Role of calcium plumbate during the formation of 2223 phase in the Bi(Pb)srcacuo system

Mat. Res. Bull., Vol. 27, pp. 1-8, 1992. Printed in the USA. 0025-5408/92 $5.00 + .00. Copyright © 1991 Pergamon Press plc.

ROLE OF CALCIUM PLUMBATE DURING THE FORMATION OF 2223 PHASE IN THE Bi(Pb)-Sr-Ca-Cu-O SYSTEM Asok K. Sarkar University of Dayton Research Institute 300 College Park Avenue Dayton, Ohio 45469-0170 Y. J. Tang, X. W. Cao and J. C. Ho Wichita State University Wichita, Kansas 67208-1595 G. Kozlowski Wright Laboratory Wright-Patterson Air Force Base, Ohio 4 5 4 3 3 - 6 5 6 3 (Received October 28, 1991; Communicated by W.B. White) ABSTRACT There are conflicting reports regarding the melting point of Ca2PbO4 in the literature. By performing differential thermal analysis u n d e r various atmospheres, we found that the melting point of Ca2Pb04 is severely depressed due to the reduced partial p r e s s u r e of oxygen. At an oxygen partial pressure of one atm., Ca~PbO 4 melts at 1055°C and this temperature is reduced to a b o u t 833°C in zero oxygen partial pressure (e.g. in nitrogen). CehPbO 4 is formed as an impurity in the initial stage during the preparation of pure 2223 phase from the Bi(Pb)-Sr-Ca-Cu-O system in air. However, the u s u a l sintering temperatures (840°-850°C} at which the 2223 p h a s e is synthesized in air are not high enough to melt Ca2PbO 4. Thus, the melting of Ca2PbO4 is not responsible for the growth of 2 2 2 3 p h a s e from a liquid medium. Ca2PbO 4 and the lead-enriched 2223 p h a s e were found to be compatible with each other at 845°C in air. Utilization of reduced partial pressure of oxygen will inhibit the growth of Ca2PbO 4 and promote the formation of the lead-contalning, 2223 phase by modifying the reaction p a t h s that would not otherwise be possible if higher oxygen partial p r e s s u r e were used. MATERIALS INDEX:

calcium, bismuth, plumbates, cuprates, superconductors

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Introduction Considerable effort h a s been devoted to obtaining single-phase, high-To, Bi2Sr2Ca2Cu301o~ (To - 110K) c o m p o u n d (herein after called 2223) in the Bi-SrCa-Cu-O system since the discovery of superconducting p h a s e s in this system. Of all the techniques reported to date, partial substitution of Bi b y Pb in the system h a s been found to be the most successful ~. A d j u s t m e n t s of cation stoichiometries of the starting compositions coupled with processing via solid state reactions u n d e r low oxygen pressure have been claimed to yield single-phase sample b y Endo et al. 2'3 and Koyama et alJ. Since addition of Pb greatly modifies the p h a s e relationships of this system, m a n y investigators have also sought to u n d e r s t a n d its role in facilitating the formation of the 2223 phase. There are m a n y theories regarding the exact role played b y Pb. One aspect that has received general c o n s e n s u s is the fact that an o p t i m u m concentration of Pb is n e c e s s a r y to synthesize the single-phase 2223 compound. Presence of excess Pb invariably leads to the formation of calcium plumbate, Ca2PbO 4, p h a s e along with the 2223 phase. Since this p h a s e is an insulator, its presence is unwanted. Even in the presence of the optimum Pb concentration, Ca~PbO4 is ubiquitously formed at temperatures as low as 550°C during the formation reaction of the final 2223 p h a s e starting from oxides and carbonates in the present quinary system s. As a result of this, some a u t h o r s have commented on the role of Ca2PbO4 in the parent Bi-Sr-Ca-Cu-O system while proposing the reaction m e c h a n i s m s for the formation of the 2223 phase. In m o s t cases, to increase the volume fraction of the 2223 phase, the processing of the Pb-doped formulations were carried out at a temperature near 850°C in air. Unfortunately, some of the proposed reaction s c h e m e s a n d / o r m e c h a n i s m s mentioned b y the a u t h o r s 6'7 are incorrect and m a y have been delineated due to m i s u n d e r s t a n d i n g of the published results. The major discrepancy arises from the a s s u m p t i o n of the incongruent melting of the Ca~PbO4 p h a s e at 822°C reported in the p h a s e diagram of the CaO-PbO system b y K u x m a n n and Fischer a in 1974. There seem to be two problems with this assumption. First, the crystal structure for this p h a s e was published b y Tromel 9 in 1969, along with the procedure for synthesizing single-phase Ca2PbO 4 b y conventional solid state reaction of CaCOa and PbO at ~900°C in air. Second, a careful examination of the K u x m a n n and Fischer paper revealed that their p h a s e diagram w a s constructed with the data obtained from the cooling curves recorded during differential thermal analysis of various premelted CaO-PbO mixtures performed in nitrogen atmosphere. The Tromel results indicate unmelted Ca~PbO 4 at ~900°C in air. The K u x m a n n and Fischer results indicate molten Ca2PbO 4 at 822°C in nitrogen. Thus, it appears that the nature of the environment or partial p r e s s u r e of oxygen can have a drastic effect on the melting behavior of the Ca2PbO 4 phase. Recently, a revised CaO-PbO p h a s e diagram h a s been published b y Kitaguchi et aljo where they s h o w that the incongruent melting point of the stoichiometric Ca2PbO 4 p h a s e in air is at 980 2°C. This temperature is m u c h higher t h a n the u s u a l processing temperatures (usually 840 ° to 870°C) for the Pb-doped b i s m u t h systems. Therefore, a m e c h a n i s m where simple melting of the Ca2PbO4 c o m p o u n d in alr to

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create the liquid p h a s e from which the growth of the 2223 p h a s e occurs is improbable. Ca2PbO4 m u s t participate in the reaction s e q u e n c e s in some other fashion. The importance of oxygen stoichiometry in the high-T c ceramic s u p e r c o n d u c t o r s and the lowering of melting t e m p e r a t u r e of these s u p e r c o n d u c t o r s u n d e r reduced oxygen partial pressure led u s to reexamine the thermal/melting characteristics of the Ca2PbO 4 p h a s e in several different atmospheres. Additionally, the effect of this phase on the melting behavior of the 2223 p h a s e was also studied to explore its role in the formation of a n y lowt e m p e r a t u r e peritectic liquid in this system. Experimental Ca~PbO4 w a s synthesized in the laboratory b y reacting stoichiometric a m o u n t s of reagent grade CaCO 3 and PbO, following the s t a n d a r d solid state reaction technique of repeated heating and grinding in air. The heating of the powder w a s carried out in a covered alumina crucible and the m a x i m u m firing t e m p e r a t u r e w a s 900°C. The resulting product w a s characterized b y conventional powder X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques. The precursor powder of the off-stoichiometric 2223 c o m p o u n d having a nominal composition of Bil.72Pbo.34Sr~.83Ca197Cu3.~3Oy w a s prepared from Bi203, PbO, SrCO3, CaCO 3 and CuO by solid state reactions below 840°C with intermediate grinding. The precursor powder, alone, and thoroughly mixed with additional 10, 20 and 30 wt% of Ca~PbO4, was then subjected to four s u b s e q u e n t sintering process in air at 845°C for a period of 50h for each h e a t treatment. The specimens were reground and pelletized between each consecutive sintering process. The final p h a s e compositions and the melting behavior of the pellets were analyzed b y both powder XRD and DTA techniques. Results and Discussion During the powder preparation there was negligible weight loss (<0.02%) and no sign of melting in the crucible after the h e a t treatment at 900°C. Also, the powder did not s h o w any sign of compaction u n d e r this treatment and w a s very easy to pulverize indicating the absence of any liquid formation. The XRD p a t t e m of this synthetic Ca2PbO 4 is shown in Fig. 1 and m a t c h e s very well with the published pattern for Ca~PbO 4 byTromel 9. It is t h u s shown that Ca~PbO4 is likely to have a melting point above 900°C in air. To explore the effect of various a t m o s p h e r e s on the melting behavior of Ca2PbO 4, DTA and TGA curves were obtained u n d e r static air and flowing (30 cc/min) air, oxygen and nitrogen at a heating rate of 10°C/min. The effect of heating Ca~PbO4 in static air is shown in the DTA and TGA curves of Fig. 2, where it can be seen (l~Ig. 2a) that the onset of melting is at 980°C with an endothermic peak at 1020°C. On the other hand, the

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FIG. I Powder XRD pattern for synthetic Ca2PbO4. corresponding TGA curve (Fig. 2b) shows no m e a s u r a b l e weight loss of this p h a s e until 950°C. The onset of melting of Ca2PbO+ at 980°C in s t a g n a n t air agrees verby well with the incongruent melting point reported b y Kitaguchi and coworkers i . However, a small increase in the onset melting t e m p e r a t u r e s from 980 ° to 990°C is observed w h e n Ca2PbO4 is heated in flowing air as seen in Fig, 3. This increase in temperature clearly points to the effect of equilibrium oxygen partial p r e s s u r e on the melting behavior of CasPbO4. A more dramatic effect of oxygen partial pressure on the melting of the calcium p l u m b a t e p h a s e is seen in Fig. 4, where the DTA/TGA curves in flowing oxygen are shown. It is observed in the DTA

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FIG. 3 a) DTA and b) TGA curves for Ca~PbO4 in flowing air; flow rate 30cc/min, heating rate 10°C/rain. curve of Fig. 4a that increasing oxygen partial pressure to 1 atm. h a s increased the onset of melting temperature to 1055°C moving the endothermic p e a k u p to 1080°C. The weight loss behavior, however, is also affected b y the increase in oxygen partial p r e s s u r e (Fig. 4b), since measurable loss starts to occur at a r o u n d 980°C. These results show that Ca2PbO4 is more stable in pure oxygen t h a n in air. The situation changes completely when the experiments are performed in pure nitrogen or in zero oxygen partial pressure. As can be seen in the DTA/TGA curves of Fig. 5, the onset of melting/weight loss begins very gradually at a r o u n d 780°C and the DTA peak (Fig. 5A) becomes very broad with the m i n i m u m being at 955°C.

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FIG. 5 a) DTA and b) TGA curves for Ca2Pb04 in flowing nitrogen; flow rate 30cc/min, heating rate 10°C/min. It is thus emphasized that the reaction mechanisms which were proposed to involve the melting of Ca2PbO4 to form the liquid phase from which the high-To, 2223 phase crystallized in the Pb-doped Bi-Sr-Ca-Cu-O system in air are incorrect. The temperature was never high enough to induce melting of this phase in air. Only in pure nitrogen atmosphere is the melting point of the plumbate phase lower than the usual processing temperatures of these superconductors. We also conducted several isothermal experiments in the DTA apparatus to check the melting point of Ca2PbO4 in flowing nitrogen atmosphere. Based on these experiments, we believe that Ca2PbO4 is unstable above 833°C in nitrogen atmosphere. The instability arises due to the process of losing oxygen from the plumbate phase; this process begins at 833°C and the rate of this oxygen loss ultimately determines the time and temperature at which Ca2PbO4 will eventually melt. Furthermore, it is now well known ~I that the formation of the liquid phase in the system plays a major role in optimizing the growth of the 2223 phase and utilization of the reduced partial pressure of oxygen during processing of these superconductors may in fact enhance the formation of the 2223 phase by inhibiting the formation of the Ca2Pb04. In order to observe the compatibility of the 2223 phase with the Ca2PbO4 phase, an off-stoichiometric synthetic 2223 phase was mixed with various percentages (up to 30 wt%) of Ca2PbO4 and the mixtures were reacted several times to assure homogenization as described in the experimental section. Powder XRD patterns of all these samples showed the presence of only the 2223 and the added Ca2PbO4 phases. The DTA curves of these various samples as shown in Fig. 6 do not show the formation of any liquid phase below 850°C. In fact, Ca2PbO4 is seen to have very marginal effect on lowering the melting point of the 2223 phase. For example, the peak temperature of the largest endotherm in the DTA thermogram (taken to be the temperature of first major melting a n d / o r liquid formation) of pure 2223 phase was at 898°C, and it was observed to occur at 892°C when 10 and 20 wt% Ca2PbO4 was added, and at 887°C when 30 wt% Ca2PbO4 was added to the system.

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Thus, it can be seen that once the plumbate p h a s e h a s formed, it is not likely to react with the 2223 p h a s e to form any liquid during sintering. So the ongoing formation of 2223 phase will occur via the reaction of this p h a s e with the low-Tc p h a s e s s u c h as 2201 and 2212 c o m p o u n d s formed in the system in the presence of other copper-rich p h a s e s that m a y or m a y not involve a liquid phase. This liquid p h a s e m a y also be rich in lead and b i s m u t h so that it forms at t e m p e r a t u r e s below the u s u a l sintering temperature for proper growth of the 2223 phase. The kinetics for the formation of the 2223 p h a s e during this stage are quite high. Once this liquid phase is exhausted through reaction, the formation of additional 2223 p h a s e will occur only via solid state diffusion reaction of all the solid impurity p h a s e s including the Ca2PbO 4 p h a s e and its formation kinetics will be very sluggish. This is in fact what is observed in practice during the preparation of the 2223 phase through repeated grinding and sintering b y the conventional solid state reaction. Usually, the formation of 2223 p h a s e during first sintering is very rapid and the rate slows down considerably u n d e r s u b s e q u e n t sintering steps. Since the presence of Ca2PbO 4 p h a s e is unwanted, the concentration of Pb in the system and oxygen in the sintering atmosphere should be controlled to inhibit its formation. However, some oxygen (~0.1 atm)

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should be present in the atmosphere, since 2223 phase is not stable at temperatures greater than 800°C in zero oxygen partial pressure, Conclusion We have observed that the melting temperature for the pure Ca2PbO4 phase is very sensitive to the oxygen partial pressure. The melting point of Ca2PbO4 can vary from 833°C in zero oxygen partial pressure to 1055°C in oxygen partial pressure of one atm. Thus, the melting of this compound, in particular, is not responsible for creating the liquid phase form which the solid, high-Tc, 2223 phase grows during the solid state reaction of the Bi(Pb)-Sr-Ca-Cu-O system. It is very difficult to avoid the formation of CazPbO4 during preparation of the 2223 phase by solid state sintering of the Bi(Pb)-Sr-Ca-Cu-O system in air. Only a prolonged reaction time in air will diminish the amount of this impurity phase. Preparation of the phase-pure 2223 compound thus requires careful control of not only the starting cation stoichiometry but also the sintering conditions. References . .

3. 4. 5. 6. . .

9. I0. 11.

A. K. Sarkar, I. Maartense, T. L. Peterson and B. Kumar, J. Appl. Phys., 66, 3717 (1989). U. Endo, S. Koyama and T. Kawai, Jpn. J. Appl. Phys., 2__7,L1476 (1989). U. Endo, S. Koyama and T. Kawai, Jpn. J. Appl. Phys., 28, L190 (1989). S. Koyama, U. Endo and T. Kawai, Jpn. J. Appl. Phys., 2__7,L1863 (1988). G. Zorn, B. Seebacher, B. Jobst and J. Gobel, Physica C, 1_17_77,494 (1991). T. Uzumaki, K. Yamanaka, N. Kamehara and K. Niwa, Jpn. J. Appl. Phys., 2__8, L75 (1989). Y. I. Chen and R. Stevens, Paper 7-EP-89, 92nd Annual Meeting and Exposition, Amer. Ceram. Soc., April 22-26, 1990, Dallas, Texas. U. Kuxmann and P. Fischer, Erzmetall, 27, 533 (1974). M. Tromel, Z. Anorg. Allg. Chem., 371,237 (1969). H. Kitaguchi, J. Takada, K. Oda and Y. Miura, J. Mater. Res., 5, 929 (1990). P. E. D. Morgan, R. M. Housley, J. R. Porter and J. J. Ratto, Physica C, 176, 279 (1991).