A. Kursumovic

Self-assembled, rare earth tantalate pyrochlore nanoparticles for superior flux pinning in YBa2Cu3O7-δ films

Addition of pyrochlore rare earth tantalate phases, RE3TaO7 (RTO, where RE = rare earth, Er, Gd and Yb) to YBa2Cu3O7?? (YBCO) is shown to vastly improve pinning, without being detrimental to the superconducting transition temperature. The closely lattice matched to RTO phase provides a lower interfacial energy with YBCO than BaZrO3 (BZO) and produces very fine (~5?nm) particles with high linearity in their self-assembly along c. Critical current densities of 0.86, 0.38?MA?cm?2 at 1 and 3?T (for fields) parallel to the c axis were recorded at 77?K in 0.5?1.0??m thick films and a transition temperature of 92?K was observed even in the highest level doped sample (8?mol%).

Pulsed laser deposition of epitaxial YBa2Cu3O7−y / oxide multilayers onto textured NiFe substrates for coated conductor applications

Pulsed laser depositions of double-buffer and triple-buffer YBa2Cu3O7−y (YBCO)/Y2O3(YSZ)/CeO2 heterostructures have been performed in situ onto commercially available biaxially textured NiFe 50%/50% tape. The deposition in the forming gas (4% H2/Ar) from a CeO2 target and the deposition in vacuum from a CeO2:Pd composite target have been explored as two possible routes for cube-on-cube growth of the first buffer layer. The influence of the critical processing parameters on the texture is investigated and some of the issues involved in the reduction of NiO (111) and the formation of cube-on-cube NiO (200) growth are discussed. X-ray diffraction has been used for texture evaluation of the substrate and subsequent deposited layers. The substrate–buffer interface region has been studied by focused ion beam cross section electron microscopy. Both the buffers and YBCO layers show biaxial alignment with ω and scans having optimum YBCO full width at half maximum (FWHM) values of 4.3° and 8.8°, respectively. The morphology has been characterized using atomic force microscopy and scanning electron microscopy. The value of Tc (onset) has been measured at 90 K (ΔTc = 10 K). The critical current density, Jc, has been measured by transport measurements and magnetic measurements performed in a dc SQUID magnetometer.

Vortex pinning landscape in YBa2Cu3O7 films grown by hybrid liquid phase epitaxy

The influence of film thickness and growth rate on the vortex pinning in hybrid liquid phase epitaxy (HLPE) films was explored. Film growth rates as high as 12 nm s−1 (0.7 µm min−1) produced high Jc films. Weak or no thickness dependence was found in films of thickness ranging from 0.4 to 3 µm. Field and angular measurements of the critical current density (Jc) and the power-law exponent (N) of the current–voltage curves were used to determine the nature of pinning. Films thinner than 0.6 µm showed a higher density of correlated defects parallel to the ab plane than thicker films. Using HLPE, it was possible to achieve very strong pinning in films ~3 µm thick, yielding critical currents over 300 A cm−1 width at self-field, and as high as 35 A cm−1 width at μ0H = 3 T at T = 75.5 K. Decreasing the deposition rate allowed improving the high field performance, opening up the possibility to engineer the pinning landscape of the HLPE films.

Hybrid liquid phase epitaxy processes for YBa2Cu3O7 film growth

A number of liquid phase epitaxy (LPE) related growth methods have been investigated. These hybrid-LPE processes enable high rate liquid assisted growth of epitaxial YBa2Cu3O7 films without the many disadvantages of classical LPE. Growth occurs by diffusive transport of Y through a thin liquid flux layer. This layer may be pre-deposited onto the substrate by various means including vacuum and non-vacuum techniques, or deposited at the growth temperature. The composition of the liquid layer is maintained during film growth by feeding YBa2Cu3O7, or the separate components, either from the vapour or by a powder route. Growth rates up to 10?nm?s?1 have been demonstrated. Deposition of c-axis oriented epitaxial YBa2Cu3O7 is reported on both seeded and non-seeded substrates; the process is tolerant of a high substrate mismatch. Films 1?2??m thick with ?K and a critical current density Jc> 2?MA?cm?2 have been grown on a range of single crystal substrates as well as on buffered textured metallic tapes. The mechanism of nucleation and growth from a thin liquid layer is described within the general theoretical framework of crystal growth. Particular features of the growth are the short time constant for equilibration of transients in the deposition conditions, the wide range of relative supersaturation spanned by the process, and dominance of interface kinetic effects compared to volume diffusion in the liquid flux.

Investigation of the growth and stability of (1 0 0)[0 0 1] NiO films grown by thermal oxidation of textured (1 0 0)[0 0 1] Ni tapes for coated conductor applications during oxygen exposure from 700 to 1400 °C

Abstract Thin ( nucleation rather than competitive grain growth controls the texture in this case. Nevertheless, the removal of the native oxide layer is crucial. Further oxidation of these templates in the temperature range from 800 to 1400 °C produced single crystal-like NiO films with a thickness up to ~70 μm and an improved cube texture. Comparison with the literature data indicates that volume diffusion of Ni contributes predominantly to the NiO growth over the whole temperature range. These NiO films can act as a good oxidation barrier at lower temperatures (~800 °C) for coated conductor applications.

Study of the rate-limiting processes in liquid-phase epitaxy of thick YBaCuO films

Abstract Liquid-phase epitaxy was used to grow c -axis YBa 2 Cu 3 O 7−δ thick films on NdGaO 3 (1xa01xa00) substrates in an Y supersaturated BaO–CuO solvent. In the case of stationary substrates, the familiar √ t growth kinetics is found. Substrate rotation was employed in order to induce forced convection in the liquid, thereby reducing the thickness of the diffusion boundary layer. The resulting growth kinetics showed transient and steady-state growth regimes. The transient regime extends to about 200xa0s with √ t growth kinetics. In the steady-state regime, diffusion across an established diffusion boundary layer led to a linear increase of film thickness with time. This diffusion boundary layer was estimated to be ∼35xa0μm for a substrate rotating at 200xa0rpm. Detailed study of the film growth suggested the presence of an interface kinetics that limited the growth on a competitive basis with volume diffusion in the liquid. The phenomenological kinetic coefficient is found to be ∼2.5×10 −6 xa0mxa0s −1 , which is significantly higher than theoretically predicted. The diffusion coefficient of Y in the solution was estimated from viscosity (Stokes–Einstein and Sutherland relations) to be around 2.9–4.3×10 −10 xa0m 2 xa0s −1 at 990°C, while the growth data gave a value of ∼4×10 −10 xa0m 2 xa0s −1 .

Ambient/low pressure synthesis and fast densification to achieve 55 K Tc superconductivity in NdFeAsO0.75F0.25

55xa0K superconductivity in NdFeAsO0.75F0.25 is demonstrated using two stage, ambient pressure synthesis (APS) with two zone temperature control for maintaining the As vapour pressure. The density of the APS material was subsequently increased by spark plasma sintering (SPS) for short times (60xa0s) at relatively low pressures (<80xa0MPa), giving density values above 95% of the theoretical value while maintaining very high slopes of upper critical field of up to 8.5xa0 Txa0K−1. The APS+SPS method developed here is ideal for fabricating tapes in a rapid and safe way.

Study and modelling of oxygen diffusion in YBa2Cu3O7-delta under isothermal conditions

This paper presents a study of oxygen diffusion in YBa2Cu3O7- (YBCO) using isothermal thermogravimetric analysis. Comparing the results of a solution of Ficks second law to the experimental data, it has been possible to calculate an expression for the chemical diffusion coefficient of oxygen in YBCO. The good agreement between the theoretical and experimental data suggests that the diffusion rate is not determined by the formation of a highly oxygenated layer on the outer surface of the sample. The analysis depends on a finite element model developed to study the diffusion process. The model both calculates the fractional weight change of the sample during the isothermal oxygenation stage and determines the change in distribution of oxygen within the sample during the oxygenation process. The treatment presented provides a convenient basis for preliminary analyses of any proposed oxygenation procedure, to check oxygenation times and the final distribution of oxygen within the sample.

High Ic, YBa2Cu3O7−x films grown at very high rates by liquid assisted growth incorporating lightly Au-doped SrTiO3 buffers

YBa2Cu3O7−x (YBCO) thick films were grown by hybrid liquid phase epitaxy (HLPE) on (001) SrTiO3 (STO) substrates. In the presence of a 100xa0nm thick, 5xa0mol% Au-doped STO buffer, self-field critical current densities, Jcsf, at 77xa0K of ~2.4xa0MAxa0cm−2 and critical currents, Icsf, up to 700xa0Axa0(cm-width)−1 were achieved. The Jc value is virtually independent of thickness and the growth rates are very high (~1xa0µmxa0min−1). From transmission electron microscopy (TEM), Y2O3 nanocloud extended defects (~100xa0nm in size) were identified as the pinning defects in the films. Enhanced random pinning was induced by the presence of Au in the buffer.

YBCO/Nd2CuO4/NiO/Ni coated conductors fabricated by liquid phase epitaxy based techniques

Liquid phase epitaxy (LPE) has a great potential in producing low-cost, high-current, coated conductors due to its fast growth rate in excess of 1 µm min−1 and the capability of growing thick films up to 10 µm without degrading the structural perfection or Jc. The main problem for LPE is the chemical reaction between the films and substrates at elevated growth temperatures. Former efforts have been focused on the reduction of growth temperatures. This has been proved to be unsuccessful due to the limited degree of temperature reduction; reducing the growth temperature also reduces the solubility of YBCO in the liquid, making the growth even more difficult. An alternative solution to this problem is to search for a special buffer which is particularly resistant to the attack of the high-temperature liquid. We have recently developed a new buffer, Nd2CuO4, which was very stable in the cuprate solution at temperatures around 950 °C and therefore extremely useful for LPE. Initial growth of YBCO on Nd2CuO4 buffered, surface oxidized Ni substrates showed a sharp superconducting transition at 90 K and transport Jc over 105 A cm−2 (77 K). Although YBCO could be grown on Nd2CuO4 by LPE without any other intermediate seed layer, growth of Nd2CuO4 on both sides of the NiO/Ni substrate with 100% coverage was not easy and had a low success rate. Complete coverage of Nd2CuO4 buffer on the NiO/Ni substrates was the critical step to avoid Ni contamination and achieve a high Jc.