Oxide bronze compositions and textured articles manufactured...

Details Oxide bronze compositions and textured articles manufactured... Oxide bronze compositions and textured articles manufactured...

2000-05-26

2002-08-20

Kopec, Mark (Department: 1751)

Compositions

Electrically conductive or emissive compositions

Metal compound containing

C505S230000, C505S231000, C505S238000, C505S701000, C505S704000, C252S520500, C174S125100

Reexamination Certificate

active

06436317

ABSTRACT:

TECHNICAL FIELD
The present invention generally relates to oxide bronze compositions. The invention more particularly relates to methods of making oxide bronze compositions and articles formed in accordance therewith.
BACKGROUND OF THE INVENTION
Oxide bronzes, as described in A. F. Wells,
Structural Inorganic Chemistry,
5
th
ed., pp. 612-625 (copyright Oxford University Press 1974, 1984), incorporated herein by reference, typically include solid oxide phases of transition metals such as tungsten, molybdenum, niobium, rhenium, titanium, vanadium, or the like that are doped with cations from the alkali, alkaline earth or lanthanide group elements. The oxide bronzes are characterized by intense color, metallic conductivity or semiconductivity and resistance to attack by non-oxidizing acids. As discussed herein, the terminology “oxide bronze” is used in accordance with the present invention to refer to this subgroup of solid oxide phases, distinguishing them from other transition metal oxides and from the purely metallic copper-based alloys called simply “bronze.”
Oxide bronzes have been extensively studied, and properties of interest for applications have been found. For example, superconductivity has been found in certain compositions at very low temperatures (i.e. helium boiling point range, approximately 4.2 K). More recently, an observation has been made as to the possibility of superconductivity in sodium tungsten bronze phase formed near the surface of a WO
3
crystal (“Na
0.05
WO
3
,” High-Tc Update,
Vol. 13, No. 9, p.1 May 1, 1999; Reich et al., “Possible Nucleation of a 2D Superconducting Phase on WO
3
Single Crystals Surface Doped With Na
+
” The European Physical Journal B,
vol. 9, 1-4; both of which are incorporated herein by reference). This suggestion specifically relates to the observation of possible superconducting characteristics and a superconducting transition temperature (T
c
) of approximately 91 K in Na-doped WO
3
crystals having a surface composition of Na
0.05
WO
3
. The possible superconductivity is suggested to occur near the surface of the sodium-doped crystal. Other properties of the oxide bronzes of interest for applications include dielectric, piezoelectric and electro-chromic properties.
For applications requiring large volumes or areas of material, single crystals are usually not practical, and some kind of polycrystalline structure is required. In addition, oxide bronzes in which the grains are not textured or aligned to reduce or eliminate weak link couplings between grains can pose significant limitations on the fabrication and practical use of certain devices formed of such materials. For example, for long-length polycrystalline superconducting wires, it can be essential to avoid weak link couplings between grains which limit supercurrent density.
In the class of high temperature superconductor (HTS) cuprates (all of which are not in the class of oxide bronzes), various techniques for polycrystalline texturing are utilized. It is generally sufficient, for example, in the case of the BSCCO-based HTS materials to align the grains such that only the c-axis is aligned in the same direction (uniaxial alignment). It is generally not necessary that the a-b planes be aligned. The required c-axis texture in the BSCCO materials can be obtained utilizing various deformation processes, e.g. a thermal-mechanical deformation process.
In the YBCO HTS systems, it is typically necessary to align the grains in the a-, b-, and c-axis directions (biaxial alignment), but the [100] direction must be aligned in the current direction. This can be accomplished for example by growing the YBCO grains on a textured template structure that has a unique crystallographic orientation called “cube texture”. The texture requirement in this case has been shown to lie within 10 and preferably 5 degrees of misorientation angle between grains, if no significant diminution of current carrying capacity is to occur. The oxide bronzes may also need to be biaxially aligned for the highest current carrying capacity. In contrast to the YBCO systems, however, the oxide bronzes can have many other textures besides cube texture.
In the context of high temperature superconductor (HTS) applications, it would therefore be desirable to provide polycrystalline structures that are advantageous over the prior art. In particular, fine-grained materials would be desirable for excellent homogeneity if grain boundary weak links are not an obstacle. It would also be desirable to provide oxide bronze superconducting compositions having grains sufficiently textured or aligned as to overcome weak link characteristics of the grain boundaries, thereby allowing transport of high current across such grain boundaries. Other properties of oxide bronze compositions may also be optimized by providing textured polycrystalline or granular structures. For example, the dielectric breakdown field of oxide bronze capacitors may be increased by texturing.
SUMMARY OF THE INVENTION
The present invention provides methods for making oxide bronze compositions and articles incorporating such compositions. More specifically, the present invention describes procedures for the synthesis of oxide bronzes into forms that can be useful for the fabrication of practical devices.
As used herein, “oxide bronze” in accordance with the present invention include materials having the general formula A
x
BO
y
, in which A is an alkali metal (e.g., Li, Na, K, Rb or Cs), an alkaline-earth metal (e.g. Mg, Ca, Sr or Ba), a lanthanide metal (e.g. La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb or Lu) or one of: In, Cu, Sn, Pb, Tl or Ag and in which A has a valence, m, of 1, 2 or 3; B is a transition metal (e.g. Ti, V, Nb, Ta, Mo, W, Re or Ru) and has a valence, n, of less than or equal to 6; 0<x<1; and y=[(x)(m)+n]/2. In some embodiments, 0.001<x<0.3 and 2.8<y<3.05.
In some embodiments, the oxide bronzes are based on a host structure having a composition of BO
2
or BO
3
, with the valence, n, of B being respectively less than or equal to 4 or 6. The cation A
m
(where m is the valence of A and is typically 1, 2 or 3) can be incorporated into the BO
2
or BO
3
structure if a portion, (x)(m), of the B
n
atoms in BO
y
are reduced to B
n−1
. The electrical properties of the oxide bronzes are associated with the fact that no distinction can be made between the B
n
and B
n−1
atoms in the lattice. The extra electrons, (x)(m), per mole are delocalized over the lattice.
The present invention thus relates to unique oxide bronze compositions and structures and methods of making such structures. Mere preparation of oxide-based bronzes (e.g., Na
x
WO
3
) in bulk form, however, is not sufficient to render them useful for the applications described herein. In order to produce useful articles and devices, the present invention therefore provides, in one aspect, polycrystalline oxide bronze structures having very small, fine grains (e.g., typically in the micron or submicron range in their maximum dimension), which can be for example, rod-like, or equi-axed. Small grains provide the polycrystalline material with mechanical robustness, and improved sintering and density characteristics. In the case of oxide superconductors in which the current capacity within each grain (crystal) varies systematically within the grain relative to position or direction, fine grains tend to reduce the maximum length scale of inhomogeneity, providing for a macroscopically more homogeneous superconductor with improved properties. Grains of larger dimensions (i.e., a few hundred &mgr;m) can also be satisfactory for some utilities of the present invention. In coated conductors, single crystal may be the most suitable form, and small grain sizes may not be favorable. Therefore, in some embodiments, the polycrystalline superconducting layer is a thin continuous sheet disposed on the substrate, in which the average grain size is epitaxially related to the grain size of the underlying substrate. In oth

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