Superconductor technology: apparatus – material – process – High temperature – per se – Bismuth containing
Reexamination Certificate
2002-06-06
2004-01-13
Gupta, Yogendra N. (Department: 1751)
Superconductor technology: apparatus, material, process
High temperature , per se
Bismuth containing
C252S519130, C252S519150, C252S518100, C505S121000, C505S125000, C505S501000, C505S510000, C505S782000, C029S599000
Reexamination Certificate
active
06677278
ABSTRACT:
This invention relates to mixed oxide powder mixtures with enhanced reactivity as presursor materials for superconductors and to a process for its manufacture.
In special the present invention relates to precursor powders produced by spray pyrolysis that are then further heat treated such that they show considerable advantage in forming the Pb—Bi—Sr—Ca—Cu—O 2-2-2-3 superconducting phase over powders produced using other routes.
Superconductors are materials that loose all resistance to the flow of electricity below a critical transition temperature. Today superconducting metals and alloys are being used in a variety of commercial applications in Electronics and Medicine, for example the use of Low Temperature Superconducting Magnets in MRI (Magnetic Resonance Imaging) systems and as Electromagnets in High Energy Physics. The discovery of Superconductivity in the Ceramic Oxide System La—Sr—Cu—O system in 1986 (ref Berdnoz und Müller: Z. Physik B 64, 189, 1986) sparked renewed activity in the search for Super-conductivity at temperatures above those only then accessible with liquid helium. Within one year the ‘High Temperature’ Y—Ba—Cu—Oxide system (HTS) had been discovered in which the critical transition temperature had been raised above 77K (ref Chu: EP341266, 12.01.87), allowing liquid nitrogen to be used as a coolant. Many new High Temperature Superconductors have been discovered and evaluated for potential commercial application over the last ten years, but still the two systems regarded as having the greatest chance of commercial success in the near future are those systems discovered the first, the ‘Re’(Rare Earth) —Ba—Cu Oxides and those based on the Bi—Sr—Ca—Cu Oxide Sytem.
The current market share for the High Temperature Superconductors within the Superconductor Industry is small at ca 3% (15M) of the total (ref BCC Market Research Study: GB-106R, The Superconductor Industry). This is because these Materials and Systems are still in the development and prototype stage, real market growth expected only after the systems have proved themselves to have advantage both in cost and quality (performance and environmental impact). The largest market segment for HTS is currently in elec- tronics and communications where they are used as microwave filters and resonators. Other segments where both the Re—Ba—Cu—O and Bi—Sr—Ca—Cu—O systems are under development include Current Leads to provide low loss energy supply to LTS systems, Magnetic Energy Storage devices able to supply energy when routine supply has been interrupted, Fault Current Limiters to reduce energy supply to a load during a fault, Magnet Systems, Power Cables,Motors and Generators. Wires and Tapes, for use in a number of the afore-mentioned applications, continue to be developed based on the Pb—Bi—Sr—Ca—Cu—O 2-2-2-3 system. In competition, the IBAD and RaBITs Process using the Re—Ba—Cu Oxides are also now attracting more attention (X. D. Wu et al, Applied Physics Letters 65, 1961, 1994).
The current prefered method to manufacture Pb—Bi—Sr—Ca—Cu—O 2-2-2-3 tapes for energy applications is to use the oxide Powder in Tube method (PIT Method). This process involves the packing of a precursor form into a silver alloy tube that is then subject to repeated mechanical deformation and high temperature thermal treatments. The final product consists of thin filaments of sintered ceramic within the silver-alloy host. During the heat treatment process the precursor form within the silver matrix undergoes conversion to the desired superconducting phase, the Pb—B—Sr—Ca—Cu—O 2-2-2-3 phase (ref EP0330305, Jan. 19, 1989; U.S. Pat. No. 4,880,771, Feb. 26, 1988). The quality of the final tapes is dependent upon many process parameters, including the intrinsic nature of the precursor powder, the density of the powder preform, the nature of the silver alloy sheath, the reduction on passing through the drawing die, pressing versus rolling, number of filaments, the calcination atmosphere, the calcination temperature, the calcination time and the heating and cooling cycles. As the processes within the tape during the heat treatments involve chemical conversion to the prefered 2-2-2-3 phase, the intrinsic properties of the precursor powder determine, to a degree, the choice of processing parameters. The conversion of the powder to the 2-2-2-3 phase is thought to proceed via one of two routes, either by growth of the 2-2-2-3 phase from a liquid phase (Flukiger et al, SST, 10, A68, 1997) or by intercalation of calcium and copper oxide layers into the 2-2-1-2 crystal grains already present in the precursor powder (J Jiang, S Abel, SST 10,1997, 678 and references therein).The role of the deformation step in the processing of these tapes is to both texture the grains and to increase the density within the ceramic core. However this step can also introduce cracks and defects in the ceramic which have to be healed during the thermal processing. The liquid phase, produced by melting of particular phases, for example induced through reaction with calcium plumbate, is thought to accelerate phase conversion as well as to both heal these microcracks and remove unwanted grain bounday phases. Therefore the relative amounts of these phases need to be controlled, both in the original precursor itself and through the processing stages to supply enough liquid phase to the system and thereby produce clean grained, well aligned 2-2-2-3 grains at the end of the process. One of the first stages during this conversion is the incorporation of lead from the plumbate phases into the Bi-2-2-1-2 grains. This can be followed by monitoring the change in the phase composition of the material, by for example X-Ray Diffraction. As the reaction proceeds, the calcium plumbate levels decrease as lead enters into the Bi-2-2-1-2 grains. This also results in a change of the lattice type that can be followed with X-Ray Diffraction, changing from a tetragonal unit cell to an orthorhombic one. Too much calcium plumbate is thought to be detrimental to the formation of well textured 2-2-2-3 grains. Excess calcium plumbate can give rise to the growth of undesirable alkaline earth phases during the processing which cannot be removed at a later stage, can disrupt the layering of the 2-2-2-3 grains during mechanical deformation and can decompose, releasing gas, that is trapped momentarily within the silver alloy sheath and results in irrecoverable deformation of the tape. It is also thought that too little calcium plumbate is detrimental in PIT processing because not enough liquid phase can then be supplied during the heat processing stages to heal all the microcracks, clean all the grain boundaries and facilitate the final conversion of residual phases to 2-2-2-3. The final reaction step involves conversion of the lead containing 2-2-1-2 into the 2-2-2-3 phase (R. Flukiger et al, Superconducting Science and Technology 10(1997) A68-A92.
Those powders commonly considered for use as precursors can be manufactured by a number of routes. These include routes starting from solid components, eg the ‘mix and grind’ process and those routes starting from solutions, eg: citrate gel (U.S. Pat. No. 5,066,636), co-precipitation (U.S. Pat. No. 5,147,848) and spray pyrolysis (GB 8723345).
Although these processes have been applied to the synthesis of superconducting powders there is no systematic procedure for a controlled conversion into the 2-2-2-3 phase of powders produced by any of these routes.
Therfore, there was a need for a method for the synthesis of superconducting powders comprising a controlled conversion into the 2-2-2-3 phase of powders produced.
A solution has been found by a process to manufacture a mixed Pb
u
Bi
v
Sr
w
Ca
x
Cu
y
oxide powder in which a mixed-metal solution is sprayed as a fine mist into a heated reactor between 600° C. and 1200° C. and the collected powder subsequently calcined between 700 ° C. and 850° C. under an atmosphere of between 0.1% oxygen and 21% oxygen for a total heating time at maximum temperature between 4 and 180 hours.
A part of the present inventio
Hsueh Ya-Wei
Liu Ru-Shi
Raeth Sebastian
Schmahl Wolfgang Wilhelm
Woodall Lee
Gupta Yogendra N.
Merck Patent GmbH
Millen White Zelano & Branigan P.C.
Vijayakumar Kallambella
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