Composite Materials

Abstract

A composite material having a plasma frequency comprising a random mixture of conductive and non-conductive particles. A material having smaller conductive than non-conductive particles and a concentration of conductive particles approximately at, close to or above the percolation threshold for mixtures of the conducting and non-conducting particles may show a plasma frequency well below plasma frequencies for conventional bulk materials.

1 . A composite material comprising a proportion of a electrically non-conductive material and a proportion of a randomly distributed electrically conductive material, and wherein:
 a) the electrically conductive material is particulate with an average particle size which is not more than 1 μm;   b) the electrically non-conductive material is not particulate;   c) the electrically conductive material proportion is sufficiently large to provide for the composite material to exhibit a plasma-like response corresponding to at least an onset of a percolation threshold; and   d) the composite material has a plasma frequency which is below the plasma frequencies of conventional bulk metals.   
   
   
       2 . A composite material according to  claim 1  wherein the electrically conductive material comprises gold or silver particles with average particle size in the range 1 μm to 1 nm. 
   
   
       3 . A composite material according to  claim 1  wherein the electrically conductive material has an average particle size of 100 nm. 
   
   
       4 . A composite material according to  claim 1  wherein the plasma frequency is not greater than 10 12  Hz. 
   
   
       5 . A method of influencing propagation of electromagnetic radiation having a radiation frequency by arranging for the radiation to be incident upon a composite material, and wherein:
 a) the composite material comprises an electrically non-conductive material and an electrically conductive material;   b) the electrically conductive material is randomly distributed in the composite material;   c) the composite material exhibits a plasma-like response and has a plasma frequency which is below conventional bulk metals' plasma frequencies; and   d) the plasma frequency is located relative to the radiation frequency such that radiation propagation at the radiation frequency in the composite material Is affected by the plasma-like response.   
   
   
       6 . A method according to  claim 5  wherein the radiation and plasma frequencies are in at least one of the ranges 10 3  to 10 15  Hz, 10 8  to 10 15  Hz and 10 8  to 10 12  Hz. 
   
   
       7 . A method according to  claim 5  wherein the radiation and plasma frequencies are microwave frequencies. 
   
   
       8 . A method according to  claim 5  wherein:
 a) the electrically non-conductive and conductive materials are both particulate;   b) the electrically conductive material has a particle size which is less than one tenth that of the electrically non-conductive material;   c) the electrically non-conductive material has an average particle size which is not more than 100 μm; and   d) the electrically conductive material is a proportion of the composite material which is sufficiently large for the composite material to exhibit a plasma-like response indicating that it is at least at an onset of a percolation threshold.   
   
   
       9 . A method according to  claim 5  wherein:
 a) the electrically conductive material is particulate;   b) the electrically non-conductive material is not particulate;   c) the electrically conductive material has an average particle size which is not more than 1 μm; and   d) the electrically conductive material proportion is sufficiently large to provide for the composite material to exhibit a plasma-like response indicating that it is at least at an onset of a percolation threshold.   
   
   
       10 . A method according to  claim 5  wherein the electrically conductive material has an average particle size of substantially 100 nm. 
   
   
       11 . A method according to  claim 5  wherein the electrically conductive material exhibits no long range order. 
   
   
       12 . A method according to  claim 11  wherein the electrically conductive material exhibits no long range order over a region having a dimension in the range 3 mm to 3 m. 
   
   
       13 . A method according to  claim 12  wherein the region's dimension is substantially 3 cm. 
   
   
       14 . A method according to  claim 5  wherein the electrically conductive material exhibits no long range order over a region having a dimension of the order of a wavelength in the material corresponding to the plasma frequency. 
   
   
       15 . A method according to  claim 5  wherein the composite material contains sufficient electrically conductive material to form therein at least one electrically conductive network extending between opposite sides of the composite material. 
   
   
       16 . A method according to  claim 5  wherein the electrically non-conductive and conductive materials are both particulate and have larger and smaller average particle sizes respectively relative to one another, and a ratio of the average particle sizes being greater than or equal to 100 or 1000. 
   
   
       17 . A method according to  claim 16  wherein the electrically conductive material comprises one of an oxidation resistant metal, a metallic alloy, an electrically conductive coating on electrically non-conductive particles and a conducting ceramic, and the average particle size of the electrically conducting material is in the range 1 nm to 1 μm. 
   
   
       18 . A method according to  claim 17  wherein the electrically conductive material has a conductivity greater than 1 S/m. 
   
   
       19 . A method according to  claim 17  wherein the electrically conductive material comprises gold or silver particles. 
   
   
       20 . A method according to  claim 5  wherein the electrically non-conductive material comprises either one of, or alternatively a mixture of at least two of, PTFE, paraffin wax, a thermosetting material, a thermoplastic material, a polymer, air, an insulating ceramic material and glass. 
   
   
       21 . A method according to  claim 20  wherein the electrically non-conductive material is PTFE with an average particle size of substantially 100 μm, and the electrically conductive material is gold or silver with an average particle size of substantially 100 nm. 
   
   
       22 . A method according to  claim 5  wherein the electrically conductive material comprises either one of, or alternatively a mixture of at least two of, a metal, metal alloy, an oxidation resistant metal, a conductive coating on non-conductive particles, an electrically conductive metal oxide, an intrinsically conductive polymer, an ionic conductive material, and a conducting ceramic. 
   
   
       23 . A method according to  claim 5  wherein the composite material has an effective conductivity exceeding at least one of 10 S/m, 30 S/m and 100 S/m. 
   
   
       24 . A method according to  claim 5  including switching the composite material between radiation propagating and attenuating states by altering its plasma frequency. 
   
   
       25 . A method according to  claim 5  wherein the electrically conductive material comprises regions of electrically conductive particles and the electrically non-conductive material comprises regions of electrically non-conductive particles, and the composite material has a degree of electrical connectivity between the regions of electrically conductive particles determining electrical properties, and the method includes applying a stimulus to the composite material to change the degree of connectivity. 
   
   
       26 . A method according to  claim 25  wherein the stimulus is pressure, temperature, chemical absorption, electric field or electric current, and applying the stimulus switches the composite material between radiation propagating and attenuating states. 
   
   
       27 . A method according to  claim 5  wherein the composite material is in the form of any one of a directional coupler lens, filter, transparent electrode, absorbing electrode, capacitor, inductor, waveguide, sensor, remote interrogation sensor package, active electromagnetic shutter, radome, switch, shield, fuse and anechoic chamber. 
   
   
       28 . A method according to  claim 5  wherein the composite material is one of a series of composite materials with differing concentrations of electrically conductive and non-conductive materials, and wherein for the series a graph of conductivity against electrically conductive material concentration on logarithmic axes has a slope which is less than 100 for an insulator to metal transition.