WO2012076281A1

WO2012076281A1 - Electrical contact element and an electrical contact

Info

Publication number: WO2012076281A1
Application number: PCT/EP2011/069790
Authority: WO
Inventors: Åke ÖBERG; Martin WÅHLANDER; Gunnar Mattsson
Original assignee: Abb Research Ltd
Priority date: 2010-12-06
Filing date: 2011-11-10
Publication date: 2012-06-14

Abstract

The present invention relates to an electrical contact element for providing electrical contact in an electrical circuit, where the contact element includes a Me- 16/17 compound acting as a lubricant of at least part of at least one contact surface of the contact element, a Me- 16/17 compound being a compound formed by at least one metal and an element from group 16 or 17 of the periodic table. At least part of at least one contact surface is formed from a Me-16/17 compound composite material comprising a matrix of a metallically conducting material and particles formed from a Me-16/17 compound. By using a Me-16/17 compound composite material as a surface material, a contact surface of high durability and low, stable contact resistance can be achieved. The matrix of the composite material ensures good conductivity, so that a composite material thickness which ensures desirable durability properties can be provided, while keeping the contact resistance within acceptable limits.

Description

ELECTRICAL CONTACT ELEMENT AND AN ELECTRICAL CONTACT

Technical field

The present invention relates to the field of electrical circuits, and in particular to electrical contacts elements for making or breaking of an electrical circuit. Background

Electrical contacts are of vital importance to most electrical circuitry and are omnipresent in modern society. Low contact resistance, good thermal properties and high durability are desirable properties of a contact surface. In many applications, the number of contact operations performed during a contact's lifetime is vast. In order to reduce wear of the contact, contact surfaces are often lubricated. Lubrication of contact surfaces is especially relevant for contacts which are frequently operated and wherein a surface of a contact element is arranged to slide against a contact surface of a counterpart contact element.

Lubrication of contact surfaces can for example be made by use of oil- or grease based lubricants. However, oil- or grease based lubricants have many drawbacks: They are difficult to apply in a layer of suitable thickness; they are often volatile and may react with other compounds in the contact area; they are sticky and may therefore absorb

contaminants such as dust which deteriorate the contact surface; there is a risk that components which should not be lubricated are exposed to the lubricant; the durability of oil- or fat based lubricants is often reduced by contaminants, etc. In some applications, such as where the electrical contact is submersed into transformer oil or similar, the use of oil- or fat based lubricants is not at all feasible.

Solid lubricants, such as graphite, molybdenum disulphide or suitable types of plastic may alternatively be used. However, many types of solid lubricants are poor electrical conductors, and are often worn off when contact surfaces slide against each other. In US 5,316,507, a contact having a surface layer formed of a composition including a noble metal component and graphite is disclosed. The surface layer is manufactured by sintering a mixture of noble metal particles and graphite particles, so that graphite particles are formed within solid noble metals, and by rolling the sintered body into a strip in a number of rolling steps with intermediate heat-treatments. Apart from involving a complicated manufacturing process, the surface layer disclosed in US 5,316,507 suffers from a low resistance against mechanical wear.

A contact element wherein at least one contact surface on a metallic body of the contact element is at least partially coated with a friction reducing layer of a metal salt, such as e.g. silver iodide, is disclosed in US 6,565,983 as well as in "Silver Iodide as a Solid Lubricant or Power Contacts", S. Arnell & G. Andersson, Proc 47^th IEEE Holm Conf. on Electrical Contacts, Montreal, 2001. The contact element disclosed in US 6,565,983 exhibits a low friction at the contact surface, a high corrosion resistance and an improved durability compared to previously known contact elements. However, the initial contact resistance of such contact element is high, and the contact element must generally be operated several times before an acceptable contact resistance level is reached. Summary

A problem to which the present invention relates is how to provide a highly durable contact element which exhibits an acceptable initial contact resistance.

One embodiment provides an electrical contact element for providing electrical contact in an electrical circuit, wherein at least part of at least one contact surface of the contact element is formed from a Me-16/17 compound composite material comprising a matrix of a metallically conducting material and particles formed from a Me-16/17 compound. A Me-16/17 compound is a compound formed by at least one metal and an element from one of group 16 or 17 of the periodic table, i.e. a metal halogenide, metal chalcogenide or a metal oxide.

By using a Me-16/17 compound composite material as a surface material, a contact surface of high durability and low, stable contact resistance can be achieved. The initial contact resistance of the composite material is approximately the same as the contact resistance when parts of the composite material have been worn off, since the surface which is exposed when parts of the composite material have been worn off will have a similar composition to that of the initial surface. Furthermore, the matrix of the composite material ensures good conductivity, so that a composite material thickness which provides desirable durability properties may be used. Hence, a contact element of sufficient durability may be achieved also for applications wherein the number of contact operations is very large. The composite material could be applied as a surface layer on a contact body of the contact element, or could form the bulk of the contact element. The thickness of a surface layer could for example be within the range of 0.1-10 000 μπι. In for example sliding contacts in switches and breakers, the thickness of the surface layer will often be within the range of 5-200 μπι. In applications where the number of contact operations will be very large, such as in some tap changers, a larger thickness, for example around 2000 μπι, is often beneficial.

In one embodiment, the Me- 16/17 compound content of the composite material is below 25 volume percent. In many applications, it may be advantageous to use a Me- 16/17 compound content which is below 10 volume percent, e.g. within the range of 0.5 - 5 volume percent.

In one embodiment, the majority of the Me- 16/17 compound particles in the composite material have a diameter in the range of 5 nm - 10 μπι. In many applications, the majority of the Me- 16/17 compound particles in the composite material have a diameter in the range of 0.1 - 1 μπι.

The matrix of the composite material can for example be formed from silver, tin, copper, aluminium, gold or any other suitable metallic material. Alternatively, the matrix could be formed from a metallically conducting ceramic material.

In one embodiment, at least a majority of the particles of the composite material are formed from a metal halogenide, such as silver iodide. The particles of the composite material may all be of the same Me- 16/17 compound, or particles of different Me- 16/17 compounds may be provided.

In one embodiment, the concentration of at least one type of Me- 16/17 compound particles show a gradient along a direction parallel to a contact surface of the contact element. Hereby can be achieved that the properties of the contact surface, such as resistivity or friction coefficient, can vary along the surface of the contact element.

A contact including one or more contact elements according to the invention may also be provided. The contact element of the invention may for example be used in a tap changer, a breaker, a switch, a plug-in contact, in a contact in an electric motor, etc.

Further aspects of the invention are set out in the following detailed description and in the accompanying claims.

Brief description of the drawings

Fig. la is a schematic illustration of an example of an electrical contact of plug-in type.

Fig. lb is a schematic illustration of an example of an electrical contact of spiral spring type.

Fig. Ic is a schematic illustration of an example of an electrical contact forming part of a tap changer.

Fig. 2a schematically illustrates a contact element having a contact surface layer made from a composite material of Me- 16/17 compound particles in a matrix of a metallically conducting material.

Fig. 2b schematically illustrates a contact element made from a composite material of

Me- 16/17 compound particles in a matrix of a metallically conducting material.

Fig. 3 schematically illustrates an example of a tap changer having a contact surface made from a composite material of Me-16/17 compound particles in a matrix of a metallically conducting material.

Fig. 4 schematically illustrates an example of a rotating electrical contact.

Fig. 5 schematically illustrates a contact element comprising particles of two

different types, of which at least one is a Me-16/17 compound particle type, and wherein the concentration of the two particle types shows a gradient along the contact surface. Detailed description

There are a vast number of designs of electrical contacts 100. An electrical contact 100 can e.g. be of plug-in type, a sliding contact or a stationary contact; it can be designed for low voltage, medium voltage or high voltage; it can be designed for use in different electrical applications, such as electrical devices, control systems, power transmission- or distribution systems, etc. Figs, la-lc show a non-limiting selection of different electrical contact designs. Only a very limited number of the possible contact designs can be shown here, and three different designs have been selected for illustration purposes only. Fig. la shows an electrical contact 100 of traditional plug-in type, which comprises a first contact element 105a and a second contact element 105b. First contact element 105a of Fig. la has two pins 110 in the form of a jaw, while second contact element 105b has a contact rail 115. Each pin 110 has a contact surface 117a which is arranged to be in contact with a contact surface 117b of the contact rail 115. The width of the opening of the jaw formed by the two pins 110 is such that when the contact rail 115 is entered into the j aw of the first contact element 110a, the pins 110 will apply a suitable force on the contact rail 115 so that electrical contact between the contact surfaces 117a and 117b is established.

Fig. lb schematically illustrates an example of an electrical contact 100 of intermediate spiral spring type design. The electrical contact 100 in Fig. lb is a spiral contact where a first contact element 105c is in the form of a resilient annular body, e.g. a ring of a spiralized wire, a second contact element 105d is in the form of an inner sleeve or a pin, and a third contact element 105e is in the form of an outer sleeve or tube. The first element 105 c is, in contacted state, compressed such that a first contact surface 117c of the first contact element will be clamped against a contact surface 117d of the second contact element 105d, and so that a second contact surface 117f of the first contact element 105c will be clamped against a contact surface 117e of the third contact element 105e, thereby making electrical contact between contact elements 105c, 105d and 105f. Fig. lc illustrates an electrical contact 100 in the form of a tap changer 130 for a transformer. Electrical contact 100 of the tap changer 130 comprises a first contact element 105g which is arranged as a moveable part of the tap changer 130, as well as four contact elements 105h arranged at different locations on a transformer winding 135. The first contact element 105g is slideably arranged so that the first contact element 105g can be moved between the four contact elements 105h. In this way, a variable number of turns 140 can be included in the electrical loop of the transformer winding, depending on which of the four contact elements 105h is in electrical contact with first contact element 105g. The first contact element 105g has a contact surface 117g, arranged to make contact with one of the four contact surfaces 117h of the four contact elements 105h, respectively.

In the following, when referring to contact elements in general, the reference numeral 105 will be used; when referring to contact surfaces in general, the reference numeral 117 will be used, and so forth.

The electrical contacts 100 shown in Figs, la-lc all have at least two contact surfaces 117, which are arranged so that a current transmission path can be made through at least two contact surfaces 1 17, and so that a made current transmission path may be broken.

Oftentimes, a first contact surface 117 of a contact 100 is provided on a first contact element 105 of the electrical contact 100, and a second contact surface 117 is provided on a second contact element 105 of the contact, where the first and second contact elements 105 are mechanically movable in relation to each other to facilitate the making and breaking of electrical contact between the contact elements 105. However, there are electrical contacts 100 which only have one contact element 105, and which are arranged to co-operate with another electrical contact 100 having at least one contact element 105— a simple example of such an electrical contact 100 is the standard wall socket used in homes. As mentioned above, low contact resistance, good thermal properties and high durability are desirable properties of a contact surface 117. A contact surface as disclosed in

US6,565,983, where a metal salt layer is provided on at least part of a contact surface of an electrical contact to provide lubrication of the contact surface, fulfils these expectations to a high degree. However, the initial contact resistance of such contact surface is very high. Furthermore, in applications where the number of contact operations is high, there is a risk that the friction-reducing layer shown in US6,565,983 will be worn out before the electrical contact 100 has reached its expected lifetime. According to the invention, a low initial contact resistance can be achieved by providing, at at least part of at least one contact surface 117 of a contact element 105, a composite material comprising particles of a Me- 16/17 compound in a matrix of metallically conducting material. The term "Me- 16/17 compound" is herein used to refer to compounds including at least one metal and an element from group 16 or 17 of the periodic table, the Me- 16/17 compounds thus including metal halogenides, metal chalcogenides and metal oxides. The term Me- 16/17 compounds thus includes metal salts, as well as other metal compounds. By providing a Me- 16/17 compound composite material on a contact surface, the initial electrical properties of the contact surface will be similar to those of the surface when the composite material has been partly worn down. Thus, a contact surface of approximately stable contact resistance and friction coefficient can be achieved until the composite material has been worn away. Furthermore, the durability of the contact surface can be increased with maintained low contact resistance by providing a sufficiently thick layer of composite material, since the metallically conducting matrix ensures good conductivity also when the thickness of the composite material layer increases.

Suitable materials for the Me-16/17 compound particles are for example the silver halogenides, including silver iodide (Agl), silver chloride (AgCl) and silver bromide (AgBr), as well as the silver chalcogenides, including silver sulphide (Ag₂S), silver selenide (Ag₂Se) and silver telluride (Ag₂Te). Metal halogenides, chalcogenides or oxides wherein the metal component is another metal than silver could also be used, such as Cu-, Sn-, A1-, Au-, W-, V-, Ta- or Nb- halogenides/chalcogenides/oxides (e.g. NbSe₂, WSe₂, MoSe₂, TaSe₂, SnSe₂, copper silver selenide and copper tungsten selenide, etc). Examples of suitable metal oxides are: Magneli-phases (T1₄O₇, T1₅O₉ and Ti₆On), ITO - Indium Tin Oxide (ln₂0₃ + Sn0₂),

and Zr0₂. As will be seen below, less stable Me-16/17 compounds which are rarely found in nature could also be used, such as Ag₂I and Agl₂. In case Agl is used as the Me-16/17 compound, all three phases which appear under atmospheric pressure (α, β, and γ Agl) are likely to exist in a Agl composite at room temperature. The metallically conducting matrix 215 of the composite material could for example be formed from a metal forming a metal component of the Me- 16/ 17 compound, or from a different material. Suitable materials for the metallically conducting matrix 215 are materials of low resistivity, preferably of resistivity of 50 μΩαη or less. Materials of suitably low resistivity include metals such as Ag, Cu, Sn, Al, Au, as well as metallically conducting ceramic materials such as oxides, carbides, nitrides, carbo-nitrides and MAX phases.

An example of a contact element 105 wherein such a composite material is provided at a contact surface 117 is schematically illustrated in Fig. 2a. A surface layer 200 of thickness d is provided on a contact body 205, which can e.g. be made from a metallically conducting material. The surface layer 200 comprises a plurality of Me-16/17 compound particles 210 dispersed in a matrix 215 of a metallically conducting material. The metallically conducting material of the matrix 215 could be the same as, or different to, a metallically conducting material of the contact body 205.

Another example of a contact element 105 wherein such a composite material is provided at a contact surface 117 is schematically illustrated in Fig. 2b. In the contact element 105 of Fig. 2b, no separate body 205 is provided, but the body of the contact element is made from the composite surface layer 200.

By providing a composite material comprising particles 210 of a Me-16/17 compound in a matrix 215 of a metallically conducting material (such composite material referred to as a Me-16/17 compound composite), the initial contact resistance will be approximately the same as the contact resistance after a number of operations, since the composition of the initial surface will be similar to the composition of the surface when the composite material has been partly worn down. Furthermore, the thickness d of the surface layer 200 which can be allowed without the resistance of the surface layer 200 taking an

unacceptably high value will typically be drastically higher than if the surface layer 200 were made from a homogenous layer of Me-16/17 compound - in the Me-16/17 compound composite, the metallically conducting matrix 215 will provide a current path, so that no or very little current will have to go through the Me-16/17 compound particles 210. The resistance of an electrical contact 100 having a surface layer 200 of a Me-16/17 compound composite can in many applications be considered to be unaffected by the wearing down of the surface layer 200 (until the surface layer 200 has been completely worn away), since the majority of the resistance of the contact 100 originates from other parts of the contact 100.

The Me- 16/17 compound particles 210 of the Me- 16/17 compound composite provides lubrication of the contact surface, so that the friction coefficient of the surface layer 200 takes a low value compared to the friction coefficient of the material of the matrix 215. The friction coefficient of the Me-16/17 compound composite is typically similar to, or slightly higher than, the friction coefficient of a pure Me-16/17 compound film. As the surface layer 200 is worn, the friction coefficient will stay approximately constant until the surface layer 200 has been completely worn away, since if some of the particles 210 that used to be part of the contact surface 117 have been worn off, other particles 210, which were previously located in the interior of the surface layer 200, will appear at the contact surface 117 and provide similar lubrication.

In order to ensure that the friction coefficient remains in principle unaffected by the wearing down of the surface layer 200, the distribution of the particles 210 in the matrix 215 should be fairly uniform throughout the surface layer 200 in the direction

perpendicular to the contact surface 117. Although a uniform particle distribution would be ideal, minor variations in the particle distribution could be accepted. Similarly, if uniform properties are desired along the contact surface 117, the particle distribution should be fairly uniform also along the directions parallel to the contact surface 117. The ratio of the particle volume to the matrix volume could advantageously lie within the range of 5- 10^"5 to 0.33, so that the particles 210 will occupy somewhere in the range of 0.1 - 25 % of the volume of the surface layer 200. Within this particle concentration range, the conductivity of the Me-16/17 compound composite will not be unacceptably reduced as compared to the conductivity of the matrix material, while the lubrication properties will be significantly enhanced. Generally, the smaller the concentration of particles, the smaller is the negative effect of the particles 210 on the conductivity of the Me-16/17 compound composite. The positive effect of the particles on the friction coefficient of the surface, on the other hand, is generally such that the friction coefficient initially decreases rapidly with increasing particle concentration, while levelling out asymptotically as the particle level increases above a certain level. In many applications, a particle concentration within the range of 0.1-10 volume percent will be sufficient, thus keeping the conductivity of Me- 16/17 compound composite at a high level.

The average diameter of the particles 210 could advantageously be in the order of 0.01 - 10 μπι, and oftentimes, an average diameter of less than 1 μπι will be advantageous. The smaller the particle size, the larger the surface area of the particles 210, and the better the compatibility between the matrix 215 and the particles 210. The adhesion force will be stronger with smaller particles, and material properties will be more homogeneous.

Furthermore, the average diameter of the particles 210 should advantageously be less than around 5 % of the thickness d of the surface layer 200, in order to maximize the conductivity of the surface layer 200. Hence, for surface layers 200 having a thickness d larger than approximately 200 μπι, an average particle size in excess of 10 μπι could be beneficial for a cost efficiency point of view.

The thickness of the surface layer 200 of a Me- 16/17 compound composite influences the durability, as well as the resistance, of the surface layer 200. The thickness d of the surface layer 200 can thus be adapted to the requirements of durability and conductance of the contact surface 117. In most applications, the thickness of the surface layer will exceed 20 μπι, inter alia to avoid diffusion of material from the contact body 205 into the surface layer 200, although thicknesses as small as 5 μπι may be contemplated in certain embodiments where few sliding operations and/or low contact force is expected. In many applications, the thickness of the surface layer will be a thin film, the thickness of which will lie within the range of 20 to 100 or 200 μπι. A thickness d within this range provides a surface which can withstand a large number of operations and/or a high contact force. However, in some applications, for example when the sliding actions cause extreme wear, and/or when it is combined with arcing, such as in current commutation, the thickness d of the Me-16/17 compound composite surface layer 200 could be considerably larger, e.g. 2000 μπι or more. In this thickness range, the surface layer 200 will exhibit properties which are more bulk-like than thin-film-like. In one embodiment, the majority of the contact element 200 is made from the Me- 16/17 compound composite (cf. for example the contact element shown in Fig. 2b). The thickness and shape of the body 205 of a contact element 105 could advantageously be adjusted to the thickness of the surface layer 200 and the requirements of a particular electrical contact 100. However, since the Me-16/17 compound composite surface layer 200 can be made to have a large thickness d without significantly influencing the conductance of the contact element 105, the body 205 could, in one implementation, be dispensed with, as illustrated in Fig. 2b. In another implementation, the thickness of the body 205 could be thin compared to the thickness d of the surface layer 200, for example if the body 205 is merely used as a substrate onto which the Me-16/17 compound composite may be deposited.

A surface layer 200 of a Me-16/17 compound composite can for example be applied onto a body 205 by means of physical vapour deposition (PVD), where the metallically conducting matrix material and the Me-16/17 compound are for example sputtered or evaporated onto a body 205 to form a suitable Me-16/17 compound composition. Reactive sputtering using an iodine-containing gas, such as CH₂I₂, can for example be employed to form Agl. The surface layer 200 of a Me-16/17 compound composite can alternatively be deposited onto a body 205 by means of thermal spraying. For surface layers 200 of larger thicknesses, thermal spraying typically provides a more durable material.

A contact element 105 having a Me-16/17 compound composite contact surface 117 can alternatively be achieved by sintering of a mixture of a powder of the metallically conducting matrix material and a powder of the Me-16/17 compound, in order to make thicker coatings or bulk contact elements 105 of composite material.

Alternative methods by which a Me-16/17 compound composite contact surface 117 may be achieved include Chemical Vapour Deposition (CVD), Sol-Gel methods and

electrochemical multi-layer deposition. Other manufacturing techniques could also be used.

Fig. 3 illustrates an embodiment of a tap changer 130 having six electrical contacts in the form of different tap selector switches 300i, 300vi, as well as an electrical contact in the form of a diverter switch 310. Two connection points 313i and 313ii are also provided for connecting the winding 135 to an electrical circuit/network.

Each of the tap selector switches 300 represent an electrical contact 100 having two switching elements 105k and 105m. The diverter switch 310 has a first contact element

105n, as well as four different contact elements 105p, arranged in pairs, where each pair is connected so that an impedance 315 lies between the contact elements 105p of the pair. The first contact element 105n is pivotally movable around a pivot point 320, so as to allow for making and breaking of electrical contact with the contact elements 105p. To select a suitable number of turns of the winding 135, a particular one of the tap selector switches 300 is closed, while the others are open, and the diverter switch 310 is positioned so that the closed selector switch 300 is in electrical contact with the connection points 313i and 313ii. Each of the contact elements 105k, 105m, 105n and 105p of the tap changer 130 has a contact surface 117, which may or may not have contact surface formed from a Me- 16/17 compound composite. In Fig. 3, the first contact element 105n of the diverter switch 300 is shown to have a contact surface 117 having a surface layer 200 comprising Me-16/17 compound particles 210 in a matrix 215 of a metallically conducting material (see the partial enlargement 325). In the embodiment shown in this drawing, the Me-16/17 compound composite is deposited on a body 205 in the form of a thin substrate.

In Fig. 4, an embodiment of an electrical contact 100 having a set of at least one rotating arm(s) 400 having contact elements 105q and a set of at least one fixed contact element(s) 105r, where the rotating arms 400 are rotatably mounted on an axis 405, so that, by rotating the rotating arms 400 around the axis 405, contact between the contact elements 105q and 105r will be made or broken. The rotating contact elements 105q of the rotating arms 400 of Fig. 4 are of bulk composite material (indicated as shaded areas), and are in the shape of contact buttons. Contact elements 105q of bulk composite will ensure lubrication of the contact surfaces 117 during a large number of contact operations. The bulk composite contact elements 105q could in another implementation be of other shapes, such as rivets or discs. The bulk composite material contact elements 105q could be made with or without a body 205 - in the example of Fig. 4, no body 205 is present. The contact surfaces of the fixed contact elements 105r could have a composite contact surface 117, or, alternatively, lubrication of the contact surface 117 could be supplied by the rotating contact elements 105q only. One or two of the contact elements 105, by means of which electrical contact between can be made or broken, could have a contact surface 117 which is formed of the Me- 16/ 17 compound composite material. If the Me- 16/17 compound composite is applied on only one of the contact elements 105, lubrication will still be achieved from the presence of the Me- 16/17 compound particles 200 on the contact surface 117 of the other contact element 105.

The Me- 16/17 compound composite could be arranged to cover parts of, or the entire, contact surface 117 of a surface element 105. A contact surface 117 which is partly covered by a Me- 16/17 compound composite could for example be useful if a part of the surface is likely to be exposed to arcing, since the particles 210, depending on the composition of the Me- 16/17 compound, may not be thermally stable at the temperatures caused locally by the arcing. Hence, in this embodiment, the parts of contact surface 117 where arcing is not likely to occur could advantageously be formed of the Me- 16/17 compound composite, while no Me-16/17 compound composite is provided at the arcing- prone parts.

In one embodiment of a contact element 105, the concentration of Me-16/17 compound particles 210 and/or the composition of the Me-16/17 compound particles 210 varies in the composite material along a direction parallel to the contact surface 117. In applications wherein different requirements of conductivity and/or friction coefficient apply to different parts of a contact surface 117, a resistivity and/or friction coefficient gradient could be formed over the contact surface 117 by varying the composition and/or concentration of the Me-16/17 compound particles. A composite material having particles 210 of a mixture of different Me-16/17 compounds, or wherein particles of other composites than Me-16/17 compounds are used locally in the composite material, could in some applications be advantageous. For example, in a contact 100 wherein only parts of the contact surface 117 will have a high exposure to arcing, the arcing exposed part(s) of the surface 117 could include particles of lower conductivity than the Me-16/17 compound particles at the other parts of the surface 117. The lower conductivity will ensure a lower current density through this part of the contact surface 117, and thus, the arcing effects will be mitigated. Examples of particles of lower conductivity include oxides, which generally also have the advantage of being thermally more stable than the metal halogenides or chalcogenides at the temperatures caused locally by the arcing. Examples of suitable conducting oxides are: Magneli-phases (T1₄O₇, T1₅O₉ and Ti₆On), ITO - Indium Tin Oxide (ln₂0₃ + Sn0₂), Μθ₉θ₂₆ and Zr0₂. In Fig. 5 is shown an example of a contact element 105 having a particle gradient along a direction x parallel to the contact surface 117. The contact element 105 of Fig. 5 comprises two different particle types 210a and 210b, where the particles 210b are concentrated to one end of the contact element 105 in the x direction, this end for example being the last part of the contact element 105 to separate from the counterpart contact element upon opening of the contact in a current commutating application (e.g. in a tap changer or breaker), thus being more exposed to arcing than the rest of the contact element 105. Surface layers having a concentration and/or composition gradient can for example be made by means of sputtering.

The inventive contact surface 117 formed from a Me-16/17 compound composite could be useful in electrical contacts of all sorts of design, such as for example on the contact surfaces 117 of the contact elements 105g-h shown in Figs, la, lb or lc, or in any other electrical contacts 100. Furthermore, contact surfaces 117 formed from a Me-16/17 compound composite could be used in any contact application, such as for example in disconnectors, tap changers, contacts in electrical motors, breakers (for example HVDC breakers and high voltage AC breakers, as well as in breakers of more moderate voltages), switches, plug-in systems, etc.

The use of a Me-16/17 compound composite contact surface 117 is, as mentioned above, particularly advantageous in electrical contacts 100 for which a large number of contact operations is expected during the contact lifetime. Furthermore, the contact surface wear caused by the undesirable phenomenon of fretting, which sometimes occurs in contacts which are exposed to mechanical vibrations, cyclic thermal loads or high contact pressure etc., can be mitigated by use of a contact surface of a Me-16/17 compound composite. Although various aspects of the invention are set out in the accompanying independent claims, other aspects of the invention include the combination of any features presented in the above description and/or in the accompanying claims, and not solely the combinations explicitly set out in the accompanying claims.

One skilled in the art will appreciate that the technology presented herein is not limited to the embodiments disclosed in the accompanying drawings and the foregoing detailed description, which are presented for purposes of illustration only, but it can be

implemented in a number of different ways, and it is defined by the following claims.

Claims

1. An electrical contact element for providing electrical contact in an electrical circuit, wherein

at least part of at least one contact surface of the contact element is formed from a Me-16/17 compound composite material comprising a matrix of a metallically conducting material and particles formed from a Me-16/17 compound.

2. The electrical contact element of claim 1, wherein

the contact element further comprises a body of a metallically conducting material; and

the Me-16/17 compound composite forms a layer on said body, where the layer has a thickness of 0.1 - 10 000 μιη.

3. The electrical contact element of claim 2, wherein

the Me-16/17 compound composite layer has a thickness of 5-200 μιη.

4. The electrical contact element of claim 1, wherein

the bulk of the electrical contact element is formed by the Me-16/17 compound composite.

5. The electrical contact element of any one of the above claims, wherein

the Me-16/17 compound content of the Me-16/17 compound composite is below 25 volume percent.

6. The electrical contact element of claim 5, wherein

the Me-16/17 compound content of the Me-16/17 compound composite is below 10 volume percent.

7. The electrical contact element of claim 6, wherein

the Me-16/17 compound content of the Me-16/17 compound composite lies in the range of 0.5 - 5 volume percent.

8. The electrical contact element of any one of the above claims, wherein the majority of the Me- 16/17 compound particles in the Me- 16/17 compound composite have a diameter in the range of 5 nm - 10 μιη.

9. The electrical contact element of claim 8, wherein

the majority of the Me- 16/17 compound particles in the Me- 16/17 compound composite have a diameter in the range of 0.1 - 1 μιη.

10. The electrical contact element of any one of the above claims, wherein

the matrix of the Me- 16/17 compound composite is formed from one of the following: silver, tin, aluminium, gold or copper.

11. The electrical contact element of any one of the above claims, wherein

at least a majority of the particles of the Me-16/17 compound composite are formed from a metal halogenide.

12. The electrical contact element of any one of the above claims, wherein

at least a majority of the particles of the Me-16/17 compound composite are formed from a metal chalcogenide.

13. The electrical contact element of any one of the above claims, wherein

at least a majority of the particles of the Me-16/17 compound composite are formed from a metal oxide.

14. The electrical contact element of any one of the above claims, wherein

the Me-16/17 compound composite material comprises at least two types of particles formed from different Me-16/17 compounds.

15. The electrical contact element of any one of the above claims, wherein

the concentration of at least one type of Me-16/17 compound particles show a gradient along a direction parallel to a contact surface of the contact element.

16. The electrical contact element of any one of the above claims, wherein the Me- 16/ 17 compound composite is formed by means of one of the following techniques: a physical vapour deposition technique such as sputtering or evaporation; a chemical vapour deposition technique; sintering; thermal spraying; a Sol-Gel method; or electrochemical multi-layer deposition.

17. An electrical contact comprising a contact element of any one of the above claims.

18. An electrical device, such as for example a tap changer, a breaker or an electrical motor, comprising a contact element of any one of claims 1-17.