Thomas Rendall Askew

A New High-Temperature Superconductor: Bi2Sr3-x Cax Cu2O8+y

A new superconductor that displays onset behavior near 120 K has been identified as Bi2Sr3-xCaxCu2O8+y, with x ranging from about 0.4 to 0.9. Single crystal x-ray diffraction data were used to determine a pseudo-tetragonal structure based on an A-centered orthorhombic subcell with a = 5.399 �, b= 5.414�, and c = 30.904 �. The structure contains copper-oxygen sheets as in La2CuO4 and YBa2Cu3O7, but the copper-oxygen chains present in YBa2Cu3O7 do not occur in Bi2Sr3-xCaxCu2O8+y. The structure is made up of alternating double copper-oxygen sheets and double bismuth-oxygen sheets. There are Ca2+ and Sr2+ cations between the adjacent Cu-O sheets; Sr2+ cations are also found between the Cu-O and Bi-O sheets. Electron microscopy studies show an incommensurate superstructure along the a axis that can be approximated by an increase of a factor of 5 over the subcell dimension. This superstructure is also observed by x-ray diffraction on single crystals, but twinning can make it appear that the superstructure is along both a and b axes. Flux exclusion begins in our samples at about 116 K and is very strong by 95 K. Electrical measurements on a single crystal of Bi2Sr3-xCaxCu2O8+y show a resistivity drop at about 116 K and apparent zero resistivity at 91 K.

Crystal Structure of Tl2Ba2Ca2Cu3O10, a 125 K Superconductor.

There is now a new series of high-temperature superconductors that may be represented as (AIIIO)2A2IICan-1CunO2+2n where AIII is Bi or Tl, AII is Ba or Sr, and n is the number of Cu-O sheets stacked consecutively. There is a general trend toward higher transition temperatures as n increases. The highest n value for a bulk phase is three and is found when AIII is Tl. This compound, Tl2Ba2Ca2Cu3O10, has the highest transition temperature(∼125 K) of any presently known bulk superconductor. The structure of Tl2Ba2Ca2Cu3O10 has been determined from single-crystal x-ray diffraction data and is tetragonal, with a = 3.85 � and c = 35.9 �. No superstructure is observed, and the material is essentially twin-free. Electron microscopy in the Tl/Ba/Ca/Cu/O system has revealed intergrowths where n = 5; such regions may well be responsible for the superconducting onset behavior observed in this system at about 140 K.

Bulk Superconductivity up to 122 K in the Tl-Pb-Sr-Ca-Cu-O System

New high-temperature superconductors based on oxides of thallium and copper, but not containing barium, have been prepared. A transition temperature (Tc) of about 85 K is found for (Tl0.5Pb0.5) Sr2CaCu2O7 whereas (Tl0.5Pb0.5)Sr2Ca2Cu3O9 has a Tc of about 120 K. Both materials possess tetragonal symmetry with a = 3.80 �, c = 12.05 � for (Tl0.5Pb0.5)Sr2CaCu2O7, and a = 3.81 �, c = 15.23 � for (Tl0.5Pb0.5)Sr2Ca2Cu3O9. A structure refinement of the latter phase has been carried out with single-crystal x-ray diffraction data.

Superconductivity near liquid nitrogen temperature in the PbSrRCaCuO system (R=Y or rare earth)

Abstract Superconductivity close to the boiling point of liquid nitrogen has been discovered in the P-Sr-R-Ca-Cu-O system where R is Y or rare earth. The superconducting compound has been identified as Pb 2 Sr 2 (R,Ca) 1 Cu 3 O 8+ y and the structure of the R=Y compound has been solved by single-crystal X-ray diffraction methods. The structure consists of double CuO 2 sheets interleaved by (Ca,R), a unit which is common to most of the high- T c copper oxide superconductors. A new structural feature found in this system is a double PbO layer separated by a sheet of copper atoms. A non-superconducting oxide of approximate composition (Y,Ca)Sr 2 (Cu,Pb) 3 O 7− y formed as a second phase in some preparations has been also characterized; its structure is related YBa 2 Cu 3 O 7 .

Superconducting and Magnetic Behavior in La2-xNaxCuO4

New phases of the type La2-xAxl+CUO4-y have been prepared where Al+ is sodium or potassium. The sodium phases are superconducting for x values from 0.2 to 0.5 at temperatures up to about 40 K. In addition, there are unusual magnetic properties below about 10 K that may be indicative of spin glass behavior. Phases of the type La2-xKxCuO4-y could only be prepared with x values up to about 0.1, and these phases are not superconducting above 4.2 K

Local magnetic field distribution in polycrystalline YBa 2 Cu 3 O 7 and its influence on bulk critical currents

The transport critical current density J c has been measured in high quality polycristalline YBa 2 Cu 3 O 7 samples made from extruded preforms. The hysteretic response of J c to applied magnetic fields was studied as a function of temperature (77-87 K) and sample morphology for various field sweeps in the 0-5 KOe range. A variety of samples show a basic J c ∼H −n behavior where n depends on temperature, but is independent of the other variables. A simple model of the local magnetic field distribution is presented and compared to te data, and the theoretical implications of the power law behavior are considered

New oxide superconductors

Abstract Crystals of the new high temperature superconductor Bi 2 Sr 3−x Ca x Cu 2 O 8+y have been grown and studied. Single crystal x-ray diffraction data were used to determine a pseudo tetragonal structure based on an A-centered orthorhombic cell with a = 5.399A,b= 5.414A,and c= 30.904A. The structure contains the copper-oxygen sheets as in La 2 CuO 4 and YBa 2 Cu 3 O 7 , but the copper-oxygen chains present in YBa 2 Cu 3 O 7 do not occur in Bi 2 Sr 3−x Ca x Cu 2 O 8+y . Electrical measurements on our single crystals show a resistivity drop at about 116 K and apparent zero resistivity at 91 K. We find superconductivity at 120 K in the Tl/Ca/Ba/Cu/O system. Also, we have for the first time prepared phases of the type La 2−x Na x CuO 4 . These materials are superconducting with T c s as high as 40 K, and they have some unusual magnetic behavior starting at about 10 K.

The production of high performance YBa 2 Cu 3 O 7 using nitrogen dioxide

Energy dispersive x-ray spectroscopy (EDS) was used to show that the elemental homogeneity of YBa{sub 2}Cu{sub 3}O{sub 7} powders can be improved substantially by heating the powder in a nitrogen dioxide-containing atmosphere (e.g., 950 {degree}C), followed by annealing in oxygen at 950 {degree}C, and slow-cooling to room temperature. The improved homogeneity results in a substantially larger flux exclusion signal for the NO{sub 2}-treated powder, as measured by both ac and dc techniques. The experimental results suggest a mechanism which involves the formation of a small amount of molten Ba(NO{sub 3}){sub 2}, which acts as a flux that dissolves the constituents and reprecipitates them as high purity YBa{sub 2}Cu{sub 3}O{sub 7}.

The production of high performance YBa sub 2 Cu sub 3 O sub 7 using nitrogen dioxide

Energy dispersive x-ray spectroscopy (EDS) was used to show that the elemental homogeneity of YBa{sub 2}Cu{sub 3}O{sub 7} powders can be improved substantially by heating the powder in a nitrogen dioxide-containing atmosphere (e.g., 950 {degree}C), followed by annealing in oxygen at 950 {degree}C, and slow-cooling to room temperature. The improved homogeneity results in a substantially larger flux exclusion signal for the NO{sub 2}-treated powder, as measured by both ac and dc techniques. The experimental results suggest a mechanism which involves the formation of a small amount of molten Ba(NO{sub 3}){sub 2}, which acts as a flux that dissolves the constituents and reprecipitates them as high purity YBa{sub 2}Cu{sub 3}O{sub 7}.

The effect of synthesis methods on the processing and properties of YBa2Cu3O6+x

Abstract A variety of synthesis routes for YBa2Cu3O6+x are described, and the resulting powders are compared in terms of their phys ical properties and densification behavior. It is shown that the processibility of the powders depends on their physical characteristics. Thus, fine reactive powders made from solution-derived routes are easily sintered to high density. On the other hand, chemical homogeneity of the starting powder and the sintered body determine the critical current, Jc. In the second part of the study, one solution route was selected and the synthesis conditions (calcination time and temperature) were varied. The structural, superconducting and physical properties of the starting powder were found to be strong functions of the synthesis conditions. In particular, it is shown that although milder synthesis conditions result in fine, easily sinterable powder, this powder is not highly crystalline and it contains structural defects. The poor crystallinity and defects are shown to degrade the superconducting properties.