Piezoresistive Device

Abstract

The present invention relates to piezoresistive devices and pressure sensors incorporating such devices. At its most general, the invention provides a piezoresistive device, comprising a piezoresistive material positioned between an upper conductive layer and a lower conductive layer, wherein the piezoresistive material comprises carbon nanoparticles (most preferably graphene nanoplatelets, graphene or carbon nanotubes) dispersed in a polymer matrix material. The invention also relates to methods of manufacturing and using such devices.

Claims

1. A piezoresistive device, comprising a piezoresistive material positioned between an upper conductive layer and a lower conductive layer, wherein the piezoresistive material comprises carbon nanoparticles dispersed in a polymer matrix material.

2. A piezoresistive device according to claim 1, wherein the carbon nanoparticles comprise graphene nanoplatelets, graphene, or carbon nanotubes.

3. A piezoresistive device according to claim 2, wherein the carbon nanoparticles comprise graphene nanoplatelets.

4. A piezoresistive device according to claim 1, wherein the carbon nanoparticles are functionalised carbon nanoparticles.

5. A piezoresistive device according to claim 1, wherein the lower conductive layer comprises a plurality of conductive traces.

6. A piezoresistive device according to claim 5, wherein the piezoresistive material bridges adjacent conductive traces within the lower conductive layer.

7. A piezoresistive device according to claim 6, wherein the conductive traces are raised features provided on a substrate, with intervening channels between said raised features, and wherein the piezoresistive material fills said channels.

8. A piezoresistive device according to claim 6, wherein the piezoresistive material is applied to the lower conductive layer as a continuous layer of piezoresistive ink.

9. A piezoresistive device according to claim 6, wherein the thickness of piezoresistive material between the upper and lower conductive layers is less than the thickness of piezoresistive material between adjacent conductive traces in the lower conductive layer.

10. A piezoresistive device according to claim 5, wherein the upper conductive layer comprises a plurality of conductive traces, and the piezoresistive material bridges adjacent traces in both the lower conductive layer and the upper conductive layer, as well as the gap between the lower conductive layer and upper conductive layer.

11. A piezoresistive device according to claim 10, wherein the thickness of piezoresistive material between the upper and lower conductive layers is less than the thickness of piezoresistive material between adjacent conductive traces in the upper conductive layer.

12. A piezoresistive device according to claim 5, wherein the lower conductive layer comprises two sets of linear conductive traces which are interdigitated with one another.

13. A piezoresistive device according to claim 5, further comprising resistance measuring equipment having positive and negative terminals, wherein the positive and negative terminals are connected to different conductive traces in the lower layer.

14. A piezoresistive device according to claim 1, wherein the loading of carbon nanoparticles in the polymer matrix material is less than 50 wt. % as a percentage of the total weight of the piezoresistive material.

15. A piezoresistive device according to claim 1, wherein the loading of carbon nanoparticles in the polymer matrix material is less than 20 wt. % as a percentage of the total weight of the piezoresistive material.

16. A piezoresistive device according to claim 1, wherein the piezoresistive material comprises multiple layers.

17. A piezoresistive device according to claim 16, wherein the piezoresistive material comprises three to six layers.

18. A piezoresistive device according to claim 1, wherein the thickness of the piezoresistive material between the upper conductive layer and lower conductive layer is less than 300 μm.

19. A piezoresistive device according to claim 1, wherein the polymer matrix material is an elastic material.

20. A piezoresistive device according to claim 1, wherein the upper conductive layer is provided on an upper substrate and the lower conductive layer is provided on a lower substrate.

21. A piezoresistive device according to claim 20, wherein the substrate is made from a polymer material, glass, fabric, metal or a composite material.

22. A piezoresistive device according to claim 1, comprising: a lower substrate, comprising said lower conductive layer; an upper substrate, comprising said upper conductive layer; and said piezoresistive material positioned between the upper and lower substrate, wherein the piezoresistive material comprises carbon nanoparticles dispersed in a polymer matrix material, and wherein said conductive layers on the lower and upper substrate overlay one another, and the piezoresistive material fills substantially all of the volume between the upper and lower substrates in the region where the conductive layers overlie one another.

23. A piezoresistive device according to claim 1, comprising said piezoresistive material positioned between said upper conductive layer and said lower conductive layer, wherein the piezoresistive material comprises carbon nanoparticles selected from carbon nanotubes, graphene and graphitic nanoplatelets dispersed in a polymer matrix material, and wherein the upper and lower conductive layers each comprise a plurality of spaced conductive traces with piezoresistive material filling the channel between conductive traces on both the upper and lower conductive layers.

24. A pressure sensor, comprising a piezoresistive device according to claim 1.

25. A method of manufacturing a piezoresistive device according to claim 1, comprising: (i) providing a first conductive layer; (ii) depositing one or more layers of piezoresistive material, comprising carbon nanoparticles dispersed in a polymer matrix material, over the first conductive layer; and (iii) bringing a second conductive layer into contact with the piezoresistive material.

26. A method according to claim 25, wherein the first conductive layer comprises a plurality of conductive traces provided on a substrate, and the one or more layers of piezoresistive material are all deposited as continuous layers over said traces.

27. A method according to claim 26, wherein the first conductive layer comprises a first set of interconnected linear conductive traces and a second set of interconnected linear conductive traces, and the traces of the first and second set of traces are interdigitated.

28. A method according to claim 25, wherein the step of providing a first conductive layer involves depositing a conductive ink on a substrate.

29. A method according to claim 25, wherein the step of depositing one or more layers of piezoresistive material involves printing one or more layers of a piezoresistive ink.

30. A method according to claim 25, wherein the step of bringing a second conductive layer into contact with the piezoresistive material involves overlaying a second substrate, having the second conductive layer, onto the first substrate.

31. A method according to claim 25, comprising: preparing a lower part by providing a conductive layer and depositing one or more layers of said piezoresistive material over the conductive layer; and bringing an upper part into contact with the lower part, wherein the upper part is identical to the lower part.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0084] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings in which:

[0085] FIG. 1A shows a conventional commercially available piezoresistive pressure sensor;

[0086] FIG. 1B shows the behaviour of the sensor shown in FIG. 1A upon application of a force;

[0087] FIG. 2 is a cross-sectional view of a known piezoresistive pressure sensor described in U.S. Pat. No. 4,856,993;

[0088] FIG. 3 shows a top view of the electrode arrangement for the device of FIG. 2;

[0089] FIG. 4 is a cross-sectional view of a piezoresistive device of the present invention set up for measurement according to the “different layer measurement mode” described above;

[0090] FIG. 5 is a cross-sectional view of a piezoresistive device of the present invention set up for measurement according to the “same layer measurement mode” described above;

[0091] FIG. 6 is a top view of the lower electrode arrangement of the device of FIG. 5;

[0092] FIG. 7 is a bottom view showing the lower electrodes overlaying the upper electrodes in the device of FIG. 5;

[0093] FIG. 8 is a schematic diagram showing the flow of current in the device of FIG. 5;

[0094] FIG. 9 is a logarithmic plot showing measured resistance vs applied force for a device according to the present invention, having a single upper and lower conductive layer and six piezoresistive layers;

[0095] FIG. 10 is a plot showing the effect of the number of layers of piezoresistive ink on measured resistance;

[0096] FIGS. 11 and 12 are logarithmic plots showing measured resistance vs applied force for devices incorporating piezoresisitve ink with different loadings of carbon nanoparticles. The device in FIG. 11 had a higher loading than that in FIG. 12; and

[0097] FIG. 13 is a logarithmic plot showing measured resistance vs applied force for a device having the configuration shown in FIG. 5, having four layers of piezoresistive ink.

DETAILED DESCRIPTION

[0098] FIG. 1A shows a conventional commercially-available sensor 1, consisting of a lower substrate 3 and flexible upper substrate 5 separated by an air gap created by a ring-shaped spacer 7. The lower substrate includes interdigitated positive and negative electrodes 9, positioned opposite to a conductive layer 11 on the upper substrate. The electrodes of the same polarity are interconnected, and terminate in a common terminal (not shown). As shown in FIG. 1B, when a force is applied to upper substrate 5, the substrate deforms so as to bring the conductive layer 11 into contact with a subset 13 of the electrodes 9. This allows current to flow between positive and negative electrodes via the conductive layer 11. The overall resistance measured between the positive terminal and negative terminal depends on the number of electrodes in contact with the conductive layer. Thus, the measured resistance can be related to the deformation of the upper substrate, which will depend on the applied force.

[0099] FIG. 2 shows another prior art sensor 101, as described in U.S. Pat. No. 4,856,993. Sensor 101 comprises a lower substrate 103 and upper substrate 105 each bearing conductive traces 107 and 109 separated by layers of piezoresistive material 111 and 113. The conductive traces of the upper and lower substrates overlay one another so as to form a grid arrangement, as shown in FIG. 3. The piezoresistive material comprises carbon-molybdenum disulphide in an acrylic binder. Resistance is measured between “drive” trace 109 and one of the “sensed” traces 107. However, to produce a functioning device with this material it is necessary for adjacent “sensed” traces to be electrically isolated from one another. Thus, resistive material 111 takes the form of individual stripes separated by an air gap.

[0100] FIGS. 4 to 7 show piezoresistive devices according to the present invention. FIG. 4 shows a piezoresistive sensor 1001, consisting of a lower substrate 1003 and upper substrate 1005 each bearing discrete silver traces 1007 and 1009 (again, a single trace is shown, in cross section, for the upper substrate) separated by layers of piezoresistive material 1011 and 1013. The distance between silver traces in the same layer D.sub.H is greater than the distance between conductive traces in different layers D.sub.V (although, note that the device in FIG. 4 is not to scale).

[0101] In contrast to the prior art device of FIG. 2, the piezoresistive material of the device in FIG. 4 includes high aspect ratio carbon nanoparticles, in this case 5 wt. % functionalised graphene nanoplatelets in a vinyl polymer matrix. The use of this particular material in the context of the device having D.sub.H greater than D.sub.V means that adjacent conductive traces are effectively electrically isolated from one another without an intervening insulating material (e.g. air). Thus, piezoresistive ink layers 1011 and 1013 are completely overprinted on both the upper and lower substrates, meaning that the ink fills the channel between adjacent silver traces (illustrated for the lower layer by cross-hatched region 1011a). This makes the device particularly simple to manufacture, since the upper and lower parts of the device can be manufactured in the same way. For example, a single starting substrate can be overprinted with silver traces and one or more layers of the piezoresistive ink, and subsequently cut in half and overlaid to form the device shown in FIG. 4.

[0102] Similarly to the device shown in FIG. 2, the pressure across the device in FIG. 4 can be determined by measuring the resistance between a silver trace 1007 on the lower substrate and a silver trace 1009 on the upper substrate.

[0103] The device in FIG. 5 is similar to that shown in FIG. 4, but in this case the resistance is measured between adjacent silver traces on the same substrate. In the device of FIG. 5, the lower substrate has two electrodes—positive electrode 1015 and negative electrode 1017—as shown in FIG. 6. The positive electrode is formed from interconnected linear conductive traces 1007a having a common positive terminal 1019. The negative electrode has a similar arrangement of silver traces 1007b and terminates in a negative terminal 1021, with the silver traces of the negative electrode interdigitating and alternating with those of the positive electrode. In this embodiment, the upper substrate has a single silver layer 1009 overlying all of the conductive traces of the lower substrate (as shown in FIG. 7). However, the form of the silver layer 1009 is not limited in this embodiment, provided that the layer overlays the silver traces 1007a and 1007b of the positive and negative electrodes, and could take the form of rows of silver traces (e.g. as shown in FIG. 3) or any other regular or irregular pattern.

[0104] In this device, resistance is measured between the positive terminal 1019 and negative terminal 1021. Electron flow between adjacent electrodes occurs via the silver layer 1009, with direct current flow between adjacent conductive traces being negligible (or even prevented) due to the use of the high aspect ratio carbon nanoparticle-based ink and the spacing of the traces. More specifically, as shown in FIG. 8, electrons flow between adjacent traces by flowing “up” through the piezoresistive material, across the silver layer 1009 and “down” through the piezoresistive material. Advantageously, this means that electrons must travel through a longer path of piezoresistive material in the device of FIG. 5 compared to the device of FIG. 4, meaning the resistance values measured in FIG. 5 are higher than those for FIG. 4 (all other things being equal). Since a higher starting resistance helps to improve the dynamic range of the device, this increased electron path length helps to improve sensitivity without increasing height.

EXPERIMENTAL RESULTS

Example 1

[0105] A piezoresitive device was produced and measured according to the different layer measurement mode described above, in order to assess the piezoresistive properties of a high aspect ratio carbon nanoparticle ink.

[0106] Two 10 mm by 9 cm strips of a conductive silver ink (AG 500, Conductive Compounds PE) were screen printed onto a PET substrate (175 μm thickness) using a DEK 248 screen printer. Each strip was then overprinted with three layers of piezoresistive ink containing carbon nanoparticles including functionalised GNPs (Haydale Graphene Industries plc) in a polymer matrix and solvent, whilst leaving a small area of the conductive silver exposed at one end. Each piezoresistive ink layer was dried before the application of subsequent ink layers. The assembly was then cut in half (each bearing a silver strip), and the two halves overlaid with the piezoresistive ink layers facing one another, so as to form a piezoresistive sensor.

[0107] To measure the resistance behaviour of the device, a multimeter (Agilent RMS) was attached to the exposed silver on each half of the device, and pressures of between 1 and 3000 N were applied using a Housfield extensometer. To achieve an even distribution of force, the sensor was attached to flat acrylic blocks and a spacer with the same area as the active sensor area was placed on top of the acrylic block. Measurements were repeated four times.

[0108] The results of these experiments are shown in FIG. 9. As is clear from FIG. 9, the results for the second, third and fourth run are in excellent agreement, but differ from those obtained for the first run. It is believed that this could be due to irreversible physical changes occurring within the sensor the first time force is applied. In this regard, it is interesting to note that after a force of 2000 N is applied the measured resistance for run 1 falls into agreement with that of runs 2 to 4. This suggests that, in this case, a force of at least 2000 N is required to achieve the irreversible physical changes which occur between 1 and 3000 N.

[0109] It is noted that there is a slight divergence in the measured resistance values for runs two, three and four at forces below around 40 N—this is probably due to artefacts caused by the extensometer since forces lower than 50N are close to instrument limits (hence there is an increased instrument error in this range).

[0110] The large dynamic range, repeatability, and interpolatable nature of the pressure response of the device means that it is particularly well-suited to use as a sensitive pressure sensor. In particular, the sensor appears to work effectively at high pressures, in contrast to prior art pressure sensors based on piezoresistive carbon-based inks where resistance plateaus at relatively low pressures.

Example 2

[0111] Experiments broadly following a similar protocol to that described for Example 1 were carried out using piezoresistive ink containing carbon nanoparticles including functionalised GNPs in a vinyl chloride copolymer based binder.

[0112] In this case silver traces of 20 mm width, 150 mm length and ˜8 μm height were screen printed with a 54/64 mesh onto a 330 μm thick PET substrate using a DEK 248 screen printer. Three layers of piezoresistive ink were printed as continuous blocks over the silver traces using a 54/64 mesh to give a total height of ˜9 μm. A piezoresistive device was then formed following the same approach as in Example 1, and the resistance measured over a circular area with a diameter 15 mm (1.77 cm.sup.2 area of compression). Three different devices were produced having “high”, “medium” and “low” loadings of carbon nanoparticles in the piezoresistive material. The devices having medium and low loadings had four and eight times less carbon than the high loading respectively.

[0113] The device made using the piezoresistive material with high carbon nanoparticle loading displayed resistances of less than 1Ω at all applied pressures, demonstrating that this ink is not piezoresistive and hence not suitable for measuring applied pressures. In contrast, devices produced using piezoresistive materials having medium and low loadings of carbon nanoparticles showed a repeatable and interpolatable variation in measured resistance with applied pressure, and a suitably high resistance at zero applied force to be useful pressure sensors. The device having low carbon nanoparticle loading performed better than the device having medium carbon nanoparticle loading.

Example 3

[0114] Experiments were carried out to determine the effect of increasing the number of layers of piezoresistive ink on resistance, with results shown in FIG. 10.

[0115] A series of lower substrates were produced by screen printing an indium tin oxide sheet with one, two or three layers of a piezoresistive ink containing 3.5 wt. % functionalised graphene nanoplatelets (with negligible content of other types of carbon particle) dispersed in vinyl chloride copolymer based binder and solvent (15 parts binder to 85 parts solvent). The dried ink had a GNP content of ˜20 wt. %. Piezoresistive devices were formed by combining the lower substrates with an upper substrate, consisting of a further indium tin oxide sheet optionally bearing a single layer of the same piezoresistive ink as the lower substrate. The resistance of these devices under an applied pressure of 2000 N was measured using a Housfield extensometer with a circular area of compression of 1.77 cm.sup.2 (diameter 15 mm)

[0116] As can be seen in the results shown in FIG. 10, the resistance of the devices increased with the number of layers, and the devices having a piezoresistive ink layer on the upper substrate (the square data points in FIG. 10) had a higher resistance than those lacking a layer of piezoresistive ink (the diamond data points in FIG. 10—i.e. devices in which the upper substrate did not have a piezoresisitve layer deposited on the silver).

[0117] The resistance values measured in these tests were particularly high. It is thought that this is due to the use of inks incorporating low loadings of functionalised GNPs with negligble content of other carbon particles.

Example 4

[0118] A piezosensitive device was produced and measured according to the “same layer measurement mode”. In this example, a lower substrate was produced by overprinting a PET sheet (330 μm thickess) with interdigitated silver positive and negative electrodes having the pattern shown in FIG. 6. The silver traces were printed using a 100/34 mesh, to produce interconnected linear traces of width ˜600 μm wide, length ˜15 mm and height ˜7 μm, with a separation of ˜400 μm between adjacent traces of the positive and negative electrodes.

[0119] An upper substrate was produced by screen printing a continuous sheet of silver (15 mm length, 16 mm width, ˜8 μm height) on a further PET substrate using a 54/64 mesh, and subsequently coating this with two continuous layers of a piezoresistive ink containing carbon nanoparticles including functionalised GNPs in a liquid medium containing 15:85 vinyl chloride copolymer based binder:solvent (corresponding to the “low” loading ink from Example 2). A piezoresistive device was created by bringing the electrodes of the lower substrate into contact with the piezoresistive ink of the upper substrate.

[0120] A second device incorporating three layers of piezoresistive on the upper substrate was produced following the same procedure described above.

[0121] The resistance of the devices was then measured by measuring the resistance between the positive and negative electrodes on the lower substrate. The results of measurements for the two layer and three layer devices are shown in FIGS. 11 and 12 respectively.

Example 5

[0122] A further piezoresistive device was produced and measured according to the “same layer measurement mode”. In this example, a lower substrate was produced by overprinting a PET sheet with interdigitated silver positive and negative electrodes following the method described in Example 4, and subsequently coating this with two continuous layers of a piezoresistive ink containing carbon nanoparticles including functionalised GNPs in a liquid medium containing 15:85 vinyl chloride copolymer based binder:solvent (corresponding to the “low” loading ink from Example 2) using a 100/34 mesh to give a piezoresistive material of height ˜4 μm. The resistance between the positive and negative electrodes was measured and found to be 220 kΩ in the absence of an applied force.

[0123] An upper substrate was produced by screen printing a silver electrode on a further PET sheet, and subsequently coating this with two layers of the same piezoresistive ink used for the lower substrate. A piezoresistive device was created by bringing the piezoresistive ink layers of the lower substrate into contact with the piezoresistive ink of the upper substrate. Resistance between the positive and negative electrodes was measured under varying applied forces, producing the results shown in FIG. 13.

[0124] The resistance of the lower substrate in the absence of the upper substrate was significantly greater than that measured for the device incorporating the upper substrate. This difference is such that the contribution of electron flow directly between adjacent conductive traces (i.e. not mediated by the upper conductive layer) is negligible in the piezoresistive device incorporating the upper and lower substrates.

[0125] In respect of numerical ranges disclosed in the present description it will of course be understood that in the normal way the technical criterion for the upper limit is different from the technical criterion for the lower limit, i.e. the upper and lower limits are intrinsically distinct proposals.

[0126] For the avoidance of doubt it is confirmed that in the general description above, in the usual way the proposal of general preferences and options in respect of different features of the piezoresistive device and methods described above constitutes the proposal of general combinations of those general preferences and options for the different features, insofar as they are combinable and compatible and are put forward in the same context.