High Tc superconductivity of a TlBaCaCuO compound

A TlBaCaCuO_4.5+x compound has been investigated from the point of view of superconductivity. Depending on the heat treatment, one part of the samples exhibits superconductivity with an onset of 121 K and a zero resistivity of 106 K, while the other part of them shows only a sharp drop in resistivity at 130 K which hints at the existence of superconducting domains. This picture was confirmed by magnetic and ESR measurements giving possibility for estimation of the critical magnetic fields.     

Study of the crystalline structure of the superconducting phase of the YBaCuO ceramic compound

We present the results of a study of the crystalline structure of the high-temperature superconducting (above 110K) phase in the single-phase ceramic compounds YBaCuO. We have concluded that the crystal symmetry of this HTS phase with chemical composition Y_1.2Ba_0.8CuO_4−δ is orthorhombic type with crystallographic lattice constants a=0.3815 nm, b=0.3457 nm, c=2.650 nm.     

Electrical properties of LiBSe ternary system

Amorphous products were obtained in the LiBSe ternary system by quenching melts of Li₂Se, B and Se mixture prepared at 100°C in sealed silica tubes. The vitreous region was slightly lower selenium composition than that of the Li₂SeB₂Se₃ tie-line. The amorphous products were lithium ionic conductors and most of them showed contributions to their total conductivity. The amorphous product of composition Li₂₅B₃₆Se₃₉ has the least electronic contribution to its total conductivity of 6.0 × 10⁻⁶ S/cm at room temperature. A new crystalline compound and crystalline LiBH₄ were also obtained in LiBSe ternary. Both of them were lithium ionic conductors having conductivities of about 1 × 10⁻⁶ S/cm at room temperature.     

Superconductivity of TiZrNi alloys containing quasicrystals

Superconductivity of rapidly quenched ribbons Ti₅₃Zr₂₇Ni₂₀ at 1.94 K and the beginning of the transition to the superconductivity in ribbons Ti₄₅Zr₃₈Ni₁₇ below 1.5 K were found. X-rays diffractometry measurements showed that the icosahedral quasicrystal phase is dominant in all samples.     

The superconductivity,structure and microstructure of BaYScCu oxides

We added small amount of Sc (0.05–0.35at.%) to BaYCu oxide and found that the superconductivity deteriorated drastically when the Sc content reached 0.325at.%. X-ray and electron diffraction showed that the orthorhombic perovskite structure changed to multi-phase structure with unidentified phases. The microstructures observed by TEM are of needle like morphology. Their phases are not yet identified.     

The effect of Sc on the superconductivity of the BaYCu oxides

The oxide compounds of Ba_0.6Y_0.4−xSc_xCu (x=0.05−0.35) have been prepared. It was found that the superconducting transition temperatures of the samples vary with the contents of Sc and when Sc>0.325, the samples become nonsuperconducting.     

Electron diffraction and electron microscopic study of BaYCuO superconducting materials

Electron microscopy and electron diffraction have been applied to show that the orthorhombic phase in the compound Ba₂YCu₃O_7−δ is responsible for the high superconducting transition temperature. A positive correlation is found between the volume fration of the orthorhombic phase and the superconducting transition temperature. By means of an “in-situ” heating experiment it is found that the orthorhombic phase is formed on cooling from a high temperature tetragonal phase with disordered vacancies. It is suggested that the low temperature tetragonal phase that occurs in the same specimens as the orthorhombic phase also contains an ordered arrangement of vacancies different from that present in the orthorhombic phase. The order-disorder transition associated with the structural vacancies is shown to be reversible, provided there has been no oxygen loss.     

Compositionally dependent superconducting transition temperature of YBaCu oxides

The superconducting transition temperatures of a family of YBaCu oxides have been explored by dc resistivity measurements. Substitutions of magnetic and non-magnetic ions on Y, Ba and Cu sites are reported. For some compositions, achieving high superconducting transition temperatures is strongly dependent on growth conditions for the material.     

Superconductivity of LaMCuO system (MBa,Sr and Ca

Superconductivity of the compounds of LaMCuO system with K₂NiF₄ type structure has been studied. For the sintered compounds with MBa, Sr and Ca, the highest onset transition temperatures, T_ci, estimated from resistivity measurement are about 36.9K, 43.0K and 23.5K, respectively and the highest temperatures where the resistivity vanishes, T_cf, are about 27.8K, 34.0K and 15.5K, respectively. Almost complete Meissner effect has been confirmed by low frequency ac susceptibility measurement. Crystals of the compounds with MBa and Sr have been prepared. As the result of the preliminary experiment for a crystal with Ba, it is found that T_cf of the crystal is higher than any one of the sintered specimens with MBa, while the T_ci of the crystal is nearly the same as the highest one of the sintered specimens.     

Electromotive force of a COCO2 sensor in COCO2H2H2O atmospheres and simultaneous determination of partial pressures of CO and CO2

The electromotive force (EMF) of the COCO₂ sensor using Na₂CO₃ and NASICON (Na₃Zr₂Si₂PO₁₂) as solid electrolytes has been examined in COCO₂Ar atmospheres. The EMF is related to the partial pressures of CO and CO₂ and proportional to log(P²_CO2P⁻¹_CO). The simultaneous use of the oxygen sensor of stabilized zirconia gives the EMF proportional to log(P_CO2P⁻¹_CO). The EMF's of two sensors permit to determine individually partial pressures of CO and CO₂. The existence of H₂ with high concentration does not affect the EMF's. This fact proves the applicability of the two-sensor system to the monitoring and the controlling of reducing atmospheres in industrial processes.     

Magnetic properties of TiZr alloys

Magnetic susceptibility measurements of TiZr alloys are reported. Zirconium, by alloying, acts on the Néel temperature giving rise to a complicated magnetic phase diagram. The main mechanism in the T_N shift is the change of the Fermi level. The experimental dependence between T_N and X₀ is in good agreement with the theoretical dependence obtained on the Fedders-Martin model by taking into account the effect of nonmagnetic impurity on the Fermi level.     

X-ray photoemission studies of PtSn and PtPb bimetallic systems

The Pt(5d) and Pt(4f) in PtSn and PtPb composites produced by sequential vapor deposition have been examined by X-ray photoelectron spectroscopy. At low concentrations of Pt, the Pt(5d) becomes a single, narrow resonance at about 4 eV below the Fermi level. Its density of states at the Fermi level is small. This leads to a large suppression of the Pt(4f) lineshape asymmetry. As the Pt concentration increases, d-d interactions split the Pt(5d) resonance and increase the 5d density of states at the Fermi level by pushing one of the split resonances to the Fermi level. Consequently, the Pt(4f) lineshape asymmetry increases. The evolution of the Pt(4f) and Pt(5d) in PtSn and PtPb are very similar but different from that of small Pt particles supported on carbon. The close similarity of the electronic properties of Pt in PtSn and PtPb suggests that the poisoning of Pt by Pb is not entirely due to electronic effects although there are indications of stronger interaction between Pt and Pb than between Pt and Sn atoms. The strong surface enrichment of Pb on PtPb points to the importance of the physical blockage of Pt atoms by surface Pb in determining the poisoning effect of Pb.     

Lattice dynamics and debye temperature of AlCu,AlSi and AlGe alloy systems

The lattice dynamics and specific heat at constant volume for AlCu alloy system is studied using our previous treatment based on the microscopic electronic theory. Considering the volume and electron density effect on the dynamical matrix of the pure constituent, we obtain the band and local mode frequencies at the temperature-dependent specific heat in the Al_1−x Cu_x solid solution. Then, using the data about the mean elastic wave and the specific heat extrapolated to the absolute zero temperature, the Debye temperature is presented for the Al_1−xCu_x, Al_1−xSi_x and Al_1−xGe_x alloy systems. The Debye temperature decreases as a function of the Cu atomic fraction x for Al_1−xCu_x alloy, and remarkably for Al_1−xSi_x and Al_1−xGe_x solid solutions.     

Molecular structure of cyclopropenylidene,HCCCH from the millimeter wave spectra of its isotopomers

The rotational spectra of the two ¹³C and of the D monoisotopic substitutions of cyclopropenylidene C₃H₂ have been measured up to 300 GHz. The determination of the rotational and centrifugal distortion constants of four isotopomers has led to a complete substitution structure. The values of the two CC bond lengths which define the three-membered ring have been improved by using Watson's “mass-dependence” (r_m) method.     

Corrosion characteristics of NdFeB sintered magnets containing various alloying elements

The effect of small additions (1 at%) of some elements (P, Cr, Ti, Zr, Pb or Sn) on the corrosion behaviour and magnetic properties of Nd₁₅Fe₇₇B₈ sintered magnets has been investigated. It was established that most advantageous was the addition of 1 at% Cr which distinctly inhibited both acid and atmospheric corrosion processes while it did not deteriorate the magnetic characteristics of the magnet. Addition of Cr accelerates, however, the dissolution of the magnet at strongly cathodic polarization.     

Anisotropy of the superconducting critical magnetic field HC2 of LaMCuO system(M = Sr and Ba)

Preliminary measurement of the superconducting critical magnetic field H_c2 of LaMCuO system (M = Sr and Ba) with layered perovskite structure has been carried out. The results show a quite large anisotropy of H_c2 indicating that the present system has a two dimensional character of the electron transport. The measured conductivities seem to be consistent with the values estimated from the observed H_c2 by use of an ideal two dimensional band with free electron mass.