Chemistry of inorganic compounds – Carbon or compound thereof – Elemental carbon
Reexamination Certificate
2001-12-12
2004-08-31
Hendrickson, Stuart L. (Department: 1754)
Chemistry of inorganic compounds
Carbon or compound thereof
Elemental carbon
C516S032000, C208S014000
Reexamination Certificate
active
06783746
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
Methods are described and surfactants are identified which can disperse carbon nanotubes in aqueous and petroleum liquid medium utilizing selected dispersants and mixing methods to form stable carbon nanotube dispersions.
2. Description of the Prior Art
Carbon nanotubes are a new form of the material formed by elemental carbon, which possess different properties than the other forms of the carbon materials. They have unique atomic structure, very high aspect ratio, and extraordinary mechanical properties (strength and flexibility), making them ideal reinforcing fibers in composites and other structural materials.
Carbon nanotubes are characterized as generally to rigid porous carbon three dimensional structures comprising carbon nanofibers and having high surface area and porosity, low bulk density, low amount of micropores and increased crush strength. The instant process is applicable to nanotubes with or without amorphous carbon.
The term “nanotube” refers to elongated structures having a cross section (e.g., angular fibers having edges) or diameter (e.g., rounded) less than 1 micron. The structure may be either hollow or solid. Accordingly, the term includes “nanofibrils” and “bucky tubes”. Such structures provide significant surface area when incorporated into a structure because of their size and shape. Moreover, such fibers can be made with high purity and uniformity.
Preferably, the nanotube used in the present invention has a diameter less than 1 micron, preferably less than about 0.5 micron, and even more preferably less than 0.1 micron and most preferably less than 0.05 micron.
The term “internal structure” refers to the internal structure of an assemblage including the relative orientation of the fibers, the diversity of and overall average of fiber orientations, the proximity of the fibers to one another, the void space or pores created by the interstices and spaces between the fibers and size, shape, number and orientation of the flow channels or paths formed by the connection of the void spaces and/or pores. The structure may also include characteristics relating to the size, spacing and orientation of aggregate particles that form the assemblage. The term “relative orientation” refers to the orientation of an individual fiber or aggregate with respect to the others (i.e., aligned versus non-aligned). The “diversity of” and “overall average” of fiber or aggregate orientations refers to the range of fiber orientations within the structure (alignment and orientation with respect to the external surface of the structure).
Carbon nanotubes can be used to form a rigid assemblage or be made having diameters in the range of 3.5 to 70 nanometers. The nanotubes, fibrils, bucky tubes and whiskers that are referred to in this application are distinguishable from continuous carbon fibers commercially available as reinforcement materials. In contrast to nanotubes, which have desirably large, but unavoidably finite aspect ratios, continuous carbon fibers have aspect ratios (L/D) of at least 10
4
and often 10
6
or more. The diameter of continuous fibers is also far larger than that of nanotubes, being always >1.0 micron and typically 5 to 7 microns. Continuous carbon fibers are made by the pyrolysis of organic precursor fibers, usually rayon, polyacrylonitrile (PAN) and pitch. Thus, they may include heteroatoms within their structure. The graphitic nature of “as made” continuous carbon fibers varies, but they may be subjected to a subsequent graphitization step. Differences in degree of graphitization, orientation and crystallinity of graphite planes, if they are present, the potential presence of heteroatoms and even the absolute difference in substrate diameter make experience with continuous fibers poor predictors of nanofiber chemistry.
Carbon nanotubes are typically hollow graphite tubules having a diameter of generally several to several tens nanometers. Carbon nanotubes exist in many forms. The nanofibers can be in the form of discrete fibers or aggregate particles of nanofibers. The former results in a structure having fairly uniform properties. The latter results in a structure having two-tiered architecture comprising an overall macrostructure comprising aggregate particles of nanofibers bonded together to form the porous mass and a microstructure of intertwined nanofibers within the individual aggregate particles. For instance, one form of carbon fibrils are characterized by a substantially constant diameter, length greater than about 5 times the diameter, an ordered outer region of catalytically grown, multiple, substantially continuous layers of ordered carbon atoms having an outside diameter between about 3.5 and 70 nanometers, and a distinct inner core region. Each of the layers and the core are disposed substantially concentrically about the cylindrical axis of the fibril. The fibrils are substantially free of pyrolytically deposited thermal carbon with the diameter of the fibrils being equal to the outside diameter of the ordered outer region.
Moreover, a carbon nanotube suitable for use with the instant process defines a cylindrical carbon fibril characterized by a substantially constant diameter between 3.5 and about 70 nanometers, a length greater than about 5 times the diameter and less than about 5000 times the diameter, an outer region of multiple layers of ordered carbon atoms and a distinct inner core region, each of the layers and the core being disposed concentrically about the cylindrical axis of the fibril. Preferably the entire carbon nanotube is substantially free of thermal carbon overcoat. The term “cylindrical” is used herein in the broad geometrical sense, i.e., the surface traced by a straight line moving parallel to a fixed straight line and intersecting a curve. A circle or an ellipse are but two of the many possible curves of the cylinder. The inner core region of the nanotube may be hollow, or may comprise carbon atoms which are less ordered than the atoms of the outer region. “Ordered carbon atoms,” as the phrase is used herein means graphitic domains having their c-axes substantially perpendicular to the cylindrical axis of the nanotube. In one embodiment, the length of the nanotube is greater than about 20 times the diameter of the nanotube. In another embodiment, the nanotube diameter is between about 7 and about 25 nanometers. In another embodiment the inner core region has a diameter greater than about 2 nanometers.
Dispersing the nanotubes into organic and aqueous medium has been a serious challenge. The nanotubes tend to aggregate, form agglomerates, and separate from the dispersion.
Some industrial applications require a method of preparing a stable dispersion of a selected carbon nanotube in a liquid medium.
For instance, U.S. Pat. No. 5,523,006 by Strumban teaches the user of a surfactant and an oil medium; however, the particles are Cu—Ni—Sn—Zn alloy particles with the size from 0.01 micron and the suspension is stable for a limited period of time of approximately 30 days. Moreover, the surfactants don't include the dispersants typically utilized in the lubricant industry.
U.S. Pat. No. 5,560,898 by Uchida et al. teaches that a liquid medium is an aqueous medium containing a surfactant; however, the stability of the suspension is of little consequence in that the liquid is centrifuged upon suspension.
U.S. Pat. No. 5,853,877 by Shibuta teaches dispersing disentangled nanotubes in a polar solvent and forming a coating composition with additives such as dispersing agents; however, a method of obtaining a stable dispersion is not taught.
U.S. Pat. No. 6,099,965 by Tennent et al. utilizes a kneader teaching mixing a dispersant with other reactants in a liquid medium using a high-torque dispersing tool, yet sustaining the stability of the dispersion does not appear to be taught nor suggested.
None of the conventional methods taught provide a process for dispersing and maintaining nanotubes in suspension as described and claimed in the instant invention as follows.
SUMMARY O
Lockwood Frances E.
Zhang Zhiqiang
Ashland Inc.
Carrithers David W.
Carrithers Law Office PLLC
Hendrickson Stuart L.
Lish Peter J
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