High Temperature Superconducting Thick Films by use of EPD Method (II)

High temperature superconducting (HTS) thick films deposited on metal Ag wire were prepared with YBCO and BSCCO powders by electrophoretic deposition method. I2 was used as additives for surface charge of YBCO and BSCCO particles. When 2-3 wt.% BaF2 was added in the YBCO suspension, the pores and cracks of film surface were decreased and film density could be increased. In case of YBCO films, the critical current density (Jc) was calculated at the value of 1458 A/cm2 (77 K, 0 K) by 4 point prove method. BSCCO films had less cracks than YBCO films, and the critical current density (Jc) was obtained more than 104 A/cm2.


Introduction
The preparation of superconducting wire is the important techniques for a wide range of application of these materials. To date, various approaches, such as powder in tube (PIT), ion beam assisted deposition (IBAD), rolling assisted bi-axially textured substrate (RABiTS), Spraying and Dip coating techniques have been investigated to prepare superconducting wires and tapes [1]. Electrophoretic deposition (EPD) is a promising technique for small and large-scale devices as well as electronic and electrical devices. In this technique, fine particles, typically a few microns, are suspended in an organic dielectric solvent. The application of a dc voltage of 50-400 volts between two electrodes causes the particles to move and deposit. EPD is a combination of two processes: electrophoresis and deposition. Electrophoresis is the motion of charge particles in a suspension under the influence of an electric field. Deposition is the coagulation of particles to a dense mass. The present paper researched both electrophoresis and deposition. The kinetics and applications of EPD to HTS films also was examined. The advantages of this method are: (1) obtaining an uniform and dense coating, (2) controlling easily the coating thickness by the deposition voltage and deposition time, (3) obtaining various shapes and sizes of coating on various substrates, (4) the thickness of the silver need only be sufficient to allow the round fiber to be rolled. In the PIT case, it must be thick and strong enough to allow the drawing process to be performed. Thus, the superconductor to silver ratio can be much higher for the electrophoretically formed tapes compared to those made using PIT processing, (5) to fabricate as-deposited superconducting thick films with highly c-axis oriented grains (the particle orientation is controlled right during the particle deposition) [2][3][4][5].
In this report, the thick films of YBCO and BSCCO were prepared by electrophoretic deposition method using powder dispersed in acetone or alcohol. The conditions and processes of electrophoretic deposition for (HTS) thick films were studied in detail and those were used for preparing BSCCO wires [6].
Morphologies of the calcined and sintered films were examined with a scanning electron microscope (SEM), (S-800, Hitachi, Ltd.). Phases generated in the film and products as a result of the reaction of the film and substrate were identified by X-ray diffraction (XRD) with a Rigaku Denki model RU-300 Xray diffracto-meter using CuK α radiation. Resistivity of the film was measured by a DC 4-probe method using a silver metal electrode with a current of 1 mA and the zero resistivity range was determined by the criterion of 0.1 µV between potential electrodes.

Electrophoretic deposition
YBCO and BSCCO superconducting thick films were deposited by electrophoresis in suspension solution with superconducting powders of which were prepared by solid-state reaction and commercial use. Average particle size distribution of these powders was in range of 1 to 5µm. Silver wire (0.8 mm φ ) and Ag coated alumina plate were used as substrates ("− ") electrode and stainless steel meshwork as counter ("+") electrode in the suspension solution. The distance between two electrodes was 10mm. To decide the best conditions and processes for YBCO thick films in acetone suspension, the amount of deposit of YBCO powder were measured by use of electronic balance at different applied voltages, content of iodine, content of YBCO powder and temperatures. In this work we propose that electrostatic stabilization of HTS particles in butanol and acetone involves three steps: I. Adsorption of solvent molecules in undissociated form onto the basic surface sites of the HTS particles II. Dissociation of the adsorbed solvent molecules by hydrogen ions transferring to the basic surface sites III.Desorption of C 4 H 9 Oanions or C 2 H 5 COinto the solution, leaving the particles positively charged. The iodine I 2 (as a cationic surfactants) is sparingly soluble in a polar solvent and therefore, tends to adsorb at the particle/solvent interfaces, thus charging the particles more positively. In our previous works [7] we reported that, in organic (non-aqueous) suspensions, the significance and importance of electrostatic stabilization mechanism for EPD. Stable suspensions provide a dense, homogeneous deposit on the electrode.
The rate at which a deposit forms during EPD is very important if the thickness of the deposited layer is to be controlled. It has been found that the current was reduced quickly with increasing deposition time at different applied voltages, and the deposition current was enhanced with increasing applied voltages. It has been shown that the amount of deposition of YBCO powder was enhanced with increasing applied electrical potential. When the applied voltage was lower than 500 V, the amount of deposits of YBCO was increased almost linearly with increasing applied voltage. When iodine content was less than 200 mg/ l in suspension, the amount of deposits was increased with increasing iodine content, but iodine content was more than 200 mg/l in suspension, the amount of deposits was reduced with increasing iodine content. So the optimum value of I 2 content could be determined with less than 200 mg/l for good preparation of YBCO thick film by EPD.
The 0-5 wt.% of BaF 2 was added in YBCO-acetone suspension as a additives for improving the surface uniformity and density of superconducting powder deposition, and reducing the crack on the surface, which was deposited at same time on the cathode [8]. The thickness of YBCO films was 30-80 µm.

Oxygen absorption process for YBCO thick film
Because the micro-cracks on YBCO and the weak links between grain boundaries have important influence on superconductivity of YBCO superconductor, the YBCO thick film samples were not superconductive using usual heat treatment process. However, due to a large difference of the thermal expansion rate of Ag layer (21.1×10 -6 /°C in 27-527°C) and YBCO thick film (14.4×10 -6 /°C in 30-500°C), micro-cracks could be developed on the YBCO thick film after heat treatment resulting in a lower critical current density. Because the melting point of Ag was 961°C, so the temperature of heat treatment should be lower than 940°C. It could be obtained by suitable heat treatment conditions, which was shown as follow; heat the samples from room temperature to 790°C for 5-8 hours, then heat it up to 910°C at the rate of 0.5-2.0 °C/min, stabilize them in the temperature for 12-24 hours, reduce the temperature to 650°C for 5 hours, stabilize them again at 650°C for 5 hours, reduce temperature to 450°C for 4 hour, stabilize it for 48 hours in oxygen flow, and then reduce it to room temperature for 2 hours. After this long procedure the YBCO thick film samples were with Meissner effect.

XRD analysis for YBCO thick film.
The X-ray diffraction patterns of YBCO thick film and sintered YBCO pellet were measured by PW1710 BASED X-ray Diffractometer (CuK α ), the results were shown in Fig.1 (a) and (b). It could be seen that the sintered YBCO pellet sample exhibited a preferred orientation with the c axis perpendicular to the sample surface.
Through comparing the data of the X-ray diffraction of standard YBCO powder and the experimental results, it was confirmed that the YBCO thick film has the almost same pattern with sintered YBCO bulk sample.

SEM and optical microscope observation
After the heat treatment procedure, by the SEM and the optical microscope for the structure of the surfaces and cross section views of YBCO thick film samples was observed. In the case of the optical microscope observation, cracks on the YBCO layer were found when the content of BaF 2 was lower than 2 wt.%. When the content of BaF 2 was higher than 2 wt.%, for example, 2 or 3 wt.%, no cracks were found on YBCO thick film samples. Figure 2 shows the surface of BaF 2 -enriched YBCO thick film wire deposited on Ag and the cross section with thickness about thick of YBCO plate deposited on Al 2 O 3 substrate. The Fig. 2 (f) shows double coated YBCO thick film, which was deposited on the area of 1 st coated layer with lots of cracks on film surface after sintering. The 2 nd coated layer was formed to prevent crack surface, which could be occurred after sintering process of 1 st coated layer. With this double coating method, cracks on 1 st coated layer could be removed by this way.
Therefore, it was prepared successfully by this double coating method on Ag wire. Silver clading was formed on YBCO thick film by same EPD method.
The thick film wire formed by this process was showed in Fig. 3 (a) and (b) with partial cross section and angle cut cross section views of single layered wire. It can be seen that the YBCO thick film and Ag clad was firmly contacted on Ag substrate wire without cracks.
Instead of BaF 2 , a nonionic polymeric, such as  poly-ethylene-glycol (PEG) was used as a new binder. The primary purpose of this polymer phase is to increase strength and toughness of the green body.
Polymer chains can adsorb simultaneously on the surface of particles, leading to "bringing" between them. Although this bridging effect is desired in the dried green state, it also may occur to some extent in the suspension, thereby promoting undesirable bridging flocculation. The effect of added polymers is highly dependent on the adsorption behavior of the polymer chains on the particles The influence of PEG on the viscosity of the suspension, on the compact behavior of HTS powders, and on the strength of as-formed green bodies, as well as the tendency of PEG to migrate during the drying process, has been the subject of several studies. This study has been designed to investigate the adsorption of PEG in colloidal superconducting powder during electrophoretic deposition and then during the drying process.
The adsorption behavior of neutral polymers, such as PEG, as solid surfaces is controlled mainly by the degree of polymerization, polymer concentration in solution, solvency, and net adsorption energy. In this study, a commercially available, almost fully hydro-lyzed PEG of moderate molecular weight (6000) was used. It was applied without further fraction. In practical application, the binder addition is usually in the range of 0 to 3 wt.% (1-10 g/l).
Binder migration during drying is caused by a combined process: transport of PEG by the solvent to the surface during the constant-rate period of the drying process and diffusion of the polymer in the opposite direction due to the developing concentration gradient. The solvent vaporizes at the surface. However, the nonvolatile PEG remains and, with time, forms a hard surface layer. Since the polymer film is a plastic or visco-elastic solid, its tendency to crack during drying and firing is substantially reduced. By this method, thick HTS films up to 50 µm thickness have been made. The solution with a crosslinking agent loses weight gradually and a crack-free film can be easily obtained. Homogeneously distributed organic was burned off below 600°C prior to texturing under controlled conditions and the remaining pores were very small and can easily be dense at lower temperature. In our case we used for 50 ml of suspension 0-3 wt.% of PEG. As increasing with PEG, the surface state of wire was dense and crack was decreased. Fig. 4 was shown the surface state of samples.   Another type of wire and tape was prepared by same way of above for the BSCCO superconductor. Preparation of BSCCO wire was entirely same method except suspension solution and sintering process. BSCCO tape was fabricated by electrophoretic deposition on Ag wire as same as above, but after deposition and thermal treatment of deposited wire it was then pressed or roll and heated for several times to make it pressed type of tapes. The superconducting wires were prepared in butanol or ethanol suspension. They were treated with 10 -3 Torr, and atmo-  sphere at 200°C, 24 h to remove existing solvent (see Fig. 5, 6). From the figure, it was proved that the surface of wire treated under vacuum system was more dense and oriented. In Fig. 7 and 8, the samples were treated with same condition except adding PEG.

(a) (b)
As a result, the crack of surface added with PEG was decreased but the particle density was decreased. The sintering temperature of above-mentioned samples was varied and the melting point was different from that of common samples. The liquid phase was appeared to the surface of samples added with PEG. In case of the samples without PEG, there is no variation on its surface phase whether it was treated under vacuum or atmosphere. From the above investigation the vacuum process has no effect to the samples added with PEG.
The Fig. 9 shows the cross section views of single and double layer BSCCO tape. The cross section of single layer tape is clearer than that of double layer. It is because that the double layer tape should be pressed or rolled more than 2 times for the single layer tape.
Therefore, the Ag clad deposited on BSCCO thick film was distorted partially and not so clear. The thickness of the tapes was 200-300 µm and that of thick film layer was 20-50 µm, respectively.

Superconductivity and current density
The critical current density, J c was determined by the standard four-probe method and it was plotted in Fig. 10. The transition temperature of the YBCO wire samples were 96 K (T c,onset ) and 91 K (T c,zero ), respectively. The critical current densities were 1215 A/ cm 2 (2 wt.%), 1458 A/cm 2 (3 wt.%) and 1921 A/cm 2 (bulk), respectively. Those were calculated using measured value of critical current at the point of 1 µV criterion of the superconductivity for the BaF 2enriched YBCO wires of 2 wt.% and 3 wt.%, and bulk superconductor. However, the shape of the I-V curves in Fig.10 (b) and (c) were not same as that of (a). It means that some of differences were there between bulk and thick film samples, but it is not analyzed in this research. samples, J c was measured at the value of 6491 A/cm 2 and 13600 A/cm 2 , respectively (at 77 K, 0T).
However, if the thick films with fine YBCO and BSCCO powder could be obtained with uni-axially oriented texturing by EPD, the critical current density, J c would be sufficiently increased with more than 10 4 -10 5 A/cm 2 on the Ag based superconductor wire. We are very interested in our future research of rotating electric field assisted EPD process for making oriented and textured superconductor wire [9]. As the same way of determining critical current density of samples, the J c of BSCCO wire and tape shaped sample were obtained at the values of 6491 A/cm 2 (d) and 13600 A/cm 2 (e), respectively (at 77 K, 0 T).

Conclusions
The YBCO and BSCCO thick film wire and tape could be also prepared well enough by EPD. The suitable conditions and processes for preparing thick film wires/tapes and oxygen absorption heat treatment were obtained successfully in research.
The optimum conditions obtained for preparing YBCO thick films were as follows: 200 mg/l of iodine content, 300-500V of applied voltage, 10-20 g/ l of YBCO powder content in acetone suspension.
After heat treatment of oxygen absorption, the YBCO thick film and bulk samples were well enough with Meissner effect, and the T c,zero was 91 K.
By the electrophoretic deposition method, the YBCO and BSCCO superconducting wire and tape could be obtained successfully, and the critical current densities, J c were obtained more than 10 3 A/cm 2 for the prepared YBCO samples.
As increasing with PEG, the surface state of wire was dense and crack was decreased.
In the case of prepared BSCCO wire and tape Received 27 September 2002.