Patent Publication Number: US-2020299868-A1

Title: Microscale composite carbon fiber ferrite microwave absorbers

Description:
BACKGROUND 
     There are several different types of microware absorber material systems available on the market today for the absorption and/or transfer of electromagnetic noise. Materials in the market include lossy foam block pyramidal type absorbers, which are one of the highest performance microwave surface reflection attenuators. Multiple sheets of carbon impregnated foam can also be used to create flat laminate absorbers. Reflections can occur at any foam surface, the magnitude of the reflection being dependent on the density of the material. Variations include high-power absorbers of a honeycomb substrate made from a phenolic-based material. Lossy films are coated on the walls of the honeycomb for effective absorption of incident electromagnetic waves. 
     Embodiments of the disclosure address these and other deficiencies of the prior art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects, features and advantages of embodiments of the present disclosure will become apparent from the following description of embodiments in reference to the appended drawings in which: 
         FIG. 1  is a flow chart illustrating a process for harvesting coated carbon fibers according to embodiments of the disclosure. 
         FIG. 2  illustrates electrostatically applying carbon fibers to a substrate. 
         FIG. 3  illustrates applying an insulative material on the carbon fibers. 
         FIG. 4  illustrates coating the insulated carbon fibers with a magnetic and/or conductive material. 
         FIG. 5  illustrates harvested carbon fibers when the substrate of  FIG. 4  is dissolved. 
         FIG. 6  illustrates an example Fresnel plate. 
         FIGS. 7A and 7B  illustrates an example Fresnel plate according to embodiments of the disclosure. 
         FIG. 8  illustrates an adjustable focus microwave lens according to embodiments of the disclosure. 
         FIG. 9  illustrates another adjustable focus microwave lens according to embodiments of the disclosure. 
         FIG. 10  illustrates a horn antenna according to some embodiments of the disclosure. 
     
    
    
     DESCRIPTION 
     Embodiments disclosed herein relate to the formation and application of microscale magnetic ferrite chokes on carbon fibers for microwave energy absorption. Carbon fiber is conductive in microwave frequencies, such as 300 MHz through 300 GHz. As such, carbon fibers have historically been used for microwave antennas. Embodiments of this disclosure, however, use carbon fibers to create a microwave absorber material and/or coating. 
       FIG. 1  is a flow chart illustrating a process for forming a treated carbon fiber, according to embodiments of the disclosure. Initially, Polyacrylonitrile (PAN) carbon fibers, approximately 1 mm to 10 mm in length, are electrostatically applied  100  to a substrate backing having an adhesive layer thereon. The carbon fibers may be applied such that they are in a near-vertical orientation. Further, the carbon fibers may be applied in such a manner that at least that at least two or more of the carbon fibers extend from the substrate backing at different angles. 
     In operation  102 , an insulative layer may be applied over the carbon fibers to cover at least 90% of the carbon fibers, forming insulated carbon fibers. The insulative layer is applied while the carbon fibers are still attached to the substrate backing. The insulative layer may be, for example, a dielectric material, such as, but not limited to, polyvinyl chloride (PVC), PTFE, polyvinyl acetate (PVA), epoxy, silicone, polyimides, urethanes, acrylics, cyanoacrylates, rubber, neoprene, isoprene, etc. 
     In operation  104 , a material, such as a magnetic and/or conductive material, may be coated on at least a portion of the insulative layer, such that beads of the material form at various positions along the length of the insulative layer. The material may be any magnetic and/or conductive material, such as, but not limited to, carbonyl iron, ceramic iron ferrites, cobalt, nickel, boron, Niobium, neodymium, or other members of the Lanthanum series. That is, some of the insulated carbon fibers may have more than one bead of magnetic and/or conductive material  300  coated on the insulative layer  200 . In some embodiments, in optional operation  106 , the magnetic and/or conductive material  300  may also be insulated with a dielectric material to prevent corrosion. 
     In operation  108 , the substrate backing and the adhesive layer may then be dissolved with a suitable solvent, such as, but not limited to, heat, alcohol, water, methyl ethyl ketone (MKE), etc. to harvest the coated or treated carbon fibers. The coated carbon fibers have a “dumbbell” structure. That is, the harvest coated carbon fibers  400  consists of insulated carbon fibers with beads of magnetic and/or conductive materials surrounding the carbon fibers at various positions along the length of the fiber. The harvested coated carbon fibers  400  may act as antennas, while the magnetic material beads  300  act like chokes. The harvested coated carbon fibers  400  can then absorb microwave energy through both dielectric and magnetic losses. 
     In operation  110 , the treated carbon fibers may be added to other carrier materials to create a microwave absorber material and/or coating. For example, the treated carbon fibers may be added to paints, coatings, epoxy, silicone, rubber, injection molding plastic feed materials, neoprene, polyurethanes, melamine foams, Fresnel plate lens material, etc. Because the treated carbon fibers are magnetic, the carbon fibers may be aligned in the carrier material by application of a magnetic field to optimize the microwave absorption properties of the carrier material with the added carbon fibers. 
       FIG. 2  illustrates operation  100  for forming carbon fibers according to embodiments of the disclosure. As can be seen in  FIG. 2 , the carbon fibers  200  can be applied in such a manner that at least two or more of the carbon fibers  200  extend in a near-vertical manner from a substrate backing  202  having an adhesive layer  204  at different angles. 
       FIG. 3  illustrates coating or covering the carbon fibers  200  with an insulative layer  300 , while the carbon fibers  100  are still attached to the substrate backing  202 . As mentioned above, the insulative layer  200  may be applied to cover at least 90% of the carbon fibers to form insulated carbon fibers. 
     As illustrated in  FIG. 4 , magnetic and/or conductive materials  400  may be coated on the insulative layer  400 , such that various beads form at various positions along the length of the insulative layer  400 . That is, some of the insulated carbon fibers may have more than one bead of magnetic and/or conductive material  400  coated on the insulative layer  300 . 
       FIG. 5  illustrates the harvested treated fibers  500 . The harvested treated carbon fibers  500  consists of insulated carbon fibers with beads of magnetic and/or conductive materials surrounding the carbon fibers at various positions along the length of the fiber. The harvested coated carbon fibers  500  may act as antennas, while the magnetic material beads  400  act like chokes. The harvested treated carbon fibers  500  can then absorb microwave energy through both dielectric and magnetic losses. 
     Further, harvested treated carbon fiber  500  has an anisotropic effect with respect to the applied electromagnetic wave. The incident wave has both an electric field (E-field) and a magnetic field (H-field). Electromagnetic waves will tend to pass along the axis of the harvested treated carbon fibers  500 . E-field waves co-linear to the harvested treated carbon fibers  500  will be pass through with insertion loss. Waves tend to not pass through when harvested treated carbon fibers  500  are perpendicular to the E-field—rather, they will tend to reflect. Conventional absorber materials are not anisotropic. 
     As mentioned above, the treated carbon fibers  500  may be used in a Fresnel plate, also referred to herein as zone plate, lens material. A zone plate lens is a device which may be used to focus electromagnetic wave energy. Unlike lenses or curved mirrors, however, zone plates use diffraction instead of refraction or reflection. A zone plate having the treated carbon fibers  500 , as discussed in more detail below, may be used for microwave radiation in the 1 GHz to 1 THz frequency range. Conventional zone plates are not capable of changing a focal point of the zone plate or lens. 
     As illustrated in  FIG. 6 , a zone plate  600  consists of a set of one or more radial symmetric rings, known as Fresnel zones, which alternate between absorptive  602  and transparent  604 . In zone plate  600 , the microwaves hitting the zone plate  600  will diffract around the absorptive zones  602 . The zones  602  and  604  may be spaced so that the diffracted microwaves constructively interfere at the desired focus, creating a region of concentrated microwave energy. 
     Embodiments of the disclosure include constructing a microwave zone plate lens  700  using the harvested treated carbon fibers  500 , discussed above. The harvested treated carbon fibers  500  may be used to create diffractive zones within a dielectric material. 
       FIGS. 7A and 7B  illustrate the microwave zone plate lenses  700  according to embodiments of the disclosure. The harvested treated carbon fibers  500  are encapsulated in a liquid elastomeric material  702 , such as, but not limited to, silicone, epoxy, polyurethane, polyvinyl acetate, neoprene, melamine foam, etc. The harvested treated carbon fibers  500  may be placed in an orientation within the liquid elastomeric material  702 . That is, the treated carbon fibers  500  may be arranged in random orientations or all in the same direction. 
     In some embodiments, after being encapsulated in the liquid elastomeric material  702 , the harvested treated carbon fibers  500  may be subjected to a magnetic field. When subjected to the magnetic field, the harvested treated carbon fibers  500  align with the magnetic field. Accordingly, the harvested treated carbon fibers  500  may be aligned in any one of the x-axis, y-axis, or z-axis, depending on how the magnetic field is applied. The different zones of the zone plate lenses  700  may also have harvested treated carbon fibers  500  in different orientations. For example, one zone may have randomly oriented harvested carbon fibers  500 , while another zone may have harvested carbon fibers  500  aligned in the z-axis. That is, each of the zones of each of the zones plates may have harvested treated carbon fibers  500  in different or the same orientations. The harvested treated carbon fibers  500  may be subjected to the magnetic field until the liquid elastomeric material  702  is cured or nearly cured. 
     With Fresnel lenses  700  constructed or structured with an elastomeric material, an adjustable focus microwave lens may be created, as illustrated in  FIG. 8 . A chamber  800  may include zone plates  802  and  804 , similar to those discussed above with respect to  FIGS. 7A and 7B , which are constructed or structured with an elastomeric material. 
     The chamber  800  includes an opening  806  structured to mate with a pump (not shown). The pump may be used to pump air into the chamber  800 , expanding and stretching the space between the zones in each of the zone plates  802  and  804 . The distance between the zone plates  802  and  804  is also increased. This allows for the focal lens of the zone plates  802  and  804  to vary with the amount of pressure inside the chamber. 
     Although two zone plates are illustrated in  FIG. 8 , only a single zone plate or up to four zone plates may be provided in the chamber  800 . For example, four different zone plates may be provided on a chamber to vary the absorption and transparent regions of the zone plates. In some embodiments, only three chambers have zone plates, so a user may decide between using a single zone plate, which may be expanded to a desired focal point, or using two zone plates in conjunction, such as shown in  FIG. 8 . 
       FIG. 9  illustrates another example of a Fresnel lens, which may be used, for example, with a horn antenna. The Fresnel lens  900  is similar to the Fresnel lens  800  and like components are given the same reference numbers. The Fresnel lens  900 , however, includes an outer layer  902  of carbon fibers around the exterior portion of the chamber  800  between the two zone plates  802  and  804 . That is, the outer layer  902  is comprised of a number of harvested treated carbon fibers  500 , similar to the zones of the zone plates. 
     Fresnel lens  900  may be used in conjunction with a horn antenna  1000 , as illustrated in  FIG. 10 . A horn antenna or microwave horn is an antenna that consists of a flaring metal waveguide shaped like a horn to direct radio waves in a beam. Horns are widely used as antennas at UHF and microwave frequencies, above 300 MHz. 
     As can be seen in  FIG. 10 , the Fresnel lens  900  may be placed at the end of the horn antenna  1000 . The Fresnel lens  900  may have air added to adjust the focus of the zone plates, as needed and as discussed above. An edge of the horn antenna creates a discontinuity of the electromagnetic wave propagating through a horn wave guide section of the horn antenna. 
     The outer layer  902  of the Fresnel lens  900  is perpendicular to the electromagnetic wave on the edge of the horn antenna  1000 . This results in the outer layer  902  directing energy radiated from the side to the front of the horn antenna  1000 . Since the harvested treated carbon fibers  500  are not coupled with the edge of the horn antenna  1000  itself, edge coupling can be reduced. 
     In this disclosure, the term “using” means “using at least in part.” The singular forms “a,” “an,” and “the” include plural referents unless the context dictates otherwise. The term “exemplary” is used in the sense of “example” rather than “ideal.” The term “or” is meant to be inclusive and means either, any, several, or all of the listed items. The terms “comprises,” “comprising,” “includes,” “including,” or other variations thereof, are intended to cover a non-exclusive inclusion such that a process, method, or product that comprises a list of elements does not necessarily include only those elements, but may include other elements not expressly listed or inherent to such a process, method, article, or apparatus. Relative terms, such as “about,” “approximately,” “substantially,” and “generally,” are used to indicate a possible variation of ±10% of a stated or understood value 
     The previously described versions of the disclosed subject matter have many advantages that were either described or would be apparent to a person of ordinary skill. Even so, these advantages or features are not required in all versions of the disclosed apparatus, systems, or methods. 
     Additionally, this written description makes reference to particular features. It is to be understood that the disclosure in this specification includes all possible combinations of those particular features. Where a particular feature is disclosed in the context of a particular aspect or example, that feature can also be used, to the extent possible, in the context of other aspects and examples. 
     Also, when reference is made in this application to a method having two or more defined steps or operations, the defined steps or operations can be carried out in any order or simultaneously, unless the context excludes those possibilities. 
     Although specific examples of the invention have been illustrated and described for purposes of illustration, it will be understood that various modifications may be made without departing from the spirit and scope of the invention. Accordingly, the invention should not be limited except as by the appended claims.