Ex-situ growth of epitaxial YBCO layers

  1. Introduction into physics and chemistry of the ex-situ process
  2. Accomplishments of our group
  3. Recent results
  4. Publications

1. Introduction

Application of YBCO coated conductors requires a fast and inexpensive deposition technique, capable of producing high-quality YBCO layers 5 - 10 micron thick. In this respect the ex-situ or barium fluoride process seems attractive. The process is not sensitive to the deposition rate and long pieces of tape may be processed in a batch mode.

The ex-situ process, also known as the BaF2 process, consists of preparing a precursor film, a mixture of fine grain Y, Cu, and BaF2 in the correct stoichiometric ratio and subsequently processing the film in a controlled-atmosphere furnace to convert it to a c-axis textured YBCO film. An attractive aspect of this process is the separation of the deposition of the precursor layer from the heat treatment for the formation of the YBCO. This separation minimizes the requirements for in situ monitoring as well as allows the use of different methods for fabricating the precursor layers, e.g., electron beam evaporation and sol gel processes.

The chemical reaction for the decomposition of the precursor material and the formation of YBCO can be written schematically as;

Y-Ba-Cu-O-F(precursor) + H2O = YBCO + 2HF (1)

which is basically the conversion of an oxy-fluoride precursor into YBCO. It was shown by our group that rate of reaction (1) is determined by removal of HF by both gaseous convection and diffusion. The absolute value of HF partial pressure, P(HF), on the growth front is determined by equilibrium condition P(HF)/P(H2O)1/2 = K, where K is the equilibrium constant of reaction (1). We have constructed the following equation for YBCO growth rate G :

G = A P(H2O)1/2/Pt (2)

 where Pt is the total pressure in the reactor and A is a geometrical factor, which describes the impedance of HF out-diffusion from the film surface. Equation (2) allows predicting capacity of an ex-situ reactor of a given geometry. We have shown that conventional tubular ex-situ reactor has very small capacity for practical applications. Partial vacuum reactor appears to be more attractive in this respect.

2. Accomplishments of our group.

  1. We have found that the YBCO growth rate was limited by the removal of HF and partial pressure of HF is determined by H2O-HF equilibrium in reaction (1). We also gave an estimate of the equilibrium constant. (Applied Physics Letters, vol. 76, No 14, (April 2000), pp. 1911-1913, PDF(147 KB).
  2. We have shown that the rate of removal of HF from the surface by gaseous diffusion and convection is a primary rate-limiting mechanism. (Physica C vol. 353, No 1&2, (May 2001), pp. 269-274., PDF (122 KB).
  3. We have constructed growth rate-total pressure diagramm, proposed and realized a concept of partial-vacuum ex-situ reactor (IEEE Transactions on Applied Superconductivity, vol. 11, No 1, (March 2001), pp. 2939- 2942, PDF (379 KB).

3. Recent results

We are working on a novel technique based on spray pyrolysis deposition of inorganic precursor with subsequent fluorination.

4. Publications

Click on the PDF link in the right colomn to download hardcopy of the paper.

Applied Physics Letters, vol. 80, No 3 (January 2002), pp. 419-421, "Mechanisms for hetero-epitaxial nucleation of YBa2Cu3O6.1 at the buried precursor/SrTiO3 interface in the postdeposition reaction process", L. Wu, V. F. Solovyov, H. J. Wiesmann, Y. Zhu, and M. Suenaga.

PDF(183 KB)

Proceedings of 2001 International ISTEC-MRS Workshop on Superconductivity, Honolulu, Hawaii, June 24-27 2001, pp. 225-228, "The effects of HF partial pressure and pressure gradients on YBCO growth in the BaF2 process", Vyacheslav F. Solovyov, Harold J. Wiesmann, Masaki Suenaga .

PDF (486 KB)

Physica C vol. 353, No 1&2, (May 2001), pp. 269-274."Growth rate limiting mechanisms of YBa2Cu3O7 films manufactured by ex-situ processing", Vyacheslav F. Solovyov, Harold J. Wiesmann, Masaki Suenaga .

PDF (122 KB)

Proceedings of Applied Superconductivity Conference, ASCí2000, Virginia Beach, Virginia, September 17-22 2000. IEEE Transactions on Applied Superconductivity, vol. 11, No 1, (March 2001), pp. 2939- 2942, "Ex-situ Post-deposition Processing for Large Area Y1Ba2Cu3O7 Films and Coated Tapes", Vyacheslav F. Solovyov, Harold J. Wiesmann, Li-jun Wu, Masaki Suenaga .

PDF (379 KB)

Proceedings of Applied Superconductivity Conference, ASCí2000, Virginia Beach, Virginia, September 17-22 2000. IEEE Transactions on Applied Superconductivity Superconductivity, vol. 11, No 1, (March 2001), pp. 3876-3879, "Superconducting and Microstructural Properties of YBCO Thick Film Grain Boundaries on SrTiO3 Tilt Bycrystals", Qiang Li, V. F. Solovyov, H. J. Wiesmann, Y. Zhu, M. Suenaga.

PDF (405 KB)

Applied Physics Letters, vol. 76, No 14, (April 2000), pp. 1911-1913, "Kinetics of YBa2Cu3O7 film growth by postdeposition processing.", V. F. Solovyov, H. Wiesmann, Y. Zhu, L-J. Wu, M. Suenaga.

PDF(147 KB)

Proceedings of International Workshop on Critical Currents, IWCC-99, Madison, Wisconsin, July 7-10, 1999, pp. 64-65.  "Ex-situ Processing of High-Jc Thick YBCO Films on Flexible Metallic Substrates ", V. F. Solovyov, L-J. Wu, H. Wiesmann, Y. Zhu, M. Suenaga, D.P. Norton and K.R. Marken.

PDF(31 KB)

Proceedings of Applied Superconductivity Conference, ASCí98, Palm Springs, California, September 14-18 1998. IEEE Transactions on Applied Superconductivity vol. 9, No2, (June 1999) pp. 1467 - 1470. "High Rate Deposition of 5 um Thick YBa2Cu3O7 Films Using the BaF2 Ex-situ Post Annealing Process", Vyacheslav F. Solovyov, Harold J. Wiesmann, Li-jun Wu, Masaki Suenaga and Ron Feenstra.

PDF(405 KB)

Physica C vol. 309, No 3&4, (December 1997), pp. 269-274. "Thick YBa2Cu3O7 Films by Post Annealing of the Precursor by High Rate e-beam Deposition on SrTiO3 Substrates", Vyacheslav F. Solovyov, Harold J. Wiesmann, Li-jun Wu, Masaki Suenaga and Ron Feenstra.

PDF(138 KB)