Novel Preparation Methods for High T c Oxide Superconductors
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ticizer to provide flexibility, a dispersant to prevent particle agglomeration and a solvent.10 After mixing all these components together, the solution is poured into a "doctor blade" device which allows a controlled thickness of material to be deposited on a plastic film carrier that passes underneath the doctor blade. Following evaporation of the solvent, the tape composed of powder, binder, plasticizer, and dispersant is separated from the carrier film. This "green" tape, (actually black because of the YBa2Cu307,, powder) is flexible but not electrically superconducting because of the nonconductive organic components. Typically, the organics are burnt out during a slow heating up to 300°C whereupon the powder is weakly bound together. Heating to 900-1000°C sinters the tape into a dense but now brittle ceramic structure. Nearly fully dense material with zero resistance at 92 K and critical current density Jc at 77 K in zero applied magnetic field H of 1000 A/cm2 has been achieved using this method.5 Cast tapes of composite YBa2Cu307 + Ag sintered on Ag substrates have showed exceptional flexibility as well as relatively high Tc and Jc values." A variant of the tape casting process is the extrusion of ceramic wires.12"14 A tape casting type slurry is partially dried to a viscous clay-like consistency, extruded through a die, and then coiled on a mandrel. Following a firing sequence similar to that for tapes yields a solid ceramic superconducting wire. Such a technique has been used extensively at Argonne National Laboratory13 to produce greater than 90% dense wires less than 100 /xm in diameter with 77 K criti-
cal current densities of hundreds of A/cm2. A similar technique has produced a coil operating at 1300 A/cm2 at 77 K which has produced a dc magnetic field of 21 gauss.14 Mixing fine ceramic oxide powder with an organic vehicle and printing the resultant paste through a screen onto a substrate forms the basis of the screen printing method. Both thin (~10 /xm) and thick (~50-100 /xm) films have been deposited on substrates such as yttriastabilized zirconia (YSZ), magnesia, sapphire, and alumina. 715 Zero resistance has been achieved at temperatures as high as 89 K with Jc(H=0, 77 K) = 70 A/cm2.15 For thin films deposited on single-crystal YSZ, substantial c-axis orientation of the ceramic deposit was noted.15 Another technique for fabricating thick (10-100 /xm films of ceramic superconductor is plasma spraying. Instead of a liquid carrier for the superconductor powder, the carrier is a flow of gas. The gas/powder mixture is transported through a plasma arc, which may partially melt the ceramic powder, and is sprayed onto a substrate at high speeds. Arbitrary shapes can be coated using plasma spraying, and deposition rates are typically high, —0.1-1 /im/s. Films deposited on Ni alloy substrates heated to 650°C and post-annealed at 950°C for 1 hour have attained Tc=90 K and Jc (77 K, H=0) = 690 A/cm2, and were dense and strongly bonded to the substrate.16
Metallurgical Processing Using cold drawing technolog
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