Kasumi Yanagisawa

The crystal structure of the superconducting copper oxide carbonate (Ba1−xSrx)2Cu1+yO2+2y+z(CO3)1−y: Defects in charge reservoirs and superconductivity

Abstract The crystal structure of a new superconductor in the Ba-Sr-Cu-C-O system has been determined by neutron powder diffraction with the auxiliary use of electron diffraction and high-resolution transmission electron microscopy. This compound is a member of a solid solution with the general formula (Ba1-xSrx)2Cu1+yO2+2y+zCO3)1-y, where x=4/9 , y=1/9, and z=0.10. It is tetragonal with a space group P4212 or P 4 21m and lattice parameters of a=5.56405 (18) A and c=7.8570(4) A . It is closely related in structure to Sr2CuO2CO3, which is an end member of the solid solution: x=1, y=0, and x=0. Two-dimensional CuO2 sheets and carbonate slabs, (Ba1-xSrs)2CuyO2y+z(CO3)1-y, alternate with each other along the [001] directions. As the formula of the slab shows, about 11% of C atoms are substituted by excess Cu atoms coordinated to O atoms which amount to 2y+z in the formula unit. These substitutional and interstitial defects enable hole carriers to move from the carbonate slab to the CuO2 conduction sheet.

Carbonization of oxidized mesophase pitches originating from petroleum and coal tar

Carbonization of oxidized mesophase pitches originating from petroleum and coal tar was examined by thermogravimetry, FT-i.r. spectroscopy, and quantitative analysis of evolved gases. Polymerization and condensation of aromatic rings through loss of aromatic hydrogen are shown to be major chemical reactions in carbonization of both oxidized pitches. Characteristic of oxidized pitches in carbonization is the evolution of considerable amounts of CO, CO2 and H2O caused by decomposition of oxygen-containing groups introduced by oxidation. Thermal decomposition of the oxygen-containing groups probably initiates polymerization of pitch molecules before the major chemical reactions. The polymerization promotes the conversion of a fusible fibre into an infusible one. Of the oxygen-containing groups, carboxyl is decomposed at the lowest temperature. The oxidized petroleum pitch contains a larger number of carboxyl groups than the oxidized coal-tar pitch because the former has more aliphatic structures which can be converted into carboxyl groups by oxidation. The results strongly suggest that the petroleum mesophase pitch is made infusible more easily than the coal-tar mesophase pitch.

High-resolution transmission electron microscope study of effects of cobalt substitution on the stability and perfection of YBa2Cu4Oy superconductors

Abstract A small amount of Co is doped in the starting mixtures for preparing YBa 2 Cu 4 O y (1-2-4) type of superconductors and the resultant materials are examined by HRTEM, in order to examine the effects of Co substitution on the structural stability and perfections of the 1-2-4 structure. Up to about 2% substitution, the 1-2-4 phase is obtained as the major phase. The stability of (CuO) 2 double-chains (D), however, decreases gradually with further substitutions and partial conversions of double-chains into (CuO) single-chains (S) are induced. With 3% substitution, domains of the 2-4-7 structure with (D S D S D) sequences are formed together with those of the 1-2-4 one with a (D D D D) sequence. Small domains of a new ordered structure (D D S D D S D D S) are also observed in Co 3% specimen, suggesting the existence of a new Y 3 Ba 6 (Cu, Co) 11 O y (3-6-11) phase. The stability of the 2-4-7 phase increases by further substitution and, finally, it becomes the major phase at a 5% substitution by Co.

Effects of Fe and Ni substitutions on the 1-2-4 structure of YBCO superconductors studied by high-resolution transmission electron microscopy

Abstract Structures and defects of YBa 2 Cu 4 O y (1-2-4) type of superconductors, doped with Fe as well as Ni, are examined by HRTEM and the results are compared with those of the Co-substituted system reported previously. By Fe substitution, a part of the (CuO) 2 double chain layers of the 1-2-4 structure is replaced with (CuO) single chain layers, as was the case in Co substitution. The single chains are induced rather randomly by Fe substitutions, while ordered structures such as 2-4-7 or 3-6-11 were obtained by Co substitutions. Contrary to Fe and Co substitutions, Ni substitution has little structural effects on the 1-2-4 structure.

ζ-potential on carbons and carbides

Abstract A ζ-potential behavior of close to zero in high pH solutions is observed when carbons and carbides are treated with acetone and ultra-pure water in turn in order to improve the wetting of samples. The abnormal ζ-potential behavior can not be explained by the view of the dissociation of acidic functional groups, but could be accounted for by the effect of H2O and atmospheric CO2. The H2O effect contributes to the difference of ζ-potential between boiled and vacuum-dried carbons at pH 11. In addition, the rate of approaching zero for ζ-potential is relatively faster in the CO2-free aqueous solution than in an ambient aqueous solution. According to the complementary effect of H2O and atmospheric CO2, it allows us to postulate that atmospheric CO2 is adsorbed onto surface and ζ-potential is dependent on the dissociation of CO2. However, CO2 is replaced by H2O when H2O diffuses into the surface which causes the ζ-potential to approach zero. In addition, the atmospheric CO2 peak is detected in API-MS. The API-MS patterns exhibit no more than the broad CO2 peak at around 533 K for either active carbon or SiC, with a negligible number of carboxylic groups. An idea regarding the mechanism of the abnormal ζ-potential behavior is discussed.

Modulated structures of (Bi1−xPbx)2Sr2CoOy examined by high-resolution transmission electron microscopy

Abstract The Pb-doped Bi-Sr-Co-O complex oxides, (Bi 1- x Pb x ) 2 Sr 2 CoO y with x =0.0 to 0.5, are examined by means of electron diffraction and high-resolution transmission electron microscopy (HRTEM) in order to clarify the effects of Pb-doping on the commensurate modulated structure of Bi 2 Sr 2 CoO y . Specimens with Pb content up to 10% ( x ≤0.1) have a commensurate modulated structure with modulation periodicity b s =4 b . At 15% substitution ( x =0.15), a stepwise increase of the value of b s is observed and the specimens with Pb content from 20 to 30% (0.2≤ x ≤0.3) have incommensurate modulations with b s ≈4.5 b . Modulated structures are absent for specimens with Pb more than 40% (0.4≤ x ). All of the modulated structures have body-centered orthorhombic symmetries (Mode I) and no transition to base-centered symmetry (Mode II) is observed even by high Pb-substitution in the present (Bi 1- x Pb x ) 2 Sr 2 CoO y system, although transition to Mode II is generally observed for superconducting Pb-doped Bi-Sr-Cu-O systems.

Effects of Fe-doping on the modulated structures of Bi-Sr-Cu-O superconductors

Abstract The effects of Fe-doping on the modulated structures in Bi 2.5 Sr 1.5 CuO y (2.5/1.5-phase), as well as Bi 2.2 Sr 1.8 CuO y (2.2 /1.8-phase), are examined by electron diffraction and high-resolution electron microscopy. The Fe-free Bi 2.5 Sr 1.5 CuO y as well as the specimens doped with Fe up to 20% of Cu in the initial ratio, crystallizes in an orthorhombic superlattice with modulation periodicity b s =4.2 b . Deviation from the orthorhombic symmetry accompanied by a shortening of the modulation periodicity is observed in the range of Fe from 20% to 40%. Specimens with Fe from 40% to 50% crystallize in a monoclinic commensurate superlattice with b s =4.0 b . The Fe-free Bi 2.2 Sr 1.8 CuO y , on the other hand, has a monoclinic superlattice with b s =4.8 b . Partial substitution of Cu with Fe results in a gradual decrease of the modulation periodicity down to b s =4.2 b for 50% substitution. At the same time, transition of the symmetry is induced by Fe substitutions and an orthorhombic superstructure is observed with Fe from 30% to 50%.

Effects of metal-substitutions on the crystal structure of YBa2Cu4O8

Abstract Effects of metal-substitutions on the structural stability and perfection of YBa 2 Cu 4 O 8 are examined by HRTEM. Substitution of Cu with Fe or Co, as well as Y with Ca, Sm or Gd, brings intergrowth of “123” unit. A small amount of this type of defects are also induced by substitutions of Ba with Sr. Planar defects due to 90°-rotational twinning of (CuO) 2 double chain layers along the c-axis are observed in Y/Ca substituted specimens. Substitution of Cu with Ni, as well as Y with Ho or Er, has little structural effects on the 124 structure.

High-Resolution Transmission Electron Microscopy of Initial Stage of Radiation Damage of YBa2Cu4Oy Induced by 1 MV Electron Beam

Radiation damage of the YBa2Cu4Oy (124) superconductor induced by a 1 MV electron beam is examined by ultra high-resolution high-voltage electron microscopy. Small spots with nearly amorphous structure are formed mainly at the (CuO)2 double-chain layers at an early stage of irradiation, and their area gradually increases with irradiation time. It is considered that the amorphization is induced by direct knock-on displacements of atoms by collisions with a high-energy electron beam.

Crystal structure and effects of Fe doping of a non-superconducting phase (B-phase) in the Bi-Sr-Cu-O system

Abstract Crystal structure and effects of Fe doping of a non-superconducting phase (B-phase) in the Bi-Sr-Cu-O system are examined by electron-diffraction and high-resolution transmission electron microscope measurements. The B phase has a long-range periodic structure closely related to the modulated structure of the superconducting phase and it crystallizes in a monoclinic superlattice with a s =0.54, b s =2.0, c s =2.5 nm and α =104°. The (BiO) 2 double layer is not a complete continuous sheet but suffers from a periodic displacement along the c -direction to form a terraced (BiO) 2 layer. Each BiO terrace consists of seven Bi atoms, and at every eighth lattice plane a vertical shear plane containing few Bi atoms is inserted. Fe doping has an effect on the periodicity of the terraced structure. Intergrowth of narrow (BiO) 2 terraces is induced in crystals doped with Fe.