Palmer N. Peters

Observation of enhanced properties in samples of silver oxide doped YBa2Cu3Ox

Samples of YBa2 Cu3Ox doped with silver oxide (Tc ∼93 K) have exhibited attractive forces in gradient magnetic fields, both normal and tangential to the surfaces, which are more than twice the sample weight. This allows sample suspension below a rare earth magnet. Critical current density was increased by ∼102 at 77 K. Persistent fields, flux pinning, magnetization, and modeling are described.

MAGNETIC HYSTERESIS OF HIGH-TEMPERATURE YBa2Cu3Ox-AgO SUPERCONDUCTORS: EXPLANATION OF MAGNETIC SUSPENSION

We have measured the magnetization M of superconducting YBa2Cu3Ox-AgO composites with Tc approximately equal to 92K as a function of an applied magnetic field H at 77 and 87K. A very pronounced M-H hysteresis loop occurs even at 87K, indicating the presence of extremely strong pinning centers. The results of these measurements, together with a simple model, explain quantitatively why these superconductors could be suspended below a magnet.

Magnetization and magnetic suspension of YBa2Cu3Ox-AgO ceramic superconductors

Abstract The magnetic force which accounts for the newly-discovered suspension of a superconductor below a permanent magnet is determined by the magnetization of the superconductor and the magnetic-field gradient. Magnetization measurements were carried out on a series of YBa 2 Cu 3 O x -AgO ceramic superconductors, with T c ≈93 K. The samples were from the set of samples in which the magnetic-suspension phenomenon was first discovered. Magnetization data were taken at 4.2 and 77 K in magnetic fields up to 180 kOe. Hysteresis loops at low fields, up to 1.2 kOe, were also studied at 4.2, 77 and 87 to 88 K. The magnetization and hysteresis in most of the samples are among the largest observed to date in ceramic high- T c superconductors. In most of our samples, the remanent moment at 4.2 K is about 80 emu/g, and about 3 emu/g at 77 K. The large magnetization and hysteresis indicate the presence of strong pinning forces. The strong hysteresis at 77 K results in an appreciable positive magnetization, parallel to the field, when the field H is decreased from a finite value (above≈0.5 kOe). This positive magnetization increases with decreasing H . The positive magnetization can be produced by bringing a permanent magnet close to the superconductor, and then withdrawing it slowly. This leads to an attractive magnetic force between the superconductor and the permanent magnet. Calculations, based on a realistic model, show that at 77 K this magnetic attraction can be sufficiently strong to balance the gravitational force. As a result, the superconductor can be suspended below a permanent magnet. The expected damped oscillatory motion near the suspension point, following the application of a vertical impulse to the superconductor, is discussed. This motion is more complicated than that near the bottom of a conventional potential well. Some remaining problems associated with the magnetic-suspension phenomenon are outlined.

Oxidation of copper by fast atomic oxygen

The surface composition of the oxide formed on thin-film and solid OFHC copper samples exposed to a fast-atomic-oxygen environment in a low-earth orbit on NASAs Long Duration Exposure Facility (LDEF) was investigated using X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES), thin-film X-ray diffraction (TF-XRD), high-resolution profilometry, and reflectance measurements. The results confirm that it is easier to form thick copper oxide layers in the atomic oxygen ambient than is normally possible with other laboratory-based techniques.

High Tc composite silver/oxide superconductors

Factors involved in the strong flux pinning effect of high-Tc YBa2Cu3O7/AgO (Y-123/AgO) composite and other REE-123/AgO composites were investigated. Samples of superconducting REE-123 and REE-123/AgO (where REE was Nd, Sm, Eu, Gd, Dy, Ho, Er, or Y) were prepared and used to obtain magnetic moments, critical field, and microstructure data. The optimum heat treatment conditions for the formation of strong flux-pinning REE-123/AgO composites were found to be different for different REE-123 compounds. It was found that the annealing temperature depends on the ionic size of the REE, with larger rare-earth ions requiring higher temperature. It was also found that strong flux-pinning REE-123/AgO composites form only in a narrow annealing temperature range.

Surface processing of semiconductor materials with fast atomic oxygen

In a series of space flight experiments and laboratory simulations of the hyperthermal oxygen atom environment of space, we have investigated several oxidation reactions occurring on materials of major significance to the semiconductor manufacturing industry. The materials on which we have reaction rate data and chemical composition measurements under various conditions include: Cu, Si, Ge, GaAs, GaP and SiC. In general, we observe accelerated growth of oxide films of thickness far greater than those obtainable at low temperatures (300 K to 600 K) with molecular oxygen or thermal atomic oxygen. Furthermore, the oxides are usually of good stoichiometry and involve the metals in their highest normal oxidation state. Some representative results we have obtained on the formation and characterization of these oxide films will be reviewed and references to more complete descriptions are cited.

Germanium‐overcoated niobium Dayem bridges

Overcoating constriction microbridges with semiconducting germanium provides additional thermal conductivity at liquid‐helium temperatures to reduce the effects of self‐heating in these Josephson junctions. Microwave‐induced steps were observed in the I‐V characteristics of an overcoated Dayem bridge fabricated in a 15‐nm‐thick niobium film; at 4.2 K (Tc−T=2.6 K), at least 20 steps could be counted. No steps were observed in the I‐V characteristics of the bridge prior to overcoating. In addition, the germanium overcoat can protect against electrical disturbances at room temperature.

Determination of the Reactivity of Copper with Atomic Oxygen

In low Earth orbit (LEO), the composition of the atmosphere is dominated by atomic oxygen which is a major determinant of material degradation of external surfaces and a critical determining factor of spacecraft life. To simulate the effects of atomic oxygen in LEO, thin films of copper were exposed to a laboratory atomic oxygen beam. Copper films were characterized both before and after exposure by a variety of surface sensitive techniques including thin film x-ray diffraction, optical reflectance measurements, high resolution profilometry, and x-ray photoelectron spectroscopy. The results of these ground based experiments were compared to actual flight data acquired from the Space Shuttle based CONCAP-II experiment which exposed various materials in LEO on ambient and 320°C hot plates. The similarity of results generated from exposure of copper in an atomic oxygen beam to actual flight results prove that ground based atomic oxygen experiments can be good simulations of the LEO environment.