original 50 or so, to more than 300 phases. Since half of the 342 MAX phases have been discovered after 2018, an overview of the progress made in the field is timely. Currently, 28 M elements, 28 A elements, and 6 X elements have been incorporated in the MAX phases, alloys included. We further categorize MAX phases based on the synthesis route used to make them; if made via a one-step approach in bottom-up synthesis or formed through elemental replacement reactions in top-down synthesis. This classification is also correlated to theoretical phase stability predictions, that in turn, can be used to identify novel synthesizable MAX phase compositions as well as to suggest suitable synthesis routes. Furthermore, using phase stability predictions we identify 182 new theoretically stable MAX phases awaiting experimental confirmation. Notably, as MAX phases are precursors for MXenes, the dramatically increased interest in the latter for a large host of potential applications renders the former even more valuable. © 2023 The Author(s)
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10.
  • Eklund, Per, et al. (författare)
  • Homoepitaxial growth of Ti-Si-C MAX-phase thin films on bulk Ti3SiC2 substrates
  • 2007
  • Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 304:1, s. 264-269
  • Tidskriftsartikel (refereegranskat)abstract
    • Ti3SiC2 films were grown on polycrystalline Ti3SiC2 bulk substrates using DC magnetron sputtering. The crystallographic orientation of the film grains is shown to be determined by the respective substrate-grain orientation through homoepitaxial MAX-phase growth. For a film composition close to Ti:Si:C=3:1:2, the films predominantly consist of MAX phases, both Ti3SiC2 and the metastable Ti4SiC3. Lower Si content resulted in growth of TiC with Ti3SiC2 as a minority phase. Thus, MAX-phase heterostructures with preferred crystallographic relationships can also be realized.
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11.
  • ElMeligy, Tarek Aly, et al. (författare)
  • Synthesis, characterization, properties, first principles calculations, and X-ray photoelectron spectroscopy of bulk Mn5SiB2 and Fe5SiB2 ternary borides
  • 2021
  • Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 888
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein, we synthesize fully dense, bulk, predominantly single-phase, polycrystalline samples of the layered ternary transition metal borides Mn5SiB2 and Fe5SiB2 by reactively hot-pressing Mn, Fe, FeB, Si, and B powders. The atomic structures were imaged using high-resolution scanning transmission electron mi-croscopy and revealed high-crystal quality. Elongated striped defects, confined below the nanometer in width, were observed. Selected area electron diffraction further accentuates the high-crystal quality by discrete spots of pattern, that is expected from a tetragonal crystal structure along the [001] zone axis. With Vickers hardness values of 12.1 +/- 0.4 GPa, and 12.7 +/- 0.1 GPa, for Mn5SiB2 and Fe5SiB2 respectively, these borides are relatively soft. The room temperature electrical resistivities were 1.5 +/- 0.1 and 1.2 +/- 0.1 mu Omega m, for Mn5SiB2 and Fe5SiB2, respectively. The binding energies of the Mn, Fe and Si measured by X-ray photoelectron spectroscopy bolster the idea that the bonds are quite metallic in character. Density functional theory (DFT) calculations confirm that the ground states of both compounds are ferromagnetic as observed experimentally. We also use DFT to predict the elastic and electronic properties. In both compounds, the density of states at the Fermi level are dominated by the d-orbitals of the transition metals. Neither material was readily machinable with conventional tooling, but is so with sharp cobalt steel bits or electro-discharge machining (EDM). (C) 2021 Elsevier B.V. All rights reserved.
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12.
  • Ghidiu, Michael, et al. (författare)
  • Alkylammonium Cation Intercalation into Ti3C2 (MXene): Effects on Properties and Ion-Exchange Capacity Estimation
  • 2017
  • Ingår i: Chemistry of Materials. - : AMER CHEMICAL SOC. - 0897-4756 .- 1520-5002. ; 29:3, s. 1099-1106
  • Tidskriftsartikel (refereegranskat)abstract
    • Ti3C2Tx MXene intercalated with Li+ ions was produced and ion-exchanged with a series of trimethylalkylammonium (AA) cations of increasing alkyl chain length. A discontinuous expansion in the MXene layer spacing was observed, attributed to complete packing of the interlayer space at a critical chain length. The latter was used to estimate the number of cations per Ti3C2 formula unit, which was found to be in good agreement with a similar quantification obtained from X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and elemental analysis. The system was also modeled using density functional theory and molecular dynamics, arriving at cation concentrations in the same range. The intercalated AA cations led to tunable increases in resistivity of the normally highly electrically conductive MXene and were investigated as interlayer pillars in electrochemical capacitors.
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13.
  • Haddad, Noël, et al. (författare)
  • Dielectric properties of Ti2AlC and Ti2AlN MAX phases : The conductivity anisotropy
  • 2008
  • Ingår i: Journal of Applied Physics. - : AIP Publishing. - 0021-8979 .- 1089-7550. ; 104:2
  • Tidskriftsartikel (refereegranskat)abstract
    • The optical properties of Ti2AlN and Ti2AlC were determined in the 2-80 eV energy range by electron energy loss spectroscopy and in the visible-ultraviolet range, from 1.6 to 5.5 eV, by spectroscopic ellipsometry. Both experimental techniques are angular resolved and in very good agreement over their overlapping energy range. We observe a dependence of the dielectric function as a function of the crystallographic orientation of the crystals. In particular, we notice a shift of the energy position of the plasmon absorption of Ti2AlC with respect to Ti2AlN. Moreover, a drastic change is also observed in the shape of the dielectric function as a function of the composition (or valence electron concentration). The dielectric functions are fitted to an empirical semiclassic Drude-Lorentz model to obtain physical parameters such as the relaxation times. These microscopic parameters are then used in a macroscopic model to yield the transport properties such as the static conductivity as function of the crystal orientation. Ti 2AlN is found to be a better conductor than Ti2AlC in all orientations, which is consistent with experimental measurements. A comparison of the electrical and optical properties of these two compounds is made in terms of different electronic properties and interband-intraband transitions deduced from our model. © 2008 American Institute of Physics.
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14.
  • Halim, Joseph, et al. (författare)
  • Electronic and optical characterization of 2D Ti2C and Nb2C (MXene) thin films
  • 2019
  • Ingår i: Journal of Physics. - : IOP PUBLISHING LTD. - 0953-8984 .- 1361-648X. ; 31:16
  • Tidskriftsartikel (refereegranskat)abstract
    • Two-dimensional (2D) transition metal carbides and/or nitrides (MXenes) are a new class of 2D materials, with extensive opportunities for property tailoring due to the numerous possibilities for varying chemistries and surface terminations. Here, Ti2AlC and Nb2AlC MAX phase epitaxial thin films were deposited on sapphire substrates by physical vapor deposition. The films were then etched in LiF/HCl solutions, yielding Li-intercalated, 2D Ti2CTz and Nb2CTz films, whose terminations, transport and optical properties were characterized. The former exhibits metallic conductivity, with weak localization below 50 K. In contrast, the Nb-based film exhibits an increase in resistivity with decreasing temperature from RT down to 40K consistent with variable range hopping transport. The optical properties of both films were determined from spectroscopic ellipsometry in the 0.75 to 3.50 eV range. The results for Ti2Clz films confirm the metallic behavior. In contrast, no evidence of metallic behavior is observed for the Nb2CT(z) film. The present work therefore demonstrates that one fruitful approach to alter the electronic and optical properties of MXenes is to change the nature of the transition metal.
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15.
  • Halim, Joseph, et al. (författare)
  • Structure and thermal expansion of (Cr-x,V1-x)(n+1)AlCn phases measured by X-ray diffraction
  • 2017
  • Ingår i: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 37:1, s. 15-21
  • Tidskriftsartikel (refereegranskat)abstract
    • MAX phases in the (Crx,V1-x)(n+1)AlCn,system were synthesized by reactive sintering or hot isostatic pressing of elemental powders at temperatures between 1400 degrees C and 1600 degrees C. For n=1, a complete range (0 amp;lt;= x amp;lt;= 1) of solid solutions was found; for n=2 and 3 the solubility ranges were 0.25 amp;lt;= x amp;lt;= 0.75 and 0 amp;lt;= x amp;lt;= 0.5, respectively. Powder X-ray diffraction revealed that the lattice parameters of all (Cr-x,V1-x)(n+1)AlCn solid solutions followed Vegards law. The thermal expansion coefficients of the various compounds were determined from Rietveld refinements of X-Ray patterns obtained at temperatures between ambient and 800 degrees C. For the n=1 and 3 phases the thermal expansion coefficients were almost isotropic; those for the n=2, however, were quite anisotropic with the expansion along the a-axis being significantly larger than along the c-axis. As a general trend, vanadium rich compounds have smaller thermal expansion coefficients than their Cr-rich counterparts. (C) 2016 Elsevier Ltd. All rights reserved.
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16.
  • Halim, Joseph, et al. (författare)
  • Synthesis and Characterization of 2D Molybdenum Carbide (MXene)
  • 2016
  • Ingår i: Advanced Functional Materials. - : WILEY-V C H VERLAG GMBH. - 1616-301X .- 1616-3028. ; 26:18, s. 3118-3127
  • Tidskriftsartikel (refereegranskat)abstract
    • Large scale synthesis and delamination of 2D Mo2CTx (where T is a surface termination group) has been achieved by selectively etching gallium from the recently discovered nanolaminated, ternary transition metal carbide Mo2Ga2C. Different synthesis and delamination routes result in different flake morphologies. The resistivity of free-standing Mo2CTx films increases by an order of magnitude as the temperature is reduced from 300 to 10 K, suggesting semiconductor-like behavior of this MXene, in contrast to Ti3C2Tx which exhibits metallic behavior. At 10 K, the magnetoresistance is positive. Additionally, changes in electronic transport are observed upon annealing of the films. When 2 mu m thick films are tested as electrodes in supercapacitors, capacitances as high as 700 F cm(-3) in a 1 M sulfuric acid electrolyte and high capacity retention for at least 10,000 cycles at 10 A g(-1) are obtained. Free-standing Mo2CTx films, with approximate to 8 wt% carbon nanotubes, perform well when tested as an electrode material for Li-ions, especially at high rates. At 20 and 131 C cycling rates, stable reversible capacities of 250 and 76 mAh g(-1), respectively, are achieved for over 1000 cycles.
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17.
  • Halim, Joseph, 1985- (författare)
  • Synthesis and transport properties of 2D transition metal carbides (MXenes)
  • 2018
  • Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
    • Since the isolation and characterization of graphene, there has been a growing interest in 2D materials owing to their unique properties compared to their 3D counterparts. Recently, a family of 2D materials of early transition metal carbides and nitrides, labelled MXenes, has been discovered (Ti2CTz, Ti3C2Tz, Mo2TiC2Tz, Ti3CNTz, Ta4C3Tz, Ti4N3Tz among many others), where T stands for surface-terminating groups (O, OH, and F). MXenes are mostly produced by selectively etching A layers (where A stands for group A elements, mostly groups 13 and 14) from the MAX phases. The latter are a family of layered ternary carbides and/or nitrides and have a general formula of Mn+1AXn (n = 1-3), where M is a transition metal and X is carbon and/or nitrogen. The produced MXenes have a conductive carbide core and a non-conductive O-, OH- and/or F-terminated surface, which allows them to work as electrodes for energy storage applications, such as Li-ion batteries and supercapacitors.Prior to this work, MXenes were produced in the form of flakes of lateral dimension of about 1 to 2 microns; such dimensions and form are not suitable for electronic characterization and applications. I have synthesized various MXenes (Ti3C2Tz, Ti2CTz and Nb2CTz) as epitaxial thin films, a more suitable form for electronic and photonic applications. These films were produced by HF, NH4HF2 or LiF + HCl etching of magnetron sputtered epitaxial Ti3AlC2, Ti2AlC, and Nb2AlC thin films. For transport properties of the Ti-based MXenes, Ti2CTz and Ti3C2Tz, changing n from 1 to 2 resulted in an increase in conductivity but had no effect on the transport mechanism (i.e. both Ti3C2Tx and Ti2CTx were metallic). In order to examine whether the electronic properties of MXenes differ when going from a few layers to a single flake, similar to graphene, the electrical characterization of a single Ti3C2Tz flake with a lateral size of about 10 μm was performed. These measurements, the first for MXene, demonstrated its metallic nature, along with determining the nature of the charge carriers and their mobility. This indicates that Ti3C2Tz is inherently of 2D nature independent of the number of stacked layers, unlike graphene, where the electronic properties change based on the number of stacked layers.Changing the transition metal from Ti to Nb, viz. comparing Ti2CTz and Nb2CTz thin films, the electronic properties and electronic conduction mechanism differ. Ti2CTz showed metallic-like behavior (resistivity increases with increasing temperature) unlike Nb2CTz where the conduction occurs via variable range hopping mechanism (VRH) - where resistivity decreases with increasing temperature.Furthermore, these studies show the synthesis of pure Mo2CTz in the form of single flakes and freestanding films made by filtering Mo2CTz colloidal suspensions. Electronic characterization of free-standing films made from delaminated Mo2CTz flakes was investigated, showing that a VRH mechanism prevails at low temperatures (7 to ≈ 60 K). Upon vacuum annealing, the room temperature, RT, conductivity of Mo2CTx increased by two orders of magnitude. The conduction mechanism was concluded to be VRH most likely dominated by hopping within each flake.Other Mo-based MXenes, Mo2TiC2Tz and Mo2Ti2C3Tz, showed VRH mechanism at low temperature. However, at higher temperatures up to RT, the transport mechanism was not clearly understood. Therefore, a part of this thesis was dedicated to further investigating the transport properties of Mo-based MXenes. This includes Mo2CTz, out-of-plane ordered Mo2TiC2Tz and Mo2Ti2C3Tz, and vacancy ordered Mo1.33CTz. Magneto-transport of free-standing thin films of the Mo-based MXenes were studied, showing that all Mo-based MXenes have two transport regimes: a VRH mechanism at lower temperatures and a thermally activated process at higher temperatures. All Mo-based MXenes except Mo1.33CTz show that the electrical transport is dominated by inter-flake transfer. As for Mo1.33CTz, the primary electrical transport mechanism is more likely to be intra-flake.The synthesis of vacancy ordered MXenes (Mo1.33CTz and W1.33CTz) raised the question of possible introduction of vacancies in all MXenes. Vacancy ordered MXenes are produced by selective etching of Al and (Sc or Y) atoms from the parent 3D MAX phases, such as (Mo2/3Sc1/3)2AlC, with in-plane chemical ordering of Mo and Sc. However, not all quaternary parent MAX phases form the in-plane chemical ordering of the two M metals; thus the synthesis of the vacancy-ordered MXenes is restricted to a very limited number of MAX phases. I present a new method to obtain MXene flakes with disordered vacancies that may be generalized to all quaternary MAX phases. As proof of concept, I chose Nb-C MXene, as this 2D material has shown promise in several applications, including energy storage, photothermal cell ablation and photocatalysts for hydrogen evolution. Starting from synthetizing (Nb2/3Sc1/3)2AlC quaternary solid solution and etching both the Sc and Al atoms resulted in Nb1.33C material with a large number of vacancies and vacancy clusters. This method may be applicable to other quaternary or higher MAX phases wherein one of the transition metals is more reactive than the other, and it could be of vital importance in applications such as catalysis and energy storage.  
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18.
  • Halim, Joseph, et al. (författare)
  • Tailored synthesis approach of (Mo2/3Y1/3)(2)AlC i-MAX and its two-dimensional derivative Mo1.33CTz MXene: enhancing the yield, quality, and performance in supercapacitor applications
  • 2021
  • Ingår i: Nanoscale. - : ROYAL SOC CHEMISTRY. - 2040-3364 .- 2040-3372. ; 13:1, s. 311-319
  • Tidskriftsartikel (refereegranskat)abstract
    • A vacancy-ordered MXene, Mo1.33CTz, obtained from the selective etching of Al and Sc from the parent i-MAX phase (Mo2/3Sc1/3)(2)AlC has previously shown excellent properties for supercapacitor applications. Attempts to synthesize the same MXene from another precursor, (Mo2/3Y1/3)(2)AlC, have not been able to match its forerunner. Herein, we show that the use of an AlY2.3 alloy instead of elemental Al and Y for the synthesis of (Mo2/3Y1/3)(2)AlC i-MAX, results in a close to 70% increase in sample purity due to the suppression of the main secondary phase, Mo3Al2C. Furthermore, through a modified etching procedure, we obtain a Mo1.33CTz MXene of high structural quality and improve the yield by a factor of 6 compared to our previous efforts. Free-standing films show high volumetric (1308 F cm(-3)) and gravimetric (436 F g(-1)) capacitances and a high stability (98% retention) at the level of, or even beyond, those reported for the Mo1.33CTz MXene produced from the Sc-based i-MAX. These results are of importance for the realization of high quality MXenes through use of more abundant elements (Y vs. Sc), while also reducing waste (impurity) material and facilitating the synthesis of a high-performance material for applications.
  •  
19.
  • Halim, Joseph, et al. (författare)
  • Transparent Conductive Two-Dimensional Titanium Carbide Epitaxial Thin Films
  • 2014
  • Ingår i: Chemistry of Materials. - : American Chemical Society. - 0897-4756 .- 1520-5002. ; 26:7, s. 2374-2381
  • Tidskriftsartikel (refereegranskat)abstract
    • Since the discovery of graphene, the quest for two-dimensional (2D) materials has intensified greatly. Recently, a new family of 2D transition metal carbides and carbonitrides (MXenes) was discovered that is both conducting and hydrophilic, an uncommon combination. To date MXenes have been produced as powders, flakes, and colloidal solutions. Herein, we report on the fabrication of similar to 1 x 1 cm(2) Ti3C2 films by selective etching of Al, from sputter-deposited epitaxial Ti3AlC2 films, in aqueous HF or NH4HF2. Films that were about 19 nm thick, etched with NH4HF2, transmit similar to 90% of the light in the visible-to-infrared range and exhibit metallic conductivity down to similar to 100 K. Below 100 K, the films resistivity increases with decreasing temperature and they exhibit negative magnetoresistance-both observations consistent with a weak localization phenomenon characteristic of many 2D defective solids. This advance opens the door for the use of MXenes in electronic, photonic, and sensing applications.
  •  
20.
  • Halim, Joseph, et al. (författare)
  • Variable range hopping and thermally activated transport in molybdenum-based MXenes
  • 2018
  • Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 98:10
  • Tidskriftsartikel (refereegranskat)abstract
    • The magnetotransport of freestanding, vacuum filtered, thin films of Mo2CTz, Mo1.33CTz, Mo2TiC2Tz, and Mo2Ti2C3Tz was measured in the 10-300-K temperature (T) range. Some of the films were annealed before measuring their transport properties. Analysis of the results suggest that-with the exception of the heavily defective Mo1.33CTz composition-in the 10- to 200-K temperature regime, variable range hopping between individual MXene sheets is the operative conduction mechanism. For Mo1.33CTz it is more likely that variable range hopping within individual flakes is rate limiting. At higher temperatures, a thermally activated process emerges in all cases. It follows that improved fabrication processes should lead to considerable improvements in the electrical transport of Mo-based MXenes.
  •  
21.
  • Halim, Joseph, et al. (författare)
  • X-ray Photoelectron Spectroscopy Characterization of Two-Dimensional Titanium Metal Carbides (MXenes)
  • 2014
  • Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
    • Herein, we report X-ray Photoelectron Spectroscopy (XPS) analysis for cold pressed exfoliated 2D nanocrystals of transition metal carbides, MXenes. MXenes are a recently discovered family of 2D materials produced by selective chemical etching of the A element from MAX phases which are ternary metal carbides and nitrides. The latter has the formula of Mn+1AXn, where M is an early transition metal, A is an A-group element, and X is C and/or N. This study is a comparison between two MXenes, Ti3C2Tx and Ti2CTx, where Tx stands for surface termination groups such as –O, –OH, and –F. Ti3C2Tx and Ti2CTx were prepared by immersion of Ti3AlC2 and Ti2AlC powders in 50% conc. HF. A thorough XPS analysis was performed through peak fitting of high resolution XPS spectra and valence band, VB, spectra analysis. The effect of Ar sputtering as well as the number of layers n was the primarily interest of this study. According to the peak fitting analysis, both phases contain the following species, Ti–C, C–C, Ti–F, Ti–O and Ti–OH resulting in the following chemical formulas: Ti3C2(OH)x(O)y(F)z and Ti2C(OH)x(O)y(F)z. Comparing the VB spectra with the DOS calculations show the valance band spectra is actually a mixture of MXene with various terminations of OH, O and F. Before Ar+ sputtering both phases show a large percentage of fluorinated-TiO2 which is due to MXene surface oxidation as well as CHx, C-O and COO groups arising from either surface contaminations or due to drying the etched powders in ethanol after washing the powder of the HF acid. According to the VB spectra, it is shown that the fluorinated TiO2 is actually a mixture of anatase and rutile. The number of layers, n, also plays a role; the lower n, the more the MXene is prone to oxidation.
  •  
22.
  • Halim, Joseph, 1985-, et al. (författare)
  • X-ray photoelectron spectroscopy of select multi-layered transition metal carbides (MXenes)
  • 2016
  • Ingår i: Applied Surface Science. - : ELSEVIER SCIENCE BV. - 0169-4332 .- 1873-5584. ; 362, s. 406-417
  • Tidskriftsartikel (refereegranskat)abstract
    • In this work, a detailed high resolution X-ray photoelectron spectroscopy (XPS) analysis is presented for select MXenes a recently discovered family of two-dimensional (2D) carbides and carbonitrides. Given their 2D nature, understanding their surface chemistry is paramount. Herein we identify and quantify the surface groups present before, and after, sputter-cleaning as well as freshly prepared vs. aged multi layered cold pressed discs. The nominal compositions of the MXenes studied here are Ti-3 C2Tx,Ti3CNTx, Nb2CTx and Nb4C3Tx where T represents surface groups that this work attempts to quantify. In all the cases, the presence of three surface terminations, O, OH and F, in addition to OH-terminations relatively strongly bonded to H2O molecules, was confirmed. From XPS peak fits, it was possible to establish the average sum of the negative charges of the terminations for the aforementioned MXenes. Based on this work, it is now possible to quantify the nature of the surface terminations. This information can, in turn, be used to better design and tailor these novel 2D materials for various applications. Published by Elsevier B.V.
  •  
23.
  • Halim, Joseph, et al. (författare)
  • XPS of cold pressed multilayered and freestanding delaminated 2D thin films of Mo2TiC2Tz and Mo2Ti2C3Tz (MXenes)
  • 2019
  • Ingår i: Applied Surface Science. - : ELSEVIER. - 0169-4332 .- 1873-5584. ; 494, s. 1138-1147
  • Tidskriftsartikel (refereegranskat)abstract
    • MXenes, transition metal carbides and/or nitrides, that are synthesized from the top down by etching of their 3D parent layered solids, the MAX phases, are the latest family of the two-dimensional solids discovered. When the A layers - mostly Al - are etched they are replaced by surface terminations, Tz mainly comprised of O-, OH- and F-terminations. One of the few techniques capable of quantifying these surface terminations is X-ray photo-electron spectroscopy, XPS. Herein, we undertook an XPS study of the out-of-plane ordered MXenes, Mo2TiC2Tz and Mo2Ti2C3Tz, in both multilayered, ML, cold pressed and delaminated thin film forms. The harsh conditions needed to etch these MAX phases into MLs, results in their partial oxidation. The hydroxide used to delaminate the MLs results in further oxidation and a reduction in the F-content. In all cases, etching resulted in a decrease in the Ti to Mo ratio implying that the former atoms are selectively etched. In all but the ML Mo2TiC2Tz case, the Mo to C ratio was also reduced. It follows that the loss of Ti also results in the loss of C atoms. Again with the exception of the ML Mo2Ti2C3Tz case, the number of termination moles per formula unit, z, was amp;gt; 2, which is energetically unfavorable and thus unlikely. However, if one assumes that not all of the O signal is coming from terminations but rather from O atoms that replace C atoms in the MX blocks, then z similar to 2. This work is an important step in quantifying and understanding the effects of etching on terminations and structure in these Mo and Ti-based MXenes.
  •  
24.
  • Hanner, Luke A., et al. (författare)
  • Synthesis, characterization and first principle modelling of the MAB phase solid solutions: (Mn1-xCrx)(2)AlB2 and (Mn1-xCrx)(3)AlB4
  • 2021
  • Ingår i: Materials Research Letters. - : Taylor & Francis. - 2166-3831. ; 9:2, s. 112-118
  • Tidskriftsartikel (refereegranskat)abstract
    • The MAB phases are a family of layered ternary transition metal borides, with atomically laminated crystal structures comprised of transition metal boride (M-B) layers interleaved by single, or double, Al (A) layers. Herein, density functional theory is implemented to evaluate the thermodynamic stability of disordered (Mn1-xCrx)(2)AlB2, and disordered and ordered (Mn1-xCrx)(3)AlB4 quaternaries. The (Mn1-xCrx)(2)AlB2 solid solutions were synthesized over the entire range of substitution. A (Mn1-xCrx)(3)AlB4 solid solution was produced, on the base of Cr3AlB4, to form (Mn0.33Cr0.66)(3)AlB4. Powder X-ray diffraction shows lattice parameter shifts and unit cell expansions indicative of successful solid solution formations.
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25.
  • Hu, C., et al. (författare)
  • Mo2Ga2C: a new ternary nanolaminated carbide
  • 2015
  • Ingår i: Chemical Communications. - : Royal Society of Chemistry. - 1359-7345 .- 1364-548X. ; 51:30, s. 6560-6563
  • Tidskriftsartikel (refereegranskat)abstract
    • We report the discovery of a new hexagonal Mo2Ga2C phase, wherein two Ga layers - instead of one - are stacked in a simple hexagonal arrangement in between Mo2C layers. It is reasonable to assume this compound is the first of a larger family.
  •  
26.
  • Hu, Yong-Jie, et al. (författare)
  • Structural and electronic properties of two-dimensional titanium carbo-oxides
  • 2023
  • Ingår i: 2D Materials. - : IOP Publishing Ltd. - 2053-1583. ; 10:1
  • Tidskriftsartikel (refereegranskat)abstract
    • This work was inspired by new experimental findings where we discovered a two-dimensional (2D) material comprised of titanium-oxide-based one-dimensional (1D) sub-nanometer filaments. Preliminary results suggest that the 2D material contains considerable amounts of carbon, C, in addition to titanium, Ti, and oxygen, O. The aim of this study is to investigate the low-energy, stable atomic forms of 2D titanium carbo-oxides as a function of C content. Via a combination of first-principles calculations and an effective structure sampling scheme, the stable configurations of C-substitutions are comprehensively searched by templating different 2D TiO2 polymorphs and considering a two O to one C replacement scheme. Among the searched stable configurations, a structure where the (101) planes of anatase bound the top and bottom surfaces with a chemical formula of TiC1/4O3/2 was of particularly low energy. Furthermore, the variations in the electronic band structure and chemical bonding environments caused by the high-content C substitution are investigated via additional calculations using a hybrid exchange-correlation functional.
  •  
27.
  • Intikhab, Saad, et al. (författare)
  • Stoichiometry and surface structure dependence of hydrogen evolution reaction activity and stability of MoxC MXenes
  • 2019
  • Ingår i: Journal of Catalysis. - : ACADEMIC PRESS INC ELSEVIER SCIENCE. - 0021-9517 .- 1090-2694. ; 371, s. 325-332
  • Tidskriftsartikel (refereegranskat)abstract
    • The exploration of non-precious catalysts for the hydrogen evolution reaction (HER) remains critical in the commercialization of electrochemical energy storage and conversion technologies. Two-dimensional transitional metal carbides called MXenes have been found to have great potential as electrocatalysts for HER. In this work, we synthesize two molybdenum-based MXenes: Mo1.33CTz and Mo2CTz, and measure their HER activity and operational durability. The ordered divacancies on the basal planes of Mo1.33CTz cause a marked decrease in HER activity compared to Mo2CTz. The stoichiometry and atomic surface structure of MXenes is found to be critically important for catalytic activity while having less of an impact on operational durability. This work provides insight for the development of active 2D materials, in general and MXenes in particular for HER and other technologically relevant electrochemical reactions. (C) 2019 Elsevier Inc. All rights reserved.
  •  
28.
  • Karlsson, Linda, et al. (författare)
  • Atomically Resolved Structural and Chemical Investigation of Single MXene Sheets
  • 2015
  • Ingår i: Nano letters (Print). - : American Chemical Society (ACS). - 1530-6984 .- 1530-6992. ; 15:8, s. 4955-4960
  • Tidskriftsartikel (refereegranskat)abstract
    • The properties of two-dimensional (2D) materials depend strongly on the chemical and electrochemical activity of their surfaces. MXene, one of the most recent additions to 2D materials, shows great promise as an energy storage material. In the present investigation, the chemical and structural properties of individual Ti3C2 MXene sheets with associated surface groups are investigated at the atomic level by aberration corrected STEM-EELS. The MXene sheets are shown to exhibit a nonuniform coverage of O-based surface groups which locally affect the chemistry. Additionally, native point defects which are proposed to affect the local surface chemistry, such as oxidized titanium adatoms (TiOx), are identified and found to be mobile.
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29.
  • Kota, Sankalp, et al. (författare)
  • Magnetic properties of Cr2AlB2, Cr3AlB4, and CrB powders
  • 2018
  • Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 767, s. 474-482
  • Tidskriftsartikel (refereegranskat)abstract
    • The MAB phases are ternary, atomically laminated compounds that crystallize with orthorhombic structures and consist of transition metal (M) boride sublattices interleaved with single or double layers of Al. To date, the magnetic properties of ferromagnetic Fe(2)AIB(2) have been well-studied experimentally, but those of most end member MAB phases with other transition metals remain poorly understood. Herein, predominantly single-phase Cr(3)AIB(4) and Cr(2)AIB(2) powders, with minor amounts of CrB impurities, were synthesized by heating mixtures of chromium monoboride (CrB), aluminum (Al), and boron or CrB and Al under flowing Ar. These reactants were found to substantially suppress the formation of undesired phases in the Cr-Al-B system. The magnetic properties of Cr(3)AIB(4), Cr(2)AIB(2) and CrB powders were characterized. All compounds exhibit paramagnetic behavior down to 4 K, with susceptibilities that are quite weakly temperature dependent at higher temperatures and a Curie-Weiss-like component prominent at low temperatures. Magnetization isotherms of the three compounds, showed approximately linear behavior above 5 kOe and no saturation of the magnetic moment up to 40 kOe, which is consistent with paramagnetism. The magnitude of the magnetic moments showed little variation with composition, on a per Cr atom basis. While the exact nature of the Curie-Weiss component is not entirely clear, we tentatively attribute it to dilute extrinsic impurities. (C) 2018 Elsevier B.V. All rights reserved.
  •  
30.
  • Kota, Sankalp, et al. (författare)
  • Synthesis and Characterization of an Alumina Forming Nanolaminated Boride: MoAlB
  • 2016
  • Ingår i: Scientific Reports. - : NATURE PUBLISHING GROUP. - 2045-2322. ; 6:26475
  • Tidskriftsartikel (refereegranskat)abstract
    • The MAlB phases are nanolaminated, ternary transition metal borides that consist of a transition metal boride sublattice interleaved by monolayers or bilayers of pure aluminum. However, their synthesis and properties remain largely unexplored. Herein, we synthesized dense, predominantly single-phase samples of one such compound, MoAlB, using a reactive hot pressing method. High-resolution scanning transmission electron microscopy confirmed the presence of two Al layers in between a Mo-B sublattice. Unique among the transition metal borides, MoAlB forms a dense, mostly amorphous, alumina scale when heated in air. Like other alumina formers, the oxidation kinetics follow a cubic time-dependence. At room temperature, its resistivity is low (0.36-0.49 mu Omega m) and - like a metal - drops linearly with decreasing temperatures. It is also a good thermal conductor (35 Wm(-1)K(-1) at 26 degrees C). In the 25-1300 degrees C temperature range, its thermal expansion coefficient is 9.5 x 10(-6) K-1. Preliminary results suggest the compound is stable to at least 1400 degrees C in inert atmospheres. Moderately low Vickers hardness values of 10.6 +/- 0.3 GPa, compared to other transition metal borides, and ultimate compressive strengths up to 1940 +/- 103 MPa were measured at room temperature. These results are encouraging and warrant further study of this compound for potential use at high temperatures.
  •  
31.
  • Lai, Chung-Chuan, et al. (författare)
  • Structural and chemical determination of the new nanolaminated carbide Mo2Ga2C from first principles and materials analysis
  • 2015
  • Ingår i: Acta Materialia. - : PERGAMON-ELSEVIER SCIENCE LTD. - 1359-6454 .- 1873-2453. ; 99, s. 157-164
  • Tidskriftsartikel (refereegranskat)abstract
    • Following our recent discovery of a new nanolaminated carbide, Mo2Ga2C, we herein present a detailed structural and chemical analysis of this phase based on ab initio calculations, X-ray photoelectron spectroscopy, high resolution scanning transmission electron microscopy, and neutron powder diffraction. Calculations suggest an energetically and dynamically stable structure for C in the octahedral sites between the Mo layers, with Ga bilayers - stacked in a simple hexagonal arrangement - between the Mo2C layers. The predicted elastic properties are below those of the related nanolaminate Mo2GaC. The predicted structure, including lattice parameters and atomic positions, is experimentally confirmed. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
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32.
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33.
  • Lane, Nina J, et al. (författare)
  • First-order Raman scattering of the MAX phases Ta4AlC3, Nb4AlC3, Ti4AlN3, and Ta2AlC
  • 2012
  • Ingår i: Journal of Raman Spectroscopy. - : John Wiley and Sons. - 0377-0486 .- 1097-4555. ; 43:7, s. 954-958
  • 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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34.
  • Lane, Nina J, et al. (författare)
  • First-principles study of dislocations in hcp metals through the investigation of the (11(2)over-bar1) twin boundary
  • 2011
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 84:18, s. 184101-
  • 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.
  •  
35.
  • Lane, Nina J., et al. (författare)
  • High-temperature stability of alpha-Ta(4)AlC(3)
  • 2011
  • Ingår i: Materials research bulletin. - : Elsevier Science B.V., Amsterdam.. - 0025-5408 .- 1873-4227. ; 46:7, s. 1088-1091
  • 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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36.
  • Lane, Nina J., et al. (författare)
  • 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
  • Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X. ; 86:21
  • 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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37.
  • Lane, Nina J., et al. (författare)
  • Structure of a new bulk Ti5Al2C3 MAX phase produced by the topotactic transformation of Ti2AlC
  • 2012
  • Ingår i: Journal of the European Ceramic Society. - : Elsevier. - 0955-2219 .- 1873-619X. ; 32:12, s. 3485-3491
  • 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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38.
  • Lapauw, T., et al. (författare)
  • Synthesis of the new MAX phase Zr2AlC
  • 2016
  • Ingår i: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 36:8, s. 1847-1853
  • 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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39.
  • Lapauw, T., et al. (författare)
  • Synthesis of the novel Zr3AlC2 MAX phase
  • 2016
  • Ingår i: Journal of the European Ceramic Society. - : ELSEVIER SCI LTD. - 0955-2219 .- 1873-619X. ; 36:3, s. 943-947
  • 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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40.
  • Lu, Jun, et al. (författare)
  • Atomic structure and lattice defects in nanolaminated ternary transition metal borides
  • 2017
  • Ingår i: Materials Research Letters. - : TAYLOR & FRANCIS INC. - 2166-3831. ; 5:4, s. 235-241
  • Tidskriftsartikel (refereegranskat)abstract
    • We use analytical aberration-corrected high-resolution scanning transmission electron microscopy to image the atomic structure of the layered ternary transition metal (M) borides, Cr2AlB2, Fe2AlB2, and MoAlB. In these ternaries, MB layers and Al single or double atomic layers are interleaved. The atomic positions of the M elements and Al are clearly resolved by Z-contrast images. The following structural defects are also found and described herein: a 90 degrees twist boundary along [010] in Cr2AlB2, a tilt boundary in Fe2AlB2, and Mo2AlB2-like stacking faults in MoAlB, where some of the MB-based structures are intercalated by one (instead of two) Al layer(s).
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41.
  • Lukatskaya, Maria R., et al. (författare)
  • Room-Temperature Carbide-Derived Carbon Synthesis by Electrochemical Etching of MAX Phases
  • 2014
  • Ingår i: Angewandte Chemie International Edition. - : John Wiley & Sons. - 1433-7851 .- 1521-3773. ; 126:19, s. 4977-4980
  • Tidskriftsartikel (refereegranskat)abstract
    • Porous carbons are widely used in energy storage and gas separation applications, but their synthesis always involves high temperatures. Herein we electrochemically selectively extract, at ambient temperature, the metal atoms from the ternary layered carbides, Ti3AlC2, Ti2AlC and Ti3SiC2 (MAX phases). The result is a predominantly amorphous carbide-derived carbon, with a narrow distribution of micropores. The latter is produced by placing the carbides in HF, HCl or NaCl solutions and applying anodic potentials. The pores that form when Ti3AlC2 is etched in dilute HF are around 0.5 nm in diameter. This approach forgoes energy-intensive thermal treatments and presents a novel method for developing carbons with finely tuned pores for a variety of applications, such as supercapacitor, battery electrodes or CO2 capture.
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42.
  • Mendoza-Galvan, Arturo, et al. (författare)
  • Spectroscopic Ellipsometry of Bulk MAX-phases
  • 2009
  • Ingår i: Proceedings of the AVS 56th International Symposium & Exhibition, 2009.
  • Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
    • MAX-phases are compounds with the chemical formula Mn+1AXn where M is a transition metal, A is an element from column 13 to 16 in the periodic system and X represents C or N and n = 1, 2 or 3. These materials are potentially technologically important as they show unique refractory and other physical properties due to the combination of metals and ceramics. From a fundamental point of view the band structure of these materials are of interest and optical reference data are important to determine. Herein we report, for the first time, on the optical properties of NbTiAlC, Nb2AlC, TiSC2, Cr2GeC, TiGeC2, Ti2AlC, and Ti2AlN by spectroscopic ellipsometry in the 0.03 to 6.0 eV spectral range. The ellipsometric data in the infrared range show features corresponding to Fano modes indicating the presence of a thin oxide layer a few nm thick. The optical response of these MAX-phases is represented by a Drude-Lorentz model. Thus, in the low energy range the electrical conductivity through the Drude term is evaluated and two or three interband electronic transitions which are compositional dependent can be identified at photon energies in the visible-ultraviolet range 1.0-6.0 eV.
  •  
43.
  • Meshkian, Rahele, et al. (författare)
  • Synthesis of two-dimensional molybdenum carbide, Mo2C, from the gallium based atomic laminate Mo2Ga2C
  • 2015
  • Ingår i: Scripta Materialia. - : Elsevier. - 1359-6462 .- 1872-8456. ; 108, s. 147-150
  • Tidskriftsartikel (refereegranskat)abstract
    • We report on the synthesis of a two-dimensional transition metal carbide, Mo2C, (MXene) obtained by immersing Mo2Ga2C thin films in hydrofluoric acid. Experimental evidences for neither synthesis of a Mo-based MXene nor selective etching of Ga from an atomic nanolaminate have previously been presented. MXene formation is verified through X-ray diffraction, transmission electron microscopy, and energy dispersive X-ray spectroscopy. This discovery unlocks new potential applications for Mo-based MXenes in a host of applications, from thermoelectrics to catalysis and energy storage.
  •  
44.
  • Meshkian, Rahele, et al. (författare)
  • W-Based Atomic Laminates and Their 2D Derivative W1.33C MXene with Vacancy Ordering
  • 2018
  • Ingår i: Advanced Materials. - : Wiley. - 0935-9648 .- 1521-4095. ; 30:21
  • Tidskriftsartikel (refereegranskat)abstract
    • Structural design on the atomic level can provide novel chemistries of hybrid MAX phases and their MXenes. Herein, density functional theory is used to predict phase stability of quaternary i-MAX phases with in-plane chemical order and a general chemistry (W 2/3 M 2 1/3 ) 2 AC, where M 2 = Sc, Y (W), and A = Al, Si, Ga, Ge, In, and Sn. Of over 18 compositions probed, only two—with a monoclinic C2/c structure—are predicted to be stable: (W 2/3 Sc 1/3 ) 2 AlC and (W 2/3 Y 1/3 ) 2 AlC and indeed found to exist. Selectively etching the Al and Sc/Y atoms from these 3D laminates results in W 1.33 C-based MXene sheets with ordered metal divacancies. Using electrochemical experiments, this MXene is shown to be a new, promising catalyst for the hydrogen evolution reaction. The addition of yet one more element, W, to the stable of M elements known to form MAX phases, and the synthesis of a pure W-based MXene establishes that the etching of i-MAX phases is a fruitful path for creating new MXene chemistries that has hitherto been not possible, a fact that perforce increases the potential of tuning MXene properties for myriad applications.
  •  
45.
  • Naguib, Michael, et al. (författare)
  • New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries
  • 2013
  • Ingår i: Journal of the American Chemical Society. - : AMER CHEMICAL SOC, 1155 16TH ST, NW, WASHINGTON, DC 20036 USA. - 0002-7863 .- 1520-5126. ; 135:43, s. 15966-15969
  • Tidskriftsartikel (refereegranskat)abstract
    • New two-dimensional niobium and vanadium carbides have been synthesized by selective etching, at room temperature, of Al from Nb2AlC and V2AlC, respectively. These new matrials are promising electrode materials for Li-ion batteries, demonstrating good capability to handle high charge-discharge rates. Reversible capacities of 170 and 260 mA.h.g(-1) at 1 C, and 110 and 125 mA.h.g(-1) at 10 C were obtained for Nb2C and V2C-based electrodes, respectively.
  •  
46.
  • Naguib, Michael, et al. (författare)
  • On the Topotactic Transformation of Ti2AlC into a Ti-C-O-F Cubic Phase by Heating in Molten Lithium Fluoride in Air
  • 2011
  • Ingår i: Journal of The American Ceramic Society. - : Wiley-Blackwell. - 0002-7820 .- 1551-2916. ; 94:12, s. 4556-4561
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein we report on the formation of a TiCOF phase via the topotactic transformation of Ti2AlC by immersion in molten lithium fluoride, LiF, at 900 degrees C in air for 2 h. The Al diffuses out of the structure and reacts with LiF to form Li3AlF6. X-ray diffraction, Raman spectroscopy, optical, scanning and transmission electron microscopy, with energy-dispersive spectroscopy, showed the selective etching of Al from the structure and the formation of a cubic, rock-salt, TiCOF phase. The transformation is topotaxial, involves de-twinning of the hexagonal Ti2AlC structure and results in domains that are of the order of 10 nm. The reaction rate is significantly higher when the reaction is carried out in air than when it is carried out in vacuum.
  •  
47.
  • Naguib, Michael, et al. (författare)
  • Synthesis of a new nanocrystalline titanium aluminum fluoride phase by reaction of Ti(2)AlC with hydrofluoric acid
  • 2011
  • Ingår i: RSC ADVANCES. - : RSC Publishing. - 2046-2069. ; 1:8, s. 1493-1499
  • Tidskriftsartikel (refereegranskat)abstract
    • A new nanocrystalline phase of titanium aluminum fluoride, with a stoichiometry of Ti(2)AlF(9)-exact stoichiometry measured herein is Ti(2.1)Al(0.9)F(9),-was synthesized by the fluorination of Ti(2)AlC in anhydrous hydrofluoric acid at 55 degrees C for 2 h. The results from X-ray diffraction, scanning and high-resolution transmission electron microscopy and selected area diffraction, together with crystal structure solution via simulated annealing, confirmed the formation of a nanocrystalline material with a trigonal structure belonging to the R3 space group. The structure can also be viewed as being comprised of corner-sharing octahedra that are arranged in a distorted simple cubic arrangement. Thermogravimetric analysis, in argon, indicates that this phase is stable up to approximate to 850 degrees C. Density functional theory calculations estimate the band gap to be approximate to 5 eV and the bulk modulus to be approximate to 109 GPa.
  •  
48.
  •  
49.
  • Naguib, Michael, et al. (författare)
  • Two-Dimensional Transition Metal Carbides
  • 2012
  • Ingår i: ACS Nano. - : American Chemical Society. - 1936-0851 .- 1936-086X. ; 6:2, s. 1322-1331
  • Tidskriftsartikel (refereegranskat)abstract
    • Herein we report on the synthesis of two-dimensional transition metal carbides and carbonitrides by immersing select MAX phase powders in hydrofluoric acid, HF. The MAX phases represent a large (andgt;60 members) family of ternary, layered, machinable transition metal carbides, nitrides, and carbonitrides. Herein we present evidence for the exfoliation of the following MAX phases: Ti2AlC, Ta4AlC3, (Ti-0.5,Nb-0.5)(2)AlC, (V-0.5,Cr-0.5)(3)AlC2, and Ti3AlCN by the simple immersion of their powders, at room temperature, in HF of varying concentrations for times varying between 10 and 72 h followed by sonication. The removal of the "A" group layer from the MAX phases results in 2-D layers that we are labeling MXenes to denote the loss of the A element and emphasize their structural similarities with graphene. The sheet resistances of the MXenes were found to be comparable to multilayer graphene. Contact angle measurements with water on pressed MXene surfaces showed hydrophilic behavior.
  •  
50.
  • Nikolaevsky, Mark, et al. (författare)
  • Possible monoclinic distortion of Mo2GaC under high pressure
  • 2020
  • Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 127:14
  • Tidskriftsartikel (refereegranskat)abstract
    • In this work, we present high-pressure diffraction results of the Mo-based M-n (+) (1)AX(n) phase, Mo2GaC. A diamond anvil cell was used to compress the material up to 30 GPa, and x-ray diffraction was used to determine the structure and unit cell parameters as a function of pressure. Somewhat surprisingly, we find that, at 295 +/- 25 GPa, the bulk modulus of Mo2GaC is the highest reported of all the MAX phases measured to date. The c/a ratio increases with increasing pressure. At above 15 GPa, a splitting in the (1 0 0) reflection occurs. This result, coupled with new density functional theory calculations, suggests that a second order phase transition to possibly a mixture of hexagonal and monoclinic structures may explain this splitting. Such experimentally and theoretically supported phase transitions were not predicted in previously published calculations.
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