JPH046713A - Aluminium stabilization superconductive wire - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to an aluminum stabilized superconducting wire, and particularly to a conductor configuration for reducing eddy current loss in a superconducting wire.

[Conventional technology]

Conventionally, aluminum stabilized superconducting wires with improved stability of superconducting wires have been disclosed, for example, in Japanese Patent Application Laid-open No. 51-13892,
As described in Japanese Unexamined Patent Publication No. 60-39706, monolithic superconductors or
There are some methods in which superconducting wires are embedded.3However, consideration has not been given to reducing eddy current loss when using large current and large capacity conductors for fluctuating magnetic fields.

[Problem to be solved by the invention]

Conventionally, high-purity aluminum has been used as a stabilizing material for superconducting wires due to its low electrical resistance at low temperatures, high thermal conductivity, ease of processing, and small magnetoresistive effect. .

However, since high-purity aluminum material has weak mechanical strength against electromagnetic force, superconductors have been coated with a composite aluminum material in which high-purity aluminum material is coated with an aluminum alloy material. However, when it comes to large magnets with large current conductors, eddy currents are generated in the high-purity aluminum stabilizing material due to the magnetic field generated by the conductor itself and the influence of external fluctuating magnetic fields from adjacent conductors, and this eddy current generates heat. There were eight interconnections that caused a normal conduction transition and decreased the stability of the superconducting wire.

An object of the present invention is to provide a superconducting wire that suppresses eddy currents in a stabilizing material, reduces eddy current loss, and improves stability.

[Means to solve the problem]

In order to achieve the above object, the present invention provides a superconductor such as a monolithic superconductor or a superconducting strand bundle with partial slits in the longitudinal direction or circumferential direction, or an insulating layer, a high-resistance metal layer, or , a high-purity aluminum material with an insulating film or a high-resistance metal film is arranged on the dividing boundary surface in a spiral shape, and a superconductor is arranged in a spiral shape, and the spiral direction of the superconductor and the spiral direction of the high-purity aluminum material are opposite to each other. It is wrapped and coated.

Furthermore, for those for high magnetic fields with strong surrounding external fluctuating magnetic fields, slits or high resistance metal coatings to suppress eddy currents are placed around high purity aluminum materials with high resistance metal coatings, for example.
It was coated with stainless steel, cypronickel, etc., which have a higher resistance than copper, and then coated with an aluminum alloy material.

[Effect]

A monolithic superconductor is formed by providing a slit, an insulating layer, a high-resistance metal layer, or an insulating film or a high-resistance metal film on the dividing boundary surface in the high-purity aluminum material of the stabilizing material covering the superconductor. , or spirally coated superconducting wire bundles,
Alternatively, by making the helical angle of the monolithic superconductor or superconducting wire bundle different from the helical angle of the high-purity aluminum material, or by coating one side with a spiral in the opposite direction, eddy currents in the high-purity aluminum material can be suppressed and the eddy currents can be reduced. Since current loss can be reduced, stability can be improved. Also, large current.

For high magnetic field magnets, by covering the periphery of the high-purity aluminum material with a high-resistance metal material, and then covering the surrounding area with an aluminum alloy material, penetration of external magnetic fields can be suppressed, resulting in stability. An aluminum-stabilized superconducting wire with high resistance can be obtained.

〔Example〕

An embodiment of the present invention will be described below using a design example. Table 1 shows the design specifications when a superconducting coil is used as a storage coil with a rated current of 707 kA and a stored energy of 5 G-〇.

Table 1 The superconducting material for the coil conductor is an Nb3Sn compound that is highly economical and has a proven track record in mass production. Oxygen-free copper is used for the outer sheath of the Nb3Sn compound, and the copper-coated Nb3Sn wires are bound with solder. High-purity aluminum is used as the stabilizing material for superconductors bound with solder because of its low electrical resistance of tJs at low temperatures, high thermal conductivity, and ease of deformation and workability. As a reinforcing material for this high-purity aluminum material, we selected an aluminum alloy material that satisfies the above conditions and has high yield strength, and designed it as a conductor for high-power current. According to this, the outer dimensions of the copper-coated NhsSn wire are approximately 10X
It becomes 10X50. Further, the dimensions of the conductor including the high purity aluminum material as the stabilizing material and the aluminum alloy material as the reinforcing material surrounding the copper coating Nb3Sn are 120. It becomes X430mn2. The total conductor weight at this time is 129 kg/m. The operation cycle of this 5G W h energy storage coil with this large conductor is such that the magnetic field at the location where the maximum magnetic field is generated is 2.29T, (
224kA) to 8T (707kA) in about 5 hours, and then returned to 2.29T in 5 hours.

Calculating the AC loss at this time, - 51k per cycle
It also becomes W. This AC loss includes hysteresis loss, eddy current loss, coupling loss, etc., but looking at the breakdown, hysteresis loss is 0.64kW, and eddy current loss is 47.6kW.
becomes.

When trying to manufacture a high-power energy storage coil using a superconducting coil, most of the loss is eddy current loss that occurs in the high-purity aluminum material that is the stabilizing material that surrounds the superconductor, so it is necessary to reduce this eddy current loss. Unless the coating composition of the high-purity aluminum material used as the stabilizer is improved, there is concern that the superconducting properties of the conductor will deteriorate significantly. Therefore, it is possible to partially slit a monolithic superconductor or a superconductor such as a bundle of superconducting wires in the length direction or circumferential direction, or to divide an insulating layer, a high-resistance metal layer, or a high-purity aluminum layer. ,
A high-purity aluminum material with an insulating film or a high-resistance metal film is arranged in a spiral on the dividing boundary surface, and a superconductor is arranged in a spiral, so that the helical direction of the helical superconductor and the helical direction of the high-purity aluminum material are A configuration in which the direction is reversed is conceivable. In addition, for high magnetic fields with strong external fluctuating magnetic fields, a high resistance metal material with a slit or a high resistance metal coating to suppress eddy current is used around the high purity aluminum material, such as a high resistance metal material of copper or higher. It is conceivable to cover it with stainless steel, cypronickel material, etc., which has a hard coating, and then cover the surrounding area with an aluminum alloy material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings.

In FIG. 1, an aluminum stabilized superconducting wire 1 has a core material 2 made of an aluminum alloy material or a composite material made of an aluminum alloy material coated with a high-purity aluminum material, and a plurality of superconducting wires around the core material 2. A superconducting strand bundle 4 made of strands 3 or a monolithic superconductor 5 is arranged, and the superconducting material of the superconducting strand bundle 4 or monolithic superconductor 5 is partially cut into longitudinal slits 6 and circumferential slits 7. and radial slit 8, and
The longitudinal slit 6, the circumferential slit 7, and the radial slit 8 are covered with insulating layers 9a, 9b, 9c, or a high-purity aluminum material 10 provided with a high-resistance metal layer, and the outer periphery is covered with an aluminum alloy material 11. The aluminum stabilized superconducting wire 1 is formed by coating.

In this way, a slit is partially provided in the superconducting wire bundle 4 or the superconductor such as the monolithic superconductor 5, and the insulating layer 9
a, 9b, 9c, or the aluminum stabilized superconducting wire 1 coated with a high purity aluminum material 10 provided with a high resistance metal layer etc.
The high-purity aluminum material 10 covering the superconductor has a longitudinal slit 6, a circumferential slit 7, or an insulating layer 9a.
, 9b.

The arrangement of 9c virtually serves as a stabilizing material for the split structure, and eddy currents in the high-purity aluminum material 10 are suppressed. Therefore, the deterioration in stability due to eddy current loss of the high-purity aluminum material 10, which has been a concern until now, is significantly improved, and an aluminum stabilized superconducting wire with improved stability can be provided.

In the above embodiments, the superconducting strand bundle 4 or.

High purity aluminum material lO covering the monolithic superconductor 5,
We have attempted to solve this problem by creating a virtual divided coating structure by creating countless longitudinal and circumferential slits and disposing insulating layers and high-resistance metal layers. However, it is feared that such a stabilizer structure would be complicated, difficult to manufacture, and expensive. A spiral coating structure made of high-purity aluminum material can be considered as a method that can solve these problems and improve stability while simplifying the structure.

FIG. 2 shows another embodiment of the invention. In the embodiment shown in FIG. 2, a monolithic superconductor or a superconducting wire bundle 4a is coated with a spiral high-purity aluminum material 12.
An insulating coating 1 is provided at the boundary 13 between the spiral high-purity aluminum material 12 and the spiral high-purity aluminum material 12.
4, the helical high-purity aluminum material 12 and the insulating coating 14 are integrally coated with an aluminum alloy material 11a, thereby forming an aluminum-stabilized superconducting wire of the helical stabilizing material.

As described above, in this embodiment, since the superconducting wire bundle 4a is covered with a strip of spiral high-purity aluminum material 12, the stabilizing material seen from the superconducting wire bundle 4a is divided and arranged along the length direction. A stabilizing material, that is, a spiral high-purity aluminum material 12, is formed by the current flowing through the superconducting wire bundle 4a.
eddy currents are significantly suppressed, and a superconducting wire suitable for reducing eddy current loss can be constructed. In addition, in the manufacturing process, it is possible to improve the complicated work by creating slits in the length direction or circumferential direction as in the above-mentioned example, or by creating countless layers of foreign materials, improving the ease and stability of the manufacturing process. An aluminum stabilized superconducting wire suitable for achieving this can be obtained.

The examples described above include an example in which a slit is provided in a high-purity aluminum material and an insulating layer or a high-resistance metal layer is arranged, and a high-purity aluminum material is virtually divided and arranged, and a -layer spiral high-purity Although we have described an example in which the superconductor is continuously arranged through an insulating film, it is also possible to create an aluminum-stabilized superconducting wire with a divided spiral coating structure in which the superconductor is covered with multiple divided high-purity aluminum materials. .

FIG. 3 shows a third embodiment of the present invention. In the embodiment shown in FIG. 3, aluminum alloy material is used as the core material 2a.

A plurality of superconducting wires 3a are arranged around the core material 2a,
A superconducting wire bundle 4b is formed by molding with solder 15, and the molded superconducting wire bundle 4b is spirally coated with a plurality of divided high-purity aluminum materials 16 and an insulating material 17, and the surrounding area is covered with an aluminum alloy material 11.
b to form an aluminum stabilized superconducting wire 1.

This embodiment differs from the arrangement structure of the high-purity aluminum material as the stabilizing material described in FIGS. Since the superconducting wire bundle 4b is coated in a shape, the superconducting wire bundle 4b
Since the high-purity aluminum material 16 divided in the length direction is divided into small pieces and coated, the eddy current of the high-purity aluminum material that is the stabilizing material is greatly suppressed, and the eddy current loss is
Since it can be reduced to about 70% compared to the design example shown in the table, it is possible to obtain an aluminum stabilized superconducting wire with significantly improved stability.

In the above embodiment, the high-purity aluminum material covering the superconducting wire bundle 4b has been divided into divided high-purity aluminum materials 16 in the circumferential direction via the insulating material 17, and the structure has been described in which the insulating material A high-resistance metal material was provided in place of 17, and the high-resistance metal material was also provided partially in the length direction. The same effects as in the above-mentioned embodiments can also be obtained by using a spirally coated aluminum stabilized superconducting wire made of split high-purity aluminum material with a high-resistance metal material interposed therebetween.

FIG. 4 shows a fourth embodiment of the present invention. The embodiment shown in Fig. 4 is an example in which the high-purity aluminum material covering the straight superconducting wires is divided into high-resistance metal materials and the high-purity aluminum material is divided into high-resistance metal materials, and the superconducting wire bundle 4C Aluminum alloy material 18a and high purity aluminum material 19 are placed at equal intervals in the circumferential direction.
Alternately, the high-resistance metal material divided high-purity aluminum material 20 is spirally coated with the high-resistance metal material divided high-purity aluminum material 20 in which the aluminum alloy material 18b is provided at constant intervals in the length direction of the high-purity aluminum material 19, and the high-resistance metal material divided high-purity aluminum material 20 is coated with an aluminum alloy material 18c.

In this way, the straight superconducting wire bundle 4c is arranged with aluminum alloy materials 18a, 1 at the dividing boundary in the longitudinal direction and the circumferential direction.
The aluminum stabilized superconducting wire spirally coated with the high-resistance metal material 20 divided by the high-resistance metal material 8b has the high-purity aluminum material 19 as the stabilizing material divided not only in the circumferential direction but also in the length direction. The eddy current generated in the high-purity aluminum material 19 of the high-resistance metallic material divided high-purity aluminum material 20 is suppressed to a greater extent than in the example, and the eddy current loss can be reduced, so it is possible to obtain an effect equal to or greater than that of the example. can.

In the above embodiment, a superconductor such as a monolithic superconductor or a bundle of superconducting wires is arranged straight, and a high-purity aluminum material as a stabilizing material covering the superconductor in the straight arrangement is divided into divided members or spirally arranged. The aim was to improve the stability of superconducting wires by coating them in a similar manner to suppress eddy currents generated in the high-purity aluminum material used as a stabilizing material. However, to suppress the eddy current of the stabilizing material, there are methods such as forming the superconductor in a spiral shape, or forming the superconductor and the stabilizing material in a spiral shape, in addition to the coating structure of high-purity aluminum material.

FIG. 5 shows a fifth embodiment of the present invention. In the embodiment shown in FIG. 5, the superconductor is spirally shaped, and the aluminum alloy material 21
A high purity aluminum material 22 is coated on the core material 23 to form a core material 23.
A plurality of monolithic superconductors 3 or a plurality of divided high-purity aluminum materials 27 having a high-resistance metal coating 26 surround a superconducting wire bundle 25 in which superconducting wires 24 are spirally formed.
The eddy current of the stabilizing material due to the fluctuating magnetic field of the superconducting strand bundle 25 is suppressed, and the eddy current loss is reduced to improve the stability of the superconducting wire. We are working to improve this.

Moreover, in the above-mentioned embodiment, the superconducting wire bundle 25 is made into a spiral,
We have attempted to suppress eddy currents by using a high-purity aluminum material as a stabilizing material in a split structure, but by forming a bundle of superconducting wires and a high-purity aluminum material as a stabilizing material into a spiral shape, the aluminum material used as a reinforcing material It may also be cylindrical molded from alloy material.

FIG. 6 shows a sixth embodiment of the present invention. In the embodiment shown in FIG. 6, a plurality of monolithic superconductors or a superconducting strand bundle 25a composed of superconducting strands 24a are formed into a spiral shape around a straight-arranged core material 23a and molded into a cylindrical shape with solder. A superconductor 29 is constructed, and this helical superconductor 29 is helically coated with a plurality of divided high-purity aluminum members 27a, and its periphery is coated with a reinforcing material 30 made of an aluminum alloy material.

In this way, the superconducting wire bundle 25a is spirally surrounded by dividing high-purity aluminum material 27a at a steep angle.

Alternatively, by spirally coating the superconducting wire bundle 25a with a spiral angle that is gentler than the helical angle of the superconducting wire bundle 25a, the divided high-purity aluminum material 27a of the stabilizing material has a structure in which it is divided into finer pieces in the length direction, which makes it difficult to stabilize the eddy current generation. Since the aluminum stabilized superconducting wire is arranged in a structure in which eddy currents are significantly suppressed, it is possible to obtain an aluminum stabilized superconducting wire with extremely low eddy current loss.

Further, although the embodiment has described a configuration in which the helical angle is in the same direction, the same effect as that of the embodiment can be obtained even if the helical direction of the superconducting strand bundle 25a and the helical direction of the divided high-purity aluminum material 27a are opposite. can get.

FIG. 7 shows a seventh embodiment of the present invention. In the embodiment shown in FIG. 7, a plurality of monolithic superconductors or superconducting wires 24b are arranged around a core material 23b made of an insulating material or a high-resistance metal coating.
The left helical superconductor 31 is formed by molding the superconducting strand bundle 25b composed of The spiral coating is applied, and the surrounding area is coated and molded with an aluminum alloy material 33.

In this way, by reversing the helical directions of the superconductor and the stabilizing material in which the left-handed helical superconductor 31 is coated with the right-handed helical high-purity aluminum material 32, partial superconductivity in the same direction as in the example is achieved. The inclination of the body and the inclination of the stabilizer, that is, the split high-purity aluminum material 27a, do not match, and the smaller the helical angle is, the more the high-purity aluminum material 32
This results in an aluminum stabilized superconducting wire that is divided into parts in the length direction, which is very effective as a structure for suppressing eddy currents in the stabilizing material, reduces eddy current loss, and improves stability significantly. be able to.

In the example, we only considered the influence of the fluctuating magnetic field of the internal superconductor coated with high-purity aluminum material as a stabilizing material.
No consideration was given to countermeasures against magnetic fields entering from the outside in conductors with large current capacity or in high magnetic fields with high surrounding magnetic fields. Therefore, as a method of constructing a superconducting wire that is strong against both the magnetic field generated inside the conductor and the intruding magnetic field from the outside, we developed a method for constructing a superconducting wire that is resistant to both the magnetic field generated inside the conductor and the intruding magnetic field from the outside. It is conceivable to provide a high resistance metal layer, for example a metal layer of stainless steel or cupronickel.

FIG. 8 shows a fourth embodiment of the present invention. FIG. 8 shows a monolithic superconductor or a plurality of superconducting strand bundles 35 made by bundling a plurality of superconducting strands 34, which are formed into a cylindrical shape using a low melting point metal material 1, for example, solder 36, and the periphery is and partially circumferentially slits 37a, 37b or an insulating layer,
Alternatively, the composite high purity aluminum material 38 may be coated with a high purity aluminum material 38 having a high resistance metal material, and the composite high purity aluminum material 38 may be covered with a high purity aluminum material 39.
The outer periphery of the molded material was covered with a high-resistance metal material 40, such as stainless steel material or cupronickel material, and the periphery thereof was covered with an aluminum alloy material 41. This is a superconducting wire 42 coated with multiple layers of aluminum.

In this way, a plurality of monolithic superconductors or superconducting wire bundles 35 made up of superconducting wires 34 are partially covered with an insulating material or a high-resistance metal material in the slits 37a and 37b or the slits 37a and 37b. Furthermore, the slits 37a and 37b are covered with a composite high-purity aluminum material 38 with an aluminum alloy material interposed therebetween, and the periphery thereof is covered with a high-resistance metal material 40 and an aluminum alloy material 41. Therefore, the eddy current in the composite high-purity aluminum material 38 due to the fluctuating magnetic field of the superconducting wire bundle 35 inside the superconductor is suppressed by the slits 37a, 37b and the aluminum alloy material included in the inclusion, and the heat generation temperature due to eddy current loss is reduced. Ru. on the other hand,
Composite high-purity aluminum material 3 protects against the intrusion of external fluctuating magnetic fields.
This is largely suppressed by the high-resistance metal material 40 covering 8. Furthermore, the high-purity aluminum material used as the stabilizing material is a composite high-purity aluminum material 38 finely divided by slits and inclusions.
Since it has a double suppressing structure against external magnetic fields, it is possible to construct aluminum stabilized superconducting wires that are resistant to fluctuations in magnetic fields inside and outside the surroundings, and to improve stability. An effective aluminum stabilized superconducting wire is obtained.

〔Effect of the invention〕

According to the present invention, a monolithic superconductor or a superconducting wire bundle is formed into a slit, or an insulating material is applied to the slit, or
A high-resistance metal material is coated straight or spirally with a high-purity aluminum material with an insulating material or a high-resistance metal material interposed at the dividing boundary, or a monolithic superconductor or a bundle of superconducting wires, etc. The superconductor is made into a spiral, and the helical angle of the superconductor and the helical angle of the high-purity aluminum material are different.
Furthermore, the helical direction of the superconductor and the helical direction of the high-purity aluminum material were coated in opposite directions. This suppresses eddy currents generated in the high-purity aluminum material of the stabilizing material and reduces eddy current loss, which is effective in improving stability.

In addition, since the periphery of the spiral high-purity aluminum material is coated with a high-resistance metal material, and the periphery is coated with an aluminum alloy material, penetration of magnetic fields from the outside can be suppressed, resulting in a superconductor that is less susceptible to external fluctuating magnetic fields. It is effective.

[Brief explanation of the drawing]

Fig. 1 is a perspective view of an aluminum stabilized superconductor wire showing one embodiment of the present invention, Fig. 2 is a perspective view showing a second embodiment of the invention, and Fig. 3 is a perspective view of a third embodiment of the invention. FIG. 4 is a perspective view showing a fourth embodiment of the present invention, FIG. 5 is a perspective view showing a fifth embodiment of the present invention, and FIG. 6 is a perspective view showing a sixth embodiment of the present invention. FIG. 7 is a perspective view showing a seventh embodiment of the present invention in which the helical directions of the superconductor and the stabilizing material are opposite;
FIG. 8 is a perspective view showing a fourth embodiment of the present invention.

Claims (1)

[Scope of Claims] 1. An aluminum-stabilized superconducting wire characterized in that a superconducting conductor is partially covered with an aluminum stabilizing material provided with slits, and the periphery of the superconducting conductor is covered with a high-strength reinforcing material. 2. In claim 1, the superconducting wire bundle is made of a high-purity aluminum material having an insulating layer, a high-resistance metal layer, or a different material layer partially in the length direction and circumferential direction, or a high-purity aluminum material Aluminum stabilized superconducting wire coated with a composite material of steel and aluminum alloy material. 3. The aluminum stabilized superconducting wire according to claim 2, wherein a high purity aluminum material with slits is spirally arranged around a superconductor such as the superconducting strand bundle. 4. The aluminum-stabilized superconducting wire according to claim 3, wherein an insulating coating or a high-resistance metal coating is provided at a boundary where the high-purity aluminum materials surrounding the superconductor are in contact with each other. 5. In an aluminum stabilized superconducting wire comprising a superconductor, a high-purity aluminum material covering the superconductor, and an aluminum alloy material reinforcing the high-purity aluminum material, the high-purity aluminum material is divided into a plurality of parts. An aluminum stabilized superconducting wire characterized by a spiral coating of superconductor. 6. The aluminum-stabilized superconducting wire according to claim 5, wherein the superconductor is spirally coated with the high-purity aluminum stabilizing material with a high-resistance metal material interposed in the circumferential direction and length direction. 7. A core material of a high-purity aluminum material, an aluminum alloy material, or a composite material of an aluminum alloy material and a high-purity aluminum material, a superconductor such as a plurality of monolithic superconductors or a bundle of superconducting strands, and a superconductor coated with the superconductor. In an aluminum-stabilized superconducting wire made of a high-purity aluminum material and an aluminum alloy material reinforced with the high-purity aluminum material, the plurality of superconductors are formed into a spiral shape, covered with a plurality of divided high-purity aluminum materials, and the surroundings are made of aluminum. An aluminum stabilized superconducting wire characterized by being coated with an alloy material. 8. The aluminum stabilized superconducting wire according to claim 7, wherein the helical superconductor is spirally coated with a plurality of high-purity aluminum materials. 9. The aluminum stabilized superconducting wire according to claim 7, wherein the helical angle of the high-purity aluminum material is smaller than the helical angle of the superconductor. 10. A high-purity aluminum core material with an insulating coating or a high-resistance metal coating, a monolithic superconductor or a superconductor such as a superconducting wire, a high-purity aluminum material as a stabilizing material for the monolithic superconductor, and An aluminum stabilized superconducting wire made of an aluminum alloy material reinforcing a pure aluminum material, characterized in that the helical direction of the superconductor and the helical direction of the high purity aluminum material are opposite to each other. 11. The aluminum stabilized superconducting wire according to claim 10, wherein the helical superconductor is cylindrically molded with solder or high-purity aluminum material, and then the high-purity aluminum material is spirally arranged, and the outer periphery of the spiral superconductor is coated with an aluminum alloy material. 12. In claim 10, a plurality of the monolithic superconductors or a superconductor made of a bundle of superconducting wires, etc. is partially covered with a composite high-purity aluminum material having slits or a high-resistance metal material, and the composite high-purity An aluminum-stabilized superconducting wire, which is a multi-aluminum coated superconducting wire, in which a pure aluminum material is molded and covered with a high-purity aluminum material, then covered with a high-resistance metal material, and the high-resistance metal material is covered with an aluminum alloy material. 13. In claim 12, the composite high-purity aluminum material of the superconductor coating is a composite high-purity aluminum material in which high-purity aluminum materials and aluminum alloy materials are alternately arranged in the circumferential direction, and the composite high-purity aluminum material is made of stainless steel. An aluminum stabilized superconducting wire that is coated with a high-resistance metal material such as steel or cypronickel, and the surrounding area is covered with an aluminum alloy material.