| 1. |
- Aboulfadl, Hisham, 1986, et al.
(författare)
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Atomic-scale characterization of diffusion kinetics in Ru/Al multilayer thin films
- 2019
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Ingår i: Materials Letters. - : Elsevier BV. - 1873-4979 .- 0167-577X. ; 254, s. 344-347
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Tidskriftsartikel (refereegranskat)abstract
- The microstructure properties of reactive multilayers play a key role in their chemical reactions. Ru/Al multilayers are investigated in as-deposited and annealed conditions using atom probe tomography to probe the interdiffusion and early phase formation. Performing atom probe analysis was accompanied with difficulties stemming from weak adhesion of the bi-layers and reconstruction artifacts. Nonetheless, it was possible to obtain valuable qualitative information concerning diffusion kinetics. Asymmetric interdiffusion during deposition is found. Grain boundary diffusion is shown to have a major contribution on enhancing mass-transport across the layers as well as on the nucleation and growth of the RuAl6 intermetallic phase.
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| 2. |
- Aboulfadl, Hisham, 1986, et al.
(författare)
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Interdiffusion in as-deposited Ni/Ti multilayer thin films analyzed by atom probe tomography
- 2019
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Ingår i: Materials Letters. - : Elsevier BV. - 1873-4979 .- 0167-577X. ; 236, s. 92-95
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Tidskriftsartikel (refereegranskat)abstract
- The nanoscale design of metallic multilayer thin films is one crucial factor that greatly influences the kinetics, often inducing unusual phase transformations. Metastable or amorphous phases may directly form in as-deposited films of certain thicknesses, which is common for Ni/Ti multilayers. Atom probe tomography and X-ray diffraction analyses are performed here to study the interdiffusion and structural changes as a function of the bilayer thickness in Ni/Ti multilayers. The films are deposited by DC magnetron sputtering with near 50:50 compositions. Multilayers with 5 nm bilayer thickness are found to be highly intermixed, with compositions up to ∼25 at.% for both diffuser metals, inducing amorphization reactions during deposition.
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| 3. |
- Barrirero, Jenifer, 1981-
(författare)
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Eutectic Modification of Al-Si casting alloys
- 2019
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aluminum alloys with silicon as the major alloying element are the most widely used aluminum casting alloys. The eutectic phase in these alloys is formed by hard and brittle silicon plates in an aluminum matrix. Such silicon plates can act as crack propagation paths deteriorating the toughness of the material. To enhance ductility, silicon can be modified to a coral-like microstructure by addition of a modifying agent. Amongst the elements proposed as modifiers, only strontium, sodium and europium induce a plate-tocoral transition, while others such as ytterbium, only refine the silicon plates. The exact mechanism for the remarkable plate-to-coral change, and the reason why certain elements only refine the structure, is still not completely understood.In this investigation, atom probe tomography and transmission electron microscopy were used to analyze and compare the crystal structure and the distribution of solute atoms in silicon at the atomic level. An unmodified alloy and alloys modified by strontium, sodium, europium and ytterbium were studied. Elements inducing silicon plate-to-coral transition were found to contain nanometer sized clusters at the defects in silicon with stoichiometries corresponding to compounds formed at the ternary eutectic reaction of each system. In contrast, the addition of ytterbium, that only refines the silicon plates, is unable to form clusters in silicon. We propose that the formation of ternary compound clusters AlSiNa, Al2Si2Sr and Al2Si2Eu at the silicon / liquid interface during solidification restrict silicon growth. The formation of clusters on silicon facets create growth steps and increase growth direction diversity. The incorporation of clusters in silicon explains the high density of crystallographic defects and the structural modification from plates to corals.The parallel lattice plane-normals 011Si // 0001Al2Si2Eu, 011Si // 610Al2Si2Eu and 111Si // 610Al2Si2Eu were found between Al2Si2Eu and silicon, and absent between Al2Si2Yb and silicon. We propose a favorable heterogeneous formation of Al2Si2Eu on silicon. The misfit between 011Si and 0002Al2Si2X interplanar spacings shows a consistent trend with the potency of modification for several elements such as strontium, sodium, europium, calcium, barium, ytterbium and yttrium.
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| 4. |
- Benavides, Vicente, et al.
(författare)
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Raman spectroscopy and hardness study of C60 transformation into nanoclustered graphene phase at high pressure/high temperature
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- In this paper, a study of the C60 – nanoclustered graphene phase (NGP) transformation via characterization of non-completely transformed carbon particles is presented. High-resolution (∽ 1 µm) Raman spectroscopy and nanoindentation measurements were performed on the same pre-selected sample area. The results evidence different steps of the transformation that allows establishing correspondence between the NGP/C60 ratio and the nanohardness: an abrupt increase in nanohardness from 2 GPa to 20 GPa for a stepwise NGP/C60 ratio change in the transformation zone. These results demonstrate that (I) at a micro-level (1 µm), the transformation of C60 into NGP does not occur simultaneously in the entire volume and (II) the residual C60 polymer is not desirable in superhard amorphous carbon materials. This work demonstrates importance of advanced experimental methodologies to characterization of disordered carbon phases.
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| 5. |
- Benavides, Vicente
(författare)
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Synthesis and characterization of nanocarbons as reinforced particles in metal composites
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this work, several scientific problems related to high pressure–high temperature (HP–HT) synthesis of new materials using fullerite as a precursor were studied: first, the mechanism of the transformation of C60 crystal into a nano-clustered graphene phase (NGP) at a pressure of 8 GPa; and second, the effect of disorder introduced into C60 crystals by ball milling prior to HP–HT synthesis on the structure and properties of the NGP. A separate set of experiments was devoted to compression of C60 precursor at unexplored before pressure of 25 GPa and elevated temperatures in search for new type of disordered carbon-based materials.In the first study, Raman spectroscopy, HRSTEM-EELS, and indentation hardness demonstrate that, under pressure, C60 exhibits a path of transformation from polymerized C60 to NGP. This phase exhibits a short-range order and preferential orientation of nano-clusters of graphene assembled in a highly disordered carbon matrix. In our studies, we observe that the mechanism of C60 transformation into NGP could be understood in terms of nucleation and growth mechanism as opposed to the pseudomartensitic mechanism. Changes in Raman intensity of the Ag(2) C60 mode monitored in polished incompletely transformed carbon particles reveal different steps of transformation. Moreover, the polishing reveals the distribution of shear bands resulting from plastic deformation of the C60 monomer and following the direction of the <110> slip planes in FCC system.HRSTEM analysis reveals the presence of disorder as an intermediate state between the parent C60 and the nano-graphene units. EELS spectra show that C60 molecules in such state are present as monomers, and the intermediate phase is an sp2–sp3 disordered phase, in which the sp2 fraction is by up to 20% lower than that of graphene nanoclusters. The findings suggest that, after the collapse, the polymer structure breaks down with the formation of a disordered (sp2–sp3) carbon phase containing some fraction of residual C60 molecules. The graphene nanoclusters further nucleate and grow in the intermediate disordered phase. Thus, a nucleation and growth mechanism is proposed for the formation of NGP phase from C60 upon HP-HT action.For the second problem, highly disordered systems were obtained from ball-milled C60 through HP–HT demonstrating a promising technique to create hard (hardness > 30 GPa) disordered carbons at relatively low pressure (up to 8 GPa).The nanoarchitecture of NGP and disordered systems was studied using multi-wavelength Raman spectroscopy, HRSTEM, and indentation techniques. The Raman data treatment was carefully studied following the three-stage amorphization trajectory of amorphous carbon. The Raman model consists of G and D bands and data from semi-empirical models that include peak position, FWHM, and intensity ratio. A new approach proposed by the research team includes the presence of carbon pentagons (F band) and carbon heptagons as defects in the graphene clusters and are eventually present in the disordered carbon matrix as well. A peak deconvolution considering the G, D, F and heptagon bands is the model that allows building an empirical correlation between the Raman spectra features and hardness. Using peak deconvolution model based on G, D, F heptagon and sp3 carbon-derived bands allowed us to build an empirical correlation that can be used for a semi-quantitative estimation/prediction of hardness of an arbitrary disordered sp2 carbon-based system based on their spectroscopic (Raman) data.Finally, experiments on compressed C60 at 25 GPa, previously unexplored pressure, produce superhard 3D-C60 polymers at temperatures below 600 oC. As the temperature increases, sp3 carbon starts dominating the disordered structures. The synthesized materials are semiconductors exhibiting ultra-high hardness that in a particular case exceeds that of single crystalline diamond. UV-Raman spectroscopy reveals a high intensity of T band and a G band position typically observed in tetrahedral amorphous carbon (ta-C)-based thin films. The phase has a residual fraction of sp2 carbons, mainly linear chains and fused aromatic rings.In summary, the results demonstrate that a whole class of novel materials with outstanding physical properties - superelastic-hard and ultrahard semiconducting carbons - can be produced for demanding technological applications at HP-HT by using C60 as a precursor and tuning its microstructure.
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| 6. |
- Benavides, Vicente, et al.
(författare)
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Tuning structure and mechanical properties of nanoclustered graphene phase by controlled disorder of precursor C60 fullerite
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The paper compares the Raman spectroscopy, HRSTEM, and indentation hardness results of disordered systems synthesized by squeezing (8 GPa, 850 °C) C60 and ball-milled C60. Mechanical activation introduces substantial damage to the C60 crystal leading to the rupture of van der Waals interaction between the C60 units and a chemical reaction between the balls creating C60 dimers. The multi-wavelength Raman spectra of both, the compressed mechanical activated phase (MA-Phase) and without the mechanical activation phase (wMA phase) reveal that fused aromatic rings with a low fraction of nanographene clusters dominate the MA phase, whereas wMA phase is composed of nanographene clusters. Moreover, a Raman model is presented which introduces fullerene-like structures because of fivefold (F-band) and sevenfold carbon rings-like defects for the wMA phase and part of fused aromatic rings for the MA phase. HRSTEM-EELS data confirm that: nanographene clusters present in wMA (I) are smaller and not abundant in the MA phase (II). (III) EELS data reveal a higher fraction of sp3 bonds in the MA phase compared to that in wMA. The hardness of the MA Phase (37 GPa) is twice its value (18 GPa) in the wMA (IV). The extensive analysis of the Raman data yielded empirical dependences of Hardness vs ID/IG/Hardness vs ID/IF that can be useful for prediction of the hardness of sp2-dominant disordered carbon systems based on their spectroscopic data.
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| 7. |
- Broering Chaar, Ana Beatriz, et al.
(författare)
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Microstructural influence of the thermal behavior of arc deposited TiAlN coatings with high aluminum content
- 2021
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Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 854
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the microstructure on the thermal behavior of cathodic arc deposited TiAlN coatings was studied as a function of isothermal annealing. Two compositionally similar but structurally different coatings were compared, a Ti0·34Al0·66N0.96 coating with a fine-grain structure consisting of a mixture of cubic (c) and hexagonal (h) phases, and a Ti0·40Al0·60N0.94 coating with a coarse-grain structure of cubic phase. By in situ wide-angle synchrotron x-ray scattering, spinodal decomposition was confirmed in both coatings. The increased amount of internal interfaces lowered the decomposition temperature by 50 °C for the dual-phase coating. During the subsequent isothermal anneal at 1000 °C, a transformation from c-AlN to h-AlN took place in both coatings. After 50 min of isothermal annealing, atom probe tomography detected small amounts of Al (∼2 at.%) in the c-TiN rich domains and small amounts of Ti (∼1 at.%) in the h-AlN rich domains of the coarse-grained single-phase Ti0·40Al0·60N0.94 coating. Similarly, at the same conditions, the fine-grained dual-phase Ti0·34Al0·66N0.96 coating exhibits a higher Al content (∼5 at.%) in the c-TiN rich domains and higher Ti content (∼15 at.%) in the h-AlN rich domains. The study shows that the thermal stability of TiAlN is affected by the microstructure and that it can be used to tune the reaction pathway of decomposition favorably.
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| 8. |
- Broering Chaar, Ana Beatriz, 1990-, et al.
(författare)
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The Effect of Cathodic Arc Guiding Magnetic Field on the Growth of (Ti0.36Al0.64)N Coatings
- 2019
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Ingår i: Coatings. - : MDPI. - 2079-6412. ; 9:10
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Tidskriftsartikel (refereegranskat)abstract
- We use a modified cathodic arc deposition technique, including an electromagnetic coil that introduces a magnetic field in the vicinity of the source, to study its influence on the growth of (Ti0.36Al0.64)N coatings. By increasing the strength of the magnetic field produced by the coil, the cathode arc spots are steered toward the edge of the cathode, and the electrons are guided to an annular anode surrounding the cathode. As a result, the plasma density between the cathode and substrate decreased, which was observed as a lateral spread of the plasma plume, and a reduction of the deposition rate. Optical emission spectroscopy shows reduced intensities of all recorded plasma species when the magnetic field is increased due to a lower number of collisions resulting in excitation. We note a charge-to-mass ratio decrease of 12% when the magnetic field is increased, which is likely caused by a reduced degree of gas phase ionization, mainly through a decrease in N2 ionization. (Ti0.36Al0.64)N coatings grown at different plasma densities show considerable variations in grain size and phase composition. Two growth modes were identified, resulting in coatings with (i) a fine-grained glassy cubic and wurtzite phase mixture when deposited with a weak magnetic field, and (ii) a coarse-grained columnar cubic phase with a strong magnetic field. The latter conditions result in lower energy flux to the coating’s growth front, which suppresses surface diffusion and favors the formation of c-(Ti,Al)N solid solutions over phase segregated c-TiN and w-AlN.
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| 9. |
- de Melo, Claudia, et al.
(författare)
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Semi-Transparent p‑Cu2O/n-ZnO Nanoscale-Film Heterojunctions for Photodetection and Photovoltaic Applications
- 2019
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Ingår i: ACS Applied Nano Materials. - : American Chemical Society (ACS). - 2574-0970. ; 2:7, s. 4358-4366
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Tidskriftsartikel (refereegranskat)abstract
- Transparent nanoscale-film heterojunctions based on Cu2O and ZnO were fabricated by atomic layer deposition and reactive magnetron sputtering. The constitutive layers exhibit high crystalline quality and a local epitaxial relation between Cu2O and ZnO was achieved with [110] Cu2O || [001] ZnO and [001] Cu2O || [010] ZnO as evidenced by high resolution transmission electron microscopy and. Cu2O films show very low resistivity and high mobility values of 9–150 Ω cm and 19 cm2/V s, respectively. The Cu2O/ZnO heterojunctions exhibit a nonlinear rectifying behavior characteristic of a p–n junction, self-powered photoresponse under 1 Sun illumination and an average transmittance of 73% in the visible region of the electromagnetic spectrum. These results are promising for all-oxide transparent electronics, photodetection and photovoltaic applications.
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| 10. |
- de Melo, Claudia, et al.
(författare)
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Tunable Localized Surface Plasmon Resonance and Broadband Visible Photoresponse of Cu Nanoparticles/ZnO Surfaces
- 2018
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Ingår i: ACS Applied Materials and Interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 10:47, s. 40958-40965
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Tidskriftsartikel (refereegranskat)abstract
- Plasmonic Cu nanoparticles (NP) were successfully deposited on ZnO substrates by atomic layer deposition (ALD) owing to the Volmer–Weber island growth mode. An evolution from Cu NP to continuous Cu films was observed with an increasing number of ALD cycles. Real and imaginary parts of the NP dielectric functions, determined by spectroscopic ellipsometry using an effective medium approach, evidence a localized surface plasmon resonance that can be tuned between the visible and near-infrared ranges by controlling the interparticle spacing and size of the NP. The resulting Cu NP/ZnO device shows an enhanced photoresponse under white light illumination with good responsivity values, fast response times, and stability under dark/light cycles. The significant photocurrent detected for this device is related to the hot-electron generation at the NP surface and injection into the conduction band of ZnO. The possibility of tuning the plasmon resonance together with the photoresponsivity of the device is promising in many applications related to photodetection, photonics, and photovoltaics.
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