Patent Description:
Magnesium diboride is an inorganic compound with the formula MgB<NUM>. Magnesium diboride's superconducting properties were discovered in <NUM>. MgB<NUM> becomes superconducting at <NUM> (-<NUM>), which is the highest operating temperature amongst conventional superconductors. This allows the functioning as superconductor without the need for using liquid helium.

In order to keep its superconducting properties, the very low electrical resistance must also be present at joints, where two ends of superconducting wire are joined. Having regard to the very fragile properties of the superconducting material, this is not a straightforward task.

The prior art has disclosed various alternatives for realizing a superconducting joint.

<CIT> discloses a superconductive connection for two end pieces of superconductors using a sheath or bushing where MgB<NUM> as superconductive contacting material is inserted. The superconductors are multi-wire superconductors. The connection shows a region inside the sheath or bushing where the superconducting wires overlap, i.e. the cross-section shows the presence of superconducting wires coming from the two end pieces of superconductors.

<CIT> discloses a superconductive connection for two end pieces of superconductors of the single wire type. The superconductor of the single wire type is a round wire with a MgB<NUM> superconducting core surrounded by a stabilization layer and a metal sheath. In order to realize the connection, the end pieces of the superconducting wires are flattened to a very large degree to increase the bonding area. The width to thickness ratio ranges from <NUM> to <NUM>. The stabilization layer and metal sheath are removed at one side of the flattened ends so that the superconducting cores become open at one side. The open sides of the superconducting cores are then brought in contact with each other. The contacting ends are introduced in a bonding container where sintered powder of Mg and B or of MgB<NUM> is introduced and put under pressure.

The prior art refers to heat treatment as a step to restore superconductivity and/or ductility in connecting joints.

<CIT> discloses a method of restoring superconductivity of a superconducting wire (joint) having at least one core (e.g. multiple filaments) of reacted MgB<NUM>, said method comprises subjecting said wire (joint) to a single-phase heat treatment by heating in a range of <NUM>-<NUM> during <NUM>-<NUM> minutes, for example, at <NUM> for <NUM> minutes.

In <CIT> a contacting joint between conductor leads can be improved by means of an annealing process, preferably at relatively low temperatures of less than <NUM>° C.

<NPL>) describe similar MgB<NUM> wire (joint) single-phase recovery annealing processes, such as using <NUM>-<NUM> for <NUM> hours or <NUM> for <NUM>-<NUM> hours.

The object of the invention is to provide a method of restoring superconductivity of a wire having a core of reacted MgB<NUM>. The method comprises the following steps.

Subjecting the wire to a heat treatment of two phases:.

The mentioned heat treatment is appropriate to restore the superconductivity of a single wire with a MgB<NUM> core where the superconductivity was lost due to damage or other incident.

The mentioned heat treatment is also applicable to restore superconductivity in a plurality of single wires (cabled construction) or in a single wire with multiple MgB<NUM> cores (multifilament wire).

The mentioned heat treatment is valid to restore general defects entered into the MgB<NUM> phase due to:.

The method of restoring superconductivity can be applied to a wire having a core of reacted MgB<NUM> that has been bent, flattened, or twisted.

The mentioned heat treatment to restore superconductivity is independent from the shape of the superconducting wire. Said superconducting wire can have a circular or shaped cross section.

This method can also be applied to restore superconductivity in a cable comprising a plurality of wires with a MgB<NUM> core where the superconductivity was lost in one or more wires due to damage or other incident.

This method can also be applied to restore superconductivity in a joint of superconducting wires.

As an illustration to the application of the method of restoring superconductivity of a wire having a core of reacted MgB<NUM>, there is provided a joint of superconducting wires. The joint and method described below are not part of the invention.

The joint comprises at least two superconducting wires each with a sheath and with a MgB<NUM> superconducting core inside the sheath. At least one first superconducting wire has a first flattened end and at least one second superconducting wire has a second flattened end. The joint further comprises a tubular metal connector. The connector has a centre being filled with magnesium or boron or MgB<NUM> material. The first flattened end of first superconducting wire is inserted at one side of the connector until it is in contact with the magnesium or boron or MgB<NUM> material. The second flattened end of the second superconducting wire is inserted at the other side of the connector until it is also in contact with the magnesium or boron or MgB<NUM> material. The connector is pressed at both sides to fix the first superconducting wire and the second superconducting wire. The centre of the connector is pressed to compact the magnesium or boron or MgB<NUM> material.

This joint does not need the superconducting wires to have an overlap. In addition, there is no need to remove the sheath of the superconducting wires.

The first flattened end and the second flattened end have a width-to-thickness ratio in order to get rid of the cavities and to provide a stable pressed wire that is no longer deformed during pressing of the connector. Preferably these flattened ends have a width to thickness ratio ranging from <NUM> to <NUM>, preferably from <NUM> to <NUM>, e.g. from <NUM> to <NUM>, preferably from <NUM> to <NUM>.

Since the first superconducting wire and the second superconducting wire comprise a core of reacted superconducting MgB<NUM> they also comprise voids. The reason is that the volume of reacted MgB<NUM> is roughly <NUM>% less than the original Mg powders and B powders. The ends of the superconducting wires are pressed to such a degree that the number and volume of voids decrease. Later pressing of these already pressed ends of superconducting wires will no longer lead to large cracks which prevent electrical current.

The tubular metal connector preferably comprises low carbon steel.

The tubular metal connector may also comprise titanium.

The tubular metal connector may have a titanium barrier radially inside and a low carbon steel radially outside.

The tubular metal connector may preferably be made of low carbon steel only.

There is provided a method of joining at least two superconducting wires having a superconducting core of reacted MgB<NUM>. This method comprises the following steps:.

Preferably the flattening of the ends of the superconducting wires are done to a degree where the width to thickness ratios of these ends range from <NUM> to <NUM>, preferably from <NUM> to <NUM>, most preferably from <NUM> to <NUM>, e.g. from <NUM> to <NUM>.

Preferable values of the critical distance Lcr range from <NUM> to <NUM>, preferably from <NUM> to <NUM>, e.g. from <NUM> to <NUM>.

To create superconductivity within the tubular metal connector and to restore the superconductivity in the flattened ends, the tubular metal connector and the flattened ends are subjected to a heat treatment. It is a second object of the invention to provide the heat treatment to recover superconductivity in a joint of superconducting wires.

This heat treatment preferably comprises two phases:.

Referring to <FIG> the subsequent steps or joining a first superconducting wire <NUM> and a second superconducting wire <NUM> will be described. The numerical values are given by way of non-limiting example.

The first superconducting wire <NUM> has a sheath <NUM> of low carbon steel and a core <NUM> of reacted MgB<NUM>.

The second superconducting wire <NUM> has a sheath <NUM> with a barrier layer of titanium and an outer layer of low carbon steel and a core <NUM> of reacted MgB<NUM>.

A tubular metal connector <NUM> is provided and contains unreacted boron powder and magnesium powder. The boron powder is preferably a nano boron powder and the magnesium powder is preferably a spherical magnesium powder. By way of example, the external diameter of the tubular metal connector <NUM> is <NUM> to <NUM> and the metal tubular connector <NUM> is about <NUM> long.

The first end <NUM> and the second end <NUM> of the metal tubular connector <NUM> are first deburred.

Then, the first end <NUM> of the tubular metal connector <NUM> and the second end <NUM> of the tubular metal connector are immersed in a diluted HCl solution for a couple of seconds and thereafter dried under vacuum.

Both the first and <NUM> and the second end <NUM> are polished followed by alcohol washing and vacuum drying.

A hole of <NUM> is drilled in both the first end <NUM> and the second end <NUM> to match the dimensions of the first and the second superconducting wires <NUM> and <NUM>. The length of the hole is L<NUM> at the sided of the first end <NUM> and L<NUM> at the side of the second end <NUM>. Both L<NUM> and L<NUM> can be about <NUM>.

After drilling, the tubular metal connector <NUM> is flattened over its whole length Ltot.

The first superconducting wire <NUM> and the second superconducting wire are flattened over a length that is greater than the drilling lengths L<NUM> and L<NUM>.

The thus flattened ends of the first and second superconducting wire <NUM> and <NUM> are ground with an angle in order to increase the surface area. This grinding operation is again followed by alcohol washing and vacuum drying. Thereafter, the flattened ends of the first and second superconducting wires <NUM> and <NUM> dipped in a diluted solution of HCl, followed by washing in alcohol and drying in vacuum.

The flattened ends of the first and second superconducting wires <NUM> and <NUM> are inserted in the tubular metal connector <NUM> until they have contact with the boron and magnesium powder in the centre.

Pressure is exercised at both ends <NUM> and <NUM> of the tubular metal connector <NUM> in order to fix the superconducting wires <NUM> and <NUM> and to seal the ends <NUM> and <NUM>.

The centre part of the tubular metal connector <NUM> is pressed over a length Lcenter by means of a hydraulic press <NUM>. The length Lcenter before flattening is for example <NUM>.

A critical length Lcr of <NUM> remains at both sides between the flattened centre part of the tubular metal connector <NUM> and the drilled holes.

Finally, a heat treatment is applied to the assembly of the first and second superconducting wires <NUM>, <NUM> and the tubular metal connector <NUM> with the Mg and B powder <NUM>.

As mentioned, the heat treatment comprises two phases.

During the first phase the assembly is heated at <NUM> during <NUM> minutes. In this first phase, the following reaction takes place:.

During the second phase immediately following the first phase, the assembly is kept at a temperature of <NUM> during <NUM> minutes. During this second phase, the following reaction takes place:.

This double phase heat treatment not only create superconductivity in the centre of the tubular metal connector <NUM> but also restores the superconductivity in the flattened ends of the first and second superconducting wires <NUM> and <NUM>.

<FIG> gives a schematical representation of a realized joint. More particularly, <FIG> shows the pressed ends <NUM> and <NUM> of the tubular metal connector <NUM> and the flattened central part <NUM>. Typical dimensions in the finalized joint, i.e. after the flattening operations, are a total length Ltot of <NUM>, a length Lcenter of the flattened central part of <NUM>, a length of insertion of first superconducting wire of <NUM> and a length of insertion of second superconducting wire of <NUM>.

As already mentioned, and out of the context of joining two superconducting wires, the heat treatment can also be used to restore the superconductivity in MgB<NUM> wires where superconductivity was lost. Two examples below show how superconductivity was recovered in damaged samples.

Reference samples of superconducting wires having a superconducting core of reacted MgB<NUM> and having a diameter of <NUM> were provided. The core of MgB<NUM> had been sintered at <NUM> during <NUM> minutes. The critical current, Ic, was measured at different temperatures. A linear regression was used to normalize the critical current values measured in different samples at different temperatures. Results obtained at respectively <NUM>, <NUM> and <NUM> are reported in table <NUM>.

REF <NUM> in Table <NUM> refers to the reference sample.

A damage was introduced by subjecting the reference sample REF <NUM> to a compression under a press of <NUM> tonnes during <NUM> minutes. REF <NUM> refers to the damaged sample and shows a drop in the critical current values, indicating a loss of superconductivity.

REF <NUM> was then subjected to different heat treatments to recover superconductivity.

COMP <NUM> and COMP <NUM> in Table <NUM> refer to comparison samples that were deformed as for REF2, then subjected to a two-phase heat treatment with durations outside the range of the present invention.

As an example, the second phase was too short for the comparative sample COMP <NUM> to recover superconductivity, as shown by the remaining low Ic values.

As a second example, superconductivity was only partially recovered in the comparative sample COMP <NUM> due to a too short duration of the first phase of the recovery heat treatment, as shown by the increased Ic values, yet too low compared to the reference sample REF <NUM>.

Samples INV <NUM> and INV <NUM> were deformed as for REF <NUM>, then subjected to the two-phase heat treatment of the invention, i.e. with a first phase at <NUM> during <NUM> minutes or <NUM> minutes, followed by a second phase at <NUM> for <NUM> minutes.

Both samples INV <NUM> and INV <NUM> have recovered superconductivity, as shown in table <NUM> with Ic values close to the original values measured in sample REF <NUM>.

Another sample not reported in Table <NUM>, sample INV <NUM>, was obtained, starting from REF <NUM> and bending it over a tube of diameter of <NUM> diameter and straightening it.

Another sample, sample INV <NUM> was obtained starting from REF <NUM> and bending it over a tube of <NUM> diameter and straightening it, then pressing it uniaxially to obtain a flat wire with a width over thickness (w/t) ration of <NUM>.

Samples INV <NUM> and INV <NUM> were subjected to the recovery heat treatment of the invention, i.e. samples were subjected to a heat treatment of two phases, a first phase of heating at <NUM> during <NUM> minutes and a second phase of heating at <NUM> during <NUM> minutes.

During the first phase the deformed wires were heated at <NUM> during <NUM> minutes. In this first phase, the following reaction takes place:.

During the second phase immediately following the first phase, the deformed wires were kept at a temperature of <NUM> during <NUM> minutes. During this second phase, the following reaction takes place:.

The critical current, Ic was measured samples INV <NUM> and INV <NUM> after heat treatment, and the critical current was measured to be Ic = <NUM> A at <NUM> in both samples. This value was found to be close to the value measured in the reference sample REF <NUM> before deformation.

Additionally, a sample obtained in the same conditions as sample INV <NUM>, i.e. by bending it over a tube of diameter of <NUM> diameter and straightening it, was heated at <NUM> for <NUM> hours, followed by heating at <NUM> for <NUM> hours. The critical current measured after this heat treatment was Ic=<NUM> A, meaning that superconductivity was not recovered.

Claim 1:
A method of restoring superconductivity of a wire having a core of reacted MgB<NUM>, said method comprising the following steps:
subjecting said wire to a heat treatment of two phases, a first phase of heating in a range of <NUM> to <NUM> during <NUM> minutes to <NUM> minutes and a second phase of heating in a range of <NUM> to <NUM> during <NUM> minutes to <NUM> minutes.