P. Lambert

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.

Texturing of thick films on a metallic substrate

Low-temperature melt-texturing of a laminated composite tape was carried out. No reaction or contamination of the superconducting phase was detected and the interface was very clean. The application of the authors procedure for obtaining well-connected textured material is discussed.

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.

Mass transfer in a ErBa2Cu3O7 pellet partially melted in a thermal gradient

Abstract Melt texturing in high thermal gradients promotes the crystallization of oriented RBa2Cu3Ox grains which improves the critical current densities. However, the presence of high thermal gradients induces a transport of material that modifies the composition of the melt and disrupts the crystallization process. In this study, material transfer phenomena have been examined by crystallizing a ErBa2Cu3Ox pellet in a static thermal field. The influence of material transfer on the microstructure of textured pellets has been studied. These microstructures indicate that the material is transferred mainly by thermocapillarity. We also determined that the temperature of the characteristic ridge formed during crystallization was about 892 ± 10°C which corresponds closely to the solidification of the binary and ternary eutectic temperatures.

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.

Metallic alloys suitable for YBCO melt-texturing at low temperature

Ag-Au-Pd alloys, having a gradient in the concentration of the elements (called diffusion alloys), were investigated in order to find homogeneous alloys suitable for melt-processing of YBCO superconductor. Interface aspects between the superconductor and diffusion alloys and composition profiles of the diffusion alloys after melt texturing are presented. The results obtained with the diffusion alloy led to the development of new ternary homogeneous Ag-Au-Pd alloys of composition (at %) Ag-(18–23)Au-(2–10)Pd. The interface between homogeneous alloys and the YBCO superconductor after melt-processing was characterized and the resistivity-temperature curve obtained with electrical contacts on the metallic part of the composite is shown.

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 ...

Effect of precursors on the size of 2-1-1 in melt-processed ErBa2Cu3O7−x

Abstract In order to study the influence of the precursors on the size of 2-1-1 inclusions, three types of precursors based on stoichiometric ErBa2Cu3O7−x, were melt-processed: a synthesized powder (SP), a melt-quenched powder (MQP) and a mixture of Er2O3 and Ba-Cu-O compounds (MP). It is demonstrated that a fine dispersion of Er2O3 in a Ba-Cu-O liquid (MP) does not explain the finer 2-1-1 inclusions that form in the MQP material. The refinement of the 2-1-1 phase in MQP samples is attributed to the presence of platinum in the ErBa3Cu2Ox phase found in melt-quenched precursors before melt-processing.

Interactions between nickel and melted R-Ba-Cu-O (R=Y, Nd)

R-Ba-Cu-O (R = Y and Nd) compounds were melted and crystallized in nickel crucibles. The interaction between these materials was studied by examining the physical aspect of the nickel interface. Interaction mechanisms between nickel and melted R-Ba-Cu-O compounds are proposed to explain the formation of an interface, constituted of a top layer of Ni0.8Cu0.2Ox and of an NiOx underlayer as well as the presence of nickel oxide-rich particles in the melt.