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Three – Dimensional Graphene Foam - Polymer Composite with Superior Deicing Efficiency and Strength Jenniffer Bustillos, Cheng Zhang, Benjamin Boesl, and Arvind Agarwal ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b18346 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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ACS Applied Materials & Interfaces

Three – Dimensional Graphene Foam - Polymer Composite with Superior Deicing Efficiency and Strength Jenniffer Bustillos, Cheng Zhang, Benjamin Boesl, Arvind Agarwal* Plasma Forming Laboratory Department of Mechanical and Materials Engineering Florida International University, Miami, FL 33174, USA KEYWORDS: graphene foam, joule heating, deicing, thermal transport, thermoelectrical stability, deicing efficiency.

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ABSTRACT

The adhesion of ice severely compromises the aerodynamic performance of aircrafts operating under critically low-temperature conditions to their surfaces. In this study, highly thermally and electrically

conductive

graphene

foam

polymer

composite

is

fabricated.

GrF

–

polydimethylsiloxane (PDMS) deicing composite exhibit superior deicing efficiency of 477 % and electrical conductivities of 500 S∙m-1 with only 0.1 vol. % Graphene foam addition as compared to other nanocarbon – based deicing systems. The three-dimensional interconnected architecture of GrF allows the effective deicing of surfaces by employing low power densities (0.2 W∙cm-2). Electrothermal stability of the GrF-PDMS composite was proven after enduring 100 cycles of the DC loading –unloading current. Moreover, multifunctional GrF – PDMS deicing composite provide simultaneous mechanical reinforcement by the effective transfer and absorption of loads resulting in a 23% and 18% increase in elastic modulus and tensile strength respectively as compared to pure PDMS. The enhanced efficiency of the GrF – PDMS deicing composite is a novel alternative to current high – power consumption deicing systems.


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1. INTRODUCTION The aerodynamic performance of aircrafts, wind turbines, rotor blades and other structures operating under critically low temperatures ( < 0 °C), is severely compromised by the adhesion of ice to their surfaces.1 For instance, the continuous accumulation of ice on aircraft surfaces is known to result in the disruption of the airflow. As a result, increases in drag forces and higher losses of energy are experienced due to the mass-imbalance in the structures.1, 2 Current efforts to reduce and mitigate the ice formation in aircraft surfaces include the dispersion of chemicals, mechanical removal and electrical heating of surfaces.1,

2

Electrical

heating systems have proven their suitability as deicing systems due to the facile induction of heat to promote the continuous removal of ice.1 However, the implementation of metals and alloys in current – induced heating elements results in high power consumptions. Consequently, recent studies3-10 have focused on the inclusion of 1D and 2D carbon-based conductive nanoparticles such as carbon nanotubes (CNT) and graphene to develop efficient deicing systems. Due to their lightweight, superior electrical and thermal performance these conductive particles are considered beneficial as compared to conventional metal-based systems.7,

11

The

effectiveness of such systems depends on the intrinsic electrical and thermal conductivity of the filler, as well as the content of the conductive component. For instance, Wang et al.5 demonstrated the potential of functionalized graphene nanoribbons (FDO/GNR) as an anti-icing and active deicing film. The resulting FDO/GNR film is capable of preventing the formation of ice in surfaces up to temperatures of −14°C. Moreover, deicing of surfaces is achieved by resistive heating of the film in periods of 90 s by implementing power densities of 0.2 W∙cm-2. However, the removal of water remnants in the surface requires the introduction of a lubricating liquid.5 Despite their success in the deicing of surfaces, the performance relies strongly on the


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distribution of conductive particles in the matrix/surface effectively generating an electrically conductive network.5 Therefore, high concentrations of conductive fillers (GNR, CNT, Graphene sheets) in the range of 2.5 – 100 vol. % are required to form a complete electrically conductive network.3, 5, 12, 13 Their application is limited due to their tendency to form agglomerates resulting in the poor dispersion. Such behavior in carbon-based nanofillers is attributed to their high surface energy and strong − interactions.6, 14-16 In efforts to address the limitations of 1D and 2D conductive nanofillers in deicing systems, this study introduces three – dimensional (3D) Graphene foam (GrF) as a conductive component to develop Graphene foam - polymer composites with deicing capabilities for the first time. Graphene foam consists of a three-dimensional interconnected architecture which serves as an intrinsic uniform electrical and thermal transport path. It combines an extremely low density (4 mg∙cm-3) with graphene's high electrical (106 S∙m-1) and thermal conductivity (2000 – 4000 W∙m1

∙K-1).17,

18

The interconnected network of branches and nodes are desirable for applications

where the uninterrupted transfer of electrons and heat transfer is required. Thus, eliminating losses due to

agglomerations and filler interfacial resistances that are detrimental to the

electrical/thermal conductivity of the composite. The large surface area provided by the 3D structure of GrF is not limited to enhancing the electrical performance of polymer composites.17, 19

For instance, Chen et al.20 demonstrated the excellent mechanical stability of a porous GrF –

PDMS composite by proving its unchanged electrical conductivity (2∙cm-1) after being subjected to 10000 bending cycles.20 Similarly, the superior thermal response of Grf was shown by Zhang et al.21, where a fluoroalkyl-silane (FS) modified – GrF exhibited maximum heating rates of 2.6 °C∙s-1 at powers of 3 W and a highly hydrophobic nature.21 A recent study performed by our group demonstrated the addition of 2 wt. % GrF to an epoxy matrix resulted in the increase of its


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glass transition temperature by 53 % improving its thermal stability, while simultaneously presenting superior damping and electrical properties.22 This makes GrF a great candidate as a reinforcement to manufacture light-weight composite with enhanced multifunctional (electrical, thermal, and mechanical) properties to serve as active deicing components in aircraft structures. In this study, we introduce the development of a highly conductive multifunctional Graphene foam – PDMS composite with high-efficiency current – induced deicing capabilities. The deicing composite is fabricated by the infiltration of the interconnected architecture of Graphene foam (GrF) by a low viscosity polydimethylsiloxane (PDMS) matrix. PDMS polymer is mainly found in aircraft applications as a coating due to its ability to absorb stresses induced by thermal cycling.1,

2

The strong chemical bonding of PDMS elastomers in their siloxane chain is

responsible for the excellent heat and oxidation resistance found in PDMS. Also, methyl groups present in the Si-O backbone result in a highly hydrophobic nature.2 Therefore, PDMS is highly favored as a matrix for the development of GrF – polymer composites with deicing capabilities. The high surface area of Graphene foam in the deicing composite demonstrates high deicing efficiency at merely 0.1 vol. % GrF concentration, representing two orders of magnitude higher than previously reported deicing systems of dispersed carbon-based fillers.3 The suitability of GrF – PDMS composites as deicing systems was evidenced by its superior stability after enduring continuous cyclic electrical and thermal loading and unloading up to 100 cycles. Also, enhanced tensile strength by 18 % for mere 0.1 vol. % GrF – PDMS proves its multifunctionality with excellent deicing efficiency. 2. EXPERIMENTAL SECTION


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2.1. Materials. Free-standing Graphene foam used in this study was procured from Graphene Supermarket

(Calverton,

NY,

USA)

and

used

as-received.

A

two-component

polydimethylsiloxane (PDMS) with a viscosity of 1000 cps (SilGel 612) were obtained from Wacker Chemie AG (Munich, Germany). Wetting and infiltration behavior between Graphene foam and low viscosity PDMS were studied by sessile drop method (KYOWA Contact Angle Meter (DM-CE1), Saitama, Japan). A droplet of uncured PDMS was deposited on the surface of Graphene foam, and the contact angles were measured using ImageJ software (Figure S1). 2.2. Synthesis of Graphene foam – PDMS deicing composite. Graphene foam with dimensions of 10 mm in length, 4 mm width, and 1.2 mm in thickness was initially connected to a platinum wire (Pt) of 0.1 mm diameter (Surepure Chemetals, LLC, New Jersey, USA) using conductive Pelco colloidal silver paste (Clovis, CA, USA) and cured at 100°C for 30 min. The colloidal silver paste was added at the interface of the Pt wire and Graphene foam to reduce the contact resistance during current – induced heating experiments. Upon reaching complete curing of the conductive adhesive, 3D graphene foam was infiltrated by low viscosity PDMS matrix by casting method on a glass slide to produce free-standing specimens. Casting and infiltration consisted in the pouring of liquid PDMS onto the GrF – electrode configuration. Similarly, Graphene foam – PDMS composites were cast as coatings on Ti-6Al-4V substrates with dimensions of 25 mm in length, 10 mm in width and 1.8 mm thickness to evaluate the performance of the deicing composite simulating its application in aircraft structures. To promote the adherence of the deicing composite as a coating to the substrate, the surface of the Ti-6Al-4V substrate was prepared by grit blasting before casting resulting in an average roughness of 1.50 ± 0.05 µm. The PDMS matrix was made by mechanical stirring of crosslinking-containing component (A) and platinum catalyst-containing component (B) in a


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2A:1B ratio. The resulting cast Graphene foam – PDMS deicing composite was cured at 100°C for 30 min. 2.3. Electrical Measurements. Electrical properties of the deicing composite were evaluated by a Keithley 2401 digital source meter (Cleveland, OH, USA). Resistances () of the Graphene foam – PDMS composite were measured by four-point probe method. For which, the corresponding current – voltage response across the composite to a constant DC current results from two pairs of probes attached to the Platinum wire side-electrodes. Sheet resistance was used to relate the geometry and resistance in the composite; = ×

(1)

where and refer to the width and length of Graphene foam in the composite, corresponding to the spacing between probes. The respective conductivity of the composites was computed as =

(2)

×

For which represents the cross-sectional thickness of the Graphene foam in direct contact with the Pt wire. 2.4. Current-induced heating and deicing experiments. Current-induced heating experiments were carried out by supplying DC currents through the deicing composite following the set-up described for electrical measurements. In this study, platinum wire is used to serve as electrical leads due to the thermal stability and oxidation resistance it provides resulting suitable for thermal cycling applications such as those encountered in heating elements. Changes in temperature on the surface of the Graphene foam – PDMS deicing composite were measured with the aid of a Raytek MX4+ Infrared (IR) Pyrometer (FLUKE, Santa Cruz, CA). Accuracy of the measured surface temperature is of ± 1 ºC.


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Similarly, deicing experiments were carried out inside a Styrofoam box under a controlled ambient temperature of −20°C achieved and maintained with the aid of dry ice. Freestanding Graphene Foam – PDMS samples were placed on top of a glass slide for the stability of flexible composite during deicing experiments. Droplets of deionized water of 0.2 mL were placed on the surface of deicing composite until the solid state was reached. Both, 0.1 vol. % Graphene foam – PDMS/Ti-6Al-4V and free-standing 0.1 vol. % Graphene foam – PDMS deicing composite, was tilted at an angle and supplied a constant current of 0.4 A during deicing experiments. The corresponding change in surface temperature was captured by IR pyrometer and a FLIR ONE thermal imaging camera (Wilsonville, OR, USA). 2.5. Mechanical Characterization. The tensile behavior of 0.1 vol. % Graphene foam – PDMS composites and pure PDMS was evaluated by an MTS universal tester (Series 40) (Eden Prairie, MN, USA), at a constant crosshead speed of 0.1 mm s-1. Tensile specimens of pure PDMS and GrF – PDMS composite had dimensions of 30 mm gauge length, 7 mm width and 2 mm in thickness. Analysis of the fracture surfaces was performed by using a JEOL JSM 6330F field emission scanning electron microscopy (FE-SEM) (JEOL, Tokyo, Japan). 3. RESULTS AND DISCUSSION 3.1. Microstructural Characterization. The deicing composite fabricated in this study introduces Graphene foam (GrF) as an electrically and thermally conductive component. Freestanding GrF consists of an interconnected graphene three – dimensional architecture with pore sizes in the range of 100 – 300 µm in diameter (Figure 1). The GrF – Polydimethylsiloxane (PDMS) deicing composite fabrication process is schematically illustrated in Figure 1. The assembly consisted of the direct connection of GrF to Platinum (Pt) wire to act as thermally stable electrodes. It was followed by the casting and infiltration of 3D Graphene foam by a low


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viscosity PDMS polymer (1000 cps). PDMS polymers are well-known elastomers used in the aerospace industry due to their excellent oxidation and thermal resistance. Freestanding GrFPDMS composites were cast on a glass slide to evaluate their current-induced heating and deicing performance as deicing components. Similarly, GrF – PDMS deicing composites were cast on Ti-6Al-4V substrates acting as coatings to demonstrate the deicing capabilities of the composite under conditions encountered in its potential application in rotors and fuselage of aircraft structures. Surface preparation of Ti-6Al-4V substrate was performed by grit blasting to a final roughness of 1.50 ± 0.05 µm, to ensure the elimination of oxide layers and promote the adhesion of the GrF-PDMS deicing composite as a coating. The cross-sectional structure of the GrF – PDMS deicing composite is shown in the inset of Figure 1. The 3D interconnected architecture of graphene foam is demonstrated to remain intact after being infiltrated by the PDMS matrix. A combination of the intrinsic low viscosity and low surface energy (Figure 1S) of the PDMS polymer allows for an almost complete infiltration of the GrF branches.23 Strong interfacial interactions between the PDMS matrix and GrF result in a mechanically robust and flexible structure that can accommodate to variable surfaces as a deicing composite without altering the 3D architecture of GrF. The interconnected conductive network provided by GrF results in GrF – PDMS composites exhibiting high electrical conductivities 400-480 S∙m-1 at merely 0.1 vol. % GrF content. The superior electrical conductivity in GrF – PDMS composites as compared to pure PDMS (1 10-15 S∙cm-1)24 is attributed to the fast and uninterrupted electron transport provided by the 3D architecture of GrF. Also, the infiltration of GrF branches results beneficial in the heat dissipation mechanisms between PDMS and GrF. As a result, such highly conductive and robust composite is expected to result in high deicing efficiency.


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3.2. Current-induced heating of Graphene foam – PDMS deicing composites. The drive for developing a high efficiency thermoelectric deicing system to replace high– power consuming processes, requires a deeper understanding of the underlying thermal transport mechanisms that Graphene foam – PDMS deicing composites will undergo. A definite limiting factor in 1D and 2D carbon-based nanoparticle reinforced polymer composites as deicing systems originates from the inherent resistance to heat flow experienced. Heat dissipation in such composites is affected by the restriction of phonon transport by the interfaces and boundaries of the randomly dispersed filler and the non-conductive polymer chains.25-27 As a result, high loadings of conductive filler are required to reduce the restraining of phonon transport.28,

29

Figure 2a shows the heating profiles of free-standing 0.1 vol. % GrF – PDMS composite under current – induced heating experiments performed at room temperature (25°C). Samples were characterized by a surface resistance of 2.64 Ω∙sq-1, and an increasing stair direct current sweep (1 mA – 0.65 A) was supplied with the step of 0.05 A and time steps of 5 s. Thermal response of the deicing composite as a function of current was recorded and used to identify the required power to achieve deicing of surfaces. The superior electrical and thermal transport in GrF – PDMS deicing composites resulted in surface temperatures sufficient to boil water (~ 100°C) by supplying merely 0.6 A. The exposure of aircraft surfaces to T≤0°C is considered to undergo the formation and adherence of frost

by the

U.S.

Department

of Transportation

(Federal

Aviation

Administration).30 Therefore, for aircraft surfaces exposed to in-flight ambient temperatures of 20°C,26 a current of 0.4 A was identified as a necessary condition to achieve increase of surface temperatures by ∆T ≈ 30°C in 0.1 vol. % GrF – PDMS deicing composites and result in the deicing of surfaces.


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Heating profiles of the free-standing GrF – PDMS composite at a current of 0.4 A is shown in Figure 2b. The increasing surface temperatures in the GrF- PDMS deicing composite plateaus at T = 78 ± 0.02 °C indicating that thermal equilibrium has been reached. The corresponding heating rate (0.87° ± 0.004 C∙s-1) of the composite was computed during the initial 25 s, which resulted in a power input of 1 Watt. Due to the direct conversion of electrical to thermal energy within the three-dimensional structure of GrF, deicing of surfaces under freezing conditions can be achieved in a period of 34 s. As shown schematically in Figure 2c, the excellent thermal transfer capabilities of the composite are attributed to the intrinsic high thermal conductivity of graphene sheets (2000 – 4000 W∙m-1∙K-1).17, 18 The intrinsic high thermal conductivity of graphene results from the electronic and lattice contributions. Pettes et al.31 demonstrated that the thermal conductivity of GrF is primarily governed by the phonon-phonon transport (lattice contributions), where merely 0.2 - 3.6% of the thermal conductivity in GrF is contributed by the electrical properties.31 Newly developed Graphene foam – PDMS deicing composite benefits from the interconnected structure of GrF by allowing the uninterrupted transport of transport of phonon vibrations within the basal plane of graphene sheets resulting in the rapid transfer of heat to the PDMS matrix.32 As a result, the internal structure of the GrF – PDMS deicing composite experiences non–uniform temperature gradients. The areas of PDMS surrounding and infiltrating GrF branches will experience lower thermal gradients than its surface. The interfacial thermal resistance of GrF can be found to be in the range of 10-5 – 10-9 K∙m2∙W1 33

.

Therefore, the interfacial contact resistance between GrF and PDMS matrix has proven to

have a minimized effect on the ability of the GrF – PDMS composite to conduct thermal energy.


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Therefore, the continuous network of graphene sheets interconnected by Van der Waals interactions in GrF act as bridges for the ideal transport of phonons and thermal energy over extended areas.34, 35 3.3. Electrical and thermal stability of GrF – PDMS deicing composites. Electrical/ thermal stability of the GrF – PDMS composite is of crucial importance as a deicing system, as they will experience cyclic electrical and thermal loadings. The GrF – PDMS composites were evaluated for their capability to withstand cyclic electrical loadings. Samples of 0.1 vol. % GrF with a sheet resistance of 1.98 Ω∙sq-1 were subjected to 100 cycles of direct current (0.1 – 0.4 A), each cycle had a period of 60 s. Changes in surface temperature as a function of variable current were recorded throughout the experiment and are shown in Figure 3. High repeatability in the change of surface temperature experienced by the GrF – PDMS deicing composite accounts for the superior electrical sensitivity and thermal response of the composite. Changes in resistance of the composite are limited to the initial 16 -20 cycles, remaining cyclic loading result in highly constant temperature changes (∆T≈ 9°C). Such behavior is attributed to the sudden increase and decrease of temperature experienced as a response to the electrical cycling, upon reaching thermal equilibrium this behavior is minimized.34 Also, thermal expansion in the sample could result in morphological and interfacial contact variations affecting the electrical performance of the deicing composite.34 Therefore, GrF – PDMS deicing composite proves superior reliability and long-term stability up to 100 thermal and electrical cyclic loadings proving its suitability to perform as a deicing component in aircraft structures. 3.4. Deicing experiments on Graphene foam – PDMS deicing composites. Figure 4a and b show photographs of the free-standing GrF- PDMS, and GrF – PDMS/Ti-6Al-4V deicing


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composites before deicing experiments. Deicing experiments were carried out in samples with GrF dimensions of 1 × 4 × 1.2 mm3 at a controlled ambient temperature of −20ºC. Droplets of deionized (DI) water of ~0.2 mL were placed on the surface of the GrF – PDMS deicing composite until solidifying (Figure 4a and b). Upon reaching freezing of DI water on the surface, current-induced heating of the composite was initiated by applying a constant current of 0.4 A, previously identified as the ideal current to increase surface temperature enough to de-ice the surface (∆T≈30°C). The energy required by the GrF–PDMS composite to ensure that the surface reaches deicing temperature is evaluated by relating the required power, the deicing composite’s resistance, and its geometry: =

!

(3)

where " represents the current implemented during the heating experiment, is the measured resistance of the composite, and & represent the width and length of the composite respectively. Time-dependent heating profiles during deicing experiments are shown in Figure 4c for both free –standing and coating GrF – PDMS deicing composites. Remarkably, free – standing 0.1 vol. % GrF – PDMS deicing composites required merely 0.21 W∙cm-2 power density to complete deicing of its surface and rapidly achieve ∆T ≈ 30°C in 40 s. It is clear that as a result of the high electrical conductivity of the GrF – PDMS composite, uniform heating of the surface is achieved without implementing high power densities (Figure 4c). Also, to prove the capabilities of the novel GrF – PDMS composite in its application as a deicing component in aircraft structures, GrF – PDMS was adhered to Ti-6Al-4V surface as a coating. Heating profiles demonstrate an


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evident surface temperature differential of ~ 10°C between free-standing GrF – PDMS deicing composites and those adhered to the surface of Ti-6Al-4V substrate. Even though heat losses are experienced by conduction through the Ti-6Al-4V substrate, deicing of the surface is achieved with power densities of 0.30 W∙cm-2 by inducing heating of the GrF – PDMS composite with 0.4 A current. The insignificant temperature difference found in GrF – PDMS/Ti-6Al-4V deicing composites is attributed to the reduced contact resistance between the deicing composite and the substrate. Subjecting the substrate to a uniform roughening treatment such as grit blasting allows the contact thermal resistance (

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