Patent Description:
The present invention relates generally to interconnectors for solid oxide fuel cells (SOFC), and methods of manufacturing SOFC interconnectors using pressed powder metallurgy. Additionally and/or alternatively, the present invention relates to the controlled oxidation of porous chromium alloys such as interconnectors for SOFCs.

SOFCs directly produce electricity by oxidizing a fuel. In a typical planar geometry SOFC, an electrolyte layer (solid oxide or ceramic) is sandwiched between two electrodes (a cathode layer and an anode layer). Fuel flows past the outside of the anode layer (the oxidizing side) to provide H<NUM> to the anode. Air flows past the outside of the cathode layer (the reducing side) to provide O<NUM> to the cathode layer. The H<NUM> and an O- from the O<NUM> react to produce H<NUM>O, which is exhausted on the fuel side of the anode. The reaction causes electron flow from the anode to the cathode, which provides electricity.

Individual SOFCs are typically stacked so that their electrical output is combined in series. An interconnector (also known as an interconnector plate or separator plate) separates adjacent SOFCs. As a result, opposing sides of an interconnector are exposed to the fuel side/oxidizing side of one SOFC and the air side/reducing side of an adjacent SOFC. The interconnector is typically designed to be substantially impermeable to the gaseous phase air and fuel so as to minimize uncontrolled combustion and catastrophic failure of an SOFC stack. An elevated temperature oxidation process step is often used in the PM manufacturing process whereby growth of an oxide layer is encouraged on the walls of the internal porosity such that internal pore channels become blocked by the formed oxide films and hence the oxidation process provides a desirable reduction in permeability relative to the un-oxidized condition.

End-plates are disposed at the end of an SOFC stack, and function as one-sided interconnectors. For ease of reference, an end plate is defined herein to be an interconnector.

Typical operating temperatures of SOFCs are between <NUM> and <NUM>.

<CIT>, <CIT>and <CIT> and <CIT> describe various interconnectors and methods of manufacturing interconnectors.

<NPL>) discloses a study of the oxidation behaviour of chromium in a temperature range of <NUM> to <NUM> degrees. A number of atmospheres were used, such as Ar-O<NUM>, Ar-H<NUM>-H<NUM>O, Ar-H<NUM>-H<NUM>O and N<NUM>-O<NUM>-H<NUM>O.

Powder metallurgy (PM) manufacturing methods have been used to manufacture interconnectors due to PM's available net shape forming capability. However, the components produced can contain residual internal porosity which poses problems with associated manufacturing methods and with final component function.

The presence of the chromium nitrides (CrN's) in an interconnector tends to be undesirable for two reasons. First, the formation of the nitrides may cause a dimensional change to interconnectors. Excessive nitride formation may lead to warping of the interconnectors beyond allowable product dimensional tolerances and hence can reduce manufacturing yield. Second, even though lower levels of nitride may not have a significant effect on manufactured dimensions, even lower levels of nitrides yet may be undesirable with respect to SOFC function. In normal SOFC operation, the interconnectors are exposed to elevated temperatures and air for extended periods of time. In such an environment, nitrides originally present within the interconnector material may grow and introduce dimensional changes to the interconnector in-situ during operation of the SOFC. Such dimensional changes may impair the contact uniformity within the SOFC stack and hence lead to accelerated degradation of electrical efficiency over time of operation of the SOFC. The present invention provides a method of oxidising a porous component, in accordance with appended claim <NUM>.

One or more embodiments of the present invention provide an oxidation process for porous chromium components (e.g., PM components such as interconnectors) that reduces the formation of nitrides in the component.

The present invention provides a method of oxidizing a porous component comprising at least <NUM> weight % chromium. The method includes: oxidizing the component in a furnace so as to expose the component to an oxidation temperature range for a predetermined time period; and during said oxidizing, feeding a controlled atmosphere into the furnace. The controlled atmosphere comprises at least <NUM> volume % nitrogen, at least <NUM> volume % oxygen, and at least <NUM> volume % water vapor. The oxidizing increases a nitrogen content of the porous component by less than <NUM> weight %.

According to one or more of these embodiments, after said oxidizing, the component comprises less than <NUM>, <NUM>, <NUM>, and/or <NUM> weight % nitrogen.

According to one or more of these embodiments, the controlled atmosphere comprises at least <NUM> volume % ambient air.

According to the invention, the controlled atmosphere comprises at least <NUM> volume % water vapor.

According to one or more of these embodiments, the controlled atmosphere comprises <NUM> volume % water vapor.

According to one or more of these embodiments, the method also includes adding water vapor to ambient air to create the controlled atmosphere.

According to one or more of these embodiments, the oxidation temperature range is above <NUM> and the predetermined time period is at least <NUM> hours.

According to one or more of these embodiments, the method also includes feeding the component through the furnace in a travel direction during said oxidizing, wherein the controlled atmosphere is fed into the furnace in the travel direction.

According to one or more of these embodiments, the method also includes feeding the component through the furnace in a travel direction during said oxidizing, wherein the controlled atmosphere is fed into the furnace in an opposite direction as the travel direction.

According to various embodiments, the component and controlled atmosphere may be fed through the furnace in the oxidation step in concurrent or counter flow directions.

According to one or more of these embodiments, the component comprises an SOFC interconnector.

Conventional wisdom in the interconnector industry was that PM interconnector density should be maximized in order to obtain maximum air/fuel impermeability. Because coarser iron particles are more compressible, the industry has conventionally relied on such coarser iron particles in an effort to maximize interconnector density, and thereby maximize air/fuel impermeability. In contrast, the present inventors discovered that good impermeability could be achieved at lower densities through the use of finer iron particles. It is believed that the use of finer iron particles results in an interconnector microstructure that is more easily sealed through oxidation than the microstructure that results from a denser interconnector made from coarser iron particles. According to various embodiments, the ability to achieve good impermeability at lower interconnector densities using finer iron particle sizes enables less expensive manufacturing techniques (e.g., avoiding a more expensive double-press procedure, using reduced sintering temperatures and/or sintering times because smaller iron particle size enhances chromium-to-iron diffusion which more readily achieves in a target coefficient of thermal expansion (CTE)) and reduces material cost by using less chromium per interconnector. The reduced chromium content requirement is advantageous because chromium is expensive, and interconnectors comprise a major fraction of the SOFC hardware cost. Reducing the total mass of the interconnectors may provide a significant cost advantage.

One or more embodiments of the present invention provide a faster, less expensive method for manufacturing an SOFC interconnector with good impermeability and dimensional characteristics.

One or more methods of the present invention provide an SOFC interconnector that utilizes a reduced amount of chromium per interconnector, thereby reducing the interconnector's material cost.

One or more embodiments of the present invention provide a Powder Metal (PM) process that enables fabrication of SOFC interconnectors with a high chromium content (e.g., over <NUM>%), precise dimensional tolerances, thermal expansion properties that match the thermal expansion properties of adjacent electrolytes, and/or good impermeability. This combination is not readily manufactured by other methods such as stamping or rolling. The PM process according to one or more embodiments may provide a very precise, cost effective fabrication of parts, to very precise dimensional tolerances.

These and other aspects of various embodiments of the present invention, as well as the methods of operation and functions of the related elements of structure and the combination of parts and economies of manufacture, will become more apparent upon consideration of the following description and the appended claims with reference to the accompanying drawings, all of which form a part of this specification, wherein like reference numerals designate corresponding parts in the various figures. In one embodiment of the invention, the structural components illustrated herein are drawn to scale. In addition, it should be appreciated that structural features shown or described in any one embodiment herein can be used in other embodiments as well. As used in the specification and in the claims, the singular form of "a", "an", and "the" include plural referents unless the context clearly dictates otherwise.

For a better understanding of embodiments of the present invention as well as other objects and further features thereof, reference is made to the following description which is to be used in conjunction with the accompanying drawings, where:.

<FIG> illustrate an SOFC stack <NUM>. The SOFC stack <NUM> includes a plurality of SOFCs <NUM>. Each SOFC <NUM> includes an electrolyte plate <NUM> sandwiched between two electrodes (an anode plate <NUM> and a cathode plate <NUM>). A fuel side passage <NUM> (i.e., a series of channels) for the passage of fuel <NUM> is disposed adjacent each anode plate <NUM>. An air side passage <NUM> (a series of channels) for the passage of air <NUM> is disposed adjacent each cathode plate <NUM>. An interconnector <NUM> separates the fuel side passage(s) <NUM> for one SOFC <NUM> from the air side passage(s) <NUM> of an adjacent SOFC <NUM>.

The interconnector <NUM> may have any shape and size suitable for use in an SOFC stack. In the interconnector illustrated in <FIG> and <FIG>, each side of the interconnector <NUM> includes a series of alternating ridges <NUM> and valleys <NUM>. As shown in <FIG>, the ridges <NUM> on opposite sides of the interconnector <NUM> abut the electrodes <NUM>,<NUM>, respectively, of adjacent SOFCs <NUM> such that the spaces formed between the ridges <NUM>, valleys <NUM>, and respective electrodes <NUM>,<NUM> create the fuel and air side passages <NUM>, <NUM>, respectively.

<FIG> is a plan view of the fuel side of the interconnector <NUM>. The fuel side passage <NUM> is defined by a depression <NUM> in the interconnector <NUM>. The depression <NUM> defines the valleys <NUM>, and the ridges <NUM> rise up from the depression <NUM>. Fuel supply and exhaust plenum regions <NUM>, <NUM> are defined on upstream and downstream sides of the ridges <NUM>/valleys <NUM>, respectively. A fuel supply hole <NUM> leads into the fuel supply plenum <NUM>. A fuel exhaust opening <NUM> leads from the exhaust plenum region <NUM>. Fuel <NUM> flows into the fuel side passage <NUM> and supply plenum <NUM> from the supply opening <NUM>, through the valleys <NUM> into the exhaust plenum <NUM> (along with produced water), and out of the exhaust opening <NUM>.

Corresponding air side depression <NUM>', valleys <NUM>, ridges <NUM>, and air supply and exhaust plenums140', <NUM>' and holes <NUM>', <NUM>' are disposed on the opposite side of the interconnector <NUM> and are shown in phantom dotted lines in <FIG>.

The interconnector <NUM> includes a flow field that encompasses the regions of the interconnector <NUM> over which fuel or air are designed to flow. In the interconnector <NUM> illustrated in <FIG>, the flow field of the interconnector <NUM> is bounded by the perimeter of the depressions <NUM>, <NUM>', and is generally + shaped. The perimeter of the interconnector <NUM> outside of the depressions <NUM>, <NUM>' are not part of the flow field. In interconnectors where the air/fuel passages <NUM>, <NUM> extend beyond the edges of the interconnector (i.e., to the top, bottom, left, and/or right of the interconnector as viewed in <FIG>), the flow field of the interconnector extends to that edge. As explained in greater detail below, it is typically important that the flow field portion of the interconnector <NUM> be impermeable to fuel <NUM> and air <NUM>.

In the interconnector illustrated in <FIG> and <FIG>, the ridges <NUM> and valleys <NUM> on one side of the interconnector <NUM> extend perpendicularly relative to the ridges <NUM> and valleys <NUM> on the other side of the interconnector <NUM>. As a result, as illustrated in <FIG>, the fuel side passages <NUM> extend into the sheet and the air side passages <NUM> extend left to right. Consequently, the SOFC stack <NUM> is designed so that the fuel <NUM> flows in one direction, while the air <NUM> flows in a perpendicular direction. However, alternatively, the fuel and air side passages <NUM>,<NUM> may be parallel (e.g., as shown in the alternative interconnector <NUM>' illustrated in <FIG>) or run in any other suitable direction relative to each other.

Hereinafter, methods of making the interconnector <NUM> according to various embodiments are described with reference to <FIG>.

Chromium (Cr) base powder <NUM> is produced from coarse chromium feedstock of about <NUM> to <NUM> x down by grinding with hammer mills, pin mills, and/or other suitable grinding machinery and then classified. The coarse chromium feedstock according to various embodiments comprises at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>% chromium (e.g., aluminothermic chromium, chromium powder produced using another suitable method).

Unless otherwise stated, all percentages disclosed herein are weight percentages. Unless otherwise stated particle sizes refer to screen classification using square openings. For example, particles smaller than <NUM> mean particles that fall through a <NUM> x <NUM> square opening. In contrast any dXX values (e.g., D50) refer to the XX% distribution particle by number of particles (not by weight). Thus, a powder with a D50 of <NUM> means that <NUM>% of the particles (by number of particles, not mass) are larger than <NUM> and <NUM>% are smaller.

The chromium powder may be classified to under <NUM> (i.e., substantially all particles fall through a <NUM> x <NUM> opening) via a suitable screen, with a D50 of somewhere between <NUM>-<NUM> and/or between <NUM>-<NUM>, and a maximum of <NUM>%, <NUM>%, <NUM>%, and/or <NUM>% chromium particles smaller than <NUM> to create the chromium base powder <NUM> (where values in the present specification are provided as "um" or "microns", <NUM> or micron is equal to <NUM>). According to various embodiments, the chromium base powder <NUM> comprises no more than <NUM>% chromium particles larger than <NUM>, no more than <NUM>% chromium particles larger than <NUM>, as much as <NUM>% chromium particles larger than <NUM>, and no more than <NUM>% chromium particles smaller than <NUM>. According to various other disclosed methods, the chromium base powder <NUM> comprises no more than <NUM>% chromium particles larger than <NUM>, at least <NUM>% chromium particles larger than <NUM>, and no more than <NUM>% chromium particles smaller than <NUM>. According various disclosed methods, the chromium base powder <NUM> comprises no more than <NUM>% chromium particles larger than <NUM>, no more than <NUM>% chromium particles larger than <NUM>, <NUM>-<NUM>% and/or <NUM>-<NUM>% chromium particles larger than <NUM>, and no more than <NUM>% chromium particles smaller than <NUM>.

Iron (Fe) powder <NUM> may be blended with a lubricant (e.g., an organic lubricant, an organo-metallic lubricant, or any other type of suitable lubricant that can be used in pressed PM) <NUM> to create a master iron/lubricant blend <NUM>. The iron powder <NUM> may comprise at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>% pure iron. The Iron powder <NUM> may comprise at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>% iron particles smaller than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>. The iron powder <NUM> may comprise any combination of these percentages and size limitations (e.g., anywhere from <NUM>% being smaller than <NUM> to <NUM>% being smaller than <NUM>).

The iron powder <NUM> may be heterogeneous such that, for example, at least <NUM>% of iron particles are smaller than <NUM> and at least <NUM>% are smaller than <NUM>. Again, any combination of sets of the above-listed percentages and size limits may be used. A combination of coarser and finer iron particles may be used to provide the better flow and compression characteristics of larger iron particles, while still providing the improved impermeability characteristics of smaller iron particles.

The iron powder <NUM> may comprise a high purity, fine iron powder such as a powder having a typical screen analysis of d10 <NUM>, d50 <NUM> microns, and d90 <NUM> microns and a chemical analysis (wt %) of <NUM>+% iron, <NUM>% carbon, <NUM>% oxygen, <NUM>% sulphur, and <NUM>% phosphorus, or a powder having a typical chemical analysis of <NUM>% iron, <NUM>% iron-met, <NUM>% O-tot, <NUM>% C, <NUM>% S, <NUM>% P, <NUM>% Si, <NUM>% Mn and a typical sieve analysis of <NUM>% over <NUM> microns, <NUM>% between <NUM>-<NUM> microns, <NUM>% <NUM>-<NUM> microns, <NUM>% <NUM>-<NUM> microns, and <NUM>% under <NUM> microns, or a mixture of such powders (e.g., <NUM>/<NUM>, <NUM>/<NUM>, <NUM>/<NUM>).

The master iron/lubricant blend <NUM> may have an organic lubricant <NUM> weight percentage of between <NUM> and <NUM>%, between <NUM> and <NUM>%, between <NUM> and <NUM>%, and/or between <NUM> and <NUM>%. Methods using iron powders <NUM> with smaller particles sizes may be first separately combined with larger amounts of lubricant than in methods with coarser iron powders because ease of flow tends to be inversely proportional to particle size. However, lubricant <NUM> may be omitted altogether. For example, methods using coarser iron powder may not use any lubricant <NUM>.

The chromium base powder <NUM> and master iron/lubricant blend <NUM> may then be blended to create a final blend powder <NUM>. The final blend powder <NUM> may comprise at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>% base chromium powder. The balance of the final blend powder <NUM> preferably comprises the master iron/lubricant blend <NUM>. The final blend powder <NUM> may comprise at least <NUM>% organic lubricant <NUM>. The final blend powder <NUM> may comprise between <NUM> and <NUM>% iron. The final blend powder <NUM> may comprise about <NUM>-<NUM>% chromium, at least <NUM>% and/or <NUM>% iron, and at least <NUM>%, <NUM>%, <NUM>%, and/or <NUM>% lubricant <NUM>. The final blend powder <NUM> may comprise <NUM>% organic lubricant.

According to various disclosed methods, the chromium base powder <NUM> and master iron/lubricant blend <NUM> are blended at about room temperature (e.g., between <NUM> and <NUM> and/or about <NUM>) to form the final blend powder <NUM>. A double cone blender and <NUM> minute blending cycle may be used.

According to other disclosed methods, the chromium base powder <NUM> and master iron/lubricant blend <NUM> are blended at temperatures above room temperature (e.g., above <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>, and below <NUM>, <NUM>, <NUM>, and/or <NUM>) to form the final blend powder <NUM>. A jacketed DC blender and a <NUM> hour cycle (including heating time and blending time) may be used. Blending at elevated temperatures proves good flow characteristics. The blending temperature may be kept below a melting temperature of the lubricant <NUM>.

A die-cavity having the desired cavity shape of the final interconnector <NUM> may then be appropriately filled with the final blend powder <NUM>. After the die-cavity is filled with the final blend powder <NUM>, the final blend powder <NUM> is single-stage compacted/pressed in a closed die to form a green interconnector <NUM>. The green interconnector <NUM> may have essentially the final shape and size of the final interconnector <NUM> (except for minor size and shape changes that result from post-pressing elastic rebound, sintering, further heat treatments, and/or oxidation). The single stage compaction may create the ridges <NUM>, valleys <NUM>, depressions <NUM>, <NUM>', plenums <NUM>, <NUM>', <NUM>, <NUM>', and holes <NUM>, <NUM>', <NUM>, <NUM>'. The compaction may be carried out at <NUM>-<NUM> MPa (<NUM>-<NUM> Tsi) and/or <NUM>-<NUM> MPa (<NUM>-<NUM> Tsi) using a press (e.g., a hydraulic press, a hybrid press, or any other suitable press).

The compaction/pressing may be carried out via a single pressing procedure, as opposed to a conventional two-stage pressing procedure (e.g., the two-stage pressing procedure disclosed in <CIT>).

The green interconnector <NUM> may have a green strength of at least <NUM>, <NUM>, <NUM>, and/or <NUM> kPa (<NUM>, <NUM>, <NUM>, and/or <NUM> psi). The green interconnector <NUM> may have an average green density within the flow field of at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>/cc and/or less than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>. The green density may be about <NUM>/cc on average in the flow field (where <NUM>/cc is equal to <NUM>/cm<NUM>).

If lubricant <NUM> was used, the green interconnector <NUM> may be delubricated in air at between <NUM> and <NUM> (e.g., about <NUM>) for <NUM> to <NUM> hours to substantially remove the lubricant <NUM> and form a delubricated green interconnector <NUM>. However, depending on the lubricant <NUM> properties and content and the size and dimensions of the green interconnector <NUM>, alternative temperatures and/or delubricating times may be used.

The delubricated green interconnector <NUM> (or green interconnector <NUM> if lubricant was not used) is then sintered to form a sintered interconnector <NUM>. The delubricated green interconnector <NUM> may be sintered in a furnace maintained within a sintering temperature range (e.g., at temperatures that are at least <NUM> and/or <NUM> and are less than <NUM>, <NUM>, and/or <NUM>) over a sintering cycle time that is between <NUM> minutes and <NUM> hours, <NUM> minutes and <NUM> hours, and/or <NUM> and <NUM>½ hours to metallurgically bond the chromium and iron particles together and diffuse the chromium into the iron. The sintering cycle time may be less than <NUM>, <NUM>, and/or <NUM> hours at the sintering temperature range. The sintering environment may comprise at least <NUM>%, at least <NUM>%, and/or up to <NUM>% H<NUM>. The delubricated green interconnector may be sintered for a cycle time of <NUM> minutes in a furnace with a sintering temperature that ranges from <NUM> to <NUM> over the course of the <NUM> minute cycle in a sintering environment that comprises about <NUM>% H<NUM> and about <NUM>% Ar. Sintering may be carried out in a pusher furnace with two sealed exit doors and at least <NUM> zones of thermal control.

For a given chemistry interconnector (e.g., <NUM>% chromium / <NUM>% iron), coarser iron particles result in fewer chromium/iron contact points through which diffusion can occur. Coarser iron particles also result in longer pathways into the center of each iron particle. The fewer contact points and longer pathways typically require high sintering temperatures (e.g., over <NUM>) and/or longer sintering times to achieve the desired diffusion levels and associated target coefficient of thermal expansion (CTE) levels. Higher sinter temperatures and longer processing times tend to result in higher manufacturing costs. In contrast, the use of smaller iron particle sizes according to various embodiments facilitates lower sintering temperatures and sintering times while still achieving desired diffusion/CTE levels.

The atmosphere flow in the sintering furnace may reduce the surface iron and chromium oxides, which are barriers to diffusion, allowing particle bonding and diffusion to proceed.

A thermal profile of the sintering step may result in a level of chromium into iron diffusion of at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>% throughout the sintered interconnector <NUM>. The sintering may result in <NUM>-<NUM>% diffusion of the chromium into the iron.

The sintered interconnector <NUM> may have an average density in the flow field of at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>/cc, and/or less than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>/cc. The sintered density may be about <NUM>/cc on average in the flow field. Some densification may be achieved through sintering (e.g., a <NUM>-<NUM>% density increase from the green interconnector density).

The sintering process may result in a sintered interconnector <NUM> with a nitrogen content of less than <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>%. The low nitrogen content may prevent or limit distortion of the final interconnector <NUM>. Nitrogen content of the interconnector may be reduced by reducing the nitrogen content of the atmosphere to which the interconnector is exposed (e.g., before, during, or after sintering).

Because SOFCs experience a wide temperature range during use (e.g., from startup, through operation, and then through shutdown), it is typically preferable for the final interconnector <NUM> to have a coefficient of thermal expansion (CTE) that is about equal to the CTE of the electrolyte plate <NUM> so that they synchronously expand and contract during startup, operation, and shutdown of the SOFC stack <NUM>. The combination of chromium/iron ratio and sintering protocol (which controls the resulting degree of chromium-into- iron diffusion) may impact the resulting CTE of the final interconnector <NUM>. Consequently, the chromium/iron ratio and sintering protocol may be tailored to match the CTE of the final interconnector <NUM> with the CTE of electrolytes commonly used in SOFCs. For example, an interconnector <NUM> with a <NUM>% chromium / <NUM>% iron content and over <NUM>% diffusion has a CTE that is well suited to one or more commonly used types of electrolyte plates <NUM>.

The sintered interconnector <NUM> is thermally stabilized and sealed by oxidation at oxidation temperatures of between <NUM> and <NUM> (e.g., at least <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>, and/or between <NUM> and <NUM>, and/or less than <NUM>, <NUM> and/or <NUM>) for at least <NUM>, <NUM>, <NUM>, and/or <NUM> hours and less than <NUM>, <NUM>, <NUM>, and/or <NUM> hours. Oxidation may begin to take place at a reasonably fast rate at temperatures of <NUM> and above. Oxidation may be carried out by keeping the sintered interconnector <NUM> in a <NUM> oxidation environment for <NUM>-<NUM> hours. <FIG> illustrates an oxidation process in which the furnace atmosphere to which the interconnectors are exposed ramps from ambient temperature (e.g., <NUM>) to <NUM> over <NUM> hours. The environment is maintained at about <NUM> for <NUM> hours. The environment is then ramped back down to ambient temperature over about <NUM> hours.

As shown in <FIG>, sintered interconnectors <NUM> may be oxidized in a continuous process in which the sintered interconnectors <NUM> are stacked on a furnace mesh belt <NUM> on ceramic setters that may help to maintain flatness of the resulting final interconnectors <NUM>. A controlled atmosphere <NUM> (described in greater detail below) may be fed into the oxidizing furnace <NUM> in the direction that the mesh belt <NUM> and sintered interconnectors <NUM> flow to provide the reaction gas (oxygen) to the environment around the interconnectors <NUM> within the furnace <NUM>. Such concurrent flow direction may help facilitate oxidation as the interconnectors <NUM> heat up (e.g., between <NUM> and <NUM>) and before nitridation might otherwise take over at higher temperatures (e.g., at or above <NUM>). Such concurrent flow may additionally or alternatively improve the oxidation cycle by moderating the temperatures to which the interconnectors are exposed (e.g., perhaps by causing the temperatures experienced by the interconnectors in the beginning of the oxidation cycle to ramp up more slowly and/or uniformly). Alternatively, the interconnectors <NUM> may be oxidized in a batch furnace instead of a continuous flow furnace. The controlled atmosphere <NUM> may be fed through the batch furnace over the course of the oxidation batch process to maintain an available supply of oxygen for the oxidation process.

Alternatively, the controlled atmosphere <NUM> may be provided to the furnace <NUM> in a counter flow direction, rather than a concurrent flow direction. In various counter flow embodiments, the controlled atmosphere enters the furnace <NUM> at or around the portion of the furnace <NUM> where the oxidized interconnectors <NUM> exit, and exhausts out of the furnace <NUM> at or around the portion of the furnace where the sintered interconnectors <NUM> enter the furnace <NUM>). This alternative counter flow process is similar to the process shown in <FIG>, but with arrows <NUM> and <NUM> shown in flipped directions and positions, and the humidifier <NUM> being repositioned accordingly.

The controlled atmosphere <NUM> may be fed into the furnace <NUM> continuously throughout the entire oxidation cycle starting as soon as the interconnectors <NUM> are initially fed into the furnace <NUM>. Alternatively, the controlled atmosphere <NUM> is only fed into the furnace <NUM> while the interconnectors <NUM> are exposed to an oxidizing temperature environment (e.g., when the interconnectors are exposed to an environment with a temperature that is above <NUM>, <NUM>, and/or <NUM>).

As shown in <FIG>, when oxidizing sintered interconnectors <NUM> in ambient air (e.g., air with about <NUM>-<NUM>% water vapor content) it has been observed that nitrides of chromium can form within the internal microstructure. After oxidation of a porous Cr alloy interconnector in ambient air the microstructure tends to show areas of enrichment of nitrogen in the areas surrounding the pores and also within the inner material grain boundaries as shown in <FIG> after exposure to the oxidation process in ambient air.

The formation of nitrides is a result of the combination at elevated temperature of the Cr base metal and nitrogen contained in the ambient air. The amount of such nitrides in the interconnector is preferably reduced. Thus, one or more embodiments of the present invention provide an oxidation process for porous chromium components (e.g., PM components such as interconnectors) that reduces and/or minimizes the formation of nitrides in the component. Reducing the extent of nitride formation in the interconnector may increase the overall interconnector yield during manufacturing (e.g., because more of the interconnectors <NUM> are within dimensional tolerances) and may result in interconnectors with improved life-long dimensional accuracies during use in an SOFC stack. Methods of reducing nitrogen absorption have been suggested in the literature for fully dense Cr materials, for example Michalik used a mixture of (<NUM>) nitrogen with <NUM>% H<NUM>O, and <NUM>% H<NUM>, or (<NUM>) nitrogen with <NUM>% H<NUM>O to supress nitride formation. However, as shown in <FIG>, those methods were found to be ineffective when applied to porous Cr alloys (e.g., PM interconnectors) where nitrogen content after oxidation actually increased to in excess of <NUM> wt%. See <NPL>).

As shown in <FIG>, according to one or more examples of a method not defined by the present invention, oxidation in a nitrogen-free Argon/Oxygen mixture may maintain nitrogen content to the pre-oxidized level of around <NUM>%. Accordingly, various examples of methods not in accordance with the invention utilize a substantially nitrogen-free Ar/O atmosphere during the oxidation process. However, the use of an Ar/O atmosphere is not always practical due to high cost of process atmosphere or the need to use complex and costly manufacturing equipment with atmosphere recycling capability.

According to embodiments of the invention, the interconnectors <NUM> are oxidized in a controlled atmosphere <NUM> comprising ambient air <NUM> and an elevated level of water-vapor <NUM>. According to various embodiments, as shown in <FIG>, the controlled atmosphere <NUM> is created by humidifying ambient air <NUM> in a humidifier <NUM> to create the controlled atmosphere <NUM>. According to various embodiments, the water vapor content of the controlled atmosphere <NUM> that is pumped into the furnace <NUM> during the oxidation step comprises ambient air <NUM> with a water vapor content (by volume) of at least <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>%, less than <NUM>%, <NUM>%, and/or <NUM>%, and or between <NUM>% and <NUM>%, between <NUM>% and <NUM>%, and/or between <NUM>% and <NUM>%. According to one or more embodiments the water vapor content in the controlled atmosphere is <NUM>%. In accordance with the invention, the water vapor content in the controlled atmosphere is at least <NUM>%. The controlled atmosphere <NUM> according to the method of the invention has been found to provide an effective means of controlling/limiting the final nitrogen content in the oxidized Cr alloy. According to various embodiments, the ambient air <NUM> to which the water vapor <NUM> is added already includes (by volume %):.

The amount of water vapor <NUM> to be added to the ambient air <NUM> will depend on the starting humidity of the ambient air <NUM>. According to various embodiments, less water vapor <NUM> is added to more humid air <NUM> to create the controlled atmosphere.

According to various methods, some of which not falling within the scope of the invention as defined by appended claim <NUM>, the controlled atmosphere <NUM> comprises:.

According to the method of the invention, the controlled atmosphere comprises at least <NUM>% nitrogen, at least <NUM>% oxygen, and at least <NUM>% water vapor, by volume.

Unless otherwise specifically stated, all atmospheric percentages are volume percentages based on the atmosphere being at standard ambient temperature and pressure (SATP) (i.e., <NUM> and <NUM> kPa). All atmospheric percentages may alternatively be considered to be molar percentages at SATP. Thus, according to various embodiments, the controlled atmosphere comprises <NUM>-<NUM>%, <NUM>-<NUM>%, <NUM>-<NUM>%, <NUM>-<NUM>%, and/or about <NUM>% water vapor (H<NUM>O) by volume and/or by molar concentration. In accordance with the invention, the controlled atmosphere comprises at least <NUM>% water vapor (H2O) by volume. According to various embodiments, the controlled atmosphere <NUM> being injected into the furnace <NUM> is actually injected at approximately SATP, such that the volume percentages may be measured as they are injected into the furnace <NUM>. According to alternative embodiments, the controlled atmosphere <NUM> may be injected into the furnace <NUM> at other temperatures or pressures (though the atmospheric percentages are still measured at SATP).

The water vapor content of the controlled atmosphere <NUM> may alternatively be measured in terms of dew point. According to various embodiments, the dew point of the controlled atmosphere <NUM> (at standard ambient pressure of <NUM> kPa) is at least <NUM>, <NUM>, <NUM>, and/or <NUM>, and/or between <NUM> and <NUM>, between <NUM> and <NUM>, between <NUM> and <NUM>, between <NUM> and <NUM>, between <NUM> and <NUM>, and/or about <NUM>.

According to various embodiments, the ambient air <NUM> may be altered in other ways in addition to having water vapor <NUM> added to form the controlled atmosphere <NUM>. For example, oxygen may also be added to the ambient air <NUM> to form the controlled atmosphere <NUM>. Added oxygen may increase the oxidation rate and allow a reduction in the oxidation cycle time.

According to various embodiments, the nominal flow rate of the controlled atmosphere <NUM> into the furnace <NUM> during the oxidation process is <NUM> cubic feet per hour per inch of furnace belt <NUM> width, with a minimum of <NUM> cubic feet per hour per inch of furnace belt <NUM> width and a maximum of <NUM> cubic feet per hour per inch of furnace belt <NUM> width (where <NUM> cubic foot per hour per inch is <NUM>. 097e-<NUM><NUM>/s). According to various embodiments, the controlled atmosphere <NUM> is fed into the furnace <NUM> during the oxidation process at at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM> cubic feet per hour per inch of furnace belt <NUM> width, and/or between <NUM> and <NUM>, between <NUM> and <NUM>, between <NUM> and <NUM> cubic feet per hour per inch of furnace belt <NUM> width. According to one or more embodiments, the furnace belt <NUM> is <NUM> (<NUM> inches) wide. According to one or more embodiments, an array of <NUM> sintered interconnectors <NUM> are stacked 3x3x3 on ground alumina setter plates and then oxidized to form the final interconnectors <NUM>. According to one or more alternative embodiments, the sintered interconnectors <NUM> are stacked <NUM> high and three across the mesh belt.

According to various embodiments, the desired humidification is accomplished using a humidifier <NUM> with a <NUM>/s (<NUM> lb. /hour) capacity to support a <NUM> cubic feet per hour flow of the controlled atmosphere <NUM> into the furnace (where <NUM> cubic foot per hour, of cfh, is <NUM>. 866e-<NUM><NUM>/s). According to one or more embodiments, the controlled atmosphere <NUM> is fed into the furnace <NUM> at a rate of at least <NUM>, <NUM>, <NUM>, <NUM> cubic feet per hour (cfh), and/or between <NUM> and <NUM> cfh, between <NUM> and <NUM> cfh, and/or between <NUM> and <NUM> cfh.

As shown in <FIG>, after flowing into the furnace <NUM> and providing reaction gas for the oxidation step, the used controlled atmosphere <NUM> (less used reaction gas and other lost components) is exhausted from the furnace <NUM> as exhaust gas <NUM> where the belt <NUM> exits the furnace <NUM>. According to various embodiments, the exhaust gas <NUM> may be recycled and reinjected (e.g., by re-humidifying the exhaust gas <NUM> to form the controlled atmosphere <NUM>).

According to one or more embodiments, the manufacturing process results in the final interconnector <NUM> having a nitrogen content after the oxidation step of no more than <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, <NUM>%, and/or <NUM>%. As shown in <FIG>, after oxidation in the controlled atmosphere containing water vapour, the resulting nitrogen content is substantially reduced relative to the values observed after oxidation in ambient air. The measured nitrogen content is similar to that seen when oxidized in the nitrogen free argon/oxygen atmosphere. According to various embodiments, the oxidation process increases the nitrogen content of the interconnector by less than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM> and/or <NUM> wt % of the final interconnector <NUM>.

According to various embodiments, the oxidation step results in the formation of an oxide layer on the surface of the interconnector, wherein the oxide (e.g., chromium oxide, Cr<NUM>O<NUM>) is at least <NUM>, <NUM>, and/or <NUM> thick and/or between <NUM> and <NUM> thick.

According to various embodiments, the oxidizing step results in the final interconnector <NUM>. According to various embodiments, the final interconnector <NUM> has an average flow field density of at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>/cc, and/or less than <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>/cc. According to one or more embodiments, the final interconnector <NUM> has an average density within the flow field of about <NUM>/cc. According to one or more embodiments, the final interconnector <NUM> is flat to within <NUM>, <NUM>, and/or <NUM> microns. According to various embodiments, an overall thickness of the interconnector plate <NUM> is between <NUM> and <NUM> (depending on the embodiment), with a thickness variation of <NUM>, <NUM>, <NUM>, and/or <NUM> microns maximum (not including in the depressions <NUM>, <NUM>').

According to various embodiments, the final interconnector <NUM> is subjected to further manufacturing steps (e.g., coatings, etc.) before being used in the SOFC stack <NUM>.

While the above oxidation process is described with respect to particular interconnectors, the oxidation process may additionally or alternatively be used on a wide variety of other components without deviating from the scope of the present invention as defined by the appended claims. For example, the above described oxidation process may be used with interconnectors made using other manufacturing techniques (e.g., interconnectors made using double-press manufacturing techniques). The oxidation process according to one or more embodiments of the present invention may be used to oxidize/passivate porous PM components (e.g., high chromium content PM components).

Conversely, while the interconnector manufacturing process is described as using various particular oxidation steps, the manufacturing methods in accordance with the invention and resulting interconnectors <NUM> may alternatively be made using any other suitable steps, where in accordance with the invention as defined by appended claim <NUM>.

According to various embodiments, iron particle size, chromium particle size, density, surface oxidation, and/or other aspects of the manufacturing process make the interconnector <NUM> impermeable to air from the cathode side <NUM> and fuel from the anode side <NUM>. According to various embodiments, the final interconnector <NUM> thereby provides the dimensional accuracy, impermeability, and CTE that are suited for good function as an SOFC interconnector <NUM>.

According to various embodiments, the final interconnector <NUM> consists essentially of chromium and iron. According to various embodiments, chromium and iron comprise at least <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, <NUM>, and/or <NUM>% of the interconnector <NUM>.

As used herein, the term "impermeable to SOFC fuel and air" and similar terms means impermeability as that term is understood in the SOFC interconnector art. SOFC interconnector impermeability does not require absolute impermeability to fuel and air. Rather, "impermeable" merely requires the interconnector to be sufficiently impermeable to provide good function to an SOFC without failure over an extended period of time.

While embodiments of the invention have been described above with respect to SOFC interconnectors <NUM>, embodiments of the invention may also be applied to other types of components. Various embodiments are particularly applicable to components in which a high density and/or impermeability is desired and/or components with complex finished shapes.

Claim 1:
A method of oxidizing a porous component comprising at least <NUM> weight % chromium, the method comprising:
forming the porous component by:
compacting a powder blend to form a green component; and
sintering the green component to form the porous component;
oxidizing the porous component in a furnace so as to expose the component to an oxidation temperature range for a predetermined time period; and
during said oxidizing, feeding a controlled atmosphere into the furnace,
wherein the controlled atmosphere comprises:
at least <NUM> volume % nitrogen,
at least <NUM> volume % oxygen, and
at least <NUM> volume % water vapor;
wherein said oxidizing increases a nitrogen content of the porous component by less than <NUM> weight %.