LIQUID PHASE EPITAXY THROUGH A POROUS MATERIAL

Abstract: A method of coating a substrate (14) by liquid phase epitaxy, comprises applying a flux (12), e.g. within a crucible (11), onto the substrate through a porous material (13) comprising one or more rare earth elements or compounds thereof that are soluble in the flux.

Description: LIQUID PHASE EPITAXY THROUGH A POROUS MATERIAL Field of the Invention This invention relates to a method for producing a liquid flux highly saturated with rare earth elements. More particularly, it relates to the growth of a textured superconducting oxide phase by liquid phase epitaxy on a suitable substrate material. Background of the Invention Known high temperature superconductors include those of the type Re-123, Re being a rare earth element or mixture of rare earth elements. One such superconductor is YBa-Cu 3 0 7 (Y-123). If such a material is to carry the large critical currents necessary for technological application, it is essential for the grains of the material microstructure to have a strong bi-axial texture. In the fabrication of coated conductor tapes, this may be achieved by depositing the superconductor on a textured substrate. The substrate can be pre-textured during fabrication. Alternatively, special techniques may be used, to apply a textured buffer layer to an untextured substrate. A particular problem of current technology relates to need to deposit a relatively thick layer of the superconductor. A vapour deposition technique such as 'pulsed laser ablation' or 'sputter deposition' is generally used. However, vapour deposition techniques are slow and costly, owing to the inherently low impingement rate of vapour atoms at the growth interface. A further particular problem of LPE growth of Re-123 is the rapid depletion of the rare earth concentration in the flux at the growth interface. This leads to a massive reduction in the growth rate. Summary of the Invention According to one aspect of the present invention, a method of applying a coating of Re-123 to a substrate comprises applying a flux onto the substrate through a porous material comprising one or more rare earth elements or compounds thereof that are soluble in the flux. According to another aspect of the invention, apparatus comprises the porous material defined above, means for moving a substrate in contact with the porous material, and means for applying a liquid flux onto the substrate through the porous material. The present invention provides a method for the continuous high rate growth of the superconducting layer by LPE where, in the liquid phase, the impingement rate of atoms at the growth interface can be six to nine orders of magnitude greater than for growth from the vapour phase. This fabrication method may be applied to the continuous coating of suitably prepared metallic tape substrates , or to the coating of large area oxide substrates. The novel technique is particularly suitable for use in the growth of Re-123 from a liquid flux based on BaCuO or BaCuFO, that is supersaturated with a rare earth element or mixture of rare earth elements . Growth from such a flux is usually slow because of the low solubility of rare earths, particularly yttrium (e.g. molar solubility 0.6%), in the flux. However, rare earths such as Nd and Sm have a higher solubility (e.g. molar solubilities of 4% and 3%). Furthermore, mixtures of rare earths collectively form a solid solution richer still in rare earth elements. Other rare earth elements that can be used include Er, Eu, Yb, Ho and Gd. Y will often be present. The technique described is a novel method that ( 1 ) achieves a high saturation of the flux with respect to a mixture of rare earth elements , and ( 2 ) provides a method for continuously presenting supersaturated flux to the growth interface for a moving substrate. Description of the Invention The invention will be described in part with reference to the accompanying drawings, in which: Figures 1 and 2 are each schematic representations of respective embodiments of apparatus of the invention, for the delivery of flux to the surface of a substrate, in accordance with the method of the invention. Description of the Invention The present invention allows high saturation of a flux. For example, a liquid flux of BaCuO or BaCuFO is passed through a sacrificial porous ceramic component made up of sintered powders of the refractory Re 2 BaCu0 5 (Re-211) phase family. The component is made from a mixture of Re-211 powders based on a number of different rare earth elements (e.g. Sm-211, Y-211, Ho-211). The proportions of rare earths in the mixture are typically chosen in the ratio of the solubilities of the different rare earths in the BaCuO flux. Other considerations also determine the chosen ratio (e.g. prevailing cost of rare earths, optimisation of the microstructure and grain boundary critical currents of the superconductor). Although Nd readily dissolves in the flux, phases based on Nd are atypical, thus the powder included in the porous component is Nd-422 and the amount used is determined by the acceptability of Nd site exchange with Ba in the growing Nd-123 phase. The porous component may alternatively be fabricated using mixtures of rare earth oxides (e.g. Re 2 0 3 ) instead of Re-211 powders, or Re-211 powders and rare earth oxide powders may be mixed to aid sintering. In another embodiment, the porous component may include native rare earth metal powders or partially oxidised rare earth powders and additives such as Cu or Ag. The composition of the porous component is adjusted to optimise the sintering characteristics and morphology and to control the oxygen activity in the component during the dissolution process. When the liquid flux passes through the porous component, it becomes fully saturated with the combination of rare earth constituents. This is a novel dynamic process for achieving saturation of the flux. An important feature of the process is that the sacrificial porous Re-211 component is preferentially dissolved at the point where the liquid flux enters the component. When the liquid is fully saturated with rare earth, the dissolution ceases. This ensures the integrity and dimensional stability of the surface or nozzle where the flux emerges from the porous component. The level of saturation in the flux depends both on temperature and the partial pressure of oxygen; both variables need to be closely controlled to ensure that the flux becomes supersaturated with rare earth elements when it emerges from the porous component. By differential control of the partial pressure of oxygen on each side of the porous element, it is possible to carry out the liquid phase epitaxy process isothermally. In another embodiment, a temperature gradient is arranged between the main body of the flux and the surface or nozzle where the saturated flux emerges from the porous component. The porosity of the porous component is chosen to give an adequate flow rate of the flux through the component for the particular system design. A typical porosity is in the range of 40%-60% volume. The size of the nozzle, or exit point for flux from the porous component, depends on the dimensions of the substrate being coated. For a moving tape system, the width of the nozzle may be equal to the width of the tape. It may however be considerably longer than the tape width, to give a large surface area for liquid phase epitaxy. Alternatively, a number of separate porous components may be arranged sequentially in a 'modular' way along the substrate tape. The longer the growth interface with the flux, the thicker the Re-123 layer grown for a given tape spooling speed. In a modular system, it is possible to replace individual modules in turn as the sacrificial porous material becomes depleted; in this way the method can be a fully continuous process. There are various methods for the delivery of supersaturated flux to the substrate surface. In one method, the porous component is in the form of a porous plug. A particular embodiment of this technique is shown in Fig. 1. Fig. 1 shows a crucible 11 containing a flux 12 above a porous plug 13. The flux passes through the plug, e.g. under the influence of p0 2 , onto a tape-like substrate 14 moving in the longitudinal direction, as illustrated, to provide a coated substrate 15. Flux is removed in the direction shown by the three arrows . As shown in Fig. 1, the porous plug may be inserted into the wall or base of a crucible containing the liquid flux. The plug may have any convenient shape (e.g. a tapered cylinder) . The liquid flux may be gravity-fed from the crucible onto the moving substrate or may be forced through the plug by gas pressure applied to the crucible. Alternatively, the porous plug may be a unit by itself that is fully charged with liquid flux at the start of the LPE process (thus a 50% dense plug may have an included volume of liquid equal to 50% of the overall volume of the plug). Subsequently, the saturated liquid may be expressed from the plug by applied gas pressure either upwards or downwards or in any chosen direction. The plug may be stationary or rotating about an axis orthogonal to the substrate. The designed shape and detailed structure of the exit surface (or nozzle) for the fluid flux that is in contact with the moving substrate, are of importance. The nozzle surface may be planar with the substrate pressed against it as it moves. Alternatively, it may be radiused and the substrate tensioned across the nozzle as it moves. The detailed structure may be grooved to control the flow of the flux (e.g. to induce local turbulent flow) so as to continually refresh the liquid at the growth interface. In another delivery method, the porous component is in the shape of a rotating cylindrical mandrel. A particular embodiment of this technique is shown in Fig. 2. Fig. 2 shows a crucible 21 containing a flux 22 and a porous component in the form of a rotatable mandrel 23. The substrate 24, its movement and its coating, at 25, and the removal of flux are analogous to the corresponding parts of Fig. 1. Thus, the porous component may be cylindrically symmetrical, and adapted to rotate about the axis of symmetry during LPE growth. The cylinder may have a central channel for supply of flux depleted of rare earth elements to the porous component; alternatively, the lower part of the cylinder may be immersed in a bath of liquid flux. Methods to expel flux from the surface of the cylinder at the interface with the substrate include gas pressure, gravity or centrifugal forces as the cylinder rotates. The substrate is in contact with the cylinder surface, either over a short length of the circumference or over a large distance. The contact may even be for more than one full circumference of the cylinder. The cylinder surface may be slotted, grooved or otherwise patterned, to control the flow of the flux at the growth interface. The following Examples illustrate the invention. Example 1 A porous plug is made by sintering a mixture of equal quantities by weight of Y-211, Sm-211 and Gd-211 powders. The mean powder size is 10 to 15 μm. The component is sintered at 1200°C, to form a 55% dense cylindrical unit. After sintering, the plug is ground to a cylinder 15 mm long and 12 mm, tapering to 10 mm, in diameter. The plug is inserted into a tapered hole in the base of a BaZr0 3 crucible. The crucible has an internal diameter of 20 mm and a length of 50 mm. The crucible is charged with 50 g (3BaO + 5CuO) lump material. The process is carried out under isothermal conditions with an oxygen partial pressure differential. The atmosphere within the crucible is an argon-2% oxygen mixture and the atmosphere outside in the nozzle region is pure oxygen. The crucible is pressed against a suitably textured and buffered substrate tape moving at 1 mm.s "1 , and the temperature is raised to 950- 975°C. The depleted excess flux has a melting point of 850°C and is removed in two stages. Firstly, there is a close fitting mechanical baffle adjacent to the nozzle region; beyond the baffle, any remaining flux is displaced by a pulsed oxygen jet directed at the Re-123 coated tape. Example 2 The procedure of Example 1 was followed, but using Y- 211, Sm-211 and Nd-211 powders (mean size 20-50 μm) , and raising the temperature, on pressing the crucible against the tape, to 1050°C.

Claims: CLAIMS 1. A method of coating a substrate with a coating of Re-123, by liquid phase epitaxy, which comprises applying a flux onto the substrate through a porous material comprising one or more rare earth elements or compounds thereof that are soluble in the flux. 2. A method according to claim 1, wherein the porous material is a sintered ceramic. 3. A method according to claim 1 or 2, wherein the porous material comprises rare earth oxides. 4. A method according to claim 3, wherein the porous material comprises Re-211, Re being one or more rare earth elements, and/or Nd-422 if Nd is present. 5. A method according to any preceding claim, wherein the one or more rare earth elements comprise Y. 6. A method according to any preceding claim, wherein the one or more rare earth elements comprise Er, Eu, Yb, Ho, Gd, Nd and/or Sm. 7. A method according to any preceding claim, wherein the flux comprises BaCuO. 8. A method according to any of claims 1 to 6, wherein the flux comprises BaCuFO. 9. Apparatus comprising a porous material as defined in any of claims 1 to 6, means for moving a substrate in contact with the porous material, and means for applying a liquid flux onto the substrate through the porous material.

Inventors: EVETTS JAN EDGAR, GLOWACKI BARTLOMIEJ ANDREJ, HENSON RONALD, KURSUMOVIC AHMED, EVETTS, JAN, EDGAR, GLOWACKI, BARTLOMIEJ ANDREJ, HENSON, RONALD, KURSUMOVIC, AHMED
Applicants: CAMBRIDGE ADVANCED MATERIALS, EVETTS JAN E, GLOWACKI BARTLOMIEJ ANDREJ, HENSON RONALD, KURSUMOVIC AHMED, CAMBRIDGE ADVANCED MATERIALS LTD., EVETTS, JAN, EDGAR, GLOWACKI, BARTLOMIEJ ANDREJ, HENSON, RONALD, KURSUMOVIC, AHMED
Application year: 1998
Main classification: C30B19
IPC codes: C30B19