| 1. |
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| 2. |
- Joffrin, E., et al.
(författare)
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Overview of the JET preparation for deuterium-tritium operation with the ITER like-wall
- 2019
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Ingår i: Nuclear Fusion. - : IOP Publishing. - 1741-4326 .- 0029-5515. ; 59:11
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Forskningsöversikt (refereegranskat)abstract
- For the past several years, the JET scientific programme (Pamela et al 2007 Fusion Eng. Des. 82 590) has been engaged in a multi-campaign effort, including experiments in D, H and T, leading up to 2020 and the first experiments with 50%/50% D-T mixtures since 1997 and the first ever D-T plasmas with the ITER mix of plasma-facing component materials. For this purpose, a concerted physics and technology programme was launched with a view to prepare the D-T campaign (DTE2). This paper addresses the key elements developed by the JET programme directly contributing to the D-T preparation. This intense preparation includes the review of the physics basis for the D-T operational scenarios, including the fusion power predictions through first principle and integrated modelling, and the impact of isotopes in the operation and physics of D-T plasmas (thermal and particle transport, high confinement mode (H-mode) access, Be and W erosion, fuel recovery, etc). This effort also requires improving several aspects of plasma operation for DTE2, such as real time control schemes, heat load control, disruption avoidance and a mitigation system (including the installation of a new shattered pellet injector), novel ion cyclotron resonance heating schemes (such as the three-ions scheme), new diagnostics (neutron camera and spectrometer, active Alfven eigenmode antennas, neutral gauges, radiation hard imaging systems...) and the calibration of the JET neutron diagnostics at 14 MeV for accurate fusion power measurement. The active preparation of JET for the 2020 D-T campaign provides an incomparable source of information and a basis for the future D-T operation of ITER, and it is also foreseen that a large number of key physics issues will be addressed in support of burning plasmas.
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| 3. |
- Murari, A., et al.
(författare)
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A control oriented strategy of disruption prediction to avoid the configuration collapse of tokamak reactors
- 2024
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Ingår i: Nature Communications. - 2041-1723 .- 2041-1723. ; 15:1
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Tidskriftsartikel (refereegranskat)abstract
- The objective of thermonuclear fusion consists of producing electricity from the coalescence of light nuclei in high temperature plasmas. The most promising route to fusion envisages the confinement of such plasmas with magnetic fields, whose most studied configuration is the tokamak. Disruptions are catastrophic collapses affecting all tokamak devices and one of the main potential showstoppers on the route to a commercial reactor. In this work we report how, deploying innovative analysis methods on thousands of JET experiments covering the isotopic compositions from hydrogen to full tritium and including the major D-T campaign, the nature of the various forms of collapse is investigated in all phases of the discharges. An original approach to proximity detection has been developed, which allows determining both the probability of and the time interval remaining before an incoming disruption, with adaptive, from scratch, real time compatible techniques. The results indicate that physics based prediction and control tools can be developed, to deploy realistic strategies of disruption avoidance and prevention, meeting the requirements of the next generation of devices.
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| 4. |
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| 5. |
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| 6. |
- Cedervall, Johan, et al.
(författare)
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Magnetocaloric effect in Fe2 P: Magnetic and phonon degrees of freedom
- 2019
-
Ingår i: Physical Review B. - 2469-9969 .- 2469-9950. ; 99:17
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Tidskriftsartikel (refereegranskat)abstract
- Devices based on magnetocaloric materials provide great hope for environmentally friendly and energy efficient cooling that does not rely on the use of harmful gasses. Fe2P based compounds are alloys that have shown great potential for magnetocaloric devices. The magnetic behavior in Fe2P is characterized by a strong magnetocaloric effect that coexists with a first-order magnetic transition (FOMT). Neutron diffraction and inelastic scattering, Mossbauer spectroscopy, and first-principles calculations have been used to determine the structural and magnetic state of Fe2P around the FOMT. The results reveal that ferromagnetic moments in the ordered phase are perturbed at the FOMT such that the moments cant away from the principle direction within a small temperature region. The acoustic-phonon modes reveal a temperature-dependent nonzero energy gap in the magnetically ordered phase that falls to zero at the FOMT. The interplay between the FOMT and the phonon energy gap indicates hybridization between magnetic modes strongly affected by spin-orbit coupling and phonon modes leading to magnon-phonon quasiparticles that drive the magnetocaloric effect.
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| 7. |
- Dahlqvist, Martin, et al.
(författare)
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Magnetically driven anisotropic structural changes in the atomic laminate Mn2GaC
- 2016
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Ingår i: Physical Review B. - : AMER PHYSICAL SOC. - 2469-9950 .- 2469-9969. ; 93:1
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Tidskriftsartikel (refereegranskat)abstract
- Inherently layered magnetic materials, such as magnetic M(n+1)AX(n) (MAX) phases, offer an intriguing perspective for use in spintronics applications and as ideal model systems for fundamental studies of complex magnetic phenomena. The MAX phase composition M(n+1)AX(n) consists of M(n+1)AX(n) blocks separated by atomically thin A-layers where M is a transition metal, A an A-group element, X refers to carbon and/or nitrogen, and n is typically 1, 2, or 3. Here, we show that the recently discovered magnetic Mn2GaC MAX phase displays structural changes linked to the magnetic anisotropy, and a rich magnetic phase diagram which can be manipulated through temperature and magnetic field. Using first-principles calculations and Monte Carlo simulations, an essentially one-dimensional (1D) interlayer plethora of two-dimensioanl (2D) Mn-C-Mn trilayers with robust intralayer ferromagnetic spin coupling was revealed. The complex transitions between them were observed to induce magnetically driven anisotropic structural changes. The magnetic behavior as well as structural changes dependent on the temperature and applied magnetic field are explained by the large number of low energy, i.e., close to degenerate, collinear and noncollinear spin configurations that become accessible to the system with a change in volume. These results indicate that the magnetic state can be directly controlled by an applied pressure or through the introduction of stress and show promise for the use of Mn2GaC MAX phases in future magnetoelectric and magnetocaloric applications.
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| 8. |
- Ericsson, A., et al.
(författare)
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Crystallization of a Zr-based metallic glass produced by laser powder bed fusion and suction casting
- 2021
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Ingår i: Journal of Non-Crystalline Solids. - : Elsevier. - 0022-3093 .- 1873-4812. ; 571
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Tidskriftsartikel (refereegranskat)abstract
- The crystallization behaviour during low temperature annealing of samples of the Zr59.3Cu28.8Al10.4Nb1.5 (at%) bulk metallic glass produced by suction casting and the laser powder bed fusion (LPBF) process were studied with small angle neutron scattering (SANS), X-ray diffraction and scanning electron microscopy. The in-situ SANS measurements during isothermal annealing reveals that the phase separation in the LPBF processed material proceeds at a smaller characteristic length-scale than the cast material. Quantitative analysis of the SANS data shows that, while the crystallization process in both materials proceed through rapid nucleation followed by diffusion limited growth, the LPBF processed material crystallizes with a smaller cluster size and at a higher rate. The smaller cluster size is attributed to the elevated oxygen content in the LPBF processed material which reduces the nucleation barrier and thus the thermal stability.
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| 9. |
- Ericsson, A., et al.
(författare)
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Transient nucleation in selective laser melting of Zr-based bulk metallic glass
- 2020
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Ingår i: Materials & design. - : Elsevier. - 0264-1275 .- 1873-4197. ; 195
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Tidskriftsartikel (refereegranskat)abstract
- The crystallization rate during selective laser melting (SLM) of bulk metallic glasses (BMG) is a critical factor in maintaining the material's amorphous structure. To increase the understanding of the interplay between the SLM process and the crystallization behavior of BMGs, a numerical model based on the classical nucleation theory has been developed that accounts for the rapid temperature changes associated with SLM. The model is applied to SLM of a Zr-based BMG and it is shown that the transient effects, accounted for by the model, reduce the nucleation rate by up to 15 orders of magnitude below the steady-state nucleation rate on cooling, resulting in less nuclei during the build process. The capability of the proposed modelling approach is demonstrated by comparing the resulting crystalline volume fraction to experimental findings. The agreement between model predictions and the experimental results clearly suggests that transient nucleation effects must be accounted for when considering the crystallization rate during SLM processing of BMGs.
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| 10. |
- Lindwall, Johan, et al.
(författare)
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Simulation of phase evolution in a Zr-based glass forming alloy during multiple laser remelting
- 2022
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Ingår i: Journal of Materials Research and Technology. - : Elsevier. - 2238-7854 .- 2214-0697. ; 16, s. 1165-1178
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Tidskriftsartikel (refereegranskat)abstract
- Additive manufacturing by laser-based powder bed fusion is a promising technique for bulk metallic glass production. But, reheating by deposition of subsequent layers may cause local crystallisation of the alloy. To investigate the crystalline phase evolution during laser scanning of a Zr-based metallic glass-forming alloy, a simulation strategy based on the finite element method and the classical nucleation theory has been developed and compared with experimental results from multiple laser remelting of a single-track. Multiple laser remelting of a single-track demonstrates the crystallisation behaviour by the influence of thermal history in the reheated material. Scanning electron microscopy and transmission electron microscopy reveals the crystalline phase evolution in the heat affected zone after each laser scan. A trend can be observed where repeated remelting results in an increased crystalline volume fraction with larger crystals in the heat affected zone, both in simulation and experiment. A gradient of cluster number density and mean radius can also be predicted by the model, with good correlation to the experiments. Prediction of crystallisation, as presented in this work, can be a useful tool to aid the development of process parameters during additive manufacturing for bulk metallic glass formation.
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| 11. |
- Pasquini, Luca, et al.
(författare)
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Magnesium- and intermetallic alloys-based hydrides for energy storage : modelling, synthesis and properties
- 2022
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Ingår i: Progress in Energy. - : Institute of Physics Publishing (IOPP). - 2516-1083. ; 4:3
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Forskningsöversikt (refereegranskat)abstract
- Hydrides based on magnesium and intermetallic compounds provide a viable solution to the challenge of energy storage from renewable sources, thanks to their ability to absorb and desorb hydrogen in a reversible way with a proper tuning of pressure and temperature conditions. Therefore, they are expected to play an important role in the clean energy transition and in the deployment of hydrogen as an efficient energy vector. This review, by experts of Task 40 'Energy Storage and Conversion based on Hydrogen' of the Hydrogen Technology Collaboration Programme of the International Energy Agency, reports on the latest activities of the working group 'Magnesium- and Intermetallic alloys-based Hydrides for Energy Storage'. The following topics are covered by the review: multiscale modelling of hydrides and hydrogen sorption mechanisms; synthesis and processing techniques; catalysts for hydrogen sorption in Mg; Mg-based nanostructures and new compounds; hydrides based on intermetallic TiFe alloys, high entropy alloys, Laves phases, and Pd-containing alloys. Finally, an outlook is presented on current worldwide investments and future research directions for hydrogen-based energy storage.
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| 12. |
- Tidefelt, Mattias, et al.
(författare)
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In Situ Mapping of Phase Evolutions in Rapidly Heated Zr-Based Bulk Metallic Glass with Oxygen Impurities
- 2024
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Ingår i: Advanced Science. - : John Wiley & Sons. - 2198-3844. ; 11:16
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Tidskriftsartikel (refereegranskat)abstract
- Metallic glasses exhibit unique mechanical properties. For metallic glass composites (MGC), composed of dispersed nanocrystalline phases in an amorphous matrix, these properties can be enhanced or deteriorated depending on the volume fraction and size distribution of the crystalline phases. Understanding the evolution of crystalline phases during devitrification of bulk metallic glasses upon heating is key to realizing the production of these composites. Here, results are presented from a combination of in situ small- and wide-angle X-ray scattering (SAXS and WAXS) measurements during heating of Zr-based metallic glass samples at rates ranging from 102 to 104 Ks-1 with a time resolution of 4ms. By combining a detailed analysis of scattering experiments with numerical simulations, for the first time, it is shown how the amount of oxygen impurities in the samples influences the early stages of devitrification and changes the dominant nucleation mechanism from homogeneous to heterogeneous. During melting, the oxygen rich phase becomes the dominant crystalline phase whereas the main phases dissolve. The approach used in this study is well suited for investigation of rapid phase evolution during devitrification, which is important for the development of MGC. Oxygen impurities impact on phase-transformations during rapid heating of Zr-based metallic glass Zr59.3Cu28.8Al10.4Nb1.5 is thoroughly investigated using a multi-technique approach. During devitrification, the extracted phase evolutions reveal that the phase fraction hierarchy correlates with the oxygen impurity concentration. Numerical simulations with a heterogeneous nucleation mode capture the experimental observations. During melting, the oxygen-rich phase becomes the dominant phase. image
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| 13. |
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| 14. |
- Asfaw, Habtom D., et al.
(författare)
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Boosting the thermal stability of emulsion–templated polymers via sulfonation : an efficient synthetic route to hierarchically porous carbon foams
- 2016
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Ingår i: ChemistrySelect. - : Wiley. - 2365-6549. ; 1:4, s. 784-792
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Tidskriftsartikel (refereegranskat)abstract
- Hierarchically porous carbon foams with specific surface areas exceeding 600 m2 g−1 can be derived from polystyrene foams that are synthesized via water-in-oil emulsion templating. However, most styrene-based polymers lack strong crosslinks and are degraded to volatile products when heated above 400 oC. A common strategy employed to avert depolymerization is to introduce potential crosslinking sites such as sulfonic acids by sulfonating the polymers. This article unravels the thermal and chemical processes leading up to the conversion of sulfonated high internal phase emulsion polystyrenes (polyHIPEs) to sulfur containing carbon foams. During pyrolysis, the sulfonic acid groups (-SO3H) are transformed to sulfone (-C-SO2-C-) and then to thioether (-C−S-C-) crosslinks. These chemical transformations have been monitored using spectroscopic techniques: in situ IR, Raman, X-ray photoelectron and X-ray absorption near edge structure spectroscopy. Based on thermal analyses, the formation of thioether links is associated with increased thermal stability and thus a substantial decrease in volatilization of the polymers.
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| 15. |
- Borisov, Vladislav, et al.
(författare)
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Tuning skyrmions in B20 compounds by 4d and 5d doping
- 2022
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Ingår i: Physical Review Materials. - : American Physical Society (APS). - 2475-9953. ; 6:8
-
Tidskriftsartikel (refereegranskat)abstract
- Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions similar to 50 nm are predicted for Co0.75Os0.25Si, compared to similar to 148 nm for Fe0.75Co0.25Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.
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| 16. |
- Caron, L., et al.
(författare)
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Magnetocrystalline anisotropy and the magnetocaloric effect in Fe2P
- 2013
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Ingår i: Physical Review B. Condensed Matter and Materials Physics. - 1098-0121 .- 1550-235X. ; 88:9, s. 094440-
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Tidskriftsartikel (refereegranskat)abstract
- Magnetic and magnetocaloric properties of high-purity, giant magnetocaloric polycrystalline and single-crystalline Fe2P are investigated. Fe2P displays a moderate magnetic entropy change, which spans over 70 K and the presence of strong magnetization anisotropy proves this system is not fully itinerant but displays a mix of itinerant and localized magnetism. The properties of pure Fe2P are compared to those of giant magnetocaloric (Fe,Mn)2(P,A) (where A = As, Ge, Si) compounds helping understand the exceptional characteristics shown by the latter, which are so promising for heat pump and energy conversion applications.
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| 17. |
- Casillas Trujillo, Luis, et al.
(författare)
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Interstitial carbon in bcc HfNbTiVZr high-entropy alloy from first principles
- 2020
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Ingår i: Physical Review Materials. - : AMER PHYSICAL SOC. - 2475-9953. ; 4:12
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Tidskriftsartikel (refereegranskat)abstract
- The remarkable mechanical properties of high-entropy alloys can be further improved by interstitial alloying. In this work we employ density functional theory calculations to study the solution energies of dilute carbon interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our results indicate that carbon interstitials in tetrahedral sites are unstable, and the preferred octahedral sites present a large spread in the energy of solution. The inclusion of carbon interstitials induces large structural relaxations with long-range effects. The effect of local chemical environment on the energy of solution is investigated by performing a local cluster expansion including studies of its correlation with the carbon atomic Voronoi volume. However, the spread in solution energetics cannot be explained with a local environment analysis only pointing towards a complex, long-range influence of interstitial carbon in this alloy.
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| 18. |
- Cedervall, Johan, et al.
(författare)
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Influence of cobalt substitution on the magnetic properties of Fe5PB2
- 2018
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Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 57:2, s. 777-784
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Tidskriftsartikel (refereegranskat)abstract
- In this study the effects of cobalt substitutions in Fe5PB2 have been studied. An increased cobalt content reduces the magnetic exchange interactions. This has been concluded from a large, linear decrease in both the Curie temperature as well as the saturated magnetic moment. At high cobalt concentrations, cobalt prefers to order at the M(2) position in the crystal structure. A tunable Curie transition like this shows some prerequisites for magnetic cooling applications.The substitutional effects of cobalt in (Fe1–xCox)5PB2 have been studied with respect to crystalline structure and chemical order with X-ray diffraction and Mössbauer spectroscopy. The magnetic properties have been determined from magnetic measurements, and density functional theory calculations have been performed for the magnetic properties of both the end compounds, as well as the chemically disordered intermediate compounds. The crystal structure of (Fe1–xCox)5PB2 is tetragonal (space group I4/mcm) with two different metal sites, with a preference for cobalt atoms in the M(2) position (4c) at higher cobalt contents. The substitution also affects the magnetic properties with a decrease of the Curie temperature (TC) with increasing cobalt content, from 622 to 152 K for Fe5PB2 and (Fe0.3Co0.7)5PB2, respectively. Thus, the Curie temperature is dependent on composition, and it is possible to tune TC to a temperature near room temperature, which is one prerequisite for magnetic cooling materials.
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| 19. |
- Cedervall, Johan, et al.
(författare)
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Low temperature magneto-structural transitions in Mn3Ni20P6
- 2016
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Ingår i: Journal of Solid State Chemistry. - : Academic Press Inc.. - 0022-4596 .- 1095-726X. ; 237, s. 343-348
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Tidskriftsartikel (refereegranskat)abstract
- X-ray and neutron powder diffraction has been used to determine the crystal and magnetic structure of Mn3Ni20P6. The crystal structure can be described as cubic with space group Fm3¯m (225) without any nuclear phase transformation within studied temperature interval from room temperature down to 4 K. The magnetic structure of Mn3Ni20P6 is complex with two independent magnetic positions for the Mn atoms and the compound passes three successive magnetic phase transitions during cooling. At 30 K the spins of the Mn atoms on the Wyckoff 4a site (Mn1) order to form a primitive cubic antiferromagnetic structure with propagation vector k=(0 0 1). Between 29 and 26 K the Mn atoms on the Wyckoff 8c site (Mn2) order independently on already ordered Mn1 magnetic structure forming a commensurate antiferromagnetic structure with propagation vector k=(0 0 12) and below 26 K, both Mn positions order to form an incommensurate helical structure with propagation vector k=(0 0 ~0.45). Magnetization vs. temperature curve of Mn3Ni20P6 shows a steep increase indicating some magnetic ordering below 230 K and a sharp field dependent anomaly in a narrow temperature range around 30 K.
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| 20. |
- Cedervall, J., et al.
(författare)
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Magnetic and mechanical effects of Mn substitutions in AlFe 2 B 2
- 2019
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Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853. ; 482, s. 54-60
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Tidskriftsartikel (refereegranskat)abstract
- The mechanical and magnetic properties of the newly discovered MAB-phase class of materials based upon AlFe 2 B 2 were investigated. The samples were synthesised from stoichiometric amounts of all constituent elements. X-ray diffraction shows that the main phase is orthorhombic with an elongated b-axis, similar to AlFe 2 B 2 . The low hardness and visual inspection of the samples after deformation indicate that these compounds are deformed via a delamination process. When substituting iron in AlFe 2 B 2 with manganese, the magnetism in the system goes from being ferro- to antiferromagnetic via a disordered ferrimagnetic phase exhibited by AlFeMnB 2 . Density functional theory calculations indicate a weakening of the magnetic interactions among the transitions metal ions as iron is substituted by manganese in AlFe 2 B 2 . The Mn-Mn exchange interactions in AlMn 2 B 2 are found to be very small.
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| 21. |
- Cedervall, Johan, et al.
(författare)
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Magnetic and mechanical effects of Mn substitutions in AlFe2B2
- 2019
-
Ingår i: Journal of Magnetism and Magnetic Materials. - : Elsevier BV. - 0304-8853 .- 1873-4766. ; 482, s. 54-60
-
Tidskriftsartikel (refereegranskat)abstract
- The mechanical and magnetic properties of the newly discovered MAB-phase class of materials based upon AlFe2B2 were investigated. The samples were synthesised from stoichiometric amounts of all constituent elements. X-ray diffraction shows that the main phase is orthorhombic with an elongated b-axis, similar to AlFe2B2. The low hardness and visual inspection of the samples after deformation indicate that these compounds are deformed via a delamination process. When substituting iron in AlFe2B2 with manganese, the magnetism in the system goes from being ferro- to antiferromagnetic via a disordered ferrimagnetic phase exhibited by AlFeMnB2. Density functional theory calculations indicate a weakening of the magnetic interactions among the transitions metal ions as iron is substituted by manganese in AlFe2B2. The Mn-Mn exchange interactions in AlMn2B2 are found to be very small.
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| 22. |
- Cedervall, Johan
(författare)
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Magnetic Materials for Cool Applications : Relations between Structure and Magnetism in Rare Earth Free Alloys
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- New and more efficient magnetic materials for energy applications are a big necessity for sustainable future. Whether the application is energy conversion or refrigeration, materials based on sustainable elements should be used, which discards all rare earth elements. For energy conversion, permanent magnets with high magnetisation and working temperature are needed whereas for refrigeration, the entropy difference between the non-magnetised and magnetised states should be large. For this reason, magnetic materials have been synthesised with high temperature methods and structurally and magnetically characterised with the aim of making a material with potential for large scale applications. To really determine the cause of the physical properties the connections between structure (crystalline and magnetic) and, mainly, the magnetic properties have been studied thoroughly.The materials that have been studied have all been iron based and exhibit properties with potential for the applications in mind. The first system, for permanent magnet applications, was Fe5SiB2. It was found to be unsuitable for a permanent magnet, however, an interesting magnetic behaviour was studied at low temperatures. The magnetic behaviour arose from a change in the magnetic structure which was solved by using neutron diffraction. Substitutions with phosphorus (Fe5Si1-xPxB2) and cobalt (Fe1-xCox)5PB2 were then performed to improve the permanent magnet potential. While the permanent magnetic potential was not improved with cobalt substitutions the magnetic transition temperature could be greatly controlled, a real benefit for magnetic refrigeration. For this purpose AlFe2B2 was also studied, and there it was found, conclusively, that the material undergoes a second order transition, making it unsuitable for magnetic cooling. However, the magnetic structure was solved with two different methods and was found to be ferromagnetic with all magnetic moments aligned along the crystallographic a-direction. Lastly, the origin of magnetic cooling was studied in Fe2P, and can be linked to the interactions between the magnetic and atomic vibrations.
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| 23. |
- Cedervall, Johan, et al.
(författare)
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Magnetic structure of the magnetocaloric compound AlFe2B2
- 2016
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Ingår i: Journal of Alloys and Compounds. - : Elsevier. - 0925-8388 .- 1873-4669. ; 664, s. 784-791
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Tidskriftsartikel (refereegranskat)abstract
- The crystal and magnetic structures of AlFe2B2 have been studied with a combination of X-ray and neutron diffraction and electronic structure calculations. The magnetic and magnetocaloric properties have been investigated by magnetisation measurements. The samples have been produced using high temperature synthesis and subsequent heat treatments. The compound crystallises in the orthorhombic crystal system Cmmm and it orders ferromagnetically at 285 K through a second order phase transition. At temperatures below the magnetic transition the magnetic moments align along the crystallographic a-axis. The magnetic entropy change from 0 to 800 kA/m was found to be - 1.3 J/K kg at the magnetic transition temperature.
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| 24. |
- Cedervall, Johan, et al.
(författare)
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Magnetostructural transition in Fe5SiB2 observed with neutron diffraction
- 2016
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 235, s. 113-118
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Tidskriftsartikel (refereegranskat)abstract
- The crystal and magnetic structure of Fe5SiB2 has been studied by a combination of X-ray and neutron diffraction. Also, the magnetocrystalline anisotropy energy constant has been estimated from magnetisation measurements. High quality samples have been prepared using high temperature synthesis and subsequent heat treatment protocols. The crystal structure is tetragonal within the space group I4/mcm and the compound behaves ferromagnetically with a Curie temperature of 760 K. At 172 K a spin reorientation occurs in the compound and the magnetic moments go from aligning along the c-axis (high T) down to the ab-plane (low T). The magnetocrystalline anisotropy energy constant has been estimated to 0.3 MJ/m3 at 300 K.
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| 25. |
- Cedervall, Johan, et al.
(författare)
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Mössbauer study of the magnetocaloric compound AlFe2B2
- 2016
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Ingår i: Hyperfine Interactions. - : Springer Science and Business Media LLC. - 0304-3843 .- 1572-9540. ; 237
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Tidskriftsartikel (refereegranskat)abstract
- Mössbauer spectroscopy in the ferromagnetic AlFe2B2 reveals Tc=299 K and shows good agreement with magnetic measurements. The crystals are plate-shaped. The flakes are found from X-ray diffraction to be in the crystallographic ac-plane in the orthorhombic system. The axes of the principle electric field gradient tensor are, by symmetry, colinear with the crystal a-, b- and c-axes. By using information about the quadrupole splitting and line asymmetry in the paramagnetic regime together with the quadrupole shift of the resonance lines in the ferromagnetic regime the magnetic hyperfine field direction is found to be in the ab-plane having an angle =40° to the b-axis.
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| 26. |
- Cedervall, Johan, et al.
(författare)
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On the structural and magnetic properties of the double perovskite Nd2NiMnO6Nd2NiMnO6
- 2019
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Ingår i: Journal of Materials Science: Materials in Electronics. - : Springer Science and Business Media LLC. - 1573-482X .- 0957-4522. ; 30:17, s. 16571-16578
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Tidskriftsartikel (refereegranskat)abstract
- The structural, electronic and magnetic properties of phase pure and stoichiometric samples of the double perovskite Nd 2NiMnO 6 have been investigated with a combination of X-ray and neutron diffraction, X-ray photoelectron spectroscopy and magnetometry. It is found that the monoclinic space group P2 1/ n best describes the crystal structure of Nd 2NiMnO 6. Photoectron spectroscopy revels a mixed valence of the transition metal sites where Ni has 3+/2+ oxidation states and Mn has 3+/4+. The compound orders ferromagnetically at ∼195 K. The magnetic structure was determined from the refinement of the neutron diffraction data. The results suggests that the B-site magnetic moments align along the crystallographic a-direction.
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| 27. |
- Cedervall, Johan, et al.
(författare)
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Phase stability and structural transitions in compositionally complex LnMO 3 perovskites
- 2021
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Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 300
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Tidskriftsartikel (refereegranskat)abstract
- Entropy stabilised materials have possibilities for tailoring functionalities to overcome challenges in materials science. The concept of configurational entropy can also be applied to metal oxides, but it is unclear whether these could be considered as solid solutions in the case of perovskite-structured oxides and if the configurational entropy plays a stabilising role. In this study, compositionally complex perovskite oxides, LnMO3 (Ln = La, Nd, Sm, Ca and Sr, M = Ti, Cr, Mn, Fe, Co, Ni, and Cu), are investigated for their phase stability and magnetic behaviour. Phase-pure samples were synthesised, and the room temperature structures were found to crystallise in either Pnma or R3¯c space groups, depending on the composition and the resulting tolerance factor, while the structural transition temperatures correlate with the pseudo cubic unit cell volume. The techniques used included diffraction with X-rays and neutrons, both ex- and in-situ, X-ray photoelectron spectroscopy, magnetometry as well as electron microscopy. Neutron diffraction studies on one sample reveal that no oxygen vacancies are found in the structure and that the magnetic properties are ferrimagnetic-like with magnetic moments mainly coupled antiferromagnetically along the crystallographic c-direction. X-ray photoelectron spectroscopy gave indications of the oxidation states of the constituting ions where several mixed oxidation states are observed in these valence-compensated perovskites.
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| 28. |
- Cedervall, Johan
(författare)
-
Structure-Magnetism Relations in Selected Iron-based Alloys : A New Base for Rare Earth Free Magnetic Materials
- 2015
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Materials for energy applications are of great importance for a sustainable future society. Among these, stronger, lighter and more efficient magnetic materials will be able to aid mankind in many applications for energy conversion, for example generators for energy production, electric vehicles and magnetic refrigerators. Another requirement for the materials is that they should be made from cheap and abundant elements. For these reasons temperature induced magnetic transitions for three materials were studied in this work; one for permanent magnet applications and two magnetocaloric materials.Fe5SiB2 has a high Curie temperature and orders ferromagnetically at 760 K, providing possible application as a permanent magnet material. The ordering of the magnetic moments were studied and found to be aligned along the tetragonal c-axis and Fe5SiB2 undergoes a spin transition on cooling through a transition temperature (172 K), where the spins reorient along the a-axis in an easy plane.AlFe2B2 orders ferromagnetically at 285 K, making it a candidate for the active material in a magnetic refrigerator. The order of the magnetic transition has been studied as well as the magnetic structure. It was found that the magnetic moments are aligned along the crystallographic a-axis and that the magnetic transition is of second order.FeMnP0.75Si0.25 undergoes a first order magnetic transition around 200 K and the transition temperatures on cooling are different for the first cooling/heating cycle than for following cycles. This so called ”virgin effect” has been studied and found to originate from an irreversible structure change on the first cooling cycle through the ferromagnetic transition temperature.
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| 29. |
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| 30. |
- Chou, Chia-Ying, et al.
(författare)
-
Precipitation kinetics during post-heat treatment of a L-PBF processed SS446 ferritic stainless steel
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- The microstructure response to post-heat treatments of SS446 ferritic stainless steel processed by laser powder bed fusion is investigated, with focus on the precipitation kinetics of Cr2N. Precipitation simulations are conducted using the precipitation module (TC-PRISMA) within the Thermo-Calc Software Package. The annealed microstructure is characterized by scanning electron microscopy and the results are used for model validation and calibration. The microstructure and hardness responses to post heat treatments is discussed in terms of optimal solid-solution strengthening in SS446 ferritic stainless steel alloys processed by laser-powder bed fusion.
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| 31. |
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| 32. |
- Chou, Chia-Ying, et al.
(författare)
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Precipitation Kinetics During Post-heat Treatment of an Additively Manufactured Ferritic Stainless Steel
- 2022
-
Ingår i: Metallurgical and Materials Transactions. A. - : Springer Nature. - 1073-5623 .- 1543-1940. ; 53:8, s. 3073-3082
-
Tidskriftsartikel (refereegranskat)abstract
- The microstructure response of laser-powder bed fusion (L-PBF)-processed ferritic stainless steel (AISI 441) during post-heat treatments is studied in detail. Focus is on the precipitation kinetics of the Nb-rich phases: Laves (Fe2Nb) and the cubic carbo-nitride (NbC), as well as the grain structure evolution. The evolution of the precipitates is characterized using scanning and transmission electron microscopy (SEM and TEM) and the experimental results are used to calibrate precipitation kinetics simulations using the precipitation module (TC-PRISMA) within the Thermo-Calc Software package. The calculations reproduce the main trend for both the mean radii for the Laves phase and the NbC, and the amount of Laves phase, as a function of temperature. The calibrated model can be used to optimize the post-heat treatment of additively manufactured ferritic stainless steel components and offer a creator tool for process and structure linkages in an integrated computational materials engineering (ICME) framework for alloy and process development of additively manufactured ferritic steels.
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| 33. |
- Clulow, Rebecca, et al.
(författare)
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Magnetic and Structural Properties of the Fe5Si1-xGexB2 System
- 2022
-
Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 316
-
Tidskriftsartikel (refereegranskat)abstract
- A series of compounds with compositions Fe5Si1-xGexB2 were synthesised and their structural and magnetic properties were investigated. The Mo5SiB2-type structure with tetragonal I4/mcm space group is maintained for all compounds with x < 0.15, which is estimated as the compositional limit of the system. The unit cell pa-rameters expand with Ge content before reaching a plateau of a = 5.5581(1) and c = 10.3545(1) angstrom at x = 0.15. The saturation magnetisation (MS) decreased slightly with increasing Ge content whilst the magnetocrystalline anisotropy energy (MAE) remains almost unaffected. The Curie temperature for all compounds studied is at 790 K whilst the spin-reorientation temperature shows suppression from 172 K to 101 K where x = 0.15. Ab Initio calculations reveal an increase in MAE for compositions up to x = 0.25 and a decreased magnitude of MAE of-0.14 MJ/m3 for the hypothetical compound Fe5GeB2 relative to the parent compound Fe5SiB2.
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| 34. |
- Clulow, Rebecca, et al.
(författare)
-
Phase Stability and Magnetic Properties of Compositionally Complex n = 2 Ruddlesden-Popper Perovskites
- 2024
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 63:15, s. 6616-6625
-
Tidskriftsartikel (refereegranskat)abstract
- Four new compositionally complex perovskites with multiple (four or more) cations on the B site of the perovskites have been studied. The materials have the general formula La0.5Sr2.5(M)2O7−δ (M = Ti, Mn, Fe, Co, and Ni) and have been synthesized via conventional solid-state synthesis. The compounds are the first reported examples of compositionally complex n = 2 Ruddlesden-Popper perovskites. The structure and properties of the materials have been determined using powder X-ray diffraction, neutron diffraction, energy dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, and magnetometry. The materials are isostructural and adopt the archetypal I4/mmm space group with the following unit cell parameters: a ∼ 3.84 Å, and c ∼ 20.1 Å. The measured compositions from energy dispersive X-ray spectroscopy were La0.51(2)Sr2.57(7)Ti0.41(2)Mn0.41(2)Fe0.39(2)Co0.38(1)Ni0.34(1)O7−δ, La0.59(4)Sr2.29(23)Mn0.58(5)Fe0.56(6)Co0.55(6)Ni0.42(4)O7−δ, La0.54(2)Sr2.49(13)Mn0.41(2)Fe0.81(5)Co0.39(3)Ni0.36(3)O7−δ, and La0.53(4)Sr2.55(19)Mn0.67(6)Fe0.64(5)Co0.31(2)Ni0.30(3)O7−δ. No magnetic contribution is observed in the neutron diffraction data, and magnetometry indicates a spin glass transition at low temperatures.
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| 35. |
- Dahlqvist, Martin, et al.
(författare)
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Complex magnetism in nanolaminated Mn2GaC
- 2014
-
Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- We have used first-principles calculations and Heisenberg Monte Carlo simulations to search for the magnetic ground state of Mn2GaC, a recently synthesized magnetic nanolaminate. We have, independent on method, identified a range of low energy collinear as well as non-collinear magnetic configurations, indicating a highly frustrated magnetic material with several nearly degenerate magnetic states. An experimentally obtained magnetization of only 0.29 per Mn atom in Mn2GaC may be explained by canted spins in an antiferromagnetic configuration of ferromagnetically ordered sub-layers with alternating spin orientation, denoted AFM[0001]. Furthermore, low temperature X-ray diffraction show a new basal plane peak appearing upon a magnetic transition, which is consistent with the here predicted change in inter-layer spacing for the AFM[0001] configuration.
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| 36. |
- Dobrovetska, Oksana, et al.
(författare)
-
Electrocatalytic activity of Pd-Au nanoalloys during methanol oxidation reaction
- 2020
-
Ingår i: International journal of hydrogen energy. - : Elsevier BV. - 0360-3199 .- 1879-3487. ; 45:7, s. 4444-4456
-
Tidskriftsartikel (refereegranskat)abstract
- Methanol fuel cells are very promising power source due to its high efficiency and low emissions of pollutants but their commercialization is hindered by development of the effective catalysts. Bimetallic nanostructured catalysts have been used to increase the effectiveness of methanol electrooxidation. Their high electrocatalytic activity can be accounted largely by the difference in electronegativity of two metals (e.g. Pd and Au), that resulting in gradual Auδ+→Auδ– transition with the increase in Pd content. Therefore, gold-enriched bimetallic Pd-Aunano were recommended as catalysts for oxidation processes since they are characterized by the presence of Auδ+ on their surface. Deposition of Pd, Au and Pd–Au nanoparticles (~50–350 nm) were carried out in dimethyl sulfoxide by pulsed mode of electrolysis directly on electrode surface. Cyclic voltammetry was the main method to study catalytic properties of the modified electrode in the anode oxidation process of methanol. It was found that oxidation rate on the electrode surface modified by bimetallic Pd–Au nanoparticles is ~1.5 times higher as compared to that in the case of electrodes modified by Pd or Au monometallic nanoparticles individually. In order to find highly active, selective, and stable catalysts for methanol electrocatalytic oxidation reaction additional studies are needed to understand the role of electrode surface charge and local OH− ions concentration from alkali solution.
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| 37. |
- Ek, Gustav, et al.
(författare)
-
Elucidating the Effects of the Composition on Hydrogen Sorption in TiVZrNbHf-Based High-Entropy Alloys
- 2021
-
Ingår i: Inorganic Chemistry. - : American Chemical Society (ACS). - 0020-1669 .- 1520-510X. ; 60:2, s. 1124-1132
-
Tidskriftsartikel (refereegranskat)abstract
- A number of high-entropy alloys (HEAs) in the TiVZrNbHf system have been synthesized by arc melting and systematically evaluated for their hydrogen sorption characteristics. A total of 21 alloys with varying elemental compositions were investigated, and 17 of them form body-centered-cubic (bcc) solid solutions in the as-cast state. A total of 15 alloys form either face-centered-cubic (fcc) or body-centered-tetragonal (bct) hydrides after exposure to gaseous hydrogen with hydrogen per metal ratios (H/M) as high as 2.0. Linear trends are observed between the volumetric expansion per metal atom [(V/Z)fcc/bct – (V/Z)bcc/hcp]/(V/Z)bcc/hcp with the valence electron concentration and average Pauling electronegativity (χp) of the alloys. However, no correlation was observed between the atomic size mismatch, δ, and any investigated hydrogen sorption property such as the maximum storage capacity or onset temperature for hydrogen release.
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| 38. |
- Ek, Gustav, 1992-
(författare)
-
Hunting Hydrogen : Structure-property relations in High Entropy Alloy-based metal hydrides
- 2021
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Metal hydrides have many uses when switching the energy system from fossil fuels to renewable sources, such as rechargeable batteries, hydrogen storage, hydrogen compression and thermal storage. State of the art materials for these applications such as LaNi5 and TiFe, however, suffer certain limitations such as degradation during repeated hydrogen cycling and harsh activation conditions for initial hydrogen uptake, promoting the need for novel materials. One class of materials that are interesting options are High Entropy Alloys (HEA), which are solid solutions where typically four or more different elements occupy a single crystallographic site in a simple structure such as body centered cubic (bcc) or cubic close packed (ccp). Due to the random distribution of the elements, there is a large variety of local environments for hydrogen, potentially unlocking sites that are unavailable in conventional transition metal hydrides. There is also the possibility of vast chemical tunability when using this many principal elements. It is therefore imperative to establish design rules to enable tuning of the hydrogen sorption properties of these materials by changing the composition. The effect of having many differently sized metals on the crystal structure is also not fully understood, and is believed to have a high impact on the bulk properties such as hydrogen sorption in these materials.This thesis covers the experimental synthesis of a wide range of HEAs and subsequent evaluation of their structural and hydrogen sorption properties. Several new design rules have been established, such as that the atomic size mismatch between the constituent metals has no effect on the maximum hydrogen capacity, that the addition of large elements like Zr leads to phase separation and that controlling the valence electron concentration, VEC, destabilizes the HEA-based metal hydrides. Based on these findings, the material TiVCrNbH8 has been identified as a candidate with properties rivaling that of TiFeH2.
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| 39. |
- Ek, Gustav, et al.
(författare)
-
Hydrogen induced structure and property changes in Eu3Si4
- 2019
-
Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 277, s. 37-45
-
Tidskriftsartikel (refereegranskat)abstract
- Hydrides Eu3Si4H2-X were obtained by exposing the Zintl phase Eu3Si4 to a hydrogen atmosphere at a pressure of 30 bar and temperatures from 25 to 300 degrees C. Structural analysis using powder X-ray diffraction (PXRD) data suggested that hydrogenations in a temperature range 25-200 degrees C afford a uniform hydride phase with an orthorhombic structure (Immm, a approximate to 4.40 angstrom, b approximate to 3.97 angstrom, c approximate to 19.8 angstrom), whereas at 300 degrees C mixtures of two orthorhombic phases with c approximate to 19.86 and approximate to 19.58 angstrom were obtained. The assignment of a composition Eu3Si4H2+x is based on first principles DFT calculations, which indicated a distinct crystallographic site for H in the Eu3Si4 structure. In this position, H atoms are coordinated in a tetrahedral fashion by Eu atoms. The resulting hydride Eu3Si4H2 is stable by -0.46 eV/H atom with respect to Eu3Si4 and gaseous H-2. Deviations between the lattice parameters of the DFT optimized Eu3Si4H2 structure and the ones extracted from PXRD patterns pointed to the presence of additional H in interstitials also involving Si atoms. Subsequent DFT modeling of compositions Eu3Si4H3 and Eu3Si4H4 showed considerably better agreement to the experimental unit cell volumes. It was then concluded that the hydrides of Eu3Si4 have a composition Eu3Si4H2+x (x
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| 40. |
- Ek, Gustav, et al.
(författare)
-
Vibrational properties of High Entropy Alloy based metal hydrides probed by inelastic neutron scattering
- 2021
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 877
-
Tidskriftsartikel (refereegranskat)abstract
- The vibrational properties of several High Entropy Alloy (HEA) based metal hydrides are investigated by inelastic neutron scattering (INS). HEAs have recently emerged as a new type of materials with a wide range of intriguing properties and potential applications such as hydrogen storage. The special properties of HEAs are believed to originate from the disordered lattice and internal strain that is introduced from the differences in atomic radii. This makes HEA hydrides provide an intriguing situation for the local H coordination, of several different transition metals. INS spectra were collected on a series of HEA-based metal hydrides starting with TiVNbHx and subsequently adding Zr and Hf to increase the atomic size mismatch. A general feature of the spectra are the optical peaks centered around an energy loss of 150 meV that can be attributed to hydrogen vibrations in a tetrahedral environment. Upon the addition of Zr and Hf, a shoulder appears on the optical peak at lower energy transfers that after comparison with in silico calculated INS spectra is indicative of hydrogen also occupying octahedral sites in the structure.
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| 41. |
- Eriksson, Rickard, et al.
(författare)
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Formation of Tavorite-Type LiFeSO4F Followed by In Situ X-ray Diffraction
- 2015
-
Ingår i: Journal of Power Sources. - : Elsevier BV. - 0378-7753 .- 1873-2755. ; 298, s. 363-368
-
Tidskriftsartikel (refereegranskat)abstract
- The tavorite-type polymorph of LiFeSO4F has recently attracted substantial attention as a positive elec- trode material for lithium ion batteries. The synthesis of this material is generally considered to rely on a topotactic exchange of water (H2O) for lithium (Li) and fluorine (F) within the structurally similar hy- drated iron sulfate precursor (FeSO4·H2O) when reacted with lithium fluoride (LiF). However, there have also been discussions in the literature regarding the possibility of a non-topotactic reaction mechanism between lithium sulfate (Li2SO4) and iron fluoride (FeF2) in tetraethylene glycol (TEG) as reaction medium. In this work, we use in situ X-ray diffraction to continuously follow the formation of LiFeSO4F from the two suggested precursor mixtures in a setup aimed to mimic the conditions of a solvothermal autoclave synthesis. It is demonstrated that LiFeSO4F is formed directly from FeSO4·H2O and LiF, in agreement with the proposed topotactic mechanism. The Li2SO4 and FeF2 precursors, on the other hand, are shown to rapidly transform into FeSO4·H2O and LiF with the water originating from the highly hygroscopic TEG before a subsequent formation of LiFeSO4F is initiated. The results highlight the importance of the FeSO4·H2O precursor in obtaining the tavorite-type LiFeSO4F, as it is observed in both reaction routes.
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| 42. |
- Fang, Hailiang, et al.
(författare)
-
Directly obtained tau-phase MnAl, a high performance magnetic material for permanent magnets
- 2016
-
Ingår i: Journal of Solid State Chemistry. - : Elsevier BV. - 0022-4596 .- 1095-726X. ; 237, s. 300-306
-
Tidskriftsartikel (refereegranskat)abstract
- The metastable tetragonal iota-phase has been directly obtained from casting Mn0.54Al0.46 and (Mn0.55Al0.45)(100)C-2 using the drop synthesis method. The as-casted samples were ball milled to decrease the particle size and relaxed at 500 degrees C for 1 h. The phase composition, crystallographic parameters, magnetic properties and microstructure were systematically studied. The results reveal that the iota-phase could be directly obtained from drop synthesis. The highest M-s of 117 emu/g was achieved in the (Mn0.55Al0.45)(100)C-2 where the iota-phase was stabilized by doping with carbon. Carbon doping increased the c/a ratio of the tau-phase as it occupies specific interstitial positions (1/2, 1/2, 0) in the structure. Furthermore, ball milling increases the coercivity (H-c) at the expense of a decrease in magnetic saturation (M-s). The increase in coercivity is explained by a decrease of grain size in conjunction with domain wall pinning due to defects introduced during the ball milling process.
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| 43. |
- Fang, Hailiang, et al.
(författare)
-
Insights into formation and stability of tau-MnAlZ(x) (Z = C and B)
- 2017
-
Ingår i: Journal of Alloys and Compounds. - : Elsevier BV. - 0925-8388 .- 1873-4669. ; 692, s. 198-203
-
Tidskriftsartikel (refereegranskat)abstract
- The tau-phase MnAl alloys are promising candidate for rare earth free permanent magnets. In this study, In order to better understand the MnAl epsilon ->tau phase transition mechanism in a continuous cooling process and metastable MnAl tau-phase high temperature stability, Mn0.54Al0.46, Mn0.55Al0.45C0.02 and Mn0.55Al0.45B0.02 alloys were systematically studied by in situ synchrotron X-ray powder diffraction (SR-XRD). The relationship between tau-phase formation tendency and different cooling rates of Mn0.55Al0.45C0.02 was investigated. Besides, the high temperature stabilities of undoped tau-MnAl and carbon/boron doped tau-MnAl were studied. Differential thermal analysis (DTA) was also employed to study the phase transformation as well. The research results show that a high cooling rate of 600 degrees C/min leads to a 50/50 wt% mixture of epsilon- and tau-phase; almost pure tau-phase was obtained when cooled at a moderate cooling rate of 10 degrees C/min; while for a slow cooling rate of 2 degrees C/min, the tau-phase partially decomposed into beta and gamma(2) phases. No intermediate epsilon'-phase was observed during the epsilon ->tau phase transition during the experiments. For the boron and carbon doped tau-MnAl, the 800 degrees C high temperature stability experiments reveal that C stabilizes the tau-MnAl while doped B destabilises the tetragonal structure and it decomposes into beta- and gamma(2)-phases.
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| 44. |
- Fang, Hailiang, et al.
(författare)
-
Insights into phase transitions and magnetism of MnBi crystals synthesized from self-flux
- 2019
-
Ingår i: Journal of Alloys and Compounds. - : ELSEVIER SCIENCE SA. - 0925-8388 .- 1873-4669. ; 781, s. 308-314
-
Tidskriftsartikel (refereegranskat)abstract
- To effectively synthesize high purity ferromagnetic low temperature phase (LTP) MnBi with optimal microstructure is still a challenge that needs to be overcome for the system to reach its full potential. Here, the phase transitions and magnetic properties of MnBi crystals are reported. The phase transition between the low and high temperature structure of MnBi was systematically investigated at different heating/cooling rates using in situ synchrotron radiation X-ray diffraction. The material crystallizes in a layered hexagonal structure giving a platelike microstructure. The magnetic characterization of the crystals reveal that the saturation magnetization varies from 645 kA/m at 50 K to 546 kA/m at 300 K. Magnetization measurements also show that the sample upon heating becomes non-magnetic and transforms to the high temperature phase (HTP) at similar to 640 K, and that it regains ferromagnetic properties and transforms back to the LTP at similar to 610 K upon subsequent cooling.
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| 45. |
- Fang, Hailiang, et al.
(författare)
-
Structural, microstructural and magnetic evolution in cryo milled carbon doped MnAl
- 2018
-
Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 8:1
-
Tidskriftsartikel (refereegranskat)abstract
- The low cost, rare earth free τ-phase of MnAl has high potential to partially replace bonded Nd2Fe14B rare earth permanent magnets. However, the τ-phase is metastable and it is experimentally difficult to obtain powders suitable for the permanent magnet alignment process, which requires the fine powders to have an appropriate microstructure and high τ-phase purity. In this work, a new method to make high purity τ-phase fne powders is presented. A high purity τ-phase Mn0.55Al0.45C0.02 alloy was synthesized by the drop synthesis method. The drop synthesized material was subjected to cryo milling and followed by a fash heating process. The crystal structure and microstructure of the drop synthesized, cryo milled and flash heated samples were studied by X-ray in situ powder diffraction, scanning electron microscopy, X-ray energy dispersive spectroscopy and electron backscatter diffraction. Magnetic properties and magnetic structure of the drop synthesized, cryo milled, flash heated samples were characterized by magnetometry and neutron powder diffraction, respectively. The results reveal that the 2 and 4hours cryo milled and flash heated samples both exhibit high τ-phase purity and micron-sized round particle shapes. Moreover, the fash heated samples display high saturation magnetization as well as increased coercivity.
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| 46. |
- Fang, Hailiang, 1985-
(författare)
-
Structural Studies of Mn-X (X=Al, Bi): Permanent Magnetic Materials without Rare Earth Metals
- 2019
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- How to generate and use electricity in a more efficient way is a major challenge for humankind. In this context, permanent magnets play an important role within a very broad range of electric power applications. The strongest magnets used today are mainly based on alloys that contain rare-earth metals, which are neither economical nor sustainable. The search for new alternative alloys with satisfactory magnetic properties is the major motivation for the investigations summarized in this thesis. Interesting candidates for alternative rare-earth free alloys were selected with τ-MnAl as the basis. Theoretical studies suggest that such alloys may show good magnetic properties after chemical modifications to optimize them. Another compound with promising magnetic properties is MnBi, included in this study.MnAl-Z (Z= C, B, Ga as doping elements) and MnBi compounds were synthesized through carefully devised high-temperature methods, followed by various milling and annealing steps. The structural phase analysis of the samples was based on X-ray and neutron diffraction. A systematic microstructural investigation was also performed for selected samples. The phase transitions of MnAl and MnBi during heating and cooling at different rates were studied by in situ X-ray diffraction from a synchrotron source. The magnetic properties were characterized by various methods.By strict control of experimental parameters, the metastable τ-MnAl was found to be directly obtainable using a "drop synthesis” process. A cooling rate of 10 K/min yielded an almost pure ferromagnetic τ-MnAl phase. A microstructural characterization of similarly synthesized MnAl-C samples revealed the presence of phase segregation, a Mn-rich region and an Al-rich grain boundary phase.A cryomilling process was employed which decreased the particle size of the MnAl-C sample. Neutron diffraction data disclosed accompanying amorphous features, related to changes in Mn and Al atom occupancies during the milling process. A flash heating procedure regenerated the structural ordering between Mn and Al in the structure, where the initial magnetic properties were recovered.The MnBi compound was synthesized by a self-flux method in order to isolate single crystals. As for τ-MnAl, in situ diffraction studies were applied for following phase transitions and the magnetic properties were studied.
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| 47. |
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| 48. |
- Gebresenbut, Girma Hailu, 1982-
(författare)
-
Quasicrystal Approximants in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge) : Syntheses, structures and properties
- 2016
-
Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, new Tsai-type 1/1 quasicrystal approximants (ACs) in the RE-Au-SM systems (RE = Gd, Tb, Ho, Yb; SM = Si, Ge) were synthesized using high temperature synthesis techniques such as self-flux, arc-melting-annealing and novel arc-melting-self-flux methods. The syntheses not only provided appropriate samples for the intended structural and physical property measurements but could also be adapted to other systems, especially where crystal growth is a challenge. The newly developed arc-melting-self-flux method uses a temperature program that oscillates near the nucleation and melting points of the intended phase in order to obtain large single crystals. Self-flux methods employed to synthesize Ho-Au-Si and Tb-Au-Si ACs using a precursor alloy ≈Au79Si21 resulted in 100 mm3 and 8 mm3 single crystals, respectively.The crystal structures of the compounds are determined by either one or combinations of the following diffraction techniques; single crystal x-ray, powder x-ray, powder neutron and single crystal neutron diffraction methods. The crystal structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. In the present compounds there are some structural variations at the cluster center and in the so-called cubic interstices located at (¼, ¼, ¼).For the current ACs; either thermoelectric, magnetic or both properties were investigated. The measured properties were understood further by correlating the properties with the atomic structures of the ACs. Significant differences are observed in the thermoelectric properties, particularly on the lattice thermal conductivities (Kphonon) of Gd-Au-Si, Gd-Au-Ge and Yb-Au-Ge ACs. The difference is attributed to the presence of chemical and positional disorder.Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd-Au-Si and Tc ≈ 13.1 K for Gd-Au-Ge. For a Tb-Au-Si AC with 14 % central-Tb occupancy, a ferrimagnetic-like transition was observed at Tc ≈ 9 K. Later, it was noted that the Tc and other magnetic properties depend on the occupancy of the central-RE site. Consistent decrease of Tc with increasing central-Tb occupancy is observed. The dependency of magnetic behavior with central-RE occupancy was clarified by solving the magnetic structure of the Tb-Au-Si AC.
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| 49. |
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| 50. |
- Gebresenbut, Girma Hailu, 1982-
(författare)
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Syntheses, structure determination, magnetic and thermoelectric properties of quasicrystal approximants in RE-Au-SM systems (RE = Gd, Tb and Yb and SM = Si, Ge)
- 2014
-
Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In this study, new compositions of Tsai-type 1/1 quasicrystal approximants Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19), Tb(14)Au(70)Si(16) and Yb(16)Au(65)Ge(19)are synthesized using both self-flux and arc-melting-annealing techniques. Both syntheses routes resulted single phase samples. The crystal structures of the compounds are determined by collecting single crystal X-ray and/or powder X-ray and powder neutron diffraction intensities. The atomic structure refinements indicated that the compounds are essentially iso-structural with the prototype Tsai-type 1/1 approximant crystal, YbCd6. However, there are subtle structural variations at their cluster centers and in the so-called cubic interstices which affects some of their physical properties.Thermoelectric and magnetic properties of the compounds are investigated. Significant differences are observed in the thermoelectric properties of Gd(14)Au(70)Si(16), Gd(14)Au(67)Ge(19) and Yb(16)Au(65)Ge(19) compounds which are explained on the bases of their crystal structures and chemical compositions. Magnetic susceptibility and specific heat measurements revealed ferromagnetic transitions at low temperatures, Tc ≈ 22.5 K for Gd(14)Au(70)Si(16) and Tc ≈ 13.1 K for Gd(14)Au(67)Ge(19), whereas, for the Tb(14)Au(70)Si(16) compound a ferrimagnetic-like transition is observed at Tc ≈ 9 K. Moreover, a re-entrant spin-glass transition is observed at TRSG ≈ 3.3 K for Gd(14)Au(67)Ge(19) compound. Finally, the magnetic structure of the Tb(14)Au(70)Si(16) compound was determined from powder neutron diffraction data which is to our knowledge the first magnetic structure refinement report in the family of quasicrystals and approximants.
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