Preparation and growth of (Bi,Pb)SrCaCuO superconductor fibersWilder CarrilloCabrera, Wolfgang Göpel, Germán F. de la Fuente, and Horacio R. Verdún Citation: Applied Physics Letters 55, 1032 (1989); doi: 10.1063/1.102455 View online: http://dx.doi.org/10.1063/1.102455 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/55/10?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Bi(Pb)SrCaCuO superconducting composite tapes prepared by the powder method using an Ag sheath J. Appl. Phys. 67, 3443 (1990); 10.1063/1.345331 Highdensity BiPbSrCaCuO superconductor prepared by rapid thermal melt processing Appl. Phys. Lett. 55, 1466 (1989); 10.1063/1.101584 Effects of Fe on the superconductivity of 110 K phase superconductor BiPbSrCaCuO Appl. Phys. Lett. 55, 1247 (1989); 10.1063/1.101667 Magnetization and flux creep in textured Bi(Pb)SrCaCuO oxide superconductors Appl. Phys. Lett. 55, 185 (1989); 10.1063/1.102401 Preparation and magnetic properties of BiPbSrCaCuO superconducting ceramics Appl. Phys. Lett. 54, 2253 (1989); 10.1063/1.101140

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Preparation and growth of (Bi,Pb)wSr~Ca .. Cu~O superconductor fibers Wilder Carrillo-Cabrera and Wolfgang Gapel institute o/Physical and Theoretical Chemistry, University of Tiibingen, auf der 1iforgenstelle 8, D-7400, Tubingen, Federal Republic of Germany

German F. de la Fuentea) and Horacio R. Verdun Fibertek, Inc., 5JO-A Herndon Parkway, JIerndon. Virginia 22070

(Received 23 January 1989; accepted for publication 12 July 1989)

The laser floating zone (LFZ) method has been used to grow (Bi,Pb)-Sr-Ca-Cu-O superconductor fibers from solution-derived ceramic precursors (nominal Hi J.8 Pb035 Sf I.S7 Ca2 CU3 Ox), prepared under different sintcring conditions. The as-grown LFZ fibers were semiconducting. After annealing at 855 and 800 °C in air, the fibers were metallic. A second anneal at these temperatures optimized the superconducting properties of the fibers [1~ (midpoint) = 111 K and Tc (zero) = 106 K]. The LFZ-grown fibers contain highly oriented superconductor grains and their electrical properties depend strongly on precursor thermal processing.

The laser floating zone (LFZ) method has recently been used to grow fibers of several high Tc oxides. Attempts to grow YBa2Cu30 7 Ii fibers] resulted in mUltiphasic samples due to loss of CuO from the melt during growth. These fibers superconducted upon subsequent thermal annealing. The Bi-Sr-Ca-Cu-O system offered a very stable moUcn zone and greater ease of processing without apparent component melt 105s/ and superconducting Bi2Sr2CaCu20 x fibers have been obtained without post-growth thermal annealing. 3 A study4

of the transformation of a well-characterized Bi2Sr2CaCu20 x superconductor ceramic precurso~ into polycrystalline fibers of similar composition suggests growth rates,;; 6 mm h - I to obtain superconducting fibers of composition similar to the precursor, Faster growth rates (50 mm h -1) yield nonsuperconducting fibers with Bi2Sr2CuO(, as the major phase. An grown fibers, regardless of growth history, become superconductors upon subse­quent thermal annealing. The superconducting fibers are composed of highly oriented platelets with the c axis perpen­dicular to the fiber surface, suggesting high critical current density values along the fiber (in the crystallographic ab plane). Because of the fundamental and practical impor­tance of obtaining highly oriented fibers, the growth of (Hi,Pb) -Sr-Ca-Cu-O superconductor6 (Tc;:::; 110 K) fibers by LFZ is under study in om laboratories and this letter reports preliminary results.

The polycrystaHine ceramic precursors were prepared by codecomposition of a mixture of the corresponding metal nitrates. 7 Powdered Bi]O, (99.999%, Cerac) , PbO(99.99 + %), Sr(NO~')2(99.9965%), Ca(N03 )2(99.9995%), and CuO(99.999%), all from Johnson Matthey, were dissolved in a hot HN03 aqueous solution under constant stirring. After heating several hours with controlled evaporation, r:. light biue solid was obtained and calcined at 800 °C for 1 h under an oxygen pressure of 0.2 bar. The resulting solid was ground, pelletized, and sintered at 830"C under an oxygen pressure of 0.08 bar. Two pellets (~8 g each), A and B, with the same nominal composition but sintering times of 16 and 30 h, respectively, were prepared. To obtain the (Bi,Pb )2Sr2Ca2Cu,OlO I- t; (denoted also as 2223) com-

'" A uthor to whom correspondence should be addressed.

pound as the major phase (Tc ;:::; 110 K), a starting composi­tion with the ratio Bi:Pb:Sr:Ca:Cu/1.8:0.35:1.87:2:3 was uscd.7

The precursors were cut into square cross-section feed rods, mounted on graphite holders, and inserted into the two-beam CO2 laser floating zone apparatus. The bottom chuck was rotated to anew efficient distribution of the fo­cused laser radiation (form a stable molten zone). The top chuck was not rotated. An 8% O2 in Ar mixture flowed through the sealed growth chamber. The melting tempera­ture (optical pyrometer) was ~ 850°C for both samples un­der 0.08 bar oxygen pressure. This temperature is lower than observed for similar samples prepared in air ( - 880 CC) . The fiber pulling rate determines the growth rate, and was set at 15 mm h - I for both types of fibers. Ceramic feeding rates were adjusted to maintain a constant volume molten zone, which allows uniform diameter fiber growth. In the present case, the cross sections ofthe as-grown fibers were somewhat irregular, containing uniform (- 1 mm diam) and nodular ( 1.2 mm max. diam) sections. The nodes were due to sudden changes in the melt temperature, consistent with second phase precipitation, observed during the growth process and related to continuous melt enrichment with a phase which is not incorporated into the growing fiber at the fiber pulling rate. This phenomenon is typical during LFZ processing of incongruently melting materials. To improve the fiber prop­erties thermal annealing cycles were performed (t = 430-855°C, in air). The samples were air quenched after each cycle.

Two fibers (-1 mm diam, 15 and 30 mm length) were cut into 12 mm bars for resistivity measurements. Fiber sec­tions of 5 mm (length) were used for most of the post-an­nealing experiments. Resistivity measurements used the standard four-contact de method (constant current of 3-15 mA) in liquid N2 with the sample inserted in a double-tube system. Nitrogen gas was used as a heat-exchanger. X-ray powder films of the precursors were obtained using a Guin­ier Camera (Cu Ka 1 radiation). X-ray powder diffraction patterns of the fibers were obtained with a commercial stan­dard powder diffractometer (Cu Ka I radiation). Scanning electron microscopy (SEM) photomicrographs were ob­tained using a JEOL 25S microscope. These results will be

1032 Appl. Phys. Lett. 55 (10), 4 September 1989 0003-6951/89/361032-03$01.00 @ 1 SS9 American Institute of PhysiCS 1032

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TABLE I. Selected data for the (Bi,Pb)-Sr-Ca-Cu-O LFZ fibers grown in an atmosphere of 8% 0, -+ 92% AI' at a growth rate of 15 mm h- 1 and a melt temperature of ",,980'C (m.p. ~·850 'C).

Sized Precursor Precursor phase composition T, (fiber)h Fiber (dXlmm7) annealing (a,b,cinA) [ T, (precursor) J

A 1-l.2>< 30 16 h at 830'C 2212 + Ca,CuO, <83K (e) 1<83 KJ

B 1-1.2 X IS 30 II at 830'C 2212(5.38,5.41,30.76) 83-· III K 2223(5.40,5.40,37.16) ['14K]

Ca,CuO, (l2.31, 3.80, 3.29}

"d ~, diameter; I = length. ;, 1: (midpoint) measured after fib!!r post-annealing (see text and Figs. 1 ami 2 for more details). 'Not determined.

detailed elsewhere. Precursor phase analysis by x-ray powder diffraction

(Table n suggests that sintering cycles were not long enough to transform the metal nitrate mixture into a single­phase (Bi,Pb) 2Sr 2Ca2Cul0 10 1- b superconductoro Precur­sor A ( 16 h) did not contain the 2223 superconductor phase, while precursor B(30 h) consisted ofa mixture of 2223 and 2212 superconductor phases and Ca2CuO). The combi.ned solution mixing/solid-state reaction method used requires long sintering to yield the 2223 high T" phase, as does the conventional solid-state route previously followed. H.'> Pre­cursor B had a T, (zero) = 99 K after the 830 "C sintering cycle. Further annealing of precursor B in air at 800 ·C (28 h) resulted in a Tc (zero) = 105 K. The highest Tc (zero resistance) value obtained for this ceramic after a second annealing treatment at 855 ·C(72 h) and 800°C( 19 h) was 107K.

The thermal histories of precursors A and B affected the phase content, and the properties of the LFZ-grown fibers Cfable I). The resistivity temperature dependence of both as-grown fibers indicates a semiconductor with fiber B resis­tivity being smaller. This result was not unexpected, consid­ering that these (high growth rates) fibers were quenched from the melt. This is a result of the very steep axial tempera­ture gradients found between the molten zone and the grow­ing fiber, characteristiC of LFZ growtho Semiconducting be­havior has been found in quenched pellets containing the

Temperature (K)

FIG. I. Resistivity in Ihe 83-300 K range of (a) .~ 12 mm section of fiber A after healing at 800 T for 14 h in air (last annealing of the first series of thermal treatments at T< 830 "C), (b) a smaller section (~5 mm long) of the fiber in ia) aftcr further annealing at 855°C for 40 h followed by 20 hat ROO 0c; (c) a small section (- 5 mm long) oftlber B after annealing itl air at 800·C for 13 h, and after subsequent annealing at 855 T for 27 h fllllowed by 20 hat 800'C {d).

1033 Appl. Phys. Lett., Vol. 55, No.1 0,4 September 1989

2223 andlor the 2212 phase,'! Annealing below the sintering temperature induced superconducting properties in these pellets. The transition temperature and room-temperature resistivity values for this system depend on the rate of cool­ing from the sintering temperature. 10

The top section of fiber A (- 12 mm long) was annealed in air (t = 430-830 °C) 0 At temperatures below 700°C its semiconducting behavior did not change, but its resistivity decrea.sed with increasing annealing temperature. After a 20 min anneal at 830°C in air, the cooled fiber displayed metal­lic behavior and a T" (midpoint) = 83 K was observed. Further annealing of this fiber at 800-830 °C only decreased the magnitude of the resistance, without any change in 1~ .. Figure 1 (a) shows the temperature dependence of the resis­tivity of fiber A after annealing (14 h, 800 CC). When the middle section of fiber B (~5 mm long) was annealed at 800·C for 13 h, a Tc (midpoint) = 83 K was found [Fig. 1 (c)}. Onsets at 100 K indicate the presence of a small amount of the 2223 superconductor phase in both fibers. The top 5 mm section of fiber A and the same section of fiber B were annealed at 855°C for 40 and 27 h, respectively, and subsequently at 800 °C for 20 h; their corresponding resistiv­ity versus temperature curves are shown in Figso 1 (b) and l(d). As in (Pb, Ca, Cu)-rich ceramic pellets,'> an extra an­nealing treatment at 800 °C( 1-2h) was required to observe optimum metallic and superconducting propertieso The fi­bers require a much longer annealing time (up to 20 h) due to their higher density. I I

After long annealing the Tc (midpoint) for fiber A ap-

1.2 ,--------- ------.---- .---- -----:"1 e j

'0, FiberS ,.' I 't -. I

~ 0.81 ," .! j ""

(l.ti.

j) ,,'. .... .

o-=: goll" .~ 04 I .,;; . I

'" i .:;) ., 0.2. .

~ 1; I ooo-r--c'~---,___...._...,_~.....,..-~,~.-.- -r'~~r--r .,-.,-r'""T"'~,_.-__l

o 50 100 ,Sf) 200 250 300

Temperature (K)

FIG. 2. Resistivity as a function oftempemture in the 83- 300 K range ofthe same section of fiber JJ shown in Figs. l( c )-1 (c!) after a second annealing cycle in air at 855 T for 14 h followed by 29 h at 800 T.

Carrillo-Cabrera et al. 1033

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FIG. 3. SEM micrograph obtained i~Jr an axially fractured LFZ-grown fiber derived from precursor B. The long, well-orit:nted grains are parallel to the fiber growth axis.

peared at ~ 110 K and no zero resistance was measured down to 80 K, indicating the presence of the 2212 (Tc ~ 73 Kin LFZ fibers) phase.4 This behavior could be linked to the relatively short sintering time in the preparation of precur­sor A, considering that fiber B yields, after similar thermal treatment, a Tc (midpoint) = 104 K with zero resistance at 98 K. The main diffel'ence between fibers A and B is the thermal history of their precursors (Table I). The supercon­ducting properties of fiber B were optimized by subsequent annealing at 855°C for 14 h followed by 29 h at 800 °C (Fig. 2): the T,. (midpoint) value increased to 111 K, while zero resistivity was measured at 106 K. Fiber B, after annealing in air for 41 hat 855°C followed by 62 h at 800 °C, contains the 2223 superconducting phase as its major component.

The microstructural ch"racteristics of the grown fibers have been studied by SEM. The orientation of the material in a typical fiber grown from precursor B above, and fractured along its growth axis, is shown in Fig. 3. The large grains in the figure are oriented paraHel to the growth axis, as expect­ed for anisotropic LFZ-processed materials due to the very steep axial temperature gradients present during growth. 12

The observed orientation of the grains is expected to yield much higher Jc values than those obtained for polycrystal­line fibers derived from other processing methods. In fact, recent magnetic hysteresis measurements performed on LFZ fibers of the related 2212 bismuth cuprate phase sug­gest Je values greater than 1 X 10' A em - 2 (4 K, 1 T) along the growth axis ofthe fibers. 13

In conclusion, the preliminary results presented in this letter indicate that superconducting fibers with Tc (zero) as

1034 Appl. Phys. Lett., Vol. 55, No. 10,4 September 1989

high as 106 K, displaying favorable orientation, can be ob­tained by LFZ after annealing. As-grown fibers show semi­conducting behavior, most likely as a consequence of their mUltiphasic precursors. The combined nitrate solution/sol­id-state reaction route used for the preparation of these pre­cursors can yield ceramics with a high percentage ofthe 2223 superconducting phase only after a long sintering period, In order to improve the properties of the as-grown fibers and to understand the ceramic to fiber transformation chemistry, it will be more convenient to use single-phase 2223 precursors, as well as to study the effect of the oxygen partial pressure and the pulling rate during growth. These studies are cur­rently under way in our laboratories.

The work performed at Ttibingen was partially support­ed by the Bundesminister fUr Forschung und Technologie, Federal Republic of Germany, the Landesforschungssch­werpunkt 39 Baden Wiirttemberg, and the Fonds der Che­mischen Industrie. The authors are indebted to Dr. Rafael Ibanez and Dr. Daniel Beltran (Universitat de Valencia) for performing SEM experiments, to Dr. Conrado Rillo (lCMA, CSIC-Universidad de Zaragoza) for providing re­sults from magnetization measurements, and to Harald Rit­ter for help with x-ray experiments. Support and encourage­ment by Dr. Walter Koechner is also acknowledged.

'x. P. Jiang, G. W. Qiao, J. G. Hiang, Y. Yu, M. Jiang, Y. L. Ge, Z. Q. Hu, and C. X. Shi, in High Temperature Superconductors II. edited by D. W. Capone n, W. H. Butler, and C. W. Chu (Materials Research Society, Pittsburgh.I'A 1988), Extended Abstracts No. EA-14, p. 125.

OR. S. Feigelsoll, D. Gazi!, D. K. Fork, and T. H. Geballe, Science 240. 1642 (988).

'D. Gazit and R. S. Feigelson, J. Cryst. Growth 91, 318 (1988). 'G. F. de Ill. Fuente, D. Beltran, R. Ibanez, E. Martinez, A. Beltran, and A. Segura, J. Less-Common Met. 150,253 (1989).

'D. Beltran, M. T. Caldes, R. Ibal'iez, E. Martinez, E. Escriba, A. Seltnin, A. Segura, V. Munoz, and J. Martinez, J. Less-Common Met. 150, 247 (1%9).

"M. Takano, J. Takada, K. Oda, H. Kitaguchi, Y. Miura, Y. Ikeda, Y. lomi!. and H. Mazaki, lpn.I. App!. Phys. 27, L1041 (1988). 'U. Endo, S. Koyama.andT. Kawai, Jpn. J. App!. Phys. 27, Ll476 (1988); S. Koyama, U. Elldo, and T. Kawai, ibid. 27, US6! (1988).

'N. Kijima, H. Endo. J. Tsuchiya, A. Sumiyama, M. Mizuno, and Y. OgurL Jpn. J. Appl.l.'hys. 27, Ll852 (1'l88).

"W. Carrillo-Cabrera and W. Gijpel, presented at the 1988 Fal1 MRS Meet­ing, Boston, MA, Nov. 28-Dec. 3, 1988 (to be published).

IOU. Balachandran, Dongill Sbi. D. 1. Dos Santos, S. W. Graham, M. A. Patel, B. Tani, K. Vandervoort, H. Claus, and R. B. Poeppd, Physica C 156, 649 (1988).

"This is consistent with our belief that the ROO"C anneal adjusts the re­quired oxygen content in the volume of the fiber, in order to obtain the high T, superconductor phases. A detailed investigation of the annealing process is in progress.

12R. S. Feigelson, MRS Bull. 13,47 (1988). "C. RiHo, F. Lera, L. M. Floria, R. Navarro, J. Bartolome, R. Ibanez, A.

Beltran, D. Beltran, and G. P. de la Fuente, Solid State CommUl1. (in press).

Carrillo-Cabrera et ai. 1034

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