GB2308127A

GB2308127A - Radiation absorbing materials

Info

Publication number: GB2308127A
Application number: GB9625975A
Authority: GB
Inventors: Scott Allen Kuehl
Original assignee: AMS POLYMERS
Current assignee: AMS POLYMERS
Priority date: 1995-12-15
Filing date: 1996-12-13
Publication date: 1997-06-18
Also published as: DE29621804U1; GB9625975D0

Description

TITLE OF INVENTION Radiation absorbing materials.

FIELD OF INVENTION The present invention relates to radiation absorbing materials.

BACKGROUND TO INVENTION in electromagnetic measurement the radiation pattern of a scattered field emanating directly from a primary source is measured. It is advantageous to measure radiation patterns in the absence of electromagnetic interference, which is typically produced by secondary fields arising from reflection or scattering from nearby bodies also illuminated by the primary source. These fields interfere with the measurement ot the field of the primary source thus degrading the accuracy of the measurement or prohibiting the correct interpretation of the measurement.

An interference free measurement of a radiation pattern can be achieved by suspending an object, which is radiating, directly or by scattering in'free space', otherwise known as the sky. In many practical applications this is not feasible and the measurement must be made on the ground. The earth itself and any objects nearby the object being measured can produce unwanted electromagnetic interference via direct scattering or by multipath.

To eliminate this unwanted interference, the lendscape around or between the measurement device and radiating or scattering object can be covered with a suitable material to absorb the radiation, which might otherwise interfere with the desired signal thus imitating the free space condition.

It 13 an object of the invention to provide a radiation absorbing material to achieve the purpose as set out above.

SUMMARY OF INVENTION According to the invention, a radiation absorbing material, includes a particulate component mixed with an electrically conducting component ; and a bonding component bonding the particulate component and the electrically conducting component together.

The particulate component may be a low density polymer, glass, mineral or ceramic material The particulate component may include expanded polystyrene beads.

The polystyrene beads may have a density of about 17 to 25 kgim3.

The particulate component also may include glass microspheres, expanded polyvinylchloride or any other kind or combination of impervious light weight foamed ceramic, mineral or polymer in particulate form.

The electrically conducting component may be a carbonaceous material, e. g. graphite powder.

The bonding component may be a slightly non-wetting binder, such as a phenolic resin.

The resin may have a viscosity such so as to allow mixing of particulate carbonaceous material into the resin and subsequently the carbon mixed resin with the particulate component.

The resin may not be able to fully wet the carbonaceous material.

The resin may be urea-formaidehyde, phenol-formaldehyde, or furan.

Also according to the invention, a method of producing a radiation absorbing material, includes the steps : a) of mixing a bonding component and an electrically conducting component to form an initial mixture ; b) of mixing a particulate component and the initial mixture to form a final mixture bonding component ; and c) of allowing the final mixture to cure.

In one practical form the method may inclue the procedure of blending a particulate carbon into resin at a specific weight or volume fraction, then blending a specific amount of expanded polystyrene into the carbon loaded resin. The resin catalyst can be added at almost any time during the blending operation.

The final mixture may bc moulded as required. it can be packed into a mould of arbitrary shape and left to cure, if required with the aid of mild heat. In one practical example it has been found that there is only a mild exotherm, which allows the passage of considerable time before the body sets or cures. This exotherm is mild enough so as not to thermally degrade the polystyrene component.

After demoulding, the moulded body can be sanded, machined or cut to any desired shape.

The material can also be combined in various ways with the aid of an adhesive, which mll not damage the polystyrene component. The material could be left in the mould which could be a plastic film or a composite skin, which also could be a part of a larger object, like an aircraft wing.

In one example, the body resembles a conglomerate consisting of expanded polystyrene beads whose surface has been wetted with the carbon loaded resin. The resin does not penetrate the expanded polystyrene bead. This resin bonds all the beads together while at the same time produces a tortuous but continuous phase which conducts electricity ohmically at all frequencies. tt has been found in practice that a body produced in accordance with the invention has an open porosity, which can be varied with the packing pressure, choice of resin to polystyrene volume fraction and choice of the particule size or particle size distribution of the expanded polystyrene.

It is preferable to use expanded polystyrene as this material is cheap, has a low density (17 to 25 kgim3) and is availabie in a wide variety of shapes and sizes.

It must be mentioned that in a practical example the body is not a foamed carbon loaded resin but a carbon loaded resin blended with a prefoamed carbon free material.

Although expanded polystyrene is preferred, glass microspheres, expanded polyvinylchloride or any other kind or combination of impervious light weight foamed ceramic, mineral or polymer in particulate form can be used.

It is desirable to use resins free of solvent or components damaging to the other constituents of the radiation absorbing material. The resin preferably has a viscosity low enough so as to allow mixing of the particulate carbon into the resin and subsequently the carbon loaded resin with the prefoamed particulate material. It is also advantageous that the resin used is one whereby the resin is not able to fully wet the carbon particulate.'Wetting' resins which, through the manifestation of surface tension and surface energy, wet the carbon throughout the blending stage to full cure while ultimately preventing interparticle contact and thus prohibiting efficient OhmicBelectrical continuity between conductive filler particles.

Interparticle contact is a desirable but not a necessary attribute of a microwave absorber. Phenolic resins and their related resin systems, such as urea-formaldehyde, phenol-formaldehyde, furan, etc. not only exhibit low viscosity and good strength even when loaded with filler after cure, they also do not damage the polystyrene component and allow for electrical percolation at low volume fractions of conductive filler even after full cure.

It is desirable to use nondamaging solvents to reduce the viscosity of the resin carbon mixture when blending with the prefoamed material if this is necessary. Phenolic resins exhibit characteristics which allow them to be diluted with water, ethanol, and methanol. These solvents are also not damaging to foamed polystyrene.

Carbon particulate is desirable as the conductive filler but not necessarily the onty additive. Metal particles may also be used. However, it should be noted that most metals contain an electrically insulating layer of stable oxide on their surface thus preventing the desired Ohmic percolation phenomenon. Although a body made using metal particulate is lossy, they do not exhibit the same attributs as one made from carbon. Platinum and other noble meta Is would be far too expensive. Carbon particulate need not be the only form used in the manufacture of the body. Conducting carbon fibre such as graphite or phenolic resin pyrolized to various temperatures could be used in addition to carbon particulate. Many kinds of r-arhnn partirtilate Ore available for use as the conductive filler. Such carbon particulate are generally dassified by the particle size, shape, crystallinity, morphology and intrinsic conductivity.

The carbon particuiate should be clean of surface treatments or additives which might have been added to aid in the dispersability of the material into resin systems. These treatments or additives may degrade the onset of percolation in the host phenolic resin system or otherwise may not be required in this system. It is therefore advantageous to use, as the predominant conductive filler, graphite powder of a nominal particle size of between 44 and 71 microns. It is advantageous that the graphite powder has a purity above 95%, although lower purities are permissible with a decrease in performance.

In addition to graphite powder, carbon black can be used simultaneously.

Depending upon the type of carbon black, its intrinsic conductivity, particle or agglomerate size, the volume fraction of carbon black has to be kept to a small percentage because the particle size of carbon black tends to increase the viscosity of the resin system dramatically as the volume fraction increases.

It is advantageous to have filler in the phenolic resin system to increase strength. The graphite powder performs a strengthening function as well as the electromagnetic function. A similar state of affairs exists if other fillers are incorporated exclusively or simultaneously.

The size of expanded polystyrene bead and the volume fraction of expanded polystyrene is important and a fine balance is to be found between strength and desirable electromagnetic properties versus volume fraction.

It is advantageous to use a bimodal distribution of expanded polystyrene beads. The packing efficiency is improved if a mixture of large and small beads are used and this produces an increase in strength and rigidity.

Other additives, which could play the role of the prefoamed material. have to exhibit optimal volume fractions.

Although a mixture of large and small expanded polystyrene beads facilitates an increase in strength, useful relative amounts of these two sizes should be used. Fine beads have an increased surface area compared to the same volume of large beads.

The graphite powderlresin mixture preferably covers a larger surface area compared to the same volume fraction of monodisperse beads.

Strength could be compromised in that there is a thinner layer of resin coating the beads and bonding them together. It has been found that as this layer becomes thinner, the intrinsic electromagnetic properties of the bulk resin/graphite mixture change and tend to reflect properties indicative of surface conduction.

It has been found that at sufficiently high volume fractions, the resin forms a continuous phase around the polystyrene. At lower volume fractions, the interb a energy between the resin and the polystyrene exceeds the surface energy of the resin itself thus producing a discontinuous phase of resin over the particles in general, and electromagnetic properties tend towards that of nascent unbonded expanded polystyrene beads, a situation which may detract from optimal electromagnetic properties.

DESCRIPTION OF EXAMPLES The invention will now be described by way of example without limiting the field ot protection.

Example 1 A radiation absorbing material was prepared from the following constituents : 12 grams phenolic resin . 6 grams phenolic catalyst 6. 6 grams 96% (Kropfmuhl) graphite powder 10 grams 2-2. 5 mm expanded polystyrene beads (precursor commercially available as BASF type 4031423).

The final product was packed in a mould, cured and a measurement was conducted at a frequency of 11-17 GHz resulting in ct = 1. 8-j. 5 (e^ = complex dielectric constant) and at a frequency of 2. 6-4. 2 GHz resulting in e*= 2. 2j. 4.

Example A radiation absorbing material was prepared from the following constituents : 12 grams phenolic resin . 5 grams phenolic catalyst 6. 7 grams (Kropfmuhl) 96% graphite powder 7. 3 grams 2-2. 5 mm expanded polystyrene beads (precursor commercially available as BASF type 403/423) 3 grams 5-1 mm expanded polystyrene bead precursor (commercially available as BASF type 655).

The final product was packed in a mould, cured and a measurement was conducted at a frequency of 11-17 GHz resulting in 1. 8-j. 5 (e* = complex dielectric constant).

Exampie 3 A radiation absorbing material was prepared from the following constituents : 8 grams phenolic resin . 6 grams phenolic catalyst 7 grams (Kropfmuhl) 99. 99% graphite powder . 55 grams (3M) glass microspheres (C22) 3 grams ethanot The final product was packed in a mould, cured and a measurement was conducted at a frequency ot 11-17 GHz resufting in * = 2 2-j [l 35 (11 GHz, 1 1 (17 GHz)] (e = complex dielectnc constant).

Exempte 4 A radiation absorbing material was prepared from the following constituents : 12 grams phenolic resin . 5 grams phenolic catalyst . 6 grams Ketjen Black EC 6. 2 grams (Kropfmuhl) 99. 99% graphite powder 1. 0 grams (3M) glass microspheres (C22) 5 grams 2-2. 5 mm expanded polystyrene beads (precursor commercially available as BASF type 403/423).

4 grams. 5-1 mm expanded polystyrene beads (precursor commercially available as BASF type 655).

4 grams ethanol The final product was packed in a mould, cured and a measurement was conducted at a frequency of 11-17 GHz resulting in e* = 1. 65-j. 3 (~* = complex dielectric constant).

It has been found that the bonding resin does not penetrate the prefoamed material, is able to hold a specific amount of carbon particulate without becoming too viscous, is able to be diluted with solvent which does not damage the prefoamed material, and has properties which allow for sufficient interparticle contact to remain between conductive filler particles as the resin cures.

It also has been found that the conductive particulate filler is one which can reach the percolation threshold at low volume fractions in the bonding resin, does not have a stable oxide layer which would prevent Ohmic electrical contact between particles in the cured resin, has a sufficient intrinsic electrical conductivity so that a desired and specific effective medium electrical conductivity is produced at the frequency of radiation in question, is able to produce the desired Ohmic conductivity at low volume fractions and at the same time does not impart to the body a dielectric constant which is too high.

Example 5 A radiation absorbing material was prepared from the following constituents : 39. 3 weight % phenolic resin 1. 18 weight % phenolic resin catalyst 21. 96 weight % (Kropfmuhl) 96% graphite powder 27. 11 weight % polystyrene bead precursor expanded to 17 kgim3 density. 2 to 2. 5 mm bead diameter, commercially available as BASF type 403/423).

9. 83 weight % polystyrene bead precursor expanded to 17 kgim3 density,. 5 to 1 mm bead diameter, commercially available as BASF type 655.

. 52 weight % 99% ethanol The final product was packed and cured in large wooden moulds.

From a large block of cured material, dimensioned at 600X300X250 mm, a sample was measured between 11 and 17 GHz resulting in a complex dielectric constant of e'= 1. 8-j [. 27 (11 GHz),. 17 (17 GHz) !.

Profiles of the cured material, which exhibited a density of 53 kg/m3, were cut with a band saw to the following dimensions : Two bases 600X300X50 mm Six wedges : 100 mm base, 250 mm high, 600 long.

Three of the wedges were glued with polystyrene adhesive onto each of the bases. Sections were then cut from blocks of polystyrene foam insulating material having a density of 16 kg/m3. The'white'polystyrene was cut as (1) Four pieces of truncated white polystyrene wedges having a 100 mm base, 600 mm long and 80 mm height.

(2) Four pieces of white polystyrene wedges having a 35 mm base, 600 mm long and a height of 80 mm.

The 100 mm wide wedges were inverted and glued so as the fit in a confonnal way into the regions between the active absorbing wedges which were giued to the base previously. The white polystyrene was glued to the sides of adjacent active absorber wedges. The two small white polystyrene wedges were glued to the outside surfaces of the outer two active absorbing wedges on each block. The resultant structure resembled a trapezoid in cross-section having a bottom width of 300 mm and a top width of 270 mm.

This block was then covered in white. UV stebilised polyethytene shrink- wrap film 170 microns thick. After shrinking the film, the films thickness increased to between 220 and 320 microns.

The film was impervious to water and water vapour and rendered the material completely weatherproof.

Two blocks were placed together forming a square of 600 x 600 mm and measured at normal incidence between 2 and 18 GHz. Both polanzations with the microwave electric field parapet and perpendicular to the long axis of the active absorber wedges were used. For both polarizations, the reflectivity loss down from a 600 x 600 mm square of aluminium was found to be between-25 to-55 dB/m2 between 2 and 18 GHz.

Claims

CLAIMS 1. A radiation absorbing material, which includes a particulate component mixed with an electrically conducting component ; and a bonding component bonding the particulate component and the electrically conducting component together.
2. A material as claimed in claim 1, in which the particulate component is a low density polymer, glass, mineral or ceramic.
3. A material as claim in claim 2, in which the particulate component includes expanded polystyrene beads.
4 A material as claimed in claim 3, in which the polystyrene beads have a density of about 17 to 25 kg/m3.
5. A material as claimed in any one of the preceding claims, in which the particulate component indudes glass microspheres, expanded polyvinylchloride or any other kind or combination of impervious light weight foamed ceramic, mineral or polymer in particulate form.
6. A material as claimed in any one of the preceding claims, in which the electrically conducting component is a carbonaceous material, e. g. graphite powder.
7 A material as claimed in any one of the preceding cX ^, In whîc the bonding component is a slightly non-wetting binder, such as a phenolic resin.
8. A material as claimed in claim 7, in which the resin has a viscosity such as to allow mixing of particulate carbonaceous material into the resin and subsequently to allow mixing of the carbon mixed resin with the particulate component.
9. A material as claim in claim 8, in which the resin is not able to fully wet the carbonaceous material.
10. A material as claimed in any one of the claims 7 to 9, in which the resin is uroa-formaldehyde, phenol-formaldehyde, or furan.
11. A method of producing a radiation absorbing material, which includes the steps : a) of mixing a bonding component and an eledrically conducting component to form an initial mixture : b) of mixing a particulate component and the initial mixture to form a final mixture ; and c) of allowing the final mixture to cure.
12. A method as claimed in claim 11, in which the bonding component is a slightly non-wetting binder, such as a phenolic resin.
13 A method as claimed in claim 11 or claim 12, in which the particule component is a low density polymer, glass, mineral or ceramic.
14. A method as claimed in claim 13, In which the particulate component includes expanded polystyrene beads.
15. A method as claimed in any one of claims 11 to 14, in which the electrically conducting component is a carbonaceous material, eg. graphite powder.
16. A radiation absorbing material substantially as described in any of the specific Examples.
17. A radiation absorbing material according to claim 1 and substantially as herein described.
18. A method of producing a radiation absorbing material substantially as described in any of the specific Examples.
19. A method according to claim 11 and substantially as herein described.