U.S. patent application number 10/935652 was filed with the patent office on 2005-03-24 for carbon nanotubes.
This patent application is currently assigned to Nano-Proprietary, Inc.. Invention is credited to Fink, Richard Lee, Mao, Dongsheng, Yaniv, Zvi.
Application Number | 20050064167 10/935652 |
Document ID | / |
Family ID | 34316529 |
Filed Date | 2005-03-24 |
United States Patent
Application |
20050064167 |
Kind Code |
A1 |
Mao, Dongsheng ; et
al. |
March 24, 2005 |
Carbon nanotubes
Abstract
Carbon nanotubes can be self-aligned by making composites of
carbon nanotube powders with particles and organic and/or inorganic
carriers such as water or other solvents. After the mixture is
applied onto a substrate by whatever ways, such as brushing,
screen-printing, ink-jet printing, spraying, dispersing,
spin-coating, dipping, and the like and combinations, a
fragmentation process occurs when the composite material is dried
or cured by certain ways to eliminate some or all of the carrier
material. This results in microcracks forming between the
fragments. CNT fibers that are bonded or set in the fragments on
either side of a crack are aligned in the crack area, either by
stretching the fibers or by allowing the fibers to spool out from
one or both fragments.
Inventors: |
Mao, Dongsheng; (Austin,
TX) ; Fink, Richard Lee; (Austin, TX) ; Yaniv,
Zvi; (Austin, TX) |
Correspondence
Address: |
Kelly K. Kordizk
Winstead Sechrest & Minick P.C.
P.O. Box 50784
Dallas
TX
75201
US
|
Assignee: |
Nano-Proprietary, Inc.
Austin
TX
|
Family ID: |
34316529 |
Appl. No.: |
10/935652 |
Filed: |
September 7, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60502464 |
Sep 12, 2003 |
|
|
|
Current U.S.
Class: |
428/292.1 |
Current CPC
Class: |
B82Y 30/00 20130101;
Y10T 428/249924 20150401; C09J 1/00 20130101; C09J 11/04
20130101 |
Class at
Publication: |
428/292.1 |
International
Class: |
D04H 001/00 |
Claims
What is claimed is:
1. A composition comprising carbon nanotubes and an inorganic
adhesive material.
2. The composition as recited in claim 1, wherein some of the
carbon nanotubes are exposed within microcracks formed in the
composition.
3. The composition as recited in claim 2, wherein at least one of
the exposed carbon nanotubes bridges across the microcrack.
4. The composition as recited in claim 2, wherein at least one
carbon nanotube exposed within the microcrack is broken in two.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] The present invention claims priority to the following:
[0002] Provisional Patent Application Ser. No. 60/502,464, entitled
"SELF-ALIGNMENT OF CARBON NANOTUBES," filed on Sep. 12, 2003.
TECHNICAL FIELD
[0003] The present invention relates in general to carbon
nanotubes, and in particular, to a process for manufacturing a
carbon nanotube composition.
BACKGROUND INFORMATION
[0004] Carbon nanotubes (CNTs) are being investigated by a number
of companies and researchers because of their unique physical,
chemical, electrical, and mechanical properties (P. M. Ajayan, O.
Z. Zhou, "Applications of carbon nanotubes," Top Appl. Phys. 80,
391-425(2001)). Aligned carbon nanotubes have been demonstrated to
play an excellent role in logical circuits, high performance
structural and functional composites, electronic devices, etc. For
example, CNTs can be used as cold electron sources for many
applications such as displays, microwave sources, x-ray tubes, etc.
because of their excellent field emission properties and chemical
inertness (Zvi Yaniv, "The status of the carbon electron emitting
films for display and microelectronic applications," The
International Display Manufacturing Conference, Jan. 29-31, 2002,
Seoul, Korea). Aligned carbon nanotubes with excellent field
emission properties can be fabricated using chemical vapor
deposition (CVD) techniques on catalytically-activated substrate
surfaces with process temperatures over 500.degree. C. (Z. F. Ren,
Z. P. Huang, J. W. Xu et al., "Synthesis of large arrays of
well-aligned carbon nanotube on glass," Science 282,
1105-1107(1998)). CNTs can also be aligned by a taping process (so
called "activation") after screen-printing a CNT paste onto a
substrate (Yu-Yang Chang, Jyh-Rong Sheu, Cheng-Chung Lee, "Method
of improving field emission efficiency for fabricating carbon
nanotube field emitters," U.S. Pat. No. 6,436,221). Other methods
have also been attempted to align CNTs, but they have some of the
following disadvantages:
[0005] 1. It is difficult to achieve high uniformity illumination
required for display applications using a CVD process to grow CNTs
over large areas.
[0006] 2. CVD growth of CNTs requires a high process temperature
(over 500.degree. C.), limiting the use of low-cost substrates such
as sodalime glass.
[0007] 3. The organic residue on the substrate after activation
processes may give off residual gases in the sealed glass display
envelope during field emission operation. Furthermore, it is
difficult to uniformly activate the substrate over a large area.
For example, many display applications may require 40" to 100"
diagonal plates.
[0008] In summary, using CNT materials that require CVD growth
processes directly on the substrate material or that require
activation of the CNT material over a large area have disadvantages
that can be overcome with the materials and processes of the
present invention disclosed herein.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] For a more complete understanding of the present invention,
the advantages thereof, reference is now made to the following
descriptions taken in conjunction with the accompanying drawings,
in which:
[0010] FIG. 1 shows a digital image of a microcrack and aligned CNT
fibers;
[0011] FIG. 2 shows another digital image of a microcrack and
aligned CNTs;
[0012] FIG. 3 shows another digital image of a microcrack and
aligned CNTs;
[0013] FIG. 4 shows broken CNTs between two fragments;
[0014] FIG. 5 shows aligned CNTs;
[0015] FIG. 6 illustrates a schematic diagram of a CNT-Resbond
coating before and after a shrinking process;
[0016] FIG. 7 illustrates field emission I-V curves of samples of
the present invention;
[0017] FIG. 8 shows an optical image of CNT dots;
[0018] FIG. 9 illustrates a process for dispensing CNT composites
in accordance with an embodiment of the present invention;
[0019] FIG. 10 illustrates an I-V curve of a sample made in
accordance with embodiments of the present invention;
[0020] FIG. 11 shows a field emission image of the sample of FIG.
10;
[0021] FIGS. 12A-12C illustrate a process in accordance with
embodiments of the present invention;
[0022] FIG. 13 illustrates an I-V curve of a composite made in
accordance with embodiments of the present invention;
[0023] FIG. 14 shows a field emission image of a sample made in
accordance with embodiments of the present invention;
[0024] FIG. 15 illustrates an I-V curve of a composite made in
accordance with embodiments of the present invention; and
[0025] FIG. 16 shows a field emission image of a sample made in
accordance with embodiments of the present invention.
DETAILED DESCRIPTION
[0026] In accordance with the present invention, carbon nanotubes
can be self-aligned by making composites of carbon nanotube powders
with particles and organic and/or inorganic carriers such as water
or other solvents. After the mixture is applied onto a substrate by
whatever ways, such as brushing, screen-printing, ink-jet printing,
spraying, dispersing, spin-coating, dipping, and the like and
combinations, a fragmentation process occurs when the composite
material is dried or cured by certain ways to eliminate some or all
of the carrier material. This results in microcracks forming
between the fragments. CNT fibers that are bonded or set in the
fragments on either side of a crack are aligned in the crack area,
either by stretching the fibers or by allowing the fibers to spool
out from one or both fragments. The CNTs align and are parallel to
each other and to the substrate. Some CNTs may also be
perpendicularly aligned on the face of the fragments. In some cases
where the crack is large, some of the CNT fibers are also broken in
the crack area resulting in dangling fibers that emanate from both
fragments on either side of the crack. It may also be the case that
some fibers are pulled out from one of the two fragments on either
side of the crack. This process has several advantages:
[0027] 1. Very easy and low cost process to align CNTs.
[0028] 2. The CNTs can be aligned on a very large area.
[0029] 3. No activation processes are required on the CNT composite
material after the curing step to achieve good field emission
properties for cold cathode applications.
[0030] The section below describes one embodiment that may be used
to make aligned CNTs.
[0031] 1. Source of Materials
[0032] Single-wall carbon nanotubes (SWNTs) were obtained from
CarboLex, Inc., Lexington, Ky., U.S.A. These SWNTs were in the
range of from about 1 nm to 2 nm in diameter and in the range of
from about 5 .mu.m to 20 .mu.m in length. Single-wall, double-wall,
or multi-wall carbon nanotubes (MWNTs) from other vendors and
prepared by other methods, and with other diameters and lengths,
can also be used with similar results.
[0033] The other components of the composite prepared were
contained in an inorganic adhesive material obtained from Cotronics
Corp., Brooklyn, N.Y., U.S.A. having a name/identifier of Resbond
989 ("Resbond") that is a mixture of Al.sub.2O.sub.3 particles,
water, and inorganic adhesives. Composites that contain other
particles may also be used, such as SiO.sub.2. These particles may
be insulating, conducting or semiconducting. The particle sizes are
less than 50 .mu.m in size. Sizes may be much smaller than this.
The carrier in the Resbond is water, but other carrier materials
may be used and they may also be organic or inorganic. Other
materials that promote other properties of this material, such as
binders (e.g., alkali silicates or phosphate) may also be present
in the composite in small quantities.
[0034] 2. Preparation of the Mixture of Carbon Nanotubes with the
Resbond and Deposition onto Substrate
[0035] 1) Grinding of the Mixture
[0036] A 1 gram quantity of CNT powders (40 wt. %) and a 1.5 gram
quantity of Resbond (60 wt. %) were put together into a mortar. The
mixture was ground using a pestle for half an hour in order that
the mixture looks like a gel, meaning that the CNTs and
Al.sub.2O.sub.3 particles did not separate with each other. Please
note that a different weight ratio of CNTs to Resbond may also
work. Additionally, water or other carrier materials may also be
added into the mixture to dilute it in order to adjust the
viscosity. The mixture was then ready for depositing onto the
substrate.
[0037] 2) Applying the Mixture onto the Substrate and Curing
[0038] A brush was used to paint the mixture onto a conventional Si
substrate (10-100 k.OMEGA. cm) with an area of 2.times.2 cm.sup.2.
Other substrate materials such as ceramics, glass, metals, alloys,
polymers, or other semiconductors may also be used. Other ways to
put the mixture onto the substrate such as screen-printing,
spraying, spin-coating, ink-jet printing, dipping, and dispensing
may also be utilized or performed. The thickness of the coating was
about 20 .mu.m to 40 .mu.m. The substrate was dried at room
temperature in the air, but it may also be dried/cured in an oven
at increased temperature (approximately 100.degree. C. or higher)
in order to more quickly eliminate the water. If the solvent
contains organic(s), then even higher temperatures may be set to
remove it. For example, up to 300.degree. C. will be set to remove
epoxy. The oven or curing vessel may contain a vacuum pump to
exhaust the air out of the oven and form a vacuum inside the oven
during the drying/curing process. The oven or curing vessel may
also provide a gas environment or flow around the sample that
further promotes curing or drying. This gas environment or flow may
or may not be partially or completely from inert gases such as the
noble gases or nitrogen. Ultraviolet or infrared light may also be
used to aid the curing process.
[0039] 3. Microstructure of the CNT-Resbond Coating
[0040] Scanning electron microscopy (SEM) was used to analyze the
surface morphology of the sample. A JEOL made, JSM6320F &
JSM-35 model SEM was used for the experiment. Because
Al.sub.2O.sub.3 is an insulating material, a 20 nm-thick Au thin
film was evaporated on the top of the coating before testing. FIGS.
1 through 5 show SEM images of the sample created using a process
of the present invention.
[0041] FIG. 1 shows an SEM image of a microcrack within the sample
and aligned CNT fibers between two fragments. One can see fibers
that are aligned and attached to both fragments and some fibers
that are dangling from one of the fragments, often in the same
picture.
[0042] FIG. 2 shows an SEM image of another microcrack within the
sample and aligned CNTs between two fragments.
[0043] FIG. 3 shows an SEM image of further microcracks in the
sample and aligned CNTs among the three fragments. No dangling CNT
fibers are seen in this image.
[0044] FIG. 4 shows broken CNTs between two fragments in the
sample. This image shows mostly dangling or broken fibers with
possibly one fiber that is stretched across the gap between the
fragments.
[0045] FIG. 5 shows perpendicularly aligned CNTs against the
substrate.
[0046] FIGS. 1 and 2 show SEM images of aligned CNTs between the
nearest two fragments. Most of the CNTs are parallel to each other.
They are also parallel to the substrate. The microcracks occurred
during the fragmentation of the CNT-Resbond mixture, during the
drying/curing step. FIG. 3 shows aligned CNTs among the three
nearest fragments with three different aligned directions. If the
fragmentation process was more dramatic, the CNTs could be
extracted from one fragment and remain on the other fragment, or
broken with the ends of the fiber remaining in the fragment on both
sides of the crack (see FIG. 4). There are several aligned CNTs
contacting both sides of the fragments. Other CNTs are either
extracted or broken and left on one or both sides of the fragments.
Overall, those CNTs are still aligned. FIG. 5 shows the
perpendicularly aligned CNTs against the substrate. Those
extracted/broken CNTs are particularly useful for field emission
applications because one end of the CNT fiber is exposed to the air
or vacuum. Furthermore, as shown in the photographs, the density of
the dangling fibers is not so great that they shield the applied
electric fields needed for field emission applications from each
other. The CNT-Resbond coating may be separated to many islands
after the shrinking process. This is also good for field emission
properties of the CNT-Resbond coating because it can minimize the
screening effect during the field emission of the CNTs and expose
more CNT fibers outside the Resbond matrix without further needed
activation processes.
[0047] FIG. 6 illustrates a magnified schematic diagram of a
CNT-Resbond coating before (a) and after (b) a shrinking process.
The film after shrinking shows the nanotubes in the cracks between
the islands. Some nanotubes may extend across the crack and others
may extend only partially into the crack.
[0048] 4. Field Emission Test of the CNT-Resbond
[0049] 1) To find the best CNT content in the mixture, the
following different weight ratios of the CNTs to Resbond were
designed to find which concentration was the best for field
emission:
[0050] 10 wt. % CNTs+90 wt. % Resbond
[0051] 25 wt. % CNTs+75 wt. % Resbond
[0052] 40 wt. % CNTs+60 wt. % Resbond
[0053] 75 wt. % CNTs+25 wt. % Resbond
[0054] The above mixtures were prepared as described above and were
brushed onto ITO/glass substrates with an area of 1 cm.times.1 cm
and were dried in the air for 10 minutes. The thickness of the
coatings were in the range of from about 20 microns to about 30
microns. After drying, the samples were ready for field emission
testing. To compare field emission properties, a CNT sample without
any Resbond content was also made using the same brush process as
the other samples.
[0055] All the samples were tested by mounting them with a phosphor
screen in a diode configuration with a gap of about 0.63 mm between
the anode and cathode. The test assembly was placed in a vacuum
chamber and pumped to 10.sup.-7 Torr. The electrical properties of
the cathode were then measured by applying a negative, pulsed
voltage (AC, 2% duty factor) to the cathode and holding the anode
at ground potential and measuring the current at the anode from
field emitted electrons from the cathode. A DC potential could also
be used for the testing, but this may damage the phosphor screen. A
graph of the emission current vs. electric field for the samples is
shown in FIG. 7. It can be seen that the sample with 40 wt. % CNTs
had the lowest extraction field for a given current, which is
desirable for field emission properties. All the samples that
contained Resbond had better field emission properties than CNTs
without Resbond.
[0056] 2) Demonstration of Applying the CNT Composite by Dispensing
Process
[0057] The mixture of 40 wt. % CNT and 60 wt % Resbond was utilized
and dispensed onto a substrate and the field emission was tested.
Dispensing is an excellent process to deposit very small dots onto
the substrate over large areas. The definition of such size dots is
suitable for making CNT high resolution field emission displays. A
Musashi-made dispenser (model: SHOT mini.TM.) was employed to
deposit the mixture onto conductive ITO (Indium tin oxide). Other
dispenser machines can be used, including ink-jet approaches.
Patterns are made by moving the dispensing head and/or the
substrate relative to each other and dispensing dots or lines of
material at pre-defined locations.
[0058] The diameter of the nozzle was 300 .mu.m. Six rows of the
mixture were dispersed with 51 dots on each of them. It can be seen
below in FIG. 8 that the CNT dots on the substrate were larger
(around 750 .mu.m to about 800 .mu.m), but smaller dots can be
achieved by adjusting the viscosity of the mixture and using
smaller nozzles. FIG. 9 shows the dispensing process. Methods of
making the nozzle and dispensing apparatus are well known to those
who practice the art and not discussed further here.
Multiple-nozzle heads may also be used to improve speed and
throughput of the machine.
[0059] Field emission of the sample (having 51.times.6 dots on it)
was tested. The I-V curve is shown in FIG. 10. An electric field as
low as 3 V/.mu.m was achieved at an emission current of 40 mA. FIG.
11 shows a field emission image of the sample, where a phosphor
covered anode is positioned in proximity to the sample cathode, and
an electric field is applied to induce field emission. FIG. 11
shows a digital image of the emitted light.
[0060] It can be seen that there is no edge emission and that
emission site density is excellent. Also note that no other
activation process was utilized to improve the field emission
properties.
[0061] 3) Field Emission from Screen-Printed CNT-Resbond
Mixture
[0062] Screen-printing is a well-known technology that has been
applied in various fields. This method was used to print the
mixture onto a selective area of the substrate through a mask.
FIGS. 12A-12C show a schematic diagram of the substrate coated with
CNT-Resbond mixture. The 3'.times.3' glass plate was screen-printed
with 10 .mu.m-thick Ag feedlines and then it was coated with a 50
.mu.m-thick black insulating layer on selective areas so it
contained in total 64 pixels. Every pixel had 3 sub-pixels. The
size of every sub-pixel was 1.times.6.6 mm.sup.2.
[0063] FIGS. 12A-12C show a schematic diagram of the steps used to
screen-print the device described above. The test results of this
type device are shown below in FIGS. 13, 14, and 15.
[0064] In FIG. 12A, Ag feedlines 1201 are screen-printed onto the
glass substrate 1202. In FIG. 12B, screen-printing of a 50
.mu.m-thick insulating overcoat 1203 is performed onto the Ag
feedline-printed glass substrate 1202. In the next step illustrated
in FIG. 12C, CNTs 1204 are screen-printed onto the Ag feedline
opening.
[0065] A sample was prepared using CNT-Resbond deposited by a
screen-printing process. A glass substrate was used with a 35
micron-thick black insulating overcoat (glass frit glaze) layer
printed on printed silver feedlines and the CNT material was
printed onto the pixels using a stencil mask (stainless steel
sheet, with no mesh in the openings). The CNT coating was about 50
microns to 70 microns. After printing and drying/curing, the sample
was then tested.
[0066] FIG. 13 illustrates an I-V curve of the sample with
CNT-Resbond deposited by printing process. FIG. 14 shows a digital
image of a field emission image of a set of pixels using the
CNT-Resbond composite.
[0067] The CNT-Resbond composite was also printed with very small
pixels (300.times.1700 .mu.m.sup.2) using a stencil mask. In the
screen-printing process, a TiW thin film-coated glass was employed
as the substrate. Printing was on top of the TiW thin film that was
thick enough to be highly conductive. The stencil mask was a
stainless steel sheet with Teflon coating on the surface. There
were 26.times.24 pixels on the substrate. Field emission of the
sample was tested. The I-V curve is shown in FIG. 15 and a field
emission image is shown in FIG. 16. There is no edge emission
observed in the image in FIG. 16. Emission site density is
reasonable.
[0068] FIG. 6 illustrates that during the drying/curing process,
the composite material may shrink and crack as a result of the
shrinking. The fibers along the crack are stretched or spooled out
of the fragments on either side of the crack. This aligns them in
the crack. In some cases the fibers are broken as a result of the
crack formation or they are pulled out of one fragment on one or
both sides of the crack.
[0069] (1) CNT-Resbond were coated onto the substrate
[0070] (2) Fragmentation process begins when the coating is curing
or drying
[0071] (3) Cracks were widened and CNTs were aligned
[0072] (4) CNTs were broken or extracted from one fragment if the
fragmentation process is dramatic.
[0073] In summary, CNT composites are made from CNT fibers, other
particles, carrier materials and possibly other materials for
binding. After the composite is dispensed or printed or placed onto
the substrate, the composite is dried or cured (carrier material is
taken out), resulting in fragmentation of the composite on the
substrate and a crack(s) is formed between the fragments. The CNT
fibers in the cracks are stretched or spooled from the fragments to
align them in the crack region. Some fibers are broken or pulled
out of the fragments to create dangling fibers that may be ideal
field emitter structures in many cases.
* * * * *