Bar Oxygen Pressure Research Articles

Effect of praseodymium substitution in YBa 2Cu 4O 8 (1:2:4)

Pr substituted YBa 2Cu 4O 8 (1:2:4) has been synthesized by a solid-state reaction at 930°C–950°C in 200 bar oxygen pressure. Pr could be substituted only up to 20% i.e., Y 0.8Pr 0.2Ba 2Cu 4O 8. T c decreased with increasing Pr; however, T c could be restored by co-substitution of Ca along with Pr which was possible up to Pr=0.2 and Ca=0.1. XANES indicated the presence of Pr 3+ but not Pr 3+. In 1:2:4, T c depends on the rare-earth ion size, and Pr 3+ ions cause only a small additional suppression of T c below that expected from the size of Pr 3+. The large “Pr effect” in the suppression of T c of 1:2:3 is thus a special property of the 1:2:3 structure.

Conversion of technical lignins into slow-release nitrogenous fertilizers by ammoxidation in liquid phase

Technical lignins derived from kraft, organosolv, soda, and ASAM pulping as well as a fermented lignosulfonate from a sodium sulfite process have been subjected to oxidative ammoniation (ammoxidation) aimed at producing slow-release nitrogenous fertilizers. Reactions were carried out in liquid phase using a 1-liter, stirred, batch reactor. The effect of time, temperature, and oxygen pressure on the amount of fixed nitrogen and the C/N ratio was studied. High nitrogen contents were obtained with kraft and organosolv lignin followed by lignosulfonate. At 150°C, 15 bar oxygen pressure and 90 min of reaction time, 13–14% nitrogen (based on lignin) could be fixed in the lignin macromolecule. Changes in molecular structure were monitored by FTIR spectroscopy. The crop yield of sorghum plants in pot experiments with ammoxidized kraft lignin as fertilizer was 82% of that obtained with ammonia fertilization.

Synthesis of YBa 2Cu 4O 8 under one bar oxygen pressure by the citrate pyrolysis method

The citrate pyrolysis method is used to investigate the synthesis process and the superconducting transitions of the YBa 2Cu 4O 8 (124) phase. DC susceptibility measurements show that the intrinsic Tc, both for the one bar sintering route and the 90 bar treatment, is close to 82 K. In the former case, however, the reaction temperature must not exceed 800°C, and χ ec and resistive transitions are severely degraded due to the extremely small grain size.

Superconducting behaviour of the T/O structural type phase La 2−xNd xCuO 4+δ(0< x

The crystallographic and analytical data relative to some compositions of the La 2− x Nd x CuO 4solid solution, with the T/O type structure, prepared under 0.2 bar oxygen pressure, are reported. After a treatment under high oxygen pressure or electrochemical oxidation, a significant increase of the deviation from oxygen stoichiometry is observed for the phase of composition x=0.30. for increasing values of δ, an evolution from a semiconducting to a superconducting behaviour has been evidenced ( T c≅30 K) and confirmed by magnetic measurement.

Characterization and superconductivity of YBa 2Cu 4O 8 and Y 2Ba 4Cu 7O 14+x and their intergrowths

Pure single phase Y 2 Ba 4 Cu 7 O 14+x (“247”) and two phase compositions containing either “123” or “124” inclusions were prepared under 85 bar oxygen pressure at 1000°C. The samples were characterized by X-ray diffraction, microprobe analysis and microstructural examination. The measurements of the a.c. susceptibility, the field cooling magnetization and the resistivity indicate that T c of “247” is actually higher than that of “124” but suggest a more pronounced influence of oxygen defects in “247” under the present conditions of synthesis.

Oxygen Pressure Effect on the Y2O3—Bao—Cuo Liquidus

Concurrent DTA, TGA, and oxygen pressure measurement were used to determine liquidus temperature for compositions near YBa2Cu3Ox. Liquidus temperatures were measured for oxygen partial pressures from 2 × 10−4 to 1 bar. Liquidus temperatures decreased for lowered oxygen pressure. The ternary eutectic is 930°C for 1 bar and 854°C for 0.02 bar oxygen pressure. Incongruent melting of YB2Cu3Oxoccurs at 1034°C for 1 bar and 938°C for 2 × 10−4 bar oxygen pressure. Some implications for processing are also discussed.

Disappearance of high temperature superconductivity induced by high carrier concentrations

Abstract It is shown that samples of La2−xSrxCuO4 prepared under 100 bar oxygen pressure exhibit a metallic conductivity which increases as x increases up to x=0.34. In spite of this behavior, superconductivity disappears with Tc decreasing from x=0.15 to x=0.25. This decrease in Tc is shown to be continuous (and not dominated by phase separation). Such decreases in Tc are shown to be a common feature of high temperature superconductors, being observed in YBa2Cu3Oy-like systems as well as in doped Bi 2212 and Tl 1201 compounds.

A Study of the Superconducting La2-xSrxCuO4 System by X-Ray Powder Diffraction

AbstractRoom temperature crystalline structures of La2-xSrxCuO4 samples with uncommonly high hole concentrations (p) up to p=0.4, annealed under 1 and 100 bars of oxygen pressure have been analyzed by X-ray diffraction. Results show that the x=0 sample with p=0.09 and superconducting below 20K is orthorhombic at room temperature. The orthorhombic distortion at 300K decreases with increasing x and becomes tetragonal when x reaches 0.08. This orthorhombic-to-tetragonal phase transformation is consistent with previous work. Samples with x≥0.28 and p≥0.31 remain tetragonal but are nonsuperconducting down to 5K. Lattice dimension anomalies have also been observed and are correlated with the sudden appearance of oxygen vacancies, as seen by the hole concentration measurements. Initially, the value of the lattice dimension a decreases and c increases monotonically with increasing x. A sudden increase in a and decrease in c begins when x reaches 0.28 for samples prepared under 1 bar of oxygen indicating a sudden loss of oxygen from the structure. This sudden reversal in lattice dimensions a and c is not present in samples annealed under 100 bars of oxygen pressure suggesting essentially no oxygen vacancies. The increase in c with x of the La2-xSrxCuO4 samples with no oxygen vacancies can be attributed mainly to the result of substituting smaller La+3 ions by larger Sr+2 ions. A study of the variation of c with the hole concentration reveals that the c lattice dimension drops sharply with no change in the hole concentration when oxygen is continuously removed from the lattice caused by an increase of the Sr content in La2-xSrxCuO4.

Equilibrium pressures of oxygen above YBa2Cu3O7±x up to 2,000 bar

The interaction of oxygen with the high-temperature superconducting oxides is very important because several phenomena that influence the superconducting properties, such as the orthorhombic to tetragonal phase transformation, are dependent on the oxygen stoichiometry. In spite of this, many of the basic thermodynamic pressure data remain unpublished. The existing information indicates that the decomposition of YBa2Cu3O7–x starts at T = 400 °C at Po2 = 1 bar (ref. 1), and that the compound melts incongruently2. Pressure data and corresponding melting points are therefore needed, to ascertain if it is possible to change the phase diagram at higher oxygen pressures so that the compound melts congruently. If this were feasible, then the growth of large single crystals from a stoichiometric melt could be possible. Here we present the equilibrium oxygen pressure curve for the YBa2Cu3O7±x decomposition (P–T phase diagram) up to 2,200 bar and 1,200 °C. To construct the three-dimensional P–T–x phase diagram of the pseudo-binary system O2–YBa2Cu3O7±x, we have also developed methods for measuring the T–x diagram at high pressure, and we present the first data at 10 bar oxygen pressure.