Carbon Nanotubes as Novel High Performance Damping Materials Principal Investigator: Nikhil Koratkar (Project Supported by National Science Foundation)

We have demonstrated that carbon nanotubes can be used to augment damping in composite

structures. This damping is related to two distinct energy dissipation mechanisms; 1) inter-tube friction in densely packed networks (thin-films) of carbon nanotubes and 2) interfacial “slip-stick” friction between nanotubes and polymer chains in nanotube filled polymer systems. In this way, we take a direct advantage of interfacial sliding at the nanotube-nanotube and nanotube-polymer interfaces to enhance structural damping without sacrificing mechanical properties or structural integrity.

We believe that this research can lead to a new class of nanostructured damping materials that show dramatically improved properties compared to traditional viscoelastic damping polymers. With their nanoscale dimensions (high surface area to volume ratio), large aspect ratio, high stiffness, low density, excellent thermal stability and extremely high thermal conductivity, carbon nanotubes can provide very high damping with minimal weight penalty. While commercially available damping polymers rapidly degrade in performance with temperature, nanotube films can offer superior performance at elevated temperatures. Nanotube films are also minimally-intrusive and may be seamlessly integrated (co-cured) with the host composite. While traditional damping polymers are very soft (low storage modulus), nanotube films offer the promise of enhancing mechanical properties (strength, stiffness) in addition to structural damping. Such nanostructured damping treatments are expected to be widely used, in a variety of aerospace, mechanical and civil applications for vibration and noise control, stability augmentation, improved fatigue life of structural components, better affordability and lower maintenance costs.

Fig. 1: Damping increase in composites using embedded CNT thin films A) micro-structure of CNT film at 2500X and 8500X (insert), B) bending test results

In order to show the feasibility of nanotube (CNT) thin-films for damping applications, bending

experiments were performed on composite beams both with and without embedded CNT films. First, a film of aligned multi-walled carbon nanotubes was fabricated on a silica substrate using catalytic chemical vapor deposition (CVD) of a xylene/ferrocene mixture. Then a piezoelectric sheet was bonded to the CNT film (anchored on Si/SiO2 substrate) with an acrylic adhesive and cured under high pressure. Figure 1a shows how the CNTs are densely interconnected together. This indicates that interactions between individual tubes in the film are a significant mechanism of load transfer and energy dissipation. The beams were clamped at one end, creating a cantilever system. The piezoelectric sheet was excited to create a bending moment in the beam. The strain response of the beams to a frequency sweep test is shown in fig. 1b. The peaks correspond to the first bending mode of the system. An increase in damping can be seen in the CNT reinforced beam by the reduction in peak strain at the bending mode, which corresponds to a 200% increase in damping compared to the baseline condition. In addition, the shift in frequency of the bending mode from 700 Hz to 840 Hz is a result of increased stiffness of the beam.

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Damping in nanotube thin-films (Fig. 1) is governed by inter-tube friction arising from atomic level contacts between densely packed nanotubes. We have also explored the possibility of improving the damping properties of bulk viscoelastic polymer systems by uniform dispersion of nanotube fillers in low concentrations (~1-3% by weight). Once purified and separated (via ultra-sonication and by using various surfactants) nanotubes were mixed with polymer directly via melt mixing. The high exposed surface area to volume ratio and high aspect ratio of the nano-fillers implies that very significant enhancement in damping is expected due to slippage at the interfaces. The low density of the fillers also implies that the bulk damping properties can be controlled with minimal weight penalty. This is demonstrated for Silane Elastomer (used in commercially available 3M-ISD-112 damping tapes). Test coupons with and without nanotube reinforcement were dynamically strained in the shear mode (0-50 Hz frequency range). Figure 2 shows the material loss factor for both the neat elastomer as well as the filled elastomer (with 2% multiwalled nanotubes by weight). Even though the neat elastomer has a very high baseline loss factor (~0.9), the nanotube filler material causes a significant increase in damping, particularly at the higher frequencies (Fig. 2). This energy dissipation results from interfacial friction as well as the telescoping or sliding that occurs within the concentric multiwalled nanotubes.

Fig. 2: Performance characterization of multiwalled nanotube (MWCNT) filled Silane Elastomer. Note that Silane is a high loss factor, high performance, damping polymer extensively used for commercial applications. MWCNT fillers are shown to significantly enhance the damping performance of the bulk elastomer.

Select publications resulting from NSF Award: 1) N. Koratkar, B. Wei, P. Ajayan, “Carbon nanotubes for damping applications”, Advanced

Materials, Vol. 14, No. 13-14, pp. 997-1000, (2002). 2) N. Koratkar, B. Wei and P. Ajayan, “Multifunctional structural reinforcement featuring carbon nanotubes”, Composites Science and Technology, Vol. 63, No. 11, pp. 1525-1531, (2003).

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Carbon Nanotubes as Ionization Detectors for Chemical Sensing Principal Investigator: Nikhil Koratkar (Project Supported by US Army Research Office)

This research was supported in part by a NER award (CMS 0210124) with Dr K. Chong as the technical monitor. Our work is directed towards developing carbon nanotube films as ionization micro-sensors for chemical sensing. This technique can offer several advantages over traditional ionization sensors (such as flame ionization (FID), photo ionization (PID) or electron capture (ECD) detectors) that are routinely used in gas chromatography sensors. FID has poor selectivity and requires bulky and hazardous hydrogen storage tanks during operation, PID with a better selectivity is limited to a small range of analytes, and ECD detectors are hazardous because they contain radioactive electron emitters. In contrast, the nanotube sensor is compact, very safe to use and requires low power to operate. Since every gas has a characteristic breakdown electric field, a nanotube detector can be applied to a broad range of analytes with good selectivity. We also show that with appropriate design of the chromatography arrangement (including choice of mobile phase, stationary phase and process parameters) detection of analytes in the low ppm range appears feasible by this technique. Compact, low-power nanotube detectors coupled to miniature separation columns could potentially lead to truly field-portable gas-chromatographs that could be used during emergency response and counter-terrorism situations that require definitive identification of contaminants in near real-time.

Figure 1, shows how the device is configured by simply placing a counter-electrode (say Al plate),

several tens of microns above an aligned multiwalled nanotube (MWNT) film, in a parallel plate condenser arrangement. The control voltage is then applied between the anode (MWNT film) and cathode (Al sheet). We take advantage of the nanoscale tip curvature of nanotubes (R ~ 15 nm for MWNTs) to create very high electric fields near sharp nanotube tips. This enables the efficient breakdown (ionization) of target gas species at relatively low voltages; allowing for a compact, portable, low-power ionization detector.

The sensor (shown in Fig. 1) was first tested in air with anode-cathode separation of 150µm. Electrical breakdown of air occurred at 346 V, with discharge current of 460µA. Tests were repeated with metal

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electrodes (no nanotubes) while still maintaining the electrode separation of 150µm. For this case breakdown of air occurred at 960 V with current discharge of 69µA. This shows that by the use of MWNTs as the anode, the breakdown voltage of air can be reduced nearly 65%. The discharge current is also increased from 69µA to 460µA (6-fold increase), leading to high sensitivity of the sensor device (discharge current at breakdown indicates concentration of the detected species, therefore a high discharge current enables detection of dilute concentrations of the unknown gas). We believe that this effect is related to the high density of MWNTs that constitute the surface of the electrode (see Fig. 1C). The billions of aligned nanotubes covering the substrate produce- a consistent nano-scale surface topology unprecedented for conventional planar electrodes or micro-machined electron emitters. A significant number of these tubes participate in ionization, leading to a more extensive conduction path and consequently higher discharge current at breakdown.

Fig. 2 (A) Identification of gas species: for a fixed electrode separation, each gas has a unique breakdown voltage (B) Discharge current indicates the concentration of the detected species (discharge is proportional to gas concentration).

The nanotube ionization sensor was used to detect the identity of several gas species such as helium, argon, nitrogen, oxygen, carbon dioxide, ammonia and air. Figure 2A shows the breakdown voltages of several gases at room temperature (300° K) and at a gas concentration of 4×10-2 moles/liter. Note that each gas exhibits a distinct breakdown behavior; helium displays the lowest (164 V) and ammonia shows the highest (430 V) breakdown voltage. The breakdown voltages shown in Fig. 2A did not vary significantly over a wide range of gas concentrations. This implies that for a fixed electrode spacing of the device (150 microns for this case), the breakdown voltage of each gas is unique and can be conveniently used to establish the identity of the gas. Figure 2B shows the self-sustaining current discharge at breakdown for four different gas species; argon, nitrogen, oxygen and air. Note that the discharge current varies logarithmically with gas concentration. This trend is valid over a wide range of concentrations ranging from 10-7 to 10-1 moles/liter. This indicates that the self-sustaining discharge current generated at breakdown is dependent on the number of gas molecules per unit volume that are available for conduction. Therefore discharge current at breakdown provides a convenient means to quantify the concentration of the gas specie being detected.

The breakdown voltage was found to go down as the inter electrode spacing was reduced. By reducing the electrode gap to 25µm, we were able to bring the device operating voltages down to ~100 V. Such voltages can be easily obtained by connecting four 22.5V (AAA size) commercially available Eveready carbon-zinc batteries in series, suggesting that portable nanotube chemical sensors based on the above idea can be made and utilized.

Select publications resulting from NSF Award: � Modi, N. Koratkar, E. Lass, B. Wei, and P. Ajayan, “Miniaturized gas ionization sensors using carbon nanotubes”, Nature, Vol. 424, No. 6945, pp. 171-174, (2003).

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