Sains Malaysiana 37(3)(2008): 223–225
Enhanced Critical Current Density in MgB
2with Dy
2O
3Particle Additions
(Peningkatan Ketumpatan Arus Genting dalam MgB2 dengan Penambahan Zarah Dy2O3) SOO KIEN CHEN, KEAN PAH LIM,
MOHD FAISAL MOHD ARIS & ABDUL HALIM SHAARI
ABSTRACT
This paper reports on the effect of oxide particle Dy2O3 additions on the superconducting properties of MgB2. The polycrystalline samples were prepared by well mixing magnesium and boron elemental powders, followed by heat treatment for in situ reaction in inert gas environment. All the samples showed MgB2 as the main phase with MgO and DyB4 as impurities. Magnetization measurements showed that the superconducting transition temperature, Tc remained largely unchanged (37.5 - 38K) even for Dy2O3 additions up to 15.0 wt.%. However, the transition curve was broadened in samples with larger amount of additions. The best sample with only 0.5 wt.% Dy2O3 additions had a Jc of around a factor of 4 higher compared to the pure sample at 6K(1T). TEM imaging showed the distribution of nano precipitates of DyB4 and MgO within the grain which may improve the pinning leading to enhancement in critical current density, Jc. Keywords: MgB2; critical current density; nano precipitates
ABSTRAK
Kertas ini melaporkan kesan penambahan serbuk oksida Dy2O3, terhadap ciri-ciri kesuperkonduksian MgB2. Sampel polikristal telah disediakan dengan mencampur serbuk magnesium and boron diikuti dengan rawatan pemanasan bagi menghasilkan tindak balas in situ dalam persekitaran gas lengai. Semua sampel menunjukkan MgB2 sebagai fasa dominan dengan kehadiran MgO dan DyB4 sebagai fasa-fasa ketaktulenan. Pengukuran kemagnetan menunjukkan suhu genting, Tc tidak mengalami perubahan besar (37.5 - 38K) walaupun dengan penambahan Dy2O3 sehingga 15.0 wt.%. Walau bagaimanapun, lengkung peralihan suhu genting bertambah luas untuk sampel dengan penambahan serbuk oksida yang tinggi. Sampel yang terbaik dengan hanya penambahan 0.5 wt.% Dy2O3 mempunyai Jc pada 6K(1T) sebanyak empat kali ganda lebih tinggi berbanding dengan sampel tulen. Imej TEM menunjukkan taburan mendakan DyB4 dan MgO yang bersaiz nano dalam butiran yang memperbaiki kesan pengepinan dan seterusnya meningkatkan ketumpatan arus genting, Jc.
Kata kunci: MgB2; ketumpatan arus genting; mendakan nano
INTRODUCTION
MgB2 has a relatively high superconducting transition temperature, Tc (~ 39K) compared to the low temperature superconductors (Nagamatsu et al. 2001). The weak link free nature (Eom et al. 2001) of this intermetallic compound has prompted enormous efforts on improving its properties for various electrical and electronic applications. Some early reports have shown that the rapid drop of critical current density at applied field is mainly originated from the weak pinning (Buzea & Yamashita 2001). On the other hand, simple chemical doping has greatly improved the in field Jc. Both oxides and non-oxide particles can be act as effective pinning centers (Chen et al. 2006; Dou et al. 2002;
Wang et al. 2002; Wang et al. 2004). In addition, nano particles doping has yielded much enhanced Jc (Dou et al.
2002; Wang et al. 2004). The oxide particle doping is of particular interesting (Chen et al. 2006; Wang et al. 2002).
MgO which usually present in MgB2 compound has been shown to act as effective pinning centres (Serquis et al.
2002). However, excess oxide phase will reduce the current carrying path especially if they reside at the grain boundaries (Kovác et al. 2004). Hence, it is desirable to have dopants inside the grains.
Oxide particle of Dy2O3 was added into Mg + 2B during in situ reaction to form a pinned material (Chen et al. 2006).
METHODOLOGY
The precursor powders for sample preparation were crystalline magnesium (99.8 %, 325 mesh) from Alfa Aesar, amorphous boron (95 - 97 %) from Fluka, and Dy2O3 (1 - 3 μm, 99.99 %) from Reacton. Stoichiometric proportion of Mg and B in the ratio of 1:2 (MgB2) was weighted and well mixed with 0.5 - 15.0 wt.% of Dy2O3 by hand grinding. The mixture was later pressed into pellets. The pellets, together with some excess Mg shavings were then wrapped with Ta foil and sintered at 900ºC for
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15 min. using heating and cooling rates of 15 ºC/min. Phase formation was checked by x-ray powder diffraction (XRD). High resolution transmission electron microscopy (TEM)
was undertaken on thinned samples using a JEOL 4000EX MK II microscope operating at 400 kV. Tc and Jc were determined by using a commercial Quantum Design DC
Magnetic Properties Measurement System (MPMS-XL). Magnetic critical current density was estimated based on the critical state model (Chen & Goldfarb 1989).
RESULTS AND DISCUSSION
The XRD pattern shows that MgB2 is the main phase with MgO as secondary phase in all the samples (Figure 1).
MgO is inevitably present due to MgO and boron oxide in the starting precursor of Mg and B powders. Except the pure sample, some of the peaks can be indexed to DyB4 and Dy2O3. By comparing the relative intensity, the quantity of DyB4 and Dy2O3 were increased with the amount of Dy2O3 additions indicating that not all the Dy2O3 reacted completely with boron powders. The amount of MgO is also expected to increase as a result of Dy2O3 additions.
The a- and c-axis remain almost constant for additions level up to 2 wt.% with slight expansion in c for 5 wt.% of Dy2O3 additions (Chen et al. 2006). However, the lattice parameter calculations yielded larger error for additions level of 5 wt.% - 15 wt.% due to the complication of impurity phases, i.e. a = 3.0836±0.0008Å, c = 3.5244±0.0008Å for 10 w.%
and a = 3.083±0.002Å, c = 3.524±0.001Å for 15 wt.%, respectively.
The bright-field TEM imaging (Figure 2a) of a 0.5 wt.%
Dy2O3 added sample shows the distribution of nano-sized (~ 10 nm) precipitates of DyB4 and MgO within the grain.
Some larger precipitates of the same kind with the size of
~ 70 nm were also observed in other regions of the sample (Figure 2b). The nano sized precipitates can act as effective pinning centres. Figure 2c shows the (210) lattice fringe of a nanosized DyB4 precipitate.
Figure 3 shows the temperature dependence of normalized magnetic moment of the pure and Dy2O3 added samples. Tc remains largely unchanged at 38K. However,
samples with 5 wt.% - 15 wt.% Dy2O3 additions showed a slight reduction in Tc (37.5K). As mentioned above, the c- axis for this sample was also marginally higher suggesting there is some perturbation of lattice structure occurred at higher additions level.
Figure 4 shows the magnetic critical current density versus field (1 - 5T) measured at 6K. Jc is increased with Dy2O3 additions especially in the low field region, and the best result is achieved for the 0.5 wt.% Dy2O3 added sample.
Jc was degraded by further additions. For 5 wt.% of Dy2O3 additions, Jc is still higher than the pure sample.
However, Jc of the 15 wt.% added sample is largely decreased and it is comparable to the pure sample. Jc(1T) at 6K of the 0.5 wt.% sample was increased by a factor of more than 4 compared to the pure sample, i.e Jc(6K, 1T) = 6.5 × 105 Acm-2 and 1.5 × 105 Acm-2 for the 0.5 wt.% Dy2O3 added and pure samples, respectively.
CONCLUSION
Small amount of oxide phase additions into MgB2 during the in situ reaction by using Dy2O3 particles has been shown to led to the increase of Jc significantly, particularly at low
FIGURE 2. (a) Bright-field TEM image of MgBw matrix with strain field effect (b) larger precipitate of
~ 70 nm was also observed in other region of the sample and (c) HRTEM image of the (210) lattice fringe of a DyB4 precipitate
(a) (b) (c)
FIGURE 1. XRD pattern with 2θ of 20 - 100º 2θ (degree)
Intensity (a.u.)
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field (<4T) while maintaining Tc ~ 38K. The enhanced pinning is attributed to the distribution of nano-scale precipitates of DyB4 and MgO within the grains as shown by transmission electron microscopy.
ACKNOWLEDGEMENT
Funding from Universiti Putra Malaysia is gratefully acknowledged. S.K.C would like to thank J. L. MacManus- Driscoll for the research facilities and Ming Wei for his help with TEM imaging.
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Physics Department Faculty of Science Universiti Putra Malaysia 43400 Serdang, Selangor D.E.
Malaysia
Received : 12 June 2007 Accepted : 19 November 2007
FIGURE 4. Magnetic Jc versus field measured at 6K Applied field (T)
Jc(Acm-2)
FIGURE 3. Temperature dependence of normalised magnetic moment (emu) measured for the pure
and Dy2O3 added samples Temperature (K)
Normalised emu
