Thallium-Based High-Temperature Superconducting Thick Films and Their Applications.

Thallium-based high-temperature superconducting thick films were deposited on polycrystalline MgO substrates; a trapezoidal firing profile was used and varied to find the best firing parameters. A new quantity, line critical current density (J_{rm LC}), was defined to evaluate the current-carrying capacity of the thick films. The optimum firing parameters are: temperature = 883^circ{rm C}, firing time = 0 min, cooling rate = 3.15 ^circ{rm C/min}. Samples fired under these optimum conditions have a J _{rm LC} of 17 A/cm. Up to 50 weight percent (wt.%) Ag powder was added to Tl-based powder. All samples with Ag content from 0 wt.% to 50 wt.% have a T_{rm zero} of about 112 K. Compared with the pure Tl-based sample, proper Ag addition can almost triple the line critical current density J_{rm LC}, while excessive Ag addition degrades the superconductivity. A statistical regression and numerical optimization show that optimum Ag content is 23.8 wt.%. Twenty different materials were tested as substrates for Tl-based superconducting thick films. The intensity of the reactions between the film and each substrate differed. Ranking the substrates tested against current-carrying capacity, the best substrates are single crystal MgO (100) (T _{rm zero}= 111.9 K, J _{rm LC} = 26 A/cm) and polycrystalline MgO (97%) (T_{rm zero} = 116.0 K, J_{rm LC} = 17 A/cm), followed by ZrO_2, NaGaO _3, Al_2{rm O}_3 (96%), Al_2{ rm O}_3+Ag buffer layer, Al _2{rm O}_3 (99%), YSZ (100), LaAlO_3 (100), SrTiO _3 (100) and Al_2 {rm O}_3+Au buffer layer. The superconductivity of the films on AlN, BeO, Si and all metals tested was destroyed completely. The current-carrying capacity increased exponentially with film thickness. When the film thickness exceeds 200 mu{rm m}, increasing the thickness further does not cause a significant increase of I_{rm C}. However, I_{rm C} was improved by multi-layer and Ag-sandwiched structures. With each new Tl layer added, I_{rm C} increased by 50% for the Tl-Tl-Tl structure and 100% for the Tl-Ag-Tl-Ag-Tl structure. Thallium-based thick films were coated on the outer surface of polycrystalline MgO hollow cylinders and crucibles to make magnetic shields. The shielding factor (SF) of the shields agrees with the theoretical formula SF = 76.6 z/(rln10) dB.