J O U R N A L OF M A T E R I A L S SCIENCE LETTERS 8 (1989) 383-386

Bi-Ca-Sr-Cu-O superconductors from nitrate precursors

RAM SRINIVASAN*, MARIBETH SAUM*, ROBERT J. De A N G E L I S * , J. O. DEASY:~, J. W. BRILL:~, CHARLES E. HAMRIN Jr § *Department of Materials Science and Engineering, ~ Department of Physics and Astronomy and § Department of Chemical Engineering, University of Kentucky, Lexington, Kentucky 40506, USA

Recently the Bi -Ca-Sr -Cu-O system has attracted much attention and extended the range of high-To superconductors to non-lanthanide cuprates [1-9]. Onset T c values have varied from 8 to 110 K. In order to make the compounds, Bi203, SrCo3, CaCO 3 and CuO have been reported to be the starting materials. The structure has been described as orthorhombic structure of alternating perovskite and Bi202 layers by Hazen et al. [1], while a pseudo-tetragonal unit cell was reported by Tarascon et al. [2]. Also an ortho- rhombic structure with space group Amaa was given by Subramanian et al. [6]. There are a number of disadvantages of using oxides and carbonates as pre- cursors for producing the bismuth-based supercon- ductors. The final material appears to be very sensitive to the solid-state reaction temperature and the ratio of the starting materials [8]. Problems such as (i) inad- equate mixing of carbonates and oxides and (ii) the formation of hard, large agglomerate thwart densifi- cation of the particles [8]. This might lead to inhomo- geneities in composition, incorrect stoichiometry etc., thus affecting the final product. These problems can be avoided by using the nitrate solution technique because nitrates yield a uniform homogeneous starting material prior to sintering of powders at high temperatures, and recently the advantages of preparing La2 xSGCuO4 and Ba2YCu307 from the nitrate precursors were described [10]. Our goal was to prepare the Bi -Ca-Sr- Cu superconductors from their nitrate precursors in order to obtain bismuth-based superconductors of fine-grained structure. Our initial results are presented in this letter.

The starting materials were bismuth nitrate, copper nitrate (Fisher Scientific Co.), calcium nitrate (J. T. Baker Chem. Co.) and strontium nitrate (Aldrich Chem. Co.). Appropriate amounts of these nitrates to make 4334 (Sample A), 4336 (Sample B) and 4156 (Sample C), in the cation ratio of Bi-Ca-Sr-Cu, were placed in a beaker of deionized water. This solution was mixed thoroughly with a magnetic stirrer at 80 ° C for half an hour. Bismuth nitrate did not dissolve completely but was slurried with the other dissolved nitrates, which produced a blue solution. It was found that bismuth nitrate is soluble in concentrated HNO3 if dissolved in a 1 : 5 ratio, but this procedure was not used. The contents after drying at 120°C overnight was ground, calcined at 850 to 860°C for 10h, reground, pelletized and retired at 860 ± 15°C over- night. The second firing for Sample B was done in an oxygen atmosphere and for Samples A and C in air. All the samples were cooled very slowly at I°C min ' to 400°C when the furnace was turned off. Although

0261-8028/89 $03.00 + .12 © 1989 Chapman and Hall Ltd.

the incompatible solubilities of these nitrate salts were well known to us, it did not stand as a deterrent factor for our goal was to obtain a final product of homo- geneous, relatively uniform fine-grained structure. Recently Kayser et al. [11] acknowledged that for the preparation of YBa2Cu307 x superconductors by the metal nitrate method, atomic-scale mixing could not be fully achieved due to different solubilities of the nitrates.

X-ray diffraction analysis was carried out using a Rigaku X-ray diffractometer fitted with a graphite- diffracted beam monochromator. The radiation used was CuKc~ with 2 = 0.154 18 nm. The scan was run on the samples with a step scan of 0.02 ° (20) and with a counting time of 10 sec per step. The data were collected using a gas proportional detector and stored in a PDP 11/23 computer.

The X-ray diffraction patterns collected from Samples A and B are presented in Fig. 1, and the calculated and observed d spacings with the observed relative intensity ratios are given in Table I. Sample A was prepared according to the formula 4334, while Sample B was prepared with excess CuO (4336)

T A B L E I Calculated and observed d spacings and intensity ratio of diffraction lines for 4334 compound sintered at 860 ° C for 16h (a = 0.542 _+ 0.03nm and c = 3.071 ± 0.015nm)

h k l dob~ (nm) dc.lc (nm) (I/lo)ob ~

0 1 1 0.5328 0.5338 16 0 0 8 0.3837 0.3839 39 1 0 6 0.3736 0.3721 17 1 1 3 0.3585 0.3589 57 0 1 7 0.3447 0.3410 22 1 I 5 0.3249 0.3251 100 1 0 8 0.3139 0.3133 20 0 0 10 0.3067 0.3071 36 - - - 0.3013 - 30 1 1 7 0.2882 0.2886 75 2 0 0 0.2703 0.2710 91 2 0 2 0.2660 0.2669 22 0 0 12 0.2549 0.2559 35 0 2 6 0.2436 0.2395 21 2 0 8 0.2209 0.2214 17 2 0 10 0.2027 0.2032 35 1 1 13 0.2007 0.2010 20 2 2 0 0.1911 0.1916 44 2 2 4 0.1854 0.1859 18 1 1 15 0.1803 0.1806 23 3 1 3 0.1686 0.1690 15 3 l 5 0.1646 0.1651 25 3 0 8 0.1630 0.1635 18 3 1 7 0.1592 0.1596 24 3 0 10 0.1574 0.1557 17 3 1 9 0.1529 0.1532 23 4 0 0 0.1358 0.1355 17

383

.,n l =

'°° t 24.0 4"

160

120

60

it}

"i

__J

o o

o 0

/ °

.o .oil -/; ;II~, o l--llNi °l~l ~

o o

0 o~ ,,.I

(b)

0 I I I I I I Z0 30 40 50 60 70

2e (deg)

Figure 1 X-ray diffraction patterns from annealed samples of (a) 4334 and (b) 4336. The patterns show a tetragonal structure with lattice pa ramete r sofa = 0.542 4- 0 .003nm and e = 3.071 _ 0.015nm.

following a suggestion of Tarascon et al. [2]. Due to the presence of excess CuO in Sample B (4336), CuO peaks were observed in the X-ray diffraction pattern of this sample (see Fig. lb). Except for the additional CuO lines for Sample B, the X-ray diffraction patterns for Samples A and B are very similar. The major component present in these samples is the 4334 super- conducting phase. The diffraction patterns were indexed on the basis of a tetragonal unit cell with a = 0.542 4- 0.003nm and c = 3.071 4- 0.015nm. Assuming a chemical formula Bi2Cal.sSq.sCu208 and four formula units per unit cell, one obtains a crystal- lographic cell density of 6.37 g cm -3 for Sample A and

6.95 g cm 3 for Sample B, assuming a chemical formula Bi2Cal.sSrl.sCu309 for Sample B (4336). The lattice parameters, determined using a least-squares fit [12], closely correspond to the values given by Hazen et al.

[1] (a = 0.541 nm, b = 0.544nm and c = 3.078nm), but differ from those of Tarascon et aI. [2] (a = 0.382 nm and c = 3.06 nm). Our results also compare with those of Kajitani et al. [13], whose neutron and X-ray diffraction analysis yielded structural par- ameters of the sub-cell for Bil.sCal.2Sq.sCuz2Os.22 of a = b = 0.539nm and c = 3.0725nm. Recently Kijima et al. [14] indexed the X-ray diffraction pattern for the 4338 compound, on the basis of a pseudo-

180.

T

I=

144

108

72

36

I I 20 30

I I I I 40 50 60 70

Z8 (deg)

Figure 2 X-ray diffraction pattern from Sample C, having 4156 com- position.

384

1.2

0.9

~ ' 0.6

0.3

0 --

I I I I I ! I I

C

~°5 I 0.3 . . . . . . . .

70 II0 150 T (K) A

[ I I I i I I I I 30 60 90 120

T(K) 150

Figure 3 Curves showing the relationship between R/Ro and temperature for typical samples (R 0 = room-temperature resis- tivity): (A) 4334, (B) 4336, (C) 4156.

tetragonal cell with lattice parameters of a = 0.54 nm and c = 3.07 nm. Recently Takayama-Muromachi et al. [t5] reported lattice parameters of a = 0.54nm and c = 3.065 nm, with a calculated cell density of 6.55 gcm -3 for a 4334 compound, prepared from ox- ides and carbonates, while in our studies nitrate salts were employed to make the compounds.

Sample C showed a more complex diffraction pattern (Fig. 2) than Samples A and B; however, there are eight peaks present in this sample that correspond within a d-spacing error limit of + 0.0015 nm to peaks in the X-ray patterns of Samples A and B. The presence of the 0.3009rim line from the pattern of Sample C in the patterns of Samples A and B indicates that a small amount of C phase is present in Samples A and B. Attempts to solve the C pattern were not successful. Since such a complex X-ray diffraction pattern has not been reported in the literature, we have presented this pattern in Fig. 2 and the d spacings for major peaks with the observed relative intensity ratios in Table II.

Pellets were cut with a diamond saw into (~ 1 m m x 1 mm x 1 cm) rods for four-probe d.c. resistance measurements. Contact, made with conducting silver

T A B L E II The d spacings of major X-ray diffraction profiles for Sample C with their relative intensity ratios

d (nm) (I/Io)obs

0.5269 25 0.4109 43 0.4069 38 0.3452* 85 0.3331 40 0.3083* 62 0.3035 62 0.3009* 100 0.2950 62 0.2854 73 0.2706* 73 0.2686 89 0.2646* 36 0.2446* 33 0.2024* 43 0.1899" 41 0.1606 36

*Peaks present in 4334 compound (Table I).

paint, had temperature-dependent contact resistances of ~ 10~. The typical room-temperature resistivity (R0) was 0.01 f~cm.

Several samples of each pellet were cooled in helium exchange gas. The samples were cooled to at least 10 K below reaching zero resistance and their critical cur- rents checked; critical currents were typically 10 mA. The temperature dependence of the resistivities was then checked, using currents less than 25% of I~ for a given sample. Each sample of a given preparation exhibited very similar resistance-temperature depen- dences, indicating that the pellets were homogeneous on a gross (e.g. 1 mm) length scale. However, each composition had different properties, as shown in Fig. 3.

Samples from 4336 had low superconducting onsets of ~ 80 K and very broad transitions, reaching zero resistance between 50 and 60 K. A small dip is also clearly seen at 108 K, indicating a higher-temperature superconducting minority phase, as reported by others [1, 16]. This may be due to excess CuO layers as reported by Torardi et al. [16] for the T1-Bi-Ca-Cu-O system. Above 80 K the resistivity is metallic, varying linearly to its room-temperature value. Metallic con- ductivity was also exhibited by samples from the 4334 composition, with a superconducting onset of 85K and reaching zero resistance at 66K; no higher- temperature dip was observed for these samples (AR/ R < 1%). For these two samples the 20 K transition breadth suggests (fine scale) heterogeneity; the tran- sition temperatures are considerably lower than the 85 K reported by others. Samples from the 4156 com- position, on the other hand, had a broad resistance maximum at ~ 150 K, no high-temperature dip and a steep superconducting onset above 90K. However, the resistance stops falling steeply near 80 K, and then proceeds linearly to zero at 30 K. The initial drop with a midpoint near 85 K is similar to the transitions reported by others [9], but apparently a large fraction of the grains have much lower transition temperatures.

Samples of the 4334 powders were examined using a scanning electron microscope. The powder is very fine, with an average grain size of 12/tm (Fig. 4), which is in good agreement with the value of 10#m

385

Figure 4 Scanning electron micrograph of 4334 powder.

reported for YBa2Cu307_ x superconductors prepared from metal nitrates [11]. There was some agglomeration of the powder, but this is expected due to the presence of small particulates. However, the powder is largely homogeneous in size. Barboux et al. [17] recently reported that a controlled precipitation process for the preparation of YBazCu307_x superconductors yielded an average grain size of 3 #m, and a sol-gel process produced grains of about 1 #m, when sintered at 500 ° C. These grains become larger upon sintering at higher temperatures.

These initial results indicate that nitrate precursors, despite their incompatible solubilities, can produce relatively uniform fine-grained Bi-Ca-Sr-Cu-O high- temperature superconductors with an average grain size of ~ 12 #m for compositions 4334, 4336 and 4156. The transition temperature onsets and widths deter- mined by resistance methods at this time do not repre- sent a significant improvement over solid-state methods.

Acknowledgement A portion of this investigation was supported by the National Science Foundation on Grants Nos MSM- 8718385 and DMR-8615463 (Solid State Physics).

References 1. R. M. HAZEN, C. T. PREWITT, R. J. ANGEL, N. h.

ROSS, L. W. FINGER, C. G. HADID1ACOS, D. Ro

VEBLEN, P. J. HEANEY, P. H. HOR, R. L. MENG,

Y. Y. SUN, Y. Q. WANG, Y. Y. XUE, Z. J. HUANG,

L. GAO, J. BECHTOLD and C. W. CHU, Phys. Rev. Lett. 60 (1988) 1174.

2. J. M. TARASCON, Y. LePAGE, P. BARBOUX, B. G. BAGLEY, L. H. GREENE, W. R. McKINNON, G. W.

HULL, M. GIROUD and D. M. HWANG, submitted to Phys. Rev. (1988).

3. H. MAEDA, Y. TANAKA, M. F U K U T O M I a n d T. ASANO, Jpn. J. AppL Phys. 27(2) (1988) L209.

4. M. K. WU, J. R. ASHBURN, C . A . H|GG1NS, C. FELLOWS, B. H. LOO, D. H. BURNS, A. IBRAHIM

and T. ROLIN, submitted to Appl. Phys. Lett. (1988).

5. C. POLITIS, Appl. Phys. A45 (1988) 261. 6. M. A. SUBRAMANIAN, C. C. TORARDI , J. C. CAL-

ABRESE, J. GOPALAKR1SHNAN, K. J. MORRISSEY, T. R. ASKEW, R. B. FLIPPEN, U. C H O W D H R Y and A. W. SLEIGHT, Science 239 (1988) 1015.

7. A. M. C H I M P I N D A L E , S. J. HIBBLE, J. A. HRIL- JAC, L. COWEY, D. M. S. BAGGULEY, P. DAY and A. K. CHEETHAN, Physica C152 (1988) 154.

8. J. M. TARASCON et al., in Proceedings of the Inter- national Conference on High-Temperature Superconductivity, Materials and Mechanisms of Superconductivity, February/ March, Interlaken, in press.

9. Y. SYONO, K. HIRAGA, N. KOBAYASHI, M. KIKU- CHI, K. KUSABA, T. KAJITANI , D. SHINDO, S. HOSOYA, A. TOKIWA, S. TERADA and Y. MUTO, Jpn. J. Appl. Phys. 27(4) (1988) 87.

10. M. L. KAPLAN and J. J. HAUSER, Mater. Res. Bull. 23 (1988) 287.

11. M. H. KAYSER, B. BORGLUM, G. ANTONY, S. G. SHYU and R. C. BUCHANAN, preprint.

12. M. H. MUELLER, L. HEATON and K. T. MILLER, Acta Crystallogr. 13 (1960) 828.

13. T. KAJITANI , K. KUSABA, M. KIKUCH] , N. KOBA- YASHI, Y. SYONO, T. B. WILLIAMS and M. HIRA-

BAYASHI, Jpn. J. Appl. Phys. 27(4) (1988) 67.

14. Y. KIJIMA, J. TANAKA, Y. BANDO, M. ONODA and F. IZUMI, ibid. 27(3) (1988) L369.

15. E. T A K A Y A M A - M U R O M A C H I , Y. UCHIDA, A. ONO, F. IZUMI, M. ONODA, Y. MATSUI, K. KOSUDA, S. TAKEKAWA and K. KATO, ibid. 27(3)

(1988) L365. 16. C. C. TORARD] , M . A . SUBRAMANIAN, J . C .

CALABRESE, J. G O P A L A K R I S H N A N , T. R. ASKEW,

R. B. FLIPPEN, U. CHOWDHRY and A . W .

SLEIGHT, Science 240 (1988) 631.

17. P. BARBOUX, J. M. TARASCON, L . H . GREENE, G. W. HULL and B. G. BAGLEY, submitted to J. Appl. Phys.

Received 27 September and accepted 11 October 1988

386