No. 93 · OCTOBER 2021

SUMITOMO ELECTRIC TECHNICAL REVIEW

INDUSTRIAL MATERIALS

Ultra-High Strength Carbon Nanotube Yarn
Made by New Growth Method
Soichiro OKUBO*, Takeshi HIKATA, Toshihiko FUJIMORI,
Daisuke TANIOKA, Daiji YAMASHITA, and Takamasa ONOKI---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------We have been focusing on carbon nanotubes (CNTs) as a material for next-generation electric wires that are lighter and more
conductive than copper and aluminum wires. CNT single fiber has a greater conductivity than copper and the highest tensile
strength of any known material. Aiming at the practical application of CNT electric wires, we have discovered the effectiveness ofapplying tensile stress to CNTs during the growing process from iron catalysts. In addition, at the University of Tsukuba, where ajoint research was conducted, the growth of centimeter-class single fibers was observed in high-speed airflow, suggesting thatstress applied during the growth contributes to the lengthening of CNTs. Using this principle as a basis for the growth method, we
have created a metric-class CNT yarn that aggregates these long CNT single fibers. The new yarn has several times the strengthof the conventional CNT yarn and exceeds the tensile strength of commercially available carbon fibers. This yarn will not only
replace carbon fibers but also create new applications that have never been seen before.
---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------Keywords: carbon nanotube, yarn, honeycomb, high-velocity gas flow

1. Introduction

100

Tensile strength (GPa)Carbon nanotubes (CNTs) are made only from carbon.
A single fiber has a cylindrical structure, which is made byrolling a sheet of six-membered rings of carbon atoms.
CNTs are lighter than aluminum, and their tensile strengthis 20 times or more that of steel. They are also expected tohave properties, such as high electrical conductivity,
surpassing that of copper. CNTs are lightweight because
their specific gravity is one fifth that of copper. Theirchemical durability is also high. Deformation with a large
curvature, which could not be attained with conventional
carbon fibers, is feasible.(1) Thus, CNTs are expected to be
used as next-generation electric wire materials to cope withthe shortages of copper and reduce the weight of electric
wires used in vehicles. Studies have been conducted to use
CNTs as electric wire materials, but the electrical andmechanical properties of CNT yarns have not yet reached alevel equivalent to those of CNT single fibers. The biggest
problem is that it has been difficult to increase the length of
single fibers. There is a report that CNT single fibers of up
to several dozen centimeters have been fabricated.
However, the technique is not suitable for mass production
because the yield is very low.(2) There is a mass production
technique to fabricate CNTs of 1 cm long. However, it isdifficult to control the quality of CNT fibers. In addition,
there is a trade-off between quality and length. Namely,
CNT single fibers of good quality grow only about several
dozen micrometers. There is a different example ofproducing yarns with continuous growth, but it has beendifficult to maintain the orientation of the fibers.
The mechanical strength of CNT yarns has beenstudied. Numerical values reported by various researchgroups are shown in Fig. 1, which indicates that thestrength of conventional CNT yarns is far below that of asingle fiber and even inferior to that of conventional carbon
fibers.
To attain practical application for electric wires,

CNT single fibers (up to 100 GPa)

16
14 GPa14
Resin cladded
12108
Carbon fibers
6
Aramid fibers
4
Metals (e.g., Steel)2
CNT02000 2005 2010 2015 2020
Year of release

Fig. 1. Changes in the tensile strength of CNT yarns reported in papers andcomparison of their strength against Sumitomo Electric’s CNT yarns

Sumitomo Electric Industries, Ltd. has studied CNT growthmethods that meet the requirements of both long growth ofhigh quality CNTs and orientation of single fibers. CNT
yarns fabricated using a newly developed growth techniquehave attained the highest tensile strength in the world, far
exceeding that of previously reported CNT yarns and evenexceeding that of carbon fibers. This paper reports thedetails.

2. Conventional Growth Methods
In 1991, S. Iijima et al. determined the structure of CNTs
using an electron microscope and revealed how peculiar itwas.(3) Previously, Endo et al. discovered a technology to
grow thin carbon fibers in the 1970s. In this technique,(4)metal catalyst microparticles, which served as the starting
points, were placed on a substrate and CNTs were allowed
Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method58（1）SUMITOMO ELECTRIC TECHNICAL REVIEW

to grow by thermal CVD.*1 This technique is used in many
mass production processes at present. The substrate
method, which was developed under the initiative of the
National Institute of Advanced Industrial Science andTechnology (AIST), achieved growth of long CNTs of one
centimeter, which was the longest at that time. However, it
was accompanied by a trade-off between quality andlength.(1) The eDIPS method, which is another techniquedeveloped by AIST, achieved CNTs with fewer defects
with a length of less than 100 μm.(5) However, the yield
was below the level for mass production. All of thesemethods have advantages and disadvantages. At Sumitomo
Electric, we discovered that high-purity graphite fibers
grew like bridges across thermal cracks generated during
gas carbonization of an oxidized iron foil. This wasreported as the “bridge growth method.”(6) In this growthphenomenon, carbon, which is subject to saturated carburizing in bulk iron, is withdrawn by the tensile force whencracks propagate on a surface. When a pure iron foil isseparated during carburizing, high-purity graphite fibers ofseveral millimeters can be withdrawn by optimizing theconditions. This gave us a hint about the phenomenon ofcontinuously extending fiber length in the newly developed
CNTs growth method.

3. Overview of the New CNT Growth Method
3-1 Discovery of a new growth phenomenon
To identify the working principle of the abovementioned bridge growth method, we conducted basic researchon the carburizing phenomenon jointly with the Universityof Tsukuba. During the carburizing experiment at theuniversity, we discovered a phenomenon in which manyhigh-quality CNTs of few centimeter with minimal defects
grew on a quartz glass substrate placed in a narrow channel(see Fig. 2).

No. 93 · OCTOBER 2021

Verification was conducted on this phenomenon, andit was found by simulation that the gas flowed at a highvelocity in the narrow channel. The results suggested thattensile stress was applied to CNTs during growth, causing
CNTs to grow very long (few centimeter) due to the significant difference in flow velocity from the wall surface tothe center of the channel. To further verify the working
principle and take a step closer to practical application, weapplied for a project under the auspices of the New Energy
and Industrial Technology Development Organization(NEDO) jointly with University of Tsukuba, and our
proposal was accepted. In the NEDO project, we measured
the strength of CNT single fibers at the University ofTsukuba. Regarding the measurement technique, CNTs
which grew across grooves on a silicon substrate were
exposed to a gas blow. The rupture strength was calculated
based on the deflection of the single fibers at the time ofrupture.(7) It was found that the strength of the single fibers
was equivalent to 100 GPa,(8) which was calculated. In thequality evaluation based on Raman spectroscopy, there
were few signals attributed to defects (D peak). This
proved that the quality was very high. Based on the peak
attributed to the diameter, the diameter of the CNT fibers
was found to be about 1 to 2 nm.
3-2 Key to the new CNT growth technology developedby Sumitomo Electric
After it was found that very long high-quality CNTs
could be fabricated by using the phenomenon discovered at
the University of Tsukuba, Sumitomo Electric started todevelop equipment to achieve mass production. We decided
to use a ceramic honeycomb, which has been often used asa reduction catalyst support for automotive exhaust gas, as
one of the methods for providing many narrow channels.
Such honeycombs are inexpensive and commercially available, and can be readily procured. Figure 3 shows the overview of the experiment system. Ten patents have been
applied for regarding this technology.

Ceramic honeycomb<Experimental Setup>
furnace

Furnace heaterFenanoparticles

CNT growth

SUS
CH 4 +Ar

Fe foilquartz plate

CNTs
SinglefibersFluid Dynamics in the Furnace

Stranded
yarn

Yarns

Velocity distribution
gas inlet

Φ25 mmFe

50 µm

quartz
CNT

Fig. 3. Schematic diagram of the new growth technique and structure of
CNT yarns in respective positions (electron microscopic images)

plate*Calculated by the FEM
Fig. 2. Schematic diagram of growth of long CNTs in a narrow channel (top)and flow velocity distribution based on calculation (bottom)
Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method

59（2）SUMITOMO ELECTRIC TECHNICAL REVIEW

Photo 1. Cross section of a honeycomb used for growth of CNT yarns

G peakIntensity (arb. Unit)

3-3 Raw materials
Transition metals with high carburizing capacity, suchas iron (Fe), nickel (Ni), and cobalt (Co), are often used as
catalyst metals, which serve as the starting points of growthof CNTs. The process of Sumitomo Electric also used Fe asa catalyst, as in the case of conventional techniques. A
ferrocene solution was used as the Fe source. The ferrocene
solution was atomized from the inlet of the electric furnace.
Methane (CH4) and hydrogen (H2) were fed as a raw material gas and a carrier gas, respectively, at a certain flow
rate. The atomized ferrocene solution mixture was carried
with the gas flow to the high-temperature reaction zone andwas turned into Fe metal nanoparticles through thermal
decomposition reaction. The raw material gas was thermally decomposed for carburizing.
3-4 CNT growth process
The basic growth process is a thermal CVD growthmethod, which is conventionally used for CNT growth(so-called “Floating Catalyst Method”). An Fe raw material
solution, which would turn into catalyst nanoparticles, wasatomized in a tubular electric furnace at about 1,000°C. A
hydrocarbon-based raw material gas and a carrier gas were
fed at the same time and were thermally reacted in the hightemperature zone. CNTs were continuously taken out fromthe outlet on the opposite side.
Regarding the working principle of CNT growth in
this technique, carburizing is performed using the CH4 gas
as a carbon source against catalyst particles (e.g., metalFe), which are present as nanoparticles at high temperature.
Surplus carbon grows as CNTs in stages from particles thatare subjected to saturated carburizing. Carbon, which isremoved from particles as CNTs, is supplied from the raw
material gas. This cycle enables continuous growth of
CNTs. The diameter of the CNTs is the same as the diameter size of the nanoparticles. In the conventional catalystic
CVD method, amorphous carbon accumulates on thesurface of nanoparticles concurrently with carburizing.
Thus, the supply of carbon from the gas decreases gradually, and the growth of CNTs stops. This is considered tobe the reason why only short single fibers can be obtained.
In our technique, CNTs, which are in a growth
process, grow while being carried with catalyst nanoparticles in a high-velocity gas flow in a narrow channel. The
CNTs are subjected to tensile stress due to the significant
difference in the gas velocity that is in the direction perpendicular to the flow direction. Thus, carbon is withdrawn
continuously, and carbon concentration in particles isbalanced at the saturated level. This is considered to inhibit
the growth of amorphous carbon on the surface.
As discussed above, a commercially available ceramichoneycomb was used as a method of providing many
narrow channels to achieve continuous CNT growth inlarge quantities. A ceramic honeycomb of about φ50 mm
was used to fit into the furnace core tube. The channel
shape was 1 mm to 3 mm square (see Photo. 1).
The quality of the CNTs which were allowed to growthrough the honeycomb was identified by Raman spectroscopy. The profile showed that the G peak attributed to thesix-membered rings of carbon was very high and that the Dpeak attributed to defects was very low. It was found thatthe quality was as high as that of CNTs that were allowed
to grow in the basic study (Fig. 4).

No. 93 · OCTOBER 2021

D peakR a m a n s h i f t ( c m -1)
Fig. 4. Results of Raman spectroscopy on CNT yarns
manufactured using a honeycomb

The element yarns were confirmed to be oriented in
one direction, as shown in Fig. 5, based on electron microscope observation of the surface. CNTs obtained using thehoneycomb and without using the honeycomb were
subjected to electron microscope observation, and element
yarns were subjected to orientation evaluation by polarized
Raman spectroscopy. The honeycomb was found to beeffective in promoting the orientation of CNT single fibers.
The orientation evaluation by polarized Raman spectroscopy aims to obtain a profile at certain angle intervals
while emitting a linearly polarized exciting light in a direction perpendicular to the longitudinal direction of a yarn
and rotating the polarization plane, and to determine theangle dependence of the G peak strength. This technique
was used to measure yarns which were stranded in thefollowing process. A strength peak was observed every 60°.
Thus, it was considered that the CNT fibers were oriented
in the longitudinal direction and that the rotation angle
when the yarns were stranded was reflected (see Fig. 6).

Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method60（3）SUMITOMO ELECTRIC TECHNICAL REVIEWHigh orientation

Rate of orienta�on

1

Random
0.8

No. 93 · OCTOBER 2021

fabrication of yarns demonstrated that tensile strength andelongation are controllable.(9) Subsequent process development also achieved winding of yarns up to about 1 m.
Appropriate post-processing attained tensile strength of 10
GPa or more with good reproducibility. This process has
made it possible to produce CNT yarns of up to 1 m bycontinuous growth in manual operation. We have taken thefirst step toward a mass production process (Photo 2). We
also conducted a demonstration to lift a weight by bundlingthe yarns (Photo 3).
Based on these results, larger-scale equipment isunder development with support from the Security
Technology Research Promotion System of theAcquisition, Technology & Logistics Agency (ATLA).

0.6

Using ahoneycomb

Without ahoneycomb

14 GPaTensile stress (GPa)Fig. 5. Evaluation of the rate of orientation of CNT element yarns(measurement by polarized Raman spectroscopy)

60°

Sumitomo’s CNT
yarn(resinimpregnated)

2 µm

Sumitomo’s
CNT yarn
50 µm
Conventional
carbon fibers

Stranded
CNT yarn

1.6 GPa

60°

Tensile elongation (%)
Fig. 7. Results of a tensile strength test on CNT yarns

Fig. 6. Results of orientation analysis of stranded yarns
by polarized Raman spectroscopy

10 cm

Photo 2. CNT yarn resin composite material (1 m long)

3-5 Measurement of tensile strength
The strength of the obtained CNT yarns was measured
based on a method specified in JIS R1657:2003 “Testing
Methods of Continuous Ceramic Fibers for Continuous
Fiber-Reinforced Ceramic Matrix Composites.” The results
are shown in Fig. 7. The tensile strength of CNT yarns
which was reported previously was up to 3 GPa, which was
below the strength of conventional carbon fibers (7 GPa at
the maximum). However, the CNT yarns fabricated using
the newly developed technique attained 6 GPa or morewith good reproducibility. Resin-impregnated yarns
attained about 14 GPa, which far exceeded the tensile
strength of conventional carbon fibers. The highest strength
in the world was attained for CNT yarns. Stranded yarns
exhibited a decrease in tensile strength, but it was foundthat elongation can be increased. Post-processing after

Photo 3. CNT yarn of φ0.4 mm capable of lifting a load of 1 kg
Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method

61（4）SUMITOMO ELECTRIC TECHNICAL REVIEW

4. Application of CNT Yarns as Yarn Materials
and Composite Materials
Based on information obtained through interviews
with experts, when the strength of fibers used for fiber-reinforced plastics (FRPs)*2 is increased, this strength isconsidered to be added to the overall strength of the FRP
composites. By using Sumitomo Electric’s high-strength
CNT fibers, it is possible to achieve FRPs with reduced
thickness for the same strength. Such FRPs can be knotted
like fishing lines because they can be bent more sharply
than carbon fibers (see Fig. 8).

Carbon fiber

achieved by CNTs due to the specific strength in design.(11)There are high expectations for ultra-high-strength CNT
yarns (Fig. 9).

CNT yarn

2
φ7 µm

Bending
ruptureFig. 9. Conceptual figure of a space elevator(Courtesy of Obayashi Corporation)

2 µm

φ8 µm

0.5 mm

0.5 mm

No. 93 · OCTOBER 2021

No bending
rupture0.5 mmFig. 8. Difference in flexibility between carbon fibers and CNT yarns

Regarding conventional carbon FRPs (CFRPs),
carbon fiber woven cloth is laminated in layers manually
into complicated shapes, such as bicycle frame joints, andis impregnated with resin. CNT woven cloth is resistant to
deformation into irregular shapes by pressing. This
increases the flexibility of the molding process.
The use of CNT yarns for current applications ofaramid fibers is expected to increase their strength significantly. Steel cords are used for the reinforced fibers oftires. Because aramid fibers are also used in part, there is apotential market to replace these fibers. Steel is used as theconcrete reinforcement material for buildings. The use of
CNT yarns offers the advantages of weight reduction andhigh strength.
The market for existing carbon fibers is expected toreach about 400 billion yen annually in 2025. The market
for CFRPs is expected to grow to about three trillion yen,mainly for structural materials for cars and aircraft. These
markets are expected to grow significantly into the future.(10)The aviation industry has been shrinking recently due tothe COVID-19 pandemic, but the market is expected torecover after the pandemic. There is another potential
application in the future. The main structural material for aspace elevator*3 called a “tether” (cable) can only be

We have been involved in technology development
under a national project. Figure 10 shows the future
deployment of technology to Sumitomo Electric products
and the product development schedule. The technology toachieve high-quality, ultra-long, and high-orientation CNT
single fibers for high-strength wires is an essential
elemental technology to achieve high strength and highelectrical conductivity for CNT electric wires. Properties
that are equivalent to conventional electric wires are
expected to be attained by overcoming such hurdles asimproving and equalizing the electrical conductivity ofsingle fibers and improving the density of yarns. We areworking on such development tasks concurrently.

2018

2020

NEDO-PJ• High-strength
CNT yarns
Strength:
10 GPa/attained2023
Acquisition, Technology& Logistics Agency(ATLA)-PJ

2025

(FY)

Joint development Market IN
with users• High-strength CNT yarns• Lightweight high-strength
materials
1 km long• Impact-resistant lightweight Impact-resistant materials
high-strength materials
Development of components
and Set to achieve practicalapplication(CFRPs: 3 trillion yen in 2025)

Development of CNT electric wires
Market IN
market:20 trillion yen for electric wires/cables in 2017,
5.7 trillion yen for harnesses in 2016

Fig. 10. National Projects and Sumitomo Electric’s productdevelopment schedule

5. Conclusion
To achieve next-generation electric wires, we havedeveloped a basic technology that can be used for a mass
production process by applying a new CNT growthphenomenon. We have succeeded in fabricating CNT yarnsof at least one meter. The CNT yarns have attained highstrength exceeding that of conventional carbon fibers. We
Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method

62（5）SUMITOMO ELECTRIC TECHNICAL REVIEW

made a breakthrough in the application of CNTs as structural materials. The CNT yarns offer a lot of potential for
Sumitomo Electric’s structural materials business and for
application in automotive parts. We plan to commercialize
such yarns as new lightweight high-strength wires. A study
on electrical conductivity will also be conducted concurrently to open the way for commercialization of next-generation electric wire materials.

6. Acknowledgments
We would like to express our deep appreciation toProfessor Junichi Fujita and the laboratory staff of theDoctoral Program in Applied Physics, Graduate School ofPure and Applied Sciences, University of Tsukuba, whohave offered generous guidance since the commencementof the development of CNT growth technology. This
research was conducted with support from the FY2018
Advanced Research Program for Energy and Environmental
Technologies “Research and Development of Innovative
Next-generation Lightweight and High-strength Structural
Materials” (P14004), a project under the auspices of the
New Energy and Industrial Technology Development
Organization (NEDO). We would like to express our thanks
to the researchers of the University of Tsukuba and the
Research Organization for Information Science &Technology (RIST), which also participate in this project,
as well as the Promotion Committee members and relevant
departments of NEDO. Currently, the technology development is supported by the Security Technology Research
Promotion System of the Acquisition, Technology &Logistics Agency (ATLA) (JPJ004596). We express our
deepest appreciation to the staff of all relevant departments.

No. 93 · OCTOBER 2021

References(1)	K. Hata, D. N. Futaba, K. Mizuno, T. Namai, M. Yumura, and S. Iijima,
Science 306, 1362 (2004)(2)	R. Zhang, Y. Zhang, Q. Zhang, H. Xie, W. Qian, and Fei Wei, ACS
Nano, 7, 6156 (2013)(3)	
S. Iijima, “Helical microtubules of graphitic carbon,” Nature, 354(6348): 56–58 (1991)(4)	M. Endo, T. Koyama, Japanese Patent, Prtent publication No.1987-242(7 January 1987)(5)	Kazuyoshi Tanaka, J. Vac. Soc. Jpn. Vol. 51, No. 4 (2008)(6)	Takeshi Hikata, Soichiro Okubo, Risa Utsunomiya, Yugo Higashi, andTeruaki Matsuba, “Novel Bridged Carbon Nanofibers Grown byCleaving Carburized Iron Substrates,” SEI TECHNICAL REVIEW,
No.79, pp.102-106 (October 2014)(7)	Report of NEDO Feasibility study program 2018 Advanced Research
Program for Energy and Environmental Technologies, “Research andDevelopment of Innovative Next-generation Lightweight and Highstrength Structural Materials” (P14004)(8)	Akira Takakura, et al., Nature Communications vol.10, 3040 (2019)https://www.nature.com/articles/s41467-019-10959-7(9)	Toshihiko Fujimori, Ryosuke Araki, Kanade Matsuo, Hideki Masuda,
Jun Otsuka, Daisuke Tanioka, Takeshi Hikata, Soichiro Okubo,
Yoshikazu Ito, and Jun-ichi Fujita, Mechanical Properties of Carbon
Nanotube Yarns Produced by Floating-Bridge Method The 80th JSAP
Autumn Meeting 2019, 18p-E307-12(10)	FUJI KEIZAI CO., LTD., The report of prospects of market with thecarbon fiber composite (CFRP/CFRTP) material and related technique.
2018 (January 2018)(11)	Obayashi Corporation Project team, “Construction concept of 100,000
km connecting from the earth to space tower “Space Elevator,”
Yoji Ishikawa, “2050 Space Elevator Journey,” Quarterly Obayashi
Vol 53, “Tower” 30-61 (2013)

Technical Terms＊1	Thermal CVD (vapor phase synthesis) method: Thisis a type of Chemical Vapor Deposition (CVD). A
raw material gas (a hydrocarbon, such as methane, inthe case of CNTs) is decomposed and reacted by theheat of the electric furnace to obtain a desired
material.
Fiber-reinforced plastics (FRPs): Fiber-reinforced＊2	plastics are materials whose strength is enhanced byhybridizing plastics with fibers. The strength ofcomposite materials using carbon fibers isparticularly high. They are used as structural
materials in cars and aircraft.
Space elevator: A space elevator refers to a＊3	transportation system for moving between the Earth’s
surface and space along a tether (i.e., cable), which
extends from a geostationary satellite to the Earth’s
surface. Such a transportation system would reducethe cost of space transport to one hundredth that ofrockets. CNTs are considered to be the only material
that can be used for a tether.(11)Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method

63（6）SUMITOMO ELECTRIC TECHNICAL REVIEW

No. 93 · OCTOBER 2021

Contributors The lead author is indicated by an asterisk (*).

S. OKUBO*• ‌Specialist
Assistant General Manager, Frontier Technologies
Laboratory

T. HIKATA• ‌Specialist
Ph.D.
Assistant General Manager, Frontier Technologies
Laboratory

T. FUJIMORI• ‌Ph.D.
Assistant General Manager, Frontier Technologies
Laboratory

D. TANIOKA• ‌Engineer, Frontier Technologies Laboratory

D. YAMASHITA• ‌Researcher, Frontier Technologies Laboratory

T. ONOKI• ‌Ph.D.
Project Leader, Frontier Technologies Laboratory

Ultra-High Strength Carbon Nanotube Yarn Made by New Growth Method64（7）