FIELD OF INVENTION;
The present invention relates to a method for growing array of carbon
nanotubes and converting them into a mechanically stable nanotube yarn.
More particularly the present invention relates to preparing yarn in the shape
of a conventional bare wire or cable for current conductor applications and
specially, a method in which carbon nanotubes grown on large area flat
substrates in the form of aligned structure is converted to carbon nanotube
yarn by dry drawing through guides and twisting by twister to achieve in the
form/shape of conventional bare wire or cable.
BACKGROUND OF THE INVENTION;
Carbon is one of the important elements in chemistry and also one of the most
important elements to life on earth. It exists in various forms including
fullerenes, carbon nanotubes (CNT's), grapheme etc. Among all the allotropes of
carbon, CNT's are technologically very important. CNT's are tubules of carbon
formed by wrapping a graphene sheet. CNT's are generally 5 um to 1 cm in
length and 5-100 nm in diameter. CNT's exist either as single-walled (SWNT),
double-walled (DWNT), thin-multi-walled (T-MWNT) or multi-walled (MWNT).
Individual CNT's have exceptional mechanical, electronic, photonic, and optical
properties, which could be important to several applications. In-addition
conversion of nanostructured CNT's into macroscopic from i.e. yarn is an
intriguing concept which can realize macro-scale applications of CNT's in
electrical, defence and aerospace [1-3].
Several inventors have disclosed methods to prepare nanotube yarn and also
methods to improve the electrical conductivity of the yarn. Some of the most
relevant disclosures related to present invention are mentioned here. Invention
US8602765B2 relates to the same, and more particularly to a carbon nanotube
(CNT) yarn and method for making the same. CNT yarn was prepared from
nanotube array grown on a substrate. Inventions US 2013/0183439 Al and
WO 2013/109442 Al disclose the methods to improve the electrical
conductivity of CNT yarn by chemical doping by bromine, iodine, chloroauric
acid, hydrochloric acid, hydroiodic acid, nitric acid and potassium
tetrabromoaurate. Invention US20090282802A1 disclose the method to
prepare yarn by spinning of 1 nm to 1 cm CNT. Invention US20080170982A1
discloses the method of making nano-fiber yarns from nanotube array.
Invention WO 2012118836 A1 disclose methods to improve electrical
conductivity of yarn by doping.

Even though growth of nanotubes have been extensively reported by several
groups, however the characteristic of direct spinnability for a CNT forest is
uniquely difficult to achieve and until now only a few research groups have
succeeded. Majority of the inventions mentioned here utilize aligned nanotubes
array grown on substrates to prepare the yarn, the process is referred as dry
drawing or spinning. In the said process, array of aligned nanotube array that
is suitable for yarn preparation is performed by atmospheric chemical vapor
deposition on catalyst coated wafers with iron (Fe) catalyst on a SiO2 buffer
layer and acetylene as feedstock gas. Iron catalyst is deposited either by
sputtering or e-beam evaporation.
Although the inventions and publications on CNT yarn report the parameters
to prepare aligned nanotubes, it is mostly on small area substrates of 0.5 to 1
inch in atmosphere chemical vapor deposition systems with 1-2 inch diameter
quartz tubes. Hence, a method or process to grow CNT arrays for yarn
application on larger substrates of 4 inch diameter or higher is required.
Further, catalyst thickness and its deposition method, carbon source gas
purification before entering the process chamber and its ratio during nanotube
growth, process pressure during nanotube growth are to be addressed for
achieving aligned carbon nanotube arrays.
OBJECTS OF THE INVENTION;
It is therefore, an object of the present invention to grow array of aligned
carbon nanotubes by low pressure chemical vapour deposition (LPCVD)
method on catalyst coated flat substrates.
Another object of the present invention is to grow array of nanotubes on
substrates of four inch diameter or on substrates of width four inch and length
eight inch.
Yet another object of the present invention is to convert the LPCVD grown
carbon nanotube array into mechanically stable carbon nanotube yarn by dry
drawing of carbon nanotubes parallel, to the substrate containing nanotube
array, through guides and twister and collection of the yarn on spindle.
SUMMARY OF THE INVENTION;
According to this invention, a method is provided for the growth of carbon
nanotube array on large area substrate by low pressure chemical vapour
deposition method. Further a method is provided for converting the said array

into a mechanically stable nanotube yarn in the shape of a conventional bare
wire or cable.
BRIEF DESCRIPTION OF ACCOMPANYING DRAWINGS;
Figure 1 - Schematic of low pressure chemical vapour deposition system with a
tube to accommodate four inch diameter substrates.
Figure 2 - Schematic of yarn drawing or pulling machine.
Figure 3 - Photograph of four inch diameter silicon wafer showing array of
aligned nanotubes grown uniformly over the substrate.
Figure 4 - Yarn being prepared from highly aligned carbon nanotube array
grown on four inch diameter wafer close-up view of yarn drawing and SEM
image of carbon nanotube yarn
DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS;
The present invention relates to the growth/synthesis of array of aligned
carbon nanotubes and its conversion to yarn by dry spinning. Such inventions
have already been reported for small size substrates usually 1-2 inch in
atmospheric chemical vapour deposition (APCVD) system. However, up-scaling
those methods to say substrates with size of four inch diameter or higher is not
straight forward due to the carbon feed stock flow dynamics etc. involved in
up-scaling APCVD systems. For large scale synthesis i.e. synthesis of nanotube
arrays suitable for preparing yarn on large size substrates, the most promising
method is low pressure thermal chemical vapour deposition (LPCVD). In this
method array of nanotubes which are relatively clean and having less
amorphous carbon can be grown resulting in the conversion of entire array of
nanotubes into yarn. An embodiment of this invention describes the synthesis
process of nanotubes by LPCVD.
A common problem in any CVD and particularly involving growth on large scale
substrates is achieving uniform distribution of catalyst particle size over the
entire substrate surface. Further achieving the fine distribution of catalyst size
to achieve relatively uniform diameter nanotubes in LPCVD is complex. This
requires careful annealing of thin film coated catalyst in vacuum preferably in

the range on 10-6 to 10-7 Torr in the presence of reducing gas such as hydrogen
of 30-50 seem for 30-45 min in ramping step from room to growth temperature
of 800-850 degree-C for single-walled, 750-800 degree-C for double-walled and
700-750 degree-C for multi-walled. An embodiment of this invention describes
the annealing process of the catalyst in high vacuum to achieve uniform
catalyst distribution along the surface of the substrate used for growth of
carbon nanotubes.
For growth of nanotubes in APCVD or LPCVD carbon feed stock gas is
essential. Among the carbon feed stock gases acetylene (C2H2) gas has been
extensively used for the growth of nanotubes and many inventions have
reported it. Among all the parameters involved in growth of nanotubes with
C2H2 feed stock gas, its purity is the most important. Availability of C2H2
variants varies from country to country. In countries where there are no strict
regimes for quality control of C2H2 gas, growth of carbon nanotubes using C2H2
is relatively tricky. Hence an alternative arrangement such as filters having a
combination of molecular sieves and silica gel is required to convert the C2H2
gas suitable to achieve growth of aligned array of carbon nanotubes by LPCVD.
An embodiment of this invention describes the synthesis process to purify C2H2
gas i.e. used for carbon nanotubes growth by LPCVD.
The following examples are presented to further illustration the invention, but
it is not be considered as limited thereto.
Example 1
Multi-walled carbon nanotube array on 5nm iron coated catalyst on four inch
silicon wafer with 30 nm SiO2 buffer layer is described. Iron of 5 nm is
deposited by RF sputtering with argon as the process gas on p-type silicon
wafers having a native oxide 30 nm. The catalyst coated wafer is placed on
quartz sample holder inside the quartz tube 1 of LPCVD. The quartz tube 1 of
LPCVD has a dimensions of length: 1500 mm and internal diameter of 100
mm. The split three zone furnace 2 provides a uniform heating along 900 mm
length of the quartz tube and sample temperature is monitored through
thermocouple 3. The gas flow rate of argon, hydrogen and acetylene 4 is
controlled through the MFC's 7, gas valve 6 and main valve 5. The process
pressure of the LPCVD is controlled through a capacitance manometer 8, 10
and the controller 9 maintains the temperature of the three zones. After placing
the catalyst at around 680 mm from the left size of the LPCVD, the flanges are

closed and the quartz tube is evacuated to 10-7 Torr with a combination of
mechanical and turbo molecular pump. After reaching 10-7 Torr the furnace is
ramped 20 degrees per minute from room temperature to 750 degree-C.
Simultaneously during the ramping, 30 seem hydrogen is also passed. After
reaching 750 degree-C the iron catalyst coated wafer, is annealed for 10
minutes. Consequently after annealing flow of hydrogen gas is stopped and
acetylene gas 100 seem is released into the LPCVD and growth is carried out
for 15-20 minutes resulting in an array 19 of aligned multi-walled carbon
nanotubes of -300 urn length and 15-20 nm in diameter. The acetylene gas
before entering the LPCVD is passed through a filter consisting a layered
structure of molecular sieves and silica gel. The filter is designed to contain
consecutive layers of molecular sieve and silica gel up-to four layers. After
growth step the LPCVD is cooled to room temperature under the flow of 30
seem of hydrogen and 30 seem of argon. As seen in figure 4 the carbon
nanotube array 19 is uniformly achieved over the entire surface of the 4 inch
diameter substrate. Carbon nanotube yarn form the array is drawn by
plunging the sharp tungsten tip 14 into the carbon nanotube substrate 19
placed on the wafer holder 11. After plunging the tungsten tip is pulled back
with nanotube 20 attached to it. After drawing the tungsten tip the wafer
holder is adjusted by moving the table 11 to achieve certain level of stiffness in
the yam and avoid sagging. The drawn yarn is passed through the primary
guide 16, twister 17 and secondary guide 18. Movement of the tungsten tip is
controlled by guide 15. To achieve certain mechanical strength the twister is
rotated at a speed of 6000 rpm and yam is drawn back and the twisted yarn is
collected on yarn thread collector 13.
Example 2
In example 1 the entire growth process was conducted wherein pressure in the
LPCVD was controlled by flow of hydrogen and acetylene gases. In this example
process pressure is controlled by capacitance manometer 8, 10. The 4 nm iron
coated silicon dioxide (30 nm) and silicon wafer are placed on the substrate
holder which is placed in the middle of the furnace. Later the LPCVD is
evacuated to 10-3 Torr and hydrogen 200 seem and argon 100 seem is passed
into the LPCVD at the same time the temperature is increased to 720 deg-C
with a ramp rate of 50 degree-C. The pressure inside the LPCVD is maintained
at 10 Torr with the help of a capacitance manometer which controls the
opening of the exit orifice. After reaching the growth temperature both gases
are stopped for a minute, before the acetylene gas required for nanotube

growth is passed into the LPCVD. Growth step is conducted for 15 minutes
with a flow of 500 seem of acetylene and 50 seem of hydrogen with a process
pressure of 10 Torr. After completion of growth step acetylene gas flow is
stopped and the substrates are cooled in hydrogen and argon, both with a flow
rate of 300 seem each. The resultant multi-walled carbon nanotube array
consisted of nanotubes with a length of 280 um and diameter of 15 nm. Similar
yarn drawing process as explained in example 1 was utilized for preparing the
carbon nanotube yarn.
Although various embodiments of this invention have been shown and
described, it should be understood that various modifications and
substitutions, as well arrangements and combinations of the preceding
embodiments can be made by those skilled in the art, without departing from
novel spirit and scope of invention.

WE CLAIM:
1. A method of growing carbon nanotube array (19) by a low pressure
chemical vapour deposition (LPCVD) method and drawing of yarn from
the carbon nanotube array.
2. The carbon nanotube array (19) as claimed in claim 1, wherein the array
comprises of single-walled, doubled-walled and multi-walled carbon
nanotubes.
3. The low pressure chemical vapour deposition method as claimed in claim
1, wherein the process pressure is 7-12 Torr.
4. A method of growing nanotube array (19) as claimed in claim 1, wherein
the growth temperature range is 800-850 degree-C for multi-walled
carbon nanotubes (19).

5. The low pressure chemical vapour deposition (LPCVD) as claimed in
claim 1, wherein the argon and hydrogen gases are preheated to 50-100
degree-C before entering the LPCVD.
6. The low pressure chemical vapour deposition (LPCVD) as claimed in
claim 1, wherein a carbon feed-stock gas comprise of acetylene (C2H2) is
used for the growth of the nanotubes in LPCVD.
7. The carbon feed-stock gas for growing carbon nanotube array (19) as
claimed in claim 6, wherein C2H2 is passed through a filter consisting
molecular sieves and silica gel before entering the LPCVD.
8. The carbon feed-stock gas for nanotube growth as claimed in claim 6,
wherein the flow rate of the C2H2 and H2 during growth cycle was 300-
700 and 300-700 sccm.
9. The process carbon nanotube array (19) as claimed in claim 6, wherein
thin films of catalyst are deposited on flat substrates of four inch
diameter or on substrates of width four inch and length eight inch.

10. The catalyst for growing carbon nanotube array (19) as claimed in claim
9, wherein the catalyst includes transition metals of iron, cobalt, nickel
or mixtures or alloys of the same with thickness 5-8 nm.
11. The catalyst as claimed in claim 9, wherein the catalyst was deposited
by sputtering or e-beam evaporation and then deposited on 20-30 nm
SiO2 buffer layer.
12. The catalyst for nanotube array (19) synthesis as claimed in claim 9,
wherein the catalyst is annealed in 10-7 Torr in LPCVD during ramp-up
from room to growth temperature for 30-45 minutes.
13. A method of making a carbon nanotube yarn comprises growing aligned
array of nanotubes on flat catalyst coated substrate by LPCVD and
drawing the nanotubes from the array by cantilever or sharp tungsten tip
(14).

14. The carbon nanotube yarn as claimed in claim 13, wherein the carbon
nanotube yarn comprises a plurality of interwined nanotubnes pulled by
cantilever or sharp tungsten that are twisted in a parallel configuration
with one another by passing through guides and twisted by twister (17)
at 5000-10000 rpm (or appropriate speed, to sustain the mechanical
integrity of the yarn) and collected on a spindle.
15. The carbon nanotube yarn as claimed in claim 13, wherein the yarn
drawing direction is parallel to nanotube array (19) substrate.
16. The carbon nanotube yarn as claimed in claim 13, wherein the carbon
nanotube yarn is in the shape of conventional bare wire or cable.