Preparation and characterisation of Os doped MgB2

Physica C 507 (2014) 70–74

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Preparation and characterisation of Os doped MgB2 J.-C. Grivel ⇑, S. Namazkar, A. Alexiou, O.J. Holte Department of Energy Conversion and Storage, Technical University of Denmark, 4000 Roskilde, Denmark

a r t i c l e

i n f o

Article history: Received 8 August 2014 Received in revised form 10 October 2014 Accepted 19 October 2014 Available online 28 October 2014 Keywords: MgB2 Doping Osmium

a b s t r a c t Polycrystalline samples with Mg1 xOsxB2.04 nominal stoichiometry were made by reacting elemental powders at 800 °C under argon atmosphere. Based on XRD diffraction patterns, EDS analysis and magnetisation measurements, it is found that Os can replace up to about 1 at.% Mg in the MgB2 lattice. Beyond this doping level, unreacted Os and Mg-rich Mg–Os impurity phases are formed. The a-axis parameter contracts upon doping while the superconducting transition temperature decreases at a rate of 2.1 K/ at.% Os substitution. At 10 K, Os doping induces an improvement of the normalised critical current density under applied magnetic fields in excess of 0.5 T, indicating a modest enhancement of flux pinning in this range. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Owing to its relatively high critical temperature (Tc  40 K), relative abundance of its constituent elements and low activity under neutron irradiation [1,2], MgB2 is foreseen as a strong candidate for many large scale applications such as coils in cryogen free MRI-systems [3,4], induction heating devices [5] and other applications as diverse as fault-current limiters, generators, transformers, magnetic shields for manned deep-space missions, etc. [6,7]. However, the critical current densities achievable in MgB2 wires under magnetic fields need to be enhanced for enabling the realisation of most of the above applications. In order to improve the performance of MgB2 in high magnetic fields, two strategies are commonly used: introducing nanometer-sized impurities as artificial flux pinning centres or doping with foreign elements. In view of the latter, carbon, which can replace up to about 30 at.% B in the MgB2 lattice, is remarkably efficient but further improvements are still desirable [8–11]. For this reason, research activities are ongoing to find other potential options for enhancing the flux pinning strength of MgB2 like for example dual doping on both B and Mg sites. Except for the case of Al that can be substituted for Mg up to at least 40% [12–15], doping on the Mg sites is usually limited to few atomic percent or even less [16–33]. Among the transition metal elements, only few results have been published on the possibility of substituting platinum group metal elements in MgB2. Besides Pt, which was studied in association with SiC doping [34], only Ir and Ru appear to have been reported to have a limited but measurable solubility and result in a decrease of Tc [28,32]. The ⇑ Corresponding author. Tel.: +45 4677 4739; fax: +45 4677 5758. E-mail address: [email protected] (J.-C. Grivel). http://dx.doi.org/10.1016/j.physc.2014.10.010 0921-4534/Ó 2014 Elsevier B.V. All rights reserved.

present contribution reports on the possible substitution of Os for Mg in MgB2 and its effects on structure and Tc.

2. Experimental details Mg (Alfa Aesar, 99.8% purity), amorphous boron (Aldrich, 95–97%) and Os (Alfa Aesar, 99.8%) were used as starting reagents. The elemental powders were mixed in Mg1 xOsxB2.04 (0.000 6 x 6 0.040, Dx = 0.005) nominal ratios and homogenised by manual grinding in an agate mortar. The choice of a small B excess in the nominal compositions is based on a previous study showing that for this specific grade of amorphous boron powder, which contains a slight amount of Mg as impurity, the best results in terms of phase purity were obtained for this particular composition in undoped MgB2 [35]. After grinding, the powders were pressed into pellets (1 g powder, 12 mm diameter under a pressure of 1.8 kbar). The pellets were wrapped into Ti foils of 32 lm thickness that act as oxygen getter and reduce the risk of Mg losses during heat treatment. After a first sintering at 700 °C for 1 h, the samples were sintered twice during 1 h at 800 °C with intermediate grinding and repressing. All heat treatments were performed under Ar atmosphere. XRD patterns were recorded in a STOE X-ray diffractometer with Cu Ka radiation (k = 1.5406 Å) on powdered samples after the final treatment at 800 °C. Silicon powder was mixed to the samples after grinding as an internal standard for lattice parameters calculations using the UnitCell least square refinement programme [36]. The microstructure of the samples and the composition of the various phases present after sintering at 800 °C were studied by means of scanning electron microscopy

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Intensity [arbitrary units]

The XRD patterns of all samples after the second sintering treatment at 800 °C are presented in Fig. 1. The Os-free sample contains a minor amount of MgO but consists otherwise of MgB2 only. Increasing the nominal Os content results in the appearance of new reflections, with elemental Os clearly visible from x = 0.025 and peaks from an unidentified phase emerging from the background around x = 0.015. The lattice parameters of the MgB2 phase are plotted in Fig. 2 and listed in Table 1. The a-axis parameter sharply decreases up to 0.015 < x < 0.020 before becoming constant within the standard deviation limits. In contrast, the c-axis parameter is not clearly affected by Os doping. It seems to follow a trend for decreasing values, but this continues up to the maximum doping level (x = 0.040). The ion radius of Os4+ is significantly lower than that of Mg2+ (63 pm versus 72 pm in VI-fold coordination [37]) and this could explain the decrease of the a-axis lattice parameter up to a maximum solubility of about x = 0.015. As for the slight decrease of the c-axis parameter beyond x = 0.015, it could be an indirect consequence of the formation of an impurity phase containing Mg and/or B in an atomic ratio different from MgB2, possibly in association with Os, because this would lead to a change of the stoichiometry of the remaining MgB2 matrix and it has been previously shown that modifications of the Mg:B ratio in MgB2 results in subtle changes of the lattice parameters [38–42]. The critical temperature (Fig. 3) decreases rapidly up to x = 0.010 and seems to become almost constant beyond x = 0.012 approximately, as estimated by extrapolating the initial slope and the Tc values for the highest doping levels. The onset of the diamagnetic transition, down to 50% of the complete transition is also shown in Fig. 3. It appears that the transition is sharp but tends to broaden beyond the solubility limit. The apparent slow decrease of Tc for x > 0.010 can be explained in a similar way as for the decrease of the c-axis parameter discussed above. By comparison

a-axis parameter [Å]

3. Results and discussion

3.084

3.082

3.080

3.078

3.076 0.00

0.01

0.02

0.03

0.04

0.03

0.04

x in Mg1-xOs x B 2.04

3.522

c-axis parameter [Å]

(SEM) in a table-top TM3000 microscope from HITACHI equipped with a QUANTAX 70 EDS analyser. The critical temperature (Tc) was determined as the onset of the diamagnetic transition from the real component of ac-susceptibility measurements performed between 5 K and 45 K with an applied magnetic field of 0.1 mT rms and 17 Hz frequency in a CRYOGENIC Ltd Mini-CFMS. A vibrating sample magnetometry (VSM) setup was used in the same instrument to record M(B) loops at 10 K and 30 K on powders loaded in sample holders with 4.5 mm internal diameter.

3.520

3.518

3.516 0.00

0.01

0.02

x in Mg1-xOs x B2.04 Fig. 2. Unit cell parameters of MgB2 as a function of nominal composition. Dashed lines are guides to the eye.

Table 1 a-Axis and c-axis parameters as well as critical temperature (Tc) of the Mg1 xOsxB2.04 samples. x in Mg1 xOsxB2.04

a-Axis (Å)

c-Axis (Å)

Tc (K)

0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

3.0836(5) 3.0823(8) 3.0804(6) 3.0797(7) 3.0784(8) 3.0783(7) 3.0785(6) 3.0782(10) 3.0776(12)

3.5214(6) 3.5216(10) 3.5192(8) 3.5191(9) 3.5177(9) 3.5173(8) 3.5197(7) 3.5194(11) 3.5170(14)

38.9(3) 37.9(3) 36.8(3) 36.9(3) 36.4(3) 36.5(3) 36.1(3) 35.9(3) 36.1(3)

x = 0.040 x = 0.035 x = 0.030 x = 0.025 x = 0.020 x = 0.015 x = 0.010 x = 0.005 x = 0.000

20

25

30

35

40

45

50

55

60

65

2θ [degrees] Fig. 1. X-ray diffraction patterns of the samples with Mg1 xOsxB2.04 nominal composition after sintering twice at 800 °C for 1 h in Ar. d: MgB2; s: MgO; : Os; .: unidentified phase.

with the lattice parameter evolution and the XRD patterns, it can be concluded that the solubility limit of Os in MgB2 is situated between x = 0.010 and x = 0.015. This is further confirmed by EDS measurements performed on a polished cross-section of the sample with Mg0.96Os0.04B2.04 nominal stoichiometry, which yield an average Mg:Os atomic ratio of 98.96:1.04 with a standard deviation r = 0.38 for a total of 20 local analyses performed on areas corresponding to MgB2 grains that do not show evidence for secondary phase particles precipitates down to a scale of 0.5 lm. As evidenced in Fig. 4, which shows the EDS mapping of a 80  60 lm2 area of the Mg0.96Os0.04B2.04 sample after sintering twice at 800 °C, Os-rich particles with relatively large diameters (up to 12 lm) are present within the ‘‘MgB2’’ matrix. EDS quantification suggests in a first instance that these particles consist of a Mg–Os intermetallic alloy containing between 13 and 36 at.% Os.

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χ' [arbitrary units]

39.0 38.5

Tc [K]

38.0 37.5 37.0

Table 2 Reported solubility limits and rate of Tc suppression (dTc/dx) for various transition elements substituting for Mg.

x in Mg1-xOsx 0.000 0.005 0.010 0.020 0.030 0.040

28

30

32

34

36

38

40

42

Temperature [K]

36.5

Doped element

Solubility limita

dTc/dx (K/at.%)

Refs.

Sc Y Ti Zr Hf Cr Mo Mn

0.03 0 0.002 0 0.22 0.02 0.0028 0.07 0.03 0.0046 0.02 0.0048 0.03 0.0026 0.0044 0.10 0.015 0.015 ± 0.005 0.06

0.0 – 7.2 – 0.25 1.5 5 10 9 13.1 9 14.2 0.9 7.6 4.8 < 0.1 1.0 2.1 1.6

[29] [16] [16] [16] [30] [24] [16] [33] [26] [16] [20] [16] [26] [16] [16] [21] [32] This work [28]

36.0 35.5 0.00

Fe

0.01

0.02

0.03

0.04 Co Cu Ag Zn Ru Os Ir

x in Mg 1-xOsx B2.04 Fig. 3. Superconducting critical temperature (Tc) versus Os content in the nominal stoichiometry. Dashed lines are guides to the eye. Inset: real component of the acsusceptibility of samples with Mg1 xOsxB2.04 nominal composition. For clarity, only a data sets for a few compositions have been plotted. a

Given as x in Mg1 xMxB2.

Jc(B) / Jc (B=0T)

1

a

x in Mg 1-xOsx 0.000 0.005 0.010 0.015 0.020 0.025 0.030 0.035 0.040

0.1

0.1

0.3

0.4

B [T]

Fig. 4. Sem picture with superimposed EDS map showing the relative concentrations of Mg (blue) and Os (red) on the surface of a polished cross-section (sample with Mg0.96Os0.04B2.04 nominal composition). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

1

0.24

Jc(B) / Jc (B=0T)

0.22

Jc(B) / Jc (B=0T)

The Mg–Os phase diagram does not appear to have been experimentally assessed. In order to check for the possibility of forming either a solid solution or a compound with a stoichiometry in the range found by EDS for the Mg–Os particles, a mixture of Mg and Os powders with Mg3Os stoichiometry was pressed into a pellet and heat treated under conditions similar to those used for the Mg1 xOsxB2.04 sample. However, XRD patterns only showed evidence for a biphasic mixture consisting of Mg and Os. This indicates that no intermetallic phase is formed between these two elements at T 6 800 °C and direct reaction between Mg and Os cannot account for the unidentified reflections observed in the XRD patterns (Fig. 1). It is not excluded that some B is also present in the impurity phases formed in the Os-rich Mg1 xOsxB2.04 samples, but quantification of B is not straightforward with the instrument used in the present study. On the other hand, pure Os or Os-rich particles were not detected by EDS down to a scale of 1 lm, indicating that the Os diffraction lines observed in the XRD patterns (Fig. 1) are probably due to smaller grains dispersed in the sample. The solubility limit of Os in the MgB2 lattice is similar to that of Ru, but the rate of Tc reduction is higher for Os than Ru doping (2.1 K/at.% versus 1.0 K/at.%) [32]. An additional contribution to

0.2

0.2 0.18 0.16 0.14 0.12 0.80 0.85 0.90 0.95 1.00 1.05 1.10 1.15 1.20

B [T]

0.1

b 0.5

1.0

1.5

2.0

B [T] Fig. 5. Normalised magnetic jc at 30 K (a) and 10 K (b). The inset in panel (b) shows an expanded view between 0.8 T and 1.2 T.

Tc depression in Os-doped MgB2 might originate in the lattice contraction, which was found to be only marginal in the case of Ru doping. However, direct comparisons with high-pressure experiments [43,44] are difficult due to the fact that hydrostatic pressure results in a stronger relative contraction of the c- than the

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a-parameter [45], which is different from the observations for Os doping (Fig. 2). The main contribution to Tc depression is probably the hole band filling resulting from the substitution of divalent Mg by Os dopant ions with higher valence, like for example in the case of Ir doping, which results in a similar rate of Tc suppression [28]. Table 2 summarises the Tc depression rate (dTc/dx) of various transition element dopants in MgB2 and their reported solubility limits. The dTc/dx of Os is relatively low in comparison with elements like Fe, Mn, Cu, Ti, Mo, etc. This makes Os an interesting candidate for doping in wires as long as its amount does not result in prohibitive increases in materials costs. In order to check if Os has any influence on flux pinning in MgB2, the normalised critical current density at 30 K and 10 K obtained from magnetisation loops (M(B)) is plotted in Fig. 5a and b. Clearly, at 30 K, Os-doping is detrimental since the slope of the normalised jc versus field increases in absolute value with increasing doping level. However, at 10 K, there is a slight, albeit systematic improvement above 0.5 T. Further work is in progress to understand the mechanism inducing the observed modifications of the flux pinning properties in Os-doped MgB2 as well as its origin, i.e. intrinsic doping effect or impurities acting as flux pinning centres. 4. Conclusions As evidenced from lattice parameter calculations based on XRD diffraction patterns, EDS analysis and magnetisation measurements, Os can be introduced in the MgB2 lattice up to a maximum solubility limit of about 1 at.% in samples with a Mg1 xOsxB2.04 nominal stoichiometry. For larger doping levels, unreacted Os and Mg-rich Mg–Os impurity phases are formed. The a-axis parameter contracts upon doping, while the superconducting transition temperature decreases at a rate of 2.1 K/at.% Os substitution, which is similar to that observed for Ir. Acknowledgements The authors acknowledge financial support from the Accelerated Metallurgy Project, which is co-funded by the European Commission in the 7th Framework Programme (contract NMP4-LA-2011263206), by the European Space Agency and by the individual partner organisations. References [1] J. Nagamatsu, T. Muranaka, Y. Zenitani, J. Akimitsu, Superconductivity at 39 K in magnesium diboride, Nature 410 (2001) 63–64. [2] Y. Hishinuma, A. Kikuchi, K. Matsuda, K. Nishimura, Y. Kubota, S. Hata, S. Yamada, T. Takeuchi, Microstructure and superconducting properties of Cu addition MgB2 multifilamentary wires using boron isotope powder as the boron source material, Phys. Proc. 36 (2012) 1486–1491. [3] M. Razeti, S. Angius, L. Bertora, D. Damiani, R. Marabotto, M. Modica, D. Nardelli, M. Perrella, M. Tassisto, Construction and operation of cryogen free MgB2 magnets for open MRI systems, IEEE Trans. Appl. Supercond. 18 (2008) 882–886. [4] S. Hahn, J. Bascunan, H. Lee, E.S. Bobrov, W. Kim, M.C. Ahn, Y. Iwasa, Operation and performance analyses of 350 and 700 MHz low-/high-temperature superconductor nuclear magnetic resonance magnets: a march toward operating frequencies above 1 GHz, J. Appl. Phys. 105 (2009) 024501. [5] M. Runde, A. Stenvall, N. Magnusson, G. Grasso, R. Mikkonen, MgB2 coils for a DC superconducting induction heater, J. Phys: Conf. Ser. 97 (2008) 012159. [6] M. Putti, G. Grasso, MgB2, a two-gap superconductor for practical applications, MRS Bull. 36 (2011) 608–613. [7] R. Battiston, W.J. Burger, V. Calvelli, V.I. Datskov, S. Farinon, R. Musenich, Superconducting magnets for astroparticle shielding in interplanetary manned missions, IEEE Trans. Appl. Supercond. 23 (2013) 4101604. [8] T. Takenobu, T. Ito, D.H. Chi, K. Prassides, Y. Iwasa, Intralayer carbon substitution in the MgB2 superconductor, Phys. Rev. B 64 (2001) 134513– 134516. [9] A. Bharathi, S. Jemima Balaselvi, S. Kalavathi, G.L.N. Reddy, V. Sankara Sastry, Y. Hariharan, T.S. Radhakrishnan, Carbon solubility and superconductivity in MgB2, Physica C 370 (2002) 211–218.

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