JP5632767B2 - Precursor for producing Nb3Sn superconducting wire and Nb3Sn superconducting wire - Google Patents

Description

The present invention relates to a precursor for producing an Nb ₃ Sn superconducting wire by a bronze method (a precursor for producing a superconducting wire), and an Nb ₃ Sn superconducting wire produced by such a precursor, and in particular, a precursor. The Nb ₃ Sn superconducting wire is capable of exhibiting excellent superconducting characteristics while ensuring good workability.

  A superconducting magnet that generates a strong magnetic field by flowing a large current through a coil wound with a superconducting wire is aimed at applications such as power storage and fusion reactors in addition to nuclear magnetic resonance (NMR) analyzers and physical property evaluation devices. The development is ongoing.

As a metal-based superconducting wire used for the superconducting magnet, an Nb ₃ Sn wire has been put into practical use, and the bronze method is mainly employed for manufacturing the Nb ₃ Sn superconducting wire. In this bronze method, as shown in FIG. 1 (schematic diagram of a precursor for producing a Nb ₃ Sn superconducting wire), a plurality (7 in the figure) of Nb or Nb base alloys in a Cu—Sn base alloy (bronze) matrix 1 is used. A primary stack material 3 is configured by embedding a core material 2 made of (for example, an Nb—Ta alloy). The primary stack material 3 has a hexagonal cross section as shown in FIG.

  The core material 2 is reduced in diameter by subjecting the primary stack material 3 to a surface reduction process such as wire drawing or extrusion to form a filament (hereinafter sometimes referred to as “Nb-based filament”). A plurality of primary stack materials 3 made of bronze are bundled to form a wire group, which is inserted into a pipe-shaped Cu-Sn alloy 5 wound with an Nb sheet or Ta sheet as a diffusion barrier layer 4, or a primary stack material A secondary multi-core billet 7 is assembled by directly winding an Nb sheet or a Ta sheet around a group of wires 3 in which 3 are bundled, and disposing a stabilizing copper 6 on the outer periphery thereof.

  The above-mentioned secondary multi-core billet 7 is hydrostatically extruded and subsequently subjected to surface reduction by drawing or the like, and a precursor that is maintained while maintaining the cross-sectional shape of FIG. Processed into a precursor (not shown).

By performing diffusion heat treatment (Nb ₃ Sn generation heat treatment) for about 80 to 150 hours at a temperature of about 650 to 720 ° C. for the precursor (wire material after wire drawing) as described above, the Nb-based filament and the bronze matrix An Nb ₃ Sn compound layer is formed at the interface to obtain an Nb ₃ Sn superconducting wire.

  In the precursor as described above, as shown in FIG. 1, a portion in which a plurality of Nb-based filaments are arranged in a Cu—Sn-based alloy (bronze) matrix 1 (hereinafter referred to as “superconducting matrix portion”). In general, the diffusion barrier layer 4 is disposed between the stabilized copper 6 and the stabilized copper 6 (for example, Patent Document 1). This diffusion barrier layer 4 is composed of, for example, an Nb layer or a Ta layer, or two layers of an Nb layer and a Ta layer, and prevents the Sn in the superconducting matrix portion from diffusing to the outside during the diffusion heat treatment. The effect of suppressing the diffusion of Sn into the copper 6 is exhibited.

By the way, the high magnetic field magnet using the Nb ₃ Sn superconducting wire as described above has a large accumulated energy, and a high stress (electromagnetic stress) is applied to the superconducting wire constituting the magnet at the time of excitation. For this reason, the superconducting wire itself is required to have enough strength to withstand high stress. In particular, the critical current of the Nb ₃ Sn superconducting wire is sensitive to strain, and when the strain exceeds 0.2%, the critical current decreases with strain. Therefore, the strength of the Nb ₃ Sn superconducting wire is increased. Various technologies have been proposed so far.

As such a technique, for example, Patent Document 2 proposes an Nb ₃ Sn superconducting wire in which a Ta alloy containing an alloy element such as Nb, V, Zr, or Hf is disposed as a reinforcing member. Patent Document 3 proposes an Nb ₃ Sn superconducting wire using an alloy such as an Nb-based alloy, a V-based alloy, a Cu—Nb alloy, or a Cu—V alloy as a reinforcing member.

JP-A-51-61794 JP-A-9-82149 Japanese Patent Laid-Open No. 10-255563

  In the conventional technology, the material proposed as the reinforcing member is basically an alloy. Such a reinforcing member is excellent in strength because of the alloy, but on the other hand, since the movement of lattice defects (dislocations) introduced by processing is hindered by the solute element, the work hardening progresses remarkably as compared with pure metal.

  Moreover, in the precursor of the above structure, since the work hardening of a Cu-Sn base alloy (bronze) part is intense, it is common to perform the annealing process several times in the middle of a manufacturing process. Although this annealing is usually performed at about 500 to 600 ° C., the Cu—Sn base alloy (bronze) portion can be softened by this annealing.

  On the other hand, since all of the reinforcing members (alloys) proposed so far are refractory metals, the degree of softening of these materials during annealing is small. In addition, after work hardening has progressed (after processing to the thin diameter of the wire), the reinforcing member becomes the starting point, and the breakage tends to occur. In order to avoid such a situation, it is conceivable to anneal the reinforcing member.

However, in order to soften the refractory metal, heat treatment at 800 ° C. or higher is usually performed. On the other hand, when heat treatment is performed at a high temperature, Nb ₃ Sn is formed at the interface between the Nb-based filament and the bronze matrix. The production reaction of the superconducting phase will proceed. Since the Nb ₃ Sn superconducting phase cannot be plastically deformed, disconnection occurs in the subsequent processing steps. That is, in order to prevent disconnection, annealing for the purpose of softening the reinforcing member leads to the result of disconnection.

  For these reasons, there is a demand for the realization of a precursor that can ensure good workability without annealing at high temperatures and can ensure high strength in the superconducting wire. In addition, when manufacturing superconducting wires, it is also an important requirement that basic superconducting characteristics such as high critical current density and reduction of AC loss can be exhibited.

The present invention has been made under such circumstances, and the object thereof is to provide a precursor (Nb ₃ N) capable of ensuring good workability and exhibiting the high strength and excellent superconducting properties of the Nb ₃ Sn superconducting wire. ₃ Sn superconducting wire production precursor), and to provide the Nb ₃ Sn superconducting wire as described above.

The precursor for producing a superconducting wire of the present invention capable of achieving the above object is a precursor for producing a superconducting wire used when producing a Nb ₃ Sn superconducting wire, and a plurality of Cu-Sn based alloys In a superconducting matrix manufacturing precursor having a superconducting matrix portion in which Nb-based filaments made of Nb or Nb-based alloys are arranged, and a diffusion barrier layer and a stabilizing copper layer on the outer periphery thereof,
In the superconducting matrix portion, a reinforcing member made of pure Nb is arranged, a Ta layer is formed on the outer peripheral surface of the reinforcing member, and the area ratio of the reinforcing member in the precursor cross section is 8 It has a gist in that it is -30%.

  In the precursor having such a configuration, the reinforcing member is preferably disposed at the center of the cross section of the superconducting matrix portion.

  The diffusion barrier layer may employ any structure including an Nb layer, a Ta layer, or two layers of an Nb layer and a Ta layer. Of these, a layer containing at least a Ta layer (Ta layer, or Nb layer and Ta layer) 2 layers) is preferred.

The Nb ₃ Sn-based superconducting phase formed by subjecting the precursor for manufacturing a superconducting wire as described above to Nb ₃ Sn generation heat treatment at a temperature of 700 ° C. or lower forms Nb ₃ Sn exhibiting desired characteristics. A superconducting wire can be obtained.

According to the present invention, the reinforcing member made of pure Nb is disposed in the superconducting matrix portion, the Ta layer is formed on the outer peripheral surface of the reinforcing member, and the area ratio of the reinforcing member in the precursor cross section is defined. Therefore, a precursor (Nb ₃ Sn superconducting wire manufacturing precursor) capable of producing an Nb ₃ Sn superconducting wire gold that can ensure good processability and has high strength and excellent superconducting properties. ) Was realized.

It is sectional drawing which showed typically the example of a structure of the precursor for superconducting wire manufacturing applied to the bronze method. A configuration example of a Nb 3 _Sn superconducting wire precursor for manufacturing of the present invention is a cross-sectional view illustrating. It is a graph which shows the influence which a pure Nb area ratio has on a critical current density and 0.2% yield strength.

  The present inventors have studied from various angles with the aim of realizing a precursor capable of ensuring good workability and ensuring high strength in a superconducting wire without annealing at high temperature. As a result, in the configuration of the precursor shown in FIG. 1, it was found that the above-described basic problem can be solved by adopting a configuration in which reinforcing members made of pure Nb are arranged at a predetermined ratio in the superconducting matrix portion. .

  However, the arrangement of a rod-shaped reinforcing member made of pure Nb in the superconducting matrix portion is also suggested in the above-mentioned Patent Document 3, and it is another problem in terms of superconducting characteristics simply by adopting such a configuration. It became clear that this could not be a fundamental solution.

That is, when pure Nb is arranged as a reinforcing member, if a diffusion heat treatment is performed in a state where the reinforcing member and the bronze are in contact with each other, an Nb ₃ Sn superconducting phase is also generated on the outer peripheral surface of the reinforcing member. When used, the AC loss, which is important as a superconducting characteristic, is increased. This is considered to be because the “effective filament diameter” increases due to the influence of the Nb ₃ Sn phase generated around pure Nb. This effective filament diameter means the effective diameter when the filament behaves as a unit, and serves as an index of magnetic stability. The smaller this value, the higher the stability of the superconducting properties. It is. Further, as shown in the following formula (2), the filament diameter (corresponding to the “effective filament diameter”) is a factor affecting the AC loss. The smaller the filament diameter, the smaller the AC loss. Become.

  The present inventors have also studied to solve these problems. As a result, it has been found that if the Ta layer is formed on the outer peripheral surface of the reinforcing member made of pure Nb, a superconducting wire that does not cause the above-described problem (that is, reduced AC loss) can be realized. completed. Hereafter, the structure of the precursor of this invention is demonstrated.

FIG. 2 is a cross-sectional view showing a configuration example of the precursor of the present invention (precursor for producing a Nb ₃ Sn superconducting wire), and the basic configuration as the precursor is similar to that shown in FIG. Common portions are given the same reference numerals). In short, in the superconducting matrix portion in which a plurality of Nb-based filaments made of Nb or Nb-based alloys are arranged (in this figure, a cross section of the superconducting matrix portion) Center), a reinforcing member 8 made of pure Nb (in this figure, a rod shape) is disposed, and a Ta layer 9 is formed on the outer peripheral surface of the reinforcing member 8.

By adopting a configuration in which a reinforcing member made of pure Nb is arranged, the workability of the precursor can be improved, and an Nb ₃ Sn superconducting phase is formed in the vicinity of the outer peripheral surface of the reinforcing member at the stage of diffusion heat treatment. Can be prevented by the presence of the Ta layer 9. As a result, the increase in effective filament diameter described above can be prevented, and the AC loss can be reduced.

The thickness of the Ta layer that exerts such action is not particularly limited, and is as thin as possible from the viewpoint of maintaining good workability (Ta has better workability than Ta alloys). However, in consideration of the point that the Nb ₃ Sn superconducting phase formation preventing effect is exhibited after the diameter reduction processing such as wire drawing, it is preferably about 0.1 to 0.5 mm at the stage of the primary stack material. . The simplest method for forming the Ta layer on the outer peripheral surface of the reinforcing member is to wrap the sheet-like Ta around the outer peripheral surface of the reinforcing member.

  The reinforcing member used in the precursor of the present invention is made of pure Nb, and this pure Nb includes a material containing a trace amount (about 0.5%) of impurities. From the viewpoint of being relatively easily available, it is preferable to use pure industrial Nb (for example, ASTM B392 Commercial Grade Unalloyed Niobium).

  The Ta layer used in the precursor of the present invention is also assumed to be composed of pure Ta, but a layer containing a small amount (about 0.5%) of impurities can be used. From the same viewpoint as Nb, it is preferable to use industrial pure Ta (for example, ASTM B708 Electron-Beam Cast, Vacuum-Arc Cast Unalloyed Tantalum).

The mechanical strength required for the superconducting wire depends on the usage of the superconducting wire, but when using an Nb ₃ Sn superconducting wire as a normal solenoid coil, the wire is made of the strong magnetic field generated by the entire coil. Electromagnetic force is generated. It is necessary to set the strength so that the superconducting characteristics do not deteriorate even under such electromagnetic force.

Here, when the magnetic flux density at a certain coil position is B (T: Tesla), the current density is J (A / m ² ), and the coil radius is R (m), the stress σ generated by the electromagnetic force is respectively Which is expressed by σ = B · J · R. That is, the stress σ is maximized at a portion where the magnetic flux density B and current density J are high and the coil radius R is large. Wires that require mechanical characteristics are used in the inner layer side coil of the superconducting magnet (the individual coil on the inner layer side has a smaller coil radius but a higher magnetic flux density B than the coil on the outer layer side) ) In particular, it is necessary to balance the superconducting characteristics and strength in the superconducting wire used for such applications.

  Increasing the amount of the reinforcing member that employs the above-described configuration (pure Nb with a Ta layer formed on the outer peripheral surface) leads to a decrease in the contact area between the Cu-Sn and the Nb-based filament. Excessive incorporation of the reinforcing member leads to deterioration of superconducting characteristics which are originally required. For these reasons, the area ratio of the reinforcing member to the precursor cross section needs to be 30% or less. However, in order to exhibit the effect as a reinforcing member, the area ratio needs to be 8% or more (see FIG. 3 described later). In addition, the preferable minimum of the said area ratio is 10%, and a preferable upper limit is 25%.

  In addition, although the said area ratio assumes the area ratio in the stage integrated in a composite member (primary stack material), after wire drawing processing (namely, after wire drawing processing and before heat treatment) and It will be almost equal.

  The precursor of the present invention is supposed to be applied to the bronze method, and has a superconducting matrix portion in which a plurality of Nb-based filaments are arranged in a Cu—Sn alloy. The Sn content in the Sn alloy is preferably about 13 to 16% by mass. By setting it as such content, critical current density Jc can be raised as much as possible. When the Sn content is less than 13% by mass, the effect of increasing the Sn concentration is not exhibited. When the Sn content exceeds 16% by mass, a large amount of Cu—Sn compound is precipitated, and uniform processing of the wire becomes difficult.

  As the Nb-based alloy that may be used for the Nb-based filament and the diffusion barrier layer, an alloy containing an additive element such as Ta, Hf, Zr, Ti or the like up to about 4% by mass can be used.

In addition, the diffusion barrier layer may employ any structure of an Nb layer, a Ta layer, or two layers of an Nb layer and a Ta layer, and of these, a layer containing at least a Ta layer (Ta layer or Nb layer) And two layers of Ta layer). By adopting such a configuration, it is possible to prevent the Nb ₃ Sn superconducting phase from being formed in the vicinity of the diffusion barrier layer at the stage of the diffusion heat treatment, and to further reduce the AC loss (preventing increase of the effective filament diameter). It becomes something that can be planned.

  A superconducting wire exhibiting good characteristics can be obtained by constituting the precursor as described above, performing a wire drawing process on the precursor, and then performing a diffusion heat treatment to form a superconducting phase. When a material having a melting point higher than Nb, such as Ta, is used as the reinforcing member, a sufficient reinforcing effect is exhibited even if diffusion heat treatment is performed at a temperature exceeding 700 ° C., but pure Nb is used as the reinforcing member. In some cases, if diffusion heat treatment is performed at a temperature exceeding 700 ° C., the softening of Nb proceeds and there is a concern that a desired reinforcing effect cannot be obtained. For these reasons, it is preferable to set the temperature during the diffusion heat treatment to 700 ° C. or lower. The diffusion heat treatment time is suitably about 100 to 200 hours.

  EXAMPLES Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited by the following examples, but may be appropriately modified within a range that can meet the purpose described above and below. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

[Example 1]
Seven holes with a diameter of 12 mm are formed in the center of the Cu-15% Sn alloy with a diameter of 60 mm and the periphery thereof, Nb bars with a diameter of 11.8 mm are inserted into the holes, and both ends are vacuum sealed by welding. And the extrusion billet for primary multifilamentary wires was produced. This extruded billet was extruded to a diameter of 20 mm by the hydrostatic extrusion method, and this was drawn by drawing. Then, the hexagonal die was used to finish a hexagonal cross-sectional shape with an opposite side length of 1.5 mm to produce a primary multi-core wire (primary stack material) having a hexagonal cross section.

  A Ta sheet having a thickness of 0.2 mm is wound around an industrial pure Nb having an outer diameter of X mm, the Y primary multi-core wires are bundled around the outer periphery, and a thickness of 0.2 mm is further formed around the outer periphery. Nb sheet is wound twice (diffusion barrier layer), these are integrated, inserted into a Cu pipe (stabilized copper layer) having an outer diameter of 60 mm and an inner diameter of 54 mm, and the end is sealed by welding, A secondary multi-core billet was used (see FIG. 2).

The obtained billet (secondary multi-core billet) was extruded to a diameter of 20 mm by the hydrostatic extrusion method, and this was drawn by drawing to produce a bronze method Nb ₃ Sn superconducting wire having a diameter of 0.8 mm. A precursor was prepared.

  When drawing primary and secondary multi-core wires containing Cu-Sn alloy, annealing is performed multiple times during the processing so that disconnection due to work hardening of Cu-Sn alloy does not occur. did. The annealing conditions are 500 ° C. and 5 hours.

The obtained precursor was subjected to a heat treatment (diffusion heat treatment) at 650 ° C. for 100 hours in a vacuum to obtain a Nb ₃ Sn superconducting wire. In the Nb ₃ Sn superconducting wire thus obtained, 0.2% yield strength, AC loss Ph and critical current density Jc were measured under the following conditions. Further, the area ratio (pure Nb area ratio) of the reinforcing member in the stage before the heat treatment (final shape after the surface reduction processing) was obtained based on the following formula (1).
Pure Nb area ratio = (area of reinforcing member / total cross-sectional area of wire) × 100 (%) (1)

[Measurement of 0.2% yield strength]
A tensile test was performed in a state where the heat treated wire was immersed in liquid helium (temperature 4.2 K), and 0.2% yield strength was measured. The acceptance criterion for 0.2% proof stress is 220 MPa or more.

[Measurement of AC loss Ph]
The AC loss Ph (loss per volume of the superconducting part) was measured in a variable magnetic field of ± 3 T (Tesla) in liquid helium (temperature 4.2 K). The AC loss Ph at this time is obtained by the following equation (2). The acceptance criterion for AC loss is 750 mJ / cm ³ or less.
Ph = (8 / 3π) × f × λ × Jc × d _f × B _m (2)

f: Frequency of externally varying magnetic field (Hz)
λ: occupied area ratio of superconducting portion in superconducting wire Jc: critical current density (A / m ² )
d _f : Filament diameter (m)
B _m : Amplitude of externally varying magnetic field (T: Tesla)

[Measurement of critical current density Jc]
In liquid helium (temperature 4.2K), the critical current is measured by the four probe method under an external magnetic field of 15T (Tesla), and this current value is divided by the cross-sectional area per non-Cu part in the wire cross section. The critical current density Jc was determined. The acceptance criteria for the critical current density is 220 A / mm ² or more.

  These results (Test Nos. 1 to 10) are collectively shown in Table 1 below.

  From these results, it can be considered as follows. Test No. 1 to 6 satisfy the requirements specified in the present invention, and by reducing the effective filament diameter, AC loss can be suppressed, and the critical current density Jc and strength (0.2% proof stress). ) Is also expensive. In contrast, test no. 7 to 10 lack any of the requirements defined in the present invention (comparative example), and any of the characteristics is deteriorated.

  Among the comparative examples, Test No. Nos. 7 and 8 do not contain pure Nb as a reinforcing member, or the pure Nb area ratio is insufficient, and the strength (0.2% proof stress) is insufficient. Test No. In No. 9, the pure Nb area ratio is excessive, and the critical current density Jc is lowered. Test No. No. 10 does not form a Ta layer, and the effective filament diameter increases and the AC loss increases.

  Based on these results, the influence of the pure Nb area ratio on the critical current density Jc and 0.2% yield strength is shown in FIG. 3 (in the figure, ◆ indicates the critical current density, and □ indicates the 0.2% yield strength). . It can be seen that the critical current density Jc and the 0.2% proof stress can be controlled in appropriate ranges by adjusting the pure Nb area ratio in appropriate ranges.

[Example 2]
Seven holes with a diameter of 12 mm are formed in the center of the Cu-15% Sn alloy with a diameter of 60 mm and the periphery thereof, Nb bars with a diameter of 11.8 mm are inserted into the holes, and both ends are vacuum sealed by welding. And the extrusion billet for primary multifilamentary wires was produced. This extruded billet was extruded to a diameter of 20 mm by the hydrostatic extrusion method, and this was drawn by drawing. Then, the hexagonal die was used to finish a hexagonal cross-sectional shape with an opposite side length of 1.5 mm to produce a primary multi-core wire (primary stack material) having a hexagonal cross section.

  A Ta sheet having a thickness of 0.2 mm is wound around an industrial pure Nb having an outer diameter of X mm, the Y primary multi-core wires are bundled around the outer periphery, and a thickness of 0.2 mm is formed around the outer periphery. A sheet made of Nb is wound, and a sheet made of Nb having a thickness of 0.2 mm is wound around the outer periphery (a diffusion barrier layer composed of two layers of a Ta layer and a Nb layer), and these are integrated to obtain an outer diameter of 60 mm. Inserted into a Cu pipe (stabilized copper layer) having an inner diameter of 54 mm, and sealed by welding to form a secondary multi-core billet (see FIG. 2).

The obtained billet (secondary multi-core billet) was extruded to a diameter of 20 mm by the hydrostatic extrusion method, and this was drawn by drawing to produce a bronze method Nb ₃ Sn superconducting wire having a diameter of 0.8 mm. A precursor was prepared.

  When drawing primary and secondary multi-core wires containing Cu-Sn alloy, annealing is performed multiple times during the processing so that disconnection due to work hardening of Cu-Sn alloy does not occur. (The annealing conditions are the same as in Example 1).

The obtained precursor was subjected to a heat treatment (diffusion heat treatment) at 650 ° C. for 100 hours in a vacuum to obtain a Nb ₃ Sn superconducting wire. In the Nb ₃ Sn superconducting wire thus obtained, the 0.2% yield strength, the AC loss Ph, and the critical current density Jc were measured in the same manner as in Example 1. Further, the area ratio (pure Nb area ratio) of the reinforcing member in the stage before the heat treatment (final shape after the surface-reduction processing) was obtained in the same manner as in Example 1 [based on the formula (1)].

  These results (Test Nos. 11 to 20) are collectively shown in Table 2 below.

  As apparent from the comparison with the result of Example 1, it is understood that the AC loss can be further suppressed by interposing at least a Ta layer as the diffusion barrier layer.

DESCRIPTION OF SYMBOLS 1 Cu-Sn base alloy matrix 2 Core material which consists of Nb or Nb base alloy 3 Primary stack material 4 Diffusion barrier layer 5 Pipe-shaped Cu-Sn alloy 6 Stabilized copper 7 Secondary multi-core billet 8 Reinforcement member 9 Ta layer

Claims (4)

A superconducting wire manufacturing precursor used when manufacturing a Nb ₃ Sn superconducting wire, and a superconducting matrix portion in which a plurality of Nb-based filaments made of Nb or Nb-based alloys are arranged in a Cu-Sn base alloy; In the precursor for producing a superconducting wire having a diffusion barrier layer and a stabilized copper layer on its outer periphery,
In the superconducting matrix portion, a reinforcing member made of pure Nb is also arranged, a Ta layer is formed on the outer peripheral surface of the reinforcing member, and an area ratio occupied by the precursor cross section of the reinforcing member is 8 A precursor for producing a Nb ₃ Sn superconducting wire, characterized in that it is ˜30%. 2. The precursor for producing an Nb ₃ Sn superconducting wire according to claim 1, wherein the reinforcing member is disposed at the center of the cross section of the superconducting matrix portion. _{3. The} precursor for producing a Nb ₃ Sn superconducting wire according to claim 1, wherein the diffusion barrier layer is composed of an Nb layer or a Ta layer, or two layers of an Nb layer and a Ta layer. The Nb ₃ Sn superconducting phase is formed by subjecting the Nb ₃ Sn superconducting wire manufacturing precursor according to any one of claims 1 to 3 to a Nb ₃ Sn generation heat treatment at a temperature of 700 ° C. or lower. An Nb ₃ Sn superconducting wire.

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