Hightemperature xray diffraction analysis for Bi2Sr2CaCu2O yT. Hasegawa, T. Kitamura, H. Kobayashi, H. Kumakura, H. Kitaguchi, and K. Togano Citation: Applied Physics Letters 60, 2692 (1992); doi: 10.1063/1.106875 View online: http://dx.doi.org/10.1063/1.106875 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/60/21?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Directional isothermal growth of highly textured Bi2Sr2CaCu2O y Appl. Phys. Lett. 63, 982 (1993); 10.1063/1.109865 Optimization studies of high current Bi2Sr2CaCu2O x superconducting tape systems AIP Conf. Proc. 251, 408 (1992); 10.1063/1.42095 Structural changes upon electrochemical insertion of lithium into the Bi2Sr2CaCuO8+y high Tcsuperconductor AIP Conf. Proc. 251, 366 (1992); 10.1063/1.42090 Temperature dependence of energy gap in Bi2Sr2CaCu2O8 by high resolution electron energy lossspectroscopy AIP Conf. Proc. 273, 401 (1992); 10.1063/1.43587 In situ xray investigation of the melting of BiSrCaCuO phases Appl. Phys. Lett. 59, 3640 (1991); 10.1063/1.105605

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High-temperature x-ray diffraction analysis for Bi,Sr2CaCu20y T. Hasegawa, T. Kitamura, and H. Kobayashi Showa Electric Wire and CabIe Co., Ltd., 2-I-I Odasakae h7awasaki-ku, hFawasaki, Kanagawa 210, Japan

H. Kumakura, H. Kitaguchi, and K. Togano National Research Institute for Metals, 1-2-I Sengen, Tsukuba, Ibaraki 305, Japan

(Received 22 November 1991; accepted for publication 30 March 1992)

In the case of Bi-2212/Ag composite tapes, the partial-melting technique is useful to obtain a highly textured microstructure. The liquid phase plays an important role during this process. In this letter, we studied the phase transition of oxide phases in the Bi-2212/Ag composite tapes during partial melting and solidification by using the high-temperature x-ray diffraction technique. In the partially molten state, the liquid phase coexists with various kinds of (Ca,Sr)-Cu oxides whose Cu content decreases with increasing temperature. The Bi-2212 phase forms directly from the coexistent state of the liquid phase and the (Ca,Sr) &!u03 compound. c-axis alignment of Bi-2212 phase starts subsequently to the beginning of precipitation. Bi-2212 phase decomposes and the Bi-2201 phase appears below 800 “C. At lower temperature, the Bi-2201 phase is more stable than Bi-2212 phase and can easily form in the super-cooled liquid phase.

The partial melting and solidification technique for Bi,Sr,CaCu,O, (22 12) /silver composite tape is widely known as a useful method to obtain a c-axis oriented mi- crostructure and an excellent critical density (J,) value.“2 In this fabrication process, the composite tape is heat treated once up to the partially molten state where Bi-22 12 decomposes into liquid and solid phases. In the solidifica- tion subsequent to the heating, Bi-2212 crystallizes with a c-axis orientation. The particles thus obtained are closely connected and each grain boundary carries high intergrain superconducting current. The liquid phase plays an impor- tant role in this way. Although Polonka et al. of the Ames Laboratory has reported a detailed study on the phase change during the partially molten state,3 information on the precipitation and alignment of Bi-2212 particles still remains incomplete. We present, in this letter, a study on the phase transformation in the partially molten state and the subsequent solidification process by using the high tem- perature x-ray diffraction (HTXRD) technique, scanning electron microscopy (SEM), and energy dispersive x-ray analysis (EDX) .

Bi-2212/Ag composite tapes were prepared by using the pyrolysis technique of organic acid salts.4 As for the starting reagents, bismuth octylate Bi(C,H&OO) 3, strontium octylate Sr(C7H&OO),, calcium octylate Ca(C!,H1,COO)a and copper octylate CU(C,H&OO)~ were used. An appropriate amount of those octylates with the cation ratio of Bi:Sr:Ca:Cu= 2:2: 1:2 were perfectly dis- solved into the adequate quantity of an organic solvent. Oxide coating layer on silver tape of 2 mm in width and 0.1 mm in thickness was formed by painting with this organic solution and calcined at 500 “C in air. This process was repeated for several times and the thickness of the coating layer was controlled by changing the number of times of repetition.

HTXRD measurements were performed for calcined tapes on a platinum heater with Cu k’cr source. The heater was covered with silver tape to avoid the influence of plat-

inum on the specimen. The specimen was heated at the rate of 5 “Urnin up to 880 or 900 ‘C, and then cooied at 0.2 “C’min. The linear position sensitive detector was set to correct data for 28 range of 4”9” and 20”58”. Temperature was determined by using a Pt-Pt( Rh) thermocouple set to the platinum heater. An all-HTXRD measurement was carried out in air.

The results of the HTXRD study were summarized in Fig. 1. The measurement was carried out for the tape spec- imen of 30 pm in the oxide layer thickness. The specimen was heated at the rate of 5 “Urnin up to 880 “C!, and then cooled at 0.2 “C/min. In this figure, we plotted the peak intensities of major phases; (006) for the 2201 phase, (008) for the 2122 phase, and (002) for the (C!a,Sr),CuOs phase. In the as-calcined state, the oxide layer is com- posed of intermediate products, which give microcrystal- linelike broad peaks in the x-ray diffraction pattern. Dur- ing heating, the peak for the 220 1 phase appears initially at about 700 “C and its intensity increases with the increase of temperature. However, the intensity starts to decrease gradually when the temperature exceeds 770 “C. At 850 “C, which is 20 “C below the partial melting temperature (870 “C), the peak for the 2201 phase completely disap- pears, while the peak of the 2212 phase appears and grows until the partial melting starts. In the partially molten state, almost all peaks of solid phases are identified as those for the (Ca,Sr)-Cu-0 phases and Ag. When the tem- perature reached 880 “C!, we started the cooling process. At the initial stage of cooling, the peak intensity of the (Ca,Sr)-Cu-0 gradually increases without any sign of the appearance of other phases. However, at about 840 “C, the peak for the 2212 phase suddenly appears and rapidly grows to much higher intensity as can be seen in the figure. This rapid increase of (008) peak height cannot only be explained by the increase of volume fraction and indicates that the 2212 particles grow and align in a c-axis orienta- tion simultaneously. The (Ca,Sr)-Cu-0 phase produced by the partial melting is retained into the solid state, although

2692 Appl. Phys. Lett. 60 (21), 25 May 1992 0003-6951/92/212692-03$03.00 @ 1992 American Institute of Physics 2692 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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0 E-2201 (006) 0 Bi-2212 (008)

700 750 800 880 800 750 700

Temperature (c)

0 Liquid + Solid (Sr-Ca-Cu-0) m Bi-2212 + Sr-Ca-Cu-0

FIG. 1. Summary of the result for HTXRD measurement for the E-2212/ Ag composite tape of 30 pm in oxide layer thickness.

the peak intensity slightly decreases through the solidificai tion reaction. During further cooling in the solid state, the peak for the 2201 phase appears again at around 800 “C in addition to those for the 2212 and (Sr,Ca)-Cu-0 phases. This suggests that the 2212 phase already becomes unsta- ble at this temperature and tends to transform into the more stable 2201 phase.

In order to have more detailed study on the phase change in a partially molten state, another sample contain- ing the thicker oxide layer of 60 ,um in thickness was heated to a higher temperature of 900 “C and then cooled down. Figures 2 and 3 show actual diffraction patterns during heating and cooling, respectively. Just as in the above-mentioned experiment, 2212 phase was observed at 850 and 860 “C during heating (Fig. 2), however, it com- pletely disappears when the partial melting starts. In the partially molten state (870-900 “C), the diffraction pattern changes with raising the temperature as shown in the fig- ure, however, all peaks other than those for Ag can be identified as (Ca,Sr) -Cu-0 compounds with different stoi- chiometry, (Ca,Sr),C+Os [ (3,5) phase], (Ca,Sr) CuO, [( 1,l) phase], (Ca,Sr),Cu03 [(2,1) phase], and (Ca,Sr)O [( 1,O) phase]. Those compounds appear in the above se- quence with increasing temperature, i.e., the compound with higher (Ca,Sr)/Cu ratio seems to be stable at higher temperature. This behavior qualitatively agrees with the results of HTXRD for Bi-2212/Ag composite reported in Ref. 3. The Ca/Sr ratios of these (Ca,Sr)-Cu-0 com- pounds determined by the EDX analysis are 1/0.5-l, l/l, and l/2-10 for the (3,5), (l,l), and (2,l) phases, respec- tively. At the highest temperature of 900 “C, only the ( 1,O) phase exists in the liquid except for the Ag, and this state is maintained until 870 “C! during cooling as shown in Fig. 3. However, the (2,l) phase appears at 865 “C with the decrease of the (1,O) phase. At the temperatures below

(yf.,jz, ~,,) ,, ‘f’ 8: --.+vs&~.~+-.~-~~~.~~,J -..+

partial melting point

FIG. 2. HTXRD peak protiles at various temperatures in heating pro- cess for the Bi-2212/Ag composite tape of 60 pm in oxide layer thick- ness. Solid circle, (ll), (35), (21), and (10) represent Bi-2212 phase, (Ca,Sr)CuO,, (Ca,Sr),Cu,Os, (Ca,Sr),Cu03, and (Ca,Sr)O, respec- tively.

835 “C, the 2212 phase appears, however, both (1,0) and (2,l) phases are retained into the solid state. In the previ- ous run in Fig. 2, where the highest temperature was 880 “C, the (Sr,Ca)-Cu-0 phases observed in the solid state were (3,5) and ( 1,l) phases. Therefore, the compo- sition of the (Sr,Ca)-Cu-0 phase, which exists in the 2212 matrix as an impurity phase after solidification, is influ- enced by how high the sample is heated in the partially molten state.

In addition to the HTXRD study, microstructural ob- servations with SEM were performed for the fractured cross sections of some specimens which were prepared as follows. The specimen was heated up to 900 “C! with the ratio of 10 “C/min in air by using a usual electrical furnace and cooled with the ratio of 0.2 Wmin to 890, or 880, or 870 “C and then quenched to the room temperature. Figure 4 shows the SEM photographs on the fractured cross sec- tion and x-ray diffraction patterns of the samples quenched from 890, 880, and 870 “C. The sample quenched from 870 “C shows a highly oriented grain structure in the whole of the oxide layer, while the samples quenched from 890 and 880 “C show less grain alignment. All major peaks in the x-ray diffraction profiles for the sample quenched from 870 “C are identified as c-axis oriented 2212 phase, while for the samples quenched from 890 to 880 “C, the 2201

2693 Appl. Phys. Lett., Vol. 60, No. 21, 25 May 1992 Hasegawa et al. 2693 This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP:

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Quenched from 890°C

&j&ew-&y-+$f~ 830” 30 40 SO 60

28 . : 2212. (ll):(srca)cuox , (35):(srca)3cu50~

(21):(SrCa)zCuOx , (1o):(sIca)ox

t solidification

FIG. 3. Phase change with decreasing temperature for the same specimen as shown in Fig. 2. Solid circles (21) and (10) represent Bi-2212 phase, (Ca,Sr),CuO,, and (Ca,Sr)O, respectively.

phase has stronger peaks than the 22 12 phase. The 2201 phase is considered to be formed directly from the super- cooled liquid phase. Those observations indicate that the 2212 phase is formed in a narrow temperature range just before the solidification complete, which is around 870 “C in this case.

The phase and microstructural changes during heating and slow cooling was studied by using the HTXRD tech- nique for the Bi-2212/Ag composite tape prepared by py- rolysis of organic acid salts. In the partially molten state, several (Ca,Sr)-Cu-0 compound phases with different stoichiometric composition exist in the liquid phase; the (Ca,Sr):Cu ratio increasing with the increase of

Quenched from 870°C

ii;

0~~12phase 0:22.01 phase

FIG. 4. SEM photographs for the cross sections and XRD peak profiles for the specimens quenched from 890, 880, or 870°C in the cooling pro- cess subsequent to the heating up to 900 “C. Solid circle and open circle represent Bi-2212 phase and Bi-2201 phase, respectively.

the temperature. During the cooling process, the growth and the alignment of the 2212 particles start just before the solidification complete and rapidly progresses. The (Ca,Sr)-Cu-0 phases existed in the liquid phase are re- tained even after the solidification as the impurity phase in the 2212 matrix.

‘J. Kase, K. Togano, H. Kumakura, D. R. Dietderich, N. Irisawa, T. Morimoto, and H. Maeda, Jpn. J. Appl. Phys. 29, L1096 (1990).

‘5. Kase, N. Irisawa, T. Morimoto, K. Togano, H. Kumakura, D. R. Dietderich, and H. Maeda, Appl. Phys. Lett. 56, 970 ( 1990).

3J. Polonka, M. Xu, Q. Li, A. I. Goldman, and D. K. Finnemore (un- published) _

41. Amemiya, H. Kobayashi, T. Nakamoto, and T. Hasegawa, IEEE Trans. Magn. MAC-27, 905 ( 1991).

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