Effects of carbon impurities in high Tc superconducting tapes using Auger electron spectroscopy

196

Thin Solid Films, 228 (1993) 196-200

Effects of carbon impurities in high Tc superconducting tapes using Auger electron spectroscopy E. W. Seibt Kernforschungszentrum Karlsruhe, lnstitut .l~r Technische Physik, Postfach 3640, IV-7500 Karlsruhe 1 (Germany)

A. Jeremie and R. Fliikiger Universitd de GdnOve, Ddpartement de Physique de la Matibre Condens~e, CH-1211 GenOve 4 (Switzerland)

Abstract High critical current densities are necessary for major bulk applications of ceramic oxide superconductors. In this paper it is shown by high resolution Auger electron spectroscopy (AES) investigations that carbon impurities (originating from the carbonates) trapped in the microstructure of dense polyerystalline silver-sheathed B i - P b - S r C a - C u - O (BPSCCO) tape materials are present during sintering processes. The studies include measurements of the critical current density Jc as a function of retained carbon concentration detected by AES techniques. In Ag/BPSCCO tapes with jc values varying from 1500 to 33 000 A crn -2 at 77 K and zero field the retained carbon content indicates a drastic change from about 11 at.% to overall concentrations at the grain boundaries smaller than 0.8 at.% for maximum j¢ values. These results prove that a carbon content as low as possible is an essential requirement for obtaining ceramic oxide superconductors with relatively high critical current-carrying capacities.

1. Introduction In the last few years, the critical current densities o f Ag-sheathed B i - P b - S r - C a - C u - O (BPSCCO) tapes have been considerably enhanced [1, 2]. However, the highest reported value in zero field at 77 K, Jc = 5.3 x 104 A cm -2 [1], is still below the critical current densities required for industrial applications. It is now accepted that a high degree of texturing, a minimum content o f finely distributed foreign phases, a high density o f the superconducting layer and a good homogeneity of the latter over long lengths are necessary requirements for obtaining high j¢ values in BPSCCO tapes. In a recent high resolution Auger electron spectroscopy (AES) investigation [3] we have found that the critical current density of BPSCCO tapes is seriously affected by carbon impurities originating from the carbonates in the precursors. It is the aim of the present paper to give more details about the distribution of the carbon impurities in the tapes as determined by AES. In addition, the correlation found in ref. 3 between a minimized C content and higher critical current densities is confirmed and extended to BPSCCO tapes with jc (77 K, 0 T) up to 3.3 x 104A cm -2.

2. Sample preparation The tapes were prepared by the usual 'powder-intube' processing as in ref. 3. The powders were all

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prepared using coprecipitated precursors with the nominal compositions given in Table 1. These powders were calcined up to three times at 800 and 820 °C with intermediate grinding. Some o f the powders underwent a preliminary heat treatment at 750 °C for 5 h in order to eliminate the large amounts of organic compounds and of CO2 which are present after the coprecipitation process. After each calcination step, the C content was measured by means o f chemical analysis. As shown in Table 1, there is a considerable reduction in the overall C content with repeated calcinations at 800 and 820 °C in air, the amount after three cycles becoming as small as 0.28, 0.1 or 0.06 wt.% given by chemical analysis for series I, II and III respectively. The preliminary calcination step at 750 °C for powders of series II and III led to the lowest C content. It can be seen that calcinations at 820 °C are more effective in removing residual C than calcinations at 800 °C are. The critical current density measurements in the three tape series show that the series with the smaller amount of C correspond in general to higher Jc values. All these observations raise the questions as to where the small amount of C left in the oxide layer inside the tape will be situated and what influence it will have on the critical current densities of BPSCCO tapes. In order to find the answers an extensive high resolution AES study has been carried out on various tapes with very different jc values, from 1500 to 33 000 A cm -2 at 0 T and 77 K. The measurements were performed on BPSCCO grains after stripping away the Ag sheath.

© 1993 -- Elsevier Sequoia. All rights reserved

E. W. Seibt et al. / C impurities in high T c superconducting tapes using A E S

197

TABLE 1. Chemicallyanalysed C contents after successivecalcination steps at 800 and 820 °C in air (G, grinding; P, pressing) Sample series

Bi:Pb:Sr:Ca:Cu ratio

Calcination steps

C content (wt.%)

I

1.72:0.34:1.83il.97:3.13

II

i.72:0.34:1.83:1.97:3.13

24 h, 800 °C, air +G + 24 h, 800 °C, air + G + 24 h, 800 °C, air 5 h, 750 °C, air + G + P + 24 h, 800 °C, aii

1.38 0.55 0.28 0.85 0.4

+ G + P + 48 h, 800 *C, air

0.1

+ G + P + 24 h, 800 °C, air 15 h, 750 °C, air + G + P + 2 4 h , 820"C, air + G + P + 2 4 h , 820 °C, air + G + P + 2 4 h , 820 °C, air

0.12 1.24 0.17 0.06 0.06

III

1.8:0.4:2:2.2:3

The observation by high resolution AES is particularly well suited to this study because of the very small dimensions of the grain boundaries or precipitations, and because of the need for a quantitative determination of the elemental composition in the superconducting layer.

3. Auger Electron Spectroscopy

High resolution AES mesurements using a PHI 600 Auger scanning multiprobe (Perkin-Elmer) were carded out on differently calcined BPSCCO tape samples to determine quantitatively the residual C content and to localize the C impurities in the microstructure using mapping and line scan techniques. For quantitative AES measurements an alternating sputtering and multiplex technique was applied in combination with a small-spot duoplasmatron ion source (Perkin-Elmer) for 6 keV argon ions [4]. After cleaning the surface contaminations by sputtering, the integral determination of the residual C content was performed by Auger survey measurements scanning over typical raster sizes of 25 Ima x 20 ~tm after alternating Ar+-sputtering periods (raster size, 1501ma x 1001xm). These sputtering periods (sputter times between 1 and 3 min) were continued until the C content detected by AES multiplex measurements reached a constant level. Typical sputter depths reached with this procedure lie between 50 and 100 nm. The AES multiplex measurements were used here to improve the statistics. The influence of shadowing effects during argon sputtering on the detected carbon concentration has been estimated and lies within the experimental errors of + 8°,6. The main experimental parameters used for the present AES studies were as follows: for the electron beams, 10 keV, 0.1-0.3 gA (resulting in a maximum electron beam diameter of 0.6 ~tm; practical lateral reso-

lution below 0.25 lam at 10 nA) and an incidence angle of 0°; for the Ar + ions, 6 keV, 2 A m -2, an incidence angle 0 of 77 ° and a sputter rate of typically 30 nm min -~ related to Y - B a - C u - O thin films with known thicknesses measured by Rutherford backscattering spectroscopy. For BPSCCO tape samples produced by differently calcined powders the quantitative AES measurements over grains and grain boundary regions indicate a correlation between the number of calcination steps and the residual C content ranging from 11 to 0.8 at.% within an experimental error of + 8%. These C concentration values have also been correlated with those ofjc, as will be shown later (see Fig. 3). For depth profiling of single grains the C content reaches nearly a constant level of about 0.5 at.% and is almost independent of the processing steps. It is not thought that this residual C is incorporated in the BPSCCO structure, but its presence is rather interpreted as the existence of very small regions of inhomogeneity within the grains, which have also been detected by transmission electron microscopy. In order to show the variation in the C content inside and between the BPSCCO grains, the AES line scans for Bi, Ca, Cu and C of two tapes with jc (77 K, 0 T) = 1500 and 33 000 A cm -2 are presented in Fig. I, the other elements (Sr, O and Pb) showing only small variations. These tapes have been prepared in an identical way, but the number of calcination steps varied from 1 for the lower Jc sample to 3 for the higher j~ tape. As mentioned above, the C content inside the grains is of the order of 0.5 at.% or less but rises sharply in the intergrain region. For the sample with lower j¢, the C content in the intergrain region reaches 11.5 at.% C, while it remains at about 1 at.% for the tape with 33 000 A cm -2 (lower part of Fig. 1). The study of the grain boundary region w i t h a typical width of 1 Ima for tapes with j¢ (77 K, 0 T) = 1 5 0 0 A c m -2 and even less than 0.5 ~rn for j¢ (77 K,

198

E. IV. Seibt et al. / C impurities in high T c superconducting tapes using A E S

RES

:[

L i n e Scan HLt:

IOkeV

IOOnR

Oc= B*

ELEMENTS:

Bi

Ca

Cu

C

.-r-..r- 1

-Pbi-Sr-Ca.:--Cu-O

Jc(OT, FTK; C ) = 1.5 x lO7fl/M 2 jc(OT, FFK; Cx) = 3.3 x lOSA/n 2

"tu....... ./Z......~...................... tu

Cu

Cu

¢N Z

Ca

Ca

Ca

F--

lit

lit

_

Bt

•

Bi

Z

tad Z

......... J .........

2

( ca. lpM )

C

C~

0

Geair Boundaeg

4

G

8

Ct

10

12

14

LENGTH [microns] Fig. 1. Quantitative AES line scan across a grain boundary in Ag/BPSCCO tapes withjc(77 K, 0 T) = 1500 A c m -2, after argon sputtering. The lower curve for the C content indicated by C* has been added for comparison and corresponds to the tape with j¢(77 K, 0 T) = 33 000 A cm -z.

0 T) = 20 000 A cm -2 is particularly interesting when including the concentrations of the other elements. In Fig. 1, one can see that this narrow region contains not only the C impurities but also other phases. The presence of foreign phases at the grain boundaries is a direct consequence of the BPSCCO formation process in the tapes. The starting point is indeed basically a non-stoichiometric composition mainly composed of the (Bi:Sr:Ca:Cu = ) 2:2:1:2 phase, which is transformed to the (Bi:Sr:Ca:Cu = )2:2:2:3 phase under the effect of a liquid phase. Since it is particularly difficult to study the precise amount of phases at the grain boundaries, because of the reduced dimensions of this region, neither energy-dispersive X-ray nor X-ray diffraction measurements can be performed, which is a further argument in favour of AES. The location of the C impurities on a BPSCCO surface after sputtering can be seen in Fig. 2. A scanning electron microscopy (SEM) image of the surface of the sample with j¢(77 K, 0 T) = 1500 A cm -2 is shown in Fig. 2(a), and the corresponding Auger map of carbon (the white spots indicating higher C contents) in Fig. 2(b). Comparing both images in Fig. 2, one can readily observe that most of the C is localized at the grain boundaries, thus confirming the findings in Fig. 1.

Therefore it can be concluded that the presence of residual CO2 in the initial powder mixtures introduced in the Ag tubes leads to the presence of a very thin C-rich layer over the whole BPSCCO grain surface, with an enhanced concentration at the grain boundaries.

4. The effect o f carbon on the critical current density

The present systematic AES study on a large number of samples revealed that there are significant differences between the C contents in the BPSCCO superconducting phases of tapes with different calcination histories, even for identical reaction conditions. The results are summarized in Fig. 3 and illustrate the limiting effect of the C content on j¢ of Ag-sheathed tapes prepared under identical pressing and sintering conditions. It can be recognized that the tapes with the lowest C content correspond to the highest critical current density values. For example, the tapes with jc(77 K, 0 T) = 1500 A cm -2, 10 000 A cm -2, 20 000 A cm -2 and 33 000 A cm -2 have C contents found by AES of about 11 a t . ° 4 at.%, 2 at.% and less than 1 at.% respectively. It has to be noted, however, that the correlation in Fig. 3 only means that the presence of C acts as a limiting

199

E. W. Seibt et al. / C impurities in high T c superconducting tapes using A E S

Je (77K,0T) [A/cm2] lOS

-

'

I

'

I'

'

I

'

I

~

I

'

m

104 "

I

•

{

103

,

0

I

2

,

I

4

,

I

6

I

I

8

,

I

,

10

12

C content [at.%] Fig. 3. C content in various Ag/BPSCCO tapes of jc(0T, 77 K) values from 1500 to 33 000 A cm -2 illustrating the limiting effect of carbon on Jc-

5. Conclusion

Figl 2. (a) SEM image of the BPSCCO grains after stripping away the Ag sheath. (b) Auger map of carbon on the same sample, illustrating the enhanced C content at the grain boundaries.

factor; for a given C content, Jc cannot exceed certain values. It is important to mention that the absence of C alone in a tape is not sufficient for obtaining a high jc value. As shown in Fig. 1, the amount of foreign phases at the grain boundaries also has to be taken into account when considering the critical current density. The latter depends on many factors, e.g. deformation parameters, degree of texturing, details of the heat treatment (temperature and time), number of annealing and pressing cycles, initial composition and others. The minimization of the foreign phase content at the BPSCCO grain boundaries is still a subject of investigation, the corresponding mechanisms being very complex.

Quantitative high resolution AES was used to determine the elemental distribution in the superconducting BPSCCO phases of Ag-sheathed tapes after stripping away the sheath. Within a narrow region (of the order of 1 lam) at the grain boundaries, foreign phases as well as a considerable amount of carbon were detected. The carbon is introduced with carbonate precursors and can be eliminated by a series of successive calcination heat treatments. The average C content of the superconducting BPSCCO phase was found to decrease with the number of calcination steps; its lowest content in the present work was determined by chemical analysis to be 0.06 wt.%. The Ag sheath limits the elimination of the CO2 during the heat reaction treatment of BPSCCO, so that all grain surfaces and grain boundaries become contaminated with carbon. After stripping away the Ag sheath, the C content at the surface of the BPSCCO grains in Ag sheathed tapes was found to vary from 11 to less than 1 ati%, depending on the calcination history.

200

E. W. Seibt et al. / C impurities in high T c superconducting tapes using AES

A correlation between the C content and the values o f j c in the tapes was found. For tapes with j¢ (77 K, 0 T) = 1500 A cm -2, 10 000 A cm -2, 20 000 A cm -2 and 33 0 0 0 A c m -2, the C contents at the surface of the BPSCCO grains determined by AES were about 11 at.%, 4 at.%, 2 at.% and 0.8 at.% respectively. This correlation is well established, but the microscopic effect of C on the foreign phases formed at the grain boundaries (and finally influencing jc) is not known yet. As a summary, we have confirmed not only that the achievement of high Jr values in Ag/BPSCCO tapes depends on the degree of texturing and on the minimum content of foreign phases at the grain boundaries, but also that the highest Jr values for a given tape can only be reached if the residual C content in the precursor powders is eliminated.

Acknowledgment

The authors would like to thank Mr. J. Pytlik for the technical assistance of AES measurements.

References 1 K. Sato, N. Shibuta, H. Mukai, T. Hikata, M. Ueyama and T. Kato, Physica C, 190 (1991) 50.

2 R. Flfikigor, B. Hens¢l, A. Jeremi¢, M. Decroux, H. Kfipfer, W. Jahn, E. W. Seibt, W. Goldackcr, Y. Yamada and J. Q. Xu, Supercond. Sci. Technol., 5 (1992) $61. 3 R. Flfikigor, A. Jcremi¢, B. Hensel, E. W. Seibt, J. Q. Xu, and Y. Yamada, Adv. Cryog. Eng., 38 (1992) 1073. 4 E. W. Seibt and A. Zalar, Mater. Lett., 11 (1991) 1.