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Author Topic: QTC - quantum tunneling composite. Can we make our own sensors?  (Read 1366 times)

OddBot

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QTC - quantum tunneling composite. Can we make our own sensors?
« on: October 07, 2018, 12:53:49 AM »
If you are unfamiliar with QTC then read this first: https://en.wikipedia.org/wiki/Quantum_tunnelling_composite

I am working on a robot hand and want to add force sensors to measure how tight the hand is gripping an object. Originally I bought some strain gauges and HX711 breakout boards. The HX711 chip is a 24 bit ADC with additional circuitry designed specifically for measuring strain gauges.

Unfortunately it is all too big to fit in a robot hand. Especially when you want at least 1 sensor for each finger and at least 2 for the thumb.
QTC pills were the obvious solution however they are expensive and very hard to find these days. Also their shape was not what I wanted.
I want to make a washer that will fit on a thrust bearing to measure the force on the bearing.



I did try to contact the company that makes them (Peratech) but I got no reply.

While trying to find the QTC pills I came across this article: http://practicalphysics.org/qtc-%e2%80%93-discovery-novel-material.html

"QTC was discovered by David Lussey, working in Darlington (UK). At the time, he was trying to make a conductive adhesive for use in a security system. Computers would be attached by a wire to an alarm; the glue joining the wire to the computer would be conducting, so that if the wire was detached, the alarm would sound.
 
David Lussey describes himself and what he was trying to achieve:
 
I’m not a scientist but I am a practical person with a technical background from the military. When I needed a conductive adhesive and found there wasn’t one available, I decided to make one.
 
To make a conducting adhesive, David mixed metal powders with adhesives in different combinations. One turned out to be very special. When two metal plates were glued together, they did not conduct – the glue between them acted as an insulator. However, when he tried to pull the plates apart, they started to conduct.
 
This was very strange and not what I was looking for. So I put that on one side (in fact I threw it on one side!) and it wasn’t until some little while later that I thought, ‘Well, that was a strange reaction.’ I went back and measured it with a meter and found I got something very unusual.
 
It was not obvious at this stage that the material had commercial possibilities; nor did David understand how the material worked to produce this strange behaviour."


After reading this I decided it might be worth trying to make my own QTC sensors. The article states that:

"QTC is a material made from particles of a metal (nickel) embedded in a polymer. Its resistance changes dramatically when it is compressed. Uncompressed, it is an almost perfect electrical insulator. When a force is applied, it conducts as well as a metal."

I have bought 3 different types of nickel powder - pulverized, ultra fine and electrolytic.
For the polymer I bought some silicone gasket sealant. From what I have read I don't think it matters too much what you use as the "polymer" too much. It is simply an insulator that separates the nickel particles.



I chose silicone gasket sealant because it is rubbery and compresses easily. I suspect that David Lussey may have also uses a silicone rubber glue because it sticks to glass as well as most other materials and does not crack with age. Remember he was trying to make a conductive glue for security systems.

I will need to experiment with different percentages of nickel powder and silicone so I bought some small electronic scales with a resolution of 0.01g to measure the amounts I use. I am just waiting for some small disposable plastic cups to arrive which I will use to mix different batches in.



I will update this post once my first test results are it.



I finally got around to mixing up some batches yesterday and tested them this morning. SUCCESS!

I measured out some different amounts of each powder and mixed them into the silicone rubber gasket glue. Although the mixtures had varying amounts of nickel powder and different types of nickel powder they all worked! As pressure increase, the resistance gradually went from open circuit down to just a few ohms.



As the electrolytic nickel powder is the cheapest and the finest I will do some more experiments to see exactly how much nickel powder we need for the QTC to work.

All my mixtures had more than 50% nickel powder to silicone (measured by weight, not volume).
When the silicone set, all my samples were very hard with little flexibility.
I want to try and create a softer, more flexible mixture with less powder.



After some more experimenting I found that 7g of electrolytic nickel powder to 8g of silicone was the lowest I could go with the nickel powder. Below that, The mixture would not conduct electricity at all. After a bit more research I found out about the "percolation threshold".
https://en.wikipedia.org/wiki/Percolation_threshold

Based on my results so far I think anyone should be able to make their own QTC pills. The nickel powder is easy to buy on the internet and most hardware stores sell silicone rubber sealant of some sort. You will need to experiment a bit to find the percolation threshold depending on the type of nickel powder you buy and polymer used. You do not need to use silicone rubber glue.

Molds:

I found my QTC mixture did not stick to nylon very well so I machined a mold from PA66. I suspect Other plastics could be used so it's possible you could use a 3D printer to make molds. My QTC mixture is not very strong so I found it was easy to damage the parts when digging them out of a 1 piece mold.

I then made a 2 piece mold which made it much easier to remove the parts without damage. The design of the mold will depend on the shape you want. I wanted a doughnut shape piece of QTC so my 2 part mold allowed me to remove the center of the mold first. I could then poke the parts out easily without damaging them.


Good luck and enjoy!



I bought the materials on TaoBao. If you live in China then these links might be useful.

Nickel powder:
https://item.taobao.com/item.htm?spm=a1z0d.7625083.1998302264.6.686c4e69CZHf4N&id=566854304530

Black Silicone:
https://item.taobao.com/item.htm?spm=2013.1.20141001.4.35c14b75XcEKCc&id=560580647699&scm=1007.12144.81309.42296_0&pvid=6cfba43b-4eec-47ca-b9c9-c4cf4a9bf21a&utparam=%7B%22x_hestia_source%22%3A%2242296%22%2C%22x_object_type%22%3A%22item%22%2C%22x_mt%22%3A0
« Last Edit: October 24, 2018, 07:41:43 AM by OddBot »

Bajdi

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #1 on: October 07, 2018, 04:10:20 AM »
Interesting stuff. Some time ago I fitted a gripper (on the service droid) with a pressure sensor. Was difficult to get good consistent results.

OddBot

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #2 on: October 07, 2018, 04:28:33 AM »
Yes it is hard to get consistent results with pressure sensors although I read that NASA used QTC for their robonaught.
https://news.thomasnet.com/companystory/peratech-presented-with-award-by-nasa-for-its-qtc-touch-technology-623451

I believe QTC gives more consistent results than something like conductive foam or thin film pressure sensors. I had some once but never got around to testing it for repeatable results.
« Last Edit: October 07, 2018, 04:33:47 AM by OddBot »

MEgg

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #3 on: October 08, 2018, 03:53:25 PM »
Interesting stuff.
Do I see that correct, that the layers have to be very thin?

Also read this one:
https://www.electronicdesign.com/embedded-revolution/11-myths-about-qtc-touch-sensors
and this:
https://www.peratech.com/qtc-single-point-sensors/
https://www.peratech.com/touch-development-kit.html

I am trying to get their development kit (hopefully for free).
:-)

You probably already found these:
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5621037/
https://www.dur.ac.uk/cmp/themes/opto-semi/psm/research/
« Last Edit: October 08, 2018, 04:07:58 PM by MEgg »
1st project: Dagu 5 Rover + Dagu - 4 Channel DC Motor + Red Back Spider robot controller + Raspberry B+
Chassis + wheels: https://picload.org/image/dggroior/20150831_028.jpg
current: https://www.keepandshare.com/userpics/m/a/r/k/usegg/2016-04/sb/img_3480-79682018.jpg

OddBot

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #4 on: October 10, 2018, 12:32:20 PM »
G'day MEgg,

Thanks for the links, I'll check them out. As far as hurdles go I can see a few.

1. The mixture will need to be very consistent otherwise I will get patches that short out or remain insulating.
2. The ratio of polymer to metal powder probably needs to be very precise.
3. The size of the metal particles will need to be very uniform.


OddBot

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #5 on: October 10, 2018, 12:47:01 PM »
@MEgg, your last link was helpful as it confirmed that silicone rubber was the polymer used in the original discovery of QTC.

David Lussey's composite, loaded above the percolation threshold with Ni powder, should have been conductive but was insulating until deformed. It became conductive when not only when compressed but also when stretched or bent. Our studies revealed that the silicone elastomer matrix intimately coated the Ni particles, which retained sharp surface features with nanometre dimensions1.

MEgg

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #6 on: October 23, 2018, 04:15:47 PM »
Hello Oddbot,
can you post a bit more e.g. sources where you bought your parts?
Where did you buy the powder, where the silicon?

Did you simply mix both or use some "special" way to make the mixture?

Thanks.
1st project: Dagu 5 Rover + Dagu - 4 Channel DC Motor + Red Back Spider robot controller + Raspberry B+
Chassis + wheels: https://picload.org/image/dggroior/20150831_028.jpg
current: https://www.keepandshare.com/userpics/m/a/r/k/usegg/2016-04/sb/img_3480-79682018.jpg

OddBot

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #7 on: October 24, 2018, 07:46:02 AM »
I have posted the material links but they probably won't be helpful unless you live in China.
There is no special method of mixing. Just put ingredients in a cup and mix thoroughly with a stick (bamboo chopsticks work well).

Gareth

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #8 on: October 27, 2018, 11:06:08 AM »
This sounds a pretty easy way to make squashable sensors.... Nice one OB1.
I did not know you could get zinc powder.....
Also off the bat  maybe graphite powder would work as you can get it in very small particles size (it is used a lubricant when added to grease).
I have used conductive ink before which was only graphite/water & gum arabic (as a binder).

I have tried making 3D printed molds for silicon (i.e. the bath sealant viriety) out of PLA, however the silicon sticks like glue to it, could not get a clean demold... I even tried oiling the mold before hand (nada). So maybe it would have to be printed with Nylon as you have pointed out in your above description.

OddBot

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Re: QTC - quantum tunneling composite. Can we make our own sensors?
« Reply #9 on: October 27, 2018, 02:14:52 PM »
G'day Big G,

I did experiment with graphite powder once before (with polymorph, not silicone). It worked like a variable resistor but the resistance was very high.
From the research I have done, the quantum tunneling effect depends on the "spikey" shape of the nickel particles.

I would still be interested in any results you get.

I suspect if you mixed the nickel powder with TPU or TPE then you could make QTC filament for 3D printers. The hard part would be to ensure you had a consistent mix throughout the entire filament.

I have read about other filaments using metal powder.
« Last Edit: October 27, 2018, 02:24:39 PM by OddBot »

 

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