Mechanical and superconducting properties of nanosize MgO added dip-coated Bi2Sr2CaCu2O8 superconducting tape

doi:10.4236/ns.2011.36067

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Vol.3, No.6, 484-487 (2011) Natural Science

http://dx.doi.org/10.4236/ns.2011.36067

Mechanical and superconducting properties of nanosize

MgO added dip-coated Bi2Sr2CaCu2O8 superconducting

tape

Nasri A. Hamid*, Mohd Yusri Abd Rahman, Noor Fairuz Shamsudin

Department of Engineering Sciences & Mathematics, College of Engineering, Universiti Tenaga Nasional, Kajang, Selangor, Malay-

sia; *corresponding author: [email protected]

Received 2 February 2011; revised 10 March 2011; accepted 20 March 2011.

ABSTRACT

In this work, 3% to 8% (in weight) of nanosize

MgO particles was added to Bi2Sr2CaCu2O8 (Bi-

2212) high-temperature superconductor to fab-

ricate Bi-2212 superconducting material with

superior mechanical properties. The mechanical

strength of the samples was studied by con-

ducting the compression test at room tempera-

ture, and the addition of 5% nanosize MgO par-

ticles produced the highest strength when com-

pared with other samples. The sample with 5%

MgO addition also exhibited superior super-

conducting properties. The Bi-2212 powder with

5% nanosize MgO addition was used to fabricate

Bi-2212 tapes through the dip-coating-then-

stacking method. The fully processed tapes

were investigated via dc electrical resistance

measurements, XRD patterns, SEM micrographs,

transport critical current density and tensile

tests. The tensile tests were conducted at room

and cryogenic (77 K) temperatures. Results of

tensile tests and Young’s modulus for the tapes

showed that the Bi-2212 tapes with nanosize

MgO addition recorded better mechanical prop-

erty compared to the non-added samples both at

room and cryogenic temperatures. The double-

core tape with 5% MgO addition recorded the

highest failure point at 160 MPa. In addition to

the strengthening effect that was observed in

the nanosize MgO added Bi-2212 superconduc-

tor tapes, the superconducting properties re-

mained intact in the tapes.

Keywords: Bi-2212 Superconductor; Electrical

Properties; Mechanical Properties; High Current

Density; Cryogenic Temperature

1. INTRODUCTION

For practical applications of high-temperature super-

conductor materials in power engineering, where ther-

mo-mechanical and electromagnetically induced stresses

are expected to occur, the high-temperature supercon-

ductor materials need to have excellent superconducting

properties and higher mechanical strength. Bismuth

based high-temperature superconductors such as

Bi2Sr2CaCu2O8 (Bi-2212) is one of the most promising

materials due to its high critical current density, rapid

phase formation and phase stability. Nevertheless the

Bi-2212 superconductor, like other Bi-based high-tem-

perature superconductors is generally has poor me-

chanical properties such as low stiffness, strength and

toughness despite their excellent superconducting prop-

erties [1-4]. Low strength and low irreversible strain are

among the factors that hindered the application of

Bi-2212 superconductor in power industry. MgO fibers

have been used to reinforce Bi-2212 superconductor and

are able to improve the mechanical properties of the

compounds [4,5]. Furthermore, the addition of nanosize

MgO particles has shown to be an effective pinning cen-

ter for Bi-2212/MgO compounds and samples with na-

nocrystalline microstructures are found to exhibit better

plasticity under stress compared with samples of larger

grains [6-10].

In the literature, there is no specific report that focuses

on the mechanical strength of single-core and double-

core dip-coated Bi-2212 superconducting tapes with

nanosize MgO addition both at room and cryogenic

temperatures. In the present work, we report our study

on the mechanical and superconducting properties of

such tapes at room and cryogenic temperatures and the

data from this study is important in assessing the per-

formance of the tapes especially at cryogenic tempera-

ture. Throughout this study, the tapes were fabricated

using the dip-coating-then-stacking (DIS) method [11].

N. A. Hamid et al. / Natural Science 3 (2011) 484-487

485

2. EXPERIMENTAL PROCEDURES

The Bi2Sr2CaCu2O8 (Bi-2212) with nanosize MgO

addition tapes were prepared through the DIS method.

The silver foil that was used in the fabrication of the

tapes was product of Alfa Aesar with 0.1 mm in thick-

ness and 99.998% purity. In the process, slurry with an

optimum composition was prepared before employing

the dip-coated process. In preparation of the slurry, or-

ganic solution comprises of Span 85 or sorbitane tri-

oleate (C60H108O8), 1,3-propandiol (C3H8O2), polyvinyl

butyral and trichloroethylen (C2HCl3) were used. The

Bi-2212 powder (with and without nanosize MgO addi-

tion) was then added with the solution in the ratio as

shown in Table 1.

The slurry was stirred for 24 hours to obtain a homo-

geneous solution before coating. The coated tape was

then heated at temperature of 80˚C for 48 hours to re-

move any oxide layers before further heating at 500˚C in

flowing oxygen for one hour to remove the organic ma-

terials. After heating, the tape was stacked and conse-

quently rolled to increase the packing density of the ox-

ide core before it is subjected to partial melt process.

During the heat treatment process, the samples were

heated up from room temperature for 2.88 hours until it

reached the partial melt temperature at 865˚C. The tem-

perature was hold for 6 minutes and then cooled down

from 865˚C to 830˚C in 3.5 hours. They were annealed

at 830˚C for 48 hours and then cooled down to room

temperature in 2.77 hours. Partial melt process was em-

ployed to overcome the problem of weak-link within the

Bi-2212 superconducting grains.

The fully processed tapes were characterized through

dc electrical resistance measurements, XRD patterns,

SEM micrographs, and transport critical current density,

Jc. In the dc electrical resistance measurements, Tc, onset

corresponds to the transition onset temperature which is

the point on the R(T) curve where the resistance starts

decreasing dramatically, and Tc, zero corresponds to tran-

sition zero temperature where the resistance is zero. The

XRD patterns were recorded on a Siemen D5000 dif-

fractometer using Cu K radiation. The SEM analysis of

the longitudinal and broad-face cross sectional of the

tapes were conducted using the Philips XL30 environ-

mental scanning electron microscope with EDX analysis.

The transport critical current (Ic) of short samples was

Table 1. Formulation of slurry used to fabricate dip-coated

tapes.

Component Material Fractions (Weight %)

Oxide powder Bi-2212/ nanosize MgO 22

Solvent Trichloroethylene 68

Binder Polyvinyl butyral 6

spersant Sorbitant triolate 4

measured in a cryostat at 77 K and zero electric field

using the four-probe method with a 1 V/cm criterion.

The transport critical current density (Jc) was calculated

by dividing Ic with the cross-sectional area of the super-

conducting core. The mechanical properties of the tapes

were studied using tensile tests that were conducted at

room and cryogenic (77 K) temperatures. The tests were

conducted using the Instron Material Testing System

model 5567. The slope of the stress-strain curve in the

elastic region represents the stiffness of the material. In a

tensile test, a sample is extended at a constant rate and

the maximum load is measured. The Young’s modulus

was estimated from the linear part of the stress-strain

curve.

3. RESULTS AND DISCUSSIONS

Figure 1 shows a typical resistivity curve of the tape

samples. The curve shows a metallic behavior due to the

silver tape until the onset temperature where the resis-

tance propitiously dropped to zero. The sharp drop of the

curve indicates high purity of Bi-2212 phase in the su-

perconducting core of the tape.

From our observation, the XRD patterns for both the

non-added and nanosize MgO added Bi-2212 tapes show

a well defined peaks all of which could be indexed on

the basis of a Bi-2212 structure. The (115) peak, which

is the characteristic of the Bi-2212 phase, can be ob-

served clearly in all the samples. Patterns of the nanosize

MgO added tape samples show a more distinct and sharp

Bi-2212 phase peaks, indicating a highly crystalline and

well textured microstructure. A few peaks which corre-

spond to secondary phases such as Bi-2201 phase and

other impurities could also be observed. Those unidenti-

fied peaks are probably due to inappropriate heat treat-

ment during the sintering process which resulted in uni-

Temperature (K)

50100 150 200 250 300

R(T)/R(293 K)

0.0

0.2

0.4

0.6

0.8

1.0

0% MgO

5% MgO

Figure 1. Temperature dependence of resistance of dip-coated

Bi-2212 superconducting tapes (with and without nanosize

MgO addition).

N. A. Hamid et al. / Natural Science 3 (2011) 484-487

486

dentified secondary phases.

Figure 2 shows a typical SEM micrograph of the

longitudinal cross-sectional of the dip-coated Bi-2212

superconducting tape with nanosize MgO addition. The

micrograph exhibits a well aligned microstructure of the

Bi-2212 superconducting core with closely connected

grain boundaries.

Table 2 shows the critical current density, Jc, zero

transition temperature, Tc zero and onset transition tem-

perature, Tc onset of the tapes. For both the single-core and

double-core tapes, there is enhancement in the Jc after

addition of nanosize MgO. From the observation of

XRD patterns and SEM micrograph, the enhancement is

most likely due to better texturing and well connected

grain boundaries of the superconducting core. The Jc of

the tapes is very much dependent on microstructural

features such as phase purity, grain alignment and grain

boundaries.

Mechanical properties such as stiffness, strength and

toughness are important parameters to indicate the ver-

satility of superconducting tapes [12,13]. Those proper-

ties at room temperature are able to predict such behav-

iors at cryogenic temperature. Figure 3 shows a typical

stress-strain curve for a double-core dip-coated Bi-2212

superconducting tape with nanosize MgO added both at

room and cryogenic temperatures. The non-linear curve

at the initial load is due to low rigidity in which the tape

Figure 2. Typical SEM micrograph of the longitudinal cross-

sectional of the dip-coated Bi-2212 superconducting tape.

Table 2. The critical current density, Jc and transition tempera-

ture, Tc of the dip-coated Bi-2212 superconducting tape.

Nanosize MgO Addition Jc (A/cm2) ± 10 Tc zero (K) Tc onset (K)

0% (single-core) 1800 94 101

5% (single-core) 2400 82 95

0% (double-core) 1900 82 88

5% (double-core) 2300 89 98

100

120

140

160

0 0.250.50.75 1 1.251.51.75 2

Strain

Stress (MPa)

273 K77 K

Figure 3. The stress-strain curve for a double-core dip-coated

Bi-2212 superconducting tape with nanosize MgO addition at

room temperature () and 77 K (■).

sample was not completely attached to the tester. As the

load increases, a more linear relationship is observed

until deformation and eventually to the failure point.

From our results, a more distinct linear curve was ob-

served in the MgO added samples before deformation

both at room and cryogenic temperatures. The presence

of MgO particles resulted in different mechanical be-

havior between the MgO added and non-added tapes.

Table 3 shows the tensile test measurements for the

single-core and double-core tapes both at cryogenic and

room temperatures. At cryogenic temperature, all the

tapes exhibit higher strength when compared to room

temperature. The double-core tape added with nanosize

MgO recorded the highest strength at about 160 MPa. In

addition, the Young’s modulus for each tape can be es-

timated based on the linear part of each curve. In con-

trast to the tapes’ strength, the Young’s modulus of the

MgO added tape slightly decreased at cryogenic tem-

perature. This is mostly due to lower stiffness of the tape

initiated by microcracks that developed between the

MgO particles and the Bi-2212 superconductor matrix

during cooling to cryogenic temperature [14]. Neverthe-

less, the Young’s modulus is still higher compared to the

non-added tape sample. As such, the Bi-2212 supercon-

ducting tapes with nanosize MgO addition show better

resilience at cryogenic temperature.

Table 3. The tensile test results for double-core and single-core

tapes both at room and cryogenic temperatures.

Nanosize MgO

Addition

Max. Strength

at 77 K

(MPa) ± 0.01

Max. Strength at

Room Tempera-

ture (MPa) ± 0.01

0% (single-core) 144.91 117.59

5% (single-core) 146.22 144.83

0% (double-core) 151.01 116.65

5% (double-core) 159.81 146.68

N. A. Hamid et al. / Natural Science 3 (2011) 484-487

487

4. CONCLUSIONS

We have successfully fabricated nanosize MgO added

Ag-sheathed Bi-2212 superconducting tapes using the

dip-coating-then-stacking (DIS) method. The samples

were subjected to a combination of partial melt and slow

cooling during heat treatment and annealing processes.

The addition of nanosize MgO in both single-core and

double-core tape samples enhanced the Jc and this was

attributed to the presence of MgO that acted as pinning

centers. The double-core tape with 5% nanosize MgO

addition recorded the highest strength. As such, the ad-

dition of nanosize MgO contributed to the improvement

of the mechanical properties of the tapes without de-

grading their superconducting properties.

5. ACKNOWLEDGEMENTS

This project is supported by the Ministry of Science, Technology

and Innovation, Malaysia through eScience fund project no.

03-02-03-SF0034.

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