Klaus Schulze

Preparation and Characterization of Ultra-High-Purity Niobium

This paper presents a review of the efforts directed to the preparation of ultra-high-purity Nb published during the last 15 years. The interest in high-purity Nb has risen from the need for materials in high temperature and vacuum technology, superconductivity, solid state physics, and its potential in atomic energy fields. Depending on its many applications, the “purity” of Nb is understood in different ways. Based on results of known purification processes for Nb (e.g., liquid-liquid extraction of fluorides, distillation of halogenides, electrolysis in molten salts, electron beam float zone refining, electrotransport, and degassing in ultra high vacuum), an optimized method has been developed, consisting of electrolytic refining which results in single crystals, zone melting, and ultra high vacuum treatments. Material produced by the process has maximum impurity concentrations of 10−l ppm Ta, 2×10−3 ppm W, 4×10−6ppm Co, 2×10−3 ppm Fe, 10−5 ppm Cr, etc., with less than 1 ppm interstitials (C,N,O, except H) and a residual electrical resistivity ratio (RRR) of >104. To accurately determine residual impurity levels, improved analytical techniques were developed. Doping of the produced Nb with selected metallic (Cr,Mo,Hf,Ir) and nonmetallic elements (C, 0) on the ppm level served as both a calibration of the analytical techniques and an effort to tailor the physical properties. General aspects for the development of purification processes for refractory metals, sources of contamination during purification, sample preparation, and application of Nb, as well as the limiting bulk and surface purity of reactive metals are presented here.

The influence of gas atmospheres on the first-stage sintering of high-purity niobium powders

Niobium and tantalum surfaces easily absorb oxygen. With decreasing particle size the content of oxygen increases. The role of this surface oxygen and oxygen in the sintering atmospheres on the first-stage sintering is not well established. Therefore the sintering behavior of high-purity niobium powders was studied by annealing cylindrical powder compacts (particle size <63 μm) in the temperature range from 1000°C to 1600°C in ultra-high vacuum and under low oxygen partial pressures, as well as in inert gas atrnospheres with low oxygen contents. The specific surface of the samples was determined by metallographic methods, adsorption, and capacitance measurements. Low oxygen partial pressures (10-3 Pa) lead to a slight enhancement of the surface diffusion which is controlling first-stage sintering. High heating rates (0T > 3000 min-1) to temperatures above the melting point of Nb2O5 (Tm = 1495 °C) enhances the neck growth due to the formation of a liquid oxide phase on the surface of the powder particles.

Characterization of polycrystalline YBa2Cu3O7-x by bulk and microdistribution analysis

When analysing various YBa2Cu3O7-x powders (some commercially available) for main, subsidiary and trace components by emission spectrometry (OES-ICP), significant deviations in some from the nominal stoichiometric composition were found, as well as considerable concentrations of carbon and water. Optical micrographs of sintered and post-annealed samples prepared from an off-stochiometric powder show second phases at grain boundaries and inclusions within the grains. In addition, a strongly reduced twin formation in the grain occurs, compared with that obtained from a sample with a stoichiometric composition, which can be traced to reduced oxygen diffusion. In the grain boundary phase a higher concentration of Cu (CuO segregation) along with a lower concentration of Ba and, in inclusions, higher Ba and relatively high C concentrations (BaCO3 segregation) as opposed to the 1-2-3 phase were found by SEM-EDX/WDX analysis. The results of the analytical investigations are correlated with superconducting properties (Tc,Jc).

Elastic twins in YBa2Cu3O7 crystals

Abstract On cooling from sintering temperatures YBa2Cu3O7 undergoes a tetragonal-to-orthorhombic phase transformation induced by oxygen ordering. The resulting lattice strain is relieved by twin formation. Besides residual twin lamellae also elastic twins are formed. Using optical microscopy observations of twin geometries, we show that elastic twins can be described by dislocation theory. This allows us to express twin geometries as a function of constants of plastic deformation, such as shear modulus, critical stress, and twin plane energy. From this expression we obtain γ≈1 J/m2 for the twin plane energy. This value is compared with the twin plane energy of other ceramic and metallic materials.

The phase equilibria of Bi2Sr2Ca2Cu3O10 in the system Bi2O3SrOCaOCuO

Abstract The phase equilibria of the system Bi2O3SrOCaOCuO with emphasis on the High-Tc superconducting compound Bi2Sr2Ca2Cu3O10 have been studied. For the preparation of single phase Bi2Sr2Ca2Cu3O10-samples the exakt stoichiometry is not likely, because it is not stable. For the processing of Bi2Sr2Ca2Cu3O10-samples the preparation of samples exhibiting a little excess of CuO and Bi2O3 is recommended.

The preparation of pure niobium for neutron dosimetry purposes

A technique is described for the preparation of high purity niobium for use in fast neutron dosimetry. Based on results of known purification processes for niobium, an optimized method has been developed, consisting of: (1) a double electrolytic refining in an eutectic lithium-, sodium-, potassium-fluoride melt, containing fluoro-potassium niobate (K2NbF7), (2) electron beam float zone melting (EBFZM) in ultra high vacuum (UHV) and (3) UHV treatments. Starting with EBFZM of niobium of commercial quality (140 μg/g Ta, 35 μg/g W) the tantalum and tungsten contents were reduced by a first electrolysis to approximately 4 and 4 × 10−2 μg/g, respectively. For a second electrolytic refining using a salt bath with extremely low tantalum and tungsten contents, this material was subjected to an additional EBFZM process. The niobium metal produced by this step was three times zone melted to reduce those elements (e.g. Fe, Co, Ni, O, N) which increased during the electrolyses. Material produced by this technique has impurity concentrations below 0.4 μg/g of tantalum and 10−2 μg/g of tungsten. The concentration of the interstitials (C, O, N except H) is below the detection limit of classical analytical methods. A further reduction of the interstitials by annealing treatments in UHV of this material resulted in an electrical residual resistivity ratio (RRR) ρ(295 K)/ρ(4.2 K) = 24 500 indicating an impurity concentration far below 1 μg/g.

Internal friction measurements on polycrystalline YBa2Cu3O7−x

Abstract The internal friction (IF) of polycrystalline YBa 2 Cu 3 O 7−x was studied from 4 to 300K for frequencies between 2 and 19 Hz. The samples were prepared by the standard solid state reaction method. The oxygen content was varied by annealing the samples in air at various temperatures and subsequent cooling in flowing Ar or O 2 . For YBa 2 Cu 3 O 6·9 two closely related and reproducible IF peaks appear: peak A at 68 K and peak B at 87 K (f= 8 Hz). Both peaks are shifted to higher temperatures with increasing frequency as expected for thermally activated (stress induced) reorientation of atomic defects. The parameters for the temperature dependence of the relaxation time, τ = τ (∞) =exp(H/kT), were determined as H A = 0.15eV, τ A (∞) = 10 −13 s; H B = 0.17eV, τ B (∞) = 10 −12 s. We assume that peaks A and B are connected with jumps of oxygen vacancies. The influence of oxygen content will be discussed in this context. Other peaks at about 40 to 50 K do not shift with frequency and are probably due to phase transitions.

High-Temperature Interactions off Refractory Metals with Gases

The thermodynamics and kinetics of high-temperature gas/metal reactions are important for the processing and application of the group IVa, Va, VIa and VIIa refractory metals—Zr, Hf V, Nb, Ta, Mo, W and Re. This article surveys the reactions of these metals with O, N, C and H. Along with various types of gas/metal reactions, the equilibria and thermodynamics for the range of solid solution of the nonmetals are discussed. The kinetics and mechanisms of reactions with N2, O2, H2O, CO and hydrocarbons are treated with an emphasis on the high-temperature regime since refractory metals are most frequently applied in high-temperature environments. Degassing reactions and steady states are also examined, as are some technical aspects of gas/metal reactions, including aspects of purification, doping and high-temperature treatments arid applications of refractory metals.

Influence of microstructure on the superconducting properties of polycrystalline YBa2Cu3O7−x

Abstract Microstructural development and shrinkage during sintering of YBa 2 Cu 3 O 7−x is determined not only by the sintering parameters but also by the starting components and calcination procedure. During sintering the residual carbonate content in calcined powders causes gas porosity and inhomogeneous microstructural development. Small amounts of second phases lead to reduced sintering rate. The critical current density of sintered samples is related to the average grain size. A fine-grained microstructure enhances the current transport capability remarkably. After identical post-annealing treatment, fine-grained samples show comparatively higher values of oxygen concentration. The enhancement of critical current density by a fine-grained microstructure is traced back to the favorable distribution of the non-superconducting phases present, fewer cracks, and an increased and more homogeneous oxygen concentration.

Processing and superconducting properties of Bi-Sr-Ca-Cu-O compounds

Abstract Samples of various compositions in the system Bi-Sr-Ca-Cu-O were prepared and screened for signs of the 110 K superconductive phase. Samples having nominal Bi-Sr-Ca-Cu composition 1112 show the steepest drop in resistivity near 110 K when prepared near the partial melting point of 870°C in air as determined by DTA. Optical and scanning electron microscopy as well as EDX were used to characterize and to identify the phases, two of which do not contain Bi. Separate high-temperature processing of carbonate mixtures at 1400°C proved to be of advantage for a low residual carbon content of the samples.