- Email: [email protected]
Controlled microstructure formation in high temperature superconductors by melt processing
Journal of the Less-Common Metals, 144 & 165 (1990) 1413-1419
1413
CONTROLLED MICROSTRUCTURE FORMATION IN HIGH TEMPERATURE SUPERCONDUCTORS BY MELT PROCESSING G. J. SCHMRZ, H. WEISS, Ch. WOLTERS Aachen Center for Solidification in Space, ACCESS e.V, Intzestr. 5, D-5100 Aachen, FRG
1.
INTRODUCTION
Physical properties of most materials are already largely determined during the phase transition liquid to solid, the solidification of the material. Solidification technologies allow to control the formation of microstructure in bulk, polycrystalline materials to a very large extent and - in addition - are the basis for a technological production with respect to shaping and contineous processes. Solidification of high temperature superconductors with a controlled microstructure thus seems to be a method to transfer the good superconducting properties known from single crystalline superconductors to their polycrystalline counterparts.
2.
EXPERIMENTAL
Y~BazCu307., and Bi,CuzCa,CuzO, were solidified at various cooling rates (IO-3 K/s - 106 K/s) using Bridgman type furnaces, centrifugal casting, a meltspinning device, a drop tube and a gas atomization set-up. These experiments mainly followed two different objectives: a)
low cooling rates in directional solidification
-
orientation of grains parallel to the high jc ab-planes avoiding grain boundaries perpendicular to the transport current providing large contact areas between grains parallel to the transport current
0022-5088/90/.$3.50
0 Elsevier Sequoia, Printed in The Netherlands
1414
b)
high cooling rates in rapid solidification controlling grain size introduction of solid precipitates at a pinning effective scale introduction of bubbles and/or voids for the same purpose, but providing an even stronger elementary vortex pin interaction
The directional solidification experiments were mainly carried out with the Y-Ba-Cu-0 system. The Bi-Sr-Ca-CuO system also yielded some interesting results with respect to texutre, the main research work on this system, however, was on rapid solidification. 2.1 DIRECTIONAL SOLiDiFiCATiON Several methods of texturing high T, superconductors already yielded significantly enhanced transport current densities in bulk, poiycrystaiiine materials. The highest values of transport jc have been reached by melt texturing’ and solidification at very low cooling rates2 and were observed in highly textured regions of some mm3. Between such regions, however, the transport current density is severely limited by cracks evoked by the tetragonai to orthorhombic phase transformation, fig. 1.
Fig. 1: Highly textured regions in boundaries YIBazCu307. The between these regions acting as weak links severely limit the critical transport current density.
Methods of directional solidification using a seeding technique can help avoiding these cracks and provide highly textured regions on a macroscopic lengthscale.
1415
Y1Ba2Cu30,_x precursor powder was compacted into cylindrical alumina crucibles, partially melted and directionally solldlfled in a Bridgman type furnace at temperature gradients G between 10 K/cm and 50 K/cm and solidification velocities v of about mm/h, keeping the cooling rate (dT/dt = G x v) constant at about 1 K/h, which is essential to achieve high current densitlesz. One of the highly textured regions achieved by this method (see fig. 1) was used as a seeding crystal for an additional directional solidification keeping the other process parameters unchanged. Some investlgations on directional solidification of BlzSr,Ca,C%O, were carried out at higher solldiflcatlon velocities (3 mm/h - 5 mm/mln) and higher temperature gradients (50 Wcm - 150 K/cm). All samples were oxygen treated to restore superconductivity (48 h, 5OO”C, 1 bar 0-J 2.2 RAPID SOLIDIFICATION
OF BizSrzCaiCuzOe
Precipitates produced in BlzSrzCa&u208 by rapid solidification of Bi2Sr2Ca2Cu3010 (2223) have been reported to significantly Increase the critical current density of this materiaP. By means of a melt spinning device, a drop tubes and a melt atomization set up6 Bi,Sr,CaICuzOs melts saturated with excess Cu (Ag) In the molten state have been rapidly solidified at cooling rates between 102 - 106 K/s. In some experiments the concentration nominal composition of the 2212 material were used as an additional parameter.
of the copper exceeding the and the concentration of silver
Using the melt atomization setup also pure 2212 with either gaseous nitrogen or gaseous helium
material
was atomized
The resulting specimen were oxygen treated’ to allow precipitation ripening of the precipitates and to restore superconductivity.
and
2.3 SAMPLE CHARACTERIZATION Metallographic analysis of the samples (both directionally and rapidly solidified) was carried out by SEM, EDX DTA, DSC, optical microscopy and x-ray texture analysis. Superconducting properties were determined as well by magnetic methods as also by resistive measurements. The reslstive measurements were carried out at 77K using a standard four probe technique (PpVlcm
1416
3.
RESULTS
The main result on directionally solidified Y,Ba,Cu30T_, is to be seen in a sample prepared by the seeding technique, fig. 2, which yields critical transport current densities exceeding 1000 A/cm2 (at 77K, OT) and only weak field dependance (450 A/cmz, 77 K, 40 mT) on a macroscopic scale.
Fig. 2: Cross section of a seeded sample. Textured regions exhibit a very high aspect ratio on a remarkable length scale as well as a high transport current density
Bi,Sr&a,f&O, samples prepared by directional solidification also yielded a high degree of texture, fig. 3. Preliminary transport current density determinations in these Samples indicate a rather low jc ( 100 A/cmz, 77 K, 0 T), which is probably due to contaminations by the Alumina crucible.
1417
Rapid solidification of the Bi 2212 system with excess copper or silver led to precipitation of Cu-rich (Ag-rich) phases, fig.4, The density of the precipitates (i.e. the number of precipitates per unit volume) can be influenced by the cooling rate applied. The mean diameter of the precipitates can be controlled either via ripening (heat treatment duration) fig. 5, or by the initial composition of the melt, fig 6.
Fig. 4: Cu-rich precipitates in BizSr2CaICu2@ produced by melt spinning
already can be seen in
the as quenched state.
1
1.0
-lz 3 1-a
b b
b
t
l
0.5
I0.l;::::::::
./’
300 --
_,’
l
1
5
zoo--
:c rIhl
‘0
d: meandiameter of predpitales 1: duration of heaf treatment
71:
foo-~
OL0
---1 2
4 c I
I
I
3
n
M
(2
wghtlb AQ
Fig. 5: Mean diameter of Cu-rich-preci-
Fig. 6: Mean diameter of silver-rich precipita-
pitates as a function of heat precipitates treatment duration. A heat treat-
tes after a heat treatment
ment of minimum 4 hours is neccessary to restore superconductivity.
of 4 hours. The
mean diameter obviously can be influenced by the initial melt composition.
1418
Melt atomization fo the pure Bi 2212 melt yielded an extended solubility of both He and NZ (e.g. 30 ppm Helium) in the atomized powders. Oxygen heat treatment of these powders led to the formation of bubbles (voids) on a micrometer scale, fig. 7.
Fig. 7:
Bubbles or voids in
SE&r2CaiCu2@ atomization
produced by melt
might
lead to even
more effective pinning compared to
solid
precipitates
as
known
from conventional superconductors. Phase pure 2212 could always be restored in these powders, which were amorphwus after atomization. Superconducting properties being determined by magnetic measurements indicate a slight increase in jc (applying the Bean model) compared to the precursor powder. CONCLUSIONS Melt processing of high Tc sup~condu~tors Ss a powerful tool to enhance the supercondu~ing (and most probably simu~an~usly also the mechanical) properties of these materials. Especially methods of directional solidification already yield a high transport current density on a cm length scale. Both - current density and sample dimensions- can still be increased. Methods of rapid soildificaiton are most useful to introduce precipitates into the superconducting material, which might enhance both superconducting and mechanical properties via pinning of flux lines and/or dislocatlons, respectively. The actual size of the precipitates being too large by a factor of 5 to provide effective pinnlng (if pinning at interfaces Is considered) can still be decreased varying other parameters of the rapid solidification process (e.g. superheating of the melt)
1419
ACKNOWLEDGEMENTS The authors are grateful to 2. Physikaiisches institut and to institut fur Metaiikunde und Metaiiphysik der RWTH Aachen for carrying out the magnetic measurements and the texutre analysis respectively. Thanks are also due to Hoechst AG for providing large quantities of the 2212 material. This work is supported by BMFT under grant No. FKZ 13 N 55 65. REFERENCES /l/
S. Jin et. al., Phys. Rev. B 37 (1988)
7850
/2/
K. Saiama
/3/
D. Shi et. al., Phys. Rev. 640
/4/
W. Hug, P.R. Sahm, GieRereiforschung
/5/
L. Kaiiien et. al., GieBereiforschung
/6/
L. Kaliien, VDI Fortschrittsberichte dorf, FRG
/7/
J. Bock, E. Preisier, “Preparation of Single Phase 2212 BiSrCaCuO by Annealing of Cast Solid Compact Material” MRS Spring Meeting, San Diego, CA, 1989
et.ai., Appi. Phys. Lett. 54 (1988)
2352
(1989) 5255 40 (1988) 138
56
(1986)
5, 157, VDI Veriag,
IS/ G.J. Schmitz et. al., “improved Contact . . Drppmg of Y~Ba2Cu307.:, this volume
Resistances
73 Dussei-
by Melt
1413
CONTROLLED MICROSTRUCTURE FORMATION IN HIGH TEMPERATURE SUPERCONDUCTORS BY MELT PROCESSING G. J. SCHMRZ, H. WEISS, Ch. WOLTERS Aachen Center for Solidification in Space, ACCESS e.V, Intzestr. 5, D-5100 Aachen, FRG
1.
INTRODUCTION
Physical properties of most materials are already largely determined during the phase transition liquid to solid, the solidification of the material. Solidification technologies allow to control the formation of microstructure in bulk, polycrystalline materials to a very large extent and - in addition - are the basis for a technological production with respect to shaping and contineous processes. Solidification of high temperature superconductors with a controlled microstructure thus seems to be a method to transfer the good superconducting properties known from single crystalline superconductors to their polycrystalline counterparts.
2.
EXPERIMENTAL
Y~BazCu307., and Bi,CuzCa,CuzO, were solidified at various cooling rates (IO-3 K/s - 106 K/s) using Bridgman type furnaces, centrifugal casting, a meltspinning device, a drop tube and a gas atomization set-up. These experiments mainly followed two different objectives: a)
low cooling rates in directional solidification
-
orientation of grains parallel to the high jc ab-planes avoiding grain boundaries perpendicular to the transport current providing large contact areas between grains parallel to the transport current
0022-5088/90/.$3.50
0 Elsevier Sequoia, Printed in The Netherlands
1414
b)
high cooling rates in rapid solidification controlling grain size introduction of solid precipitates at a pinning effective scale introduction of bubbles and/or voids for the same purpose, but providing an even stronger elementary vortex pin interaction
The directional solidification experiments were mainly carried out with the Y-Ba-Cu-0 system. The Bi-Sr-Ca-CuO system also yielded some interesting results with respect to texutre, the main research work on this system, however, was on rapid solidification. 2.1 DIRECTIONAL SOLiDiFiCATiON Several methods of texturing high T, superconductors already yielded significantly enhanced transport current densities in bulk, poiycrystaiiine materials. The highest values of transport jc have been reached by melt texturing’ and solidification at very low cooling rates2 and were observed in highly textured regions of some mm3. Between such regions, however, the transport current density is severely limited by cracks evoked by the tetragonai to orthorhombic phase transformation, fig. 1.
Fig. 1: Highly textured regions in boundaries YIBazCu307. The between these regions acting as weak links severely limit the critical transport current density.
Methods of directional solidification using a seeding technique can help avoiding these cracks and provide highly textured regions on a macroscopic lengthscale.
1415
Y1Ba2Cu30,_x precursor powder was compacted into cylindrical alumina crucibles, partially melted and directionally solldlfled in a Bridgman type furnace at temperature gradients G between 10 K/cm and 50 K/cm and solidification velocities v of about mm/h, keeping the cooling rate (dT/dt = G x v) constant at about 1 K/h, which is essential to achieve high current densitlesz. One of the highly textured regions achieved by this method (see fig. 1) was used as a seeding crystal for an additional directional solidification keeping the other process parameters unchanged. Some investlgations on directional solidification of BlzSr,Ca,C%O, were carried out at higher solldiflcatlon velocities (3 mm/h - 5 mm/mln) and higher temperature gradients (50 Wcm - 150 K/cm). All samples were oxygen treated to restore superconductivity (48 h, 5OO”C, 1 bar 0-J 2.2 RAPID SOLIDIFICATION
OF BizSrzCaiCuzOe
Precipitates produced in BlzSrzCa&u208 by rapid solidification of Bi2Sr2Ca2Cu3010 (2223) have been reported to significantly Increase the critical current density of this materiaP. By means of a melt spinning device, a drop tubes and a melt atomization set up6 Bi,Sr,CaICuzOs melts saturated with excess Cu (Ag) In the molten state have been rapidly solidified at cooling rates between 102 - 106 K/s. In some experiments the concentration nominal composition of the 2212 material were used as an additional parameter.
of the copper exceeding the and the concentration of silver
Using the melt atomization setup also pure 2212 with either gaseous nitrogen or gaseous helium
material
was atomized
The resulting specimen were oxygen treated’ to allow precipitation ripening of the precipitates and to restore superconductivity.
and
2.3 SAMPLE CHARACTERIZATION Metallographic analysis of the samples (both directionally and rapidly solidified) was carried out by SEM, EDX DTA, DSC, optical microscopy and x-ray texture analysis. Superconducting properties were determined as well by magnetic methods as also by resistive measurements. The reslstive measurements were carried out at 77K using a standard four probe technique (PpVlcm
1416
3.
RESULTS
The main result on directionally solidified Y,Ba,Cu30T_, is to be seen in a sample prepared by the seeding technique, fig. 2, which yields critical transport current densities exceeding 1000 A/cm2 (at 77K, OT) and only weak field dependance (450 A/cmz, 77 K, 40 mT) on a macroscopic scale.
Fig. 2: Cross section of a seeded sample. Textured regions exhibit a very high aspect ratio on a remarkable length scale as well as a high transport current density
Bi,Sr&a,f&O, samples prepared by directional solidification also yielded a high degree of texture, fig. 3. Preliminary transport current density determinations in these Samples indicate a rather low jc ( 100 A/cmz, 77 K, 0 T), which is probably due to contaminations by the Alumina crucible.
1417
Rapid solidification of the Bi 2212 system with excess copper or silver led to precipitation of Cu-rich (Ag-rich) phases, fig.4, The density of the precipitates (i.e. the number of precipitates per unit volume) can be influenced by the cooling rate applied. The mean diameter of the precipitates can be controlled either via ripening (heat treatment duration) fig. 5, or by the initial composition of the melt, fig 6.
Fig. 4: Cu-rich precipitates in BizSr2CaICu2@ produced by melt spinning
already can be seen in
the as quenched state.
1
1.0
-lz 3 1-a
b b
b
t
l
0.5
I0.l;::::::::
./’
300 --
_,’
l
1
5
zoo--
:c rIhl
‘0
d: meandiameter of predpitales 1: duration of heaf treatment
71:
foo-~
OL0
---1 2
4 c I
I
I
3
n
M
(2
wghtlb AQ
Fig. 5: Mean diameter of Cu-rich-preci-
Fig. 6: Mean diameter of silver-rich precipita-
pitates as a function of heat precipitates treatment duration. A heat treat-
tes after a heat treatment
ment of minimum 4 hours is neccessary to restore superconductivity.
of 4 hours. The
mean diameter obviously can be influenced by the initial melt composition.
1418
Melt atomization fo the pure Bi 2212 melt yielded an extended solubility of both He and NZ (e.g. 30 ppm Helium) in the atomized powders. Oxygen heat treatment of these powders led to the formation of bubbles (voids) on a micrometer scale, fig. 7.
Fig. 7:
Bubbles or voids in
SE&r2CaiCu2@ atomization
produced by melt
might
lead to even
more effective pinning compared to
solid
precipitates
as
known
from conventional superconductors. Phase pure 2212 could always be restored in these powders, which were amorphwus after atomization. Superconducting properties being determined by magnetic measurements indicate a slight increase in jc (applying the Bean model) compared to the precursor powder. CONCLUSIONS Melt processing of high Tc sup~condu~tors Ss a powerful tool to enhance the supercondu~ing (and most probably simu~an~usly also the mechanical) properties of these materials. Especially methods of directional solidification already yield a high transport current density on a cm length scale. Both - current density and sample dimensions- can still be increased. Methods of rapid soildificaiton are most useful to introduce precipitates into the superconducting material, which might enhance both superconducting and mechanical properties via pinning of flux lines and/or dislocatlons, respectively. The actual size of the precipitates being too large by a factor of 5 to provide effective pinnlng (if pinning at interfaces Is considered) can still be decreased varying other parameters of the rapid solidification process (e.g. superheating of the melt)
1419
ACKNOWLEDGEMENTS The authors are grateful to 2. Physikaiisches institut and to institut fur Metaiikunde und Metaiiphysik der RWTH Aachen for carrying out the magnetic measurements and the texutre analysis respectively. Thanks are also due to Hoechst AG for providing large quantities of the 2212 material. This work is supported by BMFT under grant No. FKZ 13 N 55 65. REFERENCES /l/
S. Jin et. al., Phys. Rev. B 37 (1988)
7850
/2/
K. Saiama
/3/
D. Shi et. al., Phys. Rev. 640
/4/
W. Hug, P.R. Sahm, GieRereiforschung
/5/
L. Kaiiien et. al., GieBereiforschung
/6/
L. Kaliien, VDI Fortschrittsberichte dorf, FRG
/7/
J. Bock, E. Preisier, “Preparation of Single Phase 2212 BiSrCaCuO by Annealing of Cast Solid Compact Material” MRS Spring Meeting, San Diego, CA, 1989
et.ai., Appi. Phys. Lett. 54 (1988)
2352
(1989) 5255 40 (1988) 138
56
(1986)
5, 157, VDI Veriag,
IS/ G.J. Schmitz et. al., “improved Contact . . Drppmg of Y~Ba2Cu307.:, this volume
Resistances
73 Dussei-
by Melt