Critical current density enhancement by phase decomposition of YBa2Cu4O8 into YBa2Cu3O7−σ and CuO

PHYSICA ELSEVIER

Physica C 251 (1995) 366-370

Critical current density enhancement by phase decomposition of YBa2Cu408

into Y B a 2 C u 3 0 7 _ 8 and C u O

J. Krelaus a, K. Heinemann a,*, B. Ullmann b, H.C. Freyhardt

a

a Institutftir Metallphysik der Universitiit Gi~ttingen, G&tingen, Germany b Institut ftir Technische Physik, KFK Karlsruhe, Karlsruhe, Germany Received 1 April 1995; revised manuscript received 8 May 1995

Abstract Bulk YBa2Cu408 (Y-124) is prepared from YBa2Cu307_ 8 (Y-123) and CuO by a powder-metallurgical method. The superconducting features of the Y-124, in particular critical current densities and activation energies, are measured resistively using a four-probe technique and magnetically using a Faraday magnetometer. In a second step the Y-124 is decomposed at high temperatures. The intragranular critical current density is measured at different annealing times, tA, in order to determine and discuss the characteristics of the jc(tA) curves.

I. Introduction The intragranular critical current density, j~, of a superconductor is determined by the microstructure of the single grains. Defect structures are commonly expected to serve as effective pinning centres for flux lines if their size is comparable to the coherence length ~, which is for high-temperature superconductors (HTSC's) in the range of 1 to 100 A [1]. Suitable defect structures within this range of magnitude are e.g. oxygen vacancies, dislocations, stacking faults, grain boundaries but also small normal conducting or insulating precipitates. The pinning behaviour of precipitates is not well investigated because it cannot be observed separately from other pinning mechanisms. Additionally, it is difficult to

* Corresponding author.

introduce suitably sized precipitates into superconducting grains. One way to achieve this is the decomposition of homogeneous YBa2Cu40 8. Y-124 is a superconducting phase (Tc = 81 K) in the Y - B a - C u - O system. It exhibits an additional CuO e layer in the ab plane leading to a temperature-independent oxygen content. Therefore, Y-124 is the (composition-dependent) equilibrium phase of the Y - B a - C u - O system. It gets unstable at high temperatures and low oxygen partial pressures and decomposes into Y-123 and CuO [2]. A heat treatment of Y-124 samples above the decomposition temperature ( = 850°C in air) should therefore be a suitable means to produce Y-123 with small CuO precipitates. An enhancement (about one order of magnitude compared to pure Y-123) of the intragranular jc of Y-123 + CuO prepared via the decomposition of Y-124 had been reported [3].

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J. Krelaus et al. / Physica C 251 (1995) 366-370

This paper is organized as follows: In a first part the preparation of Y-124 sinter samples is described. Then, the superconducting properties of the non-decomposed Y-124, such as critical current densities and Kim-Anderson activation energies, are presented. In the last part of the paper the decomposition process is examined; the critical current density of the decomposed material is compared to that of the Y-124 precursor material.

2. Sample preparation The preparation of the Y-124 precursors follows the route given by Jin et al. [4]. A 1:1 mixture of commercially available Y-123 and CuO powders is solved in an appopriate amount of nitric acid. The solution consists of nitrates of the metals. It is dried at about 180-200°C to decompose the strongly hygroscopic Cu(NO3) 2 into Cu2(OH)aNO 3 [5]. After being denitrified at 960°C (in air) for 4 h [6], the powder is pressed into pellets at ~ 400 bar and sintered at the optimized sintering temperature of 815°C in flowing oxygen. A Y-124 phase content of more than 80% after two days of sintering is found. In order to prepare pure samples a total sintering time of 8 days is required. The formation of the Y-124 phase is observed by XRD and by criticaltemperature (T~) measurements. According to these measurements the resulting samples consist mainly of YBa2Cu40 8. Small amounts of Y2BaCuO5 are detected by X-ray diffraction (XRD). A residual Y-123 phase content cannot be detected by XRD, but is seen by measuring T~. It has to be emphasised that this residual Y-123 phase is not expected to impact the results of the decomposition experiments which will be described later since the microstructure of Y-123 is not changed (apart from reversible oxygen loss) by the decomposition process.

3. Superconducting features of the Y-124 For comparison with other HTSC systems, typical superconducting parameters of the sintered material are presented (for more details see also Ref. [7]). Measurements of the irreversible magnetisation of the samples are performed at different temperatures

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B [T] Fig. 1. Intragranularcriticalcurrentdensityof YBa2Cu4Os, calculated from magnetic-hysteresisloopsby using Bean's critical-state model.

T < Tc in applied fields 0 ~t 40 K, it drops to values ~< 10 4 A / c m 2. In order to determine activation energies, U0, the relaxation behaviour M(t, B, T) was measured for t = 0 . . . 1500 s. Uo(B, T) was calculated applying the Kim-Anderson model [11]. In this model the magnetisation follows - in the logarithmic decay regime - the equation (a): M(t) = M~ - M 1 ln(t/t o) [12]. Mc is the Bean critical magnetisation and M 1 is the slope of the M versus In(t) curve with M 1 = M c k B T / U o. t o is a microscopic time constant in the range of 10 -° to 10 -12 s. In a typical relaxation experiment, however, the decay of the magnetisation is described by the following equation (b): M(t, B, T ) = M o - M 1 ln(1 + t / c ) . Here z is a macroscopic time constant (.~ 10 s in our experiments), describing the initial region of non-logarithmic relaxation [13] and M 0 = M(t = O, B, T). For t >> ~- Eq. (b) becomes (b'): M --- M 0 - M 1 I n ( t / z ) which has the form of Eq. (a) with the parameters

J. Krelaus et al./Physica C 251 (1995) 366-370

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Fig. 2. Kim-Anderson activation energies Uo from relaxation measurementson YBazCu408. The different curves correspondto different assumptions of "c/t o. r / t o = 101° is assumed to be most adequate to our measurements.

M 0 and z instead of M c and t o. Eliminating M e from Eq. (a) by using Eq. (b') then leads to Eq. (c): U o = k B T ( M o / M 1 + I n ( r / t 0 ) ) . This elimination is useful since the critical magnetization M c is experimentally not accessible whereas M 0 is. The time constant t o is not accessible in our relaxation experiments. Therefore the calculation of Uo was performed assuming several values of to, respectively, the relation z / t o. Fig. 2 shows the values obtained in this way. They amount to 100 meV and are comparable to activation energies of other HTSC systems (see, e.g. Ref. [14]). The different curves show the dependence of the calculated activation energies on different assumptions of the

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ratio z / t o , as explained above. With increasing temperature this ratio becomes more and more important. Thus, Kim-Anderson activation energies can only be compared with specification of the assumption of ~'/ t o. For 0 < B ~< 6 T and 4.2 K ~< T ~< 77 K, the critical transport current density j~r(B, T ) is measured across a sintered Y-124 sample, using a resistive four-point technique, see Fig. 3. The values for j tr are small ( = 102 A / c m 2 at 4.2 K), due to the polycrystalline nature of these samples. From the broadening of the resistive transition, p ( T ) , in applied magnetic fields activation energies, U~)eS(B), are calculated via p ( T , B ) ot e x p ( U / k a T ) . In Fig. 4 they are compared to similarly measured values for Y-123 [15]. The activation energies U0~es for Y-124 are about one order of magnitude smaller than those of Y-123. U0~ must not be confused with U0 from the relaxation measurements. It may rather be regarded as an empirical parameter describing the characteristics of sintered Y-124 (mainly due to weak links of the grain boundaries), see e.g. Ref. [16].

4. Decomposition

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The decomposition experiment is performed at 960°C and 900°C, respectively. In contrast to other investigations the samples are heated in normal air by putting thin Y-124 platelets (thickness ~< 0.5 mm) on a pre-heated A120 3 substrate plate. Similarly they are quenched to room temperature by moving them onto a cold Al203 plate. The advantage of this method compared to heating both fur-

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Fig. 6. jc(tA) at B = 2 T, T = 4.2, 10, 40 K for the samples from Fig. 5 (annealing at T = 960°C).

Fig. 5. jc(B) at T = 4.2 K for samples annealed up to different annealing times at the decompositiontemperature 960°C.

nace and sample to the desired temperature is a much better definition of annealing time and annealing temperature. This allows one to avoid long temperature ramps due to an inertia of the furnace (like in Re/. [3]). A numerical calculation which assumes a homogeneous thermal diffusion coefficient, k = 10 -6 m2/s, shows that the annealing or quenching temperatures are reached within ~< 2 s leading to heating/quenching rates of = 400 K / s [17]. A first set of samples has been decomposed at 960°C. Annealing times were 10, 50, 100, 500, and 1000 s. Short annealing times are not observed to enlarge the average grain size during the decomposition process, and so the grain size is assumed to stay constant ( D = 3 Izm). In Fig. 5 the field-dependent Jc measurements are shown at 4.2 K. Similar curves are obtained for T = 1 0 K and 40 K. For B = 2 T, the Jc values versus annealing times, tA, are shown in Fig. 6. A maximum of Jc is observed already for a short annealing time of 10 s. The maximal value of jc is --~ 5 times higher than j~ for the initial Y-124. For tA ~> 50 s, j~ drops down to values which are only = 2 . 5 - 3 times higher than the starting Y-124 value and stays nearly constant (a slight increase with increasing tA may be explained by a growth of the grains which has been disregarded for calculating j~). This saturation value of j~ is comparable to jc of pure Y-123 [18]. Thus, Fig. 6 clearly demonstrates that during the decomposition of Y-124 into Y-123 and CuO the intragranular critical current density passes through a maximum. For TA = 960°C, this

maximum is reached within the first 50 s after the start of the annealing. In order to investigate the influence of the decomposition temperature on jc(tA) the decomposition temperature is lowered to TA = 900°C. The result of the Jc measurement is shown in Fig. 7. Again, Jc is plotted versus t A for B = 2 T. Temperatures were 4.2 K, 40 K and 77 K, respectively. As in Fig. 6, Jc passes through a maximum during annealing. For low temperatures the effect is weak, whereas at higher temperatures it becomes more pronounced. This behaviour may be explained by strong pinning centres (e.g. small precipitates) arising and vanishing again during the process. In contrast to Fig. 6 the annealing time for maximum Jc is much longer (between 30 and 90 s), due to the lower temperature which reduces the reaction velocity. At this stage of our investigations the true origin of the Jc enhancement was not yet determined. Maybe, as proposed by Re/. [19], the enhancement

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370

J. Krelaus et al. /Physica C 251 (1995) 366-370

takes place in the Y-124 itself by the introduction of stacking faults or dislocations which precedes the decomposition process. On the other hand, it seems likely that the Y-123 formed during the process is strongly perturbed by defects. Thus, it is inappropriate to ascribe the Jc enhancement only to precipitations within the Y-123 material. In order to clarify these problems, transmission electron microscopy (TEM) investigations of decomposed samples are in progress. One preliminary result of these investigations is the presence of Y-123 inclusions within Y-124 grains even for the initial Y-124. It seems probable that these inclusions serve as nuclei for the very fast decomposition and also as pinning centres in the Y-123. This model could explain the observed maximum in the j~(t) curves, because after having grown to more extended regions, the Y-123 domains lose their ability to serve as pinning centres in the Y-124. Then the intrinsic j~ of these Y-123 regions becomes observable.

5. Summary In this paper superconducting parameters of bulk Y-124 material, such as critical current densities and activation energies are presented. Jc is measured at different stages of the decomposition of Y-124 into Y-123 and CuO. jc exhibits a maximum at very short annealing times. For the decomposition temperature TA = 960°C a j~ enhancement by a factor of 5 is found compared to the Y-124 material (i.e. a factor --2.5 compared to Y-123), after a 10 s annealing. For the decomposition temperature TA = 900°C only a very slight enhancement is observed. The annealing time for maximal current is shifted to larger values ( 3 0 - 9 0 s). The high enhancements found by Ref. [3] could not be confirmed. In contrast to Ref. [3], we do not see the origin of the j~ enhancement in CuO precipitates. It seems much more probable that during the decomposition process many defects are introduced into the Y-124, as pro-

posed by Ref. [19]. Our suggestion basing on recent TEM studies is a pinning on small Y-123 inclusions within the Y-124 material.

Acknowledgement This work was supported by the B M F T / B M B F under grant number 13N5493A.

References [1] See e.g.A.M. Campbell and J.E. Evetts, Adv. Phys. 21 (1972) 21. [2] E. Kaldis and J. Karpinski, Eur. J. Solid State Inorg. Chem. 27 (1990) 143. [3] S. Jin, T.H. Tiefel, S. Nakahara, J.E. Graebner, H.M. O'Bryan, R.A. Fastnacht and G.W. Kammlott, Appl. Phys. Lett. 56 (1990) 1287. [4] S. Jin, H.M. O'Bryan, P.L. Gallagher, T.H. Tiefel, R.J. Cava, R.A. Fastnacht and G.W. Kammlott, Physica C 165 (1990) 415. [5] L. Bonoldi, M. Sparpaglione and L. Zini, Appl. Phys. Len. 61 (1992) 964. [6] A. Maignan, M. Hervieu, C. Michel and B. Raveau, Physica C 208 (1993) 116. [7] J. Krelaus, B. Ullmann, K. Heinemann and H.C. Freyhardt, paper presented at the EUCAS, G6ttingen (1993), in: Applied Superconductivity, vol. 1, ed. H.C. Freyhardt (DGM Informationsgesellschaft, Oberursel), p. 783. [8] K. Heinemann, PhD. thesis, Universitlit Gfttingen (1987). [9] W. Mexner and K. Heinemann, Rev. Sci. Instr. 64 (1993) 3336. [10] C.P. Bean, Phys. Rev. Lett. 8 (1962) 250. [11] P.W. Anderson and Y.B. Kim, Rev. Mod. Phys. 36 (1964) 39. [12] M.R. Beasley, R. Labusch and W.W. Webb, Phys. Rev. 181 (1969) 682. [13] A. Gurevich and H. Kiipfer, Phys. Rev. B 48 (1993) 6477. [14] W. Mexner, PhD. thesis, Universit~it G6ttingen (1995). [15] B. Ullmann, PhD. Thesis, Universifiit G6ttingen (1992). [16] M. Tinkham, Phys. Rev. Lett. 61 (1988) 1658. [17] J. Krelaus, DiPloma Thesis, Universitllt G6ttingen (1994). [18] M. Ullrich, PhD. Thesis, Universit~it G6ttingen (1993). [19] A.K. Srivastava, T.V.C. Rao and O.N. Srivastava, Supercond. Sei. Technol. 7 (1994) 551.