QTC™ Material Science
The Physics of QTC™ Material Operation
Quantum Tunnelling vs Percolation
Conventional composites, consisting of polymers containing carbon have been researched for many years. Carbon particles within these composites usually have a smooth, rounded surface - consequently particles are always in contact with one another creating a constant conduction path. As pressure is applied, more particles come into contact and therefore more conduction pathways build up. This conduction process is known as percolation.
What gives QTC™ Material a different property from percolative composites is that the metal particles are given an irregular structure with a spiked surface which is wetted i.e. electrically insulated by the silicone rubber. The wetting allows the metal particles to get close but not touch even when the QTC™ Material is squeezed or densely loaded. The spikes on the surface allow a higher concentration of electron charge to build up at their tips.
The principle effect of the increased charge on the spikes is to decrease the effective width of the potential barrier in quantum tunnelling, thus reducing the distance and energy required for the electron charge to tunnel through. By this means the tunnelling regime gives a varying conductivity to the QTC™ Material under dynamic conditions.
As QTC™ Material is deformed the conductive metal particles come closer, allowing the material to conduct through the potential barriers.
Principles of Quantum Tunnelling: Classical physics and Quantum mechanics
According to classical physics, an electron will pass through a potential barrier if it possesses enough kinetic energy to overcome the barrier.
If it has less kinetic energy than the height of the potential barrier then it will be unable to pass through the barrier under any conditions.
Quantum mechanics shows that electrons can be described as waves under certain conditions, and a finite probability exists of an electron tunnelling through a classically forbidden barrier due to its wavelike properties.
When a wave meets a potential barrier, the wave does not instantly go to zero, but starts to decay exponentially within the potential barrier. If the wave has not reached zero by the time it has reached the other side of the barrier then there is a finite probability that it will be found on the other side of the barrier - the wave has effectively "tunnelled" through the non-conductive barrier.
The amount of tunnelling that occurs in a composite is determined by the energy of the electrons, as well as the thickness and electrical characteristics of the barrier.
Quantum tunnelling diagram
In QTC™ Material, quantum tunnelling is the dominant conduction mechanism. QTC™ Material works as follows:
For tunnelling conduction to occur, tunnelling probability needs to be high. To increase probability, the width, or apparent width, of the tunnelling barrier needs to be lowered. This is achieved in QTC™ Material due to the shape of the conductive filler particles and their high loading in the barrier material which is a non-conductive, elastomer binder.
Spikes on the conductive filler particles produce a localized increase in the electric field at the tips which effectively reduce the barrier ‘s width and allows conduction to occur. This is known as field-assisted tunnelling.
As QTC™ Material is compressed, the conductive particles are brought closer together and barrier widths reduced further. This leads to an exponential increase in the probability of tunnelling and an exponential decrease in electrical resistance. The ability to vary the width of the barrier through compression, tension or torsion gives QTC™ Material its uniquely controllable electrical properties.
The spikes on the metal filler particles cause localized high electric fields at their tips.
As QTC™ Material is compressed the metal filler particles are brought closer together allowing electrical conduction.
Influence of voltage on QTC™ Material
At applied low voltages, electrons have little energy and are unable to tunnel across the sizeable gaps between metal particles in any significant numbers. Therefore, QTC™ Material in its unstressed state is a near-perfect insulator.
Increasing the voltage however enables the electrons to carry more energy and at a high enough voltage, the probability of tunnelling increases to a significant level allowing a current flow through even an unstressed QTC™ Material. There is a relationship between the voltage applied and the level of conductivity change in the QTC™ Material sample. If the voltage is sufficient, the QTC™ Material will switch into a conductive state which can be maintained even after removal of the initiating voltage. For general switching and sensing, QTC™ Material is operated at voltages lower than 40v – which normally lie outside this switching regime.