Doping effects of ZrO2 nanoparticles on the superconductingproperties of Bi-2212 tapes

Chengshan Li • Shengnan Zhang • Ling Gao •

Qingbin Hao • Lifeng Bai • Pingxiang Zhang

Received: 12 December 2014 / Accepted: 23 February 2015 / Published online: 28 February 2015

� Springer Science+Business Media New York 2015

Abstract Ag sheathed Bi-2212 single filament tapes with

different content of ZrO2 nanoparticles addition were fab-

ricated with powder-in-tube process. X-ray diffractions

were performed to examine the lattice structure and phase

composition changes of these tapes. With the increase of

ZrO2 content, the lattice parameter c decreased, suggesting

the change of chemical composition in Bi-2212 phase.

Detectable Zr-containing particles can be observed in the

tape with ZrO2 content of 3.0 wt% in both XRD patterns

and SEM images due to the saturation of Zr4? irons in Bi-

2212 matrix. The thermopower values at room temperature

were measured to estimate the carrier concentrations. At-

tributed to the doping effect of Zr4?, the carrier concen-

tration decreased due to the introduction of excess

electrons. Meanwhile, the resistivity measured at room

temperature increased and the carrier mobility decreased,

suggesting the enhanced carrier scattering mechanism. Due

to the change of superconducting properties with ZrO2

doping, the current capacity of Bi-2212 tapes decreased.

However, the (Ca, Sr)ZrO3 precipitations with proper size,

which can work as effective pinning centers enhanced the

flux pinning properties of Bi-2212 tapes. Thus the tape with

ZrO2 content of 3.0 wt% regained the high current capacity

under magnetic field and a higher Jc can be expected under

the magnetic field higher than 25 T.

1 Introduction

Although many high temperature superconductor (HTS)

systems have been discovered since the first report of

cuprates HTS at 1986, only YBCO [1] and Bi-based su-

perconductors [including Bi2Sr2Ca2Cu3O8 (Bi-2223) and

Bi2Sr2CaCu2O8 (Bi-2212)] [2–4] exhibit great potentials

for the practical applications. Bi-2212, as the only HTS

material so far, which can be made into round wires with

isotropic cross sections, is attracting more and more at-

tentions [5–7], because the round wires configuration can

greatly simplify the winding process of cables and mag-

nets. Meanwhile, the high upper critical field and weak

field dependence of current capacity can both ensure the

applications of Bi-2212 under high magnetic field. Nowa-

days, Bi-2212 insert coils are considered to be a necessary

part for the manufacturing of high field (*30 T) magnets

[8–11].

However, due to the intrinsically layered structure of Bi-

2212, the critical current densities Jc act quite differently

under the magnetic field with different directions. When

the magnetic field is parallel to the ab plane of Bi-2212, the

strong intrinsic Josephson pinning properties between su-

perconducting layers contribute to the high current capacity

under high magnetic field. On the other hand, when the

magnetic field is perpendicular to the ab plane, the weak

Abrikosov pinning leads to a rapid decrease of Jc in the low

magnetic field, which greatly limits the practical perfor-

mance of Bi-2212. Therefore it is quite necessary to en-

hance the flux pinning properties on the superconducting

layers of Bi-2212 especially under perpendicular field.

Doping is considered to be an effective approach for the

improvement of current capacity of HTSs. By introducing

BaZrO3 (or BaHfO3) nanoparticles into REBCO systems,

the formation of BaZrO3 nanorods or nanodots greatly

C. Li � S. Zhang (&) � Q. Hao � L. Bai � P. ZhangSuperconducting Materials Research Center, Northwest Institute

of Nonferrous Metal Research, Xi’an 710016, China

e-mail: snzhang@c-nin.com

L. Gao

College of Material Science and Engineering, Xi’an

Technological University, Xi’an 710048, China

123

J Mater Sci: Mater Electron (2015) 26:3583–3588

DOI 10.1007/s10854-015-2872-z

enhanced the flux pinning properties of REBCO thin films.

Therefore, the in field Jc, upper critical field, Hc2 as well as

the anisotropy properties were all obviously improved [12–

16]. Similar effects were expected in Bi-system materials

as well [17, 18]. Azhan et al. [19] reported that the in-

corporation of Zr ions into the Bi-2223 lattice, which could

only happen with the annealing temperature of higher than

860 �C, influenced the phase composition in the final bulks

and the critical temperature decreased with ZrO2 addition.

The doping effect of ZrO2 nanoparticles on the flux pinning

properties of Bi-2223 bulks were studied by Zouaoui et al.

[20]. With the appearances of nano-sized Zr-rich regions,

the point pinning mechanisms were enhanced, which lead

to the enhancement of Jc for [15 %. Many efforts were

also made to enhance the in field current capacities of Bi-

2212 materials with the introduction of ZrO2 related

phases. Recently, Elsabawy et al. [21] fabricated the nano-

Zr added (Bi, Pb)-2212 powders with freeze drying tech-

nique to study the effect of Zr doping on Bi-2212 phase. It

is reported that Zr4? ions have entered into the lattice of

Bi-2212, and with the introduction of Zr4?, the critical

temperature, Tc of Bi-2212 phase slightly decreased. As we

know that the doping effect may vary a lot between bulks

and Ag sheathed conductors, due to the different heat

treatment process. Therefore, it is quite necessary to study

the doping effect in the form of wires or tapes. Holesinger

et al. [22] reported the influence of different secondary

phase particles, including Al2O3, ZrO2, and SrCa2Zr2O7,

etc. on the Bi-2212 round wires fabricated at OST. By

measuring the critical current density with transport

method, it was noticed that the embedded ZrO2 and

SrCa2ZrO7 could both enhance the current capacity of

these Bi-2212 round wires. It seemed that ZrO2 is a

promising dopant for Bi-2212 systems. However, no de-

tailed study and analysis was performed in this study.

Therefore, the aim of our study is to perform a systematical

study on the influences of ZrO2 nanoparticles on the lattice

structures, electrical properties and superconducting prop-

erties of Ag sheathed Bi-2212 single filament tapes.

2 Experimental

ZrO2 nanopowders were synthesized by co-precipitation

process with the starting materials of ZrOCl2�8H2O. After

the filtration, the precipitates were sintered at 600 �C for

2 h. ZrO2 powders with the mixed tetragonal and

monoclinic phases have been obtained, and the average

particles size is about 20 nm as shown in Fig. 1.

Bi2.1Sr1.96CaCu2.0O8?d precursor powders were prepared

by a modified co-precipitation process [23] with the start-

ing materials of Bi2O3, SrCO3, CaCO3, and CuO

([99.9 %). After a series of calcination processes in air at

800 �C/12 h, 820 �C/20 h, and 850 �C/20 h with interme-

diate grinding, the precursor powders were mixed with

different content of ZrO2 powders with the weight ratio of

0, 0.5, 1.0, and 3.0 wt%. The mixtures were ball milled for

2 h to ensure the uniformity, and then densely packed into

Ag tubes, respectively. With the drawing and rolling pro-

cess step by step, single filament tapes with the thickness of

*300 lm, width of 4 mm were obtained. The partial

melting process was performed with the O2 flowing ve-

locity of 0.4 m3/h, and the maximum heat treatment tem-

perature, Tmax of 892 �C was adopted. The tapes were kept

at Tmax for 20 min, then cooled down with the cooling rate

of 5 �C/h to 840 �C with the dwell time of 20 h, then

cooled down to room temperature with furnace.

Polycrystalline X-ray diffraction (XRD) patterns were

taken on an X-ray diffractor (XRD, Bruker D8

ADVANCED) with Cu-Ka radiation (k = 0.154 nm). The

lattice parameters were obtained by fitting the XRD pat-

terns with Rietveld method using the software Fullprof�.

Transmission electron microscope (TEM, JEM-2100 FS)

was performed to examine the morphology of ZrO2 parti-

cles. The morphology of the cross section of tapes was

observed with field-emission scanning electron microscopy

(FESEM, JSM-6700F). The compositional analysis was

taken by Inca-X-Stream Energy dispersive X-ray spec-

troscopy (EDX). The thermopower values were measured

on ZEM-2 from room temperature (25 �C) to 110 �C and

each value at certain temperature were obtained by mea-

suring the data for four times and calculating the average to

confirm the accuracy of measurement. The critical current,

Ic, was measured at liquid helium temperature (4.2 K) on a

computer-aided apparatus using a DC four-probe method

with the criterion of 1 lV/cm under the magnetic field of

0–20 T in high magnetic field facility in LNCMI-Grenoble.

And the critical current density, Jc was calculated as

Fig. 1 Typical XRD pattern and TEM image of ZrO2 nanoparticles

3584 J Mater Sci: Mater Electron (2015) 26:3583–3588

123

Jc = Ic/A, where A is the cross section of superconducting

core.

3 Results and discussion

After the partial melting process, single filament tapes with

the preferred orientation of (00l) were obtained. As shown

in Fig. 2, strong (00l) diffraction peaks can be observed

representing the high c-axis texture degree of over 98 %.

With the increase of ZrO2 content, no detectable change of

FWHM as plotted in the inset of Fig. 2a is observed, which

suggests that the ZrO2 doping does not obviously influence

the crystallization process of Bi-2212 phase during the

partial melting process. On the other hand, in the tapes with

low ZrO2 content, no detectable Zr-containing phase can be

observed. While with the ZrO2 content increased to

3.0 wt%, diffraction peaks can be detected as marked with

arrows, which could be indexed into Zr-containing phases

like ZrO2 (JCPDS #49-1746) or (Ca, Sr)ZrO3 (JCPDS #23-

0561). Considering the low intensity of these peaks, it is

hard to distinguish the phases accurately with XRD pat-

terns only.

Meanwhile, with the introduction of ZrO2 additions, the

(00l) diffraction peaks of the obtained Bi-2212 phase shift

towards higher degrees, which suggests the decrease of

lattice parameter c, as shown in Fig. 2b. There are two

factors which contribute to the change of lattice pa-

rameters. One is the introduction of excess oxygen caused

by the doping of Zr4? ions into Bi-2212 matrix. It is well

known that for Bi-2212 systems, the electron doping al-

ways leads to the introduction of excess oxygen. These

oxygen ions incorporated between the Bi–O double layers

to balance the electrons introduced by Zr4? [24–26], and

reduced the net positive charge and the repulsion force

between the Bi–O layers, therefore causing the slab se-

quence SrO–BiO–BiO–SrO to shrink and lattice parameter

c decrease [27, 28]. Thus, it can be assumed that since the

ZrO2 nanoparticles have very high reaction activity with

lower melting point, the partial melting process could ac-

celerate the doping process of Zr4? ions into Bi-2212 phase

when the ZrO2 doping content is low. Thus Zr4? ions could

enter into the lattice of Bi-2212 and cause the changed of

oxygen content. With the increase of ZrO2 content, Zr4?

doping saturated and Zr-containing phase appeared as al-

kaline-earth zironate, which lead to the decrease of Sr2?

and Ca2? content in Bi-2212 phase. Thus the decreasing

slope of lattice parameter become larger with the ZrO2

doping content increase from 1.0 to 3.0 wt%. The satura-

tion ratio of ZrO2 in Bi-2212 phase is consistent with that

reported in Bi-2223 system by Azhan et al. [19] (in which

the molar ratio of Zr4? at which ZrO2 peak showed up is

higher than 0.15, and the molar ratio in our study is be-

tween 0.07 and 0.22 as the doping content is between 1.0

and 3.0 wt%).

The microstructures of these single filament tapes were

observed with the back scattering electron (BSE) images,

and the BSE images of the cross section of single filament

tape with 0.5 and 3.0 wt% ZrO2 are shown in Fig. 3a, b,

respectively. In these tapes, typical textural structures can

be observed representing the highly preferred orientation.

Secondary phases, including Bi2Sr2CuO6?d (Bi-2201),

copper free phase (CF) and alkaline-earth cuprates (AEC),

are observed as white stripes, light gray stripes and black

dots, which are all formed during the partial melting pro-

cess. These secondary phases are not detected in XRD

patterns due to the extremely high intensity of Bi-

2212 (00l) peaks, and the small contents of these secondary

phases. Besides these secondary phases, certain dark gray

area which can be interpreted as Zr-containing phase is

observed in the tape with ZrO2 content of 3.0 wt%, as

circled out in Fig. 3b. The EDX pattern is shown in the

Fig. 2 a X-ray diffraction patterns of Bi-2212 single filament tapes

with different ZrO2 content; b lattice parameter change with ZrO2

content

J Mater Sci: Mater Electron (2015) 26:3583–3588 3585

123

inset with the atomic ratio of Sr, Ca, and Zr elements,

correspondingly. Combining with the XRD results, it can

be recognized as the (Ca, Sr)ZrO3 precipitates, with the

size of *1 9 5 lm. It suggests that after the saturation of

Zr4? ions in Bi-2212 matrix, (Ca, Sr)ZrO3 precipitations

can be obtained in the tapes.

One important effect of doping is the changing of carrier

concentration. In cuprates HTSs, the main carriers are

holes [29]. According to the previous report by Tallon et al.

[30], for small hole concentration per Cu ion, p (p\ 0.2),

the following equation can be used to estimate the hole

concentration values by the room temperature ther-

mopowers with the equation:

S ¼ kB

eln1� p

2p� ln 2

� �ð1Þ

where, kB and e are the Boltzmann constant and electron

charge, respectively, S is the thermopower with the unit of

lVK-1, the extra ln2 term comes from the orbital degrees

of freedom (assuming twofold orbital degeneracy). This

equation is derived using a single-band Hubbard model

with the assumption that the bandwidth is much less than

kBT. Therefore, by measuring the thermopower values, it is

possible to study the relationship between both the carrier

concentration change and electrical transport behavior of

normal state with ZrO2 doping qualitively.

As shown in Fig. 4a, the thermopower values were

measured with the temperature increase from 25 to 110 �C.It can be noticed that all the thermopowers of these samples

decrease with temperature, which behave similar with

semiconductors. Meanwhile, the room temperature ther-

mopowers increases with ZrO2 content, which implies the

change of carrier concentration with Zr4? entering into the

Bi-2212 lattice. With Eq. (1), the carrier concentration p of

samples with different ZrO2 content are estimated and

plotted in Fig. 4b. The carrier concentration decreases

monotonically from 0.177 to 0.168 with the ZrO2 content

increase from 0 to 3.0 wt%. The change of carrier con-

centration can be attributed to the entering of Zr4? ions into

Fig. 3 Back scattering image of the cross section of Bi-2212 tapes

with ZrO2 content of a 0.5 wt% and b 3.0 wt%, where the light gray

part is Bi-2212 grains, white stripe, light gray stripe and black dots

are Bi-2201, CF and AEC phases respectively; certain dark gray dot

can be observed in (b), inset of (b) shows the EDX patterns of the

dark gray area circled with circle representing a Zr-containing area

and the atomic ratio of Sr, Ca and Zr, respectively

Fig. 4 a Temperature dependence of thermopowers of Bi-2212 with

different ZrO2 content from 25 to 110 �C; b Carrier concentrations

calculated from thermopowers for Bi-2212 tapes with different ZrO2

content, resistivity and mobility at room temperature with the increase

of ZrO2 content

3586 J Mater Sci: Mater Electron (2015) 26:3583–3588

123

Bi-2212 lattice with the introduction of excess electrons. It

should also be noticed that the decreasing of carrier con-

centration is fast with the increase of ZrO2 content from 0

to 1.0 wt%. While, with the saturation of Zr4? doping in

Bi-2212 matrix and the precipitation of (Ca, Sr)ZrO3 phase,

the alkaline-earth content in Bi-2212 phase decreases,

which leads to the formation of holes. Thus, the slope of

carrier concentration dependence on the ZrO2 content de-

creases from 1.0 to 3.0 wt%. The change of carrier con-

centration can influence the superconducting critical

temperature therefore affect the superconducting properties

of these tapes.

On the other hand, the room temperature resistivities

were also measured as plotted in Fig. 4b, and the hole

mobilities l are calculated as l = 1/enq, where n is the

carrier concentration, q is the resistivity at room tem-

perature and e is the electron charge. As shown in Fig. 4b,

with the increase of ZrO2 content, the mobility values also

decreases monotonically. This phenomenon can be at-

tributed to the increasing electron transport scattering

centers, which proves the increasing density of lattice de-

fects after doping.

The critical current of these tapes were measured with

four-probe method at 4.2 K under the magnetic field from 0

to 20 T and the critical current density values are calcu-

lated and plotted in Fig. 5. It can be observed from Fig. 5a

that the maximum critical current density at zero field is

obtained on the tape without ZrO2 doping. The decrease of

critical current density can be observed with the doping

content of ZrO2 In order to isolate the influence of doping

on flux pinning properties, normalized Jc/J0 is calculated

and plotted with magnetic field in Fig. 5b, where J0 is the

zero field critical current density. Normalized Jc of the

sample with 3.0 wt% ZrO2 doping is larger than that

without doping, which suggests an improvement of flux

pinning properties under this doping level. The decrease of

in-field current capacity can be attributed to the doping of

Zr4? ions into the matrix of Bi-2212, which disturbs the

superconducting properties. And the possible reason for the

improvement of current capacity is the precipitation of (Ca,

Sr)ZrO3 particles, which can act as effective pinning cen-

ters to enhance the point pinning for normal phase [17, 20].

Based on these measurement, the current capacity of tapes

with 3.0 wt% can be higher than the undoped samples in

the higher magnetic field over 25 T. And higher current

capacity can be expected by further optimize the doping

content and fabrication process.

4 Conclusions

Bi-2212 tapes with different ZrO2 contents were fabricated

by powder in tube process. Due to the high activity of

nanosized ZrO2 particles, the Zr4? ions entered into Bi-

2212 lattices during the partial melt process, which lead to

a systemically change of lattice structures, carrier con-

centration and electrical transport properties. Although the

in-field current carrying capacity decreased with ZrO2

content, the flux pinning properties were improved due to

the (Ca, Sr)ZrO3 precipitations with proper size which

worked as effective pinning centers. Based on the mea-

surement results, higher current capacities can be predicted

in higher magnetic field than 25 T by ZrO2 doping. And

further optimization of doping ratio and heat treatment

process are on the way.

Acknowledgments Sincerely acknowledge the equipment supports

from EMFL during the magnetic field measurement with 20 T magnet

in Grenoble. This research was supported by the National Basic Re-

search Program of China (973 Program) under Contract No.

2011CBA00104, the International Scientific and Technological

Fig. 5 a Normalized critical current density and b Critical current

density in the magnetic field from 0 to 20 T of Bi-2212 with different

ZrO2 content

J Mater Sci: Mater Electron (2015) 26:3583–3588 3587

123

cooperation projects of China under Contract No. S2010GR0518, the

National Natural Science Foundation of China under Contract No.

51472206, and Program for Innovation Research Team in Shaanxi

Province, No. 2013KCT-07.

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