A spectroscopic comparison of YBCO superconductors synthesised by solid-state and co-precipitation methods

Raman spectroscopy, X-ray diffraction (XRD), and scanning electron microscopy (SEM) have been used to compare samples of YBa2Cu3O7 (YBCO) synthesised by the solid-state method and a novel co-precipitation technique. XRD results indicate that YBCO prepared by these two methods are phase pure, however the Raman and SEM results show marked differences between these samples.

FT Raman spectroscopic characterization of oxalate precursors to YBCO superconductors

FT Raman spectroscopy has been used to characterise the composition of the oxalate precursor to YBCO superconductors. By comparison to spectra of barium, copper and yttrium oxalate it is concluded that the co-precipitate incorporates not only the individual oxalate species but also a species ascribed to a mixed oxalate system. Significantly, Raman spectroscopy demonstrated that the precursor was not amorphous as previously deduced from XRD studies. In contrast, it is hypothesised that the sample consists of very small crystalline particles.

Bulk manufacture of YBCO powders by coprecipitation

This work presents an assessment of the coprecipitation technique for the reliable production of high-temperature superconducting (HTS) copper-oxide powders in quantities scaled up to 1 kg. This process affords precise control of cation stoichiometry (< 4% relative), occurs rapidly (almost instantaneously) and can be suitably developed for large-scale (e.g. tonne) manufacture of HTS materials. The process is based upon a simple control of the chemistry of the cation solution and precipitation with oxalic acid. This coprecipitation method is applicable to all copper-oxides and has been demonstrated in this work using over thirty separate experiments for the following compositions: YBa2Cu3O7-δ, Y2BaCuO5 and YBa2Cu4O8. The precursor powders formed via this coprecipitation process are fine-grained (∼ 5-10 nm), chemically homogeneous at the nanometer scale and reactive, Conversion to phase-pure HTS powders can therefore occur in minutes at appropriate firing temperatures. © 1995.

YBCO thin films on Yttria stabilized Zirconia and LaAlO3-growth and properties

Laser deposition was used to deposit YBaCuO thin films on Yttria-stabilized Zirconia substrates, at substrate holder temperatures of 710-765 °C. We observed a transition from singlecrystalline to polycrystalline growth at a temperature of ∼750 °C. All films were highly c-axis oriented and had critical temperatures between 89.5 and 92 K. In the twinned singlecrystalline films, the lowest measured microwave surface resistance was 0.37 mΩ at 4.2 K and 21.5 GHz, and the highest critical current 5×106 A/cm2 at 77 K. The polycrystalline films had up to a factor of 50 higher surface resistance and a factor of 10 lower critical current. A meander line resonator made of a film on a LaAlO3 substrate, showed a microwave surface resistance of 5μΩ at 4.2 K and 2.5 GHz. © 1991.

Study of in situ laser deposited YBCO thin films

YBCO thin films were fabricated by laser deposition, in situ on MgO substrates, using both O2 and N2O as process gas. Films with Tc above 90 K and jc of 106 A/cm2 at 77 K were grown in oxygen at a substrate temperature of 765 °C. Using N2O, the optimum substrate temperature was 745 °C, giving a Tc of 87 K. At lower temperatures, the films made in N2O had higher Tc (79 K) than the films made in oxygen (66 K). SEM and STM investigations of the film surfaces showed the films to consist of a comparatively smooth background surface and a distribution of larger particles. Both the particle size and the distribution density depended on the substrate temperature.

Studies of the phase evolution of YBCO materials with different additives

Y123 samples with varying amounts of added Y211, PtO 2 and CeO 2 have been melt processed and quenched from temperatures between 960°C and 1100°C. The microstructures of the quenched samples have been characterized using a combination of x-ray diffractometry, optical microscopy, scanning electron microscopy, microprobe analysis, energy-dispersive x-ray spectroscopy and wavelength-dispersive x-ray spectroscopy. The Ba-Cu-O-rich melt undergoes complex changes as a function of temperature and time. A region of stability of BaCuO 2 (BC1) and BaCu 2O 2 (BC2) exists below 1040°C in samples of Y123 + 20 mol% Y211. Ba 2Cu 3O 5 is stabilized by rapid quenching but appears to separate into BC1 and BC2 at lower quenching rates. PtO 2 and CeO 2 additions affect the distribution and volume fractions of the two Ba-Cu-oxide phases.

Melt-textured YBa2Cu3O7-x wires having plate-like grains

YBCO wires which consist of well oriented plate-like fine grains are fabricated using a moving furnace to achieve higher mechanical strength. Melt-texturing experiments have been undertaken on YBCO wires with two different compositions: YBa1.5Cu2.9O7-x, and YBa1.8Cu3.0O7-x. Wires are extruded from a mixture of precursor powders (formed by a coprecipitation process) then textured by firing in a moving furnace. Size of secondary phases such as barium cuprate and copper oxide, and overall composition of the sample affect the orientation of the fine grains. At zero magnetic field, the YBa1.5Cu2.9O7-x wire shows the highest critical current density of 1,450 Acm-2 and 8,770 Acm-2 at 77K and 4.2K, respectively. At 1 T, critical current densities of 30 Acm-2 and 200 Acm-2, respectively, are obtained at 77K and 4.2K. Magnetisation curves are also obtained for one sample to evaluate critical current density using the Bean model. Analysis of the microstructure indicates that the starting composition of the green body significantly affects the achievement of grain alignment via melt-texturing processes.

Microstructural characterization of quenched melt-textured YBa2Cu3O7-δ materials

The microstructure of YBa2Cu3O7-δ (YBCO) materials, melt-textured in air and quenched from the temperature range 900-990°C, has been characterized using a combination of x-ray diffractometry, optical microscopy, scanning electron microscopy, transmission electron microscopy, and energy dispersive x-ray spectrometry. BaCu2O2 and BaCuO2 were found to coexist in samples quenched from the temperature range 920-960°C. The formation of BaCu2O2 preceded the formation of YBCO. Once the YBCO had formed, BaCu2O2 was present at the solidification front filling the space between nearly parallel platelets of YBCO. Large Y2BaCuO5 particles at the solidification front appeared divided into smaller ones as a result of their dissolution in the liquid that quenched as BaCu2O2.

Effects of substrate temperature on the microstructure of YBa2Cu3O7-δ films grown on (001) Y-ZrO2 substrates

High-quality YBa2Cu3O7-δ films grown on (001) single-crystal Y-ZrO2 substrates by pulsed laser deposition have been studied as a function of substrate temperature using transmission electron microscopy. A transition from epitaxial films to c-axis oriented polycrystalline films was observed at 740°C. An intermediate, polycrystalline, BaZrO3 layer was formed from a reaction between the film and the substrate. A dominant orientation relationship of [001] YBCO//[001]int. layer//[001]YSZ and [110] YBCO//[110]int. layer//[100]YSZ was observed. The formation of grain boundaries in the films resulted in an increased microwave surface resistance and a decreased critical-current density. The superconducting transition temperature remained fairly constant at about 90 K.

Early stages of growth of YBa2Cu3O7-δ high Tc superconducting films on (001) Y-ZrO2 substrates

The early stages of growth of high quality YBa2Cu 3O7-δ (YBCO) films grown on (001) Y-ZrO2 (YSZ) substrates by pulsed laser deposition have been studied using a combination of atomic force microscopy and transmission electron microscopy. A one unit cell thick YBCO layer and relatively large CuO particles formed in the initial stages. Additional YBCO grew on top of the first layer in the form of one or a few unit cell high c-axis oriented islands about 30 nm in diameter. The rounded islands subsequently coalesced into faceted domains. Elongated Y 2BaCuO5 particles nucleated after the first layer of YBCO. A highly textured BaZrO3 layer formed between the YSZ and the YBCO with a cube-on-cube dominant orientation relationship with respect to the YBCO film.

Analysis and prediction of the critical current density across [001]-tilt YBa2Cu3O7- grain boundaries of arbitrary misorientation angles

A qualitative analysis of the expected dilatation strain field in the vicinity of an array of grain-boundary (GB) dislocations is presented. The analysis provides a basis for the prediction of the critical current densities (jc) across low-angle YBa2Cu3O7- (YBCO) GBs as a function of their energy. The introduction of the GB energy allows the extension of the analysis to high-angle GBs using established models which predict the GB energy as a function of misorientation angle. The results are compared to published data for jc across [001]-tilt YBCO GBs for the full range of misorientations, showing a good fit. Since the GB energy is directly related to the GB structure, the analysis may allow a generalization of the scaling behavior of jc with the GB energy. © 1995 The American Physical Society.

Microstructure of an artificial grain boundary weak link in an YBa2Cu3O(7-δ) thin film grown on a (100)(110), [001]-tilt Y-ZrO2 bicrystal

The microstructure of an artificial grain boundary in an YBa2Cu3O7-δ (YBCO) thin film grown on a (100)(110), [001]-tilt yttria-stabilized-zirconia (YSZ) bicrystal substrate has been studied using transmission electron microscopy (TEM). The orientation relationship between the YBCO film and the YSZ substrate was [001]YBCO∥[001]YSZ and [110]YBCO∥[100]YSZ for each half of the bicrystal film. However, the exact boundary geometry of the bicrystal substrate was not transferred to the film. The substrate boundary was straight while the film boundary was wavy. In several cases there was bending of the lattice confined within a distance of a few basal-plane lattice spacings from the boundary plane and microfaceting. No intergranular secondary phase was observed but about 25% of the boundary was covered by c-axis-tilted YBCO grains and a-axis-oriented grains, both of which were typically adjacent to CuO grains or surrounded by a thin Cu-rich amorphous layer.

Microstructure and properties of artificial grain boundaries in epitaxial YBA2Cu3O7-δ thin films grown on [001] tilt YZrO2 bicrystals

The microstructure of artificial grain boundaries in YBa2Cu3O7-δ (YBCO) thin films grown on [001] tilt YZrO2 (YSZ) bicrystal substrates has been characterized using transmission electron microscopy and atomic force microscopy. Despite a relatively straight morphology of the substrate boundaries, the film boundaries were wavy. The waviness was a result of the combined effects of grooving at the substrate boundaries prior to the film deposition and an island-growth mechanism for YBCO on YSZ substrates. The dihedral angle of the groove walls varied with the misorientation angle and depended on the symmetry of the substrate boundary. The amplitudes of the film boundary waviness compared well with the widths of the grooves. In addition, the grooves induced local bending of the YBCO lattice planes and additional tilt components perpendicular to the c-axis close to the film boundaries. © 1995.