Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2000-02-22
2003-12-02
Gulakowski, Randy (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C427S115000, C427S419300, C029S623400, C156S089120, C065S033500, C428S426000
Reexamination Certificate
active
06656625
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention is concerned with a method of forming a glass-ceramic coating on a substrate and to coatings produced thereby, such coatings having particular but not exclusive utility in the provision of sealing arrangements between non-porous separator plates of fuel cells, particularly planar solid oxide fuel cells (SOFC's), and includes methods of producing such sealing arrangements.
The present invention has particular advantages when used to produce seals between separator plates which are metal or metallic: in the context of this specification the terms ‘metal’ and ‘metallic’ are to be interpreted as meaning not just plates made of metals and exclusively metal alloys, but also of oxide dispersion strengthened metal alloys which include a relatively small percentage of an oxide or oxides incorporated therein.
A planar SOFC comprises a stack of vertically spaced impermeable separator plates. These separator plates separate the reactant gases and also provide electrical connection between adjacent cells. In the space between each adjacent pair of plates is held one or more cells each comprising a solid electrolyte having an anode and a cathode. Clearly, in view of their separator function, the separator plates must not be porous as they comprise part of a gas-tight assembly. The reactant gases comprise a fuel gas (e.g. hydrogen or carbon monoxide) and an oxidant (e.g. oxygen or air) and are respectively supplied to the anode and the cathode by suitable ducts which may, for example, be provided by channels in the upper and lower surfaces of the adjacent separator plates. As is known, the reactions at the electrode cause a voltage. Connection between the electrodes and adjacent separator plates can be either by direct contact or via an electrically conducting interlayer. For example a current collector (e.g. a nickel grid) may be provided adjacent the anode and a conductive porous sheet may be provided adjacent the cathode or the cathode may contact a conductive coating on the separator plate.
SOFC's usually operates at temperatures in the range 750° C.-1000° C., though it is envisaged that they could operate at lower temperatures, possibly as low as 650° C. In a planar SOFC stack, high-performance seals between adjacent plates are required to ensure separation and containment of the reactant gases. It is known to use glass-ceramic materials to produce such seals, since glass ceramics can be formulated to be (a) stable in the oxidizing and reducing atmospheres of the stack at high temperatures and (b) un-reactive towards adjoining components during operation of the stack. However, difficulties have been experienced in creating seal arrangements which are additionally capable of bonding with high integrity to the separator plates without raising high stresses due to differing thermal expansion characteristics of the seals and the adjoining materials. A requirement has also emerged to facilitate the creation of glass-ceramic seals of sufficient thickness to accommodate the thickness of the cells and current contacts.
In a stack where a plurality of laterally adjacent cells (e.g., an array of four cells) are sandwiched between adjacent separator plates in the stack, the seal should also provide high temperature electrical insulation between adjacent bipolar plates. However, such electrical insulation is not required in stack designs in which only one cell is sandwiched between adjacent plates, because in such stacks the electrolyte separates the entire area of the bipolar plates, thereby providing the required electronic insulation.
One of the problems in the manufacture of planar SOFC's using glass ceramics as a means of sealing between adjacent separator plates in the stack is the need to ensure that the cell components remain in electrical contact in all parts of the stack throughout the process of assembling and sealing the stack. This can be difficult, because a glass-ceramic, once it has crystallized, does not deform appreciably, whereas the rest of the manufacturing process can involve volume changes at elevated temperatures in the layers of the stack. This is because the oxide mixtures used to form the anodes and the anode contacts are partially reduced by passing a reforming gas such as hydrogen through the stack.
SUMMARY OF THE INVENTION
An aim of the invention is to provide a glass-ceramic coating with improved bonding to a substrate, particularly a metallic substrate of the type used for separator plates in planar SOFC's.
Another aim of the invention is to provide an effective high-performance non-porous glass-ceramic seal between adjacent non-porous separator plates of planar SOFC's.
A further aim is to produce such a seal capable of electrically insulating adjacent biploar plates from each other and preventing electronic leakage therebetween.
Another aim is to provide such a seal which accommodates change in dimension of the stack during its manufacture.
A further aim of the invention is to provide a method of applying a glass-ceramic coating to a separator plate for a solid oxide fuel cell, so providing a base layer for at least one further layer required to complete a seal between confronting surfaces of adjacent separator plates.
It is to be understood that a glass-ceramic is an inorganic, polycrystalline material formed by the controlled crystallization of a glass; a glass on the other hand is an inorganic material formed by fusion but wherein the material has cooled to a rigid condition without crystallizing.
According to a first aspect of the invention, a method of providing a glass-ceramic coating having improved bonding to a substrate comprises the steps of: depositing a first bonding layer of glass powder mixed with a binder directly onto the substrate (preferably using a screen printing or spraying process); adhering a laminar body comprising glass powder mixed with a binder to the first layer to form a second layer which is substantially thicker than the first layer, the glass powder in both layers being of a composition such as to form a glass-ceramic on heat treatment; and heat treating the resultant green coating on the substrate to drive off the binder and convert the glass powder layers to glassceramic layers.
According to a second aspect of the invention, in a fuel cell, a high-performance seal between confronting faces of adjacent non-porous separator plates comprises at least one glass-ceramic layer on at least one of the confronting faces and at least one glass seal layer interposed between the at least one glass-ceramic layer and the other separator plate.
Preferably, the at least one glass-ceramic layer is a duplex layer, comprising a first glass-ceramic layer for bonding the seal to the separator plate and a second glass-ceramic layer superimposed on the first glass-ceramic layer, the glass seal layer being interposed between the second glass-ceramic layer and the adjacent separator plate, the second glass-ceramic layer being substantially thicker than the first glass-ceramic layer.
Glass-ceramic layers are may be provided on both confronting faces of the separator plates, the glass seal layer being interposed therebetween.
The at least one glass-ceramic layer may for example comprise compositions in the SiO
2
—CaO—MgO—Al
2
O
3
system, the composition being adjusted to optimize its ability to bond with the separator plate surface and/or to optimize its thermal expansion coefficient with respect to the thermal expansion coefficient of the separator plate to which it is attached. Where a duplex glass-ceramic layer is utilized, the first and second layers are preferably of different compositions to optimize bonding of the seal to the separator plate surface in the first layer while also optimzing the thermal expansion coefficient of the second layer.
The at least one glass seal layer may for example comprise compositions in the SiO
2
—BaO—CaO-Al
2
O
3
system.
The invention further includes a method of forming a glass-ceramic coating on a substrate comprising a separator plate of a solid ox
Pyke Stephen
Thompson Christopher
Wood Anthony
Alstom UK Ltd.
Crepeau Jonathan
Gulakowski Randy
Kirschstein et al.
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