E. J. Peterson

Effects of very thin strain layers on dielectric properties of epitaxial Ba0.6Sr0.4TiO3 films

We have epitaxially grown Ba0.6Sr0.4TiO3 (BST-0.4) thin films on MgO(001) substrates. By inserting a very thin Ba1−xSrxTiO3 (x=0.1–0.7) interlayer between the MgO substrate and the main layer of BST-0.4, we are able to manipulate the degree of the stress in BST-0.4 films. We have controlled the stress states, i.e., the lattice distortion ratio (D=in-plane lattice constant/out-of-plane lattice constant) of the BST-0.4 films by varying the chemical composition of the interlayers. We have found that small variations of D value can result in significantly large changes of dielectric properties. A BST-0.4 film under small tensile stress, which has a D value of 1.0023, shows the largest dielectric permittivity and tunability.

Effect of lattice strain and defects on the superconductivity of MgB2

The influence of lattice strain and Mg vacancies on the superconducting properties of MgB2 samples has been investigated. High quality samples with sharp superconducting transitions were synthesized. The variations in lattice strain and Mg vacancy concentrations were obtained by varying the synthesis conditions. It was found that high strain (∼1%) and the presence of Mg vacancies (∼5%) resulted in lowering the Tc by only 2 K.

Target modification in the excimer laser deposition of YBa2Cu3O7−x thin films

Under conditions used for pulsed laser deposition (308 nm, 20 ns, 2–8 J/cm2) of Y‐Ba‐Cu‐O superconducting thin films, we have measured a pronounced decrease in deposition rate with cumulative laser exposure of the target. This decay in rate is accompanied by evolution on the target surface of microscopic columnar structures, having yttrium‐enriched surfaces, which are aligned in the direction of the incident laser beam (45°). Neither the vapor plume direction nor film stoichiometry is affected by the presence of these oriented, chemically altered surface features.

Magnetism and heavy fermion-like behavior in the RBiPt series

Members of the RBiPt (R=Ce–Lu with the exceptions of Pm and Eu) series have been grown as single crystals. Magnetic susceptibility and electrical resistance have been measured on all members of the series, and specific heat measurements have been performed on representatives. The high temperature resistance uniformly changes from that of a small‐gap semiconductor or semimetal seen in NdBiPt to that of a heavy‐fermion metal seen in YbBiPt, which shows a linear coefficient of specific heat at low temperatures of 8 J/K2 mole. Further, the lighter rare earth members show an unusually sharp increase in their resistance associated with antiferromagnetic ordering at low temperatures.

Influence of deposition rate on the properties of thick YBa 2 Cu 3 O 7–δ films

To investigate potential limits to the rate at which high-quality YBa 2 Cu 3 O 7–δ can be deposited, we have produced a series of 1 μm thick films by pulsed laser deposition on single-crystal SrTiO 3 substrates at average rates ranging from 2 A/s to 240 A/s. The critical current density of low-rate films was over 2 MA/cm 2 at 75 K, self field, but dropped linearly with rate to about 1 MA/cm 2 at the upper end of the range. In addition, the superconducting transition temperature, resistivity above the transition, and performance in an applied magnetic field were all degraded by increasing the deposition rate. A change in c -axis lattice parameter suggests that possible causes for this degradation are oxygen deficiency or cation disorder with the latter being the more likely. Annealing high-rate films at 790 °C for as little as 20 min improved critical current density to within 20% of low-rate values, and resulted in dramatic improvements in other film properties as well.

Surface area of respirable beryllium metal, oxide, and copper alloy aerosols and implications for assessment of exposure risk of chronic beryllium disease

The continued occurrence of chronic beryllium disease (CBD) suggests the current occupational exposure limit of 2 microg beryllium per cubic meter of air does not adequately protect workers. This study examined the morphology and measured the particle surface area of aerodynamically size-separated powders and process-sampled particles of beryllium metal, beryllium oxide, and copper-beryllium alloy. The beryllium metal powder consisted of compact particles, whereas the beryllium oxide powder and particles were clusters of smaller primary particles. Specific surface area (SSA) results for all samples (N=30) varied by a factor of 37, from 0.56 +/- 0.07 m(2)/g (for the 0.4-0.7 microm size fraction of the process-sampled reduction furnace particles) to 20.8 +/- 0.4 m(2)/g (for the </=0.4 microm size fraction of the metal powder). Large relative differences in SSA were observed as a function of particle size for the powder of beryllium metal, from 4.0 +/- 0.01 m(2)/g (for the particle size fraction >6 microm) to 20.8 +/- 0.44 m(2)/g (for the particle size fraction </=0.4 microm). In contrast, little relative difference in SSA (<25%) was observed as a function of particle size for the beryllium oxide powder and particles collected from the screening operation. The SSA of beryllium metal powder decreases with increasing particle size, as expected for compact particles, and the SSA of the beryllium oxide powders and particles remains constant as a function of particle size, which might be expected for clustered particles. These associations illustrate how process-related factors can influence the morphology and SSA of beryllium materials. To avoid errors in predicting bioavailability of beryllium and the associated risks for CBD, the mechanisms of particle formation should be understood and the SSA of beryllium particles should be measured directly.

Advances in YBCO-coated conductor technology

Processes for producing both the YSZ template film by IBAD and the YBCO superconducting film by PLD at 1 m lengths have achieved Ic values of 122 A and Jc values of 1 MA/cm2. Improvements have been made in all stages of the process. Variations of Ic along the length of the 1-m samples stimulated development of a new in-field Ic measurement capability. The use of MgO as an IBAD template film has made great progress and can potentially decrease the time to produce the template film by more than an order of magnitude. A combination of electrical and microstructural investigations are being made to understand and improve the properties of the YBCO coated

Ion-beam assisted deposition of bi-axially aligned MgO template films for YBCO coated conductors

We report the results of experiments with ion-beam-assisted deposition (IBAD) of MgO using in-situ monitoring with Reflected High-Energy Electron Diffraction (RHEED). Strips of polished Haynes 242 and Inconel 625 nickel-based super-alloys have been used as substrates for these experiments. The in-plane texture of the MgO, as measured by X-ray /spl phi/ scan, resulted in FWHM values between 11 and 15/spl deg/. Using pulsed-laser deposition, the IBAD MgO template films were then overcoated with buffer layer films and a final superconducting film of YBa/sub 2/Cu/sub 3/O/sub 7-/spl delta//. The best superconducting transport properties measured on these YBCO films were: an Ic (75 K, self-field, 1 cm wide) of 41.6 A, and a narrow-bridge J/sub c/ (1.35 /spl mu/m thick film) of 0.46 MA/cm/sup 2/.

High critical current density YBa 2 Cu 3 O 7δ thick films using ion beam assisted deposition MgO bi-axially oriented template layers on nickel-based superalloy substrates

Nickel-based superalloys have been coated with magnesium oxide (MgO) using ion-beam-assisted deposition (IBAD). This technique produced a well-oriented bi-axially textured MgO template layer with a Ф scan full width half maximum of 6.4°. The layer architecture for these samples was as follows: polished hastelloy C276/amorphous Si 3 N 4 /IBAD MgO/ pulsed laser deposited (PLD) Y 2 O 3 –ZrO 2 /PLD CeO 2 /PLD YBa 2 Cu 3 O 7?δ . The subsequent heteroepitaxial PLD of 1.5-mm-thick YBCO showed a nominal critical current density of over 1 MA/cm 2 (75 K, self-field) along a microbridge and had an in-plane mosaic spread of 4.8° and an out-of-plane spread of 1.3°. These results compare well with our earlier work using IBAD yttria-stabilized zirconia (YSZ) as a template layer and indicate that IBAD MgO is a suitable substitute. Furthermore, these results suggest that IBAD MgO could be adapted to and increase the feasibility of a continuous process to fabricate longer lengths of coated conductors at speeds 100 times faster than that previously realized with IBAD YSZ.

Development of Tl-1223 superconducting tapes

Abstract A superconducting tape based on the TlBa 2 Ca 2 Cu 3 O 9 (Tl-1223) phase sheathed in silver was fabricated using the powder-in-tube method. Optimal conditions to synthesize powder ( T c =110 K ) and anneal composite tapes were examined. Microstructural examination indicates that cracks are easily healed during a 3 h anneal at the relatively low temperature of 775°C. A transport critical current density J c of 6200 A/cm 2 at 75 K was measured on short lengths of the annealed rolled and pressed 0.20 mm thick tape. The J c value drops to about 250 A/cm 2 at 75 K and 1 T due to the presence of weak links; however, a J c of 1000 A/cm 2 at 45 K is maintained constant to above 6 T. Measurements of magnetic hysteresis loops indicate that the Tl-1223 tape exhibits a high degree of flux pinning. Very little texturing of the superconducting core was evident.