Microstructural connectivity in sintered ex-situ MgB2 bulk superconductors

Shimada, Yusuke; Hata, Satoshi; Ikeda, Ken; Nakashima, Hideharu; Matsumura, Shuichi; Tanaka, Hiroya; Yamamoto, Akiyasu; Shimoyama, J.; Kishio, K.

doi:10.1016/j.jallcom.2015.09.253

Cited by 21 publications

(16 citation statements)

References 40 publications

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“…Simultaneously, a uni-axial pressure is applied. The main difference between SPS and conventional or hot-pressing methods is that SPS allows for the preparation of highly dense samples with grain growth control and a decreased processing time [ 13 , 14 , 15 , 16 ]. Concerning MgB 2 , the samples processed by SPS additionally show excellent mechanical properties [ 17 ].…”

Section: Introductionmentioning

confidence: 99%

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

et al. 2022

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In situ MgB2 superconducting samples were prepared by using the spark plasma sintering method. The density of the obtained bulks was up to 95% of the theoretical value predicted for the material. The structural and microstructural characterizations of the samples were investigated using X-ray diffraction and SEM and correlated to their superconducting properties, in particular their critical current densities, Jc, which was measured at 20 K. Extremely high critical current densities of up to 6.75 × 105 A/cm2 in the self-field and above 104 A/cm2 at 4 T were measured at 20 K, indicating that vortex pinning is very strong. This property is mainly attributed to the sample density and MgB2 nanograins in connection to the presence of MgO precipitates and areas rich in boron.

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Section: Introductionmentioning

confidence: 99%

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

et al. 2022

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“…The electron backscatter diffraction technique (EBSD) has recently proven to yield important results concerning the crystallographic orientation of the grains (Mikheenko, 2012;Wong et al, 2015;Koblischka-Veneva et al, 2016;Shimada et al, 2016). Due to the small size of the MgB 2 grains, it turned out that the newly developed transmission EBSD (t-EBSD) technique is better suited for the investigation of such samples, enabling a higher spatial resolution and being less influenced by charging effects etc.…”

Section: Introductionmentioning

confidence: 99%

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

Koblischka-Veneva

Koblischka

Schmauch

et al. 2019

Journal of Microscopy

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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.

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“…It was also found that the sample density increased for excess Mg content up to 5 mol%. However, T c decreased slightly (* 1.5 K) with the addition of excess Mg [17].…”

Section: Introductionmentioning

confidence: 96%

“…It is known that the coupling of grains across grain boundaries (grain coupling) for ex situ samples is weaker than that of in situ ones, though the former has a higher packing factor of * 75% [3]. Unlike the stronger grain coupling in in situ MgB 2 , the pre-reacted ex situ MgB 2 grain aggregates are poorly connected, leading to the reduced effective cross-sectional area for current transport, thus lower J c [14,17]. If the grain coupling of the ex situ samples is improved, their J c should outperform that of the in situ MgB 2 .…”

Section: Introductionmentioning

confidence: 99%

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

Hapipi

Chen

Shaari

et al. 2022

J Mater Sci: Mater Electron

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In this work, ex situ MgB2 was mixed with 0.5 mol of Mg and sintered. The sintering conditions were varied over a temperature range of 600–1000 °C for 1, 3, and 7 h, respectively. The addition of Mg during the sintering increased the partial pressure of Mg and thus suppressed the decomposition of MgB2. Onset of critical temperature, Tc, was retained at ∼ 38 K even after the addition of Mg. By increasing the sintering temperature, magnetic critical current density, Jc at self-field, and 20 K of the ex situ samples increased consistently. With the addition of Mg for 1 h sintering, self-field Jc (20 K) was enhanced more than 20 times to 104 A cm−2 as the sintering temperature was increased. Such significant enhancement in the Jc is mainly due to the improved grain coupling aided by Mg during the short sintering.

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Microstructural connectivity in sintered ex-situ MgB2 bulk superconductors

Cited by 21 publications

References 40 publications

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

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

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

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Microstructural connectivity in sintered ex-situ MgB2 bulk superconductors

Cited by 21 publications

References 40 publications

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

High Critical Current Density of Nanostructured MgB2 Bulk Superconductor Densified by Spark Plasma Sintering

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

Enhanced critical current density of ex situ MgB2 via control of Mg assisted sintering

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