J O U R N A L O F M AT E R I A L S S C I E N C E : M AT E R I A L S I N E L E C T RO N I C S 1 3 ( 2 0 0 2 ) 3 5 3 ± 3 5 6

Secondary phase formation during the Bi-2223 phase calcination process V. GARNIER, A. AMBROSINI, G. DESGARDIN CRISMAT/ISMRA, 6 Bd MareÂchal Juin, 14050 Caen Cedex, France E-mail: [email protected]

In this work, we report the secondary phase evolution that occurs during calcination of Bi2223 precursor powder prepared by the polymer-matrix method. The effect of the calcination conditions (time, temperature, and intermediate milling) on the formation of secondary phases has been studied. The samples were characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) analysis. The calcined powder phase assemblage is sensitive to the calcination conditions, and control of the calcination parameters is necessary to form the adequate reactive secondary phases (Bi-2212, Ca2 PbO4 , Ca2 CuO3 , CuO). These phases are necessary for the proper synthesis of Bi-2223 during the sintering step. The optimized calcination conditions necessary to obtain these phases are 24 h at 820  C. # 2002 Kluwer Academic Publishers

1. Introduction The Bi-Pb-Sr-Ca-Cu-O system is complex, consisting of many different phases whose formation depends on stoichiometry, heating temperature, and time [1]. In this system, (Bi, Pb)2 Sr2 Ca2 Cu3 O10 ‡ d [2] is the most interesting phase for applications considering its good physical and mechanical properties. A considerable amount of research dealing with the development of synthetic processes to obtain single-phase Bi-2223 has been performed. Many groups do not synthesize their own precursor powder and instead use commercial Bi2223 precursor powder. Therefore, they have no control over the powder grain size and phase composition. It is well known that the phase composition is of prime importance to the formation of Bi-2223 during the sintering step, since the reaction of Bi-2223 is sensitive to the pre-reactive conditions, such as the powder precursor preparation and the calcination step. The powder precursor has been synthesized by many methods: solid-state synthesis [3±5], sol±gel method [6, 7], aerosol pyrolysis technique [8], coprecipitation of oxalate [9, 10], spray-dried nitrate precursors [11], freeze-dried precursors [12], and the polymer-matrix method [13]. In this study, the polymer-matrix method, which until now has not been as intensively studied as the other methods, was used to prepare the powder precursor. Secondary-phase formation during the calcination step was then studied as a function of calcination temperature, time, and intermediate milling. The evolution of several secondary phases, which are not precursors for the formation of Bi-2223, are discussed herein.

2. Experimental Starting from the cationic stoichiometry of Bi1:85 Pb0:35 Sr2 Ca2 Cu3:1 , and using the corresponding 0957±4522

# 2002 Kluwer Academic Publishers

metal acetates, the powder precursor was prepared by the polymer-matrix method [13], a liquid-phase process that allows a homogeneous mix of the powder to be maintained at the atomic scale without problems of precipitation due to the pH control. This powder preparation method is described in detail in Garnier et al. [14] The resulting powder was ground by hand in an agate mortar to form a ®ne and reactive powder, which was then subjected to the calcination procedure. Eleven types of powder were prepared according to the different calcination conditions given in Table I. The temperature did not exceed 820  C in order to avoid the formation of Bi-2223 during the calcination step. Temperatures above 820  C cause decomposition of the Ca2 PbO4 phase …Tf ˆ 822  C† [15], which forms CaO and a liquid phase rich in Pb; CaO reacts with CuO to form Ca2 CuO3 [16], which in turn accelerates the undesired formation of Bi-2223 [17] during the calcination step. Thus, a powder calcined above 820  C generates a number of Bi-2223 nuclei that hinder the growth of the Bi-2223 grains during the subsequent sintering steps [18, 19]. The calcined powders were analyzed by X-ray diffraction (Philips PW 3710, lCu [Ka1 ]†. The microstructures of the calcined powders were observed using a scanning electron microscope (SEM) (Philips XL30).

3. Results and discussion XRD spectra of the eleven calcined powders are shown in Fig. 1(a) and (b). Typical phases observed are Bi-2212 and (Ca,Sr) oxides. It is known that these phases are observed depending on the starting composition, but this study also shows that their appearance is also dependent on the calcination conditions. Control of the secondaryphase formation is important because such phases have 353

T A B L E I Description of calcinations conditions and phase assemblage after the calcinations Type of calcinations

Calcination conditions

Phases composition from major to minor

Calcin. 1 Calcin. 2 Calcin. 3 Calcin. 4 Calcin. 5 Calcin. 6 Calcin. 7. Calcin. 8 Calcin. 9 Calcin. 10 Calcin. 11*

800  C/12 h 750  C/12 h 750  C/10 h ‡ 800  C / 550  C/10 h ‡ 750  C / 750  C/12 h ‡ 800  C / 750  C/12 h ‡ 820  C / 800  C/24 h 800  C/48 h 810  C/24 h 820  C/24 h 820  C/12 h ‡ 820  C /

2212; Ca2 PbO4 ; CuO; Bi10 Ca7 O22 ; 2201; Ca2 CuO3 ; SrCO3 ; Cu2 SrO2 Bi10 Ca7 O22 ; CuO; Ca2 PbO4 ; 2201; Bi1:6 Pb2:4 Sr2:8 Ca2:1 O5 ; SrCO3 ; Ca2 CuO3 ; 2212 2212; Ca2 PbO4 ; CuO; Sr14 Cu24 O41 ; Sr0:15 Ca0:85 CuO2 ; Cu2 SrO2 2212; Ca2 PbO4 ; CuO; Sr0:15 Ca0:85 CuO2 ; Cu2 SrO2 2212; Ca2 PbO4 ; CuO; Sr0:15 Ca0:85 CuO2 ; Cu2 SrO2 ; Sr14 Cu24 O41 2212; Ca2 PbO4 ; CuO; Cu2 SrO2 ; Ca2 CuO3 2212; Ca2 PbO4 ; CuO; Sr14 Cu24 O41 ; Cu2 SrO2 ; Sr0:15 Ca0:85 CuO2 ; Ca2 CuO3 2212; Ca2 PbO4 ; CuO; Cu2 SrO2 ; Sr0:15 Ca0:85 CuO2 ; Ca2 CuO3 2212; Ca2 PbO4 ; CuO; Ca2 CuO3 ; Cu2 SrO2 ; Sr0:15 Ca0:85 CuO2 ; Sr14 Cu24 O41 2212; Ca2 PbO4 ; Ca2 CuO3 ; CuO; Sr14 Cu24 O41 ; Sr2 CuO3 ; Cu2 SrO2 2212; Ca2 PbO4 ; Ca2 CuO3 ; CuO; Sr2 CuO3 ; Cu2 SrO2

10 h 10 h ‡ 800  C / 10 h 12 h 12 h

12 h

*With an intermediate milling.

an adverse effect on the supercurrent ¯ow in the oxide core by degrading the orientation of the Bi-2223 platelets. It is important to synthesize the Bi-2223 phase rapidly during the sintering step, to prevent the growth of secondary phases, which become less reactive as they increase in size. Therefore, in order to have a maximum conversion to the Bi-2223 phase from the calcined material, this reaction must be quick to avoid the segregation and the growth of unnecessary secondary phases. To allow rapid Bi-2223 phase formation, an adequate phase composition (Bi-2212, Ca2 PbO4 , Ca2 CuO3 , CuO) [18±21] must be obtained during calcination. From our study of the calcination procedures, we can determine the growth conditions concerning the

Sr14 Cu24 O41 phase. The appearance of this phase depends completely on the thermal processing. For Calcin. 1 and Calcin. 2, calcined respectively at 800 and 750  C for 12 h, the presence of Sr14 Cu24 O41 is not detected. In these two calcinations, the phase assemblage is unsatisfactory relative to the adequate phase composition discussed above. However, in Calcin. 3, after a total of 20 h of calcination at temperatures of 750 and of 800  C, a large quantity of Sr14 Cu24 O41 is present, and the phase assemblage is closer to the ideal one for the formation of Bi-2223. Calcin. 5, 7, 9, and 10 con®rm the importance of the calcination duration. At equivalent thermal processing times (24 h), the Sr14 Cu24 O41 phase is detected in small quantities in the XRD spectra of each of these calcinations, independent of the temperatures used

Figure 1 XRD patterns of powders calcined using calcination conditions described Table I, and labeled as follows: 1: Cu2 SrO2 ; 2: 2212; 3: 2201; 4: CuO; 5: Ca2 PbO4 ; 6: Ca2 CuO3 ; 7: Sr0:15 Ca0:85 CuO2 ; 8: Bi1:6 Pb2:4 Sr2:8 Ca2:1 CuOx ; 9: Bi10 Ca7 O22 ; 10: SrCO3 ; 11: Sr14 Cu24 O41 ; 12: Sr2 CuO3 .

354

Figure 2 SEM micrograph of calcined powder number 10 (820  C/24 h).

(between 750 and 820  C). This implies the processing time has an important effect on the existence of this phase in the temperature range studied. After 20 h of calcination, the amount of Sr14 Cu24 O41 present tends to decrease, disappearing altogether after 30 h (Calcin. 4 and 8). However, intermediate milling (Calcin. 11) allows re-homogenization, thus facilitating the reactivity of the powders. Consequently, no trace of Sr14 Cu24 O41 phase is observed in the powder XRD pattern of Calcin. 11, although the total time of calcination is 24 h. In the calcination XRD spectra of the Fig. 1(a) and (b), the overlapping of a Bi-2212 peak at 2y ˆ 36.81 with an Sr0:15 Ca0:85 CuO2 peak at 2y ˆ 36.76 is observed. By considering the relative intensities of the Bi-2212 peaks, it is noted that the peak located at 2y ˆ 36.76±36.81 is ascribable to both the Bi-2212 phase as well as the Sr0:15 Ca0:85 CuO2 phase for all of the XRD spectra, except those of Calcin. 6, 10, and 11, which were performed at a temperature of 820  C. The peak situated at 2y ˆ 36.76±36.81 tends to become less intense as the calcination temperature increases, indicating the disappearance of Sr0:15 Ca0:85 CuO2 phase. There is also a superposition of the Ca2 PbO4 peak at 2y ˆ 31.96 with the Sr2 CuO3 peak at 2y ˆ 31.95 in some of the XRD spectra. By considering the relative intensities of the Ca2 PbO4 peaks located at 2y ˆ 17.72 and 2y ˆ 31.96 , it is revealed that the relative proportions of these peaks are constant for Calcin. 7, 8, and 9. On the other hand, the intensity of the peak located at 2y ˆ 31.96 in the patterns for Calcin. 10 and 11, increases compared to the peak located at 2y ˆ 17.72 ; this increase is attributable to the presence of another phase, namely Sr2 CuO3 which has a peak at 2y ˆ 31.95 .

One can con®rm the presence of the Sr2 CuO3 phase in Calcin. 10 and 11 by the detection of another peak corresponding to this phase located at 2y ˆ 34.40 . The Sr2 CuO3 phase is detected after a long calcination performed at high temperature, 24 h at 820  C. The Bi-2212 phase is the most predominant phase in each calcination, except for that performed at 750  C (Calcin. 2). However, the Bi-2212 phase has a better crystallinity (sharper peak) when the calcination temperature is higher or when the calcination time is longer (  24 h). In reference to the post-calcination phase composition desired for the formation of Bi-2223 [18±21], it appears that the best calcined powder is obtained after 24 h at 820  C. The phase assemblage obtained for this thermal treatment is, from the major to minor: Bi-2212, Ca2 PbO4 , Ca2 CuO3 , CuO, Sr14 Cu24 O41 , Sr2 CuO3 , and Cu2 SrO2 . The grain size of this powder has been observed using SEM (Fig. 2). It appears that the average grain size is relatively large, around 10 mm, and that the grain morphology corresponds to a well-de®ned platelet shape.

4. Conclusion The polymer-matrix method is, therefore, an effective process to synthesize a precursor powder allowing, with adequate calcination conditions, the formation of the necessary secondary phases for the Bi-2223 phase formation. This powder preparation method allows an homogeneous and reactive powder precursor to be formed easily and with a good reproducibility. This work provides a better understanding of the evolution of the secondary phases in the Bi-2223 system and their dependence on the calcination conditions. Control of the 355

calcination parameters is important to obtain the proper precursor phases necessary for the formation of Bi-2223, namely Bi-2212, Ca2 PbO4 , Ca2 CuO3 , CuO. The phase assemblage is sensitive to the calcination conditions. A brief calcination ( 5 24 h) is inadequate to diminish the presence of Sr14 Cu24 O41 , while a calcination temperature that is too low (750  C) results in a poor Bi-2212 phase quantity. On the other hand, as the calcination temperature increases, the Sr0:15 Ca0:85 CuO2 phase decreases, and a high calcination temperature (820  C) may also induce the crystallization of unwanted Sr2 CuO3 . It appears that the best compromise between the calcination temperature and time is obtained after 24 h at 820  C; the phase assemblage thus obtained for this thermal treatment is, in order of decreasing concentration: Bi-2212, Ca2 PbO4 , Ca2 CuO3 , CuO, Sr14 Cu24 O41 , Sr2 CuO3 , and Cu2 SrO2

6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16.

Acknowledgment

17.

The authors would like to acknowledge J. Lecourt for his help with sample preparation.

18. 19.

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S. NHIEN

Received 31 August 2001 and accepted 13 February 2002