Physica C 412–414 (2004) 651–656

www.elsevier.com/locate/physc

Effects of Dy2BaCuO5 contents on microstructureand mechanical strength of Ag-added Dy–Ba–Cu–O

bulk superconductors

S. Nariki a,*, N. Sakai a, M. Murakami a,b, I. Hirabayashi a

a Superconductivity Research Laboratory, ISTEC, 1-10-13 Shinonome, Koto-ku, Tokyo 135-0062, Japanb Shibaura Institute of Technology, 3-9-14 Shibaura, Minato-ku, Tokyo 108-8548, Japan

Received 29 October 2003; accepted 5 January 2004

Available online 19 May 2004

Abstract

We investigated the microstructure and mechanical strength of Ag-added DyBa2Cu3Oy (Dy123) bulk supercon-

ductors with various Dy2BaCuO5 (Dy211) contents. Single-grain Dy123 bulk samples 32 mm in diameter with the

addition of 5-40 mol% of Dy211 and 10 wt.% of Ag2O were fabricated in air. The sample with 5 mol% Dy211 contained

many macro-cracks in the ab-plane. The amount of cracks decreased with increasing Dy211 content. Three-point

bending tests were performed at room temperature to measure the mechanical properties. The average bending strength

of the sample with 5 mol% Dy211 was 73 MPa. The strength was improved to 95 MPa with an addition of 40 mol%

Dy211.

� 2004 Elsevier B.V. All rights reserved.

PACS: 74.25.Ld; 74.72.Bk; 74.80.Bj

Keywords: Melt-textured bulk; Mechanical strength; Microstructure; DyBa2Cu3Oy; Dy2BaCuO5

1. Introduction

Large single-grain Dy–Ba–Cu–O bulk super-conductor has an excellent field trapping capabil-

ity. The trapped magnetic field of the sample 48

mm in diameter reaches 2 T at 77 K, which exceeds

those of Y–Ba–Cu–O bulk materials [1]. Thus,

Dy–Ba–Cu–O is one of the promising candidates

for engineering applications.

* Corresponding author. Tel.: +81-3-3536-5716; fax: +81-3-

3536-5705.

E-mail address: nariki@istec.or.jp (S. Nariki).

0921-4534/$ - see front matter � 2004 Elsevier B.V. All rights reserv

doi:10.1016/j.physc.2004.01.084

We previously reported that critical current

density ðJcÞ of Dy–Ba–Cu–O is largely influenced

by the amount of Dy2BaCuO5 (Dy211) inclusions[2,3]. The Dy–Ba–Cu–O bulk samples containing a

small amount of Dy211 exhibits a large secondary

peak effect in the Jc–B curve, which was presum-

ably ascribed to the presence of oxygen deficiency.

For industrial applications of bulk superconduc-

tors, however, it is necessary to improve their

mechanical strength. The mechanical properties of

bulk superconductors are dependent on themicrostructure such as pores, cracks and second-

ary phase particles. In the present work, we

ed.

[100]

[110]c-axis

652 S. Nariki et al. / Physica C 412–414 (2004) 651–656

investigated the microstructure and the mechanical

strength of Ag-added Dy–Ba–Cu–O bulk super-

conductors with various Dy211 contents.

AB

C

A 1A 2A 3

B1B2B3

C 1C 2C 3

Bulksample

c-axis

Fig. 1. Location of specimens for three-point bending test cut

from the melt-textured Dy–Ba–Cu–O sample 32 mm in dia-

meter.

0.91 Tx = 5

y (mm)

x (mm)-20 -10 0 10 20

20

10

0

-10

-20

x = 20 1.40 T

y (mm)

x (mm)-20 -10 0 10 20

20

10

0

-10

-20

Fig. 2. Trapped field distribution of the Dy–Ba–Cu–O samples

of 32 mm diameter with Dy211 contents of x ¼ 5 and 20 at 77

K. The trapped magnetic field was measured by scanning a Hall

probe sensor at 1.2 mm above the top surface of the bulk

sample in liquid nitrogen.

2. Experimental

Ag-added Dy–Ba–Cu–O bulk samples with

various Dy211 contents were synthesized using

mixed powders of commercial DyBa2Cu3Oy

(Dy123), sintered Dy211, 0.5 wt.% Pt and 10 wt.%

Ag2O. The Dy211 powder was prepared by the

calcination of commercial Dy2O3, BaO2 and CuO

powders at 900 �C for 4 h. The mixtures withmolar ratios of Dy123:Dy211 ¼ 100:x (x ¼ 5, 10,

20 and 40) were uni-axially pressed into pellets 40

mm in diameter and 25 mm in thickness and then

consolidated using cold-isostatic pressing (CIP)

under a pressure of 200 MPa. Melt-processing was

performed in air. Nd123 (0 0 1) bulk crystal was

used as a seed. The details of the melt-process are

described elsewhere [3]. The diameter of the ob-tained samples was 32 mm. The oxygen annealing

was performed at 400–450 �C for 300 h. In order

to check the quality of bulk materials, the mea-

surements of trapped field distribution at liquid

nitrogen temperature were carried out by magne-

tizing the bulk samples using a 10 T super-

conducting magnet [3]. Microstructure of the

oxygenated samples was observed with an opticalmicroscope and a scanning electron microscope

(SEM).

The three-point bending tests were carried out

to measure mechanical properties. As illustrated in

Fig. 1, nine bar-shape specimens with dimensions

of 3 · 4 · 25–30 mm3 were cut from each oxygen-

ated bulk sample. The surfaces of the specimens

were polished using abrasive papers with the sur-face flatness of ±3 lm. The load was applied alongthe c-direction of the bulk material. The strength

was measured with 16 mm span and a crosshead

speed of 0.5 mm/min at room temperature.

3. Results and discussion

Fig. 2 shows the trapped field distribution of

Dy–Ba–Cu–O samples with x ¼ 5 and 20 at 77 K

characterized in the present study. All the samples

exhibit symmetric field distribution, which reveals

that the samples are free from macroscopic defects

along c-direction such as large cracks.

In general, melt-textured bulk superconductors

have two kinds of cracks; macro-cracks and mi-

cro-cracks. Such cracks are believed to result from

a large thermal anisotropy between aðbÞ-directionand c-direction, tetragonal to orthorhombic phase

S. Nariki et al. / Physica C 412–414 (2004) 651–656 653

transition with oxygen-annealing, and the differ-

ence in the thermal expansion coefficients between

211 inclusions and the 123 matrix. Fig. 3 shows

optical micrographs of (1 0 0) cross section of

oxygen-annealed bulk samples with Dy211 con-

tents of x ¼ 5 (Fig. 3(a–1), (a–2)) and x ¼ 40 (Fig.

Fig. 3. Optical micrographs of the polished surfaces of Dy–Ba–

Cu–O bulk samples; (a–1) the position at the distance of 3 mm

from the seed crystal in x ¼ 5 sample, (a–2) inner region of

x ¼ 5 sample, (b) inner region of x ¼ 40 sample.

3(b)). The white particles and the black spots are

Ag and pores, respectively. It is well-known that

the addition of Ag is effective in depressing the

formation of macro-cracks [4–6]. However, a large

number of macro-cracks in the cleavage plane (ab-plane) existed in the sample with x ¼ 5 as shown inFig. 3(a–1) and (a–2). The number of cracks was

reduced with increasing Dy211 addition. A few

macro-cracks were found in the sample with

x ¼ 40 as shown in Fig. 3(b). Fig. 4 shows the

SEM photographs for the samples with x ¼ 5 and

40. The light gray particles are Dy211 phase. The

micro-cracks in ab-plane were found in all sam-

ples, and the sample with x ¼ 5 contained moremicro-cracks than the other samples. Conse-

quently, the formation of macro- and micro-

cracks is strongly affected by the amount of

Dy211.

Next, we notice the pores and Ag particles. The

distribution of pores depends on the location of

the sample. In the case of the sample with x ¼ 5,

large pores with 50–100 lm in size were observedin the inner region of the sample as shown in Fig.

3(a–2), while the low porosity region as displayed

Fig. 4. SEM photograph of the polished surfaces of the Dy–

Ba–Cu–O samples with Dy211 contents of (a) x ¼ 5 and (b)

x ¼ 40.

654 S. Nariki et al. / Physica C 412–414 (2004) 651–656

in Fig. 3(a–1) was found at the distance of

approximately 5 mm from the top and side sur-

faces. It was reported that the pores in RE123 bulk

materials are formed due to the oxygen bubbles

generated during the peritectic decomposition of

RE123 phase [7]. The oxygen bubbles generatednear surface are easily released from the sample. In

the inner region of the sample, the oxygen bubbles

are entrapped and grow to large pores. Some inner

pores are filled with molten Ag at high tempera-

tures, leading to the formation of large Ag parti-

cles. A low porosity region near the surface was

also observed in other samples, however, the area

was narrowed with increasing Dy211 contents.The thickness of low porosity region in the sample

with x ¼ 20 and 40 is less than 2 mm. The distri-

bution of pores and Ag was more homogeneous in

the sample with x ¼ 20 and 40, for which the pore

size was relatively small (between 10 and 30 lm).The amount of oxygen bubbles decreases with

decreasing Dy123 contents (increasing Dy211

contents). In addition, the presence of manyDy211 particles prevents the motion and coars-

ening of oxygen bubbles, thus inhibiting the

enlargement of pores.

Fig. 5 shows the bending strength of the speci-

mens with various Dy211 contents. The average

strength of the sample with x ¼ 5 was 73 MPa at

0

20

40

60

80

100

120

0 10 20 30 40

Amount of Dy211 [x]

Bend

ing

stre

ngth

(MPa

)

Fig. 5. Relationship between the three-point bending strength

measured at room temperature and the amount of Dy211 ðxÞ.

room temperature. In this sample, the distribution

of pores was different from the test pieces; that is,

the specimens cut from the top surface (A1, B1 and

C1 illustrated in Fig. 1(a)) had a low porosity,

while the other specimens contained many pores.

The average strength of the sample with lowporosity was 79 MPa. This value was higher than

that of the other specimens with large pores (70

MPa). Therefore, the porosity is one of the

important factors determining the mechanical

strength. The bending strength increased with

increasing Dy211 contents. It was improved to 95

MPa when Dy211 content is x ¼ 40, which reflects

a reduction in the amount of cracking with Dy211addition. It has been reported in Y–Ba–Cu–O [8,9]

that finely dispersed Y211 particles enhance the

fracture resistance of Y123 through the energy

dissipation by interfacial delamination and crack

bridging. In this work, an increase of Dy211 con-

tents will enhance the fracture toughness, leading

to the reduction of pre-existing cracks formed

during sample preparation and the enhancementof resistance to crack formation during the bend-

ing test.

Fig. 6 shows the typical fracture surface for the

samples with x ¼ 5 and 40. The fractographs

consist of the steps divided by cracks parallel to

the ab-plane. In the sample with low Dy211 con-

tents, steps were small and the surface was rela-

tively smooth as shown in Fig. 6(a). In contrast,some large steps were found in the sample with

large Dy211 contents as shown in Fig. 6(b). Such a

difference in the fracture surface morphology is

correlated with the mechanical strength of the

samples. Similar results were reported in the tensile

tests for Sm–Ba–Cu–O [10], (Sm, Gd)–Ba–Cu–O

[11] and (Nd, Eu, Gd)–Ba–Cu–O [12].

In order to evaluate the reliability of the sam-ples, the results of the bending tests were analyzed

based on the Weibull distribution function. Fig. 7

reveals the Weibull plots of the bending strength.

The Weibull coefficients of the samples ranged

from 8.3 to 10.4. These values are relatively high

compared to previous reports on the bending

strengths for Y–Ba–Cu–O with/without Ag addi-

tion [13,14]. The results of trapped field measure-ment presented in Fig. 2 suggested that the

samples are free from serious defects along the

Fig. 6. SEM photographs of the fracture surface of the Dy–Ba–

Cu–O samples with Dy211 contents of (a) x ¼ 5 and (b) x ¼ 40.

40 60 80 1001

5

10

20

50

90

99

m =

8.8

m =

9.9

m =

8.3

m =

10.

4Dis

tribu

tion

func

tion,

F(%

)

Strength (MPa)

x = 5x = 10x = 20x = 40

Fig. 7. Weibull plot of the bending strength for the Dy–Ba–

Cu–O samples with different Dy211 contents.

S. Nariki et al. / Physica C 412–414 (2004) 651–656 655

c-direction, which otherwise leads to a large drop

in the bending strength. This fact contributed to

relatively high Weibull coefficients of the present

specimens.

4. Conclusions

We investigated the microstructure and

mechanical strength of Ag-added Dy–Ba–Cu–O

bulk superconductors with Dy211 contents of 5–40

mol%. The distribution of cracks and pores was

largely influenced by the Dy211 contents. The

sample with 5 mol% Dy211 contained manymacro-cracks in the ab-plane. The amount of

cracks decreased with increasing Dy211 content.

The average bending strength of the sample with 5

mol% Dy211 was 73 MPa. The strength was im-

proved to 95 MPa with increasing the amount of

Dy211 to 40 mol%.

Acknowledgements

This work is partially supported by the New

Energy and Industrial Technology Development

Organization (NEDO).

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