US20060062985A1

US20060062985A1 - Nanotube-containing composite bodies, and methods for making same

Info

Publication number: US20060062985A1
Application number: US11/090,259
Authority: US
Inventors: Prashant Karandikar
Original assignee: Individual
Current assignee: Individual
Priority date: 2004-04-26
Filing date: 2005-03-25
Publication date: 2006-03-23
Also published as: WO2006080936A1

Abstract

A composite material featuring comminuted or otherwise well dispersed and separated nanotubes reinforcing a matrix featuring metal, ceramic and/or polymer. In a preferred embodiment, the nanotubes feature elemental carbon, and the composites can be produced using a molten silicon metal infiltration technique, which may be pressurized or not, for example, a siliconizing or a reaction-bonding process. In this preferred embodiment, carbon nanotubes may be prevented from chemically reacting with the silicon infiltrant by an interfacial coating disposed between the carbon nanotubes and the infiltrant. A reaction-bonded composite body containing even a small percentage of carbon nanotubes possessed a significant increase in electrical conductivity as compared to a reaction-bonded composite not containing such nanotubes, reflecting the high electrical conductivity of the nanotubes. When the nanotubes are well dispersed throughout the preform, mechanical property enhancements start to become noticeable, such as fracture toughness enhancement.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-Part of Commonly Owned U.S. patent application Ser. No. 10/832,823, filed on Apr. 26, 2004. The entire contents of this commonly owned patent application are expressly incorporated herein by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with Government support under Contract No. FA9453-04-M-0255 awarded by the U.S. Air Force Research Laboratory. The Government has certain rights in the invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to metal and/or ceramic containing composite materials featuring a particular kind of fibrous reinforcement. In particular, the invention relates to composites having a matrix featuring metal, ceramic and/or polymer, and being reinforced with nanotubes, and preferably carbon nanotubes.
2. Discussion of Related Art
It has been known for a long time to add fibrous reinforcement to metals to increase mechanical properties such as specific strength and specific stiffness. One of the early such reinforcements was carbon or graphite fiber, produced from polymer precursors. The resulting composite material offered double or triple the strength or stiffness compared to the bulk, unreinforced metal. Processing was difficult, however, as the metals either tended not to wet the carbon fibers, or reacted with the carbon. Considerable energy has been devoted to developing ways to preserve the chemical and physical integrity of the fibers while rendering them more chemically compatible with the metal matrix.
Carbon fibers can be manufactured with high degrees of anisotropy. The graphite form of carbon in particular features a hexagonal crystallographic structure, with the covalent bonds within the {001} planes being strong, and the bonds between the {001} planes consisting of weak van der Waals bonds. It is possible to preferentially align the crystallographic planes in a graphite fiber such that the {001} planes tend to be parallel to the graphite fiber axis. By increasing the relative amount of covalent bonds in the fiber axis direction, a fiber possessing high strength and high elastic modulus in the direction of the fiber axis is produced. An interesting phenomenon that accompanies the alignment of the high strength, high modulus directions is that this particular direction also possesses a (rare) negative CTE. Thus, instead of expanding upon heating like most materials, these fibers actually shrink in the axial direction. In the radial direction of such fibers, however, the strength and elastic moduli are relatively low and the CTE is positive and relatively high. Because of the axial stiffness, the properties of the composites tend to be dominated by the axial properties.
When a reinforcement material having a negative CTE is incorporated into a composite material whose matrix component has a positive CTE, the individual CTE's tend to offset or cancel one another, yielding a composite or overall CTE somewhere between the two values. Because of this counterbalancing or offset effect, it is theoretically possible to engineer a metal matrix composite material, such as a metal-ceramic composite material, to have a net overall CTE of zero.
U.S. Pat. No. 3,807,996 to Sara teaches a carbon fiber reinforced nickel matrix composite material. Sara discloses the use of high strength, high modulus carbon fibers, as well as various geometrical arrangements of the fibers, such as arrays (plates) of parallel fibers and cross-plies (laminates) of such arrays.
In U.S. Pat. No. 4,791,076 Leggett et al. discloses a graphite fiber/silica matrix composite composition having a near-zero overall CTE. In addition to silica, the matrix contains boron phosphate and beta-spodumene, and Leggett states that the composite CTE is tailorable between −1 and +1 ppm/K by varying the matrix composition. As a consequence of the low CTE, very little thermal distortion occurred in for example, a laser mirror application, particularly at low coolant flow rates. This glass matrix composite material exhibited much less thermal distortion than did other laser mirror materials such as single crystal molybdenum or silicon. Although the cooling requirements were reduced, active cooling techniques involving heat transfer media flowing through channels in the mirror still were required.
As mentioned above, glass matrix composites have been used in environments where low expansion polymer composites would be insufficiently durable. Many of these applications, however, require high thermal conductivity, and most glasses are deficient in this area. Thus, composites workers have attempted to address the thermal conductivity problem by relying on the carbon fibers to carry this responsibility, the carbon fibers possessing relatively high thermal conductivity in the fiber axis direction. Another problem with glass matrix composites, though, is that they tend to be brittle. In many applications in which such composites are subjected to accelerations and stresses, such as with semiconductor fabrication equipment, it would be preferable to have a tougher, more impact resistant material.
Silicon carbide composites have been produced by reactive infiltration techniques for more than thirty-five years. In general, such a reactive infiltration process comprises contacting molten silicon with a porous mass containing silicon carbide plus carbon in a vacuum or an inert atmosphere environment. A wetting condition is created, with the result that the molten silicon is pulled by capillary action into the mass, where it reacts with the carbon to form additional silicon carbide. This in-situ silicon carbide typically is interconnected. A dense body usually is desired, so the process typically occurs in the presence of excess silicon. The resulting composite body thus comprises silicon carbide and unreacted silicon (which typically also is interconnected), and may be referred to in shorthand notation as Si/SiC or RBSC (denoting “reaction-bonded silicon carbide”).
In one of the earliest demonstrations of this technology, Heyroth (U.S. Pat. No. 2,431,326) subjected a carbon body, in which at least a substantial portion of the carbon formed a continuous skeletal structure, to the action of elemental silicon at a temperature well above the melting point of silicon. The silicon rapidly infiltrated the whole carbon body. Furthermore, it reacted with the carbon to form a body featuring a continuous, reticular, skeletal body of crystalline silicon carbide, with the interstices of the silicon carbide skeletal structure substantially filled with silicon-rich material.
In another early demonstration of this technology, Popper (U.S. Pat. No. 3,275,722) produced a self-bonded silicon carbide body by infiltrating silicon into a porous mass of silicon carbide particulates and powdered graphite in vacuo at a temperature in the range of 1800 to 2300° C.
More recently, it has become known to alloy the infiltrant metal used to make a reaction-formed silicon carbide body so that the metal phase of the formed body includes a constituent other than silicon. For example, commonly owned U.S. Pat. No. 6,503,572 teaches that the infiltrant may comprise an alloy of silicon and aluminum to yield a phase in the formed silicon carbide body comprising metallic aluminum or aluminum plus silicon. Such bodies containing an alloy infiltrant phase advantageously permit certain properties of the body to be tailored to meet specific needs. For example, by replacing some of the residual silicon infiltrant with aluminum, the thermal conductivity and the fracture toughness of the composite body each may be increased.
Over the years, there has been a significant amount of work directed to reinforcing SiC composites, e.g., reaction-bonded SiC composites, with carbon fibers. See, for example, U.S. Pat. Nos. 4,118,894; 4,944,904 and 6,248,269. Because the matrices of these composite materials consist of low CTE substances, i.e., SiC, typically interconnected, and typically also some residual, unreacted Si, also typically interconnected. More recently, work on carbon fiber reinforced Si/SiC composites has been undertaken with an eye toward brake components, e.g., brake discs and brake pads, for vehicle applications. For example, U.S. Pat. No. 6,248,269 to Dietrich et al. discloses a reaction-bonded SiC composite suitable for braking applications, e.g., disk and pad, for motor vehicles, consisting of carbon fibers arranged isotropically and embedded in a matrix of 40-50 volume percent SiC and not more than 15 volume percent Si. U.S. Pat. No. 6,261,981 to Dietrich et al. discloses a process for making such composites whereby short carbon fibers, optionally coating with a sizing, are bundled together and the bundles infiltrated or at least coated on their exterior surfaces with a binder that is suitable for pyrolysis, drying this binder, mixing the binder impregnated fiber bundles with a first filler and a second binder, pressing the mixture to form a green body, pyrolyzing the body to produce a porous fiber-reinforced carbon body, and then infiltrating the body with molten silicon. Dietrich claims substantial retention of individual filaments within the fiber bundles, and the short fiber bundles are surrounded by a coating of carbon that has reacted partially or completely with the metallic matrix material.
U.S. Pat. No. 6,030,913 to Heine discloses a similar technology. Here, the long or short high-strength graphite fibers are impregnated or coated with synthetic resin to form a pre-preg, which is then carbonized. The carbonized pre-preg is then re-subjected to further resin infiltration, and recarbonization, followed by graphitization, which is then followed by comminution to yield a dry material, which is then mixed with a high carbon content binder and compression molded to form a green article. The green article is then carbonized once more, and then infiltrated with molten silicon. The formed composite body features a matrix substantially consisting of SiC, and being reinforce with short graphite fibers. The fibers are enclosed by at least two shells of graphitized carbon, the outermost shell being partially converted into SiC.
U.S. Pat. No. 4,944,904 to Singh et al. discloses a similar composite material system intended mostly for high temperature, aerospace applications, such as a turbine engine component, but also mentioning applications such as wear parts and acoustic parts. The matrix comprises at least 5 volume percent SiC but preferably at least 45 percent, and 1-30 volume percent Si but preferably 1-2 percent. The fibers may be carbon or SiC, but are not disclosed as being arranged isotropically or quasi-isotropically. The fibers are protected from attack by the molten silicon using boron nitride plus an overcoat of a silicon-wettable material such as carbon or metal carbides such as SiC. The BN also provides a debond layer so that the fibers can move relative to the matrix under mechanical loading, thereby providing a toughening aspect to the resulting composite body.
Nanomaterials such as carbon nanotubes, sometimes abbreviated as “CNTs”, are relatively new materials that have attracted the attention of materials investigators, in part due to the potential for achieving extremes of properties. A review of the carbon nanotube literature indicates or at least suggests elastic modulus approaching that of diamond, thermal conductivity being about double that of diamond, strength being one to two orders of magnitude larger than that of steel, and electric current carrying capacity being three orders of magnitude greater than that of copper. See, for example:

1. P. G. Collins, and P. Avouris, “Nanotubes for electronics,” Scientific American, 283 (6) 2000, pg. 62-69.
2. M M J Treacy, T. W. Ebbesen, and T. M. Gibson, “Exceptionally high Young's modulus observed for individual carbon nanotubes,” Nature 381 (1996) pg. 680-687.
3. E. W. Wong, P. E. Sheehan, C. M. Lieber, “nanobeam mechanics:elasticity, strength and toughness of nanorods and nanotubes,” Science 277 (1997) pg. 1971-1975.
4. R. S. Ruoff and D. C. Lorents, “Mechanical and thermal properties of carbon nanotubes,” Carbon 33 (7) 1995 pg. 925-930.
5. M. Dresselhaus, G. Dresselhaus, P. Elkund, and R. Saito, “Carbon Nanotubes,” Physics World January 1998 (http://physicsweb.org/article/world/11/1/9/1)

Table I compares some physical properties of carbon nanotubes to macroscopic carbon fibers. The carbon fibers and carbon nanotubes are considered fairly representative of their respective species.

TABLE 1


Comparison of axial properties of carbon fibers
and carbon nanotubes (CNTs)

Carbon Fibers

	T300	P120 Pitch
Property	PAN based	Based	Carbon Nanotubes

Diameter (μm)	7	10	0.05
Density (g/cc)	1.76	2.17	˜2.0
Elastic Modulus (GPa)	231	827	1000-1400
Ultimate Tensile	3.75	2.41	7-10
Strength (GPa)
Thermal Conductivity	8	640	>2000
(W/mK)
Coefficient of Thermal	−0.6	−1.45	−1 (isotropic)
Expansion, CTE
(ppm/K)
Electrical Resistivity	18	2.2	<0.1
(micro-ohm-m)

T300 and P120 are BP Amoco carbon fibers.
Carbon Nanotubes are from Iljin Nanotech Co. Ltd., Seoul, S. Korea.

In addition to the obvious differences, it should be pointed out that while carbon fibers have low CTE in axial directions, they have high CTE in radial directions (12 ppm/K). CNTs, on the other hand, have low CTE in both axial and radial directions. Moreover, CNTs also have much higher strain capability and as a result, they should have higher toughness than carbon fibers.

Carbon nanotubes have the graphitic structure, e.g., each carbon atom having three nearest neighbors, and can be thought of as a graphite sheet rolled up but with the ends offset to produce a helicity or chirality.
Workers in the field of composite materials have attempted to incorporate carbon nanotubes into other materials, e.g., making composites featuring such carbon nanotubes, to make useful products having novel properties. Chang, et al, in U.S. Pat. No. 6,420,293, for example, describes the use of carbon nanotubes for reinforcing a ceramic matrix composite wherein the matrix consists of a nanocrystalline ceramic material such as a metal oxide, metal carbide, nitride, oxycarbide, oxynitride, carbonitride or oxycarbonitride, carbonate or phosphate, or a mixture of these. Curiously, when the nanotube is a carbon nanotube, the matrix cannot be silicon carbide. Chang et al. envision bearings or at least bearing surfaces, wear surfaces, cutting tools and load-bearing structural articles such as prosthetic devices, made from this nanocomposite material.
Ma et al formed a composite of 10 wt % carbon nanotubes in a silicon carbide matrix via a hot pressing route, and achieved a ten percent increase in strength and fracture toughness versus monolithic SiC ceramics.
Chen et al. produced a carbon nanotube reinforced metal matrix composite by an electroplating route.
So far, the work on composite materials featuring carbon nanotube reinforcements is still relatively scant. The above work notwithstanding, most of the reported composite work has involved polymer matrix composites. There, workers noted that one of the problems associated with the carbon nanotubes was in properly dispersing them in the polymer matrix. The production of a composite material containing both carbon nanotubes and metal, particularly molten metal, is even more of a challenge. While a coating on a typical carbon fiber may be adequately protective against chemical reaction with a metal, particularly a molten metal, one cannot simply scale the coating/fiber system down to the typical size of a carbon nanotube because the coating will likely have insufficient thickness to be protective of the underlying nanotube. Thus, another technique(s) may be required. FIG. 1 illustrates the problem. Specifically, FIG. 1 shows the relative sizes of coated carbon fibers and carbon nanotubes in cross-section. The sizes or thicknesses of the fiber, coating and nanotube of about 7 microns, 500 nanometers and about 50 nanometers, respectively, are typical for these bodies. FIG. 1 clearly shows that even the protective coating on the carbon fibers is an order of magnitude larger than the carbon nanotubes. A small fiber-matrix reaction could be permitted in the case of fibers; however, the slightest reaction would annihilate the carbon nanotubes. The CNTs are often only tens of atoms in diameter.

OBJECTS OF THE INVENTION

Thus, in view of the present state of materials development, it is an object of the present invention to produce a composite body, particularly a metal and/or ceramic matrix composite body featuring nanotubes, particularly carbon nanotubes, as a reinforcement.
It is an object of the invention to produce a nanotube-reinforced composite body in which the nanotubes are well dispersed, and well distributed throughout the body, and not agglomerated or clumped into high concentration clusters of nanotubes.
It is an object of the invention to produce a nanotube-containing composite material in which the carbon nanotubes are protected from their environment, particularly during processing.
It is an object of the invention to produce a metal-ceramic composite material by an infiltration process, e.g., in which a matrix for the composite is formed as a result of infiltrating a substance into a porous mass containing the reinforcement component.
It is an object of the invention to produce a nanotube-containing composite body, and in which one or more properties of the resulting composite material has been significantly influenced by the presence of the nanotubes.
It is an object of the present invention to produce a material having a relatively high electrical conductivity.
It is an object of this invention to produce composite materials that are tougher and/or more impact resistant than similar composites without nanotubes.
It is an object of this invention to produce composite materials that show promise as ballistic materials, e.g., armor or projectiles.

SUMMARY OF THE INVENTION

These and other objects of the present invention are achievable by producing a composite material featuring a matrix component and a reinforcement component distributed throughout the matrix composite featuring nanotubes. The nanotubes of the present invention are preferably dispersed, detached, and otherwise rendered predominantly distinct and separate from one another, for example, by comminuting or milling, prior to organizing them into a preform or other porous mass. The matrix component may be metal, ceramic and/or polymer, and in a preferred embodiment is made by infiltrating the matrix component, or one or more matrix precursors, into a porous mass or preform containing the nanotubes. The infiltrating process may be conducted in a pressureless manner, or with the assistance of pressure or vacuum to assist in the infiltration. In a preferred embodiment, the infiltrant features molten silicon metal or an alloy thereof which infiltrates by capillarity into a porous mass or preform containing the nanotubes to form a matrix containing silicon metal or alloy, and optionally silicon carbide. In a particularly preferred embodiment, the nanotubes feature elemental carbon, and the carbonaceous nanotubes are at least partially protected from chemical reaction with the molten silicon metal by one or more substances such as carbon disposed between the nanotubes and the developing matrix component. The carbon nanotubes may feature one or more coatings that serve to prevent chemical reaction with the molten infiltrant material, which coating(s) may be the above-mentioned carbon. In the free carbon embodiment, the carbon source, which may also make up the nanotube protective material, reacts with the silicon of the infiltrant to form at least some silicon carbide phase in-situ in the developing composite body.
The nanotubes can be mixed with one or more filler materials, such as silicon carbide or boron carbide particulate, and with any binders necessary for making a self-supporting preform. The one or more protective coatings may be applied before, during or after mixing with the filler(s).
The resulting preferred embodiment composite body contains the carbon nanotubes as a reinforcement component of the composite, optionally one or more coatings protective of the nanotubes, a matrix featuring the silicon-containing infiltrant, and optionally one or more filler materials, also belonging to the reinforcement component. When the preform contains free carbon, the resulting composite also contains as part of the matrix component some in-situ formed silicon carbide replacing some, up to substantially all, of the elemental silicon constituent of the infiltrant component. Where the one or more coatings are not entirely protective of the carbon nanotubes, chemical reaction could ensue, converting a carbon nanotube partially or completely to a silicon carbide nanotube, which might represent a desirable outcome.
Using the processing methods of this invention, a Si/SiC/CNT composite was obtained with a fracture toughness that was 50% higher than the baseline Si/SiC, while maintaining the other mechanical properties such as modulus and flexural strength. The existence of nanotubes in the final composite was proven by scanning electron microscopy and electrical resistivity measurements. Thus, the method of this invention successfully protected the nanotubes during processing and obtained CNT composites with enhanced properties.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is an illustration that shows the relative diameters of carbon fibers and carbon nanotubes.
FIG. 2 is a scanning electron micrograph of carbon nanotubes in an “as-received” condition.
FIGS. 3A and 3B are photographs of the composites formed in Example II and Comparative Example II, respectively, showing the macroscopic visual effect of the presence and absence, respectively, of carbon nanotubes on the outward appearance of the composite body.
FIG. 4A is an optical photomicrograph taken at about 300× magnification of a polished cross section of the composite material produced in accordance with Example XVIII.
FIG. 4B is a scanning electron micrograph of a fracture surface taken at about 50,000× magnification of the composite material produced in accordance with Example XVIII.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The genesis of the present work was an effort to produce low thermal expansion composite materials, and preferably also possessing relatively high thermal conductivity. This earlier work concerned how to reinforce metals such as aluminum alloys with macroscopic, high modulus, negative CTE (“coefficient of thermal expansion”) fibers such as high modulus carbon fibers. This work is described in commonly owned U.S. patent application Ser. No. 09/378,367, filed on Aug. 20, 1999, which has since issued as U.S. Pat. No. 6,355,340. The entire contents of this commonly owned patent are expressly incorporated herein by reference. As the work progressed, the investigators focused more attention on silicon as a candidate metal. Although intrinsically brittle, silicon possesses very low thermal expansion coefficient, which is particularly advantageous for applications requiring very low or near-zero CTE. With the inherently low CTE matrix material, one could obtain a low CTE composite material without having to resort to expensive, negative CTE carbon fibers. Carbon fibers having a positive (but still small) CTE were advantageously employed to yield a composite material having a CTE less than that of the silicon matrix, at greatly reduced cost compared to the negative CTE carbon fibers. This work is described in the commonly owned U.S. patent application having Attorney Docket No. M-102-A.
The instant invention focuses on a reinforcement component for the composite that is in the form of “nanotubes” and preferably those containing elemental carbon. Carbon nanotubes in particular have been garnering attention recently in the materials science community. As the name suggests, the tubes are hollow and much smaller than a typical fiber, at least in terms of fiber diameter. Nanotubes typically have diameters on the scale of nanometers. They often have very high aspect ratios; that is, the ratio of their length to their diameter can be quite large. The long lengths, perhaps on the order of millimeters, can have utility for, among other properties, toughening, rapid heat transfer, and enhanced electrical conductivity.
Although it is probably possible to incorporate carbon nanotubes into metal and/or ceramic based composite systems by techniques such as sintering or hot pressing, etc, what is preferred according to the instant invention is an infiltration involving a molten metal such as a silicon-containing metal, such as used to make reaction-bonded silicon carbide or siliconized silicon carbide, for instance. For example, there is likely to be at least some damage done to the carbon nanotubes during a hot pressing operation, as this typically occurs to carbon fibers that are hot pressed. Protective coatings on the fibers are especially vulnerable. Further, composite bodies made by infiltration typically exhibit much less dimensional change upon infiltration than do composite bodies made by sintering. Using this approach, nanotube volumetric loadings in the preform ranging from less than 0.1% up to 35% or more are possible, with about 1% to about 10% being particularly desirable from an economic perspective.
A particularly preferred infiltrant in the instant invention is silicon metal or silicon alloy. Not only does silicon possess low CTE, important for many of the applications contemplated by the instant inventor, but, depending on the other processing conditions, one can use this infiltrant system to produce predominantly metal-based composite bodies, or predominantly ceramic-based composite bodies. For example, molten silicon can spontaneously wick into a porous mass of silicon carbide without the need to draw the molten silicon in under vacuum or to force it in under pressure. See, for example, U.S. Pat. No. 3,951,587 to Alliegro et al. The instant invention refers to this product as “siliconized SiC”, and to the process by which it is made as “siliconizing.” This infiltration works best in a vacuum atmosphere. When the porous mass contains some free or reactable carbon that is available to react chemically with the molten silicon, the infiltration usually is enhanced and the process generally is more robust. At least at some point during the infiltration, the silicon component of the infiltrant chemically reacts with at least a portion of the carbon in the porous mass to form silicon carbide. The body thus formed features the porous mass material, typically silicon carbide, distributed throughout the in-situ silicon carbide formed from the chemical reaction. Typically, some infiltrant material remains in the infiltrated body, and distributed throughout. The instant invention refers to this metal-ceramic composite material product as “reaction-bonded SiC”, and to the process by which it is made as “reaction-bonding”, although other terms have been used in the literature over the years to mean substantially the same thing. These terms include “reaction forming”, “reaction sintering”, and “self bonding”. Both the siliconizing and the reaction-bonding processes are within the scope of the instant invention.
A common technique for producing metal composites by infiltration, at least for example, for aluminum or magnesium-based metal composites, utilizes an externally applied force such as external pressure to help the molten metal permeate the preform. This, too, is within the scope of the instant invention.
In the prior work with carbon fibers (not nanotubes), one of the issues that arose with attempts to make metal or ceramic composites by infiltration that are reinforced with such carbon fibers was that the principal constituent of the infiltrants of the present invention, namely silicon, tends to react with the carbon fibers. (Strictly speaking, silicon is a semimetal or “metalloid”, but in the context of the present invention, silicon will be considered a metal.) In the embodiment in which the silicon is one of two or more constituents of the molten infiltrant metal, the same is true for the most common alloying element for the instant silicon infiltrants, namely, aluminum. Aluminum, for example, reacts with carbon to form aluminum carbide. Not only does this represent a chemical degradation of the graphite reinforcement fibers, but also aluminum carbide is hygroscopic. The chemical conversion of aluminum carbide to aluminum hydrate from exposure to water (such as water vapor, e.g., humidity) produces acetylene gas plus a large positive volume change that can cause cracking in the composite body.
There exists the possibility that, due to their highly ordered graphitic structure, the carbon nanotubes may possess higher-than-normal resistance to chemical attack by molten silicon, or at least a resistance that is higher than that of carbon fibers. However, the instant invention has proceeded on the assumption (without further analysis) that the carbon nanotubes will not have increased corrosion resistance compared to macroscopic carbon fibers. Thus, the instant inventor expects that, the nanotube diameter being many times smaller than that of a typical carbon fiber, a carbon nanotube would be completely reacted upon contact with molten silicon. Accordingly, the present work has focused on providing one or more protective coatings to the nanotubes.
There also exists the possibility that a carbon nanotube that reacted with silicon metal would not be completely destroyed and useless as a reinforcement, but rather might still provide some utility. Specifically, the carbon nanotube could be converted in this way to a silicon carbide nanotube (“SNT”). These SNTs themselves could provide the benefits in terms of enhanced mechanical, thermal, ballistic and other properties. Thus, this may represent one way of adding SiC nanotubes to a composite material composition, e.g., add the nanotubes in the form of CNTs and convert them to SNTs during silicon infiltration-based processing. It remains to be seen, however, whether a SNT formed in this way could still exhibit the desirable properties of debonding and pull-out from the matrix upon during fracture.
In the prior case of the macroscopic carbon fibers, the one or more coatings are placed directly onto the fibers. Sometimes, coatings can be found that also permit the infiltrant metal (e.g., silicon or aluminum) to wet the reinforcement better. Wetting of the metal to the reinforcement may improve the mechanical properties of the resulting metal composite, and the wetting condition may also permit the metal to infiltrate a porous mass of the reinforcement material without the need to force the molten metal in under pressure, or to pull it in under applied vacuum. For silicon or silicon alloy melts, a desirable additional coating is silicon carbide, and a widely known technique for depositing a coating of silicon carbide onto macroscopic carbon fibers is by chemical vapor deposition (CVD). It may be possible to similarly coat or encapsulate carbon nanotubes in such CVD SiC.
A technique that is particularly preferred according to the instant invention for protecting the carbon nanotubes when the infiltrant includes silicon, is to coat the nanotubes with, or embed them in, additional carbon prior to infiltration. Carbonaceous precursor materials such as pitch, phenolic resin, furfural alcohol, epoxy resin, etc. are acceptable choices in this regard. One embodiment of this technique for coating nanotubes is to stir in the nanotubes into the liquid resin. Another embodiment would be to provide the nanotubes to a porous mass or preform using standard techniques, and then soak the liquid resin into the porous mass. One may adjust viscosity as needed for the type of processing used by “thinning” the mixture with a low viscosity liquid into which the resin is soluble, e.g., an organic solvent. In this embodiment, it is practically inevitable that some carbon nanotubes will end up better protected than others, and that the others will react to form SNTs, which may or may not be useful. However, this is a numbers game, and what the nanotubes lack in size, they compensate in terms of their numbers, and thus the goal is that enough carbon nanotubes survive infiltration as to produce a noticeable record of their presence in the formed composite body.
After the mixture has been molded or cast or otherwise processed to the desired preform shape, any solvents that may have been added are removed, for example, by a drying operation, and then the carbonaceous substance (e.g., resin) is decomposed to carbon, first by curing or crosslinking the resin, and then with further heating, generally in a non-oxidizing environment, to pyrolyze the resin to drive off the non-carbon constituents of the resin, leaving substantially pure carbon behind as a residue. The residue carbon generally is porous, which may be important for a silicon infiltration operation. However, the pores are of such a size and amount and arrangement as to permit molten silicon infiltration into the pore space of the preform but not to permit complete chemical reaction of the silicon with carbon. Thus, it may be inevitable that some of the pyrolyzed carbon reacts with molten silicon during infiltration, but typically not all of this carbon is so reacted. That the pyrolyzed carbon limits the degree of reaction during infiltration of molten silicon is a manifestation of its protective function. It is not necessary that the pyrolyzed carbon be provided in multiple layers, or be in graphitized form for it to serve this chemical protective function.
Optionally, it may be desirable to add a supplemental source of carbonaceous material not containing nanotubes to the developing preform that does contain nanotubes. Specifically, this supplemental carbon may be provided for the purpose of reacting with the silicon metal of the infiltrant to form in-situ silicon carbide and/or for further reducing the propensity for the silicon to react with the nanotubes by saturating the silicon with carbon. One means of accomplishing this carbon addition to the preform is to soak the preform in a carbonaceous resin such as furfuryl alcohol, and then pyrolyze the resin in a non-oxidizing atmosphere to decompose the resin to essentially elemental carbon. This soak-and-pyrolyze step can be repeated one or more times to increase the amount of carbon and decrease the amount of pore space in the preform.
Moreover, it may be the case that the residual carbon also serves to permit some movement in the axial direction of the nanotubes relative to the matrix, e.g., serve as a nanotube debond layer or interface, should such behavior be desirable, for instance, if toughening is desired. A composite body whose matrix is based on silicon metal typically is brittle, and thus any toughening mechanism that can be imparted to such a composite generally is desirable.
In another embodiment, it may be possible to coat or encapsulate carbon nanotubes with a silicon-containing polymer such as a polysilazane, for example, by stirring nanotubes or clumps/clusters of nanotubes into a polysilazane resin, or soaking bulk resin into a preform containing the nanotubes.
In another technique for protecting the nanotubes, it may be the case that it is not necessary to encapsulate the nanotubes in carbon, but rather to merely saturate the molten infiltrant metal in carbon. Thus, any form of carbon in the preform (e.g., particulate) that is available to dissolve and/or react with the molten infiltrant may be helpful in preventing the nanotubes from being corroded by molten infiltrant metal.
Upon contact with the molten silicon or its alloy, the latter can infiltrate the porous preform, and the silicon can react with at least a portion (generally only a portion when done correctly) of the carbon constituent of the preform to form at least some SiC in the resulting composite body, and typically leaving behind some residual Si or Si alloy. Often times, the free carbon is at least partially interconnected, which typically results in the in-situ formed SiC being at least partially interconnected. The interconnected SiC generally is thought of as forming a component of the matrix of the composite body. Ideally, the carbon nanotubes are protected from the molten silicon by the free carbon of the preform, which is generally interconnected. This leaves a composite body featuring as a reinforcement component the carbon nanotubes, and (optionally) one or more other filler materials, and as a matrix component the SiC and/or Si (or a Si-containing metal) and usually also some residual carbon, mostly at the interface between the reinforcement material(s) and the Si/SiC matrix. The residual carbon coating also provides for toughening of the composite material by making a weak bond with the Si/SiC matrix, thereby permitting movement of the carbon nanotubes relative to the matrix upon application of mechanical stress.
Without wishing to be bound to any particular theory or explanation, it may be that the free carbon that embeds the carbon nanotubes can protect the underlying carbon nanotubes as a result of the large volume change associated with chemically converting carbon to silicon carbide. Upon chemical reaction with silicon, a unit volume of carbon forms 2.3 unit volumes of SiC. The space occupied by the formed SiC can help block off unreacted carbon from further ingress of this molten silicon. In other words, a relatively small amount of reaction of the carbon can help “can off” the infiltration, and thwart continued reaction of carbon by molten silicon. Here, the carbon does not need to be in the “graphitic” form to provide adequate protection from the molten infiltrant.
Engineering the composite body such that at least some carbon remains unreacted by silicon can be accomplished through attention to, and control of, factors such as the relative amounts (volumetric loading) of reactable carbon and filler, the type or form of the carbon, the relative amount of infiltrant provided, the time required for infiltration and the processing temperature during infiltration. Factors that are conducive to the intentional incomplete reaction of the supplied carbon include supplying relatively large quantities of carbon to the preform, using sources of carbon that have a high char yield such as furfuryl or phenolic resins, and minimizing the processing time and temperature of infiltration.
If necessary or desirable, the carbon nanotubes of the reinforcement component of the composite body may be supplemented with one or more other filler materials. Filler materials are often provided in composite bodies to perform one or more functions such as tailor one or more properties of the composite body in a direction toward that property of the filler material, and/or to minimize the amount of space that must be occupied by matrix material. The filler material(s) could be other forms of carbon not intended to react with the infiltrant to form silicon carbide, other non-elemental carbon materials such as metal carbides, or non-carbon-containing materials such as refractory metals, borides, nitrides or oxides, or complex compounds, e.g., oxycarbides.
A popular filler morphology is that of particulate because particulate is economical and readily available, but other non-limiting morphologies include fibers, spheres, platelets and flakes. Useful sizes of particulate for metal-ceramic composite bodies made by infiltration of silicon-containing melts range from about 1 micron to about 1 millimeter.
Using techniques known in the art, the total volumetric loading of filler (e.g., nanotubes plus other optional fillers) can range from about 5 or 10 vol % up to almost the limits where the pore space between bodies of filler begin to close off, about 90 or 95 vol %. More typically, a metal-infiltrated composite body that is highly loaded in filler might be expected to have between about 60 and 80 vol % filler.
Where the carbon nanotubes are in discontinuous form, for example, “as provided” from the manufacturer, the nanotubes and the other fillers may simply be placed into a common container and mixed together, such as by roll mixing or dry milling. The mixture can then be shaped as a preform using techniques known in the art. However, a more sophisticated approach might take into account that the bodies of filler material generally are going to be much larger in size than the diameter of a nanotube (recall FIG. 1). Accordingly, it may make sense to first mix the nanotubes into a liquid component if one is to be used in the preforming process. Even in a dry pressing operation, there is often a liquid component used, such as the binder. (Note that in “as received” form, the dry nanotubes can be in the form of a mass of tangled nanotubes. See, for example, FIG. 2.) The mixture can then be processed in the usual way to make a preform, according to preforming techniques well-known in the art, e.g., pressing, casting (slip, sediment, tape, thixotropic, for example), injection molding, etc. If the preform is to contain free or reactable carbon, such can also be added at this time (if it has not been added already), such as by dipping the preform into a carbon-containing paint or slurry. If a preform is to be made via the prepreg route, it should be possible to incorporate the one or more filler materials into the prepreg in the same way that the nanotubes are incorporated, that is, by mixing or stirring into carbonaceous resin. Alternatively, the filler(s) may be placed between adjacent plies of prepreg material during preform development. Other incorporation techniques may occur to those skilled in the art.
The carbon nanotubes will have their greatest effect in terms of exhibiting their characteristic properties if they are dispersed or distributed throughout the resulting composite body as well and as evenly as is possible. Scanning electron microscopic examinations have confirmed that the nanotubes in the “as-received” condition from the manufacturer often are in the form of a mass of densely packed tangles. Breaking up such a tangled mass will help disperse the nanotubes. One technique for dispersing the nanotubes is to comminute them, such as in a mill, which could be a ball mill. Other forms of comminuting or milling, such as jet milling, Muller mixing, etc., will readily occur to those skilled in the art. It is sufficient to simply ball mill the as-received nanotubes in a dry condition with standard ball milling media. It should also be sufficient to mill the nanotubes in a wet condition, perhaps using the same carbon-containing liquid that one might use as a preform binder, or as a carbonaceous protective coating or layer on the nanotubes. It should also be possible to achieve good results with one or more other composite reinforcement materials present in the mill as well. This should then adequately mix the nanotubes and the reinforcement material(s). It may be possible that the reinforcement bodies by themselves can do an adequate job of breaking up nanotube tangles, that is, without the presence of the stones or ball milling media. In other words, it may not be necessary to mill per se, but merely to roll mix the nanotubes with the reinforcement bodies.
In an alternate route to preform development, a developing preform containing nanotubes and which is self-supporting, for example, as a result of the presence of skeletal carbonaceous material such as resin, may be infiltrated with one or more fillers that are carried into the preform with a liquid carrier fluid. For instance, a nanotube-containing preform can be infiltrated with a slurry or slip containing one or more fillers.
Preforms are useful because they more exactly define the final desired shape, and thus contribute to reducing the amount of final machining required to produce a nanotube reinforced composite article of some specific shape. A preform usually is thought of as a porous mass containing the filler or reinforcement of the composite body that has been rendered self-supporting, such as with a binder or coating of some kind. Often times, the preform has the shape, or is made to conform to the shape of the final object desired, such as by molding or so-called “green” machining. Additionally, and particularly for complex shaped articles, one may build up the larger, more complex structure from two or more smaller, simpler-shaped preforms, for example by adhesive bonding the component preforms to one another. Carbonaceous adhesives such as phenolic resin, epoxy, cyanate ester, etc. are well suited for this purpose. Further, one can add one or more filler materials, e.g., carbon fibers, SiC particulate, etc., including nanotubes, to the carbonaceous adhesive, for example, to more closely match one or more physical properties of the adhesive (e.g., CTE) to those of the component preforms.
When carbon fibers have been arranged in a sheet or film of a matrix material that is carbonaceous such as a resin, and the resin is cured, this product is sometimes referred to as a “prepreg”. Prepregs are often pliable. When the resin of the sheet or film has been pyrolyzed, the resulting product is called a “zero stage” body. Such a zero stage body is usually rigid. If a resin re-infiltration and pyrolysis step is repeated “n” times, the resulting body is referred to as an “n stage” body, where n=0, 1, 2, 3, etc. For example, for two resin re-infiltration/pyrolysis cycles, the resulting body is a “two stage” body. It should be possible to make such prepregs/n-stage bodies containing carbon nanotubes. In particular, it should be possible to mix or stir the carbon nanotubes into carbonaceous resin, and then prepare a sheet or film of the resin in a cured (“prepreg”) or pyrolyzed (“n stage”) condition in the usual way. From there, a preform such as a three-dimensional structure may be produced, again, in the usual way, e.g., laminating a plurality of such sheets or films. Again, where prior art composites have been made by the reactive infiltration of silicon into a preform containing carbon to produce silicon carbide, the objective usually has been to maximize the amount of SiC and to minimize the amount of Si, e.g., for refractory applications. Since the present invention embraces the infiltration of molten silicon-containing metal into a porous preform to produce a Si-containing composite body, the practitioner has wide latitude in the kind of composite body that he or she can process. For instance, one can maximize the amount of SiC produced by providing a preform that is highly loaded in SiC reinforcement and/or producing large quantities of in-situ SiC from reaction of molten Si with reactable carbon in the preform. However, the former approach may be more desirable than the latter. Among the problems that result from excessive reaction during the infiltration process are temperature spikes due to the exothermic nature of the chemical reaction of silicon and carbon. Such temperature spikes can cause cracking due to localized thermal expansion. In addition, (and as mentioned previously) the conversion of elemental carbon to silicon carbide entails a volumetric expansion of about 2.3 times. Thus, large amounts of reaction are also detrimental from the standpoint that the large volumetric change may cause cracking.
On the other hand, the instant invention is also amenable to producing composite materials having less ceramic and more metal in the matrix, for example, where something more akin to a metal matrix composite (“MMC”) or dispersion strengthened metal is the objective. In particular, such composites can be produced according to the instant techniques by providing a preform that is not highly loaded in ceramic reinforcement material and by minimizing the degree of reaction that the molten Si undergoes with carbon sources as it infiltrates the preform. Where the amount of free carbon in the preform is low or is substantially completely reacted, the amount of in-situ SiC formed will be relatively low, perhaps on the order of 1 to 10 vol % of the volume of the formed composite body. On the other hand, where the amount of free carbon is greater and extensive reaction ensues, the amount of residual infiltrant metal remaining in the formed body typically will be low, perhaps on the order of 5 to 20 vol % of the formed composite body. Where the free carbon in the preform is interconnected and is incompletely reacted during infiltration, the situation is complex, as the matrix of the resulting composite body can contain carbon, silicon carbide and residual infiltrant metal.
Prior investigators tended to avoid silicon as a matrix metal, favoring other, tougher metals for making MMCs. Nevertheless, where some degree of toughness is required, there are techniques for increasing toughness of a silicon matrix composite, e.g., fiber debonding or alloy modifications to the matrix phase.
One way for enhancing the toughness of the Si-based matrix composites is to enhance the toughness of the matrix, e.g., by alloying the silicon. Commonly Owned U.S. Pat. No. 6,503,572 (discussed previously) discloses that aluminum may be alloyed with the silicon in amounts ranging from about 0.1 percent by weight or less up to about 90 percent. The resulting alloy can still infiltrate by capillarity into a porous mass of reinforcement material containing some interconnected carbon to form a reaction bonded silicon carbide composite body. The alloy generally does not need to be heated to a temperature greater than about 100° C. above its liquidus temperature. If the body to be infiltrated contains no elemental carbon, however, the process may have to be carried out at higher temperatures, for example, in the 1600° C. to 1800° C. range. The entire disclosure of this Commonly Owned Patent is hereby incorporated by reference.
Another technique to impart toughening is the technique commonly employed in CMC's—namely, to cause the fibers to debond from the matrix under applied load, or at least to be able to move axially with respect to the matrix under the influence of an applied load. This technique is commonly achieved with a debond layer such as carbon (e.g., pyrolytic carbon) or boron nitride applied to the fibers. However, with Si/SiC composite systems, such carbon coatings are often reactive with the molten Si infiltrant, typically resulting in the loss of the debonding property. On the other hand, some coatings, such as boron nitride in particular, may not be wetted by molten Si, thus preventing infiltration. However, since it was stated earlier that the carbon nanotube reinforcement itself is reactive with many of the candidate matrix metals and thus needs to be protected, for example, with a coating, the debond coating may be located between the nanotube and the coating that is protective of the nanotube. In this way, the protective coating may protect both carbon nanotube and debond coating. The debond coating can be applied or deposited by techniques known in the art, such as chemical vapor deposition (CVD). For macroscopic fibers, the protective coating also may be applied by CVD. U.S. Pat. No. 5,015,540 discloses such a multiple fiber coating system. It has been observed, however, that for a CVD SiC coating to be protective of NICALON® SiC fibers at about 35 vol % loading in a preform, the coating needed to be on the order of about 2 microns in thickness. (See, for example, U.S. Pat. No. 5,682,594.) This size is much larger than the diameter of a single nanotube, so this approach may not be practical, unless the CVD SiC coating is of a bundle of nanotubes such that the coating layer surrounds or envelops the entire bundle and not individual nanotubes. It bears noting that since carbon was earlier identified as a candidate protective coating, under the proper conditions it is possible that a single material, e.g., carbon, can serve the dual role of acting to chemically protect the underlying nanotube, as well as acting as the debond material for toughening purposes. Again, it seems as though carbon coating thicknesses that are merely on the order of the thickness of a single nanotube will not be sufficiently protective against chemical attack by molten silicon. Accordingly, the carbon “coating” may be provided in sufficient thickness and sufficient quantity that it might be thought of as forming a part of the matrix phase of the preform (e.g., interconnected), and not merely as a discrete coating on individual nanotubes.
The following examples illustrate with still more specificity several preferred embodiments of the present invention. These examples are meant to be illustrative in nature and should not be construed as limiting the scope of the invention. Some of the examples report physical properties of the formed composite materials. Property characterization was as follows: Composite densities were measured by Archimedes method (ASTM C-135-86). Elastic moduli were measured using an ultrasonic technique (ASTM E494-95) at three locations and an average value was reported. Flexural strengths were measured using the four-point bend method (ASTM C1161-90). Typically, 3-5 specimens were tested and average strength and standard deviation were reported. Fracture toughness was measured by the Chevron notch method. Specimen volume electrical resistivities were measured by the four-point-probe method. Some of the newly made specimens were also subjected to microstructural characterization. Small samples were cut from the specimens, mounted in epoxy, and were ground and polished with successively finer diamond compounds for microstructural observations. FIG. 4A is an optical photomicrograph of a polished cross-section of a CNT composite specimen (from Example XVIII) that is fairly representative of all of the CNT-containing materials made according to the Examples.

EXAMPLE I

This Example, demonstrates, among other things, the successful incorporation of carbon nanotubes (CNTs) into a metal-ceramic composite material and in particular, the survivability of the CNTs during infiltration processing with the metal in a molten condition.
About 3.86 g of chemical vapor deposition (CVD) grown multi wall carbon nanotubes (Iljin Nanotech Co. Ltd., Seoul, Korea) were mixed with about 42.64 g of phenolic (SC1008 from Borden Chemical Inc., Louisville, Ky.) to make a mixture. The CNTs diameter ranged from about 10-50 nm.
The mixture was poured in a rubber mold with a cavity measuring about 5 by 5 by 1.3 cm. The rubber mold was placed on a vibrating table for about 12 hours. A thin TEFLON® sheet measuring about 5 cm square was placed on the mixture in the mold. A graphite block measuring about 5 by 5 by 2.5 cm (and having a mass of about 225 g) was placed on top of the TEFLON® sheet. The mold was then placed in a curing oven and heated to about 140° C. for about 3 hours and then cooled to room temperature (about 20° C.). A cured, stand-alone preform was produced after demolding. This preform was placed in a retort and heated to about 650° C. for about 2 hours in an inert atmosphere to carbonize the phenolic. Carbonization of the phenolic left a pyrolytic carbon coating on the CNTs, which coating protects them during subsequent infiltration by molten Si.
This preform was then placed in a vacuum furnace, vacuum was drawn (<100 microns) and the preform was then heated to about 1520° C. and brought into contact with molten silicon.
Molten silicon quickly infiltrated into the preform. The pyrolytic carbon reacted with molten Si and provided the driving force for the infiltration. However, it is postulated that, after initial reaction, a dense SiC coating was formed which prevented further reaction and protected the CNTs from reacting with molten Si.
A fully dense composite resulted containing SiC, CNT, residual C and Si.
Thus, this example proves that a molten metal infiltration process can be used to make CNT/SiC composites and the pyrolytic carbon coating protects the CNTs during melt infiltration or reaction bonding.

EXAMPLE II

This Example, demonstrates, among other things, the successful incorporation of carbon nanotubes (CNTs) into a metal-ceramic composite material also containing another filler or reinforcement material.
About 50 g of SiC powders consisting of a 70:30 weight ratio of 240 and 500 grit particulates (Saint Gobain/Norton Industrial Ceramics, Worcester, Mass.), 2 g CNTs, about 20.93 g phenolic and about 15.6 g THF (solvent) were hand mixed in a beaker to make a mixture. The mixture was poured in a rubber mold with a cavity measuring about 5 by 5 by 1.3 cm. The rubber mold was placed on a vibrating table for about 12 hours. A thin TEFLON® sheet measuring about 5 cm square was placed on the mixture. A graphite block measuring about 5 by 5 by 2.5 cm was placed on top of the TEFLON® sheet. The mold was then placed in a curing oven and heated to about 140° C. for about 3 hours and then cooled to room temperature. A cured, stand-alone preform was produced after demolding. This preform was placed in a retort and heated to about 650° C. for about 2 hours in an inert atmosphere to carbonize the phenolic. Carbonization of the phenolic leaves a pyrolytic carbon coating on the CNTs which protects them during infiltration by molten Si.
This preform was then placed in a vacuum furnace, vacuum was drawn (<100 micron) and then heated to about 1520° C. and brought in contact with molten silicon. Molten silicon spontaneously infiltrated into the preform. The pyrolytic carbon reacts with molten Si and provides the driving force for the infiltration. After initial reaction, a dense SiC coating is formed which prevents further reaction and protects the CNTs from reacting with molten Si.
A fully dense composite was obtained containing SiC (both as reinforcement and as matrix material), CNTs, Si and C. The density of the composite was found to be 2.65 g/cc.
Thus, this Example proves that a metal infiltration process can be used to make a CNT-containing composite and that the pyrolytic carbon coating protects the CNTs during melt infiltration or reaction bonding. And further, the Example shows that a filler can be incorporated into the composite body without interfering with the CNTs.

COMPARATIVE EXAMPLE II

This Example demonstrates, for comparative purposes, the manufacture of a SiC composite body not containing carbon nanotubes.
The technique of Example II was substantially repeated to produce a composite body containing SiC and Si. The mixture to be cast had the following composition: about 100 grams SiC powder, about 15 grams of the phenolic resin, and about 15 grams of ethanol. This composition had about the same viscosity as the nanotube-containing mixture of Example II.
The bulk density of this composite material was about 3.04 g/cc. FIGS. 3A and 3B show a visual comparison of the composite bodies with and without nanotubes, respectively.
Right rectangular beams were machined from the composite blocks of Example II and Comparative Example II for electrical resistivity measurements. The ends of the beams were painted with a silver paste to assure good contact to the probes of a commercially available multimeter. The resistivity of the carbon nanotube-containing SiC composite material of Example II was about 2 orders of magnitude lower than that of the SiC composite not containing nanotubes, 0.0038 ohm-cm versus 0.37 ohm-cm. This should serve as additional evidence that the carbon nanotubes survived the composite-making conditions.

EXAMPLE III

Examples III and IV through X demonstrate, among other things, various methods for blending and distributing carbon nanotubes throughout a mass of preform material.
To a plastic jar were added a 100 g mixture of 240 and 500-600 grit SiC (Saint Gobain Ceramics and Plastics, Inc. Worcester, Mass.) in a 60:40 weight ratio, about 5 g of CVD grown multi-walled carbon nanotubes (Iljin Nanotech, Soeul, South Korea), about 15.25 g of phenolic (SC1008 from Borden Chemical Inc., Louisville, Ky.), and about 40 g of Reagent Alcohol. The jar was sealed and placed on a rolling mill.
After roll mixing for about 12 hours, the mixture was poured out of the jar and into a rubber mold of nominal cavity size about 71 mm square by about 13 mm deep. The rubber mold was placed on a vibrating table for about 18 hours, during which time, the excess liquid pooled at the top, and it was periodically removed. The mold was placed in an oven and the temperature was raised to about 140° C. at a rate of about 10° C. per hour, with about two 2-3 hour holds for temperature equilibration. After holding at about 140° C. for about 3 hours, the oven and its contents were cooled at about 200° C. per our to ambient temperature. The preform was then demolded and placed on a graphite plate and was then placed in an inert atmosphere retort. The temperature was raised above 650° C., held for about 2 hours and reduced to room temperature to carbonize the preform.
The preform was then placed in a graphite boat along with chunks of silicon. The boat was placed in a vacuum furnace. The furnace was evacuated below 100 microns pressure. The temperature of the furnace was raised to about 1450° C. and held for 1 hour. The temperature was then reduced to room temperature. As a result of this thermal processing, the silicon had melted and wicked into the preform forming a composite.
Some properties of this composite were as follows: Density—2.8 g/cc, Elastic Modulus—274 GPa, Flexural strength—103 MPa.

COMPARATIVE EXAMPLE III

This example is substantially same as Example I except the ingredients were:

- a. 100 g mixture of 240 and 500-600 grit SiC from Saint Gobain Ceramics in 60:40 proportion by weight
- b. 15 g phenolic (grade info.)
- c. 15 g Reagent Alcohol
  In other words, this Example does not have nanotubes in its composition. Properties of this composite were as follows: Density—3.04 g/cc, Elastic Modulus—375 GPa, Flexural strength—187 MPa, Electrical resistivity 1.097 Ohm-cm, and Knoop hardness—1889 kg/mm2.

EXAMPLE IV

To a plastic jar were added a 100 g mixture of 240 and 500-600 grit SiC (Saint Gobain Advanced Ceramics, Worcester, Mass.) in a 60:40 weight ratio, about 5 g of CVD grown multi-walled carbon nanotubes (Iljin Nanotech). The jar was sealed and placed on a rolling mill.
After roll mixing for about 3 hours, the mixture was poured out of the jar and into a mixing bowl. To the mixing bowl were added about 25 g of phenolic (Durite SC-1008, Borden Chemical) and about 25 g of reagent alcohol. After hand mixing thoroughly, the admixture was hand pressed into a rubber mold of nominal cavity size about 71 mm square by about 13 mm deep. The balance of the processing was substantially the same as in Example 1 from the drying step forward.
Properties of this composite were as follows: Density—2.66 g/cc, Elastic Modulus—191 GPa, Flexural strength—77 MPa.

EXAMPLE V

To a plastic jar were added a 100 g mixture of 240 and 500-600 grit SiC (Saint Gobain Advanced Ceramics) in a 60:40 weight ratio, about 5 g of CVD grown multi-walled carbon nanotubes (Iljin Nanotech). The jar was sealed and placed on a rolling mill.
After roll mixing for about 3 hours, the mixture was poured out of the jar and into a mixing bowl. To the mixing bowl were added about 20 g of phenolic (Durite SC-1008) and about 40 g of reagent alcohol. After hand mixing thoroughly, the admixture was placed back into the plastic jar and roll mixed for another hour. Then, the admixture was poured into the approximately 51 mm square cavity of a dry press (Carver Inc., Wabash, Ind.) and pressed with a 20,000 pound (9100 kgf) load for about 5 minutes. The pressed preform was removed from the die and placed onto a graphite setter tray. The balance of the processing was substantially the same as in Example 1 from the 650 C carbonization step forward.
Properties of this composite were as follows: Density—2.83 g/cc, Elastic Modulus—290 GPa, Flexural strength—136 MPa.

EXAMPLE VI

About 5 g of CVD grown multi-wall carbon nanotubes CNTs (Iljin Nanotech, Seoul, Korea) were placed in a plastic jar along with alumina milling cylinders (1 inch diameter×1 inch long). The jar was placed on a jar mill for 4 hours. The milled CNTs were mixed with 100 g SiC powders, from Saint Gobain (a mixture of 240 and 500/600 grit in 70:30 weight proportion) by rolling together in a plastic jar for 3 hours. 50 g phenolic (details) was added to the mix and the mixed by hand. The mix was screened through a wire mesh having openings of about 1.6 mm square. The mix was poured in a 2×2 inch (51 mm square) cold pressing die and pressed at 30,000 lb load for 5 minutes. The pressed preform was removed from the die and placed on a graphite setter tray. The rest of the processing was the same as in Example 1 from the carbonization step forward.
Properties of this composite were as follows: Density—2.91 g/cc, Elastic Modulus—304 GPa, Flexural strength—173 MPa, fracture toughness—4.4 MPa m^1/2, electrical resistivity—0.156 ohm-cm.

EXAMPLE VII

This Example was conducted in substantially the same manner as Example VI except after the 600 C carbonizing step, the preform was dipped in the phenolic for 2 hours to back soak it; that is, to add more carbonaceous resin to it. The back soaked preform was placed in an inert atmosphere retort and the temperature was raised above 600° C., held for 1 hour and then reduced to room temperature, to re-carbonize the preform. After that, the preform was infiltrated with molten silicon substantially as described in Example 1.
Properties of this composite were as follows: Density—2.86 g/cc, Elastic Modulus—302 GPa, Flexural strength—171 MPa, electrical resistivity—0.0587 ohm-cm, and Knoop hardness—1789 kg/mm².

EXAMPLE VIII

This Example was performed in substantially the same manner as Example VI except the ball milling step with the alumina cylinders was carried out for 24 hours and only 35 g phenolic was added to make the pressable admixture.
The properties of this composite were as follows: Density—3.05 g/cc, Elastic Modulus—371 GPa, Flexural strength—219 MPa, fracture toughness—5.7 MPa m^1/2electrical resistivity—0.251 ohm-cm.

EXAMPLE IX

Same as Example VIII except after first carbonizing, the preform was dipped in the phenolic for 2 hours to back soak it. The back soaked preform was placed in an inert atmosphere retort and the temperature was raised above 600° C., held for 1 hour and then reduced to room temperature, to re-carbonize the preform. After that, the preform was infiltrated with molten silicon in substantially the same manner as described in Example 1.
The properties of this composite were as follows: Density—3.05 g/cc, Elastic Modulus—369 GPa, Flexural strength—194 MPa, electrical resistivity—0.235 ohm-cm.

EXAMPLE X

About 5.5 g of CVD grown multi-walled carbon nanotubes (Iljin Nanotech), about 35 g of phenolic and about 20 g of reagent alcohol were placed in a plastic jar along with alumina cylinders (about 25 mm diameter by about 25 mm in height) and milled for about 4 hours on a jar mill. This jar-milled mixture (minus the milling media) was then combined with about 100 g SiC powders, from Saint Gobain Advanced Ceramics (a mixture of 240 and 500/600 grit in 70:30 weight proportion) by hand mixing. This admixture was then screened through a wire screen having openings of about 1.6 mm square. The mix was then poured into a 51 mm square die cavity of a cold pressing die and pressed at about 30,000 lb (1400 kgf) load for about 5 minutes. The pressed preform was removed from the die and placed on a graphite setter tray. The rest of the processing was the same as in Example IX from the carbonizing step forward.
Properties of this composite were: Density—2.77 g/cc, Flexural strength—89 MPa, Electrical resistivity 0.141 ohm-cm.

EXAMPLE XI

5.5 g of CVD grown multi-wall carbon nanotubes CNTs (Iljin Nanotech, Seol, Korea), 35 g phenolic, 20 g reagent alcohol, 100 g SiC powders from Saint Gobain (a mixture of 240 and 500/600 grit in 70:30 weight proportion) and alumina milling cylinders (1 inch diameter×1 inch long) were placed in a plastic jar. The jar was placed on a jar mill for 4 hours. This mixture, minus the milling cylinders, was screened through a wire mesh (1.6×1.6 mm). The mix was poured in a 2×2 inch cold pressing die and pressed at 30,000 lb load for 5 minutes. The pressed preform was removed from the die. After this, carbonizing, back-soaking, second carbonizing, and infiltration steps were carried out similar to those in Example IX.
Properties of this composite were: Density—3.00 g/cc, Flexural strength—90 MPa, Electrical resistivity 0.461 ohm-cm

EXAMPLE XII

About 5.3 g of CVD grown multi-wall carbon nanotubes CNTs (Iljin Nanotech, Seol, Korea), 30 g phenolic, and 24 g reagent alcohol, were placed in a plastic jar. The jar was placed on a jar mill for 4 hours. This mixture was added to 100 g SiC powders from Saint Gobain (a mixture of 240 and 500/600 grit in 70:30 weight proportion) and hand mixed. This mixture was screened through a wire mesh (1.6×1.6 mm). After this, carbonizing, back-soaking, second carbonizing, and infiltration steps were carried out similar to those in Example IX.
Properties of this composite were: Density—2.78 g/cc, Flexural strength—92 MPa, Electrical resistivity 0.213 ohm-cm

EXAMPLE XIII

This example is substantially same as example VII except that hand-mixing was used instead of rolling.
Properties of this composite were: Density—3.01 g/cc, sonic modulus 325 GPa, Flexural strength—207 MPa, Electrical resistivity 0.327 ohm-cm and fracture toughness 4.4 MPa m^1/2

EXAMPLE XIV

This example is substantially same as Example XIII except, milling in step one was carried out for 24 hours, and no back soaking and second carbonizing was done.
Properties of this composite were: Density—2.95 g/cc, sonic modulus 345 GPa, Flexural strength—199 MPa, Electrical resistivity 0.075 ohm-cm and fracture toughness 4.6 MPa m^1/2.

EXAMPLE XV

This example is substantially same as Example XIV except, 10 g of CNT were used instead of 5 g.
Properties of this composite were: Density—2.83 g/cc, sonic modulus 336 GPa, Flexural strength—215 MPa, Electrical resistivity 0.184 ohm-cm and fracture toughness 4.3 MPa m^1/2.

EXAMPLE XVI

This example is substantially same as Example XIV except, 20 g of CNT were used instead of 5 g.
Properties of this composite were: Density—2.96 g/cc, sonic modulus 348 GPa, Flexural strength—219 MPa, Electrical resistivity 0.391 ohm-cm and fracture toughness 4.8 MPa m^1/2.

EXAMPLE XVII

This example is substantially same as Example XIV except, 50 g of CNT were used instead of 5 g.
Properties of this composite were: Density—3.04 g/cc, sonic modulus 373 GPa, Flexural strength—291 MPa, Electrical resistivity 0.324 ohm-cm, fracture toughness 6.1 MPa m^1/2, and Knoop hardness 1705 kg/mm².

EXAMPLE XVIII

This example is substantially same as Example XIV except, 75 g of CNT were used instead of 5 g.
Properties of this composite were: Density—3.04 g/cc, sonic modulus 372 GPa, Flexural strength—205 MPa, Electrical resistivity 0.501 ohm-cm, fracture toughness 5.4 MPa m^1/2, and Knoop hardness 1758 kg/mm². An optical photomicrograph of a polished cross section of this specimen taken at about 300× magnification is shown in FIG. 4A. Fracture surfaces of this specimen were examined via scanning electron microscopy (SEM). A photo taken at about 50,000× magnification on one of these fracture surfaces is shown in FIG. 4B. Here, several individual pulled out nanotubes are visible (marked by white arrows), and are seen to have been pulled out of the fracture surface. This nanotube pull-out mechanism is probably responsible for the enhanced toughness of this composite. This SEM photo also confirms the existence of the nanotubes in the formed composite material, and thus proves that the nanotubes survived the molten metal infiltration process.

EXAMPLE XIX

This example is substantially same as Example XIV except, 100 g of CNT were used instead of 5 g.
Properties of this composite were: Density—2.95 g/cc, sonic modulus 336 GPa, Flexural strength—117 MPa, and fracture toughness 3.8 MPa m^1/2.

EXAMPLE XX

This example is substantially same as Example XIII except, milling in step one was carried out for 24 hours.
Properties of this composite were: Density—3.02 g/cc, sonic modulus 358 GPa, Flexural strength—208 MPa, and fracture toughness 5.0 MPa m^1/2.

EXAMPLE XXI

This example is substantially same as Example XX except, 10 g of CNT were used instead of 5 g.
Properties of this composite were: Density—3.03 g/cc, sonic modulus 360 GPa, Flexural strength—169 MPa, Electrical resistivity 0.088 ohm-cm and fracture toughness 5.0 MPa m^1/2.

EXAMPLE XXII

This example is substantially same as Example XX except, 20 g of CNT were used instead of 5 g.
Properties of this composite were: Density—3.08 g/cc, sonic modulus 393 GPa, Flexural strength—323 MPa, Electrical resistivity 0.312 ohm-cm and fracture 5.3 MPa m^1/2.

EXAMPLE XXIII

About 5 g of CVD grown multi-wall carbon nanotubes CNTs (Iljin Nanotech, Seoul, Korea) and 100 g SiC powders from Saint Gobain (a mixture of 240 and 500/600 grit in 70:30 weight proportion) and alumina milling cylinders (1 inch diameter×1 inch long) were placed in a plastic jar. The jar was placed on ajar mill for 24 hours. The milling cylinders were removed and 25 g phenolic was added and hand mixed. Above mixture was screened through a wire mesh (1.6×1.6 mm). The mix was poured in a 2×2 inch (51 mm square) cold pressing die and pressed at 30,000 lb load for 5 minutes. The pressed preform was removed from the die. After this, carbonizing and infiltration steps were carried out similar to that in Example III.
Properties of this composite were: Density—2.91 g/cc, sonic modulus 341 GPa, Flexural strength—150 MPa, fracture toughness 3.5 MPa m^1/2.

EXAMPLE XXIV

This example is substantially same as Example XXIII except 10 g CNTs were used instead of 5 g.
Properties of this composite were: Density—2.91 g/cc, sonic modulus 345 GPa, Flexural strength—212 MPa, fracture toughness 4.1 MPa m^1/2.

EXAMPLE XXV

This example is substantially same as Example XXIII except 20 g CNTs were used instead of 5 g.
Properties of this composite were: Density—2.88 g/cc, sonic modulus 336 GPa.

EXAMPLE XXVI

This example is substantially same as Example XXIII except back-soaking and second carbonizing were carried out followed by infiltration similar to that in Example VII.
Properties of this composite were: Density—3.01 g/cc, sonic modulus 351 GPa, Flexural strength—198 MPa, electrical resistivity 0.110 and fracture toughness 5.0 MPa m^1/2.

EXAMPLE XXVII

This example is substantially same as Example XXVI except 10 g of CNT were added instead of 5 g.
Properties of this composite were: Density—3.03 g/cc, sonic modulus 369 GPa, Flexural strength—231 MPa, and fracture toughness 5.0 MPa m^1/2.

EXAMPLE XXVIII

This example is substantially same as Example XXVI except 20 g of CNT were added instead of 5 g.
Properties of this composite were: Density—2.99 g/cc, sonic modulus 346 GPa, Flexural strength—214 MPa, electrical resistivity of 0.084 ohm-m and fracture toughness 4.9 MPa m^1/2.

EXAMPLE XXIX

This example is substantially same as Example XVII except B₄C particulates were used in place of SiC.
Properties of this composite were: Density—2.79 g/cc, sonic modulus 330 GPa, Flexural strength—149 MPa, fracture toughness 4.5 MPa m^1/2, and Knoop hardness 1547 kg/mm²
Specimen volume resistivity measurements were made on the instant samples using the four probe technique. The volume resistivities of the specimens with CNTs were less than half of the volume resistivity of the composite without CNTs. CNTs have very high electrical conductivity (or lower electrical resistivity). Thus, the lower electrical resistivity of the composites indicates that CNTs are present in these composites.
Several composites were obtained with flexural strength and fracture toughness higher than the base line Si/SiC composite of Comparative Example II and Comparative Example III. In fact, for one composition, a fracture toughness of 6 MPa m^1/2was measured as against 4.0 for the baseline Si/SiC. This represents a 50% increase in fracture toughness over the baseline composite, while keeping other properties, e.g., elastic modulus and bend strength the same or better. This is a significant achievement and demonstrates the ability to enhance toughness of reaction bonded composites using carbon nanotube (CNT) reinforcement.
Finally, fracture surfaces of the mechanical test specimens were observed by scanning electron microscopy to find the mechanisms responsible for the increased toughness of the CNT composites. A number of nanotubes were observed extending out of the fracture surface. This observation, along with the resistivity measurements, proved that the nanotubes survived the infiltration process. Thus, the methods of this invention successfully protected the nanotubes during processing and obtained CNT composites with enhanced properties.
Based on the fracture surface observation, the enhanced toughness of CNT composites most likely is due to (1) pull-out of the CNT from the matrix; (2) bridging of cracks by the CNT; and/or (3) deflection of cracks by the CNT.

INDUSTRIAL APPLICABILITY

The methods and compositions of the present invention should find utility in applications requiring or benefiting from the unusual properties offered by nanotubes, particularly carbon nanotubes, e.g., applications where high specific stiffness, low thermal expansion coefficient, enhanced toughness, high electrical conductivity, and/or high thermal conductivity are important
For certain friction products such as brakes and clutches, the carbon nanotube reinforced SiC composites of the instant invention seem to possess many of the desirable properties.
The previously expected toughening effect of the carbon nanotubes has now been documented. This toughening should also improve the characteristics of ballistic armor, such as personnel armor (“body armor”) made from carbon nanotube reinforced composites, such as those based on Si/SiC, Si/B₄C and Si/SiC/B₄C. The high electrical conductivity of carbon nanotubes should also have the effect of increasing thermal conductivity of the composite bodies into which they are incorporated. Similarly, the very low, and even negative, CTE of the nanotubes should also have the effect of reducing the CTE of composite bodies into which they are incorporated.
The instant nanotube-composites should also find application in large structures that must maintain size and shape within exacting tolerances, such as mirrors, e.g., land or space-based mirrors.
Other applications in such industries as the precision equipment, robotics, tooling, aerospace, electronic packaging and thermal management, and semiconductor fabrication industries, among others, will occur to those skilled in those arts.
An artisan of ordinary skill will appreciate that various modifications may be made to the invention herein described without departing from the scope or spirit of the invention as defined in the appended claims.

Claims

1. A method for making a composite body containing nanotubes, comprising:

(a) providing a plurality of nanotubes;

(b) comminuting said nanotubes;

(c) organizing said comminuted nanotubes into a porous mass;

(d) infiltrating a molten infiltrant from a source into said porous mass; and

(e) solidifying said molten infiltrant.

2. The method of claim 1, further comprising:

(f) supplying at least one carbon-containing liquid to said plurality of nanotubes, to substantially coat the external surfaces of the nanotubes;

(g) removing volatiles from said carbon-containing liquid; and

(h) wherein the molten infiltrant comprises silicon metal.

3. A method of making a carbon nanotube-containing composite body, comprising:

(a) mixing a plurality of nanotubes with at least one filler material to disperse said nanotubes throughout said filler material;

(b) supplying at least one carbon-containing liquid to said dispersion of nanotubes and filler material, and stirring sufficiently to substantially coat at least all of the external surfaces of the at least one filler material and the nanotubes, thereby forming an admixture;

(c) organizing said admixture as a porous mass to be infiltrated;

(d) heating at least said carbon-containing liquid to remove volatile constituents, thereby leaving behind as a residue substantially pure elemental carbon;

(e) contacting a molten infiltrant comprising silicon to said porous mass;

(f) infiltrating said porous mass to a desired extent with said molten infiltrant to form an infiltrated mass; and

(g) cooling said infiltrated mass to form a composite body.

4. The method of claim 1, further comprising arranging said porous mass into a desired bulk shape.

5. The method of claim 1, wherein prior to infiltrating with molten infiltrant, said porous mass is green machined.

6. The method of claim 1, wherein said porous mass further comprises at least one filler material.

7. The method of claim 1, wherein said nanotube comprises elemental carbon.

8. The method of claim 2, wherein said molten infiltrant comprises at least one metal other than silicon.

9. The method of claim 8, wherein said at least one metal comprises aluminum.

10. The method of claim 1, wherein said nanotubes make up about 1% to about 15% by volume of said porous mass.

11. The method of claim 2, wherein said mixture of nanotubes and carbon-containing liquid is shaped or rendered in the form of a prepreg.

12. The method of claim 6, wherein said at least one other filler material comprising a plurality of finely divided bodies that are infiltrated into said preform by means of a carrier fluid.

13. The method of claim 6, wherein said at least one filler material comprises at least one substance selected from the group consisting of silicon carbide and boron carbide.

14. The method of claim 3, wherein said at least one filler material comprises at least one substance selected from the group consisting of silicon nitride, aluminum oxide and titanium diboride.

15. The method of claim 1, wherein said infiltrating occurs via capillarity.

16. The method of claim 1, wherein said infiltrating occurs with the assistance of an externally applied force.

17. A composite body made according to the method of claim 1.

18. A composite body, comprising:

(a) a reinforcement component comprising a plurality of nanotubes, wherein at least a weight majority of said nanotubes are not in the form of a tangled mass of nanotubes; and

(b) a matrix component comprising at least one of elemental silicon and silicon carbide.

19. The composite body of claim 18, further comprising at least one coating that substantially shields or isolates said nanotubes from said matrix.

20. The composite body of claim 18, further comprising at least one zone of carbon disposed between said nanotubes and said matrix.

21. The composite body of claim 18, comprising from about 1% to about 90% by volume of said nanotubes, about 1% to about 90% of said elemental silicon, about 10% to about 90% of said silicon carbide, and about 0.1% to about 90% of boron carbide.

22. The composite body of claim 18, wherein said at least one coating comprises silicon carbide.

23. The composite body of claim 18, wherein said carbon nanotubes make up about 0.1% to about 35% by volume of said composite body.

24. The composite body of claim 18, wherein said nanotubes have a diameter that is less than about 500 nanometers.

25. The composite body of claim 18, wherein said nanotubes have a diameter in the range of about 10 to 100 nanometers.

26. The composite body of claim 18, wherein said reinforcement component further comprises at least one other filler material.

27. The composite body of claim 26, wherein said at least one other filler material comprises at least one of carbon fibers, silicon carbide and boron carbide.

28. The composite body of claim 26, wherein said at least one other filler material comprises a morphology selected from the group consisting of particulate, fiber, platelets and flakes.

29. The composite body of claim 26, wherein said reinforcement component makes up at least about 10 vol % and no more than about 90 vol % of said composite body.

30. The composite body of claim 18, wherein said nanotubes comprises elemental carbon.

31. The composite body of claim 18, wherein said nanotubes comprise silicon carbide.

32. A composite body, comprising:

(b) a matrix component comprising elemental aluminum.

33. The composite body of claim 32, wherein said matrix further comprises at least one of elemental silicon and silicon carbide.

34. A method for making a composite body comprising silicon carbide nanotubes, comprising:

(a) providing at least one carbon nanotube;

(b) comminuting said at least one carbon nanotube to form a plurality of carbon nanotubes;

(c) organizing said comminuted carbon nanotubes into a porous mass;

(d) contacting a source of molten infiltrant comprising silicon to said porous mass;

(e) infiltrating molten infiltrant into said porous mass;

(f) reacting at least a portion of said silicon with at least a portion of said nanotubes to form silicon carbide nanotubes; and

(g) solidifying said molten infiltrant.