Quantum tunnelling composites: Making a switch

There aren't many companies that can lay claim to an entire area of material science. UK company Peratech can. Paul Fanning finds out more.

Wherever you are reading this, take a moment to count the number of switches and items that require electrical switching around you. Whether on telephones, lights, computers, remote controls, monitors or televisions – everything that has a switch. Now imagine they're gone, replaced by a material able to fulfil their function more effectively. This is the future Peratech envisages with its Quantum Tunnelling Composite (QTC).

First produced in 1996, QTC is a composite material that varies its electrical resistance according to the force being applied to it. It is made from metallic or non-metallic filler particles combined with an elastomeric binder, such as silicone rubber. The method of combining these materials results in a composite that exhibits significantly different electrical properties compared to other electrically active materials in that its electrical resistance varies according to surface pressure. What this enables is solid-state, long-life replacements for traditional switches and variable resistors that have none of the drawbacks such as mechanical breakdown or sparking between contact points.

Peratech says its first licensing deal in 2010 with Japanese advanced digital surface technology firm Nissha was worth around $1.4million. Takao Hashimoto, director, chief technology officer and general manager of technology research and development at Nissha made clear the potential of the material, saying: "QTC will be a disruptive technology for mobile phones, enabling thinner phones to be designed with amazing new input interfaces."

This potential was also apparent to Philip Taysom, Peratech's joint chief executive, when he first encountered it in 2006. He says: "When I first saw Peratech and its technology, I realised it was the most revolutionary thing I'd seen in my entire career."

This 'revolutionary thing' had its origins in 1996, when former RAF Specialist Officer David Lussey was trying out different formulas to make a conductive adhesive. He says: "As I was dismantling one particular experiment I noticed that that the resistance dropped dramatically when I attempted to pull apart two metal strips bound together with one particular formulation. I was intrigued with this counter-intuitive resistance change and investigated the material further. Putting metal particles into a polymer to make a conductive material has been known for years, but I had mixed the metal particles and the adhesive binder in a polythene mortar and pestle which imparted low shear forces to the mix. The result was QTC and its unique properties."

QTC is made from a polymer that has nanoscale conductive particles, each with a 'spiky' surface, evenly distributed throughout. The spikes do slightly touch, but when the material has a force applied, the spikes move closer together and a quantum effect occurs as electrons begin to 'leap' or 'tunnel' from one spike tip to the next. Current then flows until the pressure is removed. Thus QTC provides a change in resistance that is proportional to the pressure applied, from almost infinite resistance when no force is applied to almost zero when pressed.

Initially, says Lussey, QTC was a solution looking for a problem to solve, with the first few years of the company's life being spent investigating the material and finding ways to make it reliably. The other facet the company investigated was how changes to the polymer, the conductive material and the size or shape of particles resulted in very different performance characteristics.

For example, the overall resistance range can be chosen, and the sensitivity can range from being so sensitive that a thin film of QTC can act as a microphone, to being so insensitive that it takes the weight of a tank to activate it. It can also respond with a smooth variable change in resistance in the same manner that a conventional variable resistor performs, or have a threshold pressure where it changes from a very high resistance to virtually nothing; just like a switch. Once the pressure is removed, the resistance returns to what it was originally. This cycle can be reliably repeated time and again as there are no mechanical parts to wear out. Also, the anisotropic properties of QTC can be controlled to impart unique functionality to white boards and touchscreens.

A key feature of a QTC pressure sensor is that there is no air gap, so there is no possibility of a contaminant getting between two contact points. Similarly, as there is no make and break between contact points, there is no possibility of arcing, making it interference free and safe for potentially explosive environments.

The search for highly-profitable, high-volume applications has obviously been a key mission for Peratech since its foundation. Early applications were interesting, but often only small-volume. One such example was in clothing, where controls for iPods and similar devices could be integrated into a jacket. Effectively a flexible, textile, solid-state switch, it could be washed and dry cleaned, crimped and stretched without any adverse effect.

Some of the other applications of QTC give an idea of the versatility of the material. As QTC is effectively the next stage in the evolution of the switch, almost any application where a switch is used could have a QTC replacement. Some of the more unusual applications include NASA using QTC for the fingertips of a robotic astronaut, MIT using QTC to create a touch-sensitive skin to cover a robot (see box item), and automotive manufacturers for a variety of in car uses such as QTC sensors in seats to provide not only information that a person is in a seat but also how much they weigh so that an airbag can be deployed appropriately.

Perhaps the most unusual application, however, is the QTC 'nose'. The material changes its resistance when a force is applied and, in this case, swells when exposed to Volatile Organic Compounds (VOCs). Peratech's sensor uses a granular form of­­ QTC that provides a high surface area for absorption, enabling it to detect levels below 100ppm. The large surface area also means that it can rapidly reset itself once the VOCs have gone from the surrounding atmosphere.

These are obviously fascinating applications, but the company's real breakthrough came when it approached mobile phone manufacturers. The growing popularity of smartphones created a problem in that the Human Machine Interface (HMI) was not keeping up with advances in features.

Many phones use a small joystick-like device to navigate a cursor around the screen. However, this is rather crude, having four switches for up, down, left and right that move the cursor in the required direction at one speed. By replacing the on/off switches with QTC sensors, a variable response is possible. The harder one pushes, the faster the cursor moves, or the faster it is possible to scroll through a list of contacts. Peratech designed a QTC-based navigation keypad that was licensed by several phone manufacturers including Samsung Electro-mechanics in 2010 and is commercially available in a number of mobile phones.

This novel feature of being able to have a variable response that depended on the amount of pressure applied opens up ways of interacting with the phone by effectively giving a third dimension of input. This really excited mobile phone manufacturers, who are always looking for ways to make their phones different with better features. However, mobile phone technology evolves very quickly and more and more phones have touchscreens for input. The challenge is now how to bring this third dimension of input to a touchscreen when QTC is a black or grey opaque material.

Peratech's solution was to print a small set of QTC dots 10-20µm thick around the periphery of the screen. When pressure is applied to the screen, the dots are compressed against the phone case, enabling this third dimension of input to be achieved. This solution was licensed to Nissha, one of the largest manufacturers of touchscreens in the world, to supply mobile phone manufacturers and mobile device OEMs.

This technology evolves rapidly, however. Says Lussey: "Since the original discovery of Quantum Tunnelling Composites, Peratech has continually researched this rich area of material science. From the original 'bulk' elastomeric QTC materials, we have extended our range of QTC materials to include QTC Inks – emulsive materials that are applied by silk screen process."

The most recent development of QTC functionality comes with QTC Clear, a transparent version of QTC Inks with a huge potential to revolutionise the rapidly-growing touchscreen industry. The company wanted a way to have QTC over the whole screen to give not only pressure sensitivity, but also the co-ordinates of where that pressure is being applied. Peratech realised that the solution was to use the structure of the resistive touchscreen, but replace the air gap and spacer bumps with an ultra-thin layer of QTC.

The challenge was to create a version of QTC that would work at only 6-8µm thick and at a very low density of particulates. Peratech finally created the right combination of polymer and metal particle size and the resulting film of QTC Clear, as it is called, is almost transparent such that the resulting touchscreen is on a par with existing technologies. Thus, the structure for the new QTC touchscreen is a layer of QTC sandwiched between two layers of conductive material such as silver nano wires or ITO (Indium Tin Oxide), which is in turn sandwiched between two transparent sheets of plastic or thin glass.

The top layer can be more rigid than those materials traditionally used because the QTC can detect tiny deflections of a couple of microns from as little as 5 grams and also detect multiple touches on the surface. A harder front surface, such as a sheet of thin glass, also means the problems resistive technology has with scratches are greatly reduced and screens can be produced that have compound curves. QTC's unique properties mean that virtually no current flows unless a force is applied, which is important for battery-operated devices and overcomes the drawbacks of capacitive designs that constantly draw current and thereby create design challenges.

QTC touchscreens also offers high levels of resolution because, as the material gives a proportional response to touch, the responses from adjacent intersections on an 'x' and 'y' matrix can be interpolated to provide a highly accurate position. For example, using a 10-bit sampling of the resistance gives 1024 levels of resistance change for the 'x' co-ordinate and for the 'y' co-ordinate.

A pressure map of this process shows a finger pad pressing down has a centre point of pressure with decreasing pressure as you move away from the centre point. This accuracy enables QTC touchscreens to provide multi-touch capabilities that are greater than capacitive screen in that there is no limit to the number of simultaneous finger touches that can be detected. This opens up the possibility of greatly extending the current library of fingertip gesture controls.

As QTC's applications grow, so does the number of its licences and the number of patents associated with it. Already, the company has been the recipient of more than 40 awards for its technology. As that innovation is increasingly translated into large-scale, high-volume applications, it seems clear that this is a material technology with the capacity to change design as we know it.

Paul Fanning

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This article is excellent, and I commend the author, Paul Fanning for it.

Comment John Addo, 11/02/2014

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