Through-thickness superconducting and normal-state transport properties revealed by thinning of thick film ex situ YBa2Cu3O7−x coated conductors

Abstract

A rapid decrease in the critical current density (J c ) of YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ films with increasing film thickness has been observed for multiple YBCO growth processes. While such behavior is predicted from two-dimensional collective pinning models under certain assumptions, empirical observations of the thickness dependence of J c are believed to be largely processing dependent at present. To investigate this behavior in ex situ YBCO films, 2.0-and 2.9-m-thick YBCO films on ion beam assisted deposition-yttria stabilized zirconia substrates were thinned and repeatedly measured for (T) and J c (H). The 2.9 m film exhibited a constant J c ͓77 K, self-field (SF)͔ through thickness of ϳ1 MA/cm 2 while the 2.0 m film exhibited an increase in J c (77 K, SF) as it was thinned. Neither film offered evidence of significant dead layers, suggesting that further increases in critical current can be obtained by growing thicker YBCO layers. © 2003 American Institute of Physics. ͓DOI: 10.1063/1.1625103͔ Coated conductors ͑CCs͒, or second generation high temperature superconducting wires, hold great promise for commercial applications and electric power applications, such as transmission lines, transformers and motors. To meet the technical requirements of these applications, CCs need to deliver large critical current (I c ) values.1 An obvious approach to improving I c is to increase the thickness t of the superconductor layer, but for several YBa 2 Cu 3 O 7Ϫx ͑YBCO͒ growth processes, [2][3][4][5][6][7] there is a strong decrease in the critical current density J c with increasing film thickness t.Most knowledge of the thickness dependence of the critical current density of YBCO films comes from studies of multiple films produced with different YBCO layer thickness.2,3,6,7 A complementary approach to investigate J c (t) was employed by Foltyn et al. 4 Using ion milling, thick YBCO films were successively thinned and J c measured at multiple thickness on a single sample. This second approach also produces a variable thickness dataset, however, with reduced emphasis on thickness dependent trends in the growth, but enhanced emphasis on through-thickness homogeneity. The YBCO in Ref. 4 was grown by pulsed laser deposition ͑PLD͒ on metal tapes buffered with yttriastabilized zirconia ͑YSZ͒ textured by ion-beam-assisted deposition ͑IBAD͒. Strong structural degradation was observed in the films, most significantly in the form of ''deadlayers'' (I c ϭJ c ϭ0) at both the film surface and the buffer layer interface. Subsequent work by Foltyn and others has shown that these ''dead'' layers are not endemic of thick YBCO coated conductors, but instead dependent on the growth conditions. [8][9][10][11] In this letter, we report ion-milling studies on YBCO grown by the BaF 2 ex situ process on IBAD-YSZ buffered metal substrates. The details of the BaF 2 ex situ growth process 7,12 and preparation of IBAD-YSZ buffered metal tapes 13 are given elsewhere. Briefly, precursor layers of the desired thickness were prepared by vacuum deposition ...

Progress in Nanoengineered Microstructures for Tunable High‐Current, High‐Temperature Superconducting Wires

“…In these experiments, I c measurements are performed on patterned bridges after which a defined amount of the film is removed by ion-milling and all the measurements are repeated. [89,90] The bulk pinning force, defined as F p = J c (H) × H with Hʈc, is normalized (F p /F p,max ) and plotted as a function of the reduced field (H/H irr ), where H irr is the irreversibility field defined by extrapolation of the linear sections of the high-field regime to zero. The roughly linear increase in I c suggests a uniform distribution of current flow through thickness.…”

Influence of growth temperature on the vortex pinning properties of pulsed laser deposited YBa2Cu3O7−x thin films

“…Transport critical current density measurements were made at 77 K at an electric field criterion of 1 V / cm. To measure the angular dependence of J c , the film was progressively tilted by an angle from the magnetic field axis, always keeping a maximum Lorentz force configuration with the current flowing perpendicular to H. To better check for thermal fluctuation depinning effects, 29 we studied the J c ͑ ,77 K͒ properties of the 760°C sample by milling it from 210 to 95 nm using low energy Ar ions while the sample was cooled to ϳ230 K. 26,30 Transmission electron microscopy ͑TEM͒ and high resolution electron microscopy ͑HREM͒ imagings were performed in a Philips CM200UT and a JEOL JEM2011. All of these observations were carried out on cross sections viewed along the current flow direction so as to show the principal pinning defects capable of resisting the Lorentz force.…”

Importance of low-angle grain boundaries in YBa2Cu3O7−δcoated conductors

“…In fact the situation is rather complex as the extent of lateral overgrowth depends on YBa 2 Cu 3 O 7−δ film thickness. Thick BaF 2 -processed films have been known for some time to have a bimodal structure [62], as discussed for growth on RABiTS substrates and has been shown to have lateral layers separated by Ba 2 Cu 3 O y layers. More recent EBSD measurements comparing the structures 900 nm from the base of films grown to different thicknesses had very different microstructures.…”