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.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority to the following:

Provisional Patent Application Ser. No. 60/502,464, entitled “SELF-ALIGNMENT OF CARBON NANOTUBES,” filed on Sep. 12, 2003.

TECHNICAL FIELD

The present invention relates in general to carbon nanotubes, and in particular, to a process for manufacturing a carbon nanotube composition.

BACKGROUND INFORMATION

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° 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:

• • 1. It is difficult to achieve high uniformity illumination required for display applications using a CVD process to grow CNTs over large areas.
  • 2. CVD growth of CNTs requires a high process temperature (over 500° C.), limiting the use of low-cost substrates such as sodalime glass.
  • 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.

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

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:

FIG. 1 shows a digital image of a microcrack and aligned CNT fibers;

FIG. 2 shows another digital image of a microcrack and aligned CNTs;

FIG. 3 shows another digital image of a microcrack and aligned CNTs;

FIG. 4 shows broken CNTs between two fragments;

FIG. 5 shows aligned CNTs;

FIG. 6 illustrates a schematic diagram of a CNT-Resbond coating before and after a shrinking process;

FIG. 7 illustrates field emission I-V curves of samples of the present invention;

FIG. 8 shows an optical image of CNT dots;

FIG. 9 illustrates a process for dispensing CNT composites in accordance with an embodiment of the present invention;

FIG. 10 illustrates an I-V curve of a sample made in accordance with embodiments of the present invention;

FIG. 11 shows a field emission image of the sample of FIG. 10;

FIGS. 12A-12C illustrate a process in accordance with embodiments of the present invention;

FIG. 13 illustrates an I-V curve of a composite made in accordance with embodiments of the present invention;

FIG. 14 shows a field emission image of a sample made in accordance with embodiments of the present invention;

FIG. 15 illustrates an I-V curve of a composite made in accordance with embodiments of the present invention; and

FIG. 16 shows a field emission image of a sample made in accordance with embodiments of the present invention.

DETAILED DESCRIPTION

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:

• • 1. Very easy and low cost process to align CNTs.
  • 2. The CNTs can be aligned on a very large area.
  • 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.

The section below describes one embodiment that may be used to make aligned CNTs.

1. Source of Materials

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 μm to 20 μm 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.

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 Al₂O₃ particles, water, and inorganic adhesives. Composites that contain other particles may also be used, such as SiO₂. These particles may be insulating, conducting or semiconducting. The particle sizes are less than 50 μm 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.

2. Preparation of the Mixture of Carbon Nanotubes with the Resbond and Deposition onto Substrate

1) Grinding of the Mixture

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 Al₂O₃ 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.

2) Applying the Mixture onto the Substrate and Curing

A brush was used to paint the mixture onto a conventional Si substrate (10-100 kΩ cm) with an area of 2×2 cm². 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 μm to 40 μm. 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° 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° 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.

3. Microstructure of the CNT-Resbond Coating

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 Al₂O₃ 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.

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.

FIG. 2 shows an SEM image of another microcrack within the sample and aligned CNTs between two fragments.

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.

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.

FIG. 5 shows perpendicularly aligned CNTs against the substrate.

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.

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.

4. Field Emission Test of the CNT-Resbond

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:

• • 10 wt. % CNTs+90 wt. % Resbond
  • 25 wt. % CNTs+75 wt. % Resbond
  • 40 wt. % CNTs+60 wt. % Resbond
  • 75 wt. % CNTs+25 wt. % Resbond

The above mixtures were prepared as described above and were brushed onto ITO/glass substrates with an area of 1 cm×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.

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 10⁻⁷ 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.

2) Demonstration of Applying the CNT Composite by Dispensing Process

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™) 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.

The diameter of the nozzle was 300 μm. 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 μm to about 800 μm), 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.

Field emission of the sample (having 51×6 dots on it) was tested. The I-V curve is shown in FIG. 10. An electric field as low as 3 V/μm 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.

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.

3) Field Emission from Screen-Printed CNT-Resbond Mixture

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′×3′ glass plate was screen-printed with 10 μm-thick Ag feedlines and then it was coated with a 50 μm-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 1×6.6 mm².

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.

In FIG. 12A, Ag feedlines 1201 are screen-printed onto the glass substrate 1202. In FIG. 12B, screen-printing of a 50 μm-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.

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.

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.

The CNT-Resbond composite was also printed with very small pixels (300×1700 μm²) 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 26×24 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.

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.

• (1) CNT-Resbond were coated onto the substrate
• (2) Fragmentation process begins when the coating is curing or drying
• (3) Cracks were widened and CNTs were aligned
• (4) CNTs were broken or extracted from one fragment if the fragmentation process is dramatic.

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.

Claims

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.