Analysis of the microstructure of bulk MgB₂ using TEM, EBSD and t‐EBSD

Abstract

Summary EBSD analysis can provide information about grain orientation, texture and grain boundary misorientation of bulk superconducting MgB2 samples intended for supermagnet 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 analysed within the SEM, operating with a custom‐built sample holder. To obtain multiphase 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. Lay Description 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 supermagnets, 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 magnetised 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 nanometre 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.

The discrepancies in different facets of MgB2 bulk superconductors prepared under various sintering durations by spark plasma sintering

“…We can find similar phenomena from other studies [12,[35][36][37][38]. Since the existence of the second phase of MgB 4 particles has been confirmed from the XRD analysis, and MgB 4 particles can be effective pinning centers [9], we can speculate that the MgB 4 particles provide PP centers in this study. The h 0 and k n values rising with the temperature is not a monotonous trend, but rather a fluctuation, which can be verified from some studies with small temperature intervals [12,35].…”

“…However, samples prepared by conventional sintering methods cannot show outstanding performance because of the low density, the presence of impurity, etc. To solve these problems, many improved preparation methods are used to fabricate high-performance MgB 2 superconductors with high purity and large size, such as hot pressing sintering, hot isostatic pressing and spark plasma sintering (SPS) [5][6][7][8][9]. In this study, we fabricated MgB 2 bulk samples by the SPS technique, which combines the pulsed current and the uniaxial pressure.…”

Feasibility of THT-Induced Microstructure Gradients and Their Effects on Fatigue Properties in Additively Manufactured Cylindrical and in Conventionally Manufactured Tube-Like Ti-6Al-4V Specimens

“…As outlook, the V-rich phase regions in Figure 12 should be investigated using the selected area diffraction (SAD) method in the TEM to check whether they can actually be the αs phase [51]. Moreover the determination of the phase fractions can be done by t-EBSD [52].…”

Pinning Force Scaling Analysis of Polycrystalline MgB2

“…All samples were pure samples without any additions. The X-ray analysis and electron backscatter diffraction (EBSD) investigations revealed the presence of small amounts of MgO and MgB 4 particles in the samples prepared at reaction temperatures above 850 • C. The MgB 4 particles in the SPS sample were found to be located to a large extend along the grain boundaries, and not within the MgB 2 grains, so these particles cannot provide additional flux pinning [25]. This is important as the goal of this contribution is to study the basic flux pinning properties of MgB 2 , which would otherwise be overlaid with the strong pinning provided by the secondary particles embedded in the MgB 2 matrix.…”