The room-temperature magnetic properties of ball-milled strontium hexaferrite particles consolidated by spark-plasma sintering are strongly influenced by the milling time. Scanning electron ...microscopy revealed the ball-milled SrFe12O19 particles to have sizes varying over several hundred nanometers. X-Ray powder-diffraction studies performed on the ball-milled particles before sintering clearly demonstrate the occurrence of a pronounced amorphization process. During sintering at 950 °C, re-crystallization takes place, even for short sintering times of only 2 minutes and transformation of the amorphous phase into a secondary phase is unavoidable. The concentration of this secondary phase increases with increasing ball-milling time. The remanence and maximum magnetization values at 1T are weakly influenced, while the coercivity drops dramatically from 2340 Oe to 1100 Oe for the consolidated sample containing the largest amount of secondary phase.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
In a novel process to enhance the coercivity we have electrophoretically deposited DyF3 powder onto the surface of an as-sintered Nd–Fe–B magnet as the initial step in the grain-boundary diffusion ...process. After a conventional heat treatment at 850 and 500 °C the coercivities were higher than in the case of simple dipping after a typical 10 h, with Hci values exceeding 1600 kA/m for a 200 μm-thick deposited layer. The electrophoretic deposition (EPD) process is quick, easily controllable in terms of thickness and can be used to deposit the rare earth fluoride powder on the surface of complex and irregularly shaped magnets. Since the amount of deposited powder can be tailored to maximise the coercivity while minimising the quantity of expensive heavy rare earth there is no wasted powder, making the diffusion process, which takes place after the sintering process, more environmentally friendly and potentially cheaper than conventional dipping.
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► A novel technique to enhance coercivity via the GBDP. ► Controllable layer thickness for heavy rare earths. ► Maximised coercivity with minimum losses of material. ► Environmentally friendly and low-cost.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
While a lot is known about the deformation of metallic surfaces from experiments, elasticity theory and simulations, this investigation represents the first molecular-dynamics-based simulation of ...uniaxial deformation for the vicinal surfaces in a comparison of copper and nickel. These vicinal surfaces are composed of terraces divided by equidistant, mono-atomic steps. The periodicity of vicinals makes them good candidates for the study of the surface steps' influences on surface dynamics. The simulations of tensile and compressive uniaxial deformations were performed for the (1 1 19) vicinal surfaces. Since the steps on the surfaces serve as stress concentrators, the first defects were expected to nucleate here. In the case of copper, this was found to be the case. In the case of nickel, however, dislocations nucleated beneath the near-surface layer affected by the displacement field generated by the steps. Slip was hindered at the surface step by the vortex in the displacement field. The differences in the deformation mechanisms for the Ni(1 1 19) and Cu(1 1 19) surfaces can be linked to the differences in their displacement fields. This could lead to novel bottom-up approaches to the nanostructuring of surfaces using strain.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
Ferrite‐based, hard‐soft magnetic nanocomposites with the composition (100%−x)SrFe12O19–xCoFe2O4, where x = 5, 10, and 15 wt%, were prepared by mixing the constituent powders, followed by spark ...plasma sintering. In order to control the particle size of the constituent materials, the SrFe12O19 and CoFe2O4 powders were synthesized using the hydrothermal method, mixed and then consolidated with spark plasma sintering. The conditions during the spark plasma sintering process (sintering temperature, time, and applied pressure) were varied in order to prepare composites with a high density and exchange‐coupled hard and soft magnetic phases, leading to an increase in the maximum energy product, when compared with pure SrFe12O19. The microstructural analysis revealed that the relative density of the sintered composite exceeded 90% of the theoretical value and that the CoFe2O4 was uniformly distributed in the SrFe12O19 matrix. Magnetic measurements of the sintered composites showed a single‐phase magnetic behavior. When compared with the single‐phase SrFe12O19 used in this study, the SPS composites exhibited a 22% increase in the maximum energy product (26.1 kJ/m3).
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BFBNIB, DOBA, FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UILJ, UKNU, UL, UM, UPUK
► SiN film hydrogen permeability at 400°C was determined as P=1×10−17molH2/msPa0.5. ► We presumably report the first quantitative data on H permeation through SiN. ► Permeation reduction factor of ...2000 was achieved with only 500nm thick SiN layer. ► High SiN barrier efficiency was found only when H is incorporated in SiN forming strong bonds. ► SiN containing ∼7at.% H is ∼100 times less permeable than the film without H.
Amorphous silicon nitride films, 500 and 700nm thick, were deposited on Eurofer substrates by applying reactive radio-frequency magnetron sputtering from pure Si targets in an argon/nitrogen atmosphere. The hydrogen permeation through such double-layered, 40mm diameter membranes at 400°C and 1bar upstream pressure involved the use of a conventional technique with enhanced sensitivity. The extremely high barrier efficiency for these films with respect to hydrogen, expressed as a permeation-reduction factor in excess of 2000, was only achieved with films containing 6–7at.% of hydrogen. The achieved permeation-reduction factor at 400°C corresponds to the permeability of silicon nitride, which is as low as P=1×10−17molH2/msPa0.5. The hydrogen concentration was determined with an Elastic Recoil Detection Analysis, which indicated that this high concentration represents only the strongly bound hydrogen that is not mobile at this low temperature, but impedes the mobility of the diffusive hydrogen. A silicon nitride film with a low hydrogen content is a far less efficient barrier, which supports the role of the strongly bound hydrogen.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
Fe-Mn alloys were produced for bioresorbable applications using the laser powder bed fusion (LPBF) process with varying process parameters. The feedstock alloy powder for LPBF was derived from ...conventional cast/forged bars using plasma ultrasonic atomization. Additionally, a conventionally produced Fe-Mn alloy with the same composition was investigated to compare material properties. The influence of the processing route and LPBF process parameters on microstructure evolution, particularly the formation of Σ boundaries, was examined and correlated with the observed corrosion rate obtained from potentiodynamic curves in Hank's solution. The concentration of released Fe and Mn ions after immersion tests in lactic acid was also evaluated. The initial corrosion mechanism of the LPBF alloy was elucidated through X-ray photoelectron spectroscopy (XPS). Furthermore, in vitro tests were conducted using MG-63 human osteoblast-like osteosarcoma cells to assess the biocompatibility response. The present study established a correlation between microstructure and corrosion rate, while the biocompatibility tests affirmed the suitability of additively manufactured Fe-Mn alloys for bioresorbable applications.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Any future fusion reactor will require a structurally sound vacuum chamber that is able to withstand the attack of hydrogen produced in the transmutation reactions, while at the same time exhibiting ...the rapid decay of any induced radioactivity. However, the Eurofer '97 steel, which has already been chosen for the DEMO reactor, although possessing reduced activation, remains very susceptible to hydrogen. In this study we have looked at the effectiveness of thin, TiAlN coatings with respect to the permeability of hydrogen at 400
°C. Our results reveal that the coating forms a columnar structure, with evidence of epitaxy at the substrate–coating interface, and that this coating can produce a permeation reduction factor for hydrogen of up to 20,000. This is substantially higher than any other coating reported for this type of steel. Furthermore, the relatively low costs associated with such films and the breadth of knowledge that already exists about their characteristics suggest that such a combination of a TiAlN coating and the Eurofer '97 steel could be a very promising material for reactor technologies.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Anisotropic sintered magnets based on the Nd2Fe14B phase doped with Tb were prepared using a grain-boundary diffusion process (GBDP) in order to enhance their coercivity. A FEGSEM microstructural ...analysis revealed that these GBDP magnets had a core-shell structure, where thin, Tb-rich, (NdTb)2Fe14B shells are formed on the original matrix Nd2Fe14B grains after diffusion of the Tb. This shell thickness varies from a few tens of nanometres in the middle of the magnet up to a few micrometers near the edge. The exact chemical composition of these shells was determined using EDS and WDS electron-probe microanalyses, which were modified and optimized for submicrometer scale analyses. When analyzing the common Nd–Lα, Tb–Lα and Fe–Kα lines a mutual multiple overlap in the EDS spectra is present and, as a result, an accurate quantitative analysis was only feasible when using WDS. Using this technique we were able to achieve a lateral analytical resolution of 0.4μm. A further improvement in resolution, down to 0.15μm, was realized with a dedicated set-up using low-voltage EDS, analyzing the “atypical” low-energy Nd–Mα, Tb–Mα and Fe–Lα lines. Quantitative analyses confirmed that the reaction phase (NdxTb1−x)2Fe14B is formed after the diffusion of Tb with the equilibrium concentration of Tb being equal to x≈0.5, i.e., with the atomic ratio of Nd/Tb equal to 1/1. We also found that a relatively sharp Tb concentration gradient from the shell to the core occurs within a length of ≈0.5μm, while the Fe concentration remains unchanged. In terms of magnetic properties, the Tb-doping significantly increased coercivity by ≈30% while the remanence remained at the same value as in the undoped Nd–Fe–B.
► Nd–Fe–B sintered magnets were doped with Tb using grain-boundary diffusion process. ► A tiny core-shell reaction phase was formed around the Nd2Fe14B matrix grains. ► EDS and WDS analyses confirmed (Nd0.5Tb0.5)2Fe14B equilibrium shell composition. ► Coercivity of Tb-doped Nd–Fe–B increases by 30% without a drop in remanence.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We have fabricated Co-Pt cylindrical nanostructures comprised of a pair of nanotube and nanowire segments via direct electroplating into anodic alumina (AAO) membranes. The fabrication of such ...nanostructures is possible due to the penetration of sputtered gold (Au) nanoparticles inside the template, which serve as nucleation spots. The current transient monitored during the deposition process allowed us to distinguish between the nanotube and nanowire formation regime during the electrodeposition. In this paper, we report on the systematic changes in the angular dependence of the coercivity behavior due to the change in the length of the nanowire segment of these nanostructures, while keeping the nanotube segment constant. Considering the range of parameters studied, we found that the tube segment always reverses its magnetization through a vortex domain wall, whereas the nanowire segment reverses its magnetization through a transverse domain wall. Our results are in good agreement with literature. The possibility to alter the magnetization reversal mode in such nanostructures provides an attractive way to control the motion of the magnetic domain walls.
We report on the influence of the magnetocrystalline anisotropy on the easy magnetization axis, magnetization reversal and magnetic domain configurations of electrodeposited Co–Pt nanowires with ...lengths in the range of 4–6 µm and a diameter of 250 nm. The transmission electron microscopy and the X-ray diffractions revealed that the nanowires are composed of an intermixture of
hcp
- and
fcc
-textured crystal structures. The crystallographic orientations of both phases were such that the
00
1
¯
of the
hcp
phase and the 111 of the
fcc
phase are pointing almost perpendicular to the nanowire axis. This observation allows us to understand the perpendicular easy magnetization axis of the nanowire arrays measured with vibrating sample magnetometry. Analytical calculations of the angular dependence of the coercivity revealed that the magnetization reversal changes from vortex to transverse mode at the applied field angle
θ
= 30°. Fitting of the experiment to these calculations results in a perpendicular
effective
anisotropy constant (
K
eff
= 2.6 × 10
4
J/m
3
) in nanowires which can be ascribed to the strong magnetocrystalline anisotropy. Furthermore, the magnetic domain configurations of individual nanowires of length range 4 <
L
< 6 µm are studied using magnetic force microscopy. This reveals a spatial magnetization modulation along the length of the nanowires, which was found to be length dependent. Such an intrinsic modulation is attributed to the competition between the magnetocrystalline anisotropy and the shape anisotropy in the nanowires. We believe that this interplay between anisotropies gives rise to a magnetic configuration involving vortex-like structure with alternating chirality along the length of the nanowires.
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