Microtechnology Focus


A New Paradigm for Improving the Superconducting Upper Critical Magnetic Field of Nanocrystalline Niobium Carbonitride (NbC0.3N0.7) for Fusion Energy and Healthcare


When an electrical current flows through a normal conductor, such as copper, it encounters a resistance that transforms the current’s electrical energy into heat energy. It is therefore necessary to apply a permanent voltage to replenish the energy lost to the resistance and so maintain a steady current flow. There are at least three shortcomings with this situation; firstly, a constant supply of energy is required; secondly, energy is wasted in the form of heat; thirdly, the heating can itself be a problem. If, however, the electrical resistance can be removed from the conductor then these problems disappear.


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Superconductivity has immediate applications in electromagnet design. All moving charges produce associated magnetic fields.


Author Details:


Mark J. Raine & Damian P. Hampshire, Superconductivity Group, Department of Physics, Durham University.


Below a certain critical temperature (Tc) a superconductor abruptly enters a superconducting state that is devoid of all


electrical resistance. This is the primary reason why there has been an avid search for superconducting materials with ever-higher critical temperatures. The hope is that a room temperature superconductor will one day be found. This would remove the need for expensive cooling apparatus and the associated difficulties in maintaining the cooling over long distances. In the case of power transmission lines, for example, lossless transmission of electrical power would increase efficiency, reduce energy costs and reduce the burden on fossil fuels.


The search for high temperature superconductors is therefore an active one that has been augmented by interest from the popular press - though this is by no means the whole story. Superconductivity has immediate applications in electromagnet design. All moving charges produce associated magnetic fields. If a current is made to pass through a coiled wire, called a solenoid, a uniform magnetic field can be setup in its core. If the solenoid is made from superconducting wire and is cooled below its critical temperature, the circuit will remain resistance-free, the supply voltage can be removed, there will be no energy lost from the system and the current and field will remain constant. This lack of energy loss due to the resistance-less state ensures that the only energy cost is in setting up the super-current and in maintaining the required temperature.


SUPERCONDUCTING MAGNETS


Superconducting magnets are able to generate much higher magnetic fields than conventional electromagnets. In a conventional magnet system the electrical resistance in the coils causes the windings to run hot. This heat energy must be removed and controlled if damage is to be avoided. However, more importantly, since the magnitude of the electric current is a determining factor in the amount of heat that is produced, the heat problem limits the maximum current that can be carried by the windings. This in turn restricts the maximum magnetic field strength that can be produced. Superconducting magnets, however, do not create this heating and so their windings can be subjected to larger current densities that produce larger field strengths.


Whilst this lack of heat generation removes one particular limit on the amount of current that can be carried by a superconducting coil, the magnetic field produced by the current is unfortunately an additional limiting factor. Not only does a superconductor need to be kept under its transition temperature for it to remain superconducting but it must also be kept under a certain current density


called the critical current density (Jc). This is because superconductivity is also destroyed by large magnetic fields. All superconductors have an upper critical


magnetic field (Bc2) above which they become normal conductors. When the critical current is reached the magnetic field at the surface of the superconductor reaches the upper critical magnetic field and the superconductor is driven into the normal state. This situation provides the motivation to find superconducting materials with ever-higher critical magnetic fields that can be used to produce more powerful superconducting magnets. Of course, other important limits cannot be ignored, such as the fact that increased magnetic fields produce increased mechanical forces, which can eventually lead to a magnet’s destruction. Fortunately, increased magnetic field capabilities do not necessarily have to be used to produce magnets with increased field strengths; they can be used to produce smaller magnets for a given field strength at a reduced cost.


EXISTING MATERIALS


So, the search for new high temperature superconductors and high field superconductors goes on but there are also efforts being made in attempting to improve existing materials. Such is the case with the low temperature superconducting material


niobium-carbonitride (NbC0.3N0.7). It has a maximum critical temperature in bulk form of ~ 17.8 K and an upper critical magnetic field of ~ 12 T. The most common superconducting


materials used in magnet design are niobium-tin (Nb3Sn, Tc ~ 18 K and Bc2 ~ 30 T) and niobium-titanium (NbTi, Tc ~ 9.5 K and Bc2 ~ 12 T) [1]. The reason for this is not only due to their respectable superconducting characteristics but also because


wires can be formed from them using a number of technologies including powder-in-tube technology, which enables brittle materials to be manipulated into wire form. So, what is the motivation to pursue niobium-cabonitride? Well, there is evidence to suggest that its upper critical magnetic field could be increased to ~ 42 T in powder form [2]. Furthermore, Dew-Hughes has shown that the B1 rock salt structure, of which niobium-carbonitride is composed, is resistant to radiation damage [3]. This is in contrast to the A15 compounds niobium-tin and niobium-titanium whose superconductivity is suppressed after being irradiated. This means that it might be advantageous to use niobium-carbonitride in radioactive environments such as tokomaks for nuclear fusion energy extraction. Furthermore, this material could be a viable alternative in applications such as levitating train technology, high-energy particle accelerators, energy storage devices and magnetic resonance imaging scanners (MRI) if sufficient improvements in its upper critical field can be made.


NANOCRYSTALLINE SUPERCONDUCTORS


Increasing the normal state resistivity ρn of a superconductor has been shown to significantly improve its upper critical magnetic field [4]. The relationship between the upper critical field and the normal state resistivity is given by,


where α is a pre-factor that includes the Ginzburg-Landau parameter and a strong-coupling correction, A and B are


constants, γ is the electronic specific heat coefficient and S is the Fermi surface area. Whilst this equation suggests that changes in resistivity directly affect the critical field, as indeed they do,


changes in resistivity also affect γ and Tc; both of which are suppressed by increases in ρn due to the broadening of the density of states at the Fermi energy, which complicates the


behaviour of Bc2. However, theoretical work carried out in Durham University’s Superconductivity Group has provided a


means by which the variation in Bc2 can be predicted using microscopic theory [5]. This can be seen in Figure 1 for niobium


and niobium-tin and can be used to predict the maximum critical field obtainable through changes in the normal state resistivity.


Figure 1. Theoretical upper critical field as a function of critical temperature for increasing resistivity of niobium and niobium- tin. The experimental data for Nb and Nb3Sn are included [2].


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