Fe–Se–Te composites

Physica C 471 (2011) 721–724

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Synthesis and superconductivity of MgB2/Fe–Se–Te composites J.-H. Ahn ⇑, J. Park Department of Materials Engineering, Andong National University, Songchun-dong, Andong, Gyungbuk 760-749, South Korea

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Article history: Available online 13 May 2011 Keywords: Fe-based superconductor MgB2 Sintering Densification Composites

a b s t r a c t MgB2/FeSe0.75Te0.25 composites were synthesized to see the effect of the co-existence of superconducting phases on the superconductivity. MgB2 powders were mixed with pre-alloyed FeSe0.75Te0.25 or elemental powder mixture, followed by sintering at 400 °C. The resulting materials were superconducting MgB2/ FeSe0.75Te0.25 composites without impurity phase when using pre-alloyed FeSe0.75Te0.25 as a starting powder. When FeSe0.75Te0.25 was prepared by in situ reaction from the elemental powder mixture of Fe–Se–Te, on the other hand, non-superconducting impurity phases partly formed after sintering. No double superconducting transition was observed in MgB2/FeSe0.75Te0.25 composites in spite of coexistence of two superconducting phases. Ó 2011 Elsevier B.V. All rights reserved.

1. Introduction Following the discovery of superconducting MgB2 [1], recently discovered Fe-based compounds such as ReFeAsO [2], AeFe2As2 (Ae = alkaline earths) [3], AFeAs (A = alkali metal) [4], and FeSe [5], have attracted much attention among scientists due to scientific and engineering interests. These materials have a higher Tc than conventional A-15 compounds, showing a potential application in many fields. Fe-based superconductors are scientifically important to understand the relationship between superconductivity and magnetism. However, the critical density (Jc) of Fe-based compounds is known too low to be used in various applications. MgB2 also has the problems of insufficient flux pinning and low critical current density, although the materials can be used in applications at moderate magnetic fields such as magnetic resonance imaging (MRI). The low Jc of MgB2 is mainly due to insufficient densification, resulting in a poor connectivity between MgB2 grains. Many research efforts have been conducted to solve this problem such as by employing infiltration, high-pressure consolidation or the addition of third element [6–10]. In particular, the addition of third phase seems to be effective to fill void spaces between MgB2 grains, enhancing densification and grain connectivity. In the present, we have added FeSe0.75Te0.25 to MgB2 to see the possibility of enhanced densification as well as an eventual proximity effect of superconducting phases in composite microstructures [11]. Recently, the effect of composite with two

⇑ Corresponding author. Tel.: +82 54 820 5648; fax: +82 54 820 6126. E-mail address: [email protected] (J.-H. Ahn). 0921-4534/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2011.05.037

superconducting phases, Type I (Pb) and Type II (MgB2), was reported [12]. This result showed that a double transition of superconductivity, but the critical density of MgB2 was degraded by flux creep caused by a phase with a lower Tc (Pb). In this work, we prepared composites consisting of two superconducting phases of Type II, MgB2 and FeSe0.75Te0.25. The purpose of the present work was to see the effect of composite on the microstructural changes and the resulting superconducting properties.

2. Experimental procedure The starting materials were elemental powders of Se (>99.9%, 20 lm), Te (>99%, 20 lm), alloyed FeSe0.75Te0.25, and pre-alloyed MgB2. The MgB2 and Fe–Se–Te powders were mixed to give final compositions of MgB2/FeSe0.75Te0.25. For FeSe0.75Te0.25, elemental powder mixture or pre-alloyed powder was used as a starting powder. The pre-alloyed FeSe0.75Te0.25 was prepared by an in situ reaction of elemental powders at 650 °C for 50 h in a sealed quartz tube. The powder mixture of MgB2/FeSe0.75Te0.25 was then diecompacted at a pressure of 300 MPa, and put in a sealed quartz tube. Sintering was carried out at 400 °C for 10 h. The details of composition and starting powders are listed in Table 1. The phase identification was examined both by a Rigaku D/ Max-800 and Rapid S (microbeam XRD) using Cu Ka radiation. The powder morphology and microstructures of samples were examined by a field-emission scanning electron microscope (JSM-6700F). The critical temperature (Tc) and the critical current density (Jc) were measured by a superconducting quantum interfere device (SQUID) magnetometer at temperatures of 5–100 K. The Tc was determined as the onset of the diamagnetism. The mag-

J.-H. Ahn, J. Park / Physica C 471 (2011) 721–724

Specimen no.

Composition

Starting Fe–Se–Te powder

Mf 1 Mf 2 Mf 3

MgB2 + 50 wt.% FeSe0.75Te0.25 MgB2 + 10 wt.% FeSe0.75Te0.25 MgB2 + 50 wt.% FeSe0.75Te0.25

Pre-alloyed FeSe0.75Te0.25 Pre-alloyed FeSe0.75Te0.25 Elemental powder mixture

netic Jc values were derived from the height of the magnetization loop M using the Bean’s critical state.

3. Results and discussions Fig. 1 shows the X-ray diffracted patterns of composite samples which obtained after sintering at 400 °C for 10 h. When using prealloyed FeSe0.75Te0.25 as a starting powder together with MgB2 (Fig. 1a), no reaction took place between MgB2 and FeSe0.75Te0.25. The resulting materials were the composites of MgB2/FeSe0.75Te0.25 without the presence of non-superconducting phases, regardless of the mixing composition of two materials: 50% and 10% FeSe0.75Te0.25 for the samples Mf 1 and Mf 2, respectively. Only difference with the variation of mixing composition was the change of the relative XRD peak intensity of both phases. In the case of the use of elemental powder mixture of Fe–Se–Te as a starting powder (Fig. 1b, sample Mf 3), on the other hand, non-superconducting FeSe2, FeTe0.9 as well as un-reacted Te and Se peaks appeared in XRD patterns. However, no detrimental reaction between MgB2 and Fe–Se–Te was detected by XRD. The sintering temperature employed in the present work was as low as 400 °C to avoid such reaction, but thought be insufficient for the completion of reaction to form FeSe0.75Te0.25. Fe-SEM fractographs of superconducting MgB2/FeSe0.75Te0.25 composites which were sintered at 400 °C for 10 h, is shown in Fig. 2. As shown in Fig. 2a and b, the samples Mf 1 and 2 contain many pores, and the poor grain connectivity of MgB2 was not apparently improved by addition of FeSe0.75Te0.25. Increasing the volume fraction of FeSe0.75Te0.25 only slightly improved the grain connectivity when comparing Fig. 1a (50% FeSe0.75Te0.25) with Fig. 1b (10% FeSe0.75Te0.25). On the other hand, the sample Mf 3 where elemental powder mixture of Fe–Se–Te was used for in situ reaction, a quite dense microstructure was obtained after sintering at 400 °C (Fig. 2c). This is due to the presence of liquid phases such as Se (m.p. 221 °C) and eutectic Se–Te which probably fill the interstices between solid particles during in situ reaction. At a higher magnification (Fig. 2d), angular-shaped crystallites of MgB2 are visible. It is well-known that low Jc in MgB2 is mainly due to the poor grain connectivity with the random and loose packing of MgB2 grains. The addition of a third phase can improve the densification of sintered bulks. The distribution of MgB2 and FeSe0.75Te0.25 phases was qualitatively examined by a back-scattered electron images (BEI). The phases with larger atomic numbers (Mg and B in this case) are darker than those of lesser atomic numbers (Fe, Se and Te) in BEI. As shown in Fig. 3, the bright spots which correspond to Fe–Se–Te are more homogeneously distributed in the sample Mf 3 where elemental powder mixture were in situ reacted, than the samples Mf 1 and 2. It seems that the presence of liquid phases facilitates to fill interparticular interstices during the in situ reaction of elemental powder mixtures. Superconducting properties of MgB2/FeSe0.75Te0.25 composites were examined by Tc and Jc tests. Tc values were determined by the onset of diamagnetism, and shown in Fig. 4. For comparison, a pristine MgB2 bulk was prepared by ex-situ reaction at 950 °C for 2 h. The Tc values of both the pristine MgB2 and MgB2/FeS-

e0.75Te0.25 composites were almost identical: 37.5 K. However, MgB2/FeSe0.75Te0.25 composites exhibited less sharp superconducting transition than the pristine MgB2, in particular, for the sample Mf 3 where non-superconducting and un-reacted phases remained after in situ reaction. The observed Tc stems from MgB2, but not from FeSe0.75Te0.25. The Tc of the latter phase is 14 K [13]. Double superconducting transition such as observed in MgB2/Pb composites [12] was not observed in this MgB2/FeSe0.75Te0.25 composite. The values of critical current density (Jc) of MgB2/FeSe0.75Te0.25 composites are presented in Figs. 5 and 6. At 20 K, the pristine MgB2 generally exhibits a higher Jc than MgB2/FeSe0.75Te0.25 composites. The degradation of Jc by the addition of FeSe0.75Te0.25 to MgB2 might be attributed to two facts. First, the presence of nonsuperconducting or detrimental phases in the composites (e.g. in the sample Mf 3) might lower Jc. Second, flux creep might be induced by a lower Tc phase (FeSe0.75Te0.25) at temperatures between 13 and 39 K. Surprisingly, however, at magnetic fields higher than 3 T (20 K), MgB2/50% FeSe0.75Te0.25 composite (the sample Mf 2) exhibited a higher Jc than MgB2. This might be partly due to the improved densification by addition of 50% FeSe0.75Te0.25, as shown in Fig. 2a. The Jc values measured at 5 K showed a similar tendency as those at 20 K, but the enhancement of Jc for MgB2/50% FeSe0.75Te0.25 composites at higher fields was less apparent than Jc at 20 K.

(a)

Pre-alloyedMgB2 + FeSe0.75 Te0.25

MgB2(JCPDS : 38-1369) FeSe 0.75Te0.25

50% MgB 2 + 50% FeSe 0.75Te 0.25

Intensity (a.u.)

Table 1 The composition and the condition of starting powders.

90% MgB 2 + 10% FeSe 0.75Te 0.25

20

30

40

50

60

Two theta (deg.)

MgB2 + 10wt% FeSe0.75Te0.25(in-situ)

(b) MgB2(JCPDS : 38-1369) FeSe0.75Te0.25 FeSe2(JCPDS : 12-0290)

Intensity (a.u.)

722

Te(JCPDS : 36-1452) Se(JCPDS : 47-1516) FeTe0.9(JCPDS : 07-0140)

20

30

40

50

60

Two theta (deg.) Fig. 1. XRD results of superconducting MgB2 + FeSe0.75Te0.25 composites, sintered at 400 °C for 10 h: (a) samples Mf 1 and 2, and (b) sample Mf 3.

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(a) Mf 1

(b) Mf 2

(c) Mf 3

(d) Mf 3

Fig. 2. Fractographs of superconducting MgB2 + FeSe0.75Te0.25 composites, sintered at 400 °C for 10 h: (a) sample Mf 1 (using 50 wt.% pre-alloyed FeSe0.75Te0.25), (b) sample Mf 2 (using 10 wt.% pre-alloyed FeSe0.75Te0.25), (c) sample Mf 3 (using 50 wt.% powder mixture of FeSe0.75Te0.25), and (d) higher magnification image of sample Mf 3.

(a) Mf 1

(b) Mf 2

(c) Mf 3

Fig. 3. BEI (back-scattered electron image) of superconducting MgB2 + FeSe0.75Te0.25 composites, sintered at 400 °C for 10 h: (a) sample Mf 1, (b) sample Mf 2, and (b) sample Mf 3.

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We have initially expected that the poor grain connectivity of MgB2 can be improved by filling interparticular interstices with a second superconducting phase of FeSe0.75Te0.25. We also expected that the presence of a second superconducting phase improves the superconductivity of MgB2 by ‘proximity effect’ [11]. We observed an opposite effect. It is thought that the FeSe0.75Te0.25 with lower Tc induces a flux creep at temperatures below Tc of MgB2 (38 K), resulting in a local heating by the Lorentz force on MgB2.

MgB 2(ex-situ)

0.03

MgB2(ex-situ)+ 50wt% FeSe 0.75Te0.25(pre-alloyed) MgB2(ex-situ)+ 10wt% FeSe0.75Te0.25 (pre-alloyed)

Normalized emu

MgB2(ex-situ)+ 50wt% FeSe 0.75Te 0.25 (in-situ)

0.00

-0.03

4. Conclusions -0.06 0

5

10

15

20

25

30

35

40

45

Temperature (K) Fig. 4. Critical temperature (Tc) of superconducting MgB2 + FeSe0.75Te0.25 composites, sintered at 400 °C for 10 h.

MgB 2(ex-situ)

MgB2(ex-situ)+ 50wt% FeSe0.75Te0.25(pre-alloyed)

Critical current density (Acm -2 )

MgB2(ex-situ)+ 10wt% FeSe0.75Te0.25(pre-alloyed) MgB2(ex-situ) + 50wt% FeSe 0.75Te0.25 (in-situ)

4

10

We have synthesized and examined the superconducting properties of MgB2/FeSe0.75Te0.25 composites. Superconducting composites without impurity phase were successfully synthesized in the case of the use of pre-alloyed FeSe0.75Te0.25 as a starting powder. However, both grain connectivity and superconductivity was not markedly improved. On the other hand, although non superconducting impurity phases formed in the case of in situ reaction of elemental powder mixture of Fe–Te–Se, the grain connectivity was markedly improved. Further study is thus needed to employ a higher temperature (>400 °C) or a longer duration (10 h) for the completion of in situ reaction to form FeSe0.75Te0.25 in the examined composites. Acknowledgment This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (20100009978).

3

10

T = 20 K

References 2

10

0

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3

4

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Applied magnetic field (T) Fig. 5. Critical current density (Jc) of superconducting MgB2 + FeSe0.75Te0.25 composites, sintered at 400 °C for 10 h (20 K).

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MgB2(ex-situ)

MgB2(ex-situ)+ 10wt% FeSe0.75Te0.25(pre-alloyed)

-2

Critical current density (Acm )

MgB2(ex-situ)+ 50wt% FeSe0.75Te0.25(pre-alloyed) MgB2(ex-situ)+ 50wt% FeSe0.75Te0.25 (in-situ) 4

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T=5K

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Applied magnetic field (T) Fig. 6. Critical current density (Jc) of superconducting MgB2 + FeSe0.75Te0.25 composites, sintered at 400 °C for 10 h (5 K).

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