Composites of Graphene Nanoribbon Stacks and Epoxy for Joule

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Composites of Graphene Nanoribbon Stacks and Epoxy for Joule Heating and Deicing of Surfaces Abdul-Rahman O. Raji, Tanvi Varadhachary, Kewang Nan, Tuo Wang, Jian Lin, Yongsung Ji, Bostjan Genorio, Yu Zhu, Carter Kittrell, and James M. Tour ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11131 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 25, 2016

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ACS Applied Materials & Interfaces

Composites of Graphene Nanoribbon Stacks and Epoxy for Joule Heating and Deicing of Surfaces Abdul-Rahman O. Raji,1 Tanvi Varadhachary,1,2 Kewang Nan,3 Tuo Wang,1 Jian Lin,3,4 Yongsung Ji,4 Bostjan Genorio,3,5 Yu Zhu,1,6 Carter Kittrell,4 and James M. Tour1,3,4,* 1

Department of Chemistry, 3Department of Materials Science and NanoEngineering, 4Richard E. Smalley Institute for Nanoscale Science and Technology, Rice University, 6100 Main Street, Houston, Texas 77005-1892. 2St. John's School, 2401 Claremont Lane, Houston, Texas 77019, 5

Faculty of Chemistry and Chemical Technology, University of Ljubljana, Aškerčeva cesta 5, 1000 Ljubljana, Slovenia, 6Department of Polymer Science, The University of Akron, Ohio 44325-3909 *Email: [email protected]

ABSTRACT A conductive composite of graphene nanoribbon (GNR) stacks and epoxy is fabricated. The epoxy is filled with the GNR stacks, which serve as a conductive additive. The GNR stacks are on average 30 nm thick, 250 nm wide and 30 µm long. The GNR-filled epoxy composite exhibits a conductivity >100 S/m at 5 wt% GNR content. This permits application of the GNR-epoxy composite for deicing of surfaces through Joule (voltage-induced) heating generated by the voltage across the composite. A power density of 0.5 W/cm2 was delivered to remove ~1-cm-thick (14 g) monolith of ice from a static helicopter rotor blade surface in a ̶ 20 °C environment.

KEYWORDS: graphene, deicing, Joule heating, graphene nanoribbons, composite 1 ACS Paragon Plus Environment


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INTRODUCTION

Ice accumulation on surfaces such as on helicopter rotor blades, aircraft wings and tails, wind turbines, and transmission lines is a prevalent problem that degrades performance of the structures upon snow/ice accumulation.1,2 For instance, helicopter rotor blades are precisely designed and machined to generate airflow that supports buoyancy, but the presence or accumulation of snow and ice on the blade can cause surface roughness or shape modification that can compromise aerodynamic performance by perturbing airflow around the blade.3,4 Methods of ice removal include sprayed chemicals or hot fluid,5 mechanical force, infrared radiation,6 and Joule heating.7-9 Joule or resistive heating is an electrothermal technique that results from dissipation of electrical power generated in an electrically conductive material upon applied voltage, and it is an effective, energy-efficient, and versatile method of ice removal that is also capable of real-time deicing.10 During Joule heating, a thin layer of ice near the interface between the ice and a heated surface can melt to generate a layer of water beneath the ice, thereby weakening the ice adhesion to facilitate removal by gravity, wind, centrifugal force or inflated pneumatic wrap.11 Lightweight, flexible, robust, and stable polymers could serve as an alternative to rigid, heavier metals or metal alloys as Joule heating elements if they were electrically conductive. However, such robust and stable polymers, for example epoxy, polyimide, and polyurethane, are insulating. Inclusion of additives, such as carbon black,12 carbon nanotubes,13 graphene,14 and silver nanowires,15 in the polymers generates an electrical network through the polymers to create a conductive polymer composite. Because the conductivity depends on interconnection of the additives to form an electrical pathway, large aspect ratio conductive additives are desirable for conductive polymer composites.

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The GNR stacks used here are multilayered with ~ 30 nm thickness obtained from unzipping of multiwalled carbon nanotubes. They are a quasi one-dimensional sp2-carbon nanostructure and an analogue of graphene that has high electrical conductivity. GNR stacks embedded in an epoxy-based polymer matrix could provide tunable electrical conductivity for the otherwise insulating polymer. GNR stacks are desirable conductive additives because of their good electrical16,17 and thermal18,19 conductivities. Their high aspect ratios permit small amounts to form percolative networks. If long enough, they need only be a relatively small fraction of a polymer composite to provide significant conductivity enhancement to the composite. Here, we demonstrate that GNR-filled epoxy composites have sufficient electrical conductivity with inter-stack resistance at relatively low GNR content (≤5 wt%) to perform voltage-induced heating. The GNR stacks were successfully applied as a stable additive for Joule heating and deicing of helicopter rotor blade surfaces. This was demonstrated in a helicopter rotor blade system which consists of a rotor blade and a typical protective metal sleeve bonded with epoxy composite. By adding GNR stacks to the interlayer epoxy composite, the composite was made conductive, and it generated thermal energy upon applied voltage for deicing of the blade surface. This development will likely translate to other composite aircraft components and surfaces. It can yield immediate translation for formation of deicing coatings that can also mitigate lightning-induced charging of composite aircraft skins and electromagnetic shielding due to the conductive nature of the GNR-epoxy composite. EXPERIMENTAL SECTION GNR-Epoxy Composite Fabrication. Pristine GNR stacks were obtained from AZ Electronic Materials Corp., now EMD-Merck (H-GNRs, Batch no: 2699-119), and were used without any 3 ACS Paragon Plus Environment


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further treatments. To prepare a GNR-epoxy composite, 12 to 290 mg GNRs were added and blended in 3.9 mL epoxy (Loctite) matrix with a spatula. 1.1 mL hardener (Loctite) was added and further blended. GNR-epoxy composites containing GNRs of ~0.2 to 5 wt% (of total composite mass) were thus produced. Larger samples were prepared by scaling according to the above ratio of GNRs, epoxy and hardener weights. The composites were heated on a hot plate or in an oven at 70 °C for 3 h for curing. Characterization. Transmission electron micrographs (TEMs) of samples prepared on an amorphous carbon-coated TEM grid were acquired with a JEOL 2100F field emission gun transmission electron microscope (TEM). TEM samples were prepared by placing ~ 1 mg of the sample in ortho-dichlorobenzene (ODCB) (Sigma-Aldrich), sonicating to form a dispersion and drop casting on the grid. Scanning electron micrographs (SEMs) of powder samples placed on a double-sided carbon tape were acquired with a JEOL 6500 field emission gun scanning electron microscope. Raman spectral plots of powder samples placed on a glass slide were acquired with a Renishaw inVia Raman microscope equipped with 514 nm Ar ion laser and WiRe software. Xray diffractograms of powder samples mounted on a grooved zero background holder were acquired with a Rigaku D/Max Ultima II Powder X-ray diffractometer equipped with a Cu Kα radiation source (λ = 1.5418 Å) and JADE 2009 software. Electrical Measurements of GNR-Epoxy Composites. ~0.2 to 5 wt% GNR in epoxy samples were prepared as described above. The composites were cast in a silicone mold with a rectangular groove. The surfaces were smoothed with a spatula. The samples were then heated on a hot plate at 70 °C for 3 h. The molded rectangular bar of cured GNR-epoxy composite (dimensions, lwh: 2.5 × 0.6 × 0.5 cm3) were used in all conductivity measurements. Colloidal silver paste (Pelco Colloidal Silver Liquid, Ted Pella) was applied on two ends of the sample to 4 ACS Paragon Plus Environment

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reduce contact resistance between the composite and the probes during resistance measurement. The two probes of a Cen-Tech digital multimeter were placed on the silver-coated ends of the composite bar to measure its resistance (). A dimensionless resistance (Rs) was calculated based on the measured resistance and composite geometry with = × / where w and l are the width and length of the composite bar, respectively. The DC conductivity, , was calculated with =

×

where h is the height of the composite bar.

Joule Heating and De-icing Experiments. Joule heating experiments were conducted as described under the relevant sections by applying the reported constant voltages across the device (see Results and Discussion for specifics and photograph of the experimental set-up). 5 wt% GNR stacks were used in the GNR-epoxy composite for the experiments. DC voltages were used in all cases. The deicing experiment was conducted at −20 °C in a Styrofoam™ box by clamping the rotor blade segment on a ring stand. The box was cooled to −20 °C by placing chunks of dry ice in it. Thereafter, cold water was sprayed on the rotor blade segment from a wash bottle to form ice. The temperature was stabilized at ~ −20 °C by adding or removing dry ice chunks. A constant voltage was applied across electrodes attached on both sides of the rotor blade segment (see Results and Discussion for specifics and photograph of the experimental setup). RESULTS AND DISCUSSION



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1000 1500 2000 2500 3000

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Figure 1. Structural characterization of GNR stacks. (a) Schematic of a GNR stack (the actual GNR stack in this report contains more layers than shown in the schematic). (b) TEM and (c) SEM images of GNR stacks. (d) XRD pattern and (e) Raman spectrum of GNR stacks. (f) TGA curve of GNR stacks under air atmosphere at a rate of 10 °C/min.

In Figure 1a-c, the GNR stacks are shown to have up to 60 layers. There are up to 30 nm thick, 350 nm wide, and 50 µm long.20 The length-to-width aspect ratio is ~140. The (002) diffraction peak at 26.3° reveals a d-spacing of ~3.34 Å for the GNR stacks (Figure 1d). The Raman spectrum exhibits characteristic well-defined G and 2D bands at ~1587 cm-1 and ~2688 cm-1, respectively (Figure 1e). The G band is the stretching mode of the sp2 carbon atoms whereas the 2D band is the breathing mode of the sp2 rings, emerging from the two-phonon double resonance scattering,21 thus signifying a very high basal plane π-conjugated sp2-carbon structure. In the case of the D band, it is produced from a one-phonon double resonance process where the Raman fundamental selection rule of momentum conservation only becomes satisfied 6 ACS Paragon Plus Environment

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with the presence of defects unlike the two-phonon double resonance 2D band.21 The pronounced intensity of the D band in the Raman spectrum of the GNR stacks can be attributed to the relatively large amount of edges whereby the edges allow laser-excited electrons in the material to be backscattered21 in order to satisfy the Raman fundamental selection rule.22 The edges of the GNR stacks provide defect sites for the observation of an intense D band since the GNR stack width (up to 350 nm) is smaller than the laser spot diameter (a few microns). In addition, the GNR stacks are stable in air at room temperature, and oxidative decomposition only commences above 400 °C by thermogravimetric analysis (TGA) (Figure 1f). The material’s structural and chemical stability make it suitable for Joule heating and deicing applications.



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GNR Weight Fraction (wt%) Figure 2. Electrical conductivity of GNR-epoxy composites. (a) A bar-shaped GNR-epoxy composite. (b) SEM image of cross-section of GNR-epoxy composite. (c) Resistance of the GNR-epoxy composite. (d) Electrical conductivity as a function of GNR weight fraction. The inset shows the electrical conductivity as a function of GNR volume fraction.

The fabricated composites are used in conductivity measurements by applying voltage across the bar from the contacts at the ends (Figure 2a). The GNR stacks form a network of percolating electrical channels inside the crosslinked epoxy matrix to generate an electrically 8 ACS Paragon Plus Environment

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conducting composite material (Figure 2b). The dark regions represent the interconnected GNR stacks. The brighter regions signify charge accumulation by the insulating epoxy under the electron beam of the SEM. The composite produces resistance that reaches as low as 1 Ω at 5 wt% GNR content (Figure 2c). The GNR-epoxy composite produces conductivity of 10-4 S/m at 0.2 wt% GNR content, and the conductivity is >100 S/m at 5 wt% (2.5 vol%) (Figure 2d). The conductivity of the GNR-epoxy composite rises sharply initially, but after 2 wt% GNR content, the conductivity of the composite rises more slowly with increasing amount of the GNR stacks. This behavior of the electrical conductivity of conductive GNR-epoxy composites follows the power law typical of polymer composites containing conductive fillers, such as graphene and silver nanowires.14-15 The distinction between the two conductivity regimes can be explained by the different conduction mechanisms in both regimes. At low GNR concentrations, the GNR stacks cannot form a continuous network so transport between GNR stacks through the epoxy matrix is more prominent. As the concentration increases, a continuous percolative network is formed and conduction occurs directly between the GNR stacks, though still influenced by contact resistance between the GNR stacks. Despite the differing conductivity regimes, there is an increase in the conductivity throughout the range of the GNR contents studied (Figure 2d) and the conductivity is expected to increase with even higher GNR content. Moreover, we and others have shown that the tensile strength of composites, including epoxies, increased by more than 20% and the Young’s modulus increased by more than 30%, with 0.3 wt% GNR addition. But at 5 wt% GNR, which is the amount use in our current work, both the tensile strength and Young’s modulus were reported to decrease by ~30%.23-25 The quality of the filler dispersion is crucial for mechanical properties especially at high loadings where the filler is more susceptible to aggregation, thereby decreasing the composite mechanical performance. This is a prevalent



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problem in nanocomposites, but the mechanical properties are expected to improve with better dispersion of the filler. Joule Heating of GNR-Epoxy Composite

Figure 3. Joule heating of GNR-epoxy composite. (a) Schematic of a Joule-heated GNR-epoxy composite device. (b) Photograph of Joule-heated GNR-epoxy composite device. The resistances across each portion are shown. Scale: cm. (c–e) Heating profile of the GNR-epoxy composite at different applied voltages. The top surface temperature was measured using an infrared


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thermometer with a spot size of ~ 2 cm. The three sets correspond to the respective resistance regions shown in (b). The tests were conducted at room temperature and the experiment was conducted in a well-ventilated laboratory hood.

The GNR-epoxy composite was divided into segments with silver contacts deposited at the ends of the sample and internal segments for resistance measurement (Figure 3a,b). The GNR-epoxy composite contained 5 wt% GNR stacks because the conductivity does not increase significantly beyond that percentage (Figure 2d). However, the viscosity increased with more GNR content which made the composite difficult to blend. Voltage is applied across the entire sample from the two ends and the temperature of each segment is measured to evaluate the temperature across the composite during Joule heating (Figure 3c–e). Each segment of the device has a different resistance value due to a slight variation in average thickness. Since the current through the entire composite is the same, the higher resistance segment is expected to exhibit higher temperature due to higher power ( = ). The heating profiles of the composite segments are shown in the time-dependent temperature profiles in (Figure 3c–e). Each profile shows that the right end has higher temperature than the left end because of higher resistance. Though the left segment has a higher resistance (and thus higher power) than the middle segment, its temperature is comparable to that of the middle segment because of greater heat dissipation. The temperatures increase with applied voltage for all segments (Figure 3c–e). As the voltage increases, the power delivered through the composite increases according to =

, generating

higher surface temperature. However, since there is higher heat dissipation with increased power, the effect is non-linear. For the first segment at 20 V, the temperature plateaus at 45 °C with an applied voltage of 20 V (Figure 3c). When the voltage is increased to 40 V, the power 11 ACS Paragon Plus Environment


increases 4× but the temperature rises 2.5×. Similar behavior is demonstrated in all three segments. This shows that more heat is dissipated at higher power (Figure 3c–e). Such segmental tunability of temperature could be advantageous for removing varying amounts of ice on a structure such as a rotor blade with higher ice accretion at the leading edge.

By TGA (Figure 4), the onset of decomposition of the GNR-epoxy composite is at ~300 °C in air. The TGA curve for the GNR-epoxy composite coincides with that of neat epoxy composite below 400 °C. After 400 °C, decomposition of the GNR stacks contribute to the GNR-epoxy weight loss. Because the GNR additive is more stable toward oxidative decomposition than the epoxy used here, it gives room for use of more thermally stable epoxy in an oxygen-containing environment.

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Figure 4. TGA studies of thermal stability of GNR-epoxy composite in air at a heating rate of 10 °C/min.

A deicing experiment conducted in a Styrofoam box held at ̶ 20 °C (Figure 5) of GNRepoxy composite is demonstrated as a deicing heating layer that holds together a rotor blade and its nickel sleeve while delivering heat for deicing of the nickel surface. A segment of commercial rotorblade and its epoxy overlayer, and its standard nickel abrasion shield were used in this experiment (Carson Helicopter). Since nickel is thermally conductive, heat can be transferred from the GNR-epoxy heater to the nickel surface for deicing. The only thermal barrier is a layer of bare epoxy coated under the nickel to prevent electrical shorting between the GNR-epoxy composite and the nickel. 14 g of ice on this rotor blade segment melted off the blade in 15 min. If the blade had been spinning, centrifugal force would have immediately removed the ice once an underlayer of water formed, thus greatly reducing the time for ice removal. It should be noted that the GNR-composite was intact and no delamination was observed from the rotor blade after cooling and heating cycles. However, no further mechanical tests were done to see how the GNRs might have influenced the adhesion properties.



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Figure 5. Joule heating and deicing of helicopter rotor blade segment. (a-b) Schematics of the GNR-epoxy composite adhesive fabrication on a helicopter rotor blade. (c-g) Schematic of the coating of a rotor blade segment with the GNR-epoxy composite, followed by complete assembly. (h-i) Photographic image of deicing through Joule heating of a 20.3 cm-long segment of the rotor blade. Resistance across composite: 34 Ω; applied voltage: 45 V; current: 1.3 A; power: 44 W; heated area: ~20.3 × 5.1 cm2. Atmosphere held at −20 °C. The ice was formed on the rotor blade surface by first adding dry ice to the Styrofoam box until it reached −20 °C and stabilized, followed by spraying of cold de-ionized water from a wash bottle to result in a frozen


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layer of 14 g of ice on the rotor blade segment. (h) The set-up before ice formation. (i) After ice formation and before applying a voltage. (j) After ice removal.

4. Conclusion The work here demonstrates the efficacy of GNR stacks in epoxies to effect Joule-heating of composite structures for deicing applications. The tunable conductivity of the GNR-epoxy composites and low GNR content required make this an attractive method to use in aircraft composites. Though a metallic shield is used as an overcoat component in the helicopter blade, here such an over-structure would not be needed in fix-wing aircraft composites. Hence, the use of GNR-epoxies could have widespread application in the aircraft industry, as well as in power lines and other fixed or mobile platforms. Further, use in paints, and other coating structures might further extend the utility of this GNR-active material. Acknowledgements We thank Carson Helicopter for kindly providing the rotor blade section, the epoxy and the nickel abrasion shield, and for their helpful guidance with this research. Funding was provided by the AFOSR (FA9550-14-1-0111). Competing Financial Interests A.-R. O., Y. Z., and J. M. T. are co-inventors on the patent application (US Patent App. 13/985,458) owned by Rice University that disclose the methods of making GNR-based deicing composites.



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TOC Figure

GNR-epoxy composite heater

helicopter rotor blade segment

deicing set-up

before

after


Composites of Graphene Nanoribbon Stacks and Epoxy for Joule

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