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Grain size dependence of critical current density in MgB 2 at 20 K and magnetic field of 2 T. Inset shows the data in double-logarithmic scale. Black filled squares are for the samples with nanoparticles of external phases and open red circles are for pure MgB 2 . The red lines in the plot show reciprocal J c dependence on D. 4. Conclusions Electron backscatter diffraction was successfully applied to MgB 2 produced by the advanced methods of resistive sintering and hot isostatic pressing. EBSD allowed to obtain important information about the micro and nano-structure of the samples and clarified nature of the pinning in the material. The EBSD findings can be useful in designing new processing techniques that would be able to increase critical current density in MgB 2 . These techniques could be based on the treatment of pure MgB 2 or, alternatively, on the nano-additions of external phases. References 1 Shields T C, Kawano K, Holdom D and Abell J S 2002 Supercond. Sci. Technol. 15 202 2 Dancer C E J, Mikheenko P, Bevan A, Abell J S, Todd R I, Grovenor C R M 2009 J. Europ. Ceramic Soc. 29 1817 3 Mikheenko P, Martínez E, Bevan A, Abell J S and MacManus-Driscoll J L 2007 Supercond. Sci. Technol. 20 S26 4 Mikheenko P 2011 Journal of Physics: Conference Series 286 012014
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MgB2 is one of important new functional materials, which can play significant role in emerging Hydrogen Economy. If adopted, Hydrogen Economy would be a low-CO2 fossil-fuels-free answer to the current and future energy demands. To contribute to this economy, advanced methods of MgB2 preparation and an insight into its properties are required. This...
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... An important tool to achieve this goal is provided by the electron backscatter diffraction (EBSD) technique. However, up to now there are only some reports using this technique in the literature on MgB 2 material [11][12][13], which is strongly linked to the problems arising in the sample preparation stage. Here, we present a solution to these problems by means of the newly developed transmission EBSD technique (t-EBSD) [14,15], which is based on the fabrication of TEM slices by means of focused ion-beam milling. ...
The grain orientation, the texture and the grain boundary misorientations are important parameters for the understanding of the magnetic properties of the bulk MgB2 samples intended for super-magnet applications. Such data can be provided by electron backscatter diffraction (EBSD) analysis. However, as the grain size of the MgB2 bulks is preferably in the 100-200 nm range, the common EBSD technique working in reflection operates only properly on highly dense samples. In order to achieve a reasonably good Kikuchi pattern quality on all samples, we apply here the newly developed transmission EBSD (t-EBSD) technique to several bulk MgB2 samples. This method requires the preparation of TEM slices by means of focused ion-beam milling, which are then analyzed within the SEM, operating with a specific sample holder. We present several EBSD mappings of samples prepared with different techniques and at various reaction temperatures.
... 12. Tension strains in mono-axial direction increased the LAB fraction, while tensile strains in biaxial directions restricted the increase. It was known that the LAB was composed by arrangement of dislocations [29]. With increasing strains in mono-axial direction, dislocation density increased essentially and some subboundaries were formed by high density dislocations, resulting in the increasing fraction of LAB. ...
The grain boundary misorientation and dislocation structure for Alloy 690TT under mono-tension and biaxial tension at room temperature was investigated by using electron backscatter diffraction technique (EBSD) and transmission electron microscopy (TEM). The strain amplitudes for mono-axial tension were X3%, X5%, X7% and X9% respectively, and biaxial tension followed X3%, X5%, Y3%, X7%, Y7%, X9%, and Y9% tensile strains. The results showed that the mono-tension could decrease the amount of Σ3 twin boundaries, increased the amount of the low angle boundary and random grain boundary. The biaxial tension recovered some random boundaries partially back to Σ3 boundaries. Tension strains in Y direction restricted the decease of Σ3 boundaries. For the mono-tension specimen the dislocation had high energy pile-ups structure and for the biaxial tension specimen had low energy tangle structure. According to mechanochemical effect theory, the biaxial tension could retard stress corrosion cracking of Alloy 690TT.
pt The electron backscatter diffraction (EBSD) technique operating in the scanning electron microscope provides information on the crystallographic orientation the material by recording Kikuchi patterns. In polycrystalline samples, it becomes possible to analyse the orientations of the grains to each other. The metallic superconductor with the currently highest superconducting transition temperature, MgB2 with a Tc of 38.5 K, can be used in applications in polycrystalline form. One such application of interest are trapped field magnets or super‐magnets, where the superconductor cooled in an applied magnetic field can trap the magnetic field as vortices at numerous flux pinning sites in the sample. When the external magnetic field is removed, the sample will stay magnetized as long as it is kept cool, and importantly, the trapped magnetic fields can be much higher as for any permanent magnet. However, the small size of the MgB2 grains in the 100 – 200 nanometer range requires a different approach when using the EBSD technique on such samples. The recently developed EBSD technique working in transmission mode (t‐EBSD) helps considerably to image such materials. In this approach, a tiny TEM slice has to be milled out from the original sample by using focused ion beam milling. To understand the properties of the flux pinning in the spark‐plasma sintered MgB2 sample, we had to identify the Kikuchi pattern of MgB4, which is another, non‐superconducting phase appearing at higher reaction temperatures required to compact the material. Using this information, we could perform EBSD scans using three different phases, MgB2, MgB4 and MgO. The EBSD mappings enable to see where the secondary phase particles are located in the sample, and to judge if the particles could work as flux pinning sites.
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Abstract
en EBSD analysis can provide information about grain orientation, texture and grain boundary misorientation of bulk superconducting MgB2 samples intended for super‐magnet applications. However, as the grain size of the MgB2 bulks is preferably in the 100–200 nm range, the common EBSD technique operating in reflection mode works only properly on highly dense samples. In order to achieve reasonably good Kikuchi pattern quality on all types of MgB2 samples, we apply here the newly developed transmission EBSD (t‐EBSD) technique to spark‐plasma sintered MgB2 samples.. This method requires the preparation of TEM slices by means of focused ion‐beam milling, which are then analyzed within the SEM, operating with a custom‐built sample holder. To obtain multi‐phase scans, we identified the Kikuchi pattern of the MgB4 phase which appears at higher reaction temperatures and may act as additional flux pinning sites. We present here for the first time EBSD mappings of multiple phases, which include MgB2, MgB4 and MgO.
We report the first application of the emerging transmission Kikuchi diffraction technique in the scanning electron microscope to investigate nano-grain structures in polycrystalline MgB2 superconductors. Two sintering conditions were considered, and the resulting differences in superconducting properties are correlated to differences in grain structure. A brief comparison to X-ray diffraction results is presented and discussed. This work focusses more on the application of this technique to reveal grain structure, rather than on the detailed differences between the two sintering temperatures. © 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
