307 research outputs found
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Boundary selectivity of crack paths in corrosion fatigue of stainless steel
Stress corrosion and corrosion fatigue cracks are frequently very branched and there have been extensive attempts to define the characteristics of crack-stopping features.
EBSD has been used to examine the full length (~8mm) of a corrosion fatigue crack in stainless steel. The grain boundary character distribution of the cracked boundaries is compared to that of the rest of the material and observations presented on the effect of grain boundary character on the choice of crack path at grain boundary junctions of different configurations and orientations with restect to the principle stres
Correlation between microstructure and superconducting properties of MgB2 bulk samples with Mg addition and Mg/hBN co-additions
The microstructure of polycrystalline MgB2 has a strong influence on the current carrying ability, with grain boundaries and non-superconducting nanoparticles acting as good flux pinning centres which improve the local (intrinsic) critical current density (Jc) of the material, whereas porosity and poor connectivity between grains or particles adversely affect macroscopic current transport. Previous studies have found that hexagonal boron nitride (hBN) doping improves intrinsic Jc by introducing nanoscale flux pinning centres, and Mg doping improves extrinsic Jc by liquid-assisted sintering. Here we investigate the effect of co-doping with 5 wt.% Mg and 1 wt.% hBN with the aim of combining the improved intrinsic and extrinsic properties in bulk MgB2 samples fabricated using field assisted sintering. Additionally, the influence of ball milling and processing temperatures on MgB2 samples with only Mg additions is reported. By correlating microstructure with superconducting properties, we show that the presence of Mg liquid during processing of Mg-doped samples accelerates the reaction between BN and MgB2, forming an impurity phase, MgNB9, the presence of which is detrimental to superconducting performance. Nevertheless, we have achieved a considerable improvement in performance of samples doped only with Mg by increasing the sintering temperature
Preparation, microstructure and microwave dielectric properties of sprayed PFA/barium titanate composite films
Frequency dependence of the dielectric properties of polymer-ferroelectric composites at different bands of microwave frequencies was investigated in this work. Perfluoroalkoxy (PFA)/barium titanate (BaTiO3) nanocomposite films were prepared by spray deposition. The spraying process was scaled up to fabricate large area (max. 160 mm × 160 mm) uniform composite sheets out of which a controlled bonding process was introduced to form composite blocks. The microstructure of the composite films was examined by SEM with a microtome sample preparation method to evaluate the effectiveness of the spraying process at producing uniform particle distributions. The dielectric properties of the composite films with various BaTiO3 loadings were characterised by an Impedance Analyzer at frequencies between 10 Hz and 1 MHz and Vector Network Analyzer at 12–18 GHz respectively. The Lichtenecker mixing rule was incorporated to fit the measured dielectric constant data, which gives estimates of dielectric constant of the BaTiO3 nanometer sized particles to be 895 and 571 at 10 kHz and 15 GHz respectively. In comparison, the composite effective dielectric constant was approximately reduced by 25% at 10 kHz than that at 15 GHz
High resolution characterisation of microstructural evolution in RbFeSe crystals on annealing
The superconducting and magnetic properties of phase-separated
AFeSe compounds are known to depend on post-growth heat
treatments and cooling profiles. This paper focusses on the evolution of
microstructure on annealing, and how this influences the superconducting
properties of RbFeSe crystals. We find that the minority phase in
the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb
content and increased Fe content compared to the matrix. The microstructure is
rather complex, with two-phase mesoscopic plate-shaped features aligned along
{113} habit planes. The minority phase are strongly facetted on the {113}
planes, which we have shown to be driven by minimising the volume strain energy
introduced as a result of the phase transformation. Annealing at 488K results
in coarsening of the mesoscopic plate-shaped features and the formation of a
third distinct phase. The subtle differences in structure and chemistry of the
minority phase(s) in the crystals are thought to be responsible for changes in
the superconducting transition temperature. In addition, scanning photoemission
microscopy has clearly shown that the electronic structure of the minority
phase has a higher occupied density of states of the low binding energy Fe3d
orbitals, characteristic of crystals that exhibit superconductivity. This
demonstrates a clear correlation between the Fe-vacancy-free phase with high
c/a ratio and the electronic structure characteristics of the superconducting
phase.Comment: 6 figures v2 is exactly the same as v1. The typesetting errors in the
abstract have been correcte
Individual grain boundary properties and overall performance of metal-organic deposition coated conductors
have investigated single grain boundaries (GBs) isolated in coated
conductors produced by metal-organic deposition. When a magnetic field is swept in the film plane, an angle-dependent crossover from boundary to grain limited critical current density J(c) is found. In the
force-free orientation, even at fields as high as 8 T, the GBs still limit Jc. We deduce that this effect is a direct consequence of GB meandering. We have employed these single GB results to explain the dependence of Jc of polycrystalline tracks on their width: in-plane measurements become flatter as the tracks are narrowed down. This result is consistent with the stronger GB limitation at field configurations close to force-free found from the isolated boundaries. Our study shows that for certain geometries even at high fields the effect of GBs cannot be neglected.This work was supported by the Engineering and Physical Sciences Research Council [grant numbers EP/C011546/1 and EP/C011554/1
Microstructural analysis of phase separation in iron chalcogenide superconductors
The interplay between superconductivity, magnetism and crystal structure in
iron-based superconductors is a topic of great interest amongst the condensed
matter physics community as it is thought to be the key to understanding the
mechanisms responsible for high temperature superconductivity. Alkali metal
doped iron chalcogenide superconductors exhibit several unique characteristics
which are not found in other iron-based superconducting materials such as
antiferromagnetic ordering at room temperature, the presence of ordered iron
vacancies and high resistivity normal state properties. Detailed
microstructural analysis is essential in order to understand the origin of
these unusual properties. Here we have used a range of complementary scanning
electron microscope based techniques, including high-resolution electron
backscatter di raction mapping, to assess local variations in composition and
lattice parameter with high precision and sub-micron spatial resolution. Phase
separation is observed in the Csx Fe2-ySe2 crystals, with the minor phase
distributed in a plate-like morphology throughout the crystal. Our results are
consistent with superconductivity occurring only in the minority phase.Comment: Accepted for publication in a special edition of Supercond. Sci.
Techno
Initiation of dendritic failure of LLZTO via sub-surface lithium deposition
The occurrence of lithium deposition in occluded spaces within ceramic electrolytes due to electronic leakage currents can jeopardise the commercialization of power-dense solid-state batteries. Here, we utilize plasma-FIB serial sectioning to visualize the surface and sub-surface of a garnet solid electrolyte (LLZTO) after lithium plating. We study the morphology of surface spallation cracks, which represent the initial stage of dendrite formation. Employing a LiMg anode, we track the magnesium diffusion around these surface cracks with EDS. The absence of magnesium in early-stage cracks suggests they form due to the pressure build-up from the deposition of pure lithium in occluded pores near the electrolyte surface. These spallation cracks act as current focusing and stress concentration hot spots. Electron beam induced current imaging demonstrates that short-circuiting lithium dendrites grow from the spallations during plating. Thus, the sub-surface deposition of lithium is a possible explanation for the initiation of lithium dendrites in LLZTO
A Trapped Field of >3T in Bulk MgB2 Fabricated by Uniaxial Hot Pressing
A trapped field of over 3 T has been measured at 17.5 K in a magnetised stack
of two disc-shaped bulk MgB2 superconductors of diameter 25 mm and thickness
5.4 mm. The bulk MgB2 samples were fabricated by uniaxial hot pressing, which
is a readily scalable, industrial technique, to 91% of their maximum
theoretical density. The macroscopic critical current density derived from the
trapped field data using the Biot-Savart law is consistent with the measured
local critical current density. From this we conclude that critical current
density, and therefore trapped field performance, is limited by the flux
pinning available in MgB2, rather than by lack of connectivity. This suggests
strongly that both increasing sample size and enhancing pinning through doping
will allow further increases in trapped field performance of bulk MgB2.Comment: 10 pages, 4 figures. Accepted as a Rapid Publication in
Superconductor Science and Technology (Final version after peer review
Specimen preparation methods for elemental characterisation of grain boundaries and isolated dislocations in multicrystalline silicon using atom probe tomography
Multicrystalline silicon (mc-Si) is a cost effective feedstock for solar photovoltaic devices but is limited by the presence of defects and impurities. Imaging impurities segregated to nanometre-scale dislocations and grain boundaries is a challenge that few materials characterisation techniques can achieve. Atom Probe Tomography (APT) is a 3-dimensional time-of-flight microscopy technique that can image the distribution of elements at the atomic scale, however one of the most challenging factors when using APT is the complexity of specimen preparation for specific regions of interest. Atom probe specimen preparation methods have been developed in a dual FIB/SEM system that enable a specific extended defect such as an isolated dislocation or a section of a grain boundary to be selected for APT analysis. The methods were used to fabricate APT specimens from an isolated dislocation and a grain boundary in mc-Si samples. Complementary TEM images confirm the presence of the defects in both specimens, whilst APT analyses also reveal segregation of impurities to the defects
Removal and Reoccurrence of LLZTO Surface Contaminants under Glovebox Conditions
The reactivity of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) solid electrolytes to form lithio-phobic species such as Li2CO3 on their surface when exposed to trace amounts of H2O and CO2 limits the progress of LLZTO-based solid-state batteries. Various treatments, such as annealing LLZTO within a glovebox or acid etching, aim at removing the surface contaminants, but a comprehensive understanding of the evolving LLZTO surface chemistry during and after these treatments is lacking. Here, glovebox-like H2O and CO2 conditions were recreated in a near ambient pressure X-ray photoelectron spectroscopy chamber to analyze the LLZTO surface under realistic conditions. We find that annealing LLZTO at 600 °C in this atmosphere effectively removes the surface contaminants, but a significant level of contamination reappears upon cooling down. In contrast, HCl(aq) acid etching demonstrates superior Li2CO3 removal and stable surface chemistry post treatment. To avoid air exposure during the acid treatment, an anhydrous HCl solution in diethyl ether was used directly within the glovebox. This novel acid etching strategy delivers the lowest lithium/LLZTO interfacial resistance and the highest critical current density
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