Hybrid Fiber Fabrication Using an AC Electric Field ...

Proceedings of IMECE2007 2007 ASME International Mechanical Engineering Congress and Exposition November 11-15, 2007, Seattle, Washington, USA

IMECE2007-42305

HYBRID FIBER FABRICATION USING AN AC ELECTRIC FIELD AND CAPILLARY ACTION Woonhong Yeo Department of Mechanical Engineering, University of Washington, Seattle, WA Kyong-Hoon Lee NanoFacture, Inc. Ann Arbor, MI

Jae-Hyun Chung Department of Mechanical Engineering, University of Washington, Seattle, WA

Yaling Liu Department of Mechanical Engineering, Northwestern University, Evanston, IL

ABSTRACT We present a novel hybrid fiber fabrication method for nanostructured hybrid-materials, using an AC electric field and capillary action. Through this fabrication process, hybrid fibers composed of single walled carbon nanotubes (SWCNTs) and silicon carbide (SiC) nanowires were systematically manufactured. It was demonstrated that both diameter and length of hybrid nanofibers could be controlled by manipulating parameters, such as the mixing ratio of SWCNTs to SiC nanowires, concentration of solution, immersion time, volume of solution, and withdrawal rate. In the fabricated hybrid fibers, the SiC nanowires functioned as a structural frame (host filler materials), while SWCNTs were employed for their extraordinary mechanical and electrical properties as a binder or net. Using this method, the fabrication speed of the hybrid fiber was increased by 20 fold compared to the existing method[1]. According to the simulation and modeling results, the fibers are formed by the following three steps; (1) nanowire bridge formation by dielectrophoresis in solution (2) nanowire fiber formation by compression due to capillary action (3) alignment by the torque due to the capillary action. The proposed processing technology may provide an ample opportunity for fabricating a long hybrid-nanofiber. INTRODUCTION The need for multifunctional materials is driven by recent trends in engineering industries; such materials are especially demanded in aerospace vehicles, space applications, and electro-mechanical systems. As a result, a number of investigations have been successful to achieve the multifunctionality of nanocomposites[2-7]. The nanomaterials could demonstrate the superb properties due to the extremely large interface in different domains (e.g., filler and matrix material in a composite). This provides an opportunity to effectively tailor the properties of the nanostructures by manipulating the

Wing Kam Liu Department of Mechanical Engineering, Northwestern University, Evanston, IL

interfacial characteristics through chemical- or physical surface treatments. In particular, carbon nanotube (CNT) additives have been successfully used to engineer the mechanical, electrical and thermal properties of nanocomposite materials. Considering the essential contribution of CNTs, the CNT based nanocomposites have demonstrated strong potential for multifunctional material systems. Therefore, the development of the CNT based nanocomposites is urgently required. Recent study[8] has demonstrated that the CNT/polymer composites can be fabricated by infiltrating polymer matrix materials into vertically aligned multi walled carbon nanotubes (MWCNTs) grown on a pre-patterned silicon oxide (SiO2) substrate. This fabrication process of the nanotube structure skips the tedious dispersion process. Bulk structures, however, may not be fabricated in this way because it is still challenging to grow CNTs over a centimeter in length. Until now, only millimeter-long MWCNTs have been grown by the chemical vapor deposition (CVD) method. In fact, centimeter-long CNT fibers have been formed by using an electric field. An AC electric field has been used to fabricate a tip for an atomic force microscope (AFM)[9-12]. It has been shown that the CNTs were assembled by an AC field to fabricate an AFM tip. During the fabrication process, dielectrophoretic force was used to attract CNTs around a sharp AFM tip. Surface tension, however, changed the assembly pattern while drying the CNT solution. Ironically, this negative aspect of surface tension was utilized to fabricate a sharp tip. In this way, Tang et al.[1, 13] fabricated a 1mm-long, continuous fiber. Using a uniform withdrawal rate of 0.85µm/s and a DC electric field with a constant current, a 3cm-long fiber with a larger diameter (between 26 and 42µm) also formed[1]. To fully utilize the CNT fibers as filler materials, it is essential to develop a new technique to manufacture an array of continuous CNT fibers, similar to a fabric of carbon fibers in traditional composites. Here, we propose a novel fabrication technology for nanostructured hybrid materials (SWCNTs +

1

Downloaded From: http://proceedings.asmedigitalcollection.asme.org/ on 11/11/2013 Terms of Use: http://asme.org/terms

Copyright © 2007 by ASME

SiC nanowires) by using an AC electric field in conjunction with capillary action. Through the proposed process, a long, well-ordered hybrid fiber was systematically manufactured. The additive material, SiC nanowires, has dimensions of a few orders larger than those of SWCNTs, which can dramatically increase the production speed of the fibers. The combination of the SWCNTs and the SiC nanowires can improve mechanical strength, stiffness, and thermal conductance of the fiber. In the hybrid fiber, the SiC nanowires function as a structural frame while SWCNTs function as a binder (or net). To verify the structure of the fibers, hybrid fibers from different concentrations were studied by Scanning Electron Microscopy (SEM). Our modeling results revealed the hybrid fiber formation and fabrication process from nanowire bridging by dielectrophoresis to nanofibril formation. This unique fabrication technique can potentially eliminate the tedious dispersion process and intensively overcome the current process issues for nanocomposites. EXPERIMENTAL Material preparation To fabricate SWCNT based fibers, SWCNTs were mixed with SiC nanowires. Since the fiber formation is mainly performed in a solution, the nanomaterials were suspended in a solvent.

Fig. 1 Solutions of SWCNT + SiC nanowire mixture with different concentrations. The preparation process of the mixture is as following: A small cluster of SWCNTs (Carbon nanotechnologies, Inc. Houston TX) is weighed in a scale to measure the cluster’s mass. The cluster is dispersed in Dimethylformamide (DMF) with a sonicator for 10 hours. The SiC nanowires (Advanced Composite Materials Corporation, Greer, SC) were dispersed in DMF in the similar way. For comparison, five types of solutions having different SiC nanowire concentration were prepared. In Fig. 1, the concentration of SiC nanowires was increased; from the far left bottle, the concentration of SiC nanowires was (1) 0mg/L (2) 62.5mg/L (3) 125mg/L (4) 250mg/L (5) 250mg/L. All bottles, except bottle 5, contained a concentration of SWCNTs fixed at 120mg/L. Thus, pure SWCNTs were suspended in the bottle 1 while pure SiC nanowires were dispersed in the bottle 5. As shown in the Fig. 1, the colors of the solutions varied depending on the concentration of SiC nanowires. A 2µL drop of each solution was dried on 100nm-thick oxide layer of a Si wafer and the samples were observed under a scanning electron microscope (FEI Sirion SEM).

Fig. 2 SEM images of nanomaterials of bottles 1~5 in Fig. 1; (a), (b), (c), (d), and (e) respectively. In Fig. 2, the SEM images are shown with the corresponding number of the bottles. The images, Fig. 2(a) and (e), are pure SWCNTs and pure SiC nanowires, respectively. In Fig. 2 (b), (c), and (d), SWCNTs formed a network with SiC nanowires such that SWCNTs covered the surface of SiC nanowires. The SWCNTs formed fibrils whose diameters ranged from 5nm to 10nm. Since SiC nanowires were chemically inert, the network between SWCNTs and SiC nanowires was maintained by van der Waals interaction. In order words, the large surface energy of SWCNTs contributed to forming the network. It was interesting to note that the coverage of SWCNTs on a single SiC nanowire appeared to be uniform, meaning one SiC nanowire could bring a similar amount of SWCNTs regardless of concentrations. In addition, neighboring SiC nanowires were linked with SWCNTs, which potentially exhibited strong, but flexible hybrid structures. To fabricate a fibril, W (tungsten) wires (50µm in diameter and 5cm in length) were prepared by tensile fracture. A W wire was pulled until fractured by axial force, which fabricated a W wire having an end diameter of ~40µm. The W-tip was employed to form nanofibers. Compared to commercially available W-tips having a tip diameter of 4µm, the fabricated W-tip was suitable for a tip array due to easy handling with low cost.

(a) (b) Fig. 3 Experimental setup; xyz stage with a rotation is positioned under an optical microscope. The hybrid fiber fabrication under a light microscope was also performed with the similar experimental setup[13]. For

2



continuous fibril formation, an x-y-z stage with a rotation was constructed. Fig. 3 shows the whole experimental setup including the microscope, fiber-optic illuminator, imaging system, signal generator, and a home-made x-y-z stage. Shown on the monitor in Fig. 3(a), a W-tip with a solution hanging in a metal coil is shown. Fig. 3(b) shows the xyz manipulator under the microscope. Single fiber formation In this experiment, the process parameters producing SWCNT-based hybrid fibers are introduced. The parameters related to fiber formation include: (1) mixing ratio of SWCNTs to SiC nanowires, (2) concentration, (3) immersion time, (4) withdrawal rate, (5) volume of solution, and (6) electric field.

(a)

(b)

(c)

(d) (e) Fig. 4 Various fiber lengths due to mixing ratios; (a), (b), (c), (d) and (e) are results from bottle 1~5. The number in each picture is the corresponding numbers of the bottles in Fig. 1 and Fig. 2. And the length of each fiber is described in the inset of each figure.

Fig. 5 SEM microphotos; the numbers correspond to numbers in Fig. 4. (1) Mixing ratio: A mixing ratio in weight is a crucial parameter to determine mechanical stiffness and electrical conductance of a fiber. Also the dimensions of a fiber are dependent on the mixing ratio. When the five mixtures in Fig. 1 were used for fiber formation, various lengths of fibers were

formed. The experimental conditions were fixed; withdrawal rate (16µm/second), AC voltage (20Vpp at 5MHz / 57mVpp/µm considering the distance between the W-tip and coil), immersion time (30seconds), immersion length (100µm), and volume of each solution (2µL). Fig. 4 shows the various lengths of nanofibers according to the variation of mixing ratios of SWCNTs to SiC nanowires. Fig. 4(a) shows W-tip having a SWCNT fibril. Due to the small length (10 nm) of the SWCNT fibril, only W-tip is observed. The length was verified under a SEM (Fig. 5). It is speculated that all SWCNTs were removed from the tip in withdrawal process because of the sudden change of meniscus at the W-tip. In Fig. 4(b), (c) and (d), length of hybrid fibers was increased with the increase of the concentration of SiC nanowires. A larger number of SiC nanowires covered with SWCNTs were attracted with the increase of the number of SiC nanowires. Each SiC nanowire was linked by SWCNT bundles, which provided a flexible fiber structure. As a result, the role of SiC nanowires in fiber formation could be clarified as follows; tens of micrometers long SiC nanowires mixed with SWCNTs readily worked as backbone structures to avoid the collapse of attracted SWCNT fibers. Fig. 4(e) shows the result from pure SiC nanowires. As shown in the figure, the fiber diameter was rapidly decreased with drawing. This was due to the high resistivity of SiC nanowire. This pure SiC nano-fiber was highly fragile such that the structure could be collapsed with small vibration. Thus, it is evident that SWCNTs were working as an adhesive for the hybrid structure. (2) Concentration: The concentrations of SWCNTs and SiC nanowires are important to produce a continuous fiber as shown in Fig. 4, 5. The highest concentration (250mg/L) of SiC nanowires was determined when SiC nanowires formed bundles in DMF due to high density. Accordingly, the concentrations of SiC nanowires tested were 62.5, 125, and 250mg/L. In case of SWCNTs, the concentration of SWCNTs can be determined by the estimation of surface area of SiC nanowires relative to that of SWCNTs. (3) Immersion time: Immersion time is a duration time to dip a W-tip in the dropped solution before withdrawal. When this immersion time was increased, a W-tip could attract more SWCNTs with SiC nanowires, which resulted in a larger length and diameter of nanofibers. (4) Withdrawal rate: Withdrawal rate is directly related to production speed of fibers. In the result of Annamalai et al.[1], a withdrawal rate of 0.85µm/second was found to be optimal for a SWCNT fiber. With the withdrawal rate, about 10 hours was required to fabricate a 3cm long fibril. In our result, the withdrawal rate could be increased to 16µm/second. The withdrawal rate was increased by ~20 fold. The withdrawal rate was determined by both the approaching velocity of nanowires to a W-tip and the length of nanowires. The withdrawal rate should not exceed the approaching velocity of nanowires induced by an AC electric field. Otherwise, the formation of a nanofiber could be halted. A uniform withdrawal rate turned out to be important to achieve uniform dimensions of hybrid fiber. (5) Volume of solution: The amount of solution used in fabrication of a fiber should include a proper volume of SWCNTs and SiC nanowires. Excessive volume of solution can waste nanomaterials.

3



To estimate a proper amount of solution drop, a mixture of SiC nanowire (62.5mg/L) and SWCNTs (120mg/L) was considered to calculate. In 2µL drop of solution, the masses of SiC nanowires (density: 3.21g/cm3)[14] and SWCNTs (density: 1.33g/cm3)[15] were 62.5ng and 120ng, respectively. Assuming that the combined volume of two materials in the drop was converted to a volume of a fiber having a diameter of 4µm, the expected fiber length was computed to be 8.67mm. In our result, a ~6mm-long nanofiber was formed using 2µL of solution with an AC field (Fig. 6), which means that the amount of mixture was almost used for fabricating the nanofiber in the drop. Thus, the proposed method could provide an economic way fabricating a hybrid nanofiber.

Immersion time Withdrawal rate Volume of solution Electric field

Fiber’s length and diameter Production speed of fibers Material utilization efficiency Selective attraction and orientation

Electrical Characterization Electrical properties of the hybrid fibers were characterized by using an I-V measurement. For electrical measurement, two fibers connected in a junction were fabricated as Fig. 7(a). In Fig. 7(b), the electrical measurement shows linear relationship for the I-V curve. Based upon the I-V curve, the resistance of 1.8mm-long needle was ~77kΩ. Thus, the fabricated hybrid fiber was electrically conductive.

(a) (b) Fig. 7 Electrical Measurement (a) two fibers electrically connected, (b) I-V characteristic.

Fig. 6 Fabricating process accordance with time (a) nanofiber is drawing from a hanging solution (b) fabricating nanofiber after 2min (c) fabricated ~6mm long nanofiber after 6min. (6) Electric field: An AC electric field generates dielectrophoretic force that attracts and orients rod-shaped objects in solution. In addition, SWCNTs (or nanowires) can be selectively attracted by AC field in the mixture of unwanted particles. To achieve higher electrical conductance of a fiber, AC electric field is more advantageous, because metallic SWCNTs are selectively attracted due to higher dielectric constant than that of semiconducting SWCNTs. The various controllable process parameters and the aimed out-going fiber properties are summarized in Table 1. Table 1. Summary of process parameters and the controlled out-going fiber properties Process input parameters Controlled output Mixing ratio Concentration

Fiber’s mechanical stiffness, electrical conductance, and length Continuous fiber formation

(a)

(b)

(c) (d) Fig. 8 SEM study (a) fiber on W-tip (scale bar: 50µm) (b) magnified view (scale bar: 1µm) (c) the tip end (scale bar: 200nm) (d) SiC nanowires with SWCNTs dried on oxide layer in air (scale bar: 200nm). SEM Characterization The fabricated nanofibers were being investigated by SEM. Fig. 8(a) shows a fabricated nanofiber on W-tip. In Fig. 8(b), SiC nanowires were wrapped around and linked with the SWCNTs. Fig. 8(c) shows a tip end showing oriented SWCNTs to withdrawal direction. When the same solution was dropped and dried on an oxide layer without an electric field, random orientation of the SWCNTs was observed as expected [Fig.

4



8(d)]. The orientation of SWCNTs on a SiC nanowire may affect the mechanical strength of a hybrid fiber. Two fibril formation Two hybrid fibers were fabricated in the same way described above. The critical challenge was to avoid mechanical and electrical interference between micromachined tips. The mechanical interference included a capillary action between neighboring W-tips. When two tips were close to affect the meniscus of a solution, the contact angles on both sides of one tip were changed to generate nonsymmetric pressure on fibers. It interfered the formation of continuous fibers. In our experimental result, if the distance between two tips was larger than 100µm, a symmetric shape of meniscus was observed [Fig. 9].

Fig. 10 Illustration of the assembly process of the nanofiber formation under both electric field and capillary force.

(a) (b) Fig. 9 Two nanofiber fabrication (a) two W-tips in the mixture (b) two nanofibers (bar: 300µm) Electrical interference could be generated among attracted SWCNTs and SiC nanowires. Upon approach of the nanomaterials to W-tips, a cloud of the mixture was formed near W-tips. The diameter of the cloud was approximately ~500µm. In Fig. 9(b), the one fiber fabricated on a W-tip was much longer than the other. This difference was originated from the overlap of the clouds, which was induced by the electric field on both tips. When three W-tips were used in a solution, the interference was increased, which generated larger difference in lengths of fibers. Modeling and analysis of the hybrid fiber formation process The Capillary and Electrokinetic induced Particle Assembly When W-tip is pulling out of the dropped solution in a metal coil, nano-particles are under both surface tension and electric forces. The total energy is a summation of surface potential (surface tension), repulsive potential (geometry constrain, preventing particles from overlapping), and electrical potential (attracting particles toward the pulling tip) [Fig.10]:

E = γ D ∫ dS + γ DP D

∫ dS + γ ∫ dS + U P

DP

R

+ φtotal

P

where D, DP, and P refer to the droplet surface, dropletparticle interface, and particle surface, respectively, U R is the repulsive energy that stops particles from overlapping, and φ is the electrical potential. The kinetic energy is ignored due to the small size of particles. Moreover, to satisfy the Young’s law, γ P − γ D cos θ − γ DP = 0 .

Compression from capillary action Fig. 11 Simulation of the nanowire formation A simulation of the assembly of nanowires with only electric force is shown in Fig. 11. Randomly dispersed nanowires were attracted toward the W-tip and assembled into a fibril. Capillary action provided a compressive effect that closed the gap between branched fibrils. As a result, a straight single nanofiber was formed. Numerical simulations of surface tension could be also performed using Brakke’s Surface Evolver, a program which determined the equilibrium configuration of deformable surfaces. The air-fluid interface was modeled as an evolving interface while the pulling tip was represented as moving boundary. An initial simulation was performed on a single nanowire with different contact angle to the water surface. As shown in Fig. 12, a tilted nanowire would experience a torque due to the surface tension that rotates it to be vertical to the water surface. In the assembly of nanowire fibril, this surface tension-induced rotation was crucial. This was because under pure electrical field, simulation showed that nanowires tended to be attracted and deposited on the W-tip in a branched pattern, as shown in Fig. 11. Meanwhile, the capillary action acted to compress the branched pattern into a straight fibril. Thus, the combined effect of the electric field and capillary action was the key to form a straight fibril having consistent diameter.

5



[3]

[4]

Fig. 12 Simulation by Surface Evolver shows that a tilted nanowire will experience a torque from surface tension that pushes the nanowire to be vertical to the solution surface.

[5]

The various physical factors involved in the hybrid fibril fabrication process are summarized in Table 2. Table 2. Summary of various physics involved in hybrid fibril fabrication Process

Physics

Attraction toward W-tip Fibril confinement and alignment Hybrid structure formation Continuous fibril growth

Dielectrophoresis Surface tension Van der Waals interaction Dielectrophoresis and conductive SWCNT

CONCLUSION Using an AC electric field in conjunction with capillary action, we have successfully fabricated hybrid nanofibers having desired properties of SWCNTs and SiC nanowires. At the same time, the proposed study eliminated the tedious dispersion process and overcame the current process issues for nanocomposites. With this technology, the fabrication speed of the hybrid fiber was increased by 20 fold compared to the existing method[1]. The newly studied fabrication process also showed that we could control the diameter and length of the composite nano materials while manipulating parameters; the mixing ratio of SWCNTs and SiC nanowires, concentration of solution, immersion time, volume of solution and withdrawal rate. The hybrid fiber structure, formation and fabrication process have been characterized with SEM study, electrical measurement, modeling and analysis. The developed hybrid nanofibers will provide an ample opportunity for a long fiber fabrication in the future. ACKNOWLEDGMENTS The support of Wing Kam Liu and Yaling Liu by the National Science Foundation (NSF), Office of Naval Research (ONR), and the NSF Summer Institute on Nano Mechanics and Materials is gratefully acknowledged. REFERENCES [1] R. Annamalai, J. D. West, A. Luscher, and V. V. Subramaniam, "Electrophoretic drawing of continuous fibers of single-walled carbon nanotubes," Journal of Applied Physics, vol. 98, Dec 2005. [2] R. H. Baughman, A. A. Zakhidov, and W. A. de Heer, "Carbon nanotubes - the route toward applications," Science, vol. 297, pp. 787-792, Aug 2002.

[6]

[7]

[8]

[9]

[10]

[11]

[12]

[13]

[14] [15]

A. Y. Cao, V. P. Veedu, X. S. Li, Z. L. Yao, M. N. Ghasemi-Nejhad, and P. M. Ajayan, "Multifunctional brushes made from carbon nanotubes," Nature Materials, vol. 4, pp. 540-545, Jul 2005. H. Choi, A. C. Sofranko, and D. D. Dionysiou, "Nanocrystalline TiO2 photocatalytic membranes with a hierarchical mesoporous multilayer structure: Synthesis, characterization, and multifunction," Advanced Functional Materials, vol. 16, pp. 10671074, May 2006. Y. Liu, X. H. Qu, H. W. Guo, H. J. Chen, B. F. Liu, and S. J. Dong, "Facile preparation of amperometric laccase biosensor with multifunction based on the matrix of carbon nanotubes-chitosan composite," Biosensors & Bioelectronics, vol. 21, pp. 2195-2201, Jun 2006. V. P. Veedu, A. Y. Cao, X. S. Li, K. G. Ma, C. Soldano, S. Kar, P. M. Ajayan, and M. N. GhasemiNejhad, "Multifunctional composites using reinforced laminae with carbon-nanotube forests," Nature Materials, vol. 5, pp. 457-462, Jun 2006. M. Zhang, K. R. Atkinson, and R. H. Baughman, "Multifunctional carbon nanotube yarns by downsizing an ancient technology," Science, vol. 306, pp. 1358-1361, Nov 2004. Y. J. Jung, S. Kar, S. Talapatra, C. Soldano, G. Viswanathan, X. S. Li, Z. L. Yao, F. S. Ou, A. Avadhanula, R. Vajtai, S. Curran, O. Nalamasu, and P. M. Ajayan, "Aligned carbon nanotube-polymer hybrid architectures for diverse flexible electronic applications," Nano Letters, vol. 6, pp. 413-418, Mar 2006. J. E. Kim and C. S. Han, "Use of dielectrophoresis in the fabrication of an atomic force microscope tip with a carbon nanotube: a numerical analysis," Nanotechnology, vol. 16, pp. 2245-2250, Oct 2005. H. W. Lee, S. H. Kim, Y. K. Kwak, E. S. Lee, and C. S. Han, "The effect of the shape of a tip's apex on the fabrication of an AFM tip with an attached single carbon nanotube," Sensors and Actuators a-Physical, vol. 125, pp. 41-49, Oct 2005. H. W. Seo, C. S. Han, D. G. Choi, K. S. Kim, and Y. H. Lee, "Controlled assembly of single SWNTs bundle using dielectrophoresis," Microelectronic Engineering, vol. 81, pp. 83-89, Jul 2005. M. C. Strus, A. Raman, C. S. Han, and C. V. Nguyen, "Imaging artefacts in atomic force microscopy with carbon nanotube tips," Nanotechnology, vol. 16, pp. 2482-2492, Nov 2005. J. Tang, B. Gao, H. Z. Geng, O. D. Velev, L. C. Qin, and O. Zhou, "Assembly of ID nanostructures into sub-micrometer diameter fibrils with controlled and variable length by dielectrophoresis," Advanced Materials, vol. 15, pp. 1352-+, Aug 2003. "CRC Handbook of CHEMISTRY and PHYSICS," vol. 87th Edition, 2006-2007. P. G. Collins and P. Avouris, "Nanotubes for electronics," Scientific American, vol. 283, pp. 62-+, Dec 2000.

6

