"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 Jc 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 Jc normalised by Jc 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