Title:
Processing of powders of a refractory metal based alloy for high densification
Kind Code:
A1


Abstract:
A powder metallurgy method of making a chromium base alloy includes blending a first powder comprising a chromium powder and a second powder comprising at least one of titanium, titanium hydride, zirconium or zirconium hydride, annealing the first powder and the second powder in a reducing atmosphere after the step of mixing, compacting a blend of the first and the second powders, and sintering the compacted blend to form a chromium base alloy. The chromium alloy may be used as an interconnect for a solid oxide fuel cell, and includes least one of iron or nickel greater than zero and equal to or less than 7 weight percent, yttria greater than zero and equal to or less than 2 weight percent, at least one of titanium or zirconium greater than zero and equal to or less than 1 weight percent and at least 90 weight percent chromium.



Inventors:
Sreedhara, Sudhakara Sarma (Hyderabad, IN)
Sundaresan, Ranganathan (Hyderabad, IN)
Application Number:
11/898065
Publication Date:
03/12/2009
Filing Date:
09/07/2007
Assignee:
BLOOM ENERGY CORPORATION
Primary Class:
Other Classes:
419/31, 419/32
International Classes:
C22C27/06; B22F1/00
View Patent Images:



Primary Examiner:
KESSLER, CHRISTOPHER S
Attorney, Agent or Firm:
THE MARBURY LAW GROUP, PLLC (RESTON, VA, US)
Claims:
What is claimed is:

1. A powder metallurgy method of making a chromium base alloy, comprising: blending a first powder comprising a chromium powder and a second powder comprising at least one of titanium, titanium hydride, zirconium or zirconium hydride; annealing the first powder and the second powder in a reducing atmosphere after the step of mixing; compacting a blend of the first and the second powders; and sintering the compacted blend to form a chromium base alloy.

2. The method of claim 1, wherein the step of annealing is conducted before the step of compacting.

3. The method of claim 1, wherein the step of annealing is conducted after the step of compacting.

4. The method of claim 1, wherein the step of annealing is conducted before and after the step of compacting.

5. The method of claim 1, wherein the step of annealing is conducted in a temperature range of about 800 to about 1200° C. in a hydrogen atmosphere.

6. The method of claim 1, wherein the first powder comprises chromium powder having a bimodal distribution comprising a blend of coarse and fine chromium powders.

7. The method of claim 1, wherein the blend contains 1 weight percent or less of the second powder.

8. The method of claim 1, further comprising blending the first powder and the second powder with at least one of iron powder, nickel powder and yttria powder to form the blend.

9. The method of claim 8, wherein the annealing is conducted after the step of blending the first powder and the second powder and before the step of blending the first powder and the second powder with at least one of iron powder, nickel powder and yttria powder.

10. The method of claim 8, wherein the yttria powder has an average particle diameter of less than one micron.

11. The method of claim 8, wherein the blend comprises at least one of iron and nickel powder greater than zero and equal to or less than 7 weight percent, yttria powder greater than zero and equal to or less than 2 weight percent, at least one of titanium, titanium hydride, zirconium or zirconium hydride powder greater than zero and equal to or less than 1 weight percent and at least 90 weight percent chromium powder.

12. The method of claim 1, wherein: the step of compacting comprises cold compacting at least the first powder and the second powder; and the step of sintering is conducted in an atmosphere containing hydrogen at a temperature of about 1300 to about 1500° C.

13. The method of claim 1, wherein the sintered alloy comprises an interconnect for a solid oxide fuel cell, such that the interconnect has a coefficient of thermal expansion from 30° C. to 1000° C. of between about 11×10−6/° C. and about 13×10−6/° C.

14. The method of claim 13, further comprising providing the interconnect into a solid oxide fuel cell stack.

15. A powder metallurgy method of making a chromium base alloy, comprising: blending a first powder comprising a chromium powder and a second powder comprising yttria powder having an average particle size of less than 1 micron; compacting a blend of the first and the second powders; and sintering the compacted blend to form a chromium base alloy.

16. The method of claim 15, further comprising: blending the first powder and a third powder comprising at least one of titanium, titanium hydride, zirconium or zirconium hydride before, after or during the step of blending the first powder and the second powder; and annealing at least the first powder and the third powder in a reducing atmosphere after the step of blending the first powder and the third powder.

17. A chromium alloy interconnect for a solid oxide fuel cell comprising least one of iron or nickel greater than zero and equal to or less than 7 weight percent, yttria greater than zero and equal to or less than 2 weight percent, at least one of titanium or zirconium greater than zero and equal to or less than 1 weight percent and at least 90 weight percent chromium.

18. The alloy interconnect of claim 17, wherein the alloy comprises 2 to 5 weight percent iron and 0.25 to 2 weight percent nickel.

19. The alloy interconnect of claim 17, wherein the alloy comprises 3 to 7 weight percent of either iron or nickel.

20. The alloy interconnect of claim 17, wherein the interconnect is located in a solid oxide fuel cell stack and the interconnect comprises gas flow channels.

Description:

FIELD OF INVENTION

The invention relates to a chromium alloy in general and to an alloy for use as an interconnect in high temperature fuel cell systems, such as solid oxide fuel cell (SOFC) systems and methods of making thereof.

BACKGROUND OF THE INVENTION

One of the major constraints to producing cost effective SOFC systems or stacks is the cost of functional interconnects. In planar SOFC stacks, a planar or plate shaped interconnect is located between adjacent SOFCs. The interconnect provides reactant gas separation and containment, mechanical support to the cells, and a low resistance path for electrical current between adjacent SOFCs. Moreover, the reactant gas flow channels on either side of the interconnect are designed to ensure distribution of the fuel and the oxidant with minimal pressure drop in the overall SOFC stack, especially in respect to the air flow channels of the interconnect because of the relatively high air flow rates employed to dispose of heat from the stack. In addition, each interconnect within the stack should be resistant to deleterious reactions (such as corrosion), free from interconnected porosity to ensure separation of the reactant fuel gases on the one side and oxidant on the other, and should possess a coefficient of thermal expansion (CTE) compatible with those of the SOFC electrode and electrolyte materials in order to minimize the effects of displacement caused by differential thermal expansion.

Due to the high temperatures of operation of the SOFC system, initially ceramic materials such as lanthanum chromite had been used as interconnects. While these materials had provided some amount of success, the cost had been prohibitive, and the reliability in the various aspects of their function had been less than adequate. While electronic conductivity in the interconnect must be high for its function, most of the ceramic materials employed possess high ionic conductivity. They are also prone to chemistry changes during their life cycle due to loss of oxygen ions in the reducing atmosphere of fuel, such as hydrogen. Many of the alternatives considered among the ceramic materials also possess unacceptable values of CTE.

Metallic materials, particularly wrought alloys, such as nickel base superalloys and high temperature stainless steels, considered for interconnects in SOFC systems can provide pore-free structure. However, many of these fail on account of inadequate strength at the temperatures of operation of the SOFC, typically 700 to 1000° C., or by growth of an oxide layer that constrains electrical conductivity. Chromium based alloys have been used as interconnects with success since the oxide layer that grows on the chromium alloys at the SOFC operating temperatures is actually conducting. However, there are metallurgical limitations to producing such alloys by any process involving casting and subsequent metal working. These alloys are more amenable to production by powder metallurgy (PM) processing. Conventional PM processing comprising of consolidation by compaction of the powder, typically in a die system, followed by a high temperature treatment of sintering generates porous materials. In order to remove pores, these alloys may be manufactured by a process of hot consolidation, such as hot pressing, hot isostatic pressing of canned powder, hot rolling of canned powder, etc. All such hot consolidation processes, however, lead to slow production rates and result in excessively high costs.

Chromium based alloys provide advantages in their application as interconnects because it is possible, by judicious alloying, to realize a CTE in these alloys compatible with the other components of SOFC. The oxide layer that forms under high temperature oxidation conditions prevalent in the SOFC can be self limiting, and further the oxide is electrically conducting. In view of these factors, chromium based alloys are among the most attractive systems for use in SOFC.

In view of the favorable cost consideration, it is desirable to incorporate a processing sequence for the material that is less complex and less expensive than the hot consolidation processes for such a chromium alloy. However, the simple, comparatively inexpensive PM processing of cold compaction in a die followed by high temperature sintering has the limitation of leading to a material that is porous in nature and therefore generally unacceptable for use as an interconnect.

SUMMARY

One embodiment of the invention provides a powder metallurgy method of making a chromium base alloy, comprising blending a first powder comprising a chromium powder and a second powder comprising at least one of titanium, titanium hydride, zirconium or zirconium hydride, annealing the first powder and the second powder in a reducing atmosphere after the step of mixing, compacting a blend of the first and the second powders, and sintering the compacted blend to form a chromium base alloy.

Another embodiment of the invention provides a chromium alloy interconnect for a solid oxide fuel cell, comprising least one of iron or nickel greater than zero and equal to or less than 7 weight percent, yttria greater than zero and equal to or less than 2 weight percent, at least one of titanium or zirconium greater than zero and equal to or less than 1 weight percent and at least 90 weight percent chromium.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a micrograph of the morphology of a ready-to-press powder blend of the alloy Cr—Fe—Ni—Ti-nanoY2O3 made according to the embodiments of the invention. The bimodal distribution of the chromium powder is visible in the figure.

FIG. 2 is a plot of percent green density versus temperature which shows the effect of annealing the powder with the addition of cobalt or titanium on its compressibility.

FIG. 3 is a micrograph which shows the nano yttria powder used in the powder blend made according to the embodiments of the invention.

FIG. 4 shows a particle of base chromium powder with other finer particles embedded in it (identified as nickel, Y2O3, and iron) made according to the embodiments of the invention.

FIG. 5(a) shows a scanning electron micrograph of blended powder containing nano Y2O3 while FIG. 5(b) shows an EDS pattern for Y in the scan showing uniform distribution of Y2O3.

FIG. 6 is a plot of mass change percent versus temperature which illustrates a comparison of the oxidation resistance of four alloys measured by thermogravimetry.

FIG. 7 shows the effect of lubricant addition on the compressibility of the powder made according to the embodiments of the invention.

FIG. 8 shows the dilatogram of the sintered alloy made according to the embodiments of the invention indicating the CTE at different stages.

DETAILED DESCRIPTION OF THE EMBODIMENTS

One or more embodiments of the invention provide ready-to-press powders of a chromium base alloy that can result in high compacted density, thus enabling high sintered density with minimum interconnected porosity in the sintered component. Additional embodiments of the invention provide a high oxidation resistant chromium based alloy such that the alloy may be applied in the high temperature environment in SOFC. Preferably, the chromium base alloy of the embodiments of the invention has high compacted density and high oxidation resistance.

The high compacted density in the powder blend may be achieved by selecting a desired particle size distribution in the base chromium powder by blending of different powder size fractions. Furthermore, the hardness of the base chromium powder is reduced by including an element or compound, such as titanium, zirconium and/or their hydride that has a two-way reaction with hydrogen at elevated temperatures in the powder blend followed by a thermochemical treatment by annealing in a hydrogen atmosphere.

In another embodiment, the alloy's oxidation resistance is improved by incorporation of yttrium oxide (also referred to as yttria) in the alloy. More specifically, the oxidation resistance is enhanced by the inclusion of nano sized yttria powder rather than yttria powders in the micron size range.

The alloy can be used as an interconnect in a SOFC system. Alloying elements, such as iron and/or nickel may be added to the alloy to provide a CTE which is compatible with that of the zirconia, such as yttria or scandia stabilized zirconia or the SOFC electrolyte.

The chromium and alloying element powder blend is amenable to cold compaction in a die arrangement commonly employed in powder metallurgy processing. Such compaction made with the application of a compaction pressure in any suitable range, such as less than 800 MPa, such as about 700-750 MPa, leads to a green density of the compact of over 90% theoretical. Green compacts made from the powder blend may be sintered in the temperature range 1300-1500° C. in an atmosphere containing dry hydrogen to result in a sintered body that can be used as an interconnect or in other suitable applications.

In a first embodiment, the starting chromium powder has a bimodal distribution and comprises a blend of coarse and fine chromium powders (i.e., a blend of two chromium powders, one having a larger average size or diameter than the other one). This bimodal powder particle size distribution in a powder mass improves densification in cold compaction. In one example of the first embodiment, two batches of chromium powder from two different sources with different particle size characteristics as given in Table 1 were mixed in different proportions and subjected to compaction. The coarse powder is labeled “W” and the fine powder is labeled “D” in Table 1. Furthermore, Table 1 provides the general size ranges for each mesh size as well as the actual values used in the example.

TABLE 1
Chromium powder size distribution characteristics
Fraction (mesh size)Coarse Powder “W”Fine Powder “D”
−100 + 150 0-5%, such as 3.7%
−150 + 20035-50%, such as 41.9%
−200 + 32540-50%, such as 44.5% 0-10%, such as 4.2%
−325 0-10%, such as 9.2%90-100%, such as 95.8%

An idealized size distribution in powders for maximum density based on packing of particles into the voids between larger particles was considered. Accordingly a proportion of approximately a third each (30-35%) of particles in the size ranges 74-104 microns, 43-74 microns and <43 microns was realized by blending the two powders W and D in selected proportion. As shown in FIG. 1, the powder blend has an optimum distribution of the chromium powder. The preferred distribution has 0-5% of chromium powder in the size range 104-149 microns, 30-35% in the range 74-104 microns, 25-35% in the range 43-74 microns, and 30-40% of size less than 43 microns.

The effect on green density of the compact made with under a load of 600 MPa when the two powders were blended in different proportions is given in Table 2. The first column in Table 2 lists the sample number, the second column lists the sample composition, with the powder blend components in weight percent, and the last column lists the green density as percent of theoretical.

TABLE 2
Effect of blending chromium powders in the powder blend
on the compact density
SampleGreen Density-
NumberComposition% theoretical
S90(65W + 35D) Cr blend + 5Fe + 1Y2O3 +Poor compact
0.25 Lubricantintegrity
S91(85W + 15D) Cr blend + 4Fe + 0.5Ni +90.76
0.5Ti + 1Y2O3 + 0.25 Lubricant
S92(85W + 15D) Cr blend + 5Ni + 1Y2O3 +90.86
0.25 Lubricant
S93(75W + 25D) Cr blend + 5Fe + 1Y2O3 +91.2
0.25 Lubricant
S94(75W + 25D) Cr blend + 4Fe + 0.5Ti +91.73
0.5Ni + 1Y2O3 + 0.25 Lubricant

As can be seen in Table 2, the two chromium powders, W and D, were blended with powders selected from Fe, Ti, Ni and yttria powders and a lubricant. Table 3 shows the size characteristics of the iron, nickel and yttria powders used in the blends shown in Table 2. The iron powder is classified by mesh size, while the nickel and yttria powders are classified by average particle size as measured by FSSS/XRD.

TABLE 3
Size characteristics of iron, nickel and yttria powders
Fraction (mesh
size)IronNickelYttria
 −80 + 1005 max
−100 + 150 5-10
−150 + 20030-50
−200 + 32510-25
−32515-30
Average particle2.5-3.5 μm15-25 nm
size, FSSS/XRD

In a second embodiment of the invention, the effect of annealing of the chromium powder on the hardness, and hence on the compact density after green compaction was studied. The annealing was carried out at 1000° C. under hydrogen. It was observed that pure chromium powder did not respond to the annealing and no reduction in its hardness was observed. Experiments were carried out on blends of chromium powder with 0.5% addition of powders of cobalt, titanium metal and titanium hydride. Table 4 shows the characteristics of the cobalt, titanium and titanium hydride powders used in the blends. The chromium powder included the bimodal blend of powders W and D from the first embodiment.

TABLE 4
Size characteristics of powders blended
with chromium prior to annealing
Fraction (mesh
size)TitaniumTitanium hydrideCobalt
 +80 0-10
 −80 + 100 5-15
−100 + 14015-20
−140 + 20035-4515-20
−200 + 32540-5015-20
−32510-2020-30
Average particle1.5-2.5 μm
size, FSSS

Table 5 shows the effect of annealing of the blend of chromium powder with these additional powder. It can be seen from Table 5 that blending the chromium powder with the cobalt powder and subjecting the blend to annealing at 1000° C. in hydrogen did not lead to any significant lowering of the hardness of the blend. However, the blends of chromium powder with titanium and titanium hydride showed a decrease in the hardness of the blend after annealing in hydrogen at 1000° C.

TABLE 5
Effect of annealing on hardness of chromium powder
Hardness on particle, HV
Cr + 0.5CoCr + 0.5TiCr + 0.5TiH2
Sl. No.BlendedAnnealedBlendedAnnealedBlendedAnnealed
1214217225260287261
2215213225239287260
3208218225256282250
4195203428241274243
5192212265248289221
6213222239220289248
7209203255228289390
8204199258241313217
9197186238236404258
10 200201254200241238
11 202210221194199366
12 182201263197282250
Average203206258230284267

Without wishing to be bound by a particular theory, the reduction in hardness when chromium powder annealed in the presence of Ti or TiH2 may be attributed to a lowering of the oxygen content of the chromium powder during the hydrogen anneal. Annealing under hydrogen both pure chromium and chromium blended with cobalt powder did not show a reduction in the hardness, since the oxide layer on chromium is not susceptible to reduction under gaseous hydrogen. This is to be expected on the basis of the extremely high stability of chromium oxide. However, in the presence of Ti or TiH2, the condition of hydrogen can be different, since titanium can absorb hydrogen to form a hydride which can also release hydrogen at the annealing temperature, and titanium hydride can dissociate and subsequently continue an absorption-dissociation cycle. Such hydrogen release from cyclic formation of hydride and dissociation would result in the availability of nascent hydrogen in the vicinity of chromium. Such hydrogen as dissociated would be extremely pure with little content of moisture, providing an adequately low dew point (i.e. high H2/H2O ratio) which enables some reduction of chromium oxide, and thereby a lowering of the hardness. Similar effect of nascent hydrogen can also be expected from the addition of Zr and/or ZrH2.

Thus, a powder metallurgy method of making a chromium base alloy of the second embodiment includes blending a first powder comprising a chromium powder and a second powder comprising at least one of titanium, titanium hydride, zirconium or zirconium hydride, annealing the first powder and the second powder in a reducing atmosphere after the step of mixing, compacting a blend of the first and the second powders, and sintering the compacted blend to form a chromium base alloy.

The annealing step may be conducted before and/or after the step of compacting. Preferably, the powder blend is annealed in hydrogen and then compacted. Alternatively, the powder blend may be compacted and then annealed in hydrogen, or the blend may be annealed in hydrogen before and after compacting.

The annealing may be conducted in a temperature range of about 800 to about 1200° C., such as about 1000° C. However, other suitable temperatures may be used. Any suitable hydrogen atmosphere may be used, such as pure hydrogen, forming gas (a mixture of hydrogen and nitrogen), a mixture of hydrogen and a noble gas, such as argon, etc.

Preferably, the first powder is a chromium powder having a bimodal distribution comprising a blend of coarse and fine chromium powders, such as for examples powders D and W described in the first embodiment. However, a single mode distribution chromium powder may also be used.

The step of compacting preferably comprises cold compacting at least the first powder and the second powder. However, other compacting methods may also be used. If desired, a second hydrogen annealing step (which can be referred to as a presintering step) at a temperature of about 800 to about 1200° C., such as about 1000° C., may be conducted after the step of compacting. An optional calibration or sizing step, such as a pressing step, may be added before and/or after the sintering step. The step of sintering is also preferably conducted in an atmosphere containing hydrogen at a temperature of about 1300 to about 1500° C. However, other sintering temperatures may also be used.

While titanium and titanium hydride are described in the examples illustrated in Table 5, other chromium alloying elements which have a two way reaction with chromium, such as zirconium or zirconium hydride, may also be used in addition to or instead of titanium or its hydride. Preferably, the blend contains 1 weight percent or less, such as about 0.5 weight percent of the second powder, such as Ti, Zr or their hydride.

If desired, the chromium powder and the Ti, Zr or their hydride powder may also be blended with at least one of iron powder, nickel powder and/or yttria powder to form the blend. The step of annealing in hydrogen may be conducted after the step of blending the Cr with the Ti, Zr or their hydride powder to form a first blend, and before the step of blending the first blend the with at least one of iron powder, nickel powder and yttria powder. In other words, the chromium and Ti, Zr or their hydride blend may be annealed in hydrogen prior to adding the iron powder, nickel powder and/or yttria powder to the blend. Alternatively, the iron powder, nickel powder and/or yttria powder may be added to the blend before the step of annealing in hydrogen.

The powder blend preferably includes at least one of iron and nickel powder greater than zero and equal to or less than 7 weight percent, yttria powder greater than zero and equal to or less than 2 weight percent, at least one of titanium, titanium hydride, zirconium or zirconium hydride powder greater than zero and equal to or less than 1 weight percent and at least 90 weight percent chromium powder.

For example, the powder blend may include both iron and nickel powder, with the iron powder comprising 2 to 5 weight percent and the nickel powder comprising 0.25 to 2 weight percent, such as for example 4 to 5 weight percent iron and 0.5 to 1 weight percent nickel. Alternatively, the powder blend may contain 3 to 7 weight percent, such as 4 to 5 weight percent of either iron or nickel powder. The powder blend may contain between 0.25 and 2 weight percent, such as between 0.5 and 1.5 weight percent, for example 1 weight percent yttria. Preferably, the yttria powder has an average particle diameter of less than 1 micron. The powder blend may contain between 0.25 and 1 weight percent, such as 0.5 to 0.75 weight percent of at least one of titanium, titanium hydride, zirconium or zirconium hydride powder. The blend preferably contains between 90 and 96 weight percent, such as between 93 and 95 weigh percent, for example 94 weight percent chromium powder. The powder blend may also contain other additives, such as a lubricant (0.1 to 1 weight percent, such as about 0.25 to 0.5 weight percent). The lubricant is removed during subsequent processing and is not included in the sintered alloy composition.

Even if titanium hydride or zirconium hydride is used in the powder blend, due to the subsequent hydrogen anneal, the final sintered chromium alloy should have trace or no hydrogen content. Thus, the sintered chromium alloy preferably contains at least one of iron or nickel greater than zero and equal to or less than 7 weight percent, yttria greater than zero and equal to or less than 2 weight percent, at least one of titanium or zirconium greater than zero and equal to or less than 1 weight percent and at least 90 weight percent chromium.

For example, the sintered alloy may include both iron and nickel, with the iron comprising 2 to 5 weight percent and the nickel comprising 0.25 to 2 weight percent, such as for example 4 to 5 weight percent iron and 0.5 to 1 weight percent nickel. Alternatively, the alloy may contain 3 to 7 weight percent, such as 4 to 5 weight percent of either iron or nickel. The alloy may contain between 0.25 and 2 weight percent, such as between 0.5 and 1.5 weight percent, for example 1 weight percent yttria. The alloy may contain between 0.25 and 1 weight percent, such as 0.5 to 0.75 weight percent of at least one of titanium or zirconium. The alloy preferably contains between 90 and 96 weight percent, such as between 93 and 95 weigh percent, for example 94 weight percent chromium. The shaped alloy interconnect includes gas flow channels or grooves and may have a chromium oxide layer or coating on its surface. The interconnect may be used in a solid oxide fuel cell stack.

FIG. 2 shows the effect of annealing temperature and compacted blend composition on the density of the compact made from the powder blend. Specifically, FIG. 2 shows the compacted density (green density as percent of theoretical) after the powder was subjected to different annealing treatments. Chromium powder, without any additives, did not reflect any improvement in compact density after any annealing temperature in the range 700-1000° C. For example, FIG. 2 shows that a Cr powder compact without Ti or Zr annealed at 800° C. and 1000° C. exhibited a relatively low green density. Likewise, a Cr and Ti compact annealed at only 650° C. also exhibited a relatively low green density. In contrast, significant improvement in compact density was obtained by annealing the Cr and Ti compact at above 800° C., such as at 1000° C. Annealing a compact of Cr and Co at 1000° C. also resulted in an improved green density which was about 1 percent lower than that of the Cr and Ti compact. Thus, annealing at 800° C. and above is preferred.

In a third embodiment of the invention, the chromium powder is blended with nanosized yttria powder. Nanosized yttria includes yttria powder having an average particle size or diameter of less than 1 micron, such as 5 to 100 nm, for example 10 to 30 nm. FIG. 3 shows a scanning electron micrograph of yttria of size in the nanometer size range. The average particle/crystallite size was verified as about 23 nanometer by measurement of the FWHM (full width at half maximum) width of x-ray diffracted peaks following standard practice. The nano yttria powder was blended with chromium-titanium powder annealed as described above along with alloying additions of iron and nickel. The primary benefit from nano yttria powder can be seen in the form of uniformity of blend. FIG. 4 shows chromium powder with other finer particles embedded in it uniformly. The embedded particles were identified as nickel, Y2O3, and iron by EDAX on the selected spots following standard practice. The uniformity of Y2O3 distribution in the overall blend was evaluated by EDS analysis of the powder in a scanning electron microscope. The distribution of the element Y on the total area of powders shown in FIG. 5(a) was seen to be extremely uniform in FIG. 5(b). Such uniformity in the blend is a major advantage since the small percentage of Y2O3 added would cover a large surface area of chromium powders without using mechanical alloying. Such distribution can be achieved in a blend containing coarser (larger than micron) sized powders by using additional mechanical alloying steps which increase process cost and complexity.

The yttria enhanced the high temperature strength and the oxidation resistance of the alloy. The adhesion between scale and alloy is markedly improved and this increases the alloy's resistance to thermal cycling exposure. Furthermore, the actual growth rate of the oxide can also be reduced. Such nano sized oxide particles also provide preferential nucleation sites for oxidation of chromium and reduce oxide growth rate. In addition, the yttria also modifies the chromium oxide layer microstructure, and hence modifies diffusion rates and stresses in the oxide layer.

The powder metallurgy method of making a chromium base alloy according to the third embodiment includes blending a first powder comprising a chromium powder and a second powder comprising yttria powder having an average particle size of less than 1 micron, compacting a blend of the first and the second powders, and sintering the compacted blend to form a chromium base alloy. The chromium powder, compacting conditions and sintering conditions are described in the prior embodiments. The method may also optionally include blending the first powder and a third powder comprising at least one of titanium, titanium hydride, zirconium or zirconium hydride before, after or during the step of blending the first powder and the second powder, followed by annealing at least the first powder and the third powder in a reducing atmosphere after the step of blending the first powder and the third powder according to the process described in the second embodiment.

The effect of yttria on the oxidation rate of the alloy is illustrated in FIG. 6. This figure shows a comparison of the oxidation resistance of the alloy measured by thermogravimetry. Curves (1) and (2) in FIG. 6 pertain to alloys made by the process described in the second and third embodiments of the invention and having the following composition: Cr-4Fe-0.5Ni-0.5Ti-1Y2O3. The alloys corresponding to curves (1) and (2) have a different density because the alloy which corresponds to curve (2) was repressed after sintering while the alloy which corresponds to curve (1) was not repressed. Curve (3) pertains to a fully dense commercial alloy Ducrolloy and curve (4) pertains to a simpler powder metallurgy version of the alloy Cr-5Fe-1Y2O3. This alloy (4) was made by compaction and sintering of powder that did not incorporate the processing refinement of the embodiments of the current invention. The weight gain by oxidation of the four alloys are provided in Table 6 below. The lower weight gain shown by the alloys (1) and (2) made by the process of the embodiments of the present invention confirms a significantly higher oxidation resistance compared to the commercial Ducrolloy alloy (3) with density close to theoretical, even though the alloys (1) and (2) contained some porosity. In alloy (4), the larger porosity level presumably resulted in some breakage of the sample in oxidation as indicated by a sudden decrease in mass followed by increase due to further oxidation of the remnant sample.

TABLE 6
Thermogravimetric readings on mass change on heating
SampleTotal mass
No.Sample Descriptionchange, %
(1)Cr—4Fe—0.5Ni—0.5Ti—1Y2O3 sample2.1
as sintered
(2)Cr—4Fe—0.5Ni—0.5Ti—1Y2O3 sample1.2
sintered and repressed
(3)Ducrolloy Cr—Fe—Y2O3 sample, full density3.9
(4)5Fe—1Y2O3 sample, low densityErratic

It was possible to further enhance the compact density by suitable selection and addition of the lubricant. In the fourth embodiment, the powder blends of the alloy made according to the prior embodiments incorporated either one of the two commercially available lubricants employed widely in powder compaction. The selection of either one of the lubricants, Acrawax C or Kenolube, as shown in FIG. 7, enables the attainment of the projected green density by addition of as low a proportion of the lubricant as 0.25%. In conventional compaction, lubricant additions of about 0.5 to 0.75% are common. In FIG. 7, the first bar shows the density of the compact of Kenolube with CrTi powder, the second bar shows the density of the compact of Kenolube with CrTiH2 powder and the third bar shows the density of the compact of Acrawax C with CrTiH2 powder. The benefits of reducing the amount of lubricant include ease of removal of the lubricant prior to sintering, and the densification of the volume occupied by the lubricant during sintering being reduced to the corresponding extent. Of the two commercial lubricants, Acrawax C and Kenolube, the latter appears to be slightly superior.

FIG. 8 shows the coefficient of thermal expansion of the alloy of the embodiments of the invention as evaluated by dilatometry. The values of CTE obtained in the different temperature ranges are shown in Table 7.

TABLE 7
Coefficient of Thermal Expansion
SampleCoefficient of
NumberSample Compositionthermal expansion
1Alloy30-250° C.: 10.11 × 10−6/
° C.
Cr—4Fe—0.5Ni—0.5Ti—1Y2O330-500° C.: 10.33 × 10−6/
° C.
30-750° C.: 10.85 × 10−6/
° C.
30-1000° C.: 12.07 × 10−6/
° C.
2YSZ (from literature, for10 × 10−6/° C.
comparison)
3LSM (from literature, for11 × 10−6/° C.
comparison)
4Ni-YSZ (from literature, for13 × 10−6/° C.
comparison)

The data for sample numbers 2, 3 and 4 were taken from Azra Selimovic, Miriam Kemm, Tord Torisson, Mohsen Assadi, “Steady state and transient thermal stress analysis in planar solid oxide fuel cells”, Journal of Power Sources 145 (2005) 463-469. Thus, the sintered alloys of the embodiments of the invention have a coefficient of thermal expansion from 30° C. to 1000° C. of between about 1×10−6/IC and about 13×10−6/° C. This value is similar to the values of YSZ electrolytes, LSM cathode electrodes and Ni—YSZ anode electrodes that are widely used in solid oxide fuel cells. Thus, when the alloy is used as a SOFC interconnect, it is CTE matched to the components of the SOFC.

The foregoing description of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The description was chosen in order to explain the principles of the invention and its practical application. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.





 
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