Supercond. Sci. Technol. 13 (2000) 602–611. Printed in the UK

PII: S0953-2048(00)10206-4

Optimization of calcination conditions on the Bi-2223 kinetic formation and grain size V Garnier†, I Monot and G Desgardin Laboratoire CRISMAT, 6, Bd. Mar´echal Juin, 14050 Caen cedex, France Received 8 December 1999 Abstract. The effects of the calcination conditions (time, temperature, intermediate milling)

on the kinetics of formation of Bi-2223 and on the grain size were studied using a powder precursor synthesized by the polymer matrix method. The samples were characterized by XRD and SEM analysis. The grain size and the kinetics of formation of Bi-2223 strongly depend on the calcination conditions and thus on the phase assemblages formed at the end of the calcination that determines the reactivity during sintering. The higher the calcination temperature is the larger the grain size. We have observed that 24 h of calcination at 820 ◦ C without intermediate milling allows one to obtain 79% of Bi-2223 phase in 60 h of sintering at 835 ◦ C.

1. Introduction

Since the work of Michel et al [1] and Maeda et al [2] on superconductivity in the Bi–Sr–Ca–Cu–O (BSCCO) system, a considerable amount of research concerning the development of synthetic processes in order to obtain the purest Bi-2223 phase, which possesses the best possible values of the critical temperature (Tc ) and critical current density (Jc ), has been undertaken. Many applications were thus opened up with Bi-2223-like tapes, wires, fault current limiters and transformers, due to the relatively high Jc of this compound at liquid nitrogen temperature. However, the Jc of Bi-2223 strongly depends on parameters such as temperature, thermal processing time, milling, texturing, synthesis atmosphere and powder precursor. Although it seems that most of these parameters have been optimized, better control of the powder precursor is still needed in order to improve reproducibility and transport properties. The relations between the different phases of the system Bi–Pb–Sr–Ca–Cu–O are very complex, due to the presence of a large number of elements, and also variables according to the selected powder precursor. The formation of Bi-2223 which takes place during sintering is thus very sensitive to the assemblage of the initial phases and to the prereactive conditions. To contribute to the comprehension and the optimization of the synthesis of Bi-2223, many methods of development of the powder precursor have been implemented: solid-state method [3–5], sol–gel method [6, 7], aerosol pyrolysis technique [8], coprecipitation of oxalate [9, 10], spray-dried nitrate precursors [11], freezedried precursors [12], and the polymer matrix method [13]. † Author to whom correspondence should be addressed.

0953-2048/00/050602+10$30.00 © 2000 IOP Publishing Ltd

The study of all these methods of preparation of the precursor powder has led to improvement in the kinetics of formation and in the purity of the Bi-2223 phase, as well as to an understanding in the mechanisms of its formation. However, according to the literature, the mechanisms of formation of the Bi-2223 phase are not yet clear and several of them have been proposed, such as: • a disproportionation reaction of the Bi(Pb)-2212 phase to form the Bi-2223 and Bi-2201 phases [14, 15]; • an intercalation process in which the pre-existing Bi(Pb)-2212 crystals transform directly into Bi(Pb)2223 platelets via the insertion of the CuO2 /Ca bi-layers into the CuO2 /Ca/CuO2 blocks of 2212 [16, 17]; • a Bi-2223 precipitation from a partially molten phase [18]; • a formation of Bi/Pb-rich mobile liquid droplets which migrate over growing platelets [19]; • a dissolution–precipitation process where the Bi-2223 phase is formed from both the 2212 phase, CaO and a liquid, the last two components arising from the decomposition of Ca2 PbO4 [20]; • a two-dimensional growth with decreasing nucleation rate. A liquid phase generated in the powder reacts with the 2212 matrix via the diffusion of Ca, Cu and Pb ions, resulting in the nucleation of the Bi-2223, which diffuses into the 2212 matrix, maintaining the outline of the original crystal shape [21–25]. We chose the polymer matrix method, which had not been largely studied up to now compared to the other methods, for the synthesis of the powder precursor. This method appears promising due to its high kinetics of formation of the phase Bi-2223 and, also, due to the large grain size, which are routinely obtained using this route

Calcination optimization for Bi-2223 formation

[26, 27]. A larger grain size allows, in principal, a better grain orientation during the texturation by rolling or sinter forging. Moreover, a large grain size has an influence on the superconductive properties of bulk bismuth ceramics. It has been shown that the critical current density increases when grain size increases [28]. Feasibility studies [29] have shown that one can control and obtain small grain size particulates by adding particle seeds of Bi-2223 in the powder precursor. In this direction mechanical crushing has also been shown to reduce the size of the grains. On the other hand, obtaining large grain size is not yet well controlled, even if a low-level mobile liquid droplet mechanism [19] has been proposed to allow the development of large Bi-2223 plates. There is also the possibility to perform very long sintering times in order to obtain maximum growth of the grains, but the kinetics of formation of Bi-2223 is slowed down more and more because of the problems of diffusion, due, in particular, to generated porosity (retrograde densification [30]) and also to the losses of elements such as lead, which are responsible for stabilizing the Bi-2223 phase. By this study we seek to optimize and understand the influence of the three parameters of calcination, which are the temperature, time and intermediate milling. 2. Experimental method

The powder precursor was prepared by the polymer matrix method by using a nominal composition Bi1.85 Pb0.35 Sr 2 Ca2 Cu3.1 O10+δ suggested by Maeda et al [31] and adopted by many other groups [32–34]. Starting materials Bi(CH3 COO)3 (Aldrich, 99.99+%), Pb(CH3 COO)2 · 3H2 O (Aldrich, 99+%), Sr(CH3 COO)2 · 1 H O (Aldrich, 99.9%), Ca(CH3 COO)2 · H2 O (Aldrich, 2 2 99+%) and Cu(CH3 COO)2 .H2 O (Aldrich, 98+%) were dissolved in a mixture of acetic acid (SDS, 99%) and distilled water to obtain a pale blue solution. To this solution, another solution of polyethyleneimine (PEI) (Aldrich, 50 wt% H2 O) in distilled water was added and the resulting solution immediately turned royal blue. This dark blue solution was then introduced into a rotary evaporator to reduce the volume to approximately 10% of the initial volume. The concentrated solution was then introduced into a crucible and placed on a hot plate, slowly increasing the temperature until total evaporation of the solvent occurs. The thermoplastic paste that forms was subsequently fired on a hot plate at approximately 400 ◦ C. Due to the exothermic reaction which takes place, the temperature of the crucible measured with an IR pyrometer (Williamson 9220PS-S-C), is around 750 ◦ C. The resulting powder is then crushed by hand in an agate mortar and is calcined under air at different temperatures in the range of 750–820 ◦ C during 12–48 h. After cooling, the powder is milled, pelletized (200 MPa, 16 mm diameter, 3 g), and sintered under air at a temperature of 835 ◦ C (a temperature which will be justified in the beginning of the section results) measured with a thermocouple Pt–PtRh placed close to the sample. The thermal events around the temperature of sintering of a calcined starting powder were examined by using differential thermal analysis (DTA) (Setaram LabSys DTA/DSC). The phases present in the calcined powder and in

the sintered pellets were analysed by x-ray diffraction (Philips PW3710, λCu Kα1 ) and the volume fraction of the Bi-2223 phase was calculated by using the following equation [35]: Volume fraction of the phase Bi-2223 (%) µ IH (0010) IH (002) + = IH (002) + IL(002) IH (0010) + IL(008) ¶ IH (115) 100 + IH (115) + IL(115) 3 in which IH (002) , IH (0010) , IH (115) are, respectively, the surfaces of the peaks (002), (0010) and (115) of the Bi-2223 phase; and IL(002) , IL(008) , IL(115) are, respectively, the surfaces of the peaks (002), (008) and (115) of the Bi-2212 phase. The microstructure of the calcined powders and of the sintered pellets was observed using a scanning electron microscopy (SEM) (Philips XL30); for the sintered pellets the observation was performed on the fractured part of the sample. 3. Results

It is significant to study the calcination by coupling it with a standard sintering, in order to be able to compare the influence of the calcination parameters. We have thus to define an unique sintering schedule for all our tests of calcination. According to previous studies [13, 26], dealing with precursors synthesized by the polymer matrix method, the sintering temperature is 10–15 ◦ C lower than those commonly used for the other methods. We thus tested after calcination, Calc1, (800 ◦ C for 12 h), three temperatures of sintering: 830, 835 and 840 ◦ C (figure 1 curves A, B and C, respectively). From the curves it is clear that sintering at 835 ◦ C leads to better kinetics of formation of the Bi-2223 phase, and we chose this temperature of sintering to follow the evolution of the rate of the Bi-2223 phase at the end of all the various types of calcination tested. Moreover, DTA measurement (figure 2) performed on Calc1 shows that 835 ◦ C corresponds to the middle of the peak of decomposition of Ca2 PbO4 , which begins around 822 ◦ C, and to the beginning of the incongruent fusion peak of the Bi-2212 phase, which mainly composed Calc1 (figure 3). The corresponding liquid is supposed to facilitate the diffusion process and thus to improve the kinetics of formation of the Bi-2223 phase. 3.1. Characterization of calcined powders Eleven types of powder were prepared according to the different calcination conditions given in table 1. We chose to not exceed 820 ◦ C, in order to avoid the decomposition of Ca2 PbO4 (Tf = 822 ◦ C) [36] which forms CaO and a liquid phase rich in Pb, because CaO reacts with CuO to form Ca2 CuO3 [37], which in turn accelerates the formation of the Bi-2223 phase. Indeed, the initial formation of the Bi-2223 phase occurs around 820 ◦ C [38] and a powder calcined above this temperature generates a great number of Bi-2223 nuclei, thus opposing the growth of the Bi-2223 grains during sintering [39, 40] and resulting in the formation of small Bi-2223 grains and, consequently, a weaker Jc 603

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Figure 1. Volume fraction of the Bi-2223 phase (%) as a function of sintering time after Calc 1 for three sintering temperatures: A, 830 ◦ C; B, 835 ◦ C and C, 840 ◦ C and after D, Calc 2 at 835 ◦ C.

Figure 2. DTA curve of the Calc 1 powder calcined for 12 h at 800 ◦ C.

of the ceramic. The XRD patterns of the 11 calcined powders are shown in figure 3. The main crystallized phases present in these calcined powders are Bi-2212, Ca2 PbO4 , CuO, Sr 14 Cu24 O41 , Ca2 CuO3 and Cu2 SrO2 . Because of the significant overlapping of the peaks among the various crystallized phases, the characteristic peaks of each phase were identified and marked on the different patterns for the purpose of clarity. The detected phases based on XRD analysis for each calcined powder are summarized in table 2. Characteristic morphologies of the calcined powders are shown in figure 4, and show the large differences in size and morphology according to the selected calcination conditions. 3.2. Characterization of sintered powders The 11 above calcined powders were used as precursors to synthesize after grinding and compaction the Bi-2223 604

phase during sintering. Confirmation of the formation of this phase during sintering was carried out by XRD diffraction. Figure 5 shows the XRD spectra of the pellets sintered for 60 h at 835 ◦ C, as previously justified. This pellets only differ by the type of calcination which they underwent beforehand, i.e. Sint 1 follows Calc 1, Sint 2 follows Calc 2 etc. Just as for the XRD spectra after calcination, the characteristic peaks of each phase are identified on the different patterns, and the composition of the phases present in these sintered powders, based on XRD analysis, is summarized in table 2. Characteristic morphologies of these sintered powders, presented in figure 6, show that the different calcination cycles leads to different grain size. A follow-up of the rate of formation of the Bi-2223 phase against time during sintering is shown in figure 7, and indicates the large influence of calcination parameters on the kinetics of formation of the Bi-2223 phase.

Calcination optimization for Bi-2223 formation

4. Discussions

The influence of calcination parameters such as temperature, time and intermediate milling was studied by comparison of the different calcination processes, thus leading to a better understanding of the Bi-2223 formation and to an optimization of these parameters in order to obtain better kinetic of Bi-2223 formation during sintering and to obtain the largest grain size. 4.1. Comparison of Calc 1 (800 ◦ C for 12 h) and Calc 2 (750 ◦ C for 12 h): influence of dwell temperature In the calcinations Calc 1 and Calc 2, the dwell time (12 h) was kept the same, but a difference of 50 ◦ C on the dwell temperatures was employed in order to observe the effect of the calcination temperature on the precursor reactivity. The XRD spectra of Calc 1 and Calc 2 in figure 3 reveal the presence of Ca2 PbO4 , CuO, Ca2 CuO3 , 2201 and 2212 recognized as intermediate phases for the formation of Bi-2223. However, for Calc 1 the 2212 content is much larger than that of 2201, whereas for Calc 2, a 50 ◦ C lower calcination temperature results in an inversion of the respective amounts of these two phases, due to the lower kinetics of formation of the 2212 phase at 750 ◦ C. This difference in kinetics is confirmed by the formation of Bi-2223 during sintering (cf figure 1 by comparing B and D corresponding respectively, to Sint 1 and Sint 2). At the beginning of sintering, one notes a similar time of incubation for these two samples, from 15 to 20 h. Given the SEM observations, indicating a slightly larger grain size for Calc 1 compared to Calc 2, one can conclude that up to 30 h reactional advance of Calc 1 is compensated by a better reactivity of Calc 2, centred around 5 µm. Then, during sintering, due to the intermediate phases of Calc 1 which were already largely formed, the kinetics of formation of Bi-2223 is accentuated compared to Calc 2. Indeed, one obtains after 200 h of sintering at 835 ◦ C, 73% of Bi-2223 for Sint 1 and only 40% for Sint 2. In the same way, after 60 h of sintering, the granulometry observed with the SEM is still slightly larger for Sint 1 compared to Sint 2, with 7 µm and 5 µm on average, respectively. It is thus significant to use during the calcination, a temperature reaching at least 800 ◦ C for better formation of the intermediate phases and a larger grain size. 4.2. Comparison of Calc 3 (750 ◦ C for 10 h + 800 ◦ C for 10 h) and Calc 4 (550 ◦ C for 10 h + 750 ◦ C for 10 h + 800 ◦ C for 10 h): influence of a first dwell at 550 ◦ C

Figure 3. XRD patterns of powders calcined using calcination

conditions described table 1, and marked as follows: 1, 2223; 2, 2212; 3, 2201; 4, CuO; 5, Ca2 PbO4 ; 6, Ca2 CuO3 ; 7, Sr 0.15 Ca0.85 CuO2 ; 8, Bi1.6 Pb2.4 Sr 2.8 Ca2.1 CuOx ; 9, Bi10 Ca7 O22 ; 10, SrCO3 ; 11, Sr 14 Cu24 O41 ; 12, Sr 2 CuO3 ; and 13, Cu2 SrO2 .

By comparison between Calc 3 and Calc 4, we point out the effect of an additional dwell at 550 ◦ C during 10 h in Calc 4, a temperature corresponding to the beginning of formation of the 2201. In figure 3, XRD spectra at the end of these calcinations do not indicate any noticeable difference in the nature and quantity of the phases present, except that Calc 3 reveals a significant quantity of Sr 14 Cu24 O41 which is absent in Calc 4. In comparing the corresponding SEM photographs (figures 4(A) and 4(B)) of these calcined powders, one can observe agglomerated grains of small size of about 2–3 µm 605

V Garnier et al Table 1. Description of calcination conditions and phase composition after the calcinations. Note that condition Calc 11 had an

intermediate milling step. Type of calcination Calc 1 Calc 2 Calc 3 Calc 4 Calc 5 Calc 6 Calc 7 Calc 8 Calc 9 Calc 10 Calc 11∗

Calcination conditions

Phases composition, from major to minor

800 ◦ C for 12 h 750 ◦ C for 12 h 750 ◦ C for 10 h + 800 ◦ C for 10 h 550 ◦ C for 10 h + 750 ◦ C for 10 h +800 ◦ C for 10 h 750 ◦ C for 12 h + 800 ◦ C for 12 h 750 ◦ C for 12 h + 820 ◦ C for 12 h 800 ◦ C for 24 h 800 ◦ C for 48 h 810 ◦ C for 24 h 820 ◦ C for 24 h 820 ◦ C for 12 h + 820 ◦ C for 12 h

2212, Ca2 PbO4 , CuO, Bi10 Ca7 O22 , 2201, Ca2 CuO3 , SrCO3 , Cu2 SrO2 Bi10 Ca7 O22 , CuO, Ca2 PbO4 , 2201, Bi1.6 Pb2.4 Sr 2.8 Ca2.1 O5 , SrCO3 , Ca2 CuO3 , 2212 2212, Ca2 PbO4 , CuO, Sr 14 Cu24 O41 , Sr 0.15 Ca0.85 CuO2 , Cu2 SrO2 2212, Ca2 PbO4 , CuO, Sr 0.15 Ca0.85 CuO2 , Cu2 SrO2 2212, Ca2 PbO4 , CuO, Sr 0.15 Ca0.85 CuO2 , Cu2 SrO2 , Sr 14 Cu24 O41 2212, Ca2 PbO4 , CuO, Cu2 SrO2 , Ca2 CuO3 2212, Ca2 PbO4 , CuO, Sr 14 Cu24 O41 , Cu2 SrO2 , Sr 0.15 Ca0.85 CuO2 , Ca2 CuO3 2212, Ca2 PbO4 , CuO, Cu2 SrO2 , Sr 0.15 Ca0.85 CuO2 , Ca2 CuO3 2212, Ca2 PbO4 , CuO, Ca2 CuO3 , Cu2 SrO2 , Sr 0.15 Ca0.85 CuO2 , Sr 14 Cu24 O41 2212, Ca2 PbO4 , Ca2 CuO3 , CuO, Sr 14 Cu24 O41 , Sr 2 CuO3 , Cu2 SrO2 2212, Ca2 PbO4 , Ca2 CuO3 , CuO, Sr 2 CuO3 , Cu2 SrO2

∗ With an intermediate milling. Table 2. Phases composition after sintering, depending on the former calcination conditions.

Type of sintering

Phases composition from major to minor

Sint 1 Sint 2 Sint 3 Sint 4 Sint 5 Sint 6 Sint 7 Sint 8 Sint 9 Sint 10 Sint 11

2212, 2223, Ca2 CuO3 , Ca2 PbO4 and traces of CuO, 2201, Cu2 SrO2 2212, 2223, Ca2 CuO3 , Ca2 PbO4 and traces of Cu2 SrO2 , 2201, CuO, Sr 2 CuO3 2212, 2223, Ca2 PbO4 , Ca2 CuO3 and trace of Cu2 SrO2 2212, 2223, Ca2 PbO4 , Ca2 CuO3 and traces of Cu2 SrO2 , CuO 2223, 2212, Ca2 PbO4 , Ca2 CuO3 and traces of CuO, Cu2 SrO2 , Sr 2 CuO3 2223, 2212, Ca2 PbO4 , Ca2 CuO3 and trace of Cu2 SrO2 2212, 2223, Ca2 PbO4 , Ca2 CuO3 and traces of Cu2 SrO2 , Sr 14 Cu24 O41 2212, 2223, Ca2 PbO4 , Ca2 CuO3 and traces of Cu2 SrO2 , Sr 14 Cu24 O41 , Sr 2 CuO3 2212, 2223, Ca2 PbO4 , Ca2 CuO3 and traces of Cu2 SrO2 , Sr 2 CuO3 , Sr 14 Cu24 O41 2223, 2212 , Ca2 CuO3 and traces of Sr 14 Cu24 O41 , Sr 2 CuO3 , 2201, Ca2 PbO4 2212, 2223, Ca2 CuO3 and traces of Cu2 SrO2 , Ca2 PbO4 , Sr 2 CuO3 , 2201

for Calc 4 and grains approximately twice as large (5–6 µm) for Calc 3. This difference can of course be explained by the 10 h of additional dwell at 550 ◦ C in Calc 4, leading to the formation of numerous nucleation sites of the 2201 phase. One thus forms a multitude of small grains of 2201, which thereafter during the calcination will be used as a basis for the formation of the 2212, which consequently will consist of small grains. During sintering, one observes (figure 7) better kinetics of the beginning of formation of Bi-2223 for the sample resulting from Calc 4, compared to that resulting from Calc 3. The phases present before sintering for these two types of calcination being in similar proportions, the kinetic difference is ascribable to the grain size, which, being smaller for Calc 4, improve, their reactivity with their weak surface energy. Then, during sintering, one reaches the same limits of diffusion of matter and reactivity for these two types of samples: the rate of Bi-2223 reaches 47% and 49% for Calc 4 and Calc 3, respectively, after 60 h of sintering. XRD spectra (figure 5) of Sint 3 and Sint 4 carried out under the same measurements conditions indicate a more significant quantity of Ca2 PbO4 for Sint 4 than for Sint 3. This difference is more clear than at the end of the calcination, and indicates that a sufficiently long calcination time (over 20 h) is necessary for the good formation of this essential phase. Moreover, the longer duration of Calc 4 leads to a better crystallinity of the formed phases, as indicated by the intensity and by the full-width half-maximum of the peaks. The size of the grains obtained after 60 h of sintering (figure 6(A) and 6(B)) indicates that the larger grain size of Calc 3 compared to 606

Calc 4 before sintering is recovered after sintering. It is, thus, the first dwell calcination temperature at 550 ◦ C which determined the grain size. Thus, to obtain larger grains, it will be necessary, during the calcination, to use a temperature of at least 750 ◦ C without being detrimental to the rate of formation of Bi-2223. 4.3. Comparison of Calc 3 (750 ◦ C for 10 h + 800 ◦ C for 10 h) and Calc 5 (750 ◦ C for 12 h + 800 ◦ C for 12 h): influence of dwell time With Calc 5 we increased the dwell times 20% compared to Calc 3, in order to observe the relative tendency for such a purpose. SEM photographs of figures 4(A) and 4(C) reveal a slightly larger grain size for Calc 5 compared to Calc 3, due to the longer time of dwell, which allows the grains in Bi-2212 to grow and, consequently, reach a more significant size. The nature and the quantity of the phases obtained (figure 3) after these two calcinations are, on the whole, equivalent. However, the amount of Sr 14 Cu24 O41 is much more significant in Calc 3 than in Calc 5, and more CuO is present in Calc 5 than in Calc 3. These secondary phases are thus sensitive to the duration of the calcination for these tested temperatures, and this will influence the kinetics of formation of Bi-2223. Indeed (figure 7) Sint 5 has a double Bi-2223 rate compare to Sint 3 after 24 h sintering. Thus the Sr 14 Cu24 O41 phase appears detrimental for the formation of Bi-2223. After 60 h of sintering, XRD spectra (figure 5) of Sint 3 and Sint 5 differ only by the traces of secondary

Calcination optimization for Bi-2223 formation

Figure 4. SEM micrographs of calcined powders: (A), Calc 3; (B), Calc 4; (C), Calc 5; (D), Calc 6; (E), Calc 7; (F), Calc 8; (G), Calc 10

and (H), Calc 11.

phases. It is difficult to deduce anything from these negligible differences. However, one notices a more significant quantity of Bi-2223 for Sint 5, about 56% (figure 7), than for Sint 3,

about 49%. SEM observations indeed show that the grain size obtained for Sint 5 is larger than that obtained for Sint 3, and explain the slow down of the kinetics of formation of 607

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Bi-2223 observed between 24 and 60 h sintering. Thus, the result of having lengthened the dwells of the calcination is that it gives more advance and more reactive secondary phases for the formation of Bi-2223, due to a longer period for the grains to grow. 4.4. Comparison of Calc 5 (750 ◦ C for 12 h + 800 ◦ C for 12 h) and Calc 6 (750 ◦ C for 12 h + 820 ◦ C for 12 h): influence of second dwell temperature By comparing Calc 5 and Calc 6, we seek the influence which has the temperature of second dwell increased to 820 ◦ C. The XRD spectra (figure 3) of Calc 5 and Calc 6, show in both cases, a Bi-2212 phase predominantly present and well crystallized. On the other hand, the intermediate phases such as Ca2 PbO4 and CuO are more predominant in Calc 5 than in Calc 6, and the secondary phases Sr 14 Cu24 O41 and Sr 0.15 Ca0.85 CuO2 are detected only in Calc 5. It is only by the 20 ◦ C difference of between 800 and 820 ◦ C on the temperature of the second dwell, that we reduce the quantity of the secondary phases. In the SEM photographs in figures 4(C) and 4(D), one observes a small grain size for the both calcined powders (Calc 5 and Calc 6). These observations show that the temperature of the second dwell does not affect the grain size. It is thus the first dwell temperature which will determine the grain size, because at 750 ◦ C, we are located in a zone where the nucleation of Bi-2212 on Bi-2201 is dominating, thus a long period of time at this temperature prevents further growth of large grains even with a second step at high temperature. One can conclude that a second dwell at 820 ◦ C is beneficial to reduce secondary phase formation and an initial calcination temperature higher than 750 ◦ C should thus allow less nucleations and a better growth of the grains. 4.5. Comparison of Calc 1 (800 ◦ C for 12 h), Calc 7 (800 ◦ C for 24 h) and Calc 8 (800 ◦ C for 48 h): optimization of calcination time

Figure 5. XRD patterns of pellets sintered for 60 h at 835 ◦ C depending on the former calcination conditions, and marked as figure 3.

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Comparisons of Calc 1, Calc 7 and Calc 8 enable one to follow the influence of the calcination time during one-step calcination at 800 ◦ C. The XRD spectra (figure 3) after these calcinations show Bi-2212 as the major phase. However, in Calc 1 secondary phases such as 2201, Bi10 Ca7 O22 and SrCO3 are still present. In Calc 7 and Calc 8, the phases recognized as intermediates for the formation of Bi-2223 are present in large quantity with traces of Cu2 SrO2 . The XRD spectra of Calc 7 and Calc 8 are almost superimposable, the nature and the quantity of the phases present for these two calcinations are similar, except the presence of traces of Sr 14 Cu24 O41 phase in Calc 7 which vanish with a longer calcination time. However, the SEM observations (figures 4(E) and 4(F)) show that due to the longer calcination time the grain size obtained for Calc 8 is larger than that obtained for Calc 7, leading to a lower surface energy for Calc 8 and, consequently, to a lower reactivity than Calc 7 during sintering. In figure 7, one observes that the volume fraction of the Bi-2223 phase for Sint 7 is double than that for Sint 8 after 24 h; however, after a longer sintering time (60 h) one obtains, for these two samples, 43% and 41%, respectively, of Bi-2223 for Sint 7 and Sint 8. For Sint 1, the

Calcination optimization for Bi-2223 formation

Figure 6. SEM micrographs of the fracture part of pellets sintered for 60 h at 835 ◦ C: (A), Sint 3; (B), Sint 4; (C), Sint 6; (D), Sint 7;

(E), Sint 8; (F), Sint 9; (G), Sint 10; (H), Sint 11.

delay in the formation of the intermediate phases at the end of the calcination is not made up for during sintering, and only 26% of Bi-2223 was formed in 60 h of sintering. In Sint 1,

only a very small amount of Ca2 PbO4 is present, besides some CuO and Bi-2201 phases (figure 5). The Ca2 PbO4 phase is not totally formed after 12 h of calcination, so 609

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Figure 7. Volume fraction of the Bi-2223 phase (%) as a function

of sintering time depending on the former calcination conditions.

during sintering, time must be spent for its formation and further conversion into the Bi-2223 phase. The XRD spectra (figure 5) indicate any noticeable difference between Sint 7 and Sint 8 concerning the nature of the present phases, except Ca2 PbO4 which is in greater quantity in Sint 7 than in Sint 8, indicating that Sint 7 can, potentially, form more Bi-2223 and more quickly than Sint 8. Thus, the longer calcination time of Calc 8 is rather detrimental to rapid Bi-2223 formation. However, SEM photographs (figures 6(D) and 6(E)) show that the largest grain size is still obtained for Sint 8 compared to Sint 7, as it was at the end of the calcinations. Thus, when a calcined powder shows larger grains compared to another powder calcined with the same intermediate phases, it is observed once more that this difference of grain size is maintained after sintering. Finally, a too long calcination time is detrimental to rapid Bi-2223 formation and the grain size difference between Calc 7 and Calc 8 is not so important, so instead of increasing the time, we chose to increase the calcination temperature, with 24 h dwell time. 4.6. Comparison of Calc 7 (800 ◦ C for 24 h), Calc 9 (810 ◦ C for 24 h) and Calc 10 (820 ◦ C for 24 h): optimization of dwell temperature Calc 7, Calc 9 and Calc 10 allow one to observe the influence of calcination temperature on the grain size and the formation of Bi-2223 during sintering. The observation of the corresponding XRD spectra (figure 3) after calcination, does not reveal any large differences in the nature and the quantity of the phases present: Bi-2212 is always the major phase, accompanied by intermediate phases such as Ca2 PbO4 and CuO. One can notice, however, that the quantities of the Sr 0.15 Ca0.85 CuO2 and Ca2 PbO4 phases tend to decrease when the calcination temperature increases from 800 to 820 ◦ C. Also, Sr 0.15 Ca0.85 CuO2 is no longer detected at 820 ◦ C. The melting point of Ca2 PbO4 was estimated to be 822 ◦ C, therefore, one must enter a zone of partial melting of this phase. In contrast, the Ca2 CuO3 phase increases with the calcination temperature and Sr 2 CuO3 begins to be detected at 820 ◦ C. The SEM photographs in figures 4(E) and 4(G) confirm that the grain size clearly increases when the calcination temperature increases. The grain size of Calc 9 is intermediate with those of these two later calcinations. During sintering, (figure 7) similar evolutions for Sint 7 and 610

Sint 9 are observed with about 23% of Bi-2223 in 24 h and about 45% in 60 h. On the other hand, Sint 10 shows much better reactivity with 50% and 79% Bi-2223 formed in 24 and 60 h, respectively. SEM photographs (figures 6(D), 6(F) and 6(G) corresponding, respectively, to Sint 7, Sint 9 and Sint 10) show that the grain size has increased as the calcination temperature is raised, leading to a larger grain size for Sint 10. XRD spectra (figure 5) of Sint 7, Sint 9 and Sint 10 indicate a larger quantity of Ca2 PbO4 for Sint 7 and Sint 9 than for Sint 10. This observation is coherent with the large quantity of Bi-2223 formed for Sint 10 which consumes Ca2 PbO4 for its formation. Other phases such as Ca2 CuO3 , Sr 14 Cu24 O41 and Sr 2 CuO3 which were already present in Calc 10, and in greater quantity than in Calc 7 and Calc 9, are still present in Sint 10 and in an always prevalent rate compared to Sint 7 and Sint 9. Ca2 CuO3 known to be a direct precursor for the formation of Bi-2223, in contrast to Sr 14 Cu24 O41 , tends to show that we can raise the Bi-2223 phase content. These results are all the more interesting because the small quantity of Bi-2201 detected at the end of this sintering, and formed in balance with Bi-2223, can be retransformed in Bi-2223 by suitable thermal processing [41]. The results obtained by Sint 10 are what we had anticipated, i.e. a significant grain size (greater than 10 µm) and a large quantity of Bi-2223 (79%) synthesized in 60 h. 4.7. Comparison of Calc 10 (820 ◦ C for 24 h) and Calc 11 (820 ◦ C for 12 h + 820 ◦ C for 12 h with an intermediate milling): influence of an intermediate milling Intermediate milling is generally used to rehomogenized the powder and to improve its reactivity for the next stage of thermal processing. In order to optimize Calc 10 as much as possible, we tested Calc 11 which differs only by one intermediate milling. After Calc 10 and Calc 11, the powders, which were then milled and pressed into pellets for sintering, presented a clear difference in texture. Indeed, whereas Calc 10 showed a compact and agglomerated powder, it was noted that Calc 11 was, in contrast, made up of a fine powder without agglomerate, as if it had not reacted after the intermediate milling. However, in figure 3, the XRD spectra after Calc 10 and Calc 11 do not present a noticeable difference on the nature and quantity of the phases present, except Sr 14 Cu24 O41 which is present in Calc 10, but is no longer detected in Calc 11; this is due to the intermediate milling, which improves the phase reactivity and thus Sr 14 Cu24 O41 decomposition is facilitated. The grain size observed by SEM (figures 4(G) and 4(H)), clearly shows that intermediate milling had a harmful effect on obtaining large grains. The grain size of Calc 11 is small and remained the same as at the end of intermediate milling. Thus there was no crystal growth during the second stage of this calcination, because the calcination temperature of the powder did not increase. In figure 7, one observes an evolution radically different from the volume fraction of the Bi-2223 phase for Sint 10 and Sint 11, with 79% and 7%, respectively. XRD spectra (figure 5) do not show any appreciable difference. The Bi-2212 phase of Sint 11 almost seems to not have reacted to give Bi-2223. However, it seems that its reactivity was inhibited due to the second stage of calcination

Calcination optimization for Bi-2223 formation

or intermediate milling. These conclusions have been confirmed by testing a calcination at 820 ◦ C for 12 h, which gave an intermediate evolution of the volume fraction of the phase Bi-2223 between Sint 10 and Sint 11, with 26% in 60 h of sintering at 835 ◦ C. In figure 6(G), the SEM observation shows a grain size of 10–15 µm, with well formed platelets for Sint 10. On the other hand for Sint 11 (figure 6(H)), one observes a closed porosity of the Bi-2212 grain platelets in formation, from the small calcined Bi-2212 grains. 5. Conclusions

The effects of calcination conditions (time, temperature and intermediate milling), of the powder precursor synthesized by the polymer matrix method, on the kinetic formation of Bi-2223 and on its grain size have been studied. Various calcination conditions, which gave rise to various phase assemblages and to various grain sizes, determine the rate of formation of Bi-2223 and its grain size. In order to accelerate the Bi-2223 formation during sintering, it appears significant that the powder precursor calcined just below the formation temperature of Bi-2223, without exceeding its initial temperature formation, is the most favourable. A calcined powder showing larger grain size compared to another calcined powder, with the same phase assemblage, will maintain this larger grain size after sintering. To obtain the largest grain size at the end of the calcination (and thus at the end of sintering), it is necessary to use the highest possible temperature close to 820 ◦ C. Depending of the calcination conditions, it is observed that the faster the kinetics of formation of Bi-2223 are during sintering, the larger the grain size becomes. The optimum time of calcination correspond to 24 h without intermediate milling. A temperature of 820 ◦ C for 24 h gave the best results with fast kinetics of formation of Bi-2223 during sintering (79% in 60 h at 835 ◦ C) and a large grain size (10–15 µm). The optimization of the sintering conditions (time, temperature, intermediate milling and atmosphere) are currently under study. Acknowledgments

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