Multiscale hybrids
On the page "Carbon Fiber and Nanotube Overview," we described the role of carbon fibers in the creation of composite materials. We also noted that the interactions between the reinforcement - the carbon fiber - and the matrix - the epoxy resin or other polymer - are weaker than ideal. So while our carbon-fiber-reinforced polymer materials are strong and durable, they could be stronger, more durable, and ultimately better.
To solve the problem of weak matrix-fiber interactions, scientists have turned to carbon nanotubes as a potential solution. They want to use nanotubes' incredible strength and other unique properties to help form a strong, stable "bridge" in between the carbon fibers and the matrix. If nanotubes could reliably bond to or stably interact with both sides, the carbon fiber side as well as the matrix side, the interface between reinforcement and matrix would be improved. Furthermore, the addition of carbon nanotubes to fibers allows nanotubes to lend their extremely good strength to the fibers, thereby improving tensile strength.
The resultant materials, carbon fibers coated with nanotubes, are known as multiscale hybrids because they contain materials on both the micro (10^-6) and nano (10^-9) scales: the carbon fibers and nanotubes, respectively.
There are several different ways of incorporating nanotubes onto the surface of carbon fibers, and while scientists have developed many methods, some are more effective than others. An ideal covering would include nanotubes uniformly distributed in a thin layer across the entire surface of a carbon fiber. The other side of the "bridge," or the nanotubes' interaction with the epoxy matrix, would also be stable. The layer of nanotubes helps to increase the interaction surface area between the fibers and the matrix; to picture this, imagine using a grappling hook on a mountainside versus using it on a smooth slope. The many irregularities and increased surface area on the rock wall allow the grappling hook to grip and hold more easily. The analogy of Velcro hooks and loops is also useful here.
Another issue with the addition of nanotubes onto carbon fibers is the problem of "aggregation," or grouping up. A process that attracts or bonds nanotubes to carbon fibers may bond large clumps of the tubes to a fiber. Though they increase the interaction surface area, these clumps can congest an area and end up decreasing total interaction efficiency; think of throwing the grappling hook from above at a huge boulder instead of a small rock that the hook could fit around. Furthermore, the uneven distribution that draws the tubes into bunches can keep them from covering the entire surface of a carbon fiber. The clumps do not interact very well with the epoxy resin, and thus the "bridge" that the nanotubes form is of poor quality.
This page will examine five methods of combining nanotubes with carbon fibers. Each method strives to overcome problems such as the ones mentioned above in order to create the best, strongest, and most stable composites possible. We will include a short description of each method and its results, and at the bottom of the page, there is a chart that summarizes this information. After examining the methods, we can draw comparisons and analysis.
Various Methods of Production
I. ULTRASONICALLY-ASSISTED ELECTROPHORETIC DEPOSITION
Guo et al. (2011) used ultrasonics in an attempt to overcome a problem found in electrophoretic deposition (EPD), which is a process that can be used to attach nanotubes to carbon fibers. EPD allows for the attaching of materials (in this case, nanotubes) to conductive surfaces, but the process involves electric charges in a water medium, and the electrolysis of water takes place. Electrolysis, which splits water molecules apart, produces tiny bubbles that attach themselves onto the surface of the carbon fibers and get in the way of the nanotubes. When ultrasonics were introduced into the system, the bubbles would be immediately shaken off the fiber, and nanotubes were able to attach themselves in more places and with less hindrance than before. The figure below shows the unaltered carbon fibers, carbon fibers plus nanotubes without ultrasonics, and carbon fibers plus nanotubes in the presence of ultrasonics.
This method resulted in an increase of 16% in tensile strength of the carbon fibers. The study also provides a good illustration of the basics involved in the EPD process, including a clear outline of the drawback caused by the presence of tiny bubbles that latch onto the carbon fiber’s surface and impede the attaching process.
II. DECOMPOSITION OF ACETYLENE FOLLOWED BY THERMAL CHEMICAL VAPOR DEPOSITION
The Chemical Vapor Deposition method discussed earlier in the overview of nanotubes can also be used to grow nanotubes on the surface of carbon fibers. Sharma and Lakkad (2011) immersed carbon fibers in a bath of catalytic salts to cover them in catalyst for the CVD process. After further preparations, they reduced the catalyst salt to the appropriate size and properties for deposition before introducing acetylene into the system. The acetylene decomposed and deposited its carbons onto the catalytic surfaces, and thus nanotubes began to grow onto the carbon fibers.
Tests revealed that the carbon fibers’ tensile strength had increased by up to 69% due to this procedure. One main difference between the EPD process described above and this CVD process is that for this experiment, Sharma and Lakkad grew the tubes on the carbon fibers instead of attaching pre-existing tubes onto fibers.
III. ELECTROPHORETIC DEPOSITION FOLLOWED BY PYROLYSIS
In 2014, Moaseri et al. used the electrophoretic deposition method described in part I of this section, but with some modifications. After attaching carbon nanotubes to the surface of carbon fibers, they dried the fibers before immersing them in a solution of petroleum pitch in toluene. Finally, they pyrolized the fibers; pyrolysis is a process that involves heating a substance at increasing temperatures to a cap. These scientists pyrolized their fibers “in a tubular furnace to 400 degrees C at 20 degrees/h in nitrogen atmosphere. Similar hybrids were pyrolized to 500, 600, 700, 700, 800, and 900 degrees C.” The goal of this pyrolysis was to “strengthen the attachment of CNTs to CF through carbonized residue of the petroleum pitch.” (note that CNT is an abbreviation for carbon nanotube(s) and that CF for carbon fiber(s)
III. ELECTROPHORETIC DEPOSITION FOLLOWED BY PYROLYSIS
In 2014, Moaseri et al. used the electrophoretic deposition method described in part I of this section, but with some modifications. After attaching carbon nanotubes to the surface of carbon fibers, they dried the fibers before immersing them in a solution of petroleum pitch in toluene. Finally, they pyrolized the fibers; pyrolysis is a process that involves heating a substance at increasing temperatures to a cap. These scientists pyrolized their fibers “in a tubular furnace to 400 degrees C at 20 degrees/h in nitrogen atmosphere. Similar hybrids were pyrolized to 500, 600, 700, 700, 800, and 900 degrees C.” The goal of this pyrolysis was to “strengthen the attachment of CNTs to CF through carbonized residue of the petroleum pitch.” (note that CNT is an abbreviation for carbon nanotube(s) and that CF for carbon fiber(s)
The researchers discovered that temperatures past 700 degrees caused damage to the structure of the carbon fibers. However, the multiscale hybrids that resulted from the EPD/pyrolysis process using multi-walled nanotubes at safe pyrolysis temperatures had improved in ultimate tensile strength by over 120%.
IV. RADIALLY GRAFTING VIA SIMPLE CHEMICAL VAPOR DEPOSITION
The researchers in this study, Song et al. (2012), were concerned with the alignment of their nanotubes relative to the carbon fibers on which they were grown. Radially-grafted tubes, they argued, would achieve superior matrix interactions to those between the resin and non-radially grafted nanotubes. To achieve their desired result, they employed the CVD process in order to grow nanotubes onto carbon fibers in the desired pattern. Before the catalyst was added, the researchers coated the fibers in a layer of chemical covering with the goal of protecting the carbon fibers from metal catalyst remnants falling into and negatively affecting them. The metal catalyst coating was added, and the CVD process took place – it is worth noting that methane gas provided the carbons in this experiment, which is different from the acetylene in the previous CVD process that we have examined.
IV. RADIALLY GRAFTING VIA SIMPLE CHEMICAL VAPOR DEPOSITION
The researchers in this study, Song et al. (2012), were concerned with the alignment of their nanotubes relative to the carbon fibers on which they were grown. Radially-grafted tubes, they argued, would achieve superior matrix interactions to those between the resin and non-radially grafted nanotubes. To achieve their desired result, they employed the CVD process in order to grow nanotubes onto carbon fibers in the desired pattern. Before the catalyst was added, the researchers coated the fibers in a layer of chemical covering with the goal of protecting the carbon fibers from metal catalyst remnants falling into and negatively affecting them. The metal catalyst coating was added, and the CVD process took place – it is worth noting that methane gas provided the carbons in this experiment, which is different from the acetylene in the previous CVD process that we have examined.
The researchers then tested improvements in out-of-plane and in-plane compressive strengths, as well as interlaminar shear strength, which had improved by “275%, 138%, and 206%, respectively” as a result of the nanotubes' influence on the fibers' material strength.
V. AMINO FUNCTIONALIZATION AND ALTERNATING ELECTRIC FIELD ALIGNMENT
This final method is one of the most involved that we have examined, and its placement at the end of our list is meant to encourage readers to read it last, after the others, since it combines several concepts already seen in simpler form above. Researchers Moaseri et al. (2013), whose work form 2011 we have already described, took not only the alignment of carbon nanotubes on the fibers into account, but also incorporated bonding potential across both sides of the “bridge.” They used nanotubes that were functionalized to include amino groups in this study and made use of their electric and chemical properties.
The researchers employed a variation of the EPD method in order to affix their functionalized nanotubes onto the carbon fibers in a specific alignment. After suspending the MWNTs in water, an AC voltage was run through the solution in order to align the nanotubes in the desired formation for addition to the fibers. Next, the researchers added HCl and NaNO2 to allow formation of a diazonium salt, which can bond to carbon fibers (adding stability on the fiber-nanotube side of the “bridge”). A constant voltage then moved the aligned tubes onto the carbon fibers in the desired arrangement. This step completed the EPD process, but it is also important to mention that the animo functionality allowed the tubes to link to epoxy resin at the other end of the “bridge” and provide further reinforcement and stability. To understand the alignment created by the AC voltage, refer to the figure below:
The resulting materials had a 126% increase in tensile strength and showed “considerable improvement in the fatigue life of reinforced composites” thanks to the addition of the animo-functionalized nanotubes and their positive interactions on both sides of the “bridge.”
Summary
We have described several different methods of creating multiscale materials for use in highly-reinforced composites. Each one focuses on the problem of the nanotubes as a "bridge" between layers in a composite in a different way, and they all improve strength or functionality in some way. Below is a graph that summarizes the five methods we examined and provides details about the specifics in how they approach the problem. We encourage you to consider the methods in light of one another; which one improves tensile strength the most? What attribute of the problem (alignment, chemical stability and bond reinforcement, surface area covered by nanotubes, etc) do you think is the most important? A combination of different considerations, such as the "Amino Functionalization" method, which focuses on both alignment in space and reinforcement through chemical bonds, may ultimately prove to create the absolute best multiscale polymer materials possible, but all of these improvements and experiments shed additional light on the puzzle.
SOURCES:
Díez-Pascual, A., Naffakh, M., Marco, C., Gómez-Fatou, M., and Ellis, G. (2014, April) Multiscale fiber-reinforced thermoplastic composites incorporating carbon nanotubes: A review. Current Opinion in Solid State and Materials Science 18(2), 62-80. http://ac.els-cdn.com.libezp.lib.lsu.edu/S1359028613000399/1-s2.0-S1359028613000399-main.pdf?_tid=93eaaefc-10bc-11e4-aac8-00000aacb35f&acdnat=1405936465_2ccaa87de5d79222228cc28fb665d882
Guo, J., Lu, C., An, F., and He, S. (2012, January) Preparation and characterization of carbon nanotubes/carbon fiber hybrid material by ultrasonically assisted electrophoretic deposition. Materials Letters 66(1), 382-384. http://ac.els-cdn.com.libezp.lib.lsu.edu/S0167577X11010457/1-s2.0-S0167577X11010457-main.pdf?_tid=15bd1f62-10ba-11e4-9e5e-00000aacb35d&acdnat=1405935394_b4d4e423452be1a3ab8754f746a98d94
Moaseri, E., Karimi, M., Maghrebi, M., and Baniadam, M. (2014, February) Two-fold enhancement in tensile strength of carbon nanotube–carbon fiber hybrid epoxy composites through combination of electrophoretic deposition and alternating electric field. International Journal of Solids and Structures 51(3-4), 774-785. http://ac.els-cdn.com.libezp.lib.lsu.edu/S0020768313004423/1-s2.0-S0020768313004423-main.pdf?_tid=c9f8f702-10bb-11e4-a098-00000aacb361&acdnat=1405936126_bca2efe68e094d0ddc418674f4c6cdbb
Moaseri, E., Karimi, M., Maghrebi, M., and Baniadam, M. (2014, May) Fabrication of multi-walled carbon nanotube–carbon fiber hybrid material via electrophoretic deposition followed by pyrolysis process. Composites Part A: Applied Science and Manufacturing 60, 8-14. http://ac.els-cdn.com.libezp.lib.lsu.edu/S1359835X14000232/1-s2.0-S1359835X14000232-main.pdf?_tid=bd5f89b2-10ba-11e4-949c-00000aacb35f&acdnat=1405935675_fc91cbe9086d4b100a603bf7be976451
Sharma, S. P. and Lakkad, S. C. (2011, January) Effect of CNTs growth on carbon fibers on the tensile strength of CNTs grown carbon fiber-reinforced polymer matrix composites. Composites Part A: Applied Science and Manufacturing 42(1), 8-15. http://ac.els-cdn.com.libezp.lib.lsu.edu/S1359835X10002563/1-s2.0-S1359835X10002563-main.pdf?_tid=48ca3b88-10ba-11e4-a31e-00000aacb360&acdnat=1405935480_2f30be48d6dd6337802cb3ff284e344a
Song, Q., Li, K., Li, H., and Ren, C. (2012, August) Grafting straight carbon nanotubes radially onto carbon fibers and their effect on the mechanical properties of carbon/carbon composites. Carbon 50(10), 3949-3952. http://ac.els-cdn.com.libezp.lib.lsu.edu/S0008622312002710/1-s2.0-S0008622312002710-main.pdf?_tid=69c53666-10bb-11e4-ad25-00000aab0f27&acdnat=1405935964_a7ef2f44da5dd332879a9ea8ef6b9ebd
Synthesis of vertically aligned carbon nanotubes on carbon fiber. (2013, April). Applied Surface Science 271, 424-428. http://ac.els-cdn.com.libezp.lib.lsu.edu/S0169433213002821/1-s2.0-S0169433213002821-main.pdf?_tid=5252522e-10bc-11e4-a594-00000aab0f27&acdnat=1405936355_9464433756f6e0c17ece268a5c55a476