AC losses in polycrystalline YBa2Cu3Oy doped with Ag and Te

PHYSICA

Physica C 195 (1992) 345-351 North-Holland

AC losses in polycrystalline

YBa2Cu3Oydoped with Ag and Te

E.S. V l a k h o v a, K.A. N e n k o v a a n d M. C i s z e k b a Institute of Solid State Physics, Bulgarian Academy of Sciences, 72, Trakia Blvd., 1784 Sofia, Bulgaria b Institute of Low Temperature and Structure Research, Polish Academy o f Sciences, 50-950 Wroclaw, PO Box 93 7, Poland

Received 4 February 1992

The energy dissipation in polycrystalline YBa2Cu3Oydoped with Ag and Te has been investigated under superposed AC and DC magnetic fields using a lock-in technique. Bean's critical state model is applied to estimate intergrain Jc. The AC loss behaviour is qualitatively explained by a theoretical model accounting for the Bean-Livingston barrier in a system of superconducting grains and steps in Jc(H) dependence. The influence of Ag and Te additions on intergrain critical current density and its magnetic field dependence is discussed.

1. Introduction The problem o f energy dissipation has been intensively investigated in low-To superconductors because of their application in electrical machines and power transmission lines [ 1 ]. AC loss measurements were confirmed as a useful technique for testing new superconducting materials as Laves phase c o m p o u n d s [2] and allow the possibility of estimating critical current density, its magnetic field dependence, surface barrier etc. The loss behaviour o f high-To ceramics is characteristic o f granular superconductors, where two stages o f flux penetration were observed [ 3 - 7 ] . The first one is related to the intergrain region, and the second to the individual grains. Minima o f AC losses in high-T~ ceramics, when a DC magnetic field is additionally applied, were observed in refs. [ 5,8,9 ]. It is of great interest to obtain such information from similar investigations on polycrystalline YBa2Cu3Oy doped with silver and tellurium.

2. Experimental The investigated samples were obtained by ceramic technology described in detail elsewhere [ 10 ]. The additives were introduced as TeO2 and AgzO. Sample 1 contains both silver and tellurium addi-

tives and has a nominal composition (YBao.92Teo.o8Cu3Oy) 0.95(Ag20) 0.05. Sample 2 was doped with tellurium only, with nomial composition (YBa2Cu3Oy)0.97 (TeO2)o.03. Sample 3 with nominal composition YBazCu3Oy (undoped) is obtained by the well-known standard procedure. Crystal phases were determined by X-ray powder analysis. The morphology of the samples was investigated by a SEM-TESLA-BS-540 scanning electron microscope. Content of the components in the different phases was determined by electron-probe microanalysis ( P H I L I P S 505 E D A X ) . The AC losses were measured on the plate-type samples (20 m m in length and 2 × 3 m m 2 cross-section) as a function o f the sinusoidally varying applied AC magnetic field bo, with a frequency of 87 Hz, and a superimposed DC-bias field Bo, at temperatures 4.2 K and 77 K. The voltage signals from a single layer pick-up coil (160 turns of 25 ~tm diameter copper wire) were fed to the lock-in amplifier. From the signal component in the phase with respect to the AC magnetic field, and the value of the latter, the total losses were computed. The accuracy of loss measurements is of the order o f 10 -8 J / m 2. In order to avoid the influence of the trapped magnetic flux on the results, before each experiment the sample was heated up to room temperature and then cooled down in zero field. The earth's magnetic field was not shielded.

0921-4534/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

E.S. Vlakhov et al. / A C losses in YBCO ceramics doped with Ag and Te

346

3. R e s u l t s

........

The investigated Te-doped samples were multiphased after calcination and sintering irrespective of whether the tellurium was introduced by substitution of Ba (sample 1 ) or as an additive to the 123 phase (sample 2). X-ray diffraction patterns and SEM micrographs of samples 1 and 2 were shown in ref. [1 I] (samples FITEM and FTe3DM respectively). It was found in ref. [ 10] that introduction of Te led to the appearance of a new oxygen-containing phase. This phase (further named as phase A) consists of Y, Te, Ba, Cu and O. It was shown by EDAX analysis that the component content of phase A varied in a wide region. The contents of phase 123 and phase A are comparable in sample 2. A small quantity of 211 phase was registered in this sample too. In sample 1 the phase 123 predominates. The impurity phases are 211, CuO and phase A. The silver separates as an independent phase between grains. The loss behaviour of samples 1 and 2 versus amplitude bo of AC magnetic field is shown in figs. 1 and 2, respectively. Both samples exhibit a linear behaviour with slope n ~ 3 in log L versus log bo coordinates (L - losses in J / m 2) at low values of bo. This region is more pronounced for sample 2. It expands to about bo=10 mT (4.2 K run) and bo=3 mT (77 K run). These values for sample 1 are about

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Fig. 1. AC losses L vs. a m p l i t u d e bo of the AC m a g n e t i c field for s a m p l e 1 at T = 7 7 K - o o o o o , a n d at 4.2 K - A A A A A .

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Fig. 2. AC losses L vs. a m p l i t u d e bo of the AC m a g n e t i c field for s a m p l e 2 at T = 7 7 K - o o o o o , a n d 4.2 K - A A A A A .

one order of magnitude smaller - 1 mT and 0.3 mT, respectively. With further increase of the AC field amplitude, the two samples show quite different behaviour. The sample doped with Te (sample 2) exhibits wide transition to a new linear region with smallest slope n ( n < 2 ) . Sample 1 (with Ag and Te additives) possesses a region 0.3 mT < bo < 6 mT ( 77 K run ) of very strong loss increase - n ~ 4.1. One sees an inflection point of L(bo) correlation at bo= 1 mT (77 K run) and 6.5 mT (4.2 K run). After that region, a new one of correlation L ~ b 3 is registered. It is worth noting that it is equal to the same correlation as in the first region at lowest bo values. The fourth region at bo> 30 mT is observed at the liquid nitrogen temperature run only for sample 1. It is characterized by the lowest value of the slope n ~ 1.2. In order to compare the influence of Ag and Te additions on the AC losses, investigation of the undoped sample (sample 3) was carried out. Figure 3 shows AC loss behaviour of the undoped sample at liquid nitrogen temperature. The influence of DC magnetic field superposition on AC losses is shown in figs. 4-7. It is seen (figs. 4, 5, sample 1 ) that the loss behaviour is very different. Two linear regions with different slope are observed. A very important feature, due to the presence of the

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Fig. 3. AC losses L vs. amplitude bo of the AC magnetic field for sample 1 - o o o o o, and sample 3 (undoped) - • • • • • at T= 77

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Fig. 5. AC losses L vs. amplitude bo of the magnetic field for sample 1 at T=4.2 Kwith superposition of the DC magnetic field Bo, Bo=0-50 mT (see marked symbols in the figure).

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Fig. 4. AC losses L vs. amplitude bo of the AC magnetic field for sample l at T= 77 K with superposition of DC magnetic field B0, Bo=0-50 mT (see marked symbols in the figure).

Fig. 6. AC losses L vs. amplitude bo of the magnetic field for sample 2 at T=77 K with superposition of DC magnetic field Bo, Bo=0-50 mT (see marked symbols in the figure).

D C field, is t h e e x i s t e n c e o f t h e m i n i m a o f A C l o s s e s like t h o s e r e g i s t e r e d in refs. [ 5 , 8 , 9 ] . F o r s a m p l e 2

4. D i s c u s s i o n

( s e e figs. 6 a n d 7) t h i s e f f e c t is w e a k l y p r o n o u n c e d , in a n a r r o w e r r e g i o n o f bo, a n d s h i f t e d to h i g h e r valu e s o f t h e latter.

A c c o r d i n g t o t h e B e a n critical s t a t e m o d e l [ 12] t h e h y s t e r e t i c A C losses in a slab are e x p r e s s e d in t h e following formula: P = (2/3/to)fbo3 J~ -I , f o r bo < H * and

( 1)

348

E.S. Vlakhov et al. /AC losses in YBCO ceramics doped with Ag and Te

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~TW . . . . . . . . , 10 10 2 b0(roT)

Fig. 7. AC losses L vs. amplitude boof the AC magneticfield for sample 2 at T= 4.2 K with superposition of the DC magneticfield Bo, Bo=0-50 mT (see marked symbolsin the figure).

P = 2 × 10-8d2fboJc, for b o > H * ,

(2)

where P is dissipated power per unit surface of superconductor,/to is the magnetic constant, f t h e frequency, bo the amplitude value of the magnetic induction, 2d the thickness of the slab, Jc the critical current density and H* the magnetic field of full penetration. Let us analyse the AC loss behaviour of the investigated samples. Two penetration fields are clearly seen in fig. 7 for the undoped sample 3 at 77 K. The first one corresponds to full penetration of the field into intergrain areas and the second to full penetration into the grain volume. The AC loss behaviour of sample 1 at 77 K is shown in fig. 1. As for the undoped sample, there are two fields of full penetration shifted to some higher bo value. However, very attractive differences are observed. The cubic dependence at lowest bo tends to become stronger (n ~ 4.1 ), goes through an inflection point ( n = 3 at bo= 1.2 mT) and then smoothly decreases. At 6.5 mT < bo < 30 mT dissipation has the same cubic dependence on bo as at the lowest amplitude of AC field bo. Such behaviour can be understood if step dependence of intergrain critical current density versus magnetic field is considered. However, Ekin et al. [ 13 ] established experimentally a double step char-

acteristic at 76 K in the transport critical current as a function of magnetic field in bulk sintered Y-, Biand Tl-based high-To superconductors. For the YBa2Cu3Oy sample, Jc is constant up to the first step, which occurs at about 0.3-30 roT. By increasing the magnetic field B, critical current density decreases proportionally to B--3/2, and farther on a plateau region is observed. The second step starts at about 100 mT and is deeper than the first one. It is seen in fig. 1 that the first deviation from cubic dependence occurs at about 0.3 mT, which is the value obtained by Ekin et al. [ 13 ] for the first step of critical current density. Taking into account eq. ( 1 ) and that in the step region Jc~b( 3/2, w e obtain L ~ b g 5. This estimated value of n=4.5 is close to that registered experimentally at the region of strong increasing losses in fig. 1, where slope n ~ 4.1. The inflectional point corresponds to the start of the plateau region of Jc(bo) dependence. The value of intergrain critical current becomes very small and the sample bulk cannot be shielded any more, which leads to full magnetic field penetration into the intergrain area. By exceeding the first critical magnetic field of the grains a new contribution to the losses, due to dissipation in the grain volume, takes place. In our previous investigation of magnetization of Agand Te-doped samples [11 ] we estimated the first critical magnetic field of grains BL = 5.9 mT for sample FITEM with a nominal composition such as sample 1 in the present work. It is seen that the region with new cubic dependence of AC losses starts at about 7 roT, in good agreement with our considerations. As was mentioned above, the AC loss behaviour at 4.2 K is similar to that at 77 K and one can suppose the existence of a step in Jc(B) dependence at liquid helium temperature, too. This step is shifted to higher magnetic field 0.8 m T < b 0 < 10 mT. The critical current density of sample 2 (see fig. 2) has to be constant up to about 2.5 mT (77 K run) and 20 mT (4.2 K run) where the cubic dependence of losses is valid. The region of magnetic field penetration into the intergrain area is wide but there is no evidence for full penetration (the slope will be n = 1 after eq. (2)) up to the highest values achieved in our experimental set-up of bo= 100 mT. It is interesting to note that sample 2 with tellurium additive does not exhibit any plateau region in its loss

E.S. Vlakhov et al. /AC losses in YBCO ceramics doped with Ag and Te dependence L versus bo like some melt-textured samples reported very recently by Orehotsky et al. [ 14 ]. The lack o f the plateau region is an indication that there are no weak links in the material, after the authors of ref. [ 14 ]. One can estimate intergrain critical current density o f investigated samples using eq. ( 1 ), valid at lowest magnetic field amplitude bo. Values of j~,te, (77 K ) = 2 1 0 0 , 530, 220 A / c m 2 are obtained for samples 1, 2 and 3, respectively. Figure 8 represents the magnetic field dependence of intergrain critical current density for the investigated samples at 77 K and 4.2 K, also taking into account considerations about a step existence in Jc(bo) dependence. It is worth noting that values o f critical current density estimated by eq. ( 1 ) ( j A C = 2100 A / c m 2 and 530 A / c m 2 for samples 1 and 2, respectively, at 77 K) are considerably higher than those obtained by the resistive method. Our estimation o f J¢ by magnetization o f hollow cylinder-shaped samples with such a nominal composition and preparation technology [ 11 ] showed J~ = 70 and 130 A / c m 2 for samples 1 and 2, respectively. A similar discrepancy between AC and DC critical current densities o f about one order of magnitude has been registered by Yang et al. [ 15 ] in melt-grown Y B C O superconductors. The analysis o f energy dissipation characteristics carried out above shows that tellurium as an addi-

349

tive (sample 2) leads to some increase o f intergrain critical current density j~tc, and there is no plateau region in its loss dependence L versus bo up to the value o f bo = 100 mT. Both tellurium and silver as additives (sample 1 ) lead to a higher value o f j~nte, (about 10 times greater than that o f the undoped sample) but the step in J c ( H ) dependence is shifted to lower bo. Kugel and R a k h m a n o v [ 16 ] have investigated the influence o f the Bean-Livingston barrier on flux penetration into the system of superconducting granules. The nature of the Bean-Livingston surface barrier is that of an interaction between the attractive force to a flux line near the superconductor surface (due to its image attraction) and the repulsive force due to the Meissner current. The relation between the intergrain field Ho and the external applied one H, has been found in the form: Ho = ( lx/22 ) { H - B g ( 1 -

(3)

221lx) } ,

where Be= qbo.N/lx.ly is the intragrain induction, 2 is the London penetration depth, Ix, ly, lz are grain dimensions ( H is along the z-axis). For B e >> B~ the triangular flux lattice exists in a platelike grain with lattice constant d << 2 << Ix. In the range of sufficiently low B e the lattice constant d > 2. Figure 9 represents eq. (3) schematically. The initial growth (curve 1 in fig. 9 ( a ) ) of r i o ( H ) results

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1

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H b)

r

10

Fig. 8. Intergrain critical current density J¢ vs. magnetic field amplitude bo (an estimation by eq. ( 1): curve 1 - sample l, 77 K run; curve 2 - sample 2, 77 K run; curve 3 - sample 1, 4.2 K run; curve 4 - sample 2, 4.2 K run.

Fig. 9. (a) Intergrain magnetic field Ho vs. external magnetic field Hafter eq. (3) of the theoretical model [ 16]. (b) Intergrain critical current density Jc vs. external magnetic field H - predicted behaviour by accounting for the Bean-Livingston barrier [ 16 ]; curve 1 - Ho=n, curve FC - for sample cooled in magnetic field H>H,.

350

E.S. Vlakhov et aL / A C losses in YBCO ceramics doped with Ag and Te

from increasing flux line density and from the repulsive interaction between vortices. It is clear that derivative OHo/OH has to be small at d>>2 (or Bg< H~ ) when this repulsion is negligible. Thus OHo/ OH tends to zero for H close to Hs (Hs is the field at which flux lines enter the superconductor surface) and the dependence Ho(H) has a plateau at H~Hs (see dashed line region 2). Further increasing of H leads to values close to Ho = H dependence. If H begins to decrease from value Hmax>Hsthen Ho drops drastically and vortices begin to leave granules only at Ho < Hcl. Using the Ho(H) dependence, the possible form of J c ( H ) has been proposed in ref. [ 16] and shown in fig. 9(b). Curve 1 (dashed line) is assumed to depict the function Jc(H) for the true equilibrium situation H = Ho. Then, at H < Hs, the actual Jc ( H ) plot (curve 2 ) appears to be much lower than curve 1 due to the steep increase of H0 and, at H>H~, J c ( H ) decreases rather slowly (curve 3). The decreasing of H results in an appreciable hysteresis of Jc, due to the almost vanishing intergrain field Ho (curve 4). If a sample is cooled in a field H t r a p > H s , the Jc(H) behaviour is as shown in curve FC in fig. 9(b). The experimental data of critical current density obtained by Askew et al. [ 15 ] for unoriented polyc r y s t a l l i n e Y B a 2 C u 3 O y in magnetic field up to 1 T are in good accordance with that proposed in fig. 9(b) (for example, see figs. 4 and 8 of ref. [17]). The authors have assumed a power law approximation. Sample set B dependence, shown in fig. 8 of ref. [ 17 ], exhibits 3 regions with different values of the slopes: - 1.76, -0.43, and - 1.10, which are very similar to curves 2, 3 and 4 in fig. 9 (b), respectively. "Such a hysteresis behaviour of arc was observed by many other authors in ceramics obtained by different techniques. Majoros et al. [ 18 ] have reported J~ hysteresis for silver-doped YBCO ceramics, too. Let us analyse the influence of the DC-bias magnetic field superposition on AC losses using a theoretical model [16] accounting for the Bean-Livingston barrier in ceramics. Application of the constant bias magnetic field Bo can be considered as equal to the trapping of magnetic field Bo by the highT~ superconductor. In the present investigation, the value of Bo varies from 10 to 50 mT, or Bo>B~. For such a case, the model of Kugel et al. [ 16 ] predicts J~(H) behaviour like that given by the curve FC in

fig. 9 (b). At lowest bo values, superposition of the DC-bias field Bo leads to the increasing of AC losses (see figs. 6 and 7, sample 2). This can be explained by the lower values of Jc on curve FC in comparison to that before the step in Jc(H) dependence of ceramics. By further increasing the AC field amplitude bo (bo exceeds the value of the step in Jc(H) dependence) this situation has to change (see fig. 10) and, averaged for a cycle, JcFc will become higher than averaged Jc without DC-bias field - curves 3 and 1 respectively. Thus we can explain the existence of the regions in which AC loss minima appear. It is worth noting a very important feature of the AC critical current density in high-T¢ ceramics. This is the circumstance that the moment value Of Jc during the cycle can vary significantly. Particularly, for the part of the cycle in which the magnetic field decreases, intergrain field Ho (after the theoretical model accounting for the Bean-Livingston barrier [ 16 ] ) almost vanishes and Jc significantly increases. This behaviour has been experimentally confirmed by many authors as "critical current hysteresis". To our minds, this may be a possible reason for observed discrepancies in values of j ~ c and j ~ c for polycrystalline ceramics doped with silver and tellurium in the present work and for textured samples in ref. [15].

5. Conclusions

The energy dissipation in high-Tc superconductors ( p o l y c r y s t a l l i n e Y B a 2 C u 3 O y doped with Ag and Te)

JC J

,

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b°3

I I ~\\

',

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,

I

t

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E.S. Vlakhov et al. /AC losses in YBCO ceramics doped with Ag and Te

u n d e r s u p e r p o s e d A C a n d D C m a g n e t i c field h a v e b e e n i n v e s t i g a t e d u s i n g a lock-in t e c h n i q u e . B e a n ' s critical state m o d e l is a p p l i e d to e s t i m a t e i n t e r g r a i n critical c u r r e n t density. T h e s u p e r p o s i t i o n o f t h e D C m a g n e t i c field results in the a p p e a r a n c e o f regions w i t h m i n i m a in d i s s i p a t i o n w h i c h is e x p l a i n e d by a t h e o r e t i c a l m o d e l a c c o u n t i n g for t h e B e a n - L i v i n g s t o n b a r r i e r in a s y s t e m o f s u p e r c o n d u c t i n g grains. T h e i n f l u e n c e o f Ag a n d Te a d d i t i o n s o n i n t e r g r a i n critical c u r r e n t d e n s i t y a n d its m a g n e t i c field d e p e n d e n c e is discussed.

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[ 5 ] K.H. Miiller and A.J. Pauza, Physica C 161 ( 1989 ) 319. [ 6 ] Y. X u, W. Guan and K. Zeibig, Appl. Phys. Lett. 54 (1989) 1699. [ 7 ] J. Orehotsky, M. Garber, Y. Xu, Y.L. Wang and M. Suenaga, J. Appl. Phys. 67 (1990) 1433. [81 S. Zanella, L. Jansak, D. Lee and K. Salama, Physica C 180 (1991) 373. [9] M. Ciszek, A.J. Zaleski, J. Olejniczak and L.J.M. van de Klundert, Physica C 185-189 ( 1991 ) 2135. [10]Y. Dimitriev, Y. Ivanova, E. Gattef, E. Kashchieva, E. Vlahov, V. Dimitrov and A. Staneva, J. Mat. Sci. Lett. 10 ( 1991 ) 394. [ 11 ] E.S. Vlakhov, V.T. Kovachev, M. Polak, M. Majoros, Y.B. Dimitriev, S. Jambazov, E. Kaschieva and A. Staneva, Physica C 175 (1991) 335. [ 12 ] C.P. Bean, Phys. Rev. Lett. 8 ( 1962 ) 250. [13]W. Ekin, T.M. Larson, A.M. Hermann, Z.Z. Cheng, K. Togano and H. Kumakura, Physica C 160 (1989) 489. [14]J. Orehotsky, K.M. Reilly, M. Suenaga, T. Hikata, M. Ueyama and K. Sato, Physica C, submitted. [15] Y. Yang, S. Ashworth, R.G. Scurlock, R. Webb and Z. Yi, Supercond. Sci. Technol. 3 (1990) 282. [16] K.I. Kugel and A.L. Rakhmanov, Superconductivity physics, chemistry and technics 4 ( 1991 ) 2078 (in Russian ). [ 17 ] T.R. Askew, R.B. Flippen, K.J. Leary and M.N. Kunchur, J. Mater. Res. 6 ( 1991 ) 1135. [18] M. Majoros, F. Hanic, M. Polak and M. Kedrova, Mod. Phys. Lett. B 3 (1989) 981.