Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-12-31
2004-12-07
Zimmerman, John J. (Department: 1775)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C428S672000, C428S929000, C428S629000, C428S680000, C427S115000
Reexamination Certificate
active
06828052
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to electrical conductivity in metal elements, including metal and alloy components, and is particularly concerned with metal substrates which have been treated to improve or maintain their electrical conductivity and with a method of improving the electrical conductivity of electrically conductive metal substrates in some circumstances.
2. Description of the Related Art
Many components of electrically conductive metals and alloys suffer reduced electrical conductivity over time due to the formation in adverse conditions, such as damp and/or oxidising atmospheres, of surface layers having effectively no electrical conductivity or less electrical conductivity than the substrate material. This is true of, for example, nickel, copper, mild steel and other alloys including stainless and other heat resistant steels.
The present invention has applicability to metal elements being used at low and/or elevated temperatures, but is especially useful, in at least some embodiments, for electrically conductive components in fuel cells.
In a solid oxide fuel cell the electrolyte, anode and cathode are usually ceramic or ceramic-type, such as cermet, materials. However, the surrounding components of a fuel cell stack may be of any material which can provide the desired mechanical strength, heat transfer and other properties at the temperatures necessary for operation of the fuel cell, usually in excess of 700° C. Some of these components, for example separator plates (also known as interconnect or bipolar plates), are required to provide electrical connection between adjacent fuel cells and/or elsewhere in the stack. Sophisticated electrically conductive ceramics have been developed for this purpose but these materials are expensive, mechanically fragile and are poor thermal conductors when compared with many metals and alloys which might be considered suitable.
The operating conditions in a solid oxide fuel cell are very severe on most metals, causing them to degrade via loss of mechanical strength, oxidation or other form of corrosion, distortion, erosion and/or creep. Various heat resistant metals have been developed to cope with many of these forms of degradation. Most such metals are alloys based on iron or nickel with substantial additions of chromium, silicon and/or aluminium plus, in some alloys, more expensive elements such as cobalt, molybdenum and tungsten. Chromium-based heat resistant metals are also available.
A significant feature of heat resistant alloys is the oxide layer which is formed when the alloy is exposed to mildly or strongly oxidising conditions at elevated temperatures. They all form tight, adherent, dense oxide layers which prevent further oxidation of the underlying metal. In heat resistant steel, these oxide layers are composed of chromium, aluminium or silicon oxides or some combination of them depending upon the composition of the steel. They are very effective in providing a built-in-resistance to degradation of the underlying steel in high temperature oxidising conditions.
However, while this feature is used to great advantage in many applications, the presence of the oxide layer is highly deleterious to the use of these steels in key components of solid oxide fuel cells. These oxides, especially those of silicon and aluminium, are electrically insulating at all temperatures and this is a major problem for those fuel cell components which must act as electrical current connectors or conductors. For these heat resisting steels to be useful for electrically conducting components in fuel cell assemblies, it is imperative that the insulating effect of the oxide layer be alleviated at least in selected locations.
SUMMARY OF THE INVENTION
According to the present invention there is provided an electrically conductive metal element comprising an electrically conductive metal substrate having a layer of Ni—Sn alloy overlying an electrically conductive surface of the substrate and at least one layer of Ag or of Ag containing Sn overlying the Ni—Sn alloy layer. A layer of SnO
2
may also be provided, overlying the at least one layer of Ag or of Ag containing Sn.
Also according to the present invention there is provided a method of improving the electrical conductivity of an electrically conductive metal substrate which forms a less electrically conductive surface oxide layer in oxidising conditions, the method comprising forming a layer of Ni—Sn alloy on at least a portion of a surface of the substrate which does not have said surface oxide layer, and forming at least one layer of Ag or of Ag containing Sn on at least a portion of the Ni—Sn alloy layer.
The invention also extends to components formed from the electrically conductive metal element. The element may be coated as defined on one or both/all sides depending upon user requirements, or only on part of one or more surfaces of the substrate.
It will be appreciated from the following that the at least one layer of Ag may contain substantial amounts of Sn. For convenience, however, hereinafter throughout this description the at least one layer of Ag or of Ag containing Sn may be referred to as the at least one layer of Ag (or equivalent—ie. Ag layer) unless at least one layer of Ag containing Sn is specifically being referred to, in which case this may be identified as Ag+Sn, Ag+Sn mixture or Ag—Sn. Ag+Sn and Ag+Sn mixture shall be understood to mean any mixture, alloy or other combination of Ag and Sn, whereas Ag—Sn shall be understood to be a reference specifically to the silver-tin alloy system.
By the present invention we have found that the Ni—Sn alloy, in addition to being a good metallic conductor, also acts as (i) a relatively stable oxygen barrier layer to restrict oxygen access to the substrate metal and (ii) a diffusion barrier to Fe, Cr, Al and other elements from the substrate. Accordingly the Ni—Sn alloy layer with the at least one overlying Ag layer can alleviate the loss of electrical conductivity of the metal element by restricting the formation of an oxide surface layer on the substrate metal and by allowing electrical conduction therethrough.
While the Ni—Sn alloy is relatively stable, it has a tendency to oxidise over time, particularly at temperatures above 300° C., and thereby gradually lose its conductivity, and the optional SnO
2
layer on the at least one Ag layer may be provided to slow such oxidation. The at least one Ag layer provides excellent electrical conductivity not only directly through the Ag layer(s), but also laterally through the layer(s). Thus, the electrical connection to the at least one Ag layer may be through a point contact, but the silver greatly enhances the spread of the electrical conduction laterally to the at least one Ni—Sn alloy layer, and thereby reduces the resistance to current flow to and through the metal substrate. In some embodiments the at least one Ag+Sn layer may also act as a source of Sn for the formation of the optional SnO
2
layer, and/or as a source of Sn for the Ni—Sn alloy layer.
The Ni—Sn alloy and Ag layers, and the optional SnO
2
layer, need only be provided at one or more selected locations on the metal substrate, particularly those locations where it is desired to electrically connect the metal substrate to an adjacent component or otherwise to transmit electricity to or from the metal substrate. If the metal substrate is formed of a heat resisting alloy, the remaining portion or portions of the metal substrate surface may be protected by the natural oxide layer. In other circumstances, the remaining portion(s) of the metal substrate surface may if necessary be protected by, for example, the Ni—Sn alloy alone or by some other acceptable coating.
It has previously been proposed to apply Sn—Ni mixtures to steel as a decorative, corrosion and wear resistant surface layer and as a layer that inhibits the interdiffusion of Cu and Sn and/or Pb at ambient temperatures. However, the composition of such Sn—Ni layers, as well as their stru
Birch & Stewart Kolasch & Birch, LLP
Ceramic Fuel Cells Limited
Zimmerman John J.
LandOfFree
Surface treated electrically conductive metal element and... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Surface treated electrically conductive metal element and..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Surface treated electrically conductive metal element and... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3325872