N. E. Elliott

Microwave surface resistance and power dependence of thallium-based films deposited onto large-area silver substrates

Microwave surface resistance (Rs) measurements on large-area (11.4 cm2) Tl-based films deposited onto Ag (Consil 995) substrates have been made at a frequency of 18 GHz as a function of temperature. Deposition onto unoriented Ag substrates yields unoriented films characterized byRs values of 8.2 and 33.6 mΩ at 10.6 and 77 K, respectively. In contrast, similar deposition onto oriented Ag substrates yields oriented films with values of 12.6 and 14.6 mΩ at 11.2 and 77 K, respectively; corresponding Cu values are 9 and 21 mΩ. Additionally, it is found that the dependence ofRs on microwave surface magnetic fieldHs is weaker for the oriented films. These results suggest that the effect of orienting thec-axis in Tl films is twofold: (1) the high-frequency superconducting transition is made considerably sharper, resulting in a lowerRs value at 77 K, and (2) the rate of increase inRs with appliedHs is reduced. Both effects have significant ramifications for the potential application of these materials to high-frequency accelerating cavities.

Microwave surface resistance of magnetron-sputtered Tl-Ba-Ca-Cu-O films on silver substrates

Surface resistance measurements on Tl‐Ba‐Ca‐Cu‐O thick films (∼15 μm) magnetron sputtered onto BaF2‐buffered, silver‐based (Consil 995) substrates have been made at a microwave frequency of 22 GHz. The relatively large‐area films (∼5 cm2) are characterized by surface resistance values of 6.9±2 mΩ at 11.3 K and 30.2±1 mΩ at 77 K; the corresponding values for Cu are 10 and 22 mΩ, respectively. These results demonstrate that Tl‐Ba‐Ca‐Cu‐O can be deposited onto large‐area, metallic substrates with characteristic surface resistance values lower than Cu at 4 K. Orientation of the film should improve the surface resistance at 77 K, thereby making the fabrication of microwave cavities that are superior to Cu possible.

Microwave surface resistance of YBa2Cu3O7 thin films on LaAlO3 substrates

The surface resistance Rs of YBa2Cu3O7 (YBCO) thin films (0.6±0.1 μm) deposited onto 2.5‐cm diam (100) LaAlO3 substrates has been measured at 22 GHz using both Cu and Nb cavities. The surface resistance falls precipitously at the superconducting transition (Tc =90 K) from a normal state value of approximately 2 Ω to a 77 K value of 13.7±1 mΩ, which is 1.6 times lower than Cu. At 4 K the surface resistance is 1±0.1 mΩ, as measured in a Nb superconducting cavity, which is an order of magnitude lower than Cu. The critical current density at 77 K is 4.5×104 A/cm2. Pole figure analyses show the ratio of c‐axis to a‐axis‐oriented material in the film is 2.4:1. YBCO films deposited onto either LaGaO3 or LaAlO3 substrates with varying c/a ratios yield surface resistance values at 77 K that are crudely correlated with Rs. Therefore, the principal effect of orienting the material is to improve the sharpness of the high‐frequency superconducting transition, consistent with the notion that the sharpness is associated...

Microwave surface resistance of YBa sub 2 Cu sub 3 O sub 7 thin films on LaAIO sub 3 substrates

The surface resistance Rs of YBa2Cu3O7 (YBCO) thin films (0.6±0.1 μm) deposited onto 2.5‐cm diam (100) LaAlO3 substrates has been measured at 22 GHz using both Cu and Nb cavities. The surface resistance falls precipitously at the superconducting transition (Tc =90 K) from a normal state value of approximately 2 Ω to a 77 K value of 13.7±1 mΩ, which is 1.6 times lower than Cu. At 4 K the surface resistance is 1±0.1 mΩ, as measured in a Nb superconducting cavity, which is an order of magnitude lower than Cu. The critical current density at 77 K is 4.5×104 A/cm2. Pole figure analyses show the ratio of c‐axis to a‐axis‐oriented material in the film is 2.4:1. YBCO films deposited onto either LaGaO3 or LaAlO3 substrates with varying c/a ratios yield surface resistance values at 77 K that are crudely correlated with Rs. Therefore, the principal effect of orienting the material is to improve the sharpness of the high‐frequency superconducting transition, consistent with the notion that the sharpness is associated...

High-frequency properties of Tl-based superconductors deposited onto large-area metallic substrates

Microwave surface resistance measurements were made on large-area (11.4-cm/sup 2/) Tl-Ba-Ca-Cu-O thick films/ss ( approximately 10 mu m) which were magnetron sputtered onto oriented Ag alloy (Consil 995) substrates. Initial films were prepared by depositing a BaF/sub 2/ buffer layer between the superconducting film and the substrate to control the film/substrate interaction; however, films exhibiting the best microwave properties were prepared without a buffer layer. Surface resistance measurements were made by replacing the end wall of an 18-GHz Cu cavity (TE/sub 011/ mode) with the superconducting film. The best surface resistance values obtained on the unbuffered film were 4 and 14 m Omega at 10 K and 77 K, respectively; the corresponding Cu values are 8 m Omega and 21 m Omega . The dependence of surface resistance on surface magnetic field was measured in a similar way except that an Nb cavity was used instead of a Cu one. Films exhibiting the highest degree of c-axis texturing show the weakest dependence of surface resistance on magnetic field and also exhibit the sharpest high-frequency transition into the superconducting state. These results are important for potential cavity applications of these materials.

A Tl-based high-temperature superconducting X-band cavity

A nonplanar X-band cavity amenable to coating with high-temperature superconducting films by conventional physical vapor deposition processes has been designed, fabricated, and tested. The cavity geometry resembles a symmetric clamshell configuration. It consists of two truncated cones joined at their bases. The specific dimensions of the cavity were calculated using the SUPERFISH computer code. Cavities were constructed using a silver-based alloy, Consil 995. The separate cavity sections were coated with Tl-based high-temperature superconducting material using a two-step deposition and annealing process. The unloaded Q values for this coated clamshell cavity are 66,000 and 105,000 at 77 and 20 K, respectively.

Electrophoretic deposition of Tl-Ba-Ca-Cu-O and Bi2Sr2CaCu2Ox thick films

Abstract Thick films of Bi-2212 and Tl-2212 superconductor were electrophoretically deposited onto Consil substrates. Tl series starting powder containing BaCuO 2 impurity phase resulted in films with higher Tl content. X-ray diffraction showed that Bi-2212 films were well oriented only if melt textured. Melt textured Bi-2212 films were characterized in a TE 011 resonant cavity at 17.46 GHz and compared to a magnetron sputtered Tl-2212 thick film. It was found that the surface resistance and surface reactance of the Bi-2212 were equally sensitive to external DC magnetic fields.

Microwave power absorption by thallium‐based superconductors on metallic substrates

Tl‐based superconducting thick films deposited on large‐area (11.4 cm2) Ag alloy (Consil 995) substrates have been characterized by surface resistance measurements yielding values of 4 and 14 mΩ at 4 and 77 K, respectively, as measured in an 18 GHz Cu cavity; corresponding Cu values are 8 and 21 mΩ. Utilizing a Nb cavity, the dependence of surface resistance on microwave power was also measured. The surface resistance begins to rise in microwave magnetic fields of 1–10 Oe and saturates in 20–60 Oe, the exact values depending on the degree of c‐axis texturing of the polycrystalline films. Enhanced texturing sharpens the high‐frequency transition into the superconducting state and also weakens the dependence of surface resistance on microwave magnetic field. These results are significant for high temperature superconductor applications such as microwave cavities.

Highly-Textured Tl-Ba-Ca-Cu-O Polycrystalline Superconducting Films on Ag Substrates

Thick (8 to 10 µm) Ba-Ca-Cu-O films have been rf magnetron sputtered onto Ag alloy (Consil 995) substrates. The films were given a post-deposition anneal in an over pressure of Ti in order to form the superconducting phases. Annealing protocols were done which result in predominantly the 1212 and 2212 phases. The substrate orientation was varied to determine its effect on film orientation. Material properties of the films were characterized by x-ray diffraction (XRD), ion beam backscattering spectroscopy, energy dispersive x-ray analysis (EDAX), and scanning electron microscopy (SEM). Electrical characterization of the films was done using dynamic impedance (DI) at 10 kHz and rf surface resistance (Rs) at 18 GHz in a TE011 fundamental mode cavity.

Radio frequency surface resistance of Tl‐Ba‐Ca‐Cu‐O films on metal and single‐crystal substrates

Films of Tl‐Ba‐Ca‐Cu were dc magnetron sputtered from a single multielement target. The films were deposited onto substrates of: (1) magnesium oxide, (2) a silver based alloy (Consil 995), (3) a nickel based alloy (Haynes 230), and (4) buffer layers of barium fluoride or copper oxide on Consil. To form superconducting phases, post‐deposition anneals were made on these films using an alumina crucible with an over pressure of thallium and flowing oxygen. After annealing, the film phases were determined using x‐ray diffraction. The film surface resistances (Rs) were measured at 22 GHz in a TE011 cavity.