GB2415093A - Method of producing composite materials - Google Patents

Abstract

A method of producing an element having pre-determined properties, the method including the steps of selecting an user-determined pattern (5) corresponding to the pre-determined properties, and then making an element comprising a pre-determined material in the user-determined pattern (5). In an embodiment of the invention, a composite material comprising electrically conductive particles in an insulating or dielectric matrix or filler is made by printing particles of electrical conductive material (5) onto an insulating substrate (4).

Description

24 1 5093 Method of Producing Composite Materials The present invention is

concerned with a method of producing a composite material. The present invention is also concerned with a composite material and apparatus for making such a material.

Composite materials are created to have desirable physical properties.

Composite materials are, for example, engineered to have particular d.c. and/or a.c. electrical, electromagnetic, magnetic, thermal and/or mechanical properties.

The ensuing discussion and description concentrates on composite materials engineered for their electrical and electromagnetic properties. It will, however, be readily appreciated that the invention in its various aspects is equally applicable to the engineering, design and/or creation of any composite material comprising particles of a first material dispersed in a filler or matrix of a different material.

The ensuing discussion concentrates on electrical properties as an example only of the application of the invention. The invention is, however, of broader 1 5 application.

Materials can either support (or allow) the propagation of an electromagnetic wave through their bulk or they cannot. All materials contain electronic charges and so respond, to varying degrees, to the application of an electric field. The response of materials to an applied field is dependent on the frequency of the applied field. The plasma frequency (cup) is the frequency marking the boundary for a material between positive and negative permittivity.

Many applications, devices and/or methods rely on the control of electromagnetic radiation. For example, enclosures (radomes) are necessary to provide environmental protection for antenna systems. In mobile communications and other similar applications, there is a need to separate electromagnetic signals of different frequency. There is also a need to dissipate electromagnetic energy at the walls of anechoic chambers used in radio and microwave measurements, and to confine, within specific bounds, unintentionally emitted electromagnetic energy to meet electromagnetic compliance regulations and prevent electromagnetic interference between electrical and electronic equipment.

Materials are used to provide the means of control, either in bulk form, as coatings or as components in devices. For example, radomes tend to be fabricated from bulk materials such as plastics and fibre-reinforced polymer composites; frequency separation can be achieved on a component level in guided wave communications or by using coatings (for example on radomes) for free-field propagation; dissipation tends to be achieved by coating an existing structure (the walls and floor of an anechoic chamber) ; and electromagnetic shielding can be achieved either through coating an equipment enclosure or by fabricating the enclosure from an appropriate material.

At the simplest level, the role of the material can be to modify the propagation characteristics of incident radiation. Modification could include transmitting, filtering, absorbing or reflecting incident electromagnetic radiation as in radomes, frequency separation, coatings for anechoic chambers and equipment enclosures for electromagnetic compatibility.

It is clear that identifying and then making materials with different distributions and/or types of electronic charges and hence pemmiffivities can enable the design of components and devices with different electromagnetic functionality (for example, different levels of reflection, transmission and absorption) operating over specific regions of the electromagnetic spectrum. However, the range of naturally occurring pemmiffivities has become restrictive to the design engineer.

For example, either because the desired real permiffivity value is not available or absorption mechanisms do not exist at a required frequency, or in a material that has the required processibility, mechanical, environmental or visual properties.

For these reasons, engineers have sought to form composite media with tailored complex permiffivity. A further benefit to the design engineer would be accrued if it were possible to produce composite media with a tailored plasma frequency.

Particularly, if in a solid material, the plasma frequency could be tailored to exist at lower frequencies than naturally occur in metals.

In co-pending UK patent application no 0327412.3, the inventor of the subject application discusses how the plasma frequency can be tailored using composite materials comprising electrically conductive particles in an insulating or dielectric matrix or filler where the concentration of the electrically conductive particles is such that the composite material is near, at or above its percolation threshold.

Percolation theory is a way of describing the processes, properties and phenomena in random or disordered systems. The amount of disorder is defined by the degree of electrical connectivity between particles. If p is a parameter that defines the degree of connectivity between various particles in a material, then if p = 0, none of the particles are connected, and if p = 1, all the particles are connected to the maximum number of neighbouring particles. There is a point, Pc (the percolation threshold), where each of the particles is connected to the minimum number of neighbouring particles, such that there is an unbroken path of that type of particle from one side of the material to another. For example, in a metal matrix composite, where aluminium particles are dispersed in a ceramic matrix, the percolation threshold for d.c. current is reached when there is a conducting path defined by a continuous or almost continuous (current can flow between slightly separated particles due to tunnelling effects between particles) path of aluminium from one side of the matrix to the other. At this point, the material may begin to exhibit metallic characteristics, for example, an electric current may flow.

The inventor has appreciated that in order to design appropriate composite media it is necessary to be able to repeatedly produce composite materials with consistent and predictable properties. The known composite media are produced from mixtures of, typically, a dielectric or insulating filler, and conductive particles which are created by mixing together.

The inventor of the subjected application is the first to appreciate that the use of mixing to produce the composite material, or to produce feed stock from which the material is made does not consistently and repeatedly produce mixtures having sufficiently similar (macroscopic) physical properties such as electrical conductivity even when the same overall concentrations of the constituent materials are used. The statistical variability in conductivity for a given overall concentration of conductive elements is due to the large number of different configurations that may be adopted by the conductive elements at a microscopic level.

The problems caused by the statistical variation is particularly serious where one is seeking to produce materials which are intended to operate at or near a sharp transition in properties such as the percolation threshold for a composite material including electrically conductive elements in non-conductive filler (e.g., as described in GB 0327412.3).

The composite materials produced by mixing or dispersion of suitable particles in a host medium or filler have an inherent statistical variability and their properties can be influenced by processing conditions. The inventor has appreciated that this statistical variability is potentially highly problematic for the production and design of materials at or near their percolation threshold as the transition from insulating to conductive behaviour is typically a sharp one (see Figure 1). This sharp response is intolerant of statistical variations in the distribution of the conductive particles in the host medium or filler.

A possible range of conductivities for a composite material prepared by the known mixing or dispersion methods is marked on figure 1 as AS (prior art). The range is sufficiently wide for different samples of, apparently, the same concentrations of material produced by the same method to behave as insulator, conductor or somewhere in the transition.

The inventor is the first to appreciate that it is possible to repeatedly and consistently provide composite materials exhibiting particular defined properties by placing a pre-determined pattern of conducting material onto a dielectric, insulating or non-conductive substrate.

The present invention, in its various aspects, provides method, material and apparatus as defined in independent claims 1, 13, 18 and 21 to 23 to which reference is now made. Preferred or alternative features of embodiments of the invention are set out in the dependent claims to which reference is now made.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the attached figures in which: Finure 1 illustrates the percolation threshold for a composite material, Figures 2a to 2c illustrate three different conductive patterns made up of circular conductive elements for placing on a dielectric substrate.

Figure 3 illustrates an alternative conductive pattern made up of crossed dipoles or crosses; and Figures 4 and 5 illustrate two possible methods of making a two-dimensional composite material using conductive patterns of the type shown in figures 2 and 3.

Figure 1 is a schematic graph of conductivity in relation to conductive filler concentration for a composite material comprising conductive particles in an insulating or non-conductive filler. The graph illustrates that the conductivity of the samples falls into 3 distinct regions, marked A, B and C. In region A, the filler concentration level is low, and the material does not conduct any electrical current. There are no connected pathways of conducting elements in the composite.

In region B. an insulator-conductor transition occurs. This transition is prompted by the formation of the first network of conducting elements within the material.

For do use, this network must span the entire material. For ac use, the network need only span a region of the material. The steepness of the gradient in region B is determined by the difference in conductivity between the constituent materials, the concentration of the conducting elements at which the first network fomms and the concentration of the conductivity elements at which the overall conductivity becomes limited by the contact resistance between adjacent conductive elements.

The gradient of the insulator/conductor transition (region B) can be influenced by the degree of randomness in the distribution of the conducting elements and the nature of electrical charge transport across the contact interface. For example, the gradient can be influenced if the electrical charge transport is dominated by charge hopping or tunnelling rather than essentially free-electron movement.

In the transition region B. the conductivity continues to increase rapidly as additional parallel paths of conducting elements are created in the principal network thought the successive addition of conducting elements. This is the percolation region.

Eventually, the gradient reduces to a plateau or saturation region C in which the further addition of conducting elements does not significantly increase the conductivity of the composite. In region C, the filler concentration is high enough for the composite to conduct electricity at a level similar to that at the conductivity elements. Typically, in this region the composite is useful as an electrical conductor.

A composite material is produced by printing or placing a pattern of conductive elements onto an insulating film substrate. The conducting elements could be fommed from any conductive material, including metals, conducting metal oxides, graphitic material, fullerenes, organic conductors or ionic conductors. The insulating film substrate could be formed from any insulating material including natural or synthetic papers, cloth, fabrics or thin polymer films.

The pattern of conductive elements or particles may be printed or placed using any pattern transfer mechanism or method whereby a thin layer of the conducting material can be placed in a controlled manner on a surface to form a user defined pattern. The possible methods involve inkjet printing, screen printing, block-foil patterning or autocatalytic deposition such as described in WO 02/099162 and WO 02/099163, or physical or chemical disposition methods. In the case of printing methods, conducting particles would be dispersed in a low viscosity binder to enable deposition on the substrate. Alternatively, conducting material could be removed from an initially complete conducting film to produce a similar pattern of conducting material. The possible removing methods include etching or hole punching.

The size of the conducting elements making up the pattern is of secondary importance and would be chosen to be smaller than the area of the substrate or area over which the composite is to be used, whichever is the smaller. Typically, the element size would be less than one tenth of this size limit, and preferably less than one hundredth.

A pre-determined pattern representing a selected concentration of conductive material is stored as part of a library of pre-determined patterns each representing selected concentrations of conductive materials. These pre- determined patterns may be determined either empirically or theoretically. A combination of both theory and experience in which a basic pattern is generated theoretically before being empirically checked is a possible way of generating pre-detemmined patterns.

The pre-determined patterns are chosen or selected so as have particular properties in particular circumstances. For example, the library of patterns may include patterns which when used to print or place an ink comprising elements of a particular conductor (e.g. copper) of a particular size and shape (e.g. discs of diameter 1.6mm - see figure 2a) on a particular substrate (e.g. synthetic paper) have a conductivity falling within a particular small range AS (see Figure 1).

There are likely even with the method of the present invention to be statistical variations from one sample to the next but they will be significantly smaller than the variations in the properties of the materials made by the known mixing methods. In other words the standard deviation of the conductivity of sample composite materials of a particular conductor concentration produced by the method of this application will be significantly smaller than the standard deviation of the same apparent composite material produced by the known methods. This means that the behaviour of different samples will be closer and therefore materials can be made with more confidence that properties will be repeatable.

Figures 2a to 2c illustrate a number of pre-determined patterns made up of a 1 OOx100 array including discs 1 of circular material, corresponding to, respectively, 20%, 50% and 70% loadings of conductive elements.

Figure 3 illustrates a pre-determined pattern made up of crossed dipoles 2 and corresponding to a loading concentration of 50%. The aspect ratio of the crosses could be used, for example, to control the percolation threshold of a composite.

Figures 4 and 5 illustrate the three stage autocatalytic deposition methods described in WO 02/099162 and WO 02/099163 to which reference should be made. The contents of these two publications are herein incorporated by way of reference and as illustrations of how the preferred embodiments of invention might be implemented or created.

Turning to Figure 4, an ink jet printing system 3 coats a substrate 4 with an ink formulation containing a deposition promoting material in a user determined pattern 5. The treated substrate 4, 5 is then immersed in an autocatalytic deposition solution 6 to produce a user determined metalised pattern 7.

Ink jet printers operate using a range of solvents normally in the viscosity range 1 to 50 centipoise.

Turning to Figure 5, a screen printing system 8 coats a substrate 4, with an ink formulation containing a deposition promoting material in a user determined pattern 5 (like numerals being used to denote like features between Figures 4 and 5). The treated substrate 4,5 is once again immersed in an autocatalytic deposition solution 6 to produce a user determined metalised pattern 7.

A range of ink fommulations are possible. Criteria suitable for printing may include the following: 1) They contain materials that are able to pass through the chosen printing mechanism (for example, either an Epson 850 inkjet system or a Dek screen printer); 2) They contain liquids with the correct properties for the printing process, for example suitable viscosity, boiling point, vapour pressure and surface wetting; 3) Where suitable they contain binders and fillers affecting either the viscosity or physical printing properties of the printed ink.

The patterns of conductive material may also be transferred onto a non conductive substrate using a straightforward printing technique such as that described by Messrs Schwartz and Ludwena in An experimental method for studying two-dimensional percolations. [Am.J.Phys 72(3), March 2004 2004 American Association of Physics Teachers] Messrs Schwartz and Ludwena describe an experimental technique for analysing a range of twodimensional problems. The method is based on the printing of computer generated patterns using conducting ink. The metal-insulator transition is measured from the print out of the conductive patterns, and the conductivity critical component and the percolation threshold are calculated from these measurements.

Three-dimensional composite materials may be made by placing a second layer of insulating material over the material of figures 4c or 5c and then repeating the printing process. The process may be repeated as many times as are necessary to achieve the desired material thickness or properties. Such a material will, essentially, be three dimensional in terms of its physical shape but as the insulating layers are continuous it will only be two-dimensional in so far as its electrical properties are concerned.

The present invention allows for increased confidence in the manufacturing of composites having particular properties. This has a number of clear advantages including the reduction of scrap.

Embodiments of the invention can, as discussed above, be used to engineer composites having, inter alla, desirable electrical, magnetic, thermal and/or physical properties. Possible applications of composites including active materials (e.g. photo sensitive, piezoelectric, chemical sensitive, thermally sensitive) include sensors, actuators or switches. Composites embodying the invention could also be used as reference materials (for e. g. absorbing) in metrology in support of national and/or international traceability claims. The ability to produce something having a known and pre-determined property or behaviour could also be used in support of security and anti-counterfeiting measures.

Claims (23)

1. Claims 1. A method of producing an element having pre-determined

   properties, the method including the steps of: a) selecting an user determined pattern corresponding to the pre-determined properties; and b) making an element comprising a pre-determined material in the user-determined pattern.
2. 2. A method according to claim 1 for producing a composite material including two different constituent materials and having pre-determined properties, the method including the steps of: a) selecting an user determined pattern corresponding to the pre determined properties; and b) placing or transferring a pre-determined pattern of a first material onto a support or substrate of a second material to create the user determined pattern.
3. 3. A method according to claim 2 for producing a composite material having pre-determined electrical properties, wherein the pre-determined electrical properties are determined by an user determined pattern of electrically conductive material.
4. 4. A method according to claim 3 wherein a pre-determined pattern of conductive material is placed or transferred onto or into a nonconductive support or substrate.
5. 5. A method according to claim 3 wherein a pre-determined pattern of nonconductive material is placed or transferred onto or into a conductive support or substrate.
6. 6. A method according to claim 4 including the additional step of placing a layer of non-conductive material over the user determined pattern of conductive material, and then placing or transferring a pre-determined pattern of conductive material onto or into the layer of non-conductive material.
7. 7. A method according to claim 5 including the additional step of placing a layer of conductive material over the user determined pattern of nonconductive material, and then placing or transferring a pre-determined pattern of non- conductive material onto or into the layer of conductive material.
8. 8. A method according to claim 6 or claim 7 further including the step of repeating the additional step of claim 6 or 7 at least once more.
9. 9. A method according to any of claims 2 to 8 wherein the material is printed onto the support or substrate.
10. 10. A method according to claim 9 wherein the material is printed using inkjet printing.
11. 11. A method according to claim 9 wherein the material is printed using screen printing.
12. 12. A method according to any of claims 2 to 8 wherein the material is placed or transferred onto the support or substrate by block foil patterning.
13. 13. A composite material having user-determined properties and including an user-determined pattern of a first material on or in a support or substrate of a second different material.
14. 14. A composite material according to claim 13 having user-determined electrical properties and including an user-determined pattern of conductive material in or on a support or substrate of non-conductive material.
15. 15. A composite material according to claim 14 comprising a conductive material selected from the group of materials comprising metals, conducting metal oxides, graphitic material, fullerenes, organic conductors and ionic conductors.
16. 16. A composite material according to any of claims 13 to 15 comprising a non-conductive support or substrate selected from the group of materials comprising natural papers, synthetic papers, cloth, fabric and thin polymer films.
17. 17. A composite material according to claim 13 having user-detemmined magnetic properties and including an user-determined pattern of magnetic material in or on a support or substrate of non-magnetic material.
18. 18. Apparatus for making an element having pre-determined properties, the apparatus comprising a) a memory storing at least one pattern corresponding to the pre-determined properties; b) means for selecting a on the stored pattern corresponding to the pre-determined properties; and c) means for making an element comprising a selected material in the selected pattern.
19. 19. Apparatus according to claim 18 for making a composite material including means for placing or transferring the selected material onto a second material to create the selected pattern.
20. 20. Apparatus according to claim 19 including a printer for placing or transferring the selected material onto a second material.
21. 21. A method substantially as hereinbefore described with reference to figures2to5.
22. 22. A material substantially as hereinbefore described with reference to figures 1 to 5.
23. 23. Apparatus substantially as hereinbefore described with reference to figures 2 to 5.

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