Blaise Champagne

Calculation of magnetic flux profiles and deduction of critical current densities for type II superconductors

The critical state concept for the calculation of internal flux profiles in type II superconductors is used to model the magnetization behavior of high-T/sub c/ materials. In this application of the model, the field-dependent reversible magnetization and critical current density are incorporated flexibly into numerical solutions of the basic equations. The solutions are compared to results obtained by using the Bean (constant J/sub c/) and Kim (field dependent J/sub c/) approximations. The authors use theoretically calculated magnetization curves to analyze experimental magnetization data for bulk, textured Y/ErBaCuO samples obtained from their own work and from recently published work. Pitfalls and guidelines when using such measurements to obtain intrinsic material critical current densities are discussed.

X-ray characterization of Nd2BaCuO5

Abstract The synthesis and the X-ray powder pattern of Nd2BaCuO5 are reported. This compound crystallizes in space group P4/mbm with cell parameters a = 6.6977(4) and c = 5.8209(4) A . It is shown to be isostructural with La2BaCuO5, as observed diffracted intensities match well those calculated from the known structure of La3.6Ba2.4Cu1.8O9.6.

Post-deposition treatments of plasma-sprayed YBaCuO coatings deposited on nickel

Abstract As-sprayed YBaCuO coatings do not exhibit superconductivity because of the non-equilibrium solidification conditions of molten particles on the substrate and to the deposits loss of oxygen. Therefore post-deposition treatments are required to restore the superconductivity. In this study, post-deposition treatments were carried out on thick YBaCuO coatings (200 micro) deposited on cold nickel substrates to modify their microstructure, to restore the oxygen content and to improve their superconducting properties. These treatments consist in heating the coatings at various temperatures above 950 °C followed by controlled solidification cycles. The effect of these treatments on the microstructure of the coatings was assessed and the interaction between the coatings and the nickel substrate was also examined. Solidification cycles including a low cooling rate near the non-congruent melting temperature of YBa 2 Cu 3 O x and involving a temperature gradient were carried out to create a texture.

Occurrence of BaCu2O2 in plasma-sprayed YBaCuO coatings

Abstract BaCu 2 O 2 (tetragonal, a = 5.722, c = 10.064 A, space group I4 1 /amd, Z =4) was identified by X-ray powder diffraction in plasma-sprayed YBaCuO coatings, together with Y 2 O 3 , YBa 2 Cu 3 O 7−δ , Y 2 BaCuO 5 and BaCuO 2 . In atmospheric plasma spraying, YBaCuO particles are melted at very high temperature and undergo a rapid solidification under low oxygen pressure which leads to the formation of a large amount of BaCu 2 O 2 . This interpretation also explains the experimental results of previous investigations in which the X-ray lines of BaCu 2 O 2 had been ascribed to “an unknown BaO-CuO phase” or to “a simple cubic phase”.

Zinc-nickel coatings for improved adherence and corrosion resistance☆

Abstract Plasma-sprayed ceramic coatings may spall catastrophically in service because corrosion products are developed at the coating-substrate interface. This phenomenon is likely to occur with ceramic coatings on steel substrates even when the substrates have first been coated with metallic bond coats. This work was aimed at developing zinc-nickel thermal-spray powders for cathodic protection of steel substrates. Powders were produced from zinc-30 wt.% nickel alloy with a particle size distribution of 90 - 150 μm and a study of their spraying parameters on steel substrates was carried out. The performance of these zinc-nickel coatings was determined by subjecting plasma-sprayed alumina and chromia coatings with and without a zinc-nickel underlayer to aqueous corrosion for various periods of time according to the ASTM B117-85 test. The effect of degradation by corrosion was measured by the ASTM C633-79 procedure. A drastic loss of bond strength, as a function of corrosion time, was noticed in ceramic coatings deposited directly on steel substrates. Similar behavior was also observed for ceramic coatings plasma-sprayed on to nickel-chromium bond coat. Results of electrochemical, salt-spray and bond strength tests showed that zinc-30 wt.% nickel coatings provide a cathodic protection. This cathodic protection confers an active corrosion protection on the substrate and is not affected by the presence of porosity in the ceramic coating. The adhesion strength of these zinc-30 wt.% nickel coatings is in the neighborhood of 40 MPa.

Microstructural investigation of RBa2Cu3O7−δ (R=Y, Gd, Nd)

Abstract RBa 2 Cu 3 O 7−δ perovskites, where R is Y, Gd and Nd, were synthesized using the same experimental conditions. The resistivity curves of the three compounds were similar but their microstructures were different. Secondary phases (CuO, BaCuO 2 ) were formed in the compounds containing Y and Gd while no such phases were detected in the Nd perovskite material.

Interactions of various substrates with melted ErBa2Cu3Ox

The interactions of partially melted Er-Ba-Cu-O superconductors with various substrates including MgO, TiO2, SiC and PSZ were studied with the objective of finding an adequate buffer layer for a nickel-HTSC composite. Bulk ErBa2Cu3Ox pellets were melt-textured on the substrates and the superconductive properties of the composites were assessed by AC susceptibility. In the present paper, microstructure of the textured superconductors and the interfaces are described. Contamination of 123 grains by the various materials and the composition of the reaction products are presented.

Technical note: Agglutination process for coating ceramic particles

Summary In the fabrication of high performance metal-ceramic composites and plasma-sprayed coatings containing dispersed ceramic particles, the use of metal-coated ceramic powders can be expected to ensure better bonding between constituents and provide a more uniform microstructure than the mixing of ceramic and metal powders could do. Various coating methods have been used to coat ceramic powders. However, these methods are limited in terms of coating materials and thicknesses. A new inexpensive method based on an agglutination process is proposed to coat different kinds of ceramic particles (40 – 100 pm) with various metallic elements. The process variables are discussed with examples concerning Al 2 O 3 and SiC. It was found that there is no limitation in terms of ceramic cores and metal coatings and that it is possible to produce thick coatings depending upon the nature of metal powders and ceramic particles.

Laser processing of plasma-sprayed R-Ba-Cu-O coatings

Thick Y-Ba-Cu-O and Er-Ba-Cu-O coatings were deposited by plasma spraying onto nickel substrates. These plasma-sprayed coatings were laser melted to modify their microstructures. The effects of primary processing conditions, such as linear energy and number of passes, on microstructural modifications were assessed. The microstructure of the as-sprayed coatings was largely transformed to produce a fine Y2O3 dispersion in a Ba-Cu-O matrix. A very low level of coating/substrate interactions can be maintained by appropriate laser processing conditions.

REACTIVITY AND PHASE EQUILIBRIA OF Y-Ba-Cu-O PARTLY MELTED UNDER ARGON IN NICKEL

ABSTRACT Considerable improvement in the processing techniques of superconductors is necessary to control the reactivity of cuprates towards potential substrate materials. In this study, the reactivity of YBa2Cu2Ox, melt with nickel was assessed. YBa2Cu3Ox was partly melted in nickel at 1080°C under argon. Crystallized compounds indicated that a significant reduction of cuprates occurred due to a low oxygen potential which changes the formal copper valency (Cu2+→Cu+). A substantial nickel contamination of the melt by the attack of the nickel took place with the formation of a nickel oxide skeleton which was selectively infiltrated by a Ba-Cu-O rich liquid. Such a selective mass transport produced a significant chemical demixing and an important modification in the stoichoimetry of the melt. This preferential mass transport, fractional crystallization as well as nickel contamination caused the formation of phases which resulted from an extensive evolution of the compositional path as well as the formation ...