WO2005002841A1

WO2005002841A1 - Composite dielectric materials

Info

Publication number: WO2005002841A1
Application number: PCT/AU2004/000896
Authority: WO
Inventors: Richard Donelson; John Kot; Michael Shane O'shea; Gary Peeters
Original assignee: Commonwealth Scientific And Industrial Research Organisation
Priority date: 2003-07-02
Filing date: 2004-07-02
Publication date: 2005-01-13
Also published as: AU2003903409A0

Abstract

The present invention relates to a composite dielectric material comprising a ceramic and/or metal having a shape with a high aspect ratio which is carried by a gas-containing structure. This material is useful for high frequency optics applications.

Description

COMPOSITE DIELECTRIC MATERIALS

The present invention relates to composite dielectric materials and methods for their production. More specifically, the present invention relates to composite structures of polymer, ceramic, metal and gas having controllable dielectric constants for applications where low manufacturing costs, light weight, flexible shapes and low dielectric losses are desirable such as in high frequency optics (radio frequency, microwave or infrared) , for example, microwave lenses, the Luneburg lens, spherical lenses formed from a number of discrete uniform shells and horn antennas .

BACKGROUND Composite dielectric materials (sometimes referred to as "artificial dielectrics") are mixtures of materials with different dielectric, physical, thermal and/or mechanical properties and having useful dielectric properties. A' composite dielectric material consisting of small metal particles, more specifically magnesium particles, dispersed in a polymerised plastic matrix is described in US 2,716,190. This patent further states that the polymer is either solid or expanded. These materials are said to have a controllable dielectric constant by varying the amount of metal particles and/or the density of the polymer, have dielectric losses on the order of 0.001 and may be used in microwave lenses . The composite described in US 2,716,190 uses fine, possibly flammable metal particles, which is seen to have some disadvantages with regard to safety of manufacturing. The methods described tend to have poor uniformity leading to anisotropic properties. In some cases, the dielectric losses are seen to be too large for applications where measurement of low strength signals is required. Furthermore, composites of this type can have dielectric losses that vary with frequency. A dielectric material consisting of individual in ection-moulded polymer pieces where the pieces are either solid or hollow spheres is described in

WO 93/10572. This application further describes the use of these dielectric materials in various shaped antennas. The method of WO 93/10572 requires the processing of a large number of in ection-moulded pieces that would lead to high costs. Furthermore, when the dielectric constant is controlled only through the density of a polymer, the resulting materials would be of a relatively high weight or density and they would therefore have high materials cost and limit the size of the lens. EP 632522 describes a dielectric lens produced by fusing pre-expanded polymer beads which also contains a ceramic to modify or condition the dielectric constant of the moulded material . Application of the mixing rules for ceramic dielectrics with polymers indicates that the use of ceramic powders will result in the use of' large quantities of the ceramic to affect the dielectric constant thus adversely affect the cost, weight, and dielectric losses. A useful high frequency optical device is the spherical Luneburg lens which is a spherically symmetrical lens having a graded dielectric constant ranging from 2 in the centre to 1 at the outer radius in a parabolic manner. Such a lens focuses an incoming plane wave to a point on the surface at the outer radius. In practice, the lens may be made from a series of concentric shells rather than a continuous graded dielectric constant in order to expedite manufacturing. Moreover, it is possible to focus an incoming plane wave to a point on a surface at a distance greater than the outer radius of the lens by changing the dielectric constant profile such that the dielectric constant at the centre of the lens is less than 2. SUMMARY We have now produced a composite dielectric material which can be made using a low cost production process where the resulting material has a low density, uniform dielectric properties, a controllable dielectric constant, and low dielectric losses. This composite material will be of use in high frequency optics such as a microwave lens, a spherical Luneberg lens or a horn antenna. According to the present invention there is provided a composite dielectric material comprising a ceramic having a shape with a high aspect ratio which is carried by a gas-containing structure. The composite dielectric material may be produced by extrusion, casting, moulding or spraying, preferably extrusion as it is the most cost effective method. Thus, further according to the present invention there is provided an extruded composite dielectric material comprising a ceramic and/or metal having a shape with a high aspect ratio which is carried by a gas- containing structure. The shape of the ceramic and/or metal is preferably a flake, disc, fibre or hollow sphere such that either one or two of the three possible dimensions is significantly smaller than the other dimension (s) . Preferably, the gas-containing structure is composed of one or more polymers in which a gas is contained. Suitable examples include a foamable, low loss (co) polymer such as polystyrene, polypropylene, polyethylene or polysiloxanes . The (co) polymer may be cross-linkable and/or cross-linked. Preferably, the gas-containing structure forms a lightweight scaffolding creating a self-supporting structure. The ceramic and/or metal particles are then held by the gas-containing structure, possibly forming part of the structure itself, or in contact with the gas-containing structure, or in the voids within the structure or any combination of these. The present invention also provides a method for the production of the composite dielectric material defined above which comprises extruding, casting, moulding and/or spraying the ceramic and/or metal and the gas-containing structure . When the gas-containing structure is a (co) olymer in which a gas is contained, it will be appreciated that the (co) polymer may be foamed either before, during or after the extrusion, casting, moulding and/or spraying step(s). Preferably, the composite dielectric material is moulded into a suitable shape using known techniques such as heat or a binder after the extrusion, casting, moulding and/or spraying and optionally foaming steps. The present invention further provides use of the composite dielectric material defined above in high frequency optics, such as a microwave lens, a spherical Luneburg lens or a horn antenna.

DETAILED DESCRIPTION For the purposes of this specification it will be clearly understood that the word "comprising" means "including but not limited to", and that the word "comprises" has a corresponding meaning. The composite dielectric material has a dielectric constant which can be tailored to have a value preferably of about 1 to about 2, although it will be appreciated that higher dielectric constants are possible. It is also preferable that the composite dielectric material is high in gas and of low dielectric loss in the desired frequency range. The density of the dielectric material is preferably about 0.02 to about 0.6 g/cc. Preferably the ceramic has a low dielectric loss and a high dielectric constant. Suitable ceramics include metal oxides such as titanium dioxide, aluminium oxide or barium titanate. A particularly preferred ceramic is titanium dioxide in its rutile form which suitably has a dielectric constant of about 100. The rutile titanium dioxide is advantageously doped, preferably with aluminium oxide, to substantially prevent electronic conduction which may result in increased dielectric loss. Suitable metals include aluminium, copper, silver or any other metal with a high electrical conductivity at the frequency of operation. Preferably the metal is aluminium, copper or silver and has a protective layer, such as, an insulating layer, for example, an oxide layer or a polymer film. The ceramic and/or metal phase acts as the primary basis for the dielectric properties of the composite material and is preferably held by the polymer structure and distributed relatively .evenly in space. The ceramic and/or metal has a shape with a high aspect ratio so as to maximise the dielectric effect and minimise the cost and weight of the composite dielectric material . The term "high aspect ratio" is used herein in its broadest sense and refers to a flake, disc, fibre or hollow sphere such that either one or two of the three possible dimensions is significantly smaller than the other dimension (s) or a combination thereof. Suitable examples are shown in Figure 1. The high aspect ratio shape is desirable to maximise the increase in dielectric constant and therefore minimise the quantity of ceramic and/or metal used resulting in a reduction in cost and weight. The structure uniformity is preferably maintained below about l/10^th the minimum wavelength so as to substantially avoid Bragg diffraction issues and other undesirable effects that occur when the size of the inclusions and/or their spacings become a significant fraction of the operating wavelength. In such cases the material properties may appear to be frequency dependent and the performance of the resulting device will be compromised. The ceramic and/or metal may be shaped using any suitable known method such as slurry milling, tape casting, firing and/or crushing. The term "carried by" is used herein in its broadest sense and refers to the ceramic and/or metal being contained by and/or supported by the gas-containing structure . The gas-containing structure may be any suitable structure which preferably has a low dielectric loss of less than about 0.0001 such as (co) polymers or glass in which a gas is contained. A suitable example includes a foamed polymer with ceramic and/or metal platelets contained in the faces of the cells as shown in Figure 2. The intention is to minimise the amount of polymer or glass and_. maximise the amount of gas thereby reducing the weight and cost of the overall composite material.

Preferably, the gas-containing structure is a polymer which will also participate in the dielectric effect without significantly increasing dielectric losses, such as, for example polypropylene, polyethylene, polystyrene and/or polysiloxane . Suitable polysiloxanes include end capped polyalkyl siloxane, alkyl cyclotetrasiloxane and end capped polyalkyl hydrogen siloxane. The polymer is preferably foamable so as to reduce the weight and cost of the material and its production. The polymer foam holds the high aspect ratio shaped ceramic and/or metal in a well distributed state so as to yield the desired dielectric behaviour at microwave frequencies . Other preferred properties of the polymer include structural integrity, weather resistance (if used outside) and being readily extrudable and castable. A suitable outer layer may be used to prevent ingress of moisture if desired. Alternatively, the gas-containing structure may be a polymer which is shaped in such a manner that the gas is contained therein. Examples include layered or corrugated sheets or stacked tubes in which the gas is located in between the layers, corrugations or tubes. In another arrangement, extruded tubing could be used to create a porous structure. This may be achieved by chopping and moulding the extruded tubes. The size of tube would need to be small relative to the wavelength. The gas is preferably chosen so that it is cost effective, inert and has a low dielectric loss. Suitable gases include inert gases such as nitrogen or air. It will be appreciated that the composite dielectric material may contain additives known in the art of composite manufacture, such as binders, dispersants, initiators, inhibitors, modifiers, plasticisers, catalysts, nucleants, lubricants, foaming agents, agents for controlling gas solubility or moisture solubility and compatibilisers . The additives are chosen not to have a major detrimental effect on the dielectric performance of the material . Such additives may be used in the method of the present invention to enhance (co) polymer formation and/or foaming. The extrusion step of the method of the present invention generally involves compounding the ceramic and/or metal with a polymer and foaming or forming the compounded ceramic and/or metal and the structure into the dielectric material which may be performed in one or two steps. Preferably, the extrusion occurs in one step. The extrusion may be performed in any suitable known apparatus such as a single or twin screw extruder, preferably a twin screw extruder. The foaming can be carried out using either physical or chemical foaming agents . The foaming agent is chosen to have a solubility, decomposition temperature (for chemical) and a vapour pressure appropriate to form the foamed structure as it is extruded from the die of the extruder . Physical foaming agents such as pentane or C0₂ are preferred as chemical foaming agents can leave polar residues that can exhibit lossiness. The most preferred foaming agent is pentane for the control of cell size including the nucleating effect of the ceramic and/or metal and density. When the gas-containing structure is a (co) polymer it may be either a preformed (co) polymer or formed or modified in-situ during the method to form the gas- containing structure. One means to form the (co) polymer in-situ is by the coupling of (co) olymers of appropriate functionalities either via inter-reaction of the (co) polymers or by the reaction of the (co) polymers with a coupling agent. For example, if the (co) polymer is a two part polymer capable of reacting to form a high molecular weight crosslinked or branched (co) polymer, the ceramic and/or metal may be pre- mixed with one or both of the reactive (co) polymer components, or mixed together with the (co) polymers . If a catalyst is used then this may be added after the ceramic and/or metal are mixed with the (co) polymers to ensure good dispersive and distributive mixing of the ceramic and/or metal in the (co) polymer. One advantage of mixing the ceramic and/or metal in a lower molecular weight (co) polymer mixture is that the ceramic and/or metal is more easily dispersed and there is less chance of breakage. In order to improve the ease of extrusion foaming or the properties of the composite dielectric material, various methods known to those skilled in the art may be utilised. For example, it is difficult to extrusion foam polypropylene. If the polypropylene is branched or has a broad molecular weight distribution the material can be extrusion foamed more easily. Various commercial foamable polypropylene grades having high melt strength or increased extensional viscosity over conventional linear polypropylenes which are suitable for extrusion foaming include PF814 suppled by Basell and Daploy 130D supplied by Borealis . Alternatively branched polypropylene could be produced by the addition of appropriate initiators or modifiers such as those disclosed in WO 99/27007 and AU 732342. It is perceivable that the ceramic and/or metal may be added during the formation of the branched / chain extended polypropylene to produce a foamable composite dielectric material. It is also known that silanes containing moisture condensable groups can be melt grafted or solid state grafted onto polyolefins or incorporated during polymerisation of polyolefins to produce materials that can either form branched polymers to enhance the foaming performance or subsequently crosslinked after foaming to produce a material having superior mechanical performance . This technology is described in: "Moisture crosslinkable silane modified polyolefins" in Reactive Modifiers for Polymers" by D. Muntaenu 1997, US5371144, US4247667, JP2000219764 and WO9802483. The incorporation of the ceramic and/or metal, branching or functionalisation of the polypropylene to in- situ form a more readily foamed matrix or crosslinkable matrix and incorporation of a chemical or physical blowing agent to produce the final foamed composite dielectric material could be undertaken in one extrusion process . In another aspect the gas containing structure may be formed by polymerisation of monomer (s) to form (co) polymers in the presence of the ceramic and/or metal. The (co) polymer may also be attached to the ceramic and/or metal . Additionally the reaction of the (co) polymers to form the supporting structure may create a foaming agent in- situ. For example, the reaction of a silanol containing (co)polymer with a silyl-hydride containing (co)polymer forms siloxy linkages (Si-O-Si) and eliminates hydrogen gas which can act in part as a foaming agent. The self foaming silicone materials may be similar to those produced by Dow Corning: 3-6548 or General Electric: TOSFOAM 5700. Similar materials may be used, but further optimised by those skilled in the art to improve properties such as dielectric properties, foaming and mechanical performance . An alternative means to produce foams from solid (co) polymers involves infusing a formed (co) olymer with a blowing agent under supercritical conditions and the (co)polymer foams on returning to ambient conditions. This technology could be extended to include a solid article consisting of a mixture of (co) polymer and ceramic and/or metal to produce foamed articles. The density and structure of the extrudate may be controlled via control of the screw profile, die size, die profile, processing temperatures, screw speed, loading and size of the ceramic/metal, feed rate of the foaming agent and form of the (co) polymer (i.e., powder addition for smaller single screw extruders) . In a preferred embodiment, the ceramic and/or metal is added to the molten (co) olymer in an extruder to minimise the breakdown of the ceramic and/or metal and to provide a more gentle means to mix the materials together. The foaming agent (if physical) can be introduced further along the extruder, or if a chemical foaming agent is employed, the temperature could be raised to initiate the decomposition to release the foaming agent. The use of a twin screw extruder allows for a controlled addition of the (co) polymer and ceramic and/or metal to better control the balance between the structure of the foamed material and the minimisation of chip breakup and control of the mixing addition of the foaming agent. Preferably, there is control of density, cell size and the amount of ceramic and/or metal during extrusion. The extrudate is preferably mouldable or sinterable. If the extrudate does not have this characteristic, then the extrudate can be immersed in a liquid chemical which is absorbed into the structure which can plasticise the foamed structure to enhance the sinterability and/or assist with a second stage foaming of the material which assists with the expansion of the foamed material to sinter and fill a mould (e.g. pentane, ethanol, isopropanol) . The material may be chopped to a size that allows for the moulded structure to be uniform at a dimension less than 1/lOth of the wavelength of use to substantially prevent diffraction effects that would cause frequency dependent behaviour to be exhibited by the composite dielectric material. The moulding can be carried out using any known method, such as steam or heat moulding. Binders could be used to hold the material together. The density of the moulded material is preferably less than about 0.6 g/cc. The composite dielectric material can be moulded into any suitable shape including blocks and layered spheres . Alternatively, the composite dielectric material can be moulded into blocks and cut or machined to a specific shape using suitable known techniques, such as, hot wire cutting or NC machining. The mould-then-machine approach has the benefit of allowing the blocks to be moulded with a uniform cross section to yield better isotropy. The dielectric material may be used in high frequency optics, for example, microwave lenses, spherical Luneburg lenses and horn antennas .

BRIEF DESCRIPTION OF THE DRAWINGS In the examples, reference will be made to the accompanying drawings, in which: Fig. 1 is a diagram showing the possible shapes of the ceramic and/or metal; Fig. 2 are micrographs of the composite dielectric material of Example 1; Fig. 3 is a concept diagram of the composite dielectric material; and Fig. 4 is a diagram showing a layered spherical

Luneberg lens configuration with the outer layer created from 8 tiles (octants) . EXAMPLES

Example 1 : Production of the Composite Dielectric Material

The composite dielectric material was made up of thin flakes of doped, rutile titanium dioxide and foamable commercial grade polypropylene. The two components were placed in an extruder where mixing occurred and the polymer was melted or softened. A foaming agent pentane was injected into the mix and this was then extruded through a circular die of around 2 mm in diameter. The resulting continuous foam rod was around 10 mm in diameter and had a density of around 0.10 g/cc. This rod was then chopped into cylinders of around 2 - 5 mm long and then packed into a metal mould of the desired shape. The mould was then heated with steam, or air, to around 158°C for around 5 minutes for steam heat, and 60 minutes for air heat. The doped, rutile titanium dioxide was produced by mixing titanium oxide, fine aluminium oxide, a film forming polymer polyvinyl butyral and standard tape casting additives such as diisooctyl phthalate . The mixture was milled in a solvent and cast onto a suitable glass or plastic substrate to around 20-30 microns thick using a doctor blade. This was then allowed to dry and peeled off in strips of around 10 mm wide. The resulting sheets were crumpled into a loose ball and fired at around 1450°C for 30 minutes. The ball was removed and crushed gently and passed through a 1 mm sieve, onto a 0.5 mm sieve. The material that did not pass through the 0.5 mm sieve was then collected and used in the extruder. Example 2 : Application of the Dielectric Material to a Luneburg Lens

The lens is envisioned to be 1 - 7 meters in diameter. It will be formed in layers (at least 3 layers for the 1 meter diameter and up to 20 layers for the 7 meter lens) . The layers are moulded in pieces and then adhered using an appropriate low loss polymer adhesive or welded in place. The inner layer is produced as a full sphere or as two hemispheres. The outer layers are built up from shapes which tile the surface such as those described as icosahedron and dodecahedron derived tilings . Fig. 4 shows an example of a spherical-based tile which could be used to build the outer layer of the sphere as shown in Fig. 5. Any number of layers could be built up in this way. The tiles could be derived from hemispheres, wedges, octants or any other shape which tiles which cover the sphere completely. It is also envisaged that tiles could be selected as matched pairs or any other combination which is deemed suitable.

Example 3 : Dielectric Measurements

A number of dielectric blocks were produced using the dielectric material of Example 1 and tested in a microwave waveguide apparatus . Extruded and moulded blocks of nominally 150 mm by 15 mm by 35 mm had dielectric constants and dielectric losses as shown in Table 1.

Table 1

Example 4 : Production of Dielectric Blocks containing Silicone Elastomers and doped rutile titanium dioxide

A number of dielectric blocks can be produced using the doped rutile titanium dioxide of example 1. The foamable silicone formulation consists of (A) a 3500 centipoise @ 25°C vinyl dimethyl end capped polydimethyl siloxane to which is added 30 ppm of platinium in the form of Karstedt catalyst; (B) water; (C) methylcyclotetrasiloxane; and (D) trimethyl end capped polymethyl hydrogen siloxane having 1.5% silicone bonded hydrogen (viscosity 20 centipoise at 25°C) . Table 2 shows various compositions containing the silicone formulation and the doped, rutile titanium dioxide

Table 2

The doped rutile titanium dioxide is premixed with composition A using a liquid dispersion mixer. The other compositions B, C and D are added and mixed together for approximately 30 seconds. After mixing, the catalysed formulation begins to foam and starts to gel. The expansion of the foam is completed within two minutes and cured completely after 1 hour. The formulations shown in Table 2 produce foams within minutes of mixing. To further control the foam density and dielectric properties of the crosslinked siloxane foam, it will be appreciated that the amount of doped rutile titanium dioxide and composition of the silicone foaming mixture can be altered (including the composition, molecular weight and degree of functionality of the siloxane and silane components) . It is also possible to alter the rate of foaming by altering the amount of catalyst, amount of reaction inhibitors (if added) as well as the mixing and reaction temperature . The density of the foam can also be lowered by forming the foam in a vacuum vessel . Example 5 The composite dielectric material was composed of alumina ceramic plates and polystyrene foam and prepared by mixing expanded polystyrene beads and alumina platelet powder together with a small amount of low-dielectric constant wax, to cause the alumina platelets to adhere to the polystyrene. Dielectric measurements were then performed as described in Example 3. The results are shown in Table 3.

Table 3

It will be appreciated by persons skilled in the art that numerous variations and/or modifications may be made to the invention as shown in the specific embodiments without departing from the spirit or scope of the invention as broadly described. The present embodiments are, therefore, to be considered in all respects as illustrative and not restrictive.

Claims

CLAIMS :

1. A composite dielectric material comprising a ceramic having a shape with a high aspect ratio which is carried by a gas-containing structure.

2. An extruded composite dielectric material comprising a ceramic and/or metal having a shape with a high aspect ratio which is carried by a gas-containing structure.

3. A material according to claim 1 or claim 2, in which the shape with a high aspect ratio is a shape such that either one or two of the three possible dimensions is significantly smaller than the other dimension (s) .

4. A material according to any one of claims 1 to 3 , in which the shape with a high aspect ratio is a flake, disc, fibre or hollow sphere.

5. A material according to any one of claims 1 to 4, in which the gas-containing structure has a dielectric loss of less than about 0.0001.

6. A material according to any one of claims 1 to 5 , in which the gas-containing structure is a (co) polymer or glass in which a gas is contained.

7. A material according to claim 6, in which the (co) olymer is selected from one or more of polystyrene, polypropylene, polyethylene and polysiloxane .

8. A material according to claim 7, in which the polysiloxane is selected from one or more of end capped polyalkyl siloxane, alkyl cyclotetrasiloxane and end capped polyalkyl hydrogen siloxane.

9. A material according to any one of claims 6 to 8, in which the (co) olymer is cross-linkable and/or cross- linked.

10. A material according to any one of claims 6 to 9 , in which the polymer is foamable.

11. A material according to any one of the preceding claims, in which the ceramic and/or metal is contained by and/or supported by the gas-containing structure.

12. A material according to claim 11, in which the gas- containing structure is formed from layers, corrugated sheets or stacked tubes in which the gas is located in between the layers, corrugations or tubes, respectively.

13. A material according to claim 11, in which the gas- containing structure is formed from extruded tubing to create a porous structure.

14. A material according to any one of the preceding claims, in which the gas is inert.

15. A material according to any one of the preceding claims, in which the gas is nitrogen or air.

16. A material according to any one of the preceding claims, in which the ceramic is a metal oxide.

17. A material according to claim 16, in which the metal oxide is titanium dioxide, aluminium oxide or barium titanate.

18. A material according to claim 17, in which the titanium dioxide is rutile titanium dioxide.

19. A material according to claim 18, in which the rutile titanium dioxide is doped with aluminium oxide.

20. A material according to claim 19, in which the doped rutile titanium dioxide is present in an amount of about

20 to about 80% based on the total weight of the material.

21. A material according to any one of the preceding claims, in which the metal is aluminium, copper or silver.

22. A material according to any one of claims 19 to 21, in which the metal has a protective layer.

23. A material according to claim 22, in which the protective layer is an oxide layer or a polymer film.

24. A material according to any one of the preceding claims, which further comprises additive(s).

25. A material according to claim 24, in which the additives are selected from one or more of binders, dispersants, initiators, inhibitors, modifiers, plasticisers, catalysts, nucleants, lubricants, foaming agents, agents for controlling gas solubility or moisture solubility and compatibilisers .

26. A material according to any one of the preceding claims, which has a density of about 0.02 to about 0.6 g/cc.

27. A method for the production of the composite dielectric material defined in any one of claims 1 to 26, which comprises extruding, casting, moulding and/or spraying the ceramic and/or metal- and the gas-containing structure .

28. A method according to claim 27, in which the gas- containing structure is a (co) polymer in which a gas is contained.

29. A method according to claim 28, in which the (co) polymer is foamed either before, during or after the extrusion, casting, moulding and/or spraying step(s) .

30. A method according to claim 29, in which the foaming is carried out using either physical or chemical foaming agents .

31. A method according to any one of claims 28 to 30, in which the gas-containing structure is either a preformed (co) polymer or a (co) polymer formed or modified in-situ during the method.

32. A method according to any one of claims 28 to 31, in which the gas containing structure is formed by polymerisation of monomer (s) to form (co) polymers in the presence of the ceramic and/or metal.

33. A method according to any one of claims 28 to 31, in which the (co) polymer is attached to the ceramic and/or metal .

34. A method according to any one of claims 28 to 33, in which the extrusion step involves compounding the ceramic and/or metal with a polymer and foaming or forming the compounded ceramic and/or metal and the gas-containing structure which is performed in one or two steps.

35. A method according to claim 34, in which the extrusion is performed in one step.

36. A method according to any one of claims 28 to 35, in which the extrusion is performed in a single or twin screw extruder .

37. A method according to any one of claims 28 to 36, in which the material is moulded using heat or a binder after the extrusion, casting, moulding and/or spraying and optionally foaming step(s).

38. Use of the composite dielectric material defined in any one of claims 1 to 26 in high frequency optics.

39. Use according to claim 38, in which the high frequency optics is a Luneburg lens, spherical lenses formed from a number of discrete uniform shells or a horn antenna .