C-axis Grain Research Articles

Influence of porosity on the critical currents of trifluoroacetate-MOD YBa/sub 2/Cu/sub 3/O/sub 7/ films

The influence of porosity on the superconducting properties have been investigated on YBa/sub 2/Cu/sub 3/O/sub 7/ thin films deposited on LaAlO/sub 3/ [100] substrates by the so-called Trifluoroacetate (TFA) route. Micro-Raman spectroscopy have been used to determine the concentration of c-axis grains /spl delta/ in different samples and their influence on the final film porosity as observed from SEM imaging. This has been compared with measurements of resistivity and critical currents in the same samples. We prove that this /spl delta/ fraction is the main parameter controlling the porosity and hence the normal-state resistivity of the thin films. The optimization of the microstructure of these YBa/sub 2/Cu/sub 3/O/sub 7/ TFA films allow to have high critical currents : J/sub c/ = 3 /spl times/ 10/sup 6/ A/cm/sup 2/ at 77 K.

Thickness dependence of microstructure and critical current density of Yba2Cu3O7−δ on rolling-assisted biaxially textured substrates

The change in microstructure associated with the decrease in critical current density (Jc) of Yba2Cu3O7−δ (YBCO) films with increasing thickness was examined. Samples of pulse laser deposited YBCO films varying in thickness from 0.19 to 3.0 μm on rolling-assisted biaxially textured substrates with an architecture of CeO2/YSZ/CeO2/Ni were prepared by tripod polishing for cross-sectional electron microscopy. More randomly oriented grains in the upper portion of the YBCO film surface were observed with increasing film thickness, resulting in less cube texture. In addition, increases in mismatch across the boundaries of the c-axis grains with increasing time during deposition, along with the development of BaCeO3 and Y2BaCuO5 phases at the YBCO/CeO2 interface, contributed to the degradation of film properties. Surface outgrowths of the YBCO film were examined as well as the defect structures and second-phase formations within the films.

Processing, microstructure and characterization of artificial joins in top seeded melt grown Y–Ba–Cu–O

A technique based on the presence of an unreacted barium-cuprate phase within the melt-processed microstructure (predominantly at the platelet boundaries), which melts below the peritectic temperature of the YBa2Cu3O7–δ (Y123) phase has been developed to join large Y–Ba–Cu–O (YBCO) grains. On heating under an applied pressure of 0.1 MPa, the Ba–Cu–O liquid phase is released to the grain interface. This technique has been investigated for YBCO a–b-plane-aligned grain boundaries with misorientation angles of up to 45° and an aligned c-axis grain boundary. Optical and transmission electron microscopy studies indicate that the low-angle a–b plane grain boundaries are epitaxial in nature and are generally of high microstructural quality. The electrical and magnetic properties of the grain boundaries fabricated by this technique have been characterized by I–V and transport current measurements as a function of magnitude and orientation of the applied field. The temperature dependence of the irreversibility field of low-angle a–b grain boundaries is similar to that of the grains, providing direct evidence for the quality of the join. A join of nominally zero misorientation between the a–b planes was found to support a critical current density, Jc, exceeding 2000 A cm−2 at 77 K in fields of up to 4 T.

Microstructure and superconducting properties of powder-in-tube processed MgB 2 tapes

We fabricated MgB 2 tapes using a powder-in-tube method without any heat treatment. Commercial MgB 2 powder was put into stainless steel, Cu–Ni and pure Cu tubes, and the powder/tube composites were cold worked into tapes. The packing density of the MgB 2 layers and J c increased with increasing the mechanical hardness of the tube. A very dense and hard MgB 2 layer was obtained in the stainless-steel sheathed tape, and a high J c of ∼4.5×10 5 A/cm 2 at 4.2 K and zero field was obtained by extrapolating the J c– B curves. Some c-axis grain alignment of the MgB 2 layer was obtained for the tape conductors. The degree of this grain alignment increased with increasing the hardness of the sheath material. This grain alignment brought about the anisotropy in J c with respect to the field direction. A large anisotropy factor of about 10 was obtained in a field of 7 T for the MgB 2/(SUS 316) tape.

Microstructural dynamics of Bi-2223/Ag tapes annealed in 8% O2

The microstructural dynamics of Bi-2223 tapes are investigated in situ during annealing in 8% O2 by means of 100 keV x-ray diffraction. A green mono- and a green multi-filamentary tape are annealed at 829.5 °C. During ramp-up (Ca,Sr)2PbO4 decomposes above 750 °C, resulting in an incorporation of Pb in Bi-2212. The associated grain growth of Bi-2212 is the main cause of the strain relief and the c-axis grain alignment of the Bi containing phases. Above 825 °C the Bi-2212 partly dissociates into (Ca,Sr)14Cu24Ox and a liquid phase. The linewidth of Bi-2212 is constant during the transformation to Bi-2223, indicating no strain or finite-size broadening. The most probable transformation mechanism is found to be nucleation and growth with a fast decomposition of the individual Bi-2212 grain, followed by a growth of Bi-2223 from the Bi-2212 melt reacting with (Ca,Sr)14Cu24Ox. The multi-filamentary tape transforms faster and exhibits a much better degree of c-axis alignment. Results are compared to similar studies in air. During a final annealing of a fully converted tape no change is found in the average concentration, stoichiometry and grain misalignment. A post-annealing experiment at 650 °C on quenched tapes shows a reaction over 3 weeks, whereby randomly oriented 3222 grains are formed and Bi-2223 is depleted of Pb. Simultaneously, the critical current decreases by a factor of 2.

Bi-2212/Ag線材における高<I>J</I><SUB>C</SUB>化のための組織制御

The relationships between a critical current density (JC) and microstructure of Bi-2212/Ag composite tapes prepared by the melt-solidification under a temperature gradient have been investigated. We used a tube furnace with a temperature gradient for the heat treatment. The critical current density (JC) of the order of 4×109 A/m2 at 4.2 K under 10 T were obtained for the tape fabricated under a temperature gradient. We optimized the tape width, temperature gradient and cooling rate in melt-solidification process. The optimum tape width, temperature gradient and cooling rate were 2-4 mm, 200 K/m, and 2 K/h, respectively. JC enhancement of these tapes fabricated by melt-solidification under temperature gradient was attributed the improvement of the c-axis grain alignment, the in-plane texturing near the oxide/silver interface, and large grain size.

Preparation and orientation control of RMnO 3 (R=Y, Yb) thin film by chemical solution deposition

RMnO 3 (R=Y, Yb) thin films were prepared by chemical solution deposition using alkoxy-derived precursors. The crystallization behavior, orientation and morphology of YMnO 3 films depended on the annealing conditions. It was confirmed that heat treatment in vacuum at the initial stage of crystallization affected the restraint of the perovskite phase and the formation of the hexagonal phase, and the following heat treatment in oxygen promoted c-axis orientation and grain growth. In contrast, YbMnO 3 films crystallized in the hexagonal phase both in vacuum and oxygen. The orientation of the YbMnO 3 films did not strongly depend on the annealing ambience compared with that of YMnO 3 films. The substitution of Y ion by Yb ion made it easy to form the hexagonal structure. However, the surface roughness became larger. The lattice constant of c-axis increased and that of a-axis decreased as the Yb ratio increased.

Microstructure and superconducting properties of Bi-2212/Ag tapes fabricated using melt-solidification under a temperature gradient

We fabricated Bi-2212/Ag composite tapes by applying a melt-solidification process under a temperature gradient. We used a tube furnace with a temperature gradient for the heat treatment. The critical current density ( J c) of the tape fabricated under a temperature gradient of 1.5°C/cm was 4.4×10 5 A/cm 2 at 4.2 K under 10 T. This J c value was much higher than the J c value of the tapes fabricated by using the conventional melt-solidification process. We clearly observed quadruple rotation symmetry peaks in the (1 1 5) pole figure of Bi-2212 near the Ag substrate. These results show that the in-plane textured Bi-2212 oxide was obtained near the oxide/silver interface of the Bi-2212/Ag tape. J c enhancement was achieved due to this in-plane texturing together with the improvement of the c-axis grain alignment.

Film thickness dependence of microwave surface resistance and microstructure in YBa 2Cu 3O 7−δ thin films

Film thickness dependence of microwave surface resistance, morphology and microstructures of YBa 2Cu 3O 7− δ thin films were studied by X-ray diffraction (XRD), AFM and transmission electron microscopy (TEM). There was no direct correlation between the microwave surface resistance and film thickness, although full width at half maximum of XRD rocking curve of (0 0 5) peak increased with increase in film thickness. AFM morphology investigation showed no significant difference on film surface roughness and particulate density. Through the TEM cross-section microstructure analysis, it was found that the c-axis film became more regular from the bottom to the top, which could explain the abnormal phenomenon observed. It was very interesting to note that the c-axis films contained few stacking faults and tilting c-axis grains with a-axis grains occurring immediately. We have proposed that the occurrence of small volume fractional a-axis grains with high-angle grain boundary might benefit microwave dissipation loss.

Low-temperature synthesis in vacuum of c-axis oriented ferroelectric YMnO3 thin films using alkoxy-derived precursors

Hexagonal YMnO3 thin films were prepared on Pt(111)/TiOx/SiO2/Si and Pt(111)/Al2O3(0001) substrates using alkoxy-derived precursor solutions. The films were prepared by spin coating the YMnO3 precursor solutions and then, the films were calcined and crystallized via rapid thermal annealing in oxygen or vacuum ambient. The annealing conditions and substrates were critical for crystallization of ferroelectric YMnO3 films. When annealed in vacuum, the films both on Pt(111)/TiOx/SiO2/Si and Pt(111)/Al2O3(0001) substrates crystallized to hexagonal YMnO3 and the orientation depended on the substrates. The film on Pt(111)/Al2O3(0001) had c-axis orientation and the film on Pt(111)/TiOx/SiO2/Si had no preferred orientation. In addition, it was found that crystallization behavior, orientation and morphology of YMnO3 films on Pt(111)/TiOx/SiO2/Si substrates depended on the annealing condition. The heat-treatment in vacuum at initial stage for crystallization affected the restraint of perovskite phase and formation of hexagonal phase. The following heat-treatment in oxygen promoted the c-axis orientation and grain growth. The optimum annealing procedure for crystallization of the c-axis oriented YMnO3 films on Pt(111)/TiOx/SiO2/Si was addressed.

Critical current characteristics of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films on [110] SrTiO/sub 3/

The material and electrical characteristics of YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) thin films deposited by inverted cylindrical magnetron sputtering on [110] SrTiO/sub 3/ (STO) were investigated. X-ray diffractometry shows the grain orientations to be predominantly the YBCO [110] and [103] with no evidence of c-axis grains, Electron micrographs show the film surface to consist of coupled elongated grains parallel to the [110] STO edge. The films were patterned into small 2.5 mm squares parallel to the substrate edges for electrical characterization. Transport currents parallel and perpendicular to the [110] substrate edge showed a 945:1 anisotropy in film resistance and a factor of two in critical current density for temperatures below 60% of the transition temperature (T/sub c/). The temperature dependence of the critical current near T/sub c/ was quadratic-like and strongly dependent on the value of T/sub c/ used in the analysis. For the two orientations, there was nearly a 6 K difference in T/sub c/ as determined by the point at which the critical current became zero. The response of the critical current to small magnetic fields was greater for transport current along the c-axis direction and was observable over a temperature interval nearly four times greater than for current along the basal plain. These YBCO thin films have good response to small magnetic fields and are suitable for vortex flow device development.

Synthesis and Interface Structure of Sol Gel YBa2Cu3Ox Thin Films for Conductor Development

ABSTRACTIn our previous work we have obtained YBCO thick films on silver alloy substrate via the peritectic solidification. To further develop grain-textured YBCO thin films for conductor development, in this study, we have used a sol gel approach to deposit YBCO thin films on two different substrates, namely, yttrium-stabilized zirconia (YSZ) and a silver alloy containing 10 % of palladium (Ag –10%Pd). We have found that the sol gel film on YSZ exhibits an epitaxial growth. This is confirmed by both x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). XRD data also indicate that the YBCO film on Ag-10%Pd has a c-axis grain orientation. The details are reported on sol gel synthesis and XRD and HRTEM characterization of the YBCO thin films.

The dielectric properties and optical propagation loss of c-axis oriented ZnO thin films deposited by sol–gel process

The c-axis oriented ZnO thin films were prepared on various substrates by sol–gel processes. The stability of solution was examined through solvent and stabilizer. The c-axis orientation and grain size of films were increased with increasing of heat treatment temperature. The optical propogation losses of ZnO films deposited SiO 2/Si(111) substrates were measured using end-coupling method. The losses result in the scattering of the interface of ZnO/SiO 2, and the ZnO grain. Dielectric constant and resistivity of thin films deposited on Pt/SiO 2/Si(111) substrates are, respectively, in the range of 7–13 and 1.7×10 4∼9.8×10 5Ω cm.

Fabrication of high current carrying Bi-2212/Ag superconducting tapes by the pre-annealing and cold rolling process

The pre-annealing and rolling process is employed to improve the microstructure and J c for Bi-2212/Ag tapes fabricated by using the dip-coating method and melt–solidification process. It is found that J c has been enhanced significantly in tapes processed by the combination of pre-annealing at 800–880°C and cold rolling at the deformation rate of 20–30%. The increase of J c is assigned to the enhancement of c-axis grain alignment and the uniformity of the Bi-2212 layer that brings about the improvement of grain coupling. This method is promising for fabricating long BSCCO tapes with homogeneous and high transport J c.

Deposition of YBCO thin films over large areas by a 90° off-axis sputtering technique

We report the deposition of YBCO thin films with uniform thickness (87.5 K) and critical current density (J/sub c/ /sub 77/ /sub K/>1/spl times/10/sup 6/ A/cm/sup 2/, J/sub c/ /sub 4.2/ /sub K/>1/spl times/10/sup 7/ A/cm/sup 2/) over an 8" diameter area. Considering only the better optimized samples the films displayed high critical current densities (J/sub c/ /sub 77 K/ >3/spl times/10/sup 6/ A/cm/sup 2/, J/sub c/ /sub 4.2 K/ >3/spl times/10/sup 7/ A/cm/sup 2/). The variations in the film properties have been correlated to the variations in crystalline quality, amount of c-axis grains and substrate temperature during deposition.

Evidence of a “notch effect” in microwave surface resistance versus deposition temperature for Y 1Ba 2Cu 3O 7− x thin film on MgO (100) substrate

The effect of deposition temperature of Y 1Ba 2Cu 3O 7−x thin film on the microwave surface resistance R s has been studied for films deposited in-situ by cathodic sputtering on MgO (100) single crystal substrate and compared to results obtained on LaAlO 3 (100) single crystal substrates. The measurements of R s were performed at 10 GHz and 77 K using the dielectric resonator method. It was found that YBa 2Cu 3O 7−8 films where R s values are lower than 0.5 mΩ were obtained on MgO substrate within an extremely narrow temperature range (1–2 °C). The presence of the lowest R s values were found to be correlated with the lowest RBS yield in channelling geometry ( χ min) and with the lowest quantity of misaligned c-axis grains.

In-plane and out-of-plane dissipation in c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3O x silver-sheathed tapes

The temperature dependence of the out-of-plane and in-plane resistance of a c-axis-oriented (Bi,Pb)2Sr2Ca2Cu3O x silver-sheathed tape with high critical current density (Jc>2×104 A/cm2, 77 K, 0 T) has been investigated under applied fields up to 0.9 T. It is found that the in-plane and the out-of-plane resistance transitions are quite different: (1) The out-of-plane zero resistance temperature is much higher than the in-plane one, i.e., Tcab(H)≪Tcc(H), and the difference increases with magnetic field; (2) the out-of-plane zero resistance temperature corresponds to the c-axis decoupling temperature. These phenomena are attributed to different dissipation mechanisms. The in-plane dissipation at low temperature results from the thermally activated flux–flow, while the out-of-plane dissipation originates from the Josephson junction dissipation of weakly coupled c-axis grain boundaries and/or intrinsic Josephson junctions.

Growth features and intergranular connectivity of melt processed YBCO

Abstract Large grain melt processed Y-Ba-Cu-O (YBCO) samples have been prepared by seeded and unseeded growth techniques. The current carrying ability within individual grains and across grain boundaries has been investigated and correlated with features in the microstructure of samples fabricated by both techniques. The development of an inhomogeneous, cell-like growth microstructure in seeded samples at distances ≈4 mm from the seed is related to a saturation in inclusion density and volume proportion of Y2BaCuO5 (Y211) particles. A significant decrease in critical current density is associated with this change over a wide temperature range. The resistance of high angle c-axis grain boundaries is observed to depend critically on the magnitude of the injection current whereas the intra-granular resistance is not influenced significantly by this variable. Jc of a grain boundary fabricated by unseeded melt growth is estimated to be less than 100 A cm−2 at 77 K in zero applied field, which is more than two orders of magnitude lower than the intragranular Jc. Field screening measurements suggest that low angle grain boundaries do not form weak links between grains in modest magnetic fields and hence do not present a significant barrier to current flow.

Fabric domains in quartz mylonites: localized three dimensional analysis of microstructure and texture

Fabric domains of a quartz vein deformed under greenschist facies conditions are analysed with domainal X-ray texture goniometry, optical microscopy, computer integrated polarization microscopy (CIP) and shape analysis in two mutually perpendicular sections. The analysed domains consist of large non-recrystallized deformed clast fragments, prolate non-recrystallized ribbon grains and prolate recrystallized domains. In terms of texture, monomaximal and polymaximal domains have been distinguished. Pronounced partitioning of deformation is observed between these domains. This strain partitioning asks for caution when making kinematic inferences for fault zones based on a study of domainal fabrics. Domains persist as fabric elements up to high strains. This emphasizes the often neglected role of a non-random initial fabric. Neighbouring domains with alternating c-axis orientations and grain boundary preferred orientations, indicating opposing senses of shear, occur in both sections.The bulk texture is controlled by clast and ribbon domains, rather than being strictly deformation-induced. We infer that prolate elongation of clasts and ribbons is achieved mainly by combined prism and basal slip. Recrystallized domains are characterized by different textures. They are deformed by intracrystalline slip, combined with grain boundary migration and sliding. Inclined preferred grain boundary orientations are continuously maintained by grain boundary migration.

Fabrication and characterization of a-axis / c-axis oriented YBa 2Cu 3O y bilayer thin film

We have investigated YBa 2Cu 3O y bilayer thin films consisting of different cyrstal-axis orientations. A c-axis oriented bottom layer is deposited on a SrTiO 3 (100) substrate by pulsed-laser deposition, and then an a-axis oriented top layer is deposited by off-axis RF magnetron sputtering after reduction of the bottom layer. This reduction process contributes to the growth of the a-axis layer. At the interface between these layers, however, pyramidal c-axis oriented grains are observed. The a-axis oriented grains tilt by 2° from the [010] and [001] azimuths of the substrate. This structure is suitable for good lattice matching between the a-axis grains and the pyramidal c-axis grains. This bilayer film exhibits good electrical properties having T c = 80 K and J c = 2 × 10 6 A/cm 2 at 60 K. It is considered that the tilted structureof the a-axis oriented grains contributes to the superconductivity.