Field of the Invention

The present invention relates to a force or pressure sensing composite material, a force or pressure sensor using said composite material, and applications thereof.

Background of the Invention

Force sensing composite materials comprising electrically conducting and/or semiconducting particles disposed in an insulating matrix, such as a polymer, are known. The resistance of such composite materials, and the variation of that resistance as a function of applied force or pressure, may depend on one or more different factors as described below. The resistance may be measured between a pair of electrodes, with force or pressure being applied on one of the electrodes in a direction towards the other electrode.

One factor is the volume fraction of the composite material, i.e. the volume of particles as a fraction of the total volume of the composite material. At low volume fractions, the particles are electrically insulated from each other and there is no electrically conductive path between the electrodes. As the volume fraction increases, so does the number of electrically conductive paths between the electrodes due to contact between the particles, and the resistance decreases due to a percolation effect. Since the elastic moduli of polymers is much less than that of metals or inorganic oxide crystals, the effect of increasing pressure on the composite material at volume fractions higher than 0.25 is equivalent to an increase in the volume fraction and gives a mixture that decreased in resistance with pressure or force, as disclosed for example in GB-A-2541288 and GB-A-2564737.

The percolation resistance is largely dependent on the particle material resistivity, which will be high for high energy gap materials such as oxides. However, such materials also have a high work function, which reduces the probability of conduction through the polymer, so preventing the composite material having a low resistance at high pressures.

Another factor is the distance between particles. At short distances between particles, quantum tunnelling of carriers between adjacent particles can occur, thus reducing the electrical resistance. The application of force or pressure reduces the distance between particles, thereby increasing the quantum tunnelling effect and reducing the resistance. Quantum tunnelling falls off exponentially with distance, so this effect is only relevant for very small particle separation distances. In a composite material with equal volumes of polymer and particles, the mean polymer thickness around a particle is one-third of the particle radius, so even for particles as small as 1 micron in diameter, the polymer thickness will on average exceed 100 nm, and the tunnelling effect would be very small.

The quantum tunnelling effect increases with electrical field strength. The localised electrical field strength may be increased by using acicular or 'spiky' particles, as disclosed for example in GB-A- 2462920. However, additional processing steps or specific conditions may be required to produce acicular particles.

In composite materials using nickel particles, such as disclosed in US 6495069, the particles are likely to have a thin surface layer of insulating oxide, sufficiently thin that tunnelling may occur. This effect is described by Bloor et al in J Phys D:App Phys 38, 2851-2860, 2005 for nickel particles and Webb et al in Nanotechnology 24, 165501, 2013 for acicular particles of tin oxide and antimony oxide. In both of these structures it is thought the acicular spikes lead to thin layers of polymer, through which carriers can tunnel. If the particles are not acicular, it is still possible for an insulating film to pass current, as described in Chiu, A Review on Conduction Mechanisms in Dielectric Films, Abstracts in Materials Science and Engineering 2014, Article ID578168, 2014.

Another factor is the contact between one of the electrodes and the particles. In some designs, a layer of the composite material is applied to a first electrode and a second, floating electrode is placed on the upper surface of the layer, or force is applied to the upper surface of the layer by a metal probe acting as a second electrode. The particles at the upper surface project from the layer to varying degrees and may come into contact with the second electrode in a discontinuous fashion as force or pressure is applied. Force sensors that exploit this effect are disclosed in US 4314227, US 4489302, US 5296837, US 5302936 and US 9546859.

A summary of pressure sensitive resistance sensors is provided in S J Dempsey et al, Tactile Sensing in Human-Computer interfaces, Sensors and Actuators A Physical 232, 279-250, 2015, in which the existing sensors described are all based on particles of metals, such as nickel, or oxides, such as tin oxide or magnetite. To obtain a large working range of resistance, the particles comprise a mixture of two or more materials. It would be desirable to have a sensor which employed just one type of particle, since this would make processing simpler and cheaper.

The conductivity mechanism of the composite material may vary with the applied force or pressure. This is examined in detail by Bloor et al in J Phys D:App Phys 38, 2851-2860, 2005 where the departures from Ohm's Law are shown to be large for nickel dispersed in silicone. It would be preferable if the departures from Ohm's Law were small. Summary of the Invention

According to one aspect of the present invention, there is provided a composite material having a force- or pressure-dependent resistance, the composite material comprising particles of inorganic chalcogenide dispersed in an insulator such as a polymer. The term 'chalcogenide' is not intended to include an oxide.

Preferably, the chalcogenide comprises a sulphide, and most preferably a pyrite, such as iron pyrite (FeS ₂ ) or copper iron pyrite (chalcopyrite, CuFeS ₂ ), or a mixture of particles of both materials. For such compounds, the resistivity is high but the work function is comparatively low. For iron pyrite, the surface has an energy gap less than half that of the body (see Flerbert et al Surface Science 618, 53-61, 2013), making it much easier for electrons to pass from the particle into and through the polymer. As a result, a layer of such a composite material may have a resistance that is strongly dependent on force over a large force range. The effect may be due to low energy surface states, as measured by Von Oertzen et al in Molecular Simulation, 32, 1207- 1212, 2006. That paper showed there were similar states in chalcopyrite, which is therefore also a good candidate for this type of sensor. Flowever, the present invention is not limited to the application of any particular theory.

Zinc sulphide (ZnS) has also been shown to have surface states that are low in energy and may be suitable for certain applications, although its resistivity is high. Bornite (Cu ₅ FeS ₄ ) may also be suitable for certain applications, although its resistivity is low. Preferably, the resistivity of the inorganic chalcogenide is in the range 10 ohm-cm to 10,000 ohm-cm.

The particles may be substantially all (e.g. >95% and preferably >99%) of inorganic chalcogenide, without other particle components such as metal or carbon particles.

The particles need not be acicular, but may be approximately cubic or spherical, such as obtained from powdered crystal. According to another aspect of the present invention, there is provided a force or pressure sensor comprising a layer of the above composite material arranged in electrical contact between first and second electrodes, such that force or pressure between the electrodes reduces the resistance of the layer. The layer may be formed on a surface of metal, plastic or textile.

According to another aspect of the present invention, there is provided a wearable sensor comprising the above sensor having a flexible substrate forming part of a wearable device. Brief Description of the Drawings

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

Figure 1 shows an experimental arrangement for measuring resistance of a composite material, as a function of pressure.

Figure 2 is a graph of the dependence of resistance on force of a sample of a composite material in an embodiment.

Figure 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

Figure 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 x 6 cm, with two holes 2 of area 1 cm 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 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. Figure 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 Figure 2 is shown below in Table 1. Table 1 - Data for Figure 2

It is often convenient to maintain the polymer as a liquid, and similar results to those shown in Figure 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. The iron pyrite powder was obtained from Right Rocks, Texas. 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. cm.

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.

Tests were also made with CuFeS 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 Figure 2, as may be expected from the low basic resistance.

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. 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 Figure 3. In this Figure, the two dotted lines indicate calibrations with standard resistances of 2kO and 10 kO. 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 2 - Data for Figure 3

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

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

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.

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

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.