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Carbon Nanotube Alignment and Application as Field Effect Transistors

Jacob Feste

University of Arkansas, Biomedical Engineering, jtfeste@email.uark.edu

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Abstract

The objective of this experiment was to determine how the frequency-dependent DEP and electro-thermal forces influence the alignment process for CNTs. The impacts were determined by measuring the I-V curves of aligned CNTs diluted 5 times at different frequencies (0.5 MHz, 1 MHz, 1.5 MHz), and also by correlating them to a CNT stock solution at 1.5 MHz. A 20 V peak to peak voltage was applied four times to each sample with the corresponding current values measured. The results supported an increase in current values with dilution. Of the diluted samples, the 1 MHz measurements gave the smallest current values with the 0.5 MHz measurements giving slightly higher values due to increased DEP forces. The 1.5 MHz measurements gave significantly higher current values as a result of reaching the crossover frequency.

Introduction

Carbon nanotubes are cylindrical nanoparticles composed of a desired number of carbon layers. These nanoparticles have a wide range of application due to their unique properties. Carbon nanotubes exhibit exceptional strength, contain unique electrical properties, and are efficient conductors of heat [2]. The alignment of such a nanoparticle, however, is considered a complex process due to the small nature of the nanometer sized particles involved. Nanoparticles are unable to be seen by the human eye, are associated with an inherent degree of randomness and clumping, are very sensitive to the environment, and are generally considered difficult to control in order to form a desired, uniform structure. Therefore, the formation of nanostructures is typically done by utilizing the chemical and electrical properties of the nanoparticles. Carbon nanotubes are long, cylindrical nanoparticles that may be connected together via alignment. Such alignment is made possible using dielectrophoresis. Ultimately, this process allows the nanoparticles to be carried, or aligned, as a result of their dielectric properties [3]. Dielectrophoretic alignment of carbon nanotubes begins with the placement of a dissolved carbon nanotube source between two triangular electrodes on an electrode chip. A voltage difference is then applied between the two triangular electrodes to generate a circular, non-uniform electric field beginning from one triangular electrode and converging towards the outer point of the other triangular electrode. A dielectrophoresis (DEP) force is then generated and is given by [1]:

Equation (1): FDEP=Γ εmℜ FCM ∇¿Erms∨¿2¿

When this force is positive, the nanoparticles will migrate towards the high electric field gradient, resulting in alignment [1]. Alignment accuracy is increased with an increasing DEP force and decreasing triangular electrode angles for a more directional electric field. While the nanoparticles in the dissolved solution begin to align, the remaining solution is intended to be removed gradually in order to reduce mobility at the electrodes and allow for proper structure

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formation. Dielectrophoresis also allows this process. The remaining solution, typically water and surfactant for molecule separation, also has a force acting on it during the process. This force is termed the electro-thermal force and is given by [1]:

Equation (2): F e=−M (εmσmV rms

4

2kT π3 rc3T

)(1−2θπ

)

This force also depends on whether the frequency of the generated field is greater than or less than the solution’s crossover frequency. Above this frequency, the electro-thermal force will be positive. When the electro-thermal force is positive, the remaining fluid solution will be forced away from the electrodes [1]. The processes of both alignment and solution removal are highly dependent on the frequency of the generated field. More specifically, these processes are dependent on the crossover frequencies of the nanoparticles. At or below the crossover frequency, the electro-thermal forces given off by the electrodes are too weak to repel the attracted fluids [6]. On the other hand, the DEP force will decrease as the frequency approaches the crossover frequency [6]. It is therefore desired to maintain frequencies just above the crossover frequency to allow for solution removal and to reduce the impact of the decreasing DEP forces. The DEP alignment process is complete when the nanoparticles are properly aligned and the remaining solution is removed.

The electrical properties of aligned carbon nanotubes give rise to a wide variety of unique applications. The resulting alignment may primarily be a semi conductive material if single-walled nanotubes are produced, or a conductive material if multi-walled nanotubes are produced [1]. The semi conductive nature of single-walled carbon nanotubes (SWCNTs), for instance, provides the potential application of SWCNTs as field effect transistors. A field effect transistor (FET) is a transistor that controls the electrical conductivity of a channel of one type of charge carrier by using an electrical field to control the shape of the channel for a semi conductive material [4]. As a FET, aligned SWCNTs may be used for the detection of DNA. These FETs include an electron source at one end (i.e. gold/chrome) and an electron drain at the other end (i.e. gold/chrome), of which are connected by a semi-conductive channel (aligned SWCNTs). This channel is attached to a silicon oxide layer that separates it from a P-type substrate/backgate (silicon). A voltage difference is applied between the source and drain to allow an electron current to flow through the channel. A voltage is also applied to the backgate in order to further control conductivity. When a voltage is applied, the conductivity of the channel changes due to conformational changes [5]. DNA detection may be performed with such a device by attaching various single-stranded DNA (ssDNA) molecules to the aligned SWCNT channel. When a complimentary strands of DNA reaches the device, DNA hybridization will occur to form completed, double-stranded DNA structures. Soon after this process occurs, the negative charges associated with some of the phosphate groups on the DNA strands are transferred towards the positively charged electrodes (gold/chrome). The negative charges are then donated to the carbon nanotubes at either the source or drain regions. The binding process increases resistivity and decreases conductivity at those regions by decreasing the

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cross-sectional area at which current is able to flow due to conformational changes. Binding of one region of DNA to the nanotubes brings the remaining DNA structure near the SWCNT channel. Close proximity of the entire DNA strand is maintained following this process and is oriented in a “wrapping” manner due to intermolecular forces, with binding occurring at the source and drain electrode regions. Ultimately, the process will reduce the current and increase resistivity. Detection of these measured changes is then able to provide that DNA hybridization has occurred with a complimentary DNA strand.

Procedure

Materials

1 mg Carbon Nanotubes (NanoLab PD15L5-20) 4 µl AQ Nanosperse surfactant 10 ml distilled H2O 15 ml centrifuge tubes Wax Paper Micropipettes Electrode Microchip Conductive Wire

Equipment

Sonicator NIS software Microscope Solder Machine Voltage Source Meter Laptop BK Precision® 4005DDS Function Generator Agilent 54622A Oscilloscope Weight Scale

Procedure

5x Dilution Solution Preparation:

1. Measure 1 mg of carbon nanotubes using wax paper.2. Transfer the nanotubes to a 15 ml centrifuge tube.3. Using a micropipette, add 4 µl of surfactant to the centrifuge tube.

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4. Mix the solution by pumping the solution with the micropipette and slowly retracting three times.

5. Add 10 ml of distilled H2O to the centrifuge tube using a micropipette.6. Sonicate the solution for 10 minutes.

Note: Steps 3-6 may be repeated as needed until the solution is completely dissolved.

Electrode Microchip Preparation:

1. Pull the microchip off the blue tape very carefully.2. Place and tape the microchip on the microscope film.3. Transfer the film to the microscope.4. Under the microscope, solder conductive wire from one electrode on the microchip to

the other.

I-V Curve Measurements:

1. Prepare the stock solution of carbon nanotubes and the 5x diluted solution.2. Clean the electrode chip using an appropriate acidic solution.3. Deposit approximately 2 µl of solution on the microchip, repeated for each solution.4. Using the function generator and oscilloscope, apply 1.5 MHz with 20 peak-peak voltage

to the electrodes. Run four tests for each sample. For the 5x solution, repeat with 500 kHz and 1 MHz.

5. Observe carbon nanotube alignment and turn off the voltage before the sample is dry.6. Measure I-V curves using a source meter and a laptop.

Results

Figure (1): Illustration of the CNT alignment process from beginning to end.

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Figure (2): I-V curve results for 5x diluted CNT solution at 500 kHz frequency.

Figure (3): I-V curve results for 5x diluted CNT solution at 1 MHz frequency.

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Figure (4): I-V curve results for 5x diluted CNT solution at 1.5 MHz frequency.

Figure (5): I-V curve results for CNT stock solution at 1.5 MHz frequency.

Y= 0.0015x - 6E-6

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Figure (6): Average I-V results for each sample and at each frequency for comparison.

Discussion and Conclusion

Our results include I-V curves for both stock and 5x diluted CNTs at different frequencies. By analyzing this data, assumptions can be made about the DEP and electro-thermal forces, crossover frequencies, dilution impact, and alignment widths. While tests were not performed involving CNTs as FET devices, some properties associated with CNT-FET devices may be correlated with our results. The conformational changes of the SWCNT channel and their effects on an I-V curve, for instance, may be analyzed by determining the impact that alignment width has on the I-V curves of the results. For the diluted CNT solution, the impact that signal frequency has on its I-V curve is illustrated by figures 2-4 and figure 6. Each of these graphs are associated with a generally linear change in current with respect to voltage. The 500 kHz I-V curve resulted in peak currents around 0.00065 A while the 1 MHz I-V curve resulted in slightly less peak currents around 0.00048 A. The 1.5 MHz I-V curve resulted in significantly higher peak currents around 0.01 A, however the measured peak to peak voltages were around 13 V as opposed to the 20 V peak to peak voltages associated with the other frequencies. Based on the trend line in figure 4, it can be suggested that peak currents would likely reach around 0.015 A for a peak to peak voltage of 20 V. However, the reduced peak to peak voltage may have been due to reduced DEP forces. If the DEP forces were not strong enough, some of the CNTs may have remained unaligned; leaving them disconnected from the channel and attached to the outer regions of the electrodes. It is possible that the applied voltage was shared among these nanoparticles and therefore reduced. Due to the rapid increase in current, it can be suggested that the crossover frequency for the diluted sample is likely between 1-1.5 MHz. In other words, the electro-thermal forces at 500 kHz and 1 MHz are likely negative and push the

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remaining solution towards the electrodes, greatly increasing resistivity and decreasing current. The large difference in current values between 1-1.5 MHz suggests that this force becomes positive at some instance between these frequencies, allowing the remaining solution to be expelled away from the electrodes. The decrease in current from 500 kHz to 1 MHz is most likely due to decreasing DEP forces as the crossover frequency is approached. Therefore, the 500 kHz measurements likely gave the best alignments. The electrodes remained attracted to the fluid, however, significantly increasing resistance and decreasing current. The 1 MHz measurements also had their electrodes resisted by the fluid, with decreasing currents resulting in decreased DEP forces as the crossover frequency is approached. The difference can also be explained by the changes in CNT width. As frequency increases, CNT width also increases relative to its height [1]. A width-to-height ratio of 1 is desired to provide minimal current constriction. As the ratio increases further away from this value, resistance increases due to conformational changes. This phenomena is the primary detection mechanism for CNT-FET devices and is supported by the results. The 500 kHz measurements likely had width-to-height ratios near the desired value while the 1 MHz measurements may have had ratios beyond the desired value. For the 1.5 MHz measurements, the decrease in resistivity associated with fluid removal was significantly greater than the resistivity increase associated with width-to-height ratios and decreased DEP forces; suggesting that these increases are insignificant by comparison. The I-V curve results for the CNT stock solution at 1.5 MHz are displayed in figure 5, with the comparison data for each sample type’s average I-V values illustrated in figure 6. The I-V curve of the CNT stock solution resulted in peak currents around 0.00025 A with voltages ranging from 0 V to 10 V. These values primarily represent the true conductivity of the CNTs without alignment. By diluting the CNTs and aligning them, the conduction channel becomes much larger, resulting in increased currents at each frequency. For instance, the diluted and aligned solution at the same frequency resulted in currents around 40 times that of the CNT stock. In conclusion, the results suggest much more efficient conduction channels for dissolved and aligned CNTs. They also suggests that this large increase in efficiency is only made possible at the correct frequencies, making them highly dependent on their crossover frequencies. As long as the frequency conditions are ideal, aligned CNTs may serve as effective conductive channels with various application possibilities.

References

[1] Tung, Steve. "Aligned Carbon Nanotube Based Field Effect Transistor."Department of Mechanical Engineering (2015): n. pag. University of Arkansas. Web. 10 Nov. 2015.

[2] "Carbon Nanotubes." ScienceDaily. N.p., n.d. Web. 10 Nov. 2015.

[3] Pethig, Ronald. "Review Article—Dielectrophoresis: Status of the Theory, Technology, and Applications." Biomicrofluidics. American Institute of Physics, 29 June 2010. Web. 10 Nov. 2015.

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[4] Lesurf, Jim. "Field Effect Transistor." Scots Guide. University of St. Andrews, n.d. Web. 10 Nov. 2015.

[5] "Depletion-type IGFETs." : Insulated-gate Field-effect Transistors. All About Circuits, n.d. Web. 10 Nov. 2015.

[6] Kakac, S. Microfluidics Based Microsystems: Fundamentals and Applications. Dordrecht: Springer, 2010. Google Books. Google. Web. 10 Nov. 2015.

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The Impact of Scaling Effects on Nano Composite MEMS Devices: Mechanical and Electrical Properties

Jacob Feste

University of Arkansas, Biomedical Engineering, jtfeste@email.uark.edu

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Abstract

The goal of this experiment was to determine the mechanical and electrical properties of a PDMS nanoparticle composite with various percentages of carbon black. These properties are also measured in order to relate them to characteristics associated with a composite material, such as the Rule of Mixtures (ROM). Mechanical properties were measured via tensile testing with carbon black percentages ranging from 0%-20% (in multiples of two), with brass, polypropylene, and steel measured for reference. The results indicated a weak material and a linear increase in tensile strength as carbon black percentage increased, satisfying the ROM for this composite. Electrical properties were determined by measuring output voltages relating to changes in resistance in order to determine changes in resistivity. A Wheatstone bridge circuit staged by an instrumentation amplifier and measured through an oscilloscope was used with carbon black percentages ranging from 14%-19%. The results included a large degree of error for the 14%-16% samples, however the results from the remaining samples provided enough accuracy to support an increase in resistivity as carbon black percentage decreases.

Nomenclature

R= Resistance (Ohms)

L= Length (m)

A= Cross-sectional Area (m2)

ρ= Resistivity (Ohm*m)

V= Voltage (V)

I= Current (A)

σ= Stress (Pa)

F= Force (N)

ԑ= Strain

E= Elastic Modulus (Pa)

Introduction

A composite material is defined as a single material composed of a mixture of two or more materials. Composite materials have a wide range of application due to the dependence of their properties on the individual properties of the various materials they are composed of. This relationship allows for the engineering of a composite material with desired properties based on the combined properties included in its composition. However, for composite materials, properties such as mechanical properties may be negatively influenced by the addition of one material to the surface of another. When two materials of different shapes and sizes are combined, a small region of space is created where their dimensions are not perfectly identical, resulting in a less stable and less uniform structure. The “porous” identity, for

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example, interrupts conduction paths, decreases mechanical strength, and other properties that favor a single, uniform structure. It is therefore desired to minimize this aspect in order for a composite material to maximize its desired, combined properties and minimize the negative impacts inherently involved with the combination of different materials. Minimizing the degree of porosity in composites is performed by increasing the impact that an added material’s surface (area) has compared to the overall volume. The impact that a material’s surface has on its total volume increases as the size of a material decreases due to the effect of scaling laws. These laws support an increased area-to-volume ratio with a decrease in length due to the exponential relationship between length, area, and volume [1]. Therefore, the addition of particles of smaller sizes are desired for the production of effective composite materials. Nanoparticles fulfill this requirement, making nanoparticle composites effective at retaining the combined material properties.

The nanoparticle composites involved in this experiment are aimed to highlight the properties of a material with an elastomeric matrix phase (PDMS) and carbon black. The composites are formed using DDPOST, a process that allows the formation of a thick layer of polymer composite using micro- and nano- particles with polymer matrix [1]. The resulting polymer composite can be made electrically, mechanically, or chemically active by selecting specific particle and matrix materials [1]. The nanoparticle composites of this experiment were formed to allow chemical activation in order to serve as Micro-Electro-Mechanical Systems (MEMS) based corrosion sensors. The carbon black nanoparticle inclusions are considered electrically conductive nanoparticles and are applied to the PDMS matrix. When swelling and etching agents are applied to this mixture, the PDMS matrix swells to a certain degree dependent on the concentration of the swelling agents or chemical vapor exposure. Upon swelling, the PDMS volume expands and extends the electrical pathways of the conductive carbon black suspension [2]. The resistivity changes are then measured upon swelling equilibrium in order to determine vapor concentration. Resistivity is given by the following relationship:

Equation (1): R=ρ( LA

) or ρ=R ( AL

)

PDMS has a high resistivity around 1*1013-1*1015 ohm*m [3]. When swelling occurs, the resistivity value increases due to the extended electrical pathways [2]. These changes may be evaluated to determine concentration due to the percolation theory. This theory claims that clusters of particles attached to the surface of a material, such as the clusters of carbon black nanoparticles attached to the PDMS matrix, accurately represent a uniform structure until the randomness and separation of these clusters reaches a percolation limit at a certain percentage of clustered particles [4]. At this limit, there is no longer a possible path connecting each cluster, where this probability increases exponential as the percentage of clusters decreases. The small sizes of the nanoparticles give an advantage in this aspect. Therefore, the concentrations of the swelling and etching agents may be represented by the concentration of removed nanoparticles until the percolation limit is reached and are given by the changes in measured and known resistivity. Chemical vapor will provide effects similar to those of the swelling and etching agents, allowing its concentration to be evaluated. Changes in resistance

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must be measured in order to measure the changes in resistivity to make concentration assumptions. This process is done with the use of a complex circuit and resistance changes measured by an LCR meter. The relationship between voltage, current, and resistance is given by:

Equation (2): R=VI or V=IR

The derivation of this relationship:

Equation (3): dV=RdI+ IdR

Is utilized in order to form a circuit with variable resistance only. By maintaining a constant current, this equation becomes further reduced to:

Equation (4): dV=IdR

Therefore, a changing input voltage and constant current are necessary for the circuit to measure changes in resistance. However, the voltage values must remain constant in order to measure these changes. A “constant” voltage may be manipulated from a variable voltage source by using voltage dividers and op amps. The circuit begins with the use of a voltage divider in order to output a reference voltage close to 1 volt for the rest of the circuit. The voltage divider equation is as:

Equation (5): V out=V ¿

R1R1+R2

By using an R1 (10 kΩ) resistor with resistance significantly higher than that of R2 (10 Ω), the output voltage will always remain about 1 volt (0.999V). This circuit is followed by an op amp voltage follower in order to maintain the constant voltage, followed by a 10 kΩ resistor in order to return the output voltage to its true value by countering the effects of the much higher R1 resistor of the voltage divider and maintain constant current. This component is then followed by another op amp with one input side grounded and the other connected to the LCR device for measurements. The grounded component serves to form as an adequate zero voltage reference level while also having the ability to absorb as much current as possible without disturbing the voltage potential, ultimately maintaining the desired constant values of the circuit. Our device uses a 4-point probe that measures the voltage difference between the middle two probes. The outer two probes connect to the non-grounded side of the op amp and the op amp’s output, sending the constant current through the two ends. The middle two probes are separated by a specimen acting as a resistor, where these voltage values are measured following the addition of another voltage follower for the two probes to maintain constant voltage for each path. The change in voltage across these two probes are measured to determine the change in resistance, made possible by the constant current and voltage values that are maintained throughout the circuit and differ only between these two points. The final circuit is illustrated by figure (1) below.

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In addition to the electrical properties of a nanoparticle composite, the mechanical properties of these composites are also important. As stated previously, the combined mechanical properties may be negatively influenced by the formation of such composites due to inherent size differences between the components. Also, nanoparticles often reside in clusters and are difficult to attach uniformly, giving rise to inherent randomness. The resulting composites are often anisotropic in nature and therefore require estimates to determine properties such as their overall mechanical strength. The Rule of Mixtures (ROM) may be analyzed to provide these estimates. The ROM states that stress values are directly proportional to the ratio of the volume of nanoparticles to that of the total composite [5]. Nanoparticles have an advantage in that they have more of a surface area effect in a composite and therefore the remainder of the composite retains most of its volume and therefore individual stress values. However, this property may eliminate the possible use of the ROM in order to estimate the stress values for these composites types. The stress values may be measured using a tensile test, where the samples are elongated until failure or until a certain length, with the stress and strain values measured along the way. Stress and strain values are given by the following relationships:

Equation (6): σ=Eԑ

Equation (7): σ=FA

Equation (8): ԑ=∆lL

By comparing this data to the relationship given by the ROM, it is made possible to determine whether or not the ROM remains accurate for the composite. Proportionality should be seen

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between the stress values and different volume percentages. If this pattern is not given, it can be suggested that the ROM does not apply. This experiment will measure the mechanical properties of nanoparticle composites composed of PDMS and different volumes of carbon black. By comparing the data for the different percentages, it is possible to determine their proportionality and whether or not the composite follows the ROM.

Procedures

Materials

1. Carbon nanoparticles (Alfa Aesar, 45527[42nm, 100%], 39724 [42nm, 50%], H30253)2. Sylgard® 184 silicone elastomer kit, Dow Corning (Midland, MI)3. PASCO Scientific Plastic (AP-8222) and Metal (AP-8223) tensile test specimens4. Mixing cups and stirrer5. Isopropanol alchohol and De-ionized water6. Metal Spatula7. Glass Beakers (10mL)8. Microscope glass slide (1”x3”)9. Gold/Silver nanoparticles from synthesis lab (D. Chen)10. Microfabricated electrodes with SU-8 molds11. Tensile test specimen mold12. Instrumentation amplifier with Wheatstone bridge circuit13. Chemical vapor mixing and injection system

Equipment

1. Dell Inspiron 1764 Laptop2. Tenma 72-9365 200Mhz Oscilloscope3. BK Precision Power supply-Model93104. Agilent Technologies U1733P LCR Meter5. PASCO Scientific Stress-Strain Apparatus (Tensile Tester)6. Extech EX540 Multimeter/Thermocouple Reader (with Type-K thermocouple probe)7. Digital Hot-plate8. Digital Scale9. Microsoft LifeCam Studio Webcam

Software

1. LabView2. Instrument drivers/software

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Procedures

Electrical:

Preparation:

1. Weigh each particle of carbon black for desired volume ratios of 14%, 15%, 16%, 17%, 18%, and 19% when mixed with PDMS.

2. Convert the mass ratios from a systemic set of PDMS to the desired volume ratios given in step 1.

3. Manually mix the particles and PDMS at 5 minutes each in a disposable plastic mixing cup.

4. Squeegee the material into the pre-fabricated micro electrode SU8 mold by using a standard microscope glass slide.

5. Release the mold after it has been cured for a couple of days.

LCR Measurements:

1. Connect either the multi-meter (Extech EX540), LCR meter (Agilent U1733P), or Wheatstone bridge circuit staged by an instrumentation amplifier and measured through an oscilloscope to the laptop and open their associated link software.

2. Use the software to record all data digitally.3. Begin with the multi-meter and measure the resistance and capacitance of

each sensor device.4. Repeat step 3 with the LCR meter.5. Connect the sensors to the bridge circuit, power up the circuit with the

power supply, and feed the output to the oscilloscope. 6. Repeat all the measurements with the oscilloscope while on a hotplate with

temperatures of (RT+10oC increments up to 100oC). 7. Monitor the temperature with the type-K thermocouple connected through

the multi-meter.

Note: Only the Wheatstone bridge method was performed for measurements.

Swelling and Resistance Measurements:

1. For each of the samples in preparation step 1, place a small disk of the material into a 50mL beaker filled to the 40mL mark with one of the swelling agents.

2. Use a 50:50 ratio of swelling and etching agents (12mL to 12mL) toluene and acetic acid.

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3. Report the swelling process using a webcam (Microsoft LifeCam Cinema 720p) that has been calibrated for its pixel resolution and controlled by the LabView™ Vision® software.

4. Record the resistance when the swelling reaches equilibrium.

Note: This procedure was demonstrated but not performed for this experiment.

Mechanical:

Preparation:

1. Mix PDMS with various carbon black densities (42nm; 50%, 100%).2. Pour each sample into the prefabricated aluminum molds for natural curing

over one week.3. Weigh each particle of Carbon Black for desired volume ratios of 0%, 2%, 4%,

6%, 8%, 10%, 12%, 14%, 16%, 18%, and 20% when mixed with PDMS.4. Convert the mass ratios from a systemic set of PDMS to the desired volume

ratios given in step 1. 5. Manually mix the particles and PDMS at 5 minutes each in a disposable

plastic mixing cup.

Tensile Test Measurements:

1. Connect the PASCO Scientific Stress-Strain Apparatus’s Passport Rotary and Force Sensors to the individual USB Link, then connect it to the laptop.

2. Start the DataStudio software and select the stress-strain apparatus experiment.

3. Load the PASCO tensile test specimens and crank the rotary handle at a steady rate until the specimen breaks.

4. Save the data and repeat for each sample.5. Remove the nanoparticle PDMS composite specimen from its mold by

unscrewing the mold covers and load it into the Stress-Strain Apparatus. 6. Pull the specimen until breakage.

Results

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LCR Measurements for PDMS/Carbon Black (%) Composites Vin=0.9999 V Vsource=12 V

PDMS Composites with % Carbon Black 17% 18% 19%Vout (V) Vout (V) Vout (V)

Sample 1 11.6 6.9 5.1Sample 2 8.2 6.5 4.1Sample 3 7.3 7.4 5.9Sample 4 7.3 5.8 4.8 Sample 5 10.5 4.9 5.7Average 8.98 6.3 5.12

Figure (2): LCR Measurements using the Wheatstone bridge circuit measurement method for PDMS composites with %17-19 Carbon Black.

Figure (3): Tensile test results for PDMS composites with % Carbon Black.

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Figure (4): Tensile test results for total PDMS composite averages and different materials to serve as references.

Discussion

The results of this experiment include results for LCR and tensile test measurements. The swelling and resistance procedure was demonstrated but not performed. The LCR measurements were taken using a Wheatstone bridge circuit staged by an instrumentation amplifier and measured through an oscilloscope. The results are given by figure 2. According to equation 2 and equation 4, these results suggest an average increase in resistivity for a PDMS and carbon black composite as the percentage of carbon black decreases. This relationship was expected and likely due to the more resistant, or less conductive, nature of PDMS compared to carbon black. These measurements were intended to be taken for samples of 14%-19% carbon black. While some successful results remained, measurements taken for the composites with carbon black percentages less than 17% included a significant degree of error and therefore inconclusive results. This error was likely due to the large possibility of error associated with such a complex measurement system. It could also be due to errors involved in the preparation process in which inaccurate volume percentages were produced. The results of the tensile tests were much more accurate and are illustrated in figure 3 and figure 4. According to figure 3, there was a general increase in ultimate tensile strength, or maximum stress before rupture, as carbon black percentage increased. The stress values upon rupture were between 40,000-60,000 Pascals for the samples with no carbon black while they were between 110,000-140,000 Pascals for those with 20% carbon black. The 10% samples had stress values between 80,000-

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100,000 Pascals before rupture, suggesting a linear relationship between carbon black percentage and ultimate tensile stress. Most of the samples ruptured between stress values of 4-5. Many samples did not break, however, making it impossible to make correlations between the carbon black percentages and rupture point strain values. Alone, PDMS has a relatively low ultimate tensile strength around 15,000-90,000 Pascals. When a fraction of its volume is replaced by carbon black, the combined properties should increase this value to satisfy the rule of mixtures. The tensile test results exhibit a somewhat linear increase in stress values as carbon black percentage increases, suggesting a valid ROM relationship. Figure 4 illustrates the average of the tensile test values combining each sample of each percentage, and compares them to materials such as steel, brass, and polypropylene. Steel had the highest tensile strength but was much more brittle, or broke with less strain, than the other materials. Brass also had a high tensile strength but mimicked the ductility of the polymer materials, polypropylene and the PDMS composite. Overall, the composite material was much weaker than the other materials, likely due to its composition and identity as a nanoparticle composite. However, the negative mechanical impacts associated with a composite material were minimized due to the results suggesting ROM applicability. Error was also a possibility for the tensile test measurements as well due to testing being done manually. However, the measurements were accurate enough to generate conclusive assumptions and disregard this error.

References

[1] Huang, Adam. "Experiencing Scaling Effects via Nano Composite MEMS Devices." University of Arkansas, n.d. Web. 1 Nov. 2015.

[2] Huang, Adam, Victor Tak Sing Wong, and Chih-Ming Ho. "Silicone Polymer Chemical Vapor Sensors Fabricated by Direct Polymer Patterning on Substrate Technique (DPPOST)." Sensors and Actuators B: Chemical116.1-2 (2006): 2-10. Web.

[3] "Polydimethylsiloxane (PDMS)." CiDRA Precision Services. N.p., n.d. Web. 01 Nov. 2015.

[4] Gastner, Michael T. "Percolation Theory." Michael Gastner: Percolation Theory. Imperial College London, n.d. Web. 01 Nov. 2015.

[5] Kopeliovich, Dmitri. "Estimations of Composite Materials Properties."Substech. N.p., 2 June 2012. Web. 1 Nov. 2015.

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