Force Or Pressure Sensing Composite Material

Abstract

A composite material having a force- or pressure-dependent resistance comprises particles of inorganic chalcogenide dispersed in a polymer. The chalcogenide may be a pyrite such as iron pyrite, copper iron pyrite or a mixture of the two. The composite material may be used in a force or pressure sensor, for example in a wearable device.

Claims

1. A force or pressure sensor comprising a composite material having a force- or pressure-dependent resistance, the composite material including particles of one or more inorganic pyrite dispersed in an insulating material.

2. The sensor of claim 0, wherein the one or more inorganic pyrite includes iron pyrite, FeS.sub.2.

3. The sensor of claim 0, wherein the one or more inorganic pyrite includes chalcopyrite, CuFeS.sub.2.

4. (canceled)

5. (canceled)

6. The sensor of claim 1, wherein the composite material includes particles of bornite, Cu.sub.5FeS.sub.4.

7. A force or pressure sensor comprising a composite material having a force- or pressure-dependent resistance, the composite material including particles of bornite, Cu.sub.5FeS.sub.4.

8. The sensor of claim 1, wherein the particles have a resistivity in the range 10 Ohm-cm to 10,000 Ohm-cm.

9. The sensor of claim 1, wherein the material has an ohmic resistance.

10. The sensor of claim 1, wherein the particles are not acicular.

11. The sensor of claim 1, wherein the particles have a diameter of 100 microns or less.

12. The claim 1, wherein the volume fraction of the particles is greater than 0.25.

13. The sensor of claim 1, wherein the insulating material comprises one or more polymers.

14. The sensor of claim 13, wherein at least one said polymer comprises an elastomer.

15. The sensor of claim 13, wherein at least one said polymer comprises polyurethane.

16. The sensor of claim 1, wherein the composite material is connected between first and second electrodes arranged so that force or pressure may be applied therebetween so as to apply force or pressure to the composite material and thereby change the resistance thereof.

17. The sensor of claim 16, wherein the composite material is deposited on the second electrode.

18. The sensor of claim 17, wherein the second electrode comprises a metal.

19. The sensor of claim 17, wherein the second electrode comprises metal-coated plastic.

20. The sensor of claim 17, wherein the second electrode comprises a textile having a metal coating.

21. (canceled)

22. A method of manufacturing the sensor of claim 1, comprising forming the composite material by dispersing the particles in a fluid polymer and curing or drying the fluid polymer.

23. The method of claim 22, wherein the fluid polymer is a water-dispersed polymer.

24. (canceled)

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Specific embodiments of the present invention are described below with reference to the accompanying drawings, in which:

[0019] FIG. 1 shows an experimental arrangement for measuring resistance of a composite material, as a function of pressure.

[0020] FIG. 2 is a graph of the dependence of resistance on force of a sample of a composite material in an embodiment.

[0021] FIG. 3 is a graph of current as a function of voltage for a sample of the composite material in an embodiment, for different forces.

DETAILED DESCRIPTION OF EMBODIMENTS

[0022] FIG. 1 shows a structure for testing the dependence of resistance on force for a sample composite material. A base 1 is made of mica sheet 0.2 mm thick and dimensions 2.5 cm×6 cm, with two holes 2 of area 1 cm.sup.2 passing through the mica sheet. A rectangle of copper tape 2.5 cm wide and 7 cm long, coated on one side with conducting adhesive, is secured to the base 1 so that there is an overlap of 5 mm at each end. These are folded over to the top of the base 1 and secured there. In this way the holes 2 each comprise a cylindrical pot of height 0.2 mm and area 1 cm.sup.2, available to be filled by the sample composite material. The copper tape provides the bottom electrode 3, which is preferably earthed. A probe contact (not shown) may be applied to the upper surface of the composite material within one of the holes 2, providing both the second electrode, and a means of exerting force/pressure. The force may be applied either by calibrated weights, or, for continuous variation, by passing current through a solenoid with a metal core in contact and in line with the probe. The resultant force is measured by a digital scale, preferably accurate to 0.1 g, on which the base sits. In one experimental procedure, 4 bases are used, giving 8 samples for comparison.

[0023] FIG. 2 shows the dependence of resistance on force of iron pyrite powder dispersed in Varathane, a water-based polyurethane manufactured by Rust-Oleum, and conveniently available in a range of Vallejo™ floor paints, which hardens in a few hours at room temperature. The data for FIG. 2 is shown below in Table 1.

TABLE-US-00001 TABLE 1 Data for FIG. 2 Force in grams Resistance in Ohms 0 1 2,000,000 25 250,000 50 100,000 75 70,000 100 37,000 150 20,000 200 12,000 250 7,300 300 4,700 350 3,000

[0024] It is often convenient to maintain the polymer as a liquid, and similar results to those shown in FIG. 2 were obtained using Sericol Polyplast PY283. It is easier to spread this mixture if thinned with TS16. The final structure is made by curing at 80° C. for 30 minutes.

[0025] The iron pyrite powder was obtained from Right Rocks, Tex. The powder as supplied contains some particles larger than 200 microns, and to ease subsequent screen printing, the powder may be filtered through a 100 micron mesh gauze before use. The powder has a resistivity of about 10,000 Ohm.Math.cm.

[0026] A range of candidates for the polymer is available commercially, as paint varnishes or protective coatings. It is preferable to use an elastomer, since in some applications such as touch-sensitive sensors there should be some yielding under small forces. The polymers mentioned herein give good results, but similar results may be obtained with other polymer types.

[0027] Tests were also made with CuFeS.sub.2 (chalcopyrite), as a fine powder. This was obtained from SS Jewellery Findings, Tasmania, and had a higher conductivity than the iron pyrite samples. Tests made with Varathane as a polymer showed a large variation of resistance with force, but the range was lower than shown in FIG. 2, as may be expected from the low basic resistance.

[0028] In the test samples, the composite material was made from approximately equal volumes of solid particles in powder form and fluid polymer, with added water if the polymer is a water-dispersed polyurethane, or for other types of polymer, a solvent appropriate for thinning that particular polymer. The preferred ratio of volumes will depend on the resistivity of the solid particles and the desired pressure range of the sensor. The composite material should be thoroughly stirred before it is applied to the base material of the sensor, which can be metal, plastic, or textile. The composite material may be applied by printing, such as screen printing.

[0029] The conductivity mechanism of the samples shows marked differences from the prior art composite materials, because of the surface states mentioned above. One consequence of the different physical mechanisms is a current-voltage dependence that is close to Ohm's Law, as shown in FIG. 3. In this Figure, the two dotted lines indicate calibrations with standard resistances of 2 kΩ and 10 kΩ. The measured points are for a force of 300 grams and 130 grams as labelled on the graph, and as shown below in Table 2.

TABLE-US-00002 TABLE 2 Data for FIG. 3 Current Measured values Ohm's Law Lines % Deviation Volts R = 2K R = 10K F = 130 gms F = 300 gms 130 gms 300 gms 130 gms 300 gms 0 0 0 0 0 0 0 1 4.7 0.94 1.8 3.2 2 3.6 10 11 2 9.4 1.9 3.5 6.7 4 7.2 12 7 3 14.1 2.8 5.5 10.3 6 10.8 10 5 4 18.8 3.8 7.7 14 8 14.4 4 3 5 4.7 9.9 18.0 9.9 18.0

[0030] Current in Table 2 and FIG. 3 is measured in arbitrary linear units, but calibration using the standard resistances shows that these units are approximately 0.1 mA.

[0031] FIG. 3 includes straight lines between the values for the extreme voltages of 0 and 5 V, for both forces. Hence, it can be seen that the measured points correspond closely to Ohm's Law, to within approximately 10%.

[0032] Composite materials in embodiments of the invention may be used to manufacture a touch-sensitive sensor, in which force or pressure is applied to the second electrode by touch.

[0033] Composite materials in embodiments of the invention may be used to manufacture a wearable force or pressure sensor, in which the composite material is applied as a liquid or paste to a textile, for example so as to impregnate the textile, and the liquid or paste is then dried or cured. The textile may have a conductive (e.g. metal) coating provided therein, forming an electrode of the sensor.

ALTERNATIVE EMBODIMENTS

[0034] Alternative embodiments may be envisaged on reading the above description, which may nevertheless fall within the scope of the present invention. The description of embodiments is provided purely by way of example and should not be construed as limiting on the scope of the invention.