Method for making a superconductive metal-matrix composite

Abstract: A superconductor metal matrix composite formable into an electrical current carrying material. A superconductive particulate is intermixed with a normal metal matrix, pressed into form and heated to form the composite. The metal matrix surrounds the superconductive particulate to prevent loss of oxygen from the superconductive particulate so the particulate retains its superconductive properties. The metal matrix also becomes superconductive due to proximity effect.

Description: The invention relates to a method of forming a superconductive metal matrix composite. Recent discoveries of ceramic oxide superconductive materials has significantly raised the critical temperature of superconductors and decreased the cost of the coolant material needed to cool the superconductors below the critical temperature. A disadvantage of the various ceramic oxide materials is that they are brittle and breakable when fire hardened which limits the forms in which they can be shaped after firing. Also, if superconductivity is lost because of a rise in temperature or for other reasons a ceramic superconductor could lose conductivity and cause damage to other elements in the associated electrical circuit. WO-A-89 01 706 and "A novel metal matrix high T c superconducting composite" published in Advanced Ceramic Materials, Special Issue, Vol. 2 (3B), July 1987, pages 319 to 321 disclose a superconductive composite material comprising a superconductive ceramic particulate and a normal metal matrix and the use of silver as a matrix metal. In one embodiment the concentration of superconductive ceramic particles is less than the percolation concentration, so that the metal matrix encapsulates the superconductive ceramic particles. In the process disclosed in these documents heating, specifically sintering and/or oxygen annealing is a requirement to producing the composite. The problem underlying the invention is to provide a method of forming a superconductive metal matrix composite which does not require the application of heat either for sintering or for oxygen annealing to produce a metal matrix composite with superconducting properties. According to the invention this problem is solved by means of a method according to claim 1. Further advantageous developments of the inventive method are mentioned in claims 2 to 4. The invention provides a method of forming a superconductive metal matrix composite which includes a metal particulate having a superconductive particulate dispersed throughout the metal particulate. Heat and pressure are applied to the mixture to densify the matrix of the composite. The dispersion of superconductive particulate in the metal matrix causes the metal to also become superconductive due to a proximity effect. The metal matrix encapsulates the superconductive particulate which inhibits degradation of the superconductive particulate which can result because of chemical reactions between the superconductive particulate and the external environment. The metal also provides a degree of conductivity for the composite if superconductivity should be lost due to a rise in temperature or for other reasons. In addition, oxygen annealing after compaction is not required in the method of this invention, because the ceramic is allowed to retain its superconducting properties. That means that it is not subjected to heating that would result in oxygen depletion. It follows an explanation of the invention by means of the attached drawing. Fig. 1 is a perspective of an electrical conductor which includes a superconductive composite of the present invention. Fig. 2 is a cross-sectional view taken along line 2 - 2 of Fig. 1. Fig. 3 is a graph showing the superconducting regime of the superconductive composite of the present invention as related to temperature. current density and applied magnetic field. A conductor 10 which can be formed from superconductive metal matrix composites of the present invention is disclosed in Fig. 1. The conductor 10 is formed by using a superconductive ceramic material, such as yttrium-barium-copper oxide, finely ground to form a superconductive particulate. A metal particulate, such as silver, or silver oxide is mixed which the superconductive particulate so a plurality of superconductive particles 11 (Fig. 2) are dispersed throughout a metal particulate 12. The mixture is placed under pressure and heated to incorporate the superconductive particulate in a dense continuous metal matrix. The resulting composite can be formed into superconductive wires and coils for high field magnets used in numerous power generation and transmission devices. When conductor 10 is cooled below a critical temperature Tc (Fig. 3) the superconductive particulate has a sharp drop in the resistance to electrical current flow. Due to the well-known proximity effect the resistance of the metal particulate also drops sharply so the entire composite is superconductive below temperature Tc. In addition to forming ductile wires and becoming a superconductive material, due to proximity effect, the metal 12 of Fig. 2 also provides an electrical current path through conductor 10 if the ceramic material should loose its superconductive properties. This current path could prevent damage of electrical elements in a circuit if the ceramic material is no longer superconductive. The metal 12 (Fig. 2) also provides better thermal dissipation than the ceramic portion and may prevent the conductor temperature from rising about the critical value. The metal matrix also provides faster switching from a superconductive condition to a non-superconductive condition of conductor 10 (Fig. 1). As seen in Fig. 3, there are three critical parameters of the superconductive material which determine if the material is a superconductor. First, the temperature of the material must be lower than a critical temperature Tc. Second, the magnetic field in which the material resides must be less than a critical value Hc2. Third, the current density in the conductor must be less than a critical value Jc. A problem with processing ceramic superconductive material is that heat can cause oxygen from the superconductive particulate to be lost, which, in turn causes the ceramic material to loose its superconductive properties. In the present invention the metal particles 12 (Fig. 2) surrounding the ceramic particles 11 are formed into a dense continuous matrix by the application of heat and pressure such that the temperature of the ceramic particles does not exeed a level which results in a significant loss of oxygen. The following is one example of a composite which can be used as a superconductor: The composite can be made by first mixing, superconductive yttrium-barium-cuprate powder of 1.77 x 10⁻¹ mm and 3.45 x 10⁻¹ mm (45 to +80 mesh) with pure silver powder of 2.4 to 4.0 µm particle size. The mixture is uniaxially cold dry pressed at about 282.7 MPa (41,000 psi) in a 12.70 mm (half inch) diameter steel die. The die is then preloaded to about 35.2 MPa (5,100 psi) prior to initiating the hot pressing cycle. The material is hot pressed by heating the die to 385 degrees C in about 10 minutes and holding the temperature at 385 degrees C for an additional 40 minutes. During the heat-up portion of the cycle the applied pressure is allowed to rise from 35.2 to 47.6 MPa (5,100 to 6,900 psi). During the 40 minute soak portion of the cycle the pressure is allowed to decrease from 47.6 MPa psi to 34.5 MPa (6,900 psi to 5,000 psi). After the 40 minute soak at 385 degrees C, the die and material are rapidly cooled to room temperature. A variety of other materials can be used to form a composite. Thus, mixing a metal particulate with a ceramic superconductive particulate provides a stronger, more ductile material which can be made into the form of a wire conductor. The metal particulate also becomes superconductive due to the proximity effect, and provides an electrical current path through the wire conductor if the ceramic material should become non-superconductive. Although the best mode contemplated for carrying out the present invention has been herein shown and described, it will be apparent that modifications and variations may be made without departing from the subject-matter of the invention as defined in the appended claims.

Claims: A method of forming a superconductive metal matrix composite including the steps of: preparing a superconductive material into a particulate; adding a powered metal material to said particulate; mixing said superconductive particulate with said powered metal material; uniaxially cold dry pressing said mixture to a preload of about 35.2 MPa (5,100 psi); heating the material to 385°C over an interval of about 10 minutes; raising the applied pressure from 35.2 MPa (5,100 psi) to 47.611 MPa (6,900 psi) over said 10 minutes interval; maintaining said mixture at a temperature of 385°C for 40 minutes; and rapidly cooling said mixture to room temperature. The method of claim 1, in which said superconductive material is prepared into a particulate of between 1.77 x 10⁻¹ and 3.45 x 10⁻¹ mm (45 and 80 mesh). The method of claim 1 in which said powered metal material is silver powder of between 2.4 and 4.0 µm particle size. The method of claim 2 in which said superconductive particulate is yttium-barium-cuprate powder.

Inventors: WILSON CHARLES N, WILSON, CHARLES N.
Applicants: FMC CORP, FMC CORPORATION
Application year: 1989
Main classification: H01L39
IPC codes: C01G1, C04B35, H01B12, C23C20, H01B13, H01L39