Patent Grant
7880582
The present disclosure belongs to the field of electrically conducting materials and generally relates to an electrical resistor. More specifically, it relates to an electrical resistor having an electrically conductive stack, which comprises, in particular, a plurality of metal layers.
Many power and automation technology applications require resistive materials whose resistance or specific resistivity can be adjusted. Depending on the application, a resistive material should carry e.g. nominal and fault currents up to at least a few tens of kA and more, and support voltages of more than 1 kV. Resistances ranging between a 1 mΩ and a few Ω, say 5Ω, may be required.
Graphite materials have a specific resistivity that can be adjusted by adding suitable materials. However, Graphite can overheat locally when inhomogeneous currents are applied, since its capacity for distributing the current homogeneously is poor. Thus, hot-spots are formed and the material deteriorates or may even disintegrate.
FR 940 438 discloses a layered electrical resistance with great power dissipation. The resistor is built from resistive elements that have metal-coated faces for contacting each other. In addition, wing-like cooling elements made from metal can be interposed in an alternating sequence with the resistive layers. A low contact resistance is achieved by arranging discs made of soft metal between the resistive elements and/or the cooling elements. Very high overall resistance values are achieved owing to the bulk resistance of the resistive elements.
U.S. Pat. No. 1,956,859 discloses a stacked electrical resistor with resistor blocks spaced apart by washers and connector strips and with the blocks being clamped or bolted together. Again, large resistances are obtained owing to the bulk resistances of the resistor blocks.
U.S. Pat. No. 3,227,983 relates to a stacked electrical resistor of similar type with certain improvements, such as resistor elements having different thickness, being made of carbon powder, being coated e.g with powdered copper, being held together by clamping means, or being bonded together via their confronting high conductivity coatings. The overall resistance is again determined by the sum of the bulk resistances of the resistor elements.
Thus, a material is needed that is able to support high current loads, even when inhomogeneous current is applied, and that has a resistance that is adjustable over a wide range.
An the electrical resistor is disclosed and a method of manufacturing an electrical resistor is disclosed.
An electrical resistor is disclosed comprising an electrically conductive stack, the electrically conductive stack comprising a plurality of electrically conductive first layers and a plurality of electrically conductive second layers, wherein the total number of layers is three or more, the first layers are metal layers, and at least some of the first layers and second layers are arranged in an essentially alternating order, which is obtained from an alternating order by some further layer or layers possibly being inserted at arbitrary stack positions, characterized in that the contact resistance between two neighboring first and second layers is larger than the bulk resistance of one of the second layers.
A moveable electrical contact arrangement is disclosed, comprising an electrical resistor which comprises an electrically conductive stack, the electrically conductive stack comprising a plurality of electrically conductive first layers and a plurality of electrically conductive second layers, wherein the total number of layers is three or more, the first layers are metal layers, and at least some of the first layers and second layers are arranged in an essentially alternating order, which is obtained from an alternating order by some further layer or layers possibly being inserted at arbitrary stack positions, and the layers are arranged in substantially parallel planes, wherein the stack further has a contact surface substantially orthogonal to the planes of the layers and the contact arrangement further comprises a movable contacting element that can be moved over a portion of the contact surface.
In another aspect, a method of manufacturing an electrical resistor is disclosed based on an electrically conductive stack. Such a method comprises the following steps of: providing a plurality of electrically conductive first layers, which are metal layers; providing a plurality of electrically conductive second layers; choosing a total number of layers to be three or more; and arranging at least some of the first layers and the second layers in an essentially alternating order, which is obtained from an alternating order by some further layer or layers possibly being inserted at arbitrary stack positions, such as to form an electrically conductive stack of the electrical resistor, wherein the contact resistance between two neighboring first and second layers is larger than the bulk resistance of one of the second layers.
The disclosure will be better understood by reference to the following description of exemplary embodiments of the disclosure taken in conjunction with the accompanying drawings, wherein:
According to an aspect of the disclosure, the electrical resistor comprises an electrically conductive stack, which has a plurality of electrically conductive metal first layers, and a plurality of electrically conductive second layers. The total number of layers is three or more. These layers are arranged in an essentially alternating sequence. This means that at least some of the layers are arranged in an alternating sequence, and that further layers may be inserted into the alternating sequence. The contact resistance between two neighboring first and second layers is larger than the bulk resistance of one of the second layers. The term “electrically conductive” generally refers to materials having a resistivity of typically less than approx. 1014 Ωm. Note that other layers, such as third, fourth etc. layers, may also be present in the stacked resistor according to the disclosure.
The presence of layers allows a variable design of the conductive resistor, which can thus be tailored to have desired properties. For example, electric properties and in particular the electrical resistance of the material can be adjusted or fine-tuned according to the desired application by choosing an appropriate thickness and/or material of the layers. Further, particularly due to the metal layers, a current transmitted through the stack may be made more homogenous.
According to a further aspect of the disclosure, a method of manufacturing an electrical resistor is provided. In the method, a plurality of electrically conductive first metal layers is provided; a plurality of electrically conductive second layers is provided; the total number of layers is three or more; and the first layers and the second layers are arranged in an essentially alternating sequence, wherein the contact resistance between two neighboring first and second layers is larger than the bulk resistance of one of the second layers, such as to form an electrically conductive stack of the electrical resistor.
In
The high resistance in the direction perpendicular to the layers 12, 14 can, in an example arrangement, be due to the high contact resistance between neighboring layers. The high contact resistance can be the result of a high constriction resistance, if the effective contacting surface, though which current can flow from one layer to the other, is small; an additional contribution may arise from resistance due to surface contaminations or surface coatings, such as oxide coatings (so called film resistance).
In order to produce a high constriction resistance, the layers 12, 14 can be provided as separate sheets that are mechanically pressed against each other by a pressing force FP. It is advantageous that one of the sheets, e.g. the further sheet 14, is much softer than the other sheet, e.g. the metal sheet 12. Further, a relatively high pressing force FP may be applied. Then, the common mechanical surface, by which the sheets 12 and 14 are pressed against each other, is made large because the softer surface adapts to micro-bumps and -recesses of the harder surface. Consequently, mechanical stresses and heat can be distributed over a large area. Hence, even if a part of the surface is softened due to overheating, the effective contacting surface is essentially unchanged. In summary, the large mechanical surface can result in a long-time stable arrangement that supports large currents. In spite of the large mechanical surface, the effective contacting surface can be small, e.g. if portions of the mechanical surface are oxidized and therefore badly conducting.
The high contact resistance generally results in a highly anisotropic resistance of the stack 10. Namely, in a direction parallel to the layer planes, the layers are connected in parallel. Therefore the stack resistance is of the order of the smallest layer resistance (e.g. of the resistance of the metal layer 12). In contrast, in the direction orthogonal to the layer plane, the layers are connected in series. Therefore, the stack resistance is roughly of the order of the contact resistance between neighboring layers. Thereby, for example, a resistance ratio between a maximum-resistance direction and a minimum-resistance direction can be achieved that is larger than 2, or larger than 10, or even larger than 50.
In an alternative exemplary arrangement, the anisotropy of the resistor can be caused by the further layer 14 having a much higher resistance than the metal layer 12 and/or than the contact resistance between neighboring layers. In this case, the stack resistance in the direction orthogonal to the layer plane is roughly of the order of the highest layer resistance, i.e. of the resistance of the further layer 14.
In either exemplary arrangement, the anisotropy allows the current to be made homogenous, i.e. distributed over the entire layer surface, even if it is input to the stack in an inhomogenous manner, as is shown in more detail in connection with
In the following, the exemplary arrangement of the resistor of
The metal layers 12 can be made of aluminium, copper, steel, silver, tin or of any other metal. A metal is defined as a material having metal elements. Here, metal elements do not include metalloids. According to this definition, e.g. so-called metal polymers and organic metals are not considered to be metals.
The further layers 14 are made of an electrically conductive material. The further layers 14 preferably have a higher resistance than the metal layers 12. Independently of the shown exemplary embodiments, the further layers 14 can be made of
a metal,
a non-metal,
a material that is substantially more resistive, or substantially less resistive, than the metal layer material, or
a material that is substantially softer, or substantially harder, than the metal layer material.
For example, and independently of the shown exemplary embodiments, the further layers 14 can be made of a material that differs, in Vickers hardness, from the metal layer material by less than 20% of the Vickers hardness of the metal layers 12. Further, the material of the further layers 14 can have a Vickers hardness that is different from the Vickers hardness of the metal layers 12, preferably by more than 20% of the Vickers hardness of the metal layers 12. Preferably, the further layer 14 has a lower Vickers hardness than the metal layers 12.
The further layers 14 may include an electrically conductive material selected from the group consisting of carbons, such as graphite; metals, preferably soft metals, such as lead and aluminium; conductive plastics, such as carbon fiber reinforced plastic; conductive epoxy; and/or of electrically conductive ceramics, such as Boron carbide and Tungsten carbide; metals including metal alloys, such as steel, titanium alloys or nickel alloys; sintered materials, in particular sintered metals; constantan or constantan alloys; metal oxides, such as titanium oxide, vanadium oxide or barium titanate; conductive plastics, such as carbon fiber reinforced plastic; cermet; and doped silicones. A ceramic is generally an inorganic non-metal formed under heat.
The layers 12, 14 can all be made of the same material or of different materials. The metal layers 12 and/or the further layers 14 can be coated, e.g. by using a metal coating.
Independently of the shown exemplary embodiment, the metal layers 12 and/or the further layers 14 can have each a thickness that is preferably less than 5 mm or 2 mm or even 1 mm, and/or that is preferably more than 0.01 mm, 0.05 mm, or even 0.1 mm.
The layers 12 and 14 are arranged in an essentially alternating sequence on top of each other such as to form the stack 10. For example, the layers 12, 14 of
In the stack 10 shown in
Each of the layers 12, 14 defines a plane. The planes are substantially parallel to each other. The layers 12, 14 all have the same shape and are arranged on top of each other. However, the layers 12, 14 could also have different shapes, and they could be arranged such that at least some of the neighboring planes overlap only partially. Further, all layers have substantially the same thickness. However, the layers could as well have mutually different thickness, and they could not be parallel.
The electrical resistor 1 can be adapted to have desired electrical properties, which are in the following described in more detail. The resistor 1 has in general a total resistance of more than 1 mΩ in at least one direction, preferably including the direction perpendicular to the layer planes. Generally, the resistance of the stack 10 can be made highly anisotropic (see above). This anisotropy allows to homogenize the current density (see
Preferably, the bulk resistance of a metal layer 12 is lower than the bulk resistance of a further layer 14. In particular, the bulk resistance of one metal layer 12 can be less than 50%, or less than 20% or even less than 10% of the bulk resistance of one further layer 14. Alternatively, the bulk resistance of one metal layer 12 may be more than 100%, preferably more than 140% or even more than 200% of the bulk resistance of one further layer 14.
Preferably, the further layers 14 each have a bulk resistivity of more than 10−8 Ωm, more preferred of more than 10−6 Ωm or even more than 10−5 Ωm. Further, they preferably have a bulk resistivity of less than 1 Ωm, more preferred of less than 10−2 Ωm.
In some exemplary embodiments, the contact resistance between neighboring layers can be larger than the bulk resistance of the metal layer 12, preferably by a factor of more than 2 or even more than 10. Alternatively, the contact resistance may be less than 20% of the bulk resistance of any further layer 14.
Preferably, the contact resistance between neighboring layers 12, 14 is more than 10−5Ω or even more than 10−4Ω; it is preferably less than 10−2Ω or even less than 10−3Ω.
The layered structure of stack 10 allows adjusting the electrical resistance according to the desired application. This can be done by choosing an appropriate material of the layers 12, 14, and, in particular, of the further layer 14.
For example, if the metal layers 12 are metal sheets and the further layers 14 are graphite sheets, the resistance in the vertical direction is usually dominated by the contact resistance between neighboring graphite sheets 14 and metal sheets 12. While details depend on the nature of the surface of the graphite and the metal sheet, this contact resistance is preferably in the range of 100 to 500 μΩ, assuming a stack cross section of approx. 10 cm2. The total resistance for a given height can be tailored by choosing an appropriate average thickness of the sheets 12 and 14. For example, the thickness of both sheets may individually be varied between 0.1 mm and a few mm, say 3 mm. Then, assuming the above stack cross section of approx. 10 cm2, a resistance of approx. 0.0003 to 0.05Ω per cm stack thickness (vertical height) can be achieved.
As a further example, if the further layers 14 are ceramic layers, the resistance in the vertical direction is usually dominated by the bulk resistance of these layers 14. In this case, the total resistance for a given height can be tailored by choosing an appropriate average ratio of the ceramic layer thickness over the metal layer thickness. For example, the thickness of the ceramic layers 14 may be between 0.1 and 1 times the thickness of the metal layers 12. Then, similar resistances as for the graphite case (see above) can be achieved.
The resistance can be adapted by varying other material parameters as well. For example, the stack cross section (area of the layers) can be varied. Further, the hardness of the surface of the metal layer 12 can be varied, e.g. by tempering or coating the metal, e.g. by using silver, nickel, or chromium. A coating by a metal of relatively low hardness, e.g. silver, can reduce the contact resistance between neighboring layers. Further, the coating can protect against oxidation, diffusion and corrosion, and can thus further increase the long-time stability of the contact resistance. Independently of the shown exemplary embodiment, the metal layers can be coated using a metal coating. The coating may include a metal that is different from the metal of the metal layers. The coating may comprise e.g. silver, nickel, or chrome.
The stack 10 shown in
It is advantageous to sandwich the stack 10 between the bottom plate 2 and the top plate 3, as is shown in
There are alternative methods for assembling the stack 10, For example, each layer can be formed on top of another layer using a deposition technique, e.g. vapour deposition or a galvanisation method. The application of the metal layers 12 may require a different deposition technique than the application of the further layers 14, in which case the stack may have to be transferred between different chambers. Alternatively, sheets comprising pairs of layers can be produced, e.g. by coating a sheet with a coating, such that the sheet and/or the coating is formed as a metal layer. Then, the coated sheets can be arranged on top of each other, in order to produce the stack, and be mechanically pressed against each other, as discussed above. Optionally, during production of the stack, the stack may be heated in order to create a permanent contact between neighboring sheets. This may result in a sort of sintering the stack together. During this heating, the stack may optionally be pressed together.
In
The second electrode 20 is a contact that is moveable over a portion of the contact surface 11 in a direction perpendicular to the layers of the stack 10, and that is in electrical contact with the contact surface 11. If the second electrode 20 is moved away from the bottom plate 2, the current has to travel a relatively long way through the stack 10, resulting in a high resistance. If the second electrode 20 is moved towards the bottom plate 2, the current has a relatively short way through the stack 10, resulting in a low resistance. Thus, a tunable resistance 1 is provided.
Alternatively, a tap-changer (not shown) can be provided. The tap changer has a finite number of fixed contacts to the stack 10 at various distances from the bottom plate 2, and a switching arrangement that can variably select one or more of the fixed contacts to be contacted to the external lead. The fixed contacts could be, for example, formed by some of the metal layers that extend outwardly from the stack 10.
It is preferred that the size of the moveable second electrode 20 is equal to or larger than a layer thickness of a graphite layer 14. For this purpose, the contact surface of the second electrode 20 can be made sufficiently large such that it contacts more than one layer 12, 14, or at least one metal layer 12 regardless of the position of the contacting element at the contact surface 11 of the stack 10.
Preferably, the second electrode 20, i.e. the moveable contacting element 20, contacts at least one metal layer 12 and at least one graphite layer 14 regardless of its position at the contact surface 11 of the stack 10. The electrical conductivity at the contact of the stack 10 with the moveable contacting element 20 is usually dominated by the contact between the metal layer 12 and the moveable contacting element 20.
This homogenization is possible, because the resistance of the sheet is anisotropic. This anisotropy results from the relatively high conductivity of the metal layers 12. If the metal layers 12 were absent, i.e. in a resistor having only graphite layers, the homogenization would be far less efficient. The dashed line 32 illustrates schematically how the region of high current, delimited by line 30 in the presence of the metal layers 12, would be modified, if the metal layers 12 were absent. In this case, the graphite layers only could not homogenize the current sufficiently. Thus, even in the layers at a relatively large vertical distance from the moveable contacting element 20, or liquid metal second electrode respectively, the region of high currents would remain small. In other words, the current would be concentrated in a small portion of the resistor. As the current in graphite concentrates in hot spots, even already at moderately high current densities, the graphite would disintegrate at least in the vicinity of the hot-spots and/or of the moveable contacting element 20. Thus, the tolerable current density would be low. In contrast, in the presence of the metal layers 12 the current is homogenized more efficiently, and the maximum allowable current is increased. Thus, in some exemplary embodiments, currents of up to 10 to 100 kA can be supported by the resistor 1. For the above-described sizes and materials, up to 100 to 1000 kJ of energy can be dissipated.
Likewise, the maximum allowable voltage between two neighboring layers, i.e. over a contact resistance, can be increased by using suitable materials for the layers 12 and 14. For contacts between metal sheets of similar hardness, this voltage, also known as maximum contact voltage, is preferably around 0.1 V. If the maximum contact voltage is exceeded, the material heats up and softens or melts at the contact spots. As a consequence, the contact resistance decreases and is not long-time stable. By using a graphite layer or a soft material layer as the further layer 14, the maximum contact voltage between two neighboring layers 14, 12 can be designed to increase to 0.5 V and more, and the contact resistance remains long-time stable under such voltage loads.
The electrically conducting resistor 1 can be used for other purposes than the ones mentioned above. For example, it can be used as a non-tunable resistor or, e.g., as a pressure sensor. Further, any range of values given herein may be extended or modified without losing the principal effects achieved by the disclosure, as will be apparent to the skilled person.
It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be embraced therein.
This application claims priority as a continuation application under 35 U.S.C. §120 to PCT/EP2006/000568 filed as an International Application on Oct. 12, 2006 designating the U.S., the entire content of which is hereby incorporated by reference in its entirety
| Number | Name | Date | Kind |
|---|---|---|---|
| 1956859 | Everett | May 1934 | A |
| 3227983 | Braun | Jan 1966 | A |
| 5414403 | Greuter et al. | May 1995 | A |
| 6021564 | Hanson | Feb 2000 | A |
| 7192654 | Andresakis et al. | Mar 2007 | B2 |
| Number | Date | Country |
|---|---|---|
| 0 576 836 | Jan 1994 | EP |
| 940.438 | Dec 1948 | FR |
| Number | Date | Country | |
|---|---|---|---|
| 20090206979 A1 | Aug 2009 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/CH2006/000568 | Oct 2006 | US |
| Child | 12419419 | US |
