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Sintering of YBa2Cu3O7−x compacts
Volume 6. number
MATERIALS
7
SINTERING
LETTERS
April 1988
OF YBa2Cu307_x COMPACTS
D. SHI, D.W. CAPONE II, G.T. GOUDEY, N.J. ZALUZEC and KC. GORETTA
J.P. SINGH,
Argonne National Laboratory, Argonne, IL 60439, USA Received
17 February
1988
Heating of YBazCu,O,_ licompacts above about 930°C is shown to induce liquid formation. Presence ofthe liquid phase results in excellent densitication, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities ( c 500 A/cm’) have been obtained in these solid-state sintered samples.
1. Introduction YBaZCu307_-1 (“123”) powder can be synthesized by several disparate routes [ 11. The powder must then be shaped and consolidated in order to form useful superconductors. Shaping can be accomplished by methods such as tape casting, extrusion [ 21, swaging [ 31 or explosive forming [ 41. Only swaging and explosive forming are capable of consolidation to high density without need of sintering. Sintering is therefore an important step in the fabrication sequence for many ceramic superconductors. Several sintering schedules have been reported with temperatures from about 900 to 1000” C, and widely varying times at temperature [ 5 1. Reported properties are similar, but little systematic understanding of the effects of sintering on properties has emerged. The effects of sintering temperature on density, microstructure, and the electrical properties, namely, transition temperature (T,), resistivity at the onset of superconductivity (p), and critical current density (J,) are reported here. It was found that for T> 930’ C, liquid-phase sintering yielded dense specimens for which T, and J, were virtually unaffected by increasingsintering temperature. For 930°C and lower, sintering is dominated by solid-state diffusion and the resulting specimens are very porous. These samples have the highest J, values obtained in
this study (~500 A/cm’). The resistivity correlate with density for these specimens.
and J,
2. Experimental details “123” powder was sysnthesized by mechanically mixing Yz03, BaC03 and CuO powders followed by calcination. The powders were calcined from loose raw powders packed into a ZrOz crucible. The calcining temperature used was 950°C for 4 h followed by intermediate grinding. This was repeated three times with the final powder being milled after grinding in a polyethylene jar with ZrOz media and isopropyl alcohol. The average particle size of the final “ 123” powder was = 5 urn. The powder was pressed into 1 g, 12.7 mm diameter discs at a pressure of 138 MPa. The discs were placed on ZrO, plates and heated to the sintering temperature at a rate of 300”C/h, held for a given time at temperature, and then cooled at 300”C/h to about 700°C. Further cooling to 400°C was accomplished at a rate of = 35”C/h, after which the furnace was shut off and allowed to cool to = 100°C in ~4 h. Oxygen was passed through the furnace during the entire heating cycle. Upon removal from the furnace, each specimen was.weighed and its dimensions were measured in order to calculate density. Bars = 10 mm long and 1 217
Volume 6, number 7
MATERIALS LETTERS
mm2 were cut from each pellet for four-terminal measurements of the normal-state resistivity, and the superconducting transition temperature. These measurements were made using a liquid-nitrogen-temperature dipping with probe carbon-glass thermometry, accurate to about 0.5 K. The sample current was 10 mA for all samples measured. .I, was measured on the samples using the same probe fully immersed in the liquid nitrogen. A 1.0 uV/cm voltage criterion was used to define the .I, values obtained.
3. Results and discussion Most specimens were sintered in oxygen for 2 h. The effect of sintering temperature on the geometric density (accurate to about I!I3%) of the pellets is shown in fig. 1. The specimens sintered at 950°C are 92Ohof the theoretical density of 6.3 g/cm3, and the specimens sintered at the higher temperatures are nearly 100% dense. The densities for all specimens sintered at 9 10 and 930°C were < 72%. We believe that the large increase in density above 930°C is caused by a change from solid-state diffusion to liquid-phase sintering at the higher temperature. This is the most likely explanation for such a large change in density. The powder used here was phase pure by 1.0
n
rn8
l
.
0.6 '
900
950
1000
I 1050
T [“Cl Fig. I. Pellet density as a function of sintering temperature ( n indicates 2 h at temperature, A indicates 6 h at temperature and 0 indicates I2 h at temperature).
218
April 1988
X-ray diffraction. However, the difficulty in producing a homogeneously mixed product using the solid-state reaction technique makes it likely that the observed melting is attributable to the presence of a small amount of second phase resulting from Ba-rich regions in the final powders before firing. The occurrence of liquids resulting from Ba-rich materials is well documented in the literature [ 6-8 1. Liquid-phase sintering is rapid relative to solidstate diffusion, and thus 2 h at temperature are sufficient to produce nearly complete densification. Fig. 2a shows an SEM secondary electron image of a sample sintered at 990°C for 2 h. The photo shows a highly densified ceramic with the elongated grains typical of those resulting from liquid-phase sintering. Samples sintered at temperatures from 950 to 1020” C have similar morphologies. The significant residual porosity within the disc sintered at 950°C is probably the result of less liquid formation, although the lower temperature would also be expected to increase the viscosity of the liquid, and hence reduce the sintering kinetics [ 9 1. Two hours are clearly insufficient for complete sintering at 9 10 ’ C. Sintering for 6 and 12 h at 9 10 ’ C increased density from = 64 to zz66 and z 70% of theoretical, respectively (fig. 1). Figs. 2b and 2c show SEM photos for the samples sintered at 9 10” C for 2 and 6 h, respectively. These samples are very porous, consistent with the geometrical density, with resulting poor contact between grains. The sample shown in fig. 2b has a J, of only 76 A/cm* as a result of the poor contact. However, J, was substantially increased to 400 A/cm* when the sintering time was extended to 6 h. The contact between grains is markedly improved in this sample, as is evident in fig. 2c. Further improvements in contact area could be achieved and would result in even higher J, values. Unfortunately, our studies have shown that additional time at 9 10°C does not increase density substantially. This suggests that alternative densification schemes which enhance density at low temperature would be profitable. The effects of sintering temperature on the electrical properties of the samples are shown in figs. 3a and 3b. Sintering temperature had no appreciable effect on T,. This result is expected since the T, is determined by the oxygen content of the samples which is controlled during the post-sintering anneal
Volume 6. number 7
April 1988
MATERIALS LETTERS
E
2
-
1000
n
Q
soo-
3
n
8
:
m
n
.
0
!I00
1000
950
mR8 1050
T [“Cl 600
900
b
950
1000
1050
Fig. 3. (a) Resistivity at the onset of superconductivity as a function of sintering temperature; (b) transport critical current density as a function of sintering temperature (symbols as in fig. 1).
Fig. 2. Scanning electron micrographs showing the morphology of the “123” samples prepared under various sintering conditions: (a) 990”Cfor2h: (b) 910”Cfor2 h;and (c) 910”Cfor 6h.
[ 10,111. Resistivity is approximately constant for T& 950°C but increases sharply as temperature decreases from 950°C. This increase in p for the lower temperatures can be greatly reduced by longer sintering (see fig. 3a) as a result of the increased density. J, is also nearly independent of temperature for 7% 950°C. The highest Jc is obtained from sintering at 9 10’ C for 12 h. The values of p and .I, at the higher temperatures seem to be determined by the presence of the liquid phase. The composition of the liquid is such that it does not solidify to “ 123”. This produces insulating grain boundaries which limit current transport for all of the high-temperature specimens. Transmission electron microscopy (TEM) of spec219
Volume 6, number 7
MATERIALS LETTERS
April 1988
Fig. 4. TEM photograph of grain boundary containing second phase deficient in Y (marked by arrows). The marker has a length .oflO nm.
imens heated above 9 50 ’ C has indicated that a phase which is deficient in Y is present in layers about 100-300 8, thick on about l/4 of the grain boundaries (fig. 4 ) . This second phase was observed to grow epitaxially to the (00 1) plane of the “ 123”. TEM of specimens sintered at 910°C is pending. Resistivity and J, for the discs heated to 910 and 930°C seem to be limited by the very low densities, and hence poor contact between grains. As indicated above, sintering for longer times increases the density and concomitantly improves the contact between grains. This produces both a substantial decrease in p and an increase in J,. For specimens sintered by solid-state diffusion, J, is likely to improve substantially as density increases. The highest J, obtained to date in a bulk ceramic superconductor ( 11000 A/cm* reported by Sadakata et al. [ 31) was for an iz:95% dense wire sintered at 900 ’ C. This high density was achieved, in large part, through cold compaction to a density of = 80% [ 3 1. The results discussed here are for one specific “ 123” powder, Variations in particle shape, size, and 220
size distribution will affect sintering markedly. Small particles which are well compacted will be most likely to sinter to high densities at temperatures below that at which liquid forms. The formation of liquid phase at temperatures above about 93O”C, which appears to limit J, is, however, probably a general effect. Thus, it is likely that alternative densification techniques other than high-temperature sintering will be necessary in order to achieve the high transport J, values necessary for large conductor applications of the high-T, materials.
Acknowledgement
This work was funded by the U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under contract W31-109-ENG-38. GTG was supported by the Division of Educational Programs of Argonne National Laboratory.
Volume 6. number
7
MATERIALS
References [ I] Advan. Ceram. Mater. 2 (1987) 327-556. J.T. Dusek and I.D. [2 ] R.B. Poeppel, B.K. Flandermeyer, Bloom, in: Chemistry of high-temperature superconductors, eds. D.L. Nelson, M.S. Whittingham and T.F. George (American Chemical Society, Washington, 1987) p. 261. K. Gotoh and 0. ]3 ] N. Sadakata, Y. Ikeno, M. Nakagawa, Kohno, Mater. Res. Sot. Symp. Proc. 99 (1988). to be published. [4] L.E. Murr, A.W. Hare and N.G. Eror, Nature 329 (1987) 37. [ 51 Advan. Ceram. Mater. 2 (1987) 3B; Mater. Res. Sot. Symp. Proc. 99 (1988), to be published. [ 61 Z. Gabelica, G. Demortier, G. Deconninck, F. Bodart, A.A. Lucas, M. Renier, Ph. Lambin, J.P. Vigneron and E.G. Derouane, Solid State Commun. 64 (1987) 1137.
LETTERS
April 1988
[ 71 A. Bhargava,
M. Heuberger and R.L. Snyder, Mater. Letters 5 (1987) 495. [8]R.S. Roth, K.L. Davis and J.R. Dennis, Advan. Ceram. Mater. 2 (1987) 303. ]9 W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to ceramics (Wiley-Interscience, New York, 1976). 110 R.J. Cava, B. Batlogg, R.B. van Dover, D.W. Murphy, S. Sunshine, T. Siegrist, J.P. Remeika, E.A. Rietman, S. Zahurak and G.P. Espinosa, Phys. Rev. Letters 58 ( 1987) 1676. R. Beyers, G. Lim, E.M. Engler, V.Y. Lee, M.L. Ramirez, [ll R.J. Savoy, R.D. Jacowitz, T.M. Shaw, S. La Planca, R. Boehme, C.C. Tsuei, S.I. Park, M.W. Shafer and W.J. Gallagher, Appl. Phys. Letters 5 1 ( I987 ) 6 14.
221
MATERIALS
7
SINTERING
LETTERS
April 1988
OF YBa2Cu307_x COMPACTS
D. SHI, D.W. CAPONE II, G.T. GOUDEY, N.J. ZALUZEC and KC. GORETTA
J.P. SINGH,
Argonne National Laboratory, Argonne, IL 60439, USA Received
17 February
1988
Heating of YBazCu,O,_ licompacts above about 930°C is shown to induce liquid formation. Presence ofthe liquid phase results in excellent densitication, but limited superconducting properties. Sintering below 930°C occurs primarily by solid-state diffusion. Although the density of these samples is low, the superconducting properties are similar to those of the dense materials produced via liquid-phase sintering. The highest current densities ( c 500 A/cm’) have been obtained in these solid-state sintered samples.
1. Introduction YBaZCu307_-1 (“123”) powder can be synthesized by several disparate routes [ 11. The powder must then be shaped and consolidated in order to form useful superconductors. Shaping can be accomplished by methods such as tape casting, extrusion [ 21, swaging [ 31 or explosive forming [ 41. Only swaging and explosive forming are capable of consolidation to high density without need of sintering. Sintering is therefore an important step in the fabrication sequence for many ceramic superconductors. Several sintering schedules have been reported with temperatures from about 900 to 1000” C, and widely varying times at temperature [ 5 1. Reported properties are similar, but little systematic understanding of the effects of sintering on properties has emerged. The effects of sintering temperature on density, microstructure, and the electrical properties, namely, transition temperature (T,), resistivity at the onset of superconductivity (p), and critical current density (J,) are reported here. It was found that for T> 930’ C, liquid-phase sintering yielded dense specimens for which T, and J, were virtually unaffected by increasingsintering temperature. For 930°C and lower, sintering is dominated by solid-state diffusion and the resulting specimens are very porous. These samples have the highest J, values obtained in
this study (~500 A/cm’). The resistivity correlate with density for these specimens.
and J,
2. Experimental details “123” powder was sysnthesized by mechanically mixing Yz03, BaC03 and CuO powders followed by calcination. The powders were calcined from loose raw powders packed into a ZrOz crucible. The calcining temperature used was 950°C for 4 h followed by intermediate grinding. This was repeated three times with the final powder being milled after grinding in a polyethylene jar with ZrOz media and isopropyl alcohol. The average particle size of the final “ 123” powder was = 5 urn. The powder was pressed into 1 g, 12.7 mm diameter discs at a pressure of 138 MPa. The discs were placed on ZrO, plates and heated to the sintering temperature at a rate of 300”C/h, held for a given time at temperature, and then cooled at 300”C/h to about 700°C. Further cooling to 400°C was accomplished at a rate of = 35”C/h, after which the furnace was shut off and allowed to cool to = 100°C in ~4 h. Oxygen was passed through the furnace during the entire heating cycle. Upon removal from the furnace, each specimen was.weighed and its dimensions were measured in order to calculate density. Bars = 10 mm long and 1 217
Volume 6, number 7
MATERIALS LETTERS
mm2 were cut from each pellet for four-terminal measurements of the normal-state resistivity, and the superconducting transition temperature. These measurements were made using a liquid-nitrogen-temperature dipping with probe carbon-glass thermometry, accurate to about 0.5 K. The sample current was 10 mA for all samples measured. .I, was measured on the samples using the same probe fully immersed in the liquid nitrogen. A 1.0 uV/cm voltage criterion was used to define the .I, values obtained.
3. Results and discussion Most specimens were sintered in oxygen for 2 h. The effect of sintering temperature on the geometric density (accurate to about I!I3%) of the pellets is shown in fig. 1. The specimens sintered at 950°C are 92Ohof the theoretical density of 6.3 g/cm3, and the specimens sintered at the higher temperatures are nearly 100% dense. The densities for all specimens sintered at 9 10 and 930°C were < 72%. We believe that the large increase in density above 930°C is caused by a change from solid-state diffusion to liquid-phase sintering at the higher temperature. This is the most likely explanation for such a large change in density. The powder used here was phase pure by 1.0
n
rn8
l
.
0.6 '
900
950
1000
I 1050
T [“Cl Fig. I. Pellet density as a function of sintering temperature ( n indicates 2 h at temperature, A indicates 6 h at temperature and 0 indicates I2 h at temperature).
218
April 1988
X-ray diffraction. However, the difficulty in producing a homogeneously mixed product using the solid-state reaction technique makes it likely that the observed melting is attributable to the presence of a small amount of second phase resulting from Ba-rich regions in the final powders before firing. The occurrence of liquids resulting from Ba-rich materials is well documented in the literature [ 6-8 1. Liquid-phase sintering is rapid relative to solidstate diffusion, and thus 2 h at temperature are sufficient to produce nearly complete densification. Fig. 2a shows an SEM secondary electron image of a sample sintered at 990°C for 2 h. The photo shows a highly densified ceramic with the elongated grains typical of those resulting from liquid-phase sintering. Samples sintered at temperatures from 950 to 1020” C have similar morphologies. The significant residual porosity within the disc sintered at 950°C is probably the result of less liquid formation, although the lower temperature would also be expected to increase the viscosity of the liquid, and hence reduce the sintering kinetics [ 9 1. Two hours are clearly insufficient for complete sintering at 9 10 ’ C. Sintering for 6 and 12 h at 9 10 ’ C increased density from = 64 to zz66 and z 70% of theoretical, respectively (fig. 1). Figs. 2b and 2c show SEM photos for the samples sintered at 9 10” C for 2 and 6 h, respectively. These samples are very porous, consistent with the geometrical density, with resulting poor contact between grains. The sample shown in fig. 2b has a J, of only 76 A/cm* as a result of the poor contact. However, J, was substantially increased to 400 A/cm* when the sintering time was extended to 6 h. The contact between grains is markedly improved in this sample, as is evident in fig. 2c. Further improvements in contact area could be achieved and would result in even higher J, values. Unfortunately, our studies have shown that additional time at 9 10°C does not increase density substantially. This suggests that alternative densification schemes which enhance density at low temperature would be profitable. The effects of sintering temperature on the electrical properties of the samples are shown in figs. 3a and 3b. Sintering temperature had no appreciable effect on T,. This result is expected since the T, is determined by the oxygen content of the samples which is controlled during the post-sintering anneal
Volume 6. number 7
April 1988
MATERIALS LETTERS
E
2
-
1000
n
Q
soo-
3
n
8
:
m
n
.
0
!I00
1000
950
mR8 1050
T [“Cl 600
900
b
950
1000
1050
Fig. 3. (a) Resistivity at the onset of superconductivity as a function of sintering temperature; (b) transport critical current density as a function of sintering temperature (symbols as in fig. 1).
Fig. 2. Scanning electron micrographs showing the morphology of the “123” samples prepared under various sintering conditions: (a) 990”Cfor2h: (b) 910”Cfor2 h;and (c) 910”Cfor 6h.
[ 10,111. Resistivity is approximately constant for T& 950°C but increases sharply as temperature decreases from 950°C. This increase in p for the lower temperatures can be greatly reduced by longer sintering (see fig. 3a) as a result of the increased density. J, is also nearly independent of temperature for 7% 950°C. The highest Jc is obtained from sintering at 9 10’ C for 12 h. The values of p and .I, at the higher temperatures seem to be determined by the presence of the liquid phase. The composition of the liquid is such that it does not solidify to “ 123”. This produces insulating grain boundaries which limit current transport for all of the high-temperature specimens. Transmission electron microscopy (TEM) of spec219
Volume 6, number 7
MATERIALS LETTERS
April 1988
Fig. 4. TEM photograph of grain boundary containing second phase deficient in Y (marked by arrows). The marker has a length .oflO nm.
imens heated above 9 50 ’ C has indicated that a phase which is deficient in Y is present in layers about 100-300 8, thick on about l/4 of the grain boundaries (fig. 4 ) . This second phase was observed to grow epitaxially to the (00 1) plane of the “ 123”. TEM of specimens sintered at 910°C is pending. Resistivity and J, for the discs heated to 910 and 930°C seem to be limited by the very low densities, and hence poor contact between grains. As indicated above, sintering for longer times increases the density and concomitantly improves the contact between grains. This produces both a substantial decrease in p and an increase in J,. For specimens sintered by solid-state diffusion, J, is likely to improve substantially as density increases. The highest J, obtained to date in a bulk ceramic superconductor ( 11000 A/cm* reported by Sadakata et al. [ 31) was for an iz:95% dense wire sintered at 900 ’ C. This high density was achieved, in large part, through cold compaction to a density of = 80% [ 3 1. The results discussed here are for one specific “ 123” powder, Variations in particle shape, size, and 220
size distribution will affect sintering markedly. Small particles which are well compacted will be most likely to sinter to high densities at temperatures below that at which liquid forms. The formation of liquid phase at temperatures above about 93O”C, which appears to limit J, is, however, probably a general effect. Thus, it is likely that alternative densification techniques other than high-temperature sintering will be necessary in order to achieve the high transport J, values necessary for large conductor applications of the high-T, materials.
Acknowledgement
This work was funded by the U.S. Department of Energy, Basic Energy Sciences-Materials Sciences, under contract W31-109-ENG-38. GTG was supported by the Division of Educational Programs of Argonne National Laboratory.
Volume 6. number
7
MATERIALS
References [ I] Advan. Ceram. Mater. 2 (1987) 327-556. J.T. Dusek and I.D. [2 ] R.B. Poeppel, B.K. Flandermeyer, Bloom, in: Chemistry of high-temperature superconductors, eds. D.L. Nelson, M.S. Whittingham and T.F. George (American Chemical Society, Washington, 1987) p. 261. K. Gotoh and 0. ]3 ] N. Sadakata, Y. Ikeno, M. Nakagawa, Kohno, Mater. Res. Sot. Symp. Proc. 99 (1988). to be published. [4] L.E. Murr, A.W. Hare and N.G. Eror, Nature 329 (1987) 37. [ 51 Advan. Ceram. Mater. 2 (1987) 3B; Mater. Res. Sot. Symp. Proc. 99 (1988), to be published. [ 61 Z. Gabelica, G. Demortier, G. Deconninck, F. Bodart, A.A. Lucas, M. Renier, Ph. Lambin, J.P. Vigneron and E.G. Derouane, Solid State Commun. 64 (1987) 1137.
LETTERS
April 1988
[ 71 A. Bhargava,
M. Heuberger and R.L. Snyder, Mater. Letters 5 (1987) 495. [8]R.S. Roth, K.L. Davis and J.R. Dennis, Advan. Ceram. Mater. 2 (1987) 303. ]9 W.D. Kingery, H.K. Bowen and D.R. Uhlmann, Introduction to ceramics (Wiley-Interscience, New York, 1976). 110 R.J. Cava, B. Batlogg, R.B. van Dover, D.W. Murphy, S. Sunshine, T. Siegrist, J.P. Remeika, E.A. Rietman, S. Zahurak and G.P. Espinosa, Phys. Rev. Letters 58 ( 1987) 1676. R. Beyers, G. Lim, E.M. Engler, V.Y. Lee, M.L. Ramirez, [ll R.J. Savoy, R.D. Jacowitz, T.M. Shaw, S. La Planca, R. Boehme, C.C. Tsuei, S.I. Park, M.W. Shafer and W.J. Gallagher, Appl. Phys. Letters 5 1 ( I987 ) 6 14.
221