Jorulf Brynestad

Grain‐boundary compositions in YBa2Cu3O7−x from Auger electron spectroscopy of fracture surfaces

Comparison of Auger electron spectra obtained from intergranular and transgranular areas exposed on fracture surfaces of sintered YBa2Cu3O7−x specimens indicates that most grain‐boundary surfaces are deficient in oxygen and rich in copper compared to the bulk. The thickness of this region of altered composition is estimated to be in the range of 15–50 A. No evidence of segregation of impurities to grain boundaries was seen. The results suggest that the grain‐boundary layer is nonsuperconducting and a likely contributor to the problem of low critical current densities in these materials. It is believed that carbon at grain boundaries which has been reported results from incomplete calcination in material which is slightly off stoichiometry.

Structure and superstructure of the superconductor Tl2Ca1Ba2Cu2O8 by neutron and electron diffraction

Abstract Neutron powder diffraction has been used to refine the structure of the high- T c superconductor Tl 2 Ca 1 Ba 2 Cu 2 O 8 to determine the precise oxygen co-ordination and the effective valence of copper. The results are in excellent agreement with the Dupont X-ray structure, confirming the possible substitution of about 9% of the Ca-sites with Tl in our sample. The Tl-sites may be either 3% Tl-deficient or 7% Ca-substituted. The apparent valence of copper is 2.22. Oxygen in the center of the Tl square implies an average valence of less than 3 for Tl, and it is therefore not suprising that this oxygen is desordered toward the mid-point of the shortest Tl-Tl distance. High resolution electron images and diffraction reveal a quite different superstructure to the 5 x 5.4 A superstructure seen in Bi 2 Sr 1 Ca 2 Cu 2 O 8 . Very weak and diffuse superlattice spots may be due to a charge density wave in the TlO plane with period 6 x 3.8 A.

Synthesis and magnetic characterization of the high-Tc superconducting compound HgBa2CuO4+δ

Bulk superconducting samples of HgBa2CuO4+δ (hereafter denoted as Hg-1201) were prepared by solid state reaction of stoichiometric quantities of HgO and Ba2CuO3 precursors. The “as-synthesized” Hg-1201 samples had a Tc of 95 K, as defined by the onset of diamagnetism. The crystal symmetry was found to be tetragonal with lattice parameters, a=3.876(2) A, and c=9.515(3) A, similar to the literature values. Finely ground X-ray specimens were stable upon exposure to ambient air for a period of ≁24 h. Magnetic characterizations were conducted using a SQUID based DC magnetometer. Studies of the transition region in low applied fields showed complete screening and evidence for weak intergrain coupling of the randomly oriented crystallites. High field magnetic hysteresis studies showed rapid decay of hysteresis of the magnetization (proportional to the current density), both with applied field and temperature. These findings are attributed to the presence of substantial thermally activated flux motion.

Temperature Dependence of the Absorption Spectrum of Nickel(II)‐Doped KMgCl3 and the Crystal Structure of KMgCl3

Temperature dependence from 80° to 763°K is reported for the intraconfigurational 3d8↔3d8 transitions of octahedrally coordinated [NiCl6]4− in single crystals of K(Mg1−x, Nix)Cl3 with small x. The crystal structure of the host material, KMgCl3, is reported at 298°, 383°, and 448°K. Band energies are treated in terms of the Liehr—Ballhausen model of d2,8 electronic systems with Dq, B, C, and λ used as empirical parameters. At all temperatures the data are satisfactorily represented by B=850 cm−1, C/B=3.986 (free‐ion value), and λ=−275 cm−1. Thus, B, C, and λ are all reduced to about 82% of their free‐ion values. The parameter λ is estimated from band splittings at 80°K. The parameter Dq varies between −570 cm−1 at 80° and −500 cm−1 at 763°K. Oscillator strengths of the 3A2g→3T1g(F) and 3A2g→3T1g(P) transitions increase by a large factor between 80° and 763°K, while that of 3A2g→3T2g increases by a small factor. The compound KMgCl3 has a cubic perovskite‐type structure at 448°K. At lower temperatures it has...

Characteristics of Bi-Pb-Sr-Ca-Cu-O powders produced by aerosol decomposition and their rapid conversion to the high-Tc phase

Abstract Bi-Pb-Sr-Ca-Cu-O powders were produced by aerosol decomposition of nitrate solutions. The effects of reactor temperature and residence time on particle morphology and evaporative Pb loss from particles were demonstrated, and conditions necessary to control Pb loss established. Pb loss was roughly proportional to residence time, and minimal loss occurred with short residence times (3s) and T≤800°C. Particles produced at 700°C typically contained significant porosity, while those produced at T≥800°C were solid. Mixtures of the Bi2Sr2CuOy (2201) and Bi2Sr2CaCu2Oy (2212) phases were produced at 700–900°C in nitrogen and air. However, after hearing in air for 16 h at 850°C, pellets of powder produced at 700°C with nominal composition Pb0.44Bi1.8Sr2Ca2.2Cu3Oy converted to approximately 79 vol.% of the Bi2Sr2Ca2Cu3Oy (2223) phase and displayed a Tc (onset) of 110 K. Rapid conversion to 2223 was promoted by powder synthesis conditions, leading to controlled Pb loss and a homogeneous fine-grained dispersion of mixed-oxide precursor phases within particles.

CO2 solubility in YBa2Cu3O7−x

Abstract The interaction of CO 2 with YBa 2 Cu 3 O 7− x (123) was studied at 1173 K. The 123 compound appears to include ≈ 0.011 mol CO 2 even when prepared in the absence of CO 2 and protected from further interaction with the environment. Increasing the concentration of CO 2 in O 2 leads to increased solution of CO 2 , with a limiting composition of YBa 2 Cu 3 O 6.38 (CO 2 ) 0.036 a t 0.0014 MPa CO 2 in 0.0986 MPa O 2 . The solid-solution CO 2 may be rejected to form CO 2 -containing BaCuO 2 (011) upon slow cooling. The carbon contents of other compounds in the Y-Ba-Cu-O system were also determined.

Oxidation induced decomposition of YBa2Cu3O7−x

At low oxygen potentials, YBa2Cu3O7−x decomposes by chemical reactions involving reduction. The results of this study show that the compound also decomposes at higher oxygen potentials. The initial decomposition products were found to be Y2BaCuO5 and a Ba‐Cu oxide phase. The Ba‐Cu phase was found to be a peroxide‐type and the decomposition can be described by a chemical reaction involving oxidation: 4(YBa2Cu3O7−x)+(1/2 − 3/2δ +2x)O2⇄2Y2BaCuO5+3Ba2Cu3O6−δ+CuO. At 800 °C the equilibrium pressure for this reaction is slightly <1 bar, and the enthalpy change is ∼80‐kJ/mol Y123. This result is not consistent with the conclusions drawn from some other studies of the stability of the superconducting Y‐Ba‐Cu‐O compounds, and the differences are discussed.

Preparation of titanium diboride powder

Finely-divided titanium diboride or zirconium diboride powders are formed by reacting gaseous boron trichloride with a material selected from the group consisting of titanium powder, zirconium powder, titanium dichloride powder, titanium trichloride powder, and gaseous titanium trichloride.

Some observations of the effects of high pressures and temperatures on the stability of YBa2Cu3O7−x

Hot isostatic pressing and heat treatments in high‐pressure oxygen were used to study the effect of pressure on the stability of YBa2Cu3O7−x. At 875 °C the compound decomposes at an oxygen pressure of about 7 bars. The decomposition involves formation of Y2BaCu3O5 and a Ba‐Cu oxide phase. Formation of the latter phase involves additional oxidation because this phase can contain as many as 1.5 oxygen atoms per barium atom. The decomposition can most simply be visualized by considering the chemical reaction 2(YBa2Cu3O7−x)+( (1)/(4) +x−y/2)O2→Y2BaCuO5+Ba3Cu5O9.5−y. Encapsulated samples also decompose during hot isostatic pressing, but the process appears to be more complex. The compound YBa2Cu4O8 was detected in addition to the two products observed in oxygen‐treated samples. The reaction above, with YBa2Cu3O7−x serving as an oxygen source, aids in understanding the microstructural observations for encapsulated samples. Decomposition can be completely reversed by annealing in air for 20 h at 875 °C.

Electronic Spectra and Coordination Geometry in Molten Mixtures of CsCl and NiCl2 Containing up to 60 Mole% NiCl2

Optical absorption spectra (5–26 kK) of fused mixtures of CsCl and NiCl2 containing 2–60 mole % NiCl2 were measured at about 860°C and a mixture containing 20 mole % NiCl2 was studied at 560°—862°C. Up to 20 mole % NiCl2 the Bouguer—Beer law was accurately obeyed, and each nickel was tetrahedrally coordinated to four chlorides. At higher NiCl2 contents, nickel coordination was partitioned between tetrahedral and other (unidentified) geometries. The fraction with tetrahedral coordination progressively decreased with increasing NiCl2 content but was still appreciable at 60 mole % NiCl2. The most prominent feature of the spectrum associated with the unidentified geometries was a band between 18 and 19 kK. These results indicate that nickel has very similar coordination geometries in the liquid and crystalline phases of Cs3NiCl5 but very different coordination geometries in crystalline and liquid CsNiCl3.