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Physica C 411 (2004) 180–188
www.elsevier.com/locate/physc
Effect of low temperature short time annealing on oxygencontent and surface quality of Bi2Sr2CaCu2O8+ d single crystals
P. Kumar a, B. Kumar a,*, D.N. Kumar b, G.K. Chadha a
a Department of Physics and Astrophysics, University of Delhi, Delhi 110007, Indiab Department of Chemistry, University of Delhi, Delhi 110007, India
Received 22 March 2004; received in revised form 8 July 2004; accepted 8 July 2004
Available online 14 August 2004
Abstract
High temperature superconducting single crystals of 2212 phase of Bi-system have been grown by self-flux technique.
They are annealed in different gas atmospheres of O2, N2 and Ar to obtain overdoped and underdoped samples. Both
time as well as temperature of the annealing has been optimized to get different Tc-value in the range of 81–94 K. The
oxygen contents d for overdoped and underdoped crystals have been precisely determined by iodometric titration tech-
nique. The change in Tc and DTc have been discussed in terms of change in O-value in as-grown as well as in the opt-
imally annealed crystals due to in- and out-diffusion of oxygen. The effect of annealing on surface morphology and
defects features like growth steps, slip lines/bands and etch pits have also been studied by scanning electron microscopy.
� 2004 Elsevier B.V. All rights reserved.
Keywords: Bi2Sr2CaCu2O8+d HTSC; Single crystal growth; Annealing; Oxygen content d; Iodometric titration; SEM
1. Introduction
Since the discovery of high Tc superconducting
oxide, attempts are being made to grow good qual-
ity single crystals of these systems and to improve
0921-4534/$ - see front matter � 2004 Elsevier B.V. All rights reserv
doi:10.1016/j.physc.2004.07.008
* Corresponding author. Tel.: +91 11 27662026; fax: 91 11
27667061.
E-mail addresses: [email protected], b3kumar69@
yahoo.co.in (B. Kumar).
the superconducting onset temperature and crystal
quality by the proper heat treatment [1,2]. For the
growth of Bi-2212 single crystals, various methods
have been employed viz. melt, TSFZ, TSSG etc. [3–
6]. Out of these, melt growth is most-convenient,
since large number of single crystals can be grownin one run having similar properties. However,
crystals obtained in the melt growth technique are
rather smaller in size, normally not more than 2
mm across [7]. Different methods viz. various heat
treatment, application of pressure, change of flux,
ed.
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P. Kumar et al. / Physica C 411 (2004) 180–188 181
crucible quality etc. have been employed to im-
prove the quality of the crystals [8–10]. We have
tried various temperature profiles, charge to flux
ratio and shape of crucible to grow crystals of big-
ger size.The effect of annealing on HTSC is a subject of
investigation for long. In YBa2Cu3O7+d, a large
number of works have come up highlighting the
role of oxygen-content in it [11–13]. Transition
temperature Tc and transition width DTc were
found to depend critically on O-contents. A sys-
tematic study on the dependence of transition
temperature on the oxygen-content in Bi2Sr2Ca-Cu2O8+d single crystals has been carried out by
many workers [5,8,14]. In Bi2Sr2CaCu2O8+d
crystals, the superconducting properties are found
to vary with O-content. However in this case
the dependence is less sensitive. Investigation of
annealing effect on HTSC has been carried out in
air for higher temperatures (upto 750 �C) for
longer duration (upto few hours) to observechanges in Tc and DTc [8,15]. Effect of annealing
(temperature range 20–875 �C) on structural
changes was studied by Bdikin et al. [7]. A marked
change has been reported in the transition temper-
ature Tc and transition width DTc, which has been
explained in terms of in- and out-diffusion of oxy-
gen [8]. However, in most of the cases, the content
of O2 were expressed as �overdoped� and �under-doped� in a general way, the exact value of �d�has not been determined. We have annealed the
crystals in specific gas atmospheres O2, N2 and
Ar so that the aforesaid changes can be brought
about even at lower temperature and in lesser time.
We have also determined the precise value of O-
content in different overdoped and underdoped
crystals by iodometric titration technique.Although scanning electron microscopy (SEM)
studies on Bi-2212 crystals had been carried out
by many workers, it was mostly confined to show
the crystal size and the smoothness [10,15,16]. We
have observed features like growth steps, slip lines/
bands, etch pits, over growths etc., from which
supplementary information have been gained
regarding defect surface structure of the crystals.The effect of annealing on the aforementioned sur-
face features have also been analysed by scanning
electron microscopy (SEM).
2. Experimental
For the growth of Bi2Sr2CaCu2O8+d supercon-
ducting single crystals by self-flux technique, the
starting materials (Bi2O3, SrCO3, CuO, CaCO3
purity >99.99%, ALDRICH) were mixed in an
agate mortar and pestle for several hours, then
transferred into a high purity alumina cylindrical
crucible (purity >99.0%, CERAC) and subjected
to heat treatment in a box-type programmable
furnace (HERAEUS K-1252). The soaking tem-
perature was kept at 1000 �C and cooling rate
was maintained at (1 �C/h). The resistive measure-ments have been carried out manually by stand-
ard four-probe method using air-drying silver
paste and Kiethley setup (181 Nano-voltmeter,
195 A Digital mutimeter and 224 Programmable
current source). The as-grown crystals were an-
nealed in O2, N2 and Ar gas atmospheres in a
tubular furnace, which was designed and fabri-
cated by us. The crystal surface was examinedby scanning electron microscopy (SEM), employ-
ing a JEOL JSM-840 electron microscope. The
samples were mounted on copper stubs with car-
bon electro-conducting paint and a thin uniform
gold film was sputtered on them, using an ion
sputterer (JFC-1100).
The oxygen content has been determined for
as-grown and annealed crystals by standard iodo-metric titration technique. The superconducting
transition temperature Tc critically depends on the
degree of oxidation, i.e. on the average [Cu–O]+p
charge (or formal oxidation state [Cu]+2+p of Cop-
per). The principal variable controlling the value of
Tc has been found to be �p� on each [Cu–O]+p species
[17,18]. The formal copper valency is simply (2 + p).
The value of �p� has been determined by standardiodometric titration method, which consists of
two parts. In first part, 40mg (W1) of sample and
7 g of KI was dissolved in 30 ml of distilled water.
Then 3 g of KI and 10 ml of 2 N HCl were added
and the solution was thoroughly mixed by using
magnetic stirrer after adding few drops of starch
(solution is now black). The whole process was
performed in nitrogen atmosphere. All the Cu+
will precipitate as CuI. The quantity of iodine
liberated is proportional to the degree of oxidation,
in excess of Cu+, as follows:
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182 P. Kumar et al. / Physica C 411 (2004) 180–188
½Cu–O�þp þ ð2þ pÞI� ! CuIþ 1=2ðp þ 1ÞI2The titration was performed with 0.01 N sodium
thiosulphate (Na2S2O3 Æ 5H2O). Let V1 volume of
sodium thiosulphate is required to titrate the solu-
tion, as follows:
2S2O3�3 þ I2 ! S4O
6�2 þ 2I
In the second part, 30 g (W2) of the sample was
dissolved into 10 ml of 1 N HCl. The solutionwas heated for 10–15 min, and then cooled and
10 g of KI was added to it. Let V2 is the volume
of sodium thiosulphate used in the titration. In this
case the degree of oxidation is then given by
p ¼ V 1=W 1
V 2=W 2
� 1 ð1Þ
The oxygen content has been calculated by using
the charge neutrality principle. The valanciesof the Bi, Sr, Ca, Cu and O are +3, +2, +2, 2+p
and �2, respectively, in Bi2Sr2CaCu2O8+d super-
conducting system. Hence we have calculated the
value of oxygen content by using following
formula:
2� 3ðBiþ3Þ þ 2� 2ðSrþ2Þ þ 1� 2ðCaþ2Þþ 2� ð2þ pÞ½Cuþ2þp� � 2ð8þ dÞ ¼ 0 ð2Þ
The iodometric titration have been carried out
three times for each case and the average value
of volumes V1 and V2, measured with the help of
microburatte (least count 0.01 ml), for as-grown
and annealed crystals have been given in Table 1.
The value of W1 and W2 has been kept as 40and 30 mg (measured accurately upto five decimal
places of gram) for all the cases. The smaller least
count of microburatte enables us to measure the
Table 1
Microburatte readings for used sodium-thiosulphate in the titration a
Volume of sodium-thiosulphate As-grown Oxygen-
V1 (ml) 9.52 9.87
V2 (ml) 8.93 9.03
p ¼ V 1=W 1
V 2=W 2� 1 �0.200±0.004a �0.180±
a W1 = 40 mg, W2 = 30 mg, least count = 0.01 mg; V1 = V2 = 10 m
value of �p� upto third decimal place as given in
the third row of the Table 1. Since the arithmetical
mean value at the third decimal place is, inciden-
tally �zero� in all the three cases, we have ignored
it for further calculation of �d�. Further it is notconvenient to carry the error value (0.004) at dif-
ferent places during discussion, we have expressed
the value of oxygen content upto second decimal
place without showing the error value in the fol-
lowing discussion.
3. Results and discussion
The as-grown crystals had shiny flat surfaces
measuring upto 5 mm across (mostly 3 · 2 mm2)
and thickness upto 0.3 mm. The observed sharp
reflections in the oscillation photographs con-
firmed the single crystalline nature of the crystals.
The transition temperature Tc was determined to
be 83 K for most of the crystals with transitionwidth DTc nearly 1.5 K. The lattice parameters
of the crystals were determined to be a = b =
5.412 A, c = 30.842 A by X-ray diffraction.
The as-grown crystals have been annealed at
different temperatures (150, 200, 250, 300, and
400 �C) and for different durations (5, 10, 15
and 20 min) in different gas atmospheres viz.
O2, N2, and Ar. It is found that annealing thecrystals at a temperature 200 �C for the duration
of 5 min is sufficient to bring about a change in
transition temperature for which the Tc-value re-
mains within liquid N2 temperature. Under these
conditions of annealing, samples were over-
doped/underdoped without losing its supercon-
ducting nature. If the duration of annealing is
increased beyond 30 min and temperature is in-creased beyond 400 �C, the sample�s surface is
highly corroded and the change in Tc is more
nd �p� values for different crystals
annealed Nitrogen-annealed Argon-annealed
8.86 9.14
8.52 8.91
0.004a �0.220±0.004a �0.230±0.004a
l, least count = 0.01 ml.
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P. Kumar et al. / Physica C 411 (2004) 180–188 183
pronounced. As the annealing in O2-atmospheres
starts, the Tc value started decreasing from 83 K
and attains a value of 81 K in 5 min. For further
annealing, the Tc value keep on decreasing gradu-
ally and after nearly 20 min, the sample fails toshow superconducting transition in liquid nitro-
gen range. In the case of N2-annealing, the change
in Tc is rapid. In the first 4–5 min Tc increases
rapidly to 94 K, and then starts decreasing very
rapidly; in 9–10 min the sample does not show
Tc upto 77 K. The effect of longer annealing is
also manifested in increased roughness and cracks
on the surface in SEM studies. In the case ofargon-annealing no change in Tc value is observed
for annealing for 5 min. It is found that longer
duration of annealing (30 min or more) does re-
sult in decrease in Tc value. However, the degree
of surface roughening and cracking is very high.
Therefore, we have kept the time of annealing
as 5 min for all the cases to get overdoped and
underdoped samples so that the effect of atmos-phere may be assessed. It may be noted that in
most of the earlier annealing work on Bi-2212
Fig. 1. Resistance versus temperature curves for as-grown and anneal
annealed for 5 min at 200 �C in different gas atmospheres viz. oxygen
crystals, much higher temperatures (700–800 �C)and longer duration (few hours) have been em-
ployed to get the desired superconducting and
structural changes. In the present work, however,
much lower temperature and time have beenfound sufficient to bring about desired changes
(discussed later), thus saving both energy and
time. It has become possible because crystals have
been annealed in high purity specific gas atmos-
pheres of oxygen, nitrogen and argon, instead of
air.
The resistivity of the as-grown and annealed
crystals has been measured by four-probe tech-nique. The superconducting transition tempera-
ture Tc was determined to be 81, 94 and 83 K
for the crystals annealed in the O2, N2 and Ar
gas atmospheres with transition width DTc as 2,
4 and 3 K, respectively. Fig. 1 shows the resistance
verses temperature curves for the as-grown and an-
nealed crystals. The larger transition width DTc,
particularly in case of N2 annealed crystal indi-cates that the oxygen content is not completely
homogeneous. We have discussed that in the
ed crystals. Inset shows the variation near R! 0. Samples were
, nitrogen and argon.
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184 P. Kumar et al. / Physica C 411 (2004) 180–188
case of N2-annealing, the changes in Tc (so in the
value of O-content) takes place rapidly. The faster
rate of out-diffusion leaves the sample more oxy-
gen-inhomogeneous. For better homogenization
of oxygen content, the annealing time should beincreased [19,20]. But for longer duration of
annealing, the problem of surface corrosion and
decaying Tc-value increases (particularly in Ar-
gas atmospheres). It is noteworthy that in the case
of argon annealing although DTc and surface qual-
ity was found to change markedly, the transition
temperature remained at 83 K after annealing at
200 �C for 5 min.The critical temperature Tc of the Bi2Sr2Ca-
Cu2O8+d superconducting single crystals are very
sensitive to the oxygen content, because the excess
oxygen, which is inserted into crystals by anneal-
ing (O2-gas atmospheres) distorts CuO2 layers;
the distortion of CuO2 layer behaves as pining cen-
ters in the crystal structure of the HTSC [21]. The
superconducting layers (Ca–Cu–O Blocks) areweakly coupled in the crystal structure of Bi-
2212 system [19,22]. The region with excess oxygen
is expected to act as the field induced-pining cen-
ters, where its Tc is lower than that of the sur-
rounding region due to the strong distortion of
the lattice. It is reported that the doping state of
Bi-2212 results in the change of the critical temper-
ature Tc from the overdoped region to the under-doped region reversibly by annealing in various
atmospheres.
The oxygen contents d for the as-grown and an-
nealed crystals were found to be negative and the
values are 0.20, 0.18, 0.22, and 0.23 (column �E�in Table 2), thus giving the values of O-content in
Bi 2Sr2CaCu2O8+d as 7.80, 7.82, 7.78, and 7.77 (col-
umn �F�) for as-grown and for the crystals annealedin O2, N2 and Ar gas atmospheres, respectively.
Table 2
Tc, DTc and d-values in Bi2Sr2CaCu2O8+d single crystals annealed at
Annealing
atmosphere
(A)
Tc (K)
(B)
Change in
Tc (K)
(C)
DTc (K)
(D)
d-values(E)
As-grown 83 – 1.5 �0.20
Oxygen 81 02# 2" �0.18
Nitrogen 94 11" 4" �0.22
Argon 83 0 3" �0.23
The maximum value for Tc (94 K) was obtained
in the case of N2-annealed crystals for which the
O-content was found to be 7.78 (marked as bold
in column �F�), which we treat as optimum value
of O-content in our Bi-2212 crystal. It suggestedthat the as-grown crystals were grown as over-
doped since the O-content (7.80) in them was high-
er than the optimum value (7.78). Hence annealing
such overdoped crystals in N2 resulted in �out-diffu-sion� of O2 from Cu–O plane. This explains the in-
crease of Tc-value, since in the process; O-value is
optimized during annealing in N2 atmosphere. It
may be added that for longer duration of annealingin N2, the Tc-value decreases. It may be visualized
on the basis that if the �out-diffusion� of oxygen
continues for long, it will make the sample increas-
ingly �underdoped�. This explains the observed
non-superconducting behavior of samples (above
77 K) annealed for 10 min or more in N2. On the
other hand, when the samples are annealed in O2,
the O-content is further increased to 7.82 due to�in-diffusion� of oxygen and the samples become
more overdoped, resulting in further decrease of
Tc to 81 K. It is also significant that while a change
of Tc equal to 11 K is effected in the case of out-dif-
fusion of oxygen by an amount 0.02, in the case of
the in-diffusion of O2 by the same amount (viz.
0.02), the change in Tc is only 2 K (columns C
and G). It suggests that the dependence of Tc-valueon O-content is more sensitive near its optimum
value. Hence, when the O-value is close to opti-
mum value, a change in O-content affects the
Tc-value in a big way e.g. a change in O2-value
from its optimum 0.22–0.20 has changed the
Tc-value by 11 K. On the other hand, the same
change in O-value viz. 0.2, (but from 0.20 to 0.18)
could change the Tc value by merely 2 K when dis not so close to its optimum value.
200 �C for 5 min in different gas atmospheres
O-content
(F)
Change in O-value
w.r.t as-grown
(G)
Change in O-value
w.r.t optimum value
(H)
7.80 – 0.02"7.82 0.02" 0.04"7.78 0.02# –
7.77 0.03# 0.01#
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P. Kumar et al. / Physica C 411 (2004) 180–188 185
Further it may be noted that the similar condi-
tions of temperature and time have changed the
value of oxygen-content by same amount in differ-
ent gas atmospheres. In both the cases the change
in d-value is 0.02 (from 0.20 to 0.18 in O2-anneal-ing and from 0.20 to 0.22 in N2-annealing, column
�G�). We conclude that there is no effect of atmos-
pheres on the rate of in- or out-diffusion of oxy-
gen. It has been suggested [8] that the rate of
out-diffusion of oxygen from Cu–O plane is faster
than the in-diffusion thereby effecting different
amount of changes in O-value in the two cases.
However, it is not observed in the present study.It may be due to the fact that the oxygen diffusion,
both in- and out-, is very slow [23]. Hence due to
much reduced time of annealing in the present case
(viz. 5 min as against few hours), the different rates
of in- and out-diffusion could not generate a meas-
urable difference in the change in O-content in the
two cases.
Different growth and defect features on the sur-face of the crystals were examined by scanning
electron microscopy. Fig. 2 shows the entire sur-
face of a crystal. The features of two dimensional
layer growth, etch pits along parallel and intersect-
ing rows, pattern of slip-lines and bands are ob-
served in the study which are similar to those
Fig. 2. Scanning electron micrograph of an as
reported earlier [24]. A marked change in these
surface features has been observed due to anneal-
ing of crystals, confirming a close correlation be-
tween the changes in Cu–O layers with the
surface quality.In the as-grown crystals the reported slip lines/
bands are nearly parallel and sharp [24,25]. How-
ever in O-annealed crystals they are not strictly
parallel and also cracks are seen to be developed
on the surface of the crystals along the slip line
(Fig. 3). In the case of the N2-annealed crystals,
on the other hand, more smooth surfaces are ob-
served (Fig. 4). These changes in surface featureswell conform to the changes in the structure due
to oxygen in- and out-diffusion during annealing.
As we have discussed that as-grown crystals are
slightly overdoped, hence the in-diffusion of oxy-
gen creates more distortion in CuO2 layers, which
are manifested on the surface as increased rough-
ness and cracks as evident in Fig. 3.
In case of annealing in N2 atmosphere, how-ever, the extra oxygen is removed, thus relieving
the crystals from such distortion and, as a result,
the surface quality is also improved (Fig. 4; mag-
nification in Figs. 3 and 4 are kept same for bet-
ter comparison). In the case of annealing at
higher temperatures and for longer duration,
-grown Bi2Sr2CaCu2O8+d single crystal.
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Fig. 3. Scanning electron micrograph of oxygen-annealed crystal. Cracks developed along slip-lines are visible which are not strictly
parallel.
Fig. 4. Scanning electron micrograph of a nitrogen-annealed crystal. The surface of a crystal has become smoother and the slip lines
are straighter.
186 P. Kumar et al. / Physica C 411 (2004) 180–188
cracks are seen to be developed on the surface of
the crystals. Such degradation of surface is more
prominent in the case of argon-annealed crystals.
Even the crystallites grown at the surface of the
crystals are cracked due to over annealing
(Fig. 5).
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Fig. 5. Scanning electron micrograph of an over annealed crystal. Crystallites on the surface are seen to be fully cracked. Similar fate
of the whole crystal are observed in many cases irrespective of atmosphere but more prominently in argon-annealed crystal.
P. Kumar et al. / Physica C 411 (2004) 180–188 187
4. Conclusion
The conditions of annealing in different gas
atmospheres are optimized. The annealing tem-
perature of 200 �C and time 5 min are found to
be sufficient to bring about a change in Tc in
the range 81–94 K. The Tc and DTc are foundto be dependent upon O-content and its homoge-
neity. The rate of in- and out-oxygen diffusion is
found to be slow and independent of annealing
atmosphere. The optimum value of oxygen is
found to be 7.78 for which Tc is maximum (94
K). The oxygen and nitrogen atmosphere are
found to be very effective to bring about a change
in Tc while argon atmosphere proved to be inef-fective in smaller duration of annealing. The
change in O-content has marked effect on the
structure and the surface features. The excess of
O-value renders poor surface quality.
Acknowledgments
The financial assistance from the University
Grants Commission (UGC) under the research
project ‘‘Superconductivity R&D Programme’’ is
thankfully acknowledged. We are thankful to
Prof. G.C. Trigunayat and Prof. B.S. Garg for
necessary support and discussion.
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