Growth of a high Tc phase in the Bi1.7Pb0.3Ca2Sr2Cu3Oy system

Muteri& Chem~t~

117

and Phykcs, 33 (1993) 117-123

Growth of a high Tc phase in the Bi~.~Pb,,Ca~Sr~Cu~~~ system M. Muralidhar, Dept

of Physics,

(Received

K. Nanda Kishore and V. Hari Babu

Osmania

University,

April 13, 1992; accepted

Hyderabad,

500007

(India)

June 26, 1992)

Abstract The effect of sintering time on the Bi,.,Pbo.3Ca2Sr,Cu,0 sample was investigated through XRD, DC resistance, AC magnetic susceptibility, SEM and critical current density (JJ. The XRD patterns revealed that the intensity of the low angle line at 28~4.7” which corresponds to the high Tc phase, increases gradually with sintering time and reaches a maximum for nearly 200-240 hours. It is also found that the percentage of the high T, phase is around 94% and that of the low T, is approximately 6%. DC resistance results also showed an increase in T, with sintering time and reached a maximum of 110 K. The real part of the AC susceptibility showed a step which corresponds to the transition from a normal to a superconducting state which gradually shifts to higher temperatures with an increase in sintering time. The magnitude of the step, which is a measure of the Meissner signal, also increased gradually with sintering time. The x” vs. T plots showed that with increase in sintering time, the width at half maxima decreased and the height of the peak increased. For still longer sintering times (200-240 h) the loss peak became sharper. The 1, value increased linearly with increasing sintering time. SEM studies revealed that the grain size increases and an increase in Jc may therefore be due to a reduction in the number of grain boundaries and other defects such as voids.

Introduction

In the high T, bismuth system, three types of superconducting phases [l-3] B&$r&uO, (20211, Bi2Sr2CalCu20y (2212) and Bi,Sr,Ca,Cu,O,, (2223), with superconducting transition temperatures (7’=‘s) around - 20, 80 and 110 K respectively are known. The structure of the 2201 phase is a 3 dimensional stacking of Bi,SrzCuOX slabs along the c-axis. Each slab has a centre CuO, layer surrounded on each side first by a SrO layer and then by a double BiO layer, with adjacent slabs sharing BiO double layers. The addition of one or two CuOZ units (n) between the CuOZ layers and one SrO layer produces the 2212 and 2223 phases respectively. The n = 1 and II =2 phases are relatively easy to synthesize in a single phase compound, for II = 3 it has been difficult to obtain a single phase 2223 compound. The first attempt by Sunshine et crt. [3] to partially substitute bismuth by lead has been found to favour the development of the 2223 phase. Although extensive efforts [4-71 have been made to synthesize BISCCO (2223) superconductors the role of lead and the mechanism of the formation of the 2223 phase at the expense of the 2212 phase is not clear. Hatuno et al. [8] prepared the participation of the partially melted liquid phase in the growth process of the

0254-0584/93/$6.00

2223 phase. On the other hand Shi et al., [9] have pointed out that the formation of the 2223 phase should result from the calcium and copper diffusion into regions of the 2212 phase, and that in order to enhance the diffusion process, an excess of Ca and Cu in the starting composition is necessary. According to Endo et cd. [lo] a long sintering period under lower pressure is necessary to single out the 2223 phase. Pierre et aE. fll] obtained a single phase compound without resorting to either the introduction of excess in the constituents or to a specific oxygen pressure. Wang et al. 1121 prepared nearly single phase Bi1.6Pb0.4Ca2Sr2Cu30y high I;: superconductor using a precursor matrix method. They found that the volume fraction of the impurity phase was about 3%. Our previous results 113-151 showed a systematic increase in the volume fraction of a high T, phase with an increase in the concentration of lead and the sintering time. Our results also showed that above a certain concentration of Pb, quenching rapidly and aging at room temperature decreases the volume fraction of the high T, phase (2223) with corresponding increase in the low I;: phase (2212). In this paper systematic investigations have been made as a function of sintering time on the Bil.7Pbo.3Ca2Sr2Cu30y sample using a variety of

0 1993 - Elsevier Sequoia.

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118 TABLE Annealing

1 conditions

Nominal initial composition Bi-Pb-Ca-St=Cu 1.710.31212/3 (A) (B) (C) (D) :z

of Bi,.rPb,&r,Ca,Cu,O, Step 1 Time

&)

(h)

superconductor Step 2 Time

T (“C)

(b)

Step 3 Time (h)

step 4 Time

Step 5 Time

(h)

(h)

&)

16 16 16 16

800 800 800 800

20 20 20 20

810 810 810 810

24 24 24 24

820 820 820 820

24 24 24 24

825 825 825 825

25 50 7s 100

850 850 850 850

16 l6

800 800 800

20 20 20

810 810 810

24 24 24

820 820 820

24 24 24

825 825 825

150 125 200 240

850 850 850

experimental techniques. The critical current density has also been measured and it is found that T,, J, and the Meissner signal increased with increasing time. The results are explained on the basis of microstructure and grain growth.

Results and discussion

Figure 1 shows X-ray d~action patterns of the samples annealed at 850 “C for 25 h (A), 50 h (B), 75 h (C), 100 h (D), 125 h (E), 150 h (F)

Experimental

Samples were prepared as reported in [16] starting from high purity PbO, B&OS, SrC&, CaO and CuO. Annealing conditions are given in Table 1. The phase purity of the samples were checked by Rigaku X-ra diffractometer (Japan) using CuKcr (A = 1.5418 K ) radiation. The critical current density J, at 77 K was measured by a standard four probe resistive method. DC, resistivity measurements were carried out by employing Vander Pauw’s technique [16]. Compositional and microstructural analysis were performed with a scanning electron microscope (SEM). The AC magnetic susceptibility measurements were carried out using a home built mutual inductance bridge. A sinusoidal AC current at a fixed frequency of 83 Hz was applied to the primary coil, and the resultant voltage in the pick-up coil was measured by a Stanford Research System SR-530 lock-in-amplifier. The primary AC field was 0.25 Oe. The resistance of the pickup coil (secondary) was compensated so that there was no voltage in the absence of the sample. This ensured that the induced voltage is directly proportional to the susceptibility. The samples with sintering time 25, 50, 75, 100, 125, 150, 200 and 240 hours were designated as A, B, C, D, E, F, G and H respectively.

20

30 2 0

40

__

50

(Degrees)

Fig. 1. X-ray diffraction perconductorswithA=25 h, F= 150 h, H= 240 h. correspond to the high respectively.

pattern for Bi,.,Pb,,~Sr2CazCZu30r suh, B=50 h, C=75 h, D- 100 h, E= 125 The peaks denoted by (H) and (L) T, (2223) and low T, (2212) phases

119

100 -

(22231

Slnterlng

Phase

time

(Hours)

Fig. 2. Values of the volume fraction of the high T, phase for different denotes the low T, (2212) phase.

and 240 h (H) in air. The intensity and peak positions of (002), (OOlO), (115), (109), (0012) (119), (200) and (0014) reflections are in good agreement with the values reported in the literature [17, 181 for the high T, 2223 phase. Figure l(a) shows the broad agreement both in d values and the intensity ratios of prominent peak viz. (002) (008) (113) and (115) was found with the values reported by Maeda et al. [19], Hazen et al. [20], Chavira et al. [21], Muralidhar et al. [16], Bansal et al. [22] and Pierre et al. [ll] who determined the lattice constants assuming the structure to be orthorhombic. Maeda et al. [19] obtained a = 5.399, b = 5.414 and c = 30.90 A. These parameters has been related to the simple cubic perovskite cell (a cubic with a =3.85 A) by the ratios a:5 and &&EL They identified this as the low T, (80 K) 2122 phase. Bansal et al. [22] obtained the prominent diffraction peaks of the low T, (2122) hase at 15.491 A, 3.855 A, 3.579 A and 3.246 R have also obtained values of 15.492 A, 3.85Iy 3.759 8, and 3.242 8, corresponding to the (002): (008), (113) and (115) reflections of the low T, (2122) phase. The intensity of reflections corresponding to high T, phases increased and those corresponding to low T, phases decreased with increasing sintering time as shown in Fig. 1. The ‘c’ value of the high T, phase is 37 8, and that of low T, phase is 31 A and peaks at 28=4.8” and 28=5.7” correspond to the (002) reflections of the high T, and low T, phases respectively. In the sample sintered for 25 h, distinct peaks at 28= 4.8” and 28= 5.7” appeared as shown in Fig. l(a) indicating that the sample consisted of both low T, and high T, phases. With increasing sintering

sintering

times: (0) denotes

the high T, (2223) phase; (0)

time one can notice a gradual increase in the intensity of the peak at 28=4.8” corresponding to the (002) reflection of the high T, phase with a decrease in the intensity of the peak at 28= 5.7”. For sample H sintered for 240 h, the low T, phase peak at 20=5.7” is almost absent and the peaks corresponding to the high T, phase are relatively sharper. These results show that the volume fraction of the high T, phase increases with increasing sintering time. A small peak at 28= 17.75” corresponding to the Ca,PbO, impurity phase can be seen only in a sample sintered for 25, 50 and 75 h. For longer sintering times the impurity phase disappeared. The diffraction peaks of the sample H coincided with those of the 110 K hase whose a and c lattice parameters are 5.41 x and 37.10 A respectively. The relative volume fraction of the two phases are calculated taking into account only the surface characteristics rather than bulk. The volume fraction can be calculated on the basis of the intensities of different characteristic reflections corresponding to the high T, and low T, phases, especially in the low 28 angle, with the (002) high T, reflection and the (002) low T, reflection which are clearly separated and appear at 28=4.8” and 5.7” respectively. Several authors [ll, 22, 231 have calculated the volume fraction of low T, and high T, phases on the basis of the (002) reflection. We have also calculated volume fraction of low T, 2122 and high T, 2223 phases as a function of sintering time using the formula 1,(002)/1,(002)

+1,(002)

IO

25

I

I

I

I

I

I

50

75

100

125

I50

175

Annealing

time

I”

I

200

Temperature

(Kf

vs. temperature of Bil.rPba,Ca$r~CuaOy superconductors with A=25 h, B=SO h, C=75 h, D=lOO h, E=125 h, F=150 h and II=240 h.

TABLE

2

Experimental Sample

data of Bir.7Pb&SrzCa&u10y

superconductor

From resistivity 7.c (0)

From AC susceptibility T, (On Set)

Q Peak position

J, at 77 K (A cm-2)

1OlK 104 104 105 106 106 108 110

lO9K 109 110 110 111 112 114 115

1OOK 103 104 104 105 107 108 109

20 23 26 29 32 35 45

:

0

hours

Fig. 4. Annealing time dependence of the current BiI,,Pba.3CazSr,Cu30, superconductor.

Fig. 3. RTiR,,

!

225

density J, of

where ~~(~2) and IL(O02) are the peak intensities of (002) reflections corresponding to the high T, and low T, phases. Bansal et al. [223 also used the same formula for the determination of the percentage of high T, (2223) and low T, (2122) phases. The volume fraction of the high T, phase (2223) increased with increasing sintering time (up to 240 h) while the low T, phase (2122) decreased and these results are shown in Fig. 2. Earlier [16] we found that the high T, phase started degrading after 250 h of sintering time and T,‘s also decreased. From Fig. 2 one can also notice that the percentage of the high T, phase is around 94% and that of the low T, is approximately 6%. Figure 3 shows the temperature dependence of of the Bi,.,Pb&a,Sr,Cu,O,, superthe R&,,, conductor sintered for different times. All the plots depict a positive slope indicating metallic behaviour. It is commonly observed [24] that for these ceramics, the higher the slope in the normal state of the sample, the better the superconducting behaviour. All samples A, B, C, D, E, F and H exhibit a single step superconducting transition with a onset around - 120 K and T,(O) varied from 10-110 K. Sr,(O)s are given in Table 2. Figure 4 shows the annealing time dependence of critical current density (JJ at 77 K under zero magnetic field. From the figure one can notice a clear increase in J, with sintering time. Scanning electron micrographs of A and H are shown in Fig. 5. From the photographs one can notice an increase in the size of the grain. From EDAX the block spots have been identified by Wang et al.

(4

60

70

80

90

IO0

iI0

Temperalure

Fig. 6. AC susceptibility C, D, E, F, G and H.

Fig. 5. SEM photographs of the Bi,.,Pb,.,Ca2Sr,Cu30,, ductors with A=25 h and I-I=240 h.

vs. temperature

PO

I

I

130

I40

/

150

[K‘

for the samples A, B,

supercon-

[25] to impurity phase. From Fig. 5 one can also notice the decrease in the impurity phase. The real (x’) and imaginary (x”) parts of the AC complex susceptibility have been measured for samples A to H and are shown in Fig. 6. For these studies the same quantity (100 mg) was used and care was taken to put it in the same position in the secondary coil. For all the samples one can notice only one step in the x’ vs. T plots. The step which corresponds to the transition from normal to superconducting state gradually shifts to a higher temperature with an increase in sintering time. The magnitude of the step (Meissner signal) also increased gradually with sintering time and is shown in Fig. 7. Since the Meissner signal is proportional to susceptibility one can notice a gradual increase in its magnitude. These results show that the fraction of the high T, phase increases with increasing sintering time. This can also be seen from the X-ray diffraction patterns which

Fig. 7. Diamagnetic signal of the samples A, B, C, D, E, F, G and H measured at 77 K.

show an increase in the high T, phase and a corresponding decrease in the low T, phase with increasing sintering time. x” vs. T plots are shown in the same figure for all the samples. One can observe two broad peaks with low intensity for sample A which has been sintered for 25 h. From the XRD results, we have seen that the sample contains both low T, and high T, phases. These

122 TABLE 3 Details of a.c. loss peaks of Bi,,,Pba,SrZCa,CuSO~ superconductor S. No

Sample

Peak height (cm)

Width of the peak at half maximum

Area under the peak (cm*)

Acknowledgements

10.00 10.60 8.95 7.60 5.80 4.55 4.25 4.10

The authors gratefully acknowledge the financial support of the Department of Science and Technology, Govt. of India, New Delhi. We also thank Dr R. Somasundaram and Mr G. Swaminathan, BHEL (R&D), Hyderabad for their help in extending their critical current density (Jc) measuring facilities. M. M. Dhar would like to thank CSIR, New Delhi for awarding a Senior Research Fellowship.

(K) 1. 2. 3. 4. 5. 6. 7. 8.

A B C D E F G H

1.6 3.2 3.4 4.3 4.8 5.5 5.7 6.0

62 23 20 14 12 7.5 6.0 5.8

also demonstrated that the elimination of a second phase will lead to an improvement of J,.

broad loss peaks may then be due to the low T, (2122) and high T,(2223) phases. With an increase in sintering time, the widths at half maxima decreased and the height of the peak increased. For still longer sintering times the loss peak becomes sharper. The peak height, the width of the peak at half the maximum and the area of the peaks are given in Table 3. The AC loss peaks are generally attributed to the defects present in the samples. Since our samples are polycrystalline in nature, one can expect voids, large angle grain boundaries, dislocations etc., to be present. The area under the loss peak is a direct measure of the defect nature of the sample. As can be seen from Table 3 the area and half width of the peaks have decreased with an increase in sintering time, suggesting that the number of defects were decreased. This can also be seen from the SEM photographs which show an increase in grain size with the sintering time. All these results suggest that sintering for a sufficiently long time ensures the formation of a single high T, phase and also grain growth. This probably leads to an increase in both T,(O) and J,. Matsuzaki et al. [26] also observed low J, values in bismuth cuprates and they suggested that this may be due to the presence of a second .phase and voids. Our results presented above suggest that both second phase (2212) and voids are reduced due to longer sintering times and this leads to both an increase in T, and in

Jc.

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