J Supercond Nov Magn (2012) 25:1683–1688DOI 10.1007/s10948-012-1501-9

O R I G I NA L PA P E R

The Effect of Cu Addition on the Phase Formation and CriticalCurrent Density in the Sugar Doped MgB2 Superconductor

Zongqing Ma · Yongchang Liu · Qi Cai

Published online: 2 March 2012© Springer Science+Business Media, LLC 2012

Abstract With the aim of improving the critical currentdensity (Jc) in the MgB2 superconductor, minor Cu (3 at%)was doped to the MgB2 samples in-situ sintered with Mgpowder and sugar-coated amorphous B powder. Combinedwith thermal analysis, phase identification, microstructureobservation and Jc measurement, the effect of minor Cu ad-dition on the sintering mechanism, microstructure and criti-cal current density of sugar-doped MgB2 superconductorswere investigated. It is found that the minor Cu additioncould obviously accelerated the MgB2 phase formation andimprove the growth of MgB2 grains during the sintering pro-cess of sugar-doped MgB2 due to the appearance of Mg–Culiquid at low sintering temperature. On the other hand, theMg–Cu liquid hindered the reactive C released from sugarentering in the MgB2 crystal lattice. Hence, the connectiv-ity between MgB2 grains was improved accompanying withthe C substitution for B is decreased. At 20 K, the Jc of co-doped samples at low fields was further increased whereasit is decreased at high fields, compared with the only sugar-doped samples.

Keywords Sintering · Critical current density ·Superconductors

1 Introduction

The discovery of superconductivity in magnesium diboride(MgB2) at around 39 K has attracted much attention of

Z. Ma · Y. Liu (�) · Q. CaiTianjin Key Lab of Composite and Functional Materials,School of Materials Science & Engineering, Tianjin University,Tianjin 300072, P.R. Chinae-mail: licmtju@163.com

researchers around the world due to its wide advantagescompared to high T c cuprate superconductors [1]. How-ever, the critical currents in MgB2 are still smaller com-pared to expectations for an optimized material. Doping ofvarious elements and compounds has been carried out forthe fabrication of MgB2 bulks and wires in the past yearsin order to obtain higher critical current density [2–10].Among these dopants, the carbon-based chemical dopinghas achieved a record high in-field Jc–B , Hc2 and Hirr inMgB2 [3–7]. Enhanced Jc was obtained in the C-dopedMgB2 synthesized using carbohydrates or coated B powder[5, 11], which opened up a new direction for manufacturingC-doped MgB2 with excellent Jc in a solution route with-out using expensive nanometer carbon-based chemical addi-tions. Furthermore, this method can guarantee that the dop-ing level of carbon-based chemical is more homogeneous inthe MgB2.

With the aim of further improving the performance of Jc,some metallic elements were also tried to add into the C-doped MgB2. These results indicated that higher Jc couldbe obtained in the MgB2 samples with metals and C-basedchemical co-doping [12–14]. However, there is still no studyon the metal and sugar co-doped MgB2 superconductors tillnow.

Based on these backgrounds, whether metal and sugarmulti-doping could further improve the superconductiveproperties of MgB2 compared to the only sugar doping orthe only metal doping? In order to clarify this question, westudy the effect of Cu addition on the sintering process andsuperconductive properties of sugar-coated MgB2 samples.Hereby, Cu was chosen as the proper metallic dopant forthe reason that the minor Cu doping was recently found toenhance Jc of MgB2 superconductors at low fields togetherwith tiny decreasing Tc [15–20]. Since the MgB2 samplewith 3 at% Cu addition was reported to obtain the optimum

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result in enhancing the Jc at low fields [16, 20], in presentstudy, the 3 at% Cu was selected as the appropriate additionto the sugar-doped MgB2 sample and how the effect of Cuaddition on the sintering process and superconductivity ofsugar doped MgB2 samples was investigated based on thephase identification and microstructural observation.

2 Experiment Details

Undoped, sugar-doped and Cu and sugar co-doped MgB2

samples were prepared by a solid-state sintering methodusing amorphous boron powder (99% purity), magnesiumpowder (99.5% purity), copper powder (99.7% purity) andcommercial white sugar powder. Firstly, 5 wt% sugar wasfirstly dissolved in small quantity of water and then mixedtogether with B powders thoroughly forming the slurry inan agate mortar. All the slurry was dried in a vacuum cham-ber at about 80 °C for 4 h to remove the water. Then thesugar coated B powders was mixed with Mg powder and Cupowder in the molar ratio of MgB2 and (MgB2)0.97Cu0.03,respectively, and then pressed into pellets. All the pressedsamples were sintered at 850 °C for 0.5 h under flowinghigh-purity Ar gas and then cooled down to room tem-perature within the furnace. As a reference, the standardundoped MgB2 sample was also prepared by sintering at850 °C for 0.5 h.

In order to explore the effect of Cu addition on the sin-tering process of MgB2 and sugar-doped MgB2, the thermalanalysis of the pressed samples through all the sintering pro-cess was registered with a high-resolution differential ther-mal analysis apparatus.

The phase composition and the microstructure of the sin-tered samples are identified by the X-ray diffraction and

scanning electron microscopy, respectively. The magnetiza-tion of the bulk samples was measured by a MPMS-5 su-perconductivity quantum interference device (SQUID) mag-netometer. Then critical current density, Jc, was calculatedfrom the width of magnetization hysteresis loops (�M)based on the extended Bean model.

3 Results and Discussions

The measured differential thermal analysis (DTA) curves ofundoped and doped MgB2 samples are illustrated in Fig. 1under heating rate of 20 °C/min. It can be seen that threeobvious thermal peaks appear in the sintering process ofpure MgB2, which is consistent with the previous studies[21, 22]. It was reported that the three thermal peaks or-derly corresponded to the solid-solid reaction between Mgand B, Mg melting and solid-liquid reaction between Mgand B during the sintering process [23]. The DSC curves ofsugar-doped samples in present work perform similar withthat of the pure MgB2 sample except that the peak tempera-ture of first peak is decreased obviously. It is explained thatthe carbonization of sugar occurs firstly producing reactiveC during the sintering process, and these reactive C gener-ally exhibit more excellent thermal conductivity than amor-phous B and thus can lower the peak temperature of the firstthermal peak. For the Cu and sugar co-doped MgB2 sample,the peaking temperature of first peak further decreases com-pared to undoped and sugar doped MgB2 samples. It wasproved that Cu addition could form local Mg–Cu liquid, in-crease diffusion rate of Mg atoms into B and then acceleratethe reaction between Mg and B during the sintering of un-doped MgB2 in previous studies [9, 20]. The present resultindicates that Cu addition could also improve the formation

Fig. 1 The measured DTAcurves of undoped and dopedMgB2 samples under heatingrate of 20 °C/min

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Fig. 2 The XRD patterns of theMgB2, sugar-doped MgB2, Cuand sugar co-doped MgB2samples sintered at 850 °C for0.5 h

Fig. 3 The enlarged X-raydiffraction patterns around thepeak of (100) crystal plane ofMgB2 phase of the sinteredsamples

of MgB2 phase in the sugar doped sample following thesame mechanism. As a result, most of Mg was run out inthe solid reaction with B before it was melt. Hence, no obvi-ous thermal peak corresponding to Mg melting is observedin the DSC curve of Cu and sugar co-doped MgB2 sample.

Figure 2 illustrates the X-ray diffraction patterns of theMgB2, sugar-doped MgB2, Cu and sugar co-doped MgB2

samples sintered at 850 °C for 0.5 h. It is found that theMgB2 is the main phase in the all sintered samples. In bothundoped and sugar-doped MgB2 sample, there is only someMgO as the main impurity. On the other hand, the MgCu2

phase is easily recognized in the diffracted patterns of theCu and sugar co-doped sample, which suggests that the Cuaddition reacted with Mg during the sintering process.

Figure 3 illustrates the enlarged X-ray diffraction patternaround the peak of (100) crystal plane of MgB2 phase in thesintered samples. With the addition of sugar into the sam-ples, the (100) diffraction peak of MgB2 phase shifts to thedirection of high angles, which indicates that there was someC substitution for B occurring in the MgB2 crystal structure.Besides, the full width at half maximum (FWHM) of themain MgB2 peak becomes broadened as the sugar additionincreasing, which might be attributed to the smaller MgB2

grains in the sugar-doped samples. On the other hand, in theCu and sugar co-doped MgB2 sample, the main diffractionpeak of MgB2 phase shifts to the direction of low anglesagain while the FWHM of the main MgB2 peak becomessharper again compared to that of sugar-doped MgB2 sam-

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Fig. 4 Secondary electronimages of the microstructures ofthe pure and doped MgB2samples sintered at 850 °C for30 min with (a) MgB2 sample,(b) sugar-doped sample and(c) Cu and sugar co-dopedsample

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Fig. 5 Dependences of Jc onmagnetic field at 20 K ofundoped and doped samplessintered at 850 °C for 30 min

ple. This result shows that the amount of C substitution of Bis depressed and the size of MgB2 grains is increased withthe addition of Cu in the co-doped sample. Moreover, weestimate the amount of C substitution of B according to thefollowing formula [24].

X = 7.5 × �

(c

a

)(1)

Among them, the change of crystal lattice parameter wascalculated from XRD data. The calculated amount of C sub-stitution of B is about 0.032 and 0.015 for 5% sugar dopedsample and Cu and sugar co-doped sample, respectively.This result confirms again that C substitution of B is de-pressed in co-doped sample compared to sugar doped sam-ple.

We have indicated that the Cu addition first reacted withMg and formed the local Mg–Cu liquid at about 485 °C dur-ing the sintering process of MgB2 sample [20]. The presenceof Mg–Cu liquid can improve the formation of MgB2 phaseby accelerating the diffusion rate of Mg atoms at relativelylow temperature during heating process from room tempera-ture to 850 °C. In fact, according to DSC result, one can findthat most of MgB2 phase was formed at temperature belowMg melting point in co-doped sample. At such low tempera-ture, the C released from sugar is not active enough to enterinto MgB2 grains formed at this stage. In previous study, itis reported that the C substitution of B can only occur at thesame time with formation of MgB2 phase at high sinteringtemperature in most of carbon-based chemical doped sam-ple [6, 11]. As a result, the C substitution of B is depressedin co-doped sample compared to sugar doped sample.

Secondary electron images of the microstructures of thepure and doped MgB2 samples sintered at 850 °C for 30 minare shown in Fig. 4. In the undoped sample, the size of MgB2

grains is about 200 nm and the individual grains are easy

to distinguish. On the other hand, the sugar-doped sampleis consisted of very refined MgB2 grains and it is hard todistinguish the individual grains (see Fig. 4b). The result isconsistent with the XRD pattern (see Fig. 3) and they bothconfirm that the sugar addition decrease the crystallinity ofMgB2 phase and refine the MgB2 grains. The reason whythe MgB2 grains are refined in the sugar-doped sample hasbeen discussed previously in Refs. [5, 23]. In Cu and sugarco-doped sample, the size of MgB2 grains is even larger thanthat in the undoped MgB2 sample. Moreover, these MgB2

grains contain fewer voids and are in better connection witheach other compared to undoped MgB2 sample (see Fig. 4c).The main reason for this result is that the presence of Mg-Culiquid could accelerate the growth of MgB2 grains.

Dependences of Jc on magnetic field at 20 K of all sam-ples sintered at 850 °C for 30 min are shown in Fig. 5. It isfound that the Jc of sugar-doped sample is higher than thatof undoped sample at high fields, which mainly results fromthe C substitution for B. Besides, the refined MgB2 grainsin the sugar-doped sample can provide more grain boundarypinning and are also contributed to the enhancement of Jc athigh fields. The level of enhancement in Jc of sugar-dopedsamples prepared with present method is similar to that ofthe sugar-doped samples [5, 23] and nano-SiC-doped sam-ples [3, 7, 8, 25]. In the Cu and sugar co-doped sample, TheJc at low fields was increased whereas it is decreased at highfields, compared with the only sugar-doped samples. On onehand, the Cu addition decreases the amount of C substitu-tion for B and thus depresses the Jc of sugar-doped samplesat high fields. On the other hand, the Cu addition increasesthe crystallinity of MgB2 grains, improves the grain connec-tivity and thus enhances Jc of sugar-doped samples at lowfields. Accordingly, the corresponding Tc of co-doped sam-ple is higher than sugar doped sample but a little lower thanpure MgB2 sample, as shown in Fig. 6.

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Fig. 6 Temperaturedependences of ZFCmagnetization for the sinteredsamples

4 Conclusions

In summary, the microstructure, phase composition and Jc

performance of the undoped, sugar-doped and Cu and sugarco-doped MgB2 samples were studied. It is found that theminor Cu addition could obviously accelerate the MgB2

phase formation and improve the growth of MgB2 grainsduring the sintering process of sugar-doped MgB2 due tothe appearance of Mg–Cu liquid. On the other hand, it couldalso reduce the relatively content of reactive C introducedinto newly formed MgB2. In the Cu and sugar co-dopedsample, The Jc at low fields was increased whereas it is de-creased at high fields, compared with the only sugar-dopedsamples. On one hand, the Cu addition decreases the amountof C substitution for B and thus depresses the Jc of thesugar-doped samples at high fields. On the other hand, theCu addition increases the crystallinity of MgB2 grains, im-proves the grain connectivity and thus enhances Jc of thesugar-doped samples at low fields.

Acknowledgements The authors are grateful to the National Natu-ral Science Foundation of China (Grant No. 51077099), Program forNew Century Excellent Talents in University and Seed Foundation ofTianjin University for grant and financial support.

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