A current affair, this!

ELECTRICAL SCIENCE

Work by scientist Mahadevan Krishnan and associates from the United States demonstrates that use of thin film devices with superconducting qualities in facilities like CERN’s Large Hadron Collider will cut costs substantially, writes S Ananthanarayanan

Superconducting materials have effectively zero resistance to the flow of electricity. Using these materials could mean saving a lot of energy. It would also make possible services that have been unthinkable because of the bulky electrical parts and heat from high currents.

Another advantage of superconductors is that even a thin one is as good as a thick one.

Again, at high frequencies, the current anyway stays at the surface. There is hence much
interest in finding ways to coat ordinary materials with a superconducting layer. The ordinary material would provide the mechanical support while the thin covering carries the electricity.

Mahadevan Krishnan and associates from Alameda Applied Sciences Corp, California and Jefferson Lab and State University of Norfolk, Virgina, have described in the Institute of Science journal, Superconductor Science and Technology, their work of coating the metal niobium as a thin film on a magnesium oxide base, to show equal and higher drop in electric resistance than displayed by the bulk niobium that has been the norm. 

Superconductivity 

The explanation for superconductivity is mainly that at low temperatures, electrons form pairs, which enables them to act as particles that can all occupy the same energy state, and the mass of electrons begins to act like a ‘superfluid’ without each coming in the others’ way.  
 
Zero resistance to electric current means that a current stream once set up would not die for a long, long time. A dramatic illustration of the effect is when a magnet is lowered on to the surface of a superconductor. The magnet sets up an electric circulation in the material, to counteract the magnetic field, so that the lowering of the magnet is resisted. As the current keeps going and does not diminish, the force on the magnet persists and it can hold the magnet up in the air indefinitely! Apart from such ‘levitation’ displays, zero resistance materials can be formed in coils to create very strong electromagnets, which have applications in medical equipment, particle accelerators and possibly in high speed transport.

Superconductivity, where current can flow without a driving voltage, also allows the construction of very sensitive electronic devices and has a central role in the development of quantum computers. An important use of superconducting magnetic coils is energising the electromagnets that drive particle accelerators. High-energy atomic particles, like electrons and protons, are useful in research and in industry. The particles are speeded up with the help of electric fields and their path is kept within limits by turning them round in circles, with magnetic fields. As the speeds approach that of light, the magnets need to be powerful and they consume power, both for magnetisation as well for cooling the arrangement, as high electric current creates great heat.

One comparatively conventional solution would be to use superconductors for electromagnet coils. With the dimensions of large accelerators, this is neither feasible nor economical.

An alternative is the superconducting radio frequency (SRF) accelerator. In this arrangement, electromagnetic standing waves are allowed to build up within cavities that resonate at particular frequencies, typically from 200 million cycles to three billion cycles a second. A resonating cavity is like the body of a violin that vibrates at the frequencies of the string, and which takes up and magnifies the sound of a string that is bowed. Metallic resonating cavities can similarly respond to a specific electromagnetic frequency and store huge energy in the tuned standing waves. A charged particle entering such a cavity would then be strongly accelerated and deflected by the intense electrical and magnetic fields. When the cavity is made of superconducting material, very high amplitude waves can be maintained with minimal need for cooling. 

Side-stepping the need for cooling is not important, and then, there is still the need for low temperatures to create superconductivity. More important reasons for preferring superconductor cavities are, firstly, that very high fields, at which ordinary materials would melt, are possible and secondly that the low electrical loss with superconductors allows geometry where the charged particle beam can be larger in cross section. But for all this, superconducting RF accelerators are difficult to employ, particularly in numbers for a ‘linear accelerator’, as it is difficult to manufacture consistent series of quality superconducting RF cavities, which are made by stamping and welding pure niobium sheets. 

Superconducting films

This is where the work of Krishnan and group becomes relevant. Krishnan and colleagues grew single crystal films of niobium on a base of magnesium oxide (MgO), which in turn may be coated on the copper resonating cavity.

The method used was to set up a pulsed electric discharge, or flashing like in arc welding, between an electrode of niobium, and a molybdenum electrode in the form of a cylindrical mesh. The electric discharge created plasma, or a gas of ionised niobium, which sped through the mesh to be sprayed on the MgO base. The temperature of the MgO base, on which the niobium deposits, the voltage of the electrical arc, the pulse-rate, the operating pressure, voltage applied to the MgO base and an applied magnetic field could be varied and controlled.

In practice, the discharge ran as a helix, or a spiral, over the MgO base in about a millisecond, once every four seconds and in some 4,000 such passes, a one micron thick coat of niobium was formed.

A measure of the effectiveness of the material is the residual resistivity ratio, or how many times the electrical resistance drops when the material is cooled from room temperature to absolute zero. The second temperature, of course, cannot be reached and an approximation is made. The ratio for ordinary copper is some 40-50, against the ratio of 300 and better for cavities made of the best bulk niobium. Past efforts to grow niobium films have resulted in the ratio rising to 100, and in the case of a deposit on a sapphire base, to 199. The results of deposit on MgO have been poor and MgO has generally not been considered a suitable base.

But considering that MgO is suitable for use with copper, the present study has included niobium deposit on MgO.

The result of the process used, of deposit on a specifically oriented single-crystal MgO base, has been a resistivity drop as high as 541, a level rarely reached without heroic efforts with bulk niobium.

Further study, using X-ray analysis of the niobium layer, has shown that the orderliness of crystal orientation of the layer depends on the temperature of deposition and that the same also correlates with the resistivity drop ratio. There is similar dependence on the manner in which the MgO base was cooled when deposited. It is also found that the best resistivity drop ratio occurs with the niobium crystal orientation that has the least suitable match with MgO, which suggests that there are other factors to consider. It appears that the electric arc process, at 50,000˚C, drives niobium ions moving at 10 km/sec into the substrate surface and promotes interlock between the crystals of the base and the covering, a result that less energetic coating processes could not achieve.

But the work demonstrates that effective superconducting action can be developed in cavities for use in particle accelerators, by coating the cavity components with superconducting material, rather than constructing the components with the material itself. The process covers the gamut of technologies, from plasma to a defect-free surface that works at a temperature near absolute zero!

“Use of these thin film devices in facilities like CERN’s LHC and other upcoming facilities would cut the costs substantially,” says M Krishnan, founder of Alameda Applied Sciences Corporation and lead author of the paper.

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