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784 1038
LIBRARY
Michigan State
University
k 1

 

This is to certify that the

thesis entitled

The Effect of Halogen Ion Substitution
on the Superconducting Properties of

Y—Ba-Cu-Oxygen System
presented by

Jaimoo Yoo

has been accepted towards fulfillment
of the requirements for

Master's degree in Material Science

 

 

  

/'{” Joe/5a
\.

Major profess

Date February 16, 1990

 

07639 MS U i: an Affirmative Action/Equal Opportunity Institution

 

PLACE IN RETURN BOX to remove this checkout from your record.
TO AVOID FINES return on or before due due.

    

DATE DUE DATE DUE DATE DUE

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSU Is An Afflrmdlve Action/Equal Opportunity Institution
emmh'

 

THE EFFECT OF HALOGEN ION SUBSTITUTION ON THE
SUPERCONDUCTING PROPERTIES OF Y-Ba-Cu-Oxygen SYSTEM

BY

Jaimoo Yoo

A THESIS

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

MASTER OF SCIENCE

Department of Metallurgy, Mechanics, and Materials Science

1990

ABSTRACT

THE EFFECT OF HALOGEN ION SUBSTITUTION ON THE
SUPERCONDUCTING PROPERTIES OF Y—Ba-Cu-Oxygen SYSTEM

BY

Jaimoo Yoo

Since the discovery of high critical temperature in the
Y-Ba-Cu—O compound, a number of investigators have suggested
that the critical temperature of this compound can be in
creased by halogen ion, such as F and Cl, additions.

These results still remain controversial. At Michigan State
University, a systematic study was undertaken to investigate
and evaluate iodine ion substitution in this system by using
CuI as iodine agent. It.was found that the iodine substituted
compounds of Y-Ba-Cu-O had a critical temperature of 100K and
an onset temperature of 110K. A small amount of iodine was
detected by EDAX analysis. In this thesis, method of sample
preparation, optimum composition and electrical properties of
these compounds are presented. Also discussed are microstruc-

tural aspects of this high temperature superconductor.

ACKNOLBDGBMBNTS

I would like to express my most sincere thanks to Dr.
K. Mukherjee and my parents for their constant support
and guidance during this research. I would like to thank Dr.
P.A.A. Khan for his valuable suggestions. I like to express
personal appreciation to my lovely wife Jungsook Kim for
her never ending love and encouragement. I would also like to
thank Mr. S.H. Nam, Mr. C.S. Kim, Mr. C.S. Jang, Mr. I.S. Oh

and Mr. C.W. Chen for their timely help and support.

ii

TABLE OF CONTENTS

LIST OF FIGURES

LIST OF TABLES

1.

2.

INTRODUCTION

LITERATURE SURVEY

Crystal Structure of Y-Ba-Cu-O Compound
Anisotropy Nature of Y-Ba-Cu-O Compound

Substitution of Cu with 3-d Metal
in Y-Ba-Cu-O Compound

Fluorine Controversy

EXPERIMENTAL PROCEDURE

Sample Preparation

Magnetic Separation and Sintering

Critical Temperature Measurement Set-Up
Critical Current Density Measurement Set-Up
EDAX and X-Ray Diffraction

Morphological Examination

3-6-1 Electron Microscopy

3-6-2 Optical Microscopy

RESULT AND DISCUSSION

Critical Temperature Measurement
Critical Current Density Measurement
EDAX and X-Ray Analysis
Microstructure of Iodine Substituted

1:2:3 Sample

iii

Page

vii

10

14

20
21
24
30

33

34

34

35
39

42

50

S.

6.

CONCLUSIONS

REFERENCES

iv

56

57

LIST OF FIGURES

Figure

1.

Crystal structure of YBflfithpx.
Dashed circles represent vacant
sites (Ref. 16)

Magnetization hysteresis loops at 4.5K
for a single crystal of 1:2:3 with the
Cu-O planes oriented (a) perpendicular
to the magnetic field and (b) parallel
to the applied field (Ref. 17)

Critical current densities deduced from
magnetization hysteresis as a function

of magnetic field applied either parallel

or perpendicular to the Cu-o planes (Ref. 17)

Temperature dependence of the normalized
resistance of YBaZCu(Cuo.9Ao_1)306.2 ,
where A = Cr, Mn, Fe, Co, Ni and Zn (Ref. 5)

Resistance vs temperature measurement of
nominal composition YBazCu3onz. Curve A
shows the resistance upon initial cooling:
curve B, data obtained upon second cooling;
and curve C, data from warming after second
cooling (Ref. 7)

Powder x-ray diffraction patterns of
fluoride doped samples of YBaZCu3Fsz.
Value of X: (a) 0.0165, (b) 0.066 (c)
0.825 (d) 1.65 (Ref. 8)

Schematic diagram of the magnetic separation
procedure. (a) Before doing magnetic
separation (b) Under moving magnet

Schematic of 4 wire AC resistance measurement

Schematic of (a) 4 gold plated wire
(b) with superconducting sample

10. Sample holder assembly for 4 contact

direct AC resistance measurement

11. Schematic diagram of resistance measurement

V

Page

11

15

17

22

25

26

27

12.

13.

14.

15.

16.

17.

18.

19.

20.

21.

22.

23.

24.

25.

vi
set-up
Schematic diagram of Jc measurement set-up
Temperature effect on resistance of
YBaZCu3Isz (X=1,2,3), where I, represents
YBaZCu3I1055 etc

The cooling and heating cyclic measurement
of the resistance of YBaZCu3I1055

Current-voltage curve of YBaZCu3I1055

The EDAX spectra of YBazCu3I105_5 sample
The EDAX spectra of YBaZCu3IZO45 sample
The EDAX spectra of YBaZCu3I3O35 sample

X-ray diffraction patterns of a) 1:2:3
sample b) YBaZCu3I105_5 sample

X-ray diffraction patterns of YBaZCu3IZO45
sample '

X-ray diffraction patterns of YBaZCu3I3O35
sample

Optical microscopy of YBaZCu3I105,5

Optical microscopy of a) YBaZCu3I2045
b) Y382CU3I3035

SEM micrograph of YBaZCu3I1055

SEM micrograph of a) YBaZCu3I2045
b) YBaZCu3I3O35

29

31

36

38

4o

43

44

45

46

48

49

51

52

53

54

LIST OF TABLES

Table Page

1. I}, Measured Susceptibility at 100K
and Calculated Values of Magnetic Moment
(Ref. 5) 13

2. Phase Distribution in Samples with Nominal
Compositions of YBaZCu3FxClyOz (Ref. 9) 16

3. Critical Temperature and Transition Width
of Composition of YBaZCu3Fsz (Ref. 8) 18

4. The Superconductivity Data of Nominal
YBaZCU3FxC1102 (Ref. 9) 18

vii

1. INTRODUCTION

In 1987, Wu et a1. [1] first found a high critical
temperature (I; = 90 to 95K) superconductor, based upon the
replacement of La by Y of a RBaZCu307 (R represents rare
earth material). This has generated widespread experimental
and theoretical research work related with high values of
IQ. A number of researchers have tried various element
substitutions in Y-Ba-Cu-O compound, to further increase the
critical temperature. Substitution in Y sites of Y-Ba-Cu-O
compound by rare earth elements, such as Gd, or substitution
of Ba with Sr, do not affect critical temperature [2,3], be-
cause, the rare earth element sites and Ba sites are not
much involved in superconductivity of Y—Ba-Cu-O compound.

Several authors have reported that substitution of Cu
with 3-d transition metals such as Ni, Zn or Co can sharply
reduce critical temperature [4,5,6]. A clear explanation of
why reduction in 1} takes place when substitutions are made
for Cu, was not provided.

Recently, it was reported that fluorine substituted Y-
Ba-Cu-O compound has a very high critical temperature [7]
(up to 155K). Other researchers, however do not support the
claim that higher critical temperature is obtained by F or
Cl substitution [8,9]. Narottam et al.[8] reported that
critical temperature was slightly increased and the transi-

tion behavior was sharpened with low fluorine concentra-

2
tions. Other paper which investigated fluorinated and chlor-
inated Y-Ba-Cu-O compound, using Ban and BaClz as the
fluorination and chlorination agents, reported no such
increase in T}. To the contrary, they reported that higher
substitution levels of fluorine and chlorine sharply decrea-
sed Tc [9]..

In the present study, a systematic investigation was
undertaken to evaluate the effects of iodine ion substitu-
tion in Y-Ba-Cu-O compound. In this study, CuI is used as
the iodine agent. Copper iodide was selected, for addition
to Y-Ba-Cu-O compound, because of the following reasons:

1. Among the halogen elements, iodine has the highest diama-
gnetic susceptibility and thus it was speculated that this
element might contribute to the net diamagnetism associated
with the superconducting phase.

2. Copper source originated from the dissociation of CuI is
available to form Cu-O chain which is important for superc-
onductivity.

3. The melting and boiling temperatures of CuI are such that
at the Sintering temperature of the superconducting oxide,
no adverse effect is produced.

In this investigation, The critical temperature and
current density were measured by using a standard four point
technique and optical and scanning electron microscopes were
used to study microstructural aspects of this high tempera-

ture superconductor. Furthermore, EDAX analysis was used to

3
determine the presence of iodine in 1:2:3 superconducting

compound in which various proportions of CuI were added.

2. LITERATURE SURVEY
2-1 Crystal Structure of Y-Ba-Cu-O Compound

Though the structure of Y-Ba-Cu—O compound, based on x-
ray diffraction and neutron diffraction studies, is well do-
cumented [10-14], the mechanism of conductivity in high cri-
tical temperature superconductors is not yet clearly under-
stood. The superconducting phase YBafihuOrx (subscript X
represent unknown oxygen content) is an orthorhombic, oxygen
deficient, perovskite like structure which can be visualized
as a stack of alternating perovskite structures: (BaCuog):
(YCuO3) : (BaCuO3) [14,15]. Compounds with the perovskite
structure have the formula of A803, where A is the rela-
tively large ion in the center of unit cell, B is a small
ion at the corners and O is the oxygen ions at the edge of
the unit cell [15].

However, there are some differences between Y-Ba-Cu-O
structure and the three stacked perovskite structure as men-
tioned above. In a perovskite structure, copper ions are lo-
cated in the center of an oxygen octahedron. In YBagngOrx
structure, copper ions occupy two symmetry sites [13,16].
Copper ions are located in approximately the (003) plane and
have five oxygen neighbors due to the vacancies of oxygen
atom in the (002) yttrium atom plane. The Cu-O polyhedron is

a square pyramid with the copper ion positioned slightly

5
above the Cu plane as shown in Fig. 1. Copper ions in the
basal plane have only four oxygen atoms due to the vacancies
of oxygen atom in the basal plane.

Fig. 1 presents the Y-Ba-Cu-O crystal structure deter-
mined by neutron diffraction and x-ray studies [10-14]. Unit
cell dimensions for this structure at room temperature are
a=3.8187A, b=3.8833A, C=1l.6687A [13]. In the (002) yttrium
atom plane, all oxygen sites are vacant. In the basal plane,
the vacancies of oxygen atom along the a axis lead to an

orthorhombic deformation (a<b=c/3).

 

 

Fig. 1. Crystal structure of YBaZCu3O7_x.
Dashed circles represent vacant
sites (Ref. 16).

2-2 Anisotropic Nature of Y-Ba-Cu-O Compound

The anisotropic behavior of single crystal YBafithrx
has been studied by magnetization of the crystal with the
field applied both parallel and perpendicular to the Cu-O
planes by several investigators [17,18].

Fig. 2 shows magnetic hysterisis loops at 4.5K for the
Cu-O plane perpendicular to the field lines [Fig. 2(a)] and
parallel to the field lines [Fig. 2(b)] [17]. In Fig. 2(a),
actually the induced screening currents flow parallel to the
Cu-O plane. Fig. 3 shows the field and temperature depen-
dence of the critical current density determined from the
hysterisis curves in Fig. 2. At low temperatures, in fields
up to about 40KG, critical currents along the directions of
Cu-O planes were in excess of 106 A/cm2 [17]. These critical
current and critical field measurements of 1:2:3 single
crystals demonstrate a strong anisotrpy with the good con-
ducting directions being along the Cu-O planes which is also
supported by other papers [19,20]. Chaudhuri et a1. first
reported that high critical current density in excess of 10S
A/cmz was found in epitaxially grown films of YBaZCu307 with
c axis primary perpendicular to the plane of the film [21].
The current was flowing in the Cu-O plane. This practical
application and magnetization of 1:2:3 single crystal measu-
rements revealed strong anisotropy in critical current by a

factor of about 10.

 

0-04 I *r I I r 1 I r

(102

 

 

 

 

 

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Applied Field (k6)

Fig. 2. Magnetization hysteresis loops at 4.5K for
a single crystal of 1:2:3 with the Cu-O
planes oriented (a) perpendicular to the
magnetic field and (b) parallel to the
applied field(Ref. 17).

 

   

 

 

 

 

 

  

 

 

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Applied Field (kG)
Fig. 3. Critical current densities deduced from

magnetization hysteresis as a function of
magnetic field applied either parallel or
perpendicular to the Cu-O planes (Ref. 17).

2-3 Substitution of Cu with 3-d Metals in Y-Ba-Cu-O

Compound

Many investigators [13,17,18] have implied that Cu-O
chain is essential for the high temperature superconduc-
tivity. Thus, the effect of replacement of Cu with 3-d tran-
sition metals, could produce a substantial change in super-
conducting properties.

The superconducting properties of the mixed compound
YBa2(Cu3.xAx)O(,.z, where A = Ti, Cr, Mn, Fe, Co, Ni, Zn and Z =
unknown oxygen contents, were investigated by several resea-
rchers [4,5,6]. The ionic size and orbital structure of the
3-d transition metal are close to those of Cu; they expected
that 3-d transition metal will occupy the Cu sites if tran-
sition metal are substituted into 1:2:3 structure. Fig. 4
shows the temperature dependence of the normalized resis-
tance for several samples. Substitutions by 3-d transition
metals produce a reduction in Tc which is dependent on the
substituted elements. It can be seen that for A = Ni, Fe,
Co, Zn, the behavior is semiconductor like and the conduc-
tion electrons appear to become weakly localized before
superconductivity starts [5].

1; drops very sharply as 10% of the Cu is substituted
by Zn as shown in Fig. 4. Gang Xiao et al. [5] proposed a
possible explanation. The divalent Zn ions have ten 3-d
electrons and thus the 3—d state is very stable, compared

with the divalent Cu ion which has 9 electrons. Substitution

10

11

 

 

l.5 r g\\ 1

.[\;~\ 4

1.2M} .
Zn :

P Cm Fe ‘ I, ‘

RtT)/R(300 K)

 

 

 

 

0.8” . : IaIII"-"_ a
F : 5 Ni. :Ti

5 5 - .'. Cr l

0.4L g f 1

r 5 5 . 5 5 YBa2‘c"0.9A0.1)3°6+z i

OLLJ: I) J

0 100 200 300

'T (K)

Fig. 4. Temperature dependence of the normalized
re51stance of YBa2(Cu0 9A0 1)30I61

2!
where A = Cr, Mn, Fe, Co, Ni, a a Zn (Ref. 5).

12

of Cu by Zn provides an extra electron which fills up an-
tibonding d band and reduces the density of states (EF) at
those sites [5]. Tarascon et al. have suggested four inter-
related explanations; structural disorder, oxygen vacancy,
the dopants inducing a different oxidation state in the
copper and magnetic depairing, for the reduction of I} with
3-d metal substitution in the Cu-O planes [4].

Though a clear explanation is not available yet, it is
generally believed, from the Arbrikosov and Gorkov theory
[4], that the direct interactions of magnetic ions with the
superconducting electrons breaks the cooper pairs and reduc-
es critical temperature. The reduction relation between Tc
and magnetic impurity concentration looks like coincedence
with Arbrikosov and Gorkov's theory qualitatively and large
local paramagnetic moment in the mid 3-d elements as shown
in Table 1 prefers magnetic depairing [5].

However, the supression of’Tg due to magnetic scatter-
ing in this class of metals is not systematic. Especially,
I; reduction effect for substitution of Cu with Co is larger
than that of Fe, eventhough the magnetic moment of Fe is

larger than that of Co.

13

Table 1

T , Measured Susceptibility at 100K and

Calgulated Values of Magnetic Moment (Ref. 5)

 

 

I/q-

z... (100 K)

 

T, (K) X IO3 cmu/mol PA
Ti 75.0 0.4l9 0-0.45
Cr 84.5 0.988 == 0
Mn 78.9 1.821 1.35-2.01
Fe 38.0 5.004 3.29-3.61
C0 2l.2 4.735 2.87-3.20
Ni 66.3 1.923 1.40- 2.05
Cu 94.1 0.490 0.22
Zn < 3.0 1.378 0.49'

 

 

2-4 Fluorine Controversy

There are many reports of critical temperature above
150K in Y-Ba-Cu-O compound [22,23]. Ovshinsky et a1. claimed
superconductivity at 155K for the first time in bulk 1:2:3
material [7]. They fabricated five element compound YBagnr-
3Ffih (subscript Z represent unknown oxygen contents) by
substituting part of oxygen with fluorine. The temperature
dependence of the resistance of a portion of X=2 sample is
shown in Fig. 5. During heating and cooling (cyclic) measu-
rements, the critical temperature was reported to vary
between 168K -148K.

However, Yan et al., who investigated fluorinared
1:2:3 sample, reported that they didn't find such a high
critical temperature [9]. Table 2 shows crystalline phase
distribution in the sintered composition as observed by them
[9] . The concentration of the YBaZCu3O-, superconducting
phase, in the fluorinated samples, decreased with nominal
fluorine concentration. This data is consistent with X-ray
diffraction measurements for fluorinated 1:2:3 materials as
reported by Narottam et a1. [8] and shown in Fig. 6. From
Fig. 6, it can be seen that samples with X 5 0.066 contain
nearly 100% superconducting orthorhombic perovskite phase.
However, compositions with X 2 0.165 are multiphase, and
consist of BaF}, CuO, and other unknown phases in addition

to the superconducting perovskite phase [8]. From Table 2,

14

15

 

 

 

 

 

3i 2.,
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g! l
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1 .

r e

O
O
”“150 '1'75 A A 9200* K
TEMPERATURE

Fig. 5. Resistance vs temperature measurement of
- nominal composition YBa Cu F 02. Curve A
shows the resistance upon initial cooling;
curve B, data obtained upon second cooling;
and curve C, data from warming after second
cooling (Ref. 7).

 

16

 

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18
it can be seen that incomplete decomposition of BaF} leads
to inhomogeneous phase formation as the amount of Ban
increases [9]. Since BaF} remained intact at the sintering
temperature, part of the barium source was unavailable to
form the superconducting phase.

Although Narottam et a1. [8] find no proof for the very
high Tg's reported by Ovshinsky et al. [7], their results
indicate that superconducting critical temperature is both
slightly increased and sharpened with low levels fluorine
concentration. Data presented in Table 3 indicates that the
value of'Tg increases systematically with concentration of
F, reaches a maximum value of 93.4K for X = 0.066, and then
drops with further increase in fluorine concentration. The
transition is sharpest for X = 0.066 and the 10 to 90% tra-
nsition is within 0.7 i 0.1K of the T; for this composition
[8].

In Table 4, critical temperatures reported by Yan et
a1. [9] for F and Cl substituted samples are shown. Most
samples showed diamagnetic susceptibility starting at a
temperature between 89 and 94K. For the samples of high
fluorine concentration,X=4, diamagnetic susceptibility was
not observed above 4.2K. It is interesting to note that the
samples with a relatively high fluorine concentration (i.e,
X=2) still have a critical temperature of 93K, though the
superconducting phase is reduced to 15% at that substitution

level as shown in Table 2.

19

Table 3

Critical Temperature and Transition Width
of Composition of YBaZCu3FXOz (Ref. 8)

 

 

 

7:. (midpoint) A7: ( l0-90%)

Sample Value nix (K) ( K)

Yl 0.0105 90.8 0.9

Y2 0.033 92.1 0.8

Y3 0.050 92.5 0.8

Y4 (1060 934» ()7

Y5 0J05 912 O8

Y0 0.330 91.0 [.4

Y7 0.825 90.5 3

Y8 L05 903 14

 

 

Table 4

' The Superconductivity Data of Nominal
YBaZCu3FXC1YOZ Compositions (Ref. 9)

 

 

4‘ 7200* Tolcl" TM 2: 0)
0 0 94 94 92

l ' 0 94 93.8 9|

2 o 93 93.3 89.6

4 0 < 4 - ' T

0 l 92 94.3 88.4

o 2 < 4 l

O 4 < 4 T

4/3 4/3 89 93.0 < 79

l l 93 93.8 89.8

 

3. EXPERIMENTAL PROCEDURE
3-1 Sample Preparation

Samples were prepared with three different composi-
tions, i.e., nominal compositions of YBaZCu3Isz (where
X = 1,2,3 and Z = 5.5, 4.4 and 3.0 respectively ) by mixing
stoichiometric quantities of high purity YZO3, BaCO3, CuO and
CuI powders. The powders were weighed on a calibrated met-
tler micro balance.

The powdered mixtures were first mixed in a pestle and
mortar, the mixed powder was then placed in an alumina
crucible and calcined in oxygen atmosphere at 930°C (1203K)
for about 12 hours. The furnace was then turned off and the
calcined mixtures were allowed to cool slowly with the
furnace door closed. The resulting powder had a dark gray to
black appearance like a lump of charcoal at room tempera-
ture. The calcined powder was reground and recalcined under
similar conditions to achieve a well calcined compound. The
chemical reaction for calcination and sintering of the three

compounds used in present study are the following:

*5Y203 + ZBaCO3 + ZCUO 4' C111 Y332CU3I1055 + ZCOz (1)

2.11203 + 213aco3 + CuO + 2CuI = YBaZCu3IZO‘5 + 2c02 (2)

5.11203 + ZBaCO3 + 3CuI urea-$113130}.s + mo; (3)

20

3-2 Magnetic Separatiton and Sintering

After double calcination, magnetic separation was
carried out to enrich the superconducting powders under
liquid nitrogen temperature.

The magnetic separation procedure is summarized in Fig.
7, where the double calcined powder was placed at one end of
a pyrex beaker. The pyrex beaker was then cooled in liquid
nitrogen which was stored at larger pyrex vessel and then a
permanent magnet was moved over the pyrex beaker. The super-
conducting phase, pushed away by the magnetic field, accumu-
lated at one end of pyrex beaker. The non-superconducting
phase, unreacted powder, does not respond to the field and
remains stand still. During the magnetic separation, very
fine particles of calcined powders were not responsive to
the field and stuck together with some moisture. Barsoum et
al. [24] reported another possible problem related to separ-
ation, namely the very fine impurity can be embedded in the
relatively large superconducting particles and are carried
along with the superconducting powders. To prevent very fine
impurity from getting embedded and to obtain further purifi—
cation, the magnetically separated superconducting particles
were grounded and separated again under same conditions, as
mentioned above.

As discussed later, the magnetic separation plays a

possible role in removing impurity as judged from x-ray dif-

21

22

 

 

 

 

 

 

 

 

 

 

 

 

 

 

?/ Liquid nitrogen

 

 

 

a)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Moving magnet

b)

Fig. 7. Schematic diagram of the magnetic separation
procedure. (a) Before doing magnetic separation
(b) Under moving magnet

23
fraction studies.
The magnetically separated powders, mixed with a bind-
er, were cold pressed into one inch diameter pellets at a
pressure of 10,000 psi and then sintered at 950°C (1223K)
for 12 hours in an oxygen atmosphere. Amyl acetate was used
as the binder. The sintered pellets were slowly cooled to

room temperature in the furnace by turning the furnace off.

3-3 Set-Up for Critical Temperature Measurement

A continuous measurement of temperature dependence of
resistance was carried out by means of a LR-400, four wire
AC resistance bridge and a Houston instrument 200, X-Y
recorder. Liquid nitrogen placed in a deep, wide mouth dwer
flask was used for cooling the sample.

Fig. 8 shows a schematic of the 4 wire AC resistance
measurement compound. Four gold plated wire pins were atta-
ched to the sample. The two outer ones were used for current
supply and the inner two were used for voltage measurement.

Four standard gold plated wire wraped socket pins, laid
against one face of the sample, as shown in Fig. 9, were
used to make contact for measuring the AC resistance of the
superconducting sample. A copper-constantan thermocouple was
attached to the center of specimen. The sample and pins were
wrapped between two polyvinylchloride (PVC) blocks. All were
then clamped together with two small plastic clamps, as
shown in Fig. 10. The entire assembly was immersed very
slowly in the liquid nitrogen flask.

Temperature was monitored by a digital thermometer. The
uncertainity in the temperature measurement was estimated to
be approximately 1 0.5K.

The electrical resistance was monitored by using the
LR-400 AC resistance bridge in the temperature range from

room temperature to liquid nitrogen temperature (77K). The

24

25

 

 

 

 

 

 

 

 

 

 

 

 

LR-4OO
I high
/\
V high
Preamp
_—_d/\/\V low circuitry
I low
”0 A

 

 

 

 

 

 

 

Contact resistance

 

 

Bulk AC resistance

 

Superconducting sample

Fig. 8. Schematic of‘4 wire AC resistance measurement

26

Cu over trace on P C board

 

 

 

 

 

 

 

 

 

 

{I \ J \l/ .1 inch spacing
pin to pin
_> LCl 4 /|\
('31- J

 

C J J 4 gold plated wire
. V 4\ wrap pins
Insulated Cu wire ’ l‘

Printed circuit board section

 

 

 

 

 

Face of

/ superconducting

 

 

Insulated Cu wire

 

 

P C board

13)

Fig. 9. Schematic of (a) 4 gold plated wire
(b) with superconducting sample

 

27

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Stand

Superconducting

sample
j
/“x
P V C block
Gold plated wire wrap pin

P C board Clamp

Fig. 10. Sample holder assembly for 4 contact
direct AC resistance measurement

28
resolution of that machine is 1 micro ohm. The excitation
current used for this study was 3 mili ampere. Fig. 11 shows

the schematic diagram of electrical resistance measurement

set-up.

29

 

 

 

 

 

 

 

 

 

 

 

AC resistance

bridge

 

 

 

 

 

Sample cooling
heating system

 

 

 

 

 

 

 

 

Digital thermoter

 

 

 

 

 

X-Y recorder

 

 

 

 

 

 

‘

Fig. 11. Schematic diagram of resistance measurement

set-up.

 

3-4 Set-Up for Critical Current Density Measurement

The YBaghgloij samples were cut into rectangular bars
with cross sectional area of 0.01tm3 by using a diamond
blade. The contacts sulfaces of the cut samples were painted
with silver. After drying for one hour, leads were attached
to the silver contacts by using the same silver paint.

Upon measuring critical current density, it was ob-
served that the contacts were partially melted due to high
contact resistance resulting from poor contact. Thus, a set
of new samples were prepared under same condition except for
the contact method. Silver paint contacts were painted on
the top surface of sample and then the samples were annealed
at 950°C for 3 hours in the furnace in order to reduce the
contact resistance. Recent studies indicate that the silver
could fill the pores of surface contact and lower the inter-
grain resistances during high temperature sintering [25,26].

After 3 hours of heat treatment, the samples were
slowly cooled to room temperature in the furnace. Leads were
attached to the heat treated silver contacts by using the
same silver paint, as discussed.

The critical current density, J} ,measurement was per-
formed by using a conventional two probe technique. J; is
determined by the sudden voltage rise due to the increase of
resistance associated with the superconducting to normal

state transition at the current density equal to Jg. Fig. 12

3O

31

 

 

DC power
supplier

 

 

 

 

 

.1 ohm

 

 

 

 

 

 

 

 

 

 

 

{is

Liquid
nitrogen

 

‘nC/-r~«rxv~—

 

 

Fig. 12.

 

X-Y recorder

 

Schematic diagram of JC measurement

set-up

 

32
shows a schematic diagram of the critical current density
measurement set-up.
The critical current density values were measured at
liquid nitrogen temperature (77K). The applied current range

was from 0.1 to 6 ampere and the critical current density

was calculated to be Amp/sz.

3-5 EDAX and X-Ray Measurement

To confirm the presence of iodine in samples, EDAX
analysis was carried out. A Tracor 5500 EDAX system was used

for this purpose.
X-ray diffraction experiments were conducted by using

Cu Ka radiation. sintered pellet samples were used for this

purpose.

33

3-6 Morphological Examination
3-6-1 Scanning Electron Microscopy

Transverse fractured sections of specimens were mounted
on cylindrical aluminium stubs. To avoid surface charge, the
mounted sample surfaces were coated with pure gold by argon
sputtering of gold target at a pressure of 0.1 torr. Silver
paint was used for electrical contact to ground and also to
provide a better mechanical support.

A Hitachi 415C scanning electron microscope was used

for microstructural investigations.
3-6-2 Optical Microscopy

The samples were metallographically mounted on Lucite
blocks and initially ground on 600 grade emery paper. For
further polishing, a 5 micron alumina powder, dispersed in
methanol (in stead of distilled water) was used to prevent
any possible degredation of the superconductor due to mois-
ture up take. The samples were polished on a nylon cloth,
using diamond paste of the size from 2 micro to 0.5 microme-
ter. Photo micrographs were taken at magnifications as high

as 2000X.

34

4. RESULTS AND DISCUSSION

4-1 Critical Temperature Measurement

The temperature effect on electrical resistance for
samples with iodine stoichiometry of 1,2 and 3 are shown in
Fig. 13. It is observed that the resistance of composition
with 1 iodine showed zero resistance state at a temperature
of 100K and onset of rapid loss of resistance occured at
110K.

The samples with iodine stoichiometries of 2 and 3 pos-
sess a slightly higher critical temperature than that for
the conventional 1:2:3 materials:(97K for iodine 2 sample,
96K for iodine 3 sample). The maximum critical temperature
of conventional 1:2:3 samples was reported to be in the
range of 90 to 95K [1,27]. Our own measurements of Tc (using
our measuring system) also indicate a I; of 93 to 95K. The
overall shape of the curve is similar to a metallic to
"semiconductor-like" transition which is often found when a
3-d transition metal is substituted in Y-Ba-Cu-O compound.

It should be noted that a high substitution level of
iodine (eg. iodine 3) does not degrade the superconductivi-
ty, unlike F and Cl substitutions. To the contrary, it was
observed that even at a level of iodine 3, the critical
temperature is slightly higher than that for no iodine

substitution.

35

36

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can m . m . aux muons: ~oansO~mm> mo mcofifimomeoo
Hmcflao: mo oocmumflmou co vooumo ousuuuomfioa .ma .mwm

A a v 333383.

 

 

 

 

33.:
. . _ 2 ME.
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37

In order to determine reproducibility of results,
resistance measurements conducted with consecutive cooling
and heating cycles. Result from one such measurements for
composition of iodine stoichiometry of 1 is shown in Fig.
14. Upon cooling, the resistance starts to decrese gradually
and then decrease abruptly at 110K. The sample with one
iodine looses almost 60% of its resistance at this onset
temperature and finally zero resistance state is reached at
100K. Upon heating, the sample remains in the zero resis-
tance state up to about 102K and then the resistance begins
to increase very sharply and the normal state is reached at
110K. After coming back to room temperature, a very small

hysteresis of resistance was observed.

38

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mo ucoemusmooe oflaoxo mcflumo: Ucm mcflaooo one .vH .mflm

A s v 33209809

 

 

 

 

 

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4-2 Critical Current Density Measurement

The critical current density measurements were per-
formed at liquid nitrogen temperature (77K). The current—
voltage curve-of sample YBaflthOi5,'which is rectangular bar
with cross sectinal area of 0.01Cm5, is shown in Fig. 15. It
can be seen that the overall shape of current-voltage curve
is similar to that of a conventional 1:2:3 compound except
for the increase in voltage due to contact resistance.
Ideally, the base line should be nearly parallel to the x-
axis.

As the applied current increases, first the voltage
increases gradually and then rises abruptly at the limiting
value of current, which is defined is the critical current
for this sample. Measured value of critical current density
is found to be around 320 Amp/cm?.'This value is slightly
higher than that for the bulk YBagthrx samples without any
special sample preparation techniques, i.e., field oriented
grain method, or texture formation etc. [25,28]. As stated
earlier, samples with silver contacts, which are not heat
treated and thus have high contact resistance, show either
partially melted or broken contact. These high contact
resistance samples initially show an abrupt rise in voltage
on a current-voltage (I-V) curve. Thus for the best value of
J}, the contact resistance must be reduced as much as pos-

sible.

39

40

 

 

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A<v_

 

ma

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and

d v.0

v90

and

(Aux)

41

The heat treated silver contact samples give us the
most reasonable critical current value, even though these
samples still have a small contact resistance(0.08 miliohm).
The best measured I-V characteristics are shown in Fig. 15.

Chen et a1. [25] reported that the silver essentially
fills the surface pores of granular YBafithrm samples as
judged from their microscopic studies. This pore filling
improves the contact between grains and hence reduces the
contact resistance.

It is interesting to note that the measured value of Jc
for iodine 1 samples is 320 Amp/cm2 and this value is com-
parable to that for silver oxide substituted 1:2:3 compound

repoted by Peters et al. [26].

4.3 EDAX And X-Ray Measurements

Fig. 16 shows the result of energy dispersive X-ray
analysis for the YBafinqul5 sample, and fig. 17 and 18 show
the EDAX spectra from I; and I3 compositions respectively.
Data label represent the intensity of elements in terms of
counts. These results are qualitative only since no correc-
tions (such as absorption correction etc.) were made. It
was, however, observed that the number of counts of iodine
systematically increased, in the ratio of 362: 563: 709
which is approximately 1: 1.5: 2 as the concentration of
iodine increased. It is not known why the small peak of
sulfur (which is around 2.3keV in Fig. 17-18) was detected
for Izland 13 compositions while that peak was not detected
for I, sample. Sulfur, however, is known for improve superb
conducting properties of 1:2:3 compound such as the critical
field, and the sharpness of transition temperature range
[29] .

Fig. 19.(a) shows the x-ray diffraction patterns of
1:2:3 compound and 19.(b) shows that for the I1 sample.
Comparing x-ray diffraction patterns for 1:2:3 compound and
the I. sample, it is observed that all the peaks are similar
except that the low angle back ground intensity of the mag-
netically separated iodine 1 sample is noticeable less. It

can be considered that the magnetic separation helps to

42

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43

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GROSS:

GROSS:

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NET=

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Fig. 16. The EDAX spectra of )(Ba2Cu9’IO5.5 sample

S466

362

18096

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44

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Cursor: 0.0‘Q®ke'v‘ = O ROI (4) 0.0003 0.000

 

 

 

 

 

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LABEL:EXEC(7—C) DATA LABELI-E

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Fig. 17. The EDAX spectra of YBAgnhlghj sample

 

45

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Cursor: 8.888kev = 8 ROI (:3 7.838: 8.288

 

 

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Fig. 18. The EDAX spectra of YBagh%I§%5 sample

Intensity

Intensity

46

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

100

Fig.

19. X-ray diffraction patterns of a) 1:2:3 sample
b) YBaZCu3I05.5 sample

47
remove noncrystalline(glassy) or microcrystalline particles,
which is unseparated samples contribute to the low angle
back ground intensity. Fig. 20 and 21 show the x-ray diffr-
action patterns of I; and I3 samples. Once again magnetical—
ly separated samples show lower back ground intensity at

lower angles.

48

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ma

cow

0:

 

 

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49

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032mm enoansosmmcfl mo mcuwuuma cofluompmmwo >muux .3. .8:

cop o—p

 

 

2... g a. i, €55.

 

 

 

 

 

 

 

 

 

 

 

 

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4-4 Microstructures of Iodine Substituted 1:2:3 sample

The optical micrographs and SEM micrographs of YBaghg-
lg» (X=1,2,3 and Y=5.5,4.5,3.5 respectively) are shown in
Fig. 22-25. The microstructure of iodine substituted 1:2:3
samples show well formed twinning within the grains result-
ing from tetragonal to orthorhombic transformation. It has
been suggested that the role of these twins is to provide
strain accomodation during transformation which is of the
martensitic type [30]. Optical micrographs show relatively
thick and irregular twins in some grains as shown in Fig.
22. Comparative transformation twins in 1:2:3 samples are
much finer.

A correlation between these twins and superconducting
properties, such as Tc and Jc, has been suggested by many
authors but the subject is controversial [20,31]. Deutscher
et al. [20] claimed that twin boundaries play a role in
blocking supercurrent, supported by single crystal current
density measurements [17] and other magnetization measure-
ments of powdered 1:2:3 materials [32]. They explained that
for currents flowing in the Cu-O plane, currents do not have
to cross (001) twin boundaries while currents must cross
both (001) and (110) twin boundaries for currents flowing
perpendicular to the Cu-O plane. However, Chauhari et al.
claimed that twin boundaries act as strong pinning sites for

the flux lines [31], and thus improve Jc.

50

51

 

Fig. 22. Optical microscopy of YBaZCu3I05_5

52

 

117)

Fig. 23. Optical microscopy of a) YBa2Cu3I204~5
b) YBa2Cu3I303j

53

 

Fig. 24. SEM micrograph of YBa2Cu3I055

54

 

\.
@349 15K 2%mfiun
b)

Fig. 25. SEM micrograph of a) YBaZCuzI204.5
b) YBa2Cu3I3035

55

It can be speculated from our results being a help in

 

reducing barrier to supercurrent flow. Although that theory
can not explain weak Josephson junction between grain boun-
daries, it might explain the existence of a small fraction
of particles (internal boundary free) with have higher than

average Tc.

5. SUMMARY

In the present study the superconducting properties of
iodine substituted Y-Ba-Cu-O compounds were investigated
using CuI as the agent for iodine substitution. The critical
temperature and critical current density measurements, the
EDAX and x-ray analysis and the SEM and optical microscopy
examinations were employed to study the electrical proper-
ties, to determine the iodine stoichiometry and to examine
the microstructures of these compounds respectively. Follow-
ing are the main conclusions of this study:

1. EDAX analysis show that the relative intensity of
iodine in the lattice increases as the nominal concentration
of iodine increases in these compounds.

2. Unlike Fluorine and Chlorine substituted Y-Ba-Cu-O
compounds, the high substitution levels of iodine, eg.,
YBaZCu3I3O35 do not destroy superconductivity.

3. A relatively higher critical temperature ( critical
temperature = 100K, onset temperature = 110K ) is observed
for iodine substituted YBafinhI0&5 superconducting compound.

4. Microstructures of these compound show presence of
thick and regulary spaced twinned structure which is indica-

tive of the orthorhombic superconducting phase.

56

10.

11.

12.

13.

14.

15.

16.

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