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This is to certify that the

thesis entitled

Bi and PbO Addition and Liquid
Phase Sintering of 123 Compound

presented by

Il-Sung Oh

has been accepted towards fulfillment
of the requirements for

Master' 3 degree in Metallurgy

 

 

Major professoy

Date 5/’//99

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Bi AND PbO ADDITION AND LIQUID PHASE SINTERING
OF Y-Ba-Cu-O COMPOUND

BY
Ilsung Oh
A THESIS

Submitted to
Michigan State University
in partial fullfillent of the requirements
for the degree of

MASTER OF SCIENCE

Department of metallurgy, Mechanics and Material science

1990

ABSTRACT

Bi AND PbO ADDITION AND LIQUID PHASE SINTERING
OF Y-Ba-Cu-O COMPOUND

BY

Ilsung Oh

The relatively low sintered density of Y-Ba-Cu-O
superconducting compound is the typical of consolidation
and sintering of its powdered components. Such a porous
structure further degrades mechanical stability of intrin-
sically brittle Y-Ba-Cu-O compound. A study was conducted
to develop an economically attractive processing technique
based on liquid phase sintering, which can reduce porosity
and upgrade mechanical properties without degrading super-
conducting properties. Bi and PhD were added in Y-Ba-Cu-O
compound to employ liquid phase sintering. The mixture was
compacted and sintered at 920 and 950 C. It was observed
that addition of PhD reduced porosity by 7 to 8% without
affecting transition temperature while addition of Bi did
not reduce porosity. It was also found that both of Bi and
PhD reacted with 1:2:3 compound to form non-
superconducting phases.

In this thesis, the details of sample preparation,
experimental results of x-ray analysis, transition tempera-
ture and density measurements, optical and scanning

electron microscopy and EDAX analysis are presented.

ACKNOWLEDGEMENTS

First of all, I would like to express my deepest
thanks to professor Dr. K. Mukherjee and my parents for
their constant support and kind guidance during this re—
search. I would like to thank Dr. P.A.A. Khan for his
valuable suggestions and comments. I would also like to
thank to Mr. G. Jang, J. Yoo and C. Chen for their timely
support and help. Finally, I would like to express per-
sonal appreciation to my wife Youngah Jung for her

continuous encouragement.

ii

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

1.

2.

INTRODUCTION

LITERATURE SURVEY

2-1

2-2

The Effect of Added Impurities on Y-Ba-Cu-O
Compound

Some Processing Techniques to Decrease
Porosity

EXPERIMENTAL PROCEDURE

3-1
3-2
3-3
3-4

3-5

Sample Preparation

X-ray Measurements

Transition Temperature Measurements

Density Measurements

Morphological Examination and EDAX Analysis
3-5-1 Optical Microscopy

3-5-2 Scanning Electron Microscopy and EDAX
Analysis

RESULTS AND DISCUSSION

4-1
4-2
4-3
4-4

4-5

X-ray Diffraction Data

Critical Transition Temperature Measurements
Density Measurements

Optical Microscopy

Scanning Electron Microscopy and EDAX
Analysis

SUMMARY

REFERENCES

111

Page
iv

vi

10

15
16
17

22

24

24

25

34

36

43
53
54

LIST OF FIGURES

Figure

1.

10.

11.

12.

13.

The normalized (at 100 K) resistivity as a
function of temperature for 1:2:3 compound
doped with Cr Mn, Ti, V, Fe and Zn (Ref 7)

Superconducting transition temperarture, T
(midpoint),T (90% of the sigmoid) and Tc1
(10%) in dopea 1:2: 3 plotted against the
dopant valence (Ref 7)

C

T (solid line) and magnetic susceptibility at

180 K (dashed line) of 1: 2. 3 compound doped
with 3d element, as a function of the number of
valence electrons (Ref 5)

Resistance curve as a function of temperature
for different specimens. E1 correspond to
specimen after pressing and ElA correspond to
specimen after pressing and postannealing
(Ref 11)

Magnetic susceptibility vs. temperature curve
for melt processed 1:2:3 compound (Ref 12)

Schematic diagram of resistance measurement
set-up

Schematic diagram of four wire AC resistance
measurement

Schematic diagram of sample holder assembly
for 4 wire AC resistance measurement

X-ray diffraction patterns of pure 1:2:3
sample and 1:2:3 mixed with PbO, sintered
at 950 C

X-ray diffraction patterns of pure 1:2:3
sample and 1:2:3 mixed with PbO, sintered
at 920 C

X-ray diffraction patterns of pure 1:2:3
sample and 1:2:3 mixed with Bi, sintered
at 950 C

Temperature effect on resistance of pure
1: 2: 3 sample and 1:2: 3 mixed with Bi or
PbO - 1

Temperature effect on resistance of 1:2:3
samples mixed with Bi or PbO - 2

iv

Page

12

13

18

19

20

26

27

29

32

33

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

26.

Optical microscopy of pure 1:2:3 sample,
sintered at 920 C (a) 500x (b) 2000x

Optical microscopy of PbO 5 wt% mixed sample,
sintered at 920 C (a) 500x (b)2000x

Optical microscopy of PbO 10 wt% mixed sample,
sintered at 920 C (a) 500x (b)2000x

Optical microscopy of PbO 15 wt% mixed sample,
sintered at 920 C (500x)

Optical microscopy of Bi mixed samples,
sintered at 950 C (500x) (a) Bi 5 wt%
(b) Bi 10wt%

SEM micrographs of (a) pure 1:2:3 sample
(b) PbO 5 wt% mixed sample, sintered at 920 C

SEM micrographs of (a) PbO 10 wt% mixed sample
(b) PbO 15 wt% mixed sample, sintered at 920 C

EDAX spectra obtained from pure 1:2:3 sample,
sintered at 920 C

EDAX spectra obtained from PbO 5 wt% mixed
sample, sintered at 920 C

EDAX spectra obtained from PbO 10 wt% mixed
sample, sintered at 920 C

EDAX spectra obtained from the selected grain
marked by arrow in Fig 19b

EDAX spectra obtained from the selected grain
marked by a in Fig 20a

EDAX spectra obtained from the selected grain
marked by b in Fig 20a

37

38

40

41

42

44

45

46

47

48

49

50

51

LIST OF TABLES

Table Page
1. Various preparation conditions and transition

temperatures of pure 1:2:3 samples and 1:2:3

mixed with Bi or PbO 31
2. Measured density and porosity of pure 1:2:3

samples and 1:2:3 mixed with PbO 35

vi

1. INTRODUCTION

Since the discovery of new class of high transition
temperature (Tc = 90 to 95 K) ceramic superconductor,
YBaZCu3O7_x by Wu et al.[1] in 1987, enormous world wide
efforts have been made both to enhance the superconducting
properties and to develop the practical applications of
this new material. Up to now, a large amount of work has
been performed to understand the origin of superconduc-
tivity and to observe the effect of various element
substitution on superconducting properties. It has been
found that Y in YBaZCu307_x can be replaced by almost all
the rare-earth elements without deteriorating Tc [2,3] and
the substitution in the Ba site by 25% Sr also does not
affect Tc [4]. On the contrary, even small amount of
substitution of Cu by 3-d elements degrades the supercon-
ducting properties [5,6]. There are several reports that
the addition of oxide, nitride, carbonate and some transi-
tion metal impurities also affect the superconductivity
depending on the types of the reactions between additives
and superconducting phase [7,8].

Y-Ba-Cu-O superconductors are known to have some
undesirable properties. For instance, the relatively low
sintered-density of Y-Ba-CU-O compound is the typical of
consolidation and sintering of powdered components. Such a
porous structure further degrades not only the mechanical

stability of intrinsically brittle Y-Ba-Cu-O compound for

many applications but also the critical current density due

2
to poor connectivity between individual superconducting
grains. Efforts have also been made to remove some of the
undesirable characteristics such as high porosity, brittle-
ness, and residual impurity of Y-Ba-Cu-O compound [9,10].
Processing techniques such as HIP (Hot isostatic pressing),
hot pressing and melt processing have been tried to improve
mechanical properties and critical current density of Y—Ba-
Cu-O compound by reducing porosity [11-13].

The purpose of this reserch is to develop an economi-
cally attractive processing technique which can reduce
porosity and thereby improve mechanical properties without
degrading superconducting properties. The technique is
based upon liquid phase sintering, in which different
amounts of low melting point additives are mixed in Y-Ba-
Cu-O compound. In this research, the effect of Bi and PbO
addition in Y-Ba-Cu-O compound on reduction in porosity,
the superconducting properties and microstructure was
investigated. Bi and PbO were selected as additives to
employ liquid phase sintering since they had lower melting
points than Y-Ba-Cu-O compound and thus were expected to
act as fluxes during the sintering stage.

In this research, x-ray diffraction data was collected
to identify phases present in samples. The transition
temperature was measured by using a standard four probe
technique and the sintered density was measured by using
the buoyance method. Optical and scanning electron micros-
copy were used to examine the morphology of the material.

Elemental analysis with an energy dispersive X-ray analyzer

3
(EDAX) was also carried out to observe the presence of the
additives. The details of the experiments and the results

of this reserch are discussed in this thesis.

2. LITERATURE SURVEY
2-1 The Effect of.Added.Inpurities On Y—Ba-Cu-O Compound

The effect of the addition of various elements on the
superconducting properties of Y-Ba-Cu-O compound was inves-
tigated by several researchers to understand its
interaction with different additives for the developement
of practical applications [7,8,14]. The study provided
useful information about 1:2:3 processing such as the
possibility of contamination from crucibles, possible
electric and thermal contact materials, potential den-
sification aids and acceptable contamination levels.

Jarvinen et al. [8] investigated the interaction of 22
different additives with Y-Ba-Cu-O compound; 10 mol% of
each impurity (approximately 1 to 4 wt% depending on the
molecular weight) was added to Y-Ba-Cu-O powder as oxides
or carbonates. According to their results, large number of

materials such as ZrO v 0 Nb 0 wo , Bi In 0

2' 2 5' 2 5' 3 2 3' 2 3'
2, BaO, and szo5 have been shown to produce only

0
5102, T10
a small change in resistance vs. temperature behavior and
indicate a similar x-ray diffraction pattern of the orthor-
hombic phase as compared to that of pure Y-Ba-Cu-O
compound. On the contrary, several materials such as
C0203, M003, Fe203, and A1203 were found to totally destroy
the orthorhombic superconducting phase and transform it to

tetragonal phase.

5

The effect of transition-metal impurities on the
superconducting properties of Y-Ba-Cu-O compound was
reported by Don et a1. [7]. Samples with the composition
of YBaZCu3MzO7_x were fabricated by adding transition metal
oxides in the Y-Ba-Cu-O powders ( where M = Fe, Ni, Zn, Sc,
Ti, V, Cr, Mn and z = 0.06). Fig 1 shows the temperature
dependence of the normalized resistance for these samples.
It can be seen that samples with Fe and Zn drops Tc sharply
and have semiconductor-like temperature-resistivity be-
havior with a negative temperature derivative of
resistivity whereas other samples have metal-like behavior.
It is also noted that the non-magnetic Zn degrades TC more
strongly than magnetic Fe. For the reduction of Tc by
transition metal other than Fe and Zn, they proposed that
it depends on the oxidation potential of these cations;
i.e. dopant element with higher oxidation state reduce Tc
less than that with a lower oxidation state. Fig 2 shows

5+ 4+ 4+ 3+
I

TO values against valencies for V Ti , Mn , Cr

N12+.

and

The similar phenomenon was observed by Xiao et al. [5]
who studied the effect of 3-d element substitution for Cu
in Y-Ba-Cu-O compound. They suggested that Tc is strongly
correlated with the size of paramagnetic moments of doped
elements and Tc suppression arises from the breakage of
conducting Cooper-pairs by d-electron scattering at the
paramagnetic site. Fig 3 shows the values of TC and suc-
ceptibility at T = 100 K for the different 3-d elements.

It can be seen that the large paramagnetic moment of Fe

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Fig. 1. The normalized (at 100 K) resistivity as
a function of temperature for 1:2:3 compound

doped with Cr, Mn, Ti, V, Fe and Zn (Ref 7).

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Fig. 2. Superconducting transition temperarture, T c
(midpoint), T (90% of the sigmoid) and T
(10%) in dopea 1:2:3 plotted against the fidpant
valence (Ref 7) .

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4 6 8 10 12
N (valence electrons)

T (solid line) and magnetic susceptibility at
180 K (dashed line) of 1:2:3 compound doped
with 3d element, as a function of the number
of valence electrons (Ref 5).

9
degrades Tc more strongly than other elements. For the

non-magnetic Zn which has the stronger influence on Tc than
Fe, they explained the Tc reduction in terms of the filled
3-d shell of Zn; i.e. the substitution of Cu by Zn provide
an extra electron for the divalent Cu shell (9 electrons),
which fill up the antibonding d-band and reduce the density
of state at the Fermi-level.

On the contrary, other authors [6,15] proposed that Tc
reduction is not necessarily of magnetic origin. For
instance, Maeo et al. [6] suggested that the valencies of
the doped ion or properties related to the valency such as
differences in ionic radii are more important in determin-
ing the impurity effect. However, a clear explanatin is not
yet available.

Addition of Ag in Y-Ba-Cu-O compound drew special
interests of many researchers since it had been shown to
enhance the critical current density and mechanical charac-
teristics while leaving other superconducting properties
unchanged [16-18]. Pavuna et al. [17] reported that super-
conductivity of Y-Ba-Cu-O compound is preserved even up to
50 wt% of silver addition and the magnitude of electrical
resistivity at 300 K rapidly decreases as the amount of
silver increases. Ag addition was also found to have an
effect on decreasing porosity, improving critical current

density, and reducing the contact resistance [18].

2-2 Sole Processing Techniques to Decrease Porosity

It has been found that it is difficult to obtain high
density of polycrystalline Y-Ba—Cu-O compound by solid
state sintering since_sintering temperature is limited by
phase stability and incipiant melting considerations.
Kumahura et al. [19] reported that although a drastic
reduction in porosity in Y-Ba-Cu-O compound occurred when
heat treated above 1030 C due to partial melting of the
sample, high temperature heat treatment caused the forma-
tion of a large amount of non-superconducting phases in
addition to YBaZCu307_x, and consequently degraded the
critical current density due to weak coupling between the
superconducting grains. Thus, several processing tech-
niques have been applied to make highly dense bulk Y-Ba-Cu-
0 compound and thereby enhance mechanical characteristics
and critical current density.

HIP (hot isostatic pressing) densifies powder
paticles by massive movement of material through hot defor-
mation instead of the diffusion of individual atoms as in
sintering. Consequently, rapid densification can be ob-
tained at relatively lower temperature and shorter times
than in sintering. Tien et a1. [20] reported that fully
dense Y-Ba-Cu-O compound can be obtained with many combina-
tions of pressure, temperature and time during HIP, and
furthermore the electrical and mechanical properties of the
superconductor can be improved by controlling the

mechanisms of powder deformation during HIP which allow the

10

ll

densification to take place through a chosen deformation
process.

The effect of hot isostatic pressing on transition
temperature of Y-Ba-Cu-O compound was investigated by
Sadananda et al. [11]. Fig 4 shows temperature dependence
of resistance before and after post-annealing for the
pressed samples. It can be seen that subsequent pressing,
although it compacted the sample to almost theoretical
density, reduced TO to 65 K as compared to 92 K after
initial sintering and annealing in the non-pressed sample.
They suggested that Tc reduction for the pressed sample
resulted from either some oxygen loss during encapsulation
of sample in vacuum or partial disorder occurred during
heating, and consequently appropriate post-annealing is
necessary to recover this Tc reduction.

Fabrication of Y-Ba-Cu-O superconductor by an oxide
melting method in place of conventional sintering was also
attemped by several authors [12,21,22]. Jin et al. [12]
reported that melt processing such as melt drawing or melt
spinning could be a convenient fabrication method for
obtaining superconducting wires or ribbons with greatly
improved density and electrical properties. Fig 5 is
magnetic succeptibility vs. temperature curve which shows
the effect of melt processing on transition temperature.
It can be seen that while as-sintered and solidified sample
is non-superconducting, superconductivity is fully

recovered after a homogenization heat treatment followed by

R/R(300K)

Fig.

4.

12

 

0.12

 

 

 

 

 

 

 

1 0.00

I I
60 80 100 120 140

T (K)

Resistance curve as a function of temperature
for different specimens. E1 correspond to
specimen after pressing and ElA correspond to

specimen after pressing and postannealing
(Ref 11).

13

x vs T (MELT-PROCESSED BozYCU307-8)

 

    

 

 

 

 

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TEMPERATURE (K)

Fig. 5. Magnetic susceptibility vs. temperature curve
for melt processed 1:2:3 compound (Ref 12).

14
oxygen heat treatment. Therefore, special post heat treat-

ment is essential in melt processing for recovering
superconductivity which disappears during high temperature
process.

It was also reported [22] that melt-textured growth of
Y-Ba-Cu-O compound from a supercooled melt produced an
nearly 100% dense structure consisting of locally aligned,
long, needle-shaped grains, and this new structure has a
sharply higher critical current density than the conven-
tionally sintered materials and furthermore much reduced
field dependence of critical current density. It was
suggested that the increase in critical current density for
the melt processed sample can be explained by three com-
bined factors such as (1) the formation of dense structure,
(2) grains alligned parallel to the a-b conduction band in
this anisotropically conductive layered compound and (3)

the formation of cleaner grain boundary area.

3. EXPERIMENTAL PROCEDURE

3-1 Sample Preparation

YBaZCu3O7_x powder was prepared from high purity

(99.99%) of Y203, BaCO and CuO powders. These powders were

3
weighed on a calibrated micro balance and mixed to have
Y:Ba:Cu cation ratio of 1:2:3. The powder mixed with
methanol was thoroughly ground with a pestle and mortar to
ensure complete mixing. The mixed powder was then'dried in
air to evaporate the methanol. The dried powder was placed
in an alumina crucible and calcined at 930 C (1203 K) in
air atmosphere for 24 hours. The calcined powder was
slowly cooled down to room temperature inside the furnace.
The YBaZCu307“x powder was again regrounded with a
Cu O

2 3 7-x
various amounts of Bi (99.99% pure) or PbO (99.9% pure)

pestle and mortar. In regrounded YBa powder,
powders were added to obtain (YBaZCu3O7_x)1_YAy, where

A : Bi and PbO, y = 5 wt% and 10 wt% for Bi and 5 wt%, 10
wt% and 15 wt% for PbO. The mixtures were uniaxially
pressed into one inch diameter pellets under a pressure of
10,000 psi and then sintered for 12 hours at two different
temperatures of 920 C (1193 K) and 950 C (1223 K) in air
atmosphere. The sintered pellets were slowly cooled down
to room temperature inside the furnace. For comparison,
the parent sample of YBaZCu3O7_x unmixed with Bi or PbO was

also made under similar processing conditions.

15

3-2 x-ray Measurements

For the structual analysis and phase identification,
X-ray diffraction experiment was carried out. X-ray dif-
fraction data was obtained by using Ni-filtered Cu-Ka
radiation with a Scintac XDS 2000 diffractometer. A tube
voltage of 36 kV and tube current of 25 mA was used for

these measurements. As-sintered surface was used for this

purpose.

16

3-3 Transition.Temperature Measurements

A continuous measurement of resistance as a function
of temperature was carried out by means of an auto balanc-
ing AC bridge with a lock-in amplifier using a standard
four-probe technique. Fig 6 shows the schematic diagram of
electrical resistance measurement set-up. A LR-400, four
wire AC resistance bridge and a Houston instrument 200, X-Y
recorder were used for these measurements. Liquid nitrogen
was used for cooling the sample.

Fig 7 shows a schematic of the four wire AC resistance
measurement system. In this diagram, four gold-plated pins
were attached to the test sample. The two outer ones were
for current supply and the inner two were for voltage
measurement.

A copper-constantan thermocouple was attached to the
center of the specimen. This kind of bimetallic joint
produces small electrical voltage and the exact value of
this small voltage varies with the temperature of the area
around the thermocouple junction. Therefore, temperature
was determined by converting this voltage to temperature by
using conversion table [23]. Contacts were made by insert-
ing the superconductor pellet and gold-plated pins between
two PVC blocks as was seen in Fig 8. All were then clamped
with two plastic clamps and were immersed in the liquid
nitrogen reservoir for cooling.

The electrical resistance was monitored by the LR-400

AC resistance bridge in the temperature range from room

17

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Fig. 6. Schematic diagram of resistance measurement
set-up.

19

LR-400

 

 

circuitry

 

 

 

 

 

 

 

/ Sample

I; I “ XI
/ \/ \

Current probe Voltage probe Current probe

 

 

 

 

 

 

Fig. 7. Schematic diagram of four wire AC resistance
measurement.

2O

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

‘— Stand
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____. "-~“~“‘_ .
__ *j Sample
L PVC block \ I
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PCboard Gold plated wire wrap pin

Fig. 8. Schematic diagram of sample holder assembly
for 4 wire AC resistance measurement.

2]
temperature to liquid nitrogen temperature (77 K). The
resolution of the machine was 1 micro-ohm and an excitation

current of 3 mili ampere was used for these measurements.

3-4 Density Measurements

The density of sintered samples was measured by using
the buoyancy method following ASTM B 328-73 [24]. First,
samples were cleaned with acetone and methanol and then
completely dried in air. After obtaining the dry weights
of the samples (weight, Wa), they were immersed in SAE 10W-
40 motor oil (viscosity of approximately 200 SUS at 100 F),
held for 6 hours at 182 C and then cooled to room tempera-
ture by immersion in oil at room temperature. This
procedure allows the oil to be impregnated through inter-
connected pores of the sintered samples.

After weighing oil impregnated samples (weight, Wb),
they were tied with a fine wire (0.09 mm in diameter) and
suspended from the beam hook of the balance. A distilled
water filled beaker and bridge was placed underneath the
beam hook and sample was completely immersed in the water.
The water covered the sample by at least a quarter inch
(6.4 mm). Care was taken to ensure that no air bubbles
adhere to the sample and the wire. The wired sample was
weighed (weight, Wc) and then wire without sample was
immersed in water again to the same point as before, and
was reweighed (weigh, We).

The density of the sample was calculated by the fol—

lowing equation:

23
where:
D = density, g/cm3,

A = weight in air of oil-free specimen, 9,

B = weight of oil-impregnated specimen, g,
C = weight of oil-impregnated specimen and wire in water,
9, and

E a weight of wire in water, g.

3-5 Mbrphological Examination and EDAX Analysis

3-5-1 Optical Microscopy

The microstruture was observed on the polished surface
of the sintered samples. Samples were mounted in lucite,
mechanically ground with abrasive grit papers of 240 to 600
grit and 600 grade emery paper and then polished on a cloth
using alumina powder and diamond powder of the size from 2
to 0.5 micrometer. Methanol was used instead of water in
order to prevent possible degradation of the superconductor
samples due to moisture. Photographs were taken at the

magnification of 500 and 2000K using Leco Neophot 21.

3-5-2 Scanning Electron Microscopy and EDAx Analysis

A Hitachi S-2500C SEM with Link energy dispersive x-
ray spectroscopy was used to observe microstructure and
presence of the additives in the samples. For this pur-
pose, fractured section of specimens were mounted on
cylindrical aluminum stubs and silver paint was used for

electrical contact to ground.

24

4. RESUDTS AND DISCUSSION
4—1 x-ray Diffraction Data

Fig 9 shows x-ray diffraction patterns for the
reference pure 1:2:3 compound and 1:2:3 mixed with PbO
sintered at 950 C. It can be seen that PbO 5 wt% mixed
sample shows almost identical x-ray pattern to the
reference pure 1:2:3 sample. However, some additional
peaks (the strongest one at around 30 degree) were found in
samples with more than 10 wt% of PbO. The intensity of
these additional peaks increases whereas the intensity of
peaks from orthohombic superconducting phase decreases as
the amount of PbO increases from 5 to 15 wt%. These addi-
tional peaks are considered to come from non-
superconducting phases which might result from the reaction
between 1:2:3 compound and PbO since they are not coinci-
dent with the x-ray diffraction pattern of pure PbO. This
was confirmed by observing Meissner effect; samples with
more than 10 wt% of PbO showed a weak Meissner effect as
compared to that of reference pure 1:2:3 sample, which
suggests that these samples contain a considerable amount
of non-superconducting phases.

A similar behavior was observed in PbO mixed samples
sintered at a lower temperature of 920 C as can be seen in
Fig 10. Once again, PbO 5 wt% mixed sample shows almost
identical x-ray pattern to the reference pure 1:2:35ample.

‘ However, in the case of samples with more than 10 wt% PbO,

25

26

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28
the intensity of additional peaks from non-superconducting
phase was decreased as compared to that of samples sintered
at 950 C. Therefore, it can be assumed that the reaction
between 1:2:3 compound and PbO is more active at 950 C than
at 920 C.

Fig 11 shows x-ray diffraction patterns for pure 1:2:3
sample and 1:2:3 mixed with Bi sintered at 950 C. It can
be seen that even 5 wt% Bi mixed sample shows strong addi-
tional peaks from non-superconducting phases. Therefore,

data on low temperature sintering, i.e. 920 C is not dis-

cussed here.

29

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4-2 Critical Transition Temperature Measurements

Table 1 shows various preparation conditions and
transition temperatures of the reference pure 1:2:3 and
1:2:3 samples with various amounts of Bi or PbO additives.
It can be seen that all the samples have transition tem-
peratures between 87 and 92 K and a transition width of 4
to 5 K. These measurements are coincident with the transi-
tion temperatures of 90 to 95 K for conventional 1:2:3
materials as reported by other researchers [1,5]. Although
a considerable amount of non-superconducting phases was
observed in samples with Bi or over 10 wt% PbO as seen in
x-ray diffraction data, these phases do not significantly
affect the transition temperature. However, it is expected
that presence of non-superconducting phases may degrade
bulk superconductivity such as critical current density and
Meissner effect.

The electrical resistance as a function of temperature
for reference pure 1:2:3 samples and samples with Bi or PbO
sintered at 950 and 920 C are shown in Fig 12 and Fig 13.
It can be seen that all the samples show metallic behavior
with a positive temperature derivative of resistance.
Furthermore, it can be seen that the room temperature
resistance increases with the amount of additives and the
sintering temperature. The increase in room temperature
resistance can be due to an increase of the amount of non-
superconducting phases which resulted from the reaction

between 1:2:3 compound and additives.

30

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4-3 Density Measurements

Table 2 shows the measured density and porosity of
reference pure 1:2:3 and PbO mixed samples. The porosity
was calculated from the ratio between the apparent and
theoretical densities. A theoretical density of 6.375
g/cm3 was adopted for YBaZCu3O7_x where x = 0 [25]. The
theoretical density of 1:2:3 mixed with PbO was calculated
by using the law of mixture. Density of Bi mixed samples
was not measured since x-ray data showed that even 5 wt% of
Bi addition produces significant amounts of non-
superconducting phases, which suggests that Bi is not a
suitable element to aid in sintering.

The measured porosity of about 16 to 19% for reference
1:2:3 samples are coincident with the values (15 to 20%)
for 1:2:3 sintered conventionally, without any precursor
powder preparation technique such as ball milling and
pulverizing [18,26]. As can be seen in the table, porosity
was improved by about 7 to 8% by the addition of 5 wt% PbO
as compared to that of reference pure 1:2:3 samples. The
measured value of abOut 10% porosity for PbO 5 wt% mixed
samples is comparable to 7% for sintering-aid doped 1:2:3
compound reported by Patel et al. [26]. However, a further

reduction_in porosity was not observed in other samples in

spite of the increase of PbO addition.

34

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4-4 Optical Microscopy

Fig 14 shows the optical micrograph for reference pure
1:2:3 sample sintered at 920 C. A significant amount of
pores can be seen in Fig 14a. The micrograph taken at
higher magnification (Fig 14b) shows plate-like grain
structure and superconducting grains which contain well
formed twin bands resulting from high temperature
tetragonal to orthohombic phase transition [27]. In addi-
tion to this superconducting phase, a small amount of non-
superconducting phase (marked by NS in Fig 14b) was also
observed between some grains, which resulted from off-
stoichiometry introduced during mixing process. This non-
superconducting phase does not contain twin bands inside
the grains.

The optical micrograph for 1:2:3 sample mixed with 5
wt% PbO presented in Fig 15 shows similar structure as pure
1:2:3 except for the reduction in porosity and somewhat
inoreased grain size. However, it can be seen in Fig 15b
that the amount of non-superconducting phase was increased
as compared to pure 1:2:3 sample. This might have resulted
from either off-stbichiometry or reaction between PbO and
superconducting 1:2:3 phase. Therefore, it can be assumed
that while the formation of these non-superconducting phase
decreases the porosity by filling pores, it can affect the
critical current density by separating superconducting

grains.

36

37

 

(b)

Fig. 14. Optical microscopy of pure 1:2:3 sample, sintered
at 920 C (a) 500x (b) 2000x.

38

 

(b)

Fig. 15. Optical microscopy of PbO 5 wt% mixed sample,
sintered at 920 C (a) 500x (b)2000x.

39

Figs 16 and 17 show optical micrographs for 1:2:3
sample mixed with 10 and 15 wt% of PbO, respectively. It
can be seen that the amount of non-superconducting phases
are further increased as compared to pure 1:2:3 and 1:2:3
mixed with 5 wt% of PbO.

Fig 18 shows the optical micrographs for 1:2:3 mixed
with 5 and 10 wt% of Bi sintered at 950 C. It can be seen
that the porosity was not improved with the addition of Bi
and significant amounts of non-superconducting phases were

found even with an addition of 5 wt% Bi.

40

 

(b)

Fig. 16. Optical microscopy of PbO 10 wt% mixed sample,
sintered at 920 C (a) 500x (b)2000x.

41

 

A

Fig. 17. Optical microscopy of PbO 15 wt% mixed sample,
sintered at 920 C (500x).

42

 

(b)

Fig. 18. Optical microscopy of Bi mixed samples, sintered
at 950 c (500x) (a) Bi 5 wt% (b) Bi 10wt%.

4-5 Scanning E1ectron.Microscopy and EDAx.Analysis

Scanning electron micrographs for reference pure 1:2:3
and PbO mixed samples sintered at 920 C are shown in Fig 19
and Fig 20. It can be seen in Fig 19 that some grains in
the PbO 5 wt% mixed sample show well-grown plate-like shape
while most of grains in the 1:2:3 sample show rounded
shape. In addition to this superconducting phase, a small
amount of second phase was also observed in PbO 5 wt% mixed
sample, which was identified as Pb-rich non-superconducting
phase by EDAX analysis. It can be seen in Fig 20 that the
amount of these non-superconducting phase is further in-
creased as the amount of PbO increases. These phases with
relatively small grain size and cloud-like shape Can be
eaily distinguished from the plate-like superconducting
phase.

Fig 21 shows EDAX spectrum obtained from pure 1:2:3
sample. Data label represent the intensity of elements in
terms of counts. From qualitative analysis, no other
elements than Y, Ba, Cu were detected. Figs 22 and 23 show
EDAX spectrum obtained from PbO 5 and 10 wt% mixed samples,
respectively. It can be seen that some additional peaks
from Pb were detected in addition to Y, Ba and Cu. The
intensity of these peaks increased as the amount of PbO
addition increased.

Fig 24 shows EDAX spectrum obtained from the selected
grain marked by arrow in Fig 19b, and Figs 25 and 26 show

EDAX spectrum obtained from the selected grains marked by

43

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WINDOW START END WIDTH GROSS NET EFF. SAGE
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BA 4.36 4.58 12 21327 14655 1.00 28.32
BA 4.72 4.98 14 14682 8487 1.00 ‘16.40
EA 5.08 5.26 10 5557 2257 1.00 4.36
8A 5.44 5.62 10 3183 973 1.00 1.88
CU 7.90 8.20 16 25689 21001 1.00 40.59
CU 8.78 9.06 15 5132 2777 1.00 5.37
Y 14.84 15.08 13 2264 646 1.00 1.25

Fig. 21. EDAX spectra obtained from pure 1:2:3 sample,
sintered at 920 C.

 

 

 

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BA 4.36 4.58 12 17764 12322 - 1.00 27.68
BA ‘ 4.72 4.98 14 12433 6910 1.00 15.52
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CU 8.78 9.06 15 4605 2393 1.00 5.37
PB 10.42 10.66 13 2411 721 1.00 1.62
PB 12.54 12.70 9 1106 188 1.00 .42
Y 14.84 15.08 13 2159 612 1.00 1.37
Fig. 22. EDAX spectra obtained from PbO 5 wt% mixed

sample, sintered at 920 C.

 

48

 

 

 

 

 

 

 

 

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Y 1.82 2.06 13 3372 1273 I 1.00 2.62
PB 2.22 2.54 17 8528 4882 .1.00 10.06
BA 4.32 4.60 15 25392 18050 1.00 37.21
BA 4.72 4.98 14 16201 8438 1.00 17.39
BA 5.08 5.26 10 6791 2016 1.00 4.16
BA 5.44 5.64 11 4822 1066 1.00 2.20
CU 7.90 8.20 16 16263 11351 1.00 23.40
CU 8.78 9.06 15 4367 1435 1.00 2.96
Fig. 23. EDAX spectra obtained from PbO 10 wt% mixed

sample,

sintered at 920 C.

 

49

 

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marked by arrow in Fig 19b.

50

 

 

 

 

 

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Fig. 25. EDAX spectra obtained from the selected grain

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51

 

 

 

 

 

 

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Fig. 26. EDAX spectra obtained from the selected grain
marked by b in Fig 20a

52
'a' and 'b' in Fig 20a respectively. Strong Pb peaks were
observed in all of these selected grains while these Pb
peaks were not detected in superconducting grains in these
samples. Therefore, it is considered that PbO reacts with
1:2:3 superconducting phase, which results in the formation
of Pb-rich non-superconducting phase. It can also be seen
that while grain marked by 'a' in Fig 20acontains all of
Y, Ba, Cu and Pb, grain marked by 'b' is extremly rich in
Pb with a very small amount of Cu. Thus, it can be assumed
that non-superconducting phases resulted from the reaction

between 1:2:3 and PbO do not have homogeneous chemical

composition.

5. SUMMARY

In this study, the effect of Bi and PbO addition in
the Y-Ba-Cu-O compound on reduction in porosity and the
superconducting properties was investigated. Bi and PbO
were selected as additives with an objective to employ
liquid phase sintering. Followings are the main conclu-
sions of this study:

1. Bi addition in the Y-Ba-Cu-O compound does not
reduce the porosity and the x-ray diffraction data shows
that even 5 wt% of Bi addition produced a considerable
amount of non-superconducting phases which resulted from
the reaction between Bi and 1:2:3 compound.

2. On the contrary, 5 wt% of PbO addition in Y-Ba-Cu-O
compound reduces the porosity by 7 to 8% without affecting
the transition temperature. EDAX analysis however shows
that PbO also reacts with 1:2:3 compound and forms a negli-
gible amount of Pb-rich non-superconducting phases.

1 3. While this improvement in porosity is expected to
enhance the mechanical properties, the formation of non-
superconducting phases might affect the critical current
density by separating superconducting grains. Therefore,
to observe the possibility of using PbO as a sintering aid,
some supplemental experiments such as critical current

density and mechanical test should be carried out.

53

6.

1.

2.

10.

11.

12.

13.

14.

15.

16.

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55

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