BaFz POST:DMPOSHION REACTION PROCESS FOR THICK YBCO FILMS

The basic processes of the so-called BaFz process for the formation of YBaXh07, YBCO, films as well as its advantages over the in situ formation processes are discussed in the previous chapter. The process and the properties of YBCO films by this process were also nicely described in earlier articles by R. Feenstra, et al.1 Here, we will discuss two pertinent subjects related to fabrication of technologically viable YBCO conductors using this process. These are (1) the growth of thick (>> 1 p) c-axis-oriented YBCO films and (2) their growth~rates. Before the detail discussions of these subjects are given, we first briefly discuss what geometrical structure a YBCO-coated conductor should be. Then, we will provide examples of simple arguments for how thick the YBCO films and how fast their growth rates need to be. Then, the discussions in the following two sections are devoted to (1) the present understanding of the nucleation and the growth process for YBCO, and why it is so M13cult to grow thick c-axis-oriented films (> 3 yin), and (2) our present understanding of the YBC~ growth-limiting mechanism and methods to increase the growth rates. The values of critical-cunent densities JOin these films are of primary importance for the applications, and t~e above two subjects are intimately related to the control of Jc of the films. In general, the lower the temperatures of the Y13C0 formation are the higher the values of JCof the films. 1’2Thus, the present discussion is limited to those films which are reacted at -735 “C. This is the lowest temperature at which c-axis-oriented YBCO films (l-3 pm thick) nre comfortably grown. It is also well known that the non-c-axis oriented Y13C0 platelets are extremely detrimental to the values of Jc such that their effects on J. dwarf essentially all of other microstructural effects which control J.. Hence, the discussion given below is mainly focused on how to avoid the growth of these crystallite when the fiims are thick and/or the growth rates are high.