J O U R N A L O F M A T E R I A I , S S C I E N C E : M A T E R I A L S I N E L E C T R O N I C S 4 (19931 77 82

M icrostructural properties of Pt-doped YBa2Cu307_x high- Tc superconductor prepared by melting method

J O N G - H Y U N PARK Korea Atomic Energy Research Institute, Daejeon 305-606, Korea

H Y O U N G - W O O K KIM, J I N - T A E SONG Department of Materials Engineering, Hanyang University, Seoul 133-791, KOrea

We have studied the effect of platinum addition on the superconducting properties of YBa=Cu307_, (1 23) compound and elucidated from the metallurgical point of view the mechanism of formation of the fine dispersion of Y2BaCuO~ (211) particles in YBa2Cu307 superconductor prepared by a melting method. In this study, the amounts of BaCu02 and CuO-rich phases unreacted during the peritectic, reaction were markedly decreased by the 2111 powder addition. The 211 particles of Pt-free sintered samples were 8-10 lam in size, but in 1 wt% Pt-added samples 211 particles were finely dispersed in the 123 matrix and the size of 211 particle was about 1 2 pro. The critical temperature (T~.,~ro) of Pt-doped samples was 91.5 K and the transport critical current density (Je) of Pt-doped samples was much more than 104A cm 2 The high J~ and fine dispersion of 211 particles of Pt-doped YBa2Cu307_ , superconductor are attributed to Ba4CuPt20~ compounds formed during the partial melting, which were considered as nucleation sites of 211 particles, rather than Pt itself,

1. I n t r o d u c t i o n Since Bednorz and Miiller [1] reported possible high- T= superconductivity in the L ~ B ~ C u O system, oxide superconductors have been the subject of tremendous research and developmental imerest throughout the world. Thcrcaftcr, sevcrM super- conducting materials with high critical temperature (T~) and critical magnetic field [2~t] were found and they became of great interest, because of their own feasibility for the application of superconductors in the field of power transmission lines, superconducting magnets, electronic devices, and so on. However, as is well known, these superconducting materials have a very low critical current density (d~).

This low Jr is associated with the intrinsic aniso- tropy of the superconducting current, weak linking of grain boundaries, and inhomogeneity and low density of sintered bulk superconductors. It was reported that these problems have been diminished by new pro- cessing techniques such as melt-textured growth (MTO) [5], quench and melt growth (QMG) [6], me l t~owder melVgrowth method (MPMG) [71, and so on. All these methods are techniques to increase J~ with attempts to bring about crystallographic texturing or to introduce effective pinning centres of magnetic flux [8, 9] into the superconducting matrix. However, these processes are difficult to control and use in mass production because they include a melt- quenching process.

Recently, Yoshida etal . [ t0] and Morita et aL [11] reported that a small amount of platinum addition

0 9 5 7 - 4 5 2 2 © t993 Chapman & Hall

to YBa2Cu3OT_x (t23) superconductors made thc YzBaCuOs (211) phase very fine and almost ulzi- formly distributed in the superconducting t 23 matrix, with the result that the critical current density in- creased much more compared to 123 superconductor. However, the mechanism of formation of a fine dis- tribution of 211 particles and of the large increase of J~ still remains to be solved. It seems that many re- searches must be earned out hereafter in order to clarify these mechanisms, in this study, we examined the effect of platinum addition on the superconducting properties of 123 compound and the mechanism of fine dispersion of 211 particles in this melting process from the metallurgical point of view.

2. Experimental procedure First, in order to elucidate the mechanism of fine distri- bution of 211 particles in thc Pt-added YBa2Cu30 7_ superconductors° we prepared Y2BaCuO5 phase and Pt compound (Ba4CuPt209) by solid-state reaction and mixed them with YBa2Cu307. x powders. To prepare their precursor powders, Y203, BaCO 3, Cu,O reagents and platinum dioxide (PtO2) were used and thoroughly mixed in the atomic ratios of Y:Ba:Cu =2 :1 :1 and B a : C u : P t - - 4 : l : 2 (0412 compound).

After mixing, precursor powders were calcined at 950 °C for 5 h, followed by sintered at 1150 °C for 10 h in the ease of 211 phase and, in the case of 0412 Pt compound, calcined at 800 °C for 24 h.

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Next, 211 powder and various amount of Pt, i.e. 0, 0.1, 1 and 5 w t % , were added to 123 powder and mixed. Thereafter, for the preparation of melt-textured YBa2Cu307_ ~ (123) bulk sample, these were com- pacted under a pressure of 3 tonne cm 2 and sintered at 950 °C in air for 10 h. Sintcred samples were par- tially melted at 1100 °C for 30 mm and subsequently solidified with a very stow cooling rate between 1010 and 960 °C for the growth of 123 grains. Considering the structural change, a directionally solidified sample was annealed at 930"C for 2 h and also annealed at 550-°C for more than 48 h followed by slow cooling to room temperature under an atmosphere of oxygen. Structural analysis was carried out by X-ray diffrac- t ion (XRD) and microstructure was observed by op- tical microscope and scanning electron microscope (SEM). The critical temperature (T~) and critical cur- rent density (J¢) were measured by the four-probe method.

3. R e s u l t s a n d d i s c u s s i o n 3.1. Effect of PtO 2 addition on the properties

of melt-textured YBa2Cu307 x Fig. [ shows XRD patterns of Y2BaCuO 5 (211) and Ba4CuPteO 9 (0412) phases, synthesized by the solid- state reaction process for this study. They coincide well with the patterns of Yoshita et at. [10] and wc thus confirmed that they were synthesized exactly. Next, we examined XRD patterns of melt-textured 123 samples without Pt and with Pt addition. Fig. 2 shows X-ray diffraction patterns of Pt-free (123 + 10wt % 211) and 123 samples. It was observed that CuO-rich phase unreactcd during the periteetie reaction disap- peared following the 211 addition accorditlg to the reaction 211 phase (Y2BaCuOs) + liquid phase (CuO, BaCuO2) -~ YBaaCu307 x. It was thought that more

131

~11

090 142

320 = 133 004

221 340 34.2

10 2 0 3 0 4 0 5 0 6 0 70 8 0

20 (deg)

Figure 1 XRD powder patterns of(a} Y~BaCuO~, (b) Ba4CuPt209.

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addition of 211 powder (but less than 30wt%) en- hanced the peritectic reaction occurring in 123 oxide superconductors and then, as Fig. 2 displays, (001) peaks become larger compared to the 123 sample alone.

We also observed that the partial melting process remarkably improved the crystal texture of 123 oxide superconductor, thc a-b plane of which is oriented parallel to the growth direction. Although, we ex- pected that (123 + 211 + Pt) samples have the same considerable promotion of (0 01) preferred orientation such as (003), (005), (006), etc. it was not observed strongly. However, we think PrO 2 addition does not destroy the detinitc .growth texturing during the melting process.

Fig. 3 shows the electrical rcsistance of all samples prepared by the partial melting process as a fnnction

DO5 0 16

OO3

II 1o3 002 II ~110 007

_~-.

'b) ~ "

10 20 30 40 50 60 70 80 20 Cdeg}

Figure 2 XRD patterns (ff melt-textured samples: (a) 123 + 211 (10wt %), (b) 323 alone_ (V) 21 I, (FI) CuO.

'].0-

0.9

0.B-

0.7- c)

0,5

1 o.4L

0.3

0.2-

0.1-

0 8o 9'o t6o 11o lao l io 1go 17o 18o Temperature [ K)

Figure 3 Normalized resistance versus temper~tt~tre c~trve for Pt- free and Pt-added melt-textured samples: (-..) 0 w l % Pt, ( - - - ) 0.1% Pt, ( - - - ) 1 wt % Pt, ( ) 1 wt % 0412.

of temperature. The onset transition temperatures were 91-92 K for both melt-textured samples without Pt and with Pt addition, but the offset temperatures (T~ .. . . . ) are not the same, i.e. in the former the trans- ition width (ATe) was about 6 K, and in the latter the resistance drops very sharply and ATo was 0.5 1 K.

Fig. 4 shows a voltage versus current curve for the melt-textured sample (t23 + 30 wt % 211 + 1 wt % Pt). The critical current density (J~) of melt-textured 123 sample was about 201)0 A mn -% but in the case of 1 wt % Pt-added sample, J, could not be determined bccausc it was too high for our measuring apparatus.

1 0 - "

~o ~ / 0 wrapt (2~ A c~ -~)

10-o / o ,~

C ~ IA)

Figure 4 Critical current (I~) versus voltage at 77 K for Pt-free and Pt-added melt-textured samples.

l wt~?l

(~u& mrc ~an 1(111)0 A eft2)

3~0 ~ . _ _ l _ _ 40 46 50

Probably the value of J¢ was much more than 104 A c m - ~ when we consider the favourable micro- structures for high Jr, which wcrc shown later.

3.2. Effect of Pt02 addit ion on the microstructures of melt-textured YBa2Cu307 x

As previously stated, the addition of 211 powder greatly reduced the CuO-rich phase in the 123 matrix and the orientation factor of texture was increased with increasing addition of 211 powder. Fig. 5 shows SEM micrographs of melt-tcxtured samples mixed with both 3 0 w t % 211 and various amount of platinum (0, 0.1 and t wt %) and 1 wt % Ba4CuPt20 9. It was observed that the particle size of the 211 phase bccame much finer on the addition of Pt and de- creased as the amount of Pt increased. Fig. 6a and b show the energy-dispersive X-ray (EDX) analyses for YBa2CuaO~_~ and Y2BaCuOs, respectively.

tn the case of 5 wt % Pt-addcd sample, a large-sized complex Pt compound appeared herc and there in the 123 matrix. Fig. 7a shows the morphology of this Pt compound and its EDX spectrum is shown in Fig. 7b. Saito et al. [:21 reported that BaO may react rapidly with platinum black due to the strong basicity of Ba 2+,. and y3÷ and Cu 2+ arc expected to have a

Ftzure 5 SEM micrographs of Pt and Ba4CuPt2Og-added melt-textured samples: Pt (a) 0 wt % (b) 0.1 wt % (e) 1 wt %; (d) Ba,~CuPt20¢ l w t % _

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Ba C u B a

C u

p L I i i = I i L i i r E l 0 . 0 10 .0 0 0 10 ,0

Figure6 Energy-d~spersive X-ray spectra: (a) YBazCu_~OT_., (b) YsBaCuO~.

Pt

J . . . . . . . . . . . . . . . . . . I I ~ I l I I I I 1

o.o l O . 0 (b)

l,'~gure 7 Microstructure of 123 + 211 (30wt %) + Pr (5wt %) spe- cimen: (a) SEM micrograph showing Pt compound, (b) energy- dispersive X-ray spectrum of Pt compound.

weaker stabilizing effect on Pt 4+. In addition, Pt reacts with Ba and Cu cations in the melt to form the Ba¢CuPt209 compound and Y cations gradually react with Ba~CuPt209 to form Y2Ba3Cu2PtO10 and YzBazCuPtOs.

Considering our results above and the report of Saito et al. [12], this large-sized compound will prob- ably be a certain Pt compound which is mainly com- posed of Ba and Pt components. Yoshita et aL [10] suggested that Ba4CuPt209 (0412) phase among vari- ous Pt compounds may play an important role in improving the critical current density of 123 oxide superconductor. Therefore, instead of Pt we added the 0412 compound which we prepared bcforchand to 123

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superconductor, and investigated its effect on the microstructure. Its microstructure is shown in Fig. 5d. As this shows, the addition of Ba~CuPtzO 9 compound made 211 particles in the 123 matrix very fine, just like Pt addition, but a little more fine.

In order to elucidate the mechanism of fine disper- sion of 211 particles observed above, we prepared 21 t only, (211 + Pt), and (2tl + Ba4CuPt2Og) sintered compacts and examined the kinetic characteristics of 211 particle growth. Fig. 8 shows their appearances. As shown in Fig. 8, Pt-free sintered samples showed an 8 10 pm size of 211 particle, but in the sample with l w t % Pt addition, the size of 211 particle was 1 2 gin. Here, some facts are to be noted. Firstly, although there were many large and small 211 par- ticles in the Pt-free sample, 211 particles in Pt-added samples had almost the same fine size. Secondly, there were large differences in the growth behaviour of 211 particles in Pt-free and Pt,added samples. For the Pt- free sample, the size of 211 particles became gradually larger as the sintering time increased. In contrast, there was no coarsening of 211 particles in the 1 wt % Pt-added samples, i.e. they were almost the same as at the beginning. For example, even after being sintered for 10 h, the size of 211 particles showed no difference with those of a sample sintercd for 3 h. Thirdly; the addition of0412 compound made 211 particles a little finer.

As Yoshida et aI. [10] reported, there may be two ways to explain the mechanism by which the size of 211 phase particles decreased substantially and fine dispersion occurred due to the Pt addition. One is the mechanism to suppress grain growth due to the change of the inter'facial energy of the 21l fiquid interface, and the other is the mechanism by which added Pt or its compound acts as the nucleation site of 211 particles. Considering our results, it was thought that the fine dispersion of 211 particles results from the latter mechanism. McGinn et aL [ i3 ] reported that in their study of the microstructure of 123 com- pound (Y2BaCuO~) with BaSnO 3 additions, the addi- tion of BaSnO3 brought about a substantial reduction in the size of211 particles, and suggested that it might be caused by the reduction of interracial energy in the 211 liquid interface. However, we could not find ap- preciable growth of 211 particles in the Pt-added sample, even though the sintering time was increased to 15 h at 1150°C. Saito et aL_[12] reported that Pt added to 123 superconductor did not react with 211 phase. This also seems to support our suggestion.

From tllese results, it seemed that added ptatinum provides the nucleation site of 211 particles rather than reducing the interracial energy between 211 phase and liquid 123 phase. Therefore, the fine dis- tribution of 211 particle probably results from the role of Pt or Pt compound as a nucleation site. It is important to note that this suggestion is also sup- ported by the fact that when we added PtOz to 211 sintered powder, i.e. after the formation of 211 'phase, the refinement of 211 particles could not occur.

However, it is not clear whether or not the nucle- ation site is Pt or Pt compound, tn our experiment when the 211 precursor, to which I wt % Ba4CuPtzO 9

Figure 8 SEM micrographs showing 211 particle size after sintering at 1150 °C: (a) Y_~O5 + BaCO 3 + CuO ~ 211 compact sintered for 3 h; (hi a~ (~) but sintered for I0 h; (c) Y203 + BaCO3 + CuO + Pt (1 wt %) ---, {211 + Pt) compact sintered for 3 h; (d) as (c) but sintered for 10h; (e) YzO_~ + BaCOa + CuO + Ba4CuPI209 (1 wt %)

(211 + Ba,CaPt2Og) compact sintcred for 10 h.

compound was added, was calcined and sintered at t 150°C, 21t particles were very line and almost uni- formly distributed with a size of about I gin, as shown in Fig. Be. In addition, Ba4CuPt20 9 cmnpotmd is formed at low temperature (less than 700 °C) and has nearly the same lattice constant as the 211 phase. F rom these facts, it was considered that Ba4CuPtaO 9 compound played a role as the nucleation site of 211 phase. Additional work is in progress in order to elucidate this problem in more detail.

4. Conclusion From the results mentioned above, we conclude:

1. PtOz much improved the critical current density (J,) of YBa2Cu_~O 7 _~ superconductors. This improve- ment results from the fine dispersion of 211 partMes due to the addition of platinum.

2. With the addition of 1 wt % Ba4CuPt~O 9 com- pound, 211 particles became very fine and almost uniformly distributed with a size of about 1 pm.

3. The fine dispersion of 211 particles in the 123 matrix is attributed to Ba~CuPt20~ compound which acts as a nucleation site of211 particles, rather than Pt itself.

Acknowledgement The authors wish to acknowledge the support of the Korean Ministry of Science and Technology for this research.

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Received 17 July and accepted 26 August 1992

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