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Recrystallization of 110 K high-Tc Bi2Sr2Ca2Cu3Ox superconducting phase from the molten state and characterizations
Volume 9, number
MATERIALS
12
RECRYSTALLIZATION SUPERCONDUCTING
B.M. MOON,
OF 110 K HIGH-T,
August
LETTERS
1990
Bi2Sr2Ca2Cu30x
PHASE FROM THE MOLTEN
STATE AND CHARACTERIZATIONS
G. KORDAS
Science and Technology Centerfor Superconductivity. and Department University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
of Materials
Science and Engineering,
Ceramics
Dlvlsion.
and B. LALEVIC Department of Electrical and Computer Engineering, Piscataway, NJ 08855-0909, USA Received
Rutgers - The State University ofNew Jersey,
26 May 1990
We report the formation of the 110 K (Bi, Pb)-Sr-Ca-Cu-0 superconducting phase from a molten state via rapid thermal melt processing techniques. Sintered superconducting specimens were melted at 1200°C for 2 min using a rapid thermal furnace. Following the melting, the specimens were heat treated at 861 “C for different periods of time in order to optimize recrystallization conditions. We achieved a zero transition temperature of 105 K with a AT, of 10 K for an annealing time of 240 h. The J, of meltgrown samples is twice the value of conventionally sintered samples. An increase in the bulk density from 4.75 to 5.60 g/cm3 has been achieved using this process.
Bi-based copper oxide superconducting ceramics have been extensively investigated since the discovery of the Bi-Sr-Ca-Cu oxide system by Maeda et al. [ 11. The discovery of high-temperature superconductivity in Bi-based compound was significant because it contains no rare-earth element with T, values of more than 100 K [ 2 1. This BiSrCaCuO superconductor can be produced without oxygen annealing unlike the YBaCuO superconductor. These advantages will offer the potential to be utilized in many applications of high-T, superconductors. However, this system may contain several superconducting phases with transition temperatures of 80 K and 110 K. Furthermore, the materials so far exhibited high porosity and low strength. For a technological application, a processing procedure must be developed for the production of the BiSrCaCuO superconductor leading to the 110 K phase with optimized properties. This Letter expands upon our previous rapid thermal melt processing (RTMP) work [ 3 ] in which we 0167-577x/90/$
03.50 0 Elsevier Science
Publishers
report an optimization of our processing procedures leading to a significant improvement of the formation of the 110 K Bi (Pb)-Sr-Ca-Cu-0 phase and properties. Powder was prepared by ball milling Biz03, PbO, and SrCO, with Hz0 for 3 h. CuO and CaCO, were then added and the entire mixture was ball-milled again for 3 h. The Bi: Pb: Ca : Sr : Cu cation proportions were chosen to be 0.7:0.3: 1.0: 1.0: 1.8 [4]. After 20 h of drying at 120°C the mixture was calcined in air at 780” for 12 h, followed by additional heat treatment at 820°C for 6 h in air. Samples were prepared by solid state sintering of calcined powders at 860°C for 120 h. RTMP was performed in an oxygen atmosphere on previously sintered samples. The specimens were rapidly melted in platinum crucibles at 1200°C for 2 min with a heating rate of lOOO”C/min and cooled to 875°C within 1 min. After holding at 875°C for 15 min, the samples were cooled to room temperature in 5 min. Following the RTMP process, the
B.V. (North-Holland
)
533
Volume 9, number
12
MATERIALS
samples were heat treated at 861 “C for annealing times ranging between 24 and 360 h. The specimens bccamc superconducting after this annealing process. The dependence on the annealing time of the T, and AT, is summarized in fig. I. For 36 h of annealing, the 7; and AT, arc 110 K and 43 K, respcctivcly. As the annealing period was increased from 36 h to 240 h. the onset T, rose to I 15 K while AT, decreased down to 2 IO K. However, a small drop off in these properties for annealing times longer than 240 h was observed. We noticed a reduction of T, and an increase of ATc for an annealing exceeding 240 h. Thus, these properties arc best for 240 h of annealing. Longer annealing time may cause significant volatilization of Pb and the formation of other phases. The electrical rcsistivity of these samples is shown in fig. 1. The shape of the various curves indicatcs the cocxistcnce of the high-l; I10 K phase with other low-T, phases. These measurements indicate that the low-7; phases dominate the transition for short annealing time. AS annealing times are increased, the amount of high-Tc phase in these samples is concurrently increased. Fig. 2 shows a series of the XRD patterns at the individual processing step. The X-ray diffraction pattern of the solid state reacted samples (fig. 2a) exhibits the high-T, phase. the low-I; phase, and also a small amount of the Ca>PbO, phase. The XRD of the RTMP/annealed sampled (fig. 2b) reveals the 009
LETTERS
August 1990
b) RTMP:annealed
0
x
L 2
t
t
t
I
10
20
30
40
2223 Phase 2212
I
Phase
1
I
SO
60
Fig. 2. X-ray diffraction pattern obtained from the same (Bi. Pb)Sr-<‘a-Cu-0
superconductor
after processing; (a) convention-
ally sintered at 860 ‘C/ 120 h and (b) RTMP/annealed
at 86 I ‘C/
240 h.
high-l; phase with small amounts of the low-T, phase. The XRD of the RTMP/annealed samples show a reduction of impurity and low-T, phases for longer annealing times. The 2-2-2-3 phase concentration was increased more than 95% after 240 h of treatment. A better estimate was obtained by the temperature dependence of the magnetic susccptibility data of these melt-grown specimens (86 I “C/ 240 h). We may determine the volume fraction ratio of the low-T,/high-T, phase of less than I / 10. The bulk density of the RTMP/annealed sample at 861 “C for 240 h was 5.60 g/cm’ which is considerably higher
0 08 oc7
(Bi, Pb) -SFCa-Cu.0 RTMP-Annealing at 661C in Air
F
(Bi. Pb)_Sr-Ca-Cu-0 Superconductor
32
E 8l P >”
50
75
100
125
150
175
200
31
225
TEMPERATURE(K)
Fig. Cu-0
I. dc resistance
00 versus temperature
superconductor
20
plot of (Bi. Pb)-Sr-Ca-
RTMP/post-annealed
40
63 Current
Fig. 3. I-V
transition width (AT;)
ductor after following processing;
function
samples annealed at 861 ‘C’ in air.
534
ofannealing
: 30
-1 i 23
(mA)
at 861 .C for 36 h,
72 h. 120 h. and 240 h. Inset indicates the onset 7;. zero 7;. and asa
80
time
for RTMP
characteristics
and (b) RTMP/annealed
of (Bi. Pb)-Sr-Ca-Cu-0
(a)
at 861 ‘C/240
supercon-
sintercd at 86O”C/ I20 h h measured at 77 K.
Volume 9, number
12
Fig. 4. SEM micrographs 6 h), (b) conventionally
MATERIALS
of (Bi, Pb)-Sr-Ca-Cu-0 superconductor at each processing step: (a) calcined powder sintered at 86O”C/ 120 h, (c) RTMP, and (d) RTMP/annealed at 861”C/240 h.
than the density of the conventionally sintered samples (4.75 g/cm3). Fig. 3 presents the current-voltage (Z-V) characteristics of (Bi, Pb)-Sr-Ca-Cu-0 superconductors measured at 77 K, (a) sintered at 86O”C/120 h and (b) RTMP/annealed at 861 “C for 240 h. For these measurements, samples were cut into a rectangular shape with the dimensions: 0.1 cmx 0.1 cm x 0.4 cm. The cross-sectional area was 0.01 cm2. The measurements were performed at 77 K with an applied current in the range between 0 and 100 mA. A conventional four-terminal resistivity method was
.August 1990
LETTERS
(78O”C/
I2 h, 820-C/
used for current-voltage measurements. Z-V characteristics provide information on critical current density, J,, as well as current-voltage behavior of the superconducting sample. When the experiment is attempted at room temperature, an Ohmic voltage arises between the two contacts as soon as current is injected into the sample. At 77 K (fig. 3a), the sintered sample exhibits superconductivity for applied currents < 15 mA. However, as the current is raised above 15 mA, a distinct voltage can be measured, implying a loss of superconductivity. A substantial improvement was observed in the RTMP/annealed 535
Volume 9. number I2
MATERIALS LETTERS
sample (fig. 3b). Electrical resistance does not appear until an application of 40 mA. The absolute values of .I, from the I-V curve are 1.5 and 4.0 A/cm* for the as-sintered and RTMP/annealed samples, respectively. Other groups reported higher J, at 77 K [ 51. Considering differences in sample geometry, contacts, instrumental sensitivities, a comparison cannot be made between measurements of different groups, at present. We only attempt to indicate that the _I, value of RTMP/annealed sample is 2.7 times greater than that of the conventionally sintered sample. Similarly, Jin et al. [6] achieved a significant improvement of J, in melt textured growth of YBaCuO samples which was found to be very close to the .I, value reported for single crystals. Thus, RTMP treatments of sintered samples may enhance the .I, value of these materials. Figs. 4a-4d display SEM micrographs taken at 1000 x magnification, showing the microstructure of (a) calcined powder, (b) conventionally sintered sample, (c) after RTMP and (d) RTMP/annealed samples. The calcined sample consists of grains with an average value of 10 pm. The sintered sample is composed of large and flat randomly oriented grains. Figs. 4c and 4d show that the grains appear to be directionally grown. The amount of porosity has been substantially reduced due to improved grain packing. Bulk density measurements suggest a reduction of the porosity for melt-processed versus sintered materials.
536
August 1990
For high-T, superconductors, the production of high-quality bulk material is essential for engineering applications. Melt-processing techniques allow careful control of microstructure which, in turn, potentially results in enhanced overall properties. Based on the results of this study, rapid thermal melt processing can be considered to be a viable fabrication technique for the synthesis of high-quality, dense (Bi, Pb)-Sr-Ca-Cu-0 superconductors. This work has been supported by the NSF-STC 8809854, Science and Technology Center for Superconductivity at University of Illinois at UrbanaChampaign. The authors wish to acknowledge Dr. A. Safari, Dr. B.H. Kear and Dr. L.E.McCandlish for their assistance and useful scientific discussions.
References [ 1] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 ( 1988) L209. [2] WE. Pickett, Rev. Mod. Phys. 61 ( 1989) 460. [3] B.M. Moon, B. Lalevic, B.H. Kear, L.E. McCandhsh,A. Safari and M. Meskoob, Appl. Phys. Letters 55 ( 1989) 1466. [4] E. Yanagisawa, D.R. Dietderich, H. Kumakura, K. Togano, H. Maeda and K. Takahashi, Japan J. Appl. Phys. 27 ( 1988) L1460. [ 51T. Komatsu, K. Imai, R. Sato, K. Matusita and T. Yamashita, Japan. J. Appl. Phys. 27 (1988) L533. [6] S. Jin, T.H. Tiefel, R.C. Sherwood, R.B. van Dover, M.E. Davis, G.W. Kammlott and R.A. Fastnacht, Phys. Rev. B 37 (1988) 7850.
MATERIALS
12
RECRYSTALLIZATION SUPERCONDUCTING
B.M. MOON,
OF 110 K HIGH-T,
August
LETTERS
1990
Bi2Sr2Ca2Cu30x
PHASE FROM THE MOLTEN
STATE AND CHARACTERIZATIONS
G. KORDAS
Science and Technology Centerfor Superconductivity. and Department University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
of Materials
Science and Engineering,
Ceramics
Dlvlsion.
and B. LALEVIC Department of Electrical and Computer Engineering, Piscataway, NJ 08855-0909, USA Received
Rutgers - The State University ofNew Jersey,
26 May 1990
We report the formation of the 110 K (Bi, Pb)-Sr-Ca-Cu-0 superconducting phase from a molten state via rapid thermal melt processing techniques. Sintered superconducting specimens were melted at 1200°C for 2 min using a rapid thermal furnace. Following the melting, the specimens were heat treated at 861 “C for different periods of time in order to optimize recrystallization conditions. We achieved a zero transition temperature of 105 K with a AT, of 10 K for an annealing time of 240 h. The J, of meltgrown samples is twice the value of conventionally sintered samples. An increase in the bulk density from 4.75 to 5.60 g/cm3 has been achieved using this process.
Bi-based copper oxide superconducting ceramics have been extensively investigated since the discovery of the Bi-Sr-Ca-Cu oxide system by Maeda et al. [ 11. The discovery of high-temperature superconductivity in Bi-based compound was significant because it contains no rare-earth element with T, values of more than 100 K [ 2 1. This BiSrCaCuO superconductor can be produced without oxygen annealing unlike the YBaCuO superconductor. These advantages will offer the potential to be utilized in many applications of high-T, superconductors. However, this system may contain several superconducting phases with transition temperatures of 80 K and 110 K. Furthermore, the materials so far exhibited high porosity and low strength. For a technological application, a processing procedure must be developed for the production of the BiSrCaCuO superconductor leading to the 110 K phase with optimized properties. This Letter expands upon our previous rapid thermal melt processing (RTMP) work [ 3 ] in which we 0167-577x/90/$
03.50 0 Elsevier Science
Publishers
report an optimization of our processing procedures leading to a significant improvement of the formation of the 110 K Bi (Pb)-Sr-Ca-Cu-0 phase and properties. Powder was prepared by ball milling Biz03, PbO, and SrCO, with Hz0 for 3 h. CuO and CaCO, were then added and the entire mixture was ball-milled again for 3 h. The Bi: Pb: Ca : Sr : Cu cation proportions were chosen to be 0.7:0.3: 1.0: 1.0: 1.8 [4]. After 20 h of drying at 120°C the mixture was calcined in air at 780” for 12 h, followed by additional heat treatment at 820°C for 6 h in air. Samples were prepared by solid state sintering of calcined powders at 860°C for 120 h. RTMP was performed in an oxygen atmosphere on previously sintered samples. The specimens were rapidly melted in platinum crucibles at 1200°C for 2 min with a heating rate of lOOO”C/min and cooled to 875°C within 1 min. After holding at 875°C for 15 min, the samples were cooled to room temperature in 5 min. Following the RTMP process, the
B.V. (North-Holland
)
533
Volume 9, number
12
MATERIALS
samples were heat treated at 861 “C for annealing times ranging between 24 and 360 h. The specimens bccamc superconducting after this annealing process. The dependence on the annealing time of the T, and AT, is summarized in fig. I. For 36 h of annealing, the 7; and AT, arc 110 K and 43 K, respcctivcly. As the annealing period was increased from 36 h to 240 h. the onset T, rose to I 15 K while AT, decreased down to 2 IO K. However, a small drop off in these properties for annealing times longer than 240 h was observed. We noticed a reduction of T, and an increase of ATc for an annealing exceeding 240 h. Thus, these properties arc best for 240 h of annealing. Longer annealing time may cause significant volatilization of Pb and the formation of other phases. The electrical rcsistivity of these samples is shown in fig. 1. The shape of the various curves indicatcs the cocxistcnce of the high-l; I10 K phase with other low-T, phases. These measurements indicate that the low-7; phases dominate the transition for short annealing time. AS annealing times are increased, the amount of high-Tc phase in these samples is concurrently increased. Fig. 2 shows a series of the XRD patterns at the individual processing step. The X-ray diffraction pattern of the solid state reacted samples (fig. 2a) exhibits the high-T, phase. the low-I; phase, and also a small amount of the Ca>PbO, phase. The XRD of the RTMP/annealed sampled (fig. 2b) reveals the 009
LETTERS
August 1990
b) RTMP:annealed
0
x
L 2
t
t
t
I
10
20
30
40
2223 Phase 2212
I
Phase
1
I
SO
60
Fig. 2. X-ray diffraction pattern obtained from the same (Bi. Pb)Sr-<‘a-Cu-0
superconductor
after processing; (a) convention-
ally sintered at 860 ‘C/ 120 h and (b) RTMP/annealed
at 86 I ‘C/
240 h.
high-l; phase with small amounts of the low-T, phase. The XRD of the RTMP/annealed samples show a reduction of impurity and low-T, phases for longer annealing times. The 2-2-2-3 phase concentration was increased more than 95% after 240 h of treatment. A better estimate was obtained by the temperature dependence of the magnetic susccptibility data of these melt-grown specimens (86 I “C/ 240 h). We may determine the volume fraction ratio of the low-T,/high-T, phase of less than I / 10. The bulk density of the RTMP/annealed sample at 861 “C for 240 h was 5.60 g/cm’ which is considerably higher
0 08 oc7
(Bi, Pb) -SFCa-Cu.0 RTMP-Annealing at 661C in Air
F
(Bi. Pb)_Sr-Ca-Cu-0 Superconductor
32
E 8l P >”
50
75
100
125
150
175
200
31
225
TEMPERATURE(K)
Fig. Cu-0
I. dc resistance
00 versus temperature
superconductor
20
plot of (Bi. Pb)-Sr-Ca-
RTMP/post-annealed
40
63 Current
Fig. 3. I-V
transition width (AT;)
ductor after following processing;
function
samples annealed at 861 ‘C’ in air.
534
ofannealing
: 30
-1 i 23
(mA)
at 861 .C for 36 h,
72 h. 120 h. and 240 h. Inset indicates the onset 7;. zero 7;. and asa
80
time
for RTMP
characteristics
and (b) RTMP/annealed
of (Bi. Pb)-Sr-Ca-Cu-0
(a)
at 861 ‘C/240
supercon-
sintercd at 86O”C/ I20 h h measured at 77 K.
Volume 9, number
12
Fig. 4. SEM micrographs 6 h), (b) conventionally
MATERIALS
of (Bi, Pb)-Sr-Ca-Cu-0 superconductor at each processing step: (a) calcined powder sintered at 86O”C/ 120 h, (c) RTMP, and (d) RTMP/annealed at 861”C/240 h.
than the density of the conventionally sintered samples (4.75 g/cm3). Fig. 3 presents the current-voltage (Z-V) characteristics of (Bi, Pb)-Sr-Ca-Cu-0 superconductors measured at 77 K, (a) sintered at 86O”C/120 h and (b) RTMP/annealed at 861 “C for 240 h. For these measurements, samples were cut into a rectangular shape with the dimensions: 0.1 cmx 0.1 cm x 0.4 cm. The cross-sectional area was 0.01 cm2. The measurements were performed at 77 K with an applied current in the range between 0 and 100 mA. A conventional four-terminal resistivity method was
.August 1990
LETTERS
(78O”C/
I2 h, 820-C/
used for current-voltage measurements. Z-V characteristics provide information on critical current density, J,, as well as current-voltage behavior of the superconducting sample. When the experiment is attempted at room temperature, an Ohmic voltage arises between the two contacts as soon as current is injected into the sample. At 77 K (fig. 3a), the sintered sample exhibits superconductivity for applied currents < 15 mA. However, as the current is raised above 15 mA, a distinct voltage can be measured, implying a loss of superconductivity. A substantial improvement was observed in the RTMP/annealed 535
Volume 9. number I2
MATERIALS LETTERS
sample (fig. 3b). Electrical resistance does not appear until an application of 40 mA. The absolute values of .I, from the I-V curve are 1.5 and 4.0 A/cm* for the as-sintered and RTMP/annealed samples, respectively. Other groups reported higher J, at 77 K [ 51. Considering differences in sample geometry, contacts, instrumental sensitivities, a comparison cannot be made between measurements of different groups, at present. We only attempt to indicate that the _I, value of RTMP/annealed sample is 2.7 times greater than that of the conventionally sintered sample. Similarly, Jin et al. [6] achieved a significant improvement of J, in melt textured growth of YBaCuO samples which was found to be very close to the .I, value reported for single crystals. Thus, RTMP treatments of sintered samples may enhance the .I, value of these materials. Figs. 4a-4d display SEM micrographs taken at 1000 x magnification, showing the microstructure of (a) calcined powder, (b) conventionally sintered sample, (c) after RTMP and (d) RTMP/annealed samples. The calcined sample consists of grains with an average value of 10 pm. The sintered sample is composed of large and flat randomly oriented grains. Figs. 4c and 4d show that the grains appear to be directionally grown. The amount of porosity has been substantially reduced due to improved grain packing. Bulk density measurements suggest a reduction of the porosity for melt-processed versus sintered materials.
536
August 1990
For high-T, superconductors, the production of high-quality bulk material is essential for engineering applications. Melt-processing techniques allow careful control of microstructure which, in turn, potentially results in enhanced overall properties. Based on the results of this study, rapid thermal melt processing can be considered to be a viable fabrication technique for the synthesis of high-quality, dense (Bi, Pb)-Sr-Ca-Cu-0 superconductors. This work has been supported by the NSF-STC 8809854, Science and Technology Center for Superconductivity at University of Illinois at UrbanaChampaign. The authors wish to acknowledge Dr. A. Safari, Dr. B.H. Kear and Dr. L.E.McCandlish for their assistance and useful scientific discussions.
References [ 1] H. Maeda, Y. Tanaka, M. Fukutomi and T. Asano, Japan. J. Appl. Phys. 27 ( 1988) L209. [2] WE. Pickett, Rev. Mod. Phys. 61 ( 1989) 460. [3] B.M. Moon, B. Lalevic, B.H. Kear, L.E. McCandhsh,A. Safari and M. Meskoob, Appl. Phys. Letters 55 ( 1989) 1466. [4] E. Yanagisawa, D.R. Dietderich, H. Kumakura, K. Togano, H. Maeda and K. Takahashi, Japan J. Appl. Phys. 27 ( 1988) L1460. [ 51T. Komatsu, K. Imai, R. Sato, K. Matusita and T. Yamashita, Japan. J. Appl. Phys. 27 (1988) L533. [6] S. Jin, T.H. Tiefel, R.C. Sherwood, R.B. van Dover, M.E. Davis, G.W. Kammlott and R.A. Fastnacht, Phys. Rev. B 37 (1988) 7850.