IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE

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1 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE Growth Mechanism During Firing Process of Single-Coated Thick YBCO Films by TFA-MOD Takeshi Araki, Mariko Hayashi, and Hiroyuki Fuke Abstract Single-coating technology for metal organic deposition using trifluoroacetate has various advantages: no inhomogeneous interface in the case of multilayers, no random orientation from the interface, and no deterioration of material through two or more heat treatments. However, single-coated thick film inevitably has many voids. In order to minimize voids, both a calcining process and a firing process are required. This paper focuses on the firing process. During the firing process, balance of V d and V g is important. V d and V g are the drop rate of quasi-liquid and the growth rate of YBa 2 Cu 3 O 7 x, respectively. We found that the volume of voids can be reduced in the condition of V d >V g, where gas flow is low, temperature high, and humidity low. We prepared gel films whose thickness was about 50 µm and calcined them to realize thick calcined films. The films are fired to realize 5.2-µm-thick superconductor on LaAlO 3 single crystal. Transmission electron microscopy observation revealed oriented layers. The film is still 20% void by volume. However, the film must be 40% void by volume without optimal firing process based on a quasi-liquid network model. The quasi-liquid network model is effective for reducing voids. Optimization of the calcining process will spur further development. Index Terms Growth mechanism during firing process, single coated thick film, trifluoroacetate metal organic deposition (TFA-MOD), YBa 2 Cu 3 O 7 x (YBCO). I. INTRODUCTION METAL-ORGANIC deposition using trifluoroacetic acid (TFA-MOD) is one of the most promising candidates for the fabrication process of second-generation superconducting tape and wire [1]. TFA-MOD has various advantages: uniformity of coated layer, scalability of the process, and a low-cost non-vacuum approach. TFA-MOD has one drawback, namely, the difficulty of preparing thick films. In the conventional metal-organic deposition process, (CH 2 ) n -based chemicals are adopted as crack preventing chemicals (CPCs) during heat treatment. If a (CH 2 ) n -based chemical is used for TFA-MOD, chemical reaction between fluorine of trifluoroacetates and hydrogen of the CPC causes strong stress, which leads to cracks. During the calcining process in TFA-MOD, carbon content vaporizes [2] as CF 2 O, CF 2 CO, etc. Melting points of these chemicals are C. If bond between carbon and fluorine atoms is broken in CPC, the possibility of carbon residue in resulting films increases. Carbon residue fatally deteriorates the Manuscript received October 5, 2012; accepted January 21, Date of publication January 29, 2013; date of current version May 4, The authors are with the Corporate Research and Development Center, Functional Materials Laboratory, Toshiba Corporation, Saiwai-Ku, Kawasaki , Japan ( takeshi2.araki@toshiba.co.jp; mariko2.hayashi@ toshiba.co.jp; hiroyuki.fuke@toshiba.co.jp). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TASC superconductivity [3]. The (CH 2 ) n -based chemical increases stress of the film and the stress makes it difficult to prepare single-coating film whose thickness is greater than 1 micrometer. Therefore, repeated-coating technology has been developed to increase critical current (I c ) of the resulting film. With a view to overcoming the above-mentioned drawback of the chemical reaction, single-coating technology [4] was reported in 2006, in which the CPC structure is (CF 2 ) n - chain instead of (CH 2 ) n - chain. For example, H (CF 2 ) 8 COOH is one of the CPCs. High fluorine ratio of fluorine and hydrogen atoms avoids chemical reaction between fluorine and hydrogen atoms. Stress is decreased because few chemical reactions occur during the calcining process and single-coated thick film with thickness of over 1 micrometer can be routinely realized. The segregation of Ba and Cu is also suppressed because few chemical reactions occur. Our group reported single-coated 1.3 micrometer-thick superconductor and 2.9 micrometerthick seamless calcined film in 2008 [5], [6]. Recently, the single-coating process seems to have become predominant in TFA-MOD. The single-coating technology has a drawback in that CPC inevitably forms voids in calcined film. During the calcining process, Cu trifluoroacetate decomposes at the lowest temperature. Y and Ba trifluoroacetates decompose at higher temperature. At that time, volumes of the three trifluoroacetates greatly decrease and consequently it is supposed that the volume ratio of the CPCs is approximately 30 vol%, in the case that 15 vol% of CPCs is mixed at the initial stage. At the abovementioned temperature of 250 C, CPCs may be liquid phase and facilitate coarsening of decomposed material derived from trifluoroacetates. Large numbers of voids inevitably form in the case of single-coating technology. Despite this drawback, single-coating technology remains attractive because of no segregation of Ba and Cu contents, no rough interface of repeated-coating area, stability of the coating process, and simple low-cost process compared with repeated coatings. The voids decrease total critical current (I c ) of the resulting superconducting tape, physically. To increase the I c, it is necessary to develop condensed superconductor with this singlecoating technology. There are two recipes to decrease the voids in resulting films. One is preparation of calcined film with small volume of voids by selecting CPCs and their volume ratio, thereby establishing the proper calcining profile. We are now developing the process. The other is a firing process to minimize the voids. Our group believes liquid phase forms a network during the firing process of TFA-MOD. During the firing process, the network enables rapid movement of material between growth front and film surface. We called this the /$ IEEE

2 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013 quasi-liquid network model [2], [7]. Prior to single-coating technology, we only have 300 nm thick films, with which concrete data of drip rate of quasi-liquid by gravity (V d, nm/sec) and growth rate of YBCO (V g, nm/sec) are difficult to obtain. Thanks to single-coating technology, we can obtain concrete data of V d and V g in the model. The data would enable us to establish the proper firing process. In this paper, we present an initial report on the quasi-liquid network model with V d and V g. We fabricated YBCO film with thickness of over 5 micrometers and observed it by Transmission Electron Microscopy (TEM) to detect the thickness of the oriented layer. We present Energy Dispersive Spectroscopy (EDS) map and report Secondary Ion Mass Spectroscopy (SIMS) results to detect segregation of metal contents and seamless structure. II. EXPERIMENT Coating solutions for YBa 2 Cu 3 O 7 x were prepared by the solvent-into-gel method, reported elsewhere [7]. The total impurity of water and acetic acid in the coating solution is less than 1 wt%. The total metal ion concentrations of the coating solutions are 1.86 mol/l. The CPCs selected were H(CF 2 ) 4 COOH and H(CF 2 ) 8 COOH vol% of each CPC was added to metal trifluoroacetates in the coating solution. Density of mixed metal trifluoroacetates is 2.41 g/cm 3.The mixed coating solution was stirred for over 15 minutes. Gel films were deposited on 10 mm 30 mm (001) LaAlO 3 single crystals with a dip coater. Withdrawal speeds were from 70 mm/sec to 200 mm/sec. All gel films were calcined at C with 4.2% (relative humidity) humidified oxygen using the profile shown in [7, Fig. 10]. Elapsed time between at 200 and 250 C was 7.5 hours. The calcined films were fired at C with % humidified argon gas mixed with 1,000 ppm oxygen and annealed at 525 C in dry oxygen under the profile shown in [7, Fig. 12] to yield YBa 2 Cu 3 O 7 x films. TEM image is observed after trimming by focused ion beam on porous film. Some areas are deficient in terms of sample preparation. Total void area is overestimated compared with the image. Void area is calculated based on area of TEM observation. EDS map is obtained to examine Ba and Cu segregation. SIMS measurement is applied to confirm seamless inner layer and total volume of thick film by a single deposition. III. RESULTS AND DISCUSSION Role of the CPC is illustrated in Fig. 1(a) (c). CPC is directly mixed in purified coating solution shown in Fig. 1(a). We can obtain gel film on buffered metal tape or single crystals with the mixed coating solution. The gel film contains small amount of methanol. Such confined methanol absorbs humidity and captured water may decompose the trifluoroacetates. Gel film is heat treated to become calcined film. During the calcining process, Cu trifluoroacetates decompose at around C. Y and Ba trifluoroacetates decompose at around C. It should be noted that these temperatures depend on the ramp rate of the calcining process. As shown in Fig. 1(c), CPC is not decomposed at around 250 C. CPC may be liquid phase at this temperature. If so, decomposed CuO and oxyfluorides merge and consequently many large voids form. Fig. 1. Illustration of (a) coating solution, (b) gel film, and (c) film during calcining process. Fig. 2. TEM observation of calcined films derived from coating solution with (a) H (CF 2 ) 4 COOH and (b) H (CF 2 ) 8 COOH. Fig. 2(a) and (b) are TEM images of the films derived from coating solutions whose CPCs are H(CF 2 ) 4 COOH and H(CF 2 ) 8 COOH, respectively. Decomposing temperature of H(CF 2 ) 8 COOH is higher than that of H(CF 2 ) 4 COOH, because of molecular weight. In the case of H(CF 2 ) 8 COOH, state of Fig. 1(c) persists longer than in the case of H(CF 2 ) 4 COOH. Accumulation of data is necessary to establish the model. The film derived from CPC of H(CF 2 ) 8 COOH has larger voids. We focused on the firing process in this work. We prepared calcined film with many large voids such as those shown in Fig. 2(b) to establish the growth scheme. A. Quasi-Liquid Network Model Various groups have proposed growth schemes for TFA- MOD. McIntyre implied the existence of quasi-liquid [8], [9]. Our group thinks that the quasi-liquid forms all over the film during the firing process [7], [10]. With this model, we can understand various peculiar phenomena in TFA-MOD. In this model, V d and V g are important. V g is estimated with high-resolution fluorine ion detector. But V d is very difficult to estimate because the film thickness without CPC is only nm and there are no voids in the film. TEM images provide no information on V d. Thanks to single-coating technology [4], we can obtain porous and extremely thick films. Such films provide hints for establishing the quasi-liquid network model. Porous structure derives from imbalance of V g and V d.inthe case of V g >V d, porous structure remains in resulting films. On the other hand, in the case of V d >V g, condensed structure is expected. V d >V g is the key to obtaining high I c film.

3 ARAKI et al.: GROWTH MECHANISM DURING FIRING OF SINGLE-COATED THICK YBCO FILMS BY TFA-MOD Fig. 3. V d and V g dependence of firing condition. B. Other Publication To reduce voids during the firing process, we know of three parameters: gas flow, temperature, and humidity. Quasi-liquid formation is independent of gas flow [7]. A report on the electron beam process, in which a similar chemical reaction is expected, suggests V g dependence on gas flow [11]. V g increases with temperature, which was reported elsewhere [12]. Dependence of humidity on V g is reported in [13]. All the above-mentioned relationships are summarized in Fig. 3. Dependence of temperature and humidity on V d is unknown. We confirm the relationship of gas flow experimentally by lowresolution TEM observation. C. Basic Data for Quasi-Liquid Network Model Fig. 4(a) shows superconducting film, fired at 800 C, 1.0 L/min gas flow, and 1.26% vapor. Fig. 4(b) shows the other film, fired with 0.5 L/min gas flow. Compared with Fig. 4(a), density of the film has apparently increased. Although V g is low at low gas flow, V d is independent of gas flow. V d must be determined mainly by temperature and humidity. Therefore, low gas rate is effective for obtaining dense superconductor. However, we have to pay attention to low growth rate, which increases the production cost. We prepared a film fired at the low temperature of 750 C, which is shown in Fig. 4(c). At low temperature, we can suppress V g but V d must be decreased. Fig. 4(c) shows many isolated grains and growth direction of the grains is inconsistent with that of the single crystal. These results show V d dramatically decreases with the decrease of firing temperature. Quasi-liquid in upper layer could not retrieve the orientation of growth front and finally formed isolated grains. From Fig. 4(a) and (c), high firing temperature is desirable to yield dense superconductor. We obtained dependence of humidity on V d. Dependence of V g on humidity is reported in [13]. According to the quasiliquid network model, state of quasi-liquid depends on starting materials: CuO, Y-Ba-O-F, and vapor. Among these materials, only total amount of vapor can be controlled during the firing process. At the same firing temperature, constant of equilibrium chemical reaction doesn t change and total amount of quasiliquid increases with vapor. V d must have a relationship to viscosity of quasi-liquid. Dependence of the viscosity on vapor is unclear. From the fluorine ion measurement for the firing process, quasi-liquid forms at a wide range of humidity, which implies that state of the quasi-liquid never dramatically changes with the vapor. Viscosity of the quasi-liquid may not change greatly. We prepared three films to clarify the above issue. Fig. 4. TEM observation of fired films: (a) standard firing condition, (b) prepared under low gas flow, and (c) fired at 750 C. We fired three films at 800 C with 4.2%, 1.26%, and 042% humidity. Fig. 5(a) (c) show superconductor prepared with 4.2%, 1.26%, and 042% humidity, respectively. Porosity increases with the humidity. These results confirm dependence of V d on humidity is much lower than that of V g. As a result, low humidity is desirable for fabricating dense structure of the resulting films. All these results are summarized in Fig. 3. Low gas flow, high temperature, and low humidity are desirable for obtaining single-coated thick dense films. D. Demonstration of Single Coated 5.2 Micron Film Limitation of the film thickness derived from the simple dip coating process is about 1.7 micrometers. The thickness is estimated for the fully dense state of superconductor. To prepare thicker film on single crystals, derivatives of the dip-coating process or another process are required. We directly prepared gel film by dripping the coating solution on single crystal and drying it. The process is equivalent to the screen coating process. 15 volume % of CPC is mixed in coating solution,

4 IEEE TRANSACTIONS ON APPLIED SUPERCONDUCTIVITY, VOL. 23, NO. 3, JUNE 2013 Fig. 5. TEM images of fired films. Films were fired at 800 C under (a) 4.2%, (b) 1.26%, and (c) 0.42% humidified gas. Porosity of the films was 40%, 31%, 22%, respectively. Fig. 7. EDS map of the 5.2-µm-thick film. No segregation is observed from Cu and Ba maps. Fig. 6. TEM images of single-coated 5.2-µm-thick film. Nominal thickness is 5.2 µm. Porosity is 20%. 4.2-µm-thick material is deposited on the substrate. which has concentration of 1.20 mol/l in metal contents and is stirred for 15 minutes. We drip the 7.0 micro litters coating solution on 10 mm-square LaAlO 3 single crystals and dried the drop under dry oxygen gas for 15 minute. The dried drop was subsequently calcined to yield calcined films. Then the calcined films are fired for 12 hours to realize superconducting films and a cross-sectional TEM image of a film is shown in Fig. 6. From the figure, porosity of the film is estimated to be 20% and the nominal thickness is 5.2 micrometers. As a condensed material, 4.1 micrometer-thick material was deposited on the single crystal. Orientation of the film is confirmed from the entire area of the film in Fig. 6. Dense 5.2 micrometer-thick calcined film is more easily obtained than in the case of the porous film, because cracks generate from the bridge area (nonvoid area) of calcined film. The obtained film had superconducting phase in XRD but showed almost no superconductivity by inductive method. Fig. 6 shows extraordinary large amount of a/b-axis oriented grains. Such grains lead to the above result. EDS map was obtained and SIMS measurement performed for the 5.2 micrometer-thick film. EDS map in Fig. 7 shows typical uniform film, which has no segregation of Ba and Cu contents. SIMS results shown in Fig. 8 indicate the seamless na- Fig. 8. SIMS results of single-coated 5.2-µm-thick film. Seamless signals are measured. ture of the results. Thickness is estimated to be micrometers from the results. The results indicate that 5.2 micrometerthick film is approximately 20% void. IV. CONCLUSION Porous calcined films can be condensed by the proper firing process. Important parameters during the firing process are drip rate of quasi-liquid (V d ) and growth rate of YBa 2 Cu 3 O 7 x (V g ) to realize condensed superconductor. Low gas flow, high temperature, and low humidity lead to V d >V g, where porous calcined films are converted to condensed ones. H (CF 2 ) 4 COOH is an effective crack preventing chemical and yields film whose thickness is about 5 7 micrometers when the chemical is mixed with purified coating solution.

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