FIELD OF THE INVENTION

[0001] The invention relates to electromagnetic interference (EMI) shielding for wires and cables.

BACKGROUND OF THE INVENTION

[0002] Electromagnetic shielding is often required to minimize noise and/or cross-talk in wires and cables, such that the data signal remains uncorrupted. Some examples of cables include twisted-pair cables, such as MIL-STD-1553, and coaxial cables, such as RG-316. Wire harnesses and data bus cabling also often require EMI shielding. Coaxial cables are used with RF, microwave, and millimeter wave components and are typically constructed, from interior to exterior, of an elongated conductive core (e.g. silver coated copper wire), a dielectric coating (e.g. Teflon), a second conductive layer for shielding (e.g. silver coated copper braid), and a jacket (e.g. fluorinated ethylene propylene). The core conductor transmits electrical signals (electromagnetic waves) along its length. The dielectric coating isolates electrically the conductive core from the shielding layer, creating a transmission line for the data signal to propagate. The impedance of the cable can be tuned by varying the thickness of the dielectric layer. The shielding reduces or suppresses the electromagnetic field that is generates around the conductive core when electrical current flows along the conductive core. The shielding also reduces or eliminates electromagnetic interference (EMI) from electromagnetic (EM) fields external to the data cable, to prevent "cross-talk" between data or signal cables. It is important to reduce and exclude any electronic noise from external EM fields to maintain the integrity of the data signal on the data cable. The jacket provides mechanical durability and integrity, and holds the sub-layers and members in place while insulating the cable. The shielding tape consists typically of metallic tape and/or metallic braid, or metallic coated thermoplastic threads or films, and provides good low-frequency shielding effectiveness (> 60 dB below 1 GHz). The drawbacks of metallic tapes and braids are their heavy basis weight, and a "skin effect", which reduces shielding performance at high frequencies (greater than 1-10 GHz). SUMMARY OF THE INVENTION

[0003] The present invention provides a conductive cable, comprising (i) an elongated conductive core, (ii) a dielectric material surrounding the conductive core, (iii) a CNT shielding wrap in one or more layers around the dielectric material, and (iv) a casing or jacket surrounding the CNT shielding wrap.

[0004] The present invention also provides a conductive cable, comprising (i) elongated conductive cores, (ii) a dielectric material surrounding the conductive cores, (iii) a CNT shielding wrap in one or more layers around the dielectric coated conductive cores, and (iv) a casing or jacket surrounding the CNT shielding wrap. The dielectric coated conductive cores can optionally be ordered in twisted pairs which helps to cancel out EMI from external sources.

[0005] The CNT shielding wrap comprises a nonwoven structure comprising CNTs and a porous substrate layer.

[0006] The CNT shielding wrap alternately comprises a film structure comprising CNTs and a polymer binder.

[0007] In an aspect of the invention, the substrate layer can include a porous carrier material. Examples include carbon fiber veil, conductive polymer veil, a polymer film, a metalized polymer film, metallic foils, expanded foils, and a meshes/wovens layer.

[0008] In a further aspect of the invention, the CNTs can include single-wall or multi- wall carbon nanotubes, and preferably single-wall CNTs.

[0009] In another aspect of the invention, the typical aspect ratio of the CNTs are at least about 10 ^x :l, where x is selected from the group consisting of 3 (1000:1), 4 (10,000:1), 5 (100,000:1), and 6 (1,000,000:1), and preferably at least about 100,000:1.

[0010] The present invention also provides a shielding tape consisting of a roll of a CNT shielding wrap comprising a nonwoven structure comprising CNTs and a substrate layer or CNT/polymer film comprising CNTs and a polymer binder. In an aspect of the invention, the shielding tape has a tensile strength of at least 1 N/ram.

[0011] In another aspect of the invention, the shielding tape comprises CNTs can include single-wall or multi-wall carbon nanotubes, and preferably single-wall CNTs. [0012] In another aspect of the invention, the typical aspect ratio of the CNTs are at least about 10 ^x :l, where x is selected from the group consisting of 3 (1000:1), 4 (10,000:1), 5 (100,000: 1), and 6 (1,000,000:1), and preferably at least about 100,000:1.

[0013] In yet another aspect of the invention, the substrate layer can include a layer selected from the group consisting of a carbon fiber veil, conductive polymer veil, a polymer film, a metalized polymer film, a metallic foil, an expanded foil, and a meshes/wovens layer.

BRIEF DESCRIPTION OF THE FIGURES

[0014] Figure 1 shows the basic principles to reflection of electromagnetic waves.

[0015] Figure 2 is Table A which shows the CNT nonwoven structures which are wet- laid and include a carbon fiber veil layer.

[0016] Figure 3 shows the thickness and tear strength properties of various CNT-carbon fiber structures as the basis weight of the carbon fiber veil increases.

[0017] Figure 4 shows the shielding effectiveness of 6" x 6" specimens of CNT shield structures P3. P4, and P6 from Table A.

[0018] Figure 5 is Table B which shows the results of EMI resistance testing of RG-316 configured conductors using a varying number of wrap layers of a nonwoven CNT structure (PI) that comprises SWCNTs (22 gsm) on a carbon fiber veil (7 gsm).

[0019] Figure 6 shows the effectiveness in EMI shielding of a nonwoven CNT structure (PI) shield wrappings of the invention.

[0020] Figure 7 shows the effectiveness in EMI shielding of a nonwoven CNT structure (P4) shield wrappings of the invention.

[0021] Figure 8 shows the effectiveness in EMI shielding of the nonwoven CNT structure (P4) shield wrappings of the invention with the addition of a single Ag/Cu braid.

[0022] Figure 9 is Table C which shows the weight savings of single braid Ag/Cu RG- 316 cables with the addition of P4 CNT shield tape compared to double braided RG-316 coaxial cable.

[0023] Figure 10 is Table D which shows the structures of various shield layers comprising one or more nonwoven CNT structures or CNT/polymer film structures, along with physical properties (thickness and basis weight of the shield), performance (resistance, conductivity and shielding effect), and tear strength (as maximum load, N).

DETAILED DESCRIPTION OF THE INVENTION

[0024] The present invention provides a CNT shielding wrap comprising a CNT nonwoven deposited onto a layer of porous carrier substrate such as carbon fiber veil. The CNT nonwoven shielding wrap provides effective shielding with a significantly reduced weight basis (22 gsm) as compared to conventional metallic tapes (322 gsm) and single braids (1347 gsm). For a RG-316 cable, greater than 60 dB shielding can be achieved with CNT shielding wrap with a cable weight of 1.6 g/ft compared to 5.4 g/ft using CoTS (Ag/Cu braid) shielding. A nonwoven CNT structure comprising CNTs carried on the carbon fiber veil layer can be made by a wet-laid nonwoven process as described in US provisional application 62/030,860, filed July 30, 2014, and US provisional application 62/111,624, filed February 3, 2015, the disclosures of which are incorporated by reference in their entirety. An example CNT used in the nonwoven CNT structure has an aspect ratio of at least about 10 ^x :l, where x is selected from the group consisting of greater than 3 (1000:1), at least 4 (10,000:1), at least 5 (100,000:1), and at least 6 (1,000,000:1). The CNTs can include single wall CNTs (SWCNTs), double wall CNTs (DWCNTs), multi-wall CNTs (MWCNTs), and mixtures and combinations thereof. Single wall CNTs having an aspect ratio where x is at least 5 (100,000:1) provides good results.

[0025] A CNT nonwoven shielding wrap can include a laminate of a plurality of nonwoven CNT structures, and a laminate of one or more nonwoven CNT structure and a layer of veil fabric and/or thermoplastic film. A thermoplastic film layer can provide tear strength to the laminate CNT nonwoven shielding wrap for commercial processing of shielded cable. A tensile strength of at least 1 N/mm, including a tensile strength in the range 1.2 - 6 N/mm, provides adequate processing. Laminates can be formed using conventional processing facilities, including a heated nip roll for thermally bonding a thermoplastic film to one or more layers of the nonwoven CNT structure.

[0026] A master roll of the nonwoven CNT structures and a laminate thereof can be slit normal to its axis into individual rolls of CNT shielding tape, typically about 0.25 inch (6 mm) or more in width, and typically up to about 2 inches (5 cm). The CNT shielding tape can then be spiral wrapped in overlapping layers along the length of a conventional dielectric-coated conductive core or provide shielding to wires and cables such as wire harnesses, the twisted-pair MIL-STD-1553, or coaxial RG-316.

[0027] Shielding effectiveness is the ratio of incident electric (and magnetic) field to the transmitted electric (and magnetic) field. Equation 1 shows the relationship(s) in decibels (dB), where E is electric field, H is magnetic field, and P is power.

[0028] The shielding effectiveness depends upon the thickness and electrical properties of the shielding material. The following equation (2) depicts the three losses which contribute to the total shielding effectiveness, where A is absorption losses, R is reflection losses, and B is correction factor. These losses (Equations 3, 5, and 7) can be modeled using plane wave shielding theory for reflection, absorption, and transmitted electromagnetic waves.

[0029] The absorption losses are driven by frequency, electrical properties, and thickness of the shield. Because the CNT nonwoven shield wrap is thin and nonmagnetic, the contribution of absorption losses to shielding effectiveness will be nominal.

[0030] Where d is the thickness of the shield (m), and δ is the skin depth (m). The skin depth can be calculated as a function of frequency (Hz), shown in Equation 4, where ω is the angular frequency (radian/s), μ is the magnetic permeability of the material (H/m), and σ is the electrical conductivity of the material (S/m).

[0031] The reflection losses are driven the intrinsic impedance of the shielding material; these losses can be approximated as followed, where Zo is the intrinsic impedance of free space (~ 377 Ω) and Z _s is the intrinsic impedance of the shielding material. ¾ can be calculated according to Equation 6.

[0032] For thin shields or composite shields the correction factor needs to be considered. Equation 7 outlines these correction factor losses. With large absorption loss, the correction factor is small; with small absorption loss, the correction factor is large.

[0033] The shielding effectiveness can also be approximated using the impedance method for reflected and transmitted waves. Equation 8 shows the relationship between impedance of the shield tape and shielding effectiveness, where Z ₁ is the open space impedance and Z ₂ is the impedance of the shielding material. Low sheet resistance is desirable to increase shielding effectiveness.

[0034] Another feature of a CNT shielding wrap is its tensile strength. Tensile strength can be measured in Newtons (N). Tensile strength is crucial to the manufacturing of shielded cables and wires, where automated systems wrap a stretched shielding tape in the processing of shielded wire or cable. The stronger the tensile strength, the more reliable and effective the shielding or wrapping process. The use of a melt spun nonwoven as the carrier veil can provide a much stronger (higher tensile strength) veil than that of a wet-laid veil material.

[0035] The CNT nonwoven shielding wrap of the present invention provides effective shielding with a significantly reduced weight basis as compared to conventional metallic tapes and braids. On an RG316 configured cable, the CNT tape wrap can provide a linear weight of about 1.6g/ft (an areal weight of about 30 gsm), compared to the conventional Ag/Cu single braids having a linear weight of about 3.44g/ft (an areal weight of about 1347 gsm).

Examples:

I. Nonwoven CNT structures

[0036] Nonwoven (NW) CNT structures comprising CNTs, including upon a porous substrate layer, which can include a carbon fiber veil layer, were made by a wet-laid nonwoven process, and are shown in Table A (Figure 2). The CNTs comprised single-wall carbon nanotubes (SWCNT). Basis weight is in grams per square meter (gsm). Without a carrier (veil) material, the SWCNT nonwoven does not have the mechanical strength to be handled.

[0037| The bulk of the mechanical properties come from the substrate layer (the carrier veil material). Examples of a substrate layer can include a carbon fiber veil, polymer film layer, metalized polymer film layer, a conductive woven fabric, metallic foils, expanded foils, and meshes, and combinations and laminates thereof. Non-limiting examples of a polymer layer can include a polyethylene terephthalate (PET) film and a polyimide film, and including a thin- gauge (10-12 micron) mPET film. The polymer layer can be meltspun or meltblown. A non- limiting example of a meltspun or meltblown polymer layer is a meltspun polyamide. A non- limiting examples of a metalized film layer is a nickel acrylic laminate film. In the selection of a substrate layer or veil material, there can be a trade-off in basis weight, thickness, and tear strength, depending upon type and material chosen. For example, Figure 3 shows that as a carbon fiber veil becomes heavier, it provides more tensile strength, but at the cost of thickness and basis weight of the nonwoven.

II. Nonwoven CNT Structure - Shielding Performance

[0038] A nonwoven CNT structure was used as a shield wrapping for a coaxial cable having a configuration of an RG-316 cable, with the nonwoven CNT structure shielding replacing the single or double Ag/Cu braid. Several of the nonwoven CNT structures (P3, P4, and P6) were tested for shielding effectiveness, per ASTM D4935. Figure 4 outlines the results of these panel tests. The nonwoven (PI) CNT structure comprises SWCNTs (22 gsm) on a carbon fiber veil (7 gsm) made by the wet laid process described in the US provisional applications mentioned above. Several configurations of a shield wrap for use on RG-316 configured conductors were made, using varying numbers of wrap layers. Conventional double braided RG-316 cables provide > 60 dB shielding. Conventional single braided RG-316 cables provide > 40 dB shielding. The results of EMI shielding effectiveness testing of PI tape wrapped RG-316 cables are shown in Table B (Figure 5).

[0039] The shielding effectiveness of numerous CNT nonwoven structure shield (PI) wrappings is shown in Figure 6. Figure 6 shows that a conventional double silver/copper braiding (RG-316) provides better shielding at low EM frequencies. As EM frequency increases, the effective diameter of the copper wires in the braid for conduction is reduced by the skin effect, and shielding performance decreases.

[0040] The shielding effectiveness of numerous CNT nonwoven structure shield (P4) wrappings is shown in Figure 7. Figure 7 shows that a conventional double silver/copper braiding (RG-316) provides better shielding at low EM frequencies. As EM frequency increases, the effective diameter of the copper wires in the braid for conduction is reduced by the skin effect, and shielding performance decreases. The low frequency performance can be increased by keeping one Ag/Cu braid and wrapping the CNT shield tape over said single braid. Figure 8 compares the shielding effectiveness of a single braided RG-316 cable, a single braided RG-316 cable with 2 wraps of P4 CNT shield tape, a single braided RG-316 cable with 3 wraps of P4 CNT shield tape, and a double braided RG-316 cable. Table C (Figure 9) outlines a summary of P4 shielded RG-316 cables and weight savings compared to double braided RG- 316 CoTS. Adding a single wrap of P4 increases shielding effectiveness by ~ 20 dB and only adds 3% weight; adding 2 wraps of P4 yields shielding effectiveness greater than or equal to double braided RG-316 (> 200 MHz) with almost 25% weight savings.

HI. CNT Structures and Laminates - Shielding Performance

[0041] Nonwoven CNT structures and laminates thereof were used as shield wrappings for a conductive cable, such as an RG-316 cable configuration. Several configurations of shield wrap were made, including CNT nonwoven structures, and laminates of CNT nonwoven structures and thermoplastic films.

[0042] Table D (Figure 10) shows the structure of various shield layers, along with physical properties (thickness and basis weight of the shield), performance (resistance, conductivity and shielding effect), and tensile strength (as maximum load, N). Shield-6 has a CNT structure (P7) that comprises a CNT/polymer film and is made by a coating process. CNTs are dispersed in polymer, optionally with solvent, and said CNT/polymer solution is coverted to a film with a coating process. Tensile strength testing was based on ASTM D5034, for nonwoven materials. Mechanical characterization was performed using an Instron testing unit with 100 N load cell. All mechanical data was tabulated, measuring at least 5 sample specimens for each CNT shield tape product. The test specimen consists of a 7.9 mm wide by 60 mm long strip; thickness of the samples were measured using a digital micrometer. For tensile strength testing, the specimens were mounted with a 25 mm spacing; the rate of pulling during testing was maintained at 5 mm/min, measuring stress versus strain.