Structural and Electrical Properties of Niobium Doped Y0.6Gd0.4Ba2-xNbxCu3O7-y Superconductors

doi:10.4236/msa.2011.28147

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Materials Sciences and Applicatio n, 2011, 2, 1090-1096

doi:10.4236/msa.2011.28147 Published Online August 2011 (http://www.SciRP.org/journal/msa)

Structural and Electrical Properties of Niobium

Doped Y0.6Gd0.4Ba2-xNbxCu3O7-y Superconductors

Mucahit Yilmaz1*, Oguz Dogan1

1A. Keleşoğlu Education Faculty Department of Physics, Selçuk University, Meram-Konya, Turkey

Email: *[email protected]

Received August 9th, 2010; revised January 8th, 2011; accepted May 30th, 2011

ABSTRACT

Polycrystalline samples of Y0.6Gd0.4Ba2-xNbxCu3O7-y (YGBNCO) with different Nb contents (x = 0.05, 0.10, 0.15, 0.20,

and 0.25) were prepared using the usual solid state reaction technique. The structure for all samples was characterized

by XRD and SEM. The electrical properties were measured by the FPP method in the temperature range from 70 to 130

K. The lattice constant of b remains almost unchanged and a and c increases with the increase of Nb content with x ≤

0.10. The zero resistance transition temperature and Jc decrease with increasing Nb content. But superconductivity did

not suppress. As the Nb content in the samples increases, it gives a diffused phase indicating a niobium perovskite

phase and it is a small amount of unidentified phase.

Keywords: Y0.6Gd0.4Ba2Cu3O7-Y Cuprates, Gd and Nb Co-Doping, Structural and Superconducting Properties

1. Introduction

YBCO is one of the most widely studied compound

among the cuprate superconductors, owing to simplicity

of synthesis procedure by solid-state reaction, the easy

availability of the starting powders, and the non-toxity of

the material compared to the other high-Tc superconduc-

tors such as Tl and Hg based oxides [1-5].

One of the most fascinating challenges of the material

science is to develop YBa2Cu3O7-x (YBCO) and RE-

Ba2Cu3O7-x (REBCO: RE = Er, Nd, Gd, Sm) supercon-

ductors for practical applications [6]. Several attempts

have been made to study the effects of substituting Nb

for YBCO [7-13]. In a study by Suresha et al. [14], a

resistivity for YBa2Cu3-xNbxO7-y sample with x = 0.3,

0.45 and 0.6 indicated that Tc was in the range 85 - 90.3

K. They showed a possible change in crystal structure

and composition with changing of x. Kuwabara and Ku-

saka [13] concluded that Cu and Nb did not co-exist in

the same compound containing Y and Ba. Another study

by Abdullah and Tan [7] mentioned that the added Nb

formed perovskite YBa2NbO6 instead of a compound

containing Cu. On the other hand, in a study by Eguchi et

al. [15] it was shown that Nb substitute to Cu in the sam-

ple with nominal composition x = 0.01. Some researches

[9,16] reported the existence of two phases in Y-Ba-Cu-

Nb-O compound. One is the cubic Nb perovskite YBa2-

NbO6, and the other is YBa2Cu3Oy phase. Some previous

works indicated that at a small amount of Nb content of

transition temperature, Tc, of the Y-Ba-Cu-Nb-O com-

pound was nearly constant [9,14,17] or slowly higher

[10,18].

The effect of Gd substitution in YBCO polycrystalline

stabilizes an orthorhombic phase while the lattice pa-

rameters are found to increase due to a larger size of the

Gd atoms than Y atoms. There is no effect in the transi-

tion temperature regardless of the concentration of Gd

atoms introduced in YBCO [18], but Tc in Gd123 and

Nd123 decreases rapidly with only little oxygen defi-

ciency from fully oxygenated state, while in Y123 high-

est Tc is retained even with certain oxygen deficiency (up

to δ ≈ 0.2). This may be one of the reasons why the

Gd123 material is not yet considered for various applica-

tions as widely as Y123 [19]. The results of X-ray dis-

tribution maps of x = 0.4 composite indicate that the

RE123 matrix is homogeneous and Y and Gd elements in

the Y1-xGdxBa2Cu3O7-δ a perfect solid solution [20]. For

the Gd diffused-doped samples, magnetization and resis-

tivity measurements show that the critical transition

temperature, Tc, increased from 88 to 91 K and the criti-

cal current density, Jc, which was calculated from M-H

loops taken at 77 K, increased from 55 to 122 A·cm−2 in

comparison with those of undoped Y123. Such en-

hancement, which is considered to represent a character-

istic strength of inter-grain coupling, is more clearly rec-

Structural and Electrical Properties of Niobium Doped YGd Ba Nb Cu O Superconductors1091

0.60.4 2-xx 37-y

ognized when critical current densities are compared

[21].

We aimed that Nb doped to the compound which

oxygen concentration is high and it is only a little af-

fected from absence of oxygen. Because when the Nb

substitutes for Cu in cuprates, Cu-O chains and CuO2

planes are distorted, oxygen concentration reduces, thus

superconductivity does not disappear. Therefore, we

mixed 60% Y2O3 and 40% Gd2O3. Then we doped Nb to

the (Y0.6Gd0.4) BCO compound. In Section 2, we gave

some details about the sample preparation and experi-

ments. Results of the dependences of the superconductiv-

ity properties and structural parameters upon the content

of Nb in Y0.6Gd0.4Ba2-xNbxCu3Oy compound are summa-

rized and discussed in Section 3. Some conclusions have

been drawn in Section 4.

2. Experimental

The samples were prepared using the standard solid-state

reaction technique. The detailed procedures for sample

preparation were as follows: pure cation oxides of Y2O3

(99.995%), Gd2O3 (99.99%), Nb (99.8%), BaCO3 (99%),

and CuO (99%) were weighted and mixed according to

the chemical formula of YBa2Cu3O7-y (YBCO) and

Y0.6Gd0.4Ba2-xNbxCu3O7-y (YGBNCO) with x = 0.05, 0.10,

0.15, 0.20 and 0.25, individually. Each of these mixtures

was ground ten minutes for several times. They were put

into a furnace (Nabartherm-N11/R) and calcined at

925˚C in air at 48 h. This process was repeated two times.

The calcined powders were pulverized and reground then

pelletized into disk-shaped pellets. In the sintering proc-

ess, the pellets were then heated up to 925˚C in the tube

furnace (Carbolite 201) with 10˚C/min and kept at this

temperature for 24 h in air. Afterwards the temperature

was decreased down to about 550˚C with 4˚C /min and

kept at this level for 12 h in flowing oxygen (≈ 2.l/min).

Finally the products were cooled down to room tempera-

ture.

The structural characterization was performed by

X-ray diffraction (XRD) and scanning electron micros-

copy (SEM) which is equipped with an energy dispersive

spectrum (EDS). A JEOL JSM-6390LV scanning elec-

tron microscope and an EDS were used. EDS analysis

was carried out to displayed region with SEM for 1300

count/min. XRD experiments were carried out on Rigaku

Multiflex powder diffractometer. At the XRD measure-

ments, CuKα beam were used at 10˚ < 2θ < 60˚, 5˚/min

scan speed and 0.02˚ sampling space. a, b, c, unit cell

dimensions and, V, unit cell volumes were calculated

from XRD data using Lapod Program which uses Co-

hen's method of least square. Electrical properties were

measured by a standard four point probe method with

silver and indium soldering contacts. The resistivity de-

pendence of temperature in the range of 70 - 130 K was

measured under 10 - 6 Torr pressure with CTI-Cyro-gen-

ics Cyrodyne Refrigerator System. Critical current den-

sity of samples were measured 77 K and under 0 T ex-

ternal magnetic field. Jc value of samples was calculated

for 2 μV/cm from I-V curves.

3. Results and Discussion

All samples were examined by powder XRD. Figure 1(a)

and Figure 1(b) show the measured XRD patterns for

samples (a) YBa2Cu3O7 (Y123), and (b) Y0.6Gd0.4Ba2-x-

NbxCu3Oy (YGBNCO) with x = 0.05, 0.10, 0.15, 0,20

and 0.25 respectively. The peaks of the undoped sample

(YBCO) and Gd doped sample (YGBCO) were well

matched to the orthorhombic Y123 structure. As can be

seen there is a slight difference in the patterns for the

sample with x = 0.00 compared to the Nb doped samples.

The XRD patterns show that the peaks marked with ar-

row exist in the spectrum of the samples in Figure 1(b).

The peaks were observed at about 29.8 degrees and about

53.1 degrees and the intensity of these peaks increased

gradually with the increasing of Nb content. These peaks

were identified as due to niobium perovskite. With in-

creasing Nb content samples gave a diffused phase indi-

cating a niobium perovskite phase [22] and a small

amount of unidentified phase. The results showed that

the intensity of peaks owing to the YBCO decreased,

while those for YBa2NbO6 increased with Nb content.

The results of XRD analysis indicate that niobium does

not go into the lattice of YBCO, but forms a secondary

phase which uniformly distributes in the YBCO com-

pound with improved microstructure. Also, there are

some reports that YBa2Cu3-xNbxOy could be owing to the

compound of this composition was not stable, leading to

the formation of three components (YBCO, YBa2NbO6,

and CuO), and Cu and Nb never coexist in the same

compound containing Y and Ba [7].

Figure 2(a) and 2(b) illustrate typical back-scattered

electron SEM micrographs of the samples with x = 0.10

and 0.20. The SEM studies proved that the samples with

x ≤ 0.10 are homogeneous. The SEM micrographs show

the homogeneous stone-like grains with typical size of

several microns (Figure 2(a) and 2(b)). The SEM mi-

crographs show that the stone-like grains marked with A

and the sponge-like grains marked with B co-exist in the

surface of the samples (Figure 2(b)). X-ray structural

analysis of the samples showed that the sponge-like

grains are distributed statistically inhomogeneous. We

believe that the B grains are composed of Nb impurity

phase which is located on the grain boundaries and space

of between the grains.

The lattice parameters (a, b and c) were calculated

from ten and upper peaks using least square methods.

Structural and Electrical Properties of Niobium Doped YGd Ba Nb Cu O Superconductors

1092 0.60.4 2-xx 37-y

(a)

(b)

Figure 1. (a) XRD patterns for YBa2Cu3O7-y; (b) XRD patterns for Y0.6Gd0.4Ba2-xNbxCu3O7-y.

Structural and Electrical Properties of Niobium Doped YGd Ba Nb Cu O Superconductors 1093

0.60.4 2-xx 37-y

(a)

(b)

Figure 2. (a) SEM pictures of the surface structure for sam-

ples of YGBNCO with the nominal composition of x = 0.10;

(b) SEM pictures of the surface structure for samples of

YGBNCO with the nominal composition of x = 0.20.

The lattice parameters of a, b, c, V orthorhombicity

parameters a/b and c/b are plotted in Figure 3 versus the

Nb content, x, for YGBNCO samples. The lattice con-

stants of the YBCO phase for samples were found to be a

= 3.829 – 3.864 Å, b = 3.887 – 3.900 Å, c = 11.692 –

11.736 Å respectively. While b lattice parameter almost

remain unchanged. a and c rapidly increases with the

increase of Nb content with x ≤ 0.10 and then rapidly

decreases with x ≤ 0.10. We believe that the variations of

the lattice parameters are related to the co-doping of Gd

and Nb in YGBNCO.

The resistivity dependence of temperature in the range

of 70 - 130 K for all samples, which are respectively

normalized to that of the value of the room temperature

(130 K) are shown in Figure 4. Good linear behavior and

the metallic behavior for all samples were observed with

T > 94 K. The resistivity at the normal state (T > 94 K) is

measured with x = 0.25, bigger than with x = 0.05, 0.10,

0.15 and 0.20. The room temperature resistivity’s in-

creases in the doping ranges 0.00 ≤ x ≤ 0.25. This result

is similar to that of obtained by Strukova et al. [10], and

can be explained that the Nb impurity phase on the grain

boundaries promotes an increase in the resistivity value

at room temperature.

The dependences of the zero resistance temperature,

, and transition temperature, , upon the con-

tent of Nb in Y0.6Gd0.4Ba2-xNbxCu3Oy are shown in Fig-

ure 5, which smoothed with adjacent-averaging method

versus Nb content x. It can be found that there is slowly

increase of with the increase of Nb content (from

92 K to 95 K). This result is good agreement to compare

with the other results [16-22], and can be explained as a

small amount of Nb can be caused by the increase of

oxygen index because of higher Nb affinity for oxygen

than of Y, Ba and Cu.

offset

Tonset

onset

All the same, it can be found that there is a slowly de-

crease of zero-resistance temperature, , with the

increase of Nb content. almost remain unchanged

as x ≤ 0.05 (~91.5 K), and then decreases with the in-

crease of x as x ≥ 0.05. drops down to 84 K with

x = 0.25 in Figure 5. The transition width, Tc, which is

defined as the temperature difference between 100% and

0% in the extrapolated normal-state resistivity value

(and , respectively), increases with the in-

crease of Nb content. The larger transition width may

result from the YBa2NbO6 phase, impurity, and unidenti-

fied phases of the sample due to the Nb doping. The im-

purity, YBa2NbO6 and unidentified phases of the samples

were proved by the XRD and SEM experiments which

were discussed above. Also, inhomogeneities impurity of

the sample may enlarge the width of the superconducting

transition, too. The oxygen content and charge transfer

were believed to be an important factor for the super-

conductivity of YBCO.

offset

onset

Toffset

Critical current density (Jc) values measured in the

YBCO and YGBNCO for x = 0.00 are 89.9 and 85.9

A/cm2, respectively. Critical current density dependence

on the Nb content is shown in Figure 6. Jc decreases

with the increase of Nb content. We think that this result

is related to the YBa2NbO6 particles which gather be-

tween grains.

4. Conclusions

In summary, effect of Nb doped on the superconductivity

properties and the structural parameters at Y0.6Gd0.4Ba2-x

NbxCu3O7-y system with doping range (0 ≤ x ≤ 0.25) were

investigated. The lattice constant of b remains almost

unchanged and a and c increases with the increase of Nb

content with x ≤ 0.10. However, they decrease with the

increase of Nb content, x, with x ≥ 0.10. The zero resis-

tance transition temperature and Jc decrease with increa-

Structural and Electrical Properties of Niobium Doped YGd Ba Nb Cu O Superconductors

1094 0.60.4 2-xx 37-y

Figure 3. Lattice constants of YGBNCO versus Nb content (x).

Figure 4. The temperature dependence of the normalized resistivity for YGBNCO.

178

Structural and Electrical Properties of Niobium Doped Y0.6Gd0.4Ba2-xNbxCu3O7-y Superconductors

1095

Figure 5. The and as a function of the Nb content for YGBNCO.

onset

Toffset

Figure 6. The critical current density (Jc) as a function of the content of Nb for YGBNCO.

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