HTS Technology Update

Not including the MetOx Single Pass System which is described elsewhere.

In order to develop appropriate conductor coating schemes for the continuous processing and manufacturing of long wires, or tapes, or ribbons, additional steps, not previously investigated, need to be included in the different deposition processes. In general, reel-to-reel types of continuous manufacturing schemes developed out of any of the above techniques, would consist of at least the following operations

  • Preparation of metal substrate material

  • Preparation and application of the seed and buffer layer(s)

  • Preparation and application of the HTS material and required post-annealing

  • Preparation and application of the passivation/stabilization/insulation layer

Exhibit 3.1 shows the materials and engineering related parameters to be considered for each of the four operations in continuous processing of coated conductors. Conceptually, the manufacture of HTS thin film conductors or tapes will consist of a means of epitaxially growing the HTS on a suitable starting substrate. The substrate must provide a suitably textured surface to promote the required ordered growth of the HTS grains such that low angle grain boundaries result. This, in turn, implies that the surface lattice dimensions should approximate and be compatible with those of the HTS thin film. In addition, the substrate should possess the strength and flexibility required of a conductor and have a favorable match of its thermal expansion with that of the superconductor. The substrate material should also be thermally and chemically compatible with all of the intermediate deposition or growth processes involved in creating the superconducting film on its surface.

Options for preparing textured substrates

Three processes for producing metallic substrates are currently being studied. In the first process, described as the Rolling Assisted Biaxially Textured Substrate (RABiTS) process, the starting metal strip is rolled and annealed to promote a textured surface as shown in Exhibit 3.2. Many metals are amenable to such mechanical texturing but the present metals of choice are high purity nickel or nickel alloys (Ni-Cr or Ni-W). A suitable buffer layer system is applied on the rolled and annealed metal strip. Although, in Exhibit 3.2 this buffer layer application is shown being carried out using pulsed laser deposition, it can equally be carried out using sputtering, electron beam evaporation, or a sol-gel process. In the literature, scientists have applied yttria stabilized zirconia (YSZ), cerium oxide (CeO2), lanthanum aluminate (LaAlO3), barium zirconate (BaZrO3), and other ceramics in single constituent or multi-constituent layers to provide a buffered, textured substrate for the YBCO.

 

The other option for producing textured metallic substrate is called the Ion Beam Assisted Deposition (IBAD) process and is shown in Exhibit 3.3. The IBAD process differs from the RABiTS process in that no texture is forced on the starting metal strip, but rather the first buffer layer laid on the metal strip is forced to have a preferred texture independent of the underlying metal strip. This is accomplished by using an ion beam impinging on the thin film buffer surface as it is being laid down by vapor phase deposition. Thus, in the IBAD process, the required surface texture results from the initial buffer layer deposition rather than using the metal surface as an initial growth template. Various metals could potentially be used as the substrate base metal, but a high nickel content super alloy, is the preferred choice because of its easy availability, high-temperature strength, and good thermal expansion match with the buffer and YBCO materials. The IBAD-YSZ process is a slow process and by its general nature is carried out only as a vapor phase deposition process. IBAD-MgO, in contrast is very fast due to the need for a thin IBAD layer followed by a homo-epitaxial MgO layer. Although in Exhibit 3.3, an ion beam is shown as the source for vaporizing the buffer material (YSZ), a laser powered or sputtering scheme could be considered as well. Like RABiTS, IBAD represents a family of possible options involving various buffer and base metal materials.

The third option for producing textured metallic substrates is called inclined substrate deposition (ISD) and is shown in Exhibit 3.4. Like the IBAD process, the ISD process differs from the RABiTS process in that no texture is forced on the starting metallic substrate, but rather the first buffer (seed) layer laid on the metal strip is forced to have preferred texture independent of the underlying metal substrate. This is accomplished by inclining the substrate at a prescribed angle to the plume. Various metals/alloys could potentially be used as the substrate material. The ISD process is very fast (»100 A/sec). Although, in Exhibit 3.4, ISD buffer layer application is

shown being carried out using electron beam evaporator, a PLD or sputtering scheme could be considered as well.  As described above, RABiTS, IBAD and ISD all provide a textured substrate that consists of a starting metal strip with appropriate buffer material(s) laid down with a preferred orientation.  This textured substrate then provides the foundation over which the HTS film is deposited using various physical and/or chemical methods.

Options for applying HTS material onto textured substrates

The textured substrate obtained from the RABiTS, the IBAD or the ISD processes provides a starting material over which the epitaxial layer of YBCO (HTS) can be applied using various candidate options. The process schematics for such YBCO deposition options are shown in Exhibits 3.5-3.13, and brief descriptions for each of these flow sheets are as follows.

Pulsed Laser Deposition/Ablation (PLD/PLA). A process schematic incorporating pulsed laser based YBCO deposition is shown in Exhibit 3.5. The primary characteristic of this process is that YBCO targets, which have been processed off-line by a variety of possible methods, are vaporized or ablated by a laser source. The vaporized YBCO is deposited as a film on the substrate at about 800oC in a low pressure atmosphere containing an O2-N2 mixture. PLD is effectively a transfer of YBCO from a target source to the substrate surface.

E-Beam Based Deposition. A process schematic for YBCO deposition by the E-beam technique is shown in Exhibit 3.6. The features of E-beam deposition of YBCO are that the E-beam vaporizes elemental Y, Ba (or BaF2 for the ex-situ process) and Cu which are deposited in oxide form at very low pressures (~10-5 mm Hg), at 740oC under an atomic oxygen atmosphere. The E-beam technique synthesizes YBCO on the substrate from its constituent elements or in the case of the ex-situ approach, enables deposition of YBCO precursor. The ex-situ process is thought to be very slow (1A/sec conversion rate) but is done in a relatively inexpensive and non-vacuum furnace. The in-situ process is unproven for tapes although it has been used for wafers. This process does not require the costly and time consuming post water heat treatment to remove fluorine.

Metal Organic Chemical Vapor Deposition (MOCVD). In the MOCVD scheme depicted in Exhibit 3.7, the Y, Ba and Cu are introduced as vaporized forms of highly pure organic precursors. It is anticipated that a mixture of Y(TMHD)3, Ba(TMHD)2 and Cu(TMHD)2 (where TMHD stands for 2,2,6,6-tetramethyl-3-5 heptanedionate) would be prepared in an organic solvent mix that consists of tetrahydrofuran (THF), isopropanol and tetraglyme. The product mix is believed to be very sensitive to trace levels of contaminants and is correspondingly costly for high purity. Application of the precursor is carried out in a MOCVD chamber maintained at about 600-850oC and at a pressure of 1-10 mm Hg. The required N2O/O2 plasma is introduced from a plasma generator tube and vaporized Y-, Ba-, and Cu- containing precursors are conveyed by flowing N2 at about 230oC to the deposition chamber.

Sol-Gel. The sol-gel method is a solution growth technique and is devoid of energetic particles for transport or vaporization of precursor material but rather uses a conventional dip coating of a precursor bearing liquid which is subsequently dried and reacted. A process schematic for the 

sol-gel technique is shown in Exhibit 3.8. A gel solution containing precursors is prepared by mixing organic solutions of Ba-alkoxide and Y-alkoxide in 2-methoxy ethanol, with a solution of copper oxide in pyridine. Vacuum distillation and partial hydrolysis are used to convert the organic solution to a gel solution of desired concentration and flow characteristics for dip coating. It is believed that repetitive dip coating processes with the gel will be required to produce film of uniform crystalline structure and desired overall thickness. Between successive coatings, the deposited gel containing the precursors is pyrolyzed in an oxygen atmosphere at 150-250oC to vaporize organic solvent and to oxidize the precursors.

Chemical Vapor Deposition (CVD). This particular chemical coating option uses commonly available halide salt precursors: Y (from YCl3), Ba (from BaI2), and Cu (from CuCl) and the process is depicted in Exhibit 3.9. The precursor salts are thermally vaporized and conveyed in stoichiometric proportions to a common mixing point by flowing N2. Coating of the textured substrate by the vapor mixture is carried out in a chamber maintained at 750-950oC, and 20 mm Hg pressure under a moist O2 atmosphere.

Aerosol/Spray Pyrolysis. Another chemical coating technique that uses commercially available salt precursors is the aerosol/spray pyrolysis technique which is shown in Exhibit 3.10. Aqueous solutions of Y (NO3)3, Ba (NO3)2 and Cu (NO3)2 in stoichiometric amounts are prepared from high purity nitrates and water. The texturized substrate is coated with the atomized precursor solution in a spray chamber where the substrate speed, chamber temperature and O2 partial pressure are process variables of importance. Like the dip coating step in the sol-gel process described above, it may be necessary to use a multiplicity of spraying operations to acquire the desired YBCO film thickness and epitaxy.

Metal Organic Decomposition (MOD). The MOD process, depicted in Exhibit 3.11 is similar to the aerosol spray pyrolysis described above. In the MOD process, stoichiometric proportions of Ba, Y and Cu acetates are mixed in aqueous trifluoroacetic acid. The resulting mixture is dried and re-dissolved in methanol to form an organic solution of trifluoroacetates which is used to dip coat a textured substrate. The coated tape is oven heated at about 200-400oC under O2 atmosphere to remove water of crystallization and excess organic solvent and to convert the trifluoroacetate film to oxyfluoride form. Repetition of the coating and baking operations may be required to achieve the desired film thickness.

Electrodeposition. This is an electrochemical method whereby dissolved material in an ionized form is uniformly deposited over the substrate maintained at the appropriate polarity. In the literature, no information was found related to suitable buffer coating over a metal substrate, however, it is assumed here that this technique would work on any suitably textured substrate. An electrodeposition process schematic is shown in Exhibit 3.12. The precursor solution is prepared by mixing stoichiometric quantities of Ba(NO3)2, Cu(NO3)2·2.5 H2O, and Y (NO3)3·6 H2O in deionized water (acidified with dilute HNO3 solution) and then by further diluting with isopropanol to provide the desired nitrate concentration. The textured substrate is coated with the precursor material in an electrolytic cell and subsequently dried at about 150oC to remove adsorbed water and convert some metal hydroxides to appropriate oxide form. They are then subjected to high-temperature treatment to complete the YBCO conversion.

Electrophoresis. Electrophoresis is the only candidate coating option discussed herein that has been used to produce kilometer lengths of conductor at rates up to 1 m/min. This work was done on silver tapes and yielded low Jc YBCO. An improved version utilizing a textured substrate is proposed in the process schematic presented in Exhibit 3.13. The process uses finely ground YBCO in a non-aqueous liquid which is transferred to the substrate in an electrophoretic cell. The coated substrate is then sintered in a furnace maintained at 950-1030oC under O2/N2 (low pO2 atmosphere) and with a residence time of about 1-30 min to ensure compete oxidation. Of the nine processes described above, PLD/PLA, E-beam based deposition and electrophoresis are physical deposition processes and the others are chemical deposition methods. In addition, PLD/PLA and E-beam based depositions are the only processes that result in a thin film of YBCO in the substrate and require no additional post coating thermal treatment. The other processes, MOCVD, sol-gel, CVD, aerosol/spray pyrolysis, MOD, electrodeposition and electrophoresis all required post coating thermal treatment to complete the annealing and oxidation of the deposited film to the desired YBCO phase. At the various steps within each of the processes described waste streams of material are generated as results of decomposition of precursor materials and inefficiencies of the coating processes. In some cases, as in case of the halide salts used in chemical vapor deposition, the waste streams can be highly corrosive and environmentally unfriendly.  Finally, the end step in all nine processes would be a common final step comprised of a passivation/stabilization/insulation layer to provide the final finished conductor product in a usable form.