Effects of Graphene Nanopetal Outgrowths on Internal Thermal

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Effects of Graphene Nanopetal Outgrowths on Internal Thermal Interface Resistance in Composites Anurag Kumar, Nikhil Ayyagari, and Timothy S. Fisher ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11796 • Publication Date (Web): 22 Feb 2016 Downloaded from http://pubs.acs.org on March 1, 2016

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ACS Applied Materials & Interfaces

Effects of Graphene Nanopetal Outgrowths on Internal Thermal Interface Resistance in Composites Anurag Kumar1,2, Nikhil Ayyagari1,2, Timothy S.Fisher1,2,* 1: Birck Nanotechnology Center, Purdue University, Lafayette, IN, 47907,USA 2: School of Mechanical Engineering, Purdue University, Lafayette, IN, 47907, USA

Abstract The thermal resistance at the interface between fiber and matrix is often the determining factor influencing thermal transport in carbon fiber composites. Despite its significance, few experimental measurements of its magnitude have been performed to date. Here, a 3-omega method is applied to measure the interfacial thermal resistance between individual carbon fibers and an epoxy matrix. The method incorporates bulk and interfacial regions to extract interfacial characteristics. Measured values indicate an average thermal interface resistance of 18 mm2K/W for an interface between bare fiber and epoxy, but the average value drops to 3 mm2K/W after a microwave plasma chemical vapor deposition of two-dimensional graphene nanopetals on the carbon fiber surface.

Keywords: carbon fiber composites, microwave plasma chemical vapor deposition, graphene nanopetals, 3-omega method, thermal interface resistance

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1. Introduction Carbon fiber composites, in which carbon fibers are dispersed in a matrix (either polymer, ceramic or metal), have found numerous applications that demand low weight with good mechanical as well as thermal performance1-2. The performance of composites is often limited by poor coupling between fibers and matrix.

For example, thermal transport through a

composite can be dominated by thermal resistance at the interface2-5 that results primarily from phonon scattering due to mismatched phonon spectra in the two phases (fiber and matrix) and from interfacial defects. The resistance is further increased by process-induced interfacial irregularities such as voids and interfacial stress, which result from fiber surface roughness, poor adhesion between fiber and matrix and dissimilarities in thermal expansion coefficient between a fiber and the surrounding matrix. Aside from the possibility of improving the overall mechanical properties of the composite, improved fiber-matrix coupling would reduce interfacial thermal resistance and enhance heat conduction. Such an enhancement would have positive implications for a variety of applications, ranging from aircraft skins to lightweight heat exchangers. Owing to difficulty in directly probing an interface in the interior of a composite, most prior work on this subject has focused on measuring the effective thermal conductivity of composites.6-10 While theory and computations have highlighted qualitatively the significance of interfacial resistance,4, 11-12 few direct experimental measurements of local interface resistances have been reported.3, 13-14 In some cases the thermal resistance at an interface has been extracted by assuming the validity of a particular model (e.g., an effective medium model) for the overall composite thermal conductivity.15-16 Only a few direct measurements of interfacial thermal resistance have been reported for such materials. Chapelle et al.13 measured the resistance


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between Ni wire and polymer matrix using a hot-wire method and reported resistances in the range of 3 to 10 mm2K/W. In another study, Neubauer et al.14 mimicked the interface in a copper-based metal-matrix/carbon-fiber composite by depositing 1 µm thick Cu film on a smooth glassy carbon surface and measured a thermal interface resistance of 7.5 mm2K/W using photothermal IR radiometry. Multiscale composites for which in-plane reinforcement by fiber is complemented by out-ofplane reinforcement by nanostructure (usually carbon nanotubes, CNTs) on the fiber surface have emerged as a concept for advanced composites.17-18 However, nanostructures such as carbon nanotubes are typically grown with the help of a catalyst and are not strongly bonded to the fiber. Conversely, Bhuvana et al.19 demonstrated the catalyst-free growth of covalently bonded contiguous multi-layer graphene-based petals emerging from a carbon fiber surface. In comparison with non-covalently bonded nanostructures, the petals are expected to be more effective in enhancing out-of-plane composite transport properties. In this work we examine this hypothesis by studying the effectiveness of graphene petals in enhancing thermal transport across a fiber/matrix interface. Here, we use a 3ω method for measuring the interfacial thermal resistance to test the hypothesis that these petals cause an improvement in interfacial thermal transport. Subsequent sections first describe growth of graphene petals on a single carbon fiber, followed by a description of the 3ω method as applied to an individual carbon fiber embedded in an epoxy. The sensitivity of temperature rise of the fiber to physical properties of the fiber and epoxy is also discussed. Finally, we present experimental results along with associated analysis and discussion.


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2. Experimental methods A microwave plasma chemical vapor deposition (MPCVD) process (based on previously reported work19-20) was used for growth of multi-layer graphene petals on a single fiber. First, a single carbon fiber (YSH 50A, by Nippon Graphite Fiber Corp., after stripping off the sizing 20) supported by two ceramic stands on a molybdenum puck is loaded on MPCVD (SEKI AX5200S) growth chamber’s stage (Figure 1 (a) and (b)). YSH 50A is light weight, high thermal conductivity fiber supplied particularly for satellite and aerospace applications. The portion of the fiber suspended between the ceramic supports was approximately 25 mm in length. The fiber is elevated by approximately ~12 mm from the puck. Growth chamber is then evacuated to base pressure of 2 Torr and then filled with H2 (flowing at 50 sccm) while allowing the pressure to reach a constant value of 10 Torr. Microwave plasma at an excitation frequency of 2.45 GHz is then ignited at 300 W. The stage height is adjusted such that the fiber is soaked in plasma with a good apparent coupling (estimated visually) between the two. Figure 1 (c) and (d) contain scanning electron microscopy (SEM) images of a carbon fiber under different growth conditions. At the initial stage of growth (Figure 1(c)), nucleating petals on the surface are apparent. The inset shows a bare fiber before the growth. Figure 1(d) reveals that after 15 min of growth twodimensional petals are dispersed on the fiber along its entire length. We found growth of graphene-petals on single fibers (as opposed to fiber tow) to be particularly sensitive to process parameters (Supporting Information, section S1). A small change in plasma power and pressure resulted in a transition from good growth to no growth.


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Figure 1. Schematic of the growth setup in the microwave plasma CVD (MPCVD) chamber showing (a) cross-section and (b) isometric view. Carbon fibers are elevated on two ceramic pieces placed on a Mo puck inside the growth chamber. (c) and (d) Scanning electron microscopy (SEM) images of a carbon fiber under different growth conditions. At the initial stage of growth (Figure 1(c)), nucleating petals on the surface are apparent. The inset shows a bare fiber before the growth. Figure 1(d) reveals that after 15 min of growth two-dimensional petals are dispersed on the fiber along its entire length. We also obtained simultaneous growth on small groups of fibers (approx. 5) placed next to each other on the ceramic support, and also on a fiber tow containing 6000 fibers. For growth on the small group of individual fibers, plasma power had to be raised to 200 W at a pressure of 15 Torr. Growth on a tow required 1000 W at pressure of 40 Torr. In all cases H2 and CH4 were maintained at 50 sccm and 10 sccm respectively. We postulate that the differences in power and pressure required for growth on different numbers of fibers relates to the variations in local plasma concentration and carbon deposition caused by differing substrate surface areas. Ideally,


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such differences would be assessed by non-invasive plasma diagnostics, but such techniques today offer spatial resolution on the order of 1 mm21 . For 3ω experiments, carbon-fiber-in-epoxy samples were prepared. The schematics in Figure 2 compare the sample used in a typical 3ω method (Figure 2(a)) with the fiber-in-epoxy approach used in this study (Figure 2(b)). A sample employed in the conventional method consists of a line heater deposited on top of a flat substrate. A thermal wave front generated in the heater has an approximately cylindrical symmetry at long distances from the heat source that is not strictly satisfied near the heater. Thus, a Cartesian 1-D heat diffusion model used to obtain an expression for the temperature response and 3ω voltage is valid only under certain conditions such as a large heater width compared to the thermal penetration depth and negligible edge effects. The effect of heater thickness is often ignored. In the high-frequency regime when the wave is confined closer to the heater, thermal properties of the heater would influence 3ω voltage. On the other hand, in the case of a fiber in epoxy where the fiber serves as the heating element, the cylindrical symmetry is preserved. This geometry significantly simplifies the analysis and allows a detailed theoretical study of the effect of heater/fiber properties on 3ω voltage.


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Figure 2. Schematics of: (a) conventional 3ω sample on a flat substrate and (b) a carbon fiber embedded in epoxy, as used in this study. (c) A schematic showing a carbon fiber suspended on Cu wires wrapped around a Teflon ring. Fiber and the Teflon ring are submerged in an epoxy (d) SEM image of a fiber suspended between middle two Cu wires. The inset shows a high magnification image revealing the diameter and rough surface of the suspended fiber. For preparing the samples a support for suspending the fibers was developed. The support consisted of four parallel Cu wires wrapped around a circular Teflon ring (Figure 2 (c)). A single fiber was pulled out of a bundle of YSH 50 fibers. The fiber was then gently placed over the Cu wires around the Teflon ring. In the 3ω experiment, the outer two wires are connected to a current source, while the middle two wires serve as voltage probe. In order to bond the fiber to Cu wires, a drop of electrically conductive Ag epoxy (H2OE Epotek, two-part Ag epoxy) was applied that was subsequently cured at 175 oC for 5 minutes. Figure 2 (d) shows a SEM image of a carbon fiber suspended between two Cu wires in the middle. The inset shows a high magnification image revealing more clearly the surface and diameter of the fiber.


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3. Temperature rise in the heater/fiber: effect of interfacial thermal resistance and physical properties A typical 3ω experiment consists of a line heater deposited on a thin film of interest (Figure 2 (a))22-23. A sinusoidal current (I=I0 sin(ωt)) applied at frequency ω through the heater results in Joule heating at frequency 2ω. Temperature fluctuation due to Joule heating causes a resistance fluctuation, also at frequency 2ω. This fluctuation in the electrical resistance at frequency of 2ω when combined with the current at frequency of 1ω results in a small voltage fluctuation at frequency 3ω. The magnitude of the temperature fluctuation in the heating line depends on the thermal properties of the substrate beneath it. A substrate with high thermal diffusivity conducts heat away from the heater more efficiently, thus reducing the amplitude of 2ω temperature fluctuations. This effect in turn decreases the amplitude of the 3ω voltage. Thus, the 3ω voltage is sensitive to substrate’s thermal properties. By sweeping the current over a range of frequency the thermal diffusivity of the substrate can be obtained. The method has been used extensively for measuring thermal conductivity of solids and thin films22-23. A modified 3ω process has also been reported for measuring thermal properties of filament-like specimens (such as Pt wire)

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fluids 25 and gases 26. For a fiber (radius ‘a’, length ‘l’) embedded in a matrix of infinite thickness, the Joule heat produced by a sinusoidal current through the fiber would result in a periodic temperature rise of both fiber as well as the surrounding matrix. If the temperature rise at time ‘t’ at a distance ‘r’ from the centerline of fiber is given by θf(r,t) and θm(r,t) for the fiber and the matrix respectively and if the sensitivity of electrical resistance (R) of fiber to temperature is given by β= dR/dT, the 3ω voltage can be shown to be (see section S3 for complete derivation)


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V3ω =

I 3 R ( dR / dT )  K0 ( qm r )    2π al  km qm K1 ( qm a ) 

(1)

where K0 and K1 are modified Bessel functions of the second kind and r is the radial distance from the centerline of the fiber. The complex quantity q is given by

qm =

i 2ω

αm

(2)

Eq. 1 highlights the important cubic proportionality between third harmonic voltage and current, V3 ω ∝ I 3

(3)

If we consider an interfacial thermal resistance Ri’’ between the heated element (the carbon fiber in this case) and the surrounding matrix (epoxy), then the temperature rise of the fiber can be shown to be ∧

θf =

 I 2 R  K 0 ( qm a ) + Ri''   2π al  k m qm K1 ( qm a ) 

(4)

Thus, an interface resistance (Ri’’) causes a frequency-independent constant shift in the in-phase component of the temperature fluctuation. The above expression is obtained using an analysis that ignores the physical properties of the heater/fiber (e.g., specific heat, density, thermal conductivity). This can be justified in the low frequency regime when the penetration depth is much larger than fiber diameter. The penetration depth (λ) of the thermal wave in the matrix is given by,

λ=

αm 2ω

(5)

where α m is the thermal diffusivity of matrix and ω the frequency of sinusoidal current. In the frequency regime where penetration depth is comparable to the fiber diameter, the effect of


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fiber/heater properties may no longer be ignored. If the physical properties of the heater are accounted for, the temperature-rise of the heater (fiber in this case) can be shown to be

q ''' θ f = η I 0 (q f r ) + k f q 2f ∧

(6)

And the temperature-rise in the matrix surrounding the fiber can be shown as ∧

θm = ε K0 (qm r )

(7)

The constants (η and ε) can be obtained from the heat diffusion equations by applying the two boundary conditions of constant heat flux at the fiber/matrix interface and a temperature jump at the interface due to interfacial thermal resistance Ri'' . Applying the two boundary condition of continuous one obtains

 a K 0 (qm a)  1  η = q  Ri'' − I ( q a ) + k q I ( q a ).  0 1  f f f f  2 km qm K1 (qm a)   2 k f q f  

−1

'''

(8)

and

ε =−

ηk f q f I1 (q f a) km qm K1 (qm a)

(9)

Substituting η into Eq. 6 gives the temperature rise of the fiber. If properties of the fiber and epoxy are known, the experimentally measured temperature rise can be fitted to Eq. 6 to obtain both thermal interface resistance ( Ri'' ) and thermal conductivity of the matrix ( km ). Figure 3(a) shows the effect of interfacial thermal resistance on the temperature rise of fiber for the case of a line heater where the physical properties of the fiber are ignored. As evident from Eq. 4, interfacial resistance results in a constant upward shift in the in-phase component while the out-of-phase component remains unaffected. Figure 3(b) shows the effect of interface resistance for the case of fiber/heater with finite diameter. For the case of a line heater (Figure


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3(a)), the interface resistance leads to a constant upward shift of the in-phase temperature rise. The resistance had no effect on the out-of-phase component of temperature rise. With properties of heater included in the analysis (Figure 3(b)), we observe a similar upward shift in the in-phase component. However, a splitting of the out-of-phase component at higher frequencies is also observed. Nevertheless, it is apparent from this analysis that below a certain threshold frequency (~100 Hz in this case), the out-of-phase temperature rise is unaffected by the interface resistance. Thus, for an experiment based on an analysis that ignores interfacial resistance as well as the heater’s thermal properties, the out-of-phase component would provide a more reliable estimate of thermal properties of the surrounding matrix. However, it should also be noted that in the low frequency regime the out-of-phase component is much weaker in magnitude than the in-phase component and would therefore require a more sensitive data collection in the 3ω experiment.


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Figure 3. Effect of interfacial thermal resistance on the temperature rise of the fiber when (a) Physical properties of fiber are ignored (case of a line heater). (b) Physical properties of fiber are included. The effects of physical properties of the heart/fiber become pronounced at high frequencies when the heat wave is localized in the vicinity of the fiber. For line-heater case, the out-of-phase component remains unaffected by interfacial thermal resistance. For finite-diameter fiber case, interface resistance leads to a gradually increasing splitting of the out-of-phase component as frequency increases. Effect of (c) fiber thermal conductivity and (d) fiber specific heat on the temperature rise. Temperature rise in the heater/fiber is also influenced by thermal conductivity and specific heat of the fiber and surrounding matrix (epoxy in this case). In order to assess the effect of the fiber’s radial thermal conductivity on the second harmonic temperature rise, the radial conductivity was varied as 1 W/mK (an arbitrarily low value), 10 W/mK (cross plane thermal conductivity of graphite) and 120 W/mK (axial thermal conductivity of the fiber used in this study). Figure 3(c) shows the effect of radial thermal conductivity and specific heat of the fiber on temperature rise. For the properties of the fiber and epoxy used in this study radial thermal


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conductivity of the fiber has a negligible effect on the temperature rise. Specific heat of the fiber (Figure 3 (d)) has a small non-negligible effect only beyond 100 Hz of frequency. Figure 4 (a) shows the effect of thermal conductivity of epoxy on the temperature rise. The thermal conductivity of epoxy affects the slope of the initial linear part of the in-phase component as well as the magnitude of the out-of-phase component. The specific heat of epoxy has a less pronounced effect on the temperature rise (Figure S4), and the effect of the thermal properties of epoxy in general is most significant in the low-frequency regime when the heat wave penetrates much deeper into the epoxy.

Figure 4. Effect of (a) epoxy thermal conductivity on the temperature rise of fiber and (b) Effect of temperature coefficient of resistance of the fiber on the third harmonic voltage.

Figure 4 (b) shows the effect of temperature coefficient of resistance of the fiber on the third harmonic voltage. The third harmonic voltage is highly sensitive to the temperature coefficient of resistance. Hence its accurate determination is essential for precisely predicting the interface resistance.


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4. Results and discussion The YSH50A fiber used in this study has a high thermal conductivity of 120 W/mK along the axial direction. Radial thermal conductivity of the fiber has a negligible effect on the temperature rise (Figure 3(c)). For the purpose of fitting experimental data to the model, a fiber density of 2100 Kg/m3 (value provided by supplier) and specific heat of 0.71 kJ/kgK 27(value for graphite) were used. The specific heat of epoxy (1.42 kJ/kgK) was obtained using differential scanning calorimetry (DSC). Fibers behaved as Ohmic conductors (linear current vs voltage profile) with a negative temperature coefficient of resistance (Figure S5). Owing to stronger signal and also the stronger influence of the interfacial thermal resistance on the temperature rise of the fiber in the low frequency regime, data was collected in the 1 Hz to 100 Hz range. Figure 5 (a) shows the thermal interface resistance for bare fiber and graphene-petal decorated fibers. Uncertainty bars correspond to uncertainty obtained from 95% confidence bounds on the fitted values. The interface resistance varies over a wide range of values for bare fiber samples. Despite significant sample-to-sample variation, the values provide a useful range of this important parameter. Thermal resistance across an interface is sensitive to surface roughness, interfacial stress and entrapped inclusions (such as air/gas) at the interface. The observed variation in the interface roughness across different samples could be attributed to these factors. Nevertheless, a general trend of improvement after petal decoration is apparent where the average interface resistance reduces from 18 mm2K/W for bare fibers to 3 mm2K/W for fibers with petal growth. Further, the petal-decorated fiber also shows reduced variability in resistance measured across the three samples.


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Figure 5. (a) Thermal interface resistance for (a) bare fiber and graphene-petal decorated fiber samples. (c) Epoxy conductivity as obtained by the 3ω experiment from bare fiber samples and graphene-petal decorated fiber samples. Uncertainty bars correspond to uncertainty obtained from 95% confidence bound on the fitted values. The decrease in thermal resistance after petal growth can be attributed to several factors. First, petal growth on the fiber increases the fiber surface area. 500 nm tall petals grown on a fiber with a separation of 200 nm between them would lead a three-fold increase in the surface area of a fiber. This would reduce the interface resistance by a similar factor, assuming that the petals act as highly conductive thermal fins. Secondly, edges of graphitic nanopetals protruding from the fiber surface possess defect and dangling bonds which make them chemically active28-30. This increased chemical activity would further enhance bonding with the surrounding matrix. Thirdly, petals are grown in a plasma CVD process. Plasma treatment of fibers is a known technique for improving fiber/matrix adhesion31 as the process increases fiber surface activity. Thus it can be expected that the petals grown under conditions requiring coupling between plasma and fiber during growth also have activated surface sites. These would further enhance adhesion with the matrix. Finally, the morphology of the petals also imparts roughness to the fiber surface, an additional factor which has been demonstrated to improve fiber/matrix adhesion.31 The


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cumulative effect of these factors leads to a decrease in the interfacial resistance as evidenced by the measurements. Figure 5 (b) shows epoxy conductivity as obtained from bare fiber samples and petal-decorated samples. The epoxy conductivity increases from about 0.3 W/mK to about 0.4 W/mK after petal decoration. Graphene-petals emerging from the fiber surface form a thermally conductive graphene/epoxy composite layer around the fiber. This conductive layer would lead to an increase in the apparent thermal conductivity of the epoxy as detected in the experiment. The porous morphology of the petals results in non-uniformity in material distribution around the fiber surface. Increased uncertainty in the fitted parameters observed after petal growth could be attributed to this increased non-uniformity in the material distribution around the fiber.

5. Conclusion 3ω measurements conducted on carbon fiber embedded in an epoxy indicate an interfacial thermal resistance larger than 10 mm2K/W for an interface between bare fiber and epoxy. Plasma CVD assisted growth of graphene-petals offers a viable route towards composites with reduced interfacial resistance, with thermal interface resistance dropping consistently to less than 10 mm2K/W after petal growth. Reduced interfacial thermal resistance could also be indicative of stronger coupling between fiber and matrix. Thus, petal decoration is also expected to improve mechanical properties of the composite. The developed technique using the 3ω method can also serve as a useful tool to study matrix properties in the vicinity of the fiber and also to probe the effect of various fiber treatment processes on interfacial transport. Low yield in sample preparation observed in this study can be improved by using less crystalline and hence less brittle


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fibers. However, such fibers would also have lower thermal conductivity and may not be the best choice for composites intended to enhance thermal transport. Corresponding Author *Tel/Fax: (765) 494-5627, E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Funding Sources The authors are thankful to the U.S. Air Force Research Laboratory (AFRL), and its Office of Scientific Research (AFOSR, FA9550-12-1-0037) under the MURI program on Nanofabrication of Tunable 3D Nanotube Architectures (PM: Dr. Joycelyn Harrison), for financial support of this work. ACKNOWLEDGMENT AK gratefully acknowledges helpful discussions with Dr. John Ferguson (Air Force Research Lab, Ohio, USA) in the initial stages of the work. AK also gratefully acknowledges help from Anurag Das with sample preparation and Dr. Rajib Paul with DSC measurements. Authors also gratefully acknowledge help from Roger H. Gerzeski with supplying carbon fiber for the experiments. REFERENCES 1. Chand, S., Review Carbon Fibers for Composites. J. Mater. Sci. 2000, 35 (6), 1303-1313. 2. Roy, A. K.; Farmer, B. L.; Varshney, V.; Sihn, S.; Lee, J.; Ganguli, S., Importance of Interfaces in Governing Thermal Transport in Composite Materials: Modeling and Experimental Perspectives. ACS Appl. Mater. Interfaces 2012, 4 (2), 545-563.


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20. Gerzeski, R. H.; Sprague, A.; Hu, J.; Fisher, T. S., Growth of Contiguous Graphite Fins from Thermally Conductive Graphite Fibers. Carbon 2014, 69, 424-436. 21. Tuesta, A. D.; Bhuiyan, A.; Lucht, R. P.; Fisher, T. S., Laser Diagnostics of Plasma in Synthesis of Graphene-Based Materials. J. Micro Nano-Manuf. 2014, 2 (3), 031002-031002. 22. Cahill, D. G.; Fischer, H. E.; Klitsner, T.; Swartz, E. T.; Pohl, R. O., Thermal Conductivity of Thin Films: Measurements and Understanding. J. Vac. Sci. Technol., A 1989, 7 (3), 1259-1266. 23. Dames, C., Measuring the Thermal Conductivity of Thin Films: 3 Omega and Related Electrothermal Methods. Annu. Rev. Heat Transfer 2013, 16 (16), 7-49. 24. Lu, L.; Yi, W.; Zhang, D. L., 3ω Method for Specific Heat and Thermal Conductivity Measurements. Rev. Sci. Instrum. 2001, 72 (7), 2996-3003. 25. Wang, Z. L.; Tang, D. W.; Liu, S.; Zheng, X. H.; Araki, N., Thermal-Conductivity and Thermal-Diffusivity Measurements of Nanofluids by 3ω Method and Mechanism Analysis of Heat Transport. Int. J. Thermophys. 2007, 28 (4), 1255-1268. 26. Yusibani, E.; Woodfield, P. L.; Fujii, M.; Shinzato, K.; Zhang, X.; Takata, Y., Application of the Three-Omega Method to Measurement of Thermal Conductivity and Thermal Diffusivity of Hydrogen Gas. Int. J. Thermophys. 2009, 30 (2), 397-415. 27. DeSorbo, W.; Tyler, W. W., The Specific Heat of Graphite from 13° to 300°K. J. Chem. Phys. 1953, 21 (10), 1660-1663. 28. Claussen, J. C.; Kumar, A.; Jaroch, D. B.; Khawaja, M. H.; Hibbard, A. B.; Porterfield, D. M.; Fisher, T. S., Nanostructuring Platinum Nanoparticles on Multilayered Graphene Petal Nanosheets for Electrochemical Biosensing. Adv. Funct. Mater. 2012, 22 (16), 3399-3405. 29. Rout, C. S.; Kumar, A.; Xiong, G.; Irudayaraj, J.; Fisher, T. S., Au Nanoparticles on Graphitic Petal Arrays for Surface-Enhanced Raman Spectroscopy. Appl. Phys. Lett. 2010, 97 (13), 133108. 30. Rout, C. S.; Kumar, A.; Fisher, T. S., Carbon Nanowalls Amplify the Surface-Enhanced Raman Scattering from Ag Nanoparticles. Nanotechnology 2011, 22 (39), 395704. 31. Tang, L. G.; Kardos, J. L., A Review of Methods for Improving the Interfacial Adhesion between Carbon Fiber and Polymer Matrix. Polym. Compos. 1997, 18 (1), 100-113.

A 3ω method is applied to measure the interfacial thermal resistance between individual carbon fibers and an epoxy matrix. Measured values indicate a thermal interface resistance larger than 10 mm2K/W for an interface between bare fiber and epoxy. Values drop to less than 10 mm2K/W after a microwave plasma chemical vapor deposition of two-dimensional graphene-nanopetals on carbon fiber surface.


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Figure 1 254x190mm (300 x 300 DPI)



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Figure 3 254x190mm (300 x 300 DPI)



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Figure 5 254x190mm (300 x 300 DPI)



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Effects of Graphene Nanopetal Outgrowths on Internal Thermal

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