The preparation of “1223” Tl-Ca-Ba-Cu-oxide superconducting films via the reaction of silver-containing spray deposited Ca-Ba-Cu-Oxide with thallium oxide vapor

PHYSICA

Physica C 205 ( 1993 ) 21-31 North-Holland

The preparation of"1223" T1-Ca-Ba-Cu-oxide superconducting films via the reaction of silver-containing spray deposited Ca-BaCu-oxide with thallium oxide vapor John A. DeLuca, Pamela L. Karas, J.E. Tkaczyk, Peter J. Bednarczyk, Mary F. Garbauskas, Clyde L. Briant and Donald B. Sorensen General Electric Corporate Research and Development, Schenectady, NY 12301, USA

Received 7 July 1992 Revised manuscript received 2 September 1992

A process is described for the preparation of superconductingfilms of "1223" TlxCa2Ba2Cu3Oy(0.65 < x < 1.00) via the reaction at 860 °C of thallium oxide vapor with spray deposited Ca-Ba-Cu-oxide films containing silver. The thallium oxide vapor reaction was carried out in oxygen in a two-zone reactor which permits the independent control of sample temperature and thallium oxide partial pressure. The films have a textured microstructure exhibiting a preferential orientation of the crystallographic c-axis perpendicular to the surface of the polycrystallineyttria stabilized zirconia substrates. Liquid phase formation induced by the presence of silver is believed to be responsiblefor the accelerated film growth observed. Films are prepared routinely with Tc (0) values of 104-107 K and zero-fieldcritical current densities at 77 K in excess of 20 000 A/cmz. Valuesas high as 110 000 A/cm2 have been measured. The excellentin-field characteristics of the "1223" films are reflected in the behavior of a sample having a zero-fieldJ¢ value of 105 000 A/cm2at 77 K for which a Jc> 10 000 A/cmz was measured at 60 K-2 T with the magnetic field applied parallel to the crystallographicc-axisof the film.

1. Introduction Oxide superconductors hold promise for achieving significant i m p r o v e m e n t s in the efficiency, size, and weight of a wide variety of electrical equipment, such as motors, generators, transformers, magnets, and transmission lines. For m a n y of these applications considerable lengths of conductor must be fabricated that have critical current densities of at least 104-105 A / c m 2 in magnetic fields of one T or greater at operation temperature. The need to produce large quantities of conductors economically has stimulated effort worldwide to develop processes for fabricating polycrystalline wires and tapes from oxide superconductors belonging to the Y - B a - C u - o x i d e ( Y B C O ) , B i - C a - S r - C u - o x i d e (BCSCO), and T I C a - B a - C u - o x i d e ( T C B C O ) families [ 1-11 ]. Key to the success of this work is the ability to prepare polycrystalline conductors which have both excellent intergranular connectivity and excellent magnetic flux p i n n i n g characteristics.

The in-field behavior of the YBCO superconductors is superior to that of the BCSCO and TCBCO materials [ 12]. However, despite intense activity, no one has been able to achieve a high degree of intergranular connectivity in polycrystalline forms of the YBCO-type materials in the practical lengths required for use in electrical power applications. On the other hand, significantly greater success has been realized in achieving good intergranular connectivity in long lengths of conductors fabricated from "double B i - O layer" BCSCO-type materials by "powder-in-tube" ( P I T ) methods [ 4,13 ]. However, the in-field behavior of these materials may restrict their use to temperatures < ~ 20 K. Good intergranular connectivity can also be achieved in polycrystalline TCBCO materials [ 14-17 ], although much less work has been reported on the fabrication of wires and tapes from these materials than has been reported for either the YBCO or BCSCO materials [ 18-21 ]. However, interest in the TCBCO materials has been stimulated by reports [ 18,22-26 ] empha-

0921-4534/93/$06.00 © 1993 ElsevierScience Publishers B.V. All rights reserved.

22

./.A. DeLuca el a L / Preparation (?l " 1223" 77-( "a-Ha-( 'u-O lih~

sizing the intrinsically superior in-field behavior of the "single T I - O layer" materials relative to that or" thc "'double B i - O layer" and "'double T I - O layer" superconductors, a b e h a v i o r attributed to stronger caxis coupling between the groups o f C u - O planes in the "'single TI-O layer" structures. This interest is heightened further by the fact that there is no known "'single B i - O layer" BCSCO phase, a situation which may be an unavoidable consequence of the crystal chemistry of the Bi +3 ion [ 2 7 - 2 9 ] . Since the publication of our original report [ 30 ] on the preparation of"2223"" T C B C O films, wc have found that the incorporation o f silver into our spra,~ deposited C a - B a - C u - o x i d e films not o n b d r a m a t ically improves our "'2223"" process but also resulls m a process well suited to the preparation of'" 1223"" films. In this paper we report details o f our "'silvera d d i t i o n " process for the fabrication o f " 1 2 2 3 " thick films which exhibit both the attractive in-field behavior reported by others for "single T I - O layer" phases and excellent intergranular connectivity as evidenced by their DC transport characteristics.

2. Experimental 2. 1. S a m p l e p r e p a r a t i o n

The spray deposition apparatus is illustrated schematically in fig. 1. In the nebulization chamber shown in the diagram an aqueous solution of metal nitrates is converted into a mist by the ultrasonic transducer ( T D K model NB-32B, T D K C o r p o r a t i o n o f America) built into the bottom o f the t e m p e r a t u r e controlled water bath ( 2 0 ° C ) . A Dyna-Sense-7186 controller (Scientific Instruments, Inc. ) in c o m b i n a t i o n with a teflon solenoid valve is used in the liquid level control system. A spraying action is achieved by entraining the mist in a 12.5 SLPM flow o f nitrogen gas and passing the mixture through the 5 m m id exit nozzle. The nozzle to sample distance is 9 cm. The substrates are fastened with stainless steel clips 1o a t e m p e r a t u r e controlled heater block oriented vertically as shown. The heater block is m o u n t e d on an ,T-Ytranslation stage driven by two C o m p u m o t o r LL3C linear motors controlled by a C o m p u m o t o r 3000 Indexer ( P a r k e r Hannifin Corporation, C o m p u m o t o r D i v i s i o n ) . The Indexer is p r o g r a m m e d to

move the heater block at a velocity of 17 c m / s to Form a 7.6 c m X 7.6 cm "'spray pattern" consisting of a series of 24 back-and-forth 7.6 cm horizontal translations spaced - 0 . 3 cm apart (0.1 s dwell between translations) folloveed b} a similar set of translations in the vertical direction. 111 addition to the number of "'spra} patterns", the arnount o f material deposited depends upon the sprayer output which we ha~c found can change with time. Because of the nced lo periodically adjust this drift in output, we monitor the mist density b'~ an optical transmission measurement using fiber optic light guides ( " m i s t densit} sensor" in fig. I) coupled to a "'SMARTEYES A L G " photoelectric source/sensor module (TriTronics Company. Inc. ) Precursor oxide films weighing ~().001 g wcrc prepared on 12.5 r a m × 8 r a m × 0 . 5 m m polycr.~stallinc ytlria-stabilized zireonia subslrates [31] using a 0.18M (total mctal ions) aqueous spray solulion o f barium, calcium, copper, and silver nitrates having a C a : B a : C u : A g mole ratio o f 2 : 2 : 3 : 0 . 3 -7 The spray solution was prepared from calcium carbonate {Johnson Malthey-puratronic grade), barium carbonate (Johnson Matthey-puratronic grade ), silver nitrate (Johnson Matthey-puralronic grade). 5N copper shot (,lohnson Matlhe> ). semiconductor grade nitric acid, and deionized water. The mist output was adjusted so that the required amount of material was deposited aficr a repetition of 12 "'spra~ patterns". The subslratc heater block was mainrained at 2 7 5 ( " during the deposition after which it was heated to and held at 650 C for 5 min and then cooled to 275-C. The samples were transferred lo a movable tray positioned inside an annealing t'urnacc combustion tube through which was maintained a flow o f ultra high purity oxygen. The tray was moved into the furnace hot zone which had been preheated to 5 0 0 C . The sample temperature was p r o g r a m m e d to rise from 500 C t o 845 C in 30 rain at which point the furnace was turned off and the sample allowed to cool under oxygen to 3 0 0 : C ( ~ 2 h). The sample tray was then moved within the combustion tube to a position outside the furnace where it was allowed to cool under oxygen before removal. The precursor oxide films were converted to superconducting TCBCO films in an oxygen ambient via reaction with thallium oxide vapor in the twozone reactor previously described [30]. The "two-

J.A. DeLuca et al. /Preparation of"122Y' Tl-Ca-Ba-Cu-O films

23

~---Metal Nitrate Solution Reservoir 5 mm id

\S\ubstrate

Nozzle \ ~-] d Valve -- \ ~1 I,- Heater Block On j.~m~ ~ II [ Computer Controlled / ~ r / I ~ I x'YTranslati°nStage [

~ Mist Density ~ J Nebulization Chamber

Level Controller

N2 Inlet

Liquid Level Sensor

Metal Nitrate Solution 4-- Temperature Controlled Water Bath

1/2 mil FEP Teflon Membrane

Ultrasonic Transducer

Fig. 1. Schematic diagram of the spray deposition apparatus used to prepare the Ca2Ba2Cu307:0.37Ag precursor films.

step" process schedule shown in fig. 2 was used for this series o f experiments, as we wanted to retard the conversion reaction by m a i n t a i n i n g a reduced partial pressure of thallium oxide until the sample had reached the desired process temperature. It will be noted that the samples and the thallium oxide boat were allowed to cool at a rate o f ~ 5 ° C / r a i n at the end o f the process. All films were prepared at a sample temperature o f 860°C. The thallium oxide content of the samples was d e t e r m i n e d by the temperature o f the thallium oxide source used in the second " s t e p " of the process ( 7 3 5 - 7 5 0 ° C to achieve x=0.65-1.00). After the two-zone reactor process the films were patterned with a test bridge structure as described below and then heated in an oxygen ambient. The sample t e m p e r a t u r e was p r o g r a m m e d to rise from room t e m p e r a t u r e to 600°C at 100°C/h, hold at 600°C for 8 h, and then d r o p to 100°C at 100°C/

h. Tests on 12 samples in which measurements were m a d e both before and after this "oxygen anneal" revealed a 2- to 6-fold increase in Jc ( Z F 77 K ) in every case. Little ( < 2 K ) or no effect on Tc(0) was observed. 2.2. Characterization

ICP analyses have verified that both the precursor oxide films and the final superconducting films retain the Ca: Ba: Cu: Ag cation stoichiometry o f the spray solution, and that the thallium oxide content o f the films agrees with that calculated using the m e t h o d described below. We have found the m e a s u r e m e n t o f weight gain to be a rapid non-destructive m e t h o d for determining the a m o u n t of thallium oxide incorporated into the films. The weights o f the uncoated ( U C ) , precursor oxide coated ( P O C ) , and superconducting oxide

24

J.A. DeLuca el al. / Preparation o/ " 1223" 7 7 - C a - B a - ( ' u - O /ilms

1000

800 STEP 2 STEP

o2 600

I

/ SOURCE

/

400

'

i

200 /

\!

ii

i 1

0 i! 0

20 40 60 80 100 120 140 160 180 200 220 Time (min.)

Fig. 2. A "two-step'" process schedule was used for the conversion of the CaaBa2Cu3OT:0.37Agfilms via their reaction with thallium oxide vapor. The plateau in the sample temperature schedule corresponded to 860 C in all cases. The "first step" in the source (thallium oxide boat) temperature corresponded to 690°C in all cases while the "second step" temperature was varied from 735 C to 750°C depending upon the thallium content desired in the film. coated (SOC) substrates were determined with a microbalance. With the knowledge that the films retain the 2 : 2 : 3 : 0 . 3 7 C a : B a : C u : A g stoichiometry of the spray solution, we represent the average composition of the superconducting films as T1,Ca2Ba2Cu30( 7.5+ J.s~,) : 0.3 7 Ag where, 2[weight S O C - w e i g h t POC] × [697.38] [weight P O C - w e i g h t UC] x [456.74]

bridge. The larger bridge was patterned to permit measurement across the full 4 m m bridge length and each of four I m m segments. The resistance versus temperature characteristic was determined by a lour point prove DC measurement. From this plot we determined the temperature, 7',.(0), at which the resistance became zero. Measurement of the zero-field DC transport critical current density, J,,(ZF 77 K), was made with the same apparatus using a 1 laV/cm criterion. A surface profilometer was used to determine both film thickness and the apparent cross-sectional area of the bridge. The superconducting oxide films were typically - 3 lain thick. ,I~. was calculated by dividing the measured critical current by the apparent cross-sectional area, no attempt being made to correct for sample porosity. Additional transport measurements were made in a variable temperature cryostat containing a c) T superconducting magnet. Contact to the sample was made using silver-paint applied over annealed silver pads. Measurement of the critical current versus magnetic field at each temperature was made having first cooled in zero-field from above T~.. An electric field criterion of l ~ V / c m was used to define .I,. Measurements were made both with the field applied parallel and perpendicular to the substrate. The angular resolution was of the order of a few' degrees. X-ray diffraction ( X R D ) patterns were obtained on a Siemens D500 diffractometer using Cu Kit radiation. SEM micrographs were obtained using a Cambridge Stereoscan 240 scanning electron microscope which included a Tracor Northern energy dispersive spectrometer attachment for obtaining chemical information.

+0.02, and _+0.02 reflects estimated weighing errors. Assumptions made in deriving this expression are that: ( 1 ) silver exists as the metal, (2) thallium exists in the + 3 state, (3) the weight gain of the precursor oxide film is due only to the incorporation of T1203, and (4) errors related to deviations in oxygen stoichiometry from that assumed are negligible. The films were patterned with a test bridge structure using the photoresist process previously described [30]. For this study we used both a "small" 0.4 m m × 0 . 1 m m bridge and a larger 4 m m ×0.2 m m

3. Results Samples with compositions .v< ~ 0 . 6 are multiphase, have poorly developed microstructures, and have unimpressive superconducting properties. Films with ~ 0.6 < . v < ~ 1.0 are superconducting and yield X R D patterns showing predominately the presence of a textured "1223" TCBCO phase in which the crystallographic c-axis is oriented perpendicular to the substrate surface. For samples with ,x~ 1.0 the X R D pattern shows the presence of both the "'1223"" and the "2223'" phases. The X R D pattern of the

J.A. DeLuca et al. / Preparation of"1223"" T I - C a - B a - C u - O films

"2223" phase becomes more dominant as x increases, until at x ~ 1.3-1.5 it consists predominantly of lines from a textured "2223" phase accompanied by weaker reflections identified as belonging to the "2122" phase. The changes in the XRD pattern with x are evident in fig. 3. The XRD pattern shown for a film with x = 0.85 is typical of those obtained for samples with 0.65 < x < 1.00, the composition range of principle interest in this study. It should be noted that XRD patterns for samples prepared by the above process to have the 0 . 6 < x l . 0 composition, but using silver-free precursor oxide films, show no evidence of TCBCO superconducting phases, but rather consist of lines attributed to BaCuO2 plus other unidentified phases. SEM micrographs characteristic of films having 0 . 6 5 < x < 1.0 are shown in figs. 4, 5, and 6. As seen in fig. 4 the films in this composition range have a textured microstructure with plate-like growth features visible on the film surface. The cross-sectional

25

view shown in fig. 5 indicates that the material comprising the films is dense. The nodule-like features apparent in the micrographs, particularly in fig. 4, have been identified as silver rich regions. The variations in the growth features and the voids apparent in the lower magnification micrograph shown in fig. 6 show that the films are not perfectly uniform. Shown in fig. 7 is a micrograph of a silver-free sample (x=0.85). In the initial phase of this study of 10 samples having compositions in the 0 . 6 5 < x < 1.00 range were patterned with the 0.4 m m × 0.1 m m bridge. The Tc values of these samples ranged from ! 04 to 107 K. Shown in fig. 8 is a resistance versus temperature plot that is typical of these as well as all other samples prepared in this composition range. The Jc(ZF 77 K) values of these 10 samples ranged from 8800 to 105 000 A / c m 2. The Jc versus field behavior at different temperatures for the sample with Jc(ZF 77 K) = 105 000 A / c m 2 is shown in fig. 9 for the case

(oo14) (00 16)

z,o2

(002)

(004)

II

(OOS) (000)

I 1oo01 (ool)

.____7

(ooa) I

12.00

ZrI02

(004) I

32100

22.0O

tl

(007)

i 42.00

2O Fig. 3. X-ray diffraction patterns of: (a) a textured "1223" film having the cation composition Tlo.asCa2Ba2Cu3Ago.37. The (00l) lines of the "1223" phase are labeled. (b) A film having the cation composition Tll.o3Ca2Ba2Cu3Ago.37 which shows the presence of both the "1223" and "2223" phases, and (c) a film having the cation composition Tli asCa2Ba2Cu~Ago.37 which shows the presence of the "2223" material as the predominant phase. The (001) lines of the "'2223" phase are labeled. Lines from the substrate (ZrO2) and silver (Ag) are labeled in (a) and (c).

26

J.A. DeLuca et al. / Preparation qf'" 1223" 7"/-C'a-Ba-( u-O./ilm.~

Fig. 4. SEM micrograph of a typical "1223" superconductingfilm prepared by the silver-addition process.

Fig. 5. SEM micrograph of a cross section (snapped sample l of a "'1223" film prepared by the silver-addition process.

of the field applied perpendicular to the substrate surface (parallel to the c-axis). The data for the case of the field applied parallel to the substrate surface (parallel to the a - b planes ) is shown in fig. 10. In fig. I 1 are shown plots of./,, versus field at 77 K for four samples with differing ,Ic(ZF 77 K) values. The variation in the ,I,,(ZF 77 K) values observed for the 10 samples described above and the n o n - u n i form film morphology evident in SEM micrographs such as that shown in fig. 6 prompted us to evaluate our samples from the point of view of the uniformity of J,. over the film area. To this end 4 to 6 samples were prepared at each of the n o m i n a l compositions, x = 0 . 7 , x = 0 . 8 , and x = 0 . 9 , and were patterned with the 4 m m segmented test bridge. The thallium oxide source-boat temperatures used in the "'second step"

Fig. 6. SEM micrograph of a typical"'1223" superconductingfilm prepared by the silver-additionprocess. The micrographwas taken to show the inhomogeneitypresent in the film morphology.

Fig. 7. SEM micrograph of a .vih'cr:l?ee film prepared using the same process schedule that results in textured "1223"" fih'nsfor samples conlaining silver. of the process for each ofthc sample sets wcrc 735 ('. 740~'C and 7 4 7 C respectively. The 7~,(0 ) values and ,I,.(ZF 77 K) values obtained on the segmented test bridges are listed in table 1.

4. Discussion

It is clear from the SEM micrographs, X R D patterns. T~,(0) values, and Jc characteristics discussed above, that the addition of silver to the precursor oxides dramatically affects the growth kinetics and morphology of the TCBCO films formed via the thallium oxide vapor conversion reaction. In our

J.A. DeLuca et aL / Preparation of"1223" TI-Ca-Ba-Cu-O filrns

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D ' " ' O " ' - , - O " " ' O " - O 20K

~A-- ~mm"~ ~A

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-~ 0

O 40K

t~-..~A

60K

104

:.~ 0

103

~

W~emper~ur; iK]2Oo. . . .

....

H\\ab E c = 1.uV/cm

2~0' ' '

Fig. 8. A typical R vs. Tplot for"1223"" samples prepared by the silver-addition process and having a cation composition of TlxCa2Ba2Cu3Ago.37 (0.65 < x < 1.0).

10 s

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1 Field (T)

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Fig. 10. Critical current density vs. field at various temperatures for a textured "1223" sample having Jc(ZF 77 K) = 105 000 A / cm 2. The field was applied parallel to the substrate surface (Ilab plane).

4.2K

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o 104 E 0 103 H\\C E c = 1p.Vfcm 102 , , ............... 0.01 0.1

z~ \ 60K ~'77K m~

~ \90K

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k ~

m~ •

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Fig. 9. Critical current density vs. field at various temperatures for a textured "1223" sample having Jc(ZF 77 K) = 105 000 A/ cm 2. The field was applied perpendicular to the substrate surface (lie-axis).

previous work [ 30 ] we demonstrated that the thallium oxide vapor reaction of silver-free Ca: Ba: Cuoxide films in an oxygen ambient to form "2223" superconducting films requires sample temperatures of at least 895°C. In the present study we see that such films form readily at 860°C. Of even greater significance is the fact that textured thick films of the "1223" phase material exhibiting impressive superconducting properties can be formed in a relatively short time at 860°C by our silver-addition process. The rounded features evident in the micrograph shown in fig. 4, especially those in the vicinity of the voids, suggest that the formation of a liquid phase is responsible for the accelerated growth kinetics. This

(.5

102 0.01

. . . . . . . . . . . 0.1

~ .....

m 1

...... 10

Field (T)

Fig. 11. Critical current density vs. field at 77 K for four textured "1223" having different Jc(ZF 77 K) values. The field was applied perpendicular to the substrate surface (IIc-axis).

liquid may be similar in nature to those believed to form when YBCO and BCSCO materials are processed in the presence of silver [ 32,33]. Indeed, the silver-rich nodules found in our films may form by a mechanism similar to that proposed by Deslandes et al. [32]. Additional studies are required to better characterize both the film growth mechanism and the microstructural characteristics responsible for the high transport Jc values. For example, the plate-like growth features apparent on the film surface in the micro-

28

J.A. DeLuca et al. / Preparation Off" 1223" Tl-('a-Ba-('u-Ojihn,s

Table 1 Superconducting "1223" films were prepared at each of the nominal compositions x = 0,7 ( 735('), a = 0.8 (740C), and x = 0.9 (747(') and patterned with a 4 mm × 0.2 mm test bridge, A-B-C-D-E, provided with test points lhat permitted the determination of.l,: for the full 4 mm bridge, AE, as well as for each of four 1 mm segments, AB, BC, CD, DE. The values given above in parentheses refer to the thallium oxide source boat temperature used in the "'second step" of the thallium oxide vapor reaction process Sample

.v

7~:(0 )

.I,.(ZF 77 K) in A/cm 2 AE

Nominal ,r=0.7 SS-263 SS-265 SS-266 SS-268 Nominal x=0.8 SS-258 SS-259 SS-260 SS-261 SS-262 SS-264 Nominal x=0.9 SS-269 SS-270 SS-273A SS-275 SS-276

AB

BC

CD

DE

0.68 0.64 0.67 0.68

105 K 104 K 107 K 105 K

4400 6900 700 8900

4600 30 000 13 500 31 500

1() 800 13 900 17 000 15 400

13 000 6700 700 15 000

6800 16 300 7700 8900

0.81 0.80 0.77 0.75 0.78 0.77

106 K 107 K 106 K 107 K 106 K 107 K

2200 2800 6800 3100 14 900 6300

3100 6700 7400 21 300 20 800 7200

2700 2800 6800 42 900 14 500 6500

2200 3500 9100 3800 25 000 6700

2400 5300 890(I 110 000 t5 500 (~30(1

0.88 0.88 0.86 0.9(t

105 K 105 K 106 K 106 K 107 K

13 400 23 800 3000 11 800 4200

17 500 49 500 9600 II 800 5700

19 600 23 500 6000 12 10(} 12 700

13 2130 29 200 3400 22 500 4400

20 200 48 600 2900 17 600 550(/

g r a p h s h o w n in fig. 4 a n d t h e v i r t u a l l y f e a t u r e l e s s c r o s s - s e c t i o n a p p a r e n t in fig. 5 m i g h t i n d i c a t e t h e p r e s e n c e o f a t e x t u r e d s t r u c t u r e f o r m e d by t h e d e n s e , l o w - a n g l e - g r a i n - b o u n d a r y i n t e r g r o w t h o f flat cryst a l l i t e s e a c h c o n s i d e r a b l y t h i n n e r t h a n t h e t o t a l film t h i c k n e s s . O n t h e o t h e r h a n d , w h a t a p p e a r to be ind i v i d u a l p l a t e - l i k e c r y s t a l l i t e s m a y b e steps in a g r o w t h p l a n e , a n d t h e a v e r a g e c r y s t a l l i t e size in dir e c t i o n s parallel to t h e s u b s t r a t e surface m a y be m u c h l a r g e r t h a n t h e d i m e n s i o n s o f t h e flat s u r f a c e f e a t u r e s v i s i b l e in fig. 4. In a d d i t i o n , t h e f i l m s m a y in fact b e "'one c r y s t a l " t h i c k . We h a v e n o t e d t h a t t h e T o ( 0 ) v a l u e s o f o u r "" 1223'" f i l m s are e s s e n t i a l l y u n c h a n g e d b y t h e 6 0 0 ° C o x y g e n h e a t t r e a t m e n t u s e d a f t e r p a t t e r n i n g t h e films. T h i s b e h a v i o r is n o t u n e x p e c t e d for a single T 1 - O l a y e r superconductor [34-36], especially one which has b e e n p r e p a r e d a n d slowly c o o l e d in a n o x y g e n a m b i e n t p r i o r to t h e e x t e n d e d o x y g e n a n n e a l , O n t h e o t h e r h a n d , t h e 2- to 6-fold i n c r e a s e in t h e J c ( Z F 77 K ) v a l u e s o f o u r " 1 2 2 3 " f i l m s a f t e r t h e 6 0 0 ° C oxygen t r e a t m e n t is o f c o n s i d e r a b l e i n t e r e s t a n d des e r v i n g o f f u r t h e r study, O u r i n i t i a l e x p e r i m e n t s in-

d i c a t e t h a t t h e c h a n g e s i n d u c e d in t h e f i l m s are o f a n a t u r e m o r e f u n d a m e n t a l t h a n effects s u c h as t h e rem o v a l o f d a m a g e c a u s e d by t h e p a t t e r n i n g a n d e t c h ing process. It is u n k n o w n to us at t h i s t i m e w h e t h e r t h e i m p r o v e m e n t in .I,, is r e l a t e d to c h a n g e s in t h e i n t e r g r a n u l a r or i n t r a g r a n u l a r .1~, c h a r a c t e r i s t i c s ( o r b o t h ). As i n d i c a t e d in fig. 11 i m p r o v e d zero-field critical c u r r e n t is a c c o m p a n i e d by i m p r o v e d b e h a v i o r in an a p p l i e d m a g n e t i c field. T h e low-field slopes o f t h e s e c u r v e s d e c r e a s e as t h e z e r o - f i e l d Jc i m p r o v e s . H o w ever, t h e h i g h - f i e l d b e h a v i o r o f t h e t w o best s a m p l e s suggests a n i n t r i n s i c limit. T h e critical c u r r e n t o f t h e best s a m p l e s v a n i s h e s at a p p l i e d fields > - 1-2 T at 77 K, A n i n t e r p r e t a t i o n o f t h e s e d a t a r e q u i r e s c o n s i d e r a t i o n o f at least t h r e e factors. F i r s t is a field ind e p e n d e n t p o r o s i t y f a c t o r w h i c h r e p r e s e n t s t h e eff e c t i v e cross s e c t i o n a l a r e a a v a i l a b l e for c u r r e n t t r a n s p o r t . S e c o n d is t h e n a t u r e o f t h e c o u p l i n g bet w e e n g r a i n s w h i c h are in g o o d p h y s i c a l c o n t a c t . W e a k - l i n k i n t e r g r a n u l a r b e h a v i o r is a c h a r a c t e r i s t i c o f m i s a l i g n e d g r a i n b o u n d a r i e s in H T S m a t e r i a l s [ 3 7 ] a n d is s t r o n g l y field d e p e n d e n t [ 3 8 ] . T h e de-

J.A. DeLuca et al. / Preparation of"1223'" TI-Ca-Ba-Cu-O films

creased low-field slopes in fig. 11 may be associated with a greater fraction of strongly coupled grains in the higher Jc(0) samples. Finally, intragranular pinning strength can dominate in cases where intergranular effects have been minimized. The abrupt decrease above 1-2 T in the best samples is consistent with the intragranular behavior of TI (1223) probed by magnetization measurements [ 26,39 ]. Figures 9 and 10 show the magnetic field dependence of J~ at various temperatures typical of the best films both with the field applied perpendicular and parallel to the film surface. At each temperature, a field B * ( T ) can be chosen above which an abrupt decrease in critical current is noted. For example, an appropriate criterion might be the field at which the critical current falls below 100 A / c m 2. Independent of the specific criteria, it is clear that B * ( T ) decreases nearly exponentially with temperature. An analysis of this data with the model o f K i m et al. [ 12 ] has been recently presented [40]. As typically observed in the oxide superconductors, the critical current performance at high fields is superior for the field applied along the a - b plane. This has been taken in support of an intrinsic pinning mechanism [41]. However, it is also consistent with a higher upper critical field associated with the smaller coherence length for this direction. The results of this study demonstrate the considerable potential of our silver-addition process for the fabrication of practical superconducting wires and tapes. Of critical importance to the realization of that potential will be the ability to achieve over an entire film the high Jc values measured in some of our test segments. The promise and challenge of achieving this goal in evident in the data given in table 1 for sample SS-261 in which the J¢(ZF-77 K ) s of the four test bridge segments range from 3800 to 110 000 A/ cm 2. Improving the "J¢ uniformity" depends upon the development of a process for ( 1 ) preparing films of uniform thickness and density, and (2) preparing films consisting of superconducting material having uniform intergranular and intragranular properties. Our J¢ values have been calculated using the apparent cross sectional areas of the test bridges. As seen in fig. 6 inhomogeneities are present in the form of voids which are not distributed homogeneously

29

throughout the films. Even if the superconducting properties of the film material were uniform, the presence of non-uniformly distributed voids would result in a variation in the Jc values as determined by our method. This inhomogeneity must clearly be avoided if uniform properties are to be achieved over the large areas required for practical applications. However, we have not been able to identify any significant differences in the morphological features over a given test bridge which suggests that other causes of the non-uniformity in Jc observed for our test bridges are likely to be important. Variations in J¢ may be consequence also of characteristics of both the precursor deposition method and the film growth mechanism that affect more than the gross features of film morphology. Our results suggest that liquid phase formation is important for the accelerated film growth observed. We consider it likely that the film forms as the result of the simultaneous growth of superconducting material from sites distributed throughout the precursor film as thallium oxide is incorporated. In such a case one would expect the intergranular properties to depend not only on the manner in which the phases grow at each site (including effects of secondary phases which may interfere with the growth of the superconductor or contaminate grain boundaries), but also upon the manner in which the advancing growth fronts from various sites interact with one another. Additionally, considering the relatively short process times of the silver-addition method, we expect that the phases formed are characterized by the presence of defects such as vacancies, mixing of ions on lattice sites, both cation and anion compositional variations (within a grain as well as from grain to grain), and some level of syntactic intergrowths. Any of these growth anomalies may result in variations in the properties of the superconducting crystallites comprising the film [4245 ] in addition to any variations in the intergranular connectivity. That our films do not consist of single phase materials of the ideal "1223" composition and structure is supported by the evidence of secondary phases in the XRD patterns, film-average Tlx values x < 1, and Tc values lower than the 110-117 K values reported for bulk and single crystal samples of the "1223" phase [ 46,47 ]. Compositional variations in the precursor film would be expected to make things even worse. This is a definite possibility for spray

30

J.,4. DeLuca et al. /Preparation ~/" 1223" TI-Ca-Ba-( "u-O /ilms

deposited films, for, because of their differing solubilities some degree of segregation of the calcium, barium, copper, and silver nitrates may occur as the spray solution evaporates. Work is in progress in our laboratory to better u n d e r s t a n d and control the mechanisms which determine film uniformity. Polycrystalline YSZ was selected for this study to demonstrate, as we have, that TCBCO thick films with good superconducting properties can be prepared on other than oriented single crystal substrates. An important next step will be the fabrication of thick films with comparable properties on flexible ceramic or metal substrates better suited for the fabrication of c o m p o n e n t s for electrical power applications.

ments made on segmented-bridge test structures. Of particular interest is the DC-transport ,It versus field behavior of a "'1223'" thick film measured under the "worse-case" conditions with the field applied perpendicular to the substrate surface. Crilica[ current densities exceeding 100000 A / c m 2 were measured at 4.2 K in a 9 T field, and values exceeding 10 000 A / c m 2 were measured at 60 K in a 2 T field. The high critical current densities measured in these films are believed to be a consequence of the excellent intergranular connectivity thal is achieved in our process. The impressive in-field behavior observed is consistent with that reported b~ others for ~'single T I - O layer" TCBCO superconductors.

5. Summary References

A process is described for the preparation of high quality thick films of TCBCO superconductors. Films of the "'1223" phase superconductors, having the cation stoichiometry Tl~Ca2Ba2Cu3Ago.37, have been formed at 860°C on polycrystalline yttria stabilized zirconia substrates via the reaction of spray deposited silver-containing C a - B a - C u - o x i d e films with thallium oxide vapor in a two-zone reactor. The thallium oxide content and phase composition of the films is dependent upon the thallium oxide partial pressure in the reaction vessel. Films prepared in the composition range, 0 . 6 5 < x < 1.00, consist predominately of the "1223" phase while those in which x ~ 1.3-1.5 consist predominately of the "2223" phase. Heating the "1223" films in oxygen at 600°C has been found to result in a 2- to 6-fold increase in the J c ( Z F 77 K) values, although it has lille effect on the To(0) values. Our data suggest that the effect of the silver addition to the films may be to induce the formation of a liquid phase during processing which results in accelerated growth kinetics, Films of "1223" T C B C O can be prepared routinely that have J~(ZF 77 K) values in 10 0 0 0 100 000 A / c m 2 range as measured in small test bridges. Both the range of J¢ values measured and the morphology of the films evident in SEM micrographs suggest that the observed variations in Jc are related, at least in part, to variations in Jc within a given film. This has been confirmed by Jc measure-

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