Carbon Nanotube Hybrid Materials

Carbon Nanotube Hybrid Materials

CHAPTER CARBON NANOTUBE HYBRID MATERIALS 4 Guangfeng Hou*, Vianessa Ng*, Rui Chen*, Devika Chauhan†, Chenhao Xu*, Sergey Yarmolenko‡, Svitlana Fial...

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Guangfeng Hou*, Vianessa Ng*, Rui Chen*, Devika Chauhan†, Chenhao Xu*, Sergey Yarmolenko‡, Svitlana Fialkova‡, Mark J. Schulz* Department of Mechanical and Materials Engineering, University of Cincinnati, Cincinnati, OH, United States* Department of Aerospace Engineering, University of Cincinnati, Cincinnati, OH, United States† Center for Advanced Materials and Smart Structures, North Carolina A&T State University, Greensboro, NC, United States‡

CHAPTER OUTLINE 1 Introduction ......................................................................................................................................77 2 Gas Phase Pyrolysis Method ..............................................................................................................78 2.1 Gas Phase Pyrolysis Synthesis Process .............................................................................. 78 2.2 CNT Sock Dynamics ......................................................................................................... 78 3 Carbon Nanotube Hybrid Material .......................................................................................................84 3.1 CNT Hybrid Material Formation ......................................................................................... 84 3.2 Structure and Applications of CNT Hybrid Material ............................................................. 85 4 Conclusion .......................................................................................................................................88 Acknowledgment ...................................................................................................................................88 References ...........................................................................................................................................88 Further Reading ....................................................................................................................................90

1 INTRODUCTION Carbon nanotubes (CNTs) have found various engineering applications [1–5] such as electronics, composites, biosensors, and in the energy fields. CNTs can be produced by different methods, among which the gas-phase pyrolysis method demonstrated unprecedented advantage of synthesizing high-quality CNTs in a large scale [6–9]. The related research focuses on understanding the synthesis mechanism, structure enhancement, property improvement, and applications. In this chapter, a new hybrid structure material has been developed, which is composed of CNTs and nanoparticles (NPs). This hybrid configuration is different from other structures that only overlays different materials together providing one interface. For this CNT hybrid material, the NPs are uniformly distributed inside the macro-CNT entity (yarn or sheet) with numerous interacting interfaces between CNTs and NPs. In this chapter, the fundamentals of gas-phase pyrolysis method are introduced first, including Nanotube Superfiber Materials. https://doi.org/10.1016/B978-0-12-812667-7.00004-5 Copyright # 2019 Elsevier Inc. All rights reserved.

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its nature and CNT sock dynamics. This provides the bases where the CNT hybrid material is formed. Then, the CNT hybrid material formation process, structure, and application will be discussed.

2 GAS PHASE PYROLYSIS METHOD The gas-phase pyrolysis process includes different stages such as catalyst formation, chemical feedstock decomposition, CNT growth, and multiscale sock formation. Billions of CNTs are grown on top of the catalyst particles when they flow through the high-temperature growth zone. These CNTs then self-assemble into an aerogel-like sock in the later stage. CNT yarn or sheet can be easily produced from this sock assembly [10–14]. Studies have focused on different synthesis variables [8, 15–17], such as growth promoter, feedstock injection, gas flowrate, and synthesis temperature. Meanwhile, great efforts have been put on the synthesis mechanism and the sock formation process, which are critical to optimization and the hybrid material development.

2.1 GAS PHASE PYROLYSIS SYNTHESIS PROCESS The gas-phase pyrolysis process is multiscale in time and length and involves gas-phase synthetic chemistry phenomena (Fig. 1). Inside the reactor tube, fluid dynamics dominates the catalyst and CNT transport, which is a combination of multiphase flow and heat transfer phenomena. In the first part of the reactor, catalyst nucleates from angstrom iron atoms through diffusion and agglomeration. The CNT growth region is located at the center high-temperature zone. In this growth zone, the hydrocarbon gas decomposes into free carbon atoms that dissolve into catalyst particles to produce CNTs. A newly discovered phenomenon of plasma dynamics also happens in the growth zone, providing measurable electric voltage signals directly from the carrier gas. Close to the reactor outlet, the grown CNTs assemble into an aerogel-like sock, which bridges the nanoscale CNT with macroscale thread/sheet products. It should be noted that at the growth zone, multiple phenomena overlap and happen simultaneously, which significantly increase the complexity.

2.2 CNT SOCK DYNAMICS Among the different phenomena, the CNT sock formation process is the critical link bridging the nanoscale CNTs with macroscale products. There are some studies on the sock formation mechanism, including thermophoresis or inertial migration [18], van der Waals attraction [19], and electrostatic attraction [20]. Recently, a new mechanism based on vortex flow has been proposed, which is supported by simulation and experiment results. The aerogel-like CNT sock inside the reactor has a multiscale hierarchical structure (Fig. 2). Individual CNTs are attracted to each other by van der Waals and Casimir forces, generating bundles that are carried downstream by the fluid flow. The bundles then interact with each other and form microscale networks. These networks further entangle into the CNT sock. The CNT volume fraction inside the reactor can be estimated from the sock dimension and dynamics. For a typical sock, its diameter D is equal to the inner diameter of the ceramic tube. Its moving velocity u can be calculated by the carrier gas flowrate. This sock is continuously collected on a drum with process yield of Y (mCNT/Δt). To calculate the CNT volume fraction, we will consider the mass of the sock mCNT produced during time period Δ t. The volume of the sock at time Δ t equals to 2 Vsock ¼ u  Δt  πD4 , where u is the carrier gas velocity and D is the reactor inner diameter. The volume

The multiscale process of CNT synthesis in the gas-phase pyrolysis method.

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FIG. 2 CNT sock and bundles: (A) CNT sock outside the reactor, (B) μm scale network, and (C) TEM image of CNT bundles. Reproduced with permission from G. Hou, V. Ng, C. Xu, L. Zhang, G. Zhang, V. Shanov, et al., Multiscale modeling of carbon nanotube bundle agglomeration inside a gas phase pyrolysis reactor, MRS Adv. (2017) 1–6, doi:10.1557/adv.2017.371. Copyright 2017 Cambridge University Press.

of the CNTs can be estimated as VCNT ¼ mρ CNT ¼ Y  ρΔt , where ρCNT is the CNT density (1.8  106 g/m3 CNT

CNT

4Y [21]). From these two equations, the CNT volume fraction can be calculated as ΦCNT ¼ VVCNT ¼ πD2 uρ sock 7

. CNT

Using the standard run values, the CNT volume fraction ΦCNT is around 2.36  10 inside the reactor tube. By assuming a CNT bundle diameter D of 50 nm and length l of 500 μm, the CNT bundle number density nV is around 2.4  1011 per m3. To investigate the CNT bundle interaction during sock formation, these CNT bundles can be modeled as spherical particles with the same volume, where the spheres have an equivalent diameter. The CNT bundle motion inside the reactor can be modeled using Newton’s second law and considering the drag force, thermophoretic force, and interparticle force [22]. In the study, a commercial software COMSOL was used for the simulation. For the dilute case (nVl3 ¼ 0.3), the simulation result agrees qualitatively with the experimental observation (Fig. 3). The calculated drag force on the individual particle is at a range of 3.9  1010–9.7  106 μN, and the thermophoretic force is 4.9  1012–1.7  1010 μN. In general, the drag force dominates, but the higher range of thermophoretic force is comparable with the lower range of drag force. Thus, both forces contribute to the agglomeration process. For the semidilute case (nVl3 ¼ 30), an advanced model is required to capture the long-range bundle interaction.

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FIG. 3 CNT bundle agglomeration for the dilute case: (A) experimental observation, side view; (B) simulation, side view; (C) experimental observation, front view; and (D) simulation, front view. Reproduced with permission from G. Hou, V. Ng, C. Xu, L. Zhang, G. Zhang, V. Shanov, et al., Multiscale modeling of carbon nanotube bundle agglomeration inside a gas phase pyrolysis reactor, MRS Adv. (2017) 1–6, doi:10.1557/adv.2017.371. Copyright 2017 Cambridge University Press.

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(B) FIG. 4 Convection vortex-induced sock formation: (A) gas streamline in the outlet region and (B) cross-sectional arrow volume line showing the region where the sock closed end forms. In part figure (A), the arrow indicates the highvelocity area. The top part shows flow toward the harvest box; the bottom part shows the flow in reverse direction. In the simulation, the heating zone temperature is 1400°C for our system, with gas flowrate 1000 sccm. Reproduced with permission from G. Hou, R. Su, A. Wang, V. Ng, W. Li, Y. Song, et al., The effect of a convection vortex on sock formation in the floating catalyst method for carbon nanotube synthesis, Carbon N. Y. 102 (2016) 513–9, doi:10.1016/j. carbon.2016.02.087. Copyright 2016 Elsevier.

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Based on the simulation and experimental work, a convection vortex model [23] has been proposed to explain the sock formation mechanism. Around the outlet region of the reactor tube, there is gas flowing from the harvest box back into the tube (Fig. 4A, bottom), which interacts with the carrier gas flowing out of the tube into the harvest box (red arrows). In the region where the two flows interact (Fig. 4B), CNT flows upward. Later around the tube outlet, strong velocity flow (around 0.5 m/s) is generated, which is quite high compared with the average flow speed of 0.01 m/s in the middle of the tube. It is noted that the CNT flow path is intertwined (Fig. 4A) instead of laminar flow, which is responsible for the vortex generated. This localized high-velocity flow and complex flow pattern could lead to connected CNT networks, formed by CNTs coming into close proximity. The high velocity and ensuing high-drag force cause the CNT to align and self-connect, which creates the closed end of the sock. With more CNTs joining the sock end by the carrier gas flow, this closed end serves as a seed for further sock shell formation. Once the sock is complete, carrier gas flow will push the sock out, forming a continuous CNT sock. It is observed that when the sock forms, a closed-end cap forms first, which flows upward and move toward the tube outlet; then, the CNT sock continuously comes out of the reactor. Fig. 2 shows an example of the entire sock formation process. In the process, the CNTs connect with each other forming a closed end around the vortex region. Later, this closed end flows out of the reactor tube, followed by continuous sock flows, with pattern affected by the vortex flow (Fig. 5). The sock dynamics at different carrier gas flowrate are shown below (Fig. 6). The sock morphology changes dramatically when flowrate was increased from 600 to 2000 sccm. Firstly, the sock is able to

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FIG. 5 Sequential images showing sock formation by convection vortex: (A) before feedstock injection; (B–D) CNT forming closed-end networks, which flow upward around the vortex region; (E) sock end comes out of the reactor tube; and (F) continuous sock. Experimental conditions: ferrocene concentration is 2 wt%, gas flowrate is 1000 sccm, and feedstock injection rate is 16 mL/h. Reproduced with permission from G. Hou, R. Su, A. Wang, V. Ng, W. Li, Y. Song, et al., The effect of a convection vortex on sock formation in the floating catalyst method for carbon nanotube synthesis, Carbon N. Y. 102 (2016) 513–9, doi:10.1016/j. carbon.2016.02.087. Copyright 2016 Elsevier.

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FIG. 6 Sock dynamics under different carrier gas flowrate: (A) 600 sccm, (B) 800 sccm, (C) 1000 sccm, (D) 1200 sccm, (E) 1400 sccm, (F) 1600 sccm, (G) 1800 sccm, and (H) 2000 sccm. The temperature is 1400°C, feedstock injection rate is 32 mL/h, and ferrocene is 1 wt%. Reproduced with permission from G. Hou, V. Ng, Y. Song, L. Zhang, C. Xu, V. Shanov, et al., Numerical and experimental investigation of carbon nanotube sock formation, MRS Adv. (2016) 1–6, doi:10.1557/adv.2016.632. Copyright 2016 Cambridge University Press.

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continuously come out of the reactor at flowrates between 800 and 1800 sccm (sock formation zone). At a 600 sccm flowrate, the CNTs accumulated in the reactor, and curly CNT strands formed. At a 2000 sccm flowrate, the sock broke and could not maintain any pattern, probably due to high-drag force. Secondly, the change of sock pattern in the formation zone (800–1800 sccm) is related to CNT-flow field interaction. This is a complex multiphase flow problem including assembly of CNTs into sock under the influence of heat and flow field and two-way interaction. The two stages (i) catalyst nucleation and (ii) CNT growth all have influence on the sock dynamics. In general, the catalyst nucleation process determines the size distribution of the catalyst particles and their spread throughout the growth zone, which affects the number of active catalyst particles growing CNTs. In CNT growth stage, the residence time affects the length of CNTs. The length and number of CNTs in the reactor are related to the CNT assembly and sock dynamics.

3 CARBON NANOTUBE HYBRID MATERIAL Carbon nanotubes (CNTs) have been combined with different materials to produce composite-like materials, including metals, ceramics, and polymers. These materials have promise of enhancing mechanical and electric properties of the final mixture. Traditionally, the mixture can be produced by sintering [24], infiltration [25], and electrodeposition [26–31]. However, these methods could only produce structure with limited integration, usually one interface between different materials. Here, based on the gas-phase pyrolysis method, various NPs can be fully integrated into the CNT sock, creating a new CNT hybrid material.

3.1 CNT HYBRID MATERIAL FORMATION The CNT hybrid material can be produced by injecting different NPs into the reactor (Fig. 7). NPs are injected in a carrier inlet gas at different concentrations. Injecting particles at the inlet produces mixing with the fuel vapor and aids in integrating the particles with the nucleating CNT. The fundamental process is that NPs are injected into the gas-phase pyrolysis reactor and assembled with the growing CNTs into a sock- or aerogel-like material. The CNT hybrid sock may be twisted into yarn or rolled into sheet. Hybrid materials containing single-walled carbon nanotube (SWCNT), metals, and/or ceramics in variable combinations can be synthesized. In the process, the NPs travel through the hightemperature zone where CNT growth happens. Due to the existence of NPs during both the CNT growth and sock formation process, the CNT hybrid material is created in situ. This is different with other methods that often are implemented as post processing and could not produce fully integrated mixture.

FIG. 7 Integration of NPs into CNT sock to produce fully integrated CNT hybrid material.

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The NPs used can have different forms, such as copper, activated carbon (AC), gold nanowire, graphene, and even CNTs. With different NPs incorporated, this method could produce CNT hybrid structures of different functionalities. An example of CNT-Cu hybrid material is shown below (Fig. 8). Fig. 8A shows a standard black CNT sock exiting the nanotube reactor; Fig. 8B shows a CNT-Cu brown (light gray in print versions) sock exiting the reactor with Cu NP injection; Fig. 8C shows Cu NPs integrated into the CNT material; Fig. 8D shows Cu is present chemically detected by EDS.

3.2 STRUCTURE AND APPLICATIONS OF CNT HYBRID MATERIAL The CNT hybrid material integrates powdered NPs into CNT-based material, which can produce nanotube sheet and yarn materials with tailored structure and properties. Two examples of CNT-Cu (Fig. 9) and CNT-activated carbon (Fig. 10) have been successfully produced. As shown by the SEM images, the NPs are fully integrated into the CNT matrix with uniform distribution. Potentially, the CNT hybrid material can be used to produce smart materials with tailorable properties, which will improve the performance of electric actuators, motors, new smart materials, structural composites, electric conductors, and many applications where metals/ceramics can be replaced by nanotube hybrid materials. CNT hybrid materials that conduct electricity or magnetic flux may replace

FIG. 8 CNT-Cu hybrid material: (A) CNT sock exiting the reactor without Cu NP injection, (B) CNT-Cu brown (light gray in print versions) sock exiting the reactor with Cu NP injection (sock is floating in air, the Cu NPs have a large surface area and make the sock look like Cu, but the Cu volume percent is small), (C) Cu particles on the CNT after 25 nm coated Cu NP injection, and (D) EDS showing Cu (circles on figure) with CNT.

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FIG. 9 CNT-Cu hybrid material: (A–C) pristine CNT sheet and (D–F) CNT-Cu hybrid.

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metal wiring and iron core materials to produce lightweight electric systems including wires, transformers, solenoid actuators, and rotary and linear electric motors for use in smart structures, robotics, and many other applications.

4 CONCLUSION In this chapter, a CNT hybrid material has been analyzed, including the formation process, structure characterization, and application. This hybrid material combines the high aspect-ratio carbon nanotube with different NPs. The final structures and properties are readily tailorable based on the selected NPs. In comparison with pristine CNT material, this hybrid material provides great opportunity of property enhancement. Further study is underway to explore the design space, optimizing various parameters such as NP type, NP morphology, loading ratio, chemical modification, and post processing. Depending on the target application, different hybrid structures will be developed, providing advanced performances.

ACKNOWLEDGMENT This research was broadly supported by Office of Naval Research Award N00014-15-1-2473, the NSF ERC EEC0812348, the UCT Seed Grant under ESP TECH 15-0160, the University of Cincinnati’s Education and Research Center Targeted Research Training program (UC ERC-TRT Program), the Water Environment & Reuse Foundation, and the NSF I/UCRC Center for Intelligent Maintenance Systems (IMS).

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FURTHER READING [32] G. Hou, V. Ng, Y. Song, L. Zhang, C. Xu, V. Shanov, et al., Numerical and experimental investigation of carbon nanotube sock formation, MRS Adv. (2016) 1–6, https://doi.org/10.1557/adv.2016.632.