1962 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 5, NO. 2, JUNE 1995

Superconducting T1-Pb-Ba-Sr-Ca-(3-qAg) Thick Films (5-20 pn) Prepared Using a Commercial Spray Pyrolysis System and 2-Zone Furnace Annealing

Douglas L. Schulz, Philip A. Parilla, and David S. Ginley National Renewable Energy Laboratory, Golden, CO USA

James A. Voigt, E. Peter Roth, and Eugene L. Venturini Sandia National Laboratories, Albuquerque, NM USA

A b s t r a c t - A s p r a y pyro lys i s r o u t e t o superconducting Pb-, Sr-, and Ag-substituted T1- Ba-Ca-Cu-0 thick films has been developed. First, a precursor powder with the stoichiometry ~ b 0 . 4 6 ~ ~ 0 . 4 0 ~ ~ 1 . 5 2 ~ ~ 1 . 8 6 ~ ~ 3 . 0 0 ~ ~ ( ~ ~ 0 . 3 7 ) (PBSCCO) was synthesized with the particle size of this powder being subsequently reduced to 4-6 p m using a ball mill. Next, this powder was mixed with ethyl cellulose binder and sprayed with a n a i r b r u s h using a n N2 ca r r i e r gas. Flexible polycrystalline Y 2 0 3 : Z r 0 2 (YSZ) as well as single-crystal MgO, SrTiOg, NdGaOg, a n d LaA103 substrates were mounted on a hot plate with typical growth temperatures of 80-100 "C. Growth times of -10 min provided films which were 5-20 p m in thickness. After a n intermediate 0 2 anneal to remove the organic binder, these films were subjected to a flowing 2- zone thallination process. Very well g- a x i s oriented ~ ~ 0 . 7 0 ~ ~ 0 . 3 6 ~ ~ 0 . 3 9 ~ ~ 1 . 5 8 ~ ~ 1 . 9 8 ~ ~ 3 ~ ~ (TP-1223) phase material was obtained for films grown on single-crystal LaA103. Films grown on all other substrates surveyed in the study gave rise to i m p u r i t y phase f o r m a t i o n a n d / o r incomplete reaction. The PBSCCO and TP-1223 films were characterized by 8/28 and o - r o c k i n g curve X-ray diffraction (XRD) analyses, scanning electron microscopy (SEMI, variable temperature magnetic measurement, and inductively coupled plasma atomic emission spectroscopy (ICPAES).

I. INTRODUCTION

High current applications, such as superconducting magnets and underground power transmission cables, have been one of the main goals for the use of high-temperature superconducting (HTS) materials. A requirement for these applications is the ability to maintain high critical current densities (Jc) while in the presence of significant magnetic field. An optimum high critical temperature (T,) is also desirable as is a film thickness in the 10-20 pm range. Of the HTS systems, the TIiBa2Ca2Cu309 phase is a viable candidate for these applications due to its high Tc and the position of its irreversibility line versus Y-Ba-Cu-0, Bi-Sr- Ca-Cu-0, and other TI-Ba-Ca-Cu-0 materials [ 11. Indeed, 2-3 pm thick films of Ag-doped TliBa2Ca2Cu309 have been

shown to exhibit Tc = 104-107 K with zero-field critical current density (J , (zF~~K)) > lx105 A/cm2 and J c ( 6 0 ~ ) > lx104 A/cm2 in a 2 T field (parallel to the c-axis) [2]. The addition of Ag in these films reduces the processing temperature by -35 "C and enhances liquid phase formation thereby accelerating T11Ba2Ca2Cu309 phase growth kinetics and improving phase purity [3].

Pb and Sr substitution of T1-Ba-Ca-Cu-0 materials has been reported to favor the formation of single-T1 layer phases vs. double-T1 layer phases with the TP-1223 phase possessing a Tc = 114 K [4], [ 5 ] . Material of this composition has been successfully used for powder-in-tube (PIT) processing of a flexible 24 m superconducting tape with J , ( z F ~ ~ K ) = lx104 A/cm2 [6]. However, the Jc properties of this PIT material degrade severely under magnetic fields [6]. This weak-linked behavior, caused by a lack of texturing and absence of low-angle grain boundaries, has severely impeded development of TI-based PIT conductors

The synthesis of open-faced tapes provides an alternative to the PIT process. For example, c-axis textured TliBa2Ca2Cu309(Ag) films 1 pm in thickness have recently been fabricated on Ag foil by an electrodeposition method with J c ( z F 7 7 ~ ) - 7x104 Ncm2 and J c ( 7 7 ~ ) -lx104 A/cm2 at 5 T (field II to the tape) [8]. A key question is the ability to extend the results reported for 1-3 pm films to the thicker 10-20 pm thick films of interest for conductors.

We have recently reported the formation of c-axis oriented TP-1223 films on LaAlO3 by spray pyrolysis [9]. Electrical characterization of these 5-21 pm thick films gave Tc onset values of 106-1 15 K with Tc zero reached by 99-101 K and transport J c ( 7 7 ~ ) up to 2 . 9 ~ 1 0 ~ A/cm2. A mixed stronglweak-linked magnetic field dependence was observed for these films at 77K and 0.4 T. We report in this communication the extension of this work on thick TP-1223 films by spray pyrolysis.

[71.

II. EXPERIMENTAL

Single crystal (100)-oriented LaA103 (DOWA, Tokyo, Japan), (1 1 1)-oriented SrTi03 (Ceres, North Billerica, MA), (001)-oriented NdGa03 (GFI Corp.), (1 00)-oriented MgO (MarketTech Intl., Pittsburgh, PA), and flexible polycrystalline CeraflexTM YSZ (MarketTech Intl., Pittsburgh, PA) substrates were rinsed in methanol prior to film deposition. Precursor powder with nominal

1051-8223/95$04.00 0 1995 IEEE

1963

stoichiometry Pbo.46B ao.40Sr 1.52Ca 1.86cu 3 .OOOX, prepared as previously reported [lo], was ground with a ball mill to a particle size of 4-6 pm (U.S. Mesh -230/+325). The spray pyrolysis ink was prepared by mixing 2 g of the precursor powder and 1 g of ethylceHulose/diglyme/ethanol HTS vehicle binder (ZYP Coatings, Oak Ridge, TN) into 7g absolute ethanol. This ink was sonicated for 30 min immediately prior to the spray deposition. A Paasche VL- Setm airbrush was employed for film depositions with a nitrogen pressure of 105 kPa that aspirated the ink and formed the spray mist. A substrate temperature of 80-100 "C and growth times of -10 min provided films 5-20 p m in thickness. These precursor films were next subjected to an oxygen anneal at 700-800 "C for 60-75 min to remove the binder. Two films were assayed at this point with stoichiometry and crystalline phase development being determined (vide infra). Thallination was performed in a flowing 2-zone reactor process which is described elsewhere [8]. The variables for the thallination process were held constant for the samples reported in this study with a 1 2 0 3 source zone temperature of 770 "C, a film zone temperature of 93 1 'C, and a 2 h reaction period.

HI. RESULTS AND DISCUSSION

A. PBSCCO Precursor Films

Two PBSCCO films on LaA103 were assayed for crystalline phase and chemical composition using XRD and ICPAES analyses. After spray deposition, both films were subjected to a flowing oxygen anneal; one at 700 'C for 75 min and another at 800 "C for 60 min. XRD analyses of both these PBSCCO precursor films showed the formation of the same phases, i.e., BaPb03, Ca2PbO4, Cao.4Sr0.602, Ag, and Ca2Cu03 (Table I), even when subjected to different annealing conditions. ICPAES analyses showed no Pb-loss during either of these anneals with the original P b 0.4 6 Bfa 0.40 S r 1 .5 2 C a 1 .8 6 C U 3 .000 stoichiometry maintained. The suppression of Pb evolution, i.e., formation of volatile PbO (g), is presumably due to the crystallization of stable Pb-alkaline earth oxides. Furthermore, the formation of BaPb03 precludes the development of BaC03; a phase which is known to impede superconductor phase formation. SEM analysis showed these films consist of < 1 pm particles connected in a porous network.

B. TP-1223 Films on YSZ

The synthesis of TP-1223 films on flexible materials is desirable for conductor development. Therefore, flexible YSZ (Ceraflexm) was employed as a substrate for film deposition. After the binder oxygenation anneal and flowing 2-zone thallination, these films were characterized by 8/28 XRD analyses. Phases observed were unoriented TP- 1223,

TABLE I

XRD 8/28 ANALYSES OF TP-BASED THICK FILMS. Substrate Crystalline Phase Formation PBSCCO precursor Ca2CuO3

Mgo (100) srTi03 (l l ) NdGaOg (001)

BaPbOg > Ca2PbOq - Cao.4Sro.602 - Ag >

poly-YSZ PbZrOg > cCiO83cU02 - TP-1223a > Ag TP-1223a > Ca0.83Cu02 > TP-1212 > Ag

TP-1223a - c ~ & u 0 2 > TP-1212 > c u o - Ag TP-1223 >> TP-1212 - Cao83CuO2 - CaO -

Ag LaA103 (100) TP-1223 >>> TP-1212 - Ag a non-oriented TP-phases. All others are c-axis oriented.

Ca0 .83Cu02 , Ag, and PbZrOg (Table I). Substrate outdiffusion and concomitant formation of PbZrO3 appears to preclude the use of YSZ as a substrate. The extent of this substratelfilm reaction was probably enhanced in the present example versus a single-crystal YSZ substrate due to the larger surface areas associated with the CeraflexTM.

C. TP-1223 Films on MgO and SrTiOj

The phase and morphological development of TP- 1223 films on MgO and SrTiO3 were quite similar. Figure 1 is an SEM photograph of a TP-1223 film on MgO. As evidenced by this photo and a similar one for TP-1223 on SrTi03, there is a lack of texturing of the TP-1223 phase on these single-crystal substrates. Furthermore, 8/28 XRD analyses indicate the presence of unoriented TP-1223, TP-1212, Ca0.83Cu02, and Ag for the films on both substrates with CuO additionally observed in the film on SrTi03 (Table I). The lack of texturing for films on MgO and SrTiO3 might be explained as a consequence of lattice-mismatch between the TB-1223 film and the substrates, 10.2% and 11.6%, respectively (Table II). This evaluation infers a substrate- induced growth phenomenon which will be discussed (vide infra).

~~

Fig. 1. SEM photograph of a TP-1223 film on MgO.

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TABLE I1

Physical Properties of Substrates and TP-1223 Materials Phase a(& b(A) c(A) % m

~ ~ 1 0 3 5.377 5.377 5.377 0.6 1 1

SrTiO3 6.764 6.764 6.764 11.6 10 NdGaOg 5.417 5.499 7.717 0.2/1.7 9 TP-1223 3.824 3.824 15.41 (>l l )

mismatch (ppm/K)

MgO 4.213 4.213 4.213 10.2 13

- D. TP-1223 Films on NdCaOj

8/28 XRD analysis of a film on NdGa03 shows &-axis oriented TP-1223 phase formation (Table I). Also present at lower levels are E-axis oriented TP-1212 (- 5 %), Ca0 .83Cu02 (- 10 %), CaO (- 5 %), and Ag. SEM characterization of this film shows some regions where large (> 100 pm2) platelets formed on the film surface. However, large voids were also observed. The observations presented here, i.e., the formation of mostly c-axis TP-1223 superconductor with evidence for partial melting, are preliminary with the conditions employed for thallination optimized for TP-1223 films on LaAlO3 films and not for TP-1223 films on NdGa03.

E. TP-1223 Films on LaAlO3

Nearly phase-pure c-axis oriented TP- 1223 phase formation was observed for films on single-crystal LaAlO3. A 8/28 XRD pattern shown in Figure 2 indicates highly c- axis oriented TP- 1223 phase formation with trace signals from TI-1212 and Ag. As a qualitative comparison, the ratio of the peak heights of the TP-1223 (006) to the TP-1212 (005) reflections in this example is 117:l; less than 1 %. o- rocking curve analysis of the TP-1223 (006) peak gives a full-width at half-maximum (FWHM) value of 0.8-0.9 O

indicating extremely c-axis textured material. By way of comparison, a blank single-crystal LaA103 substrate gives a FWHM values of 0.2 '. A very well-melted TP-1223 film morphology is observed for this 10 pm thick film by SEM (Fig. 3) with excellent film densification as compared to the

Fig. 2. 8/28 XRD pattern for a TP-1223 film on LaA103.

Fig. 3. SEM photo of a 10 pn thick "-1223 film on LaA103.

porous PBSCCO precursor film. The surface morphology of these thicker films (-10 pm) in many respects is superior to that observed for thinner (1-3 pm) TI-based films [ l l ] , [12]. However, microcracking is also evident in the SEM photograph (Fig. 3). This phenomenon is probably due to a mismatch in the coefficient of thermal expansion (CTE) between the TP-1223 material and the LaAlO3 substrate. As a consequence of the microcracking, it can be inferred that the CTE of TP-1223 is larger than that of LaA103 (Table 11).

Determination of the electrical transport properties was essentially precluded by the microcracks within the film. As a consequence, both Tc and Jc for these TP-1223 on LaAlO3 films were determined magnetically. One 10 pm thick TP- 1223 on LaAlO3 film exhibited a Tc = 103 K as evidenced by the onset of diamagnetism. Magnetic critical current density (Jcm) values were determined using the Bean Model [I31 as applied to a 125 pm x 250 pm geometry [14]. The area over which the model was applied, i.e., 125 pm x 250 pm, is similar to an average size of a region without microcracks as determined by SEM analysis. Table I11 shows the Jcm data as a function of variable temperature and applied magnetic field. The zero-field 5K data gives an extraordinarily high Jcm value of almost IO7 A/cm2. The film exhibits good in- field behavior at 5K with Jcm remaining greater than lo6 A/cm2 under 4.75 T. The 20 K variable field data follows the same trend with less than one order of magnitude decrease in Jcm up to 4.75 T. Even at a temperature of 40 K, Jcm remains above lo4 Akm2 out to 4.75 T. However, at higher temperatures of 60 K and 76 K, a strong field dependency is noted with a Jcm becoming less than lo3 Ncm2 under fields of 2.75 T and 1.00 T, respectively. This Jcm data is quite encouraging and could be used to support one of the two following perspectives: either, the grains within the areas bounded by microcracks are strongly linked or, the areas bounded by microcracks are essentially single grains of high Jc material. Further evaluation of the microstructure of these

1965

N. CONCLUSION TABLE III

Jcm DATA FOR A TP- 1223 FILM ON LaA103 Temp 5K 20K 40K 6 0 K 76K OT I 9x106 3x106 1x106 6x105 3 x 1 6

0.50 T 1.00 T 1.75 T 2.75 T 3.75 T 4.75 T

7x106 1x106 3x105 7x104 6 ~ 1 0 ~ 5x106 9x105 3x105 4x104 <lo3 4x106 9x105 3x105 lx104 <lo3

3x106 8x105 8x104 <lo3 <lo3 2x1O6 7x105 3x104 <lo3 < I O 3

3x106 9x105 2 x 1 ~ 5 <lo3 <lo3

H Field I Jcm (A/cm2)

films shall be determined using microanalytical X-ray diffraction.

F. Pb-Loss Upon Thallination

The stoichiometries of two TP-1223 films were determined using ICPAES analysis. An identical composition of T10.70Pb0.36Ba0.39Sri .58Cal.98Cu3.000~ was measured for a highly c-axis oriented, nearly phase-pure (TP-1223:TP-1212 = 58) TP-1223 on LaA103 film and a non-oriented TP-1223 + impurities on MgO film. This means that versus the post-binder annealed PBSCCO films also assayed in this study, there is -20 % Pb-loss upon thallination in the flowing 2-zone system. In marked contrast are the results of Aselage et al., who report zero Pb- loss for TP-1223 pellets annealed for 72-120 h at 920 "C [15]. The observed stoichiometric differences for Pb in TP- 1223 films prepared using nearly identical precursors [ 161 may be explained as a consequence of differences between the two thallination systems as follows: first, the static 2-zone approach in [ 151 provides a quasi-equilibrium where PbO (g) is contained by diffusion barriers; second, for the flowing 2- zone approach described herein, no such diffusion barriers exist and, in fact, the reaction ambient is actively purged thereby allowing outdiffusion of PbO (g).

G. TP-1223 Phase Growth Mechanism

Under the growth conditions employed, excellent morphological and phase development was only observed for TP-1223 films on LaA103 single-crystal substrates. The function of substrate type upon TP-1223 film quality, as assessed qualitatively by SEM and XRD, might be arranged in the following order: LaA103 > NdGa03 > SrTiO3 - MgO > YSZ. With the exception of YSZ which is not included in the comparison due to interfacial reaction, there seems to be a direct correlation between this qualitative film quality measure and lattice mismatch (Table 11) with film quality degrading as the lattice mismatch increases. This lattice matching requirement coupled with the observation of microcracks provides a prospective path to high transport Jc TP-1223 materials where a lattice-matched buffer layer might be deposited on a CTE-matched substrate prior to TP-1223 film synthesis.

An ink spray pyrolysis route to thick (5-20 pm) TP-1223 films has been established. Precursor PSBCCO films show no loss of Pb upon annealing in oxygen at temperatures up to 800 "C for 60 min. There seems to be a correlation between lattice mismatch and quality for the substrates employed in this study with high quality TP- 1223 films observed only for those synthesized on LaAlO3 (001). Although transport electrical properties are limited by microcracking, magnetically-determined Jcm values for these 10 pm thick TP-1223 on LaA103 films approach those measured for thin epitaxial films less than 300 A in thickness.

ACKNOWLEDGMENT

The authors wish to gratefully acknowledge Douglas J. Arent and Anna Duda for assistance with X-ray diffraction characterizations. This research was funded by the Department of Energy (Contract No. DOEKH10093-264).

(Manuscript received October 17, 1994.)

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[I61 The precursor films in [15] have no Ag-substitution.