We have reviewed the convenience and possibility of utilizing thallium-based HT c superconductors for power applications. Some basic properties of Tl-based systems such as the crystal structure, phase stability, weak-links, pinning problems and irreversibility lines are discussed first. Two basic approaches to conductor preparation are then described, the closed and the open one. Special attention is focused on the fabrication of Tl-1223 and Tl-2223 superconductors. In the case of the more successful open approach a two-step technological procedure is usually chosen: first, the basic precursor material without Tl is synthesized, then second, the precursor is thallinated in a one-or two-zone reaction chamber. The most important precursor preparation techniques are described including aerosol deposition from either solution or ink, electro-deposition, sol-gel methods such as spin-and dip-coating and screen-printing and painting. Finally, some properties of the produced conductors important for power applications are presented.

In the following survey a rather promising, simple, non-vacuum method of preparation of high-& (HT,) superconducting films synthesized by means of chemical deposition from an aerosol is described. Thin HT, films of YBCO, BSCCO and TBCCO systems were prepared by this method. Some details of the deposition process as well as the deposition technique are given. Properties of prepared YBCO, BSCCO and TBCCO films are presented. The best films, suitable eventually for high-temperature, high-current and high-field applications, are so far polycrystalline films of the TBCCO 1223 system prepared on polycrystalline YSZ substrates having Tc = 104-107 K, and J, = 110000 A cm-*/77 K, B = 0; Jc = lo4 A cm-*/60 K, B = 2 T, or Jc = I O 5 A ~m -~/ 4 . 2 K, B = 9 T. Possible mechanisms of this high-current ability are discussed. Contents 2.5. Substrates used for aerosol deposition 2.6. The aerosol deposition technique 3.1. Sample preparation and characterization 3.2. Properties of prepared films 4.1. Sample preparation and characterization 4.2. Properties of prepared films 5.1. Sample preparation and characterization 5.2. Properties of prepared films 3. YBCO thin films 4. BSCCO thin films 5. TBCCO thin films 6. Conclusion * I

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Thin YECO films were prepared from aerosol by a low-temperature deposition process consisting of two steps-the deposition at atmospheric pressure and firing in vacuum degraded by partial pressure of oxygen-both at temperatures not exceeding 600 "C. A stoichiometric 1-24 aqueous nitrate solution of Y. Ba and Cu constituents was used as a source of aerosol. MgO and AI,O, substrates were heated to 140 to 160°C during deposition. The obtained films were 1 to 10pm thick with T, > 80 K and J, = lo3 to 10' Acm-'. However, detailed TG, OTG and OTA studies performed by us and others showed that thermal decomposition of t h e nitrates used only starts at temperatures higher than 200°C. The use of low substrate temperatures during deposition (140 to 160 "C in our case) is apparently t h e reason for rather low J. values.J. values at processing temperatures not exceeding 600 to 700°C. Such temperatures are still interesting for substrates with elevated diffusivity into prepared films.

Superconducting TI-Ba-Ca-Cu-0 (TBCCO) 2212 films have been prepared by annealing the spray deposited precursor Ba-Ca-Cud/MgO layers with crude TBCCO pellets. Properties of both films and bulks are described. To deposit the precursor films, an aerosol generated ultrasonically from aqueous nitrate solution was used. The prepared 2212 films were 2-3 pm thick; the highest 7, = 102 K was obtained in both film and pellet annealed at 850 "C for 30 min. The Jo values measured on some films were in the intelval 103-104 A the XRPD data show rather low grain orientation both in films and in bulks. The x-ray patterns revealed that the film samples contain more than 95 vol.% of 2212 phase; the bulks contain more than 9O.vol.% of 2212.The AES depth profiles in the surface layers of films show a slight drop in the TI content towards the film-substrate interface. From the Raman spectra, XRPD data, structural investigation and physical properties of obtained 2212 phases, correlations among cationic disorder, c axis parameters, J, values and frequency of the Raman modes were deduced. Results of detailed thermogravimetric TG, DTG and DTA measurements of homogenized precursoi Ba-Ca-Cu nitrate and oxide mixtures as well as those of the crude TCBa-Ca-Cu oxide mixture are presented.

The elemental composition and depth profiles of MgB2 films prepared by successive e-beam evaporation as well as by thermal co-deposition of Mg and B components were investigated by Rutherford backscattering spectrometry (RBS). In the case of films deposited by e-beam evaporation we studied both Mg-B precursors and appropriate MgB2 films grown on glassy carbon, Si(100) and J-sapphire substrates annealed in situ. For the films co-deposited by thermal evaporation on R-sapphire substrates and annealed ex situ we investigated superconducting MgB2 films only. The Tco values of all MgB2 films ranged from 21 to 30 K. Because of a very fine granular structure of the annealed films, confirmed also by SEM observations, we could not identify any MgB2 phase from x-ray diffraction (XRD) patterns. On the other hand, Mg2Si phase has been detected by XRD on the film–substrate interface for the superconducting film deposited on Si(100) substrate. The RBS measurements were performed with a 3.1 MeV 4He+ beam. Under these conditions, the 16O(α,α)16 elastic resonance allowed us to detect oxygen in all studied samples especially in B layers. The depth profiles of precursors prepared by successive e-beam evaporation showed clearly the multilayer film structure consisting of B and Mg layers. A strong interdiffusion between B and Mg layers may be observed after an in situ annealing, but still some degree of non-homogeneous component distribution may be observed. On the other hand, the MgB2 films co-deposited by thermal evaporation and annealed ex situ are much more homogeneous, but a higher content of oxygen is present.

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