J.C. Moore

Grain boundary misorientation and thermal grooving in cube-textured Ni and Ni-Cr tape

AFM and EBSD characterization has been carried out to measure the degree of grain boundary grooving in cube-textured Ni and Ni-10wt%Cr substrates. Low angle grain boundary grooves are found to be consistently shallower than grooves at high angle boundaries. Our results suggest that the recrystallization of pure Ni substrates in a Ar-H/sub 2/ atmosphere rather than vacuum may be beneficial in minimizing groove depth. Grain boundary grooves in Ni-10wt%Cr tape were found to be deeper than in pure Ni, consistent with the higher temperature used for recrystallization.

Fabrication of biaxially textured Ni substrates and LaNiO/sub 3/ buffer layers for Tl-1223 thick films

We have fabricated cube texture nickel substrates and examined the relationship between percentage rolling deformation and annealing conditions on the degree of texture obtained using XRD and EBSD techniques. We have found that unaligned grains may be avoided by rolling to 95% reduction and annealing at 800/spl deg/C for 4 hours. We have fabricated LaNiO/sub 3/ films on YSZ and lanthanum aluminate by spray pyrolysis and achieved cube texture in thin coatings.

Fabrication of cube-textured Ag-buffered Ni substrates by electro-epitaxial deposition

Cube-textured silver buffer layers have been fabricated using the easily scalable technique of electro-epitaxial deposition. Continuous 150 nm thick silver films with in-plane FWHM values of 8° have been epitaxially deposited directly onto cube-textured Ni, Ni-10 wt% Cr and NiFe substrates using no intermediate buffer layer and no post-deposition heat treatment. Studies of the thermal stability of the coatings show that the texture is stable up to 900 °C and that continuous films are retained up to 800 °C. We believe that electro-epitaxial deposition offers a promising method for the fabrication of low-cost, long length biaxially textured substrates for superconducting tape.

Microstructure and phase development in melt processed Tl-1223 tapes

Abstract The effect of processing conditions on microstructure and phase development has been investigated for TlBi-1223 tapes made by a melt processing route. Increasing loss of crystallinity and phase decomposition into Tl-1212 and SrCaCu-oxides is seen for melt temperatures in the range 910–925°C. The 1223 phase is reformed during a slow cool and subsequent annealing. It has been established that for samples of composition (Tl 0.78 Bi 0.22 )(Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O δ a melt temperature of around 925°C and a cooling rate of 20°C/h results in a much increased grain size of up to 60–70 μm. Use of fully reacted 1223 powder was found to result in a larger increase in grain size and better phase purity than a partly reacted precursor.

Limitations To The Performance Of PIT Tl(Bi)-1223 Tapes

We have investigated the possibility of improving the weak link properties of Tl-1223 tape by melt processing, and also studied the technical limitations of coil fabrication using multimeter lengths of PIT tape. We conclude that melt processing does offer some potential for grain growth, but that grain alignment is only achieved in some places at the Ag/1223 interface. Until we can establish more control of the local grain alignment by altering the geometry of the samples, the properties of coils in high fields remains rather poor.

Optimization of phase formation and microstructure in Tl-1223 thick films

Tl-1223 films have been fabricated using spray pyrolysis on silver and zirconia substrates. We have investigated the effect of a Tl-2212/TlF source powder and observed a significant increase in melting and, consequently, a reduction of around 20/spl deg/C in the optimum reaction temperature and increased grain growth. We have found that the effect of the fluorine is greatest in the presence of silver. We have also observed that films grown on polycrystalline silver tape have several preferred in-plane orientations.

Bending strain tolerance of critical current values in Tl-based superconducting tapes

Mono- and multifilamentary silver-sheathed Tl-1223 powder-in-tube (PIT) tapes have been strained by bending at 77 K. The changes in the critical current of the samples due to the bending strain have been related to the tape microstructure. The shapes of the curves are similar to those found in the literature for Bi-2223 PIT samples. In situ reacted samples show a significant improvement in bending strain tolerance compared to samples prepared using pre-reacted powder.

Development of a melt processing route for the fabrication of Tl-1223 coils

Tl-1223 tapes with engineering critical current densities, (E-J/sub c/) values of 2500 Acm/sup -2/ can be routinely fabricated but the magnetic field properties are poor due to weak link effects. We have developed a partial melt processing route which results in increased grain growth with greater potential for achieving grain alignment. We have investigated the effect of Tl-1223 phase composition on the melt process and found that Bi substitutions lead to an increased grain size in tapes made using unreacted powder but a decrease in grain growth in tapes made with reacted powder.

Microstructural Comparison Between Melt Processed, In-situ Processed and Sintered Tl(Bi)-1223/Ag Tapes Using TEM.

We have compared the microstructure of sintered, in-situ and melt processed Tl(Bi)-1223/Ag tapes using TEM. Melt processing of the tapes increases grain connectivity and removes amorphous phases leading to an improvement in the IC/B characteristics of the material. No long range grain alignment, which is the main requirement for improving the properties, was observed in the tapes.

Processing of BiSrCaCuO materials with improved superconducting properties

The authors have been investigating a number of processing techniques for introducing texture into Bi2Sr2Ca1Cu2Ox samples. These include casting onto ceramic substrates followed by simple heat treatments and hot pressing. They present results obtained from these experiments and suggest how the properties of the materials may be improved by better control over the processing parameters.