Robert C. Dye

Epitaxial CeO2 films as buffer layers for high-temperature superconducting thin films

We have prepared epitaxial (100)CeO2 thin films on LaAlO3, sapphire, and yttria‐stabilized zirconia using pulsed laser deposition. It is demonstrated in this letter that the CeO2 films are chemically and structurally compatible to the high‐temperature superconductor YBa2Cu3O7−δ (YBCO). Epitaxial YBCO films on CeO2/LaAlO3 had a zero resistance temperature and critical current density in a zero field of 90 K and 5.9×106 A/cm2 at 75 K, respectively. Furthermore, epitaxial multilayers of CeO2/YBCO were prepared. This work demonstrated that CeO2 is an excellent buffer layer material for the high‐temperature superconductors.

Pulsed laser deposition of thick YBa2Cu3O7−δ films with Jc≥1 MA/cm2

Using pulsed laser deposition, YBa[sub 2]Cu[sub 3]O[sub 7[minus][delta]] (YBCO) films ranging in thickness from 0.065 to 6.4 [mu]m have been deposited on yttria-stabilized zirconia substrates with an intermediate layer of CeO[sub 2]. The thinnest films have critical current densities of over 5 MA/cm[sup 2] at 75 K with zero applied field; as film thickness is increased, [ital J][sub [ital c]] decreases asymptotically to 1 MA/cm[sup 2]. X-ray analysis of a 2.2-[mu]m-thick film shows that the YBCO is predominantly [ital c]-axis oriented and textured in-plane, while a Rutherford backscattering spectrometry minimum channeling yield of [approx]75% indicates that the film contains disordered material at this thickness.Using pulsed laser deposition, YBa2Cu3O7−δ (YBCO) films ranging in thickness from 0.065 to 6.4 μm have been deposited on yttria‐stabilized zirconia substrates with an intermediate layer of CeO2. The thinnest films have critical current densities of over 5 MA/cm2 at 75 K with zero applied field; as film thickness is increased, Jc decreases asymptotically to 1 MA/cm2. X‐ray analysis of a 2.2‐μm‐thick film shows that the YBCO is predominantly c‐axis oriented and textured in‐plane, while a Rutherford backscattering spectrometry minimum channeling yield of ≊75% indicates that the film contains disordered material at this thickness.

Properties of epitaxial SrRuO3 thin films

SrRuO3 thin films were deposited on (100) LaAlO3 using pulsed laser deposition. The films were (001) oriented normal to the substrate surface with a high degree of in‐plane orientation with respect to the substrate’s major axes. An ion beam minimum yield of 2.5% was obtained for the films, indicating high crystallinity. The films exhibited metallic behavior with a room temperature resistivity of ∼200 mW cm. A kink in the resistivity, corresponding to a ferromagnetic phase transition, was observed at ∼160 K. It was found that SrRuO3 is structurally and chemically compatible with the YaB2Cu3O7−d (YBCO) superconductors. High quality YBCO films were obtained on SrRuO3 LaAlO3. Multilayers of YBCO/SrRuO3 were successfully fabricated.

Composite PdTa metal membranes for hydrogen separation

The fabrication of a composite metal membrane using vapor deposition techniques is reported. Commercially obtained tantalum foils are cleaned by ion milling and subsequently coated with palladium metal. The test foil was 13 μm tantalum sheet coated with 1 μm of palladium on each side. For comparison, several other foils of differing tantalum and palladium thicknesses were investigated. These membranes having an area of about 2 cm2 provide a hydrogen flux ranging from 18.7 standard cubic centimeters per minute (sccm) at 300°C to 27.4 sccm at 365°C with a hydrogen differential pressure of about 355 torr. The deposited palladium films were oriented along the (111) crystal axis. Gas flux versus pressure measurements revealed that these membranes do not show the P0.5 dependence upon applied differential pressure expected for foils in which bulk diffusion is rate limiting. This indicates that surface effects are becoming significant in determining hydrogen flux.

a‐axis oriented YBa2Cu3O7−x thin films on Si with CeO2 buffer layers

We have grown a‐axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x‐ray diffraction, x‐ray pole‐figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical‐current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a‐axis oriented YBCO films.

Target modification in the excimer laser deposition of YBa2Cu3O7−x thin films

Under conditions used for pulsed laser deposition (308 nm, 20 ns, 2–8 J/cm2) of Y‐Ba‐Cu‐O superconducting thin films, we have measured a pronounced decrease in deposition rate with cumulative laser exposure of the target. This decay in rate is accompanied by evolution on the target surface of microscopic columnar structures, having yttrium‐enriched surfaces, which are aligned in the direction of the incident laser beam (45°). Neither the vapor plume direction nor film stoichiometry is affected by the presence of these oriented, chemically altered surface features.

Effect of deposition rate on properties of YBa2Cu3O7−δ superconducting thin films

Superconducting thin films of YBa2Cu3O7−δ on (100) SrTiO3 are prepared in situ by a pulsed laser deposition technique at deposition rates from 1 to 145 A/s. Crystallinity of the films is examined by Rutherford backscattering in the channeling mode using a 2.2 MeV He+ ion beam. The backscattering minimum yield (χmin) increases with the deposition rate. A χmin of 3% is observed in the films deposited at the lowest deposition rate. Even at a deposition rate of 145 A/s, the films show good crystallinity with χmin of 15%, indicating epitaxial growth. The x‐ray diffraction measurements show that the films have strong c‐axis orientation normal to the substrates. The films have metallic resistance versus temperature behavior with zero resistance temperatures of 90 K. The results indicate that the pulsed laser deposition technique could be used to deposit large‐area films efficiently with adequate substrate movement.

Multilayer metal membranes for hydrogen separation

Abstract A novel multilayer metal membrane has been developed that can be used for the separation of ultra-high purity hydrogen from impure feed streams. The membrane is comprised of very thin layers of dense palladium film deposited on both sides of a thin Group V metal foil, ion-milled prior to the deposition of the palladium. This membrane operates at elevated temperatures on the order of 300 °C and is capable of high rates of hydrogen flow. Flows are dependent on the pressure differential applied to the membrane, but flows of 100 standard cubic centimeters of hydrogen per minute per square centimeter of membrane and higher are regularly observed with differentials of under one atmosphere. Testing of the membrane for a period of 775 h showed stable flows under constant conditions. A membrane system has been successfully applied to a proton exchange membrane fuel cell and was tested using a pseudo-reformate feed stream containing 1% carbon monoxide without any degradation in performance.

Large‐area, two‐sided superconducting YBa2Cu3O7−x films deposited by pulsed laser deposition

The in situ excimer‐laser deposition process is demonstrated, for the first time, to be an effective technique for producing superconducting films on both sides of substrate wafers of up to 5 cm in diameter. By exploiting the directed nature of the laser produced plume and controlling its off‐axis composition, thickness and stoichiometric variations of less than ±5% have been obtained over the entire surface. Coatings on 5‐cm‐diam LaAlO3 substrates have uniform transition temperatures of ≥ (R18)90 K on both sides, with a critical current density at 77 K and zero field of 2.5×106 A/cm2. The 10 GHz surface resistance for samples cut from a 5 cm wafer is 40–60 μΩ at 4 K.

a -axis oriented YBa sub 2 Cu sub 3 O sub 7 minus x thin films on Si with CeO sub 2 buffer layers

We have grown a‐axis oriented YBa2Cu3O7−x (YBCO) thin films on Si(100) substrates with (110) oriented insulating buffer layers of cerium dioxide (CeO2) using the pulsed laser deposition technique. The films are highly oriented and textured as determined by θ–2θ x‐ray diffraction, x‐ray pole‐figure scan, scanning electron microscopy, Rutherford backscattering spectroscopy, and ion channeling. No diffusion at the interface has been found at growth temperatures up to 760 °C, indicating the CeO2 is a chemically stable and structurally compatible intermediate material for the growth of YBCO on Si. A zero resistance superconducting transition temperature of 87 K and a critical‐current density (Jc) of 1.5×105 A/cm2 at 75 K have been measured; Jc obtained represents the highest value for the a‐axis oriented YBCO films.