WO2014001332A1

WO2014001332A1 - Method for forming a body comprising a particle structure fixated in a matrix material

Info

Publication number: WO2014001332A1
Application number: PCT/EP2013/063276
Authority: WO
Inventors: Henrik HØYER; Matti Knaapila; Geir Helgesen; Mark Buchanan; Arne Torbjørn SKJELTORP
Original assignee: Institutt For Energiteknikk
Priority date: 2012-06-25
Filing date: 2013-06-25
Publication date: 2014-01-03
Also published as: NO20120739A1

Abstract

There is provided a method for forming a body comprising a particle structure fixated in a matrix material, the method comprising: - providing an amount of particles; - providing a viscous matrix material to include said particles, - forming a particle structure of at least a portion of said amount of particles, - fixating said viscous matrix so as to fixate said particle structure in the matrix material, characterized by - at least a portion of said amount of particles are paramagnetic or ferromagnetic; wherein the formation of the particle structure includes the step of: - Subjecting the particles to a magnetic field of a Kittel's open domain structure, from a magnet system comprising two permanent magnets being arranged adjacent to one another along respective adjoining surfaces, and each magnet having end surfaces in a mutual end plane perpendicular to said adjoining surfaces, the permanent magnets being arranged with opposite directions of the polarity of their magnetic fields, such that said magnetic field of a Kittel's open domain structure appears adjacent the end surfaces of the permanent magnets and centered over a junction formed by said adjoining surfaces at said end surfaces.

Description

METHOD FOR FORMING A BODY COMPRISING A PARTICLE STRUCTURE FIXATED IN A MATRIX MATERIAL

TECHNICAL FIELD

The present invention relates to a method for forming a body comprising a matrix material and particles, comprising providing a viscous mixture including the matrix material and the particles, wherein at least a portion of said particles have magnetic susceptibility. The invention also relates to a body obtained by said method.

BACKGROUND OF THE INVENTION

Anisotropic materials are used in a wide and increasing range of applications. Typically, such materials include conductive particles in a non-conductive matrix material. The conductive particles are intended to form conductive pathways in the matrix material, so as to enable the anisotropic material to be, at least under certain circumstances, electrically conductive.

Depending on the selection of particles and matrix materials, the anisotropic materials may be formed to be suitable for various applications, such as for sensors (e.g. stress sensors), in solar cell applications, printed electronics etc. Prior art methods for forming anisotropic materials often involve providing a viscous mixture including the matrix material and conductive particles, applying an electric field over the viscous mixture so as to cause the conductive particles to align to form conductive pathways in the mixture, and thereafter curing the viscous mixture.

Alternatively, it has been proposed to use the magnetic properties of the particles to cause the particles to align to form conductive pathways. WO 2008/153679 is such an example, where a viscoplastic material including a plurality of magnetic particles is subject to a magnetic field for a time sufficient to at least partially align a portion of the magnetic particles to a predetermined position. To increase the versatility of the anisotropic materials formed, and to enable industrial production thereof, there is a need for alternative methods for forming materials in this field.

It is an object of the invention to provide a method fulfilling said need.

SUMMARY OF THE INVENTION

The above-mentioned need for alternatives is fulfilled by a method for forming a body comprising a particle structure fixated in a matrix material, the method comprising: - providing an amount of particles; providing a viscous matrix material to include said particles, forming a particle structure of at least a portion of said amount of particles, fixating said viscous matrix so as to fixate said particle structure in the matrix material, characterized by at least a portion of said amount of particles is paramagnetic or ferromagnetic; wherein the formation of the particle structure includes the step of:

Subjecting the particles to a magnetic field of a Kittel's open domain structure, from a magnet system comprising two permanent magnets being arranged adjacent to one another along respective adjoining surfaces, and each magnet having end surfaces in a mutual end plane perpendicular to said adjoining surfaces, the permanent magnets being arranged with opposite directions of the polarity of their magnetic fields, such that said magnetic field of a Kittel's open domain structure appears adjacent the end surfaces of the permanent magnets and centered over a junction formed by said adjoining surfaces at said end surfaces, so as to arrange at least a portion of said particles into particle assemblies, each particle assembly comprising a plurality of particles and extending along a flux direction of said magnetic field, said particle assemblies at least partly bridging said junction. In this document, "particle structure" is intended to mean any desired configuration or structure of particles which is or is to be fixated in matrix material.

Fields from Kittel's open domain structures, although previously known, have not previously been used to create particle assemblies for fixation in a matrix material.

An advantage with these fields is that they may provide magnetic fields having relatively large separation strengths.

With "separation strength" is meant herein the product B VB, where B is the magnetic induction and VB is the gradient of the magnetic field. In prior art, focus has often been on the size of the magnetic flux. With magnetic field is meant B-field or induction. However, it has been understood that the separation strength may have a greater impact on the capacity of the field to displace particles, in particular when mixed in a viscous matrix material. Using a magnetic field with relatively high separation strength will moreover enable relatively fast displacement of the particles.

As speed is an important factor when it comes to enabling industrial production, the use of a field having a relatively large separation strength which is enabled by the method proposed herein, may be a key factor to succeeding with industrial applications.

For example, advantageously the particles may be subject to the magnetic field for a time period being less than 5s, preferably less than 3 s, most preferred less than 1 s to form said particle assemblies. Using a magnetic field having an open Kittel structure may be advantageous to enable formation of particle assemblies in time periods being less than 5s, preferably less than 3 s, most preferred less than 1 s.

Although it is possible to estimate the separation strength of the magnetic field with an open Kittel structure, it is difficult to determine exact values thereof. Various analytic calculations of the separation strengths have been attempted, but no conclusive method is available. Moreover, practical measurements are also difficult to perform. However, to give an idea of what is a relatively high separation strength, it may be referred to works from ll'yashenko et al. in Phys. Stat. Sol. (a) 203, No. 7, 1556-1560 (2006), suggesting a separation strength of 4.2 · 10⁵ T²/m, or by Inge B. Roth in master thesis, University of Oslo, May 2009, instead arriving at 5 · 10⁴ T²/m, both at a distance of 10 μηη above the mask of an open Kittel structure as generally described in EP 1 842 596.

Moreover, an open Kittel structure may be advantageous in that the particles to be subject to the field may conveniently be arranged above the junction of the Kittel structure.

Preferably, the particles shall be positioned close to said junction, for example the particles may be positioned in a plane at a distance from said junction being less than 3 mm, preferably less than 1 mm, most preferred less than 0.5 mm.

Advantageously, the magnet system further comprises a mask arranged at the end surfaces of said permanent magnets, said mask having a gap corresponding in location to said junction, for affecting the form and gradient of the magnetic field. The mask could preferably be formed by thin sheets of a soft magnetic material placed on the end surfaces of the magnets. The size of the gap could be adjusted in order to achieve a suitable form and gradient of the magnetic field. Moreover, the magnet system may comprise a yoke arranged at the surfaces of the magnets opposing said end surfaces. The yoke will hence close the surface of the magnets opposite the end surfaces. Such a yoke may advantageously be made by a soft magnetic material.

Advantageously, the particles are subject to the magnetic field so as to form a plurality of particle assemblies arranged continuously next to each other, forming a pathway of particles extending in a longitudinal direction perpendicular to the flux direction of the magnetic field, along said junction.

Advantageously, at least a portion of said particles are electrically conductive, such that the pathway is an electrically conductive pathway. A pathway extending through the matrix material is such that the ends of said pathway may be connected to external devices. Preferably at least a portion of said particles are conductive such that the pathway is a conductive pathway. The method is hence suitable for forming a body having a conductive pathway through a matrix material, which may in turn have uses e.g. as a sensor. The particle assemblies as formed by the magnetic field may be directly fixated in the matrix material to form the particle structure.

Alternatively, the particle assemblies as formed by the magnetic field are moved and/or rotated before being fixated in the matrix material to form the particle structure. In this case, the particle assemblies may be subject to a second field so as to move and/or rotate the particle assemblies to form the particle structure, before fixation of the particle structure in the matrix material.

The second field may be an electric field, in which case at least a portion of the particles should be electrically conductive. Alternatively, the second field may be a magnetic field, having a different flux direction than the initial magnetic field.

However, it is also possible to add additional fields for creating particle assemblies and/or for moving and/or rotating particle assemblies, in order to attain the desired particle structure. Hence, the particle assemblies could be subject to a third field, a fourth field and so on. Moreover, the first and second field as described in the above may be alternately applied to achieve a final particle structure.

In accordance with one embodiment of the method, the formation of the particle structure includes:

First, the particles are provided separate from the matrix material,

- Second, the particles are subject to the magnetic field so as to form the particle assemblies,

Third, the viscous matrix material is applied to the particle assemblies,

In this embodiment, the particle assemblies are obtained by letting the particles be subject to the magnetic field. Thereafter, viscous matrix material is applied to the particle assemblies. For example, the matrix material may be poured over the particle assemblies.

Thereafter, the viscous matrix material including the particle assemblies may be fixated, or it may be subject to a second field. The second field may be a magnetic or an electric field. Under the influence of the second field, the particle assemblies may be moved and/or rotated in the matrix material before fixation thereof. In another embodiment, the formation of the particle structure includes:

First, the particles are provided in a mixture with the viscous matrix material, Second, the viscous mixture is subject to the magnetic field to form the particle assemblies in the viscous matrix material,

In this embodiment, the particles are initially provided in a mixture with the viscous matrix material, and the mixture is subject to the magnetic field to form the particle assemblies. Thereafter, the matrix material may be fixated, or a second field may be applied to the viscous mixture, so as to further move and/or rotate the particle assemblies before fixation thereof.

When the particles are in a viscous mixture with the viscous matrix material, it is preferred that the particles have a concentration in the viscous matrix material being less than the percolation threshold.

For conductive mixtures a "percolation threshold" is defined as the lowest concentration of conductive particles necessary to achieve long-range conductivity in the random system. In a system formed by a method according to the invention the concentration of conductive particles necessary for achieving conductivity in a predefined direction is not determined by the percolation threshold and the concentration can be lower. For practical reasons the concentration of particles is determined by the requirements on the conductive paths, there usually being no reason to have excess amounts of conductive particles not arranged into the conductive paths.

The concentration of particles in the viscous matrix could be up to 10 times lower than the percolation threshold or even lower. Concentrations of particles may be in the range of 0. 0.01 - 10 vol%, or 0.01 - 2 vol%, or 0.01 -1 .5 vol% For example, the particles may have a concentration in the viscous matrix material in the range 0.01 to 1 vol%.

To be displaceable by use of a magnetic field, the particles may advantageously be paramagnetic or ferromagnetic, preferably ferromagnetic.

To be displaceable by use of an electric field, the particles may advantageously be electrically conductive. This electrical conductivity mechanism may be based on electrons and/or holes. Alternatively, the electrical conductivity mechanism may be based on ions or protons.

The particles may be homogenous particles, i.e. a particle consists of a single material or material mixture throughout the particle. However, the particles may also be

heterogeneous particles, i.e. a particle consists of several materials, for example the particle may have a core of one material, and a sheath of another material.

The particles to be subject to the fields in the proposed method may comprise only one type of particles, but may also be a mixture of different types of particles. Particles may be para-/ferromagnetic and/or electrically conductive. Advantageously, at least some particles may be both para- and/or ferro magnetic, and electrically conductive. Such particles will be displaceable by both magnetic and electric fields.

Advantageously, the amount of particles includes particles of metal and/or metal alloys, preferably nickel or iron oxide. The size of the particles, i.e. the largest linear dimension of the particles, may

advantageously be in the range 10 nm to 100 μηη.

The matrix material should be material having a viscous form which is capable of being fixed. Fixation may be achieved by any suitable method, such as, for example, cooling, curing, ceramisation, cross-linking, gelling, irradiating, drying, heating, sintering, or firing. Advantageously, the matrix material comprises a polymer material.

In particularly useful embodiments, the viscous matrix material may be UV-curable, and the fixating of the matrix material comprises UV curing thereof.

In other useful embodiments, the viscous matrix material may be humidity-curing, and the fixating of the matrix material comprises exposing the mixture to moisture, preferably in air at room temperature.

Advantageously the matrix material, when fixated, is an elastomeric material. This enables creation of bodies being useful for applications such as strain sensors, where the elastic properties of the matrix material is used together with the properties of the particle structure to achieve a desired function. In a second aspect of the invention there is provided a body comprising a particle structure fixated in a matrix material, wherein the particle structure comprises at least one pathway extending through the matrix material, characterized in that at least a portion of the particles in said pathway is ferromagnetic, and the direction of the magnetic polarity of the ferromagnetic particles in the pathway is perpendicular to the extension of the pathway in the matrix material.

Advantageously, the particles of the body include particles comprising metal or metal alloys, preferably nickel or iron oxide.

Advantageously, the matrix material is a polymer material. Preferably, the matrix material is an elastomeric material.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples and embodiments of the invention will now be described with reference to the accompanying drawings wherein: Fig. 1 is a schematic illustration of an embodiment of a magnet system to be used with the method of the invention;

Fig. 2 is a schematic illustration of particle assemblies formed by an embodiment of the method of the invention;

Fig. 3 is a schematic illustration of an embodiment of a continuous method for

manufacturing a body comprising more than one pathway of particles;

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

As mentioned in the above, a magnetic field of an open Kittel structure may be used with the present invention. EP 1 842 596 describes an example of a magnet system which has been used to create the magnetic fields of the embodiments of the invention. A similar magnet system is schematically illustrated in Fig. 1. The magnetic system 1 comprises one permanent magnet 2a with polarisation in a first direction (up) and another permanent magnet 2b with polarisation in a direction opposite to the first direction (down). The magnets 2a, 2b are joined together along adjacent surfaces, forming a junction 6 between the magnets 2a, 2b. Moreover, the magnets 2a, 2b are mounted on a base of magnetic material, the yoke 3. On top of both magnets is mounted a thin plate of high permeability magnetic material (e.g. permendure, permalloy, etc.), denoted the "mask" 4.

A thin gap 5 is formed between the portions of the mask 4 covering the "up" magnet 2a and the "down" magnet 2b, the location of the gap 5 corresponding to the junction 6 between the adjacent surfaces of the up and down magnets 2a, 2b.

The purpose of the mask 4 is to collect the magnetic flux lines and steer them toward the gap 5 where both the flux and the flux gradient will be very high. Typically, the size of the gap 5 would be of a size approximately corresponding to the thickness of the mask 4. Fig. 2 is a schematic illustration of particle assemblies 7 formed by application of the magnetic field from a magnet system as illustrated in Fig. 1. In Fig. 2, the magnet system 1 is seen from the above. The mask 4 covers the majority of the upper surfaces of the magnets 2a, 2b, but form a gap 5 which is located over the junction 6 between the two magnets 2a, 2b. The arrow illustrates the direction of the magnetic field created by the magnet system 1.

In Fig. 2, a situation is illustrated where para- or ferromagnetic particles have been subject to the magnetic field, and as a consequence thereof the particles have been arranged into linear particle assemblies 7. The particle assemblies 7 each comprise a plurality of particles and extend along the flux direction of the magnetic field, bridging the junction 6. Moreover, the particle assemblies 7 extend over the gap 5, hence also bridging said gap 5.

As illustrated by the smaller arrows in Fig. 2, the direction of the magnetic polarity of the particles is parallel to the magnetic field, and hence perpendicular to the longitudinal extension of the gap 5 in the mask. As already explained in the above, if a sufficient number of particle assemblies 7 are created side by side along said gap 5, the result will be a pathway of particles extending along said gap 5.

Such alignment may be used for forming a body comprising a particle structure, wherein the direction of the magnetic polarity of the ferromagnetic particles in the pathway is perpendicular to the extension of the pathway in a matrix material, as outlined in the above.

Fig. 3 illustrates schematically the continuous production of a body including several pathways in a matrix material. In such production, uncured matrix material 8 wherein particles T are mixed may be fed from a side towards a magnet system 1. In the illustrated embodiment, the magnet system 1 comprises four different magnets, 2a, 2b, arranged side by side and each magnet having an opposite direction of polarization than the neighboring magnets. With four different magnets, three junctions 6 between adjacent magnet surfaces will be formed. In fig. 3a it is illustrated how the matrix material 8 with the magnets T is brought over the magnet system 1 , resulting in the formation of particle structures and the formation of pathways 7 along the junctions 6. From the magnet system 1 , the matrix material 8 is continuously fed towards a curing station or the like, allowing the matrix material 8 to be cured, whereby the pathways 7 are fixated in the matrix material 8. It is envisaged that such production may be performed continuously, with matrix material 8 i.e. in a form similar to a film being continuously fed over the magnet system 1 .

Fig. 3b illustrates how the magnetic polarity of the particles 7 is aligned with the magnetic field.

Fig. 3c illustrates the magnet system 1 in more detail, in particular suggesting that it includes a mask 4 forming gaps 5 over the junctions 6.

Fig. 3d is a top view of a body formed by the method illustrated in Figs. 3a to 3c. In particular the body comprises a matrix material 8 wherein particles forming more than one pathway 7 is fixated. The pathways 7 would, in this case, have a direction of the magnetic polarity of the particles therein being perpendicular to the extension of the pathway in the matrix material. However, a continuous method as described in the above, may naturally be used also for creating only one single pathway in the material.

It should be noted that the described features of the various embodiments may be combined with each other. Accordingly, no embodiment is intended to limit any combination of features which are presented in the embodiments, but rather to illustrate examples of embodiments.

The invention is illustrated, but not limited, by the following Examples.

EXAMPLES Example 1

Particles of Ni 123 were dispersed in the liquid polymer precursor Dymax 3094. The concentration of the particles of Ni 123 was 0.1 vol%. The thus formed mixture was smeared on top of the junction of a GIAMAG magnet. With a GIAMAG magnet is meant a magnet system in accordance with Canadian patent CA2595721 as delivered by the Norwegian company Giamag Technologies AS. The particles are aligned by a magnetic field. The matrix of aligned mixture was cured under a mercury lamp, which lead to a solid polymer composite with aligned particle strings. The resistance was measured before and after curing. The resistance before curing was 2.2 ΜΩ. The resistance after curing was 145 Ω. Thus, the formed polymer composite was a better electrical conductor after alignment using the magnetic field resulting from the GIAMAG magnet. It can be concluded that application of the magnetic field results in excellent alignment of the particles.

Example 2

Particles of Ni 123 were dispersed in the liquid polymer precursor Dow Corning 734. The concentration of the particles of Ni 123 was 0.1 vol%. The thus formed mixture was smeared on top of the junction between the two magnets being part of the GIAMAG magnet. The particles are aligned by a magnetic field. The matrix of aligned mixture was cured in humid air, which leads to a solid polymer composite with aligned particle strings. The resistance was measured before and after curing. The resistance before curing was 2.2 ΜΩ. The resistance after curing was 218 Ω. Thus, the formed polymer composite was a better electrical conductor after alignment using the magnetic field resulting from the GIAMAG magnet. It can be concluded that application of the magnetic field results in excellent alignment of the particles.

Claims

1 . Method for forming a body comprising a particle structure fixated in a matrix

material, the method comprising: providing an amount of particles; providing a viscous matrix material to include said particles, forming a particle structure of at least a portion of said amount of particles, fixating said viscous matrix so as to fixate said particle structure in the matrix material, characterized by at least a portion of said amount of particles are paramagnetic or

ferromagnetic; wherein the formation of the particle structure includes the step of:

Subjecting the particles to a magnetic field of a Kittel's open domain structure, from a magnet system comprising two permanent magnets being arranged adjacent to one another along respective adjoining surfaces, and each magnet having end surfaces in a mutual end plane perpendicular to said adjoining surfaces, the permanent magnets being arranged with opposite directions of the polarity of their magnetic fields, such that said magnetic field of a Kittel's open domain structure appears adjacent the end surfaces of the permanent magnets and centered over a junction formed by said adjoining surfaces at said end surfaces, so as to arrange at least a portion of said particles into particle assemblies, each particle assembly comprising a plurality of particles and extending along a flux direction of said magnetic field, said particle assemblies at least partly bridging said junction.

2. Method in accordance with any one of the previous claims, wherein the magnet system further comprises a mask arranged at the end surfaces of said permanent magnets, said mask having a gap corresponding in location to said junction, for affecting the form and gradient of the magnetic field.

3. Method in accordance with any one of the previous claims, the magnet system further comprising a yoke arranged at the surfaces of the magnets opposing said end surfaces.

4. Method in accordance with any one of the previous claims, wherein the particles are subject to a magnetic field so as to form a plurality of particle assemblies arranged continuously next to each other, forming a pathway of particles extending in a longitudinal direction perpendicular to the flux direction of the magnetic field, along said junction.

5. Method in accordance with claim 4, wherein at least a portion of said particles are electrically conductive, such that the pathway is an electrically conductive pathway.

6. Method in accordance with claim 4 or 5, wherein the particle structure includes at least one pathway of particles.

7. Method in accordance with any one of the preceding claims, wherein the particle

assemblies as formed by the magnetic field are fixated in the matrix material to form the particle structure.

8. Method in accordance with any one of the claims 1 to 6, wherein the particle

assemblies as formed by the magnetic field are moved and/or rotated before being fixated in the matrix material to form the particle structure.

9. Method according to claim 8, wherein the particle assemblies are subject to a second field so as to move and/or rotate the particle assemblies to form the particle structure.

10. Method according to claim 9, wherein at least a portion of the particles are electrically conductive, and the second field is an electric field.

1 1 . Method according to claim 9, wherein the second field is a magnetic field, having a different flux direction than the initial magnetic field.

12. Method in accordance with claim 1 , wherein

First, the particles are provided separate from the matrix material,

Second, the particles are subject to the magnetic field so as to form particle assemblies,

5 Third, the viscous matrix material is applied to the particle assemblies.

13. Method in accordance with any one of the claims 1 to 12, wherein

First, the particles are provided in a mixture with the viscous matrix material,

Second, the viscous mixture is subject to the magnetic field so as to form 10 particle assemblies in the viscous mixture.

14. A method in accordance with any one of the previous claims, wherein the particles have a concentration in the mixture being less than the percolation threshold.

15. A method in accordance with any one of the previous claims, wherein the particles have a concentration in the mixture in the range 0.01 to 2 vol%.

15 16. A method in accordance with any one of the previous claims, wherein the amount particles includes particles comprising metal or metal alloys, preferably nickel or iron oxide.

17. A method in accordance with any one of the previous claims, wherein the amount of particles includes particles having sizes in the range 100 nm to 100 μηη.

20 18. A method in accordance with any one of the preceding claims, wherein the magnet system is positioned in relation to said particles such that a distance between said junction and a plane including the particles is less than 3mm, preferably less than 1 mm, most preferred less than 0,5 mm.

19. A method in accordance with any one of the previous claims, wherein the magnetic field is applied for a time period less than 5s, preferably less than 3 s, most preferred less than 1 s.

20. A method in accordance with any one of the previous claims, wherein the matrix comprises a polymer material.

21 . A method in accordance with any one of the previous claims, wherein the viscous matrix material is UV-curable, and the fixation comprises UV curing thereof.

5 22. A method in accordance with any one of the previous claims, wherein the fixated matrix material is an elastomeric material.

23. Body comprising a particle structure fixated in a matrix material, wherein the particle structure comprises at least one pathway extending through the matrix material, characterized in that at least a portion of the particles in said pathway is

10 ferromagnetic, and the direction of the magnetic polarity of the ferromagnetic particles in the pathway is perpendicular to the extension of the pathway in the matrix material.

24. Body according to claim 23, wherein said particles include particles comprising metal or metal alloys, preferably nickel or iron oxide.

25. Body according to claim 23 or 24, wherein said matrix material is a polymer material.

15 26. Body according to any one of the claims 23-25, wherein said matrix material is an elastomeric material.