Li-Jun Wu

Kinetics of YBa2Cu3O7 film growth by postdeposition processing

The ex situ growth of thick textured YBa2Cu3O7 films on SrTiO3 substrates was investigated in order to find the factors, which influence growth kinetics. It was found that the growth rate of YBa2Cu3O7 films remains constant during heat treatment and is proportional to the square root of the H2O partial pressure in the processing atmosphere. Using transmission electron microscopy, we observed that the growth of the YBa2Cu3O7 films proceeds from a thin, ∼7-nm-thick, layer of liquid located at the growth front. Chemical analysis of this layer and the adjacent material suggests that decomposition of an oxifluoride compound in the precursor film occurs at the liquid layer-precursor film interface. The rate of this decomposition reaction defines the growth rate of YBa2Cu3O7. Using a simple model we show that rate of this reaction is limited by the out-diffusion of the decomposition product HF.

High rate deposition of 5 /spl mu/m thick YBa/sub 2/Cu/sub 3/O/sub 7/ films using the BaF/sub 2/ ex-situ post annealing process

We have investigated the crystallization of thick YBCO films under various annealing conditions. The films were deposited by high-rate co-evaporation of Y, Cu and BaF/sub 2/ onto room-temperature SrTiO/sub 3/ substrates at rates exceeding 10 nm/s. TEM microscopy was performed to establish the microscopic structure of partially processed films and help elucidate the growth mechanism. In-situ resistivity measurements were used to monitor the crystallization of the YBCO films during the annealing process. Resistivity measurements and TEM microscopy of samples at different stages of the film growth are compared. Our results indicate that, in addition to oxygen partial pressure, water vapor pressure is an important parameter which defines the dynamics of film growth and crystallization. 5 /spl mu/m thick films with J/sub c/>2/spl times/10/sup 5/ A/cm/sup 2/ (1 T H/spl par/c) have been fabricated.

Ex-situ post-deposition processing for large area Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ films and coated tapes

Ex-situ post-deposition reaction processing is considered a promising alternative to in-situ physical vapor deposition techniques for coated Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ conductor development. It was reported recently that attempts at ex-situ processing of long Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ precursor tapes resulted in an inhomogeneous growth rate over the tape length and rather poor properties for the whole sample. We performed a set of experiments in order to clarify the mechanisms for the ex-situ processing of large area films and estimate properties of long coated conductors manufactured by the ex-situ technique. It was found that rate limiting step of ex-situ growth of Y/sub 1/Ba/sub 2/Cu/sub 3/O/sub 7/ is removal of the reaction product, hydrofluoric acid, by both gas diffusion and convection in the reaction atmosphere. We report on a quantitative model that well describes the observed growth rates for films with various areas.

Mechanisms for hetero-epitaxial nucleation of YBa2Cu3O∼6.1 at the buried precursor/SrTiO3 interface in the postdeposition reaction process

The mechanisms have been identified for the hetero-epitaxial nucleation of YBa2Cu3O∼6.1 (YBCO) at the buried interface between a precursor film and SrTiO3 for the so-called BaF2 process which is a postdeposition reaction process for the synthesis of epitaxial YBCO films. It is shown that the preferential nucleation of YBCO at the interface is due to (1) the strong chemical affinity of the (Y,u200aBa)-oxy-fluoride, an intermediate phase, to SrTiO3 and (2) the epitaxial alignment of its (111) planes with the (001) surface of the SrTiO3 which reduces the activation barrier for the formation of YBCO. In thin films (<2–3 μm) the YBCO nuclei, whose c axes are perpendicular to the SrTiO3 surface, form directly from this aligned oxy-fluoride. In thick films (5 μm), however, this oxy-fluoride decomposes into a disordered transitory cubic phase which then orders to form YBCO nuclei with three orientational variants, one with its c axis perpendicular and two with their c axes parallel to the (001) plane of SrTiO3.

Comparative studies of MgB/sub 2//Mg nano-composites and press-sintered MgB/sub 2/ pellets

We present our studies of superconducting and microstructural properties of bulk MgB/sub 2//Mg nano-composites and press-sintered MgB/sub 2/. TEM investigation revealed that the composites are very dense, consisting of nano-sized MgB/sub 2/ grains connected by the well-defined clean grain boundaries (GB). Both amorphous and structural intact GBs were observed in the press-sintered samples. Magnetization and transport measurements showed that the composite carried over 1 MA/cm/sup 2/ critical current density (J/sub c/) at 5 K and self-field, while the pressed sintered MgB/sub 2/ has J/sub c/ /spl sim/ 0.25 MA/cm/sup 2/ at 5 K and self-field. The origin of substantially higher J/sub c/ found in the composites is mostly due to their nano-sized grains and clean grain boundaries.

A method for improving nucleation of thick YBCO films in the ex-situ process

Some coated conductor applications require a YBCO layer at least 5 /spl mu/m thick with high J/sub c/. The growth of thick c-axis oriented epitaxial layers using the barium fluoride ex-situ technology is not always possible. Films over 3 /spl mu/m thick have more undesirable a-oriented and random grains than thinner ones. We present an analysis of thick film nucleation and conclude that thick precursor layers impede out-diffusion of the ex-situ reaction product, HF. High impedance for HF diffusion results in large variations of the chemical potential of the growth reaction and disruption of the nucleation process. We conclude that a solution for c-axis growth is control of the precursor permeability. A method is described for controlling the permeability of precursor layers. Using this technique we were able to grow c-axis oriented 5-/spl mu/m thick films on oxide-buffered Ni tape with J/sub c/(0 T) = 4 /spl times/ 10/sup 5/ A/cm/sup 2/ and J/sub c/(1 T) = 8 /spl times/ 10/sup 4/ A/cm/sup 2/.

Twin boundaries and critical current densities of YBa2Cu3O7 thick films fabricated by the BaF2 process

2 µm thick epitaxial YBa2Cu3O7, YBCO, films on SrTiO3 substrates were prepared at the reaction temperatures of 725, 750 and 800 °C by the BaF2 process. The dc critical current densities of these films were found to vary by a factor of 2 at 77 K in perpendicular dc magnetic fields measured up to 1.5 T. The twin boundary spacing was essentially unchanged among these films while the size of Y2O3 precipitates in the films varied by a factor of ~5. Also, the measured ratios of the lattice parameters, a/b, of the films were surprisingly larger than those found in standard fully oxygenated YBCO. This unusual property of the films appeared to be caused by the presence of the Y2O3 precipitates which provided a strain constraint in limiting the full tetragonal-to-orthorhombic phase transformation due to the oxygenation. Differences in the degree of the constraint possibly led to the observed difference in Jc among these films.

BAF(2) POST-DEPOSITION REACTION PROCESS FOR THICK YBCO FILMS.

The BaF2 process is a very attractive fabrication process for YBCO conductors since it allows the deposition and the formation steps of YBCO films to be separately performed, and it has been demonstrated that the films with high critical current densities can be fabricated on the buffered metallic substrates. However, we still face one major challenge for this process to be made as a commercially viable fabrication method. This is the avoidance of non-c-axis-oriented growth of YBCO when the thickness and the growth rates for the films are increased significantly beyond what are currently achieved. Meeting both the thickness and the growth-rate requirements simultaneously is extremely difficult. One possible way to make the rate requirement reduced is to find winding methods of the tapes in such ways that the both-sided tapes can be reacted in a batch process. Although the winding fixture will be more complicated than a simple dram, I believe that there are ways to construct such fixtures if the subatmospheric-pressure processing is incorporated in the design for reacting the wide precursor tapes. Then, we are only faced with the problem of growing the c-axis-oriented thick YBCO films. In order to overcome the latter difficulty, we need a detailed understanding of the YBCO nucleation kinetics in thick films under various atmoshperic conditions. Such an understanding is likely to lead to the development of new or modified precursor-deposition methods and/or reaction-heat treatment procedures to facilitate the growth of the thick YBCO layers without the non-c-axis-oriented YBCO growth.