conductive polymeric composites and the quantum tunneling...

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NANOLAB – Educational Nanoscience- www.nanolab.unimore.it

Conductive polymeric composites and the Quantum Tunneling Composite

The boundary between electrical insulators and conductors is not so clear-cut as it is traditionally thought of. Today, thanks to modern technologies, the conductivity range is extremely varied. Research brings towards lighter, less expensive and more versatile conductive materials. Actually you can now talk of glass, ceramics, polymers and polymeric composites which are all conductive. Polymers are extremely interesting and feature new and exciting applications ranging from optoelectronics to pressure sensors for embedding into synthetic skin.

Traditional conductive plastics are based on percolation. QTC© on the other hand exploits quantum tunneling phenomena and is able to modulate its own electrical resistance within a wide range of values covering up to 14 power orders.

Version 20/06/2013

All NANOLAB materials, included this guide, are property of NANOLAB (www.nanolab.unimore.it) authors and distributed under Creative Commons 3.0 licence.

http://www.nanolab.unimore.it/it/?page_id=4398

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Contents

Contents ............................................................................................................................................... 2

Notes .................................................................................................................................................... 3

INTRODUCTION .................................................................................................................................... 4

1 Linking to the curriculum .......................................................................................................... 4

2 Module Guide............................................................................................................................ 4

EXPERIMENTS ....................................................................................................................................... 5

1 –QTC: from perfect insulator to extremely good conductor ....................................... 5

2 –Piezoresistive materials and pressure sensors ............................................................... 15

Resources ........................................................................................................................................... 17

Finding materials and equipment ……………………………………………………………………………………………… 17

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Notes

This Teacher Guide describes the experiments on conductive polymeric composites and QTC which you can find on Nanolab website www.nanolab.unimore.it/en/ at this page

Home > Labs > Conductive Polymers

In addition to the detailed description of both the set up and the actual experiment implementation, the guide offers examples on how and where to link the experimental protocols to high school science curricula, links to background materials, tips on buying samples or any other equipment which may not be ordinary in school labs. Activities are numbered (1, 2, …) and match the corresponding experiments at www.nanolab.unimore.it/en/

On the website www.nanolab.unimore.it/en/, you can also find video guides, student lab sheets, presentations for classroom use plus an extensive collection of background reading for teachers. All of them can be downloaded.

In this guide the following symbols are used.

Qualitative demonstration. These experiments are particularly easy and need very little and simple equipment. They are suitable to be used in the classroom also outside the lab.

Quantitative experiment. These experiments involve data acquisition. The number of Erlenmeyer flasks states the difficulty level.

Safety tips, either regarding people or equipment (tools, samples).

Technical notes: technical tips, suggestions on possible alternative implementations.

Didactical notes. Teaching tips and didactical analysis

QR codes make the lab videoguide page, or alternatively videos of data sampling, accessible through tablets and smartphones

http://www.nanolab.unimore.it/

http://www.nanolab.unimore.it/en

http://www.nanolab.unimore.it/en/?page_id=470



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INTRODUCTION

1 Linking to the curriculum

The activities on the conductive polymeric composites are particularly suited to be linked to traditional curricular topics supplementing and completing them in a new perspective, namely matter studied at the nanoscale, opening the way to interesting interdisciplinary connections between chemistry, physics and biology .

Below you will find suggestions on possible didactical paths

a) Conductors and insulators, charge transport mechanisms - the boundary line between electric insulators and conductors is much more subtle than students usually think. Thanks to modern technologies the range of conductivity is nowadays extremely varied, sometimes even within the same material. The experimental investigation of different charge transport mechanisms integrates and completes the analysis of traditional conductivity in metals.

b) Quantum tunneling effect - QTC© (Quantum Tunneling Composite) is a new material exploiting electron quantum tunneling for charge transport. The study of electrical resistance versus applied pressure and the following analysis of IV characteristic curves offers the opportunity to see quantum physics at work and explore in a hands-on approach one of the basic principles of quantum theory. The experimental data, easily collected within ordinary school labs, are in fact consistent with the hypothesis of a conductive mechanism based on quantum tunneling.

c) Pressure sensors - Measuring instruments, their properties and calibration process are a topic typically dealt with at the beginning of any Physics course. Investigating the properties of good sensors (accuracy, precision, promptness, dynamic range, reliability) and subsequently designing and testing one, integrates such a topic in a strongly operative context with a potentially very high impact on student motivation thanks also to the connections with cutting edge research and driving technology. Piezoresistive materials are particularly interesting as they envision possible applications as pressure sensors integrated either in “touch screens” of the newest generation electronic devices or within artificial skin. The issue regarding sensors and neural connections integration in prosthetic limbs is particularly appealing for Life Science lessons.

2 Module Guide

The topic can be easily introduced through some videoclips1 on prosthetic limbs and robotic hands. This approach is particularly suitable for first years students. The motivational impact is high: students curiosity is strongly stimulated by a ‘real’ issue to which their investigations will finally try to find an answer.

The first question is how tactile integrated sensors may transform into an electric signal (namely the resistance R) the pressure exercised by fingers in grip control. Different piezoresistive

1 Such as “The magic touch” at www.youris.com/Nano/NANOTV . DVD available on request.

http://www.youris.com/Nano/NANOTV

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materials2, among which QTC©, are tested. From the experimental data (resistance VS applied mass) a theoretical model describing the response of each type of sample is found. The different charge transport mechanisms at the base of the different materials are studied with a focus on consistency between theory and experimental data.

For a general introduction on polymers and conductive polymeric composites, particularly QTC, see

Home > Labs > Conductive Polymers > Background reading

If students are already acknowledged with metals conductive mechanism and are familiar with Ohm’s law it may be interesting to have them plot I-V characteristic curves of the samples at different applied pressure. The QTC© graph is definitively anomalous and to a careful inspection gives further support to the hypothesis of electron quantum tunneling as main charge transport mechanism (see background reading). However this is not an elementary task and it’s highly suggested for last year students followed by a guided discussion led by the teacher.

Testing good sensors properties and calibration activities on the different samples is on the contrary suitable also for young students. Afterwards pupils are shown quite a number of applications for such piezoresistive materials and, provided there’s enough time, divided in groups will devise innovative possible applications for the tested materials. Finally they will implement one of such applications. Time requirements: Between 4 and 5 lessons: 10 ‘ for watching videos; 60' for the experimental investigation of resistance versus applied mass; 60’ to plot I-V curves; 60’ -120’ to test sensor properties. It’s almost impossible to predict the exact amount of time needed to design and implement a sensor application since it all depends on the class final goals and, of course, specific time constraints. Students are supposed to work on their own projects outside curricular hours too.

EXPERIMENTS

1 –QTC: from perfect insulator to extremely good conductor

Lab goals

Investigate the behavior of piezoresistive materials based on different charge transport mechanisms.

Introduce electron quantum tunneling with a hands-on approach

What’s to be observed

Investigating resistance VS pressure, you may observe in QTC©, differently from other

2 If you have time you can extend the analysis of piezoresistive plastic such as Velostat by 3M, to the new generation

of nanotech piezoresistive textiles, such as Eontex by Eonyx, produced thanks to thin film coating technology. Conductive polymers coat the single fibers .




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piezoresistive materials, an exponential curve. This is coherent with the hypothesis of quantum tunneling charge transport within the polymer (‘assisted ‘ quantum tunneling).

The hypothesis is further supported by comparing and contrasting I-V (Current-Voltage) curves. At intermediate pressure, QTC© exhibits a clearly not linear hysteretic behavior, negative resistance regions and current fluctuations. These characteristic features can be related to QTC© specific structure and electron quantum tunneling.

Equipment (for one working group only)

QTC© pill 3, Velostat3, Eontex3 2 copper strips 4

scale (± 0,1 gr) multimeter (as ohmmeter)

sellotape

Lab masses or sand (3 Kg)

Link to the videoguide

Read the QR code on the right or go to the page

Home > Labs > Conductive Polymers > 1 – Quantum Tunneling Composite (QTC©): from perfect insulator to extremely efficient conductor > Videoguide

Background reading


Experimental protocol

N.B. QTC© can be purchased in “pills” about 4 mm2 in size3. This is why, to make comparing and contrasting easier, all materials have been studied through samples of the same dimensions although some of them can be easily purchased in larger sheets and are commonly used in applications in pieces of a few cm2.

A – Connecting samples into the circuit

To connect the sample into the circuit, sandwich it between two thin metal strips (“embossing copper” sheets used at school in Arts and Crafts do work fine. The sample should make contact with the silvery side!). Fix them with tape directly on the table or on any other hard and stiff electrically insulating surface (cardboard is ok too). Careful! The two strips must not be touching in any way: current should

flow from one to the other only through the sample!

Building the electrodes with cardboard and copper strips

3 See in Finding materials and equipment.

4Any other metal is ok. The strips should not be larger than the QTC© pill while they can be as long as you like :

approx. 0.4 cm X 10 cm strips are ok .

a

b









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B – The circuit

Fix horizontally a lab bar ending with a pair of tongs (2) to a lab post (1) with a double screw clamp (3). Tighten the tongs around a hollow cylinder. The stem of the mass holder plate (4) should be able to slide smoothly through the hole. Position the copper sandwiched sample under the

point of the stem. Carefully center the sample. Connect the two copper strips to a multimeter in ohmmeter modality. The measured resistance is in the Mohms range. Check that the contacts are working properly by gently pressing on the plate with your hand: resistance should undergo a dramatic drop (particularly with QTC©).

a

b

a)Materials; b) Copper strips and cardboard pieces: dimensions. hg

c)-h) Assembling. (i)(ii) Possible contact arrangement: red: copper; black: QTC. The arrow shows the force direction.

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You may use plates taken from other lab equipment: similar specimen are sometimes found in optics kits; alternatives are Volta electrophorus disks or any plate with a stem, no matter the plate shape. The ending tip of the stem should be small enough without being really sharp. The higher the pressure on the sample and the greater the sensitivity allowing to record and acknowledge very small masses (this is particularly true of QTC©)

C- Resistance versus pressure

To increase pressure you can use either lab masses or a beaker progressively filled with either sand or water (add a constant quantity each time). At first, when the exerted pressure is still low, resistance may fluctuate and reach a fixed value only after a while. This is why, when adding a new mass, it’s extremely important to have all the readings after the same time has lapsed (say 0,5 or 1

sec). Repeat the experiment with different conductive polymers and compare results.

The first tested material is Velostat, a traditional piezoresistive material based on percolation. Similar measurements are subsequently taken with QTC©, a new material whose conductive mechanism is based on electron quantum tunneling. Lastly you may

want to test Eontex, a new piezoresistive textile whose conductivity is due to the fact that its fibers are uniformly covered with polypyrrole (PPY) an intrinsically conductive polymer, with a “thin film coating technology”.

Details of the experimental apparatus setup.

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In the picture you can see all the necessary materials (left) and the already set up apparatus (right). The easiest way consists in measuring R directly with a multimeter in ohmmeter mode. Otherwise you’ll need a power pack and a multimeter to read either I or V (one of the two values is read on the multimeter, the other one from the power

pack display) and finally calculate R=V/I. We like the first way best since, owing to its simplicity, it can be implemented also in labs with very little equipment. It’s actually possible to reach a wide compression range (from 50 g to approximately 3Kgs). Unfortunately one of the drawbacks is that it’s not possible to monitor the effect of varying voltage on the Resistance VS Pressure curve. However this goal is quite an advanced one and may be pursued later or with senior students in the context of a more detailed and in-depth investigation.

On comparing Velostat with QTC© the limited range of resistance values of the first material shows immediately. Velostat initial resistance is high and even upon applying 3 Kgs it doesn’t drop beyond one thousand Ohm (this is even more true for Eontex ). QTC© has an initially very high resistance which drops dramatically to less than 1 ohm under the same range of masses used for Velostat.

Moreover QTC© is much more sensitive to tiny variations in mass. Its exponential behaviour may not come out so clearly when operating with a low mass sensitvity. This is the reason why sand is better than usual lab masses, since the values can be finely modulated. Sand is extremely useful particularly in the initial and steeper part of the graph to plot it in more detail and to evaluate QTC©sensitivity at minimal pressure variations. From a didactical point of view it’s preferable that students work first with lab masses and only afterwards with sand to refine measurements.

D – Searching for the best fitting model

Once you have collected data, search for the theoretical model that fits the resistance VS pressure data best. Repeat the procedure for each sample and compare results. You can make use of the best-fit curves functionality in the electronic spreadsheet.

Owing to the wide range of R in the different materials it’s not easy to plot the three graphs all together unless you revert to semi logarithmic scale. Therefore the graphs will be first analyzed separately. For each of them different best fit curves will be tested,

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among them the exponential one.

First of all it is possible to show to students that the three sets of data are very similar and only a more in-depht analysis comes out better and more easily defined. With just a few experimental data (approximatively ten) of resistance vs applied mass Velostat and Eontex behavior already clearly shows that the best fitting curve is not exponential (third grade polynomial and power curve). QTC© behavior with so little data is unclear: exponential and power curve are almost overlapping. With more data the exponential nature comes out more clearly even if the difference with the power curve is not as striking as with the other two materials. On the right a picture of Velostat data: the blue line (power curve) fits much better than the red one (exponential). Plus it is possible once again to appreciate the relatively limited range of values if compared with QTC©.

In case you want to pursue the topic further with students and not just have them use the dedicated spreadsheet tool automatically generating the best fit curves, you can remind them that linear laws are expected whenever arithmetic progressions are transformed into arithmetic progressions whereas exponential laws have to be expected when arithmetic progressions are

transformed into geometric ones. All this can be tested through the spreadsheet. Such an activity is accessible even to first years students.

Even more sophisticated and definitively beyond our scope is the issue regarding how to check whether the best-fit curve is either a polynomial or a power curve (the last one transforms geometric progressions in geometric progressions therefore in order to test this it would be far better to apply masses which increase geometrically rather than with a constant quantity as we actually did). This topic will be partially tackled in the second activity about characterization and calibration of pressure sensors where it will be shown that the theoretical model chosen as locally best fitting can be totally inadequate in extrapolation (e.g. a fourth grade polynomial can fit

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locally very well the data set but at both ends the curve goes to infinity with the same sign so it may not be a good choice for the description of a decreasing trend).

Advanced - If students are already acknowledged with logarithms the above issue may be solved by introducing data representation in log form. In the pictures below you have the regular representations of QTC© and Velostat data (left), the semilogarithmic representation of both sets in the same plane: QTC© appears linear, mostly at the beginning, a clear sign of the original exponential trend (centre). Velostat data set however exhibits a linear trend when plotted in log log format: this proves the original power trend (right).

On the other hand it may be simpler and more effective to compare conductance C=1/R (Ω-1) graphs, rather than resistance ones. The anomalous behavior of QTC© stands out even better. While Velostat and EonTex conductance is acceptably linear, QTC© one is once again an exponential however an increasing one (this confirms the mathematical rule that the inverse law of a decreasing exponential is still an exponential though an increasing one) In black, red and green respectively the best-fit linear, exponential and power curves.

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During the experiment what immediately strikes the observer is that R takes quite a bit to settle. Students are asked to decide the time interval between mass positioning and ohmmeter reading. They are absolutely free to choose whatever interval they do think best but then they are asked to stick to this very slot throughout the whole measuring process. For further research they can be asked to investigate how the choice of a particular time slot may impact data collection. In the right graph above data corresponding to blue dots have been read immediately after positioning the mass, while the red ones have been taken after 60 secs. Both curves are clearly exponentials.

The experimental apparatus is extremely sensitive to even the slightest collision or external movement. Avoid leaning against the desk during data collection and possibly use a very stable desk. Even sudden current peaks or drops may influence results: avoid turning on and off other electrical gear and do not connect or disconnect multimeters in the circuit while measuring since it all may bring to sudden and not immediately reversible changes in QTC resistance due to voltage drops.

In positioning masses on the plate try to put them symmetrically centered (see fig.1) and keep the plate perfectly parallel to the table so that the stem tip will exert a normal force on the sample. This is also the reason why it’s important that the stem is able to move smoothly without friction through the sustaining hollow cylinder but still keeping perfectly straight.

Tool sensitivity can be increased mechanically by decreasing the tip dimension (which ultimately means increasing pressure while keeping a constant force). Be careful not to pierce the sample with too sharp a tip. Moreover since the conductive volume decreases, maximum endurable current without sample damaging decreases too. On the other hand an augmented sensitivity can be obtained electrically by voltage increase or current decrease.

M (g) R

(o

hm

)

QTC

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If you are not interested in obtaining very precise quantitative data but just to point out QTC exponential trend you may pile 500g masses directly on the QTC covered with a paper disk large enough to avoid contact with the electrods (see above picture)

D – Current VS voltage

Disconnect the ohmmeter from the circuit and connect a power pack in its place. Choose a suitable mass to put on the plate and keep it constant throughout the whole experiment. Applying a mass A will produce a specific initial resistance RA. Use current (I) and voltage (V) probes for on line data acquisition and then plot the sample I-V curve. Repeat the procedure with different initial resistance values (e.g. different applied masses). Investigate whether QTC (or alternative

samples) has an ohmic behaviour and whether the result depends on initial pressure .

It’s in the I-V plot even more than in the relationship resistance versus applied mass that QTC blatantly exhibits the novelty of its conductive mechanism. Scientific literature

describes a)an ohmic behaviour at either an extremely low pressure (initial R0>1M or

quite an high one (initial R0 of a few b) a strongly non ohmic (e.g. not linear) behaviour at intermediate pressure, with hysteresis and negative resistance 5.

This last configuration is the most interesting one to study in order to support the hypothesis of QTC quantum tunneling conductive mechanism. Limitations to the maximum range of voltage sensors may result in a partially cut off IV plot. It’s still possible however to outline many important features such as negative resistance areas, the hysteresis, the continually fluctuating current (particularly intense where a rearrangement of charge distribution takes place) and fluctuating resistance too, sometimes reaching even extremely high peaks. Resistance can be obtained at each and every point in the I-V plot as the inverse of the tangent line gradient.

5 Although there are no totally negative resistors, a few devices may locally exhibit such

phenomena (this can be clearly appreciated in the I-V curve of tunnel diode or in conductive polymers.

M= 3 Kg ; R0 = 1,2 Ohm.

Leybold Sensors

M= 3 Kg ; R0 = 1,2 Ohm

Vernier Sensors

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In the above picture a few examples of plots obtained with sensors with different ranges. Below a comparison of Velostat and QTC data

I-V curve for Velostat (green) and QTC (rossa). The first part of the red line is clearly an exponential (blue line). For voltages higher than 10 V it saturates. The IV plots produced are all very similar in their shape for each of the samples

The fact that initial and final resistance are different in QTC is due to charge accumulation and subsequent redistribution in their geometry owing to voltage modification and to “pinch off” phenomena (see background reading”). Usually final R is higher than the initial one (this can be observed as well directly from the plot by tracing the two tangent lines in the origin and comparing the two gradients: the less steep line corresponds to a an higher resistance since R= V/I). However this resistance will decrease in time due to a natural dissipation of the electrostatic charge. In the graphic in the following page you can see IV curves obtained in a continuous cycle: overlapping in Velostat, quite different in QTC where a progressive increase in the initial R can be clearly appreciated together with the curve downshifting and a decreasing in the maximum I

current reached. This is probably due to the fact that too short a time has been left before running the next cycle for the trapped charges to slip away.

In the case of Velostat and EonTex the characteristic curve doesn’t exhibit any hysteresis whatever and even on repeated cycling it

appears substantially linear

Eontex

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(in accord to the percolation conduction model where the percolative paths can actually be considered as a sort of wires). At the bottom the circuit diagram for on-line data acquisition setup.

2 –Piezoresistive materials and pressure sensors

One of the driving applications of conductive polymers is actually represented by “printed electronics” consisting in printing on various substrates electrical devices ranging from resistors to microphones and many types of sensors. Although not yet comparable in their performance to superconductors based devices, printed electronics is however expected to facilitate widespread, low-cost, low-performance electronics for applications such as smart clothes and labels, health monitoring applications, flexible displays and so on. Moreover printing on flexible substrates allows electronics to be placed on curved surfaces and a much higher integration level.

Lab goals

Identify the necessary requirements for good sensors Design, implement and test pressure sensors

What’s to be observed

All scientific instruments, sensors among them, should exhibit a few characteristics such as accuracy, promptness, repeatability, resolution, which can be tested on the field

Although commercially sold mostly as switches, printed piezoresistive sensors can be calibrated for specific uses.

Equipment (for one working group only)

1 printed Flex sensor and/or Force Electric cables+ alligator clips)

Optional

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sensor 6 multimeter (as ohmmeter) cylinders of different diameters7

Lab masses

Link to the videoguide

Read the QR code on the right or go to the page

Home > Labs > Conductive Polymers > 2 – Piezoresistive materials and pressure sensors> Videoguide>

Background reading


Experimental protocol

A – Connecting the sensor into the circuit

Either use a motherboard or just connect with electric cables to make the circuit with sensor and ohmmeter. Put the round disk part (sensing part) on a hard surface such as the table and pile masses centered on it. For each mass let the reading settle before writing it down. Plot conductance VS mass and test all the other sensors characteristics.

Students often learn about instruments characteristics and calibration mostly from a theoretical point of view. Let them investigate and understand how the sensor works,

devise a possible application and test the most important characteristics needed for this specific application.

B – Flex sensor

Build the circuit as in step A, then gently tape the flex sensor around a beaker. Be careful to fix the connecting end quite tight otherwise the contacts may break by repeated bending.

Resistance changes according to the degree the sensor is bent. N.B. Notice that some types of sensor measure a “bending moment”. Some sensors are bidirectional, others just sense when bent in one direction. Once the mechanism of the specific sensor has been investigated, using a set of tins and beakers of different diameter Resistance VS applied load may be plotted.

Make students think of valid alternatives provided they do not bend the sensor too roughly and sharply. For instance tape the flex sensor on a long flexible metal strip (or any

6See Finding materials and equipment .

7 Beakers of different diameters are ok










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other rigid material) and actually bend it by applying varying masses. See pictures. Again one of the possible approaches is to have student devise a possible application and then test the most important characteristics needed for this specific application.

Some ideas for sensor application and possible implementation in the school lab in a simplified version may be as following

Newborn breathing monitoring

Coin park metres

Discern between grip and casual touch: security switches

Detect presence or movement

Detect pipe occlusions

Resources


Finding materials and equipment

QTC can be purchased at www.mindsetsonline.co.uk £ 0.40 each pill + shipping costs (autumn 2012) – Text “qtc “ in the “quick search” slot in the left column.

From the same website you can also buy both a DVD and the SEP booklet -“QTC: a remarkable new material to control electricity”

Peratech [1] offers evaluation kits to potential customers interested in developing new applications . In the kit QTC sheets and QTC cables are included (approx. 300 £).

Copper electrodes are cut out of copper sheets of the kind used for Arts and Crafts at school (approx. 1 euro at stationery stores). From each sheet you can get more than forty electrodes.

Velostat for sale at 3M, or at http://www.plugandwear.com (in the left column: ProductsFabrics

11

11.2

11.4

11.6

11.8

0 20 40 60

R (

kW)

m (g)

Flex Sensor on deformed metal blade




http://www.mindsetsonline.co.uk/

http://www.plugandwear.com/

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Conductive. cost: 9,60 euros per meter (h 91 cm). It’s a traditional piezoresistive material based on percolation.

Eontex piezoresistive fabric can be purchased at http://www.eeonyx.com/. Eontex is a conductive textile based on “thin film coating technology”. Its fibers are coated with polypyrrole (PPY), an intrinsically conductive conveniently drugged polymer.

Printed Flex and force sensors can be purchased at http://www.imagesco.com/ , http://www.simplelabs.co.in in USA. In Italy at Futura elettronica http://www.imagesco.com/. Cost between 8-12 euro

http://www.eeonyx.com/

http://www.imagesco.com/

http://www.simplelabs.co.in/

http://www.imagesco.com/

conductive polymeric composites and the quantum tunneling...

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