TECHNICAL FIELD The present invention relates to a device for making or breaking electric contact between at least two electrodes.

BACKGROUND OF THE INVENTION An ideal electrical switch or circuit breaker would be one having zero resistance when making electrical contact between two electrodes and having an infinite resistance when breaking contact. It would change from its conducting to non-conducting position instantaneously and would be able to handle large currents when conducting and withstand large voltages when not conducting.

Most circuit breakers consist of a switch mechanically connected to an electromagnetic circuit. These switches have very favourable resistance characteristics but are relatively slow to operate ; they are subject to wear and prone to arcing problems particularly when used in high voltage applications. More advanced circuit breakers include a semiconductor device such as a MOSFET, thyristor or IGBT. Semiconductor switches are fast acting but they are expensive, have relatively large on-state losses and need to be cooled during use.

SUMMARY OF THE INVENTION It is an object of this invention to provide a high speed, simple device for making or breaking electric contact between at least two electrodes, having favourable electrical resistance characteristics, requiring minimum maintenance and which is suitable for use in both high voltage applications,

such as in transmission and distribution systems as well as for switching low-power signals with milliampere and millivolt requirements.

This and other objects of the invention are achieved by a device according to claim 1, namely a device comprising magnetic nanostructures dispersed in a dielectric liquid contained between said at least two electrodes. The device also comprises a first controllable magnetic field means, comprising a permanent magnet, a coil or an electromagnet for example, to control the movement of the magnetic nanostructures. According to a preferred embodiment of the invention the first controllable magnetic field means are arranged so as to align the magnetic nanostructures into conducting paths between the electrodes when the device is in conducting position.

Each magnetic nanostructure magnetized in single domain state or magnetized in an external field, behaves like a magnetic dipole comparable to a tiny permanent rod magnet having a magnetic north and south pole.

The direction of the magnetisation in the particle is determined by the interaction between the shape and the magnetocrystalline anisotropy of the nanostructure and the magnitude of the external magnetic field.

When a magnetic field is applied across the dielectric liquid containing the magnetic nanostructures the magnetic dipoles align themselves parallel to the external magnetic field. Unlike poles attract to form chains of nanostructures similar to iron powder under the influence of an external magnetic field. The repulsion of like poles holds the chains apart so that substantially parallel conducting paths between said at least two electrodes are formed during the application of the magnetic field.

When the magnetic field is removed, Brownian motion of the nanostructures resulting from the impact of molecules of the dielectric liquid surrounding the nanostructures as well as the nanostructures'own thermal

energy displaces the magnetic nanostructructures out of their conducting paths. According to a preferred embodiment of the invention stirring means are provided to displace the magnetic nanostructures out of the conducting paths once the first controllable magnetic field means are de-energized.

Alternatively the device comprises draining means that drain the liquid containing the nanostructures from between the said at least two electrodes. In a preferred embodiment of the invention the gap between the electrodes may then be filled with dielectric fluid.

According to another preferred embodiment of the invention multi-domain nanostructures are used which enclose the magnetic field generated by the nanostructure within the particle when the external field is de-energized.

According to a preferred embodiment of the invention the device comprises a second controllable magnetic field means to control the movement of the magnetic nanostructures. The first controllable magnetic field means are activated when the device is in its conducting position and the second controllable magnetic field means are activated when the device is in its non-conducting position. According to a further preferred embodiment the same means provide both the first and second magnetic fields i. e. a permanent magnet providing a first magnetic field is rotated to provide the second magnetic field for example.

In the device according to the present invention electrical contact between the electrodes is broken without giving rise to arcing because the conducting paths are broken at a tens to billions of locations instantaneously. As well as operating in an arc-less manner, such a device withstands high voltage and has very low on-state losses.

The magnetic field provided to manipulate the nanostructures in the dielectric liquid does not decay entirely when the controllable magnetic field

means are de-energized. A magnetic induction, i. e. remanence, remains.

However if at least one magnetic pulse in the reversed direction is provided by said controllable magnetic field means after de-activation, part of the remaining remanence can be removed, which aids the displacement of the magnetic particles out of the conducting paths. Another way to displace the magnetic particles is to heat the particles above their Curie temperature so that the magnetic interaction disappears, which aids the displacement of the magnetic particles out of the conducting paths. In a preferred embodiment of the invention this is achieved using a radio frequency field.

The term nanostructures includes all structures having a diameter in the range 0.1 to 100 nm or larger, up to tens of, um however the structures . must be small enough to avoid sedimentation due to gravitation when submersed in the dielectric liquid. Such nanostructures can be synthesized by chemical vapour deposition, physical vapour deposition, electrolysis, sol-gel technology or by a reverse micelle colloidal reaction.

According to a preferred embodiment of the invention the magnetic nanostructures comprise at least one constituent of the following : nanoparticies, whiskers, open or closed single-or multi-wall nanotubes, fullerenes, nanospheres, nanocrystals, nanorods, nanorope, nanostrings, nanoribbons, nanowires, nanoropes, nanoribbons or nanofibres.

According to another preferred embodiment of the invention the magnetic nanostructures comprise at least one of the following, a ferromagnetic material such as a metal like nickel, iron, cobalt, a rare earth metal such as neodymium or samarium or a magnetic metal oxide, nitride, carbide or boride.

According to-another preferred embodiment of the invention the magnetic nanostructures comprise a superparamagnetic material such as Pd-Ni nanostructures, magnetic spinel particles of y-Fe203, Fe304and CoFe204.

According to yet another preferred embodiment of the invention materials that are normally ferromagnetic are transformed into a superparamagnetic state by utilising a particular temperature and/or reducing their size.

According to another preferred embodiment of the invention carbon nanotubes are filled with ferromagnetic material. Alternatively the magnetic particles are chemically or physically bonded to the ends of the tube, to aid the tube's alignment in an external magnetic field and also to function as electrical contacts to the tube.

Ferromagnetic materials heated above their Curie temperature lose their ferromagnetic behaviour and become paramagnetic. Pure bulk iron for example is ferromagnetic up to 768°C. Above this temperature iron exhibits only a week magnetisation. This means that if the nanostructures are subjected to a temperature higher than their Curie temperature, due to excess current for example, the magnetic nanostructures lose their ferromagnetic behaviour and move out of the conducting paths. Since the size of the nanostructures directly influences their Curie temperature, sub- micro to nanometers diameter nanostructures having a significantly lower Curie temperature, the Curie temperature may be tuned to the desired temperature by choosing the corresponding size of nanostructure.

In this way the device acts as a fuse when the current reaches unsafe levels. However most fuses work only once as they disintegrate when they are heated above a certain level. According to a preferred embodiment of the invention magnetic material such as iron is enclosed inside carbon nanotubes or chemically or physically bonded to at least one end of the

carbon nanotubes. If excess current heats up the magnetic material inside or attached to the carbon nanotubes above its Curie temperature the nanotubes will move out of the conducting paths and electric contact will be broken between the electrodes. However once the magnetic material cools down the nanotubes containing said material are ready for use again. Even if the excess current causes the magnetic material contained in the carbon nanotubes to melt, once the magnetic material has re-solidified, the device is ready for use again.

According to a further preferred embodiment of the invention the magnetic nanostructures comprise an electrically conducting coating such as gold or another metal, an oxide, a bromide, a nitride, a carbide, an organic coating or a polymer. According to a yet further preferred embodiment of the invention the nanostructures comprise an oxidation resistant coating.

According to another preferred embodiment of the invention the magnetic nanostructures are at least partly coated with a surfactant or dispersing agent, to help the nanostructures defy the force of gravity and prevent spontaneous agglomeration of nanostructures and thus promotes the formation of a stable colloid. According to a yet further embodiment of the invention the liquid comprises additives to protect the nanostructures from degradation by oxidation for example. According to another embodiment of the invention the liquid comprises additives to prevent discharges between the conducting electrodes when contact between the electrodes has been broken.

According to a further embodiment of the invention the dielectric liquid comprises at least one of the following: water, an acid or base, silicone or liquid in a gel state, a liquid hydrocarbon, liquefied gas or an oil such as transformer oil. The viscosity of the dielectric liquid must be chosen so as to allow the nanostructures to be manipulated into conducting paths

immediately on application of a magnetic field and to break up these conducting paths as soon as the magnetic field is de-activated or withdrawn.

The device according to any of the embodiments described above is suitable for use in a circuit breaker, a surge diverter, an electronic or electrical switch, or in an electric motor for example to a replace commutator brush, in either high or low voltage applications.

BRIEF DESCRIPTION OF THE DRAWING The invention will now be described by way of example and with reference to the accompanying drawing in which: figure 1 depicts anisotropic magnetic nanostructures under the influence of a magnetic field. figure 2 is a schematic diagram of a device according to a preferred embodiment of the invention in its conducting position, and figure 3 is a schematic diagram of a device according to a preferred embodiment of the invention in its non-conducting position.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 shows anisotropic magnetic nanostructures 1, for example cobalt nano-rods having a length of about 70nm and a diameter of about 4nm, which are aligned in parallel chains 2. The nanostructures assemble into these chains under the influence of a magnetic field, H. Unlike poles 3 of the nanostructures'magnetic dipoles attract and like poles 4 repel forming

substantially parallel continuous chains that are capable of conducting an electric current. It is possible that conducting paths may form between the substantially parallel continuous chains due to the attraction of unlike poles in different chains however this will not adversely affect the conductivity of the nanostructures between the electrodes.

Figure 2 shows a device for making or breaking contact between two electrodes 5 in its conducting position. The electrodes are attached to external conductors 6. The electrodes 5 are separated by a gap filled with a low viscosity dielectric liquid 7 contained in an electrically insulating chamber 8. The dielectric fluid contains magnetic nanostructures, such as nickel whiskers.

Under the influence of a magnetic field, H, the nanostructures are manipulated into conducting paths 2 so that current can flow between the two electrodes. Two current-carrying coils, 9 and 10, energized by a DC power source for example, are arranged adjacent to the chamber to create a magnetic field in the chamber when energized. Alternatively the coils are incorporated into the electrically insulating chamber walls 8. The device comprises a second pair of coils, 11 and 12, that are de-energized while the device is in its conducting position.

Figure 3 shows the same device in its non-conducting position. In this position coils 9 and 10 are de-energized and coils 11 and 12 are energized. The magnetic field supplied by coils 11 and 12 manipulate the magnetic nanostructures into paths that are substantially parallel to the electrode plates 5. No current is therefore conducted between the electrodes.

The controllable magnetic field means need not be supplied by current- carrying coils as in this example. Movable permanent magnets may be

used with mechanical means to move the magnets towards and away from the dielectric liquid housing in order to apply and withdraw a magnetic field.

While only certain preferred features of the present invention have been illustrated and described, many modifications and changes will be apparent to those skilled in the art. It is therefore to be understood that all such modifications and changes of the present invention fall within the scope of the claims.

For example the device could be used to make or break contact between several electrodes connected in series or in parallel simultaneously, or a chamber containing magnetic nanostructures suspended in a dielectric liquid could be divided into several zones to make or break contact between pairs of electrodes located in each zone independently of electrodes in other zones.