Partenaires

Logo tutelle CNRS
Logo tutelle Ensicaen
Logo tutelle Unicaen
Logo CNRT
Logo Labex EMC3
Logo Carnot


Rechercher

Sur ce site

Sur le Web du CNRS


Intranet Webmail

Accueil du site > Divers > equipes de recherche > Matériaux Fonctionnels et de Structure (MFS) > Activités > Céramiques supraconductrices >

28 juin 2010

 

Observe : Superconducting exhibition : SUPERLIFE

 

Introduction

 

 

We are working on two bulk superconducting material families.

 

- RE-Ba2Cu3O7-δ (RE=Rare Earth : Pr, Nd, Sm, Eu, Gd,...) and YBa2Cu3O7-δ (Y123).

 

Y123 derives from the ABO3 perovskite structure. It is constituted by a stacking of 3 perovskite cells presenting an oxygen deficit. This structure can be represented as the A3B3O9 form as following :

 

(YBa2)Cu3(O2+z 1-z)3 i.e. YBa2Cu3O6+3z

 

The resulting structure YBa2Cu3O7-δ can be described as 6 parallel planes aligned along the c axis with respective compositions CuO1-δ, BaO, CuO2, Y, CuO2, BaO. The crystalline structure as the superconducting properties depend on the oxygen stoechiometry. Indeed, at low oxygen content (large d) the compound presents a tetragonal symmetry and is non superconducting although at higher oxygen content (d<0.65) the compound adopts the orthorhombic symmetry and becomes superconducting. Note that an optimal oxygen amount exists around O6.92.
 






- (Bi,Pb)2Sr2Ca3Cu3O10+x (Bi2223)

 

This phase is in the orthorhombic symmetry.This cristallographic structure is based on the intragrowth of quadruple sheets of NaCl type (SrO/BiO/BiO/SrO) with sheets of perovskite type (CuO/Ca/CuO/Ca/CuO). So, its cristallographic structure is anisotropic and this compound cristallizes naturally in a lamellar symmetry.

 

 


Problem

The superconducting currents circulating preferentially in ab planes of both materials families (anisotropic properties), it is essential to obtain a grains preferential orientation, that is to say to texture the material. The texturation of these ceramics allows also to diminish the grains boundaries effect which is a barrier for the current. The texturation method depends on the material. In both following parts, we present methods used in the laboratory for both types of superconducting ceramics.

 

 

Re-BaCuO

 

Y123 compounds have suscited a big interest when they were discovered. Their principal quality lies in the fact that they present superconducting properties in liquid nitrogen (77K). Actual researches on bulk material focus on the elaboration and on the shaping in a view to industrialization. The material geometry depends on the application envisaged : a bar for current transport or a large disk for levitation or field trapped applications. In both cases, important critical current values are required. However, grains boundaries and desorientation between grains of the sintered material are barriers opposite to the good current flow. Hence, it is absolutely necessary to texture Y123, that is to say to produce samples containing one domain. This compound being highly anisotropic, the single domain orientation versus sample form must be also controlled to optimize his capacities. Different texturing technical ways have been developed in CRISMAT laboratory.

Crystalline growth methods can be resumed by the following equation :

Depending on the method used, the texturing force will change.

 

 

TSMTG (Top Seeding Melt Texturing Growth)

This method uses a seed (texturing force) which has a melting point higher than the Y123’s one and cristallographic parameters close to Y123’s ones.

Classical seeds are textured samples of SmBa2Cu3O7-δ or NdBa2Cu3O7-δ . Their cristallographic parameters are close to Y123 parameters. (001) oriented samples are put on the upper surface of Y123 pellets . Y123 recristallisation occurs by epitaxy on these seeds during a very slow cooling (<1°C/h) in a temperature range extending from 5 to 15°C. The domain grows then by sympathetic nucleation from the first Y123 nucleus. If the thermal process is well adapted and controlled, the c axis of the grown domain is imposed by the seed and is parallel to the pellet axis. In this configuration, the induced currents are circuling in ab planes which is the best configuration regarding the compound anisotropy. Pellets properties are then optima.

 

   
 
 


A single domain is obtained by this technique without grain boundary.

The textured domain orientation is very well controlled with this process. 10 cm diameter pellets have already been synthesized.

 

 

Bridgman furnace (Melting zone)


This method is used to texture Y123 bars, which are intended for current transport or as current limitors. It consists in translating ( 1mm/h) a bar in a furnace which presents a thermal gradient (texturing force) as can be seen on the following figure. In the middle of the furnace, the temperature is higher than Y123 melting point. After melt, the material recristallizes to form a single domain from the first nucleus (homogeneous nucleation). The scheme and the image below illustrate the process.

   
   Four Bridgman
 single - domain  




Elaboration by infiltration of YBa2Cu3O7 ceramics from Y2BaCuO5 (Y211) bulks and "sponges"



Infiltration
By combining infiltration-growth (IG) process (following figure) and Top Seeding Melt Texture Growth (or TSMTG), Y123 single domain samples have been obtained from Y211 impregnated by a liquid phase Ba3Cu5Ox or "Y035" phase . (a) and (b) figures present both methods used in the laboratory.

This process allows :
· to keep the initial form of the material
· to obtain materials which have less macro or microcracks
· a fine and homogeneous distribution of Y2BaCuO2 inclusions in YBa2Cu3O7 matrix without dopants addition such as CeO2 , PtO2 …)

 

Sponges
In collaboration with ACCESS e.V Materialsforshung, Y211 porous ceramics have been elaborated from polyurethane foams. Polyurethane is impregnated by a viscous mix prepared from commercial Y211 in a solution containing 10% of polyvinyl alcohol. By capillarity, the liquid phase (Y035) infiltrates in Y211 foam. Nucleation and growth of the textured domain is controlled by a Nd123 seed placed upon a Y2O3 gauze.

The Y123 porous ceramic presents some advantages :
· a saving of time for oxygenation. His "alveolate" structure is good for oxygen diffusion
· the faster cooling in liquid nitrogen. This material could be used as resistive element in current limitation, because his morphology is adequate for more efficient thermic transfers with the refrigerant. This should reduce the formation probability of "hot spots" frequently observed in Y123 bulks.

 

 

  (a) polyurethane sponge

(b) Y211 sponge

(c) Y123 sponge

(d) macrostructure of Y123 sponge

(e) microstructure of Y123 sponge

 

Perforated Y123
In collaboration with CRETA-Grenoble, Y123 perforated superconducting materials with oriented grains are obtained. Starting materials are sintered (Y123 or Y211) ceramics. They are perforated mechanically along a regular network. After that, either a conventionnal texturing treatment (TSMTG) or infiltration-growth (IG) is performed. One of the goals of the preformed structure is to obtain a more stable structure presenting the same advantages than "sponges" materials. The video shows the crystalline growth of a perforated material identical to conventionnal samples.

 

 

 
 Perforated Y123 examples

 

 

 

(Bi,Pb) SrCaCuO

 

The lamellar structure of this compound leads researchers to use an original process named "Sinter-Forging". This method consists in applying a pressure during the temperature rise in order to align the grains along (ab) planes. The following scheme presents you the process.

The application of a mechanical constraint s associated to the high temperature leads to the grain alignement.

 

 
 Before : isotropic sintered ceramic  After : textured ceramic


The decrease of grains boundaries effect allows the superconducting currents to flow easily through the whole sample. So, the material properties are clearly increased.

 

Contact : Christelle HARNOIS

Publications

 

 


Back to home page