The comparison of dynamic processes of the Bi2Sr2Ca2Cu3Ox phase formation

December

1997

Materials Letters 33 (1997) 185-189

ELSEVIER

The comparison of dynamic processes of the Bi,Sr,Ca,Cu,O, phase formation Xiaoming Tang a, Minquan Wang b, Guohong Xiong b3*, Zhanglian Hong b, Xianping Fan b a Center of Analysis and Measurement, Zhejiang University, Hangzhou 310027, People’s Republic of China b Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, People’s Republic of China Received 25 November

1996; revised 27 March 1997; accepted 27 March 1997

Abstract The differences of the dynamic processes of the 2223 phase formation by multi-step method (MSM) and direct synthesizing method (DSM) are discussed in the paper. It reveals that the formation of the 2223 phase crystal nucleus is very easy by the multi-step method, which differs greatly from the direct synthesizing method. The formation of the 2223 phase is a diffusion-controlled process in a multi-step method. By the multi-step method together with adding a nucleator, regrinding and resintering, the :formation of the 2223 phase is obviously accelerated. The acceleration mechanism is also discussed. 0 1997 Elsevier Science B.V. Keywords: High T, superconductor;

Dynamic

process; Synthesis

1. Introduction Bi-based superconductors have three superconducting phases described by a general formula Bi,Sr,Ca,_,Cu,O, (n = 1, 2, 3). The desirable phase is the n = 3 phase, Bi,Sr,Ca,Cu,O, (22231, because of its higher T,. However, by the usual direct synthesizing method (DSM - mixing all components in a single step and then sintering), it is still difficult to synthesize its pure phase although the heat treatment period lasts several hundreds of hours. The reason is that the formation of the 2223 phase is a very slow process taking place within a very limited temperature range [ 1,2]. In air, this range is about 5°C. The addition of Pb stabilizes the 2223 phase, widens this temperature range and pro* Corresponding

author.

motes its formation [3,4]. The oxygen partial pressure (PO,) of the atmosphere plays an important role in the formation and the stability of the 2223 phase [5]. Very low or high PO, values are inappropriate for the formation and stability of the 2223 phase. The optimum heating temperature is close to the solidus temperature. A small amount of the liquid phase assists the 2223 phase formation, but too much leads to the emergence of 2201 and other phases. Intermediate grinding is also good for the 2223 phase formation. As discussed above, there are many factors affecting the formation of the 2223 phase. To explain these phenomena and to get the single 2223 phase conveniently, it is necessary to establish a dynamic model for the formation of the 2223 phase. The authors have proposed a nucleation and growth model for the formation of the 2223 phase and also have

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studied in detail the reaction process of the Bi-SrCa-Cu-0 system [6,7]. The results indicate that the Bi,Sr,Ca,Cu,O, (2212) phase is formed by the reaction of an intermediate phase BiSr,Ca, pxO2,5 (x = 0.00-0.75) together with CuO, SrO and CaO, while the 2223 phase is formed through the 2212 phase reacting with CuO, SrO and CaO. Based on this, a multi-step method (MSM) is presented to prepare the 2212 and 2223 phases with the presynthesized BiSr,Ca, _xO2,5 and 2212 phases, respectively. BiSr,Ca, pxO2,5 is the solid solution of Sr and Ca (x = 0.00-0.75) [8]. By MSM, it is easy to adjust the Sr solid solubility of the 22 12 and 2223 phases which affects their electromagnetic properties. In this paper the authors use MSM to study the formation of the 2223 phase. It is found that this method can greatly accelerate the formation of the 2223 phase. It only needs as long as 50 h to get the single 2223 phase. The dynamic process of the 2223 phase formation of this method differs very much from the one of the direct synthesizing method (DSM). The differences of the dynamic processes of these two methods are discussed in the paper.

2. Experimental Analytically pure Bi,O,, SrCO,, CaCO,, CuO and PbO were used as raw materials. The mixture in molar ratio Bi:Sr:Ca = 1:0.5:0.5 was ground and mixed in agate ball mill for 3 h. After taking shape at the pressure of 300 MPa, it was sintered at 790°C for 1 h and 830°C for 15 h, and the single BiSr,,,Ca,.,O,,, phase was obtained (XRD showed). Both the 2212 and 2223 phases were prepared by MSM. The BiSr,,,Ca,,,O,,, phase was used as starting material to prepare the Bi,Sr,Ca,Cu,O, phase (2212) by adding &CO, and CuO, and sintering at 790°C for 1 h and 830°C for 10 h. Then this 2212 phase was used as starting material to prepare the 2223 phase (nominal composition is Bi,.,,Pb,,3,Sr,,,,Ca,Cu,O,) by adding PbO, CaCO,, SrCO, and CuO, and sintering at 800°C for 5 h and 855°C for different periods. The pure 2223 phase as nucleator was also used to accelerate the 2223 phase formation. All sintering temperatures were selected according to the DTA curves of the mixture. Calcining at lower temperatures for short periods was used

Letters 33 (1997) 185-189

0

-

2223 phase

------

2212 phase

-h

50 Sintering

100 Period

150

200

(h)

Fig. 1. The curves of XRD intensity of the 2223 and 2212 (002) peak of samples sintered at 843°C vs. sintering periods. 0 calcined at 790°C for 15 h; 0 - 0 sintered for 100 h and then reground and resintered; T - calcined at 800°C for 15 h; 0 - v 5 wt% 2223 phase added as nucleator; A - calcined at 810°C for 1.5 h.

(described as above) to avoid the formation of the liquid phase through the lower melting point materials reacting with other components at lower temperatures. Phase analyses were carried out with a D/max-y A X-ray diffractometer using CuKa radiation. The conversion fraction of the 2223 phase was indicated by the ratio of XRD integrated intensity of the 2223 phase (002) peak to the sum of the one of the 2212 and 2223 phase (002) peaks. The mixture was subjected to differential thermal analyses (DTA) with a heating rate of lO”C/min. AC magnetic susceptibility was measured.

3. Results and discussion The authors have studied the dynamic process of the 2223 phase formation by direct synthesizing method (DSM) in Ref. 161, and Fig. 1 is cited from it, which shows the dependence of the amount of the 2223 phase on the sintering periods. The formation curves of the 2223 phase (except the case of adding nucleator) show an S-shape. Thus, the formation of the 2223 phase follows a nucleation and growth process. The inducing period of the formation of the 2223 phase crystal nucleus is very long (50-100 h), while the growth process goes considerably fast. That is to say, the formation of the 2223 crystal nucleus by DSM is very difficult, which controls the

X. Tang et al./Materials 100 8

;! 0 k g 2 b 2 8

80

60

i

40

I

20

/

0

.

-7-./

0

/

.

.

.!O

40

Wintering

60 Time

80

100

(h)

Fig. 2. The fraction of the 2223 phase vs. sintering periods at 855°C by MSM. 0 - not adding a nucleator; n - adding a nucleator; - - - regrinding and resintering.

whole process of the 2223 phase formation. Therefore, the key of accelerating the formation of the 2223 phase is to shorten the inducing period of the formation of the 21223 crystal nucleus. It can be reached by multi-step method (MSM). As shown in Fig. 2, the formation curve of the 2223 phase made by MSM (not adding nucleator) is parabolic. When sintering periods are within 50 h, the formation speed is considerably fast. The fraction of the 2223 phase reaches 33% when the sample is sintered for 10 h, and reaches 90% when sintered for 50 h. After 50 h, the formation of the phase tends to saturation. Thus, the formation of the 2223 phase by MSM is a diffusion-controlled process, which greatly differs from the one by DSM. The formation of the 2223 phase crystal nucleus by MSM is very easy. There is almost no inducing period of the nucleus formation (Fig. 2). Why is the forma.tion of the 2223 crystal nucleus in MSM much easier than in DSM? Firstly, this is because the structure of the 2212 phase made by MSM differs from the one formed by DSM. As discussed in Ref. [a], the BiSr,Ca,_,O,., phase is the solid solution of Sr and Ca. It can be considered to be formed by substituting Ca with Sr in the BiCaO,,, phase. Therefore, the distribution of Sr and Ca in BiSr,Ca, _xO2.5 is disordered, which makes the distribution of Sr and Ca of the 2212 phase prepared with BiSr,.,Ca,,,O,,, differ from the one prepared by DSM. The authors [9] have studied the distribution of Sr and Ca of the 2212 phase with XPS and found out that there is a higher average

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atomic number of (Sr + Ca) in the Ca site between two CuO layers of the 2212 phase made by MSM compared with the one made by DSM. Because of this, there is a stress between the two-CuO layers, which causes this area a little distorted, and the energy of this area a little higher. The crystal nucleus of the 2223 phase is formed by inserting porovskitelike Ca-Cu-0 cuboids into this area of the 2212 matrix [6]. This process can release the excess energy of the area between two-CuO layers of the 2212 matrix. Therefore, the crystal nucleus formation is easier by MSM than by direct synthesizing method. Secondly, the sintering temperature of MSM is 855”C, 12°C higher than the one (843°C) of DSM. These two temperatures are optimum for the formation of the 2223 phase by the two methods (discussed below). If higher, more liquid phase appears and the 2201 phase is formed. The higher sintering temperature is good for the inserting process and the diffusion of ions. So the crystal nucleus formation of the 2223 phase is easy and the formation speed is fast by MSM. Fig. 3 shows the DTA curves of the mixed powders with nominal composition Bi 1,80Pb,,,, Sr, ,90-

750

800

850

Temperatwe(

900 C )

Fig. 3. The DTA curves of the mixed powders with nominal composition Bi,.,, Pb,,J,Sr,,,,CazCu,O,. A: the mixture of all components mixed in a step and calcined at 790°C for 15 h; B: the mixture of the presynthesized 2212 phase mixed by MSM with CaO, SrO and PbO and calcined at 800°C for 5 h.

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Ca,Cu,O, (A: the mixture of all components mixed in a step and calcined at 790°C for 15 h; B: the mixture of the presynthesized 2212 phase mixed by MSM with CaO, SrO and PbO and calcined at 800°C for 5 h). Below 8OO”C, there is no peak in the two curves. This means that some reactions, including the decomposition of CaCO,, SrCO,, the formation of the 2212 and other phases, are completed at the calcining step. The peak S represents the formation of the 2223 phase and the peak T represents the decomposition of the 2212 phase into the 2201 and liquid phases [ 10,111. The two curves are similar, but curve B shifts to a higher temperature by 10°C. So, the appearing temperature of the liquid phase rises by MSM. Because the formation rate of the 2223 phase is very slow, the sintering temperatures are always selected as high as possible to approach the liquid appearing temperatures. The authors selected several temperatures (15’C or so below peak T’s temperatures) to sinter samples by the two methods, and found out that 843 and 855°C are the two optimum sintering temperatures for the two methods, respectively. Sintering at a little higher temperature results in the appearance of a large amount of the liquid phase. The liquid phase is important for the formation of the 2223 phase. But it always leads to the appearance of the 2201 phase by DSM. The melting point (832°C) of Bi,O, is the lowest in the (Bi, Pb)-SrCa-Cu-0 system. The liquid phase always contains a large amount of the Bi3+ ion. And its amount is difficult to control in this ternary system. So the 2223 phase always coexists with the 2201 phase by DSM. However, with the MSM, in the first step of the formation of BiSr,Ca,_,O,,,, the Bi,O, is almost all used up to establish the crystal lattice of the BiSr,Ca, _XO2.5. In the second and third step, no Bi,O, is added. Therefore, in the third step of the formation of the 2223 phase, the liquid phase does not contain many Bi3+. Its amount is determined by the one of PbO added in the third step and is easy to control. So, by MSM, it is easy to avoid the coexistence of the 2223 and 2201 phases. In our experiments, the 2201 phase isnot found in the samples. In the third step of the formation of the 2223 phase, except the 2212 and 2223 phases, the third phase is Ca,CuO,, and it decreases gradually with the progressing reaction and vanishes at last.

Letters 33 (19971 185-189 3K

3

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20 @eg) Fig. 4. The XRD pattern of the sample, added with a nucleator, sintered for 30 h and then reground and resintered for 20 h (by MSM).

Fig. 2 also shows that the formation process of the 2223 phase by MSM in the case of adding the pure 2223 phase as nucleator is similar to that of not adding a nucleator. But the formation rate of adding a nucleator is much faster than that of not adding a nucleator, especially in the initial sintering period. The fraction of the 2223 phase reaches 68% in the sample sintered for 10 h, and reaches 93% in that sintered for 30 h. Next, the formation tends to saturation. Prolonging the sintering time is not effective in increasing the amount of the 2223 phase. This is because with the reaction going, the layer of object (the 2223 phase) becomes thicker and thicker, and the further reaction needs the long-distance diffusion of ions through the thick layer of object. By regrinding and resintering, the residue 2212 phase can be exposed and directly take part in the reaction, which can accelerate the formation rate of the 2223 phase and produces the pure 2223 phase more easily [6]. The sample, added with a nucleator, sintered for 30 h and then reground and resintered for 20 h (by MSM), is nearly the pure 2223 phase, and the 2212 phase is exhausted entirely (as shown in Fig. 4). Fig. 5 shows its T, is about 105 K. The MSM accelerates the formation of the 2223 phase, but does not degrade its superconductivity. Now we lay emphasis on the differences of the 2223 phase formation rate between DSM and MSM in the case of adding a nucleator. From Figs. 1 and

X. Tang et al./Materials

TO

Temperature

(K)

Fig. 5. The susceptibility of the sample, added with a nucleator, sintered for 30 h and then reground and resintered for 20 h (by MSM).

2, it is easy to see that though the inducing period of nucleus formation disappears by adding a nucleator in DSM, the formation rate of the 2223 phase is much smaller than that in MSM. For instance, the fraction of the 2223 phase is only 75% in the sample sintered for 50 h by DSM, while the fraction reaches 85% in the sample sintered for 20h by MSM. The reasons for this are the structure distortion of the 2212 phase made by MSM and the higher sintering temperature. Both are advantageous to the diffusion of ions. From the above, the multi-step method (MSM) together with adding the pure 2223 phase as nucleator and regrinding and resintering is a very effective way to synthesize the single 2223 phase.

4. Conclusion The formation of the 2223 phase by the multi-step method (MSM) is a process controlled by diffusion.

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By this method, the 2223 phase crystal nucleus is easy to form, which differs very much from the one by the direct synthesizing method (DSM). The structure distortion of the 2212 phase made by MSM and the higher sintering temperature accelerate the ions diffusion which speeds the formation of the 2223 phase. By this method, all Bi,O, (the raw material with the lowest melting point) is incorporated into the matrix (the BiSr&aa.sO,, phase) in the first step. So, the amount of liquid phase is easy to control in the third step, which is very favorable for the formation of the 2223 phase. Therefore, MSM together with adding the pure 2223 phase as nucleator, as well as regrinding and resintering, is a very effective way to synthesize the single 2223 phase. References HI K. Song, H. Lin, S. Don, C. Sorrell, J. Am. Ceram. Sot. 73 (1990) 1771.

[21 B. Seebacher,

B. Jobst, G. Zorn, in: G. de With, R.A. Terpstra, R. Metselaar (Eds.), Proc. 1st Europ. Ceramic Sot. Conf., Maastricht, Euro-Ceramics, vol. 2, 1989, p. 2456. 131 U. Endo, S. Koyama, T. Kawai, Jpn. J. Appl. Phys. 27 (1988) L1476. 141 Sh. Narumi, H. Ohtsu, I. Iguchi, R. Yoshizaki, Jpn. J. Appl. Phys. 28 (1989) L27. [51 W. Zhu, P.S. Nicholson, J. Mater. Res. 7 (1992) 38. t61 M. Wang, G. Xiong, X. Tang, Z. Hong, Physica C 210 (1993) 413. [71 G. Xiong, M. Wang, X. Fan, X. Tang, Appl. Phys. A 56 (1993) 99. [a1 G. Xiong, M. Wang, X. Fan, G. Lu, J. Mater. Sci. 30 (1995) 167. [91 M. Wang, Z. Hong, G. Xiong, The study of distribution of Sr and Ca in lattice of Bi-based superconductor by XPS, Physica C, to be submitted. [lOI H. Yao-Tung, W. Wang-Nan, W. Sheng-Feng, S. Cheng-Yei, H. Weir-Mirn, L. Wun-Hsin, W. Pieng-Tien, J. Am. Ceram. sot. 73 (11) (1990) 3507-3510. [Ill Wai Lo, B.A. Glowacki, Physica C (1992) 253-263.