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Preparation, crystallization and properties of rapidly solidified YBa2Cu3O7−δ
Volume 5. number
MATERIALS
IO
LETTERS
September
1987
PREPARATION, CRYSTALLIZATION AND PROPERTIES OF RAPIDLY SOLIDIFIED YBa2Cu307_6 N.K. KIM, L. DROZDYK,
D.A. PAYNE
Department ofMaterials Science and Engineering, and Materials Research Laboratory, Unrversity ofIllinois at Urbana-Champaign, 105 S. Goodwin Avenue, Urbana, IL 61801, USA
T.A. FRIEDMANN,
W.H. WRIGHT
and D.M. GINSBERG
Department of Physics and Materials Research Laboratory, 105 S. Goodwin Avenue, Urbana, IL 61801, USA Received
29 June 1987
YBazCu30,_B was prepared down to liquid-He temperature. A well-defined superconducting
in the glassy state by rapid solidification from the melt. The quenched material was paramagnetic Diamagnetic behavior developed after crystallization of the glass at high temperatures ( > 800°C). transition occurred at 90 K for material heat treated and crystallized above 900°C.
1. Introduction
tallization behavior, and superconducting ties, are reported below.
Earlier this year, Chu reported on superconductivity in the Yz03-BaO-CuO system [ 11, with critical temperatures ( Tc) greater than 90 K. Since this announcement, numerous investigators have worked in the system [ 21, and it is generally recognized that the principal superconducting phase is YBazCu30, _+ Most of the early measurements were made on polycrystalline specimens, prepared by the mixed-oxide route, many were heterophasic by nature, and some contained a glassy phase. The possibility of a superconducting glass or amorphous phase, with intergranular connectivity, has already been suggested. In this communication we report on the preparation of “glassy YBa2Cu30,_6” via a rapid solidification method. The method involves splat-cooling of the melt between rapidly revolving twin-rollers. No evidence of superconductivity was found for the “asquenched” material. Superconductivity only developed after heat treatments at high temperatures ( > 8OO”C), followed by annealing at lower temperatures ( > 400°C). The heat treatment involved crystallization of the quenched material and the development of long-range order. Details of the crys0167-577x/87/$ (North-Holland
University of Illinois at Urbana-Champaign,
03.50 0 Elsevier Science Publishers Physics Publishing Division)
proper-
2. Experimental A twin-roller quencher was used for the splat-cooling of the melt. Fig. 1 illustrates the equipment used. An oxygen-acetylene flame was used to melt droplets from a YBaZCu307_6 source. The source was
.ylene
1-z ;r-+I_ Collector
Fig. I. Rapid solidification
B.V.
F,okes
Pan
ofYBaZCu30,_6.
387
Volume 5, number 10
MATERIALS LETTERS
prepared by reacting appropriate proportions of reagent grade chemicals Y203, BaCO,, and CuO at 920°C for 24 h, followed by size reduction and pressing into a bar. X-ray diffraction determined the calcined powders to be YBa&u@_~, which after slow cooling (1 “C/min ) and annealing in oxygen (600-4OO”C), was found to be highly diamagnetic at liquid-nitrogen temperature, as judged by a simple magnetic repulsion test. Rapid solidification occurred as the molten droplets were allowed to fall between, and have intimate contact with, two counter-rotating metal rollers. The separation distance and speed of revolution were adjusted to give the best quenching results. No glassformers were used in the present study. Estimates of the material’s quench rate were of the order of lO”C/s. The quenched flakes were IO-20 urn thick and several millimeters wide. A SQUID magnetometer was used to measure magnetic susceptibility characteristics at reduced temperatures. The magnetic field was carefully reduced to zero before insertion of the samples, so they were “zero-field cooled” from room temperature to the lowest temperatures. The measured parameters were the sample’s magnetic moment and mass, the magnetic field H (21 Oe), and temperature. The susceptibility X=&I/H was found by multiplying the magnetic moment by the density (as determined from X-ray diffraction data) and dividing by H. All magnetic measurements were made on specimens which had been equilibrated at 430°C for 6 h in flowing oxygen. Crystallization behavior was investigated by X-ray diffraction, differential scanning calorimetry, and electron microscopy.
3. Results The as-quenched flakes were black in color, which gradually changed to orange on standing in ambient conditions. The color change could be reversed by heating, or prevented by storing in a desiccator. Magnetic susceptibility measurements determined the black flakes were paramagnetic down to liquidhelium temperature. The black flakes were mainly amorphous to X-ray diffraction, but with some microcrystallinity (fig. 2). On heating to 600°C in air, differential scanning calorimetry determined 388
September 1987
l
YBa2Cu307-6
600”C,4h
As-quenched
60
1
I
I
I
50
I
I
30 28
Zqrees)
Fig. 2. Room-temperature X-ray diffraction data, for specimens processed under various heat-treatment conditions.
three diffuse and irreversible exotherms at 270, 335, and 365°C. The only phase which could be identified after initial crystallization was Y20) (fig. 2). This was an intermediate phase, which decreased in concentration with increasing temperature. Fig. 2 illustrates the development of YBaZCu307_-6 with increasing temperature. After heat treatment at 820°C for 4 h the grains had grown with uniform shape and size to 0.4 urn (fig. 3). Fig. 2 also illustrates the increasing development of the major YBazCu,07_d phase after heat treatment at 920°C for 4 h. The grain shape changed with increasing temperature. After heat treatment at 950°C for 12 h the grains had a more platelet morphology and were of a larger size (e.g., 15 urn X 2 urn; fig. 4). Magnetic moment measurements showed the development of diamagnetism for specimens heat treated above 820°C (fig. 5). The superconducting transition was clearly established at 90 K for specimens crystallized at 920°C. The extent of the Meissner effect increased with increasing heat-treatment temperature (fig. 5). This appeared to be asso-
Volume 5, number
10
MATERIALS
September
LETTERS
I-
I
,
I
,
I
,
7
,
1987
0
As-Quenched /
O-4$ -I -
4
A
4
+”
G
R
X
Fig. 3. SEM photomicrograph of ceramic heat treatment at 820°C for 4 h.
microstructure,
after
-6-
-7-X” 0
9999nno-
i 820°C
x 7 20
X
’
.950x
-
X
’ 40
’
’ 60
’
’ 00
’
100
T(K)
Fig. 5. Magnetic susceptibility
Fig. 4. SEM photomicrograph of ceramic heat treatment at 950°C for 12 h.
microstructure,
after
ciated with the degree of crystallinity and the extent of grain growth. For example, material crystallized at 950°C for 12 h had 87% of the theoretical value (1/4x ) associated with a full Meissner effect when measured at the lowest temperature. Superconductivity was never detected in glassy or amorphous specimens of YBarCu307_a.
4. Summary
Rapidly solidified YBa2Cu307_-6 was prepared by splat-cooling of the melt in a twin-roller quencher. The as-quenched flakes were principally amorphous
measurements
at low temperatures.
to X-ray diffraction, but with some microcrystallinity. The “glassy” material was paramagnetic down to liquid-helium temperature. Diamagnetic behavior only developed after heat treatment at high temperatures ( > SOO’C). This was associated with crystallization and grain growth of the ceramic A well-defined superconducting microstructure. transition developed at 90 K for material heat treated above 900°C. Superconductivity appeared to be associated with well-crystallized ceramic microstructures having long-range order. No evidence of superconducting behavior was detected for the glassy or amorphous phase.
Acknowledgement
This work was supported by the US Department of Energy, Division of Materials Sciences, under contract DE-AC023-76ERO1198, and by the National Science Foundation, Division of Materials Research under contracts DMR 85-01346 and DMR 86-12860. The former contract supported materials preparation, X-ray diffraction, electron microscopy and differential scanning calorimetry by NKK, LD and DAP. The latter contracts supported the magnetic susceptibility measurements of TAF, WHW and DMG. 389
Volume 5. number 10
MATERIALS LETTERS
References [ 1] C.W. Chu, Superconductivity above 90 K, presented at the National Academy of Sciences Seminar, March 23, 1987, Proc. Natl. Acad. Sci. US, to be published.
390
September 1987
[ 21 See for example, preprint announcements in High T, Update. Division of Materials Sciences, Office of Basic Energy Sciences, US DOE.
MATERIALS
IO
LETTERS
September
1987
PREPARATION, CRYSTALLIZATION AND PROPERTIES OF RAPIDLY SOLIDIFIED YBa2Cu307_6 N.K. KIM, L. DROZDYK,
D.A. PAYNE
Department ofMaterials Science and Engineering, and Materials Research Laboratory, Unrversity ofIllinois at Urbana-Champaign, 105 S. Goodwin Avenue, Urbana, IL 61801, USA
T.A. FRIEDMANN,
W.H. WRIGHT
and D.M. GINSBERG
Department of Physics and Materials Research Laboratory, 105 S. Goodwin Avenue, Urbana, IL 61801, USA Received
29 June 1987
YBazCu30,_B was prepared down to liquid-He temperature. A well-defined superconducting
in the glassy state by rapid solidification from the melt. The quenched material was paramagnetic Diamagnetic behavior developed after crystallization of the glass at high temperatures ( > 800°C). transition occurred at 90 K for material heat treated and crystallized above 900°C.
1. Introduction
tallization behavior, and superconducting ties, are reported below.
Earlier this year, Chu reported on superconductivity in the Yz03-BaO-CuO system [ 11, with critical temperatures ( Tc) greater than 90 K. Since this announcement, numerous investigators have worked in the system [ 21, and it is generally recognized that the principal superconducting phase is YBazCu30, _+ Most of the early measurements were made on polycrystalline specimens, prepared by the mixed-oxide route, many were heterophasic by nature, and some contained a glassy phase. The possibility of a superconducting glass or amorphous phase, with intergranular connectivity, has already been suggested. In this communication we report on the preparation of “glassy YBa2Cu30,_6” via a rapid solidification method. The method involves splat-cooling of the melt between rapidly revolving twin-rollers. No evidence of superconductivity was found for the “asquenched” material. Superconductivity only developed after heat treatments at high temperatures ( > 8OO”C), followed by annealing at lower temperatures ( > 400°C). The heat treatment involved crystallization of the quenched material and the development of long-range order. Details of the crys0167-577x/87/$ (North-Holland
University of Illinois at Urbana-Champaign,
03.50 0 Elsevier Science Publishers Physics Publishing Division)
proper-
2. Experimental A twin-roller quencher was used for the splat-cooling of the melt. Fig. 1 illustrates the equipment used. An oxygen-acetylene flame was used to melt droplets from a YBaZCu307_6 source. The source was
.ylene
1-z ;r-+I_ Collector
Fig. I. Rapid solidification
B.V.
F,okes
Pan
ofYBaZCu30,_6.
387
Volume 5, number 10
MATERIALS LETTERS
prepared by reacting appropriate proportions of reagent grade chemicals Y203, BaCO,, and CuO at 920°C for 24 h, followed by size reduction and pressing into a bar. X-ray diffraction determined the calcined powders to be YBa&u@_~, which after slow cooling (1 “C/min ) and annealing in oxygen (600-4OO”C), was found to be highly diamagnetic at liquid-nitrogen temperature, as judged by a simple magnetic repulsion test. Rapid solidification occurred as the molten droplets were allowed to fall between, and have intimate contact with, two counter-rotating metal rollers. The separation distance and speed of revolution were adjusted to give the best quenching results. No glassformers were used in the present study. Estimates of the material’s quench rate were of the order of lO”C/s. The quenched flakes were IO-20 urn thick and several millimeters wide. A SQUID magnetometer was used to measure magnetic susceptibility characteristics at reduced temperatures. The magnetic field was carefully reduced to zero before insertion of the samples, so they were “zero-field cooled” from room temperature to the lowest temperatures. The measured parameters were the sample’s magnetic moment and mass, the magnetic field H (21 Oe), and temperature. The susceptibility X=&I/H was found by multiplying the magnetic moment by the density (as determined from X-ray diffraction data) and dividing by H. All magnetic measurements were made on specimens which had been equilibrated at 430°C for 6 h in flowing oxygen. Crystallization behavior was investigated by X-ray diffraction, differential scanning calorimetry, and electron microscopy.
3. Results The as-quenched flakes were black in color, which gradually changed to orange on standing in ambient conditions. The color change could be reversed by heating, or prevented by storing in a desiccator. Magnetic susceptibility measurements determined the black flakes were paramagnetic down to liquidhelium temperature. The black flakes were mainly amorphous to X-ray diffraction, but with some microcrystallinity (fig. 2). On heating to 600°C in air, differential scanning calorimetry determined 388
September 1987
l
YBa2Cu307-6
600”C,4h
As-quenched
60
1
I
I
I
50
I
I
30 28
Zqrees)
Fig. 2. Room-temperature X-ray diffraction data, for specimens processed under various heat-treatment conditions.
three diffuse and irreversible exotherms at 270, 335, and 365°C. The only phase which could be identified after initial crystallization was Y20) (fig. 2). This was an intermediate phase, which decreased in concentration with increasing temperature. Fig. 2 illustrates the development of YBaZCu307_-6 with increasing temperature. After heat treatment at 820°C for 4 h the grains had grown with uniform shape and size to 0.4 urn (fig. 3). Fig. 2 also illustrates the increasing development of the major YBazCu,07_d phase after heat treatment at 920°C for 4 h. The grain shape changed with increasing temperature. After heat treatment at 950°C for 12 h the grains had a more platelet morphology and were of a larger size (e.g., 15 urn X 2 urn; fig. 4). Magnetic moment measurements showed the development of diamagnetism for specimens heat treated above 820°C (fig. 5). The superconducting transition was clearly established at 90 K for specimens crystallized at 920°C. The extent of the Meissner effect increased with increasing heat-treatment temperature (fig. 5). This appeared to be asso-
Volume 5, number
10
MATERIALS
September
LETTERS
I-
I
,
I
,
I
,
7
,
1987
0
As-Quenched /
O-4$ -I -
4
A
4
+”
G
R
X
Fig. 3. SEM photomicrograph of ceramic heat treatment at 820°C for 4 h.
microstructure,
after
-6-
-7-X” 0
9999nno-
i 820°C
x 7 20
X
’
.950x
-
X
’ 40
’
’ 60
’
’ 00
’
100
T(K)
Fig. 5. Magnetic susceptibility
Fig. 4. SEM photomicrograph of ceramic heat treatment at 950°C for 12 h.
microstructure,
after
ciated with the degree of crystallinity and the extent of grain growth. For example, material crystallized at 950°C for 12 h had 87% of the theoretical value (1/4x ) associated with a full Meissner effect when measured at the lowest temperature. Superconductivity was never detected in glassy or amorphous specimens of YBarCu307_a.
4. Summary
Rapidly solidified YBa2Cu307_-6 was prepared by splat-cooling of the melt in a twin-roller quencher. The as-quenched flakes were principally amorphous
measurements
at low temperatures.
to X-ray diffraction, but with some microcrystallinity. The “glassy” material was paramagnetic down to liquid-helium temperature. Diamagnetic behavior only developed after heat treatment at high temperatures ( > SOO’C). This was associated with crystallization and grain growth of the ceramic A well-defined superconducting microstructure. transition developed at 90 K for material heat treated above 900°C. Superconductivity appeared to be associated with well-crystallized ceramic microstructures having long-range order. No evidence of superconducting behavior was detected for the glassy or amorphous phase.
Acknowledgement
This work was supported by the US Department of Energy, Division of Materials Sciences, under contract DE-AC023-76ERO1198, and by the National Science Foundation, Division of Materials Research under contracts DMR 85-01346 and DMR 86-12860. The former contract supported materials preparation, X-ray diffraction, electron microscopy and differential scanning calorimetry by NKK, LD and DAP. The latter contracts supported the magnetic susceptibility measurements of TAF, WHW and DMG. 389
Volume 5. number 10
MATERIALS LETTERS
References [ 1] C.W. Chu, Superconductivity above 90 K, presented at the National Academy of Sciences Seminar, March 23, 1987, Proc. Natl. Acad. Sci. US, to be published.
390
September 1987
[ 21 See for example, preprint announcements in High T, Update. Division of Materials Sciences, Office of Basic Energy Sciences, US DOE.