Niobium bronzes

The compound K0 3NbF3 has an average niobium valency of 2.7 and forms a crystal structure that is referred to as hexagonal tungsten bronze [239]. 

Finally, a number of other mixed oxides that do not have the perovskite structure have also been examined. For example, niobium titanates with the rutile structure,tetragonal tungsten bronze... 

Figure 9.1 Tetragonal tungsten-bronze structure of SiiBai-xNboOf, (Strontium barium nio-bate, SBN). Characteristic are the corner-linked niobium-oxygen-octahedra and 3 types of vacancies A,Bi,2,3 and C. Sr2+-ions are embedded in the A- and B2-positions and Ba2+ in the Bi-positions according to their ionic radii. The C-positions and B3-position remain empty.

Figure 3. Stacking pure niobium rods into a drilled billet of bronze prior to drawing down to wire.

Zireonium has been determined in niobium by using Pyrocatechol Violet [71]. Hafnium has been determined in uranium alloys with the use of PAN [27]. 2-(2-Pyridylmethylenamino)phenol has been applied for determination of Zr in the presence of Cr (in bronzes) by derivative spectrophotometry [108]. 

NbsSn filament conductors are generally fabricated by means of a solid-state diffusion process ( ). In this the niobium rods are placed into a bronze matrix and jointly extruded. Once the final dimension is attained, the wire is subjected to heat treatment upon which tin selectively diffuses from the copper-tin matrix into niobium. This process is often executed after solenoid winding since the heat-treated wire can only to a limited extent be mechanically deformed. 

A series of monofilament (Nb in Cu plus 13 wt.% Sn) bronze wires was prepared. Each wire had approximately the same Nb core diameter, 1.3 mm, but the wires had different overall sizes, i.e., different bronze to niobium ratios. 

Influence of bronze-to-niobium ratios on Tc of stressed and unstressed wires. 

Fig. 5. Effect of bronze-to-niobium ratio on strain at the maximum T.

The matrix-to-core diameter ratios correspond to the following bronze-to-niobium (filament) area ratios ... 

A more detailed investigation of the mechanical and electrical properties was made using a series of conductors in which the v/o of stabilizing copper was increased to 62 v/o. These conductors were all made from a monolithic 102,163-filament composite rod consisting of 19 tantalum-clad modules in a copper matrix (Fig. 3). Each module contained 5,377 niobium filaments in a Cu-14 w/o Sn bronze matrix. The initial composite contained 34v/oCu, 4v/oTa, 15.9 v/o Nb, and 46.1 v/o bronze. The types of conductors were ... 

Samples of the four configurations have been made, and lengths sufficient for small coils are now being produced. A cross section of the strand presently being drawn to produce a conductor of configuration 9 is shown in Fig. 3. The core for this and all of the other conductors described consists of 4453 Nb filaments in a bronze matrix with a 2.8 1 bronze-to-filament ratio. At the final core diameter of 0.254 mm, the filaments will be approximately 2/um in diameter, with critical current densities in the bronze and niobium of about 70kA/cm, as previously reported for a 2.54-cm bend at 10 T and 4.2 K [%... 

Fig. 3. Cross section of a copper-stabilized composite strand at 3.86-mm diameter during the wire-drawing sequence. The finned tantalum barrier surrounds a 4453-filament bronze and niobium core.

B. A. Zeitlin, A. Petrovich, J. R. Hughes, and M. S. Walker, Fabrication of a High Filament Density Bronze and Niobium Composite for Multifilament NbsSn Conductors, paper presented at the Manufacture of Superconducting Materials Conference, Port Chester, New York, November 1976. 

Hashimoto et al. [ ] have described a multifilamentary NbaSn superconductor produced by heat treatment of a composite consisting of niobium cores and a two-component matrix of pure copper and a high-tin bronze (Sn-20 wt.% Cu). The technical feasibility and economic advantages of this technique have been evaluated in the present study. This study included consideration of the likely cost per A m and the overall current densities achievable with this new conductor design. Particular attention was devoted to high current (10 kA at 12 T) conductors suitable for application in magnets intended for experimental controlled thermonuclear reaction devices, such as the Fusion Engineering Research Facility (FERF) [ ] at Lawrence Livermore Laboratory and the tokamak-type experimental reactors. 

There are, however, limitations on the degree to which the matrix/NbaSn volume ratio can be reduced in these high-tin bronze composites. For example, the niobium cores need to be spaced sufficiently for uniform reaction throughout the filament groups and niobium filaments must not be engulfed by Kirkendall voids or non-a-bronze phases [ ] during the homogenization heat treatment, which distributes the tin prior to reaction. A purpose of this study was to determine the quantity of matrix material necessary to fulfill these requirements. 

Spacing between niobium filaments must be sufficient to produce a uniformly reacted composite. Experience with a variety of bronze/niobium composites indicates that a distance of nearest approach of approximately one filament diameter ( 3 to 4/im) is sufficient. 

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