Electron. Mater. Lett., Vol. 10, No. 3 (2014), pp. 597-600

The Influence of Heat Treatment on the Formation and Transport Current Density of DyBa2Cu3O7-δ Ceramics Superconductor Synthesized from

Nano-Coprecipitated Powders

Imad Hamadneh1,2,*

1Department of Chemistry, Faculty of Science, The University of Jordan, 11942 Amman, Jordan2Material Science and Nano Technology Lab, Hamdi Mango Center for Scientific Research,

The University of Jordan, 11942 Amman, Jordan

(received date: 11 August 2013 / accepted date: 6 September 2013 / published date: 10 May 2014)

DyBa2Cu3O7-δ superconducting ceramic has been prepared from nano metal oxalate precursors with averagegrain size <30 nm using the coprecipitation method. Four sintering temperatures were applied for the same timeperiod. XRD showed a single phase of an orthorhombic structure for all samples with small amount of 211 phasewhich was detected for samples sintered above 930°C. The sintered samples also exhibited metallic behaviourwith TC(R=0) = 89 K - 90 K and the transport critical current density (JC) values enhanced from 4.35 A/cm2

to12.9 A/cm2 as the sintering temperature increased. The SEM micrographs showed grains of large size that arerandomly distributed and have irregular structure. The compaction between the grains was improved as sinteringtemperature increased and thus resulted in reducing the weak links effect. In conclusion, Reducing the gapsbetween the grain and the formation of 211 phase as pinning centers might be explain the enhancement in thetransport current properties of Dy123 in a way to be used in the HTSC device applications.

Keywords: x-ray powder diffraction, coprecipitation method, sintering temperature, transport properties

1. INTRODUCTION

Polycrystalline DyBa2Cu3O7-δ (Dy123) superconductor isa bulk material with low porosity and it is widely investigateddue to its excellent superconducting properties and lowthermal conductivity. This makes it essential for practical usefor current leads, current fault limiter and also for quasi-permanent magnets.[1,2] Different synthesis techniques suchas conventional solid state route (SSR)[3,4] and coprecipitationmethod (COP)[5] were utilized to produce this material. SSRmethod normally requires high temperature treatment thatexceeds 1000°C, long calcination and sintering durations(>100 hours) with intermediate grinding to improve thereaction.[3]

COP method has the capability to obtain starting powderswith nano grain size, better purity and homogeneity than thepowders produced by the SSR method.[5-10] The presence ofthe initial mixture of starting cations in the atomic scale inthe solution enhanced the reaction during the heat treatmentand the resulting powders are more homogenous and requireshorter thermal and time processing.

In this paper, we report the effect of sintering temperaturevariation on the formation of Dy123 superconductor usingCOP technique. Systematic investigations on the supercon-

ducting properties were performed using x-ray diffraction(XRD), DC electrical resistance-temperature measurements,Field Emission Scanning Electron Microscopy (FESEM)and Scanning Electron Microscopy (SEM) are reported.

2. EXPERIMENTAL PROCEDURE

The nano-powder precursor with nominal composition ofDyBa2Cu3O7-δ was synthesized via oxalate co-precipitationmethod as described in Ref.[3] The blue precipitated slurrywas obtained and filtered after 5 minutes of reaction followedby the drying stage at 80°C over night. The blue precipitatedpowders which are slightly aggregated with particle size<30 nm (as shown in Fig. 6(a)) were heated up to 900°C inair for 12 hours to remove the remaining volatile materials.The calcined powders were reground in a marble mortar for10 minutes and pressed into pellets of ~12.5-mm diameterand 2 mm thickness under a load of 6 tons using a hydraulicpress model Carver. The pellets were sintered at differenttemperatures (920°C (A), 930°C (B), 940°C (C) and 950°C(D)) for 15 hours under oxygen environment (100 mL/min)and slowly cooled to room temperature at 1°C/minute.Thesamples were examined by x-ray powder diffractometerwith Cu Kα radiation (λ = 1.5418 Å) using PANalytical'sX'Pert PRO x-ray diffraction system at 40 kV and 30 mAwith a step of 0.02° over the range 4° - 60°. The resistivitymeasurements in the range 50 - 250 K were performed by

DOI: 10.1007/s13391-013-3250-8

*Corresponding author: i.hamadneh@ju.edu.jo©KIM and Springer

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Electron. Mater. Lett. Vol. 10, No. 3 (2014)

the four-probe technique using 30 mA (DC) in a closed cyclerefrigerator system model ARS-EA202A.Transport criticalcurrent density, JC, was performed in zero magnetic field andconstant temperature of 77 K. All samples were cut inrectangular shape with cross section areas 0.138 cm × 0.375 cm(A), 0.137 cm × 0.294 cm (B), 0.136 cm × 0.359 cm (C) and0.139 cm × 0.401 cm (D). Field emission microscope (FESEM)and Scanning electron microscope (SEM) micrographs of astarting powder and fractured surface of the sintered sampleswere recorded using FEI QUANTA 200 and Hitachi S3400,respectively.

3. RESULTS AND DISCUSSION

Figure 1 shows the XRD patterns for the calcined andsintered samples A, B, C and D. The orthorhombic structureof Dy123 phase (JCPDS, # 01-082-2293) was dominant forall samples. The space group, Pmmm, No.47, Z = 1, α = β =γ = 90° with lattice parameters, a = 3.824 ± 0.009, b = 3.892± 0.014, c = 11.664 ± 0.007 Å. The calcined sample alsoshowed a dominant 123 phase. It can be concluded thatusing nano oxalate powders enhanced the diffusion reactionand produced the orthorhombic structure of (Dy123) phasewith shorter time and temperature required. Sample Ashowed extra peaks due to the incomplete reaction. Almostno extra peaks belonging to the impurities were detected forsample B. The extra peaks were clearly observed forsamples C and D due to the formation of impurity phase and

was assigned to Dy2BaCuO5 (Dy211) phase (JCPDS, # 01-081-0799). The volume fraction of 211 phase can be estimatedfrom the intensities of 123 and 211 phase peaks Eq. (1):[11]

Dy211(%) = (1)

where I is the peak intensity of the observed phases. TheDy211 phase is found to be 4%, 0.5%, 12% and 15% forsamples A, B, C, and D, respectively (Fig. 2). The averagecrystallite size for the resulted materials was estimated usingSheerer equation Eq. (2).[12]

Dhkl = kλ/βcosθ. (2)

Where k is Shape factor (~ 0.9), λ-wavelength of the radia-tion Cu Kα radiation (λ = 1.5418 Å) and β is full width athalf maximum (FWHM) in radians (Fig. 3). The biggest

ΣI211

ΣI123

ΣI211

ΣIothers+ +-------------------------------------------------- 100%×

Fig. 1. X-ray diffraction patterns for samples Dy123calcined at900°C and sintered at temperatures (A) 920°C, (B) 930°C, (C) 940°Cand (D) 950°C. (hkl) : Dy-123, • : 211 phase, *: others.

Fig. 2. Dy123% as a function of sintering temperature.

Fig. 3. The average crystallite size as a function of sintering tempera-ture.

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crystalline size was obtained for sample sintered at 930°C.Above that temperature the crystalline size started todecrease gradually due to the decomposition process andformation of 211 phase.

Figure 4 shows the DC electrical resistance as a functionof temperature (R-T) for the sintered Dy-123 pallets. Allsamples showed a typical metallic behavior with a sharpdrop of the curve at the on-set resistance temperature (TC-onset)as an indication of good grain connectivity. However, thezero resistance temperature (TC-R=0) is 89.0 ± 0.1 K forsamples A and B, 90.0 ± 0.1 K for samples C and D. Thetransport critical current density, JC, was calculated usingEq. (3):[13]

(3)

where IC is the maximum current passing through the crosssection area of sample (A) before the sample loses its super-conducting behaviour at 77 K and zero magnetic field. Figure5 showed the JC values of 4.35 ± 0.11 A/cm2, 6.35 ± 0.43 A/cm2, 7.89 ± 0.71 A/cm2 and 12.9 ± 0.89 A/cm2 for samplesA, B, C and D, respectively. These results are in good agree-ment with the Ref. 14 & Ref. 15 where addition of smallamount of 211 phases (>10%) in R123 superconducting sys-tem enhanced the transport properties.

The agglomerated metal oxalate nanoparticles with averagegrain size bellow 30 nm are shown in Fig. 6(A). Large grains(20 - 50 µm), that are highly compacted and randomlydistributed were observed for all sintered samples (Fig. 6B-E). The size of gaps between the grains decreased as sinteringtime increased and thus improved the compaction betweenthe grains. The enhancement in the transport properties canbe explained by reducing the weak links between the grains

and the formation of pinning centers as confirmed by XRDanalysis for samples sintered above 930°C. It can be concludedthat the formation of nano-sized precursor powders by using

JC

IC

A----=

Fig. 4. Resistance as a function of temperature for samples sinteredat (A) 920°C (B) 930°C, (C) 940°C and (D) 950°C.

Fig. 5. Critical current density, Jc, as a function of sintering tempera-ture.

Fig. 6. SEM micrographs showed (A) an aggregated nano-metaloxalate precursor and (B, C, D, E) for samples sintered at 920°C,930°C, 940°C and 950°C, respectively.

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coprecipitaion method is a crucial technique to enhance thegrowth of the orthorhombic structure of 123 during thepreparation of Dy123 superconductor with just a singlesintering step.

4. CONCLUSIONS

DyBa2Cu3O7-δ superconducting ceramic has been preparedvia coprecipitation method extracted from the metal acetateprecursors. Four sintering temperatures were applied for thesame time period. The results showed an increase in thetransport critical current density as the sintering temperatureincreases. This is due to the good grain connectivity whichreduces the weak link effect. In addition, the formation ofDy211 phase in the samples sintered above 930°C mighthave contributed to the enhancement of transport currentproperties. The nano-sized precipitated powders simplifiedthe preparation method and enhanced the formation of Dy-123 superconducting phase with just a single step sintering.This enhanced the growth of the orthorhombic phase. Theseresults could contribute to the enhancement of the quality ofDy123 to be used in the HTSC device applications.

ACKNOWLEDGEMENTS

The author would like to thank the Deanship of Researchat the University of Jordan and Hamdi Mango Center forScientific Research (HMCSR) for their financial support.The author would also like to extend his appreciation toUniversity Putra Malaysia for the accessibility to equipments.

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