quantum tunnelling composite

qtc switch
A simple switch made with QTC sheet.

Quantum tunnelling composite (QTC) is a smart flexible polymer, with extraordinary electrical properties, used for pressure switching and sensing. In its normal state it’s a near-perfect electrical insulator, but when deformed QTC becomes a metal-like conductor capable of passing very high currents. In fact, a QTC button measuring 4mm square and 1.5mm thick can pass up to 10 amps when squeezed! Also, the change from insulator to conductor is dramatic and can be obtained with only the tiniest pressure.

Oh, and did I mention it’s inexpensive too?

-> Update: I recently realized that QTC pills are magnetic too. Nothing like having some magnets laying around on the table while you work :)

What is it exactly?
Unlike carbon loaded polymers, such as resistive foam, that require a lot more pressure and conduct minute currents through a percolation process (the effect of carbon particles touching within the polymer structure), QTC is made of metal filler particles combined with an elastomeric binder, typically silicone rubber. Instead of percolation this material owes its extraordinary properties to a quantum tunneling phenomenon: electrons tunnel through the material, i.e. conduct, when their physical structure is slightly changed by pressure.

QTC usually comes in the form of pills or sheet, but I’ve also encountered references to cable, ink/coating, and granule. QTC pills are just tiny little pieces of the material. The sheets are composed of one layer of QTC, one layer of a conductive material, and a third layer of a plastic insulator. While QTC sheets switch quickly between high and low resistances, QTC pills are pressure sensitive variable resistors.

What for?
– Touch switches (sheet)
– Force/pressure sensors (pills)
– Motor speed control using force (pills)

The small size of QTC pills and their rubbery nature make them particularly well suited for soft circuits. Some interesting applications include a Double Sided Fabric Keyboard and an FM Radio w/ Textile Controls.

How?
:: Making a pressure/force sensor with a QTC pill
The video below shows how to use a 4x4x1.5mm QTC button (in this case 3 connected buttons, since one was too small to be visible on the video) as a pressure sensor. A QTC button is placed on a piece of copper sheet, though it could be another conductive material, which in turn is connected to 9V. The DC motor is connected to ground and an alligator clip that is used to press the QTC button, closing the circuit.


Apologies for the awful video quality :( I need to get some new equipment.

:: Making a simple switch out of QTC sheet
Simply place your piece of QTC sheet, with the plastic side facing up, across two pieces of conductive material (see illustration above). This is your button. You can hold it in place with some tape, but don’t forget that this is a very sensitive material and too much pressure exerted by the tape will cause the switch to be on at all times. If you need to put anything else on top of it, such as a label, add a piece of foam in between the button and the label. A 3K3 resistor will produce a switch that requires moderate force and will not be triggered by light pressure.

:: Making QTC sheets from pills
The illustration above shows the 3 layers of QTC sheet. Start with a bottom layer of some insulating plastic. Glue a sheet of a conductive material on top. It doesn’t matter which or how much glue you use to attach the polymer to the conductive material, the plastic is used here only as a ‘cushion’ and insulator between the hand pressing the switch and the current. I use conductive adhesive copper track with the adhesive part facing up and superglue on the back. And then place the QTC pill or pills, depending on how large a surface you need, onto the adhesive of the copper track. If you use more than one pill they’ll have to be connected to each other.

Resources
About QTC Technology @ Peratech
QTC Applications @ Peratech

Products & Supplier
I’ve only been able to find this material at MUTR Teaching Resources (UK). They supply:
QTC Pills
QTC Sheet Holders
QTC Testing Kit
QTC CDRom
Science of QTC Booklet

Share your knowledge
If you’d like to contribute content or corrections regarding QTC, please use the comment form below or add them directly to the openMaterials wiki:
materials/pressure sensitive/quantum tunneling composite

>> about the materials 101 series.



3 responses to “quantum tunnelling composite”

  1. Alex Sinn says:

    Hello

    Want to buy some QTC Pills in stripes.

    How can I order it and what is the price ?

    Shippingcosts to Germany ?

    Best regards Xandre

  2. catarina says:

    Hi Xandre. Open Materials is strictly an educational website, we don’t sell materials. Best approach is to contact the manufacturer or retailer directly.

  3. […] Quantum tunneling composites are rubber-like polymers filled with metal fibers that in their normal state are too far apart to conduct a current. But when compacted, the fibers come into contact, conducting the electrons in a phenomenon called quantum tunneling. This response to pressure makes quantum tunneling composites useful for keyboards and touch screens. […]

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