| 1. |
- Gangaprasad Rao, Smita, 1992-
(author)
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Cantor-Alloy-Based Multicomponent Nitride Thin Films
- 2023
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Doctoral thesis (other academic/artistic)abstract
- In this Thesis, I have investigated multicomponent alloy based thin films synthesized by magnetron sputtering. The studies in the thesis are centered around the phase diagram of the CrFeCoNi nitrogen containing system. Theoretical and experimental methods were employed to understand the phase formation in this system which is related to the archetypical Cantor alloy (CrMnFeCoNi). CrFeCoNi thin films of approximately equimolar composition crystallize with fcc structure when grown at room temperature. This structure, however, is not retained when nitrogen (x) is added into the lattice. Density functional theory calculations together with the experimental investigation on the (CrFeCoNi)Nx system revealed the stabilization of the metallic fcc when x ≤ 0.22 and the stabilization of the NaCl B1 structure when x > 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films. The importance of the phase diagram is realized when attempting to grow much more complex structures for application-oriented research such as irradiation resistance, corrosion resistance as well as epitaxial films for a fundamental understanding of the material system. The phase diagram of the CrFeCoNi system indicated that higher stability of the single-phase solid solution Cantor nitride lay in a limited temperature range of 200 to 300 °C. In order to compensate for the higher deposition temperature required to grow epitaxial films magnetic field assisted dc magnetron sputtering was used. This technique allows for the control of the flux of Ar ions bombarding the substrate during growth thereby providing the growing film with kinetic energy as opposed to thermal. The results from the study indicated that the quality of epitaxy can be improved by increasing low ion energy, high ion-flux bombardment. The thesis in whole, gives a fundamental understanding of the nitride cantor alloy material system in terms of crystal structure, mechanical and electrical properties.
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| 2. |
- Gangaprasad Rao, Smita, et al.
(author)
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Low temperature epitaxial growth of Cantor-nitride thin films by magnetic field assisted magnetron sputtering
- 2023
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In: Journal of Vacuum Science & Technology. A. Vacuum, Surfaces, and Films. - : American Institute of Physics (AIP). - 0734-2101 .- 1520-8559. ; 41:5
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Journal article (peer-reviewed)abstract
- Low-temperature epitaxial growth of multicomponent alloy-based thin films remains an outstanding challenge in materials science and is important for established fundamental properties of these complex materials. Here, Cantor nitride (CrMnFeCoNi)N thin films were epitaxially grown on MgO(100) substrates at low deposition temperature by magnetic-field-assisted dc-magnetron sputtering, a technique where a magnetic field is applied to steer the dense plasma to the substrate thereby influencing the flux of Ar-ions bombarding the film during growth. Without ion bombardment, the film displayed textured growth. As the ion flux was increased, the films exhibited epitaxial growth. The epitaxial relationship between film and substrate was found to be cube on cube (001)film parallel to(001)MgO, [100]film parallel to[100]MgO. The epitaxy was retained up to a thickness of approximately similar to 100 nm after which the growth becomes textured with a 002 out-of-plane orientation. The elastic constants determined by Brillouin inelastic light scattering were found to be C-11 = 320 GPa, C-12 = 125 GPa, and C-44 = 66 GPa, from which the polycrystalline Young's modulus was calculated as 204 GPa and Poisson's ratio = 0.32, whereas available elastic properties still remained very scarce.
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| 3. |
- Gangaprasad Rao, Smita, et al.
(author)
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Mechanical properties of Xe-ion-irradiated high-entropy-alloy-based multilayers
- 2024
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In: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 124:6
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Journal article (peer-reviewed)abstract
- In this Letter, we investigate the mechanical stability of HEA-based multilayers after Xe-ion irradiation. CrFeCoNi/TiNbZrTa metallic and nitride thin films with a bilayer thickness of 30 nm were grown by reactive dc-magnetron sputtering on Al2O3(0001) substrates for irradiation studies and on Si(100) substrates for other characterizations. The films were subjected to 3-MeV Xe-ion irradiation at room temperature (RT) and at 500 degrees C. The crystal structure and mechanical properties of the films before and after irradiation were studied by x-ray diffraction and nanoindentation. Before irradiation, both the metallic and nitride multilayers displayed a lower hardness (7 and 20 GPa, respectively). Annealing at 500 degrees C for 150 min increased the hardness of the multilayer samples, but it also induced intermixing of elements between the sublayers of the metallic multilayer. Irradiation hardening was observed only in the metallic multilayer at room temperature. When comparing the effects of irradiation damage vs the effects of annealing on the mechanical properties, it was observed that annealing the multilayers had a more pronounced effect.
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| 4. |
- Gangaprasad Rao, Smita, 1992-, et al.
(author)
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Phase formation and structural evolution of multicomponent (CrFeCo)Ny films
- 2021
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In: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 412
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Journal article (peer-reviewed)abstract
- The Cantor alloy (CoCrFeMnNi) and its variants, in bulk as well as thin films, have been extensively studied. They are known to exhibit cubic crystal structures and thermodynamic stability regardless of their complex chemical composition. Therefore, they may find use as hard, wear-resistant, corrosion and oxidation-resistant coatings. The addition of light elements, such as nitrogen, is known to help improve these properties further through processes such as amorphization and nitride compound formation. Here, we investigate the ternary CrFeCo system to study the effects of nitrogen addition. (CrFeCo)Ny multicomponent thin films are grown on silicon substrates by DC magnetron sputtering. Changes in crystal structure, morphology, mechanical and electrical properties with gradual increases of nitrogen in the film are described and discussed. Increased addition of nitrogen from 14 at.% to 28 at.% in the film leads to a transformation from an fcc to a bcc crystal structure, affects both the mechanical and electrical properties. XPS analysis shows the tendency of nitrogen to bond with Cr over other metals. The films display hardness values between 7 and 11 GPa with resistivities values ranging between 28 and 165 μΩ cm.
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| 5. |
- Gangaprasad Rao, Smita, et al.
(author)
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Phase formation in CrFeCoNi nitride thin films
- 2023
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In: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 7:4
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Journal article (peer-reviewed)abstract
- As a single-phase alloy, CrFeCoNi is a face centered cubic (fcc) material related to the archetypical highentropy Cantor alloy CrFeCoNiMn. For thin films, CrFeCoNi of approximately equimolar composition tends to assume an fcc structure when grown at room temperature by magnetron sputtering. However, the single-phase solid solution state is typically not achieved for thin films grown at higher temperatures. The same holds true for Cantor alloy-based ceramics (nitrides and oxides), where phase formation is extremely sensitive to process parameters such as the amount of reactive gas. This study combines theoretical and experimental methods to understand the phase formation in nitrogen-containing CrFeCoNi thin films. Density functional theory calculations considering three competing phases (CrN, Fe-Ni and Co) show that the free energy of mixing, Delta G of (CrFeCoNi)(1-x)N-x solid solutions has a maximum at x = 0.20-0.25, and AG becomes lower when x < 0.20 and x > 0.25. Thin films of (CrFeCoNi)1-xNx (0.14 >= x <= 0.41) grown by magnetron sputtering show stabilization of the metallic fcc when x <= 0.22 and the stabilization of the NaCl B1 structure when x > 0.33, consistent with the theoretical prediction. In contrast, films with intermediate amounts of nitrogen (x = 0.22) grown at higher temperatures show segregation into multiple phases of CrN, Fe-Ni-rich and Co. These results offer an explanation for the requirement of kinetically limited growth conditions at low temperature for obtaining single-phase CrFeCoNi Cantor-like nitrogen-containing thin films and are of importance for understanding the phase-formation mechanisms in multicomponent ceramics. The results from the study further aid in making correlations between the observed mechanical properties and the crystal structure of the films.
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| 6. |
- Gangaprasad Rao, Smita, 1992-
(author)
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Phase formation in multicomponent films based on 3d transition metals
- 2021
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Licentiate thesis (other academic/artistic)abstract
- The need for materials that enhance life span, performance, and sustainability has propelled research in alloy design from binary alloys to more complex systems such as multicomponent alloys. The CoCrFeMnNi alloy, more commonly known as the Cantor alloy, is one of the most studied systems in bulk as well as thin film. The addition of light elements such as boron, carbon, nitrogen, and oxygen is a means to alter the properties of these materials. The challenge lies in understanding the process of phase formation and microstructure evolution on addition of these light elements. To address this challenge, I investigate multicomponent alloys based on a simplified version of the Cantor alloy.My thesis investigates the addition of nitrogen into a Cantor variant system as a step towards understanding the full Cantor alloy. Me1-yNy (Me = Cr + Fe + Co, 0.14 ≤ y ≤0.28 thin films were grown by reactive magnetron sputtering. The films showed a change in structure from fcc to mixed fcc+bcc and finally a bcc-dominant film with increasing nitrogen content. The change in phase and microstructure influenced the mechanical and electrical properties of the films. A maximum hardness of 11 ± 0.7 GPa and lowest electrical resistivity of 28 ± 5 μΩcm were recorded in the film with mixed phase (fcc+bcc) crystal structure.Copper was added as a fourth metallic alloying element into the film with the mixed fcc + bcc structure, resulting in stabilization of the bcc phase even though Cu has been reported to be a fcc stabilizer. The energy brought to the substrate increases on Cu addition which promotes surface diffusion of the ions and leads to small but randomly oriented grains. The maximum hardness recorded by nanoindentation was found to be 13.7 ± 0.2 GPa for the sample Cu0.05. While it is generally believed that large amounts of Cu can be detrimental to thin film properties due to segregation, this study shows that small amounts of Cu in the multicomponent matrix could be beneficial in stabilizing phases as well as for mechanical properties.This thesis thus provides insights into the phase formation of nitrogen-containing multicomponent alloys.
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| 7. |
- Gangaprasad Rao, Smita, 1992-, et al.
(author)
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Plasma diagnostics and film growth of multicomponent nitride thin films with magnetic-field-assisted-dc magnetron sputtering
- 2022
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In: Vacuum. - : Elsevier. - 0042-207X .- 1879-2715. ; 204
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Journal article (peer-reviewed)abstract
- During direct current magnetron sputtering (dcMS) of thin films, the ion energy and flux are complex parameters that influence thin film growth and can be exploited to tailor their properties. The ion energy is generally controlled by the bias voltage applied at the substrate. The ion flux density however is controlled by more complex mechanisms. In this study, we look into magnetic-field-assisted dcMs, where a magnetic field applied in the deposition chamber by use of a solenoid coil at the substrate position, influences the energetic bombardment by Ar ions during deposition. Using this technique, CrFeCoNi multicomponent nitride thin films were grown on Si(100) substrates by varying the bias voltage and magnetic field systematically. Plasma diagnostics were performed by a Langmuir wire probe and a flat probe. On interpreting the data from the current-voltage curves it was confirmed that the ion flux at the substrate increased with increasing coil magnetic field with ion energies corresponding to the applied bias. The increased ion flux assisted by the magnetic field produced by the solenoid coil aids in the stabilization of NaCl B1 crystal structure without introducing Ar ion implantation.
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| 8. |
- Gangaprasad Rao, Smita, et al.
(author)
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The effects of copper addition on phase composition in (CrFeCo)1-yNy multicomponent thin films
- 2022
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In: Applied Surface Science. - : Elsevier. - 0169-4332 .- 1873-5584. ; 572
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Journal article (peer-reviewed)abstract
- The Cantor alloy CrFeCoMnNi is generally fcc structured, but moderate changes in the composition can have a large influence on the phase formation. The aim of this study was to understand the changes brought on in lownitrogen-containing (CrFeCo)1-yNy thin films with y = 0.19 on the addition of copper, an interesting metal in terms of atomic size and nitride formation enthalpy. (CrFeCoCux)1-yNy films were grown by reactive magnetron sputtering. The amount of copper in the films was increased from x = 0 to x = 0.15 to study competitive phase formation. Without Cu, two-phase fcc + bcc films were obtained. The addition of Cu was found to stabilize the bcc structure despite the fact that Cu as a pure metal is fcc. Nanoindentation tests showed slight increase in hardness with initial Cu addition from 11 GPa to 13.7 +/- 0.2 GPa. The occurrence of pile up as opposed to cracking is an indication of the films ductility.
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| 9. |
- Gangaprasad Rao, Smita, et al.
(author)
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Thin film growth and mechanical properties of CrFeCoNi/TiNbZrTa multilayers
- 2022
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In: Materials & design. - : Elsevier Science Ltd. - 0264-1275 .- 1873-4197. ; 224
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Journal article (peer-reviewed)abstract
- Multilayers of high entropy alloys (HEA) are picking up interest due to the possibility of altering material properties by tuning crystallinity, thickness, and interfaces of the layers. This study investigates the growth mechanism and mechanical properties of CrFeCoNi/TiNbZrTa multilayers grown by magnetron sputtering. Multilayers of bilayer thickness (A) from 5 nm to 50 nm were grown on Si(1 0 0) substrates. Images taken by transmission electron microscopy and energy-dispersive X-ray spectroscopy mapping revealed that the layers were well defined with no occurrence of elemental mixing. Multilayers with A < 20 nm exhibited an amorphous structure. As A increased, the CrFeCoNi layer displayed a higher crystallinity in comparison to the amorphous TiNbZrTa layer. The mechanical properties were influenced by the crystallinity of the layers and stresses in the film. The film with A = 20 nm had the highest hardness of approximately 12.5 GPa owing grain refinement of the CrFeCoNi layer. An increase of A >= 30 nm resulted in a drop in the hardness due to the increase in crystal domains of the CrFeCoNi layer. Micropillar compression induced shear in the material rather than fracture, along with elemental intermixing in the core of the deformed region of the compressed micropillar.
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| 10. |
- Linder, Clara, et al.
(author)
-
Cobalt thin films as water-recombination electrocatalysts
- 2020
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In: Surface & Coatings Technology. - : Elsevier B.V.. - 0257-8972 .- 1879-3347. ; 404
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Journal article (peer-reviewed)abstract
- Catalysts and electrocatalysts are crucial for energy production and storage. To develop cost-efficient systems taking advantage of functionalized surfaces, the catalysts can be synthesized as nanomaterials or thin films. In this work, cobalt thin films were deposited on low-alloyed steel using magnetron sputtering. The films are uniform with a smooth surface and a thickness of ~400 nm. The films were electrochemically oxidized via anodization to a mix of cobalt oxides, one of them being Co3O4, at room temperature in an alkaline solution. The electrocatalytic performances of the anodized films were evaluated in 1 M KOH electrolyte saturated with oxygen. Cathodic currents in −0.5 mA/cm2 range, corresponding to oxygen reduction reaction (ORR) activity, were measured with cyclic voltammetry. The catalytic activity of the films was evaluated as a function of time. The anodized Co coating exhibited three times higher activity than the steel substrate. The kinetics for the ORR were evaluated through Tafel plots and a slope of 226 mV/decade was found. Post-ORR characterization of the films revealed hexagonal plate-like oxide particles on the surface. After 50 cyclic voltammograms, the film was further oxidized, indicating that the ORR activity also affects the overall surface state of the film. This study demonstrates that thin films, after electrochemical modification, can be electrocatalysts for the oxygen reduction reaction and potentially used for applications in energy production and storage. © 2020 The Authors
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| 11. |
- Linder, Clara, et al.
(author)
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Corrosion Resistance and Catalytic Activity toward the Oxygen Reduction Reaction of CoCrFexNi (0 ≤ x ≤ 0.7) Thin Films
- 2022
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In: ACS Applied Energy Materials. - : American Chemical Society. - 2574-0962. ; 5:9, s. 10838-10848
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Journal article (peer-reviewed)abstract
- Corrosion resistance and catalytic activity toward the oxygen reduction reaction (ORR) in an alkaline environment are two key properties for water recombination applications. In this work, CoCrFexNi (0 ≤ x ≤ 0.7) thin films were deposited by magnetron sputtering on polished steel substrates. The native passive layer was 2-4 nm thick and coherent to the columnar grains determined by transmission electron microscopy. The effect of Fe on the corrosion properties in 0.1 M NaCl and 1 M KOH and the catalytic activity of the films toward ORR were investigated. Electrochemical impedance spectroscopy and potentiodynamic polarization measurements indicate that CoCrFe0.7Ni and CoCrFe0.3Ni have the highest corrosion resistance of the studied films in NaCl and KOH, respectively. The high corrosion resistance of the CoCrFe0.7Ni film in NaCl was attributed to the smaller overall grain size, which leads to a more homogeneous film with a stronger passive layer. For CoCrFe0.3Ni in KOH, it was attributed to a lower Fe dissolution into the electrolyte and the build-up of a thick and protective hydroxide layer. Scanning Kelvin probe force microscopy showed no potential differences globally in any of the films, but locally, a potential gradient between the top of the columns and grain boundaries was observed. Corrosion of the films was likely initiated at the top of the columns where the potential was lowest. It was concluded that Fe is essential for the electrochemical activation of the surfaces and the catalytic activity toward ORR in an alkaline medium. The highest catalytic activity was recorded for high Fe-content films (x ≥ 0.5) and was attributed to the formation of platelet-like oxide particles on the film surface upon anodization. The study showed that the combination of corrosion resistance and catalytic activity toward ORR is possible for CoCrFexNi, making this material system a suitable candidate for water recombination in an alkaline environment.
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| 12. |
- Linder, Clara, et al.
(author)
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Effect of Mo content on the corrosion resistance of (CoCrFeNi)1−xMox thin films in sulfuric acid
- 2024
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In: Thin Solid Films. - : Elsevier B.V.. - 0040-6090 .- 1879-2731. ; 790
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Journal article (peer-reviewed)abstract
- (CoCrFeNi)1−xMox thin films with various Mo content (0–10 at.%) were grown by magnetron sputtering on a stainless steel substrate. The films with 0–2 at.% presented two crystal structures: one FCC phase and one sigma phase, while films with higher Mo content only had the FCC structure. All films have a (111) texture and follow the topography of the substrate. The corrosion resistance of the films was evaluated in 0.05 M H2SO4 at room temperature and at 80 °C. It was observed that the corrosion current densities considerably decreased for Mo > 2 at%, and that the current densities were higher at the elevated temperature. Scanning Kelvin Probe Force Microscopy showed a large potential difference between the main FCC phase and sigma phase for the Mo0–2 films. This would suggest that preferential dissolution of the FCC phase occurs over the sigma and reduces the corrosion resistance. Such preferential dissolution does not occur for the higher Mo content films with only the FCC phase. The high corrosion resistance was also attributed to the inhibition of Fe and Cr dissolution by Mo and the stabilisation of the Cr enriched oxide by incorporating Mo oxides into the passive film, identified by X-ray photoelectron spectroscopy. The low corrosion current densities (below 1 µA/cm2) make these thin films possible candidates for protective coatings of bipolar plates in PEM fuel cells.
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| 13. |
- Shu, Rui, 1990-, et al.
(author)
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Effect of nitrogen content on microstructure and corrosion resistance of sputter-deposited multicomponent (TiNbZrTa)Nx films
- 2020
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In: Surface & Coatings Technology. - LAUSANNE, SWITZERLAND : Elsevier BV. - 0257-8972 .- 1879-3347. ; 404
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Journal article (peer-reviewed)abstract
- Multicomponent (TiNbZrTa)Nx films were deposited on Si(100) substrates at room temperature using magnetron sputtering with a nitrogen flow ratio fN [fN = N2/(Ar + N2)], which was varied from 0 to 30.8%. The nitrogen content in the films varied between 0 and 45.2 at.%, i.e., x = 0 to 0.83. The microstructure was characterized by X-ray diffraction and electron microscopy. The metallic TiNbZrTa film comprised a dominant bcc solid-solution phase, whereas a single NaCl-type face-centred cubic structure was observed in all nitrogen-containing films (TiNbZrTa)Nx. The mechanical, electrical, and electrochemical properties of these films varied with nitrogen content. The maximum hardness was achieved at 22.1 ± 0.3 GPa when N = 43.0 at.%. The resistivities increased from 95 to 424 μΩcm with increasing nitrogen content. A detailed study of the variation of morphology and chemical bonding with nitrogen content was performed and the corrosion resistance of the TiNbZrTa nitride films was explored in 0.1 M H2SO4. While all the films had excellent corrosion resistances at potentials up to 2.0 V vs. Ag/AgCl, the metallic film and the films with low nitrogen contents (x < 0.60) exhibited an almost stable current plateau up to 4.0 V vs. Ag/AgCl. For the films with higher nitrogen contents (x ≥ 0.68), the current plateau was retained up to 2.0 V vs. Ag/AgCl, above which a higher nitrogen content resulted in a higher current. The decrease in the corrosion resistance at these high potentials indicate the presence of a potential-dependent activation effect resulting in an increased oxidation rate of the nitrides (present under the passive oxide film) yielding a release of nitrogen from the films. TEM results indicate that the oxide layer formed after this corrosion measurement was thick and porous for the film with x = 0.76, in very good agreement with the increased corrosion rate for this film. The results demonstrate that an increased nitrogen content in (TiNbZrTa)Nx system improves their mechanical properties with retained high corrosion resistance at potentials up to 2.0 V vs. Ag/AgCl in 0.1 M H2SO4. At even higher potentials, however, the corrosion resistance decreases with increasing nitrogen concentration for films with sufficiently high nitrogen contents (i.e. x ≥ 0.68).
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| 14. |
- Shu, Rui, 1990-, et al.
(author)
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Effects of alloying and deposition temperature on phase formation and superconducting properties of TiZrTaNb-based high entropy-alloy films
- 2022
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In: Applied Physics Letters. - : American Institute of Physics (AIP). - 0003-6951 .- 1077-3118. ; 120:15
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Journal article (peer-reviewed)abstract
- A variety of bulk high-entropy alloy superconductors have been recently discovered; however, for thin films, only the TaNbHfZrTi highentropy alloy system has been investigated for its superconducting properties. Here, (TiZrNbTa)1-xWx and (TiZrNbTa)1-xVx superconducting films have been produced by DC magnetron sputtering at different growth temperatures. The phase formation and superconducting behavior of these films depend on the content of alloying x and deposition temperature. A single body-centered cubic (bcc) phase can be formed in the low x range with enough driving energy for crystallinity, but phase transition between amorphous or two bcc structures is observed when increasing x. The highest superconducting transition temperature Tc reaches 8.0 K for the TiZrNbTa film. The superconducting transition temperature Tc of these films deposited at the same temperature decreases monotonically as a function of x. Increasing deposition temperature to 400 °C can enhance Tc for these films while retaining nearly equivalent compositions. Our experimental observations suggest that Tc of superconducting high entropy alloys relate to the atomic radii difference and electronegativity difference of involved elements beyond the valence electron number.
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| 15. |
- Shu, Rui, et al.
(author)
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Microstructure and mechanical, electrical, and electrochemical properties of sputter-deposited multicomponent (TiNbZrTa)N-x coatings
- 2020
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In: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 389
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Journal article (peer-reviewed)abstract
- A series of (TiNbZrTa)Nx coatings with a thickness of similar to 1.1 mu m were deposited using reactive magnetron sputtering with segmented targets. The deposition temperature was varied from room temperature to 700 degrees C resulting in coatings with different microstructures. The coatings were characterized by electron microscopy, atomic force microscopy, compositional analysis, and X-ray diffraction. Effects of the deposition temperature on the electrical, mechanical and corrosion properties were studied with four-point probe, nanoindentation and potentiodynamic polarization measurements, respectively. X-ray photoelectron spectroscopy (XPS) analyses reveal a gradual change in the chemical state of all elements with increasing growth temperature from nitridic at room temperature to metallic at 700 degrees C. A NaCl-type structure with (001) preferred orientation was observed in the coating deposited at 400 degrees C, while an hcp structure was found for the coatings deposited above 400 degrees C. The resistivities of the TiNbZrTa nitride coatings were found to be around 200 mu Ocm. In 0.1 M H2SO4 aqueous solution, a corrosion current density of 2.8 x 10(-8) A/cm(2) and a passive behaviour up to 1.5 V vs. Ag/AgCl were found for the most corrosion resistant coating. The latter corrosion current is about two orders of magnitude lower than that found for a reference hyper-duplex stainless steel.
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| 16. |
- Shu, Rui, 1990-, et al.
(author)
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Stoichiometry Effects on the Chemical Ordering and Superconducting Properties in TiZrTaNbNx Refractory High Entropy Nitrides
- 2023
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In: Annalen der Physik. - : Wiley-VCH Verlagsgesellschaft. - 0003-3804 .- 1521-3889.
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Journal article (peer-reviewed)abstract
- High-entropy materials, an exciting new class of structural materials involvingfive or more elements, are emerging as unexplored ground forsuperconductors. Here, the effects of nitrogen stoichiometry are investigatedon local chemical structure of TiZrNbTa-based thin films by variousX-ray-based techniques. Lattice distortion and short-range order of a set ofTiZrNbTaNxsamples, including bond lengths of different atomic pairs andcoordination numbers of substituting atoms are quantitatively studied. Themaximum superconducting transition temperature Tcis found at 10 K for anear-stoichiometric (TiZrNbTa)N1.08film, which is>8 K measured for ametallic TiZrNbTa film. The underlying electronic structure and chemicalbonding in these high entropy nitrides thus influence the superconductingmacroscopic properties.
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| 17. |
- Wang, Ji, et al.
(author)
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Structural stability under Xe-ion irradiation of TiZrNbTaV-based high-entropy alloy and nitride films
- 2023
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In: Surface & Coatings Technology. - : ELSEVIER SCIENCE SA. - 0257-8972 .- 1879-3347. ; 454
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Journal article (peer-reviewed)abstract
- Refractory high-entropy protective coatings are of interest for nuclear fuel cladding applications due to their corrosion resistant properties and irradiation resistance at elevated temperature. Here, TiZrNbTaV metallic and (TiZrNbTaV)N films were deposited by magnetron co-sputtering. The metal elemental contents of both films were nearly equiatomic. These films were irradiated by Xe ions at room temperature and 500 degrees C, and examined by X-ray diffraction and transmission electron microscopy. The as-deposited (TiZrNbTaV)N film showed a single NaCl-type fcc phase and a pronounced columnar growth structure, which could remain intact after irradiation treatments. In contrast, the as-deposited TiZrNbTaV film exhibited an amorphous structure and formed a bcc phase structure after irradiation at 500 degrees C. The TiZrNbTaV film after irradiation at 500 degrees C composed of depth -dependent size of grains. This distribution of grain size is consistent with simulated displacement damage. The stable structure of (TiZrNbTaV)N film under high temperature irradiation indicates that these materials have potential for use as protective coatings for nuclear fuel claddings.
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| 18. |
- Wang, Ji, et al.
(author)
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Xe-ion-irradiation-induced structural transitions and elemental diffusion in high-entropy alloy and nitride thin-film multilayers
- 2022
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In: Materials & design. - : Elsevier Science Ltd. - 0264-1275 .- 1873-4197. ; 219
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Journal article (peer-reviewed)abstract
- The study aims to understand the irradiation behavior of multilayer coatings composed of high-entropy materials. Here, we report the structural stability and elemental segregation of high-entropy TiNbZrTa/CrFeCoNi metallic and nitride multilayer coatings under 3-MeV Xe20+ ion-irradiation at room temperature and 500 degrees C, respectively. Transmission electron microscopy analysis shows that the microstructure of nanocrystalline CrFeCoNi high-entropy-alloy sublayers are not stable and readily transforms into amorphous state at 500 degrees C and/or under irradiation conditions. The elemental distribution, acquired by energy-dispersive X-ray spectroscopy under scanning transmission electron microscopy mode, shows preferential diffusion of Co and Ni into TiNbZrTa sublayers, while Fe and Cr preferentially remain within the previous CrFeCoNi sublayers. TiNbZrTaN/CrFeCoNiNx nitride multilayers exhibit a higher crystallinity and structural stability as well as resistance to diffusion at high-temperature and/or irradiation conditions than their TiNbZrTa/CrFeCoNi metallic multilayer counterparts. These findings are explained by atomic size differences, the difference in Gibbs free energy of the mixing system, and interstitial-solute-induced chemical heterogeneity. Our findings thus provide a design strategy of high entropy nitride for nuclear fuel cladding. (C) 2022 The Author(s). Published by Elsevier Ltd.
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