- Email: [email protected]
Superconducting YBa2Cu3O7 prepared by arc melting and laser melting
Physica C 153 155 (1988) 405 406 North-Holland, Amsterdam
SUPERCONDUCTING
X.Z.
WANG,
Department
K.
YBa2Cu307
DONNELLY,
of P u r e
PREPARED
BY ARC
M. M c L O U G H L I N ,
and Applied
Physics,
MELTING
J. L U N N E Y Trinity
AND LASER
and J.M.D. College,
MELTING
COEY
Dublin
2,
Ireland
M e l t i n g o r t h o r h o m b i c Y B a 2 C u 3 0 7 e i t h e r in an arc f u r n a c e or w i t h 7ms l a s e r p u l s e s of w a v e l e n g t h 1 . 0 6 ~ m (7 x 103 W . c m -2) l e a d s to s i m i l a r , c o m p l e x m i x t u r e s of o x i d e s . In b o t h c a s e s the p u r e o r t h o r h o m b i c s u p e r c o n d u c t i n g p h a s e c a n be r e s t o r e d b y an oxygen anneal. The l a s e r m e l t d e p t h (15~m) is d i s c u s s e d in t e r m s of t h e t h e r m a l p a r a m e t e r s of the Y - B a - C u o x i d e .
A g o a l in the m a t e r i a l s t e c h n o l o g y of the copper perovskite superconductors is p r o d u c t i o n of f u l l y - d e n s e p o l y c r y s t a l l i n e material with good electrical contact between individual crystallites. Here we d e m o n s t r a t e that the p u r e s u p e r c o n d u c t i n g p h a s e can be r e c o n s t i t u t e d f r o m a m i x t u r e of oxides in melts produced in two d i f f e r e n t ways. The ternary orthorhombic oxides RBa2Cu30 x have optimum superconducting properties when x ~ 7 [i~, which corresponds f o r m a l l y to one Cu ~+ or one hole in t h e O(2p) band per formula. Neither is stable at elevated temperature; o x y g e n loss t o t h e e x t e n t 6.0 ~ x ~ 6.5 ( e s s e n t i a l l y f r o m ib sites) corresponds to transformation of the orthorhombic superconductor (space g r o u p Pmmm) to a tetragonal semiconductor (space group P4/mmm) [2]. A n n e a l i n g in o x y g e n in the r a n g e 400 - 600 °C is k n o w n to restore the orthorhombic phase. However, melting, at temperatures in excess of 1040 °C leads to disproportionation of the o x i d e i n t o a m i x t u r e of p h a s e s . H e r e we s t u d y the e f f e c t s of m e l t i n g YBa2Cu307, b o t h in an a r c f u r n a c e a n d w i t h 7 ms p u l s e s f r o m a N d - Y A G laser. We s h o w t h a t t h e m e l t p r o d u c t s are s i m i l a r in b o t h cases, a n d t h a t t h e o t h o r h o m b i c superconducting p h a s e w i t h x - 7 can be restored by annealing in oxygen. Microstructures and densities of the p r o d u c t s are e x a m i n e d . The s t a r t i n g m a t e r i a l was Y - B a - C u o x i d e w i t h a 1:2:3 m e t a l ratio, p r e p a r e d by coprecipitation of t h e o x a l a t e s and firing in a i r at 940°C [3]. X-ray d i f f r a c t i o n s h o w e d that Y B a 2 C u 3 0 x was the m a i n p h a s e (a = 3 . 8 2 1 ( 4 ) i , b = 3 . 8 9 2 ( 4 ) i , c : II.679(12)A) , and that it was partially oriented at t h e surface. A minor amount of Y2BaCuO5 was also present. Scanning electron micrographs (Fig. la) s h o w an o p e n m i c r o s t r u c t u r e ,
0921 4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
a
b
C
F i g u r e i. S c a n n i n g e l e c t r o n m i c r o g r a p h s showing the microstructures of a) s t a r t i n g Y - B a - C u oxide, b) the same a f t e r arc m e l t i n g a n d o x y g e n a n n e a l a n d c) the s a m e a f t e r l a s e r m e l t i n g . The s c a l e b a r r e p r e s e n t s 25 ~m. w i t h s l a b - l i k e c r y s t a l l i t e s up to a b o u t 5~m thick. The density, measured by weighing in a c e t o n e a n d air, m a k i n g no a l l o w a n c e for f i l l e d p o r e s in the sample, was 5.74 g . c m -3 w h i c h is 89% of t h e theoretical density (6.42 g . c m - 3 ) . The superconducting transition temperature Tsc determined by ac susceptibility m e a s u r e m e n t s (Fig. 2a) was 90(1) K.
X.Z. Wang et al. / YBa2Ctt307 prepared by arc melting and laser melting
406
!
I-
•~
l
I
I
I
6 X
!
b/ I I 80
I
I 120
I T(K)
I 160
F i g u r e 2. AC m a g n e t i c s u s c e p t i b i l i t y d a t a on a) s t a r t i n g Y - B a - C u o x i d e a n d b) t h e same a f t e r arc m e l t i n g a n d o x y g e n anneal. Five-gram pellets were arc-melted in a i r at a p r e s s u r e of 500 m b a r . The m e l t has a low s u r f a c e tension, a n d fills the depression in t h e c o p p e r h e a r t h of the furnace. D i s c s a m p l e s 14 m m d i a m e t e r a n d 1 m m t h i c k w e r e i r r a d i a t e d w i t h 7 ms l a s e r l a s e r p u l s e s at 1.06 ~m. M e l t i n g was a c h e i v e d w i t h a p u l s e e n e r g y of 3.4 J w e a k l y f o c u s s e d t o a 3 m m d i a m e t e r spot g i v i n g a t h r e s h o l d i r r a d i a n c e for m e l t i n g of 7 x 1 0 3 W . c m -2. A continuous melted s u r f a c e l a y e r was o b t a i n e d b y r a s t e r i n g the sample in the beam. X-ray diffraction of the arc-melted and l a s e r - m e l t e d m a t e r i a l s h o w e d a m i x t u r e of phases: Y203 , tetragonal YBa2Cu3Ox, Cu403, Y B a 3 C u ~ O x a n d B a C u O 2. The l a t t e r f o r m e d an o r l e n t e d l a y e r on t h e u p p e r s u r f a c e of the a r c - m e l t e d b u t t o n s . A n n e a l i n g of the m e l t e d s a m p l e s was carried out for 20 h o u r s in flowing o x y g e n . X - r a y p a t t e r n s of the a r c - m e l t e d button and powder as well as the laser-melted s u r f a c e t h e n s h o w a pure, orthorhombic 1:2:3 phase with a = 3.820(4)A, b = 3.889(4)A and c = ii. 6 7 4 (12) A. The superconducting t r a n s i t i o n t e m p e r a t u r e (Fig 2b) was 97(1) K . Thermopiezic analysis [4] t r a c e s (Fig.3) indicate thermal desorption behaviour that is typical of the stoichiometric superconducting phase[5] . Micrographs t a k e n of t h e s a m p l e s a f t e r t h e o x y g e n a n n e a l are s h o w n in F i g . l b & c. The m a t e r i a l is s o m e w h a t m o r e c o m p a c t after arc melting, a n d its d e n s i t y was measured as 5.98 g . c m -3, or 93% of t h e t h e o r e t i c a l d e n s i t y . In the l a s e r - t r e a t e d sample, a dense surface layer a p p r o x i m a t e l y 15 ~ m t h i c k was o b t a i n e d . Polycrystalline YBa2Cu30 x presents a decided advantage for laser melting compared with ordinary metals thanks to a c o m b i n a t i o n of low r e f l e c t i v i t y (R - 0.2) [6], which means that the light is r e a d i l y c o u p l e d to the sample, a n d poor thermal conductivity of the polycrystalline ceramic. Taking the
400
!
!
800
"~l
I
400
I
800 T[°C)
Figure 3. T h e r m o p i e z i c analysis (TPA) traces from vacuum of a) Typical YBa2Cu30 7 superconductor and b) the arc-melted and annealed sample. Heating r a t e is 2 0 ° C . m i n -I. D u l o n g - P e t i t v a l u e for s p e c i f i c heat (C = 324 J.mole-i .K -I ) and a melting temperature of 1 2 8 0 K, t h e threshold i r r a d i a n c e for m e l t i n g (12 J . c m -2) l e a d s t o an e s t i m a t e of 0.6 W . m - I K -I for t h e thermal conductivity. This estimate may be compared with the value of 0.5 W . m - I K -I m e a s u r e d at i00 K [7]. The t h e r m a l d i f f u s i o n l e n g t h is g i v e n as k = (4~t/pC) I/2, w h e r e t is the p u l s e length. T a k i n g t = 7 ms, t h i s g i v e s k = 80 ~ m which is c o m p a t i b l e with the observed melt depth of - 15 ~ m . A detailed n u m e r i c a l a n a l y s i s of the m e l t i n g p r o c e s s w i l l r e q u i r e d a t a on t h e l a t e n t h e a t of f u s i o n as well as a c c u r a t e v a l u e s of K a n d C at e l e v a t e d t e m p e r a t u r e s . In c o n c l u s i o n , we h a v e s h o w n t h a t laser melting followed by an oxygen a n n e a l at a r e l a t i v e l y low temperature offers the prospect of m a k i n g rather dense superconducting l a y e r s some i0 ~ m in t h i c k n e s s .
REFERENCES [I] S. T a n a k a et al, Int. J. M o d e r n P h y s B1 (1987) 755; R . J . C a v a , ibid 813. [2] C . C . T o r a d i , E.M.McCarron, P.B.Bierstedt, A . W S l e i g h t a n d D . E . C o x , S o l i d S t a t e C o m m . 6 4 (1987) 497; J . D . J o r g e n s e n et al. Phys. Rev. B36 (1987) 5731. [3] X . Z . W a n g , M . H e n r y , J . L i v a g e a n d I . R o s enmen, S o l i d S t a t e Comm. 64 (1987) 881. [4] D . H . R y a n a n d J . M . D . C o e y , J. P h y s E l 9 (1986) 693 [5] J . M . D . C o e y , K . D o n n e l l y a n d F . S u p p l e , in Proc. E u r o p e a n W o r k s h o p on H i g h - T c Superconductors and Potential Applications, (CEC, B r u s s e l s 1987) p.423. [6] J . O r e n s t e i n et al, Phys. Rev. B 3 6 (1987) 729 [7] D . T . M o r e l l i , J . H e r e m a n s and D.E.Swets, Phys. Rev. B36 (1987) 3917.
SUPERCONDUCTING
X.Z.
WANG,
Department
K.
YBa2Cu307
DONNELLY,
of P u r e
PREPARED
BY ARC
M. M c L O U G H L I N ,
and Applied
Physics,
MELTING
J. L U N N E Y Trinity
AND LASER
and J.M.D. College,
MELTING
COEY
Dublin
2,
Ireland
M e l t i n g o r t h o r h o m b i c Y B a 2 C u 3 0 7 e i t h e r in an arc f u r n a c e or w i t h 7ms l a s e r p u l s e s of w a v e l e n g t h 1 . 0 6 ~ m (7 x 103 W . c m -2) l e a d s to s i m i l a r , c o m p l e x m i x t u r e s of o x i d e s . In b o t h c a s e s the p u r e o r t h o r h o m b i c s u p e r c o n d u c t i n g p h a s e c a n be r e s t o r e d b y an oxygen anneal. The l a s e r m e l t d e p t h (15~m) is d i s c u s s e d in t e r m s of t h e t h e r m a l p a r a m e t e r s of the Y - B a - C u o x i d e .
A g o a l in the m a t e r i a l s t e c h n o l o g y of the copper perovskite superconductors is p r o d u c t i o n of f u l l y - d e n s e p o l y c r y s t a l l i n e material with good electrical contact between individual crystallites. Here we d e m o n s t r a t e that the p u r e s u p e r c o n d u c t i n g p h a s e can be r e c o n s t i t u t e d f r o m a m i x t u r e of oxides in melts produced in two d i f f e r e n t ways. The ternary orthorhombic oxides RBa2Cu30 x have optimum superconducting properties when x ~ 7 [i~, which corresponds f o r m a l l y to one Cu ~+ or one hole in t h e O(2p) band per formula. Neither is stable at elevated temperature; o x y g e n loss t o t h e e x t e n t 6.0 ~ x ~ 6.5 ( e s s e n t i a l l y f r o m ib sites) corresponds to transformation of the orthorhombic superconductor (space g r o u p Pmmm) to a tetragonal semiconductor (space group P4/mmm) [2]. A n n e a l i n g in o x y g e n in the r a n g e 400 - 600 °C is k n o w n to restore the orthorhombic phase. However, melting, at temperatures in excess of 1040 °C leads to disproportionation of the o x i d e i n t o a m i x t u r e of p h a s e s . H e r e we s t u d y the e f f e c t s of m e l t i n g YBa2Cu307, b o t h in an a r c f u r n a c e a n d w i t h 7 ms p u l s e s f r o m a N d - Y A G laser. We s h o w t h a t t h e m e l t p r o d u c t s are s i m i l a r in b o t h cases, a n d t h a t t h e o t h o r h o m b i c superconducting p h a s e w i t h x - 7 can be restored by annealing in oxygen. Microstructures and densities of the p r o d u c t s are e x a m i n e d . The s t a r t i n g m a t e r i a l was Y - B a - C u o x i d e w i t h a 1:2:3 m e t a l ratio, p r e p a r e d by coprecipitation of t h e o x a l a t e s and firing in a i r at 940°C [3]. X-ray d i f f r a c t i o n s h o w e d that Y B a 2 C u 3 0 x was the m a i n p h a s e (a = 3 . 8 2 1 ( 4 ) i , b = 3 . 8 9 2 ( 4 ) i , c : II.679(12)A) , and that it was partially oriented at t h e surface. A minor amount of Y2BaCuO5 was also present. Scanning electron micrographs (Fig. la) s h o w an o p e n m i c r o s t r u c t u r e ,
0921 4534/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
a
b
C
F i g u r e i. S c a n n i n g e l e c t r o n m i c r o g r a p h s showing the microstructures of a) s t a r t i n g Y - B a - C u oxide, b) the same a f t e r arc m e l t i n g a n d o x y g e n a n n e a l a n d c) the s a m e a f t e r l a s e r m e l t i n g . The s c a l e b a r r e p r e s e n t s 25 ~m. w i t h s l a b - l i k e c r y s t a l l i t e s up to a b o u t 5~m thick. The density, measured by weighing in a c e t o n e a n d air, m a k i n g no a l l o w a n c e for f i l l e d p o r e s in the sample, was 5.74 g . c m -3 w h i c h is 89% of t h e theoretical density (6.42 g . c m - 3 ) . The superconducting transition temperature Tsc determined by ac susceptibility m e a s u r e m e n t s (Fig. 2a) was 90(1) K.
X.Z. Wang et al. / YBa2Ctt307 prepared by arc melting and laser melting
406
!
I-
•~
l
I
I
I
6 X
!
b/ I I 80
I
I 120
I T(K)
I 160
F i g u r e 2. AC m a g n e t i c s u s c e p t i b i l i t y d a t a on a) s t a r t i n g Y - B a - C u o x i d e a n d b) t h e same a f t e r arc m e l t i n g a n d o x y g e n anneal. Five-gram pellets were arc-melted in a i r at a p r e s s u r e of 500 m b a r . The m e l t has a low s u r f a c e tension, a n d fills the depression in t h e c o p p e r h e a r t h of the furnace. D i s c s a m p l e s 14 m m d i a m e t e r a n d 1 m m t h i c k w e r e i r r a d i a t e d w i t h 7 ms l a s e r l a s e r p u l s e s at 1.06 ~m. M e l t i n g was a c h e i v e d w i t h a p u l s e e n e r g y of 3.4 J w e a k l y f o c u s s e d t o a 3 m m d i a m e t e r spot g i v i n g a t h r e s h o l d i r r a d i a n c e for m e l t i n g of 7 x 1 0 3 W . c m -2. A continuous melted s u r f a c e l a y e r was o b t a i n e d b y r a s t e r i n g the sample in the beam. X-ray diffraction of the arc-melted and l a s e r - m e l t e d m a t e r i a l s h o w e d a m i x t u r e of phases: Y203 , tetragonal YBa2Cu3Ox, Cu403, Y B a 3 C u ~ O x a n d B a C u O 2. The l a t t e r f o r m e d an o r l e n t e d l a y e r on t h e u p p e r s u r f a c e of the a r c - m e l t e d b u t t o n s . A n n e a l i n g of the m e l t e d s a m p l e s was carried out for 20 h o u r s in flowing o x y g e n . X - r a y p a t t e r n s of the a r c - m e l t e d button and powder as well as the laser-melted s u r f a c e t h e n s h o w a pure, orthorhombic 1:2:3 phase with a = 3.820(4)A, b = 3.889(4)A and c = ii. 6 7 4 (12) A. The superconducting t r a n s i t i o n t e m p e r a t u r e (Fig 2b) was 97(1) K . Thermopiezic analysis [4] t r a c e s (Fig.3) indicate thermal desorption behaviour that is typical of the stoichiometric superconducting phase[5] . Micrographs t a k e n of t h e s a m p l e s a f t e r t h e o x y g e n a n n e a l are s h o w n in F i g . l b & c. The m a t e r i a l is s o m e w h a t m o r e c o m p a c t after arc melting, a n d its d e n s i t y was measured as 5.98 g . c m -3, or 93% of t h e t h e o r e t i c a l d e n s i t y . In the l a s e r - t r e a t e d sample, a dense surface layer a p p r o x i m a t e l y 15 ~ m t h i c k was o b t a i n e d . Polycrystalline YBa2Cu30 x presents a decided advantage for laser melting compared with ordinary metals thanks to a c o m b i n a t i o n of low r e f l e c t i v i t y (R - 0.2) [6], which means that the light is r e a d i l y c o u p l e d to the sample, a n d poor thermal conductivity of the polycrystalline ceramic. Taking the
400
!
!
800
"~l
I
400
I
800 T[°C)
Figure 3. T h e r m o p i e z i c analysis (TPA) traces from vacuum of a) Typical YBa2Cu30 7 superconductor and b) the arc-melted and annealed sample. Heating r a t e is 2 0 ° C . m i n -I. D u l o n g - P e t i t v a l u e for s p e c i f i c heat (C = 324 J.mole-i .K -I ) and a melting temperature of 1 2 8 0 K, t h e threshold i r r a d i a n c e for m e l t i n g (12 J . c m -2) l e a d s t o an e s t i m a t e of 0.6 W . m - I K -I for t h e thermal conductivity. This estimate may be compared with the value of 0.5 W . m - I K -I m e a s u r e d at i00 K [7]. The t h e r m a l d i f f u s i o n l e n g t h is g i v e n as k = (4~t/pC) I/2, w h e r e t is the p u l s e length. T a k i n g t = 7 ms, t h i s g i v e s k = 80 ~ m which is c o m p a t i b l e with the observed melt depth of - 15 ~ m . A detailed n u m e r i c a l a n a l y s i s of the m e l t i n g p r o c e s s w i l l r e q u i r e d a t a on t h e l a t e n t h e a t of f u s i o n as well as a c c u r a t e v a l u e s of K a n d C at e l e v a t e d t e m p e r a t u r e s . In c o n c l u s i o n , we h a v e s h o w n t h a t laser melting followed by an oxygen a n n e a l at a r e l a t i v e l y low temperature offers the prospect of m a k i n g rather dense superconducting l a y e r s some i0 ~ m in t h i c k n e s s .
REFERENCES [I] S. T a n a k a et al, Int. J. M o d e r n P h y s B1 (1987) 755; R . J . C a v a , ibid 813. [2] C . C . T o r a d i , E.M.McCarron, P.B.Bierstedt, A . W S l e i g h t a n d D . E . C o x , S o l i d S t a t e C o m m . 6 4 (1987) 497; J . D . J o r g e n s e n et al. Phys. Rev. B36 (1987) 5731. [3] X . Z . W a n g , M . H e n r y , J . L i v a g e a n d I . R o s enmen, S o l i d S t a t e Comm. 64 (1987) 881. [4] D . H . R y a n a n d J . M . D . C o e y , J. P h y s E l 9 (1986) 693 [5] J . M . D . C o e y , K . D o n n e l l y a n d F . S u p p l e , in Proc. E u r o p e a n W o r k s h o p on H i g h - T c Superconductors and Potential Applications, (CEC, B r u s s e l s 1987) p.423. [6] J . O r e n s t e i n et al, Phys. Rev. B 3 6 (1987) 729 [7] D . T . M o r e l l i , J . H e r e m a n s and D.E.Swets, Phys. Rev. B36 (1987) 3917.