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Characterizing Self-Heating in Multiwalled Carbon Nanotube Coated Piezoceramic Sheet Actuators

Dae-Bok Cho* and Nikhil Koratkar**

Department of Mechanical, Aerospace and Nuclear Engineering

Rensselaer Polytechnic Institute, Troy, NY 12180-3590, USA

ABSTRACT

In this paper we investigate the technical feasibility of minimizing self-heating in piezoceramic sheet actuators using carbon nanotube thin film coatings. Our hypothesis is that the enhanced active surface area provided by the carbon nanotube coating will significantly enhance heat dissipation to the atmosphere. To demonstrate this concept multiwalled carbon nanotube films were deposited on the surfaces of PZT-5H piezoceramic sheets. The multiwalled nanotube coating results in significant reduction in power consumption compared to the baseline uncoated actuator (for the same induced-strain level). By comparing the surface temperature of the baseline (or uncoated) and nanotube coated piezo sheet we confirmed that the observed reduction in current and power consumption is related to improved convection of heat energy from the piezo surface.

INTRODUCTION

Piezoceramic actuators are widely being used in a wide range of engineering applications1-5. Their main attractive feature is their high frequency range of operation. However high frequency actuation can result in material self heating due to internal friction in the material. This material self heating results in actuator depolarization6-8 and can also cause a severe reduction in the fatigue life of the actuator. Another --------------------------------------------------- * Undergraduate Research Assistant ** Assistant Professor, Member AIAA, AHS Copyright 2005 by D.-B. Cho and N. Koratkar. Published by the American Institute of Aeronautics and Astronautics, Inc., with permission

limitation of self-heating is that with increase in temperature, the capacitance of the piezoceramic also increases resulting in a corresponding increase in current and power consumption6. In this paper we propose to implement a technology that may help to alleviate material self heating in piezoceramic actuators operating at high frequencies. Our idea is based on exploiting the enhanced active area of carbon nantotubes. Carbon nanotubes9, were first observed by Sumio Iijima in 1991. Ever since their discovery carbon nanotubes have been extensively researched because of their remarkable mechanical, electrical and electronic properties. Carbon nanotube is a thin graphene sheet rolled into a cylinder with both ends capped. Fig 1a and b show a molecular model of a carbon nanotube. These quasi-one-dimensional carbon whiskers are perfectly straight tubules with diameters of nanometer size, and properties close to that of ideal graphite. Singlewalled nanotubes (SWNTs) with a cylindrical shell (Figure 1a-b) can be considered as the fundamental structural unit. Such structural units form the building blocks of multiwalled nanotubes (MWNTs), containing multiple co-axial cylinders of ever increasing diameter about a common axis. Figure1c shows a High Resolution Transmission Electron Microscopy (HRTEM) image of a MWNT showing concentric cylindrical shells. A SWNT can behave as a well-defined metallic, semiconducting or semi-metallic wire depending on two key structural parameters, chirality and diameter10. Ensembles of nanotubes11 have been used for field emission based flat-panel display, composite materials with improved mechanical properties and electromechanical actuators. Bulk quantities of nanotubes have also been suggested to be useful as

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Copyright © 2005 by Dae-Bok Cho and Nikhil Koratkar . Published by the American Institute of Aeronautics and Astronautics, Inc., with permission.

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high-capacity hydrogen storage media. Individual nanotubes have been used as field emission sources, tips for scanning probe microscopy and as nano-tweezers. Nanotubes also have significant potential as the central elements of nano-electronic devices including field-effect transistors, single-electron transistors and rectifying diodes. Besides many novel structural properties (such as fully reversible large bending angles, an effect associated with the flexibility of the strongly bonded planar hexagonal network), a nanotube is characterized by extremely high Young’s modulus and tensile strength. Table 1 compares the mechanical properties of carbon nanotubes with other conventional materials. One of the areas where carbon nanotube technology could have its most revolutionary impact is in the area of thermal management. The exposed surface area per unit volume of carbon nanotubes is extremely large (> 109 m-1), providing an opportunity to enhance convective heat transfer efficiency at the nanotube interfaces. In this paper we coat pizeoceramic sheets with multiwalled carbon nanotubes (MWNT) and exploit the large specific area of MWNT to enhance the convective heat dissipation from the surface of the piezo sheet to the atmosphere. We show that this results in significantly reduction in material self-heating and power consumption. PATTERNING OF NANOTUBE FILM

For this project we decided to use multiwalled carbon nanotubes since they are much easier to disperse than singlewalled nanotubes which tend to form clusters or aggregates. Multiwalled carbon nanotubes (MWNT), were prepared using a vapor-phase introduction, catalyst-enhanced, chemical vapor deposition (CVD) technique12-13, illustrated in Figure 2. The major difference of this technique in contrast to a typical CVD growth process is the use of a metallocene catalyst precursor (instead of pre deposited catalyst particles) which is carried by the carbon source into a tube furnace. Ferrocene and xylene were selected as the catalyst and carbon source, respectively. The xylene and ferrocene solution was produced by dissolving 0.4g of ferrocene into 40mL of xylene. This solution was then pre-heated to a temperature of ~165° C before being introduced into a tube furnace. Once introduced, the ferrocene decomposes, resulting in the formation of iron nanoparticles that are deposited on the substrate and act as the catalyst for MWNT

growth. The xylene also decomposes in the furnace, providing the carbon feedstock for nanotube growth.

The MWNT films were grown on silica (SiO2) substrates. Silica was used because it provides the strong catalyst-substrate interactions necessary for efficient nanotube growth. The substrates used were ~1 cm in width and ~3 cm in length. Before MWNT growth, they were ultrasonically cleaned in acetone and dried with air, then put into alumina boats and loaded into the tube furnace. The tube was sealed and pumped down to less than 500 mTorr and Argon gas was circulated through the furnace as it was heated to provide an inert atmosphere and minimize the amount of any reactive gases in the tube. At the desired temperature of 800° C, the argon flow was stopped and the preheated xylene/ferrocene solution was then introduced into the system to expose the substrate, producing the MWNT arrays.

Figure 3 shows a typical silica sheet with a thin film of densely packed and vertically aligned MWNT on the top surface. A Scanning Electron Microscopy (SEM) image of the MWNT arrays that decorate the surface of the silica sheets is also shown. Our CVD technique results in MWNT with dimensions of ~30 nm outer diameter, ~10 nm of wall thickness, and several tens of microns in length. The center-to-center distance between individual MWNT within our films has been shown to be ~50 nm14. After fabrication, the nanotubes were purified by room temperature plasma etching to remove amorphous carbon, and other impurities. The purified MWNT were then separated from the silica substrate and suspended in a tetra-hydro-furan (THF) solution. The mixture was sonicated for several minutes at room temperature using a 750W, 20 KHz sonicator (Sonics & Materials Inc., Model: VC750). This solution was then injected on to the surfaces of PZT-5H sheets14 (1”x 0.77”) and allowed to dry, resulting in the precipitation of a thin uniform film of nanotubes on the surfaces of the piezoelectric crystal. Figure 4 shows scanning electron microscopy (SEM) images of the MWNT coated PZT-5H sheet. We observe a reasonably uniform film of nanotubes on the substrate. Note that in the image MWNT have been intentionally scraped off parts of the substrate to expose the underlying PZT-5H grain structure. Based on the SEM images, the thickness of the MWNT film is estimated to be about 500 nm (less than 0.02% of the baseline PZT-5H sheet thickness). Therefore the nanotube film is relatively non-intrusive and is unlikely to interfere with the actuation dynamics of the PZT materials.

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TEST PROCEDURE The PZT-5H sheets15 were dynamically excited in the in-plane mode by the application of a sinusoidal control voltage in the thickness direction. Simultaneous measurements of the piezoelectric strain, PZT surface temperature and current and power consumption were made. Both the baseline as well as MWNT coated samples were tested for comparison. The excitation voltage was applied using a function generator connected to an AC power amplifier and the dynamic piezoelectric strain response was measured using strain gages mounted on the surface of the actuator. The voltage and current outputs of the power amplifier along with the strain gage transducer output was sampled in real time using a DSP Siglab Model 20-42 data acquisition system. The piezoelectric sheet’s surface temperature was also monitored using a K-type thermocouple probe that was directly attached to the sample surface. The samples were tested in the 70-80 Vrms range, and at excitation frequencies in the range of 10-300 Hz.

TEST RESULTS

The instantaneous strain vs. voltage, instantaneous current vs. voltage and temperature of the piezoelectric sheet were recorded for both the baseline as well as the nanotube-coated samples. The results were compared to quantify the performance benefits that accrue from nanotube film deposition. First we measured the strain response of the baseline and MWNT coated samples. As expected, the nanotube surface coating had minimal impact on the strain response of the piezoceramic sheet. Both the baseline uncoated and the MWNT coated samples showed nearly identical results for strain amplitude in the 0-300 Hz frequency range. Figure 5 and 6 show results for peak current consumption and surface temperature for the baseline and MWNT coated samples for 75 Vrms excitation in the 10-300 Hz frequency range. The results indicate that the nanotube coating significantly reduces both the current consumption and the surface temperature particularly at the higher activation frequencies. The reduced temperature confirms that the increased surface area of the piezo sheet serves to improve heat convection from the piezoceramic to the atmosphere, thereby alleviating material self-heating effects. The self-heating of the piezoceramic increases the material dielectric permittivity and hence the capacitance leading to increased current consumption. However as our results indicate, the

nanotube coating alleviates material self-heating and this in turn serves to reduce current consumption. The improvements that are seen are quite substantial. For example, for 75 Vrms activation at 300 Hz, the surface temperature and current consumption are reduced by about 25% and 15% respectively for the MWNT case. Further improvements can be expected by testing singlewalled carbon nanotube (SWNT) coatings. This is bacuase SWNT has a much greater exposed surface area per unit volume as compared to MWNT; for example it has been shown that the specific area16 of SWNT can be as high as 1000 m2/gm compared to only about 100 m2/gm for MWNT. The increased exposed surface area of SWNT compared to MWNT implies that the SWNT coated sample would be even more effective in drawing energy away from the bulk piezoceramic material and dissipating it to the atmosphere. Testing is on-going in our lab to characterize the performance of SWNT coated samples.

SUMMARY AND CONCLUSIONS

Our tests indicate that a nanotube film coating can reduce the power consumption of piezoceramic actuators for high frequency actuation. The nanotubes within the film serve to convect thermal energy away from the bulk piezoceramic material to the surrounding atmosphere. Reduced self-heating results in lower capacitive losses and reduced current and power consumption of the piezoceramic material, particularly at high excitation frequencies. Up to 25% reduction in self-heating and 15% reduction in power consumption is demonstrated in this paper. The nano-film coating is extremely lightweight, minimally intrusive and does not adversely affect the strain response of the piezoceramic material. This technology shows the potential to result in very substantial reductions in power consumption coupled with an improvement in fatigue life for a range of piezoelectric actuators.

ACKNOWLEDGEMENTS We thank the National Science Foundation for sponsoring this research under the Faculty Early Career Development (CAREER) program, with Dr Yip-Wah Chung serving as the technical monitor. The authors would also like to thank Professor Pulickel Ajayan for providing access to CVD and to J. Suhr for help with SEM characterization.

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REFERENCES

1. Koratkar, N., and Chopra, I., “Analysis and Testing

of a Froude-Scaled Rotor with Piezolectric Bender Actuated Trailing-Edge Flaps,” Journal of Intelligent Material Systems and Structures, Vol. 8, (7), July 1997, pp. 555-570.

2. Fulton, M., and Ormiston, R., “Hover Testing of a

Small-Scale Rotor with On-Blade Elevons,” American Helicopter Society 54th Annual Forum Proceedings, Washington, DC, May 20-22, 1998.

3. Prechtl, E., and Hall, S., “Design of a High

Efficiency Discrete Servo-Flap Actuator for Helicopter Rotor Control,” SPIE Smart Structures and Materials Symposium Proceedings, San Diego, CA, March 3-6, 1997.

4. Bernhard, A., and Chopra, I., “Hover Testing of an

Active Rotor Blade Tip,” Journal of Intelligent Material Systems and Structures, Vol. 9, (12), December 1998, pp. 963-974.

5. Wilbur, M., Wilkie, W., Yeager, W., Lake, R.,

Langston, C., Shin, S., and Cesnick, C., “Hover Testing of the NASA/ARL/MIT Active Twist Rotor,” 8th Army Research Office Workshop on the Aeroelasticity of Rotorcraft Systems, State College, PA, October 18-20, 1999.

6. Sirohi, J. and I. Chopra. 1998. “Investigations on

Piezoelectric Actuator Response”, in Proceedings of the SPIE Smart Structures Conference, pp. 626-646, San Diego, California.

7. Sirohi, J. and I. Chopra. 2000. “Actuator Power

Reduction using L-C Oscillator Circuits”, in Proceedings of the 41st AIAA/ASME/ASCE/AHS/ASC Structures, Structural Dynamics and Materials Conference, Atlanta, GA.

8. Koratkar, N. and I. Chopra. 2002. “Wind Tunnel

Testing of a Smart Rotor Model with Trailing-Edge Flaps”, Journal of the American Helicopter Society, Vol. 47, No. 4, pp. 263-272.

9. Iijina, S. 1991. "Helical Microtubules of Graphitic

Carbon", Nature, 354, 56.

10. Dresselhaus, M. S., G. Dresselhaus, and P. Avouris.

2001. “Carbon Nanotubes: Synthesis, Structure, Properties and Applications”, Topics in Applied Physics, Vol. 80, pp. 391-425.

11. Ajayan, P., and O. Zhou. 2001. “Applications of

Carbon Nanotubes”, Topics in Applied Physics, 80, 391-425.

12. Wei, B., Z. Zhang, G. Ramanath and P. Ajayan.

2000. “Lift-Up Growth of Patterned Aligned Carbon Nanotubes”, Applied Physics Letters, 77, 2985-2987.

13. Wei, B., R. Vajtai, Y. Jung, J. Ward, R. Zhang, G.

Ramanath and P. Ajayan. 2002. “Organized Assembly of Carbon Nanotubes”, Nature, 416, 495-496.

14. J. Drotar, B. Wei, Y. Zhao, G. Ramanath, P. Ajayan,

T. Lu, and G. Wang, Phys. Rev. B, 64, 125417, 2001. 15. Morgan Matroc, Inc. (Electro Ceramics Division)

1997. “Guide to Modern Piezo-ceramics”, Bedford, Ohio.

16. Zhou, X., E. Shin, K. W. Wang, and C. E. Bakis.

2003. “Damping Characteristics of Carbon Nanotube Based Composites”, ASME International Design Engineering Technical Conference, Chicago, IL.

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FIGURES

Material Density gm/cc

Modulus (E) Gpa

Strength MPa

Specific Strength MPa/(gm/cc)

Specific Modulus GPa/(gm/cc)

% Strain to Failure

Steel 7.85 207 300-1600 40-200 26 10 Al 2.7 69 100-600 37-220 26 15 E fiber 2.58 71 1400-2500 540-1000 28 2-3 Boron 2.57 420 4200 1630 160 - C fiber 1.8-2.15 240-550 2000-3500 1000-1700 120-160 0.4-1.5 Kevlar 1.47 154 2800 1900 105 2-3 Graphite 2.25 1000 20000 9000 450 - Carbon Nanotubes

1.31 800-1000 50000 38460 615-770 25-30

Diamond 3.51 107.6 1052 30.6 300 -

Fig. 1 (a) Molecular model of an open singlewalled carbon nanotube; (b) molecular model of a closed singlewalled carbon nanotube. (C) TEM image of a multiwalled carbon nanotube with multiple coaxial concentric cylinders.

Table 1: Comparison of mechanical properties of carbon nanotube fibers with typical fiber materials used in composites

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Figure 2: Schematic of thermal chemical vapor deposition (CVD) used for MWNT fabrication

Figure 3: Aligned array of MWNTs fabricated using thermal CVD

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CnHm FeCatalyst Support

CnHm CnHm

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CnHm

Oven Temperature: 800° C

CnHm

Catalyst: Fe Nano-Particles

FeCatalyst Support

CnHm CnHm

FeCatalyst Support: Silica

CnHm CnHm

Multi-WalledNano Tubes

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CnHm FeCatalyst Support

CnHm CnHm

FeCatalyst Support

CnHm CnHm

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CnHm

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CnHm

Catalyst: Fe Nano-Particles

FeCatalyst Support

CnHm CnHm

FeCatalyst Support: Silica

CnHm CnHm

Multi-WalledNano Tubes

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Figure 4: SEM image of the surface of PZT-5H sheet coated with MWNT

Figure 5: Results for current consumption

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Figure 6: Results for surface temperature

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