Low density dielectric having low microwave loss

Details Low density dielectric having low microwave loss Low density dielectric having low microwave loss

2000-04-06

2003-05-13

Thibodeau, Paul (Department: 1773)

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

C428S313900, C428S332000, C428S402000, C428S403000

Reexamination Certificate

active

06562448

ABSTRACT:

TECHNICAL FIELD
This invention relates to composite materials including a filler and a polymer, in which the composite has low density, low microwave loss and a predetermined dielectric constant.
BACKGROUND
Microwave lenses and antennas can be made from microwave transmissive high dielectric materials, such as plastics that contain high dielectric constant fillers.
High-density inorganic compounds, such as titanium dioxide and barium titanate, can be used as fillers in the preparation of high dielectric materials for microwave applications. As the volume loading of these fillers is increased to elevate the dielectric constant of a composite material, the density of the composite material also increases. Lower density high dielectric materials have been made by coating metal onto relatively large particles of low density materials.
Microspheres are relatively small, substantially spherical particles, either solid or hollow. Hollow microspheres are also called microbubbles. Microspheres are frequently made of glass. Microspheres with a metal coating have been incorporated into polymeric articles.
DISCLOSURE OF INFORMATION
Briefly, the present invention provides a composite material comprising a matrix material comprising a microwave transmissive polymer; and from about 1 to 65 volume percent of a particulate filler dispersed in the matrix. The particulate filler is characterized by an electrically conductive coating on the filler particles, particles selected from spheroids and ellipsoids having a major dimension below about 0.5 mm (preferably below about 0.2 mm) and particles having an aspect ratio greater than about 2 to 1 having a minor dimension below 100 &mgr;m, and a combination of particle size and coating thickness sufficient to yield a combination of a composite material dielectric constant between about 1.2 to 100, and a composite material microwave loss tangent no greater than about 0.10 at 1 GHz.
In another aspect, the present invention provides a composite material comprising a matrix material comprising a microwave transmissive polymer; and from about 1 to 65 volume percent of a filler comprising microspheres dispersed in the matrix. The filler microspheres include an electrically conducting coating, with particularly useful electrically conducting coatings including, for example, aluminum, stainless steel, titanium, and tungsten. The filler microspheres are also characterized by: an average particle size in the range of 10 to 350 &mgr;m; a density below about 3.5 g/cc; and a combination of particle size and coating thickness sufficient to yield a combination of a composite material dielectric constant between about 1.2 to 100, and a composite material microwave loss tangent no greater than about 0.10 at 1 GHz.
As used herein:
“matrix” means a network of polymeric material, which may be continuous or discontinuous and may include voids and/or a gas;
“filler” means coated or uncoated particles, which may be hollow or solid and which may be made from inorganic materials such as glass or ceramics or organic materials such as polymers and may be in various shapes such as spheres, fibers, and/or flakes;
“electrically conductive” means having a resistivity between about 10
−6
to 10
0
ohm-cm;
“major dimension” means the largest of a particle's diameter, length, width, cross-section, or thickness, and “minor dimension” means the smallest of a particle's diameter, length, width, cross-section, or thickness, where both major and minor dimensions can be directly measured or classified by standard screening techniques or particle sizing equipment;
“substantially hollow” means encompassing at least some void or gas;
“bubbles” means substantially hollow particles, usually having a spherical shape;
“microbubble” and “microsphere” mean a sphere below about 350 &mgr;m particle size;
“density” means the ratio of weight to a volume of particulate material such as described in ASTM D2840;
“melt processable” as applied to hollow microspheres, means capable of withstanding pressures such as typically encountered during mixing with a polymer melt or forming operations to make lenses or antennas, without crushing a majority of the microspheres;
“metal coated” means substantially encapsulated by an electrically continuous metal layer;
“chemical blowing agent” means a material that can be incorporated into an extrudable polymer and that decomposes at temperatures above the temperature used during extrusion, to produce a gas, such as nitrogen or carbon dioxide, so that the gas introduces voids in a polymer matrix when the material is heated above its decomposition temperature; and
“significantly ruptured” means microspheres damaged to an extent that renders them ineffective as a low density filler.
It is an advantage of the present invention to provide a low density, predetermined dielectric constant, composite material with low dielectric loss in the microwave frequency region. The composite material dielectric constant can be selected within a wide range. Increasing the volume fraction of the preferred filler in the composite material increases the dielectric constant of the composite material. In some embodiments, increasing the volume fraction of filler reduces, rather than elevates, the density of the composite material. These inventive materials are useful, for example, in microwave antennae and microwave lens applications, especially where weight is an important consideration.


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