Final Report on RFxCNT Material

BackgroundResorcinol-formaldehyde (RF) Aerogels are known to be porous, low-density material whose mechanical properties can range from a stiff, compression resistive material to an elastic and flexible material. These diverse mechanical properties are possible by adding varying quantities of resorcinol, formaldehyde, and water in the creation of the material. Carbon nanotubes (CNT) are tube-like structures made of carbon atoms that are known to be conductive. Given the mechanical properties of RF aerogels and the electrical properties of CNTs, in this research we combine RF aerogel with cross-linked carbon nanotubes in order to create a novel material with high elasticity and tunable conductivity. A material with such mechanical and electrical characteristics would have potential for e-skin applications.

ProcedureResorcinol Formaldehyde (RF) AerogelResorcinol (1 g) was added to distilled water (20 mL) and stirred for 5 min. Then Na2CO3 (19.3 mg) was dissolved into the mixture and stirred for another 5 min with a pH meter attached. After this additional stirring, HNO3 (2 N) was added drop-wise until the pH reached a level between 5.4 and 5.6. When the mixture reached the specified pH level it was stirred for another 60 minutes at 150 rpm and at room temperature, then cured for a week at 80oC. Once the mixture was cured it was freeze-dried.

Cross-Linked Carbon Nanotubes (CNT)CNT (90 mg) had its moisture removed in a Schlenk flask and was heated for 30 mins at 100 - 120oC. The CNT was then placed in a vacuum for 2 hr. In the meantime, Na (45 mg) and Napthalene (250.87 mg) was dissolved in DMAC (22.5 mL) in another flask and the solution was stirred for 2-3 hrs. 6.46 mL of this NaNap solution was added to the CNT in the Schlenk flask and the flask was sealed and stirred overnight. After this the resulting CNT solution was split into several vials, each containing 13.5 mL of the solution. In a separate vial p-DIB (185.4 mg) and DMAC (1.85 mL) are added and stirred for 2-3 hrs. Then 0.371 of this p-DIB solution is added to each of the CNT vials dropwise. The vials are then sealed and left to set for 3 days.

Resorcinol Formaldehyde crossed with Carbon Nanotubes (RFxCNT)The procedure follows similarly to the preparation of the RF aerogel. Resorcinol (1 g) was added to distilled water (20 mL) and stirred for 5 min. Then Na2CO3 (19.3 mg) was dissolved into the mixture and stirred for another 5 min with a pH meter attached. After this additional stirring, HNO3 (2 N) was added drop-wise until the pH reached a level between 5.4 and 5.6. When the mixture reached the specified pH level it was stirred for another 60 minutes at 150 rpm. Then

cross-linked CNT was infused into the mixture and cured for a week at 80oC. Once the mixture was cured it was freeze-dried.

ExperimentalMechanical Properties TestingThe mechanical properties were tested using a dynamic mechanical analysis machine (DMA Q800). This DMA machine performed a frequency sweep on the RF aerogel and CNT material to obtain data on the storage and loss modulus, and a stress-strain test on the RF aerogel to obtain the elastic modulus. Both tests were performed at room temperature.

For the frequency sweep test, a range from 0 to 200 Hz was used and was cycled through twice. The RF aerogel sample size was 4.2978 mm in thickness and 12.155 mm in diameter, while for the CNT it was 7.381 mm in thickness and 16.275 mm in diameter.

For the stress-strain test, a ramp displacement of 100 µm/min and a maximum displacement of 8000 µm was used. The sample size of the RF aerogel was 8.63 mm in thickness and 53.24 mm2 in surface area.

Fig 1: RF aerogel between the clamps of the DMA machine

Electrical Properties TestingIn order to test the electrical properties (impedance of the material, the DMA machine was used along with an LCR meter (BK 891 Precision). Two pieces of Pyralux (3 cm by 2 cm) were cut from a sheet and soldered to copper wires. The material tested was placed in between the sheets of pyralux and then clamped by the DMA machine. Two clips from the LCR meter were

attached to the copper wires in order to measure the impedance of the material under compression.

Three tests were performed to measure the impedance of the material. The first test was a 0.5-1.5 N compressional sweep, in which the DMA machine exerted a compressional force ramping up from 0.5 N to 1.5 N at a rate of 0.5 N/min, then held a compressive force of 1.5 N for 1 min, and ramped the force down from 1.5 N to 0.5 N at a rate of 0.5 N/min. As this was happening the impedance of the material was measured throughout the entire test. The second test was a 0.5-2.5 N compressional sweep, following the exact same procedure as the first test but going at a rate of 1 N/min. The final test was a 0.5-5.5 N compressional sweep followed the procedure of the second test but going to a maximum force of 5.5 N.

The same RFxCNT sample was used for all of the tests performed. The sample size was 9.864 mm in thickness and 15.0 mm in diameter.

Fig 2: RFxCNT material in between the Pyralux foils and DMA clamp

Results and DiscussionMechanical Properties

Fig 3: Range of storage moduli of RF aerogel and CNT material

Data on the storage moduli (Fig 3) show that the RF aerogel and CNT material have a similar storage modulus, ranging from about 0.005 to 0.06 MPa. These materials have a lower storage modulus compared to similar porous polymer substances. For example, the porous polymer polyHIPE has a storage modulus of around 10 MPa at room temperature.1 This means that the RF aerogel and CNT material are not as elastic as other porous materials similar to RF aerogel.

0 10 20 30 40 50 60 70 80 900.00E+00

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Stress vs Strain

Strain (%)

Stre

ss (M

Pa)

Elastic modulus = 0.0064 MPa

Fig 4: Stress/Strain curve for RF Aerogel

NOTE – Elastic modulus value is based on the points taken from a strain of 0-40%

The stress-strain curve (Fig 4) for the RF aerogel shows the relationship between stress and strain for this material and the linear regression line used to calculate the elasticity modulus of it. The shape of the graph curves upward, meaning that as the strain increases the stress on the material increases as well. The material shows this characteristic because as the material is compressed, the porous structures of the material break, and it splits apart into very small pieces. A calculation of the elasticity modulus using the data from 0 to 40% strain shows a value of 0.0064 MPa.

Electrical Properties

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.50

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Impedance vs Force (0.5 - 1.5N Test)

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Fig 5: Graph of impedance vs force (for 0.5 – 1.5N test)

0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.750

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Impedance vs Force (0.5 - 2.5N Test)

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Fig 6: Graph of impedance vs force (for 0.5 – 2.5N test)

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Impedance vs Force (0.5 - 5.5N Test)

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Fig 7: Graph of impedance vs force (for 0.5 – 2.5N test)

Note – Arrow indicates direction of graph

As shown in Fig 5 – 7, all of the tests done on the RFxCNT material show a slight hysteresis. For the 0.5 – 1.5N and 0.5 – 2.5N tests, the impedance of the material at the end of the test is lower than when it first started. This happened because the at the end of each test, the material is more deformed than at the beginning of each test, as shown by the change in the material’s strain value in Fig 6. However, the 0.5 – 5.5N test shows that the impedance of the material is higher than when it first started, which is contrary to the data for the 0.5 – 1.5N and 0.5 – 2.5N tests. This may have occurred from a crack formed in the material due to the compressional force on it.

Material Force (N) Change in Impedance/Resistance (ohms) Percent ChangeRFxCNT (0.5-1.5N) 1.5 255.683 59.59%RFxCNT (0.5-2.5N) 2.5 252.672 63.57%RFxCNT (0.5-5.5N) 5.5 357.643 77.27%FSR Interlink 4022 1.0 24000 80.0%

Fig 8: Table of Change in Impedance/Resistance

From the data in Fig 8, as the force on the material increases the resistance increases as well. For all tests the impedance dropped to 40% or less than its original impedance value, and as the force on the material increases the change in impedance decreases. This is due to the increase in strain of the material, which causes the material to pack more closely together and thus lowers its impedance. When compared to a standard resistive force sensor, the change in impedance is a lot smaller than the force sensor’s change in resistance, but when compared to their respective starting resistances, the change in impedance of the RFxCNT is closer to that of the force sensor (compare 59.59% to 80.0%).

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Impedance vs Strain

0.5 - 2.5N0.5 - 1.5N0.5 - 5.5N

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Fig 9: Graph of Impedance vs Strain for all tests

The impedance vs strain graph (Fig 9) follows similar trends to the impedance vs force graphs of all three tests. However, it should be noted that the strain percentage does not return to zero for each test. This strain value is increased after every test, showing that the material has a slight deformation due to the compressional force. The data shows a general inverse relationship between strain and impedance such that when the strain increases, the impedance decreases. However, this inverse trend is broken in the 0.5 – 5.5N test, where the impedance is greater than the starting impedance despite having a higher strain value. This contradicts the trend shown by the data in the 0.5 – 1.5N and 0.5 – 2.5N tests. Another 0.5 – 5.5N test should be conducted on the RFxCNT material for data precision and consistency.

0 0.25 0.5 0.75 1 1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.250

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Stress vs Strain (0.5 - 1.5N)

Strain (%)

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Elastic modulus = 0.16625 MPa

Fig 10: Graph of stress vs strain (0.5 – 1.5N test)

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Stress vs Strain (0.5 - 2.5N)

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Elastic modulus = 0.25453 MPa

Fig 11: Graph of stress vs strain (0.5 – 2.5N test)

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Stress vs Strain (0.5 - 5.5N)

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Elastic modulus = 0.324830 MPa

Fig 12: Graph of stress vs strain (0.5 – 5.5N test)

Note – The calculated elastic modulus given in each graph is based only on the portion of data where compressional force is increases and hits its peak.

The stress/strain graph of all three tests (Fig 10 – 12) follow a similar shape, but differ in the slope (elastic modulus) and end strain value. As the compressional range increases, the elastic modulus increases as well. This result was contrary to what was predicted, which was that the modulus would remain the same value for all three tests. This can be explained in that the material’s strain value was affected in the 0.5 – 2.5N and 0.5 – 5.5N tests due to previous tests conducted on the material that slightly deformed it. As such it would have affected the readings of the strain values, and hence the elastic modulus. The current stress/strain data on RFxCNT shows that the material has a low elastic modulus (even lower than that of silicone rubber, which has an elastic modulus ranging from 1 – 50 MPa3), but a more thorough stress/strain test needs to be conducted on the material.

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Impedance vs Time (0.5 - 1.5N)

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Fig 13: Graph of impedance vs time when material is under acompressional force of 1.5N for 30 sec

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Impedance vs Time (0.5 - 2.5N)

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Fig 14: Graph of impedance vs time when material is under acompressional force of 2.5N for 30 sec

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Impedance vs Time (0.5 - 5.5N)

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Fig 15: Graph of impedance vs time when material is under acompressional force of 5.5N for 30 sec

The RFxCNT material is able to hold its impedance value well for longer periods of time under a sustained compressional force. The value of the impedance generally decreases over time, but this decrease is small, with the largest drop at about 25 ohms. As the compressional force on the material increases, its impedance drop decreases.

ConclusionGiven the results of the impedance tests the RFxCNT material shows potential for tunable resistivity and elasticity, but more tests need to be done on the material before a definite conclusion can be reached. The material’s resistance does decrease when a compression force is applied on it, but compared to manufactured resistive tactile sensors, the drop in impedance of the material is significantly lower than that in tactile sensors. However, this does not mean that the material cannot be used for tactile sensing applications. As mentioned before, the current stress/strain data on RFxCNT show that the material has a low elastic modulus, but further examination of the mechanical properties need to be conducted.

Improvements to ExperimentThere are several improvements to be made to the experiment that would better ensure the accuracy of the data and better test the material to its full potential. One such improvement includes conducting the same test multiple times on the same sample. Such a test would show how the RFxCNT’s properties change after multiple uses, which is significant if such a material is used in e-skin applications. Also, if enough samples are produced, different samples should be used for each test, so that the conditions under which the sample is tested are standardized and that the sample is not affected by previous tests.

Another concern with the current experiment setup is the size of the samples. While the cylindrical shape of the sample is ideal, the sample size as created by the Teflon molds is too small for testing its mechanical and electrical properties. A new sample should be created with the same form, but with its dimensions doubled. A new Teflon mold would have to be created in order to create a bigger sample size.

Finally, an experiment with the RFxCNT material in which the material is held under a constant force for longer periods of time, like 1-2 mins. While the data shows that the material’s change in impedance under a specified compressional force for 30 sec is low, the material shows a trend of decreasing impedance throughout the 30 sec it is subjected to the force. Another test using the same or similar compressional forces and holding the material under these forces for a longer period of time will show whether the impedance of the material continues to decrease over the entire period of time, in which case may be a concern if the change in impedance drops to a significantly lower value.

References1 – M. S. Silverstein, Polymer, Elsevier, 55 (2014), pp. 304-3202 - https://www.adafruit.com/product/166?gclid=EAIaIQobChMI2eqA8fDj1QIVFIJ-Ch20VgRlEAQYBCABEgJYf_D_BwE3 - https://www.azom.com/properties.aspx?ArticleID=920