Compositions – Electrically conductive or emissive compositions – Metal compound containing
Reexamination Certificate
2002-07-25
2003-05-27
Gupta, Yogendra N. (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Metal compound containing
C252S518100, C505S120000, C505S121000, C505S124000, C029S599000, C174S125100, C174S126200, C174S12900B
Reexamination Certificate
active
06569360
ABSTRACT:
BACKGROUND OF INVENTION
The present invention relates to a method of preparing a metal matrix composite material with a textured compound. In particular, it relates to a method of preparing a textured superconducting composite wire.
Some important advanced materials have layered structures. For example, high temperature oxide superconductors and the superconducting intermetallic compound MgB
2
are materials with a layered structure. The layered structure causes the anisotropy in physical properties, and texture formation is necessary to achieve superior performance. As the layered compounds are brittle, the layered compounds are often prepared as metal matrix composites to provide better mechanical properties.
One well-known method to prepare such composite materials is the “powder-in-tube” method. This method has been used to prepare the low temperature superconducting compound Nb
3
Sn, the high temperature oxide superconductors, and the recently discovered intermetallic compound MgB
2
. According to the formation of the compound in the powder-in-tube method, the method is also classified into two categories: the in situ technique and the ex situ technique. For the in situ technique, the components in the elemental forms are used as the starting powder, and the compound is formed inside the tube after mechanical deformation of the composite wire. For the ex situ technique, a powder of the compound is used as the starting powder.
For the high temperature oxide superconductors, both metal precursor and oxide precursor were tested, and the best results so far were achieved with the Bi2223 and Bi2212 prepared by the oxide powder-in-tube method (See, for example, a summary by D. Larbalestier, “The road to conductors of high temperature superconductors: 10 years do make a difference”, IEEE Trans. Appl. Supercond. 7(2) 1997, p90-97. Also refer to a review by H. Kitaguchi and H. Kumakura, “Advance in Bi-based high T
c
Superconducting Tapes and Wires”, MRS BULLETIN, February 2001, p121-125). This is attributed to the weakly-bonded, double Bi—O layer in the structures. The easy cleavage between the Bi—O layers introduces texturing of the phase in the deformation process. Attempts to make superconducting wires for the rare earth 123 compounds have not been very successful in achieving a high critical current density for the powder-in-tube methods. For a recent effort in this area, refer to U.S. Pat. No. 6,202,287 by A. Otto which describes an attempt to make a bi-axially aligned 123 wire from a metallic precursor powder Although the melt-texture growth method has produced high critical current densities in bulk RBaCuO-123 superconductors, the very slow growth rate is not practical for the production of long length wires. Current efforts for the rare earth 123 compounds have been focused on the coated thin-film conductors (See a review by Finnemore et al., Physica C, 320, 1999, 1). Other related developments are discussed in the following references: S. Annavarapu et al., “Progress Towards a Low-cost Coated Conductor Technology”, Physica C, 341-348, 2000, p2319-2322; L. R. Motowildlo et al., “Recent Progress in High-temperature Superconductors at Intermagnetics General Corporation”, Physica C, 335, 2000, p44-50; R. L. Meng et al., “Tape processing of HBCCO, BSCCO, and YBCO Thick films on Metallic Substrates with High J
c
by the Spray/Compress Technique”, Physica C, 341-348, 2000, p2315-2318; M. Zhou et al., “Properties of YBa
2
Cu
3
O
x
films on textured Ag tapes”, Physica C, 337, 2000, p101-105; S. P. Athur et al., “Melt-processing of Yb123 tapes”, Physica C, 341-348, 2000, p2421-2424.
Besides the powder-in-tube method, there are many other methods for the preparation of a metal matrix composite. Thin film methods have been used to prepare all types of superconducting materials mentioned above either through physical deposition or chemical deposition. Dip coating has been used to form a layer of compound on a metal substrate. The present invention is related to a bulk method using a powder as the starting material for a layered compound.
The challenges in preparing a composite with a textured compound by a powder method include:
(a) The flow compatibility of the metal and the powder. Poor compatibility will cause difficulty in the deformation process and the formation of sausage in the composite.
(b) Powder packing. A high powder density before sintering is preferred.
(c) Densification. The decrease in density of Bi2223 during heat treatment seems to be mainly caused by the growth of non-aligned oxide grains. International Application Publication WO 01/22436A1 by Li et al., entitled “Simultaneous constraint and phase conversion processing of oxide superconductors”, and the references cited disclose various ways to apply pressure during various stages of heat treatment to deal with the desintering problem to certain degree of success.
(d) Texture formation. It is desirable to develop a high degree of texture in the layered compound. For certain superconducting materials with grain boundary weak link problems, good grain boundary connectivity is also required. In the prior art, texture in Bi2212 is formed through a melt-texture growth method.
Texture in Bi2223 depends on the rolling deformation and the easy cleavage of the 2212 phase. Therefore, a high critical current density is obtained only in the tape form for Bi2223. However, round wire or wires with a low aspect ratio are more desirable for many applications.
A processing method should provide solutions to all the challenges simultaneously in order to be commercially viable. Although superconductors, and especially superconducting wires, are used as examples in the following discussion, the present invention should be applicable to other metal matrix composites with textured compounds.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a method of preparing metal matrix composite with a textured compound, which has better flow compatibility, higher packing density, better densification and texture formation. As a result, improved performance of the compound will be obtained.
Another object of the present invention is to provide a method of preparing composite wires of layered superconducting materials with a better processing condition and improved superconducting properties such as a high critical current density.
Still another object of the present invention is to provide a method of preparing composite wires of layered superconducting materials with a low aspect ratio.
Still another object of the present invention is to provide a method of preparing bulk layered superconducting materials with improved superconducting properties.
According to the present invention, the starting powder for preparing a layered compound should comprise of
(a) A plate-like powder of the layered compound or an intermediate phase (metastable phase) for the layered compound which retains its shape at least at the initial sintering stage. This plate-like powder serves as the template for texture formation of the layered compound. The “plate-like powder”, as the term used in this specification, has a phase with a layered structure and the particles have a minimum basal plane dimension that is at least 1.62 times greater than the thickness dimension. The minimum basal plane dimension is the shortest line segment on the basal plane through its geometry center, which is the diameter of a circular shape, the side length of a square, or the length of the shorter side of a rectangular. Because of the natural variation in powder preparation, it should be understood that the majority (>50% by volume) of the particles should meet the requirement, and preferably 80%, and more preferably 90% of the particles should meet the requirement.
(b) The remaining powder with a particle size smaller than half the median minimum basal plane dimension of the plate-like powder. Preferably this powder has a near-sphere shape, or the dimensions in different directions are similar. This second powder may contain any phases which in combinati
Gupta Yogendra N.
Vijayakumar Kallambella
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