Timothy L. Peterson

Holographic interferometry of crazes in polycarbonate

Holographic interferometry has been used to determine the differential stress concentration at the tip of a craze as a function of over‐all strain. The differential stress concentration factor δKσyy is unity at low strains. Upon further strain increase, δKσyy increases, passing through two maxima before it finally decreases again to approximately a value of unity before fracture. In addition, the differential craze opening displacements of, and stress increments borne by, various regions along the craze have been determined as a function of strain. It is found that the craze opens unevenly, with the base of the craze opening at lower applied stresses than the craze tip. As a result, the differential stress concentration in the craze at a point well behind the craze tip can be greater than that at the craze tip. The strain dependence of δKσyy is in qualitative agreement with that expected, given the nonlinear stress‐strain curve of similar crazes measured by Kambour and Kopp. A detailed explanation of the ...

Development of nickel alloy substrates for Y-Ba-Cu-O coated conductor applications

Fabrication of long-length, textured substrates constitute a critical step in the successful application of coated High Temperature Superconductors (HTS). Substrate materials stronger than nickel are needed for robust applications, while substrates with non-magnetic characteristics are preferred for AC applications. The present work is thus focused on development of texture in high strength, non-magnetic substrate materials. As the development of cube texture is easier in medium to high stacking fault energy materials, binary alloys based on nickel were evaluated for the present application. High purity alloys were melted and hot/cold worked to obtain thin tapes. The development of texture in these alloys as a function of processing parameters was studied by X-ray diffraction and metallographic techniques. Orientation Imaging Microscopy (OIM) was used to quantify the extent of texture development in these substrates. Results to date on the development of texture by thermo-mechanical processing of these alloys are presented.

Transport and magnetic properties of high- Tc YBa sub 2 Cu sub 3 O sub 7 minus x films

Thin superconducting films of YBa2Cu3O7−x were deposited onto (100) SrTiO3 substrates at 750 °C in 100 and 200 mTorr of O2, by ArF excimer laser ablation. The as‐deposited films were strongly oriented with the c axis perpendicular to the surface when they were cooled to room temperature in oxygen. The highest values of transport critical current density (Jc≳105 A/cm2 below 80 K) were observed in films cooled in flowing O2. We find that, in all films, Jc follows a power law (1−T/Tc)n with n≊2, near the critical temperature, Tc, when Tc is defined as the temperature at which Jc=0. It appears that this behavior is governed by the short coherence lengths in this high‐Tc material, as suggested by G. Deutscher and K. A. Muller [Phys. Rev. Lett. 59, 1745 (1987)], and not by the specific nature of the large‐scale structure of the films.

Self-heating hotspot effects in HTS thin films

The current-voltage (I-V) characteristics associated with self-heating hotspots in HTS thin films patterned into several different geometries have been investigated under both current-biased and voltage-biased conditions. The hysteretic behavior of the I-V characteristics as a function of sample temperature are in agreement with behavior predicted by resistive hotspot models for superconducting bridges. Additionally, the effect of magnetic fields on the current required to generate, as well as maintain, a normal hotspot has been investigated. Finally, the characteristics of voltage pulses resulting from a step increase in current have been examined to study the propagation of hot spots in HTS films.<<ETX>>

Microstructure and properties of YBa/sub 2/Cu/sub 3/O/sub 7/ thin films grown on vicinal LaAlO/sub 3/ substrates

Thin films of YBa/sub 2/Cu/sub 3/O/sub 7/ have been grown on vicinal LaAlO/sub 3/ substrates by pulsed-laser deposition. In most cases, the substrate surface, nominally [001], has been rotated /spl sim/6/spl deg/ about an axis parallel to the <110> direction. Films grown on these surfaces were found to have substantially improved properties compared to those obtained for films deposited on [001]-oriented substrates under equivalent conditions at our facility. The surface morphology of the films exhibited an elongated granular structure which differed markedly from the more equiaxed grain structure found in films grown on [001]-oriented substrates. Furthermore, this particular elongated morphology and improvement in properties seems to occur only for films deposited at higher laser energies (/spl sim/30J/cm/sup 2/).<<ETX>>