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Effect of the Ga-doping on the growth and superconducting properties of the Bi2Sr2CaCu2Oy whiskers
PhysicaC 235-240 (1994)473-474 North-Holland
PUYSlCA
EFFECT OF THE Ga-DOPING ON THE GROWTH AND SUPERCONDUCTING PROPERTIES OF THE Bi2Sr2CaCu2Oy WHISKERS L. DIMESSO, I. MATSUBARA, T. OGURA, R. FUNAHASHI, H. YAMASHITA and A. TAMPIERI& Osaka National Research Institute, Midorigaoka, Ikeda, Osaka 563, Japan &CNR/Research Institute Ceramics Technology, via Granarolo 64, 48018 Faenza (RA), Italy The effect of the Ga-doping on the growth mecchanism and s u p e r c o n d u c t i n g properties of whiskers in the BiSrCaCu2GaxOy system p r e p a r e d by annealing a glassy melt-quenched plate u n d e r flowing oxygen is described. The thermal behaviour of the glassy samples was f o u n d to be d e p e n d e n t on the Ga concentration; whiskers of BSCCO 2212 with a diamagnetic onset temperature Tc,on=76 K, were grown for x=0.6 after annealing at 855°C. 1.
Introduction
Superconducting whiskers in the Bi-SrCa-Cu-O (BSCCO) system have been grown recently from as-cast specimens [1]. Many factors have to be taken into account for the growth of Bi2SreCaCu2Oy whiskers, such as glassy p r e c u r s o r , a n n e a l i n g in flowing oxygen and presence of dopants. The effect of A1, Li and Pb on the growth mecchanism and on the s u p e r c o n d u c t i n g properties of 2212 whiskers has been reported {2]. In the Al-doped system, whiskers as long as 20 mm were grown and the presence of a small fraction of Bi2Sr2CaeCu3Ox (2223 phase) was detected; the effect of Ga-doping and its concentration on the chemical and physical properties of the BSCCO 2212 whiskers is also investigated in order to u n d e r s t a n d the growth mecchanism. 2.Experimental
The p r e p a r a t i o n of the a s - q u e n c h e d samples in the BiSrCaCuzGaxOy (with O
Elsevier Science B.V. All rights
s u p e r c o n d u c t i n g properties were checked by magnetization m e a s u r e m e n t s p e r f o r m e d in zero field cooled (H=IO Oe). 3.Results
and
discussion
The DTA curves in flowing air for the asq u e n c h e d glassy samples are shown in Fig. 1. At lower t e m p e r a t u r e s (T<600°C) the t h e r m a l b e h a v i o u r is i n d e p e n d e n t of the value of x, on the c o n t r a r y at h i g h e r temperatures the DTA patterns are strongly a f f e c t e d by the G a - c o n t e n t : in t h e temperature range 860-875°C the growth of 2212 whiskers is expected, the peak at 880°C corresponds to the melting point of the 2212 phase, while the peak above 900°C c o r r e s p o n d s to the melting of the 2201 phase. The presence of a liquid phase and a starting glassy state seem to be i m p o r t a n t factors for the whiskers growth. The meltquenched samples were annealed in flowing 02 gas in the t e m p e r a t u r e range in which the presence of solid and liquid phases are e x p e c t e d . After a n n e a l i n g , p r e l i m i n a r y analysis u n d e r optical microscope revealed that in the samples for x=0.2 and 0.4 only a crystallization process occurred, while the presence of whiskers as long as 1 mm were etected in the Ga-richer samples (x=0.6 and 0.8). In o r d e r to u n d e r s t a n d the growth mechanism of the whiskers, a study on the reserved.
474
I,, IJimessu et a/. 'Phvsica (' 235 240 (1994) 473 474
a s m a l l e r one at l ti3 K w h i c h s u g g e s t e d the p r e s e n c e of a small f r a c t i o n of 2223 p h a s e in l h e w h i s k e r s .
untreated I T
/ "f-'~
51Q°C
~
................
ao0
l
x__
500
J
~
-
700
'
JL.
>
T
630C
.
T
700°C
~
°
?
.........
l
,L ....
........
\ ~ ~l ~ x
I >
.45
X L4,5
~
-
,
~
]
C _.
,
900
T ("C)
T
Fig. 1: DTA p a t t e r n s of t h e as q u e n c h e d BiSrCaCu2GaxOy s a m p l e s . c r y s t a l l i z a t i o n b e h a v i o u r was c a r r i e d out on t h e G a - r i c h e r s a m p l e s . T h e XRD p a t t e r n s of t h e s a m p l e for x=0.6, a f t e r a n n e a l i n g for 15h u n d e r flowing O2 at d i f f e r e n t t e m p e r a t u r e s , a r e s h o w n in fig. 2. H e a t i n g at 5 1 0 ( : y i e l d s C u 2 0 as c r y s t a l l i n e p h a s e : the 63() (: a n n e a l e d s a m p l e r e v e a l e d t h e p r e s e n c e ot t h e Cu20, Sr-Ca-Cu-O, Bi-Sr-Cu-O a n d 2201 p h a s e s . A n n e a l i n g at h i g h e r t e m p e r a t u r e s (T>700°C) yields the 2212 p h a s e a n d the p r e s e n c e o f CuO was also o b s e r v e d . A d e e p e r a n a l y s i s o n t h e m o r p h o l o g y of t h e s a m p l e , performed b y SEM-EDX t e c h n i q u e s [3], s u p p o r t e d t h e XRD d a t a a n d t h e p r e s e n c e of m i c r o c r y s t a l s c o m p o s e d b y Bi-Sr-Ca-Ga-O (Cu-free p h a s e ) was d e t e c t e d [4]. "['he g r o w t h of the Cu-free phase, was explained supposing the presence of a liquid phase w h o s e f o r m a t i o n i n v o l v e s t h e a b s o r p t i o n of energy; moreover the microqualitative data r e v e a l e d an a t o m i c ratio G a / C u > > 1 in the Cuf r e e p h a s e a n d an a t o m i c r a t i o G a / C u < < l m the whiskers. The data suggested thai a l i q u i d - l i q u i d s e p a r a t i o n o c c u r r e d : a Garich l i q u i d f r o m w h i c h t h e C u - f r e e p h a s e grows and a Ga-poor one from which the 2212 whiskers crystallize. Two steps were o b s e r v e d in t h e m a g n e t i z a t i o n c u r v e of t h e s a m p l e for x=0.6: a b r o a d o n e at 76 K a n d a
_ ....
830°C
3
~~
-
3 i~-
20
>
'
40
2~-)(de~rec~ Fig. 2: XRI) p a t t e r n s oI the s a m p l e for x tLo a f t e r a n n e a l i n g at d i f f e r e n t t e m p e r a t u r e s : (1) Cu2(), (2) Cu(), i3) Sr-Ca-Cu-O, 14) 22()1, (X) Bi-Sr-(;u-(), t+) B i - S r - C a - ( ) a n d (5~ 2 2 1 2 phases.
4.
Conclusions
T h e i n f l u e n c e ol t h e Ga d o p i n g on t h e g r o w t h a n d s u p e r c o n d u c t i v i t y of t h e BSCC() 22 l 2 w h i s k e r s has b e e n e x a m i n e d . F r o m the d a t a o b t a i n e d , it was a r g u e d t h a t the g r o w t h o f t h e w h i s k e r s c a n b e r e l a t e d to a liquid liquid s e p a r a t i o n . F r o m t h e G a - r i c h liquid a f t e r t h e g r o w t h of a C u - f r e e p h a s e ~.as o b s e r v e d ; o n t h e o t h e r h a n d f r o m t h e Ga poor liquid the formation of the 2212 w h i s k e r s t o o k place. REFERENCES
1. 1. M a t s u b a r a et al., Jpn. J. Appl. Phys. 28 (1989) L1121 2. [. M a t s u b a r a et al., J. of Cryst. Growth, 110(1991)973 3, L. D i m e s s o et al,, a c c e p t e d for p u b l i c a t i o n on J. Mater. Res. 4. L. D i m e s s o et al., s u b m i t t e d for p u b l i c a t i o n on Physica C
PUYSlCA
EFFECT OF THE Ga-DOPING ON THE GROWTH AND SUPERCONDUCTING PROPERTIES OF THE Bi2Sr2CaCu2Oy WHISKERS L. DIMESSO, I. MATSUBARA, T. OGURA, R. FUNAHASHI, H. YAMASHITA and A. TAMPIERI& Osaka National Research Institute, Midorigaoka, Ikeda, Osaka 563, Japan &CNR/Research Institute Ceramics Technology, via Granarolo 64, 48018 Faenza (RA), Italy The effect of the Ga-doping on the growth mecchanism and s u p e r c o n d u c t i n g properties of whiskers in the BiSrCaCu2GaxOy system p r e p a r e d by annealing a glassy melt-quenched plate u n d e r flowing oxygen is described. The thermal behaviour of the glassy samples was f o u n d to be d e p e n d e n t on the Ga concentration; whiskers of BSCCO 2212 with a diamagnetic onset temperature Tc,on=76 K, were grown for x=0.6 after annealing at 855°C. 1.
Introduction
Superconducting whiskers in the Bi-SrCa-Cu-O (BSCCO) system have been grown recently from as-cast specimens [1]. Many factors have to be taken into account for the growth of Bi2SreCaCu2Oy whiskers, such as glassy p r e c u r s o r , a n n e a l i n g in flowing oxygen and presence of dopants. The effect of A1, Li and Pb on the growth mecchanism and on the s u p e r c o n d u c t i n g properties of 2212 whiskers has been reported {2]. In the Al-doped system, whiskers as long as 20 mm were grown and the presence of a small fraction of Bi2Sr2CaeCu3Ox (2223 phase) was detected; the effect of Ga-doping and its concentration on the chemical and physical properties of the BSCCO 2212 whiskers is also investigated in order to u n d e r s t a n d the growth mecchanism. 2.Experimental
The p r e p a r a t i o n of the a s - q u e n c h e d samples in the BiSrCaCuzGaxOy (with O
Elsevier Science B.V. All rights
s u p e r c o n d u c t i n g properties were checked by magnetization m e a s u r e m e n t s p e r f o r m e d in zero field cooled (H=IO Oe). 3.Results
and
discussion
The DTA curves in flowing air for the asq u e n c h e d glassy samples are shown in Fig. 1. At lower t e m p e r a t u r e s (T<600°C) the t h e r m a l b e h a v i o u r is i n d e p e n d e n t of the value of x, on the c o n t r a r y at h i g h e r temperatures the DTA patterns are strongly a f f e c t e d by the G a - c o n t e n t : in t h e temperature range 860-875°C the growth of 2212 whiskers is expected, the peak at 880°C corresponds to the melting point of the 2212 phase, while the peak above 900°C c o r r e s p o n d s to the melting of the 2201 phase. The presence of a liquid phase and a starting glassy state seem to be i m p o r t a n t factors for the whiskers growth. The meltquenched samples were annealed in flowing 02 gas in the t e m p e r a t u r e range in which the presence of solid and liquid phases are e x p e c t e d . After a n n e a l i n g , p r e l i m i n a r y analysis u n d e r optical microscope revealed that in the samples for x=0.2 and 0.4 only a crystallization process occurred, while the presence of whiskers as long as 1 mm were etected in the Ga-richer samples (x=0.6 and 0.8). In o r d e r to u n d e r s t a n d the growth mechanism of the whiskers, a study on the reserved.
474
I,, IJimessu et a/. 'Phvsica (' 235 240 (1994) 473 474
a s m a l l e r one at l ti3 K w h i c h s u g g e s t e d the p r e s e n c e of a small f r a c t i o n of 2223 p h a s e in l h e w h i s k e r s .
untreated I T
/ "f-'~
51Q°C
~
................
ao0
l
x__
500
J
~
-
700
'
JL.
>
T
630C
.
T
700°C
~
°
?
.........
l
,L ....
........
\ ~ ~l ~ x
I >
.45
X L4,5
~
-
,
~
]
C _.
,
900
T ("C)
T
Fig. 1: DTA p a t t e r n s of t h e as q u e n c h e d BiSrCaCu2GaxOy s a m p l e s . c r y s t a l l i z a t i o n b e h a v i o u r was c a r r i e d out on t h e G a - r i c h e r s a m p l e s . T h e XRD p a t t e r n s of t h e s a m p l e for x=0.6, a f t e r a n n e a l i n g for 15h u n d e r flowing O2 at d i f f e r e n t t e m p e r a t u r e s , a r e s h o w n in fig. 2. H e a t i n g at 5 1 0 ( : y i e l d s C u 2 0 as c r y s t a l l i n e p h a s e : the 63() (: a n n e a l e d s a m p l e r e v e a l e d t h e p r e s e n c e ot t h e Cu20, Sr-Ca-Cu-O, Bi-Sr-Cu-O a n d 2201 p h a s e s . A n n e a l i n g at h i g h e r t e m p e r a t u r e s (T>700°C) yields the 2212 p h a s e a n d the p r e s e n c e o f CuO was also o b s e r v e d . A d e e p e r a n a l y s i s o n t h e m o r p h o l o g y of t h e s a m p l e , performed b y SEM-EDX t e c h n i q u e s [3], s u p p o r t e d t h e XRD d a t a a n d t h e p r e s e n c e of m i c r o c r y s t a l s c o m p o s e d b y Bi-Sr-Ca-Ga-O (Cu-free p h a s e ) was d e t e c t e d [4]. "['he g r o w t h of the Cu-free phase, was explained supposing the presence of a liquid phase w h o s e f o r m a t i o n i n v o l v e s t h e a b s o r p t i o n of energy; moreover the microqualitative data r e v e a l e d an a t o m i c ratio G a / C u > > 1 in the Cuf r e e p h a s e a n d an a t o m i c r a t i o G a / C u < < l m the whiskers. The data suggested thai a l i q u i d - l i q u i d s e p a r a t i o n o c c u r r e d : a Garich l i q u i d f r o m w h i c h t h e C u - f r e e p h a s e grows and a Ga-poor one from which the 2212 whiskers crystallize. Two steps were o b s e r v e d in t h e m a g n e t i z a t i o n c u r v e of t h e s a m p l e for x=0.6: a b r o a d o n e at 76 K a n d a
_ ....
830°C
3
~~
-
3 i~-
20
>
'
40
2~-)(de~rec~ Fig. 2: XRI) p a t t e r n s oI the s a m p l e for x tLo a f t e r a n n e a l i n g at d i f f e r e n t t e m p e r a t u r e s : (1) Cu2(), (2) Cu(), i3) Sr-Ca-Cu-O, 14) 22()1, (X) Bi-Sr-(;u-(), t+) B i - S r - C a - ( ) a n d (5~ 2 2 1 2 phases.
4.
Conclusions
T h e i n f l u e n c e ol t h e Ga d o p i n g on t h e g r o w t h a n d s u p e r c o n d u c t i v i t y of t h e BSCC() 22 l 2 w h i s k e r s has b e e n e x a m i n e d . F r o m the d a t a o b t a i n e d , it was a r g u e d t h a t the g r o w t h o f t h e w h i s k e r s c a n b e r e l a t e d to a liquid liquid s e p a r a t i o n . F r o m t h e G a - r i c h liquid a f t e r t h e g r o w t h of a C u - f r e e p h a s e ~.as o b s e r v e d ; o n t h e o t h e r h a n d f r o m t h e Ga poor liquid the formation of the 2212 w h i s k e r s t o o k place. REFERENCES
1. 1. M a t s u b a r a et al., Jpn. J. Appl. Phys. 28 (1989) L1121 2. [. M a t s u b a r a et al., J. of Cryst. Growth, 110(1991)973 3, L. D i m e s s o et al,, a c c e p t e d for p u b l i c a t i o n on J. Mater. Res. 4. L. D i m e s s o et al., s u b m i t t e d for p u b l i c a t i o n on Physica C