WO2003049208A2

WO2003049208A2 - Magnesium diboride superconductors

Info

Publication number: WO2003049208A2
Application number: PCT/GB2002/005527
Authority: WO
Inventors: Lisa Cowey; Judith Louise Driscoll
Original assignee: Imperial College Innovations Limited
Priority date: 2001-12-07
Filing date: 2002-12-06
Publication date: 2003-06-12
Also published as: GB0129410D0; AU2002347355A1; WO2003049208A3

Abstract

A method of manufacturing polycrystalline bulk MgB2 with enhanced superconducting properties, by introducing structural defects forming pinning sites for quantised magnetic vortices, so as to prevent them from moving under the action of the Lorentz force. Chemical dopants like Si, Cu, Zn, Al, C, Li, or N or inert or nano-boride forming particles like Y2O3 can be used.

Description

"Magnesium Diboride Superconductors" Magnesium Diboride (MgB₂) was discovered to have superconducting properties in early 2001 [1]. The pure material is a type II superconductor with a superconducting transition temperature Tc of ~ 39K.

Magnesium Diboride falls between the classic low temperature superconductors (LTS) e.g. NbTi and NbSn with Tc's under 20°K, and the newer High Temperature

Superconductors (HTS) e.g.YBaCuO and BiSrCaCuO with Tc's over 90°K. LTS superconductors are most frequently operated at or near 4°K using liquid

helium as a coolant. HTS superconductors have been targeted at an operating temperature as high as 77°K where liquid nitrogen could be used as the coolant.

Magnesium Diboride shows promise for use at about 20°K, where relatively inexpensive

closed cycle refrigerator cooling could be employed. Unlike many of the HTS materials, supercurrent in the MgB₂ system is not significantly hindered by grain boundaries. This means that long lengths of high performance polycrystalline materials may be easier to fabricate. In addition, the starting and sheathing materials of the MgB₂ conductor are less expensive than those involved in the production of polycrystalline HTS conductors of the Bi based compounds, and much more simple than for the HTS conductors of the RE based compounds. The possibility of use at about 20°K, combined with the possible low cost in conductor

fabrication would make MgB₂ an attractive material for the Magnetic Resonance Imaging market (MRI) operated on 20°K cryocoolers, and other high current conductor applications. Fragments of the pure material have been shown to support critical current density (Jc) of over 10⁶ A/cm² at 20°K and zero applied field. However, in such fragments the critical current density falls to values below 10³ A/cm² at relatively low fields of between 3.5 and 4 T at 20°K [2]. This fall off in the Jc in bulk material with applied field appears to be more rapid that it is in the HTS materials at the same temperature.

The magnitude and field dependence of Jc are related to the presence of structural defects that can "pin" the quantised magnetic vortices that permeate the material and prevent them from moving under the action of the Lorentz force. Vortex studies suggest that it is the lack of suitable pinning sites in MgB₂ that causes the rapid decay of Jc with field. [2]

If the vortex pinning could be strengthened so as to increase Jc in magnetic fields of a few Tesla then MgB₂ could compete both on price and performance with the HTS alternatives when operated at about 20°K.

It has been shown that low levels of atomic disorder, introduced by proton irradiation enhance the pinning and so increase Jc significantly at high fields. [3] Irradiation is not a viable technique for large scale conductor fabrication. Accordingly, the present application provides a method of enhancing the superconducting properties of polycrystalline bulk MgB₂ by modifying the process of manufacturing the material in such a way as to give rise to structural defects forming pinning sites for quantised magnetic vortices, so as to prevent them from moving under the action of the Lorentz force. A first preferred method according to the present invention comprises the step of introducing specific chemical impurities, chosen on the basis of their ionic radius and ionic charge. Suitable materials include Si, Cu, Zn and Al, C, Li, and N

A second preferred method according to the present invention comprises the step of introducing particles of a material chosen for its nano-particle starting size. The additive should be a material which remains in the same form after reaction, or something that reacts to form a nano-boride phase. A preferred material for this purpose is Y₂0₃ [4]

In the first preferred method, the additive is preferably introduced in nominal concentrations between 1% and 5% atomic percent during a solid state reaction between Mg, B and the additive. Preferably the concentration is about 3 at.%. In the second preferred method, the additive is preferably introduced in concentration of 5-15 wt. %. Preferably the concentration is about 10 wt. %.

Preferably B, Mg and alloying additive are mixed, pressed into pellets and then sintered in an inert atmosphere (5% H₂ in Ar or in N/Ar ) in a reaction which involves a 15-minute anneal at 900°C. Excess Mg is present in the reaction chamber to ensure the

formation of MgB₂ .

A third preferred method according to the invention is characterised by the time of reaction and the precise form of the B starting precursor. MgB₂ made using commercially obtained amorphous B powder appears to yield a shallow dependence of J_c on field, similar to the effect of the alloying additions and Y₂O₃ nano-particle additions. A short reaction time is also beneficial, with 15 minutes at 900°C better than longer reaction times (e.g. 60 minutes).

Figure 1 shows the form of the precipitates which result when 10 wt. % Y₂O₃ is reacted with the Mg+B. A nano-phase of YB₄ results in a very fine and even distribution. The graph of Figure 2 shows the critical current J_c normalised by J_c at 2T field

20°K of the MgB₂ + various dopants measured using a magnetisation method. Also shown is the Jc behaviour for an undoped MgB₂ made by a standard reaction route as well as MgB₂ made by a rapid reaction route (15 min. 900°C) using an amorphous B precursor powder. The graph shows the following different samples:

MgB₂ + 3 at. % Zn, MgB₂ + 3 at.% Cu, MgB₂ + 3 at.% Al, MgB₂ MgB₂ + 10wt % Y₂0₃ nanoparticles

MgB₂ undoped sample made by a standard route (60min anneal at 900°C) MgB₂ undoped sample made by a 'special' route (15min anneal at 900°C) Because the pellets are not fully dense and unlikely to be fully connected it is not possible to directly compare the exact values of Jc as obtained by the method of measurement. Hence the normalised method is more informative. What is of importance is the rate at which the Jc is being diminished with increasing field i.e. the gradient of the Jc(B) plot.

The addition of the dopants, and in particular the Al and the Y₂0₃, as well as rapid reaction of undoped material to induce disorder, prolongs to much higher fields the value at which Jc drops below 1000 A/cm². This suggests that the pinning has been strengthened considerably by the addition of the dopants. [1] Nagamatsu, J., Nakagawa, N., Muranaka, Y., Zenitani, Y. & Akimitsu, J. Superconductivity in MgB2. Nature 410, 63-64 (2001).

[2] Y. Bugoslavsky, G. K. Perkins, X Qi, L. F. Cohen, and A. D. Caplin. Vortex dynamics in superconducting MgB2 and prospects for applications. Nature 410 563-565, 2001.

[3] Y. Bugoslavsky, L. F. Cohen, G. K. Perkins, M. Polichetti, T. J. Tate, R. Gwilliam, and A. D. Caplin. Enhancement of the high-magnetic-field critical current density of superconducting MgB₂ by proton irradiation. Nature 411 :561-563, 2001.

[4] J. Wang, Y. Bugoslavsky, A. Berenov, L. Cowey, A.D. Caplin, L.F. Cohen, J.L.

MacManus-Driscoll, L.D. Cooley, X. Song, D.C. Larbalestier, " High Critical Current Density and Improved Irreversibility Field in Bulk MgB₂ Made by a Scaleable, Nanoparticle Addition Route, Applied Physics Letters 81 (11 ) 9 September 2002

Claims

1. A method of manufacturing polycrystalline bulk MgB₂ with enhanced superconducting properties, by introducing structural defects forming pinning sites for quantised magnetic vortices, so as to prevent them from moving under the action of the Lorentz force.

2. A method according to claim 1 in which the structural defects are formed by including a specific chemical impurity chosen for its ionic radius and ionic charge.

3. A method according to claim 2 in which the impurity comprises one or more of Si, Cu, Zn , Al, C, Li, or N.

4. A method according to claim 3 in which the invention is introduced in a nominal concentration of between 1 and 5 at .% during a solid state reaction between Mg, B and the additive.

5. A method according to claim 1 in which the structural defects are formed by including an additive chosen on the basis of its inert nature or ability to react with B to form a nano-boride phase of nano particle starting size .

6 A method according to claim 5 in which the additive is introduced in a concentration of 5-15 wt.% during a solid state reaction between Mg. B and the additive.

7. A method according to claim 5 or claim 6 in which the additive comprises Y₂0₃.

8. A method according to claim 4 in which the concentration is 3 at.%.

9. A method according to claim 6 in whicn the concentration is 10 wt.%.

10. A method according to claim 1 in which structural defects are formed by utilising commercially grade amorphous B powder and annealing for a relatively short time to produce an inherently disordered structure.

1 1. A method according to any one of the preceding claims in which the materials are mixed, pressed into pellets and then sintered in an inert atmosphere in a reaction which includes annealing in an Mg -rich environment.

12. A method according to claim 10 or claim 11 in which the annealing is performed for 15 minutes at 900°C.

13. A superconducting material comprising bulk polycrystalline MgB₂ made by the method of any one of the preceding claims.