| 1. |
- Anasori, Babak, et al.
(författare)
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Two-Dimensional, Ordered, Double Transition Metals Carbides (MXenes)
- 2015
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Ingår i: ACS Nano. - : AMER CHEMICAL SOC. - 1936-0851 .- 1936-086X. ; 9:10, s. 9507-9516
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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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| 2. |
- Chun, JS, et al.
(författare)
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Synchrotron x-ray diffraction and transmission electron microscopy studies of interfacial reaction paths and kinetics during annealing of fully-002-textured Al/TiN bilayers
- 2001
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Ingår i: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - 0734-2101 .- 1520-8559. ; 19:1, s. 182-191
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Tidskriftsartikel (refereegranskat)abstract
- Dense fully-002-textured polycrystalline TiN layers, 110 nm thick with a N/TI ratio of 1.02+/-0.03, were grown on SiO2 by ultrahigh vacuum magnetically unbalanced magnetron sputter deposition at T-s = 450 degreesC in pure N-2 utilizing high N-2(+)/Ti Aux ratios and low energy (E-N2(+) = 20 eV) ion irradiation of the growing film. Al overlayers, 160 nm thick and possessing a strong 002 texture inherited from the underlying TiN, were then deposited at T-s = 100 degreesC without breaking vacuum. Synchrotron x-ray diffraction was used to follow interfacial reaction paths and kinetics during postdeposition annealing as a function of time (t(a) = 200 - 1200 s) and temperature (T-a = 500 - 580 degreesC). Changes in bilayer microstructure and microchemistry were investigated using transmission electron microscopy (TEM) and scanning TEM to obtain compositional maps of cross-sectional and plan-view specimens by energy dispersive x-ray analysis. The initial bilayer reaction step during annealing involves the formation of a continuous AIN interfacial layer which, due to local epitaxy with the TIN underlayer, grows with the metastable zinc-blende structure up to a thickness x similar or equal to3-5 nm, and with the wurtzite structure thereafter. Ti atoms released during AIN formation diffuse into the Al layer leading to supersaturation followed by the nucleation of dispersed regions of tetragonal Al3Ti with inherited 002 preferred orientation. The aluminide domains grow rapidly until they reach the free surface, thereafter growth is two dimensional as Al3Ti grains spread radially. The overall activation energy for Al3Ti formation and growth is 1.8+/-0.1 eV. In situ synchrotron x-ray diffraction analyses during thermal ramping show that the onset temperature for interfacial reactions was increased by more than 100 degreesC for fully dense completely 002-textured bilayers compared to Ill-textured bilayers deposited by conventional reactive sputter deposition. (C) 2001 American Vacuum Society.
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| 3. |
- Mei, A. B., et al.
(författare)
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Reflection thermal diffuse x-ray scattering for quantitative determination of phonon dispersion relations
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : AMER PHYSICAL SOC. - 1098-0121 .- 1550-235X. ; 92:17, s. 174301-
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Tidskriftsartikel (refereegranskat)abstract
- Synchrotron reflection x-ray thermal diffuse scattering (TDS) measurements, rather than previously reported transmission TDS, are carried out at room temperature and analyzed using a formalism based upon second-order interatomic force constants and long-range Coulomb interactions to obtain quantitative determinations of MgO phonon dispersion relations (h) over bar omega(j) (q), phonon densities of states g((h) over bar omega), and isochoric temperature-dependent vibrational heat capacities c(v) (T). We use MgO as a model system for investigating reflection TDS due to its harmonic behavior as well as its mechanical and dynamic stability. Resulting phonon dispersion relations and densities of states are found to be in good agreement with independent reports from inelastic neutron and x-ray scattering experiments. Temperature-dependent isochoric heat capacities cv (T), computed within the harmonic approximation from (h) over bar omega(j) (q) values, increase with temperature from 0.4 x 10(-4) eV/atom K at 100 K to 1.4 x 10(-4) eV/atom K at 200 K and 1.9 x 10(-4) eV/atom K at 300 K, in excellent agreement with isobaric heat capacity values c(p) (T) between 4 and 300 K. We anticipate that the experimental approach developed here will be valuable for determining vibrational properties of heteroepitaxial thin films since the use of grazing-incidence (theta less than or similar to theta(c), where theta(c) is the density-dependent critical angle) allows selective tuning of x-ray penetration depths to less than or similar to 10 nm.
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| 4. |
- Muraro, A., et al.
(författare)
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Neutron radiography as a non-destructive method for diagnosing neutron converters for advanced thermal neutron detectors
- 2016
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Ingår i: Journal of Instrumentation. - : Institute of Physics (IOP). - 1748-0221. ; 11:C03033
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Tidskriftsartikel (refereegranskat)abstract
- Due to the well-known problem of He-3 shortage, a series of different thermal neutron detectors alternative to helium tubes are being developed, with the goal to find valid candidates for detection systems for the future spallation neutron sources such as the European Spallation Source (ESS). A possible He-3-free detector candidate is a charged particle detector equipped with a three dimensional neutron converter cathode (3D-C). The 3D-C currently under development is composed by a series of alumina (Al2O3) lamellas coated by 1 mu m of B-10 enriched boron carbide (B4C). In order to obtain a good characterization in terms of detector efficiency and uniformity it is crucial to know the thickness, the uniformity and the atomic composition of the B4C neutron converter coating. In this work a non-destructive technique for the characterization of the lamellas that will compose the 3D-C was performed using neutron radiography. The results of these measurements show that the lamellas that will be used have coating uniformity suitable for detector applications. This technique (compared with SEM, EDX, ERDA, XPS) has the advantage of being global (i.e. non point-like) and non-destructive, thus it is suitable as a check method for mass production of the 3D-C elements.
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| 5. |
- Tao, Quanzheng, et al.
(författare)
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Rare-earth (RE) nanolaminates Mo4RE4Al7C3 featuring ferromagnetism and mixed-valence states
- 2018
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 2:11
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Tidskriftsartikel (refereegranskat)abstract
- Rare-earth-based (RE) nanolaminates have attracted attention recently because of their complicated magnetism and their potential as precursors for strongly correlated two-dimensional materials. In this work, we synthesized a class of nanolaminates with a Mo4RE4Al7C3 chemistry, where RE = Ce or Pr. Powder samples of both phases were characterized with respect to structure and composition. Single crystals of Mo4Ce4Al7C3 were used for magnetization measurements. The crystal structure was investigated by means of x-ray diffraction and scanning transmission electron microscopy. Magnetization analysis reveals a ferromagnetic ground state with a Curie temperature of similar to 10.5 K. X-ray absorption near-edge structure provides experimental evidence that Ce is in a mixed-valence state. X-ray magnetic circular dichroism shows that only the Ce atoms with 4f(1) configuration occupying one of the two possible sites are ferromagnetically coupled, with a saturation moment of similar to 1.2 mu(B) per atom. We thus classify Mo4Ce4Al7C3 as a ferromagnetic, mixed-valence compound.
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| 6. |
- Alling, Björn, et al.
(författare)
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Dynamic and structural stability of cubic vanadium nitride
- 2015
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - : American Physical Society. - 1098-0121 .- 1550-235X .- 2469-9950 .- 2469-9969. ; 91:5, s. 054101-
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Tidskriftsartikel (refereegranskat)abstract
- Structural phase transitions in epitaxial stoichiometric VN/MgO(011) thin films are investigated using temperature-dependent synchrotron x-ray diffraction (XRD), selected-area electron diffraction (SAED), resistivity measurements, high-resolution cross-sectional transmission electron microscopy, and ab initio molecular dynamics (AIMD). At room temperature, VN has the B1 NaCl structure. However, below T-c = 250 K, XRD and SAED results reveal forbidden (00l) reflections of mixed parity associated with a noncentrosymmetric tetragonal structure. The intensities of the forbidden reflections increase with decreasing temperature following the scaling behavior I proportional to (T-c - T)(1/2). Resistivity measurements between 300 and 4 K consist of two linear regimes resulting from different electron/phonon coupling strengths in the cubic and tetragonal-VN phases. The VN transport Eliashberg spectral function alpha F-2(tr)(h omega), the product of the phonon density of states F(h omega) and the transport electron/phonon coupling strength alpha(2)(tr)(h omega), is determined and used in combination with AIMD renormalized phonon dispersion relations to show that anharmonic vibrations stabilize the NaCl structure at T greater than T-c. Free-energy contributions due to vibrational entropy, often neglected in theoretical modeling, are essential for understanding the room-temperature stability of NaCl-structure VN, and of strongly anharmonic systems in general.
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| 7. |
- Anasori, Babak, et al.
(författare)
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Experimental and theoretical characterization of ordered MAX phases Mo2TiAlC2 and Mo2Ti2AlC3
- 2015
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Ingår i: Journal of Applied Physics. - : AMER INST PHYSICS. - 0021-8979 .- 1089-7550. ; 118:9, s. 094304-
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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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| 8. |
- Anastasopoulos, M., et al.
(författare)
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Multi-Grid detector for neutron spectroscopy : Results obtained on time-of-flight spectrometer CNCS
- 2017
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Ingår i: Journal of Instrumentation. - : IOP PUBLISHING LTD. - 1748-0221. ; 12:4
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Tidskriftsartikel (refereegranskat)abstract
- The Multi-Grid detector technology has evolved from the proof-of-principle and characterisation stages. Here we report on the performance of the Multi-Grid detector, the MG.CNCS prototype, which has been installed and tested at the Cold Neutron Chopper Spectrometer, CNCS at SNS. This has allowed a side-by-side comparison to the performance of 3He detectors on an operational instrument. The demonstrator has an active area of 0.2 m2. It is specifically tailored to the specifications of CNCS. The detector was installed in June 2016 and has operated since then, collecting neutron scattering data in parallel to the He-3 detectors of CNCS. In this paper, we present a comprehensive analysis of this data, in particular on instrument energy resolution, rate capability, background and relative efficiency. Stability, gamma-ray and fast neutron sensitivity have also been investigated. The effect of scattering in the detector components has been measured and provides input to comparison for Monte Carlo simulations. All data is presented in comparison to that measured by the 3He detectors simultaneously, showing that all features recorded by one detector are also recorded by the other. The energy resolution matches closely. We find that the Multi-Grid is able to match the data collected by 3He, and see an indication of a considerable advantage in the count rate capability. Based on these results, we are confident that the Multi-Grid detector will be capable of producing high quality scientific data on chopper spectrometers utilising the unprecedented neutron flux of the ESS.
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| 9. |
- Bakhit, Babak, 1983-, et al.
(författare)
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Multifunctional ZrB2-rich Zr1-xCrxBy thin films with enhanced mechanical, oxidation, and corrosion properties
- 2021
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Ingår i: Vacuum. - : Elsevier BV. - 0042-207X .- 1879-2715. ; 185
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Tidskriftsartikel (refereegranskat)abstract
- Refractory transition-metal (TM) diborides have high melting points, excellent hardness, and good chemical stability. However, these properties are not sufficient for applications involving extreme environments that require high mechanical strength as well as oxidation and corrosion resistance. Here, we study the effect of Cr addition on the properties of ZrB2-rich Zr1-xCrxBy thin films grown by hybrid high-power impulse and dc magnetron co-sputtering (Cr-HiPIMS/ZrB2-DCMS) with a 100-V Cr-metal-ion synchronized potential. Cr metal fraction, x = Cr/(Zr+Cr), is increased from 0.23 to 0.44 by decreasing the power Pzrb2 applied to the DCMS ZrB2 target from 4000 to 2000 W, while the average power, pulse width, and frequency applied to the HiPIMS Cr target are maintained constant. In addition, y decreases from 2.18 to 1.11 as a function of Pzrb2, as a result of supplying Cr to the growing film and preferential B resputtering caused by the pulsed Cr-ion flux. ZrB2.18, Zr0.77Cr0.23B1.52, Zr0.71Cr0.29B1.42, and Zr0.68Cr0.32B1.38 2 films have hexagonal AlB2 crystal structure with a columnar nanostructure, while Zr0.64Cr0.36B1.30 and Zr0.56Cr0.44B1.11 are amorphous. All films show hardness above 30 GPa. Zr0.56Cr0.44B1.11 alloys exhibit much better toughness, wear, oxidation, and corrosion resistance than ZrB2.18. This combination of properties makes Zr0.56Cr0.44B1.11 ideal candidates for numerous strategic applications.
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| 10. |
- Beckers, Manfred, et al.
(författare)
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Formation of basal plane fiber-textured Ti2AlN films on amorphous substrates
- 2010
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Ingår i: PHYSICA STATUS SOLIDI-RAPID RESEARCH LETTERS. - : John Wiley and Sons, Ltd. - 1862-6254. ; 4:05-Jun, s. 121-123
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Tidskriftsartikel (refereegranskat)abstract
- The synthesis of fiber-textured Ti2AlN(0001) films on SiO2 was characterized by in-situ and ex-situ X-ray scattering and Rutherford backscattering spectrometry. Ti2AlN was formed by solid-state reaction between sequentially deposited Ti and AlN layers. A deposition at 275 degrees C yields a Ti(0001) out-of-plane orientation which is maintained for the following AlN(0001)/Ti(0001) layers. Annealing to 600 degrees C yields AlN decomposition and diffusion of Al and N into Ti, with consecutive transformation into (TiAlN)-Al-3(111) and Ti2AlN(0001) plus AlN residuals. Despite preferred Ti2AlN(0001) out-of-plane orientation, the in-plane distribution is random, as expected from the self-organized pseudo-epitaxial growth.
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