Electromagnetic field absorbing composition

Abstract

This invention relates to the field of an electromagnetic (EM) field absorbing composition, in particular, those capable of providing absorbance in the frequency of commercial radar. The composition finds particular use as a radar absorbing coating for wind turbines, in particular for use in onshore and offshore environments. There are further provided coated surfaces comprising the composition, methods of absorbing EM radiation, and methods of use of such a composition, such that a surface coated in the composition is capable of absorbing EM radiation. There is provided an electromagnetic radiation absorbing composition comprising elongate carbon elements with an average longest dimension in the range of 50 to 1 000 microns, with a thickness in the range of 1 to 15 microns and present in the range of from 0.5 to 20 volume % dried, in a non conductive binder.

Claims

1. A method of providing absorption of electromagnetic radiation at a selected frequency on a surface structure or body or portions thereof, comprising the steps of determining the selected frequency, applying at least one coat of electromagnetic radiation absorbing composition at a thickness which selectively absorbs at said frequency or an appliqué film comprising the electromagnetic radiation absorbing composition, to a first side of said surface structure or body or portions thereof, and drying the electromagnetic radiation absorbing composition, wherein the electromagnetic radiation absorbing composition comprises a carbon filler substantially consisting of machined elongate carbon elements with an average longest dimension in the range of 100 to 150 microns, with a thickness in the range of 1 to 15 microns, wherein the total carbon filler content is present in the range of from 2 to 10 volume % dried, in a non conductive binder, and wherein the binder is selected from an acrylate, an epoxy binder, an acrylic, a urethane and epoxy-modified acrylic, a polyurethane, an alkyd, a modified alkyd, a fluoropolymer, and mixtures thereof, wherein the electromagnetic radiation absorbing composition after the drying step has a dielectric loss tangent that is configured to absorb microwave radiation, the dielectric loss tangent being a ratio of an imaginary component of complex permittivity to a real component of complex permittivity, wherein the method further comprises applying at least one or more sub-layers, each of which have been separately applied in an orthogonal direction to the preceding layer.

2. The method according to claim 1, wherein the elongate carbon elements are cylindrical and have a diameter in the range of from 5 to 10 microns.

3. The method according to claim 1, wherein the elongate carbon elements have an average thickness to average longest dimension ratio of from 1:10 to 1:25.

4. The method according to claim 1, wherein the binder is selected from a water based dispersion comprising a binder selected from an acrylic, or polyurethane based latex.

5. The method according to claim 1, wherein the composition is a liquid formulation.

6. The method according to claim 1, wherein the thickness of said coating is one quarter of the wavelength (λ/4) of the resonant frequency of the incident radiation to be absorbed.

7. The method according to claim 1 further comprising providing an electromagnetic reflective backplane between the surface, structure or body and the at least one dried coating.

8. The method according to claim 1, wherein the machined elongate carbon elements are milled carbon fibers.

9. The method according to claim 5, wherein the composition comprises a solvent.

10. The method according to claim 1, further comprising applying at least one coat of electromagnetic radiation absorbing composition at a thickness which selectively absorbs at said frequency or an appliqué film comprising the electromagnetic radiation absorbing composition, to a second side of said surface structure or body or portions thereof.

Description

(1) Embodiments of the invention are described below by way of example only and with reference to the accompanying drawings in which:

(2) FIG. 1a and FIG. 1b show graphs of the real component of permittivity and the imaginary component of permittivity (dielectric loss), respectively for three different aspect ratio carbon elements.

(3) FIGS. 2a to 2e show graphs of the permittivity of milled carbon fibres dispersed in polyurethane (PU) at various percentage fills by volume.

(4) FIG. 3 shows a graph of reflection and transmission through a sample composed of milled carbon fibres dispersed in PU at 0.5% by volume.

(5) FIG. 4 shows a graph of reflection and transmission through a sample composed of milled carbon fibres dispersed in PU at 20% by volume.

(6) FIG. 5 shows a graph of reflectivity of a 3 GHz absorber.

(7) FIG. 6 shows a graph of reflectivity of a 9.4 GHz absorber.

(8) FIG. 7 shows a structure and a film coating the structure.

(9) Turning to FIGS. 1a and 1b, FIG. 1a shows a graph of the real component of permittivity for (i) spherical particles 20 vol % in wax, line 1a, (ii) carbon fibres according to the invention, 6 vol % in PU, line 2a and (iii) chopped fibres 1 vol % in PU, line 3a. The use of wax, rather than PU, as the inert binder for the spherical particles does not alter the permeability/permittivity, and so does not change the formulations effectiveness as an absorber.

(10) FIG. 1b shows a graph of the imaginary component of permittivity (dielectric loss) for (i) spherical particles 20 vol % in wax, line 1b, (ii) carbon fibres according to the invention, 6 vol % in PU, line 2b and (iii) chopped fibres 1 vol % in PU, line 3b. The results are discussed in Experiment 1, below.

(11) FIGS. 2a to 2e show graphs of the permittivity of milled carbon fibres dispersed in PU over a range of frequencies, with different rates of inclusion at 0.5 vol %, 2 vol %, 3 vol %, 5 vol % and 6 vol %, respectively. The graphs 2a to 2e show that as the vol % of carbon fibre increases, both the real ∈′ (upper lines) and imaginary ∈E″ (lower lines) components of permittivity increase.

(12) However, at lower levels of inclusion, such as FIG. 2a, shows that when the loading is reduced to 0.5 vol %, poor levels of loss (imaginary permittivity) are exhibited. This means there is no effective mechanism for energy dissipation within the layer and therefore low vol % may be considered to be ineffective for the production of radar absorbing materials.

(13) FIG. 3 shows a graph of reflection, line 5, and transmission, line 4, through a sample composed of milled carbon fibres dispersed in PU at 0.5% by volume (sample XC4343), FIG. 3, shows that when the sample is loaded with very low levels of carbon fibre (even in the highly preferred length range) the composition possess low reflectance, line 5, and is highly transparent to the incident radiation, i.e. due to the lack of absorption.

(14) FIG. 4 shows a graph of reflection line 15, and transmission, line 14, through a sample composed of milled carbon fibres dispersed in PU at 20% volume (sample XC4344). As can be seen a 20 volume % loading produces a near metal-like performance, leading to a reflective material (high reflectance value, as indicated by line 15), with only a low level of absorption. As the percentage volume increases beyond 20 vol %, the composition will move towards a perfect reflector, and so will provide little or no absorbance.

(15) FIG. 5 shows a graph of reflectivity of a composition which has been formulated and deposited at a selected thickness to specifically absorb at 3 GHz. The composition (sample XC4332) comprises milled carbon fibres dispersed in PU at 5.5 volume %. The composition was deposited onto the test surface at a thickness of 4 mm (λ/4). The graph shows good absorption at greater than 99% (see Table 5), with the maximum absorption occurring in the 3 GHz region.

(16) FIG. 6 shows a graph of reflectivity of a composition which has been formulated to specifically absorb at 9.4 GHz. The composition (sample XC4288) comprises milled carbon fibres dispersed in PU at 5.0 volume %. The composition was deposited onto the test surface at a thickness of 1.5 mm (λ/4). The graph shows good absorption at greater than 99.9% (see Table 5), with the maximum absorption occurring in the 9.4 GHz region.

(17) FIG. 7 shows a structure 720 and a film 710 coating the structure 720. Structure 720 may be a wind turbine, e.g., a wind turbine located in a marine environment. The film may reduce radar reflections. The reduction of these reflections may reduce the structure's impact on the operation of nearby air traffic control (ATC), air defense (ADR), meteorological (MR) and Marine navigational radars (MNR).

Experiment 1

(18) Three compositions each containing a different shaped carbon particles were prepared, according to Table 1, below.

(19) TABLE-US-00001 TABLE 1 Different shaped carbon elements in a non conductive binder vol % FIG. 1a Element element and 1b type Average dimension dried binder Line 1 Spherical 2-12 micron(diameter) 20 vol %  wax particles Line 2 Milled 7 micron (diameter) 6 vol % Poly- carbon 100-150 micron(length) urethane Line 3 Chopped 7 micron (diameter) 1 vol % Poly- carbon 6000 micron (length) urethane

(20) The results of the above formulations are shown in the graphs in FIGS. 1a and b. The graphs show that as the aspect ratio increases, i.e. from spherical to milled to chopped fibre lengths, the dielectric loss tangent (ratio of imaginary to real component, ∈″/∈′) increases and the loading required to achieve absorbance decreases due to improved connectivity.

(21) To produce an effective absorber requires the correct values of real and imaginary components of permittivity, for example, materials with low values imaginary permittivity produce low conduction loss and therefore do not possess a mechanism for absorbing effectively. This is shown by the results for spherical carbon particles, lines 1a and 1b, in FIGS. 1a and 1b, respectively.

(22) Conversely, materials with high values of the real component of permittivity produce high impedance relative to air. The impedance mismatch at the material surface causes the electromagnetic radiation to be reflected. Likewise, materials possessing high loss tangents (∈″/∈′>1), similar to the results for chopped carbon fibres, lines 3a and 3b in FIGS. 1a and 1b, respectively, are not ideally suited to microwave absorption.

(23) Whereas elongate carbon elements that are provided in the dimensions (length) and inclusion ranges according to the invention, provide the optimum trade-off between real and imaginary components of permittivity, as shown in, lines 2a and 2b, in FIGS. 1a and 1b respectively.

(24) The absorption of a composition according to the invention comprising elongate elements when provided in the preferred range is demonstrated by the microwave absorption results given in FIGS. 5 and 6. The properties of said elongate elements in the composition according to the invention can be attributed to the selection of the narrow range of the length of the fibres in combination with their vol % inclusion to optimise their resulting coupling to the applied electromagnetic field. The coupling increases as fibre size increases with a resulting change in the permittivity. However, at lengths approaching 4 mm to 6 mm the primary mode of interaction will be one of reflection, as shown in the chopped fibres line 3a and 3b, in FIGS. 1a and 1b, respectively.

Experiment 2

(25) Preparation of Sample XC4332

(26) The composition was prepared with milled carbon fibres, whose average length was 100-150 microns, diameter 7 microns. The fibres were incorporated 5.5% by volume within a base polyurethane binder material.

(27) TABLE-US-00002 Component Equivalent Ratio Equivalent Weight PTMEG 1000 1.0 501.79* Trimethylol propane 0.2 44.7 Isonate M143 1.26 144.83*

(28) Table 2 showing formulation of the base polymer

(29) TABLE-US-00003 As a % by weight of PTMEG As a % by volume Additive and TMP in Part B of Polymer Silcolapse 0.12 — Milled Carbon 15.95 5.5 Fibres

(30) Table 3 showing additional components of the material system

(31) *typical values

(32) The material is manufactured using a “quasi-prepolymer” route. The Part B consists of two parts Isonate M143 to one part PTMEG by mass. The remaining (Polytetramethylene Glycol 1000) PTMEG is added to the Part A to aid with mixing.

(33) TABLE-US-00004 Formulation XC4332 Weight in grams PART A PTMEG 1000 192.53 Trimethylol propane 5.34 Silcolapse 430/BYK 085 0.30 Milled Carbon Fibres 40.10 PART B PTMEG 1000 54.55 Diphenylmethane diisocyanate (Isonate M143) ® 109.09 Part A is mixed with the Part B in the ratio 100:56 by weight.
Blending Part A

(34) The blending is performed using a low shear blender (e.g. Molteni planetary mixer). The TMP may be pre-dissolved in a small amount of the PTMEG to assist with the blending of Part A.

(35) The mixture is placed under a vacuum of at least 5 mbar until fully degassed. The mixing time depends on the type of equipment and the amount of material, but should be sufficient to achieve an evenly dispersed product, free from solid agglomerates. Care must be taken to ensure that the mixing process does not significantly affect the final density of the material.

(36) Blending Part B

(37) The dry PTMEG is heated to 60° C. and degassed for 2 hours at a reduced pressure of 5 mbar immediately before use. The PTMEG is then added to the Isonate®, with stirring, and the mixture is heated at 60° C. for 4 hours at a reduced pressure of 5 mbar.

(38) The composition was deposited as an appliqué film with a thickness of 4 mm (300×300 mm panel).

Experiment 3

(39) The measurements, as shown in Table 4 and 5 below, were undertaken using a focussed horn system arrangement. The compositions were manufactured by casting, i.e. forming an appliqué film at the desired thickness, but may alternatively be applied using spray painting technology or trawling, as hereinbefore defined. The appliqué test sample materials were made to dimensions of 300 mm×300 mm, with different thicknesses.

(40) The equipment comprised of an Anritsu 37397C vector network analyser connected to corrugated microwave horns. The horns were focused by mirrors to the mid-plane, where the test samples were positioned. The focussed horn set up was used to measure the complex scattering, S, parameters associated with transmission and reflection from the test samples, from which the permittivity, ∈, was obtained using the Nicholson and Ross method [Pitman K C, Lindley M W, Simkin D and Cooper J F 1991 Radar absorbers: better by design IEE Proc.—F 138 223].

(41) For reflectivity measurements, such as those in FIGS. 5 and 6 respectively, a metal backing plane was applied to the test samples and a similar set of measurements carried out, to determine the degree of absorption (i.e. reduced reflectivity) from the test samples.

(42) The elongate carbon elements were the same milled carbon fibres as defined in experiment 2 above. The following compositions were prepared in an analogous manner to those in experiment 2, with different vol % inclusion of milled fibres.

(43) TABLE-US-00005 Carbo nfibre Coating Sample (vol %) thickness/ number in PU mm ε′ ε″ XC4343 0.5 .sup. 1(±0.2) 4 at 15 GHz 0.5 at 15 GHz (FIG. 2a) FIG. 3 XC4285 2 1.2(±0.2) 14 at 10 GHz 1.7 at 10 GHz (FIG. 2b) XC4286 3 1.2(±0.2) 26 at 10 GHz 3.8 at 10 GHz (FIG. 2c) XC4288 5 1.4(±0.3) 33 at 10 GHz 6.1 at 10 GHz (FIG. 2d) XC4332 5.5 .sup. 4(±0.3) (FIG. 5) XC4297 6 4(±1) 38 at 10 GHz 13.3 at 10 GHz (FIG. 2e) XC4344 20 1.7(±0.5) (FIG. 4)

(44) Table 4 showing different vol % (dry) inclusions of milled carbon fibres in a PU mix.

(45) TABLE-US-00006 Sample measured with metal Sample backing number Sample measured without metal Peak (carbon Coating backing (Results at 15 GHz) loss Peak fibre vol % thickness/ Reflection Transmission Absorption position Reflectivity Absorption in PU) mm dB % dB % % GHz dB % % XC4343   1(±0.2) 7.6 17 1.4 72 11 (0.5 vol %) XC4285 1.2(±0.2) 2.0 63 5.8 27 10 16 9.2 12 88 (2 vol %) XC4286 1.2(±0.2) 1.8 66 7.3 18 16 12 15 3 97 (3 vol %) XC4288 1.4(±0.3) 4.7 34 7.0 20 46 9.4 33 0.05 99.95 (5 vol %) XC4332   4(±0.3) 3 21 0.8 99.2 (5.5 vol %) XC4297 4(±1) 3.4 46 19.4 1 53 (6 vol %) XC4344 1.7(±0.5) 1 80 36 0.03 20 (20 vol %)

(46) Table 5 showing reflection and transmission for different sample types.

(47) The results in Table 4 and Table 5 above, show that the optimal results for an absorber are achieved by selecting a narrow range of inclusion of said elongate carbon elements, namely greater than 0.5 vol % inclusion and 20 vol % or less. A reflectivity of 20 dB corresponds to 99% of the incident signal being absorbed.

(48) FIGS. 5 and 6 and the % absorbance values in Table 5 show that the milled carbon fibres, which have an average length and volume % inclusion, provided in the ranges according to the invention, give rise to effective absorbers.

(49) The compositions when provided at the prerequisite thickness to provide 3 GHz and 9.4 GHz absorbers, are mere examples of selected narrow frequency absorbers, and therefore the composition according to the invention is not limited to these frequencies. The composition may be deposited at other thicknesses in order to produce optimum performance at alternative frequencies.