Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Processes of preparing a desired or intentional composition...
Reexamination Certificate
1999-02-12
2001-07-24
Cain, Edward J. (Department: 1714)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Processes of preparing a desired or intentional composition...
Reexamination Certificate
active
06265466
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates generally to electromagnetic (EM) radiation absorbing composites containing nanotubes.
The need for electromagnetic shielding materials is enormous. Applications of EM shielding material are found in, for example, EM-sensitive electronic equipment, stealth vehicles, aircraft, etc., having low radar profiles, protection of electronic components from interference from one another on circuit boards, protection of computer equipment from emitting RF radiation causing interference to navigation systems, medical life support systems, etc. Metal shielding has long been known for these functions. However, with the replacement of metals by a wide variety of new materials, e.g. polymeric, there has been a loss of the metals' inherent EM shielding characteristics. Some attempts at improving the EM shielding characteristics of plastics have been made. However, these approaches suffer from substantial drawbacks. Thus, new and improved methods and materials are needed to effect the desired shielding.
SUMMARY OF THE INVENTION
This invention represents a new approach to electromagnetic shielding. It is not derived from conventional concepts related to conductivity-based approaches. It has been discovered that conductivity is not required for the composite of this invention to provide very effective EM shielding. The latter term has its conventional meaning herein. In fact, composites having essentially no or low bulk conductivity, i.e., conventionally being classifiable as insulators, have excellent EM shielding properties. Without being bound by theory, it is believed that in composites of this invention which have such low bulk conductivity, EM shielding is achieved through absorption of radiation rather than reflection. By “low bulk conductivity” in this context is meant general macroscopic low conductivity, but it also includes anisotropically low conductivity in at least one dimension, e.g., in a sheet-type composite, low conductivity across the plane (thickness) of the sheet and not necessarily across the length or width of the sheet. Thus, both isotropic and anisotropic low or essentially no bulk conductivity (e.g., insulating properties) are included. Such low conductivities can be achieved for example by not including processing steps which would enhance isotropic or random electrical contact among the nanotubes.
In another preferred embodiment of this invention, the nanotubes do not substantially increase the bulk conductivity (as discussed above) of the polymer which forms the base of the composite. Thus, polymers which are conventionally classified as insulators remain insulators. In one embodiment the nanotubes are primarily not in isotropic contact with each other and for nanotubes which are in contact with each other, e.g., in general alignment along the nanotubes' longitudinal axes, they are not bonded or glued to each other (other than by virtue of being copresent in the base polymer formulation). For example, when the composites are subjected to a shearing treatment as described herein, the nanotubes become aligned and/or disentangled as a result of which the EM shielding properties of the composites are enhanced or optimized. Without wishing to be bound by theory, it is believed that such alignment or disentanglement increases the effective aspect ratio of the nanotubes collectively. For instance, in disentangling and/or alignment of the nanotubes, some of the nanotubes become in contact with each other more or less along the their longitudinal axes whereby they act effectively as a single nanotube having a length in such direction longer than that of either of two individual contacting nanotubes. Typically, the effective aspect ratios will be at least about 100:1, 500:1, 1000:1 etc. or greater.
In an especially preferred aspect of this invention, the composite will have both high EM shielding properties and also low radar profile due to the high absorptiveness of the composites and correspondingly low reflectance to electromagnetic radiation.
Thus, in one aspect, this invention relates to an electromagnetic (EM) shielding composite comprising a polymer and an amount of nanotubes effective for EM shielding, e.g., of RF and microwave and radiation of higher frequencies.
In a further aspect, this invention relates to an electromagnetic (EM) shielding composite comprising a polymer and an amount of substantially aligned nanotubes effective for EM shielding.
In a further aspect, this invention relates to an EM shielding composite comprising a polymer and an amount of nanotubes effective for EM shielding, wherein said composite has been subjected to shearing, stretching and/or elongation, which aligns and/or disentangles nanotubes contained therein.
In a further aspect, this invention relates to a method for preparing an EM shielding composite comprising a polymer and an amount of nanotubes effective for electromagnetic shielding comprising formulating said polymer and nanotubes and shearing, stretching, or elongating the composite.
In a further aspect, this invention relates to an electromagnetic shielding composite, e.g., energy absorbing composite, comprising a non-carbonizable polymer and nanotubes in an amount effective for EM shielding, e.g., energy absorption. This invention does not require carbonization to induce EM shielding properties.
In a further aspect, this invention relates to an EM shielding composite comprising an inner space and a surface defining said space, the improvement wherein said surface comprises a layer of nanotubes according to the invention effective for EM shielding.
In a further aspect, this invention relates to a method of lowering the radar observability of an object comprising partially or entirely surrounding said object with a layer of nanotubes according to the invention effective for lessening radar observability.
In a further aspect, this invention relates to a method of electromagnetic (EM) shielding an object or space comprising partially or entirely surrounding said object or space with a layer of composite of this invention.
In a further aspect, this invention relates to an electromagnetic shielding composite, comprising nanotubes mixed in a polymer, wherein the composite is absorptive and effective for shielding broadband electromagnetic radiation, e.g., in a range of 10
3
Hz to 10
17
Hz.
In a further aspect, this invention relates to an electromagnetic radiation absorbing composite, comprising nanotubes mixed in a polymer, wherein the composite is absorptive, e.g., to RF and microwave radiation and higher frequencies in dependence also on the properties of the base polymer, and, thus, effective for shielding from broadband electromagnetic radiation, e.g., in a range of 10
3
Hz to 10
17
Hz, and for generating heat.
The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying examples, in which reference characters refer to the same parts throughout the various views.
Primary components of the electromagnetic shielding composites of this invention are the base polymeric material and the nanotubes.
Suitable raw material nanotubes are known. The term “nanotube” has its conventional meaning as described; see R. Saito, G. Dresselhaus, M. S. Dresselhaus, “Physical Properties of Carbon Nanotubes,” Imperial College Press, London U.K. 1998, or A. Zettl “Non-Carbon Nanotubes”
Advanced Materials
, 8, p. 443 (1996). Nanotubes useful in this invention, include, e.g., straight and bent multi-wall nanotubes, straight and bent single wall nanotubes, and various compositions of these nanotube forms and common by-products contained in nanotube preparations. Nanotubes of different aspect ratios, i.e. length-to-diameter ratios, will also be useful in this invention, as well as nanotubes of various chemical compositions, including but not limited to carbon, boron nitride, SiC, and other materials capable of forming nanotubes. Typical but non-limiting
Conroy Jeffrey L.
Glatkowski Paul
Mack Patrick
Piche Joseph W.
Winsor Paul
Brobeck Phleger & Harrison LLP
Cain Edward J.
Eikos, Inc.
Wyrozebski Katarzyna
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