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NASA NTRS1988

Electronic materials high-T(sub c) superconductivity polymers and composites structural materials surface science and catalysts industry participation

The fifth year of the Center for Advanced Materials was marked primarily by the significant scientific accomplishments of the research programs. The Electronics Materials program continued its work on the growth and characterization of gallium arsenide crystals, and the development of theories to understand the nature and distribution of defects in the crystals. The High Tc Superconductivity Program continued to make significant contributions to the field in theoretical and experimental work on both bulk materials and thin films and devices. The Ceramic Processing group developed a new technique for cladding YBCO superconductors for high current applications in work with the Electric Power Research Institute. The Polymers and Composites program published a number of important studies involving atomistic simulations of polymer surfaces with excellent correlations to experimental results. The new Enzymatic Synthesis of Materials project produced its first fluorinated polymers and successfully began engineering enzymes designed for materials synthesis. The structural Materials Program continued work on novel alloys, development of processing methods for advanced ceramics, and characterization of mechanical properties of these materials, including the newly documented characterization of cyclic fatigue crack propagation behavior in toughened ceramics. Finally, the Surface Science and Catalysis program made significant contributions to the understanding of microporous catalysts and the nature of surface structures and interface compounds.

Source record↗

NASA NTRS1989

Edge geometry superconducting tunnel junctions utilizing an NbN/MgO/NbN thin film structure

An edge defined geometry is used to produce very small area tunnel junctions in a structure with niobium nitride superconducting electrodes and a magnesium oxide tunnel barrier. The incorporation of an MgO tunnel barrier with two NbN electrodes results in improved current-voltage characteristics, and may lead to better junction noise characteristics. The NbN electrodes are preferably sputter-deposited, with the first NbN electrode deposited on an insulating substrate maintained at about 250 to 500 C for improved quality of the electrode.

Hunt, Brian D.↗

NASA NTRS1990

Performance and modeling of superconducting ring resonators at millimeter-wave frequencies

Microstrip ring resonators operating at 35 GHz were fabricated from laser ablated YBCO thin films deposited on lanthanum aluminate substrates. They were measured over a range of temperatures and their performance compared to identical resonators made of evaporated gold. Below 60 Kelvin the superconducting strip performed better than the gold, reaching an unloaded Q approximately 1.5 times that of gold at 25 K. A shift in the resonant frequency follows the form predicted by the London equations. The Phenomenological Loss Equivalence Method is applied to the ring resonator and the theoretically calculated Q values are compared to the experimental results.

Bhasin, K. B.↗

NASA NTRS1990

A superconducting quenchgun for delivering lunar derived oxygen to lunar orbit

The development of a parametric model for a superconducting quenchgun for launching lunar derived liquid oxygen to lunar orbit is detailed. An overview is presented of the quenchgun geometry and operating principles, a definition of the required support systems, and the methods used to size the quenchgun launcher and support systems. An analysis assessing the impact of a lunar quenchgun on the OEXP Lunar Evolution Case Study is included.

Nottke, Nathan↗

NASA NTRS1990

The superconducting state of the high-transition temperature superconductors: Experimental basis

Experiments on the high-T(sub c) cuprate superconductors continue to narrow the possible theoretical explanations of the phenomenon. Experimental evidence to date points to a BCS-like state, with pairs in singlet s-states, the familiar gap in the excitation spectrum, type 2 behavior in a magnetic field and a normal state with fermi liquid origins. Several other features of the superconducting state in the cuprates, however, appear to differ from those of conventional alloy superconductors (these relate to the detailed structure of the gap and to the nature of the coupling mechanism). Recent experiments have helped clarify what these differences are, and together with the earlier experiments, they now impose still stronger constraints on theories of these superconductors. These and other developments will be reviewed.

Little, W. A.↗

NASA NTRS1990

Laser ablated high T(sub c) superconducting thin YBa2Cu3O(7-x) films on substrates suitable for microwave applications

The development of high temperature superconducting YBa2Cu3O(7-x) thin films on substrates suitable for microwave applications is of great interest for evaluating their applications for space radar, communication, and sensor systems. Thin films of YBa2Cu3O(7-x) were formed on SrTiO3, ZrO2, MgO, and LaAlO3 substrates by laser ablation. The wavelength used was 248 nm from a KrF excimer laser. During deposition the films were heated to 600 C in a flowing oxygen environment, and required no post annealing. The low substrate temperature during deposition with no post annealing gave films which were smooth, which had their c-axis aligned to the substrates, and which had grains ranging from 0.2 to 0.5 microns in size. The films being c-axis aligned gave excellent surface resistance at 35 GHz which was lower than that of copper at 77 K. At present, LaAlO3 substrates with a dielectric constant of 22, appears suitable as a substrate for microwave and electronic applications. The films were characterized by resistance-temperature measurements, scanning electron microscopy, and x ray diffraction. The highest critical transition temperatures (T sub c) are above 89 K for films on SrTiO3 and LaAlO3, above 88 K for ZrO2, and above 86 K for MgO. The critical current density (J sub c) of the films on SrTiO3 is above 2 x 10(exp 6) amperes/sq cm at 77 K. The T(sub c) and J(sub c) are reported as a function of laser power, composition of the substrate, and temperature of the substrate during deposition.

Warner, J. D.↗

NASA NTRS1990

Phenomenological theory of the normal and superconductive states of Cu-O and Bi-O metals

The universal normal state anomalies in the CuO metals follow from a marginal Fermi liquid hypothesis: there exists a contribution to the polarizability over most of momentum space proportional to omega/T for omega/T much less than 1 and constant thereafter up to a cutoff omega(sub c). Using the same excitation spectrum, the properties of the superconductive state were calculated. The right order of T(sub c) can be obtained, the zero temperature gap, 2 delta (0)/T(sub c) and the nuclear relaxation rate near T(sub c). The possible microscopic physics leading to the marginal Fermi liquid hypothesis is discussed.

Varma, C. M.↗

NASA NTRS1990

Dynamics of the iron spins in superconducting YBa2(Cu(1-x)Fe(x))O7

The dynamics of the iron spins in YBa2(Cu(1-x)Fe(x))3O7 alloys (0 = to or less than 0.12) was studied by the means of inelastic neutron scattering. Measurements were performed using the time of flight technique with an excellent resolution of 50 micro eV, in a temperature range of 1.8 to 300 K. The doped samples show an elastic and a quasielastic intensity strongly varying with temperature. A spin glass like freezing is revealed at low temperature by a sudden decrease of the quasielastic intensity, an increase of the elastic or resolution limited intensity and a minimum in the quasielastic width. The freezing temperature corresponds to the one already determinated by a magnetic splitting in Mossbauer experiments. Above freezing, the occurrence of superconductivity slightly modifies the characteristics of the spin relaxation in the paramagnetic state, as shown by measurements in two x = 0.06 samples. In the whole temperature range of measurement, the dependence of the quasielastic intensity with the scattering vector q, mainly reflects the variation of the Iron form factor, which shows that the spins are almost uncorrelated.

Mirebeau, I.↗

NASA NTRS1990

Superconducting film on metallic wire

For technological applications of high T(sub c) superconductors, it will be necessary to overcome the inherent problem of brittleness, to develop materials with high current carrying capacity, and to devise ways of joining superconductors with other materials. These issues will be addressed in the context of superconducting films on metallic wires. These composite systems are expected to produce flexible wires with desirable properties.

Akbar, Sheikh A.↗

NASA NTRS1990

Low frequency electrical noise across contacts between a normal conductor and superconducting bulk YBa2Cu3O7

Virtually every device that makes use of the new ceramic superconductors will need normal conductor to supercondutor contacts. The current-voltage and electrical noise characteristics of these contacts could be become important design considerations. I-V and low frequency electrical noise measurements are presented on contacts between a normal conductor and superconducting polycrystalline YBa2Cu3O7. The contacts were formed by first sputtering gold palladium pads onto the surface of the bulk superconductor and then using silver epoxy to attach a wire(s) to each pad. Voltage across the contacts was found for small current densities. The voltage spectral density, S sub v(f), a quanity often used to characterize electrical noise, very closely followed an empirical relationship given by, S sub v(f) = C(VR)sq/f, where V is the DC voltage across the contact, R is the contact resistance, F is frequency, and C is a contant found to be 2 x 10(exp -10)/Omega sq at 78 K. This relationship was found to be independent of contact area, contact geometry, sample fabrication technique, and sample density.

Hall, J.↗

NASA NTRS1990

Effects of grain size and grain boundary on critical current density of high T(sub c) superconducting oxides

By means of adding impurity elements in high T sub c oxides, the effects were studied of grain size and grain boundary on the critical current density of the following systems: YBa2Cu3O(7-y) and Bi-Pr-Sr-Ca-Cu-O. In order to only change the microstructure instead of the superconductivity of the grains in the samples, the impurity elements were added into the systems in terms of the methods like this: (1) substituting Y with the lanthanide except Pr, Ce, and Tb in YBa2Cu3O(7-y) system to finning down grains in the samples, therefore, the effect can be investigated of the grain size on the critical current density of 1:2:3 compounds; (2) mixing the high T sub c oxides with the metal elements, such as Ag, according to the composition of (high T sub c oxide)1-xAgx to metallize the grain boundaries in the samples, studying the effect of the electric conductivity of the grain boundaries on the critical current density; (3) adding SiO2, PbO2, and SnO2 into the high T sub c oxide to form impurity phases in the grain boundaries, trying to find out the effects of the impurity phases or metalloid grain boundaries on the critical current density of the high T sub c superconductors. The experimental results indicate that in the case of of the presence of the metalloid grain boundaries finning down grains fails to enhance the j sub c, but restrains it strongly, the granular high T sub c superconductors with the small size grains coupled weakly is always the low j sub c system.

Zhao, Y.↗

NASA NTRS1990

Microwave conductivity of laser ablated YBaCuO superconducting films and its relation to microstrip transmission line

The discovery of high temperature superconductor oxides has raised the possibility of a new class of millimeter and microwave devices operating at temperatures considerably higher than liquid helium temperatures. Therefore, materials properties such as conductivity, current density, and sheet resistance as a function of temperature and frequency, possible anisotropies, moisture absorption, thermal expansion, and others, have to be well characterized and understood. The millimeter wave response was studied of laser ablated YBa2Cu3O(7-x)/LaAlO3 thin films as a function of temperature and frequency. In particular, the evaluation of their microwave conductivity was emphasized, since knowledge of this parameter provides a basis for the derivation of other relevant properties of these superconducting oxides, and for using them in the fabrication of actual passive circuits. The microwave conductivity for these films was measured at frequencies from 26.5 to 40.0 GHz, in the temperature range from 20 to 300 K. The values of the conductivity are obtained from the millimeter wave power transmitted through the films, using a two fluid model.

Bhasin, K. B.↗

NASA NTRS1990

Superconductivity in 2-2-3 Y2Ba2Cu3O(sub 8+ delta)

Researchers synthesized a new high T(sub c) 2-2-3 superconductor (Y2Ba2Cu3O8+delta) by a special preparation technique and characterized it by ac-susceptibility measurements. Diamagnetism and Meissner effect sets in at low fields and superconducting transition onsets at 90 K. The systematic investigation of the real and imaginary components of ac-susceptibility as a function of temperature and applied ac magnetic field reveals that the magnetic behavior is that of a granular type superconductor.

Joshi, H. H.↗

NASA NTRS1990

Microstructures and properties of superconducting Y-Er-BaCu-O thin films obtained from disordered Y-Er-BaF2-Cu films

Since the first reports on superconducting thin films obtained by evaporating BaF2, Cu and Y(sup 1), or Yb or Er(sup 2), several others have followed. All these reports describe thin films prepared by means of molecular beam cells or electron guns. Researchers show that films with similar properties can be obtained by radio frequency sputtering of a single mosaic target composed by Y-Er, BaF2 and Cu. Process steps are described.

Cikmach, P.↗

NASA NTRS1990

Spatial conductivity measurements on high T(sub c) superconducting films

High T(sub c) superconducting thin and thick films have potential applications in future NASA flight projects. In anticipation of film use, the Materials Branch is developing a nondestructive, non-contact method of measuring the spatial variation of conductivity across a film sample. This method uses a computer-controlled, X-Y positioning table to scan a conventional eddy current probe across the surface of a film. The induced changes in impedance caused by variations in film conductivity are recorded during the scanning process. Ultimately the two-dimensional data set is displayed using imaging equipment on a personal computer.

Vansant, J. Timothy↗

NASA NTRS1990

Composite superconducting wires obtained by high-rate tinning in molten Bi-Pb-Sr-Ca-Cu-O system

The preparation of high-T(sub c) superconducting long composite wires by short-time tinning of the metal wires in a molten Bi-Pb-Sr-Ca-Cu-O compound is discussed. The application of this method to the high-T(sub c) materials is tested, possibly for the first time. The initial materials used for this experiment were ceramic samples with nominal composition Bi(1.5)Pb(0.5)Sr2Ca2Cu3O(x) and T(sub c) = 80 K prepared by the ordinary solid-state reaction, and industrial copper wires from 100 to 400 microns in diameter and from 0.5 to 1 m long. The continuously moving wires were let through a small molten zone (approximately 100 cubic mm). The Bi-based high-T(sub c) ceramics in a molten state is a viscous liquid and it has a strongly pronounced ability to spread on metal wire surfaces. The maximum draw rate of the Cu-wire, at which a dense covering is still possible, corresponds to the time of direct contact of wire surfaces and liquid ceramics for less than 0.1 s. A high-rate draw of the wire permits a decrease in the reaction of the oxide melt and Cu-wire. This method of manufacture led to the fabrication of wire with a copper core in a dense covering with uniform thickness of about h approximately equal to 5 to 50 microns. Composite wires with h approximately equal to 10 microns (h/d approximately equal to 0.1) sustained bending on a 15 mm radius frame without cracking during flexing.

Grozav, A. D.↗

NASA NTRS1990

Novel superconducting phases of Tl-based compounds

Several new superconducting phases of the Tl-based compounds were prepared. Structural studies revealed cell lengths of 31.9 A and longer. Properties of Ce-containing compounds are also discussed.

Kostadinov, I. Z.↗

NASA NTRS1990

Magnetic properties of the 110K superconducting phase in Pb-doped Bi-Sr-Ca-Cu-O thin films

The relaxation of the remnant moment induced in a nearly single phase high T(sub c) thin film of Pd-doped Bi-Sr-Ca-Cu-O was investigated. Measurements reveal that the relaxation obeys a logarithmic time dependence for observation times up to 2000 seconds. The temperature dependence of the initial magnetization of the film and its decay rate are obtained. The initial magnetization monotonically decreases. However, the decay rate normalized by initial magnetization has a peak at approximately 14 K for an applied field of 500 gauss. The peak shifts to lower temperature for a stronger magnetic field. These data are compared with existing data on other high T(sub c) superconducting materials.

Tanaka, Atsushi↗