U.S. patent application number 10/377137 was filed with the patent office on 2005-03-31 for ceramic materials reinforced with single-wall carbon nanotubes as electrical conductors.
This patent application is currently assigned to THE REGENTS OF THE UNIVERSITY OF CALIFORNIA, a California corporation. Invention is credited to Kuntz, Joshua D., Mukherjee, Amiya K., Zhan, Guodong.
Application Number | 20050067607 10/377137 |
Document ID | / |
Family ID | 32926328 |
Filed Date | 2005-03-31 |
United States Patent
Application |
20050067607 |
Kind Code |
A1 |
Zhan, Guodong ; et
al. |
March 31, 2005 |
CERAMIC MATERIALS REINFORCED WITH SINGLE-WALL CARBON NANOTUBES AS
ELECTRICAL CONDUCTORS
Abstract
Composite materials formed of a matrix of fused ceramic grains
with single-wall carbon nanotubes dispersed throughout the matrix
and a high relative density, notably that achieved by electric
field-assisted sintering, demonstrate unusually high electrical
conductivity in combination with high-performance mechanical
properties including high fracture toughness. This combination of
electrical and mechanical properties makes these composites useful
as electrical conductors in applications where high-performance
materials are needed due to exposure to extreme conditions such as
high temperatures and mechanical stresses.
Inventors: |
Zhan, Guodong; (Davis,
CA) ; Kuntz, Joshua D.; (Lafayette, CA) ;
Mukherjee, Amiya K.; (Davis, CA) |
Correspondence
Address: |
TOWNSEND AND TOWNSEND AND CREW, LLP
TWO EMBARCADERO CENTER
EIGHTH FLOOR
SAN FRANCISCO
CA
94111-3834
US
|
Assignee: |
THE REGENTS OF THE UNIVERSITY OF
CALIFORNIA, a California corporation
Oakland
CA
|
Family ID: |
32926328 |
Appl. No.: |
10/377137 |
Filed: |
February 26, 2003 |
Current U.S.
Class: |
252/502 |
Current CPC
Class: |
C04B 2235/3217 20130101;
Y10S 977/776 20130101; C04B 35/6261 20130101; C04B 35/64 20130101;
C04B 2235/77 20130101; C04B 2235/322 20130101; B82Y 30/00 20130101;
C04B 2235/785 20130101; C04B 35/80 20130101; C04B 2235/5454
20130101; Y10S 977/75 20130101; C04B 2235/96 20130101; B82Y 10/00
20130101; C04B 2235/666 20130101; C04B 2235/3229 20130101; A61F
2002/30968 20130101; C22C 2026/002 20130101; C04B 35/443 20130101;
C04B 2235/6581 20130101; C04B 35/645 20130101; C22C 26/00 20130101;
C04B 35/50 20130101; C04B 35/46 20130101; C04B 35/488 20130101;
C04B 2235/5445 20130101; C04B 2235/5288 20130101; C04B 35/053
20130101; C04B 35/62615 20130101; C04B 35/119 20130101; C04B
2235/549 20130101 |
Class at
Publication: |
252/502 |
International
Class: |
H01B 001/00 |
Goverment Interests
[0001] This invention was made with Government under Contract No.
G-DAAD 19-00-1-0185, awarded by the United States Army Research
Office. The Government has certain rights in this invention.
Claims
1. In an application requiring the conduction of an electric
current as the result of a voltage applied between two terminals,
the improvement comprising interposing between said terminals a
composite material comprised of metal oxide and single-wall carbon
nanotubes, said composite material having a density of at least 95%
relative to a volume-averaged theoretical density, said composite
material being the product of a process comprising consolidating a
mixture of ceramic particles of less than 500 nm in diameter and
single-wall carbon nanotubes into a continuous mass by compressing
said mixture at a pressure of from about 10 MPa to about 200 MPa
and a temperature of from about 800.degree. C. to about
1,500.degree. C. while passing a sintering pulsed direct electric
current of from about 250 A/cm.sup.2 to about 10,000 A/cm.sup.2
through said mixture.
2. The improvement of claim 1 in which said density is at least 98%
relative to said volume-averaged theoretical density.
3. The improvement of claim 1 in which said density is at least 99%
relative to said volume-averaged theoretical density.
4. The improvement of claim 1 in which said metal oxide is a member
selected from the group consisting of alumina, magnesium oxide,
magnesia spinel, titania, cerium oxide, and zirconia.
5. The improvement of claim 1 in which said metal oxide is
alumina.
6. The improvement of claim 1 in which said single-wall carbon
nanotubes constitute from about 1% to about 50% of said
composite.
7. The improvement of claim 1 in which said single-wall carbon
nanotubes constitute from about 5% to about 25% of said
composite.
8. The improvement of claim 1 in which said single-wall carbon
nanotubes constitute from about 5% to about 20% of said
composite.
9. The improvement of claim 1 in which said composite material is
the product of a process comprising consolidating a mixture of
ceramic particles of less than 500 nm in diameter and single-wall
carbon nanotubes into a continuous mass by compressing said mixture
while passing a sintering electric current through said
mixture.
10. The improvement of claim 9 in which said process comprises
compressing said mixture at a pressure of from about 10 MPa to
about 200 MPa and a temperature of from about 800.degree. C. to
about 1,500.degree. C., and said sintering electric current is a
pulsed direct current of from about 250 A/cm.sup.2 to about 10,000
A/cm.sup.2.
11. The improvement of claim 1 in which said process comprises
compressing said mixture at a pressure of from about 40 MPa to
about 100 MPa and a temperature of from about 900.degree. C. to
about 1,400.degree. C., and said sintering electric current is a
pulsed direct current of from about 500 A/cm.sup.2 to about 5,000
A/cm.sup.2.
12. In an application requiring the conduction of an electric
current as the result of a voltage applied between two terminals,
the improvement comprising interposing between said terminals a
composite material having a density of at least 95% relative to a
volume-averaged theoretical density, said composite material being
the product of a process comprising consolidating a mixture of
alumina particles of less than 500 nm in diameter and single-wall
carbon nanotubes into a continuous mass by compressing said mixture
while passing a sintering electric current through said mixture,
and in which said single-wall carbon nanotubes constitute from
about 5% to about 25% of said composite.
13. The improvement of claim 12 in which said density is at least
98% relative to said volume-averaged theoretical density.
14. The improvement of claim 12 in which said density is at least
99% relative to said volume-averaged theoretical density.
15. The improvement of claim 12 in which said single-wall carbon
nanotubes constitute from about 5% to about 20% of said composite.
Description
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention resides in the field of electrically
conductive ceramics, and incorporates technologies relating to
nanocrystalline materials, carbon nanotubes, and sintering methods
for densification and property enhancement of materials.
[0004] 2. Description of the Prior Art
[0005] The ability of ceramics to withstand extreme conditions of
temperature, mechanical stress, and chemical exposure without
failure or with a very low failure rate has led to the use of
ceramics in applications that require high-performance materials,
such as heat engines, cutting tools, wear and friction surfaces,
and space vehicles. In recent years, the use of ceramics has
extended into the fields of microtechnology and nanotechnology,
since the increasing demands of nano-scale electronics and
microelectromechanical systems (MEMS) for example have prompted
researchers to investigate the use of ceramics in these areas as
well.
[0006] An unfortunate characteristic of nanocrystalline ceramics is
brittleness. To reduce the brittleness, composites have been
developed in which secondary materials are dispersed throughout the
ceramic matrix. In some of the more recent developments, carbon
nanotubes, specifically multi-wall carbon nanotubes, have been used
as the secondary material. A description of "ceramic matrix
nanocomposites containing carbon nanotubes" is found in Chang, S.,
et al. (Rensselaer Polytechnic Institute), U.S. Pat. No. 6,420,293
B1, issued Jul. 16, 2002 on an application filed on Aug. 25, 2000.
While the description encompasses both single-wall and multi-wall
carbon nanotubes, the only carbon nanotubes for which test data is
presented in the patent are multi-wall carbon nanotubes. To form
the composites, the starting powders in the patent are sintered
into a dense continuous mass by hot isostatic pressing. Single-wall
carbon nanotubes, although not investigated to the extent of
multi-wall carbon nanotubes for this purpose, are known to have
both high stiffness (a Young's modulus of 1,400 GPa) and high
strength (a tensile strength well above 100 GPa).
[0007] In addition to applications where their mechanical
properties are needed, ceramics are of increasing interest in
electronics since various kinds of electrical devices are being
designed for use in environments that require a combination of high
temperature resistance, toughness, and chemical inertness. In the
microelectronics industry, for example, materials with the
qualities demonstrated by ceramics are sought for use as silicon
substitutes, as trays and wafer carriers, as ruggedized microchip
substrates, and as components with electrostatic discharge
protection. In the microwave industry, the high-temperature
environments that are frequently encountered require
high-performance materials that can shield components from, or
absorb, electromagnetic interference. In the automotive industry,
high-temperature, high-strength, chemically inert materials that
conduct electricity are needed for components such as fuel injector
assemblies. The need for these qualities extends to medicine as
well, where a wide variety of medical devices, such as implants,
prostheses, and surgical devices, would benefit from a combination
of electrical functionality, high strength and chemical inertness.
In electrical power supplies such as batteries and solid oxide fuel
cells, electrodes that possess these properties are needed. The
need also exists in analytical and testing devices for materials
used as chemical sensors, gas separation materials, and materials
for hydrogen absorption. And in the aerospace and defense
industries, materials with these properties are needed for aircraft
and aircraft engines as well as for thermal management materials in
human spaceflight applications.
[0008] Ceramics are electrically insulating materials. To make them
electrically conductive, ceramics have been formulated as
composites with electrically conductive fillers. Carbon nanotubes
have been investigated as conductive fillers since carbon nanotubes
are known to possess both high electrical conductivity and high
thermal conductivity. Studies of the electrical characteristics of
ceramic composites that contain carbon nanotubes, notably
composites of alumina, iron and carbon nanotubes, composites of
magnesium oxide, cobalt and carbon nanotubes, and composites of
MgAl.sub.2O.sub.4, iron, cobalt, and carbon nanotubes have been
reported by Flahaut, E., et al., "Carbon Nanotubes-Metal-Oxide
Nanocomposites: Microstructure, Electrical Conductivity, and
Mechanical Properties," Acta Mater. 48: 3803-3812 (2000); Laurent,
Ch., et al., "Carbon Nanotubes-Fe-Alumina Nanocomposites. Part II:
Microstructure and Mechanical Properties of the Hot-Pressed
Composites," J. Euro. Ceram. Soc. 18: 2005-2013 (1998); Peigney,
A., et al., "Carbon Nanotubes-Fe-Alumina Nanocomposites. Part I:
Influence of the Fe Content on the Synthesis of Powders," J. Euro.
Ceram. Soc. 18: 1995-2004 (1998); Peigney, A., et al., "Carbon
Nanotubes in Novel Ceramic Matrix Nanocomposites," Ceram. Inter.
26: 677-683 (2000); Peigney, A., et al., "Carbon Nanotubes Grown
in-situ by a Novel Catalytic Method," J. Mater. Res. 12: 613-615
(1997); Peigney, A., et al., "Aligned carbon nanotubes in
ceramic-matrix nanocomposites prepared by high-temperature
extrusion," Chem. Phys. Lett. 352: 20-25 (2002). The nanocomposites
in these reports were produced by hot pressing nano-sized powders.
The composites were electrically conductive to a moderate degree
with an electrical conductivity within the range of 0.2-4.0 S/cm.
The fracture strengths and fracture toughnesses of the composites
were generally lower however than those of the metal-ceramic
composites (lacking carbon) and only marginally higher than those
of the pure ceramics. The Peigney et al. 2002 paper (Chem. Phys.
Lett. 352: 20-25 (2002)) also reports that the carbon nanotubes in
the composites can be aligned in the bulk ceramic matrix of the
composite by a high-temperature extrusion technique to produce
materials that show an anisotropy of electrical conductivity. The
best conductivity however was only obtainable in the center of the
extrusion since the carbon nanotubes in other parts of the
composite had been damaged during the extrusion.
[0009] Of further relevance to this invention is the literature on
electric field-assisted sintering, which is also known as spark
plasma sintering, plasma-activated sintering, and field-assisted
sintering technique. Electric field-assisted sintering is disclosed
in the literature for use on metals and ceramics, for consolidating
polymers, for joining metals, for crystal growth, and for promoting
chemical reactions. The densification of alumina powder by electric
field-assisted sintering is disclosed by Wang, S. W., et al., J.
Mater. Res. 15(4) (April 2000): 982-987.
SUMMARY OF THE INVENTION
[0010] It has now been discovered that a composite material formed
of a matrix of fused ceramic grains with single-wall carbon
nanotubes dispersed throughout the matrix and a high relative
density demonstrates unusually high electrical conductivity in
combination with favorable mechanical properties including a high
fracture toughness. The density is within the range of density that
is achievable by electric field-assisted sintering, which is a
preferred method of densification. This invention thus resides in
electrical devices, electrical systems, and electrical applications
in general in which a composite of this description is interposed
between two terminals between which a voltage has been applied to
provide an electrical conduction path and to thereby serve as an
electrical conducting medium between the terminals. The electrical
conductivity of the composite far exceeds the electrical
conductivities of composites of similar composition but with lower
relative densities that are typically achieved by means other than
electric field-assisted sintering. The electrical conductivity of
the composite also far exceeds that of composites with lower
relative densities that contain conductive metals dispersed
throughout the ceramic matrix in addition to the carbon nanotubes.
The invention thus finds application in each of the various
electrical systems and applications listed above.
[0011] In the preferred embodiments of the invention that achieve
densification by electric field-assisted sintering, the invention
offers the further advantage of a reduction in processing time due
to the speed with which electric field-assisted sintering can be
performed relative to other sintering methods.
[0012] These and other features, advantages and objects of this
invention will be apparent from the description that follows. All
literature references cited in this specification are incorporated
herein by reference for their descriptions of the subject matter
that is addressed in the contexts in which the citations are
made.
BRIEF DESCRIPTION OF THE FIGURE
[0013] FIG. 1 is a plot of the electrical conductivity of certain
composites of alumina and carbon nanotubes vs. temperature, listing
test results generated by the inventors herein together with test
results reported in the literature.
DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS
[0014] The ceramic materials that form the major component of the
composites of this invention include any known ceramics, although
preferred ceramics for use in this invention are metal oxides.
Examples of metal oxide ceramics are alumina, magnesium oxide,
titania, cerium oxide, yttria, and zirconia. Further examples are
combinations of two or more of these metal oxides, and combinations
that include other oxides such as silica and other metal and
non-metal oxides. Still further examples are mixed metallic oxides
such as SiAlON, AlON, spinels such as magnesia spinel, and calcium
aluminate. A metal oxide that is currently of particular interest
is alumina, either .alpha.-alumina, .gamma.-alumina, or a mixture
of both.
[0015] Carbon nanotubes are polymers of pure carbon. Both
single-wall and multi-wall carbon nanotubes are known in the art
and the subject of a considerable body of published literature.
Examples of literature on the subject are Dresselhaus, M. S., et
al., Science of Fullerenes and Carbon Nanotubes, Academic Press,
San Diego (1996), and Ajayan, P. M., et al., "Nanometre-Size Tubes
of Carbon," Rep. Prog. Phys. 60 (1997): 1025-1062. The structure of
a single-wall carbon nanotube can be described as a single graphene
sheet rolled into a seamless cylinder whose ends are either open or
closed. When closed, the ends are capped either by half fullerenes
or by more complex structures such as pentagonal lattices. The
average diameter of a single-wall carbon nanotube is within the
range of 0.5 to 100 nm, and more typically, 0.5 to 10 nm, 0.5 to 5
nm, or 0.7 to 2 nm. The aspect ratio, i.e., length to diameter, can
range from about 25 to about 1,000,000, and preferably from about
100 to about 1,000. Thus, a nanotube of 1 nm diameter may have a
preferred length of from about 100 to about 1,000 nm. (All ranges
stated herein are approximate.) Nanotubes frequently exist as
"ropes," which are bundles of 10 to 500 single-wall nanotubes held
together along their lengths by van der Waals forces. Individual
nanotubes often branch off from a rope to join nanotubes of other
ropes. Multi-walled carbon nanotubes are two or more concentric
cylinders of graphene sheets. The cylinders are of successively
larger diameter to fit one inside another, forming a layered
composite tube bonded together by van der Waals forces, the
distance between layers typically being approximately 0.34 nm as
reported by Peigney, A., et al., "Carbon nanotubes in novel ceramic
matrix nanocomposites," Ceram. Inter. 26 (2000) 677-683.
[0016] Carbon nanotubes are commonly prepared by arc discharge
between carbon electrodes in an inert gas atmosphere. The product
is generally a mixture of single-wall and multi-wall nanotubes,
although the formation of single-wall nanotubes can be favored by
the use of transition metal catalysts such as iron or cobalt.
Single-wall nanotubes can also be prepared by laser ablation, as
disclosed by Thess, A., et al., "Crystalline Ropes of Metallic
Carbon Nanotubes," Science 273 (1996): 483-487, and by Witanachi,
S., et al., "Role of Temporal Delay in Dual-Laser Ablated Plumes,"
J. Vac. Sci. Technol A 3 (1995): 1171-1174. A further method of
producing single-wall nanotubes is the HiPco process disclosed by
Nikolaev, P., et al., "Gas-phase catalytic growth of single-walled
carbon nanotubes from carbon monoxide," Chem. Phys. Lett. 313,
91-97 (1999), and by Bronikowski M. J., et al., "Gas-phase
production of carbon single-walled nanotubes from carbon monoxide
via the HiPco process: A parametric study," J. Vac. Sci. Technol.
19, 1800-1805 (2001).
[0017] The composites of this invention can further include
conductive metals such as iron, aluminum or copper, dispersed
throughout the composite to further enhance the electrical
conductivity of the composite. In certain preferred embodiments of
the invention, however, such metals are not included, and the
composite consists entirely of a ceramic matrix with carbon
nanotubes dispersed throughout the matrix as the sole non-ceramic
component.
[0018] The starting materials for the composites of this invention
are preferably powder mixtures of the ceramic material and the
single-wall carbon nanotubes. The composites of this invention may
contain multi-wall carbon nanotubes in addition to the single-wall
carbon nanotubes. It is preferred however that the mixtures, and
the final product as well, be free of multi-wall carbon nanotubes,
or if multi-wall carbon nanotubes are present, that their amounts
relative to the amount of single-wall nanotubes be so small that
the presence of the double-wall or multi-wall nanotubes does not
obliterate or significantly reduce the beneficial properties
attributable to the single-wall nanotubes.
[0019] The relative amounts of ceramic material and single-wall
carbon nanotubes can vary, although the mechanical properties and
possibly the performance characteristics may vary with the
proportions of the single-wall carbon nanotubes. In most cases,
best results will be achieved with composites in which the
single-wall carbon nanotubes constitute from about 1% to about 50%,
preferably from about 5% to about 25%, and most preferably from
about 5% to about 20%, by volume of the composite. The volumes used
in determining the volume percents referred to herein are
calculated from the weight percents of the bulk starting materials
and the theoretical density of each component.
[0020] The ceramic material used as a starting material is
preferably in the form of particles in the micro- or nano-size
range, particularly particles that are less than 500 nm in diameter
on the average. The carbon nanotubes can be dispersed through the
ceramic powder by conventional means to form a homogeneously
dispersed powder mixture, although a preferred method is one in
which the materials are mixed by being suspended together in a
common liquid suspending medium. Any readily removable, low
viscosity, inert suspending liquid, such as a low molecular weight
alcohol (ethanol, for example), can be used. Carbon nanotubes are
available from commercial suppliers in a paper-like form, and can
be dispersed in ethanol and other liquid suspending agents with the
assistance of ultrasound.
[0021] Once the ceramic powder and carbon nanotubes are combined,
the powders are preferably mixed prior to electric field-assisted
sintering. Mixing can be done on the powders alone or on a
suspension of the powders. Mechanical mixing can be performed by
ball-milling in conventional rotary mills that mix the powder
mixture with the assistance of tumbling balls. The sizes of the
balls, the number of balls used per unit volume of powder, the
rotation speed of the mill, the temperature at which the milling is
performed, and the length of time that milling is continued can all
vary widely. Best results will generally be achieved with a milling
time ranging from about 4 hours to about 50 hours. The degree of
mixing may also be affected by the "charge ratio," which is the
ratio of the mass of the balls to the mass of the powder. A charge
ratio of from about 5 to about 20 will generally provide proper
mixing.
[0022] The consolidation of the product can be enhanced by
mechanical activation of the ceramic particles prior to
consolidating them into the composite. Mechanical activation can be
achieved by high-energy ball milling utilizing ball mills as
described in the preceding paragraph but at an increased intensity
and for an extended period of time. Included among the beneficial
effects of high-energy ball milling are a reduction in particle
size.
[0023] The composite materials can be consolidated into a
continuous mass by conventional means of compression. The benefits
of the invention will be most evident when the composite is
densified to a high density, i.e., one that approaches full
theoretical density, which is the density of the material as
determined by volume-averaging the densities of each of its
components. A density of at least 95% of the theoretical density is
sought, preferably at least 98%, and most preferably at least 99%.
The term "relative density" is used herein to denote the actual
density expressed as a percent of the theoretical density.
[0024] The preferred method of densification is electric
field-assisted sintering. One method of performing this type of
sintering is by passing a pulsewise DC electric current through the
dry powder mixture or through a consolidated mass of the mixture
while applying pressure. A description of electric field-assisted
sintering and of apparatus in which this process can be performed
is presented by Wang, S. W., et al., "Densification of
Al.sub.2O.sub.3 powder using spark plasma sintering," J. Mater.
Res. 15(4), 982-987 (2000). While the conditions may vary, best
results will generally be obtained with a densification pressure
exceeding 10 MPa, preferably from about 10 MPa to about 200 MPa,
and most preferably from about 40 MPa to about 100 MPa. The
preferred current is a pulsed DC current of from about 250
A/cm.sup.2 to about 10,000 A/cm.sup.2, most preferably from about
500 A/cm.sup.2 to about 1,500 A/cm.sup.2 The duration of the pulsed
current will generally range from about 1 minute to about 30
minutes, and preferably from about 1.5 minutes to about 5 minutes.
Preferred temperatures are within the range of from about
800.degree. C. to about 1,500.degree. C., and most preferably from
about 900.degree. C. to about 1,400.degree. C. Densification is
typically performed by uniaxial compression under vacuum, and
preferred vacuum levels for the densification are below 10 Torr,
and most preferably below 1 Torr.
[0025] The prefix "nano-" as used herein generally refers to
dimensions that are less than 100 nm. The ceramic powders used as
starting materials in the practice of this invention are preferably
in the nano-size range although powders with particle sizes above
100 nm can be used as well. In addition, the particles in many
cases undergo grain growth during sintering. The resulting
composites may therefore have grain sizes that exceed the nano-size
range by several hundred nanometers.
[0026] The composites of this invention are useful as conducting
media in any application requiring an electrical conduction path in
a material that is capable of withstanding extreme conditions of
temperature, mechanical stress, or both. The path can assume the
form of a coating on an electrically insulating substrate, a lead
joining components of an electrical circuit or system of circuits,
a wire, a conductive line on printed circuit boards, and any other
circuitry application in high-performance applications. The range
of possibilities will be readily apparent to those skilled in the
art.
[0027] The following examples are offered for purposes of
illustration and are not intended to limit the scope of the
invention.
EXAMPLES
[0028] Materials, Equipment, and Experimental Procedures
[0029] Purified single-wall carbon nanotubes produced by the HiPco
process with more than 90% of the catalyst removed were obtained
from Carbon Nanotechnologies Incorporated (Houston, Tex., USA). The
nanotubes were in paper form, and once obtained were dispersed in
ethanol with the assistance of ultrasound. The ceramic material was
nanocrystalline alumina powder. A mixture consisting of 80%
.alpha.-Al.sub.2O.sub.3 and 20% .gamma.-Al.sub.2O.sub.3 with
particle sizes of 300 nm (40 nm crystallite size) and 20 nm,
respectively, was obtained from Baikowski International Corporation
(Charlotte, N.C., USA). In addition, .gamma.-Al.sub.2O.sub.3 with
average particle sizes of 15 nm and 32 nm and synthesized by gas
condensation was obtained from Nanophase Technologies Corporation
(Darien, Ill., USA).
[0030] For certain experiments, the alumina or alumina mixture was
first mechanically activated by high-energy ball milling prior to
being combined with the carbon nanotubes. The high-energy ball
milling was performed on a Spex 8000 mixer mill (Spex Industries,
Metuchen, N.J., USA) in a zirconia vial with 1 weight percent
polyvinyl alcohol, a dry milling agent, to prevent severe powder
agglomeration. Milling was performed at room temperature for 24
hours, followed by vacuum heat treatment at 350.degree. C. for 3
hours to remove the polyvinyl alcohol. Thus milled, the alumina was
mixed with the single-wall carbon nanotube dispersion, and the
combined dispersion was sieved using a 200-mesh sieve, then
ball-milled for 24 hours (still in ethanol) using zirconia milling
media, then dried to form a dry powder mixture.
[0031] All dry powder samples, including those containing alumina
mechanically activated by high-energy ball milling and those
containing alumina that had not been activated, were placed on a
graphite die of inner diameter 20 mm and cold-pressed at 200 MPa.
The cold-pressed powder mixture was then sintered on a Dr. Sinter
1050 Spark Plasma Sintering System (Sumitomo Coal Mining Company,
Japan) under vacuum. Electric field-assisted sintering was then
performed at an applied pressure of 80 MPa with a pulsed DC current
of 5,000 A maximum and a maximum voltage of 10 V. The pulses were
12 ms in duration separated by intervals of 2 ms. Once the pressure
was applied, the samples were heated to 600.degree. C. in 2 minutes
and then heated further at rates of 550.degree. C./min to
600.degree. C./min to 1,150-1200.degree. C. where they were held
for 3 minutes. The temperature was monitored with an optical
pyrometer focused on a depression in the graphite die measuring 2
mm in diameter and 5 mm in depth.
[0032] The final densities of the sintered compacts were measured
by the Archimedes method using deionized water as the immersion
medium. The density of the single-wall carbon nanotubes used as a
starting material was 1.8 g/cm.sup.3. Microstructure determinations
of the sintered compacts were performed with an FEI XL30-SFEG
high-resolution scanning electron microscope with a resolution
better than 2 nm. Grain sizes were estimated by high-resolution
scanning electron microscopy of fracture surfaces and x-ray
diffraction profiles. Additional characterization by analytical
electron microscopy and high-resolution transmission electron
microscopy was performed on a Philips CM-200 with a field emission
gun operating at 200 kV. Indentation tests were performed on a
Wilson Tukon hardness tester with a diamond Vickers indenter. Bulk
specimens were sectioned and mounted in epoxy, then polished
through 0.25-micron diamond paste. The indentation parameters for
fracture toughness (K.sub.IC) were a 2.5 kg load with a dwell time
of 15 seconds. The fracture toughness was calculated by the Anstis
equation as disclosed by Anstis, G. R., et al., "A Critical
Evaluation of Indentation Techniques for Measuring Fracture
Toughness: I, Direct Crack Measurement," J. Am. Ceram. Soc. 64(9):
533-538 (1981). At least ten measurements were performed for each
sample.
[0033] An Agilent 34420A nanoVolt/microOhm meter (Agilent
Technologies, Palo Alto, Calif., USA) was used for conductivity
measurement using a four-wire probe technique. To remove the effect
of extraneous voltages such as those arising due to thermal EMF
caused by dissimilar materials in the circuit, two measurements
were made for every reading: one with the current on and the other
with the current off. Using this configuration the meter has a
resolution of 0.1 .OMEGA.. The four-point probe electrical
conductivity (.sigma.) of the dense materials was measured at four
different temperatures: -196.degree. C., -61.degree. C., 25.degree.
C., and 77.degree. C.
[0034] A composite prepared from 15-nm .gamma.-alumina particles
that had been activated by high-energy ball milling and 15 volume
percent single-wall carbon nanotubes was prepared by the procedure
described above. The same procedure was used to prepare a composite
starting with 32-nm .gamma.-alumina particles that had not been
activated by high-energy ball milling and 10 volume percent
single-wall carbon nanotubes, and another composite starting with
an alumina mixture consisting of 80% 300-nm .alpha.-alumina and 20%
20-nm .gamma.-alumina, also without activation by high-energy ball
milling, and 5.7 volume percent single-wall carbon nanotubes. The
procedure was also performed on pure alumina (a mixture consisting
of 80% 300-nm .alpha.-alumina and 20% 20-nm .gamma.-alumina), also
without activation by high-energy ball milling. Since pure
.alpha.-alumina nanopowders can be consolidated to full density by
electric field-assisted sintering at 1,150.degree. C. for three
minutes, those specimens containing a:-alumina were sintered under
these conditions. By contrast, .gamma.-alumina requires a sintering
temperature of 1,200.degree. C. to achieve full density, and
accordingly the specimen containing .gamma.-alumina was sintered at
this higher temperature.
[0035] Results
[0036] The relative density, grain size, and specific conductivity
were determined on the sintered alumina and the sintered samples of
each of the three composites, and the results are listed in Table
1, together with data for nanocomposites and other materials
selected from the prior art as representative of the state of the
art preceding this invention. For each entry, the table lists the
composition, expressed in terms of the components of the composite
(where "SWCN" denotes single-wall carbon nanotubes. "MWCN" denotes
multi-wall carbon nanotubes,"and "CNT" denotes carbon nanotubes
that are unspecified as either single-wall or multi-wall),
including the type and grain size of the starting alumina (where
known), and the volume percent of each component; the processing
conditions including high-energy ball milling ("HEBM") when used
and the sintering method, expressed as either electric
field-assisted sintering ("SPS" as an abbreviation for "spark
plasma sintering"), hot pressing ("HP"--sintering in the absence of
an electric field), or high-temperature extrusion ("HTE"); the
grain size of the sintered product; the specific conductivity at
25.degree. C.; and the source of the data, i.e., either the
inventors herein or the literature citation (identified in the list
following the table). Analysis of the alumina-containing products
by x-ray diffraction after sintering revealed that the alumina in
all of these products was exclusively .alpha.-alumina. To obtain
the relative density, the theoretical density is first calculated
as the total of the densities of the components multiplied by their
volume percents, and the measured density is then expressed as a
percent of the theoretical density.
1TABLE I Compositions, Processing Conditions, Relative Densities,
Grain Size and Specific Conductivities (.sigma.) of Composites
Within the Scope of the Invention vs. Materials of the Prior Art
Composition - additive(s) and Relative Grain .sigma. volume %;
matrix Processing Density Size (S/cm at No. in parentheses
Conditions (%) (nm) 25.degree. C.) Source (1) 0% (pure
.alpha.-Al.sub.2O.sub.3) SPS/1150.degree. C./3 min 100 349
10.sup.-12 to 10.sup.-14 herein (2) SWCN: 5.7% SPS/1150.degree.
C./3 min 100 .about.200 10.50 herein (.alpha.-Al.sub.2O.sub.3) (3)
SWCN: 10% SPS/1200.degree. C./3 min 97.5 .about.100 15.11 herein
(.gamma.-Al.sub.2O.sub.3, 32 nm) (4) SWCN: 15% HEBM/SPS/ 99.2
.about.100 33.45 herein (.gamma.-Al.sub.2O.sub.3, 15 nm)
1150.degree. C./3 min (5) CNT: 8.5%; HP/1500.degree. C./15 min 88.7
.about.300 0.4-0.8 (i) Fe: 4.3% (.alpha.-Al.sub.2O.sub.3) (6) CNT:
10%; HP/1500.degree. C./15 min 87.5 .about.300 2.8-4.0 (i) Fe: 4.3%
(.alpha.-Al.sub.2O.sub.3) (7) Aligned CNT: 10%; HTE/1500.degree.
C./15 min 90 500 0.8-1.6 (ii) Fe: 4.3% (.alpha.-Al.sub.2O.sub.3)
(8) Aligned CNT: 9.8%; HTE/1500.degree. C./15 min 90 800 0.6-2.0
(ii) Fe/Co: 3.2% (MgAl.sub.2O.sub.4) (9) .beta."-Al.sub.2O.sub.3
ceramic -- -- -- .about.0.25 (iii) ion conductor (10) MWCN: 20% --
-- -- .about.0.3 (iv) (polymer) (11) 0% (aluminum) -- -- -- 294.12
(v) (12) CNT: 1 weight % HP/520.degree. C./30 min -- -- 204.08 (v)
(aluminum) (13) CNT: 4 weight % HP/520.degree. C./30 min -- --
151.52 (v) (aluminum) (14) CNT: 10 weight % HP/520.degree. C./30
min -- -- 181.82 (v) (aluminum)
Literature Sources:
[0037] (i) Flahaut, E., et al., "Carbon Nanotubes-Metal-Oxide
Nanocomposites: Microstructure, Electrical conductivity, and
Mechanical Properties," Acta Mater. 48: 3803-3812 (2000)
[0038] (ii) Peigney, A., et al., "Aligned carbon nanotubes in
ceramic-matrix nanocomposites prepared by high-temperature
extrusion," Chem. Phys. Lett. 352: 20-25 (2002)
[0039] (iii) Ceramic Innovations in the 20.sup.th Century,
Wachtman, J. B., Jr., ed., pp. 152-154, published by the American
Chemical Society (1999)
[0040] (iv) Yoshino, K., et al., "Electrical and optical properties
of conducting polymer-fullerene and conducting polymer-carbon
nanotube composites," Full. Sci. Technol. 7: 695-711 (1999)
[0041] (v) Xu, C. L., et al., "Fabrication of aluminum-carbon
nanotube composites and their electrical properties," Carbon 37:
855-858 (1999)
[0042] The specimens represented by rows (2) through (4) of Table I
are the only specimens among those in the Table that fall within
the scope of the present invention. The data show that alumina,
which is normally an electrical insulator with high electrical
resistance, becomes electrically conductive when formed into
composites that incorporate small amounts of single-wall carbon
nanotubes and densified by electric field-assisted sintering, and
that the electrical conductivity of such a composite increases with
the carbon nanotube content. Thus the conductivity at room
temperature of a composite containing 5.7 volume percent carbon
nanotubes that has been sintered by electric field-assisted
sintering is 10.50 S/cm (second row of data in the Table). At the
10% level, the composites of this invention (third row of data)
exhibit more than three times the conductivity of the 10% carbon
nanotube composite of the Flahaut et al. reference that has been
sintered by means other than electric field-assisted sintering
(sixth row of data), and by increasing the carbon nanotube level to
15% (fourth row of data) the result is more than eight times the
conductivity of the 10% composite of the Flahaut et al. reference.
This value (fourth row of data) is more than 100 times the value of
the .beta."-alumina ceramic ion conductor (eighth row of data).
This increase of conductivity with increasing carbon nanotube
content is contrary to reports in the literature, which teaches
that the conductivity decreases with carbon nanotube content rather
than increasing in metal-matrix composites (Xu et al. 1999, source
(v)).
[0043] The variation of electrical conductivity with temperature is
shown in FIG. 1 for the three composites of Table I that are within
the scope of the invention together with the values for other
composites from the Table. The symbols used in FIG. 1 are as
follows:
[0044] open squares: composite of the present invention with 15%
single-wall carbon nanotubes, alumina pre-activated by high-energy
ball milling, sintered by electric field-assisted sintering
[0045] open diamonds: composite of the present invention with 10%
single-wall carbon nanotubes without pre-activation of alumina,
sintered by electric field-assisted sintering
[0046] open circles: composite of the present invention with 5.7%
single-wall carbon nanotubes without pre-activation of alumina,
sintered by electric field-assisted sintering
[0047] filled triangle (pointing upward): data generated by Flahaut
et al. above, source (i), on composites containing 10% carbon
nanotubes of unspecified type and 4.3% iron and in which sintering
was performed by hot pressing at 1,500.degree. C. for 15 minutes in
the absence of an electric field
[0048] filled inverted triangle (pointing downward): data generated
by Yoshino et al. above, source (iv), on composites containing 20%
multi-wall carbon nanotubes in a polymer matrix
[0049] filled circle: alumina with 5.7% carbon black, generated by
the inventors herein for comparison
[0050] FIG. 1 demonstrates that the conductivity of the 15%
single-wall carbon nanotube composite sintered by electric
field-assisted sintering increases from 20.62 S/cm at -194.degree.
C. to 33.75 S/cm at 77.degree. C. When combined with the high
fracture resistance of these composites, these composites are
superior to those of other reported carbon nanotube-containing
ceramic composites where the carbon nanotubes produce either no
increase in toughness of the ceramic or a only a marginal increase,
the toughness limited by damage to the carbon nanotubes during
high-temperature sintering.
[0051] To demonstrate that the composites of this invention have
superior mechanical properties in addition their high electrical
conductivity, the fracture toughnesses of the materials represented
by the first two rows of Table I are shown in Table II.
2TABLE II Fracture Toughness of a Composite of the Present
Invention vs. Alumina Composition - additive(s) and Relative Grain
Fracture volume %; matrix Processing Density Size Toughness No. in
parentheses Conditions (%) (nm) (MPam.sup.1/2) Source (1) 0% (pure
.alpha.-Al.sub.2O.sub.3) SPS/1150.degree. C./3 min 100 349 3.3
herein (2) SWCN: 5.7% SPS/1150.degree. C./3 min 100 .about.200 7.9
herein (.alpha.-Al.sub.2O.sub.3)
[0052] Scanning electron microscopy, transmission electron
microscopy, and high-resolution transmission electron microscopy
indicated that the composites within the scope of the present
invention consisted of a network of alumina grains with ropes of
carbon nanotubes entangled with the grains. The result is a
conductive material that is lightweight and resistant to corrosion
and exhibits high strength and high toughness, rendering it unique
in its usefulness in creating electrical circuitry in a wide range
of applications.
[0053] The foregoing is offered primarily for purposes of
illustration and explanation. Further variations, modifications and
substitutions that, even though not disclosed herein, still fall
within the scope of the invention may readily occur to those
skilled in the art.
* * * * *