Improved critical current densities in bulk FeSe superconductor using ball milled powders and high temperature sintering

Abstract

This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.The present study is investigating the effect of high temperature sintering combined with ball milled powders for the preparation of FeSe material via solid state sintering technique. The commercial powders of Fe (99.9% purity) and Se (99.9% purity) were mixed in a nominal ratio Fe:Se ¼ 1:1 and thoroughly ground and ball-milled in a glove box during 6 h. Then, the powder mixture was pressed into pellets of 5 mm in diameter and 2 mm thickness using an uniaxial pressure of 100 MPa. The samples were sealed in quartz tubes and sintered at 600 8C for 24 h. Then, the pellets were again thoroughly ground and ball-milled in the glove box and pressed into pellets, and the final sintering was performed at two different temperatures, namely at 900 8C for 24 h and at 950 8C for 24 h. X-ray diffraction results confirmed that both samples showed mainly of the b-FeSe with tetragonal structure. The temperature dependence of magnetization (M-T) curves revealed a sharp superconducting transition T c, onset ¼ 8.16 K for the sample sintered at 900 8C. Further, scanning electron microscopy observations proved that samples sintered at 900 8C show a platelike grain structure with high density. As a result, improved irreversibility fields around 5 T and the critical current density (J c ) values of 6252 A cm À2 at 5 K and self-field are obtained. Furthermore, the normalized volume pinning force versus the reduced field plots indicated a peak position at 0.4 for the sample sintered at 900 8C. Improved flux pinning and the high J c values are attributed to the textured microstructure of the material, produced by a combination of high temperature sintering and ball milling.

A simple method for preparing superconducting FeSe pellets without sealing in evacuated silica tubes

“…According to the phase diagram assessed by Okamoto [6], tetragonal FeSe is stable up to 457 °C and decomposes peritectically into α-Fe and hexagonal FeSe above this temperature. Since the reaction temperature was lower than this transition point, it is unlikely that the tetragonal FeSe has formed by reaction between α-Fe and hexagonal FeSe during cooling, as it is the case in processes involving reaction at T > 457 °C [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22].…”

“…So far, most reported preparation conditions involve sealing in evacuated quartz ampoules, heating most often well above the temperature, below which tetragonal FeSe is stable, followed or not by annealing at 300 °C ≤ T ≤ 460 °C [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22]. In some cases, several heat treatments are performed, requiring re-sealing in new evacuated ampoules [14][15][16][17][18][19][20][21][22]. Although this processing route usually results in good samples, it is time and resources consuming, because the silica tubes are generally broken and disposed of at the end of each heat treatment, and thus not optimized for potential large-scale production.…”

Comparison of Temperature and Field Dependencies of the Critical Current Densities of Bulk YBCO, MgB<inline-formula> <tex-math notation="LaTeX">$_2$</tex-math> </inline-formula>, and Iron-Based Superconductors

“…Figure 3 presents finally the results of the two IBS samples. 37.1 7.5 no information [8], [9] HIP-processed, polycrystalline, 10 mm dia., 92% dense The first sample is a FeSe sample plus 4 wt.-% Ag [11], [25]. The Ag-addition proved to be effective to improve the superconducting properties of FeSe as well as the grain connectivity, allowing the use of the simple sintering process.…”

“…For the application as superconducting permanent magnets, dubbed supermagnets or trapped field (TF) magnets, there are three different candidate materials available: (i) bulk, melttextured single crystalline materials of YBa 2 Cu 3 O x (YBCO or RE-123, where RE indicates rare-earths) [3], [4], and a superconducting YBCO foam [5] prepared in two different ways [6], [7] and (iii) bulk, polycrystalline iron-based superconductors (IBS) of Ba 0.6 K 0.4 Fe 2 As 2 (122) [8], [9] and FeSe with addition of Ag [10], [11] were chosen for comparison. The latter two types of material offer a much easier (and hence, cheaper) way of producing the samples as the grain boundaries (GBs) were found not to be weak-links, but do provide flux pinning like in the case of conventional superconductors like NbTi or Nb 3 Sn.…”

“…For MgB 2 , we selected a sample prepared via solid-state sintering [6], a spark-plasma sintered sample [7], [22] and a sintered sample with added carbon-encapsulated boron [23], [24]. Finally, for the IBS, we selected sintered FeSe with 4 wt.-% Ag [11], [25] and we took data of hot-isostatic processed (HIP) 122 sample from Weiss et al [8], [9], where already data of TF measurements were published. For comparison, all j c data are plotted versus the reduced temperature, t = T /T c .…”