METHOD OF CONTROLLING THE ELECTRICAL PROPERTIES OF MAGNETITE PARTICLES

Abstract

A method of controlling the electrical properties of a quantity of magnetite particles comprises the step of oxidising at least some of the quantity of magnetite particles by heating the said quantity of magnetite particles in an oxygen rich environment for a period of time.

Claims

1. A method of controlling the electrical response sensitivity of a quantity of magnetite particles, the magnetite particles each having a plurality of planar faces, adjacent planar faces connected at a vertex, each particle having a plurality of vertices wherein the magnetite particles are irregular in shape and have a low aspect ratio, the method characterised by the step of oxidising at least some of the quantity of magnetite particles by heating the said quantity of magnetite particles in an oxygen rich environment for a period of time, wherein the electrical response sensitivity of the so oxidised quantity of magnetite to a force stimulus is reduced relative to an electrical response sensitivity to the same force stimulus of quantity of said magnetite particles which have not been so oxidised.

2. A method according to claim 1, wherein, the quantity of magnetite particles is heated to a selected temperature of one of: 200 C; up to 375 C; between 200 C and 250 C; between 200 C and 400 C; between 375 C and 550 C; and between 550 C to 575 C.

3. A method according to claim 1, wherein the force stimulus is a selected one of: mechanical; electrical; and mechanical and electrical.

4. (canceled)

5. (canceled)

6. (canceled)

7. (canceled)

8. A method according to claim 1, wherein the oxygen rich environment is air or an environment that is enriched with oxygen, the oxygen rich environment having a greater proportion of oxygen than air.

9. A method according to claim 1, wherein the magnetite particles are heated for a selected period of time of: between 1 minute and 240 minutes; between 5 and 120 minutes; between 5 and 60 minutes; between 5 and 45 minutes; 10 minutes; 30 minutes; and 45 minutes.

10. A method according to claim 1, wherein the electrical response sensitivity that is reduced is selected from the group comprising: a rate of change of resistance of the quantity of so oxidised magnetite in response to the force stimulus; and a resistance range of the quantity of so oxidised magnetite in response to the force stimulus.

11. A method according to claim 1, wherein the quantity of magnetite particles includes a distribution of particle sizes between sub-micron and tens of microns.

12. A method according to claim 11, 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.

13. An electrically anisotropic material responsive to applied force, the material comprising at least a first electrically conductive filler and a non-conductive filler containment matrix, wherein the conductivity of the material in an unstressed state is related to the conductivity of the non-conductive filler containment matrix and in a stressed state to the conductivity resulting from the presence of the at least first electrically conductive filler in the material, characterised in that the first electrically conductive filler is comprised of magnetite particles and wherein at least some of the magnetite particles are the product of the method of controlling the electrical response sensitivity of a quantity of magnetite particles, the magnetite particles each having a plurality of planar faces, adjacent planar faces connected at a vertex, each particle having a plurality of vertices wherein the magnetite particles are irregular in shape and have a low aspect ratio, the method characterised by the step of oxidising at least some of the quantity of magnetite particles by heating the said quantity of magnetite particles in an oxygen rich environment for a period of time, wherein the electrical response sensitivity of the so oxidised quantity of magnetite to a force stimulus is reduced relative to an electrical response sensitivity to the same force stimulus of quantity of said magnetite particles which have not been so oxidised.

14. A material according to claim 13, wherein the non-conductive filler containment matrix is one of: a binder; a textile; a textile in the form of a non-woven assembly of fibres; a textile in the form of a non-woven assembly of fibres which is a yarn or a roving; a surface to which the electrically conductive filler may adhere; or an open or closed cell foam.

15. A material according to claim 14, wherein the material is formed by loading a selected one of: an open cell foam with the electrically conductive filler prior to foaming a closed cell foam with the electrically conductive filler prior to foaming; by applying a coating of the electrically conductive filler to a finished foam.

16. A material according to claim 14, wherein the binder is of a selected one of: a polymer binder; a grease; an oil; a gel and a wax.

17. A material according to claim 13, wherein the at least one first electrically conductive filler is provided on the non-conductive filler containment matrix as a thin film.

18. A material according to any of claim 13, wherein the applied force to which the material is responsive is a selected one of: mechanical; electrical; and mechanical and electrical.

19. (canceled)

20. (canceled)

21. (canceled)

22. (canceled)

23. (canceled)

24. (canceled)

25. (canceled)

26. (canceled)

27. An electrically anisotropic material according to claim 13 laid down on a substrate as a thin film, wherein the minimum depth of the film is the dimension of the largest magnetite particle measured in the direction of the depth of the film.

28. An electrically anisotropic material according to claim 27, wherein the thin film has a maximum thickness of 0.25 mm.

29. An electrically anisotropic material according to claim 27, wherein the thin film is laid down on the substrate in a selected one of: a single layer, and multiple layers.

30. A touch screen comprising an electrically anisotropic material according to claim 27, the thin film forming a layer of the touch screen.

31. A touch screen according to claim 30, wherein the layer is substantially transparent.

32. A method according to claim 1, comprising the further step of incorporating the so oxidised quantity of magnetic into a matrix.

33. The combination of magnetite particles produced according to the method of claim 1, and a matrix, the magnetite particles incorporated into the matrix.

Description

[0047] FIG. 1 is a graph showing the relationship between resistance and pressure for compositions formed using LKAB's M10 magnetite (example 1, table 1) when heated to 250 C for different periods of time.

[0048] FIG. 2 is a graph showing the relationship between resistance and pressure for compositions formed using LKAB's M10 magnetite (example 1, table 1) respectively when heated to 250 C for 10 minutes and when the same magnetite is not oxidised.

[0049] FIG. 3 is a graph showing the relationship between resistance and pressure for compositions formed using LKAB's M10 magnetite (example 1, table 1) respectively when heated to 250 C for 30 minutes and 45 minutes and when the same magnetite is not oxidised.

[0050] FIG. 4 is a graph showing the relationship between resistance and pressure for compositions formed using LKAB's M25 magnetite (example 2, table 1) respectively when heated to 250 C for 10 minutes and when the same magnetite is not oxidised.

[0051] FIG. 5 is a graph showing the relationship between resistance and pressure for compositions formed using LKAB's M25 magnetite (example 2, table 1) respectively when heated to 250 C for 30 minutes and 45 minutes and when the same magnetite is not oxidised.

[0052] In each example the magnetite was mixed with a binder in a low shear mixing regime, the binder being a water-based polyurethane binder.

[0053] The proportion of magnetite to binder was 3:1 by weight. The resulting composition was laid down on an open weave mesh in a layer approximately 0.25 mm thick. Forces were applied to the sample by a gold plated electrode rod of a circular cross-cross-section having a diameter of 6 mm against a stainless steel electrode plate, the composition being between the stainless steel base plate and the electrode.

[0054] LKAB of Sweden provide natural magnetite of different particle sizes which has been used in this invention. Alternatively, natural magnetite from New Zealand has been found to work in the invention when comminuted and sized and sorted by sieving.

Table 1 below sets out four different types size distributions of magnetite available from LKAB.

TABLE-US-00001 TABLE 1 Particle size distribution Example 1 Example 2 Example 3 Example 4 (cyclosizer Magnetite - Magnetite - Magnetite - Magnetite - method) Magnif 10 Magnif 25 Magnif 50 Magnif EX014 d10 (micron) 5 6 9 3 d50 (micron) 10 22 63 7 d90 (micron) 25 50 180 13 particle irregularly shaped, irregularly shaped, irregularly shaped, irregularly shaped, characteristics low aspect ratio low aspect ratio low aspect ratio low aspect ratio

[0055] It can be seen from the graphs that by increasing the oxidation of magnetite the sensitivity of the composition formed therewith is reduced. It can also be seen from the graphs that where the particle size of the magnetite is smaller there is a greater reduction in sensitivity by heating the magnetite for a longer time period.

[0056] The increased range of response sensitivity which results from this oxidising process provides a large increase in the number of achievable mixing ratios and sensitivities of composites and allows the formulation of thinner FvR composite lay-downs.

[0057] The LKAB magnetite particles used in this invention range in size between sub-micron and tens of microns at D50. The particles are produced by a pulverisation process and have irregular shapes described as each having a plurality of planar faces, adjacent planar faces connected at a vertex, the particles each having a plurality of vertices.

[0058] Although the overall range of the resistance change in response to applied force may be reduced somewhat, the reduction in response sensitivity can still provide a large range of resistance change in response to an input of force which can be useful for the detection of larger applied forces and activation by different voltages.

[0059] There are a number of ways of heating the magnetite include heating it in an electric element oven or using induction or microwave heating. An electric element oven was used for the oxidation of the magnetite used for making the graphical examples.

[0060] Polymers with different mobilities can be used with different magnetite particle sizes to produce composites with different final sensitivities. The thickness and mobility of the matrix also has an effect on the working sensitivity of the composite. In thin matrixes with low particle loadings individual magnetite particles may be some distance apart and it is possible to use polymers with a lower mobility as the sensitivity of the composite will increase as the composite laydown is reduced in thickness. Thin lay-downs of composites with very low loadings of the smaller magnetite particles can use clear polymer binders to provide the active pressure component of transparent and translucent touch-panels and screens. By using an electrically anisotropic pressure sensitive material, it is possible to measure the force a user applies, and the x, y location of the applied force, when touching the screen.

[0061] A substantially transparent thin film can be obtained with a proportion of 2% magnetite to 98% binder. However, loadings of magnetite in thin composite lay-downs can be as low as 0.5% (by weight) of the composite. In thicker composites, high-mobility matrixes such as gels can be loaded with magnetite up to levels of approximately 97% (by weight) which is well beyond the accepted percolation level of magnetite. Mixing of both high and low loadings of magnetite into the matrixes is done using controlled, low-shear mixing regimes to reduce or eliminate the effect of aggregation of the particles on the electrical qualities of the composite.

[0062] In order to provide a composite having the desired sensitivity, that is change in resistance in response to the application of pressure, the particles can be sorted into individual sizes and/or mixed ranges of sizes prior to incorporation into a matrix, typically a binder. The invention provides a simple process which alters the magnetite's inherent range of sensitivity prior to its incorporation in a matrix. This is achieved by heating the magnetite in an oxygen rich environment for a period of time.