The achievement of low cost deposition techniques for high critical current YBa2Cu3O7 coated conductors is one of the major objectives to achieve a widespread use of superconductivity in power applications. Chemical solution deposition techniques are appearing as a very promising methodology to achieve epitaxial oxide thin films at a low cost, so an intense effort is being carried out to develop routes for all chemical coated conductor tapes. In this work recent achievements will be presented towards the goal of combining the deposition of different type of buffer layers on metallic substrates based on metal–organic decomposition with the growth of YBa2Cu3O7 layers using the trifluoroacetate route. The influence of processing parameters on the microstructure and superconducting properties will be stressed. High critical currents are demonstrated in 'all chemical' multilayers.

http://ematweb.cmi.ua.ac.be/emat/pdf/1256.pdf

Chemical solution deposition: a path towards low cost coated conductors

The influence of three processing parameters, temperature, gas flow rate and water pressure, on the YBa2Cu3O7 film growth on LaAlO3 single-crystal substrates from trifluoroacetate precursors has been investigated and the optimal film processing conditions to achieve high critical currents have been determined. We have found that the growth conditions maximizing the critical current density are those where the nucleation of a-axis oriented grains is minimized, as determined by μ-Raman spectroscopy. Under these conditions the normal state resistivity is very near to that of single crystals because a vanishingly small film porosity is achieved. Transmission electron microscopy analysis of films quenched from the growth temperature gives some hints for understanding the mechanism linking the film porosity with the concentration of a-axis grains. A cross-linked influence of different processing parameters, such as temperature and water pressure, or water pressure and gas flow, has been demonstrated. The optimal growth temperatures are 790–830 °C, but at these growth temperatures, the critical current density is still dependent on the gas flow rate and water pressure. The optimal processing ranges are a compromise between two different competing phenomena influencing the quality of the films: inhomogeneous film formation due to HF gas stagnancy at small nominal growth rates (low gas flow rate or water pressure) and perturbed crystallinity at high gas flow rates or water pressures.

http://iopscience.iop.org/article/10.1088/0953-2048/18/8/020/pdf

The influence of growth conditions on the microstructure and critical currents of TFA-MOD YBa2Cu3O7 films

Chemical solution growth of epitaxial YBa2Cu3O7 thin films on single crystalline substrates has been investigated using trifluoroacetate precursors. The concentration of the starting solution was selected to achieve a final film thickness of 250 nm. First, it is shown that high quality films can only be achieved if an homogeneous nanocrystalline film is obtained after the spin coating deposition and pyrolysis steps. Secondary phases such as BaCuO2, Y2BaCuO5 and CuO remain after the high temperature growth process, when macrosegregation is detected after these initial processing steps. A local degradation of texture associated with macrosegregation is detected by μ-Raman spectroscopy. The influence of growth temperature on the film quality has been analysed and the reaction times have been optimized at each temperature according to the reaction kinetics. In-situ fluoride analysis and μ-Raman spectroscopy have been used to determine the advancement of the formation reaction. The most apparent microstructural modification of these thin film samples with optimized annealing times has been found to be an enhanced porosity for low processing temperatures. The influence of porosity on the normal state resistivity and the critical currents has been evidenced. Optimized processing parameters lead to samples with very high critical currents (Jabc = 3.2 × 106 A cm−2 at 77 K and 2.7 × 107 A cm−2 at 5 K) which demonstrates the capability of the trifluoroacetate metalorganic deposition method for thin film and coated conductor preparation.

https://iopscience.iop.org/article/10.1088/0953-2048/16/1/309/pdf

High quality YBa2Cu3O7 thin films grown by trifluoroacetates metalorganic deposition

We report on measurements of the surface resistance of YBa2Cu3Ox thin films at frequencies between 0.087 and 2 THz and temperatures between 50 and 120 K by time-domain terahertz-transmission spectroscopy (TDTTS) and resonant microwave spectroscopy. The determination of the surface resistance of superconducting thin films by TDTTS is extended to higher frequencies and thicker films than previously by numerically solving the complex transmission coefficient. The numerical solution also provides the dielectric function of the YBa2Cu3Ox thin films. The temperature and frequency dependence of the surface resistance of YBa2Cu3Ox thin films in the THz range is successfully explained by a weak coupling model of d-wave superconductivity which incorporates inelastic and elastic scattering. The surface resistance of YBa2Cu3Ox thin films at THz frequencies is compared to the surface resistance of gold and niobium. The advantages of YBa2Cu3Ox thin films for superconducting THz electronic devices are discussed.

https://hp.physnet.uni-hamburg.de/wilke/wilke_jap_0300.pdf

Terahertz surface resistance of high temperature superconducting thin 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.

/pdf/influence-of-porosity-on-the-critical-currents-of-3oh0vxoibg.pdf

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

We present a model that describes the epitaxial quality of YBa2Cu3O7 films by three parameters — the c-axis oriented fraction of the film, its degree of in-plane orientation and the degree of in-plane orientation of the remaining a-axis oriented fraction. These parameters can be specified by the relative phonon intensities in polarized Raman spectra. We have determined the epitaxy parameters of a-axis and c-axis oriented laser-deposited YBa2Cu3O7 films on MgO (100) substrates. The accuracy of these parameters has been tested by performing Raman measurements at various rotation angles. The ratio of the intensities of the B2g and B3g modes observed on the a-axis oriented film indicates that a-axis growth is preferred to b-axis growth.

M. Löhndorf

Papers

Epitaxial quality of c-axis and a-axis oriented YBa2Cu3O7 films. Characterization by Raman spectroscopy