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The low-temperature synthesis of YBa2Cu3O7−δ under reduced oxygen pressure
PhysicaC 173 (1991) North-Holland
245-250
The low-temperature oxygen pressure
synthesis of YBa2Cu307_-6 under reduced
R.R. Schartman and E.E. Hellstrom of MaterialsScience and Engineering,
Department Received
12 September
University of Wisconsin-Madison,
Madison,
WI 53706, USA
1990
Thermogravimetric data on the synthesis of YBa2Cu30,_6 below 800°C under reduced pressure are presented. It is demonstrated that the reaction pathway and rate are determined by the CO*, 02, and total pressure in the reaction vessel. The importance of these variables to the impurities generated during the synthesis is discussed.
1. Introduction the critical current density of Although YBazCu,O,_d as measured by magnetic methods (J,,) is quite high, the transport critical current density (J,,) in bulk samples remains low. Seuntjens and Larbalestier suggested that improvements in J,, may come through reducing the particle size [ 11. This has led to our interest in the low temperature synthesis of YBazCu30,_+ Obtaining phase-pure YBazCu307_s ( 123 ) at temperatures below 800°C has proved difficult. What follows is a presentation of our results in conjunction with a discussion of previous work on the importance of oxygen pressure (P( O2 ) ), total pressure (P,) and the CO2 pressure (P( CO1 ) ). The reactants Y203, BaC03 and CuO were chosen because the vast majority of synthetic routes to 123 (solid-state synthesis from the carbonates, coprecipitation, and solgel techniques) involve the formation of these compounds prior to the formation of 123.
2. Experimental
procedures
Thermogravimetric experiments were performed using a Cahn 1000 microbalance. Samples were supported with a quartz hangdown fiber and bucket. The heating schedule for all of these experiments was 25600” C over 4 hours, 600-800°C over 10 h, and a 092 l-4534/91
/$03.50
0 1991 - Elsevier Science Publishers
hold at 800°C. The mass changes presented in this paper have been normalized to the mass of BaCO, initially present. Low total pressure was generated by using a Welch vacuum pump with a 160 1/min pump rate. The purity of the reactants was: Y20) 99.99%, BaCO, 99.99% and CuO 99.9%. Powder X-ray diffraction was used to identify phases. Since the detection limit of this technique is only 3-5%, some impurities may have escaped detection. The oxygen pressure was fixed by flowing O2 or OJAr mixtures using Tylan General mass flow controllers and an MKS power supply/readout device. The quoted flow rates are for gas flowed at standard temperature and pressure. The work tube in the TGA had a 2.2 cm inside diameter. This fact is noted because linear flow rates determine whether or not gas mixtures will remain mixed or will segregate to different regions of the furnace [ 2 1. Darken and Gurry suggest a linear flow rate of 0.9 cm/s is required to maintain the mixed condition. Our flow rates are somewhat smaller than this. However, we positioned a zirconia cell directly below the sample in the TGA apparatus and measured the oxygen fugacity directly. Table I summarizes the reactions and experimental conditions we used in this study.
B.V. (North-Holland)
246
R. R. Schartman,
Table I TGA syntheses
carried
E.E. Hellstrom
/Low-temperature
out in this study.
Experiment no.
Reactants
TGA 007 TGA 008 TGA 010 TGAOll TGA 012 TGA 015 TGA 016
Yz03, BaCO,, CuO Y203, BaCO,, CuO Y203, BaC03, CuO Yz03, BaCO,, CuO BaCO, BaCO,, CuO BaCO,
Sample size
3. CO2 pressure Forming 123 from a BaCO, precursor necessarily involves losing COZ. However, 123 does not necessarily form directly from the reactants according to: $Y203 + 2BaC03 + 3CuO =YBa2Cu-,065 The reaction may instead composing the carbonate:
proceed
+2C02.
(1)
by initially
de-
BaCO, = BaO + CO, . Alternatively, intermediates:
(2)
the reaction
may
proceed
BaCO, + CuO = BaCuO, + CO1 , Y*O3 +2cuo=Y,cu~o~
)
through
(3) (4)
which then react to give 123: tY,CuzOS
synthesrs of YBalCu307_-s under reduced P(OJ
+2BaCu02
=YBazCu,06.5.
(5)
Other possible reaction pathways have been addressed by Ruckenstein et al. [ 31. However, their results are not directly applicable to the present study due to their high reaction temperature (94O”C), which is above the BaCuO,-CuO eutectic. This undoubtedly influences the reaction kinetics through liquid phase reactions and sintering [ 41. The results of a thermogravimetric experiment, TGA 008, with stoichiometric amounts of Y203, BaCO, and CuO to form 123 are shown in fig. 1. This run, which used a 0.35 12 g sample had a gas flow rate of 125 cc/min, P(0,) ~3.3 Torr, and a total pressure of 1 atm. Using these facts in conjunction with the measured mass loss, the CO2 pressure was
(g)
P(O,) (Torr)
p, (Torr)
0.3505 0.3512 0.3534 0.3543 0.1859 0.2602 0.1857
3.3 3.3 0.78 0.82 3.4 3.5 3.5
3.3 760 3 760 3.4 760 760
estimated to be 1.2~ 1O-4 atm during the first ten hours of the reaction after the sample reached 800°C. This is almost four times the decomposition pressure of BaCO, at 800°C 3.16x 10e5 atm [5]. Thus, reaction 2 was not thermodynamically possibly during a large part of the reaction period. We checked whether BaCO, decomposed according to reaction 2 under the conditions of TGA 008. The results, TGA 016, shown in fig. 2, confirm that the COZ loss observed in TGA 008 could not have occurred via reaction 2. The X-ray diffraction pattern of the powder from TGA 008 showed 123 and BaCuOz (01 1 ), suggesting that reaction 3 may be the dominant reaction route for loss of CO*. Data from TGA 0 15 are displayed in fig. 1. In this experiment, BaCuOz was synthesized under conditions similar to those used in TGA 008 (P,=l atm, P(02)=3.5 Torr, flow rate = 125 cc/min ). Note the loss of CO2 in TGA 0 15 and TGA 008 proceeds at nearly the same rate during the first part of the reaction period, consistent with the above hypothesis. Differences occur after the first 24 h, suggesting that other factors are involved in the decomposition of BaCO, to form 123 at longer times. Though we did not detect it, Y,BaCuOS (2 11) may have been an intermediate involved in the formation of 123. Of course, the direct reaction (eq. ( 1 ) ) may have also occurred. The likelihood of any given pathway will depend on a number of factors including the oxygen and COZ pressures. Resolving this problem will require data on the stability of 0 11, 2 11, and 123 as a function of P(COZ) and P(0,).
R.R. Schartman,
E.E. Hellstrom
/Low-temperature
synthesis of YBazCu307-6
under reduced P(0 j
TGA xxxxx TGA . . . . . TGA
00000
x x x
247
008 007 015
x x x
x x
4 Ld OL
80.0
75.0
Fig. 1. Relative mass change is due to loss of CO2 for each run. TGA 007 formation of 123 at P,= P(0,) = 3.3 Torr. TGA 008 formation of 123 at P,=760Torr, P(02)=3.3 Torr. TGA015 formation of01 1 at P,=760Torrand P(Oz)=3.3 Torr. 102.0
6
E+? _ 98.0
w
94.0
>
F 4
E
92.0 90.0
ooooo TGA xxxxxTGA
016 012
!
Fig. 2. Relative mass change due to loss of CO2 for each run. TGA 012 decomposition of BaCO, at P,=P(02) =3.5 Torr. TGA 016 decomposition of BaCO, at P,= 760 Torr and P(0,) ~3.5 Torr. Initial mass increase in TGA 012 was due to an imperfect bucket buoyancy correction.
4.
Total pressure As noted by Balachandran
et al., reducing the total
pressure increases the reaction rate [ 141. Experiment TGA 007, shown in fig. 1 is consistent with this claim. The conditions of TGA 007 were similar to
248
R.R. Schartman, E.E. Hellstrom /Low-temperature synthesis of YBaZCu,07_8 under reduced P(OJ
those of TGA 008: P(0,) = 3.3 Torr and sample mass of 0.3505 g. However, the low pressure in TGA 007 was obtained by flowing pure O2 at 92.5 cc/min and simultaneously vacuum pumping on the TGA system to reduce the total pressure. Although the reaction rate increased with a lowered total pressure, we always observed the impurity phase BaCuOz in our product. The reason for the increased reaction rate is twofold. First, the reduced pressure gives rise to a much larger effective gas volume thereby reducing the CO, pressure over the sample. This is readily seen by using the ideal gas law. Assume 0.005 moles of gas were flowed past the sample per minute. Under conditions of standard temperature and pressure this corresponds to a flow rate of 112 cc/min. However, if instead 0.005 moles of gas flow per minute under conditions of standard temperature but at a pressure of 3.3 Torr, the flow rate increases to 25.8 x lo3 cc/ min, an increase by a factor of about 230 times. Thus, under the low pressure conditions, a sample would have to evolve CO2 230 times as fast as a sample synthesized at 1 atm in order to maintain the same CO* pressure. Or to put it another way, the vacuum calcined sample is free to evolve CO1 230 times faster than the other sample. A second reason for the increased reaction rate is seen by calculating the P(C02) from the measured mass loss, the total pressure, and the flow rate. For the conditions of TGA 007 during the first ten hours at 800°C the P(COZ) was approximately 2.5~ 10e6 atm. This is more than a factor of ten lower than the decomposition pressure of BaC03 at 800°C. Thus, at reduced total pressure, reaction 2 is possible and 123 can form via an additional pathway. That BaCO, can decompose under reduced total pressure was confirmed in experiment TGA 012, whose conditions were similar to TGA 007 (fig. 2). The initial mass increase in TGA 0 12 was due to an imperfect bucket buoyancy correction.
P(0,) 20.2 atm produces an unwanted phase [ 6lo]. This phase is similar in structure to 123. It has the proper cation stoichiometry and the oxygen to metal ratio is sufficiently large that superconducting orthorhombic 123 material is expected. However, the phase that forms is neither superconducting nor orthorhombic. It is readily identified by its c-axis, which is shorter than that of the desired superconductor. The cause of the undesired phase remains unclear. Goodenough has suggested that disorder on the oxygen sublattice gives rise to the unwanted phase [ 91. Lay has suggested in his summary of the problem that the phase is produced by yttrium which has substituted for some of the barium [ 10 1. Irregardless, the phase appears to be metastable and can be converted to orthorhombic superconducting 123 either by high temperature anneals or simply by synthesizing 123 at these reduced temperatures under reduced oxygen pressure [ 9, lo]. Oxygen pressure also influences the reaction pathway by reducing CuO to CuZO. At sufficiently low P(O,), BaC03 decomposition can then proceed via the following route: BaC03 + Cu20 = BaCuzOz + CO* .
(6)
This fact appears to have been noted as early as 1976 by Migeon et al. in their attempts to synthesize stoichiometric BaCu02 [ 111. Recently, a more detailed study was performed by Grader et al. and it was found that reaction 6 proceeds at temperatures as low as 550” C [ 121. Unfortunately, this has not proved useful to lowering the synthesis temperature of 123 because the required ratio of Ba to Cu in 123 is 2 : 3 and reaction 6 has a Ba to Cu ratio of 1: 2, which necessarily leaves some BaC03 unreacted. Thus, higher temperatures are still needed to complete the conversion to 123. An illustration of the importance of reaction 6 will be given in the following section.
6. Impurity phases 5. Oxygen pressure The importance of the oxygen pressure is twofold. First, the P(OZ) appears to be important in achieving the desired 123 phase when preparing the material below approximately 800°C. Synthesis in
Part of our interest in this study was to explain why different authors have reported different impurities in their 123 samples prepared under different conditions. Their experimental conditions are summarized in table II. Lay has reported the presence of Y,Cu205 (202 ). Uno et al. have reported the pres-
249
R.R. Schartman, E.E. Hellstrom /Low-temperature synthesis of YBaZCujO,_d under reduced P(OJ Table II Impurity phases observed
under different
Study
P(Gz) (Torr)
p, (Torr)
Lay 1101 Unoetal. [13] Balachandranet al. Fjellvag et al. [ 15 ]
conditions.
760
600 C
780
BaCO,, CuO, yzcu205 211,012(?) 211 202, BaCO, 202 011 011 011 012,011
25-950 25-800 300-900 25-800 25-800 25-800 25-800
atm [ 15 1. They have shown that if the CO2 pressure is sufficiently high 123 decomposes to 202, BaC03 and CuO. This would explain the results of Lay. If the unidentified phase in the work of Uno et al. is indeed 0 12, then they may have simply crossed the reduction boundary of 123. Ahn et al. indicate that upon reduction, 123 decomposes to 0 12, 2 11 and a third phase, YBa3Cu206 [ 161. Although Uno et al. did not report this last phase, it is expected to be present in levels below 10 mol% and so it may have escaped detection.
ence of (2 11) and an unknown phase containing Ba and Cu in the ratio of 2: 3 [ 131. We note however, their X-ray diffraction pattern resembles that of BaCu,Oz (0 12 ). Balachandran et al. have reported the presence of 2 11 in their samples [ 141. They suggested that this was a result of a build up of CO2 and when the P(0,) :P(CO*) > 50 the unwanted phase did not form. We on the other hand, routinely observe 011. Thermodynamic measurements have been performed by Fjellvag et al. at oxygen pressure near 1 105.0
Ar
Major impurities
(“C)
02 OJAr WAr WAr
3.3 3.3 0.78 0.82
3.3 760 3.3 760
TGA 007 TGA 008 TGA 010 TGAOll
Temp.
Air GZ WCG2
0.1-0.001 2.0 735-760
0.1-0.001 2.0 760
[ 141
Gas mix
800 c
A
ER 00000
xxxxx
70.0
,I 0
I,
I
I
I
I,
I,
I,,
,
,
,
,
,
,
,
,
/
,
,
,
,
,
,
,
,
,
10
TIM?
(
HO&
Fig. 3. Relative mass change due to loss of CO2 for each run. TGA 0 10 formation formation of 123 at P,= 760 Torr and P(0,) =0.82 Torr.
)
,
,
/
TGA TGA
,
,,
010
011
1 50
4o
of 123 at P,= 3 Torr and P( Oz ) = 0.78 Torr. TGA 011
250
R.R. Schartman. E.E. Hellstrom /Low-temperature
The applicability of the thermodynamic work by Fjellvag et al. [ 15 ] to low pressure synthesis is unclear. The oxygen pressure used was different in each case. Experiments to determine the thermodynamic stability of 123 in CO* at reduced oxygen pressure are needed. It is also possible that the impurity phases present not a thermodynamic but a kinetic problem. Each author generated a different set of reaction conditions throughout the course of the synthesis. Although thermodynamically 123 will be the stable phase once the P(COZ) drops sufficiently, kinetically the impurity phases may remain. This is illustrated in the following two TGA experiments to prepare 123 (fig. 3). The two experiments were performed under identical heating rates and used the same batch of reactants. The oxygen pressure in both experiments (0.8 Torr) was just below the CuO-Cu20 redox boundary at 800°C ( 1.2 Torr) [ 171. Flow rates for TGA 010 and TGA 011 were 109 cc/min and 125 cc/min, respectively. The main difference was that TGA 0 10 was carried out at 3.3 Torr total pressure whereas TGA 0 11 was carried out at 1 atm. Since the material synthesized at low pressure was almost fully reacted before the CuO-CuzO boundary was crossed, the major impurity detected was BaCuOz. This was not true for the material reacted at P,= 1 atm where both BaCuOz and BaCuzOz impurities were detected.
7. Conclusions The importance of CO*, O2 and total pressure to the low temperature synthesis of 123 was illustrated. These variables influence the kinetic pathway taken in forming 123 and thus determine the reaction rate and the impurities generated. A complete knowledge of the thermodynamic variables involved in the low temperature synthesis is lacking. This, plus the kinetic hindrance to removing the impurities gener-
synthesis of YBaZCu307_d under reduced P(0.J
ated at low temperatures have blocked efforts at synthesizing phase pure 123 at low temperatures.
Acknowledgement This work was supported Research Institute.
by the Electric
Power
References [ 1] J.M. Seuntjens
and D.C. Larbalestier, J. Appl. Phys. 67 ( 1990) 2007. [2] L.S. Darken and R.W. Curry, J. Am. Chem. Sot. 67 ( 1945) 1398. [3] E. Ruckenstein, S. Narain and N.L. Wu, J. Mater. Res. 4 (1989) 261. [4] R.S. Roth, K.L. Davis and J.R. Dennis, Adv. Ceram. Mater. 2 (1987) 303. [5] J.J. Lander, J. Am. Chem. Sot. 73 (1951) 5794. [ 61 Y. Nakazawa, M. Ishidawa, T. Takabatake, K. Koga and K. Terakura, Jpn. J. Appl. Phys. 26 (1987) L796. M.A. Subramanian, H.S. ]7 C.C. Torardi, E.M. McCarron, Horowitz, J.B. Michel and A.W. Sleight, in: Chemistry of High-Temperature Superconductors (Am. Chem. Sot., Washington, 1987) pp. 152-163. Solid [8 X.2. Wang, M. Henry, J. Livage and I. Rosenman, State Commun. 64 ( 1987) 88 1. [9] A. Manthiram and J.B. Goodenough, Nature 329 (1987) 701. [IO] K. Lay, J. Am. Ceram. Sot. 72 (1989) 696. [ I 1 ] H.N. Migeon, F. Jeannot, M. Zanne and J. Aubry, Rev. Chim. Minerale I3 ( 1976) 440. [ 121 G.S. Grader, P.K. Gallagher and D.A. Fleming, Chem. Mater. I (1989) 665. [ 131 N. Uno, N. Enomoto, Y. Tanaka and H. Takami, Jpn. J. Appl. Phys. 27 (1988) L1003. [ 141 U. Balanchandran, R.B. Poeppel, J.E. Emerson, S.A. Johnson, M.T. Lanagan, C.A. Youngdahl, D. Shi and K.C. Goretta, Mater. Lett. 8 ( 1989) 454. [ 15 ] H. Fjellvag, P. Karen, A. Kjekshus, P. Kofstad and T. Norby, Acta. Chem. Stand. A 42 ( 1988) 178. [ 161 B.T.Ahn, V.Y. Lee, R. Beyers,T.M. Gurand R.A. Huggins, Physica C 167 ( 1990) 529. [ 171 F.H. Smyth and H.S. Roberts, J. Am. Chem. Sot. 42 ( 1920) 2582.
245-250
The low-temperature oxygen pressure
synthesis of YBa2Cu307_-6 under reduced
R.R. Schartman and E.E. Hellstrom of MaterialsScience and Engineering,
Department Received
12 September
University of Wisconsin-Madison,
Madison,
WI 53706, USA
1990
Thermogravimetric data on the synthesis of YBa2Cu30,_6 below 800°C under reduced pressure are presented. It is demonstrated that the reaction pathway and rate are determined by the CO*, 02, and total pressure in the reaction vessel. The importance of these variables to the impurities generated during the synthesis is discussed.
1. Introduction the critical current density of Although YBazCu,O,_d as measured by magnetic methods (J,,) is quite high, the transport critical current density (J,,) in bulk samples remains low. Seuntjens and Larbalestier suggested that improvements in J,, may come through reducing the particle size [ 11. This has led to our interest in the low temperature synthesis of YBazCu30,_+ Obtaining phase-pure YBazCu307_s ( 123 ) at temperatures below 800°C has proved difficult. What follows is a presentation of our results in conjunction with a discussion of previous work on the importance of oxygen pressure (P( O2 ) ), total pressure (P,) and the CO2 pressure (P( CO1 ) ). The reactants Y203, BaC03 and CuO were chosen because the vast majority of synthetic routes to 123 (solid-state synthesis from the carbonates, coprecipitation, and solgel techniques) involve the formation of these compounds prior to the formation of 123.
2. Experimental
procedures
Thermogravimetric experiments were performed using a Cahn 1000 microbalance. Samples were supported with a quartz hangdown fiber and bucket. The heating schedule for all of these experiments was 25600” C over 4 hours, 600-800°C over 10 h, and a 092 l-4534/91
/$03.50
0 1991 - Elsevier Science Publishers
hold at 800°C. The mass changes presented in this paper have been normalized to the mass of BaCO, initially present. Low total pressure was generated by using a Welch vacuum pump with a 160 1/min pump rate. The purity of the reactants was: Y20) 99.99%, BaCO, 99.99% and CuO 99.9%. Powder X-ray diffraction was used to identify phases. Since the detection limit of this technique is only 3-5%, some impurities may have escaped detection. The oxygen pressure was fixed by flowing O2 or OJAr mixtures using Tylan General mass flow controllers and an MKS power supply/readout device. The quoted flow rates are for gas flowed at standard temperature and pressure. The work tube in the TGA had a 2.2 cm inside diameter. This fact is noted because linear flow rates determine whether or not gas mixtures will remain mixed or will segregate to different regions of the furnace [ 2 1. Darken and Gurry suggest a linear flow rate of 0.9 cm/s is required to maintain the mixed condition. Our flow rates are somewhat smaller than this. However, we positioned a zirconia cell directly below the sample in the TGA apparatus and measured the oxygen fugacity directly. Table I summarizes the reactions and experimental conditions we used in this study.
B.V. (North-Holland)
246
R. R. Schartman,
Table I TGA syntheses
carried
E.E. Hellstrom
/Low-temperature
out in this study.
Experiment no.
Reactants
TGA 007 TGA 008 TGA 010 TGAOll TGA 012 TGA 015 TGA 016
Yz03, BaCO,, CuO Y203, BaCO,, CuO Y203, BaC03, CuO Yz03, BaCO,, CuO BaCO, BaCO,, CuO BaCO,
Sample size
3. CO2 pressure Forming 123 from a BaCO, precursor necessarily involves losing COZ. However, 123 does not necessarily form directly from the reactants according to: $Y203 + 2BaC03 + 3CuO =YBa2Cu-,065 The reaction may instead composing the carbonate:
proceed
+2C02.
(1)
by initially
de-
BaCO, = BaO + CO, . Alternatively, intermediates:
(2)
the reaction
may
proceed
BaCO, + CuO = BaCuO, + CO1 , Y*O3 +2cuo=Y,cu~o~
)
through
(3) (4)
which then react to give 123: tY,CuzOS
synthesrs of YBalCu307_-s under reduced P(OJ
+2BaCu02
=YBazCu,06.5.
(5)
Other possible reaction pathways have been addressed by Ruckenstein et al. [ 31. However, their results are not directly applicable to the present study due to their high reaction temperature (94O”C), which is above the BaCuO,-CuO eutectic. This undoubtedly influences the reaction kinetics through liquid phase reactions and sintering [ 41. The results of a thermogravimetric experiment, TGA 008, with stoichiometric amounts of Y203, BaCO, and CuO to form 123 are shown in fig. 1. This run, which used a 0.35 12 g sample had a gas flow rate of 125 cc/min, P(0,) ~3.3 Torr, and a total pressure of 1 atm. Using these facts in conjunction with the measured mass loss, the CO2 pressure was
(g)
P(O,) (Torr)
p, (Torr)
0.3505 0.3512 0.3534 0.3543 0.1859 0.2602 0.1857
3.3 3.3 0.78 0.82 3.4 3.5 3.5
3.3 760 3 760 3.4 760 760
estimated to be 1.2~ 1O-4 atm during the first ten hours of the reaction after the sample reached 800°C. This is almost four times the decomposition pressure of BaCO, at 800°C 3.16x 10e5 atm [5]. Thus, reaction 2 was not thermodynamically possibly during a large part of the reaction period. We checked whether BaCO, decomposed according to reaction 2 under the conditions of TGA 008. The results, TGA 016, shown in fig. 2, confirm that the COZ loss observed in TGA 008 could not have occurred via reaction 2. The X-ray diffraction pattern of the powder from TGA 008 showed 123 and BaCuOz (01 1 ), suggesting that reaction 3 may be the dominant reaction route for loss of CO*. Data from TGA 0 15 are displayed in fig. 1. In this experiment, BaCuOz was synthesized under conditions similar to those used in TGA 008 (P,=l atm, P(02)=3.5 Torr, flow rate = 125 cc/min ). Note the loss of CO2 in TGA 0 15 and TGA 008 proceeds at nearly the same rate during the first part of the reaction period, consistent with the above hypothesis. Differences occur after the first 24 h, suggesting that other factors are involved in the decomposition of BaCO, to form 123 at longer times. Though we did not detect it, Y,BaCuOS (2 11) may have been an intermediate involved in the formation of 123. Of course, the direct reaction (eq. ( 1 ) ) may have also occurred. The likelihood of any given pathway will depend on a number of factors including the oxygen and COZ pressures. Resolving this problem will require data on the stability of 0 11, 2 11, and 123 as a function of P(COZ) and P(0,).
R.R. Schartman,
E.E. Hellstrom
/Low-temperature
synthesis of YBazCu307-6
under reduced P(0 j
TGA xxxxx TGA . . . . . TGA
00000
x x x
247
008 007 015
x x x
x x
4 Ld OL
80.0
75.0
Fig. 1. Relative mass change is due to loss of CO2 for each run. TGA 007 formation of 123 at P,= P(0,) = 3.3 Torr. TGA 008 formation of 123 at P,=760Torr, P(02)=3.3 Torr. TGA015 formation of01 1 at P,=760Torrand P(Oz)=3.3 Torr. 102.0
6
E+? _ 98.0
w
94.0
>
F 4
E
92.0 90.0
ooooo TGA xxxxxTGA
016 012
!
Fig. 2. Relative mass change due to loss of CO2 for each run. TGA 012 decomposition of BaCO, at P,=P(02) =3.5 Torr. TGA 016 decomposition of BaCO, at P,= 760 Torr and P(0,) ~3.5 Torr. Initial mass increase in TGA 012 was due to an imperfect bucket buoyancy correction.
4.
Total pressure As noted by Balachandran
et al., reducing the total
pressure increases the reaction rate [ 141. Experiment TGA 007, shown in fig. 1 is consistent with this claim. The conditions of TGA 007 were similar to
248
R.R. Schartman, E.E. Hellstrom /Low-temperature synthesis of YBaZCu,07_8 under reduced P(OJ
those of TGA 008: P(0,) = 3.3 Torr and sample mass of 0.3505 g. However, the low pressure in TGA 007 was obtained by flowing pure O2 at 92.5 cc/min and simultaneously vacuum pumping on the TGA system to reduce the total pressure. Although the reaction rate increased with a lowered total pressure, we always observed the impurity phase BaCuOz in our product. The reason for the increased reaction rate is twofold. First, the reduced pressure gives rise to a much larger effective gas volume thereby reducing the CO, pressure over the sample. This is readily seen by using the ideal gas law. Assume 0.005 moles of gas were flowed past the sample per minute. Under conditions of standard temperature and pressure this corresponds to a flow rate of 112 cc/min. However, if instead 0.005 moles of gas flow per minute under conditions of standard temperature but at a pressure of 3.3 Torr, the flow rate increases to 25.8 x lo3 cc/ min, an increase by a factor of about 230 times. Thus, under the low pressure conditions, a sample would have to evolve CO2 230 times as fast as a sample synthesized at 1 atm in order to maintain the same CO* pressure. Or to put it another way, the vacuum calcined sample is free to evolve CO1 230 times faster than the other sample. A second reason for the increased reaction rate is seen by calculating the P(C02) from the measured mass loss, the total pressure, and the flow rate. For the conditions of TGA 007 during the first ten hours at 800°C the P(COZ) was approximately 2.5~ 10e6 atm. This is more than a factor of ten lower than the decomposition pressure of BaC03 at 800°C. Thus, at reduced total pressure, reaction 2 is possible and 123 can form via an additional pathway. That BaCO, can decompose under reduced total pressure was confirmed in experiment TGA 012, whose conditions were similar to TGA 007 (fig. 2). The initial mass increase in TGA 0 12 was due to an imperfect bucket buoyancy correction.
P(0,) 20.2 atm produces an unwanted phase [ 6lo]. This phase is similar in structure to 123. It has the proper cation stoichiometry and the oxygen to metal ratio is sufficiently large that superconducting orthorhombic 123 material is expected. However, the phase that forms is neither superconducting nor orthorhombic. It is readily identified by its c-axis, which is shorter than that of the desired superconductor. The cause of the undesired phase remains unclear. Goodenough has suggested that disorder on the oxygen sublattice gives rise to the unwanted phase [ 91. Lay has suggested in his summary of the problem that the phase is produced by yttrium which has substituted for some of the barium [ 10 1. Irregardless, the phase appears to be metastable and can be converted to orthorhombic superconducting 123 either by high temperature anneals or simply by synthesizing 123 at these reduced temperatures under reduced oxygen pressure [ 9, lo]. Oxygen pressure also influences the reaction pathway by reducing CuO to CuZO. At sufficiently low P(O,), BaC03 decomposition can then proceed via the following route: BaC03 + Cu20 = BaCuzOz + CO* .
(6)
This fact appears to have been noted as early as 1976 by Migeon et al. in their attempts to synthesize stoichiometric BaCu02 [ 111. Recently, a more detailed study was performed by Grader et al. and it was found that reaction 6 proceeds at temperatures as low as 550” C [ 121. Unfortunately, this has not proved useful to lowering the synthesis temperature of 123 because the required ratio of Ba to Cu in 123 is 2 : 3 and reaction 6 has a Ba to Cu ratio of 1: 2, which necessarily leaves some BaC03 unreacted. Thus, higher temperatures are still needed to complete the conversion to 123. An illustration of the importance of reaction 6 will be given in the following section.
6. Impurity phases 5. Oxygen pressure The importance of the oxygen pressure is twofold. First, the P(OZ) appears to be important in achieving the desired 123 phase when preparing the material below approximately 800°C. Synthesis in
Part of our interest in this study was to explain why different authors have reported different impurities in their 123 samples prepared under different conditions. Their experimental conditions are summarized in table II. Lay has reported the presence of Y,Cu205 (202 ). Uno et al. have reported the pres-
249
R.R. Schartman, E.E. Hellstrom /Low-temperature synthesis of YBaZCujO,_d under reduced P(OJ Table II Impurity phases observed
under different
Study
P(Gz) (Torr)
p, (Torr)
Lay 1101 Unoetal. [13] Balachandranet al. Fjellvag et al. [ 15 ]
conditions.
760
600 C
780
BaCO,, CuO, yzcu205 211,012(?) 211 202, BaCO, 202 011 011 011 012,011
25-950 25-800 300-900 25-800 25-800 25-800 25-800
atm [ 15 1. They have shown that if the CO2 pressure is sufficiently high 123 decomposes to 202, BaC03 and CuO. This would explain the results of Lay. If the unidentified phase in the work of Uno et al. is indeed 0 12, then they may have simply crossed the reduction boundary of 123. Ahn et al. indicate that upon reduction, 123 decomposes to 0 12, 2 11 and a third phase, YBa3Cu206 [ 161. Although Uno et al. did not report this last phase, it is expected to be present in levels below 10 mol% and so it may have escaped detection.
ence of (2 11) and an unknown phase containing Ba and Cu in the ratio of 2: 3 [ 131. We note however, their X-ray diffraction pattern resembles that of BaCu,Oz (0 12 ). Balachandran et al. have reported the presence of 2 11 in their samples [ 141. They suggested that this was a result of a build up of CO2 and when the P(0,) :P(CO*) > 50 the unwanted phase did not form. We on the other hand, routinely observe 011. Thermodynamic measurements have been performed by Fjellvag et al. at oxygen pressure near 1 105.0
Ar
Major impurities
(“C)
02 OJAr WAr WAr
3.3 3.3 0.78 0.82
3.3 760 3.3 760
TGA 007 TGA 008 TGA 010 TGAOll
Temp.
Air GZ WCG2
0.1-0.001 2.0 735-760
0.1-0.001 2.0 760
[ 141
Gas mix
800 c
A
ER 00000
xxxxx
70.0
,I 0
I,
I
I
I
I,
I,
I,,
,
,
,
,
,
,
,
,
/
,
,
,
,
,
,
,
,
,
10
TIM?
(
HO&
Fig. 3. Relative mass change due to loss of CO2 for each run. TGA 0 10 formation formation of 123 at P,= 760 Torr and P(0,) =0.82 Torr.
)
,
,
/
TGA TGA
,
,,
010
011
1 50
4o
of 123 at P,= 3 Torr and P( Oz ) = 0.78 Torr. TGA 011
250
R.R. Schartman. E.E. Hellstrom /Low-temperature
The applicability of the thermodynamic work by Fjellvag et al. [ 15 ] to low pressure synthesis is unclear. The oxygen pressure used was different in each case. Experiments to determine the thermodynamic stability of 123 in CO* at reduced oxygen pressure are needed. It is also possible that the impurity phases present not a thermodynamic but a kinetic problem. Each author generated a different set of reaction conditions throughout the course of the synthesis. Although thermodynamically 123 will be the stable phase once the P(COZ) drops sufficiently, kinetically the impurity phases may remain. This is illustrated in the following two TGA experiments to prepare 123 (fig. 3). The two experiments were performed under identical heating rates and used the same batch of reactants. The oxygen pressure in both experiments (0.8 Torr) was just below the CuO-Cu20 redox boundary at 800°C ( 1.2 Torr) [ 171. Flow rates for TGA 010 and TGA 011 were 109 cc/min and 125 cc/min, respectively. The main difference was that TGA 0 10 was carried out at 3.3 Torr total pressure whereas TGA 0 11 was carried out at 1 atm. Since the material synthesized at low pressure was almost fully reacted before the CuO-CuzO boundary was crossed, the major impurity detected was BaCuOz. This was not true for the material reacted at P,= 1 atm where both BaCuOz and BaCuzOz impurities were detected.
7. Conclusions The importance of CO*, O2 and total pressure to the low temperature synthesis of 123 was illustrated. These variables influence the kinetic pathway taken in forming 123 and thus determine the reaction rate and the impurities generated. A complete knowledge of the thermodynamic variables involved in the low temperature synthesis is lacking. This, plus the kinetic hindrance to removing the impurities gener-
synthesis of YBaZCu307_d under reduced P(0.J
ated at low temperatures have blocked efforts at synthesizing phase pure 123 at low temperatures.
Acknowledgement This work was supported Research Institute.
by the Electric
Power
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