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 Insti tute

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 45433-6563

(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 under various atmospheres, we found that the melting point of Ca2Pb04 is severely depressed due to the reduced partial pressure of oxygen. At an oxygen partial pressure of one atm., Ca~PbO 4 melts at 1055°C and this temperature is reduced to abou 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 preparat ion of pure 2223 phase from the Bi(Pb)-Sr-Ca-Cu-O system in air. However, the usua l sintering tempera tures (840°-850°C} at which the 2223 phase is synthesized in air are not high enough to melt Ca2PbO 4. Thus, the melting of Ca2PbO4 is not responsible for the growth of 2223 phase from a liquid medium. Ca2PbO 4 and the lead-enriched 2223 phase 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 pa ths tha t would not otherwise be possible if higher oxygen partial pressure were used.

MATERIALS INDEX: calcium, bismuth, plumbates, cuprates, superconductors

2 A.K. SARKAR et al. Vol. 27, No. 1

Introduction

Considerable effort has been devoted to obtaining single-phase, high-To, Bi2Sr2Ca2Cu301o ~ (To - 110K) compound (herein after called 2223) in the Bi-Sr- Ca-Cu-O sys tem since the discovery of superconduct ing phases in this system. Of all the techniques reported to date, partial subst i tu t ion of Bi by Pb in the sys tem has been found to be the most successful ~. Adjus tments of cation stoichiometries of the starting compositions coupled with processing via solid state reactions under low oxygen pressure have been claimed to yield single-phase sample by Endo et al. 2'3 and Koyama et alJ. Since addition of Pb greatly modifies the phase relationships of this system, many investigators have also sought to unders tand its role in facilitating the formation of the 2223 phase.

There are many theories regarding the exact role played by Pb. One aspect that has received general consensus is the fact that an opt imum concentrat ion of Pb is necessary to synthesize the single-phase 2223 compound. Presence of excess Pb invariably leads to the formation of calcium plumbate , Ca2PbO 4, phase along with the 2223 phase. Since this phase is an insulator, its presence is unwanted. Even in the presence of the opt imum Pb concentration, Ca~PbO4 is ubiqui tously formed at tempera tures as low as 550°C during the formation reaction of the final 2223 phase starting from oxides and carbonates in the present quinary system s. As a result of this, some authors have commented on the role of Ca2PbO4 in the parent Bi-Sr-Ca-Cu-O sys tem while proposing the reaction mechanisms for the formation of the 2223 phase. In mos 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 schemes a n d / o r mechanisms mentioned by the authors 6'7 are incorrect and may have been delineated due to misunders tanding of the published results. The major discrepancy arises from the assumpt ion of the incongruent melting of the Ca~PbO4 phase at 822°C reported in the phase diagram of the CaO-PbO system by Kuxmann and Fischer a in 1974. There seem to be two problems with this assumption. First, the crystal s t ructure for this phase was published by Tromel 9 in 1969, along with the procedure for synthesizing single-phase Ca2PbO 4 by conventional solid state reaction of CaCOa and PbO at ~900°C in air. Second, a careful examination of the Kuxmann and Fischer paper revealed that their phase diagram was const ructed 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 Kuxmann and Fischer results indicate molten Ca2PbO 4 at 822°C in nitrogen. Thus, it appears that the nature of the environment or partial p ressure of oxygen can have a drastic effect on the melting behavior of the Ca2PbO 4 phase. Recently, a revised CaO-PbO phase diagram has been publ ished by Kitaguchi et a l jo where they show that the incongruent melting point of the stoichiometric Ca2PbO 4 phase in air is at 980 2°C. This temperature is much higher than the usua l processing tempera tures (usually 840 ° to 870°C) for the Pb-doped b i smuth systems. Therefore, a mechanism where simple melting of the Ca2PbO4 compound in alr to

Vol. 27, No. i CALCIUM PLUMBATE

create the liquid phase from which the growth of the 2223 phase occurs is improbable. Ca2PbO4 mus t participate in the reaction sequences in some other fashion.

The importance of oxygen stoichiometry in the high-T c ceramic superconductors and the lowering of melting tempera ture of these superconductors under reduced oxygen partial pressure led us to reexamine the thermal /mel t ing characterist ics of the Ca2PbO 4 phase in several different a tmospheres . Additionally, the effect of this phase on the melting behavior of the 2223 phase was also studied to explore its role in the formation of any low- tempera ture peritectic liquid in this system.

Experimental

Ca~PbO4 was synthesized in the laboratory by reacting stoichiometric amoun t s of reagent grade CaCO 3 and PbO, following the s tandard solid state reaction technique of repeated heating and grinding in air. The heating of the powder was carried out in a covered alumina crucible and the max imum firing tempera ture was 900°C. The resulting product was characterized by conventional powder X-ray diffraction (XRD), thermogravimetric analysis (TGA) and differential thermal analysis (DTA) techniques.

The precursor powder of the off-stoichiometric 2223 compound having a nominal composition of Bil.72Pbo.34Sr~.83Ca197Cu3.~3Oy was 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 subsequen t sintering process in air at 845°C for a period of 50h for each hea t t reatment. The specimens were reground and pelletized between each consecutive sintering process. The final phase compositions and the melting behavior of the pellets were analyzed by both powder XRD and DTA techniques.

Resul ts and Discussion

During the powder preparat ion there was negligible weight loss (<0.02%) and no sign of melting in the crucible after the hea t t rea tment at 900°C. Also, the powder did not show any sign of compaction under this t rea tment and was 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 matches very well with the publ ished pat tern for Ca~PbO 4 byTromel 9. It is thus shown that Ca~PbO4 is likely to have a melting point above 900°C in air. To explore the effect of various a tmospheres on the melting behavior of Ca2PbO 4, DTA and TGA curves were obtained under static air and flowing (30 cc/min) air, oxygen and nitrogen at a heating rate of 10°C/min.

The effect of heat ing Ca~PbO4 in static air is shown in the DTA and TGA curves of Fig. 2, where it can be seen (l~Ig. 2a) tha t the onset of melting is at 980°C with an endothermic peak at 1020°C. On the other hand, the

A.K. SARKAR et al. Vol. 27, No. 1

l - z

>. n, < n, I -

n- <

>. I--

Z uJ I-- z m

I I I

4 14 :)4 3 4 4 4 2 8 (DEGREE)

FIG. I Powder XRD pat tern for synthetic Ca2PbO4.

corresponding TGA curve (Fig. 2b) shows no measurable weight loss of this phase until 950°C. The onset of melting of Ca2PbO+ at 980°C in s tagnant air agrees verby well with the incongruent melting point reported by Kitaguchi and coworkers i . However, a small increase in the onset melting tempera tures from 980 ° to 990°C is observed when Ca2PbO4 is heated in flowing air as seen in Fig, 3. This increase in temperature clearly points to the effect of equilibrium oxygen partial pressure on the melting behavior of CasPbO4. A more dramatic effect of oxygen partial pressure on the melting of the calcium plumbate phase 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. 2 a) DTA and b) TGA curves for Ca2PbO4 in static air, heat ing rate lOoClmm.

Vol. 27, No. 1 CALCIUM PLUMBATE

o I , - <~

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TEMPERATURE (oc)

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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.

100

curve of Fig. 4a that increasing oxygen partial pressure to 1 atm. has increased the onset of melting temperature to 1055°C moving the endothermic peak up to 1080°C. The weight loss behavior, however, is also affected by the increase in oxygen partial pressure (Fig. 4b), since measurable loss s tar ts to occur at a round 980°C. These resul ts show that Ca2PbO4 is more stable in pure oxygen than in air. The si tuation 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 melt ing/weight loss begins very gradually at a round 780°C and the DTA peak (Fig. 5A) becomes very broad with the minimum being at 955°C.

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FIG. 4 a) DTA and b) TGA curves for Ca2PbO 4 in flowing oxygen;

flow rate 30cc/min, heating rate 10°C/min.

A . K . S A R K A R e t a l . V o l . 27 , N o . 1

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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 Ca2PbO 4 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 Ca2PbO 4 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, Ca2PbO 4 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 and/or 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.

Vol. 27, No. 1 CALCIUM PLUMBATE

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FIG. 6 DTA traces (heating rate 10°C/min) for the synthetic, single-phase, 2223 compound with various calcium plumbate concentrations: a) 0 wt%, b) 10 wt%, c) 20 wt%, and d) 30 wt%.

Thus, it can be seen that once the p lumbate phase has formed, it is not likely to react with the 2223 phase to form any liquid during sintering. So the ongoing formation of 2223 phase will occur via the reaction of this phase with the low-T c phases such as 2201 and 2212 compounds formed in the sys tem in the presence of other copper-rich phases that may or may not involve a liquid phase. This liquid phase may also be rich in lead and b i smuth so that it forms at tempera tures below the usua l sintering temperature for proper growth of the 2223 phase. The kinetics for the formation of the 2223 phase during this stage are quite high. Once this liquid phase is exhausted through reaction, the formation of additional 2223 phase will occur only via solid state diffusion reaction of all the solid impuri ty phases including the Ca2PbO 4 phase and its formation kinetics will be very sluggish. This is in fact what is observed in practice during the preparat ion of the 2223 phase through repeated grinding and sintering by the conventional solid state reaction. Usually, the formation of 2223 phase during first sintering is very rapid and the rate slows down considerably under subsequen t sintering steps. Since the presence of Ca2PbO 4 phase is unwanted, the concentrat ion of Pb in the system and oxygen in the sintering a tmosphere should be controlled to inhibit its formation. However, some oxygen (~0.1 atm)

8 A.K. SARKAR et al. Vol. 27, No. 1

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-T c, 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.

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