Influence of fast neutron irradiation on critical currents and irreversibility lines in MPMG-processed YBa2Cu3O7 superconductors

Influence of fast neutron irradiation on critical currents and irreversibility lines in MPMG-processed YBa2Cu307 superconductors* M. W a c e n o v s k y , R. Miletich, H . W . Weber and M. Murakami t Atominstitut der 0sterreichischen Universit~ten, A-1020 Vienna, Austria t International Superconductivity Technology Center, Superconductivity Laboratory, Tokyo 135, Japan

Research

Melt-powder melt-grown samples with different amounts of artificially introduced 211 particles were exposed to fast neutron irradiation up to a fluence of 4 x 1021 m -2 (E > 0.1 MeV). The influence of the irradiation on the critical current characteristics and on the irreversibility lines is investigated. Shifts of the irreversibility lines and enhanced critical current densities are observed. The results are compared with similar experiments on high quality single crystals.

Keywords: 123 superconductors; melt processing; critical currents

Many applications of high Tc materials require high critical current densities in the presence of strong magnetic fields. To achieve high Jc values it is of fundamental importance to introduce pinning centres for flux lines into the superconductor. Although several candidates for defect sites have been suggested (such as twin planes, structural defects, oxygen defects etc.), the actual flux pinning mechanism in 123-materials is still controversial. Sufficiently high values of Jc have been found only in epitaxial thin films or in tiny single crystals t2. From a technological point of view, however, much bigger samples would be desirable. Sintering represents a simple method for preparing samples of larger dimensions, but granularity has been found to be the main obstacle for a large scale current flow in these materials. Josephson coupling between adjacent grains, misorientations and impurities suppress the 'intergrain' critical current densities by orders of magnitude below the relatively high intrinsic ('intragrain') values, even in moderate magnetic fields. Therefore, the reduction (or even elimination) of this kind of defect seems to be the key problem for achieving high J,, values. Several technologically successful ways to manipulate the microstructure in a positive way have been described recently. Different kinds of melt processing seem to be most promising. Jin et al. 3 reported on a process in which sintered YBa2Cu307 was melted and slowly cooled in a temperature gradient. They obtained large grains with preferential orientations resulting in significantly improved critical currents.

A similar technique described by Salama et al. 4 resulted in highly c-axis oriented platelike grains on a millimetre scale. No sign of granularity was observed in these samples by magnetization measurements. In the present work samples prepared by a special liquid phase technique 5 were investigated. As a result of several melting and grinding steps (melt-powder melt-growth, MPMG) fairly large grains are obtained in a nonsuperconducting matrix. According to previous investigations, weak link structures are almost absent. Moreover, optical micrographs revealed a fine dispersion of non-superconducting 211 particles within the 123 matrix. The density of these inclusions is determined by changing the starting composition. Although their size (1-10/~m) is large compared with the coherence length, it is assumed that they can act as pinning centres 6. Previous experimental work has demonstrated clearly that neutron induced defects are very effective in increasing the magnetically measured Jc in single crystals by at least one order of magnitude, depending on temperature, field and neutron fluence 7'8. In order to study such irradiation effects on melt-textured materials, several samples were exposed to fast neutron irradiation up to a fiuence of 4 x 10 2'm -2. The influence of irradiation on critical currents and irreversibility lines will be reported in this contribution.

* Paper presented at the c o n f e r e n c e 'Critical Currents in High Tc Superconductors', 22-24 April 1 9 9 2 , Vienna, Austria

MPMG samples with two different amounts of 211 precipitates were prepared. Powders of Y203, BaCO 3

0011 - 2275/93/010070 - 07 © 1 9 9 3 B u t t e r w o r t h - H e i n e m a n n Ltd

70

Cryogenics 1993 Vol 33, No 1

Experimental

Samples

Influence of fast neutron irradiation: M. Wacenovsky et al. and CuO were mixed, calcined and heated to 1400°C for 20 min in a Pt crucible. The starting compositions were Y:Ba:Cu = 1:2:3 for one sample (MPMG-1-B; 4 vol% 211) and 1.8:2.4:3.4 for the other (MPMG-3; 10 vol% 211). The melt was rapidly quenched using copper plates. The quenched samples were ground and mixed with small amounts of AGO2. Pressed pellets of the powders were heated to 1100°C in flowing oxygen and cooled slowly to room temperature. Large single grains (3 x 3 × 3 mm 3) of the 123-phase were found in a nonsuperconducting matrix and shaped into parallelepipeds with their sides parallel to the major crystallographic directions. The (001) axes were determined by X-ray diffraction while the orientation of the a and b axes was determined from the orientation of the twin boundaries along (110). Polarized optical microscopy was used extensively to investigate the complicated microstructure of the samples. The chemical compositions of the inclusions were determined by energy-dispersive X-ray analysis (EDX) performed in a scanning electron microscope. The different weight fractions of the included phases were obtained from X-ray fluorescence analysis (XRF) 9. Experimental set up All magnetic measurements reported in this paper are based on a mutual inductance system for a.c. susceptibility measurements. A.c. fields of up to 20 mT can be employed. A computer controlled two-channel lock-in amplifier is used to record the magnetic response of the sample. The whole coil system is located in a variable temperature insert (2 - 150 K). A d.c. magnetic field up to 17 T can be applied with a superconducting solenoid. The a.c. frequency was set to 9 Hz in order to avoid eddy currents in the surrounding metal parts. Values for critical current densities were obtained by a.c. inductive flux-profile measurements, which are based on a modeldependent analysis of the non-linear pick-up response. Details of this measuring method are given elsewhere to. The irradiation was performed in the central irradiation facility of the TRIGA MARK II nuclear research reactor in Vienna at ambient reactor temperatures ( < 50°C). All fluences are quoted for neutron energies E > 0.1 MeV.

Results Samples According to Figure la the crystals form mosaic structures consisting of mutually slightly misoriented blocks of sizes _> 100 #m, separated by low angle grain boundaries ( < 8°). As shown in Figure lb large amounts of second phases are present in the samples. According to our analysis, these inclusions are Y2BaCuO5 (211), Ba4Cu2PtO9 (421) and metallic silver. A summary of the different weight fractions is presented in Table 1. The presence of 421 is caused by severe reactions of the melt with the crucible. Particles of this phase with lengths of up to 150/zm have been observed. Cracks, which were found in all the samples, are caused by the shrinkage of the c-axis of the 123 phase due to oxygen incorporation during the last heat treatment. In a previous paper 9 a clear correlation between the Ag con-

Figure 1 (a) Typical microstructure of polarized light. The mosaic structure with can be seen; (b) microstructure of sample inclusions (Ag, 421 and 211) are marked

a 123 crystal under low-angle boundaries MPMG-3. The various by arrows.

tent and the crack formation was reported. It was proposed there, that the decomposition of AgO2 into metallic silver and oxygen results in a melt with enhanced oxygen content. Therefore, less oxygen has to be absorbed by the solidifying crystals during annealing, which leads to reduced crack formation. Since during the second melting step the 123 phase is grown by a peritectic reaction between the melt and the 211 phase Y2BaCuO5 + L(BaCuO 2, CuO) -- 2YBa2Cu3065 Table 1 Average composition (a) and calculated phase contents (b) in w t % of MPMG YBa2Cu307-~ crystals. The standard deviations are quoted in parentheses (a)

MPMG-1

MPMG-3

Y203 BaO CuO Ag20 Pt03

19.0 43.8 30.3 5.0 1.8

(0.8) (0.7) (0.6) (0.7) (1.1)

20.3 41.5 28.8 7.9 1.4

(0.7) (0.7) (0.6) (0.7) (1.1)

74.5 12.8 8.0 4.7

(2.4) (2.0) (1.5) (0.6)

69.0 17.5 6.1 7.4

(2.3) (1.6) (1.5) (0.6)

(b) 123 211 421 Ag

Cryogenics 1993 Vol 33, No 1

71

Influence of fast neutron irradiation: M. Wacenovsky et al.

an incomplete reaction of the components (depending on the starting composition) causes some amount of 211 to remain in the 123 matrix in the form of finely dispersed precipitates on a /zm scale. Recent comparisons of crystals with different amounts of these inclusions revealed a clear reduction of crack formation with increasing amount of 211. The increased connectivity allows high critical currents to flow in the whole sample area, which results in improved screening capabilities. In addition to the introduction of pinning centres, the latter fact represents the second beneficial effect of the precipitates.

Oe5

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MPMG-3, HIIc

I

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4xlO=I/m =

0.4 0.3 0.2

Irreversibility lines

The maximum of the out-of-phase peak was used to define the so called irreversibility temperatures for different magnetic fields (Figure 2). This method has been widely used for the determination of the 'irreversibility lines'. However, because of the complex vortex dynamics in the transition region the results obtained in this way are still under discussion. Dependences on frequency, time scale and a.c. amplitude are observed in most cases. Figure 3 shows the two components X' and X" of the complex susceptibility X = X" + iX" plotted against each other for different magnetic fields. Corrections for demagnetization were made in the usual way, which converts the measured susceptibility Xm (with respect to the applied a.c. field) to the actual susceptibility X (with respect to the effective field) X ' + ix" =

0.5

X" + ix" 1 - DX'm - iDx~

I

(1)

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~ O

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Bean-model diffusive I

~

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model I

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Figure 3 A.c. loss component X" as a function of the in-phase component ×' plotted for the d.c. fields in Figure 2. Theoretical predictions for a critical state model and for a diffusive model (as explained in the text) are also s h o w n

(The demagnetizing factor D can be estimated from simple formulae.) In the graphical representation of Figure 3 all the data points collapse fairly well onto one master curve, which seems to be characteristic for the sample. Two limiting curves are also plotted on the same graph, namely the corresponding results for a critical state model without any form of relaxation and for a simple linear diffusive model based on skin depth considerations. Within Bean's critical state model exact analytical solutions can be obtained for the shape of the a.c.-driven minor hysteresis loops. In the case of harmonic excitation fields h(t)= ho sin o~t, the pick-up response contains higher harmonics. Hence, the susceptibility is written in a generalized form

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(2)

i=0

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0.0

-0"170

I

75

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80

I

85

I

90

95

T(K) Figure 2

Typical loss peaks, as obtained by a.c. susceptibility measurements. The maximum is used to define Tirr for different fields

72

Cryogenics 1993 Vol 33, No 1

u(t)

dm(t) dt

o~ - -

(3)

these susceptibility coefficients can be obtained as a function of the normalized penetration depth

Influence of fast neutron irradiation: M. Wacenovsky et al.

x = ho/(JcR). Special attention has to be paid to the first

18

harmonic component, since all higher harmonics are usually suppressed by the lock-in filtering technique. A maximum in the out of phase signal X~' is obtained, when the flux front related to the a.c. field just reaches the centre of the sample (x = 1). The value of this maximum is found to be 0.21. The amplitude dependence of the peak temperature is easily explained within this model. In order to keep x constant, while h0 is increased (decreased), the proper Jc is found at a lower (higher) temperature. The diffusive model is based on the assumption that flux movement in the sample is governed by a simple diffusive relation, as also obtained for screening effects in normal metals

16

02b(x, t) OX2

+ D

Ob(x, t) 3t

- 0

(4)

D can be related to a flux-flow resistivity PFF" Within TAFF theory ll,t2, this resistivity is caused by the hopping of flux-line bundles over pinning potential barriers due to thermal activation processes PFF = po(T, B)exp( - U/kB T)

(5)

A simple solution is found for a long sample with slab geometry (Ixl < d/2). In this case, the susceptibility is given by tanh(u) #-=X + 1 - - u

(6)

The complex argument u depends on the diffusion constant, the sample dimension and the measuring frequency: u = (1 + i)(o~d2/2D), D = PFF//~0. The maximum value of the out-of-phase peak (about 0.42) occurs at l u I --- 1.5 and is definitely higher than the critical state model. It should be noted that the observed frequency dependence can be qualitatively explained by this model. For higher frequencies the value of resistivity PFF must also increase, in order to keep u constant. Since the flux-flow resistivity increases with temperature, a shift to higher values is observed. As can be seen in Figure 3, the actual data points for the susceptibility are located between these two models. Consequently, more sophisticated models are required to explain the complicated amplitude- and frequencydependence simultaneously. From this point of view, a comparison between irreversibility lines obtained by different methods (SQUID magnetometry, a.c. susceptibility) seems to be only of limited use. Nevertheless, it is reasonable to compare similar samples or changes on specific samples, if the measurements are made under well defined and constant conditions. Results for both the unirradiated and the irradiated samples are shown in Figure 4. All the measurements were made with an a.c. amplitude of 0.1 mT at 9 Hz. The influence of the a.c. amplitude was found to be very small for H II c. The frequency dependence of the irreversibility lines was studied between 0.1 Hz and 10 kHz. A nearly logarithmic increase of Tir r by about 0.5 K decade -~ was found even at frequencies as low

, r

Irreversibility Lines MPMG-3

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Figure 4 Irreversibility lines of sample MPMG-3 (a.c. amplitude O. 1 mT, frequency 9 Hz). The results for the unirradiated and the irradiated sample are shown for both orientations

as 0.1 Hz. The irreversibility lines are strongly anisotropic (in good agreement with results on high quality single crystals). The irreversibility lines can be fitted by. a power law according to #0H~rr = A ( 1 T/T~)% Power exponents of about 1.5 and 2 are observed for H II c and H II (a, b) respectively. By increasing the amount of 211 precipitates, a shift of the irreversibility lines towards higher temperatures and fields has been observed. The position of the irreversibility lines is changed by neutron irradiation. For H IIc, shifts to higher temperatures and fields were observed after irradiation. Saturation seems to set in already at the second irradiation step (4 x 102' m-2). Different behaviour was observed in the perpendicular direction H II (a, b). In this case the lines shifted to lower temperatures and fields after irradiation. It should be noted that these results are in agreement with recent results on single crystals 7. Critical current densities

The critical current densities were obtained inductively from flux profile measurements. Results for MPMG-3 and MPMG-1-B are presented in Figure 5. The correlation of Jc with the amount of 211 inclusions is obvious for all fields and temperatures. Very large critical currents were found especially in sample MPMG-3, which contains a high amount (17.5 wt%) of 211 inclusions: 6 x 10 9 A m -2 at 4.2 K and 1.5 x 108 A m-2 at 77 K and/z0H = 1 T II c. The values for sample MPMG-1-B (12.8wt% 211 inclusions) are definitely smaller: 2.8 x 10 9 A m -2 at 4.2 K and 6 × l0 7 A m-2 at 77 K for the same field. Different Jc characteristics are observed for the two major orientations. For H II c, Jc increases slightly with magnetic field within a certain field range and develops a more or less pronounced maximum (as can be seen for MPMGl-B, T = 60 K, H II c). This peak characteristic is commonly found in magnetization curves and known as 'fish-tail behaviour'. It has been Suggested that this feature might be caused by oxygen disordering or oxygen deficient regions within the samples t3. Indeed, some evidence for a non-homogeneous oxygen distribution was found in the present samples on optical

Cryogenics 1 9 9 3 Vol 33, No 1 7 3

Influence of fast neutron irradiation: M. Wacenovsky et al. I

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on(T) Fig=me, 5 (a) Critical c u r r e n t d e n s i t i e s f o r t h e u n i r r a d i a t e d s a m p l e M P M G - 3 (H I c). 'Fish tail b e h a v i o u r ' is o b s e r v e d at i n t e r m e d i a t e t e m p e r a t u r e s ; (b) critical c u r r e n t d e n s i t i e s f o r t h e u n i r r a d i a t e d s a m p l e M P M G - 3 (H Ua,b); (c) critical c u r r e n t d e n s i t i e s f o r t h e u n i r r a d i a t e d s a m p l e M P M G - 1 - B (H H c). T h e 'fish tails' are e v e n m o r e p r o n o u n c e d t h a n in s a m p l e M P M G - 3

74

Cryogenics 1993 Vol 33, No 1

Influence of fast neutron irradiation: M. Wacenovsky et al.

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micrographs. Some 'greyish' areas, which can be seen only under crossed polarizers 9, were-observed occasionally. These areas of about 100 #m length turned out to contain less oxygen than the surrounding 123 material 14. For the perpendicular direction, H IIa, b, however, no sign of fish tail behaviour can be found at any temperature: the Jc values decrease continuously with increasing field. Moreover, because of the much higher irreversibility fields in this direction, the critical currents depend more weakly on the magnetic field. At fields far away from the irreversibility lines, the anisotropy factor (j~nc/j~lab) is about 3. This value is much smaller than for 'good' single crystals, where anisotropies of up to 102 were found recently 7. The first irradiation step led to significant changes in the critical current characteristics. The fish-tail behaviour for H II c was removed and smoothly deereas'ing Jc characteristics obtained for all temperatures (Figure 6). As a consequence, Jc decreases after irradiation nearly exponentially with field for H II c. The enhancement factors with respect to the unirradiated state depend on orientation, temperature and field. For H II c, a maximum increase of the critical current densities is found especially at high temperatures and low fields, e.g. Jc/Jco = 5 at 7 7 K and 0 . 5 T ( J c = 7 X 108A m -2) for sample MPMG-3 (Figure 7a). For fields close to the irreversibility lines, Jc remained nearly unchanged. For H II (a,b) enhancement factors between 1.5 and 2.5 are found (Figure 7b). The second irradiation step (fluence = 4 × 1021 m -2) caused only minor changes with respect to the state after the first

4

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Cryogenics 1993 Vol 33, No 1

75

Influence of fast neutron irradiation: M. Wacenovsky et al. irradiation for H II c. This result is in contrast to irradiation studies on single crystals, where no sign of saturation is observed at comparable fluences. In the perpendicular direction (H IIa,b) the critical current densities are further increased only at T < 50 K and start to decrease for T > 50 K.

Summary Melt processed 123 materials have been examined with regard to microstructure, irreversibility lines and critical current densities. A clear correlation of these parameters with the volume fraction of non-superconducting 211 inclusions was observed. The irreversibility lines shift to higher temperatures and fields with an increasing number of 211 precipitates. Moreover, 211-rich samples exhibit larger critical current densities than samples with fewer 211 inclusions. This is explained by a significant contribution of the 211 inclusions to flux-pinning. Further improvement of the irreversible behaviour has been observed after irradiation, but in a less pronounced way-than for single crystals. The saturation of the irradiation-induced changes occurs already at relatively low fluences and may be attributed to the different pre-irradiation defect structures of the samples. Further systematic studies on the subject are currently under way.

Acknowledgements This work was supported in part by Fonds zur F6rderung der Wissenschaftlichen Forschung, Wien,

76 Cryogenics 1993 Vol 33, No 1

under contract Nos 7098 and 7970. The authors wish to thank Dr E. Seidl and Mr H. Niedermaier for useful discussions and technical help during the experiments.

References 1 Roas, B., Schultz, L. and Saemann-Ischenko, G. Phys Rev (1990) 64 479 2 Crabtree, G.W., Liu, J.Z., Umezawa, A., Kwok, W.K., Sowers, C.H., Malik, S.K., Veal, B.V., Lam, D.J., Brodsky, M.B. and Downey, J.W. Phys Rev B (1987) 36 4021 3 Jin, S., Tiefel, T.II., Sherwood, R.C., van Dover, R.B., Davis, M.E., Kammlott, G.W. and Fastnacht, R.A. PhysRev B (1988) 37 7850 4 Salama, K., Selvamanickam, V., Gao, L. and Sun, K. Appl Phys Len (1989) 54 2352 5 Murakami, M., Morita, M., DOi, K. and Miyamoto, K. Jpn JAppl Phys (1989) 28 1189 6 Murakami, M., Gotoh, S., Fujimoto, H., Yamaguchy, K., Koshizuka, N. and Tanaka, S. Supereond Sci Teehnol (1991) 4 43 7 Sanerzopf, F.M., Wiesinger, H.P., Kritscha, W., Weber, H.W., Frischherz, M.C. and C,erstenberg, H. Cryogenics in press 8 Weber, H.W. Supercond Sci Technol (1992) 5 19 9 Miletieh, R., Murakami, M., Preisinger, A. and Weber, H.W. Jpn J Appl Phys in press 10 Campbell, A.M. J Phys C (1969) 2 492 11 Kes, P.H., Aarts, J., van den Berg, J., van der Beek, C.J. and Mydosh, J.A. Supercond Sci Technol (1988) 1 242 12 Brandt, E.H. Supercond Sci Technol (1992) 5 25 13 Daeumling, M., Seuntjes, J.M. and Larbalestier, D.C. Nature (1990) 346 332 14 Murakami, M., Yoshida, M., Fujimoto, H., Gotoh, S., Koshizuka, N. and Tanaka, S. Jpn J Appl Phys (1991) 30 796