Catherine E. Vallet

Sol-Gel Synthesis of Rare Earth Aluminate Films as Buffer Layers for High Tc Superconducting Films

Cubic rare-earth aluminate perovskites of the general form REAIO 3 have been identified as possible buffer layers for high current carrying cuprate superconductors deposited on roll-textured metals. This paper describes on-going research to develop solution routes which would avoid the use of slow and costly vacuum processing. Our research plan includes the development of an appropriate solution chemistry, studies of the crystallization behavior of powders under reducing and oxidizing conditions, deposition of polycrystalline films on silver substrates, deposition of epitaxial films on single-crystal oxide substrates, and deposition on roll textured nickel. The method is illustrated with the example of PrAlO 3 .

Epitaxial growth of gadolinium oxide on roll-textured nickel using a solution growth technique

Chemical solution epitaxy was used to deposit an epitaxial film of Gd 2 O 3 on roll-textured nickel. A 2-methoxyethanol solution of gadolinium methoxyethoxide was used for spin-coating and dip-coating. Films were crystallized using a heat treatment at 1160 °C for 1 h in 4% H 2 /96% Ar. Single-layer films were approximately 600 A in thickness, and thicker films could be produced using multiple coatings. θ/2θ x-ray diffractograms revealed only (0041) reflections, indicating a high degree of out-of-plane texture. A pole-figure about the Gd 2 O 3 (222) reflection indicated a single in-plane epitaxy. Scanning electron microscopy showed that the films were smooth, continuous, and free of pin holes. Atomic force microscopy revealed an average surface roughness of 53 A. Electron diffraction indicated that the misalignment of the majority of the grains in the plane was less than 10°. High-current (0.4 MA/cm 2 ) Yba 2 Cu 3 O 7–δ films were grown on roll-textured nickel substrates using Gd 2 O 3 as the base layer in a three-layer buffer structure.

Ni overlayers on biaxially textured Ni-alloy and Cu substrates by DC sputtering

Use of many Ni-based and Cu-based alloys requires deposition of a Ni overlayer prior to the deposition of the seed layer in order to obtain better epitaxy. In this work, Ni overlayers have been deposited by DC sputtering on biaxially textured NiW and Cu substrates. Films were characterized by x-ray diffraction and atomic force microscopy as a function of temperature, film thickness and deposition rate. The optimum temperature range for deposition was found to be around 400?500??C, which results in relatively smooth films with a good cube texture on both substrates.

Preparation of textured YBCO films using all-iodide precursors

Abstract We developed an all-iodide precursor approach to grow epitaxial YBa 2 Cu 3 O 7− y (YBCO) films on single crystal substrates. The respective metal iodides were used as the starting materials. Initially, yttrium and barium iodides were dissolved in 2-methoxyethanol, whereas copper iodide was dissolved in ammonium iodide/dimethylformamide solution. The solutions were mixed together and concentrated to give a 2.5-M total cation precursor solution. The precursor solutions were spin coated on SrTiO 3 (100) substrates and heat-treated at 800–830°C in 300 ppm oxygen atmospheres. X-ray diffraction (XRD) results from the theta–2theta (θ–2θ) scan, omega (ω) and phi (φ) scans for the films revealed a (100) cubic texture. The full-width-at-half-maximum (FWHM) values for YBCO (105) and YBCO (005) were 1.3° (in-plane epitaxy, Δ φ ) and 0.42° (out-of-plane epitaxy, Δ ω ), respectively. The YBCO films had a T c of 90.3 K. The measured transport J c , critical current density was 1.3×10 5 A/cm 2 at 77 K and self-field.

MOCVD of very thin films of lead lanthanum titanate

Films of lead lanthanum titanate were deposited using metal-organic chemical vapor deposition (MOCVD) at temperatures between 500 and 550{degrees}C in a hot-wall reactor. The precursors used were Pb(THD){sub 2}, La(THD){sub 3}, and Ti(THD){sub 2}(I-OPr){sub 2} where THD = 2,2,6,6-tetramethyl-3,5-heptanedionate, O{sub 2}C{sub 11}H{sub 19}, and I-OPr = isopropoxide, OC{sub 3}H{sub 7}. The three precursors were delivered to the reactor using a single solution containing all three precursors dissolved in tetraglyme and the precursor solution was volatilized at 225{degrees}C. Films were deposited on Si and Si/Ti/Pt substrates, and characterized using Rutherford Backscattering Spectroscopy (RPS) and X-ray diffraction(XRD). Films deposited at 550{degrees}C had a composition which was close to that of the precursor solution while films deposited at 500{degrees}C were deficient in lanthanum. Even at 500{degrees}C, the desired perovskite phase showed an increase in the intensity of the X-ray lines, but did not change the width of these lines, implying the grain sizes had remained unchanged.

Growth of TlBa2Ca2Cu3O9− y superconducting films with local biaxial alignment extending up to 5 mm on Ag substrates using a spray-pyrolysis technique

We report superconducting properties of TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 9{minus}y} (Tl-1223) thick films grown on smooth and rough Ag substrates. Thallium-free precursor films of the type Ba{sub 2}Ca{sub 2}Cu{sub 3}Ag{sub 0.37}O{sub 7} were deposited on Ag substrates using a spray pyrolysis technique followed by post-thallination in a two-zone furnace. XRD results showed the presence of a c-axis aligned Tl-1223 films and some degree of local in-plane texture (often referred as a colony microstructure). The microstructure of the Ag surface dictates the growth behavior of Tl-1223 films. The transport critical-current densities (J{sub c}) for Tl-1223 films grown on smooth and rough Ag substrates were {approximately}65000 A/cm{sup 2} and {approximately}37000 A/cm{sup 2}, respectively, at 77 K and zero field. The films grown on smooth Ag substrates were strongly linked and had good in-field properties with a J{sub c} value of {approximately}10{sup 4} A/cm{sup 2} at 77 K and 0.5 T. {copyright} {ital 1997 Materials Research Society.}

High-J c YBCO coatings on reel-to-reel dip-coated Gd 2 O 3 seed buffer layers epitaxially fabricated on biaxially textured Ni and Ni-(3at%W-1.7at%Fe) alloy tapes

A low-cost, non-vacuum reel-to reel dip-coating system has been used to continuously fabricate epitaxial Gd2O3 buffer layers on mechanically strengthened, biaxially textured Ni(3at.%W-1.7at%Fe), defined as Ni-alloy, metal tapes. X-ray diffraction analysis of the seed Gd2O3 layers indicated that well textured films can be obtained at processing temperatures (Tp) between 1100 and 1175 o C. Processing speed did not significantly affect the crystalline quality of the Gd2O3. Scanning electron microscopy revealed a continuous, dense and crack-free surface morphology for these dip-coated buffers. The Gd2O3 layer thickness led to remarkable differences in the growth characteristics of the subsequent YSZ and CeO2 layers deposited by rfmagnetron sputtering. Epitaxial YBCO films grown by pulsed laser deposition on the short prototype CeO2/YSZ/Gd2O3/Ni-(3at%W-1.7at%Fe) conductors yielded self-field critical current densities (Jc) as high as 1.2x10 6 A/cm 2 at 77 K. Pure Ni tapes were used to asses the viability of dip-coated buffers for long length coated conductor fabrication. The YBCO films, grown on 80 cm long and 1 cm wide CeO2/YSZ/Gd2O3 buffered Ni tapes by the industrially scalable ex-situ BaF2 precursor process, exhibited end-to-end self-field Jc of 6.25x10 5 A/cm 2 at 77 K.

Atomic force microscopy study of bicrystal SrTiO3 substrates and YBCO thin films

Single-crystal substrates allow the epitaxial deposition of high-T c superconductor (HTS) films that exhibit high critical current densities (J c ). The decrease in J c observed for films deposited on polycrystalline crystals is attributed to misorientation between adjacent grains. To understand better this phenomenon, deposition of HTS films on bicrystal substrates is used to model the effects of a single grain boundary. The surfaces of strontium titanate (001) bicrystals with tilt boundary orientations of 7°, 15° and 24° were analyzed by atomic force microscopy (AFM). Annealing conditions were found to be a key factor for obtaining atomically smooth surfaces. The grain boundary morphologies were analyzed from sections of the AFM images. Images of YBa 2 Cu 3 O 7-x (YBCO) films produced on these substrates by pulsed-laser deposition are shown for regions that include the grain boundaries. Different morphologies of YBCO thin films are presented.

Solution Synthesis of Epitaxial Rare-Earth Oxide Thin Films on Roll-Textured Nickel

Using solution chemistry, epitaxial films of rare-earth oxides of the general formula RE 2 O 3 (where RE = Sm to Lu) were prepared on cubic-textured nickel tapes. Solutions of metal methoxyethoxides or metal acetate/methoxyethoxides in 2-methoxyethanol were used to coat rolltextured nickel tapes using either spin-coating or dip-coating. Coated tapes were subsequently heated in a reducing atmosphere at temperatures between 950 °C and 1160 °C for varying lengths of time. Film quality was determined using X-ray diffraction and electron and surface probe microscopy. Films were found to be oriented both in and out-of-plane of the substrate, free of pinholes and gross defects, and of sufficient quality for use as substrates for high T c , superconductors.

Photoacoustic Spectroscopy and the In-Situ Characterization of the Electrochemical Interface

Photoacoustics is a new spectroscopic method which has been used for in situ characterization of the electrochemical interface during the past ten years. The basic principles of the photoacoustic effect and the principal results of the Rosencwaig-Gersho theory are discussed in light of the usefulness of the method in electrochemical studies. Different experimental arrangements suitable for in situ electrode studies are presented. A review of the use to date of photoacoustics in electrochemistry includes studies of electrochromic systems, semiconductor electrodes, passivation layers, and of electrocatalytic mixed oxides. These works demonstrated that, with relatively simple apparatus, it was possible to detect and to characterize very thin layers formed on electrodes. It is still not clear whether in most cases photoacoustic spectroscopy has an overwhelming advantage over well-established optical methods for absorption measurements; however, all the potentialities of the method have yet to be explored.