Note: Descriptions are shown in the official language in which they were submitted.
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PATENTS 43612 USA 6A
MICROWAVE ABSORsER EMPLOYING ACICULAR
MAGNETIC METALLIC FILAMENTS
s
Technical Fi eld
This invention involves electromagnetic radiation
absorbers which comprise magnetic metallic filaments
embedded in dielectric binders.
Background
Electromagnetic radiation absorbers typically are
non-conductive composites of one or more kinds of
dissipative particles dispersed through dielectric binder
materials. The absorption performance of the composite
absorber depends predominantly on the electromagnetic
interactions of the individual particles with each other
and with the binder. For example, U.S. Patent 4,538,151
(Hatakeyama et al.) discloses an absorber comprising a
mixture of: metal or alloy fibers having high electric
conductivity, a length from 0.1mm (100 microns) to 50mm and
a length to diameter ratio ("aspect ratio") larger than 10;
ferrite or a ferromagnetic material; a high molecular
weight synthetic resin; and, optionally, carbon black.
The term "whiskers" is often used confusingly for
both monocrystalline and polycrystalline fibers. For this
invention, relatively long fibers are called acicular
("needle-like") whiskers if monocrystalline in structure,
or acicular filaments if polycrystalline.
Thickness, weight, and ease of application of the
composite absorber are important practical considerations.
Accordingly, absorbing paints have also been developed for
certain applications. The paints are typically dispersions
of the metal/binder composites. For example, U.S. Patent
4,606,848 (Bond) teaches a paint comprising stainless
steel, carbon, or graphite fibers in polyurethane, alkyd,
or epoxy binders. The fibers range in length from 10
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micron to 3 cm (30,000 micron~ as the diameter ranges from
0.01 micron to 30 micron, thus the aspect ratio is a
constant 1000.
Summary of Invention
The invention is a non-conductive microwave
radiation absorber, comprising acicular magnetic metallic
filaments with an average length of about 10 microns or
less, a diameter of about 0.1 micron or more, and aspect
10 ratios between 10:1 and 50:1. The filaments are dispersed
in a dielectric binder. An absorbing paint may be formed
by dispersing the filaments into a base liquid, such as by
dissolving the filament/binder dispersion in the base
liquid. The absorber or the paint may be applied to a
conductor such as a metal foil, plate or wire.
srief Description of the Drawing
Figures 1-4 are graphs of the real and imaginary
parts of the permittivity and permeability of four
embodiments of the invention, as a function of incident
radiation frequency.
Figure 5 is a graph of the predicted absorption
response of one embodiment of the invention, and of the
actual absorption response of another embodiment of the
invention, as a function of incident radiation frequency.
Figure 6 is a cross sectional view of another
embodiment of the invention.
Detailed Description
One embodiment of the invention is a
non-conductive composite absorber having at least two major
components. The first component is acicular magnetic
metallic polycrystalline filaments (or simply "filaments")
having an average length of less than about 10 micron, a
diameter greater than about 0.1 micron, and length to
diameter ratios ("aspect ratios") between 10:1 and 50:1.
The second component is dielectric binder in which the
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filaments are dispersed, and which contributes to the
absorption performance of the composite absorber.
Another embodiment of the invention is an
absorbing paint for direct application to either a
conductive or insulating surface. This embodiment may be
made by dispersing either the filaments themselves into a
base liquid, or by forming a pigment comprising the
composite absorber and dissolving the pigment in a base
liquid. In either case the paint must remain
non-conductive. For this reason, dissolving the composite
absorber pigment is preferred, as the dielectric binder
substantially surrounds the filaments and prevents them
from electrical contact with each other. If an absorber is
used as a pigment, a polymeric binder material is preferred
for ease of preparation and use, although the choice of
binder depends on the choice of base liquid.
Another embodiment of the invention includes a
conductor adjacent the composite absorber. The conductor
may be an object which the absorber is designed to shield,
or it may be a conductive layer intended to promote
microwave absorption.
To form an effective absorbing structure, the
composite should be in a form which has a thickness in the
direction of radiation propagation greater than about
one-fortieth (2.5 percent) of the wavelength to be
absorbed. The composites of this invention absorb
radiation over a broad incident frequency range in the
microwave region of approximately 2 to 20GHz, implying a
thickness greater than about 0.0375 cm.
Also for any embodiment of the invention,
impedance matching of the absorber to the incident medium
(usually air) is preferred but not required. Typically the
match is done by a material having permeability and
permittivity values that minimize reflection of microwaves
at the surface of incidence. Usually a layer of such
impedance matching material is added to the absorber or
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dried absorbing paint, and the dimensions, weight, etc. of
the layer are considered in the complete design.
All the embodiments employ magnetic metallic
polycrystalline filaments. Presently available filaments
typically range in length from 50-500 microns and in
diameter from 0.1 to 0.5 microns; to preserve the filament
shape, the aspect ratios generally are maintained between
500:1 to 1000:1. These filaments can be shortened for use
in the invention by milling and grinding. The average
sizes of the filaments may be determined from individual
measurements performed with a scanning electron microscope.
The reduction in length of the magnetic metallic
filaments broadens the absorption performance of the
composite material in which they are embedded. Long
filaments produce only narrowband absorption response
because of their conductivity, although it is generally
stronger than that of, for example, the carbonyl iron
spheres known in the art, due to the dipole moments of the
filaments. However, the shortened, low aspect ratio
magnetic metallic filaments used in the present invention
produce effective and versatile absorbers, exhibiting
strong absorption magnitude over a broad frequency range.
We believe at this time that the dissipative performance of
the filaments is due in part to the magnetic and metallic
natures of the filaments, in addition to their length and
aspect ratio.
Also, the inventive absorber has a reduced volume
loading factor (absorbing particle volume as a percentage
of total absorber volume), which leads to a reduction in
weight of the final product. For example, volume loading
factors for compo ites based on carbonyl iron microspheres
typically range from 40 to 65 percent. In the present
invention, the volume loading may be as low as 25 to 35
percent with no decrease in absorption performance.
The reduced acceptable volume loading factor also
helps ensure that the composite absorber i6 an insulator,
i.e., it has a high bulk resistivity, despite the
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conductivity of the individual filaments. If the bulk
resistivity is too low, the composite absorber effectively
becomes a conductive sheet, which reflects microwaves
instead of absorbing them. The resistivity of iron, for
5 example, is about 10- 5 ohm-cm at room temperature.
Insulators typically have bulk resistivities of 1Ol 2 ohm-cm
or more. Samples of the invention with 25 percent volume
loading of iron filaments have measured bulk resistivity of
approximately 1.5xlO1 3 ohm-cm at room temperature,
indicating an insulator.
Several types of filaments may be used in the
invention. Iron, nickel, and cobalt filaments are
suitable, as are their alloys. For example, iron-nickel,
nickel-manganese, and iron-chromium alloys are acceptable,
if they form acicular magnetic metallic polycrystalline
filaments of the proper size. More than one type of
filament may be used in a single absorber, and other
absorbing materials (e.g., carbonyl iron) may be added to
the composite material to tailor the absorption versus
frequency characteristics to a particular application.
The dielectric binder may be ceramic, polymeric,
or elastomeric. Ceramic binders are preferred for
applications requiring exposure to high temperatures, while
polymeric and elastomeric binders are preferred for their
flexibility and lightness.
Many polymeric binders are suitable, including
polyethylenes, polypropylenes, polymethylmethacrylates,
urethanes, cellulose acetates, epoxies, and
polytetrafluoroethylene (PTFE). The polymeric binder may
be a thermosetting polymer, a thermoplastic polymer, or a
conformable polymer which changes shape to assume a final
applied configuration. For example, a heat-shrinkable
binder may be formed from cross-linked or oriented
crystallizable materials such as polyethylene,
polypropylene, and polyvinylchloride; or from amorphous
materials such as silicones, polyacrylates, and
polystyrenes. Solvent-shrinkable or mechanically
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stretchable binders may be elastomers such as natural
rubbers or synthetic rubbers such as reactive diene
polymers; suitable solvents are aromatic and aliphatic
hydrocarbons. Specific examples of such materials are
taught in U.S. Patent No. 4,814,546 (Whitney et al.).
Suitable elastomeric binders are natural rubbers
and synthetic rubbers, such as the polychloroprene rubbers
known by the trade name "NEOP~ENE."
The binder may be homogenous, or a matrix of
interentangled fibrils, such as the PTFE matrix taught in
U.S. Patent 4,153,661 (Ree et al.).
An electrical conductor with a microwave
absorbing coating may be made by extruding a composite
absorber onto the conductor. Many polymeric binders are
suitable for extrusion, especially polyvinylchlorides,
polyamides, and polyurethanes. The conductor may be a
wire, cable, or conductive plate.
The exact choice of binder depends on the final
absorption versus frequency characteristics desired and the
2Q physical application required. The choice of binder also
dictates the procedure and materials required to assemble
the composite absorber, paint, or coated conductor. The
basic procedures are illustrated by the following examples.
Example 1
Four samples of the invention, labeled A-D, were
prepared, differing only in the lengths of filaments
produced. In each sample, 100 parts by weight of
commercially available iron filaments, typically 50-200
microns in length and 0.1 to 0.5 microns in diameter, were
wetted with methylethylketone and pulverized to shorter
lengths in a high speed blade mixer for one hour. After
the shortened filaments settled, the excess solvent was
decanted away. The filaments were milled again, in
methylethylketone with 800 grams of 1.3 millimeter diameter
steel balls at 1500 revolutions per minute in a sand mill
supplied by Igarashi Kikai Seizo Company Ltd. Each of the
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four samples was milled for a different amount of time.
The milling times were: Sample A, 15 minutes; Sample B, 30
minutes; Sample C, 60 minutes; and Sample D, 120 minutes.
Inspection of the milled particles by scanning
electron microscopy (SEM) showed that some individual
filaments were pressed together into larger particles.
This effect was most pronounced in Sample D. Generally,
the filaments were not pressed together end-to-end as much
as they were pressed together to form wider filaments. No
attempt was made to separate these pressed filaments, and
their lengths and diameters were measured as if they were
single filaments. SEM also confirmed that the filaments
were not aligned in any preferred direction.
The distributions of filament length in microns
as a percentage of total filaments measured for each sample
is shown in Table I. The percentages do not add to 100 due
to rounding. Approximately 150 filaments were measured for
each sample.
Table I
Percentage of Total Filaments by Sample
Size Range A B C D
0-5 60 74 82 99
5-10 30 17 9
11-15 6 6 5 0
16-20 2 1 2 0
21-25 1 1 1 0
26-50 1 1 2 0
51-100 1 1 0 0
30101-150 0 0 0 0
151-200 0 0 0 0
The longest length, average length, average
diameter, and aspect ratio of the samples are shown in
Table II, the first three measured in microns. The average
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length calculation used the average length of each size
range, weighted by the percentage distribution in each size
range.
Table II
Sample
A B C D
Longest Length 55 60 35 10
Avg. Length 6.2 5.4 4.7 2.6
Avg. Diameter 0.25 0.25 0.25 0.25
Aspect Ratio 24.8 21.6 lB.8 10.4
The diameters of the filaments were essentially
unchanged by the milling, i.e., they ranged from 0.1 to 0.5
microns. Because Table 1 shows that substantially all of
the filaments in the samples have lengths of 10 microns or
less, the diameter range of 0.1 to 0.5 microns implies that
the filaments in each sample have aspect ratios between
20:1 and 50:1. The preferred aspect ratio range is 10:1 to
25:1, using the average length and diameter values of Table
.
For each sample, a paint containing the milled
filaments was made from two major components. The first
component was (by weight) 198.0 parts of methylethylketone,
50.0 parts of toluol, 43.6 parts of a polyurethane
("ESTANE" type 5703 supplied by B.F. Goodrich Company), and
2.5 parts of a suitable dispersing agent ("GAFAC" type
RE-610 supplied by GAF Corporation). This component was
stirred until the polyurethane dissolved. The second
component was (by weight) 100 parts of the shortened iron
filament samples, 2.7 parts of diphenylmethane
diisocyanate, and 1.8 parts of propylene glycol methylether
acetate. The two components were mixed in a blade mixer to
form a homogeneGus paint. Each mixture was degassed and
cast onto a flat surface, then allowed to dry in air to
remove the volatile vehicle chemicals.
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After sufficient drying and curing (about 1-3
days), the resulting radiation absorber was machined into
circular toroidal ("donut-shaped") samples for coaxial
microwave absorption measurements. The inner and outer
diameters of the sample were 3.5 + 0.0076mm and 7.0 +
0.0076mm, respectively. Each sample was placed, at a
position known to + O.lmm, in a 6cm long coaxial airline
connected to a Hewlett-Packard Model 8510A precision
microwave measurement system. The substrates used had a
permittivity of 2.58 and a permeability of 1.00.
Two hundred one step mode measurements from 0.1
to 20.1 GHz were made on each sample. Measurements of the
transmission and reflection of the microwaves by the
samples were used to calculate the real and imaginary parts
Of the permittivities and permeabilities of the samples as
a function of incident frequency, as shown in Figures 1-4.
The errors in the calculation of the imaginary parts of the
permittivity and permeability are typically 5 percent of
the measurement. In Figures 1-4, the real parts are solid
lines and the imaginary parts are dashed lines. The
letters A-D identify the values from Samples A-D.
Figures 1-4 show that filament length strongly
affects both the real and imaginary parts of permittivity.
The real part of the permittivity decreases significantly
faster than the imaginary part, thus the ratio of the
imaginary part to the real part (a measure of the
absorption ability of the composite) increases with
decreasing filament length. The effect of the varying
filament length on the measured absorber permeability is
generally weak, but in Sample D the imaginary part of the
permeability shows a significant decrease compared to that
of Samples A-C, especially at low frequencies. For this
reason, Sample C (average filament length about 5 microns)
is preferred, although each of the samples is an acceptable
microwave absorber.
Based on our data and the known performance of
absorbers employing much longer filaments (e.g., the
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greater than 100 micron filaments of U.S. Patent
4,538,151), we believe the improved performance of the
present invention lies in part in the use of filaments with
an average length of 10 micron or less, preferably about 5
micron, diameter greater than about 0.1 misron, and aspect
ratios between 50:1 and 10:1, preferably between 25:1 and
10:1.
Example 2
A stock formulation containing iron filaments was
made as follows. First, 52.49 grams of synthetic rubber
("NEOPRENE" type W as supplied by E.I. du Pont de Nemours
Company) was banded on a two roll rubber mill and mixed for
five minutes to reach an elastic phase. Then 0.52 grams
benzothiazyl disulfide, 13.12 grams stearic acid, and 2.62
grams white mineral oil were added, and mixing continued
for another five minutes. After 147.38 grams of commercial
length iron filaments were added, mixing continued until
the average length of the filaments was approximately 6.5
microns and the average diameter approximately 0.26
microns, for an aspect ratio of 25:1. Next a curing
accelerator was made, comprising 0.25 grams
hexamethylenetetramine, 0.26 grams tetramethylthiuram
disulfide, and 0.52 grams polyethylene glycol. The
accelerator was mixed into the iron filament/binder mixture
to produce the stock formulation. The volume loading of
the filaments into the binder was determined to be 35~. To
reduce premature cure, the stock formulation was kept below
30C.
A thin calipered sheet of the stock formulation
was dissolved in a base mixture of e~ual parts butylacetate
and toluene, followed by agitation for two hours. This
formed a paint designated Sample E. A 16.5cm square
aluminum plate was repeatedly sprayed with thin coats of
the paint, allowing typically 15 to 30 minutes drying time
between each spraying. To keep the solid content of the
paint at approximately 15% by volume, the same
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butylacetate/toluene base mixture was thinned into the
paint as needed. Once a final sprayed thickness of about
lmm was reach~d, the coat was allowed to dry and cure at
room temperature for three days.
The coated aluminum plate was mounted in a
measurement chamber with microwave radiation normally
incident on the coated side. ~ctual measurements of the
transmission and reflection coefficients were used to
calculate the predicted absorption for transverse magnetic
(TM) radiation incident upon the plate at a 65 angle from
normal, as a function of incident frequency. The predicted
results are graphed in Fiqure 5 and show the desired broad
and strong absorption response, at least 10dB over a 13 GHz
range from 6 to 19 GHz and at least 20dB over a 3dB wide
range from 10.5 to 13.5 GHz.
A paint designated Sample F was made by the same
procedures as for Sample E above with the following
ingredients: "NEOPRENE" Type w, 69.99 grams; benzothiazyl
disulfide, 0.70 gram; stearic acid, 17.50 grams; white
mineral oil, 3.50 grams; iron filaments, 196.50 grams;
hexamethylenetetramine, 0.35 gram; tetramethylthiuram
disulfide, 0.35 gram; polyethylene glycol, 0.70 gram. The
volume loading of the iron filaments was 25%. After
painting the conductive plate, actual measurements were
made of the absorption coefficient for TM radiation
incident upon the plate at a 65 angle from normal, as a
function of incident frequency. The results are also
graphed in Figure 5 and confirm the desired broad and
strong absorption response, at least 10dB over a 11 GHz
30 range from 5 to 16 GHz, at least 20dB over a 3.5dB wide
range from 9 to 12.5 GHz, and at least 30dB over a 1 dB
wide range from 10.6 to 11.6 GHz.
Example 3
The construction shown schematically in Figure 6
was made as follows. Iron filaments 43 were dispersed in a
1.2mm thick calipered sheet 42 made from the stock
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formulation which was used to form Sample E of Example 2.
A conductive layer 48 of aluminum, vapor coated on one side
of a polyester support sheet 46, was adhered to sheet 42
with an ethylene acrylic acid ( EAA) type internal adhesive
44 between the polyester support sheet 46 and the stock
formulation 42. This produced a radiation
absorber/conductive metal layer construction, sometimes
known as a Dallenbach construction.
In another sample, aluminum foil, 0.0085mm thick,
was used for conductive layer 48 and applied directly to an
absorbing sheet of the same composition without a polyester
support 46. The polyester support 46 for the vapor coated
aluminum also would not be required if the internal
adhesive 44 adheres to both conductive layer 48 and
absorbing sheet 42. Several types of internal adhesives 44
may be used, depending on the choice of materials made in
constructing the tile and the conditions in which it will
be applied. Any conductive metal is suitable for the
conductive layer 48.
In fact, for some choices of binder material, the
absorbing composite may be coated directly on the
conductive layer without any internal adhesive at all. For
example, an absorbing paint could be made and applied to a
suitable conductive layer, as in Example 2.
In this embodiment as in any embodiment of the
invention, an impedance matching layer 56 is preferred but
not required. Suitable materials for this layer include
polymeric materials with high volumes of trapped air, such
as air-filled glass microbubbles embedded in the binder
materials described above.
Example 4
An absorber comprising iron filaments in a
matrix of interentangled polytetrafluoroethylene (PTFE)
fibrils was made according to the process of U.S. Patent
4,153,661 (Ree et al.). A water-logged paste of 10.0 grams
of iron filaments and 4 cc of an aqueous PTFE dispersion
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(5.757 grams of PTFE particles) was intensively mixed at
about 75C, biaxial calendered at about 75C, and dried at
about 75C. The lengths of the filaments were reduced by
the mixing and calendering steps to an estimated range of 1
to 10 microns. The volume loading of the whiskers in the
total volume of the absorber was calculated to be 32.7
percent. Measurements of the real and imaginary parts of
the permeability indicated that the real part decreased
from about 4.0 to about 1.5 over a 2 GHz to 8 GHz range;
the imaginary part was greater than 1.0 over the entire
range of 2 GHz to 20 GHz, and about 2.0 in the range of 5
GHz to 8 GHz.
While certain representative embodiments and
details have been shown to illustrate this invention, it
will be apparent to those skilled in this art that various
changes and modifications may be made in this invention
without departing from its full scope, which is indicated
by the following claims.
PFAPP5.5
