Oxford team exploits properties of metamaterials

Many researchers are exploring the field of metamaterials, which gain their unique properties from their structure, rather than the fundamental characteristics of the material from which they are composed. While much research to date has focused on the use of such materials for the manipulation of high-frequency electrical fields, less attention has been paid to how they might be used in power conversion equipment at lower frequencies in the hundreds of kilohertz range.

But Dr Chris Stevens, an Oxford University fellow and tutor in engineering science, believes there is a great opportunity to exploit metamaterials in such applications. His research aims to demonstrate the advantages and benefits of using them to build lightweight transformers and electric motors.

Traditional electrical transformers work through the principle of electromagnetic induction. A simple transformer comprises two windings that are wrapped around a core of a ferromagnetic material, such as iron. A varying current in the primary winding creates a varying magnetic flux in the transformer’s core and thus a varying current in the secondary winding.

“Such an approach could potentially produce a core at least 50 per cent lighter than one made of iron”

’The iron cores of such devices have limitations,’ said Stevens. ’First, they are often bulky and heavy, which limits their use in certain applications. Second, they reach a fundamental saturation limit, at which any increase in applied external field cannot increase the magnetisation of the material further.’

He thinks that using metamaterials to structurally engineer the cores of such transformers would help to alleviate both issues. He has designed a transformer from metamaterials whose electromagnetic properties alone provide an alternative to traditional ferromagnetic materials, where performance is defined solely by their chemical composition.

The transformer itself was constructed from resonators formed from short coils of copper wire, with polystyrene capacitors connected across them. The resonant circuits were then stacked together in a toroidal structure that could guide the magnetic flux to a secondary winding.

’Such an approach could potentially not only produce a core at least 50 per cent lighter than one made of iron, but also one that has no problems of magnetic saturation although in practice, the core’s limitations may be constricted by the performance of the materials from which it is comprised,’ Stevens added.

Because metamaterials exhibit strongly resonant behaviour at a relatively narrow range of frequencies, the transformers naturally produce a filtering effect meaning any frequencies outside a narrow band will not be guided through the resonant material that comprises the core of the transformer. In addition, the magnetic permeability of Stevens’ metamaterials is frequency dependent. This varies between an attractive, positive (or ferromagnetic) state and a negative (or diamagnetic) state, where the metamaterial creates a magnetic field in opposition to an externally applied magnetic field, causing a repulsive effect.

As such, Stevens believes they could also prove useful in the design of electric motors. His research team is building a rotating machine, using similar resonant circuitry used in the transformer, to exploit the metamaterial’s ability to generate both attractive and repulsive forces.

’The motor’s core will comprise a cylindrical barrel free to rotate about its axis, the surface of which will be covered by a number of resonant metamaterial plates. A number of external fixed-drive coils driven by frequency-modulated signals will then alternatively create positive paramagnetic forces and negative diamagnetic forces on the plates, creating a force of attraction and repulsion at specific times in the rotational cycle of the machine,’ said Stevens.

A number of other rotating machines, such as a simple homopolar unit, could also be constructed using the metamaterial.