High-Performance and Lightweight Thermal Management Devices by

High-Performance and Lightweight Thermal Management Devices by 3D Printing and Assembly of Continuous Carbon Nanotube Sheets. Nam Nguyen* ...
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High-performance and Lightweight Thermal Management Devices by 3D Printing and Assembly of Continuous Carbon Nanotube Sheet Nam Nguyen, Songlin Zhang, Abiodun Oluwalowo, Jin Gyu Park, Kang Yao, and Zhiyong Liang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b07556 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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High-performance and Lightweight Thermal Management Devices by 3D Printing and Assembly of Continuous Carbon Nanotube Sheet Nam Nguyen*, Songlin Zhang, Abiodun Oluwalowo, Jin Gyu Park*, Kang Yao, Richard Liang. High-Performance Materials Institute, Florida State University, 2005 Levy Ave., Tallahassee, FL 32310, USA * Email: [email protected] (Nam Nguyen). * Email: [email protected] (Jin Gyu Park). KEYWORDS: 3D printing; embedded 3D printing; direct ink writing; buckypaper; carbon nanotube sheet; thermal management.

ABSTRACT Free standing carbon nanotube films or buckypaper can provide a significant platform to develop practical applications of nanocarbon materials. For this research, buckypaper with high thermal conductivity (20 W/mK) and large surface area (350 m2/g) was mass produced in-house to investigate for use in lightweight thermal management devices. Floating catalyst chemical vapor deposition (FC-CVD) carbon nanotube sheets were also studied in this work. We introduced two manufacturing techniques to use the sheets for heat dissipation: 1) printing conductive composite ink on the sheets to make lightweight thermal devices, such as heat sinks, and 2) assembling the sheets directly into 3D structures that were mounted on the back of heat generating devices.

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These manufacturing techniques resulted in extremely lightweight, high-performance heat dissipation devices compared to other heat sink materials.

1. INTRODUCTION Lightweight and highly thermal conductive materials are important in modern electronics, aircrafts, and autonomous devices that require thermal management while minimizing weight and size.1-2 Several widely studied nanostructured materials, such as silver nanowires, carbon nanotubes (CNTs), and boron nitride nanotubes, have demonstrated the potential to meet various requirements of thermal management devices. Among these materials, CNTs are known to have the potential to provide superior properties, including lightweight, high electrical/thermal conductivity, and excellent mechanical properties.3 Researchers have attempted to transfer the excellent thermal properties of CNTs for heat sinks,4 thermal interfaces,5 oil spill cleaning6 or fillers to increase the thermal conductivity of composites.1 However, harnessing the properties of individual CNTs into macroscale assemblies for engineering applications is still challenging due to their dispersing issues and fabrication technique constraints.7-8

Recently, CNT sheets and fiber materials have been integrated to make macroscale devices and products, such as hybrid composites with multifunctional properties,9 actuators,10 and electromagnetic interference shielding, etc.11-13 Results have demonstrated the potential to transfer nanoscale properties of CNTs by using fibers and sheets for potential engineering applications.3 Unfortunately, scalable processing methods to fully implement CNT sheets for thermal management has not been established.

This research addresses two approaches to produce lightweight thermal management devices by 3D printing and by an assembly method using in-house fabricated buckypaper from continuous 2 ACS Paragon Plus Environment

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infiltration technique14 and using commercial CNT sheets. By using these effective manufacturing methods, we successfully fabricated thermal devices with high performance and extreme lightweight.

2. MATERIALS AND CHARACTERIZATION Multi-walled carbon nanotubes (MWCNTs) were purchased from General Nano Inc. and dispersed in water with the aid of a surfactant, TritonTM X-100 (Alfa Aesar) and sonicated to make CNT suspension. This CNT suspension was continuously filtered and dried to form free standing buckypaper.14-15 Nanocomp Technologies Inc. provided nanotube sheets produced using a floating catalyst chemical vapor deposition (FC-CVD) process. Thin graphite sheets (Panasonic PGS) were purchased from Digikey Inc. A hybrid composite ink consisting of epoxy with graphite nanoplatelets and chopped carbon fibers was used for 3D printing. Details regarding the composite ink and printing techniques can be found in a previous paper.16 The microstructures of samples were observed by scanning electron microscope (SEM, JEOL JSM-7401F). An infrared thermal camera (E40, FLIR) was used to obtain thermal images to investigate temperature distribution. We used an AbaqusTM 6.13 for finite element analysis (FEA) of the heat transfer. In the heat transfer simulation, 8-node linear heat transfer brick (DC3D8) elements were used for meshing the model. Heat transfer was governed by equation:17           +  +  + + hA(T −  ) =      where k is the thermal conductivity (W/mK), q is the heat generation per unit volume (W/m3), h is the convective heat transfer coefficient (W/m2K), A is the area (m2), T is the temperature (°C), Cp (J/K) is the specific heat and  is the density (kg/m3). 3 ACS Paragon Plus Environment

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Laser flash apparatus (LFA 457 Microflash, NETSZCH, Germany) was used to measure in-plane thermal conductivity. The thermal diffusivity (α) of the samples were measured from LFA using half-rising time of detector temperature reading. The density (ρ) was measured using the Archimedes method, and the specific heat (Cp) was found in literature.18 From α, ρ and Cρ, the thermal conductivity (κ) was determined using κ = α × ρ × Cρ. Three tests for each sample type were conducted. A Q50 (TA Instrument Inc.) was used for thermogravimetric analysis (TGA) of nanotube sheets before and after heat treatment at a 10 °C/min rate.

The porosity of the samples was measured using an automated gas adsorption analyzer (Quantachrome Autosorb-iQ) with nitrogen gas adsorption-desorption isotherms at 77 K. All samples were outgassed at 180 ºC for 2 hours prior to testing. The Brunauer-Emmett-Teller (BET) specific surface areas were determined from the linear portion of the adsorption isotherm.19

3. RESULTS AND DISCUSSION 3.1 Continuous Buckypaper, Heat Treatment, and Thermal Properties High quality CNT sheets from scalable manufacturing method could be beneficial for many applications, such as multifunctional composites9, flexible heaters20, energy related devices21, and flexible conductors.22 Two promising methods for producing the sheets involve a vacuum assisted filtration and float catalyst chemical vapor deposition.23 In this work, we used an inhouse continuous buckypaper prototype that involves a roll-to-roll manufacturing system.14 This approach could pave the way for in-line characterization and functionalization of the continuous CNT sheets. Using this process, we produced sheets on a large scale with an average thickness of 20 µm and an aerial density of 10 g/m2 of 6 to 12 inches wide. 4 ACS Paragon Plus Environment

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Figure 1 (a-c) shows the morphology of the continuous buckypaper. Subsequently, we heated the CNT sheets to 400 °C for 2 hours to remove surfactant residues. TGA results show that the residual surfactant in the sheets was removed after heat treatment (Figure 1g). Various posttreatment processes are available to improve their mechanical and electrical properties of CNT sheets, including mechanical stretching,24-25 acid treatment26-27 or functionalization.28

For this study, we selected heat treatment to enhance the thermal conductivity of the CNT sheets, which is a simple technique and suitable for large sample fabrication. The CNT sheets fabricated by FC-CVD method (Figure 1 d-f) contain more catalyst particles in the sheets (Figure 1f), therefore, heat treatment was not a suitable method to remove the catalyst because of high vapor temperature of metallic catalyst (Figure 1h).

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Figure 1. Morphology of (a-c) continuous buckypaper and (d-f) FC-CVD CNT sheet. Thermogravimetric analysis (TGA) of (g) raw and heat-treated continuous CNT sheets, (h) raw and heat-treated FC-CVD CNT sheets. (i) Comparison of specific surface area of CNT sheets before and after heat treatment.

Materials with large specific surface areas draw great interest in catalyst support, heat exchangers, and electrochemical applications.29-31 Single-walled carbon nanotubes have large surface areas of up to 1315 m2/g due to their 1D nanostructure.32 The specific surface area of CNTs is affected by several parameters, such as type of CNT, diameter, impurity, and surface

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functionalization.33 Figure 1i shows that the specific surface area of buckypaper increasing from 100 m2/g to 350 m2/g after the heat treatment. The increase was attributed to removal of surfactant residue and more accessible CNT surfaces were exposed for nitrogen adsorption during BET measurements.33 The specific surface area of FC-CVD CNT sheets was relatively higher than that of as prepared continuous buckypaper with surfactant but lower than that of heat treated buckypaper, which may be due to more iron catalyst residue and amorphous carbon impurities found in the FC-CVD process.

After heat treatment of FC-CVD at 400 °C, the surface area increased only slightly since no significant change in amorphous carbon and catalyst occurred due to the temperature not high enough to remove the amorphous carbon. Although higher temperatures and additional acid treatments could reduce the impurities, we used 400 °C for surfactant removal to avoid oxidation.27

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Figure 2. (a) Schematic of in-plane thermal conductivity measurement set-up. (b) In-plane thermal conductivity and thermal diffusivity of aluminum foil as a baseline measurement. (c) Thermal diffusivity and (d) thermal conductivity of CNT sheets before and after the heat treatment. We used a laser flash method with an in-plane sample holder (Figure 2a) to measure the thermal properties of CNT sheets. First, we measured aluminum foil as a baseline (Figure 2b) to ensure measurement reliability. The results showed that the thermal conductivity of the aluminum was ~200 W/mK, which is consistent with literature reports.34 The thermal diffusivity and thermal conductivity of the continuous buckypaper were 10 mm2/s and 10 W/mK at room temperature, respectively. After heat treatment, the in-plane thermal conductivity of the sheets was 20 W/mK,

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which was 100% improvement from room temperature (Figure 2c, 2d). This is largely attributed to the improvement of the dense stacking of CNTs and removal of surfactant residue.35-36 For FC-CVD CNT sheets, thermal diffusivity and thermal conductivity were reduced after heat treatment. A possible reason is the oxidation of metal catalyst in CNT network, which may introduce thermal resistance in the CNT network. Hence, we decided not to use heat treated FCCVD CNT sheets for the following thermal management application study.

3.2 Embedding CNT Sheets into Thermal Devices by 3D Printing. Embedding nanostructures, such as CNT films and yarns, into products should provide the product with novel properties.37-40 In the previous section, we reported that the continuous buckypaper has thermal conductivity of 10-20 W/mK at room temperature with a high specific surface area (100 m2/g to 350 m2/g). This indicates the CNT sheets could be a potential candidate for various thermal management applications. To facilitate transferring the nanotube properties into macroscale devices, we adopted 3D printing technique for embedding buckypaper into the composite structure to make a lightweight heat sink. In this process, lightweight and thermally conductive composite ink were used to print the 3D structure, and buckypaper was integrated into structure to improve its performance while minimizing the weight (Figure 3a, 3b). The process is illustrated in Figure 3 (c-e). We printed the initial layer of the heat sink structure (Figure 3c), and the buckypaper was placed on top of the printed layer (Figure 3d), then an additional layer was printed on the buckypaper (Figure 3e). After curing, the printed composite structure was heated at 120 oC for 2 hours. The buckypaper embedded heat sink was made without sacrificing its large surface area. By using printing technique, we can overcome the difficulty in incorporating CNT sheets into engineering application due to buckypaper’s flexibility. 9 ACS Paragon Plus Environment

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Figure 4 shows different type of samples fabricated by 3D printing using composite ink and buckypaper. This concept could provide a new way of integrating pre-produced nanostructures, such as CNT sheets, for thermal management applications, sensors, or multifunctional composites.

Figure 3. Schematic of devices fabrication using buckypaper and 3D printing technique: (a) Hybrid composite ink made of epoxy filled with graphite-nanoplatelets and chopped carbon fiber. (b) Large scale buckypaper fabricated using continuous filtration method. (c) Printing first layer of heat sink structure using composite ink (d) Placing buckypaper on top of printed layer. (e) Printing next layer. Printed structure undergoes curing process at 120 oC for 2 hours.

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Figure 4. Different structure fabricated by direct ink printing technique using hybrid composite ink. (a) Printing thin layer on CNT sheets. (b) 3D printed square to show ability to print 3D structure. (c) 3D printed composite structure with embedded buckypaper layer in site. (d) Printing thin and smooth structure. (e) Printing the heat sink. (f) 3D printed heat sink with embedded buckypaper. SEM images of (g) fracture section of cured composite and (h) buckypaper surface in heat sink. Inset show the high magnification image of each sample. Scale bar: 5mm. 11 ACS Paragon Plus Environment

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We compared the heat dissipation of 3D printed heat sink with and without embedded heattreated buckypapers (Figure 5a). Two samples were placed on a hot plate and the temperature profile was monitored as a function of time. The 3D printed heat sink with buckypaper reached a steady state temperature faster, as shown in Figure 5b-5f. To verify this experimental result, we built heat transfer FEA models using thermal conductivity value from experimental data (k = 20 W/mK for buckypaper and k = 2 W/mK for composite ink41). Convective heat transfer coefficient (h = 20 W/m2K) was applied to the sheet area for carbon nanotube sheets. Figure 6a shows the temperature profile of the heat sink at steady state, which was similar with thermal image result (Figure 6b).

The simulation results indicate the buckypaper area had a high heat flux due to high thermal conductivity and convective coefficient (Figure 6c). The temperature of heat sink as a function of time (marked spot in Figure 6a) also show that heat sink with buckypaper reached a steady state temperature faster than the control sample

(Figure 6d). This trend was consistent with

experiment results. The experiment and simulation results demonstrated that combining buckypaper using a printing technique effectively improved the performance of heat sink while minimizing weight. These findings indicate potential use for a wide range of applications, including personal thermal management, thermal conductive textiles, or thermoelectric devices.

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Figure 5. (a) Printed composite heat sink with and without BP. (b-f) IR camera image of heat sink at different time frame to show the heat dissipation performance of samples.

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Figure 6. Heat transfer simulation of samples with and without buckypaper layer embedded between fin. (a) Temperature profile from simulation. (b) Temperature profile from experiment. (c) Heat flux contour. (d) Temperature at the end of heat sink fin as a function of time. 3.3 Assembly of CNT Sheets into Lightweight Heat Sink This section introduces another way of using CNT sheets for heat dissipation application. We assembled CNT sheets to make an extremely lightweight and flexible heat sinks harnessing the high thermal conductivity and large surface area of buckypapers (Figure 7c). This approach is advantageous over other works by providing scalability.4

While reported heat sinks at the microscale was fabricated by laser-assisted surface patterning and CVD growth of vertical CNTs, our simple and effective technique can be applied for macroscale products and devices. To the best of our knowledge, this was the first attempt to use large CNT sheets to make heat sinks.

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Figure 7. Thermal image of lighting LED attached to different type of heat sinks: (a) LED without heat sink, (b) commercial aluminum heat sink, (c) buckypaper heat sink fabricated by assembly of CNT sheets, (d) continuous buckypaper heat sink, (e)heat treated continuous BP, (f) FC-CVD CNT sheets, (g) graphite sheet. We evaluated the performance of assembled buckypaper heat sinks by monitoring the temperature of a commercial LED (1W, 3.0-3.6V, 350 mA, White – Uxcell) with a DC bias. All the thermal images were captured after LED was on for 2 minutes. Figure 7a shows a thermal image of the temperature close to the limit of LED without a heat sink. Using a commercial aluminum heat sink to dissipate the heat of LED, the temperature reduced significantly by 13.7 o

C (from 44.9 oC to 31.2 oC), as shown in Figure 7b. Figure 7 (d-g) compares the heat dissipation

performance of different types of carbon sheet based heat sinks that were made of continuous buckypaper, heat treated buckypaper, FC-CVD CNT sheet, and commercial graphite sheet, respectively. Figure 7d shows that the temperature of LED with the continuous buckypaper heat sink was approximately 4 oC lower than that without a heat sink (Figure 7a). However, no decrease was observed for the temperature of LED with the heat-treated buckypaper heat sink even though it has higher thermal conductivity (~ 20 W/mK) and surface area (350 m2/g). We assume that the difference in thermal conductivity was not significant enough to overcome the thermal contact resistance in thermal interface layer used to attach the assembled buckypaper heat sink onto the LED.42 As a result, the temperature difference between two samples was not significant. When FC-CVD CNT sheets and graphite sheets were used, the LED temperature decreased by 11.4 oC (from 44.9 oC to 31.2 oC). These results demonstrated that the performance of FC-CVD CNT and graphite sheet heat sink were equal to 81% of commercial heat sink in terms of heat dissipation. Although the thermal conductivity of FC-CVD CNT sheets was 60 15 ACS Paragon Plus Environment

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W/mK, which is lower than that of graphite, 500 W/mK, there was no apparent difference in the heat dissipation performance. Considering the large surface area of CNT sheets, 152 m2/g compared to the graphite surface area up to 5 m2/g43, the high surface area of CNT sheets appears to play a critical role in convective heat transfer coefficient in the heat sink applications.

We further investigated the effects of thermal conductivity and convective heat transfer coefficient by conducting simulations for different case studies. Figure 8a shows simulation results performed with different thermal conductivity (10, 20, 60, 200 and 500 W/mK) from our case studies, while convective coefficient was kept constant (20 W/m2K). Overall temperature reduction with increased thermal conductivity matched well with experimental results. Slight differences might be originated from the convective coefficient mismatch. Figure 8b shows the simulation to explore the effect of convective coefficient while thermal conductivity was kept constant at 10 W/mK. Interestingly, convective coefficient also has profound effect on heat dissipation, which explains why the performance of relatively low thermal conductive FC-CVD CNT sheet (60 W/mK) with high surface area was comparable to the high thermal conductive graphite sheet44-46 (>500 W/mK). Therefore, heat sinks using CNT sheets alone, as shown in Figure 7, is possible and its performance was comparable to the commercial aluminum heat sinks while weighing approximately 50 times less. Additional works on optimization and growing of CNT structure to achieve the large surface area should increase the heat dissipation efficiency further.

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Figure 8. Heat transfer simulation of LED temperature with heat sinks for different (a) thermal conductivity and (b) convective heat transfer coefficients.

4. CONCLUSION We attempted to harness the high thermal properties of CNT sheets for macroscale thermal management applications. For this purpose, two different techniques were introduced in this work: printing thermally conductive composite inks on CNT sheets and assembling CNT sheets to serve as lightweight effective thermal devices. Results indicate that both techniques are effective ways to transfer the properties of CNT sheets into lightweight thermal devices for thermal management applications. Both experimental and simulation results show that convective coefficient is also important as well as thermal conductivity for heat dissipation. Therefore, CNT sheets appear to be promising materials for high-performance and lightweight thermal management applications.

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ACKNOWLEDGEMENT

This work was partially supported by NSF Scalable Nanomanufacturing Program (SNM 1344672) and AFOSR FA9550-17-1-0005 project.

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(39) Richter, C.; Schmulling, S.; Ehrmann, A.; Finsterbusch, K. In Design, Manufacturing and Mechatronics (ICDMM 2015), Shahhosseini, A. M., Ed.; World Scientific: 2015; pp 961-969. (40) Muth, J. T.; Vogt, D. M.; Truby, R. L.; Menguc, Y.; Kolesky, D. B.; Wood, R. J.; Lewis, J. A. Embedded 3D Printing of Strain Sensors Within Highly Stretchable Elastomers. Advanced materials 2014, 26, 6307-6312. (41) Nguyen, N.; Melamed, E.; Park, J. G.; Zhang, S. L.; Hao, A. Y.; Liang, R. Direct Printing of Thermal Management Device Using Low-Cost Composite Ink. Macromol Mater Eng 2017, 302, 1700135. (42) Ralphs, M. I.; Kemme, N.; Vartak, P. B.; Joseph, E.; Tipnis, S.; Turnage, S.; Solanki, K. N.; Wang, R. Y.; Rykaczewski, K. In Situ Alloying of Thermally Conductive Polymer Composites by Combining Liquid and Solid Metal Microadditives. ACS Appl Mater Interfaces 2018, 10, 2083-2092. (43) Shornikova, O. N.; Kogan, E. V.; Sorokina, N. E.; Avdeev, V. V. The Specific Surface Area and Porous Structure of Graphite Materials. Russ J Phys Chem a+ 2009, 83, 1022-1025. (44) Xiang, J. L.; Drzal, L. T. Thermal Conductivity of Exfoliated Graphite Nanoplatelet Paper. Carbon 2011, 49, 773-778. (45) Kong, Q. Q.; Liu, Z.; Gao, J. G.; Chen, C. M.; Zhang, Q.; Zhou, G. M.; Tao, Z. C.; Zhang, X. H.; Wang, M. Z.; Li, F.; Cai, R. Hierarchical Graphene-Carbon Fiber Composite Paper as a Flexible Lateral Heat Spreader. Advanced Functional Materials 2014, 24, 4222-4228. (46) Wu, H.; Drzal, L. T. Graphene Nanoplatelet Paper as a Light-Weight Composite with Excellent Electrical and Thermal Conductivity and Good Gas Barrier. Carbon 2012, 50, 11351145.

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