ELECTRICALLY CONDUCTIVE COMPOSITION

Abstract

A pressure sensitive electrically conductive composition comprises a contained quantity of magnetite particles, wherein the quantity of magnetite particles includes a distribution of particle sizes between sub-micron and tens of microns, and wherein the magnetite particles have a plurality of planar faces, adjacent planar faces connected at a vertex, the particles each having a plurality of vertices wherein the magnetite particles are irregular in shape and have a low aspect ratio.

Claims

1. A pressure sensitive electrically conductive composition comprising a contained quantity of first electrically conductive particles, which first electrically conductive particles are magnetite particles, wherein the quantity of magnetite particles includes a distribution of particle sizes between sub-micron and tens of microns, and wherein the magnetite particles have a plurality of planar faces, adjacent planar faces connected at a vertex, the particles each having a plurality of vertices wherein the magnetite particles are irregular in shape and have a low aspect ratio, and wherein the resistance of the electrically conductive composition changes anisotropically immediately around the point of exertion of pressure on the composition, the change in resistance being in accordance with the pressure exerted thereon, the resistance reducing with increased applied pressure and increasing with reduced applied pressure, between a quiescent state in the absence of pressure and an electrically conducting state when the composition is subject to an applied pressure.

2. A pressure sensitive electrically conductive composition according to claim 1, wherein the shape of the first electrically conductive particles in the distribution fall under the particle shape definitions of, oblate, that is tabular, and/or bladed, that is a flat or elongated shape form.

3. A pressure sensitive electrically conductive composition according to claim 1 or 2, wherein the distribution of particle size of the first electrically conductive particles at d.sub.50 is one of: between 50 and 75 micron; between 60 and 65 micron; between 20 and 25 micron; between 5 and 15 micron; and 10 micron.

4. (canceled)

5. (canceled)

6. (canceled)

7. (canceled)

8. A pressure sensitive electrically conductive composition according to claim 1, wherein the distribution of particle sizes between sub-micron and tens of microns in the quantity of magnetite particles includes sub-micron sized particles and particles that are tens of microns in size.

9. A pressure sensitive electrically conductive composition according to claim 1, further comprising a second type of electrically conductive particle of a different shape to the first electrically conductive particle.

10. A pressure sensitive electrically conductive composition according to claim 9, wherein the second type of electrically conductive particle has one of the following shapes: void bearing, plate like, needle like and spherical.

11. A pressure sensitive electrically conductive composition according to claim 9 or 10, wherein the second type of electrically conductive particle is selected from the group comprising: silver; nickel; copper and iron.

12. A pressure sensitive electrically conductive composition according to claims 9 to 11, wherein the particle size of the second type of electrically conductive particles is between 10 and 15 micron.

13. A pressure sensitive electrically conductive composition according to claim 1, wherein the resistance of the composition decreases by more than one order of magnitude with increased applied pressure and increases towards a quiescent state as the applied pressure is reduced.

14. A pressure sensitive electrically conductive composition according to claim 1, further including a binder, the electrically conductive particles contained in the binder.

15. A pressure sensitive electrically conductive composition according to claim 14, wherein the binder is a polymer binder.

16. A pressure sensitive electrically conductive composition according to claim 15, wherein the polymer is one of: a silicone polymer, a polyurethane polymer; polyethylene and an acrylic polymer.

17. A pressure sensitive electrically conductive composition according to claim 11, wherein the binder forms a thin layer around each particle in the particle distribution.

18. A pressure sensitive electrically conductive composition according to claim 17, wherein the thickness of the thin layer is in the order of 10 nano-metres.

19. A pressure sensitive electrically conductive composition according to claim 13, wherein the proportion of magnetite particles to binder is one of: greater than or equal to 33% by weight; greater than or equal to 75% by weight; and greater than or equal to 90%.

20. (canceled)

21. (canceled)

22. A sensor comprising two layers of semi-conductor separated by a layer of pressure sensitive electrically conductive composition comprising a contained quantity of first electrically conductive particles, which first electrically conductive particles are magnetite particles, wherein the quantity of magnetite particles includes a distribution of particle sizes between sub-micron and tens of microns, and wherein the magnetite particles have a plurality of planar faces, adjacent planar faces connected at a vertex, the particles each having a plurality of vertices wherein the magnetite particles are irregular in shape and have a low aspect ratio, and wherein the resistance of the electrically conductive composition changes anisotropically immediately around the point of exertion of pressure on the composition, the change in resistance being in accordance with the pressure exerted thereon, the resistance reducing with increased applied pressure and increasing with reduced applied pressure, between a quiescent state in the absence of pressure and an electrically conducting state when the composition is subject to an applied pressure, each layer of semi-conductor having at least one electrode attached thereto and wherein the at least one electrode of one layer of semi-conductor is arranged orthogonal to the at least one electrodes of the other semi-conducting layer.

23. A sensor according to claim 22, wherein each layer of semi-conductor has two or more two electrodes attached thereto, and wherein the two or more electrodes of one layer of semi-conductor are arranged orthogonal to the two or more electrodes of the other semi-conducting layer.

24. A method of manufacturing a pressure sensitive electrically conductive composition comprising a contained quantity of first electrically conductive particles, which first electrically conductive particles are magnetite particles, wherein the quantity of magnetite particles includes a distribution of particle sizes between sub-micron and tens of microns, and wherein the magnetite particles have a plurality of planar faces, adjacent planar faces connected at a vertex, the particles each having a plurality of vertices wherein the magnetite particles are irregular in shape and have a low aspect ratio, and wherein the resistance of the electrically conductive composition changes anisotropically immediately around the point of exertion of pressure on the composition, the change in resistance being in accordance with the pressure exerted thereon, the resistance reducing with increased applied pressure and increasing with reduced applied pressure, between a quiescent state in the absence of pressure and an electrically conducting state when the composition is subject to an applied pressure, further including a binder, the electrically conductive particles contained in the binder, the method comprising the step of: providing the electrically conductive particles and the binder; and mixing the electrically conductive particles with the binder until the mixture thereof is homogeneous.

25. A method of manufacturing a pressure sensitive electrically conductive composition according to claim 24, including the additional step of heating the binder to reduce the viscosity thereof, prior to and/or during the step of mixing the electrically conductive particles with the binder.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0046] In the Drawings, which illustrate preferred embodiments of the invention:

[0047] FIG. 1 is a schematic representation of a first arrangement of a sensor;

[0048] FIG. 2 is a schematic representation of an alternative arrangement of a sensor;

[0049] FIG. 3 is a micrograph of a distribution of the magnetite particles;

[0050] FIG. 4 is a graph illustrating the relationship between applied force and resistance for three example compositions according to the invention; and

[0051] FIG. 5 is a graph illustrating the relationship between applied force and resistance for another example composition according to the invention.

[0052] FIG. 6 is a graph illustrating the relationship between applied force and resistance for another example composition according to the invention; and

[0053] FIG. 7 is a schematic representation of the test electrode used to collect the data represented in the graphs of FIGS. 4 to 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0054] FIGS. 1 and 2 both illustrate pressure sensors.

[0055] The sensor shown in FIG. 1 is able to detect both the x-y position of a force applied to the surface of the sensor and also the force component in the direction z.

[0056] The sensor illustrated in FIG. 1 comprises a first layer which includes a pair of spaced apart conductive elements 1, which may comprise electrode strips of a metal such as silver. The conductive elements 1 are joined together by layer of semi-conductive material 5. The semi-conductive material 5 may be a polymer filled with a semi-conductor, a film or paper coated or impregnated with semi-conductor or a textile filled, coated or impregnated with semi-conductor. In this embodiment the semi-conductor is carbon. The resistance between the conductive elements across the semi-conductor material 5 is approximately 5 k Ohms, although this resistance could vary greatly depending on the input requirement of the electronic interface to which the conductive element is attached.

[0057] The sensor comprises a second layer of the same construction as the first layer. However in the assembled sensor the conductive elements 2, which are situated at the edges of the semi-conductive material 3, lie orthogonal to the conductive elements 1 of the first layer.

[0058] A third layer 4 is situated between the first and second layers, the third layer 4 being formed of a sheet of polymer material, such as polyethylene, loaded with magnetite particles of the type described above and in Table 1 below. The third layer 4 is anisotropically conductive. When a pressure is exerted on or removed from the third layer 4 it alters its resistance according to the pressure applied. Conduction occurs only in a very small region around the point of application of the force. Typically, a force is exerted in the z direction on the third layer 3 by means of a force acting on the outside of one of the first and second layers, which force is reacted through the other of the first and second layer, resulting in compression of the third layer 4 around the point of application of a force in the z direction.

[0059] A current that is passed, for example, through the electrodes 2 is conducted by layer 3 and is detectable via the electrodes 2. The sensitivity of the material of the third layer to pressure means that not only can the position of the force in the z direction be established, but also an indication of the magnitude of the force component in the z direction.

[0060] The sensor shown in FIG. 2 differs from that show in FIG. 1 in that more electrodes 6, 7 are present in the first and second layers respectively. Instead of electrodes being located at the outer edges of the semi-conductive material 8, 10, electrodes 6, 7 are provided at regular spaced apart locations across the sheets of semi-conductive material 8, 10. The result is a grid of cells. Such a sensor may be used to measure the load variation across the sensor when a distributed load is placed on the sensor. This is because a location and force measurement can be determined within each cell of the sensor.

[0061] Table 1 below illustrates the particle distributions for the different specifications of magnetite used in Examples 1 to 3 below:

TABLE-US-00001 Particle size distribution (cyclosizer Example 1 Example 2 Example 3 method) Magnetite Magnetite Magnetite d10 (micron) 5 6 9 d50 (micron) 10 22 63 d90 (micron) 25 50 180 particle characteristics irregularly shaped, low aspect ratio

[0062] The magnetite used in each of the examples 1 to 3 is sourced from LKAB Minerals and comprises a magnetite power manufactured from the natural iron oxide known as magnetite. The magnetite contains at least 98.1% Fe3O4 and not more than 0.3% SiO2.

[0063] A typical distribution of particles is shown in FIG. 3.

[0064] In a first set of samples for each of examples 1 to 3, the magnetite particles were coated with a binder comprising a thin layer of water-based polyurethanes thinned with up to five parts of water to one part of polyurethane (by weight). The polyurethane used was Witcobond 781 dispersion thinned 1:1 with water. The relative proportion by weight of polymer (after thinning with water as described) to magnetite was: for example 1, 7% polymer to 93% magnetite; for example 2, 6% polymer to 94% magnetite; and for example 3, 5% polymer to 95% magnetite.

[0065] Coating of the particles was achieved by mixing together the coating material and the particulate magnetite. Magnetite particles are significantly less susceptible to shear forces than spiky nickel particles and hence compositions utilising magnetite may be mixed for longer and with greater shear forces without incurring damage. Nevertheless, a low shear mixing regime was used.

[0066] The magnetite and the binder are mixed only long enough until a homogeneity of the mix is visible to the eye. At that point the resulting composite was found to be electrically anisotropic with a very large resistance range which with the composite in the quiescent state starts in the region of an insulator and can be changed over many orders of magnitude by the application of a force to the composite. The polymers used in the invention have inherent flexibility and revert to their quiescent state when the operating force is removed.

[0067] Mixing the water based polymer composites is achieved by adding the magnetite to the liquid polymer in a container under the action of a stirrer (a blade stirrer was used). The mixing is ended as soon as the components are seen to be totally combined. The resulting composite liquid is useful as an ink, coating or feedstock version of the invention. Witcobond 781 is a thermoset based polymer used in this invention.

[0068] A mixing regime suitable for mixing magnetite with thermoplastic polymers such as polythene consists of a heated single powered metal roller loaded with a known quantity of thermoplastic polymer. A measured amount of magnetite is dropped onto the surface of the rotating polymer and worked into the mix with a blade. When the components are seen to be combined thoroughly the resulting composite can be stripped from the roller using a scraper blade. This or similar methods may be used to make versions of the composite with anisotropic properties suitable for use as heat forming feedstock and hot melt adhesives.

[0069] The methods above describe mixing magnetite with a polymer until the components are seen to be totally combined. By repeating the mixing process using sight to determine the degree of mixing, it is possible to define the required mixing for known quantities of known materials under known mixing conditions in terms of time or other parameters related to mixing the composition, thereby removing the need for observation.

[0070] It was found that the electrical response to pressure was greater for the magnetite particle size distribution of example 2 than example 1, and greater still for example 3. This is illustrated in FIG. 4, which demonstrates the effect on resistance of force applied to the three different compositions shown in Table 1 above, the particles of each example within and coated by a thin layer of polyurethane polymer binder as described above. Also evident from the graphs shown in FIG. 4 is that for each of the examples 1 to 3, there is a very rapid fall in resistance upon application of only a small force.

[0071] FIG. 5 demonstrates the effect on resistance of force applied to a composition where the composition of example 1 has a proportion of dendritic silver powder added thereto in a proportion by weight of two parts magnetite of example 1 to one part of silver powder. The particle size of the silver powder was predominantly in the range 10 to 15 micron. The relative proportion by weight of polymer to the magnetite/dendric silver powder was 7% polymer to 93% magnetite/dendric silver powder.

[0072] The magnetite/silver composition exhibits a large resistance range, from an open circuit when no force is applied to a resistance of less than 1 Ohm when a force of 7255 grammes is applied. Further, this composition exhibits very low noise, less noise than the compositions of Examples 1 to 3 which include electrically conductive particles of only one material, namely: magnetite.

[0073] For each of Examples 1 to 3 and the example where silver powder and the magnetite particles or Example 1 are mixed together, the electrically conductive particles were mixed together in a polyurethane polymer binder as described above, and were laid down onto a 5050 mm piece of a fine nylon net having a thickness of 0.255 mm. The finished thickness of the samples was approximately 0.30 mm.

[0074] FIG. 6 illustrates demonstrates the effect on resistance of force applied to a composition where the composition of example 1 has a proportion of spherical magnetite added thereto. The graph shows that the synthetic magnetite reduces the sensitivity of the MAG 1. The spherical synthetic magnetite is Bayferrox 4330 and the ratio of synthetic mag to MAG 1 is 1:1. As with the examples described above the polymer binder was Witcobond 781 mixed 1:1 with water applied to the same type of textile test piece, that is a fine nylon net having a thickness of 0.255 mm. The finished thickness of the samples was approximately 0.50 mm. The relative proportion by weight of polymer to the MAGI/synthetic magnetite was 10% polymer to 90% MAGI/synthetic magnetite.

[0075] The test electrode used in the Examples is illustrated in FIG. 7 and comprised an upper electrode 1 in the form of a 5 mm diameter gold plated rod conductor, a lower electrode 2 in the form of a silver plated metal disc 10 mm in diameter and the sample 3. The upper and lower electrodes are brought into contact with the sample 3, and a potential difference was applied through the electrodes 1, 2 to the sample. An increasing force was applied to the sample 3 by the electrodes 1, 2. The applied force was measured. The change in current was measured and the changing resistance calculated.

[0076] The electrically conductive compositions described herein are advantageous over the prior art because: they use less costly materials in their manufacture; the particles are more robust and therefore the requirement to control applied shear forces during mixing is reduced; they provide an anisotropic resistance change response to applied pressure and with reduced noise when compared to the prior art materials. Both magnetite and silver are safe for use in contact with human skin and food products; nickel and many other things are not.