WO1993025063A1

WO1993025063A1 - Ultralight electromagnetic interference shielding and method for making same

Info

Publication number: WO1993025063A1
Application number: PCT/US1993/005134
Authority: WO
Inventors: James R. Weir, Jr.; David P. Stinton
Original assignee: Martin Marietta Energy Systems, Inc.
Priority date: 1992-06-01
Filing date: 1993-05-28
Publication date: 1993-12-09
Also published as: CA2134887A1; AU4398093A; KR950700679A

Abstract

A novel ultralight electromagnetic interference shielding material and a method for making the same is disclosed. A cloth fabric is subjected to a chemical vapor deposition process resulting in fibers (56) of the cloth being coated with a sheath of an electrically conductive metal (58). Coated layers of cloth are then fabricated in an epoxy-matrix composite as necessary to provide a material having a selected structural strength EMI capability.

Description

ULTRALIGHT ELECTROMAGNETIC INTERFERENCE SHIELDING AND METHOD FOR MAKING SAME

This invention was made with Government support under Contract No. DE- AC05-84OR21400 awarded by the U.S. Department of Energy to Martin Marietta Energy Systems, Inc. The Government has certain rights in this invention.

BACKGROUND OF INVENTION

Field of Invention

The invention relates generally to a novel technique for electromagnetic interference (EMI) shielding and particularly to an ultralight EMI shielding material and method for making same.

Description of Prior Art

Electromagnetic radiation, whether naturally occurring or artificial, frequently inhibits effective operation of electronic equipment unless effective measures are taken to prevent such EM radiation from either entering or escaping from such equipment. For modern telecommunication, navigation, systems control and medical equipment, false signals can be induced in the equipment by unprotected exposure to external EM radiation which may even be generated by other equipment in close proximity. Electronic equipment and electric power generation systems operating at high power or high frequency may even render nearby systems designed to function at very low EM signal levels inoperable. Additionally, such stray EM fields can destroy information stored on magnetic media. Finally, EM radiation from unshielded electronic equipment is susceptible to unwanted electronic interception and surveillance causing potentially damaging security problems.

It is apparent that the effective solution to protect areas and/or equipment from the emission or reception of unwanted EM rac. m is to entirely enclose the area and/or equipment to be protected in a ser ,ιess, electrically conductive enclosure. The apparent solution, however, is generally not practical or feasible for a number of reasons, e.g. complexity, expense, space etc. For EMI shielding of areas, it is known to construct double-walled room- within-a-room enclosures where the outer room is made of a electrically conductive material. Such a technique is complex, expensive, quite heavy and generally not mobile. Alternative approaches are known involving conductive coverings such as 5 metal foils and laminate sheet materials for surfaces of the enclosure. Metal foils for applications where extremely light weight is desired are adequate for EMI shielding purposes but are generally quite fragile and require considerable care in handling. It is also known to make cloths for the purpose of EMI shielding wherein the cloths have metal-plated fibers, metal fibers interwoven with the cloth fibers or

)0 winding a fiber sheath about a thin metal fiber as a conjugated yarn and then weaving a cloth from the conjugated yarn. However, with all metal fiber cloth composites, careful consideration must be given to matching characteristics of the fiber, e.g. elongation, tensile strength, shrinkage, diameter, etc., with those of the fiber or the metal fiber tends to break causing loss of electrical continuity of the

15 overall cloth and penetration of the cloth fabric by the broken ends of the metal fibers. It is also known to incorporate carbon cloth for strength and metal foils for EMI shielding into carbon-metal/epoxy composites. Additional weight reductions could be achieved by reducing the amount of metal in such composites since even the thinnest foils are considerably thicker and heavier than is necessary for adequate 0 EMI shielding. However, the strength of the foil remains the limiting factor in constructing such composites.

The technique of the present invention provides a new EMI shielding material that is flexible, more easily handled in fabrication than conventional EMI materials, ultralight and can be precisely tailored for desired applications by 5 controlling the thickness of the shielding material and the amount of metal in the material.

SUMMARY OF INVENTION

A principal object of invention is to provide a new ultralight EMI shielding material. A further object of invention is to provide a new method for fabricating 0 ultralight EMI shielding materials. These and other objects of invention are achieved by chemical vapor deposition of layers of an electrically conductive metal onto fibers of a suitable cloth material. The thickness of the metal layer deposited and the number of cloth layers may be matched to the strength and the EMI shielding needed. Multiple layers of cloth are fabricated in an epoxy-matrix composite to produce structural strength in the EMI shielding.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically illustrates the method of fabrication of an EMI shielding material according to the present invention.

Figure 2 is a magnified photograph of cloth fibers having a metal layer surrounding each fiber in an EMI shielding material fabricated according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In order to encapsulate individual fibers of a fabric with a metallic sheath, a chemical vapor deposition (CVD) technique is employed. By way of example and not limitation, one such technique for providing such an electromagnetic interference (EMI) shielding material is shown with reference to Figure 1.

A source of a carrier gas 10 such as argon is connected through carrier lines 12, 14 and 16 having a first mass flowmeter 18 and a bidirectional valve 20 to an enclosure 22 containing the material 24, for example a carbon cloth, to be coated with a selected metal. The cloth may have all fibers of the same substance and of approximately the same diameter. Alternatively, selected ones of the fibers may be of different substances and different diameters.

The source of carrier gas 10 is also connected through a second carrier line 26 having a second mass flowmeter 28 and a bidirectional valve 30 to a gaseous source 32 of the metal to be deposited on said material 24. The gaseous source 32 of metal may, for example, be a Fe(CO)₅ bubbler generating iron pentacarbonyl vapor. The gaseous source 32 is connected to enclosure 22 through a bidirectional valve 34 and carrier line 16 which has at least a pair of inlets 17 into the enclosure 22. Another bidirectional valve 34 is connected between carrier lines 14 and 26 to assist in equalizing the pressure of the carrier gas across the gaseous source 32. The pressure of the combined carrier gas and metal vapor in carrier line 16 is constantly monitored by means of a compound gauge 34.

A vacuum pump 36 is connected via vacuum line 38 and a bidirectional valve 40 to the enclosure 22 containing the material 24 to be coated. By way of example and not limitation, decomposition of the metal vapor inside the enclosure 22 resulting in deposition of a layered metal sheath about the individual fibers of the material 24 is achieved by application of a radiant or other heat source 42 to said material 24 in said enclosure 22. Air is exhausted from the enclosure 22 prior to the deposition process by means of exhaust line 44 and bidirectional valves 40 and 46. After deposition of the desired thickness of the metal about selected ones of the individual fibers at selected depths in the material 24 is obtained, the gases in the enclosure 22 are exhausted through line 50 and bidirectional valve 52 to a burn- off furnace which removes residual metal before entering the building exhaust. The metal coated material is then removed from the enclosure and multiple layers of such metal coated materials may then be fabricated in an epoxy-matrix composite by conventional techniques to produce an EMI shielding material having a particular structural strength.

Referring to Figure 2, an enlarged photograph is shown of a material 24 having individual fibers 56 coated with a metal sheath 58 according to the teachings of the present invention. Such a material provides significant weight reduction and enhanced structural strength over EMI shielding materials prepared by conventional techniques. Further, the CVD process used to prepare such materials may be operated to optimize the sheath thickness and hence the weight of the material for specific applications.

While there has been shown and described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims.

Claims

We claim:

1. An electromagnetic interference shielding material comprising: at least one layer of cloth having a plurality of individual fibers; and a layered metal sheath formed about selected ones of said fibers at selected depths in said cloth.

2. The material of Claim 1 wherein all said fibers of said cloth are the same substance.

3. The material of Claim 1 wherein selected ones of said fibers are of a different substance than the rest of said fibers.

4. The material of Claim 1 wherein all of the fibers of said cloth have approximately the same diameter.

5. The material of Claim 1 wherein selected ones of the fibers of said cloth may have different diameters.

6. The material of Claim 1 wherein said metal sheath is composed of an electrically conductive metal.

7. The material of Claim 1 wherein said metal sheath is composed of iron.

8. The material of Claim 1 wherein multiple ones of said cloth layers are fabricated into an epoxy-matrix composite.

9. A method of making an ultralight electromagnetic interference shielding material comprising the steps of: providing at least one cloth layer having a plurality of fibers; and depositing a layered metal sheath about each of said fibers of said at least one cloth layer by a chemical vapor deposition.

10. The method of Claim 8 wherein the step of depositing includes the steps of: combining a carrier gas with metal vapor; providing an enclosure about said cloth; generating a vacuum within said enclosure; exposing said cloth layer to said carrier gas and said metal vapor; heating said cloth layer in said enclosure to decompose said metal vapor and cause deposition of the metal in the vapor about the fibers in the cloth; exhausting said carrier gas and metal vapor from said chamber after the deposition of the metal achieves a sheath of desired thickness about each of said fibers; and removing said cloth layer from said enclosure.

11. The method of Claim 9 wherein said carrier gas is an inert gas.

12. The method of Claim 10 wherein said inert gas is argon.

13. The method of Claim 9 wherein the metal in said metal vapor is electrically conductive.

14. The method of Claim 9 wherein the metal vapor is iron pentacarbonyl.

15. The method of Claim 9 wherein the fibers of said cloth are all of the same substance.

16. The method of Claim 9 wherein selected ones of the fibers of said cloth are of different substances.

17. The method of Claim 9 wherein all of the fibers of said cloth have approximately the same diameter.

18. The method of Claim 9 wherein selected ones of the fibers of said cloth have different diameters.

19. The method of Claim 9 further including the step of fabricating multiple ones of said metal coated cloth layers in an epoxy-matrix composite.