U.S. patent application number 12/403608 was filed with the patent office on 2009-08-13 for microwave-attenuating composite materials, methods for preparing the same, intermediates for preparing the same, devices containing the same, methods of preparing such a device, and methods of attentuating microwaves.
This patent application is currently assigned to Science Applications International Corporation. Invention is credited to Robert F. Brady, JR., Bor-Sen Chiou, Walter J. Dressick, Dana Leamann, Ann Mera, Ronald R. Price, Joel M. Schnur, Paul E. SCHOEN, Daniel Zabetakis.
Application Number | 20090202719 12/403608 |
Document ID | / |
Family ID | 36943082 |
Filed Date | 2009-08-13 |
United States Patent
Application |
20090202719 |
Kind Code |
A1 |
SCHOEN; Paul E. ; et
al. |
August 13, 2009 |
MICROWAVE-ATTENUATING COMPOSITE MATERIALS, METHODS FOR PREPARING
THE SAME, INTERMEDIATES FOR PREPARING THE SAME, DEVICES CONTAINING
THE SAME, METHODS OF PREPARING SUCH A DEVICE, AND METHODS OF
ATTENTUATING MICROWAVES
Abstract
The present invention provides microwave attenuating, filled
composite materials which contain a polymer or ceramic matrix and
metallic tubules and processes for making the same and devices
which contain such materials.
Inventors: |
SCHOEN; Paul E.;
(Alexandria, VA) ; Price; Ronald R.;
(Stevensville, MD) ; Schnur; Joel M.; (Burke,
VA) ; Zabetakis; Daniel; (Brandywine, MD) ;
Brady, JR.; Robert F.; (Gaithersburg, MD) ; Mera;
Ann; (Huntingtown, MD) ; Leamann; Dana;
(Frederick, MD) ; Chiou; Bor-Sen; (Pinole, CA)
; Dressick; Walter J.; (Fort Washington, MD) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
Science Applications International
Corporation
San Diego
CA
Department of the Navy
Arlington
VA
|
Family ID: |
36943082 |
Appl. No.: |
12/403608 |
Filed: |
March 13, 2009 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11223263 |
Sep 12, 2005 |
7525497 |
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12403608 |
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10353952 |
Jan 30, 2003 |
7125476 |
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11223263 |
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Current U.S.
Class: |
427/241 ;
427/304; 427/305 |
Current CPC
Class: |
C08G 59/226 20130101;
Y10T 428/269 20150115; C08G 81/00 20130101; C08G 18/12 20130101;
C08L 21/00 20130101; C08L 63/00 20130101; Y10T 428/249924 20150401;
C08G 18/12 20130101; C08G 18/48 20130101; C08L 63/00 20130101; C08L
2666/22 20130101 |
Class at
Publication: |
427/241 ;
427/304; 427/305 |
International
Class: |
B05D 3/10 20060101
B05D003/10; B05D 3/12 20060101 B05D003/12 |
Claims
1.-35. (canceled)
36. A process for producing a metallic halloysite microtubule
comprising, (a) suspending halloysite clay tubules in an aqueous
palladium catalyst solution or dispersion at a pH near neutral,
wherein said palladium catalyst is selected from the group
consisting of PD1A and PD1B, to obtain a suspension; (b)
centrifuging or filtering the suspension; and (c) electroless
coating the halloysite clay tubules.
37. The process according to claim 36, wherein said PD1A is added
in the form of a solution or dispersion which comprises 3.5 to 3.75
mM [Pd(II)], 120 to 130 mM [Cl.sup.-], and 15 to 20 mM [MES] at a
pH of 5.
38. The process according to claim 37, wherein said solution or
dispersion comprises about 3.67 mM [Pd(II)], about 127 mM
[Cl.sup.-], and about 18 mM [MES] at a pH of 5.
39. The process according to claim 36, wherein said PD1B is added
in the form of a solution or dispersion which comprises 3.5 to 3.75
mM [Pd(II)], 115 to 120 mM [Cl.sup.-], and 15 to 20 mM [MES] at a
pH of 5.
40. The process according to claim 39, wherein said solution or
dispersion comprises 3.67 mM [Pd(II)], 118 mM [Cl.sup.-], and 18 mM
[MES] at a pH of 5.
41. The process according to claim 36, wherein said electroless
coating comprises adding a formulation comprising a metal selected
from the group consisting of copper, nickel, cobalt, iron, and a
permalloy metal.
42.-45. (canceled)
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to microwave-attenuating
composite materials. The present invention also relates to methods
for preparing such microwave-attenuating composite materials. The
present invention also relates to intermediates useful for
preparing such microwave-attenuating composite materials. The
present invention further relates to devices containing such
microwave-attenuating composite materials and methods for preparing
such devices. The present invention additionally relates to methods
for attenuating microwaves. More specifically, the present
invention relates to methods for absorbing microwave radiation, for
signal attenuation at appropriate wavelengths, and for isolating
transmitting antennas and receiver components in radar sets.
[0003] 2. Discussion of the Background:
[0004] Electronic devices are becoming increasingly compact and
complex. At microwave frequencies, cross-talk and ringing are
becoming serious problems that need to be addressed. In particular,
devices such as cellular telephones, pagers, and palm- or lap-size
computers contain many circuits that require isolation from each
other for maximum performance. In addition, military platforms such
as ships and aircraft operate a variety of radio and radar systems
that may interfere with each other, thus requiring isolation
measures. These systems may consist of passive devices, active
devices, or mixtures thereof. The number, sophistication, and cost
of these systems continue to rise, while the demand for improved
performance moves forward at an even a faster pace.
[0005] In order to prevent cross-talk, it is necessary to isolate
the components of electronic devices. This may be accomplished by
placing a material of defined and controllable shape, which absorbs
interfering frequencies of radiation, between the components.
However, the gap where the isolating material is placed may be
small, complex in shape, and hard to reach. In addition, the
isolating material is preferably dimensionally stable, compact and
lightweight, chemically inert and mechanically robust, and
unaffected by its operating environment.
[0006] At present, numerous microwave-absorbing composite materials
are made from organic matrix resins and carbonyl iron powder.
Typically, the carbonyl iron powder has a density near 7.9
g/cm.sup.3, and the composite has a density near 5 g/cm.sup.3. The
high density of the conventional microwave-absorbing materials
presents the serious drawback of imparting a dramatic increase in
the weight of any device incorporating such a material, which is
especially undesirable in portable, or hand-held devices, such as
mobile telephones, lap-top computers, or aircraft.
[0007] Moreover, conventional microwave-attenuating materials are
not as effective as desired. In this regard, it is noted that on a
commercial or industrial scale even small improvements in absorbing
microwave radiation, signal attenuation, and isolation of signal
devices are economically significant.
[0008] Therefore, there remains a critical need for
microwave-attenuating composite materials, which do not suffer from
this drawback. There also remains a need for methods for preparing
such microwave-attenuating composite materials and intermediates
useful for preparing such microwave-attenuating composite
materials. In addition, there remains a need for devices, which
contain such a microwave-attenuating composite material, and
methods for preparing such devices. There also remains a need for
improved methods of attenuating microwaves.
SUMMARY OF THE INVENTION
[0009] Accordingly, it is one object of the present invention to
provide novel composite materials, which are useful for attenuating
microwaves.
[0010] It is another object of the present invention to provide
novel methods for preparing such composite materials.
[0011] It is another object of the present invention to provide
novel intermediates which are useful for preparing such composite
materials.
[0012] It is another object of the present invention to provide
novel devices, which contain such a composite material.
[0013] It is another object of the present invention to provide
novel methods for preparing such devices.
[0014] It is another object of the present invention to provide
novel methods for attenuating microwaves.
[0015] These and other objects, which will become apparent during
the following detailed description, have been achieved by the
inventors' discovery that composite materials, which comprise:
[0016] (a) a polymer or ceramic matrix; and
[0017] (b) a plurality of metal microtubules dispersed within said
matrix, are effective for the attenuation of microwaves.
[0018] The inventors have also discovered that such composite
materials may be prepared by a method comprising:
[0019] (1) incorporating a plurality of metal microtubules in a
polymer or ceramic matrix.
[0020] The inventors have also found that such composite materials
may be prepared from an intermediate, which comprises: [0021] (a) a
polymer matrix precursor or a ceramic matrix precursor; and [0022]
(b) a plurality of metal microtubules dispersed within said polymer
matrix precursor or said ceramic matrix precursor.
[0023] The inventors have also discovered that electronic devices,
which comprise a microwave-attenuating composite, said composite
comprising
[0024] (a) a polymer or ceramic matrix; and
[0025] (b) a plurality of metal microtubules dispersed within said
matrix, exhibit a reduction in the problems associated with or
caused by ineffective attenuation of microwaves.
[0026] The inventors have additionally found that such devices may
be prepared by a process, which comprises: [0027] (1) incorporating
a microwave-attenuating composite material in an electronic device,
wherein said microwave-attenuating composite material
comprises:
[0028] (a) a polymer or ceramic matrix; and
[0029] (b) a plurality of metal microtubules dispersed within said
matrix.
[0030] The inventors have also found that microwaves may be
effectively attenuated between a source point and a detection point
by a method, which comprises:
[0031] (1) placing a microwave-attenuating composite material
between said source point and said detection point, wherein said
microwave-attenuating composite material comprises:
[0032] (a) a polymer or ceramic matrix; and
[0033] (b) a plurality of metal microtubules dispersed within said
matrix.
[0034] As will be recognized from the following detailed
description, the flexibility, density, frequency response, and
environmental stability of the present microwave-attenuating
composite materials may be varied as desired over wide ranges.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0035] Thus, in the first embodiment, the present invention
provides composite materials, which comprise:
[0036] (a) a polymer or ceramic matrix; and
[0037] (b) a plurality of metal microtubules dispersed within said
matrix, which are effective for the attenuation of microwaves.
[0038] In the context of the present invention, the term
"microwaves" refers to electromagnetic radiation having a frequency
of 100 MHz to 200 GHz.
[0039] In the case of a polymer matrix, the matrix may be formed of
any suitable polymer material. A wide variety of polymers may be
used as the matrix resin in this invention. These include but are
not limited to vinyl, styrene-butadiene, natural rubber, nitrile
rubber, and acrylic resins, in addition to a polyurethane, an
epoxy, or a siloxane. These polymers may be used singly or in
combination; when used in combination the amount of each may be
adjusted to optimize the performance of the composite. In general,
the polymer matrix resin may be obtained by premixing the base
component, followed by the addition of a curing agent. Preferred
curing agents include diisocyanate prepolymers containing a
plasticizer, polyoxypropylenediamine, and copolymers of
methylhydrosilane and dimethylsiloxane. In this case, none of the
ingredients of the resin are volatile, and shrinkage during curing
is less than 0.1%. Although, each of the resins cures at room
temperature, heating the liquid mixture to 60.degree. C. may
accelerate curing.
[0040] Especially preferred elastomeric polymers include epoxy
resins and polyurethanes.
[0041] In the case of a ceramic mixture, the matrix may be likewise
any conventional ceramic, provided that the ceramic matrix may be
formed from a mixture of ceramic matrix precursor(s) and metal
microtubules under conditions which do not destroy the desired
attenuation. Suitable ceramics are described in Kirk-Othmer,
Encyclopedia of Chemical Technology, 4.sup.th Ed., Wiley
Interscience, NY, vol. 5, pp. 599-728 (1993), which is incorporated
herein by reference. Ceramics which are prepared from a sol/gel
precursor(s) are particularly preferred.
[0042] The term metal microtubule refers to any metal or metallized
cylinder having a length of 2 to 200 micrometers (.mu.m),
preferably 5 to 100 .mu.m, more preferably 10 to 30 .mu.m; an
outside diameter of 0.5 to 3.0 .mu.m, preferably 0.5 to 2.0 .mu.m,
more preferably 1.0 to 2.0 .mu.m; an inside diameter of 0 to 0.5
.mu.m, preferably 0.01 to 0.5 .mu.m, more preferably 0 to 0.4
.mu.m, even more preferably 0.01 to 0.4 .mu.m, still more
preferably 0 to 0.3 .mu.m, even more preferred 0.01 to 0.3 .mu.m.
Preferably, the metal microtubule is electrically conductive and
resistant to oxidation. In this case, the use of microtubules with
a Ni overcoat to protect against oxidation is preferred.
[0043] The preparation of such metal or metallized microtubules is
described in detail in U.S. Pat. Nos. 4,911,981; 5,049,382;
5,342,737; 5,814,414; and 6,280,759, all of which are incorporated
herein in their entirety by reference.
[0044] Two types of microtubules, lipid and halloysite, are
especially preferred for use in the present invention. Lipid
microtubules self-assemble from water-alcohol solutions of
biologically-derived diacetylenic lipids and precipitate at the
appropriate temperature and concentration. Electroless
metallization processes deposit an electrically conductive coating
of copper, nickel, iron, other electroless metals, or combinations
thereof upon the lipid microtubules. The lipid may then be
withdrawn from the center of the microtubule and reused. The
resulting metallized microtubules have high aspect ratios (20-200),
low density, and impart appreciable permittivity when used in
organic matrix resins. These materials are described more fully in
U.S. Pat. No. 6,013,206, which is incorporated herein by reference
in its entirety.
[0045] A further method to form the electroless-plated metallic
microtubules is to process the lipid microtubules in such a manner
to yield a suspension of the lipid at a concentration of about 2.5
mg/ml in a mixed solvent consisting of about 70% of an alcoholic
mixture and about 30% distilled water, wherein the alcoholic
mixture consists of about 20 to 80% methanol and about 20 to 80%
ethanol. Once formed, the microtubules are subjected to a polyanion
mixture, such as polystyrenesulfonate, at a dilution of 0.1 to 1.0
mg/ml in distilled water.
[0046] The polyanion is allowed to bind to the lipid microtubules
for 15 to 45 minutes. After the incubation period, the mixture is
diluted with an equivalent volume of a polycation mixture, such as
polyethylenimine or a chitosan, at a dilution of 0.5 to 1.5 mg/ml.
It may be advantageous to cross-link the polycationic layer to
promote stability by the use of gluteraldehyde, or another suitable
crosslinking agent, at concentrations sufficient to cross-link the
polyion layers.
[0047] In order to prevent the accumulation of an excess amount of
polyelectrolyte and therefore excess bundling and bridging of the
microtubules it is necessary to use a slightly deficit amount of
the polyion so that the polyion in solution will be 100% bound to
the tubule surface and not found in excess in the supernatant
solution. One approach to calculate the amount of polyion to be
added is to determine the total surface charge available for
binding by titrating the tubule surface with a charged dye complex
such as Toluidine Blue O for the determination of the net negative
charge available or the use of a dye such as FastUSOL Red 50L for
polyanions. In order to facilitate the titration, a known quantity
of the tubule suspension or dispersion is titrated with a known
dilution of the dye stuffs such that the dye is added to the
tubule/polyion suspension and allowed to bind for 15 minutes. Then
the tubules may be spun out of solution at 2000 g and the
supernatant is analyzed by visible spectrophotometry for the
presence of unbound dye in solution. The titration is complete when
the dye is found to remain in solution following the binding
period. The amount of dye added is calculated and the amount of
charge determined. Then the appropriate amount of the oppositely
charged polyion is added to the solution and allowed to bind, and
the process is repeated until the required number of layers is
obtained.
[0048] The microtubule suspension is then allowed to settle by
gravity or may be filtered to remove the excess polyion solution
and the excess alcohol. However, this step is not necessary, if the
concentration of alcohol at this point is insufficient to
precipitate the metal salts. The microtubules are then resuspended
in distilled water to the original volume used for microtubule
production. The final concentration of lipid should be 2.0 to 2.5
mg/ml. A catalytic mixture of palladium salts is added at a volume
ratio of 1:500 of catalyst solution or dispersion to microtubule
suspension. The catalyst is allowed to bind to the polyion
complexes for a minimum of 30 minutes followed by immediate
plating.
[0049] The resultant catalyzed microtubules may be electroless
coated with simple formulations of copper, nickel, cobalt or iron
or permalloy metal to a thickness that is sufficient to result in a
conductive coating. Especially preferred are copper-coated,
nickel-coated, cobalt-coated, and iron-coated lipid tubules, nickel
or cobalt over copper-coated lipid tubules, and iron and nickel
coatings over lipid tubules.
[0050] The methods of electroless coating are described more fully
in U.S. Pat. No. 5,089,742, which is incorporated herein by
reference in its entirety.
[0051] The present composite materials typically comprise the
polymer or ceramic matrix in an amount of 40 to 99% by weight,
preferably 48 to 99% by weight, more preferably 50 to 90% by
weight, based on the total weight of the composite material. The
tubules are suitably present in an amount of 1 to 60% by weight,
preferably 1 to 52% by weight, more preferably 10 to 50% by weight,
based on the total weight of the composite material.
[0052] Other types of materials which may be present in the
composite material include stabilizers (e.g., light absorbers such
as carbon black and/or UV absorbers), plasticizers, fungicides,
bactericides. Such ingredients may be present in any amount such
that the desired level of attenuation is retained.
[0053] The composite material may further contain one or more
magnetic powdered metals and/or metal oxides, such as iron powder
and/or ferrite powder.
[0054] It is also to be understood that the present composite
material can be in the form of any suitable shape. Thus, when the
composite material is used on the inside of a device, it may be
preferred that the composite material have a shape which conforms
to the open space(s) or interstice(s) between components within the
device or the space(s) or interstice(s) between one or more
components in the device and all or part of a housing or casing of
the device.
[0055] The present composite materials may also be applied to the
outside of a device. In this case, it may be preferred that the
composite material be flexible so that it will conform to the
outside of the device. If desired, the rigidity or flexibility of
the composite material can be modified using conventional
techniques to fit the application. In this case, it may also be
preferred to initially form the composite material in the form of a
flexible sheet of arbitrary length and width, which may be cut to
shape at the time of application. In this type of application, the
composite material will typically have a thickness of 0.025 to 2.5
millimeters (mm), preferably 0.25 to 2.0 mm, more preferably 0.5 to
1.5 mm.
[0056] When the composite material is to be applied to the outside
of a device, it may be preferred to incorporate the composite
material in a laminated sheet, in which either one or both of the
major surfaces of a sheet of the composite material is coated with
a sheet or film of a different material. Materials suitable for
coating such a sheet of the composite material include UV-cured
acrylates and UV-cured thiol-enes. It may be preferred that the
composite material take the form of a laminate which contains two
or more layers which contain the metal microtubules and that the
microtubules are predominately aligned in distinct directions in at
least two of the layers. Alignment of the microtubules in a layer
may be carried out by flow alignment during the formation process
for non magnetic metals or by forming the matrix sheet or layer
while in a magnetic field of known orientation followed by
crosslinking of the polymer matrix to lock the orientation in
place.
[0057] It may also be preferred to provide at least one side of
such a sheet of the composite material or such a laminated sheet
comprising the composite material with an adhesive or other
fastening means, such as velcro.
[0058] In an especially preferred embodiment, the composite
material forms a conformal coating which encapsulates and isolates
components on a printed circuit board. According to this object,
external microwave radiation that otherwise would be emitted or
absorbed by the electronic components or conductive traces on the
board would be contained or eliminated.
[0059] In another preferred embodiment, the composite material
comprises or consists essentially of (a) a polyvinylbutryl or epoxy
matrix; and (b) lipid or halloysite microcylinders which have been
coated with a coating of nickel, cobalt, or an alloy of nickel,
iron and boron (permalloy) by electroless deposition and in which
the microcylinders are loaded into the matrix in an amount of 0.5
to 15% by weight, based in the total weight of the matrix and the
microcylinders.
[0060] In another embodiment, the present invention provides a
method for making such composite materials, which comprises:
[0061] (1) incorporating a plurality of metal microtubules in a
polymer or ceramic matrix.
[0062] In this embodiment, the polymers, the ceramics, the
microtubules, other components, relative amounts of components, and
the shape of the composite material are as described above.
[0063] The exact means for effecting the step of incorporating the
metal microtubules in the polymer or ceramic matrix will depend at
least in part on the type of polymer or ceramic which forms the
matrix. For example, if the polymer matrix is formed of a
thermoplastic polymer, then the metal microtubules may be
incorporated by first forming a melt of the thermoplastic polymer
and then incorporating the metal microtubules into the melt. The
microtubule-containing melt may then be cooled to afford the
composite material. Any suitable apparatus for forming the melt and
for incorporating the metal microtubules into the melt may be used.
Of course, it is to be understood that the cooling step may be
carried out in such a way to afford the composite material in the
form of a desired shape. For example, the melt may be extruded or
cast into a mold or extruded as a sheet.
[0064] It is preferred that the viscosity of the melt be maintained
at a sufficiently high value to prevent or reduce any settling of
the microtubule while cooling in the melt. The viscosity of the
thermoplastic melt may be controlled by any conventional method,
such as controlling the temperature of the melt and/or controlling
the amount of plasticizer, if any, added to the melt.
[0065] When the polymer or ceramic matrix is formed by curing
polymer precursor(s) or ceramic precursor(s), then the metal
microtubules may be conveniently incorporated into the polymer or
ceramic matrix by simply adding the metal microtubules to the
polymer or ceramic precursor(s) prior to completion of the curing.
Curing of the microtubule-containing mixture then affords the
composite material. Once again, this step may be carried out to
afford the composite material in the form of a desired shape. For
example, the precursor components and the metal microtubules may be
cured in a mold of the desired shape. Alternatively, the final or
desired shape may be obtained by cutting, sanding, or machining the
composite material.
[0066] In one preferred embodiment, the polymer matrix is a resin
which is comprised of two liquid components, a base component and a
curing agent. The components of the matrix resin are mixed
immediately before use and will eventually harden to form a solid
matrix. A liquid mixture of tubules and freshly-mixed resin may be
placed within a mold and cured. By this method, a dry, flexible
matrix of defined dimensions may be formed. The composite is
subsequently removed from the mold and sanded to the desired size
and shape or used as is. Alternatively, the liquid mixture may be
poured, or pulled by vacuum forces, directly into an electronic
device where it fills the cavities of the device. Accordingly, the
mixture then assumes the shape of the internal voids as it cures.
In either embodiment, the mixture cures in place without
appreciable shrinkage.
[0067] In another embodiment, the present invention provides
intermediates, which are useful for forming the present composite
materials and which comprise: [0068] (a) a polymer matrix precursor
or ceramic matrix precursor; and [0069] (b) a plurality of metal
microtubules dispersed within said polymer matrix precursor or
ceramic matrix precursor.
[0070] In this embodiment, the present intermediates, the polymer
precursor(s), the ceramic precursor(s), the microtubules, other
components, relative amounts of components, and the shape of the
composite material are as described above. Thus, when the matrix is
a polymer resin, the polymer matrix precursor may be the uncured
resin. When the matrix is a ceramic, the ceramic matrix precursor
may be the uncured ceramic.
[0071] In another embodiment, the present invention provides
electronic devices, which comprise a microwave-attenuating
composite, said composite comprising
[0072] (a) a polymer or ceramic matrix; and
[0073] (b) a plurality of metal microtubules dispersed within said
polymer or ceramic matrix.
[0074] In the present devices, the polymers, the ceramics, the
microtubules, other components, relative amounts of components, and
the shape of the composite material are as described above. The
present devices may be any in which it is desired to attenuate
microwaves originating from the device itself or external to the
device. Examples of such devices include antennas, repeaters,
amplifiers, circuit boards, platform fuselages, wing tips,
removable panels, multichip modules, antennas for cell phones,
microwave ovens, NMRs (shielding), computers (shielding), etc.
[0075] In one preferred embodiment, the composite containing the
metal microtubules takes the form of an article of clothing, which
may be connected to an electronic device, such as a cellular
telephone. In this case, the article of clothing can be used as an
inconspicuous antenna for the electronic device. In this
embodiment, the metal microtubules may be contained in either
electrospun polymeric fibers or in a sheet of highly flexible
polyurethane in an amount of between 1% and 50% by weight to
provide a personal communications antenna for transmission over a
range of frequencies when added to clothing to form a flexible
antenna for wear.
[0076] When it is desired to shield components within the device
from one another, then it will be preferred to place the composite
material inside the device and between those components. When it is
desired to protect the environment (including the user(s) and other
devices) from radiation emitted from the device, then it will be
preferred to fully or partially surround the components of the
device with the composite material. In this case, the composite
material may be placed inside and/or outside any housing or casing.
When it is desired to protect the device from radiation originating
from the environment (including ambient radiation and/or other
devices), then it will be preferred to fully or partially surround
the components of the device with the composite material. In this
case as well, the composite material may be placed inside and/or
outside any housing or casing.
[0077] In a particularly preferred embodiment, the device contains
a printed circuit board, and the composite material is a
non-conductive polymer matrix with a suitable loading of metal
microtubules (1 to 20% by weight) which forms a conformal coating.
This conformal coating encapsulates and isolates the components on
the printed circuit board. In this embodiment, external microwave
radiation that otherwise would be emitted or absorbed by the
electronic components or conductive traces on the board is
contained or eliminated.
[0078] In another embodiment, the present invention provides a
method for preparing such devices by: [0079] (1) incorporating a
microwave-attenuating composite material in an electronic device,
wherein said microwave-attenuating composite material comprises:
[0080] (a) a polymer or ceramic matrix; and [0081] (b) a plurality
of metal microtubules dispersed within said polymer or ceramic
matrix.
[0082] In this method, the polymers, the ceramics, the
microtubules, other components, relative amounts of components, the
shape of the composite material, and devices are as described
above.
[0083] In a particularly preferred embodiment, the device contains
a printed circuit board, and the matrix precursor is a liquid
mixture of a non-conductive polymer matrix with a suitable loading
of microtubules (1 to 50% by weight) which is used to form a
conformal coating. This conformal coating encapsulates and isolates
components on the printed circuit board. In this embodiment,
external microwave radiation that otherwise would be emitted or
absorbed by the electronic components or conductive traces on the
board is contained or eliminated.
[0084] In another embodiment, the present invention provides a
method for attenuating microwaves between a source point and a
detection point by:
[0085] (1) placing a microwave-attenuating composite material
between said source point and said detection point, wherein said
microwave-attenuating composite material comprises:
[0086] (a) a polymer or ceramic matrix; and
[0087] (b) a plurality of metal microtubules dispersed within said
polymer or ceramic matrix.
[0088] In this method, the polymers, the ceramics, the
microtubules, other components, relative amounts of components, and
shapes of the composite may be as described above. Preferably, the
method involves the use of a sufficient thickness or amount of the
present composite material to achieve a 10 dB (i.e., 90%) or higher
attenuation of the radiation.
[0089] The metal tubule-filled composite materials described in
this disclosure effectively attenuate or absorb a broad spectrum of
the radio frequency spectrum and microwave radiation. The materials
are low in density, effective at a broad range of frequencies even
when very thin, able to be cast or molded into a variety of shapes
and sizes, and stable at temperatures between -20.degree. C. and
120.degree. C. and at any relative humidity.
[0090] These materials are suitable for use in electronic equipment
on military platforms because of their lightweight, high
performance, and durability in the operating environment. Weight
savings translate directly into decreased fuel consumption and
extended duty cycles.
[0091] Potential uses extend beyond commercial applications to many
military applications. With the technological advances in
electronics, more and more equipment and appliances are emitting
electromagnetic radiation. The Federal Communications Commission
has established rules and regulations to control and enforce limits
on electromagnetic interference (EMI) and radio frequency
interference (RFI). A microwave oven is an example of a household
device which may leak microwaves and which will benefit from the
use of lightweight, thin microwave absorbing materials. Such
materials may also be used in the EMI filters and low frequency
chokes of input and output filters in switched mode power
supplies.
[0092] Prior to the present invention, it was not recognized that
composite materials with the aforementioned properties may be made
from an elastomeric polymer such as an epoxy, polyurethane or
siloxane, wherein the polymer may contain hollow cylinders of a
metal or metallic-coated substrates measuring 1 to 100 microns long
by 0.5 microns in diameter or may contain conductive particles
consisting of tubular morphologies and/or other complementary
geometric shapes.
[0093] One important feature of the use of the mixed components of
elastomeric resins (e.g., a polyvinylbutryl or epoxy matrix) to
prepare the composite according to the present invention is that
they have a sufficiently low viscosity, and the addition of up to
14 percent by weight of tubules produces a fluid mixture which may
be poured or aspirated with ease. Halloysite tubules offer the
ability to use far more energetic means of mixing the metallic
tubules and the resin without the problem of shear-induced aspect
ratio reduction.
[0094] A second important feature of the use of the mixed
components of the resins described above is that they contain no
volatile materials. Thus, there is no evaporation during curing and
the resulting composite closely resembles the shape of the
mold.
[0095] The mixed components may be poured into an open mold. Curing
the resin produces a solid, which conforms to the shape of the mold
surfaces but may be domed or dished on the open face.
[0096] The mixed components may be poured or forced under pressure
into a mold, which is then closed. Once cured, the resulting resin
is a solid, which conforms to the exact shape of the mold.
[0097] Alternatively, the mixed components may be sucked into or
forced under pressure into the internal cavities of a complex
device such as a cellular telephone. The resin surrounds and
encapsulates the internal parts and cures in place without
shrinkage.
[0098] The desired quantities of metal particles and matrix
material may be simply weighed out and combined in a beaker. In
order to lower viscosities and improve mixing, the matrix materials
may be warmed to 40-60.degree. C. By this method, the time
available before the matrix begins to cure is reduced. Accordingly,
this time must be taken into account (e.g., pot life for the
commercial urethanes is usually from 30 to 90 minutes). Antifoaming
agents may be added to the mixture to prevent the formation of
bubbles. Other additives may also be mixed in at this time to
improve either the mechanical or the electrical properties of the
composite. In addition, small amounts of additives may be added to
improve the ease of application, storage stability, air release,
tubule wetting, tubule flocculation, ease of processing, or other
properties of the composite.
[0099] Since the quantities of metal particles may be as high as
60% percent by weight, the resulting mixtures may have very high
viscosities. In this case, the mixing can be laborious and time
consuming, and also can cause breakage of the metallized tubules.
Therefore, it is preferred that the minimum amount of stirring with
the minimum amount of force be used to avoid breakage.
[0100] If lower amounts of metal particulate additives are used,
the mixture may have a relatively low viscosity, which may allow
the mixture to be readily poured into a mold. Therefore, a panel
may be formed in an open container, i.e., without a lid. In this
case, the mold must be carefully balanced to insure uniform
thickness.
[0101] When more heavily loaded mixtures with high viscosities are
utilized, pouring and self-leveling are not practical options.
Accordingly, the mixture may be spread with a spatula in a mold.
The mixture may then placed in a vacuum oven at about 40 to
60.degree. C. and pumped until most of the bubbles are removed
(about 5 minutes). Then the mold and contents may be covered with a
lid and allowed to cure at room temperature. Spacers at the edge of
the mold establish the thickness of the mixture, as it is
hardening. An antistick surface may used in the mold to allow easy
demolding of the cured composite.
[0102] The present composite is a mixture of electrically
conductive metal particles in an insulating matrix. Addition of
metal powder to a mixture eventually results in sufficient metal to
metal contacts to produce a conductive composite. This phenomenon
is described as percolation and constitutes a phase transition in
the material. The loading with metal, which corresponds to the
onset of percolation, is termed threshold loading. Along with the
onset of percolation there is also a sharp increase in the
permittivity of the composite. The "real" part of the dielectric
constant increases steadily with loading density, reaching a
maximum near the threshold loading. The "imaginary", or lossy, part
of the dielectric response increases slowly with loading, and near
the threshold jumps suddenly to large values, becoming larger than
the "real" part above the threshold.
[0103] Panels of the composite can be placed on the surface
separating two antennas, one a receiving and one a sending antenna.
The change in coupling between the antennas due to the tubule based
film between them is termed attenuation. Attenuation is a function
of the loading and the thickness of the composite. In order for the
energy which couples the two antennas to be absorbed it must travel
inside the composite, effectively being wave-guided by the
composite. There is a maximum wavelength (minimum frequency) that
is allowed to be guided by a layer; effectively equal to a half
wavelength in the material taking into account the index of
refraction of the composite. Thus, to accommodate long wavelengths,
the composite panel either must be physically thick or it must have
a high permittivity.
[0104] Panels made according to the present invention more
typically have a thickness of 1.1 to 1.4 mm with tubule loading
densities of about 15 volume percent and permittivities of about 30
to 50, that produced attenuation of antenna coupling of about 10
fold (10 dB) at 10 GHz.
[0105] Other features of the invention will become apparent in the
course of the following descriptions of exemplary embodiments,
which are given for illustration of the invention and are not
intended to be limiting thereof.
EXAMPLES
[0106] In the following examples, and throughout this
specification, all parts and percentages are by weight, and all
temperatures are in degrees Celsius, unless expressly stated to be
otherwise. Where the solids content of a dispersion or solution is
reported, it expresses the weight of solids based on the total
weight of the dispersion or solution, respectively. Where a
molecular weight is specified, it is the molecular weight range
ascribed to the product by the commercial supplier, which is
identified. Generally this is believed to be weight average
molecular weight.
1. Tubule Preparation.
[0107] In order to ensure the proper final concentration sufficient
for activation of the polyion reinforced lipid, the colloidal
Pd(II) catalyst dispersions suitable for use in the invention were
prepared in the following manner.
[0108] A stock 1.0 M aqueous solution of NaCl was first prepared by
dissolving 58.44 g NaCl (Fisher Scientific Cat. # S-271; M.W.=58.44
g/mole; Certified A.C.S. Reagent; Lot #865603) in approximately 750
mL deionized (DI) water (Nanopure II.RTM. Water Still; 17.6
M.OMEGA. resistivity). The solution was then diluted to 1 L with DI
water. An aqueous 0.1 M morpholinoethane sulfonate (MES), pH 5
buffer stock solution was separately prepared by dissolving 2.13 g
of morpholinoethane sulfonic acid (98%; Aldrich Chemical Cat.
#16,373-2; M.W.=213.26 g/mole; Lot # EF04410EF) in approximately 50
mL DI water and titrating dropwise with a freshly prepared 2 M
aqueous solution of NaOH (99.99%; Aldrich Chemical Cat. #30,657-6;
M.W.=40.00 g/mole; Lot #LG02424KG) to pH 5.0. The resulting
solution and aqueous washings were transferred to a 100-mL
volumetric flask and diluted to 100 mL with DI water to yield a
stock buffer solution.
[0109] For the preparation of the metallization catalyst, 60 mg of
Na.sub.2PdCl.sub.4.3H.sub.2O (98%; Aldrich Chemical Cat. #20,581-8;
M.W.=294.19 g/mole; Lot #TF13323 PF) was transferred to a clean 50
mL volumetric flask. A 1.5-mL aliquot of the 1.0 M NaCl (aq) stock
solution was added to the flask to completely dissolve the
Na.sub.2PdCl.sub.4.3H.sub.2O. A 10-mL portion of 0.1 M, pH 5 MES
(aq) stock buffer was immediately added to the flask and the
solution was diluted to the 50 mL mark with DI water. The contents
of the flask were mixed by inversion of the flask 25 times. The
flask was then incubated for 20 hours in a temperature-controlled
water bath at 23.+-.1.degree. C. Subsequent to incubation, a 5-mL
aliquot of the resulting straw-yellow colored dispersion was
removed by pipet and discarded. A 5-mL aliquot of the 1.0 M NaCl
(aq) stock solution was added to the contents of the flask by pipet
and the flask was mixed by inversion as described above to produce
the final metallization catalyst dispersion, hereinafter referred
to as PD1A.
[0110] The concentrations of Pd(II), chloride ion, and total MES in
the final pH 5 PD1A dispersion are: [Pd(II)] .about.3.67 mM;
[Cl.sup.-] .about.127 mM; [MES] .about.18 mM (approximate values).
This metallization catalyst exhibited a useful lifetime of
approximately 14 days at room temperature before destabilization
and bulk precipitation of Pd(II) salts occurred. This metallization
catalyst promoted essentially complete electroless metallization
(i.e., >95% coverage) on more than 90% of the lipid tubules
treated, as described below.
[0111] A preferred metallization catalyst for tubule metallization
may be produced with a slight modification of the above catalyst
formulation. The preferred metallization catalyst formulation,
hereinafter referred to as catalyst PD1B, was prepared exactly as
described for catalyst PD1A with one modification: the solid
Na.sub.2PdCl.sub.4.3H.sub.2O was initially dissolved using 1.0 mL,
rather than 1.5 mL, of 1.0 M NaCl (aq) stock solution. This
resulting metallization catalyst solution was a dark yellow-brown
to orange-brown color after preparation. The concentrations of
Pd(II), chloride ion, and total MES in the final pH 5 PD1B
dispersion are: [Pd(II)] .about.3.67 mM; [Cl.sup.-] .about.118 mM;
[MES] .about.18 mM (approximate values). Although the useful
lifetime of this metallization catalyst before destabilization and
bulk precipitation of the Pd(II) salts is reduced to approximately
10 days at room temperature, use of PD1B catalyst leads to
essentially complete electroless metallization (i.e., >95% metal
coverage) on more than 95% of the treated lipid tubules, as
described below.
[0112] The advantage of using these palladium metallization
catalysts (i.e., PD1A or PD1B) to catalyze the electroless
metallization of the lipid tubules over other methods using Pd/Sn
catalysts (see, e.g., U.S. Pat. No. 6,013,206) is that it (1)
eliminates the need to remove the alcoholic solvent mixture prior
to the electroless metalization step by dialysis, (2) eliminates
the need for acidic salts that tend to disrupt the structure of the
microtubules, and (3) eliminates the need to conduct an activation
step for the stannous palladium commercial catalyst system used in
previous methods as elucidated in U.S. Pat. No. 6,013,206, which is
incorporated herein by reference herein. Another advantage is that
the number of filtrations (which are typically used after each step
in the Pd/Sn catalysis method) is reduced. As few as one filtration
to remove the PD1A or PD1B metallization catalyst is required. This
is beneficial, because filtrations are the source of much of the
breakage of the tubules, which is detrimental to the intended
use.
[0113] In another embodiment, halloysite, a clay which occurs
naturally as hollow cylinders, may be metallized by first treating
the surface of the clay with an active metallization catalyst as
described above. In this case, the clay need not be withdrawn from
the center of the metallic tubule before use.
[0114] Washing and pin milling until a reduced particle size is
achieved may be carried out to first prepare the clay. It is
preferred that the clay be prepared from a deposit of halloysite
that is comprised of essentially all tubular materials. These
materials are blunged to reduce the particle size and washed with
Calgon salts to remove any exogenous allophate materials. This is
followed by the Pd(II) catalysis step outlined above, which entails
simple suspension of the clay at a near neutral pH followed by the
addition of the aqueous palladium metallization catalyst.
Subsequently, centrifugation or filtration may remove the excess
catalyst and the clay is resuspended in water. The process is
repeated until all unbound catalyst is removed. Following removal
of the catalyst, the halloysite is capable of being electroless
plated by the same process as the lipid microtubules. The
halloysite clay is available in a range of sizes up to an aspect
ratio of 40-50. Specific note may be made of the following: Copper
Coated, Nickel Coated, Cobalt Coated, or Iron Coated halloysite
tubules, Nickel or Cobalt over Copper coated halloysite tubules,
and Iron, Nickel and Boron alloy coatings over halloysite tubules.
The halloysite materials are described more fully in U.S. Pat. No.
5,651,976, which is incorporated herein by reference in its
entirety.
Elastomeric Matrix Resin Example 1
Elastomeric Polyurethane Matrix Resin
[0115] A polyether diol, a polyether triol, metal-plated tubules
and a curing catalyst are mixed and then cured with a diisocyanate
prepolymer containing a plasticizer. In a preferred embodiment, a
base component and a curing agent are made separately as
follows:
Base Component: Mix thoroughly the following:
[0116] Diol: 2.54 g, poly(propylene glycol) [C. A. 25322-69-4],
having a weight-average molecular weight of about 425, a viscosity
of 80 centistokes at room temperature, and a density of 1.004
g/mL.
[0117] Triol: 0.41 g, trimethylolpropane propoxylate [C. A.
25723-16-4], having a molecular weight of 308 and a density of
1.040 g/mL.
[0118] Curing Catalyst: 0.03 g, dibutyltin dilaurate [C. A.
77-58-7], having a molecular weight of 631 and a density of 1.066
g/mL.
[0119] Metallized Tubules: metal microtubules with a wall thickness
of 10 to 2000 nm or halloysite microcylinders with a metallic
coating thickness of 10 to 1000 nm are mixed to a final weight
percentage of 1 to 50 percent of the base component with loadings
from 10 to 20 percent preferred.
Curing Agent: Mix thoroughly the following:
[0120] Diisocyanate prepolymer, 9.60 g, poly(propylene glycol)
terminated with toluene-2,4-diisocyanate [C. A. 9057-91-4], having
a weight-average molecular weight of about 1000, a viscosity of
12,500 centipoise at 40.degree. C., and an average isocyanate
content of 8.4 weight percent.
[0121] Plasticizer, 0.24 g, dibutyl adipate [C. A. 105-99-7],
having a molecular weight of 258 and a density of 0.962 g/mL.
[0122] The base component and curing agent are then blended
together thoroughly, poured into a mold, and allowed to cure at
room temperature. In this manner a tough, elastomeric composite
with a Shore A Hardness of 52 is produced.
Elastomeric Matrix Resin Example 2
Elastomeric Polyurethane Matrix Resin
[0123] The elastomeric matrix resin may be made with a commercial
polyurethane elastomer as the base component. A suitable product is
LS-40 from BJB Enterprises, Inc. (14791 Franklin Avenue, Tustin,
Calif. 92780). According to this method, 90 g of the base component
is mixed with the curing agent (100 g), poured into a mold, and
allowed to cure at room temperature. The resulting resin has a
Shore A hardness of 40, a tensile strength of 490 pounds per square
inch, and an elongation of 800%.
Elastomeric Matrix Resin Example 3
[0124] Elastomeric epoxy matrix resin. The diglycidyl ether of
bisphenol A, diluted with a monoepoxide, is mixed with a polyglycol
diglycidyl ether and cured with a polyoxypropylenediamine. In a
preferred embodiment, a base component and a curing agent are made
separately as follows:
Base Component Mix thoroughly the following:
[0125] A liquid epoxy resin (46 g) containing the diglycidyl ether
of bisphenol A and butyl glycidyl ether. The mixture has a
viscosity of 5-7 Poise, a specific gravity of 1.13, and an epoxy
equivalent weight of 175-195.
[0126] A polyglycol diepoxide resin (31 g) having a viscosity at
25.degree. C. of 30-60 centipoise, a specific gravity at 25.degree.
C. of 1.14, and an epoxy equivalent weight of 175-205.
Curing Agent:
[0127] A polyoxypropylenediamine (23 g) having a viscosity at
25.degree. C. of 9 centipoise, a specific gravity at 20.degree. C.
of 0.948, and an average molecular weight of 230.
[0128] The base component and curing agent are then thoroughly
blended together, poured into a mold, and allowed to cure at room
temperature. A tough, elastomeric resin with a Shore A hardness of
65 is obtained.
Elastomeric Matrix Resin Example 4
[0129] Elastomeric siloxane matrix resin. A vinyl-terminated
polydimethylsiloxane and a copolymer of methylhydrosilane and
dimethylsiloxane are mixed with a curing catalyst. In a preferred
embodiment, a base component and a curing agent are made separately
as follows:
Base Component: Mix thoroughly the following:
[0130] A vinyl-terminated polydimethylsiloxane, 50 g, having a
molecular weight of 28,000, a viscosity of 1000 centistokes, a
specific gravity of 0.97, and containing 0.18 to 0.26 weight
percent of vinyl groups.
[0131] A curing catalyst solution, 15 microliters, formed from 39
parts of n-hexane and one part of 1,2-divinyltetramethyldisiloxane
containing 3.0 to 3.5 weight percent of platinum, which has a
viscosity of 0.7 centistokes and a specific gravity of 0.81.
Further, one milliliter of this solution contains one milligram of
platinum.
[0132] Curing Agent:
[0133] A copolymer of methylhydrosilane and dimethylsiloxane, 1.5
g, having a molecular weight of 1900-2000, a viscosity of 25-35
centistokes, a specific gravity of 0.98, and containing 25-30 mole
percent of methylhydrosilane moieties.
[0134] The base component and curing agent are thoroughly blended
together, poured into a mold, and allowed to cure at room
temperature. By this method a tough, elastomeric composite is
produced.
Elastomeric Matrix Resin Example 5
[0135] Elastomeric siloxane matrix resin. The elastomeric matrix
resin may be made using a commercial siloxane elastomer. Smooth-Sil
900 from Smooth-On, Inc. (2000 St John Street, Easton, Pa. 18042)
is suitable for this purpose. According to this method, 100 g of
the base component is mixed with the curing agent (100 g), poured
into a mold, and allowed to cure at room temperature. The resulting
resin has a Shore A hardness of 35, an elastic modulus of 77, and
an elongation of 175%.
[0136] Obviously, numerous modifications and variations on the
present invention are possible in light of the above teachings. It
is therefore to be understood that within the scope of the appended
claims, the invention may be practiced otherwise than as
specifically described herein.
[0137] Unless otherwise defined, all technical and scientific terms
used herein have the same meaning as commonly understood by one of
ordinary skill in the art of polymers. Although methods and
materials similar or equivalent to those described herein can be
used in the practice or testing of the present invention, suitable
methods and materials are described herein. All publications,
patent applications, patents, and other references mentioned herein
are incorporated by reference in their entirety. In case of
conflict, the present specification, including definitions, will
control. In addition, the materials, methods, and examples are
illustrative only and are not intended to be limiting.
* * * * *